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Columbia ?3ntbers;itu Hectures

HEREDITY AND SEX

THE JESUP LECTURES
1913

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COLUMBIA UNIVERSITY LECTURES

HEREDITY AND SEX

BY

THOMAS HUNT MORGAN, Pn.D,

PROFESSOR OF EXPERIMENTAL ZOOLOGY
IX COLUMBIA UNIVERSITY

SECOND (REVISED) EDITION

at/.

gork

COLUMBIA UNIVERSITY PRESS
1914

All rights reserved

COPYRIGHT, 1913,

BY COLUMBIA UNIVEK8ITY PRESS.
Set up and electrotyped. Published November, 1913.

Nortoooti

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

INTRODUCTION

Two lines of research have developed with surpris-
ing rapidity in recent years. Their development has
been independent, but at many stages in their progress
they have looked to each other for help. The study
of the cell has furnished some fundamental facts
connected with problems of heredity. The modern
study of heredity has proven itself ta be an instrument
even more subtle in the analysis of the materials of
the germ-cells than actual observations on the germ-
cells themselves.

In the following chapters it has been my aim to point
out, wherever possible, the bearing of cytological
studies on heredity, and of the study of heredity on the
analysis of the germinal materials.

The time has come, I think, when a failure to recog-
nize the close bond between these two modern lines of
advance can no longer be interpreted as a wise or
cautious skepticism. It seems to me to indicate rather
a failure to appreciate what is being done at present,
and what has been accomplished. It may not be desir-
able to accept everything that is new, but it is cer-
tainly undesirable to reject what is new because of its
newness, or because one has failed to keep in touch with
the times. A hypercritical spirit in science does not
always mean greater profundity, nor is our attitude
toward science more correct because we are unduly

vi INTRODUCTION

skeptical toward every advance. Our usefulness will,
in the long run, be proven by whether or not we have
been discriminating and sympathetic in our attitude
toward the important discoveries of our time. While
every one will probably admit such generalities, some of
us may call those who accept less than ourselves con-
servatives ; others of us who accept more will be called
rash or intemperate. To maintain the right balance
is the hardest task we have to meet. In attempting to
bring together, and to interpret, work that is still in the
making I cannot hope to have always made the right
choice, but I may hope at least for some indulgence
from those who realize the difficulties, and who think
with me that it may be worth while to make the
attempt to point out to those who are not specialists
what specialists are thinking about and doing.

What I most fear is that in thus attempting to for-
mulate some of the difficult problems of present-day
interest to zoologists I may appear to make at times
unqualified statements in a dogmatic spirit. I beg to
remind the reader and possible critic that the writer
holds all conclusions in science relative, and subject
to change, for change in science does not mean so much
that what has gone before was wrong as the discovery
of a better strategic position than the one last held.

HEREDITY AND SEX

CHAPTER I

THE EVOLUTION OF SEX

ANIMALS and plants living to-day reproduce them-
selves in a great variety of ways. With a modicum of
ingenuity we can arrange the different ways in series
beginning with the simplest and ending with the more
complex. In a word, we can construct systems of
evolution, and we like to think that these systems reveal
to us something about the evolutionary process that
has taken place.

There can be no doubt that our minds are greatly
impressed by the construction of a graded series of
stages connecting the simpler with the complex. It is
true that such a series shows us how the simple forms
might conceivably pass by almost insensible (or at
least by overlapping) stages to the most complicated
forms. This evidence reassures us that a process of
evolution could have taken place in the imagined order.
But our satisfaction is superficial if we imagine that
such a survey gives much insight either into the causal
processes that have produced the successive stages, or
into the interpretation of these stages after they have
been produced.

Such a series in the present case would culminate
in a process of sexual reproduction with males and

i

2 t : .: k*: .-.••: : HEREDITY AND SEX

females as the actors in the drama. But if we are
asked what advantage, if any, has resulted from
the process of sexual reproduction, carried out on
the two-sex scheme, we must confess to some un-
certainty.

H>

The most important fact that we know about living
matter is its inordinate power of increasing itself. If
all the fifteen million eggs laid by the conger eel were
to grow up, and in turn reproduce, in ten years the
sea would be a wriggling mass of fish.

A single infusorian, produced in seven days 935 de-
scendants. One species, stylonichia, produced in 6}^
days a mass of protoplasm weighing one kilogram.
At the end of 30 days, at the same rate, the number of
kilograms would be 1 followed by 44 zeros, or a mass of
protoplasm a million times larger than the volume of
the sun.

Another minute organism, hydatina, produces about
30 eggs. At the end of a year (65 generations), if all the
offspring survived, they would form a sphere whose
limits would extend beyond the confines of the known
universe.

The omnipresent English sparrow would produce in
20 years, if none died except from old age, so many de-
scendants that there would be one sparrow for every
square inch of the State of Illinois. Even slow-
breeding man has doubled his numbers in 25 years.
At the same rate there would in 1000 years not
be standing room on the surface of the earth for
his offspring.

I have not gone into these calculations and will

THE EVOLUTION OF SEX 3

ot vouch for them all, but whether they are en-
tirely correct or only partially so, they give a rough
idea at least of the stupendous power of growth.

There are three checks to this process: First, the
food supply is insufficient — you starve ; second, ani-
mals eat each other — you feed ; third, substances are
produced by the activity of the body itself that inter-
fere with its powers of growth — yoji poison yourself.
The laws of food supply and the appetites of enemies
are as inexorable as fate. Life may be defined as a
constant attempt to find the one and avoid the other.
But we are concerned here with the third point, the
methods that have been devised of escape from the
limitations of the body itself. This is found in repro-
duction. The simplest possible device is to divide.
This makes dispersal possible with an increased chance
of finding food, and of escaping annihilation, and at
the same time by reducing the mass permits of a more
ready escape of the by-products of the living machine.

Reproduction by simple division is a well-known pro-
cess in many of the lower animals and plants; it is
almost universal in one-celled forms, and not unknown
even in many-celled organisms. Amoeba and para-
mcecium are the stock cases for unicellular animals;
many plants reproduce by buds, tubers, stolons, or
shoots ; hydroids and sea-anemones both divide and
bud ; many planarians, and some worms, divide trans-
versely to produce two new individuals. But these
methods of reproduction are limited to simple structures
where concentration and division of labor amongst the
organs has not been carried to an extreme. In con-
sequence, what each part lacks after the division can be

4 HEREDITY AND SEX

quickly made good, for delay, if prolonged, would
increase the chances of death.

But there is another method of division that is almost
universal and is utilized by high and by low forms alike :
individual cells, as eggs, are set free from the rest of
the body. Since they represent so small a part of the
body, an immense number of them may be produced on
the chance that a few will escape the dangers of the
long road leading to maturity. Sometimes the eggs
are protected by jelly, or by shells, or by being trans-
parent, or by being hidden in the ground or under
stones, or even in the body of the parent. Under these
circumstances the animal ventures to produce eggs with
a large amount of food stored up for the young embryo.

So far reaching were the benefits of reproduction
by eggs that it has been followed by almost every
species in the animal and plant kingdom. It is ad-
hered to even in those cases where the animals follow
other grosser methods of separation at the same time.
We find, however, a strange limitation has been put
upon the process of reproduction by eggs. Before the
egg begins its development it must be fertilized. Cells
from two individuals must come together to produce
a new one.

The meaning of this process has baffled biologists
ever since the changes that take place during fertili-
zation were first discovered ; in fact, long before the
actual processes that take place were in the least un-
derstood. There is a rather extensive and antiquated
literature dealing with the part of the male and of
the female in the process of procreation. It would
take us too far to attempt to deal with these questions

(TO

,

THE EVOLUTION OF SEX 5

in their historical aspects, but some of their most
modern aspects may well arrest our attention.

In the simplest cases, as shown by some of the one-
celled organisms, two individuals fuse into a single
one (Fig. 1) ; in other related organisms the two in-
dividuals that fuse may be unequal in size. Some-
times we speak of these as male and female, but
it is questionable whether we should apply to these
unicellular types the same names that we use for the

FIG. 1. — • Union of two individuals (Stephanos phcera pluvialis) to

form a single individual. (After Doflein.) ^

many-celled forms where the word sex applies to the
soma or body, and not to the germ cells.

One of the best known cases of conjugation is that
of paramcecium. Under certain conditions two in-
dividuals unite and partially fuse together. An in-
terchange of certain bodies, the micronuclei, then takes
place, as shown in Fig. 2, and in diagram, Fig. 3. The
two conjugating paramcecia next separate, and each
begins a new cycle of divisions. Here each individual
may be said to have fertilized the other. The process
recalls what takes place in hermaphroditic animals of
higher groups in the sense that sperm from one indi-
vidual fertilizes eggs of the other.

We owe to Maupas the inauguration of an epoch-
making series of studies based on phenomena like this
in paramcecium.

6

HEREDITY AND SEX

FIG. 2. — Conjugation in Paramo3cium. The micronucleus in one indi-
vidual is represented in black, in the other by cross-lines. The macro-
nucleus in both is stippled. A-C, division of micronucleus into 2 and
4 nuclei; Cl-D, elongation of conjugation nuclei, which interchange and
reeombine in E\ F-J, consecutive stage in one ex-conjugant to show three
divisions of new micronucleus to produce eight micronuclei (J). In lower
part of diagram the first two divisions of the ex-conjugant (J) with eight
micronuclei are shown, by means of which a redistribution of the eight
micronuclei takes place. See also Fig. 100.

THE EVOLUTION OF SEX

II

ffl

FIG. 3. — The nu:-lei of two individuals of paramcecium in I (homozygous
in certain factors, and heterozygous in other factors) , are represented as divid-
ing twice ( in II and III ) ; the first division, II, is represented as reducing,
i.e. segregation occurs ; the second division, III, is represented as equational,
i.e. no reduction but division of factors, as in the next or conjugation division,
IV, also.

8 HEREDITY AND SEX

Maupas found by following from generation to
generation the division of some of these protozoa that
the division rate slowly declines and finally comes to
an end. He found that if a debilitated individual
conjugates with a wild individual, the death of the race
is prevented, but Maupas did not claim that through
conjugation the division rate was restored. On the
contrary he found it is lower for a time.

He also discovered that conjugation between two
related individuals of these weakened strains produced
no beneficial results.

Butschlihad earlier (1876) suggested that conjugation
means rejuvenation or renewal of youth, and Maupas'
results have sometimes been cited as supporting this
view. Later work has thrown many doubts on this
interpretation and has raised a number of new issues.

In the first place, the question arose whether the
decline that Maupas observed in the rate of division
may not have been due to the uniform conditions under
which his cultures were maintained, or to an insuffi-
ciency in some ingredient of these cultures rather than
to lack of conjugation. Probably this is true, for
Calkins has shown that by putting a declining race
into a different medium the original division rate may
be restored. Woodruff has used as culture media a
great variety of food stuffs and has succeeded in keep-
ing his lines without loss of vigor through 3000 gen-
erations. Maupas records a decline in other related
protozoa at the end of a few hundred generations.

Butschli's idea that by the temporary union (with
interchange of micronuclei) of two weak individuals
two vigorous individuals could be produced seems

THE EVOLUTION OF SEX 9

mysterious ; unless it can be made more explicit, it
does not seem in accord with our physico-chemical
conceptions. Jennings, who has more recently studied
in greater detail the process of division and conjugation
in paramoecium, has found evidence on which to base
a more explicit statement as to the meaning of rejuve-
nescence through conjugation.

Jennings' work is safeguarded at every turn by care-
ful controls, and owing in large part to these controls
his results make the interpretations more certain. He
found in a vigorous race, that conjugated at rather
definite intervals, that after conjugation the division
rate was not greater than it had been before, but on
the contrary was slower — a fact known, as he points
out, to Maupas and to Hertwig. Conjugation does
not rejuvenate in this sense.

Jennings states that, since his race was at the be-
ginning vigorous, the objection might be raised that
the conditions were not entirely fulfilled, for his pred-
ecessors had concluded that it is a weakened race that
was saved from annihilation by the process. In order
to meet this objection he took some individuals from
his stock and reared them in a small amount of culture
fluid on a slide. After a time they became weakened
and their rate of division was retarded. He then al-
lowed them to conjugate, and reared the conjugants.
Most of these were not benefited in the leafet by the
process, and soon died. A few improved and began
to multiply, but even then not so fast as paramcecia hi
the control cultures that had been prevented from con-
jugating. Still others gave intermediate rates of
division.

10

HEREDITY AND SEX

He concludes that conjugation is not in itself bene-
ficial to all conjugants, but that the essence of the pro-
cess is that a recombination of the hereditary traits
occurs as shown in the diagram, Fig. 3 and 4. Some

FIG. 4. — Illustrating conjugation between two stocks, with pairs of
factors A, B, C, D, and a, b, c, d ; and union of pairs into Aa, Bb, Cc, Dd.
After these separate, their possible recombinations are shown in the 16
smaller circles. (After Wilson.).

of these new combinations are beneficial for special
conditions — others not. The offspring of those con-
jugants that have made favorable combinations will
soon crowd out the descendants of other conjugants
that have made mediocre or injurious combinations.
Hence, in a mass culture containing at all times large

THE EVOLUTION OF SEX

11

numbers of individuals, the maximum division rate is
kept up, because, at any one time, the majority of the
individuals come from the combinations favorable to
that special environment.

There are certain points in this argument that call
for further consideration. In a mass culture the fa-
vorable combinations for that culture will soon be made,
if conjugation is taking place. At least this is true if
such combinations are homogeneous (homozygous, in
technical language). Under such circumstances the
race will become a pure strain, and further conjugation
could do nothing for it even if it were transferred to a
medium unsuited to it.

In the ordinary division of a cell every single de-
terminer divides and each of the new cells receives
half of each determiner. Hence in the case of para-
mcecium all the descendants of a given paramoecium
that are produced by division must be exactly alike.
But in preparation for conjugation a different pro-
cess may be supposed to take place, as in higher
animals, among the determiners. The determiners
unite in pairs and then, by division, separate from
each other, Fig. 4. In consequence the number of
determiners is reduced to half. Each group of deter-
miners will be different from the parent group, pro-
vided the two determiners that united were not
identical. If after this has occurred conjugation
takes place, the process not only restores the total
number of determiners in each conjugant, but gives
new groups that differ from both of the original
groups.

The maintenance of the equilibrium between an

12 HEREDITY AND SEX

organism and its environment must be a very delicate
matter. One combination may be best suited to one
environment, and another combination to another.
Conjugation brings about in a population a vast num-
ber of combinations, some of which may be suited to
the time and place where they occur. These survive
and produce the next generation.

Jennings' experiments show, if I understand him
correctly, that the race he used was not homogeneous
in its hereditary elements ; for when two individuals
conjugated, new combinations of the elements were
form 3d. It seems probable, therefore, that the chemi-
cal equilibrium of paramcecium is maintained by the
presence of not too much of some, or too little of other,
hereditary materials. In a word, its favorable com-
binations are mixed or heterozygous.

The meaning of conjugation, and by implication,
the meaning of fertilization in higher forms is from this
point of view as follows : — In many forms the race, as a
whole, is best maintained by adapting itself to a widely
varied environment. A heterozygous or hybrid con-
stitution makes this possible, and is more likely to
perpetuate itself in the long run than a homozygous
race that is from the nature of the case suited to a more
limited range of external conditions.

What bearing has this conclusion on the problem of
the evolution of sex and of sexual reproduction ?

This is a question that is certain to be asked. I am
not sure that it is wise to try to answer it at present,
in the first place because of the uncertainty about the
conclusions themselves, and in the next place, because,
personally, I think it very unfair and often very unfor-

THE EVOLUTION OF SEX 13

tunate to measure the importance of every result by
its relation to the theory of evolution. But with this
understanding I may venture upon a few suggestions.

If a variation should arise in a hermaphroditic
species (already reproducing sexually) that made cross-
fertilization more likely than self-fertilization, and if,
as a rule, the hybrid condition (however this may be
explained) is more vigorous in the sense that it leaves
more offspring, such a variation would survive, other
things being equal.

But the establishment of the contrivance in the
species by means of which it is more likely to cross-
fertilize, might in another sense act as a drawtack.
Should weak individuals appear, they, too, may be
perpetuated, for on crossing, their weakness is concealed
and their offspring are vigorous owing to then- hybrid
condition. The race will be the loser in so far as re-
cessive or weak combinations will continue to appear,
as they do in many small communities that have some
deficiency hi their race ; but it is a question whether the
vigor that comes from mixing may not more than com-
pensate for the loss due to the continual appearance of
weakened individuals.

This argument applies to a supposed advantage
within the species. But recombination of what already
exists will not lead to the development of anything
that is essentially new. Evolution, however, is con-
cerned with the appearance and maintenance of new
characters. Admitting that sexual reproduction proved
an advantage to species, and especially so when com-
bined with a better chance of cross-fertilization, the
machinery would be at hand by means of which any

14 HEREDITY AND SEX

new character that appeared would be grafted, so to
speak, on to the body of the species in which it appeared.
Once introduced it would be brought into combination
with all the possible combinations, or races, already
existing within the species. Some of the hybrid com-
binations thus formed might be very vigorous and would
survive. This reasoning, while hypothetical, and, per-
haps not convincing, points at least to a way in
which new varieties may become incorporated into
the body of a species and assist in the process of
evolution.

It might be argued against this view that the same
end would be gained, if a new advantageous variation
arose in a species that propagated by non-sexual
methods or in a species that propagated by self-fertili-
zation. The offspring of such individuals would trans-
mit their new character more directly to the offspring.
Evolution may, of course, at times have come about
in this way, and it is known that in many plants self-
fertilization is largely or exclusively followed. But in
a species in which cross-fertilization was the estab-
lished means of propagation, the new character would
be brought into relation with all the other variations
that are found in the component races 'and increase
thereby its chances of favorable combinations. We
have in recent years come to see that a new heritable
character is not lost by crossing, or even weakened by
" blending," as was formerly supposed to be the case;
hence no loss to the character itself will result in the
union with other strains, or races, within the species.

If then we cannot explain the origin of sexual re-
production by means of the theory of evolution, we

THE EVOLUTION OF SEX 15

can at least see how the process once begun might be
utilized in the building up of new combinations ; and
to-day evolution has come to mean not so much a
study of the origination of new characters as the method
by which new characters become established after they
have appeared.

THE BODY AND THE GERM-PLASM

As I have said, it is not unusual to speak of the uni-
cellular animals and plants as sexual individuals, and
where one of them is larger than the other it is some-
times called the female and the smaller the male. But
in many-celled animals we mean by sex something
different, for the term applies to the body or soma, and
not to the reproductive cells at all. The reproductive
cells are eggs and sperm. It leads to a good deal of
confusion to speak of the reproductive cells as male
and female. In the next chapter it will be pointed out
that the eggs and sperm carry certain materials ; and
that certain combinations of these materials, after fer-
tilization has occurred, produce females ; other combi-
nations produce males ; but males and females, as such,
do not exist until after fertilization has taken place.

The first step, then, in the evolution of sex was taken
when colonies of many cells appeared. We find a
division of labor in these many-celled organisms ; the
germ-cells are hidden away inside and are kept apart
from the wear and tear of life. Their maintenance
and protection are taken over by the other cells of the
colony. Even among the simplest colonial forms we
find that some colonies become specialized for the pro-
duction of small, active germ-cells. These colonies

16 HEREDITY AND SEX

are called the males, or sperm-producing colonies. The
other colonies specialize to produce larger germ-cells —
the eggs. These colonies are called females or egg-pro-
ducing colonies. Sex has appeared in the living world.

To-day we are only beginning to appreciate the far-
reaching significance of this separation into the immor-
tal germ-cells and the mortal body, for there emerges
the possibility of endless relations between the body on
the one hand and the germ-cells on the other. What-
ever the body shows in the way of new characters
or new ways of reacting must somehow be represented
in the germ-cells if such characters are to be perpetu-
ated. The germ-cells show no visible modification to
represent their potential characters. Hence the classi-
cal conundrum — whether the hen appeared before the
egg, or the egg before the hen ? Modern biology has
answered the question with some assurance. The egg
came first, the hen afterwards, we answer dogmati-
cally, because we can understand how any change in
the egg will show itself in the next generation — in
the new hen, for instance ; but despite a vast amount
of arguing no one has shown how a new hen could get
her newness into the old-fashioned eggs.

Few biological questions have been more combated
than this attempt to isolate the germ-tract from the
influence of the body. Nussbaum was amongst the
first, if not the first, to draw attention to this distinc-
tion, but the credit of pointing out its importance is
generally given to Weismann, whose fascinating specu-
lations start from this idea. For Weismann, the germ-
cells are immortal — the soma alone has the stigma of
death upon it. Each generation hands to the next

THE EVOLUTION OF SEX 17

one the immortal stream unmodified by the experience
of the body. What we call the individual, male or
female, is the protecting husk. In a sense the body is
transient — temporary. Its chief " purpose" is not
its individual life, so much as its power to support and
carry to the next point the all important reproductive
material.

Modern research has gone far towards establishing
Weismann's claims in this regard. It is true that the
germ-plasm must sometimes change — otherwise there
could be no evolution. But the evidence that the germ-
plasm responds directly to the experiences of the body
has no substantial evidence in its support. I know, of
course, that the whole Lamarckian school rests its
argument on the assumption that the germ-plasm re-
sponds to all profound changes in the soma ; but despite
the very large literature that has grown up dealing with
this matter, proof is still lacking. And there is abun-
dant evidence to the contrary.

On the other hand, there is evidence to show that
the germ-plasm does sometimes change or is changed.
Weismann's attempt to refer all such changes to recom-
binations of internal factors in the germ-plasm it-
self has not met with much success. Admitting that
new combinations may be brought about in this
way, as explained for paramcecium, yet it seems un-
likely that the entire process of evolution could have
resulted by recombining what already existed; for
it would mean, if taken at its face value, that by re-
combination of the differences already present in the
first living material, all of the higher animals and plants
were foreordained. In some way, therefore, the germ-

18 HEREDITY AND SEX

plasm must have changed. We have then the alter-
natives. Is there some internal, initial or driving im-
pulse that has led to the process of evolution ? Or has
the environment brought about changes in the germ-
plasm ? We can only reply that the assumption of an

•§•

FIG. 5. — Schematic representation of the processes occurring during
the fertilization and subsequent segmentation of the ovum. (Boveri, from
Howell.)

internal force puts the problem beyond the field of
scientific explanation. On the other hand, there is a
small amount of evidence, very incomplete and in-
sufficient at present, to show that changes in the en-
vironment reach through the soma and modify the
germinal material.

THE EVOLUTION OF SEX 19

It would take us too far from our immediate subject
to attempt to discuss this matter, but it has been nec-
essary to refer to it in passing, for it lies at the founda-
tion of all questions of heredity and even involves, as
we shall see later, the question of heredity of sex.

This brings us back once more to the provisional
conclusion we reached in connection with the experi-
ments on paramcecium. When the egg is fertilized
by the sperm, Fig. 5, the result is essentially the same
as that which takes place when two paramoecia fer-
tilize each other. The sperm brings into the egg a
nucleus that combines with the egg-nucleus. The new
individual is formed by recombining the hereditary
traits of its two parents.

It is evident that fertilization accomplishes the same
result as conjugation. If our conclusion for paramce-
cium holds we can understand how animals and plants
with eggs and sperm may better readjust themselves
now to this, now to that environment, within certain
limits. But we cannot conclude, as I have said, that this
process can make any permanent contribution to evolu-
tion. It is true that Weismann has advanced the hy-
pothesis that such recombinations furnish the materials
for evolution, but as I have said there is no evidence
that supports or even makes plausible his contention.

I bring up again this point to emphasize that while the
conclusion we arrived at — a provisional conclusion at
best — may help us to understand how sexual repro-
duction might be beneficial to a species in maintaining
itself, it cannot be utilized to explain the progressive
advances that we must believe to have taken place
during evolution.

20 HEREDITY AND SEX

THE EARLY ISOLATION OF THE GERM-CELLS

There is much evidence to show that the germ-cells
appear very early in the development of the individual
when they are set aside from the cells that differentiate
into the body cells. This need not mean that the germ-
cells have remained unmodified, although this is at

FIG. 6. — Chromatin diminution and origin of the germ-cells in Ascaris.
(After Boveri.)

THE EVOLUTION OF SEX

21

first sight the most natural interpretation. It might be
said, indeed, that they are among the first cells to
differentiate, but only in the sense that they specialize,
as germ-cells.

FIG. 7. — Origin of germ-cells in Sagitta.
Korschelt and Heider.)

(From

In a parasitic worm, ascaris, one of the first four
cells divides differently from the other three cells. As
seen in Fig. 6, this cell retains at its division all of its
chromatin material, while in the other three cells some
of the chromatin is thrown out into the cell-plasm. The

''-*'•'*£•••&

i

:'M<

o

FIG. 8. — Origin of germ-cells in Miastor. Note small black proto-
plasmic area at bottom of egg into which one of the migrating segmentation
nuclei moves to produce the germ-cells. (After Kahle.)

22

HEREDITY AND SEX

single cell that retains all of the chromatin in its nucleus
gives rise to the germ-cells.

In a marine worm-like form, sagitta, two cells can
easily be distinguished from the other cells in the wall of
the digestive tract (Fig. 7). They leave their first posi-
tion and move into the interior of the body, where they
produce the ovary and testes.

FIG. 9. — Origin of germ-cells in certain vertebrates, viz. turtle, frog,
gar-pike and bow-fin. The germ-cells as darker cells are seen migrating from
the digestive tract (endoderm). (After Allen.)

In several of the insects it has been shown that at a
very early stage in the segmentation, one, or a few cells
at most, lying at one end of the egg develop almost in-
dependently of the rest of the embryo (Fig. 8). Later
they are drawn into the interior, and take up their
final location, where they give rise to the germ-cells.

Even in the vertebrates, where, according to the

THE EVOLUTION OF SEX 23

earlier accounts, the germ-cells were described as appear-
ing late in embryonic development, it has been shown
that the germ-cells can be detected at a very early stage
in the walls of the digestive tract (Fig. 9). Thence they
migrate to their definitive position, and give rise to
the cells from which the eggs or the sperm arise.

The germ-cells are in fact often the earliest cells to
specialize in the sense that they are set aside from the
other cells that produce the soma or body of the in-
dividual.

THE APPEARANCE OF THE ACCESSORY ORGANS OF
REPRODUCTION

As animals became larger the problem of setting free
the germ-cells was a matter of great importance. Sys-
tems of outlets arose — the organism became piped, as it
were. In the lower animals the germ-cells are brought
to the surface and set free directly, and fertilization is a
question of the chance meeting of sperm and egg ; for
there is practically no evidence to show that the sperm
is attracted to the egg and much evidence that it is
not. Later, the copulatory organs were evolved in all
the higher groups of animals by means of which the
sperm of the male is transferred directly to the female.
This makes more certain the fertilization of the egg.

In the mollusks, in the insects and crustaceans, and
in the vertebrates the organs of copulation serve to
hold the individuals together during the act of mating,
and at the same time serve to transfer the semen of the
male to the oviduct, or to special receptacles of the
female. Highly elaborated systems of organs and
special instincts, no less elaborate, serve to make the

24

HEREDITY AND SEX

union possible. In some types mating must occur for
each output of eggs, but in other cases the sperm is
stored up in special receptacles connected with the ducts
of the female. From these receptacles a few sperm at
a time may be set free to fertilize each egg as it passes
the opening of the receptaculum. In the queen bee
enough sperm is stored up to last the queen for five or
six years and enough to fertilize a million eggs.

FIG. 10. — Squid : Two upper right-hand figures illustrate two methods
of copulation. Lower right-hand figure dissected to show spermatophore
placed in mantle cavity of female. Left-hand figure (below), spermatophore
pocket behind mouth of male; upper figure, section of same. (After Drew.)

There are a few cases where the transfer from the
male to the female is brought about in a different way.
The most striking cases are those of the squids and
octopi, and of the spiders.

In the squid, the male and female interlock arms
(Fig. 10). The male takes the packets of sperm (that
are emitted at this time from the sperm-duct) by means
of a special arm, and transfers the packets either to a

THE EVOLUTION OF SEX

25

special receptacle within the circle of arms of the female,
or plants them within the mantle chamber itself of the
female. Each packet of spermatozoa is contained in a
long tube. On coming in contact with sea water the
tube everts at one end, and allows the sperm to escape.

FIG. 11. — Octopus, male showing hectocotyl arm (ha). Cop-
ulation (below), small male, A; large female, B.

After separation the female deposits her strings of
eggs, which are fertilized by the sperm escaping from
the spermatophores. In octopus and its allies, one
arm, that is used to transfer the spermatophores, is
specially modified at the breeding season (Fig. 11).

26 HEREDITY AND SEX

This arm is inserted by the male, as shown in the figure,
within the mantle chamber of the female. In some
species, Argonauta argo for instance (Fig. 12), the arm

FIG. 12. — Argonauta showing developing (A) and developed (B)
hectocotyl arm, which, after being charged with spermatophores, is left in
mantle of female.

is broken off, and remains attached by its suckers in-
side the mantle of the female. The eggs are later fer-
tilized by sperm set free from this " hectocotylized " arm.

THE SECONDARY SEXUAL CHARACTERS

In the most highly evolved stages in the evolution
of sex a new kind of character makes its appearance.
This is the secondary sexual character. In most cases
such characters are more elaborate in the male, but
occasionally in the female. They are the most aston-
ishing thing that nature has done : brilliant colors,
plumes, combs, wattles, and spurs, scent glands (pleas-
ant and unpleasant) ; red spots, yellow spots, green
spots, topknots and tails, horns, lanterns for the dark,
songs, howlings, dances and tourneys — a medley of
odds and ends.

The most familiar examples of these characters are
found in vertebrates and insects, while in lower forms

THE EVOLUTION OF SEX 27

they are rare or absent altogether. In mammals the
horns of the male stag are excellent examples of second-
ary sexual characters. The male sea cow is much
greater in size than the female, and possesses long tusks.
The mane of the lion is absent in the lioness.

FIG. 13. — Great bird of Paradise, male and female.
(After Elliot.)

In birds there are many cases in which the sexes differ
in color (Figs. 13 and 14). The male is often more
brilliantly colored than the female and in other cases
the nuptial plumage of the male is quite different from
the plumage of the female. For example, the black
and yellow colors of the male bobolink are in striking
contrast with the brown-streaked female (Fig. 15).
The male scarlet tanager has a fiery red plumage with
black wings, while the female is olive green. The male

28

HEREDITY AND SEX

of the mallard duck has a green head and a reddish
breast (Fig. 16), while the female is streaked with brown.
In insects the males of some species of beetles have
horns on the head that are lacking in the female (Fig.
17). The males of many species of butterflies are col-
ored differently from the females.

FIG. 14. — White-booted humming bird, two males
and one female. (After Gould.)

The phosphorescent organ of our common firefly,
Photinus pyralis, is a beautiful illustration of a second-
ary sexual character. On the under surface of the male
there are two bands and of the female there is a single
band that can be illuminated (Fig. 18). At night the
males leave their concealment and fly about. A little
later the females ascend to the tops of blades of grass

THE EVOLUTION OF SEX

29

s

FIG. 15. — Male and female bobolink. (From
"Bird Lore.")

FIG. 16. — Male and female mallard duck. (From
" Bird Lore.")

30 HEREDITY AND SEX

and remain there without glowing. A male passes by
and flashes his light ; the female flashes back. In-
stantly he turns in his course to the spot whence the
signal came and alights. He signals again. She re-
plies. He ascends the blade, and if he cannot find her,
he signals again and she responds. The signals con-

FIG. 17. — Male and female Hercules
beetle. (After Kingsley.)

tinue until the female is found, and the drama of sex
is finished.

Mast has recently shown that the female firefly does
more than simply respond to the signal of the male.
If a male flies above and to the right of the female, she
bends her abdomen so that its ventral surface is turned
upward and to the right. If the male is above and to
the left, the light is turned in this direction. If the male

THE EVOLUTION OF SEX 31

is directly above, the abdomen of the female is twisted
almost upward. But if the male is below her, she emits
her light without turning the body. In the firefly the
evidence that the phosphorescent organ is of use in
bringing the sexes together seems well established.

FIG. 18. — Male and female firefly.

Whether all secondary sexual organs are useful in
mating is a question that must be referred to a later
chapter.

THE SEXUAL INSTINCTS

Side by side with the evolution of these many kinds
of structural difference the sexual instincts have evolved.
It is only in the lowest forms that the meeting of the
egg and sperm is left to chance. The instincts that
bring the males and females together at the mating
season, the behavior of the individuals at this time in

32 HEREDITY AND SEX

relation to each other, forms one of the most curious
chapters in the evolution of sex, for it involves court-
ship between the males and females; the pairing or
union of the sexes and subsequently the building of
the nest, the care, the protection and feeding of the
young, by one or both parents. The origin of these
types of behavior is part of the process of evolution of
sex ; the manner of their transmission in heredity and
their segregation according to sex is one of the most
difficult questions in heredity — one about which noth-
ing was known until within recent years, when a
beginning at least has been made.

