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Monographs on Experimental Biology

EDITED BY

JACQUES LOEB, Rockefeller Institute

T. H. MORGAN, Columbia University

W. J. V. OSTERHOUT, Harvard University

THE PHYSICAL BASIS OF HEREDITY

BY

THOMAS HUNT MORGAN

MONOGRAPHS ON EXPERIMENTAL

BIOLOGY

PUBLISHED

FORCED MOVEMENTS, TROPISMS, AND ANIMAL

CONDUCT

By JACQUES LOEB. Rockefeller Institute

THE ELEMENTARY NERVOUS SYSTEM

By G. H. PARKER. Harvard University

THE PHYSICAL BASIS OF HEREDITY

By T. H. MORGAN. Columbia University

INBREEDING AND OUTBREEDING: THEIR GENETIC
AND SOCIOLOGICAL SIGNIFICANCE

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

PURE LINE INHERITANCE

By H. S. JENNINGS. Johns Hopkins University

THE EXPERLvIENTAL MODIFICATION OF THE
PROCESS OF INHERITANCE

By R. PEARL. Johns Hopkins University

LOCALIZATION OF MORPHOGENETIC SUBSTANCES

IN THE EGG

By E. G. CONKLIN. Princeton University

TISSUE CULTURE

By R. G. HARRISON. Yale University

PERMEABILITY AND ELECTRICAL CONDUCTIVITY

OF LIVING TISSUE

By W. J. V. OSTERHOUT, Harvard University

THE EQUILIBRIUM BETWEEN ACIDS AND BASES IN
ORGANISM AND ENVIRONMENT

By L. J. HENDERSON. Harvard University

CHEMICAL BASIS OF GROWTH

By T. B. ROBERTSON, University of Toronto

COORDINATION IN LOCOMOTION

By A. R. MOORE. Rutgers College

THE NATURE OF ANIMAL LIGHT

By E. N. HARVEY, Princeton University
OTHERS WILL FOLLOW

Monographs on Experimental Biology

r

THE PHYSICAL BASIS
OF HEREDITY

BY

THOMAS HUNT MORGAN

PROFESSOR OF EXPERIMENTAL ZOOLOGY IN COLUMBIA UNIVERSITY

117 ILLUSTRATIONS

PHILADELPHIA AND LONDON
J. B. LIPPINCOTT COMPANY

COPYRIGHT. 19 19. BY J. B. LIPPINCOTT COMPANY

Electrolyped and Printed by J. B. LippincoH Company
The Washington Square Press, Philadelphia, U. S. A.

EDITORS' ANNOUNCEMENT

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

Biology, which not long ago was purely descriptive
and speculative, has begun to adopt the methods of the
exact sciences, recognizing that for permanent progress
not only experiments are required but quantitative experi-
ments. It will be the purpose of this series of monographs
to emphasize and further as much as possible this develop-
ment of Biology.

Experimental Biology and General Physiology are one
and the same science, in method as w^ell as content, since
both aim at explaining life from the physico-chemical
constitution of living matter. The series of monographs
on Experimental Biology wilL therefore include the field
of traditional General Physiology.

Jacques Loeb,
T. H. Morgan,

W. J. V. OSTERHOUT.

5

CONTENTS

CHAPTER PAGE

I. Introduction * 15

II. Mendel's First Law — Segregation of the Gexes 19

III. The Mechanism of Segregation 39

IV. Mendel's Second Law — The Independent Assortment of

the Genes 59

V. The Mechanism of Assortment 73

VI. Linkage 80

VII . Crossing Over 87

VIII. Crossing Over and Chromosomes 96

IX. The Order of the Genes 118

X. Interference 126

XI. Limitation of the Linkage Groups 133

XII. Variation in Linkage 139

XIII. Variation in the Number of the Chromosomes and its Re-
lation TO the Totality of the Genes 147

XrV. Sex-Chromosomes and Sex-linked Inheritance 165

XV. Parthenogenesis and Pure Lines 204

XVI. The Embryological and Cytological Evidence that the

Chromosomes are the Bearers of the. Hereditary Units . . 212

XVII. Cytoplasmic Inheritance 219

XVIII. Maternal Inheritance 227

XIX. The Particulate Theory of Heredity and the Nature of

the Gene 234

XX. Mutation 247

»

ILLUSTRATIONS

Fig. page

1 . Cross Between a Tall and a Short Race of Garden Peas 20

2. Cross Between WTiite and Red Flowered Four-o'clocks 24

3. Cross Between Splashed-WTiite and Black, in Andalusian 26

4. Male and Female Vinegar Fly 28

5. Normal and Abnormal Abdomen of D. melanogaster 29

6. Relation of Black Body Color to Wild Type as Shown by Classes

of Fhes 30

7. Normal, Heterozygous, and Bar Eye of the Vinegar Fly 31

8. Relation of Bar Eye to Normal Eye 31

9. Relation of Andalusian to Splashed White and to Black as Shown

by Classes of Birds 32

10. Relation of Tall to Short Peas 32

11. Relation of Normal to Abnormal Abdomen as Shown by Classes

of Flies 32

12. Relation of Normal to Duplicate Legs of Flies 33

13. Notch Wings in the Vinegar Fly 35

14. Oocyte of Ancyracanthus; Growth Period; Nucleus with Tetrads. . . 40

15. Egg of Ancyracanthus 40 ■

16. Eggs of Ancyracanthus within Membrane 41

17. Spermatogenesis of Ancyracanthus 42

18. Last Spermatogonial Division of Tomopteris and Stages Before and

During Synapsis 45

19. Thin-Thread Stage of Tomopteris Spermatoc5^e ; Tetrads, and First

and Second Spermatocyte Divisions 47

20. Synaptic Stages and Those Immediately Following in Batracoseps ... 48

21. Synaptic Stages and Those Immediately Following in the Egg of

Pristiurus 50

22. Sister Blastomeres of Ascaris Preparatory to Another Division .... 52

23. Normal and Reduced Chromosomes of Biston 53

24. Division Figures in Egg of Ctenolabrus Fertihzed by Fundulus 54

25. Female and Male Chromosome Groups of Protenor 55

26. Reduced Chromosome Group; and Extrusion of Polar Bodies in

Protenor ^^

9

10 ILLUSTRATIONS

27. Reduced Chromosome Group of Male; and Spermatogenesis in

Protenor 56

28. Diploid and Haploid Chromosome Groups of Drosophila buschii and

D. melanica (negleda) 57

29. Cross Between Wingless and Ebony Vinegar Fly 65

30. Miniature Wing, Dumpy, and Miniature Dumpy 66

31. Combs of Fowls 69

32. Eight Chromosome Groups of Twelve Chromosomes Each of

Trimerotropis 77

33. Back-cross of Male (Out of Black Vestigial by Wild) to Black

Vestigial 81

34. Back-cross of Male (Out of Gray Vestigial by Black) to Black

Vestigial 83

35. Scheme Showing the Inheritance of the X-Chromosome in

Drosophila 84

36. Back-cross of Female (Out of Black Vestigial by Wild) to Black

Vestigial Male 89

37. Back-cross of Female ( Out of Gray Vestigial by Black) to Black

Vestigial Male 90

38. Scheme to Illustrate Double Crossing Over Between White and

Forked 93

39. Curve Showing the Influence of Temperature on Crossing Over Control 98

40. Curve Showing the Influence of Temperature on Crossing Over .... 98

41 . Diagram Showing Crossing Over of Two Chromosomes at Four-strand

Stage and the Subsequent Opening Out of the Tetrad 101

42. Scheme Showing the Opening Out of the Strands of the Tetrad 102

43. Scheme Showing Crossing Over Involving Both Strands of Each

Chromosome 103

44. Spermatogonia! Cells in the Last Phase of Division and the Following

Resting Stages 105

45. Cells Emerging From the Resting Stages Preparatory for the Next

Spermatogonia! Division 106

46. Cells Emerging From Their Last Spermatogonia! Division 106

47. Formation of a Thick Thread after Synapsis and the Following

Condensation of a Tetrad 107

48. A Pair of Chromosomes in Conjugation 109

ILLUSTRATIONS 11

49. The Same Chromosome Pair in Conjugation from Thirteen Different

Cells 110

50. Conjugation of an Unequal Pair of Chromosomes and Their Subse-

quent Separation Ill

51. Two Schemes Illustrating the Idea of Reduplication by Bateson

and Punnett 1 16

52. Scheme Illustrating How Double Crossing Over Between Two

Distinct Genes takes Place 121

53. Chromosome Groups of Pea, Wheat, and Primula 135

54. Types of Chromosome Groups Found in Drosophila 136

55. Haploid Group of Chromosomes of the Silkworm Moth 137

56. Curve Showing Influence of Crossing Over at Different Temperatures 142

57. Diagram Illustrating the Effect on Crossing Over Due to the Presence

of Crossover Genes 143

58. Chromosome Group of (Enothera Lamarckiana and 0. gigas, and

Triploid Group 149

59. Life Cj^cle of Moss 152

60. Diagram Illustrating the Formation of Individuals from the Regener-

ation of the Sporophyte in a Dioecious Species 153

61. Diagram Illustrating the Formation of Individuals from the Regener-

ation of the Sporophyte in a Hermaphroditic Species 153

62. Somatic Chromosomes Groups of (Enothera sciniillans 156

63. Scheme Showing the Probable Relation Between the Extra Chromo-

some Pieces of Fig. 62, and the Normal Fifteen Chromosomes of
This Mutant 158

64. An Egg of Ascaris hivalens FertiUzed by Sperm of A. univalens .... 160

65. Diploid and Haploid Groups of the Sundew Drosera 160

66. A Scheme Illustrating the Fertilization of the Egg of One Species of

Moth by the Sperm of Another 161

67. Scheme Illustrating the History of the Chromosomes, and the Back-

cross Between a Hybrid Male and One or the Other Parent 162

68. Scheme Showing the Relation of the Sex- Chromosome to Sex-De-

termination 1""

69. Cross Between WTiite-Eyed IVIale and a Red-Eyed Female of the

Vinegar Fly 1^8

12 ILLUSTKATIONS

70. Cross Between White-Eyed Female and a Red-Eyed Male of the

Vinegar Fly 169

71. Cross Between a Yellow White-Eyed Female and a Wild-Type

("Gray"), Red-Eyed Male 171

72. The Results from the Reciprocal Cross of That Shown in Fig. 71 . . . 173

73. Scheme Showing the Relation of the Sex-Chromosomes of the Moth

in Sex Determination 174

74. Cross Between Abraxas lacticolor Female, and Grossulariata Male .... 175

75. Cross Between Abraxas grossulariata Female and Lacticolor Male. . . 176

76. Cross Between Barred Plymouth Rock Male and Black Langshan

Female 178

77. Scheme Showing the Transmission of the Sex- Linked Characters .... 178

78. Cross Between Black Langshan Male and Barred Plymouth Rock

Female 178

79. Scheme Showing the Transmission of the Sex-Linked Characters

Shown in Fig. 78 179

80. First and Second Spermatocyte Divisions in the Bee 181

81. First and Second Spermatoc3^e Divisions in the Hornet 182

82. Life Cycle of Phylloxera caryoecaulis 182

83. Extrusion of the Polar Body from a Male-Producing Egg 183

84. First and Second Spermatocyte Divisions in the Bearberry Aphid 184

85. Hydatina senta: Adult Female, Young Female Soon After Hatching,

Adult Male, Parthenogenetic Egg, Male-Producing Egg, Resting
Egg 186

86. Diagram Showing How a Continuous Diet of Polytoma Through

Twenty-Two Months Yielded Only Female-Producing Females ... 187

87. A Gynandromorph of Drosophila melanogaster that was Female on

the Right Side and Male on the Left 190

88. Diagram Showing EUmination of X' at an Early Cell Division 191

89. Caterpillars of the Silkworm Moth 192

90. Diagram Illustrating How a Heterozygous Egg With Two Nuclei

Fertilized by Two Sperms Might Produce a Gynandromorph like
that Shown in Fig. 89 193

91. Scheme Showing the Transmission of a Lethal Sex- Linked Factor

in an X-Chromosome 199

92. Normal Female and Male Groups of Chromosome of the Vinegar

Fly 200

ILLUSTRATIONS 13

93. Non-Disjunction. Egg Fertilized by X-Sperm 201

94. Non-Disjunction. Egg Fertilized by Y-Sperm 202

95. A Wingless Apliid and a Winged One 207

96. Curve Showing the Non-effect of Selection for the First Twelve

Generations for Increase in Body Length 208

97. Curve Showing the Effect of Selection for the Second Score of

Generations 209

98. Scheme Showing Dispermic FertiHzation of the Egg of the Sea

Urchin 214

99. First Division of a Hybrid Egg 215

100. FertiHzation of an Egg Starting to Develop Parthenogenetically . . . 216

101. Larval Sea Urchin Seen in Side View 217

102. Green Leaf and Checkered Leaf of Four-o'clock 220

103. Pelargonium that Gave Rise to a \Miite Branch 221

104. Diagram to Show How a Sectorial Chimera May be Produced 221

105. Diagram to Illustrate Maternal Inheritance 228

106. Diagram to Show the Inheritance of Two Pairs of Mendelian

Characters 238

107. Normal Sebright Hen-Feathered Male and a Castrated Sebright ... 246

108. Diagram Illustrating Mutation in a Nest of Genes 252

109. Two Flies (Drosophila) with Beaded Wings 258

110. Diagram Showing the Relation of the Chromosomes 258

111. Diagram to Show how the Appearance of a Lethal Near Beaded

Causes the Stock to Produce only Beaded 259

112. Diagram Showing the Results of Crossing Over in a Stock Contain-

ing Both Beaded and Lethal 260

113. Diagram Illustrating How in the Presence of a Dominant Factor,

Dichete, and a Lethal in Its Homologous Chromosome at About
the Same Level, Together with Another Factor, Peach -Colored
Eyes, Gives the Result Shown in the Squares 261

114. Diagram Illustrating Crossing Over of Factors in Fig. 113 262

115. Rosettes of the Twin Hybrids of the Evening Primrose 263

116. Diagram Illustrating Balanced Lethals and Twin Hybrids 264

117. Diagram Illustrating Lethals and Four Types 265

THE PHYSICAL BASIS OF

HEREDITY

CHAPTER I

INTRODUCTION

That the fundamental aspects of heredity should have
turned out to be so extraordinarily simple supports us in
the hope that nature may, after all, be entirely approach-
able. Her much-advertised inscrutability has once more
been found to be an illusion due to our ignorance. This
is encouraging, for, if the world in which we live were as
compHcated as some of our friends would have us believe
we might well despair that biology could ever become an
exact science. Personally I have no sympathy with the
statement that ' ' the problem of the method of evolution
is one which the biologist finds it impossible to leave alone,
although the longer he works at it, the farther its solution
fades into the distance." On the contrary, the evidence
of recent years and the methods by means of which tliis
evidence is obtained have already in a reasonably short
time brought us nearer to a solution of some of the import-
ant problems of evolution than seemed possible only a few
years ago. That new problems and developments have
arisen in the course of the work— as they are bound to
do in any progressive science, as they do in chemistry and
in physics for example— goes without saying, but only a
spirit of obscurantism could pretend that progress of this
kind means that we see the solution of our problem fading
away into the distance.

Mendel left his conclusions in the form of two general
laws that may be called the law of segregation and the

15

16 PHYSICAL BASIS OF HEREDITY

law of independent assortment of the genes. They rest
on numerical data, and are therefore quantitative and can
be turned into mathematical form wherever it seems desir-
able. But though the statements were exact, they were
left without any suggestion as to how the processes
involved take place in the living organism. Even a purely
mathematical formulation of the principles of segregation
and of free assortment would hardly satisfy the botanist
and zoologist for long. Inevitably search would be made
for the place, the time, and the means by which segre-
gation and assortment take place, and attempts would
sooner or later be made to correlate these processes with
the remarkable and unique changes that take place in the
germ-cells. Sutton, in 1902, was the first to point out
clearly how the chromosomal mechanism, then known,
supplied the necessary mechanism to account for Mendel's
two laws.

The knowledge to which Sutton appealed, had been
accumulating between the years 1865, when MendePs
work was published, and 1900, when its importance became
generally known. An account of the chromosomal
mechanism may be deferred, but I have spoken of it here
in order to call attention to a point rarely appreciated,
namely, that the acceptance of this mechanism at once
leads to the logical conclusion that MendePs discovery
of segregation applies not only to hybrids, but also to
normal processes that are taking place at all times in all
animals and plants, whether hybrids or not. In conse-
quence we find that we are dealing with a principle that
concerns the actual composition of the material that car-
ries one generation over to the next.

Segregation and independent assortment were the two
fundamental principles of heredity discovered by Mendel.
Since 1900, four other principles have been added. These
are known as linkage, the linear order of the genes, inter-
ference, and the limitation of the linkage groups. In the
same sense in which in the physical sciences it is custo-

INTRODUCTION 17

mary to call the fundamental generalizations of the science
the "laws'' of that science, so we may call the foregoing
generalizations, the six laws of heredity known to us at
present. Despite the fact that the use of this word ' ' law ' '
has been much abused in popular biological writing we
need not apologize for using it here, because the postu-
lates in question have been established by the same scien-
tific procedure that chemists and physicists make use of,
viz., by deductions from quantitative data. Excepting for
the sixth law they can be stated independently of the chro-
mosomal mechanism, but on the other hand they are also
the necessary outcome of that mechanism.

The theory of the constitution of the germ-plasm,
to which MendePs discoveries led him, not only failed to
receive any recognition for fifty years, but the principle
of particulate inheritance to which it appeals has met
with a curious reception even in our own time, leading
a recent writer to state that particulate theories in general
"do not help us in any way to solve any of the funda-
mental problems of biology, ' ' and another writer to affirm
that if the chromatin of the sperm is * ' pictured ' ' as com-
posed of individual units that represent "some specific
unit-characters of the adult, ' ' then we should expect it to
be extremely complex, "more complex indeed than any
chromatin in the body, since it is supposed to represent
them all, ' ' but " as a matter of fact chemical examination
shows the chromatin in the fish sperm to be the simplest
found anywhere. ' ' Were our knowledge of the chemistry
of the "chromatin'' as advanced as these very positive
statements might lead one to suppose, the objection raised
might appear to be serious, but there is no evidence in
favor of the statement that the sperm-chromatin should be
expected to be more complex than the same chromatin
in the cells of the embryo or adult. And even were it
different in the germ-tract and soma the criticism would
miss its mark, because heredity deals with the constitution
of the chromatin of the germ-tract and not with that of

18 PHYSICAL BASIS OF HEEEDITY

the soma. Until physiological chemists are in position
to furnish more complete information concerning the com-
position of the chromosomes, or more illuminating criti-
cism of the situation as it exists, we need not, I think,
be over-much troubled by such views so long as we handle
our own data in a manner consonant with the recognized
methods of scientific procedure.

Other critics object for one reason or another to all
attempts to treat the problem of heredity from the stand-
point of the factorial hypothesis. It has been said, for
instance, that since the postulated genetic factors are
not known chemical substances the assumption that they
are such bodies is presumptuous, and gives a false analogy
with chemical processes. Such critics claim that the pro-
cedure is at best only a kind of symbolism. Again, it has
been said, that the factorial hypothesis is not a real
scientific hypothesis, for it merely restates its facts in
terms of factors, and then by juggling with numbers pre-
tends that something is being explained. It has been
argued that Mendelian phenomena relate to unnatural
conditions and that they have nothing to do with the
normal process of heredity in evolution that takes place
in ''nature.'^ It has been objected that such a hypoth-
esis assumes that genetic factors are fixed and stable in
the same sense that molecules are stable, and that no such
hard lines are to be found in the organic world. And
finally it has been urged that the hypothesis rests on dis-
continuous variation which, it is said, does not exist.

If the implications in any or in all of these objections
were true, the attempt to explain the traditional prob-
lem of heredity by the factorial hypothesis would
appear fantastic in the extreme. An attempt will be
made in the following chapters to present the evidence
on which our present views concerning heredity rest, in
the hope that an understanding of this evidence will go
far towards removing these a priori objections, and will
show that they have no real foundation in fact.

CHAPTER II

MENDEL'S FIRST LAW— SEGREGATION

OF THE GENES

Mendel, succeeded in discovering the principle of
segregation because he simplified the conditions of his
experiments so that he had to deal with one process at
a time. Others before him had failed because they worked
with too complex a situation. In each case Mendel picked
out for study a pair of contrasted characters of a kind
that were sharply distinguishable from each other when-
ever they appeared. He chose plants that normally self-
fertilize and are little liable to accidental cross-fertiliza-
tion, which made it possible easily to obtain in the second
generation numbers large enough to give significant
results. To MendePs foresight in arranging the condi-
tions of his work, as much as to his astuteness in interpret-
ing the data, is due his remarkable success.

Mendel used varieties of the common edible garden
pea (Pisum sativum). Many of these varieties (races)
differ from each other in a particular character. Some
races are tall, others short; some have green peas (seeds
in the pods), others have yellow peas ; some of these seeds
have a smooth surface, others are wrinkled; some of the
pods are hard, others are soft. One of the crosses made
by Mendel will serve as an illustration of his work (Fig. 1).

Pollen from a race of tall peas was put artificially on
the stigma of a plant of a short race, whose own stamens,
and therewith the pollen, had been previously removed.
The hybrid plants that came from the seed were tall.
These hybrids were allowed to self-fertilize and their
seeds collected. Some of the seeds produced tall plants,

19

20

PHYSICAL BASIS OF HEREDITY

loll g %

Short

00

FiQ. l.-Cross between a tall and a short race of garden peaa. ^he Fi generation is
tall. In the second generation, F2, there are three tails to one short. (Pi, Fx and i^s v^ere
reared from peas supplied by Dr. O. E. White.)

MENDEL'S FIRST LAW 21

others produced short plants; in the ratio of 3 tall to 1
short. In other words, the contrasted characters of the
grandparents reappeared in the grandchildren in the ratio
of 3 to 1. The experiment was carried through one more
generation, which was necessary in order to get data
for finding out what had been taking place. The short
peas were allowed to fertilize themselves. They pro-
duced only short peas. The tall peas were also allowed to
fertilize themselves. One-third of the tall peas produced
only tall offspring; two-thirds produced both tali and
short offspring in the ratio of 3 : 1, as had the first genera-
tion hybrids. Evidently then the grandchildren had been
of three kinds, one kind was pure for shortness, others
were hybrids, and the remaining kind was pure for tall-
ness. These kinds appeared in the proportion of 1 : 2 : 1.
Some factor or factors in the original tall peas must
cause the peas of that race to be always tall, and some
factor in the original short peas must cause them to be
short. The short factor may be represented by s, and
the long factor by S. When crossed, the fertilized egg
should contain both factors (sS), and since the hybrids
coming from this egg were tall, it is evident that tall must
dominate over short. Now if the two factors (sS) present
in the hybrid should separate (i.e., ''segregate'') when its
ovules and its pollen-grains are formed, half of the eggs
would contain the factor that represents the short peas
(5), and half of the eggs the factor that represents tall
peas (S); also half of the pollen grains would contain the
factor that represents the short peas (5), and half of them
would contain the factor that represents the tall peas (S).
Chance meeting between egg-cells and pollen-cells (one
ovule being always fertilized by one pollen grain), would,
on the average, give one fertilized egg containing two
factors for short (55) ; to two fertilized eggs that contain
one of each kind of factor (5^) ; to one that contains two

22 PHYSICAL BASIS OF HEREDITY

factors for tall (SS). The chance combination, just given
may be represented graphically as follows :

( Ovules Tall .^ ^ Short

fA t >< t

( Pollen Tall / \ Short

( Tall-Short.
Tall- Tall, -f ^ +Short-Short.

^/Tall-Short.

In the actual experiment that Mendel carried out, plants
of the tall race measured from 6 to 7 feet, and those of
the short plants three-quarters to one foot and a half.
The 2^1 plants were as tall as, or even taller than the tall
parent. When these F^'s were self -fertilized, the seeds
(either from the same plant or from a random collection
of seeds from different F^ plants) produced 787 long
plants and 277 short plants — a ratio of 2.84 to 1.

As a fair sample of each plant, ten seeds were taken
from each of 100 tall plants of this second (or Fg) genera-
tion. Out of the 100 plants so tested, 28 plants produced
only tall plants, while 72 of them produced some tall
and some short offspring. This means that 28 plants
were pure (homozygous) tall, whilst 72 were hybrid like
the Fi plants. Taking, then, all F2 plants together, the
results show % were short, V4 were hybrid, and % were
tall, i.e., they stand in a ratio of 1 : 2 : 1.

This relation is illustrated in the scheme below, based
on what 16 F2 plants might give. Twelve would be tall
to 4 short. If the tall plants are tested, they are found
to consist of 4 pure tails (SS) and 8 hybrid tails {sS).
Altogether, then, there are 4 tails to 8 hybrid tails to 4
short, i.e., there are three kinds of F2 peas in the ratio
of 1:2:1.

12 tall -1- 4 short

, ■ , « ,

4SS + 8sS -f- 4ss

1 2 1

The process of disjunction, or separation of the mem-
bers of a pair of factors, is known technically as segre-
gation. While we sometimes also speak of the segrega-

MENDEL'S FIRST LAW 23

tion of the characters themselves, it seems better, I think,
to avoid as far as possible this application of the word.
The factor for tall and the factor for short are said to be
allelomorphio to each other. The parents are generally
designated by Pi ; the first hybrid generation is known
as the first filial generation, or briefly F^. The next
generation, derived from F^ is called F2, etc. When one
member of the pair of contrasted characters appears in
Fi to the exclusion of the other it is said to be dominant,
the eclipsed character is said to be recessive. The hybrid
itself is said to be heterozygous, meaning that it contains
one factor or gene of each kind, while an individual con-
taining both genes of the same sort is said to be homo-
zygous for the genes involved. Mendel did not emphasize
the idea that even in pure races each character is also
represented, as a rule, by a pair of factors or genes that
segregate in the formation of the germ-cells in the same
way as do the pair of contrasted genes in the hetero-
zygotes, but at the present time this idea is accepted by
all geneticists. It was at least implied on MendePs view
that the two pure classes in F^ {SS and ss), formed by
the recombination of two like genes, are identical with
the two grandparental races (Pi).

A crucial test of the correctness of the assumption that
segregation of the members of a pair of elements takes
place in the germ-cells of the hybrid, consists in back-
crossing the hybrid (Pi) to one of the parent stock, viz.,
to the not dominant stock, here the short pea. Since short
is recessive to tall, it will not influence the height of the
offspring when a tall and a short factor are brought
together. Such a cross should show whether the germ-
cells of the hybrid are, as postulated, of two sorts, and
whether equal numbers of each sort are produced. Mendel
made such tests, and obtained equal numbers of two kinds

of offspring.

Mendel obtained results like these with tall versus
short peas for other pairs of characters, such as f asciated
versus normal stems, hard versus soft pod, yellow versus

24

PHYSICAL BASIS OF HEREDITY

green pods, gray versus white-skinned peas, yellow versus
green cotyledons (seen through the skin of the seed),
and round versus wrinkled seeds (determined by the
nature of the cotyledons within the seed coat).

The 3 : 1, i^o? ratio characteristic for a single pair of
characters is the expectation based on the chance meeting
of either one of two kinds of eggs with either one of two
kinds of pollen grains. In actual numbers this ratio is,
of course, not always exactly realized, but only approxi-
mately. For the seven pairs of characters that Men-
del examined, the F2 ratios were as follows :

Dominants

Receesivee

No's, per 4

Form of seed

7,324

8,023
929

1,181
580
858

1,064

5,474

6,022

705

882
428
651

787

1,850
2,001
224
299
152
207
277

2.99 : 1.01

Color of cotyledons

Color of seed coats

3.00 : 1.00
3.04 : 0.96

Form of ood

2.99 : 1.01

Color of Dod

2.95 : 1.05

Position of flowers

3.03 : 0.97

Length of stem

2.92 : 1.08

Totals

19,959

14,949

5,010

2.996 : 1.004

The following collective data for the inheritance of
color of the cotyledons of garden peas show that the
approximation to a 3 to 1 for the recessive character is
very close :

Yellow

Green

Total

No's, per 4

Probable errors

Mendel

Correns

Tschermak

Hurst

Bateson

Lock

Darbishire

Darbvshire

White

Correns

Tschermak

Lock

Darbishire

Correns

Lock

6,022
1,394
3,580
1,310
11,903
1,438
109,060
1,089
1,647
1,012
3,000
3,082
5,662
225
2,400

2,001
453

1,190
445

3,903
514
36,186
354
543
344
959

1,008

1,856

70

850

8,023
1,847
4,770
1,755
15,806
1,952
145,246
1,443
2,190
1,356
3,959
4,090
7,518
295
3,250

3.002 : 0.998
3.019 : 0.981
3.002 : 0.998
2.986 : 1.014

3.012 : 0.988
2.947 : 1.053
3.004 : 0.996
3.019 : 0.981
3.008 : 0.992
2.985 : 1.015
3.0;-; 1 : 0.969
3.014 : 0.986

3.013 : 0.987
3.051 : 0.949
2.954 : 1.046

±0.0130
=t 0.0272
±0.0169
±0.0279
±0.0093
±0.0264
±0.0030
±0.0308
±0.0250
±0.0319
±0.0186
±0.0183
±0.0135
±0.2151
±0.0205

Totals

152,824

50,676

203,500

3.004 : 0.996

±0.0026

PARENTS

o.

ixj

Fig. 2. — Cross between white and red flowered four-o'clocks (Mirabilis jalapa). In
the lower part of the diagram the large circles represent somatic conditions, the included
small circles the genes that are involved.

MENDEL'S FIRST LAW 25

That MendePs principles apply to animals was first
made out by Bateson and by Cnenot in 1902. Since then
many characters both in domesticated and in wild animals
and plants have been studied, and there can be no question
of the wide application of MendePs discovery.

During the years immediately following the re-dis-
covery of MendePs principles (1900) much attention was
paid to the phenomena of dominance and recessiveness.
This was due, no doubt, to the striking fact that the hybrid
sometimes resembles only one parent in some particular
trait, whereas the older observations, where many charac-
ters were generally involved in the cross, seemed to have
shown that hybrids are intermediate in regard to their
parents. We now know, however, that although there are
cases in which the dominance is as complete as in those
described by Mendel, yet in a very large number of forms
the hybrid is intermediate be*tween the parents, even
when only a single pair of characters is involved. A few
examples will serve to illustrate these relations.

The common garden four o 'clock, Mirabilis jalapa, has
a white-flowered and a red-flowered variety (Fig. 2).
When crossed, the hybrid has a pink flower, which may be
said to be intermediate in color between white and red.
Here neither color can strictly be said to dominate. When
the hybrid (F^) is self -fertilized the offspring (F2) are
in the proportion of one white, to two pink, to one red-
flowered plant. The Fg reds and the F2 whites breed true ;
the pinks when self -fertilized give white, pink and red in
the proportion of 1 : 2 : 1. In a case of this kind the color
of the F2 plants reveals the nature of the three classes
present, so that it is not necessary to test them out, as was
the case in the F2 generations of MendePs peas, where
the Fo tails were found in this way to be of two sorts.
The F2 results wdth the four o'clock also show, that
the segregation of the genes is clean, for the Fo whites
never produce in subsequent generations anything

26 PHYSICAL BASIS OF HEREDITY

but white descendants, and the F2 reds never anything
but red descendants.

In this case the color of the F^ flowers is obviously
somewhere between red and white. In so far as the F^
flower is colored, it may be said that red is dominant ; in
which case the red and the pink F2 classes (1 + 2 = 3)
are to be counted together as contrasted with the white,
giving a 3 : 1 ratio. On the other hand, if one chose to
emphasize the fact that the F^ pink flower is not red, but
affected by the white-producing element in its make-up,
then not red, but white, might be said to be the dominating
character ; in which case the white and the pink F2 classes
(1 + 2 = 3) would be counted together as contrasted with
the red giving an inverse 3 : 1 ratio. It appears then
largely a matter of choice as to what is to be called
dominance (see below). The essential fact of segrega-
tion is not affected by the decision, and it is this that is
fundamentally important.

Another example of failure of complete dominance is
shown in the race of Andalusian fowls. In this race there
are blue, splashed-white, and black birds ; the blue birds
going under the name of Andalusians. When splashed-
white is mated to black, all the offspring (F-^) are blue
(Fig. 3) ; when these blues are bred together they give
1 splashed-white : 2 blues : 1 black. Evidently the blue
birds are the heterozygous type. Their feathers show
under the microscope less black pigment, somewhat dif-
ferently distributed from that in the black birds. The
intermediate blue color is due in this case to the less dense
distribution of the pigment in the heterozygote. Lippin-
cott, who has recently examined this cross in greater detail
than heretofore, states that the colored areas or splashes
in the white males are either blue or blackish according
to the part of the body on which they occur, and that this
corresponds with the distribution of the color on the Anda-
lusian, for while the latter is said to be blue, this applies

;«-^

Fig. 3. — Cross between splashed-white and black, giving in Fi Andalusian, and in F2 one
splashed-white, two Andalusian, and one black.

MENDEL'S FIRST LAW 27

strictly only to the hen and to the lower parts of the body
in the cock whose upper surface is very dark bine or
even black.

In this case neither black nor white can be said to be
dominant. The blue brought in as splashes by the
splashed-white might indeed be regarded as dominant over
the black of the other (black) parent, but if so, then the
uniform distribution of the blue must be determined by
dominance of the allelomorphic gene brought in by the
black parent. Each parent then would contribute at the
same time a dominant and a recessive effect, each the
product of one member of the same pair of allelomorphs.

There are other cases in which the hybrid is inter-
mediate in color, and, in addition, its range of variation
is so large that the extremes overlap one or even both
of the two parental types. For example : In the vinegar
fly, Drosophila melanogaster, there is a race with ebony
wings and another race with sooty wings. When such
flies are crossed to each other, the wings of the F^ fly are
intermediate in color, ranging from wings like those of
sooty to wings as black as ebony. When the F^ flies are in-
bred they give rise to a series that at one extreme has gray
wings and at the other black wings. Separation into three
classes is difficult or impossible. Here it may appear that
the two original characters have completely blended in
F^ and in F2, but that there are in reality three classes
of flies in F2 can be demonstrated by suitable tests. If,
for instance, we pick out a sufficient number of F2 males
to give a fair sample of the population, and mate each
male first to an ebony female of pure stock, and then to
a female of sooty stock, we shall find that one-quarter of
the males mated to ebony give only ebony, one-quarter
mated to sooty give only sooty, while the remaining two-
quarters give, both in the back-cross to sooty, and in that
to ebony, a wider ranging group, which is darker on the
whole when mated to ebony, and lighter when mated to

28

PHYSICAL BASIS OF HEREDITY

sooty. These and other tests show that in the F^ hybrid
segregation of the same kind as in the preceding cases has
taken place, but the results are obscured by the wide
variability of the hybrid flies. In other words, evidence
can be obtained that the segregation of the genes has been
clean cut, even although this is obscured by the character
of the heteroz3^gous flies.

Fig. 4. — Male and female vinegar fly (,Drosophila melanogaster) .

In the preceding illustrations the character difference
between the two races is supposed to show itself in the
same environment. It has been found in a few other
cases that the dominance of one character over the other
may depend on the environment. For example, in the
normal vinegar fly the black bands of the abdomen show
great regularity (Fig. 4), but in a mutant race called
*' abnormal abdomen'^ (Fig. 5) the bands may be irregu-
larly broken up, or even absent. In cultures with abund-
ance of fresh food and moisture, all the individuals have
very irregular bands, but as the culture gets old, and the

MENDEL ^S FIRST LAW

29

food and moisture become less and less, the bands become
more and more regular until at last the flies are indistin-
guishable from normal flies. If a cross is made between
a female with abnoraial bands and a wild male, the off-
spring that first hatch under favorable conditions are all
very abnormal. Here abnormal completely dominates
normal bands. But as the culture dries up, the hybrid
offspring become more and more normal, until finally they
are all normal. At this time it might be said that normal
dominates abnormal. Both statements are correct, if we
add that in one environment abnormal banding dominates.

Fig. 5. — Normal and abnormal abdomen of D.mdanogaster.

in another environment normal banding dominates. The
genetic behavior of the pairs of genes is the same here
as in all other cases of Mendelian behavior, but this is
revealed only when the environment is one in which the
abnormal gene produces one effect, the normal a different
one. That the gene is not itself affected by the environ-
ment can be shown very simply. If a female from the
abnormal stock be picked out, at a time when the stock
has onlv normal bands, and crossed to a wild male, the
offspring will all be as '' abnormal'' as when the mother
herself is abnormal, provided the food and moisture
conditions are of the right kind. The late hatched normal
flies of abnormal stock may be bred from for several

30 PHYSICAL BASIS OF HEEEDITY

generations, but as soon as a generation hatches under
favorable conditions they are as abnormal as though all
their ancestors had been of this sort. Thus it is evident
that no fundamental importance is to be attached to domi-
nance of characters. On the other hand, it is equally
obvious that it would be entirely unwarranted to suppose
that incompleteness of dominance is due to failure of
segregation of the genes that stand for the characters.

While the problem of segregation can be studied to
greatest advantage where the characters of a pair are
sharply separated, yet even where the pair does not
possess this advantage, the cleanness of the segrega-
tion process can be just as definitely, though more
laboriously, demonstrated.

In cases where there is an overlap between the hetero-
zygous type and one of the parental types it may, simply
as a matter of convenience, be advantageous to call that
character that gives the more continuous F2 group the
dominant, thus leaving the smaller more sharply defined
group as the recessive. For example, the F2 group from
black by wild-type Drosophila may be represented by
such a scheme (Fig. 6) as the following:

Fig. 6. — Relation of black body color to wild type as shown by the classes of F2 flies.
The heavy outline includes the mutant class, the lighter line the wild type, «nd the dotted
line the heterozygous class.

Here the heterozygous flies are typically intermediates,
but their variability overlaps that of the wild type to
such an extent that separation of the intermediate from
the wild type is practically impossible. On the other hand,
there is no difficulty in making a complete separation
between the heterozygous class and the homozygous black.

MENDEL'S FIRST LAW

31

Black is accordingly treated as a recessive in nearly
all experiments.

a

Fig. 7. — Normal eye, o, a', heterozygous eye 5, 5', and bar eye c, c', of the vinegar fly.

A mutant eye shape of Drosophila, called ^'bar*' (Fig.

C/ 7, W), has an intermediate hybrid type (Fig. 7, &). The Fg

group may be represented (Fig. 8) in the following scheme :

FiQ 8. — Relation of bar eye to normal eye, as shown by the Fi classes.

In this case the hybrid, intermediate type, overlaps the
bar type, so that in Fc these two latter types give a nearly
continuous class. At the other end of the F^ series, the
round eyed normal (or wild) type can be distinguished
without difficulty from either of the other classes. Bar is
therefore normally treated as a dominant.

32

PHYSICAL BASIS OF HEREDITY

The case of Mirabilis, or of the Andalusian fowl, might
be represented (Fig. 9) in the following scheme:

A

\.

Fig. 9. — Relation of Andalusian to splashed white and to black as shown by classes

of F2 birds.

Here all three types are fnlly separable, in which case
either homozygote might be considered the dominant.

Finally, to return to the case of the tall and short
peas, the following scheme (Fig. 10) represents the F2

Fig. 10. — Relation of tall to short peas as shown by F2 classes.

group: Here the tall and the heterozygous group are
alike, and inseparable by ordinary inspection, even at
the extreme end of their variation curves, and short is
^'completely'' recessive.

In cases in which the environment enters more
obviously into the result (as in ' ' abnormal abdomen, ' ' Fig.
5), the following scheme (Fig. 11) represents the relation :

Dry

Wet

Fig. 11. — Relation of normal to abnormal abdomen as shown by classes of Fi flies. " Dry"
signifies conditions that make for normal; wet for abnormal.

In this case both the heterozygous and the parental
*' abnormal'' type may show '^ normal" abdomen like the

MENDEL'S FIRST LAW 33

wild type. The abnormal type is treated as the dominant
although only when the conditions are favorable to its
appearance is the hereditary phenomenon seen. In
another case (duplicate legs) only the homozygous form
may show the duplications (in a special environment).
The following scheme (Fig. 12) represents this relation,
reduplication of legs being treated as a recessive:

Fig. 12. — Relation of normal to duplicate legs.

There are still other relations that affect the dominance
of characters. For example, there may be internal fac-
tors, which when present, determine that a character shall
be dominant over its allelomorph, or recessive to it. In
this connection might be mentioned what has been called
''reversal of dominance. '^ An example from Davenport
will illustrate what is meant. In a certain strain of fowls
there is a tendency for the toes to be united by a web at
the base. Crossed to birds with normal feet, no birds
with united toes (syndactyls) appeared in F^^. The F^
birds inbred gave in F2 only about 10 per cent, of syndactyl
birds. It would appear that the latter character is reces-
sive, and that the recessive type overlaps largely the
dominant heterozygous type.

Davenport interpreted, however, the syndactyl as the
dominant type, because ''two syndactyls may give nor-
mals, but no true normals give syndactyls.'' In other
words, he defines the dominant type as the one that can
carry the other type, because he says dominance is due to
presence of factors, recessiveness to absence. "Now
dominance may fail to develop but recessiveness never
can do so." For this reason two syndactyls may give
3

34 PHYSICAL BASIS OF HEEEDITY

normals, because a dominant character may fail to develop,
even though its factors be present. Since normal feet
never give syndactyls, the normal type must be recessive.
But Davenport ^s definition of a recessive type as one
that never shows in the heterozygous condition is in my
opinion based on an arbitrary distinction of what is the
cause of dominance and recessiveness. The evidence may,
I think, be better interpreted as indicated in the same
diagram as that for abnormal abdomen (Fig. 11) in that
part marked *'dry,'' in which the syndactyl condition
would be represented as recessive (heavy line). In the
hybrid the character is usually seen only in a few individ-
uals, i.e., it is intermediate, overlapping both parent types.
While this case shows that it is often only a convention
as to which type is called the dominant and which the
recessive, I can see no special reason why in these cases
of syndactylism the usual convention may not be followed
which recognizes the small F2 class as the recessive.

Mendelism rests on the theory of a clean separation
of the members of each pair of factors (genes). In
every heterozygote the factor for the dominant and that
for the recessive are supposed to come into relation to
each other and then to separate at the ripening of the
germ-cells. If we think of the two genes coming together
and afterwards separating, it would seem that a favor-
able situation might exist for the two to become mixed,
and one "contaminate" the other. If any extensive
process of this kind occurred the Mendelian phenomena
would be so irregular and erratic that they would have
little interest. But even those who are inclined to appeal
to contamination as an exceptional phenomenon, grant
that clean separation of the genes is the rule. The best
critical evidence against contamination is in cases in which
for many successive generations breeding has taken place
from heterozygous forms only (which creates a favorable
situation for contamination to take place were it possible).
No influence of contamination has been found in such cases.

MENDEL ^S FIEST LAW

35

Marshall and Muller kept flies heterozygous for three re-
cessive mutant factor for about seventy-five generations,
and at the end of that time found that these factors had
not been weakened in any way as a result of juxtaposition

Fig. 13. — Notch wings in the vinegar fly, extreme condition, a; average condition, b;

nearly normal condition, c.

with their normal dominant allelomorphs. I have kept
a stock of notch-winged flies under selection for twenty-
five generations. Notch (Fig. 13) is a character varying
in the direction of normal wings (Fig. 13, c) ; in every
generation of notch, many notch flies have normal wings.
The character is dominant, and exists only in heterozy-

36 PHYSICAL BASIS OF HEEEDITY

gous condition, since a fly homozygous for notch dies. The
race is therefore necessarily maintained in a hetero-
zygous state. In each generation females that were
genetically notch, but had normal wings, were selected
and bred to normal males. The selection was away from
notch {i.e.y toward normal). After a time more than half
of the notch flies had normal wings. The effect produced
proved to be due not to a change in the notch gene through
contamination, but to modifying genes ; for at the end of
the selection the original notch could be recovered at any
time by removing the influence of the modifying factor.
It has been sometimes stated, usually by the opponents
of MendePs theory, or by advocates of doctrines of evolu-
tion that appeared to be compromised by the Mendelian
conception of ^*unit factors,'* that Mendelism deals only
with such superficial characters as the color of flowers
or the hair color of mammals. This statement contains
an element of truth in so far as it covers most of the
kinds of characters that students of heredity find most
convenient to study; but it contains an entirely false
inference as to the limitations of Mendelism. The issue
involved is this : changes in superficial characters are not
so likely to affect the ability of the organism to survive
as are changes in essential organs ; hence they are the best
kind of hereditary characters for study. But there is no
evidence that such superficial characters are inherited in
a different way from *^ fundamental" characters, and
there is evidence to the contrary. A common class of
characters showing perfect Mendelian behavior are
so-called lethals that destroy the individual when in homo-
zygous condition. There can be no question as to the
fundamental importance of such factors. Between these
extreme cases and the superficial shades of eye color,
for example, all possible gradations of structure, physio-
logical and pathological, are known. The only possible
question that might be seriously raised is whether these
characters are all losses or deficiencies, while progres-

MENDEL'S FIRST LAW 37

sive advances may belong to a different category. This
may be a serious question for the evolutionist, but has
nothing to do with the problem that concerns us here.

In recent years an entirely unexpected and important
discovery in regard to segregating pairs of genes (allelo-
morphs) has been made. In an ever-increasing number of
cases it has been found that there may be more than
two distinct characters that act as allelomorphs to each
other. For example, in mice, yellow, sable, black, white-
bellied gray, and gray-bellied gray (wild type) are allelo-
morphs, i.e., any two may be present (as a pair) in an
individual, but never more than two. In Drosophila the
eye colors white, eosin, cherry, blood, tinged, buff, milk,
ivory, coral and the normal allelomorph form a series of
multiple allelomorphs. In the grouse locust, Paratettix,
there are nine types that may be aUelomorphic, all of
which exist in the wild state (Nabours). In Drosophila,
again, there are as many as twelve other series of allelo-
morphs known at present; in rats there is a small aUelo-
morphic series, also two in guinea pigs and two in rabbits.
In plants there are a few cases known, especially in corn.
In all these series it is the same organ that is mainly
affected by the different allelomorphs, which seems ' ' natu-
ral, ' ' but was not necessarily to have been expected. The
chief interest of these series is that they appear to demon-
strate that the normal (wild type) allelomorph, and its
mutant mates need not be due to presence and absence,
but rather represent modifications of the same unit in the
hereditary material ; for, taken literally, only one absence
is thinkable, and yet in Drosophila there are eight such
''absences" in one series.

As has been stated, Mendel did not make it clear that
there exists in the normal animal or plant the same dual-
ity that comes to light when a hybrid is produced ; never-
theless this condition is implied, at least, in his paper,
and has been taken for granted in practically all of the
modern work on hereditv. The demonstration that such

38 PHYSICAL BASIS OF HEREDITY

is the case is, however, not a simple matter. It could not
have been made by Mendel or in the earlier days after the
rediscovery of Mendelism (1900). An attempt to furnish
this demonstration is given in Chapter XX. Assuming
the demonstration to be satisfactory, we reach the highly
important conclusion that segregation is not something
peculiar to hybrids, but something most readily demon-
strated by means of hybrids, and that in all probability the
germ-plasm is at first made up of pairs of elements, but
at the ripening of the germ-cells these elements (genes)
separate, one member of each pair going to one daughter
cell, the other member to the other cell. The mechan-
ism by means of which such a process might take place
had been known for several years before its relation
to MendePs principles of segregation was realized. This
mechanism is to be found in the conjugation and reduc-
tion processes that take place in the maturation of egg-
and sperm-cell. An account of this process is given in
the next chapter.

CHAPTER III
THE MECHANISM OF SEGREGATION

One of the most secure generalizations bf modern work
on the cell is that every cell of the individual contains a
constant number of self -perpetuating bodies (called chro-
mosomes), half of which are traceable to the father and
half to the mother of the individual. No matter how
specialized cells may be, they contain the same number
of chromosomes. Equally important is the fact that after
the eggs of the female and the sperm-cells of the male
have passed through the ripening or maturation divisions
the number of chromosomes is reduced to half.^ Lastly,
there is convincing evidence that the reduced number of
chromosomes is brought about as the result of a separa-
tion of such a kind that each mature germ-cell gets only a
paternal or a maternal member of each chromosome pair.

The reduction takes place in the female at the time
when the polar bodies are given off from the eg^ ; and in
the male just prior to the formation of the spermatozoa.
A characteristic process is seen in the oogenesis and sper-
matogenesis of the nematode worm Ancyr acanthus cysti-
dicola (a parasite in the swim-bladder of fresh-water
fishes) described by Mulsow. The young eggs contain
twelve chromosomes (Fig. 14, a). As the result of the
later union of these twelve in pairs, six short threads
appear in the nucleus of the egg just before it extrudes its
polar bodies. The threads contract to six short rods
(split in two planes at right angles to each other), the
tetrads (Pig. 14, c). With the dissolution of the nuclear
wall these tetrads are set free in the protoplasm, and a
spindle develops about them (Fig. 15, a). They pass to
the equator of the spindle, and there dividing lengthwise,

* Exceptions occur in certain cases of parthenogenesis.

39

40

PHYSICAL BASIS OF HEREDITY

half of each goes to one pole, and half to the other pole
of the spindle (Fig. 15, &). One end of the spindle pro-
trudes from the egg, and around it the protoplasm con-

a

Fig. 14. — Oocyte of Ancyr acanthus, a; growth period, h; nucleus with tetrads, c. (After

Mulsow.)

stricts off (Fig. 15, c) to form the first polar body. About
the six ovoidal chromosomes left in the egg a new spindle
develops; and these chromosomes become drawn into
its equator, where they divide again, half of each going

a

c

e

Fig. 15. — Egg of Ancyr acanthus with six tetrads, a; egg with first polar spindle, h;
egg after extrusion of first polar body, c; egg with second polar spindle, d; egg after the
extrusion of both polar bodies, e.

to one pole and half to the other (Fig. 15, d). A sec-
ond protrusion takes place from the surface of the egg
which pinches off to form the second polar body (Fig.
15, e). Thus, after two mitotic divisions, the egg has lost
three-quarters of its chromatin, but retains half the full

MECHANISM IN SEGREGATION

41

number of chromosomes, and as a result, the original
twelve chromosomes have been reduced to six.

Around the six chromosomes left in the egg, a nuclear
wall forms, and the chromosomes become spun out into
delicate fibres. Meanvvhile a spermatozoon has entered
the eggy and out of its head another nucleus develops. The
two nuclei, the egg nucleus and the sperm nucleus, move
toward the center of the egg (Fig. 16, a), where they
come into contact with each other. After a time, the
chromatin threads begin to condense again into rods.

Fig 16. — Eggs of Anojraranthu^ within membrane. Egg with two pronuclei, a;
egg pronucleus with six chromosomes and sperm nucleus with six chromosomes, h; egg pro-
nucleus with six chromosomes and sperm nucleus with five chromosomes, c; union of male
and female pronuclei, d. (After Mulsow.)

Six appear in the egg nucleus, and six in the male nucleus
(Fig. 16, hy. A spindle develops in the protoplasm of
the egg around the twelve chromosomes of which six have
come from the father (the paternal chromosomes) and
six from the mother (the maternal chromosomes) (Fig.
16, d). Each chromosome now splits lengthwise into
equivalent halves, and a half moves to each pole of the
mitotic spindle. The spindle rotates in the cytoplasm of
this egg until its long axis corresponds with that of the
egg. As the daughter chromosomes move towards the
poles of the mitotic spindle the egg protoplasm constricts

' Assuming a female producing sperm to have entered.

42

PHYSICAL BASIS OF HEEEDITY

between them so that two cells are formed, each cell con-
taining twelve chromosomes, six paternal and six mater-
nal. Thus, through fertilization, the whole number of
chromosomes is restored to the egg. This number remains
through all subsequent divisions of the cells of the embryo.
The male of Aiicyr acanthus has only eleven (Fig. 17, a)
chromosomes ; because the male has only one sex-chromo-

a

Fig. 17. — Spermatogenesis of Ancyr acanthus. Spermatogonia! cell, a; cell after growth
period with tetrads, h; first spermatocyte division, c; two cells resulting from first division
with six and with five chromosomes, respectively, d; four cells resulting from the next
division, e; ditto, /; mature spermatozoa, one with six, the other with five, chromosomes, g;
ditto, living spermatozoa, h. (After Mulsow.)

some, while the female has two sex-chromosomes. Both
sexes have ten other chromosomes, sometimes called auto-
somes. Just before the maturation divisions take place,
there are six rods in each sperm-cell, five of which (the
autosomes) condense into tetrads, the sixth (the sex-
chromosome) into only a double body (Fig. 17, &). A
spindle develops about these and each of the five auto-
somes divides. The sex-chromosome does not divide, but
passes to one pole of the spindle (Fig. 17, c). The result

MECHANISM IN SEGREGATION 43

is that two cells are produced, one with six, the other
with five chromosomes (Fig. 17, d).

Without a resting stage a new spindle develops in each
cell, and a new division takes place — each dumb-bell-
shaped body dividing, as well as the sex chromosome in
the cell that contains it. In all, four cells result (Fig. 17,
e and /) — two with five chromosomes each, two with six
each. Each becomes a spermatozoon, which in this worm
is a round cell with the chromosomes at one pole (Fig.
17, g). Half of the spermatozoa contain six, half
five chromosomes. They can be distinguished even in
the living sperms (Fig. 17, h). If a six-chromosome
sperm fertilizes an egg (Fig. 16, h), a female (with 12
chromosomes) is produced — if a five-chromosome sperm
fertilizes an egg (Fig. 16, c), a male (with 11
chromosomes) is produced.

The two chromosome divisions (or separations) that
take place when the polar bodies are extruded from the
egg are, for a number of reasons that need not be entered
into here, generally regarded as equivalent to the two
final divisions in the ripening of the sperm-cells. One
of the two divisions is interpreted as an ordinary cell-
division in which the chromosomes split lengthwise into
equivalent halves — half going to each pole. The other
division is interpreted as a separation of whole chromo-
somes that have come together side by side at an earlier
stage. The tetrad is, then, looked upon as a pair of
chromosomes that have conjugated in the sense that they
have come to lie side by side (with interchange of mate-
rials at times in a way to be described later). One split
is supposed to correspond to the line between the conju-
gated pairs ; the other split represents a division in each
chromosome of the pair. As a consequence when the
chromosomes move apart (at the maturation division) one
of the two divisions is said to be a '* reducing division''
because whole chromosomes are supposed to separate ; the
other division is said to be an ''equation division,'' each

44 PHYSICAL BASIS OP HEREDITY

chromosome splitting lengthwise into equivalent halves as
in ordinary cell-division.

The interpretation of these two divisions that occur
in the egg and in the sperm-cell has been the subject of
much speculation. It is apparent that the process reduces
the number of chromosomes by half, and that the whole
number is regained by fertilization. It is sometimes said
that the ''purpose" of this division is to keep the number
of chromosomes constant, for, if not reduced, they would
increase in number with each fertilization.

The ''reason" for the other, the second, division is
acknowledged to be obscure. For present purposes it is
futile to speculate concerning these two divisions, but it
should be pointed out here that the genetic evidence is in
full accord with the interpretation of these two divisions
that is generally accepted to-day by cytologists, i.e., that
one of the divisions separates the conjugating pair, and
that the other represents a longitudinal division within a
paternal and within a maternal chromosome of each pair.

