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http://www.archive.org/details/cu31924003060583

EXPERIMENTAL ZOOLOGY

THATS:

EXPERIMENTAL ZOOLOGY

BY

THOMAS HUNT MORGAN

PROFESSOR OF EXPERIMENTAL ZOOLOGY
COLUMBIA UNIVERSITY

New Work
THE MACMILLAN COMPANY

LONDON: MACMILLAN & CO., Lrp.
1907

All rights reserved

@s332, 556
COPYRIGHT, 1907,
By THE MACMILLAN COMPANY.

Set up and electrotyped. Published January, 19¢7.

Norwood Press
J.§. Cushing & Co. — Berwick & Smith Co.
Norwood, Mass., U.S.A.

PREFACE

THE great interest that has been shown during the last
fifteen years in the study of Experimental Zodlogy has led
to the rapid development of this branch of biology. An
attempt is made in the following pages to bring together the
results of this work. A series of about thirty-five lectures
formed the basis for my treatment of the subject, and this
will account, in part, for the way in which the matter has
been handled; many details have been omitted that an
exhaustive treatment would demand; and the plan has been
to select the most typical and most instructive cases for
presentation, when such a choice was possible. Neverthe-
less, I believe that the reader will find a fairly full account
of the subjects considered.

Physiology has from the beginning made use of the method
of experiment, and with notable success. Morphology has,
up to the present time, followed mainly the historical and
descriptive methods, although striking exceptions could be
cited. While the historical study of zodlogy must always
remain a legitimate field for activity, as human history has
been a time-honored study, yet there can be little doubt that
the more promising and searching method of zoological study
in the future will be found in experiment.

The central problem of morphology—the causes of the
changes in form, or at least the determination of the condi-
tions under which changes in form occur — will furnish the
main theme of the present treatise. Two fields of study that
properly fall under this head are, however, not considered,
viz. experimental embryology and the experimental study
of regeneration. Both of these subjects have in recent years
received comprehensive treatment in book form, so that it
did not seem desirable to go over the ground again. More-

v

vi Preface

over, their consideration would have demanded too much
space to be included with the present matter in a single
volume. The fascinating study of the psychical side of living
phenomena also belongs to the province of experimental
biology, especially comparative psychology; but this subject
has quite recently been fully dealt with by Loeb and by
Jennings, who have themselves been largely instrumental-in
developing the subject, so that further treatment would be
more than superfluous.

The excellent summaries and reviews of some of the topics
discussed here, that have been published in recent years,
have greatly facilitated my work. JI need only mention
Roux’s and Driesch’s analysis of the experimental method,
Grafin v. Linden’s summary of the experiments on butter-
flies, Herbst’s excellent treatment of the subjects of “ For-
mative Reiz,” Phillips’s very full review and literature on sex
determination, and Cuénot’s, Lenhossek’s, and O. Schultze’s
treatment of the same subject. I need scarcely add that,
while using these and other reviews, I have made my own
compilation almost exclusively from the original sources.

Covering as extensive a field as I have attempted to cover,
it is probable that the different subjects have received un-
equal treatment, and I fear that some omissions may have
been made. I trust, however, that no serious oversights or
mistakes will be found.

It gives me great pleasure to express here my appreciation
of the generous assistance in the correction of the manuscript
and proof rendered by my wife, by Professor E. B. Wilson,
by Professor C. B. Davenport, and by Professor C. E. Castle.

CONTENTS

EXPERIMENTAL STUDY OF EVOLUTION

CHAPTER I
INTRODUCTION
PAGE
THE EXPERIMENTAL METHOD . . F . : . . . ‘ 3
CHAPTER II
THE INFLUENCE OF EXTERNAL CONDITIONS IN CAUSING CHANGES IN THE
STRUCTURE OF ANIMALS ; : 7 F ‘ é . 12
Adaptive Responses. . : : : ‘ ‘ é » 12
Non-adaptive Responses. . . . . . . » 15
Influence of Temperature on 1 the Coloration of Butterflies . - 15
Effect of Temperature on Caterpillars a . 5 . - 24
CHAPTER III
THE INFLUENCE OF EXTERNAL CONDITIONS IN CAUSING CHANGES IN THE
STRUCTURE OF ANIMALS (Continued) . = ‘ ‘ 3 » 29
Experiments on the Influence of the Food Plant . : 29

The Influence of Light, Electricity, Centrifugal Force, Chemical Sub-
stances, and Oxygen on the Caterpillars and Pup of Moths and

Butterflies . . . - 37
The Influence of Humidity on a the Gharasters of Moths and Hutterdies 38
Experiments with Flatfish . . . : . . : 5 «40
Experiments with Crustaceans. . . A : ‘ . + 40
Changesin Mammalsand Birds . . . . . . «. « 4!

CHAPTER IV

Tue INHERITED EFFECTS OF CHANGES INDUCED BY EXTERNAL FACTORS 43

CHAPTER V
THE INHERITANCE OF ACQUIRED CHARACTERS . . . : . » 50
Telegony . 7 ¢ “ - . . . . . . » 59
Xenia . . . . . . . . . Or

Theories of Tranuaiaion of Soiatie Teipeiges . 0 ‘ : . 61
vii

vill Contents

CHAPTER VI
EXPERIMENTAL HYBRIDIZING. ‘ z 4 3 :
Mendel’s Law P P ‘ ‘i 3
Mendel’s Law and the eee Fi . ’ 7

CHAPTER VII

EXPERIMENTAL HYBRIDIZING (Condinued)
Experiments with Mice

CHAPTER VIII
EXPERIMENTS WITH OTHER MAMMALS AND WITH BIRDS .

Experiments with Guinea Pigs. é . : .
Experiments with Rabbits

Experiments with Rats

Experiments with Cats .

Data for Other Maminals and Man

Experiments with Poultry

Experiments with Pigeons

CHAPTER IX

EXPERIMENTS WITH SNAILS, MOTHS, AND BEETLES . i

Experiments with Silkworms
Experiments with Beetles fi
Experiments with the Currant Moth
Experiments with Tephrosia

CHAPTER X
OTHER KINDS OF HYBRIDIZING : . . : .
Blended Inheritance . . F ‘ , .
Mosaic Inheritance i 2 . . .
Hybridization between Time Sadie . .
Dimorphism P
Reversal of Symmetry . 5
Conclusions regarding Mendelian Diente . .
CHAPTER XI

BEHAVIOR OF THE GERM-CELLS IN CROSS-FERTILIZATION .
Experiments with Amphibia
Experiments with Echinoderms
Factors involved in the Entrance of the Ssconaianion
Artificial Helps to Fertilization and to Cross-fertilization
Polyspermy . . .

Contents ix

CHAPTER XII

PAGE

INBREEDING . 6 ‘ . . . 7 . . . . - 186
Experiments with Mice and Rats : : F - : - 188
Experiments with the Pomace Fly , , . : - - 190
Behavior of Germ-cells in Inbreeding . ‘ : . . ‘ + 195

CHAPTER XIII

INFLUENCE OF SELECTION . i F ‘i : f : . - 198
Fluctuating or Individual Differences . ‘ ‘ i é » 199
Selection of Fluctuating Differences . : z : - . + 202
Elimination within the Species. ‘ . . : - é - 204
Variation and Parthenogenesis. 206

The Results of Selection and Hy bidibation af Wild Wlenentary Spadies 207
Selection under Domestication of Mutations, Saltations, Sports, and
Discontinuous Varieties in General . ‘ : . . 208

CHAPTER XIV

THE THEORY OF EVOLUTION . : - . : ‘ . . + 213
The Analogy with Artificial Selection . : J ; é 3 - 214
The Influence of the Environment . : zs : . 215
De Vries’s Results with CEnothera : : : : , ‘ . 217
Variation and Mutation in Helix . r : : : F 221
Evolution by Means of Definite Variation . : a : . . 224
Adaptation . ‘ . 229

The Selection Theory and the Theory ot he Sucvivel of Mutations . 232

EXPERIMENTAL STUDY OF GROWTH

CHAPTER XV
INTRODUCTORY . . F : : : : . . - - 239
Normal Growth . é F : ‘ . > . : is 239
Senescence . A : : : é : j . 246
Length of Life in Different Srecits : 2 H | 248
Absorption of Parts by Larve . . : . : : ‘ + 251

CHAPTER XVI

EXTERNAL FACTORS THAT INFLUENCE GROWTH . ; o> dG + 253
Influence of Food . : . : . . : 3 - 253
Stimulants affecting Growth . z “ é : : . 256
Effects of Salton Growth. ire es Ee : : . 257

Effects of Heat on Growth . ‘ ti ‘i A Z ‘ , . 260

x Contents

PAGE

Effects of Light on Growth : 3 3 : ‘i . . - . 261
Growth toward the Light; Phototropism . . . « . . 265
Influence of Gravity on Growth . ‘ ‘ , : F F . 266
Effects of Electricity on Growth . ‘ ‘ 5 é - : . 267
Pressure and Contact . ‘ - , . - 7 : : . 267
The Formation of Galls : : 5 . . . . F + 268

CHAPTER XVII

GROWTH AND REGENERATION . «©. wee 2

EXPERIMENTAL STUDIES IN GRAFTING

CHAPTER XVIII

EXPERIMENTS IN GRAFTING a . : , a - . . 285
The Union of Different Regions . : : ‘ : « 285
The Influence of the United Parts on Each Other: Formative Factors 289
The Union of Parts of Different Species ; ji : P . 298
Special Problems of Development : : : , . : - 301

EXPERIMENTAL STUDIES OF THE INFLUENCE OF
THE ENVIRONMENT ON THE LIFE-CYCLE

CHAPTER XIX

CHANGES IN THE LIFE-CYCLE AND CHANGES IN THE ENVIRONMENT + 309
The Life Histories of Some Animals. . . , + 309
Influence of Food on the Life-cycle of Lépidepters . 313
Influence of the Environment on the Time of Ripening of the Sexual

Organs. - : E ‘ 7 : . . : + 315
CHAPTER XX

ALTERNATION OF SEXUAL AND PARTHENOGENETIC FoRMS . J « 322

Alternation of Generations in the Aphids and Phylloxerans . ‘ » 322

CHAPTER XXI

INFLUENCE OF EXTERNAL CONDITIONS ON THE LIFE-CYCLE OF THE
LOWER CRUSTACEANS . . : - 7 , + 336

CHAPTER XXII

INFLUENCE OF EXTERNAL CONDITIONS ON THE LIFE-CYCLE OF THE Ro-
TIFER, HYDATINA SENTA : . . . : é E » 346

Contents xl

CHAPTER XXIII

PAGE
THE LIFE-CYCLE OF SOME HyMENopTEROUsS INSECTS. BEES . : + 351
Other Hymenoptera. if A . : : 7 F ‘ » 356

EXPERIMENTAL STUDY OF THE DETERMINATION
OF SEX

CHAPTER XXIV

INTRODUCTION: THE DIFFERENT KINDS OF SEXUAL INDIVIDUALS . - 363
Dicecious Species, composed of Unisexual Individuals . ‘ , 365
Moneecious Species, composed of Bisexual or Hermaphrodite Tnidividwela 368
Parthenogenetic Species. 3 a . ri ‘ : a . 372

CHAPTER XXV

EXTERNAL Facrors OF SEX DETERMINATION . : . . . - 376
Influence of Food 3 < . . 376
Supposed Influence of Noutebinent in paving Sex in Nits rl

other Mammals . . 5 . ‘ . » 384
Geddes’s and Thomson’s Theory of Sex ‘ . $ 7 e . 387

CHAPTER XXVI

INTERNAL Factors OF SEX DETERMINATION . ‘ : 5 - + 391
: Age of Parents . . . 2 " . a ‘ . é - 391
Condition of the Germ-cells . : 5 . i : : - 393
Vigor of the Parents . ‘ : . . . ‘ . . - 393
Effects of Inbreeding . ‘ ‘ : . . ‘ . . » 394
Size of the Egg . : : ri ; - ‘i ‘ F - 304
Ratio of Nucleus to Gitopiacun ‘ "i 396

The Extrusion of the Polar Bodies, and the canalopaie eae in the
Sperm-cells . ‘ é . + 397
The Formation of Male and Female Producing Sperttakodon . . 401

CHAPTER XXVII

THE INTERNAL FAcToRS OF SEX DETERMINATION (Continued) . - 407
The Origin of Gynandromorphs . 7 . : : . 3 + 407
The Sex of Multiple Embryos. . . . : - 410
The Sex of Human Twins and Double ‘Monsters é . 411
Two Recent Theories of Sex Determination based on the Assninpon

of Male and Female Eggs . F F . : + 413
The Reduction Process in Parthenogenetic Ege P . : » 415

Influence of the Cytoplasm . é 7 : fs . . F . 417

Xi Contents

PAGE
Conclusions :
(1) The Morphological Conception of Sex Determination . 420
(2) The Physiological Conception of Sex Determination . » 421
(3) Analysis of the Results. r 3 2 z dj - 422

EXPERIMENTAL STUDY OF SECONDARY SEXUAL
CHARACTERS

CHAPTER XXVIII

SECONDARY SEXUAL CHARACTERS 2 . ‘ - is 5 a + 429
Introduction ‘ é ‘ 4 , ® a 3 m : - 429
Correlation between the Secondary Sexual Characters and the Essential

Organs of Reproduction. : : . . F . + 432

CHAPTER XXIX

SECONDARY SEXUAL CHARACTERS (Continued) . é . . . + 439
Theories of the Origin of Secondary Sexual Characters . . + 439

INDEX 4 . . : . % aq . a . ‘: 7 + 449

EXPERIMENTAL STUDY
OF EVOLUTION

EXPERIMENTAL ZOOLOGY

CHAPTER I
THE EXPERIMENTAL METHOD

THE study of Zodlogy by experimental methods is not a new
departure, for the method of experiment has been often applied
to special zodlogical problems. On the other hand, the recogni-
tion that only by experimental methods can we hope to place
the study of Zodlogy on a footing with the sciences of chemistry
and of physics is a comparatively new conception, and oné that
is by no means as yet admitted by all zodlogists. I do not wish
to appear to disparage those studies that deal with the descrip-
tive and with the historical problems of biology. They also
offer a wide field for activity, and the more familiar we become
with the structure and modes of development of animals, so
much the better can we apply the experimental method. In
fact, many of the problems of biology only become known to us
as the result of direct observation. The wider, therefore, our
general information, the greater the opportunity for experimen-
tation.

It is undoubtedly true that many zodlogists who have spent
their lives in acquiring a broad knowledge of the facts of their
science fail to make use of their information by testing the very
problems that their work suggests. This is owing, no doubt, to
their exclusive interest in the observational and descriptive sides
of biology, but also in part, I think, to the fact that the experi-
mental method has not been sufficiently recognized by zodlogists
as the most important tool of research that scientists employ.

3

4 Experimental Zoology

Perhaps also the fact that the historical side of biology attracts
such popular interest accounts, in part, for the neglect of more
searching scientific methods of study.

Whether the method of observation or the method of observa-
tion and experiment is followed, seems to be also a question
of the kind of interest aroused by living objects. If the number
of collectors, naturalists, zodlogists, anatomists, entomologists,
ornithologists, mammalogists, conchologists, etc., be compared
with the number of physiologists, physiological chemists, bac-
teriologists, it will be seen that the former have an enormous
advantage in numbers. It is true that a few zodlogists are
experimentalists, and that some physiologists do not experiment
at all, but the proportion remains about the same. In other
words, interest in collecting and recording the results of obser-
vation and in the artistic side of nature is much more wide-
spread than interest in the study of problems, or, if the interest
is not lacking, the will to take the initiative in the formulation
and solution of problems seems to be less cultivated in the
biological sciences than the power to observe and to describe.
In so far as the followers of the one or of the other method of
investigation have made their selection as a matter of tempera-
ment, the disproportion will probably always remain; but in so
far as the result is due to imitation, or to following the line of
least resistance, or to a failure to appreciate differences in aim
and method, the proportion may to some extent be altered; and
I think it will be generally admitted that at the present time there
is greater need for experimental work than for descriptive and
observational study.

It is sometimes said that experimental study is the analytical
study of problems, and this in a sense is true, but it is only a part
of the truth. It is rather the method of attacking problems that
is the chief characteristic of experimental work, for is not the
historical method also a study of problems? We demand in
the case of a problem in experimental science that the condi-
tions under which an event takes place be discovered, and that,
if possible, we reproduce artificially the result by controlling the

The Experimental Method 5

conditions. In fact the control of natural phenomena is the
goal of experimental work. :

In the studies of physics and chemistry the method of experi-
ment is so familiar, that we think of their advancement as
taking place by experiment alone. In biology the situation is
different, and new discoveries are looked for as often in the
field of observation as of experiment. This difference is due to
the higher stage of development that has been reached by the
physical sciences, while biology is still, in large part, in the
lower stages of its evolution where facts are insufficiently known.
Nevertheless the amount of time still given to descriptive work
is out of proportion to the present condition of development of
biology.

A few examples may serve to illustrate the differences be-
tween the descriptive and the experimental study of zodlogy.
The egg of an animal, if set free and fertilized, begins at oncc
to develop. Descriptive embryology gives us the different stages
through which the egg passes, but no matter how complete the
description, we still know little or nothing of the causes that
are operating to bring about the development. What, for
instance, does the spermatozoén bring into the egg to make it
develop? What physical and chemical changes take place
during cleavage? What makes the embryo turn in at one pole?
Why do certain cells develop cilia? These and a hundred other
questions suggest themselves. Observation has failed to answer
them.