A few samples taken almost at random will illustrate
some of the familiar features in the psychology of sex.
Birds have evolved some of the most complicated types
of courtship that are known. It is in this group, too,
as we have seen, that the development of secondary
sexual characters has reached perhaps its highest types.
But it is not necessarily in the species that have the
most striking differences between the sexes that the
courtship is most elaborate. In pigeons and their
allies, for example, the courtship is prolonged and elab-
orate, yet the males and females are externally al-
most indistinguishable ; while in the barnyard fowl and
in ducks the process is relatively simple, yet chanti-
cleer is notoriously overdressed.

Even in forms so simply organized as the fishes it is
known that the sexual instincts are well developed.
In the common minnow, fundulus, the males develop in
the breeding season elaborate systems of tactile organs.
The male swims by the side of the female, pressing
his body against her side, which causes her to set free

THE EVOLUTION OF SEX 33

a few eggs. At the same time the male sets free the
sperm, thereby increasing the chance that some of the
spermatozoa will reach the egg.

In bees, the sexual life of the hive is highly special-
ized. Mating never occurs in the hive, but when the
young queen takes her nuptial flight she is followed by
the drones that up to this have led an indolent and use-
less life in the colony. Mating occurs high in the air.
The queen goes to the new nest and is followed by a
swarm of workers who construct for her a new home.
Here she remains for the rest of her life, fed and cared
for by the workers, who give her the most assiduous
attention — an attention that might be compared to
courting were it not that the workers are not males
but only immature females. The occurrence of these
instincts in the workers that never leave or rarely at
least leave offspring of their own is a special field of
heredity about which we can do little more than specu-
late. This much, however, may be hazarded. The
inheritance of the queen and of the worker is the same.
We know from experimental evidence that the amount
of food given to the young grub, when it hatches from
the egg, is the external agent that makes the grub a
queen or a worker. In the worker the sex glands are
little developed. Possibly their failure to develop may
in part account for the different behavior of the workers
and of the queen. I shall devote a special chapter to
this question of the influence of the secretions of the
sex glands or reproductive organs on the character of
the body. We shall see that in some animals at least
an important relation exists between them.

In the spiders the mating presents a strange spectacle.

34 HEREDITY AND SEX

Let us follow Montgomery's careful observations on
Phidippus purpuratus. The male spun a small web
of threads from the floor to one side of his cage at an
angle of 45°. "Four minutes later he deposited a
minute drop of sperm on it, barely visible to the naked
eye ; then extending his body over the web reached his
palpi downwards and backwards, applying them al-
ternately against the drop ; the palpal organs were
pressed, not against the free surface of the drop, but
against the other side of the web." Later, a minute
drop of sperm is found sticking to the apex of one of the
palpi. In 1678 Lister had shown that the male applies
his palpi to the genital aperture of the female ; but not
until 1843 was it found by Menge that the palpi carry
the sperm drop.

In man, courtship may be an involved affair. Much
of our literature revolves about this period, while paint-
ing and sculpture take physical beauty as their theme.
Unsatiated with the natural differences that distinguish
the sexes, man adds personal adornment which reaches
its climax in the period of courtship, and leaves a
lasting impression on the costuming of the sexes.
Nowhere in the animal kingdom do we find such a
mighty display; and clothes as ornaments excel the
most elaborate developments of secondary sexual char-
acters of creatures lower in the scale.

I have sketched in briefest outline some of the gen-
eral and more familiar aspects of sex and the evolution
of the sexes. In the chapters that follow we shall take
up in greater detail many of the problems that have
been only touched upon here.

CHAPTER II
THE MECHANISM OF SEX-DETERMINATION

IN many species of animals and plants two kinds of
individuals are produced in every generation. This
process occurs with such regularity and persistence that
our minds naturally seek some mechanism, some sort
of orderly machinery, by which this condition is brought
about. Yet from the time of Aristetle alniQst to the
present day the problem has baffled completely all
attempts at its solution. However, the solution is very
simple. Now that we hold the situation in our grasp,
it seems surprising that no one was keen enough to
deduce it by purely theoretical reasoning. At least
the general principles involved might have been de-
duced, although we can see that without an intimate
knowledge of the changes that take place in the germ-
cells the actual mechanism could never have been
foretold.

The bodies of animals and plants are composed of
millions of protoplasm-filled compartments that are
called cells. In the middle of each cell there is a sphere,
or nucleus, containing filaments called chromosomes
(Fig. 5).

At each division of a cell the wall of the nucleus is
absorbed, and the thread-like chromosomes contract
into rod-shaped, or rounded bodies (Fig. 6). Each
chromosome splits lengthwise into halves ; the halves

35

36 HEREDITY AND SEX

are brought into relation with a spindle-shaped system
of lines, and move apart along these lines to opposite
sides of the cell. The protoplasm of the cell next con-
stricts to produce two daughter cells, each containing
a group of daughter chromosomes.

4 *

— v

$•

••* — ^m

•.

_•

• A • w • •

:^-

• «.•••

%-A. .«vV

^.j^ ••r#*.<i;%<

FIG. 19. — Fertilization and polar-body formation of Nereis. The
four smaller figures show entrance of sperm. The extrusion of the first
polar body is shown in lower left-hand figure and of the second polar body
in the two large right-hand figures. The last three also show the formation
of the sperm asters, which is the beginning of the first cleavage spindle in
the egg. (After F. R. Lillie.)

The egg is also a cell, and in its earlier stages contains
the same number of chromosomes as do the other cells of
the body; but after two peculiar divisions that take
place at maturation the number of the chromosomes is
reduced to half.

THE MECHANISM OF SEX-DETERMINATION 37

But before this time the egg-cells divide, like all the
other cells of the body. In this way a large number
of eggs is produced. After a time they cease to divide
and begin to grow larger, laying up yolk and other
materials. At this time, the chromosomes unite in
pairs, so that their number seems to be reduced to half
the original number. At the final stage in the matura-
tion of the egg, two peculiar divisions take place that
involve the formation of two minute cells given off at
one pole — the polar bodies. In some eggs, as in the
sea urchin, the polar bodies are given off while the egg
is still in the ovary and before fertilization; in other
eggs, as in the frog, one polar body is given off before
fertilization, the other after the sperm has entered ;
and in other eggs, as in nereis (Fig. 19), both polar
bodies are given off after fertilization.

The formation of the polar bodies is a true cell-
division, but one that is unique in two respects.
First, one of the cells is extremely small, as seen in
Fig. 19. The smallness is due to the minute amount
of protoplasm that it contains. Second, the number of
chromosomes at each division is the half or "haploid "
number. There is much evidence to show that at one
or at the other of these two divisions the two chromo-
somes that had earlier united are separated, and in this
respect this division differs from all other cell-divisions.
In consequence, the egg nucleus, that re-forms after the
second polar body has been produced, contains only
half the actual number of chromosomes characteristic
of all the other cells of the female.

In the formation of the spermatozoa a process takes
place almost identical with the process just described

38

HEREDITY AND SEX

for the female (Fig. 20). In their earlier history the
germ-cells of the male divide with the full number of
chromosomes characteristic of the male, which may be
one less chromosome than in the female. The early

FIG. 20. — A-B, somatic cell division with four chromosomes. C-H,
the two maturation divisions to produce the four cells (H) that become
spermatozoa. (After Wilson.)

germ-cells then cease to divide for a time, and begin
to grow, laying up yolk and other materials. At this
time the chromosomes unite in pairs, so that the num-
ber appears to be reduced to half. Later two divisions
occur (Fig. 20, D-H), in one of which the united chro-
mosomes separate. The male germ-cells differ, how-

THE MECHANISM OF SEX-DETERMINATION 39

ever, from the female, in that at each of these two di-
visions the cells are equal in size. Thus four sperm-
cells are produced from each original cell, all four pro-
duce tails, and become spermatozoa*

At the time of fertilization, when the spermatozoon
touches the surface of the egg, the egg pushes out a cone
of protoplasm at the point of contact (Fig. 19), and,
lending a helping hand, as it were, to the sperm, draws
it into the egg. The projecting cone of protoplasm
is called the fertilization cone. In a few minutes the
head of the sperm has entered. Its tail is often left
outside. The head absorbs fluid from the egg and
becomes the sperm nucleus, which passes towards the
center of the egg. Here it comes to lie by the side of the
egg nucleus, and the two fuse. The walls of the com-
bined nuclei dissolve away and the chromosomes appear.
Half of these are derived from the father through
the nucleus of the sperm, and half from the mother
through the egg nucleus. If we count the paternal
chromosomes, there are half as many of them as there
are chromosomes in each cell of the body of the fattier.
Presently I shall point out 'that this statement is not
always true, and on this little fact, thatrit is not quite
true, hangs the whole story of sex-determination.

What is the meaning of these curious changes that
have taken place in the egg and sperm ? Why has the
egg, before developing, twice thrown away its most
valuable heritage — its chromatin material ? We do
not know with certainty, but one consequence at least
stands out clearly ! Before the egg gave off its polar
bodies it had the full, or diploid, number of chromo-
somes. After this event it has only half as many. A

40 HEREDITY AND SEX

similar reduction occurs in the sperm, excepting that no
chromatin is lost, but is redistributed amongst four
spermatozoa. Egg and sperm-nucleus each have in
consequence the haploid or half number. By combin-
ing they bring up the number to that characteristic of
the species.

The history of the germ-cells, that we have just
traced, is the background of our knowledge of the pro-
cess of heredity in so far as observable changes in the
germ-cells have been made out. We owe to Weismann
more than to any other biologist the realization of
the importance of these changes. It is true that
Weismann contributed only a part of the actual facts
on which the interpretation rests. Many workers,
and a few leaders, have laboriously made out the com-
plete account. But Weismann, by pointing out the
supreme importance of the changes that take place at
this time, has furnished a stimulus that has acted like
yeast in the minds of less imaginative workers.

We are now in a position to apply this knowledge to
the interpretation of the mechanism by means of which
sex is determined.

THE CYTOLOGICAL EVIDENCE

If we study by means of modern histological methods
the body cells of the male of the insect, Protenor
belfragei, we find, when each cell is about to divide,
that a group of chromosomes appears like that shown
in Fig. 21, A. There are twelve ordinary oval chromo-
somes, and one much larger than the rest. This group
of chromosomes is characteristic of all divisions of the
cells of the body, regardless of whether the cells belong

* \si
^

THE MECHANISM OF SEX-DETERMINATION 41

to muscle, skin, gland, ganglion, or connective tissue.
The early germ-cells of the male, the so-called "sper-
matogonia, ' ' also have this same number. It is not until
a later stage in their development that a remarkable
change takes place in them. When this change occurs
the thread-like chromosomes unite in pairs. This is
the synapsis stage =>-the word means to fuse together.

It is the most difficult stage to interpret in the whole
history of the germ-cells. In a few forms where the
changes that take place have been seen to best advan-
tage it is found that chromosomes are in the form of
long threads and that these threads unite in pairs to
make thicker threads. When the process is completed,
we find half as many threads as there were before. This
statement is not quite true. In the case of the
male protenor, for instance, there are twelve ordinary
chromosomes and one large one* The twelve unite in
pairs at synapsis, so that there are six double chromo-
somes, but the large one has no mate (Fig. 21, B).
When the others have united in synapsis, it has taken
no Par^ in the process, hence the reduced number, of
chromosomes in the male is seven — the seventh is
the sex chromosome.

Two divisions now follow each other in rapid succes-
sion (Fig. 21, C, D). In the first division (C) each
chromosome divides — seven go to one pole and seven
to the other pole. Two cells, the primary spermato-
cytes, are produced. Without resting^ another divi-
sion takes place (D) irTeach of these two cells. It is
the second spermatocyte division. Each of the six
ordinary chromosomes divides, but the large sex chro-
mosome does not divide, and, lagging behind the others,

42

HEREDITY AND SEX

as shown in the figure (Z>), it passes to one pole. Each
secondary spermatocyte produces, therefore, two cells —
one with six, the other with seven chromosomes. These
cells become spermatozoa (EEf), the ones with seven
chromosomes are the female-producing spermatozoa, the
ones with six chromosomes are the male-producing

Prote/nor 3

• •

D*

FIG. 21.

spermatozoa. These two classes of spermatozoa are
present in equal numbers.

If we study the body cells of the female protenor, we
find fourteen chromosomes (Fig. 22, A). Twelve of
these are the ordinary chromosomes, and two, larger
than the rest, are the sex chromosomes. At the synap-
sis stage all of the chromosomes uni^e in pairs, including
the two sex chromosomes. When the pracess ^.finished,
there are seven double chromosomes (Fife/. 2W, 3).

THE MECHANISM OF SEX-DETERMINATION 43

When the egg sends off its two polar bodies, the chro-
mosomes divide or separate. At the first division seven
chromosomes pass out (C), and seven remain in the
egg. At the next division the seven chromosomes in
the egg divide again, seven pass out and seven remain

Profenor •

in the egg (D). Of these seven, one chromosome,
recognizable by its large size, is the sex chromosome.

All the eggs are alike (E) . There is only one kind of
egg, but there are two kinds of sperm. Any egg that
is fertilized by a sperm carrying six chromosomes pro-
duces an individual with thirteen chromosomes. This
individual is a male.

Any egg that is fertilized by a sperm carrying seven

44

HEREDITY AND SEX

chromosomes produces an individual with fourteen
chromosomes. This individual is a female.

In another species of insect, Lygaeus bicrucis, the male
differs from the female, not in having a different

FIG. 23.

number of chromosomes as in protenor, but by the
occurrence of a pair of different-sized chromosomes.

The body cells of the male have twelve ordinary
chromosomes and two sex chromosomes — one larger,
X, than the other, Y (Fig. 23, A).

After synapsis there are six double chromosomes >and
the two sex chromosomes, called X and Y (Fig. 23, D).

THE MECHANISM OF SEX-DETERMINATION 45

At the first spermatocyte division all the chromosomes
divide (C). The two resulting cells have eight chro-
mosomes, including X and Y. At the second division
(D) the double chromosomes again divide, but X and Y
do not divide. They approach and touch each other,
and are carried into the spindle, where they separate
from each other when the other" ordinary chromosomes

/

divide. Consequently there are formed two kinds of
spermatozoa — one containing X and the other Y
(Fig. 23, E).

In the body cells and early germ-tract of the female.
of lygseus (Fig. 24, A), there are twelve ordinary
chromosomes and two sex chromosomes, K and X.
After reduction there are seven double chromosomes,
the two X's having united when the other chromosomes

46 HEREDITY AND SEX

united (B). Two divisions take place (C, D), when the
two polar bodies are formed, leaving seven chromosomes
in the egg (E) . Each egg contains as a result only one
X chromosome*

Any egg of lygseus fertilized by a sperm carrying an
X chromosome produces a female that contains two

&».

tar

FIG. 25.

X's or XX. Any egg fertilized by a sperm containing
a F chromosome produces a male that contains one
X and one F, or XF.

Another insect, Oncopeltus fasciatus, represents a
third type in which the chromosome groups in the
male and in the female are numerically alike and alike
as to visible size relations.

THE MECHANISM OF SEX-DETERMINATION 47

In the body cells of the male there are sixteen chro-
mosomes (Fig. 25, A). After reduction there are nine
chromosomes — seven in a ring and two in the middle
(B). The seven are the fused pairs or double chro-
mosomes ; the two in the middle are the sex chromo-
somes that have not fused.

The evidence for this interpretation is circumstan-
tial but sufficient.

At the first reduction division all nine chromosomes
divide (C). Just before the second division the two
central chromosomes come together and remain in
contact (DDf). All the double chromosomes then
divide, while the two sex chromosomes simply sepa-
rate from each other, so that there are eight chromo-
somes at each pole (DE).

48

HEREDITY AND SEX

In this case all of the spermatozoa (EE') contain
eight chromosomes. There is no visible difference
between them. Nevertheless, there is reason for be-
lieving that here also there are two kinds of sperm.
The principal reason is that there are all connecting
stages between forms in which there is an unequal pair,

FIG. 27.

as in lygseus, and forms with an equal pair, as in oncopel-
tus. Another reason is that the two sex chromosomes
behave during the synapsis stages as do the X Y chromo-
somes in related species. Moreover, the experimental
evidence, of which I shall speak later, leads us to con-
clude that the determination of sex is not due only to

THE MECHANISM OF SEX-DETERMINATION 49

a difference in size of X and Y . The sex chromosomes
must carry a host of factors other than those that de-
termine sex. Consequently it is not surprising that in
many species the sex chromosomes appear equal or
nearly equal in size. It is a fortunate circumstance for
us that in some species there is & difference in size or

an unpaired sex chromosome ; for, in consequence, we
are able to trace the history of each kind of sperm in
these cases ; but it is not essential to the theory that
X and Y, when present, should be visibly different.

In the female of oncopeltus sixteen chromosomes
occur as in the male (Fig. 26, A). The reduced number
is eight double chromosomes (B). At one of the two
polar divisions eight chromosomes pass out, and eight
remain in the egg (C). At the second division also
eight pass out, and eight remain in the egg (D).

50 HEREDITY AND SEX

I shall pass now to a fourth condition that has only
recently come to light. It is best shown in some of the
nematode worms, for example, in the ascaris of the
horse. Here the sex chromosomes are generally at-
tached to other chromosomes. In this case, as shown
by the diagram (Fig. 27, A), there is in the male a single
X attached to one of the other chromosomes. At the
first spermatocyte division it does not divide (C),
but passes over bodily to one pole, so that two kinds
of cells are produced. At the second spermatocyte
division it divides, in the cell that contains it, so that
each daughter cell gets one X (D). Two classes of
sperm result, two with X (E), two without (Ef).

In the female there are two X'Sj each attached to a
chromosome (Fig. 28). After the polar bodies are
given off, one X only is left in each egg (C, Z), E). Sex
is determined here in the same way as in the insects,
described above, for there are two classes of sperm and
but one class of eggs.

The discovery of the sex chromosome and its rela-
tion to sex is due to several investigators. In 1891
Henking first described this body, and its unequal distri-
bution, but was uncertain even as to its relation to the
chromosomes. Paulmier (1899), Montgomery (1901),
Sinety (1901), gave a correct description of its behavior
in spermatogenesis. McClung (1902) confirmed these
discoveries, and suggested that the accessory, or odd
chromosome, as it was then called, had some relation
to sex, because of its unequal distribution in the
sperms. He inferred that the male should have one
more chromosome than the female, but he gave no evi-
dence in support of this suggestion, which as we have

THE MECHANISM OF SEX-DETERMINATION 51

seen is the reverse of the actual conditions. Stevens
(1905) made out the relations of the XY pair of chro-
mosomes to sex and Wilson in the same year (1905)
the correct relation of the accessory chromosome to sex.
The results described above for the insects are for the
most part from Wilson's studies on the chromosomes ;
those for ascaris from the recent work of Sophia
Frolowa, which confirms in the main the work of Boveri,
Gulick, Boring, and Edwards.

In the fruit fly, Drosophila ampelophila, it appears
from the recent work of Metz, that, in the male, there
is an XY pair of sex chromosomes, instead of only an
X, as Stevens supposed. The female has, of course,
two X's. An analysis of certain experimental evi-
dence has led H. J. Muller to the conclusion that prob-
ably the Y chromosome carries no factors that influence
development. If this proves true, we can better under-
stand how it might be completely lost in certain types.

The whole history of the sex chromosomes of ancyro-
canthus, a nematode worm, is strikingly shown in a
recent paper by Carl Mulsow (Fig. 29 and 29a, A).
This is a typical case in which the male has one less
chromosome than the female, as in protenor. The
case is striking because the chromosomes can be seen
and counted in the living spermatozoa. Some sperm
have six, some have five chromosomes. The sperm-
nucleus can be identified in the egg after fertilization
because it lies nearer the pole opposite to the polar
bodies. The entering sperm nuclei show in half of
the fertilized eggs six chromosomes and in the other
half five chromosomes.

An interesting confirmation of these conclusions in

52 HEREDITY AND SEX

regard to the relation between sex and the sex chromo-
somes was found in another direction. It has long been
known that the fertilized eggs of aphids or plant lice
produce only females. The same thing happens in
near relatives of the plant lice, the phylloxerans.

-y-

2

<» t

,<!t '•'".

FIG. 29. — 1 and 2 are spermatogonia ; 3, growth period ; 4-7, prophases ;
8, equatorial plate of first division, 9-10 ; 11, spermatocytes of second order ;
12-13, division of same; 14-16, the four cells or spermatids that come
from the same original cell, two with 5, two with 6 chromosomes; 17,
spermatids; 18, mature sperm; 19, living sperm. (After Mulsow.)

In these insects a study of the chromosomes shows
that the male has one less chromosome than the female.
At the first maturation division in the male (Fig. 30),
all the chromosomes divide except one, the X chromo-
some, and this passes to one cell only. This cell is
also larger than the sister cell. The small cell lacking
the X degenerates, and does not produce spermato-

THE MECHANISM OF SEX-DETERM I NATION 53

zoa. The large cell divides again, all of the chromo-
somes dividing. Two functional spermatozoa are
produced, each carrying one sex chromosome. These
spermatozoa correspond to the female-producing sper-
matozoa of other insects.

In the sexual female there is an even number of chro-

•

Oil

•)

FIG. 29a. — 20 and 21, oogonia (equatorial plate); 22, growth period;
23, before fertilization: 24-25, entrance of sperm; 26-31, prophases of
first division ; 32—33, formation of first polar body ; 34—36, extrusion of
same and formation of second polar body ; 37, two pronuclei ; 38-41, union
of pronuclei ; 42-45, cleavage. (After Mulsow.)

mosomes — one more than in the male. They unite
in pairs. When the two polar bodies of the sexual
egg are formed, all the chromosomes divide twice, so
that each egg is left with one sex chromosome.

It is now evident why only females are produced
after fertilization. The female-producing sperm alone
is functional.

54

HEREDITY AND SEX

FIG. 30. — Diagram of chromosomes in Phylloxera, carycecaulis. Top
line, somatic cell of female with 6 chromosomes and somatic cell of male
with 5 chromosomes. Second line, stages in first spermatocyte division
producing a rudimentary cell (below) with two chromosomes. Third line,
second spermatocyte division into two equal cells. Fourth line, sexual
egg (3 chromosomes) and two polar bodies ; and two functional, female-
producing sperm with three chromosomes each.

THE MECHANISM OF SEX-DETERMINATION 55
THE EXPERIMENTAL EVIDENCE

The experimental evidence, indicating that there is
an internal mechanism for sex determination, is derived
from two sources — from experimental embryology, and
from a study of the heredity of sex-linked characters.

The evidence from embryology shows that the chro-
mosomes are the bearers of materials essential for the
production of characters. The evidence from hered-
ity shows that certain characters follow^ the sex
chromosomes.

It has long been taught that the hereditary factors
are carried by the nucleus. The evidence for this was
found in fertilization. When the spermatozoon enters
the egg, it carries in, as a rule, only the head of the sper-
matozoon, which consists almost entirely of the nucleus
of the original cell from which it comes. Since the
male transmits his characters equally with the female,
it follows that the nucleus is the source of this
inheritance.

The argument has not been regarded as entirely
conclusive, because the sperm may also bring in some
of the protoplasm of the original cell — at least that part
lying immediately around the nucleus. In addition a
small body lying at the base of the sperm head seems
also to be brought in by the male, and according to
some observers it becomes the center about which the
entire division system or karyokinetic spindle develops.

The most convincing evidence that the chromosomes
are the most important elements in heredity is found in
some experimental work, especially that of Boveri,
Baltzer, and Herbst. Under certain circumstances in

56

HEREDITY AND SEX

the sea-urchin two spermatozoa may enter a single
egg. They both unite with the egg nucleus (Fig. 31).
Each brings in 18 chromosomes. The egg contributes
18 chromosomes. There are in all 54, instead of 36
chromosomes, as in normal fertilization.

FIG. 31. — Dispermy and its effects in egg of sea urchin. (After
Boveri.)

Around these chromosomes a double system of
threads develops with four poles. The chromosomes
become unequally distributed on the four spindles that
develop. Each chromosome then divides, and half of
each goes to the nearest pole. To some of the poles
many chromosomes may pass, to other poles fewer.

THE MECHANISM OF SEX-DETERMI NATION 57

In order to simplify the case let us imagine that each
sperm has only four chromosomes and the egg nucleus
only four. Let us represent these by the letters as
shown in Fig. 32. Any one of the four cells that is

FIG. 32. — Diagram illustrating the irregular distribution of the chro-
mosomes in dispermic eggs in an imaginary case with only four kinds of
chromosomes, a, b, c, d. There are here three sets of each of these in
each egg. The stippled cells are those that fail to receive one of each
kind of chromosome. (After Boveri.)

produced at the first division of these dispermic eggs
may contain a full complement of the chromosomes,
or only some of them. The possibilities for four
chromosomes are shown in the diagram. Any cell
that does not contain at least these four chromosomes
is shaded. One case is present in which all the four

58 HEREDITY AND SEX

cells contain a complete assortment. If normal devel-
opment depends on an embryo containing in every cell
at least one of each kind of chromosome, then in
our simple case only one group of four cells has this
possibility.

Boveri found that such dispermic eggs produce
normal embryos very rarely. He calculated what the
chance would be when three times 18 chromosomes
are involved. The chance for normal development
is probably not once in 10,000 times. He isolated
many dispermic eggs and found that only one in 1,500
of the tetrad type developed normally.

Boveri went still further in his analysis of the prob-
lem. It had been shown for normal eggs that if at
the two-celled stage the cells are separated, each forms
a perfect embryo. This is also true for each of the
first four cells of the normal egg.

Boveri separated the four cells of dispermic eggs and
found that the quadrants not infrequently developed
normally. This is what we should anticipate if those
cells can develop that contain one of each kind of
chromosome.

The evidence furnishes strong support of the view
that the chromosomes are different from each other,
and that one of each kind is necessary if development
is to take place normally.

The evidence that Baltzer has brought forward is
also derived from a study of sea-urchin eggs. It is
possible to fertilize the eggs of one species with sperm
of another species. The hybridizing is greatly helped
by the addition of a little alkali to the sea water.

Baltzer made combinations between four species of

THE MECHANISM OF SEX-DETERMIXATION 59

sea-urchins. We may take one cross as typical. When
eggs of strongylocentrotus are fertilized with sperm of
sphserechinus, it is found at the first division of the egg
that, while some of the chromosomes divide and pass
normally to the two poles, other chromosomes remain
in place, or become scattered irregularly between the
two poles, as shown in Fig. 33. When the division

;•

FIG. 33. — 1 and la, chromosomes in the normal first cleavage spindle of
Sphaerechinus ; 2, equatorial plates of two-cell stage of same ; 3-3a, hybrid,
Sphaerechinus by Strongylocentrotus, spindle at two-cell stage ; 4-4a, same
equatorial plates ; 5-5a, hybrid, Strong, by Sphser., cleavage spindle in telo-
phase ; 6, next stage of last ; 7, same, two-cell stage ; 8, same, later ; 9, same,
four-cell stage ; 10, same, equatorial plate in two-cell stage (12 chromosomes) ;
11, same, from later stage, 2-4 chromosomes. (After Baltzer.)

is completed, some of these chromosomes are found
outside of the two main nuclei. They often appear
as irregular granules, and show signs of degeneration.
They are still present as definite masses after the next
division, but seem to take no further part in the de-
velopment.

Baltzer has attempted to count the number of chro-
mosomes in the nuclei of these hybrid embryos. The

60 HEREDITY AND SEX

number is found to be about twenty-one. The maternal
egg nucleus contains eighteen chromosomes. It appears
that only three of the paternal chromosomes have
succeeded in getting into the regular cycle — fifteen of
them have degenerated.

Baltzer thinks that the egg acts injuriously in this
case on the chromosomes of foreign origin, especially
on the fifteen that degenerate, so that they are elim-
inated from the normal process.

The embryos that develop from these eggs are often
abnormal. A few develop as far as the pluteus stage,
when a skeleton appears that is very characteristic for
each species of sea-urchin. The plutei of these hybrids
~are entirely maternal. This means that they are
exactly like the plutei of the species to which the
mother belongs.

The conclusion is obvious. The sperm of sphserechi-
nus has started the process of development, but has
produced no other effect, or has at most only slightly
affected the character of the offspring. It is reason-
able to suppose that this is because of the elimination
of the paternal chromosomes, although the evidence
is not absolutely convincing.

Let us now examine the reciprocal cross. When the
eggs of sphserechinus are fertilized by the sperm of
strongylocentrotus, the division of the egg and of the
chromosomes is entirely normal. All the chromosomes
divide and pass to the poles of the spindle. The total
number (36) must, therefore, exist in each cell, although
in this case they were not actually counted.

The pluteus that develops has peculiarities of both
maternal and paternal types. It is hybrid in structure.

THE MECHANISM OF SEX-DETERMINATION 61

Both parents have contributed to its formation. It is
not going far from the evidence to infer that the hybrid
character is due to both sets of chromosomes being
present in all of the cells.

FIG. 34. — 1. The chromosomes of the egg lie in the equator of the
spindle, the chromosomes of the sperm lie at one side. 2. A later stage,
showing all the paternal chromosomes passing to one pole. 3 (to the right).
A later stage, a condition like the last. There is also a supernumerary sperm
in the egg (to left, in another section.) 4. Same condition as last. 5. Plu-
teus larva that is purely maternal on one side and hybrid on the other.
(After Herbst.)

The evidence that Herbst has brought forward is of
a somewhat different kind. It supplements Baltzer's
evidence and makes more probable the view that the
chromosomes are essential for the development of the
characters of the individual.

62 HEREDITY AND SEX

Herbst put the eggs of sphaerechinus into sea water
to which a little valerianic acid had been added. This
is one of the many methods that Loeb has discovered
by which the egg may be induced to develop parthe-
nogenetically, i.e. without the intervention of the sper-
matozoon. After five minutes the eggs were removed to
pure sea water and the sperm of another species, stron-
gylocentrotus, was added. The sperm penetrated some
of the eggs. The eggs had already begun to undergo
the changes that lead to division of the cell — the sperm
entered ten minutes late. The egg proceeded to
divide, the sperm failed to keep pace, and fell behind.
Consequently, as shown in Fig. 34, the paternal
chromosomes fail to reach the poles when the nuclei
are re-formed there. The paternal chromosomes form
a nucleus of their own which comes to lie in one of the
two cells. In consequence one cell has a nucleus that
contains only the maternal chromosomes ; the other
cell contains two nuclei, one maternal and the other
paternal. In later development the paternal nucleus
becomes incorporated with the maternal nucleus of its
cell. There is produced an embryo which is maternal
on one side and hybrid on the other. Herbst found in
fact that half-and-half plutei were not rare under the
conditions of his experiment.

This evidence is almost convincing, I think, in
favor of the view that the chromosomes are the es-
sential bearers of the hereditary qualities. For in
this case, whether the protoplasm of the embryo
comes from the egg or the sperm also, the fact re-
mains that the half with double nuclei is hybrid.
Even if the spermatozoon brought in some proto-

-

THE MECHANISM OF SEX-DETERMINATION 63

plasm, there is no reason to suppose that it would
be distributed in the same way as are the paternal
chromosomes!

EVIDENCE FROM SEX-LINKED INHERITANCE

The experimental evidence based on sex-linked in-
heritance may be illustrated by the following examples.

The eyes of the wild fruit-fly, Drosophila ampe-
lophila, are red. In my cultures a male appeared that
had white eyes. He was mated to a red-eyed female.
The offspring were all red-eyed j~- both males and
females (Fig. 35). These were inbred and produced
in the next generation red*-eyed females, red-eyed males,
and white-eyed males (Fig. 35). There were no white-
eyed females. The white-eyed grandfather had trans-
mitted white eyes to half of his grandsons but to none
of his granddaughters.-

Equally important are the numerical proportions
in which the colors appear in the grandchildren. There
are as many females as the two classes of males taken
together; half of the males have red eyes and half
have white eyes. The proportions are therefore 50 %
red females, 25 % red males, 25 % white males.

Only white-eyed males had appeared at this time.
It may seem that the eye color is confined to the male
sex* Hence the origin of the term sex-limited inheri-
tance for cases like this. But white-eyed females may
be produced easily. If certain of the rted-eyed grand-
daughters are bred to these white-eyed males, both
white-eyed females and males, and red-eyed females
and males, appear (Fig. 37). The white eye is there-
fore not sex-limited but sex-linked^

64

HEREDITY AND SEX

With these white-eyed females it is possible to make
the reciprocal cross (Fig. 36). A white-eyed female
was mated to a red-eyed male,. All of the daughters
had red eyes and all the sons had white eyes. These
were then inbred and gave red-eyed males and females,,

xx

FIG. 35. — Sex-linked inheritance of white and red eyes in Drosophila.
Parents, white-eyed $ and red-eyed 9 ; Fi, red-eyed $ and 9 ; F2 red-
eyed 9 > red-eyed $ and white-eyed $ . To right of flies the history of the
sex chromosomes XX is shown. The black X carries the factor for red
eyes, the open X the factor for white eyes ; the circle stands for no X.

and white-eyed males and females in equal numbers
(Fig. 36).