If we follow the history of the germ-cells further back
before the maturation divisions, we find that between the
stage when *the half number of chromosomes reappears
(tetrads) and the stage at which the full number was
present, there is a very obscure period in the history of
the germ-cells. This period has been studied chiefly in
the male. Only a few types have been found favorable
for the study of this period. One of the most favorable
ones is a marine annelid, Tomopteris, studied by the
Schreiners. The early division of the germ-cells (the
spermatogonia) of Tomopteris, when the full number
of chromosomes is present, is shown in Fig. 18, a-g.
The division is like that of all the other cells of the body.
The chromosomes appear as thick bent threads that split
lengthwise (Fig. 18,a, h). The nuclear wall disappears and
a spindle appears near the group of split chromosomes
(Fig. 18, c). As the poles of the spindle move apart the
chromosomes become arranged at the equator of the spin-

MECHANISM IN SEOEEOATION

45

die, each half of each chromosome becoming attached by
a spindle fibre to one pole (Fig. 18, ^). The halves move

Fig. is. Last spermatogonial division of Tomopteris, a-k; stages before and during

synapsis, i-l. (After Schreiner.)

apart towards their respective poles (Pig. 18, e) and as
they become separated into two groups the cell protoplasm

46 PHYSICAL BASIS OF HEREDITY

constricts between them to produce new cells (Fig. 18, /).
When the chromosomes have reached the pole they shorten
(Fig. 18, g) and appear to send out anastomosing threads.
Around this group of threads a new nuclear wall is formed
(Fig. 18, /i). All trace of the separate chromosomes is
now lost, but between the last stage just described and the
stage now to be described it is supposed that important
changes in the chromosomes take place. This new phase
is spoken of as the synizesis stage. At the beginning of
this stage (Fig. 18, i and j) faint indications of the chromo-
some appear, and soon they can be seen again (Fig. 18, U)
as long thin threads whose free ends place themselves in
parallel pairs. The pairing of the threads continues to
extend inwards from the ends (Fig. 18, 1) until they have
united throughout the length of the loops (Fig. 19, a).
There are exactly half as many of these loops as there
were original chromosomes, which is expected if they have
united in pairs. The conjugation has been accomplished.

During the stages that follow, the double chromosomes
shorten and become thicker (Fig. 19, &, c, ^Z), and con-
dense into the form of tetrads (Fig. 19, e). They begin
to separate into halves, each half is also split lengthwise.
A spindle appears, and the cells divide (Fig. 19, f,g,h).
In each cell the chromosomes show indications of passing
into a resting stage, as happens after all ordinary cell
divisions, but before this change has gone very far a new
spindle appears (Fig. 19, i), and preparations for another
division are rapidly made. The new division completes
the maturation of the sperm-cells (Fig. 19, j, k,l). Each
of the four cells resulting from the original sperm-mother-
cell differentiates into a spermatozoon.

In one of the salamanders, Batrocoseps, the matura-
tion stages of the male are particularly well shown. The
^^ essential stages in synizesis are shoAvn in Fig. 20, a-d
as worked out by Janssens. These stages are essentially
the same as those of Tomopteris. During the early multi-
plication stages the cells of the future testes divide by

MECHANISM IN SEOEEGATION

47

the ordinary mitotic process. The cells then pass into the
synaptic stage (Fig. 20, a-d). As the chromosomes begin

FiQ. 19. — Thin-thread stage of Tomopferis spermatocyte, a-d; tetrads, e; first sperma-
tocyte division, /-?'; second spermatocyte division, >-Z. (After Schreiner).

to emerge as thin threads, it is found in Batracoseps that
their ends are all pointed towards one pole (Fig. 20, (i).
This is the same pole as that towards which the two ends

48

PHYSICAL BASIS OF HEREDITY

of each V-shaped chromosome pointed as the cell went
into the resting* stage. It appears then that the chromo-
somes not only retain their original orientation, but that
the ends of homologous chromosomes have already come

\

X ■'

•^^,^ I

Fig. 20. — Synaptic stages and those immediately following in Batracoscps. (After Janasens.)

together, or are coming together, as the following
stages show clearly.

The union that begins at the ends (Fig. 20, e) grad-
ually extends along the length of the chromosomes, which

MECHANISM IN SEGREGATION 49

are now in the form of thin threads. At the point where
the two threads come togetlier (Fig. 20, /) they can often
be seen to be shaped like a Y and, at the point of meeting,
the uniting threads are often twisted about each other.

The fused part of the united threads steadily grows
shorter and thicker. They become the condensed pachy-
tene threads, and appear as represented in Fig. 20, g. The
thick threads shorten further, and the line of fusion
between them (or a new line of cleavage) appears, as
seen in Fig. 20, li. It will be noticed also that the ragged
outline that the chromosomes had during the preceding
stages is gradually lost, so that they now appear as solid
rods or cords, which finally when they have reached the
last stage in their condensation (Fig. 20, i) appear (in
Batracoseps) as rods tivisted about each other. Whether
this twisting represents the original wrapping around
each other of the leptotene threads as they conjugate, or
whether it is a new arrangement resulting from the con-
densation of the chromosomes that are not free to move at
all points, hence twist about each other as they condense,
is a question that calls for further and careful considera-
tion. For the present — since segregation alone is here
involved — this matter may be laid aside. In this con-
densed condition the chromosomes pass into the first
maturation division.

As already stated, the union of the chromosomes in
the eggs of the female has been less often studied, but
that the process is essentially the same is sufficiently evi-
dent. In one of the sharks, Pristiurus melanostomus, the
following stages described by Marechal show how similar
are the maturation stages in the female to those in the
male. When the germ-cells have reached the end of the
multiplication period they pass into the synaptic condition,
as shown in Fig. 21, a to d. Then threads appear in the
nucleus ; and soon it becomes evident that most of them
are in the form of loops, whose ends are uniting in pairs
(Fig. 21, e, /). When conjugation is finished thick loops

4

50 PHYSICAL BASIS OF HEREDITY

are present that shorten further into thick rods (Fig.
21, g) that often show a single longitudinal split. The
Qgg now begins to accumulate the enormous amount of
yolk characteristic of selachian eggs ; and during this time
the chromosomes become more and more indistinct. As
shown in the figure (Fig. 21, h-k) they appear to send out
loops laterally, which loops may be only the bendings of a
long thread. When the yolk formation is finished the
chromosomes condense into shorter threads, with lateral
branches (Fig. 21, /). When the Qgg is ripe, the nuclear
wall is absorbed, the chromosomes appear as short rods
(arranged in twos), which place themselves in the polar
spindle. Two polar bodies are given off, leaving the
reduced number of chromosomes in the ^g^.

It is obvious from the preceding account that the sperm
and the Qgg pass through essentially the same stages dur-
ing maturation, the essential feature of which is the con-
jugation of homologous chromosomes followed by their
subsequent segregation. Each sperm and each Qgg is left
with half the original number of chromosomes — one of
each kind.

Lateral Versus End -to-End FtJSiON of the Chromosomes

In the preceding account of the union of the chromo-
somes only one method of union is described, viz., side-to-
side conjugation. The tetrad as represented is due to one
division plane between the conjugating pairs, and the other
due to a longitudinal split of each conjugating member.
But according to some observers, more especially botan-
ists, another method of union also occurs, in which the split
chromosomes unite end to end. If the division planes
in such a tetrad represent respectively the plane of union
at the ends, and the longitudinal split through the united
rods, the final result of this separation would be exactly
the same so far as the four elements of the tetrad are
concerned, but the process would have serious conse-

a

/'

U

W^k

w

c

.♦ il^

/*

' w?(

^ mh m

^■^.

:ja

yn

/*

■-«*i4-

.-,<'

^ - :

xo^!':^^

/■^

>.<

\-'

i»,

-'^^

^./ : Ir

K \ ' t ^'^

■-> •■"

*V**-''^*-^..w

~:t

•

«

•'[ ■"

■■%

•

J

-,./4

^

L«

vSV..

'%■•

i-.r

''-w

'^'^^'^J/'

?;

• .... ^^'^ ^j.

t

•

-i

/

Fig 21 — Synaptic stages, and those immediately following, in the egg of Pristiurus.

(After Mar6chal.)

MECHANISM IN SEGREaATION 51

quence for genetics in so far as the chromosomes represent
the bearers of genes, for while side-to-side union offers
an opportunity for interchange between the paternal and
maternal members of a pair, no such interchange could be
postulated if end-to-end conjugation took place. So far
as segregation is concerned either method supplies all
that is called for.^ A discussion of other matters will
be left until later.

Individuality of the Chromosomes

During the period of cell-division there can scarcely
be any question concerning the persistence of the individ-
ual chromosomes, because they remain visibly distinct
elements in the cell; but when the nucleus re-forms after
each division the chromosomes spin out threads laterally,
and these appear to fuse, making a continuous network
throughout the nucleus. Whether there is actual fusion
between these threads or whether they occupy delimited
contact areas, and whether the branches represent the
essential part of the chromosome concerned in heredity,
are questions impossible to answer at present. The
genetic evidence at least consistently shows that no real
fusion of the hereditary material occurs even in cells that
have passed through many such resting periods. ^

From several other sources there are strong indica-
tions that the chromosomes retain their individuality dur-
ing the resting stage. In Ascaris, where the chromosomes
are few and long, they are often drawn out in an irregular
way in the cleavage cells as they pass to the poles of the
spindle of the dividing cells. Daughter halves of the same
chromosomes show the same identical irregularity.
Boveri has shown by an examination of a large number of
daughter cells (pairs) that are getting ready for the next
division, that when the chromosomes of sister cells reap-

^ If the pairs fused end to end and the tetrad arose by two longitudinal
divisions, the outcome would not be in harmony with the theory of segrega-
tion based on separation of maternal and paternal chromosomes at reduction.

52

PHYSICAL BASIS OF HEEEDITY

pear they show the identical irregularities (Fig, 22, a
to d) . It is probable, therefore, that each chromosome has
retained the particular form that it had when it passed into
the resting stage ; or at least that the axial thread from
which the network was spun out has remained in place.

In a few cases the chromosomes appear more or less
visible during the resting stages. This, however, is such
a rare event that it is doubtful whether it can be appealed
to in support of the view that in other cases the chromo-
somes remain intact.

a o c a

Fig. 22. — Sister blastomeres of Ascaris preparatory to another division, showing similar

arrangements of chromosomes. (After Boveri.)

The most convincing evidence comes from exceptional
cases of accidental or irregular distribution of one or
more chromosomes, so that an ^gg^ or a cell comes to have
one more chromosome than is usually present. In the
thread-worm Ascaris there are two varieties — one that
has four chromosomes in the embryonic cells (with two as
the reduced number) and another variety with two
chromosomes (with one as the reduced number). A few
females have been found in which the unfertilized eggs
contain one of these numbers, and all of the spermatozoa
that have been received from another individual the
other number. In such cases the fertilized eggs, and

MECHANISM IN SEGREGATION

53

all embryonic cells, have three chromosomes each (Fig.
64), showing that when an egg starts with three chromo-
somes, this number is retained through all subsequent
divisions, despite the fact that after each division a rest-
ing stage intervenes.

a

a'

.".•Si Y'*

• 4 • • * •

• • • • ^

Fig. 23. — Normal and reduced chromosomes of Biston. (After Doncaster.)

The evening primrose, CEnothera Lamarchiana, has
14 chromosomes (reduced number 7). Individuals are
known in which there are 15 chromosomes. As a result
of accidental displacement at a division in a germ-cell,
possibly one cell came to contain an additional chromo-
some. Such a cell combining with a normal one, at f ertil-

54

PHYSICAL BASIS OF HEREDITY

ization, would produce a plant of the 15-cliromosome
type. Here again, the additional chromosome persists
as an individual element of the cell throughout subsequent
cell-generations.

a lUnli \ i: Ml \

a

i\i ll

<^»'*V*'^

<fe:

./

/

Fig. 24. — Division figures in egg oi Ctenolabrus 9 . fertilized by Fundulus cf .

In DrosopMla sl female occasionally appears with two
X's and a Y-chromosome. There are several ways in
which this may arise, but the most common way appar-
ently is for an egg to retain both of its X elements. Such
an egg fertilized by a Y-bearing sperm produces an XXY
embryo. Such an embryo retains throughout the entire

MECHANISM IN SEGREGATION 55

development (cell-divisions) its two X's and its Y. There
is evidence for this, obtained by Bridges, both from
observation of the cells themselves and from the genetic
behavior of such an individual.

In certain crosses between moths with different num-
bers and sizes of chromosomes, Federley, and Harrison,
and Doncaster have shown that the cell of the hybrid
contains half the number of each species, even with
their characteristic size differences (Fig. 23). In crosses
between different species of fish, where the size differ-
ences are quite conspicuous, it has been shown by Moenk-
haus, Morris and Pinney (Fig. 24) that the embryonic
cells may continue through their divisions to retain
the characteristic chromosomes of both species. These
hybrid cases are particularly significant; for the chromo-
somes derived from the father are in the foreign
medium of the protoplasm of the other species. Never-
theless, in some cases they retain their own peculiarities,
through successive cell generations.

Evidence That Homologous Chromosomes Mate with

Each Other

That the mating of the chromosomes in pairs is not a
haphazard process, but that each paternal chromosome

m

Fig. 25. — Female and male chromosome groups of Protenor. (After Wilson.)

mates with a definite maternal chromosome, has been
established by evidence from several sources. In many
species the chromosomes are of different sizes, and some-
times certain ones are markedly different in size from the
others. In the bug Protenor the two sex-chromosomes

56

PHYSICAL BASIS OF HEEEDITY

of the female are conspicuously larger than the others
(Fig. 25,^). When reduction takes place the sizes of

Maturation Divisions or Protlnor Q

I

■<?(?>

Mature Eg^ and
PoUy Bodies

Fig. 26. — Reduced chromosome group; and extrusion of polar bodies in Protenor.

Oocjte

V' Polar 5pmdle 2'^ Polar 5piiidle

Maturation Divisions or Protenor cf

y./

^O o

^ly^ "

^.^

III ' 1

Metaphase oj the X^i^ O^ ^

1 ^' Spermatocyte ~;.\\" ^^ r\

Anaphase o| the

P' Spermatocyte m<^©0

Metapbsc o| the -^\ //■ "

2'^'^Sp£rmaloc)/!e '''^'

Anaphase o( the

ord Q . t Spermatozoa
/ Spermatocyte ^

Fig. 27. — Reduced chromosome group of male; and spermatogenesis in Protenor.

the fused pairs show that these two large chromosomes
must always unite with each other (Fig. 26). In the
male of certain species, as in Protenor (Fig. 27), the

MECHANISM IN SEGREGATION 57

sex chromosome has no mate, and therefore nothing to
fuse with. Its size, after the others have conjugated
(Fig. 27) shows that it remains single; while its failure
to divide twice, as do the other chromosomes, corrobo-
rates the view that having no mate of its own it never
combines with any other. At the other extreme, the two
very minute chromosomes in several of the DrosopMla
species must have united to form the smallest chromo-
some of the reduced series (Fig. 28, a-a', h-h'). In a
few cases the X and the Y are different in size. When
they fuse (in the male) the size of the fused mass is what

\^ 6 sj

Fig. 28. — Diploid and haploid chromosome groups of Drosophila busckii, a, a', and D.

tnelanica, {neglecta) b, h'. (After Metz.)

is expected, viz., the sum of the masses of X and Y, and
their subsequent separation into parts corresponding in
size to the fused bodies supports the view that conjuga-
tion amongst the chromosomes is a very definite process.
In the very exceptional case of a bug, Metapodius, there
is a pair of small chromosomes called m's. When the
other pairs enter the spindle the two m's come together,
touch, and then separate, to pass to opposite poles.

Resume

The evidence from studies of the maturation of eggs
and sperm shows that the paternal and maternal chromo-

58 PHYSICAL BASIS OF HEEEDITY

somes come together at this time in pairs, and subse-
quently separate, so that each egg comes to contain
one or the other member of a pair. The same process
takes place in the formation of the sperm-cells. It is
obvious that if one member of any pair contains material
that produces an effect on some character as one of the
end results of its activity, and the other member of the
pair contains a different material, the behavior of the
chromosomes at the time of maturation supplies exactly
the mechanism that MendePs law of segregation calls for.

CHAPTER IV

MENDEL ^S SECOND LAW— THE INDEPENDENT
ASSORTMENT OF THE aENES

Mendel proved that when races differ from each
other in two pairs of characters, each pair considered
by itself alone gives the 3 : 1 ratio, and the inheritance
of one pair is independent of that of the other. If a tall
race of peas with colored flowers is crossed to a short
race with white flowers the offspring show the two domi-
nant characters, i.e., they are tall and have colored
flowers. If these are inbred they produce tall and short
offspring {F2) in a ratio of 3: 1, and these same individ-
nals, if reclassified for pigment, are colored or white in
the ratio of 3 : 1. For example, the ideal for 12 tall peas
would be 9 colored and 3 white; and for 4 short peas
there would be 3 colored and 1 white. Expressed in a
diagram we have :

12 tall 4 short

9 colored : 3 white 3 colored : 1 white

The preceding way of stating the results deals
directly with the facts. The explanation, of these results,
based on the segregation of the members of the two
independent pairs of factors, is as follows: If we call
the gene for tallness by the same name as the character
itself, viz., tall, and the gene for shortness by the name
of this character, viz., short, and similarly for the other
pair of characters, viz., color versus white, then when
crossed the hybrid has two pairs of allelomorphs,

tall color

short white

59

60

PHYSICAL BASIS OF HEREDITY

If at the maturation (whether of egg or spemi) tall and
color go to one cell, then short and white go to the other
cell; but if one of the pairs is turned, so to speak, the
other way, thus

short color

tall white

so that short and color go to one cell, then tall and white
go to the other. Four classes of germ-cells are expected
in 2^1, namely,

tall coior, tall white, short color, short white.

Chance meeting of any one of these four kinds of pollen
grains with any one of the same four kinds of eggs will
give the sixteen recombination classes sho^vn in the fol-
lowing table:

Eggs

Sperm
tall color

tall white
short color
short white

tall color

tall white

short color short white

tall color
tall color

tall white
tall color

short color
tall color

short white
tall color

tall color
tall white

tall white
tall white

short color
tall white

short white
tall white

tall color
short color

tall white
short color

short color
short color

short white
short color

tall color
short white

tall white
short white

short color
short white

short white
short white

The four kinds of eggs are written above and the four
kinds of sperm are written to the left. There are 16 pos-
sible combinations. Since tall and color are dominant
the recombinations give : 9 tall color, 3 tall white, 3 short
color, 1 short white. In this table the genes have the
same name as the character for which they stand, and
these names are written out in full, but it is generally
more convenient to use symbols for the genes in order

MENDEL ^S SECOND LAW

61

to save space and time. It is customary to represent
the members of a pair Tby the same letter, as Mendel
himself did, and to represent the dominant member by
the capital letter, the recessive member by a small letter.
Thus if A = tall and a = short ; and B = color and
h z= white, the recombination square becomes :

Eggs

Sperm
AB

Ah
aB
ah

AB

Ah

aB

ab

AB
; AB

Ab
AB

aB
AB

ah
AB

\ AB

1 Ah

Ah
Ah

aB

Ah

ah

Ah

AB
aB

Ah
aB

aB
aB

ab
aB

\ AB

Ah
ab

aB

ab

ab
ah

Instead of using arbitrary letters for the characters
as above, it has been found more convenient to use a
mnemonic system in which the first letter of one of the
members of each pair becomes the symbol. The two
members of such a pair are then distinguished from each
other by using a capital letter for one and a correspond-
ing small letter for the other. For example, we might
let ^ = short, T=:tall, c = white, C== color. In this
case the capital letter represents the dominant character,
and the small letter represents the loss of that character,
as seen in the recessive type. But besides prejudging the
question as to what kind of a change took place in the
germ-plasm to change a dominant to a recessive by
assuming that it is due to a loss, this system is unsatis-
factory in cases where many modifications of the same
organ exist (such as the 40 eye colors of the vinegar fly),

62

PHYSICAL BASIS OF HEREDITY

and where new ones are being- found. For example, if the
symbol R (red) is nsed for the dominant wild eye color,
small r would stand for any one of 40 mutant eye colors,
and when several of these occur in the same experiment
there would be no way of telling for which one the small
letter stood. Some other system becomes imperative
in such cases, ^ and the most consistent seems to be to
use a small letter for the mutant gene in question (or
when unknown for the recessive gene), and the corre-
sponding capital letter for its allelomorph (usually the
wild type). Thus, 5 = short, <S' = tall, ^^^ white, W =
color. The recombination square for the same characters
treated above is then:

Eggs

Sperm
SW

Sw

sW

sw

sw

Sw

sW

sw

sw

sw

sw

Sw

Sw
SW

sW
SW

sw

sw

Sw
Sw

sW

Sw

sw
Sw

sw

sW

Sw
sW

sW
sW

sw
sW

sw

sw

Sw
sw

sW

sw

sw
sw

Since the large letters simply represent the wild type of
each particular character it may sometimes simplify the
f ormulaB to omit them, or since this may lead to confusion
in making up pairs of genes, some convention for wild
type, such as N (normal), T (type), or the + sign, or a
dash, or a dot may be used. Such short-hand methods
are followed by many workers, but it is not necessary to
advance the claims of any one of them here. If, for

*An even more serious objection to the system is explained in "The
Mechanism of Mendelian Heredity, " pages 233-235.

MENDEL'S SECOND LAW

63

example, the normal, meaning the wild type in each factor
pair, is represented by N, the foregoing table becomes :

Eggs NN

Sperm

NN

Nw
sN

8W

Nw

sN

sw

NN
NN

Nw

NN

sN
NN

sw

NN

NN
Nw

Nw
Nw

sN
Nw

sw
Nw

NN
sN

Nw

sN

sN
sN

sw

sN

NN
sw

Nw
sw

sN
sw

sw
sw

In the preceding illustration one grand-parent (PJ was
assumed to have had both dominant characters (tall and
colored), while the other grand-parent had both reces-
sives (short and white). Obviously the grand-parents
might have happened to be made up differently — one
might have been tall and white, the other short and col-
ored. The Fi plants (Ss, Wiv) would have been the
same in either case, and so would the F2 results. In
other words, for the principle of assortment it should
male having the wild- type color (dominant) is bred to
come. This is illustrated in the following cross (Fig. 29),
in which a wingless vestigial (recessive) Drosophila
male having the wild type color (dominant) is bred to
long- winged (dominant) female with ebony (recessive)
body color. The F^ flies have long wings and wild type
body color. Inbred, they give 9 long wild type color, 3
long ebony, 3 vestigial wild type color, and 1 vestigial
ebony. In the diagram the gene for vestigial is repre-
sented by V, and its allelomorph for long wings by V; the
gene for ebony by e, its allelomorph for wild type color
by E. The germ-cells of the two P^ flies are therefore
vE and Ve. Each contains the wild-type allelomorph
of the recessive mutant gene in the other parent. The

64 PHYSICAL BASIS OF HEEEDITY

Fi fly lias the formula vVeE, Indepeiiclent assortment
of the two pairs of factors

v_ E_

T T

give four kinds of germ-cells both in males and females,
thus :

vE Ve VE ve

Any one of the four kinds of egg may be fertilized by any
one of the same four kinds of sperm giving the same result
as in the case of the peas, viz., four kinds of F2 individuals
in the ratios of 9 : 3 : 3 : 1. In practical tests the occur-
rence of or the possession of a race with both recessives
in it is highly desirable for use in making a back-cross to
jSfl^i (instead of inbreeding F/s), because the numerical
results obtained by back-crossing furnish, for a smaller
number of individuals, more significant data. For exam-
ple, if a tall pea with colored flowers is crossed to a
short pea with white flowers, and the F^ individuals
(SsWiv) are back-crossed to short white peas (siv)y the
expected ratio will be 1:1:1:1, because the four kinds
of gametes in F^ (SW, Siv, sW, siv) will then reveal
themselves in the offspring, since the double recessive
individual (sw) used for back-crossing (having only
recessive gametes) will not ^* cover up*' any of the factors
coming from the F^ hybrid. For instance, as shown
in our type example, the JPi gametes are SW, Sw,
sW, SW, The only kind of gamete produced by 'the
double recessive, short white, is siv. When this meets
each of the above gametes only four kinds of combina-
tions are possible, viz., SWsw, Swsw, sWsw, swsw; and
these zygotes, containing only the same dominants as the
7^1 gametes, will reveal what the kinds of gametes were.
In practice an approximation to a 1 : 1 : 1 : 1 ratio is much
more likely to be evident than an approximation to a
9 : 3 : 3 : 1 in which only one double recessive individual out

MENDEL'S SECOND LAW

65

CZED

mpm CUD

3L

Camrifs.

F^ /- y^ole C5 iind ()

U VJ

_A_

Fxu. 29. — Cross between wingless and ebony vinegar fly.

66

PHYSICAL BASIS OF HEEEDITY

of 16 individuals is expected. Whenever possible, there-
fore, the back-cross experiment is preferable to the
inbred F^ cross.

In animals and in plants with separate sexes it has
been found that both F^^ males and F^ females give when
back-crossed, identically the same results, showing that
free assortment takes place in both sexes.

Fig. 30. — Miniature wing, a; and dumpy, b; and minature dumpy, c.

There is a corollary to the cross involving two pairs
of factors that is interesting, because it gives an explana-
tion of the phenomena of atavism. The wild vinegar fly,
Drosophila melanog aster (Fig. 4), has long wings. It
gave rise, through mutation, to a race with miniature
{mm) wings (Fig. 30, a), and also to another race with
short wings (Fig. 30, h) called *^ dumpy'' {dd). If a
female miniature (mmDD) is crossed to a dumpy male
(MMdd), all the offspring (MmDd) have long wings like

MENDEL'S SECOND LAW 67

those of the wild fly. The miniature fly carries the domi-
nant {DD) wild-type allelomorph of the dumpy gene, and
the dumpy carries the dominant {MM) wild-type allelo-
morph of the miniature gene. Since the hybrid contains
the two wild-type genes {DM) it ^^ reverts'' to the long-
winged fly. The proof that two pairs of factors are in-
volved is found by inbreeding an F^ male and female,
which give 9 long, 3 miniature, 3 dumpy, and 1 miniature
dumpy (Fig. 30, c) fly. ^

There are certain modifications of the two-pair ratio
that arise sometimes when different factors produce a like
effect on the same organ. Such cases have sometimes been
treated as special cases, and rather peculiar interpreta-
tions given to them on the basis that the situation is
unusual. In reality they are only interesting cases of
Mendelian behavior, the results obscured to some degree
by superficial character relations. The absence of
color, albinism, is, perhaps, the most familiar example
of this sort. There are certain recessive factors
that when homozygous interfere in some unknown way
with the development of color. Albinos of the ordinary
house mouse are white because they are homozygous for
the albino factor, although they may be pure for all other
factors that are essential for color. If a certain kind of
albino mouse is crossed to a pure black mouse the off-
spring will be gray because black (&&), being recessive to
its wild-type allelomorph {BB), brought in by the albino,
disappears; and white {ww) being recessive to its wild
type allelomorph for color {WW), brought in by the
black, also disappears, so that the color of the resulting
animal, gray, is due to the hybrid having recovered all
the factors that give this color. The two factor-pairs
involved are black {h) and its normal allelomorph {B ^=
gray), and white {w) and its normal allelomorph
(TF= color). The Fj results, put into the recombination
square, are as follows:

68

PHYSICAL BASIS OF HEREDITY

Eggs
BW

Bw

bW

bw

BW

Bw

bW

bw

BW

BW

BW

BW

gray

gray

gray

gray

BW

Bw

bW

bw

Bw

Bw

Bw

Bw

gray

white

gray

white

BW

Bw

bW

bw

bW

bW

bW

bW

gray

gray

black

black

BW

Bw

bW

bw

bw

bw

bw

bw

gray

white

black

white

Sperm
BW

Bw

bW

bw

The resulting ratio is 9 grays, 3 blacks, and 4 whites. The
last two terms of the 9:3:3:1 ratio are here united in one
class (4 whites) because when homozygous for absence of
color the individual is white, regardless as to whether
the other color-producing factors make for the wild type
of coloration or for some mutant color.

Another interesting two-pair case involves varieties
of the combs of domesticated breeds of fowls. There is a
dominant type called ^^Eose'* (Fig. 41, c), which, bred to
single (wild t^^DC, Fig. 31, a), gives Rose in F^, and 3 Rose
to 1 Single in Fg. Another dominant type called ^'Pea'^
(Fig. 31, h) likewise gives Pea in F^ and 3 Peas to 1 Single
Comb in F2. But when Rose is bred to Pea there is not
produced the wild type, as one might have anticipated, but
a comb called ^'Walnut'' (Fig. 31,d)y that differs from
both parental types. The character is due to the com-
bined action of both dominants. If two Fj birds with
Walnut combs are bred to each other they give 9 Walnut,
3 Pea, 3 Rose, 1 Single comb. This ratio shows that two
factors are involved, and that the Walnut comb appears
in all birds carrying both the Rose and the Pea genes.
The Single comb is the double recessive form.

MENDEL'S SECOND LAW

69

If the single comb be supposed to be the wild type,
then Pea and Rose represent dominant mutant types.

^

Fig. 31. — Combs of fowls, single, a; rose, b; pea, c; and walnut, d.

Neither produces any single comb, if the races are homo-
zygous for Pea or for Rose respectively, but when crossed,
the Pea comb brings in the normal recessive allelomorph

70 PHYSICAL BASIS OF HEREDITY

of Rose, and the Rose comb the normal recessive allelo-
morph of Pea: but the result is not an atavistic normal
comb, but a Walnut produced by the action of both domi-
nants that are here the mutant characters.

An important class of factors that are known as
diluters or intensifiers are often met with in genetic
work. For instance, a black mouse pure for a certain
'^diluting'' factor has a ^'blue'' color (just as a black
mouse pure for albino factors is white). Such a blue
mouse crossed to black gives F^ black mice, and in F2
three blacks to one blue. A two-factor cross results when
a blue mouse is bred to a ^ ^ chocolate ' ' (=black cinnamon)
mouse. The F^ will be black, the Fo will be 9 black, 3 blue,
3 chocolate, 1 '^silver-fawn'^ (dilute black cinnamon).
In this case, the same factor that changes black to blue also
changes chocolate to silver-fawn. If the diluter had
been a specific one affecting black only, then F2 from the
above cross would have been 9 black, 3 blue, 4 chocolate.
Such a case is found in the vinegar fly, in which the diluter
atfects only a recessive factor — eosin. This specific
diluter for eosin is called ' 'whiting. ' ' It gives the follow-
ing results: A red-eyed female homozygous for whiting
is indistinguishable from the ordinary wild type. If a
female of this kind is crossed to an eosin male the off-
spring (2^1 ) are red eyed. If they are inbred they give 12
red-eyed flies, 3 eosin, 1 eosin-whiting which is colorless.

Another modification of the 9:3:3:1 ratio appears
when the last three classes are superficially alike. For ex-
ample, Bateson and Punnett crossed two white flowering
varieties of sweet peas. The F^ had purple flowers, which,
inbred, gave 9 purple and 7 whites. Here there are two
different recessive factors which in homozygous condition
give white, ww and aa; each has a normal dominant allelo-
morph in the other white, AA and WW. The two white
parents are then wivAA and aaWW. The F^ individuals

MENDEL'S SECOND LAW

71

are WwAa, and the four gametes are WA, Wa, wA, wa.
The table below gives the sixteen recombinations
of these gametes :

Eggs

Sperm
WA

Wa

wA

wa

WA

Wa

wA

wa

WA

WA

purple

Wa

WA

purple

wA

WA

purple

wa
WA
purple

WA

Wa
purple

Wa

Wa

white

wA

Wa

purple

wa
Wa
white

WA

wA
purple

Wa
wA
purple

wA

wA
white

wa
wA
white

WA

wa
purple

Wa

wa
white

wA
wa
white

wa
wa
white

Any individual that has both recessives ivw or aa is white.
There are 7 such classes to 9 that carry both A and W,
Lastly, a 15 :1 modification of the 9 :3 :3 :1 ratio is obtained
when an individual homozygous for both pairs of recessive
genes gives a different result from any other combina-
tion. Thus, ShuU found when Bursa pastoi'is, with trian-
gular capsules, is crossed to one with round capsules, the
latter appears in F2 only once in 16 times.

Assortment of Three Factors

When three independent factor-pairs are present the
numerical expectation can be directly derived from the
9:3:3:1 ratio in the same way that the latter was derived
from the 3 : 1 ratio. Thus :

3 ' 1 One pair of factors.

1 Two pairs of factors.

3: 1 Three pairs of factors.

72 PHYSICAL BASIS OF HEREDITY

Each F2 class of the two-factor case (9:3:3:1) will con-
tain a three-to-one ratio for the third factor-pair. Thus,
in the 9 class there will be 3 dominants of the third factor
to one recessive (27:9). So for each 3 class: each con-
tains the third factor in the ratio of 3:1. So also for
the 1 class. The total result therefore is :

In actual practice the three-factor cases are almost
never used. Other methods are employed to detect the
factors present, so that these three-factor ratios have a
theoretical rather than a practical value. In cases where
multiple factors are suspected, some of them may be only
modifiers of some one of the other more conspicuous char-
acters and in such cases special methods of procedure will
recommend themselves.

# V

CHAPTER V
THE MECHANISM OF ASSORTMENT

Each pair of chromosomes, just before the reduction
division, consists of a maternal and a paternal member.
As the members of each pair are in nearly all cases identi-
cal in appearance, it is not possible to tell how they place
themselves on the mitotic spindle with respect to their
parental origin ; that is, it is not possible to tell by inspec-
tion whether at the maturation division all those of mater-
nal origin pass to one pole of the spindle, and all those of
paternal origin to the other, or whether the pairs come to
lie haphazard on the spindle, so that it is merely a matter
of chance whether a maternal or a paternal member passes
to a particular pole. For the utilization of the chromo-
somal mechanism for the theory of assortment, it is a
matter of great importance which of the preceding alter-
natives is followed, for if all of the maternal chromosomes
should go to one pole, and all the paternal to the other,
there would be no free assortment of the chromosomes,
and no free assortment of the genes if these are carried by
the chromosomes. Without random assortment there
could only be two kinds of gametes produced by the
hybrid, hence only three types possible in F2, viz., the two
grandparental tjipes and the hybrid type.

On the other hand, if the assortment of chromosomes
is a random one, then the reduction division furnishes the
mechanism that MendePs law calls for in so far as the
character-pairs lie in different chromosome pairs.

There is not a single cytological fact opposed to the
view of free assortment of maternal and paternal chromo-
somes; on the contrary, there is a general expectation
that the chromosomes should assort freely; and what is
more to the point, there are a few crucial cases that show
that free assortment takes place. Let us turn to these

73

74 PHYSICAL BASIS OF HEREDftY

cases. The most convincing evidence is that furnished
by Miss Carothers (1913) from some grasshoppers of the
genus T rimer otro pis. Here, in addition to the single sex-
chromosome (in the male), that goes to one pole of the
first maturation spindle, there is also present another pair
of chromosomes that are unequal. In some cells the
smaller member of the pair goes to the same pole as the
sex-chromosome, in other cells it goes to the opposite
pole. The assortment of the unequal pair as regards the
sex-chromosome is tlierefore a random one. Thus, in
three hundred first spermatocytes, the smaller partner
went to the same pole as the sex-chromosome in 48.7 per
cent, of cases, and into the cell without the sex-chromo-
some in 51.3 per cent. Voinov ('14), Wenrich ('14) and
Robertson have reported similar cases.

Other evidence of a different kind has more recently
('17) been described by Miss Carothers. The evidence
rests on the constancv of attachment of the fibres of the
mitotic figure to a definite point of the chromosome, as
seen when the chromosomes are moving towards the poles
of the spindle. In one of the cases she describes there
are two kinds of attachments, viz., terminal, when the fibre
is attached at the end of the rod-shaped chromosome, and
suhterminal when the fibre is attached some distance from
the end. In the latter case the end bends over, making
the chromosome J-shaped. There are certain individuals
in which one member of a pair of chromosomes may have
a terminally attached fibre, and its mate have a subter-
minally attached fibre. Throughout all the cell-divisions
of such an individual these two chromosomes show this
difference. During maturation, i.e., after conjugation of
the chromosomes, one member of this pair passes to the
pole of the spindle with a terminal attachment, and its
mate with a suhterminal to the other pole. In the male, the
single sex-chromosome passes to one or to the other pole
at one spermatocyte division. Its relation to the two
members of the pair of chromosomes in question will show

THE MECHANISM OF ASSORTMENT 75

whether random assortment or correlated movement takes
place. Observation shows that sometimes one, sometimes
the other, member of the pair goes to the same pole as the
sex-chromosomes.

It happens that in a species studied by Miss Carothers
(T rimer otropis suffusa) there are several chromosomes
that may show constantly terminal or subterminal attach-
ment of the fibres; as many as ten out of the twelve
chromosomes of the first spermatocyte division may con-
sistently show this difference. In other words, any one
of these ten chromosomes may have one or the other kind
of attachment. Each grasshopper may have any one
of ten of its pairs showing combinations of these kinds of
attachment, but of course in any one individual only two
possible arrangements exist for a given pair of chromo-
somes. It is to be remembered that for a given combina-
tion all the cells of an individual are exactly alike, which
incidentally is a strong argument in favor of the individ-
uality of the chromosomes. An example will give further
details. In Fig. 32 are shown eight groups of chromo-
somes {h, Cy d, e, /, g, h, j) from the same individual. Each
group of 12 chromosomes comes from a single cell about
to divide. Each series of 12 is here arranged in a single
horizontal line. The dividing chromosome is a tetrad, one
of whose halves is about to separate. It is significant to
note that in this case the separating halves represent the
two conjugating members of each pair ; in other words,
the reduction division is taking place. In this individual,
one of the tetrads (No. 10) has subterminal attachment
only, i.e.y for both members of the dividing pair (dyad) ;
four of the tetrads (Nos. 2, ^, 5 and 6) have terminal
attachments only, while the remaining three tetrads (Nos.
1, 7 and 8) have one end with a terminal attachment, and
the other subterminal. In addition there is the sex-
chromosome (No. 4) that is here going upwards toward
the top of the figure, and will pass with the upper half
of each tetrad into an imaginary cell above (the female-

76 PHYSICAL BASIS OF HEREDITY

producing sperm). The first double cliromosome (No. 1)
lias not only a different mode of fibre attachment to
each half, but the halves are also different in size. In
five cases the chromosome with terminal attachment is
going to the cell that will get the sex-chromosome (the
upper one here), while in three cases it goes to the pole
that will not get the sex-chromosome. Chromosomes 7
and 8 are slightly different in size, but this is not distin-
g-uishable in these figures. In the first four cells (vis.,
h, c, d, e) the halves of 7 and 8 with subterminal attach-
ments are going to opposite poles ; in the remaining four
cells if, g,h,i) they are going to the same pole. Again, if
we compare Nos. 7 and 8 with No. 1 it is found that in four
cells (/, g, h, i) the half with terminal attachment passes
into the cell with the same attachment (/ and i) (for 7
and 8), and the other half into the cell with the other
attachment {g and /i). In other words, the distribution
for four chromosomes pairs (1, 4, 7, 8) is here a random
assortment. Let A,B,C represent the chromosomes with
one kind of attachment, and a, h, c their mates with the
other kind of attachment. D is the sex-chromosome and
d its absence. There will then be sixteen possible assort-
ments of these four, all equally probable. Thus:

ABCD aBCD ahCD ahcD ahcd

AhCD AhcD ahCd

ABcD ABcd aBcd

ABCd aBcD Ahcd
aBCd
AhCd

There were 100 spermatocytes recorded by Miss Carothers
as to the distribution of their chromosomes to the two
poles, giving data for 200 cells. Their distribution as
well as the expectation for free assortment is as follows :

Only one chromosome with sub-terminal attachment.
Any three chromosomes with sub-terminal attachment.
Any three chromosomes with sub-terminal attachment.
Any four chromosomes with sub-terminal attachment.

Expected Realized

y*

200

50

48

V4

200

50

84

V4.

200

50

48

\
1 ti

200

121/^

8

THE MECHANISM OF ASSORTMENT 77

12 II 10 9 8 7 6 5 4 3 2 I

Fig. 32. — Eight chromosome groups of twelve chromosomes each of Trimerotropis. (After

Carothers.)

78 PHYSICAL BASIS OF HEREDITY

For a coant of only 100 cells the agreement with expecta-
tion is sufficiently close to show that independent assort-
ment takes place.

In addition to the differences of attachment just exam-
ined there are other differences that Miss Carothers has
studied. A constriction is found in certain chromosomes
in some individuals (Fig. 32, Nos. 3 and 5) that is absent
in other individuals. In some individuals the tetrad is
separating so that the group looks like four beads in a line.
In other cases one member of the pair is not constricted,
while its mate is constricted. Similarly for another
chromosome. In one individual both halves of the dyad
show a constriction, while another individual has one
smooth and one constricted half. These same two kinds
of chromosomes also have in some cases terminal attach-
ment, and in other cases subterminal, making possible
further combinations that can be identified.

Finally, there are two types of subterminal attach-
ment in two chromosomes of the series. In one type the
chromosome is bent further from the end than in the other.
Either of these two types may have a mate of the other
type with terminal attachment, thus giving several fur-
ther identifiable combinations. **A11 possible combina-
tions of the dyads in these two types of heteromorphic
tetrads occur and segregate [assort] freely in relation
to sex. " Miss Carothers points out that when three types
of the same chromosoine exist ^^we have a visible mechan-
ism whose behavior in the maturation divisions corre-
sponds to the segregation of triple allelomorphs.''

In addition to the 12 ordinary chromosomes certain
individuals may have a small thirteenth or even a four-
teenth chromosome. These are called supernumeraries.
In Circotettix they were found present in two of eleven
individuals examined. If present, it, or they, are constant
for all cells except that at the reduction division there may
be a new distribution. If one is present it may go to
either pole with reference to the sex-chromosome, and at

THE MECHANISM OF ASSORTMENT 79

the second spermatocyte division it divides as do the others
at this time in the cell that contains it. If two are present
they do not behave as mates, but at the first spermatocyte
division may both go to the same pole (which may be
either pole in reference to the sex-chromosome), or they
may go to opposite poles. At the second spermatocyte
division each divides independently, and halves go to
opposite poles. These bodies then also move to either
pole without respect to other chromosomes — or at least
without respect to the sex-chromosomes ; but this behavior
can scarcely be used to advantage for the question of
assortment because these chromosomes have no mates (in
the cases so far described) and are so inconstant in their
occurrence that an appeal to their behavior as bearing
on the other chromosomes might not be conceded. If they
are pieces of other chromosomes (the bent ends, for exam-
ple) that have been broken off, we might expect them to
show some relation during synapsis to the original
chromosome from which they came, but as yet nothing
of the sort has been described. If they carry factors that
influence the characters of the individual, their presence,
especially when two occur, would give rise to unexpected
genetic results.

The evidence furnished by cytology that has just been
given makes clear that whenever an opportunity has been
found to study the mode of assortment of the chromosomes
the result shows random distribution. If then the chromo-
somes carry the genes for the hereditary characters, we
should expect that the genes in different chromosome pairs
will ''assort'' independently, and this, in fact, is what
MendePs second law postulated.

CHAPTER VI
LINKAGE

Mendel ^s results involving two or more pairs of char-
acters led to tlie conclusion that distribution of the mem-
bers of one pair of genes is independent of the distribution
of the members of other pairs. This process may be
called free or independent assortment, and is what is
expected if each pair of genes is carried by a different
pair of chromosomes. If this rule held for all pairs of
characters then there could be no more pairs that assorted
independently than there were pairs of homologous
chromosomes. On the other hand, if the chromosomes
carry the genes we should anticipate from what we have
found out concerning* the individuality of the chromo-
some, and from what we know concerning the large
number of inherited characters, that many of these fac-
tors must be carried in the same chromosome. If this
is true, then MendePs second law can have only a very
limited application.

As our information about the mode of inheritance of
characters has widened, the number of cases in which free
assortment does not occur has steadily increased. Many
characters have been found to keep together in successive
generations. This tendency to keep together rather than
to assort freely is called linkage. The most extreme cases
are those where characters hold together completely;
at the other extreme are those that show only a slightly
greater probability of holding together than of assorting
freely. Between these extremes aU intermediate degrees
of linkage are found. For the sake of simplicity, cases of
complete linkage will be dealt with in this chapter; the
others will be taken up in the next chapter.

If a fly (Drosophihi) with two recessive mutant charac-

80

LINKAGE

81

ters, black body color and vestigial mngs (Fig. 33), is
mated to a fly with wild-type body color and long wings,
the offspring (i^\) are wild type. If one of the F^ sons
is back-crossed to a black vestigial female from stock,

Fig. 33. — Back-cross of F\ male (out of black vestigial by wild), to black vestigial.

the offspring {F^) are of two kinds only, half are black
vestigial, and the other half are wild type. In other
words, the two mntant characters that went in together,
black and vestigial, have come out together; and their two
6

82 PHYSICAL BASIS OF HEREDITY

normal allelomorphic characters, wild-type body color and
long wings, have also come out together. There are no F2
flies that are black and long, and none that are vestigial
and gray, as would be the case if independent assortment
took place.

In the diagram (Fig. 33) the results are worked out on
the chromosome theory. The genes for black (5) and for
vestigial {v) are represented as carried by the same
chromosome {hv) -, the homologous chromosome of the
wild-type fly carries the normal allelomorphic genes {BV).
In Fi, one of each of these two chromosomes is present,
and the fly is normal because the two normal allelomorphs
are dominant. In the F^ male these two chromosomes
{hv and BV) separate at the reduction division of the
germ-cells, one going to each gamete. If this F^ male is
mated to a black vestigial female, all of whose eggs carry
genes for black and for vestigial, the offspring should
reveal the composition of the gametes of the F^ male, since
the eggs of the black vestigial fly, containing only two
recessive factors, will not cover up the effects of the fac-
tors contained in the gametes of the F-^ male.

Unless we knew that the two characters black and
vestigial are distinct mutant characters, the preceding
experiment would not necessarily show that the char-
acters are linked, because the same result would have
followed if black and vestigial were both due to the effect
of a single gene. Other experiments, however, show that
they are independent characters.

It is interesting to compare the preceding cross with
another in which black comes in from one parent, and ves-
tigial from the other. For instance, if a black fly with
long wings is crossed to a wild-type fly with vestigial wings
(Fig. 34), the F^ offspring will be wild type both in their
color and in their wings, because the black fly brings in
the normal allelomorph of vestigial, and the vestigial fly
brings in the normal allelomorph of black. If the F^
sons are back-crossed to black vestigial females, the off-

LINKAGE

83

spring are of two kinds only, namely, black long, and wild-
type color vestigial. The combinations that went into
the cross together have come out together. The diagram,

Fig. 34. — Back-cross of Fi male (out of gray vestigial by black) to black vestigial.

based on the chromosomes, shows that the genetic results,
as before, follow the chromosome behavior, provided there
has been no interchange of genes in the male.

For the sake of simplicity only two linked factors were

84 PHYSICAL BASIS OF HEREDITY

utilized in the preceding cases. Three, four, five, or,
theoretically, any number of characters may show this
relation to each other. Thus there is a stock of Droso-
phila with five linked mutant characters, namely, black,
purple, curved, plexus, speck. In a back-cross, like the
one above, all the mutant characters, if they went in
together, will come out together in half of the second
generation (back-cross) flies, and their wild type allelo-
morphic characters in the other half.

There is another way in which linkage may be very
simply illustrated. There are certain characters, called

Diploid Nuclei XX

Gametes X X Y

Fertilization

Zygotes

Fig. 35. — Scheme showing the inheritance of the sex-chromosome in Drosophila.

sex-linked characters, because their factors follow the sex-
chromosomes, or may be said to be carried by them or to
be in them. Now in Drosophila, the female has two X-
chromosomes (Fig. 35), the male one X (and a Y). After
reduction the eggs have each one X chromosome. Any
Such ^gg fertilized by a Z-bearing sperm will produce a
male {XY), as shown in the scheme below. The single X-
chromosome that this male gets is therefore from his
mother. If her X-chromosome carried sex-linked factors,
these should be present in the son. Such, in fact, is the case.
For example, a female Drosophila with yellow wings and
white eyes mated to a wild-type male will produce wild-
type females, and yellow white-eyed sons (like the

LINKAGE 85

mother) . Here the son gets his sex-linked characters from
his mother, since his only X is derived from her. Experi-
ments have shown that this holds for any number of
sex-linked characters that are present in the mother.*

Linkage has been demonstrated in a number of animals
and plants. The first case discovered was in sweet peas.
Bateson and Punnet t (1905) found that when purple
flowers and long pollen grains went in from one parent,
and red flowers and round pollen went in from the other
parent, they tended to come out together more frequently
than would be expected on the two-factor ratio, 9:3:3:1.
In the case of these sweet peas the linkage is not com-
plete, apparently not in either sex. At present two dif-
ferent linkage groups are known in sweet peas, one made
up of three linked characters, and the other of three, pos-
sibly four. In the edible or garden pea there are two
linked characters, and two that are doubtful (Bateson
and Vilmorin, White). Mendel did not happen to make
any combinations of linked characters in this form, hence
he got free assortment. In the primrose (P. sinen-
sis) there is a group of five linked characters (Gregory,
Altenburg) ; in the snapdragon a group of five (Baur) ;
in stocks a group of three or four (Saunders). In the
groundsel {Senecio vulgaris) there are two linked charac-
ters known; other cases occur in com (Lindstrom),
tomatoes (Jones), wheat (Engledow), oats (Surface),
Oenothera (DeVries, Muller ) . In animals the largest num-
ber of linked characters is found in the vinegar fly, Droso-
phila melayiog aster, in which there are four groups— a
sex-linked group containing about 100 characters, a second
group containing 75 characters, and a third group contain-
ing about 60 characters, and a fourth group of two charac-
ters. In other species of Drosophila, linked characters
(other than sex-linked) are beginning to be reported as
more characters are studied (Metz in D. virilis, Warren

* A reservation for crossing over in a heterozygous motlier must be
added to this statement.

86 PHYSICAL BASIS OF HEEEDITY

in D. huschii, Sturtevant in D, repleta). Nabours has
found a case in one of the grouse locusts, and Castle and
Wright in rats. In the silk-worm moth, Tanaka has found
one group of linked characters. In poultry Goodale has
found one case. In the moths and poultry it appears
that linkage is complete in the female, incomplete in the
male. In this respect the situation is the reverse of
that in Drosophila. There are some other cases where
linkage is suspected but uncertain.

The fact that relatively so few cases of linkage have
been as yet reported is due in part to the fact that in
most species the heredity of only a very few charac-
ters is generally known. Where more are known each
has as a rule not been examined in relation to all the
others, so that even if some of the factors were linked it
would not have been found out. Furthermore, in Mendel-
ian crosses, the practice of mating F^'s instead of back-
crossing, tends to conceal the linkage phenomena if pres-
ent. The fact of greatest significance, however, is that
the number of cases of linkage is steadily increasing as
the inheritance of more characters in each species is
becoming known.

CHAPTER VII
CROSSINa OVER

The correlative aspect of linkage is crossing over, and
inasmuch as it involves a change in the mechanism that
gives linkage, it is entitled to rank as one of the funda-
mental principles of heredity.

In the illustration of complete linkage given in the
preceding chapter, the cases chosen were ones in which
entire chains of genes remained intact during the reduc-
tion divisions. The male of Drosophila exhibits this phe-
nomenon, as does also the female of the silk-worm moth.
On the other hand, there is an interchange of blocks of
genes between homologous pairs of chromosomes in other
cases, as in the females of Drosophila, in the males of
moths and fowls, and in both egg-cells and sperm-cells
of the plant Primula} This interchange is called crossing
over, and the evidence shows that it is not haphazard, but
gives numerical results of extraordinary constancy. A
few examples will suffice to illustrate crossing over.

When a black fly with vestigial wings is crossed to a
wild-type (^^gray'') fly with long wings (Fig. 35) the
offspring are, as we have already seen, ''gray,'' long.
If one of the F^ females is back-crossed to a black ves-
tigial male there are four kinds of offspring produced, viz.,
the two original combinations^ black vestigial, and gray
long; and in addition two recombinations of these, viz.,
black long, and gray vestigial. The two latter classes are
called the crossover classes, or more briefly, crossovers.
The percentage of crossovers is definite for a given stock,

^Crossing over in both sexes in the rat has been reported by Castle
and Wright, and in the male and female grasshopper by Nabours.

87

88

PHYSICAL BASIS OF HEREDITY

of a given age and under given environmental conditions.
In this case the percentages are as follows :

Crossovers
Black long Gray vestigial

8.5 per cent 8.5 per cent

Non-crossovers
Black vestigial Gray long

41.5 per cent 41.5 per cent

83 per cent

17 per cent

If a pair of chromosomes in the F^ fly is represented as
carrying the genes of the characters here involved, one
member of such a pair carries both a gene for black and
a gene for vestigial (Fig. 36) ; the homologous member
of the pair of chromosomes carries both of the normal
allelomorphs, viz., a gene for gray and a gene for long
wings. When crossing over takes place so that a gene
for black goes over into the other chromosome, the con-
verse phenomenon takes place, a gene for gray goes over
into the chromosome that gave up its black gene. It is the
constancy of this interchange that makes the phenomenon
reducible to exact mechanical treatment.

The interchange is independent of the way in which the
genes enter the cross. For example, if a black long-
winged fly is crossed to a gray vestigial fly (Fig. 37), the
jPi offspring will be, as before, gray long. If an F^
female (gi'ay long) is back-crossed to a black vestigial
male, there will be four kinds of offspring, namely, the two
original combinations black long, and gray vestigial;
and the two crossover combinations, black vestigial, and
gray long, in the following proportions :

Non-crossovers
Black long Gray vestigial

41.5 per cent 41.5 per cent.

83 per cent

Crossovers
Black vestigial
8.5 per cent

Gray long
8.5 per cent

17 per cent

The interchange in the last cases is in the reverse order
of that in the first case, but it is numerically identical.
In other words, it makes no difference whether the gene
for black and for vestigial enter the cross together, i.e..

CROSSING OVER

89

Fia. 36.— Back-cross of Fi female (out of black vestigial by wild) to black vestigial male.

in the same chromosome, or whether they enter the cross
in opposite chromosomes— their likelihood of interchange
is exactly the same. If the F^ male had been back-crossed
(Fig. 34) only two kinds of offspring would have been

90

PHYSICAL BASIS OF HEREDITY

Fig. 37. — Back-cross of F2 female (out of gray vestigial by black) to black vestigial male.

produced because, as was shown, there is no crossing
over in the male.

It should be pointed out here, that the interchange (or
crossing over) can of course only be recorded when two

CROSSING OVER

91

or more pairs are involved, for, obviously, unless a char-
acter that enters the cross comes in with some other known
one that is recognizable as such, there is no way of de-
termining whether interchange between the homologous
chromosomes has taken place. As will be pointed out
later, there is every reason to suppose, and practically
a demonstration of the fact, that the interchange goes on
irrespective of the presence of other genes by which it
can be observed.

Experiments with different pairs of characters show
that for each two pairs there is a definite numerical ratio.
For instance, if a female fly with yellow wings and white
eyes is crossed to a fly with gray wings and red eyes
(wild type) the daughters will have gray wings and
red eyes (wild type). If the F^ female is back-crossed to
a male with yellow wings and white eyes,, four classes of
offspring will be produced in the following proportions :

Crossovers
Yellow red Gray white

0.5 per cent 0.5 per cent

Non-crossovers
Yellow white Gray red

49.5 per cent 49.5 per cent

99 per cent

1 per cent

Here, crossing over takes place in only one case out of
a hundred. If the characters enter in a different combina-
tion, viz., yellow red and gray white, the crossover per-
centage is the same as before, viz.y

Non-crossovers

Yellow red Gray white

49.5 per cent 49.5 per cent

■fc.^ ■IIMIiH -M ■■! ~V '' —■■-■■-■■■ ■ ■*

99 per cent

Crossovers
Yellow white Gray red

0.5 per cent 0.5 per cent

1 per cent

Another combination of white eyes with a different charac-
ter shows a different linkage. If a female fly with white
eyes and miniature wings is crossed to a male with red
eyes and long wings (wild type), the F^ daughters will
have red eyes and long wings. If one of these F^ females

92

PHYSICAL BASIS OF HEREDITY

is back-crossed to a white miniature male the four classes
of offspring appear in the following proportions:

Non-crossovers
White miniature Red long

33.5 per cent 33.5 per cent

67 per cent

Crossovers
White long Red miniature

16.5 per cent 16.5 per cent

33 per cent

Here crossing over takes place in 33 out of 100 flies,
whereas in the former crosses between white eyes and
another mutant character (yellow) crossing over took
place only once in a hundred times. Based on these
numerically different ratios of crossing over, and on other
related phenomena, a theory of crossing over has been
formulated that will be discussed later. For the present
we are concerned only with the data.