Another method employed in recent years has been to attempt
to find certain physical or chemical changes that seem to be
similar to those observed in the developing egg. Thus it has
been suggested that the spermatozodn brings a ferment into the
egg; that the cleavage is due to differences of surface tension ;
and that the gastrulation is caused by osmotic pressure. Ma-
chines that behave in somewhat similar ways have even been
constructed to illustrate some of these changes. Interesting as
the ideas derived from these sources may be, their scientific
value lies only in their suggestiveness, until it can be shown

6 Experimental Zoology

that the changes in the embryo are really the outcome of similar
processes; and the only way in which certainty can be gained
on this point is by experiment on the organisms themselves.
If a ferment starts the development it is our duty to isolate it,
and to introduce it hypodermically into the egg. If this could
be done and the development thereby started exacily as in normal
fertilization, the hypothesis becomes at least probable that a fer-
ment is brought in by the spermatozo6n and starts development.
Similarly, for cleavage, for gastrulation, and for every stage
in the development, experiment alone can give a satisfactory
answer.

The essence of the experimental method consists in requiring
that every suggestion (or hypothesis) be put to the test of experi-
ment before it is admitted to a scientific status. From this point
of view the value of an hypothesis is to be judged, not by its
plausibility, but by whether it meets the test of experiment. Its
use is therefore primarily for the investigator, and not for the
layman; yet as a matter of fact the wildest speculations are
likely to be the ones that excite most popular attention and
applause.

It is sometimes said that an hypothesis is useful in proportion
to the number of facts it brings under one point of view. This
is true for the student rather than for the investigator. Such
an hypothesis may have no scientific status. It belongs rather
to a system of mnemonics. To the teacher, also, hypotheses are
useful in arousing the interest of his hearers, so that by exciting
their undeveloped imagination he can make his dry facts more
entertaining. But let us be careful to distinguish between the
forensic and the scientific value of hypotheses.

Hypotheses may be useful, and have been used in various
ways. They have been used, as just stated, to hold together a
body of isolated facts; as such they are in reality only fictions.
They have been used in the reconstruction of supposed historical
events, especially in biology in the setting up of family trees.
In this case they can only claim to be more or less plausible
suggestions. Hypotheses have been used to direct intercst

to 4 4 4

The Experimental Method 7

toward certain fields of study. As such they have often proven
stimulating and have been useful in acting as a guide for others.
But the hypothesis of real importance is the working hypothesis
of the investigator. It is the test by means of which he tries
to interpret his problem, and therefore it is essential that his
hypothesis is one having a practical bearing, z.e. an hypothe-
sis that can be shown to be true or false. It differs in this
essential respect from purely fictitious and from metaphysical
hypotheses.

The working hypothesis carries along with it its dangers as
well as its advantages; since, while it may lead to discoveries,
it may, if it is wrong in principle, blind us to the real condi-
tions. Therefore the investigator must not only be an inventor
of working hypotheses, but cultivate also a skeptical state of
mind toward all hypotheses — especially his own —and_ be
ready to abandon them the moment the evidence points the other
way. And herein lies one of the differences between the re-
corder of observations and the experimenter. The work of the
observer, if exact, is complete in itself, and stands forever as a
monument to his ability, or at least to his industry; while the
conclusions of the experimenter, if they are to bear fruit, must.
become modified with each new discovery. His results are ab-
sorbed in the current of the next advance, but his consolation
will be that he has had at least a share in the causal study of
living things, and in helping the human race toward the control
of organic phenomena.

To return to our examples. The growth of animals and
plants offers a wide field for-experimental study. Under cer-
tain conditions we see a young animal continuing to grow larger
until a certain size is reached, when growth slowly ceases. Al-
though the animal may live for many years longer, it has ceased
to grow. What makes it grow? Why does it stop growing?
We have hardly begun experimental work along these lines;
yet we shall see later that there is a promising field for work in
this direction. After a time old age comes on and the animal
dies. We say it dies a natural death, and this seems inevitable,

8 Experimental Zoology

but only because we have found that death always takes place
under ordinary conditions. Suppose, however, we change the
conditions; might we not hope to prolong the duration of life?
Improbable as this may seem, there are already experiments
afoot that indicate that, however difficult, the problems may
not be insoluble.

Why, on an average, in most animals, are equal numbers of
two forms born —male and female? Is there an internal
mechanism? If so, what regulates it? Do external or inter-
nal conditions determine that one egg becomes male, another
female? Even if an internal mechanism exists, it might be
affected by external conditions, and in any case the cause of
the production of the two types must be determined.

Observation has established that the evolution of animals and
plants has, in all probability, taken place. But what factors are
involved in the process are unknown. Only in the last few
years by means of an experimental study of the subject has
decided advance been made.

It is sometimes stated that nature has already carried out
innumerable and wonderful experiments, and that we can
never hope to excel her in this power. Is it not better, therefore,
to examine patiently and reverently what she has done, and in
this way learn how her processes have been carried out? Let
us not be blinded by rhetorical questions of this kind. No
doubt nature has carried out prodigious experiments; but we
can never be certain that we know how she has obtained her re-
sults until we can repeat the process ourselves. What would
the chemist or the physicist say if he were told that nature has
already carried out experiments on a much greater scale than
he can hope to accomplish, and that he should drop his ex-
perimental methods and. study his physics in a thunderstorm
and his chemistry in a volcanic eruption !

I have brought up this point because it illustrates one side
of the experimental method that is sometimes overlooked. Al-
most all of the phenomena with which the biologist has to deal
are so complex that he cannot determine what part each factor

The Experimental Method 9

plays in the result unless he study the effects of each under
different conditions that can be controlled. Little by little, in
this way we can hope to gain a clearer insight into the condi-
tions that, taken all together, produce the result.

So much for the experimental method. It is pertinent to ask
what is an experiment? If I cut an earthworm in two to see
what will happen, have I performed an experiment? Perhaps
not, for the actual performance of cutting the worm in two is
not the essential point. The essence of an experiment is a trial
or test, and the conditions are so arranged that an answer is ex-
pected. If the worm is cut in two in order to study the physio-
logical behavior of the two ends, as has been done in fact, with
some interesting results, or in order to see what regenerates at
the two cut ends, we have a distinct purpose in view, although no
formulated problem. If we proceed farther and remove a
definite number of segments in order to see how many come
back, and then try to determine what conditions are involved
in the results, we are clearly carrying out an experiment with a
more definite aim. This illustration will serve to show that the
most essential feature of an experiment is the anticipation of
the results of a test. The operation may be so simple, and the
conditions so little known, that to call the performance an
experiment may easily expose one to the ridicule of those un-
favorably inclined to the claims of experimental work. If the
process is carried out with scarcely a thought as to its purpose,
and in complete ignorance of all the conditions entering into
the problem, it can scarcely be called an experiment at all. At
most it is only a preliminary testing of possibilities. Much of
the pioneer work in experimental zodlogy has necessarily been
of this kind, and crude as such preliminary work must be, it
should be looked upon only as the first step toward a further
and more critical analysis.

The carrying out of an experiment implies the formulation of
a working hypothesis, and this usually presupposes some knowl-
edge of the possible conditions that control the phenomena.
The experimental work becomes more explicit and accurate the

10 Experimental Zoology

more we know beforehand of the possible conditions that may
enter into the result. The ability of the experimenter is shown
in his insight into the possible factors that may be present. His
ability may be the result of a correct estimate of the possible
conditions, but for the highest order of work there is demanded
also great imaginative power. Good judgment and accurate
observation may lead to fine work, but constructive imagination
seems to be required for the highest order of original work.
This does not imply that accuracy of observation is not as requi-
site in original work as in descriptive and observational work,
and should always be expected; but the man who sees new
and overlooked combinations may open fields of research that
will set to work an army of able “‘investigators.”

The branches of biology that have made most extensive use
of the experimental method are physiology, bacteriology, and
physiological chemistry. The zodlogist and the embryologist
have also to deal with physiological problems, and already the
beginning of important experimental work has been carried out
in this field; but the most distinctive problem of zodlogical work is
the change in form that animals undergo, both in the course of
their development from the egg (embryology) and in their develop-
ment in time (evolution). It will be granted, I think, that these
formative problems are more difficult than those relating to
function with which the physiologist has concerned himself in
the main; but this is all the greater reason why the experimental
method should be used in their study, especially after so much
purely descriptive work has been already done.

The term “ morphology” has been used in recent times to
denote the study of form, as contrasted with physiology, that
deals with functional changes. Morphogenesis has also been
employed to signify a study of the changes in form through
which organisms pass. It is mainly the experimental study of
these changes in form that I propose to examine in the follow-
ing pages. Experimental morphology would perhaps nearly
indicate the field to be examined; but since the line between
experimental physiology and experimental morphology is often

The Experimental Method II

hard to draw, and since I shall not hesitate at times to enter
upon the physiological side of many problems, I have chosen
the somewhat broader title of Experimental Zodlogy to include
the subjects to be treated.

The principal topics to be discussed fall under the following
six headings : —

I. Experimental Study of Evolution.
II. Experimental Study of Growth.
JII. Experimental Studies in Grafting.
IV. Experimental Studies of the Influence of the Environ-
ment on the Life-cycle.
V. Experimental Study of the Determination of Sex.
VI. Experimental Study of the Secondary Sexual Characters.

CHAPTER II

THE INFLUENCE OF EXTERNAL CONDITIONS IN CAUSING
CHANGES IN THE STRUCTURE OF ANIMALS

ANIMALS and plants are so constituted that one of their chief
characteristics is that they respond to their natural environment
in such a way as to insure their continued existence. These
responses are in the main physiological, and therefore in large
part transitory; but in some cases the response is structural,
involving a temporary or even a permanent change in form or
structure that persists at least so long as the external condi-
tions that called it forth remain. The question arises whether
these changes, directly induced by the environment, may not
give origin to the more fixed characters that have become the
permanent inheritance of each species. May not these have
been in the first instance adaptive responses to the environ-
ment? This leads to the further question ,of the origin of all
the characters of the species, whether adaptive or non-adaptive.
In this and in the following chapters the different sides of this
question will be considered.

ADAPTIVE RESPONSES

External conditions sometimes cause adaptive structural
changes in organisms. We are familiar with some effects of
this sort in our own bodies. Pressure on the skin, if long con-
tinued, causes it to become thicker and more capable of resist-
ing the injurious effects of pressure. Sunlight tans the skin
and protects it from “‘burning.” It is said that cold causes the
fur of some mammals to become thicker, and this change better
protects them against the cold. Conversely, it is said that horses

and dogs lose their hair to some extent in very warm climates.
12

The Influence of External Conditions 13

A number of Arctic animals become white in winter. This
change seems to be in part due directly to the cold, for it has
been found if these animals are transferred to warmer climates
they show less marked changes on the approach of winter.

Flounders and some other fish and some amphibians become
lighter in color on a light background and darker on a dark
background. The most remarkable case of this sort is that of
the pupe of certain butterflies. If the pupation takes place on a
light background, the chrysalidsare lighter; andif ona dark back-
ground, they are darker. Experiments by Poulton ' have shown
that this effect is produced directly through the skin and not
through the ocelli. Poulton thought in one case that even the
color of the silk in which the caterpillar incloses itself is influ-
enced by the color of the background, but this has been shown
not to be the case.

It is popularly supposed that the African chameleon becomes
green in green surroundings, and brown in a dark environment,
but this is probably not true; at least ithas been shown in another
lizard, Anolis, that can also change from green to brown and
the reverse, that the animal is as a rule green if warm and
brown if cold. The effects are produced by change in the pig-
ment of the dermal pigment cells. In the cold the black pigment
spreads out over the surface and conceals the stationary green
pigment. In the warmth the black pigment migrating inward
exposes the green. Light has a somewhat different effect. In
both the African chameleon and in Anolis a strong light acts
like a low temperature, causing the black pigment to migrate
to the surface, and a faint light or darkness acts like a high
temperature, causing the black pigment to wander inward,
so that the animal becomes green. Other lizards give reverse
effects in light. Parker and Starratt have shown for Anolis that
when both light and heat act together, the results are as follows:

‘ For details of the experiment, see Poulton, “The Colours of Animals,”
p. 110, 1890; ‘(Further Experiment. upon the Colour Relation,” etc., Trans.
Ent. Soc., p. 293, London, 1893; “An Inquiry into the Cause and Extent of
a Special Colour Relation,” Proc. Roy. Soc., Vol. XII, 1887.

14 Experimental Zoology

At alow temperature, 10°C., Anolis changes from green to brown,
irrespective of illumination. At a high temperature, 40° to
45° C., it turns from brown to green, irrespective of illumination.
Thus heat is the controlling factor at these extremes. Between
these extreme temperatures there is a range from 25° to 35°
through which light is the controlling factor, although heat is
not without its influence, as shown by the rate of the change.
Parker and Starratt have discovered the astounding fact that the
effects of the illumination may be produced when an area of
the skin no larger than a square millimeter is exposed, the rest
of the animal being in the dark.

The changes that have just been described, except perhaps the
last ones, seem to be of benefit to the animal, either in directly
protecting it from the agent that brings about the result, as in
the effects of pressure, cold, sunlight, etc., or in more effectually
concealing the animal from its enemies. These responses are
said to be adaptive, but it is remarkable how rare are adaptive
structural responses, when we recall the fact that adaptation of
the organism to its surroundings is one of its most characteristic
properties. The poverty of adaptive structural response does
not encourage one to look to external agents as having brought
about directly the structural adaptation of organisms to exter-
nal conditions, even if it could be shown that such influences

‘are inherited.

There are, on the other hand, many cases of physiological
responses that are adaptive; in fact, nearly all functional
changes are directly beneficial to the organism. Animals re-
spond in a most remarkable way to poisons. If certain alka-
loids are injected in ever increasing doses, the animal becomes

.immune to a dose which if given in the first instance would
have been fatal. In the case of the poisonous ptomaines pro-
duced by bacteria, the animal produces a counter poison, an
antitoxin, that nullifies the effects of the ptomaine. In a num-
ber of bacterial diseases the animal becomes more or less im-
mune after the first attack.

Equally striking are the adaptive responses shown by animals

The Influence of External Conditions 15

to temperatures higher or lower than those to which they are
normally subjected. If the change of temperature is gradual,
the organism may become adapted to a temperature that would
have been fatal if met at once.

Somewhat similar results have been found by subjecting ma-
rine animals to water containing less salt. If the change is
gradual, the animal will become adapted to the new density.
The extent to which the process may be carried differs greatly
in different species. In some cases the animal may be gradually
transferred even to fresh water. There are also other animals
that may pass at once from salt to fresh water without serious
injury. Salmon and shad leave the ocean to migrate up the
rivers, and other fish do the same thing. Conversely, the
young salmon migrates back to the sea. The number of fish
that will stand as great a change as this is, however, limited,
although many oceanic species will live in water much less salt
than that of the sea.

NON-ADAPTIVE RESPONSES

In contrast to the few cases of adaptive structural responses
to the environment there are quite a number of cases in which
definite structural responses occur that are not adaptive. One
of the interesting points connected with these responses is that
the differences effected by changes in the environment have been
shown in some cases to resemble the kind of differences that
separate species from each other; but whether species have
really originated, either directly or indirectly, in this way, must
be carefully considered later.

Influence of Temperature on the Coloration oj Butterflies

The earliest experimenters on the influence of temperature
on butterflies were Dorfmeister (1864) and Weismann (1875).
Earlier naturalists, who were familiar with seasonal dimor-
phism in butterflies, had supposed, it is true, that differences in
temperature might be responsible for the differences in color
that characterize the summer and the winter broods; but it

16 Experimental Zoology

required the experimental work of Dorfmeister and of Weis-
mann to show that this supposition was correct.

Weismann showed for Vanessa levana-prorsa that when a
pupa, destined to give rise to the summer form, is kept at a low
temperature, it may produce the winter form, V. levana, or a
type transitional between the summer and the winter forms.
He also succeeded, by raising the temperature, in changing the
winter pupa so that it gave rise to the summer type of butterfly.

More recently Merrifield, Standfuss, Fischer, Grafin von
Linden, and others have carried out extensive experiments on
the effects of temperature. The butterflies and moths used
for this work are usually those having summer and winter broods,
that differ in color and often in size, and even in the shape of
the wings. In other cases, however, similar changes have been
brought about in forms that do not show seasonal dimor-
phism. It has been found that not only the summer form can
be changed into the winter form, and vice versa, but in certain
cases the type may be changed by cold so that it resembles
northern varieties of the same species, and by heat to resem-
ble southern varieties. Temperatures that are only somewhat
higher or lower than normal produce the southern and northern
types respectively, while much higher or lower temperatures
produce effects that are rarely or never found in nature. These
latter changes are sometimes called aberrations. We may first
examine a few examples of these effects given by Standfuss.

The effect of heat on Vanessa cardui is shown in Fig. 5. The
colors are much lighter above than those of the normal butterfly.
The black bands are much reduced. Similar changes are ob-
servable on the under side of the wing. The pupa had been
kept for 60 hours at 36-37° C., and then at normal temperatures
for six to seven days, when the butterfly emerged. The effect of
cold on Vanessa cardui is shown in Fig. 6. The color is darker
and the white spots are reddish in tint as seen especially on the
under side. The pupe had been kept for 33 days in an ice
chest, then for five days in a cellar (+13°C.), and lastly for nine
days at room temperature.