The heredity of this eye color takes place with the
utmost regularity, and the results show that in some
way the mechanism that is involved is closely bound
up with the mechanism that produces sex.

THE MECHANISM OP SEX-DETERMIXATlOX 65

Other combinations -give results that are predictable
from those just cited. For instance, if the FI red-eyed
female from either of the preceding crosses is mated to
a white-eyed male, she produces red-eyed males and
females, and white-eyed males and females, as shown in

w

/v

FIG. 36. — Reciprocal cross of Fig. 35, Parents, white-eyed 9 and

red-eyed $, (criss-cross inheritance). FI, red-eyed 9. white-eyed $.

Y-, red-eyed 9 and $ ; white-eyed 9 and $ . To right, sex chromo-
somes (as in Fig. 35).

Fig. 37 (upper two lines). If the FI red-eyed male
from the first cross (Fig. 35) is bred to a white-eyed
female, he will produce red-eyed daughters and white-
eyed sons. Fig. 37 (lower two lines).

The same relations may next be illustrated by an-
other sex-linked character.

66

HEREDITY AND SEX

A male with short or miniature wings appeared in
my cultures (Fig. 38). Mated to long- winged females
only long-winged offspring were produced. When
these were mated to each other, there were produced

x

FIG. 37. — Upper series, back cross of FI 9 to white $ . Lower series
back cross of FI red-eyed $ to white 9 •

long-winged females (50%), long- winged males (25%)
and miniature-winged males (25%).

It is possible to produce, in the way described for
eye color, miniature-winged females.

When such miniature-winged females are mated to
long- winged males, all the daughters have long wings,
and all the sons have miniature wings (Fig. 39). If

THE MECHANISM OR SEX-DETERMINATION 67

these are now mated, they produce, in equal numbers,
long-winged males and females and miniature-winged
males and females.

The same relations may again be illustrated by body
color.

xx

XXXIX®

FIG. 38. — Sex-linked inheritance of short ("miniature") and long wings
in Drosophila. Parents, short -winged $ , long-winged 9 • Fi long-winged
$ and 9 • F2 long-winged 9 and $ and short-winged $ . Sex chromo-
somes to right. Open X carries short wings.

A male appeared with yellow wings and body. Mated
to wild gray females he produced gray males and
females. These mated to each other gave gray females
(50%), gray males (25%), and yellow males (25%).

As before, yellow females were made up. Mated to
gray males they gave gray females and yellow males.

68

HEREDITY AND SEX

These inbred gave gray males and females and yellow
males and females, in equal numbers.

These cases serve to illustrate the regularity of this
type of inheritance and its relation to sex. In the fruit
fly we have found as many as twenty-five sex-linked

FIG. 39. — Reciprocal cross of Fig. 38. Parents, long-winged $ and
short-winged 9 . Fi long-winged 9 . short-winged $ . F2 long-winged
9 and $ , short-winged 9 and $ • Sex chromosomes as in last.

factors. There are other kinds of inheritance found in
these flies, and at another time I shall speak of some
of these ; but the group of sex-linked factors is of special
importance because through them we get an insight
into the heredity of sex.

In the next chapter, when we take up in detail Men-
delian heredity, I shall try to go further into the ex-

THE MECHANISM OF. SEX-DETERMINATION 69

planation of these facts^ For the present it will suffice
to point out that the cases just described, and all like
them, may be accounted for by means of a very simple
hypothesis. We have traced the history of the X
chromosomes. If the factors that produce white eyes,
short (miniature) wings, and yellow body color are
carried by the X chromosomes, we can account for
these results that seem at first sight so extraordinary.
The history of the sex chromosomes is accurately known.
Their distribution in the two sexes is not a matter of
conjecture but a fact. Our hypothesis rests therefore
on a stable foundation..

At the risk of confusion I feel bound to present here
another type of sex-linked inheritance* In principle
it is like the last, but the actual mechanism, as we shall
see, is somewhat different. Again I shall make use
of an illustration, If a black Langshan hen is mated
to a barred Plymouth Rock cock, all the offspring will
be barred (Fig. 40) . If these are inbred, there are pro-
duced barred females and males, and black females.
The numerical proportion is 50 per cent barred males, 25
per cent barred females, and 25 per cent black females.

The black hen has transmitted her character to half
of her granddaughters and to none of her grandsons.
The resemblance to the case of the red-eyed, white-
eyed flies is obvious, but here black appears as a sex-
linked character in the females.

The converse cross is also suggestive. When a
barred hen is mated to a black cock, all the daughters
are black and all the sons are barred (Fig. 41). When
these are inbred, there are produced black males and
females and barred males and females in equal num-

70

HEREDITY AND SEX

bers. Again, the resemblance of the reciprocal cross
to one of the combinations for eye color is apparent.
In fact, this case can be explained on the same prin-
ciple as that used for the flies, except that in birds it is

FIG. 40. — Sex-linked inheritance in fowls. Upper line black Laugshan
hen and barred Plymouth Rock cock. Second line, Flt barred cock and
hen. Third line, F2, three barred (cock, hen, cock) and one black (hen).
(Cuts from " Reliable Poultry Journal.") F\ and F2 for color only.

THE MECHANISM OE SEX-DETERMINATION 71

the female that produces two kinds of eggs; she is
heterozygous for a sex factor while the male produces
only one kind of spermatozoon.

FIG. 41. — Reciprocal cross of Fig. 40. Upper line, black cock and
barred hen. Second line, Fi, barred cock and black hen. Third line, F2,
barred hen and cock, black cock and hen. (Cuts from " Reliable Poultry
Journal.") FI and FZ correct for color pattern only.

72 HEREDITY AND SEX

We lack here the certain evidence from cytology that
we have in the case of the insects. Indeed, there is
some cytological evidence to show that the male bird
is heterozygous for the sex chromosome. But the
evidence does not seem to me well established ; while
the experimental evidence is definite and has been
independently obtained by Bateson, Pearl, Sturtevant,
Davenport, Goodale and myself. However this may be,
the results show very clearly that here also sex is con-
nected with an internal mechanism that is concerned
with other characters also. This is the mechanism of
Mendelian heredity. Whether the chromosomes suffice
or do not suffice to explain Mendelian heredity, the
fact remains that sex follows the same route taken by
characters that are recognized as Mendelian.

To sum up : The facts that we have considered
furnish, I believe, demonstrative evidence in favor
of the view that sex is regulated by an internal mech-
anism. The mechanism appears, moreover, to be the
same mechanism that regulates the distribution of cer-
tain characters that follow Mendel's law of inheritance.

CHAPTER III

THE MENDELIAN PRINCIPLES OF HEREDITY AND
THEIR BEARING ON SEX

THE modern study of heredity dates from the year
1865, when Gregor Mendel made his famous discoveries
in the garden of the monastery of Briinn. For 35
years his paper, embodying the splendid results of his
work, remained unnoticed. It suffered the fate that
other fundamental discoveries have sometimes met.
In the present case there was no opposition to the
principles involved in Mendel's discovery, for Darwin's
great work on " Animals and Plants" (1868), that dealt
largely with problems of heredity, was widely read and
appreciated. True, Mendel's paper was printed in
the journal of a little known society — the Natural
History Society of Briinn, — but we have documentary
evidence that his results were known to one at least of
the leading botanists of the time.

It was during these years that the great battle for
evolution was being fought. Darwin's famous book on
"The Origin of Species" (1859) overshadowed all else.
Two systems were in deadly conflict — the long-ac-
cepted doctrine of special creation had been challenged.
To substitute for that doctrine the theory of evolution
seemed to many men of science, and to the world at
large, like a revolution in human thought. It was in
fact a great revolution. The problems that bore on the

73

74 HEREDITY AND SEX

question of how the higher animals and plants, and
man himself, arose from the lower forms seemed the
chief goal of biological work and thought. The out-
come was to establish the theory of evolution. The
circumstantial evidence that was gathered seemed so
fully in accord with the theory of evolution that the
theory became widely accepted. The acute stage was
passed, and biologists found themselves in a position
to examine with less haste and heat many other phe-
nomena of the living world equally as important as
evolution.

It gradually became clear, when the clouds of con-
troversy had passed, that what I have ventured to call
the " circumstantial evidence "on which the theory of
evolution so largely rested, would not suffice as a direct
proof of evolution. Investigation began to turn once
more to that field of observation where Darwin had
found his inspiration. The causes of variations and
the modes of inheritance of these variations, the very
foundations of the theory of evolution, were again
studied in the same spirit in which Darwin himself had
studied them. The return to Darwin's method rather
than to Darwin's opinions marks the beginning of the
new era.

In 1900 three botanists were studying the problem
of heredity. Each obtained evidence of the sort
Mendel had found. Happily, Mendel's paper was
remembered. The significance of his discovery now
became apparent. De Vries, Correns, and Tschermak
brought forward their evidence in the same year (1900).
Which of the three first found Mendel cannot be stated,
and is of less importance than the fact that they ap-

THE MENDELIAN PRINCIPLES OF HEREDITY 75

predated the significance of his work, and realized
that he had found the key to the discoveries that they
too had made. From this time on the recognition of
Mendel's discovery as of fundamental importance was
assured. Bateson's translation of his paper made
Mendel's work accessible to English biologists, and
Bateson's own studies showed that Mendel's principles
apply to animals as well as to plants.

THE HEREDITY OF ONE PAIR OF CHARACTERS

Mendel's discovery is sometimes spoken of as Men-
del's Principles of Heredity and sometimes as Mendel's
Law. The former phrase gives a better idea perhaps
of what Mendel really accomplished, for it is not a little
difficult to put his conclusions in the form of a law.
Stated concisely his main discovery is this : — in the
germ-cells of hybrids there is a free separation of the
elements derived from the two parents without regard to
which parent supplied them.^

An example will make this more obvious. Mendel
crossed an edible pea belonging to a race with yellow
seeds to a pea belonging to a race with green seeds
(Fig. 42). The offspring or first filial generation (Fi)
had seeds all of which were yellgw. When the plants
that bore these seeds were self-fertilized, there were
obtained in the next generation, F2r both yellow and
green peas in the proportion of 3 yellows to 1 green
(Fig. 42). This is the well-known Mendelian ratio
of 3:1.

The clue to the meaning of this ratio was found when
the plants of the second generation (F2) were selfbred.
The green peas bred true ; but the yellows were of two

76

HEREDITY AND SEX

kinds — some produced yellow and green seeds again
in the ratio of 3 : 1 ; others produced only yellow
peas. Now, if the yellows that bred true were counted,
it was found that the number was but one-third of
all the yellows.

FIG. 42. — Illustrating Mendel's cross of yellow (lighter color) and greeu
(dark color) peas.

THE MEXDELIAX PRINCIPLES OF HEREDITY 77

In other words, it was shown that the ratio of 3 yel-
lows to 1 green was made up of 1 pure yellow, 2 hy-
brid yellows, 1 pure green. This gave a clue to the
principles that lay behind the observed results.

Mendel's chief claim to fame is found in the discovery
of a simple principle by means of which the entire
series of events could be explained. He pointed out
that if the original parent with yellow (Pi) carried
something in the germ that made the seed yellow, and
the original parent with green seeds (Pi) carried some-
thing that made the seed green, the hybrid should con-
tain both things. If jboth being present^ one domi-
nates the other or gives color to the pea, all the peas in
the hybrid generation will be of one color — yellow in
this case. Mendel assumed that in the germ-cells of
these hybrids the two factors that make yellow and
green separate, so that half of the germ-cells contain
yellow-producing material, and half contain green-
producing material. This is Mendel's principle of
separation or segregation. It is supposed to occur
both in the male germ-cells of the hybrid flower, i.e.
in the anthers, and also in the ovules. If self-fertili-
zation occurs in such a plant, the following combina-
tions are possible : A yellow-bearing pollen grain may
fertilize a " yellow" ovule or it may fertilize a "green"
ovule. The chances are equal. If the former occurs,
a pure yellow-seeded plant will result ; if the latter a
hybrid yellow-seeded plant. The possible combina-
tions for the green-producing pollen are as follows : A
" green" pollen grain may fertilize a "yellow" ovule,
and produce a hybrid, yellow-seeded plant, or it may
fertilize a "green" ovule, and produce a green-seeded

78 HEREDITY AND SEX

plant. If these meetings are random, the general or
average outcome will be : 1 pure yellow, 2 hybrid
yellows, and 1 pure green.

It is now apparent why the pure yellows will always
breed true, why the yellow-greens will split again into
yellows and greens (or 1:2:1), and why the pure
greens breed true. By this extremely simple assump-
tion the entire outcome finds a rational explanation.

o

F.1

t

0

FIG. 43. — " Checker " diagram to show segregation and recombination of
factors. In upper line, a black bearing gamete is supposed to unite with a
white bearing gamete to give the, zygotes shown in FI, each of which is
heterozygous for black-white here represented as allelomorphs, etc.

The same scheme may be represented by means of
the above " checker" diagram (Fig. 43). Black crossed
to white gives hybrid black, F\, whose germ-cells are
black or white after segregation. The possible com-
bination of these on random meeting at the time of
fertilization is shown by the arrows in FI and the results
are shown in the line marked F2. There will be one
pure black, to two black-and-whites, to one pure white.

THE MENDELIAX PRINCIPLES OF HEREDITY 79

The first and the last will breed true, if self-fertilized,
because they are pure races, while the black-and-whites
will give once again, if inbred, the proportions 1:2: 1.
A better illustration of Mendel's principles is shown
in the case of the white and red Mirabilis jalapa de-
scribed by Correns. This case is illustrated in Fig. 44,

FIG. 44. — Cross between white and red races of Mirabilis Jalapa, giving
a pink hybrid in FI which when inbred gives, in F2, 1 white, 2 pink, 1 red.

in which the red flower is represented in black and the
pink is in gray. The hybrid, F^ out of white by red,
has pink flowers, i.e. it is intermediate in color. When
these pink flowers are self-fertilized they produce
1 white, 2 pink, and 1 red-flowered plant again. The
history of the germ-cells is shown in Fig. 45. The germ-

st

80 HEREDITY AND SEX

cell of the F\ pink flower segregates into " white" and
"red," which by chance combination give the white-
pink, and red flowers of F2. The white and red flowers
are pure ; the pink heterozygous, i.e. hybrid or mixed.
In this case neither red nor white dominates, so that the
hybrid can be distinguished from both its parents.

PARENTS

F'O- *-^* •

-6 o o o • •

FIG. 45. — Illustrating history of gametes in cross shown in Fig. 44. A
white and a red bearing gamete unite to form the pink zygote in F\, whose
gametes, by segregation, are red and white, which by random combinations
give the F2 zygotes, etc.

Mendel tested his hypotheses in numerous ways, that
I need not now discuss, and found in every case that
the results coincided with expectation.

THE HEREDITY OF A SEX-LINKED CHARACTER

We are now in a position to see how Mendel's funda-

THE MENDELIAN PRINCIPLES OF HEREDITY 81

mental principle of segregation applies to a certain class
of characters that in the last chapter I called " sex-
linked" characters.

Diagram 35 (page 64) will recall the mode of trans-
mission of one of these characters, viz. white eyes.

Let us suppose that the determiner for white eyes
is carried by the sex chromosome. *The white-.eyed
male has one sex chromosome of this kind. This sex
chromosome passes into the female-producing spermato-
zoon.

Such a spermatozoon fertilizing an egg of the red-
eyed fly gives a female with two sex chromosomes —
one capable of producing red, one capable of producing
white. The presence of one red-producing chromosome
suffices to produce a red-eyed individual.

When the FI female produces her eggs, the two sex
chromosomes separate at one of the two maturation
divisions. Half of the eggs on an average will contain
the "white" sex chromosome, half the "red." There
are, then, two classes of eggs.

When the FI male produces his sperm, there are
also two classes of sperm — one with the "red"
sex chromosome (the female-producing sperm), and
one without a sex chromosome (the male-producing
sperm) .

Chance meeting between eggs and sperm will give
the classes of individuals that appear in the second filial
generation (F2) . It will be observed that the Mendelian
ratio of 3 red to 1 white appears here also. Segregation
gives this result.

The explanation that has just been given rests on
the assumption that the mechanism that brings about

82 HEREDITY AND SEX

the distribution of the red- and the white-producing
factors is the same mechanism that is involved in sex
determination. On this assumption we can readily
understand that any character that is dependent on the
sex chromosomes for its realization will show sex-linked
inheritance.

The reciprocal cross (Fig. 36) is equally instructive.
If a white-eyed female is mated to a red-eyed male,
all the daughters are red-eyed like the father, and all
the sons are white-eyed like the mother. When these,
FI, flies are bred to each other there are produced red-
eyed females (25%), white-eyed females (25%), red-
eyed males (25%), and white-eyed males (25%). The
explanation (Fig. 36 ; page 65) is in principle the
same as for the other cross. If, for instance, we
assume that the two X chromosomes in the white-eyed
female carry the factors for white, all the eggs will
carry one white-producing X. The red-eyed male will
contain one X chromosome which is red-producing
and passes into the female-producing sperm. The
other sperm will not contain any sex chromosome, and
hence lacks the factors for these eye colors. When the
female-producing sperm, that carries the factor for
red, fertilizes a " white" egg, the egg will give rise to a
female with red eyes, because of the presence of one
red-producing chromosome. When the male-produc-
ing sperm fertilizes any egg, a white-eyed son will be
produced, because the single sex chromosome which
he gets from his mother is white-producing.

The production of four classes of individuals in the
second generation works out on the same scheme, as
shown in the diagram. The inheritance of white and

THE MEXDELIAX PRINCIPLES OF HEREDITY 83

red eyes in these cases is typical of all sex-linked in-
heritance. In man, for instance, color blindness, so
common in males and rare in females, follows the
same rules. It appears that hemophilia in man and
night-blindness are also examples of sex-linked in-
heritance. These cases, as already stated, were formerly
included under the term "sex-limited inheritance," that
implies that a character is limited to one sex, but we
now know that characters such as these may be trans-
ferred to the females, hence the term is misleading.
Their chief peculiarity is that in transmission they ap-
pear as though linked to the factor for sex contained in
the sex chromosome, hence I prefer to speak of them as
sex-linked characters.

If our explanation is well founded, each sex-linked
character is represented by some substance — some
material particle that we call a factor in the sex
chromosome. There may be hundreds of such materials
present that are essential for the development of sex-
linked characters in the organism.

The sex chromosomes must contain, therefore, a
large amount of material that has nothing whatever
to do with sex determination; for the characters in
question are not limited to any particular sex, although
in certain combinations they may appear in one sex
and not in the other.

What then, have the sex chromosomes to do with sex ?
The answer is that sex, like any other character, is due
to some factor or determiner contained in these chro-
mosomes. It is a differential factor of such a kind
that when present in duplex, as when both sex chromo-
somes are present, it turns the scale so that a female

84 HEREDITY AND SEX

is produced — when present in simplex, the result is
to produce a male.

In other words, it is not the sex chromosomes as a
whole that determine sex, but only a part of these chro-
mosomes. Hence we can understand how sex is deter-
mined when an unequal pair of chromosomes is pres-
ent, as in lygaeus. The smaller chromosome lacks
the sex differential, and probably a certain number of
other -materials, so that sex-linked inheritance is pos-
sible here also. Moreover, in a type like oncopeltus,
where the two sex chromosomes are alike in size, we
infer that they too differ by the sex differential. If
all the other factors are present, as their size suggests,
sex-linked inheritance of the same kind would not be
expected, but the size difference observable by the
microscope is obviously too gross to make any such
inference certain. We have come to see that it was a
fortunate coincidence only that made possible the dis-
covery of sex determination through the sex chromo-
somes, because the absence of the sex factor alone in
the Y chromosome might have left that chromosome
in the male so nearly the same size as the X in the
female that their relation to sex might never have been
suspected. When, however, one of the sex chromosomes
began to lose other materials, it became possible to
identify it and discover that sex is dependent upon
its distribution.

THE HEREDITY OF TWO PAIRS OF CHARACTERS

Mendel observed that his principles of heredity apply
not only to pairs of characters taken singly, but to
cases where two or more pairs of characters are involved.

THE MEXDELIAX PRINCIPLES OF HEREDITY 85

An illustration will make this clear. There are races
of edible peas in which the surface is round ; other races
in which the surface is wrinkled. Mendel crossed a
pea that produces yellow round seeds with one that pro-
duces wrinkled green seeds.

The result of this cross was a plant that produced
yellow round peas (Fig. 46). Both yellow and. round
are therefore dominant characters. When these F\
plants were self-fertilized, there were produced plants
some of which bore yellow round peas, some yellow
wrinkled peas, some green round peas and some green
wrinkled peas. These were in the proportion of
9:3:3:1.

The explanation of the result is as follows : One of
the original plants produced germ-cells all of which
bore determiners for yellow and for round peas, YR; the
other parent produced gametes all of which bore deter-
miners for green and for wrinkled, GW (Fig. 47).
Their combination may be represented :

YR by GW = YRGW

The germ-cells of the hybrid plant YRGW produced
germ-cells (eggs and pollen) that have either Y or G,
and R or W. Expressed graphically the pairs, the
so-called allelomorphs, are :

Y R^

G W

and the only possible combinations are YR, YW, GR,
GW. When pollen grains of these four kinds fall on
the stigma of the same kind of hybrid plant whose
ovules are also of the four kinds the following chance
combinations are possible :

HEREDITY AND SEX

YR
YR

YR
YW

YR
GR

YR
GW

YW
YR

YW
YW

YW
GR

YW
GW

GR
YR

GR
YW

GR
GR

GR
GW

GW
YR

GW
YW

GW
GR

GW
GW

FIG. 46. — Illustrating Mendel's cross of yellow-round with green-wrinkled
peas. The figures show the peas of F\ and F2 in the latter in the charac-
teristic ratio of 9 : 3 : 3 : 1.

THE MEXDELIAX PRINCIPLES OF HEREDITY 87

o

PARENTS

Y R X / G~W

O

YR GW

'YR

YW
YW

GR
GR

GW

GW.

FIG. 47. — Illustrating the history of the gametes of the cross represented
in Fig. 46. The composition of the parents YR and GW and of the FI hybrid
YRGW is given above. The four classes of ovules and of pollen are given in
the middle of the figure. These by random combinations give the kinds of
zygotes represented iri the squares below.

88 HEREDITY AND SEX

In each combination in the table the character of
the plant is determined by the dominant factors, in
this case yellow and round, hence :

9 YR : 3 YW : 3 GR : 1 GW

This result works out on the assumption that there
is independent assortment of the original determiners
that entered into the combination. The determiner
for yellow and the determiner for round peas are
assumed to act independently and to segregate from
green and wrinkled that entered from the other parent.
The 9:3:3:1 ratio rests on this assumption and is the
actual ratio realized whenever two pairs of characters
freely Mendelize.

THE HEREDITY OF TWO SEX-LINKED CHARACTERS

The inheritance of two sex-linked characters may be
illustrated by an imaginary case in which the linkage
of the factors to each other is ignored. Then the same
case may be given in which the actual results obtained,
involving linkage, are discussed.

The factors in the fruit fly for gray color, G, and for
red eye, R, are both sex-linked, i.e. contained in the
X chromosome. Their allelomorphs, viz., yellow color,
Y, and white eye, W, are also sex-linked. When a
gray red-eyed female is mated to a yellow white-eyed
male, the daughters and sons are gray-red, GR. Their
origin is indicated in the following scheme :

Gray-red $ G R X — GR X

Yellow-white <? Y W X- ...

G R X Y W X Gray-red $
G R X . . . Gray-red*

THE MENDELIAN PRINCIPLES OF HEREDITY 89

In the gray-red F\ female there will be the possibility
of interchange of the Cf and Y, and of the W and R
factors, so that gametes of four kinds will be formed,
namely, GRX - GWX YRX - YWX. For the
moment we may assume free interchange • of factors ;
and therefore these four classes of eggs will exist in
equal numbers.

In the gray-red FI male there is but one X chromo-
some that contains the factors G and R. There will
be then only one kind of female-producing sperm,
viz., GRX', and one kind of male-producing sperm,
the latter containing no X, and therefore none of the
factors in question. The chance meeting of these two
classes of sperm and the four classes of eggs gives the
following results :

Fi eggs GRX — GWX — YRX — YWX
FI sperm GRX

Females. Males.

GRXGRX gray-red.- GRX gray-red.

GRXGWX gray-red.- GWX gray-white.

GRXYRX gray-red.- YRX yellow-red.

GRXYWX gray-red.- YWX yellow-white.

All the females are gray with red eyes, since these
are the dominant characters. There are four classes
of males. These males give a measure of the kinds of
eggs produced by the females, since the male-producing
sperms, having no sex chromosomes, do not affect
the sex-linked characters derived through the sex
chromosome of the FI female. The expected proportion
is therefore :

90

HEREDITY AND SEX

GR?
4

GWZ
1 1

1

These results are illustrated by means of Fig. 48,
in which the yellow color of the fly is indicated by
stippling the body and wings, and the red eyes by
black. The X chromosome is also marked and colored

Parents

xx

XX X

FIG. 48. — Inheritance of yellow-white ( $ ) and gray-red ( 9 ) of Dro-
sophila. In F\ both sexes are gray-red. In F2 are produced 4 GR 9 —
1 GR $ — 1 GW $ — I YR $ — 1 YW $.

THE MENDELIAN PRINCIPLES OF HEREDITY 91

in the same way as the flies ; thus the two X's in the
red-eyed gray female are half black (for red) and half
gray; the single X in the white-eyed yellow male is
half white and half stippled.

In the FI generation the X chromosomes are first
represented as they came in (second line), i.e. with
their original composition. The next line gives the
three large classes that result, viz., 2 GR9 --1 GR$
- 1 YW $ . But if free interchange takes place in the
female, some of the eggs will have chromosomes like
those in the fourth line, viz. YR and GW. Such
eggs will give the classes represented in the lowest line,
viz., 2 GR9— 1 GW$— 1 YR$. Thus, as already
explained, there results one kind of female and four
kinds of males.

I said that the proportion 4:1:1:1:1 is the ideal
result in the cross between the yellow-white and the
gray-red flies. This ideal scheme is not realized because
of a complication that comes in. The complication
is due to linkage or a tendency to hang together of the
characters that go in together. We must now take up
this question. It is one of the most modern develop-
ments of the Mendelian theory — one that at first
seemed to throw doubt on the fundamental idea of
random assortment that gives Mendel's proportion
9:3:3:1. But I believe we can now offer a reasonable
explanation, which shows that we have to do here with
an extension of Mendelism that in no sense invalidates
Mendel's principle of segregation. It not only extends
that principle, as I have said, but gives us an oppor-
tunity to analyze the constitution of the germ-plasm
in a way scarcely dreamed of two or three years ago.

92 HEREDITY AND SEX

The actual numbers obtained in the GR by YW
cross are as follows. These are the figures that Dexter
has obtained :

GR? GR$ GW$ YR$ YW $

6080 2870 36 34 2373

The apparent discrepancy between the expected
and the realized ratios is due to the linkage of the factors
that went into the cross. For instance, the factors
for gray and red that went in with one chromosome are
linked ; likewise their allelomorphs, yellow and white.
As shown by the analysis, the F\ female offspring
will have two sex chromosomes, one of each sort -
one from the father, the other from the mother. But
the male will have but one sex chromosome derived
from the mother.

If in the germ-cells of the F\ females there were
random assortment of the allelomorphs in the sex
chromosomes, the ideal ratio of 4:1:1:1:1 would, as
has been said, be realized. But if the red and gray
factors tend to remain together since they go in
together in the one chromosome, and the yellow and
white in the other chromosome tend to remain together,
and if the chances are about 84 to 1 that the factors
that enter together remain together, the realized ratio
of 170 : 84 : 1 : 1 : 84 will be found.

Experiments show that, for these two factors, the
chances are about 84 to 1 that the factors that go in
together remain together; hence the departure from
Mendel's ratios for these two pairs of characters. We
may make a general statement or hypothesis that
covers cases like these, and in fact all cases where

THE MENDELIAN PRINCIPLES OF HEREDITY 93

linkage occurs : viz. that when factors lie in different
chromosomes they freely assort and give the Mendelian
expectation ; but when factors lie in the same chromo-
some, they may be said to be linked and they give
departures from the Mendelian ratios. The -extent to
which they depart from expectation will vary with
different factors. I have suggested that the departures
may be interpreted as the distance between the factors
in question.

A THEORY OF LINKAGE

In order to understand more fully what is meant
by linkage on the interpretation that I have here
followed, it will be necessary to consider certain changes
that take place in synapsis. The sex chromosomes

o*

FIG. 49. — Illustrating chiasma-type theory. 1 and 2, from Triton
cristatus, 3—46, chromosomes of Batracoseps attenuatus. Note especially
the chiasma shown in 13. (After Janssens).

94

HEREDITY AND SEX

(when two are present as in the female), like all other
chromosomes, unite in pairs at the synaptic period.
A recognized method of uniting is for like chromosomes
to come to lie side by side.

Before they separate, as they do at one of the two
maturation divisions, each chromosome may be seen
to be split throughout the length. Thus there are

FIG. 50. — Chromatin filaments in the amphitene stage from spermato-
cytes of Batracoseps. (After Janssens.)

formed four parallel strands each equivalent to a
chromosome — the tetrad group. At this time Jans-
sens has found that cross unions between the strands
are sometimes present (Fig. 49). In consequence
a strand is made up of a part of one chromosome and
a part of another. Whether this cross union can be
referred to an earlier stage — at the time when the two
like chromosomes come together, when they can be

THE MEXDELIAX PRINCIPLES OF HEREDITY 95

seen to twist around each other (Fig. 50) — is not
certain ; but the fact of the existence of cross connec-
tions is the important point. A consequence of this
condition is that the chromosomes that come out of
the tetrad may represent different combinations of
those that united to form the group. On the basis
of this observation we can explain the results of associ-
ated inheritance. For, to the same extent to which
the chromosomes that unite remain intact, the factors
are linked, and to the extent to which crossings occur
the exchange of factors takes place. On the basis
of the assumption of the linear arrangement of the
factors in the chromosomes the distance apart of the
factors is a matter of importance. If two factors
lie near together, the chance of a break occurring be-
tween them is small in proportion to their nearness.
We have found that some factors cross over not once
in a hundred times. I interpret this to mean that they
lie very near together in the chromosome.

Other factors cross over to various degrees; in the
extreme cases the chance is one to one that they cross
over. In such cases the factors lie far apart — perhaps
near the ends of the chromosome.

The strongest evidence in favor of this view is found
when the constant relation of the factors to each other
is considered. If, for instance, we know the distance
from A to B (calculated on the basis of crossing over)
and from B to C, we can predict what A and C will do
when they are brought into the hybrid from two
parents. If a fourth factor, Z), is discovered and its
distance from A is made out, we can predict before the
experiment is made what will take place when D and

96 HEREDITY AND SEX

B or D and C are combined. The prediction has been
fulfilled so many times and in so many ways that we
feel some assurance that we have discovered here a
working hypothesis of considerable interest. If the
hypothesis becomes established, it will enable us to
analyze the structure of the chromosomes themselves
in the sense that we can determine the relative location
of factors in the chromosomes. If, as seems not
improbable, the chromosomes are the most important
element in Mendelian inheritance, the determination
of the linear series of factors contained in them becomes
a matter of great theoretical interest ; for we gain
further insight into the composition of the material
on which heredity itself depends.

There is a corollary to this explanation of crossing
over that has a very direct bearing on the results. In
the male there is only one X chromosome present.
Hence crossing over is impossible. The experimental
results show that no crossing over takes place for
sex-linked factors in the male of drosophila.

Other factors, however, lie in other chromosomes.
In these cases the chromosomes exist in pairs in the
male as well as in the female. Does crossing over
occur here in both sexes? Let me illustrate this by
an example. In drosophila the factor for black
body color and the factor that gives vestigial wings
lie in the same chromosome, which we may call the
second chromosome. If a black, long-winged female
is crossed with a gray vestigial male, all the offspring
will be gray in color and have long wings, since these
are the dominant characters. If these F\ flies are
inbred, the following classes will appear:

THE MENDELIAN PRINCIPLES OF HEREDITY 97

Gray Long Black Long Gray Vestigial
2316 1146 737

It will be noted that there are no black vestigial
flies. Their absence can be explained on the assump-
tion that no crossing over in the male, between the
factors in the second chromosome, has taken place.

But if another generation (F3) is raised, some black
vestigial flies appear. With these, it is possible to test
the hypothesis just stated. If, for instance, some of
the long, gray, FI females are mated to black vestigial
males, the following classes are produced :

GL BL GV BV

578 1413 1117 307

The results are explicable on the view that crossing
over takes place in the germ-cell of the FI female,
and that the chance that such will occur is as 1 to 3.

But if the long-winged, gray, FI males are crossed
to black vestigial females, only the following classes
are produced :

BL GV

992 721

These results are in accord with the hypothesis that
no crossing over takes place between the second
chromosomes in the FI male. Why crossing over
should occur in the FI female, and not in the FI
male, we do not know at present ; and until the
synaptic stages in the males and females have been
carefully studied, we must wait for an answer to the
question.