"When more than two pairs of characters are involved
new phenomena of crossing over make themselves evident.
Some of these are more related to principles that are dis-
cussed in later chapters, but a few results may be pointed
out here. When, for example, three pairs are involved,
two may cross over, while the third does not. A female
with white eye color, miniature wings, and forked bristles
crossed to a wild-type male gives wild type in F^. An
t\ daughter back-crossed to a white miniature forked male
will give, in the next generation, not only the two original
combinations but recombinations also. As we have seen,
33 per cent, of all the offspring will be crossovers between
white and miniature; there will also be 20 per cent, of
crossing over between miniature and forked. In other
words, there will be both red miniature and white long
flies, and there will also be crossovers between white and
miniature, i.e.^ miniature wings straight spines, and long
wings forked spines. It follows that there may also be
cases in which crossing over has taken place between both
of the above combinations at the same time (Fig. 38),
that is, there will be some flies that are white long- winged
and forked and others that are red miniature and straight

CKOSSING OVER

93

spines. A list of these classes with the expectation based
on the results from a single experiment is given below.

Non-crossover
w mj 23.2

W M F23.2

Double crossover
in both ( ^^ ^/ 3.3
regions \w mF^.Z

Single crossover

in 1st (-^^^13.2

^^g^^^ \W mf 13.2

in 2nd f ^ ^ ^ ^■'^
region \^ ^ f 6.7

Inasmuch as this subject and certain peculiarities in
the results can be better understood after the evidence for

w

_j

/

1=;:

/

FiG.38. — Scheme to illustrate double crossing over between white and forked. The gene
for miniature standing between furnishes the e^^dence.

the linear order of the genes has been discussed, I shall
not press further the discussion here. It should be pointed
out, however, that the question of crossing over involves
more than the independent action of the pairs in the cases
so far considered; for, as will be shown later, when cross-
ing over takes place not single genes but great groups of
genes are involved. This block effect, as it may be called,

94 PHYSICAL BASIS OF HEREDITY

is not in evidence unless a larger number of genes than
two is studied in the same experiment. These questions
will be discussed in Chapter IX.

For the purpose of clearer exposition I spoke of link-
age, in the preceding chapter, as though the term should
be limited to cases where all the genes of a group hold
together, and have used the term crossing over to mean
the breaking of the group in one or more pairs. As a
matter of fact, it is not desirable to emphasize this sharp
distinction. There is, however, a real distinction that lies
behind the phenomena. In the male of Drosophila there
is no crossing over at all between homologous chromosome
groups, while in the female there is crossing over between
the pairs of chromosomes. The cases of the male and
female are, therefore, on a different footing here.

We speak of pairs of characters as being loosely linked,
meaning that crossing over of genes frequently takes
place, and as strongly linked when crossing over is very
infrequent. We have seen that yellow and white break
apart only once in 100 times. If characters (or the genes)
were still more closely linked, they might break only once
in a thousand times, and if closer still once in many
thousand times, in which case they would appear to be com-
pletely linked for all practical purposes. Such a grada-
tion, however, does not appear to be the case, but the lower
limit of crossing over seems to be well within the range of
human capacity to detect. This means probably that
there is a limiting value for crossing over, and if this
can be established it may give us the lower limit of size
of the gene (in terms of chromosome length), or at least
it may allow us to form some idea as to how many genes
are present in the hereditary material.

In this same connection it has been suggested that
when more than two allelomorphs occur, we may be deal-
ing only with close or even absolute linkage. For instance,
suppose in a white-eyed race of flies a mutation should
take place in a gene so closely tied up in some way to the

CROSSING OVER 95

gene for white that the two never separated, and suppose
the new mutation affected the eye so that its effect could
be observed (for if not the change would not concern us).
The new mutation would behave towards white in the
same way as do all pairs of allelomorphs and yet in a
strict sense is not allelomorphic. It is not necessary to
elaborate here this idea, because fortunately in the case
of Drosophila there is strong evidence to show that multi-
ple allelomorphs do not arise in this way. The evidence
for this statement will be given in Chapter XVII.

CHAPTER VIII
CROSSING OVER AND CHROMOSOMES

There are several occasions in the maturation period
of the germ-cells when it would seem that there might be
an opportunity for an interchange between like chromo-
somes. Such an occasion might be found at the time
when the thin threads twist around each other; or it
might be found after fusion of the threads, or possibly
after a general breaking up of the chromosomes and
reunion of the pieces. Unfortunately the cytological
evidence does not furnish explicit information as to the
stage at which interchange takes place.

It has also been suggested that crossing over may take
place at a still earlier stage in the germ-tract, i.e,, long
before the time of maturation, even in the early embryo.
Fortunately, it has been possible to obtain critical genetic
evidence showing approximately the time when crossing
over takes place. This evidence was obtained by Plough in
his work on the influence of temperature on crossing over
in Drosophila melanog aster.

The way in which Plough's experiment was carried
out was as follows : Females homozygous for three mutant
factors in the second chromosome, viz., black, purple,
curved, were mated to wild-type flies. Some of these
females were kept in an incubator, some in an ice-box,
and some were kept at room temperature; under one
or the other of these conditions they laid their eggs
which hatched and produced larvae and pupae and flies. The
daughters were then mated to black, purple, curved males,
and remained under the same temperature conditions until
their offspring hatched. It was found that there was
more crossing over in the offspring of the pairs kept at a
high and at a low temperature than in those kept at room

96

CROSSING OVER AND CHROMOSOMES 97

temperature. Later tlie crossing over values for inter-
mediate points was also obtained, and from these data
the curve shown in Fig. 56 was made.

At a low temperature (about 10° C.) crossing over is
increased as compared with a somewhat higher tempera-
ture (18-27° C). Room temperature (22° C.) lies in that
part of the curve where there is the least amount of cross-
ing over. The amount then rises suddenly until about 29°
and remains high till 31° C. is reached. The apparent
fall after this temperature, as shown by the curve, may
not be significant. The flies fail to lay eggs or may die
at about this point.

In the foregoing experiment the eggs, larvae and adult
flies had been kept continuously at the same temperatures.
If, however, the heterozygous virgin females reared at
high temperature are back-crossed to the triple recessive
males, and kept afterwards at normal temperature (22°
C.) it is found that only the first ten-day output of such
females shows the high crossing over values. The value
drops during the following ten days. If a correction
is made for a change in the crossing-over value due
to age — since age, as Bridges has shown, causes a lower-
ing in the value — still the effect of the early period is
found to have begun to disappear after ten days, and
soon completely disappears.

In still another way the influence of temperature may
be shown. Heterozygous females that had lived at nor-
mal temperatures are mated to triple recessive males, and
then exposed for the first seven days to 31.5° C. At first
the normal crossover values are found, as seen on com-
paring Fig. 40 mth Fig. 39 which is the control. The
latter drops slightly from the second to the eleventh day.
About the eighth day the heat effects begin to show (Fig.
40), and there is a sudden and considerable rise in the
curve, that lasts for ten days, when it drops back to normal,
corresponding with the removal of the flies from the high
to normal temperature, i.e., after the seven-day exposure.

7

98

PHYSICAL BASIS OF HEREDITY

From data of this kind it is possible to locate the stage
in the development of the egg when the heat is affecting it.
If, for instance, we know how long after subjecting a

Percentage of
crossing over

21

19

17
16

13
11

e

7
6

1

r

- Purple-curved

sa

1

( • Biack-puroie

4-6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

Days

after mating

FiQ. 39. — Curve showing the influence of temperature on crossing over; control.

(After Plough.)

Percentage of
crossing over

20,

Purple-curved

2 4 6 8 10 12 14 (6 18 20 22 24 26 28 30 32 34 36
FiQ. 40. — Curve showing the influence of temperature on crossing over. (After Plough.)

female to a high temperature, the effect of heat on crossing
over begins to be observed in her offspring, and also how
many eggs are laid by the female before this influence
is manifest, we can tell approximately in what stages heat

CROSSING OVER AND CHROMOSOMES 99

affects crossing over. Furthermore, if we keep eggs,
larvae and pupae in a high temperature, and then find out
how many eggs have been affected by the high tempera-
ture, we can find out to what stage the eggs must have
developed in order that crossing over may be influenced.
Plough has made this calculation, and finds that only the
eggs that have reached the stage where conjugation of
the chromosomes takes place are affected — all the earlier
stages are not influenced. It follows that the initial effect
appears at about the time of conjugation of the chromo-
somes, but whether the crossing over occurs at this critical
stage or some effect only is then produced that later
affects the crossing over is not specifically shown. Never-
theless, I am inclined to think it more probable that the
crossing over is actually changed at the time the heat acts
(rather than afterwards), because in general most reac-
tions of living things to environmental influence take place
immediately rather than after a long interval. However
this may be, the fact of prime importance in this work
is that earlier than the period of conjugation of the
chromosomes crossing over does not take place.

Expressed in numbers of eggs, the results show that in
a just-hatched virgin female there are from 125 to 175
eggs that will be laid before the effects of heat are shown.
In the females that have just hatched about 150 eggs are
present that have passed beyond the conjugation period.
This number (150) agrees with the estimated number of
eggs (125-175) first laid that are not affected, and estab-
lishes the conclusion that after conjugation of the chromo-
somes crossing over cannot be influenced any more than
it could before that period. The results clearly establish,
then, that crossing over cannot be affected earlier than the
conjugation, but can be affected at the time when the
conjugation is known to occur.

As already pointed out, the chromosomes become
drawn out into long threads at the synaptic period, and
in many animals and plants these threads have been shown

100 PHYSICAL BASIS OF HEREDITY

to place themselves at this time in pairs. The double
threads shorten later to take on the form of the ordinary
chromosome. How the earlier, long thin thread (lepto-
tene thread) is changed into a thick thread when the
chromosomes condense is not known. According to sev-
eral accounts the thread coils spirally within the wall of
the ''chromosome/' at first in a loose coil, then in a tightly
twisted coil. This idea of a coiled thread, or core, in a
condensed chromosome is one that fits in very well with
the idea that the thin thread represents the essential ele-
ment in the chromosome that retains its original continuity
even when the chromosome is condensed into a short rod
or even into a ball. Unfortunately the evidence in favor
of this view is by no means well established.

At the time when the threads conjugate, the evidence
in several forms, such as Batracoceps, Tomopteris, etc.,
shows that when the conjugating pairs are U-shaped, the
union begins apparently at both ends of the U at the same
time. When the chromosomes are rod-shaped (in the last
telophase) the evidence fails to show whether the union
begins at both ends simultaneously or at one end only.

As the union between the threads progresses the parts
not yet united can often be seen to be twisted about each
other. They not only overlap, but they seem to be wrapped
around each other.

Whether the threads are split lengthwise before their
union can not be stated for all cases. It is certain that
no splitting has been seen in several animals, but in one
case (Ascaris) the threads have been found to be split
lengthwise before they conjugate.

For a short time following the union of the threads
they come in close contact with each other, and give the
impression of having fused into a single thread. Usually
before the nuclear wall breaks down to release the thick
threads, a split can be seen again extending throughout
the length of the threads. Not infrequently another
longitudinal split appears in each half resulting from the

CROSSING OVER AND CHROMOSOMES 101

first split, so that four parallel strands appear. It is
customary to call the split, that is supposed to correspond
to the line of union of the maternal and paternal chromo-
some, the primary split or reductional split, and the split
that corresponds to the longitudinal division within the

Fig. 41. — Diagram showing crossing over of two chromosomes at the four-strand stage,
a,b, and the subsequent opening out of the tetrad, d.

maternal or the paternal chromosomes, the secondary or
equational split. Only in very special cases is it possible
to be able to say which is the primary and which is the
secondary split. In fact, whenever crossing over takes
place in the four-strand stage this distinction fails to

have much meaning.

102

PHYSICAL BASIS OF HEREDITY

There are certain questions connected with crossing
over that are illustrated by the following models (Figs.
41, 42, 43) . In these models of tetrads the dotted rod, split
lengthwise, stands for a maternal chromosome, and each
of its halves- may be called a strand. The split in the rod
is the secondary (or equational) split. The black rod,
also split lengthwise, stands for the paternal chromosome.

In Fig. 41, a, the two split rods are represented as
twisted about each other. If the two inner strands break
and the cords interchange at the levels, where they first
come into contact with each other (Fig. 41, 6), and then

Fig. 42. — Scheme showing the opening out of the strands of the tetrad, a, in two planes,

b, according to Robertson and Wenrich.

later the four strands come to lie side by side, i.e., ^'fuse,''
the result will be that shown in Fig. 41, c. Two of
the strands represent crossovers in the sense that an
interchange has taken place between a maternal and a
paternal strand ; and if at the first spermatocyte division,
when the threads begin to pull apart, the maternal sepa-
rate from the paternal threads, two threads may be seen
actually crossing each other (Fig. 41, d). They are here
the two non-crossover strands, but if the two strands
thrown to the left had been thrown to the right the two
crossover strands would cross over. The scheme is essen-
tially the same as the chiasma of Janssens, but the strands
that cross may or may not (as here) represent the cross-
over strands.

CROSSING OVER AND CHROMOSOMES 103

The next two figures (Fig. 42, a, h) show how Robert-
son and Wenrich interpret the crossed threads, that they
have observed in the spermatogenesis of some of the
grasshoppers. The four strands are represented as con-
jugating side by side in Fig. 42, a. When the strands
begin to open out preparatory to the first spermatocyte
division the two maternal separate from the paternal at the
ends of the tetrad, while in the middle of the tetrad the
opening up involves the separation of a maternal and a
paternal strand from a maternal and a paternal. In
other words, the tetrad opens up in two planes at right
angles to each other. This scheme also gives an appa-

d

Fig. 43. — Scheme showing crossing over involving both strands of each chromosome.

rent crossing of the strands at the level where the open-
ing out in one plane passes over into the opening out
in the other plane, but there has been no real crossing
over of the strands in the sense of interchange between
them. Theoretically this explanation is sound, and
moreover seems to be supported by observations in
cases where the maternal and the paternal strands can
be identified. The results undoubtedly show that the
occurrence of crossed threads in cases where the split
occurs in two planes does not necessarily imply that
crossing over has taken place ; but, on the other hand, as
has been shown (in Fig. 41) a similar figure may also
necessarily result after crossing over of the threads. In
a word, the crossed-strand stage is not ipso facto evidence
that it must have come about according to Robertson's

104 PHYSICAL BASIS OF HEEEDITY

scheme. It should also be observed that the scheme rests
on the assumption that no tmsting has preceded the stage
of the crossed threads, or, if such has taken place it
has no relation to the resulting chiasma. Yet crossing
of the threads is an observed fact.

A third scheme (Fig. 43, a, h) makes both maternal
strands interchange with both the paternal ones. This
scheme has at least one formal advantage over the other
two in that it represents the four strands, after crossing
over, as in position to lie side by side in the tetrad, so that
the tw^o longitudinal splits that reappear later lie in the
same plane throughout their length. This seems more in
accord mth many of the observations that are recorded.
If, during the following stages, the tetrads open out by the
separation of the maternal from the paternal strands the
crossed threads that result correspond to those in the first
scheme (Fig. 41). At present it is not possible to decide
between these different modes of representing crossing
over. They may all occur. Their discussion shows little
more than certain possibilities involved in the situation.

Details of Spekmatogenesis

Some of the stages in the spermatogenesis of a grass-
hopper, Phrynotettix, as described by Wenrich, are shown
in the following figures. The material furnishes certain
details concerning the ''resting stages^' of the nuclei
preceding synapsis more completely than any other, and
it serves also to illustrate clearly the relationship of the
chromosomes to the vesicles into which they pass (or
which they form) during the resting stages. The figures
also show how the threads emerge from the vesicles in
which they appear to have been contained during the
resting stages, and how the opening out of the tetrads
in two planes gives the appearance of chiasma accord-
ing to Wenrich.

During the time when the germ-cells are increasing in
number by division there is a resting stage after each

CROSSING OVER AND CHROMOSOMES 105

division during which the chromosomes expand into a
sort of vesicle, as seen by comparing Figs. 44, a and
44, h. An optical cross section of the stages shown
in the last figure is represented in Fig. 44, c. An
older stage is seen in Fig. 44, d. The stage of greatest
diffusion of the chromatin material within its vesicle is
seen in this figure, where the outlines of each vesicle are
still visible. As the nucleus gets ready for another divi-

x^^p^:^^^^^

d

Fig. 44. — Spermatagonial cells in the last phase of division, c, and the following resting

stages, 6, d. (After Wenrich.)

sion the vesicles become more distinct (Fig. 45, a, h), and
soon a coiled thread can be seen to be present in each
vesicle (Fig. 45, c). As the thread thickens (Fig. 45, d),
a longitudinal split appears in it, wliich indicates the plane
of division of each chromosome at the next division.

At the last spermatogonial division, the chromosomes
of the two daughter nuclei form vesicles, as they have done
in earlier divisions (Fig. 46, a and h). But changes begin
to take place that carry the chromosomes through a very
different series of stages from those seen in preparations

106

PHYSICAL BASIS OF HEEEDITY

Kia. 45. — Cells emerging from the resting stages preparatory for the next spermatagonia

division. (After Wenrich.)

mA

f'it

a

Fia. 46. — Cells emerging from their last spermatagonial division, a. b; passing into the

synapsis stage, c, d; (After Wenrich.)

CROSSING OVER AND CHROMOSOMES 107

for the ordinary spermatogonial (or somatic) cell-divi-
sions. Each chromosome vesicle begins to show a coiled
thread (Fig. 46, c). Each thread next becomes longer
and longer (Fig. 46, d) until the whole nucleus is filled
with them. One or both ends can often be seen at the
** distal pole'' of the cell, where deep-staining nucleoli
are present. The cells are now in the so-called thin
thread, or leptotene stages.

The threads next come together in pairs beginning
at the distal end of the chromosomes (the zygotene stage.

FiQ. 47. — Formation of a thick thread after synapsis, a, h; and the following condensation

of a tetrad, c. (After Wenrich.)

Fig. 47, a). When the fusion is complete and all the
threads are double (Fig. 47, &), the stage is called the
thick thread or pachytene stage. There are half as many
threads now present as at the beginning. A longitudinal
split is present in the chromosome throughout these stages
along the line of fusion of the two thin threads. Wenrich
identifies the split as the ^ ' primary split. ' '

Another longitudinal split at right angles to the other
one soon appears (Fig. 47, c), thus forming tetrads, each
composed of four chromosomes. The tetrads next shorten.

108 PHYSICAL BASIS OF HEEEDITY

opening out in various ways to produce figures like those
shown in Fig. 47, c.

The sex-chromosome {X) that has no mate in the
Plirynotettix male, and hence has not conjugated, has only
one longitudinal split (a dyad). The cell, the primary
spermatocyte, with its nucleus next divides. Eleven auto-
somes go to each pole, and the sex-chromosome failing
to divide at this time goes to one daughter cell only. The
secondary spermatocytes are produced — half with 12, half
with 11 double chromosomes. A short resting stage follows
— the chromosomes again becoming diffuse, ie., forming
vesicles. They soon reappear and a second division takes
place, producing the spermatids — the daughter cells of
the secondary spermatocytes. Half of these have 12, half
11 chromosomes — the X-chromosome having divided at
the second division.

Wenrich found it possible to identify certain of the
chromosomes and was thus enabled to follow a few of
them through several successive stages. Eight consecu-
tive stages in the history of chromosome ^'B^' of Pliryno-
tettix are shown in Fig. 48. Indications of the primary
split are present in a, h, c, the secondary split appears
first in d. The evolution of the thread continues as the
tetrad becomes placed in the spindle in such a way that
the first separation of the chromosomes takes place along
the secondary split, i.e., the first division is equational.
Wenrich found in several other individuals of this species
that this same chromosome pair ^'B^^ consist of unequal
members as shown in Figures 48, 2 Or-h and 3 a-d. In
48, 2 (7 a distinct crossing of the threads is present. The
shape of the contracted chromosome (f g h) and its posi-
tion on the spindle show that one of the longer, and one
of the shorter strands passes to one pole, and similarly a
longer and shorter to the other pole. The division here is
in the plane of the secondary split, i.e., equational. The
inequality in length of the conjugating pair makes this
conclusion certain in this case.

CROSSING OVER AND CHROMOSOMES 109

In the second division of this chromosome the longer
thread separates from the shorter one— the second is
therefore rednctional. It is evident, especially from this

\i b

d

f

g

h

d

f

h

a

d

a

Fig. 48. — A pair of chromosomes "B" in eonjugation. 1; thr same pair in con,iu?a-
tion in another indi\'idual in which one chromosoine is shorter than the other, 2; same
in a third individual, 3; later stage showing chiasma of threads. 4. (After \N enrich.)

last example, that the crossing of the threads is not an
indication that the division of the chromosome is neces-
sarily different from what it is when there is no such
crossing. What is more important is that the crossed

110 PHYSICAL BASIS OF HEEEDITY

threads furnisli no proof that an interchange must have
taken place earlier, but neither does it furnish any evi-
dence that interchange had not taken place. For example,
the most obvious interpretation of Fig. 48, 2 d is that
the upper end of the tetrad has separated in the plane
of the secondary split (in anticipation, as it were, of the
separation about to take place in this plane) , and has sepa-
rated in the lower part of the same tetrad in the plane of
the primary split. This interpretation does not involve
any real crossing over in the sense that the two crossed
threads had previously broken and interchanged, as Jans-

FiQ. 49. — The same chromosome pair in conjugation from thirteen different cells. (After

Wenrich.)

sens' chiasmatype assumes on the ground that the two
granules (threads) in contact at the upper end of the
tetrad must be related to each other in the same way as
are those further back in the tetrad.

This last assumption is the foundation of Janssens'
view, but has no longer sufficient evidence to support it,
even though none opposes it. Nevertheless, it should be
clearly understood that evidence such as this, derived
from Wenrich 's results, can not possibly be held to show
that an earlier interchange or crossing over has not
occurred. If it had, such a figure as this (c) would, as
explained above, be a consequence to be expected.

The constancy of the beading of the chromosomes in
each individual is most remarkable. Its significance for

...5.-

CROSSING OVER AND CHROMOSOMES 111

the linear order of the material of the chromosomes cannot
be overestimated. As a further example Wenrich gives
identical stages of the same chromosomes (Fig. 49), each
of the figures is from a different individual. The identity
in size and in location of the principal beads in the series
is obvious.

Robertson has also brought forward a case of an
unequal pair of chromosomes and interpreted the facts as
opposed to the crossing-over hypothesis. He found two
cases in a grasshopper of the genus Tettigidea in which
there was a very unequal pair of chromosomes. The
shorter piece conjugated consistently with only one part
of the longer chromosome, as shown in the next figure

a

d

I

Fig. 50. — Conjugation of an unequal pair of chromosomes and their subsequent separatic r

(After Robertson.)

(Fig. 50, a, &). At the first maturation division the two
chromosomes separated, as shown in (c, (i, e) . It Avould be
difficult to find a more excellent illustration of the per-
sistence of the individuality of the chromosomes after con-
jugation, and the case falls equally in line with the view
that conjugation takes place only between those parts of
the chromosome that are alike, ie., composed of the same
series of genes. How, then, could this case, so admirably
suited to support the chromosome theory be turned against
the chiasma theory! Only, I think, through a misconception
of the essence of the theory. Robertson says: ''In both
types of unequal tetrads we have very strong evidence that
the homologous chromosomes, on entering the side-to-side
pairing process of synapsis, remain as distinct individ-
uals, retain their identity throughout the period, and come

112 PHYSICAL BASIS OF HEREDITY

out of it with at least the same size they had on entering
it. Each pairing chromosome maintains its distinct indi-
\dduality during this period." This is opposed to the
idea of Janssens ( '09) and Morgan ( '11), as expressed in
the theory of * ' chiasmatype. ' ' In their theory they assume
that homologous chromosomes in parasynapsis twist about
each other and fuse. On splitting, a plane passes down
the fused body, regardless of the previous spiral fusion
plane, resulting in two daughter chromosomes which may
not be identical with the two chromosomes which entered
the process. Each new one may contain parts of both
original chromosomes. If such had been the case, the
separation or formation of a short and a long chromo-
some out of the first chromosome with such regular-
ity of size, etc., as we have shown, could not have
occurred. On the contrary, even if crossing over had
occurred within the region where the short and the
long pieces came together, the separation would be
expected still to be exactly that described by Robertson ;
for the genetic evidence points very clearly to the con-
clusion that the interchange involves exactly equal and
opposite parts. There is no reason to suppose that
regions outside the conjugating region would be affected ;
on the contrary, all the genetic evidence would lead us to
expect no such effects.

Summary of Evidence

If we have found Janssens' evidence inadequate as a
demonstration of crossing over, what other evidence is
there in the history of the chromosome to which an appeal
can be made? First, there is the undisputed fact that at
the time when the chromosomes come together they spin
out into long, thin threads which, as they meet, lie over
and under each other, so that the line of fusion is in a
spiral plan. Later, when the fusion is complete, it is no
longer possible to follow the plane of union, but unless
the chromosomes slip around each other after crossing

CROSSING OVER AND CHROMOSOMES 113

over — for which there is no evidence — one member of the
pair must lie on one side of its mate in one region, and
on the other side in other regions. Second, when the thick
thread splits anew just before condensing into the tetrad
it is so difficult to follow the course of the split in all cases
that it cannot be affirmed that it always lies in one plane
throughout the length of the chromosome, but if such
should turn out to be the case, as so often figured, it would
appear to mean that the crossing over had taken place and
been obliterated by the time the condensation began.
Third, evidence such as that described by Wenrich — of
which sort there are other cases but none quite so clear —
indicates that the chromosomes are enclosed in vesicles
until they begin to spin out each into a long thread. Inter-
change of the sort called for by the genetic evidence could
scarcely take place until the walls of the sacs had disap-
peared. The thin thread stage that follows would seem
best to fulfill the conditions called for by the genetic evi-
dence. The moment the primary split appears after the
two threads have fused there would seem to be precluded
any further chance for crossing over, as the genetic
evidence suggests. This analysis leads, then, to the thin-
thread stage as the most favorable stage for the requir-
ments of the genetic evidence.

It is well known that most of our information about the
maturation stages is derived from the male, because of the
greater ease of obtaining the critical stages, and in prepar-
ing material. We are handicapped in discussing crossing
over to a large extent by the fact that we must appeal
largely to the evidence of spermatogenesis. In Drosophila
at least there is no crossing over in the male. On the
other hand, Nabours has recently found evidence in one of
the grasshoppers that crossing over occurs both in the
male and female. In this case evidence from the male
would be more to the point. Whether genetic crossing
over occurs in the male of Batracoseps and To^nopteris,
we do not know.
8

114 PHYSICAL BASIS OF HEKEDITY

In the female of some insects, amphibians, selach-
ians and annelids, the thin-thread stages in the form of
U-shaped loops have been described — stages that are so
much like those of the male that the argument for one
would seem to extend to the other. But again this proves
too much, and we have yet to learn what cytological dif-
ferences exist in cases where crossing over occurs in one
sex and not in the other. On the whole, then, while the
genetic evidence is favorable in all essentials to the theory
of interchange between homologous chromosomes, it must
be confessed that the cytological evidence is so far behind
the genetic evidence that it is not yet possible to make a
direct appeal to the specific mechanism of crossing over on
the basis of our cytological knowledge of the maturation
stages. The idea that the chromosomes disappear as such
and go into some sort of suspension during the resting
stage is an old idea. O. Hertwig thought that the chromo-
somes did actually ' ^ dissolve ' ' at this time and ^ ^ recrystal-
lize'^ at each division stage. Goldschmidt elaborated a
view of crossing over that rests on the assumption that
the homologous genes are set free in the resting nucleus
and may become interchanged during reconstruction.
Aside from certain inherent contradictions in Gold-
schmidt's scheme (the most obvious ones have been
pointed out by Sturtevant and by Bridges), it stands
in contradiction to the one most certain fact that we
know about crossing over, viz., that not single genes
but whole blocks of genes are involved — in fact, the most
common sort of interchange involves the two entire pieces
of each chromosome.

The general idea that the genes become dissociated
during the resting phases is disproven by the way in
which they come together. The genetic evidence from
Drosophila shows that when crossing over occurs, let us
say at the middle of the chromosome, all of the genes of
each half of each pair hold together — and exchange as
large pieces. Now if the genes are dissolved at each rest-

CROSSING OVER AND CHROMOSOMES 115

ing stage, there can be given no explanation as to why
homologous genes should not recombine in all possible
combinations with other genes. But this is exactly what
does not happen. If it be supposed that the chromosomes
dissolve only partly into chains of genes, it is still not
obvious why the chains of one chromosome should be iden-
tical with those of the other (its homologue) as they must
be to recombine properly ; for, in neighboring nuclei other
chains are forming — as the crossing-over results indi-
cate— involving breaking at all possible levels.

Bateson and Punnett have proposed a theory of cross-
ing over that is called reduplication. It is fundamentally
different from the one here adopted. Although I think
this theory outlawed by the evidence that Plough has
obtained, and made impossible by certain other considera-
tions that will be given later, the theory is so interesting
that it may be briefly stated. Bateson suggests that at
some time early in the embryo segregation may take place
involving heterozygous pairs of factors. In the actual
case presented only two such pairs are involved. As a
^^ symbolic presentation'' of the situation Bateson gives
the diagram drawn in Fig. 51.

Although the dichotomous method of separation is
utilized in the second line of figures to show reduction of
the two pairs at once, such figures could obviously bear
no relation to the ordinary process of cell division — nor
do they, I understand, pretend to be. After separa-
tion (segregation) the cells that get AB and ab are repre-
sented as dividing faster than the cells Ab and Ba, hence
there will be more of them in proportion as the two rates
of division differ.

Bateson 's view is open to the following criticisms:

1. The evidence from Drosophila, where many linkage
ratios: are known, gives no support to the view that these
ratios fall into relatively few dichotomous schemes, such
as Bateson 's hypothesis calls for. Other forms also fail to
fit such a view. On the contrary, the ratios fall into no

116

PHYSICAL BASIS OF HEREDITY

such groups as those given by Bateson. Even were it pos-
sible to suppose that in each case a different reduplica-
tion occurred {Le., a different number of generations was
passed through), still, as said above, it is not obvious
that the linkage series stands in any such numerical (i.e.,
dichotomous) relation as the view demands.

ABxab

[ AB ab ]

AbxaB

( Ab.aB J

n-i

n-i

3&B

IBa

MB

Fio. 51. — Two schemes illustrating the idea of reduplication by Bateson and
Punnett; the three figures to the left illustrating "coupling," and the three to the right
"repulsion."

2. If reduplication occurred at an early stage in the
germ tract, we should expect to find in any organ of limited
size, as a stamen, that there would be a likelihood that it
would contain for the most part a particular kind of ceU.
Altenburg tested out this view with pollen of the primrose
and found no evidence in favor of a limited distribution —
on the contrary, he found that all the linkage combinations

CROSSING OVER AND CHROMOSOMES 117

were present in each stamen in the expected proportions.
These and other difficulties make it improbable that link-
age can be the result of this kind of reduplication.

Bateson and Punnett formulated their hypothesis at
first for only two pairs of linked factors. When it was
shown that three pairs of factors could show linkage,
Bateson and Punnett assumed that all three pairs of fac-
tors might segregate at the same time (or in three suc-
cessive divisions), the observed ratios being due, as
before, to unequal division rates later. Trow has sug-
gested that in such cases the segregation and reduplica-
tion for the third pair of factors might not occur until
that for the first two pairs was completed. This view
seemed to meet certain inadequacies of the former hypoth-
esis, but meets with certain difficulties on its own account.
One of the most obvious of these objections is, as Sturte-
vant has pointed out, that the number of cell divisions,
necessary to produce some of the higher ratios that are
known, would produce a mass of cells thousands of times
larger than the animal itself.

CHAPTER IX
THE OEDER OF THE GENES

The proof of the linear order of the genes is derived
directly from the linkage data. It is not dependent on the
chromosome theory, of heredity. Fortunately, as was
pointed out in the last chapter, there are many facts about
the maturation stages of the eggs and sperm that fit in
extraordinarily well with the theory of the linear order
of the genes, but let me repeat, the proof of the order is
not dependent on the chromosomal situation. The evi-
dence for the linear order is furnished by linkage and its
correlative phenomenon, crossing over. By linkage is
meant that certain factors that enter the cross from each
parent remain together in subsequent generations, more
often than they separate. For example, if in Drosophila
yellow wings and white eyes have entered from one parent
and gray wings and red eyes from the other, the new
(crossover) combinations, yellow and red, gray and
white, are less numerous than are the original linked
combinations, yellow and white, gray and red. The num-
ber of individuals (crossovers) that result from this
interchange, expressed as percentage of the whole number
of individuals, is called the crossover value. Such a
percentage indicates how often the linkage is broken.
Thus, if crossing over between yellow and white is shown
in 1 per cent, of the gametes, then 1 stands for the cross-
over value of yellow and white. Conversely, yellow and
white have remained together (linked) in 99 per cent, of
the gametes. We speak of the linkage relations in such
cases in terms of the crossover values, here 1 per cent.

For the proof of the linear order of the genes, it is only

118

THE ORDER OF THE GENES 119

necessary to represent one set of linked genes (a, h, c, etc),
ignoring the normal allelomorpliic series, for these follow
the same (reciprocal) changes.

If a, b, and c stand for three genes, and if the linkage
relations of a to h and of h to c are known, the relation
of a to c is a function of the sum of ab and be or of the
difference of ab and be. For example, if the crossover
value ab is expressed as 5, and that of be as 10, then ae
is a function of the sum (15), or the ditference (5) of ab
and be. It cannot be said that ae must be 5 or 15 because
another possible process may intervene to affect the sum
or the difference, viz., double crossing over in the region
involved. By making the distance so small that double
crossing over is practically excluded the sum or the dif-
ference is actually the realized result, as the following
example illustrates :

Wlien three mutant characters yellow, white and
bifid were all used together in a single experiment, it
was found that there were 1160 non-crossovers, 15 flies
representing single crossovers between yellow and white,
and 43 flies representing single crossovers between white
and bifid. There were no flies representing crossing over
in both regions at the same time, i.e., there were no double
crossovers. Thus the crossover value yellow white is
1.2, and the crossover value white bifid is 3.5. The same
data give the yellow bifid crossover value of 4.7, which is
precisely the sum of the two component values :

The simplest way in which such a relation can be
thought of is that the three genes stand in a line. Suppose
a fourth linked gene, d, is added to the series. It is then
found that bd, is a function of the sum or of the differ-

120 PHYSICAL BASIS OF HEREDITY

ence of 6 to c and c to d. Four points arranged in a
straight line still fulfill the relations here found. I know
of no other geometric configuration that covers all these
results — perhaps there is none. When we add more
and more linked genes to the series, and find the same
predictable relations continue to hold, the theory of
the linear arrangement becomes firmly established. Per-
haps the best proof of the linear order is found in the
oppoi-tunity it gives for prediction; for, when the rela-
tion oi d to h and to 6 is known its relation to a can be
foretold accurately.

It has been found when there is a large amount of cross-
ing over between two factors used in an experiment, that
the crossover value is not the same as the value deter-
mined by adding together the crossover values of inter-
mediate points between the two factors in question. What
appears here to be a contradiction proves, when under-
stood, to be one of the best pieces of evidence in support
of the theory of the linear order.

A few examples will serve to illustrate the point at
issue. When a fly with yellow wings and bar eyes is mated
to a wild-type fly, the amount of crossing over in the F^
female between yellow and bar (as determined by back-
crossing) is 43.6 per cent., but if the crossing over between
yellow and bar is calculated by adding up the crossover
values obtained by using intermediate points {ah + fee,
etc.) the value is about 57 per cent. The apparent incon-
sistency is at once cleared up by arranging the experiment
so that, while obtaining the data for yellow and bar,
there are also obtained data showing what is happening in
theregion between them. It is found that a large amountof
double crossing over occurs, and, when the correction for
this is made, the *' discrepancy'' disappears. If crossing
over may take place at any level, it is obvious that it might
occur at two points at the same time, and experience

THE ORDER OF THE GENES

121

shows that such is the case, for such double crossing over
can be detected if enough points in the series are * in-
volved'^ to catch all single crossovers. Now, as sho^\^l
in Fig. 52, whenever double crossing over takes place
between y and B, the two series that result, as marked
by their ends alone {y and B), are still y and B. The flies
Avill therefore be placed in such classes, which are the non-
crossover classes. A numerical increase in this class
will decrease the calculated percentage of crossovers.
Thus double crossing over by increasing the number of

y

B

B

Fig. 52. — Scheme illustrating how double crossing over between two distinct genes, y and
B, is not recorded, when only y and B are involved.

apparent non-crossovers, decreases the observed per-
centage of crossovers. When enough points are marked
along the series to pick up ^11 double crossovers, and
these are then referred to the proper single crossover
classes, the ^ ' piece-by-piece " per cent, estimate, and
the percentage obtained from the cross, are found in
complete agreement.

The amount of double crossing over in Drosophila is so
large that the percentage of ^'crossing over'' is rarely or

122

PHYSICAL BASIS OF HEEEDITY

never more than 50 per cent., although the actual num-
bers given for ^ ^ distances ' ' between two genes may be as
much as 107 based on summation of short distances. The
latter method of calculation is the accurate way of stating
the result, and whenever possible it is adhered to, i.e.,
the percentage numbers for crossing over are sum totals
based on results obtained with genes so near together that
double crossing over is practically excluded.

Another illustration where the difference between the
direct calculation between two factors (scute and forked)
and the ' ^ piece-by- piece ' ' estimate is greater than 50, is as
follows : At one end of the series of sex-linked genes is a
factor scute (zero) and near the other end forked. The
direct data for crossing over between them gave a cross-
over value of 48.2. Between them three other loci were
present in the same experiment, and crossing over between
them could also be detected. As shown in the table below,
the sum of these crossover values gave 61.1 units between
scute and forked.

Scute

16.5

Echinus

Cut

25.2

Garnet

14.2

Forked

48.2

The presence of the intermediate factors makes it pos-
sible to pick up most of the double crossing over that
occurred between scute and forked. When a correction
is made for these the difference between 48.2 and 61.1
entirely disappears. Another and still more extreme
example will help to make this more evident. Near one
end of the second chromosome is the gene for star (eyes),

THE OEDER OF THE GENES

123

near the other end is the gene for speck. Bridges fur-
nishes the following data in regard to crossing over
between these loci. When only these distant loci are
used the crossover value is 48.7. When the sum of the
crossover values between the following seven genes is
taken as the value for star and speck it amounts to 104.4.

star

Dachs

Black

104.4

11.8

Purple
Vestigial

Curved

30.2

48.7

Speck

In this case (as other experiments show) there are still
two units missing in the map distance given above (104.4),
because of one per cent, of double crossovers in the
region between curved and speck, that are not here
recorded, since there are here no loci within this distance
of 30.2.

Whenever cases in which double crossing over has
taken place are checked up as in the foregoing cases, it is
found that the discrepancies in the two methods are
accounted for.

It is instructive to compare the preceding case with
another one including several of the same genes, but in
addition something else (deficiency), that cuts down the
amount of crossing over in certain regions. The cross-
over value between star and speck was found to be 47.7
in this experiment. The sum of the crossover values of
the six loci involved gave 79.3 units. The difference

124

PHYSICAL BASIS OF HEREDITY

between 47.7 and 79.3 is due to double crossovers as the
data for the intermediate regions show.

star

24.1

14.5

79.3

Dachs deficiency

29.5

Black

Purple

Vestigial
Speck ^

4*— p^

There was known to be present in this case a factor called
''deficiency'' in one of the two second chromosomes
involved. It is near ' ' dachs ' ' and cuts down the crossing
over between star and speck by about 25.8 units. It
will be noticed that while the second summation value
for star speck, viz., 79.3, and the first value, viz., 104.4, are
very different, the crossover value between star and speck
is in one case 47.7 and in the other 48.7. The meaning of
this, as showm by data for intermediate loci, is that by the
addition of 25.1 units (104.4 minus 79.3 equals 25.1) the
number of double crossovers has so greatly increased that
a difference of only about 1 per cent, of apparent crossing
over is recorded in the star speck value.

''Distance'' and Linear Order

The linear order of the genes implies distance between
them, for which the crossover values stand as indices. It
is obvious that if the order of the genes remained the same
but something doubled the number of crossovers between
two loci, their "distance" apart would at the same time
appear to have been doubled. Again, if crossing over is
thought of as due to twisting of the chromosomes of a pair
about each other, then if the twisting is more likely to occur
at the ends of the chromosomes, or if the twists themselves

THE OEDER OF THE GENES 125

are shorter there, '* distance'' in these regions is on a dif-
ferent scale from distance in the middle of the same
chromosome. Factors for crossing over have been found,
by Stnrtevant, that change the values in certain parts of
the series and leave other parts unaffected. When the
influence of these special genes, that can be treated in the
same way as are all Mendelian genes, is removed, the
region that was affected gives its original crossover
values again.

It is to be understood, then, that when we substitute
the idea of distance for crossing over values the term is
not used in an absolute sense, but in a relative sense, and
that it depends always on the conditions of the experiment.
That the genes do stand at definite levels in the chromo-
somes, and that in this sense they are definitely spaced,
seems reasonable in the light of all the evidence bearing
on this point ; but even if they are so spaced that crossing
over is a function of their distance from each other in the
series, any influence that determines how often inter-
change between homologous pairs will take place would
give the appearance that the actual distances themselves
have changed.

CHAPTER X

INTEEFERENCE

One of the most significant results that a study of
crossing over has brought to light is that whole blocks
of genes go over together. Thus, if one series be A B C D
E F GHIJKLMN and its allelomorphic series be ah c
defghijklmn crossing over may give two blocks of
genes.

ABCDEfghijk I m n
a h c deFGHIJKLMN

This result can best be demonstrated in cases where a
number of loci are followed at once.

The fact that crossing over takes place in blocks is
highly significant for the phenomenon of distribution,
since it means that pairs of linked genes do not act inde-
pendently of their neighbors. This fundamental relation
was not suspected until quite recently.

The size of the blocks, when only one crossing over
occurs between the chromosome pairs, depends on the loca-
tion in the series of the breaking point. If the crossing
over occurs near the middle, the four pieces will be of the
same length as shown below:

ahcdefgHIJKLMN
ABCDEFGhijklmn

If it is near the end of the series, two of the resulting
pieces will be small, the other two large. Thus :

a h c DEFGHIJKLMN
ABC defghijklm^n

The two ''like'' pieces in all cases contain identical series
of loci.

126

INTERFERENCE 127

The data also show that the series may break at two
points, and that when this happens the three blocks of one
set always correspond to the three blocks of the other
series of genes. Thus interchange at two levels gives :

abcdEFGHijkl
ABCDefghlJKL

The same relation holds in principle for three or more
breaks in the series.

If in such a system the blocks have no commonest
length, the break in the series at one level should not
bear any relation to the place at which another break
takes place. For example, if it is true that when a break
occurred between D and E it had no influence on a break at
any other point of the series, the blocks resulting from two
breaks would not tend to be more of one length than of any
other length. But if the evidence shows that when a
break occurs between D and E the chance of another break
occurring in that vicinity is decreased, or increased, the
results would be expected to follow some definite law or
principle, rather than be simply the result of chance. This
is in fact the case. An illustration may make this clear.

Suppose when crossing over takes place within the
blocks A B C D, dindi E F G E, diU^ I J K L \i can be
recorded. If we know how often, when the break occurs
only once in the series, it takes place in the first, in the
second, or in the third block, we can then determine in
those cases where breaking occurs in the first block,
whether it is as likely to take place in the second block as
when no break occurs in the first, etc. Such tests have
been made (MuUer, Sturtevant, Bridges, Weinstein,
Growen) with Drosophila, and the same kind of results con-
sistently obtained. It has been found, for example, that
when a crossing over takes place between G and H, a sec-
ond one is less likely to take place on either side, i.e.,
between F and G or between H and / than when no cross-

128 PHYSICAL BASIS OF HEREDITY

ing over takes place between G and H. Stated in another
way, crossing over in one region protects neighboring
regions from crossing over. Moreover, this relation fol-
lows a perfectly definite law according to the '^distances,**
as determined by linkage relations of genes outside of the
region of crossing over. If we take two pairs of factors
^ closely linked together we find that the genes lying
immediately to the right and left of ^ never cross over
independently of - and ~ at the time that a crossover
separates ^ ^^^ f • I^i other words, the genes imme-
diately to the right of H always go over with H, and those
to the left of G always go over with G, when G separates
from H,

If we consider genes that are less closely linked with
G and with H, we find that while their crossing over is
interfered with by the crossing over between G-H, it is
affected to a limited extent. Genes still less linked with G
or with H are still less interfered with ; until finally there
is no relation at all between crossing over between G-H,
and other more loosely linked genes, i.e., crossing over
between G-H is found to have no relation to crossing over
between L and M. Put in another way, one may say that
crossing over at L and M is no more likely to take place
when none occurs between G-H, than when it does.

For different pairs of chromosomes the regions that
bear this relation to each other have been found to be
different. Even within the same chromosome this rela-
tion may be different at the ends and in the middle. There
are also special factors that affect special chromosomes
and special regions of chromosomes. An example will
illustrate this relation that is called interference. If in
a group of genes A B C D E F a break occurs somewhere
between A and D in 6 per cent, of cases, and if between
M and T in the same series (M N 0 P Q R S T), in 10 per
cent, of cases, a double break involving both regions simul-
taneously should, if the breaks occurred independently of

INTERFERENCE 129

each other, take place in 0.6 per cent, of the cases. But
if the regions in question are close together, that is, if the
intervening block (i.e., G F H J K L) of genes is short, it
is found that there are fewer double crossovers than the
0.6 per cent, expected on a purely random basis. This
was shown by Sturtevant in his paper on chromosome
maps. It means that a break in one region interferes
with a break in the other region when the intervening
block is short.

The ratio of the number of actual double breaks
obtained to the number of double breaks that would occur
if one of them did not interfere with the other is termed
coincidence. If in the above example only 0.3 per cent,
of the cases were double crossovers involving the regions
ABCBEF and M N 0 P Q R S T the coincidence would
be 0.3 per cent, divided by 0.6 per cent., or 0.5.

It has been found that as the distance between two
regions increases, crossing over in one of them interferes
less and less with crossing over in the other; that is, the
number of double crossovers obtained approaches the
number expected on a random basis, and coincidence rises
gradually to the value of 1. This phenomenon is shown
in all the cases where more than one block of genes has
been followed. It is especially clear in the work of MuUer,
who studied a large number of factors in the sex-chromo-
some of Drosophila simultaneously.

When the intervening block becomes sufficiently long
so that the coincidence attains the value of 1, interference
has entirely disappeared. When, however, the distance is
increased stiU further interference reappears, i.e., coin-
cidence decreases again. There was a suggestion of this
in MuUer's work; and the work of Weinstein undertaken
to get critical evidence on this point indicates clearly that
such a decrease exists. For the second chromosome a
similar rise and fall with increase of distance is indicated
by Bridges ' data.

9

130 PHYSICAL BASIS OF HEEEDITY

The fact that interference reappears, i.e., that coinci-
dence decreases after reaching a maximum, indicates that
the segment of a chromosome between the breaking points
tends to be of a particular (modal) length ; and that breaks
which are closer together or farther apart than this modal
length are less frequent. That is, genes not only stick
together in blocks, but the blocks tend to be of a definite
size, and longer and shorter blocks are less frequent.
In the sex-chromosome of Drosophila, which is 65 units
long, Weinstein's data indicate that the most frequent
length of block is about 46. In the second chromosome
(w^hich is 107 units long). Bridges' data indicate a modal
length of about 15 in the centre of the chromosome and
of about 30 on either side of the middle point.

The work on coincidence throws light on the behavior
of the chromosomes during crossing over. The cytologi-
cal evidence has not determined whether when crossing
over takes place the chromosomes are twisted loosely or
tightly. But Muller has shown that this question may be
attacked by certain calculations based on the data of inter-
ference. If, as a rule, chromosomes twist in long loops,
crossing over at two points close together would be rare,
for it would require a shorter twist than usually occurs.
The occurrence of long loops would explain the interfer-
ence of neighboring regions. Moreover the decrease of
interference as distance increases would be accounted for,
because short loops would be less frequent than longer
ones. The reappearance of interference for widely separ-
ated regions is explained by supposing that extremely
long loops are infrequent as are very short ones. That is,
on the supposition of long twists there would be a modal
length of loop, and loops of greater or lesser length would
be less frequent.

If, however, the chromosomes are tightly twisted into
short loops, the interference of neighboring regions might
be explained on the supposition that a break at one point
allows the chromosomes partly to unravel in the neighbor-

INTERFERENCE 131

hood of the break, and that this loosens the twisting and
prevents another break near by. In regions farther away
from the break, the threads would not be so much unrav-
elled, so that the greater the distance from the first point
of breaking tlie more Avould a second break be likely to
occur. That is, interference should grow less at greater
distances. But the reappearance of interference at still
greater distances seems incompatible with this scheme;
thus the actual data favor the first view of crossing over,
in which the break occurs during a stage of loose twisting.
At any rate, as Weinstein has pointed out, the variation
of coincidence with distance must be dependent on other
conditions than the mere tension due to the twisting of the
chromosomes, and any view which refers the breakage of
the threads to the tension of tight twisting must be
rejected or supplemented.

Castle has recently suggested that the difference
between the values for a long *^ distance" and summation
of short ^^ distances'^ is due to the loci not lying in a
straight line but ^ ' out of line. ' ' He suggested that when
short steps are taken as the basis for map distance
they represent the ^^long way round,'' as, for instance, in
passing from one end of a F to the other end, keeping on
the line; while when a direct cross is made, giving a
shorter ^^ distance," this is a measure of the direct or
air-line between the two ends of the F. Such a theory is
not in harmony with the following facts. The best data
{i.e., data sufficient in amount and free from crossover
variations) show that Castle's three dimensional figures
reduce to a curved line in a plane. In such a curved line
the most distant points are nearer to each other in an ^ ^ air-
line" than along the line. Such a graphic representation
of the data is possible, but leads to certain inconsistencies.

If Castle 's procedure is followed it leads to the placing
of the same locus in two or more different places on the
basis of adequate and comparable data for both positions.
The two cases that Castle says furnish the crucial evi-

132 PHYSICAL BASIS OF HEREDITY

dence for his view demonstrate just the opposite, when
complications due to crossover variations are excluded,
by using only data in which three or more loci are recorded
simultaneously. In his attempt to explain the all-import-
ant fact of rarity of double crossovers, Castle is obliged
to assume that there is a difference in frequency of cross-
ing over in different planes (directions). This assump-
tion can be shown to be inconsistent with the primary
assumption that he accepts, viz., that crossing over is
proportional to the distance between genes.

CHAPTER XI
LIMITATION OF THE LINKAGE GROUPS

It may be questioned whether we are at present justi-
fied in speaking of the limitation of the linkage groups
to the number of chromosome pairs as one of the funda-
mental principles of heredity, since the only species in
which a correspondence that is numerically significant
between the two has been proved is Drosophila melano-
gaster. But despite the absence of other positive evi-
dence, the fact that in no other animal or plant does the
number of linkage groups exceed the number of the
chromosome pairs, may be, I think, legitimately inter-
preted in favor of the view.

It may also be argued, that if the phenomena of linkage
are assumed to be due to the genes being carried by the
chromosomes,it follows that there could be no more groups
of linked genes than there are chromosome pairs ; hence
one relation is the direct outcome of the other. But the
proof of the linear order that has been developed here
rests directly on the linkage data, and is independent of
any assumption concerning the chromosomes. It has been
shown, secondarily so to speak, that the chromosomes ful-
fill all the requirements of the abstract reasoning from
the data, and therefore give a mechanism capable of per-
forming all that the theory demands. The demonstration,
then, that in Drosophila the linkage groups correspond
in number to the chromosome pairs may be taken as a con-
clusion or a discovery independent of the other relations
furnished by linkage. If then, as I anticipate ^\ill be the
case, further work in other groups should show that the
same relation holds everywhere, w^e should be fully war-
ranted in stating the result as one of the general prin-
ciples of heredity.

133

134 PHYSICAL BASIS OF HEREDITY

In Drosophila melanogaster the evidence is now very-
strong in favor of the identity in number of linkage groups
and chromosome-pairs. As the new characters coming up,
one after the other, have continued to fall into the four
kno^\^i groups, and as something like 200 characters have
been so placed, and as none of them has failed to show link-
age with one of the four established series, the probability
is enormously in favor of a causal relation between the two
events, especially in the light of the evidence from other
sources that the chromosomes are the bearers of the
hereditary factors — evidence from the sex-chromosomes,
for example.

The only other species in which the heredity of known
mutant characters approaches that of the chromosome
group is the garden pea in which about 35 mutant factors
have been studied. From the summary of what has
been so far recorded, as well as from the results of his
own work, White has recently given an account of what
is fairly well established. Of the 35 mutant factors
in this pea, seven independently inherited groups have
been recorded, i.e., each one of seven factors has been
tested out and found to assort independently of the other
six. There are seven pairs of chromosomes in the edible
pea (Fig. 53, a). The agreement between the two is to
date perfect. It is, of course, possible that the linkage
between some of the factors tested was so loose that they
appeared to give free assortment, and that until more fac-
tors have been studied the evidence is not above sus-
picion. Nevertheless, it is important to find that the
number of independent mutant factors in Pisum sativum
does not exceed the number of chromosome pairs.

White's study of the linkage of factors in the edible
peas shows further that there are four linkage groups —
three of them include factors that are also included in
those that freely assort. It is fair, perhaps, to conclude
that four of the possible seven-linked groups have been
found. There are no other forms known in which the

LIMITATION OF THE LINKAGE GROUPS 135

number of linkage groups approaches so near the number
of the chromosome pairs. In the snapdragon, Baur has
described two linked groups. He states that there are 16
pairs of chromosomes. In wheat one linked group has
been described. There are 8 pairs of chromosomes (Fig.
53, b). In Indian corn there appear to be a few linkage
groups, and probably 10 pairs of chromosomes. In oats,
Surface finds two linked genes. In Primula there is one
group composed of several linked genes, and 12 pairs of
chromosomes (Fig. 53, c).

In the silkworm moth one linked group of genes has
been found by Tanaka, and Yatsu records (Fig. 55) 20
pairs of chromosomes. In Drosophila virilis three linked
groups of genes have been found by Metz, who has also

Fig. 53. — Chromosome group, of pea,, a,, wheat, b, and primula, c.

described six pairs of chromosomes for this fly. In Droso-
phila busckii there is one group of linked genes and four
pairs of chromosomes. In D. repleta one group, and six
pairs of chromosomes. The groups of chromosomes in
some of the different species of Drosophila, as described
by Metz, are shown in Fig. 54. As indicated by the
arrangement of the figures (that correspond fairly closely
with the actual arrangement of the chromosome in the
cells themselves) it appears that one pair of chromosomes
in one species is at times represented by two pairs in
related species, and this view is borne out by the attach-
ment of the spindle fibre to the middle of the chromosomes
in the bent pairs, but to the inner ends of the two that
supposedly correspond to its halves in other species.

In the mouse one group of linked genes has been
reported. There are 20 pairs of chromosomes (Fig. 55, h).
In man no linked genes are known, if we do not count sex-

136 PHYSICAL BASIS OF HEEEDITY

lined genes, which must, however, if carried by the sex-
chromosomes, be linked to eax3h other. The number of
chromosome pairs in man is, according to Gruyer, 12 (Fig.
55, Cy d), but Winewarter describes 24 pairs (Fig. 55, e).
The difference would seem to be due to technic, rather
than to differences in different races of men.

••• II. II. IL

II. II, 11. j^.

'^.<' '^.♦ II II

Z i <f

n\

t J d>

At

^A*<^^- »"fc))r((

t K

'HI <^:\:n\

Fig. 54. — Types of chromosome groups found in Drosophila. A-H female groups;
I-L female and male groups. In A, C, F, I, J, K, and L, the X-chromosome can be identi-
fied, because, in the male (Alex. Metz), the Y-chromosome has a different shape from the X.