Fic. 1 Showing the influence of heat and cold on the pupse of some butter-
thes. Vanessa antiopa: Fig. 1, effect of heat; of cold, Figs. 2 and 3. Vanessa
cardui: Fig. 5, effect of heat: Fig. 6, effect of cold. Vanessa atalanta: Fig. 7,
effect of heat: Fig. &, effect of cold. Fig. 4, Vanessa pollychloros, aberratio
dixeyt, effect of cold. (After Standfuss.)

Lhe Influence of External Conditions ry

The effect of heat on Vanessa atalanta is shown in Fig. 7.
The blue spots on the outer edge of the wings are so much re-
duced that often two small flecks alone remain. The red cross-
band of the fore-wings is spread out. In the black, that is
somewhat brownish, red-brown shading appears near the base
of the wing. The large white spot at the anterior border of
the fore-wing and the neighboring five scattered white spots
show a reduction, and the latter may even disappear. In all
of these aspects the type approaches a variety from the Canary
Islands. The pupz were kept 172 hours at 37° C., then three
to four days at 24° C.

The effect of cold on Vanessa atalanta is shown in Fig. 8.
The ground color of the upper surface is bluish black. The
white spots are larger, and the whole of the outer tip of the fore-
wing is lighter. The red cross-band is reduced and broken.
The under surface of both wings are much changed, as the
figures show.

The influence of heat and of cold on Vanessa antiopa is shown
in Figs.1 and2. The effect of heat (Fig. 1) is to make “dusty”
the brown ground color of the upper surface, especially on the
hind wing, that may become nearly black. The blue spots of the
marginal row are reduced to half their normal size and are more
violet in color. The characteristic yellow margin is dusted with
brown. The pup had been kept 60 hours at 37° C., and then
for 12 days at 24°C. The influence of cold is shown in Fig. 2.
The blue spots are much larger, and the ground color of the
wings is darkened. The marginal yellow band is reduced and
richly dusted with black scales. The pup had been kept for
44 days in an ice chest, and then for 15 to 19 days at normal
temperatures. Another individual produced by cold (29 to 34
days) is shown in Fig. 3. It approaches Vanessa polychloros,
Fig. 4, in a number of points,

Fischer has obtained some very aberrant forms of Vanessa
antiopa by heat and by cold. Some of those produced by cold
are shown in Figs. 1-8 as seen from above, and in Figs. 9-11
as seen from below. The former set (Figs. 1-8) is arranged to

Cc

18 Experimental Zoology

show how the broadening of the yellow margin begins in the
posterior wings and finally extends forward to the same extent
on the anterior wings. The change is in an antero-posterior
direction. The conditions under which these butterflies had
been kept were as follows :—

In the first experiment 20 pupe of Vanessa antiopa, about
12 hours old, were kept for six hours at a temperature of 14°C.
and then four hours in a temperature decreasing from 14° C.
too° C. After this they were put three times daily for a short
period in a temperature of —3° for 18 days. They were then
keptin the cellar (14°C.) and finally at room temperature (22°C.).
Six pupe died, the remaining 14 began to emerge after 10 to 12
days. Figure 7 shows one of these butterflies which is the
aberration known as hygiza; another is shown in Fig. 6, in
which the blue spots and the dark border have completely dis-
appeared, but the yellow border does not extend inward so far
as in the last case. Three other individuals (Figs. 2, 3, 4, 5)
show transitional forms in some of which traces of the blue
spots could be seen. The last butterfly of this series to emerge
was like the normal antiopa with the blue spots even larger
than the normal, but less sharply defined. Thus under identi-
cal external conditions quite a range of colors result; but of
course the caterpillars themselves had probably not lived under
identical conditions, nor had they been subjected to the cold at
precisely the same stage in their pupation.

In a second experiment the conditions were the same, except
that the pupz were brought into the extreme cold (three times
daily) for only 14 days. Five of these showed the aberration
artermis (like that in Fig. 1, which developed, however, under
different conditions). The five individuals showed also aber-
rations with the hygiza characters. Two individuals were like
Fig. 4; the others were like the intermediate forms.

In a third experiment the conditions were the same as before,
except that the pupz were brought into the extreme cold for
only six days. They produced one normal butterfly; one transi-

? The butterfly had been kept at a temperature below o° and 6° C.

Fic. 2. Vanessa antiopa: Figs. 1-8 showing effects of a low temperature on
pupa. (After Fischer.)

19

20 Experimental Zovlogy

tional form like Fig. 3;

one form, the aberration artermis,

with many groups of golden scales on the brown ground color
and the black border, and even within the blue spots; the rest
for the most part belonged to the aberration hygiaa, one of
these being as extreme a form as that shown in Fig. 8.

11

Fic, 3. Vanessa antiopa, under surface:

Figs. g-11 showing influence of
(After Fischer. )

The under side of the
wings of these butterflies
is also changed, but not
always in the same way as
the upper side. As shown
in Fig. ro, not only does
the yellow border of the
wing extend inward, as it
does also on the upper side,
but at the same time the
black scales spread toward
the periphery, darkening
the broader yellow band, as
shown in Fig. to. This
figure shows the under side
of the aberration hygiza.
In the butterfly shown in
Fig. 11 the whole under
side is black, the yellow
border having disappeared.
This is the under side of
the butterfly shown in Fig.
8 that had the broadest
development of the yellow
band on the upper side.

Some further details may.
now be considered. Merri-
field did not employ very ex-

treme temperatures. The pupe were either iced (33° F.) or only
cooled (39° to 57° F.). For “ forcing,” temperatures not higher
than 70° to 80° F., or even go° F., were employed. He found

The Influence of External Conditions 21

in general that cooled or iced pup gave dark and much-spotted
moths, while “forced” pupe were pale and spotless or with
reduced spots. He states that the markings are affected by
long-continued exposure, especially during the early pupal period,
but the color is chiefly affected during the penultimate period.

Standfuss has experimented on as many as 7000 individuals
in all. He determined that the period in the life of the pupa
when it is most sensitive to heat is at the beginning, although
it is best not to try to produce the effect too soon after the cater-
pillar has become a pupa. If exposed soon to extreme cold, the
pupz die; but if exposed soon to heat the best results, z.e. the
most divergent forms, are obtained, although the mortality is
high. If the pupa is exposed as soon as it can stand it to
extreme cold and then to heat, the heat-type is produced. In
other words, the cold delays the development so that the heat
produces the greater effect; for as soon as the temperature is
raised the development goes forward rapidly. To get the best
effect from cold, Standfuss found it advantageous to expose first
to low temperatures, then for 5 to 10 days to moderate cold
(11° to 14° C.), and lastly bring the pup to room temperature.

In ‘his earlier work Fischer subjected the pup for several
days to extreme temperatures of heat or of cold, but later he
found it better to subject the pupz two or three times a day
to the extreme temperatures during a period of 11 to 20 days.
This method applies particularly to extreme temperatures. For
a low temperature he used 0° C. or — 3° C. or even —8° to — 20°
C. The high temperature was from 35° to 46°C. The more ex-
treme the temperature that the pup will stand the greater the
effect produced. Thus temperatures between 0° and — 20° gave
greater effects than those between 0° and +10°. Tempera-
tures of 42° to 46° gave more striking results than those of 35°
to 41° C.

The most important result obtained by Fischer was to show
that the same aberrations are obtained by extreme heat and by
extreme cold. The result may seem puzzling, but it must be re-
membered that the coagulation of proteids, which is probably

22 Experimental Zoology

one of the factors in the results, can be caused artificially either
by a high or by a low temperature. In a mixture of proteids
we might expect some slight differences in the results of coagu-
lation, even if the principal changes are the same, and it is not
improbable that such minor differences do exist.

An excellent general review of the effects on the colors has
recently been given by Griafin Marie von Linden. She points
out that in the:Vanessa series, a higher temperature makes the
red or yellow deeper or more fiery. The dark background
suffers a reduction. Cold gives the reverse, a brightening of
the general dark ground color, the yellow expanding at the cost
of the red. There is also a lightening of the red and increase
of white scales. Extreme heat and cold, as stated above, give
remarkably similar results. The black spots on the border run
together, so that the peripheral dark spots are lost. The dark
border zone becomes clearer (in some forms only at the tip).
Despite this peripheral clearing up, extreme temperatures cause
an increase of dark pigment elsewhere. It may be said, there-
fore, that extremes of heat and of cold do not give specific effects,
but produce the same physiological change.

As a result of these changes the differences between related
species sometimes seem to disappear to a greater or less extent.
The nearer the forms experimented upon, the more alike are
their aberrations. This result led Fischer to the conclusion
that extreme heat and cold cause an atavistic return to the primi-
tive type of all the Vanessas, 7.e. a return to the stem-form from
which they have come. He attempts to explain this result on °
the grounds that during the development of the color the butter-
fly passes in its ontogeny through phylogenetic stages. Cold
and heat cause an arrest of development, so that an ancestral
stage emerges.

Standfuss, on the other hand, looks upon the changes as some-
thing new, and points out certain contradictions to Fischer’s
idea that the aberrations are atavistic. For instance, the males
are much less prone to atavism than the females, and yet pro-
duce a much greater number of aberrations. He thinks it im-

Lhe Influence of External Conditions 23

probable that the original form of the Vanessas was darker
than the present forms, to judge by related groups. Uncertain
as arguments, pro and con, based on these phylogenetic specu-
lations necessarily are, there is fortunately experimental evi-
dence that shows how little ground there is for Fischer’s argu-
ment. Grafin Marie von Linden has examined the developing
pigment in the wing itself. She finds that the lighter ‘colors
develop first and that in the younger stages the red and yellow
tones occupy a greater area than they do later. This is the
reverse of Fischer’s primary assumption of sequence. In pupe
exposed to freezing and to heat it seems that disturbances in the
development of the color occur. Standfuss observed in these cir-
cumstances that the color develops later than in the normal; and
von Linden finds that the black color appears relatively earlier
than the others, and at times even before the red and yellow.

Grafin von Linden has also shown that changes in certain
colors similar to these shown by the pupe can be produced in
vitro. Extracts of the red color, when heated, become fiery red
or more red-brown in color; while on ice the red and the yellow-
red tone remain constant. It is likely, therefore, that some of
the effects of high and low temperature can be explained en-
tirely as due to the direct influence of the temperature on the
chemical composition of the pigment.

Not only have the summer and winter forms been changed
by changing the temperature, but in two cases, in which sexual
dimorphism exists, it has been possible to change the female
coloration into that of the male. For example, the colors of
the female of Parnassius apollo can be changed into those of the
male. In the female of Rhodocera rhammi the white colora-
tion of the wings can be changed by warmth into the intensely
yellow color of the male. It has been suggested that while
ordinary temperatures suffice to cause the development of the
deeper color in the male, it requires.a higher temperature than
that ordinarily met with to cause the same change in the tissues
of the female. Standfuss has pointed out that while in many
cases a lower temperature may cause a darker color and a higher

24 Experimental Zoology

temperature a lighter one, this is not invariably the rule,
as seen in certain dimorphic forms. Thus, in Lythria rotaria
the butterfly that hatches in the spring (from over-wintering
pupe) is darker than the summer type (the second generation,
variety L. purpuraria). On the other hand, Vanessa levana
(also from over-wintering pup) is /ighter than the summer
variety, V. prorsa. A lower temperature does not therefore
always produce darker colors and a higher temperature lighter
ones, although this is the general rule, but the reaction depends
also on the nature of the organism.

In these experiments with butterflies the more conspicuous
result is the change in color; but it should not be forgotten that
changes in size also often occur, and even constant changes in the
shape of the wings have been observed, the outline of the wing in
some cases being quite different from that of the normal animal.

Effect of Temperature on Cater pillars

Standfuss has shown that by rearing caterpillars at a higher
temperature (20°-25° C.) than normal, the characters of the
moths may be affected.'. Changes in the shape of the wings
are sometimes caused in this way. The color is also altered
to some extent. The most constant change, however, is in
the size. Standfuss gives the following rule: “The more
the feeding period of the caterpillar is shortened by raising the
temperature the smaller the butterflies. Lasiocampa quercifolia,
for example, had its weight decreased in this way to one
seventh of the normal. On the other hand, if the time of feed-
ing (or in other words the time of the caterpillar stage) is not,
or only very slightly, shortened, despite the higher temperature,
there is an increase in size, which in the case of Arctia fasceata
may be as much as half again the entire normal weight.”

Standfuss points out that results similar to these are found
in nature where the size of certain forms appears to be connected
with the time of year at which the pupe appear. Species

* Merrifield also has made observation on the effects of temperature on
caterpillars.

The Influence of External Conditions 25

whose growth occurs in late autumn (with lower temperature
and decreasing vegetation) and is completed’in the autumn so
that they winter as pup, are generally smaller in the first gen-
eration and larger in the second or summer generation; for
the caterpillars of the latter are born in summer and have plenty
to eat. Conversely, species which winter over as small cater-
pillars and develop further during the favorable conditions of
spring produce larger butterflies in the first than in the second
generation.

LITERATURE, CHAPTER II

BACHMETJEW, P. Ueber die Temperatur der Insecten nach Beobachtun-
gen in Bulgarien. Zeit. f. wissen. Zool. LXVI. 1899.
Der kritische Punkt der Insecten und das Entstehen von Schmetter-
lings-aberration. Ill. Zeit. f. Entomol. V. 1900.
Experimentelle entomologische Studien. 1x. Band. Temperaturver-
haltnisse bei Insecten. Leipzig. gor.
Kalometrische Messungen an Schmetterlingspuppen. Zeit. f. wissen.
Zool. LXXI. 1902.
Bateson, W. On Variation in the Colour of Cocoons of Eriogaster
Trans. Entomol. Soc. London. 1892.
On Variation in the Colour of Cocoons, Pupe, and Larve; Further
Experiments. Ibid. 1892.
Brucke, E. Ueber den Farbenwechsel der Chamaeleonen. Sitz. Kais.
Akad. Wiss. Wien, math.-naturw. Cl. VII. 185r.
Untersuchungen ueber den Farbenwechsel des Afrikanischen Chami-
leons. Denkschr. Kais. Akad. Wiss. Wien, math.-naturw. Cl. IV.
1852.
Caron F.C. The Colour Changes in the Skin of the so-called Florida
Chameleon, Anolis Carolinensis Cuv. Proc. of the Amer. Acad. of
Arts and Sci. XXXIX. 1903.
CHLopKovsky, N. Sur quelques Variations Artificielles du Papillon de
lOrtie (Vanessa urtice). Ann. Soc. Entomol. de France, LXX.
Igol.
DIXxEy, F. A. On the Phylogenetic Significance of the Wing marking in
certain Genera of the Nymphalida. Trans. Entomol. Soc. Lon-
don. 1891.
Mr. Merrifield’s Experiments in Temperature-Variation as Bearing
on the Theory of Heredity. Trans. Entomol. Soc. London. 1894.
DorFrMEIsTEeR, G. Ueber die Einwirkung verschiedener wihrend der Ent-
wickelungsperiode angewandter Warmegrade auf die Farbung und
Zeichnung der Schmetterlinge. Mitt. d. naturw. Ver Steiermark.
II. 1864.
Ueber den Pinfluss der temperatur bei der Erzeugung von Schmetter-
linge varietéten. Ibid. 1879.
Epwarps, W. H. An Abstract of Weismann’s paper on “The Season
Dimorphism” of butterflies, to which is appended a statement of

26 Experimental Zoology

some experiments made upon Papilio ajax. Canad. Entomol.
1875-1876.

Effects of Cold applied to the Chrysalides of Butterflies. Amer. Ento-
mologist, III. 1880.

Farkas, K. Beitrage zur Energetik der Ontogenese. Arch. ges. Physio-
logie, XCVIII. 1903.

Fickert, C. Kunstliche Kalteabartungen von Schmetterlingen. Jahres-
hefte d. ver. f. vaterlind. Naturkunde in Wurtemburg, LIII.
1897.

Pista, 2 Transmutation der schmetterlinge infolge Temperaturan-
derungen. Experimentelle Untersuchungen ueber die Phylogenese
der Vanessen. Berlin. 1894.

Neue experimentelle Untersuchungen und Betrachungen ueber das
Wesen und die Ursachen der Aberrationen in der Faltergruppe Va-
nessa. Berlin. 1896.

Zwei sonderbare Aberrationen von Vanessa antiopa und eine neue
Methode zur Erzeugung von Kalteaberrationen. Illustr. Wochen-
schr. f. Entomol. 1897.

Beitrage zur experimentellen Lepidopterologie. Ill. Zeitschr. f. En-
tomol. II. 1897. III. 1898. IV. 1899. V. 1900.

Experimentelle kritische Untersuchungen uber das prozentuale Auf-
treten der durch tiefe Kalte erzeugten Vanessa aberrationen.
Soc. Entomol. XTII. 1899.

Experimentelle Untersuchungen ueber die Vererbung erworbener Eigen-
schaften. Allg. Zeitschr. f. Entomol. VI. 1901.

Lepidopterologische Experimentalforschungen. Ill. Zeitschr. f. En-
tomol. VI. 1901, und VIII. 1903.

Weitere Untersuchungen ueber die Vererbung erworbener Eigen-
schaften. Allg. Zeitschr. f. Entomol. VII. 1902.

Fiemminc, W. Ueber den Einfluss des Lichts auf die Pigmentirung der
Salamanderlarve. Arch. f. mikr. Anat. XLVIII. 1897.