98

HEREDITY AND SEX

THREE SEX-LINKED FACTORS

When three sex-linked factors exist in the same
chromosomes, we have a method by means of which
the " crossing-over " hypothesis may be put to a further
test. Sturtevant has recently worked over the evidence

I £7

G.'

H.'

FIG. 51. — A-D, YW and GR that enter (A), crossing over to give YR
and GW as seen in D. E-Ei, no crossing over. F-Fi, crossing between WM
and RL. G-Gi, crossing between YW and GR. H-Hi, double crossing over
of YWM and GRL, to give YRM and GWL.

for a case of this kind. He analyzed the data of the
cross between a fly having gray color, red eyes, long
wings, mated to a fly with yellow color, white eyes,
and miniature wings. The relative location of these
three factors is shown in the above diagram (Fig. 51,

THE MEXDELIAN PRINCIPLES OF HEREDITY 99

E, F, £, H). The Fi flies gave the expected re-
sults. These inbred gave the following F2 significant
classes : 1

GRL YWM GWM YRL

2089 1361 17 23

GRM YWL

887 817

GWL YRM
5- 0

In these results the classes where single crossing over
is shown are GWM (17) and YRL (23) (Fig. 51, G,
Gf) and GRM (887) and YWL (817) (Fig. 51, F, F').

There are two classes, namely, GWL (5) and YRM
(0) (Fig. 51, H, Hr), which involve double crossing over.
In order that they may take place, the two sex chromo-
somes in the female must break twice and reunite
between the factors involved, as shown in the diagram.
Such a redistribution of the parts of the homologous
chromosomes would be expected to occur rarely, and
the small number of double crossovers recorded in
the results is in accord with this expectation.

In these questions of linkage we have considered
some of the most recent and difficult questions in the
modern study of heredity. We owe to Bateson and
his collaborators the discovery of this new departure.
In plants they have recorded several cases of linkage,
and other authors, notably Correns, Baur, Emerson,
East, and Trow have described further cases of the
same kind. Bateson has offered an interpretation
that is quite different from the one that I have here
followed. His view rests on the assumption that
separation of factors may take place at different times,
or periods, in the development of the germinal tissues.

1 The classes omitted are those that do not bear on the question
in hand.

100 HEREDITY AND SEX

In a word, he assumes that assortment is not confined
to the final stages in the ripening of the germ-cells,
but may take place at any time in the germ-tract.
It seems to me, however, if the results can be brought
into line with the known changes that take place
in the germ-cells at the time when the maternal and
paternal chromosome unite, that we have not only
a simpler method of dealing with these questions,
but it is one that rests on a mechanism that can be
studied by actual observation. Moreover, on purely
a priori grounds the assumptions that I have made seem
much simpler and more tangible than the assumptions
of " reduplication" to which Bateson resorts.

But leaving these more theoretical matters aside,
the evidence from a study of sex-linked characters shows
in the clearest manner that they, while following Men-
del's principle of segregation, are also undeniably asso-
ciated with the mechajjism " of sex. There is little
doubt that sex itself is inherited in much the same
way, since we can explain both in terms of the same
mechanism. This mechanism is the behavior of the
chromosomes at the time of the formation of the germ-
cells.

CHAPTER IV

SECONDARY SEXUAL CHARACTERS AND THEIR RELA-
TION TO DARWIN'S THEORY OF SEXUAL SELECTION

IN his "Origin of Species " Darwin has defined Sexual
Selection as depending "on a struggle between the
individuals of one sex, generally the males, for the
possession of the other sex. The result is not death
to the unsuccessful competitor, but few or no offspring.
Sexual selection is, therefore, less rigorous than natural
selection. Generally, the most vigorous males, those
which are best fitted for their places in nature, will
leave most progeny. But in many cases, victory
depends not so much on general vigor, as on having
special weapons, confined to the male sex. A hornless
stag or spurless cock would have a poor chance of leav-
ing numerous offspring. Sexual selection, by always
allowing the victor to breed, might surely give indomi-
table courage, length to the spur, and strength to the
wing to strike in the spurred leg, in nearly the same
manner as does the brutal cock-fighter by the careful
selection of his best cocks."

Darwin continues: "Amongst birds, the contest
is often of a more peaceful character. All those who
have attended to the subject, believe that there is the
severest rivalry between the males of many species
to attract, by singing, the females. The rock-thrush
of Guiana, birds of paradise, and some others, con-

101

102 HEREDITY AND SEX

gregate, and successive males display, with the most
elaborate care, and show off in the best manner, their
gorgeous plumage; they likewise perform strange
antics before the females, which, standing by as spec-
tators, at last choose the most attractive partner/'

Here we have two different pictures, each of which
attempts to give an account of how certain differences
between the sexes have arisen — differences that we
call " secondary sexual characters."

On the one hand we deal with a contest between
the males; on the other with choice by the female.
The modus operandi is also different. After battle
the successful male takes his pick of the females. If
the scheme is to work, he must choose one that will
leave the most offspring.

On the other hand, we have the tourney of love.
The males "show off"; the females stand by spell-
bound and "at last choose the most attractive partner."

Now, concerning this display of the males, I beg
leave to quote a paragraph from Wallace's "Natural
Selection and Tropical Nature" :

"It is a well-known fact that when male birds possess
any unusual ornaments, they take such positions or
perform such evolutions as to exhibit them to the best
advantage while endeavoring to attract or charm
the females, or in rivalry with other males. It is
therefore probable that the wonderfully varied decora-
tions of humming-birds, whether burnished breast-
shields, resplendent tail, crested head, or glittering
back, are thus exhibited ; but almost the only actual ob-
servation of this kind is that of Mr. Belt, who describes
how two males of the Florisuga mellivora displayed

SECONDARY SEXUAL CHARACTERS 103

their ornaments before a female bird. One would
shoot up like a rocket, then, suddenly expanding the
snow-white tail like an inverted parachute, slowly
descend in front of her, turning around gradually to
show off both back and front. The expanded white
tail covered more space than all the rest of the bird, and
was evidently the grand feature of the performance.
Whilst one was descending the other would shoot
up and come slowly down expanded."

There is just a suspicion in my mind that these males
were otherwise engaged, for while I know nothing
about the habits of these humming birds I find on the
next page of " Tropical Nature " this statement :

"Mr. Gosse also remarks: 'All the humming-
birds have more or less the habit, when in flight, of
pausing in the air and throwing the body and tail into
rapid and odd contortions. This is most observable
in Polytmus, from the effect that such motions have
on the long feathers of the tail. That the object of
these quick turns is the capture of insects, I am sure,
having watched one thus engaged.' '

If what I have just said implies that I take a light-
hearted or even facetious attitude toward Darwin's
theory, I trust that my position will not be misunder-
stood. Darwin brought together in his book on the
" Descent of Man" a mass of interesting observations
for which he suggested a new theory. No one can
read his wonderful book without the keenest interest,
or leave it without high admiration for the thorough-
ness with which the subject is treated ; for the ingenuity
and skill with which the theory is applied to the facts,
and, above all, admiration for the moderation, modesty,

104 HEREDITY AND SEX

and honesty with which objections to the theory are
considered.

I will let no one admire Darwin more than I admire
Darwin. But while affection and respect and honor
are the finest fruits of our relation to each other, we
cannot let our admiration for the man and an ever
ready recognition of what he has done for you and for
me prejudice us one whit in favor of any scientific
theory that he proposes. For in Science there is no
authority ! We should of course give serious considera-
tion to any theory proposed by a man of such wide expe-
rience and trained judgment as Darwin ; but he himself,
who broke all the traditions of his race, would be the
first to disclaim the value of evidence accepted on
authority.

From the definition of sexual selection with which
we started it may be said that Competition and Courtship
stand for the two ways in which Darwin supposes
the secondary sexual characters to have arisen.

Competition amongst the males is only a form of nat-
ural selection, as Darwin himself recognized (if we leave
out of account the further assumption that the victor
chooses his spoils). We may dismiss this side of the
problem as belonging to the larger field of natural selec-
tion, and give our attention mainly to those secondary
sexual characters that Darwin supposes to have arisen
by the female choosing the more ornamented suitor.

I shall first bring forward some of the more striking
examples of secondary sexual characters in the animal
kingdom. These characters are confined almost ex-
clusively to three great groups of animals — Insects,

SECONDARY SEXUAL CHARACTERS

105

Spiders, and Vertebrates. There are a few scattered
instances found in other groups, but they are rare.
In the lowest groups they are entirely absent, and are

FIG. 52. — Four species of beetles in which the male (to the left) has horns
which are absent in the female (to the right). (After Darwin.)

not found at all in plants; or rather, if character-
istic differences exist in plants, they are not called by
this name — for plants cannot see or move and there-
fore cannot court each other.

106

HEREDITY AND SEX

In fact, sight in the sense of forming visual pictures
can occur only when eyes are well developed. This

FIG. 53. — Male (to left) with long eye stalks and female (to right) of
a fly, Achia longividens. (After Wood.)

may be taken to score a point in favor of Darwin's
hypothesis.

In the group of insects the most noticeable differences
occur in the butterflies and moths, and in flies. A
few cases are found in the beetles and bugs. The
male cicada's shrill call is supposed to attract the

FIG. 54. — Male to left with horns and female to right without horns of a
fly, Elaphomyia. (After Wood.)

females. The males of certain beetles have horns -
the female lacks them (Fig. 52).

In a genus of flies the eyes are stalked, and the

SECONDARY SEXUAL CHARACTERS 107

eyes of the male have stalks longer than those of
the female (Fig. 53). In another genus of flies there
are horns on the head like the antlers of the stag
(Fig. 54).

'In the spiders the adult males are sometimes very
small in comparison with the females (Fig. 55). The
size difference may be regarded as a secondary sexual

FIG. 55. — Male (to left) and female (to right) of a spider, Argiope aurelia.
(From " Cambridge Natural History.")

character. Darwin points out, since the male is some-
times devoured by the female (if his attentions are
not desired), that his small size may be an adaptation
in order that he may more readily escape. But the
point may be raised as to whether he is small in order
to escape ; or whether he is eaten because he is small.
In one of our native spiders, Habrocestum splendida,
the adult males and females are conspicuously different

108 HEREDITY AND SEX

in color — the male more highly colored than the
female. In another native species, Maevia vittata,
there are two kinds of males, both colored differently
from the female.

Passing over the groups of fishes and reptiles in
which some striking cases of differences between the
sexes occur, we come to the birds, where we find the
best examples of secondary sexual characters.

FIG. 56. — Superb bird of paradise.
(After Elliot.)

In the white-booted humming bird (Fig. 14) two
of the tail feathers of the male are drawn out, their
shafts denuded of the vanes except at the tip where
the feather ends in a broad expansion.

In the great bird of paradise, of the Aru Islands (Fig.
13), the male has wonderful plumes arising from the
sides that can be erected to produce a gorgeous display.

SECONDARY SEXUAL CHARACTERS 109

The female is modestly clothed. In the male of the
superb bird of paradise (Fig. 56), the mantle behind
the neck, when erected, forms a striking ornament ;
and on the breast there is a brilliant metallic shield.
In the six-shafted bird of paradise (Fig. 57) the
male has on its head six feathers with wiry shafts,

FIG. 57. — Six-shafted bird of paradise.
(After Elliot.)

ornaments that occur in no other birds. In the king
bird of paradise there are remarkable fans at the
sides of the body of the male that can be expanded.
The feathers of the fan are emerald-tipped. The
two middle feathers of the tail are drawn out into
"wires" with a green web at one side of the tip.

In mammals, secondary sexual differences are very

110

HEREDITY AND SEX

common, although startling differences in color are
rather rare. In the male the coat of fur is often darker
than that of the female.

In many deer the antlers are present in the male
alone. In Steller's sea-lion the male is much larger
and stronger than the female. In a race of the Asiatic
elephant the male has tusks much larger than those
of the female.

If we fix our attention exclusively on these remarkable

FIG. 58. — Wilson's phalarope, female (in center) , male (to right and
behind). A bird in winter plumage is at the left. (From Eaton, "Birds of
New York.")

cases where differences between the sexes exist, we
get a one-sided impression of the development of
ornamentation and color differences in animals. We
must not forget that in many cases males and females
are both highly colored and exactly alike. We forget
the parrots, the cockatoos, the kingfishers, the crowned
pigeons, toucans, lories, and some of the starlings;
the " brilliant todies" and the " sluggish jacamars"
whose brilliant metallic golden-green breasts rival

SECONDARY SEXUAL CHARACTERS

111

those of the humming-birds; we forget the zebras,
the leopards ; the iridescent interiors of the shells of
many mollusks ; the bright reds and purples of starfish,
worms, corals, sea anemones, the red, yellow, and green
sponges, and the kaleidoscopic effect of the microscopic
radiolarians ; — a brilliant array of color.

FIG. 59. — (A), female of a copepod, Calocalanus plumulosus. (B), a female
of Calocalanus parous. (C), male of last. (After Giesbrecht.)

In the egret both males and females have remark-
able nuptial plumes, which, had they been present in
one sex alone, would have been classified as secondary
sexual characters. It does not appear that selection
had anything to do with their creation.

Our common screech owl exists in two colored types
sharply separated. No one is likely to ascribe these
differences to sexual selection, yet if one sex had been

112 HEREDITY AND SEX

red and the other gray, the difference would have been
put down to such selection. There are also cases like
the phalarope, shown in Fig. 58, where the female is
more highly ornamented than the male. In fact, for
these cases, Darwin supposed that the males select
the females ; and in support of this view he points out
that the females are more active, while the male con-
cerns himself with the brooding of the eggs. In some
of the marine copepods female ornamentation is car-
ried to even a higher point. In Calocalanus plumosus
the female has one of the tail setae drawn out into a long
feather-like structure (Fig. 59). In another species,
C. parva, all eight setae of the tail of the female are
feather-like (Fig. 59, B), while the male (Fig. 59, C)
lacks entirely these " ornaments."

In some butterflies also, two, three, or more types of
females are known, but only one male type. I shall
have occasion later to consider this case.

COURTSHIP

The theory of sexual selection hinges in the first
place on whether the female chooses amongst her
suitors.

It has been objected that the theory is anthropo-
morphic — it ascribes to beetles, butterflies, and birds
the highly developed esthetic sense of man. It has
been objected that the theory leaves unexplained the
development of this esthetic sense itself, for unless the
female kept in advance of the male it is not self-evident
why she should go on selecting the more highly orna-
mented. If she has advanced esthetically, what has
brought it about? In answer to this last question

SECONDARY SEXUAL CHARACTERS 113

Allen suggests that if the word conspicuousness is sub-
stituted for the word beauty, the objection may to some
extent be met. The more conspicuous male would be
more likely to attract attention and be selected.

It has been pointed out that there is more than a
suspicion that the contests of the males for the females
are sham affairs. They are like certain duels. There
is seldom any one hurt. There are very few records of
injured males, but many accounts of tremendous
battles. And he who fights and runs away will live
to mate another day.

It is clear, I think, that the case against the theory
must rest its claims on actual evidence rather than on ar-
guments or poetry pro or con. Darwin admitted that
the evidence was meager. Since his time something
more has been done. Let us consider some of this new
evidence.

It will be conceded, I think, that Alfred Wallace,
through his wide experience with animals in their
native haunts, is in a position to give weighty evidence
concerning the behavior of animals. He was with
Darwin a co-discoverer of the theory of Natural Se-
lection and cannot be supposed to be prejudiced against
the selection principle. Yet Wallace has from the
beginning strongly opposed the theory of sexual se-
lection. Let me quote him :

Referring to Darwin's theory of Sexual Selection -

"I have long held this portion of Darwin's theory to
be erroneous — and have held that the primary cause
of sexual diversity of color was the need of protection,
repressing in the female those bright colors which are
normally produced in both sexes by general laws."

114 HEREDITY AND SEX

Again, Wallace says: "To conscious sexual selec-
tion— that is, the actual choice by the females of the
more brilliantly colored males or the rejection of those
less gaily colored — I believe very little if any effect
is directly due. It is undoubtedly proved that in
birds the females do sometimes exert a choice ; but
the evidence of this fact, collected by Mr. Darwin
('Descent of Man/ chap, xiv), does not prove that color
determines that choice, while much of the strongest
evidence is directly opposed to this view.7'

Again, Wallace says: "Amid the copious mass of
facts and opinions collected by Mr. Darwin as to the
display of color and ornaments by the male birds, there
is a total absence of any evidence that the females, as
a rule, admire or even notice this display. The hen,
the turkey, and the peafowl go on feeding, while the
male is displaying his finery; and there is reason to
believe that it is his persistency and energy rather than
his beauty which wins the day."

Hudson, who has studied the habits of birds in the
field, asks some very pertinent questions in connec-
tion with their performances of different kinds. ' i What
relation to the passion of love and to the business of
courtship have these dancing and vocal performances
in nine cases out of ten ? In such cases, for instance,
as that of the scissor-tail tyrant-bird, and its pyro-
technic displays, when a number of couples leave their
nests containing eggs and young to join in a wild aerial
dance ; the mad exhibition of grouped wings ; the
triplet dances of the spur-winged lapwing, to perform
which two birds already mated are compelled to call
in a third bird to complete the set; the harmonious

SECONDARY SEXUAL CHARACTERS 115

duets of the oven-birds and the duets and choruses of
nearly all the wood-hewers, and the wing-slapping
aerial displays of the whistling widgeons, — will it be
seriously contended that the female of this species
makes choice of the male able to administer the most
vigorous and artistic slaps ?"

He continues: "How unfair the argument is,
based on these carefully selected cases, gathered from
all regions of the globe, and often not properly reported,
is seen when we turn to the book of nature and closely
consider the habits and actions of all the species in-
habiting any one district." Hudson concludes that he
is convinced that anybody who will note the actions of
animals for himself will reach the conviction, that
"conscious sexual selection on the part of the female
is not the cause of music and dancing performances in
birds, nor of the brighter colors and ornaments that
distinguish the male."

In the spiders Mr. and Mrs. Peckham have described
in detail the courtship of the males. They believe
that his antics are specifically intended to attract the
female. They point out that his contortions are of
such a sort that his brightest spots are turned toward
the female. But, as he makes in any case a hundred
twists and turns, there is some danger of misinterpret-
ing his poses. Montgomery, who has studied spiders
of other groups, reaches the conclusion that here the
male is contorted through fear of the female. The male
goes through some of the same turns if approached by
another male. The courtship of the male spider is,
he thinks, a motley of fear, desire, and general
excitement.

116 HEREDITY AND SEX

The evidence that the Peckhams have given, even if
taken to mean that the motions of the male attract
the attention of the female, — and I can see no reason
why this may not be the case, — fails nevertheless to show
that the female selects, when she has a chance, the more
highly colored male.

Mayer, and Mayer and Soule have made many ex-
periments with moths. The moth promethea, Callo-

FIG. 60. — Above, Callosamia promethia, male to left, female to right.
Below Porthetria dispar, male to left, female to right.

samia promethea, is distinctly sexually dimorphic, as
shown in Fig. 60. Mayer's experiments show that the
male finds the female entirely by the sense of smell.
The wings of some 300 males were painted with scarlet
or green. They mated as often as did the normal male
with which they competed.

Where the wings of males were stuck on the female
in place of her own wings, no disturbance in the mating
was observed. Conversely, normal females accepted

SECONDARY SEXUAL CHARACTERS 117

males with female wings as readily as they accepted
normal males.

In the gipsy moth (Porthetria dispar), the male is
brown and the female white (Fig. 60). Here again
it was found that the males are guided solely' by the
odor of the female.

The silkworm moth is also sexually dimorphic. Kel-
logg has shown that males with blackened eyes find a
female with as much precision as does a moth with
normal eyes.

If the antennae are cut off, however, the male can not
find the female unless by accident he .touches her. He
then mates. The female has scent glands whose odor
excites the male with normal antennae even at some dis-
tance. Chemotaxis and contact are the active agents
in mating. The eyes do little or nothing.

Andrews has found that touch determines mating in
the crayfish. Pearse has obtained similar results.
Chidester has shown the same thing for crabs. Holmes
found this kind of behavior in Amphipoda. Fielde and
Wheeler have also found that in ants sex-discrimination
is through smell or by what Forel calls contact-odors.

Montgomery and Porter recognize touch as the most
important factor in mating in spiders. Petrunke-
witsch has shown that in the hunting spider vision also
helps the sexes to find each other. Tower has found
that contact or odor rather than sight is the important
condition in mating in leptinotarsa.

I am able to give the unpublished results of A. H.
Sturtevant on the mating of the fruit fly, drosophila.
The male carries on an elaborate courtship in the
sense that he circles around the female, throws out one

118 HEREDITY AND SEX

wing, then the other, and shows other signs of excite-
ment. The male has sex combs on his fore legs, the
female lacks them. Lutz cut them off and gave the
female a choice between such a male and a normal
male. One was chosen as often as the other. The
wings of the male and female are wonderfully irides-
cent. Sturtevant cut off the wings of a male and
matched him against a normal male. The female
showed no marked preference. The converse experi-
ment, when a clipped female competed with a normal
female, showed no selection on the part of the males.

If instead of allowing two males (a normal and a
clipped) to compete for one female, a female is given to
each male separately, and the interval before mating is
noted, it is found that on an average this interval is 18
minutes for the normal and 40 minutes for the clipped. If
any such difference existed in the first case, when the two
males were competing, we should expect a much greater
selection in favor of the normal male than was actually
found. This would seem to mean that the female is
more quickly aroused by the normal male, and hence
when both males are present she will accept the clipped
male more quickly than when he alone is present. This
suggests that normal courtship precipitates copulation.

In the following experiments the female was offered
a choice between a new type (mutant) with white eyes
and a normal male. Conversely, the white-eyed fe-
male had a like alternative. The evidence shows that
the more vigorous male — the red-eyed male — is
more successful.

Since vision itself is here involved, for the white-
eyed flies are probably partly blind, the observations

SECONDARY SEXUAL CHARACTERS

119

RED VERSUS WHITE EYES.

GRAY VERSUS YELLOW COLOR.

Rede?

| Red 9 - 54
{ White 9-82

Gray cf

j Gray 9 - 25
\ Yellow 9-31

White J

j Red 9 - 40
| White 9-93

Yellow d"

f Gray 9 - 12
{ Yellow 9-30

Red 9

f Red d" - 53
\ White cf - 14

Gray 9

{ Gray cf - 60
{ Yellow cf - 12

White 9

{ Red cf - 62
| White d" - 19

Yellow 9

J Gray cf - 25
[ Yellow cf - 8

NORMAL VERSUS CLIPPED

GRAY-WHITE VERSUS YELLOW-

WINGS.

WHITE.

Normal J

| Clipped cf - 51
| Normal cf - 67

Gray cf

f Gray 9 - 11
( Yellow 9-4

Normal -?

{ Clipped 9-27
| Normal 9-25

Gray 9

f Gray cf - 21
( Yellow cf - 3

were repeated with a new type that had yellow wings.
The gray male is more successful and the yellow females
less resistant. The results are in accord with the as-
sumption that greater vigor is an important factor
in success.

The following mating bears on this point. Stur-
tevant used in competition a red- and a vermilion-
eyed male. The latter seems as vigorous as is the
red-eyed type. The results were :

Red

Red $
Vermilion $

11

14

showing that the red-eyed male has no advantage
when the males are equally vigorous.

This evidence, taken as a whole, seems to me to show
with some probability that sight plays a minor role in

120 HEREDITY AND SEX

courtship. It is so inferior to vigor, to the sense of
smell and to touch in the lower animals at least, that
it is very questionable whether it has had anything
more to do with mating than helping the sexes find
each other.

VIGOR AND SECONDARY SEXUAL CHARACTERS

We have seen that Darwin himself has stated ex-
plicitly that unless the secondary sexual characters
are associated with greater vigor, or productivity,
nothing can be accomplished.

It will be recalled that Wallace, who disbelieved in
Darwin's theory of sexual selection, attempted to ac-
count for the appearance of secondary sexual characters
on the ground of the greater vigor of the male (he
sometimes says vitality and again activity of the male)
at the breeding season. The vigor is assumed to be
associated with the development of the sex glands
at this time. This may be admitted, but whether the
vigor is the result of the sex glands, or the sex glands of
the vigor, is a nice point that I shall not try to decide.
It may appear that Wallace's view is in part justified
from the facts that we have examined. But I do not
think so. In the first place, he attempts the impos-
sible task of explaining the outgrowths and colors that
appear in special regions by the local activity of the
muscles (for example) in those regions. The facts
before us do not support any such interpretation. The
Peckhams easily overturn his argument, as applied to
spiders.

Second, in birds, to which Wallace mainly refers, the
sex glands of the male do not affect the secondary

SECONDARY SEXUAL CHARACTERS 121

sexual characters of the male, while the sex glands of the
female suppress these characters.

Wallace's theory leaves out of account the hereditary
factor that is also present and which acts quite apart
from the physiological effects of the sex glands.

Cunningham, who has more recently written on the
same subject, accepts the hormone hypothesis as the
basis for all cases of secondary sexual characters.
But he fails to make good his view when it is applied
to insects, for reasons that we shall take up later. He is
especially concerned, however, in the attempt to make
plausible his own hypothesis that secondary sexual
characters have arisen through the use of the parts, or
through special nervous or blood supplies to certain lo-
calities of the body which become suffused during sexual
excitement. In both cases he thinks the increased local
activity will cause the cells to produce hormones that
will be dispersed throughout the body, and absorbed
by other cells. The germ-cells will in this way get
their share and carry over the hormone to the next
generation.

Cunningham forgets one important point. If these
imaginary hormones can get out of cells and into germ-
cells, they can get out of the germ-cells again. Hence
in the long period of embryonic and juvenile existence
through which the individual passes before the second-
ary sexual characters appear they would surely be lost
from the body like any other ordinary hormone.

CONTINUOUS VARIATION AS A BASIS FOR SELECTION

And now let us turn to an entirely different aspect of
the matter. What could selection do, admitting that
^election may take place. For fifty years it has been

122

HEREDITY AND SEX

n

A-E

FIG. 61. — I. Diagram of five pure lines of beans (A, B, C, D, and E)
and a population formed by their union, A-E. II. Diagrams illustrating a
pure line of beans and two new biotypes derived from it. The upper
diagram indicates the original biotype ; the second and third diagram in-
dicate the elongated (narrower) and shorter (broader) type of beans. X
indicates the average class of the original biotype. (After Johannsen.)

SECONDARY SEXUAL CHARACTERS

123

taken for granted that by selecting a particular kind
of individual the species will move in the direction
of selection.

A few examples will bring the matter before us. If
we take a peck of beans and put all of those of one size
in one cylinder and those of other sizes in other cyl-
inders, and place the cylinders in a row, we get a result
like that in Fig. 61, A-E. If we imagine a line joining

26

+36

FIG. 62. — The normal binomial curve or the "ideal curve" of distribu-
tion. At the base line, the directions from the average value (o) are
indicated with the standard deviation (<r) as unity. (After Johannsen.)

the tops of the beans, the line gives a curve like that
shown in Fig. 62. This is known as the curve of prob-
ability. The curve can be, of course, most readily
made by making the measurements directly. Most
individuals of such a population will have the charac-
ter developed to the degree represented by the highest
point in the curve. Now if two individuals standing
at one side (let us say with the character in question
better developed than the average) become the parent

124

HEREDITY AND SEX

of the next generation, their offspring will make a new
curve that has moved, so to speak, in the direction of
selection (Fig. 63).

If again two more extreme individuals are selected,
another step is taken. The process is assumed to go
on as long as the selection process is maintained.

So the matter stood until a Danish botanist, Johann-
sen, set seriously to work to test the validity of the
assumption, using a race of garden beans for his meas-
urements. He discovered in the first place that popu-

FIG. 63. — Schematic representation of the type-shifting effect of selec-
tion from the point of view of Galton's regression theory. The * marks the
point on the curves of A, Ai, Az from which the selection is supposed to be
made. (Goldschmidt.)

lations are made up of a number of races or "pure
lines." When we select in such a population we sort
out and separate its constituent races, and sooner
or later under favorable conditions can get a pure
race. Selection has created nothing new; it has
picked out a particular preexisting race from a mixed
population.

Johannsen has shown that within a pure line selec-
tion produces no effect, since the offspring form the
same group with the same mode as the group from which
the parents came. The variability within the pure
lines is generally ascribed to environmental influences

SECONDARY SEXUAL CHARACTERS 125

which are recurrent in each generation. The germ-
plasm is homogeneous for all members of the pure line,
while in a mixed population the germ-plasm is not the
same for all individuals.

Darwin himself saw this to some extent, for he has
repeatedly pointed out that selection depends on the
materials offered to it by variation ; that in itself it
can produce nothing. Yet from Darwin to Johannsen
the teaching of the post-Darwinians has been such as to
lead most people to believe that selection is a causative
or creative principle that will explain the progressive
development of animals and plants.

DISCONTINUOUS VARIATION OR MUTATION AS A BASIS
FOR SELECTION

The second great movement since Darwin has been
to show that hereditary variations do not give a con-
tinuous series but a discontinuous one. Bateson and
De Vries brought forward some twelve years ago evi-
dence, in favor of this view, that has gone on increasing
in volume at an amazing rate.

I cannot attempt to discuss this evidence here, but
I may point out the bearing of the new point of view
on the meaning of secondary sexual characters.

In a number of butterflies there occur two or three
or even more different kinds of females. One of the most
remarkable cases of the kind is that of Papilio polytes
that lives in India and Ceylon. It has a single male
type and three types of females (Fig. 64).

Wallace, who first observed that the three types of
female belong to one male type, argued that two
of these three types owe their origin to their resem-

126

HEREDITY AND SEX

blance to butterflies of other species that are protected,
namely, Papilio aristolochia and P. hector. These

FIG. 64. — Papilio polytes ; male (upper left) and three types of female
(to right). The "models," which two of these females are supposed to
"mimic," are shown to their left. (After Punnett.)

two feed on the poisonous plant aristolochia and are
said to be unpalatable. The two aberrant types of
P. polytes bearing a close resemblance to these two

SECONDARY SEXUAL CHARACTERS 127

species have been dubbed the hector form and the
aristolochia form.

Wallace, and those who adhere to the same view,
believe that the resemblance of the model and the
mimic has come about through the accumulation of
minute variations which have survived as a result of
their advantages. In a word, the process of natural
selection is assumed to have gradually brought about
the evolution of these two new types of females.

This case has been recently examined by Punnett.

Punnett says that while in cabinet specimens the
resemblance between the model and the mimic is re-
markably close, yet in the living animals, with their
wings spread out, the resemblance is less marked, espe-
cially the resemblance between the hector model and
the polytes mimic. At a distance of a few yards the
difference between the two is easily seen.

When flying the differences are very apparent. ' ' The
mode of flight of P. polytes is similar for all three forms,
and is totally distinct from that of P. hector and P.
aristolochia." In flight the latter pursue an even
course, while the polytes form follow a lumbering
up and down course. Punnett thinks these differ-
ences are so distinct that they are " unlikely to be
confounded by an enemy with any appreciation of
color or form."

Moreover, in Ceylon at least, the distribution of the
model and its mimic is very different from what is
expected on the theory of mimicry. He concludes that
the facts relative to their distribution "are far from
lending support to the view that the polymorphic
females of P. polytes owe their origin to natural selec-

128 HEREDITY AND SEX

tion, in the way that the upholders of the theory of
mimicry would lead us to suppose."

After considering the difficulties that the theory of
mimicry has to contend with, Punnett points out that
dimorphic and polymorphic species are not uncommon
in butterflies, and that in many of these cases there can
be little or no question of mimicry having anything

FIG. 65.— Papilio turnus ; female (above) and male (below), and the variety
P. turnus glaucus (above, right) which appears only in the female.

to do with the matter. It is well known that in Lepidop-
tera the modified form commonly belongs to the female
sex. In one case (Abraxas grossulariata) it is known
that the aberrant female type appears sporadically, as a
sport, and follows Mendel's law of segregation. Punnett
shows how the recurrence of the single type of male and
the three types of females of polytes may also be ac-
counted for by the recognised methods of Mendelian in-
heritance. He points out that by the assumption that

SECONDARY SEXUAL CHARACTERS 129

these types have suddenly appeared as mutants many
of the difficulties of the older theories are avoided,
and that such an assumption is in harmony with
an ever increasing body of evidence concerning
variation and heredity. On this view " natural se-
lection" plays no part in the formation of these
polymorphic forms," nor does sexual selection. The
absence of transitional forms is explicable on this

FIG. 66. — Colias philodice, showing two female forms above and
one male form below.

view, and unaccountable on the other theory. In
fact polymorphic forms, if they appear, would be
expected to persist if they are not harmful to the
species.

We have in this country several species of butter-
flies in which polymorphism exists. In the north
the species Papilio turnus (Fig. 65) is alike in the male
and in the female. But in the south two types of
females exist — one like the male and the other a
black type.

