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GENETICS AND EUGENICS

A TEXT-BOOK FOR STUDENTS OF BIOLOGY AND

A REFERENCE BOOK FOR ANIMAL

AND PLANT BREEDERS

BY

W. E. CASTLE

PROFESSOR OF ZOOLOGY IN HARVARD UNIVERSITY AND

RESEARCH ASSOCIATE OF THE CARNEGIE

INSTITUTION OF WASHINGTON

Private Prot:jerfy of

^^ P. METCALF

»yjaaaiait«»i*r*tittfcM

/

CAMBRIDGE
HARVARD UNIVERSITY PRESS

LONDON: HUMPHREY MILFORD

Oxford University Press

1921

COPYRIGHT, 1916
HARV.\ED UNIVERSITY PRESS

First impression issued December, 1916

Second impression issued February, 1917

Third impression issued July, 1917

Second Edition

First impression issued August, 1920
Second impression issued August, 1921

PREFACE

This book is an attempt to present, in a form as simple and
readily intelligible as possible, the subject of heredity, as
related to man and his creatures, the domestic animals and
cultivated plants. To write such a book has been with the
author a long cherished ambition, but one which, as the years
went by, seemed less and less likely of realization, as knowl-
edge of the subject increased and took on more and more
complicated forms. Each year, however, he has been
forced by his responsibilities as a teacher, to make, for
students having only an elementary knowledge of biology,
an analysis and summary of our knowledge of this subject
to date. The longer he has continued to do this, the more
fully he has realized that a subject in a state of healthy
growth can never assume a final and finished form. He
makes no apology, therefore, for presenting the subject
with very unevenly and incompletely developed parts.
Such, it must be confessed, is the present state of our knowl-
edge.

It would be a great service to the student to show him
where in his subject positive knowledge stops and specula-
tion, the useful servant but dangerous master in science,
begins. This task, where possible, has been attempted in
this book. But such attempts can of necessity succeed only
partially and for the time being, for it often happens that
the speculation of today becomes the verified theory of
tomorrow. For having guessed right and proved the cor-
rectness of their guesses, we honor in this field the names of
Lamarck, Darwin, Weismann, and Mendel. Others still
living have made contributions of scarcely less importance
but to name them would be invidious. Americans may take
encouragement from the thought that all are not likely to
be named from one side of the Atlantic and later enumera-

W-^

V

19653

iv PREFACE

tions are likely to include names from Pacific lands also.
For advance in science never results merely from brilliant
guesses by the few, but takes place chiefly through the
patient, persistent efforts of numerous workers who test by
observation and experiment every suggested explanation of
the phenomena of nature. This is a task of such magnitude
and such importance that in it the cooperation of all nations
is needed and fortunately is not withheld. To promote the
common good of all is the greatest honor of each.

The author has found that interest in the subject of
heredity is not confined to college classes but is shared by
people of intelligence everywhere, because it touches and
affects the lives of all. The animal breeder and the plant
breeder have an intensified interest in the subject because it
vitally concerns the success or failure of their occupations.
The needs of this wider public have been kept in mind in the
preparation of this book, but it has not been thought neces-
sary to omit on this account discussion of questions re-
quiring thoughtful consideration for their full understanding.
A discussion which evokes no independent thinking, or even
opposition, is not likely to extend knowledge, the teacher's
prime concern.

1 am indebted to many friends and fellow biologists for
assistance in connection with the illustrations, acknowl-
edged in the legends of the figures, to Professor B. M. Davis
for a critical revision of Chapter VI, and to Professor J. A.
Detlefsen for assistance in revising the proofs. My best
thanks are due to the publishers who have spared no effort
to make their part of the work successful.

W. E. Castle.

Cambridge, Massachusetts,
December, 1916.

PREFACE TO SECOND EDITION

Rapid advance in our knowledge of the fundamental prin-
ciples of genetics has made necessary a complete rewriting of
several chapters as originally published and the addition of
several others. The more important changes and additions
relate to the subjects of blending inheritance, the pure line
principle, the nature of genetic changes, their frequency and
location in the germ-cells, linkage, inbreeding, and heterosis.

W. E. C.

March, 1920.

^

CONTENTS

INTRODUCTION

PAGE

3

PART I. GENETICS

I. Darwin's Theory of Evolution and its Evidences 7
11. Contributions of Lamarck, Weismann, and Herbert
Spencer to the Theory of Evolution; Darwin's

Theory of Pangenesis 18

III. Are Acquired Characters Inherited .^ 28

IV. Weismann's Theory of Heredity 47

V. Attempts to Classify and Measure Variation : Biom-
etry b^

VI. The Mutation Theory 71

VII. The Pioneer Plant Hybridizers : the Discovery and

Rediscovery of Mendel's Law 82

VIII. Mendel's Law of Heredity Illustrated in Animal

Breeding 88

IX. Some Mendelian Terms and their Uses 98

X. Calculating Mendelian Expectations 104

XI. Modified Mendelian Ratios; Heterozygous Char-
acters; Atavism or Reversion 109

XII. The Unit-Characters of Rodents l^^a

XIII. Unit-Characters in Cattle and Horses 130

XIV. Unit-Characters in Swine, Sheep, Dogs and Cats 137
XVv Unit-Characters in Poultry and in Plants . . . 145

XVI. Unit-Characters of Insects 154

XVII. Sex-Linked and other Kinds of Linked Inheritance

in Drosophila 159

XVIII. Drosophila Type and Poultry Type of Sex-Linked

Inheritance 164

XIX. Linkage , 107

XX. The Nature of Genes 177

XXI. Are Unit-Characters (Genes) Constant or Variable? 182
XXII. Inheritance of Size and other Quantitative Char-
acters. The Hypothesis of Multiple Factors 190

XXIII. Genetic Changes and the Chromosomes 205

XXIV. Genetic Changes in Asexual Reproduction, in Par-

thenogenesis, AND IN Self-Fertilization . . . 209

vii

Till CONTEXTS

XXV. Genetic Changes in Bisexual Reproduction ... 219

XXVI. Inbreeding and Cross-Breeding 2-27

XXVII. Hybrid A'igor or Heterosis 242

XXVIII. Galton's Law of Ancestral Heredity and his Prin-
ciple of Regression 246

XXIX. Sex Determination 248

TART II. EUGENICS

XXX. Human Crosses 20.5

XXXI. Physical and Mental Inheritance in Man ... 271
XXXII. Heredity of General Mental Ability', Insanity.

Epilepsy', and Feeble-Mindedness 279

XXXIII. The Possibility' and Prospects of Breeding a Better

Human Race C92

APPENDIX. Translation of Mendel's Paper, Experiments

IN Pl.\nt-Hybridisation 313

BIBLIOGRAPHY 355

INDEX 391

GENETICS AND EUGENICS

t

\

INTRODUCTION

Genetics may be defined as the science which deals with the
coming into being of organisms. It does not refer, however,
to the first creation of organic beings, but rather to the pres-
ent and e very-day creation of new individuals or new races.
It refers particularly to the part that parent organisms have
in bringing new organisms into being and to the influence
which parents exert on the characteristics of their offspring.
In this sense it is nearly equivalent to the term heredity.
But logically, though less immediately, it is concerned with
all agencies which in any way affect, condition, or limit the
coming into being of a new organism or a new race. All
physical and chemical changes in the world outside the organ-
ism, or in a word the environment, vitally concern genetics,
though they are the more immediate field of study of other
branches of biology.

Eugenics, from its et;yTiiology, means coming into being well.
It is used at present solely with reference to man, and means
almost literally the science of being well-born. Since man is
zoologically merely one of the higher animals, it is evident
that his reproduction is a very special case falling under the
general laws of genetics, and before we can properly under-
stand this special case we must know something of the general
laws of genetics. We shall therefore turn our attention to
genetics first and foremost, and to eugenics subsequently and
secondarily.

The term Eugenics was proposed by Francis Galton who
defines it thus: — " Eugenics is the study of agencies under
social control that may improve or impair the racial qualities
of future generations, either physically or mentally."

As thus defined it is purely an applied science, for it is con-
cerned only with those agencies which are under social con-
trol and gives no attention to any agencies, however impor-

4 ' GENETICS AND EUGENICS

tant, which are not under social control. Its scope therefore
is much narrower than that of genetics. It is concerned with
only so much of genetics as concerns man, and with only so
much of that as is under social control. To determine what
are the general principles of genetics and to what extent man
is subject to them are primarily biological problems, but to
determine how far these are socially controllable is a problem
for the sociologist, and one which I shall not attempt to
answer without help from sociologists.

The coming into being of a new organism is one of the
least understood of all natural phenomena. Even to the
trained biologist it is largely an unexplained mystery. To
understand his vie^'point concerning it, and what definite
facts he knows about it, and how he attempts to explain
them, we must be familiar with certain of the generalizations
of biology. Familiarity with the more important of these
fundamental generalizations of biology will be assumed in the
present work.

From the philosophical standpoint genetics is only a sub-
division of evolution. For the evolution theory teaches that
the organisms now existing have come into being through
descent with modification from those which existed at an
earlier time and, in general, that the world as we know it today
is different from what it has been at any previous time; that
all things, organic and inorganic, are constantly undergoing
change, yet nothing wholly new comes into being, for every-
thing new arises out of something which existed before. Thus
no new matter is created, yet new creations constantly arise
out of elements which before existed in different form.

It will be our first task to discuss the rise of the evolution
theory and in particular its relation to the subject of genetics.
Subsequently we shall discuss the known facts of genetics and
the several ways in which biologists interpret them; and
finally we shall discuss human evolution as a subdivision of
genetics, and its social control, or eugenics.

PART I

GENETICS

CHAPTER I

DARWIN'S THEORY OF EVOLUTION AND
ITS EVIDENCES

The human mind is characterized above all else by curiosity,
the source of all our wisdom as well as of our woes. This fact
the ancients portray in the tale of Pandora's box. We in-
stinctively seek an explanation of all the phenomena of
nature, unless our natural curiosity has been repressed by
convention or education (falsely so called). We demand a
reason for everything, and if none is forthcoming from an out-
side source, we straightway construct one for ourselves out of
our own imaginings. This is the attitude of mind of the child
whose perpetual " why " and " what " are so distressing to
perplexed parents. It is the attitude of mind in which all
primitive peoples and original thinkers have regarded the
phenomena of nature. It was this attitude of mind which led
to the formulation of the evolution theory, which is an attempt
to explain the present condition of the world in terms of simpler
pre-existing conditions.

When evolution is mentioned, we think of Darwin as its
originator, but in reality he did not originate it; the idea of
organic evolution had often been suggested before his time,
but he proved its reality. The principle of evolution had
long been recognized in relation to inorganic things. In
chemistry, physics, and astronomy, the constancy and inde-
structibility of matter were fully established. It was recog-
nized for example that more complex states of matter, that is,
" chemical compounds," may arise out of the simpler " ele-
ments " by their combination in definite proportions, and
that out of such compounds the elements may by suitable
means be recovered again unchanged and in the original pro-
portions.

7

8 GENETICS AND EUGENICS

In geology, the work of Lyell had shown that the present
condition of the earth's crust had come about gradually
through the action of causes still at work.

Accordingly in all the fundamental sciences which deal
with the inorganic world the reign of natural law was ac-
knowledged before the time of Darwin, and the principle of
miraculous change was no longer offered as an explanation of
existing conditions.

But in the realm of living things it was in Darwin's time
very different. The animal kingdom was not supposed to
have grown, but to have been made outright. The higher
animals were not supposed to have originated from lower
ones but to have been made in the form in which they exist
today. It was Darwin's work which dispelled this outgrown
idea, and established the principle of evolution as an explana-
tion of the organic as well as of the inorganic world. In his
time the idea was so novel as applied to animals and plants
that it aroused the greatest opposition. But the idea was not
wholly new to human thought; in forms more or less fanciful
and incomplete it had been suggested in previous centuries
from the days of the early Greek philosophers on. ^

Darwin lived in a time peculiarly inhospitable to the idea
of organic evolution, partly because of theological, and partly
because of scientific dogma. Had the idea been brought for-
ward centuries before accompanied by proofs such as Darwin
advanced in its support, it undoubtedly would have met more
ready acceptance than it found in the last century. As it
was, Darwin had to make the discovery anew for himself,
largely unaided by his predecessors, who, though they had
formulated more or less clearly the same line of explanation
which he adopted, had failed to put it to the test of long-
continued and detailed observation and experiment, which
alone sufficed firmly to establish it.

1 Professor H. F. Osborn ('94) has described in a most interesting book the
various foreshadowings of the idea of organic evolution which appear in the writings
of Darwin's predecessors, and the development of the idea in Darwin's own mind
as evidenced by his letters and other writings. One interested in the historical and
philosophical growth of the idea cannot do better than to consult Osborn's book.

DARWIN'S LIFE 9

Charles Darwin was born in 1809 and died in 1882. Both
his father and his paternal grandfather were physicians; the
grandfather, Erasmus Darwin, was also a naturalist and phi-
losopher of note, who anticipated many of the evolutionary
ideas of Lamarck and some of those of his own illustrious
grandson.

On his mother's side, Darwin's grandfather was Josiah
Wedgwood, the famous manufacturer of pottery. Francis
Galton, the founder of Eugenics, was his cousin. Those who
consider special tastes and talents hereditary find significance
in these relationships. Thus one biographer, after noting
that Darwin's father had originally intended him for the
Church, continues "but hereditary tendencies toward nat-
ural history led him in another direction." It may fairly be
questioned whether ** tendencies toward natural history "
are hereditary in the strict sense of the word any more than
tendencies toward pottery, which Darwin does not seem to
have manifested though his grandfather was Josiah Wedg-
wood. Such language as I have quoted is quite permissible
on the part of a literary biographer (indeed Darwin speaks in
like vein in his autobiography) but the student of eugenics
must be on his guard against accepting it at its face value.

What Darwin probably inherited was not a " tendency
toward natural history " but a good mind; what subjects
engaged it was probably determined not by inheritance but
by the subjects which came to his attention at the period of
life when men do their best creative thinking. In Darwin's
case, the thing which centered his attention upon the prob-
lem of the origin of species and held it there for the rest of
his lifetime was the famous voyage of the Beagle.

In school Darwin was not a distinguished student. He
attended Edinburgh University for two sessions and then the
University of Cambridge, where he took the B.A. degree in
1831. Shortly after graduation he seized the opportunity to
go as naturalist on the ship Beagle of the English navy, which
was detailed on a voyage of exploration round the world.
This voyage lasted almost five years, from December 27,

10 GENETICS AND EUGENICS

1831, to October 2, 1836. Much time was spent by this
expedition in making surveys of southern South America,
ana of oceanic islands. For a large part of this time Darwin
was brought into intimate daily contact with the animals
and plants of an unexplored part of the world. What a post-
graduate course in natural history this was! It is probably
fortunate that his previous studies of natural history had not
been more specialized and detailed, and that he had no master
at hand to guide him in his studies during the voyage. Other-
wise he would certainly have been hampered by precon-
ceived ideas and have been less inclined to depart from ac-
cepted notions. But here he was face to face with a new world
of animals and plants awaiting explanation, and his it was to
study them without assistance or let up for three years. For
an ordinary boy of twenty-two, what a perplexing and be-
wildering task, what a fate, sentenced to five years of sea-
sickness, the effects of which were to last throughout his life!
But for a Darwin, what an opportunity, to study at first hand
the animals, the plants, the peoples of all lands and of all
seas!

After Darwin had spent some three years on the Beagle he
returned home with impaired health which forced him to live
quietly at his country home in Downs, England. Here he
devoted a part of each day to working up the scientific results
of his journey, and published during the next twenty years an
attempt to correlate, to unify and to explain the various ob-
servations which he had made, an attempt which finally
found fruition in his theory of evolution through natural
selection.

It had long been known to a number of Darwin's scientific
friends that he was working on a theory of evolution when,
in 1858, he received from A. R. Wallace, then in the East
Indies, the manuscript of a paper containing precisely the
same explanation of organic adaptations which he himself
had reached. Darwin was naturally much embarrassed, but
seemed willing to throw aside his own work and give prece-
dence to Wallace's paper. On the advice of friends, however.

DARWIN'S THEORY 11

he submitted to the Linnaean Society of London an abstract
of his own conclusions, which was read and pubhshed simul-
taneously with the paper by Wallace. The work of each
author was so manifestly independent of the other and each
dealt so generously with the other that no rivalry arose
between them, and both were to the last the best of friends.
The essential points in their theory, which Darwin elabor-
ated more fully the following year (1859) in his Origin of
Species, have been summarized thus by Conn (p. 353) :

" 1. Overproduction, All animals and plants tend to
multiply more rapidly than it is possible for them to continue
to exist. More offspring are produced by even the slowest
breeding animals and plants than can possibly find susten-
ance in the world.

" 2. Struggle for existence. As a result of overproduction,
the individuals that are born are engaged in a constant
struggle with each other for the opportunity to live. This
struggle is sometimes an active, sometimes a passive one;
and sometimes it is a struggle with each other for food. It
is a struggle in which only the victors remain alive, the
vanquished being exterminated without living long enough
to leave offspring.

" 3. Variation, or diversity. All animals and plants show
a large amount of diversity among themselves, and, as a
result, some must be better fitted for the struggle for life
than others.

''4. Natural selection, or the survival of the fittest. It is a
logical result of the struggle for existence that only those
individuals best fitted for the struggle will be the ones, in
the long run, to win in the contest. Hence the " fittest "
in the long run will survive, while those less fitted to exist
will be exterminated.

"5. Heredity. By the laws of heredity, individuals trans-
mit to their offspring their own characters. Hence if one
individual survives the struggle for existence by virtue of
some special characteristic, it will transmit this characteristic
to its offspring. The offspring will inherit it, and in the

12 GENETICS AND EUGENICS

course of a few generations the only individuals left alive
will be those that have developed it, while those that did
not develop it will be exterminated by the law of natural
selection."

This theory stands today in the main as Darwin left it,
the chief advances since his time being concerned with one
or other of the two factors, variation and heredity, concern-
ing which our knowledge, though still incomplete, has made
notable advances. But before we pass to the consideration
of these, let us pause to inquire what were the lines of
evidence upon which Darwin relied to establish his theory.

These have been well summarized by T. H. Huxley (1825-
1895) who by his able championship of Darwin's views did
more than any other one man to gain for these views general
recognition and acceptance. As modified by Lock, Huxley's
summary is as follows : —

" 1. The Gradation of Organisms, Both in the animal and
vegetable kingdoms we may trace, in spite of certain gaps, a
long series of gradations in complexity of structure, so that
between the simplest and the most complicated of living
things a great number of intermediate stages are to be found.
When we pass to the lower end of the scale in either case, we
come upon a group of creatures of comparatively simple
organization. Among them we find members with regard
to which we cannot definitely say that they are either animals
or plants. Moreover, these unicellular organisms resemble
in many ways the egg-cell from which every individual among
the higher animals and plants originates.

"2. Embryology. All the members of a particular group of
animals or plants as a rule resemble one another more closely
in the early stages of their individual development than they
do in the adult condition, and in the earliest stages of all they
are often indistinguishable. These facts are explained if we
suppose that such individuals have a common origin, that
they are descended from a common ancestor, and that traces
of their pedigree are still to be observed in the developmental
stages through which each one passes. We do not find a com-

DARWIN'S EVIDENCES 13

plete parallelism between the development of the individual
and the history of the race, nor should we expect to do so,
since embryonic as well as adult stages may be modified in the
course of evolution; what we should expect is a more or less
vague historical sketch, and this is what is usually found
remaining.

"3. Morphology. On comparing together the different
members of one of the great groups or classes of animals or
plants, we find the same fundamental plan of organization
running through all of them. Series of corresponding organs
are often to be made out which are built upon the same
general scheme, although their functions may be quite dis-
similar; so that, for instance, in the hand of a man, the paw
of a dog, the wing of a bat, and the paddle of a whale, almost
identically the same series of bones can be traced. An ob-
vious explanation is to be found in the supposition that these
parts have arisen by the divergent modification of parts
which were originally identical.

"4. Geographical Distribution. Observation shows that
groups of closely allied creatures are often found living in
neighbouring districts, and that when such a barrier as an
ocean or a range of lofty mountains is passed an entirely new
fauna and fiora are usually to be met with. These facts may
be explained by the hypothesis that allied groups of species
originated by a process of descent in the same countries which
they now inhabit, and they can be explained by no other
known hypothesis.

" 5. The Geological Succession of Organisms. The general
facts regarding the distribution of allied species of animals
and plants in time point in precisely the same direction as
those relating to their distribution in space. In a few cases,
notably in that of the extinct horse of North America, a long
chain of possibly ancestral types has been found leading back
to a remote and very different progenitor. This supposed
ancestor of the horse was a creature little larger than a
moderate-sized dog. It had four separate toes to each fore-
limb, and three to each hind-limb, and its teeth were much

14 GENETICS AND EUGENICS

simpler and less specialized than those of existing horses.
The general distribution of organisms throughout the geo-
logical strata agrees, moreover, in a remarkable way with
what is to be expected on the evolution theory.
^ "6. Changes under Domestication. Among domesticated
animals and plants we know of numerous cases in which the
actual origin of new forms has been observed. These have
often differed from their predecessors by amounts quite com-
parable with the differences by which natural species or even
genera are separated. A notable example of this process is
afforded by the numerous breeds of pigeons known to have
arisen under domestication from a single wild species. We
have no reason whatever for supposing that domesticated
species are more mutable than wild species, and there is con-
sequently every reason to believe that changes of a similar
character take place in Nature. The conditions of domesti-
cation, of course, afford much better opportunities of observ-
ing such phenomena.

\ " 7. The Observed Facts of Mutation. Nevertheless, indi-
vidual specimens of particular wild species are frequently
found showing modifications which, if they occurred con-
stantly in an isolated group, would afford a basis for the
description of new species. In a few cases the actual occur-
rence of similar changes has been observed in wild species of
plants.

" We see, therefore, that the evidence in favour of the
existing species of animals and plants, having arisen by a
process of evolution, is of a most ample and convincing kind."

How some of these evidences first presented themselves
to Darwin's mind and how he came later to value them, Dar-
win states in the closing pages of the Introduction to his
Variation of Animals and Plants under Domestication,

WTien I visited, during the voyage of H. M. S. Beagle, the Galapagos
Archipelago, situated in the Pacific Ocean about five hundred miles from
South America, I found myself surrounded by peculiar species of birds,
reptiles, and plants, existing nowhere else in the world. Yet they nearly
all bore an American stamp. In the song of the mocking-thrush, in the
harsh cry of the carrion-hawk, in the great candlestick-like opuntias, I

DARWIN'S OWN ACCOUNT 15

clearly perceived the neighbourhood of America, though the islands were
separated by so many miles of ocean from the mainland, and differed much
in their geological constitution and climate. Still more surprising was the
fact that most of the inhabitants of each separate island in this small
archipelago were specifically different, though most closely related to each
other. The archipelago, with its innumerable craters and bare streams of
lava, appeared to be of recent origin; and thus I fancied myself brought
near to the very act of creation. I often asked myself how these many
peculiar animals and plants had been produced: the simplest answer
seemed to be that the inhabitants of the several islands had descended
from each other, undergoing modification in the course of their descent;
and that all the inhabitants of the archipelago were descended from those
of the nearest land, namely America, whence colonists would naturally
have been derived. But it long remained to me an inexplicable problem
how the necessary degree of modification could have been effected, and it
would have thus remained for ever, had I not studied domestic productions,
and thus acquired a just idea of the power of Selection. As soon as I had
fully realized tliis idea, I saw, on reading Malthus on Population, that
Natural Selection w^as the inevitable result of the rapid increase of all
organic beings ; for I was prepared to appreciate the struggle for existence
by having long studied the habits of animals.

Before visiting the Galapagos I had collected many animals whilst
travelling from north to south on both sides of America, and everjy'where,
under conditions of life as different as it is possible to conceive, American
forms were met with — species replacing species of the same peculiar
genera. Thus it was when the Cordilleras were ascended, or the thick
tropical forests penetrated, or the fresh waters of America searched. Sub-
sequently I visited other countries, which in all their conditions of life were
incomparably more like parts of South America, than the different parts
of that continent are to each other; yet in these countries, as in Australia
or Southern Africa, the traveller cannot fail to be struck with the entire
difference of their productions. Again the reflection was forced on me
that community of descent from the early inhabitants of South America
would alone explain the wide prevalence of American types throughout
that immense area.

To exhume with one's own hands the bones of extinct and gigantic
quadrupeds, brings the whole question of the succession of species vividly
before one's mind; and I found in South America great pieces of tesselated
armour exactly like, but on a magnificent scale, that covering the pigmy
armadillo; I had found great teeth like those of the living sloth, and bones
like those of the cavy. An analogous succession of allied forms had been
previously observed in Australia. Here then we see the prevalence, as if
by descent, in time as in space, of the same types in the same areas; and
in neither case does the similarity of the conditions by any means seem
sufficient to account for the similarity of the forms of life. It is notorious
that the fossil remains of closely consecutive formations are closely allied
in structure, and w^e can at once understand the fact if they are closely
allied by descent. The succession of the many distinct species of the same

16 GENETICS AND EUGENICS

genus throughout the long series of geological formations seems to have
been unbroken or continuous. New species come in gradually one by one.
Ancient and extinct forms of life are often intermediate in character, like
the words of a dead language with respect to its several offshoots or living
tongues. All these facts seemed to me to point to descent with modifica-
tion as the means of production of new species.

The innumerable past and present inhabitants of the world are con-
nected together by the most singular and complex affinities, and can be
classed in groups under groups, in the same manner as varieties can be
classed under species and sub-varieties under varieties, but with much
higher grades of difference. These complex affinities and the rules for
classification, receive a rational explanation on the theory of descent, com-
bined with the principle of natural selection, which entails divergence of
character and the extinction of intermediate forms. How inexplicable is
the similar pattern of the hand of a man, the foot of a dog, the wing of a
bat, the flipper of a seal, on the doctrine of independent acts of creation!
How simply explained on the principle of the natural selection of successive
slight variations in the diverging descendants from a single progenitor ! So
it is with certain parts or organs in the same indi\'idual animal or plant, for
instance, the jaws and legs of a crab, or the petals, stamens, and pistils of a
flower. During the many changes to which m the course of time organic
beings have been subjected, certain organs or parts have occasionally be-
come at first of little use and ultimately superfluous; and the retention of
such parts in a rudimentary and useless condition is intelligible on the
theory of descent. It can be shown that modifications of structure are
generally inherited bj^ the offspring at the same age at which each succes-
sive variation appeared in the parents; it can further be shown that varia-
tions do not commonly supervene at a very early period of embryonic
growth, and on these two principles we can imderstand that most wonder-
ful fact in the whole circuit of natural history, namely, the close similarity
of the embryos within the same class — for instance, those of mammals,
birds, reptiles, and fish.

It is the consideration and explanation of such facts as these which has
convinced me that the theory of descent with modification by means of
natural selection is in the main true. These facts as yet received no ex-
planation on the theory of independent Creation; they cannot be grouped
together under one point of view, but each has to be considered as an
ultimate fact. As the first origin of life on this earth, as w^ell as the con-
tinued life of each individual, is at present quite beyond the scope of
science, I do not wish to lay much stress on the greater simplicity of the
view of a few forms or of only one form having been originally created,
instead of innumerable periods; though this more simple view accords
well with Maupertuis's philosophical axiom of " least action."

In considering how far the theory of natural selection may be ex-
tended; that is, in determining from how many progenitors the inhabitants
of the world have descended, — we may conclude that at least all the mem-
bers of the same class have descended from a single ancestor. A number
of organic beings are included in the same class, because they present.

DARWIN'S FINAL VIEWS 17

independently of their habits of Hfe, the same fundamental type of struc-
ture, and because they graduate into each other. Moreover, members of
the same class can in most cases be shown to be closely alike at an early
embryonic age. These facts can be explained on the belief of their descent
from a common form; therefore it may be safely admitted that all the
members of the same class are descended from one progenitor. But as the
members of quite distinct classes have something in common in structure
and much in common in constitution, analogy would lead us one step
further, and to infer as probable that all Hving creatures are descended
from a single prototype.

I hope that the reader will pause before coming to any final and hostile
conclusion on the theory of natural selection. The reader may consult my
" Origin of Species " for a general sketch of the whole subject; but in that
work he has to take many statements on trust. In considering the theory
of natural selection, he will assuredly meet with weighty difficulties, but
these difficulties relate chiefly to subjects — such as the degree of perfec-
tion of the geological record, the means of distribution, the possibility of
transitions in organs, etc., on which we are confessedly ignorant; nor do
we know how ignorant we are. If we are much more ignorant than is
generally supposed, most of these difficulties wholly disappear. Let the
reader reflect on the difficulty of looking at whole classes of facts from a
new point of view. Let him observe how slowly, but surely, the noble
views of Lyell on the gradual changes now in progress on the earth's sur-
face have been accepted as sufficient to account for all that we see in its
past history. The present action of natural selection may seem more or
less probable; but I believe in the truth of the theory, because it collects,
under one point of view, and gives a rational explanation of, many ap-
parently independent classes of facts.

In his earlier statements of his theory, Darwin does not
seem to have paid much attention to the source of variations
or to the manner of their inheritance, but these subjects re-
ceive much attention in his great work on the Variation of
animals and plants under domestication, from which we
have just quoted. He seems to have come more and more to
hold views similar to those of Lamarck, his great French pre-
decessor, regarding the direct effect of environment as a cause
of variation, and the inheritance of effects so produced. Con-
cerning the general nature of Lamarck's views we should
therefore inform ourselves.

CHAPTER II

CONTRIBUTIONS OF LAMARCK, WEISMANN, AND HERBERT
SPENCER TO THE THEORY OF EVOLUTION; DARWIN'S

THEORY OF PANGENESIS

Lamarck (1744-1829), the greatest evolutionist before Dar-
win, was, according to his biographer, a man of great physical
and moral courage. He distinguished himself by a deed of
singular bravery in the French army, and, receiving an in-
jury, re-entered life as a doctor. He was first attracted to
botany by the rich flora near Monaco observed during his
military service. Going to Paris he gained the attention of
the great naturalist, Buff on, under whose direction he pub-
lished a " Flora of France," written in six months, which
passed through many editions. He seems to have possessed
powers of exceptionally rapid observation, with great facility
in writing and with unusual powers of definition and descrip-
tion. At the age of forty-nine (1793) he was transferred to a
Zoological chair in the Jardins des Plantes, being placed in
charge of invertebrate zoology, while at the same time Geoff-
roy Saint-Hilaire was placed in charge of vertebrate zoology.
Being at this time in his fiftieth year, Lamarck took up the
study of zoology with such zeal and success that he almost
immediately introduced striking reforms in classification, and
developed (after having reached middle life) the conception
of the mutability of species and of the origin of new species
by descent. His relation to the evolution idea was thus very
different from Darwin's. It came to Darwin almost in his
boyhood and he spent a lifetime working it out, not publishing
anything upon it until he was fifty years old. To Lamarck
the idea seems scarcely to have come before the age of fifty,
and he rapidly developed it into a system, sufficiently elabor-
ate to explain evolution, if his basic principle is true, viz.

18

LAMARCK AND HIS THEORY 19

the inheritance of acquired characters. This we shall consider
further.

Regarding Lamarck's later life, Osborn (p. 158) says:

His devotion to the study of the small forms of life, probably with in-
ferior facilities for work, for he was extremely poor, gradually deprived him
of the use of his eyes, and in 1819 he became completely blind. The last
two volumes of the first edition of his Natural history of invertehrated
animals, which was begun in 1816 and completed in 1822, was carried on
by dictation to his daughter, who showed him the greatest devotion; after
Lamar.ck was confined to his room, it is said she never left the house.
Lamarck was thus saddened in his old age by extreme poverty and by the
harsh reception of his transmutation theories, in the truth of which he
felt the most absolute conviction.

Lamarck's Theory

The factors recognized by Lamarck as concerned in evolu-
tion may be summarized as follows : —

1. The direct effect of environment. We know that a plant
in rich soil grows large and luxuriant, but that the same plant
in poor soil would remain small and stunted. This is a direct
effect of the environment. Lamarck supposed that such
effects of environment are cumulative from generation to
generation so that long-continued growing in rich soil would
produce a more luxuriant race, while continued growing in
poor soil would produce a different and smaller race. In the
case of animals Lamarck does not think that the action of
environment is quite so direct, but that animals are changed
indirectly through changes in their habits. Buff on considered
the action of environment direct in both animals and plants,
and this view Darwin seems to have adopted rather than
Lamarck's slightly different one. Darwin in his Variation
adopts this factor, the direct effect of environment, as one of
the causes, if not the chief cause of variations. He says
(p. 6):

If then organic beings in a state of nature vary even in a slight degree,
owing to changes in the surrounding conditions, of which we have abundant
geological evidence, or from any other cause, — then the severe and often-
recurrent struggle for existence will determine that those variations, how-
ever slight, which are favorable shall be preserved or selected, and those
which are unfavorable shall be destroyed.

20 GENETICS AND EUGENICS

2. Lamarck regarded new physical needs as a second factor
or cause of variations. He supposed that the need of an
organ caused the organ to be produced, that need of horns to
fight with or of teeth to chew with would cause the produc-
tion of horns and teeth respectively. Darwin never adopted
this view.

3. A third Lamarckian factor however Darwin did regard
as a genuine cause of variation, viz., use and disuse. The use
of an organ, as the arm or leg, causes it to increase in size
and strength; conversely disuse causes decrease in size and
efficiency.

4. Inheritance of acquired characters. As regards heredity,
Lamarck believed that variations of every sort are inherited.
Those which result from direct action of the environment or
from use and disuse, we now call acquired characters, and
Lamarck supposed that acquired characters are inherited. In-
deed he supposed that all variations are of this nature. Dar-
win shared Lamarck's view in part; he too probably did not
clearly distinguish between variations which we should class
as acquired characters and those of other sorts. Certainly
Lamarck did not make this distinction, for on his view all
variations are what we should call acquired.

In illustration of Lamarck's views concerning the causes of
variations and of consequent evolution, it may be well to
quote a few passages largely in his own words, as given in
translation in Osborn, pp. 164-171.

In considering the natural order of animals, the very positive gradation
which exists in their structure, organization, and in the number as well as
in the perfection of their faculties, is very far removed from being a new
truth, because the Greeks themselves fully perceived it; but they were un-
able to expose the principles and the proofs of this evolution, because they
lacked the knowledge necessary to establish it. In consideration of this
gradation of life, there are only two conclusions which face us as to its ori-
gin : — The conclusion adopted up to today : Nature (or its Author) in cre-
ating animals has foreseen all possible sorts of circumstances in which they
would be destined to live, and has given to each species a constant organi-
zation, as well as a form determined and invariable in its parts, which forces
each species to live in the places and climates where it is found, and there
to preserve the habits which we know belong to it. My personal conclusion:
Nature, in producing successively all the species of animals, and commenc-

LAMARCK, DARWTN, AND WEISMANK 21

ing by the most imperfect or the most simple to conchide its labour in the
most perfect, has gradually completed their organization ; and of these
animals, while spreading generally in all the habitable regions of the globe,
each species has received, under the influence of environment which it has
encountered, the habits which v/e recognize and the modifications in its
parts which observation reveals in it.

All that Nature has caused individuals to acquire or lose by the influ-
ences of environment to which they have been long exposed, and conse-
quently by the influence of the predominant employment of a certain organ,
or by that of the continued lack of use of the same part, — all this Nature
conserves by generation to the new individuals which arise, provided that
these acquired variations (changements) are common to both sexes, or to
those which have produced these new individuals.

But great changes in environment bring about changes in the habits of
animals. Changes in their wants necessarily bring about parallel changes
in their habits. If new wants become constant or very lasting, they form
new habits, the new habits involve the use of new parts, or a different use
of old parts, which results finally in the production of new organs and the
modification of old ones.

Darwin's later views concerning variation and heredity,
as compared with those of Lamarck, may be briefly stated
thus:

1. Variation was thought to be due either to the two
Lamarckian factors, direct action of the environment and use
or disuse, or to other as yet unknown causes, the results of
which Darwin refers to as " chance variations."

2. As regards heredity, Darwin seems to have thought
with Lamarck that variations of all sorts are inherited,
though some doubtless were inherited more strongly and per-
sistently than others.

Weismann (1834-1914). The first great advance, after
Darwin, in our knowledge of variation and heredity was made
by Weismann, a German zoologist, who within two years after
Darwin's death (viz. in 1883) brought forward a new classifi-
cation of variations and a new theory of heredity.

He showed that some variations are congenital {i. e., are
horn with us), are in the blood so to speak, while others are
acquired through the action of environment, use or disuse.
Regarding acquired characters, he showed that these, in all
probability, are not inherited. This was a wholly new idea
and called forth a hot debate which has not yet ended, but

22

GENETICS AND EUGENICS

gradually biologists have been coming to the view that Weis-
mann is right. The consequences of this view are very im-
portant not only as regards evolution in general, but also as
regards education, for if Weismann is right scholarship is not
inherited, but only capacity to learn. The son must begin in
his education, not where his father left off, but at the alpha-
bet, and he will not learn any faster because his father was
educated. I think the experience of educators justifies this
view. Children growing up in cultured homes have a certain
educational advantage due to their environment, but not to
heredity. Thus Darwin's attention was directed toward nat-
ural history, by the home environment in which he grew up.
The same is true in even greater degree of his sons, three of
whom have become distinguished scientists. It is very im-
probable that he inherited a taste for natural history, as he
supposed. More likely he acquired such a taste.

©G

Fig. 1. Diagram showing the relation of the body or soma (S) to the germ-cells

(G) in heredity. (After E. B. Wilson.)

Besides showing that there is no sufficient evidence that
acquired characters are inherited, Weismann pointed out
anatomical and physiological reasons why we should not
expect them to be inherited. In the higher animals and
plants reproduction takes place not by division of the body
but by the development of special reproductive cells, eggs,
spores, and the like. The fertiHzed egg-cell of an animal be-
gins its development by dividing into two cells; these divide
into four, and so on. Sooner or later we notice that these cells
are not all alike. Some of them develop into muscles, others
into bone, or nervous tissue; in short they become differ-
entiated to form the various parts and tissues of the body, all
except some few which remain undifferentiated like the origi-
nal egg-cell itself. These undifferentiated cells will in fact

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Results of ovarian transplantation in guinea-pigs. Ovaries from a small black guinea-pig (Fig. •i)
ivere transplanted into an albino (Fig. 3) which, mated with another albino (Fig. 4), produced black
young (Figs. 5-7).

BODY AND GERM-CELLS

23

give rise to egg-cells or sperm-cells rather than to muscle,
bone, or any other part of the body proper. Weismann called
the cells which collectively make up the body the soma (Greek
for body) ; whereas those undifferentiated cells destined for
reproduction he called germ-cells or collectively the germ-

Fig. 8. Fruits of an apple " graft-hybrid " or " chimera." Two distinct varieties are represented in
one fruit. The stem-end of the apple is russet and sour; the blossom-end is smooth-skinned, red-striped
and sweet. A sharp line of division separates the two portions. Such fruits are borne on a tree pro-
duced by grafting one variety on another, the tree-trunk having grown from a bud which arose just
where stock and scion join, and which included cells derived from both sources. But the two kinds
of cells and all their descendants have retained their original distinctness, as the composite fruits •ehow.
Hence, not only may the body and germ-cells be of unlike character (as Figs. 2-7 show), but even the
body may be composite and yet each part retain its original character. By grafting tadpoles, Harrison
has produced a frog which anteriorly was of one species and posteriorly of another. If such a frog
produced eggs, their character would depend upon which part of the body furnished the eggs. " Graft-
hybrids " between the tomato and black nightshade (Solanum nigrum) produced by Winkler and
studied by him and by Baur were found to produce as seedlings either pure tomato plants or pure
nightshade plants, depending on which species made up that part of the " chimera " from which the
germ-cells arise.

plasm. Now Weismann maintained that the germ-cells, since
they are not descended from body-cells but only from the
fertilized egg-cell, have no way of transmitting body-modifi-
cations, i. e., acquired characters. The germ-cells are guests
in the body, but not members of the household. They feed
at the common table but have no share in the other activities

24 GENETICS AND EUGENICS

of the home, and are themselves unmodified bv those activi-
ties. To show the biological soundness of Weismann's con-
clusion that soma and germ-plasm are anatomically and
physiologically distinct, I may cite an experiment performed
by Dr. John C. Phillips and myself:

A female albino guinea-pig (Fig. 3) just attaining sexual maturity was
by an operation deprived of its ovaries, and instead of the removed ovaries
there were introduced into her body the ovaries of a young black female
guinea-pig (Fig. 2), not yet sexually mature, aged about three weeks. The
grafted animal was now mated with a male albino guinea-pig (Fig. 4).
From numerous experiments with albino guinea-pigs it may be stated
emphatically that normal albinos mated together, without exception, pro-
duce only albino young, and the presumption is strong, therefore, that had
this female not been operated upon she would have done the same. She
produced, however, by the albino male three litters of young, which to-
gether consisted of six individuals, all black. (See Figs. 5-7.) The first litter
of young was produced about six months after the operation, the last one
about a year. The transplanted ovarian tissue must have remained in its
new environment therefore from four to ten months before the eggs at-
tained full growth and were discharged, ample time, it would seem, for the
influence of a foreign body upon the inheritance to show itself were such
influence possible.

Since, then, germ-cells and body are distinct, heritable
variations cannot have their origin in body-cells but only
in the germ-plasm. The problem of evolution, therefore, on
Weismann's view, becomes this — how are changes in the
germ-plasm brought about ?
Darwin's theory of ^pangenesis.

Before Weismann's time, Darwin, in common with biolo-
gists in general, had come to recognize that the germ-cells
(i. e., the egg and sperm-cells) are the sole vehicles of inheri-
tance. Darwin therefore realized that if acquired characters
are inherited, as everyone then supposed, bodily modifica-
tions must in some way be registered in the germ-cells, and
he framed an hypothesis to explain how this could come
about. This hypothesis, which he called Pangenesis, is put
forward in the closing chapters of his book on Animals and
plants under domestication. Darwin himself was not sure
of its correctness and advanced it as he says " tentatively "
only. We are very sure that it was not correct, but it has for

SPENCER AND PANGENESIS 25

us an historical interest because it had much influence upon
biological investigation and theory at that time and subse-
quently. Logically, Darwin's theory of pangenesis may be
regarded as a modification of one of Herbert Spencer's specu-
lations upon biology.

Herbert Spencer (1820-1903) was the champion of evolu-
tion from the standpoint of philosophy, as Huxley was from
the standpoint of comparative anatomy and embryology.
His ideas had much influence on the development of evolu-
tionary thought down to our own time. (See Delage and
Goldsmith, 1912.) Spencer tried to explain the structure of
living substance (protoplasm) in harmony with the chemical
explanation of lifeless substance then current. He supposed
that there are structural units of protoplasm comparable with
the molecules of chemical compounds, each kind of proto-
plasm within the body being composed of a different kind or
kinds of units. These he called physiological units.

Darwin adopting this same line of thought, but with a
more intimate knowledge of the facts of inheritance, saw that
every kind of physiological unit must be supposed to exist in
the germ-cell, since out of the germ-cell an entire body de-
velops. In his theory of pangenesis, he supposes that every
part of the body is constantly giving off its particular kinds
of units into the blood, just as a fungus gives off spores into
the air. These given off units Darwin called " gemmules,"
or little buds. He supposed further that these gemmules are
carried through the body in the blood stream, and accumu-
late in the germ-cells, in which they multiply as the germ-cell
develops. Thus out of one germ-cell comes an entire body
with its various parts, because each part was represented in
the germ by a gemmule. No one today holds this theory, as
Darwin stated it, but the underlying idea of preformed deter-
mining particles existing in the germ-cell reappears a little
later in Weismann's theory of heredity, and has wide accep-
tance today in the chromosome theory of inheritance.

We shall come to these later, but for the present let us go
back to Darwin's theory of pangenesis. Darwin's method of

26 GENETICS AND EUGENICS

reaching this theory was inductive and beyond criticism. He
first collected all the facts obtainable about inheritance and
then attempted to frame an hypothesis which would account
for them all, which would bring them all under one point of
view. Where he erred was in accepting as facts some things
which we know are not facts. In fitting a theory to them, he
framed a false theory, simply because the assumed facts were
false.

Darwin's cousin, Francis Galton, showed the unsoundness
of pangenesis by a simple experiment. He reasoned thus. If,
as Darwin assumes, gemmules circulating in the blood deter-
mine the character of the germ-cells, then blood of one animal
transfused into blood-vessels of another should carry into the
germ-cells of the second animal gemmules derived from the
first animal. Consequently offspring subsequently produced
by an animal into which blood has been transfused should
show characteristics of the animal from which the blood was
taken. Galton performed this experiment on rabbits but with
results wholly negative. The experiment, however, cannot be
regarded as altogether conclusive because (1) blood trans-
fused from one individual to another probably does not long
persist, but is replaced by new blood formed by the individual
into which transfusion occurred. Therefore the effects of
transfusion would at most be of short duration. (2) Suppos-
ing that modifications were induced in the germ-cells by
transfusion, it is not to be expected, in the light of our present
knowledge, that such modifications would in all cases appear
in the first generation offspring, but rather in the second or
later generations of offspring, but Galton did not carry the
experiment so far. Galton's experiment therefore cannot be
regarded as a complete refutation of pangenesis, but such a
refutation has become unnecessary through the development
of biological knowledge along other lines.

The theory of pangenesis was an attempt to explain the
mechanism of the inheritance of acquired characters. If
acquired characters are not inherited, as we now have reason
to think, the hypothesis of pangenesis is unnecessary and

PANGENESIS DISCARDED 27

should accordingly be discarded. This in fact is what has
actually happened. The theory as Darwin stated it has no
supporters at present. Those who now hold, in a modified
form, that acquired characters are inherited, have adopted
other ways of explaining their inheritance, or else, with De-
lage, admit the inadequacy of Darwin's explanation and
state that no satisfactory substitute has yet been found, but
entertain the hope that one will yet be discovered.

CHAPTER III

ARE ACQUIRED CHARACTERS INHERITED ?

Evidence from ovarian transplantation experiments with
guinea-pigs has been cited to show that body and germ-cells
are morphologically and physiologically distinct and that
germ-cells may be lodged in a foreign body during their de-
velopment without losing their distinctive character. But
this by no means proves that germ-cells are immune from
modification by influences which reach them through the
body. The evidence cited is negative evidence. It creates a
presumption against the inheritance of acquired characters
but does not prove a universal negative, which is impossible.
The question whether acquired characters are or are not in-
herited is therefore a question to be decided only by the care-
ful weighing of evidence. It is possible that some categories
of supposed acquired characters are more readily capable of
an alternative interpretation than are others. Several of
these may now be discussed briefly.

1. Mutilations, It is now all but universally admitted
that somatic modifications due to mutilation are not in-
herited. Nevertheless " cases " are from time to time re-
ported, in which a man or a domesticated animal which by
accident had lost a limb has produced offspring similarly de-
fective. One of the most frequently recurring of these stories
has come to me at first hand. A cat which had accidentally
lost her tail gave birth to kittens part of which were short-
tailed. It is not necessary to suppose that the report is in-
accurate. Certain races of cats are naturally short-tailed,
and a cat might produce offspring short-tailed by inheritance
quite irrespective of any injury to either parent. On the
other hand where docking of the tail has been followed up
systematically for many generations and on a large scale, as
is the case in sheep, no racial shortening of the tail is observ-
es

ACQUIRED CHARACTERS 29

able. Finally, we have the direct experimental evidence of
Weismann, who cut off the tails of mice for nineteen genera-
tions in succession without however observing any inheri-
tance of the mutilation. We have also the evidence furnished
by long-continued mutilations practiced by man upon his
own person, such for example as tatooing and circumcision.
The effects of such mutilations, as is well known, are not in-
herited in the slightest degree.

Notwithstanding all this negative evidence, Semon, who
like a drowning man catches at every straw, cites Kammerer
as having recently shown that a soft-bodied marine animal
(Ciona, an ascidian) after its siphons are cut off regenerates
new ones longer than normal, and he maintains that the
young of such animals have siphons of abnormal length. In
view of all the negative evidence furnished by other animals
this case, as yet incompletely published, seems highly im-
probable. The unsupported claim throws more light upon
the credibility of Kammerer as a witness (and he has brought
forward many cases in recent years) than upon the general
question of the inheritance of mutilations.

2. Congenital diseases. Cases of disease acquired by a
parent and by him transmitted to his offspring are frequently
reported. But all these cases are capable of other explana-
tions than that of inheritance of an acquired character.

(a) In some cases a disease-producing organism may be
present in the body of the parent and may pass directly into
the reproductive cell. Thus in silkmoths, the organism
which causes "pebrine" is transmitted as an infection within
the egg, as Pasteur showed. The same is true of Texas fever
in cattle. This disease is caused by a protozoon which is
introduced into the blood of cattle by a tick which harbors
the disease. The protozoan parasite is present in the egg-cell
of the tick, so that the young tick which develops out of such
infected eggs cannot fail to contain the parasite; but the
disease is no more inherited than a grain of sand placed within
the egg would be inherited. In a similar way in man syphilis
may be transmitted, but it is in no true sense inherited. Yet

30 GENETICS AND EUGENICS

the practical outcome is very similar; an individual once in-
fected with syphilis is racially condemned; his seed is as
truly bad as if the syphilis germ were an essential part of the
germinal substance.

(b) The intimate relationship of parent to child may give
unusual opportunities for post-natal infection, as in the case
of tuberculosis. Thus the children of tuberculous parents
are more liable to infection with tuberculosis, other things
being equal, than the children of non-tuberculous parents.
But we are not justified for that reason in speaking of tuber-
culosis as hereditary. It is probably in all cases acquired by
the patient, individually, and not inherited. Whether some
individuals are more susceptible than others is a wholly
different question. Susceptibility may well be inherited.

(c) Just as a disease-producing organism may be received
into the egg or the embryo while it is still within the body of
the mother, so chemical substances in the mother's blood may
enter the egg or embryo and affect its subsequent character.
Thus it has been shown that in guinea-pigs immunity ac-
quired by the mother (which is known to be due to the
presence of specific substances in the blood) may be trans-
mitted to her offspring, though the father has no such in-
fluence, the reason being that the sperm-cell is too small to
carry an effective quantity of antitoxin, i. e., of immunity
producing substance. In such cases as I have just mentioned
of transmitted immunity, the immunity does not last beyond
a single generation. It has not become hereditary, it has
simply been passively received by the embryo.

On the whole, we must conclude that disease transmission
furnishes no evidence in favor of the transmission of acquired
characters. The most debatable case is that of acquired
disease transmitted in the germ-cell. For practical purposes
this is heredity. For truly hereditary characters are often as
detachable and separate from the germ-cell as foreign bodies,
as we shall see when we come to study Mendelian inheritance.

3. Induced epilepsy. A famous case cited in all discussions
of this subject is the case of Brown-Sequard's guinea-pigs.

I

ACQUIRED CHARACTERS 31

From 1869 to 1891 Brown-Sequard experimented on thou-
sands of guinea-pigs, developing methods by which a certain
form of epilepsy could be induced through injury to different
parts of the nervous system, such as the spinal cord or the
sciatic nerve. In some cases the young of animals thus
rendered epileptic were themselves similarly affected. Some
persons who have repeated Brown-Sequard's experiments
confirm his results, notably Romanes; others have failed to
confirm them.

Weismann has suggested that some pathogenic organism
may have got into the wounds and, migrating into the central
nervous system, have caused the epilepsy, and this same or-
ganism may have infected the young. There is no evidence
that such was the case, however.

Guinea-pigs are said to be strongly predisposed to epilepsy,
and so the results of Brown-Sequard's experiments may be
pure coincidences, or due to the transmission of a chemical
substance. In some cases reported by Brown-Sequard the
animals gnawed off one or more toes after the sciatic nerve
had been cut. Certain of their young are reported to have
done the same. This is almost certainly pure coincidence,
since the evidence as regards the inheritance of mutilations
is unmistakable.

4. Acclimatization. It is well known that animals or plants
taken from one climate to another undergo changes of form.
The same plant divided into two parts and planted one part
upon an exposed mountain side, the other in a sheltered,
fertile valley, assumes forms very different in the two places.
The mountain form is short, compact and dwarfed; the val-
ley form is tall, spreading and luxuriant. It is assumed by
Lamarckians that these direct effects of the environment are
to some extent inherited, that if they are repeated through a
long series of generations they at last become habitual ^ so to
speak, and appear spontaneously even when the external
cause is lacking. In this way it is explained why mountain
species in general are dwarfed, and lowland species are tall
and luxuriant, even when the two are grown side by side

S2 GENETICS AND EUGENICS

under identical conditions. Lamar ckians assume that the
direct effects of the environment have accumulated and be-
come hereditary. Selectionists, on the other hand, maintain
that dwarf species were dwarfs originally and by nature, and
that they have found their way to the mountains because
they alone can survive under the harsh conditions there ob-
taining, whereas the more luxuriant forms were better
adapted to lowland conditions and have there crowded out
the dwarfs. It is evident that both explanations are logically
sound, though both cannot be true. Many experiments have
been tried to determine which best accords with fact, but the
results are not entirely conclusive because they are usually
capable of alternative interpretations, and each one inter-
prets them in accordance with the general theory which he
favors. A few typical experiments may be enumerated.

(a) To altered salinity. Paul Bert, many years ago, at-
tempted to acclimatize some Daphniae (small fresh-water
Crustacea) to salt water by gradually adding salt to the
aquarium. At the end of forty-five days, when the water
contained 1.5 per cent of salt all the adults had died; but the
eggs in their brood-chambers survived, and the new genera-
tion arising from these flourished well in the salt medium.
This case has been cited as a case of inherited modification,
but such it clearly is not, because the parents did not succeed
in becoming acclimatized ; they died without becoming modi-
fied sufficiently to exist in the salt water. But their egg-cells
did become so modified, and the animals developing out of
them were acclimatized, through direct response to the en-
vironment, not through inheritance.

Ferroniere transferred a worm {Tuhifex) from fresh water
into sea water. The animal lived there and underwent cer-
tain changes of form (loss of bristles, etc.), which became
more deeply marked in later generations. After several
generations the animals were unable to live in the original
medium. This case is cited as showing inheritance of an
acquired modification. But it can with equal propriety be
interpreted as showing power of direct adaptation to changed

ACQUIRED CHARACTERS 33

environment. It is doubtul whether any inheritance oc-
curred at all, for these animals usually reproduce by fission
and Ferroniere's " several generations " probably represent
merely regenerated fragments of one and the same original
individual. Had the transfer back to fresh water been gradual
enough there can be little doubt that it would have been
accomplished successfully.

(6) To a shorter season. Corn or other grain taken from a
southern to a northern latitude adapts itself to a shorter
growing season, maturing earlier. The change is not imme-
diate, but progressive, the period required for maturity grow-
ing shorter through several generations. This at first sight
looks like a good Lamarckian effect, but selectionists regard
it as equally good evidence in support of their view. For it
is evident that the shorter growing season in northern lati-
tudes would act as a selecting agency, killing off all variations
requiring a long growing season, so that earlier maturity
would become a racial character.

5. Effects of changed food supply. Kellogg and Bell (1903)
fed larvae of the silkmoth on a reduced quantity of mul-
berry leaves or on a diet partly of lettuce, partly of mulberry
leaves. A decrease in size of the adult moths resulted which
persisted through two subsequent generations, even when
normally fed. In this way a race of dwarf moths was pro-
duced which however died out at the end of three genera-
tions. This is not a clear case of inherited modification, but
of direct weakening of the organism through mal-nutrition or
disease, the cause whatever it was being probably transmitted
in the egg like "pebrine."

Similar but more extensive experiments were performed
by Pictet (1910-1911) upon larvae of the gipsy-moth. These
larvae feed by preference on oak leaves. Pictet fed some on
walnut leaves and thus obtained moths of modified, paler
coloration. These modifications became accentuated after
several generations had been reared on walnut leaves. In
one experiment the modified coloration persisted in spite of
a return to normal diet. The first generation was fed on

34 GENETICS AND EUGENICS

walnut leaves and presented the paler coloration; the second
and third generations were fed on oak leaves but retained the
modified coloration. In the third generation, however, the
female showed partial return to normal coloration.

Pictet observed some cases in which moths became so
completely accustomed to the diet of walnut leaves that their
coloration became normal. Delage regards this as greatly-
weakening the case for inherited modification. He interprets
the case thus. Walnut leaves are in general a poor diet for
gipsy-moth larvae. They weaken the animal. This weakness
persists through one or more generations, doubtless because
of impaired constitution of the egg, but is not certainly trans-
mitted as an acquired character. Indeed the race may re-
cover from the weakening produced by the changed diet.

6. Temperature experiments. Many experiments have been
performed with moths and butterflies in which the pupae
were subjected to abnormally low or abnormally high tem-
peratures. The effects of both extremes are in many cases
similar. In general extremely low or extremely high tem-
peratures produce darker adults. Fischer reared adults from
pupae of Arctia caja exposed to a very low temperature, 8° C.
Abnormally dark adults were obtained in this way. Some of
the darkest of these, produced under normal conditions un-
usually dark offspring. Fischer considers that the induced
modifications were transmitted. But this is far from certain
for (1) the moths vary in darkness of coloration under normal
conditions. It is not established that the supposedly induced
variations lie outside the range of normal variation. (2)
Fischer's treatment served to show what animals were nat-
urally inclined to become dark, for these under treatment
would become darkest, and from such Fischer bred. The
supposed transmission of an acquired characteristic may be
regarded in this case as nothing but the transmission of a
natural or inborn characteristic, the treatment serving as a
guide to selection.

Weismann, however, influenced by studies of his own upon
variation in color of butterflies in northern and in southern

ACQUIRED CHARACTERS 35

Europe, is willing to accept at full face value such cases as
this brought forward by Fischer, and to allow that the race
may become darker through long-continued subjection to
lower temperatures. He supposes not that the body effects
are transferred to the germ-cells, but that the low tempera-
tures act simultaneously on the body and on the germ-cells,
producing in them similar changes, the changes in the germ-
plasm affecting the hereditary character of the race per-
manently. This view under the name of ^parallel-induction
now has many adherents. It is a practical admission for a
particular case of the Lamarckian principle of evolution
guided in its course by environmental action. Whether,
however, Weismann is right in his interpretation may still
be regarded as an open question.

In this country, W. L. Tower (1896) has carried on exten-
sive experiments upon potato beetles and related insects, in
which variations in temperature and humidity of the environ-
ment have been followed by variations in pigmentation of the
insects, similar to those observed by Fischer in the case of
butterflies. Tower interprets his observations, as would Weis-
mann, as showing, not inheritance of acquired characters but
direct modification of the germ-cells, independently of the
soma. For, he claims to have obtained modification of the
germ-plasm, which accordingly resulted in inherited varia-
tions, where no parallel modification of the body of the parent
had occurred. Inheritance of an acquired character is accord-
ingly excluded because no modification was acquired. His
strongest evidence for this claim consists of cases in which
the same parents were subjected to periods of heat or cold,
alternating with periods of normal temperature, each being of
several weeks' duration. It was found that when a batch of
eggs was produced in or immediately following a period of
heat, characteristic color variations were likely to occur
among the offspring which may be called heat variations and
these proved hereditary. But when eggs were produced by
these same parents at normal temperatures, no such varia-
tions occurred. Similar effects were obtained in cold periods.

36 GENETICS AND EUGENICS

as contrasted with normal temperatures. While the bodies
of the parents remained unaffected, the coloration of their
offspring varied with conditions of temperature and moisture
during the growth and fertilization of the eggs which pro-
duced those offspring. Tower therefore concludes that the
germ-plasm was directly and permanently affected by varia-
tions in the environment during a particular sensitive growth
period of the egg. This work is therefore no argument for the
inheritance of acquired characters; nevertheless it is an argu-
ment for evolution directly guided by the environment, which
after all is the essence of Lamarckism. There are several
reasons why we should accept Tower's conclusions with some
reservation.

1. In the first place his experiments are not reported in
sufficient detail to enable us to form a critical opinion as to
their conclusiveness.

2. If the supposed temperature and moisture effects are
due solely to those conditions, they should appear equally in
all eggs subjected to the same conditions, but this is not the
case. Only certain individuals are modified. Since this is so,
it is evident that all the eggs were not alike at the outset, for
some were more sensitive than others to temperature and
moisture changes in the environment, if indeed these were
the agencies which caused the changes observed. A good
argument could therefore be made for considering the tem-
perature and moisture changes as merely selective agencies
exerted on a collection of germ-cells already inherently vari-
able in their potentialities. For Tower maintains that the
variations once obtained are perfectly stable for an indefinite
number of generations. His claim, therefore, is that by direct
action of the environment for a comparatively brief period j
permanent changes in the germ-plasm may be brought about.

It would seem that if the germ-plasm is thus directly modi-
fiable, the action ought to be reversible. Changes of environ- I
ment should unmake species as readily as they make them,
yet such a result would scarcely harmonize with Tower's

ACQUIRED CHARACTERS 37

theory, or with the known stubborn and persistent nature of
heritable variations, when once they have arisen.

Kammerer of Vienna has pubHshed in the last five years
the results of a long series of experiments with salamanders
and lizards designed to show the inheritance of acquired
characters. In this connection we will consider his experi-
ments with temperature. The coloration of several species of
lizard, with which Kammerer experimented, changes with
changes of temperature. Kammerer kept lizards at abnor-
mally high or abnormally low temperatures, and found that
the induced changes of coloration persisted to some extent
even after the animals were returned to normal conditions.
Further, while they were thus altered, the offspring which
they produced, inherited in some degree the supposedly in-
duced changes. The evidence for this case, as for many
similar cases which might be cited, is quite insufficient. Un-
doubtedly individual differences in coloration occur among
the lizards quite independently of external temperatures.
Further some probably change more readily and extensively
than do others in consequence of changed temperatures. A
corresponding variation among the offspring, plus and minus,
as compared with their parents, would then account for such
plus variations in pigmentation as Kammerer observed
among the offspring and which he ascribes to inheritance of
changes induced in the parents.

Sumner (1915) kept white mice, some in a cold room, some
in a warm room, where they multiplied. The mice which
grew up in the cold room had shorter tails and feet than those
which grew up in the warm room. Animals reared in each
room were now transferred to a common room of ordinary
temperature and allowed to produce offspring there. In three
out of four such lots of offspring studied, the cold -room
parents had young with shorter tails and feet, but in a fourth
lot these relations were reversed. It seems doubtful, there-
fore, whether the agreement between parents and offspring
in three of the four cases studied is anything but a coinci-

38 GENETICS AND EUGENICS

dence. But even supposing it to have statistical significance,
it may be due, as Sumner suggests, to differences directly
impressed upon the germ-cells while they were contained
within the body of the parent and the parent itself, being
very young, varied in body temperature with the room in
which it was born. If so, there can be no question of a
transfer of an effect from body to germ-cells, but only of
simultaneous modification of the two.

7. Pressure effects. It is well known that pressure has
direct effects upon the parts of the body. The skin on the
soles of our feet is thickened where our weight rests upon it,
and callouses form on the hand when it is used at hard work.
A long illness, during which the person does not stand upon
his feet causes the thickenings on the feet in part to disappear.
They are undoubtedly due directly to pressure. Yet all pre-
vious generations of man have been subjected to the same
action, and if acquired effects are inherited this should be. In
fact, it is found that in the foetus of man, long before birth
(from five months on) the skin is thicker on the sole of the
feet than on the back of the foot. If this is not to be regarded
as an inherited effect of use (pressure), it will be necessary to
explain how the skin came to be thickened originally in those
particular regions where use induces thickening.

The camel's hump has been cited as a character acquired
by pressure, carrying loads on its back. But this is a less
fortunate example for the Lamarckians, for the camel's hump
is not due probably to pressure at all. It represents rather a
reserve food organ, like special accumulations of fat in most
animals. For not all animals which carry loads on their backs
acquire humps, for example the ass, the horse. Further,
animals may acquire humps without carrying loads, as the
American bison and the humped cattle of India.

8. Light effects. Kammerer has experimented with the
European spotted salamander (" fire salamander ") which is
mottled with black and yellow areas. He finds that if sala-
manders are kept on a yellow background, the yellow areas
become more extensive, while if the animals are kept on a

ACQUIRED CHARACTERS 39

black background, their black areas become more extensive.
Thus there is an automatic control of the color-pattern
adapted for concealment, such as is known to occur in many
fishes. Now Kammerer bred from animals, thus rendered
extremely yellow, and reared part of the young on a yellow
background, part of them on a black background. Both lots
developed yellow spots but these were more extensive in those
animals kept on a yellow background. In some of them the
yellow was more extensive than in the parents. This result
Kammerer ascribes to inheritance of the acquired yellow
coloration added to the direct effect of the yellow background
on the young. This conclusion is a fallacious one. Spotted
animals are extremely variable in pattern, even when the
environment does not change. If a particular kind or degree
of spotting is selected in the parent animals, it may be ex-
pected that offspring will be obtained both darker and lighter
than the parents. In this way the race can by selection be
made either darker or lighter, quite irrespective of any change
in the environment. Kammerer has obtained nothing be-
yond such effects as these. There is no reason to think that
a change of illumination induced them to any greater extent
in the second generation than it did in the first.

Another light experiment carried out by Kammerer seems
to me to have more weight. This was concerned with the
degeneration of the eyes in cave animals. It is a well-known
fact that cave animals have bodies nearly or quite colorless
and possess degenerate eyes. In animals pigment formation
is an oxidation process, which frequently does not take place
in the absence of light. Therefore many animals which de-
velop in complete darkness are unpigmented. The human
skin, to be sure, develops pigment even in darkness, but it
develops much more of it in direct sunlight. The skin of a
European is fair if he stays indoors, but darkens quickly if he
spends much time outdoors in the direct sunlight. The dark-
est races of mankind are those which live where the sunlight
is strongest and the skies are clear; the fairest races live
where the sun's rays are less intense and the skies are often

40 GENETICS AND EUGENICS

overcast. This signifies to the Lamarckian that the effects of
the sun's rays on the human skin are inherited; but to the
selectionist it means only that men vary in depth of pig-
mentation and that each race has migrated to that climate
which it is best fitted to endure.

As regards the origin of cave animals the same diversity of
opinion exists. Some consider that animals which found their
way into caves lost their pigmentation and transmitted this
condition to their offspring; others hold that such animals
as were able to survive when by chance they made their way
into caves were probably animals with little pigmentation,
which could not very well exist elsewhere.

As regards the vision of cave animals, the Lamarckians
hold that the eyes have degenerated because no longer used,
whereas the selectionists hold that the animals which have
taken to living in caves have been driven to this course by
the degeneration of their eyes, and they point out that the
nearest relatives of cave animals are those with poorly
developed eyes, which live in semi-darkness.

Kammerer, very commendably, has put these alternative
views to an experimental test. He has reared in daylight the
young of the cave salamander, Proteus anguinius. Under
these circumstances the skin became pigmented and the eye
did not degenerate, as normally; but if the animals were kept
in strong light continuously the skin became so heavily pig-
mented, including that in front of the eye where the trans-
parent cornea forms in ordinary animals living in the light,
that in consequence the eye itself degenerated. To overcome
this difficulty Kammerer kept the animals in red light, which
is less favorable than daylight to pigment formation, but
suffices nevertheless to stimulate the eyes to development.
The red-light treatment was given for one week out of three
during the first eighteen months of the animals' lives. In
this way the eye, which in cave-inhabiting individuals is very
small and rudimentary, was brought to full development,
with a transparent cornea and all other parts necessary for
vision.

ACQUIRED CHARACTERS 41

This result leaves no doubt that light is a necessary stimu-
lus for full development of the eye in Proteus, and it is the
absence of this stimulus which has led in part to the present
degenerate condition of the eye. Whether or not the degen-
eration has advanced from generation to generation is of
course conjectural, but seems highly probable. Weismann
indeed considered the evidence for the progressive degenera-
tion of disused organs so strong that he framed a special
hypothesis, that of germinal selection, to account for it. To
this matter we shall return later.

9. Instincts, Instincts are among the most vital posses-
sions of animals, but the same difference of opinion exists as
regards their origin as concerning the origin of other adap-
tive characteristics of organisms. Without being taught,
animals do generation after generation the same acts in the
same way. They seem to know, without individual experi-
ence or education, exactly what to eat, and how to secure it;
how to prepare a nest or burrow of a very definite pattern;
how to care for young, though they have never seen young
cared for before; what to do as the seasons change; and
numberless other vital and necessary things. Some say this
is inherited memory, nothing less; the ancestors have learned,
their descendants remember. Just as brain cells, after re-
ceiving a variety of sensations one after another, are able to
reproduce them again in the same order and complexity
through memory, so the reproductive cells become store-
houses of racial experience or habit which they transmit as
instincts. This easy way of accounting for instincts as habits
registered like phonograph records in the germ-plasm has
even been extended to all inJieritance by a number of writers,
represented at the present time by Richard Semon. This
idea had great influence in America in the last quarter of the
last century, when a strong school of modern Lamarckians,
or neo-Lamarckians, flourished here. Many still hold to
this view, but the neo-Darwinians, or followers of Weismann,
have of late been rather in the ascendancy. In their view,
instincts arise because the structure of the germ-plasm neces-

42 GENETICS AND EUGENICS

sitates a particular response when certain external stimuli are
operative, not at all because such a response has before been
made by the ancestors. Having denied that action of the
individual can affect the germ-plasm within it, they can con-
ceive of no mechanism for the transmission of habits formed
by the individual, and so deny the existence of such trans-
mission.

On the neo-Lamarckian view a hen sits on eggs because
her ancestors have formed the habit of incubating eggs; on
the Weismannian view the hen sits on eggs because she can-
not help doing it; when she is in a certain physiological state
and the nest of eggs is there, she sits, and that is all there is
to it. Neither of these views is very satisfying. On one
hand the neo-Lamarckian fails to explain how the first hen
came to incubate, which the Weismannian glibly states is
just because she is built that way; her germ-plasm necessi-
tates it. On the other hand, the Weismannian can give us
no suggestion as to how structural conditions of the germ-
plasm can cause a hen to sit rather than to crow, when a nest
of eggs is before her, but the well-established effects of
internal secretions come here to his rescue.

The whole question of the relation of instincts to inheri-
tance is very perplexing. At present we can make very little
out of it, yet there can be no doubt that it concerns vitally
our fundamental theories of evolution and such applied fields
as Eugenics.

The correct attitude in the study of instincts is maintained
by those who are seeking to learn how much each instinct
involves, and to what extent imitation and education supple-
ment or modify it. So far as possible each instinct should be
resolved into terms of response to external chemical or physi-
cal changes, or to internal physiological states. For example
it was observed many years ago that certain small Crustacea
instinctively swim toward a light. More careful study showed
that they do so only under particular conditions. If the
temperature of the water is raised, or its salinity increased,
the animal may reverse its response and swim away from the

ACQUIRED CHARACTERS 43

source of light. These are changes of external conditions
which modify the instinctive response. Internal or physio-
logical states of the animal may also modify the instinctive
responses. Thus, if the crustacean has been subjected to
mechanical stimulation (repeated touching with a sohd ob-
ject) its response may be altered.

Again larvae of a barnacle for a few minutes after hatching
swim toward the light, then they turn and swim away from it,
a series of responses calculated to bring them to suitable
spots for attachment. The response has been modified
through some internal physiological change. Larvae of the
brown-tail moth, after their winter fast, are strongly positively
phototropic. They migrate up to the tips of the branches to
feed on the opening buds. If at this time they are brought
into the laboratory and placed in a test tube, they go toward
the window and will remain at the end of the tube toward the
window until they die, even if food is at the opposite end of
the tube a few inches away. After the larvae have fed they
are no longer phototropic. Digestion has probably destroyed
the substance in their bodies on which their phototropism
depended. (Loeb, Yale Review, July, 1915.)

By such methods of studying the instincts of animals the
problem of instinct formation and inheritance may be simpli-
fied, through the elimination from it of all non-essential and
outside elements.

As intelligence increases in the animal kingdom, we find
that instinct sinks more and more into a subordinate position.
In man there is very little inherited knowledge, if instinct
may so be regarded ; nearly everything has to be learned from
the beginning. Nevertheless it is an open question whether
intelligence has not increased through use, whether we do not
learn more easily for the reason that our ancestors have for a
million generations been learners. Of course I do not refer
here to formal education, but only to the exercise of such
intelligence as distinguishes man from other animals. May
not this have been evolved in part through use ?

44 GENETICS AND EUGENICS

Summary. Notwithstanding the fundamental nature of
the problem of the inheritance of acquired characters, and all
that has been said and done to solve it, it still remains an
unsolved problem. So far as the inheritance of mutilations,
disease, and induced epilepsy are concerned, the evidence is
negative or inconclusive. Acclimatization, the effects of
changed food supply, and temperature effects can be ex-
plained quite as well on other grounds as on that of the in-
heritance of acquired characters. Pressure and light effects
are somewhat more easily explained as cumulative from
generation to generation, i. e., as inherited acquired charac-
ters, than as due merely to germinal variation. The same is
true of instincts, which, if interpreted as inherited habits,
afford the strongest outstanding evidence for the inheritance
of acquired characters. Nevertheless even here an alterna-
tive explanation is possible.

The Lamarckian view has been shown by the critical work
of Weismann and his followers to be inapplicable to many
groups of cases to which it had previously been applied. This
is a real service on the part of Weismann. Nevertheless, in
fields where the Lamarckian principle has not yet been dis-
proved, viz., as regards the effects of use and disuse, it
affords an easier and fuller explanation of progressive evolu-
tion and of adaptation in particular than does the selectionist
view. Further, Weismann and his followers have been forced
practically to concede the existence of Lamarckian evolution,
that is evolution the course of which is guided in adaptive
directions by the environment. For Weismann admits that
the environment may cause 'parallel modifications of soma
and germ-plasm. For practical purposes this is just as effec-
tive in guiding evolution as if the soma first developed modifi-
cations and then handed them on to the germ-cells. That a
mechanism for the transmission of acquired characters from
soma to germ-cells has as yet not been demonstrated, does not
of course disprove the existence of such a mechanism. Such
phenomena as memory, having its basis in the nervous sys-
tem, and as the control of development and of behavior

ACQUIRED CHARACTERS 45

through internal secretions, give us grounds for believing that
an adequate basis will be found when our knowledge of the
organism becomes more complete.

The problem of acquired characters, after all, concerns only
the higher animals. In the lower animals and in plants no
such sharp distinction exists between body and germ-cells
as we find in the higher animals. We may reproduce the
entire plant from a cutting of root, stem, or even a leaf in
some cases. Hence there is more chance in such cases of
direct modification of the cells capable of reproduction, for
most of the cells of the plant retain this capacity. In the
lowest organisms {protozoa, bacteria) there is no distinction
whatever between body and germ-cells. Every cell is capable
of reproduction; and modifications produced in a cell by the
environment are handed on directly to the next generation.
For example medical men have learned how to decrease the
virulence of diseases at will by heat or chemicals acting
directly on the disease germs. They are thus able to confer
immunity to a virulent disease by first producing and
then introducing into the body a feeble form of the same
disease.

If in the lower organisms the potentialities of living sub-
stance can thus be altered, it seems reasonable to suppose
that the same possibility may exist in the higher animals and
plants, provided agencies capable of producing change are
allowed to act on the germinal substance. It is the sheltered
position of the germ-cells which seems ordinarily to exempt
them from direct modification, but we cannot safely assume
that they are in all cases free from such modification. Experi-
ments of Stockard show that in guinea-pigs repeatedly in-
toxicated with alcohol, the germ-cells are enfeebled so that
offspring of such parents, whether male or female, are more
likely to be feeble and sickly, and so to die. Experiments of
Hertwig show that similarly the germ-cells of frogs are
capable of being injured by emanations of radium in conse-
quence of which enfeebled or abnormal offspring may be
produced.