Weitere Bemerkungen ueber den Einfluss von Licht und Temperatur
auf die Farbung der Salamanderlarve. Arch. f. mikr. Anat.
XLVI. = 1897.

Frines, C. Experimente mit erniedrigter Temperatur im Jahre 1808.
Soc. Entomol. XIV. 1899.

Experimente mit erniedrigter Temperatur im Jahre 1899. Soc. En-
tomol. XV. 1900.

Temperaturversuche im Jahre 1900. Soc. Entomol. XVI. 1907.

Berichte ueber Temperaturexperimente im Jahre 1gor. Soc. En-
tomol. XVI. 1902.

Gauckter, H. Einfluss hoher Temperatur auf den Organismus von In-
sekten. Entomol. Nachr. XII. 1886.

ey mit niedrigen Temperaturen an Vanessenpuppen. _ Iris,
. 1896.

Ketter, R. Ueber den Farbenwechsel des Chameleons und einiger
anderer Reptilien. Arch. f. ges. Physiol. LXI. 1895.

Kropet, E. Durch Einwirkung niederer Temperaturen auf das Puppen-
stadium erzielte Aberrationen der Lycaena-Arten: Corydon Poda
und Damon Schiff. Allg. Zeitschr. f. Entomol. IX. 1904.

Linpen, M. v. Versuche ueber den Einfluss aiisserer Verhaltnisse auf die
Gestaltung der Schmetterlinge. Ill. Zeitschr. f. Entomol. IV. 1898.

Lhe Lnfluence of Lxternal Conditions 2a

Morphologische und _physiologisch chemische Untersuchungen ueber
die Pigmente der Lepidopteren, I. Die gelben und roten Farb-
stoffe der Vanessen. Arch. f.d. Ges Physiol. XCVIII. 1903.

Die Ergebnisse der experimentellen Lepidopterologie. Biol. Centralb,
xxiv. Ig04.

MERRIFIELD, F. Report of Progress in Pedigree Moth-Breeding. Trans.
Entomol. Soc. London. 1888.

Incidental Observations in Pedigree Moth-Breeding. Ibid. 1889.

Systematic Temperature Experiments on Some Lepidoptera in
all Stages. Ibid. 1890.

Conspicuous Effects on the Markings and Colouring of Lepidoptera.
Ibid. 18or.

The Effects of Artificial Temperature on the Colouring of Several
Species of Lepidoptera with an Account of Some Experiments on
the Effects of Light. Ibid. 1892.

The Effects of Temperature in the Pupa Stage on the Colouring of
Pieris napi, Vanessa atalanta, Chrysophanus phloeas, and Ephyra
puctaria. Ibid. 1893.

Temperature Experiments in 1893 on Several Species of Vanessa and
Other Lepidoptera. Ibid. 1904. j

Recent examples of the effect on Lepidoptera of extreme Temperature
applied in the Pupal Stage. Proceedings of the South London En-
tomol. and Nat. Hist. Soc. 1897.

Parker, G. H., and Starratt, S. A. The Effect of Heat on the Color
Changes in the Skin of Anolis Carolinensis (Cuv.). Proc. of the
Amer. Acad. of Arts and Sci. XL. 1904.

Pictet, A. L’Influence des changements de nourriture sur les chenilles
et sur la formation in sexe de leurs papillons. Compt. Rend. Soc.
de phys. et d’hist. Nat. Geneve, XIX. rg02.

L’influence des changements de nourriture des chenilles et sur le
developpement de leurs papillons, I. Compt. Rend. Soc. Hebret.
sc. Nat. Geneve. Arch. sc. phys. et nat.

Variations des papillons provenant des changements d’alimentation
de leurs chenilles et de Vhumidité. Arch. sc. phys. et nat. Geneve.
XVI. 1903.

Notes complementaires sur les variations des papillons provenant de
Vhumidité. Soc. de Phys. et d@’hist. Nat. Bull. d. séances, I. 1903.

Poutton, E. B. The Essential Nature of the Coloring of Phytophagous
Larve (and their Pupe), with an Account of Some Experiments
upon the Relation between the Color of such Larve and that of
their Food Plants. Proc. of the Roy. Soc. 1885.

A Further Enquiry into a Special Color-Relation between the Larva
of Sinerinthus ocellatus and its Food Plants. Proc. of the Roy.
Soc. 1886.

An Inquiry into the Cause and Extent of a special Color-Relation
between certain exposed Lepidopterous Pupe and the Surfaces
which immediately surround them. Proc. of the Roy. Soc. XLII.
1887.

The TepetentAl Proof of the Protective Value of Color and Mark-
ings in Insects in reference to their Vertebrate Enemies. Proc.
of the Zool. Soc. of London, 1887.

Further Experiments upon the Color-Relation between certain Lepi-

28 Experimental Zoology

dopterous Larve and their Surroundings. Trans. Entomol. Soc.
London. 1892.

The Experimental Proof that the Colours of Certain Lepidopterous
Larve are largely due to Modified Plant Pigments. Nature,
XLVITII. 1893.

Réaumur, M. DE. Memoires pour servir 4 l’histoire des insectes. Tome
II. 1737.

REICHENAU, W v. Die Ziichtung des Nesselfalters (Vanessa urtice L.);
ein Beweis fur den direken Einfluss des Klimas. Kosmos, V. 1882.

Ruumer, R. G. W. Die Uebergainge von Araschnia levana L. Zu var.
prorsa L. und die bei der Zucht anzuwende Kaltemenge. En-
tomol. Nachr. XXIV. 1898.

Stanpruss, M. Handbuch fur Sammler der europaischen Grosschmet-
terlinge. 1891.

Ueber der Variation und Aberration des Falter stadiums bei der
Schmetterlingen mit ausblicken auf die Entstehung der Arten.
Leipzig. 1894.

Weitere Mitteilungen ueber den Einfluss extremer Temperaturen auf
Schmetterlingspuppen. Entomol. Zeitschr. 1895.

Handbuch der palaarktischen Grosschmetterlinge. Jena. 1896.

Experimentelle zoologische Studien mit Lepidopteren. Neue Denk-
schrift d. Allg. Schweiz. Gesellsch. f. d. gesamte Naturw. 1808.

Gesamtbild der bis Ende, 1898, an Lepidopteren vorgenommenen
Temperatur-und Hybridationsexperimente. Insektenborse, XVI.
1899.

Urecu, F. Experimentelle Ergebnisse der Schniirung von noch weichen
Puppen der Vanessa urticae quer ueber die Fliigelchen. Zodl. Anz.
XX. 1897.

Ergebnisse von Temperaturesperimenten von Vanessa io, L. IIL.

Zeitschr. f. Entomol. 1808.

VariGNY, H. pe. Experimental Evolution. Lectures. 1892.

Venus, C. Pr. Ueber Varietatenzucht. Korresp. Bl. des entomol. Ve-
reins. Iris z. Dresden, I. 1888.

Weismann, A. Studien zur Deszendenztheorie, I. Ueber den Saisondi-
morphismus der Schmetterlinge. Leipzig. 1875.

Neue Versuche zum Saisondimorphismus der Schmetterlinge. Zoél.
Jahrb. Abt. f. Syst. VIII. 1895.

CHAPTER III

THE INFLUENCE OF EXTERNAL CONDITIONS IN CAUSING
CHANGES IN THE STRUCTURE OF ANIMALS (Continued)

Experiments on the Influence of the Food Plant

PicTET has studied the influence of the food of the caterpillar
on the color, the size, and other characters of the butterfly.
As a rule the caterpillars of each species are found on a par-
ticular plant, and cannot be induced to eat the leaves of a differ-
ent one, or only with great difficulty. A few species, however,
are polyphagous, 7.e. they feed on a number of different plants.
For example, the caterpillars of the Arctiidae feed upon all sorts
of herbaceous plants; many Noctuidae consume indifferently
several species of Composite; Papilio macchaon lives on dif-
ferent Umbellifere; Ocneria dispar, Porthesia chrysorrhea, and
Bombyx neustria are found on nearly all kinds of trees. Occa-
sionally caterpillars are found on plants that are not those nor-
mal for them, and the question has often been asked whether
the aberrant types of butterflies sometimes met with may not
have arisen in consequence of a change in the food plant. This
question Pictet has studied experimentally.

In captivity certain caterpillars adapt themselves readily to
very different kinds of food. As a rule a caterpillar will feed
on the flowers of its natural plant, and even the fruit may be
used, as in the case of Cossus cossus, which will eat pieces of
apple instead of the wood and the bark of the tree.

It appears that in nature, also, certain species have recently
extended their dietary. Thus Lasiocampa quercus was known
at the time of Linneus to feed on the oak tree (as its

1 These examples are given by Pictet.
29

30 Experimental Zoology

name implies) and on the leaves of certain shrubs. It is now
found on a number of other trees.

Certain aberrations have been traced directly to the food of
the caterpillar. The caterpillars of Ellopia prosapiaria living
on the pine become reddish butterflies; but if they occur on
the fir, they give rise to the aberration prasinaria, which is
green. An analogous case is that of Cidaria variata, whose
caterpillars living on the fir give rise to a form that is gray, but
on the pine produce a variety obeliscata, which is brownish red.

In certain countries where a variety almost entirely replaces
the parent species it is not uncommon to find the caterpillar on
a different plant. For example, Lasiocampa quercus lives on
different food plants in different countries. In Scotland, where
the variety callunze dominates, the caterpillars live on the
heather; in the South, where the variety roboris is found, the
caterpillars live on the oak, Quercus robur.

In changing the food, Pictet often made use of plants that
were very different from the natural one. Thus the oak was
often replaced by the esparcette, by the dandelion, by the
lettuce or by the pimprenelle.? In other cases it was replaced
by the walnut, neflier,? and the sorbier.* In some cases a food
plant closely related to the normal was the only one that could
be substituted; thus the European Evonymus europeus (spindle
tree) can be replaced by E. japonicus. Certain species refuse
all food but their natural one. Other species can be induced
to take a different nourishment, but only after much persever-
ance. Others, such as Lasiocampa quercus, when young ac-
commodate themselves to nearly all kinds of vegetation, but
once full size, many plants are rejected. The acceptation of a
new kind of food is transmitted by heredity, and individuals
whose parents have become accustomed to a strange food will
consume the same food with greater facility. Some of the

effects that Pictet obtained by changes in the food are as fol-
lows : —

? Onobrychis sativa, or holy clover. 5 Mespilus germanica, or mespit.
? Poterium sanguisorba, or salad burnet. 4 Sorbus ancuparia.

The Lnfluence of External Conditions 31

The typical male and female moth of Ocneria dispar are
shown in Fig. 1 and F ig. 2. In the male the wings are gray,
ash, or dusky, with four zigzag black lines. The female is
whitish gray or slightly yellow, with the same grayish white

Fic. 4. Ocneria dispar: male, Fig. 1; and female, Fig.2. First generation fed
on walnut leaves: male, Fig. 3; female, Fig. 4. Second generation fed on walnut:
male, Fig. 5; female, Fig. 6. First generation on walnut, second on oak: male,
Fig. 7; female, Fig. 8. First generation on walnut, second on oak, third on wal-
nut, fourth on walnut: male, Fig. 9. (After Pictet.)

pattern as the male, but more marked. The normal food is oak
or birch. The young caterpillars can be made to eat the leaves
of the walnut (Juglans regia), at first with difficulty, but the
subsequent generations eat it with avidity. The male of the first
generation is shown in Fig. 3. The wings are pale yellow, the

32 Experimental Zoology

central lines have partly disappeared, and the rest of the pattern
is less marked. The female of this first generation is shown
in Fig. 4. The wings are slightly transparent, with rarely a
darker mark on the upper surface. The insect is almost white.

The caterpillars of the next generation were also fed on the
walnut. The male is shown in Fig. 5. ‘The wings are whitish,
the marginal band has partly disappeared, and the transverse
lines are but slightly visible. The female is shown in Fig. 6.
The wings are transparent; the V and the fifth of the marginal
points alone remain of the markings.

The figures show a decided decrease in the size of the indi-
vidual, which is accompanied with a constitutional weakness.
It was not possible to obtain eggs from this generation. Pictet
resorted, therefore, to the device of feeding the first generation
on the walnut, the second on the oak, and then the third and
even the fourth on the walnut. When the first generation was
fed on the walnut and the second on the oak, the male moth
appeared as in Fig. 7, and the female as in Fig. 8. The fig-
ures show that the effects, produced by feeding the first genera-
tion on the walnut, persist during the second generation when
returned to the normal food.

In another experiment the caterpillars were fed first on wal-
nut, then on oak, and then on walnut. The male was much
like that produced by two generations of walnut, as were the
females also. Another lot was fed walnut (1st gen.), oak (2d
gen.), walnut (3d gen.), walnut (4th gen.). The male type is
shown in Fig. 9. The wings are gray ash, or dusky; the
marginal points strongly marked, as well as most of the trans-
verse lines. The marginal band is accentuated in the four
wings. The females have white wings — sometimes yellowish.
As in the male, the lines and the marginal band of the lower
wings is much accentuated. Thus while the greatest change is
effected by walnut, oak, walnut, there is a return to the darker
type if walnut is used again for the fourth generation. Pictet
interprets this as a return to the ancestral type; the result of
becoming accustomed to the walnut, and states that he has ob-

The Influence of External Conditions 23

tained similar results in other experiments. It is not entirely
clear from his figure that the darker form in the fourth gener-
ation is really a return to the ancestral type —at least the
possibility of a different interpretation must be kept open.

Another experiment showed that when the first generation
was fed on walnut, and the second and the third on oak, the
effects of the walnut were still apparent in the last generations.

When the first generation was fed on walnut, the second on
the oak, and the third on flowers of different kinds (rose and
peony), the last seemed to accentuate the effects of the walnut
and ‘‘tend to cause to disappear those of the normal food. To
the same extent as the walnut the flowers appear to be a
poor alimentation.” The caterpillars of Ocneria were also fed
on other plants, the results being in some respects like the pre-
ceding cases, in others different.

There are two known aberrant forms of the moth Psilura
monacha. The typical male form is shown in Fig. 10, and the
female in Fig. 11. The aberrant form nigra, male and female,
is shown in Figs. 12 and 13, and the form eremita, male and
female, in Figs. 14 and 15. The caterpillars are found on oak,
birch, and conifers.

Caterpillars fed on oak and on birch produced moths of all
three types, in the proportion of 58 per cent type-form; 24 per
cent ab. nigra, and 18 per cent ab. eremita; fed on walnut the
proportions were 38 per cent type-form; 23 per cent ab. nigra;
30 per cent ab. eremita. The moths were smaller than the aver-
age. The proportion of the aberrant forms is greater from the
walnut than from the normal food. In the second generation,
nourished on the walnut, the females of the type form assume
the characters of the typical males.

In all, Pictet’s experiments with feeding included 21 species,
comprising 4695 individuals. In nearly all cases some effects
of a change from the normal type could be directly traced
to the food.’

1 For somewhat similar experiments see Romanes, “Darwin and After Dar-

win,” II, pp. 217-218.
D

34 LExperinental Zoology

Pictet has also tried the effects of insufficient nourishment,
using smaller amounts of the normal food. In the case of
Pieris crategi, lack of food during the last days of the life of
the caterpillar causes the wings to become lighter in color and
more transparent. If all food is removed from the full-grown

Fic. 5. Psiluria monacha: typicai male, Fig. 10; and female, Fig. 11. Aber-
rant form P. ingra: male, Fig. 12; female, Fig. 13. Aberrant form P. eremita:
male, lig. 14; female, Fig. 15. (After Pictet.)

caterpillar of Vanessa urtice, it becomes a chrysalid in a few
days after the last moult. The butterflies are dwarfs, but show
no changes in coloration. On the other hand, if the very young
caterpillars are kept on a régime that becomes less sufficient
every day, they become pupe even before the last moult, and
produce dwarf butterflies that have the aberrant characters
much accentuated.

The Influence of External Conditions 35

It is not difficult to starve caterpillars, but it is difficult to
make them take more food than they do normally; yet by
indirect means this can be done by giving them food containing
an excessive amount of nutritive substances.

Reviewing the results of the effects produced by changes in
food, Pictet points out that, in general, the variations produced
are either in the direction of lighter (albinistic) or darker (mel-
anistic) shades. If the kinds of food that produce the former
effect are compared with those that produce the latter effect, it
will be seen that the lighter colors result from feeding on plants
which, owing to their anatomical structure, present obstacles to
mastication —either as a result of a thick epidermis or of the pres-
ence of crystals or of hairs, etc. In consequence of the imperfect
nutrition of the caterpillar, the butterfly is less pigmented. On
the other hand, the melanistic or black variations result from
those plants that offer no obstacles to mastication or nutrition.
The caterpillars develop rapidly, acquire a size greater than the

‘normal, and the butterfly shows a greater development of pig-
ment.

The influence of different food plants in causing an albinistic
or melanistic change in the moths can be traced, according to
Pictet, to the length of the pupal stage, and this in turn to that
of the caterpillar stage, which again is due to the amount of
nourishment received. Thus, those food plants that give in-
sufficient nourishment prolong the caterpillar stage; but this
leads to a shortening of the pupal stage, and the albinistic effect
is produced, because the dark pigments do not have time to
develop extensively. Conversely, an abundant nourishment
shortens the caterpillar stage; but this causes a prolongation of
the pupal stage during which the pigments have time to develop
even more fully than normally, and the melanistic variation is
the result.