It should be emphasized that it is to be expected for
new types that the number of characters that may seem
to give independent assortment will be found at first
greater than the number of chromosomes, because wher-
ever two genes in the same chromosome are far apart they
will appear to assort independently until the discovery

LIMITATION OF THE LINKAGE GROUPS 137

of intermediate genes shows their true relation. This will
be especially the case when crossing over occurs in both
sexes ; when it occurs only in one sex, the linkage relations
are more quickly determined. Moreover, in some cases
where several genes are known the mutant characters
have not yet been tested out against each other but against
different ones. Such information does not furnish the
data that are needed.

a

^

Fig. 55. — Haploid group of chromosomes of the silkworm moth (Yatsu) a. Haploid
group of chromosomes of mouse (Yocom) b. Haploid group of chromosomes of man
(Guyer), c and (von Winnewarter) d.

There are several forms in which there are two or
more chromosomes that come together in a group at the
time of segregation and move collectively to one pole.
Such groups should be expected to count as a single chro-
mosome so far as segregation is concerned, although the
crossing over relations may turn out to be something
different from anything as yet known.

138 PHYSICAL BASIS OF HEEEDITY

An extension of the principle of agreement of linkage
groups and chromosomes (if they are thought of only as
a linear order of genes) is found in the case of ^'duplica-
tion" described by Bridges, where a short series of linked
genes appear to lie at one end of the regular series, dupli-
cating their number for this region of the chromosome.
Obviously this is not to be looked upon so much as an
exception to the principle but rather as a special case due
to an accidental change in the mechanism. The number
of linkage groups is not changed, but one of them has
its genes duplicated for a short part of its length.

CHAPTER XII
VARIATION IN LINKAGE

Crossing over is not absolutely fixed in amount, but
is variable. This statement does not refer to variability
in tbe number of crossovers due to random sampling,
but to variability due to fluctuation in environmental
conditions, or due to internal changes in the mechanism of
crossing over itself. For example, it has been shown
that the amount of crossing over in Drosophila is different
at different temperatures, and it has also been shown that
there are factors (genes) carried by the chromosomes
themselves that affect the amount of crossing over. These
questions, that have already been touched upon in other
connections, may be taken up here in more detail.

The work of Plough on the influence of temperature
on crossing over in Drosophila, that has already been
utilized, was concerned with the influence of different tem-
peratures on the number of crossovers obtained. It may
be recalled that he found that when the eggs were sub-
jected to a given temperature during a certain stage in
their maturation the amount of crossing over that took
place, as shown in the kinds of flies produced, was definite
in the sense that the average results were predictable for
each specific temperature, and that there are values for
different temperatures which, when plotted, give the curve
drawn in Fig. 56.

Further details of one of the experiments may serve
to make its significance clearer. Three points (or loci)
were made use of that involved three mutant genes (and
their diagnostic characters, of course). Males, pure for
the three mutant characters, black body color, purple eyes,

139

140

PHYSICAL BASIS OF HEEEDITY

and curved wings were crossed to wild-type females. The
i^i female produced in this way would be heterozy-
gous for the three mutant factors involved in the cross.
Such an Fj^ female was then bred to a male pure for the
three recessive genes, black, purple, curved; and her
offspring were kept at a given temperature until they
emerged as flies, and then if necessary for some days
longer in order that as many eggs as possible might have
matured under the specified temperature. Controls of
sisters and brothers were made in each case and kept at
average *' normal' ' temperature. In the table that fol-
lows crossing over between black and purple is indicated
as ^^Ist crossover,'* and between purple and curved as
^^2nd crossover," and between both as double crossover.
Ten different temperatures were tested. At 5° C. the
eggs did not hatch, and at 35° C. the females were sterile.
In the seven intermediate temperatures the results were
those recorded in the next table.

pr

Temp.

Total

Female parents hatched at temperature

indicated below

Weighted
Value for

Num-

Non-

1st

2nd

Double

1st

2nd

b — pr

ber

cross-

cross-

cross-

cross-

cross-

cross-

Region

over

over

over

over

over

over

per cent

per cent

per cent

2

9°

995

643

95

218

39

13.5

25.8

13.6

3

13°

2,972

1,854

310

716

92

13.5

27.2

17.5

4

17.5°

2,870

2,021

189

610

50

8.3

23.0

8.2

5

22°

15,000

11,318

735

2,775

172

6.0

19.6

6.0

7

29°

4,269

2,993

315

898

63

8.8

22.5

8.7

8

31°

3,547

2,265

333

785

164

14.0

26.7

18.2

9

32°

4,376

2,701

513

984

178

15.7

26.5

15.4

At the two lower temperatures the crossover value is
high, i.e., little crossing over occurs. At the next three
temperatures (17.5°, 22°, 29° C.) the crossing over value
is much less, while at the last two temperatures 29° and

VARIATION IN LINKAQE

141

31° C, it is high again. The control values for sister
flies, at normal temperature (22° C.)> is given in the next
table.

Controls — female parents hatched at 22' C.

ist
cross-
over

2nd
cross-
over

Total

Non-
cross-
over

ist
cross-
over

2nd
cross-
over

Double
cross-
over

Per cent

percent

6.1

7.8
5.9

19.2
20.1
19.5

904
3,622
2,219

683
2,655
1,678

47
231
108

166
685
409

8
51
24

'5.9

20^3

4,822

3,608

231

927

56'

....

....

....

....

....

....

....

The figures given in this table were obtained as a con-
trol for the last results, and from these data the results
of crossing over are reduced to the same scale. These
weighted crossing-over values for the first regions give
the curve drawn in Fig. 56. The curve begins at a high
level and drops rapidly. The first maximum is reached at
about 13° C, and then falls to 17.5° C, where the level
remains nearly constant for ten degrees more (27° C).
It rises rapidly at about 28° and reaches a second maxi-
mum at 31° to 32° C. Afterwards it is seen to fall until
sterility occurs at 35° C.

The temperature curve of crossing over seems to show
that the phenomenon is not a simple chemical reaction,
for if it were we should expect for every rise in 10° C. the
amount of change in crossing over to be approximately
tripled. It would appear, therefore, that the phenomena
might be due to the physical state of the materials involved
in crossing over. Plough calls attention to the similarity
of this curve to that shown by the amount of contraction
of a frog's muscle. Here there is an increase from zero
to 9° C, when a maximum is reached: After this, the
amount of contraction decreases, reaching a low point

142

PHYSICAL BASIS OF HEREDITY

between 10° C. and 20° C. It then rises rapidly, reaching
a higher maximum than the first at about 28° C, after
which it decreases until rigor sets in at 38° C.

The results of crossing over between purple and curved
gave similar results, but the '^distance'' here is so great
that double crossing over complicates the results ; there-
fore they need not, for the present, be analyzed further.
Attempts to change the crossing over value by starvation,
moisture, increase in fermentation of the food, iron salts,
etc., gave no results that seemed significant. On the other

%

«

of

)3

• W

22

Z1 « 3*32

Fig. 56. — Curve showing influence of crossing over at different temperatures. (After Plough.)

hand. Bridges had already noted that a decrease in the
amount of crossing over is found in second broods as
compared with first broods — ten-day periods. Wliat
change in the environment is behind this *^age'' dif-
ference is not clear, but since most of the eggs pass
through this early prematuration stage in the larvae
and some of them may reach the maturation stage
in the pupa, it is possible that prevailing conditions in
one or the other of these physiological states may be
responsible for the difference between these states and
those that prevail after the fly has hatched.

VAEIATION IN LINKAGE 143

Not only external factors but internal factors, and
these genetic ones, may influence the amount of crossing
over that takes place. Sturtevant has discovered two such
genes in the second chromosome of a certain stock of
Drosophila. A female from a wild stock from Nova
Scotia was crossed to a male showing the characters ves-
tigial and speck. One of the daughters was tested and
gave no crossovers in 99 offspring, though the vestigial,
speck hybrid usually gives about 37 per cent, of crossing
over. All of the descendants of this female that were

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FiQ. 57. — Diagram illustrating the effect on crossing over due to the presence of crossover

genes. (After Sturtevant.)

known, through linkage relations, to have the Nova Scotia
second chromosome, gave the same result, while those of
her descendants that did not have the particular chromo-
some did not show such a change in linkage. These rela-
tions held regardless of whether the chromosome involved
had come from the father or the mother.

A number of experiments were made with females hav-
ing a Nova Scotia second chromosome, while the other
second chromosome bore the mutant genes for black, pur-
ple, curved, and in other experiments other mutant genes
were present. In Fig. 57 (upper line) all the genes stud-
ied, viz,, star (S), black (fc), purple (pr)y vestigial (vg)y

144 PHYSICAL BASIS OF HEREDITY

curved (c), and speck {s2:)) are indicated in their relative
locations, i.e., spaced in proportion to the usual amount
of crossing over between them. Correspondingly, the
short second line is based on the crossover relations of
these factors when the female is heterozygous for the two
Nova Scotia genes.

Further experiments were made with females
(obtained by crossing over) in which only the ^4eft half '^
of a Nova Scotia chromosome was present (third line),
the other half being derived from an ordinary chromo-
some. The offspring of such a female showed that cross-
ing over was decreased only in the left half.

When the right half of the Nova Scotia chromosome
was present (fourth line) that half was ' ^ shortened. ' ^ It
follows that there are two (or possibly more) factors
present, one in each half of the second chromosome of the
Nova Scotia stock, each inhibiting almost completely
crossing over in its own region, but not in the other region.

An equally surprising result was obtained from a
female so constituted that the right halves of both mem-
bers of this pair of second chromosomes were present, ie.,
when she was homozygous for the ^' right hand" pair of
factors for little crossing over. Under these circum-
stances, the crossing over was normal for this end (last
two lines). How such results are produced (quite aside
from the nature of the factor producing them) is unknown.
Almost inevitably, however, we think of the cause as a
difference in the length or shape of the chromosome con-
taining these factors, so that corresponding levels do not
come together, hence failure of interchange. When, how-
ever, both chromosomes are affected in the same way their
corresponding regions might be expected to come to-
gether and cross over.

The preceding results of Sturtevant^s suggest the
possibility that all genes may have an effect on crossing
over — possibly one might think that in some mysterious
way the crossing-over values shown by the genes are a

VARIATION IN LINKAGiE 145

function of their nature. It may be well to point out that
in the only cases where the evidence suffices to give an
answer to such a question, that answer is very clearly
against such a view. For instance, if we determine the
linkage between two factors A-M and then exchange one
of the intermediate genes for its allelomorph, we find that
in general the change has no effect on crossing over
between A and M, If we exchange factors outside of
A and M — either near them or far away — still no effect on
crossing over between A and M is observed. If we sub-
stitute one allelomorph for another, in cases where more
than two are known, we find no change in the crossing
over for that level. This and other evidence shows that
crossing over is quite independent of such genes, never-
theless there are other specific genes, as shown above,
whose sole effect, or main effect at least, is to change the
crossing-over values.

One highly important and significant result of Sturte-
vant's work on crossing-over factors should be noticed.
The order of the factors is not in any way changed by
the " shortening' ' process, as shown by the experiments
in which three or more loci are followed at the same time.

The most remarkable fact connected with crossing
over is that no crossing over at all takes place in the
male of Drosophila, and this applies not only to sex-
chromosomes {XY) but also to the other pairs or auto-
somes. When the absence of crossing over was discovered
for sex-linked genes, it seemed probable that this was due
to the presence of only one Z-chromosome in the male, for
at this time Steven's work had led us to conclude that the
male Drosophila, like some other insects, is XO. Later,
when failure to cross over in the male was found in other
chromosomes as well, it was evident that some more gen-
eral relation was behind the phenomenon in these chromo-
somes at least. It is true that other genetic evidence
has shown that the T-chromosome is *' empty" (i.e., con-
tains no genes dominant to any of the mutant genes as yet

10

146 PHYSICAL BASIS OF HEREDITY

discovered) and on this account one might still ascribe
failure to cross over in this pair to its peculiar condition.

The interest in the situation became even greater when
it was found that in the silkworm moth (in which the sex
formula is reversed, so to speak) crossing over is again
absent in the sex that is heterozygous for the sex fac-
tors— here the female. The female moth is apparently
ZW, at least in two cases.

In one of the flowering plants, Primula sinensis, cross-
ing over occurs in both sexes (Gregory, Altenburg), but
the amount of crossing over in the pollen is somewhat dif-
ferent from that in the ovules. Gowen has examined
Altenburg ^s data statistically and finds that the differ-
ence is probably significant.

That crossing over should take place in the sex that is
homozygous for the sex-chromosomes (the female in
Drosophila, the male in the silkworms) but in both sexual
elements in the hermaphrodite plant (Primula) may
appear to have a deeper significance, but more recent dis-
coveries seem to deprive the results of any such meaning.
Castle, for instance, gives data that show crossing over
in the male rat (the male is probably heterozygous for
the sex-chromosome) , and Nabours gives data for crossing
over in the male and female grouse locust, Apotettix
(in which the male is presumably heterozygous). Until
more cases are forthcoming it must seem doubtful, there-
fore, if any such relation as that mentioned above is a
general one.

CHAPTER XIII

VARIATION IN THE NUMBER OF THE CHROMO-
SOMES AND ITS RELATION TO THE TOTAL-
ITY OF THE GENES

The theory that the chromosomes are made up of inde-
pendent self -perpetuating elements or genes that compose
the entire hereditary complex of the race, and the impli-
cation contained in the theory that similar species have an
immense number of genes in common, makes the numeri-
cal relation of the chromosomes in such species of un-
usual interest. This subject is one that could best be
studied by intercrossing similar species with different
numbers of chromosomes, but since this would yield sig-
nificant results only in groups where the contents of the
chromosomes involved were sufficiently known to follow
their histories, and since as yet no such hybridizations
have been made, we can only fall back on the cytological
possibilities involved, and on the suggestive results that
cytologists have already obtained along these lines.

A good deal of attention has been paid in recent years
to the not uncommon fact that one species may have
twice as many chromosomes as a closely related one. So
frequent is this occurrence that it seems scarcely possible
that it is due to chance. The implication is that the num-
ber of the original chromosomes has either become
doubled, or else halved. If the number is simply doubled
there would be at first four of each kind of chromosome
from the point of view of genetic contents. This is what
I understand by tetraploidy. There is some direct evi-
dence that doubling may occur. If a new race or species
is ever established in this way, we should anticipate that
in the course of time changes might occur in the four iden-
tical chromosome groups so that they would come to differ

147

148 PHYSICAL BASIS OF HEEEDITY

and form two different sets.^ Theoretically, the number
of different genes in a species might in this way be in-
creased. If changes in the same gene in the same direction
sometimes occur, as the evidence indicates that they do,
then identical new mutant genes, derived from the same
kind of original ones, might later arise in different pairs.

There is, however, another way in which the number
of chromosomes may be doubled without doubling the
number of genes. If the chromosomes break in two,
double the number will be produced. It is not easy to
explain how this could occur in all of the chromosomes at
the same time if the process is supposed to be accidental.
If it be supposed that the break first occurred accidentally
in one member of the pair, it is not clear why such a
broken chromosome could establish itself on the theory
of chance, for the intermediate condition of one broken
and one intact chromosome would seem of no apparent
advantage. The same reasoning applies to the converse
process, viz., the coming together of chromosomes end
to end which would reduce the number by half. Such a
process would not increase the number of genes in
the total complex. Until we know more about the
physical or chemical forces that hold the genes in chains,
and more about the way new genes arise, it is not worth
while to speculate about the causes or probabilities
of such occurrences.

"What has just been said in regard to doubling and
halving of the whole set of chromosomes applies also to
doubling in one pair of chromosomes. If doubling
occurred in one pair of a ten-chromosome type, a twelve-
chromosome type would result ; if in two pairs, a f ourteen-
chromosome type, etc. Unless tetraploidy is the simpler
procedure we should a priori suppose that increasing (or
decreasing) in pairs would, on the theory of chance alone,

^ The question as to whether the four chromosomes involved would or
would not mate at random introduces a difficulty (as shoAvn in the
primula case).

VARIATION OF CHEOMOSOMES 149

be the more common procedure. A few examples will
illustrate what has been found out so far concerning some
of these possibilities.

The evening primrose, CEnothera lamarckicma, has 14
chromosomes as its full or somatic number, and 7 as its
reduced number (Fig. 58, a), and these numbers charac-
terize most of the mutant types that De Vries found. But
there is one mutant known as gigas, that has 28 chromo-
somes as its full number, and 14 as its reduced number
(Fig. 58, b). Stomps estimates that gigas appears about
9 times in a million cases, i.e., in 0.0009 per cent. Gigas is
distinguished from Lamarckiana in many details of struc-
ture, but chiefly in its thick stem, etc., which is associated
with larger cells.

?i5f m

IPS

a o c

Fig, 58. — Chromosome group of CEnothera lamarckiana, a; chromosome^of [group"of O.

gigas, b; triploid group, c.

The type breeds true, i.e. .it does not revert to Lamarck-
iana; thus De Vries grew a family of 450 individuals from
his original gigas, only one being a dwarf gigas, viz.,
nanella. The way in which gigas originates has been
much discussed, but no conclusion reached. De Vries
suggested that it is produced by an egg with 14 chromo-
somes (diploid), being fertilized by a sperm with 14
chromosomes, both of these diploid cells originating by
the suppression of a cytoplasmic division in the develop-
ment of the gametes. It has also been suggested that a
tetraploid condition might arise in a spore mother cell
that developed without fertilization (by apospory) . Gates
pointed out that by suppression of the first division of the
egg, after fertilization, the tetraploid condition would
arise. The only objection to this last view, that seems

150 PHYSICAL BASIS OF HEREDITY

the simplest one since such suppressed division has been
seen and can be induced in animal eggs, is that the follow-
ing division might be expected to be into four parts owing
to the doubling of the centrosomes.

Gregory has described two tetraploid races of Primula
sinensis,^ one of which arose from ordinary plants in the
course of his experiments. Since known genetic factors
were present an opportunity was given to examine into the
relation between the members of the four chromosomes
of a set. The possibilities involved are these : Assuming
the parents to be AA', and a a', and that conjugation of
chromosomes takes place in twos only, then if any one
of the four (A A' aa') chromosomes of a set may mate with
any other member, there will be six possible unions^ viz.,
AA', Aa, Aa', A'a, A'a', aa'. If the two derived from
the same parents were the only ones that can mate, only
two combinations are, possible, AA'y aa\ and if the two
derived from the opposite parents were the only ones that
mate only two (but different ones) could form, viz., Aa,
A'a. The , genetic expectation is somewhat different
for each of the three cases, since the number of different
kinds of gametes produced is different in each. The data
obtained by Gregory are not sufficient to give convincing
evidence in favor of any one of these possibilities, although
as MuUer has shown by an analysis of the evidence, they
are more in favor of the first possibility, viz., that of ran-
dom assortment. Gregory, without committing himself to
the chromosome view, follows the second possibility in his
analysis of the case. There is, however, nothing in the
chromosome theory that would support the view that
restricts the conjugation of homologous chromosomes
according to their parental origins.

There are two other species of primose. Primula flori-
hunda and P. verticillata, each with 18 chromosomes that
have, after crossing, produced tetraploid types. In a

' other giant races of P. sinensis examined by Keeble and by Gregory
are diploid.

VARIATION OF CHROMOSOMES 151

cross between these two, a hybrid called P. kewensis was
produced, which Digby has shown has also 18 chromo-
somes. It produced only thrum flowers, and was therefore
sterile. Five years later, after this plant had been multi-
plied by cuttings, one pin flower appeared which was pol-
linated by a thrum flower. It gave rise to the fertile race
of P. kewensis, that had 36 chromosomes. What connec-
tion there may have been between the hybridization and
the subsequent doubling, if there is any connection, is by
no means clear. It may be noted that in the reciprocal
cross between P. verticillata and P. florihunday a hybrid,
P. kewensis, with 36 chromosomes also appeared.

The most interesting results on tetraploidy are those
of Elie and Emile Marchal on certain mosses, for they
have been able to produce tetraploid types experimentally.
It may be recalled that in mosses there is an alternation
of generations. The diploid {2N) generation is known
as the sporophyte (Fig. 59) that develops out of and
remains attached to the other haploid generation, the
gametophyte or moss plant (IN). The sporophyte pro-
duces a large number of spores, each containing the half
number of chromosomes (IN) as a result of reduction that
has taken place in their formation, and from each spore
a young moss plant develops, beginning as a protonema of
loose threads. Wlien the moss plant produces its heads or
flowers the sexual organs appear — archegonia ( 9 ) and
antheridia (S). Thus the '' sexes'' are here represented
by the haploid generation.

The egg-cell, contained in the archegonium, is ferti-
lized by a sperm-cell, the antherozooid. The fertilized
egg-cell (2N) develops in situ into the straight stalk
imbedded at its lower end in the tissue of the moss plant,
expanding at its upper end into the cup containing the
spores. The mother-cells of the spores — like the tissue of
the sporophyte itself — contain the 2N number of chromo-
somes, which, by two divisions (similar to these already
described for the animal cells during reduction), reduces

152

PHYSICAL BASIS OF HEEEDITY

the number to IN. It is at this time, too, in mosses with
separate sexes, that sex differentiation takes place, for as
the Marchals have shown, each spore gives rise to a male

x(m

%(n)

y^'® ^ ® ®

© ® XiU)

Xy(2n) Sporophyte

XiTl) Gamehphyie

® ym)

©xm)

FiQ. 59. — Life cycle of moss. The mycelial thread and the moss plant constitute
the /n, or gametophyte generation; and the stalk and capsuie (with its contained spores),
arising after fertilization out of the moss plant, constitutes the 2n or sporophyte generation.

or to a female thread that produces archegonia or else
antheridia regardless of the condition under which the
young plants are reared. Allen has recently shown in
related plants — the liverworts — that during the reduction
division (that gives rise to the spores) an unpaired sex-

VARIATION OF CHROMOSOMES

153

chromosome is present that goes to half only of the spores.
Presumably then in liverworts, and mosses, also, there
is an internal mechanism for producing the two ^ * sexes. ' '
The Marchals have worked both with species having
separate sexes and with hermaphrodites. We may con-

©T/(n) ® x(n)

Fig. 60.

xtfiAni

FiQ. 60. — Diagram illustrating the formation of 2n individuals from the regeneration of
the sporophyte in a dicBcious species. (According to Marchal.)

Fia. 61. — Diagram illustrating the formation of 2n individuals from the regeneration of
the sporophyte in a hermaphroditic species. (According to Marchal.)

sider the former first. If the sporophyte is removed and
cut across, its cells regenerate a tangle of threads (pro-
tonema) , which is the beginning of a new moss plant (Fig.
60). Since the sporophyte had the double number (2N)
of chromosomes, it is to be expected that the young moss
plant that regenerates from its tissue (sporophyte) will
also have the double number, and such proves to be the

154 PHYSICAL BASIS OF HEREDITY

case. The new moss-plant is therefore 2N (or diploid)
instead of being IN, as in the normal mode of propaga-
tion. Since no reduction has taken place into male- and
female-producing individuals, it would seem possible that
such a plant might produce either or both sexes. Such
is the case, for when the 2N moss plant produces its
^* flowers'' some contain archegonia, others spermato-
gonia (with their contained germ-cells) and other flowers
contain both. The hermaphroditism here produced would
seem to be the sum of both the contrasted elements. The
expectation from such a 2A^ plant would be that its germ-
cells {2N) would produce a 4iV sporophyte — unfortunately
the plants proved sterile. Imperfect germ-cells were
present incapable of fertilizing or of being fertilized,
so that it was not possible to perpetuate the 2N plant by
sexual reproduction.

The results with the 2N plants derived from the regen-
erating sporophyte of the hermaphroditic species (Fig.
61) is different in one important respect. When, as
before, a diploid i2N) plant is obtained by regeneration
from the sporophyte it produces hermaphroditic flowers,
i.e., flowers containing both oogonia and spermatogonia,
and these are fertile. The sporophyte that they produce
is tetraploid {4tN), due to the union of a diploid anther-
ozooid with diploid egg. Regeneration from the tetraploid
sporophyte (42V) should produce fertile gametes, which
might give rise by their union to an octoploid sporophyte
(8iV) . So far the Mar^chals have not been able to produce
such plants, for although in a few cases the 4:N sporophyte
regenerated it failed to produce flowers.

The difference then between the results from mosses
with separate sexes and mosses that are hermaphrodite is
that the 2N plant of a race with separate sexes does not
form normal gametes, while a 2A^ plant of hermaphroditic
races forms fertile gametes. It may appear more or less
plausible that the failure of the former is due to failure
in the reduction of the spores into two alternative types,

VARIATION OF CHROMOSOMES 155

while in the latter case, since there are presumably no
such types found, there is no conflict. Some other dif-
ference would have to he appealed to to explain why the
octoploid forms fail to develop.

A triploid condition (3N) has been found to occur in
certain types of the evening primrose (Stomps, Lutz,
Gates) . De Vries has found in crosses in which Lamarch-
iana was the mother and some other species (muricata,
cruciata, etc.), the father, that triploid types appear three
times in 1000 cases. He interprets the results to mean
that three in 1000 times the egg-cell of Lamarchiana has
the double number of chromosomes (14), which being fer-
tilized by a normal pollen grain with seven chromosomes,
gives the triploid number, viz., twenty-one chromosomes.
The same result would be reached if a diploid pollen grain
fertilized a normal Qgg. That such poUen grains appear
is as probable a priori as that diploid eggs occur. It
may be recalled that one explanation of the tetraploid
evening primrose (gigas) is that it arises from a 2N pollen
grain meeting a 2^ egg-cell. How reduction takes place
in the triploid Oenotheras is uncertain, since the accounts
of the process are different. Geerts states that, as a rule,
only seven chromosomes conjugate (7 + 7), while the
remaining seven chromosomes are irregularly distrib-
uted in the dividing germ-cells. On the other hand. Gates
finds in a 21-chromosome type that the chromosomes
separate into groups of 10 and 11, or occasionally into
9 and 12. The former account fits in better with results
of the same kind obtained by others, and is more easily
understood from a general point of view, because seven
homologous pairs would correspond to the normal conju-
gation, while the seven chromosomes left over would have
no mates and fail to divide at the reduction division, hence
their erratic distribution.

It has also been shown in CEnothera that there are
three 15-chromosome types. If the 15th chromosome is

1

156 PHYSICAL BASIS OF HEREDITY

sometimes one, sometimes another chromosome, there may
be genetically several types, but as yet evidence on this
point is lacking.

Irregularities in the germ-cells of (Enothera have been
observed by Gates of such a kind that one cell gets 6, the

J(Z

^l^r

a

-% ^

^■0., >>&

d

^,

Fig. 62. — Somatic chromosomes groups of (Enothera scintiilans, showing variable numbers

of chromosomes. (AfterHanse.)

other 8 chromosomes. A pollen grain with 8 chromo-
somes fertilizing an egg with 7 would give a 15-chromo-
some type. When such a 15-chromosome plant forms its
egg-cells the supernumerary chromosome having no mate
may go to either pole of the spindle, hence eggs of two

VAEIATION OF CHROMOSOMES 157

sorts would result, vis., 7- and 8-cliromosome cells.^ Such
a plant if crossed to a normal plant should give half nor-
mal (14), half 15-chromosome types. Such plants have
been shown, in fact, to be produced (Lutz). Other com-
binations that would give 22, 23, 27, 29 chromosomes have
been reported.

A variation in the number of the chromosomes of a
somewhat different kind has been described by Hance for
(Enothera scintillans, one of the 15-chromosome types of
0. Lamarchiana. No variation in number was found in
the germ-tract of the same individuals that consistently
gave two types of pollen grains, one with 7 and the other
with 8 chromosomes. The number of chromosomes in the
somatic cells was found to vary from 15 to 21. Some of
the groups are shown in Fig. 62. When the 15 chromo-
somes of the type-group are measured, it is found that
they can be arranged in respect to length in 7 pairs, with
one odd one (marked a in the figures). There is also
found a constant length difference between the pairs. In
those cases where there are more than 15 chromosomes in
a cell, measurements show that the pieces can be assigned
to particular chromosomes. When this is done. Fig. 63, the
lengths of the chromosomes come out as in the typical
cells. There can be no doubt that the extra chromosomes
in these cases represent pieces that have broken off from
typical chromosomes. This process of fragmentation
does not destroy the ''individuality of the chromosomes''
since the increase in this way of the number of chromo-
somes would not lead to any immediate change in the
number of the genes. The peculiarity of the mutant 0.
scintillans is not connected with the increase in the number
of its chromosome bodies, but rather to the presence of a
15th chromosome.

Bridges has called attention to a peculiar case in
Drosophila (1917) in which an individual behaves as

' No pollen is produced by most of the lata plants.

158

PHYSICAL BASIS OF HEREDITY

though a piece of the X-chromosome (recognizable from
its genes that normally lie in the middle of the chromo-
some) had become attached to one end of the other X-chro-
mosome. Owing to this piece (including the region that
contains the normal allelomorphs of vermilion and sable)
the individuals give unexpected results in relation to domi-
nance or recessiveness of certain factors. For example,

A8C0EPGHI JKLUNO

ABCOCFOHI JKLMNO

! >»

ft

I

2

BCOErGHlJkLMNO

a

b e

0 h

1

il

4_6CDefGMl JKLMNO

l-l

e

t

n _

A. BCOClFGHI jklumo

k I

Fig. 63. — Scheme showing the probable relation between the extra chromosome pieces of
Fig. 62. and the normal 15 chromosomes of this mutant. (After Hanse.)

a male that contains the recessive genes for vermilion and
for sable, normally located, and having attached to this
chromosome the duplicated piece (containing the normal
allelomorphs of vermilion and sable) is in appearance a
wild-type fly, instead of being vermilion sable as it would
otherwise be without the piece. On the other hand, a
female having one such chromosome and a normal ver-
milion sable chromosome is in appearance not wild type

VARIATION OF CHROMOSOMES 159

(as might have been expected), but shows vermilion and
sable, because in this case the two recessive genes for
vermilion and for sable dominate the single normal allelo-
morphs. But a female having two such duplicated chro-
mosomes {i.e., tetraploid for the genes of certain regions
of the sex-chromosome) is now wild type in appearance,
because the two dominants dominate the two recessives.
Such a female crossed to a vermilion sable male gives wild-
type sons and vermilion sable daughters, which is criss-
cross inheritance in an opposite sense from that ordinarily
met with in DrosopMla.

A second instance discovered by Bridges, but not yet
reported, seems best explained on the assumption that a
piece taken from the second chromosome has become
attached to the middle of the third chromosome. This
condition makes possible the linkage of mutant characters
to genes in both the second and the third chromosome at
the same time. The second chromosome that lost a piece,
and the third chromosome that gained the piece (both were
of course in the same cell), have been easily kept together
in the same stock ever since, because in those cases where
they become separated through assortment every zygote
that receives the deficient (2nd) chromosome dies unless
the same zygote has received the third chromosome with
the duplicate piece.

The preceding results show that chromosomes may
not only gain genes by the attachment of pieces
(duplication), but also that chromosomes may lose
pieces ( deficiency) .

Other instances of deficiency have been reported by
Bridges which can be explained either as total losses of
certain regions, or due to their inactivation. Unless the
lost pieces happen to have been retained as in the last
case, the distinction between these possibilities is difficult.
A study of one ease has shown that no crossing over takes
place in the region of deficiency, although the rest of the
chromosome was little or not at all affected. As a result

160

PHYSICAL BASIS OF HEEEDITY

the chromosome is ^ ^ shortened" by an amount correspond-
ing to the *^ length'' of the deficient region.

It is not without interest to notice that in the first
case the duplicating piece is attached to that end of the
first chromosome where the spindle fibre is attached. In
the other case the duplicating piece is attached to the

a b

Fig. 64. — An egg of Ascarishivalem fertilized by sperm of A. univalens, a; later stage

of same, b.

middle of the third chromosome, and in this chromosome
the spindle fibre is attached to the middle.

An interesting case of triploidy has been reported in
the threadworm Ascaris (Boveri). Two varieties occur,
one with four chromosomes (haploid two), and one with
two (haploid one). Rarely a female of one variety is

•*»

Fig. 65. — Diploid and haploid groups of the sundew Drosera. (After Rosenberg.)

found that has mated with a male of the other variety.
The fertilized eggs have each three chromosomes (Fig.
64). As yet no triploid adults have been met with, so that
the method of conjugation of the chromosomes in the
triploid types is not known.

Rosenberg crossed two species of sundew, Drosera
longifolia, with 40 chromosomes (haploid 20), and D.
rotundifolia, with 20 chromosomes (haploid 10), Fig. 65.

VARIATION OF CHROMOSOMES

161

The hybrid had 30 chromosomes (20+10). He found that
when this hybrid produces its germ-cells they show, after
reduction, 20 chromosomes, which he interprets as due to
10 of the rotundifolia conjugating with 10 of the longi-
folia. This leaves 10 without mates. At the following
maturation division Rosenberg describes the 10 paired
chromosomes as reducing, sending one member of each
dyad to one pole, the other member to the other ; but the

Zygote. Conjugation. Kedudion. Gamete

Ess-

>perm.

II

ni

Fia. 66. — A acheme illustrating the fertilization of the egg of one species of moth by
the sperm of another, with reduction in I, with no reduction in II, and with partial reduc-
tion in III.

10 unpaired chromosomes are irregularly distributed at
this division. If the account is confirmed, the situation
is peculiar, for if the 20 (haploid) chromosomes of longi-
folia correspond to the 10 (haploid) of rotundifolia it is
not obvious why all 20 might not find a place alongside
of the 10, unless chance or some difference of length, etc.,
makes this impossible. This assumes, however, that longi-
folia is not tetraploid — if it is, then a further question
arises as to which chromosomes of each set of three would
be the ones most likely to conjugate, etc.

Crosses between three species of the moth PygcBraj
11

162

PHYSICAL BASIS OF HEEEDITY

having different chromosomes, were made by Federley.
The hybrids showed intermixed characters of both
parents, and their chromosome number was the sum of
the haploid numbers of their parents (Fig. 66),

No reduction in number of the chromosomes takes
place in the hybrid at the synaptic stage ( except perhaps
for one or two small ones), so that the 1st spermatocytes
contain nearly the sum of the haploid number of the

E^^-

>periiv

Zygote. Conjugation. Reduction. Gamete.

IV
F.XP

V

F.XP.

Fia. 67. — Scheme illustrating the history of the chromosomes, and the back-cross between a
hybrid male and one or the other parent; also between two such hybrid Fi individuals.

parents {A and B) after division of each chromosome
(Fig. 67). A second maturation division follows in which
each chromosome again divides. As a result each sperm
contains the full number of chromosomes, half paternal,
half maternal {A and B), The hybrid female is sterile,
but the male is fertile. If he is back-crossed to a female
of the A race his sperm, carrying both sets of chromo-
somes, will produce a 3N individual, A -}- B -{- A. It will
have two sets of the A genes to one set of B. In appear-
ance the moth is practically the same as the F^ hybrid,
because both contain both sets of chromosomes — the

VARIATION OF CHROMOSOMiES 163

double set AA with B not producing any striking differ-
ence from the single set A+B. When this second hybrid
(3^) matures its germ-cells, the two homologous series
(A + A) mate with each other, and then segregate at the
first division, while the unmated 5-series simply divides.
At the second division both the A- and the J5-series divide,
thus giving to each sperm a haploid set of chromosomes
[A + B). The sperm then is the same as the sperm of
the first hybrid. So long as the back-crossing continues
the outcome is expected to be the same.

If, instead of back-crossing the first hybrid to parent
^, it is back-crossed to parent B, the same result as
before takes place, except that the second hybrid is now
A-\-B-\-B. When it matures its germ-cells, the B's
unite and then separate, giving AB sperm as before.

Here then we find a kind of inheritance that super-
ficially appears to contradict the generality of MendePs
law of segregation. On the contrary, a knowledge of the
chromosomal behavior shows that the results are different
because the mechanism of conjugation of the chromosomes
is changed, and changed moreover in such a way that on
the chromosome theory itself the results are what are to
be expected.

These crosses are so important that some further
details may be added. The whole {2N) and half (IN)
number of chromosomes of the three species studied by
Federley are as follows :

Whole Half

Pygaera anachoreta 60 30

Pygaera curtula 58 29

Pygaera pigra 46 23

In the hybrid between the first two species the number of
spermatocyte chromosomes was found to be 59 (30 -f 29)*
No union between any of the maternal and paternal chro-
mosomes could have taken place. But in the hybrid
formed by the union of the two more nearly related spe-
cies, curtula and pigra, the number of spermatocyte chro-

164 PHYSICAL BASIS OF HEKEDITY

mosomes was found to be as a rule somewhat smaller than
the sum of the parental haploid numbers, indicating that
one or more had conjugated. To the extent to which such
union, and the consequent reduction, takes place, the
characters of the second hybrid generation may differ
from those of the first — at least if the conjugating pairs
have different factors in them.

A similar behavior of the chromosomes has been
described by Doncaster and Harrison for two species of
moth of the genus Biston (Fig. 24). The hybrids were
sterile, and no further generations were raised.

Federley later made similar crosses with three other
moths. A cross between Smerinthus ocellata (with 27
chromosomes as the haploid number) and Dilina tilice
(with 29) he regards as a cross between genera. A cross
between 8. ocellata and 8, populi (with 28) he regards as a
species cross. A cross between 8. ocellata and 8. ocellata
var. planus he regards as a racial, or varietal, cross.
As before the spermatocytes of the hybrid have the sum
of the two parental numbers of chromosomes (or a few
less at most) . In other words, conjugation of the chromo-
somes does not take place. The most unexpected result in
these combinations is that the types that are so alike as to
be classified as varieties behave as regards conjugation
like the other two combinations. The results suggest that
ordinary conjugation may not be due to the similarity
of the sets of genes carried by the chromosomes so much
as to other peculiarities of the combination.

CHAPTER XIV

SEX-CHROMOSOMES AND SEX-LINKED

INHERITANCE

The discovery that the female in certain species of
animals has two X-chromosomes and the male has only one
X-chromosome, either with a Z-chromosome in addition
(Stevens) or without the Y (Wilson), established a view
first suggested by McClung that the difference between
the sexes is connected with the distribution of particular
chromosomes. Two interpretations of the facts have been
proposed : The first, and most obvious one, was that the
presence of two sex-chromosomes (XX), in connection
with the rest of the cell complex, causes a female to
develop ; while only one sex-chromosome (X) in connection
with the rest of the cell causes a male to develop; the sec-
ond interpretation was that of XX and X are merely
indices of sex, i.e., that the sex-chromosomes follow sex
and do not determine sex.

It is now possible to show that sex follows the chromo-
somes and not the reverse, because if a * ^female produc-
ing*' sperm (X) fertilizes an egg without an X (as excep-
tionally occurs) an XO individual is produced that is a
male, whereas if this same sperm had fertilized an egg
with an X, giving an XX individual, a female would be
the result. Conversely when a *^male producing'' Y-
sperm fertilizes an egg with two X^s (as exceptionally
occurs) an individual is produced that is a female, despite
the presence in her of a Z-chromosome.

The Sex-Chromosome

It will be convenient to treat the XX-XZ type of com-
bination first. I shall follow the usual custom of calling
both X and Y sex-chromosomes.

165

166 PHYSICAL BASIS OF HEREDITY

At the time when the polar bodies are extruded from
the eggy the two X's separate, one passing out, the other
remaining in the egg. Every egg is left with one X
(Fig. 68).

In the male, the X and Y conjugate and separate at one
of the maturation divisions, so that each sperm contains
either an X- or a Z-chromosome (Fig. 68). Fertilization
of any egg (X) by an X-bearing sperm produces a female

Female XX XYM-'^

->^^

^

i^ggs yY yv ^P®"^^ Y

Female XX XY^ale

Fig. 68. — Scheme showing the relation of the sex-chromosome to sex-determination.

XX-XY type.

(XX) . Fertilization of any egg (X) by a Z-bearing sperm
produces a male (XY).

Since the two kinds of spermatozoa are produced in
equal numbers, females and males will be equal in num-
ber. The mechanism is self-perpetuating.

The Inheritance of Factors Carried by the Sex-
Chromosomes in the Drosophila Type

Since the son gets his one X-chromosome from his
mother, and the Y from his father, he inherits factors
carried by the sex-chromosomes in a different way from

SEX-CHKOMOSOMES AND INHERITANCE 167

the way in which he ihlierits the factors carried by
the other chromosomes (autosomes), because X and Y
differ from each other in a wav in which no other chromo-
somes differ.

The recessive gene for white eyes (w) in Drosophila is
carried by the X-chromosome. It is inherited in the fol-
lowing way (Fig. 69) : When a male with white eyes (w)
is mated to a red-eyed female (TFTF), the F\ sons and
daughters have red eyes. When these are bred to each
other, all the daughters have red eyes (50 per cent.), half
the sons have red eyes (25 per cent.) and half the sons
have white eyes (25 per cent.). The ratio, irrespective
of sex, is three red to one white, but the white-eyed flies
are found only amongst the males. In the diagram (Fig.
69), the relation of these results to the sex-chromosomes
is shown. The X-chromosome that carries the normal
gene (wild type) which gives red eyes is indicated by
W. The X-chromosome that carries the gene for white
eyes is indicated by w. The rod with a bent end stands
for the Z-chromosome.

The Fi daughters contain one of each kind of X-chro-
mosome. The Fj sons only one kind. The recom-
binations that give the F2 results are shown in the middle
of the lower part of the diagram. Half of the females
are seen to be homozygous for the wild-type gene (W).
They should never transmit white eyes, and they do not.
The other half of the females are heterozygous (Wtv), and
if mated to a white-eyed male should give 50 per cent,
red-eyed males and females, and 50 per cent, white-eyed
males and females. This they do. The red F2 sons (W)
should never transmit white eyes, nor the white-eyed sons
(w) transmit red eyes. These relations are also known
to hold.

The reciprocal cross (Fig. 70), viz., a white-eyed
female (luw) to a red-eyed male (W) gives red-eyed
daughters (wW) and white-eyed sons (iv). If these F^^s

168 PHYSICAL BASIS OF HEEEDITY

w

n

V-/

Fig. 69. — Cross between white-eyed male and a red-eyed female of the \4negar fly.

SEX-CHROMOSOMES AND INHERITANCE 169

If w

<J \^

^ o

vJ \J

Fia. 70. — Cross between white-eyed female and a red-eyed male of the vinegar fly.

170 PHYSICAL BASIS OF HEEEDITY

are bred together, the results are as follows: Half the
daughters have white eyes {unv)y half red eyes (wW) ;
half the sons have white eyes (iv), half red eyes (TF). It
will be seen that the red-eyed Fg daughters are all hetero-
zygous, and should give 50 per cent, white and 50 per cent,
red offspring if mated to white-eyed males. This occurs.

Similar illustrations might be given for any of the
50 sex-linked characters of Drosophila. Of these the sex-
linked lethals form the most interesting cases and will be
spoken of in another connection.

Despite the fact that the results in one of the two fore-
going crosses gave a 3 : 1 ratio, and in its reciprocal a 1 : 1
ratio, the results in both cases conform to Mendel's first
law of segregation. The peculiarity of the 1 : 1 ratio is
due to the fact that the Pj red-eyed male is in a sense
heterozygous for the wild-type eye color (since he has
but one X-chromosome that carries the factor for red
eyes). Since in the second cross the F^ male gets no red-
producing X from either parent, he is pure for white
eyes in the sense that he has an X beaiing the factor for
white eyes and a Y that bears no factor making red.
Hence this F^ cross is exactly like a back-cross of a hetero-
zygous female to a recessive male, and gives the same
numerical result, viz., 1 : 1.

Cases of sex-linked inheritance of this kind are also
known in man. Color blindness in man appears to follow
exactly the same procedure as sex-linked inheritance in the
vinegar fly — at least certain kinds of color blindness have
been shown to do so. H^maphilia also is sex-linked, and
there are four or five other defects in man that appear
to come under this head. According to several accounts
there is an unpaired sex-chromosome (or two of them)
in man, which is also called for by the genetic evidence
relating to sex-linkage in man, but since the female number
of chromosomes in man is stated by Guyer to be 24, and
by von Winiwarter to be 48, it is unsafe as yet to appeal

SEX-CHROMOSOMES AND INHERITANCE 171

FiQ. 71. — Crosa between a yellow, white-eyed female and a wild-type ("gray").

red-eyed male.

172 PHYSICAL BASIS OF HEREDITY

to this evidence as showing the identity of the sex-deter-
mining mechanism of man and the vinegar fly.

When two or more sex-linked characters are involved
at the same time, the situation is different only in so far
as crossing over may take place in the female. It will be
simpler to consider such a cross and its reciprocal in the
reverse order from that just given. If a female with
yellow wings {yy) and white eyes {wiv) is crossed to a
wild- type male, *^gray*^ wings (Z) and red eyes (TF), the
sons are yellow white and the daughters are gray red
(Fig. 71). When these are inbred there are four types
in F2 (ignoring sex), viz., the two original combinations
yellow white and gray red, and the two crossover com-
binations yellow red and gray white. They occur in the
following ratios :

Yellow white Gray red Yellow red Gray white

99 per cent. 1 per cent.

In this case the Fj male acts as a double recessive, reveal-
ing the amount of crossing over in the F^ female. Since
neither his female-producing nor his male-producing
sperms carry factors that cover up the characters carried
by the four classes of gametes in the F^ female, all four
classes of her gametes are revealed in their numerical
proportions. Reciprocally, when a male with yellow
wings {y) and white eyes {w) \^ crossed to a wild- type
female (gray {YY) red {WW), both sons and daugh-
ters are gray red, because both get the dominating genes
for these characters carried by the X-chromosome
received from the mother. If these F-^^s are inbred (Fig.
72), the 2^2 females are gray red, since each contains an
X with the two dominant genes derived from the father
whose genes have remained completely linked, as there
is no crossing over in the male. On the other hand,
there are four kinds of F2 males : yellow white ; gray red ;
yellow red; gray white; because each male shows the

SEX-CH'EOMOSOMES xVND INHERITANCE 173

character of his single Z-chromosome, and there are four
kinds of these chromosomes in his mother on account of
crossing over in the female. The other sex-chromosome,
the r, has no dominating influence.

Fig. 72. — The fa results from the reeiprccal cross of that shown in Fig. 71.

Sex-linked Inheritance oe the Abeaxas Type
In certain moths and birds it has been shown by the
genetic evidence that the female is heterozygous for sex-
linked factors. The cytological evidence, as far as it goes,
supports this evidence, but for birds the material is so

174 PHYSICAL BASIS OF HEEEDITY

difficult to interpret that Gruyer's conclusions do not seem
to me as yet to be on as secure grounds as those of Seller's
for moths. Both descriptions give, however, the bases for
a consistent explanation of sex-linked inheritance in this
type {WZ-ZZ),

Since we do not know as yet whether the same or dif-
ferent sex factors are involved in the Drosophila and in
the Abraxas types, it seems best not to use the same sym-

FeaWZ ZZm-i

r

j^.

Wfe'Z Z s

P

erm

FemaleV/Z ZZ ^al

Fig. 73. — Scheme showing the relation of the sex-chromosomes of the moth (and of the bird)

in sex-determination. WZ-ZZ type.

bols in both for the sex-factors. If in both types a single
sex-factor is concerned, and if it is the same in both, the
conditions that make for a female in one case and for a
male in the other must be due to a difference in the
rest of the hereditary complex that reverses the reaction.
It would appear simpler to assume that the sex-factor itself
is different in the two cases. If there is more than one
factor for sex, the two types may have some in com-
mon, but the theoretical situation would remain the same.
For our present purpose these possible distinctions are
of no importance.

SEX-CHROMOSOMES AND INHERITANCE 175

If the sex-chromosome that carries the sex-linked genes
in birds and moths be symbolized by Z, and its homologue
that occurs in the female by W, the scheme for sex-deter-
mination is that shown in Fig. 73 : The eggs of the female
extrude either one or the othe^ sex-chromosome. If Z
stays in, and this egg is fertilized by a sperm (Z-bearing

LACTICOLOR 9 OL

GROSSULARIATA 6 CG

O © G™ CEILS ®

GR055ULARIATA Q OG ,

GROSaiLARlATA d LG

O ® GERM CELLS,. ® ®

o ®-

LACTICOLOR <? OL

O ®' ® ®

GR0SSULAR1ATA9 OG CR0S5ULAR1ATA c5 GL

® ®

GR055ULAR1ATA 6 GC

Fig. 74. — Cross between Abraxas lacticolor female and groasulariata male.

also) a male (ZZ) is produced; if W stays in, and the egg
is fertilized by a ^-bearing sperm, a female (WZ) is
produced. The way in which sex-linked characters are
transmitted may be illustrated by the inheritance of a
color difference in the currant moth Abraxas. The wild
species (grossulariata) has a mutational variety called
lacticolor, that differs from the former by having less

176

PHYSICAL BASIS OF HEEEDITY

black pigment in the wings. When a dark (grossnlariata)
male is mated to a light (lacticolor) female, both sons and
daughters are dark (Fig. 74). If these are inbred all the
F2 sons are dark, half the daughters are dark, half light.
As the diagram shows, the distribution of the Z-chromo-
some furnishes the mechanism by means of which we can

GR05SUlJ^RlATA 9 OG

I^CTICOLOR -d LL

O ® GERM CELLS ®

LACTICOLOR 9 OL

GROSSULARIATA <5 GL

O ©.GJR1 CELLS ^„(g) ®

O ®"" O ®" © ®

GR05SULAR1ATA 9 OG LACTICOLOR 9 OL GR0S5ULARIATA d LG

® ©

LACTICOLOR 6 LL

Fig. 75. — Cross between Abraxas grossnlariata female and lacticolor male.

explain, as in Drosophila, the process of sex-linked inheri-
tance in this moth.

The reciprocal cross is shown in the next diagram (Fig.
75) in which a dark (gross.) female is mated to a light
(lact.) male. The daughters are light like the father, the
sons dark like the mother — criss-cross inheritance. The
daughters get their one Z-chromosome carrying the light

SEX-CHROMOSOMES AND INHERITANCE 177

factor from their father, the sons get in addition to a light
Z from their father a dark dominating Z from their
mother. When the F^^s are bred together four classes
result in the proportion of 1:1:1:1, when sex is taken
into consideration, or in the ratio 1 : 1 for the color differ-
ences alone.

According to Doncaster, the male and the female
Abraxas have each 56 chromosomes, i.e., the female is ZW
rather than ZO; but as yet the sex-chromosomes as such
have not been identified. That sex is connected with such
chromosomes is not only established by sex-linked in-
heritance, but is also indicated by an aberrant race of
Abraxas found by Doncaster. The males of the race had
the normal number of chromosomes (56), but the females
had only 55 chromosomes. Doncaster found that in these
females an unpaired chromosome, presumably the Z-chro-
mosome, was more often thrown out into the polar body
than left in the egg, so that most of the resulting eggs had
only 27 chromosomes. Any egg of this kind fertilized
by a spermatozoon should give a 55-chromosome indivi-
dual, i.e., a female. The few eggs that retained the
unpaired Z-chromosome, fertilized by a Z-spermatozoon,
would be expected to give rise to the rare males, which
like normal males have 56 chromosomes. The excess of
females is thus accounted for, and incidentally the results
show that the TF-chromosome carries no essential factors
for the life of the individual, since females without it
develop and look like normal females. Probably it is
empty as is the Y of Drosophila.

In poultry there are several cases of sex-linked inheri-
tance that follow the Abraxas^ type. One of the most
striking cases is the cross between Barred Plymouth Rock
and Langshan. When a barred male is crossed to a black
female, the sons and daughters are barred (Fig. 76).
Barring is dominant to black. Two such F\ 's, inbred, give
all barred males ; half the hens are barred, half are black.
It may be said here that the black grandmother transmits

12

178 PHYSICAL BASIS OF HEEEDITY

her black color to only half of her grandsons. The chro-
mosomal explanation can obviously be worked out on the
same scheme as in Abraxas (Fig. 77). But if Guyer's
recent account of spennatogenesis in birds is correct, the
situation is different. Guyer describes the ripening of
the sperm as follows: There are 18 chromosomes in the
male, including two large Z's (16 + 2). After synapsis
there are 9 double chromosomes in the first spermatocyte,
all of which, except ZZ separate at the first maturation
division, 8 going to one pole and 8 to the other. One

Barred c? Black p

^' z^z^ zV

zV zV

1 Darredc? Barred

z^z° z^z' zV zV

Barredc? Barred c? Barredo Black 9

FiQ. 77. — Scheme showing the transmission of the sex-Hnked characters B = barred, and

b =black in the cross shown in Fig. 76.

daughter cell gets both Z's {S + 2). This cell then divides
again, the Z's presumably separating so that two second
spermatocytes are produced, each with 9 chromosomes
(8+1), including the Z. These become the functional
sperm. The other spermatocyte, the one without a Z,
may divide again, but it, or its products, degenerate, and
never produce sperm. According to Guyer, there are 17
chromosomes in the female, including one Z. Presum-
ably, then, after reduction half of the eggs will contain a
Z (8 + 1), the other half will be without it (8). The egg
that carries a Z (8 + 1), fertilized by a sperm (each sperm
carries a Z (8 + 1)), will make a male with 18 chromo-

I\

Fi3. 76.— Cross between Barred Plymouth Ro"ck male and Black Langshan female.

Fi3. 73. — Cross between Black Langshan male and Barred Plymouth Rock female.

SEX-CHEOMOSOMES AND INHERITANOE 179

somes, including two Z's. The egg that lacks a Z (8), fer-
tilized by a sperm (8 + 1), makes a female with 17 chro-
mosomes, including one Z.

This scheme gives consistent results for sex-linked
inheritance in birds. Since the daughter gets her single
Z-chromosome from her father, she will show any sex-
linked characters carried by his Z-chromosome. If the
father carries a sex-linked dominant gene his sons and his
daughters will be alike. It should be noticed that while
Guyer's scheme gives the same results so far as sex-link-

Blackcf

Barred (

Pi

Fi

Darredc? Black

Barredd^ DIack c?

zV

Darredg Black o

Fig. 79. — Scheme showing the transmission of the sex-linked characters B = barred, and

b = black in the cross shown in Fig. 78.

age is concerned, as the one described by Seller for some
moths, the machinery in the male is dilferent in the two
cases, while that in the female is presumably the same. In
both the female is heterozygous for Z; in the moth the
male is homozygous {ZZ)^ but in the bird the two Z's
described by Gruyer both go to one pole at one of the
maturation divisions, and reduce at the other — a proce-
dure not known in any other animal.

In the reciprocal cross (Fig. 78) a black cock is bred
to a barred hen. The sons are barred — like their mother —
the daughters are black— like their father, criss-cross
inheritance. Wlien the barred Fj cock and the black hen

180 PHYSICAL BASIS OF HEREDITY

are inbred, there are four F^. classes with sex taken into
account in the proportion of 1:1:1:1; or ignoring sex,
1 barred to 1 black. The barred and the black races
differ by one factor difference (Fig. 79), viz., barred Z^
and its normal recessive allelomorph Z^. This seems to
mean that the Barred Plymouth Rocks is a black race
with an additional dominant factor for barring. The
Black Langshan is the same black race but without the
barring factor.

Until quite recently no cases of crossing over had been
observed in forms having the Abraxas type of sex-linked
inheritance, for, except in one or two cases in poultry,
only a single pair of sex-linked genes were known, and two
at least must be studied together in order to demonstrate
linkage. Goodale has recently studied two sex-linked
characters in poultry, and states that crossing over occurs
in the male, but whether or not in the female is not stated.