46 GENETICS AND EUGENICS i

If the germ-cells are thus capable of modification, evolu-
tion guided by the environment must be in some measure at
least a reality. The truth then lies neither in the extreme
Lamarckian view that all acquired characters are inherited
nor in the extreme Weismannian view, that no extraneous
influences modify the germ-plasm, but somewhere in
between.

CHAPTER IV

WEISMANN'S THEORY OF HEREDITY

Weismann believed that a new type of organism arises only
in consequence of the origin of a new type of germ-cell. If he
had been asked the ancient riddle, " which was created first,
the egg or the hen," he would undoubtedly have answered,
" the egg.'' He would have explained that the first bird came
from a new type of egg laid by a reptile-like ancestor.
Changed structure of the germ-plasm must result, he
thought, in changed structure of the organism developing
from it; and he would scarcely have admitted that a new
sort of organism might arise in any other way. But the
experimental study of the development of organisms has
shown that the germ-plasm forms only one of two comple-
mentary sets of agencies which determine what the adult
organism shall be. It is true that the character of the germ-
cell determines in part what the character of the adult organ-
ism shall be, but so also does the environment. If we plant
beans, we must expect to harvest beans not corn, but whether
the harvest is large or small will depend upon the soil and the
season. Sunlight, moisture, a suitable temperature, and
proper chemical substances in the soil are all indispensable
conditions to the production of any crop at all, and they con-
trol within hmits the size, vigor, and productiveness of the
plants grown. Both internal and external agencies influence
the form of organisms. These are sunmiarized in the two
words, heredity and environment. Weismann emphasized
the first almost to the neglect of the second. Lamarck had
previously gone to the opposite extreme, emphasizing the im-
portance of the environment not only in directly adapting the
organism to its surroundings but also in controlling its

47

48 GENETICS AND EUGENICS

heredity. It is coming to be recognized that the truth hes
somewhere between these extreme views.

WTiat in general were Weismann's views and how did he
arrive at them ?

Weismann's Method

Weismann's method of constructing an hypothesis to
account for heredity differed fundamentally from Darwin's.
Darwin reasoned inductively, Weismann deductively. Dar-
win tried first to ascertain what characteristics are inherited
and then to imagine a mechanism which might explain their
inheritance. The result was " pangenesis." Weismann, on
the other hand, first inquired what is the mechanism of in-
heritance and, having answered this to his own satisfaction,
proceeded to the conclusion that only such characters are
inherited as have their basis in this mechanism. The result
was the chromosome theory of inheritance. It has this fea-
ture in common with " pangenesis," the inherited character-
istics are supposed to be determined in advance and to be
represented in the germ-cell by material bodies. These are
the " gemmules " of Darwin, the " determiners " of Weis-
mann. Darwin supposed that the " gemmules " migrate
from all parts of the body into the germ-cells and so make it
inevitable that the organism which develops out of the germ-
cell shall have the same parts and properties as the parent.
As regards the origin of variations, pangenesis might be
called a centripetal theory, since determiners are supposed
by it to migrate centrally tow^ard the germ-cells.

Weismann's theory, on the other hand, is centrifugal; he
supposes that the " determiners " originate solely in the
germ-plasm and migrate thence out into the various parts of
the developing body and that thus differentiation is pro-
duced. There is on his view no centripetal movement of
determiners whatever; they never pass from soma to germ-
cells, but only in the reverse direction.

WEISMANN'S THEORY OF HEREDITY 49

Weismann's Mechanism of Heredity

Weismann had this advantage over Darwin; in his time
knowledge of the structure of the germ-cells had considerably
increased over what it was when Darwin conceived the hy-
pothesis of pangenesis.

Weismann identified his " determiners " with certain con-
spicuous structures of the germ-cell called chromosomes
(unknown in Darwin's time), and supposed that the nature
of these determines and controls the nature and activity of
the cell containing them.

It is the theoretical importance which Weismann and
others have assigned to these structures that has given them
their great prominence in the study and description of cell
phenomena in the last thirty years. In reality the chromo-
somes make up a part only of the germ-cell and we have no
certain knowledge that they form the more important part.
Nevertheless a majority of biologists, probably, at the present
time believe with Weismann that heredity is due to material
substances or determiners which are located in the chromo-
somes. The principal reasons for so thinking are:

1. The conspicuousness of the chromosomes at the tune
of cell division and the very exact manner in which as a rule
each of them divides into two equal parts, which pass into
different cell-products.

2. The constancy of the number of the chromosomes in
the same species of animal or plant. The number is different
in different species but within the same species it is very con-
stant. The only known exceptions to this rule are such as
may be cited in support of the general idea that chromosomes
are determiners of heredity.

(a) The two sexes within the same species frequently differ
as regards the number of chromosomes in their genn-cells.
When this is the case the male has the smaller number of
chromosomes, and it is assumed that the chromosome or
chromosomes which the male lacks determine femaleness.

(6) It has been shown in the case of the evening primroses
(Oenothera) that a particular heritable type of variation

50 GENETICS AND EUGENICS

(" lata mutant ") contains one more chromosome than the
parent species from which it has been observed repeatedly to
arise. Another type of mutant in this same group of plants
contains twice the ordinary number of chromosomes (" gig as
mutant," Gates, 1915). The fact that visible characters of
the organism vary simultaneously with variation in the
chromosomes creates a presumption that the relationship is
a causal one.

3. The experimental evidence shows that in general the
father is just as influential as the mother in determining the
inheritance of the children. But the egg-cell is vastly larger
than the sperm-cell. Therefore much of the substance of the
egg cannot be concerned in heredity. What the egg and
sperm-cell have in common consists more largely of chromatin
than of any other substance. This makes it seem probable that
chromatin is concerned in heredity.

4. There exists a parallelism between the behavior of the
chromosomes in the development of the germ-cells and that
of certain characteristics in heredity. It is supposed, there-
fore, that the chromosomes actually contain chemical sub-
stances necessary for the development of these inherited
characters and in this sense are determiners of heredity.

The assumption of Weismann that heredity is due to deter-
miners contained in the germ-cell, like the pangenesis theory
of Darwin, has encountered many difficulties. Consequently
numerous supplementary hypotheses have been found neces-
sary to enable it to feature as a general explanation of the
facts of inheritance.

Difficulties Encountered by Weismann's Theory

1. Development (ontogeny). The first diflSculty en-
countered lay in the explanation of the development of the
individual from the egg. Weismann assumed that each cell
owes its peculiar form and activities to the determiners which
it contains, these being located in its chromosomes. Since
the cells composing the different parts and tissues of the body
differ in their forms and activities, it was necessary to assume

WEISMANN'S THEORY OF HEREDITY 51

further that the different kinds of cells contain different de-
terminers and consequently that as the egg divides up into
cells which form the different parts of the body, these cells
must receive different determiners. But microscopic exami-
nation of the cells of the body reveals no such differences; it
shows differences in pretty much everything except chromo-
somes, which remain remarkably constant.

Boveri (1887) has described one case which seems to
support the idea that changes in the chromatin occur, as
body-cells become distinguishable from germ-cells. In the
parasitic worm, Ascaris, the chromosomes are seen partially
to break up and disintegrate in those cells of the embryo from
which the body arises, whereas the original ovarian structure
remains unmodified in the germ-cells. No similar case, how-
ever, has been described in other organisms, so that it seems
very doubtful whether the observed changes have the signifi-
cance originally attached to them by Boveri.^ There are good
reasons for believing that the chromatin content of each cell
of the body is like that of every other cell of the same body,
and that differentiation results either (a) from the position
of a cell in relation to other cells, which will accordingly regu-
late its intake and output, or (6) from an original difference
in substance contained in the cytoplasm of the cell (the extra-
nuclear part). Such cytoplasmic differences between cells
arise, during development, from the fact that the egg cyto-
plasm, at the beginning of development, is not homogeneous,
and consequently the cells into which the egg divides are not
alike in cytoplasmic content.

2. Regeneration. A man who loses a leg or an arm is de-
prived of the same for the remainder of his life, but many of
the lower animals can restore lost parts by a process which
we call regeneration. If a young salamander, a crab or a
lobster is deprived of a leg, a new leg grows out again from

1 It is true that Hegner (1914), confirming Kahle (1908), has also observed
"diminution of chromatin" occurring in the differentiation of somatic cells in an
insect, Miastor, but in numerous other animals studied by Hegner he has found
no such diminution of chromatin but has observed the germ-cells to be differen-
tiated solely by cytoplasmic changes.

52 GENETICS AND EUGENICS

the stump of the old one. Such facts as these compelled
Weismann to assume that, in cases of leg regeneration, not
all the leg determiners pass out during development into the
leg, but a supply is also held in reserve in the adjacent parts
of the body; these being latent or inactive ordinarily, but
becoming active when the leg is removed.

Experimental studies of regeneration made by Morgan,
Child, and others scarcely support Weismann's view. They
indicate that any undifferential cell of the body, if placed at
the stump of an amputated leg, might function in leg re-
generation, and so that specific leg regenerators do not exist.
It is true that, in many animals, particular groups of cells
have the ability to produce only a particular kind of struc-
ture, no matter where they are placed in the body, in a
transplantation experiment. But in such cases it is pretty
clear that we are dealing, not with the effects of specific deter-
miners, but with the consequences of cytoplasmic differentia-
tion which, in many cases at least, arose in the undivided ^gg
when no nuclear difference existed within the organism, since
it contained only a single nucleus.

3. Polymorphism. In many species of animals and plants
the form of the adult differs fundamentally according to the
environment in which it is placed. In certain amphibious
plants (e. g., Ranunculus aquatilis) the plant when growing
in the air develops flat broad leaves, but when growing under
water develops leaves dissected into numerous hairlike ap-
pendages. Weismann supposed that in such cases there exist
alternative sets of determiners in the germ-plasm, one for the
land form of leaf, one for the water form, conditions of dry-
ness or dampness during development calling one or the other
set into activity. If intermediate conditions were shown to
produce intermediate effects, he would doubtless assume a
joint and partial activity of both sets. In animals more
complicated conditions of polymorphism occur. Many spe-
cies of butterfly have spring and summer generations of off-
spring (broods as they are called), quite different in appear-
ance, corresponding to different external conditions of tem-

WEISMANN'S THEORY OF HEREDITY 53

perature or food supply. The gall insects of oak and willow
trees have summer and winter generations very different in
character. The summer generation usually feeds upon the
soft tissues of the growing leaf and produces winged adults of
both sexes; whereas the winter generation feeding on the
woody tissues produced by a stem or metamorphosed bud,
may consist of wingless females only, which lay unfertilized,
i. e., parthenogenetic eggs. In such cases Weismann sup-
poses that alternative sets of determiners exist in the germ-
plasm, which are activated by summer or by winter condi-
tions respectively.

The case of the social insects (bees and ants) is still more
complicated ; here there may exist four or ^ve different adult
forms as drones (males) queens (egg-laying females) and
workers or soldiers of various sorts. The workers and soldiers
are all imperfectly developed females, not producing eggs
ordinarily but merely taking care of the rest of the colony.
Experiment has shown that the same egg, in the case of the
honeybee, may produce either a queen or a worker, depend-
ing upon the amount and quality of the food suppHed to the
developing larva. The same is undoubtedly true of the
various sorts of soldiers, among other social insects, these be-
ing alternative forms of the female. Weismann supposes
that there are as many distinct sets of determiners in the egg
as there are different forms into which it may develop. This
line of explanation assigns to determiners located within the
nucleus of the egg, influences which demonstrably lie outside
the egg. As an explanation of polymorphism the theory of
alternative nuclear determiners is not only superfluous but
also positively erroneous.

4. Variation. Weismann supposed that all variations
originate in the germ-plasm, and subsequently find expres-
sion in the body of the offspring, reversing the idea of La-
marck and Darwin, who supposed that variations first origi-
nate in the body and are thence transferred to the germ-cells.
To account for adaptive variation, Weismann framed two
supplementary hypotheses. 1. To account for the origin of

54 GENETICS AND EUGENICS

inherited variations similar to those which the environment
directly produces in the body, he invented the hypothesis of
parallel modification of germ-plasm and soma, to which refer-
ence has already been made. 2. To account for the appar-
ent inheritance of the effects of use and disuse, he invented
the hypothesis of germinal selection. On this view the various
determiners which compose the germ-plasm are competing
with each other in a struggle for nourishment, just as animals
and plants struggle with each other for existence in the world
at large. Sometimes one determiner gets more nourishment,
sometimes another; but whichever one gets most nourish-
ment, grows largest, and would consequently give rise to a
plus variation of a corresponding part or organ of the body.
When one determiner gets more nourishment, that is, pro-
duces a plus variation, some other determiner gets less and
so produces a minus variation. Thus there is perpetual varia-
tion in the parts and organs of the body, which affords
abundant material for natural selection to act upon. For if
any essential organ gets too small, its possessor is eliminated.
But if the organ which undergoes minus variation is a use-
less one, no disadvantage results to the organism; on the
contrary, there is more nourishment left for essential organs,
which therefore grow at the expense of the useless ones.
Thus through natural selection useless organs tend to dimin-
ish and ultimately to disappear altogether, while essential
organs (those most used) grow in size and activity. An
apparent inheritance of the effects of use and disuse results.
Modern research supports Weismann's theory of nuclear
determiners to this extent. It appears highly probable that
special chemical substances necessary for the production of
particular variations are located in particular parts of the
cell, possibly in chromosomes. It is also conceivable that
these substances may vary from cell to cell in amount or
quality, and that under a constant environment variation in
particular organs affected may thus result. But it is not neces-
sary to suppose, as Weismann did, that these groups of sub-
stances are engaged in a struggle of any sort, with each other.

CHAPTER V

ATTEMPTS TO CLASSIFY AND MEASURE VARIATION:

BIOMETRY

The period from 1880 to 1900, following Darwin's death, was
marked by extreme speculation concerning evolution rather
than by inductive study of its phenomena. This speculative
tendency found its culmination in Weismann's brilliant es-
says, but his ideas, notwithstanding their brilliancy, failed to
win acceptance among such biologists as insisted on having a
substantial basis of well-ascertained facts on which to rest
their theories. Weismann's theories were accordingly dis-
tinctly on the wane when in 1900 they received support from
an unexpected source, the rediscovery of Mendel's law of
heredity, which now fully established seems to require for its
explanation some such system of determiners as Weismann
had hypothecated and located in the chromosomes.

During this period of speculation about evolution, biolo-
gists had been looking in various directions for new tools
with which to attack the study of evolutionary problems.
The facts of development were more carefully studied and
accurately described than ever before, and more precise in-
formation was sought about the influence of environment
upon development and growth. Thus experimental embry-
ology and experimental morphology were born, to be followed
a little later by experimental breeding. Meantime, Bateson
was attempting to classify variations on morphological
grounds without reference to their causation, and Pearson
was seeking to measure variability so as to determine its
direction and rate of progress.

Darwin had throughout nearly a lifetime collected all ob-
tainable facts about variation in animals and plants as a
basis for his generalizations concerning evolution and hered-
ity. Much of his data is contained in his work on the

55

56 GENETICS AND EUGENICS

Variation of Animals and Plants under Domestication.
Bateson took up this work after Darwin's death and collected
a large number of facts concerning variation, which he at-
tempted to classify, but without great success. His results
are found in a book entitled Materials for the Study of Va-
riation, published in 1894. The most important conclusion
reached by Bateson, was one which Francis Galton had al-
ready stated with great clearness in 1889 {Natural Inheri-
tance), viz., that variations fall naturally into two classes,
continuous and discontinuous. Continuous variations are
those which are graded, the extremes being connected by a
complete series of intermediate conditions; discontinuous
variations are such as are separated by gaps in which no
intermediate stages occur. Bateson believed that discon-
tinuous variations are more important in species formation
than are continuous ones, because, where variations are
discontinuous, the action of natural selection is greatly sim-
plified. In discontinuous variation selection determines the
survival of one or the other of two distinct groups, since
intermediates do not occur and it is unnecessary to assign
selectional value to each plus or minus gradation of an organ.
Galton had earlier expressed the same idea, suggesting that
evolution may be like the behavior of a polyhedron when
pushed. If pushed or tipped a little, it returns to its former
position of equilibrium, merely oscillating back and forth on
the same face as before. But if it is pushed hard enough, it
rolls over on to a new face coming to rest in a new position
of equilibrium. Galton suggested that discontinuous varia-
tions may be species forming variations, stable from the start,
whereas slight or graded variations may have no lasting
effect, like the oscillations of the polyhedron on one and
the same face. This view was strongly supported a few
years later by the botanist De Vries in his theory of muta-
tion (1900-1903).

Meanwhile variation was being studied from a new point
of view, which we may call biometry. Francis Galton (1889)
was the founder of biometry but its full development has

BIOMETRY 57

been due chiefly to the valuable work of Karl Pearson. The
underlying idea in biometry is to apply to the study of evolu-
tion the precise quantitative methods followed in the study
of physics and chemistry with such signal success.

Biometry is the statistical study of variation and heredity.
It deals with masses, not with individuals, differing in this
respect from the method of Darwin and Bateson. It seeks
to obtain a quantitative estimate, as precise as possible, of
variation in one generation, and to compare with this a
similar quantitative estimate of the next generation and then
by comparing these to learn in what direction evolution is
taking place and at what rate. In some cases it has at-
tempted to discover the direction of evolution from the
character of the variation within a single generation.

Biometry is best adapted to deal with continuous varia-
tion, but it has its uses also in dealing with discontinuous
variations. Its ideal, to make biological investigation more
accurate and comprehensive, is wholly commendable. But
mere collection and compilation of biological statistics will
not advance knowledge unless brought into relation with
other facts about living things, and it is in this respect chiefly
that biometricians have sometimes erred, drawing unwar-
ranted conclusions from their statistical data.

Biometry means literally the measurement of living things.
It is obvious that it can deal only with characteristics which
are measurable, such as linear dimensions, volume, weight,
or number of parts. One of the cases most carefully studied
by Galton was human stature. This case illustrates very
well the methods and results of biometric study.

Measurements made at the Harvard gymnasium of the
height and weight of one thousand students of ages eighteen
to twenty-five are classified in Table 1. In order that the
number of classes may not be too great for convenient sta-
tistical treatment, height classes are formed of 3 cm. each.
Thus students measuring 155, 156, or 157 cm. are all placed
in a common class, whose middle value is 156 cm. In dealing
with large numbers, the probability is that each of the three

Number of
Individ-
uals

180

If.O

HO

120

100

80

CO

40

20

0

V

1
1

^,

f

J
»

t
1

1

<
*

<

»

1

\
t

1

\

*

1

\
1

1

\
(
\

\

\
\
\

\

<

f

1

1
i

f
1

1
I

r

(

\
\
\
\

\
%
(

%
1
1

#

1

t

1

1

t
(
1

f
/

/
«
«

1

/
1

/
1
#

1

%
1

»

/
/
/

«

t

/

t
t

/

4
%
%

ir.C 15» 162 165 168 171 174 177 180 183 186 189 192 195 198

Height in OenlirMtera r\ \

Fig. 9. Frequency-polygon and curve showing variation in height of one thousand

Harvard students of ages 1&-S5.

58

BIOMETRY 59

measurements would occur as frequently as either of the
others, so that the middle value would be a fair representa-
tive of the class and could be used in statistical computations
as the class value with entire propriety and accuracy. Weight
classes are also formed of three kilos extent in classifying the
weights. The numbers of individuals found in each height
class are shown in the totals at the bottom of Table 1. The
largest number of individuals is found in the class, 173-175
cm., viz., 188. On either side of this class the numbers of
individuals (called frequencies) fall off steadily reaching a
frequency of four in the shortest class and of one in the tallest
class. In Fig. 9 the relative frequencies of the height classes
are shown graphically, each column of the figure being pro-
portional in altitude to the frequency of the class which it
represents. This method of representing variation is called
the " method of loaded ordinates." By joining the tops of
the several columns of the figure, as in the dotted line, a so-
called variation curve is obtained.

The class with greatest frequency in a group of variates is
called the mode, i. e., the fashionable class. It has, of course,
the tallest ordinate in the variation figure (class 174, Fig. 9).

A classification of the same one thousand students as re-
gards weight is given in the totals at the right of Table 1,
and a graphic presentation of the same data in Fig. 10. The
modal class is that which has as its middle value sixtv-three
kilos. This has a frequency of one hundred and fifty-four
with the two adjacent classes almost as large and more remote
classes diminishing in frequency to minima in classes forty-
five and one hundred and five. The falling off is more rapid
to the left than to the right of the modal class, so that in all
there are only six classes below the mode but there are four-
teen classes in the range of variation above the mode. This
results in a " skew " or asymmetrical curve obtained by
joining the tops of the ordinates (dotted line. Fig. 10). The
variation curve for the height measurements (Fig. 9) was
also slightly skew, but its skewness was much less than that
of the curve for weight.

60

GENETICS AND EUGENICS

A variation curve which is free from skewness resembles
what mathematicians call a " frequency of error " curve or
simply a '* curve of error " or '' normal curve " (Fig. 11).

Number

of
Individ
uals
160

140

120

100

80

/
/

/

\

\

-V-

%
t

1

t

}■

(
f
•
•

1

1

1

%

t
>

<
%

%

«
«

1

i

t

1
\

t

60

t
1

1
•

1

1
f

«

40

r

'
\

\

1
>

*
•
<
1

\
1

\
\

i
\

20

/

1

0

>

K. .

45 48 61 54 57 €0 63 66 69 72 75 78 81 84 87 90 93 96 99 102 1051

Weight in Kilograms

Fig. 10. Frequency-polygon and curve showing variation in weight of one thousand

Harvard students of ages 18-25.

BIOMETRY

61

TABLE 1

Showing the Variation in Height and Weight and the Correlation

BETWEEN Height and Weight among 1000 Harvard Students

OF Ages 18-25 Measured at the Harvard Gymnasium

IN the Years 1914-1916

Weight

H

eight

in Centimeters

ID

Kilos.

155-
157

15S-
160

161-
163

164-
166

167-
169

- 170- 17^ 176- 179- 182-
172 175 178 181 184

185- 188- 191-
187 190 193

194-
196

197-
199

To-
tals

44- 46

1

• *

1

47- 49

1

, .

3

1

, ,

, ,

, ,

, ,

. .

• •

• •

5

50- 52

1

2

1

6

4

6

2

. .

, ,

, ,

. ,

22

53- 55

1

4

8

15

12

8

7

2

, .

, ,

1

58

56- 58

1

4

10

15

19

20

11

3

2

, ,

85

59- 61

1

5

8

22

43

25

21

11

4

2

142

62- 64

1

2

8

9

31

39

29

21

10

2

2

154

65- 67

1

2

10

21

25

39

30

18

4

, ,

1

151

68- 70

1

1

9

6

30

27

32

16

13

2

1

138

71- 73

. ,

2

4

5

18

20

12

18

15

4

2

100

74- 76

• •

1

4

11

15

6

7

9

6

1

60

77- 79

• •

1

2

2

8

5

7

4

4

1

34

80- 82

, ,

• ,

• .

4

6

3

4

6

2

25

83- 85

, ,

• ,

, .

2

1

2

3

2

2

12

86- 88

, ,

2

1

2

, ,

2

, ,

7

89- 91

, ,

, ,

. ,

, ,

, ,

1

1

2

92- 94

, ,

• ,

, .

1

1

• ,

2

95- 97

• •

, ,

, ,

• •

• •

• •

, ,

98-100

a ,

• •

• •

« •

• •

• •

• •

101-103

• •

, ,

• •

• •

• •

• •

, ,

104-106

• •

• •

• •

1

1

2

Totals

4

8

26

53

89

146

188

181

125

92

60

22

4

1

1

1000

Mean height = 175.33 cm. (5 ft. 9 in.)
Mean weight = 65.66 kilos. (144.75 lbs.)
a height = 6.58 cm.

ff weight = 7.84 kilos.

C V height = 3.76 %
C P weight = 11.94%
r height-weight = .54

It expresses the result of the simultaneous action of several
independent causes, or contingencies. If, for example, I toss
ten coins in the air simultaneously, it is certain that each one
will show uppermost on landing either a head or a tail, but
the landing of one coin does not affect that of the others.
The landing of each coin is a separate contingency. If the

62

GENETICS AND EUGENICS

coins are thrown several times and a count made of the
number of heads following each throw and these results are
then combined and plotted we shall get a frequency of error
curve about the number five which will be the most frequent ,

Fig. 11. " Frequency of error " or " normal " curve M, mode. Q, Q', quartile; one-half
the area of the figure lies between Q and Q'. After Lock.

{. e., the modal result, heads being of the same frequency as
tails. See Fig. 12 and Table 2.

Biometry has established the fact that biological variation,
when measurable, is commonly of the frequency of error

TABLE 2
Probable Results of Tossing Ten Coins Simultaneously. (After Lock)

Heads
10
9
8
7
6
5

and

Tails

0 .

1 .

2 .

3 .

4 .

5 .

Relative Probability

1

10

45

120

210

252

Heads
4
3
2
1
0

Tails Relative Probability

and

6.
7.
8.
9.
10.

210

120

45

10

1

type, which means that it must be the result of several inde-
pendent contingencies or causes. Some of these causes are
doubtless environmental, others are due to heredity. Their
combined action is to produce variation of the frequency of
error type.

BIOMETRY

63

The action of several heredity factors which are indepen-
dent of each other produces a curve of the same sort; and
so do several environmental factors independent of each
other; in most cases of variation agencies of both sorts are
at work. But in some cases the causes which tend to pro-
duce plus variation may be stronger or weaker than those
which tend to produce minus variation. The result is an

Tails I 2 3 45 6 769 10

Fig. 12. A graphic presentation of the data contained in Table 2. After Lock.

unsymmetrical or " skew " variation curve. Thus among
Harvard students the causes which tend to produce variation
in weight above the normal are apparently stronger than
those causes which tend to produce weight below the nonnal,
as is indicated by Fig. 10. The same was found to be true still
more emphatically of adult males in England, according to
data tabulated by Yule.

In some cases, biological variation is exclusively in one
direction from the mode, i. e., all the causes of variation
which are operative tend in one direction. Thus the common
buttercup varies in number of petals from five upward but
very rarely in the reverse direction. Five is the commonest
or modal number, but the observed variation curve is one-
sided. See Fig. 14, H 1887.

It is evident that to describe the character of variation in
any case it will not suffice to name the mode; we must also

64 GENETICS AND EUGENICS

state whether the variation is symmetrical about the mode,
how extensive is its range, and whether the majority of the
variates cluster closely about the mode or are widely scat-
tered. To express these various features of the variation,
special statistical coefficients have been devised. It will
suffice for our purposes to discuss only the more important
of these.

1 . The mean, or average, is in a case of symmetrical varia-
tion, identical with the mode. Thus the average height of
the thousand Harvard students (Table 1) is close to 174 mm.,
the mode. But their average weight lies outside and above
the modal weight class, because their variation in weight
is decidedly skew, more men exceeding 66 kilos in weight
than fall below that weight. To find the average, multiply
the value of each class hy the number of individuals contain-
ed in it, add the products, and divide hy the entire number of
individuals.

2. Average Deviation and Standard Deviation. Two sets of
variates having the same mode and mean may nevertheless
differ widely in their variability, one being more scattered
than the other.

To express the greater spread of one curve as compared
with another, the average deviation, may be employed. That
is, we may estimate how far, on the average, an individual
taken at random differs from the mean. This is computed as
follows : Find the deviation of each class from the mean, multi-
ply this by the frequency of that class, add the products, and
divide by the entire number of variates. The quotient is the

111 D f

average deviation. Formula A D = in which 2) signi-
fies that the sum is to be taken of the products indicated,
D means the deviation of each class value from the mean of
all variates, / means the frequency (number of individuals)
of each class, and n means the total number of variates
(individuals). This measure of variability is improved,
mathematicians tell us, by the method of least squares, i. e.,
by squaring the deviation of each class, and extracting the

BIOMETRY 65

square root of the final quotient. To distinguish it from the
average deviation, this is called the standard deviation. Its

• It forms a measure of the degree of

scatter of the variates. This measure is expressed in the same
units as were employed in measuring the variates.

3. To compare one case of variation with another as re-
gards degree of scatter of the variates, another expression has
been devised which is called the Coefficient of Variation. It
is obtained by dividing the standard deviation by the mean.

Formula, CV = — 77 It is an abstract number expressing

the variability in per cent of the mean.

Judged by their coefficients of variability. Harvard stu-
dents are found to be more variable in weight than in height,
the respective coefficients (C V) for height and weight being
3.76 and 11.94. See Table 1.

4. Another important tool of the biometrician should be
mentioned, viz., the coefficient of correlation, which is a
measure of the extent to which one character varies in
agreement with another.

In order to obtain a coefficient of correlation a set of
observations may be classified simultaneously as regards
two characteristics. Thus we might inquire is there any
correlation between the height and the weight of men, and if
so how much ? Are tall men on the whole heavier than short
ones or vice versa ? To determine this matter we must first
obtain observations on the height and weight of the same
individuals. The observations may then be classified in a
correlation table (as in Table 1), which is made by ruling
paper into squares and entering the observations on height
in vertical columns, and the observations on weight in hori-
zontal rows, or vice versa. An individual 156 cm. in height
and weighing 48 kilos will be entered in the square at which
column 156 and row 48 intersect; an individual of the same
height but ten kilos heavier will be recorded in the third
square below, and so on. When all the observations have

66 GENETICS AND EUGENICS

been entered in the table, we may proceed to calculate ^ a
coefficient of correlation which will be a measure of the
extent to which men vary in weight as they vary in height.
Its numerical value will lie between 0 and 1.

It is evident that the correlation would be most complete if
men invariably increased in weight as they increase in height.
The entries in the table would then be distributed in a single
diagonal row running across the table from its upper left-hand
corner to its lower right-hand corner. We should infer that
in such a case the two completely correlated phenomena were
due to the same causes or contingencies exactly. Our numer-
ical coefficient of correlation would in such a case be -f- 1.

In reality such correlation as this rarely, if ever, occurs in
biological material. We know that men of the same height
vary in weight and vice versa. For weight does not depend
upon height alone but also upon width and thickness and
specific gravity. It does however depend somewhat upon
height, and so our table would show incomplete correlation,
which would be expressed by a coefficient less than 1 but
greater than 0.

^ The coefficient of correlation is calculated by the formula

r = .

in which r is the coefficient of correlation, Di and Dy are the deviations of each
observed group of individuals from the respective means of height and weight, S sig-
nifies that the sum of the products indicated is to be taken, n is the total num-
ber of individuals observed, and <Tx and Cy are the standard deviations for height
and weight respectively. To express in the form of a rule the procedure to be
followed in calculating the coefficient of correlation between (say) height and
weight: First find the average height and the average weight of all individuals ob-
served, then their standard deviation in height and their standard deviation in
weight. Next determine for each square of the table its deviation from the aver-
age height and average weight respectively. Find the product of these two devia-
tions (regarding signs) and multiply it by the number of individuals recorded
in the square under consideration. After such a product as this has been found
for every square in the table, the products are to be added (regarding signs) and
this sum is to be divided by the product of the two standard deviations times the
total number of individuals observed. There are several short-cuts by which the
calculation as here described may be shortened or simplified. For a description of
these the reader is referred to the special works of C. B. Davenport (1904), Eugene
Davenport (1907), and Yule (1912).

BIOMETRY 67

In the table the entries would show a tendency to group
themselves about the diagonal, but there would be a con-
siderable scattering of entries in squares not lying in the
diagonal. Compare Tables 1 and 3.

If men in general did 7iot increase in weight as they increase
in height, but actually grew lighter as they grow taller, then
we should find a negative value for the coefficient of correla-
tion. Cases of this kind are occasionally met with, but they
are of no importance since by rearrangement of the correla-
tion table (as by reversing the order of the grades for one
character) a negative result may always be converted into a
positive one of like magnitude. The essential thing, which a
coefficient of correlation does, is to show whether two ob-
served phenomena are or are not causally related to each
other. Any result other than 0 indicates that the two sets
of phenomena are so related, and the size of the coefficient
indicates the extent to which they are causally related, up to
a value of + 1 which would indicate that they are due to
identical causes.

In biometry the correlation table has found two principal
uses (1) to show what parts or processes of an organism
vary in unison and to what extent they so vary and (2) to
measure heredity. Examples of the first use are the relation
between height and weight in man already discussed and
the relation between one skeletal dimension and another,
as skull length and femur length, which in rabbits have a
correlation of 0.76, or the lengths of femur and humerus,
which in rabbits show a correlation of 0.86. See Table 3.
The correlation values for corresponding bone measurements
in men are very similar. If the correlation between two
parts is known, it is possible from a knowledge of the
magnitude of one of them to predict the magnitude of the
other, with an accuracv indicated bv the coefficient of corre-
lation. If for instance the correlation between femur and
humerus is 0.86 and I know the femur length of an individ-
ual, I can estimate his humerus length with an accuracy of
about 86 per cent.

68

GENETICS AND EUGENICS

The second use of the correlation coefficient is still more
important, viz., to measure the strength of heredity. It
affords a means of comparing the strength of a character in
successive generations and of thus measuring its heredity.
Thus the amount of white on the body of piebald rats is a
variable character (Fig. 125) to some extent inherited. The
resemblance between parents and offspring in grade of white-
ness as shown in Table 4 is about 23 per cent, the correlation
coefficient in this case being 0.233. Pearson found, for his
human data, the height of father and son to have a correla-
tion of 0.514; between brother and brother he found the
correlation to be 0.511, figures which indicate the strong
inheritance of size differences in man.

TABLE 3

Correlation Table showing the Rel.\tion between Femur-length
AND Humerus-length IN 370 Rabbits, r = 0.857

From MacDov

rell. Appendix, Table 16,

Hnmpriis

Femur, Length in mm.

Length in mm. 'j

'6- 78-
77 79

SC-
SI

S2-
83

84- 86- 88-
85 87 89

90-
91

92-
93

94-
95

Totals

60-61

1 2

2 16

9

1

13

51

13

1

4

62-63

4
32
52
10

1

35

64-65

4

96

66-67
68-69
70-71
72-73

47 4 ..

29 29 4

3 13 13

1 4

• •

4
1

3

2

116
73
33
10

74-75

2

76-77

78-79

1

1

Totals

3 27

79

99

83 47 21

5

5

1

370

Probable error is a measure of the reliability of a statistical
conclusion. The need of such a measure rests on the fact
that the number of observations on which the conclusion rests
is finite, that is the number of observations is smaller than
the class concerning which generalization is made. For ex-

BIOMETRY

69

ample, if I knew the height of each member of a college
class I could calculate the absolute average height of the
class without any possible inaccuracy, if the arithmetical
operations were free from mistakes. But if I want to know
the average height of students in the entire college and have
only the measurements of a particular class on which to base
an opinion, it is obvious that my conclusion is possibly
erroneous. Perhaps I have not a fair sample of the students
of the college as regards height. Obviously the larger my
class the less probable is any error in my conclusion. If my
class included half or more than half of all the men in the
University (unselected as to size) the probability of an error
through random sampling would be small; and if it included
all men in the University, the probability of error would
disappeai;.

TABLE 4

Correlation Table used as a Measure op Heredity. The Character

Studied is the Relative Amount of White in the " Hooded "

Pattern of Piebald Rats, r = 0.233.

From Castle and Phillips, Table 11.

Grade of

Grade of Offspring

Totals

Parents

2f

3

3J

31

31

4

4J

4i

4!

5

31

2

7

2

• •

11

31

2

7

17

87

162

41

11

3

3

. ,

333

4

, ,

3

2

25

87

65

24

6

1

1

214

4i

, ,

3

3

16

49

27

8

2

2

. .

110

4i

, ,

, ,

. .

2

13

5

3

1

1

. .

25

41

1

3

4

Totals

2

13

22

132

319

143

46

12

7

1

697

What statisticians call the probable error is a pair of values
one larger than the calculated value, one smaller, the
chances being even that the true value lies inside or outside
the limits of these values.

To understand the significance of this statement, consider
for a moment the normal curve or curve of error (Fig. 11).
On either side of its mean and mode (M) we may draw a line

70 GENETICS AND EUGENICS

(Q^ Q') so placed that between the two Hnes half the area of
the figure will be included. It is obvious that an individual
taken at random may fall in any part of the figure, but the
chances are even that it will fall inside or outside of the
probable error (Q, Q') since half the group occurs in each
position. The probable error of a determination of the mean

equals ± 0.6745 —7=- Notice in accordance with this that

the more individuals one observes the more accurate his con-
clusion, i. e., the less the probable error, but not in direct
proportion to the number observed but to its square root.

The probable error of the standard deviation is expressed
by the equation,

Ea =^ 0.6745 — ^

The probable error of the coefficient of variability is
expressed by

CV

Ec=^ 0.6745

V^

n

The probable error of the coefficient of correlation is

expressed by

r, ± 0.6745 (1 - r^)
£jr — 1=

yjn

The probable error of a determination of the cross-over
percentage between two linked characters is

P (1 — P) .

rt .6745\/ in which P is the observed cross-over

T n

percentage. (Haldane, 1919.)

Private Property of

Z. p. r^^ETCALF

iliJi'i'MJiMiWiM)'

CHAPTER VI

THE MUTATION THEORY

The theory that new races and species originate discontinu-
ously and not gradually, has received its strongest support
from the work of the Dutch botanist, Hugo de Vries, who
was one of the pioneers in the recent revival of the study of
evolution by experimental methods.

De Vries began studying the variation of species of plants
in the field, transferring these variations to his garden and
there subjecting them to selection. He found that garden
conditions, i. e., cultivation and improved nutrition, in-
creased variability as regards minor differences in size,
luxuriance and productiveness. Such variations, which Bate-
son calls continuous, De Vries speaks of as fluctuating. They
depend, he thinks, wholly upon nutrition but do not per-
manently affect the specific type. This is stable, like Galton's
polyhedron resting securely on one of its faces. Its fluctua-
tions due to nutrition are like the oscillations of the poly-
hedron. No permanent change results from them. De
Vries indeed appears to think that selection acting upon
fluctuations {i. e., upon continuous variations) may change
the average condition of the race, but that such changes will
not persist unless maintained by rigorous selection. As soon
as selection ceases, he thinks, the race begins a gradual return
to its former condition.

De Vries supported this view both with data from the his-
tory of cultivated plants and with direct experiments of his
own. He showed for example that in the history of the
cultivation of the sugar beet, the unimproved race contained
(about sixty years ago) from 7 to 14 per cent of sugar. Vil-
morin after two generations of selection of the sweetest beets
for seed obtained beets with 21 per cent of sugar. Since then
the choice of individual seed beets according to sugar-content

71

72

GENETICS AND EUGENICS

has become general. Often hundreds of thousands of beets
are tested at a single factory. De Vries has plotted a varia-
tion curve for forty thousand beets tested in 1896 at a factory
in Holland. The result (Fig. 13) was a beautiful frequency
of error curve with its mode at 15.5 per cent. The upper
limit of variation was 21 per cent, or the same per cent as

Fig. 13. Graph showing the variation in sugar-content of 40,000 sugar beets tested at a
factory in Holland. (After De Vries.) The data are as follows:

Percent sugar
12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19

Number

340 635 1,192 2.205 3,597 5,561 7,178 7,829 6,925 4,458 2,233 692 133 14 5

The broken line shows the theoretical curve for (a+ b)*".

Vilmorin obtained after two generations of selection. The
general average, to be sure, is considerably higher than when
the selection began, but De Vries believes that this is due in
part to improved methods of cultivation and more accurate
methods of determining the sugar-content. He believes that
whatever real improvement has taken place is due largely
to the elimination of the poorest sorts through selection, and
that these would speedily become reestablished if the selec-
tion were discontinued.

The fact has only recently come to light that sugar beets
are regularly cross-pollinated by a minute insect, a species of

VIEWS OF DE VRIES ON SELECTION

73

thrips, the plant being scarcely capable of self-pollination.
This explains why constant selection is required to maintain
a high standard. Hybridization constantly occurs and for
this reason fully stable types cannot be obtained.

De Vries is also led to adverse conclusions concerning se-
lection as an agency in producing racial changes by experi-
ments of his own, one of the most extensive of which was an

0 7 8 9 10 II 7^ 13

Pig. 14. Variation of the buttercup (Ranunculus bulbosus) in number
of petals preceding and following selection. H 1387, variation curve of
unselected race. E 1891 and 1892, curves for successive generations of the
selected race. A 1891, curve for parent plants of the 1892 generation.
(After De Vries.)

attempt to increase by selection the number of petals in the
common meadow buttercup (Ranunculus bulbosus). This
regularly has five-petaled flowers, but an occasional flower
contains one or more extra petals. See Fig. 14. When this
plant was cultivated in his garden, De Vries found the aver-
age number of petals to be 5,Q. After five successive selec-
tions the average was raised to 8.6, the upper limit of
variation from eight to thirty-one, and the mode (or com-
monest condition) from five to nine. De Vries concludes that
the change thus produced could be maintained only by con-
tinued selection, and that further progress could probably
not be made. This conclusion seems to me unwarranted, but
I state it as illustrative of the general view of De Vries, who
maintains that when a permanent racial change occurs it is
due to something different from fluctuating variability, viz.,
to a discontinuous variation or sport, a process which De Vries

74 GENETICS AND EUGENICS

calls mutation. Mutation, he believes, involves a change in
the nature of the germ-cells, whereas fluctuation involves
only effects due to environment. These latter may indeed
modify the soma, and also the germ-plasm temporarily, but
not permanently. Weismann, as we have seen, admits for
certain cases a direct modification of the germ-cells by the
environment, and believes that such modifications when once
produced are permanent. De Vries on the other hand is
much more ready to admit modification of the germ-plasm
by the environment, but maintains that these modifications
are not permanent. Permanent changes in the germ-plasm,
according to De Vries, have no relation to the action of the
environment. They arise spontaneously out of internal
conditions and are not necessarily adaptive in nature. Most
of them perish because not adaptive (i. e., beneficial) in
character; only those mutations survive in a ^tate of nature
which chance to be adaptive. The environment does not
cause mutations, according to De Vries, but only determines
what ones may survive. Evolution is thus due primarily to
internal causes; but its course is guided by the environment,
which selects those mutations which are capable of survival.

The Evidences of Mutation

Two lines of evidences in favor of mutation may be cited,
one general, the other special.

1. The occurrence of elementary species. Among many wild
species of plants there occur varieties quite distinct and
breeding true, but differing from each other by such minor
characteristics as ordinarily escape notice. Thus in the
common dandelion a considerable number of varieties may
be distinguished. Some have narrow leaves, some broad
leaves; on some the leaves are deeply notched, on others
almost entire. If we save the seeds of any of these peculiar
individuals and plant them we find that the characteristics of
the parent plant are inherited. They breed true like distinct
species, indeed they may be regarded as little species within
the dandelion species. De Vries calls them " elementary "

■ «'

ll

' ^B^l

i

■ i&-X~.3» /jsV-'Sti '■, r-.

1

J

I

1

»■

■

i

P

Hi

Fig. 15. Lamarckiana.

Fig. 17. Lamarckiana.

Fig. 19. Oblonga.

Fig. 16. Gigas.

Fig. 18. Gigas.

Fig. '20. Lata.

Oenothera Lamarckiana and Some of its Mutants

Fig. 15, late in season; 16, at mid-season; 17-20, in rosette stage (wintering-over stage).

From cultures and photographs by Professor B. M. Davis.

ELEMENTARY SPECIES 75

species. The same thing may be observed in the case of
violets; many distinct varieties or elementary species may
be recognized within the commonly recognized species, and
experiment has shown that these breed true.

Among cultivated plants a similar diversity of forms oc-
curs, especially among such as are self -fertilized, as for
example wheat, beans and peas. Varieties differ in shape of
leaf, hairiness, color of seed, fruit or flowers, and many other
characteristics. Varieties of the same species may in many
cases be grown together in the same field without mixing,
and even if artificially crossed may not produce an inter-
mediate character but one which is distinctive of one parent
or the other. The same thing is true of our domesticated
animals. Varieties are often discontinuous, intermediates
being unknown. De Vries joins with Bateson in urging a dis-
continuous origin for such variations and brings forward
much experimental evidence in support of this idea. He
supposes that discontinuous variations arise through internal
causes, that is by mutation.

2. " Mutation " in Oenothera. For proof of discontinuity
in variation De Vries relies principally upon a specific case
which he has studied for many years, that of Lamarck's
evening primrose {Oenothera Lamarckiana) . See Figs. 15-26.
This plant is supposed to be of American origin. It is culti-
vated in Europe (and to some extent in America) in parks
and gardens, for its showy yellow flowers. Here and there it
has escaped from cultivation and grows wild. In this condi-
tion De Vries found it in an abandoned potato field near
Amsterdam. But the plant has not been found growing wild
in the western hemisphere, original home of the Oenotheras.
For this reason some naturalists are inclined to regard it as
of hybrid and old-world origin.

The plant is a biennial, five or six feet high when fully
grown, with a stout branching stem bearing at the ends of its
branches spikes of bright yellow flowers. They open towards
evening, as the name, evening primrose, indicates and are
polHnated by bees and moths. On bright days their duration

76 GENETICS AND EUGENICS

IS confined to one evening and the following morning, but in
cloudy weather they may remain open longer.

^ATien De Vries discovered this plant growing wild in 1886
he was struck by its variability. It seemed to be producing,
in the isolated spot where he found it, new species, the thing
for which De Vries had long been looking. He says:

I visited [the spot] many times, often weekly or even daily during the
first few years, and always at least once a year up to the present time
[eighteen years later]. This stately plant showed the long-sought peculi-
arity of producing a number of new species every year. Some of them were
observed directly in the field, either as stems or as rosettes [young plants in
their first year's growth]. The latter could be transplanted into my garden
for further observation, and the stems yielded seeds to be sown under like
control. Others were too weak to live a sufficiently long time in the field.
They were discovered by sowing seed from plants of the wild locality.

By these means over a dozen new types were discovered
never previously observed or described. De Vries has given
to these distinctive names; some of them he regards as true
species, others merely as varieties; the basis of his distinction,
an arbitrary one, does not concern us. The peculiarity of the

TABLE 5

Some Mutants of Oenothera Lamarckiana

1. Smooth-leaved {laevifolia)

2. Short-styled {hrevistylis)

3. Dwarf (nanella)

4. Giant (gigas) \ .

_ Ti J . , / 7 . • \ r -rrogressive or (jam variations.

5. Ked-vemed {rubrinervis) J

•6. Pale-leaved (albida) | _, , ,

_ >-.,, , 1/71 N > reeble mutants.

7. Ublong-leaved {pblonga) J

case is, not that a group of undescribed species or varieties
was found growing together, but that they were produced
year after year from the seed of the parent species, and from
their first origin bred true (in most cases) to their distinctive
characters.

One of the mutants was distinguished by its smooth slen-
der leaves {laevifolia) \ another by the short style of its
flowers {hrevistylis) ; a third by its dwarf habit {nanella, Fig.
26), one-fourth the height of the parent species. All three
bred true to these peculiarities which De Vries considers due

' Retrogressive or Loss variations.

Fig, 21. Lamarckiana.

Fig. 23. Oblonga.

Fig. 25. Lata.

Fig.

22.

Gigas.

>

1

1

'.■■J^^^

3

FnB.

j

^

£

P

Fig. 24. Scintillans.

Fig. 26. Nanella.

Oenothera Lamarckiana and Some of its Mutants
Figs. 21-24, inflorescence and leaf from base of main stem; 25, inflorescence only;

26, entire plant. (From Davis.)

■I

\<

EVIDENCES OF MUTATION 77

to loss of something the parent possessed. For this reason
he calls them *' retrogressive varieties."

Two very vigorous mutants the giant {gig as. Figs. 16, 18,
22) and the red-veined {ruhrinervis) De Vries considers to have
acquired additional characters not present in the parent, and
for this reason he regards them as genuine *' elementary
species " (having attained a new progressive characteristic).
The giant is no taller than its parent species but much
stouter, with larger leaves and flowers. Its cells contain
twice as many chromosomes as those of the parent species,
which fact is considered very important by some cytologists.
A wide-leaved mutant {lata. Figs. 20, 25) has one extra chro-
mosome in its nucleus (14 + 1 = 15).

The red-veined mutant {ruhrinervis) has more red on its
leaves and stems than has the parent species; its stems are
also more brittle, the bast fibres having thinner walls.

Two other mutants are naturally feeble, not strong enough
to survive in a wild state. They are albida (the pale whitish
mutant), and ohlonga (having oblong leaves on feeble plants,
about half as tall as the parent species). See Figs. 19 and 23.

" These seven new forms," says De Vries, " which diverge
in different ways from the parent type, were absolutely con-
stant from seed. Hundreds or thousands of seedlings may
have arisen, but they always come true and never revert to
the original 0. Lamar ckiana-type.^* Several other mutants
have been described by De Vries, among them scintillans,
but they are less constant in character than those already
mentioned. Their behavior need not here be considered.

A fact deserving especial attention in connection with De
Vries' experiments is the repeated occurrence of the same
mutation year after year in pedigree cultures from self -fer-
tilized plants, showing that these particular variations occur
with some regularity.

Starting with nine plants transplanted from the field De
Vries carried a culture through seven subsequent generations,
always planting seed of Lamarckiana parents, with the results
shown in Table 6.

78 GENETICS AND EUGENICS

TABLE 6

Eight-generation

Pedigree Culture of Lamarck's

Evening

PRTMR

Genera-
tion Gigas

Albida

Oblonga

Rubri- Lamarcki-

nervis ana Nanella

Lata

Scintil-
lans

1

• •

• •

9

• •

« •

2

• •

• •

15,000 5

5

• ,

3

• •

• •

1 10,000 3

3

, .

4 1

15

176

8 14,000 60

73

1

5

25

135

20 8,000 49

142

6

6

11

29

3 1,800 9

5

1

7

• •

9

3,000 11

• •

• ■

8

5

1

1,700 21

1

• •

The giant mutant was obtained only once, but all the
others in at least three different generations, from Lamarcki-
ana parents.

Without going into the details of the case, to which De
Vries has devoted an entire volume, we may notice what de-
ductions or " laws " De Vries bases upon it.

1. New elementary species appear suddenly and attain full constancy at
once.

2. The same new species are produced in a large number of individuals.

This would, of course, give them a better chance and fuller
test in the struggle for existence than if they appeared but
once.

3. Mutahility is something fundamentally different from fluctuating vari-
ability. All organs and all qualities of Lamarckiana fluctuate and vary
in a more or less evident manner, and those which I had the opportunity of
examining more closely were found to comply with the general laws of
fluctuation. But such oscillating changes have nothing in common with
the mutations. Their essential character is the heaping up of slight devia-
tions around a mean, and the occurrence of continuous lines of increasing
deviations, Hnking the extremes with this group. Nothing of the kind is
observed in the case of mutations. There is no mean for them to be
grouped around and the extreme only is to be seen, and it is wholly un-
connected with the original type. It might be supposed that on closer
inspection each mutation might be brought into connection with some
feature of the fluctuating variability. But this is not the case. The dwarfs
are not at all the extreme variants of structure, as the fluctuation of the
height of the Lamarckiana never decreases or even approaches that of the
dwarfs. There is always a gap. The smallest specimens of the tall type
are commonly the weakest, according to the general rule of the relationship
between nourishment and variation, but the dwarfs according to this same
rule are of course the most robust specimens of their group.

EVIDENCES OF MUTATION 79

Fluctuating variability, as a rule, is subject to regression. The seeds
of the extremes do not produce an offspring which fluctuates around their
parents as a center, but around some point on the line which combines
their attributes with the corresponding characteristic of their ancestors,
as Vilmorin has put it. No regression accompanies mutation, and this
fact is perhaps the completest contrast in which these two great tj-pes of
variability are opposed to each other.

The offspring of my mutants are, of course, subject to the general laws
of fluctuating variabihty. They vary, however, around their own mean,
and this mean is simply the tj^De of the new elementary species.

4. The mutations take 'place in nearly all directions.

Some are larger, others smaller than the parent species;
some more vigorous and productive, others less so; some are
more heavily pigmented, others less so; some can survive in
competition with the parent form, others cannot. There is
no evidence of adaptive modification, or modification con-
trolled by the environment for the benefit of the species.
The variation is in all directions.

The facts upon which De Vries bases these generalizations
have been verified in the main by a number of workers in
different parts of the world, notably in this country where
several botanists have studied the seedlings of Lamarck's
evening primrose. But the facts are not interpreted in the
same way by all observers.

One view accepts the facts at their face value, including
the regularity of the occurrence of the same mutation in
successive generations, and its entire distinctness from the
parent form, but maintains that 0. Lamarckiana is a hybrid
plant, not a pure species, and that the so-called mutation is
only a new illustration of the splitting up of a hybrid into new
forms, many of which are constant, a thing which is known
frequently to occur following hybridization.

In support of this view it may be said that 0. Lamarckiana
has not been found growing wild in this country, its supposed
place of origin, though careful search has been made for it.
On the other hand 0. Lamarckiana has for many years been
growing wild in certain English stations, notably on the sand
hills north of Liverpool, and there are good reasons for be-
lieving that the Lamarckiana first brought out by seedsmen

80 GENETICS AND EUGENICS

about the year 1860 may have come from some English lo-
cality. The fact that several species of Oenothera are known
to have been in England previous to this date suggests that
Lamarckiana may have arisen through the crossing of other

forms.

In this connection it is of interest to note that a hybrid
has been synthesized by Davis from a cross of 0 . franciscana
with 0, biennis, which is essentially indistinguishable in its sys-
tematic characters from 0. Lamarckiana. Furthermore this
hybrid behaves like Lamarckiana in producing two classes of
progeny when crossed with certain wild species as described
in the next paragraph. This Lamarckiana-\\ke hybrid,
which has been given the name of neo-Lamarckiana, in the
fourth generation bred true for about one-third of its pro-
geny and therefore gave a very much larger percentage of
variants than Lamarckiana, but its seed fertility was very
much higher, which may account for the fact. At this stage
in the investigation neo-Lamarckiana presents a breeding
behavior at least similar to that of Lamarckiana and it will
be a matter of interest to see whether in later generations the
resemblance may not become more marked.

Another adverse view of De Vries' theory, with less concern
as to the origin of 0. Lamarckiana, maintains that however it
originated it is clearly not pure genetically; if not actually a
hybrid of recent origin, it at least has the genetic character of
a hybrid and hence the regularity of its mutations. For hy-
bridization, as we shall see, is a sure means of producing new
and stable varieties. Hybridization experiments made by
De Vries and repeatedly confirmed by others show that in
every generation 0. Lamarckiana produces different kinds of
fertile gametes. In particular, it forms two classes of hy-
brids, " twin hybrids," in approximately equal numbers, in
crosses with certain wild species, as do several of the wild
species in crosses with each other, so that it is evident that
0. Lamarckiana, as well as some wild species of Oenothera,
have the variability characteristic of hybrids. Even those
which seem to breed true, and which do breed true when

EVIDENCES OF MUTATION 81

self -pollinated, may give a variable progeny in crosses, and
they seem to breed true merely because certain classes of their
progeny are too feeble to survive. For in some cases only a
fractional part of the seeds produced contain embryos
capable of survival.

According to the views expressed above, Oenothera Lamar ck-
iana is best interpreted as an impure or hybrid species which
only breeds true in a relatively high degree because of
extensive sterility, which eliminates large numbers of gametes
and zygotes that differ from the germinal cells which repro-
duce the Lamarckiana type. The " mutants " come from
occasional seeds of different types that survive the heavy
mortality which renders sixty per cent or more of the seeds
infertile and about fifty per cent of the pollen grains abortive.
If this is the correct explanation of the peculiar breeding
behavior of Lamarckiana, this plant is very far from being
representative of a pure species, as De Vries assumed it to be,
and is hardly suitable material for experiments designed to
give evidence of mutation.

Even if we reject this explanation and consider that the
mutability of the evening primrose has no causal relation to
its hybridity, it by no means follows that mutation is a
general method of origin of new varieties and species among
animals and plants, which is the thesis of De Vries. In recent
years the expression "mutation theory" has been used in a
sense very different from that in which De Vries originally
used it, and implying merely the origin of new and stable
organic forms by change in single inheritance factors (genes) ,
whether these produce striking variations (sports) or varia-
tions so minute as to be scarcely observable. This form of
mutation theory will be discussed in a later chapter. To the
mutation theory of De Vries, as a general theory of evolution,
it seems to be a fatal objection that such mutation as it re-
cognizes is not general in occurrence. Crosses of species or
varieties as found in the wild state more often reveal the
existence of numerous minute genetic differences than a
single or a few striking differences.

CHAPTER \TI

THE PIONEER PLANT HYBRIDIZERS: THE DISCOVERY
AND REDISCOVERY OF MENDEL'S LAW

While De Vries was engaged in his studies of the evening
primrose he hit upon an idea far more important, as most
biologists now beheve, than the idea of mutation, though
De Vries himself both then and since has seemed to regard
it as of only minor importance. He called this the " law of
the splitting of hybrids.'' The same law, it is claimed, was
independently discovered about the same time by two other
botanists, Correns in Germany, and Tschermak in Austria.
Further, historical investigations made by De Vries showed
that the same law had been discovered and clearly stated
many years previously by an obscure naturalist of Briinn,
Austria, named Gregor Mendel, and we have now come to
call this law by his name, Mendel's law. Mendel was so little
known when his discovery was published that it attracted
little attention from scientists and was soon forgotten, only
to be unearthed and duly honored years after the death of its
author. Had Mendel lived forty years later than he did, he
would doubtless have been a devotee of biometry, for he had
a mathematical type of mind and his discovery of a law of
hybridization was due to the fact that he applied to his
biological studies methods of numerical exactness which he
had learned from algebra and physics. In biology he was an
amateur, being a teacher of the physical and natural sciences
in a monastic school at Briinn. Later he became head of his
monastery and gave up scientific work, partly because of
other duties, partly because of failing eyesight.

The subject of plant hybridization had received consider-
able attention from botanists for a century before it was
taken up by Mendel and the law of the splitting of hybrids
which was discovered by Mendel and rediscovered by De

8^

THE FIRST PLANT HYBRIDIZER 83

Vries had narrowly escaped discovery at the hands of their
predecessors. There was lacking only the numerical exact-
ness of a Mendel or the clear-sighted analysis of a De Vries
to bring to light the rule governing the splitting of hybrids.

By a hybrid we understand an organism produced by the
crossing of two distinct species or varieties of plant or animal,
i, e,, an organism which has an individual of one species or
variety as its mother and an individual of a different species
or variety as its father. At times and by certain naturalists
a distinction has been made between the offspring of a species
cross and that of a variety cross, the term hybrid being
limited to the progeny of a species cross, and the term mon-
grel being used to designate the progeny of a variety cross.
But it has been found quite impossible to distinguish species
from varieties sharply, for Darwin showed that varieties may
be only incipient species, and that no definition can be
framed of variety which will not also include species and
vice versa. Accordingly at present we use the terms species
and variety in a relative sense only. The differences which
exist between species are supposed to be either more numer-
ous or greater in degree than those which exist between
varieties. The terms to the majority of biologists imply
nothing more than this. If we cannot distinguish species
from varieties, it is obvious that we cannot distinguish the
products of a species-cross from the products of a variety-
cross, and so at present all cross-bred offspring, whether of
species or varieties, are called hybrids. The same law of
splitting applies to all, as we shall see.

The pioneer plant hybridizer was Joseph (Gottlieb) Kol-
reuter (1733-1806) who between the years 1760 and 1766
carried out the first series of systematic experiments in plant
hybridization which had ever been undertaken. The more
important features of Kolreuter's work have been thus
summarized by Lock, pp. 150-155.

These experiments not only established with certainty for the first time
the fact that the seeds of plants are produced by a sexual process com-
parable with that known to occur in animals, but also led to a knowledge

84 GENETICS AND EUGENICS

of the general behaviour of hybrid plants, which was scarcely bettered until
Mendel made his observations a century afterwards.

Kolreuter found that the hybrid offspring of two different plants gener-
ally took as closely after the plant which yielded the pollen as after that
upon which the actual hybrid seed was borne. Indeed, he found that it
made little or no difference in the appearance of the hybrid which of the
parental species was the pollen-parent (male), and which the seed-parent
(female) — that is to say, in the case of plants the result of reciprocal
crosses is usually identical. Thus, for the first time it was definitely shown
that the pollen-grain plays just as important a part in determining the
characters of the offspring as does the ovule which the pollen-grain fer-
tilizes. This was a wholly novel idea in Kolreuter's time, and the fact was
scarcely credited by his contemporaries.

Kolreuter had no means of discovering that the contents of a single
pollen-grain unite with the contents of a single ovule in fertilization. But
he ascertained by experiments that more than thirty seeds might be made
to ripen by the application of between fifty and sixty pollen-grains to the
stigma of a particular flower, so that, if he had had any hint of the actual
microscopic processes of fertilization, he would have been quite prepared
for the more fundamental discovery.

Kolreuter, indeed, believed that the act of fertilization consisted in the
intimate mingling together of two fluids, the one contained in the pollen-
grain, and the other secreted by the stigma of the plant. The mingled
fluids, he supposed, next passed down the style into the ovary of the plant,
and arriving at the unripe ovules, initiated in them those processes which
led to the formation of seeds. In this belief Kolreuter simply followed the
animal physiologists of his time, who looked upon the process of fertiliza-
tion in animals as taking place by a similar mingling of two fluids. Now
that we know that fertilization consists essentially in the intimate union
of the nuclei of two cells, one of which, in the case of plants, is the ovum
contained within the ovule, whilst the other is represented by one of a
few cells into which the contents of the pollen-grain divide, we can under-
stand more clearly the bearing of Kolreuter's observation. And it is
greatly to this eminent naturalist's credit that he succeeded in carrying
out his observations with so much accuracy, when the full meaning of
those observations was of necessity hidden from his comprehension.

Kolreuter was the first to observe accurately the different ways in which
pollen can be naturally conveyed to the stigma of a flower. This may
take place either by the p>ollen-grains falling directly upon the stigma, or
by the agency of the wind, or, lastly, the pollen may be carried by insects
visiting the flowers. And he recognized many features characteristic of
flowers apt to be fertilized in one or other of these ways in particular.
Thus he was aware, for example, of the nature and use of the nectar which
so many flowers produce — namely, that it is the substance from which
the bees — by far the most diligent visitors of flowers — obtain their
honey.

Curiously enough, Kolreuter was not aware of the existence of any
natural wild hybrid plants. But he was quite right in contending that

Fig. 26a. The first artificially produced plant hybrid and its parents. A, Nicotiana paniculata; B,
N. rustica var. humilis; C, Fi hybrid between A and B; D, individual flowers of the hybrid (middle),
of N. paniculata (right) , and of N. rustiea (left). Photographs by Prof. E. M. East, from his repetition
of Kolreuter's pioneer experiment.

LOCK ON THE WORK OF KOLREUTER 85

supposed examples of such hybrids required for their substantiation the
experimental proof, which could only be afforded by making actual artifi-
cial crosses between the putative parent species.

The first hybrid made artificially by Kolreuter was obtained in ITtiO
by applying the pollen of Nicotiana paniculata to the stigma of Nicotiana
rustica. The hybrid offspring of this cross showed a character intermediate
between those of the two parent species in almost every measurable or
recognizable feature, with a single notable exception. This exception was
afforded by the condition of the stamens and of the pollen grains pro-
duced by the hybrids. These organs were so badly developed that in all
the earlier experiments, self-fertilization of the hybrid plants yielded no
good seed at all, nor were the pollen grains of the hybrid any more effec-
tive when applied to the stigmas of either of the parent species. On the
other hand, when pollen from either parent was applied to the stigmas of
the hybrid plants, a certain number of seeds capable of germination was
obtained, although this number was much smaller than in the case of
normal fertilization of either parent species. This partial sterility, affect-
ing in particular the stamens and the pollen which they produce, is a
feature common to the majority of hybrids between different natural
species. Many such hybrids, indeed, are altogether sterile, so that a
further generation cannot in any way be obtained from them. On the
other hand, the members of different strains or varieties which have arisen
under cultivation \Tield, as a rule, when crossed together offspring which
are perfectly fertile.

In subsequent years Kolreuter was able to obtain a very few self-ferti-
lized offspring from hybrids of the same origin as the above. The resulting
plants were described as resembling their hybrid parent so closely as to be
practically indistinguishable from it.

The offspring obtained by crossing the hybrid plants with pollen from
either parent showed in each case a form more or less intermediate between
that of the original hybrid and that of the parent species from which the
pollen was derived. But the plants were not all alike in this respect, some
of them being much more like the parent species than others, and some,
again, varying in other directions. There were also considerable differ-
ences between the different individuals in respect of fertiHty. so that some
of the plants were more and some less sterile than the original hybrids.
Also, there was some tendency to the production of malformations of the
flowers and other parts.

One of the most noted of Kolreuter's experiments was that which con-
sisted in repeatedly crossing a hybrid plant with one of the parent species
from which the hybrid was derived. By continuing to pollinate the mem-
bers of one generation after another with the pollen of the same parent
species, plants were at last arrived at which were indistinguishable from
the parent in question. We shall return to this fact later on, when the
reader will be in a position to appreciate its importance more fully.

Kolreuter found that the result of reciprocal crosses is usually identical
— that is to say, the offspring obtained by fertilizing a plant A with ]H)llon
from a plant B are not to be distinguished from those obtained when B is

86 GENETICS AND EUGENICS

fertilized with the pollen of A. But the two opposite processes of fertili-
zation are not always equally easy to carry out. An extreme instance of
this circumstance was met with in the case of the genus Mirabilis. Mirab-
ilU jalapa was easily fertilized with pollen from M. longiflora. During
eight years Kolreuter made more than two hundred attempts to effect the
reverse cross, but without success.

It was shown by Kolreuter that hybrids between different races or
varieties of the same species are usually much more fertile than hybrids
obtained by crossing distinct species. Indeed, he believed that varieties
of a single species were in all cases perfectly fertile together, whilst hybrids
between species always showed some degree of sterility. But in this case
Kolreuter based his definition of a species upon the very point at issue,
and when he found forms, which other botanists regarded as good species,
to be perfectly fertile together, he immediately regarded them as being
only varieties of a single species.

One curious point is worth noting in this connection. Five varieties
of Nicotiana tahacum were found to be perfectly fertile with one another,
but when crossed with Nicotiana glutinosa one of them was found to be
distinctly less sterile than the rest.

Another interesting point observed by Kolreuter was the fact that
hybrid plants often exceed their parents in luxuriance of growth. Upon
this fact, as we shall see later on. Knight and afterwards Darwin based
theoretical conclusions of considerable importance in connection with the
problem of sex.

To pick out the salient features of the foregoing account
we may notice:

1. That Kolreuter established the occurrence of sexual
reproduction in plants by showing that hybrid offspring in-
herit equally from the pollen plant and the seed plant.

2. He showed that hybrids are commonly intermediate
between their parents in nearly all characters observed, such
for example as size and shape of parts.

3. Many hybrids are partially or wholly sterile, especially
when the parents are very dissimilar (belong to widely dis-
tinct species). Such hybrids often exceed either parent
species in size and vigor of growth.

4. Kolreuter did not observe the regular splitting of hy-
brids which Mendel and De Vries record, but some of his
successors did, particularly Thomas Knight (1799) ^ and John
Goss (1822) 1 in England who were engaged in the crossing of
garden peas with a view to producing more vigorous and

* For a fuller account of the work of these early plant hybridizers, see Lock.

NAUDIN MENDEL'S FORERUNNER 87

productive varieties, and Naudin (1862) in France who
made a comprehensive survey of the facts of hybridization
in plants and came very near to expressing the generalization
which Mendel reached four years later. He pointed out the
significance of the fact first observed by Kolreuter that hy-
brids may be brought back to the form of either parent by
repeated crossing with that parent. Naudin supposes that
the potentialities of each species are contained in its pollen
and ovules and the potentialities of both species are present
together in the hybrid. If species A is fertilized by species B,
the hybrid contains potentialities AB. Naudin supposes
that these potentialities may segregate from each other in
the pollen grains and ovules of the hybrid plant. An ovule
A of such a hybrid plant, if fertilized by pollen of the pure
species A, will form a plant of exactly the same nature as
pure species A. This idea of the segregation of potentialities
in the germ-cells of the hybrid was adopted by Mendel. He
added to it the conception that the segregation applies to
single potentialities or characteristics rather than to all the
potentialities of a species at once, and the result is what we
call Mendel's law. Like all great discoveries it was not made
out of hand, nor as the result of one man's work alone.
Mendel added one final touch to the work of his predecessors
as summarized by Naudin, and the result was that hybridi-
zation became for the first time an orderly and understand-
able process, capable of throwing light on normal heredity.

CHAPTER VIII

MENDEL'S LAW OF HEREDITY ILLUSTRATED IN

ANIMAL BREEDING

Mendel's law may best be explained with the aid of ex-
amples, which will be chosen, for convenience, from the
heredity of guinea-pigs. If a guinea-pig of pure race with
colored fur (say black) is mated with a guinea-pig having
uncolored (white) fur, a so-called albino, the offspring will
all have colored fur, none being albinos. See Figs. 27-30.
To use Mendel's terminology, colored fur dominates in the
cross, while albinism recedes from view. Colored fur is, there-
fore, called the dominant character; albinism, the recessive
character.

But if now two of the colored individuals produced by this
cross are mated with each other, the recessive (albino)
character reappears on the average in one in four of their
offspring (Fig. 30). The reappearance of the recessive char-
acter, after skipping a generation, in the particular propor-
tion, one fourth, of the second generation offspring, is a
regular feature of Mendelian inheritance. It may be ex-
plained as follows (see Fig. 30a) : the gametes which united in
the original mating of a pure colored individual with an
albino must have transmitted, one color (C), the other
albinism (c) . The contrasted characters were then associated
together in the offspring. But color from its nature domi-
nated, since albinism is due apparently to the lack of some-
thing necessary to the formation of color, which the other
gamete would supply.

But when the young produced by this cross have become
adult and themselves form gametes, the characters, color and
albinism, will separate from each other and pass into differ-
ent gametes, since, as regards the transmission of alternative

88

o

.9
IS

<

CO
(5<

c

a
i/
u

■c
a
c

MENDEL'S LAW

89

characters like color and albinism, a gamete is able to trans-
mit only one, its nature being simplex.

Accordingly a female hybrid will transmit the character,
color (C), in half its eggs, and the contrasted character, al-

Fig. 30a. Diagram to explain the inheritance of color (C) and albinism (c) in the

cross shown in Figs. 27-30.

binism (c), in half its eggs. A male hybrid will also transmit
color (C) in half its sperm, and albinism (c) in the other half.

90 GENETICS AND EUGENICS

If the type of egg which transmits color (C) is fertihzed as
readily by one type of sperm as by the other, combinations
will result which are either CC or Cc in character. And if the
tj^pe of egg which transmits albinism (c) is also fertilized as
readily by one kind of sperm as by the other, combinations
will result which are either Cc or cc in character. Putting
together the results expected from the fertilization of both
types, we get 1 CC : 2 Cc : 1 cc, i. e., one combination of color
with color, two combinations of color with albinism, and one
combination of albinism with albinism; or three combina-
tions which contain color (and so w^ill show it) to one combi-
nation which lacks color and so will be white. This agrees
with the observed average result.

The albino individual may be expected to transmit only
the albino character (c), never color (C), which it does not
possess. Experiment shows this to be true. Albino guinea-
pigs mated with each other produce only albino offspring.
But the colored individuals are of two sorts, CC and Cc in
character. The CC individual is pure, so far as its breeding
capacity is concerned. It can form only C gametes. But
the Cc individuals may be expected to breed exactly like
the first generation hybrids, w^hich had the same composition.
They will transmit color (C) in hg^lf their gametes, albinism
(c) in the other half. Experiment justifies these expectations
also. The test of individual animals may readily be made by
mating them one by one with albinos. The pure colored in-
dividuals (CC) will produce only colored offspring, since they
transmit color (C) in all their gametes. But the other and
more numerous class of colored individuals (Cc) will produce
offspring part of which will be colored (Cc) and the remainder
albino (cc). The two kinds of dominant individuals, those
which breed true and those which do not, w^e may call
homozygous and heterozygous, following the convenient ter-
minology of Bateson. A homozygous individual is one in
which like characters are joined together, as CC or cc; a
heterozygous individual is one in which unlike characters are
joined together, as Cc. It goes without saying that reces-

ij

wj

Fig. 31

Fig. 32

Fig. 33 Fig. 34

Figs. 31-34. Results of a cross between two varieties of guinea-pig differing in two unit-characters,
color and roughness of fur. Fig. 31, a colored and smooth-coated guinea-pig.

Fig. 32. An albino and rough-coated guinea-pig. Fig. 33. One of the Fi young, colored and rough.

Fig. 34. A smooth-coated albino, one of the four varieties occurring among the F2 young. The other
three varieties of F2 young are like the parents and grandparents respectively (Figs. 31-33).

Fip. :i5

Fi^'. .'>(i

Fig. 37 Fig- 38

Figs, 35-38. Results of a cross between two varieties of guinea-pig differing in the two unit-characters,
color and length of fur. Fig. 35, a colored and short-haired guinea-pig. Fig. 36, an albino and
long-haired guinea-pig. The Fi young were colored and short-haired like the parent shown
in Fig. 35. Fig. 37, a colored and long-haired guinea-pig. one of the new F2 varieties.
Fig. 38, an albino and short-haired guinea-pig, the other new F2 variety. The two other
F2 varieties were like the grandparents (Figs. 35 and 36).

4

MENDEL'S LAW 91

sives are always homozygous. For they do not contain the
dominant character; otherwise they would show it.

It will be observed that, in the cross of colored with albino
guinea-pigs, color and albinism behave as a pair of alternative
units which may meet in fertilization but separate again at
the formation of gametes.

Mendel's law as illustrated in this cross includes three
principles: (1) The existence of unit-characters, (2) domi-
nance, in cases where the parents differ in a unit-character,
and (3) segregation of the units contributed by the respective
parents, this segregation being found among the gametes
formed by the offspring.

The principles of dominance and segregation apply to the
inheritance of many characteristics in animals and plants.
Thus in guinea-pigs a rough or rosetted coat (Figs. 32 and 33)
is dominant over the ordinary smooth coat. If a pure rough
individual is crossed with a smooth one, all the offspring are
rough; but in the next generation smooth coat reappears in
one-fourth of the offspring, as a rule. Again, in guinea-pigs
and rabbits a long or angora condition of the fur (Figs. 36,
and 37) is recessive in crosses with normal short hair. All
the immediate offspring of such a cross are short haired, but
in the next generation long hair reappears in approximately
one-fourth of the offspring.

In cattle, the polled or hornless condition is dominant over
the normal horned condition; in man, two jointed fingers
and toes are dominant over normal three-jointed ones.

In each of the cases thus far considered a single unit-
character is concerned. Crosses in such cases involve no
necessary change in the race, but only the continuance within
it of two sharply alternative conditions. But the result is
quite different when parents are crossed which differ sunul-
taneously in two or more independent unit-characters. Cross-
ing then becomes an active agency for the production of new
varieties.

In discussing the crosses now to be described, it will be
convenient to refer to the various generations in more pre-

92 GENETICS AND EUGENICS

cise terms, as Bateson has done. The generation of the
animals originally crossed will be called the parental genera-
tion (P); the subsequent generations will be called fihal
generations, viz., the first filial generation (Fi), second fihal
(F2), and so on.

When guinea-pigs are crossed of pure races which differ
simultaneously in two unit-characters, the Fi offspring are
all alike, but the F2 offspring are of four sorts. Thus, when
a, smooth colored animal (Fig. 31) is crossed with a rough
albino (Fig. 32), the Fi offspring are all rough and colored
(Fig. 33) , manifesting the two dominant unit-characters, —
colored coat derived from one parent, rough coat derived
from the other. But the F2 offspring are of four sorts, viz.,
(1) smooth and colored, like one grandparent, (2) rough and
albino, like the other grandparent, (3) rough and colored,
like the Fi generation, and (4) smooth and albino, a new
variety (Fig. 34). It will be seen that the pigmentation of
the coat has no relation to its smoothness. The dark animals
are either rough or smooth, and so are the white ones. Pig-
mentation of the coat is evidently a unit-character indepen-
dent of hair direction, and as new combinations of these two
units the cross has produced two new varieties, — the rough
colored and the smooth albino.