130 HEREDITY AND SEX

In the Eastern States there is a butterfly, Colias
philodice, in which two types of female exist (Fig. 66).
Gerould has studied the mode of inheritance of these
two types and finds that they conform to a scheme in
which the two females differ by a single factor. The evi-
dence is strongly in favor of the view that one of these
forms has arisen as a mutation. There is no need to
suppose that sexual selection has had anything to do
with its origin, and no evidence that it owes its exist-
ence* to mimicry of any other species.

Finally, I should like to speak of a case that has come
under my own observation. One of the mutants that
appeared in a culture of drosophila had a new eye
color that was called eosin. In the female the eye is
much deeper in color than in the male. The race main-
tains itself as a bicolor type without any selection.

CONCLUSIONS

In conclusion let me try to bring together the main
considerations that seem to me to throw serious doubts
on Darwin's theory of sexual selection.

First. Its fundamental assumption that the evolution
of these characters has come about through the "will,"
"choice," or selection of the female is questionable,
because of want of evidence to show that the females
make their choice of mates on this basis. There is also
some positive evidence to show that other conditions
than selection of the more ornamented individual
(because he is the more ornamental) are responsible
for the mating.

Second. We have come to have a different concep-
tion of what selection can do than the sliding scale

SECONDARY SEXUAL CHARACTERS 131

assumption that has been current, at least by implica-
tion, in much of the post-Darwinian writings.

Third. Recent advances in the study of variations
have given us a new point of view concerning the na-
ture of variation and the origin of variations. If we
are justified in applying this new view to secondary
sexual characters, the problem appears greatly sim-
plified.

CHAPTER V

THE EFFECTS OF CASTRATION AND OF TRANSPLAN-
TATION ON THE SECONDARY SEXUAL CHARACTERS

IN several of the preceding chapters I have spoken in
some detail of sex-linked inheritance. In sex-linked
inheritance we deal with a class of characters that are
transmitted to one sex alone in certain combinations,
and have for this reason often been called sex-limited
characters ; but these same characters can be trans-
ferred by other combinations, as we have seen, to the
other sex, and are therefore not sex-limited.

In contrast to these characters Secondary sexual char-
acters appear in one sex only and are not transferable
to the other sex without an operation. For instance,
the horns of the stag and the colors and structures of
certain male birds are in nature associated with one
sex alone.

It has long been recognized in mammals and birds
that there is a close connection between sexual maturity
and the full development of the secondary sexual char-
acters. This relation suggests some intimate correla-
tion between the two. It has been shown, in fact, in
some mammals at least, that the development of the
secondary sexual characters does not take place, or
that they develop imperfectly, if the sex glands are
removed. It may appear, therefore, that we are deal-
ing here with a purely physiological process, and that

132

THE EFFECTS OF CASTRATION 133

the development of these structures and colors is a by-
product of sex itself, and calls for no further explana-
tion.

But the question cannot be so hastily dismissed.
This can best be shown by taking up at once the ma-
terial at hand.

OPERATIONS ON MAMMALS

In the deer, the facts are very simple. If the very
young male is castrated before the knobs of the antlers
have appeared, the antlers never develop.

FIG. 67. — Merino; male (horned) and female (hornless).

If the operation is performed at the time when the
antlers have already begun to develop, incomplete
development takes place. The antlers remain covered
by the velvet and are never thrown off. They are called
peruke antlers. If the adult stag is castrated when
the horns are fully developed, they are precociously

134 HEREDITY AND SEX

dropped, and are replaced, if at all, by imperfect ant-
lers, and these are never renewed.

These facts make it clear that there is an intimate
relation between the orderly sequence of development
of the horns in the deer and the presence of the male
sexual glands.

In the case of sheep, the evidence is more explicit.
Here we have carefully planned experiments in which
both sexes have been studied ; and there are breeding

FIG. 68. — Dorset; male (horned) and female (horned).

experiments also, in which the heredity of horns has
been examined.

In some breeds of sheep, as in the Merinos and
Herdwicks, horns are present in the males, absent in
the females (Fig. 67). In other breeds of sheep, as
in Dorsets, both males and females have horns (Fig.
68). In still other breeds both sexes lack horns, as
in some of the fat-tailed sheep of Africa and Asia
(Fig. 69).

Marshall has made experiments with Herdwicks -
a race of sheep in which the rams have large, coiled
horns and the ewes are hornless. Three young rams
(3, 4, and 5 months old) were castrated. The horns
had begun to grow (3, 4^, and 6 inches long) at the
time of operating. They ceased to grow after the
operation.

THE EFFECTS OF CASTRATION 135

A similar operation was also carried out on females.
Three Herdwick ewe lambs (about 3 months old) were
operated upon. After ovariotomy, the animals were
kept for 17 months, but no horns appeared, although
in one, small scurs developed, in the other two scarcely
even these. It is clear that the removal of the ovaries j
does not lead to the development of horns like those
in the male.

Now, the interpretation of this case can be made
only when taken in connection with experiments in
heredity. There is a crucial experiment that bears on
this question. Arkell found when a IVJoritiu ewj£-(a, race
with horned males and hornless females) was bred to a
ram of a hornless breed, that the sons had horns. In
this case the factor for horns must have come from the
hornless mother, while the development of the horns was
made possible by the presence of the male glands, 'lib '
is evidentJiherefore in the-iastration experiment that
a factor for horns is inherited by both sexes, but in order
that the horns may develop fully, the male glands must
be present and functional.^

In the Dorset, both sexes are horned, the horns of the
females are lighter and smaller than the horns of the
ram (Fig. 68). In the castrated males the horns are
like those of the females. In this case we must sup-
pose that the hereditary factor for horns suffices to
carry them to the point in development reached by the
females. To carry them further the presence of the sex
glands of the male is necessary.

In the case of the hornless breeds I do not know of
any evidence from castration or ovariotomy. We may
suppose, either that the factor for horns is absent ; or,

136 HEREDITY AND SEX

if present, some inhibitory factor must bring about sup-
pression of the horns. The former assumption seems
more probable, for, as I shall point out, certain experi-
ments in heredity indicate that no inhibitor is present
in hornless breeds.

The series is completed by cases like that of
the eland and the reindeer. Both males and females

FIG. 69. — Fat-tailed hornless sheep (Ovis
aries steatopyga persicci).

have well-developed horns. In this case the hereditary
factors suffice in themselves for the complete develop-
ment of horns, for even after castration the horns de-
velop.

We have anticipated to some extent the conclusions
arrived at by breeding experiments in these races of
sheep. The best-known case is that of Wood, who
crossed horned Dorsets and hornless Suffolks. As

THE EFFECTS OF CASTRATION

137

shown in the picture (Fig. 70) the sons had horns -
the daughters lacked them. When these are inbred,
their offspring are of four kinds, horned males, hornless
males, horned females, hornless females.

It seems probable that these four classes appear in
the following proportions :

Horned
3

Hornless S
I

Horned
1

Hornless
3

The explanation that Bateson and Punnett offer for
this case is as follows : The germ-cells of the horned race

FIG. 70. — 1, Suffolk (ram), hornless in both sexes; 2, Dorset (ewe),
horned in both sexes ; 3, FI ram, horned ; 4, Fi ewe, hornless ; 5-8, the four
types of F* ; 5 and 6 are rams, 7 and 8 are ewes. The hornless rams are
pure for absence of horns, and the horned ewes are pure for the presence of
horns. Figs. 5 and 6 represent lambs. (Bateson, after Wood.)

(both male and female) carry the factor for horns (H) ;
the germ-cells of the hornless race lack the factor for
horns (h). The female is assumed to be homozygous
for the sex factor, i.e. two sex chromosomes (X) are
present ; while the male has only one sex chromosome

138 HEREDITY AND SEX

carried by the female-producing sperm. The analysis
is then as follows: One "dose" of horns (H) in the
male produces horns, but two doses are necessary for
the female.

Hornless ? hX — hX

Horned 3 H X — H

Fi HXhX hornless 9

H h X horned $

Gametes
of Fi

Eggs HX — hX

Sperm HX — hX — H — h

- F2 FEMALES F2 MALES

HXHX horned HEX horned

H X h X hornless H h X horned

hXHX hornless HEX horned

h X h X hornless h h X hornless

As pointed out by Punnett a test of the correctness
of this interpretation is found by breeding the FI
hornless female to a hornless male (of a hornless breed) .
It is assumed that such a female carries the factors for
horns in a heterozygous condition ; if so, then half of
her sons should have horns, as the following analysis
shows :

Fi Hornless ? HX — hX
Hornless $ hX — h

/* X # X hornless T

h X h X hornless ?

hEX horned <J

hh X hornless c?

THE EFFECTS OF CASTRATION

139

GP

FIG. 71. — Upper figure normal male guinea pig (from below), to show
mammary glands. Lower figure, a feminized male ; i.e. castrated when
three weeks old and pieces of ovaries transplanted beneath the skin, at On.

140 HEREDITY AND SEX

The actual result conforms to the expectation. The
results of both of the experiments are consistent with
the view that one factor for horns in the male produces
horns, which we may attribute to the combined action
of the inherited factor and a secretion from the testes
which reenforces the action of the latter. This, how-
ever, should be tested by castrating the FI males. In
the females, one factor for horns fails to produce horns,
while two factors for horns cause their development.

Aside from some of the domesticated animals (horses,
cattle, dogs, cats, pigs), the only other mammals on
which critical experiments have been made — if we
exclude man — are the rat and the guinea pig. The
next case is unique in that tne ovary was transplanted
to a male.

Stginach removed the sex glands from the male
guinea pig and rat and transplanted into the same
animals the ovaries of the female, which established
themselves. Their presence brought about remarkable
effects on the castrated male. The mammary glands,
that are in a rudimentary condition in the male, be-
come greatly enlarged (Fig. 71). In the rat the hair
assumes the texture of that of the female. The skele-
ton is also more like that of the female than the male.
The size of the feminized rats and guinea pigs is less
than that of normal (or of castrated) males and
like that of the female (Fig. 72). Finally, in their
sexual behavior, the feminized rats were more like
females than like males. These cases are important
because they are the only ones in which success-
ful transplanting of the ovary into a male has been
accomplished in vertebrates.

THE EFFECTS OF CASTRATION

141

V

FIG. 72. — Two upper figures, normal male guinea pig to left, M, and
his brother, F, to right — a feminized male. Two middle and two lower
figures, a normal male at J/, and his feminized brother, F. (After Steinach.)

142 HEREDITY AND SEX

OPERATIONS ON BIRDS

In striking contrast to these results with mammals
are those with birds, where in recent years we have
gained some definite information concerning the devel-
opment of secondary sexual characters.

I am fortunate in being able to refer to several
cases — the most successful on record — carried out
by my friend, H. D. Goodale, at the Carnegie Lab-
oratory at Cold Spring Harbor. One "case" is that
of a female Mallard duck from which the ovary was
completely removed when she was a very young bird.
Figure 16 illustrates the striking difference between
the normal male and the female Mallard. In the
spayed female the plumage is like that of the male.

Darwin records a case in which a female duck in her
old age assumed the characteristics of a male, and
similar cases are recorded for pheasants and fowls.

Goodale also removed the ovary from very young
chicks. He found that the female developed the
secondary sexual plumage of the cock.

How shall we interpret these cases ? It is clear that
the female has the potentiality of producing the full
plumage of the male, but she does not do so as long as
the ovary is present. The ovary must therefore be
supposed to prevent, or inhibit, the development of
secondary sexual characters that appear therefore only
in the male.

The converse operation — the removal of the male
glands from the male — is an operation that is very
common among poultrymen. The birds grow larger
and fatter. They are known as capons. In this case

THE EFFECTS OF O^TRATION

143

/

the male assumes his full nprm^l plumage with all of

his secondary male sexual^ characters. It is said that
the comb and wattles and to soi/ie extent the spurs are
less developed in the capon thadi in the normal, male.
But aside from this it is /quite certain that the de-
velopment of the secondary sexual plumage in the

//

FIG. 73. — Male and female Seabright. Note short neck feathers and
incomplete tail cover in male. In the Seabright cock the sickle feathers on
back at base of tail are like those of the hen. (After " Reliable Poultry
Journal.")

male is largely independent of the presence of the sex
glands. ^,

The method of inheritance of the secondary sexual
characters in birds has been little studied. Daven-
port has reported one case, but I am not sure of his in-
terpretation.1 I have begun to study the question by
using Seabright bantams, in which the male lacks some

1 Because it is not evident whether the secondary sexual char-
acters as such are involved or only certain general features of
coloration.

144 HEREDITY AND SEX

of the secondary sexual characters of the domestic
races, notably the saddle feathers, as shown in Fig. 73.
A male Seabright was mated to a black-breasted game
female. The son was hen-feathered and like the Sea-
bright father in this respect. Evidently in this case
the secondary sexual character in question is dominant
and is transmitted from father to son.

In the reciprocal cross one hen was obtained which
was back-crossed to a recessive male. She produced
both hen-feathered and normally feathered sons. The
character appears therefore to be sex-limited but not
sex-linked. If hen-feathering in the Seabright be rep-
resented by S and its normal allelomorph by s, the
first cross would be as follows : —

Game ? sF — s

Seabright <J S — S

SsF female

Ss hen-feathered male

Eggs of Fl SF — sF — S — s

Sperm of FI S — s

F2 Females F2 Males
SSF SS hen-feathered

SsF Ss hen-feathered

sSF sS hen-feathered

ss cock-feathered

In conclusion, then, in mammals the secondary sexual
characters owe their development to the testes. - The
testes add something to the common inheritance.
But in birds the ovary takes something away.

THE EFFECTS OF CASTRATION 145

OPERATIONS ON AMPHIBIA

The male~triton develops each year a peculiar fin or
comb on the back and tail. Bresca has found that
after castration the comb does not develop. If present
at the time of castration, the comb is arrested, but
only after several months. Certain color marks pe-
culiar to the male are not lost after castration. If the
comb is removed in normal males, it regenerates, but
less perfectly in castrated males. If a piece of the
dorsal fin of the female is transplanted to a normal male
in normal position, it may later- produce the comb under
the influence of the testes.

In the frog there appears at the breeding season a
thickening of the thumb. Castrated males do not
produce this thickening.

If it is present in a male at the time of castration it
is thrown off, according to Nussbaum, but according to
Smith and Shuster its further progress only is arrested.
According to Nussbaum and Meisenheimer injection
of pieces of testes beneath the skin of a castrated male
cause the thumb development to take place, or to
continue, but Smith and Shuster question this con-
clusion.

Such are the remarkable relations that these experi-
ments have brought to light. How, we may ask, do
the sex glands produce their effect, in the one case to
add something, in the other to suppress something?

It has often been suggested these glands produce
their effects through the nervous system by means of
the nerves to or from the reproductive organs. This
has been disproved in several cases by cutting the

146 HEREDITY AND SEX

nerves and isolating the glands. The results are the
same as when they are left intact.

This brings us to one of the most interesting chapters
of modern physiology, the production and influence of
Internal Secretions.

INTERNAL SECRETIONS

It has become more and more probable that the effects
in question are largely brought about by internal se-
cretions of the reproductive organs. These secretions
are now called " hormones" or " exciters." They are
produced not only by glands that have ducts or outlets,
but by many, perhaps by all, organs of the body. Some
of these secretions have been shown to have very re-
markable effects. A few instances may be mentioned
by way of example.

The pituitary body produces a substance that has an
important influence on growth. If the pituitary body
becomes destroyed in man, a condition called gigan-
tism appears. The bones, especially of the hands and
\ feet and jaws, become enlarged. The disease runs a
short course, and leads finally to a fatal issue.

The thyroid and parathyroid bodies play an im-
portant role in the economy of the human body
through their internal secretions. Removal leads to
death. A diseased condition of the glands is asso-
ciated with at least six serious diseases, amongst them
cretinism.

The thymus secretion is in some way connected with
the reproductive organs. Vincent suggests that "the
thymus ministers to certain needs of the body before
the reproductive organs are fully developed."

THE EFFECTS OF CASTRATION 147

Extirpation of the adrenal bodies, another ductless
gland, leads to death. Injury to these bodies causes
Addison's disease.

Finally, the reproductive glands themselves produce
internal secretions. In the case of the male mammal it
has been shown with great probability that it is the
supporting tissues of the glands, and not the germ-cells,
that produce the secretion. Likewise, in the case of
the ovary, it appears that the follicle cells of the corpus
luteum give rise to an important internal secretion.
If the sac-like glands are removed, the embryo fails to
become attached to the wall of the uterus of the mother.
If the ovary itself is removed from a young animal,
before corpora lutea are formed, the uterus remains in
an infantile condition.

From a zoological point of view the recent experi-
ments of Gudernatsch are important. He fed young
frog tadpoles with fresh thyroid glands. " They began
very soon to change into frogs, but j^aseiLto^grow in
size. The tadpoles might begin tEeir metamorphosis
in a few days after the first application of the thyroid,
and weeks before the control animals did so."

In contrast to these effects Gudernatsch found that
tadpoles fed on thymus grew rapidly and postponed
metamorphosis. They might even, in fact, fail to
change into frogs and remain permanently in the tad-
pole condition. If thyroid extracts produce dwarfs;
thymus extracts make giant tadpoles that never become
adults.

These examples will suffice to show some of the im-
portant effects on growth that these internal secretions
may bring about.

148 HEREDITY AND SEX

OPERATIONS ON INSECTS

The Insects constitute the third great group in which
secondary sexual characters are common.

The first operations on the reproductive organs were
carried out by Oudemans on the gipsy moth, Ocneria
(Porthetria) dispar. The male anoTlemale are strik-
ingly different. Oudemans removed the testes from

FIG. 74. — Ovaries of Lymantria (Porthetria} dispar transplanted to male.
They have established connection with the sperm ducts. (After Kopec.)

young caterpillars and found no change in the color, or
size, of the male. He also removed the ovaries from
young caterpillars, and again found no effect in the fe-
male. The same experiments were later carried out on
a large scale by Meisenheimer, who obtained similar
results. Meisenheimer went further, however, and per-
formed another operation of great interest. He removed
the male glands from a male and implanted in their

THE EFFECTS OF CASTRATION

149

place the ovary of a female, while it was still in a very
immature condition. The caterpillar underwent its
usual growth, changed to a chrysalid, and then to a
moth. The moth showed the characters of the .male.
The presence of the ovary had produced no effect what-
ever on the body character of the individual. When
this individual was dissected, Meisenheimer found that
the ovary had completely developed. It contained
mature eggs, and the ovary had often established con-
nection with the outlets of the male organs that had

FIG. 75. — Testes of Lymantria (Porthetria) dispar transplanted to female.
They have connected with the oviducts. (After Kopec.)

been left behind, as seen in Fig. 74, taken from Kopec's
description.

The converse experiment was also made. The ovaries
were removed from young caterpillars, and in their
place were implanted the male sex glands from a young
male caterpillar. Again no effects were produced on the
moth, which showed the characteristic female size and
color. On dissection the testes were also found to have
grown to full size and to have produced spermatozoa
(Fig. 75).

These remarkable results, confirmed by Kopec, show

150

HEREDITY AND SEX

that in these insects the essential organs of reproduc-
tion have no influence on the secondary sexual char-
acters of the individual. They show furthermore that
the male generative organs will develop as well in
the female as in the body of the male itself, and vice
versa.

It is evident, then, in insects (there is a similar, but
less complete, series of experiments on the cricket),

FIG. 76. — Papilio Memnon. 1, male; 2, 3, 4, three types of females.
(After Meijere.)

that the heredity of the secondary sexual characters
can be studied quite apart from the influence of the
sex glands. How, then, are they inherited so that they
appear in one sex and not in the other sex? Within
the last two or three years the inheritance of the second-
ary sexual differences in insects has been studied.

First, there is the case of the clover butterfly, Colias
philodice, that Gerould has worked out, where there

THE EFFECTS OF CASTRATION 151

are two types of females and one kind of male
(Fig. 66). *

Without giving the analysis of this case I may say
that the results can be explained on a Mendelian basis.
The peculiar feature of Gerould's explanation is that
two doses of the yellow-producing determiner in the
female give yellow color — one dose gives white. In
the male, on the other hand, one dose of yellow gives
yellow.

The second case is that of PjipiJ^memnon, worked out
by de Meijere from the experiments of Jacobson. There
is one male type and three female types, Fig. 76. De
Meijere accounts for the results of matings in this
species recorded by Jacobson on the assumption of
three factors, one for each type of female. The three
factors are treated as allelomorphs, and therefore only
two of them can be present in any one individual, and
since they are allelomorphs they will pass into different
gametes. The order of dominance is Achates, Agenor,
Laomedon. The male carries these same factors, but
they are not effective in him. Baur accounts for the
results in a somewhat different way, but involving or-
dinary Mendelian conceptions.

An interesting case is that reported by Foot and
Strobell. They crossed a female of a bug, Euschistw
variolarius, the male of which has a black spot on the
end of the body (the female lacking the spot), with a
male of Euschistus servus that lacks the spot both in
the males and the females (Fig. 77). The daughters
had no spot ; the sons had a faint spot, less developed
than in variolarius. When these (Fi) offspring were
inbred, they obtained 249 females without a spot,

152 HEREDITY AND SEX

107 males with a spot (developed to different degrees),
and 84 males without a spot. The authors give no
explanation of their results — but they use the re-

FIG. 77. — To left, in 1, is male of Euschistus variolarius, to right male of
E. serous. 2 and 3 show eight F2 males; 4 shows seven F2 males from
another mating. (After Foot and Strobell.)

suits to discredit some of the explanations, that rest
on the assumption that the chromosomes are the chief
factors in Mendelian heredity. I venture, neverthe-
less, to suggest the explanation shown on the accom-

THE EFFECTS OF CASTRATION 153

panying diagram (Fig. 78). The analysis rests on the
assumption that neither one, nor two doses of S in the
female is able to produce a spot, while in the male one
dose of S suffices.

E. variolarius ? SX --SX
E. servus $ sX — s

F,

.sXSX spotless $
sXS spotted $

Gametes
ofF,

(Eggs sX — SX
(Sperm sX — SX — s — S

sXsX

sXSX
SXsX

SXSX ,

spotless ?

sXs spotless c?

sXS spotted g

SXs spotted (J

SXS spotted (J

FIG. 78. — Diagram to show inheritance of spot when E. rariplarius (?)
is mated to E. servus (cT). S = spot. s= no spot. X = sex chromosome,
that does not carry the factor S for spot.

It is very important to understand just what is meant
by this ; for otherwise it may seem only like a restate-
ment of the facts. In the F2 female with the formulae
SXSX, with two doses of the S factor, no spot is as-
sumed to appear (nor in the hybrid female SXsX). At
first sight it seems that a female having the formula
SXSX is only double the male with sXSy especially if
small s is interpreted to mean absence of spots. But
this view, in fact, involves a misconception of what the
factorial hypothesis is intended to mean.

154 HEREDITY AND SEX

To make this clearer, I have written out the case
more fully :

X ABC S X ABCS ?
X ABC S ABC s S

In this, as in all such Mendelian formulae, the result
(or character) that a factor produces depends on its
relations to other things in the cell (here ABC). We
are dealing, then, not with the relation of X to S alone,
but this relation in turn depends on the proportion of
both X and S to A B C. It is clear, if this is admitted,
that the two formulae above — the one for the male
and the other for the female — are neither identical
nor multiples.

It will be noted that in only one of these attempts to
explain in insects the heredity of the secondary sexual
characters have the factors for the characters been
assumed to be carried by the sex chromosomes. If
one accepts the chromosome basis for heredity, these
results may be explained on the assumption that the
factors lie in other chromosomes than the sex chromo-
somes.

In the next case, however, that I shall bring forward
the factors must be assumed to be in the sex
chromosomes themselves.

The mutant of drosophila with eosin eyes that arose
in my cultures is the case in question. The female
has darker eyes than the male. The experimental
evidence shows that the factor for eosin is carried
by the sex chromosomes. In the female it is present,
therefore, in duplex, or, as we say, in two doses; in
the male in one dose.

THE EFFECTS OF CASTRATION 155

The difference in color can be shown, in fact, to be
due to this quantitative relation. If, for instance, an
eosin female is mated to a white-eyed male, her
daughters have light eyes exactly like those of the
eosin male. The white-eyed fly lacks the eosin factor
in his sex chromosomes (as suitable matings show),
hence the hybrid female has but one dose of eosin,
and in consequence her eye color becomes the same as
the male.

In this case a sex-linked character is also a secondary
sexual character because it is one of the rather unusual
cases in which a factor in two doses gives a stronger
color than it does in one dose.

PARASITIC CASTRATION OF CRUSTACEA

Let us turn now to a group in which nature performs
an interesting operation.

Giard first discovered that when certain male crabs
are parasitized by another crustacean, sacculina (a .
cirriped or barnacle) , they develop the secondary sexual
characters of the female. Geoffrey Smith has confirmed
these results and carried them further in certain re-
spects. Smith finds that the spider crab, Inachus*
mauritanicus, is frequently infected by Sacculina neglecta
(Fig. 79). The parasite attaches itself to the crab and ,
sends root-like outgrowths into its future host. These
roots grow like a tumor, and send ramifications to all
parts of the body of the crab.

The chief effect of the parasite is to cause complete
or partial atrophy of the reproductive organs of the
crab, and also to change the secondary sexual charac-
ters. Smith says that of 1000 crabs infected by

156

HEREDITY AND SEX

sacculina, 70% of both males and females showed
alterations in their secondary sexual characters.

As a control, 5000 individuals not infected were ex-
amined, only one was unusual, and this one was a her-
maphrodite (or else a crab recovered from its parasite).

FIG. 79. - — A male of Inachus mauritanicus (upper left hand). Female of
Inachus scorpi (lower left hand) . Male of Inachus mauritanicus carrying on
its abdomen two specimens of Danalia curvata and a small Sacculina neglecta
(upper right hand). Male of Inachus mauritanicus with a Sacculina neglecta
on it (lower right hand). The abdomen and chelae of the host are inter-
mediate in character between those of an ordinary male and female. (After
Geoffrey Smith.)

As the figures (Fig. 80) show, the adult male has
large claws ; the female, small ones. He has a narrow
abdomen; she has a broad one. In the male there
is a pair of stylets on the first abdominal ring (and
a pair of greatly reduced appendages behind them).
The adult female has four biramous abdominal append-
ages with hairs to carry the eggs.

THE EFFECTS OF CASTRATION

157

^ &

FIG. 80, — 1, adult normal male; 2, under side of abdomen of normal

158 HEREDITY AND SEX

The infected males "show every degree of modi-
fication towards the female type." The legs are small,
the abdomen broad, the stylets reduced, and the
typical biramous appendages with hairs appear.

When the female crab is infected she does not change
" toward" the male type, although the ovary may
be destroyed. The only external change is that the
abdominal appendage may be reduced.

In a hermit crab, Eupagurus meticulosus, infected by
Peltogaster curvatus, similar results have been obtained.
The infected male assumed the ordinary sexual char-
acters of the female, but the females showed no change
towards the male.

In these cases it seems probable that the testes of
the male suppress the development of the secondary
sexual characters that appear ordinarily only in the
females. The case is the reverse of that of the birds
and different again from that of the mammals.

In birds and mammals the secondary sexual charac-
ters are in many cases directly dependent on the in-
ternal secretions of the sex glands. These secretions are
carried alike to all parts of the body, hence the absence
of bilateral gynandromorphs in these groups.

adult male; 3, male infected with sacculina, showing reduction of chela
and slight broadening of abdomen; 4, 5, showing attenuated copulatory styles
and slight hollowing out of abdomen; 6, under side of abdomen of a similar
male specimen, showing reduction of copulatory styles and presence of
asymmetrically placed swimmerets characteristic of female; 7, infected
male which has assumed complete female appearance; 8, under side of
abdomen of 7, showing reduced copulatory styles and swimmerets; 9, under
aide of abdomen of similar male specimen with well-developed copulatory
styles and swimmerets; 10, adult female, normal; 11, under side of abdomen
of 10, showing swimmerets and trough-shaped abdomen; 12, under side of
abdomen of infected female, showing reduction of swimmerets; 13, immature
female showing small flat abdomen; 14, under side of abdomen of 13,
showing flat surface and rod-like swimmerets. (After Geoffrey Smith.)

THE EFFECTS OF CASTRATION 159

CONCLUSIONS

In conclusion it is evident that the secondary sexual
characters in four great groups, viz. mammals, birds,
Crustacea, and insects, are not on the same footing.
Their development depends on a different relation to
the reproductive organs in three of the groups, and is
independent of the reproductive organs hi the fourth.
It is not likely, therefore, that their evolution can be
explained by any one theory, even by one so broad in
its scope as that of sexual selection.

If, for example, in the mammals a, mnrp vigorous
male, due to greater development of the testes, were
" selected " by a female, the chances are that his second-
ary sexual characters will be better developed than are
those of less vigorous males, but he is selected, not on this
account, but hppansp of his vigor. If a male bird were
"selected" on account of greater vigor, it does not
appear that his secondary sexual characters would
be more excessively developed than those of less vig-
orous males, provided that his vigor were due to the
early or greater development of the testes. If in
birds the male by selecting the female has brought
about the suppression of the male plumage, which is
their common inheritance, he must have done so by
selecting those females whose ovaries produced the
greatest amount of internal secretions which suppresses
male-feathering. Moreover, he must have selected,
not fluctuating variations, but germinal variations.
"Tn insects the development of the secondary sexual
characters is not connected with the condition of the
reproductive organs, but is determined by the complex

160 HEREDITY AND SEX

of factors that determines sex itself. If selection acts
here, it must act directly on germinal variations, that
are independent in origin of the sex-determining factor,
but dependent on it for their expansion or suppression.

These considerations make many of the earlier state-
ments appear crude and unconvincing ; for, they show
that the origin of the secondary sexual characters is
a much more complex affair than was formerly im-
agined.

These same considerations do not show, however,
that if a new germinal character appeared that gave
its possessor some advantage either by accelerating
the opposite sex to quicker mating or by being corre-
lated with greater vigor and thereby making more
certain the discovery of a mate, such a character would
not have a better chance of perpetuation. But in
such a case, the emphasis no longer lies on the idea
of selection with its emotional implications, but rather
on the appearance of a more effective machine that
has arisen, not because of selection, but, having arisen
quite apart from any selective process, has found itself
more efficient. Selection has always implied the idea
that it creates something. Now that the evidence
indicates that selection is not a guaranteed method
of creating anything, its efficiency as a means of easy
explanation is seriously impaired.

CHAPTER VI

GYNANDROMORPHISM, HERMAPHRODITISM,
PARTHENOGENESIS, AND SEX

THREE different sex conditions occur in animals and
plants that have a direct bearing on problems of
Heredity and Sex.

The first condition is called GyjiandrorjaLO_rphism -
a condition in which one part of the body is like the
male, and the other part like the female.
i v The second condition is called Hermaphroditism -
a condition in which the individuals of a species are all
alike — maleness and femaleness are combined in
the same body. Two sets of reproductive organs are
present.

The third condition is called Parthenogenesis -
a condition in which the eggs of an animal or plant
develop without being fertilized.

GYNANDROMORPHISM

Gynandromorphs occur most frequently, in fact
almost exclusively, in insects, where more than one
thousand such individuals have been recorded. They
are most abundant in butterflies, common in bees
(Fig. 81) and ants, rarer in other groups. They
occur relatively more often, when two varieties, or
species, are crossed, and this fact in itself is signifi-
cant. A few examples will bring the cases before us.

In my cultures of fruit flies several gynandro-

161

162 HEREDITY AND SEX

morphs have arisen, of which two examples are shown
in Fig. 82. In the first case the fly is female on one
side, as shown by the bands of her abdomen, and male
on the other side (upper right-hand drawing).

In the second case the fly looked like a female seen
from above. But beneath, at the posterior end, the
genital organs of the male are present, and normal

FIG. 81. — A gynandromorph mutillid wasp, Pseudomethoca canadensis,
male on right side, female on left side. (After Wheeler.)

in structure. In the latter case the fly is ostensibly
a female, except for the male organs of reproduction.

How can we interpret these cases? We find a
clue, I think, in the bee. It is known that if the egg
of the bee is fertilized, it produces a female — only
female-producing sperms are formed. If it is un-
fertilized, it produces a male. In the bee two polar
bodies are produced, and after their extrusion the num-
ber of chromosomes is reduced to half, as in ordinary
cases. The haploid number produces a male; the
double number produces a female.

Boveri pointed out #iat'*P through any chance the

GYNANDROMORPHISM

163

entering sperm should fail to reach the egg nucleus
before it divides, it may then fuse with one of the
halves of the egg nucleus after that divides. From the

V

FIG. 82. — Two gynandromorphs of Drosophila ampelophila. Upper
left-hand figure, female dorsally, male ventrally (as seen in third figure,
lower line). Upper right-hand figure, male on left side, female on right,
and correspondingly the under side shows the same difference (lower row,
last figure to right. Lower row from left to right; normal female, normal
male, vertical gynandromorph and lateral gynandromorph.

half of the egg containing the double nuclei female
structures will develop ; frojjri the other half, contain-
ing the half number of chromosomes, male structures
(Fig. 83, A). Here we ha^a very simple explanation
of the gynandromorphism.