Sex-determination and Natural Parthenogenesis

Variations in the ordinary sex-determining mechanism
account in some cases for the normal output of males and
females produced by parthenogenesis, and determine the
exceptional sex-ratios of such species. The honey bee
furnishes the best known example. The queen comes
from a fertilized Qgg, and has therefore the double {^N)
number of chromosomes. Her eggs give off two polar
bodies, hence have the reduced, or single number of
chromosomes. Any eg^ that is not fertilized develops
parthenogenetically into a male. If there are two X-chro-
mosomes in the bee, as in some of the other insects, the
Qg^ is expected to contain only one of them after the
extrusion of the polar bodies. Hence, if it develops with-
out doubling its chromosomes, it should give rise to a
male. That the male has the single number of chromo-
somes is also borne out by the evidence from a peculiarity
of the first spermatocyte division in which the cytoplasm
divides, but the chromosomes do not separate into two

SEX-CHROMOSOMES AND INHERITANCE 181

groups. Several stages in the maturation of the sperma-
tozoon of the bee are shown in Fig. 80. In a, the spindle
for the first spermatocyte division has appeared. A small
piece of the cytoplasm cuts off, but the chromosomes do
not separate, and they return again {h and c) to a resting

a

a e

Pro. 80. — First spermatocyte divisions a-c, and the second spermatocyte division d-g

in the bee. (After Meves.)

stage. Another spindle forms {d)^ and the chromosomes
separate into two groups, one of which is pinched off
as a rudimentary cell that never becomes a spermatozoon.
Hence only one, and not four spermatozooa as in ordi-
nary cases, is formed from each spermatocyte. In the
hornet (Fig. 81), the spermatogenesis is similar to that of
the bee in that the first division is abortive. It is different

182

PHYSICAL BASIS OF HEREDITY

C

d

f

Fig. 81. — First spermatocyte division a-c, and the second spermatocyte division d-J in

the hornet. (.After Meves.)

in that the second division produces two functional
sperms, both female producing.

Since the male comes from an unfertilized egg, the

->%/-^'3B^^ rai//t7fra//A)

JT/m /%7///W

\J

m

0

Fig. 82. — Life cycle of Phylloxera carycecaulis.

SEX-CHEOMOSOMES AND INHERITANCE 183

queen must transmit to him all her characters, thus giving
rise to a form of inheritance that has a superficial resem-
blance to sex-linked inlieritance. A queen of a pure race,
bred to a male of another race with a dominant factor,
produces daughters all showing the dominant character
of the father, and sons all showing the recessive character
of the mother. Since the son gets his entire chromosome-
complex from his mother, he must necessarily be like her,
whether the character in question is in the sex-chromo-
some, or in some other one.

yik^g^ ... Sv^l^K ■ ■ •■••■ ■'• •'•••■•:. ••.■.•••;•: :■.,•' ■. .''■Si

-*«*■■

"•■'■/•> ..'.•...'

(^

Fig. 83. — Extrusion of the polar body from a male-producing egg with lagging chro-
mosome.s on the spindle, a; and extrusion of the polar body from a female-producing
egg, 6; in Phylloxera.

In the phylloxerans there are two parthenogenetic
generations followed by a sexual one (Fig. 82) . In the sec-
ond parthenogenetic generation two whole chromosomes
leave certain eggs (Fig. 83) passing into* the single polar
body which is given off from the egg. Such eggs have two
less sex-chromosomes and develop parthenogenetically
into males. In other eggs of the same generation all four
sex-chromosomes are retained after the polar body is
produced. These eggs also develop parthenogenetically,
but produce females. Similar changes take place no doubt
in the aphids, for the males have been shown to have one
less chromosome than the female, although the loss of one

184

PHYSICAL BASIS OF HEREDITY

chromosome in the polar body has not yet been observed
in the group.

In both phylloxerans and aphids there are two classes
of sperm produced in the males as in other insects, one
with X, one without it. The latter degenerates, and only
the X or female-producing sperm remains functional. A
few stages in the spermatogenesis of the bearberry aphid

a

Fia. 84. — First and second spermatocyte division in the bearberry aphid with the formation

of one rudimentary cell.

are shown in Fig. 84, a^g. In b, the chromosomes have
divided and moved to opposite poles while the sex-chromo-
some is drawn out but has not moved yet to either pole.
In c, the sex-chromosome has been drawn into the larger
of the two cells that is produced. In d, the division into a
larger and a smaller cell is completed. In e, preparations
for another division are taking place in the larger cell, and
in / and g this is completed. The smaller cell does not
divide, and later degenerates. The two spermatozoa from

SEX-CHROMOSOMES AKD INHERITANCE 185

the two larger cells each contain one X-chromosome and
two autosomes. They correspond obviously to the female-
producing sperm of other insects. Hence only females
arise from fertilized eggs.

The rotifers, especially Hydatina senta, are the only
animals in which the transition from parthenogenetic to
sexual reproduction has so far been gotten under con-
trol by regulating the environment, and although the
evidence that the environment causes part of its effects by
influencing the chromosomal mechanism is not yet estab-
lished, there is, in my opinion, some indication that such
is the case. The common method of reproduction in
Hydatina is as follows : A parthenogenetic female (Fig.
85, A) lays eggs (D), each of which, after giving off a
single polar body, develops at once {i.e., without fertiliza-
tion) into a female like the mother. The whole number
of chromosomes is retained in the eggs. Several or many
generations may be produced in this way. Whitney has
shown that if such females are fed on a green alga,
Eiiglena, daughters appear (structurally like the others)
that produce smaller eggs {E). If these eggs develop
without fertilization they become males (C). Examina-
tion of these small eggs show that they give off two polar
bodies, and retain a reduced number of chromosomes. This
processes the same by which the male bee is produced.

If the female, that produces the small eggs just
described from which the males develop, should have been
impregnated by a male soon after she hatched, her eggs
would then grow larger and surround themselves with a
thick-walled coat. They become the winter or resting
eggs. Each such eggy after the sperm enters, gives off two
polar bodies, reducing in this way the number of its chro-
mosomes. By the addition of the sperm nucleus the full
number of chromosomes is recovered.

Whitney has recently shown that there are two classes-
of spermatozoa produced by the male, large and small ;*

186

PHYSICAL BASIS OF HEREDITY

for, owing to the few sperms produced by each male their
actual immber can be counted. There are twice as many
large as small spennatozoa, if, as may be the case, only
the large ones contain chromosomes and are functional.

Fig. 85. — Hudatina senita, adult female, A; young female soon after hatching, K; adult
male. C; parthenogenetic egg, D; male-producing egg, E; resting egg, F. (After Whitney. )

the conditions here would appear to be like those in the
hornet, provided there are no chromosomes in the small
spermatozoa. This would also explain why all fertilized
eggs produce females.

So long as the ordinary parthenogenetic females are
fed on the poor diet of Polijtoma, they continue to produce

SEX-CHROMOSOMES AND INHERITANCE 187

parthenogenetic females like themselves (Fig. 86), and
this non-sexual process continues indefinitely. If on the
contrary, parthenogenetic females are fed abundantly on a
rich diet of the green alga Euglena, their eggs develop into
individuals which, if early fertilized as explained above,
become sexual females, i.e., they lay fertilized eggs, but if
not fertilized, produce small eggs that, developing par-
thenogenetically, become males. In other words, the same
female becomes either a sexual female, or a female that

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Fig. 80. — Diagram showing how a continuous diet of Polytoma (P-P) through twenty-
two months yielded only female-producing females, but when the diet was suddenly changed
to Chlamytlomonas (at C), male-producing females appeared at once. (After Whitney.)

gives birth to males. Some recent writers, misunderstand-
ing these relations, have tried to make it appear that the
change here is one that is sex-determining, using this
expression to all appearances as it is ordinarily employed
in other cases, but in fact usifig the term in such a way
as to obscure the one important fact that the results really
show, viz., that an environmental change of a specific kind
produces a new kind of female that is either a producer
of eggs that become males (after or because two polar
bodies' are extruded), or becomes a sexual female, should
she early meet a male.

188 PHYSICAL BASIS OF HEREDITY

Sex-determination and Aetificial Parthenogenesis

Many interesting questions concerning sex-determina-
tion might be studied were it as easy for man, as it appears
to be for nature, to make eggs develop without fertiliza-
tion. Only three cases are known in which eggs developing
under artificially induced conditions have reached matur-
ity. Delage raised one sea urchin that had been produced
artificially to maturity, and determined that it was a male.
Tennent has shown that the male is heterozygous for the
sex-chromosomes. Hence, if the artificially produced
urchin has the half number of chromosomes it should, if
like the bee, be a male, but if, as Herlandt has shown, the
number of chromosomes may double before development,
a female would be expected.

In the frog, Hertwig, and later his pupil Kuschake-
witch, found that the number of males is increased up to
100 per cent, if the eggs are detained in the uterus for
one to three days before adding sperm to them. Hertwig
has attempted to explain the result as due to a relative
change in the size of the nucleus that takes place in conse-
quence of the delay, but since the chromosomes are at this
time in the metaphase of the second polar spindle, it is not
obvious how such an enlargement could be brought about,
quite aside from the question as to whether the result
imagined would follow even after such a change. I have
suggested that these eggs with deferred fertilization may
develop parthenogenetically, due either to the egg nucleus
alone giving rise to the nuclei of the embryo, or to the
sperm alone giving rise to these nuclei, in the latter case,
the polar spindle of the egg having been caught at the sur-
face and prevented from taking part in the development.
The possibility of the nuclei of the frog arising in one
or the other of these ways is shown by the work of Oscar
and Gunther Hertwig who have found evidence that after
treatment with radium, the sperm-nucleus alone may give
rise to the somatic nuclei of the embryo. Packard also

SEX-CHEOMOSOMES AND INHERITANCE 189

has shown that such kinds of androgenetic embryos
may arise in the eggs of CJicetopterus treated with radium,
and by following every stage in the process he has
determined also that the embryos have the reduced
number of chromosomes.

Other work on the egg of the sea-urchin had seemed
to show that while in most cases the egg, that begins to
develop parthenogenetically, starts with, and continues
to maintain the half number of chromosomes, yet accord-
ing to a recent observation of Brachet, a parthenogenetic
tadpole, eighteen days old, that he produced, had the
double number of chromosomes. Whether it may turn out
that when the egg nucleus gives rise to the nuclei of the
parthenogenetic individual it may sometimes double its
number of chromosomes (by failure of the first cytoplas-
mic division, for example), and that when a sperm gives
rise to these nuclei the half number is retained, cannot be
stated. Until we have farther information on these points
the expectation as to what the sex of parthenogenetically
produced frog individuals will be can only be speculative.
Loeb has raised seventeen adult, or nearly adult male
frogs and three nearly adult female frogs from eggs devel-
oping after Bataillon's puncture method of inducing par-
thenogenesis. One male frog had more than the half num-
ber of chromosomes (at least 20 and presumably the
whole number, 26!). The number of chromosomes in the
females was not determined.

GrYNANDROMORPHS AND SeX

In the group of insects especially, it has long been
known that individuals occasionally appear that are part
male, part female. In the most striking cases the line
of division runs down the middle of the body, but there
are also antero-posterior gynandromorphs, and individ-
uals with only a quadrant or even a small piece of the body
different from the rest in its sex character. Several
hypotheses have been advanced to explain these rare com-

190 PHYSICAL BASIS OF HEEEDITY

binatioiis of the two sexes, and it is probable that g^Tian-
dromorphs may arise in more than one way, but in Droso-
pliila it can be demonstrated that the great majority of
gynandromorphs result from dropping out of one of the
sex-chromosomes at some early division of the fertilized
^gg. The demonstration is made possible by using sex-
linked characters that are known to be carried by the sex-
chromosomes. For example : Yellow body color in Droso-
pJiila is due to a recessive gene carried by the X-chromo-
some. Its allelomorph (wild type) lies also, of course, in
the normal X-chromosome. If yellow is crossed to wild,
and a bilateral gynandromorph should arise, it may be
yellow on the male side (as seen in the yellow wings and
yellow hairs over half the body) and wild type on the
female side (Fig. 87).

Since the male characters arise when only one sex-
chromosome is present, it must be the yellow-bearing
chromosome in this case that gives the male side. Since
the female characters arise when two X's are present,
both must be present in the female side, which will here
be the wild type, since the gene for wild type domi-
nates the yellow-producing gene. The gynandromorph
must have arisen, therefore, at a very early nuclear divi-
sion in the egg in which one daughter X-chromosome failed
to pass into one of the daughter nuclei. The diagram
(Fig. 88) shows how such a result might be supposed to
have come about.

The diagram indicates that one daughter chromosome
X' (bearing the gray gene) has failed to become incor-
porated in its proper nucleus, which is therefore left with
only one X. From this nucleus the nuclei of the male half
are produced, while from the XX nucleus the nuclei of the
female half arise. That both of these nuclei, the XX and
the X nucleus contain other chromosomes derived from
both parents has been shown by making one of the original
parents homozygous for some recognizable autosomal

NL>

Fig. 87. — A gynandromorph of Drosophiia melanogaster that was female on the right side
and male on the left. It was also yellow on the male side and gray on the normal side.

SEX-CHROMOSOMES AND INHERITANCE 191

character. It, or its normal allelomorph, should therefore
be present in both nuclei if all the chromosomes of the
fertilized egg have divided normally except the X-chromo-
somes. This, in fact, has been found to be the case (Mor-
gan, Bridges, Sturtevant).

Nearly all of the many hybrid gjmandromorphs of
Drosophila can be explained as above. In a few cases,
when the abdomen of the fly was sufficiently female to
make mating possible, it has been found that the eggs give
the results expected for a female having the sex-linked
factors that entered the cross.

Fig. 8S. — Diagram showing elimination of X' at an early cell-division, so that the nucleus
to the right gets X and X' and that to the left only X.

In a few cases in Drosophila the explanation of chro-
mosomal dislocation will not cover the results. Some of
these cases can, however, be accounted for by another
hypothesis. Should an egg arise with two nuclei (there
are several possible ways for this to occur), one nucleus
having one set of factors, the other the other set (the
parent being heterozygous), then if each nucleus is sepa-
rately fertilized a difiPerent combination of factors is pos-
sible from that possible on the elimination theory. A
gynandromorph, described by Toyama, appears to belong
to this category. Toyama found two gynandromorphs
of the silkworm (Fig. 89) whose mother belonged to a race
with banded caterpillars, and whose father belonged to a

192

PHYSICAL BASIS OF HEREDITY

race with pale caterpillars. One of these was banded on the
left side (which side was also female) and pale on the right
side (which was also male). The sex of the two sides was
only apparent after the moth had appeared. The banded
character of the worm is known to be dominant to the pale
character, but neither is sex-linked. The case can be
explained, if as the evidence indicates, the mother was

c r>

»w- -.*.'

striped

gynatidromorph

^

plain

Fig. 89. — Caterpillars of the silkworm moth. A striped one to the left, a plain one to
the right, a hybrid gynandromorph in the middle.

heterozygous for a not sex-linked character, banded, and
if she produced an e^g with two nuclei (Fig. 90). Don-
caster has found such eggs in Abraxas, and has shown
that each nucleus extrudes separately polar bodies, and
that each reduced egg nucleus is fertilized by a separate
spermatozoon. If as showTi in the next diagram one
reduced nucleus has a TF-chromosome, and a factor for
banded carried in one of the autosomes, and the other
reduced nucleus has a ^-chromosome, and in one of the

SEX-CHROMOSOMES AND INHERITANCE 193

autosomes a factor for pale, and if a spermatozoon, carry-
ing the factor for pale, fertilizes each nucleus, the two
zygotic nuclei will he ZW female and banded, and ZZ
male and pale. This gives at least a formal explanation
of the results, and helps us to see how such a rare event,
the appearance of two gynandromorphs in the same brood,
happened to occur at the same time ; because, as Doncas-
ter's evidence shows, a double nuclear condition may be
characteristic of the eggs of certain females.

RJe + Z

Fig. 90. — Diagram illustrating how a heterozygous egg with two nuclei fertilized by two
sperms might produce a gynandromorph like that shown in Fig. 89.

* ^ Intersexes * ' and Sex Genes

The quantitative relation of one X for male and two
X's for female that has been found to hold in many of the
groups of animals might seem from a purely a priori
point of view capable of being modified in such a way that
an intermediate condition might be realized, but whether
such conditions should be expected to give rise to her-
maphrodites or to non-sex-somethings (intermediates) —
or to a mosaic of both sexes, or should rather be expected
to die could scarcely be foretold. There are three cases
in which individuals called * * intersexes * ^ have been found,
or produced; and since their interpretation has led to a
view that has appeared to contradict the ordinary sex-
determination scheme, these cases must be briefly referred
to here. Goldschmidt has studied very thoroughly ^ * inter-

13

194 PHYSICAL BASIS OF HEEEDITY

sexes'* that arise when the European and Japanese race
of gypsy moths, Lymantria dispar and L. japonica, are
crossed. Riddle has described doves obtained by crossing
the white ring dove (Streptopelia alba) and the Japanese
turtle dove (Turtur orientalis) that are intersexual in
their mating habits. Olga Kuttner and Banta have found
that certain lines of Cladocerans (Simocephalus) may
produce (parthenogenetically) ^ intersexual individuals * *
in the sense that an individual may possess some of
the secondary sexual differences of one sex and some
of the other. ^

Some of Goldschmidt's combinations between different
races of gypsy moth produce only intersexual females, i.e.,
individuals that are mostly female, but have also, in spots,
male characters. In the most extreme cases they are
almost like males, not only in color, but even in the partial
production of testes. Other racial combinations give male
intersexes, i.e., individuals that are foij the most part
males, but show, in spots, some of the characteristics of
the female. Goldschmidt explains these results by the
assumption that the sex factors have different quantita-
tive values in the different races. He represents the
female by FFMm, and the male by FFMM. If the FF ' ' fac-
torial set'* is represented by 80 units, and the *' present*'
male factor, M, by 60 units, then the above formula for the
female becomes 80-60 = + 20, and the male formula
becomes 80- (60 + 60)= -40. In the former, female units
* dominate," in the latter, the male. Values like these
can be arbitrarily set for all the different races. For
instance, to the ^^weak" European race and the '^strong"
Japanese the following values are assigned :

Weak Europe

Ban Race

Strong Japanese Race

9 FF

Mm

FF Mm

80,

60

100, 80

c? FF

MM

FF MM

80,

60,60

100, 80, 80

SEX-CHROMOSOMES AND INHERITANCE 195

If a Japanese female is crossed to a European male,
the i^i female and male may be represented in the fol-
lowing formula:

Fi 9 FF Mm F, c? FF MM

100, 60 100, 80, 60

Both ''normar' female and male oif spring are expected
in equal numbers. The reciprocal cross gives a different
result, vi^.:

Fi 9 FF Mm Fi d FF MM

"80, 80 80, 80, 60

The Fi female is FF-M = 0; and is therefore repre-
sented as intersexual. It will be observed that the so-
called ''female factors'' in these formulae are supposed
to be inherited entirely through the mother.

By assigning different values to FF and M in the dif-
ferent races it is possible to express the results in such
a way that the sexes obtained by various crosses have
different minimal values — those less or more than any
assigned value for a given sex are interpreted as inter-
sexes. In the example cited, an exact balance (=0)
between the conflicting factors produces an individual that
is represented as neither male nor female. It is not
obvious, however, why it should be made up of parts each
of which is strictly comparable to the same part in a male
or a female.

While the assignment of arbitrary values to sex fac-
tors is a legitimate procedure, it is not a quantitative
analysis in the ordinary sense, since the quantities
are not referred to some external measure, but are
purely arbitrary.

How far an erratic elimination of sex-chromosomes
in later stages of cell-division might account for the result
cannot be stated, since there are at present no facts to
go upon — the chromosome count in somatic cells of the
hybrid has not yet been reported, but Goldschmidt thinks

196 PHYSICAL BASIS OF HEEEDITY

that the mode of development of the embryo precludes
this interpretation.

Kiddle obtained his intersexual hybrids by causing
their mother to produce many more eggs than she would
ordinarily produce. This was done by removing the eggs
from the nest as soon as they were laid. Towards the end
of a series obtained in this way an overworked female
produced an excess of males. Some of these males Kiddle
regards as females that have been changed into males —
the completeness of the change being shown in their sexual
behavior towards other males, etc. But there is involved
in the cross a sex-linked factor that behaves, as K. M.
Strong had already shown several years ago, as do sex-
linked factors in other birds. It is thus possible to identify
the chromosomal make-up of Kiddle's intersexual hybrids.
His own results show that the hybrids have the expected
combination of chromosomes for males. It appears, there-
fore, that whatever it may be that aifects their behavior
their sex is determined by their possessing the ordinary
chromosome constitution for males.

Hermaphroditism and Sex

As has been shown, the sex-mechanism, whether XX-
XY or WZ-ZZ, gives rise to two kinds of individuals —
males and females. There are, however, many groups and
species of animals where both eggs and sperm are found
within the same individuals, and in typical cases there are
in such individuals special ducts that are outlets for the
male germ-cells and others for the female germ-cells. In
these hermaphrodites ^ * sex-chromosomes ' ' are not known
to be present, or if present as in Ascaris nigrovenosa, they
act as sex determinants only in alternate generations.

The usual interpretation of the determination of the
sex-cells of hermaphrodites is that their differentiation
is determined by the same kind of specific influences that
determine, for example, that certain cells of the primitive
gut develop into liver cells, others into lung cells, still

SEX-CHROMOSOMES AND INHERITANCE 197

others into pancreas cells, etc. There is nothing inconsist-
ent in such a view with the theory that in other cases a
different mechanism produces different kinds of germ-
cells. Logically, this viewpoint is consistent, but I can
sympathize with efforts that are continually being made to
find an explanation that makes use of the same kind of
process in genetic segregation and in embryonic differen-
tiation. In fact, in 1902, while still under the influence of
the then recent advances, in the field of experimental em-
bryology (developmental mechanics), I suggested that one
might attempt to treat the phenomenon of segregation
from the same theoretical standpoint (viz., the realization
of alternative states) as was then appealed to for embry-
onic differentiation. It soon became apparent to me, how-
ever, that (1) the two kinds of results depended upon
entirely different situations, and therefore need not have a
common explanation ; (2) that the genetic evidence showed
the improbability of explaining segregation and differ-
entiation in the same way; (3) that special tests that I
carried out failed to support the supposition of a common
explanation; (4) that while no detailed explanation is
possible at present for the general phenomena of specific
differentiation, yet for Mendelian segregation the reduc-
tion division supplies all that the results call for.

Sex Ratios

The theory of sex-determination has been deduced
from the evidence of equality of males and females as
well as from the cytological evidence. It remains to
explain why in some cases the machine fails to give
equality of the two sexes; why, for example, all fertilized
eggs of phylloxerans and aphids, or daphnians, or roti-
fers, or bees, are female ; why certain mutant races of flies
give twice as many daughters as sons; why other races
of flies produce nearly all sons ; why the sex ratio in man
is about 106 males to 100 females.

It is perhaps needless to point out that if, in a species

198 PHYSICAL BASIS OF HEREDITY

in which sex is determined by a chromosome mechanism,
it were possible to change the sex by other agencies in
spite of the chromosome arrangement, the latter relation
would be entirely thrown out of gear and males would
transmit sex-linked characters and sex itself like females,
and females like males. As no such cases have been
found, it is futile to discuss such a possibility.

It has been shown that only the female-producing
sperm in phylloxerans and aphids becomes functional,
hence it is obvious why all the fertilized eggs develop
into females. In daphnians and other Crustacea it is not
known whether one class of spermatozoa degenerates, but
the results are explicable on such a view. In rotifers
the production of males only by certain females is due to
the eggs developing by parthenogenesis with the haploid
number of chromosomes and this explains also the case of
the bees, wasps and other hymenoptera. If a queen bee
is unfertilized or if her supply of sperm gives out she
produces only males. If she contains sperms, then any
egg that is fertilized produces a female, and as Petrunke-
witch showed several years ago, spermatozoa are to be
found in eggs laid in worker cells — such eggs being known
to produce workers ( $ $ ). In rotifers, too, the presence
of a large and a small class of sperm suggests that only
the former is functional.

Certain females of DrosopJiila give a sex ratio of two
females to one male. By making such a female hetero-
zygous as to her X-chromosomes (each carrying different
factors) it can be determined that the half of the expected
sons that die are the ones containing one of these two
chromosomes. It is easily possible by means of linked
genes to locate a factor in the sex-chromosome (Fig. 91)
and to show that whenever it goes to a male the fly dies.
All the daughters survive because the lethal factor being
recessive does not harm a female whose other chromo-
some comes from a normal father. The scheme is sho^\Ti
on the next page.

SEX-CHROMOSOMES AND INHERITANCE 199

As many as 20 different lethals have been found in
the X-chromosomes of Drosophila. Their occurrence in
these chromosomes is first noticed by the appearance of
such exceptional sex ratios. Lethal factors like these
need not be thought of as different in kind from any
other mutant factors. They may mean only that the
changes that they cause are of such a kind, structural
or physiological, that the affected individual cannot
develop normally. Some of the lethals may be fatal in

9 cf

Fig. 91. — Scheme showing the transmission of a lethal sex-linked factor in an X-chromosome

the black one in the diagram.

the egg stages, others are known to cause the death of
the larvae, others probably act on the pupae, and a few
even allow an affected male to occasionally come through.
In man and in several other mammals there is at birth
a slight excess of males over females. Since male babies
die of tener than females, the difference has been said to be
an ' ' adaptation, ^ ' with the implication that it calls for no
further explanation. Several possible solutions suggest
themselves. The male-producing sperm bearing the sex-
chromosome may more frequently develop abnormally
than the female-producing sperm. Again, since the sper-
matozoa must, by their own activity, travel the entire

200

PHYSICAL BASIS OF HEEEDITY

length of the oviduct to reach the egg as it enters the
tube, the greater size or weight of the female-producing
sperm may give a slight advantage to the male-produc-
ing sperm in the long trip up the tube. This would lead
to an excess of males. There are still other possibilities,
which if realized, would suffice to slightly change the equal-
ity of the output of the machine.

Noi^-DISJUNCTION

Females of Drosophila are occasionally found that
give exceptional breeding results which have been
explained by Bridges on the view that these females are

FEMALE

MAtE

XXT FEMALE

Fig. 92. — Normal female and male groups of chromosomes of the vinegar fly, with the

XXY female group below.

XXY individuals (Fig. 92). It has been shown by cyto-
logical examination that such females do actually contain
an additional F-ehromosome. The four possible ways in
which these three chromosomes might be expected to
behave at the reduction division when the polar bodies

SEX-CHROMOSOMES AND INHERITANCE 201

are given off by the egg are shown in the next diagram
(Fig. 93) . One X may go out of the egg, and the other X
and the Y stay in ; or one X may stay in the egg and the
other X and the Y go out. In these two cases, X and X
may be thought of as members of a pair that conjugate, as
in the normal female, and then separate, and chance alone
determines whether the Y stays in or passes out. Again
Y may go out of the egg and X and X stay in ; or X and X

POLAB
BODY

EIGGS

SPERM

4

Fig. 93. — Non-disjunction. In the upper part of the figure the four possible modes of
elimination of the sex-chromosome from XX Y eggs are shown; the results of their fertili-
zation by an X-bearing sperm of the male is shown below.

go out and Y stay in. Here X and Y may be supposed to
be members of the conjugating pair, and the free X goes
to the same pole as the X that conjugated.

In the diagram, each of these four types of eggs is
represented as fertilized by an X-bearing sperm. In order
to make the outcome more apparent the original XXZ
female may be supposed to have had white eyes (clear
X'5) and the male that fertilized her red eyes (here repre-
sented by the black X carrying the gene for red eyes).

Four classes of individuals are expected : (1) Red-eyed
females (XXZ) ; (2) red-eyed females (XX) ; (3) red-eyed

202

PHYSICAL BASIS OF HEEEDITY

females (XXX) that die, and (4) red-eyed males (XY).
The last are exceptional, since white-eyed females nor-
mally never produce anything but white-eyed sons. Here
the exceptional male is due to an egg without an X, being
fertilized by a ' * female-producing ' ' ( or X-bearing) sperm.
The three X individuals have never been found, and
undoubtedly die, presumably from too many X's. The
remaining red females are of two kinds, one normal XX

POLAR
BODY

EGGS

SPERM

WHITE c?
5

\X/HlTEc^
6

WHITE 9 (EXCtPTION)
7

DIES

8

Fig. 94. — Non-disjunction. In the upper part of the figure the four possible modes
of ehmination of the-sex chromosome from the XXY eggs are shown, andTthe results of
their fertilization by a Y-bearing sperm of the male is shown below.

and the other (XXY), which is expected to repeat the
exceptional behavior of her mother. In fact, this is what
she does.

In the next diagram (Fig. 94) the fate of the same four
kinds of eggs is shown if they are fertilized by a Z-bearing
sperm. Four classes of individuals are expected (5) white
males (XYY) ; (6) white males (XY) ; (7) white females
(XXY) ; and (8) YY individuals. No individuals having
the last make-up have ever been found, and there can
be no doubt that an individual without at least one X dies.
The white-eyed females are exceptional, since white-eyed

SEX-CHROMOSOMES AND INHERITANCE 203

mothers by red-eyed fathers have normally only red-eyed
daughters. These exceptional white-eyed females (XXY)
must repeat the phenomena of non-disjunction, and it has
been found that they do so invariably. The white-eyed
male XY is normal ; the other male should produce some
XY sperm and thus transmit both X and Y to some of his
daughters. Such daughters as get both X and Y from
the entering sperm should show non-disjunction. This
has been proven to occur.

An analysis of the data has shown that two of the four
types of eggs are more common than the other two. As
indicated in both diagrams the types of eggs that result
after X and X have united occurs in 92 per cent, of the
cases, and since in this type the unmated Y has a random
distribution, the XY egg is found in 46 per cent, of cases
and the X egg in 46 per cent. The more uncommon type
of egg would be expected to result if X and Y united and
then separated while the other X had a random distribu-
tion.^ Eight per cent, of such cases occur, giving XX eggs
in 4 per cent., and Y eggs in the other 4 per cent, of cases.

These results not only furnish very strong proof of
the chromosome theory of sex, but serve also to show how
a knowledge of the actual mechanism involved leads to
the discovery of how a change in the mechanism gives a
new output. The conclusion that females behaving in
this way must contain a Z-chromosome was confirmed
by the cytological demonstration that showed in them two
X^s and a Y,

^ Since this was written it has been found that after XY synapsis the free
X always goes to the same pole as the synapsed X.

CHAPTER XV
PARTHENOGENESIS AND PURE LINES

In so far as parthenogenetic reproduction takes place
without reduction in number of the chromosomes, the
expectation for any character is that it will have the same
frequency distribution in successive generations, because
the chromosome group is identical in each generation.
There are a few cases where parthenogenetic inheritance
has been studied. The results conform to expectation.

The only difference between a species reproducing by
diploid parthenogenesis and one propagating vegetatively
is that in the latter a group of cells starts the new genera-
tion and in the former only one cell, viz., an egg^ that no
longer undergoes reduction, or needs to be fertilized. In
both, the chromosome complex remains the same as in the
parent. Strictly analogous to the two foregoing methods
of propagation are the cases of sexual reproduction in a
homozygous group of individuals, composed of males and
females or in a group of hermaphroditic forms that are
homozygous. Successive generations are here also
expected to have the same frequency distribution, whether
selected or not, because they have the same germ-plasm.
Johannsen's pure lines furnish an example of the last
case, for, in principle, pure lines, parthenogenetic repro-
duction, and vegetative propagation, are concerned with
nearly the same situation.

Johannsen worked with one of the garden beans
{Phaseolus vulgaris) taking the weight of the seeds, in
some cases, and measuring their sizes in other cases. It
is known that this bean regularly fertilizes itself. As a
consequence of self-fertilization there is a tendency for
the descendants of any form to become in time homozy-
gous, even when heterozygous forms were present at first.

204 »

PAETHENOGENESIS AND PURE LINES 205

In fact, in a few generations perpetuated by self-fertiliza-
tion with chance elimination of individuals, a homozygous
race will result. This comes about as follows: Starting
mth a heterozygous hermaphroditic individual, some of
its offspring will, through recombination of factors,
become homozygous, and if self-fertilization prevails they
will continue homozygous ; other offspring will be hetero-
zygous. From the latter both homo- and hetero-zygous
offspring will again be produced, the former remaining
such in later generations, the latter continuing the process
of splitting. Since only a part of each generation sur-
vives, there is in the long mn a better chance that the
homozygous individuals will be the survivors, because
those that have become such in each generation are fixed,
and those that are not will continue to produce some
homozygotes. There will be in consequence a steady proc-
ess of recurrence of homozygotes which, on chance alone,
will sooner or later win out.

The beans that Johannsen worked with had apparently
reached a homozygous condition, arid at the start there
must have been several such lines. He studied nineteen
of them. The offspring of any one plant produced beans
that gave the same frequency distribution as the beans
of the last generation. This condition continued through
all successive generations. It is to be noted that the beans
on any one plant differ in size, but any one will give the
same frequency distribution as the beans of the preceding
generation. It made no difference whether the larger
or the smaller beans were chosen for planting— they gave
the same group in the next generation.

It is interesting to compare this result with what would
have happened had the beans been propagating by cross-
fertilization at the time when Johannsen began his work
with them. If this had been their normal method of
reproduction they would probably have been heterozygous
at the start, and would have given different genetic types
for several generations, even if self -fertilized. Pure lines

206 PHYSICAL BASIS OF HEEEDITY

would have appeared only after the beans had become
homozygous through repeated inbreeding. But Johann-
sen, starting with homozygous beans, was able to obtain
his extremely important results, because if selection could
bring about any change it would have to be due to a
change in the genes themselves. Here, by means of a
crucial experiment, he exposed an error that had been
accepted by selectionists from 1859 to 1903. It would have
been difl&cult, almost impossible, to give this demonstra-
tion on any plant or animal in which self-fertilization or
asexual reproduction was not the rule ; for, if the material
had been heterozygous either for the main factors for a
character, or for modifying factors for that character,
selection in one or another direction would be expected
through recombination of factors to change the original
frequency distribution. It is true that any stock, even
such as reproduces by males and females, may be made
homozygous by inbreeding brother and sister for ten or
more generations, but even such stock would have to be
constantly watched for mutation.

Johannsen defined a pure line as a race or family of
individuals descended through an unbroken series of self-
fertilizations from an ancestor homozygous in all its
genes. By making this definition precise he made clear
the essential point of his demonstration. Now that his
point is made, it seems no longer necessary or even desir-
able, I think, to narrow the definition of a pure line to
races that self -fertilize, since this is only one form of
inbreeding, resulting in the production of homozygous
individuals. By extending the definition of a pure line
to all forms whose genes are the same in all individuals
(whether the pairs are homozygous or not), the definition
covers all cases of parthenogenesis that do not undergo
reduction, and all cases propagating by non-sexual means,
for, in these cases the same complex of genes is present
in successive generations.

Many plants are propagated by offshoots, stolons.

PAETHENOGENESIS AND PUEE LINES 207

tubers, cuttings, etc. East has studied the effect of selec-
tion of tubers of certain races of the common potato. A
race was first grown from a single tuber. By boring holes
into the tubers enough material could be obtained for a
chemical test of the amount of nitrogen in them. The
rest of each tuber could, if desired, be cut into pieces of
standard size and planted. Ten tubers, high in nitrogen,
and ten, low in nitrogen, were selected. The tubers of the
next generation showed that there was no relation found
between the amount of nitrogen in the original tuber and
in those that came from it. A repetition of the experi-

Fia. 95. — A wingless aphid to the left and a winged to the right, both belonging to
the same species. (After Webster and Phillips.)

ment in another generation gave only meagre results
owing to drought. As far as the facts went, this genera-
tion, too, showed no effect of selection.

Most of the protozoa propagate by dividing into equal
or nearly equal parts — i.e., by a process of cell-division.
Jennings has studied the effect of selection in a culture
of Paramecium, all members of which had descended from
a single individual. No change Vas induced. Later, how-
ever, working on another protozoon, Diffugia corona^
Jennings found that selection brought about changes in
the direction of selection. In this case, the method of
division may possibly include irregular distribution of the
chromatin material, and the recent work of Hegner indi-
cates that such an interpretation is not improbable. Pos-

208

PHYSICAL BASIS OF HEEEDITY

sibly, too, the irregular distribution of chromatin par-
ticles (chromidia) in the cytoplasm — aside from the
nuclear phenomena, or in connection with them — may
make the results similar in certain aspects to the distri-
bution of plastids in certain plant cells.

Many species of plant lice — aphids — (Fig. 95, a)
propagate throughout the summer by parthenogenesis.
There is no chromosomal reduction during the develop-
ment of the egg. Each egg gives off only one polar body,

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for increase in body length, the heavy solid lines represent the fluctuations of the fraternal
means; the light solid line the fluctuations of the longest variant; the broken line the
fluctuation of the shortest variant. (After Ewing.)

each chromosome splitting into two daughter chromo-
somes, so that the egg retains the whole number of chromo-
somes. Ewing has carried out an extensive experiment
with Aphis avence, selecting individuals through a num-
ber of generations for the length of the cornicles (honey-
dew tubes), for the length of the antennae, and for body
length. Considering here only the last, individuals were
selected for forty-four generations in a plus and in a
minus direction. The graph for the f ourty-fourth to the
sixty-third generation is shown in Fig. 96. The heavy
solid line represents the fluctuations of the longest vari-

PARTHENOGENESIS AND PURE LINES 209

ants, the broken line the fluctuations of the shortest
variants. It was found that much of the fluctuation
observed was connected with temperature. The tempera-
ture was therefore kept constant at about 65° F. for the
next twenty generations, and as shown in Fig. 97, the
fluctuation in the fraternal line was cut down. No in-
fluence of the selection is observable in the chart. This
evidence, in conjunction with that for other characters,
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the insect so long as the same group of chromosomes
remains. It would be difficult to find a better example
than these parthenogenetic insects to test the claim that
selection can change the germ-plasm, for here the con-
ditions are even simpler than in unisexual forms unless
they have first been made homozygous.

The aphids also furnish favorable material to illus-
trate how the environment may cause very great changes,
even when the genetic complex remains the same. The
parthenogenetic aphids appear often as winged individ-
uals (Fig. 95, &). There is an entire change in structure
involving practically every part of the body. The winged

14

210 PHYSICAL BASIS OF HEEEDITY

and wingless individuals may differ more strikingly than
do species of the same genus. The winged forms arising
from the wingless produce wingless forms again in the
next generation that may be identical with those from
which they came. It has long been believed that environ-
mental influences bring about these transitions in aphids,
but only recently has critical evidence been obtained. The
clearest evidence is that of Shinji, with the rose aphid.
By sticking twigs of the rose in sand and flooding the sand
with water containing substances in solution — a method
first suggested by W. T. Clarke— the fluid being drawn
up into the leaves is sucked out by the aphids on the leaves.
As the following table shows, young aphids reared on the

Winged Apterous

Individuals. Individuals

AgNO. 51 0

CuSo, 34 1

HgCl, 31 6

NiSO, 955 5

SbCls 41 5

PbCl, 12 2

SnCl, 579 8

ZnCl, 49 2

Mg salts 840 9

Sugar 365 160

Alcohol 2 288

Alum 3 34

Acetic acid " "'

Na salts 2 1029

Ca salts 1 433

K salts 3 324

Sr Salts 1 220

Tannin 1 14

Urea 5 153

Water, distilled 0 394

Water, tap and creek 17 461

Peptone • • 15

salts of the heavy metals as well as on magnesium salts
and sugar became winged, while those reared on the other
substances in this list remain apterous. Here we have
an excellent example of how in one environment a given
germ-plasm produces one result, and in another environ-
ment a different result without any intermediate forms.

PARTHENOGENESIS AND PUEE LINES 211

The change from wingless to winged aphids is far greater
than most mutational changes that we know, yet must
involve a different kind of change because the result is
reversible, while a mutation, having once taken place, is
relatively irreversible.

Summing up, it may be said that the evidence shows
that whenever the same chromosomal complex containing
the same genes is found, the measurements of any charac-
ter in successive generations show the same frequency
distributions of the measurements, and the form may be
said in a general sense to belong to a pure line. The
evidence shows that whether the chromosomal complex is
heterozygous or homozygous, the results are the same,
so far as the pure line is concerned ; but it is also obvious
that in most animals and plants, where redistribution
(reduction) of the chromosomes takes place in each gen-
eration, only forms already homozygous will give pure
lines. This was the special feature of the material that
Johannsen worked with, but aside from its practical value
in studying the selection problem, the limitation of the
definition of pure lines to such an exceptional situation
leaves out of sight the wider bearing of the evidence.

CHAPTER XVI

THE EMBRYOLOGICAL AND CYTOLOOICAL EVI-
DENCE THAT THE CHROMOSOMES ARE THE
BEARERS OF THE HEREDITARY UNITS

Long before the genetic evidence brought forward its
abundant data that are explicable on the theory that the
chromosomes carry the genes, embryologists had already
found other evidence that led them to regard the chromo-
somes as the bearers of the hereditary factors. Taken
as a whole, this evidence makes out a very strong case
for the chroniosomes, but since it did not establish the
relation beyond question, the genetic evidence was all
the more welcome.

The earliest evidence, sometimes cited in favor of
chromosomal inheritance, was based on the statements
that in some cases at least, only the head of the spermato-
zoon enters the egg. Since it was then thought that the
head is composed almost entirely of the nucleus, and since
the child inherits equally (in the older parlance) from its
father and from its mother, it followed that the nucleus
carries the hereditary elements. When later it became
known that the head of the sperm represents almost
exclusively the mass of condensed chromatin, it was sup-
posed that the chromosomes, in particular, must be that
part of the nucleus that is the bearer of hereditary charac-
ters. Such a conclusion received indirect support from
the facts, then becoming known, that the chromosomes
remain constant through successive generations of cells,
whereas the nuclear sap becomes lost in the gen-
eral cytoplasm each time that the nuclear wall is dis-
solved. It was also found that the spindle fibres disappear
in the resting stages, while the nuclear reticulum (chro-
matin) remains.

212

BEAEERS OF HEREDITARY UNITS 213

This evidence failed, however, in so far as there might
be present a certain amount of nuclear plasm in the sperm-
head that is carried in with the head, and if so, would be
later mixed with the egg cytoplasm. The discovery that
at the base of the sperm-head there is present in some eggs
a centrosome that becomes, through division, the dynamic
centre of the next division, opened the door to suspicion
that the sperm might bring in other things than the chro-
mosomes to influence development, and hence heredity.

In conclusion then, while it may be said that the evi-
dence that the sperm-head alone enters the egg may be
claimed as favorable for the chromosome view, it cannot
be accepted as critical proof, because it is uncertain
whether other things also may not be brought in besides
the chromatin of the sperm.

Boveri^s evidence for chromosomal heredity from di-
spermic sea urchin eggs was open to less objection. It was
known that when two sperms enter the sea urchin's egg
simultaneously, the first division of the egg is into three
or into four parts, because four (instead of two) division-
centres appear in these dispermic eggs. It was also known
that these eggs rarely produce normal embryos or larvae.
Boveri, studying the mode of division of the dispermic
eggs, found that there was an irregular distribution of
the chromosomes to the three or four poles that appear,
and consequently to the three or four resulting cells (Fig.
98). The abnormal development of the whole egg that
generally follows might be ascribed to the irregular dis-
tribution of chromosomes to different regions ; for, quite
apart from the specific nature of each chromosome or
group of chromosomes, the activity of one region being
quantitatively different from that of a corresponding
region in another part of the egg might be responsible for
the failure to develop normally. But Boveri went further
in his analysis. He shook apart the three or four blasto-
meres coming from dispermic eggs (by using Herbst's
calcium-free sea-water method), and compared the num-

214

PHYSICAL BASIS OF HEEEDITY

ber that developed into normal plutei with the number
of plutei from one-fourth normally fertilized blastomeres.
From the latter a large proportion give rise to normal
embryos, from the former normal embryos are rarer.
Their greater rarity, Boveri thought safe to attribute to
the chromosomal deficiencies present in most of such iso-

FiQ. 98. — Scheme showing dispermic fertilization of the egg of the sea urchin with the
subsequent irregular distribution of the chromosomes. (After Boveri.)

lated blastomeres. He suggested that the chance of a
blastomere developing normally depends on its having
at least one full set of chromosomes. For these triploid
sea urchin eggs with three times 18 chromosomes, the
chance of one full set of chromosomes getting into each
blastomere is, according to Boveri 's calculation, only one
to 10,000. The chance of getting at least one chromosome
of each kind in one cell is greater. He concluded that the
few embryos he obtained came from quadrants that had at
least one haploid set of chromosomes. There is, however,

BEAKERS OE HEREDITARY UNITS 215

to-day some uncertainty concerning the assumption that
normal development is to be expected if in addition to
one haploid set of chromosomes other chromosomes are
also present, because while one set alone might permit
normal development, it is by no means certain that if
there were one, two, or more additional chromosomes, the
balance might not be upset and abnormal development fol-
low. On chance distribution alone the isolation of just one
set and no more would seem a very remote possibility,

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Fig. 99._First division of a hybrid egg showing the elimination of chromosomes at the
equation of the spindle, o. The reciprocal cross, 6, shows no such elimination. (After iJaltzer) .

but if there is to some degree a tendency for a group of
daughter chromosomes to move off together as a result
of their method of division, there might be a better chance
of such a group getting into one of the three or four
blastomeres than by chance distribution alone. At pres-
ent it is not possible to make any calculation based on such
an assumption. While, therefore, Boveri's argument can-
not be accepted as demonstrative, yet it has probability
in its favor.

Baltzer has found a different kind of evidence of
chromosomal influence. When the eggs of one sea urchin.

216 PHYSICAL BASIS OF HEREDITY

Strongylocentrotus, are fertilized by the sperm of another
sea urchin, 8 phaer echinus, the segmentation nucleus,
formed by the union of the ^^g- and sperm-nucleus shows
irregularities in the movements of the daughter chromo-
somes to the poles of the spindle. While some of the
chromosomes after dividing pass normally to the poles,
others become scattered irregularly between the two poles
and fail to become incorporated in the two-daughter nuclei
(Fig. 99, a). They appear to become lost and take no

.X

y"

Fig. 100. — Fertilization of an egg that had started to develop parthenogenetically.
The belated sperm unites with one of the daughter chromosomes groups only, a; an
earlier condition of the same procedure. (After Herbst.)

part in the further development. Counts of the chromo-
some plates in the later divisions of the egg give about
21 chromosomes, whereas 36 are expected as the whole
number. It appears that 15 chromosomes are lost, and
presumably they belong to the foreign sperm. Many of
these eggs develop abnormally, but those that reach the
pluteus stage show a maternal skeleton only. This seems
to mean that the sperm has done no more than start the
development. It has contributed nothing, or little, to the
embryo, and it seems reasonable to attribute this to the

BEAREES OF HEREDITARY UNITS 217

loss of the paternal chromosomes, especially in the light
of the reciprocal cross.

In this reciprocal cross, the egg df Sph(Er echinus
is fertilized by the sperm of Strongylocentrotus, All
the chromosomes of the segmentation nucleus divide
and pass regularly to the two poles (Fig. 99, &). The
hybrid embryo shows characters of both parental species.

/5f

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o ^

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O:

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Fig 101 — Larval sea urchin seen in side view. On one side it shows hybrid characters,
on the other side it is maternal. The sizes of the nuclei on these two sides, as seen in the
figure, coincide with the view that the hybrid side is diploid and the maternal side haploid.
(After Herbst.)

The difference in the two cases can be safely attributed
to the observed differences in the fate of the chromosomes,
rather than to unrecognized differences in other elements
brought in by the sperms.

Herbst 's experiments contribute further evidence in
favor of the chromosome interpretation. He caused the
unfertilized eggs of a sea urchin to begin to develop
parthenogenetically by adding a little acid to the sea
water. After five minutes the eggs were removed to pure
sea water, and sperm of another species, Strongylocen-

218 PHYSICAL BASIS OF HEREDITY

trotus, was added. The sperm entering the ^gg after its
nucleus had started to divide, failed to reach the Qgg
nucleus until the latter had divided ( Fig. 100 ) . The sperm
nucleus then formed a nucleus of its own, that passed into
one only of the daughter cells. This cell got two nuclei.
The other cell had only one of the daughter nuclei. Such
half -fertilized eggs give rise to larvae that are maternal
on one side, and hybrid on the other — or at least larvae
of this kind are sometimes found in such cultures (Fig.
101), and Herbst believes it is safe to refer them to the
half -fertilized eggs. If so, there can be little doubt that
the hybrid half owes its peculiarities to the presence of
both sets of chromosomes in its cells, while the maternal
half owes its peculiarities to its single set of maternal
chromosomes. This in itself, however, shows little more
than do whole hybrids and whole parthenogenetic eggs
themselves, for the demonstration that it is the chromo-
somes and not other constituents of the sperm-nucleus
that make the difference in the two sides rests on the
unproven inference that if other things than the nucleus
are involved they would be distributed equally throughout
the cytoplasm, but produce no effects. There is no reason
to' suppose that they would be so distributed, and no evi-
dence that they are. Hence the proof is not cogent, how-
ever probable it may seem that only the sperm-nucleus is
responsible for those cases where there is a difference
in the two sides.

On the whole, then, while I am inclined to give much
weight to this evidence from experimental embryology as
very favorable to the hypothesis that the chromosomes
carry the hereditary characters, it is the genetic evidence
that furnishes convincing evidence in favor of this view.

CHAPTER XVII
CYTOPLASMIC INHEEITANCE

In the preceding pages so much emphasis has been
laid on the chromosomes as bearers of the hereditary
material that it may appear that no very important role
is left to the rest of the cell. Such an impression would be
quite misleading; for the evidence from embryology
appears to show that the reactions by means of which
the embryo develops^ and many physiological processes
themselves, reside at the time in the cytoplasm. Further-
more, there is also genetic evidence to show that certain
forms of inheritance are the outcome of self -perpetuating
bodies in the cytoplasm, most of which go under the name
of plastids. Recognition of plastid inheritance carries
with it the idea that if there are such materials in the
cytoplasm that are self-perpetuating they will have to
be taken into account in any complete theory of heredity.

In the case of certain chlorophyll characters there is
excellent genetic evidence to show that a peculiar kind of
inheritance is due to the mode of transmission of plastids
in the cytoplasm. There is a race of f our-o 'clocks known
as Mirabilis Jalapa alhomaciilata, whose leaves are made
up of patches of green and white. Such leaves are said
to be checkered (Fig. 102, b). The amount of green, or of
white, varies on different leaves, and on such plants there
frequently appear leaves and entire branches that are
green and others that are white. The white is due to the
absence of green in the chlorophyll grains. Some cells
have only green chlorophyll bodies, and others only white,
still others may have the two mixed in various amounts.

Correns has shown that if the flowers on a green
branch are self -fertilized they produce only green plants,
and these again only green plants. Flowers on white

219

220

PHYSICAL BASIS OF HEREDITY

branches give only white offspring. Flowers on the check-
ered branches give some checkered plants, some white
plants and some green plants. The proportions in which
these different types arise varies according to the amount
of green in the branch from which the self -fed seed came.
When the ovary of a flower on a green branch is fertil-
ized by pollen from a white branch, the plant produced
is green like the maternal branch. If the ovary of a
flower on a white branch is fertilized by pollen from a

::ir^

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m

!ir

Fig. 102. — Green leaf and checkered leaf of four-o'clock. (After Baur.)

green branch the offspring is white like the maternal
branch. These and other combinations show that this
color inheritance is only through the mother. The results
are explicable on the assumption that there are normal
(green) chlorophyll bodies and abnormal chlorophylless
bodies, both kinds propagating in the cytoplasm by divi-
sion, and that these two kinds are transmitted only through
the egg-cell. The green or white color of the leaves of a
given branch is an index of the kind of chlorophyll body
that the ovaries will probably contain. At each division
of the body-cells the chlorophyll grains present in it are
sorted out more or less at random— hence from a cell that

CYTOPLASMIC INHERITANCE 221

contains both kinds, more white granules than green ones
may at times get into a cell, and at other times only white
granules will get into one daughter cell, so that a white
branch arises.

In other species of plants that have white leaves and
branches and green leaves and branches, the cross may
give a different result. Thus in Melandrium and Antirrhi-
num, green by white gives green F^ (whichever way the
cross is made), in F2 there are 3 green to 1 white plant.
In this case the results can be explained as due to the
action of genes in the chromosome on the production of
chlorophyll in the c^^oplasm — an action of such a kind that

Fig. 103. — Pelargonium that gave rise to a white branch. (After Baur.)

the granules do not develop green color unless the (nor-
mal) gene is present, in single dose at least. In this case,
even if the eggs only transmit plastids, the F^ individual
from a white-leaved mother by a green-leaved father is
green, because the paternal nucleus introduces a gene
that causes the green color to develop in the plastids. It
is the segregation of the genes in the germ-cells of the F^
individual that leads to the 3 : 1 ratio in Fg, and not the
distribution of the plastids as in the preceding case.

The most peculiar case is that of Pelargonium de-
scribed by Baur. White leaves and branches, and green
leaves and branches occur on the same plant (Fig. 103).
Self-fertilized seeds from each breed true to color of
branch. White to green gives a different result, vis.,

222

PHYSICAL BASIS OF HEEEDITY

mosaic seedlings with patches of green and white on stems
and leaves (Fig. 104). When these seedlings grow into
plants, the color of the leaves will depend on the color
of that part of the stem from which the terminal bud, and
lateral buds grow out. If a bud lies in a green part of the
stem the new part will be green (Fig. 104, a) : if the new
bud lies in a white part of the stem the new part will be
white (Fig. 104, c) : and if it lies in a partly green, partly
white region the new part will have some white, some

a t c

Fig. 104. — Diagram to show how a sectorial chimera may be produced. If the ter-
minal bud has come from a region of the seedling entirely green, all of the future leaves
will be green, a; if from a region without chlorophyll, all the future leaves will be white, c;
but if the terminal bud lies partly in one, partly in the other region, some white and some
green leaves will arise, h. (After Baur.)

green parts (Fig. 104, h). The only explanation that is
suggested by Baur is that in this plant the plastids are
transmitted both by the eg^ and by the pollen. The white
plant with defective plastids contributes part of the plas-
tids in the fertilized Qgg, the green plant with normal
plastids the other part. The fertilized Qgg contains there-
fore both kinds of plastids. During division of the Qgg and
embryo, the granules become irregularly distributed in
the cells. Whenever a cell gets only defective granules,
it and its descendants are white, producing white parts :
when a cell gets mostly or only green granules, it and its
descendants are green, producing green parts. Hence

CYTOPLASMIC INHERITANCE 223

arise the checkered seedlings from which white or green
branches grow out.

The preceding facts and theories relating to plastid
inheritance show that if any element outside the nucleus
has the power to propagate itself it may be transmitted
through the eggy and even possibly through the sperm
(pollen) also. There is no contradic/tion here in any
sense to Mendelian inheritance but only an additional type
of inheritance that can be studied by as exact methods
as those used in Mendelian work. The chief difference
between chromosomal and plastid inheritance lies in the
orderly sequence of the distribution of the genes in all
divisions by means of the mitotic figure, whereas the plas-
tids are supposed to be shuffled about at random to the
daughter cells (partly because their division period does
not correspond with that of the cell). This haphazard
distribution of the plastids at any and aU divisions is in
striking contrast to the sorting out of the genes that occurs
only at one specific cell-division when the germ-cells pass
through the maturation stage. Hence the orderliness of
Mendelian inheritance as contrasted with the more irregu-
lar procedure in plastid inheritance.

To embryologists familiar with the fact that differen-
tiation of the egg is closely associated with the cleavage
pattern, it was a natural inference that in the cytoplasm
lay the inherited characteristics that gave form to the
embryo, and even to all of its essential features. Little
room would seem to be left for the action of the chromo-
somes except to fill in the details of the characters already
outlined by cytoplasmic activity. This view might be la-
conically referred to as the theory of the ''Embryo in the
Rough,'' or more generally as the ''Theory of the Organ-
ism as a Whole.'' Boveri discussed some such view
(1903), and at first considered it favorably. It has since
been seriously discussed by others. Boveri pointed out
that when a horse is crossed to an ass it makes no differ-
ence which way the cross is made, for both egg and sperm

224 PHYSICAL BASIS OF HEEEDITY

bring in the characteristics that make the organism first
a bilateral one, then a vertebrate, then a mammal, and,
lastly, a perissodactyl. In all these aspects, both parents
agree, and beyond these limits hybridizing is impossible.
Whatever the germ develops into must contain these com-
mon characters. The important point to determine,
Boveri thought, is whether the species characteristics are
or are not in the nucleus. He concluded, after discussing
the pros and cons, that it is doubtful if these preformed
qualities of the egg-protoplasm extend beyond the larval
periods, but that in general all characteristics that distin-
guish the individual from aU others of its species and
from the characteristics of related species are inherited
through the chromosomes. Later he restated his con-
clusion as follows: *^A11 essential characteristics of the
individual and of the species are epigenetic, and the deter-
mination is brought about through the nucleus. ^ ' Conklin
at one time expressed even more sharply the idea that
group characteristics may be inherited in a different way
from specific characters in the following paragraph:

We are vertebrates because our mothers were vertebrates and pro-
duced eggs of the vertebrate pattern; but the color of our skin and
hair and eyes, our sex, stature, and mental peculiarities were determined
by the sperm as well as by the egg from which we came. There is
evidence that the chromosomes of the egg and sperm are the seat of
the differential factors or determiners for Mendelian characters, while
the general polarity, symmetry and pattern of the embryo are deter-
mined by the cytoplasm of the egg.