Again, hair-length is a unit-character independent of hair-
color. For if a short-haired colored animal (either self or
spotted, Fig. 35) be crossed with a long-haired albino (Fig.
36), the Fi offspring are all short-haired and colored, but
the F2 offspring are of four sorts, viz., (1) colored and short-
haired, like one grandparent, (2) albino and long-haired,
like the other, (3) colored and long-haired, a new combina-
tion (Fig. 37), and (4) albino and short-haired, a second new
combination (Fig. 38).

Now the four sorts of individuals obtained from such a
cross as this will not be equally numerous. As we noticed in
connection with the simple cross of colored with albino
gumea-pigs, dominant individuals are to the corresponding
recessives as three to one. Therefore, we shall expect the

MENDEL'S LAW 93

short-haired individuals in F2 to be three times as numerous
as the long-haired ones, and colored ones to be three times
as numerous as albinos. Further, individuals which are both
short-haired and colored should be 3 X 3 or nine times as
numerous as those which are neither short-haired nor colored.
The expected proportions of the four classes of F2 offspring
are accordingly nine short colored : three long colored : three
short albino : one long albino, a proportion which is closely
approximated in actual experience.

The Mendelian theory of independent unit-characters ac-
counts for this result fully. No other hypothesis has as yet
been suggested which can account for it. Suppose that each
independent unit has a different material basis in the gamete.
Let us represent the material basis of hair-length by a circle,
that of hair-color by a square; then combinations and re-
combinations arise as shown in Fig. 39. The composition of
the gametes furnished by the parents is shown in the first line
of the figure; that of an Fi zygote, in the second line;
that of the gametes formed by Fi individuals in the third
line. S meets s and C meets c in fertilization to form an Fi
individual duplex and also heterozygous as regards hair-
length and hair-color, but these units segregate again as the
gametes of the Fi individuals are formed, and it is a matter
of chance whether or not they are associated as originally,
S with C and s with c, or in a new relationship, s with C and
S with c. Hence we expect the Fi individuals to form four
kinds of gametes all equally numerous: SC, sc, sC, and Sc.
By chance unions of these in pairs nine kinds of combinations
become possible, and their chance frequencies will be as
follows :

Short Colored Long Colored Short Albino Long Albino

1 SSCC 1 ssCC 1 SScc 1 sscc

2 SSCc 2 ssCc 2 Sscc
2 SsCC

4 SsCc

9 3 3 1

Four of these combinations, including nine individuals, will
show the two dominant characters, short and colored; two

94

GENETICS AND EUGENICS

classes, including three individuals, will show one dominant
and one recessive character, viz., colored and long; two
more classes, including three individuals, will show the other
dominant and the other recessive character, viz., short and
albino; and lastly, one class, including a single individual,
will show the two recessive characters, long and albino. The

Fig. 39. Diagram to explain the simultaneous and independent inheritance of colored fur (C) and

short hair (S) in the cross shown in Figs. 35-38.

four apparent classes, or, as Johannsen calls them, pheno-
types, will accordingly be as 9:3:3:1.

One individual in each of these four classes will, if mated
with an individual like itself, breed true, for it is homozygous,
containing only like units. The double recessive class, long
albino, of course contains only homozygous individuals, but
in each class which shows a dominant unit, heterozygous
individuals outnumber homozygous ones, as 2 : 1, or 8 : 1.

Now the breeder who by means of crosses has produced a
new type of animal wishes, of course, to " fix " it, — that is.

Fig. 40

Fig. 41

Fig. 42

Fig. 43

Fig. 44

Fig. 45

Fig. 46

Fig. 47

Figs. 40-47. Results of a crass between varieties of guinea-pig differing in three unit-characters, color,
length and roughness of fur. Fig. 40, the colored, short-haired and smooth parent. Fig. 41, the albino,
iong-haired and rough parent. Fig. 42, one of the Fi young, colored, short-haired and rough. Figs.
43-47, five new varieties occurring among the F2 young. Fig. 43, colored, long-haired and rough.
Fig. 44, colored, long-haired and smooth. Fig. 45, albino, short-haired and rough. Fig. 46, albino,
long-haired and smooth. Fig. 47, albino, short-haired and smooth. Throe other F2 varieties were
like the parents and grandparents respectively (Figs. 40-42) .

MENDEL'S LAW 95

to obtain it in a condition which will breed true. He must,
therefore, obtain homozygous individuals. If he is dealing
with a combination which contains only recessive characters,
this will be easy enough, for such combinations are invari-
ably homozygous. His task will become increasingly diffi-
cult, the more dominant characters there are included in the
combination which he desires to fix.

The most direct method for him to follow is to test by
suitable matings the unit-character constitution of each in-
dividual which shows the desired combination of characters,
and to reject all which are not homozygous. In this way a
pure race may be built up from individuals proved to be pure.
Such a method, however, though sure, is slow in cases where
the desired combination includes two or more dominant unit-
characters, for it involves the application of a breeding test
to many dominant individuals, most of which must then be
rejected. It is, therefore, often better in practice to breed
from all individuals which show the desired combination, and
eliminate from their offspring merely such individuals as do
not show that combination. The race will thus be only
gradually purified, but a large stock can be built up much
more quickly.

We may next discuss a cross in which three unit-character
differences exist between the parents, instead of two. If
guinea-pigs are crossed which differ simultaneously in three
unit-characters, color, length, and direction of the hair, a
still larger number of phenotypes is obtained in F2, namely,
eight. A cross between a short-haired, colored, smooth
guinea-pig (Fig. 40) and one which was long-haired, albino,
and rough (Fig. 41) produced offspring in Fi which were
short-haired, colored, and rough (Fig. 42), these being the
three dominant characters, two derived from one parent, one
from the other. The F2 offspring were of eight distinct t>T)es,
two like the respective grandparents, one like the Fi indi-
viduals (parents), and the other five new, shown in Figs. 43-
47. The largest of the eight apparent classes {phenotypes)
was the one which manifested the three dominant charac-

96

GENETICS AND EUGENICS

ters, short, colored, and rough, which had been the exclusive
Fi type (Fig. 42) ; the smallest class was the one which mani-
fested the three recessive characters, long, albino, and smooth

Fig. 48. Diagram to explain the simultaneous and independent inheritance of short (S)
colored (C) and rough (R) fur in the cross shown in Figs. 40-47.

(Fig. 46). Theoretically these two classes should be to each
other as 27 : 1. Of the twenty-seven triple dominants,
twenty-six should be heretozygous. The triple recessive
would of course be fully homozygous.

MENDEL'S LAW 97

A comparison of this case with the one just previously
described shows what an increasingly difficult thing it is to
fix types obtained by crossing, as the number of dominant
characters in the selected type increases. On the theory of
unit-characters the gametic combinations and segregations
in this cross are as shown in Fig. 48. The nature of the
gametes formed by the parents crossed is shown in the first
row; the composition of the Fi individuals, immediately be-
low. In the two lower rows are shown four different sorts of
gametic splittings which may occur in Fi individuals, pro-
ducing thus eight different kinds of gametes.

If, as suggested, the Fi individuals produced in this cross
form eight different kinds of gametes, each of these kinds
should, when united with a gamete having the same consti-
tution as itself, produce a homozygous and so true-breeding
zygote of a different variety, making in all eight true-breeding
varieties. Experiment has shown that in reality eight such
varieties are produced in F2. It is therefore evident that the
crossing of varieties which differ from each other by unit-
characters becomes, under the operation of Mendel's law, a
ready means of producing other new varieties different from
those crossed, and that the number of such new varieties
capable of production in this way increases rapidly with every
additional unit-character difference between the parent
varieties which are crossed.

CHAPTER IX

SOME MENDELIAN TERMS AND THEIR USES

In describing Mendelian heredity it is convenient for brevity
to use technical terms, some of which are already in general
use among biologists, but others of which have been framed
to meet needs not previously existing. The significance of
these the reader must keep clearly in mind, for which reason
it seems best brieflv to define them.

A gamete is a reproductive cell capable of uniting with
another reproductive cell to form a new individual. In all
the higher animals and plants the gametes which are capable
of union in pairs are of two unlike sorts, eggs and sperms.

An egg-cell (capable of fertilization) is the larger, non-
motile gamete, produced by the female parent, when the
parents are sexually different.

A sperm is the smaller gamete, commonly motile, and pro-
duced by the male parent, when the parents are sexually
different. Exceptions to the motility of sperms occur in the
Crustacea among animals and in all but the lowest of the
flowering plants. In the lowest flowering plants motile
sperms are found in the pollen-tube, but in the ordinary
flowering plants the two gametes which are produced in the
pollen-tube are non-motile. The pollen-tube itself transports
them by its growth toward the egg-cell of the plant.

A zygote results from the union of two gametes in fertiliza-
tion, an egg with a sperm. It is, potentially or actually,
a new individual produced by a sexual process (union of
gametes) .

A homo-zygote results from the union of gametes which
transmit the same Mendelian character, as black joined with
black, or white joined with white.

A hetero-zygote results from the union of gametes which
transmit alternative Mendelian characters, as black united
with white.

98

MENDELIAN TERMS 99

Mendelian characters exist in contrasted pairs which are
alternatives of each other, as black and white, rough and
smooth, long and short. A gamete may from its nature trans-
mit only one of a pair, either black or w^hite, but not both.
Its nature is simplex. A zygote is duplex in nature; it may
contain a character twice represented (when it is a homo-
zygote) , or contain both a character and its alternative (when
it is a heterozygote) . The same zygote may be a homozygote
as regards one character (say hair-color) and a heterozygote
as regards another (say hair-length).

Unit-character or unit-factor or gene. Such characters of
animals and plants as follow Mendel's law^ in heredity, i. e.,
are inherited as independent units, are often called unit-
characters. But it has been shown in numerous cases that
an independent factor, which follows Mendel's law in trans-
mission, may affect or condition the inheritance of a supposed
unit-character, without itself producing any other discover-
able effect. Thus the agouti (or yellow-ticked) character of
the fur of rodents is not developed unless along with the
other genetic factors which produce a black or a brown coat,
a particular " agouti " factor is present; yet we have no
other evidence of the existence of this factor, except the form
which the black or brown coat assumes when this factor is
inherited. But it can be shown unmistakably that the in-
heritance of this unseen factor is that of an independent
Mendelian character.

Some have sought to avoid the difficulty presented by such
cases by making a distinction between unit-characters and
unit-factors, the former being the recognized morphological
or physiological parts or properties of the organism, the
latter their hypothetical determiners. But this distinction
is of doubtful utility, since the only objective evidence which
we possess that unit-characters exist is the occurrence of
classes among the F2 individuals and their numerical fre-
quencies. But this same evidence also forms our only indi-
cation that determiners exist. In fact the " unit-characters "
about which we talk are the hypothetical determiners. For

100 GENETICS AND EUGENICS

no one familiar with Mendelian phenomena would venture
to classify the anatomical parts or physiological processes of
an organism as unit-characters in heredity merely because
they are distinct anatomical parts or distinct physiological
processes.

The head, the hand, the stomach, stomach-digestion, —
these are not unit-characters so far as any one knows. But if
a race without hands were to arise and this should Mendelize
in crosses with normal races, then we should speak of a unit-
character or unit-factor for " hands," loss of which or varia-
tion in which had produced the abnormal race. But in so
doing we should refer not to the hand as an anatomical part J
of the body nor to the thousand and one factors concerned
in its production but merely to one hypothetical factor to
which we assign the failure of the hand to develop in a
particular case. It is immaterial whether we call this a imit-
character or unit-factor or use both terms inter-changeably,
but it would be a mistake to suppose that they refer to differ-
ent things or that one is less abstract than the other. Histori-
cally the term unit-character has priority, though factor seems
better to express the abstract and purely hypothetical nature
of the conception involved. The application of the term
unit-character at first to certain agencies which were later
found to be complex led to the coining of a new term (unit-
factor) to apply to the newly recognized simpler agencies. If
this process were to be continued indefinitely we should have
to invent a new set of terms for every step in advance in
Mendelian analysis. It seems better to discard earlier and
imperfect analyses as knowledge advances but not to multi-
ply technical terms needlessly when no new conception is
involved.

Parental and filial generations. The manifestation of Men-
delian characters is often very different in successive gener-
ations, for which reason it is necessary to have a convenient
means of designating the different generations concerned.
The significant generation from which reckoning should be

^

MENDELIAN TERMS 101

made is that in which hybridization occurs, i. e., in which
parents of unHke character are mated with each other. Tliis,
following Bateson, we may call the parental generation or P
generation. Subsequent generations are called filial genera-
tions (abbreviated F) and their numerical order is indicated
by a subscript, as first filial (Fi), second filial (F2), etc.
\Mien pure races are crossed the first filial generation (Fi)
is usually as uniform in character as the parental races. Any
striking lack of uniformity in Fi may be taken as prima facie
evidence that one or other of the parent races is impure
(heterozygous for one or more characters) . It is in the F2
generation that recombinations are formed of the characters
in which the parent races differ from each other. The num-
bers of classes of individuals obtained in F2 and their numeri-
cal proportions are the significant features which indicate
how many Mendelizing factors distinguish the parental races
and w^hat their nature is, whether dominant or recessive.

The members of contrasted pairs of Mendelian characters
are known as allelomorphs, i. e., alternative forms. For ex-
ample, colored and albino coat are allelomorphs among
guinea-pigs, as also are rough and smooth, long and short.
The dominant allelomorph is that one which is expressed in
the heterozygote; the recessive allelomorph is that one which
is not expressed in the heterozygote. It follows that domi-
nant allelomorphs are regularly expressed in Fi while recessive
allelomorphs are as regularly suppressed in that generation,
but that both of them find expression in F2, though domi-
nants exceed recessives in F2 as three to one.

For the simplification of inheritance formulae, Mendelian
factors are commonly designated by letters of the alphabet,
members of the same allelomorphic pair being designated by
the same letter, a capital being used for the dominant allelo-
morph, a small letter for the recessive allelomorph. It will
assist the reader to choose letters which suggest descriptive
names of the characters involved. Thus for the agouti factor
we may use A, for its recessive allelomorph a; for the color

102 GENETICS AND EUGENICS

factor we may use C, and for its recessive allelomorph (found
in albinos) c, etc.

Though a gamete, from its simplex nature, may never
contain more than a single allelomorph, and a zygote, from
its duplex origin, may never contain more than two allelo-
morphs, the same race may contain three or more variations
which belong in the same allelomorphic series; i. e., which
are allelomorphs of each other. In such a race, a gamete may
transmit any one of the series, and. a zygote may contain
any two, but never more. In such cases the original termi-
nology of Mendel, which involved the use of capitals and
small letters, becomes inadequate, and it has been deemed
advisable to use in its stead a numerical or descriptive sub-
script. Thus four allelomorphic conditions of the color factor
found among guinea-pigs have been designated C, Cd, Cr,
and Ca respectively.

In calculating the result to be expected from a particular
cross it is obviously necessary to consider, not the number of
characters which the parents possess, but only the number in
which they differ, since as regards these only will heterozy-
gotes be formed in Fi, to be followed by the production of
new homozygous combinations in F2. Our inheritance
formulae therefore will contain only differential factors but
the student must not fall into the error of supposing these to
be the only factors concerned. A thousand factors held in
common by the parents are doubtless involved to every one
in which the parents are observed to differ. But factors held
in common are incapable of demonstration by the method of
experimental breeding. A factor reveals itseK only by its
disappearance or alteration in gametes produced by one of
the parents crossed.

Both from Mendelian theory and from the experience of
practical breeders, it is clear that individuals which look
alike often do not breed alike. Hence it is useful to recognize
(with Johannsen) a " phenotype " as including all individuals
which look or seem alike, and in counter distinction to this

I

MENDELIAN TERMS 103

to recognize a " genotype " which includes only such indi-
viduals as breed alike, i. e., which produce the same kind or
kinds of gametes. A single phenotype often includes two or
more categories of genotypes. Thus F2 dominants though
all may look alike (be of one phenotype) regularly include
both homozygotes and heterozygotes (wholly distinct geno-
types).

CHAPTER X

CALCULATING MENDELL^^N EXPECTATIONS

Mendelian expectations may be calculated either by the
algebraic method used by Mendel himself or by the ingenious
checkerboard method devised by Punnett. The first step
in either process consists in ascertaining what factorial com-
binations are to be expected among the gametes formed by
either parent. By the algebraic method, we ascertain the
product of the gametic combinations of the two parents,
which will give the zygotic combinations to be expected
among their Fi offspring. A repetition of this process, con-
sidering the Fi individuals now as parents, will give the
combinations to be expected among the F2 offspring, etc.

For example, if a homozygous colored guinea-pig is crossed
with an albino, the gametes formed by the parents contain
C and c respectively. The Fi zygotes will contain the two in
association, Cc. The gametes formed by the Fi individuals
will contain either C or c, or collectively will be C -f c. The
Fi female will produce gametes (eggs), C -|- c; the Fi male
will produce gametes (sperms), C + c; the F2 zygotes will
correspond with their product or CC + 2Cc -h cc, or one
homozygous colored (CC), two heterozygous colored (Cc)
and one homozygous albino (cc), or altogether three colored
to one albino, the observed average result.

Suppose now we wish to calculate the result to be expected
from a back-cross of Fi with the recessive (albino) parent.
The Fi gametes, we have assumed, are C -j- c; the gametes
of the recessive parent are all c. Their product is Cc -|- cc
or equal numbers of heterozygous colored individuals and
albinos, the observed experimental result.

The checkerboard method of calculating Mendelian ex-
pectations consists in writing the gametic contributions of
one parent in a series of horizontal squares, each combination

104

CALCULATING MENDELIAN EXPECTATIONS 105

in a different horizontal row. The contributions of the other
parent are then written in the same squares, but in vertical
rows, instead of horizontal ones (since their distribution con-
stitutes a separate contingency) each gametic combination
being entered in a different vertical row. The checkerboard
will then show (within its individual squares) what factorial
combinations are to be expected among the zygotes (progeny
of the parents in question) and with what frequencies.

For the example chosen, the cross between homozygous
colored and albino guinea-pigs, all the gametes of each parent

Egg3

Eggs

C c

01

a

a

C/2

c

c

c

c c

C c

c

c C

c c

a

C c

c c

Fig. 49, Checkerboard method of cal-
culating a Mendelian F2 expectation.

Fig. 50. Checkerboard method of calcu-
lating the result of a back-cross between
Fi and the recessive parent.

being alike, the Fi zygotes would be all of one sort, Cc. But
since the gametes formed by each Fi parent are of two sorts,
C and c, it is evident that the checkerboard must contain
two horizontal and two vertical rows, or a total of four
squares. (See Fig. 49.) Let us enter C in the upper horizon-
tal row and c in the lower row as the gametic contributions
of one parent, then enter C in the left vertical row of squares
and c in the right vertical row as the contributions of the
other parent. We then have the table as shown, one square
containing CC, two containing Cc, and one cc, the same result
given by the algebraic method.

For the back-cross of Fi with the recessive parent, only
two squares are required. (See Fig. 50.) The recessive parent
contributes always c, which we enter in the two squares
placed in a horizontal row. The Fi parent contributes C to
one square, c to the other. The resulting combinations are

106

GENETICS AND EUGENICS

obviously Cc and cc respectively. A checkerboard is scarcely
necessary for cases as simple as these, but will be found very
clarifying to thought for the beginner, particularly if he is"^
not accustomed to thinking in algebraic terms, when he comes
to deal with crosses involving simultaneously three or four
independent characters.

The essential point about which one must first of all he entirely
clear in his own mind is this — what kinds of gametes will each
parent form. If he is clear as to this question the calculation
of expectations by either method will present no difficulties.
It should be borne in mind therefore that the fundamental
Mendelian assumptions are (1) that homozygotes form only
one type of gamete but (2) that heterozygotes form two
types of gametes equally numerous, viz., dominants and re-
cessives. Further (3) double heterozygotes {i. e., individuals
heterozygous for each of two independent characters) form
four types of gametes all equally numerous, and (4) triple
heterozygotes form eight types of gametes, all equally numer-
ous. (5) In general every additional character in which the
individual is heterozygous doubles the assortment of gametes
which it would otherwise form. See Table 7.

TABLE 7

Zygotic Composition of Parents and the Expected Constitution

OF THEIR Gametes

Parent
Homozygote, AA

AABB
" AABBCC

Heterozygote, Aa
Bb
Cc
Double heterozygote, AaBb

AaCc
BbCc

Triple

AaBbCc

Gametes which it will form

all A
all AB
all ABC

A + a
B + b

C + c

AB + Ab + aB + ab

AC + Ac + aC + ac
BC + Be + bC + be
ABC + ABc + AbC + aBC
+ Abe + aBc + abC + abc

Inspection of a typical checkerboard calculation, that
for the F2 generation following a dihybrid cross, shows some

CALCULATING MENDELIAN EXPECTATIONS 107

interesting facts. All the homozygotes expected lie in the
diagonal row of squares running from the upper left to the
lower right comer of the figure. Compare Fig. 49. These
are the individuals that will "breed true," i. e., will fonn
only a single type of gamete. They are four in number, each
of a different sort and would result from the union of two like
gametes of each of the four expected types, AB + Ab + aB -f
ab (or in Fig. 49, EA + Ea + eA + ea). They represent all
the possibilities as regards true breeding ('*fixed") forms to
be expected from the cross. What the nature of the other
individuals to be expected would be would depend upon
the completeness of dominance. If dominance should be
complete, heterozygotes would be indistinguishable except
by breeding test from the four expected homozygotes; other-
wise homozygotes and heterozygotes might be distinguish-
able by appearance as well as by breeding tests. With
complete dominance, i. e., with only dominant characters
showing in the zygote, the four sorts would appear as 9 AB :
3 Ab : 3 aB : 1 ab, the typical dihybrid F2 ratio. Let the
reader make out the checkerboard and verify these state-
ments.

In a similar way one may calculate, either by algebra or by
checkerboard the F2 expected result from a trihybrid cross.
The eight kinds of gametes which the triply heterozygous Fi
individuals would produce have already been indicated, viz.,
ABC -F ABc + AbC + aBC + Abe -f aBc + abC + abc.

By the checkerboard method, each combination would
be found homozygous (united with a gamete like itself) in
a different square of the diagonal of the figure, and hetero-
zygotes containing the same dominant characters would be
found elsewhere in the table sufficient in number to bring the
totals up to 27 ABC : 9 ABc : 9 AbC : 9 aBC : 3 Abc : 3 aBc:
3 abC : 1 abc. This is the typical trihybrid F2 ratio, when
complete dominance exists.

To repeat, it is all essential to determine first the kinds of
gametes each parent to a mating is expected to produce. The
subsequent calculation is easy and certain. One soon learns

108 GENETICS AND EUGENICS

to write out F2 ratios without going through the calculation
in detail either by algebra or by checkerboard. Thus, if
we take the expected completely recessive class as 1, each
class containing one dominant factor will be 3, each class
containing two dominant factors will be 9 {i, e., 3^) each class
containing three dominant factors will be 27 {i. e., 3^) etc.
Accordingly by mere inspection of a gametic series to ascer-
tain how many dominant factors each term contains, we may
at once assign to each the proportional number of F2 zygotes
in which it will be seen. See Table 8.

TABLE 8
Relation between the Fi Gametic Series and the Expected F2 Zygotes

Fi Gametic Series F2 Zygotes

A + a 3A + la

AB + Ab + aB + ab 9 AB + 3 Ab + 3 aB + 1 ab

ABC + ABc + AbC + aBC\ f 27 ABC + 9 ABc + 9 AbC + 9 aBC
+ Abe + aBc + abC + abc J \ + 3 Abe + 3 aBc + 3 abC + 1 abe
ABCD + ABCd + etc. 81 ABCD + 27 ABCd + ete. (let the reader

supply the missing terms).

Stated in general terms, as Mendel himself showed (and
as follows from the binomial formula), when the number of
unit-character differences between the parents is n, the visibly
different classes of offspring will be 2", the total different
sorts of zygotes will be 3", and the smallest number of in-
dividuals which may be expected to contain all of them will
be 4".

TABLE 9

Differences
Between Parents

Visibly
Different Classes

Really
Different Classes

Minimum Number
of F2 Individuals
Including all Classes

n

271

3«

4"

1

2
3

2
4
8

3
9

27

4
16
64

J

Tested by

Mendel for

• Peas and

Found

Correct

4

16

81

256^

5

32

243

1024

• Calculated

6

64

729

4096

J

Table 9 shows what the size of these several classes is for
1-6 independent characters.

CHAPTER XI

MODIFIED MENDELIAN RATIOS; HETEROZYGOUS
CHARACTERS; ATAVISM OR REVERSION

In the last chapter MendeHan ratios have been calculated on
the supposition that homozygous dominants and heterozy-
gous dominants are not distinguishable from each other,
which frequently is true; but if they are distinguishable from
each other, then a larger number of F2 classes can be recog-
nized and their numerical proportions are different. A case
of this kind was early recognized among plants by Correns.
(See Fig. 51.) When a white variety of four-o'clock {Mira-
hilis) is crossed with a red variety, Fi plants are produced
which bear pink flowers, and F2 consists of whites, pinks, and
reds in the ratio, 1:2:1. Reds and also whites breed true, but
pinks again produce the three sorts. This result indicates
that both reds and whites are homozygotes (RR and rr
respectively) but that pinks are regularly heterozygotes (Rr)
and for this reason do not breed true but are " unfixable."
Pink in this case may be called a heterozygous character; it
is for that reason unfixable.

A similar but even better-known case among animals has
been described by Bateson and Punnett, that of the blue
Andalusian fowl. Birds of this race are of a slaty blue color
and are known to fanciers to be unfixable as to color. When
blues are mated with each other, chicks are obtained of three
distinct sorts as regards color, viz., blacks, blues, and
" splashed whites." The blacks breed true, as also do the
whites, but the blues invariably produce in every generation
the three sorts, of which blacks may be called homozygous
dominants (BB), whites homozygous recessives (bb), and
blues heterozygotes (Bb) . But it is clear that if we so desig-
nate them, dominance must be recognized to be imperfect.

109

no

GENETICS AND EUGENICS

Attempts of poultry men to " fix " the blue variety are mani-
festly hopeless, unless some new variation arises within the
race which can be secured in homozygous form and will yet
possess the desired appearance.

Another example of a heterozygous and so unfixable char-
acter is found among short-horn cattle. Here red is a true-

/^imbilis Jalafta

Fig. 51. A diagram to show inheritance of flower color in crosses of Mirabilis, the "four-o'clock."
Alha, white parent; rosea, red parent; alba + rosea, the unfixable Fi heterozygote, of intermediate
color, pink. I. Gen. = Fi. U. Gen. = F2. (After Correns.)

breeding type as also is white, but the heterozygote between
red and white is an unfixable roan. (See Figs. 62-64.)

The effect which the production of a recognizable hetero-
zygous form has upon the typical r2 monohybrid ratio
(3:1) is to convert it into a 1:2:1 ratio, in which each
parental type is represented by one individual while the
heterozygous type is represented by two. The typical di-

HETEROZYGOUS CHARACTERS 111

hybrid ratio (9:3:3:1) we might expect to see modified in a
similar way, if a cross were made involving simultaneously
two Mendelian characters imperfectly dominant. The num-
ber of distinguishable classes, as shown originally by Mendel
(see Appendix) would then be 9, numerically as follows: 1:1:
2:2:4:2:2:1:1. For three factors all imperfectly dominant
the modified trihybrid Mendelian ratio would be expressed
by (1 + 2 + 1) 3 and for n factors by (1 +2 + 1)'*. Hetero-
zygous characters must from definition always be unfixable.
In the foregoing cases comparison of their behavior in breed-
ing experiments with that of the corresponding homozygotes
has shown this to be true, but there exist cases in which
only one type of homozygote has been found to occur, the
other being apparently impossible of production.

The first case of this sort to be demonstrated is found
among yellow mice and to Cuenot (confirmed by Little) we
owe its demonstration. If certain strains of yellow mice are
crossed with black ones, the offspring produced are of two
sorts equally numerous, yellow and black. From this result
alone it is impossible to say which is the dominant character,
but breeding tests of the offspring show that yellow is the
dominant character. For the black offspring bred together
produce only black offspring, but the yellows bred together
produce both yellow offspring and black ones. The curious
feature of the case is that when yellows are bred with each
other no pure yellows, that is, homozygous ones, are obtained.
Hundreds of yellow individuals have been tested, but the
invariable result has been that they are found to be hetero-
zygous; that is, they transmit yellow in half their gametes,
but some other color in the remaining gametes — it may be
black or it may be brown, or gray. Non-yellows obtained by
mating yellow with yellow mice never produce yellow off-
spring if mated with each other. This shows that they are
genuine recessives and do not contain the yellow character,
which is dominant.

Now ordinary heterozygous dominants, when mated with
each other, produce three dominant individuals to one reces-

112 GENETICS AND EUGENICS

sive. Accordingly we should expect yellow mice, if, as
stated, they are invariably heterozygous, to produce three
yellow offspring to one of a different color, but curiously
enough they do not. They produce two yellows (instead of
the expected three) to every one of a different color. About
the ratio there can be no reasonable doubt. It has been
determined with great accuracy by Dr. C. C. Little, who
finds that in a total of over twelve hundred young produced
by yellow parents almost exactly two-thirds are yellow.
Instead of the regular Mendelian ratio, 3:1, we have then in
this case the peculiar ratio, 2:1, and this requires explana-
tion. The explanation of this ratio is to be found in the same
circumstance as is the total absence of pure yellow individu-
als. Pure yellow zygotes are indeed formed, but they
perish for some reason. A yellow individual produces gametes
of two sorts with equal frequency, viz., yellow and non-
yellow (let us say black). For, if yellow individuals are
mated w^ith black ones, half the offspring are black, half
yellow, as already stated. Now if yellow individuals are
mated with each other we expect three sorts of young to be
produced, numerically as 1:2:1, viz., 1 Y Y, 2 Y B, and 1
B B. But since observation shows that only two combina-
tions are formed which contain yellow to one not containing
yellow, and since further all yellows which survive are found
to be heterozygous (YB), it must be that the expected Y Y
individual either is not produced or straightway perishes.
As to which of these two contingencies happens we also have
experimental evidence. Dr. Little finds (confirming Cuenot),
that yellow mice when mated to black ones produce larger
litters of young than w^hen they are mated to yellow ones.
The average-sized litter contains something like 5.5 young
when the mate is a black animal, but only 4.7 when it is a
yellow animal. It is evident, then, that about one young one
out of a litter perishes when both parents are yellow, and this
undoubtedly is the missing yellow-yellow zygote. The
yellows which are left are heterozygous yellow-black zygotes,
and they are to those that perish as 2 :1. They are also to the

Fig. 52. Simple Mendelian inheritance in crosses of red guinea-pigs with black ones. P, parents; one
red, one black. Fi, one of the young, all heterozygous blacks. BC, young produced by a back-cross of
an Fi black with the red parent. Half are red, half are black.

UNFIXABLE CHARACTERS 113

non-yellow zygotes as 2:1, the ratio observed also among the
surviving young of yellow by yellow parents.

This interpretation of the 2:1 ratio observed in this case is
strongly supported by a similar case among plants, in which
the evidence is even more complete. A so-called " golden "
variety of snapdragon, one in which the foliage was yellow
variegated with green, was found by the German botanist,
Baur, to be unfixable, producing when self-pollinated fully
green plants as well as golden ones, in the ratio 2 golden:
1 green. The green plants were found to breed true, that
is, to be recessives, while the golden ones were invariably
found to be heterozygous. Baur found, however, by ger-
minating seeds of golden plants very carefully, that there
were produced in addition to green plants and golden ones a
few feeble seedlings entirely yellow, not variegated with
green, as the golden plants are. These, for lack of assimilat-
ing organs (green chlorophyl), straightway perished. Clearly
they were the missing pure yellow zygotes.

Frequently one of the visible characters of an organism
depends upon the combined action of two or more inde-
pendent Mendelian factors, in which case it is demonstrably
not a unit-char SLCter, as has already been pointed out, since
each of the known " factors " is indispensable to the develop-
ment of the visible character, as are probably also a great
many other as yet unknown factors. The dependence of a
visible character upon two or more simultaneously varying
factors leads to the production of modified dihybrid or poly-
hybrid F2 ratios. It also leads to a phenomenon known as
atavism or reversion, by which is meant the restoration of a
lost ancestral character, which frequently follows crossing of
unrelated varieties.

Atavism or reversion to an ancestral condition is a phe-
nomenon to which Darwin repeatedly called attention. He
realized that it is a phenomenon for which general theories
of heredity must account. He supposed that the environ-
ment was chiefly responsible for the reappearance in a species
of a lost ancestral condition, but that in certain cases the

114 GENETICS AND EUGENICS

mere act of crossing may reawaken slumbering ancestral
traits. Thus he noticed that when rabbits of various sorts
are turned loose in a warren together, they tend to revert to
the gray-coated condition of wild rabbits. And when pigeons
are crossed in captivity they frequently revert to the plum-
age condition of the wild rock pigeon, Columha livia. In
plants, too, Darwin recognized that crossing is a frequent
cause of reversion. The explanation which he gave was the
best that the knowledge of his time afforded, but it leaves
much to be desired. This lack, however, has been completely
supplied by the Mendelian principles. An illustration or two
may now be cited.

When pure-bred black guinea-pigs are mated with red ones,
only black offspring are as a rule obtained. (See Fig. 52.)
The hairs of the offspring do indeed contain some red pig-
ment, but the black pigment is so much darker that it largely
obscures the red. In other words, black behaves as an ordi-
nary Mendelian dominant. In the next generation black and
red segregate in ordinary Mendelian fashion, and the young
produced are in the usual proportions, three black to one red,
or 1:1 in back-crosses of the heterozygous black with red.
All black races behave alike in crosses with the same red
individual, but among red animals individual differences
exist. Some, instead of behaving like Mendelian recessives,
produce in crosses with a black race a third apparently new
condition, but in reality a very old one, the agouti type of
coat found in all wild guinea-pigs, as well as in wild rats, mice,
squirrels, and other rodents. In this type of coat reddish
yellow pigment alone is found in a conspicuous band near the
tip of each hair, while the rest of the hair bears black pig-
ment. The result is a brownish or grayish ticked or grizzled
coat, inconspicuous, and hence protective in many natural
situations. (See Fig. 53.)

Some red individuals produce the reversion in half of their
young by black mates, some in all, and others, as we have
seen, in none, this last condition being the commonest of the
three. It is evident that the reversion is due to the intro-

Fig. 53. Reversion in crosses of a red guinea-pig with a black one. P, parents. Fi, one of the rever-
sionary (agouti) young. BC, young produced by a back-cross of an Fi agouti with an ordinary red
individual. Half the young are red. The other half are equally divided between agoutis and blacka.

REVERSION OR ATAVISM 115

duction of a new factor, additional to simple red or simple
black. It is evident further that this new factor, which we
will call A (agouti), has been introduced through the red
parent, and that as regards this factor, A, some red indi-
viduals are homozygous (AA) in character, others are hetero-
zygous (Aa), while others lack it altogether (aa). The agouti
character becomes visible only in the presence of both black
and red, because it is a mosaic of those two pigments. If the
Fi agouti individuals are bred together they produce in the
next generation (F2) three sorts of young, viz., agouti,
black, and red, which are numerically as 9:3:4. This evi-
dently is a modification of the dihybrid Mendelian ratio
9:3:3:1, resulting from the fact that the last two classes are
superficially alike. They are red animals with and without
the agouti factor respectively; but this agouti factor is in-
visible in the absence of black, so that both sorts of reds look
alike. Together they number four in sixteen of the F2 off-
spring. Figure 54 is intended to show by the checkerboard
method how this modified dihybrid ratio is obtained.

Black and red varieties differ from each other by a varia-
tion in what has been called the extension factor (E), the
reference being to the fact that black (or brown) pigment,
found in the eyes of both varieties, extends throughout the
coat in the black variety but is restricted to the eye in the
red variety. The allelomorphic conditions of this factor are
designated E (in black) and e (in red) respectively. The
agouti factor (A) may exist in red animals without producing
visible effects because there is no black pigment in the fur
of such animals to bring out the ticking, but its existence in
animals which would otherwise be black changes the coat to
agouti. Hence the constitution of the parental gametes is:
Black parent, Ea; red parent eA. Fi is EeAa, a double
heterozygote. Its gametes are EA + Ea + eA + ea, which
with dominance complete will produce F2 zygotes, 9 EA +
3 Ea -1- 3 eA -h 1 ea. (See Figure 54.) But EA contains the
two factors which together produce agouti; Ea contains the
factors for black; eA contains the factor for agouti but with-

116

GENETICS AND EUGENICS

out the factor (E) necessary to make it visible, and so will be
red; and ea contains neither the factor for agouti nor that
for black, hence will also be red. Accordingly the expected
F2 distribution is nine agouti, three black, four red, the ratio
observed. This is a very common modification of the F2

E A

E a

e A

e a

E A

E a

e A

e a

E A

E A

E A

EA

E A

E a

e A

e a

Agouti

Agouti

Agouti

Agouti

E a

E a

E a

E a

EA

E a

e A

e a

Agouti

Black

Agouti

Black

e A

e A

e A

e A

EA

E a

e A

e a

Agouti

Agouti

Red

Red

e a

e a

e a

e a

E A

E a

eA

e a

Agouti

Black

Red

Red

Fig. 54. Checkerboard to explain the modified dihybrid F2 ratio,
9 : 3 : 4, as observed when black guinea-pigs are crossed with red ones
which transmit the agouti factor (A).

dihybrid ratio and owes its production to the fact that two
independent Mendelian factors are involved one of which pro-
duces no visible effect except in the presence of the other.

Another example of this same modified dihybrid ratio
(9:3:4) is obtained by crossing an albino rodent (rat, mouse,
rabbit or guinea-pig) derived from a black race, with a wild
(agouti) individual. Fi consists of agoutis, like the wild
parent, but F2 contains agoutis, blacks, and albinos in the
proportions, nine agouti, three black, four albino. The ex-
planation is as follows. The albino parent differs from the
wild agouti parent as regards two factors, viz., the color
factor (C) and the agouti factor (A). The albino parent is

%

Fig. 5.5. Reversion to full intensity of pigmentation on crossing a pink-eyed cream-and-white rat with
an albino. P, parents; cream-and-white at left, albino at right. Fi, one of the black-and-white young.
F2, cream-and-white at left, black-and-white in middle, albino at right. Their numerical relations are
about as 3 : 9 : 4. A slight departure from these proportions is observed on account of linkage (in this
case repulsion) between the genes for pink-eye and albinism. See chapter on Linkage.

i

MODIFIED F2 RATIOS 117

ac; the agouti parent AC. Fi is AaCc, a double hetero-
zygote. Its gametes consequently should be of four types,
viz., AC + Ac + aC + ac, and the F2 zygotes, 9 AC:3 Ac:
3 aC :1 ac. But only zygotes which contain C will develop
a colored coat, hence both 3 Ac and 1 ac will be albinos. The
9 AC individuals contain the factors of the wild parent and
hence will be agouti; the 3 aC individuals will develop a
colored coat since they contain C, but this coat will be non-
agouti (a), i. e., they will be like the wild type except for the
lack of the agouti factor and so will be black.

Precisely the same result in Fi and F2 is obtained if a black
rodent (rat, mouse, rabbit, or guinea-pig) is crossed with an
albino which transmits the agouti factor, as for example an
albino whose parents were homozygous for the agouti factor.
In this case Fi is agouti by reversion, C being derived from
the black parent, A from the albino parent. But Fi is doubly
heterozygous, precisely as in the foregoing case, and the F2
generation contains only three apparent classes of individuals
instead of the usual four for the reason that one of the two
differential factors concerned in the cross (viz.. A) is unable
to produce a visible effect except in the presence of the
other (C).

Another somewhat similar case involving reversion in Fi
with the production of the modified dihybrid ratio, 9:3:4,
in F2 is illustrated in Fig. 55. A pale-coated " cream-and-
white " rat was crossed with an albino and produced black-
and-white young, a reversion to pigmentation of full inten-
sity, though white spotting was retained, this being an
independent Mendelian character transmitted by both
parents. The F2 generation consisted of black-and-white,
cream-and- white, and albino individuals in numbers approxi-
mating the 9:3:4 ratio. Black-and-white is here the double
dominant class, 9; cream-and-white is the single dominant
class, 3; and the albinos include three which transmit the
dominant character, black-and-white, but which fail to show
it because they lack the color factor, and also one which
transmits cream-and-white but which fails to show it for the

118 GENETICS AND EUGENICS

same reason, lack of the color factor. Together the albinos
number four.

A different modification of the typical dihybrid ratio is
illustrated by the following case in which two varieties were
crossed which possessed complementary factors neither of
which is able to produce a visible effect apart from the other.
When certain white-flowered varieties of sweet peas are
crossed with each other they produce Fi plants which bear
red-colored flowers (Bateson and Punnett). F2 consists of two
apparent varieties only, viz., reds and w^hites in the ratio,
nine red to seven w^hite. This is explained as a modified di-
hybrid ratio (9:3:3:1) in which the last three terms are in-
distinguishable (all being white). The two factors involved
in this case are assumed to be a color factor found in one white
parent and a red factor found in the other, both together (in
Fi) producing a red color, but either by itself producing no
color whatever. One parent accordingly produces gametes
all Cr, the other produces gametes all cR. Fi is CcRr, a
double heterozygote; its gametes, CR -f Cr + cR -\- cr;
and the F2 zygotes containing the same assortments of fac-
tors are 9CR:3Cr:3cR :lcr. But if C and R, neither of
them, produce color apart from each other, then only the
9 CR zygotes are colored, all the others, seven in sixteen,
being white, and the observed F2 ratio (9 : 7) is thus accounted
for as the result of a dihybrid cross at the same time that the
Fi result is explained.

When some other white-flowered varieties of sweet peas
are crossed with each other, there are produced, not red-
flowered Fi plants as in the foregoing case, but those which
are purple bi-color, like the wild sweet pea, a case of rever-
sion or atavism, like those known for pigeons, rabbits and
guinea-pigs. This reversion involves a third independent
factor (a factor for blue, B) which is ineffective except in
the presence of both the color factor (C) and the red factor
(R). When in such reversionary crosses a colored Fi is pro-
duced which is heterozygous for all three factors, F2 mani-
fests a peculiar modified trihybrid ratio, less common than

MODIFIED F2 RATIOS 119

the modified dihybrid ratios just discussed. If, for example,
one white parent contributes the color factor while the other
parent contributes the red and the blue factors, then we may
represent the parental gametes as Crb and cRB respectively.
Fi will then be a triple heterozygote, CcRrBb, which from
the combined action of the three dominant characters will be
a purple bi-color. Its gametes will then be of eight sorts and
the zygotes in which corresponding groupings of the domi-
nant factors occur will be as follows: ^

27 CRB, purple 3 Crb, white

9 CRb, red 3 cRb, white

9 CrB, white 3 crB, white

9 cRB, white 1 crb, white

But only the first two of these eight groupings contain com-
binations of factors capable of producing colored flowers, viz.,
CRB, which will produce purples, and CRb, which will pro-
duce reds. All the other six combinations lack one or both
of the two factors (C and R) which must be present together
in order to produce colored flowers. Consequently all will
produce uncolored (white) flowers, and the expected classes
of phenotypes will be as follows: twenty-seven purple, nine
red, twenty-eight white, a modified trihybrid ratio.

Summary on Modified Ratios

1. When a cross involves two factors, one of which pro-
duces no visible effect except in the presence of the other,
the dihybrid r2 ratio, 9:3:3:1, is modified to 9:3:4, because
the last two classes of the typical ratio are indistinguishable.

2. When a cross involves two factors, neither of which pro-
duces a visible result in the absence of the other, the dihybrid
ratio becomes 9:7, because the last three classes of the
typical ratio are indistinguishable; if in addition a third
factor is involved which produces no visible effect except in
the presence of both the others, a modified trihybrid ratio
is obtained, viz., 27:9:28.

^ It is suggested that the reader make out the trihybrid checkerboard calcula-
tion for this cross and color the squares with crayon in accordance with the assump-
tion made.

120

GENETICS AND EUGENICS

Modification of the ratio, 9:3:3:1, due to linkage. When two Mendelian
characters are not wholly independent of each other, but show a tendency
to be inherited together, they are said to be coupled or linked to each other.
Thus, in the sweet pea, purple and red are alternative color forms, and
long pollen and short pollen are alternatives as to pollen shape. And if a
purple plant with long pollen is crossed with a red plant having round
pollen, four classes are obtained in r2, viz., purple long, purple round,
red long and red round. This being apparently a dihybrid Mendelian

TABLE 10

The F2 Ratio, 9:3:3:1, as Affected by Coupling or Linkage, A and B
Entering the Fi Zygote in the Same Gamete

Ratio, Crossover to

Proportion
Crossover
Gametes

F2 Zygotes

Non-crossover
Gametes

AB

Ab

aB

ab

Total

\:x

1

3x2 _|_ 2(2a: -j- 1)

2x+ 1

2x+ 1

X2

(2x + 2) 2

x-\- 1

1:11

1/2

9

3

3

1

16

1:2

1/3

22

5

5

4

36

1:3

1/4

41

7

7

9

64

1:4

1/5

66

9

9

16

100

1:5

1/6

97

11

11

25

144

1:6

1/7

134

13

13

36

196

1:7

1/8

177

15

15

49

256

1:8

1/9

226

17

17

64

324

1:9

1/10

281

19

19

81

400

1:99

1/100

29,801

199

199

9,801

40,000

Limiting values^

....

3

0

0

1

4

1 No coupling.

2 Not distinguishable from the case in which A and B are due to a single genetic factor.

cross, we should expect the four classes to be respectively as 9:3:3:1, but
in reality the classes purple long and red round (the parental combinations)
are in excess of these proportions. When these facts were discovered by
Bateson and Punnett, it was stated that coupling exists between the
characters purple and long and their allelomorphs red and round. Later,
however, when a cross was made between purple round and red long, it
was found that these combinations were in excess in r2. Purple and long
which in the first case were coupled, now showed repulsion. Morgan
explains both cases by supposing that the two character-pairs have deter-
miners or genes located near to each other in the germ-cell, probably in the
same chromosome, so that the parental combination has a tendency to
persist in F2. Morgan also proposes to substitute a single term, linkage,
for the two terms of Bateson, coupling and repulsion.

F2 RATIOS MODIFIED BY LINKAGE

121

It is evident that linkage will cause modification of the tj'pical dihybrid
ratio, 9:3:3:1, since the four possible classes of gametes formed by Fi
individuals will not all be equally numerous. Accordingly the stronger
the linkage, the greater will be the modification of the tj^pical ratio. Con-
versely, we may estimate the strength oj the linkage by the observed depart-
ure from the 9:3:3:1 ratio.

In so doing, tables 10 and 11 may be found useful, in which the expected
modification of the 9:3:3:1 F2 ratio is given for various integral ratios of

TABLE 11

The F2 Ratio, 9:3:3:1, as Affected by Repulsion (Negative Linkage),
A AND B Entering the Fi Zygote in Different Gametes

Ratio, Crossover to

XT

Proportion
Crossover
Gametes

F2 Zygotes

Non-crossover
Gametes

AB

Ab

aB

ab

Total

l:x

1

2(x2 4- 2x) 4- 3

x2+2x

x2+2x

1

(2x+2)2

x+1

1:13

1/2

9

3

3

16

1:2

1/3

19

8

8

36

1:3

1/4

33

15

15

64

1:4

1/5

51

24

24

100

1:5

1/6

73

35

35

144

1:6

1/7

99

48

48

196

1:7

1/8

129

63

63

256

1:8

1/9

163

80

80

324

1:9

1/10

201

99

99

400

1:99

1/100

20,001

9,999

9,999

40,000

Limiting values *

....

2

1

1

0

4

3 No repulsion.

* Not distinguishable from the case in which A and B are allelomorphs.

gametes showing the 'parental combinations, to gametes not showing them.
Morgan calls the gametes which show novel combinations crossover
gametes and those which show the original combinations non-crossover
gametes. If the latter are two, three, four, etc., times as numerous as the
former, then we get the modified F2 ratios shown in the tables, where also
formulae are given for extending the tables to any desired extent. In
making use of these tables, it is necessary only to reduce to the basis of a
common total the observed F2 zygotic series and any series of the table
with which a comparison is desired. This will be facilitated by consulting
Table 11a, in which each zygotic class of Tables 10 and 11 is expressed as
a percentage of the total population.

122

GENETICS AND EUGENICS

TABLE 11a

A Combination of Tables 10 and 11, in which the Size of each
F2 Class is expressed as a Percentage of the F2 Population.
It is assumed that the Gametic Series is the same in both Sexes

Percentage F

'i zygotes when

Ratio, cross-
over to non-
crossover
gametes

A and B enter together
(Table 10)

A and B enter separately
(Table 11)

AB

Ab (or aB)

ab

AB

Ab (or aB)

ab

1

56.2

18.7

6.2

56.2

18.7

6.2

2

61.1

13.6

11.1

52.8

22.2

2.8

3

64.0

10.9

14.0

51.5

23.4

1.5

4

66.0

9.0

16.0

51.0

24.0

1.0

5

67.3

7.6

17.3

50.7

24.3

0.7

6

68.3

6.6

18.3

50.5

24.5

0.5

7

69.3

5.8

19.3

50.4

24.6

0.4

8

69.7

5.2

19.7

50.3

24.7

0.3

9

70.2

4.7

20.2

50.2

24.8

0.2

99

74.5

0.5

24.5

50.0+

24.9

0.0+

oc

75.0

0.0

25.0

50.0

25.0

0.0

In using Table 11a to test a case of suspected linkage, the size in per cent
of each observed F2 class should first be determined and comparison made
with the corresponding class in the Table. In cases of doubt, determina-
tion of the probable error of linkage may show whether the observed de-
partures from the normal 9:3:3:1 ratio are or are not significant. The
9:3:4 ratio as affected by linkage may be obtained by combining in Tables
10 or 11, the numbers in the columns headed aB and ab.

Fig. 56. A dihybrid Mendelian cross between a wild Norway rat and the tame variety known as
black hooded. P, parents; wild gray at left, black hooded at right. Fi, a heterozygote, gray like the
wild parent, but showing traces of the recessive white spotting. Note white left fore foot. F;, the four
second-generation classes of offspring. From left to right, gray self, gray hooded, black self, black
hooded. Numerically as 9 : 3 : 3 : 1. Let the reader identify in Table 12 the unit-characters involved.

«

CHAPTER XII

THE UNIT-CHARACTERS OF RODENTS

No group of mammals has been studied as thoroughly, in
respect to heritable characters, as have the rodents. This is
particularly true as regards those striking variations of the
coat which form the basis of the many recognized domestic
varieties. In nearly every case the distinctive features of
these several varieties are found to be Mendelian unit-char-
acters. As an example we may take the varieties of the
domestic cavy or guinea-pig, probably the first of the rodents
in point of time to be domesticated. Certainly in richness of
varieties it surpasses all others. It was domesticated by the
ancient Peruvians before the discovery of America and
formerly held an important economic place among the natives
of tropical America where it was reared as an article of food
in every cabin, a practice which to some extent still continues
among the poorer classes. Its variation in color and other
coat characters has been very extensive, unequalled in
amount perhaps among mammals other than dogs. Nearly
every distinct variety is characterized by the possession of
one or more Mendelian unit-character variations. At least
ten such unit-characters are concerned in the production of
these varieties. Several of these unit-characters have al-
ready been referred to. (See Table 12.) All but one of them
(" rough ") may be regarded as recessive unit-character va-
riations from the conditions found in wild cavies generally.
Perhaps the earliest in point of time, certainly the com-
monest among rodents wild or domesticated, is the albino
variation, in which the fur is white and the eye pink. This
makes its appearance as a sport, probably originally in a
single individual and later as a recurring variation among its
descendants. Albino individuals are undoubtedly at a dis-
advantage in the struggle for existence in a wild state because
of the conspicuousness of the albino to its enemies and also

122a

UNIT-CHARACTERS OF RODENTS

123

because of its defective vision. For the eyesight of the albino
is very poor owing to the imperfect pigmentation of its eyes.
Albino sports accordingly never become very common in a
wild species but are probably among the earliest formed
domestic or tame varieties, because of their striking character

Table 12
Some Unit-Characters of Rodents

Name of Factor

Symbol,
Dominant

Appearance of Dominant

Symbol,
Recessive

Appearance of Recessive

Phase

Individual

Phase

Individual

Color

c

Colored

c

Albino

Extension

E

Black or brown

e,e'

Yellow, yellow spotted
with black or brown

Agouti

A

Gray (agouti)

a

Black or brown (non-
agouti)

Black

B

Black or black agouti

b

Brown or brown agouti

WTiite spotting

S

Self colored

s

Spotted with white

Dark eye

D

Dark eyes and coat

d

Pink eyes and coat
pale, where not yel-
low

Intensity

I

All pigments dark

i

All pigments pale

Hair length

L

Short-haired like wild
cavies

1

Hair long and silky

Rough coat

R

Coat rosetted

r

Coat smooth

Rough modi-

M

Coat slightly rosetted

m

Coat rosettes fully de-

fier

veloped

and the ease with which a distinct variety is established.
For, being recessive, the albino variation is secure as a racial
character as soon as a pair of albinos has been isolated.^

The albino variation is commonly considered to be the re-
sult of a recessive variation in a color factor whose dominant
phase is expressed by the symbol, C, its recessive or albino
phase by c. (See Table 12.)

Another color sport occasionally observed among wild ro-
dents, and which is the basis of distinct varieties among

^ The contemporary origin of an albino race of field mouse (Peromyscus) has
recently been recorded (Castle, 1912) in a species in which neither this nor any other
of the common color sports had previously been recorded.

124 GENETICS AND EUGENICS

tame ones, is a change to yellow coat. This results from a
disappearance of black pigment from the hair or its replace-
ment by yellow. But the black pigment still persists in the
eye. Hence one ma\^ speak of this change as being a restric-
tion of black to the eye, whereas in wild rodents it is regu-
larly extended throughout the coat. The factor which has
undergone change is therefore said to be the extension factor
for black (or brown) pigment. Its dominant phase may be
expressed by E, its recessive phase (found in yellow animals)
by e. (See Plate 7, Fig. 29.) An alternative recessive phase
(eO is found in yellow animals spotted or brindled with black
or brown. ^

A third sport among wild rodents is responsible for the
origin of black varieties which lack the yellow tip of the fur
found in most wild gray or ''agouti " varieties. (See Plates 6
and 7, Figs. 22-26). This yellow tip sometimes takes the
form of a subapical band of yellow on hair which is black (or
brown) both at the base and at the extreme end. This is the
case for example in the agouti varieties of the rabbit and the
guinea-pig. The optical effect of the agouti factor in either
case is to produce a protectively colored, neutral gray coat,
inconspicuous against many natural backgrounds. The black
sport may be regarded as a recessive variation in an agouti
factor possessed by most wild rodents. The dominant phase
of this factor may be expressed by A, its recessive phase (the
non-agouti variation) by a.^

Another unit-character variation found in many rodents,
as well as in some other mammals, is responsible for the re-
placement of black pigment by brown throughout the coat
and even in the eye. (See Plate 7, Figs. 27 and 28.) This

^ The occurrence of yellow sports among wild meadow mice (Microtus) has been
observed by Cole, Barrows, F. Smith and others, though no tame races of this very
common rodent have yet been established. The contemporary origin in England
of a yellow race of the Norway rat has been recorded by Castle (1914), and the
origin of a yellow race of Mus rattus by Bonhote.

2 Sometimes black varieties arise by a process other than a change in the agouti
factor, as is the case probably in a locally common black variety of the gray squirrel
of Eastern North America. This shows the agouti marking of the fur to so small an

Plates 6 and 7 are reproduced by permission from Publication
No. 2Ji^l of the Carnegie Institution without change of figure num-
bers. They show in the natural colors how a single pure-breeding
domestic type {20) crossed with a single pure-breeding wild type
{23 and 2Jt) may produce in the next generation only a single type
{22) y which however may, in the following generation, through the
operation of MendeVs law, produce half-a-dozen very distinct pure-
breeding types {25-30) . Through a knowledge of MendeVs law
the multiplication of color types among animals and plants has
ceased to be a haphazard process and has become a simple and
orderly procedure.

-J,

0

^«

"■^s^S

"ig. 20, half-grown guinea-pig, race C. Figs. 23, 24, male and female Cavia cutleri, adult
Fig

Fip-. 34^ X Cavia riitleri. adult

22, Fi hybrid, race C x Cavia cutleri, adult. Fig. 21, Fi hybrid, race B (Plate 5

%.aK oA/eiff'ii'dliifSuj.gXi. affatoiir^

F2 hybrids, race Cx Cavia cutleri. Fig. 25, agouti; 26, black; 27, chocolate; 28, cinnamon;
29, yellow; 30, albino.

i

UNIT-CHARACTERS OF RODENTS 125

change converts an ordinary gray variety into a ** cinna-
mon" variety, and black into "chocolate," while yellow with
black eyes becomes changed to yellow with brown eyes. The
factor which in such cases has undergone change we may
call the black factor, its original or dominant phase being
expressed by B, the recessive (brown) phase by b. (See
Table 12.)

Another unit-character color variation perhaps commoner
than any of those yet mentioned is found both among wild
and among domesticated mammals. It consists in spotting
with white. It takes the form among wild rodents of a white
spot in the forehead (common among wild rabbits) or a white
spot on the belly, a white foot, or a white-tipped tail. Rarely
does it go beyond these slight and inconspicuous markings,
probably for the reason that it would render the possessor
too conspicuous for his safety, though this appears to be a
consideration of no consequence in the case of skunks, which
possibly are less disturbed because of their advertisement.
But under artificial selection in captivity it is possible rapidly
to increase the extent of the white areas in the coat,which
then takes on striking and often rather definite outlines, as in
Dutch-marked rabbits, ''English" rabbits (Fig. 123), hooded
rats (Fig. 56), and black-eyed white mice, the latter being all
white except the eyes. The production of white-spotted races
from small beginnings observed in wild stocks has been ac-
complished in the laboratory by Castle and Phillips in the
case of Peromyscus and by Little in the case of the house-
mouse (unpublished data). Physiologically this variation is
quite distinct from the albino variation. It appears to be
due to a locally inhibited action of the color factor, which in

*

extent that the prevaiHng color of the coat is black. The same is true in some speci-
mens of the black rat (Mus rattus), this black character being dominant in crosses
over the true agouti character found in the gray variety of the same species which
is known as the "roof-rat" {Mus Alexandrinus of some systematists). A similar
dominant black has been discovered among domestic rabbits by Punnett, who has
shown that it owes its origin to a change, not in the agouti factor, but in the exten-
sion factor, E, which has become of such unusual strength or potency that the
agouti factor is unable in its presence to produce the usual conspicuous effect.

126 GENETICS AND EUGENICS

other parts of the body retains its full force; whereas in an
albino the action of the color factor is everywhere wanting or
greatly weakened.

The variation, ** white spotting," may be regarded as a
unit-character change from a condition of uniform action of
the color factor to a condition of locally suppressed action of
the color factor. The former may be designated S, the
latter s. Its inheritance is as sharply Mendelian as that of
any other color variation but, the precise extent to which
color development is suppressed being obviously quantita-
tively variable (Fig. 56), it is easier by selection to modify
the modal state of a white-spotted race than of races of most
other color varieties.

That this factor is genetically entirely distinct from albi-
nism is shown by the fact that w^hite-spotting is transmitted
quite as readily through albinos as through colored indi-
viduals.

In some rodents not only the color factor, but also the
extension factor is subject to locally inhibited action. Local
inhibition of the extension factor produces yellow spots in an
otherwise black, brown, or agouti coat. This color variation,
which follows Mendel's law in crosses, may be called yellow
spotting. It behaves as a third allelomorph (eO alternative
both to full extension (E) and to full restriction (e). When
yellow spotting coexists with white spotting, a tri-color con-
dition of the coat results, spots of yellow, white, and black
(or brown) being found on the same individual. Familiar
examples are found among guinea-pigs, cats and dogs.

Another unit-character variation of certain rodents greatly
reduces the production of black and brown pigments without
affecting at all the production of yellow pigment. As the
pigmentation of the eye consists almost entirely of black or
brown, it follows that in this variation the eyes become pink,
while the coat pigments other than yellow are greatly reduced
in amount. Pink-eyed blacks or browns are very pale coated,
but pink-eyed yellows are indistinguishable from other yel-
lows except by the eye-color. The changed eye-color is ac-

Fig. 57. A trihybrid Mendelian cross between a black hooded rat (top left) and an all-yellow sport
(top right) recently captured among wild Norway rats in England. Fi, one of the first-generation pro-
geny, gray by reversion, like wild rats. F2, the eight classes of second-generation young, from left to
right, black hooded, black self, gray hooded, gray self, yellow self, yellow hooded, cream (non-agouti
yellow) self, cream hooded. Numerically these classes should beas3:9:9:27:9:3 :3:1. Let
the reader determine which of the eight classes may be expected to breed true and to what extent the
other varieties will not breed true without " fixation " (elimination of heterozygotes) .

UNIT-CHARACTERS OF RODENTS 127

cordingly the most constant feature produced by this varia-
tion. The dominant phase of this unit-character, which is
regularly found in all wild races, may be designated dark-eye,
D; its recessive allelomorph, pink-eye, d. The recessive varia-
tion, pink-eye, occurs in guinea-pigs, rats, and mice. It has
not been reported as yet for any other mammal. (See Fig.
55.)

Another unit-character variation, which affects the pig-
mentation of rodents, occurs also in other mammals. This
consists in a reduced quantity of pigment and in such a
clumping of the pigment granules within the air spaces of the
hair as to produce a dilution of the pigmentation as a whole.
Black under these circumstances becomes a slaty blue, choco-
late becomes a dull muddy brown, and yellow acquires a pale
washed-out appearance. The best-known examples are found
in blue (Maltese) cats, blue rabbits and blue mice.^ This
condition may be regarded as a recessive variation of a factor
for intense pigmentation normally found in w^ild rodents. We
may designate this intensity factor by I, its recessive allelo-
morph by i (dilution).

In guinea-pigs and rabbits there has occurred a unit-char-
acter variation which affects, not the color, but the length
and texture of the hair, which in the so-called *' angora"
variety is long and silky. This results from a failure of the
hair follicle to end its activity when the hair has attained its
normal length. In the angora variety the hair keeps on grow-
ing for an indefinitely long period. The long or angora coat
of guinea-pigs and* rabbits is a recessive character in relation
to normal (short) coat. We may regard a normal and domi-
nant character for short coat, L, as having undergone varia-
tion to long coat, 1. (See Figs. 36, 37, and 41.)

Among guinea-pigs alone of rodents has occurred another
morphological unit-character variation of the coat, which,
instead of being smooth and sloping uniformly from the nose

^ This variation probably does not occur in guinea-pigs; what was at one time
described as a variation of this sort having proved to be an alternative form of the
color factor.

128 GENETICS AND EUGENICS

backward as in wild mammals, may become rough or rosetted
with the hair radiating out from centers located in various
parts of the body. (See Fig. 33.) Rough coat is dominant
over smooth coat, for w^hich reason we may consider a unit-
character, rough coat, R, to be responsible for it, the recessive
phase of which, r, is found in smooth-coated guinea-pigs.

It should be noted that both rough coat and short coat,
like the uniformity factors affecting pigmentation, obviously
vary quantitatively. For some rough guinea-pigs are rougher
than others and some long-haired guinea-pigs have longer,
silkier hair than others. Selection has undoubtedly been
concerned in producing the present high standard long-haired
and rough-coated guinea-pigs respectively. Dr. Sewall
Wright has show^n (Castle and Wright, 1916) that an inde-
pendent Mendelizing factor found in many wild cavies inter-
feres with or partially inhibits the development of the rough
coat in hybrid guinea-pigs. We may designate this factor
rough modifier (M), its recessive phase which permits full
development of the rough coat may be expressed by m. Aside
from this striking modifier of rough, it is probable that nu-
merous other factors act as slight modifiers of rough and that
the apparently continuous variation in the development of
the roughness may thus be accounted for. Continuous varia-
tion in the expression of the angora character, as regards
length of hair, may be accounted for on similar grounds.

Leaving out of consideration such quantitative variations,
it is possible to obtain by crosses a large number of different
unit-character combinations of the ten independent varia-
tions which have been mentioned as occurring in guinea-pigs.
Theoretically one thousand and twenty -four are possible, or
if we count separately homozygous and heterozygous com-
binations, fifty-nine thousand and forty-nine are possible.
Needless to say there have been produced thus far only a small
part of the varieties of guinea-pigs theoretically possible as
unit-character combinations of the ten factorial variations
known to have occurred in this species. And the variation
of the guinea-pig is not different in kind or degree from

UNIT-CHARACTERS OF RODENTS 129

that of other rodents. Its variation has probably merely
been followed up more closely by selective breeding. Among
domesticated rabbits, at least seven of the ten enumerated
variations have occurred; all except the pink-eye and the
rough-coat variations are reported for rabbits, and most of
them are well known. The house mouse has undergone at
least six of the ten variations listed in Table 12. Its yellow
varieties have apparently not arisen in the same way as yel-
low varieties of guinea-pigs and rabbits, but by a peculiar
change in the agouti factor, for yellow in mice is a third
allelomorph of agouti and non-agouti. Mice also lack long-
haired and rough-coated varieties, but in other respects the
variations of mice are parallel with those of guinea-pigs. In
the Norway rat four of the ten unit-character variations of
guinea-pigs find exact equivalents, viz., in albinism, non-
agouti, pink-eye and white spotting. A third allelomorph of
the color factor (ruby-eye) has been shown by WTiiting and
King to occur among wild rats.

A red-eyed yellow variety of rats is due to a unit-character
variation distinct from the yellow variations known in
guinea-pigs and in mice respectively. In one and the same
linkage system in the Norway rat are found (1) the color
factor and its allelomorph, ruby-eye, (2) the factor for pink-
eyed yellow and (3) the factor for red-eyed yellow.

CHAPTER XIII

UNIT-CHARACTERS IN CATTLE AND HORSES

Unit-character changes have produced new varieties
among our more important domesticated mammals as well
as among our pet rodents.

Cattle. Among cattle four or five Mendelizing color varia-
tions occur similar to those of rodents and in addition two
variations of a morphological character have been reported,
one of which has considerable economic importance. Wild
cattle existed within historic times in central Europe, the
hunting of the last-existing herds being held as a royal pre-
rogative by the kings of Poland. These cattle represented
probably the chief source from which domesticated cattle
were derived. They were of large size but of what color we
do not certainly know. It seems probable, however, that
their coat, like that of most wild ruminants, contained a
mixture of yellow and black pigments somewhat like the
coat of Jersey cattle at the present time. In most existing
domestic breeds either the black or the yellow pigments have
become predominant or white has taken their place in whole
or in part. Such is the general tendency of man's agency in
modifying the color characters of his domesticated animals.
Nature's colors are usually adapted to concealment or pro-
tection. Mixtures of pigments are common and minute color
patterns abound. Man seeks to make his domestic animals
as different as possible from the wild. He either gives pre-
ference to pure colors, black, white, or yellow, or seeks to
outdo nature in the production of color patterns in great
blotches of two or three colors. The materials for his opera-
tions consist of sports to solid black, yellow, or white,
together with white spotting and yellow spotting. All of
these have occurred among cattle and have been used to the
fullest extent. ,

130

Fig. 58. Wild white cattle from Chartley Park, England. (After Wallace.)

Fig. 59. Wild white cattle from Chartley Park. Note black individual produced by white parents.

(After Wallace.)

Fig. 60. Kerry cow, a black breed, originated in

Ireland. (Figs. 60 and 61 from photographs by

Professor C. S. Plumb.)

Fig. 61. Dexter-Kerry cow. Its short-legged

compact form is a dominant Mendelian character

according to Professor James Wilson.

Fig. 6^2. White short-horn heifer.

Fig. 63. Red short-horn heifer with a small amount of white spotting underneath.

Fig. 64. Roan short-horn cow. Beef tjT)e. The fine mosaic of red and white spots indicates that this
animal is a heterozygote between red and white (Figs. 62 and 63).

Fig. 65. Ayrshire bull. Extensive white spotting in this breed leaves only an occasional small s{X)t
pigmented. The breed is hardy, " dual purpose " but inclining more to the dairy type, yet less spe-
cialized and better adapted to a severe climate than the Jersey and Guernsey breeds. It originated in
Scotland,

il I'

I

r

UNIT-CHARACTERS OF CATTLE 131

In English parks there have existed, since Roman days and
perhaps longer, herds of all-white cattle kept in a half wild
state. Some have supposed that these white cattle represent
the unchanged original stock of European wild cattle, but it
seems much more probable that they represent a striking
sport from the original stock, which was isolated and allowed
to increase in the hunting preserves of princes, a semi-sacred
character perhaps attaching to it. These cattle differ from
albinos among rodents in that they have pigmented eyes.

TABLE 13

Some Unit-Chara.cters of Cattle

Dominant Recessive

Black. Yellow. ^ /

Z^l Polled. Horned. , f

Dexter form (short legs). Kerry form (legs normal).

Dominance Uncertain or variable

White. Colored.

Uniformly colored. Spotted with white.

Uniformly black. Black spotted with yellow.

They also have some sooty black or brownish pigment in the
skin and hair of the extremities (feet, nose, ears, and tail).
Ordinarily they breed true, but occasionally an all black calf
is produced, but whether as a recessive in the Mendelian
sense or as a reversion, through recombination of comple-
mentary color factors, is unknown. (See Figs. 58 and 59.)
In any case it seems highly probable that the white race
resulted from an ancient sport derived directly from wild
cattle. In the breed of " short-horn " cattle, which origi-
nated in England, white individuals frequently occur and
they breed true when mated with each other. In matings
with red individuals, a sort which also breeds true, roan
heterozygotes are produced (as noted on page 110). The
white of this breed was probably derived from the same
original source as the white cattle of the English parks, but
the black character which seems to inhere in the cattle of the
parks has been eliminated from the short-horn breed, which
produces only reds, whites, and their heterozygotes, with or
without admixture of white spotting. (See Figs. 62-64.)

132 GENETICS AND EUGENICS

Red cattle have an intensified yellow pigmentation. They
probably represent derivatives of an original all-yellow sport,
comparable with the yellow sports of rodents, Which originate
through restriction of black pigment to the eye. Among
cattle yellows vary in shade from a very deep red (Devons
and short-horns) to a light cream color (some South German
and Swiss breeds). The extremes in both directions w^ere
doubtless secured through repeated selection. WTiether the
different shades or intensities of yellow are alternative is un-
known, but it seems probable that in cattle as in rodents
intensity of pigmentation is independent of its specific char-
acter as black or yellow.

Black breeds of cattle are represented by the Galloway and
Aberdeen Angvis of Scotland. In them we have either deriva-
tives of an all black sport, or the end result of a gradual in-
crease of black in the coat through selection. Pure-bred
Aberdeen Angus cattle sometimes produce red calves, red
being obviously a Mendelian allelomorph recessive to black
in cattle as it is in rodents. As red is not favored in the
standard of the breed, it w^ill doubtless be entirely eliminated
in time, as seems already to be the case in the best families
of the Galloway breed. (See Fig. 73.)

In most breeds of cattle white spotting occurs and this is
a Mendelian alternative to uniform coloration, though nei-
ther condition is entirely dominant over the other. The
self-coloration of breeds which are all black or all white has
a strong tendency to prevail in the offspring. Black breeds
in which white spotting occurs are represented by the Hol-
stein-Friesian cattle originally bred in Holland and Denmark,
but now extensively kept in this country, also by the belted
cattle of Holland. (See Figs. 66 and 69.) Red-and- white
and yellow-and-white cattle are represented by Hereford and
Guernsey cattle respectively. (Figs. 68 and 67.) Black-
and-white breeds may produce red-and-white offspring as
recessives, but red-and-white breeds never produce black-
and-white calves, which shows clearly that black is dominant
over red. In the Hereford breed a definite pattern of white

Fig. 66. Holstein-Friesian cow and her triplet calves. Note the black-and-white mottling similar in
all four animals, yet with individual differences. This breed of large vigorous cattle originated on the
borders of the North Sea in Europe. It excels all other breeds in milk production. (Photograph by the
owner, N. P. Sorensen, Bellingham, Wash.)

Fig. 67. Guernsey cow, " golden yellow-and-white " in color, graceful in form, gentle in dLsposition.
producing a good quantity of milk extremely rich in butter-fat. The breed came originally from the
island of Guernsey. (Photograph from Lang water farms, N. Easton, Mass., F. L. Ames, proprietor).

i

Fig. 68. Hereford heifer. One of tlie leading beef breeds, dark red and white in color. The white
face, back stripe and underline constitute a pattern which has a tendency to dominate in crosses. (See
Fig. 80a.) Like the short-horn, its principal rival as a beef breed, this breed arose in England.

Fig. 69. Dutch belted or " Lakenfeld " bull. Bred for three centuries for this chanut eristic pattern
by aristocratic families of Holland. Probably derived from the same original general stock as the cattle
of Holstein farther east, but selected more closely for color pattern to which productiveness has been
sacrificed.

4
^

m\

Fig. 70. Polled Durham (or short-horn) cow. Produced by a hornless sport within the short-horn
breed or possibly by some unrecorded cross, as with the polled red breed.

Fig. 71. Polled Hereford heifer. A breed of English origin, ik-liornod in Aiu.rira i»y tli. .ii»i>lii-.itiuii
of genetic principles. Hornlessness is a dominant sport or " mutation." Compare Fig. 68. A com-
parison of the white spotting in Figs. 70 and 71 suggests strongly that one is only a more advanced
stage (quantitatively) of the other.

UNIT-CHARACTERS OF CATTLE 133

spotting has been so fixed by selection that it shows itself (as
a white forehead) in crosses with self-colored breeds and even
in hybrids with the American bison.

Yellow spotting on a black background is not very com-
mon among cattle, no standard breed with this characteristic
being known, but a brindling of yellow and black spots is oc-
casionally seen in mongrel animals and no doubt good black-
and-yellow spotted animals could be produced, if it were
considered sufficiently desirable, or even tri-colors with black-
yellow-and-white coats. It is possible that brindling (yellow
spotted with black) is a third allelomorph of black and of
yellow, as in guinea-pigs.

A morphological variation of cattle of some economic im-
portance is hornlessness. This has occurred among cattle of
Scotland and England for several centuries at least and is
known also to have occurred among cattle kept on the conti-
nent and still earlier to have occurred among cattle of the
ancient Egyptians. Loss of horns is a completely discon-
tinuous variation, dominant in crosses. Heterozygotes may
develop mere traces of horns, known as scurs, but never a
fully formed horn with bony core. Hornlessness has become
an established racial character (homozygous) in the Scotch
breeds of black cattle, Aberdeen Angus (Fig. 73) and Gallo-
way, also in an English breed of red cattle called Red
Polled. Within the last thirty years polled sports have
appeared in pure-bred Holstein cattle in the United States
and a breed of polled Holsteins is now being established in
this country. A breed of polled Herefords was produced in
the United States from a three-quarters Hereford, one-
quarter short-horn polled calf born in 1889. (Wallace, p.
122.) See Figs 68 and 71. Polled cattle are easier to manage
and less liable to injure each other than are horned cattle.
There can be no doubt that hornlessness had its origin as a
unit-character variation dominant in crosses.

Another morphological character, said to be a Mendelian
dominant, occurs in Dexter-Kerry cattle. They have ab-
normally short, stumpy legs. (See Figs. 60 and 61.)