164

HEREDITY AND SEX

There is another way in which we may imagine that
the results are brought about. It is known that two or

FIG. 83. — Diagram, illustrating on left (A) Boveri's hypothesis, on right
(B) the author's hypothesis, of gynandromorphism.

more spermatozoa frequently enter the egg of the bee.
Should only one of them unite with the egg nucleus,
the parts that descend from this union will be female.
If any of the outlying sperm should also develop,

GYXAXDROMORPHISM 165

they may be supposed to produce male structures
(Fig. 83, B).

The first case of the fly, in which one half the body
is male and the other female, would seem better in
accord with Boveri's hypothesis. In its support
also may be urged the fact that Boveri and Herbst
have shown that the belated sperm-nucleus may
unite with one of the two nuclei that result from the
first division of the egg nucleus.

On the other hand, the second case of the fly (where
only a small part of the body is male) may be better
accounted for by my hypothesis. It is known that
single sperms that enter an egg without a nucleus,
or even with one, may divide. The two hypotheses
are not mutually exclusive, but rather supplementary.

Toyama has described a gynandromorph in the
silkworm that arose in a cross between a race with a
banded caterpillar (the female parent) and a race
with a white caterpillar (the male parent). As shown
in Fig. 84, the gynandromorph was banded on the left
(maternal) side and white on the other (right) side.
When the adult moth emerged, the left side was female
and right side was male. Since the sperm alone bore
the white character, which is a recessive character, it
appears that the right side must have come from sperm
alone. This is in accordance with my hypothesis, but
I have also shown that Gynandromorphs may arise
through somatic dislocations of the sex chromosomes
in the early embryo. Gynandromorphs are not un-
common in insects, rare (or never present) in birds and
mammals.

The explanation K f this difference is found, I think, in

166

HEREDITY AND SEX

* *

FIG. 84. — I, a, plain, 6, striped caterpillar of silkworm. II, a, gynandro-
morph silkworm, b, moth of same. Ill, wings of last. IV, dorsal view of
same moth. V, abdomen of same. VI, end of abdomen of same moth
VII, normal female, and VIII, a normal male. (After Toyama.)

HERMAPHRODITISM 167

the relation of the secondary sexual characters to the
sex glands. In insects the characters in question are not
dependent on the presence or absence of these glands.
Hence, when such conditions occur after fertilization,
as those I have just considered, each part may develop
independently of the rest.

HERMAPHRODITISM

In almost all of the great groups of animals a condi-
tion is found in which complete sets of ovaries and testes
occur in the same individual. This condition is called
"hermaphroditism." In some groups of animals, as in
flatworms, leeches, mollusks, hermaphroditism is the
rule, and it is the common condition in flowering
plants. Sometimes there is only one system of outlets
for eggs and sperm, but not infrequently each has a
separate system.

Here there is no problem of the production of males
and females, for one kind of individual alone exists.
But what determines that in one part of the body
male organs develop, and in another part a female
system ?

Two views suggest themselves, either somatic segre-
gation, or regional differentiation. By somatic seg-
regation I mean that at some time in the development
of the embryo — at some critical division — a separa-
tion of chromosomes takes place so that an egg-produc-
ing group and a sperm-producing group is formed.
There is no direct evidence in support of this view.

Another view is that the formation of ovary and
testis is brought about in the same way as all
differentiations of body organs, as for example the

168

HEREDITY AND SEX

formation of liver and lungs and pancreas from the
digestive tract. The following case may perhaps
be considered as supporting such an hypothesis. In
a hermaphroditic worm, Criodrilus lacuum the ovaries
lie in the thirteenth and the testes in the tenth and
eleventh segments. If the anterior end be cut off, a
new one regenerates, as shown by Janda (Fig. 85),

Fig. 1.

FIG. 85. — 1, anterior end of normal criodrilus, showing reproductive
system; 2-5, regenerated anterior ends. (After Janda.)

in which the ovaries and testes reappear approximately
in their appropriate regions. It is true their location
is more liable to vary than in the normal worm, but
this is unimportant.

In the Gephyrean worm, Bonellia viridis, the degen-
erate male is parasitic on the female. Baltzer has dis-
covered that every fertilized egg is potentially a male
or a female. For, if the swimming larvae have a chance

HERMAPHRODITISM 169

to settle down on an adult bonellia they become males ;
but if the larvae have no such opportunity a long period
without further development intervenes, and later the
larvae become, for the most part, females, a few 'become
males, and a few hermaphrodites. Bonellia appears
.therefore to be a protandric hermaphrodite, — like many
plants. Somewhat similar relations are known for Hydra
viridis, as shown by Nussbaum, and by Whitney.

In cases where a sexual generation alternates with
a hermaphroditic generation, the problem of the two

FIG. 86. — Rhabditis nigrivenosa, male (left) and female (right). (After

Leunis.)

sexes reappears. There is but one case in animals
that has been adequately worked out. A nematode
worm, Rhabditis nigrovenosa, lives as a parasite in
the lungs of frogs. It is an hermaphrodite. Its
eggs give rise to another generation that lives in mud
and slime. In this generation two kinds of individuals
are present — true males and females (Fig. 86). The
females produce eggs, that are fertilized, and develop

170

HEREDITY AND SEX

into the hermaphrodites which find their way again
into the lungs of frogs.

Boveri and Schleip have worked out the history
of the chromosomes in this case. The cells of the

FIG. 87. — Chromosomes of Angiostomum. (A), oogonia, CB), equa-
torial plate of first maturation division; (C), young spermatocyte
(D) , first spermatocyte division in metaphase ; (E) , same in anaphase
(F), spermatocyte of second division; (G), and (#), division of same
(/), and (K), loss of X at plane of division ; (L), first segmentation division
of a male embryo ; two sets of chromosomes (5 and 6=11 respectively)
separate ; (M ) equatorial plate of dividing cell of female embryo = 12
chromosomes ; (N) , same from male embryo =11 chromosomes. (After
Schleip.)

hermaphrodite have twelve chromosomes (Fig. 87).
The eggs, after extruding two polar bodies, have
six chromosomes. The spermatozoa that develop
in the body of the same animal have six or five chro-
mosomes each, because one chromosome is lost in half

HERMAPHRODITISM 171

of the cells by being left at the dividing line between
the two cells. We can understand how two kinds
of individuals are produced by the hermaphrodites
from the two classes of sperm combining at random
with the eggs.

These two kinds of individuals are females with
twelve chromosomes, and males with eleven chromo-
somes. How then can we get back to the hermaph-
roditic generation? Boveri and Schleip suggest that
the males again produce two kinds of spermatozoa, -
they have shown this to be the case in fact, — and that
the male-producing spermatozoa become function-
less. Here we have at least an outline of some of
the events in the life cycle of this worm in relation
to the chromosomes, but no explanation of hermaph-
roditism.

Turning to plants, there are the interesting experi-
ments of the Marchals with mosses. They show that
the sporophyte is hermaphroditic and has the factors
for maleness and femaleness combined as a result
of fertilization ; while in the formation of the spores
the factors in question are separated.

Blakeslee has found somewhat similar relations in
certain of the molds. The spores in molds contain
more than one nucleus, therefore it is not clear how
segregation in the sense used for other cases applies
here.

In the flowering plants that are hermaphroditic
we have Correns' experiments, in which he crossed an
hermaphroditic type of Bryonia alba with a type
B. dioica in which the sexes are separate. The
cross when made one way gives only females, while

172 HEREDITY AND SEX

the reciprocal cross gives males and females in equal
numbers. Correns' interpretation is shown in the
lower part of the next diagram.

Bryonia dioica and B. alba
B. dioica $ by B. alba $ B. alba 9 by B. dioica $

\ / \ /

\ / \ /

Females Females and Males

Correns1 Explanation

F - F B. dioica £ (FM)-(FM} B. alba 9

(FM )— (FM ) B. alba $ F - M B. dioica $

F(FM) female F(FM) female

male

It is based in the first case on the assumption that
the hermaphroditic condition of B. alba is recessive to
the dioecious condition of B. dioica, and that the female

tlialeTf. -HcrmabH. FH

i>— H

f H fwotv. -PH Txemv.

"f" — H Kerriv. ovtile
'F — -p mate

FIG. 88. — Diagram to illustrate G. H. Shull's results on Lychnis dioica.
The symbols here used are not those used by Shull. Two types are assumed
not to appear, viz. HH and Hf. Third cross should give FH also.

PARTHENOGENESIS 1 73

dioica is homozygous for the sex factor. The recip-
rocal cross is explained on the basis that maleness
dominates femaleness. The sex-determining factors
must here be different from cases like the insects.

Shull obtained as a mutant a hermaphroditic plant
of Lychnis dioica. The next diagram (Fig. 88) gives
the principal facts of his crosses. When a female
plant is fertilized by the pollen of the hermaphrodite,
two kinds of offspring are produced — females and
hermaphrodites. When the hermaphrodite is self-
fertilized, the same two classes are produced. When
the ovule of the hermaphrodite is fertilized by the
pollen from the male plant, two kinds of offspring
are again produced — female and male. ShmTs inter-
pretation is too involved to give here. In the diagram
the scheme is worked out on the purely arbitrary
scheme that the hermaphrodite is FH, in which F
is a female factor, and H a modification of it which
gives hermaphroditism. This leads to the further
assumption that ovule and pollen, bearing the H
factor, cannot produce a plant nor can the combination
/ H. This scheme is only intended as a shorthand way
of indicating the results, and not as an interpretation
of actual conditions.

PARTHENOGENESIS

A third important condition in which the heredity
of sex is involved is found in parthenogenesis.

It has long been known to biologists, that in many
different species of animals and plants eggs develop
without being fertilized. This is recognized as a
regular method of propagation in some species. The

174 HEREDITY AND SEX

eggs are produced in the same way as are other eggs.
They are produced in ovaries that have the same
structure as the ovaries that give rise to ordinary
eggs. Parthenogenetic eggs differ from spores, not
only in their origin in an ovary, but in that they also
produce polar bodies like ordinary eggs. Most, but
not all, parthenogenetic eggs give rise, however, to
only one polar body. Some of them at least fail to
pass through the- stage of synapsis, and, in consequence,
they retain the full number of chromosomes.

e ;'^«..

FIG. 89. — Miastor, sexual male and female (to right). Three larvae
with young inside (to left).

A few examples will bring the main facts before us.

A fly, miastor, appears in the spring of the year
under two forms, male and female (Fig. 89) . The eggs
are fertilized and each produces a worm-like larva.
This larva produces eggs while still in the larval stage.
The eggs develop without fertilization, and produce
new larvae, which repeat the process. This method
of propagation goes on throughout the rest of the
year until finally the adult winged flies reappear.

The bee is the most remarkable instance, for here

PARTHENOGENESIS 175

the same egg will produce, if it is fertilized, a female
(queen or worker), or, if it is not fertilized, a male
(drone). If the queen deposits an egg in a cell of the
comb that has been built for a queen or a worker, she
fertilizes the egg ; if in a drone cell, the egg is not fertil-
ized. We need not conclude that the queen knows
what she is about — the difference in shape of the drone
cell may suppress the reflex, that in the other cases
sets free the sperm.

The case of the bee has attracted so much attention
that I may be allowed to pause for a moment to point
out some of the most recent results connected with the
formation of the germ-cells.

The egg produces two polar bodies — the process
being completed after the sperm has entered the fer-
tilized egg (Fig. 90). Eight chromosomes are present
at each division. Eight remain in the egg (these are
double chromosomes — therefore 16). The sperm
brings in 8 (double) chromosomes so that the female
comes to have 16 single chromosomes in her cells. There
is only one kind of spermatozoon, as shown by the figure,
for the first spermatocyte division is abortive — all
the chromosomes passing into one cell only, and the
second division gives rise to a small cell, that does not
produce a spermatozoon, and a large cell that becomes
a spermatozoon.

If the egg is not fertilized, it also gives off two polar
bodies. It has 8 chromosomes left. The male de-
velops with the half number. The formula for the
female will be XABCD XABCD and for the male
XABCD.

If the bee conforms to the ordinary type for insects,

176 HEREDITY AND SEX

we may suppose that one sex chromosome is present
in the male or at least one differential factor for sex,
and that it is present in all the functional spermato-
zoa. The female will then have two such chromo-
somes and come under the general scheme for insects.

16 <?

FIG. 90. — Oogenesis and spermatogenesis in bee. Four upper figures,
A-D, show formation of first (A), and second (B) polar bodies. Only inner
group of chromosomes remains (C) to form egg nucleus. Entrance of sperm
nucleus in D. E shows scheme of these two divisions involving eight double
(82) chromosomes. F, first and second spermatocyte divisions, the first,
a, b, abortive, leading to pinching off of a small cell without a nucleus, the
second, c, c, leading to formation of a large (functional) and an abortive
cell (above).

In the gall fly, Neuroterus lenticularis, partheno-
genetic females appear early in the spring. Their eggs
produce females and males — the second generation.
The fertilized eggs of these females give rise the follow-
ing year to the spring parthenogenetic females. Don-
caster has found that each parthenogenetic female

PARTHENOGENESIS

177

produces eggs, all of which give rise to females or else
to males. In connection with this fact he finds that
the eggs of some females do not give off any .polar
bodies but retain the full number (20) of chromosomes.

ft*

FIG. 91. — Illustrating chromosome cycle in Neuroterus. A, one type of
-spring female, whose eggs (containing 20 chromosomes) produce no polar
bodies. Only sexual females result. B, the other type of spring female
whose eggs form two polar bodies, leaving 10 chromosomes in egg. These
eggs give rise to males. C, ripening of egg of sexual female (2d generation),
and D, spermatogenesis of male (second generation).

These eggs produce sexual females (in left-hand side
of Fig. 91). From the eggs of other parthenogenetic fe-
males two polar bodies are given off, and the half (10)
number of chromosomes is left in the egg (see right-hand
side of Fig. 91). These eggs produce males. The life

178

HEREDITY AND SEX

cycle finds its explanation in these relations except that
the origin of the two kinds of parthenogenetic females
is unexplained. If we were justified in assuming that
two classes of female-producing sperm are made in the
male, even this point would be cleared up, for in this

FIG. 92. — Life cycle of Phylloxera carycecaulis.

way the two classes of parthenogenetic females could
be explained.

In another group of insects, the aphids and phyllox-
erans, the situation is different.

In the phylloxerans of the hickories there emerges
in the spring, from a fertilized egg, a female known as
the stem mother (Fig. 92). She pierces a young leaf

PARTHEXOGENESIS 179

with her proboscis, which causes a proliferation of the
cells of the leaf. Eventually the leaf cells grow so fast
that the stem mother is overarched in the gall that she
has called forth.

Inside the gall she begins to lay her eggs. From these
eggs emerge young individuals that remain in the gall
until they pass their last molt, when they become winged
migrants. Externally all the migrants are alike; but
if they are dissected, it will be found that some of them
have large eggs, some small eggs. But all the offspring
of the same mother are of one or of the other sort.

The migrants crawl out of the opening in the gall and
fly away. Alighting on other hickories, they quickly
•deposit their eggs. From the large eggs the sexual
females emerge. They never grow any bigger than the
egg from which they hatched. In fact, they have no
means of feeding, and contain only one large egg with
a thick coat — an egg almost as large as the female
herself.

From the small eggs of the migrants, minute males
are produced — ripe at their birth. They fertilize
the sexual female. She then deposits her single egg on
the bark of the hickory tree. From this egg (that lies
dormant throughout the entire summer and following
winter) there emerges next spring a female, the stem
mother of a new line.

Here we find three generations in the cycle — two
of which reproduce by parthenogenesis. The first
parthenogenetic generation gives rise to two kinds of
individuals — one makes large eggs, the other small
eggs. The large eggs produce sexual females, the small
eggs males.

180 HEREDITY AND SEX

A study of the chromosomes has explained how some
of these changes in successive generations are brought
about. It has explained, for instance, how males are
produced by parthenogenesis, and why the sexual egg
produces only females. Let us take up the last point
first.

When the spermatocytes are produced, we find, as in
many other insects, that at one division a sex chromo-
some passes to one cell only (Fig. 93) . Two classes of
cells are produced — one with three, one with two,
chromosomes. The latter degenerates, and in conse-
quence only the female-producing spermatozoa become
functional. All fertilized eggs give rise therefore to
females.

The second point that has been made out concerns
the production of the male. When the small egg
produces its single polar body, all of the chromosomes
divide, except one, which passes out entire into the
polar body. In consequence the number of chromo-
somes left in the egg is one less than the total number.
In a word, there are five chromosomes in the male,
while there are six chromosomes in the female (Fig. 93^.
By throwing out one chromosome, the change is effected.
The chromosome is the mate of the sex chromosome,
that appeared as a lagging chromosome in the spermato-
genesis.

In the large egg no such diminution takes place,
consequently the diploid number of chromosomes is
present in the female. These unite in pairs and are
reduced to three when the two polar bodies of the
sexual egg are produced.

We see that by means of the chromosomes we can

PARTHENOGENESIS

181

bring this case into line with the rest of our informa-
tion bearing on the relation of the chromosomes to sex.
One important point still remains to be explained.
What causes some of the migrants to produce large

PHYLLOXERA CAKWECAUIJS

7>lcct*

9? tern, 7?lcthv& -C

O

o

O

9 f?

O

o

o

FIG. 93. — Chromosomal cycle of P. carycecaulis.

182 HEREDITY AND SEX

v-

eggs and others small eggs ? There must be either two
kinds of stem mothers or one kind with double po-
tentiality. Inasmuch as in other parthenogenetic types
there is experimental evidence to prove that environ-
mental conditions determine which alternative state,
whether male-producing or female-producing individ-
ual, is realized, so here we may, provisionally, follow
the same interpretation. Once the course is deter-
mined the subsequent internal events follow for two
generations in a definite order. If the stem mother has
been affected in one way, all of her daughters produce
large eggs ; if in the other way, small eggs.

In another group of animals, the daphnians, parthen-
ogenetic species occur, that, in certains respects, are
like the phylloxerans ; but these species illustrate also

\ another relation of general interest.

* The fertilized winter egg produces always a female,
the stem mother, which gives rise by parthenogenesis
to offspring like herself, and the process may continue
a long time. Each female produces one brood, then
another and another. The last broods fail to develop,
and this is a sign that the female has nearly reached
the end of her life.

But a parthenogenetic female may produce one or two
large resting eggs instead of parthenogenetic females,
and the same female may at another time produce a
brood of males. The large resting eggs are inclosed
in a thick outer protecting case. They must be fer-
tilized in order to develop, yet they do not develop at
once, but pass through an enforced, or a resting, stage
that may be shortened, if the egg is dried and then
returned to water.

PARTHENOGENESIS

183

a

/t

/ 2> 3 -k 3 & 7

I

0

0

0

0

0

0

0

0

0

0

0

0

O

0

0

0

°0

O

0

o

o

0

0

0

o

0

©

o

©

0

0

°

0

(D

0

0

0

1

9.

O

0

0

°

©

CD

cb

(D

0

V

0

0

0

0

0

°0

0

0

0

FIG. 94. — Life cycle of Simocephalus ; successive broods in horizontal
lines, successive generations in vertical lines. (After Papanicolau.)

184 HEREDITY AND SEX

In this life history we do not know what changes
take place in the chromosomes. It has, however, often
been claimed in this case that the transition from par-
thenogenesis to sexual reproduction is due to changes in
the environment.

In fact, this is one of the stock cases cited in the older
literature to show that sex is determined by external
agents. It was said, that if the environment causes
males to appear, then sex is determined by the environ-
ment. But as a matter of fact, in so far as changes in
the environment affect this animal, they cause it to
cease reproducing by parthenogenesis, and induce sexual
reproduction instead. The evidence is consistent in
showing that any external change that affects the
mode of reproduction at all calls forth either sexual
eggs or males. The machinery of parthenogenesis
is switched off, and that for sexual reproduction is
turned on.

The discrepancies that appear in the older accounts
are probably due, as Papanicolau suggests, to dif-
ferent observers using females that belong to different
phases of the parthenogenetic cycle. Papanicolau,
starting in each case with a winter egg, finds that as
successive broods are produced the color of the par-
thenogenetic eggs can be seen to undergo a progressive
change from blue to violet. As the change progresses
the chance that males and sexual eggs (" females ") will
appear is greater. Until towards the end of the life of
the individual the males and females come, as it were,
of themselves (Fig. 94). If, however, individuals of
successive broods are subjected to cold, it is found that
while earlier broods do not respond, later ones respond

PARTHENOGENESIS 1 85

more and more easily and change over to the sexual
phase of the cycle.

What has just been said about the successive broods
might be said equally of the first-born offspring of the
successive generations, as Papanicolau's table shows
(Fig. 94). Later born offspring respond more readily
than do those that are historically nearer to the fer-
tilized egg.

More recently Grosvenor and Smith have found for
Moina that if females are reared alone (at 25°-30° C.)
no sexual broods appear, while parallel cultures of
females crowded together give 30 or higher per cents
of males. Agar has carried isolated females through 46
parthenogenetic generations and has found that mothers
from late brood also give only parthenogenetic offspring
in a suitable environment.

A third type, Hydatina senta (Fig. 95), an almost
microscopic wormlike animal belonging to the rotifers,
reproduces by parthenogenesis.

The resting egg always gives rise to a parthenogenetic
female, i.e. she also reproduces by parthenogenesis.
Whitney has obtained 500 generations produced in this
way. But from time to time another kind of individual
appears. She is externally like the parthenogenetic
female, but has entirely different capacities. Her
eggs may be fertilized, and if they are they become
resting eggs inclosed in a hard case. The sperm enters
when the eggs are immature and still in the ovary of
the mother. The presence of a spermatozoon in an egg
determines that the egg goes on to enlarge and to pro-
duce its thick coat. But if perchance no males are
there to fertilize the eggs, this same female produces a

186

HEREDITY AND SEX

crop of male eggs that develop into males without
being fertilized at all.

There are several facts of unusual interest in the

HYDAT/MA SEMTA

Male

FIG. 95. — Life cycle of Hydatina senta.

life history of hydatina, but we have occasion to consider
only one of them. It has been claimed in this case
also that external conditions determine the production
of males. A more striking example of the erroneous-

PARTHENOGENESIS 187

ness of this general conclusion would be hard to find ;
for, in the first place, as we have seen, the same indi-
vidual that produces males will produce out of the same
eggs females if she happens to be fertilized. In
the second place the older evidence which was supposed
to establish the view that certain specified changes in
the environment cause the production of males has
been overthrown.

The French zoologist, Maupas, is deserving of high
praise for working out some of the most essential facts
in the life cycle of hydatina, and for opening up a
new field of investigation. But the evidence which
he brought forward to show that by a low tempera-
ture a high production of males is caused has not
been confirmed by very careful and extensive repeti-
tion of his experiments by Whitney and by A. F.
Shull. The evidence that Nussbaum obtained which
seemed to him to show that food conditions de-
termined the production of males has likewise not
borne the test of more recent work by Punnett, Shull,
and Whitney.

It has been found, however, that the production of
the sexual phase of the cycle can be suppressed so that
the animals continue almost indefinitely propagating
by parthenogenesis. In several ways this may be
accomplished. If hydatina is kept in a concentrated
solution of the food culture, the sexual phase does not
appear. The result has nothing to do with the abun-
dance of food, for, if the food be filtered out from
the fluid medium, the filtrate gives the same result.
The following table given by Shull shows this very
clearly.

188

HEREDITY AND SEX

SPRING WATER

OLD CULTURE FILTRATE

One-fourth

One-half

Three-fourths

Undiluted

cf ?

9 9

<J ?

? 9

d1 ?

? 9

cf 9

9 9

d 9

9 9

26

177

25

407

15

350

8

362

0

337

%ofc?9

12.8

5.7

4.1

2.1

0.0

Showing the number of male- and female-producers in the progeny
of five sister individuals of Hydatina senta, one line being reared in
spring water, the others in various concentrations of the filtrate
from old food cultures.

Mitchell found that a sudden change in food causes
male-producing individuals to appear in Asplanchna;
and Whitney can now produce, at will, in Hydatina a
high percentage of male producers. Females feeding
on the colorless flagellate Polytoma were fed on the
green flagellate Duniella, and gave birth to 80 % of
male producers ; while the control female fed on Poly-
toma gave birth to only 9 % of male producers. Since
the eggs of the male producer give rise to sexual fe-
males, or to males, according to whether they are fer-
tilized or whether they are not, sex itself is not here
determined by the environment, but by fertilization ;
the environment determines the kind of reproduction.

ARTIFICIAL PARTHENOGENESIS

We have now considered some of the most striking
examples of natural parthenogenesis in the animal
kingdom. The facts show that fertilization of the egg
is not in itself essential for development. The in-

ARTIFICIAL PARTHENOGENESIS 189

dividuals that develop from parthenogenetic eggs are as
vigorous as those from eggs that have been fertilized.
We have seen that such eggs without being fertilized
are capable of producing sexual females and males.
In one case, at least, we have seen how the process is
accomplished.

When we review the facts of natural parthenogenesis,
we find certain relations that arrest our attention.

Most parthenogenetic eggs give off only a single
polar body, while fertilized eggs without exception give
off two polar bodies. This difference is clearly con-
nected with the fact that in parthenogenetic eggs the
full number or diploid number of chromosomes is re-
tained by the egg.1 In fertilized eggs half the chromo-
somes are thrown out in one of the two polar bodies.
The number is made good by the chromosomes brought
in by the spermatozoon.

But this difference does not in the least explain nat-
ural parthenogenesis ; for we have experimental evi-
dence to show, that an egg will develop when only half
the number of chromosomes is present — one set will
suffice.

There is another fact about parthenogenetic eggs
that has, I believe, been generally overlooked. Many
of these eggs begin to develop into an embryo before
they reach the full size of the fertilized eggs of the
same species. This is true at least of the eggs of aphids,
phylloxerans, daphnians, and rotifers. I interpret this

1 According to my observations on aphids and phylloxerans, the
synapsis stage is omitted in parthenogenetic eggs, hence there is
no union (or reduction) of the chromosomes. The omission of this
stage may have something to do with parthenogenesis, although it
is not evident what the relation may be.

190 HEREDITY AND SEX

to mean that the eggs begin their development be-
fore there has been produced over their surface a
layer that in the mature egg seems to have an im-
portant influence in restraining sexual eggs from de-
velopment.

This brings us at once to a consideration of what
keeps sexual eggs from developing until they are fer-
tilized.

In recent years a great variety of methods has been
discovered by means of which sexual eggs can be made
to develop without fertilization. This process is
called artificial parthenogenesis. We owe especially
to Professor Jacques Loeb the most successful accom-
plishment of this important feat. The discovery in
his hands has led to very great advances in our
understanding of the developmental process.

The chief importance of Loeb's work lies, in my
opinion, not only in the production of embryos with-
out fertilization (nature has long been conversant
with such methods), but in other directions as well.

First, it has thrown light on the nature of the in-
hibitory process that holds back the sexual egg from
developing until the sperm enters.

Second, the information gained in this way tells us
something of how the sperm itself may act on the egg
and start it on its course.

Third, it opens up the opportunity of studying cer-
tain problems connected with the determination of sex
that can be gained in no other way.

Let me attempt briefly to elaborate some of these
points.

In many eggs, perhaps in all, a membrane is produced

ARTIFICIAL PARTHENOGENESIS 191

at the surface of the egg immediately after the
sperm has entered. Here we have ocular evidence
that fertilization effects a change in the surface layer
of the egg.

It has been shown that after this membrane is formed,
the permeability of the egg to salts and other agents is
affected and that the processes of oxidation are greatly
accelerated.

In other words, the ulterior of the unfertilized egg is
separated by means of its membrane from many things
in the surrounding medium — oxygen and the salts hi
sea-water, for example. The egg after fertilization
lives hi a new world.

These same changes are brought about by those
external agents that cause artificial parthenogenesis.
Loeb has shown by a thorough study of the conditions
that any substance that causes cytolysis (a typical form
of cell destruction) will induce parthenogenesis if ap-
plied to the surface of the egg only.

Loeb has shown that development depends not
only on a change hi the surface of the egg, but on other
changes also. Hence his most successful methods are
those in which two agents are applied successively
to the egg — one affects primarily the surface, the other
the interior of the egg. If, for example, the eggs are
placed in a solution of a fatty acid, the membrane is
produced. The egg is then removed to pure sea
water from which oxygen has been driven out and left
there for three hours. After its return to sea water it
will produce a normal embryo.

If, instead of jnitting the egg into water without
oxygen, a hypertonic solution of salts is used (50 cc.

192 HEREDITY AND SEX

of sea water plus 8 cc. of 2^2 NaCl), the development
may be carried through.

Loeb concludes that the oxidations set up in the egg
by a change in its outer surface affect the egg itself
injuriously; and unless they are removed or the
effects are counterbalanced by some other change
(as when a hypertonic solution is used) the egg goes
to pieces. Hence he believes that the sperm has a
double r61e in fertilization. First it changes the surface
layer and increases in consequence the oxidations
in the egg ; second, the sperm brings into the egg some
substance that counteracts poison produced by the
oxidation itself.

This is what fertilization accomplishes from a
physiological point of view. In addition, we have
seen that fertilization brings into the egg certain ma-
terials whose presence affects the characters of the
individuals that develop from it. This is what fertili-
zation does from the point of view of the student of
heredity.

Let us turn for a moment, in conclusion, to the
question of sex of animals that come from artificially
parthenogenetic eggs.

In natural parthenogenesis such eggs may de-
velop into males, sexual females, or parthenogenetic
females.

But in artificial parthenogenesis the egg has already
undergone reduction in its chromosomes and is repre-
sented by half of the female formula as far as the
chromosomes are concerned. The half formula will
be XABC for the type with homozygous female.
Since the egg has one X it may be expected to become

ARTIFICIAL PARTHENOGENESIS 193

a male, but if sex is a relation of X to ABC, one cannot
be certain that it might not be a female.

In cases where the female is heterozygous for the
sex factor, as in birds and some sea urchins, the formula
for the female would be XABCD — YABCD and for
the male YABCD — YABCD. There would be two
types of eggs, XABCD and YABCD. The former
might be expected to produce a female, the latter prob-
ably a male if such eggs were incited artificially to
develop.

Concerning the sex of the embryos so far produced
by artificial parthenogenesis, we know of only two
cases. These two cases are Delages' result for the
sea urchin, in which he got one male, and Loeb's and
Bancroft's case for the frog, in which they believe that
the two young obtained were males.

What to expect on theoretical grounds is uncertain.
We have only two facts that bear on the question.
In the parthenogenetic eggs of the aphid, with the for-
mula XABC ABC we get a male. In the case of the
bee the formula is XABC, which also gives a male. All
else is hypothetical and premature, but if these two
formulae are correct, it appears that one X gives a
male and that maleness is not due to a quantitative
relation between X and one or two sets of the other
chromosomes. It is the quantity of something in X,
not the relation of this to the rest of the chromosomes.

CHAPTER VII

FERTILITY

DARWIN'S splendid work on cross- and self-fertiliza-
tion, his study of the mechanism of cross-fertilization in
orchids, and his work on the different forms of flowers
of plants of the same species, mark the beginning of
the modern study of the problem of fertility and
sterility. Darwin carried out studies on the effects
of cross-fertilization in comparison with self-fertilization
and reached the conclusion that the offspring resulting
from cross-fertilization are more vigorous than the
offspring from self-fertilization. No one can read his
books dealing with these questions without being
impressed by the keenness of his analysis and the
open-minded and candid spirit with which the prob-
lems were handled. Since Darwin's time we have not
advanced very far beyond the stage to which Darwin
carried these questions. We have more extensive
experiments and some more definite ways of stating
the results, but Darwin's work still stands as the most
important contribution that has been made to this
subject.

The credit of the second advance belongs to Weis-
mann. His speculations concerning the effects of
mixing of the germ-plasms of the two individuals,
that combine at the time of fertilization, not only
aroused renewed interest in the nature of the process
of sexual reproduction, but brought to light also the

194

FERTILITY 195

effects of recombination of the different sorts of qualities
contained in the parental strains. His attack on the
hypothesis of rejuvenation that was so generally held
at that time did very great service in exposing the
mystical nature of such an imagined effect of cross-
fertilization. In particular, Weismann's endeavor to
connect the theory of recombination with the facts
of maturation of the egg and sperm has opened our
eyes to possibilities that had never been realized before.
His work has led directly to the third advance that
has been made hi very recent years, when the results
of Mendelian segregation have been applied directly
to the study of fertility and sterility.

As I have said, Darwin's work showed that cross-
fertilization is generally beneficial. The converse
proposition has long been held that continued inbreed-
ing leads to degeneration and to sterility. This opinion
rests largely on the statements of breeders of domesti-
cated animals and plants, but there is also a small
amount of accurate data that seems to support this
view. I propose first to examine this question, and
then consider what cross-fertilization is supposed to do,
in the light of the most recent work.

Weismann inbred white mice for 29 generations,
and Ritzema-Bos bred rats for 30 generations. In
each case the number of young per litter decreased
in successive generations, more individuals were sterile
and many individuals became weakened. This evi-
dence falls in line with the general opinion of breeders.