Not only the colors of the moth or butterfly are affected by
the character of the food, but the colors of the caterpillars them-
selves may sometimes be affected, although to a less degree.
Pictet finds, in fact, that there is a correlation between the pig-

36 Experimental Zoology

mentation of the caterpillar and of the adult. The caterpillar
of the death’s head moth (Acherontia atropos) is ordinarily
yellow, but has also a black type. Dasychira pudibanda is
white, but has a green and gray variety, etc.

The color of many caterpillars is much influenced by the
food in the digestive tract that can be seen through the semi-
transparent skin. Poulton has also shown that certain of the
epidermal pigments are derived from the chlorophyll absorbed
with the food. He separated into three groups caterpillars of
Agrotis pronuba: the first was fed with the green leaves of
cabbage, the second with the yellow etiolated leaves. Both
lots developed their normal yellow-brown color. A third group
was fed on leaves deprived of the green and of the yellow
coloring matter, and only the brown pigments developed.
Standfuss has produced within a few hours color changes
in the caterpillar of Eupithecia absinthiata. It becomes ‘‘a
lemon-yellow when fed on the yellow bunches of Solidago,
green on non-flowering leaves of the same plant, rose on the
‘buttons’ of Statice armeria, white on the umbels of Pimpinella
saxifraga, brown on the bouquets of Artemisia vulgaris, and a
delicate blue on the small balls of Succisa pratensis.” Similar
results are known for other forms. Pictet finds also that the
colors in certain caterpillars that hibernate tend to disappear,
but reappear again a few days after feeding begins again in the
spring. The colors of some species change with each moult.
Difference in the color, and even in the markings of caterpillars,
occur when they are fed on food other than the normal. In
some cases the effect is direct, as explained above; in other
cases the effects are indirect, i.e. not due to the food in the
digestive tract or to the pigments directly absorbed and present
in the skin or in the blood. In the case of Ocneria dispar the
food plants that produce lighter colors in the moths have the
same effects on the caterpillar. Similarly for the dark or mel-
anistic changes. Moreover, the effects may last over for one
or two generations after the caterpillar is again fed on its
normal food.

The Influence of External Conditions a7

The Influence of Light, Electricity, Centrijugal Force, Chemical
Substances, and Oxygen on the Caterpillars and Pupe of
Moths and Butterflies

It has been shown by Grdfin von Linden that the size, the
colors, and the markings of butterflies may be altered by sub-
jecting the caterpillars or the pupe to several kinds of external
conditions other than temperature. In general, however, the
results are similar to the effects produced by higher and lower
temperatures.

Light. — Vanessa urtice and V. io were used. Some of
the caterpillars were kept, with their food, in red, or green, or
blue light. Others were kept in the dark. The principal
changes were in the ground color of the wings of the butterflies.
This was intensest and brightest in red light, dusky in green,
and paler in blue and in the dark. The butterflies of V. urtice
were largest from the caterpillars reared in the blue, and of V.
io in the dark. The changes in the markings were very slight.

Electric Shocks. — The pupe of V. urtice, in a fresh but dry
condition, were put into an iron box through which an electric
current was passed of sufficient strength to cause a pricking
sensation when applied to the hand. In other cases one elec-
trode was applied at the wing axis and the other at the tip of
the wing case. The resulting butterflies were brightly colored.
The black border of the wing was broader, and the tip of the
wing was sometimes dark. The blue and yellow scales of the
sides of the wing were little developed.

Centrifuge, — After the pupe had become hard they were
subjected to a centrifugal force for ten minutes each day. The
effects were in general similar to those caused by light.

Chemical Substances. — Contrary to the results of Standfuss,
who found changes in the food had no influence on the color
of the moth, except in one case, when salt was added to the food
of Callema, Grafin von Linden found that certain substances
given with the food produced distinct effects. Some effects, for
the most part slight, were obtained with (1) defibrinated blood ;

38 Experimental Zoology

(2) iron albuminate (officinal solution: 4 parts metallic iron to
tooo water); (3) argonin silver casein (5 per cent); (4) sugar;
(5) lupulin; (6) capsicum; (7) morphine (1 per cent); (8)atropin
(1 per cent).

The iron and the silver compounds by exciting hunger
tended to produce larger forms. The largest butterflies came
from caterpillars fed with argonin silver casein and the smallest
from those given morphine with their food. The younger the
caterpillars, when the experiment began, the better the results.
Darkening of the ground color of the wings was found after
argonin and morphine. Also much red was present. Capsi-
cum also gave a dark background. The color was strongly
developed after iron, sugar, and lupulin. The markings were
affected in much the same way as by cold and warmth. Re-
duction of the black or of the blue flecks occurred in some
cases.

Pure Oxygen. — Pupe kept in pure oxygen produced in the
usual time butterflies normally colored and marked. Young
caterpillars kept in oxygen took food for only one day, wandered
about the next day, and died on the third or fourth day. The
moist atmosphere rather than the oxygen may have been respon-
sible for the early death of the caterpillars.

The Influence of Humidity on the Characters of Moths and
Butterflies

Humidity is also supposed to have an effect on the coloration
of butterflies. Marshall thinks that in the Transvaal, where a
dry and a wet season alternate, as do summer and winter in
northern climates, the seasonal dimorphism of certain butterflies
is the direct result of the effects of moisture and of dryness, in
the same way that cold and heat cause the seasonal dimor-
phism of northern species. Similar results have been described
by Doherty for Melanites leda of India. The differences fol-
low the dry and the wet seasons. Pictet has shown that
effects of this kind can be artificially produced in certain Eu-
ropean species. Moisture on the leaves produces great mortality

The Influence of External Conditions 39

amongst the young caterpillars, but while the moths may be
small their markings show no variations. On the other hand,
although the fully formed caterpillars better resist the effects
of moisture, aberrations in the color are produced. Thus when
the caterpillars of Vanessa urticae were fed for 8 to 10 days
on leaves kept constantly wet, changes in the markings of the
butterflies were produced. Similar results were brought about
in Vanessa polychloros and in Hybernia defoliaria. In Ocne-
ria dispar variations were induced in the first generation, but
disappeared in the second, owing, Pictet thinks, to the cater-
pillars having adjusted themselves to the change, in the same
way as they do to changes in their food. The variations in-
duced in the first generation are very much like those pro-
duced by esparcette and dandelion, especially in respect to the
males, and these plants contain a great deal of water in their
tissues.

A fine spray of water at ordinary temperatures was con-
stantly applied for 36 to 48 hours to caterpillars of Vanessa
urtice that had suspended themselves preparatory to changing
into chrysalids. A marked effect was produced on the but-
terfly, a yellow band appearing across both fore and hind
wings. The blue spots became gray or violet, and the border
of the wings was clear. The variation resembles the variety
polaris of this butterfly.

Humidity also acts on the chrysalid’s stage, causing aberra-
tions in a number of moths and butterflies. If the chrysalids
are kept moist at a warm temperature, 30° to 35° C., very little
effect is produced, because the development is so much has-
tened that the chrysalids pass through the critical stages be-
fore the protecting waxy covering is worn away. But if the
chrysalids are kept cool while the moisture is applied, the effects
are much more marked; for now the development is so much
delayed that the water has time to penetrate the protecting coat
and affect the critical stages.

.

40 Experimental Zoology

Ex periments with Flatfish

There are some experiments made by Cunningham with
young flatfish which also show the effect of an external agent,
viz., light, on the development of color. Very young fish were
put into aquaria lighted from below. As the young fish under-
went their metamorphosis, the pigment gradually disappeared
on one side, as it does under normal conditions, 7.e. as when
they are lighted from above only. If, however, the fish are
still illuminated from below, the pigment begins to come back
again on the lower surface. The markings are similar in all
respects to those on the upper side of the animal.

The result shows that the lower side of flatfish in their natu-
ral environment is white, because it is not exposed to light; but
whether the result shows, as Cunningham believes, that the
lower side has become white in the course of generations, be-
cause it has been turned away from the light, is not shown
conclusively by the experiment.

Experiments with Crustaceans

There are a few cases, in other groups, where it has been
shown that external agents produce changes in form. Bran-
chippus ferox, inhabiting salt and fresh water, shows small
differences in the length of the ovigerous sac, in the form of the
segments of the body, in the length of the lobes that terminate
the abdomen, and in the disposition of the abdominal bristles.
Daphnia degenerata of salt water is only a degenerate variety
of Daphnia magna of fresh water.

A species related to Daphnia — Moina rectivoctris — oc-
curs in one form in fresh water and in another in brackish.
The two differ in general points of structure due to the indi-
viduals becoming sexually mature before the final structural
changes are completed.

The oft-cited case of Artemia may also be mentioned here.
Schmankewitsch has described the slow transformation of
Artemia salina into A. milhausenii, as the lagoon in which the

The Influence of External Conditions 41

former was contained became more concentrated by evapo-
ration. Experimentally he procured the same result by con-
centration. Furthermore Schmankewitsch claimed that by
gradually diluting the brackish water in which Artemia salina
lives he obtained a form having the characters of the genus
Branchippus.

These conclusions have been seriously questioned by Bateson
and by Samter and Heymons,' who have shown that Artemia
salina is subject to great individual variability and that there
is no close connection between the different variations and the
concentration of the water in which they occur. Especially
doubtful is Schmankewitsch’s comparison with Branchippus,
whose diagnostic features he seems to have imperfectly under-
stood. Samter and Heymons find nevertheless that the salt
content of the water has some influence on the form of Artemia
salina, although in different pools of the same concentration
a large range of variability exists. They think that other fac-
tors than concentration probably also affect the result.

Changes in Mammals and Birds

The Porto Santo rabbits, so fully described by Darwin, fur-
nish another instance of influence of the environment. It is
said that these rabbits originated from a single pregnant female
that produced a litter on board ship in the year 1418 or 1419.
Set free on the island of Porto Santo, the rabbits increased
rapidly and soon became a pest. Darwin examined these
rabbits and found that, compared with domesticated rabbits
of average size, the Porto Santo rabbits had lost three inches
in length and almost half the weight of the body. In other
points also they differed —in the skull, and especially in
color. ‘‘But here we meet with a singular circumstance: In
June, 1861, I examined two of these rabbits recently sent to
the Zodlogical Gardens, and their tails and ears were colored

1 SchimkeWitsch (Biolog. Centralblatt, XXVI, 1906) states that Anikin (1889)

and Butschinsky (1901) have obtained results contradictory to those of Schman-
kewitsch. I have not seen their papers.

42 Experimental Zoology

as just described (i.e. like the feral rabbits of Porto Santo); but
when one of their dead bodies was sent to me in February,
1865, the ears were plainly edged, and the upper surface of the
tail was covered with blackish gray fur and the whole body
was much less red, so that under the English climate this in-
dividual rabbit had resumed the proper color of its fur in rather
less than four years.” *

Darwin thinks that “from the direct action of a humid cli-
mate and poor pasture the horse rapidly decreases in size in
the Falkland Islands.”

There is a great deal of evidence to indicate that climate acts
directly in bringing about a change in the hair of animals, and
not only is the thickness of pelt affected, but also the character
of the hairs. Geoffroy St. Hilaire states that horses that have
lived for several years in deep coal mines come to have a vel-
vety hair, somewhat like that of the mole.

Wild ducks are said to change under the influence of do-
mestication. The collar around the neck of the mallard be-
comes broader and more irregular. White feathers appear in
the wings. The birds also increase in size, etc.

Certain foods affect the color of the feather of birds. Hemp
seeds cause the color of bullfinches to become darker.” Wal-
lace states that the natives of the Amazonian region feed the
common green parrot with the fat of siluroid fishes, which
causes it to become variegated with red and yellow feathers.
How far these effects, produced directly by the environment,
may become inherited will be examined in the next chapter.

1 “Animals and Plants,” Chap. V.
? Romanes, in his book on “Darwin and After Darwin,” II, gives some
further cases of this sort, p. 217.

CHAPTER IV

THE INHERITED EFFECTS OF CHANGES INDUCED BY
EXTERNAL FACTORS

Ir has been pointed out that in butterflies the changes brought
about by higher and lower temperatures give rise to forms that
resemble southern and rorthern varieties of the same butter-
flies. The question at once arises whether species may not
have originated in this way.

Fischer found that when a dark moth, produced by cold,
was paired with another similar moth, the offsprings were also
dark. His experiments extended, however, only to the first
generation, and consequently the cold may have acted directly
on the germ-cells of the parents. Highly important as this
observation is in showing that the undeveloped germ-cells may
be affected in the same way as the somatic tissues of the pupa,
so that even under altered conditions the effect persists, yet the
result as it stands does not conclusively show that permanent
racial or specific changes have been produced in this way.
Were such forms bred for several generations at a ‘warmer
temperature, it is possible that they would return again to their
original condition. The new type might persist only so long as,
or a little longer than, the external conditions are the same as
those that produced it. The change may represent only an
extreme fluctuating variation that has been caused by an ex-
ternal factor. 'The results do not appear, from this point of
view, to belong to the kind of changes by which new species
are made. Nevertheless the question still remains an open
one as to whether changed external conditions may not at
times cause more permanent effects. Some observations of
Standfuss seem to show that such may be the case, although

43

44 Experimental Zoology

the evidence is not satisfactory in all respects. He states that
certain aberrations, that occur when the external factors have
produced a sudden divergence from the parent form, are in-
herited. He shows by numerous experiments that these aber-
rant forms when crossed with the parent type do not give
intermediate varieties, but that the offspring correspond to one
or to the other parent. Thus the new type is not lost by inter-
crossing; but if adaptive, might perpetuate itself and form the
beginning of new species.

The aberrations seem to resemble in these respects mutations.
If they are really produced by a change in the environment, as
Standfuss believes, the results throw most important light on
the question of the origin of mutations — a question which at
the present time is one of the most pressing questions of the
theory of organic evolution.

Standfuss believes that effects of these kinds are inherited,
and that new species may be evolved at the limits of the range
of forms through the effect of external agents. It is not quite
clear to me, how he supposes such results come about, since
his own experiments seem to show that the effects of moderate
changes in temperature are only temporary. He appears to
believe, however, that the effects, if long enough carried out,
become in large part fixed.

In plants the effects of the environment on the form, growth,
and time of flowering have long been known; and many ex-
periments have been made by transporting plants from one
locality to another. It has been shown that many alpine plants
will flourish in the valley, and often great changes in the char-
acter of the plants have been noted. Naegeli has carried out
elaborate experiments of this sort. He collected in the Botani-
cal Garden of Munich 2500 varieties of mountain plants, and
for several years made observations on the effects of changes
in the environment. The changes are seen at once, showing
that no permanent effects have been produced by the alpine
climate. Other botanists, however, claim that for a time at
least the effect of the original habitation may be seen.

The Inherited Effects of Changes 45

Pictet, as has been shown in the preceding chapter, has also
obtained direct evidence of the inherited effects of food. It
should be noticed in this case that the young caterpillar fed on
the leaves of a new food plant produces changes of a certain
type in its somatic cells that appear in the butterfly. The
caterpillars of these butterflies, reared on their natural food
plant, produce butterflies that also show evidence of the effect
caused in the preceding generation. It is probable that this influ-
ence was directly induced in the germ-cells of the first genera-
tion, so that the effects were not inherited through the soma, but
were directly produced. The point, however, of special interest
in these cases, aside from the question of inheritance, is that the
influences that induce certain changes in the somatic cells of
the caterpillar affect the germ-cells of that caterpillar in the
same way, so that when they develop they, too, give the same
results. The effect is weakened, it is true, in the second genera-
tion; but this may be due to the counteracting influence of the
normal food. The results show that the influence of the envi-
ronment may persist for one or more generations in another
environment.

Recently de Vries has dealt with the same subject, and has
carried out certain experiments that bear on the question. He
believes that in general the effect of the environment produces
only the fluctuating variations seen in plants. The full effects
may not appear at once, but may be accumulated, through sev-
eral generations, and hence would seem to be inherited. He
speaks of these changes as acquired characters, and believes,
as I have said, that individual variations are simply acquired
characters due to differences in the environment. We are to
understand, however, that de Vries means that these characters
are acquired either by the somatic cells or by the germ-cells,
but independently of each other, z.e. the effects acquired by the
somatic cells are not supposed to affect the germ-cells except
indirectly, namely, by affecting the nourishment of the germ-

cells.
The question arises whether these new characters are inherited

46 Experimental Zoology

when the environment that has caused them is altered. De
Vries answers this question in the following way: If the seeds
of the best-nourished ordinary plants are selected and planted
under the most favorable conditions, the average plants of the
next generation will not only be as vigorous as the former, but
their seeds themselves will have a higher average of size. If
this process is continued, the average of the race will be in-
creased in the direction of selection, and in time the race, 7.e.
all the individuals, may be brought to and maintained near the
highest plane to which fluctuating variation ever reaches.

There is a counteracting principle that must also be con-
sidered, viz., what Galton has called regression toward medi-
ocrity. If variations that depart from the average of the type
are united, their descendant will tend in some degree to return
to the average or to mediocrity, as Galton has shown. If,
however, we pick out in each generation those that are most’
above the average, the average of the descendants in each gen-
eration rises, despite the regression, and in time the average may
be brought near to the highest point to which individual varia-
tions reach. In other words, the character that appeared in
the first generation as a somatic variation of one individual
has become temporarily transferred to the seeds; but unless the
same favoring external conditions are vigorously maintained,
there will be a return to mediocrity.