134 GENETICS AND EUGENICS

Horses. The original color of wild horses is probably seen
in a wild horse still existing on the plains of central Asia
(Mongolia) and known as Prevalski's ^ horse. (See Fig. 81.)
It has somewhat the appearance of an ordinary bay horse,
except that the yellow pigment is paler and the black pig-
ment more diffuse dorsally. The mane, tail and legs are black,
the back reddish or yellowish brown shading off into pale
sooty yellow below. In tame horses of the bay color variety
as compared with this, the yellow pigmentation is of a

TABLE 14

I

Some Unit-Characters of Horses

Dominant Recessive

1. Bay. Not bay (i. e., black or chestnut).

2. Black. Chestnut.

3. Gray. Not gray (any color but gray).

4. Trotting. Pacing.

Dominance Uncertain or Wanting
o. Uniformly colored. Spotted with white.

brighter and more intense sort, called red, and more free
from black dorsally, while the black markings of mane, tail,
and feet are probably more distinct, changes that seem to
have come in with careful selective breeding. For in mongrel
horses of no particular breeding the fine points of the bay are
often wanting, the yellow being of a dull shade and mixed
dorsally with black and approaching a " dun " in general
appearance. Unit-character variations are less in evidence
in domestic horses than in cattle. The bay appears to be an
improved ty^e of wild-horse coloration not produced by
abrupt changes in any particular characters but by gradual
changes in several characters. Black is a color variety reces-
sive to bay in crosses. It seems to have arisen in the same
way that black varieties of rodents usually arise, by loss of a
pattern factor. In rodents it is the agouti factor which
having disappeared produces a black (non-agouti) variety.

^ The common spelling of this name is Prejvalski, but as this makes in English
an unpronounceable combination, I take the liberty of dropping the j in the interest
of my readers, without intentional disrespect to Mr. Prejvalski or his horse.

Fig. 72. Jersey cow. One of the best strictly dairy breeds. Color light yellow (" fawn ") shaded with
diffuse black pigment, possibly a primitive type of coloration in cattle. Similar to the Guernsey in
character and source. Home the island of Jersey. A little delicate in constitution and nervous in
temperament.

Fig. 73. Polled Aberdeen Angus bull. A Scotch breed, self black in color, of beef type aiul hanly.

I

Fig. 74. Fi cow, black, polled.

Fig. 76. Selected F2 bull.

Fig. 78. Rejected F2 buU.

Fig. 75. Choice F2 heifer.

Fig. 77. Selected F2 cow.

ijected Fi heifer.

Results of crossing Jersey cows (Fig. 72) with an Angus bull (Fig. 73) in an effort to combine in one race
the dairy excellence of the former with the size, hardiness and good feeding qualities of the latter.
Figs. 74, 78 and 79 show the dominant black of the Angus, Figs. 75-77 show the recessive fawn of the
Jersey somewhat darkened. All show dominant hornlessness. (After Kuhlman.)

i

Fig. 80. A zebu bull, typical example of one of the humped cattle of India. (Photograph from Pro-
fessor Nabours, Kansas Agr. College.)

riG. 80a. Fi calf from cross of zebu bull with Hereford cow. Notice imperfect dominance of Hereford
pattern (Fig. 68). Indian cattle being more tolerant of heat and more resistant to Texas fever, the cross
IS made to combine these qualities with the beef excellence of the Hereford. (Photograph from Na-
bours.)

fl

UNIT-CHARACTERS OF HORSES 135

In horses it is a hay factor which the black variety has lost.
This factor appears to inhibit the development of black in
regions where the bay variety shows red, just as an agouti
factor inhibits the development of black pigment in certain
regions of the coat of rodents which then are yellow. Wlien
the bay factor is lacking, black pigment develops throughout
the entire coat. Whether this loss occurred originally as a
single sudden change (a sport) or whether it occurred gradually
is uncertain, but it seems clear that at present in crosses black
is a unit-character recessive to bay, and this makes it seem
probable that it arose as a discontinuous variation originally.

A unit-character difference has also been shown to exist
between black and chestnut horses, a difference comparable
to that which exists between black and brown varieties of
rodents. Chestnut is recessive to black, corresponding with
the " chocolate " varieties of rodents. " Suffolk " or " Suf-
folk Punch " horses are invariably chestnut in color. But
the term " chestnut " as here used probably includes both
brown animals which, like black, lack the bay factor and those
which possess this factor. For the latter it would probably
be better to use a term in common use, sorrel. We should
then have parallel black and brown series with and without
the bay factor. Black pigmented horses with the bay factor
are " bays," without it they are " blacks." BrowTi pig-
mented horses with the bay factor should be called " sor-
rel "; those without it, chestnut. Records compiled by
Wentworth and others indicate that such a factorial differ-
ence does exist among horses called " chestnut " in the
records. For blacks mated inter se produce some chestnut
colts (which should be possible if the black parents are
heterozygous for chestnut) with a doubtful record of a few
bays, but black mated with " chestnut " produces more bays
than anything else, which shows clearly that some at least
of the chestnut parents do transmit the bay factor.

The gray (or white) color variation of horses corresponds
roughly with the white variation in cattle. It is a dominant
unit-character in crosses, but shows itself only in the second

136 GENETICS AND EUGENICS

and later coats. For the colts are born with colored coats,
but at the iirst shedding of the hair, white hairs begin to
come in mingled with the colored ones. (See Fig. 84.) Later
white hair may almost completely replace the colored ones.
The eyes of gray horses are always colored. The term gray
as applied to horses has the same significance as when applied
to human beings. It means the occurrence of white hairs
among colored ones, more or less completely replacing them.
WTien among horses the original coat partially replaced by
white was a black one, an ordinary or '' iron " gray coat
results; but when the original coat was bay or sorrel, then a
roan coat is produced.

White spotting is of frequent occurrence among horses,
though it is usually less extensive than among cattle. In this
variation the loss of pigment from the body area affected is
complete and is present from birth on, so that its nature is
evidently very different from the gray variation already
described. (Figs. 81-85.) It corresponds physiologically
with white spotting in cattle and in rodents. The com-
monest form of white spotting is the occurrence of a white
spot in the forehead sometimes extending down over the nose,
or the possession of one or more white feet, or both. These
are regular features of the coloration of Clydesdale and Shire
horses. More extensive spotting takes the form of irregular
white areas extending across the neck or body. (Fig. 81a.)
It is less common than the former and unlike it behaves as a
dominant character in crosses. Often seen in children's
ponies, it is probably genetically distinct from the spotting
of horses with white stockings and blaze. The pacing gait in
American race horses is a character recessive to the trotting
gait, according to Bateson. In pacing the two legs of the
same side of the body move in unison or nearly so, while in
trotting the foreleg of one side moves almost simultaneously
with the hind leg of the other side. Some trotters may be
made to acquire the pacing gait and these, of course, may
produce trotters, but natural pacers produce only natural
pacing colts when bred with each other, whereas in crosses
trotting dominates.

Fig. 81. Prevalski horse in the New York Zoological Garden. (Photograph by courtesy of Director
W. T. Hornaday.) Notice large head, erect mane, absence of forelock and taillock, faint zebra-like
striping on front leg, and general pattern of " bay," with light muzzle and darker mane, tail, and legs.

^y"

Fig. 81a. Pony of uncertain pedigree on farm of Simpson Bros., Palmer, III. (Photograph by courtesy
of Professor J. A. Detlefsen.) Notice general form like that of Prevalski horse, but with white spotting
extending up over front legs and entirely around body. Spotting of hind feet also extends up over l>ody
on right side.

Fig. 82. A saddle horse (" hunter ") showing typical white markings, " white stockings " and
" blaze " (face stripe). These are manifestations of white spotting fully developed at birth and not
changed subsequently.

Fig. 83. Clydesdale, typical example of one of the breeds of heavy draft horses.

blaze of white are regularly present in this breed.

White stockings and

[

Fig. 84. Gray Percheron mare anci c-oit. ^^uc•h coits, blacK at Dinh, oecome gray later iii iite. Notice,
however, that the colt's face is already white. This is due to white spotting, as in the hunter and
Clydesdale, not to the gray factor. The two forms of white are genetically quite distinct.

Fig. 85. White mare and colt. (Photograph by courtesy of W. P. Newell, Washburn,
111.) An extreme condition of white spotting is here shown, in which the entire coat is
white from birth on.

I

CHAPTER XIV

UNIT-CHARACTERS IN SWINE, SHEEP, DOGS, AND CATS

Swine. In the wild boar of Europe, from whicli in part
domestic swine are descended, the coat is slaty black, the
individual bristles bearing a band of pale yellow like the
agouti marking of rodents. The young of the wild boar are
also marked with longitudinal body stripes, a character per-
haps correlated with the agouti-like banding of the bristles.
This banded character of both young and adult has appar-
ently been lost in all domestic breeds, which are either self
black, red, or white, or else black or red spotted with white,
yet it occasionally reappears in crosses, showing a probable
dependence upon complementary factors still found sepa-
rately in certain breeds (Severson) . In the white variety the
entire coat is colorless but the eye is colored. This is a domi-
nant variation. White spotting is possibly a distinct varia-
tion from the foregoing, and uncertain as to dominance. But
it may be that the two differ only in degree and are really
allelomorphs.

TABLE 15

Unit-Characters of Swine
Dominant Recessive

1. Wild color. Not wild color (black or red).

2. Black. Red.

3. Self white. Colored.

4. Mule-footed (syndactyl). Normal foot.

Dominance Uncertain or Wanting

5. Uniformly colored. Spotted with white.

Two forms of white spotting (which occur naturally and are
comparable with the two types of white spotting amouii
horses) are sought after by breeders and have become breed
characters, viz., (1) a condition in which a broad wliite belt
encircles the body (as in Hampshire hogs) and (2) a condition
in which white appears at the extremities, on the feet and

137

138 GENETICS AND EUGENICS

snout (as in Berkshires). It is probable that they are similar
in genetic character. Black among swine is dominant over
red, as in cattle, horses and rodents. But the dominance of
black is commonly imperfect or complicated by the presence
of a spotting factor in the red breeds known as Tamworth
and Duroc- Jersey. (See Figs. 86-93.)

A curious morphological variation, syndactylism, is a domi-
nant unit-character. In this variation the normal two hoofs
of each foot have completely fused together and the foot has
a single hoof like a ''mule." Hence the variety is called
'* mule-footed." A breed having this characteristic has been
established in the United States. Although the hoofs are
fused the bones proximal to the toe retain their original
paired character. (See Figs. 94 and 95.)

Sheep. In sheep ordinary white fleece is dominant over
black fleece, the latter occasionally cropping out in flocks as
a recessive, as indicated in the old saying *' every flock has
its black sheep." Black sheep breed true inter se. Black is
probably not a reversionary variation but a loss variation of
a pattern factor found in wild sheep and similar to the bay
pattern of horses. Wild sheep are white or whitish except at
the extremities where the pigmentation is heavier. In some
breeds of sheep the skin and wool of the extremities is dark,
similar to the coat of Himalayan rabbits, and white spotting
may affect these pigmented regions just as it does the coat
of Himalayan rabbits. (See Figs. 96-100.) Hornlessness is
a variation from the original horned condition of wild sheep
which is dominant in females but recessive in males, a matter
deserving further consideration in connection with the sub-
ject of heredity as affected by sex. (See Figs. 96-104.)

Dogs. By Darwin and most other students of the origin
of dogs, the conclusion has been reached that dogs are de-
scended from several different wild species of wolves inde-
pendently domesticated in different parts of the world.
These, it was thought, having been subsequently inter-
crossed have produced a highly variable stock from which
selection has isolated the genetically diverse modern breeds.

Fig. 86. Berkshire boar. Black with white points.

Fig. 87. Yorkshire boar. A self white breed.

Fig. 88. Fi sow from cross, Berkshire X Yorkshire, and F2 pigs. Note reappearance of recessive
blacks but with white spotting increased in amount. (After W. W. Smith.)

Fig. 89. Fi sow from cross, Berkshire X Yorkshire, and pi^^s protiuceil Ijy a l>ack-eross with
hour. Note 1 : 1 ratio and modified ^pol^tiiiL'. lAftcr W. W. ^tuilh.)

Berkshire

Fig. 90. Hampshire sow, typical example of a belted black-and-white breed.

Fig. 91. Belted red sow. This breed produced by Q. I. Simpson by crossing black belted (Hampshire)

with self red (Tamworth and Duroc) swine.

I

Fig. 92. A litter of pigs by two belted red parents. Evidently this form of white spotting is not fully
recessive, since part of the pigs are not belted. (By courtesy of Simpson and Detlefsen.)

Fig. 93. A belted red sow and her litter by a belted red boar. Note variation in belt or its total ab-
sence. (By courtesy of Simpson and Detlefsen.)

Figs. 94 and 95. Foot bones of mule-footed (syndactyl) swine. Only the hoof and nearest pair of bones

show complete fusion. (After Spillmau.)

UNIT-CHARACTERS OF DOGS 139

A different opinion as to the ancestry of dogs has recently
been expressed by G. S. Miller and particularly by G. M.
Allen, who has made a careful study of the cranial characters
of dogs kept by the aboriginees of the American continent.
Allen finds strong evidence that the native dogs of America
are not descended from American wolves but came with man
in his migration from north-eastern Asia to north-western
America. Previous to that migration there existed in Europe
and Asia both a large and a small type of dog, and both types
were introduced into America when it was peopled from
Asia. A third type, the Eskimo dog with heavy coat and
tail curled forward over the hip, occurs in the northernmost
parts of both Asia and America and doubtless came with the
Eskimos in their comparatively recent migration from Asia.
What part, if any, species hybridization has played in the
genesis of dogs can not at present be stated, but a survey of
existing breeds of dogs shows the occurrence among them of
several unit-characters and accordingly unit-character varia-
tion (mutation) may be regarded as having been an import-
ant element in their production. A case which well illus-
trates the point is the color variation of Great Danes as
worked out by Little and Jones. (See Fig. 104a.) Starting
with the self black variety (3, Fig. 104a), we have as its reces-
sive allelomorphs either brindle (4) or fawn (5) . A recessive
dilution factor, if present in a homozygous condition, gives
us dilute black (6), dilute brindle (7) and dilute fawn (8).
A dominant factor for white spotting produces the harlequin
variety (2). A recessive factor for white spotting produces
white feet or breast spot (1). These t^^^es of white spotting
remind us respectively of the English and Dutch patterns of
white spotting among rabbits. Presumably either pattern
might occur in association with dilute black, brindled, dilute
brindled, fawn, or dilute fawn coat (4-8, Fig. 104a). AVliat
are probably more developed forms of the recessive type of
white spotting are represented in Figs. 100-109. In the
breeds there shown white spotting has been selected lor,
whereas in the Great Dane it is rigidly selected against. A

140 GENETICS AND EUGENICS

more specialized form of the dominant spotting (harlequin)
is found in the coach-dog (Fig. 110). Besides the five unit-
character variations of Great Danes, several other unit-
character variations can be recognized in other breeds. (See
Table 16.) The cranial characters of dogs show their an-
cestors to have been wolf -like.

Most wolves have a protectively colored gray coat, in
which black and yellow pigments are intermingled on the
same hair somewhat as in the agouti pattern of rodents.
This pattern is wanting in most dogs, but has been retained
in some examples of the Eskimo-dog or "husky." It is
probably due to a dominant factor.

A more conspicuous pattern is seen in black-and-tan dogs.
In a black-and-tan the general body-color is yellow (tan) but
with a blanket of black extending down from the back over
the sides of the body and the outer surfaces of the legs. A
yellow spot is found also above each eye. Fox hounds and
beagles have this pattern regularly. Airedale terriers are
distinguished chiefly by this pattern from Irish terriers.
Some setters and pointers have it while others do not. Al-
though the white spotting in these breeds often obscures it,
the black-and-tan pattern can readily be recognized in the
light spot above the eye. It is apparently a recessive pattern
factor in various breeds of dogs. Since the pattern seen in
black-and-tan dogs may be transferred in crosses as a unit-
character to dogs which are brown or red pigmented, it is
probably better to adopt for it a term appropriate in different
combinations. Bi-color has been suggested by Barrows and
Phillips as such a term. Bi-color black dogs are "black-and-
tan," bi-color brown dogs are "liver-and-tan," and bi-color
red dogs are "red-and-lemon." Self black breeds of dogs
have probably originated by a loss of an original pattern
factor such as the bi-color factor; and self yellow ( or red)
breeds by independent loss (sudden or gradual) of black from
the coat. Brown ("liver") varieties have originated by a
unit-character variation from black to brown, comparable
with that of various rodents. Self white occurs in dogs

Fig. 96. " Black faced " Highland ram. (After Plumb.)

Fig. 97. Black faced Highland ram and ewes. Note white spotting of pigmented face and legs,
also sexual difference in size of horns. (After Plumb.)

Fig. 98. Malitch sheep. An Asiatic flock containing self-black, spotted black-and-white and grayish
white sheep, the last probably the primitive condition, (.\fter C. C. Young.)

Fig. 99. Cheviot ram. This Scotch breed has long and coarse
wool with face and legs bare and white. Both sexes are hornless.

Fig. 100. Hampshire Down ewe. Extremities pigmented.
Hornless in both sexes.

Fig. 101. Delaine merino ram. This breed produces abundant, fine wool. Males have
well-developed horns, females are hornless. (Figs. 99-101 after Plumb.)

UNIT CHARACTERS OF DOGS 141

either as a sport from the colored condition, or more probably
as an extreme form of white spotting. In this variety the eye
pigmentation is never entirely lost as in albino rodents; it is
largely retained, as is the case also in white cattle, horses and
swine. In crosses between the different colored breeds,
black-and-tan {i. e., bi-color black) is dominated by self black
and bi-color brown by self brown; black is dominant over
yellow (or red) and also over brown. As yellow and brown
are independent unit-character variations they may be com-
bined, a result seen in brown-eyed yellow dogs. Thus among
pointers (Little, 1914) or cocker spaniels (Barrows and
Phillips, 1915) a cross of black-eyed yellow with brown pro-
duces in Fi black dogs and in F2 blacks, browns, black-eyed
yellows and brown-eyed yellowsX The same result in both
Fi and F2 may be obtained by crossing black with brown-
eyed yellow. What appears to be self white, but is more
probably a very pale yellow, according to Barrows and
Phillips, has appeared in spaniels as a sport and is recessive
in heredity. Whether in other breeds self white is recessive
or dominant is not known at present. It is probable that in
some cases, as in bull terriers, it is only an extreme form of
white spotting, in which case we should expect the dominance
to be imperfect. A short stumpy tail is probably a dominant
unit-character variation in dogs, as it is in cats.

/

TABLE 16

Unit-Characters of Dogs

1.

Gray.

Black.

2.

Self-color.

Bi-color (black-and-tan,
brown-and-tan, red-
and-tan).

3.

Black.

Brindle, yellow, or red ^

4.

Black.

Brown (liver).

5.

Harlequin type of white

Self color.

spotting.

6.

Color intense.

Color dilute-

7.

Colored all over

Spotted with white (Dutch type)

8.

Stumpy tail.

Normal tail.

^ In Dachshunds red is not uniformly recessive; it apparently may be dominant.

142 GENETICS AND EUGENICS

Cats, Domestic cats are descended from a wild species
{Felis maniculata) still found in northern Africa. The domes-
tication was accomplished in ancient Egypt and the domestic
cat was introduced into Europe in the middle ages, since
Roman times. The wild species is similar in size and color to
the common tabby or tiger cat. This has a coat consisting
of agouti-like hairs, which contain both black and yellow
pigments, but the body is marked with stripes in which black
pigment predominates, and it is these black stripes that pro-
duce the tiger pattern, which is a dominant unit-character.
Different forms of the tiger pattern, distinguished as lined,
striped, blotched, etc., are probably multiple allelomorphs.
In the self -black variety the tiger pattern and agouti mark-
ing of the hairs have been covered up by a greatly increased
amount of black. The black variety probably originated as
a sport and it behaves as a recessive to tabby. An all yellow
variety represents another unit-character variation imper-
fectly dominant over black. Homozygous individuals are
all yellow but heterozygous females usually show both yel-
low and black (tortoise shell) though occasionally they may
be all yellow. The inheritance of yellow is sex-linked and
of the Drosophila type. (See Chapter XVIIl.) Yellow cats
usually, if not always, show the tiger pattern, which leads
to the question whether this pattern is ever lost even in the
black variety. It may be only covered up with black pig-
ment. Darwin notes the fact that black kittens often show
the tiger pattern which is not visible in them later in life.
All-white varieties of cats exist having colored eyes (either
"yellow" or blue). The relation of this variation to colored
forms, as regards dominance, is uncertain, but it probably
represents an extreme form of white spotting. Blue (or
Maltese) is a dilute form of black, recessive to the latter.
The dilution factor probably affects the appearance of tabby
and yellow also, but definite information on the point is not
available. White spotting is a character the behavior of
which as regards dominance is unknown. Yellow spotting
occurs only as a heterozygous character in the cross between

Fig. 105. Pomeranian, self-colored, and having
long silky hair. Toy variety.

Fig. 106. Boston hull terrier. Pattern in wiiili-
spotting like the Dutch marking of rahbits.

Fig. 107. Saint Bernard.

Fig. 108. Beagle. Tri-color, black-and-tan
with white.

Fig. 109. Collie. Figs. 106-109 show white spotting of the same general character.

Fig. 110. Dalmatian or coach dog. A peculiar form of white spotting, resembling that of

the English rabbit, is found in this breed.

."■■A

ii>-

^,.it^.:';^, u>

Fig. 111. Great Dane. Brindled type, with
yellow spotting on a black background.

Fig. ll^. Irish setter. Color, dark red.

Fig. 113. Dachshund. Black-and-tan.

(Figs. 105-114, by courtesy of F. G. Carnochan, from Field and Fancy.)

Fig. 114. Bull terrier. All white except nose
and eyes.

vei

In

UNIT-CHARACTERS OF CATS 143

yellow and black and then chiefly in the female sex. Lon^
(angora) hair is a recessive variation from normal coat in cats
as in rabbits and guinea-pigs. A short stumpy tail, seen in
the ''Manx" cat, represents an imperfectly dominant unit-
character variation. Homozygous dominants are tailless;
heterozygotes are short-tailed; normal (long) tail is recessive.
Polydactylism (the possession of extra toes) is an imperfectly
dominant variation.

TABLE 17

Unit-Characters of Cats

Dominant Recessive

1.

Tabby. Not tabby (black or blue).

2.

Black. Blue.

2a.

Normal (intense) pigmentation. Siamese dilution.

3.

Short hair. Long hair (angora).

Dominance Imperfect or Uncertain

4.

Colored all over. Spotted with white.

o.

White (eyes only colored). Colored all over.

6.

Yellow. Not yellow (tabby or black)

7.

Tailless (Manx). Long-tailed.

8.

Polydactyl. Toes liormal.

i-i'ivste Propsrty of

Z. p. IViETCALF:
No,.

•'■iifimfSimfdt

CHAPTER XV

UNIT-CHARACTERS IN POULTRY AND IX PLANTS

Poultry. The production of varieties by unit-character
variation is nowhere more clearly seen than among domestic
fowls. The wild ancestor is supposed to be represented at
present in the jungle fowl of India {Gallus bankiva) a small
bird of bantam size having the color character of the breed
known as brown Leghorn, and producing fully fertile off-
spring in crosses with domestic breeds.

Under long centuries of domestication size in many breeds
has been increased, though certain breeds of bantams are no
larger than the jungle fowl. Punnett and Bailey (1914) have
maintained that several unit factors are concerned in size
differences between bantam and ordinarv breeds, but there
is some doubt as to the correctness of their interpretation.
We have no information at present as to whether the bantam
represents the persistent small size of the wild ancestor or has
resulted from secondary variation in races of normal size.
The size changes from the wild jungle fowl to our large
breeds of poultry have undoubtedly been numerous and
probably gradual, involving long-continued selection.

Color variations are in fowls, as among mammals, the most
conspicuous unit-character changes. The plumage of the
jungle fowl contains both black and yellow pigments com-
bined in a pattern of some complexity. This pattern may
possibly be lost or suppressed as a unit-character variation,
but in most cases it is changes in the relative amounts of
black and yellow which give rise to self black or self yellow
(red or buff) breeds. \ATiite spotting may come in to produce
colorless patches in the plumage and if these become suffi-
ciently extensive an all-white breed results such as the white
Leghorn. The white of Leghorns is a dominant character

145

146 GENETICS AND EUGENICS

but even pure bred birds may develop an occasional colored
feather, and in crosses with brown Leghorns, which have the
ancestral color, the heterozygotes produced may show traces
of color, as for example a reddish breast. A form of white
plumage genetically distinct from the foregoing is found in
white silky fowls and in some other breeds. In this the down
plumage is colored and the adult plumage is not as clear and
pure a white as that of white Leghorns. When such recessive
whites are crossed with white Leghorns, fully colored offspring
result in F2 though not in Fi. It is probable that recessive
white is not an extreme form of white spotting, as perhaps
the white of Leghorns is, but that it is due rather to some
change which produces fainter pigmentation; to a loss varia-
tion, rather than to an inhibition. It is accordingly com-
parable with the albino or the pink-eye variation of rodents,
whereas the white of Leghorns is comparable with the black-
eyed white variation of rodents, an extreme form of white
spotting. Bateson has shown that there are two or possibly
three distinct classes of recessive white varieties, probably of
independent origin, for when two of these (one being the
white silky) were crossed, fully colored Fi offspring were ob-
tained similar in appearance to the wild Gallus bankiva.
This is a result comparable with that obtained when pink-
eyed rodents are crossed with albinos producing fully colored
young. It shows that white plumage in fowls, like pink eyes
and pale coats in rodents, may result from different genetic
changes. Pigment formation is a complex chemical process
in which several factors are concerned. Change in any one
of these may interfere with the normal pigmentation.

It seems doubtful whether the Gallus bankiva pattern is
lost in the ordinary black breeds of fowls; more probably it
is simply covered up by an excessive development of black
pigment. Indeed in some cases the pattern is faintly visible
in the black breed and can readily be brought out in crosses.
Such varieties are comparable with the blackened agouti va-
rieties of some rodents (black squirrels for example) . In self
yellow (red or buff) breeds, the pattern fails to develop

UNIT-CHARACTERS OF POULTRY 147

^ merely for lack of black pigment. Yellow varieties are im-
perfectly recessive to black in crosses, the ancestral pattern
usually resulting in Fi. Blue is a heterozygote between black
and splashed white (an impure sooty strain of white). It is
unfixable.

A color pattern of fowls, not ancestral in origin, but domi-
nant in crosses is found in breeds with barred plumage, such
as the Dominique and the barred Plymouth Rock. Its
inheritance is sex-linked. It may be transmitted through
white breeds, as for example the white Leghorn.

A black pigmented skin associated with black bones is
found in certain strains of fowls, e. g,, silkies. This is domi-
nant over normal (white or yellow) skin.

Several morphological variations of the plumage are in-
herited as unit-characters. Thus, the possession of a topknot
or crest (usually associated with cranial hernia) is an im-
perfectly dominant character; frizzled (twisted) feathers are
dominant over normal feathers; silky feathers (devoid of
barbules) are recessive to normal feathers (with barbules).
An extra or fifth toe (due to a divided hind toe) is an imper-
fectly dominant character found in Houdans and Dorkings.
The comb is also a highly variable character. Single comb is
the form found in Gallus hankiva and in the connnoner breeds
of poultry. It consists of a high serrated ridge. Pea comb
is a dominant variation from this ancestral form in which the
comb is lower and broader, without distinct serrations but
with two low lateral ridges in addition to a chief central
ridge. It is found in Indian Games and the Brahma breeds.
Rose is another form of comb, likewise dominant over single.
It consists of a broad flat comb with numerous papillae not
arranged in distinct rows. A cross of rose with pea produces
a peculiar type of comb known as walnut, which is found in
the Malay breeds. When produced by crossing, it does not
breed true without fixation, but in F2 gives rise to walnut,
rose, pea, and single comb in the ratio, 9:3:3:1. Evidently
walnut in such cases is due to the joint action of two domi-
nant factors (R and P) which act separately in pea-combed

148

GENETICS AND EUGENICS

and rose-combed varieties respectively, and when both P and
R are lacking the original type of single comb is formed.

I

TABLE 18

Unit-Chaeacters of Domestic Fowls

A. Sex-linked

Dominant

Recessive

1.

Black skin (silkies).

Normal skin (dominant in femalcjs,
imperfectly recessive in males),

2.

Silver (lacing, spangling, penciling).

Gold (lacing, spangling, penciling).

3.

Striped down of chicks, black breast
of adult male (game bantams).

Plain down, brown breast of male.

4.

Barred feathers.

Unbarred feathers.

4.(

1 Spangling (Hamburgs)

Non-spangled feathers.

B. Not Sex-linked

5.

Black plumage.

Yellow (or buff or red) plumage.
(Heterozygote often like jungle
fowl.)

6.

White of white Leghorns.

Colored.

7.

Colored.

\Miite (of silkies).

8.

Colored.

\Miite (of rose-comb bantams).

9.

Colored.

"NMiite (of white rocks).

10.

Normal feathers.

Silky feathers.

11.

Frizzled feathers.

Plain feathers.

12.

Crest.

No crest.

13.

Extra toe.

No extra toe.

14.

Yellow skin.

"VMiite skin.

15.

Rmnpless.

Normal tail.

16.

Walnut comb.

Rose, pea, or single comb.

17.

Pea comb.

Single comb.

18.

Rose comb.

Single comb.

19.

Single c(|jnib.

Combless (Breda).

Plants. No attempt will be made at a detailed survey of
unit-character variations in plants but certain general cate-
gories of variations may be indicated and examples cited.
These will serve to show that the same sorts of changes are at
work among plants as among animals to produce striking
varieties.

1. Colors of flowers. Some of the clearest cases relate to the
colors of flowers. Wild species often exhibit in their flowers
a mixture of pigments associated in a definite pattern. Loss
or suppression of the pattern, or of one or more of its com-
ponent colors, leads to the formation of self-colored flowers.

UNIT-CHARACTERS OF PLANTS 149

or those which are white. Thus in the sweet pea the wild
plant has flowers of a purple bi-eolor, resulting from the asso-
ciation of red and blue pigments in a definite pattern. Red
flowers may arise by a suppression of a factor for blue. This
change alone produces a red flower with wings lighter than
the standard (a red bi-color). Another recessive factorial
change does away with the lightness of the wings, producing
a flower with both wings and standard full red. A corre-
sponding change in pattern in purple (the original color), not
attended by suppression of blue, produces purple with both
wings and standard of full color. A quantitative change in
the color factor (a partial loss of color) produces faintly
colored varieties known as picotee, either purple or red. In
the flowers of many cultivated plants striping, mottling or
spotting with white or red comes in as a unit-character varia-
tion, as in petunias, snapdragons, etc.

4,

TABLE 19

Unit-Chakacters of Plants

1. Colors of Flowers
(Example, unit-characters of the sweet pea flower.)

Dominant Recessive

(1) Colored. White.

(2) Colored. Slightly colored (picotee).

(3) Purple. Red.

(4) Bi-color. Self.

2. Forms of Flowers

(1) Normal. Peloric.

(2) Single. Double.

3. Colors of Leaves and Stem

(1) Variegated with yellow. Normal green (dominance imperfect).

(2) Containing much red. With little red (Oenothera, Coleus, maize).

4. Colors of Fruits and Seeds

(Example, maize)

(1) Yellow endosperm. White endosperm.

(2) Aleurone black. Aleurone red or uncolored.

(3) Aleurone red. Aleurone uncolored,

(4) Endosperm starchy. Endosperm sugary.

(5) Endosperm starchy. Endosperm waxy.

- (6) Seed-coat red. Seed-coat colorless.

(7) Seed-coat variegated. Seed-coat not variegated.

150 GENETICS AND EUGENICS

5. Forms of Leaves

(1) Serrate. Entire (Urtica, Fig. 115).

(2) Normal. Laciniate (Chelidonium).

(3) Palmate. Pinnatifid or fern-leaf (Primula).

(4) Hairy. Glabrous (dominance often imperfect).

2. Forms of flowers. The forms of flowers, no less than
their colors, are subject to unit-character variation. In
sweet peas the ordinary form of flower with erect standard is
dominant over a variation in which the standard lops down
at either corner forming what is called a "hood." Symmetri-
cal forms of flowers which appear as sports in species having
normally asymmetrical flowers are a unit-character variation.
Thus a peloric (symmetrical) variation in the snapdragon is
recessive to normal (asymmetrical) shape of flower (Baur).
Double flowers, those which have an increased number of
parts (commonly petals), are in general recessive to singles.
This is the case for example in primulas, poppies and lark-
spurs. But some cases occur in which the heterozygote is
intermediate, as for example in carnations. Here a good
commercial double type is found to be regularly heterozy-
gous, producing when selfed both singles and extremely
double types (**busters"), each of which sorts breeds true,
and in addition the unstable but more valuable heterozygous
type of the parent (Norton) .

3. Colors of leaves and stems. The colors of leaf and stem
often vary abruptly in cultivated plants by unit-character
changes. Thus strains variegated with yellow arise from
local loss or inhibition of chlorophyl, a change which impairs
the assimilative power of the plant but adds to its ornamental
value in horticulture. Of course plants largely or completely
yellow because of deficiency of chlorophyl would be unable
to maintain themselves other than as parasites, such as
dodder; hence the yellow of variegated plants is usually
Hmited in amount. Some varieties of cultivated plants pos-
sess as a distinguishing character an unusual amount of red
coloring matter (anthocyan) in leaf or stem. Examples of
this are seen in purple beeches and maples, variations known

UNIT-CHARACTERS OF PLANTS

151

to have originated as sports and doubtless Mendelizing in
crosses. The cultivated celosias are good examples of i)lants
in which an excessive amount of anthocyan pigment pro-
duces brilliant red or yellow plants, the latter a probably
recessive sport from the former, just as the yellow fruit of
the tomato is known to be recessive to red fruit. In Coleus

mica
DoddTttU/iilulifera

jiilulifeTA

Dodartii

.'4 4

41 HU *il4 II

ten.

Fig. 115. A Mendelian cross between two varieties of nettle differing in shape of leaf, I. Gen. = Fi.
II. Gen. = F2. III. Gen. = F3. The diagram indicates that the serrated form is dominant, the re-
cessive form reappearing in F2 and breeding true in F.3. (After Correns.)

the red has a mosaic and highly variable distribution on the
green leaves, like that of yellow spotting in mammals.

4. Colors of fruits and seeds. The colors of fruits and seeds
vary discontinuously in the same way that the colors of
flowers, leaves and stems vary. As an example we ma}^ con-
sider some variations in the color and composition of the
seed of maize. The common varieties of corn are either
yellow or white seeded, the yellow grain containing a yellow
colored endosperm, a character dominant to white. A black
pigment which is present in the aleurone layer just under the
seed-coat is responsible for a dominant variation in some

152 GENETICS AND EUGENICS

varieties. Red aleurone color is a recessive allelomorph of
black. Both are dominant over colorless aleurone. Red
seed-coat is a character dominant over colorless seed-coat,
and a seed-coat striped with red is allelomorphic to unstriped
seed-coat. A highly starchy condition of the endosperm is
found in ordinary varieties of field corn, which have rela-
tively plump seeds. A recessive allelomorphic condition is
found in sweet corn cultivated for table use, in which sugar
predominates in the seeds so that on drying it takes on a
shriveled, wrinkled appearance. A different recessive varia-
tion is found in a variety of corn recently imported from
China, in which the endosperm is waxy rather than sweet or
starchy. If the variety with waxy endosperm is crossed with
sweet corn, starchy corn is obtained by reversion in Fi, and
in F 2 all three sorts are obtained in the ratio, nine starchy
to three waxy, and four sweet.

5. Forms of leaves. Leaf form in many cultivated plants
is known to vary by Mendelizing units. In the nettle
(Urtica) Correns has shown that the much-serrated leaves of
one natural variety possess a character dominant over the
nearly entire leaves of another variety (Fig. 115). In Cheli-
donium majus, sl laciniate leaf form is known to be recessive
to the normal form of leaf. In Primula sinensis, normal pal-
mate leaves are dominant over fern -like pinnatifid leaves. In
a great number of plants hairy or spinous leaves, stems, or
fruits, are known to be dominant (more or less completely)
over smooth ones.

6. Form of stem. One of the seven discontinuous varia-
tions with which Mendel dealt in his original paper is in-
volved in the difference between tall and dwarf races of peas
and beans. The original and the dominant form of stem is
the tall form. Dwarf form, in which the intemodes of the
plant are relatively short, segregates in regular recessive
fashion. Semi-dwarf races also exist, which indicate either
imperfect segregation or alternative forms of dwarfness.
Dwarfness occurs as a variation alternative to normal tall
form in snapdragons, nasturtiums, and many other culti-
vated plants.

UNIT-CHARACTERS OF PLANTS 153

The original much-branched condition of the annual sun-
flower and of stocks and of many other cultivated plants is
dominant over the unbranched condition found in certain
cultivated races.

These illustrations serve to show that practically all parts
and structures of plants, as well as of animals, are likely to be
affected by unit-character variations and that combining of
such variations by means of crossing is a ready means of
producing new varieties.

CHAPTER XVI

UNIT-CHARACTERS OF INSECTS

The so-called " silkworm " is the larva of an Asiatic moth
which feeds principally on the leaves of the mulberry tree.
The " worms " when full grown spin a silken cocoon (which
furnishes the silk of commerce) within which they complete
their metamorphosis into the moth stage. As moths they
mate and the females lay eggs. In some races there is only
one generation a year, the eggs laid one summer hatching the
next spring. These are said to be univoltine, having one
flight or mating period annually. In other races there are
two or more broods a year depending on temperature condi-
tions. These are said to be bivoltine or multivoltine. In
crosses between univoltine and bivoltine races the eggs laid
have the character of the mother's race, being purely ma-
ternal structures. Thus, eggs laid by a univoltine mother
refuse to hatch before the following season, whatever the
racial character of the male that fertilized the eggs. And
eggs laid by a bivoltine mother are regularly bivoltine regard-
less of the father's racial character. But the females which
hatch from cross-bred eggs are really heterozygous as regards
voltinism. Their eggs show the dominant {univoltine) char-
acter but their daughters, the F2 females, are some univol-
tine, others bivoltine, in the ratio, 3:1.

Races of silkmoths differ by numerous characters, many
of which are Mendelian. Toyama has enumerated more
than a dozen such Mendelizing characters found in the larva
alone. Some races differ in the number of larval moults,
which may be either three or four. Tri-moulting is dominant
over tetra-moulting in crosses. The blood of the larva may
or may not be yellow colored, yellow blood being dominant.
Yellow-blooded larvae spin yellow cocoons so that there is a
correlation between blood-color of the larva and the cocoon-

154

UNIT-CHARACTERS OF INSECTS 155

color. Presence of pigments in the larval skin is dominant
over uncolored skin. Various patterns of the larval pigmen-
tation (spotting, striping, etc.) are dominant over their alj-
sence. Reddish-brown color of the larva is recessive to bhick.
The possession of knob-like outgrowths of the larval skin is
dominant over smooth skin.

TABLE 20

Unit-Characters of Silkworms

1. Egg Characters, all Maternal in Origin
Dominant Recessive

(1) TJnivoltine. Bivoltine.

(2) Eggs oval. Eggs spindle-shape.

(3) Eggs normal slate color. Eggs light brown or gray.

2. Characters of the Larva or the Cocoon, of Biparental Origin

(1) Tri-moulting. Tetra-moulting.

(2) Blood (and silk) yellow.^ Blood (and silk) white.

(3) Silk white (European races) . Silk yellow.

(4) Larval skin pigmented. Larval skin unpigmented.

(5) Larva spotted or striped. Larva not spotted or striped.

(6) Larva black. Larva reddish brown.

(7) Larval skin with knob-like Larva not knobby.

outgrowths.

White cocoon-color (silk) has been found in some races to
be a recessive character and in others to be dominant. The
two kinds of white have been shown to be genetically distinct.
One is probably a loss variation like albinism in rodents, the
other a white variation due to inhibition of color, like some
forms of white spotting in mammals. Certain variations in
the color and shape of the egg have been found to Mendelize,
but with the same complication as in the variation from
univoltinism to bivoltinism. Egg characters being deter-
mined entirely by the mother, the influence of the father does
not show in the Fi generation. Which of the contrasted
characters is dominant does not become evident until eggs

^ Uda has recently shown that yellow color of the blood is due to a single domi-
nant factor but that the silk will also be yellow only when a second and independent
factor is also present. When this second factor is lacking, white silk will appear to
be dominant over yellow silk, even though the blood is yellow.

156 GENETICS AND EUGENICS

are laid by the Fi females and segregation is seen first in the
eggs laid by F2 females. Spindle-shape of egg is a recessive
variant from normal, oval shape, and light brown egg-color
and gray egg-color are recessive variations from normal
slate-color.

Bateson (1913) has brought together records for numerous
cases of unit-character color variation in moths and beetles
occurring in the wild state. These cases present nothing in
principle different from the variations of silkworms, but
show that Mendelian sports occur among insects "in nature"
as well as under artificial conditions.

The most complete and in many respects the most instruc-
tive series of unit-character variations recorded in any insect
has taken place within a very few years in a small fruit fly,
Drosophila, while it was under observation in the Zoological
Laboratory of Columbia University. For this discovery we
are indebted to Professor T. H. Morgan and his pupils.
Drosophila melanog aster is a small fly with grayish brown
body and red eyes, which lays its eggs in fermenting fruits.
Apples, peaches, grapes or bananas with broken skin afford
good conditions for its multiplication. It is sometimes known
as the vinegar or pomace fly because the alcoholic fermenta-
tion of apple juice attracts it to vinegar jugs, pickle jars, and
cider mills. This fly while breeding in Professor Morgan's
laboratory produced a white-eyed sport, which lacked en-
tirely the normal red eye-color. The sport was first observed
in a male individual, which bred to normal mates produced
only normal offspring. But when these Fi offspring were
bred together they produced white-eyed offspring as reces-
sives in the expected proportion, one-fourth. Curiously
enough, however, all were males. Nevertheless, when these
obviously recessive white-eyed males were mated with Fi
females (heterozygotes) a generation was produced consisting
of white-eyed individuals and red-eyed individuals in equal
numbers, and among both sorts the sexes were approximately
equal. White-eyed individuals bred together breed true, but
in crosses the white-eyed character seems to have a prefer-

UNIT-CHARACTERS OF INSECTS 157

ence for male individuals, which has led to its being called a
sex-linked character. White-eye has proved to be only the
first of a long series of unit-character variations, which have
appeared in Professor Morgan's cultures of Drosophila,
which have this same curious sex-linked character. Among
these may be mentioned a variation in which the entire body
is yellow, another in which the eye-color instead of being an
ordinary red, is a brilliant vermilion, and several variations in
the form of the wing known as rudimentary, miniature,
forked, etc. It is found that when a race possessing two of
these recessive sex-linked characters (as white eye and yellow
body) is crossed with another race which lacks them, there is
a tendency for the two sex-linked characters to go together
in heredity, so that whatever F2 individuals possess one of
them possess also the other. This suggests that the material
basis or '*gene'' of each lies in the germ-cell near that of the
other, that their genes are either connected directly with each
other or with a common third structure. Since there are
several of these variations which show ''linkage" with each
other and a peculiar relationship to sex, the pertinent sug-
gestion was made by Morgan that they had as a common
connecting element a structure concerned in the determina-
tion of sex, commonly known as the sex-chromose or X-chro-
mosome. The **genes" of sex-linked characters, according
to Morgan, lie in the X-chromosome and the peculiar features
of the inheritance are due to the fact that the X-chromosome
is paired in females but unpaired in males. Strong support
is given to this idea by the result of crosses in which each
parent introduces a different sex-linked character, as in the
cross between a white-eyed race and a yellow-bodied race,
each being otherwise normal. The two characters in this
case keep apart as strongly as they keep together when in-
troduced into a cross by the same parent. This is exactly
what we should expect if, as Morgan supposes, sex-linked
characters have their genes in a common cell structure (for
example an X-chromosome). For when two genes lie in the
same X-chromosome, they will go together (show linkage).

158 GENETICS AND EUGENICS

but when they lie in different X-chromosomes, as for example,
in those furnished by the father and mother respectively,
then each will go with a different X, when the paired chromo-
somes separate from each other, as they do when gametes are
formed.

But we are forced to suppose that occasionally in the eggs
of Drosophila a gene may detach itseff from one X-chromo-
some and pass over into the other, for once in a while we find
that two sex-linked characters which were repelling each
other have in some way got into the same gamete and are
now coupled, and vice versa two which were coupled may later
show repulsion. Morgan's hypothesis offers a simple expla-
nation of such occurrences. The supposed changing of a gene
from one X-chromosome to another, when repulsion gives
place to coupling or vice versa, Morgan calls a ** crossing-
over." It occurs only in female individuals, or more properly
in their eggs, for it has not been observed to occur in the
sperms of Drosophila.

CHAPTER XVII

SEX-LINKED AND OTHER KINDS OF LINKED INHERITANCE

IN DROSOPHILA

All the facts of sex-linked inheritance in Drosophila liar-
monize with Morgan's hypothesis that the genes of sex-linked
characters lie in a common cell structure (X-chromosome)
which is duplex in females, simplex in males. Accordingly,
in a race which breeds true for a sex-linked character, that
character may be transmitted by every egg, but by only half
the sperms, namely by such as possess an X-chromosome and
by virtue of that fact determine as female all zygotes into
which they enter. To male zygotes the sperm will not trans-
mit sex-linked characters. This hypothesis is supported by
some curious facts already alluded to but deserving of fuller
consideration in this connection, viz., facts observed in re-
ciprocal crosses involving a sex-linked character, as for
example white eye in Drosophila.

TABLE 21

Reciprocal Crosses of White-Eyed with Red-Eyed Drosophila

Male Female Male Female

P White X Red Red X White

Fi Red Red White Red

F2 1 Red :1 White Red 1 Red: 1 White 1 Red :1 White

It has already been stated that a white-eyed male Droso-
phila crossed with normal females has only normal children
of both sexes, while the white-eyed grandchildren are all of
the male sex. In the reciprocal cross, between a white-eyed
female and a normal male all the daughters are normal, but
the sons are white-eyed, and among the grandchildren white-
eyed individuals occur in both sexes. Diagrams will best
explain these facts on the basis of Morgan's hypothesis.
(See Figs. 116 and 117 and Table 21.)

159

160

GENETICS AND EUGENICS

To state the foregoing facts in another way, it will be ob-
served that the recessive sex-linked character in Drosophila,
when introduced in a cross by the male parent, disappears
entirely in Fi and reappears in F2 only in male individuals.

Flies

(S

Chromosomes

XX

IXI
X X $

2YO: 2X11

X ®
iXi
X 1

XX XI X

Parents

Gametes

Fi

Gametes

Fz

Fig. 116. Sex-linked inheritance of white and of red eyes in Drosophila. Parents, white-eyed male and
red-eyed female; Fi, red-eyed males and females; F2, red-eyed females and equal numbers of red-eyed
and white-eyed males. A black X indicates an X-chromosome bearing the gene for red eye, a white
X bears white eye. O indicates that an X is wanting; in recent publications Morgan replaces it by Y.
(From Conklin, after Morgan.)

But if the recessive sex-linked character is introduced by the
female parent, it appears in Fi in male individuals but in F2
in both sexes.

Suppose now a cross is made between two races, each of
which possesses a different sex-linked recessive character, as
for example white eye and yellow body. (See Table 22.) If
the white-eyed parent is a female, there will be produced
white-eyed males in Fi and white-eyed flies of both sexes in
F2. But the male parent being yellow, there will be no yellow
flies produced in Fi and only yellow males in F2. In the re-
ciprocal cross (yellow female X white-eyed male) yellow

SEX-LINKED INHERITANCE IN DROSOPHILA 161

males will be produced in Fi and yellow flies of both sexes in
F2, while white-eyed flies will not appear until F2 and then
only in the male sex. In either of the reciprocal crosses we
expect the production in F2 both of yellow-bodied males and

Flies

Chromosomes

X0

6

iXi

Parents

Gametes

<S ?

X IX ?

Fi

Gametes

M M X0 m Fz

Fig. 117. Reciprocal cross to that shown in Fig. 116. Parents, red-eyed male and white-eyed female;
Fi, white-eyed males and red-eyed females (" criss-cross inheritance " — Morgan); Fs, equal numbers
of red-eyed and white-eyed individuals in both sexes. The distribution of the sex-chromosomes is
shown at the right, as in Fig. 116, (From Conklin, after Morgan.)

of white-eyed males. Usually no other sort of male is pro-
duced throughout the experiment except these two, but occa-
sionally there is produced a male both yellow-bodied and
white-eyed, or one which is gray-bodied and red-eyed, like
wild flies. How do these arise ? If in Fi females the paired
X's were to exchange loads in part, so that G and R came
to be attached to the same X and g and r to the other X,
and if each of the eggs having such a constitution were to be
fertilized with a sperm which lacked X (male determining
sperm), this would make possible the production of Fo males
possessing both dominant characters and others possessing
both recessive characters or gray-red and yellow-white

162 GENETICS AND EUGENICS

respectively, as actually observed in about one case in a hun-
dred by Morgan.

It may add interest to the case to state parenthetically that
in man occur a number of sex-linked variations which are in-
herited in this same curious fashion. Among them may be
mentioned color blindness and bleeding {haemophilia), which

TABLE 22
Reciprocal Crosses of White-Eyed and Yellow-Bodied Flies

Male Female Male Female

P Yellow-red X Gray-white Gray-white X Yellow-red

Fi Gray-white Gray-red Yellow-red Gray-red

F2 1 Gray-white: 1 Gray-red: 1 Gray-white: 1 Gray-red:

1 Yellow-red 1 Gray-white 1 Yellow-red 1 Yellow-red

occur chiefly in males, but are never transmitted by males to
their sons but only through their daughters to their grand-
sons.

Morgan and his pupils have described between forty and
fifty characters in Drosophila which are sex-linked in hered-
ity; they also have discovered a large number of other
Mendelizing characters in Drosophila which are not sex-
linked but which nevertheless are inherited in groups, char-

$?,6

Fig. 118. Drawing showing the four pairs of chromosomes seen in the
dividing egg-ceil of Drosophila. (After Dr. C. W. Metz.)

acters in the same group showing coupling when introduced
in a cross from the same parent, and repulsion when intro-
duced from different parents. The number of these groups
exactly corresponds with the number of the chromosomes and
Morgan believes that their genes are located in the chromo-
somes, an hypothesis which seems reasonable but which
would be severely strained if an additional group of characters
should be discovered. There are three groups of the non-sex-
linked characters. (See Fig. 119.) In one of these referred
to as Group II (the sex-linked group being called Group I),

.oo YELLOW, SPOT.
:0.7 LETHAL I.
»-o W IIITE. EOSIN, COEBHY.

»o ABNORMAL.

••O BIEID.

•0.0 STREAK.

•0.0 SEPIA.

• 14.7 CLITB.

■!•.• SHIFTED.

• 20.5 LETHAL nr.

•27.3 TAN.

■BENT.

•ETELES8.

■!•.• DA CHS.

• a»« PINIC PEACH.

■33.0 VERMIUQH.
•3e.a MINIATURE.

•9*.7 BLACK.

■41,7 LETHAL V.
■43.0 SABLE.

■40.3 LETHAL IV.

-40.« PURPLE.

-40. KIDNEY.

-•a.o VESTIGUJt*

■8B.1 RUDIMENTARY.

»«-8 FORKED.
"87.0 BARRED.

.EBONY, SOOTTj

• SO.S FUSED.

. ©0.4 CURVED.

■08.9 LETHAL S.

Fig. 119. Diagram show-
ing the location, in the
four paired chromosomes
of Drosophila, of the
genes for various Men-
delizing characters, as de-
termined by Morgan and
his pupils. The X-chro-
mosome is at the left.
All characters there enu-
• merated are sex-linked.
The numerals indicate
the supposed relative dis-
tances of the genes from
the upper (zero) end of
each chromosome as de-
termined by linkage
strengths in crosses. (Af-
\%^ Morgan, Sturtevant,
Muller and Bridges.)

■ 7». BEADED.

■•44 ARC

> 80.0 SPECK.
•«.© MORULA.

-•f.OTlOUGH

[\

(

\

SEX-LINKED INHERITANCE IN DROSOPHILA 163

are found variations knowTi as black body and vestigial wings
respectively, together with some thirty-five other variations.
In Group III are found the variations known as pitik eye,
spread wings, and ebony body, together with some twenty
other variations. In Group IV are included as yet only
two characters, bent wings and eyeless, which however show
linkage with each other. No inherited characters have been
discovered in Drosophila which are not inherited in one or
another of the four linkage groups.

CHAPTER XVIII

DROSOPHILA TYPE AND POULTRY TYPE OF SEX-LINKED

INHERITANCE

1. Drosophila type. The same type of sex-linked inheri-
tance which is found in Drosophila is found also in man, in
cats (inheritance of yellow color), and in the plants, Lychnis

Fig. 120. Sex-linked inheritance of barred and of unbarred (black) plumage in poultry. P, parents,
barred male, unbarred female; Fi, barred males and females; F2, males all barred, females in equal
numbers barred and unbarred. (After Morgan.)

and Bryonia. The essential feature of this " Drosophila
type " of inheritance is this. In a race breeding true for a
sex-linked character, the female is homozygous for the
character in question while the male is heterozygous and in-

164

SEX-LINKED INHERITANCE IN POULTRY 165

capable of becoming homozygous. Reciprocal crosses with
such a race give unlike results, because the female transmits
the character to all her offspring, but the male transmits it
to only half his offspring, viz., the females.

2. Poultry type. Another type of sex-linked inheritance
exists in which the sex relations are exactly reversed. This
was first observed in the moth. Abraxas, but more familiar
cases occur in poultry, for which reason it may be called the
poultry type of sex-linked inheritance. Here the male is the

Fig. 121. Reciprocal cross to that shown in Fig. 120. F, parents, unbarred male, barred female; Fi,
barred males, unbarred females (criss-cross inheritance); F2, barred and unbarred birds equally nu-
merous in both sexes.

homozygous sex, the female being heterozygous. This condi-
tion is found in moths and in certain birds, viz., m domestic
fowls, pigeons, ducks and canaries. As an example we may
take the inheritance of the color pattern, barring, in crosses of

166 GENETICS AND EUGENICS

barred Plymouth Rock fowls. In reciprocal crosses between
pure-bred barred Plymouth Rocks and black Langshans
(or any other unbarred breed), the results are not identical. If

TABLE 23

Reciprocal Crosses of Barred and Black Breeds

\ OF Fowls

Male Female Male

Female

p

Barred X Black Black X

Barred

Fi

Barred Barred Barred

Black

F2

Barred 1 Barred: 1 black 1 Barred: 1 black

1 Barred: 1 black

See Fig. 120. See Fig

121.

the barred parent is the male (Fig. 120 and Table 23), all Fi,
offspring are barred and in F2 all males are barred, but half the
females are black and half are barred. If, however, the barred
parent is the female (Fig. 121 and Table 23), all Fi males are
barred, but all Fi females are black. In F2 barred birds and
black birds occur in both sexes. These curious facts, which
have been repeatedly verified, suggest the occurrence of a
vehicle of inheritance which is duplex in males but simplex in
females. What this is we do not know. No chromosome has
been found which has a distribution of this sort in fowls, but
it is possible that some chromosome component, or other cell
constituent, has such a distribution and may be the actual
vehicle of inheritance in such cases. The most important
character economically, which appears to be affected by some
sex-linked factor in poultry, is fecundity. Pearl has observed
that when reciprocal crosses are made between Cornish In-
dian games, a poor breed for winter egg production, and
barred Plymouth Rocks, a fairly good breed for winter egg
production, the Fi females in each case resemble the father's
race more strongly than the mother's race as regards egg
production. Pearl did not maintain, however, nor do his ex-
periments suggest, that the inheritance of fecundity depends
exclusively upon a sex-linked factor. Goodale, however, has
not been able to confirm Pearl's observations, in the case of
Rhode Island Red fowls. He finds no evidence of superior
influence of the sire in the transmission of racial fecundity.

CHAPTER XIX

LINKAGE

In ordinary Mendelian inheritance, if two characters, A and
B, enter a cross in the same gamete (either egg or sperm), it
will be wholly a matter of chance whether they continue
together or are found apart in the following generation. If
in the formation of gametes by the cross-bred, A and B
separate from each other and pass into different gametes, it
is evident that one of them has crossed-over from the gametic
group in which both originally lay to enter the alternative
group. This event may be called simply a crossover. Cross-
overs and non-crossovers will be equally numerous (50
per cent each) where no linkage occurs. Also, if A and B
enter a cross in different gametes, one in the egg, the other
in the sperm, it will in ordinary Mendelian inheritance be a
matter of chance whether they emerge from the cross to-
gether or apart. If together, it is evident that a crossover
has occurred; if apart, a non-crossover, that is a persistence
of their previous relations. Again, crossovers and non-
crossovers will be equally numerous (50 per cent each) if
no linkage occurs.

Linkage may be defined as the tendency sometimes shown
by genes to maintain in hereditary transmission their previ-
ous relations to each other. Thus if two linked genes, A and
B, enter a cross together in the same gamete, they will
oftener than not be found together in the gametes formed
by the cross-bred individual. Crossovers in that case will
be less than 50 per cent, and non-crossovers more. And if
the same two genes enter the cross separately, one in the egg,
the other in the sperm, then oftener than not they will be
found apart, in different gametes formed by the cross-bred
individual. Again crossovers will be less than 50 per cent.

The number of genes in a linkage group varies in known

167

168 GENETICS AND EUGENICS

cases from 2 to 50 or more. However many genes there are
in a linkage group, each gene shows linkage with every other
gene belonging to the same group, but the apparent strength
of the linkage varies greatly. Under uniform environmental
conditions, A and B show a fairly constant hnkage with each
other, A and C show a different and likewise fairly constant
linkage strength, and so on through the entire group. This
leads to the conclusion that the genes of a linkage system
are bound together, gene with gene, with bonds of definite
strength in each case. In order to visualize the matter and
get a more objective view of linkage relations, Morgan and
his associates have developed the chromosome__theory_of
linkage,. Its essential parts are:

1) Genes which show linkage with each other are located
in the same pair of chromosomes. It is the substance of the
chromosome which binds the genes to each other and causes
A to be inherited when B is.

(2) Genes close together in the same chromosome show
strong linkage, genes farther apa^t show less linkage.

(3) Homologous chromosomes,) those containing corre-
^sponding sets of genes, one set derived from the father, one

from the mother, lie side by side (in synapsis) previous to
the formation of gametes. At this time breaks are likely to
occur in the chromosomes and parts of one are likely to re-
place corresponding parts of the other.

(4) Such replacement is called crossing-over.

(5) Breaks are commoner in long chromosomes than in
short ones, and between distant points than between near
points on the same chromosome.

(6) The genes occur in a chromosome, like beads on a
string, in a single row and in definite order.

The supposed order of the genes in the four linkage groups
of Drosophila and their relative distances apart are shown
in Fig. 119. In these diagrams, or"maps,"when the probable
order of the genes in a system has once been determined, the
supposed end gene of the system is placed at position 0 and
the gene next to it is placed at a distance (in centimeters or

LINKAGE

169

other units) corresponding to the average cross-over percent-
age between the two, this process being repeated from gene
to gene until the whole chain is plotted. The "map" is thus
based on a summation of the distances (measured in cross-
over percentages) from gene to gene. But if we compare the
"map distances" between genes not adjacent to each other
in the chain with the observed cross-over percentages be-
tween the same genes, we find that the map distance is regu-
larly greater than the cross-over percentage, except for very
short distances (5 or less) . Thus if three genes occur in the

I

IV

M

a

IV

M

Fig. 121a. B and C illustrate Morgan's idea of
the linear arrangement of the genes in the chromo-
somes. A and D show how the composition of
chromosomes is supposed to change as a result of
a crossover. Fig. 1'22. A pair of homologous
chromosomes a, before; 6, during; c, after a double
crossover. (After Morgan.)

order A, B, C, it is usually found that AB + BC is greater
than AC. In other words, the cross-over percentage be-
tween A and B plus the cross-over percentage between B
and C is commonly greater than the cross-over percentage
between A and C, and the discrepancy increases with the
magnitude of the values involved. This fact has been
accounted for in two different ways. First, it may be sup-
posed that the arrangement of the genes is really not linear,
that B lies out of line with A and C, so that AC will be less
than the sum of AB and BC, and that the more distant
genes are no farther apart than indicated by the cross-over
percentages between them. This explanation has met with

170 GENETICS AND EUGENICS

more difficulties than it has cleared away. The second expla-
nation is that the map-distances indicate proportionate
numbers of breaks in the linkage chain between points, not
proportionate numbers of changes of relation between genes
at particular points. Thus, suppose genes AB C D E of a link-
age system meet their allelomorphs, a b c d e, in a cross and
gametes are later formed by the cross-bred as follows, (1)
A B c d e, (2) A B c d E, and (3) A b c D e. Assuming that
the arrangement is linear, we must suppose that one break in
the linkage chain has occurred in (1), two breaks in (2), and
three breaks in (3). But if we did not have genes BCD
under observation, and merely noted the relation of A to E,
we should infer that in case (1) and in case (3) a single cross-
over had occurred, but that in case (2) no crossover had oc-
curred. We should on that basis underestimate the amount
of breaking in the linkage chain. Accordingly the construc-
tion of maps on the basis of short distances summated is
justifiable, provided the arrangement is linear, as it seems
to be. But it must be borne in mind that the map distances
do not correspond with cross-over percentages (although
they are based on them) except in the case of very short
distances. Map distances often exceed 50, but cross-over
percentages can not do so, as already pointed out. To get
a distinctive name for the map units, Haldane has called
them units of Morgan or simply ** morgans." Haldane has
computed a formula for converting cross-over percentages
into ** morgans" and vice versa. He finds that the two cor-
respond only for very low values (5 or less) and diverge
more and more as the observed cross-over percentages ap-
proach 50. Haldane's formula may be stated thus. If three
genes. A, B, and C, occur in a common linkage group, and
the cross-over percentages are known between A and B and
between B and C, we may predict with a probable error of
not over two per cent, what cross-over percentage will be
found to occur between A and C. Calling the cross-over
percentage between A and B, m, and that between B and C,
n, the cross-over percentage between A and C will lie be-

LINKAGE 171

tween (m + n) and (m + n - 2mn) . It will approach the
former for amounts of 5 or less, and the latter for amounts
of 45 or over. In a useful table Haldane shows the calculated
map distances (morgans) for all cross-over percentages be-
tween 5 and 50. This table is based on the relations of the
genes observed in the sex-linked group of Drosophila, but it
applies equally well to the second linkage group of Droso-
phila and to a group of three genes in the plant, Primula.
Provisionally it may be considered to be applicable generally
to linkage systems in animals and in plants.

TABLE 24

A Table for Converting

Cross-Over Percentages

i INTO

Map Distances

("Morgans")

AND V

ICE Versa.

After Haldane

Cross-over percentage 0.0

5.0

8.0

10.0

11.0

12.0

13.0

Map distance 0.0

5.1

8.2

10.3

11.4

12.5

13.6

14.0 15.0 16.0

17.0

18.0

19.0

20.0

21.0

22.0

14.7 15.9 17.0

18.1

19.3

20.5

21.7

22.9

24.1

23.0 24.0 25.0

26.0

27.0

28.0

29.0

30.0

37.0

25.3 26.6 27.9

29.2

30.5

31.9

33.3

34.7

36.2

32.0 33.0 34.0

35.0

36.0

37.0

38.0

39.0

40.0

37.7 39.3 40.9

42.6

44.3

46.1

48.0

50.0

52.2

41.0 42.0 43.0

44.0

45.0

46.0

47.0

48.0 ■

49.0

54.4 56.8 59.6

62.6

66.0

70.1

75.1

81.9

93.0

49.5 49.7 49.8

49.9

50.0

99.2 109.4 117.7

128.1

oc

As an example of how the table may be used in predicting
undetermined linkage values, suppose that A is linked with
B, and B with C and that between A and B there are 10 pvr
cent of crossovers, and between B and C, 15 per cent of
crossovers. What will be the cross-over percentage between
A and C.^ Converting the observed cross-over percentages
into map distances with the aid of the table, we find the
distance AB to be 10.3 and the distance BC to be 15.9. On
the linear theory the distance AC will equal either the sum
or the difference of AB and BC, that is will be either '■26.''Z or
5.4. Converting these distances into cross-over percentiiges
by interpolation in the table, we find that the cross-over

172 GENETICS AND EUGENICS

percentage between A and C should be either 23.7 or 5.1,
according as the hnear arrangement is ABC or ACB.

Measurement of linkage. It will be observed that as the
strength of linkage increases, the cross-over percentage de-
creases. With a cross-over percentage of 50, there is no
linkage. With a cross-over percentage of 0, the linkage is
complete, two characters so related behaving as allelomorphs.
Accordingly we depend upon the observed cross-over per-
centage both for the detection of linkage and for the measure-
ment of its strength. But unfortunately the linkage strength
varies inversely as the cross-over percentage. This makes
the cross-over percentage directly considered, a rather poor
measure of linkage strength. It is really the amount by
which the cross-over percentage falls below 50 that measures
directly the strength of linkage. Thus with cross-over per-
centages of 50, 40, 30, 20, 10, and 0, we should have linkage
strengths of 0, 10, 20, 30, 40, and 50. We should then have
a standard for measuring linkage strength directly, on a
scale of 50. But as we are more accustomed to grading on a
scale of 100, it seems preferable to double the values indicated
above. We then have grades of linkage strength on a scale
of 100, as follows:

Cross-over Percentage
50

Linkage Strength
0

40

20

30

40

20

60

10

80

0

100

Accordingly, to estimate the strength of linkage in a particu-
lar case, we multiply by 2 the difference between the ob-
served cross-over percentage and 50.

But suppose the observed cross-over percentage were
greater than 50, what then.^ Such an occurrence would not
indicate linkage, a tendency of characters to remain grouped
as they were, but an opposite tendency, to assume new group-
ings. No such tendency has been observed. If it should be,
it would need a different name and method of measurement.

LINKAGE 173

We may now consider some further examples of linkage.
In the plant, Primula sinensis, Gregory observed the oc-
currence of linkage in a group of five characters, viz.

Dominant Recessive

1. Short style vs. long style (1).

2. Magenta corolla vs. red corolla (r).

3. Tinged corolla vs. full-colored corolla.

4. Green stigma vs. red stigma (s).

5. Pale stem vs. full red stem.

Altenburg later determined the strength of the linkage ex-
isting between three of these five pairs of characters, viz., 1,
2, and 4 of the above list. His results may be expressed in
a linkage map as follows:

0 34.0 45.6

The cross-over percentage between 1 and r was found to be
34.02, between r and s, 11.62. The sum of these two, 45.64,
is the total (uncorrected) map distance. The observed cross
over percentage between 1 and s was 40.6, which falls short
of the map distance by almost exactly the amount indicated
by Haldane's table.

In the sweet pea the earliest discovered examples of link-
age are found. Here are known two linkage groups con-
taining each three pairs of characters as follows;

Dominant Recessive

1. Blue vs. red flower color.

Group I. 2. Long vs. round pollen.

3. Erect vs. hooded standard.

1. Dark vs. light leaf-axil.
Group II. 2. Fertile vs. sterile anthers.
3. Normal vs. cretin flowers.

Results described by Bateson and by Punnett indicate that
in Group I the map relations of the three genes are:

E-B L

0 .78 12.5

The group is a compact one, with E and B very closely
linked, cross-over percentage less than one, with B and L

174 GENETICS AND EUGENICS

showing between 11 and 12 per cent crossovers, and with
E and L showing about 12.5 per cent of crossovers.

In Group II, the cross-over percentage between D and F
is about 6.2, between F and N about 25.0. Until the cross-
over percentage between D and N has been experimentally
determined, it cannot be stated whether the **map" order
is F D N or F N D. In the former case, the total map
distance will be 25, or about double the length of Group I;
in the latter case, it will be still longer, or about 31.2.

In garden peas two independent pairs of linked characters
are known and two more are suspected (Wliite). In one of
the established cases close linkage is found between round
starchy seeds and tendrils on the leaves, with about 1.5 per
cent of crossing-over. In the other case a gene for late flow-
ering is linked with red flower color with an estimated cross-
over percentage of between 12 and 16.

In the snapdragon. Antirrhinum, two factors for flower
color were found by Baur to be linked, with about 20 per
cent of crossovers occurring.

In maize three linkage groups are known, one of four
factors and two of two factors each. Group 1 includes a
factor for waxy endosperm and the factor C for aleurone
color. These show a cross-over percentage of 26.7. Group 2
includes fowr linked factors, aleurone factor R, chlorophyl
factor G, chlorophyl factor L, and aleurone spotting factor,
S. No crossovers have been observed between R and L
which behave as if they were allelomorphs, or ** completely
linked." The cross-over percentage between L and G has
been determined as 23, that between R and G has been de-
termined less accurately as 19, and that between R and S
as 12.5. The order of the genes is accordingly

RL S G.

Group 3 includes the two characters, starchy endosperm
and tunicate ("podded") seeds. The cross-over percentage
in this case is 8.3 (Jones and Gallastegui).

In the cultivated tomato two cases of linkage have been
reported. A gene for "standard" vine habit and a gene for

LINKAGE

175

TABLE 25
Cases of Linkage in Plants or in Anevlvls other than Drosophila

Species

Group

Linked Characters

Cross-over , Linkage i . »l •.
Percentage' Strength Authority

Sweet
pea

1

1
1

2

2

2

Purple flowers, long pollen
Purple flowers, erect standard
Long pollen, erect standard
Dark axil, fertile anthers
Dark axil, normal (not cretin)

flower
Fertile anthers, normal (not

cretin) flowers

11 or 12
0.78
12.5
6.2

5

*
•

25.0

76-78

98.4

75

87.6

50

Bateson

and

Punnett

Primula
sinensis

Short style, magenta corolla
Short style, green stigma
Magenta corolla, green stigma
Tinged corolla, green stigma
Pale stem, green stigma

34.0
40.6
11.6

•
•

32

18.8
76.8

Altenburg

((

<<
Gregory

Garden
pea

2

Round seeds, tendrils on leaves
Late flowering, colored flowers

1.5
12-16

97
68-76

Bateson and

Vilmorin

Hoshino

Antirrhi- 1
num

Red flower color, "picturatum "
pattern

20.0 60

Baur

Maize

1
2
2
2
3

Waxy endosperm, Aleurone C
Aleurone R, Chlorophyl G
Aleurone R, Chlorophyl L
Chlorophyl G, Chlorophyl L
Starchy endosperm, tunicate
seed

26.7
19.0

0.0
23.0

8.3

46.6
62?
100
54

83.4

Breggar
Lindstrom

it

Jones

Tomato 1

2

Vine habit, fruit shape
Green foliage, 2-celled fruit

20.0 1 60 Jones
0? 100.^

Beans j 1

Seed pattern. Vine habit ' 0?

100? Surface

Silkworm

1

Pattern Q of larva, yellow silk

26.1

47.18

Tanaka

Apotettix

1
1
1
1

1
1
1
1

Patterns G and M

M « K

K « Y

Y " R

u Y " T

R " T

" M '' R

« Y " Z

4 (in 9)

1 "

6 "
10 "
12 "

0 "
10 "

92
98
88
80
76
100
80
80

Nabours

u
u
u
u
u
u
u

Pigeon

1

Sex-linked factors I and A

40(in d") 20 1 Cole and

Kelley

Rat

1
1
1

Albinism, red-eye ' 1-0? 98?' Castle and
Albinism, pink-eye ; 21.0 58 Dunn
Red-eye, pink-eye 18.3 j 63.4

Mouse

1

Albinism, pink-eye.

14.3 71.4 1 Castle and

Dunn

1

176 GENETICS AND EUGENICS

*' constricted" fruit shape show about 20 per cent of crossing-
over. In another linkage group, no crossovers have been
observed between green foHage color and two-celled fruit, as
opposed to yellow foliage color and many-celled fruit, in a
total of 24 r2 plants. It seems probable that the linkage in
this latter case is close, though the number of observations
is too small to do more than establish a probability.

In rats a group of three linked characters has been found,
albinism (c), red-e}^e (r) and pink-eye (p), which may be
mapped, thus

c_i._______ _____-- p

0 1 20

In mice albinism (c) and pink-eye (p) are linked, as they are
in rats, but the cross-over percentage is less, viz., 14.3.

In the silkworm, linkage occurs between a factor, Q, which
gives to the larva characteristic pattern markings, and a
factor, Y, which gives to the blood of the larva and the silk
of the cocoon a yellow color. Crossing-over occurs only in
males, and in a percentage of 26.1 (in a large series of back-
crosses of Fi hybrid male with double recessive female, pro-
ducing 24,918 individuals). In Drosophila crossing-over
occurs only in the female parent, that is in the maturation
of the eggs. This is true of all linkage groups, whether they
involve sex-linkage or not. In the grouse-locust, Apotettix,
a linkage group of seven or more characters has been dis-
covered by Nabours, which have this curious feature, that
crossing-over seems to occur much more frequently in fe-
males than in males. In all other known cases of linkage,
crossing-over occurs with about the same frequency in the
gametes formed by both sexes. This accordingly is to be
regarded as the normal condition. Failure of crossing-over
to occur in the oogenesis of Drosophila and in the spermato-
genesis of the silkworm would seem to imply unusual
cytological conditions in those cases.

CHAPTER XX

THE NATURE OF GENES

When a pair of alternative characters, such as pigmentation
and albinism, is involved in a cross, we assume that the
gamete which. transmit5~T>ne of the alternative conditions
differs structurally from that which transmits the other and
that this structural difference is the cause of their difl'erent
powers of transmission. By the study of linkage relations
we find that the structural difference is confined to a par-
ticular linkage group, in mice and rats to the group which
also includes the factor for pink-eyed dilution. If we adopt
the chromosome hypothesis, we locate the structural differ-
ence in a particular chromosome and suppose that it exists
in a definite region (or locus) of that chromosome. Each
structurally different state of a locus is called a gene. The
color gene shows the alternative forms which we call C and
c. With all the residual heredity unchanged, C will cause the
development of full pigmentation, while c will leave the skin
unpigmented. For information as to what C and c are, we
may consult the biochemists, who have devoted considerable
attention to the chemical processes involved in pigment for-
mation. Wright (1917) after an exliaustive review of the
chemical evidence concludes (1) '*that melanin (pigment)
is produced by the oxidation of certain products of protein
metabolism by the action of specific enzymes, (^2) that the
reaction takes place in the cytoplasm of cells probably by
enzymes secreted by the nucleus, (3) that various chromo-
gens are used, the particular ones oxidized depending on the
character of the enzymes present, and finally that hereditary
differences in color are due to hereditary differences in the
enzyme element of the reaction." The final conclusion is of
particular interest. It indicates that the gene C is concerned
in enzyme production. Wright offers a provisional hypolhe-

177

178 GENETICS AND EUGENICS

sis to explain variations in the character or amount of pig-
ment found in the coats of mammals, which involves two
enzymes acting in succession in the oxidation of chromogens.
Enzyme I performs the initial action, and acting by itself
produces yellow pigment (known also as red or cream, ac-
cording to the amount of pigment formed) . Enzyme II can-
not act on chromogens except in connection with Enzyme I
in which relation it carries forward the oxidation to a brown
or black stage. Without the presence of Enzyme I, no pig-
ment at all will be produced, that is the albino state will
result, even though Enzyme II is present. According to this
hypothesis the gene C is concerned in the production of
Enzyme I. But we are acquainted with several allelomorphic
forms of this gene, which in guinea-pigs are effective re-
spectively in full pigmentation, dilute pigmentation, red-
eyed dilution, and Himalayan albinism. We must suppose
that in this series of mutations. Enzyme I is produced more
and more feebly, until in complete albinism (as seen in rab-
bits, rats and mice) no effective production of Enzyme I
occurs. On the chromosome theory we must accordingly
suppose that the production of Enzyme I depends upon a
structure of some sort (gene C, c, etc.) having a definite
position (locus) in a particular chromosome. At definite
positions in this same chromosome, we must, on this theory,
locate one or more genes which influence the production of
Enzyme II in the rat and in the mouse. In both the rat and
the mouse, a gene for pink-eye (or its allelomorph, dark eye)
is linked with the color gene. This gene (in the form pink-
eye) diminishes greatly the amount of black pigment pro-
duced in eye and coat, but does not diminish at all the
amount of yellow pigment formed. Hence it affects the
hypothetical Enzyme II but not Enzyme I. In the rat, an-
other gene, that for red-eyed yellow, linked still more closely
with the color gene, likewise reduces the amount of black
pigment formed in the coat and the eye, but without dimin-
ishing at all the production of yellow pigment. But it allows
of more pigment development in the eye than does the gene

THE NATURE OF GENES I79

for pink-eye, and this is indicated in the name **red-eye."
That the genes for red-eye and pink-eye are different in
chemical nature is shown by their complementary action.
When pink-eyed and red-eyed rats are crossed, black pig-
mented young result.