On the other hand, we have Castle's evidence on
inbreeding the fruit fly through 59 generations. He
found some evidence of the occurrence of sterile pairs

196 HEREDITY AND SEX

(mainly females), but we must be careful to distinguish
between the appearance of sterile individuals in these
cultures and the lessened fertility that may be shown
by the stock in general. The recent work of Hyde on
these same flies has shown that the appearance of
sterile individuals may be an entirely different question
from that of a decrease in general fertility. The
latter again may be due to a number of quite different
conditions. Castle and his co-workers found that the
sterile individuals could be eliminated if in each genera-
tion the offspring were selected from pairs that had not
produced sterile individuals. Hyde has found, in
fact, that one kind at least of sterile females owe their
sterility to a definitely inherited factor that can be
eliminated as can any other Mendelian recessive
trait. Moenkhaus, who has also extensively studied the
problem of inbreeding in these flies has likewise found
that his strains could be maintained at their normal
rate of propagation by selecting from the more fertile
pairs.

If we eliminate from the discussion the occurrence
of sterile individuals, the question still remains whether
the output of the fertile pairs decreases if inbreeding
is carried on through successive generations. There
is some substantial evidence to show that this really
takes place, as the following figures taken from Hyde's
results show.

FI F2 Fs FI F5 FQ FU

368 209 191 184 65 119 156

At the end of thirteen generations the fertility
of the stock was reduced by half, as determined in this

FERTILITY 197

case by the average number of flies per pair that
hatch. But this is not a measure of the number of
eggs laid or of those that are fertilized.

Whether inbreeding where separate sexes exist is sim-
ilar to self-fertilization in hermaphroditic forms is not
known. Darwin gives results of self-fertilization in Ipo-
mcea pur pur ea for ten generations. The effects vary so
much in successive generations that it is not possible
to state whether or not the plant has become less
fertile. His evidence shows, however, that the cross-
fertilized plants in each of the same ten generations
are more vigorous than the self-fertilized plants, but
this does not prove that the latter deteriorated.

The problem has been studied in other ways. Some
animals and plants propagate extensively by partheno-
genesis ; others by means of simple division.

Whitney and A. F. Shull kept parthenogenetic strains
of Hydatina senta for many generations. Whitney
carried a strain of this sort through 500 generations.
Towards the end the individuals became weak, the
reproductive power was greatly diminished, and finally
the strain died out. No attempt was made to breed
from the more fertile individuals, although to some
extent this probably occurred at times. If we admit
that weakened individuals appear sometimes in these
lines and their weakness is inherited, then each time
such an individual happened to be picked out a step
downward would be taken ; when the more fertile
individuals chanced to be selected, the strain would be
temporarily held at that level. But on the whole
the process would be downwards if such downward
changes are more likely to occur than upward ones.

198 HEREDITY AND SEX

This is an assumption, but perhaps not an unreasonable
one. Let me illustrate why I think it is not unreason-
able. If the highest possible point of productivity
is a complex condition due to a large number of things
that must be present, then any change is more likely
to be downward, since at the beginning the high-water
mark had been reached. In time casual selection would
be likely to pick out a poor combination — if this hap-
pened once the likelihood of return would be small.

As we have seen (Chapter I) Maupas found in a
number of protozoa that if he picked out an individual
(after each two divisions) to become the progenitor
of the next generation, the rate of division after a
time slowed down. The individuals became weaker
and finally the race died out. Calkins repeated the
experiments with paramcecium on a larger scale and
obtained similar results. The question arose whether
the results were not due to the hay infusion lacking
certain chemical substances that in time produced an
injurious effect. Calkins tested this by transferring
his weakened strains to different culture media. The
result was that the race was restored to more than
its original vigor. But very soon degeneration again
set in. A new medium again restored vigor to some
degree, but only for a short time, and finally the
oldest culture died out in the 742d generation. It
was evident, therefore, that if the slackened rate of
division and other evidences of degeneration were in
part due to the medium, yet some of the effects
produced were permanent and could not be effaced by
a return to a more normal medium. Then came
Woodruff's experiments. He kept his paramoecia on

FERTILITY 199

a mixed diet — on the kind of materials that it would
be likely to meet with in nature, alternating with hay
and other infusions. He found no degeneration, and
at his last report his still vigorous strain was in the
3000th generation.

Recently Woodruff has found in his long-lived race
that under proper conditions individuals will conjugate.
Woodruff and Erdmann found in this race, although
not allowed to conjugate, that periodically the macro-
nucleus breaks down and several micromere divisions
take place. Finally a new nuclear apparatus of micro-
nuclear origin is reconstructed. The process is com-
parable to the nuclear changes prior to conjugation
except that the last micronu clear division is omitted.
This periodic change is not peculiar to this race of
Paramcecium, but appears in other races that regularly
conjugate.

Let us turn now to the other side of the question
and see what results cross-fertilization has given.

Hyde has found that if two strains of flies with low
fertility are crossed, there is a sudden increase in the
output, as seen in the diagram (Fig. 96). The facts
show clearly an improvement. More eggs of each
strain are fertilized by sperm from the other strain
than when the eggs are fertilized by sperm from the
same strain.1 In this case the results are not due
to a more fertile individual being produced (although
this may be true) but to foreign sperm, acting better
than the strain's own sperm. The evidence, as such,

1 The upper line Fi-Fu gives the average output of flies per pair.
Below this line the percentages mean the number of isolated eggs
that hatched.

200

HEREDITY AND SEX

does not show whether this is due to each strain having
degenerated in certain directions, or to some other
kind of a change in the heredity complex.

The egg counts show that in the inbred stock many
of the eggs are not fertilized, or if fertilized (32%)
they still fail to develop. This means a decrease
in fertility in the sense in which that word is here
-used. The offspring that arise from the cross-fer-
tilization of these strains are more vigorous than their
parents, if their increased fertility be taken as the
measure of their vigor. The latter result is not shown
in the table, for here 52% and 58% are the percent-
ages of fertile eggs produced when the two strains are
crossed.

-History of Irtfrred Sfock.

Fl 2 3 45 67 8 9 10 11 \Z Fl3

368 209 <9f 184 65 H9 ------ f£fc

Cro55 of
1rwtuate9 fry Truncate d

52%

58%

FIG. 96. — The horizontal line Fi-Fi3 gives the average number of flies
per pair that emerged from inbred stock, decreasing from 368 to 156 per pair.
B-elow is shown the results of a cross between a race of Truncates (short
wings) and Fis. The percentages here give the number of eggs that hatched
in each case.

Darwin found that cross-fertilization was bene-
ficial in 57 species of plants that he studied. In the

FERTILITY

201

case of primula, which is dimorphic, he found not only
that self-fertilization gave less vigorous plants, but
that when pollen from a long-styled flower of one plant
fertilizes the pistil of another long-styled plant the
vigor of the offspring is less than when the same kind
of pollen is used to fertilize the pistil of a short-styled
flower. The next table gives the detailed results.

NATUBE OF UNION

NUMBER OF
FLOWERS
FERTILIZED

NUMBER
OF SEED
CAPSULES

MAXIMUM
OF SEEDS IN
ANY ONE
CAPSULE

MINIMUM
OF SEEDS IN
ANY ONE
CAPSULE

AVERAGE
No. OP
SEEDS PER
CAPSULE

Long-styled form by

pollen of short-
styled form:

10

6

62

34

46.5

Legitimate union.

Long-styled form by

own-form pollen:

20

4

49

2

27.7

Illegitimate union.

Short-styled form by

pollen of long-
styled form :

10

8

61

37

47.7

Legitimate union.

Short-styled form by

own-form pollen :

17

3

19

9

12.1

Illegitimate union.

The two legitimate
unions together.

20

14

62

34

47.1

The two illegitimate
unions together.

37

7

49

2

21.0

We know now that these two types of plants — long-
styled and short-styled — differ from each other by
a single Mendelian factor. We may therefore state

202

HEREDITY AND SEX

Darwin's result in more general terms. The hetero-
zygous plant is more vigorous than the homozygous
plant. Moreover, in this case it is not the presence
of the dominant factors that makes greater vigor (for
the short-styled plant containing both dominants is
less vigorous than the heterozygous), but the presence
of two different factors that gives the result.

FIG. 97. — At left of figures there are two strains of pure bred corn and
at right the hybrids produced by crossing those two pure strains. (After
East.)

The most thoroughly worked out case of the effects
of inbreeding and cross-breeding is that of Indian corn.
In recent years East and G. H. Shull have studied on
a very large scale and with extreme care the problem
in this plant. Their results are entirely in accord on
all essential points, and agree with those of Collins,
who has also worked with corn.

East and Shull find that when two strains of corn

FERTILITY

203

(that have been to a large extent made pure) are crossed,
the offspring is more vigorous than either parent (Fig.

FIG. 98. — At left an ear of Learning Dent corn, and another at right
after four years of inbreeding. The hybrid between the two is shown in the
middle ear. (After East.)

97). This is clearly shown in the accompanying pic-
tures. Not only is the hybrid plant taller and stronger,
but in consequence of this, no doubt, the yield of corn

204 HEREDITY AND SEX

per bushel is much increased, as shown in the next
figure (Fig. 98).

When the vigorous FI corn is self -fertilized, it produces
a very mixed progeny, more variable than itself. Some
of the F2 offspring are like the original grandparental
strains, some like the corn of first generation, and
others are intermediate (Fig. 99).

FIG. 99. — No. 9 and No. 12, two inbred strains of Learning Dent corn
compared with Fl and F2 (to right). (After East.)

It will not be possible for us to go into an analysis
of this case, but Shull and East have shown that the
results are in full harmony with Mendelian principles
of segregation. The vigor of the FI corn is explained
on the basis that it is a hybrid product. To the extent
to which the two parent strains differ from each other,
so much the greater will be the vigor of the offspring.

This seems an extraordinary conclusion, yet when
tested it bears the analysis extremely well.

Shull and apparently East also incline to adopt the

FERTILITY 205

view that hybridity or heterozygosity itself is the basis
for the observed vigor ; but they admit that another
interpretation is also possible. For instance, each of
the original strains may have been deficient in some of
the factors that go to make vigor. Together they give
a more vigorous individual than themselves.

Whitney ran one line of hydatina through 384 par-
thenogenetic generations, when it died (Line A). An-
other line was carried through 503 generations, and at
the last report was in a very weakened condition (Line
B). When the former line was becoming extinct, he
tried inbreeding. From the fertilized eggs he ob-
tained a new parthenogenetic female. It showed
scarcely any improvement. The other line gave similar
results. In one case he again inbred for a second time.
He found that the rates of reproduction of lines A and
B were scarcely, if at all, improved.

Whitney then crossed lines A and B. At once an
improvement was observed. The rate of reproduction
(vigor) was as great as that in a control line (reared
under the same conditions) that had not deteriorated.

The experiments of A. F. Shull on hydatina were
somewhat different. He began with the twelfth gen-
eration from a sexual egg. The line was supposedly
not in a weakened condition. He inbred the line and
obtained from the fertilized egg a new parthenogenetic
series. After a few generations he inbred again. The
results are shown in the next table. It is clear that
there has been a steady decline despite sexual repro-
duction, measured by four of the five standards that
Shull applied, namely, size of family of parthenogenetic
females, and of sexual females, number of eggs per day,

206

HEREDITY AND SEX

SHOWING DECREASE OF VIGOR, AS MEASURED BY VARIOUS CHAR-
ACTERS, IN Six SUCCESSIVELY INBRED PARTHENOGENETIC LINES
OF Hydatina senta

m

CHARACTER TO BE MEASURED

NUMBER OF PARTHENOGENETIC
LINE

1

2

3

4

5

6

I.

Size of family of parthenogenetic female . .
Size of family of fertilized sexual female . .
Number of eggs laid per day

48.4
16.7
11.0
2.27

1/11

42.5
12.8
11.4
1.66

1/3

46.8
12.8
10.3
2.25

2/4

42.5
11.5
10.0
1.93

3/16

31.0

6.3
9.2
2.25

0/4

22.6
7.3
7.5
2.12

5/8

Number of days required to reach maturity
Proportion of cases in which first daughter
did not become parent

Same in percentages

14.2

25.0

41.6

II.

Size of family of parthenogenetic female . .
Size of family of fertilized sexual female . .
Number of eggs laid per day

48.4
16.7
11.0
2.27

1/11

30.8
13.7
11.6
1.55

4/9

41.0
13.5
7.9
2.57

2/7

37.0
15.2

7.7
2.20

2/10

33.8
10.1
9.6
1.90

8/20

24.8
7.6
8.6
2.00

7/16

Number of days required to reach maturity
Proportion of cases in which first daughter
did not become parent

Same in percentages

25.0

23.5

41.6

number of times the first daughter was too weak to
become the mother of a new line. It is clear that
inbreeding did not lead to an increase in vigor.

In paramcecium there is also some new evidence.
Calkins in 1904 brought about the conjugation of two in-
dividuals of a weak race in the 354th generation. From
one of the conjugants a new line was obtained that
went through another cycle of at least 376 generations
in culture, while during the same time and under sim-
ilar conditions the weakened race from which the con-
jugants were derived underwent only 277 generations.

Jennings has recently reported an experiment in
which some paramcecia, intentionally weakened by
breeding in a small amount of culture fluid, were

FERTILITY 207

allowed to conjugate. Most of the lines that descended
from several pairs showed no improvement but soon
died out. In only one case was an individual produced
that was benefited by the process.

Jennings' results are, however, peculiar in one very
important respect. He did not use a race that had run
down as a result of a long succession of generations, but
a race that he had weakened by keeping under poor
conditions. We do not know that the result in this
case is the same as that in senile races or inbred races
of other workers. It is not certain that the hereditary
complex was affected in the way in which that complex
is changed by inbreeding. He may have injured some
other part of the mechanism.

Jennings interprets conjugation in paramoecium to
mean that a recombination of the hereditary factors
takes place. Some of these combinations may be more
favorable for a given environment than are others.
Since these will produce more offspring, they will soon
become the predominant race.

The next diagram (Fig. 100) will serve to recall the
principal facts in regard to conjugation in paramce-
cium. Two individuals are represented by black and
white circles. At the time of conjugation the small
or micronucleus in each divides (B), each then divides
again (C). Four nuclei are produced. One of these
micronuclei, the one that lies nearest the fusion point,
divides once more, and one of the halves passes into the
other individual and fuses there with another nucleus.
The process is mutual. Separation of the two indi-
viduals then takes place and two ex-con jugants are
formed. Each has a new double nucleus. This nu-

208 HEREDITY AND SEX

cleus divides (G) and each daughter nucleus divides
again (H), so that each ex-con jugant has four nuclei.

FIG. 100. — Diagram to show the history of the micronuclei of two
Paramoecia during (A-F) and after (F-J) conjugation. Compare this dia-
gram with Fig. 2.

Another division gives eight nuclei in each. The para-
mcecium itself next divides — each half gets four nuclei.
A second division takes place, and each gets two of
the nuclei. Four new individuals result. In each of

FERTILITY 209

these individuals one of the nuclei remains small and
becomes the new micronucleus, the other enlarges to
form the new macronucleus. Thus from each ex-
con jugant four new paramoecia are produced, which
now proceed to divide in the ordinary way, i.e. the
micronucleus and the macronucleus elongate and divide
at each division of the animal.

It is customary to regard some phase in this process
as involving a reduction division in the sense that a
separation of the paired factors takes place. If this
occurs prior to interchange of micronuclei (E), then each
ex-con jugant corresponds to an egg after fertilization.
It is conceivable, however, that segregation might oc-
cur in the two divisions that follow conjugation, which
would give a different interpretation of the process
than the one followed here.

On the first of these two hypotheses two new strains
result after conjugation. Each is a recombination of
factors contained in the two parents. If the two par-
ents were alike, i.e. homozygous, in many factors, and
different, i.e. heterozygous, in a few, the two individuals
would be more alike than were the original races from
which they came. This is, in fact, what Jennings has
shown to be the case, at least he has shown that on
the average the ex-con jugant s are more like each other
than were the original strains.

Calkins has obtained some new and important facts
concerning the likeness and unlikeness of the new
strains that result from conjugation. He has used
wild, i.e. not weakened, individuals, and has followed
the history of the four lines resulting from the first
four individuals produced by each ex-con jugant. The

210

HEREDITY AND SEX

history of six such ex-con jugants is shown in the next
diagram (Fig. 101). The four lines, "quadrants,"
(1, 2, 3, 4) that are descended from each of six ex-
conjugants (viz. G, #, L, M, Q, B) are shown. At
intervals large numbers of the populations were put
under conditions favorable to conjugation and the

in/ »)<jc €& cen/uynn£i

Mr a. flee. Z) c

~F

"•'&

N<,

/<

v:

1iO

-&€-

FIG. 101. — History of six (G, H, L, M, Q, B) ex-conjugants. In each
the descendants of the first four individuals (after conjugation) is shown;
the numbers indicate the pairs of conjugants counted when the test was
made. -X" indicates deaths; O indicates that no conjugation took place.
(After Calkins.)

number of conjugating pairs counted. The results
are shown in the diagram. The circles indicate no
conjugations ; X indicates the death of the strain.
In the G and in the M series many conjugations took
place. In other series conjugation did not take place
until much later. Striking differences appear in the
different quadrants although they were kept under
similar conditions.

FERTILITY 211

But even amongst the four lines descended from the
same ex-con jugant marked differences exist. These
differences cannot be attributed to constitutional dif-
ferences unless a segregation of factors takes place
after conjugation or unless it can be shown that these
differences are not significant. In the light of these
conflicting results on paramoecium it may seem unsafe
to draw any far-reaching conclusions concerning the
nature of sexual reproduction in general from the evi-
dence derived from these forms. In the higher animals,
however, the evidence that segregation takes place
prior to fertilization and that recombinations result
can scarcely be doubted.

THEORIES OF FERTILITY

Let us now try to sum up the evidence in regard to
the influence of cross-fertilization. This can best be
done by considering the three most important hypoth-
eses that have been brought forward to explain how
crossing gives greater vigor.

Shull and East explain the vigor of the hybrid by
the assumption that it contains a greater number of dif-
ferent factors in its make-up than either of its parents.
They support the view by an appeal to the next (F2)
generation from such hybrids that shows a lower
range of vigor, because, while a few individuals of this
generation will be as mixed as the hybrid (Fi), and
therefore like it, most of them will be simpler in com-
position. This interpretation is also supported by the
evidence that when pure lines (but not necessarily,
however, homozygous lines) are obtained by self-fer-
tilizing the offspring of successive generations from

212 HEREDITY AND SEX

these first hybrids, further decline does not take
place.

An alternative view, that is also Mendelian, has been
offered by Bruce and by Keeble and Pellew. Vigor, it
is maintained, is in proportion to the number of domi-
nant factors, and in proportion to the number of these
factors present whether in a hybrid or in a homozygous
(duplex) condition.

On this view the hybrid is vigorous, not because it is
hybridous, so to speak, but because in its formation a
larger number of dominant factors (than were pres-
ent in either parent) have been brought together.

A third view is also compatible with the evidence,
namely, that there may exist factors that are them-
selves directly concerned with fertility. There is one
such case at least that has been thoroughly analyzed
by Pearl.

Pearl studied for five years the problem of fertility
in two races of fowls, viz. barred Plymouth rocks and
Cornish Indian games. The main features of his
results are shown in the diagram (Fig. 102). He finds
that the winter output of eggs, which is correlated
with the total production, is connected with two factors.
One factor, designated by LI, is a non-sex-linked char-
acter. If it is present, an average of less than 30 eggs
is produced in the winter season. There is another
factor, L2, that is present in the barred rocks, but not
in the Indian game. If present alone, the winter out-
put is again about 30 eggs on an average. If, how-
ever, both LI and L2 are present, the winter output
is more than 30 and may be as great as 90, or in rare
cases 100-120 eggs.

FERTILITY 213

The peculiarity about this discovery is that the
second factor, L2, is sex-linked, which means in this case
that it is carried by the eggs that will produce the males
in the next generation, and not by the eggs that will
produce the daughters. Hence if the daughters of high-
producing hens are selected, one does not get in them

irtheriran.ee of FertlliTy in fowl.
Low 9 F.L, L, Ft* —

F. L, ^e L, ^ Loir 9

Kt;";% %$£

9 F— L*

o l^ — -tt

(L£) <3

*> i a.

LO»V 9

T

FIG. 102. — Illustrating Pearl's hypothesis. F = female factor present
in half of the eggs and determining sex. L\ = factor for low egg produc-
tion; /i, its allelomorph for zero production of winter eggs. L* = factor
for high winter production; /2, its allelomorph.

the high productiveness of the mother. It is her sons
that inherit the character, although they cannot show
it except in their offspring.

Aside from whatever practical interest these results
may have, the facts are important in showing that such
a thing as a factor for fertility itself may be present,
without otherwise being apparent, and that this factor

214

HEREDITY AND SEX

taken in connection with another (or others) gives high
productivity.

The other point to which I wish to call attention
relates to a different matter. We have met with some
cases where lowered fertility was due to eggs failing

FIG. 103. — Normal male of Drosophila (on left) and male with "rudi-
mentary" wings (on right). Note sex comb (lower left).

to a greater or less degree to be fertilized by sperm of
the same strain.

A striking case of this kind is found in a mutant of
the fruit fly that appeared in my cultures. The mu-
tant has rudimentary wings (Fig. 103). The females
are absolutely infertile with males of the same kind.

FERTILITY 215

If they are mated to any other male of a different strain,
they are fertilized. The males, too, are capable of fer-
tilizing the eggs of other strains, in fact, are quite
fertile.

The factor that makes the rudimentary winged
fly is of such a sort that it carries infertility along with
it — in the sense of self-infertility. This result has
nothing to do with inbreeding, and the stigma cannot
be removed by crossing out and extracting.

A somewhat similar factor, though less marked, is
found by Hyde in certain of his inbred stock to which
I have referred. As his experiments show, the infer-
tility in this case is not due to lack of eggs or sperm, but
to a sort of incompatibility between them so that not
more than 20 per cent of the eggs can be fertilized by
males of the same strain.

In the flowering plants where the two sexes are often
combined in the same individual, it has long been known
that there are cases in which self-fertilization will not
take place. The pollen of a flower of this kind if placed
on the stigma of the same flower or of any other flower
on the same plant will not fertilize the ovules. Yet the
pollen will fertilize other plants and the ovules may be
fertilized by foreign pollen.

Correns has recently studied that problem and has
arrived at some important conclusions. He worked
with a common plant, Cardamine pratensis. In this
plant self-fertilization is ineffectual. He crossed plant
B with plant G, and reared their offspring. He tested
these with each other and also crossed each of them back
to its parents that had been kept alive for this pur-
pose. The latter experiment is simple and more in-

216

HEREDITY AND SEX

structive. His results and his theory can best be
given together.

Correns assumes that each plant contains some factor
that produces a secretion on the stigma of the flowers.
This secretion inhibits the pollen of the same plant
from extending its pollen tube. He found, in fact,
that the pollen grains do not grow when placed on the
stigma of the same plant. All plants will be hybrid

9
rf

BG

B

FIG. 104. — Illustrating the crossing of the types Bb and Gg to give four
classes : BG, Bg, bG, bg. Each of these is then back-crossed either to B or
to G with the positive (+) or negative ( — ) results indicated in the diagram.

for these factors, hence plant B will produce two kinds
of germ-cells, B and 6. Similarly, plant G will produce
two kinds of germ-cells, G-g. If these two plants are
crossed, four types will be produced. When these are
back-crossed to the parents, the expectation is shown in
the diagram (Fig. 104). Half the combination should
be sterile and half should be fertile. This is, in fact,
what occurs, as shown in the same diagram. The
— signs indicate that fertilization does not occur, while
the + signs indicate successful fertilization.

Correns' theory is also in accord with other com-

FERTILITY 217

binations that he made. There can be little doubt
that he has pointed out the direction in which a solu-
tion is to be found.

There is a somewhat similar case in animals. In one
of the Ascidians, Ciona intestinalis, an hermaphrodite,
the sperm will not fertilize the eggs of the same indi-
vidual. But the sperm will fertilize eggs of other
individuals, and vice versa. Castle first found out this
fact, and I have studied it on a large scale. The
diagram (Fig. 105) gives an example of one such ex-
periment made recently by W. S. Adkins.

Five individuals are here used. The eggs of one
individual, A, were placed hi five dishes (horizontal
line) ; likewise those of B, C, D, E. The sperm of A,
designated by a (vertical lines) was used to fertilize
the eggs, A, B, C, D, E ; likewise the sperm 6, c, d, e.
The self-fertilized sets form the diagonal line in the
diagram and show no fertilization. The other sets
show various degrees of success, as indicated by the
percentage figures. These results can best be under-
stood, I think, by means of the following hypoth-
esis. The failure to self-fertilize, which is the main
problem, would seem to be due to the similarity in the
hereditary factors carried by eggs and sperm ; but
in the sperm, at least, reduction division has taken
place prior to fertilization, and therefore unless each
animal was homozygous (which from the nature of the
case cannot be assumed possible) the failure to fertilize
cannot be due to homozygosity. But both sperm and
eggs have developed under the influence of the total
or duplex number of hereditary factors; hence they
are alike, i.e. their protoplasmic substance has been

218

HEREDITY AND SEX

under the same influences. In this sense, the case is
like that of stock that has long been inbred, and has

jSetf and

in Gona.

Aa

Ah

A°

Aa

Ae

o

8?

£Z

84-

^>6

Ba

Bb

Bc

Bd

B*

38

o

35

^<f

•97

c-

c*

cc

Cd

Cc

92>

96

o

97

9e

Da

D*

Dc

Da

De

.p/

98

//

O

83

E3

V

Ec

Ea

Ee

9$

9Z

eo

/*

#

FIG. 105. — The oblique line of letters A°, 56, Cc, Z)d, ^e, gives the self-
fertilized sets of eggs; the rest Ab, Ac, etc., the cross-fertilized sets. A, B,
C, D, E = eggs ; a, b, c, d, e, = sperm of same individuals. (From unpub-
lished work of W. S. Adkins.)

come to have nearly the same hereditary complex. If
this similarity decreases the chances of combination be-
tween sperm and eggs, we can interpret the results. Cor-
rens' results may come under the same interpretation.

FERTILITY 219

I have tried to bring together the modern evidence
that bears on the problems of fertility and sterility.
It is evident that there are many obscure relations that
need to be explained. I fear that, owing to the diffi-
culty of summarizing this scattered and diverse ma-
terial, I have failed to make evident how much labor
and thought and patience has been expended in ob-
taining these results, meager though they may appear.

But while it is going to take a long time and many
heads and hands to work out fully these problems, there
can be little doubt that the modern method is the only
one by which we can hope to reach a safe conclusion.

CHAPTER VIII
SPECIAL CASES OF SEX-INHERITANCE

THE mechanism of sex-determination that we have
examined gives equal numbers of males and females.
But there are known certain special cases where equality
does not hold. I have selected six such cases for
discussion. Each of these illustrates how the mechan-
ism of sex-determination has changed to give a different
result ; or how, the mechanism remaining the same, some
outside condition has come in that affects the sex ratio.

It is so important at the outset to clearly recognize
the distinction between sex-determination and sex
ratio, that I shall take this opportunity to try to make
clear the meaning of this distinction. The failure to
recognize the distinction has been an unfailing source
of niisunderstanding in the literature of sex..

(1) A hive of bees consists of a queen, thousands of
workers, and at certain seasons a few hundred drones
or males. The workers are potentially females, and
these with the queen give an enormous preponderance
of females. In this case the explanation of the sex
ratio is clear. Most of the eggs laid by the queen are
fertilized, and in the bee all fertilized eggs become fe-
males, because as we have seen there is only one class of
spermatozoa produced, and not two as in other insects.

There is a parallel and interesting case in one of the
wasps described by Fabre. The female lays her eggs

220

SPECIAL CASES OF SEX-IXHERITAXCE 221

as a rule in the hollow stems of plants, each egg in a
separate compartment. In some of the compartments
she stores away much more food than in others. From
these compartments large females hatch. From com-
partments where less food is stored the smaller males
are produced. It may seem that the amount of food
stored up determines the sex of the bee. To test this
Fabre took out the excess of food from the large
compartments. The wasp that emerged, although
small for want of food, was in every case a female.
Fabre enlarged the smaller compartments and added
food. The wasp that came out was a male, larger
than the normal male.

It is evident that food does not determine the sex,
but the mother wasp must fertilize the eggs that she
lays in chambers where she has stored up more food,
and not fertilize those eggs that she deposits in com-
partments where she has accumulated less food.

(2) A curious sex ratio appeared in one race of fruit
flies. Some of the females persisted hi producing twice
as many females as males. This was first discovered
by Miss Rawls. In order to study what was taking
place, I bred one of these females that had red eyes to
a white-eyed male of another stock. All the offspring
had red eyes, as was to be expected. I then bred these
daughters individually to white-eyed males again
(Fig. 106). Half of the daughters gave a normal
ratio ; the other half gave the following ratio :

Red Red White White

$<??<?
50 0 50 50

222

HEREDITY AND SEX

It is evident that one class of males has failed to ap-
pear — the red males. If we trace their history through
these two generations, we find that the single sex chro-

FIG. 106. — Diagram to show the heredity of the lethal factor (carried
by black X). A, red-eyed female, carrying the factor in one -X", is bred to
normal white-eyed male. B, her red-eyed daughter, is bred again to a normal
white-eyed male, giving theoretically the four classes shown in C, but one of
the classes fails to appear, viz. the red-eyed male (colored black in the dia-
gram). The analysis (to right) shows that this male has the fatal X. One
of his sisters has it also, but is saved by the other X. She is the red-eyed
female. If she is bred to a white-eyed male, she gives the results shown in
D, in which one class of males is again absent, viz. the red-eyed male. In
this diagram the black X represents red eyes and lethal (as though completely
linked).

SPECIAL CASES OF SEX-INHERITANCE 223

mosome that each red male contains is one of the two
chromosomes present in the original red-eyed grand-
mother. If this chromosome contains a factor which
if present causes the death of the male that contains it,
and this factor is closely linked to the red factor,
the results are explained. All the females escape the
fatality, because all females contain two sex chromo-
somes. If a female should contain the fatal factor,
her life is saved by the other, normal, sex chromosome.
The hypothesis has been tested in numerous ways and
has been verified. We keep this stock going by mat-
ing the red females to white males. This gives con-
tinually the 2 : 1 ratio. The white sisters, on the other
hand, are normal and give normal sex ratios.

(3) Another aberrant result, discovered by C. B.
Bridges, is shown by a different race of these same fruit
flies. It will be recalled that when an ordinary white-
eyed female is bred to a red-eyed male all the sons have
white eyes. But in the race in question a different re-
sult follows, as shown by the diagram. From 90 to
95 per cent of the offspring are regular, but 5 per cent
of the females and 5 per cent of the males are uncon-
formable, yet persistently appear in this stock.

The results can be explained if we suppose that the
egg contains two X's and a Y chromosome and in con-
sequence the two X's may pass out into one of the
polar bodies, in which case the red-eyed males will
develop if the egg is fertilized by a female-producing
sperm; or the two X-chromosomes will both stay in
the egg, and give a kind of female with three sex chro-
mosomes.

Here also numerous tests can be made. They verify

224

HEREDITY AND SEX

the expectation. Thus by utilizing sex chromosomes
that carry other sex-linked characters than white eyes,
it can be shown that the results are really due to the
whole sex chromosome being involved, and not to
parts of it. The result is of unusual interest in another
direction ; for it shows that the female-producing

FIG. 107. — Non-disjunction. A non-disjunction female produces four
types of eggs, viz., XY — X — XX — Y. Such a female will give, with a
normal male, XY, the classes indicated on the diagram. Y should be sub-
stituted for 0 in the diagram. The non-disjunction 9 is XX Y.

sperm will make a male if it enters an egg from which
both sex chromosomes have been removed. It is
therefore not the female-producing sperm, as such,
that gives a female under normal conditions, but this
sperm plus the sex chromosome already present in
the egg that gives an additive result — *a female.

(4) In the group of nematode worms belonging es-
pecially to the genus Rhabditis, there are some extraor-

SPECIAL CASES OF SEX-INHERITANCE 225

dinary perversions of the sex ratios. The table gives
the ratios that Maupas discovered. Not only are the

Diplogaster robustus 0. 13 male

Rhabditis guignardi 0.15 male

Rhabditis dulichura 0.7 male

Rhabditis caussaneli 1.4 males

Rhabditis elyaus 1.5 males

to 1000 females

Rhabditis coronata 5.0 males

Rhabditis perrieri 7.0 males

Rhabditis marionii 7.6 males

Rhabditis duthiersi 20.0 males

Rhabditis viguieri 45.0 males

males extremely rare — almost reaching a vanishing
point in certain cases — but they have lost the instinct
to fertilize the female.

The females, on the other hand, have acquired the
power of producing sperm, so that they have passed
over into the hermaphroditic state. The behavior
and history of the sperm that the females produce has
only recently been made out by Miss Eva Krtiger.
It is found that a spermatozoon enters each egg and
starts the development, but takes no further part in
the development (Fig. 108). The egg may be said to
be half fertilized. It is a parthenogenetic egg and
produces a female.