In another statement, however, Conklin takes what
seems to me to be more nearly a correct view in regard to
the question, viz., that ' * There is no doubt that most of the
differentiations of the egg cytoplasm have arisen during
the ovarian history of the egg, and as a result of the
interaction of nucleus and cytoplasm ; but the fact remains
that at the time of fertilization the hereditary potencies
of the two germ-cells are not equal, all the early stages of
development, including the polarity, symmetry, type of

CYTOPLASMIC INHERITANCE 225

cleavage, and the pattern, or relative positions and pro-
portions of future organs, being foreshadowed in the cyto-
plasm of the egg-cell, while only the differentiations of
later development are influenced by the sperm. In short,
the egg cytoplasm fixes the general type of development,
and the sperm and egg nuclei supply only the details.'^
If, as implied, the egg nucleus at first has already pro-
duced its effect on the cytoplasm, it has done something
more than supply the details ; and as to the sperm nucleus
I should substitute nearly all the stages of development
later than the gastrula. Moreover, sex is certainly one
of the fundamental characters of the organism, yet it
appearsi to be determined at fertilization by the chromo-
somal combination formed at that time. Conklin later
abandoned his earlier interpretation.

Quite recently, in his book on ''The Organism as a
Whole,'' Loeb has discussed the question as to whether
the protoplasm of the egg is ''the future embryo in the
rough, ' ' the sperm furnishing only the ' ' individual charac-
ters. ' ' Loeb suggests that the ' ' specificity of the species ' '
must be due to their proteins, and that the "heredity of the
genus is determined by proteins of a definite constitution
differing from the proteins of other genera. This consti-
tution of the proteins would therefore be responsible for
the genus heredity. The different species of a genus have
all the same genus proteins, but the proteins of the species
of the same genus are apparently different again in chemi-
cal constitution and hence may give rise to the specific bio-
logical or immunity reactions. ' ' The possible relations of
these considerations to heredity are summed up in the
following paragraph :

It is thus doubtful whether or not any of the constituents of the
nucleus contribute to the determination of the species. This in its
ultimate consequences might lead to the idea that the Mendelian charaxi-
ters which are equally transmitted by egg and spermatozoon determine
the individual or variety heredity, but not the genus or species heredity.
It is, in our present state of knowledge, impossible to cause a spermato-
15

226 PHYSICAL BASIS OF HEEEDITY

zoon to develop into an embryo, while we can induce the egg to develop
into an embryo without a spermatozoon. This may mean that the
protoplasm of the egg is the future embryo, while the chromosomes of
both egg and sperm nuclei furnish only the individual characters.

The evidence from Mendelian heredity is adverse to
any such distinctions as those made by the three authors
referred to above. We find in them, I think, an echo of an
old and somewhat mystical conception of fundamental dis-
tinctions between order, family and generic characters
of animals and plants — ^distinctions that even most syste-
matic writers recognize to-day as little more than conven-
tions that change from group to group. In the second
place, since the cytoplasm of the egg has been under the
influence of its own nucleus with a paternal and a maternal
group of chromosomes there is no direct means of deter-
mining whether its characteristics are due to such an
influence or have always been free from it. The fact that
sperm of a foreign species does not change the cytoplasm
of the egg at once is to be expected even from a chemical
viewpoint. Mendelian workers can find no distinction
in heredity between characteristics that might be called
ordinal or specific, or fundamental, and those called 'indi-
vidual.'' This failure can scarcely be attributable to a
desire to magnify the importance of Mendelian heredity,
but rather to experience with hereditary characters. That
there may be substances in the cytoplasm that propagate
themselves there and that are outside the influence of the
nucleus, must, of course, be at once conceded as possible
despite the fact that, aside from certain plastids, all the
Mendelian evidence fails to show that there are such char-
acters. In a word, the distinction set up between generic
versus specific characters or even ''specificity'' seems at
present to lack any support in fact.

CHAPTER XVIII
MATERNAL INHERITANCE

There is a kind of inlieritance shown by eggs and
embryos, sometimes called maternal inheritance, that is
not the same as plastid inheritance, even although the lat-
ter is maternal in another sense. Nor is this so-called
maternal inheritance to be confused with cases of inheri-
tance in which all or some of the paternal chromosomes
fail to function, leaving the embryo at the mercy of its
maternal set alone. Nor should it be confused with sex-
linked inheritance where the son gets certain characters
only from the mother, because he gets his single sex-
chromosome from her.

^^True'' maternal inheritance relates to peculiarities
of the egg or larva that are due to materials already pres-
ent in the egg-oytoplasm w^hen the egg is laid. For exam-
ple, if pigment is scattered in the egg, it may collect in
certain regions after fertilization, and produce color in
them, as does the yellow pigment in the egg of Cynthia,
studied by Conklin. In this ascidian, much of the yellow
pigment is carried at the moment of fertilization to that
part of the egg that later goes into the muscle of the tail.
If the sperm used to fertilize such an egg should come
from a species without pigment in the egg, the inheritance
of color of the young embryo would obviously be entirely
maternal. In oases like this one, the formed material,
or any substance producing such materials, is already
present in the cytoplasm, but^^^hether it has always been
free from nuclear influence must be shown by other tests.
In only one cross, viz., in the silkworm, has a third genera-
tion been raised, and until this has been done in
others we cannot know whether we are dealing in them
with plastid or with deferred nuclear influence ('' ma-
ternal inheritance ' 0 •

227

228

PHYSICAL BASIS OF HEEEDITY

In certain races of the domesticated silkworm moth,
Toyama has shown that pigment develops in the em-
bryonic membrane (serosa) which, seen through the
egg-shell, gives a specific color character to the embryo.
It is not clear from Toyama 's account whether the pig-
ment is present at first, scattered in the cytoplasm, and
collects later at the surface, or whether it develops only
after the embryo develops. When races are crossed with
characteristic but different embryonic membranes, the
color of the hybrid is like that of the maternal race only.

Pi

Fi

?2<

Eggs and embryo .

Genetic
constitution.

Eggs and embryo .

Genetic consti-
tution of P2
individuals .

DD

$D by <f rO
D R

I

#
VR RR

O9R by <f D<

DD

P3 Eggs

D

1

2

O
R

1

D

1

RR

O
R

1

Fig. 105. — Diagram to illustrate maternal inheritance. The black circle stands for a dom-
inant character affecting the serosa coat of the embryo.

If adults (Fi) are raised from these eggs, it is found
when they in turn produce embryos, that the color of their
embryonic membrane is determined by the dominant char-
acter of the preceding generation that had been carried
in the chromosomes, irrespective of whether it came in
from the father or the mother (Fig. 105). That the result
is really chromosomal is shown by still another generation
in which some of the females show the dominant character
in the membranes of their embryos and others no color
in the ratio of 3:1.

It appears therefore in this case, the only one known
that furnishes critical evidence, that maternal inlieritance

MATERNAL INHERITANCE 229

does not differ in any essential respect from ordinary
Mendelian heredity.

A peculiar case that in some respects and in certain
combinations appears to be maternal inheritance, is
shown in the character of the seed of com {Zea mais).

The endosperm of maize is produced, as in some other
plants, at the time of fertilization — one pollen nucleus
unites with the Qgg to form the embryo, another pollen
nucleus unites with two nuclei in the embryo-sac to pro-
duce the endosperm whose cells, therefore, are triploid.
Floury corns have an endosperm, that is almost wholly
made up of cells containing * ^ sof t ^ ^ starch, while flint corns
have only a small amount of soft starch in the centre of
the seed which is surrounded by a large amount of hard
** corneous'^ starch. Hayes and East have showTx that if
a floury com be used as the mother and a flint corn as the
father, the seeds are floury like those of the pure race of
floury corn. If a flint corn be used as the mother and a
floury corn as the father, the seeds are flinty. In both
cases there is apparently maternal inheritance, at least
as far as the endosperm is involved, which is not, how-
ever, a part of the embryo proper. If the seeds from races
of the foregoing crosses are sown and the plants allowed
to self -fertilize, the following results are obtained : The
2^1 derived from floury $ by flint $ produces both floury
and flint in F2 in the ratio of 1 : 1. The F^ flinty reciprocal
cross gives exactly the same result. The explanation of
the F^ and F^ results is as follows : If the factor for flinty
be F, and that for floury be f, then in the first cross the
endosperm is ffF and in the reciprocal cross FFf. Since
ffF is floury and FFf flinty it follows that two doses of
floury dominate over one dose of flinty, and conversely
two doses of flinty dominate over one dose of floury.

The F^ embryo, however, in each of the crosses has
only one F and one / factor {Ff). Its gametes are F and
/, and so are its endosperm nuclei which, as shown by
Weatherwax have the same reduced formula as the ovules

230 PHYSICAL BASIS OF HEREDITY

in the embryo sac. Hence half the embryo sacs are F and
half /. The former, F {-\-F), fertilized by F pollen gives
FFF endosperm, by / pollen give FFf; the latter, / (+/),
fertilized by F poUen gives ffF endosperm, by / pollen
/// endosperm. The four kinds of endosperm fall into two
classes, soft and hard, in the ratio of 1 : 1 in the Fg seeds.

There are races of maize with yellow dominant endo-
sperm and others with recessive white. If the mother
belongs to a yellow race and the father to a white one,
the F^ endosperm is yellow like the mother. In the recip-
rocal cross it is also yellow. If, however, races with floury
seeds are used, the F^, yellow endosperm in the former
cross is somewhat paler than the pure yellow of the yellow
race. Races with purple or with red endosperm crossed to
white give the same results, except that in these crosses the
quantitative effects seen in the floury flint crosses are not
observed, for, one dose of the dominant (purple) to two
doses of white gives the same color as two doses of purple
to one dose of white.

There are two kinds of maize with white endosperm.
These when crossed together give F^ colored endosperm.
In these cases one race has one of the factors for color, and
the other, another complementary factor — like the two
white sweet peas. Ther^ is also a race with a dominant
white endosperm factor. The occurrence of these kinds of
whites led to some confusion in the earlier experiments of
Correns on endosperm inheritance. The word Xenia, that
had earlier a different meaning, is used to-day for these
cases of double fertilization in which the pollen has an
influence on the seed (the endosperm) that is not a part of
the Fi plant itself. East and Hayes sum up the results
given above (exclusive of the floury-flint cross) as follows :

Wlien two races differ in a single visible endosperm character in
which dominance is complete, Xenia occurs only when the dominant
parent is the male; when they differ in a single visible endosperm
character in which dominance is incomplete, or in two characters both
of which are necessary for the development of the visible difference,
Xenia occurs when either is the male.

MATERNAL INHERITANCE 231

In cases in which a foreign sperm may start develop-
ment but take no further part in it, the resulting embryo
is like the maternal race. Here we are dealing not so much
with maternal inheritance, but rather with a special kind
of parthenogenesis. Such eggs, however, rarely go beyond
the cleavage stages.

The rate of cleavage of an egg fertilized by foreign
sperm usually coincides with that of the species to which
the egg belongs. Since the cytoplasm of the egg has, prior
to fertilization, always been under the influence of its own
nucleus, this relation is what might be expected. It is
necessary to study eggs from an F^ generation in such
cases in order to judge how far paternal chromosomes
may influence the cleavage. It is thinkable, for example,
that a spermatozoon might bring in a factor dominant for
rate of cleavage, but because this factor had not had time
to influence the cytoplasm its effect would not show in the
Pi cross. In the F^, on the other hand, the paternal char-
acter might prove dominant. Both Driesch and Boveri
have shown in the sea urchin that the rate of cleavage,
the pigmentation, and the kind of gastrulation are entirely
or largely determined by the egg — they differ in opinion
only as to how soon the influence of the sperm can be seen.

At the time when the larval skeleton is formed most
observers agree that the influence of the foreign sperm
makes itself felt. Most of the accounts of the skeleton of
hybrid sea urchins describe it as intermediate in struc-
ture, but one that varies widely under different external
conditions. Tennent has shown, in fact, that the character
of the hybrid larval skeleton is so greatly influenced by
the alkalinitv or aciditv of the sea v/ater that it can be
artificially thrown towards one or the other side — mater-
nal or paternal. Loeb, King and Moore have attempted to
determine whether the larval skeleton has dominant char-
acters in certain parts and recessive ones in other parts.
They crossed the sea urchins, Strongylocentrotus Francis-
canus and 8. purpureus. Both the straight cross and its

232 PHYSICAL BASIS OF HEREDITY

reciprocal showed neither a great predominance of the
characters of the paternal race, nor of the maternal race,
but rather certain characteristic features of purpuratus
and others of Franciscanus. The larval characters
appeared to be dominant or recessive taken singly. Until
an F2 generation can be raised it is obviously hazardous
to speak here of Mendelian dominance and recessiveness
of characters that are based on F^ observations alone,
especially since it is becoming more and more apparent
that many F^ characters are more or less intermediate,
and there are no general grounds for expecting pure domi-
nance or recessiveness.

Many crosses have been made between different species
of fish, and in some of these the young, at the time of hatch-
ing, are maternal. It has generally been supposed that
such cases are due to the absorption of the paternal chro-
mosomes at the first or at later cleavage stages. Loss of
chromosomes has in fact been recorded in several of these
cases of maternal inheritance. On the other hand. Miss
Pinney's observations, summarized in the following table.

Cross Development Results Chromosomal Behavior

Ctenolabrus o X Fundulu^ o Development cases Eiarly mitotic behavior

during gastrulation. is prevailingly nor-
mal.
Ctenolabrus o X Stenoto-

mus o Many hatching em- Early mitoses are nor-

bryos of the mater- mal.
nal type.

Ctenolabrus o X Menidia o. . Advanced development. Early mitoses are nor-
mal.

Ctenolabrus o X Fundulus o One hatching embryo Abnormal nuclear be-

reported. Many axi- havior occurs,
vanced embryos —
maternal type.
Ctenolabrus o X Stenoto-

mus o Development ceases Abnormal mitosis pre-

during gastrulation. dominant.

Ctenolabrus o X Menidia o. . Two hatching embryos Abnormal mitosis is of

reported. Maternal frequent occurrence,
type.

show that the maternal type may appear not only when the
early mitoses are abnormal, but in one case at least when
they are normal. It is quite possible, therefore, that while

MATERNAL INHERITANCE 233

early loss of the paternal chromosomes may account for
some of the cases of maternal embryos, there may also
be cases where the chromatin may divide normally but
fail to produce any conspicuous effects on the cytoplasm
sufficiently soon to become apparent in the young fish.
In this connection the tobacco crosses described by
Groodspeed and Clausen may be recalled. In these cases
it was a particular group of chromosomes, regardless of
whether it was of paternal or of maternal origin, whose
*' reaction system ' dominated in the F^ hybrid.

CHAPTER XIX

THE PAETICULATE THEOEY OF HEREDITY AND
THE NATUEE OF THE GENE

The attempt to explain biological phenomena by means
of representative particles has often been made in the
past. The superficial resemblance of the theory of the
gene to some of the older theories, long since abandoned,
has furnished the opponents of the Mendelian theory of
heredity an opportunity to injure the latter by pretending
that the modern idea of the gene is the same as the older
ideas of Herbert Spencer concerning physiological units,
of Darwin relating to pangenes, and especially of Weis-
mann about biophors. There is no need for such con-
fusion, for even a little knowledge of the evidence on which
thei old and the new views rest ought to have sufficed to
make evident some important and essential differences.
It need not be denied, however, that there is an historical
connection between the mediaeval theory of preformation
and the particulate theory of heredity. Bonnet, one of the
best known adherents of preformation, believed at first
in ^ 'whole *^ germs, but later admitted that pieces of germs
might be stowed away in regions of the body likely to be
injured. Weismann, also, the most prominent modem
adherent of preformation, held that whole germs, ids, are
present in the germ-plasm, each standing for a whole
organism — each (or most or one!) becoming unravelled as
the embryonic development proceeded. In fact, Weis-
mann's entire theory was invented primarily to explain
embryonic development rather than genetics. Its connec-
tion Avith the modern idea of the germ-plasm is little more
than an analogy — for reduction in "Weismann 's original

234

PARTICULATE THEORY OF HEREDITY 235

sense meant the sorting out of the wholes of ancestral
germ-plasms with which he peopled the chromosomes.^

The danger of any appeal to a theory of representative
particles obviously lies in the ease with which by its means
any phenomenon might be accounted for, if the theorizer
is allowed to endow the particles with any and all the
attributes that he wishes to use in his explanation. It
was because Bonnet, Spencer, and Weismann assigned
arbitrarily attributes to the ultimate particles of living
matter, that these views appear to-day highly speculative.
The different kind of evidence to which the modern theory
of the gene appeals is what I wish to emphasize here.

The Evidence for the Gene

The evidence that Mendelian inheritance rests on the
distribution of separate elements has already been given.
The numerical results obtained in the second generation
from any Mendelian cross involving a pair of contrasted
characters, find their explanation on the assumption that
the two original germ-plasms (or some element in them)
separate cleanly in the germ-cells of the F^ hybrid. Tested
by back-crossing the assumption is verified. Recombining
the Pi, Fi, jPg individuals in all possible ways also gives
results consistent with the very simple assumption that
whatever it is that causes one race to produce one charac-
ter, and another race another character, the two separate
in the hybrid in such a way that equal numbers of germ-
cells of each kind are produced. Up to this point the
results do not tell us whether the two germ-plasms separ-
ate as wholes — one from the other — or whether only some
part or parts behave in this way. But when two or more

^ The nominal adoption (1904) toward the end of his career of heredi-
tary units in the Mendelian sense did not go deep. Weismann still adhered
to his view of dissociation of the ids as their' most characteristic feature —
the only one in fact for which, they were originally invented. The evidence
on which Mendelian units rest has nothing whatever to do with this
cardinal doctrine of Weismann's teaching.

236 PHYSICAL BASIS OF HEREDITY

pairs of contrasted characters are involved in the same
cross, we get further information as to the situation.

For example, Mendel showed that when peas that are
both yellow and round are crossed to peas that are both
green and wrinkled, there appear in the F2 generation not
only the original combinations, but also recombinations of
these, viz., yellow and wrinkled; and green and round
(Fig. 106). Here also the numerical results 9:3:3:1
can be explained on the theory that the representatives
of each pair of characters separate in the germ-plasm,
and that the separation of each pair is independent of
what takes place in the other pair. Obviously it can no
longer be whole germ-plasms that separate, but there
must be different pairs of elements in the germ-plasm that
assort independently of each other. It has been found that
this principle of independent assortment may apply to a
considerable number of pairs of characters segregating at
the same time. The only restriction that is found is in
the case of linked pairs of characters. This relation will
be considered later.

The independent assortment of the pairs of characters
proves that the elements that stand for the characters in
the two original germ-plasms may separate from each
other. If each such pair of characters represented one
of the pairs of homologous chromosomes, the evidence, so
far considered, would be in accord with the view that the
chromosomes were the ultimate units involved in the proc-
esses of segregation and assortment. The chromosomes
are, as has been shown, independent units in the germ-
plasm. But as Drosophila shows, there are many more
pairs of characters than there are pairs of chromosomes.
It is obvious that if the chromosomes are the ulti-
mate units involved, and remain intact, there could be no
more independent pairs of characters than there are pairs
of chromosomes. In animals and in plants there are
no cases known where there are more independent pairs
than there are chromosomes, so that, as has been pointed

PAETICULATE THEORY OF HEREDITY 237

out in another connection, this evidence may also he
appealed to as favorahle.

The behavior of linked pairs shows, however, that the
analysis must be carried further, because, despite linkage,
the elements that went in together may be separated. The
evidence shows that while some linked genes separate
almost as freely as do independent genes, so that their
linkage to each other can only be safely determined by
their relation to certain other genes, other linked genes
may separate not once in a hundred times, or even less
often. Between these extremes all intermediate linkage
values are found. These results indicate that the chromo-
somes do not represent the ultimate elements that may be
separated out of the original complex (germ-plasm).

We are led, then, to the conclusion that there are ele-
ments in the germ-plasm that are sorted out independently
of one another. The Drosophila evidence shows at least
several hundred independent elements, and as new ones
still appear as frequently as at first, the indications
are that there are many more such elements than those
as yet identified.

These elements we call genes, and what I wish to insist
on is that their presence is directly deducible from the
genetic results, quite independently of any further
attributes or localizations that we may assign to them.
It is this evidence that justifies the theory of partic-
ulate inheritance.

So far as representative elements in the germ-plasm
are concerned, we might be content to rest the case on the
preceding analysis of the results ; but recent work has now
advanced far enough to tempt us to assign further attri-
butes to the genes than those deducible from the preceding
analysis alone. Some of these attributes may appear
better established than others, but, all together, they give
a consistent body of data, and have therefore a certain
value and use.

238 PHYSICAL BASIS OF HEREDITY

It lias been pointed out that the evidence shows not
only that the genes are carried by the chromosomes, but
that there may be interchanges between paternally-
derived and maternally-derived chromosome pairs. The
evidence shows that this interchange is a normal feature
of the germ-cell, and not peculiar to hybrids, or to a
heterozygous condition of the pairs.

This analysis leads then to the view that the gene is
a certain amount of material in the chromosome that may
separate from the chromosome in which it lies, and be
replaced by a corresponding part (and by none other)
of the homologous chromosome. It is of fundamental sig-
nificance in this connection to recognize that the genes
of the pair that interchange do not .iump out of one chro-
mosome into the other, so to speak, but are changed
by the thread breaking as a piece in front of or else
behind them, but not in both places at once, as would
be the case if only a single pair of allelomorphs were
involved each time.

That the gene does not stand for the whole length of
the chromosome between two other known genes is shown
by the fact that new genes arising by mutation in the inter-
mediate region do not affect the character of the gene
already known. This fact recurring continually in Droso-
phila, where new mutations frequently appear, reassures
us that the idea of the gene as a very small part of the
thread is a legitimate conclusion, even if we can not tell
how large or how small that region is.

1. The Manifold Effects of Each Gene
If we examine almost any mutant race, such as the
race of white-eyed Drosophilay we find that the white eye
is only one of the characteristics that such a mutant race
shows. The productivity of the individual is also much
affected, and the viability is lower than in the wild fly. All
of these peculiarities are found whenever the white eye
emerges from a cross, and are not separable from the

o

PARENTS

c c

Fig. 106. — Diagram to show the inheritance of two pairs of Mendelian characters, viz.,
yellow versus green peas, and round versus wrinkled skin in garden peas.

^.-.

%^-i

PAETICULATE THEORY OF HEREDITY 239

white-eyed condition. It follows that whatever it is in
the germ-plasm that produces white eyes, also produces
other modifications as well, and modifies not only such
'' superficial'' things as color, but also such '^ fundamen-
tal ' ' things as productivity and viability. Many examples
of this manifold effect are known to students of heredity.
It is perhaps not going too far to say that any change
in the germ-plasm may produce many kinds of effects on
the body. Clearly then the character that we choose to
follow in any case is only the most conspicuous or (for
purposes of identification) the most striking or convenient
modification that is produced. Since, however, these
effects always go together, and can be explained by the
assumption of a single unit difference in the germ-plasm,
the particular difference in the germ-plasm is more sig-
nificant than the character chosen as its index.

2. The Vaeiability of the Ohakacter is Not Due to the

COEEESPONDING VaEIABILITY OF THE GeNE

All characters are variable, but there is at present
abundant evidence to show that much of this variability
is due to external conditions that the embryo encounters
during its development. Such differences as these are not
transmitted in kind — they remain only so long as the
environment that produces them remains. By inference
the gene itself is stable, although the character varies ; yet
this point is very difficult to establish. The evidence is
becoming stronger nevertheless that the germ-plasm is
relatively constant, while the character is variable.

3. Chaeactees That aee Indistinguishable May be the

Peoduct of Diffeeent Genes

We find, in experience, that we cannot safely infer
from the appearance of the character what gene is pro-
ducing it. There are at least three white races of fowls,
produced by different genes. We can synthesize white-

240 PHYSICAL BASIS OF HEREDITY

eyed flies that are somatically indistinguishable from the
ordinary white-eyed race, yet they are the combined prod-
uct of several known color-producing genes. The purple
eye color of Drosophila is practically indistinguishable
from the eye colors maroon and garnet. In a word, we are
led again to units in the germ-plasm in our final analysis
rather than to the appearance of a character.

4. Infekence That Each Chakacter is the Product of

Many Genes

We find that any one organ of the body (such as an
eye, leg, wing) may appear under many forms in different
mutant races as a result of changes of genes in the germ-
plasm. It is a fair inference, I think, that the normal
units — ^the allelomorphs of the mutant genes — also often
affect the same part. We have found about 50 different
factors that affect eye-color, 15 that affect body-color, and
at least 10 factors for length of wing in Drosophila.

If, then, it is a fair inference that the units in the wild
fly, that behave as Mendelian mates to the mutant genes,
also affect the same organ that the mutant gene affects, it
follows that many genes, and perhaps a very large num-
ber, are involved in the production of each organ of the
body. It might perhaps not be a very great exaggeration
to say that every gene in the germ-plasm affects several
or many parts of the body ; in other words, that the whole
germ-plasm is instrumental in producing each and every
part of the body.

Such a statement may seem at first hearing to amount
almost to an abandonment of the particulate conception
of heredity, but on the contrary, the statement conveys a
very important idea in the modern conception of the
nature of the genes and the way they act.

The essential point here is that even although each of
the organs of the body may he largely a product of the
entire germ-plasm, yet this germ-plasm is made up of
units that are independent of each other in at least two

PARTICULATE THEORY OF HEREDITY 241

respects, viz,, in that each one may change (mutate) with-
out the others changing, and in segregation and in crossing
over each pair is separable from the others,

5. ''The Organism as a Whole/ ^ or The Collective

Action of the Genes

Several writers have stated their objections to the
particulate theory of heredity on the grounds of their
belief that the organism is a ''whole/' If this phrase is
intended to mean that there is some sort of an entity or
entelechy that directs all processes that go on in each
living thing, there is little to be said here, except that
this very old idea has not been found profitable as a
working hypothesis. It is improbable, however, that
many biologists mean to appeal to any such vitalistic
agency when they speak of the ' ' organism as a whole, ' '
but have rather some other idea in mind. I am inclined to
think that certain phenomena of embryonic development
are responsible for the slogan of the "organism as a
whole. ' ' In the segmentation of the egg the entire chromo-
somal complex is distributed to every cell in the body.
Each cell inherits the whole germ-plasm. How then it
may be asked can the result depend on the particular
make-up of its chromosomes rather than on the action of
the whole material?

Granted that we know very little about the interactions
between the cells that cause some of them to differentiate
in one direction, others in other directions, yet if one fer-
tilized egg should begin its development with one kind of
material, and another egg with .a different material, should
we not expect the end products to be different, irrespective
of the way in which the materials were present in the
original eggl No matter where the differences may lie,
i.e., whether in the nucleus or in the cytoplasm, there is
nothing here in any way inconsistent with this particulate
theory of the composition of the germ-plasm. On the
contrary, the only conclusion that seems at all reasonable

16

242 PHYSICAL BASIS OF HEREDITY

is that if differences are present at the beginning, the end
product is expected to be correspondingly different. So
much is clear. But why, it may still be asked, are not two
organisms that are different at the start, if only in some
one difference, different later in every part, rather than
in only some one small part such as in a red or in a white
eye. The answer is, of course, that the first difference
was such that it affected principally a particular process,
viz., the formation of the red pigment of the eye, and to
a less degree, or not at all, other chemical processes. This
seems to me an entirely consistent view.

Perhaps the difficulty in accepting the particulate
theory lies in the erroneous idea that the specific effect
comes into action only at the moment when the red pig-
ment is about to form. But no one has, so far as I know,
made such a claim. It may be true, but it has not been
proven, and is moreover not in any way essential to the
assumption of the particulate theory. On the contrary,
as our knowledge of Mendelian heredity has increased
many cases have been found where a special factor-differ-
ence affects not only one part of the body but many parts.
It is true that the particulate theory as held at one time
by Roux and for a long time by Weismann was used to
explain the differentiating changes in the segmenting
egg and embryo in the sense that development was looked
upon as a process that resulted immediately in the sorting
out of the inherited chromosomal particles to the differ-
ent parts of the organism. Differentiation resulted in the
sorting out of particular genes to particular groups of
cells whose development they controlled. But the cyto-
logical evidence in regard to the chromosomes gave no
evidence in support of the view, and the evidence from
the experimental study of embryology seemed to entirely
disprove any such basis for the developmental phe-
nomena. In fact, Roux himself abandoned this view
in the light of the brilliant experiments of Driesch and
of other embryologists.

PARTICULATE THEORY OP HEREDITY 243

Our present conception of the relation of the germ-
plasm to developmental phenomena has then only a most
superficial resemblance to the older theories. The newer
point of view may be summed up in a few words, and has
in fact been stated already. First, that each gene may
have manifold effects on the organism, and second, that
every part of the body, and even each particular character,
is the product of many genes. The evidence for these two
conclusions has been so repeatedly referred to in the pre-
ceding pages that it is not necessary to go over it again,
but it may be worth while to emphasize that these two
conclusions are not pure speculations, but derived from
the evidence itself. It may also be well to point out that
even if the whole germ-plasm — the sum of all the genes —
acts in the formation of every detail of the body, still
the evidence from heredity shows that this same material
becomes segregated into two parts during the maturation
of the egg and sperm, and that at this time individual
elements separate from each other largely independently
of the separation of other pairs of elements. It is in this
sense, and in this sense only, that we are justified in speak-
ing of the particulate composition of the germ-plasm and
of particulate inheritance.

There is a further idea deducible from well-known
facts of physiology that may at first sight seem to give
an impression that the organism is a * ^ whole. '^ This
is the action of one part of the body on other parts by
means of substances set free in the blood, called hor-
mones. Many of them arise through the action of certain
so-called endocrine glands. But the relation here is so
obviously different from the problem dealt with as par-
ticulate inheritance that it callsi for little more than
passing notice. It may, however, not be without interest
to refer to one case of the kind in which an endocrine
secretion depends on a genetic factor inherited in the
same way as are other genetic factors. There is a race
of poultry known as Sebrights (Fig. 107, a) in which the

244 PHYSICAL BASIS OF HEREDITY

males are always hen-feathered. This means that the
feathers of the neck and back and the tail coverts of the
Sebright cock are nearly like those of the hen of this
breed, and not long and pointed as in the ordinary cock.
When Sebrights are crossed to game bantams (which
have ordinary males), the F^ males are hen-feathered.
Wlien these are inbred the two types reappear in the F2
males. One, or probably two, Mendelian factor differ-
ences account for the results.

It has been sho^vn that when the testes are removed
from the Sebright male, he then develops at the next moult
(or at once if some feathers are plucked out) the long
and highly colored feathers of the ordinary male (Fig.
107, h). It is probable, therefore, that the testes of the
Sebright produce an internal secretion that inhibits in the
male the full development of certain feathers. This makes
him like the hen, and in this connection it is interesting to
note that when the ovary of a hen of an ordinary breed is
removed she also develops the full plumage of the cock, as
Goodale has clearly demonstrated. Whether the testes of
a male are of the sort to develop this inhibiting substance,
depends on the presence in the cells of the testes of certain
genetic factors. These factors are present, presumably,
in all the cells of the body, but if they are, their activity
is ineffective in the absence of secretions produced by the
testes, as is sho\^^l by the castrated Sebright becoming
cock-feathered. Whether this substance belongs in the
heterogeneous group of substances called hormones —
defined by the kind of action they produce rather than by
any chemical peculiarity — or to the groups of enzymes
that have a more or less specific action, cannot be stated.

The fore.o^oing discussion touches upon the question as
to whether there is any evidence that the genes themselves
are to be regarded as enzymes.* In almost all of the

* Inadequate as is our knowledge of the physico-chemical processes that
go on in development, it is enough to indicate that many processes are
at work.

PARTICULATE THEORY OF HEREDITY 245

recent papers (Bates, Riddle, Onslow, Goldsclimidt) that
touch on this question it is argued, from the evidence of the
specific enzymes supposed or demonstrably involved in
the production of some final stage in the chemical reaction
that leads to the character in question, that the gene itself
is the same specific enzyme. The argument shifts back
and forth from unit-character to unit-factor. The reason-
able position to take in this matter is, in my opinion, that
stated by Loeb and Chamberlain (1915), ^'The hereditary
factor in this case must consist of material which deter-
mines the formation of a given mass of these enzymes,
since the factors in the chromosomes are too small to carry
the whole mass of the enzymes existing in the embryo
or adult. ^' It should not be forgotten, however, that the
evidence in favor of enzyme action as the most important
developmental process is by no means established, and
even were the evidence for this view adequate, the stages
between such action and the ultimate chemical nature of
the gene may be too great to be cleared at a single bound.
Some of the modern work on the chemical composition of
the nucleus indicates that extremely complex protein com-
pounds may be present in it — even though some of the
split products obtainable from it may be relatively simple.
It seems to me therefore that it is both premature and
highly speculative at present to tie up the genetic evi-
dence concerning the genes with hypotheses concerning
their chemical composition. I urge this, but at the same
time I realize of course that we should endeavor to obtain
as soon as possible better knowledge as to the chemical
nature of the chromatin.

Another question concerning the gene, that has been
raised, is whether it is to be regarded as something having
a definite molecular constitution, or whether the gene is to
be regarded as a quantity of material fluctuating about a
mode — its definiteness representing only a general ten-
dency for the same frequency distribution to recur in
each species. From the nature of the case such a question

246 PHYSICAL BASIS OF HEREDITY

is speculative, and would have little importance were it not
that, by imputing to the advocates of Mendelian heredity
the assumption of absolute fixity to the gene, attempts
have been made to throw the burden of proof that the
genes are ''constant'' on the advocates of Mendelism.

So far as the genetic evidence is involved, I see at pres-
ent no way of deciding whether the gene has a definite
molecular constitution, or is only something that fluctuates
under the condition of its occurrence about a mode. Inter-
esting as it might be to speculate about these alternatives,
it seems futile to do so at present, but there is one impli-
cation that I should like to examine. If the gene is a
chemical molecule it is not evident how it could change
except by altering its chemical constitution. Its influence,
i.e., the chemical effects it produces, might, however, be
altered by changing other substances with which the mate-
rial it produces reacts. This is the idea involved in the
theory of ''modifying genes.''

But if the gene is a fluctuating amount of something it
might seem that any "fluctuation" that is present at one
time might be perpetuated by selection, and that a further
fluctuation in the same direction might be utilized for a
further advance, etc. It may be pointed out that this
picture of the process is quite fanciful, and its success
would depend largely on a denial of the premise as to the
nature of the gene, viz., that it is of a fluctuating amount.
Johannsen's facts contradict an interpretation of the
fluctuations of the character being due to a new modal
position of the gene standing for that character. And his
facts furnish the only crucial evidence we have at present.

:i

Fig. 107. — Normal Sebright hen-feathered male to left, and a castrated Sebiight to
the right. The latter shows all the highly developed secondary sexual characters of the
cock, except the comb which is that of the capon.

CHAPTER XX
MUTATION

Concerning the origin of the germinal differences that
give rise to mutant characters very little is known at pres-
ent except, (1) that they appear infrequently, (2) that the
change is definite from the beginning, (3) that some of
the changes at least are recurrent, and (4) that the differ-
ence between the old character and the new one is small
in some cases and greater in others. I do not think that
any of the work purporting to produce specific mutational
changes has succeeded in establishing its claims, at least
in the sense that we can pretend at present to control the
appearance of specific mutant changes, and until this is
done we can not hope to find out very much as to the
nature of these changes. Our study of the germ-plasm
is largely confined, therefore, for the present, to a study
of transmission of the genes, to the kinds of effects they,
produce on the organism, and to the special relations of
the genes in the chromosomes where they are located.

Concerning the frequency of mutation there is a slowly
increasing body of evidence showing in some animals
and plants how often or how rarely changes of this kind
take place. The impression prevails that mutation is less
rare in some species than in others, and while I am inclined
to think that this may be true, not much value can be
ascribed to such impressions ; for it is not improbable that
the frequency with which mutations are found is often
directly in proportion to the number of individuals exam-
ined and to familiarity with the type in question, so
that the smaller changes are not overlooked. The dis-
covery of new mutant types in almost every plant and
animal that has been carefully examined indicates at least
the very general occurrence of definite mutations, and the

247

248 PHYSICAL BASIS OF HEKEDITY

great variety of types shown by nearly all of our domesti-
cated animals and plants — varieties that follow MendePs
law — appears to give further support to the view that the
process of mutation is widespread.

One of the most interesting phenomena connected with
mutation is the recurrence of the same change. It has
long been recognized that certain * ' sports ' ' such as albi-
nos and melanic forms are found again and again in
nature. In insects there are many records of the sporadic
appearance of the same type, such as the light form (lacti-
color) of the moth Abraxas. It is true that not all such
appearances are to be accepted offhand as the first appear-
ance of the mutative change, since when these are reces-
sive it is probable in most cases * that the actual mutation
occurred several generations before the mutated genes
came together to produce the mutant character. But
granting this, it is at least probable that the same type
has appeared in many cases independently. The only
evidence that can be relied upon in such cases is from
pedigreed cultures, followed up by evidence that the
mutants that look alike are really due to mutations in the
same locus. Fortunately there is actual evidence, both
for plants and for animals, that can be appealed to to show
that the same mutations recur.

The most extensive evidence is from Drosophila
melanog aster. One of the first mutants that appeared,
viz., white eyes, has appeared anew in our cultures about
three times, in cultures known to be free from it before and
not contaminated. The same mutant has been found by
several other observers. The eye-color vermilion has
appeared at least six times; the wing character called
rudimentary, five times; cut wing has been found four
times; truncate wing has frequently appeared, but has
not necessarily been always produced by the same change.
Certain characters such as notch wings, that have

* Recessive mutations in the X-chromosomes of the XX-XY type may
appear in the male in the next generation.

MUTATION 249

appeared quite often, represent, it seems, a peculiar
change whose relation to the changes that stand behind
other mutant characters is not yet worked out.

In plants the best evidence is that reported by Emerson
for Indian corn. Emerson has shown that when a race
of corn (Zea mais) having red cobs and red seeds is
crossed to a race having white cobs and white seeds only,
the two original combinations appear in the second (F^)
generation giving plants with red cobs and red seeds and
plants with white cobs and white seeds. Either a single
factor determines that both cob and seed are red in one
case and white in the other, or if the color of each part
is due to a separate factor these factors are completely
linked. Now striped seeds with white cobs sometimes
mutate to red seeds and red cobs. The new combination
(red and red) acts as a unit toward the other known com-
binations. Therefore a single factor must have changed,
for, if not, mutation must occur in two (or more) closely
linked factors, i.e., for seed and cob color at the same time,
which is highly improbable.

In forms propagating by sexual methods it cannot
be told whether mutation has occurred in one locus or in
both homologous loci at the same time, because in the egg
one of each pair of genes is lost in the polar body, and
irrespective of whether one or two mutated genes were
present only one member of the pair is left in the ripe egg ;
and in the sperm the chance of any one sperm reaching
the egg is so small that it is unlikely that the difference
between one sperm or two sperms having the mutated
locus could be detected. It is true that of the twelve domi-
nant mutants that have appeared in Drosophila each
appeared at first in a single individual — ^never two — ^which
might appear to favor the single locus view, but this evi-
dence is too meagre to be significant. Mutants from reces-
sive genes usually come to light in about a quarter of the
offspring of a given pair. This means that both parents
were heterozygous for the mutant gene, but this gene

250 PHYSICAL BASIS OF HEREDITY

must have arisen at least one generation earlier, and
have been carried over into the two heterozygous indi-
viduals in question.

It would be a point of capital importance if it could be
determined beyond doubt that at times recessive mutant
genes change back to the original (wild type) gene, or
even if a recessive gene could mutate to a dominant one.
The appearance of the wild type in a pure culture of a
mutant race can be accepted as good evidence of such a
change only when every possibility of contamination by
the wild type is excluded, and this is difficult to regulate.
In our cultures we have come across such cases, but have
not ventured to exploit them, since wild-type flies are
always present in the laboratory, and hence the discovered
form may have arisen through accidental contamination.
Thus even when a red-eyed yellow fly appeared in the
white-eyed yellow stock there is the barest chance that a
yellow red-eyed fly, or an egg of such a fly, had somehow
gotten into the stock. Certainty can be attained only when
a stock, pure for several mutant characters, reverts to the
normal in one of these characters, and not in the others.
Only one case of this kind that is above suspicion has been
as yet recorded. This is a mutant stock in which, as May
has recorded, reversion to the wild type occurs with such
frequency that there can be no chance of error. The stock
in question, bar eye, is a dominant mutant and the rever-
sion therefore is to the recessive wild type of eye (round
eye). The change back to normal is complete, since such
individuals give only normal offspring. When such a
mutant chromosome comes from the mother and goes into
a son he has normal (wild type) eyes ; when it comes from
the father, and goes to a daughter, she is heterozygous
for bar eye. Baur has recently recorded the appearance
of recessive ( ?) mutants from self -fertilized plants (snap-
dragon) that bred true at once. Punnett has described a
similar case (1919). The result can be accounted for, if a
mutation occurred in only a single chromosome far enough

MUTATION 251

back in the germ-tract to give rise, after reduction, both
to pollen and to ovules, each one carrying the mutated
genes. Such an interpretation is supported by the evi-
dence from DrosopUla, where, although mutations are
much more numerous, no such cases have been observed,
and none such would be expected if mutation occurs in a
single chromosome at a time, since here the germ-cells
come from separate individuals.

Probably the most important evidence bearing on the
nature of the genes is that derived from multiple allelo-
morphs. Now that the proof has been furnished that the
phenomena connected with these cases are not due to nests
of closely linked genes, we can properly appeal to these as
crucial cases. As already explained, in ever-increasing
numbers of animals and plants, series of genes have been
found in each of which mutant characters with the same
normal allelomorph have been found. These mutant char-
acters of each series are also allelomorphs of one another
--only two ever existing in the same individual. Ob-
viously, not all such mutants can be due to the absence
of a factor present in the germ-plasm of the wild type,
since only one kind of absence is thinkable. If to save the
situation for the theory of presence and absence it be
assumed that only a part of the original gene is absent,
and a different part in each case, then nothing is gained by
the admission; and while this may be trne it is equally
possible that the genes change in other ways. It is not
essential that we should specify the nature of the change,
but simpler to look upon the mutant gene as due to
some kind of change or changes that have taken place
in the original germ-plasm at a specific locus— there is
nothing known at present to furnish even a clue as to the
nature of this change.

The demonstration that multiple allelomorphs are
modifications of the same locus in the chromosome, rather
than cases of closely linked genes, can come only where
their origin is known, and at present this holds only in

252 PHYSICAL BASIS OF HEEEDITY

the case (just stated) for Indian corn and for the fruit
fly. If each member of such a series of allelomorphs has
arisen historically from the preceding one in the series,
by a mutation in a locus closely associated with the locus
responsible for the first, they would be expected to give
the wild type when crossed; and as the proof of their
allelomorphism turns on the failure of members of the

B

1 •

2 •

K

3 •

4 t

5 t

1 0

1 •

1 •

1 •

1 •

2 t

2 0

2 •

2 t

2 f

3 t

3 •

3 0 .

3 •

3 •

4 •

4 t

4 •

4 0

4 •

5 •

5 •

5 •

5 •

5 0

a

b

0

d

e

1 0

a 0

1 0

1 0

1 0

2 •

2 0

2 0

2 0

2 0

3 •

3 •

3 0

3 0

3 0

4 •

4 •

4 •

4 0

4 0

5 •

5 •

5 •

5 f

5 0

a

b

c

d

e

Fig. 108. — Diagram illustrating mutation in a nest of genes so closely linked that no

crossing over£takes place.

series to show the atavistic behavior on crossing, it is
necessary, as stated, to know how they arose. This may
be made clear by the following illustration :

Let the five circles of Fig. 108, A represent a nest of
closely linked genes. If a recessive mutation occurs in
the first one (line B, a) and another in the second gene
(line B, h), the two mutants a and b if crossed should give
the atavistic type, since a brings in the normal allelo-
morph (B) of h, and h that (A) of a. If a third mutation
should occur in the third gene it, too, will give the atavistic

MUTATION 253

type if crossed to a or to &. Similarly for a mutation in
the fourth and in the fifth normal gene. Now this is
exactly what does not take place when members of an
allelomorphic series are crossed — they do not give the
wild type, but one of the other mutant types or an inter-
mediate character. Evidently independent mutation in a
nest of linked normal genes will not explain the results
if the new genes arise directly each from a different nor-
mal allelomorph.

But suppose, as shown in Fig. 4 (line C) after a muta-
tion had occurred in the first gene a new mutant, h, arose
from a new gene, and from h a mutation arose in a third
gene c, and c similarly gave rise to d; then a crossed to b
will give a (or something intermediate if the heterozygote
is an intermediate type). Likewise c crossed to b will
give b, or c crossed to a will give a, etc. If mutant allelo-
morphic genes in a series such as C, a, b, c, d, e, arise as
successive steps, i.e., Ca to Cb and Cb to Cc, etc., then
the hypothesis of closely linked genes would seem to be a
possible interpretation of the data, but if they do not
arise in this way, but by independent mutations from the
wild type (or even from each other, but not seriatim), then
they must be due to mutations in the same gene : for, to
assume that they are not, requires that, when the second
mutation took place both gene a and gene b mutated at
the same time, and that when c appeared three genes
mutated, when gene d appeared four; when gene e five
genes mutated at once, four of them being mutant genes
that have already arisen independently. Such an inter-
pretation is excluded, since it is inconceivable, even in a
readily mutating form like Drosophila, that five muta-
tions could have occurred at the same time in distinct but
neighboring loci. As has been stated, the evidence from
Drosophila shows positively that multiple allelomorphs
arise at random.

Only two members of a series of multiple allelomorphs
can be present in any one individual, and in the case of

r

254 PHYSICAL BASIS OF HEEEDITY

genes carried by the sex-chromosome only one can exist
at a time in the sex that has onlv one of these chromosomes.
In the individual with two mutant allelomorphs one of
them replaces the normal allelomorph of the ordinary
Mendelian pair. The two mutant allelomorphs behave
towards each other in the same way as does the normal
towards its mutant allelomorphs. It is doubtful whether
we can conclude anything more from this relation of Men-
delian pairs than we knew before/ although there is at
least a sentimental satisfaction in knowing that both nor-
mal allelomorphs can be replaced by mutant ones without
altering the working of the machinery.

The linkage relation of each member of a series of
multiple allelomorphs to all other genes of its chromo-
some is, of course, the same. While the theory of identical
loci requires this as a primary condition it is not legiti-
mate to use this evidence as a proof of the identity of the
loci, because it is not possible to work with sufficient pre-
cision in locating genes by their relation to other linked
genes to distinguish between identical loci and close-
linked genes.

The question of lethal genes has attracted in recent
years increasing attention, both on account of their fre-
quency and because of a curious complication they may
produce in hiding the effects of other genes also present.
In Drosophila we have records of more than 20 sex-linked
lethals, and about 15 not sex-linked, and scattering records
of many others. Gametic lethal genes are those that
destroy eggs or pollen cells that contain such genes.
Zygotic lethal genes affect the embryo, the larva, or the
adult, so that it dies. In the case of the garden plant
known as double ^^ stocks," the genetic evidence obtained
by Miss Saunders indicates that certain kinds of pollen
are not produced, and presumablv die because of a con-
tained factor. The same factor does not kill the ovules,

^ The substitution bv crossincr over reallv furnishes as orood a demon-
stration of this point.

MUTATION 255

which may therefore transmit the recessive lethal gene to
half the progeny. How far the frequent occurrence of im-
perfect pollen grains in many species of plants is due to
such factors is still uncertain.

Belling found that while the Florida velvet bean
produces normal pollen grains and ovules, and the Lyon
bean, another bean of the same genus, also produces nor-
mal gametes, the F\ hybrid contains 50 per cent, abortive
pollen grains, and possibly about 50 per cent, of the ovules
are abortive. In the second generation {F2) half of the
pollen grains of half of the plants are abortive. The other
half of the plants have normal pollen grains. This is the
result expected if there are present in one of the species
the factors A Abb, and in the other species the factors
aoBB, the viable gametes in the F^ generation being
those containing Ab, Ba, and the two gametes that die
being AB, ab.

Other observers have made records of abortive pollen
in hybrids, but without knowing the condition of the
pollen in the parents the interpretation of the results is
doubtful, for, as Jeffrey has emphasized, abortive pollen
is a characteristic of many wild species. There is one
fact of capital importance recorded by several botanists,
viz., that the degeneration of the germ-cells only takes
place after the tetrad has been produced, and only in some
of the cells of each tetrad. In other words, the lethal
effect is not observed until the chromosomes have under-
gone reduction. It is obvious that if there is present a
recessive lethal for the germ-cells (or for any cells, in
fact), it causes no injury in the presence of its normal
allelomorph, but kills when the counter-effect of its part-
ner is removed.

Tischler found in a hybrid currant that tetrad forma-
tion was normal, and that the shrinking of the pollen
grains occurred afterwards. Geerts found that one-half
of the pollen grains of (Enothera Lamarckiana degen-
erate, and that half of the embryo sacs abort in the tetrad

256 PHYSICAL BASIS OF HEEEDITY

stage. Other related (wild) species and genera of the
evening primrose have also been found to have some
abortive pollen and ovules.

Complete or nearly complete abortion has been seen
in other hybrids; viz,, by Eosenberg in the sundew, by
Osawa in the Satsuma orange, by Goodspeed and others
in the hybrid tobacco {N. tahacum by N, sylvestris), by
Jesenko in the wheat-rye hybrid, and by Sutton in the
hybrid between the Palestine pea {Pisum humile) and the
edible pea. These cases may be in part the same phe-
nomenon and in part a different one connected with fail-
ure of the chromosome to conjugate or to be properly
distributed during the maturation divisions.

The ^ ^yellow mouse case^' is an example of a zygotic
lethal effect. The gene that produces the dominant yellow
color is lethal in double dose, so that all homozygous yel-
low mice die, as Cuenot first discovered, and as has been
more positively demonstrated by the work of Castle and
Little. There is some evidence indicating that these homo-
zygotes die as young embryos. Little has also shown that
black-eyed white mice carry a lethal, that acts in the same
way. In Drosophila there is a sex-linked recessive lethal
factor that causes the development of tumors in the larvae,
destroying every male larva that contains the sex-chromo-
some carrying this gene. This effect, discovered by
Bridges, has been the basis for an extensive series of
experiments by Miss Stark. The gene is present in the
X-chromosomes ; it follows the rules for all sex-linked
genes in its inheritance. The females of the stock are of
two kinds : One has the lethal in one sex-chromosome, and
its normal, dominant allelomorph in the other. Such a
female has survived because the effect of the lethal gene
is counteracted by the effect of its normal allelomorph.
Half of her sons get the affected chromosome. All such
sons develop the tumor — one or more melanitic growths
that appear in the imaginal discs or in other parts of the
larva. The other sons get the other chromosome with the

MUTATION 257

normal allelomorph. They never produce a tumor and
never transmit the disease. The same mother that gave
these two kinds of sons — having been fertilized by a nor-
mal male, since no affected males exist — produces also two
kinds of daughters, one containing the gene for the tumor
(and its normal allelomorph), the other having two nor-
mal genes. The former transmit the disease as just
explained, the latter daughters are perfectly normal and
do not transmit the disease.,

Other lethal genes kill the pupae, a few of them even
allow the fly occasionally to come through, but such flies
rarely propagate. Certain races of Drosophila have ster-
ile or nearly sterile females, other races sterile males.
The sterility is here lethal in so far as it affects the germ-
cells. Some effects on other characters are also generally
to be seen.

The presence of a lethal gene near to, i.e., linked to,
another mutant gene may affect the kinds of individ-
uals that appear because owing to the linkage the other
mutant character fails to appear, except when crossing
over takes place. Some examples of this relation may be
given. There is a mutant race called beaded (Fig. 109)
in which the margin of the wing is irregularly broken,
giving the appearance of a beaded edge. The gene for
beaded is dominant, and lethal when homozygous.

As in the case of the yellow mouse, only the hybrid
(heterozygous) combination exists, and consequently
when two beaded flies mate they produce two beaded to
one! normal fly, as shown in Fig. 110. Here the first pair
of vertical lines stand for th^ pair* of third chromosomes
present in the egg before its reduction. The two genes
here involved, that for beaded and its allelomorph for
normal, are indicated at the lower end of the vertical lines.
The two corresponding chromosomes in the male are
represented to the right of the last. After the ripening of
the germ-cells each egg and each sperm carries one or
the other of these chromosomes. Chance meetings of egg

17

258

PHYSICAL BASIS OF HEREDITY

and sperm are indicated in the fignre by the arrow-scheme
below, which gives the combinations (classes) included in
the four squares. The double dominant BB is the class
that does not come through. The result is two beaded
(heterozygous) to one normal fly.

The beaded stock remained in this condition for a long
time ; although selected in every generation for beaded, it
did not improve, but continued to throw 33 per cent, of
normal flies. Then it changed and bred nearly true.

Eggs

Sperm

////

/ // /

B

/ /^/ /

N

////

N

B

- -

-N B-

- -

-N

B

B

B ^

2 Beaded

•

: 1 Normal.

Fig. 110. — Diagram showing the relation of the chromosomes (represented by the
vreitical rods) in a cross of "beaded" by "beaded." Fhes homozygous for beaded die as
indicated by the cross-hatched square.

The change must have been due to the appearance of
another lethal factor (now called lethal three, here I) in
Fig. 111). Such a gene was found in the race when
studied later by Muller.

The lethal gene that appeared in the beaded stock was
also in the third chromosome, and in the chromosome that
is the mate of the one carrying the gene for beaded, i.e.,
in the normal third chromosome of the beaded stock. The
lethal gene lies so near to the level of the beaded-normal

•^

OS

O

p
a.
o
a-

3
'iJQ
CO

I&— »-™.»u

MUTATION

259

pair of genes that almost no crossing over takes place
between the levels occupied by the two pairs. These rela-
tions are illustrated in the next diagram, Fig. 111. Here
again the two pairs of vertical lines to the left represent
the two third-chromosome pairs in the female and to the
right in the male. The location of the two pairs of genes
involved, N-l^ and B-N, are indicated. These combina-
tions give the four classes in the squares of which two
classes die, vie., NNBB (pure for beaded) and l^l^NN

Eggs

Sperm

N --

B ■-

\ N
N B

T t»

All Beaded

KB

KB

liK

liH

Fig. 111. — Diagram to show how the appearance of a lethal near beaded causes
the stock to produce only beaded except for the small number of crossovers, as shown
by the next diagram.

(pure for lethal three). The result is that only beaded
flies come through, and since all these are heterozygous
both for B and l^, the process is self -perpetuating.

If the preceding account represented all of the facts in
the case, the stock of beaded should have bred perfectly
true, but it has been shown in Drosophila that crossing
over between the members of the pairs of genes takes
place in the female. Hence we should expect a complica-
tion due to crossing over here unless the level of the two
pairs of genes was so nearly the same as to preclude this
possibility. In fact, in addition to the beaded flies the

260

PHYSICAL BASIS OF HEEEDITY

stock in this condition alone should give 10 per cent, of
crossing over, i.e., it should still produce a small percentage
of noi-mal flies. It so happened, however, that there was
present in the stock a third gene that lowers the amount
of crossing over in the female to such an extent that, for
the two ^ ^ distances ' ' here involved, practically none takes
place. When it does, a normal fly appears, hut this is
so seldom that such an occurrence, if it happened in a
domesticated form of which the wild type was unknown,

Crossover
Eggs

Sperm

N X -

k| -

H

-B B-- --N

--I1

NB

y"^^^

IBead • INorm. 0 • SjS

KB

liB

liH

Fig. 112. — Diagram showing the results of crossing over in a stock containing both beaded

and lethal, as shown in Fig. 111.

would be set down as a mutation like that shown by the
evening primrose.