A gene which in mice influences the action of Enzyme II
has the allelomorphic forms black (B) and brown (b). It is
not linked with the color gene and so cannot lie in the same
chromosome with it (Little and Phillips, Detlefsen). Gene
b interrupts the action of Enzyme II when the pigment has
been oxidized to a chocolate brown color, B allows the oxi-
dation to continue until the black stage is reached.

Another gene which limits the action of Enzyme 11 is the
agouti factor. In mice it is not linked either with C or with
B. Hence it must lie in a third chromosome. It restricts the
action of Enzyme II to particular parts of the hair, the base
and tip of the hair in most body regions, and on the belly
the base alone, or it may exclude the action of Enzyme II
from the entire hair in the belly region. As the dominant
allelomorph of the agouti factor, the gene yellow inhibits the
action of Enzyme II more or less completely throughout the
coat of mice.

In rabbits and guinea-pigs a gene called the extension
factor (E, e) influences the production or action of Enzyme
II. As E it permits black (or brown) pigment to be pro-
duced throughout the coat, except where its production is
interfered with by the agouti factor. As e, it does not permit
Enzyme II to function in the coat, but only in the eyes and
skin. Consequently the coat is yellow through the unas-
sisted action of Enzyme I. A third allelomorph, e', in guinea-
pigs allows Enzyme 11 to act in part of the coat only, thus
producing a yellow-and-black spotted coat. The extension
factor is apparently not linked with any of the other factors
for color production, and so must be located in a fourth
chromosome.

How many other genes there are which influence the action
of Enzyme II, we do not know, nor do we know wluit their

180 GENETICS AND EUGENICS

nature is, but it would seem improbable that any one of
them is itself Enzyme II, but only that it is in some way con-
cerned in the production of Enzyme II, either locally or
generally.

As regards Enzyme I, which is produced in several grades
(qualitative or quantitative) through mutations in the color
gene resulting in multiple allelomorphs, we know that its
action may be localized by other independently inherited
genes, those not in the same linkage-group or chromosome.
Such are the factors for white spotting which in no case have
been shown to be linked with albinism. Some factors of this
sort seem to interfere with the production of Enzyme I in
particular parts of the body, others allow Enzyme I to be
produced but inhibit its action in particular body regions.
Again we have no present knowledge as to what the nature
of these modifying genes is. In Drosophila there occur in a
single linkage system (chromosome), genes affecting various
parts of the body and affecting them in various ways. Thus
in the sex-linked group of genes are found those which in-
fluence the shape of the eye, the color of the eye, the length
of the wings, the shape of the wings, the venation of the
wings, the form of the legs, the color of the body, the shape
of the bristles on head and thorax, the form of the abdomen,
and many others less easy of description.

Again, in the '* second-chromosome" linkage group of
Drosophila, are found other genes which also affect practi-
cally all regions of the body, as for example, shape, size, and
venation of wings, length of legs, color and structure of the
compound eyes, patterns of thorax, shape of abdomen, and
general body-color. No linkage system specializes in genes
of any particular sort, or affecting any particular region of
the body. Often a single gene is known to affect various
parts of the body. Thus the gene, **dachs," affects both the
length of the legs and the venation of the wing.

If any part of any chromosome of an egg of Drosophila
were removed or changed in composition, it seems probable
that some departure from normality would follow in the fly

THE NATURE OF GENES 181

which developed from the egg. In that case the chromosome
change might be regarded as a gene responsible for the ob-
served departure from normality. As such it would behave
in crosses with normal individuals. If this is true, it seems
probable that the entire chromatin, or at least so much of
it as is concerned in determining the activities of the cell,
may be regarded as composed of genes. A gene will be the
smallest part of the chromatin capable of varying by itself.
And if the gamete contains any structures not chromatin
which are concerned in heredity, that is which are repro-
duced when the cell divides, these too will constitute genes.
Further investigation alone can show whether or not genes
are found exclusively in the chromatin. At present it is
assumed that such is the case.

CHAPTER XXI

ARE UNIT-CHARACTERS (GENES) CONSTANT OR

VARLABLE?

In some of the preceding chapters we have considered facts
which show to what a large extent the varieties of animals and
plants formed under domestication owe their origin to dis-
continuous variations or sports, which, by reason of their
Mendelian behavior in heredity, may be combined in various
ways through the agency of hybridization. It is a question of
much interest, both theoretical and practical, whether these
sports or unit-character variations, are entirely stable or
whether they themselves are subject to variation. For if a
unit-character is not variable, we can only vary the combi-
nations into which it enters, the character itself being un-
affected. But if a unit-character is variable, it is important
to know whether its variation is continuous or discontinuous.
For if it varies by distinct steps only, that is discontinuously,
it would be a waste of time to try by selection to establish
any other conditions than those which arise spontaneously,
by " mutation " as De Vries would say.

The mutation idea has greatly weakened the faith of biolo-
gists in selection. Darwin had great confidence in the power
of selection gradually to modify the characteristics of races.
Practical breeders of animals and plants have always worked
by this means, and Darwin based his views concerning the
eflScacy of selection largely on the results of their experience.
But breeders do not confine their attention to the propaga-
tion of variations which they have seen arise spontaneously.
They often form ideals of uncreated varieties and then work
zealously for the production of these. Some of these ideals
may be unattainable, but too many of them have been real-
ized to make us think that all work of this sort is fruitless.
Today animal breeders hold among their unrealized ideals,
a tri-color variety of mouse; a blue variety of fowl which will

182

Fig. 123. English rabbits showing a dominant form of white spotting which fluctuates both somatically and gene-
tically. The first five figures were employed as grades 1-5 in classifying observed fluctuations. The third figure
(middle row, left) is close to the fancier's ideal English marking. The two rabbits shown at the top and the one ut
the right, bottom, were homozygous for the English pattern; the other three were heterozygous. English pattern
is allelomorphic to Dutch, Fig. 138.

ARE UNIT-CHARACTERS CONSTANT ? 183

breed true, as blue pigeons do; a race of barred Pl^-mouth
Rock fowls of the same color in both sexes. These ideals the
student of genetics says are unattainable and he can gi\'e good
reasons for so regarding them. Nevertheless breeders will
doubtless continue to try for them and it is hardly safe to say
that success is impossible. Most advances in practical affairs
are made by those who have the courage to attempt what
others with good reason think unattainable. When such
attempts have succeeded, the world simply revises its classifi-
cation of things attainable and unattainable, and makes a
fresh start.

Many students of genetics at present regard unit-characters
as unchangeable. They consider them as impossible of modi-
fication as are the atoms. To recall Bateson's comparison,
the carbon and oxygen of carbon monoxide, CO, are each
unchangeable. Adding another atom of oxygen does not
alter them, though it changes radically the compound fonned
which becomes carbon dioxide, CO2, possessed of very differ-
ent properties. But the carbon and the oxygen are still there,
unaltered and recoverable. This question is one of great
practical importance, — are unit-characters as constant as
atoms, so that we can merely recombine them, or are they
different in nature from atoms so that we can modify as well
as recombine them. Much careful work has been devoted to
the solution of this question. It was at first assumed from
chemical analogy that characters which behave as units in
heredity must, like C and O in the case of carbon dioxide,
emerge from combinations unmodified. But presently case
after case came to light in which this was not true. Albin-
ism emerged from crosses tainted with color; clear yellows
emerged from crosses intensified to red, or diluted to cream,
or sooty with minute quantities of black; patterns such as
are seen in Dutch or in English rabbits, or in hooded rats,
emerged considerably altered in appearance. Facts such as
these were interpreted in two different ways. It was as-
sumed by some that the actual unit-character, factor, or
gene involved was subject to quantitative and possibly to

184

GENETICS AND EUGENICS

qualitative change. By others it was assumed that the ob-
served character changes were not due to changes in single
genes but to the supplemental or modifying action of the
other genes. For example, the hooded pattern of rats (Figs.
124 and 125) clearly behaves as a simple unit-character alle-
lomorphic to Irish pattern or to self in crosses. But the
hooded pattern as seen either in pure-bred or in cross-bred
litters of young (Fig. 124) varies slightly, and such variations
have a genetic basis since by selecting either the whitest or

Fig. 124. Inheritance of a recessive pattern of white spotting seen in " hooded " rats. The parents
(at the left) are a homozygous hooded mother and a heterozygous " Irish " father (black with white
belly). An entire litter of their young is shown at the right. Four are homozygous hooded like the
mother, five are heterozygotes like the father. Note fluctuation in both classes. Such fluctuations
are found to be in part heritable.

the blackest individuals, one can either whiten or blacken
the average racial condition. (See Tables 24a and 25a.)
Races corresponding with the extremes of the series shown in
Fig. 125 were thus produced. The question now arose
whether the observed changes had occurred as a result of
change in the single unit-character or gene clearly concerned
in the case, or whether this was due to other agencies. To
test the matter the selected races, now modified genetically
in opposite directions, were crossed repeatedly with a non-
hooded (wild) race. The recessive hooded character dis-
appeared in Fi but was recovered again in F2 in the expected
25 per cent of this generation. Compare Fig. 56. These
extracted hooded individuals, following each cross, were less
divergent than their hooded grandparents from the ordinary
hooded pattern. After three successive crosses (six genera-

ARE UNIT-CHARACTERS (GENES) CONSTANT? 185

tions) the whitest individuals extracted from the dark hooded
race were no darker than the darkest individuals extracted
from the white hooded race. In other words repeated cross-
ing with the non-hooded (wild) race had caused the changes
in the hooded character, which had been secured by selection,
altogether to disappear. This result shows conclusively that
the changes in question had not occurred in the gene for the
hooded pattern, but in the residual heredity. Other cases of
apparent gradual change in unit-characters under the action
of selection may be explained in a similar way. Accordingly
we are led to conclude that unit-characters or genes are re-

FiG. 125. A series of grades for classifying the plus and minus variations of the white

spotting pattern of hooded rats.

markably constant and that when they seem to change as a
result of hybridization or of selection unattended by hybridi-
zation, the changes are rather in the total complex of factors
concerned in heredity than in single genes.

Nevertheless changes do sometimes occur in single genes.
Such, we assume, are the several unit-character variations
described in previous chapters, which form the basis of the
varieties of domestic animals and cultivated plants. These
occur singly and sporadically as changes each in a particular
locus or part of a system of genes. By hybridization these
isolated changes are later combined in any desired fashion.
Change in a genetic locus, that is the appearance of a new
gene, is in the terminology of Morgan called a mutation but
this use of the term differs fundamentally from that of De
Vries. There is no known means by which a mutation, in this

186

GENETICS AND EUGENICS

sense, can be brought about. Genes are discovered, not
made in laboratories, and may be manipulated by hybridiza-
tion but not changed. The suddenness of their coming and
their stability are implied in the term mutation.

Sometimes a single genetic locus may undergo several dif-
ferent mutations, but these, so far as we know, occur in-

TABLE 24a

Results of the Plus Selection of Hooded Rats Continued through

Twenty Successive Generations

Generation

Mean Grade
of Parents

Mean Grade
of Offspring

Lowest

Grade

of Offspring

Highest

Grade

of Offspring

Standard

Deviation of

Offspring

Number of
Offspring

1

2.51

2.05

+ 1.00

+3.00

.54

150

2

2.52

1.92

-1.00

+3.75

.73

471

3

2.73

2.51

+ .75

+4.00

.53

341

4

3.09

2.73

+ .75

+3.75

.47

444

5

3.33

2.90

+ .75

+4.25

.50

610

6

3.52

3.11

+ 1.50

+4.50

.49

861

7

3.56

3.20

+ 1.50

+4.75

.55

1,077

8

3.75

3.48

+ 1.75

+4.50

.44

1,408

9

3.78

3.54

+ 1.75

+4.50

.35

1,322

10

3.88

3.73

+2.25

+5.00

.36

776

11

3.98

3.78

+2.75

+5.00

.29

697

12

4.10

3.92

+2.25

+5.25

.31

682

13

4.13

3.94

+2.75

+5.25

.34

529

14

4.14

4.01

+2.75

+5.50

.34

1,359

15

4.38

4.07

+2.50

+5.50

.29

3,690

16

4.45

4.13

+3.25

+5.87

.29

1,690

17

4.81

4.48

+3.75

+5.75

• •

351

18

4.80

4.46

+3.50

+5.50

• •

420

19

4.66

4.49

+3.50

+5.50

• •

280

20

4.66

4.61

+3.75

+5.75

• •

92

Total

17,250

dependently, at different times or places, and cannot be
combined for the reason that they behave as allelomorphs
in crosses. For this reason not more than two of them can be
brought into the same zygote, nor more than one into the
same gamete. We call them multiple allelomorphs.

I

UNIT-CHARACTERS AND SELECTION 187

A good example of multiple allelomorphism is found in
the several mutations which the color factor of rodents has
undergone. This is the factor which in its best known muta-
tion assumes the form of albinism. In guinea-pigs four alle-

TABLE 25a

Results of the Minus Selection' of Hooded Rats Continued through

Twenty-one Successive Generations

Generation

Mean Grade
of Parents

Mean Grade
of Offspring

Lowest

Grade

of Offspring

Highest

Grade

of Offspring

Standard

Deviation of

Offspring

Number of
Offspring

1

-1.46

-1.00

+ .25

-2.00

.51

55

2

-1.41

-1.07

+ .50

-2.00

.49

132

3

-1.56

-1.18

0.

-2.00

.48

195

4

-1.69

-1.28

+ .50

-2.25

.46

329

5

-1.73

-1.41

0.

-2.50

.50

701

6

-1.86

-1.56

0.

-2.50

.44

1,252

7

-2.01

-1.73

0.

-2.75

.35

1,680

8

-2.05

-1.80

0.

-2.75

.28

1,726

9

-2.11

-1.92

- .50

-2.75

.28

1,591

10

-2.18

-2.01

-1.00

-3.25

.24

1,451

11

-2.30

-2.15

-1.00

-3.50

.35

984

12

-2.44

-2.23

-1.00

-3.50

.37

1,037

13

-2.48

-2.39

-1.75

-3.50

.34

1,006

14

-2.64

-2.48

-1.00

-3.50

.30

717

15

-2.65

-2.54

-1.75

-3.50

.29

1,438

16

-2.79

-2.63

-1.00

-4.00

.27

1.980

17

-2.86

-2.70

-1.75

-4.25

.28

868

18

-3.09

-2.84

-2.25

-4.00

. .

330

19

-3.10

-2.89

-2.25

-4.00

, ,

130

20

-2.81

-2.78

-2.00

-3.50

. .

79

21

-2.58

-2.74

-2.00

-3.50

35

Total

17,716

lomorphic conditions of the color factor are known, (a) in-
tense pigmentation, (6) dilute pigmentation, (c) red-eyed
dilution, and (d) Himalayan albinism. In rabbits, the color
factor occurs in three forms, (a) ordinary pigmentation,
(b) Himalayan albinism, (c) pure white albinism. In rats
also the color factor occurs in three forms, but not the same

188 GENETICS AND EUGENICS

three as in rabbits. They are (a) ordinary pigmentation,
{b) ruby-eyed dilution (\\Tiiting) — perhaps homologous with
red-eyed dilution in guinea-pigs — and (c) albinism.

In Drosophila a factor for eye-color has been discovered
in several allelomorphic forms, such as white, eosin, buff,
cherry, blood, and red, analogous with the allelomorphs of
the color factor found in guinea-pigs.

The agouti factor of rodents occurs in the rabbit in three
allelomorphic forms, (a) ordinary gray, (b) black-and-tan,
and (c) non-agouti. In mice the agouti series includes (a) or-
dinary gray, (h) gray with white belly, (c) yellow, and (d)
non-agouti. In a cavy {Cavia rufescens) and its guinea-pig
hybrids, it has three forms, (a) agouti with light belly,
{h) agouti with ticked belly, and (c) non-agouti.

The extension factor has in rabbits three allelomorphic
forms (a) ordinary extension as in gray or black rabbits,
(6) "darkened" extension (DE, Punnett) seen in steel gray
rabbits, and (c) non-extension (restriction) seen in yellow
and in tortoise rabbits. Extension in guinea-pigs assumes
three alternative forms seen in (a) black, (6) tortoise, and
(c) yellow.

AMiite spotting shows numerous allelomorphic forms. In
rats (a) hooded pattern, (6) "Irish" pattern, and (c) self
pattern are allelomorphs. In rabbits, all known forms of
white spotting behave as allelomorphs. These include at
least three different patterns of Dutch marking as well as
English marking.^

In silkworms Tanaka discovered a series of three factors
for marking of the larva, which behave as allelomorphs,
although he prefers to describe them as factors completely
coupled. The three are Q (quail), Qs (striped quail) and Qm
(moricaud quail). He also observed several minor forms of
Q, which he designated QS Q^, Q^, and Q'^, which consider-
ably extend the allelomorphic series, but which differ so

^ But in mice two forms of white spotting are known which are not allelomorphic,
nor even Imked. These are known as black-eyed white and piebald respectively.
On the chromosome theory, they must be located in diflFerent chromosomes.

MODIFYING FACTORS 189

little one from another that the variation is practically
continuous.

Some gametic factors show their influence chiefly, if not
exclusively, m the form of changed action of other factors.
Thus the ordinary extension factor in rabbits produces with
the regular agouti factor an ordinary gray coat, but the
darkened extension factor produces with the same agouti
factor a steel gray coat. We think of the character of the
gray marking as a consequence of the agouti factor but find
in reality that it is changed by a change in the extension
factor, no less than by changes in the agouti factor. It is
assumed that there are many factors whose only discoverable
function is to modify the action of other factors and when
we find that some particular character, manifestly influenced
mainly by a single gene, has undergone slight change, or
continues to change progressively under continued selection,
it is safer to assume that modifying factors are concerned in
the matter than that the principal gene is gradually changing.

The substance of our present knowledge as to changes in
genes may be summed up in the statement that such changes
come or go suddenly and in their entirety, and cannot, so far
as we know, be influenced by selection or any other control-
able process. Hence we may well call changes in genes
mutations.

CHAPTER XXII

INHERITANCE OF SIZE AND OTHER QUANTITATIVE
CHARACTERS. THE HYPOTHESIS OF MULTIPLE FACTORS

Having observed how wide-spread unit-character variations
are and what an important part they play in the formation of
varieties of domesticated animals and cultivated plants, it is
natural to inquire whether any other sort of heritable varia-
tions occur, whether in the last analysis all inheritance is
Mendelian inheritance. This view is held by many students
of genetics at the present time. The cases of doubtful inter-
pretation relate chiefly to variations in size or shape of the
organism or of its parts, cases in which the characters under
observation vary continuously.

That size may be affected by ordinary Mendelian factors
has never been questioned. One of the seven unit-character
variations studied by Mendel himself was found in the cross
between tall and short varieties of peas. Tall was found to be
dominant and the alternative conditions, tall and short, were
observed to segregate in true-breeding types in F2. In man
brachydactylism was early demonstrated to be a dominant
unit-character, by Farabee confirmed by Drinkwater. In
this peculiar condition, the skeleton is shortened throughout,
and in particular the fingers are reduced from the usual
three-jointed to the short, two- jointed condition. An analo-
gous variation in Drosophila known as ''dachs" is inherited
in the "second chromosome" group of genes.

But the ordinary size differences between races of men,
breeds of animals, or varieties of plants, are not inherited in
this simple way, with dominance of one type, followed by
complete segregation from an alternative type. As a rule
intermediates or blends are produced in Fi (see Fig. 130). In
F2 the commonest type is still the intermediate as in Fi, but
variability is considerably increased, which may be regarded

190

Fig. 126. Angora male.

Fig. 127. Lop-eared female.

Fig. 128. Fi black half-lop.

Fig. 129. F2 albino half-lop.

Fig. 130. Skulls of mother (at left), of father (at right) and of son (between).

Compare Figs. 126-128.

SIZE INHERITANCE 191

as a tendency toward segregation of the original types.
These, the well established facts, were at one time regarded
as showing the occurrence of a distinct type of inheritance
known as blending, but at present we are inclined to give
them a different explanation, the same in fact as for ordinar\"
Mendelian inheritance except that several factors, instead of
one, are supposed to be concerned in the case, and that
dominance is not in evidence.

If a large rabbit is crossed with a small one, the young are
of intermediate size and the F2 offspring show no such segre-
gation into large, small, and intermediate-sized individuals
as a simple Mendelian system would demand. For if tlie
size difference between a large and a small rabbit depended
upon one unit-character, then the F2 animals should be as
regards size in the proportions, one large, two intennediate,
one small. But in the cases thus far studied all F2 individ-
uals are intermediate in size. A specific case illustrating the
point is the following: A cross was made between a large lop-
eared rabbit and a small short-eared one. The fonner was
also a sooty yellow animal and short-haired (Fig. 127); the
latter an albino and long-haired (angora). See Fig. 126. The
character of Fi is shown in Fig. 128. Notice first the smiple
Mendelian behavior of the color characters and the hair-
length. Albinism disappeared in Fi, for all the Fi aninuils
were black. But it reappeared in Fo; one Fo albino is shown
in Fig. 129. Long hair also behaved as a Mendelian recessive
(as in guinea-pigs), disappearing in Fi but reappearing in F-
as expected, sometimes in colored individuals, sometimes in
albinos, thus showing its independent inheritance. The
black character seen in the Fi individuals was received from
the albino (angora) parent, which had black ears. The black
character (dominant in Fi) was found in a majority of the
F2 colored individuals also, as we should expect, but the
yellow character of the other grandparent reappeared as a
recessive in F2 in certain of the individuals. Three inde-
pendent coat characters were thus Mendelizing in the cross,
viz..

192 GENETICS AND EUGENICS

Color dominant over albinism.
Black dominant over yellow.
Short hair dominant over long hair.

As regards ear-length, neither dominance nor segregation
of the difference between the parents is observable. All the
Fi as well as the r2 individuals have ears of intermediate
length. The inheritance is what has been called blending.
The same is true as regards size of the body.

In Fig. 130 the skulls of the parents are shown with the skull
of the Fi individual between them. In absolute dimensions,
as well as in the proportions of its parts the Fi skull is strictly
intermediate. The same blending effect was observed in all
other parts of the skeleton.

The multiple factor hypothesis. It is clear that in blending
inheritance there is no dominance, but the suggestion has
been made that nevertheless segregation may occur, and so
the inheritance may have a Mendelian basis. This suggestion
was first made by a Swedish plant breeder, Nilsson-Ehle
(1909) who obtained some very peculiar inheritance ratios in
crosses of wheat differing in color of seed or of chaff.

When a variety having brown chaff is crossed with one
which has white chaff, the hybrid plants are regularly brown
in Fi and three brown to one white in F2, but a particular
variety of brown-chaffed wheat gave a different result. In
fifteen different crosses it gave uniformly a close approxima-
tion to the ratio 15:1 instead of 3:1. The totals are suffi-
ciently large to leave no doubt of this. They are one thou-
sand four hundred and ten brown to ninety-four white,
exactly 15 :1. This is clearly a dihybrid Mendelian ratio, and
Nilsson-Ehle interprets it to mean that there exist in this
case two independent factors, each of which is able by itself
to produce the brown coloration, though no qualitative
difference can be detected between them.

A still more remarkable case was observed in crosses be-
tween varieties of wheat of different grain-color. Red crossed
with white gave ordinarily all red in Fi and three red to one
white in F2, but a certain native Swedish sort gave only red

HYPOTHESIS OF MULTIPLE FACTORS 193

(several hundred seeds) in F2. This result was so surprising
that one cross which had yielded seventy-eight grains of
wheat in F2 was followed into F3, with the following result:

Expected
50 plants gave only red seed (being homozygous) 37

5 " " approximately 63 R : 1 W (being trihybrid) 8

15 " " " 15 R : 1 W (being dihybrid) 12

8 " " « 3 R : 1 W (being monohybrid) 6

0 " " all white 1

The interpretation given by Nilsson-Ehle is this. The red
variety used in this cross bears three independent factors,
each of which by itself is able to produce the red character.
Their joint action is not different in kind from their action
separately, though possibly quantitatively greater. The F2
generation should contain one white seed in sixty-four. It
happens that none was obtained in this generation. The
next generation should contain, in a total of sixty-four indi-
viduals, the sorts actually observed as well as a sort which
would produce only white seed, the progeny namely of the
expected white seed of F2, but as that was not obtained, the
all-white plant of F3 could not be obtained either. The ex-
pected proportions of the several classes in F3 are given for
comparison with those actually obtained. The agreement
between expected and observed is so good as to make it seem
highly probable that Nilsson-Ehle's explanation is correct.
Corroborative evidence in the case of maize has been obtained
by East, and in shepherd's-purse by Shull.

This work introduces us to a new principle which has im-
portant theoretical consequences. If a character ordinarily
represented by a single unit in the germ-plasm may become
represented by two or more such units identical in character,
then we may expect it to dominate more persistently in
crosses, fewer recessives being formed in F2 and subsequent
generations. Further, if duplication of a unit tends to in-
crease its intensity, as seems probable, then we have in this
process a possible explanation of quantitative variation in
characters which are non-Mendelian, or at any rate do not
conform with a simple Mendelian system. Consider, for

194 GENETICS AND EUGENICS

example, the matter of size and skeletal proportions in rab-
bits. It is perfectly clear from the experiments described
that in such cases no dominance occurs, and also that no
segregation of a simple Mendelian character takes place, but
it is possible to explain the observed facts by the combined
action of several similar but independent factors, the new
principle which Nilsson-Ehle has brought forward. This is
known as the principle of multiple factors. Let us apply such
an hypothesis to the case in hand.

Suppose a cross be made involving ear-lengths of approxi-
mately four and eight inches respectively, as in one of the
crosses made. The Fi young are found to have ears about
six inches long, the mean of the parental conditions, and the
F2 young vary about the same mean condition. If a single
Mendelian unit-character made the difference between a
four-inch and an eight-inch ear, the F2 young should be of
three classes as follows:

Classes 4 in. 6 in. 8 in.

Frequencies 12 1

(Compare Fig. 131, bottom left, and Table 28.) The grand-
parental conditions should in this case reappear in half the
young. This clearly does not occur in the rabbit experiment.
But if two unit-characters were involved, Fi would be un-
changed, all six inches, yet the F2 classes would be more
numerous, viz., four, ^ve, six, seven, and eight inches, and
their relative frequencies as shown by the height of the
columns in Fig. 131, middle left, one, four, six, four, one.
The grandparental states would now reappear in one-eighth
of the F2 young, while three-eights would be intermediate.
It is certain, however, that in rabbits the grandparental con-
ditions, if they reappear at all, do not reappear with any such
frequency as this.

If three independent size-factors were involved in the cross,
the Fi individuals should all fall in the same middle group,
as before, viz., six inches, but the F2 classes should number
seven, and their relative frequencies would be as shown in

HYPOTHESIS OF MULTIPLE FACTORS 195

Fig. 131, top left. For four independent size-factors, the F2
classes would be more numerous still, viz., nine (Fig. 131,
right), and the extreme ear-size of either grandparent would
be expected to reappear in only one out of two hundred and
fifty-six offspring, while considerably more than half of them
would fail within the closely intermediate classes included
between five and one-half and six and one-half inches, the

I

I

Fig. 131. — Diagrams to show the number and size of the classes of individuals to
be expected from a cross involving Mendelian segregation without dominance. One
Mendelian unit involved, bottom left; two units, middle left; three units, top left;
four units, right.

three middle classes of the diagram. With six size-characters,
the extreme size of a grandparent would reappear no oftener
than once in four thousand times, while with a dozen such
independent characters it would recur only once in some
seventeen million times. It would be remarkable if under
such conditions the extreme size were ever recovered from
an ordinary cross.

196

GENETICS AND EUGENICS

From Table 28 it will be seen that when three like factors
are concerned, fifty to one hundred individuals must be pro-
duced to insure the recovery of the parental condition in F2;

with 4 like factors, 200-300 individuals must be produced ;
" 5 " " over 1000 " " '^ " ; and

« 6 " " " 4000 " " " « .

The foregoing calculations are based on the assumption
that each of the several hypothetical factors involved has an
equal influence in determining the general result and that all

TABLE 28

Theoretical Factorial Composition of a Population Produced by a

Cross Involving more than a Single Mendelian Factor,

Dominance being wanting

Factors

Frequencies of F2 Classes

Total

(=4°)

Number

of Homo-

zygotes

(=2°)

Per Cent

of Homo-

zygotes

1

1

2

1

4

2

50.0

2

, ,

, ,

, ,

1

4

6

4

1

. .

, ,

, ,

16

4

25.0

3

, ,

^ ,

1

6

15

20

15

6

1

, ,

, ,

64

8

12.5

4

, ,

1

8

28

56

70

56

28

8

1

, ,

256

16

6.2

5

1

10

45

120

210

252

210

120

45

10

1

1024

32

3.1

6

1

12

66

220

495

792

924

792

495

220

66

12

1

4096

64

1.5

are mutually independent (not linked). If, however, one or
more of the factors had greater influence than the others, the
apparent blending would be less perfect and a "tendency
toward segregation" or "imperfect segregation" would re-
sult. It is probable that this is the correct explanation of
what at one time was called "a type of inheritance inter-
mediate between Mendelian and blending." Also if certain
of the multiple factors were linked (borne in the same chro-
mosome), this would result in a tendency of the factors to
segregate in groups as originally introduced into the cross,
although crossing-over might lead to the production of
transitional types, any of which would "breed true" as soon
as all factors involved became homozygous.

SIZE INHERITANCE 197

A considerable number of cases of size inheritance has now
been studied in both animals and plants. Their results may
be summarized thus: (1) When animals or plants are crossed
which have racial differences in size or other characters, in
respect to which each race shows continuous variation about
a different mean, the Fi progeny are of intermediate size.^
They may or may not be more variable than the races
crossed, but quite commonly are not. (2) The F2 generation
as a whole commonly varies about the same intermediate
mean as the Fi generation, but its variability as measured by
the standard deviation or the coefficient of variation is usu-
ally greater than that of the Fi generation. The increased
variability of F 2 as compared with Fi may in extreme cases
include forms larger than the larger parental race or smaller
than the smaller race, and which show a tendency to vary
in F3 about the same size as characterized the F2 parent.

Some illustrative cases may be cited. Phillips (1912, 1914)
crossed two breeds of ducks which differed markedly in size,
namely Rouens and Mallards. The average adult weight of
the Rouen race used w^as, for males, two thousand three hun-
dred grams, and for females two thousand two hundred and
thirty-seven grams. Corresponding weights for the Mallard
race were one thousand sixty-eight and nine hundred and
twenty-eight grams respectively. The Rouens accordingly
were more than tw^ice as large as the Mallards. The two
races did not overlap in weight, as appears from Table 29,
where the animals are classified by weight. In this table the
mean weight of the Mallards is taken as the center of class
2 and the mean weight of the Rouens as the center of class 10.
Each sex was classified separately but the two are combined
in classes bearing the same class number in Table 29. The
seventy Fi offspring have their mode in the intermediate

1 I leave out of consideration here such differences as exist between till ami dwarf
peas, and between brachydactylous and normal men. In such cases a simple
Mendelizing difference exists, which shows both dominance and segregation in
typical fashion. Aside from this simple difference, however, ordinary size differ-
ences exist in such cases, which I doubt not follow the ordinary rules of size in-
heritance.

198 GENETICS AND EUGENICS

class 6, though they range all the way from class 2 to class 9.
The sixty-three Fa offspring likewise have their mode in
class 6, and are slightly more variable than Fi, though only
one aberrant individual falls beyond the range of Fi. This is
a case in which apparently many independent factors of
approximately equal influence on weight are concerned and
which do not segregate in linked groups. The result is that
both Fi and F2 vary symmetrically about the same strictly
intermediate mode (class 6).

A case in which fewer factors are involved or in which the
factors are either not all of equal influence or occur in linked
groups is the following. Punnett and Bailey crossed two
breeds of fowls differing w^idely in weight, the larger breed
being represented in a gold-penciled Hamburg cock, the
smaller in silver Sebright bantam hens. The relative size
of the breeds is shown in Table 29a. As male fowls are larger
than females, the weight of each sex is tabulated separately
in absolute weight units (grams). The weight of the Fi birds
was much nearer that of the larger than that of the smaller
parent breed, an indication that one or more of the factors
for large size show dominance. An alternative interpretation
would ascribe the large size of Fi to hybrid vigor. (See
Chapter XXVII.) Possibly each explanation is in part
correct. The F2 generation showed very great variability in
weight, covering the ranges of both parent breeds, so far as
those ranges had been ascertained for the material studied.
But the variation curve for F2 was not symmetrical about an
intermediate mode,as in the case of ducks studied by Phillips.
The mode was close to the Fi mode, but the variation was
very '*skew," ending abruptly above, but sloping gradually
downward to bantam size. When the more extreme F2 in
dividuals were mated, large with large and small with small,
broods were obtained which averaged larger than pure Ham-
burgs and smaller than pure Sebrights respectively.

Punnett interprets the case as involving four independent
factors having among themselves unequal influence on the
total weight. He supposes that three of the four factors are

SIZE INHERITANCE

199

TABLE 29a

Weight Inheritance in Fowls
After Punneft and Bailey (1914)

Weight classes in grams

500-

600-

700-

800- 900- 1000-1100- 1200-

1 ' 1

1300- 1400- 1500-1600-

Females, Hamburg. . . .
Sebright

1
3

3

1

17
1

25
4

27
7

6
36

1

• *

15

3

1
8
3

26
1

1

4
5

5
19

2

1
2

2

29

4

2

• •

1
1
9

2

• •

2
1

• •

2

• •

Fi

F2

F3 (from largest Fzs).. . .
F3 (from smallest F2S) . .

Males, Hamburg

Fi

F2

* *

F3 (from largest F2S) . . .
F3 (from smallest F2S) . .

2

1

borne by the Hamburg race, and one by the Sebright race.
Recombinations which inchide all four factors produce a race
larger than Hamburg, seen in the Fss from largest F2S.
Recombinations which include the four allelomorphs of
these factors produce a race smaller than Sebright, seen in
the Fgs from smallest FsS, Table 29a. He supposes further
that two of the four hypothetical factors exert a greater in-
fluence than the other two on the total size, the influence of
the first being to the second as 66 to 30. The figures are purely
provisional and are intended to indicate a form of explanation
which may cover such cases satisfactorily. But it must be
confessed that the number of individuals studied bv Punnet t
and Bailey is small and their assumptions as to the number
and potency of the hypothetical factors is quite arbitrary.

The extensive and carefully executed studies of Emerson
and East (1913) upon crosses of maize involving difl'erences
in size and other quantitative characters afford excellent il-
lustrations of the usual consequences of size crosses. The
simplest and clearest cut cases relate to the size of the ear or
of the seeds borne upon it. The behavior of ear-diameter in
crosses is shown in Table 30.

200 GENETICS AND EUGENICS

Both Fi and F2 are intermediate in character in comparison
with the parent races, but F2 is shghtly more variable. Dif-
ferent lots of Fi progeny (combined in Table 30) give co-
efficients of variability of 8.29 and 6.88 respectively, whereas
F2 progeny have coefficients ranging from 9.66 to 11.77. The
extreme ranges of the parent races are not attained in F2.
This case is similar to that of weight inheritance in ducks,
except that F 2 is on the average less than Fi, being more
nearly intermediate between the parental races than was Fi.
The case is probably complicated by hybrid vigor in Fi,
which is not retained in F2. (See Chapter XXVII.) It is
evident that so many independent factors are involved that
no complete segregation occurs in F2.

Table 31 shows the result of crossing two races of com (A
and B) differing in seed width. In this cross also, Fi, and F2
were alike intermediate, but the latter was slightly more
variable. It was found that the F2 plants differed in genetic
character as to seed width. An F2 with low seed width (143
mm.) produced an F3 likewise low (mean 141.3 mm.); and
F2 with seed width above the average (178 mm.) produced
an F3 of like character (mean 172.9 mm.). The range of the
low selected F3 extended even lower than the range of the
uncrossed low race (B), which is similar to the result obtained
by Punnett and Bailey in the weight inheritance of fowls and
suggests a similar explanation, recombination of factors.

Some instructive cases involving multiple factors affecting
the size and shape of fruits have been studied by Gross.
See Fig. 132. It is evident that in these cases length and
width of the fruit are affected by numerous independent
factors which recombine so as to form a complete series of
intergrading forms.

In garden peas the time between germination of the seed
and flowering varies greatly in different varieties. In early
varieties the time is short, in late varieties it is relatively long.

Hoshino crossed two varieties of garden peas which had
been found to breed very true as to flowering time and flower
color. One variety was early and white flowered, the other

SIZE INHERITANCE

^201

was late and red flowered. Fi was very uniform also, bein^
red in flower color and nearly as late in flowering as the late
parent. F2 showed regular Mendelian segregation as to flower
color into three reds to one white. As regards time of flower-

TABLE 29

Weight Distribution of Ducks

After Phillips {1912 and 1914)

Class Number

1

2

3

4

5

6

7

8

9

10

11

12

13

Rouen

3

3

9

5

1

1

Mallard

23

54

23

2

. .

. .

. ,

. .

. .

. .

. .

, ,

, ,

Fi

, .

1

1

4

21

35

6

1

1

. .

, .

• •

. •

F2

• •

1

6

10

22

13

9

1

1

TABLE 30

Ear-Diameter in Crosses of Maize

After Emerson and East (1913, p. 56)

Class
means

22.5

24.5

26.5

28.5

30.5

32.5

34.5

36.5

38.5

40.5

42.5

44.5

46.5

48.5

50.5

52.5

54.5

56.5

in mm.

Race A . .

..

2

2

2

4

2

4

5

2

RaceB..

15

32

14

3

1

, ,

, ,

. .

. .

. .

. .

. .

. .

. •

• .

• •

Fi

1
35

2
38

4
69

8
50

11
43

21
30

12
17

6

8

1

• •

F2

2

6

23

Class means

in mm. (25

seeds)

Race A . . . .

Race B

Fi

F2

F3 (from Fi,

class 143)
F3 (from F2,

class 178)

113

118

TABLE 31

Width of Seed in Crosses of Maize

After Emerson and East (1913, p. 63).

123

128

133

10

138

4

5

13

143

7
15

148

1

i2

9

1

153

1

10

8
1

158

1
31

6

5

163

2
23

168

3
24

1

5

173

2
15

178

4
26

10

183

3
12

13

188

6
12

193

198'203 208 213 218 223 228,233 238

202

GENETICS AND EUGENICS

ing, F2 was intermediate but highly variable, covering prac-
tically the entire range from the flowering time of the early
to that of the late parent. F3 was also highly variable but a
few families were found to be as ''constant " in flowering time
as the parent varieties, and in r4 the proportion of constant
families had increased further. Two hundred and thirty of

Fig. 132. A cross of two varieties of peppers differing greatly in size and shape of fruits. Fruits of
the parent varieties are shown at P and P, of Fi between them, and of F2 in the four lower rows.
Each fruit is taken from a diflFerent plant and is typical for the plant. (After Gross.)

the four hundred and twenty-one F4 famihes studied by
Hoshino were found to be as "constant" in flowering time
as the parent varieties. The mean flowering time in days
from sprouting as observed by Hoshino is given in Table 32.
It will be observed that the white-flowered F4 constant
families were all early or intermediate in flowering time
whereas the red-flowered families were chiefly late. This
clearly indicates linkage, or coupling, between flower color
and time of flowering. But flower color clearly Mendelizes,
hence flowering time must also depend upon a Mendelizing
gene, which is linked with the gene for red flower color.

GAMETIC PURITY

203

Cross overs occasionally occur resulting, for example, in the
F4 pure red early family shown in Table 32. But if these two
were the only genetic factors involved in the cross, no *' con-
stant" families of intermediate flowering time could result

TABLE 32

Variation in flowering time of two pure varieties of garden peas, one Early
White, the other Late Red; and a classification, both as to color and as to flowering
time, of two hundred and thirty F4 families produced by crossing the two varieties,
these F4 families being all regarded as " constant " in flowering time because of
their low variability, as low as that of the parent varieties. Only the position of
the mean of each r4 family is given in the table, not its range as in the case of the
parent varieties.

Days to Flowering

32

33

34 35

36

37 38 39 40 41 42 43 44 45 46 47 48

Early White Parent . . .
T^ate Hed Parent . . .

1

2

11 7

9

13 7 .. 1 ..

F4 White Families ....
F4 Mixed Families,
White or Red

• •

1 13
.. 1

5

1

2 . . 3 18 12 15 14 13 12 3 . . . .
1 .. 1 1 .. 1 1 4 2

Ya Red Families

.. 1

1 1 3 12 13 4 6 2 . .

Total " Constant " F4
Families

1 15

6

3 4 20 13 19 27 30 18 9 2

25

142

Days to Flowering

49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Earlv White Parent

Late Red Parent

5 11 10 11 10 7 10 1 .. 3 2 1

Va W^Viitp T^^amilips

F4 Mixed Families, White

. 1 3

Ya Red Families

5 3 3 2 10 13 3 10 8 1 1

Total " Constant " F4
Families

5 3 3 2 11 16 3 10 8 1 1

-^

63 = 230

from the cross. As a matter of fact more than half the F4
families are of this constant intermediate type, which shows
that one or more other factors, independent of the chief
factor for flowering time, must be concerned in the result.
Hoshino supposes that a single supplementary factor (not

204 GENETICS AND EUGENICS

linked with flower color or with the chief factor for flowering
time) will account for the case. In accordance with this
view, four true-breeding combinations of the factors for
flowering time might be expected, and it is possible that their
modes fall in Table 32 on 35 days, 40 days, 44 days, and 54
days respectively, all of which show high frequencies. An-
other possibility is that several modifying factors acting in
various combinations produce the wide ranging group of 142
** constant" intermediate families and that linkage among
these modifying factors is responsible for the apparent dis-
continuity between the intermediate and the early and the
late groups. Certainly more than one supplementary or
modifying factor is in evidence. For it is to be remembered
that in Table 32, the F4 distribution is not that of individual
plants varying round particular modes, but each frequency
indicated is itself the mode of a family, ** constant as to that
particular modal length of time between sprouting and
flowering. Accordingly the ''constant" varieties resulting
from the cross are not four only, as a two factor scheme would
demand, but their number is very great, since they range
with only two apparent breaks all the way from the original
early to the original late variety. Such a result could be
produced only by numerous modifying factors, which in
action supplement, or else inhibit, the action of the chief
gene for flowering time so clearly linked with red flower
color in transmission. No other "factorial" explanation
seems admissible.

CHAPTER XXIII

GENETIC CHANGES AND THE CHROMOSOMES

In one way our views concerning heredity have been consid-
erably simplified by the discovery that blending inheritance
may be included in the category of Mendelian inheritance.
One mechanism will now suffice for all kinds of inlicri-
tance, this mechanism being found in the chromosomes.
In them, we may reasonably suppose, is found the material
basis of every inherited character. When the inheritance
is of the simplest kind, involving presence or absence of
color or some similar character, we assume that a genetic
change has occurred in a single, definite locus in a particular
chromosome, and that this single change is responsible for
the observed inherited variation. Other characters depend
on two or more genes, which may lie at different loci in the
same chromosome, or even in different chromosomes. Thus
the gray coat of a rabbit is an inherited character which de-
pends on at least five different genes, each of which appar-
ently lies in a different chromosome. These are (1) a color
factor, (2) a black factor, (3) an extension factor, (4) an
agouti factor and (5) an intensity factor. Each of these
factors or genes behaves as an independent unit in trans-
mission. We know of their existence only because each of
them has been observed to occur in two or more alternative
forms. For a gene which remains unchanged remains un-
known. We do not know how many undiscovered genes are
concerned in producing the gray coat of a rabbit, nor in what
linkage-systems (chromosomes) they he. These few have
revealed themselves by their striking variations. By various
combinations of the different forms of these five genes, we
get all the known color varieties of gray, black, yellow, and
white rabbits. When it comes to the inheritance of size
differences among rabbits, we suppose that genes affecting

205

206 GENETICS AND EUGENICS

size are involved and that they also are located in the chro-
mosomes. But it is clear that the size genes must be numer-
ous since the inheritance of size is blending, and they are
probably located in many different chromosomes or even in
all the different chromosomes. That genes do affect size is
shown by the t;yT)ical Mendelian behavior of the characters
tall and short in crosses of peas, and brachydactyl and nor-
mal in human families. The case studied by Hoshino in
which late flowering in peas was found to be coupled with
red flow^er color is important because it shows that a gene
which affects a quantitatively varying character, one which
blends in heredity, is located in the same chromosome with a
color gene. There is no reason to think that any genes occur
elsewhere in the gamete than in the chromosomes.

An apparent exception occurs in the case of a plant,
Mirabilis, the cultivated four-o-clock, studied by Correns.
A single plant of this species arose in his cultures, which had
white-margined leaves, the white areas being due to an ab-
normal condition of the normally green plastids, which are
cytoplasmic (not nuclear) structures. Sometimes entire
branches arose on this plant (or its descendants) which were
white, and which contained only colorless plastids, and others
which were green containing normal colored plastids. White
branches produced only white seedlings when self-pollinated
and green branches produced only green seedlings. When
flowers on the two sorts of branches were intercrossed, the
seeds borne on white branches still produced only white
seedlings, and those borne on the green branches produced
only green seedlings, which thereafter bred true. The white
seedlings perished because without chlorophyl they could not
live. The case has been explained as one in which inheritance
is exclusively maternal, by means of the ^gg but not by
means of the pollen. Further it is a cytoplasmic structure,
the plastids of the egg, which determine the plastid character
of the offspring, the nucleus not being concerned in the proc-
ess. Here then we seem to have a case of cytoplasmic in-
heritance in which nuclear genes are not concerned. But a

GENETIC CHANGES 207

more careful study of the case makes it seem probable tliat
we are here dealing with a pathological condition of the
cytoplasm rather than with true inheritance. Consider a
similar case in animals. The organism which produces Texas
fever in cattle is introduced into the blood of cattle b\' tlic
bite of a diseased tick. Among ticks, the disease passes from
mother to offspring in the cytoplasm of the egg. The sperm is
too small to carry the disease germ, and so the disease does
not pass from father to offspring in the sperm. In reciprocal
crosses between diseased and healthy ticks, if such could be
made, we should observe exactly the same mode of trans-
mission as in the four-o-clock crosses between white branches
and green branches. The offspring would always show the
condition of the mother, never that of the father. But we
should hesitate to describe the transmission of a foreign or-
ganism in the egg of a tick as inheritance, and the same
hesitancy should be shown regarding the transmission of
diseased chloroplastids in the cytoplasm of Mirabilis}

Leaf -variegation, quite similar in appearance to that just
described, but of truly genetic origin, occurs also in Mirabilis
and was studied simultaneously by Correns. This is trans-
mitted alike in egg-cell and pollen, as a recessive character,
which shows that the gene concerned is probably borne in
the nucleus. In certain other plants (Antirrhinum, Melan-
drium) leaf -variegation is a dominant character transmitted
equally by both sexes. In fact, in a great majority of cases,
variegation is inlierited as an ordinary Mendelian character,
either dominant or recessive, and so may be explained as
due to genes contained in the nucleus. The exceptional
cases are cases of cell pathology rather than of inheritance.

If then, as seems probable, genes located in the chromo-
somes constitute the sole vehicle of inheritance, it follows
that heritable variations can arise only from changes in the
genes. Such changes are called *' mutations," of which we
can distinguish the following varieties:

1 Similar cases of "maternal inheritance" have been studied by Baur in Antirrhi-
num, by Gregory in Primula, by Ikeno in Plantago, and by Winge in Hunmlus.

208 GENETICS AND EUGENICS

(1) Change in a single gene, ordinary unit-cliaracter varia-
tion, mutation in the sense of Johannsen and Morgan.

(2) Doubling of the normal chromosome number, pre-
sumably resulting in a duplication of every gene of the
normal gamete, the duplicate condition being handed on per-
manently from generation to generation. This is the '*gigas "
type of mutation first observed by De Vries in the case of
Oenothera, later in Primula by Gregory, and in the night-
shade and the tomato by Winkler.

(3) Addition of a single extra chromosome to the regular
number in the gamete, probably by duplication of a single
chromosome. This is the "lata" type of mutation as ob-
served in Oenothera, and it is related to the phenomenon of
non-disjunction as observed by Bridges in the case of the
sex-chromosome in eggs of Drosophila.

(4) Loss of a definite part of the sex-chromosome of Dro-
sophila, has been described by Bridges under the name ''de-
ficiency." This involved the simultaneous disappearance
from a single chromosome of at least two neighboring genes.

Of these four varieties of mutation, the last two may be
regarded as rare and more or less pathological phenomena,
the second leads occasionally to the sudden origin of a new
variety of flowering plant, and may have functioned in the
evolution of some of the lower plants (mosses, algae) and
lower animals, but beyond a doubt the first mentioned vari-
ety of mutation, spontaneous change in single genes, is the
usual. and continuously operative method by which genetic
changes arise in both animals and plants. To this we must
look for that unceasing variability of organisms which fur-
nishes the material for natural selection to operate upon and
for men to work with in the improvement of the domestic
animals and cultivated plants.

CHAPTER XXIV

GENETIC CHANGES IN ASEXUAL REPRODUCTION IN
PARTHENOGENESIS, AND IN SELF-FERTH^IZATION

The frequency of occurrence of variation in single genes ap-
parently is very different in different species of animals and
plants, and in different modes of reproduction. It is sup-
posed to be commonest in organisms which reproduce only
sexually but it must be remembered that sexual reproduction
favors the spread of any genetic change which happens to
occur, whereas under asexual reproduction a mutation, how-
ever favorable, has no chance to spread from the family in
which it originated to others of the same species. Accord-
ingly mutation (in single genes) may seem to be less common
than it really is, in organisms which are propagated asexu-
ally. It is only w^hen systematic search is made for genetic
variations that we gain any adequate idea of how com-
monly they occur. Jennings was the first to take this matter
up in connection with the asexual reproduction of the pro-
tozoan, Paramecium. He was unable to detect any genetic
changes in size in races of Paramecium reproducing by fission,
but in a soft-bodied animal like Paramecium in which body
size is constantly changing, measurement of size is not an
easy matter. Later Jennings sought more favorable ma-
terial for study and apparently found it in a shelled proto-
zoan, Difflugia. This has a definiteness and rigidity of form
which is wanting in Paramecium. Its shell can be measured
with great exactness and the number of spines which it bears
can be counted, and their length measured. In the case of
Difflugia Jennings found that differences in size, number of
spines, and length of spines may be observed among the
asexually produced descendants of a single individual tliat
in consequence of selection these differences become strength-
ened and divergent races are thus created. Hegner has ob-

209

210 GENETICS AND EUGENICS

served the occurrence of similar genetic changes in Arcella.
It is evident that on the theory that genes are the exclusive
vehicles of inheritance, it must be supposed that genes are
undergoing change rather frequently in the asexual repro-
duction of Difflugia and Arcella.

In the asexual reproduction of plants genetic changes
known as bud-variations occasionally occur. East (1910)
has observed, in the reproduction of the potato by tubers,
changes in the shape, color, or depth of eyes of the tubers,
such as are known to behave as simple unit-character varia-
tions in reproduction by seed. It seems probable therefore
that they have arisen as changes in single genes occurring in
asexual reproduction. In the propagation by budding of
citrous fruits and of prunes, according to Shamel, genetic
changes of commercial importance occur with so great fre-
quency that it seems desirable to take budding stock only
from carefully selected trees within the variety. The varia-
tions noted affect especially the shape and size of the fruit,
or the vigor and productiveness of the tree. Shamel de-
scribes thus the recent origin of a new and improved variety
of the French prune. (See Fig. 133.)

In 1904, in a French prune tree growing in an orchard near Saratoga,
Cat, one branch high up in the tree was found bearing very large fruits.
There is no question as to its being a true bud variation. Several grafts
were secured from this branch and placed in bearing peach trees in order
to secure early evidence as to whether this variation, or bud sport, could
be propagated. The fruits produced by these grafts were found to be iden-
tical to those borne by the original branch. The large fruits possessed all
of the desirable characteristics of the smaller fruits of the ordinary French
prune and, in addition, possessed the desired improvement in size.

In order to give this strain a commerical test Mr. Coates bought 10
acres containing about 1000 peach trees for experimental trials of the
large prune variety. These trees were five years old in 1914 at the time
of their purchase. The large-fruited French prune variety was budded
into every other row of the peach trees with the usual method practiced
in top-working citrus and other fruit trees.

The top-worked trees with the improved French prime strain, called
No. 1418 for convenience during the experimental stages, are in alternate
rows with the ordinary or other selected strains of the parent variety.
In other words, in the 10-acre experimental orchard there is one row of
No. 1418 followed by a row of the parent variety, and so on throughout

Fig. 133. Origin of a new and improved variety of French prune by a bud-variation. Top row, leaves,
fruit and seed of the new variety; bottom row, leaves, fruit and seed of the parent variety, shown ou
the same scale. (After Shamel.)

■CV53*

■^i

t

\ .,i

.-./-

w^

' ..- ^ ML

i^M-^

I '1

^V r \,--v

\

H

i )

-^ V

Fig. 134. Eight different types of variegated seeds of maize, which behave in general as allelomorphs
One to another. But mutation from one type to another is common, only the end types of the scries,
A and H, being as stable as ordinary Mendelian allelomorphs. (After Emerson.)

Fig. 135. Bud variation (mutation) in a plant
of maize. The lower ear bears chiefly light
colored seeds, the upper ear bears chiefly dark
colored seeds. Each kind, as a rule, reproduces
its own sort. (After Emerson.)

Fig. 136. Bud variation (mutation) in single ears
of maize. Each ear bears patches of very dark
seeds among the generally light colored s«H'd.s.
Dark seeds and light seeds reproduce each its
own sort, as a rule. (After Emersoa.)

GENETIC CHANGES 211

the entire orchard. The conditions are comparative and furnish the hasis
for a fair comparison of the No. 1418 strain trees with those of the parent
variety.

The yield of the No. 1418 trees in the experimental plant in<,' lias heen
more than double that of the comparative trees. The No. 1418 fruits are
about twice the size and weight of the comparative fruits. They are more
uniformly distributed throughout the tree than is the case with the fruits
borne by the comparative trees. Furthermore, the fruits are more uni-
form in size, shape and other characteristics than are the fruits of the
ordinary variety. So uniform are the No. 1418 fruits tliat they api)ear
to have been graded mechanically as to size as they lie on tlie ground
after falling.

The No. 1418 trees appear to be more vigorous growing and develop
larger leaves than do the comparative trees. The leaves of the trees of
this strain appear to be thicker and have a tougher feel than do the leaves
of the trees of the parent variety. In looking down the rows one notices
that the larger trees of the No. 1418 strain, with their more luxurious
and abundant foliage, stand out markedly as compared w^ith the trees
and leaves of the parent variety.

The fruits of the No. 1418 strain average about 25-30 to the pound as
compared with an average of from about 50-60 to the pound as is the
case of the fruits of the parent variety.

The increased size of leaves and fruit and the great vigor of
the tree, suggest that in this case a "gigas type" of bud-
mutation has occurred rather than change in a single gene.
Cytological study might reveal whether this is the case or not.
In plants with variegated leaves, such as Pelargonium
(Baur) and Coleus (Stout), it is easy to change the racial
proportion of green to white or green to colored areas b}'
vegetative selection, that is by selection from among the
vegetatively propagated offspring of a single mother plant.
Apparently in such cases what is varying is the plastid con-
tent of the cytoplasm of cells rather than their nuclear
structure, but the studies of Emerson and of Hayes u])()n
variegation of the seed-coat in maize show that in this case
there is a close correlation between the somatic variation
(seen in the seed-coat) and the variegated character trans-
mitted by the embryo within the seed, so that selection on
the basis of the former is attended by genetic change of ;i
corresponding sort within the gametes. It cannot be doubted
therefore that, in practically all cases of variegation in

212 GENETICS AND EUGENICS

«

plants, real genetic changes are involved whenever selection
on the basis of vegetatively produced individuals or struc-
tures is found to change the racial character. Such a rela-
tion has been observed to hold in all cases thus far carefully
studied.

In regard to variegated seed-coat in maize, Emerson and
Hayes are agreed that the chief genetic changes occur in one
and the same gene, which results in producing a series of
multiple allelomorphs. Hayes recognizes four allelomorphs
in the same series, Emerson **at least nine or ten." The num-
ber is probably limited only by the ability of the observer
to discriminate them. Besides variation in a single gene,
Hayes assumes additional '* slight germinal variations,"
probably to be understood as changes in other genetic loci,
possibly located in other chromosomes and functioning as
'* modifying factors." Emerson finds that some states of
the chief gene for variegated seed-coat in maize are appar-
ently more stable than others, since some members of the
multiple allelomorph series are observed to mutate less fre-
quently than others. Thus, ''self-colored and colorless races
are," he says, **as constant probably as most Mendelian
characters," but the truly variegated or intermediate types
mutate much more frequently, from one type of variegation
into another, or even into the more stable self-colored and
colorless types.

Parthenogenesis in animals, like vegetative reproduction
in plants, when as commonly it occurs without the forma-
tion of gametes, affords an opportunity to observe how com-
mon genetic changes are. For in such cases no reduction of
the chromosomes occurs, there is no segregation of duplicate
genes, and there is no opportunity for the production of new
character combinations as a result of union of gametes in
fertilization. Genetic changes can in such cases occur only
under conditions comparable with those of bud-variation in
plants. Banta has observed for long periods, extending into
hundreds of generations, the successive parthenogenetic gen-
erations of small Crustacea known as water fleas (Simoce-

GENETIC CHANGES 213

phalus, Daphnia) with a view to detecting genetic changes,
if such occur. His attention has been centered upon the
characters which distinguish females (the ordinary partheno-
genetic individuals) from the more rare males. He has ol)-
served the occurrence as mutations in Siviocephalus reiuhis
and in several different strains of Daphnia longis-plna, ul'
what are called ''sex intergrades," individuals intermediate
in character between males and females as regards the sex-
differentiating characters both primary and secondary, or
showing various combinations of the several characters
which ordinarily distinguish the sexes. ^ That these varia-
tions are due to real genetic changes is shown by their occur-
rence in parthenogenetic lines descended (asexually) from a
common mother individual; that their occurrence is not rare
is shown by the fact that five out of six lines of Daphnia
under observation in the year 1918 w^ere observed to give
rise to strains of sex intergrades. Further, such changes did
not occur in single lines once only and cease thereafter. SLx
lines were propagated from the descendants of a single mu-
tant sex intergrade, and selected, three toward normal female-
ness, three toward maleness. The selection is characterized
as ** somewhat effective." "In most later generations," says
Banta, '*the stock in the strains selected away from the
intergrade characters has been moderately or only slightly
intergrade, while in some cases the stock has been almost
wholly normal female. In the strains selected to make them
strongly intergrade, the stock has usually been strongly in-
tergrade. ... In general there is a fairly pronounced dif-
ference between the characters of the stock in strains selected
toward femaleness and in strains selected toward a more

1 Banta enumerates five easily recognized secondary sex-characters in Daphnia.
See Fig. 137. These are (1) Body size, greater in females than in males; (2) outline
of the head, forming a beak in the female but not in the male; (3) .fize ami character
of the first antenna, well developed in males but rudimentary in females; (4) outline
and hairiness of ventral anterior margin of carapace, whic-h in males forms almost a
right angle and is hairy, but in females is rounded and hairless; (5) character of
first thoracic appendage, in males with a hook-shaped finger-like projection, in fe-
males without hook and branched into many long terminal filaments.

214 GENETICS AND EUGENICS

strongly intergrading condition." This fact shows that minor
genetic changes have occurred subsequent to the original
mutation either in the same genetic locus, or loci, or in 1
other genetic loci. Banta has shown that the degree of in- ^
tergradeness is considerably influenced by environmental
conditions, but that the facts are as stated, when all needed
control observations are made. This leads to the strong con-
viction that genetic changes probably occur with consider-
able frequency in the parthenogenesis of animals as well as
in bud- variation among plants. ^ There is, however, some
negative evidence on record. Ewing selected for forty -four
parthenogenetic generations a species of plant louse. Aphis
avenoB, which was observed to vary as to length of body,
length of antennae, and length of cornicles (honey-dew tubes) .
All the observed variations were apparently due to environ-
mental conditions, because no permanent modification of
the race was effected by selection. The variation curve went
up and down with change in environmental conditions
(temperature and the like) but returned to normal when
normal conditions were restored. Hence it appears that
genetic variations affecting size were not occurring with any
considerable frequency, if at all, in the particular characters
studied at the particular time they were studied. This is
not surprising when we consider what a specialized organism
a plant louse is, adapted and limited to a particular host
plant. But a single positive case, like that studied by Banta,
outweighs any number of negative cases so far as concerns

^ Banta (1919) has also studied the effects of selection, in pure line cultures of
Simocephalus vetulus, upon the sensitiveness of this species to light stimulation, as
measured by its reaction time. The selection experiment was continued for 54
months, 181 parthenogenetic generations. In the first two-month period, no differ-
ence could be detected in the average reaction time of plus selected and minus
selected strains of the same pure line, but subsequently the two strains gradually
diverged in reaction time so that "in the final ten generations the strain selected
for greater reactiveness to light had a reaction time less than one-third as large as
that for the strain of the same line selected for reduced reactiveness to light." No
differences in general vigor between the selected lines could be detected. The
change was a specific one in relation to light reactiveness and had been attained
gradually.

Fig. 137. Male tabove) and female (below) of Daphina longispina. Note striking differences as
regards a, antenna; br, breast (.ventral anterior margin of carapace); bk; beak; ap, first thoracic
appendage. Sex intergrades may have any combination or intermediate condition of the male and
female characters shown. (After Banta.)

i .

]■ I

GENETIC CHANGES 215

showing the possibility of the occurrence of genetic change
outside of sexual reproduction.^

Self-fertilization among plants is almost as favorable as
parthenogenesis or as vegetative reproduction for showing
genetic changes, if they occur. For in self-fertilization both
egg and pollen gametes are furnished by the same parent
individual. Johannsen first advanced the view that when
such a parent individual is homozygous for all genetic
factors, no genetic changes will be observed among the de-
scendants, which will continue generation after generation
to constitute a '*pure line." He substantiated this view by
studies of size variation in successive generations of self-
fertilized beans. He found in a number of cases that no
change in size resulted from selecting in successive genera-
tions either the largest or the smallest beans borne on the
same mother plant and concluded that such plants were
homozygous for all genetic factors affecting size of seed, and
that the observed variations in size upon which his selec-
tions had been based were due to environmental agencies
such as the position of the bean in the pod and the consequent
amount of material available for storage in the seed, which
conditions were not subject to inheritance.

The case is very different if one selects by size beans borne
on a plant heterozygous for genetic size factors (as for ex-
ample an Fi plant from a cross between a large-seeded and
a small-seeded race of beans). Under those conditions races
differing in average seed-size are quickly segregated (Emer-
son) . Johannsen's observations show that genetic variations
affecting the seed-size of beans are not of frequent occur-
rence, yet he has himself recorded the occasional occurrence

^ A very puzzling case of genetic change in parthenogenesis is recorded by Na-
bours. He observed in grouse locusts (Apotettix) the development of offspring from
unfertilized eggs which showed unmistakable segregation of characters and even
crossing-over among linked characters for which the mother was heterozygous. All
the offspring, however, were of the female sex, indicating that the eggs from which
they developed had not undergone reduction as regards the sex determinant, thougii
it would appear that they must have undergone reduction as regards other char-
acters. Cytological study of such material should prove interesting.

216 GENETICS AND EUGENICS

of mutation within a pure line of beans. That such mutations
must occasionally occur or at least have occurred in times
past is shown by the very existence of races differing in
genetic constitution. By crossing these we can produce in-
termediates of any desired size. This shows that the genetic
differences between them are numerous (on the multiple
factor h;^^othesis) and numerous genetic differences have,
most probably, not originated at one time or place. Studies
of other self -fertilizing plants, such as peas, oats, wheat and
tobacco, support the view that genetic variations in such
species are rare as compared with the variability to be se-
cured by artificially crossing different varieties, in which the
beneficial genetic changes of centuries may have accumu-
lated. All these are immediately made available for recom-
bining in every possible way with the genetic variations ac-
cumulated in any other variety, when the two are artificially
crossed. Any advantageous genetic variations which have
made their appearance in a self-fertilizing plant, from the
time it was taken into cultivation to the present time, are
likely to be found in varieties now in cultivation, since if
such variations had survival value they would naturally in-
crease and come to predominate in the crops of successive
years, even if no conscious selection was exercised. Acci-
dental cross-pollinations, such as are known to occur occa-
sionally in any species normally self-fertilized, would give
opportunity for combination to arise of two or more advan-
tageous genetic variations, distinct in origin. Subsequent
self-fertilization for ten or more generations would establish
in homozygous lines all possible combinations of the genetic
factors introduced in the accidental cross. Thus it happens
that a field crop of any seK-fertilizing plant contains a great
number of pure lines, each considered by itself a pure-breed-
ing homozygous variety. In such cases the work of the plant
breeder is very simple. He has only to isolate the varieties
which nature gives him ready-made and test these out to
determine which can most profitably be grown in a particu-
lar region or under a particular set of field conditions. In

GENETIC CHANGES 217

farm practice elimination from the seed planted of all but
the best pure hues, may greatly increase the total yield.
This in many cases has actually been done. The work of
determining what are the best pure lines and of increasing
these to the exclusion of all others is the main work of the
plant breeder. It is a work which will have to be done over
again generation after generation, because impurities will
creep in from accidental crossing with inferior ^'a^ieties or
from the occasional origin within the pure line of new genetic
changes for the worse, for quite as many of this sort occur,
probably, as of those which are for the better. Besides dis-
covering and isolating, as pure lines, homozygous strains of
favorable genetic variations, as they occur in commercial
field crops, the plant breeder has a second important function
to perform in connection with plants normally self -fertilized.
He may, by artificially crossing varieties, combine the excel-
lent qualities which they severally possess. It often happens
that favorable variations which have arisen in the crops of
one country are unknown in those of another country. The
plant breeder may bring together the best varieties of all
countries, determine the good qualities of each and then by
suitable crosses combine these in new varieties adapted to
special conditions or particular regions. Never in the history
of the world has this been done on so extensive a scale or
with greater success than in the United States at the present
time.

To return to the point of our departure, how common in
occurrence are genetic changes in self -fertilizing plants.^ An
answer to this question can be made only in relative terms.
It is scarcely safe to assume that they never occur. The very
existence of numerous genetically different pure lines in
every self -fertilized crop shows that genetic changes have
occurred in the past, and if so they are doubtless occurring
today. Some refer all such multiplicity of varieties to past
hybridizations of species genetically different, but this is
only referring to a more remote period the genetic changes
which are uivolved in the origin of the hypothetical species

I

218 GENETICS AND EUGENICS

themselves. The genetic changes must have occurred some-
time if related species really had a common origin as we,
under the Darwinian theory, suppose.

Moreover, a cultivated plant, regularly self-fertilized, the
sweet pea, whose historic origin from a single wild species is
known, exists today in hundreds of true-breeding varieties
differing one from another in genetic constitution. All these
genetic changes have occurred within a few centuries and in
most if not in all cases within what were at the time prob-
ably *'pure lines."

A common answer to the question proposed is that genetic
changes in pure lines are comparatively rare. Rare in com-
parison with what.^ With the genetic variations already
existing in the same species. But the latter are accumula-
tions of the genetic changes of centuries, or in the case of
cultivated wheat, of thousands of years. Is it surprising that
in comparison with such accumulations of variations, the
variations observed contemporaneously to occur in pure
lines are relatively few? Practically, it would be, as has
often been said, a '* waste of time" to look for the occurrence
of favorable genetic changes within pure lines of self -fertiliz-
ing plants, so long as a wealth of untested varieties exists
ready made in every commercial variety of such crop, and
an even greater number of new varieties may be created by
crossing the best existing varieties. But this is not to be
regarded as evidence that genetic changes have not come
about in the past exactly as they are coming about today,
within lines pure or otherwise.

CHAPTER XXV

GENETIC CHANGES IN BISEXUAL REPRODUCTION

How common are genetic changes in ordinary bisexual re-
production? This question also can be answered only in
relative terms. Few organisms have been studied intensively
enough and for enough generations in succession to enable
us to answer the question intelligently. Drosophila melano-
gaster has probably been studied more thoroughly than any
other species, these studies, because of the rapid reproduc-
tion of Drosophila, extending over hundreds of successive
generations. No other organism has yielded such a great
number of known distinct genetic variations, but at first
their discovery came rather slowly. Improved technique and
training on the part of observers enabled them to recognize
more and more genetic changes. Those discovered within
ten years have mounted in number into the hundreds. There
is reason to think that a goodly proportion of these genetic
changes have actually occurred (not merely been discovered)
during the period of laboratory study of Drosophila at Co-
lumbia University. Some of them have been observed to
occur independently at different periods and in unrelated
stocks of flies. This indicates that in the best-known genet-
ically of all organisms the genes are extremely numerous and
are subject to rather frequent changes, for we are acquainted
only with such genes as have revealed themselves by under-
going change. The first discovered gene was that for white
eye. In all eight different allelomorphic forms of this gene
have now been described, viz., (1) white, (2) tinged, (3) buff,
(4) eosin, (5) cherry, (6) blood, (7) coral, and (8) red. They
form a series of grades of increasing intensity of red pigmen-
tation, each one having made its appearance independently
of the others. Bridges has made an intensive study of juinor
genetic variations in one of these seven grades, viz., c\)sin,

219

220 GENETICS AND EUGENICS

the middle one of the series. He finds that in a pure culture
of eosin, the intensity of the pigmentation may vary from a
"deep pink darker than eosin" to a **pure white," through
the modifying action of eight other factors, '*in origin en-
tirely independent of one another" and located each at a
different genetic locus, four being in chromosome II and one
in chromosome III, the others not having been definitely
located. Seven of the eight modifying factors act as diluters
or lighteners of unmodified eosin, one only acting as a dark-
ener. They are in the order of their darkening (or lightening)
effects, (1) dark, (2) pinkish, (3) cream c, (4) cream b,
(5) cream a, (6) cream III, (7) cream II, and (8) whiting.
**Each of these genes arose by mutation," while the stocks
were under continuous study, ''by the transformation of the
materials of a particular locus into a new form having a
different effect upon the developmental processes." The
eye-color mutations observed to occur in Drosophila since
Morgan's discovery of white eye are so numerous that
Bridges classifies them in per cents, as 60 per cent general or
non-specific modifiers of eosin, such as vermilion and pink,
22 per cent specific modifiers of eosin, and 18 per cent allelo-
morphs of eosin. He continues, **It is probable that muta-
tion (change in single genes) is very much more frequent
than appears, since a great many mutations are of very
slight somatic effect and would pass undetected except that
certain characters such as eosin eye-color, truncate wings,
beaded wings, and a few others, are peculiarly sensitive dif-
ferentiators for eye-color and wing-shape genes, etc." Here
we have a picture of genetic mutability in the most carefully
studied of organisms, occurring contemporaneously, which
affords all the material needed for selection either natural or
artificial to act upon in either darkening or lightening the
eye-color by a series of progressive steps, if such change
should be found advantageous or desirable.

Among organisms reproducing sexually, the evening-
primrose has probably been studied more intensively than
any other except Drosophila. But it is impossible to say in

BISEXUAL REPRODUCTION 221

the case of Oenothera to what extent variation in single
genes is occurring, because those who, following De Vries,
have studied ^'mutation" in this multifarious genus of
plants, have directed their attention, almost without excep-
tion, to the major variations which they have called *'nui-
tations," and have neglected or denied the existence of
minor genetic variations, such as have been studied in much
detail in Drosophila. In some cases, such as the *'gigiis" and
the ''lata" types of mutation, irregularities of cell-division
seem to have resulted in duplication of entire chromosomes,
or of the entire set of normal chromosomes. As a comple-
mentary phenomenon we should expect entire chromosomes
to be lost from the germ-cell in other cases, and this may
possibly be the explanation of some classes of Oenothera
mutants whose associated cytological conditions have not
been determined. In the presence of such striking genetic
changes it is not surprising that variations in single genes
have scarcely been detected, although the '*nanella" and
*'rubri-calyx" mutants may be mentioned as manifesting
simple uni-factorial Mendelian inheritance. It seems prob-
able that when the minor variations of Oenothera are studied
as intensively as its peculiar "mutations" have been studied,
they will be found to be not less frequent in occurrence.

In domesticated mammals and birds, where asexual repro-
duction, self-fertilization, and parthenogenesis are unknown,
and where so much racial or family hybridization is con-
stantly being carried on with a view to increasing vigor or
variability, it is difficult to say how much of the genetic vari-
ability is of contemporaneous origin and how much of it hiis
been handed down in the stock from previous generations.
Theoretically, it should be possible to make any stock of
animals homozygous for practically every gene by inbreed-
ing continued for twenty or more generations, mating brother {
with sister, parent with offspring, cousin with cousin, or
uncle with niece (Jennings) . If this is done and genetic varia-
tion is subsequently observed to occur, this must have orig-
inated after the stock had been purified. By this means we

222 GENETICS AND EUGENICS

get an idea of how frequently genetic changes are happening.
Such purification of stock has rarely been undertaken. Miss
King has inbred rats, brother with sister, for 25 generations
and the resulting stock has been studied as to genetic char-
acter chiefly in respect to sex ratio. In the course of the
inbreeding, selection was made in two different lines for
opposite changes in the sex ratio; in Series A for a high ratio
of males to females, and in Series B for a low ratio of males
to females. The result in 6,274 young of the 25 generations
of the A series was a ratio of 122.3 males to 100 females. In
5,893 young of the B series, the ratio was 81.8 males to 100
females. These very different results were secured within
the first 10 or 12 generations of selection, after which progress
in the direction of the selection was negligible. This indicates
that the genetic factors responsible for the changes were
already in existence in the stock at the beginning of the
selection and were gradually sorted out and rendered ho-
mozygous in the early part of the selection period, and that
new genetic changes appreciable in amount did not appear
subsequently.

Selection was made simultaneously for large size in the
course of the inbreeding experiments of Miss King and this
resulted in producing inbred races which were larger than
the unselected stocks from which they were derived. The
maximum size was attained as early as the seventh inbred
generation, possibly earlier, as the seventh generation is the
earliest one for which comparable data are available. The
inbred races maintained throughout the entire period, up to
the twenty-fifth inbred generation, their superiority in size
over the control stocks from which the inbred strains had
originated, but no evidence was found that genetic size factors
had changed in the period between the seventh and twenty-
fifth inbred generations. Observations were made also on
fecundity as indicated by size of litter in the inbred rats.
No change was observed in this character as a result of the
inbreeding. The average size of litter was for the inbred
series 7.5 young, for the stock albinos used as a control, 6.7

«^«^«>^

12

15

18

Fig. 138. Grades 1-18 of white spotting as seen in Dutch rabbits. By systematic selection the average
grade of a race of Dutch rabbits may be gradually but peruiamntly changi-d either in a plus or in a
minus direction. Dutch spotting is allelomorphic with English (Fig. liS).

I

II

Fig. 139. TjTiical examples of three races of Dutch rabbits, each having a different and characteristic
amount or distribution of white spotting. The top figure represents the " while " rao-; niiddlc figure,
the "tan" race; lowest figure, the "dark" race. Each is allelomorphic with the others in crosses but
segregates in a slightly modified form.