(5) Some very high male ratios have been reported
by Guyer in cases where birds of very different families
have been crossed — the common fowl by the guinea
hen, individuals of different genera of pheasants bred
to each other and to fowls, etc. Hybrids between
different genera gave 74 $ - - 13 9 . Hybrids between
different species of the same genus 72 $ - - 18 9 . In
most of these cases, as Guyer points out, the sex is

226 HEREDITY AND SEX

recorded from the mounted museum specimen which
has the male plumage. But it is known that the re-
productive organs of hybrids, extreme as these, are gen-
erally imperfect and the birds are sterile. It has been

Fig. 2.

Fig. 5.
Fig. 4.

Fig. 12
Fig. 7. Fig. 8. FiR-9. Fig. 10. Fig. 11. ^

FIG. 108. — Oogenesis and spermatogenesis of Rhabditis aberrans.
1-5, stages in oogenesis, including incomplete attempt to form one polar
body. Eighteen chromosomes in 1 and again in 4 and 5. In 3 the entering
sperm seen at right. 6, prophase of first spermatocyte with 8 double and
two single chromosomes (sex chromosomes). At the first division (7) the
double chromosomes separate, and the two sex chromosomes divide, giving
ten chromosomes to each daughter cell (8). At the next division the two
sex chromosomes move to opposite poles, giving two female-producing
sperm (9 and 10). Rarely one of them may be left at the division plane
and lost, so that a male-producing sperm results that accounts for the rare
occurrence of males. (After E. Kriiger.)

shown that if the ovary of the female bird is removed
or deficient, she assumes the plumage of the male.
Possibly, therefore, some of these cases may fall under
this heading, but it is improbable that they can all be
explained in this way. In the cases examined by Guyer
himself the hybrids were dissected and all four were
found to be males.

SPECIAL CASES OF SEX-INHERITANCE 227

Pearl has recently pointed out that the sex ratio
in the Argentine Republic varies somewhat accord-
ing to whether individuals of the same race, or of dif-
ferent races, are the parents. As seen in the following
table, the sex ratio of Italian by Italian is 100.77;

COMPARISON OF THE SEX RATIOS OF THE OFFSPRING OF PURE AND
CROSS MATINGS

MATINGS

SEX RATIO

DIFFERENCE

P.E. OF DIFFERENCE

Italian $
Italian $

Argentine 9
Italian 9

105.72 ±.46
100.77 ±.20

Difference

4.95 ±.50

9.9

Italian $
Argentine $

Argentine 9
Argentine 9

105.72 ±.46
103.26 ±.34

Difference

2.46 ±.57

4.3

Spanish $
Spanish $

Argentine 9
Spanish 9

106.69 ±.74
105.55 ±.36

Difference

1.14 ±.82

1.4

Spanish $
Argentine $

Argentine 9
Argentine 9

106.69 ±.74
103.26 ±.34

Difference

3.43 ±.81

4.2

Argentine by Argentine, 103.26 ; but Italian by Argen-
tine, 105.72. If, as has so often been found to be the
case, a hybrid combination gives a more vigorous
progeny, the higher sex ratio of the cross-breed may
account for the observed differences, since other data
show that the male infant is less viable and the in-
creased vigor of a hybrid combination may increase
the chance of survival of the male.

228 • HEREDITY AND SEX

(6) We come now to the most perplexing case on
record. In frogs the normal sex ratio is approximate
equality. Professor Richard Hertwig has found that
if the deposition of the eggs is prevented for two to
three days (after the eggs have reached the uterus)
the proportion of males is enormously increased -
in the extreme case all the offspring may be males.
By critical experiments Hertwig has shown that the
results are not due to the age of the spermatozoa, al-
though in general he is inclined to attribute certain
differences in sex-determination to the sperm as well
as to the eggs.

The evidence obtained by his pupil, Kuschakewitsch,
goes clearly to show that the high male sex ratio is
not due to a differential mortality of one sex.

In the following table four experiments (a, 6, c, d)
are summarized. The interval between each record

/72\

a) 47 9 : 32 $ 0 9 : 97 $

b) 34 9: 472 659:772 156?: 1942 79:482

/36\ /18\

c) 64 ?: 61 £ 1019:1392 1159:1692

d) 55 9: 52 $ 1489:872 719:702 179:1292

is written above in hours. In all cases an excess of
males is found if the eggs have been kept for several
hours before fertilization. In the first (a), second (6),
and fourth (d) cases the excess of males is very great.
Hertwig attempts to bring his results into line with

SPECIAL CASES OF SEX-INHERITANCE . 229

his general hypothesis of nucleo-plasm relation. He
holds, for instance, that sex may be determined by the
relation between the size of the nucleus and the proto-
plasm of the cell. As the value of the evidence has
been seriously called into question in general, and as
there is practically no evidence of any weight in its favor
in the present case, I shall not dwell further on the
question here. But the excessively high male ratio is
evident and positive. How to explain it is difficult
to say. It is just possible, I think, that delay may have
injured the egg to such an extent that the sperm may
start the development but fail to fuse with the egg
nucleus. Under these circumstances there is the possi-
bility that all the frogs would be males.

Miss King has also carried out extensive sets of ex-
periments with the common toad. She has studied the
eggs and the sperm under many different conditions, such
as presence of salt solutions, acids, sugar solutions, cold,
and heat. Her results are important, but their inter-
pretation is uncertain. In sugar solutions and in dry
fertilization the males decreased, in the latter from
114.10 to 29.41 per 100 ??. The normal sex ratio for
the toad is 90 $ to 100 ? . Whether the solutions have in
any sense affected the determination of sex, or acted to
favor one class of sperm at the expense of the other re-
mains to be shown, as Miss King herself frankly admits.

In the case of man there are extensive statistics
concerning the birth rate. The accompanying tables
give some of the results. There are in all parts of the
world more males born than females. The excessively
high ratios reported from the Balkans (not given here)
may be explained on psychological grounds, as failure

230

HEREDITY AND SEX

MALES

Italy 105.8

France 104.6

England 103.6

Germany 105.2

Austria 105.8

Hungary 105.0 to 100 females

Switzerland 104.5

Belgium 104.5

Holland 105.5

Spain 108.3

Russia 105.4

to report the birth of a boy is more likely to lead to the
imposition of a fine on account of the conscription.

There can be no doubt, however, that slightly more
males than females are born. Moreover, if the still-
born infants alone are recorded, surprisingly large ratios
occur, as shown in the next table.

MALES

Italy . 131.1

France ....... 142.2

Germany 128.3

Austria 132.1

Hungary 130.0

Switzerland 135.0 to 100 females

Belgium 132.1

Holland 127.7

Sweden 135.0

Norway 124.6

Denmark 132.2

And if abortive births are also taken into account, the
ratio is still higher. It seems that the male embryo
is not so strong as the female, or else less likely, from
other causes, to be born alive.

In many of the domesticated animals also, especially

SPECIAL CASES OF SEX-INHERITANCE 231

the mammals, there is an excess of males at birth, as
the next table shows.

MALES FEMALES

Horse 98.31 100 (Busing)

Cattle 107.3 100 (Wilckens)

Sheep 97.7 100 (Darwin)

Pig 111.8 100 (Wilckens)

Rat 105.0 100 (Cuenot)

Dove 105.0 100 (Cuenot)

Hen 94.7 100 (Darwin)

A little later I shall bring forward the evidence that
makes probable the view that in man the mechanism
for sex-determination is like that hi other animals,
where two classes of sperm are produced, male- and
female-producing. How then can we account in the
human race for the excess of eggs that are fertilized
by male-producing spermatozoa ? At present we do not
know, but we can at least offer certain suggestions that
seem plausible.

In mammals the fertilization occurs in the upper
parts of the oviduct. In order to reach these parts
the sperm by their own activity must traverse a dis-
tance relatively great for such small organisms. If
the rate of travel is ever so slightly different for the two
classes of sperm, a differential sex ratio will occur.

Again, if from any cause, such as disease or alcoholism,
one class of sperm is more affected than the other, a
disturbance in the sex ratio would be expected.

At present these are only conjectures, but I see
no ground for seizing upon any disturbance of the
ratio in order to formulate far-reaching conclusions
in regard to sex-determination itself. As I pointed
out in the beginning of this chapter, we may go

232 HEREDITY AND SEX

wide of the mark if we attempt to draw conclusions
concerning the determination of sex itself from devia-
tions such as these in the sex ratio, yet it is the mistake
that has been made over and over again. We must
look to other methods to give us sufficient evidence as
to sex-determination. Fortunately we are now in a
position to point to this other evidence with some
assurance. With the mechanism itself worked out,
we are in a better position to explain slight variations
or variables that modify the combinations in this way
or in that.

THE ABANDONED VIEW THAT EXTERNAL CONDITIONS
DETERMINE SEX

But before taking up the evidence for sex-determina-
tion in man I must briefly consider what I have been
bold enough to call the abandoned view that external
conditions determine sex.

Let us dismiss at once many of the guesses that have
been made. Drelincourt recorded 262 such guesses,
and Geddes and Thomson think that this number has
since been doubled. Naturally we cannot consider
them all, and must confine ourselves to a few that
seem to have some basis in fact or experiment.

The supposed influence of food has been utilized in
a large number of theories. The early casual evidence
of Landois, of Mrs. Treat, and of Gentry has
been entirely set aside by the careful observations of
Riley, Kellogg and Bell, and Cu6not. In the latter
cases the experiments were carried through two or even
three generations, and no evidence of any influence of
nourishment was found.

SPECIAL CASES OF SEX-INHERITANCE 233

The influence of food in sex-determination in man has
often been exploited. It is an ever recurrring episode
in the ephemeral literature of every period. The most
noted case is that of Schenk. In his first book he said
starvation produced more females ; in his second book
he changed his view and supposed that starvation
produces more males.

Perhaps the most fertile source from which this view
springs is found in some of the earlier statistical works,
especially that of Busing. Busing tried to show that
more girls are born in the better-fed classes of the com-
munity, in the poorer classes more boys. The effective
difference between these two classes is supposedly one of
food ! For instance, he states that the birth-rate for
the Swedish nobility is 98 boys to 100 girls, while in the
Swedish clergy the birth-rate is 108.6 boys to 100 girls.

Other statistics give exactly opposite results. Pun-
nett found for London (1901) more girls born amongst
the poor than the rich. So many elements enter into
these data that it is doubtful if they have much value
even in pointing out causes that affect the sex ratio, and
it is quite certain that they throw no light on the
causes that determine sex.

In other mammals where a sex ratio not dissimilar
to that in man exists, extensive experiments on feeding
have absolutely failed to produce any influence on
the ratio. We have, for instance, Cu6not's experi-
ments with rats, and Schultze's experiments with
mice. The conditions of feeding and starvation were
much more extreme in some cases than is likely to
occur ordinarily, yet the sex ratio remained the same.

Why in the face of this clear evidence do we find

234 HEREDITY AND SEX

zoologists, physicians, and laymen alike perpetually
discovering some new relation between food and sex?
It is hard to say. Only recently an Italian zoologist,
Russo, put forward the view that by feeding animals
on lecithin more females were produced. He claimed
that he could actually detect the two kinds of eggs
in the ovary — the female- and the male-producing. It
has been shown that his data were selected and not
complete ; that repetition of his experiments gave no
confirmative results, and probably that one of the two
kinds of eggs that he distinguished were eggs about to
degenerate and become absorbed.

But the food theories will go on for many years to
come — as long as credulity lasts.

Temperature also has been appealed to as a sex fac-
tor in one sense or another. R. Hertwig concluded
that a lower temperature at the time of fertilization
gave more male frogs, but Miss King's observations
failed to confirm this. There is the earlier work of
Maupas on hydatina and the more recent work of
von Malsen on Dinophilus apatris. I have already
pointed out that Maupas' results have not been con-
firmed by any of his successors. Even if they had been
confirmed they would only have shown that tempera-
ture might have an effect in bringing parthenogenesis
to an end and instituting sexual reproduction in its
stead. In hydatina the sexual female and the male
producing individual are one and the same. A more
striking case could not be found to show that the en-
vironment does not determine sex but may at least
change one method of reproduction into another.

There remain von Malsen's results for dinophilus,

SPECIAL CASES OF SEX-INHERITANCE 235

where large and small eggs are produced by the same
female (Fig. 109). The female lays her eggs in clus-
ters, from three to six eggs, as a rule, in each cluster.
The large eggs produce females ; the small eggs pro-

FIG. 109. — DinophUus gyrdciliatus. Females (above and to left) and
males (below and to right). Two kinds of eggs shown in middle of lower
row. (After Shearer.)

duce rudimentary males that fertilize the young fe-
males as soon as they hatch and before they have left
the jelly capsule.

Von Malsen kept the mother at different tempera-
tures, with the results shown in the table. The ratio
of small eggs to large eggs changes. But the result

TEMPERATURE

No. OF
BROODS

d"

9

SEX
RATIO

EGGS PER
BROOD

Room temp. 19° C. .

202

327

813

1:2,4

5,6

Cold, 13° C. . . .

925

973

2975

1:3,5

4,2

Heat, 26° C. . . .

383

507

886

1: 1,7

3,6

236 HEREDITY AND SEX

obviously may only mean that more of the large eggs
are likely to be laid at one temperature than at another.
In fact, temperature seemed to act so promptly accord-
ing to Von Malsen's observations that it is very un-
likely that it could have had any influence in deter-
mining the kind of egg produced, but rather the kind
of egg that was more likely to be laid. We may dis-
miss this case also, I believe, as not showing that sex
is determined by temperature.

SEX-DETERMINATION IN MAN

i

Let us now proceed to examine the evidence that
bears on the determination of sex in man. I shall
draw on three sources of evidence :

1. Double embryos and identical twins.

2. Sex-linked inheritance in man.

3. Direct observations on the chromosomes.

The familiar case of the Siamese twins 'is an example
of two individuals organically united. A large "series
of such dual forms is known to pathologists. There
are hundreds of recorded cases. In all of these both
individuals are of the same sex, i.e. both are males
or both are females. There is good evidence to .show
that these double types have come from a single fer-
tilized egg. They are united in various degrees (Fig.
110) ; only those that have a small connecting region
are capable of living. These cases lead directly to
the formation of separate individuals, the so-called
identical twins.

Galton was one of the first, if not the first, to recognize
that there are two kinds of i wins — identical twins and
"ordinary or fraternal twins.

SPECIAL CASES OF SEX-INHERITANCE 237

Identical twins are, as the name implies, extremely
alike. They are always of the same sex. There
is every presumption and some collateral evidence
to show that they come from one egg after fer-
tilization.' On ^Ene other hand, amongst ordinary
twins a boy and a girl, or two boys and two girls, occur
in the ratio expected, i.e. on the basis that their sex is

. tit it it

DIAGRAM SHOWING THE INTERRELATIONS OF THE VARIOUS SORTS OF OlPLOPAGI ANO
DUPLICATE TWINS, ILLUSTRATIVE OF THE THEORY ADVANCED IN THIS PAPER. FURTHER EX-
PLANATION IN THE TEXT.

FIG. 110. — Diagram showing different types of union of double monster
(After Wilder.)

not determined by a common external or internal
cause. Since fraternal twins and identical twins show
these relations at birth and from the fact that they
have been in both cases subjected to the same condi-
tions, it follows with great probability that sex in
such cases is determined before or at the time of
fertilization.

This conclusion finds strong support from the condi-

238 HEREDITY AND SEX

tions that have been made out in the armadillo.
Jehring first reported that all the young of a single
litter are of the same sex (Fig. 111). The statement
has been verified by Newman and by Patterson on a
large scale. In addition they have found, first, that
only one egg leaves the ovary at each gestative period ;
and second, that from the egg four embryos are pro-

FIG. 111. — Nine-banded Armadillo. Four identical twins with a
common placenta. (After Newman and Patterson.)

duced (Fig. 112). The material out of which they
develop separates from the rest of the embryonic
tissue at a very early stage. The four embryos are
identical quadruplets in the sense that they are more
like each other than like the embryos of any other
litter, or even more like each other than they are to
their own mother.

The second source of evidence concerning sex-deter-

SPECIAL CASES OF SEX-INHERITANCE 239

ruination in man is found in the heredity of sex-linked
characters.

The following cases may well serve to illustrate
some of the better ascertained characters. ~ The tables
are taken from Davenport's book on "Heredity in
Relation to Eugenics." The squares indicate males,
affected males are black squares^- the heavy circles indi-
cate females, that are supposed to carry the factors, but

FIG. 112. — Nine banded Armadillo. Embryonic blastocyst that has
four embryos on it, two of which are seen in figure. (After Newman and
Patterson.)

such females do not exhibit the character themselves.
Solid black circles stand for affected females.

Haemophilia appears in affected stocks almost ex-
clusively in males (Fig. 113). Such males, mating
with normal females, give only normal offspring, but
the daughters of such unions if they marry normal
males will transmit the disease to half of their sons.
Affected females can arise only when a haemophilious
male marries a female carrying haemophilia. If we

240

HEREDITY AND SEX

o

Q

-0

-a
o

1

i Davenport.)

-D

-a
g

-O

-a
o

-i

-•
-a
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-a

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-m
—m
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n

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— 0

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— 0

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a

D'-S

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g

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pa S

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a
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pO °

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fi
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— B

o
L-a

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&H

L-i MD

SPECIAL CASES

FIG. 114. — Diagram to indicate heredity of color blindness through
male. A color-blind male (here black) transmits his defect to his grandsons
only.

XX

50CX

9

cr cr

xx

FIG. 115. — Diagram to indicate heredity of color blindness through
female. A color-blind female transmits color blindness to all of her sons,
to half of her granddaughters and to half of her grandsons.

242

HEREDITY AND SEX

substitute white eyes for haemophilia, the scheme
already given for white versus red eyes in flies applies
to this case. If, for instance, the mother with normal
eyes has two X chromosomes (Fig. 114), and the fac-
tor for haemophilia is carried by the single X in the
male (black X of diagram), the daughter will have
one affected X (and in consequence will transmit the
factor), but also one normal X which gives normal

FIG. 116. — Pedigree of Ichthyosis from Bramwell. (After Davenport.)

vision. The sons will all be normal, since they
get the X chromosomes from their mother. In the
next generation, as shown in the diagram (third line),
four classes arise, normal females, hybrid females, normal
males, and haemophilious males. Color blindness fol-
lows the same scheme, as the above diagrams illustrate
(Figs. 114 and 115). In the first diagram tfoe color-
blind male is represented by a black eye ; the normal
female by an eye without color. The offspring from

SPECIAL CASES OF SEX-INHERITANCE 243

0

—6

Q

I-D

N

>ni Herringham. (After Davenport.)

-i
9

-i
o

r-0

D
-0

D u

-•

D D

-9

D

-a

r-0

LJ

r-0

-0

c

HI
-f

ta

D

a

-D

-a

•S

1

1

• rj

r-0

-0

C

D

3

-a

r-O

-0

a
-a

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— o

D

o

r-0
-0

a

§

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6 z

6 z
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244 HEREDITY AND SEX

two such individuals are normal, but the color blindness
reappears in one-fourth of the grandchildren, and in
these only in the males. The reverse mating is shown
in the next diagram in which the female is color-blind.
She will have color-blind sons and normal daughters
(criss-cross inheritance), and all four kinds of grand-
children.

Other cases in man that are said to show sex-linked
inheritance are atrophy of the optic nerve, multiple

T

6 • <^

ipOBiQ

. • ...

T

...•....*

FIG. 118. — Pedigree of night blindness in a negro family, from Bordley.
(After Davenport.)

**IA

sclerosis, myopia, ichthyosis (Fig. 116), muscular
atrophy (Fig. 117). Night blindness is described in
certain cases as sex-linked ; in other cases, however, a
disease by the same name appears to be a simple domi-
nant and not sex-linked (Fig. 118).

All these cases of sex-linked inheritance in man
are explained by the assumption that the factor that
produces these characters is carried by the sex chromo-
some, which is duplex (XX) in the female and simplex
(X) in the male. A simpler assumption has not yet
been found. If one is fastidious and objects to the

SPECIAL CASES OF SEX-INHERITANCE 245

statement of factors being carried by chromosomes, he
has only to say, that if the factors for the characters
follow the known distribution of the sex chromosome,
the results can be accounted for.

The culmination of the evidence of sex-determina-
tion in man is found in a study of the cell structure
of the human race itself. Strange as it may seem, we
have been longer in doubt concerning the number
of chromosomes in man than hi any other animal as
extensively studied. Four conditions are responsible :

(1) The large number of chromosomes present in man.

(2) The clumping or sticking together of the chromo-
somes. (3) The difficulty of obtaining fresh material.
(4) The possibility that the negro race has half as many
chromosomes as the white race.

Two years ago Guyer announced the discovery that
in all probability there exist in man two unpaired
chromosomes in the male (Fig. 119) that behave in all
respects like that in the typical cases of the sort in
insects, where, as we have seen, there are two classes
of spermatozoa, differing by the addition of one more
chromosome in one class. These produce females ; the
lacking class produces male's. But Guyer's evidence
was not convincing. He found in all 12 chromosomes
in one class of sperm and 10 hi the other. Mont-
gomery has also studied the same problem, but his
account, while confirming the number, is in disagree-
ment in regard to the accessory.

Jordan has gone over a number of other mammals,
in some of which he thinks that he has found indica-
tions at least of two classes of sperm.

Still more recently another investigator, von Wini-

246 HEREDITY AND SEX

warier, has attacked the problem (Fig. 120). His
material and his methods appear to have been superior
to those of his predecessors. His results, while stated
with caution and reserve, seem to put the whole
question on a safer basis.

His main results are illustrated in the diagram

>fr $$!.
I j» 4*

FIG. 119. — Human spermatogenesis according to Guyer. The sex
chromosomes are seen in 6-9.

(Fig. 120). In the male he finds 47 chromosomes.
Of these 46 unite at reduction to give 23 double
chromosomes — one remains without a mate. At the
first reduction division the pairs separate, 23 going
to each pole, the unpaired chromosome into one cell
only.

SPECIAL CASES OF SEX-INHERITANCE 247

At the next division all the chromosomes in the 23
group divide, likewise all in the 24 group divide.
There are produced two spermatozoa containing 24

•

<#»'

FIG. 120. — Human spermatogenesis according to von Winiwarter. a,
spermatogonial cell with duplex number; b, synapsis ; c, d, e,f, first spermato-
cytes with haploid number of chromosomes ; g, first spermatocyte division,
sex chromosomes (below) in advance of others ; h, two polar plates of later
stage ; i, first division completed ; /, second spermatocyte with 23 chromo-
somes ; k, second spermatocyte with 24 chromosomes ; /, second spermato-
cyte division ; m, two polar plates of later stage.

248 HEREDITY AND SEX

chromosomes, and two containing 23 chromosomes ;
all four sperms having come from the same spermato-
gonial cell (Fig. 121).

In the female von Winiwarter had difficulty in deter-
mining the number of chromosomes present. His

. v, ,- determination in /Han

~{~<l

,;•>,

^7 **

„ /

N<">IW|H»»»»l ''

•

FIG. 121. — Diagram of human spermatogenesis. A, spermatogonial
cell with 47 chromosomes ; B, first spermatocyte with reduced haploid number
and sex chromosome (open circle) ; C, first division ; D, two resulting cells
= second spermatocytes ; E, division of second spermatocytes ; F, four
resulting spermatozoa, two female-producing (above), two male-produc-
ing (below).

best counts gave 48 chromosomes for the full or duplex
number. These observations fit in with the results
from the male.

If these observations are confirmed, they show that
in man, as in so many other animals, an internal
mechanism exists by which sex is determined. It is
futile then to search for environmental changes that

SPECIAL CASE& OF SEX-INHERITANCE 249

might determine sex. At best the environment may
slightly disturb the regular working out of the two
possible combinations that give male or female. Such
disturbances may affect the sex ratio but have nothing
to do with sex-determination.

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INDEX

Abraxas, 128
Achates, 151
Achia, 106

Addison's disease, 147
Adkins, 217-218
Adrenal, 147
Agenor, 151
Allen, 113
Amphibia, 145
Amphipoda, 117
Andrews, 117
Angiostomum, 170
Antlers, 110, 133
Ants, 117
Argentine, 227
Argonauta, 26
Aristotle, 35
Armadillo, 238
Ascaris, 20, 21, -19
Ascidian, 217

Baltzer, 55, 58, 61

Bancroft, 194

Barnacle, 155

Bateson, 72, 75, 99, 100, 125

Baur, E., 99

Beans, 123

Bee, 174, 175, 176, 220

Bees, 32

Beetles, 106

Bell, 232

Belt, 102

Bird of paradise, king, 109

six-shafted, 109

superb, 109
Black, 96-97
Blakeslee, 171
Bobolink, 27
Boring, 51
Boveri, 51, 55, 58, 162, 165,

171

Bresca, 145
Bridges, 223, 224

170,

Bruce, 212
Bryonia, 171-172
Biitschli, 8

Calkins, 8, 198, 206, 209, 210

Callosamia, 116

Capons, 142, 143

Cardamine, 215-216

Castle, 195

Ceylon, 125, 127

Checker diagram, 78

Chemotaxis, 117

Chidester, 117

Cicada, 106

Ciona, 217-218

Clipped wings, 119

Colias, 129-130, 150

Collins, 202

Color-blind, 241

Color blindness, 242

Conger eel, 2

Corpus luteum, 147

Correns, 74, 79, 99, 171, 172. 215. 216

Crab, 155

Cretinism, 146

Cricket, 150

Criodrilus, 168

Cuenot, 232, 233

Cunningham, 121

Daphnians, 182-185, 189

Darwin, C., 73-74, 101, 103, 104, 107,

112-114, 120, 125, 142, 194, 197,

200-202

Davenport, C., 72, 143, 239
Deer, 110, 133, 134
Delage, 193
Dinophilus, 234
Diplogaster, 225
Doncaster, 176

Dorsets, 134, 135, 136, 137, 138
Drelincourt, 232
Drone, 175

281

282

INDEX

Drosophila, 63-68, 96, 117, 130
Busing, 233

East, 99, 202, 204, 211
Edwards, 51
Egret, 111
Eland, 136
Elaphomyia, 106
Elephant, 110
Emerson, 99
Eosirt eye, 130, 154, 155
Eupaguras, 158
Euschistus, 151

Fabre, 220, 221

Fielde, 117

Firefly, 28, 30, 31

Fishes, 32

Florisuga, 102

Foot, 151

Forel, 117

Frog, 145, 147, 228

Frolowa, 51

Fruit fly, 117, 195, 196, 221

Fundulus, 32

Gall, 179
Galton, 236
Game, 144, 212
Geddes, 232
Gentry, 232
Germ-cells, 23
Gerould, 130, 150, 151
Giard, 155
Gigantism, 146
Goldschmidt, 124
Goodale, 72, 142
Gosse, 103 :
Growth, 3
Gudernatsch, 147
Guinea hen, 225
Gulick, 51

Guyer, 225-226, 245
Gynandromorphism, 161
Gypsy moths, 117

Habrocestum, 107
Haemophilia, 239, 240, 242
Hectocotylized arm, 26
Henking, 50
Herbst, 55, 61, 62
Herdwicks, 134-135

Hermaphroditism, 161
Hertwig, R., 9, 228, 234
Holmes, 117
Hormones, 146
Horns, 133-138
Hudson, 114, 115
Humming-birds, 103, 108
Hydatina, 2, 185
Hyde, 196, 199, 215

Ichthyosis, 242
Identical twins, 236-239
Inachus, 155
Ipomcea, 197
Italian, 227

Jacobson, 151

Janda, 168

Janssens, 94

Jehring, 238

Jennings, 9, 12, 206-208

Johannsen, 122-125

Jordan, H. E., 245

Keeble, 212
Kellogg, 117, 232
King, 229, 234
Kopec, 149
Kruger, 225
Kuschakewitsch, 228

Lamarckian school, 17

Landois, 232

Langshan, 69-71

Laomedon, 151

Lethal factor, 221-223

Linkage, 93

Lion, 27

Lister, 34

Loeb, J., 62, 190, 191, 192, 193

Lutz, 118

Lychnis, 172-173

Lygseus, 44

Lymantria, 148

McClung, 50

Maevia, 108

Mallard, 28, 142

Malsen, von, 234, 235, 236

Mammals, 159

Mammary glands, 140

Man, 34, 229, 236-249

INDEX

283

Marchals, 171

Mast, 30

Maupas, 5, 8, 187, 198, 234

Mayer, 116

de Meijere, 151

Meisenheimer, 145, 148-149

Mendel, 84, 73-75, 80, 84

Menge, 34

Merino, 134, 135

Miastor, 21, 174

Mi<-<>. 233

Mimicry, 127-130

Miniature wings, 66-67

Mirabilis, 79-80

Moenkhaus, 196

Montgomery, 34, 50, 115, 117, 245

Mosquito, 51

Mosses, 171

Mulsow, 51

Myopia, 242

Xematode, 224-226
Nereis, 36

Neuroterus, 176-177
Nswmann, 238
Night blindness, 242
Non-disjunction, 223-224
Xussbaum, 16, 145

Ocneria, 148
Octopus, 25
Oncopeltus, 46, 84
Optic nerve atrophy, 244
Oudemans, 148
Ovariotomy, 135
Owl, 111

Papanicolau, 183-185

Papilio, 125-129, 151

Paramoecium, 5, 6, 12, 206-211

Parathyroid, 146

Parthenogenesis, 161

Patterson, 239

Paulmier, 50

Pea, edible, 75-78, 85-88

Pearl, R., 72, 212-213, 227

Pearse, 117

Peckham, 115-116, 120

Pellew, 212

Peltogaster. 158

Petrunkewitsch, 117

Phalarope, 112

Pheasants, 225

Phidippus, 34

Photinus, 28

Phylloxerans, 52, 54, 178, 179, 180,

181, 189
Pigeons, 32
Pituitary body, 146
Plutei, 60

Plymouth rock, 69-71, 212
Polar bodies, 37
Polytmus, 103
Porter, 117^
Porthetria, 117, 148
Primula, 201, 202
Promethea, 116
Protenor, 40
Punnett, 127, 128, 138, 233

Rawls, 221
Rat, 140, 233
Reduplication, 100
Reindeer, 136
Rhabditis, 169, 224, 226
Riley, 232
Ritzema-Bos, 195
Rotifers, 185-189
Rudimentary wing, 214, 215
Russo, 234

Sacculina, 155

Sagitta, 21, 22

Schenk, 233

Schleip, 170, 171

Schultze, 233

Sclerosis, 242

Seabright, 143-144

Sea cow, 27

Sea-lion, Steller's, 110

Sea-urchin, 56-62

Segregation, 81, 100

Sex, 83, 84

Sex chromosome, 50, 80, 83, 84

Sex determination, 84

Sex-limited, 83

Sex-linked, 81, 83, 84, 132

Sheep, 134-138

Shull, A. F., 187, 197, 205

Shull, G. H., 173, 202, 204, 211

Shuster, 145

Siamese twins, 236

Silkworm, 117, 165

Sinety, 50

284

INDEX

Skeleton, rat, 140
Smith, G., 145, 155
Soule, 116
Sparrow, 2
Spermatophores, 25
Sphaerechinus, 59-60
Spiders, 34, 107, 115, 117
Squid, 24
Stag, 133
Steinach, 140
Stephanosphaera, 5
Stevens, 51
Strobell, 151

Strongylocentrotus, 59, 60, 62
Sturtevant, 72, 98, 117, 118
Stylonichia, 2
Suffolks, 136-138
Synapsis, 93

Tadpoles, 147
Tanager, scarlet, 27
Thomson, 232
Thymus, 146-147
Thyroid, 146-147
Toad, 229
Tower, 117
Toyama, 165
Treat, 232

Triton, 145
Trow, 99
Tschermak, 74

Vermilion eye, 119
Vestigial wing, 96-97
Vigor, 120
Vincent, 146
de Vries, 74, 125

Wallace, 102, 113-114, 120, 125, 127

Wasp, 220

Weismann, 16, 17, 40, 194, 195

Wheeler, 117

White eye, 62-65, 81, 82, 88-92, 118,

119, 221-223

Whitney, 185, 187, 197, 205
Wilder, 237
Wilson, 51
Winiwarter, 245-248
Wood, 136
Woodruff, 8, 198

X-chromosome, 51, 82, 84, 242

Y-chromosome, 51, 84

Yellow body color, 67, 88-92, 119

THIS BOOK IS DUE ON THE LAST DATE
STAMPEDJiELOW

AN INITIAL FlNEToF 25 CENTS

OVERDUE.

OCT 0 1932
OCT 23 1932
JAN 31 1933

COT 21 1933

NOV 27 1933

(\ V <wO I

DF3 4 1933

OCT 2 4 1935

OCT 1 4 1936

MOV

MAR 2

MAR 9 1953

SEP
SEP 9 1968 IS

LD 21-

300295

QH43I

'9/4

THE UNIVERSITY OF CALIFORNIA LIBRAR*