This fastening, as it were, of the individual variation upon
the race is due, de Vries thinks, to the action of the nourish-
ment on the germ-cells.

If a plant is well nourished, it produces larger seeds that are
better supplied with nourishment. Hence on an average they
will give rise to more vigorous plants, and these in turn will
produce an average of larger or at least of better-nourished
seeds, and the plants from these will be still stronger. Thus
by slow degrees the seeds acquire the same sort of characteris-
tics as those shown by the best-nourished plants in any one
generation. The curious thing about this is that the seeds do
not respond completely in the first generation, but it takes sev-

The Inherited Effects of Changes 47

eral generations to produce seeds al of which contain nearly
the maximum possible food supply.

Suppose when this condition is attained we reverse the pro-
cess and plant the seeds under poorer conditions. The first
crop from such seeds will still be above the original average,
but not as much above as though they had grown under favor-
able conditions. Their seeds again will contain less nourish-
ment, and the second generation will be still smaller on the aver-
age, until finally the race reaches the lowest level shown by
fluctuating variations.

The inheritance of the acquired character in these cases is
brought about in rather a peculiar way. The favorable exter-
nal conditions, for instance, act favorably on the body-cells of
the plant, and the germ-cells are therefore well nourished and
store up more food: not all to the same extent, but the average
is higher than in the preceding generation. The next genera-
tion thus gets a better start, and if the plants are better nourished
than the average, a similar advance is again made. We must
not forget that in each generation some of the seeds may be as
good as the best, but others are not as good. It is the average
that is improved, and not the best individual seed.

It may appear from the case just given that the food condi-
tions determine only the size and vigor of the plant. This is
not the whole matter, for even the characters of certain parts
may be changed in a plus or a minus direction, provided they
are correlated with the condition of nourishment of the plant.
De Vries cites the case of the poppy, Papaver somniferum. In
this plant the stamens may be changed into supernumerary or
accessory carpels by changing the external conditions. The
number of these carpels may be as great as 150.0r more. From
the seeds of flowers with an average crown of carpels, plants
may be reared having many or few of these organs. The more
favorable the conditions, the more numerous the carpels, and
vice versa. Poorly nourished plants may have only one or two
rudimentary or accessory carpels.

If we select in each generation the most vigorous plants (hav-

48 Experimental Zoology

ing, therefore, a high average of carpels), and keep them under
favorable conditions, the average of carpels of the race can be
temporarily increased. Conversely, if we select the less vig-
orous plants, and put them under unfavorable conditions, the
accessory carpels can be made to disappear.

This connection between individual vigor and the forma-
tion of carpels is not an absolute one, for there are means by
which vigorous plants can be obtained without accessory car-
pels. If very young plants are transplanted, and then put under
favorable conditions, vigorous plants result without or with few
carpels. The interpretation that de Vries gives of this result
is that the flower buds were not laid down at the time of trans-
planting, but develop soon afterward, when the conditions are
temporarily unfavorable. Later, when the favorable conditions
begin to act, the rest of the plant responds, but it is too late to
affect the carpel formation.

It will be observed that we are dealing here with rather a
special case, viz., that of nourishment alone. Somatic cells
and body-cells alike are affected in the same way. If along
with this condition of nourishment there are certain correlated
changes, such as the formation of accessory carpels, the prin-
ciple remains the same. The point of special interest is that
the effects may be accumulated only slowly by the seeds, so
that it takes several generations to produce the best average
effects. The effect, once produced, may persist in part through
several generations subjected to the reverse conditions. The
results are not unlike those in the butterflies, in which the effects
of temperature or of feeding are marked in the first generation,
and then decline if the external conditions that produced them
are changed.

It is difficult to reach any probable conclusion from the evi-
dence given in the preceding pages in regard to the inherited
effects of the influence of the environment. Possibly we are
dealing with two distinct problems. In most cases the effects
on the body-cells and on the germ-cells are only temporary,
and persist only as long as, or a little longer than, do the condi-

The Inherited Effects of Changes 49

tions that called them forth. The result is only a special case
of the inheritance of fluctuating variations. On the other hand,
in some of the cases of aberrations given by Standfuss, it ap-
pears that a permanent change has been brought about by the
environment which persists afterward, independently of the
influences that caused it. What is still more interesting is that
the new characters may be transmitted to the offspring formed
by a cross between the new and the parent type. We shall
consider this question at greater length in a later chapter.

LITERATURE, CHAPTERS III AND IV

Bateson, W. Materials for the Study of Variation. Pages 96-101.
London. 1894.

Cuénor, L. L’influence du milieu sur les animaux.

CunnINGHAM, J. T. An Experiment concerning the Absence of Color
from the Lower Side of Flatfishes. Zool. Anz. 1891.

The Evolution of Flatfishes. Nat. Sc. I. 1892.
Researches on the Coloration of the Skins of Flatfishes. Jour.
Mar. Biol. Assoc. III. 1893.

CUNNINGHAM, J. T., and MacMunn, C. A. On the Coloration of the Skins
of Fishes, especially of Pleuronectide. Phil. Trans. CLXXXIV. B.
1893. '

Darwin, C. Animals and Plants under Domestication.

Dewitz. Zur Verwandlung der Insectenlarven. Zool. Anz. XXVIII. 1904.

GAUCKLER, H. Untersuchungen ueber beschleunigte Entwickelung ueber-
winternder Schmetterlingspuppen. IIl. Zeitsch. f. Entomologie,
IV. 1899.

Pictet, A. Des Variations des Papillons provenant des changements
d’alimentation de leurs chenilles et de ’humidité. Comptes rendus
du 6™° Congrés intern. d. Zool. Session d. Berne. 1904.

Influence de l’Alimentation et de ?Humidité sur la Variation des Papil-
lons. Mem. Soc. Phys. et Hist. Natur. de Genéve, XXXV. 1905.

PoucuET, G. Des changements de coloration sous l’influence des nerfs.
Jour. Anat. et Physiol. norm. et pathol. XII. 1876.

SamTER, M., and Hrymons, R. Variationen bei Artemia salina Leach,
und ihre Abhangigkeit von aeusseren Einfluessen. Abh. d. k.
Preuss. Akad. d. Wissen. Anhang. 1go2.

THILENIUS, G. Die Farbenwechsel von Varanus griseus, Uromastix
acanthinurus und Agame inermis. Morph. Arb. VII. 1897.

DE Vries, H. Die Mutationstheorie Versuche und Beobachtungen
ueber die Entstehung der Arten im Pflanzenreiche. I. Die Ent-
stehung der Arten durch Mutation. Leipzig. gor.

Die Mutationen und die Mutationsperioden bei der Entstehung der
Arten. Vortraz. Vers. Deutsch. Naturf. u. Arzte zu Hamburg.
Igor.

E

CHAPTER V
THE INHERITANCE OF ACQUIRED CHARACTERS

In the preceding chapters we have seen that external factors
may cause definite changes in the form, color, markings, etc.,
of animals, as well as certain changes in plants. The question
arose whether these effects are transmitted to the offspring of
the next generation. An examination of the evidence seemed to
show that in most cases the effects are inherited only in so far
as the germ-cells are also affected by the external factors.

It has been long recognized that internal factors also may
cause changes in animals. The use of an organ may increase
its size, and also its effectiveness, even when the change in form
is so slight as to escape notice. Here, also, the question arises
whether these effects are inherited. Disease or injury may
bring about changes in an organ, and again the question has
been raised as to whether the effects are transmitted to the
offspring.

The phrase ‘‘the inheritance of acquired characters” is used
to include supposed cases of inheritance of these different kinds
of effects; and it is customary to use this term to include also
cases like those described in the last chapter, in which the ex-
ternal agents act directly on the germ-cells, as well as on the
body-cells. In this chapter I shall use the term in a restricted
sense, and include under it only those cases in which the body-
cells are first affected and are then supposed to transmit their
influence to the germ-cells.

It is not necessary to consider at length the historical origin
of the idea that acquired characters are inherited. It is well
known that Lamarck based his theory of evolution on this

50

The Inheritance of Acquired Characters 51

assumption, not only in the restricted sense as defined above,
but also in the sense that external conditions may directly affect
the whole organism, i.e. germ-cells as well as the body-cells.
He made no such sharp distinction, however, between these two
sides of the question as we find it convenient to make nowa-
days.

Let us examine critically the experimental evidence on which
the theory of the inheritance of acquired characters, in the
restricted sense, rests, for, could it be shown that changes
acquired through use are transmitted to the next generation, we
might seem to be able to explain how many of the complicated
adaptations and coérdinations of animals have arisen.

Amongst modern writers the first to seriously question the
truth of the generally admitted doctrine that acquired charac-
ters are inherited was August Weismann. In his essay “On
Heredity,”’ published in 1883, and in two subsequent papers
“On the Supposed Botanical Proofs of the Transmission of
Acquired Characters” (1888), and ‘The Supposed Transmis-
sion of Mutilations’” (1888), Weismann challenged the ac-
cepted point of view. His conclusions have become common
knowledge, so that it will not be necessary to go over the ground
again. While many zodlogists and botanists were convinced
by Weismann’s argument, which seemed to show that the evi-
dence fails to support the view that acquired characters are
inherited, a few zodlogists have always insisted nevertheless
that such characters are inherited, and a few investigators have
brought’ forward experimental evidence which they believe
shows convincingly that changes brought about in the body
may be transmitted to the germ-cells. It is this experimental
evidence that I propose éspecially to consider.

Darwin believed that acquired characters are inherited, and
in the “Origin of Species,” in the ‘Animals and Plants under
Domestication,” and in the ‘Descent of Man” he accounts for
many adaptations in this way. Herbert Spencer, especially,
although not always with discrimination, used this hypothesis to
account for many structures and habits of animals and plants.

52 Experimental Zoblogy

I do not propose to consider in detail the cases that Darwin
and Spencer have brought forward (most of them will not bear
critical examination, as Weismann has so ably shown), but, as
has been said, I shall consider rather those cases, most of them
recent, in which attempts have been made by direct experiment
to show that acquired characters are inherited.

The work that has attracted most attention is that of Brown-
Séquard. A full statement of Brown-Séquard’s experiments and
results is given by Romanes in his book “Darwin and After
Darwin,” Vol. IJ, Chap. IV. The experiments were made
with guinea pigs. Epilepsy was induced by operations on
some part of the nervous system. The young of these animals
sometimes developed epilepsy, or some of its effects, in the same
part of the body as that affected in the parent. The details of
the experiments are as follows: The parents became epi-
leptic after injury to the spinal cord or by section of the sciatic
nerve. The “epileptiform habit” does not supervene until
some time after the operation and lasts only ‘‘for some weeks
or months.” The convulsions “never occur spontaneously,
but only as a result of irritating a small area of skin behind the
ear on the same side of the body as that on which the sciatic
nerve had been divided.” + The attack lasts only a few minutes,
and during this time the animal is unconscious and convulsed.
The habit is only rarely transmitted to the young, but as the
disease occurs only in guinea pigs whose parents have been
made epileptic by an operation of the sort described above,
and never in the young of guinea pigs that have been operated
on in other ways, there seems to be here something more than
a coincidence.

Another series of experiments consisted in cutting the cervi-
cal sympathetic nerve. This operation causes a change in the
shape of the ear, and a similar change is said to appear in the
young. By cutting the cervical sympathetic nerve or by re-
moving the superior cervical ganglion, the eyelids partially
close, and this closure was also seen in the young.

* Romanes, “Darwin and After Darwin,” II, 1&g5.

The Inheritance of Acquired Characters 53

If a particular spot of the restiform body of the brain is
injured, a marked protrusion of the eyeball quickly follows.
The progeny of parents thus affected show also an abnormal
protrusion. Romanes also observed this, but he found that the
young show less protrusion than do the parents, and since the
amount of protrusion of the eyeball is variable in normal guinea
pigs, Romanes is not certain that there is anything more than a
coincidence in the cases that he observed.

An injury to the restiform body may also cause dry gangrene
(and hematoma) in the ears. This disease may appear either
several weeks after the operation or even later. It affects,
Romanes says, usually the upper parts of both ears and may
gradually “eat its way down”’ until two thirds of the tissues of
the ears are affected. In the offspring from animals of this sort
a morbid condition of the ears may arise at any time in their
lives, even after they have become full grown. The disease
does not go so far as in the parents, and ‘‘almost always affects
the middle third of the ears.” Romanes points out that this
particular disease never appears amongst guinea pigs unless
their own or their parents’ restiform bodies have been injured.
. Furthermore, he tested the possibility that the results are due
to contagion by inoculating “corresponding parts of the ears
of normal guinea pigs by first scarifying those parts and then
rubbing them with the diseased surfaces of the ears of muti-
lated guinea pigs.” The disease was not communicated in
this way.

Brown-Séquard found that after cutting the sciatic nerve (or
this and the crural also) the leg became anesthetic, and the
guinea pigs would sometimes eat off two or three of their hind
toes. In the offspring of these animals he found sometimes
an absence of toes, or only a part of one or more of the toes
might be missing. The inheritance occurred in only one or
two per cent of cases. Romanes, who repeated the operation
through six successive generations, never obtained any results.

Another outcome of injury to the sciatic nerve is to induce
“morbid states of the skin and hair of the neck and face in

54 Experimental Zoology

animals.” This result is also said by Brown-Séquard to be
inherited.

I have given somewhat fully these remarkable results of
Brown-Séquard because the experiments appear to have been
carried out with such care, and the results are given in such
detail that it seems that they must be accepted as establishing
the inheritance of acquired characters.

Moreover, similar results have been obtained by other in-
vestigators. They have been corroborated in part by Ober-
steiner.* Westphal has produced epilepsy by striking the heads
of the animals with a hammer, and has found that the young
are often epileptic.” Still more important are the experiments
of Romanes. His conclusions, it is true, are much more cau-
tious, and his statements more guarded than those of Brown-
Séquard; yet on the whole they confirm Brown-Séquard’s
claims.

Weismann has attempted to discredit these results on the
ground that we are still ignorant of the cause of epilepsy. The
possibility that it is a bacterial disease must be admitted, he
claims, and, if this is the case, the bacteria themselves may be
transmitted to the young during their uterine existence. It is
supposed, in fact, that other diseases may be inherited by direct
contagion through the germ-cells of the father or the mother.
This objection is, however, purely formal, and as long as we do
not know that epilepsy is a bacterial disease that is contagious
in the way supposed, the objection may raise a doubt but cannot
set aside the results.3

There are some quite recent experiments that may have a
very direct bearing on the questions here raised. In a paper
by Charrin, Delamare, and Moussu, the inherited effects of
injury are described. The liver or the kidneys of pregnant rab-
bits and guinea pigs were injured, which caused these organs

* Oesterreichische medicinische Jahrbiicher, p. 179, 1875.

? See Weismann, “Essays,” Vol. I, p. 323.

5 It has also been suggested by some more recent authors that epilepsy occurs

as the result of a weakening of the general condition either direct or inherited.
This view will not explain the localized inheritance claimed by Brown-Séquard.

The Inheritance of Acquired Characters 55

to become diseased. The offspring sometimes showed defects
in the corresponding organs. The authors suggest that some
substance may be set free from the diseased organ which may
be carried in the blood, and by diffusion get into the blood of
the embryo, and directly affect the development of the corre-
sponding organ. They attempted to test this hypothesis by in-
jecting into the blood of a pregnant animal extracts from the
diseased kidney of another animal, and while the authors do
not appear so certain that similar effects are here also pro-
duced in the organs of the embryo, yet this seemed to be
the case.

It will be observed that this transmission of an acquired
character, if it really occurs, appears to be different from that
of transmission through the egg, for it is the developing organ
itself that is acted upon.

These results may possibly have a bearing on Brown-Séquard’s
work, since they seem to show that if an organ of an adult ani-
mal (that is viviparous) is diseased, the same organ of the young
may develop abnormally (or become diseased). Whether the
two cases are really the same we do not know. In fact, it
would be extremely hazardous to conclude even that they may
be, until we know in what way epilepsy is caused — whether
by a physical defect in the nervous system, or by bacteria, or
by some other means. It should not be overlooked that the
epilepsy was not present in the young when born, but developed
later. There is urgent need that experiments of this sort be
carried out on an extensive scale.

Many other cases of mutilations have been cited to show
that acquired characters are inherited. Defects in the parents
are said sometimes to reappear in the young, and it is
inferred that in some way the two things are connected.
We are apt to overlook the fact that thousands of injuries are
not inherited, and that a malformation appearing in the child in
the same organ that had been injured in the mother or father
may chance to occur at any time, and would be certain to arrest
attention. We might err if we concluded in such cases that

56 Experimental Zoology

there was any direct connection between the conditions in the
parent and in the offspring.

On the other hand, if the results of Charrin, Delamare, and
Moussu are confirmed, there is a chance that diseased organs
at least may affect the young in utero. This would, of course,
only allow the inheritance of acquired characters from the
mother, and not from the father. This difference might give
us a chance to test the view that the effects are produced in the
embryo in utero and not in the germ-cells. It is interesting to
note that Brown-Séquard found that epilepsy is more often
transmitted through the mother than through the father. In
the case of mutilations, the injury may have been inflicted
years before, and the wounds have completely healed before
the young are conceived; yet cases of this sort have often been
cited to show inherited effects. In such cases it is difficult to
see how such effects could become transmitted, especially
through the male.