The third factor that entered into the result is not
unique, for Sturtevant has shown that crossover factors
sire not uncommon in DrosophUa. The analysis that Mul-
ler has given for beaded, while theoretical, is backed up
by the same kind of genetic evidence that is accepted in
all Mendelian work. It makes an assumption but one that
can be demonstrated by any one who will make the neces-
sary tests. It is also possible to produce at will other bal-
anced lethal stocks that will '* mutate'' in the sense that

MUTATION

261

they will throw off a small predictable number of a
** mutant'* type — a type that we can introduce into the
stock for the express purpose of recovering it by such an
apparent mutation process.

For example, dichete is a third chromosome dominant
wing-and-bristle character and, like beaded, a recessive
lethal. Sturtevant bred flies with the gene for dichete
in one of the third chromosomes and with a gene for the
recessive eye-color, peach, in the other for several genera-

T?on crossover eggs «^^^^
(955^ of total) SP®^^'

D'

1

N --

N

■4-w

p

CHH

DUK

Klap

HI2P

Fia. 113. — Diagram illustrating how in the presence of a dominant factor, dichete, and
a lethal in its homologous chromosome at about the same level, together with another
factor, peach-colored eyes (p), gives the result shown in the squares. No peach appears
in the offspring except where crossing over takes place as shown in the next diagram.

tions. A lethal appeared by mutation in the peach-bearing
chromosome very near the level of the dichete gene in
the opposite chromosome.

The order of these genes is shown in Fig. 113. This is
then a balanced lethal stock that throws only dichete
flies,2 except for a small percentage of dichete peach flies
due to crossing over. The result for the non-crossover
classes is shown in the square to the right. Only two
of the four classes come through : the two that die are the

* Very rarely a crossover not-dicherte fly will appear.

262

PHYSICAL BASIS OF HEREDITY

one pure for dicliete and the one pure for lethal. The sur-
viving classes continue to produce the same kind of
offspring since they are, like the parents, heterozygous
for the two lethal factors. But the factors are not near
enough together to prevent crossing over, which occurs
in about 5 per cent, of cases between the lethal and peach
genes. The next diagram, Fig. 114, shows how when
crossing over takes place in the female, there result four

Crossover eggs c^evm
{bfo of total) ^P®^""

N--

I

--Iz
--K

N-- --P

EfNp

N\P
Dich. peach

NIN
CiNN

pichete

/^N^,/,

95-^95-H5 Dichete 1 ^ , w
5 Dichete Peach] "^^ /^ ' '^ '^

D^p Nlz W

txt

D'NK ^^3.3 P

Fig. 114. — Diagram illustrating crossing over of factors shown in Fig. 113.

classes (see squares), of which two die (as before), and
of the two that survive one is dichete peach. Taking both
non-crossover and crossover results together, the expec-
tation is 95 + 95 + 5 dichete to 5 dichete peach or 971/2
to 21/2. This stock then breeds true for dichete without
showing the gene it carries for peach eye-color except in
a small percentage of cases, and if the peach-eyed fly
should be unable to establish itself in nature, like some
of the (Enothera mutants, the stock would not be changed
by it, but continue to throw off a few '^ mutants" with
peach-colored eyes.

MUTATION

263

Now this process is not what is ordinarily meant by
mutation, for we mean by the latter that a new type has
suddenly arisen in the sense that some change has taken
place in the germ-plasm — a new gene has been formed.
The process here described is one of recombination of
genes shown by Mendelian hybrids, the only unusual fea-
ture being that all the phenomena involved do not come to
the surface because many classes are destroyed by lethals.

The results are interesting also in another way. It has
been assumed by those who think that 0. Lamarckiana is

Fig. 115. — Rosettes of the twin hybrids of the evening primrose, the plant to the left is
called laeta, and that to the right velutina. (After De Vries.)

a hybrid that the mutant types are only the segregation
products of the types or combinations that went in to pro-
duce the hybrid. But the Drosophila cases show that
balanced lethal stocks may arise within stocks themselves
by the appearance in them of lethal factors closely linked
to other factors — new or old ones. When new genes arise
in such lethal stocks the process may be one of true muta-
tion, but the revelation of the presence of the gene is
hindered by the lethal factors, so that when the character
appears, it appears as a ^'new'' mutant, but is in reality
due to recombination of mutant genes that had arisen in
an earlier generation. As a matter of fact, the first

264

PHYSICAL BASIS OF HEREDITY

appearance of even ordinary mutants, unless they be
dominant, must come two or more generations after the
mutation has taken place ; for, the evidence indicates that
mutation appears in only one chromosome at a time.^
In the case of sex-linked genes, however, any mutation
that takes place in one of the X-chromosomes of the mother
is revealed if the egg containing it gives rise to a son,
because he has but one X-chromosome and that comes
from his mother.

The delayed occurrence then of mutants in balanced
stocks is not different from the delay in other stocks —

\711d

9

N-- -H

Besided
cT

N-- --N H-. -1,

B

IXt

KB liN

»

Twin Hybrids,

Fig. 116. — Diagram illustrating balanced lethals and twin hybrids.

only when the recombinations occur in balanced lethal
stocks they must have been preceded by crossing over,
which diminishes the number of mutants that appears.
The number of mutants that appears is determined by
the distance of the genes for the character from the
nearest lethal gene.

One of the most interesting features of Lamarck ^s
primrose arises when it is bred to certain other species
or varieties. It gives rise to two kinds of offspring
called Twin Hybrids, to which De Vries gives the names
IcBta and velutina (Fig. 115). Now it is a feature

'If in self-fertilizing forms a mutation takes place so early in the
germ-plasm that it gets into both eggs and sperm the new character may
appear at once (isee ante).

MUTATION

265

of balanced lethal stocks like beaded that they repeat
precisely this phenomenon. For instance, if a beaded
male is crossed to wild female, two kinds of offspring are
produced, viz., beaded and normal. A similar process
would account for twin hybrids in CEnothera crosses.
There is another peculiar phenomenon that has been
described for crosses in the evening primroses, viz., the
occurrence in F^ of four types. This phenomenon, too,
can be imitated in Drosophila by crossing balanced lethal
dichete to balanced lethal beaded (Fig. 117).

Dlcheto

Beaded

If-

-- .-

N

N
I2

N

N

4-N

N
B

"N

d'n K H

d'n If H

N N K B

N N liH

Bead .Dichete

Dichete

N IjU K

N LjN N

N N N B

N N Ij N

Bea^ied

Normal

Both stocks "breed true. Four types in F^ 1:1; 1:1

Fig. 117. — Diagram illustrating lethals and four types in Fi.

Other parallels might be cited, but these, I think, will
suffice to indicate that the discovery of balanced lethal
stocks may solve some at least of the outstanding difficul-
ties of mutation and inheritance in CEnothera, and bring
it into line with other groups. There are, of course, other
peculiarities of the evening primrose that such zygotic
lethals will not explain ; such, for instance, as the 15-chro-
mosome type, and 0. gigas. But these cases are already
on the road to solution.

The occurrence of other lethals, called gametic lethals.

266 PHYSICAL BASIS OF HEEEDITY

that kill the germ-cells — gametes — before they are ready
for fertilization, has already been invoked by De Vries
and others to explain the peculiarity of ^'double recipro-
cal hybrids/'

Is THE BiKECTION OF MuTATION GiVEN IN THE CONSTITUTION

OF THE Genes 1

When writers have brought forward evidence of con-
tinuous and progressive change in a character, they have
not concerned themselves with the analysis of the change
in the germ-plasm that has brought it about — in fact, in
most of these cases the possibility of advance in a princi-
pal gene or of advance through modifying genes has not
been appreciated or even understood. Paleontologists
who have in the main been the strong advocates of ortho-
genesis have based their conclusions on the observed
advances in a character in the same series and in ^^ paral-
lel" series. They overlook the fact that to-day there is
experimental evidence demonstrating that variations as
small even as those they record have been shown to rest
on mutational stages. If the progress has been in the
direction of adaptation, natural selection of small mutant
differences will completely cover their findings. If it is
claimed that in some of these cases the orthogenetic series
is not in the line of adaptive advance, the burden of proof
lies heavily on their shoulders. Moreover, the fact, that
recent work has made clear, that genes generally have
more than a single effect on the organization, opens wide
the door of suspicion, for the observed morphological
progress might be a by-product of influences that have
other and important, though unseen or unknown, effects.
In a word, an orthogenetic series of changes does not in
itself without a closer analysis than has as yet been fur-
nished, establish that an innate principle-, urge, vis-a-tergo,
^^kicJc/^ or vital ^^ force" is causing the successive moves.
The genetic evidence concerning multiple factors must
create at least a strong suspicion against the ^'will to

MUTATION 267

believe'^ in the mystic sentiments for which these terms
always stand. That a progressive series of advances in a
gene might take place with a consequent advance in the
many characters involved is thinkable, especially if it could
be shown that environmental changes cause parallel prog-
ress in the gene, and this in turn on the character. How
probable this is the reader must decide for himself in the
light of the very clear evidence that each character is
affected by changes in many genes differently located in
the germ-plasm, and that it is not a progressive change in
one gene that makes selection possible, but changes in any
one of many genes.

Chance Mutation and Natueal Selection

The mutation process rests its argument for evolution
on the view that among the possible changes in the genes,
some combinations may happen to produce characters that
are better suited to some place in the external world than
were the original characters. Apparently this appeal to
chance, like Darwin's appeal, has offended some of the
adherents of the doctrine of organic evolution, because it
has seemed to them inconceivable that chance could ever
bring about the assembling of such an intricate piece of
machinery as a highly complex organism. The attempt
to mitigate the rude shock of the appeal to chance was
made by Darwin by pointing out that evolution had been
gradual and that the assemblage has not taken place out
of chaos, but each stage has been built up on one a little
less complex than the preceding one. Nevertheless the
fact remains that persistent efforts continue to be made
from time to time to introduce into the theory of evolution
some sort of directive mystical agency. The Lamarckian
theory has tried to bring about a more immediate relation
between the organism and its environment of such a kind
that the adaptive change that appears in the body as a
result of a reaction between the environment and the ani-
mal or plant, is reflected into the germ-plasm. Bergson

268 PHYSICAL BASIS OF HEEEDITY

has cut the knot by postulating an innate adaptive respon-
siveness of the animal to every critical situation that calls
out a response. The adherents of orthogenesis appeal,
apparently — in so far as they conunit themselves — to
some sort of innate principle that causes advance in com-
plexity along one line, and they seem to hint at times even
along directed lines of adaptation. Still more elusive are
vague appeals made to some unknown principle — some
sort of mysterious element, some '^Bion/' resident in
living material and peculiar to it that is responsible
for evolution.

We are not concerned with any of these so-called
agents, but there is a relation between chance and evolu-
tion shown by living things that has been largely neglected,
or at least vaguely referred to, even by natural selection-
ists, that is of fundamental importance when evolution is
treated as a phenomenon of chance.

This relation may be stated in a general way as fol-
lows : Starting at any stage, the degree of development of
any character increases the probability of further stages
in the same direction. The relation can better be illus-
trated by specific cases. The familiar example of tossing
pennies will serve. If I have thrown heads five times in
succession, the chance that at the next toss of a penny I
may make a run to six heads is greater than if I tossed
six pennies at once. Not, of course, because five separate
tosses of heads will increase the likelihood that at the
next toss a head rather than a tail will turn up, but only
that the chances are equal for a head or a tail, so that I
have equal chances of increasing the run to six by that
throw, while if I tossed six pennies at once the chances
of getting six heads in one throw are only once in 64 times.

Similar illustrations in the. case of animals and plants
bring out the same point. If a race of men average 5 feet,
10 inches, and on the average mutations are not more than
two inches above or below the racial average, the chance
of a mutant individual appearing that is 6 feet tall is
greater than in a race of 5-foot men. If increase in height

MUTATION 269

is an advantage, the taller race has a better chance than
the smaller one. This statement does not exclude the
possibility that a short race might happen to beat out
in height a taller race, for it might more often mutate;
but chance favors the tall. In this sense evolution is more
likely to take place along lines already followed, if further
advantage is to be found in that direction.

A rolling snowball that already weighs 10 pounds is
more likely to reach 15 pounds than is another that has
just begun to roll. The chance that a monkey could change
into a man is far greater than that an amoeba could
make the transition. The monkey has accumulated, so
to speak, so many of the things that go to make up a
man that his chance of reaching that goal is vastly greater
than the amoeba's.

There is also a peculiarity of animals and plants that
assists greatly towards progress along lines already
started. The individual multiplies itself, and a new
mutant character that is advantageous becomes estab-
lished in a large number of individuals, or even in all indi-
viduals of the race. The number of individuals increases
the chance of a new random mutation along the path
already taken. It is true that the chance of a random
variation in the opposite direction is equally great, but
as this, by hypothesis, is the less advantageous direction
it will fail to establish itself in numbers.

Darwin built up his evidence for natural selection and
even for evolution, on the artificial selection of variations
of animals and plants under domestication. It is in this
field that the student of Mendelism revels. Almost without
exception he finds that the domestic races of animals and
plants are built up by mutational differences. It is this
evidence that to-day is a hundredfold stronger for the
theory of evolution than it was in Darwin's time.

The slightest familiarity with wild species will suffice
to convince any one that they differ from each other
generally, not by a single Mendelian difference, but by

270 PHYSICAL BASIS OF HEEEDITY

a number of small differences. The student of Men-
delian heredity at least is not likely to fall into the error
of identifying single Mendelian differences with the sum
total of differences by which wild types and often even
wild varieties differ from each other, but whenever he
has had an opportunity to study these single differ-
ences in wild varieties he has found that they seem to
originate and to be inherited in the same way as other
Mendelian characters. ,

Species as Gkoups of Genes

If related species have many genes in common they
may be expected to produce at times the same mutants.
In fact, it is not at all uncommon to find even in Men-
delian literature such forms as albinos spoken of as though
they represent the same mutation wherever it arises.
Attractive as such a view appears, experience has shown
that it is very unsafe to judge as to the nature of the muta-
tion from the appearance of the character alone. Two
different white-flowered races of sweet peas are known
which give the wild purple-flowering pea when crossed,
showing that they represent different mutations. Simi-
larly, at least two recessive white races of fowls are
known, as well as a third dominant white race. Three
independent mutations have produced white birds.
Whether albino mice, rats, rabbits, squirrels and guinea
pigs have arisen through a mutation in a common gene
cannot be determined because they cannot be crossed
to each other. When we consider that many factors may
combine to produce a given pigmented animal, and that a
change in any one of them may affect the end result, it
will be evident that the expectation would be against
rather than for the conclusion that the same gene had
changed in all cases. Only when it could be shown that a
particular gene of the complex is more likely to change
in a given direction than other genes of the complex would
this interpretation become plausible.

MUTATION 271

There is evidence in Drosophila melanogaster show-
ing that the same mutation to white eyes has occurred sev-
eral times, and the additional and all-important proof
has been obtained that it is the same locus that has pro-
duced the white-eyed mutant. This may appear to give
some slight support to the view that albino mutants
appearing in other related species may be due to the same
mutative changes, but without additional evidence this
conclusion is problematical.

In the mammals melanic individuals have been fre-
quently described, but there is no direct evidence to show
that they are due all to the same change. In the roof rat
there is a black type that is dominant to the gray of this
race, while the black type of the Norway rat is recessive
to the gray of that race. It seems probable that they are
different mutations, but not necessarily so.

Yellow in the mouse is dominant and lethal ; two races
of yellow rats are known, both recessive forms. The rela-
tion of yellow to black in mice is different from the rela-
tion of either of the yellows to black in the Norway rat.
If the blacks are the same mutant the yellows are differ-
ent ; if either yellow of the rat is the same as the yellow of
the mouse, the blacks must be different, etc.

The uncertainty of reaching any conclusion in regard
to the nature of the mutation from the appearance of the
character of the mutant is' excellently illustrated in such
a group of mutants as that of the fruit fly, where a con-
siderable number of cases are known in which mutants
that are almost indistinguishable externally have been
shown to be due to mutations in different parts of the
germ-plasm. There are five kinds of black mutants, three
or more yellows and several eye colors that are practically
indistinguishable. The evidence showing their difference
is obtained from the results of crossing, where, as a rule
(except, for example, cases of complete or incomplete
dominants), reversion to the wild type occurs. In addi-

272 PHYSICAL BASIS OF HEEEDITY

tion, the localization of the gene causing the modification
shows them to be different

The method of localizing genes offers an opportun-
ity for obtaining evidence in regard to like-mutants in
related species that cannot be crossed, and a step forward
in this direction has been taken by C. W. Metz for other
species of the genus Drosophila. In one species, D. virilis,
he has found 12 mutants, and these fall into three
groups of linked genes. Three of them, yellow, forked
and confluent, resemble externally characters of D,
melcmogaster. Yellow and forked are sex-linked and look
like the same characters in melanogaster. Confluent is
like a second chromosome character of the same name in
melanogaster in three respects : first, in that the structures
are similar ; second, in that the character is dominant in
both forms ; and, third, in that it is lethal in the homo-
zygous state. The terminal position of yellow and the
large amount of crossing over with forked are, roughly
speaking, the same in both.

Even in this case further work is needed, first, because
within the same species the occurrence of similar-looking
characters due to different factors is known, e.g., there
are two genes for yellow color (yellow and lemon) in the
first chromosome of D. melanogaster and in the same part
of that chromosome, and second, because it is not to be
expected that the number of crossovers would be identi-
cally the same between the same loci in different species,
since marked variations are known within a single species.
Unless such species can be crossed, the only convincing
evidence that we can hope to get will be to establish
the same linear order in the chromosome for several
genes whose characters appear to be the same or similar.

Other evidence of a different kind also helps to make
probable that the same mutations occur in different
species. For example, in cases where a mutant gene pro-
duces a number of changes in different parts of the body,
the probability that it is the same as one in a different
species that causes the same modifications, is in propor-

MUTATION 273

tion to the number of the same kinds of change that they
produce. The two following cases recorded by Stnrtevant
illustrate this relation:

Two species, viz., Drosophila melanogaster and D,
funehris, have each produced a mutation called notch.
This character, notch, involves not only a notching at the
end of the wings but also the thickening of the second and
fifth veins of the wings, frequent reduction and roughen-
ing of the eyes, inequalities of the rows of hairs on the
thorax, frequent doubling of the anterior scutellar bristles,
and a recessive lethal effect. The character is also dom-
inant and sex-linked. It is one of the commonest muta-
tions in melanogaster and was the first to be picked out
in funebris. So many peculiarities in common make it
hard to believe that they do not represent the same genetic
change. Another mutant also found in D. funebris that
parallels one in D. melanogaster is called hairless, produc-
ing several similar effects in both. In both the factor is
an autosomal dominant; it affects the hairs, certain
bristles, and the second, fourth and fifth veins of the wings,
and has a recessive lethal effect.

One of the most interesting ideas that De Vries brought
forward in his mutation theory is that groups of ''small
species'' or of varieties are made up of many common
genes and differ in a relatively small number of genes.
The genetic analysis of a group of smaller species would
consist in finding out how the different genes are dis-
tributed amongst the members of this group. Phylogene-
tic relationship comes to have a different significance
from the traditional relationship expressed in the descent
theory ; but this point of view is so novel that it has not
yet received the recognition which we may expect that
it will obtain in the future when relationship by common
descent will be recognized as of minor importance as
compared with relationship due to a community of genes.

18

LITERATURE

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vetulus and other Cladocera. Jour. Gen., 1914, iii.

Altenbiteg^ E. : Linkage in Primula sinensis. Genetics, 1916, i.

Babcock, E. B., and R. E. Clausen : Genetics in Relation to Agriculture.
1918.

Bailey, P. Gr. : Primary and Secondary Reduplication Series. Jour.
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Baltzer, F. : Ueber die Beziehung zwischen dem Chromatin und der
Entwicklung und Vererbungsrichtung bei Echinodermenbastarden.
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Baltzer, F. : Die Bestimmung des Geschlechts nebst einer Analyse des
Geschlechts-dimorphismus bei Bonellia. Mitteil. Zool. Station
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Bartlett, H. H. : Additional Evidence of Mutation in (Enothera. Bot.
Gaz., 1915, lix.

Bartlett H. H. : The Mutations of (Enothera stenomeres. Am. Jour.
Bot., 1915, ii.

Bartlett^ H. H. : Mutations en masse. Am. Nat., 1915, xlix.

Bartlett^ H. H. : Mass Mutation in (Enothera pratincola. Bot. Gaz.,
1915, Ix.

Bartlett^ H. H. : The Experimental Study of Genetic Relationships.
Am. Jour. Bot., 1915, ii.

Bateson, W. : Materials for the Study of Variation, 1894, London.

Bateson, W. : The Methods and Scope of Genetics, 1898, Cambridge.

Bateson^ W. : The Present State of Knowledge of Color-heredity in
Mice and Rats. Proc. Zool. Soc, 1913, ii.

Batesok,, W. : Problems of Genetics. Yale Univ. Press, 1913.

Bateson_, W. : Mendel's Principles of Heredity, third impression, Cam-
bridge (Eng.) and New York, 1913.

Bateson, W. : Address of the President, British Assoc. Adv. Sci. Sci-
ence, 1914, n.s., xl, pp. 287-302; 319-334.

Bateson, W. : Root Cuttings, Chimeras and Sports. Jour. Gen., 1916, vi.

Bateson, W., and Pellew, C. : On the Genetics of " Rogues " among
Culinary Peas. Jour. Gen., 1915, v.

Bateson, W., and R. C. Punnett: On the Interrelations of Genetic
Factors. Proc. Roy. Soc, 1911, Ixxxiv.

Bateson, W., and R. C. Punnett : The Inheritance of the Peculiar Pig-
mentation of the Silky Fowl. Jour. Gen., 1911, i.
274

LITERATURE 275

Bateson, W., and R. C. Punnett : On Grametic Series Involving- Redu-
plication of Certain Terms. Jour. Gen., 1911, i.
Bateson_, W. ; Saunders^ E. R. ; Punnett, R. C. ; Hurst, C. C; et al.:

Reports (I to V) to the Evolution Committee of the Royal Society,

London, 1902-1909,
Baur, E. : Das Wesen und die Erblichkeitsverhaltnisse der " Varietates

albomarginatsB hort." von Pelargonium zonale. Zeits. Abst. u. Ver-

erb, 1909, i.
Baur, E. : Propfbastarde. Biol. Centr., 1910, xxx.
Baur, E. : Ein Fall von Faktorenkoppelung bei AntiiThinum ma jus.

Verh. naturf. Ver. Brilnn, 1911, xlix.
Baur, E. : Vererbungs- und Bastardierunssversuche mit Antirrhinum —

II. Faktorenkoppelung. Zeits. Abst. u. Vererb., 1912, vi.
Baur, E. : Ein Fall von geschlechtsbegi-enzter Vererbung bei Melan-

drium album. Zeits. Abst. u. Vererb., 1912, viii.
Baur, E. : Einf iihrung in die experimentelle Vererbungslehre 1914.
Belling, J. : Third Generation of the Cross Between Velvet and Lyon

Beans. Rept. Fla. A.E.S., 1913. ,
Belling, J. : The Mode of Inheritance of Semi-sterility in the Off-
spring of Certain Hybrid Plants. Zeit. Abst. Vererb., 1914, xii.
Belling, J. : Inheritance of Pod Pubescence and Partial Sterility in

Stibolobium Crosses. Rept. Fla. A.E.S., 1915.
Benedict, R. C. : Some Modem Varieties of the Boston Fern and Their

Source. Jour. N. Y. Bot. Gard., 1915, xvi, 189.
Benedict, R. C. : The Origin of New Varieties of Nepholepis by Ortho-
genetic Saltation. I. Progressive Variations. Bull. Torrey Bot.

Club, 1916, xliii, 5.
BiFFEN, R. H. : Mendel's Laws of Inheritance and Wheat Breeding.

Jour. Agric. Sci. Cambridge, 1905, i.
Boring, Alice M., and Raymond Pearl: The Odd Chromosome in the

Spermatogenesis of the Domestic Chicken. Jour. Exp. ZooL, 1914,

xvi.
Boring, A. M., and T. H. Morgan: Lutear Cells and Henfeathering.

Jour. Gen. Phys., 1918, i.
BovERi, Th. : Ueber den Einfluss der Samenzelle auf die Lar\^eneharak-

tere der E'chiniden. Arch. Ent. der Organismen, 1903, xvi.
Bo\t:ri, Th. : Zellen Studien. Die Entwicklung dispermer Seeigel-Eier,

etc. Jena, 1907.
BovERi, Th. : Ueber die Beziehung des Chromatins zur Geschlechtsbestim-

mung. Sit2. Phys.-Med. Gesell. Wiirzburg, 1908.
BovERi, Th. : Die Blastomerenkeme von Ascaris megalocephala und die

Theorie der Chromosomen-Individualitat. Arch. Zellf., 1909, iii.
BovERi, Th. : Ueber " Geschlechtschromosomen " bei Nematoden. Arch.

Zellf., 1909, iv.

276 PHYSICAL BASIS OF HEREDITY

BovERi, Th. : Ueber die Charaktere von Echiniden-Bastardlai'veii bei
Hermaphroditismus. Verh. Phys.-Med. Gesell. Wilrzhurg, 1911,
xli.

BovERi, Th. : Ueber die Charaktere von Ecbiniden-Bastardlarv^en bei
versehiedenem Mengenverbaltnis miitterlicber and vaterlicher Sub-
stanzen. Verh. Phys.-Med. Gesell. Wilrzhurg, 1914, xliii.

Brauer, a.: Zur Kenntniss der Spermatogenese von Aecaris megalo-
eephala. Arch. mikr. Anat., 1893, xlii.

Bridges, C. B. : Non-disjunction of the Sex-chromosomes of Drosophila.
Jour. Exp. Zool., 1913, xv.

Bridges, C. B. : The Chromosome Hypothesis of Linkage Applied to
Cases in Sweet Peas and Primula. Am. Nat., 1914, xlviii.

Bridges,, C. B. : A Linkage Variation in Drosophila. Jour. Exp. Zool.,
1915, xix.

Bridges^ C. B. : Non-disjunction as a Proof of the Chromosome Theory
of Heredity. Genetics, 1916, i.

Bridges^ C. B. : An Intrinsic Difficulty for the Variable Force Hy-
pothesis of Crossing Over. Am. Nat., 1917, li.

Bridges, C. B. : Deficiency. Genetics, 1917, ii.

Bridges, C. B. : Maroon — A Recurrent Mutation in Drosophila. Proa.
Natl. Acad. Sciences, 1918, iv, 316-318.

Bridges, C. B. : The Genetics of Purple Eye Color in Drosophila. Jour,
Exp. Zool., 1919, xxviii.

Bridges, C. B. : Vermilion-deficiency. Jour. Gen. Physiology, 1919,
i, July.

Bridges, C. B.: Specific Modifiers of Eosiu Eye Color of Droso-
phila. Jour. Exp. Zool., 1919, xxviii.

Bridges, C. B., and Morgan, T. H.: The Second Chromosome Group
of Mutant Characters. Carnegie Pub. No. 278, Part II, 1919.

Bridges, C. B., and Sturtevant, A. H.: A New Gene in the Second
Chromosome of Drosophila and Some Considerations on Differen-
tial Viability. Biol. Bull, 1914, xxvi.

Calkins, G. N. : Studies on the Life History of Protozoa. III. Biol.
Bull, 1902, iii.

Calkins, G. N. : Studies on the Life History of Protozoa. IV. Jour.
Exp. Zool, 1904, i.

Cannon, W. A.: Studies in Plant Hybrids: The Spermatogenesis of
Hybrid Peas. Bull Torrey Bot. Club, 1903, xxx.

Carothers, E. E. : The Mendelian Ratio in Relation to Certain Orthop-
t^ran Chromosomes. Jour. Morph., 1913, xxiv.

Carothers, E. E.: The Segregation and Recombination of Homologous
Chromosomes Found in Two Genera of AcrididcB (Orthoptera).
Jour. Morph., 1917, xxviii.

Castle, W. E. : Heredity in Relation to Evolution and Animal Breeding,
1911.

LITERATURE 277

Castle, W. E.: Some New Varieties of Rats and Guinea-pigs and

Their Relation to Problems of Color Inheritance. Am. Nat., 1914,

xlviii.
Castle, W. E. : Size Inheritance and the Pure Line Theory. Zeits. Ahst.

Vererh., 1914, xii.
Castle, W. E'. : Yellow Varieties of Rats. Am. Nat., 1914, xlviii.
Castle, W. E. : Genetics and Eugenics, 1916.
Castle, W. E. : Further Studies on Piebald Rats and Selection with

Observations on Gametic Coupling. Carnegie Inst., Wash., Pub. 241

Part III, 1916.
Castle, W. E. : Is the Arrangement of the Genes in the Chromosome

Linear? Proc. Nat. Acad. Sci., 1919, v.
Castle, W. E. : The Linkage System of Eight Sex-linked Characters

of Drosophila Virilis (Data of Metz). Proc. Nat. Acad. Sci.,

1919, V.
Castle, W. E., and Phillips, John C. : Piebald Rats and Selection.

Carnegie Inst., Wash., Pub. 195, 1914.
Caullery, M. : Les Problemes de la Sexualite, 1913.
Clausen, R. E., and Goodspeed, T. H. : Hereditary Reaction-system

Relations — an Extension of Mendelian Concepts. Proc. Nat. Acad.

Sci., 1916, ii.
Cole, L. J. : A Case of Sex-linked Inheritance in the Domestic Pigeon.

Science, 1912, n.s., xxxvi.
Cole, L. J. : Studies on Inheritance in Pigeons. Bhode Island A. E. S.

Bull, 158, 1914.
Cole, L. J., and Lippincott, W. A. : The Relation of Plumage to Ovar-
ian Condition in a Barred Plymouth Rock Pullet. Bi^l. Bull., 1919,

xxxvi.
Collins, G. N. : Inheritance of Waxy Endosperm in Hybrids of Chinese

Maize. Proc. Fourth Intern. Conf. Genetic, Paris, 1911.
Collins, G. N. : Gametic Coupling as a Cause of Correlations. Am.

Nat., 1912, xliv.
Collins, G. N., and Kempton, J. H. : Inheritance of Endosperm Texture

in Sweet and Waxy Hybrids of Maize Am. Nat., 1914, xlviii.
Collins, G. N. : Maize. Am. Nat., 1914, xlviii.
Collins, G. N., and Kempton^ J. H. : Patrogenesis. Jour. Hered.,

1916, xii.
CoNKLiN, E. G. : Heredity and Environment. Princeton Univ. Press,

1915.
CoNKLiN, E. G. : The Share of the Egg and Sperm in Heredity. Proc.

Nat. Acad. Sci., 1917.
CORRENS, C. : Bastarde zwischen Maisrassen. Biblioth. Botanica, 1901,

liii.
CORRENS, C. : Ueber den Modus und den Zeitpunkt der Spaltung, etc.

Bot. Zeit., 1902, \x.

278 PHYSICAL BASIS OF HEEEDITY

CoRRENS, C. : Zur Kenntniss der Rolle von Kern und Plasma bei der

Vererbung. Zeit. Ahst. Vererh., 1909, ii.
CORRENS^ C. : Vererbungsversuche mit blass (gelb) grlinen und bunt-

blattrigen Sippen bei Mirabilis Jalapa, Urtica piluHf era und Lunaria

annua. Zeit. Ahst. u. Vererh, 1909, i.
CoRRENS_, C. : Der Uebergang aus dem homozygotisehen in einen heter-

ozygotischen Zustand in selben Individuum bei buntblattrigen und

gestreiftbliihenden Mirabilis-Sippen. Ber. Deutsch. Ges., 1910.

xxviii.
CoRRENS, C, and Goldschmidt, B. : Die Vererbung und Bestimmung des

Geschlechtes, 1913, Berlin.
CuENOT^ L. : L'heredite de la pigmentation cliez les souris (2),

Heredite de la pigmentation chez les souris noires. Arch. Zool.

Exp. et Gen., 1903, i.
CuENOT^ L. : L'heredite de la pigmentation chez les souris (3), Les

formules hereditaires. Arch. Zool. Exp. et Gen., 1904, ii»
CuENOT, L. : Les races pures et leurs combinaisons chez les souris (4).

Arch. Zool. Exp, et Gen., 1905, iii.
CuENOT, L. : L'heredite de la pigmentation chez les souris (5). Arch.

Zool. Exp. et Gen., 1907, vi.
CuENOT, L. : Sur quelques anomalies apparentes des proportions men-

deliennes (6). Arch. Zool. et Gen., 1908, ix.
CuENOT, L. : Les determinants de la couleur chez les souris etude com-
parative (7). Arch. Zool. Exp. et Gen., 1911, viii.
CuENOT, L. : L'heredite chez les souris. Verh. naturf. Vereines in Briinn,

1911, xlix.

CuENOT^ L. : Rechei*ches sur I'hybridation. Proc. VII Inter. Zool. Con-
gress. 1909.
Darbishire^ a. : Note on the Result of Crossing Japanese Waltzing

Mice with European Albino Races. Biometrika, 1902, ii.
Darbishire, a. : Breeding and the Mendelian Discovery, 1911, London.
Darwin, C. : The Origin of Species by Means of Natural Selection, or

the Preservation of Favored Races in the Struggle for Life, 1859,

London.
Darwin^ C. : The Variation of Animals and Plants under Domestication,

second ed., 1868, New York.
Davenport^ C. B. : Inheritance in Poultry. Carnegie Inst., Wash., Pub.

53, 1906.
Davenport. C. B. : Inheritance of Characteristics in Domestic Fowl.

Carnegie Inst., Wash., Puh. 121, 1909.
Davenport, C. B. : Heredity in Relation to Eugenics. New York, 1911.
Davenport^ C. B. : ,Sex-limited Inheritance in Poultry. Jour. Exp. Zool.,

1912, xiii.

LITERATUEE 279

Davenport, G. C, and C. B. : Heredity of Hair Color in Man. Am. Nat.,

1909, xliii.
Davenport, C. B. : Sex-limited Inheritance in Poultry. Jour. Exp.

Zool, 1912, xiii.
Davis, B. M. : Cytological Studies on (Enothera. Annals of Botany,

1909-11, xxiii, xxiv, xxv.
Davis, B. M. : Genetical Studies on (Enothera. Am. Nat.^ xliv, xlv,

xlvi, xlvii.
Davis, B. M. : The Problem of the Origin of (Enothera lamarckiana De

Vries. New Phytol., 1913, xii.
Davis, B. M. : The Test of a Pure Species of (Enothera. Proc. Am.

Phil. Soc, 1915, liv.
Davis, B. M. : Additional E\idenee of Mutation in (Enothera. Am.

Nat., 1915, xlix.
Davis, B. M. : A Method of Obtaining Complete Germination of Seeds

in (Enothera and of Recording the Residue of Sterile Seed-like

Structures. Proc. Nat. Acad. Sci., 1915, i.
Davis, B. M. : (Enothera neo-Lamarckiana, Hybrid of 0. franciscana

Bartlett X 0. biennis. Am. Nat., 1916, 1.
Delcourt, a., and Guyenot, E. : Genetique et milieu. Bull. Scient.

France et Belg. 1911, xlv.
Detlefsen, J. A. : Genetic Studies on a Cavy Species Cross. Carnegie

Inst, Wash. Pub. 205, 1914.
Dexter, J. S. : On Coupling of Certain Sex-linked Characters in Droso-

phila. Biol. Bull., 1912, xxiii.
Dexter, J. S.: The Analysis of a Case of Continuous Variation in

Drosophila by a Study of Its Linkage Relations. Am. Nat., 1914,

xlviii.
Dexter, J. S.: Nabours' Breeding Experiments with Grasshoppers.

Am. Nat., 1914, xlviii.
DiGBY, L. : The Cytology of Primula kewensis and of Other Related

Primula Hybrids. Ann. of Bot., 1912 xxvi.
DoNCASTER, L. : Inheritance and .Sex in Abraooas grossulariata. Nature,

1907, Ixxvi.
DoNCASTER, L. : Gametogenesis and Fertilization in Nematus ribesii.

Q. J. M. S., 1907, U.
DoNCASTER, L. : On Sex Inheritance in the Moth, Abraxas grossulariata

and Its Var. Lacticolor. 4th Rep. Evol. Comm., B. Soc. Lond.,

1908.
DoNCASTER, L. : Some Stages in the Spermatogenesis of Abraxas grossu-
lariata and Its Var. Lacticolor. Jour. Genet., 1911, i.
DoNCASTER, L. : The Chromosomes in the Oogenesis and Spermatogenesis

of Pieris brassicce, and in the Oogenesis of Abraxas grossulariata.

Jour. Genet., 1912, ii.

280 PHYSICAL BASIS OF HEREDITY

DoNCASTER, L. : Note ou the Chromosomes in Oogenesis and Sperma-
togenesis of the White Butterfly, Pieris hrassicce. Proc. Camb,
Phil Soc, 1912, xvi.

DoNCASTER, L. : On the Relations between Chromosomes, Sex-limited
Transmission and Sex-Determination in Abraxas grossulariata.
Jour. Genet., 1914, iv.

DoNCASTER, L. : Chromosomes, Heredity, and Sex. Quart. Jour. Micr.
Sc, 1914.

DONCASTER, L. : The Determination of Sex, 1914.

DONCASTER, L., and Raynor, G. H. : Breeding Experiments with Lepi-
doptera. Proc. Zool. Soc. Lond., 1906.

Duncan, F. N. : An Attempt to Produce Mutations through Hybridiza-
tion. Amer. Nat., 1915, xlix.

Duncan, F. N. : A Note on the Gonads of Gynandromorphs of Dro-
sophila ampelophila. Am. Nat., 1915, xlix.

Dunn, L. C: The Genetic Behavior of Mice of the Color Varieties
'' Black and Tan " and " Red." Am. Nat., 1916, 1.

Durham, F. M. : On the Presence of Tyrosinases in the Skins of Some
Pigmented Vertebrates. Proc. Boy. Soc. Lond., 1904, Ixxiv.

Durham, F. M. : Note on Melanins. Jour. Physiol., 1907, xxxv.

Durham, F. M. : A Preliminary Account of the Inheritance of Coat
Color in Mice. Jour. Gen., 1908, i.

Durham, F. M., and Marryat, D. E. C. : Note on the Inheritance of Sex
in Canaries. 4th Bept. Evol. Comm., Boy. Soc. Lond., 1908.

East, E. M. : The Relation of Certain Biological Principles to Plant
Breeding. Conn. A.E.S. Bull, 1907, clviii.

East, E. M. : A Study of the Factors Influencing the Improvement of
the Potato. Ill A.E.S. Bull 127, 1908.

East, E. M. : The Transmission of Variations in the Potato in Asexual
Reproduction. Conn. A.E.S. Bept., 1909-10.

East, E. M. : The Distinction between Heredity and Development in In-
breeding. Am. Nat., 1909, xliii.

East, E. M. : The Genotype Hypothesis and Hybridization. Am. Nat.,

1910, xliv.

East, E. M. : A Mendelian Interpretation of Variation that is Appar-
ently Continuous, Ibid., 1910, xliv.
East, E. M. : The Genotype Hypothesis and Hybridization. Am. Nat.,

1911, xlv.

East, E. M. : A Study of Hybrids between Nicotiana bigelovii and N.

quadrivalvis. Bot. Gaz., 1912, liii.
East, E. M. : The Mendelian Notation as a Description of Physiological

Facts. Am. Nat., 1912, xlvi.

LITERATURE 281

East, E. M. : Inheritance of Flower Size in Crosses between Species of

Nicotiana. Bot. Gaz., 1913, Iv.
East, E. M.: The Chromosome View of Heredity and Its Meaning to

Plant Breeders. Am. Nat., 1915, xlix.
East, E. M. : Size Inheritance in Nicotiana. Genetics, 1915, i.
East^ E. M. : The Bearing of Some General Biological Facts on Bud

Variation. Am. Nat., 1917, li.
East, E. M., and Hayes, H. K.: Inheritance in Maize. Conn. Exp.

Sta. Bull, 1911, clxvii.
East, E. M., and Hayes, H. K.: Heterozygosis in Evolution and in

Plant Breeding. U. S. Dept. Agric, Bureau Plant Ind. Bull,

1912, ccxliii.
East, E. M., and Hayes, H. K. : A Genetic Analysis of the Changes

Produced by Selection in Experiments with Tobacco. Am. Nat.,

1914, xlviii.
East, E. M., and Hayes, H. K. : Further Experiments on Inheritance

in Maize. Connecticut Agr. Exp. Sta., Bull 188, 1915.
Emerson, R. A.: Inheritance of Color in the Seeds of the Common

Bean, Phaseolus vulgaris. Ann. Eep. Nehr. Agr. Exp. Sta., 1909,

xxii.
Emerson, R. A. : The Inheritance of Sizes and Shapes in Plants. Am.

Nat., 1910, xliv.
Emerson, R. A.: Genetic Correlation and Spurious Allelomorphism

in Maize. Ann. Rep. Nehr. Agr. Exp. Sta., 1911, xxiv.
Emerson, R. A.: The Inheritance of Certain Fonns of Chlorophyll

Reduction in Corn Leaves. Ann. Eep. Nehr. Agr. Exp. Sta., 1912,

XXV.

Emerson, R. A.: The Inheritance of the Ligule and Auricles of Com
Leaves. Ann. Rep. Nehr. Agr. Exp. Sta., 1912, xxv.

Emerson, R. A. : The Unexpected Occun^ence of Aleurone Colors in F^
of a Cross between Non-colored Varieties of Maize. Am. Nat., 1912,
xlvi.

Emerson, R. A. : The Inheritance of a Recuning Somatic Variation in
Variegated Ears of Maize. Am. Nat.^ xlviii and Neb. A.E.S. Re-
search Bull, 1914, iv.

Emerson, R. A. : A Genetic Study of Plant Height in Phaseolus vul-
garis. Neb. A.E.S. Research Bull, 1916, vii.

Emerson, R. A.: The Calculation of Linkage Intensities. Am. Nat.,
1916, 1.

Emerson, R. A. : Genetical Analysis of Variegated Pericarp in Maize.
Genetics, 1917, ii.

Emerson, R. A., and East, E. M. : The Inheritance of Quantitative
Chai-acters in Maize. Univ. Neh. Agric. Exp. Station Bull, 1913, ii.

282 PHYSICAL BASIS OF HEREDITY

EwiNG, H. E. : Eighty-seven Generations in a Parthogenetic Pure Line
of Aphis avencB. Fab. Biol. Bull., 1916, xxxi.

Federley, H. : Vererbungsstudien an der Lepidopteren-Gattung Pygcera.
Arch. Bass. Gesell., 1911.

Federley, H. : Das Verhalten der Chromosen bei der Spermatogenese
der Schmetterlinge Pygcsra anachoreta, curtula und pigra sowie
einiger ilirer Bastarde. Ein Beitrag zur Frage der konstanten inter-
mediaren Artbastarde und der Spermatogenese der Lepidopteren.
Zeitschr. f. inductive Ahstammungs- und Vererhungslehre, 1912, ix.

Federley, H. : Ein Beitrag zur Kenntnis der Spermatogenese bei Misch-
lingen zwiscben Eltern verscbiedener systematischer Verwandt-
scbaft. Ofversigt af Finska Vetenskaps-Societetens Forhandlingar.

1914, Ivi.

Foot, K., and Strobell, E. C. : Preliminary Note on the Results of
Crossing Two Hemipterous Species, etc. Biol. Bull, 1913, xxiv.

Foot, Katherine, and Strobell, E. C. : Results of Crossing Euschistus
variolarius and Euschistus servus with Reference to the Inheritance
of an Exclusively Male Character. Linn. Soc. Jour., 1914, xxxii.

Galton, F. : Inquiries into Human Faculty, 1883, New York.

Galton"_, F. : Natural Inheritance, 1889, London.

Galton, F.: Hereditary Genius, 1892, London.

Galton, F. : The Average Contribution of Each of Severail Ancestors to
the Total Heritage of the Offspring. Proc. Boy. Soc. Lond., 1897, Ixi.

Gates, R. R. : The Mtaterial Basis of Mendelian Phenomena. Am. Nat.,

1910, xliv.
Gates, R. R. : Pollen Formation in (Enothera gigas. Ann. Bot., 1911,

XXV.

Gates, R. R. : Tetraploid Mutants and Chromosome Mechanisms. Biol.

Centr., 1913, xxxiii.
Gates, R. R. : On the Modification of Characters by Crossing. Am. Nat.,

1915, xlix.

Gates, R. R. : The Mutation Factor in Evolution, with Particular Refer-
ence to (Enothera, 1915, London.
Gates, R. R. : On Pairs of Species. Bot. Gaz., 1916, Ixi.
Gates, R. R. : Vegetative Segregation in a Hybrid Race. Jour. Gen.,

1917, vi.
Gates, R. R., and Thomas, N. : A Cytological Study of (Enothera mut.

lata and CE. mut. semilata in Relation to Mutation. Quart. Jour.

Micr. Sc, 1914.
Geerts, J. M. : Beitrage zur Kenntnis der Cytologie und der partiellen

Sterilitat von (Enothera Lamarckiana. Recueil des Trav. Bot.

Neerl., 1909, v.
Gerould, J. H. : The Inheritance of Polymorphism and Sex in Colias

philodice. Am. Nat., 1911, xlv.

LITERATUEE 283

GoDLEwsKi, E, : Untei-siieliungen iiber die Bastarclierung der Echiniden-
iind Chiinoidenfamilie. Arch. Ent.-mec, 1906, xx.

GoLDSCHMiDT, R. : Einf ulirmig in die Vererbungswissenschaf t. Leipzig,
1911.

Gc)LDSCH]\nDT, R. : Erblichkeitsstudien an Sclimetterlingen. I, Zeits.
Abst. Vererh., 1912, vii.

GoLDSCHMiDT, R. : Bemei'kungen zur Vererbung des Gesehleclitspoly-
morphismus. Zeits. Abst. Vererb., 1912, viii.

GoLDSCHMiDT^ R. : A Preliminary Report on Further Experiments in
Inheritance and Determination of Sex. Proc. Nat. Ac. Sci., 1916, ii.

GoLDSCHMiDT^ R. : The Function of the Apyrene Spermatozoa. Science^
U.S., 1916, xliv.

GoLDSCHMiDT, R. : Experimental IntersexuaUty and the Sex Problem.
Am. Nat., 1916, 1.

GoLDSCHMiDT, R. : Genetic Faetoi's and Enzyme Reaction. Science,
1916, xliii.

GOLDSCHMIDT, R. : Crossing Over Ohne Chiasmatypie ? Genetics, 1917, ii.

GOLDSCHMIDT, R. I On a Case of Facultative Parthogenesis in the Gypsy-
moth Lymantria dispar., with a Discussion of the Relation of
Parthenogenesis to Sex. Biol. Bull., 1917, xxxii,

GOLDSCHMIDT, R. : A Further Contribution to the Theory of Sex. Jour.
Exp. Zool., 1917, xxii.

GooDALE^ H. D. : Studies on Hybrid Ducks. Jour. Exp. Zool., 1911, x.

GooDALE_, H. D. : Some Results of Castration in Ducks. Biol. Bull.,
1911, XX.

GooDALE^ H. D.: Sex-limited Inheritance and Sexual Dimorphism in
Poultry. Science, 1911, xxxiii.

GooDALE_, H. D. : Castration in Relation to the Secondary Sexual Char-
acters of Brown Leghorns. Am. Nat., 1913, xlvii.

GoODALE^ H. D. : A Feminized Cockerel. Jour. Exp. Zool., 1916, xx.

GooDALE^ H. D. : Gonadectomy in Relation to the Secondary Sexual
Characters of Some Domestic Birds. Carnegie Inst., Wash., Pub.
No. 243. 1916.

GooDALE, H. D., and Morgan, T. H. : Heredity of Tri-color in Guinea-
pigs. Am. Nat., 1913, xlvii.

GooDSPEED, T. H. : Quantitative Studies of Inheritance in Nicotiana
Hybrids I-IV. Uni(o. Calif. Pub. Bot., 1912-15, v.

GooDSPEED_, T. H. : On the Partial Sterility of Nicotiana Hybrids Made
with A'^. sylvestris as a Parent. Univ. Calif. Pub. Bot., 1913, v.

GooDSPEED^ T. H. : Parthenogenesis, Parthenoearpy and Phenospermy in
Nicotiana. Univ. Calif. Pub. Bot., 1915, v.

GoODSPEED, T. H., and Clausen, R. E.: Variation in Flower Size in
Nicotiana. Proc. Nat. Acad. Sci., 1915, i.

284 PHYSICAL BASIS OF HEEEDITY

GoODSPEED^ T. H., and Clausen, R. E. : Factors Influencing Flower Size

in Nicotiana with Special Reference to Questions of Inheritance.

Amer^ Jour. Bot., 1915, ii.
GooDSPEED, T. H., and Clausen, R. E. : The Nature of the F^ Species

Hybrids between Nicotiana sylvestris and Varieties of Nicotiana

Tabacum with Special Reference to the Conception of Reaction-
system Contrasts in Heredity. Univ. Cal. Pub. Bot., 1917, v.
GooDSPEED, T. H., and Clausen, R. E. : Mendelian Factor Differences

Versus Reaction-system Contrasts in Heredity. Am. Nat., 1917, 1.
GoRTNER, R. A. : Spiegler^s " White Melanin " as Related to Dominant

or Recessive White. Am. Nat., 1910, xliv.
GoRTNER, R. A.: Studies on Melanin I-III. Jour. Biol. Chem., viii;

IV. Am. Nat., 1910-11, xiv.
Gould, H. N. : Studies on Sex in the Hermaphrodite Mollusc Crepidula

plana. I. History of the Sexual Cycle, 1917, xxiii.
Gregory, R. P. : Note on the Histology of the Giant and Ordinary Forms

of Primula sinensis. Proc. Cambridge Phil. Soc, 1909, xv.
Gregory, R. P. : Experiments with Primula sinensis. Jour. Gen., 1911, i.
Gregory, R. P. : On Gametic Coupling and Repidsion in Primula sinensis.

Proc. Boy. Soc, 1911, B, Ixxxiv.
Gregory, R. P. : The Chromosomes of a Giant Form of Primula sinensis.

Proc. Cambridge Phil. Soc, 1912, xvi,
Gregory, R. P. : On the Genetics of Tetraploid Plants in Primula sinensis.

Proc. Roy. Soc, 1914, B, Ixxxvii.
Gregory, R. P. : On Variegation in Primula sinensis. Jour. Gen.,

1915, iv.
Gregory, R. P.: Note on the Inheritance of Heterostylism in Primula

acaulis Jacq.. Jour. Gen., 1915, iv.
GuLiCK, A. : Ueber die Geschlechtschromosomen bei einigen Nematoden.

Arch. f. Zellf., 1911, vi.
GuYER, M. F. : Hybridism and the Germ-cell. Univ. of Cincinnati Bull.,

1902, No. 21.
GuYER, M. F. : The Germ-cell and the Results of Mendel. Cincinnati

Lancet-Clinic, 1903.
GuYER, M. F. : The Spermatogenesis of the Domestic Chicken. Anat.

Anz., 1909, xxxiv.
GuYER, M. F. : Atavism in Guinea-chicken Hybrids. Jour. Exp. Zool.,

1909, vii.
GuYER, M. F. : Accessory Chromosomes in Man. Biol. Bui., 1910, xix.
GuYER, M. F. : Nucleus and Cytoplasm in Heredity. Am. Nat., 1911, xlv.
GuYER, M. F. : Accessory Chromosomes in Man. Science, n.s., 1914,

xxxix.
GuYER, M. F. : Studies on the Chromosomes of the Common Fowl as seen

in Testes and in Embryos. Biol. Bui., 1916, xxxi.

LITERATURE 285

GuYER^ M. F. : Being Well Bora. 1916. Indianapolis.

Hadley, p. B. : Studies on Inheritance in Poultry : I. The Ck)nstitution
of the White Leghorn Breed. B.I.A.E.S. Bull. 155. II The Fac-
tor for Black Pigmentation in the White Leghorn Breed. Ibid.,
Bull 161, 1913-14.

Hagedoorn, a. L. : The Genetic Factors in the Development of the House
Mouse, etc. Zeits. Ahst. Vererh., 1912, vi.

Hance, R. T. : Variations in the Number of Somatic Chromosomes in
(Enothera scintillans De Vries. Genetics, 1918, iii.

Harrison, J. W. H., and Doncaster, L.: On Hybrids between Moths
of the Geometrid Sub-family BistoninsB, with an Account of the
Behavior of the Chromosomes in Gametogenesis in Lycia (Biston)
hirtaria, Ithysia (Nyssia) zonaria and in Their Hybrids. Jour.
Genet., 1914, iii.

Hayes, H. K., and East, E. M. : Further Expei-iments on Inheritance
in Maize. Conn. A. E. S. Bull., 1915, clxxxviii.

Hegner, R. W. : Variation and Heredity during the Vegetative Reproduc-
tion of Arcella dentata. Proc. Nat. Acad. Sci., 1918, iv.

Hegner, R. W. : Quantitative Relations between Chromatin and Cyto-
plasm in the Genus Arcella. Proceed. Nat. Acad. Sci., 1919, v.

Henking, H. : Untersuchungen liber die ersten Entwicklungsvorgange in
den Eiern der Insekten. Zeits. f. wiss. Zool., 1891, li.

Heribert-Nilsson, N. : Die Variabilitat der (Enothera Lamarckiana
und das Problem der Mutation. Zeit. ind. Ahst. u. Vererh., 1912, viii.

Herlant, M. : Etude sur les bases cytologiques du mecanisme de la
parthenogenese expeiimentale chez les Amphibiens. Archives de
Biologie, 1913, xxviii.

Herlant, M. : Le Mecanisme de la Parthenogenese Experimentale chez
les Amphibiens et les Echinodermes. Bull. Scientiflque de la France
et de la Belgique, 1917, li.

Herlant, M. : Un Cas d'Hermaphrodisme Complet et Fonctionnel chez
Paracentrotus Lividis. Arch. Zool. Exp. et Gen., 1918, Ivii.

Herla, V. : fitudes des Variations de la Mitose chez I'Ascaride Megalo-
cephale. Arch. Biol., 1895, xiii.

Hoge, M. a. : The Influence of Temperature on the Development of a
Mendelian Cha,racter. Jour. Exp. Zool., 1915, xvui.

Hoge, M. A. : Another Gene in the Fourth Chromosome of Drosophila.
Am. Nat., 1915, xlix.

Hyde, R. R. : Fertility and Sterility in Rrosophila ampelophila. I-IV
Jour. Exp. Zool., 1914, xvii.

Hyde, R. R.: Two New Members of a Sex-linked Multiple (Sextuple),
Allelomorph System. Genetics, 1916, i.

Hyde, R. R. : Mosaics in Drosophila ampelophila. Genetics, 1916, i.

286 PHYSICAL BASIS OF HEREDITY

Ibsen, H. L. : Tricolor Inheritance, I. The Tricolor Series in Guinea-pigs.

Genetics, 1916, i.
Ibsen, H. L. : Tricolor Inheritance, II. The Basset Hound. Genetics,

1916, i.
Ibsen, H. L. : Tricolor Inheritance, III. Tortoise-shell cats. Genetics,

1916, i.
ISHiKAWA, MiTSUHARu : A list of the Number of Chromosomes. Bot.

Mag., 1916, Tokyo, xxx.
Janssens, F. a. : Evolution des auxocytes Males du Batracoseps Attenu-

atus. La Cellule, 1905, xxii.
Janssens, F. A. : La Theorie de la chiasmatypie. Nouvelle interpreta-
tion des cineses de maturation. La Cellule, 1909, xxv.
Jennings, H. S. : Assortative Mating, Variability and Inheritance of

Size in the Conjugation of Paramecium. Jour. Exp. Zool., 1911, xi.
Jennings, H. S. : Pure Lines in the Study of Genetics in Lower Organ-
isms. Am. Nat., 1^911, xlv.
Jennings, H. S. : The Effect of Conjugation in Paramecium. Jour.