No
50-

40-

30-

20-

10-

0
10-

\~VI,

0'

F,, DxW

\\ \J ^ M \ ■ <<

Grade, 0 I

16 17

Fig. 140. Graphic presentation of the variation in grade of two races of Dutch rabbits, "dark (D)
and "white" (W), and results of intercrossing them. At the top is shown the gra. e distribution of
each uncrossed race; below is shown the grade distribution of Fi animals, of F2 amma s. an.l of anmiaLs
produced by back-crosses of Fi with each parent race. Note that the extracted D or AN groups diverge
less from each other than the uncrossed D and W groups.

II

BISEXUAL REPRODUCTION 223

young per litter. At the beginning of the inbreeding experi-
ment strains of large, vigorous, rap id -growing and fecund
animals were isolated from the general stock, and those
characters seem to have been maintained w^ithout diminu-
tion under the continuous selection exercised in choosing as
breeders the largest and best nourished individuals from
each litter, but no evidence is forthcoming of further pro-
gressive genetic changes.

In hooded rats inbred, but not exclusively in brother-
sister ma tings, for twenty generations, selection has been
made successfully for change of the hooded pattern in op-
posite directions, to make the race as white as possible in one
line, and as dark as possible in another line. (See Tables 27
and 28.) Genetic variability decreased somewhat during
the first seven or eight generations, which probably suflBced
to eliminate most of the genetic variability originally present
in the stock as modifying factors. But subsequently the
variability as measured by the standard deviation showed
little change up to the end of the experiment in generation
21 when the selected races died out owing to the prevalence
of disease and infertility. The case seems to be best inter-
preted as one in which minor genetic changes are continually
occurring, so that selection utilizing these may move the
racial mode and mean either in a plus or in a minus direction
without encountering impassable limits short of an all white
or an all black condition. There is a strong parallelism be-
tween the variability of the white-spotting pattern in rats
and other mammals and the variability of variegated seed-
coat in maize and of variegated foliage in a great many
species of plants. In both sets of cases an unstable mosaic
of alternative characters exists, pigmentation and nonpig-
mentation; somatic variation in the relative proportions of
the balanced characters is constantly occurring, and germi-
nal variation of a similar nature very connnonly occurs at
the same time as the somatic Variation, so that selection on
the basis of the somatic variation effects germinal change in
the race. The variabihty (or ''mutability") in the case of

224 GENETICS AND EUGENICS

plants with variegated seed-coat or foliage extends into end
stages of the series which are wholly colored or wholly color-
less, which stages seem to be more stable than the inter-
mediate (mosaic) stages, as pointed out by Emerson. It is
to be regretted that in the selection experiments with rats
similar end stages were not reached before the selected races
perished. In the case of Dutch rabbits (Figs. 138-140) the all-
white condition has been recorded once, and the all-colored
condition is often found in animals known to be either
heterozygous or homozygous for some form of white spotting.

Two different explanations of cases of this class in animals
and plants have been suggested. (1) On one view the chief
genetic locus mutates frequently producing multiple alle-
lomorphs more or less stable (Emerson), but these multiple
allelomorphs may be supplemented in action by minor modi-
fying genes (Hayes). (2) On another view the chief gene is
as stable as other genes and the ordinary genetic variability
is due exclusively to modifying genes (MacDowell, Pearl,
Sturtevant) . If the chief gene is really less stable in the case
of these mosaic characters than in ordinary cases, as the
descriptive term used by DeVries, **ever sporting char-
acters," would seem to imply, at least in the case of plants,
it may be because a mosaic condition exists at the genetic
locus itself. In variegated plants the character of the mosaic
in particular parts of the plant corresponds roughly with
the character of variegation transmitted by flowers arising
in those same parts of the plant whether egg-cells or pollen-
cells are the vehicles of transmission, which suggests actual
variation in the genetic locus involved rather than change in
modifying genes. (See Figs. 135, 136.)

MacDowell inbred, brother with sister, a race of Droso-
phila possessing a recessive Mendelian character, extra
bristles, for 49 generations, selecting meanwhile in different
lines for high and for low number of extra bristles. For about
eight generations the selection was effective after which no
material change was observed attributable to the selection.
MacDowell concludes that at the beginning of the experi-

BISEXUAL REPRODUCTION 225

ment a number of genetic factors modifying l)ristle number
were present in the stock in heterozygous condition. Selec-
tion attending the inbreeding served to ehminate certain of
these from one race and to estabhsh them in homozygous
condition in the other race, after which no genetic changes
would be observed unless they arose de novo. As MacDow ell
was unable to detect any such changes, he concludes that
none were occurring. Payne has carried out a similar selec-
tion experiment for changed number of bristles, in another
race of Drosophila, starting with the descendants of a
single **mutant" individual with "reduced" bristle number,
which appeared in an "extra-bristle" strain. Selection was
made among the descendants of a single pair of flies and was
carried in brother-sister matings in a minus selected line for
64 generations, and in a plus selected line for 60 generations.
Toward the end of the experiment, the flies of the minus line,
in from 96 to 100 per cent of all cases, were without bristles.
This degree of purity was attained gradually during the first
seventeen generations of minus selection, after which no
further genetic change was observed. But in the plus selec-
tion, toward the normal number of bristles, four, in other
races of Drosophila, progress continued longer, reaching its
maximum in the 55th generation when 64 per cent of the
individuals possessed four bristles. The two selected lines,
plus and minus, had thus become very different as a result of
selection. Payne finds evidence that two or possibly three
genetic factors affecting bristle number were present in the
plus line, two of them being sex-linked, but that in the
minus line only a single factor was present. (Figs. 141, 14''2.)
Zeleny inbred a race of Drosophila possessing a dominant
character, bar eye, meanwhile selecting for high and for low
grades of the character (number of ommatidia). During the
course of the selection two striking mutations were observed
of the gene under study, one a reverse mutation to "full"
(normal) eye, the other a mutation in the direction of selec-
tion toward a more reduced condition of the eye, and called
*'ultrabar." The average size of the eye in these thrw alle-

£26 GENETICS AND EUGENICS

lomorphic states of the *'bar" gene is as follows: Full
(normal) eye, 849,.8 facets; bar eye, 75.6 facets; ultra-bar
eye, 23 facets. Zeleny also observed lesser mutations of the
gene for bar, which made their appearance during 42 care-
fully controlled generations of selection for low and for high
facet number in brother-sister matings. Aside from muta-
tions in the gene for bar, Zeleny observed genetic differences
between his high selected and low selected lines which he
ascribes to "accessory factors outside of the sex chromosome
in which the bar gene is located." These when present in
heterozygous state are speedily sorted out by selection,
which '* ceases to be effective after three to five generations."
*' There is, however, no limit to the possibilities of selection
if the occasional mutants are included in the series, and two
at least of these, reversal to full and ultra-bar, have been
shown to be changes in the bar gene itself."

We may conclude that the amount of genetic change
which is occurring at the present time is greater as regards
some characters than as regards others, and is probably
greater in some organisms than in others. But in any group
of organisms capable of interbreeding, which has been divided
for any length of time into non-interbreeding groups (races,
breeds, or strains) genetic differences of one sort or another
will probably be found to have arisen, when an intensive
study of the matter is made. If so, we must conclude that
genetic changes are probably occurring with appreciable
frequency in most, if not in all organisms. But it should be
stated emphatically that the amount of variability to be
detected by selection within pure lines is in all cases small
as compared with that which can be secured by crossing
different strains, breeds or varieties, which have long been
established within the species. For in pure line selection only
genetic changes occurring during the process of selection are
likely to be revealed, but following a variety cross, all pos-
sible recombinations may be expected of the genetic changes
which have occurred since the two parent groups diverged
from each other.

K^—r-. -, ,1

gen«iationt

Fig. 141. Results of selection for a reduced number of bristles continued for 64 generations among
the inbred descendants of a single pair of flies. The heavy line shows the percentage of flies with-
out bristles in each generation. Note that little change occurs after the 17th inbred generation.
(After Payne.)

I 1»' 12 M I*

8f ne ra 1 1 Da«

■nc-

FiG. 142. Results of selection for an increased number of bristles made throughout 60 inbred g*--
rations upon the same initial stock as is mentioned in the description of Fig. 141. The hea\->' line
shows the percentage of four-bristled (normal) flies in each generation. Note the progressiw
increase which continued as late as generation 55. (After Payne.)

CHAPTER XXVI

INBREEDING AND CROSSBREEDING

It is the opinion of most experienced animal breeders that
close inbreeding should be avoided because it has a tendency
to decrease the size, vigor and fecundity of the race in which
it is practiced. Many even believe that it leads to the pro-
duction of abnormal individuals or monstrosities. On the
other hand some of those who have had greatest success in
producing new or improved breeds of domesticated animals
have practiced the closest kind of inbreeding and attribute
their success in part to this fact.

In human society we find a nearly unanimous condemna-
tion of the marriage of near-of-kin. Nearly all peoples,
civilized or uncivilized, forbid it. Only exceptionally, as in
the case of the royal families of ancient Egypt and ancient
Peru, has the marriage of brother and sister been sanctioned.
The underlying reason in such cases was a belief that the
family in question constituted a superior race whose members
could find no fit mates outside their own number. There was
probably no thought that inbreeding itself was beneficial l)iit
only a desire to conserve the superior excellence believed to
reside in certain individuals. The same considerations, j^rob-
ably have led to the occasional practice of inbreeding in
animal husbandry, viz., the desire to conserve and per]:)etu-
ate the superiority of particular individuals.

If we inquire into the biological foundation of the idea that
inbreeding is harmful, we come upon seemingly conflicting
evidence. No generalization can be drawn which is applic-
able to all organisms.

By inbreeding we mean the mating of closely relatcnl in-
dividuals. As there are different degrees of relationshi]) be-
tween individuals, so there are different degrees of inbreeding.
The closest possible inbreeding occurs among plants in what

227

228 GENETICS AND EUGENICS

we call self-pollination, in which the egg-cells of the plant are
fertilized by pollen-cells produced by the same individual. A
similar phenomenon occurs among some of the lower animals,
notably among parasites. But in all the higher animals,
including the domesticated ones, such a thing is impossible
because of the separateness of the sexes. For here no indi-
vidual produces both eggs and sperm. The nearest possible
approach to self-pollination is in such cases the mating of
brother with sister, or of parent with child. But this is less
close inbreeding than occurs in self-pollination, for the
individuals mated are not in this case identical zygotes,
though they may be similar ones.

It has long been known that in many plants self-pollina-
tion is habitual and is attended by no recognizable ill effects.
This fortunate circumstance allowed Mendel to make his
remarkable discovery by studies of garden peas, in which the
flower is regularly self -fertilized, and never opens at all unless
made to do so by some outside agency. Self-pollination is
also the rule in wheat, oats, and the majority of the other
cereal crops, the most important economically of cultivated
plants. Crossing can in such plants be brought about only
by a difficult technical process, so completely adapted is the
plant to self-pollination. And crossing, too, in such plants is
of no particular benefit, unless by it one desires to secure
new combinations of unit-characters.

In maize, or Indian corn, however, among the cereals, the
case is quite different. Here enforced self-pollination results
in small unproductive plants, lacking in vigor. But racial
vigor is fully restored by a cross between two depauperate,
unproductive individuals obtained by self-fertilization, as has
been shown by Shull. This result is entirely in harmony
with those obtained by Darwin, who showed by long-con-
tinued and elaborate experiments that while some plants do
not habitually cross and are not even benefited by crossing,
yet in many other plants crossing results in more vigorous
and more productive offspring; that further, the advantage
of crossing in such cases has resulted in the evolution in

INBREEDING AND CROSSBREEDING 2!29

many plants of floral structures, which insure crossing
through the agency of insects or of the wind.

In animals the facts as regards close fertilization are similar
to those just described for plants. Some animals seem to be
indifferent to close breeding, others will not tolerate it. Some
hermaphroditic animals (those which produce both eggs and
sperm) are regularly seK-fertilized. Such is the case, for
example, with many parasitic flatworms. In other cases
seK-fertilization is disadvantageous. One such case I was
able to point out some twenty years ago, in the case of a sea-
squirt or tunicate, Ciona. The same individual of Ciona
produces and discharges simultaneously both eggs and sperm,
yet the eggs are rarely seK-fertilized, for if self-fertilization is
enforced by isolation of an individual, or if seK-fertilization is
brought about artificially by removing the eggs and sperm
from the body of the parent and mixing them in sea water,
very few of the eggs develop, — less than 10 per cent. But if
the eggs of one individual be mingled with the sperm of any
other individual whatever, practically all of the eggs are
fertilized and develop.

In plants much attention has been given to the problem of
self -sterility by East, Stout, Dorsey, and others. The case
of native American plums is as simple as any. All varieties
investigated by Dorsey were found to be self-sterile. If self-
pollinated, they set no fruit, either because the pollen grains
fail to germinate or because the pollen tubes, if formed, grow
too slowly to reach and fertilize the ovules. Not only are
all varieties self -sterile, some are also cross-sterile, i. e., sterile
when crossed with each other. It is probable that such
varieties have inherited a similar genetic constitution, so
that the pollen of one reacts toward the pistil of another as
toward pistils of its own plant. In support of this view it
may be said that East and Park found that the F. phmts
produced by crossing Nicotiana Forgeiiana with N. data fell
into four groups, all the plants in each group being mutually
cross-sterile but fertile with any plant of the other three
groups. The obvious conclusion is that the plants of each

230 GENETICS AND EUGENICS

group were similar in constitution as regards factors affect-
ing fertility, and that some dissimilarity is necessary to
enable the pollen of one individual or variety to grow vigor-
ously on the stigma of another individual or variety. The
phenomenon of self-sterility accordingly involves the prin-
ciple of heterosis. (See Chapter XXVII.)

In the great majority of animals, as in many plants, seK-
fertilization is rendered wholly impossible by separation of
the sexes. The same individual does not produce both eggs
and sperm, but only one sort of sexual product. But among
sexually separate animals the same degree of inbreeding
varies in its effects. The closest degree, mating of brother
with sister, has in some cases no observable ill effects. Thus,
in the case of a small fly, Drosophila, my pupils and I bred
brother with sister for fifty-nine generations in succession
without obtaining a diminution in either the vigor or the
fecundity of the race, which could with certainty be attrib-
uted to that cause. A slight diminution was observed in
some cases, but this was wholly obviated when parents were
chosen from the more vigorous broods in each generation.
Nevertheless crossing of two inbred strains of Drosophila,
both of which were doing well under inbreeding, produced off-
spring superior in productiveness to either inbred strain.
Even in this case, therefore, though inbreeding is tolerated,
crossbreeding has advantages.

In the case of many domesticated animals, it is the opin-
ion of experienced breeders, supported by such scientific
observations as we possess, that decidedly bad effects follow
continuous inbreeding. Bos (1894) practiced continuous in-
breeding with a family of rats for six years. No ill effects
were observed during the first half of the experiment, but
after that a rapid decline occurred in the vigor and fertility
of the race. The average size of litter in the first half of the
experiment was about 7.5, but in the last year of the experi-
ment it had fallen to 3.2, and many pairs were found to be
completely sterile. Diminution in size of body also attended
the inbreeding, amounting to between 8 and 20 per cent.

1005^

8

10

Segregating Cenerationa

Fig. 143. Graphs showing the progressive reduction of heterozygous individuals in a p«ipuIation of
self-fertilized plants, starting with all individuals heterozygoles. Four east-s are shown, in which the
number of independent allelomorphs is respectively 1, 5, 10, or 15. (After East and Jones.)

INBREEDING AND CROSSBREEDING 231

Experiments made by Weismann confirm those of Bos as
regards the falHng off in fertility due to inl)r(M'(h*ng. Vov
eight years Weismann bred a colony of mice started from
nine individuals, — six females and three males. 'Jlie experi-
ment covered twenty-nine generations. In tlie first ten gen-
erations the average number of young to a litter was 0.1;
in the next ten generations, it was 5.6; and in the last nine
generations, it had fallen to 4.2.

But recent inbreeding experiments with rats carried on at
the Wistar Institute by Dr. Helen King give results quite at
variance with those of Bos and Weismann. She finds, as
was found to be the case in Drosophila, that races of large
size and vigor and of complete fertility may be maintained
under the closest inbreeding, if the more vigorous individuals
are selected as parents. By this means she seems to have
secured races of rats which are relatively immune to injurious
effects from inbreeding. My own experience with rats inbred
within lines of narrow selection for seventeen generations is
that races of fair vigor and fecundity can be maintained
under these conditions, but that when two of these inbred
races are crossed with each other, even though they had their
origin in a small common stock many generations earlier, an
immediate and striking increase of vigor and fecimdity oc-
curs. This is quite similar to the result observed in the case
of Drosophila, and is quite in harmony with the results ob-
tained by ShuU in maize; it indicates that by careful selection
races may be secured which are vigorous in spite of inbreed-
ing, but that nevertheless an added stimulus to growth and
reproduction may be secured in such cases by crossbreeding.

In the production of pure breeds of sheep, cattle, hogs, and
horses inbreeding has frequently been practiced extensively,
and where in such cases selection has been made of the more
vigorous offspring as parents, it is doubtful whether any
diminution in size, vigor, or fertility has resulted. Never-
theless it very frequently happens that when two pure l)reeds
are crossed, the offspring surpass either pure race in size and
vigor. This is the reason for much crossbreeding in eeo-

232 GENETICS AND EUGENICS

nomic practice, the object of which is not the production of a
new breed, but the production for the market of an animal
maturing quickly or of superior size and vigor. The inbreed-
ing practiced in forming a pure breed has not of necessity
diminished vigor, but a cross does temporarily (that is in the
Fi generation) increase vigor above the normal. Now why
should inbreeding unattended by selection decrease vigor,
and crossbreeding increase it.^ We know that inbreeding
tends to the production of homozygous conditions, whereas
crossbreeding tends to produce heterozygous conditions.
Under self-pollination for one generation following a cross
(involving one unit-character only), half the offspring become
homozygous; after two generations, three-quarters of the
offspring are homozygous; after three generations seven-
eighths are homozygous, and so on. So if the closest inbreed-
ing is practiced there is a speedy return to homozygous, pure
racial conditions. We know further that in some cases at
least heterozygotes are more vigorous than homozygotes.
The heterozygous yellow mouse is a vigorous lively animal;
the homozygous yellow mouse is so feeble that it perishes as
soon as produced, never attaining maturity. Crossbreeding
has, then, the same advantage over close breeding that fer-
tilization has over parthenogenesis. It brings together differ-
entiated gametes, which, reacting on each other, produce
greater metabolic activity. East and Jones have suggested
that the superiority in vigor of crossbred over inbred indi-
viduals is roughly proportional to the number of genetic dif-
ferences between the races crossed. This idea is worthy of
an experimental test.

Inbreeding, also, by its tendency to secure homozygous
combinations, brings to the surface latent or hidden reces-
sive characters. If these are in nature defects or weak-
nesses of the organism, such as albinism and feeble-minded-
ness in man, then inbreeding is distinctly bad. Existing
legislation against the marriage of near-of-kin is, therefore,
on the whole, biologically justified. On the other hand,
continual crossing only tends to hide inherent defects, not to

INBREEDING AND CROSSBREEDING 233

exterminate them; and inbreeding only tends to bring them
to the surface, not to create them. We may not, therefore,
Hghtly ascribe to inbreeding or intermarriage the creation of
bad racial traits, but only their manifestation. Further, any
racial stock which maintains a high standard of excellence
under inbreeding is certainly one of great vigor, and free from
inherent defects.

The animal breeder is therefore amply justified in doing
what human society at present is probably not warranted in
doing, — viz., in practicing close inbreeding in building np
families of superior excellence and then keeping these pure,
while using them in crosses with other stocks. For an animal
of such a superior race should have only vigorous, strong off-
spring if mated with a healthy individual of any family what-
ever, within the same species. For this reason the production
of *' thoroughbred" animals and their use in crosses is both
scientifically correct and commercially remunerative.

The early plant hybridizers found that frequently (but not
always) hybrids produced by the crossing of distinct species
or genera are characterized by remarkably vigorous growth
and large size, superior to that of either parent. But these
same large vigorous hybrids produced little or no seed.
Vegetative and reproductive activity are to some extent
complementary and opposed activities of the plant. A vig-
orously growing young fruit tree may be brought into
bearing early if it is cut partly in two, or a ring of bark is
removed from it in the growing season, thus checking its
growth. Under such circumstances fruit buds are formed.
In many hybrid plants, in which the vegetative vigor is great,
partial or complete sterility exists. This, however, is not
invariably the case. The offspring of a cross between geo-
graphic varieties of the same species are usually both vigor-
ous and fertile, but the offspring of widely separated species
or genera may be lacking in vigor as well as fertility. A\ ith
increasing diversity of the parents the following series of
conditions obtains:

1. The mating of parents belonging to the same pure race

234 GENETICS AND EUGENICS

and closely related to each other has on the whole the same
effect as seK-fertilization. It brings together gametes which
transmit the same characters, which are doubtless chemi-
cally alike, and no particular increase of vigor results when
they unite. It is on a par with asexual reproduction by par-
thenogenesis, fission, budding, or vegetative multiplication.
There is in consequence no change in the germinal constitu-
tion, or relatively little. There is neither increase of vigor
nor loss of vigor.

2. The mating of closely related individuals within a nor-
mally intercrossing population such as a breed of domesti-
cated animals, or a human population, is apt to cause some
loss of vigor. So much of the vigor of the population as is
due to its crossed (or heterozygous) character, will tend
gradually to disappear, as homozygous conditions are ob-
tained in consequence of inbreeding. The greater the num-
ber of characters in which a population varies, the slower will
be the attainment of a fully homozygous state in consequence
of inbreeding. If suflScient vigor is retained after a fully
homozygous state has been reached, then the closest inbreed-
ing (or even self-fertilization, when this is possible) should
cause no further loss of vigor. There is no reason to think
that monstrosities are produced by inbreeding (as for ex-
ample deformities, feeble-mindedness, insanity) except in
so far as such maladies may be due (1) to the lack of suffi-
cient vigor on the part of the organism to complete its normal
development, or (2) to the appearance in a homozygous state
of a recessive condition unseen in the heterozygous parents.

3. The mating of individuals belonging to distinct geo-
graphical races of the same species of animal or plant usually
produces offspring larger or more vigorous than either parent
and fully fertile. The same result follows when distinct breeds
of domesticated animals or distinct varieties of cultivated
plants are crossed. The offspring are equal to or superior to
the parents in vigor and not less uniform in character.
But the F2 generation from such a cross does not retain the
superiority of the Fi generation, for it shows great variabil-

INBREEDING AND CROSSBREEDING 235

ity in all respects, which in economic animals or plants is
very undesirable. For the characters in which the two pure
breeds differed undergo recombination in all possible ways
in the F2 offspring. Even a back-cross of an Fi iiullvidujil
with one of the pure races would produce offspring (juite
variable and including undesirable combinations, since each
Fi individual would form the maximum number of different
kinds of gametes. Hence crossing of pure breeds of domes-
ticated animals may in special cases be advantageous but
should never be carried beyond the Fi generation unless the
breeder is setting out on the slow and tedious process of pro-
ducing and fixing a wholly new breed. In that case he must
be prepared to produce and sacrifice many worthless aninuils
for the sake of obtaining in the end a few of possibly superior
value. For such an undertaking the imagination and the
patience of an inventor are required.

4. When animals or plants of widely separated species or
genera are crossed, one of two results follows: Either the
offspring are of remarkable vigor but of impaired fertility,
or the offspring lack both vigor and reproductive capacity.
In the former category comes one very unportant economic
cross, that of the horse with the ass, producing a very valu-
able animal, the mule. The economic importance of mules
is indicated by the large numbers produced in the United
States, South America, Europe and Africa, and by the fact
that the market price of a mule averages higher than the
price of either a horse or an ass. Nevertheless a mule is
absolutely incapable of reproduction. It has well developed
sexual glands and sexual instincts, but the sexual cells de-
generate before reaching full maturity. If mules were capable
of reproduction, they would probably be less valued than
they now are, for F2 and F3 individuals would doubtless then
be produced, and these would lack the uniformity and vigor
of the Fi individuals which alone exist at present.

Crosses of cattle with the American bison produce hybrids
which are sterile in the male sex only, the femal(\s being fertile
with either parent species. By use of these fertile female

236 GENETICS AND EUGENICS

hybrids, three-fourths bloods may be produced which are
almost as variable as a true F2 generation. If the products
of this cross are shown to possess economic advantages over
domestic cattle (which seems very doubtful) a fertile hybrid
race will doubtless be established in the near future. How
this can be done is shown in experiments made by Dr. Det-
lefsen and myself in crossing the guinea-pig with a wild
Brazilian species of cavy, Cavia rufescens. The Fi individuals
surpass either parent species in size and vigor, but the males
are fully sterile, the females, however, being fertile. After
two back-crosses of female hybrids with the guinea-pig a
few fertile males were obtained, whose descendants were also
fertile. But they possess certain Mendelizing characters de-
rived from the wild parent, Cavia rufescens. The skeletal
characters of the hybrids are a blend. The great vigor of the
Fi hybrids is not shown in the fertile hybrids obtained by
back-crossing. As regards size and vigor they are not su-
perior to guinea-pigs. If the Mendelizing color characters
possessed economic value, the hybrid race could now be
easily continued. As in the case of the cattle-bison cross, the
economic value of the Fi generation is not sufficient to war-
rant the expense of its continued production.

Hybrids which are feeble as well as sterile have, of course,
no economic value. They are scientifically interesting as
showing how, when the difference between gametes becomes
too great, they can no longer form a vigorous zygote. Few,
if any, animal hybrids of this sort are known, but many
plant hybrids of this sort have been produced, among them
being some of the first produced hybrids obtained by cross-
ing different species of Nicotiana (tobacco). See Fig. 26a,
East's repetition of Kolreuter's pioneer experiment.

5. WTien organisms are crossed which differ more widely
than do ordinary species, so that they are referable to differ-
ent genera or families, the production of a hybrid organism
does not follow, apparently because the uniting gametes are
too unlike to be capable of continued existence together in the
same cell. Nevertheless a parthenogenetic development of the

INBREEDING AND CROSSBREEDING 237

egg-cell may result from its fertilization by the foreign sp<Tin.
Thus when the egg of a sea urchin is fertilized with the sperm
of a sea lily, an anhnal of a wholly different class of echino-
derms, the egg begins development following a fusion of the
sperm and egg nuclei, but the nuclear substance introduced
by the sperm soon degenerates and disappears. The eggy
however, having once started to develop, continues to do so,
producing an organism showing only characters of the ma-
ternal species. Its development is as truly parthenogenetic
as when Induced by chemical or osmotic means, as Is now
known to be possible in the case of the eggs of many marine
and of some fresh-water animals. Thus the unfertilized egg
of a frog may be made to develop by chemical means (or
even by puncturing the superficial layer of the egg with a
needle), a process we may call artificial or induced partheno-
genesis. Now in crosses of species too widely separated to
produce a hybrid Individual, the sperm may merely induce
parthenogenesis. This method of inducing parthenogenesis
is being used by plant breeders of the United States Depart-
ment of Agriculture to obtain orange seedlings which It Is
hoped may be superior to the mother plant In certain re-
spects, though the progeny will inherit none of the qualities
of the pollen plant. It Is hoped merely that there may occur
in the parthenogenetic offspring some segregations or vari-
ations of the characters found in the mother plant.

What might be called male parthenogenesis has been re-
ported In crosses of strawberries made many years ago by
Millardet and also In a cross between Mexican teoslnte, a
plant related to maize, and a coarse grass of the southern
United States. (Collins.) In such cases a cross-fertilized seeii
produces a plant which shows only characters of the pollen
parent. It is supposed that the egg nucleus has taken no
part in the production of an embryo, but that this has arisen
wholly from nuclear material of the pollen tube.

Considering all the facts, changes in heterozygosity alone
seem an insufficient explanation of the effects of crossing and
inbreeding respectively. It is necessary to suppose further

238 GENETICS AND EUGENICS

that gametes as well as zygotes vary in vigor. Some can
exist as gametes alone, so great is their natural vigor. Here
there can be no heterozygosity. Examples are found both
in animals and in plants (honeybee drone, fern gameto-
phyte). Others can exist only as zygotes, so feeble are they
(the majority of the higher animals and plants). Still others
cannot exist as homozygotes, but only as heterozygotes, be-
cause they are still feebler (the yellow mouse, the aurea snap-
dragon).

The experience of Miss King in inbreeding rats brother
with sister for twenty-five generations, shows that heterozy-
gosity is not indispensable to vigor even in bisexual repro-
duction, for she did not observe any evidences of decline in
vigor, size or fecundity, yet in all probability great increase
in homozygosity took place, since variability decreased.

Pearl (1915) has attempted to devise a precise measure
of inbreeding based on the number of times that the same
individual or individuals appear in the pedigree of a particu-
lar animal. Thus, in bi-parental reproduction each individual
has two parents, each of these also had two parents, which
may or may not be the same pairs. If the parents were
brother and sister, then their parents were one pair, not two.
Thus the maximum number of different ancestors would be
two parents, four grandparents, eight great-grandparents,
etc. Such would be the condition when no inbreeding had
occurred. But occurrence of the same individual more than
once in a pedigree would show a certain amount of inbreed-
ing, and the extent of the inbreeding would increase with
every repetition of an individual in the pedigree. Pearl
makes this the basis of his "coefficient of inbreeding," which
is intended to express the relation between the possible (max-
imum) number of different ancestors and the actual number
of different ancestors, each individual being counted only
once, no matter how many times it is mentioned in the
pedigree.

The chief utility of such a coefficient is to show what ap-
proach to homoz^^gosity of genetic factors has probably been

INBREEDING AND CROSSBREEDING 239

made in the production of a particular individual, as a con-
sequence of mating together related individuals among his
direct ancestors, but this the coefficient of inbreeding can
not do with great exactness because even with the closest
possible inbreeding (self-fertilization) the approach to honuj-
zygosity in individual cases is quite a matter of chance.
Thus, East and Jones say, forcefully and quite correctly,
*'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 generation may be either completely heterozygous
or completely homozygous or any degree in between, de-
pending upon chance. The only condition which must fol-
low in self-fertilization is that no individual can ever be more
heterozygous 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 heterozygos-
ity, or it may bring about complete homozygosity in the first
inbred generation. In other words the rate at which homo-
zygosity is approached may vary greatly in different lines."
. . . ** Although nearly complete homozygosis is theoreti-
cally brought about by seven generations of self-fertilization,
the attainment of absolute homozygosity is a difficult matter
and in practice it may never be reached." . . . "Con-
tinued selective mating is necessary to bring about homo-
zygosity. Intermittent inbreeding alternating with periods
of outcrossing, which is the prevailing state of affairs ^^•ith
many organisms, cannot maintain any high degree of homo-
zygosity." These statements show that the coefficient of in-
breeding, though it appears to be very precise, like Galton's
law of ancestral heredity, is subject to similar limitations.
It indicates what is true of populations in the mass, but has
small utility as an indicator of what happens in individual
cases. \Mien it is applied to the case of a particular Jersey
bull it may be very much less reliable as an index of prob-
able performance than the judgment of an experienced cattle
breeder.

240

GENETICS AND EUGENICS

It is nevertheless of value to know what the tendency of a
particular system of breeding is, if persistently followed, as
regards homozygosity, for homozygosity implies fidelity to
type in transmission and is probably what the animal breeder
means by "prepotency," so far as he has any clearly defined

TABLE 32a

Probable Percentage of Homozygosity, under Different

Systems of Inbreeding, in Populations at the Outset Entirely

Heterozygous. (From Data of Jennings and Fish.)

Generations of
Inbreeding

Self-fertilization, or

Back -cross to the

same homozygous

Parent

Brother-sister
Matings

Generations of
Inbreeding

Brother-sister
Matings

1

50.00

50.00

12

94.31

2

75.00

50.00

13

95.40

3

87.50

62.50

14

96.28

4

93.75

68.75

15

96.99

5

96.87

75.00

16

97.56

6

98.43

79.69

17

98.03

7

99.22

83.59

18

98.40

8

99.61

86.72

19

98.71

9

99.80

89.26

20

98.96

10

99.90

91.31

21

99.15

11

....

92.97

25

99.64

idea in mind when he uses the term. Inbreeding tends auto-
matically to replace heterozygous germinal conditions by
homozygous conditions in the inbred population and the
*' closer" the degree of inbreeding the stronger is this tend-
ency. Jennings has worked out formulae for calculating the
probable percentage of homozygosity in populations inbred
after a particular system of matings for any number of gen-
erations. The results in three systems of matings for a
series of from 10 to 25 inbred generations are sho^n in
Table 32a. The progress toward homozygosis, it will be ob-
served is rapid in self-fertilization, heterozygotes being only
one-tenth of one per cent after 10 generations of inbreed-
ing. The elimination of heterozygotes is equally rapid when

INBREEDING AND CROSSBREEDING 241

back-crosses are made in every generation with the same
homozygous parent race. In brother-sister matings, the next
nearest degree of inbreeding, progress toward homozygosity
is much slower, twenty-five generations of sucli matings
accomplishing no more than eight generations of self-
fertilization.

CHAPTER XXVII

HYBRID VIGOR OR HETEROSIS

Plants or animals which maintain normal size and vigor
mider self-fertilization or close inbreeding may nevertheless
show an added vigor when outcrossed, that is when mated
with individuals of races genetically different from their own.
This is called heterosis, because it is supposedly due to
heterozygosis i the cross-bred state of genetic factors. The
mule has already been mentioned as a familiar example
among animals, in which hybrid vigor is shown. Many
similar examples are on record for hybrid plants. For ex-
ample East and Hayes describe a cross between two dif-
ferent wild varieties of tobacco (Nicotiana rustica hrazilia
and N, rustica scahra) showing that reciprocal crosses pro-
duce Fi plants taller than either parent variety. See Table

TABLE 326

Variation in Height of Plants of Nicotiana rustica brazilia (349), of N. rustica

scahra (352), and of their Reciprocal Fi Hybrids

{After East and Hayes)

Variety or cross

Class centers in inches

24

27

30

33

36

39

42

45

48

51

54

67

60

63

66

69

72

75

78

349

4

10

22

14

7

• •

2

1

5

11
1

16
3

17
0
3

6
5
5

5

2

• •
• •

5
4

• •

6
6

• •

1

5

1
1

352

352X349, Fi
349X352, Fi

• •

• •

• •

• •

• •

• •

2

In maize, which is normally cross-fertilized and so main-
tained as a field crop in a state normally heterozygous, self-
fertilization for a number of generations serves automati-
cally to eliminate most of the heterozygosity (see Fig. 143)
and consequently produces races of size and vigor less than

242

Fig. 144. Four characteristically different inbred strains of maize after eleven generations of seiN
fertilization. Note the remarkable uniformity of each strain, (\fter East and Jones.)

^^^PMj

;

1

^^^i

-

'"'^I^S^M^

H^9hR^%^v t-

P

" -

:g|§^

HiBSit^ ^If V" '8BI

j^'

- ■: ^,^,

Bfc&i-

■^"^ "•s

-.,;i>-'-^:

&

r-'fr^s*^^

BgWlBFf^-^->v 9

■Me^^4-*fi-S

,.."< ""

^S8^£^s-^B

^BwnKuEi^^^^^^ #%^^l

••^r--e|

t^'

::- *<

^^§€p

■■T-.i*in^''S*SS

■ ••- -S"

^

•3Sr^i!3..,SE

- — Tf

, V

jP>g|^L^r3«^;<-'^^H

■■ ■'"» v-"^^^r«»Safi

. ^<S

Cgt.-£ ^

&;-

^^¥3

^^^^^ffifrFrVmiPr' ^-

ta^

^-:.

'^

^^w3

■B|^^^B^^hSipc%> V

H^Hfe. - *%

. . „j ^

Fig. 145, Two ears of maize self -fertilized for
six generations and between them an ear of
their Fi hybrid, (After East and Jones.)

Fig. 146. Two plants of maize self-ferti-
lized for eleven generations, and be-
tween them, plants of their Fi hybrid
showing greatly increased size and pro"
ductiveness.

Fig. 147. A field of maize showing remarkably uniform and vigorou^ plants representing a lirst gener-
ation cross between two inbred strains. (After Last and Jones.)

100

Gronth Curves of
Two Inbred Strains
of Maize and Their
P]^ and Fg Hybrids.

to

4)

o

c

i

A3
00

"T

60

90^

50 60 70 80

Number of Days from Planting

Fig. 148. Graphs showing growth curves of two inbred strains of maize, P (1-7) and P (1-9), ami of
their Fi and F2 hybrids. Note that both Fi and F2 are at all ages much taller than either inbred fwirent
race, but that Fi is considerably taller than F2, as the plants approach maturity. (After East and
Jones.)

Pig. 152. Diagram showing a method of double crossing maize to secure maximum yield from »^i\
plot and general crop. Four different inbred strains (shown in the top row) are cros.s«'d in pairs, pro-
ducing the two vigorous but unrelated Fi hybrids shown in the middle rt)w. By rros.sinK Ihcso with
each other, an entire crop of Fi seed of high productiveness is secured. (After East and Jones.)

fl

HYBRID VIGOR OR HETEROSIS 243

normal but very uniform in character (Fig. 144). But if two
of these inbred strains are crossed with each other a great
increase ui size results in Fi, which as it accompanies restora-
tion of the origmal heterozygosis may reasonably be ascribed
to its agency (see Figs. 145-147). If a second generation
of the crossed corn is raised by planting seeds taken from Fi
plants, there is found to be a falling off in vigor fsee Fig. 148).
The F2 plants start out well owing to the large amount of
food materials stored in the plump Fi seeds, but ultimately
they fall behind Fi plants in vigor of growth so that they
attain a height considerably less, though still much in excess

WeigM

in
Grams

800

KfRaeeB.

— ^

-^

==^

=_^_

fiOO

? h-nre B

dOO

^

^

^' ^^^^

6 CutUri.

200

^

/^

=rn

-

? CutUri

^

■

AgeinJDaya 40 80 120 160 200 240 280 320 360

Fig. 149. Growth curves of race B guinea-pigs and of Cavia cutleri.

-too

of the inbred parent races. This is in harmony wiLli the
view that heterosis is the cause of hybrid vigor, for heterosis
should be at a maximum in Fi and should decline in F2
exactly as the height of the maize plants is seen to do in this
cross. A case in which ordinary (blending) size mheritimci'
is complicated by heterosis is seen in crosses made between
Cavia Cutleri from Peru and races of guinea-pigs which ^^ e
will call B and C. The growth curve of each of the parent
stocks is shown in Fig. 149. In each case males are heavier
than females except for the first few weeks of life when the
females are heavier. Races B and C are nearly twice as heavy
in adult weight as Cavia Cutleri.

Growth curves of the Fi and Fo hybrids are shown in Figs.

244

GENETICS AND EUGENICS

150 and 151, where they can be compared with the growth
curves of the respective parent races. In each case Fi sur-
passes either parent race in size, but F2 is intermediate be-
tween them. So far as heredity is concerned, the inheritance
is blending, but Fi shows an increase in size due to hybridi-
zation. It seems to be due not to heredity at all, strictly
speaking, but to heterosis, and it begins to disappear as the
Fi hybrids are bred together producing an F2 which theo-
retically is only half as heterozygous as Fi (Table 32a). It
might be expected to decline still more in later generations.

Weight

in\
GraMs

A^e iaDaya 40 80 120lfi0200240280320360«)a

Fig. 150. Growth curves of race B and Cavia cutleri males and of their Fi and F2 male hybrids.

Animal breeders have long utilized the principle of hetero-
sis in the production of mules and in the "grading" of cattle,
hogs, and sheep for meat production. Plant breeders are like-
wise seeking to take advantage of this same principle for im-
proving field crops in quantity, quality and uniformity of
yield. In particular East and Jones have suggested the fol-
lowing novel methods of breeding maize. First, a standard
variety should be inbred (self -pollinated) for several gen-
erations, in the course of which it will automatically resolve
itself into a number of genetically different pure lines (Fig.
144). Any lines inherently weak w^ill become extinct or may
be discarded. Those which remain will contain the best com-
binations of genetic factors originally present in the variety,
but will lack any vigor due to heterosis and so will be less

HYBRID VIGOR OR HETEROSIS

24

o

productive than the original variety before it was inbred.
It will accordingly not be profitable to propiigate these pure
lines as field crops, and further the amount of seed w]iif]i
they will yield if cross-pollinated will not be large. Hence to
produce a large quantity of cross-bred seed will be expensive.
But from a small number of Fi plants a very large yield of F.
seed might be obtained at small expense, since Fi plants are
extremely productive. The aim should be therefore to cross-
breed Fi plants. This can be done by securing four different
inbred lines and crossing these in pairs, A with B, C with D.
There will result two unrelated and vigorous Fi groups, AB
and CD which may now be planted in alternate rows. One

Weight

in I
Orams

AgeinDaya 40 80 120 160 200 240 2S0 320 360 400

Fig. 151. Growth curves of race B and Cavia cutleri females and their Fi and F2 female hybrids.

of them, if detasseled, will be naturally pollinated by the
other and consequently all the seed which it produces will
be crossbred, representing combinations of factors found in
AB with the allelomorphic factors in CD. Such seed, if
planted, will produce a field crop of maximum yield, since
all plants will be cross-bred Fi individuals, though produced
by Fi plants. This last fact will keep down the cost of pro-
ducing the seed because the yield will be heav>% half the
total crop from the area planted (see Fig. 152).

CHAPTER XXVIII

GALTON'S LAW OF ANCESTRAL HEREDITY AND HIS
PRINCLPLE OF REGRESSION

Galton (1889) was the first to recognize the distinction be-
tween alternative and blending inheritance. But he sought
nevertheless to unify the two categories of cases and finally
formulated in 1897 a generalized *'law of ancestral heredity"
which he believed would include both. In seekfng such a
general law of heredity he had studied a representative case
each of blending and of alternative inheritance. The former
was found in family statistics of human stature, the latter in
the coat color of Basset hounds. The latter we should now
describe as a case of Mendelian inheritance involving simul-
taneously white spotting, and a color pattern (bi-color).
Stature inheritance is well described by Galton's term,
''blending," but is now understood to involve multiple
Mendelian factors whose action is cumulative.

In either case, Galton would have admitted that the entire
inheritance is from the parents through the two gametes
which unite to form the zygote, so that strictly speaking
there is no inheritance from generations more remote than
the parents. But he w^ould have maintained quite correctly
that a better idea can be had of what the gametes on the
average will transmit, if one knows the character of several
generations of ancestors than if one knows the character of
the parents alone, and in this sense we may be said to inherit
from ancestors more remote than our parents. Galton be-
lieved that the apparent influence of each generation of an-
cestors diminished as its remoteness increased, each more
remote generation having only half the influence of the next
later one. In his own words: "The two parents contribute
between them, on the average, one-half, or (0.5); the four
grandparents, one-quarter, or (0.5) 2; the eight great-grand-
parents, one-eighth, or (0.5) ^ and so on. Thus the sum of

246

GALTON'S LAW Q47

the ancestral contributions is expressed by the series (0.5) -f-
(0.5) 2 -f (0.5) 3, etc.] which being equal to 1, accounts for
the whole heritage."

If one attempts to make use of this law by basing upon it
predictions as to the character of the offspring in particular
kinds of matings, it works fairly well when blending charac-
ters are tinder consideration, but fails completely when ordi-
nary Mendehzing characters are under consideration. See
Castle (1903). As a useful generalization it is now pretty
generally discredited.

Regression was a name given by Galton to the apparent
going back of offsprmg from the condition of their parents
toward that of more remote ancestors, or more correctly
toward the general average of the race. Thus he observed that
very tall parents have children less tall than themselves,
while very short parents have children taller than themselves.
In either case the children regress toward the general average
of the race, and the regression is greater the more pronounced
the deviation of the parents from the general average of the
race. Also in sweet peas, Galton observed that when very
large seeds are planted, the crop harvested averages smaller
in size than the seeds planted; and that when small seeds
are planted, the crop averages larger in size. Regression
occurs in both cases toward the mean of the race. Galton
regarded regression as a feature of ancestral heredity; but
Johannsen has shown, as regards size of beans, that regres-
sion is due to a lack of agreement between somatic and
genetic variations, the latter being more conservative, ami
that when selection is made within a line pure genetically, no
regression occurs. Davenport confirms this view in the case
of human stature, showing that the children of parents
genetically pure for tall stature do not regress toward me-
diocrity, as Galton supposed all classes of a population to do.
Galton's law of ancestral heredity and his principle of regres-
sion are now chiefly of historical interest, but it is well to
keep them in mind when generalizations based on similar
reasoning are brought forward. (See Chapter XX^ I.)

CHAPTER XXIX

SEX DETERMINATION

Certain facts presented in an earlier chapter show that there
is a close connection between sex-linked inheritance and sex
determination, since only male-determining gametes or only
female-determining gametes are able to transmit sex-linked
characters in particular crosses. We must now consider more
fully the facts and theories of sex determination. In all the
higher animals and plants a discontinuous variation occurs
as regards sex, every individual being either male or female.
The distribution of males and females in successive genera-
tions presents many analogies with Mendelian inheritance.
This idea occurred to Mendel himself, as is shown in his post-
humously published letters. Bateson suggested it independ-
ently in 1902, and this idea was more fully elaborated by
Castle (1903). The view is now generally accepted that a
factor concerned in sex determination is in all the higher
animals and plants inherited in accordance with Mendel's
law. What in such cases is the distinction between male and
female individuals?

The essential difference between a female and a male indi-
vidual is that one produces eggs, the other sperm. All other
differences are secondary and dependent largely upon the
differences mentioned. If in the higher animals (birds and
mammals) the sex glands (i. e., the egg-producing and sperm-
producing tissues) are removed from the body, the superficial
differences between the sexes largely disappear. In insects,
however, the secondary sex characters seem to be for the
most part uninfluenced by presence or absence of the sex
glands. Their differentiation occurs independently, though
simultaneously, with that of the sex glands, evidently de-
pending on the genetic (chromosome) constitution of the
cells in each part of the body. When the constitution of cells

248

Fig. 155. Effects of removal or transplantation of sex glands in Brown Leghorn fowls. 1 and •?. Normal
male and female respectively. 3. Feminized male. At an early age the testes were remove*! anil re-
placed by ovaries. 4. Castrated male, three years old. Notice undevelop<Mi comb and wattles, but
characteristic male hackle feathers, tail feathers and spurs. 5. Castrated female. Notice well-developed
comb and wattles but characteristic female plumage. (After Dr. H. D. Goodale.)

SEX DETERMINATION

249

in different parts of the body differs in respect to sex-linked
characters, a sex-mosaic results known as a gynandroniorph.
Morgan and Bridges have made an exhaustive study of such
mosaic individuals found in their cultures of Drosophila.
One of the simplest types, a bilateral sex-mosaic, is shown in
Fig. 153. The right half of this fly shows male characters,
viz., shorter wing, black-tipped abdo-
men, sex-comb on first leg. The left
side of the fly shows the contrasted
female characters. The right eye was
also white, a character inherited in the
single X-chromosome derived from
the white-eyed mother of the fly. The
left eye was red resulting from the
presence (in the female part of the
body) of an X-chromosome bearing
red-eye, derived from the red-eyed
father, which is dominant over the
white-eye borne by the X-chromosome
furnished by the mother.

Three different explanations have
been offered in recent years for the
origin of sex-mosaic insects. These are
expressed diagram mat ically in (Fig.
154). The first. A, was offered by
Boveri. It suggests that an egg which
has undergone maturation, and which
accordingly retains a single X-chro-
mosome may, on account of delayed
fertilization, undergo nuclear division
before fertilization is complete, so that
it becomes binucleate before fusion of egg and sperm nuclei
has occurred. The sperm now fuses with one of the egg's two
nuclei. That nucleus and its descendants will be ^2X (female),
but the unfertilized nucleus, if it develops by itself will be
X (male). A body mosaic as to sex will result, part male,
part female. The case shown in (Fig. 153) could be accounted

Fig. 153. A ^ex-mosaic, or gynan-
dromorphic, Drosophila. The rikfht
half of the body shows male char-
acters, viz. comb on first h'^r. shi)rt
wing, and black-tipped ab<lonien.
The left half of the bo«ly shows
female characters, viz. long wing
and light-tipped ab<lomcn. Note
also that the right eye waa white,
the left eye red. See text, (.\fter
Morgan.)

250

GENETICS AND EUGENICS

for on this hypothesis, but many other cases in Drosophila
cannot, for which reason Morgan and Bridges favor a differ-
ent explanation. B and C (Fig. 154) are explanations of sex-
mosaics offered at different times by Morgan. In B it is
supposed that two sperms have entered the egg, one of
which united with the egg-nucleus and produced a female
(2X), hybrid as to sex-linked characters, the other developing
by itself produced male parts showing only characters of the
father. This explanation evidently will not fit the case of
(Fig. 153) because the male side of the fly inherits from the

Fig. 154. Three different explanations which have been offered to account for the production of gynan-
dromorphs (sex-mosaics or sex-intergrades) in Drosophila. See text. (After Morgan.)

mother, not the father. An alternative explanation, C, is
offered by Morgan for such cases as this. It is supposed that
the egg has been normally fertilized but that in a division
of the fertilized nucleus, one division product of an X-
chromosome gets left behind at the middle of the spindle.
Thus one daughter nucleus gets two X-chromosomes (fe-
male) and the other only one (male). Whether the male
part shows maternal or paternal characters will depend on
which X-chromosome (maternal or paternal) was eliminated.
Explanation C is thus an alternative to A for cases in which
the male part of the mosaic shows maternal characters, and it
also affords an explanation (alternative to B) of cases in which
the male part of the mosaic shows paternal characters.
In contrast to the case of insects, the dependence of second-

SEX DETERMINATION 0.5 1

ary sex differences in mammals and birds upon the presence
of the gonads acting through secretions (hormones) is clearly
shown by the experimental work of Steinach and Goodale.
The former castrated immature male rats and guinea- i)igs
and then introduced into the bodies of the castrated males
ovaries of the female of the same species. The transplanted
ovaries became established and caused remarkable changes
in the castrated animals. Their mammary glands, which are
rudimentary in the male, became greatly enlarged. The
body remained small as in females and the fur soft. Their
behavior too was more like that of females than of males.

Goodale (1916) performed a similar experiment on male
brown Leghorn chicks with like results. (See Fig. 155.)
Goodale (1911% 1913) found also that mere removal of the
ovaries from female birds (hens and ducks) causes them to
assume, to a considerable extent, the quite different appear-
ance of males and that castrated males fail to develop many
of the normal male characteristics. It is accordingly clear
that some secretion of the ovary normally acts as an inhibitor
against the development of male plumage in birds, and that
in males a secretion of the testis is necessary for full develop-
ment of the secondary sex characters.

Morgan has shown that what in female fowls acts as an
inhibitor to the development of male plumage is not a secre-
tion of the egg-cells proper but a secretion of certain "luteiil
cells" normally present in the ovary. He finds that in Se-
bright bantams, which breed has hen-feathered males,
"luteal cells" are present in the testis of males as well as in
the ovary of females. Consequently when Sebright bantam
males are castrated they become ** cock-feathered," that is
they grow the long tail-feathers and the hackle feathers
characteristic of males in other breeds of fowls. In crosses
of Sebright bantams, in which the cocks are hen-feathered,
with black-breasted game bantams, in which the cocks are
normal, Morgan found that hen-featliering behaved iis a
non-sex-linked dominant character probably involving two
distinct genetic factors.

252 GENETICS AND EUGENICS

To recapitulate, we have in fowls this relationship of
plumage and other secondar^^ sex characters to the gonads
or their secretions. Fowls of both sexes will develop the
same plumage characters, viz., the full plumage of normal
males, if no secretions interfere. In females such an inhibit-
ing secretion is normally produced by luteal cells present in
the ovary, and in hen-feathered males luteal cells in the
testis produce a similar secretion. If luteal cells are intro-
duced into castrated males (in transplanted ovaries) the
birds become hen-feathered. Likewise if the luteal cells
are removed (with the ovary) from a female, she becomes
"cock-feathered." If the luteal cells are removed (with the
testes) from a hen-feathered cock, he becomes cock-feathered.
Hence ** hen-feathering" in either sex is due to the secretion
of luteal cells, not to the sex-cells proper. But the developed
condition of comb and wattles normally seen in males is due
to a different secretion formed by the testis. For this con-
dition disappears in castrated males and is not attained in
feminized males into which ovaries have been introduced.

In male sheep a secretion of the testis seems to act as a
stimulant to horn development, for male sheep regularly have
larger horns than females (Fig. 97) and in some breeds, for
example the merino, males only have horns. (See Figs. 101
and 103.) Early castration of the male in such breeds results
in hornlessness.

Finally Lillie (1916) has shown that in cattle hormones
in the blood of the developing male, if allowed to enter the
circulation of the developing female, so interfere with the
growth of the ovary as to render its possessor sterile. This
is the explanation of the *'free martin," a sterile female calf
born as a twin to a male calf. The twins in this case begin
their development, each from a separate fertilized egg, but
become later so closely crowded together in the uterus of the
mother that their foetal blood vessels unite, allowing the
blood from one embryo to pass freely over into the other.
A sterilizing influence on the female results, the ova in the
body of the female embryo failing to grow, but no reciprocal

SEX DETERMINATIOX 253

influence on the male has been noted, nor is the sex of the
female changed but merely her sexual development repressed.

An interesting case of sex control through secretions litis
recently been discovered in a mollusk, Crepiduhi. The in-
dividuals of this species normally function as males when
they are small, at that time developing sperm, but when
grown to larger size they develop eggs and function as fe^
males. Since eggs and sperm are not developed simultane-
ously m the same mdividual, the eggs are regularly cross
fertilized. Gould has shown that if a smaU Crepiduhi is iso-
lated from other individuals it remains a "neuter," but that
if it is brought withui a few millimeters of a large (female)
individual, it proceeds to develop as a male and liberates
sperm. The action is supposed to result from substances
given off into the sea water from the body of the nearby
female. As the individuals of Crepidula remain in one i)lace
practically throughout their adult life, this curious adapta-
tion has manifest advantages to the species.

The egg or larger gamete (the so-called macro-gamete) in
all animals is non-motile and contains a relatively large
amount of reserve food material for the maintenance of the
developing embryo. This reserve food material it is the func-
tion of the mother to supply. In the case of some animals,
for example flatworms and mollusks, the food supply of the
embryo is not stored in the egg-cell itself, but in other cells
associated with it, which break dow'n and supply nourish-
ment to the developing embryo derived from the fertilized
egg. Again, as in the mammals, the embryo may derive its
nourishment largely from the maternal tissues, the embryo
remaining like a parasite within the maternal body during
its growth, feeding by osmosis. But in all cases alike the
mother supplies the larger gamete and the food material
necessary to carry the zygote through its embryonic stages.
The father, on the other hand, furnishes the bare hereditary
equipment of a gamete, with the motor ai)paratus nect^ssary
to bring it into contact with the egg-cell but without food for
the developing embryo produced by fertilization. The ga-

254 GENETICS AND EUGENICS

mete furnished by the father is therefore the smaller gamete,
the so-called micro-gamete.

From the standpoint of metabolism, the female is the more
advanced condition: the female performs the larger function,
doing all that the male does in furnishing the material basis
of heredity (a gamete), and in addition supplying food for the
embryo. As regards the reproductive function, the female is
the equivalent of the male organism, plus an additional func-
tion, — that of supplying the embryo with food. When we
come to consider the structural basis of sex, we find, often in
differences in chromosome number, reasons for thinking that
here, too, the female individual is the equivalent of the male
plus an additional element.^ The conclusion has very natu-
rally been drawn that if a means could be devised for increas-
ing the nourishment of the egg or embryo, its development
into a female should be thereby insured, while the reverse
treatment should lead to the production of a male.

In a few cases it has been found possible by indirect means
to control the state of nutrition of the eggs and so to control
the sex of the individual which develops from it. Thus in the
rotifer, Hydatina senta, parthenogenetic eggs of two sorts
are produced, which are either male-producing or female-
producing, the former being smaller. Whitney has shown
that when a colony of Hydatina is fed for a generation exclu-
sively on the green flagellate, Dunaliella, practically all the
mothers lay male-producing eggs, but a continuous diet of
the colorless flagellate, Polytoma, leads to the production of
female eggs. The effect in each case is seen not in the first
generation, but in the second generation of offspring. The
female fed on Dunaliella has grandsons; the female fed on
Polytoma has granddaughters. The diet of the mother is
immaterial.

In pigeons, eggs are produced in clutches of two each, and
in wild species these commonly develop, one into a male, the

^ But in the poultry type qf sex -linked inheritance it is evident that the male is
more liberally equipped with certain genes, in which he is duplex while the female
is simplex.

SEX DETERMINATIOX 255

other into a female. Riddle has shown tliat the female-pro-
ducing egg is the larger of the two and contains the larger
amount of potential chemical energy. If the eggs are re-
moved from the nest as fast as laid, the female is induced to
lay a larger number of eggs than she would otherwise have
laid and the majority of these are female-producing. Toward
the end of the season nothing but females may come from
eggs the production of which is forced in this way.

In such cases sex is subject to a certain amount of conlnjl
through the state of nutrition of the egg itself. But, neither
in this case nor in that of most other animals is the state
of nourishment of the single eggs directly affected by nourish-
ment of the mother.

In certain cases (Daphnia) poor nutrition of the mother
may diminish the number of eggs which she liberates, without
increasing the proportion of males among the offspring pro-
duced, since nourishment of the individual egg is not lessened,
for the eggs under such circumstances resort to cannibalism,
devouring one another, and those which survive are fully
nourished.

Attempts to influence the sex of an embryo or larva by
altered nutrition of the embryo or larva itself have proved
equally futile. Practically the only experimental evidence of
value in favor of this idea has been derived from the study of
insects, and this is capable of explanation on quite different
grounds from those w^hich first suggest themselves. It Inis
sometimes been observed, as by Mary Treat for example, that
a lot of insects poorly fed produce an excess of males. In
such lots, however, the mortality is commonly high, and
more females die than males, because the female is usually
larger and requires more food to complete her develoi)ment.

A delayed fertilization of the egg has in certain ciises,
notably frog's eggs, been shown to increase the i)ercentage of
male offspring. This is not due to any change in the si)er-
matozoa, as experiment clearly shows, but merely to the rela-
tive staleness of the egg. If the fertilization of the frog's egg
is delayed three or four days after its passage into the uterus,

256 GENETICS AND EUGENICS

more male offspring occur. It is possible that the chemical
composition of the egg changes when fertilization is delayed,
the total energy content decreasing and so diminishing the
probability that the egg will develop into a female. Riddle's
work with pigeons suggests such an interpretation.

Further frog's eggs may by various means be caused to
develop parthenogenetically. Loeb (1918) has raised twenty
leopard frogs from unfertilized eggs artificially stimulated
into development by the prick of a needle. Among these
frogs both sexes were represented and the chromosome num-
ber was found to be diploid. Accordingly sex differentiation
in this case would seem not to have depended upon chro-
mosome reduction of the ordinary sort. Again King (1912)
has shown that keeping toad's eggs out of water for several
hours after fertilization raises the percentage of female
young from fifty to over seventy. Hence it may be, as
Riddle thinks, that the natural sex tendencies of the gametes
may under certain conditions be overbalanced or counter-
acted by other agencies influencing metabolism, the eggs
perhaps developing parthenogenetically without passing
into the haploid chromosome state of ordinary gametes.

What are we to understand by the expression, '* natural sex
tendencies of the gametes " ? Obviously what is meant is the
genetic constitution of the gametes, that is their content of
genes. But we have seen that strong reasons exist for believ-
ing that genes are found exclusively in the chromatin. If this
is so, "natural sex tendencies of the gametes," can mean only
composition of the gametes as regards chromatin. One of the
most important generalizations reached in recent years by
cytologists is this, that the chromatin composition of the
gamete does in reality determine its natural sex tendencies.

In a great many animals, possibly in all, the chromosome
composition of the individual's cell-nuclei bears an interest-
ing relation to its sex. Thus in bees, ants, wasps, and related
insects, as well as in small Crustacea and rotifers, only females
develop from fertilized eggs, i. e., from zygotes, whereas males
develop from unfertilized eggs which have the nuclear con-

SEX DETERMINATION /

52.57

stitution of gametes, and which, in some cases at least, are
capable of actually functioning as gametes. It would seem
that in such cases the female must have a duplex chromosome
composition, since two gametes have united to pnxhice it,
whereas the male can be only simplex, since he represents
a developed gamete.

The case of the honeybee affords a familiar example, l^ie
mother bee, or "queen "of the hive, lays eggs which are
capable of development either vnth or without fertiHzatioii.
The mother is able to produce or to withhold fertlHzalion
according to circumstances, for she has in a sac connected
with the oviduct a supply of sperm received at mating. The
eggs pass the outlet of this sac as they are laid. The outlet
of the sac is controlled by muscles which relax when an egg
is to be fertilized, permitting sperm to come in contact with
the egg, but closing the outlet tightly when the egg is not to
be fertilized. Fertilized eggs are laid in cells of the reguhir
size in the wax comb, but unfertilized eggs are laid only in
cells of a larger size known as drone cells. The fertilized eggs
develop into females, even if they are moved from ordinary
cells to drone cells; but the unfertilized eggs produce males,
even if they are transferred to cells of ordinary size, in which
case, however, they will become small-sized drones because
of the limited amount of space in which they complete their
growth. Fertilized eggs developing in cells of ordinary lioney-
comb size produce female bees with imperfectly-developed
sex organs, known as workers. They are the individuals that
gather honey and pollen and feed the young of the colony.
A fertilized egg, which produces a larva that receives special
care and nourishment and develops in a cell of unusual size,
gives rise to a queen, a fully developed female cai)al)le of mat-
ing and laying great numbers of eggs, but without the struc-
tural peculiarities or instincts of workers. From these facts
it will be clear that, in the bee, fertilization detennines sex,
though environment (size of cell, food of the lars^a) may de-
termine many other characteristics of the individual. As
regards their origin, the female is a zygote produced by the

258 GENETICS AND EUGENICS

union of two gametes, the male is derived from a gamete
developing by itself. So far as chromosome constitution is
concerned, the female is duplex, the male simplex.

In small Crustacea, and rotifers, the case is slightly differ-
ent. The female here, too, is duplex and the male simplex,
but the conditions of their origin are less simple, for the
mother here produces three different kinds of eggs. The first
kind never passes into the simplex state of ordinary gametes,
but retains the duplex number of chromosomes, omits the
reducing cell-division, and begins development at once un-
fertilized and duplex. It forms a female, like the mother in
all respects. The other two t^^Des of eggs undergo reduction
and pass into the condition of gametes, with the simplex chro-
mosome number. They differ in size. The smaller-sized egg
develops unfertilized into a male (simplex) individual, which
forms simplex sperm just as the male bee does, by omitting
a reduction division in spennatogenesis. The larger-sized egg
(winter egg) is incapable of further development without the
stinmlus of fertilization. When fertilized, it develops into a
female individual, since in consequence of fertilization it con-
tains the duplex chromosome number.

The cases of bee and rotifer agree in this, that the female
regularly has the duplex chromosome condition, the male the
simplex condition, a difference completely parallel with that
between Oenothera Lamarckiana (which has fourteen chro-
mosomes) and its mutant gigas (which has twenty-eight).

In plant lice the difference between the sexes as regards
chromosome number is not so great. Here the female merely
has one or two chromosomes more than the male, recalling
the mutant Oenothera lata, which has one more chromosome
than the parent species, Lamarckiana. The male however
in plant lice develops from an unfertilized egg, partially re-
duced in chromosome nimiber. The female arises either from
an egg unreduced and so with the full duplex number of
chromosomes, and which develops without fertilization into
a female, or from a reduced egg (a true gamete) which has
been fertilized and thus brought back to the duplex condition.

SEX DETERMINATION 259

If one were inclined to be facetious, he might say that in all
these lower animals, duplicity is synonomous with femak-iiess,
simplicity with maleness!

It should be noted in passing that among plants as well as
among animals, an unfertilized gamete may undergo multi-
plication and growth while in the simplex, reduced condition.
The ordinary fern plant is a zygote with a duplex chromo-
some number. But it produces reproductive cells (spores)
containing the reduced (simplex) chromosome number, and
these after growing into a small inconspicuous little plant,
known as a prothallus, produce the functional gametes {<^gQ
and sperm-cells) without further reduction. Union of these,
egg with sperm, produces duplex zygotes again, which
develop into the ordinary fern plant.

In many animals in which males and females alike arise
from fertilized eggs, there occurs nevertheless a difference in
chromosome number between males and females, the female
always containing the higher number, as in the partheno-
genetic plant lice. One of the best-known cases is that of
the common squash bug, Anasa tristis, first worked out by
E. B. Wilson, but since fully confirmed by the obsers-ations
of others. In this animal the body-cells of the female contain
twenty-two chromosomes, those of the male twenty-one.
Historically this is a famous case, the first one in which the
mechanism of sex determination was definitely ascertained.
The egg, according to Wilson, always undergoes reduction to
the simplex chromosome number, eleven. But reduction in
the male is less simple because the male contains an odd
number of chromosomes, viz., twenty-one. All the spenu
cells cannot receive the same number of chromosomes at the
reduction division, unless the odd chromosome splits, but this
it refuses to do. The division occurs into cells with eleven
chromosomes, and those with ten. Both nietamon^hose into
sperm cells. The 10-chromosonie sperm cells, if they ferti-
lize an egg, cause it to develop into a male, since Egg 11 +
Sperm 10 = 21, the number characteristic of the male. But
the 11-chromosome sperm fertilizing an egg causes it to

260 GENETICS AND EUGENICS

develop into a female, since Egg 11 + Sperm 11 = 22, the
female number. The first man to suggest a relation between
the odd chromosome and sex determination (McClung) sup-
posed of course that the extra chromosome must go to pro-
duce a male, the more important sex, and he called it a male
sex-determining chromosome, but it turned out otherwise. The
extra chromosome is really a female sex determinant. When
a difference exists between the sexes in chromatin content,
it is regularly the female that has the larger supply. The
significance of this we may inquire into further.

In some cases, several of which are described by Morgan,
the number of chromosomes is found to be the same in both
sexes, but one of the chromosomes in the female is regularly
larger than the corresponding chromosome in the male. This
indicates that the female, in this case also, contains some
chromosome element not found in the other sex.

But Wilson and his pupils have shown that in species in
which the female contains two X-chromosomes and the male
one such chromosome, a new chromosome may appear in the
male, a so-called Y-chromosome, which the female does not
normally possess. What its precise function is has not yet
been ascertained.

Finally, in many animals no difference has been detected
between the chromosome composition of the two sexes, but
this does not preclude the existence of such a difference, even
though it has not yet been discovered.

To summarize the foregoing, there are many known facts
which support and none which contradict the idea that the
female has a greater chromatin content than the male and,
either by reason of this fact or independently of it, has
greater anabolic activity in reproduction, producing macro-
gametes, gametes stored with food. Micro-gametes, those
not stored with food but generally possessed of locomotive
ability, are the distinctive product of males.

Morgan (1913) assumes that the chromatin element, which
occurs in the female but not in the male, is the specific cause
of femaleness, that is, of egg production, and so speaks of the

SEX DETERMINATION 2G1

odd chromosome (when this occurs) as a sex-cliromosome, or
an X-chromosome. But a moment's reflection will show (as
Morgan himself once suggested) that quantity of such sub-
stance may be quite as influential as quality in detenniniiig
sex, since by hypothesis one X-chromosome produces a male
and two X-chromosomes a female, in species such as the
squash bug. The essential thing in sex detcnninalion is
probably not so much the possession of some particular sort
of material as the attainment of a particular grade of ana-
bolic capacity, femaleness implying a higher grade than niale-
ness, since in the former condition macro-gametes are pro-
duced, whereas in the latter micro-gametes are produced.

That maleness and femaleness are only different grades of
reproductive capacity is indicated by a study of organisms
in which the two functions are combined. In many of the
lower animals and in most of the higher plants, the same in-
dividual is capable of producing both macro-gametes and
micro-gametes. Sometimes these are produced simultane-
ously but in separate gonads, as in flatwomis and leeches
among animals, and in " perfect " flowering plants. Such
parents are true and simultaneous hermaphrodites. Some-
times the individual may function at first as a male and later
as a female, a condition known as successive hennaphro-
ditism. This is found in certain worms and moUusks and in
the prothallia of certain ferns and mosses. This condition is
also approached in flowering plants such as cucinnbers,
melons, and squashes, which at first produce only male blos-
soms but later produce those of both sexes. In other cases
the individual may function chiefly as of one sex but partially
as of the other sex. This condition is found in polygamo-
dioecious plants and exceptionally in such animals as cray-
fish, mollusks, worms, and even frogs and fishes, which, in a
particular part of an ovary may develop sperms, or in a
particular part of a testis may develop eggs.

Such facts as these indicate that maleness and fenudeness
are merely different grades of one and the same f onn of repro-
ductive activity. This is not inconsistent with their behavior

262 GENETICS AND EUGENICS

— \V .• V :

as Mendelian alternatives in heredity, for in color inheritance
different grades of pigmentation, of spotting, etc., frequently
)3ehave as Mejpidelian allelomorphs. So probably different
degrees of sexual distinctness behave in heredity, for in the
plant, Lychnis, Shull has shown that femaleness is allelo-
morphic not only with maleness but also with hermaphro-
ditism, the three conditions being triple allelomorphs. A
similar interpretation may perhaps be given to conditions
found in certain mosses as discussed by Collins.

!•.

PrK'ate Property of

^' P. rvlETCALF

PART II

EUGENICS

<0

CHAPTER XXX

HUMAN CROSSES

Mi^^NKiND consists of a single species; at least no races exist
so c^istinct that when they are crossed sterile progeny are pro-
duced. The widest possible human crosses are comparable
with the crossing of geographical varieties of a wild species
of animal, or with the crossing of distinct breeds of domesti-
cated animals. The race horse and the draft horse differ as
muph in bodily conformation and temperament as do the
Dio^^t diverse races of mankind.

0/fspring produced by crossing such races do not lack in
vigor, size or reproductive capacity. But these are not the
only qualities w^hich we desire either our horses or our citi-
zens ^ to possess. It is a particular combination of qualities
whic:}! makes a race horse useful, and a different combination
whicl|i makes a draft horse useful. Crossing the two will
produ^ce neither one type nor the other. The progeny will
be use.jess as race horses and they will not make good draft
horses. A second generation of offspring will be more vari-
able but will rarely approach the specialized type of either
the race hvr)rse or the draft horse, and will be too heterogene-
ous in chan^cter to serve any single purpose well. For such
reasons as Lhese, pure breeds of domesticated animals are
rarely crossed unless a new type of animal is desired to meet
special needs i^^^nd conditions. Even then many animals of
small value musi'^ be produced and discarded and this process
must be continue! i for generations before the new type can
be established. For such reasons wide racial crosses among
men seem on the whoJe undesirable. There is no question
about the physical vigeor of the offspring, provided the
parents are free from disea^se. The statement is often made
that mixed races are feeble, btut if this is ever true it is not
because they are mixed, but beca-use the specimens that mix

m \

we GENETICS AND EUGENICS

are feeble. Mating out of the race, when mates within he
race are available, is prima facie evidence that the individial
so mating is a social outcast. It is not surprising that he
progeny of such individuals are sometimes feeble. If he
parents were diseased, licentious, or feeble-minded, i^ is
natural that the children should be of like character.

Of course not all racial crossing implies such conditiois.
Frequently Europeans, when pioneers in a new country iud
without mates of their own race, have married native wonen.
Such men have not always been social outcasts; frequertly
they have been men of great energy, ability, and courige
both physical and moral, and free from disease. When in
such cases, the mothers belonged to a race with capacity for
civilization, the results have been good. Examples ma^ be
found among the Indian citizens of our southwest states.
But human racial crossing in general is a risky experiment,
because it interferes with social inheritance, which after all
is the chief asset of civilization. Physically and also intel-
lectually, according to Professor Osborn, we are no whit
superior to the men of twenty-five thousand years ago. All
the advantage which we have over them lies in the accumu-
lated experience of the human race since then.

All this we as individuals learn from our mothers and
fathers, or in the schools, the churches, the markets, or the
courts of justice. Wide racial crosses unsettle the founda-
tions of these agencies of enlightenment. At times it is
necessary that some of these agencies be distur'bed in order
that we may lay their foundations deeper and broader, but
racial crossing leads rather toward the discarding of all
foundations of civilization than to improvin.g them.

Such crosses, therefore, as of Europear^s with Asiatics or
Africans can not be recommended as age^acies for the improve-
ment of the human race. Physically T^uropeans on one hand
and Asiatics or Africans on the oth-^^r, are suflSciently diversi-
fied among themselves to allow 'the maximum benefit from
intercrossing, without resort^'^ng to crosses with a distinct
branch of the human fam^ily. Socially the effects of such

\

i

HUMAN CROSSES 2ii7

crosses on a large scale are too disturbing to be recommended.
This country has seen a suflSciently extensive experiment of
that sort in its southern states, the outcome of which we
shall not know fully for several generations yet. It is desir-
able that each nation should have the fullest intercourse with
every other in commerce and in the exchange of ideas. This is
mutually beneficial to all, but the obliteration of all racial
differences within the human family is not to be expected or
desired.

What has been said thus far refers only to crosses between
the widely separated branches of the human family and even
as regards such cases may be accepted with reservation, since
there is room for a difference of opinion concerning such
matters, which are not primarily biological, but sociological.

What opinion one holds will also depend upon his point of
view. From the viewpoint of a superior race there is nothing
to be gained by crossing with an inferior race. From the
viewpoint of the inferior race also the cross is undesirable if
the two races live side by side, because each race w ill despise
individuals of mixed race and this will lead to endless friction.
About the only conditions under which a racial cross of this
sort could be fairly tested would be those under which Pit-
cairn Island was populated. Here more than a century ago
a few English sailors and a few Polynesian women founded a
population still in existence and flourishing. Neither pure
race was present to create social distinctions or racial anti-
pathy. The story of this hybrid human race is a romantic one.

In the year 1788 the Englishman, John Bligh, who as
sailing master had been round the world with Captain Cook
on his second voyage, was commissioned by the British
Government to go to Tahiti, secure plants of the bread-fruit
tree and introduce them into the West Indies. To this end
he was given command of the ship Bounty. Bligh proved a
harsh and oppressive captain, and on his way from Tahiti to
Jamaica the crew mutinied. They put the captain with
eighteen of his crew into the ship's launch and themselves
turned back to Tahiti. The captain and his companions after

1268 / GENETICS AND EUGENICS

three months of hardship all reached land (Timor, three
thousand six hundred miles from where they started) safely,
and were taken back to England. The British Government
sent out a warship to punish the mutineers and part of them
were captured on Tahiti. But their leader and nine other
sailors had already escaped to Pitcairn Island in company
with eighteen natives, six men and twelve women. Their
place of refuge remained a secret for twenty years, when it
was accidentally discovered by an American sealing ship
which visited the island in 1808. Pitcairn Island is the
southernmost island of the Low Archipelago in latitude 25°
S. and longitude 180° W. It is about two miles long and one
mile wide, and consists of a mountain surrounded by coral
reefs. For ten years after the landing of the refugees, dis-
order and lawlessness prevailed. In 1808 the sole survivors
were one Englishman by the name of John Adams (formerly
Alexander Smith), eight or nine women, and several children.
It is related that the elements of disorder being removed
Adams instilled ideas of morality and religion into the others,
with the result that the settlement prospered. In 1815 when
the ship Britain visited the island, the captain was impressed
with the peace and good order prevailing. In 1839 the island
became a British dependency. In 1855 the number of in-
habitants had increased to two hundred and the island was
becoming too small for them. They therefore petitioned the
British government to be removed to Norfolk Island, which
was done the following year. Since then some of them have
returned to Pitcairn Island whose present population is about
one hundred and twenty -five. The population of Norfolk
Island in 1901 was eight hundred and seventy, mostly
descendants of the Pitcairn Islanders.