Weismann has given in the “Essays” referred to above an
interesting and very full discussion of the supposed cases of
inheritance of acquired characters. He himself carried out
some experiments with mice. For four generations the tails
of mice were cut off. Of the gor mice born during this time
not one had a short tail, and careful measurements showed
that there was no shortening at all of the tail of the offspring.
This experiment was, it is true, almost needless, since it is cus-
tomary to cut off the tails of certain breeds of sheep and the -
ears of dogs without these breeds ever having become tailless
or earless, and circumcision has been practiced in man for cen-
turies without any appreciable effect.

Since in these cases the organs operated upon had healed
over before the next generation was born, there would be little
chance of the injury directly affecting the embryos. Moreover,
even if such effects are inherited, it might not follow that the
tails would be shortened, but at most only diseased in some
way.

Recently Nussbaum has commented on this side of the ques-

The Inheritance of Acquired Characters 57

tion. He claims that we should not expect the inheritance of
short tails, because even if such an influence were transmitted
to the egg, the young embryo would promptly regenerate its
missing portions. Unfortunately there is no evidence that they
can regenerate in the embryo.

It has been claimed that the removal of or injury to an organ
is a different process from the modification of an organ caused by
some external condition. There are, however, cases in which
an organ has been greatly modified for several generations and
no inherited effect has been produced. The feet of Chinese
women are compressed so that their form is greatly changed,
and in the higher classes this has been kept up for generations,
yet no effect has been produced on the feet of the children.
Certain races of Indians are known to have changed the shape
of the heads of their children by compressing them between
boards, yet the effect does not seem to have been inherited.
Tight lacing may change very greatly the shape of the ribs and
to some extent the position of the viscera, yet no inherited effect
has been produced.

We may now consider the cases of the supposed inherited
effect of use and of disuse. We are familiar with the decrease
of a part in size and in function through disuse, in the case of
muscles, glands, bones, etc., and also with the enlargement of
organs through use. It is especially this class of facts that
impressed Lamarck, and which led him to assume that these
effects are inherited. The rudimentary eyes of animals living
in the dark are often cited as having resulted from disuse. The
long neck of the giraffe and the long tongue of the ant-eater, of
humming birds, and of woodpeckers are examples, that La-
marck has given, of effects produced by use; and many natu-
ralists since Lamarck’s time have cited these and other similar
cases as bearing on the question. But neither Lamarck nor
his successors have been able to demonstrate by experiment
that the effects of use and disuse of this kind are inherited by
the next generation, and until this proof is forthcoming we
must regard their view as purely speculative. It is surprising

58 Experimental Zoology

that no experimental proof of this kind has been furnished,
because it would seem that it ought not to be difficult to make
crucial experiments that would settle the question at once. It
is true that many of the latter-day Lamarckians claim that
effects of this sort are only very slowly brought about, and that
we should not expect to observe any results that are meas-
urable in the course of a few generations. It seems to me that
by taking this position the Lamarckians distinctly avoid the
real issue; for it is not evident why such effects, if produced
at all, should not appear at once in the embryo, since they
appear at once in the adult.

It is evident that if the effects of use and disuse were inherited,
many of the adaptations of organisms to their surroundings
could be quickly attained; for the parts most used would be
strengthened in successive generations, those less used would
soon decrease in proportion to their use, and the complicated
adjustments of the different parts could be accounted for. The
habits and instincts of animals would also be made to con-
form to the needs of the animal. These benefits are so obvious
that it is small wonder that the theory of the inheritance of ac-
quired character has always had its adherents. But the plausi-
bility of a theory is not a scientific proof of its worth, and the
best evidence, viz. that from experiment, that we have at pres-
ent, does not show that acquired somatic characters are inherited
through the germ-cells. Moreover, the advocates of the-theory
overlook a consideration of prime importance. If the effects of
mutilations and of diseases are also inherited, the results would
be highly injurious to animals. Considering how often animals
are injured, we should expect to find the animal kingdom in a
most dilapidated condition if the accidental injuries of all their
ancestors were transmitted to subsequent generations. It is
unfortunate that many of the best-authenticated cases of the
inheritance of acquired character are those relating to disease.

In recent years two elaborations of the principle of inheri-
tance of acquired characters have appeared. Semon has worked
out Hering’s original suggestion that heredity is racial mem-

The Inheritance of Acquired Characters 59

ory. Under the title of ‘Die Mneme,’’ Semon has analyzed
and classified the different forms of inheritance of former ex-
periences of the individual which are assumed to become in
time the herditary capital of the race. Rignano has developed
the idea of the influence of the germ-cells on the soma, and
vice versa, from a different standpoint, viz. that the germ-
cells influence the soma during development and in turn are at
times influenced by the soma. ‘These speculations are based on
the assumption that acquired characters are inherited. Since
we are concerned here only with the experimental evidence in
favor of or opposed to this assumption, it would carry us too
far to attempt to deal critically with these elaborations, that
assume at the starting point that such characters are inherited.’

Telegony

We may next examine the evidence that has been supposed
by certain writers, in the main ‘‘practical”’ breeders, to prove
that maternal impressions of various kinds can be transmitted
to the young in utero. The crudest examples are those in
which it is related how the pregnant mother, being impressed
by some unusual or revolting sight, has transmitted to her
infant a corresponding structural deformity. Somewhat less
credulous perhaps are those breeders and “fanciers” who are
firmly convinced that if a purely bred animal — the horse and:
the dog are the stock examples — has first been paired with a
mongrel animal, the subsequent offspring to a purely bred
father will show evidences of the first birth, 7.e. be impure as
to their breed.

The credulity of men who have not been trained as to the
value of evidence is a matter of everyday observation, and it is
not going too far to say that most opinions or statements of the
“practical” breeder must be put to a rigorous scientific test
before they can be trusted. This has proven to be the case
with telegony.

* For a criticism of Semon’s argument, see Weismann’s recent (1906) review.

60 Experimental Zoology

The best-known case, and one that Darwin himself believed
to have been “‘perfectly well ascertained,” is that of Lord Mor-
ton’s mare, a full account of which is given in Darwin’s “ Ani-
mals and Plants.” A pure Arabian mare was bred to a
quagga, and the first offspring was, of course, a hybrid. The
second time, the mare was bred to a stallion of pure stock.
The colt showed cross markings on the legs, that were said to
be much more developed than on colts of pure pedigree. Dar-
win thought the evidence sufficient to show that in some
unknown way the mother had been affected, either by the
sperm of the quagga or by the hybrid embryo zm wiero, and this
effect was again in some unknown way transmitted to the sec-
ond offspring. Darwin advanced this case as one of several in
support of his hypothesis of pangenests.

The statements remained practically undisputed until within
recent years. A few years ago Ewart undertook to repeat the
experiment, which he was enabled to do by having a small
experimental farm placed at his disposal. Careful series of
cross-breeding experiments, followed by pure breeding, were
carried out. Ewart discusses his results in his book entitled
“The Penycuik Experiments,” and reaches the conclusion that
no clear evidence of infection, if I may so call it, can be pro-
duced in this way. He also points out that it is not uncommon
for colts, purely bred, to show stripes as distinct as those in Lord
Morton’s case. Fortunately a picture of this colt is still in ex-
istence, and its examination shows that the markings are not
more distinct than those that sometimes occur in the case of
purely bred animals. There was, then, merely a coincidence,
and not a causal connection.

Bell has given an excellent summary of the evidence in regard
to the possible influence of a previous sire, and has carried out
a number of experiments on horses and dogs. In none of
these cases was any influence of the previous progeny.or of a

1 Two other cases were reported to Bell, one for pigeons and the other for a
cross between a negro and a white woman. In neither case were the later off-
spring, by a father of their own kind, affected by the first union.

The Inheritance of Acquired Characters 61

previous sire to be seen in the later offspring. The experiments
were made with pedigree stock, and the results are convincing,
and indicate that the belief in an influence of this kind is another
breeders’ myth. Minot has also obtained negative evidence
with guinea pigs, and I have obtained similar negative evidence
with mice.

Xenia

It has been claimed that in plants the influence of the pollen
is sometimes shown in those parts of the fruit or of the seed that
are derived from the mother plant.'| The flower of the orange,
fertilized accidentally by pollen from the lemon, is said some-
times to produce fruit that may show a stripe in the peel like
that of the lemon, although the peel is a product of the tissues
of the mother plant. Similar cases have been recorded for the
seed-color, and even for the pods of peas. The most familiar
case is that of the color of the grains of maize. .

In recent years it has been shown that some, at least, of these
cases can be explained as the result of a process of double fer-
tilization. It has been found that there enters the embryo sac
not only the sperm-nucleus, that unites with the ovum, but .
another nucleus that fuses with the “polar” nucleus or nuclei.
The latter combination gives rise to the endosperm, which is
therefore hybrid in origin, and may show the influence of the
sperm nucleus if this contains the dominant character.

In regard to the other cases, where the color is not in the endo-
sperm, the results cannot be ascribed to the sperm nucleus.
Giltay, who has made some experiments to test this point in
peas, found no instances where the color of the pods could
be assigned to the influence of pollen plant.’

Theories of Transmission of Somatic Influences

The only theory of any prominence that pretends to indicate
how changes in the somatic cells may affect the germ-cells is

1 From the nature of the case the process could not be expected to occur in

animals.
2 See Davis, B. M., American Naturalist, XXXIX, August, 1905.

62 Experimental Zoology

Darwin’s provisional hypothesis of pangenesis. Many of the
assumptions of this theory are scarcely in accord with our pres-
ent cytological knowledge. For instance, few cytologists would
be likely to admit that the germ-cells are built up of living par-
ticles representing the different tissues and organs of the body
that are collected by the reproductive organs. In a modified
form, however, Darwin’s hypothesis could no doubt be brought
up to date, if it were desirable to do so; but is it worth while to
speculate further in this direction until we have a better basis
of fact on which to rest the speculation; for, as has been pointed
out, the experimental evidence in favor of the inheritance of
acquired characters is unsatisfactory?

The idea that the cell is made up of smaller morphological
units that represent the various potentialities of the cell has been
a favorite assumption of modern writers. Thus we have the
physiological units of Spencer, the gemmules of Darwin, the
pangens of de Vries, the plasomes of Wiesner, the micelle of
Nageli, the plastidules of Haeckel, the biophores of Weismann,
the biogens of Verworn, the idioblasts of Hertwig, etc.

It is perhaps needless to point out that the kind of reasoning
on which this method of treating the problem of heredity rests
is of the sort that gives only the appearance of a real explanation,
for the responsibility is only shifted to invisible and imaginary
units that can be worked like puppets, at the will of the philoso-
pher. Grossly ignorant as we are concerning the chemical and
physical basis of cell activity, it is not probable that such guesses
can be much more than fictions or at most symbolic.

A single citation from Darwin will serve to bring the main
points of his theory of pangenesis before us. ‘“‘It is universally
admitted that the cells or units of the body increase by self-divi-
sion or proliferation. . . . But besides this means of increase
I assume that the units throw off minute granules which are
dispensed throughout the whole system, that these, when sup-
plied with proper nutriment, multiply by self-division, and are
ultimately developed into units like those from which they were
originally derived. These granules may be called gemmules.

The Inheritance of Acquired Characters 63

They are collected from all parts of the system to constitute
the sexual elements, and their development in the next genera-
tion forms a new being. . . .”

A few experiments have been made to test this view. Galton?
transfused the blood of one variety of rabbit into the veins of
both sexes of another species, and then bred together the latter.
If there are gemmules in the blood, the germ-cells of the rabbits
containing the transfused blood might possibly show the influ-
ence of the other variety. No evidence of such an influence
was found.

Darwin did not admit that this experiment was decisive, and
Galton himself admitted that the results are not convincing.
Darwin thought that the few gemmules present in the blood
at any one time might not succeed in supplanting the similar
kinds of gemmules supposed to be already present in the germ-
cells.

Another experiment was carried out by Romanes. Wild
rabbits supplied the blood, and Himalayan rabbits received it.
Several transfusions were made. In one case the blood of
three wild rabbits passed through the veins of the domesticated
individual. No evidence of any “foreign” influence was found
in the offspring. Romanes said later that he had discovered
that this experiment could not have been expected to give any
positive results, because rabbits when crossed do not produce
young having intermediate character. The force of the admis-
sion is not very convincing, however, for the offspring might
still have been expected to show the effects — if such influences
are transmitted in this way — of the dominant breed, if this had
been used to supply the transfused blood. Moreover, Castle
has shown that in some breeds of rabbits certain characters at
least are intermediate in the hybrid — the length of the ears, for
example.

Other zoélogists who have refused to accept the doctrine of
the inheritance of acquired characters — Weismann, for instance
— have nevertheless used another idea contained in Darwin’s

1 Proc. Roy. Soc., 1871.

a

64 Experimental Z oilogy

hypothesis. The elementary characters of the cell are assumed
to be contained in minute, living elements, gemmules, pan-
genes, etc., that multiply and increase independently of each
other; but their migrations are now limited to the individual
cell. The nucleus is believed to be the storehouse of these
units that issue forth at times to perform any function whatever
that may be assigned, pro tem., to the cell. De Vries’s theory of
intracellular pangenesis rests on this assumption. How such
bodies eat up or replace the rest of the cell contents to dominate
its function we are not told, and details of a chemical nature are
deplorably lacking. Since this army of immigrants is confined
within the boundaries of the cell, they have no importance to us
in this connection. The historical origin of the ideas in regard
to these bodies is, however, not without interest here. The
superficial analogy between the theory and that of the atomic
theory of the chemists has sufficed to lure many writers into
this fascinating and facile mode of speculation.

LITERATURE, CHAPTER V

BARTHELET. Expériences sur la télégonie. Compt. Rend. Paris. Tom

131. 1900.
Bet, A. L. The Influence of a Previous Sire. Journ. Anat. and Physiol.
XXX. 1896.

BRowNn-S£QuARD. Nouvelles Recherches surl’epilepsie. Archiv de Physi-
ologie Normal et Pathol. 1869.

Remarques sur l’epilepsie causée par la section du nerf sciatique.
Ibid. 1870.

Quelques faits nouvelles relatifs 4 ’epilepsie. Ibid. 1871.

Faites nouveaux établissait Vextréme fréquence de la transmission
par Vheredite d’états organiques morbides produits accidentelle-
ment chez les descendents. Compt. Rend. 1882.

Cuarrin, A., DELAMARE, G., et Moussu, G. Transmission experi-
mentale aux descendants des Iésions developpées chez les ascendents.
Compt. Rend. Acad. Sc. Paris. CXXXV. 1902.

Davis, B. M. Studies on the Plant Cell. Am. Natural. XXXIX. 1905.

Ewart, J.C. The Penycuik Experiments. London. 1899.

FiscHer, E. Experimeritelle Untersuchungen ueber die Vererbung er-
worbener Eigenschafften. Allgem. z. f. Entomologie, VI. gor.

Gatioway, T. W. The Experimental Evidence for the Inheritance of
Acquired Character in Organisms. Cumberland Presbyt. Quar-
terly, I. 1go02.

Herinc, E. Ueber das Gedichnis als eine allgemeine Function der
organischen Materie. 1870. .

The Inheritance of Acquired Characters 65

MEYER, J..DE. L’heridité de caractére acquis est-elle expérimentalement.
verifiable? Archiv de Biologie, XXI. 1905.
Minor, C. S. An Experiment with Telegony. Report 74. Meet. Brit-
ish Assoc. Cambridge. 1904.
Nussspaum, M. Die Vererbung erworbener Eigenschaften, Sitzungsber,
der Niederrheim. Gesellsch. f. Natur- u. Heilkunde zu Bonn.
1903.
Ras, K. Ziichtende Wirkung Functionelle Reize. 1904.
Ricnano, E. Sur la transmissibilité des caractéres acquis. Hypothése
d'une centro-épigénése. Paris. 1go6.
Semon, R. Die Mneme als erhaltendes Princip im Wechsel des organ-
ischen Geschehens. 1904.
Ueber die Erblichkeit des Tagesperiode. Biol. Centralb. XXV.
1905.
Strowsk1, L. Biologische Beobachtungen ueber Motten. Bull. Acad.
Science de Cracovie. 1905.
Spencer, H. Principles of Biology. 1864. Revised ed. 1898.
The Inadequacy of ‘‘Natural Selection.” Contemp. Rev. 1893.
Prof. Weismann’s Theories. Contemp. Rev. 1893.
. Weismann, A. Das Keimplasma, eine Theorie der Vererbung. Jena.

1892.
Die Allmacht der Naturziichtung, eine Erwiderungen H. Spencer.
Jena. 1893.

Ueber Germinal-selection. C. R.z. Congrés internat. Zool. Leyde
und Jena. 1896.

Richard Semon’s “‘Mneme” und die ‘“Vererbung erworbener Ei-
genschaften.” Arch. f. Rassen- und Gesell. Biologie, III. 1906.

CHAPTER VI
EXPERIMENTAL HYBRIDIZING

WITHIN the limits of the Linnzan species it has been found
that varieties, or races, or breeds, are generally perfectly fertile
when crossed, and in recent years these fertile crosses have
been much studied. In some cases it has been found that a
character that is different in two parents blends in the offspring,
and may or may not separate again; in other cases, however,
a character that differs in the parents does not blend, and all the
offspring of the first generation are like one of the parents. The
inheritance is alternate. If hybrids of this kind are bred to
each other, the original character of one of the grandparents
may reappear in some of the offspring, the contrasting char-
acter of the other grandparent in others. These cases follow
what is known as Mendel’s law.