Exp. Zool., 1913, xiv.
Jennings, H. S., and Hargitt, G. T. : Characteristics of the Diverse

Races of Paramecium. Jour. Morph., 1910, xxi.
Jennings, H. S., and Lashley, K. S. : Biparental Inheritance and the

^Question of Sexuality in Paramecium. Jour. Exp. Zool., 1913, xiv.
Jesenko, F. : Sur une hybride fertile entre Triticum sativum et Secale

cereale. Bap. IV Conf. Internat. de Genetique, 1913.
Jesenko, F. : Ueber Getreide-Speziesbastarde. Zeits. Abts., 1913, x.
Johannsen, W. : Ueber Erblichkeit in Populationen und in reineu

Linien, 1903, Jena.
Johannsen^ W. : Ueber Knospenmutation bei Phaseolus. Zeit. Ahst.

Vererb., 1908, i.
Johannsen, W. ': Elemente der exakten Erblichkeitslehre, 1909, Jena.
Johannsen, W. : The Genotype Conception of Heredity. Am. Nat.,

1911, xlv.
Jones, D. : Dominance of Linked Factors as a Means of Accounting for

Heterosis. Genetics, 1917, ii.
Keeble, F. : Gigantism in Primula sinensis. Jour. Gen., 1912, ii.
Keeble, F., and Pellew, C. : The Mode of Inheritance of Stature and

Time of Flowering in Peas (Pisum sativum). Jour. Gen., 1910, i.
Kellogg, Vernon L. : Inheritance in Silkworms, I. Stanford Tin. Pub.,

1908, i.
King, H. D. : The Sex Ratio in Hybrid Rats. Biol. Bull., 1911, xxi.
KuscHAKEwiTSCH, Sergius : Die Entwicklungsgeschichte der Keim-

driisen von Bana esculenta. Ein Beitrag zum Sexualitatsproblem.

Festschrift zum sechzigsten Geburtstag Richard Hert-^igs, II, 1910.

LITERATUEE 287

Lang, A. : Vererbungswissenschaf tliclie Miszellen. Zeits. Abst. Vererh.,

1912, viii.
Lashley, K. S. : Inheritance in the Asexual Reproduction of Hydra.

Jour. Exp. Zool., 1915, xix.
Lashley, K. S. : Results of Continued Selection in Hydra. Jour. Exp.

Zool., 1916, XX.
LiFF, Joseph : Data on a Peculiar Mendelian R^tio in Drosophila ampelo-

phila. Am. Nat., 1915, xlix. ~
LiLLiE^ F. R. : The Theory of the Free^nartin. Science, 1916, n.s., xliii.
Lillie, F. R. : Sex^determination and Sex-Differentiation in Mammals.

Proc. Nat. Acad. Sci., 1917, iii.
LHiLiE, F. R. : The Free-martin. A Study of the Action of Sex-hormones

in the Foetal Life of Cattle. Jour. Exp. Zool., 1917, xxiii.
LiPPiNCOTT, W. A. : The Case of the Blue Andalusian. Am. Nat., 1918,

Iii.
LiPSCHUTZ, A. : On the Internal Secretion of the Sexual Glands. Jour.

Physiol, 1917, li.
Little, C. C. : Preliminary Note on the Occurrence of a Sex-limited

Character in Cats. Science, 1912, xxxv.
Little, C. C. : Experimental Studies of the Inheritance of Color in Mice.

Carnegie Inst., Wash., Pub. 179, 1913.
Little, C. C. : Dominant and Recessive Spotting in Mice. Am. Nat.,

1914, xlviii.

Little^ C. C. : The Inheritance of Black-eyed White Spotting- in Mice.

Am. Nat., 1916, xlix.
Little, C. C. : The Relation of Yellow-Coat Color and Black-eyed White

Spotting of Mice in Inheritance. Genetics, 1917, ii.
Little, C. C, and Phillips, J. C. : A Cross Involving Four Pairs of

Mendelian Characters in Mice. Am. Nat., 1913, xlvii.
Lloyd-Jones^ 0. : Studies on Inheritance in Pigeons. Jour. Exp. Zool.,

1915, xviii.

Lock, R. H. : Recent Progress in the Study of Variation, Heredity and

Evolution. 1906. London and New York.
Lock, R. H. : On the Inheritance of Certain. In^dsible Characters in Peas.

Proc. Boy. Soc, 1907, B, Ixxix.
Lock, R. H. : The Present State of Knowledge of Heredity in Pisum.

Annals. Roy. Bot. Gar., 1908, iv.
Loeb, J. : Ueber den autokatalytischen Charakter der Kernsynthese bei

der Entwicklung. Biol. Cent., 1910, xxx.
Loeb, J. : Heredity in Heterogeneous Hybrids. Jour. Morph., 1912,

xxiii.
Loeb, J. : Artificial Parthenogenesis and Fertilization, 1913, Chicago.
Loeb, J. : The Organism as a Whole. 1916. New York.

288 PHYSICAL BASIS OF HEREDITY

LoEB, J.: Is Species Specificity a Mendelian Character? Science, 1917,
xlv.

LoEB, J.: Further Experiments on the Sex of Parthenogenetic Frogs.
Proc. Nat. Ac. Sci., 1918, iv.

LoEB^ J., and Bancroft, F. W. : Further Observations on Artificial Par-
thenogenesis in Frogs. Jour. Exp. Zool., 1913, xv.

LoEB, J., and Chamberlain, M. M. : An Attempt at a Physico-chemieal
Explanation of Certain Groups of Fluctuating Variation. Jour.
Exp. Zool., 1915, xix.

LoEB, J. W., King, W. 0. R., and Moore, A. R. : Ueber Dominanzer-
scheinungen bei den hybriden Plutein des Seeigels. Arch. Entw.
Org., 1910, xxix.

LoTSY, J. P. : Hybrides entre espeees d'Antirrhinum. Repts. 4th Intern.
Conf. Genet., 1911, Paris.

LoTSY, J. P. : Evolution by Means of Hybridization. 1916. The Hague.

Love, H. H.^ and Leighty, C. E. : Variation and Correlation of Oats
(Avena sativa). Cornell A.E.S., 1914.

Love, H. H., and Leighty, C. E. : Changes on Biometrical Constants.
Cornell A.E.S., Memoir 3, 1914.

LuTZ, A. M. : Triploid Mutants in (Enothera. Biol. Centr., 1912. xxxii.

LuTZ, A. M. : (Enothera Mutants with Diminutive Chromosomes. Am.
Jour. Bot., 1916, iii.

LuTZ, A. M. : Fifteen- and Sixteen- Chromosome (Enothera Mutants.
Am. Jour. Bot., 1917, iv.

LuTZ, F. E. : Experiments with Drosophila ampelophila Concerning
Evolution. Carnegie Inst., Wash., Pub. 143, 1911.

LuTZ, F. E. : Experiments Concerning the Sexual Difference in the Wing
Length of Drosophila ampelophila. Jour. Exp. Zool., 1913, xiv.

McClung, C. E.: The Accessory Chromosome — Sex-Determinant?
Biol. Bull, 1902, iii.

McClung, C. E. : Notes on the Accessory Chromosome. Anat. Anz.,
1902, XX.

McClung, C. E. : The Chromosome Complex of Orthopteran Spermato-
cytes. Biol. Bull., 1905, ix.

McClung, C. E. : A Comparative Study of the Chromosomes in Orthop-
teran Spermatogenesis. Jour. Morph., 1914, xxv.

McClung, C. E. : The Multiple Chromosomes of Hesperotettix and Mer-
miria. Jour. Morph., 1917, xxix.

MacCurdy, H. M. : Nuclear Reorganization and Its Relation to Conju-
gation and Inheritance in Arcella vulgaris. Anat. Record, 1919, xv.

MacDougal, D. T. : Alterations in Heredity Induced by Ovarian Treat-
ments. Bot. Gaz., 1911, li.

MacDougal, D. T.; Vail, A. M.; Shull, G. H.; and Small, J. K. :
Mutants and Hybrids of the (Enotheras. Carnegie Inst., Wash.,
Pub. 24, 1905.

LITEEATUEE . 289

MacDougal, D. T. ; Vail, A. M. ; and Shull, G. H. : Mutations, Varia-
tions and Relationships of the (Enotheras. Ibid., Pub. 81, 1907.
MacDowell, E. C. : Size Inheritance in Rabbits. Carnegie Inst., Wash.,

Pub. 196, 1914.
MacDowell_, E.g.: Multiple Factors in Mendelian Inheritance. Jour.

Exp. Zool., 1914, xvi.
MacDowell, E. C. : Bristle Inheritance in Drosophila. Ibid., 1915, xix.
Marchal, El. and Em. : Recherches Experimentales sur la Sexualite des

Spores chez les Mousses dioiques. Mem. couronnes, par la Classe des

sciences, dans la seance du 15 decembre 1905, 1906.
Marchal, El. and Em. : Aposporie et Sexualite chez les Mousses. Bull.

de VAcad. roy. de Belgique. {Classe de science), no. 7, 1907.
Marchal, El. and Em. : Asporie et Sexualite chez les Mousses. Bull, de

VAcad. roy. de Belgique. {Classe de science), no. 1, 1919.
Marchal, El. and Em.: Asporie et Sexualite chez les Mousses. III.

Bull, de VAcad. roy. de Belgique. {Classe de science), nos. 9-10.
Marechal, J. : Sur TOvogenese des Selaciens et de quelques autres Chor-

dates., I. Morphologic de I'Element chromosomique dans I'Ovocjrte I

chez les Selaciens, les Teleosteens, les Tuniciers et I'Amphioxus.

La Cellule, 1906, xxiv.
May, H. G. : The Appearance of Reverse Mutations in the Bar-eyed Race

of Drosophila under Experimental Control. Proc. Nat. Acad. Sci.,

1917, iii.
May, H. G. : Selection for Higher and Lower Facet Numbers in the

Bar-eyed Race of Drosophila and the Appearance of Reverse Muta-
tions. Biol. Bull., 1917, xxxiii.
DE Meijere, J. C. H. : Ueber Jacobsons Ziichtung-sversuche beziiglich

des Polymorphismus von Papilio Memnon. Zeit. Abst. Vererb.,

1910, iii.
de Meijere, J. C. H. : Ueber getrennte Vererbung der Gesehlechter.

Biol. Centr., 1910, xxx.
de Meijere, J. C. H. : Ueber getrennte Vererbung der Gesehlechter.

Arch. Rass. GeselL, 1911, viii.
Meisenheimer, J. : Experimentelle Studien zur Soma- und Gesehlechts-

differenzierung, I. Jena, 1909.
Meisenheimer, J. : Experimentelle Studien zur Soma- und Geschlechts-

differenzierung. Fests zum 60 Geburtstage von Dr. J. W. Spengel,

1912, iii.
Mendel, G. : Versuche iiber Pflanzen-hybriden. Verh. d. Naturf. Vereins

in Brilnn, 1865, iv.
Metz, C. W. : An Apterous Drosophila and Its Genetic Behavior. Am.

Nat., 1914, xlviii.
Metz, C. W. : Chromosome Studies in the Diptera, I. Jour. Exp. Zool.,

1914, xvu.
19

290 PHYSICAL BASIS OF HEREDITY

Metz, C. W.: Mutations in Three Species of Drosophila. Genetics,

1916, i.
Metz, C. W.: Chromosome Studies on the Diptera. II. The Paired

Association of Chromosomes in the Diptera, and Its Significance.

Jour. Exp. Zool, 1916, xxi.
Metz, C. W.: Chromosome Studies on the Diptera. III. Additional

Types of Chromosome Groups in the Drosophilidce. Am. Nat.,

1916, 1.
Meves, F.: Die Spermatocytenteilungen bei der Honigbiene. Arch.

mikr. Anat., 1907, Ixx.

Meves, F. : Die Spermatocytenteilungen bie der Plornisse ( Yespa crabro
L.) Arch. mikr. Anat., 1908, Ixxi.

Morgan, T. H. : An Alternative Interpretation of the Origin of Gynan-
dromorphous Insects. Science, 1905, n.s., xxi.

Morgan, T. H. : Experimental Zoology, 1907, New York.

Morgan, T H. : The Determination of Sex in Frogs. Am. Nat., 1908,
xlii.

Morgan, T. H. : A Biological and Cytological Study of Sex-Determina-
tion in Phylloxerans and Aphids. Jour. Exp. Zool., 1909, vii.

Morgan, T. H. : Hybridology and Gynandromorphism. Am. Nat., 1909,

xliii.
Morgan, T. H. : Sex-limited Inheritance in Drosophila. Science, 1910,

xxxii.

Morgan, T. H. : The Chromosomes in the Parthenogenetic and Sexual
Eggs of Phylloxerans and Aphids. Proc. Soc. Exp. Biol, and Med.,
1910, vii.

Morgan, T. H. : Chromosomes and Heredity. Am. Nat., 1910, xliv.

Morgan, T. H. : The Method of Inheritance of Two Sex-limited Charac-
ters in the Same Animal. Proc. Soc. Exp. Biol. Med., 1910, viii.

Morgan, T. H. : An Attempt to Analyze the Constitution of the Chromo-
somes on the Basis of Sex-limited Inheritance in Drosophila. Jour.
Exp. Zool., 1911, xi.

Morgan, T. H. : Mosaics and Gynandromorphs in Drosophila. Proc.
Soc. Exp. Biol. Med., 1914. xi.

Morgan, T. H. : Two Sex-linked Lethal Factors in Drosophila and Their
Influence on the Sex Ratio. Jour. Exp. Zool., 1914, xvii.

Morgan, T. H. : A Third Sex-linked Lethal Factor in Drosophila. Jour.
Exp. Zool., 1914, xvii.

Morgan, T. H. : The Constitution of the Hereditary Material. Proc. Am.
Philos. Soc, 1915, liv.

Morgan, T. H. : Localization of Hereditary Material in the Germ-cells.
Proc. Nat. Acad. Sci., 1915, i.

Morgan, T. H. : The Role of the Environment in the Realization of a
Sex-linked Mendelian Character in Drosophila. Am. Nat., 1915,
xlix.

LITERATURE 291

Morgan, T. H.: A Critique of the Theory of Evolution. Princeton

Univ. Press, 1916.
MORGAN, T. H. : The Theory of the Gene. Am. Nat., 1917, li.
Morgan, T. H. : Concerning the Mutation Theory. Set. Mon., May, 1918.
Morgan, T. H. : Changes in Factors Through Selection. Sci. Monthly,

June, 1918.
Morgan, T. H. : Evolution by Mutation. Sci. Mon., July, 1918.
Morgan, T. H. : A Demonstration of Genes Modifying the Character

'^Notch." Carnegie Pub. No. 278, Part IV, 1919.
Morgan, T. H., and Bridges, C. B. : Dilution Effects and Bicolorism in

Certain Eye Colors of Drosophila. Jour. Exp. Zool., 1913, xv.
Morgan, T. H., and Bridges, C. B. : Sex-linked Inheritance in Droso-
phila. Carnegie Inst., Wash., Pub. 237, 1916.
Morgan, T. H., and Bridges, C. B.: The Origin of Gynandromorphs.

Carnegie Pub. No. 278, Part 1, 1919.
Morgan, T. H., and Cattell, E.: Data for the Study of Sex-linked

Inheritance in Drosophila. Jour. Exp. Zool., 1912, xiii.
Morgan, T. H., and Cattell, E.: Additional Data for the Study of

Sex-linked Inheritance in Drosophila. Jour. Exp. Zool., 1913, xiv.
Morgan, T. H., and Goodale, H. D. : Sex-linked Inheritance in Poultry.

Ann. N. Y. Acad. Sci., 1912, xxii.
Morgan, T. H., and Lynch, C. J.: The Linkage of Two Factors in

Drosophila That are Not Sex-linked. Biol. Bull, 1912, xxiii.
Morgan, T. H., Payne, F., and Browne, E. N.: A Method to Test the

Hypothesis of Selective Fertilization. Biol. Bull., 1910, xviii.
Morgan, T. H. ; Sturtevant, A. H. ; Muller, H. J., and Bridges, C. B. :

The Mechanism of Mendelian Heredity, 1915, New York.
Morris, Margaret : The Behavior of the Chromatin in Hybrids between

Fundulus and Ctenolabrus. Jour. Exp. Zool., 1914, xvi.
Muller, H. J. : A New Mode of Segregation in Gregory's Tetraploid

Primulas. Am. Nat., 1914, xlviii.
Muller, H. J. : The Bearing of the Selection Experiments of Castle and

Phillips on the Variability of Genes. Ibid., 1914.
Muller, H. J.: A Gene for the Fourth Chromosome of Drosophila.

Jour. Exp. Zool., 1914, xvii.
Muller, H. J. : The Mechanism of Crossing Over. Am. Nat., 1916, 1.
Muller, H. J. : An CEnothera-like casQ in Drosophila. Proc. Nat. Acad.

Sci., 1917, iii.
Muller, H. J.: Genetic Variability, Twin Hybrids and Hybrids, in a

Case of Balanced Lethal Factors. Genetics, 1918, iii.
Mulsow, K. : Der Chromosomencyclus bei Ancyracanthus cystidicola

Rud. Arch. f. Zellf., 1912, ix.
Nabours, R. K. : Studies of Inheritance and Evolution in Orthoptera I.

Jour. Genet., 1914, iii. II. Ibid., 1917. III. Ibid., 1917.

292 PHYSICAL BASIS OF HEEEDITY

Nabours, R. K. : Parthenogenesis and Crossing Over in the Grouse

Locust Apotettix. Am. Nat., 1919, liii.
Nachtsheim, H. : Parthenogenese, Eireifung und Geschlechtsbestim-

mung bei der Honigbiene. Sitzgunsher. Gesell. Morph. u. Phys.

Miinchen, 1912.
Nakao, M. : Cytological Studies on the Nuclear Division of the Pollen

Mother-cells of Some Cereals and Their Hybrids. Jour. Coll. Agr.,

Tohoku Imp. TJni., Sapporo, Japan, 1911, v.
Newman, H. H. : Spawning Behavior and Sexual Dimorphism in Fun-

dulus heteroclitus and Allied Fish. Biol. Bull, 1907, xii.
Newman, H. H. : Further Studies of the Process of Heredity in Fun-

dulus Hybrids. Jour. Exp. Zool., 1910, viii.
Nilsson-Ehle, H.: Om lifstyper och individuell Variation. Bot. No-
User, Lund^ 1907.
Nilsson-Ehle, H. : Einige Ergebnisse von Kreuzungen bei Hafer und

Weizen. Bot. Notiser, 1908.
Nilsson-Ehle^ H. : Kreuzungsuntersuchungen an Hafer und Weizen.

Lund's Univ. Arsskrift, 1909.
Phillips, J. C. : A Further .Study of the Size Inheritance in Ducks with

Observations on the Sex Ratio of Hybrid Birds. Jour. Exp. Zool,

1914, xvi.
PiNNEY, E. : A Study of the Relation of the Behavior of the Chromatin

to Development and Heredity in Teleost Hybrids. Jour. Morph.,

1918, xxxi.
Plate^ L. : Vererhungslehre. 1913. Leipzig.
Plough, H. H. : The Effect of Temperature on Crossing Over. Jour.

Exp. Zool, 1917, xxiv.
Plough, H. H. : Linear Arrangement of Genes and Double Crossing

Over. Proceed. Nat. Acad., 1919, v.
Punnett, R. C. : On Nutrition and Sex-Determination in Man. Proc.

Cambr. Phil. Soc, 1903, xii.
Punnett, R. C. : Sex-Determination in Hydatina, with Some Remarks

on Parthenogenesis. Proc. Boy. Soc, 1906, Ixxviii.
PuNNETT, R. C. : Mendelism, third ed.. New York, 1911.
PuNNETT, R. C. : Inheritance of Coat Color in Rabbits. Jour. Gen.,

1912, ii.

PuNNETT, R. C: Reduplication Series in Sweet Peas. Jour. Gen.,

1913, iii.

PuNNETT^ R. C. : Further Experiments on the Inheritance of Coat Color
in Rabbits. Jour. Gen., 1915, v.

PuNNETT, R. C, and Bailey, P. G. : On Inheritance of Weight in Poul-
try. Jour. Gen., 1914, iv.

OsAWA, I. : Cytological and Experimental Studies in Citrus. Jour. Coll
Agr. Imp. Uni. Tokyo, 1912, iv.

LITERATURE 293

Packakd, C. : The Effect of Radium Radiations on the Development
of Chaetopterus. Biol. Bull, 1918, xxxv.

Pantel^ J.,and Sini^ty^ R. de : Snr rApparition des males et d'Hermaph-
rodites dans les pontes parthenogenetiques des Phasmes. C. R.
Acad. Sci., 1908, cxlvii.

Payne, F. : An Experiment to Test the Nature of the Variations on
which Selection Acts. Indiana Univ. Studies, 1918, v.

Payne, F. : The Effect of Artificial Selection on Bristle Number in
Drosophila ampelopJiila and Its Interpretation. Proc. Nat. Acad.
Sci., 1918, iv.

Pearl_, R. : The Mode of Inheritance of Fecundity in the Domestic Fowl.
Jour. Exp. Zool., 1912, xiii.

Rawls, E. : Sex Ratios in Drosophila ampelophila. Biol. Bull., 1913,
xxiv.

Riddle, 0. : Our Knowledge of Melanin Color Formation and Its Bear-
ing on the Mendelian Description of Heredity. Biol. Bull., 1909,
xvi.

Riddle^ 0. : Preliminary Chemical Studies on Male- and Female-Produc-
ing Eggs of Pigeons. Science, 1912, xxxv.

Riddle, Oscar: Sex Control and Known Correlations in Pigeons. Am,
Nat., 1916, 1, pp. 385-410.

RiDDLE_, Oscar : Success in Controlling Sex. Jour. Hered., 1916, vii.

Riddle, Oscar: The Theory of Sex as Stated in Terms of Results of
.Studies on Pigeons. Science, 1917, n.s,, xlvi.

Riddle, Oscar : The Control of the Sex Ratio. Joujn. Wash. Acad. Sc,
1917, vii.

Riddle, Oscar: The Theory of Sex as Stated in Tenns of Results of
Studies on Pigeons. Science, 1917, n.s., xlvi.

Robertson, W. R. B.: Chromosome Studies. III. Inequalities and
Deficiencies in Homologous Chromosomes: Their Bearing upon
Synapsis and the Loss of Unit Characters. Jour. Morph., 1915, xxvi.

Robertson, W. R. B. : Chromosome Studies. I. Taxonomic Relation-
ships Shown in the Chromosomes of Tettigidce and Acrididce: V-
shaped Chromosomes and Their Significance in Acrididce, Locustidce,
and Gryllidce: Chromosomes and Variation. Jour. Morph., 1916,
XX vii.

Robertson, W. R. B. : Chromosome Studies IV : A Deficient Super-
numerary Accessory Chromosome in a Male of Tettigidea parvi-
pennis. Kansas Uni. Sci. Bull., 1917, x. No. 14.

Rosenberg, 0.: Das Verhalten der Chromosomen in einer hybriden
Pflanze. Ber. Deutsch. hot. Gessell., 1903, xxi.

Rosenberg, 0. : Ueber die Tetradenteilung eines Drosera-Bastardes.
Ber. deutsch. hot. GeselL, 1904, xxii.

294 PHYSICAL BASIS OF HEREDITY

Rosenberg, 0.: Cji:ologische und morphologische Studien an Drosera

longifoliaXroUmdilofia. Kungl. Sevenska Vetens-Kaps. akad.

Handl., 1909, xliii.
Saunders, E. R. : Further Experiments on the Inheritance of " Double-

ness " and Other Charactei-s in Stocks. Jour. Genet., 1911, i.
Saunders, E. R. : Studies in the Inheritance of Doubleness in Flowers.

I. Petunia. Jour. Genet., 1911, i.
Saunders, E. R. : Fui-ther Contribution to the Study of the Inheritance

of Hoariness in Stocks. (Mathiola.) Proc. Roy. Soc, 1912, B,

Lxxxv.
Saunders, E. R. : On the Mode of Inheritance of Certain Characters

in Double-throwing Stocks. A reply. Zeit. Ahst. Vererh., 1913, x.
Seiler, J.: Das Verhalten der Geschlechtschromosomen bei Lepidop-

teren. Zool. Anz., 1913, xli.
Seiler, J.: Das Verhalten der Geschlechtschromosomen bei Lepidop-

teren. Archiv. fur Zellfor., 1915, xiii.
Shearer, C, Morgan, W. de, and Fuchs, H. M. : On Paternal Charac-
ters in Echinoid Hybrids. Q. J. M. S., 1912, Iviii.
Shinji, G. 0.: a Contribution to the Physiology of Wing Develop-
ment in Aphids. Biol. Bull, 1918, xxxv.
Shull, a. F. : Studies in the life Cycle of Hydatina senta, I. Jour.

Exp. Zool., 1910, viii.
Shull, a. F. : Inheritance in Hydatina senta, II. Jour. Exp. Zool.,

1915, xviii.
Shull, G. H. : The Composition of a Field of Maize. Am. Breeders'

Assoc, 1908, iv.
Shull, G. H. : The "Presence and Absence" Hypothesis. Am. Nat.,

1909, xliii.
Shull, G. H. : A Pure Line Method in Corn Breeding. Am. Breed. Ass.,

1909, V.
Shull, G. H. : Hybridization Methods in Com Breeding. Am. Breeder's

Mag., 1910, i.
Shull, G. H. : Inheritance of Sex in Lychnis. Bot. Gaz., 1910, xlix.
Shull, G. H. : Reversible Sex-mutants in Lychnis dioica. Bot. Gaz.,

1911, lii.

Shull, G. H. : Genotypes, Biotypes, Pure Lines, and Clones. Science,

1912, xxx\'.

Shull, G. H. : Hermaphrodite Females in Lychnis dioica. Science,

1912, xxxvi.
Shull, G. H. : Duplicate Genes for Capsule Form in Bursa bursa-pas-

toris. Zeits. Ahst. Vererh., 1914, xii.
Smith, G. : Famia und Flora des Golfes von Neapel. Rhizocephala.

Zool. Sta. Neapel. Monographie, 1906, xxix.
Smith, G. : Crustacea. Cam. Nat. Hist., 1909.

LITERATURE 295

Smith, G. : Studies in the Experimental Analysis of Sex. Parts 1 and 2.

Q, J. M. S., 1910, liv.
Smith, G. : Studies in the Experimental Analysis of Sex. Parts 3 and

4. Q.J. M. S., 1910, Iv.
Smith, G. : Studies in the Experimental Analysis of Sex. Part 5. Q. J.

M. S., 1911, Ivi.
Smith, G. : Studies in the Experimental Analysis of Sex. Part 6.

Q. J. M. S., 1911, Ivii.
Smith, G. : Studies in the Experimental Analysis of Sex. Part 7.

Q.J.M. S., 1911, Ivii.
Spillman, W. J. : Spurious Allelomorphism. Results of Recent Investi-
gations. Am. Nat., 1908, xlii.
Spillman, W. J. : Bamng in Barred Plymouth Rocks. Poultry, 1909, v.
Staples-Brown, R. : Second Report on the Inheritance of Color in

Pigeons, with Special Reference to Sex-limited Inheritance, 1912.
Stark^ M. B. : An Hereditary Tumor in the Fruit Fly, Drosophila.

Jour. Cancer Research, 1918, iii.
Stark, M. B. : An Hereditaiy Tumor. Jour. Exp. Zool., 1919, xxvii.
Stevens, N. M: Studies in Spermatogenesis with Especial Reference

to the "Ac<jessory Chromosome." Carnegie Inst., Wash., Pub. 36,

1905.
Stevens, N. M. : A Study of the Germ-cells of Certain Diptera. Jour.

Exp. Zool, 1908, V.
Stevens, N. M. : Heterochromosomes in the Guinea-pig. Biol. Bull.,

1911, xxi.
Stout, A. B. : The Establishment of Varieties in Coleus by the Selection

of Somatic Variations. Carnegie Inst., Wash., Pub. 218, 1915.
Strasburger, E. : Kernteilung bei der Erbse. Flora oder Allg. Bat.

Zeit. Miinchen, 1911, ii.
Strong, R. M. : Results of Hybridizing Ring-doves, Including Sex-linked

Inheritance. Biol. Bull., 1912, xxiii.
Sturtevant, a. H. : An Experiment Dealing with Sex-linkage in

Fowls. Jour. Exp. Zool., 1912, xii.
Sturtevant, a. H. : Is There Association between the Yellow and Agouti

Factors in Mice? Am. Nat., 1912, xlvi.
Sturtevant^ a. H. : The Linear Arrangement of Six Sex-linked Factors

in Drosophila, etc. Jour. Exp. Zool., 1913, xiv.
Sturtevant^ a. H. : The Himalayan Rabbit Case, with. Some Consid-
erations on Multiple Allelomorphs. Am. Nat., 1913, xlvii.
Sturtevant, a. H.: a Third Group of Linked Genes in Drosophila

ampelophila. Science, n.s., 1913, xxxvii.
Sturtevant, a. H. : The Reduplication Hypothesis as Applied to

Drosophila. Am. Nat., 1914, xl\iii.

296 PHYSICAL BASIS OF HEREDITY

Sturtevant^ a. H. : Linkage in the Silkworm Moth. Am. Nat., 1914,

xlviii.
Sturtevant, a. H. : No Crossing Over in the Female of the Silkworm

Moth. Ibid., 1915, xlix.
Sturtevant, a. H. : A Sex-linked Character in Drosophila repleta.

Ibid., 1915.

Sturtevant^ a. H. : Experiments on Sex-Recognition and the Prob-
lem of Sexual Selection in Drosophila. Jour. Am. Behav., 1915, v.

Sturtevant, a. H. : The Behavior of the Chromosomes as Studied
through Linkage. Zeit. Abst. Vererb., 1915, xiii.

Sturtevant, a. H. : Genetic Factors Affecting the Strength of Linkage
in Drosophila. Nat. Acad. Sci., 1917, iii.

Sturtevant, A. H. : Crossing Over without Chiasmatype. Genetics,
1917, ii.

Sturtevant, A. H. : An Analysis of the Effects of Selection. Carnegie
Inst., Wash., Pub. 264, 1918.

Sturtevant, A. H. : A Parallel Mutation in Drosophila funebris.
Science, 1918, xlviii.

Sturtevant, A. H. : Inherited Linkage Variations in the Second Chro-
mosome. Carnegie Pub. 278, Part III, 1919.

Sturtevant, A. H., Bridges, C. B., and Morgan, T. H. : The Spatial
Relations of Genes. Proceed. Nat. Acad., 1919, v.

Sumner, F. B. : Genetic Studies of Several Geogi-aphie Races of Cali-
fornia Deer-mice. Am. Nat., 1915, xlix.

Sumner, F. B. : Several Color Mutations in Mice of the Genus, Per-
omyscus. Genetics, 1917, ii.

Sutton, W. : On the Morphology of the Chromosome Group in Brachy-
stola Magna. Biol. Bull., 1902, iv.

Sutton, A. W. : Experiments in Crossing a Wild Pea from Palestine
with Commercial Peas. Bap. IV Conf. Internat. de Genetique.,

1913.
Tanaka, Y. : A Study of Mendelian Factors in the Silkworm, Bombyx

mori. Jour. Col. Ag. Tohoku Imp. Univ., 1913, v.
Tanaka, Y. : Gametic Coupling and Replusion in Silkworms. Ibid.,

1913, v.

Tanaka, Y. : Further Data on the Reduplication in Silkworms. Ibid.,

1914, vi.

Tanaka, Y. : Sexual Dimorphism of Gametic Series in the Reduplication.

Trans. Sapporo Nat. Hist. Soc, 1914, v.
Tanaka, Y. : Further Data on the Reduplication in Silkworms. Jour.

Coll. Agr. Sapporo, Japan, 1914, vi.
Tennent, D. H. : a Heterochromosome of Male Origin in Echinoids.

Biol. Bull., 1911, xxi.

LITERATURE 297

Tennent^ D. H. : Studies in Cytology, I and II. Jour. Exp. Zool.,

1912, xii.

Thompson, J. A. : Heredity, 1908, London and New York.

TiscHLER^ G. : Ueber Embryosack-Obliteration bei Bastardpflanzen. Bot.
Centralbl. Beihefte, 1903, xv.

TiscHLER, G. : Ueber die Entwicklung der Sexualorgane bei einem steri-
len Bryonia-Bastard. Ber. deutsch. bot. Gesell.^ 1906, xxiv.

TiscHLER, G. : Ueber die Entwicklung des Pollens und der Tapetenzellen
bei Ribeshybriden. Jahr. wiss. Bot,, 1906, xlii.

Tower, W. L. : An Investigation of Evolution in Chrysomelid Beetles of
the Genus Leptinotarsa. Carnegie Inst., Wash., Pub., 1906, xlviii.

TowER^ W. L. : The Determination of Dominance and the Modification of
Behavior in Alternative (Mendelian) Inheritance by Conditions Sur-
rounding or Incident upon the Germ-cells at Fertilization. Biol.
Bull, 1910, xviii.

ToYAMA, K. : Studies on the Hybridology of Insects, I. On Some Silk-
worm Crosses, with Special Reference to Mendel's Law of Heredity.
Bull. Coll. Agr., Tokyo Imp. Uni., 1906, vii.

ToYAMA, K. : On Certain Characteristics of the Silkworm Apparently
Non-mendelian. Biol. Cent., 1912, xxxii.

Trow, A. H. : On the Inheritance of Certain Characters in the Common
Groundsel — Senecio vulgaris — and Its .Segregates. Jour. Genet.,

1913, ii.

Trow, A. H. : Forms of Reduplication^ — Primary and Secondary. Jour.

Genet., 1913, ii.
Trow, A. H. : A Criticism of the Hypothesis of Linkage and Crossing

Over. Jour. Gen., 1916, v.
TscHERMAK_, E. : Ueber kiinstliche Kreuzung bei Pisum sativum. Zeit.

landw. Versueh. Oest., 1900.
v. TscHERMAK, A.: Ueber den Einfluss der Bastardierung auf Form,

Farbe und Zeichnung von Kanarieneiern. Biol. Centr., 1910, xxx.
V. TscHERMAK, A. : Ucbcr Veranderung der Form, Farbe und Zeichnung

von Kanarieneiern durch Bastardierung. Arch. f. Gesell. Phys.,

1912, cxlviii.
V. TSCHERMAK, E. I Der modeme Stand des Vererbungsproblems. Arch.

Bass, und Gesell., 1908, v.
De Vilmorin_, p., and Bateson^ W. : A Case of Gametic Coupling in

Pisum. Proc. Roy. Soc, 1911, B, Ixxxiv.
De VrieS;, H. : Die Mutationstheorie. 1901. Leipzig.
DeVries^H. : Species and Varieties; Their Origin by Mutation, 1905,

Chicago.
De Vries, H. : Plant-Bi'eeding ; Comments on the Experiments of Nils-
son and Burbank, 1907, Chicago.
De Vries, H. : On Twin Hybrids. Bot. Gaz., 1907, xliv.

298 PHYSICAL BASIS OF HEREDITY

De Vries, H. : Ueber die Zwillingsbastarde von (Enothera nanella. Ber.

deutsch. hot. Ges., 1908, xxvi.
De Vries^ H. : Bastarde von (Enothera gigas. Ber. deutsch. hot. Ges.,

1908, xxvi.
De Vries, H. : On Triple Hybrids. Bot. Gaz. 1909, xlix.
De Vries, H. : Ueber Doppeltreziproke Bastarde von (Enothera hiennis

and 0. muricata. Biol. Centr., 1911, xxxi.
De Vries, H. : Gruppenweise Artbildung, 1913, Berlin.
De Vries, H. : The Probable Origin of (Enothera Lamarckiana Ser.

Bot. Gaz., 1914, xvii.
De Vries, H. : Ueber kiinstliche Beschleunigung der Wasserauf nahme in

Samen dureh Druck. Biol. Centr. ^ 1915, xxxv.
De Vries, H. : (Enothera gigas nanella, a Mendelian Mutant. Bot. Gaz.,

1915, Ix.

De Vries, H. : New Dimorphic Mutants of the (Enotheras. Bot. Gaz.,

1916, Hi.

De Vries, H. : Mass Mutation in Zea Mays. Science, 1918, xlvii.

De Vries, H. : Van Amoebe tot Mensch. Laatste Les aan de Universi-
teit van Amsterdam, op 13 Juni 1918.

Weinstein, a.: Coincidence of Crossing Over in Drosophila melano-
gaster (ampelophila). Genetics^ 1918, iii.

Wenrich, D. H. : The Spermatogenesis of Phrynotettix Magnus with
Special Reference to Synapsis and the Individuality of the Chromo-
somes. Bull. Mus. Comp. Zool., Harvard Coll., 1916, Ix.

Weismann, a. : The Germ-plasm, English translation by W. N. Parker
and Harriet Ronnf eldt. New York, 1893.

Weismann, a. : Vortrage iiber Deszendenztheorie, Jena, 1913.

Wheeler, W. M. : The Origin of Female and Worker Ants from the
Eggs of Parthenogenetic Workers. Science, 1903, xviii.

Wheeler, W. M.: The Effects of Parasitic and Other Kinds of Cas-
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Wheeler, W. M. : A Gynandromorphous Mutillid. Psyche, 1910, xvii.

Wheeler, W. M. : Gynandromorphous Ants, Described During the Dec-
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Wheldale, M. : On the Nature of Anthocyanin. Proc. Camhridge Phil.
Soc, 1909, XV.

Wheldale, M. : The Colors and Pigments of Flowers, with Special Ref-
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Wheldale^ M. : Die Vererbung der Bliitsenf arbe bei Antirrhinum majus,
Zeit. Ahst. Vererh., 1910, iii.

Wheldale, M. : The Chemical Differentiation of Species. Biochem.
Jour., 1911, V.

Wheldale, M. : On the Formation of Anthocyanin. Jour. Gen., 1911, i.

LITERATUEE 299

Wheldale, M., and Bassett, H. L. : The Flower Pigments of Antirrhi-
num ma jus. Ill, The Red and Majenta Pigments. Biochem. Jour.,
1914, viii.

Wheldale^ M. : The Chemical Interpretation of Some Mendelian Fac-
tors for Flower Color. Proc. Roy. Soc, 1914, B, Ixxxvii.

White, 0. E. : Inheritance Studies in Pisum I. Inheritance of Coty-
ledon Color. Am. Nat., 1916, 1.

White, 0. E. : Inheritance Studies in Pisum IV. Interrelation of the
Genetic Factors of Pisum. Jour. Agr. Research, 1917, xi.

White, 0. E. : Inheritance of Endosperm Color in Maize. Am. Jour.
Bot., 1917, iv.

White, 0. E. : Studies of Inheritance in Pisum. II. The Present State of
Knowledge of Heredity and Variation in Peas. Proc. Am. Phil.
Soc, 1917, Ivi.

White, Thos H. : Tomato Variations Induced by Culture. Md. A.E.S.

Bull. 173, 1913.
Whitney, D. D. : Reinvigoration Produced by Cross Fertilization in

Bydatina senta. Jour. Exp. Zool., 1912, xii.
Whitney, D. D. : The Influence of Food in Controlling Sex in Hydatina

senta. Jour. Exp. Zool., 1914, xvii.
Whitney, D. D.: The Control of Sex by Food in Five Species of

Rotifers. Jour. Exp. Zool., 1916, xx.
Whitney, D. D. : The Relative Influence of Food and Oxygen in Con-
trolling Sex in Rotifers. Jour. Exp. Zool., 1917, xxiv.
Whitten, J. C. : Progress Report on Horticultural Investigations. Mo.

A.E.S. Bull. 131, 1915.
Wichura, M. : Die Bastardbefruchtung im Pflanzenreich, 1865, Breslau.
WiEMAN, H. L. : The Chromosomes of Human Spermatocytes. Am.

Jour. Anat., 1917, xxi.

Wilson, E. B. : The Cell in Development and Inheritance. 1899. New
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Wilson, E. B. : Studies on Chromosomes, I and II. Jour. Exp. Zool,
1905, ii.

Wilson, E. B. : Studies on Chromosomes, III. Jour. Exp. Zool, 1906,
m.

Wilson, E. B. : Studies on Chromosomes, IV and V. Jour. Exp Zool
1909, vi.

Wilson, E, B. : Studies on Chromosomes, VI. A New Type of Chromo-
some Combination in Metapodius. Jour. Exp. Zool, 1910, ix.

Wilson, E. B.: The Chromosomes in Relation to the Determination of
Sex. Sci. Progress, 1910, xvi.

Wilson, E. B. : The Sex-chromosomes. Arch. Mikr. Anat., 1911, Ixxvii.

Wilson, E. B. : Studies on Chromosomes, VII. Jour. Morph., 1911, xxii.

300 PHYSICAL BASIS OF HEEEDITY

Wilson, E. B. : Studies on Chromosomes, VIII, Jour. Exp. Zool., 1912,

xiii.
Wilson, E. B. : Some Aspects of Cytology in Relation to the Study of

Genetics. Am. Nat., 1912, xlvi.
Wilson, E. B. : Croonian Lecture: The Bearing of Cytological Research

on Heredity. Proc. Boy. Soc, 1914, Ixxxviii.
WiNGE, 0. : The Chromosomes. Their Numbers and General Importance.

Comp.-ren. des trav. du. Lab. de Carlsberg, 1917, xiii.
VON WiNiwAETER^ H. : Etudes sur la Spermatogenese humaine. I. Cellule

de Sertoil. II. Heterochromosome et mitoses de Fepithelium seminal.

Arch. Biol., 1912, xx\di.
Winkler, H. : Ueber die Nachkommenschaft der Solanum Pfropfbas-

tarde und die Chromosomen Zahlen ihrer Keimzellen. Zeit. fur

Botanik., ii. Rev. in Zeit. fiir indukt. Ahst.-u. Vererh., 1910, iii.
Winkler, H. : Die Chimarenf orschung als Methode experimenteller

Biologie. Phys.-Med. Gesselschaft Wiirzhurg, Jahrg., 1913-14.
WoLTERECK, R. : Ucber Veranderung der Sexualitat bei Daphniden.

Leipzig, 1911.
Wood, T. B. : Inheritance of Horns and Fac« Color in Sheep. Jour.

Agr. Sci., 1909, iii.
Woodruff, L. L. : An Experimental Study of the Life History of

Hypotrichous Infusoria. Jour. Exp. Zool., 1905, ii.
Woodruff, L. L. : The Life Cycle of Paramecium when Subjected

to a Varied Envii'onment. Am. Nat., 1908, xiii.
Woodruff, L. L. : On So-called Conjugating and Non-conjugating Races

of Paramecium. Jour. Exp. Zool., 1914, xvi.
Woodruff, L. L., and Erdmann, Rhoda : A Normal Periodic Reorganiza-
tion Process without Cell Fusian in Paramecium. Jour. Exp. Zool.,

1914, xvii.
Wright, Sewall: Duplicate Genes. Am. Nat., 1914, xlviii.
Wright, Sewall: Color Inheritance in Mammals. Jour. Hered., 1917,

viii.
Yatsu, N. : Notes on the Spermatogenesis of the wild and the Domesti-
cated Silkworms. Annotationes Zoologiccs Japonenses, 1913, viii.
Zeleny, C. : Full-eye and Emarginate-eye from Bar-eye in Drosophila

Without Change in the Bar Gene. Abst. 15th Ann. Meet. Am. Soc.

Zool, 1917.
Zeleny, C: Selection for High-facet and for Low-facet Number in

the Bar-eyed Race of Drosophila. Abst. 15th Ann. Meet. Am. Soc.

Zool, 1917.
Zeleny, C, and ]\Iattoon, E. W. : The Effect of Selection upon the

" Bar-eye " Mutant of Drosophila. Jour. Exp. Zool, 1915, xix.
Zeleny, C, and Senay, C. T. : Variation in Head Length of Spermato-
zoa in Seven Additional Species of Insects. Jour. Exp. Zool, 1915,

xix.

INDEX

Abnormal abdomen, 28-29, 32, 33
Abraxas, 175-178, 192, 248

type, 173-177, 180
Albinism, 67
Albinos, 248
AllQlomorphs, 23, 59, 60
Allen, 152
Altenburg, 85, 146
Amphibians, 114

Ancyracanthus eystidicola, 39-44
Andalusian fowls, 26, 32
Androgenetic, 189
Annelids, 114
Antlieridia, 152
Antirrhinum, 22]
Aphids, 197, 207
Aphid, bearberry, 184
Apotettix, 146
Aphis avense, 208
Archegonia, 152
Ascaris, 51, 52, 100, 160

nigrovenosa, 196
Assortment, 73-79
Atavistic type, 252 253

Baltzer, 215-217

Banta, 194

Bar-eyed, 31, 120, 121, 250

Bataiilon, 189

Bates, 245

Bateson, 25, 70, 85, 115-117

Batracoseps, 46-49, 100, 113

Baur, 85, 135, 220-222, 250

Beaded wing, 257-260

Bean, Florida velvet, 255

Lyon, 255
Beans, 204
Bearberry aphid, 184
Bee, 180,' 181, 197, 198
Belling, 255
Bergson, 267
Bifid wing, 119
Bion, 268

Biophor, 234

Bird, 174

Biston, 53, 164

Black fly, 30, 31, 63, 81-83, 87-90, 96,

123, 124, 139-144
Bonnet, 234, 235
Boveri, 51, 52, 160, 213-215, 223,

224, 231
Brachet, 189
Bridges, 55, 97, 114, 122, 127, 129,

138, 157, 159, 191, 200, 246
Bursa pastoris, 71

Car others, 74-77

Castle, 86, 87, 131, 132, 146, 256

Castration, 244

Chamberlain, 245

Chromosomes, 39-58, 73, 96-117

Circotettix, 74

Claxiocerans, 186

Clarke, W. T., 210

Clausen, 233

Cobs of corn, 249

Color blindness, 170

Confluent wing, 272

Conjugation, 49, 50

Conklin, 224, 225, 227

Contamination, 34

Com, 85, 135, 229, 249, 252

Correns, 219-220, 230

Criss-cross inheritance, 176

Crossing over, 87-95, 96-117, 139

Cu^not, 25, 256

Curved wing, 96, 124, 140-144

Cut wing, 122, 248

Ctenolabrus, 54, 232

Cynthia, 227

Cytoplasmic inheritance, 219-226

Dachs, 123

deficiencv, 124
Daphnians, 197
Darwin, 234, 267, 269

301

302

INDEX

Davenport, 33, 34

Deficiency, 159

Delage, 188

Dichete, 261, 262

Difflugia corona, 207

Digby, 151

Dilina, 164

Diluting factor, 70

Diploid, 84, 153-154

Disjunction, 23

Doncaster, 55, 164, 177, 192, 193

Dominance, 25, 60

Double crossing over, 119

Driesch, 231, 242

Drosera longifolia, 160
rotundifolia, 160

Drosophila busckii, 57, 86, 135
funebris, 273
melanica, 57

melanogaster, 27, 28, 30, 31, 37,
54, 57, 63, 66, 80, 84, 85, 87,
94^96, 113-115, 118, 127, 129,
130, 133, 134, 139, 143, 145, 146,
157, 159, 167, 170, 176, 177, 190,
191, 198-200, 236-238, 240, 248,
249, 251, 253, 256-258, 260, 263,
270, 271
repleta, 86, 135
virilis, 85, 135, 272

Dumpy wing, 66, 67

Duplication, 159

East, 229, 230
Ebony fly, 27, 63
Echinus eye, 122
Emerson, 249
Endosperm, 229, 230
Engledow, 85
Enzyme, 245
Eosin eye, 70
Equation division, 43
Euglena, 185, 187
Ewing, 208

Federley, 54, 162

Flinty corn, 229

Flowery corn, 229

Fish, 54

Forked bristles, 92, 93, 122, 272

Four-o'clock, 25, 220

Fowls, 239, 270
Fruit fly, 252, 272
Fundulus, 54, 232

Game bantam, 244

Gamete, 84

Gametic lethal, 254, 265

Garnet eye, 122

Gates, 149, 155, 156

Geerts, 155

Gene, 234-246

Genes, the order of, 118-125

Germ-plasm, 234, 239

Gigas, Oenothera, 149, 265

Goldschmidt, 245

Goodale, 86, 180, 244

Goodspeed, 233, 256

Gowen, 127, 146

Grasshopper, 74, 75

Gregory, 85, 146, 150

Groundsel, 85

Grouse locust, 146

Guinea pigs, albino, 271

Guyer, 136, 137, 170, 174, 178, 179

Gynandromorph, 190-193

Gypsy moth, 194

Haemophilia, 170
Hairless fly, 273
Hance, 157
Haploid, 55, 153, 154
Harrison, 55, 164
Hayes, 229, 230
Hegner, 207
Herbst, 213, 217, 218
Herlandt, 188
Hermaphrodite, 197

plant, 154
Hertwig, G., 188
Hertwig, O., 114, 188
Hertwig, R., 188
Heterozygous, 23
Homozygous, 23
Hornet, 181
Hydatina senta, 185

Individuality of the chromosomes,

51
Insects, 114
Interference, 126-132

INDEX

303

Internal secretion, 244
Intersexes, 193

Janssens, 46, 48, 102, 110, 112
Jennings, 207
Jesenko, 256
Johannsen, 204-206, 240
Jones, 85
Keeble, 150
King, 231

Kuschakewitsch, 188
Kuttner, O., 194
Lamarckian theory, 267
Langshan, 177-180
Leptotene threaxi, 100, 107
Lethal, 254, 257-265

factor, 198-200
Linkage, 80-86, 94

groups, 133
Lippincott, 126
Little, 256
Liverwort, 152

Loeb, J., 189, 225, 226, 231, 245
Lutz, A., 155, 157
Lymantria dispar, 194

japonica, 194

Maize, 229

Man, 137, 170, 200

McClung, 165

Marchal, Elie and Emile, 151-154

Marechal, 49

Marshall, 35

Maternal inheritance, 227-233

Maturation division, 43

May, 250

Melandrium, 221

Melanic forms, 248, 271

Men, 136

Mendel, 15-17, 19, 22, 23, 37, 38, 85,

236
Mendelism, 36
Mendel's first laAv, 19-38, 73

second law, 59-72, 79
Menidia, 232
Metapodius, 57
Metz, 57, 85, 135, 137, 272
Meves, 181, 182
Mice, albino, 271
Miniature wing, 66, 67, 91-93

Mirabilis jalapa, 25, 32, 219, 220

Mitotic division, 40

Modifying genes, 246

Moenkhaus, 55

Moore, 231

Morgan, 35, 112, 191

Morris, 55

Moss, 151-154

Moths, 55

Mouse, 67, 68, 70, 135, 137

albino, 70

blue, 70

chocolate, 70

silver-fawn, 70

yellow, 257
Muller, 35, 85, 127, 129, 130, 150,

258, 260
Mulsow, 39, 41

Multiple Allelomorphs, 251-254
Mutation, 247-273 '

Nabours 37, 86, 146
Natural selection, 267-270
Nicotiana sylvestris, 256

tabacum, 256
Non-disjunction, 200-203
Notch, 35
Notch wing, 248
Nova Scotia stock, 143, 144

Oats, 85, 135
Oenothera lafita, 264

Lamarckiana, 53, 85, 149, 155,
262-264

scintillans, 157

velutina, 264
Ouslow, 245
Oogonia, 154
Orthogenesis, 266
Osawa, 256

Pachytene thread, 49
Packard, 188, 189
Paleontologists, 266
Parthenogenesis, 39, 180, 204-211

artificial, 188, 189
Pea comb, 68, 69
Pea, Garden, 19-23, 32, 59-62, 85

Palestine, 256

wild, 270

304

INDEX

Peach eye, 261
Pelargonium, 221
Petrunke'watcli, 198
Phaseolus \nilgaris, 204
Phillips, 207
Phrynotettix, 104-110
Phylloxera caryoecaulis, 183
Phylloxerans, 197
Pinney, 55, 232
Pisum humile, 256
sativum, 19, 134
Plough, 96-99, 115, 139-142
Plymouth rock, barred, 176, 180
Polar body, 40
Polytoma, 186
Prematuration stage, 142
Primary split, 101, 102, 107, 108
Primrose, 85
Primula floribunda, 150, 151

kewensis, 151

sinensis, 85, 87, 135, 146, 150

verticillata, 150, 151
Pristiurus melanostomus, 49
Protenor, 55, 56
Protozoa, 207

Punnett, 70, 85, 115-117, 250
Pure culture, 250
lines, 204, 211
Purple eye, 96, 124, 139-144, 240
Pygsera, 161, 163

Rabbits, albino, 271
Rat, black, 271

Norway, 271

roof, 271

yellow, 271, 272
Recessive, 23, 25
Reduction division, 43, 75, 101
Reduplication, 115-117
Regeneration, 154
Reversal of dominance, 33
Riddle, 194, 196, 245
Ring dove, 194

Robertson, 74, 102, 103, 111-112
Rose comb, 68, 69
Rosenberg, 160, 161, 256
Rotifer, 185, 197, 198
Roux, 142
Rudimentary wing, 248

Sable body color, 158, 159
Saunders, 85, 254
Satsuma, 256
Schreiners, 44
Scute, 122
Sebrights, 243, 244
Secondary split, 101, 102
Sea urchin, 188, 189
Segregation, 16, 23, 39
Seller, 174, 179
Selachian egg, 50
Selachians, 114
Senecio vulgaris, 85
Sex, 196-203

chromosomes, 42, 57

determination, 180

genes, 193

linked characters, 84

ratios, 197-200
Shinji, 210
Shull, 71

Silkworm, 135, 137, 146, 186, 228
Simocephalus, 194
Smerinthus, 164
Single comb, 68, 69
Snapdragon, 85, 135, 255
Sooty fly, 27
Species, 270
Speck, 123, 144
Spencer, Herbert, 234
Spermatogonia, 154
Spermatogenesis, 104-110
Spermatozoon, 43
Sphaerechinus, 216, 217
Spores, 151
Sports, 248
Squirrels, albino, 271
Star eye, 122, 123
Stark, 256
Stenotomus, 232
Stevens, 145, 165
Stocks, 85, 254
Stomps, 149, 155
Streptopelia abla, 194
Strong, 196
Strongylocentrotus, 216, 217

Franciscanus, 231, 232

purpureua, 231, 232

INDEX

305

Sturtevant, 86, 114, 117, 125, 127,

143-145, 191, 260, 261, 272
Sundew, 256
Surface, 85, 135
Sutton, 16, 256
Sweet peas, 70, 85
Synaptic stage, 47
Syndactyls, 33, 34
Synizesis stage, 46

Tadpole, 189

Tanaka, 86, 135

Temperature, 96-97, 139-142

Tennent, 188, 231

Tetrads, 39, 43, 50, 75, 107, 114

Tetraploid, 150, 154, 159

Tettigidea, 111

Tomatoes, 85

Tomopteris, 44-46, 100, 113

Toyama, 191, 192, 228

Trimerotropsis, 74, 75, 78

Triploid, 155, 160

Trow, 117

Truncate wing, 248

Turtle dove, 194

Turtur orientalis, 194

Twin hybrids, 264

Variation — in linkage, 139-146
Variation — in number of chromo-
somes, 147-164
Vermilion eye, 158, 159, 248
Vestigial fly, 63, 81-83, 87-90, 123,
124

Vilmorin, 85

Vinegar fly, 27, 28, 70, 168-170, 172
Voinov, 74

Vries, de, 85, 149, 155, 263, 264,
266, 273

Walnut comb, 68, 69

Warren, 85

Wasps, 198

Weatherwax, 229

Weinstein, 127, 129, 131

Weisraann, 234, 235, 242

Wenrich, 74, 102, 110, 113

Wheat, 85, 135

WTiite, O. E., 20, 67. 85, 134

White-eyed fly, 91-93, 118, 167, 168,

171-173, 238, 248, 271
Whiting, 70
Whitney, 185, 186
Wilson, E. B., 55, 165
Winiwarter, 136-137, 170
Wright, 86, 87

Xenia, 230

Yatsu, 135, 137

Yellow mouse, 257

Yellow- winged fly, 91, 118, 120, 171-

173, 190, 250, 272
Yocom, 137

Zea mais, 229, 249
Zygote, 84
Zygotene stage, 107
Zygotic lethal, 254, 256

1 , ^^"0

50

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