Here then on these two islands is a race of probably one
thousand persons at the present time, originated more than
a century ago by a cross between English men and women of
Tahiti. The experiment has gone far beyond the Fi genera-
tion and would afford unique material for a study of the
effects of race-crosses uncomplicated by race-antipathies. So

HUMAN CROSSES 269

far as present information goes the results have been excellent
both biologically and sociologically. It is to be hoped that
some student of eugenics will give the case careful and
critical study.

Another successful experiment in human racial crossing has
been recently studied and described by a German, Fischer,*
who chronicles the origin of a tribe in Genu an Southwest
Africa of mixed Boer and Hottentot blood. This arose from
the intermarriage with native Hottentots of a few Boers dis-
satisfied with British rule in South Africa, who penetratecl
far northward among hostile tribes, and were thus forced to
combine with each other against a common enemy. IMieir
descendants, intermarrying, formed a distinct cultural grouj)
entirely surrounded by pure native stocks and wholly isolated
from contact with Europeans. Pride in their ancestry and
cultural inheritance held them together and prevented mix-
ing with neighboring tribes. After this had gone on for
several generations they came within the German zone of
colonial influence (again British at present under the fortune
of war). Very likely the group as such will presently dis-
appear, but the experiment has progressed far enough to
show that under conditions which do not interfere with
cultural inheritance crossing of racial stocks as widely sepa-
rated as Europeans and Africans has no evil consequences,
but produces a vigorous, sound race. Fischer finds evidence
of Mendelian inheritance of physical characters among these
people, but critically examined, this evidence is substantially
like that available from other sources. Some characters,
such as hair and eye-colors show fairly good segregation. As
regards skin-color, proportions of the skeleton, features, etc.,
the hybrids are intermediate between the parent races, but
more variable. It is probable that intelligence and other
psychic traits are inherited in this way.

Racial crosses, if so conducted as not to interfere with
social inheritance, may be expected to produce on the whole
intermediates as regards physical and psychic characters.

1 " Die Rhehobothen Bastarden," 1911.

270

GENETICS AND EUGENICS

This seems to have been the result in Central and South
America and in the West Indies, where racial crossing has
taken place to a very great extent. A similar outcome seems
likely to occur in Africa, as that continent is further overrun
by European races. The leading racial stocks of Asia seem
at the present moment to have such physical, mental, and
cultural vigor that they are not likely to amalgamate with
European races.

i

> (

CHAPTER XXXI

PHYSICAL AND MENTAL INHERITANCE IN MAN

The same laws govern inheritance in man as in other animals
and in plants, but our knowledge of human heredity is k-ss
accurate than that of animals and plants, because we are in
the human field debarred from experiment, llie best we
can do is to observe and compare the traits of individuals in
successive generations and thus to ascertain with what known
laws of heredity these cases best agree. For the discovery of
new laws of heredity, human data can have little value be-
cause of our inability to experiment. Nevertheless the inter-
est in human heredity is so general and the num})er of
competent observers so large, including as it does a great
many physicians and other men of science, that we may look
forward to a very complete cataloguing of human heredity
as fast as general categories of inheritance phenomena are
established by the experimental study of other organisms.
Already we have in hand a great amount of material bearing
on human heredity, gathered chiefly by medical men, much
of it within the last fifteen years. A considerable part of this
is unreliable because of the careless or biased way in which it
has been gathered, or the uncritical treatment which it has
received in publication. But still there remains a consider-
able body of valuable information, which shows that man is
subject to heredity in every aspect of his physical and
mental make-up.

Two comprehensive attempts have been made to gather
and analyze data concerning human inheritance, one in Eng-
land at the Eugenics Laboratory of the LTniversity of Loiulon,
founded by Galton and presided over by Karl Pearson, the
other and more recent one at the Eugenics Record Oflice,
Cold Spring Harbor, New York, directed by Dr. C. R.
Davenport. Pearson's data are recorded in the " Treasury

271

27^2

GENETICS AND EUGENICS

TABLE 33

Inherited Characters in Man

1 . Blending

General body size, stature, weight, skin-color, hair-form (in cross-section, corre-
lated with straightness, curliness, etc.) shape of head and proportions of its parts
(feat ares).

2. Mendelian
Dominant

Skin and hair '

Eyes

Dark.

Spotted with white.

Tylosis and ichthyosis (thickened
or scaly skin).

Epidermolysis (excessive forma-
tion of blisters).

Hair beaded (diameter not imi-
form).

Front of iris pigmented (eye

black, brown, etc.).
Hereditary cataract.
Night blindness (when not sex

limited).
Normal.

Recessive

Blonde or albino (probably

multiple allelomorphs).
Uniformly colored.
Normal skin.

Normal skin.

Normal hair.

Only back of iris pigmented

(eye blue).
Normal.
Normal.

Pigmentary degeneration of
retina.

r-\

Skeleton

Kidneys

K

Brachydactyly (short digits and

limbs).
Polydactyly (extra digits).
Syndactyly (fused, webbed, or

reduced number of digits).
Symphalangy (fused joints of

digits, stiff digits).
Exostoses (abnormal outgrowths

of long bones).
^ Hereditary fragility of bones.

Diabetes insipidus, (excessive pro-
duction of urine).
Normal.

Normal.

Normal.
Normal.

Normal.

Normal.

Normal.

Alkaptonuria (urine black on
oxidation).

Nervous j Huntington's chorea.
System \ Normal.

Normal.

Hereditary feeble-mindedness.

HUIVIAN INHERITANCE <27f^

3. Mendelian and Sex-Linked
(Appearing in males when simplex, but in females only when duplex.)
Dominant Recessive

Normal. Gower's muscuhir atrophy.

Normal. Haemopliiliu (liltM-ilin;?). '

Normal. Color blindness (inability to

distinguish red from ^n't'n).
Normal. Night blindn<'ss (inability to

see in faint light j,

4. Probably Mendelian but Dominance Uncertain or Imperfect

Defective hair and teeth or teeth alone, extra teeth, a double set of permanent
teeth, hare-lip, cryptorchism and hypospadias (imperfectly developed male organs),
tendency to produce twins (in some families determined by the father, in others by
the mother), left-handedness, otosclerosis (hardness of hearing owing to thickened
tympanum).

5. Subject to Heredity, but to what Extent or how Inherited Uncertain

General mental ability, memory, temperament, musical ability, literary al)ility,
artistic ability, mathematical ability, mechanical ability, congenital deafness, lia-
bility to abdominal hernia, cretinism (due to defective or diseased thjToids), defec-
tive heart, some forms of epilepsy and insanity, longevity.

of Human Inheritance" (1909). The data collected by the
Eugenics Record Office have been published in part in a
series of bulletins and monographs which is being rapidly
extended.

We may provisionally distinguish inherited human traits
as (1) blending (probably involving multiple factors); (2)
clearly Mendelian (involving a single genetic factor); (3)
Mendelian and sex-linked; (4) probably Mendelian but with
dominance imperfect or uncertain, and (5) hereditary, but
to what extent or how, uncertain.

The grounds on which a category of blending characters
may be based have already been discussed. If they are valid
for animals and plants, they are also valid for man. Here
belong characters which show intermediate inlieritance in Fi
and also in F2, but with greater variability in Fj than in F,.
Size and stature are good examples. The greater variability
of F2 shows that the blending was not perfect in F, and that
multiple factors are probably involved. Iiidieatioiis of
segregation more or less complete were observed by Daven-

274 GENETICS AND EUGENICS

port in his studies of skin-color and hair-form inheritance
in negro-white crosses, which supports the idea that mul-
tiple factors are involved, or one or more chief factors as-
sociated with modifying factors. The well known lack of
correlation between skin-color and hair-form in mulattoes of
the F2 or later generations certainly indicates the existence
of independent factors affecting these characters.

As regards shape of the head, anthropologists have long
distinguished between long-headed and round-headed races
or types within mixed races. These may be convenient terms
for purposes of classification, but it by no means follows that
the t}^es are alternative in heredity. Without positive evi-
dence to the contrary, it is safe to assume from what we know
of skull shape in animals and in negro-white crosses that
skull shape is in all cases blending (multiple factorial) in in-
heritance. Salaman (1911) himself an English Jew, has de-
scribed the Jewish tj^^e of countenance as recessive to the
Anglo-Saxon type in mixed marriages in England on classi-
fications of the offspring as of Jewish or Gentile type, made
for him by Jews, but the evidence is far from satisfactory and
not based on any clearly defined differences. If measurable
characters were considered, it is probable the inheritance
would be found to be blending, and the classification adopted
in his tables to have been based on blending in many char-
acters rather than on simple segregation in any one.

It is to be noted that in man, as in wild species of animals
and plants, characters which hlend in heredity are in no case
abnormal or monstrous conditions, but are such as distin-
guish one member of a perfectly normal population from
another.

The case is very different when we come to the category of
simple Mendelian characters, whether or not sex-linked.
Here a great majority of the characters listed refer to abnor-
malities or monstrosities. As regards variation in the color of
hair, skin and eyes, we have, in these, recessive or loss varia-
tions, similar to those of other mammals, producing a graded
series of probable allelomorphs ranging from black to albino.

HUMAN INHERITANCE 07",

z ( o

Retrogressive variation of eye pigmentation leads from
''heavily pigmented iris (back and front)" through inoiv
faintly pigmented conditions to *'iris pigmented only be-
hind," the ultimate recessive, blue. Spotting with while,
affecting skin and hair pigmentation, or affecliug only Ihc
pigmentation of the iris (Bond, 1912) are unit-character
variations completely parallel with those of rodents. Nearly
all other known Mendelizing characters in man are more or
less pathological. They include a variety of h(Teditary mal-
formations or '* diseases" affecting skin, eye, skeleton, kid-
neys or nervous system. (See Table 33.)

Many characters (mostly loss variations) are probably
Mendelian in inheritance, but not enough is known concern-
ing their behavior to permit of a positive statement in the
matter. (See Table 33, 4.)

In Section 5 of Table 33 are included many important
characters known to be to some extent hereditarv, but in
accordance with what law is still uncertain. Especially im-
portant are such characters as general mental ability, mental
capacity in special directions, hereditary epilepsy and in-
sanity, and longevity. It would be a mistake to cover up
our present ignorance concerning the inheritance of these
characters by classifying them either as unifactorial or as
multifactorial. We shall presently examine into the evidence
that the more important of these are inherited.

Hair-form. This character has been studied by Dr. and
Mrs. Davenport, whose findings may be briefly sunnnarized.
Hair having a circular cross-section is straight. But if the
hair is elliptical in cross-section, it has a tendency to become
curly. Grades of departure from the straight condition are
formed with increase in flattening of the hair in cross-section
as follows: (1) straight, (2) wavy, (3) curly, (4) kinky (Afri-
cans). Crosses produce intermediates or show imperfect
dominance of curliness, with segregation more or less com-
plete in later generations.

Hair and skin-color. Hair-color is in general correlateii
with skin-color, the darkest shades of hair-color l)eing found

276 GENETICS AND EUGENICS

only in persons with dark skin. Whole races of mankind
have only black hair and dark skin (known as ''black, brown,
red or yellow"). A dark skin is an adaptation to life in a
tropical country or one having much intense sunlight. Fair-
skinned races are unable to endure life in the tropics unless
the body is protected from the direct rays of the sun. Dark-
skinned races, however, have a natural protection against
the effects of direct sunlight. From an evolutionary stand-
point the white races are possibly retrogressive variations,
*'loss" variations. In a population of Europeans, the darker
shades of hair and skin-color are either completely or incom-
pletely dominant. It is not at all uncommon to find a
mixture of dark-haired and light-haired children in the
same family, provided one or both parents are dark-haired,
but when both parents are light-haired, the children are all
light-haired. This result shows that the lighter shades of
hair-color are recessive in relation to the darker shades. An
exact estimate is often difiScult to make because persons with
light hair in childhood often have much darker hair when
adult, and further, the hair may later become gray or even
white, which makes direct comparison with the hair of
younger persons impossible.

Extremely pale conditions of hair, skin and eye pigmenta-
tion are known as albinism and occur in all races, even in
negroes and American Indians. Albinism is clearly a reces-
sive character in relation to normal pigmentation. The vari-
ous shades of blonds probably correspond physiologically
and as regards inheritance with the graded series of albino
allelomorphs found in guinea-pigs. Each darker shade is
dominant to the lighter shades, any two in the entire series
being allelomorphs of each other. This is known to be the
case in rodents and probably holds for European races of
mankind. In other races of mankind blond variations are
rare, even more so than extreme albinism. Here again we
have a condition parallel with that found in most rodents, in
which the albino variation is known, but not other members
of the graded series of retrogressive allelomorphs.

HUMAN INHERITANCE 277

In a cross between a negro and a white person, children are
produced of an intermediate, but frequently varial)le skin-
color, and are known as mulattoes. Mulattoes mating ijifer se
produce an F2 generation of highly variable skin-color but
rarely pure white. Davenport has concluded that two inde-
pendent Mendelian factors affecting skin-color are involved.
This explanation would lead us to expect one in sixteen of the
F2 mulatto offspring to have skin as white as a Euroixan,
even though his negro ancestry might show in other charac-
teristics, such as curly hair, broad nose, thick lips, etc. It is
difficult to get any wholly satisfactory evidence either for or
against this explanation. That published by Davenport can
scarcely be considered conclusive, for the data studied are
derived from a population in which illegitimacy, by Daven-
port's own statement, is as high as 72 per cent. On the
whole, it seems probable that segregation of skin pigmenta-
tion in mulattoes is either incomplete or rarely complete, be-
cause multiple or modifying factors are involved.

A clearly and sharply defined Mendelian factor which in-
volves spotting with white occurs in many human families,
as in domesticated animals. In some families a lock of white
hair (usually above the middle of the forehead, or on top of
the head) is inherited as a Mendelian dominant (transmitted
only through affected individuals). Irregular spotting of the
body with unpigmented areas has been shown to be heredi-
tary as a dominant character in a family of Louisiana n^»ror.; ® L A c>
(exhibited in Europe and America), and a similar variation
is inherited in the same way in a white family in Minnesota,
one or more of whom have studied at the University of
Minnesota.

s

i

CHAPTER XXXIl

HEREDITY OF GENERAL MENTAL ABILITY, INSANITY,
EPILEPSY, AND FEEBLE-MINDEDNESS

One of the first investigations carried on in the la])oralory
of Pearson related to the inheritance of abiUty as indicated
by the " class lists " (rank lists) of Oxford. The investigation
of the relative rank of two thousand five hundred pairs of
fathers and sons showed that a distinct correlation exists
between them. If the father took high rank the son also
ranked high, and vice versa, in a considerable percentage of
cases. Expressed numerically the correlation in the Oxford
lists was found to be .31 where 1.00 would express exact
agreement in rank and 0 would express only chance agree-
ment. Between four thousand two hundred brothers the
agreement was closer still, viz., .40. Closer resemblance was
indeed to be expected, since in this case the mothers as well
as the male ancestors were the same. The conclusion reached
is that mental capacity, as indicated by rank attained at the
University, is inherited; that the proverb "like father, like
son " applies in the long run to scholarship, as well as to
physique. This is a conclusion which every experienced
teacher would have anticipated. It is interesting to find that
it has full statistical warrant.

But the further question arises whether success in study
has any relation to success in life outside of schools. Of this
question an investigation was made in Pearson's laboratory.
Rank in the Oxford B. A. examinations was compared with
subsequent rank in the professions, the Church and the Law.
The measure of success in the Church was taken to be the
holding of a high office in the Church or of a first-class
scholastic appointment. It was found that the higher the
classification of a man at the Oxford examinations, the

•
279

280 GENETICS AND EUGENICS

brighter were his prospects of attaining distinction in the
Church.

Rank in Percentage

Oxford Examinations Distinguished

First class 68

Second " 37

Third " 32

Fourth " 29

Pass degree 21

No degree 9

Of those who attained a first-class degree, 68 per cent ob-
tained ofl&cial distinction, etc.

The results of the investigation as regards lawyers were
found to be very similar. The measure of success here was
taken to be the holding of public office under the government.

Of the first class men, 46 % were so distinguished.

« " second" " 33%.

" « third " " 22%.

" " fourth " " 20%.
Pass degree men, 16%.

No degree men, 15 %.

The general conclusion reached is that the " promise of
youth " as indicated by scholarship is in general justified by
the " performance of manhood " in the professions. The
objection might be offered that appointments in church and
state may be influenced by a man's university rank, but this
is offset by results obtained in America, where this is cer-
tainly not true.

Insanity. Considerable work has been done in Pearson's
laboratory in the study of the heritability of insanity. David
Heron made a study of the inheritance of insanity as indi-
cated by three hundred and thirty-one family histories col-
lected during a period of thirty years by the superintendent
of an asylum patronized by middle-class people of Perth,
Scotland. See Table 34.

If insanity is treated as due to one and the same thing in
all cases, it is obvious that the inheritance is not Mendelian;
i. e., insanity does not behave as a simple Mendelian unit-
character, either dominant or recessive. But that insanity

INSANITY C2.S1

is in some way inherited is obvious, for it occurs much
oftener in these families than in the general i)oi)ulati()ii,
where it is between 1 and 2 per cent. But in these fanu'lic.s
21 per cent of the offspring of sane parents are insane, and
a still higher percentage of the offspring of insane parents
are insane.

The correlation coeflBcient used as a measure of the
strength of the inheritance of insanity lies ])etvveen .52 and
.62. For comparison it may be said that the correlation co-
eflScient between parent and child in the case of pulmonary

TABLE 34
Data on Inheritance of Insanity (Heron)

Children

Parents Insane Sanr % Insane

Both sane 314 1179 21

One insane 93 299 24

Both insane 4 4 50

tuberculosis was found by Pearson to be about .50; for deaf-
mutism 1 it was found to be .54; for stature .50; for intelli-
gence between .49 and .58.

Heron concludes that insanity on the whole is inherited
about as strongly as other mental and physical character-
istics.

But insanity cannot be regarded as a simple defect wliicli
can accordingly be eliminated from a population altogether,
as could albinism. Insanity is a general name for a great
variety of conditions of mental lack of balance and man>'
different factors may enter into it. Not every family stock
in which it occurs is to be regarded as unsound. 15ut the
intermarriage of families in which insanity occurs, and, still
more, inbreeding within a family containing insanity is likely
to increase the percentage of insane offspring and so should
be avoided.

Two American investigators (Rosanoft' and Orr) more
friendly than the biometric school to iVIendclian theory, have

1 Dr. Fay's U. S. data.

282

GENETICS AND EUGENICS

attempted to eliminate several categories of insanity and to
find out more precisely what the law of inheritance of the
remaining sort is. They eliminate cases possibly due to
injury to the brain, alcoholism, syphilis, tumors, apoplexy
and the like. Their material consisted of cases in the state
hospital for the insane at Kings Park, N. Y. Careful in-
quiry was made as to the pedigree of all patients whose in-
sanity was not referable to other than genetic causes.
Seventy-two families were thus investigated, representing
two hundred and six different matings, with a total of one
thousand ninety-seven offspring. These are tabulated to
test the hypothesis that insanity is a Mendelian recessive
unit-character, as follows:

TABLE 35
Data on Inheritance of Insanity (Rosanoff and Orr)

Mat-
ings

Children

rarents

Neuro-
pathic

Normal

Expected

Both insane

17
93
14
62

20

54
190

• •

107

10*

239

45

215

77

All insane.

Only one insane, DR X RR

1:1

Only one insane, DD X RR

All sane.

Both normal (but tainted), DR X DR

Both normal (only one or neither tainted),
DR X DD (?)

1:3
All sane.

* Eight have not yet passed " age of incidence."

The table seems in a general way to substantiate the
hypothesis advanced, chat insanity is a recessive character,
especially the first category of matings where only insane
progeny are expected. But when we look into the method of
gathering the data and of compiling the table we become
somewhat skeptical of this conclusion. The data have the
scientific value of gossip, consisting of answers made by " in-
formants " to leading questions designed to bring out any
weakness in the pedigree. Like inquiries made concerning
any individual in the community would show him an un-

INSANITY 283

mistakable victim of insanity. The authors frankly a^hnil
that " of the four hundred and thirty-seven persons classed
by them as neuropathic, only one hundred and fifteen, or '2().S
per cent, presented at any time in their lives indications for
commitment to sanitariums or hospitals for the insane."
Three-fourths, therefore, of their persons insane for pedi^Tce
purposes would be classed as fully normal, if they occurred in
families free from insane hospital patients. Such classifica-
tion has little scientific value.

In dealing with the pedigrees the authors class as neuro-
pathic persons whose only offence, aside from having an in-
sane relative, are the following: " Crank "; " easily excited,
nervous temperament'!; " very nervous " ; " erratic, excit-
able "; " nervous, little things bothered her, worried a great
deal "; but in one case, which goes beyond all others, the
individual is classed as insane on the following grounds:
" money mad, very cruel, very miserly though wealthy, left
much of his money to his housekeeper." To the layman this
does not read like the characterization of an insane person;
change the word housekeeper to hospital and it might de-
scribe a philanthropist and captain of industry.

It seems that, in the light of this investigation, if critically
viewed, and in the light of Heron's investigation, very doubt-
ful whether insanity in general is inherited as a Mendel ian
unit-character. Very likely there are different varieties of
insanity independently inherited. That insanity is inherited,
however, there can be no doubt. Heron quotes Pearson's
family records as including seventeen cases in which one or
both parents were insane. In only one case were all nienibeis
of the family who attained the age of fifty or over hoc from
insanity. When both parents were insane, Pearson's records
give 66 per cent of insane offspring; when only one parent
was insane, forty per cent of the oft'spring were insane, where-
as in the general population only 1 or 2 per cent are insane.
Hence with insanity in one or both parents, the percent-
age of insane progeny increases; on this all investigators
agree.

284

GENETICS AND EUGENICS

The practical conclusion is obvious : insane persons should
not be permitted to marry; indeed legislation forbids this in
most countries. Further it would be well to avoid marriage
into families in which insanity is common. It need not be
assumed, however, that every person who has had an insane
relative is an unfit mate. For such a conclusion, if enforced,
would soon bring human breeding to a standstill.

Epilepsy. As regards the inheritance of epilepsy and
feeble-mindedness the evidence is much clearer. By epilepsy

TABLE 36

Epilepsy and Feeble-Mindedness in Epileptic Families

{Davenport and Weeks)

Children

Parents

Number of
Matings

Epileptic

Feeble-
Minded

Normal

Both enileDtic

1
5
6
3

3

8
5

1

6

16

4

One epileptic, one feeble-minded ....

Both feeble-minded

One epileptic, one insane

9*

* One " nem'otic."

we understand such nervous troubles as manifest themselves
in the simplest cases in momentary loss of consciousness, and
in extreme cases in marked convulsions. Much so-called
epilepsy is probably due to infection with syphilis, congenital
or otherwise, in which case its inheritance would be apparent
only.

But if we leave out of account this possible complication,
the inheritance seems to be that of a simple recessive Men-
delian character. Davenport and Weeks (Eugenics Record
Office, Bull. No. 4) have tabulated records concerning in-
mates of the New Jersey State Village for Epileptics at Skill-
man, N. J., which show one case, in which, both parents
being epileptic, their three children were epileptic also. In
^Ye matings between an epileptic and a feeble-minded person
fourteen children were produced, eight epileptic and six

I

FEEBLE-MINDEDNESS 285

feeble-minded. In six cases feeble-minded persons marritnl
each other producing sixteen feeble-minded and five epileptic
offspring. These cases indicate that the epilepsy and feeble-
mindedness here dealt with were merely different manifes-
tations due to a single cause, either a common infection or
a common form of defect inherited without specific infection.

That insanity is probably due to a variety of causes and
not the same ones as epilepsy or feeble-mindedness is shown
by matings of the insane with epileptic or feeble-minded })er-
sons. Davenport and Weeks report three matings of an in-
sane person with an epileptic or feeble-minded person, which
produced fifteen adult offspring. Of these nine, or a majority,
are described as normal, one as epileptic, and four as fee))le-
minded, while one is classed as " neurotic." This result indi-
cates that the insane parent in most of these cases did not
transmit the same abnormality or pathological condition as
the epileptic or feeble-minded parent. Insanity in the family
is racially less serious than epilepsy, possibly because less
often due to congenital infection.

Feeble-mindedness. The most complete study of the inheri-
tance of feeble-mindedness that has ever been made is that
published by Dr. H. H. Goddard of the Vineland New Jersey
Training School for Feeble-minded, who has recently pub-
lished his results in book form (Macmillan & Co., 1914). He
has studied the family histories of three hundred and twenty-
seven families which sent pupils to the Vineland School.
These family histories are pubHshed in detail, though not
of course by name, and include in many cases photographs
of the pupil or of his written work. In every case the family
pedigree is charted to show the occurrence of mental or
physical peculiarities in ancestors or any pertinent facts con-
cerning their lives. The information was obtained from the
parents of pupils, from family physicians, friends or neigh-
bors, partly through printed questionnaires, partly throngh
personal interviews by trained investigators. This method
of obtaining information is of course capable of uncritical use,
as already pointed out, but seems to have been employed

286 GENETICS AND EUGENICS

with circumspection and in some cases with independent
verification by Dr. Goddard.

The importance of such an investigation as this is shown,
according to Goddard, by many facts.

First. Feeble-mindedness is much commoner than most
persons suppose, understanding the feeble-minded to include
all persons congenitally of such low intelligence that they are
either unable to care for themselves or are incapable of
managing their own affairs with ordinary prudence. God-
dard believes that the feeble-minded are individuals of
arrested or undeveloped mentality and are thus quite differ-
ent from the insane, who show pathological mentality. A
feeble-minded person has the undeveloped mind of a child;
an insane person may have attained mental maturity and
then lost it again, his mentality having degenerated. Feeble-
mindedness and insanity may coexist in the same individual
but they are due to distinct agencies. Feeble-mindedness,
according to Goddard, characterizes a large proportion of
such persons as become public charges as paupers, drunkards,
or criminals.

The method now generally employed of grading the intelli-
gence of individuals is known as the Binet test, from the
Frenchman who devised it. It consists of giving the indi-
vidual a series of standardized tasks to perform of increasing
difficulty as regards the demands on intelligence. The results
of these tests are graded in terms of the average performance
of normal children of particular ages. Thus a feeble-minded
person may show the mentality of a normal child of any age
from one year to twelve years, and is spoken of as mentally
of age one, two, three, etc. Tests of intelligence made by the
Binet method upon juvenile criminals in various state refor-
matories show that a large proportion of the inmates are of
abnormally low intelligence, i. e., are feeble-minded. In
New Jersey the proportion reported feeble-minded as indi-
cated by Binet tests is 46 per cent; in Ohio 70 per cent; in
Virginia 79 per cent; and in Illinois 89 per cent. Probably
50 per cent would be a conservatively low general estimate

FEEBLE-MINDEDNESS !287

of the youthful crimmals who are feeble-iniiuk-d. Goddard
says, " It is easier for us to realize this if we remember how
many of the crimes that are commited seem fooHsh and sill v.
One steals something that he cannot use and cannot dispose
of without getting caught. A boy is offended because llic
teacher will not let him choose what he will study, and
therefore he sets fire to the school building. Another kills a
man in cold blood in order to get two dollars. Somebody- v\sv
allows himself to be persuaded to enter a house and pass out
stolen goods under circumstances where even slight intelli-
gence would have told him he was sure to be caught. Some-
times the crime itself is not so stupid but the perijotrator acts
stupidly afterwards and is caught, where an intelligent per-
son would have escaped. Many of the * unaccountable '
crimes, both large and small, are accounted for once it is
recognized that the criminal may be mentally defective.
Judge and jury are frequently amazed at the folli/ of the
defendant — the lack of common sense that he displayed in
his act. It has not occurred to us that the folly, the crudity,
the dullness, was an indication of an intellectual trait tliat
rendered the victim to a large extent irresponsible."

This same line of explanation Goddard applies with much
plausibility to drunkenness in relation to feeble-mindedness.
It is well known that drunkenness and feeble-mindedness are
often associated, and people have concluded that drunken-
ness causes feeble-mindedness. Goddard believes the reverse
of this to be true that feeble-mindedness occasions drunken-
ness, because the individual has not enough intelligence and
will power to resist temptation when it arises.

Another social evil, prostitution, Goddard finds to l)e due
in large measure to feeble-mindedness. Binet tests nuide in
an Illinois reformatory of girls committed for innnorality
showed 97 per cent of them to be feeble-minded. A ^Fassa-
ehusetts Commission reports that Binet tests api)lie(l to
three hundred immoral women under detention in that state
proved 51 per cent of them to be feeble-minded, while tlie
rest had the mentality of children aged nine to twelve years.

288 GENETICS AND EUGENICS

If Dr. Goddard is right in the opinion that feeble-minded-
ness is responsible for much crime of various sorts, for much
drunkenness and pauperism, it would seem that the easiest
way to attempt to diminish these evils would be by attempt-
ing to diminish feeble-mindedness. Hence the importance
of his undertaking to get at the causes of feeble-mindedness.

Dr. Goddard divides his three hundred and twenty-seven
cases, as regards the probable causes of the observed feeble-
mindedness, into six groups:

1. Hereditary 164

2. Probably hereditary 34

3. Neuropathic ancestry (a possible cause) 37

4. Accident (to mother or child, as disease) 57

5. No cause assignable 8

6. Unclassified 27

327

From this table it will be seen that he regards the feeble-
mindedness as clearly hereditary in half of the families
studied, while it is " probably hereditary " in 10 per cent
more. Heredity then is the largest single discoverable cause
for feeble-mindedness. Neuropathic ancestry and accident
are also recognized as probable causes in a small percentage
of cases each, but it is not to be expected that feeble-minded-
ness so produced would prove hereditary. He can find no
evidence that hereditary feeble-mindedness is caused by a
variety of agencies to which it is frequently referred, as for
example to alcoholism, tuberculosis, syphilis, insane, epilep-
tic or paralytic ancestry, etc.

Most feeble-mindedness, then, is due to heredity, but how
did the character become hereditary ? How did it originate ?
Goddard does not attempt to answer this question, but he
does make clear his view that the feeble mind is an unde-
veloped childish mind. His observations show that the
physical vigor of the feeble-minded equals that of normal
individuals and that the feeble-minded are even more fecund
than normal individuals owing to their lack of normal pru-
dence and self-control. It might be supposed, therefore,

FEEBLE-MINDEDNESS 281)

either that they represent a primitive, animal-like eonditiuu
of the human race, which has survived down to the present
time, or that they represent a retrogressive (loss) variation.
The manner of inheritance of the condition is of iiitiTesl in
connection with this question, for evolution by loss usually
results in the production of recessive variati(jns.

Goddard's evidence indicates that feeble-mindednrss i> a
recessive unit-character. In his family records one hundred
and forty-four matings of feeble-minded inter se have j)ro-
duced seven hundred and forty-nine children of whom four
hundred and eighty-two are of ascertained mentality. Of
these, all but six are recorded as feeble-minded. These few-
exceptions to theoretical expectation might be explained as
being of ancestry other than that assigned. A case reported
from an Ohio institution illustrates the point well. "In a
white family, the father and mother are both feeble-minded.
They have twelve children, all feeble-minded but two. These
two are normal (as regards intelligence) but they are colored."

TABLE 37
Data on the Inheritance of Feeble-Mindedness

Children
Mating F-M N

F X F 476 6

F X N 193 144 (N heterozygous ?). Some families tabulated

here belong above, probably.

F X N . . 68 (N homozygous ?).

NXN 39 83 (Both heterozygous ?). Some belong above, prol>-

ably.

NXN . . 116 (One or both homozygous ?).

The data of Goddard indicate clearly that feeble-mindedness
is inherited as a recessive Mendelian character, but one which
like albinism may occur in many different grades, the higluT
grades probably tending to dominate. The feeblr-niindt-d
are frequently deficient in physical strength and vigor. II o w-
ever, many of them seem to possess unusually good physic lue.
Goddard compares them to savages with strong bodies but
childish minds. The high-grade feeble-minded, kncnvn as
" morons," with mentality of eleven or twelve years, are

290 GENETICS AND EUGENICS

capable of being useful members of society in manual or
mechanical occupations not demanding too much planning
or initiative. But it is evident that as they are easily in-
fluenced and imposed upon and more than ordinarily fecund,
since they do not exercise the prudence and self-restraint of
normal individuals, their numbers are likely to increase un-
duly, unless some restraint is put upon them. A self-govern-
ing democracy with universal suffrage is seriously threatened
by a large increase in the unintelligent portion of its popula-
tion, and is justified in adopting strong measures to counter-
act it. This is often urged as an argument for restricted immi-
gration without due regard for the distinction between low
intelligence and illiteracy. Many of our immigrants w^ho are
illiterate, because they have never had an opportunity to
attend school, are people of unusual intelligence and energy.
Their illiteracy is usually speedily removed when they get
within reach of American schools and the next generation is
represented among the most earnest students in our univer-
sities and later among the successful men in the professions.
But the person of low intelligence, whether literate or illiter-
ate is more dangerous to society than the intelligent illiterate,
because he and his descendants for all time will require
parental protection and care from the state to prevent them
from becoming criminals, paupers, idlers, and purchasable
voters.

To prevent the natural increase of the feeble-minded, God-
dard recommends their segregation, so far as possible, in
schools and institutions under state control. This is already
being done to some extent in many of the states, but alto-
gether too few individuals have yet been segregated to insure
a decrease in the proportion of feeble-minded in the popula-
tion. Many have hitherto been unrecognized as feeble-
minded, who are classed as backward pupils in school, and
later as truants, drug fiends, drunkards, criminals, tramps or
prostitutes. A proper recognition of the source from which
these classes are recruited and of what really ails them
should lead to more intelligent efforts to reduce their number.

FEEBLE-MINDEDNESS

291

When segregation is impracticable, tlie feeble-iniiKlcd should
be looked after in their homes, as children arc looked alter.
They should not be allowed to marry unless first sterilized.
In the case of males this is now possible by a very siinj)le
surgical operation, vasectomy, unattended by risk or serious
consequences to health. In the case of females se<,'regati()u
during the reproductive period is probably more to be recom-
mended than sterilization.

CHAPTER XXXIII

THE POSSIBILITY AND PROSPECTS OF BREEDING A

BETTER HUMAN RACE

The suggestion that the human race might be improved by
the methods of the stock breeder is a very old one. Plato
advanced it in his Republic as the only practicable basis for
the production of a permanent and superior governing class
within the ideal state. The family had no place in his
scheme.

It was his proposition that the best of both sexes should
be mated with each other and should be given every encour-
agement to the production of offspring, the young being
taken at birth into a state nursery and their identity lost so
far as the parents were concerned. Inferior persons, on the
other hand, were to be kept from reproducing, as far as pos-
sible, and their progeny destroyed. Realizing that such
favoritism would cause no end of trouble, if known, Plato
said that what was done should be kept a secret from all but
the magistrates themselves, and " an ingenious system of lots
must be contrived in order that inferior persons may impute
the manner in which couples are united to chance and not to
the magistrates."

The eugenics system of Plato has probably never had a
full and fair trial, but if we may believe the account of
Plutarch, in his life of Lycurgus, something very like it
actually existed in Plato's time in Sparta, and it was prob-
ably the Spartan system that Plato had in mind. Sparta was
practically an armed camp, in which a military class ruled
with great severity the subject native races, holding them in
subjection by force of arms and compelling them to work the
land for the benefit of their conquerors. The Spartans sub-
jected themselves, both men and women, to the severest
discipline. Gymnastics and war were their exclusive occu-

292

SPARTAN EUGENICS 09:]

pations. Family life scarcely existed among the Spartans.
The men lived together in a sort of camp or club, very fru-
gally, and ready for instant warfare. Marriage was recog-
nized as an institution for the production of sohh'crs merely.
The child belonged to the state, rather than to its pannts*
The magistrates decided whether it should be reared or not.
Plutarch says concerning Lycurgus, founder of the Si)artan
constitution:— "Lycurgus was of a persuasion that children
were not so much the property of their parents as of the
whole commonwealth, and therefore, would not have his
citizens begot by the first-comers, but by the best men that
could be found; the laws of other nations seemed to him very
absurd and inconsistent, where people would be so solicitous
for their dogs and horses as to exert interest and pay money
to procure fine breeding, and yet kept their wives shut up,
to be made mothers only by themselves, who might be fool-
ish, infirm, or diseased; as if it were not apparent that chil-
dren of a bad breed would prove their bad qualities first
upon those who kept and were rearing them, and well-born
children, in like manner, their good qualities."

The Spartan system of eugenics seems to have attained
its object, the production of superior children, but we must
remember that with it was combined a system of life-long
physical education and military discipline which has rarely
if ever been equalled, so that it is impossible to say how
much of the result obtained was due to breeding and how
much to training of the youth.

Further the Spartan system succeeded only so long as
Sparta was a small, isolated community, without wealth,
luxury, or leisure, and using iron for money. Foreign con-
quest was the undoing of Sparta. She could con(|uer in a
fight but she could not govern except as she governed lier
Helots — by enslaving them. Upon contact with the rest of
the world, life was found to have other attractions than
fighting, and the old discipline was relaxed.

Moreover, what the Spartan system produced wa.s a single
type of man, the soldier. The memory of Athens is sacred for

294 GENETICS AND EUGENICS

other types of manhood and achievement, art, Hterature,
philosophy and science, the greatest intellectual achieve-
ments of mankind up to that time, but in these Sparta had
no share. Her eugenics was of the same type as that of the
animal breeder. It aimed to produce a single specialized type
of superior excellence. In this it succeeded, but at the sacri-
fice of all else. In this, again, it resembles animal husbandry,
which produces a type of animal more useful to man, but
wholly dependent upon him, and unable to maintain itself
if thrust back into the struggle for existence with other
animals.

The civilization for whose continuance Plato planned came
to an end. We do not know why. Historians differ widely
in their views as to why Greece and Rome fell. But one sug-
gestion is that in their later days the inferior classes increased
more rapidly than the superior ones and the general average
was thereby lowered. Now it is conceivable that this may
have happened in one of two ways. If each class reproduced
its kind, then the lower classes must have reproduced faster
than the upper ones. This is what is assumed to have
occurred by those who consider modern nations to be
threatened in a similar way.

On the other hand it is possible that there was no real
germinal difference between the so-called upper and the
lower classes. The classification of ancient society may have
rested on economic rather than biological grounds and the
downfall have been due to economic causes rather than to
racial changes. If this is true then the more rapid reproduc-
tion of those low in the social scale was not in itself harmful
to the race, that is would not have caused a lowering of its
biological level, and economic causes must be sought to
explain the decay of ancient civilization. The question is
one for historians to deal with, but its answer must be borne
in mind when the fate of ancient civilizations is cited as a
warning to us.

A belief that biological decline is occurring or is likely to
occur among modern nations has given rise to the modern

THE MODERN EUGENICS MOVEMENT 29.5

eugenics movement. This movement was started by Francis
Galton, who, adopting Darwin's theory of evolution, souglit
to apply it to human society. His studies of family histories
had convinced him that both physical and mental traits are
largely matters of inheritance. He reasoned that the exist-
ing biological status of society could be maintained only if all
classes of society reproduced at the same rate; that improve-
ment would result if the biologically best individuals repro-
duced faster than others, but that deterioration would result
if the biologically inferior individuals reproduced faster than
others. He sought to devise measures which would encourage
early marriage and the rearing of large families by the best
and most competent members of every profession and trade.
His suggestions met chiefly with ridicule at the time, but
are coming now to be taken more seriously.

No one can deny that our country's population is increas-
ing fast enough, the only danger is that the biologically poor-
est elements in the population may increase faster than any
other. The declining birth rate is not in itself serious, but
the differential character of its decline is serious. The most
intellectual and cultured elements in the population breed
slowest. Professor Cattell says that a Harvard graduate has
on the average three-fourths of a son and a Vassar graduate
one-half of a daughter. If this continues college graduates
may look forward to the early extinction of their line as an
element in the American population.

As elements in the differentially declining birth-rate we
may recognize (1) late marriages, shortening the reproductive
period and (2) voluntary limitation of the number of children.
Voluntary limitation occurs for a variety of reasons such as
expense, health, etc., but chiefly because of selfishness and
luxury, causes which were operative in the decline of Greece
and Rome as they are among modern nations.

The more complex human life becomes, the less attention
is given to its perpetuation. In a small connnunity family
life is dominant and the rearing and education of children are
its most important occupations. But as comumnity life be-

296

GENETICS AND EUGENICS

comes more complex family life sinks into a subordinate
position. The more intellectual and cultured the individual
is, the more does he find outside the home to interest and
attract him. The consequence is that home life suffers. It
is slighted or shunned altogether by those who are best quali-
fied to be parents, and the rearing of children is left to those
considered too dull for other activities. In consequence the
majority of the children produced in a cultured and progres-
sive city population are produced by its least cultured and
progressive members. This is the condition which today
confronts the leading nations of the world and has given rise
to the eugenics movement. If this condition is interpreted
from the standpoint of the animal breeder, it means that the
average capacity of the population for intellectual pursuits,
for culture and for progress is bound to decline. For this
amounts to selecting for breeding, not the best, but the culls
of the flock, and every breeder knows that this means
deterioration.

If a great city can in each generation import a fresh stock
of youths from the country or from foreign countries, all
may go well, but it is questionable whether this can continue
indefinitely. Already many of our rural New England com-
munities are said to be running out of good human stock.
For generations they have been sending their best to the
cities and to the developing West. Many of those left be-
hind are lacking in energy or ambition, perhaps also in
intelligence, and a European peasant population is rapidly
replacing them. Will this new population be a fit substitute
for the old Anglo-Saxon stock ? Time alone will tell. If it
is a sound stock which has hitherto lacked opportunity to
rise in the social scale, we may now expect it to do so, oppor-
tunity being offered. But if it is inherently a feeble stock, it
will not replace the old New England stock in supplying our
cities with the bright youths whom they require but are un-
able to produce in sufficient numbers. A time of storm and
stress like that which now distracts the world may at some
future day decide our fitness to survive as a race.

EUGENICS IN ENGLAND 297

In England a genuine alarm is felt as regards the character
of its future citizens, for there as here the cities draw from
the country. But the country population there is not only
not regenerated by immigration but is further depleted of its
best elements by foreign emigration. The consequence i^
that a eugenics movement has there been started, which
seeks to remove the indifference on the part of the best ele-
ments in the population to marriage and the rearing of chil-
dren. Just how this can be done, or whether it can be done
at all is uncertain. But the British eugenists are very much
in earnest and they base their appeal on both patriotic and
religious grounds. Professor and Mrs. Whetham (who have
written several books devoted to this subject) discuss pri-
marily conditions in Great Britain. Their point of view
is to some extent an aristocratic one. They recognize in the
hereditary aristocracy of England a genuinely and gemiinally
superior element of the population. The younger sons of
the titled families who inherit (it is supposed) the superior
germ-plasm but not the aristocratic titles, have frequently
married into successful families of the middle class, and
are believed thus to have improved the standard of the
entire nation. This theory sounds plausible, but an out-
sider free from class prejudice might reasonably question
its validity.

If the English aristocracy is really a biologically superior
race, how are we to account for the historical steady rise in
power and influence of the Commons ^ Opportunity has
always favored the aristocratic families; in spite of this we
find the great men of the British nation usually coming from
the middle class, and not from the younger sons of aristo-
cratic families either. America's experience does not indicate
that the English aristocracy is either better or worse than the
English yeomanry as a biological human stock. What little
of aristocratic blood the colonies received went chiefly to
Virginia and previous to the Civil War an aristocracy of first
families comparable with that of England ruled \^irginia and
furnished the nation with presidents and statesmen. Since

298 GENETICS AND EUGENICS

the war the presidents have come from other sections, and
seem not to have been inferior in ability to their predecessors.
In some quarters it is the fashion to point to New England
as the source of the really superior American stock, viz., its
intellectuals, but there is no better ground for thinking the
Puritan stock superior than for thinking the Cavalier stock
superior. Circumstance has had much to do with the ad-
vancement of each in influence. In this connection it is
interesting to note the conclusions reached by Professor
Cattell {Popular Science Monthly, May, 1915) from a study of
the families of America's one thousand leading scientists. He
says :

"If men of performance could only come from superior fam-
ily lines, this would be a conclusive argument for a privileged
class and for a hereditary aristocracy. If the congenital
equipment of an individual should prescribe completely what
he will accomplish in life, equality of opportunity, education
and social reform would be of no significance. Such an ex-
treme position, though it is approached by men with so much
authority as Sir Francis Galton, Professor Karl Pearson, Dr.
F. A. Woods, Dr. C. B. Davenport and Professor E. L.
Thorndike, is untenable. Equally extreme in the opposite
direction is M. Odin's aphorism " Genius is in things not in
men," or the not uncommon opinion that almost anything
can be done with a child by training and education.

My data show that a boy born in Massachusetts or Con-
necticut has been fifty times as likely to become a scientific
man as a boy born along the southeastern seaboard from
Georgia to Louisiana. They further show that a boy is fifty
times as likely to do scientific work as a girl. No negro in
this country has hitherto accomplished scientific work of
consequence. A boy from the professional classes in New
England has a million chances to become a scientific leader
as compared with one chance for a negro girl from the
cotton fields.

" These great differences may properly be attributed in part
tO natural capacity and in part to opportunity. If the 174

CATTELL ON EUGENICS ^99

»

babies born in Massachusetts and Connecticut who became
leading scientific men had been exchanged with babies born
in the south, it seems probable that few or none of them
would have become scientific men. It may also be the case
that few or none of the babies from the south transplanted to
New England would have become scientific men, ])ut it is
probably true that a nearly equal number of scientific men
would have been reared in New England. It is certain that
there would not have been 174 leading scientific men from
the extreme southern states and practically none from Massa-
chusetts and Connecticut. If the stock of the southern
states remains undiluted, it may, as social conditions change,
produce even more scientific men per thousand of its popula-
tion than New England has hitherto produced. In the first
list [made in 1906] of the thousand leading scientific men,
Massachusetts produced 109 and Connecticut 87 per million
of their population. Of the younger men added to the list
in the second arrangement [made in 1910] under comparable
conditions, Massachusetts produced 85 and Connecticut 57.
The other North Atlantic states failed in like measure, while
the central states show a gain — Michigan from 36 to 74,
Minnesota from 23 to 59, etc. These changes must be attri-
buted to an altered environment, not to an altered racial
stock. Japan had no scientific men a generation ago and
China has none now, but it may be that in a few years their
contributions to science will rival ours.

"A Darwin born in China in 1809 could not have become a
Darwin, nor could a Lincoln born here on the same day have
become a Lincoln had there been no civil war. If the two
infants had been exchanged there would have been no Dar-
win in America and no Lincoln in England. Darwin was a
member of a distinguished family line possessing high natural
ability and the advantages of opportunity and wealth. Lin-
coln had no parental inheritance of ability or wealth, but he
too had innate capacity and the opportunity of circum-
stance. If no infants had been born with the peculiar natural
constitutions of Darwin and Lincoln, men like them could

300 GENETICS AND EUGENICS

not have been made by any social institutions, but none the
less the work they did might have been accomplished by
others and perhaps their fame would have been allotted to
others. There may have been in England other family lines
equal in natural ability to the Darwins and in this country
other individuals as well constituted as Lincoln, but undis-
tinguished from lack of opportunity. It is still more probable
that such conditions obtain in Russia and in China, in whose
graveyards there may lie innumerable *' mute inglorious "
Miltons, Lincolns and Darwins.

"The most exceptional ability may be suppressed by cir-
cumstances; but it can sometimes deal with them on equal
or perhaps superior terms. Thus the writer has pointed out
how widely distributed in race, age and performance are the
most distinguished men who have lived. When we turn
from the most eminent men to those next in rank, we may
doubt whether their natural ability has not been equaled by
thousands who have not attained distinction. Among the
two hundred most eminent men who have lived in the his-
tory of the world are: Napoleon III, Nero, Fox, Julian,
Fenelon, Clive, Alberoni, Bentley and Gerson. It is quite
conceivable that there are at present living in the United
States hundreds or thousands of men having as great natural
ability as these. There may be a hundred thousand men and
women having the natural and specific ability of the thou-
sand in this country who have accomplished the best
scientific work.

"President A. Lawrence Lowell has remarked that we have
a better chance of rearing eaglets from eagles' eggs placed
under a hen than from hen's eggs placed in an eagle's nest.
But it is equally true that we have a better chance of raising
tame eaglets in a chicken coop than in an eyrie. The differ-
ence between a man uninterested in science and a scientific
man is not that between a chicken and an eagle, but that
between an untrained chicken and a trick cock. Some
cockerels can be trained better than others, but there are in-
numerable cockerels that might be trained and are not.

CATTELL ON EUGENICS 301

**The son of a scientific man may on the average have the
inherited ability which would make him under equally favor-
able circumstances twice, or ten times, or a hundred times,
as likely to do good scientific work as a boy taken at random
from the community. The degree of advantage should be
determined. It surely exists, and the children of scientific
men should be numerous and well cared for. But we can do
even more to increase the number of productive scientific men
by proper selection from the whole community and by giving
opportunity to those who are fit. Galton finds in the judges
of England a notable proof of hereditary genius. It would be
found to be much less in the judges of the United States. It
could probably be shown by the same methods to be even
stronger in the families conducting the leading publishing
and banking houses of England and Germany. As I write,
the death is announced of Sir William White, the distin-
guished naval engineer, chief constructor of the British navy,
president of the British Association. If his father had been
chief constructor of the navy, he would have been included
among Galton's noteworthy families of fellows of the Royal
Society. The fact that his father-in-law was chief construc-
tor of the British navy throws, if only by way of illustration,
a light on the situation in two directions.

On the one hand, the specific character of performance and
degree of success are determined by family position and
privilege as well as by physical heredity; on the other hand,
marriage, chiefly determined by environment, is an import-
ant factor in maintaining family lines. The often-quoted
cases of the Jukes and Edwards families are more largely due
to environment and intermarriage within that environment
than to the persistence of the traits of one individual through
several generations. The recently published *' Kallikak
Family " by Dr. H. H. Goddard demonstrates once again the
heredity of feeble-mindedness. It would, however, have been
a stronger argument for the omnipotence of heredity if the
original ancestor had left by a healthy mother illegitimate
children who established prosperous lines of descent, and a

302 GENETICS AND EUGENICS

child by a feeble-minded wife who left degenerate lines of
descent. Two experiments have been made on a large scale
which seem fairly definite even though quantitative results
cannot at present be reached. The mulattoes may be as-
sumed to have a heredity midway between negroes and
whites, but their social environment is that of the negroes,
and their performance corresponds with their social environ-
ment rather than with their heredity. Illegitimate children
have perhaps a heredity as good as the average, but their
performance falls far below the average. If performance
were determined by heredity alone there might be expected
to be among our thousand leading scientific men some forty
mulattoes and some forty of illegitimate birth, whereas there
is probably not one of either class.

"At nearly the same time Agassiz came from abroad to
Harvard and Brlinnow to Michigan. We all know the list
of distinguished naturalists trained under Agassiz — Brooks,
Hyatt, Jordan, Lyman, Minot, Morse, Packard, Putnam,
Scudder, Shaler, Verrill, Whitman, Wilder, and many more,
directly and indirectly. From Michigan have come, as is not
so well known, one-fourth of our most distinguished astron-
omers, including Abbe, Campbell, Comstock, Curtis, Doo-
little. Hall, Hussey, Klotz, Leuschner, Payne, Schaeberle,
Watson and Woodward. Certainly the coming of Agassiz
and Brunnow was the real cause of greatly increased scientific
productivity in America. Some, but not all, of those who
worked under Agassiz would have become naturalists apart
from his influence. The astronomers from Michigan must in
the main be attributed to their environment. The men had
the necessary ability, but if Brunnow had not gone to Michi-
gan, they would not have become astronomers; if they had
gone to the University of Pennsylvania, they would have
been more likely to have become physicians than astrono-
mers; if they had not gone to a university they would not
have become scientific men.

*'It is certainly satisfactory if we can attribute the inferi-
ority of scientific performance in America as compared with

CATTELL ON EUGENICS 303

Germany, France and Great Britain to lack of opportunity
rather than to lesser racial ability. In Germany scientific
research has been made by the university rather than the
reverse. In Great Britain also the universities have been
potent, and, in addition, its leisure class has conlrihulcd
greatly. Here prior to 1876 we had no university in wliicli
research work was adequately encouraged, and we have had
no amateurs comparable to those of Great Britain. Professor
Pickering found that of the 87 scientific men who were mem-
bers of at least two foreign academies, 6 were Americans as
compared with 17 from Prussia, 13 from England and 12
from France. In so far as our scientific production is so
measured, the reference is to a generation ago, when our
universities were only beginning to develop and research
work was only beginning to be appreciated. But it is a strik-
ing fact that of the six distinguished Americans, three are
astronomers; and astronomy is the only science in which
thirty years ago the facilities for research work in this coun-
try were equal to those of the leading European nations. Of
the remaining three, two have not been engaged in teaching,
and the third has been practically freed from teaching for his
research work. We may hope that when conditions become
as favorable for other sciences as they have been for astron-
omy, the United States will assume leadership in scientific
productivity.

"In order to answer questions such as the extent to which
the scientific work accomplished in America is due to native
endowment, whether such endowment is general or specific,
how far it occurs in family lines, what part of those endowed
are able to prove their ability, the influence of education and
example, the effects of opportunity, encouragement and re-
wards, it is necessary to make a study of individual cases.
A large mass of material is at hand concerning the relatives of
scientific men who have shown scientific productivity or have
attained distinction, but these data are not in order for pub-
lication and should be supplemented by answers to many
inquiries. In the meanwhile the writer may say that it is

304 GENETICS AND EUGENICS

his opinion that while we should welcome and support a
eugenic movement tending to limit the birth of feeble-minded
and defective children and encouraging the birth of those
that are well endowed, it appears that under the existing
conditions of knowledge, law and sentiment, we can probably
accomplish more for science, civilization and racial advance
by selecting from the thirty million children of the country
those having superior natural ability and character, by train-
ing them and giving them opportunity to do the work for
which they are fit. We waste the mineral resources of the
country and the fertility of the soil, but our most scandalous
waste is of our children, most of all of those who might be-
come men and women of performance and of genius.

' "Eugenics may become the most important of all applied
sciences, but at present its scientific foundations must be laid
by the study of comparative genetics, on the one side, and the
study of human conduct, on the other. There is more im-
mediate prospect of improving our civilization than our germ-
/ plasm. It is easier to decrease or eliminate typhoid fever by
hygienic measures than to attain racial immunity, although
this is not equally the case for tuberculosis and still less for
cancer. We can increase to anv desired extent from the
existing population by proper selection and training the
number of scientific workers in the United States. The
number capable of exhibiting genius is limited, but many of
them are lost through lack of opportunity. It is our business,
it should be our principal business, to improve our civiliza-
tion by giving opportunity to those who are fit, while at the
same time investigating the conditions which will give us a
better race."

Writers on sociology have shown that himian progress is
largely limited and determined by the social environment and
that it is even possible for social progress to occur in spite of
biological deterioration. If this idea is correct, one argument
for control of human matings by the state or some other
central agency has been frequently over-emphasized. Racial
progress does not require a constantly advancing biological

LIMITATIONS OF EUGENICS 305

standard in the individual. As individuals, primitive men
were probably more than a match for us physically, and at
least our equals mentally. As regards the standard of the
individual, then, the race has not progressed. Civilization
is a matter of collective achievement; it is not a biological
inheritance at all, but a cultural one. " AVe are heirs of all
the ages " not biologically, but only culturally. Standing on
the shoulders of the last generation we see farther because
we are higher up, not because we are taller.

It is of course essential that the racial stock be kept sound
and free from taint of disease or racial poison, but granting
this, the situation is not so alarming as some persons seem to
think. For the normal unperverted instincts of the average
man have a distinctly eugenic trend. Cupid is a safer guide
in matrimony than a licensing board. The old folks always
" make a mess of it " when they interfere in the match-
making of the young folks. This is as true in real life as in
literature. Of course it is possible for young folks to make
mistakes as well as for old ones, and it is necessary that those
older persons who have been burned by the fire, or have
seen others suffer in like fashion, should see that their chil-
dren do not fall into the fire. For example, civilization has
brought into being many perils which did not exist in a
simpler and more primitive mode of living. Of these the
young must be advised. Implicit trust in the guidance of the
instincts will in a civilized community lead to endless trouble.
Sexual promiscuity has only disastrous consequences among
civilized peoples and for a very simple reason, the certainty
of contamination sooner or later with venereal disease, in
particular with gonorrhoea or syphilis.

It is probable that Polynesians, before the advent of
Europeans, were free from these diseases, and their rather
loose sexual relations, as viewed by our standards, had no
serious racial consequences. But with the advent of Euro-
peans all this has changed. Continued promiscuity means to
them now racial extermination, as it does among Europeans.
Sexual purity is necessary with us, not merely because social

306 GENETICS AND EUGENICS

standards demand it, but because avoidance of loathsome
venereal disease is impossible otherwise.

This element of venereal disease has frequently been an
important factor in determining the success or failure of race
mixtures. European men of loose morals have frequently
introduced venereal disease in race mixtures with native
populations, and this will account for the poor results ob-
served in many racial crosses. When this element is absent,
racial crosses of Europeans with native peoples have been
observed to produce offspring of complete vigor and fertility.
Racial crossing among men, as among domesticated animals,
is biologically beneficial within limits. The English people
were originally very mixed racially, and the same is pre-
eminently true of Americans today. This mixture of elements
not too dissimilar, provided the social heritage is not unduly
disturbed, is on the whole beneficial. It results in increase
of vigor and energy in the offspring, together with an in-
crease of variability, physical and mental, which favors social
progress.

It is certain that human progress depends upon two sets of
agencies, one sociological or cultural, the other biological.
In this discussion we have dealt chiefiy with the biological
agencies. Biologically the human race can be improved only
by improvement of its germ-plasm. If acquired characters
were inherited, we might hope to improve the human race
germinally by improving the environment. If as seems
more probable acquired characters are not to any consider-
able extent inherited, then environmental agencies affect man
chiefly culturally, not biologically. To change man biologi-
cally, to make a different sort of animal of him, it will be
necessary to act through heredity, that is through selection
of parents for the next generation.

Leaving aside for the present the practical difficulties arid
supposing that it were possible to manage the human race
like a stock farm, the choice of parents would necessarily
be limited by the material available. We could select parents
only for such characteristics as the human race today pos-

LIMITATIONS OF EUGENICS 307

sesses. We could not, for example, breed a human race with
wings, however desirable such a characteristic might seem.
We are limited definitely for all time to the hand type of
appendage. But there are different types and sizes of hands
among human beings among which a selection might be nuide
if this were considered desirable, as for example normal
hands, short-firigered hands (i. e,, brachydactyl), hands with
a reduced number of fingers {i. e., syndactyl), and hands with
an increased number of fingers {i. e., polydactyl). These
several types of hand are known to be hereditary. If the
unusual types were superior to the normal, we might through
heredity make them replace the normal in the race. But in
reality, the normal type of hand seems on the w^hole to be
the best type, and so we have no desire to change it. The
same is true as regards most human traits known to be in-
herited, whether physical or intellectual. Our ideal is in
I general the normal. There are certain types of abnormality
which we should be glad to see become less frequent in occur-
rence, as for example albinism, night blindness, color-blind-
ness, and haemophilia. A complete control of heredity would
render their elimination from the race possible, but it is
doubtful if they are serious enough to call for such elmiina-
tion, even if human matings w^ere wholly controllable by a
single central agency, which of course they are not. For in
discriminating against persons possessing such minor defects
as these we should be in danger of rejecting some of our
human stock which is best in regard to characteristics of
much greater consequence. The independent inheritance of
traits must ever be kept in mind in deciding who are desir-
able and who undesirable parents, weakness in one particular
being frequently offset by unusual strength in another. Those
undesirable traits which are inherited in the simplest way,
as Mendelian characters, are not likely to become very com-
mon in a freely intermarrying population. It is only when
society becomes stratified, and class distinctions arise with
castes or families closely intermarrying, that heredity is likely
to bring Mendelian recessive defects repeatedly to the sur-

308 GENETICS AND EUGENICS

face. Democracy is as safe a remedy against such evils as
state controlled marriages would be, if they were obtainable.

The most important inherited traits are probably those
which are quantitatively variable, which occur in a graded
series, like bodily size and strength, mental power, and power
of resisting disease. In regard to these, excellence is a matter
of degree and is relative. Further no particular grade breeds
true. Regression toward the normal is the universal rule.
If society could be managed like a stock farm, then it would
be possible to change the normal toward which regression
occurs, very slowly and gradually, as for example in mental
power. The average grade of intelligence could be raised by
rigid selection long continued. Possibly this has occurred in
the evolution of existing races of men. If so, it has occurred
unconsciously and through natural selection and probably
more from the struggle of one cultural group with another
than from the struggle of one individual with another. But
the modern eugenic ideal is to make a conscious selection of
parents within the group with a view to elevating the normal
within the group, a thing that has not hitherto been at-
tempted, unless in Sparta for the breeding of soldiers.

If there were a central directing agency which had the
power as well as the wisdom to control matings within the
group, something could undoubtedly be done slowly to ele-
vate the general average of bodily vigor or innate mental
power within the group. This could be done most rapidly
by polygamy which w^ould permit of a relatively rigid selec-
tion of sires; less rapidly under monogamy by a selection of
parents among both sexes, the offspring to be cared for largely
by the rest of the community. But the social consequences
of either of these methods are so tremendous, so subversive
are they of individual liberty, that no modem civilized com-
munity has been willing to contemplate either of them. The
whole movement of modem times is in an opposite direction.
Practically therefore, we are limited to such eugenic measures
as the individual will voluntarily take in the light of present
knowledge of heredity. It will do no good, but only harm, to

EUGENICS AND THE INDIVIDUAL 309

magnify such knowledge unduly, or to conceal its present
limitations. We should extend such knowledge as rapidly
as possible, but not legislate until we are very sure of our
ground.

Every young person of sound and healthy stock should
look forward to marriage and family life as the comi)k'lion
of a normal career and incidentally as fulfilling an obligation
which he owes to his country and his race. Any young per-
son who for any reason finds himself debarred from this part
in life should fulfill the racial obligation vicariously by helj)-
ing to care for and to educate the children of his more
fortunate fellows.

APPENDIX

APPENDIX

EXPERIMENTS IN PLANT-HYBRIDISATION '

By Gregor Mendel

{Read at the Meetings of the 8th February and 8th March, 1865.)

INTRODUCTORY REMARKS

Experience of artificial fertilisation, such as is effected with orna-
mental plants in order to obtain new variations in colour, has led
to the experiments which will here be discussed. The striking'
regularity with which the same hybrid forms always reajijieared
whenever fertilisation took place between the same species induced
further experiments to be undertaken, the object of which was to
follow up the developments of the hybrids in their progeny.

To this object numerous careful observers, such as Kolreuter,
Gartner, Herbert, Lecoq, Wichura and others, have devoted a part
of their lives with inexhaustible perseverance. Gartner especially,
in his work "Die Bastarderzeugung im Pflanzenreiche " (The Pro-
duction of Hybrids in the Vegetable Kingdom), has recorded ver>'
valuable observations; and quite recently Wichura pu})lished
the results of some profound investigations into the hybrids of the
Willow. That, so far, no generally applicable law governing the
formation and development of hybrids has been successfully fornui-
lated can hardly be wondered at by anyone who is acquainted with
the extent of the task, and can appreciate the difficulties witli
which experiments of this class have to contend. A final decision
can only be arrived at when we shall have before us the results of
detailed experiments made on plants belonging to the most diverse
orders.

Those who survey the work done in this department will arrive
at the conviction that among all the numerous experiments made,
not one has been carried out to such an extent and in such a way as

1 This translation was made by the Royal Horticultural Society of London, and
is reprinted, by permission of the Council of the Society, with footnotes adde<i and
minor changes suggested by Professor W. Bateson, enclosed within ( ]. The original
paper was published in the V&rh. naturj. Ver. in Brunn, Abhandlungen, iv. 1805,

which appeared in 1866.

313

314 APPENDIX

to make it possible to determine the number of different forms
under which the offspring of hybrids appear, or to arrange these
forms with certainty according to their separate generations, or
definitely to ascertain their statistical relations.^

It requires indeed some courage to undertake a labour of such
far-reaching extent; this appears, however, to be the only right way
by which we can finally reach the solution of a question the impor-
tance of which cannot be overestimated in connection with the
history of the evolution of organic forms.

The paper now presented records the results of such a detailed
experiment. This experiment was practically confined to a small
plant group, and is now, after eight years' pursuit, concluded in all
essentials. Whether the plan upon which the separate experiments
were conducted and carried out was the best suited to attain the
desired end is left to the friendly decision of the reader.

Selection of the Experimental Plants

The value and utility of any experiment are determined by the
fitness of the material to the purpose for which it is used, and thus
in the case before us it cannot be immaterial what plants are
subjected to experiment and in what manner such experiments
are conducted.

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

The experimental plants must necessarily —

1. Possess constant differentiating characters.

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

The hybrids and their offspring should suffer no marked disturb-
ance in their fertility in the successive generations.

Accidental impregnation by foreign pollen, if it occurred during
the experiments and were not recognized, would lead to entirely
erroneous conclusions. Reduced fertility or entire sterility of cer-
tain forms, such as occurs in the offspring of many hybrids, would
render the experiments very difficult or entirely frustrate them. In

^ [It is to the clear conception of these three primary necessities that the whole
success of Mendel's work is due. So far as I know this conception was absolutely
new in his day.]

APPENDIX 315

order to discover the relations in which the hybrid forms stand
towards each other and also towards their progenitors it appears
to be necessary that all members of the series developed in each
successive generation should be, without exception, subjected to
observation.

At the very outset special attention was devoted to the Legu-
minosae on account of their peculiar floral structure. Exjjeriments
which were made with several members of this family led to the
result that the genus Pisum was found to possess the necessary
qualifications.

Some thoroughly distinct forms of this genus possess characters
which are constant, and easily and certainly recognizable, and
when their hybrids are mutually crossed they yield perfectly fertile
progeny. Furthermore, a disturbance through foreign pollen
cannot easily occur, since the fertilising organs are closely packed
inside the keel and the anther bursts within the bud, so that the
stigma becomes covered with pollen even before the flower opens.
This circumstance is of especial importance. As additional advan-
tages worth mentioning, there may be cited the easy culture of these
plants in the open ground and in pots, and also their relatively short
period of growth. Artificial fertilisation is certainly a somewhat
elaborate process, but nearly always succeeds. For this purpose
the bud is opened before it is perfectly developed, the keel is
removed, and each stamen carefully extracted by means of forceps,
after which the stigma can at once be dusted over with the foreign
pollen.

In all, thirty -four more or less distinct varieties of Peas were
obtained from several seedsmen and subjected to a two years' trial.
In the case of one variety there were noticed, among a larger num-
ber of plants all alike, a few forms which were markedly dift'erent.
These, however, did not vary in the following year, and agreed
entirely with another variety obtained from the same seedsman; the
seeds were therefore doubtless merely accidentally mixed. All the
other varieties yielded perfectly constant and similar oftspring; at
any rate, no essential difference was observed during two trial years.
For fertilisation twenty-two of these were selected and cultivated
during the whole period of the experiments. They remained
constant without any exception.

Their systematic classification is difficult and uncertain. If we
adopt the strictest definition of a species, according to which only

316 APPENDIX

those individuals belong to a species which under precisely the
same circumstances display precisely similar characters, no two of
these varieties could be referred to one species. According to the
opinion of experts, however, the majority belong to the species
Pisum sativum; while the rest are regarded and classed, some as
sub-species of P. sativum, and some as independent species, such as
P. quadratum, P. saccharatum, and P. umhellatum. The positions,
however, which may be assigned to them in a classificatory system
are quite immaterial for the purposes of the experiments in ques-
tion. It has so far been found to be just as impossible to draw a
sharp line between the hybrids of species and varieties as between
species and varieties themselves.

Division and Arrangement of the Experiments

If two plants which differ constantly in one or several characters
be crossed, numerous experiments have demonstrated that the
common characters are transmitted unchanged to the hybrids and
their progeny; but each pair of differentiating characters, on the
other hand, unite in the hybrid to form a new character, which in
the progeny of the hybrid is usually variable. The object of the
experiment was to observe these variations in the case of each pair
of differentiating characters, and to deduce the law according to
which they appear in the successive generations. The experiment
resolves itself therefore into just as many separate experiments as
there are constantly differentiating characters presented in the
experimental plants.

The various forms of Peas selected for crossing showed differences
in the length and colour of the stem; in the size and form of the
leaves; in the position, colour, and size of the flowers; in the length
of the flower stalk; in the colour, form, and size of the pods; in the
form and size of the seeds; and in the colour of the seed- coats and
of the albumen [cotyledons]. Some of the characters noted do not
permit of a sharp and certain separation, since the difference is of a
** more or less " nature, which is often difficult to define. Such
characters could not be utilised for the separate experiments ; these
could only be applied to characters which stand out clearly and
definitely in the plants. Lastly, the result must show whether they,
in their entirety, observe a regular behaviour in their hybrid unions,
and whether from these facts any conclusion can be come to re-
garding those characters which possess a subordinate significance
in the type.

APPENDIX 317

The characters which were selected for experiment relate:

1. To the difference in the form of the ripe seeda. These are either
round or roundish, the depressions, if any, occur on the surface,
being always only shallow; or they are irregularly angular and
deeply wrinkled (P. quadratum).

2. To the difference in the colour of the seed albumen (endosperm) . '
The albumen of the ripe seeds is either pale yellow, l)right yellow
and orange coloured, or it possesses a more or less intense green
tint. This difference of colour is easily seen in the seeds as [ = if]
their coats are transparent.

3. To the difference in the colour of the seed-coat This is either
white, with which character white flowers are constantly corre-
lated; or it is grey, grey-brown, leather-brown, with or without
violet spotting, in which case the colour of the standards is violet,
that of the wings purple, and the stem in the axils of the leaves is of
a reddish tint. The grey seed-coats become dark brown in boiling
water.

4. To the difference in the form of the ripe pods. These are
either simply inflated, not contracted in places; or they are deeply
constricted between the seeds and more or less wrinkled (P.
saccharatum) .

5. To the difference in the colour of the unripe pods. They are
either light to dark green, or vividly yellow, in which colouring the
stalks, leaf- veins, and calyx participate. ^

6. To the difference in the position of the flowers. They are either
axial, that is, distributed along the main stem; or they are ter-
minal, that is, bunched at the top of the stem and arranged almost
in a false umbel ; in this case the upper part of the stem is more or
less widened in section (P. umbellatum) .^

7. To the difference in the length of the stem. The length of the
stem^ is very various in some forms; it is, however, a constant

^ [Mendel uses the terms *' albumen " and " endosperm " somewhat l<x).sely to
denote the cotyledons, containing food-material, within the seed.]

^ One species possesses a beautifully brownish-red coloured |)od, which when
ripening turns to violet and blue. Trials with this character were only l>egun last
year. [Of these further experiments it seems no account was published. Corrcns has
since worked with such a variety.]

^ [This is often called the Mummy Pea. It shows slight fasciatiou. The form
I know has white standard and salmon-red wings.]

^ [In my account of these experiments {R.H.S. Journal, vol. xxv. p. 54) I mis-
understood this paragraph and took '" axis " to mean the Jioral axis, instead of the

318 APPENDIX

character for each, in so far that healthy plants, grown in the same
soil, are only subject to unimportant variations in this character.

In experiments with this character, in order to be able to dis-
criminate with certainty, the long axis of 6 to 7 ft. was always
crossed with the short one of | ft. to 1| ft.

Each two of the differentiating characters enumerated above
were united by cross-fertilisation. There were made for the

1st trial 60 fertilisations on 15 plants.

2nd " 58 " " 10

3rd " 35 " " 10

4th " 40 " " 10

5th " 23 " " 5

6th " 34 " " 10

7th " 37 " " 10

From a larger number of plants of the same variety only the most
vigorous were chosen for fertilisation. Weakly plants always afford
uncertain results, because even in the first generation of hybrids,
and still more so in the subsequent ones, many of the offspring
either entirely fail to flower or only form a few and inferior seeds.

Furthermore, in aU the experiments reciprocal crossings were
effected in such a way that each of the two varieties which in one
set of fertilisation served as seed-bearer in the other set was used
as the pollen plant.

The plants were grown in garden beds, a few also in pots, and
were maintained in their naturally upright position by means of
sticks, branches of trees, and strings stretched between. For each
experiment a number of pot plants were placed during the blooming
period in a greenhouse, to serve as control plants for the main
experiment in the open as regards possible disturbance by insects.
Among the insects ^ which visit Peas the beetle Bruchus pisi might
be detrimental to the experiments should it appear in numbers.
The female of this species is known to lay the eggs in the flower,
and in so doing opens the keel; upon the tarsi of one specimen,
which was caught in a flower, some pollen grains could clearly be
seen under a lens. Mention must also be made of a circumstance

main axis of the plant. The unit of measurement, being indicated in the original
by a dash ('), I carelessly took to have been an inch, but the translation here given
is evidently correct.]

^ [It is somewhat surprising that no mention is made of Thrips, which swarm in
Pea flowers. I had come to the conclusion that this is a real source of error and I see
Laxton held the same opinion. 1

APPENDIX 319

which possibly might lead to the introduction of foreign pollen.
It occurs, for instance, in some rare cases that certain parts of an
otherwise quite normally developed flower wither, resulting in a
partial exposure of the fertilising organs. A defective development
of the keel has also been observed, owing to which the stigma and
anthers remained partially uncovered.^ It also sometimes hap-
pens that the pollen does not reach full perfection. In this e\'ent
there occurs a gradual lengthening of the pistil during the bloom-
ing period, until the stigmatic tip protrudes at the point of the keel.
This remarkable appearance has also been observed in hybrids of
Phaseolus and Lathyrus.

The risk of false impregnation by foreign pollen is, however, a
very slight one with Pisum, and is quite incapable of disturbing the
general result. Among more than 10,000 plants which were care-
fully examined there were only a very few cases where an indubi-
table false impregnation had occurred. Since in the greenliouse
such a case was never remarked, it may well be supposed that
Bruchus pisi, and possibly also the described abnormalities in the
floral structure, were to blame.

[Fi] The Forms of the Hybrids ^

Experiments which in previous years were made with ornamental
plants have already afforded evidence that the hybrids, as a rule,
are not exactly intermediate between the parental species. With
some of the more striking characters, those, for instance, which
relate to the form and size of the leaves, the pubescence of the
several parts, &c., the intermediate, indeed, is nearly always to be
seen; in other cases, however, one of the two parental characters
is so preponderant that it is difficult, or quite impossible, to detect
the other in the hybrid.

This is precisely the case with the Pea hybrids. In the case of
each of the seven crosses the hybrid-character resembles ^ that of
one of the parental forms so closely that the other either escapes

1 [This also happens m Sweet Peas.]

2 [Mendel throughout speaks of his cross-bred Peas as " hybrids," a terra whR-h
many restrict to the offspring of two distinct species. He, as he explains, hold this

to be only a question of degree.] i , • j

3 [Note that Mendel, with true penetration, avoids speaking of the hybrid-
character as " transmitted " by either parent, thus escapmg the error pervaduig the
older views of heredity.]

320 APPENDIX

observation completely or cannot be detected with certainty. This
circumstance is of great importance in the determination and
classification of the forms under which the offspring of the hybrids
appear. Henceforth in this paper those characters which are trans-
mitted entire, or almost unchanged in the hybridisation, and there-
fore in themseh^es constitute the characters of the hybrid, are
termed the dominant, and those which become latent in the process
recessive. The expression " recessive " has been chosen because the
characters thereby designated withdraw or entirely disappear in
the hybrids, but nevertheless reappear unchanged in their progeny,
as will be demonstrated later on.

It was furthermore shown by the whole of the experiments that it
is perfectly immaterial whether the dominant character belongs to
the seed-bearer or to the pollen-parent; the form of the hybrid
remains identical in both cases. This interesting fact was also
emphasised by Gartner, w^th the remark that even the most
practised expert is not in a position to determine in a hybrid which
of the two parental species was the seed or the pollen plant. ^

Of the differentiating characters which were used in the experi-
ments the following are dominant:

1. The round or roundish form of the seed with or without
shallow depressions.

2. The yellow colouring of the seed albumen [cotyledons].

3. The grey, grey-bro^m, or leather-brown colour of the seed-
coat, in association with violet- red blossoms and reddish spots in
the leaf axils.

4. The simply inflated form of the pod.

5. The green colouring of the unripe pod in association with the
same colour in the stems, the leaf-veins and the calyx.

6. The distribution of the flowers along the stem.

7. The greater length of stem.

With regard to this last character it must be stated that 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 all parts of plants when stems of very different
length are crossed. Thus, for instance, in repeated experiments,
stems of 1 ft. and 6 ft. in length yielded without exception hybrids
which varied in length between 6 ft. and 7| ft.

1 [Gartner, p. 223.]

APPENDIX 3^Zi

The hybrid seeds in the experiments with seed-coat are often
more spotted, and the spots sometimes coalesce into small bluish-
violet patches. The spotting also frequently appears even when it
is absent as a parental character.^

The hybrid forms of the seed-shape and of the al])umen [colour]
are developed immediately after the artificial fertilisation by the
mere influence of the foreign pollen. They can, therefore, be
observed even in the first year of experiment, whilst all tlie other
characters naturally only appear in the following year in such
plants as have been raised from the crossed seed.

[F2] The Generation [bred] from the Hybrids

In this generation there reappear, together with the dominant
characters, also the recessive ones with their peculiarities fully
developed, and this occurs in the definitely expressed average pro-
portion of three to one, so that among each four plants of this
generation three display the dominant character and one the
recessive. This relates without exception to all the characters
which were investigated in the experiments. The angular wrinkled
form of the seed, the green colour of the albumen, the white colour
of the seed-coats and the flowers, the constrictions of the pods, the
yellow colour of the unripe pod, of the stalk, of the calyx, and of
the leaf venation, the umbel-like form of the inflorescence, and the
dwarfed stem, all reappear in the numerical proportion given,
without any essential alteration. Transitional forms were not
observed in any experiment.

Since the hybrids resulting from reciprocal crosses are formed
alike and present no appreciable difference in their subsequent
development, consequently the results [of the reciprocal crosses]
can be reckoned together in each experiment. The relative num-
bers which were obtained for each pair of differentiating characters

are as follows:

Expt. 1. Form of seed. — From 253 hybrids 7,324 seeds were
obtained in the second trial year. Amon^; them were 5,474 round
or roundish ones and 1,850 angular wrinkled ones. Therefrom the
ratio 2.96 to 1 is deduced.

Expt. 2. Colour of albumen. — 258 plants yielded 8,023 seeds,
6,022 yellow, and 2,001 green; their ratio, therefore, is as 3.01 to 1.

1 [This refers to the coats of the seeds borne by Fi plants.)

322 APPENDIX

In these two experiments each pod yielded usually both kinds of
seeds. In well-developed pods which contained on the average six
to nine seeds, it often happened that all the seeds were round
(Expt. 1) or all yellow (Expt. 2); on the other hand there were
never observed more than five wrinkled or five green ones in one
pod. It appears to make no difference whether the pods are
developed early or later in the hybrid or whether they spring from
the main axis or from a lateral one. In some few plants only a few
seeds developed in the first formed pods, and these possessed exclu-
sively one of the two characters, but in the subsequently developed
pods the normal proportions were maintained nevertheless.

As in separate pods, so did the distribution of the characters vary
in separate plants. By way of illustration the first ten individuals
from both series of experiments may serve.

Experiment 1.

Experiment 2.

Fonn of Seed.

Color of Albumen

Plants Round

Angular

Yellow

Green

1

45

12

25

11

2

• 27

8

32

7

3

24

7

14

5

4

19

10

70

27

5

32

11

24

13

6

26

6

20

6

7

88

24

32

13

8

22

10

44

9

9

28

6

50

14

10

25

7

44

18

As extremes in the distribution of the two seed characters in one
plant, there were observed in Expt. 1 an instance of 43 round and
only 2 angular, and another of 14 round and 15 angular seeds. In
Expt. 2 there was a case of 32 yellow and only 1 green seed, but also
one of 20 yellow and 19 green.

These two experiments are important for the determination of
the average ratios, because with a smaller number of experimental
plants they show that very considerable fluctuations may occur.
In counting the seeds, also, especially in Expt. 2, some care is
requisite, since in some of the seeds of many plants the green colour
of the albumen is less developed, and at first may be easily over-
looked. The cause of this partial disappearance of the green
colouring has no connection with the hybrid-character of the plants,
as it likewise occurs in the parental variety. This peculiarity

APPENDIX 3!23

[bleaching] is also confined to the individual and is not inherited by
the offspring. In luxuriant plants this appearance was freciuently
noted. Seeds which are damaged by insects during their develop-
ment often vary in colour and form, but, with a little i)ractice in
sorting, errors are easily avoided. It is almost sui)crfiuous to men-
tion that the pods must remain on the plants until they are thor-
oughly ripened and have become dried, since it is only then that
the shape and colour of the seed are fully developed.

Expt. 3. Colour of the seed-coats. — Among 929 plants 705 bore
violet-red flowers and grey-brown seed-coats; 224 had white flowers
and white seed-coats, giving the proportion 3.15 to 1.

Expt. 4. Form of pods. — Of 1,181 plants 882 had them simply
inflated, and in 299 they were constricted. Resulting ratio,
2.95 to 1.

Expt. 5. Colour of the unripe pods. — The number of trial
plants was 580, of which 428 had green pods and 152 yellow ones.
Consequently these stand in the ratio 2.82 to 1.

Expt. 6. Position of flowers. — Among 858 cases 651 had
inflorescences axial and 207 terminal. Ratio, 3.14 to 1.

Expt. 7. Length of stem. — Out of 1,064 plants, in 787 cases
the stem was long, and in 277 short. Hence a mutual ratio of 2.84
to 1. In this experiment the dwarfed plants were carefully lifted
and transferred to a special bed. This precaution was necessary,
as otherwise they would have perished through being overgrown
by their tall relatives. Even in their quite young state they can be
easily picked out by their compact growth and thick dark-green
foliage.^

If now the results of the whole of the experiments be brought
together, there is found, as between the number of forms with the
dominant and recessive characters, an average ratio of 2.98 to 1 ,
or 3 to 1.

The dominant character can have here a double signification —
viz. that of a parental character, or a hybrid-character.'- In which
of the two significations it appears in each separate case can only
be determined by the following generation. As a parental char-
acter it must pass over unchanged to the whole of the ott'spriug; as

1 [This is true also of the dwarf or " Cupid " Sweet Peas.]

2 [This paragraph presents the view of the hybrid-character as something inci-
dental to the hybrid, and not " transmitted " to it — a true and fundamental
conception here expressed probably for the first time.)

SM APPENDIX

a hybrid-character, on the other hand, it must maintain the same
behaviour as in the first generation [F2].

[F3] The Second Generation [bred] from the Hybrids

Those forms which in the first generation [F2] exhibit the reces-
'^ V sive character do not further vary in the second generation [F3] as
regards this character; they remain constant in their offspring.

It is otherwise with those which possess the dominant character
in the first generation [bred from the hybrids]. Of these ^w;o- thirds
yield offspring which display the dominant and recessive char-
acters in the proportion of 3 to 1, and thereby show exactly the
same ratio as the hybrid forms, while only one-third remains with
the dominant character constant.

The separate experiments yielded the following results :

Expt. 1. Among 565 plants which were raised from round seeds
of the first generation, 193 yielded round seeds only, and re-
mained therefore constant in this character; 372, however, gave
both round and wrinkled seeds, in the proportion of 3 to 1. The
number of the hybrids, therefore, as compared with the constants
is 1.93 to 1.

Expt. 2. Of 519 plants which were raised from seeds whose
albumen was of yellow colour in the first generation, 166 yielded
exclusively yellow, while 353 yielded yellow and green seeds in the
proportion of 3 to 1. There resulted, therefore, a division into
hybrid and constant forms in the proportion of 2.13 to 1.

For each separate trial in the following experiments 100 plants
were selected which displayed the dominant character in the first
generation, and in order to ascertain the significance of this, ten
seeds of each were cultivated.

Expt. 3. The offspring of 36 plants yielded exclusively grey-
brown seed-coats, while of the offspring of 64 plants some had
grey -brown and some had white.

Expt. 4. The offspring of 29 plants had only simply inflated
pods; of the offspring of 71, on the other hand, some had inflated
and some constricted.

Expt. 5. The offspring of 40 plants had only green pods; of the
offspring of 60 plants some had green, some yellow ones.

Expt. 6. The offspring of 33 plants had only axial flowers; of
the offspring of 67, on the other hand, some had axial and some
terminal flowers.

APPENDIX 3^5

Expt. 7. The offspring of 28 plants inherited the long axis, and
those of 72 plants some the long and some the short axis.

In each of these experiments a certain number of the plants came
constant with the dominant character. For the determination of
the proportion in which the separation of the forms with the con-
stantly persistent character results, the two first experiments are
of especial importance, since in these a larger number of plants can
be compared. The ratios 1.93 to 1 and 2.13 to 1 gave together
almost exactly the average ratio of 2 to 1. The sixth experiment
gave a quite concordant result; in the others the ratio varies more
or less, as was only to be expected in view of the smaller numl^er of
100 trial plants. Experiment 5, which shows the greatest depar-
ture, was repeated, and then, in lieu of the ratio of 60 and 40, tliat
of 65 and 35 resulted. The average ratio of 2 to 1 appears, thereforey
as fixed with certainty. It is therefore demonstrated that, of those
forms which possess the dominant character in the first generation,
two-thirds have the hybrid-character, while one-third remains
constant with the dominant character.

The ratio of 3 to 1, in accordance with which the distribution of
the dominant and recessive characters results in the first genera-
tion, resolves itself therefore in all experiments into the ratio of
2:1:1 if the dominant character be differentiated according to its
significance as a hybrid-character or as a parental one. Since the
members of the first generation [F2] spring directly from the seed
of the hybrids [Fi], it is now clear that the hybrids form seeds having
one or other of the two differentiating characters, and of these one-half
develop again the hybrid form, while the other half yield plants which
remain constant and receive the dominant or the recessive characters
[respectively] in equal numbers.

The Subsequent Generations [bred] from the Hybrids

The proportions in which the descendants of the hybrids develop
and split up in the first and second generations presumably hold
good for all subsequent progeny. Experiments 1 and 2 have
already been carried through six generations, 3 and 7 through five,
and 4, 5, and 6 through four, these experiments being continued
from the third generation with a small number of plants, and no
departure from the rule has been perceptible. The offspring of the
hybrids separated in each generation in the ratio of 2:1:1 into
hybrids and constant forms.

326 APPENDIX

If A be taken as denoting one of the two constant characters, for
instance the dominant, a, the recessive, and Aa the hybrid form in
which both are conjoined, the expression

A + ^Aa + a

shows the terms in the series for the progeny of the hybrids of two
differentiating characters.

The observation made by Gartner, Kolreuter, and others, that
hybrids are incHned to revert to the parental forms, is also con-
firmed by the experiments described. It is seen that the number of
the hybrids which arise from one fertilisation, as compared with the
number of forms which become constant, and their progeny from
generation to generation, is continually diminishing, but that
nevertheless they could not entirely disappear. If an average
equality of fertility in all plants in all generations be assumed, and
if, furthermore, each hybrid forms seed of which one-half yields
hybrids again, while the other half is constant to both characters
in equal proportions, the ratio of numbers for the offspring in each
generation is seen by the following summary, in which A and a
denote again the two parental characters, and Aa the hybrid forms.
For brevity's sake it may be assumed that each plant in each
generation furnishes only 4 seeds.

Generation

A

Aa

a

1

1

2

1

2

6

4

6

3

28

8

28

4

120

16

120

5

496

32

496

n

Ratios

A

: Aa

a

1

: 2

1

3

: 2

3

7

; 2

7

15

: 2

15

31

: 2

31

2"— 1

: 2

r-1

In the ten.th generation, for instance, 2" — 1 = 1023. There
result, therefore, in each 2,084 plants which arise in this generation
1,023 with the constant dominant character, 1,023 with the reces-
sive character, and only two hybrids.

The Offspring of Hybrids in which several Differen-
TL\TiNG Characters are Associated

In the experiments above described plants were used which
differed only in one essential character.^ The next task consisted

^ [This statement of Mendel's in the Hght of present knowledge is open to some
misconception. Though his work makes it evident that such varieties may exist.

/'

VPPENblX 8*27

in ascertaining whether the law of development discovered in these
applied to each pair of differentiating characters when several
diverse characters are united in the hybrid by crossing. As regards
the form of the hybrids in these cases, the experiments showed
throughout that this invariably more nearly approaches to that one
of the two parental plants which possesses the greater miin})er of
dominant characters. If, for instance, the seed plant has a short
stem, terminal white flowers, and simply inflated pods; tlio pollen
plant, on the other hand, a long stem, violet-red flowers distributed
along the stem, and constricted pods; the hybrid resembles the
seed parent only in the form of the pod; in the other characters it
agrees with the pollen parent. Should one of the two parental
types possess only dominant characters, then the hybrid is scarcely
or not at all distinguishable from it.

Two experiments were made with a considerable number of
plants. In the first experiment the parental plants differed in the
form of the seed and in the colour of the albumen; in the second
in the form of the seed, in the colour of the albumen, and in the
colour of the seed-coats. Experiments with seed characters give
the result in the simplest and most certain way.

In order to facilitate study of the data in these experiments, the
different characters of the seed plant will be indicated by A, B, C,
those of the pollen plant by a, 6, c, and the hybrid forms of the
characters by Aa, Bb, and Cc.

Expt. 1. — AB, seed parents; ab, pollen parents;

Ay form round; a, form wrinkled;

B, albumen yellow. 6, albumen green.

The fertilised seeds appeared round and yellow like those of the
seed parents. The plants raised therefrom yielded seeds of four
sorts, which frequently presented themselves in one pod. In all,
55Q seeds were yielded by 15 plants, and of these there were:

315 round and yellow,
101 wrinkled and yellow,
108 round and green,
32 wrinkled and green.

it is very unlikely that Mendel could have had seven pairs of varieties such that the
members of each pair differed from each other in cnhj one considerable character
{wesentliches Merkmal). The point is probably of little theoretical or practical
consequence, but a rather heavy stress is thrown on " wesentlichr ]

328 APPENDIX

All were sown the following year. Eleven of the round yellow seeds
did not yield plants, and three plants did not form seeds. Among
the rest:

38 had round yellow seeds AB

65 round yellow and green seeds ABb

60 round yellow and wrinkled yellow seeds . . , . AaB
138 round yellow and green, wrinkled yellow and

green seeds AaBb

From the wrinkled yellow seeds 96 resulting plants bore seed, of

which:

28 had only wrinkled yellow seeds aB

68 wrinkled yellow and green seeds aBb.

From 108 round green seeds 102 resulting plants fruited, of which:

35 had only round green seeds Ab

67 round and wrinkled green seeds Aab.

The wrinkled green seeds yielded 30 plants which bore seeds all of
like character; they remained constant ab.

The offspring of the hybrids appeared therefore under nine
different forms, some of them in very unequal numbers. When
these are collected and co-ordinated we find :

38 plants with the sign AB

35 " " " " Ab

28 " " " " aB

30 " " " " ab

65 " " " " ABb

68 " " " " aBb

60 " " " " AaB

67 " " " " Aab

138 " " " " AaBb.

The whole of the forms may be classed into three essentially
different groups. The first includes those with the signs AB, Aby
aB, and ab : they possess only constant characters and do not vary
again in the next generation. Each of these forms is represented
on the average thirty-three times. The second group includes the
signs ABb, aBb, AaB, Aab: these are constant in one character and
hybrid in another, and vary in the next generation only as regards
the hybrid-character. Each of these appears on an average sixty-
five times. The form AaBb occurs 138 times: it is hybrid in both

APPENDIX 3>9

characters, and behaves exactly as do the hybrids from wliich it is
derived.

If the numbers in which the forms belonging to these classes
appear be compared, the ratios of 1, 2, 4 are unmistakably evident.
The numbers 32, 65, 138 present very fair approximatioas to the
ratio numbers of 33, 66, 132.

The developmental series consists, therefore, of nine classes, of
which four appear therein always once and are constant in })ot}i
characters; the forms AB, ab, resemble the parental forms, the
two other present combinations between the conjoined cliaracters
Ay a, B, b, which combinations are likewise possibly coiLstant.
Four classes appear always twice, and are constant in one cliaracter
and hybrid in the other. One class appears four times, and is
hybrid in both characters. Consequently the offspring of the
hybrids, if two kinds of differentiating characters are coml)ined
therein, are represented by the expression

AB-j-Ab-^aB-{-ab + ^ABb + 2aBb + ^AaB + ^Aab + 4^AaBb.

This expression is indisputably a combination series in which the
two expressions for the characters A and a, B and b are combined.
We arrive at the full number of the classes of the series by the
combination of the expressions:

A 4- 2Aa + a
B -\-2Bb + b.
Expt. 2.

ABC, seed parents; abc, pollen parents;

A, form round; a, form wrinkled;

B, albumen yellow; 6, albumen green;

C, seed-coat grey-brown. c, seed-coat white.

This experiment was made in precisely the same way as the
previous one. Among all the experiments it demanded the most
time and trouble. From 24 hybrids 687 seeds were obtained in all :
these were all either spotted, grey-brown or grey-green, round or
wrinkled.^ From these in the following year 639 plants fruited,
and, as further investigation showed, there were among them:

1 [Note that Mendel does not state the cotyledon-colour of the first crosses in
this case; for as the coats were thick, it could not have been seen without opening or
peeling the seeds.]

330

APPENDIX

8 plants ABC

22]

slants ABCc

45 plants ABbCe

14 '

' ABc

17

u

AbCc

36

u

aBbCc

9 '

' AbC

25

u

aBCc

38

u

AaBCc

11 '

' Abe

20

u

abCc

40

u

AabCc

8 '

' aBC

15

a

ABbC

49

u

AaBbC

10 '

' aBc

18

u

ABbc

48

u

AaBbc

10 '

' abC

19

(I

aBbC

7 '

' abc

24

u

aBbc

14

u

AaBC

78

u

AaBbCi

18

u

AaBc

20

u

AabC

16

<<

A abc

The whole expression contains 27 terms. Of these 8 are constant
in all characters, and each appears on the average 10 times; l!2 are
constant in two characters, and hj^brid in the third; each appears
on the average 19 times; 6 are constant in one character and hybrid
in the other two ; each appears on the average 43 times. One form
appears 78 times and is hybrid in all of the characters. The ratios
10, 19, 43, 78 agree so closely with the ratios 10, 20, 40, 80, or 1, 2,
4, 8, that this last undoubtedly represents the true value.

The development of the hybrids when the original parents differ
in three characters results therefore according to the following
expression :

ABC + ABc + AhC + Abc + aBC + aBc + abC + abc
+ 2 ABCc + 2 AbCc + 2 aBCc + 2 abCc + 2 ABbC
+ 2 ABbc + 2 aBbC + 2 aBbc + 2 AaBC + 2 AaBc
+ 2 AabC H- 2 Aabc + 4 ABbCc + 4 aj56Cc + 4 ^a5Cc
+ 4 ^a6Cc + 4 ^a56C + 4 AaBbc + 8 AaBbCc.

Here also is involved a combination series in which the expres-
sions for the characters A and a, B and 6, C and c, are united.
The expressions

A -{- 2Aa + a
B + ^Bb + b
■ C + 2Cc + c

give all the classes of the series. The constant combinations w^hich
occur therein agree with all combinations which are possible
between the characters A, B^ C, a, 6, c; two thereof, ABC and abc,
resemble the two original parental stocks.

In addition, further experiments were made with a smaller num-
ber of experimental plants in which the remaining characters by

APPENDIX 381

twos and threes were united as hybrids: all yielded ai^proximately
the same results. There is therefore no douht that for the whole
of the characters involved in the experiments the principle applies
that the offspring of the hybrids in which several essentially different
characters are combined exhibit the terms of a series of combinations^
in which the developmental series for each pair of differentiating
characters are united. It is demonstrated at the same time that the
relation of each pair of different characters in hybrid union is inde-
pendent of the other differences in the two original parental stocks.

If n represents the number of the differentiating characters in the
two original stocks, 3" gives the number of terms of the c-oinbina-
tion series, 4" the number of individuals which belong to the series,
and 2" the number of unions which remain constant. The series
therefore contains, if the original stocks differ in four characters,
3^ = 81 classes, # = 9.5Q individuals, and 2^ = 16 constant forms;
or, which is the same, among each 9,5^ offspring of the hybrids
there are 81 different combinations, 16 of which are constant.

All constant combinations which in Peas are possible by the
combination of the said seven differentiating characters were
actually obtained by repeated crossing. Their number is given by
2^ = 128. Thereby is simultaneously given the practical y)ro()f
that the constant characters which appear in the several varieties of a
group of plants may be obtained in all the associations which are pos-
sible according to the [mathematical] laws of combinationy by means of
repeated artificial fertilisation.

As regards the flowering time of the hybrids, the experiments are
not yet concluded. It can, however, already be stated that the
time stands almost exactly between those of the seed and pollen
parents, and that the constitution of the hybrids with respect to
this character probably follows the rule ascertained in the case of
the other characters. The forms which are selected for experiments
of this class must have a difference of at least twenty days from tlic
middle flowering period of one to that of the other; furthermore,
the seeds when sown must all be placed at the same depth in tlie
earth, so that they may germinate simultaneously. Also, during
the whole flowering period, the more important variations in tem-
perature must be taken into account, and the partial hastening or
delaying of the flowering which may result therefrom. It is clear
that this experiment presents many difficulties to be overcome and
necessitates great attention.

332 APPENDIX

If we endeavour to collate in a brief form the results arrived at,
we find that those differentiating characters, which admit of easy
and certain recognition in the experimental plants, all behave
exactly alike in their hybrid associations. The offspring of the
hybrids of each pair of differentiating characters are, one-half,
hybrid again, while the other half are constant in equal proportions
having the characters of the seed and pollen parents respectively.
If several differentiating characters are combined by cross-fertili-
sation in a hybrid, the resulting offspring form the terms of a com-
bination series in which the combination series for each pair of
differentiating characters are united.

The uniformity of behaviour shown by the whole of the char-
acters submitted to experiment permits, and fully justifies, the
acceptance of the principle that a similar relation exists in the other
characters which appear less sharply defined in plants, and there-
fore could not be included in the separate experiments. An experi-
ment with peduncles of different lengths gave on the whole a fairly
satisfactory result, although the differentiation and serial arrange-
ment of the forms could not be effected with that certainty which
is indispensable for correct experiment.

The Reproductive Cells of the Hybrids

The results of the previously described experiments led to further
experiments, the results of which appear fitted to afford some con-
elusions as regards the composition of ^ the egg and pollen cells of
hybrids. An important clue is afforded in Pisum by the circum-
stance that among the progeny of the hybrids constant forms
appear, and that this occurs^, too, in respect of all combinations of
the associated characters. So far as experience goes, we find it in
every case confirmed that constant progeny can only be formed
when the egg cells and the fertilising pollen are of like character, so
that both are provided with the material for creating quite similar
individuals, as is the case with the normal fertilisation of pure
species. We must therefore regard it as certain that exactly similar
factors must be at work also in the production of the constant forms
in the hybrid plants. Since the various constant forms are pro-
duced in one plant, or even in one flower of a plant, the conclusion
appears logical that in the ovaries of the hybrids there are formed
as many sorts of egg cells, and in the anthers as many sorts of
pollen cells, as there are possible constant combination forms, and

APPENDIX 338

that these egg and pollen cells agree in their internal composition
with those of the separate forms.

In point of fact it is possible to demonstrate theoretically tliat
this hypothesis would fully suffice to account for the development
of the hybrids in the separate generatioas, if we miglit ;it the sanic
time assume that the various kinds of egg and pollen cells were
formed in the hybrids on the average in equal numbers'.^'

In order to bring these assumptions to an experimental pr«M)f,
the following experiments were designed. Two forms which were
constantly different in the form of the seed and the colour of the
albumen were united by fertilisation.

If the differentiating characters are again indicated as A, ii,
a, 6, we have:

AB, seed parent; ah, pollen parent;

A, form round; a, form wrinkled;

B, albumen yellow. 6, albumen green.

The artificially fertilised seeds were sown together with several
seeds of both original stocks, and the most vigorous examples were
chosen for the reciprocal crossing. There were fertilised :

1. The hybrids with the pollen of AB.

2. The hybrids " " " " ah.

3. AB " " " " the hybrids.

4. ah " " " " the hybrids.

For each of these four experiments the whole of the flowers on
three plants were fertilised. If the above theory be correct, there
must be developed on the hybrids egg and pollen cells of the forms
AB, Ah, aB, ah, and there would be combined:

1. The egg cells AB, Ah, aB, ah with the pollen cells AB.

2. The egg cells AB, Ah, aB, ah with the pollen cells ah.

3. The egg cells AB with the pollen cells AB, Ah, aB, ah.

4. The egg cells ah with the pollen cells AB, Ah, aB, ah.

From each of these experiments there could then result only the
following forms:

1. AB, ABh, AaB, AaBh. 3. AB, ABh, AaB, AuBb.

2. AaBh, Aah, aBh, ah. 4. AaBh, Aah, aBh, ah.

1 [This and the preceding paragraph contain the essence of the Mendcliaa
principles of heredity.]

334 APPENDIX

If, furthermore, the several forms of the egg and pollen cells of
the hybrids were produced on an average in equal numbers, then
in each experiment the said four combinations should stand in the
same ratio to each other. A perfect agreement in the numerical
relations was, however, not to be expected, since in each fertilisa-
tion, even in normal cases, some egg cells remain undeveloped or
subsequently die, and many even of the well-formed seeds fail to
germinate when sown. The above assumption is also limited in so
far that, while it demands the formation of an equal number of the
various sorts of egg and pollen cells, it does not require that this
should apply to each separate hybrid with mathematical exactness.

The first and second experiments had primarily the object of
proving the composition of the hybrid egg cells, while the third and
fourth experiments were to decide that of the pollen cells. ^ As is
shown by the above demonstration the first and third experiments
and the second and fourth experiments should produce precisely
the same combinations, and even in the second year the result
should be partially visible in the form and colour of the artificially
fertilised seed. In the first and third experiments the dominant
characters of form and colour, A and B, appear in each union, and
are also partly constant and partly in hybrid union with the reces-
sive characters a and b, for which reason they must impress their
peculiarity upon the whole of the seeds. All seeds should therefore
appear round and yellow, if the theory be justified. In the second
and fourth experiments, on the other hand, one union is hybrid in
form and in colour, and consequently the seeds are round and
yellow; another is hybrid in form, but constant in the recessive
character of colour, whence the seeds are round and green; the
third is constant in the recessive character of form but hybrid in
colour, consequently the seeds are wrinkled and yellow; the fourth
is constant in both recessive characters, so that the seeds are
wrinkled and green. In both these experiments there were conse-
quently four sorts of seed to be expected — viz. round and yellow,
round and green, wrinkled and yellow, wrinkled and green.

The crop fulfilled these expectations perfectly. There were
obtained in the

1st Experiment, 98 exclusively round yellow seeds;
3rd " 94 " " " "

^ [To prove, namely, that both were similarly differentiated, and not one or
other only.]

APPENDIX 3,3

o

In the 2d Experiment, 31 round and yellow, 2G round and green,
27 wrinkled and yellow, 26 wrinkled and green seeds.

In the 4th Experiment, 24 round and yell<nv, 25 round and green,
22 wrinkled and yellow, 26 wrinkled and green seeds.

There could scarcely be now any doubt of the success of the
experiment; the next generation must afford the iinal jjroof.
From the seed sown there resulted for the first experiment 90
plants, and for the third 87 plants which fruited: these yielded
for the

1st Exp. 3rd Exp.

20 25 round yellow seeds AB

23 19 round yellow and green seeds ABh

25 22 round and wrinkled yellow seeds AaB

22 21 round and wrinkled green and yellow seeds .... AaBh

In the second and fourth experiments the round and yellow
seeds yielded plants with round and wrinkled yellow and green
seeds, AaBb.

From the round green seeds, plants resulted with round and
wrinkled green seeds, Aab.

The wrinkled yellow seeds gave plants with wrinkled yellow and
green seeds, aBb.

From the wrinkled green seeds plants were raised which yielded
again only wrinkled and green seeds, ab.

Although in these two experiments likewise some seeds did not
germinate, the figures arrived at already in the previous year were
not affected thereby, since each kind of seed gave plants which, as
regards their seed, were like each other and different from the
others. There resulted therefore from the

2d Exp. 4th Exp.

31 24 plants of the form AaBh

26 25 " " " " '1«^

27 22 " " " " aBb
26 27 " " " " ob

In all the experiments, therefore, there appeared all the forms
which the proposed theory demands, and they came in nearly

equal numbers.

In a further experiment the characters of flower-colour and
length of stem were experimented upon, and selection was so made
that in the third year of the experiment each character ought to
appear in half of all the plants if the above theory were correct.
A, B, a, b serve again as indicating the various characters.

336 APPENDIX

A, violet-red flowers. a, white flowers.

B, axis long. 6, axis short.

The form Ab was fertilised with ab, which produced the hybrid
Aab. Furthermore, aB was also fertilised with ab, whence the
hybrid aBb. In the second year, for further fertilisation, the hybrid
Aab was used as seed parent, and hybrid aBb as pollen parent.

Seed parent, Aab. Pollen parent, aBb.

Possible egg cells, Ab,ab. Pollen cells, aB^ab.

From the fertilisation between the possible egg and pollen cells
four combinations should result, viz.,

AaBb + aBb + Aab + ab.

From this it is perceived that, according to the above theory, in
the third year of the experiment out of all the plants

Half should have violet-red flowers (Aa), Classes 1, 3

" " " white flowers (a) ' " 2, 4

« « " a long axis (Bb) " 1,2

" " "a short axis (6) " 3, 4

From 45 fertilisations of the second year 187 seeds resulted, of
which only 166 reached the flowering stage in the third year.
Among these the separate classes appeared in the numbers fol-
lowing:

Class

Color of flower

Stem

1

violet-red

long

47 times

2

white

long

40 "

3

violet-red

short

38 "

4

white

short

41 "

There subsequently appeared

The violet-red flower-colour (Aa) in 85 plants.
" white " " (a) in 81 "

" long stem {Bb) in 87 "

" short " (6) in 79 "

The theory adduced is therefore satisfactorily confirmed in this
experiment also.

For the characters of form of pod, colour of pod, and position of
flowers, experiments were also made on a small scale, and results
obtained in perfect agreement. All combinations which were pos-
sible through the union of the differentiating characters duly
appeared, and in nearly equal numbers.

APPENDIX 337

Experimentally, therefore, the theory is confirmed that the pea
hybrids form egg and pollen cells which, in their constitution, represent
in equal numbers all constant forms which result from the combination
of the characters united in fertilisation .

The difference of the forms among the progeny of the hybrids,
as well as the respective ratios of the numbers in which they are
observed, find a sufficient explanation in the principle above
deduced. The simplest case is afforded by the developmental series
of each pair of differentiating characters. This series is repre-
sented by the expression A + ^Aa + a, in which A and a signify
the forms with constant differentiating characters, and Aa the
hybrid form of both. It includes in three different classes four
individuals. In the formation of these, pollen and egg cells of the
form A and a take part on the average equally in the fertilisation;
hence each form [occurs] twice, since four individuals are formed.
There participate consequently in the fertilisation

The pollen cells A -\- A -{- a ■\- a
The egg cells A -\- A -\- a -]r a.

It remains, therefore, purely a matter of chance which of the two
sorts of pollen will become united with each separate egg cell.
According, however, to the law of probability, it will always hap-
pen, on the average of many cases, that each pollen form, A and a,
will unite equally often with each egg cell form, A and a, conse-
quently one of the two pollen cells A in the fertilisation will meet
with the egg cell A and the other with an egg cell a, and so likewise
one pollen cell a will unite with an egg cell A, and the other with

egg cell a.

Pollen cells A A a a

\ .\ i

Egg cells A A a a

The result of the fertilisation may be made clear by putting the
signs for the conjoined egg and pollen cells in the form of fractions,
those for the pollen cells above and those for the egg cells below the

line. We then have

A A a a

_ I — _i_ — J- - .

A^ a^ A^ a

In the first and fourth term the egg and pollen cells are of like kind,
consequently the product of their union must be constant, viz. A

338 APPENDIX

and a; in the second and third, on the other hand, there again
results a union of the two differentiating characters of the stocks,
consequently the forms resulting from these fertilisations are
identical with those of the hybrid from which they sprang. There
occurs accordingly a repeated hyhridisaiion. This explains the strik-
ing fact that the hybrids are able to produce, besides the two

A a

parental forms, offspring which are like themselves; — and —

a A

both give the same union Aa, since, as already remarked above,
it makes no difference in the result of fertilisation to which of the
two characters the pollen or egg cells belong. We may write then

-- + - + -4-- = ^ + 2^a + a.
A a A a

This represents the average result of the self -fertilisation of the
hybrids when two differentiating characters are united in them.
In individual flowers and in individual plants, however, the ratios
in which the forms of the series are produced may suffer not in-
considerable fluctuations.^ Apart from the fact that the numbers
in which both sorts of egg cells occur in the seed vessels can only be
regarded as equal on the average, it remains purely a matter of
chance which of the two sorts of pollen may fertilise each separate
eg,g cell. For this reason the separate values must necessarily be
subject to fluctuations, and there are even extreme cases possible,
as were described earlier in connection with the experiments on the
form of the seed and the colour of the albumen. The true ratios
of the numbers can only be ascertained by an average deduced
from the sum of as many single values as possible ; the greater the
number, the more are merely chance effects eliminated.

The developmental series for hybrids in which two kinds of
differentiating characters are united contains, among sixteen
individuals, nine different forms, viz.,

AB-]-Ah^ aB + ah-\- 2ABb + ^aBb -f 2AaB + Mah + 4AaBb.

Between the differentiating characters of the original stocks, Aa
and Bby four constant combinations are possible, and consequently
the hybrids produce the corresponding four forms of egg and pollen
cells ABy Aby aB, ab, and each of these will on the average figure

^ [Whether segregation by such units is more than purely fortuitous may perhaps
be determined by seriation.]

APPENDIX 339

four times in the fertilisation, since sixteen individuals are inrluded
in the series. Therefore the participators in the fertilisation are

Pollen cells AB + AB -{- AB -{- AB -f Ab -f Ah + Ab + Ab

+ aB -{- aB -\- aB -\- aB + ab + ab + ab -\- aL

Egg ceUs AB + AB + AB + AB + Ab + Ab + Ab + Ab

+ aB-\-aB-{-aB-{-aB-\-ab-^ab-\-abi- ab.

In the process of fertilisation each pollen form unites on an average
equally often with each egg cell form, so that each of tlie four pollen
cells AB unites once with one of the forms of egg cell Ali, Ab, aB,
ab. In precisely the same way the rest of the pollen cells of the
forms Ab, aB, ab unite with all the other egg cells. We obtain
therefore

AB AB AB AB Ab Ab Ab Ab
AB Ab aB ab AB Ab aB ab

_L. £:?. 4- £^ 4- £^ _i_ £^ 4_ _^ , ab ab ah
AB Ab aB ab AB Tb '^i ab'
or

AB + ABh + AaB + AaBb + ABb + Ab + AaBb + Aab + AaB

+ AaBb + aB + aBb + AaBb + Aab + aBb + ab = AB

-f- ^6 + aB + a6 + '^ABb + ^aBb + '^AaB + 2.4a6 + A AaBb}

In precisely similar fashion is the developmental series of hybrids
exhibited when three kinds of 'differentiating characters are con-
joined in them. The hybrids form eight various kinds of egg
and pollen cells — ABC, ABc, AbC, Abe, aBC, aBc, abC\ abc — and
each pollen form unites itself again on the average once with eacli
form of egg cell.

The law of combination of different characters, which governs
the development of the hybrids, finds therefore its foundation and
explanation in the principle enunciated, that the hybrids j)r()duce
egg cells and pollen cells which in equal numbers represent all con-
stant forms which result from the combinations of the chanicters
brought together in fertilisation.

1 [In the original the sign of equality ( = ) is here represented by -f , evidently a
misprint.]

340 APPENDIX

Experiments with Hybrids of other Species of Plants

It must be the object of further experiments to ascertain whether
the law of development discovered for Pisum applies also to the
hybrids of other plants. To this end several experiments were
recently commenced. Two minor experiments with species of
Phaseolus have been completed, and may be here mentioned.

An experiment with Phaseolus vulgaris and Phaseolus nanus gave
results in perfect agreement. Ph. nanus had, together with the
dwarf axis, simply inflated, green pods. Ph. vulgaris had, on the
other hand, an axis 10 feet to 12 feet high, and yellow-coloured
pods, constricted when ripe. The ratios of the numbers in which
the different forms appeared in the separate generations were the
same as with Pisum. Also the development of the constant com-
binations resulted according to the law of simple combination of
characters, exactly as in the case of Pisum. There were obtained

Constant
combinations

Axis

Colour of
the unripe pods

Form of
the ripe pods

1

2

long

u

green

u

inflated
constricted

3
4

u
u

yellow
a

inflated
constricted

5
6

short

green

U

inflated
constricted

7
8

a
u

yellow

a

inflated
constricted

The green colour of the pod, the inflated forms, and the long axis
were, as in Pisiim, dominant characters.

Another experiment with two very different species of Phaseolus
had only a partial result. Phaseolus nanus, L., served as seed
parent, a perfectly constant species, with white flowers in short
racemes and small white seeds in straight, inflated, smooth pods;
as pollen parent was used Ph. multiflorus, W., with tall winding
stem, purple-red flowers in very long racemes, rough, sickle-shaped
crooked pods, and large seeds which bore black flecks and splashes
on a peach-blood-red ground.

The hybrids had the greatest similarity to the pollen parent, but
the flowers appeared less intensely coloured. Their fertility was
very limited; from seventeen plants, which together developed
many hundreds of flowers, only forty-nine seeds in all were obtained.
These were of medium size, and were flecked and splashed similarly

APPENDIX 341

to those of Ph. multiflorusy while the ground colour was not niJi-
terially different. The next year forty-four plants were raised
from these seeds, of which only thirty-one readied the flowering
stage. The characters of Ph. nanuSy which had been altogetluT
latent in the hybrids, reappeared in various combinations; their
ratio, however, with relation to the dominant plants was neces-
sarily very fluctuating owing to the small number of trial jjlants.
With certain characters, as in those of the axis and the form of pod
it was, however, as in the case of Pisum, almost exactly 1 :3.

Insignificant as the results of this experiment may be as regards
the determination of the relative numbers in which the various
forms appeared, it presents, on the other hand, the phenomenon of
a remarkable change of colour in the flowers and seed of the hybrids.
In Pisum it is known that the characters of the flower- and seed-
colour present themselves unchanged in the first and further
generations, and that the offspring of the hybrids display exclu-
sively the one or the other of the characters of the original stocks.
It is otherwise in the experiment we are considering. The white
flowers and the seed-colour of Ph. nanus appeared, it is true, at
once in the first generation [froTn the hybrids] in one fairly fertile
example, but the remaining thirty plants developed flower-colours
which were of various grades of purple-red to pale violet. The
colouring of the seed-coat was no less varied than that of the
flowers. No plant could rank as fully fertile; many produced no
fruit at all; others only yielded fruits from the flowers last pro-
duced, which did not ripen. From fifteen plants only were well-
developed seeds obtained. The greatest disposition to infertility
was seen in the forms with preponderantly red flowers, since out of
sixteen of these only four yielded ripe seed. Three of these had a
similar seed pattern to Ph. multiflorus, but with a more or less pale
ground colour; the fourth plant yielded only one seed of plain
brown tint. The forms with preponderantly violet-coloured
flowers had dark brown, black-brown, and quite black seeds.

The experiment was continued through two more generations
under similar unfavorable circumstances, since even among the
offspring of fairly fertile plants there came again some which were
less fertile or even quite sterile. Other flower- and seed-colours
than those cited did not subsequently present themselves. The
forms which in the first generation [bred from tlie hybrids] con-
tained one or more of the recessive characters remained, as regards

342 APPENDIX

these, constant without exception. Also of those plants which
possessed violet flowers and brown or black seed, some did not vary
again in these respects in the next generation; the majority, how-
ever, yielded, together with offspring exactly like themselves, some
which displayed white flowers and white seed-coats. The red
flowering plants remained so slightly fertile that nothing can be
said with certainty as regards their further development.

Despite the many disturbing factors with which the observations
had to contend, it is nevertheless seen by this experiment that the
development of the hybrids, with regard to those characters which
concern the form of the plants, follows the same laws as in Pisum.
With regard to the colour characters, it certainly appears difficult
to perceive a substantial agreement. Apart from the fact that from
the union of a white and a purple-red colouring a whole series of
colours results [in F2], from purple to pale violet and white, the
circumstance is a striking one that among thirty-one flowering
plants only one received the recessive character of the white colour,
while in Pisum this occurs on the average in every fourth plant.

Even these enigmatical results, however, might probably be
explained by the law governing Pisum if we might assume that the
colour of the flowers and seeds of Ph. multiflorus is a combination of
two or more entirely independent colours, which individually act
like any other constant character in the plant. If the flower-
colour A were a combination of the individual characters Ai
-|- ^^2 + • • • which produce the total impression of a purple colora-
tion, then by fertilisation with the differentiating character,
white colour, a, there would be produced the hybrid unions Aia -\-
A^a + • • . and so would it be with the corresponding colouring of
the seed-coats.^ According to the above assumption, each of these
hybrid colour unions would be independent, and would conse-
quently develop quite independently from the others. It is then
easily seen that from the combination of the separate develop-
mental series a complete colour-series must result. If, for instance,
A = ^1+ Ai, then the hybrids Aia and A^a form the develop-
mental series —

Ai + '^Aia + a, ^2 + ^A^a + a.

1 [As it fails to take account of factors introduced by the albino this represen-
tation is imperfect. It is however interesting to know that Mendel realized the fact
of the existence of compound characters, and that the rarity of the white recessives
was a consequence of this resolution.]

APPENDIX 343

The members of this series can enter into nine different combina-
tions, and each of these denotes another colour —

1 A,At

2 AiaA2

1 yl2«

2 AiA^a

4 AiaA^a

2 A2aa

1 Aia

2 Aiaa

1 aa.

The figures prescribed for the separate combinations also indicate
how many plants with the corresponding colouring ijelong to the
series. Since the total is sixteen, the whole of the colours are on the
average distributed over each sixteen plants, but, as the series itself
indicates, in unequal proportions.

Should the colour development really happen in this way, we
could offer an explanation of the case above described, viz. that the
white flowers and seed-coat colour only appeared once among
thirty-one plants of the first generation. This colouring aj)pears
only once in the series, and could therefore also only be developed
once in the average in each sixteen, and with three colour char-
acters only once even in sixty -four plants.

It must, nevertheless, not be forgotten that the explanation here
attempted is based on a mere hypothesis, only supported by the
very imperfect result of the experiment just described. It would,
however, be well worth while to follow up the development of
colour in hybrids by similar experiments, since it is probable that
in this way we might learn the significance of the extraordinary
variety in the colouring of our ornamental flowers.

So far, little at present is known with certainty beyond the fact
that the colour of the flowers in most ornamental plants is an
extremely variable character. The opinion has often been ex-
pressed that the stability of the species is greatly disturbed or
entirely upset by cultivation, and consequently there is an inclina-
tion to regard the development of cultivated forms as a matter of
chance devoid of rules; the colouring of ornamental plants is indeed
usually cited as an example of great instability. It is, however, not
clear why the simple transference into garden soil should result in
such a thorough and persistent revolution in the plant organism.
No one will seriously maintain that in the open country the
development of plants is ruled by other laws than in the garden
bed. Here, as there, changes of type must take place if the condi-
tions of life be altered, and the species possesses the cai)acit>' of
fitting itself to its new environment. It is willingly granted that

344 APPENDIX

by cultivation the origination of new varieties is favoured, and that
by man's labour many varieties are acquired which, under natural
conditions, would be lost; but nothing justifies the assumption
that the tendency to the formation of varieties is so extraordinarily
increased that the species speedily lose all stability, and their
offspring diverge into an endless series of extremely variable forms.
Were the change in the conditions the sole cause of variability we
might expect that those cultivated plants which are grown for
centuries under almost identical conditions would again attain
constancy. That, as is well known, is not the case, since it is pre-
cisely under such circumstances that not only the most varied but
also the most variable forms are found. It is only the Leguminosaey
like Pisum, Phaseolus,^ Lens, whose organs of fertilisation are pro-
tected by the keel, which constitute a noteworthy exception. Even
here there have arisen numerous varieties during a cultural period
of more than 1000 years under most various conditions; these
maintain, however, under unchanging environments a stability as
great as that of species growing wild.

It is more than probable that as regards the variability of culti-
vated plants there exists a factor which so far has received little
attention. Various experiments force us to the conclusion that our
cultivated plants, with few exceptions, are members of various
hybrid series, whose further development in conformity with law is
varied and interrupted by frequent crossings inter se. The circum-
stance must not be overlooked that cultivated plants are mostly
grown in great numbers and close together, affording the most
favourable conditions for reciprocal fertilisation between the varie-
ties present and the species itself. The probability of this is sup-
ported by the fact that among the great array of variable forms
solitary examples are always found, which in one character or
another remain constant, if only foreign influence be carefully
excluded. These forms behave precisely as do those which are
known to be members of the compound hybrid series. Also with
the most susceptible of all characters, that of colour, it cannot
escape the careful observer that in the separate forms the inclina-
tion to vary is displayed in very different degrees. Among plants
which arise from one spontaneous fertilisation there are often some
whose offspring vary widely in the constitution and arrangement
of the colours, while that of others shows little deviation, and

^ [Phaseolus nevertheless is insect-fertilised.]

APPENDIX 345

among a greater number solitary examples occur which transmit
the colour of the flowers unchanged to their offspring. The culti-
vated species of Dianthus afford an instructive exami)l(' of this.
A white-flowered example of Dianthus caryophyllii.s, which itself
was derived from a white-flowered variety, was shut up during its
blooming period in a greenhouse; the numerous seeds obtained
therefrom yielded plants entirely white-flowered like itself. A
similar result was obtained from a sub-species, with red flowers
somewhat flushed w4th violet, and one w4th flowers white, striped
with red. Many others, on the other hand, which were similarly
protected, yielded progeny which were more or less variously
coloured and marked.

Whoever studies the coloration which results. In ornamental
plants, from similar fertilisation, can hardly escape the conviction
that here also the development follows a definite law, which
possibly finds its expression in the combination of several inde-
pendent colour characters.

Concluding Remarks

It can hardly fail to be of interest to compare the observations
made regarding Pisum with the results arrived at by the two
authorities in this branch of knowledge, Kolreuter and Gartner,
in their investigations. According to the opinion of both, the
hybrids in outward appearance present either a form intermediate
betw^een the original species, or they closely resemble either the
one or the other type, and sometimes can hardly be discriminated
from it. From their seeds usually arise, if the fertilisation was
effected by their own pollen, various forms which differ from the
normal type. As a rule, the majority of individuals obtained In-
one fertilisation maintain the hybrid form, while some few others
come more like the seed parent, and one or other individual
approaches the pollen parent. This, however, is not the case with
all hybrids without exception. Sometimes the offsi)ring have
more nearly approached, some the one and some the other of the
two original stocks, or they all incline more to one or the other
side; while in other cases they remain perfectly like the hybrid and
continue constant in their offspring. The hybrids of varieties
behave like hybrids of species, but they possess greater variability
of form and a more pronounced tendency to revert to the original
types.

346 APPENDIX

With regard to the form of the hybrids and their development,
as a rule an agreement with the observations made in Pisum is
unmistakable. It is otherwise with the exceptional cases cited.
Gartner confesses even that the exact determination whether a
form bears a greater resemblance to one or to the other of the two
original species often involved great difficulty, so much depending
upon the subjective point of view of the observer. Another cir-
cumstance could, however, contribute to render the results fluctu-
ating and uncertain, despite the most careful observation and
differentiation. For the experiments, plants were mostly used
which rank as good species and are differentiated by a large number
of characters. In addition to the sharply defined characters, where
it is a question of greater or less similarity, those characters must
also be taken into account which are often difficult to define in
w^ords, but yet suffice, as every plant specialist knows, to give the
forms a peculiar appearance. If it be accepted that the develop-
ment of hybrids follows the law which is valid for Pisum, the series
in each separate experiment must contain very many forms, since
the number of the terms, as is known, increases, w^ith the number
of the differentiating characters, as the powers of three. With a
relatively small number of experimental plants the result therefore
could only be approximately right, and in single cases might
fluctuate considerably. If, for instance, the two original stocks
differ in seven characters, and 100 or 200 plants were raised from
the seeds of their hybrids to determine the grade of relationship of
the offspring, we can easily see how uncertain the decision must
become, since for seven differentiating characters the combination
series contain 16,384 individuals under 2187 various forms; now
one and then another relationship could assert its predominance,
just according as chance presented this or that form to the observer
in a majority of cases.

If, furthermore, there appear among the differentiating char-
acters at the same time dominant characters, which are transmitted
entire or nearly unchanged to the hybrids, then in the terms of the
developmental series that one of the two original parents which
possesses the majority of dominant characters must always be pre-
dominant. In the experiment described relative to Pisum, in
which three kinds of differentiating characters were concerned, all
the dominant characters belonged to the seed parent. Although
the terms of the series in their internal composition approach both

APPENDIX 347

original parents equally, yet in this experiment the type of the seed
parent obtained so great a preponderance that out of each sixty-
four plants of the first generation fifty-four exactly reseniljled it, or
only differed in one character. It is seen how rash it must be under
such circumstances to draw from the external reseml)lances of
hybrids conclusions as to their internal nature.

Gartner mentions that in those cases w^here the develoi)incnt was
regular, among the offspring of the hybrids, the two original si)ecies
w^ere not reproduced, but only a few individuals which approaclied
them. With very extended developmental series it could not in
fact be otherwise. For seven differentiating characters, for
instance, among more than 16,000 individuals — offspring of the
hybrids — each of the two original species w^ould occur only once.
It is therefore hardty possible that these should appear at all among
a small number of experimental plants; with some probability,
however, we might reckon upon the appearance in the series of a
few forms which approach them.

We meet with a.n essential difference in those hybrids which re-
main constant in their progeny and propagate themselves as truly as
the pure species. According to Gartner, to this class belong the
remarkably fertile hybrids^ Aquilegia atropurpurea canadensis,
Lavatera pseudolbia thuringiaca, Geum urbano-rivale, and some
Dianthus hybrids; and, according to Wichura, the hybrids of the
Willow family. For the history of the evolution of plants this cir-
cumstance is of special importance, since constant hybrids acquire
the status of new species. The correctness of the facts is guaran-
teed by eminent observers, and cannot be doubted. Gartner had
an opportunity of following up Dianthus Armeria deltoides to the
tenth generation, since it regularly propagated itself in the garden.

With Pisum it was shown by experiment that the hybrids form
egg and pollen cells of different kinds, and that herein lies the reason
of the variability of their offspring. In other hybritls, likewise,
whose offspring behave similarly we may assume a like cause; for
those, on the other hand, which remain constant, the a-ssum})tion
appears justifiable that their reproductive cells are all alike and
agree with the foundation-cell [fertilised ovum] of the hybrid, lii
the opinion of renowned physiologists, for the purpose of proi)aga-
tion one pollen cell and one Qgg cell unite in Phanerogams ^ into a

1 In Pisum it is placed beyond doubt that for the formation of the new embryo a
perfect union of the elements of both reproductive cells must take place. How

348

APPENDIX

single cell, which is capable by assimilation and formation of new
cells to become an independent organism. This development fol-
lows a constant law, which is founded on the material composition
and arrangement of the elements which meet in the cell in a vivify-
ing miion. If the reproductive cells be of the same kind and agree
with the foundation cell [fertilised ovum] of the mother plant, then
the development of the new individual will follow the same law
which rules the mother plant. If it chance that an egg cell unites
with a dissimilar pollen cell, we must then assume that between
those elements of both cells, which determine opposite characters,
some sort of compromise is effected. The resulting compound cell
becomes the foundation of the hybrid organism, the development
of which necessarily follows a different scheme from that obtaining
in each of the two original species. If the compromise be taken to
be a complete one, in the sense, namely, that the hybrid embryo
is formed from two similar cells, in which the differences are
entirely and permanently accommodated together, the further result
follows that the hybrids, like any other stable plant species, repro-
duce themselves truly in their offspring. The reproductive cells
which are formed in their seed vessels and anthers are of one kind,
and agree with the fundamental compound cell [fertilised ovum].

With regard to those hybrids whose progeny is variable we may
perhaps assume that between the differentiating elements of the
egg and pollen cells there also occurs a compromise, in so far that
the formation of a cell as foundation of the hybrid becomes possible;
but, nevertheless, the arrangement between the conflicting ele-
ments is only temporary and does not endure throughout the life of
the hybrid plant. Since, in the habit of the plant, no changes are
perceptible during the whole period of vegetation, we must further
assume that it is only possible for the differentiating elements to
liberate themselves from the enforced union when the fertilising
cells are developed. In the formation of these cells all existing

could we otherwise explain that among the offspring of the hybrids both original
types reappear in equal numbers and with all their peculiarities ? If the influence
of the egg cell upon the pollen cell were only external, if it fulfilled the role of a nurse
only, then the result of each artificial fertilisation could be no other than that the
developed hybrid should exactly resemble the pollen parent, or at any rate do so
very closely. This the experiments so far have in no wise confirmed. An evident
proof of the complete union of the contents of both cells is afforded by the experi-
ence gained on all sides that it is immaterial, as regards the form of the hybrid,
which of the original species is the seed parent or which the pollen parent,

APPENDIX 349

elements participate, in an entirely free and equal arrangement, by
which it is only the diflPerentiating ones which nmtiially sei)arate
themselves. In this way the production would be rendered i)o.ssible
of as many sorts of egg and pollen cells as there are combinations
possible of the formative elements.

The attribution attempted here of the essential difference in the
development of hybrids to a permanent or temporanj union of the
differing cell elements can, of course, only claim the vahie of an
hypothesis for which the lack of definite data offers a wide scope.
Some justification of the opinion expressed lies in the evidence
afforded by Pisum that the behaviour of each pair of difl'ercntiating
characters in hybrid union is independent of the other difi"erences
between the two original plants, and, further, that the hybrid i)ro-
duces just so many kinds of egg and pollen cells as there are possible
constant combination forms. The differentiating characters of
two plants can finally, however, only depend upon differences in
the composition and grouping of the elements which exist in
the foundation-cells [fertilised ova] of the same in vital inter-
action.^

Even the validity of the law formulated for Pisum requires still
to be confirmed, and a repetition of the more important experiments
is consequently much to be desired, that, for instance, relating to
the composition of the hybrid fertilising cells. A differential [ele-
ment] may easily escape the single observer,^ which although at the
outset may appear to be unimportant, may yet accumulate to such
an extent that it must not be ignored in the total result. Whether
the variable hybrids of other plant species observe an entire agree-
ment must also be first decided experimentally. In the meantime
we may assume that in material points an essential difference can
scarcely occur, since the unity in the developmental plan of organic
life is beyond question.

In conclusion, the experiments carried out by Kolreuter, Gartner,
and others with respect to the transformation of one species into
another by artificial fertilisation merit special mention. Particular
importance has been attached to these ex])erimcnts and (liirtner
reckons them among '* the most diflScult of all in hyl)ridisation."

If a species A is to be transformed into a species B, both must be
united by fertilisation and the resulting hybrids then be fertilised

1 " Welche in den Grundzellen derselben in lehendiger Wechselwirkung stchen."

2 *' Dem einzelnen Beobachter kann leicht ein Dijjcrenziale entgchcn."

350 APPENDIX

with the pollen of B; then, out of the various offspring resulting,
that form would be selected which stood in nearest relation to B
and once more be fertilised with B pollen, and so continuously until
finallj^ a form is arrived at which is like B and constant in its prog-
eny. By this process the species A would change into the species
B. Gartner alone has effected thirty such experiments with plants
of genera Aquilegiay Dianthus, Geum, Lavatera^ Lychnis, Malva,
Nicotiana, and Oenothera. The period of transformation was not
alike for all species. While with some a triple fertilisation sufficed,
with others this had to be repeated five or six times, and even in the
same species fluctuations were observed in various experiments.
Gartner ascribes this difference to the circumstance that " the
specific [typische] power by which a species, during reproduction,
effects the change and transformation of the maternal type varies
considerably in different plants, and that, consequently, the periods
within which the one species is changed into the other must also
vary, as also the number of generations, so that the transformation
in some species is perfected in more, and in others in fewer genera-
tions." Further, the same observer remarks '' that in these trans-
formation experiments a good deal depends upon which type and
which individual be chosen for further transformation."

If it may be assumed that in these experiments the constitution
of the forms resulted in a similar way to that of Pisnm, the entire
process of transformation would find a fairly simple explanation.
The hybrid forms as many kinds of egg cells as there are constant
i' combinations possible of the characters conjoined therein, and one
of these is always of the same kind as that of the fertilising pollen
cells. Consequently there always exists the possibility with all
such experiments that even from the second fertilisation there may
result a constant form identical with that of the pollen parent.
Whether this really be obtained depends in each separate case upon
the number of the experimental plants, as well as upon the number
of differentiating characters which are united by the fertilisation.
Let us, for instance, assume that the plants selected for experiment
differed in three characters, and the species ABC is to be trans-
formed into the other species abc by repeated fertilisation with the
pollen of the latter ; the hybrids resulting from the first cross form
eight different kinds of egg cells, viz.,

ABC, ABc, AbC, aBC, Abc, aBc, abC, abc.

/,

APPENDIX 351

These in the second year of experiment are united again w illi the
pollen cells abcy and we obtain the series

AaBbCc + AaBbc + AabCc + aBbCc + Aabc + ciBbc -f abCc + ahr.

Since the form abc occurs once in the series of eight terms, it is
consequently little likely that it would be missing among the experi-
mental plants, even were these raised in a smaller number, and the
transformation would be perfected already by a second fertiHsa-
tion. If by chance it did not appear, then the fertilisation nuist })e
repeated with one of those forms nearest akin, Aabc, ciBhr, abCc.
It is perceived that such an experiment must extend the farther the
smaller the number of experimental plants and the larger the number
of differentiating characters in the two original species; and that,
furthermore, in the same species there can easily occur a delay of
one or even of two generations such as Gartner observed. The
transformation of widely divergent species could generally only l)e
completed in five or six years of experiment, since the numl)er of
different egg cells which are formed in the hybrid increases, as the
powers of two, with the number of differentiating characters.

Gartner found by repeated experiments that the respective
period of transformation varies in many species, so that frec^uently
a species A can be transformed into a species B a generation sooner
than can species B into species A. He deduces therefrom that
Kolreuter's opinion can hardly be maintained that " the two
natures in hybrids are perfectly in equilibrium." It appears, how-
ever, that Kolreuter does not merit this criticism, but that Giirtner
rather has overlooked a material point, to which he himself else-
where draws attention, viz. that '* it depends which individual is
chosen for further transformation." Experiments which in this
connection were carried out with two species of Pisum demon-
strated that as regards the choice of the fittest indiAiduals for the
purpose of further fertilisation it may make a great difi'erence which
of two species is transformed into the other. The two experi-
mental plants differed in five characters, while at the same time
those of species A were all dominant and those of species B all
recessive. For mutual transformation .1 was fertilised with pollen
of B, and B with pollen of A, and this was repeated with both

hybrids the following year. With the first experiment - there were
eighty-seven plants available in the third year of experiment for

352 APPENDIX

selection of the individuals for further crossing, and these were of

A

the possible thirty-two forms; with the second experiment —

JB

seventy -three plants resulted, which agreed throughout perfectly in
habit with the pollen parent; in their internal composition, however,
they must have been just as varied as the forms in the other experi-
ment. A definite selection was consequently only possible with the
first experiment; with the second the selection had to be made at
random, merely. Of the latter only a portion of the flowers were
crossed with the ^ pollen, the others were left to fertilise themselves.
Among each five plants which were selected in both experiments
for fertilisation there agreed, as the following year's culture showed,
with the pollen parent :

1st Experiment

2nd Experiment

2 plants

in all characters

3 "

"4 "

2 plants

" 3 "

2 "

u a u

1 plant

" 1 character

In the first experiment, therefore, the transformation was com-
pleted; in the second, which was not continued further, two or
more fertilisations would probably have been required.

Although the case may not frequently occur in which the dom-
inant characters belong exclusively to one or the other of the
// original parent plants, it will always make a difference which of the
two possesses the majority of dominants. If the pollen parent has
the majority, then the selection of forms for further crossing will
afford a less degree of certainty than in the reverse case, which
must imply a delay in the period of transformation, provided that
the experiment is only considered as completed when a form is
arrived at which not only exactly resembles the pollen plant in
form, but also remains as constant in its progeny.

Gartner, by the results of these transformation experiments, was
led to oppose the opinion of those naturalists who dispute the
stability of plant species and believe in a continuous evolution of
vegetation. He perceives ^ in the complete transformation of one
species into another an indubitable proof that species are fixed
within limits beyond which they cannot change. Although this

^ [" Es sieht " in the original is clearly a misprint for " Er sieht."]

APPENDIX '35S

opinion cannot be unconditionally accepted, we find on the other
hand in Gartner's experiments a noteworthy confirmation of that
supposition regarding variability of cultivated plants which has
already been expressed.

Among the experimental species there were cultivated plants,
such as Aquilegia atropurpurea and canadensis, Dianthus caryophyl-
lus, chinensisy and japonicus, Nicotiana rustica and paniciilaia, and
hybrids between these species lost none of their stability after four
or five generations.

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Woodruff, L. L., and R. Erdmanx, 1914. X normal p(!ri<>dic rcor^^aniza-
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17.
Woods, E. A., 1906. Mental and moral heredity in royalty. New York.
Wright, S., 1914. Duplicate genes. Am. Nat., 48.

1915. The albino series of allelomorphs in guinea-pigs. Ibid., 40.
1917. The probable error of Mendelian class frequencies. Am. Nat.»
51.

1917, 1918. Color inheritance in mammals. Jour. Her., 8, 9.

1918. On the nature of size factors. Genetics. 3.

Yerkes, R. M., 1913. The heredity of savageness and wildness in rats.

Jour. An. Behavior, 3.
Yule, G. U., 1912. An introduction to the study of statistics. London

and New York.
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INDEX

Acclimatization, 31.

Acquired characters, gO, 22, 28, 45.

Agouti, 115. 124, 188.

Albino, 24, 88, 122a.

Alcohol, effects of, on germ-cells, 45.

Allelomorph, 101 .

Allelomorphs, multiple, 186.

Allen, 139.

Andalusian fowl, 109.

Angora, 91, 127.

Ants, 53.

Apotettix, 176.

Apple graft-hybrid, 23.

Arcella, 210.

Ascaris, 51.

Atavism, 113.

Average deviation, 64

Barnacle, light reactions of, 43.

Barrows, 140.

Basset hounds, 246.

Bateson, 55, 92, 109, 120, 136, 156, 173.

Baur, 23, 113, 211.

Beagle J voyage of , 9, 14.

Beans, selection for size in, 215.

Bees, 53.

Bert, 32.

Binet test, 286.

Biometry, 55, 57, 62.

Birthrate, differential, 295.

Bison, 133, 236.

Black coat, 124.

Blending inheritance, 192, 246.

Bond, 275.

Bonhote, 124.

Bos, 230.

Bounty, mutineers of the, 207.

Bo veri, 51,249.

Bridges, 219, 249.

British aristocracy and eugenics, 297.

Brown coat, 124.

Brown-Sequard, 30.

Buffon, 18.
Buttercup, 63, 73.
Butterfly, 52.

Castration, effects of, 2.')1.
Cats, short-tailed, 2H, 143

unit-characters of, 142.
Cattell, 29.), 298,
Cattle, polled, 91, 133.

short-horn, 110.

unit-characters of, 130.

white, of English parks, 131,

wild, 130.
Cave animals, 40.
Cavia Cutleri, 243.
Cavia rufescens, 256.
Checkerboard method, 105, 116.
Cheledonium, 152.
Child, 52.
Chimera, 23.
Chromosomes, 49, 51.

changes in, 205.
Chromosome map, 168.
Ciona, 29, 229.
Coefficient of correlation, 65.
of inbreeding, 238.
of variation, 65.
Coleus, 149, 211.
Collins, 262.
Colorblindness, 162.
inheritance, 88.
Colors of flowers, 148.

of fruits, 151.
Columba, 114.
Congenital disease, 29.
Conn, 11.
Correlation, 65.
Correns, 82.
Crepidula, 253.
Crossing-over, 168.
Cuenot, 111.
Curve of error, 60, 62*, 72.

391

392

INDEX

Daphnia, 32, 213, 255.
Darwin, Charles, 7, 48, 55, 83, 113, 228.
Darwin, Erasmus, 9.
Davenport, 271, 275, 277, 284.
Davis, 80.
Delage, 34.

Determiners, 48, 49, 50, 53, 54.
Detlefsen, 179.
Diffliigia, 209.
Dihybrid ratio, 107, 116.
Dilute pigmentation, 127.
Disease, inheritance of, 29.
Dogs, unit-characters of, 138.
Domestication, changes under, 14.
Dominant, 88.
Drinkwater, 190.

Drosophila, 156, 176, 180, 188, 219, 224,
230, 249.

eye colors of, 219.
Ducks, size inheritance in, 197.

East, 193, 199, 210, 242, 244.

Egg-cell, 98.

Egypt, royal family of, 227.

Elementary species, 74.

Embryology, 12.

Emerson, 199, 211, 224.

Environment, direct effect of, 20.

Epilepsy, 30, 284.

Eugenics defined, 3.

Eugenics Laboratory, 271.

Eugenics Record Office, 271.

Evening primrose, 49, 75, 220.

Evolution defined, 4, 7.

history of, repeated in devel-
opment, 13.

Extension factor, 188.

Factor, modifying, 189.

multiple, 192.
Farabee, 190.
Feeble-mindedness, 285.
Ferroniere, 32.
Filial generation, 100.
Fischer, 34, 269.
Fixation of new varieties, 95.
Flower colors, 148.

forms, 150.

Fluctuations, 71, 78.
Free martin, 252.
Fruit colors, 151.

Galapagos islands, 14.

Gall insects, 52.

Gallus bankiva, 145.

Galton, 3, 9, 26, 56, 71, 246,, 295.

Galton's law of ancestral heredity, 239, 246.

Gamete, 98.

Gates, 50.

Gene, 99.

Genes, linear arrangement of, 168.
nature of, 177.

Genetic changes, 205.

in bisexual reproduc-
tion, 219.

in parthenogenesis,
212.
in self-fertilization, 215.

Genetics defined, 3.

Genotype, 102.

Geographical distribution, 13.

Geological succession, 13.

Germ-cells, 23, 47.

Germinal selection, 54.

Gipsy moth, 33.

Goddard, 285.

Goodale, 251.

Goss, 86.

Gould, 253.

Gradation of organisms, 12.

Graft hybrid, 23.

Greek philosophers, 8.

Gross, 202.

Guinea-pig, 24, 30, 88, 91, 114, 122a, 187.

Gynandromorph, 249.

Haemophilia, 162.

Hair form, inheritance of, 275.

Haldane, 170, 173.

Harrison, 23.

Hegner, 51, 209.

Height of Harvard students, 57, 61.

Heron, 281, 283.

Hermaphroditism, 261.

Hertwig, 45.

Heterosis, 242.

INDEX

393

Heterozygote, 98.
Heterozygous, 90.
Homozygote, 98.
Homozygous, 90.
Hormones, 252.
Horse, ancestor of, 13.

Clydesdale, 136.

Prevalski's, 134.

Shire, 136.

unit-characters of, 134.
Hoshino, 200.

Human crosses, English-Polynesian, 268.
Boer-Hottentot, 269.
Jew-Anglo Saxon, 274.
Negro-white, 271.
Huxley, 12.
Hybrid, 79, 83.
Hydatina, 254.

Illiteracy, 290.

Immigration, 290.

Inbreeding, 222, 223, 224, 227, 230.

Insanity, 280.

Instinct, 41.

Intense pigmentation, 127.

Jennings, 209, 240.
Johannsen, 94, 208, 247.
Jones, 244.
Jungle fowl, 145.

Kammerer, 29, 37.

Kellogg, 33.

King, 129, 222, 231, 238, 256.

Knight, 86.

Kolreuter, 83.

Lamarck, 17, 18, 47.
Leaves, forms of, 152.
Light effects, 38.
Lillie, 252.
Linkage, 120, 167, 180.

chromosome theory of, 168.

measurement of, 172.
Little, 111, 125, 139, 179.
Lizard, 37.
Lock, 12, 83.
Loeb, 43, 256.

Lychnis, 262.
Lyell, 8.

MacDowell, 224.

Macroganiele, 253.

Maize, 151, 174, ]f)l, 211, 223, 228.

Malthus, 15.

Marriage of noar-of-kin, 227.

McClung, 200.

Mean, 64.

Memlcl, 82, 190, 248.

Mendel's law, 55, 82, 88.

Mendelian expectations, 104.

ratios, nioflified, 109, 110, 119.
Mental ability, inheritance of, 279.
Miastor, 51.
Microgamete, 254.
Microtus, 124.
Miller, 139.
Mirabilis, 86, 110,206.
Mode, 59.
Mongrel, 83.

Morgan, 52, 120, 249, 251.
Morphology, 13.
Moth, brown-tail, 43.
Mulatto, 277.
Mule- footed swine, 138.
Multiple allelomorphs, 187, 219.
Multiple factor hypothesis, 192.
Mus alexandrinus, 125, .

rattus, 124.
Mutants of Oenothera, 76*
Mutation, 14, 71, 182, 185, 189.

varieties of, 208.
Mutilations, 28.

Nabours, 176.

Natural selection, 11, 17, 56.

Naudin 87.

Negro, white-spotted, 277.

Nicotiana, 85, 229, 236, 242.

Nilsson-Ehle, 193,

Oenothera, 49, 75, 221.
Origin of Specieti, 11, 17.
Osborn,8, 19,266.

Ovarian transi)hintalion in fowls, 251.
in guinea-pigs, 24.

394

INDEX

Ovarian transplantation in rats, 251.
Overproduction, 11.
Oxford "class lists," 279.

Pangenesis, 24, 26, 48.
Parallel induction, 35, 54.
Paramecium, selection for size in, 209.
Parental generation, 100.
Parthenogenesis, 212.

in distant crosses, 236.

in frog, 256.

in honey-bee, 257.

in orange, 257.

in sea-urchin, 237.

male, in strawberry,

237.

in teosinte, 237.
Pasteur, 29.
Payne, 225.
Pearl, 238.
Pearson,57, 68, 279.
Pearson, 57, 68, 279.
Peas, dwarf and tall, 152.

flowering time of, 200.
linkage in, 174.
Pebriue, 29.
Pelargonium, 211.
Peppers, size inheritance in, 202.
Peromysciis, 123, 125.
Phenotype, 94, 102.
Phillips, 24, 125, 140, 179, 197.
Pictet, 33.

Pink-eyed rodents, 126.
Pitcairn Island, 2G8.
Plants, unit-characters of, 149.
Plato's Eepublic, 292.
Plums, self-sterility in, 229.
Polymorphism, 52.

Polynesians and venereal disease, 305.
Potato beetle, 35.
Poultry, unit-characters of, 145.
Pressure effects, 38.
Prevalski's horse, 134.
Prunula, 150, 173.
Probable error, 68.
Proteus, 40.
Prune, new variety of, 210.

Punnett, 104, 109, 120, 125, 145, 173,

198.
Pure line hypothesis, 215.

Rabbit, 26, 67, 191.

Dutch-marked, 125, 224.

ear-length of, 194.

Enghsh, 125, 183.

gray coat of, 205.

Himalayan, 138.

size inheritance in, 191.
Race mixture and venereal disease, 306.
Rat, 117, 176, 222, 230.

hooded, 184, 223.
Regeneration, 51.
Regression, 247.
Reversion, 113.
Riddle, 255.
Roan cattle, 110.
Rock pigeon, 114.
Rodents, unit-characters of, 122.
Romanes, 31.
Rosanoff and Orr, 281.
Rough coat of guinea-pigs, 128.
Ruby eye of rats, 129.

Saint-Hilaire, 18.

Salaman, 274.

Salamander, 37.

Salinity, altered, 32.

School rank and success in later life,

279.
Sebright bantam, 251.
Segregation, 91.
Selection, 71, 184, 189, 222, 223.

in Aphis avenae, 214.

in beans, 215.

in Daphnia, 213.

in hooded rats, 184.

in Paramecium, 209.

in parthenogenesis, 213.

in pure lines, 215.

in Simocephalus, 214.
Self-fertilization, 75, 223, 228.
Self-pollination, 228.
Self-sterility, 229.
Semon, 29, 41.
Sex-chromosome, 157.

I

I

INDEX

31),

Sex-determination of bee, 257.

of Crustacea and roti-
fers, 258.
of squash-bug, 259.

Sex-linked inheritance, 159.

Drosophila t^TJe,

1G4.

poultry type, 165

Sex-intergradcs, 213.

Sex-mosaic, 24'9. ,

Sex ratio in rats, 222.

Shamel, 210.

Sheep, docking of, 28.

unit-characters of, 138.

Shepherd's-purse, 193.

Shull, 193, 228, 262.

Silkworm, unit-characters of, 154.
linkage in, 176.

Simocephalus, 213.

Skin-color, inheritance of, 277.

Snapdragon, 113, 174.

Soma, 23.

Spartan eugenics, 292.

Spencer, Herbert, 25.

Sperm, 98.

Standard deviation, 64.

Steinach, 251.

Sterility in crosses, 233.

Stockard, 45.

Stout, 211.

Struggle for existence, 11.

Sugar beet, 71.

Sumner, 37.

Survival of the fittest, 11.

Sweet peas, 118, 120, 149, 173, 218.

Swine, unit-characters of, 137.

Syphilis, 30.

Temperature effects, 34.
Texas fever, 29, 207.
Tomato, linkage in, 174.
Tower, 35.
Toyama, 154.
Transfusion of blood, 26.
Transplantation of ovaries, 24, 28, 251.
Treasury of Human Inheritance, 271

Tri-colorcoat, 12G.
Tri-hybrid ratio, 107, 119.
Tsrhcrni.'ik, 82.
Tuberculosis, 30.
Tubifex, 32.

Unit-character, 91, 99, 182.
Unit-factor, 99.
Use and disuse, 20.

Variation, 11, 19, 21, 53, 55, 71.
Variation of Animals and Plants under

Domestication, 14, 17, 19, 24, 56.
Variation, coefficient of, 05.

continuous or. fliscontinuous,
56.
Variation curve, 59.
Vasectomy, 291.
Vries, H. de, 56, 71, 82.

Wallace, 10.

Wedgewood, 9.

Weight of Harvard students, 57, 61, 63.

Weismann, 21, 29, 34, 41, 47, 231.

Wentworth, 135.

Wheat, crosses of, 192.

Whetham, 297.

White spotting, 125, 188.

Whiting, 129.

Whitney, 254.

Wilson, 259.

Winkler, 23.

Wislar Institute, 231.

Wolves, coat color of, 140.

Wright, 128, 177.

X-chromosome, 157, 159, 260.

Y-chromosome, 260.
Yellow coat, 124.
Yellow mice, 111.
Yellow spotting, 126.
Yule, 63.

Zeleny, 225.
Zygote, 98.

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