It is sometimes stated that Mendel’s law applies only to
crosses between varieties, and this is true for many cases; but
characters that are entirely new to the race may also follow
Mendel’s law; and if the appearance of one new character suf-
fices to characterize a new form as an elementary species, we
must conclude that the characters of some elementary species
also follow Mendel’s law of alternate inheritance.

Mendel’s Law

In 1865 Mendel published the results of an elaborate series
of experiments that he had made with varieties of peas. It is
strange that so important a discovery should have been entirely

neglected for thirty-five years, especially since the question of
66

Experimental Hybridizing 67

heredity and evolution were being actively discussed during
that time. It was not until 1900 that de Vries, and simul-
taneously Correns and Tschermak, independently obtained re-
sults that brought to light.again the long-forgotten discoveries
of Mendel.

Mendel found that when the flowers of one race of peas are
fertilized artificially with pollen from anotHer race, the hybrid
offspring (f,) of the first generation are like. one of the parents
in each particular character, and not intermediate in character.
If, however, these hybrids were self-fertilized or inbred, both
grandparental types reappeared in their offspring (F,), and
in definite proportions. The character of one of the parents
that appears in the first hybrid generation (F,) is called the
dominant, and the contrasted character of the other parent that
disappears in the first hybrid generation is called the recessive.
When these first hybrids are inbred as stated above, there ap-
pears in the second generation of hybrids (F,) three individuals
showing the dominant character to one individual showing the
recessive. This, however, is by no means the whole discovery ;
for Mendel found that the recessives of this second generation,
if inbred, give always recessives and nothing else. Those that
show the dominant character, on the other hand, do not all
breed true. A third only are pure and give rise only to domi-
nants, while two-thirds of them produce both dominants and
recessives. The matter can be graphically expressed as fol-
lows : —

If we call the dominant character A, the recessive B, then the
first generation (F,) of hybrids will be A(B). This means that
while the hybrids show outwardly only the dominant character
A, the recessive character (B) is also present in an undeveloped
condition. When these hybrids (F,) are inbred, the A-char-
acter dominates in one fourth of the offspring, the B-character
in one fourth, and the A(B) character in two fourths, 7.e. in
the proportion of 1: 2:1. Mendel showed by a simple

1 In practice A(B) can only be distinguished from A by the kind of progeny
that each produces.

68 Experimental Zoology

assumption how this numerical relation could be explained.
When the male, A, and the female, B, germ-cells unite, every
cell of the hybrid will contain both A and B; in which case
one dominates, namely, A, giving A(B). If we assume that in
the germ-cells of the hybrid the characters A and B separate
again, and go to different cells, half of the germ-cells will con-
tain the one character only, and the other half the other char-
acter. This is supposed to take place both in the male and in
the female individual. The male germ-cells containing A may
meet egg-cells containing A or B, and conversely the male-cells
B may meet egg-cells containing A or B. The possible com-
binations that result are shown in the following diagram : —

Lass |

AYNB

The chances are that, on the average, A will meet B twice as often
as A meets A, or that B meets B. Hence the combination A(B)
will occur twice as often as AA or BB. The outcome will be
1 AA, 2 A(B), 1 BB. Thus according to the assumption of
two kinds of germ-cells in the hybrid the numerical results agree
with the actual results of the experiments. For this reason
Mendel’s assumption of two kinds of gametes has been generally
accepted. Furthermore the theory can be tested in several
ways, as will be shown later, and it has so far, on the whole,
stood the test. When in addition to this it was found that in
the germ-cells a mechanism exists that seemed capable of carry-
ing out the postulated process of purification, it appeared to a
number of modern zodlogists that Mendel’s assumption of two
kinds of germ-cells in hybrids of this sort is a real and not a
fictitious explanation of the results.

An actual example may make clearer Mendel’s principle and
its interpretation. If a gray house mouse, A, is crossed with a
white albino mouse, B, the offspring, (F,), will be all gray like
the house mouse. If these gray hybrids' (F,) are bred to each

* Following Mendel the cross between two races or varieties is called a hybrid,
although this term has been usually employed for crosses between species.

Experimental Hybridizing 69

other, their offspring (F,) will be gray or white in the proportion
of 3:1. If the white individuals are inbred, they will give
only white, and this is true for all of their descendants. They
are said’ therefore to be “pure.” The gray individuals, how-
ever, show themselves to be of two kinds; one third of them,
if inbred, produce only gray, and all of their descendants will
be gray. They, too, are said to be “pure.” The other two
thirds, if inbred, produce both white and gray mice. If these
offspring are examined by further crossing, it is found that the
whites are pure and give only whites; that some of the grays
are “pure” gray, but the others are gray-dominant-white-
recessives, A(B), and again in these we find the proportion 1 A :
2AB:1B. The following scheme will show at a glance
the succession of generations : —

A B

/ i ‘i =o a

Fe ee N

7 i a ris \ _ :
a A A LA 2A iB 3B B B

A practical consideration of some importance is obvious from
these results. Pure races can be obtained from the hybrid,
A(B), by selecting the offspring with “pure” germ-cells, A or B.
On: the other hand, the A(B) hybrids always produce some
A(B), so that all their offspring do not return entirely to the
two parental types, but in every succeeding generation they will
continue to split off some ‘‘pure” A’s and B’s.

As has been stated, Mendel’s assumption in regard to the two
kinds of germ-cells has been tested in other ways and found to

70 Experimental Zoology

conform to expectation. One way has been to breed back the
first hybrids A(B) to the parent form, either A or B; the other
way has been to apply the rule to more than one character.
These two methods may now be illustrated.

If ahybrid, A(B), is bred back to the parent type, B, half of the
offspring should be A(B) and half BB. This must occur be-
cause, on the assumption, the germ-cells of A(B) are A and B,
while those of BB are B and B; thus—

A B

B B
A(B) BB
A(B) BB

_If, on the other hand, A(B) is bred back to the other parent
type, A, all the offspring will be like A, although only half are
pure A’s, the others being A(B); thus —

A B
A A
AA A(B)
AA A(B)

The most interesting test that Mendel made of his theory
involves the heredity of two dominant characters and two re-
"cessive characters. Thus, if two varieties, AB and ab, are
crossed, the first hybrids (f) will be AaBb. Since A and B
dominate, these individuals will all resemble AB externally.

The germ-cells of the hybrid individual, AaBb, will be of as
many kinds as there are possible combinations of A, a, B, 8,
provided that each combination contains some A (or a) and
some B (or 6), 7.e. one or the other kind of the two characters,
Thus the only possible combinations are AB, Ab, aB, ab.’

"Tt may seem that these four combinations do not exhaust all the possible
combinations of the letters, because AA, Aa, aa, BB, Bb, bb, might be sup-
posed to appear, but this is not the case, because on the assumption of paired
characters A (or a) must always be accompanied in the germ-cell with B (or

b) characters. Similarly, B (or 6) must always be accompanied by A (or a)
characters. Hence the six combinations just given are excluded.

Experimental Hybridizing 71

Each of the four kinds of egg-cells may be met by any one of
these same four kinds of male cells, giving in all nine combina-
tions; viz. AABB, AABb, AaBB, AaBb, AAbb, Aabb, aaBb,
aaBB, aabb. But since these are combinations of both dominant
and recessive characters, the offspring will appear to be of only
four kinds. ‘Thus the first four terms will belong to the type
AB; the next two kinds will be Ad; the next two aB; and the
last ab. It will be found by making all the possible combina-
tions that the AB type occurs nine times, Ab three times, aB
three times, and ab only once.

Mendel carried out an experiment of this sort in which two
contrasted characters were involved. The results fulfilled the
expectations of the hypothesis. He used two races of peas, in
one of which the form of the seed was round, A, in the other
angular, a. In the round seeds the albumen was yellow, B;
in the angular seeds the albumen was green, b, thus —

AB, seed parent ab, pollen parent
A, form round a, form angular
B, albumen yellow b, albumen green

The outcome of the experiment conformed to the scheme given
above. It should be noted that in the first generation, A Bab,
the hybrid will contain all of the possibilities, although A domi-
nates @ and B dominates 6. In the germ-cells the characters
separate on the assumption of pure gametes, but in such a way
that A will always be associated with B or 8, 7.e. the round
form will always be yellow or green. Similarly, @ will always
be associated with B or b. Hence, as stated above, the only
combinations possible will be —

AB, round and yellow,
Ab, round and green,
aB, angular and yellow,
ab, angular and green.

From the foregoing account it will be obvious that the prob-
lem will become more complex when three contrasted charac-

72 Experimental Zoology

ters are involved. Mendel found that the results with three
characters agree also with the expectations. As the number of
characters increases further, the results will be very complex
and difficult to detect except by an exhaustive series of experi-
ments, although each single character can easily be traced and
found to follow the Mendelian law. Under these circumstances
we might anticipate that types differing in many characters
would give results too complicated for analysis, especially if
some of the characters follow Mendel’s law and others follow
other laws of inheritance. The generally accepted statement
that species hybrids are intermediate in character between
the parental types does not appear to hold in all cases critically
examined for all the characters. It is evident that in the future
the heredity of each character must be studied by itself.

Mendel’s Law and the Germ-cells

On the assumption that the characters of the animal or plant
are represented by primordia or elements or unit-characters in
the chromosomes, the following attempt to account for the
purity of the germ-cells, assumed on Mendel’s hypothesis, has
been suggested by Sutton.

In the early germ-cells, the spermatogonia and odgonia, the
number of chromosomes is the same as the number in the
body-cells, z.e. the somatic number; but just before the two
maturation divisions there is a synapsis stage, in which the
chromosomes come into closer connection with each other,
and, as Montgomery has shown, it is probable that at this
time the chromosomes pair with each other in such a way
that each paternal chromosome unites with its homologous '
maternal chromosome; and for the working out of Sutton’s
scheme it is essential that each paternal unites with its
homologous maternal, 7.e. that the paternal do not unite with
any other maternal or with each other.

* Homologous chromosomes are those that have the same form, or, according
to some writers, similar characters.

Experimental Hybridizing as

At one of the two. maturation divisions the united pairs of
chromosomes separate again and move into opposite cells (Figs.
rt and 1 A), so that one cell gets one and the other cell the other
of each of the homologous chromosomes. Thus each cell will
contain some paternal and some maternal chromosomes, but
the number of the maternal may be different from the number

I Ta I ie

Ne Nz L—=—, bb
; ( a
on

IN
4

BS:
VA

Fic. 6. Scheme to illustrate the two maturation divisions as seen in the
spindle and chromosomes. The clear circles represent paternal chromosomes
and the black dots the maternal chromosomes. In the first division, represented
by I and I a, some of the maternal and some of the paternal chromosomes move
toward each pole of the spindle. In the second division each paternal and each
maternal chromosome divides into equal parts. In II the chromosomes of the
cell derived from the upper end of I and Ia is represented; in IIa those from
the lower end.

of the paternal. At the other maturation division’ each chro-
mosome divides equally Figs. II and II A, so that the daughter
cells are exactly alike.* Thus there will be two cells of one kind
in regard to the single character (or group of united characters)
contained in’ each chromosome, and two cells of the other kind.

The same process occurs both in the egg and in the sperm-
cells. In the egg three of the cells, the three polar bodies, are

1 In some species the first, in others, the second, is the equation division.
? The meaning of this equation division has been much discussed, but nothing
is known about it.

74 Experimental Zoology

incapable of developing, and only the egg continues the history
of the race. Half of the eggs, however, will on the average con-
tain a particular kind of chromosome, and the other half the
homologous kind, as shown in Figs II and II A.

It is of special importance to notice in this connection that the
pairs of chromosomes are assumed to lie haphazard on the spindle,
so that while in one pair the maternal chromosome may be turned
toward a given pole, in another the paternal chromosome may
be turned toward the same pole. In other words, there are
no grounds for assuming that all the paternal chromosomes
turn toward one pole, and all the maternal toward the other,
but ‘‘accident” alone determines which way they come to lie
on the spindle. Hence the possibility of various combinations
of chromosomes in the different cells is given. The evidence
in the favor of the assumption of the accidental position of the
chromosomes is indirect, and is deduced from the way in which
the characters appear in the offspring of Mendelian hybrids
when more than a single character is taken into account.
Without this assumption the chromosomal hypothesis given
above will not apply for more than one character. Whether
this assumption is entirely satisfactory, will be considered later.

A special case may make this discussion clearer. Let us
assume that the character albinism of a white mouse is contained
in one chromosome, and the gray character of the gray mouse
in the homologous chromosome of the gray mouse. When
these individuals are bred together the white chromosome, so
to speak, and the gray chromosome are both present in the
fertilized egg, which gives rise to the gray hybrid of the first
generation, because the gray dominates the white. In the germ-
cells of these gray hybrids the changes described above take
place. At the synapsis stage the white chromosome pairs with
the gray chromosome. Later, at one of the maturation divi-
sions, the two separate and go to separate cells. Hence each
germ-cell becomes “pure” and carries only one kind of
color. If these hybrids (F,) with white and with gray germ-cells
are paired, there will be formed by chance unions of the germ-

Experimental Hybridizing 75

cells the three kinds of individuals of the second generation. A
white spermatozoén meeting a white egg, as one may say for
brevity, produces a “pure” recessive individual; a gray sperma-
tozodn meeting a gray egg gives a “‘pure”’ dominant individual ;
but when a white spermatozobn meets a gray egg, the off-
spring will be mixed, a heterozygote, and like the first
hybrid (F;).

If this hypothesis is true, we ought to be able to determine
which groups of characters — where several are involved —
are contained in a given chromosome, for the number of chro-
mosomes is often small, and therefore the actual number of
combinations is limited. All characters contained in the same
chromosome should “‘Mendelize’’ together.

If we consider two characters, the principles stated above
will apply. Let us take the case of the round and yellow,
angular and green. peas. The round form, A, may become
combined with the yellow, B, or with the green color, 6, giving
AB or Ab; the angular form a with B or 8, giving aB or ab.
To work this out we must suppose the color to be contained
in a different chromosome from the form, for otherwise they
could not shift over. When the chromosomes unite in synapsis
the A-bearing chromosome can only unite with @, and B can only

. may be turned either way

unite with 6. Hence since x and

on the spindle, A may pass into a cell with B or 0, and also a
into a cell with B or 8.

Simple and logical as Mendel’s assumption appears to be on
the hypothesis of each character being contained in only one of
the chromosomes, yet I do not think it can be accepted in this

- form because the primary assumption that each character is
contained in a single chromosome and not in others is highly
arbitrary, and also because there are certain actual results that
are difficult to explain on the assumption of “pure” germ-cells.
In fact, it has not been shown beyond question that the chro-
mosomes are the bearers of the hereditary qualities. The evi-
dence that is generally supposed to establish the assumption of

76 Experimental Zoology

the hereditary nature of the chromosomes is that the spermato-
zoon brings into the egg only the chromosomes of the male
germ-cells. While it is undoubtedly true that the largest part
of the sperm-head is made up of chromatin material derived
from the nucleus, it is also true that the protoplasm of the origi-
nal germ-cell is not lost. It is partly used up in the formation
of the tail of the spermatozo6én, but also forms a condensed
layer around the sperm-head. There is every reason to assume
that the latter may become incorporated as a part of the cyto-
plasm of the egg. There is also another serious objection to the
explanation of the purity of the germ-cells given above, for, even
if the chromosomes be the bearers of the hereditary qualities of
the egg and sperm, it does not follow that each unit character
would be contained in only one chromosome. If it be assumed
that each chromosome carries all the hereditary qualities, it is im-
possible to account for the purity of the germ-cells on this as-
sumption. For instance, if we assume that each of the chro-
mosomes contains all of the hereditary characters, the germ-cells
of the hybrid, A(B), will contain, before synapsis, half of their
chromosomes bearing the character A, and half bearing the
character B. If these unite at the synapsis in pairs, and then
come to lie, haphazard, on the spindle, some turned one way,
some the other, the resulting germ-cells will contain all
mixtures of A and B, and hence be impure. If we give up
the idea of “purity” and assume that the relative number of
A or of B chromosomes determine the character of the result-
ing cells, the three types of the Mendelian ratio might be ac-
counted for, provided the reduced number of the chromosomes
is an odd number. If it were an even number, it must often
happen that equal numbers of a character, of A or B, would be
contained in the same germ-cell, and hence there would be an
exact balance, which on the theory should give neither result.
This, however, is not in harmony with the facts.

In the light of these theoretical difficulties it seems to me that
the chromosomal theory must be applied to Mendel’s law with
caution, and that while at first sight it appears to offer an

Experimental Hybridizing 77

explanation of the assumed purity of the germ-cells in the Men-
delian cases, yet more careful consideration shows that in order
to do so certain assumptions are necessary that are not above
suspicion.

It may be seriously questioned, I think, whether the germ-cells
of Mendelian hybrids are pure. It is true that the Mendelian
proportion, 1:2:1, in the second (F,) generation can be most
easily accounted for by assuming two kinds of male and
two kinds of female germ-cells, each kind existing in equal
numbers; but the assumption that the two kinds must be pure
germ-cells meets with serious difficulties when certain results
are considered. It will suffice to point out here that the main
difficulty lies in the behavior of the so-called extracted reces-
sives which ought to be a pure strain on.the assumption of
“pure’’’ germ-cells, but which have been shown on the con-
trary to contain in a latent condition the dominant character.
I have tried to show that the results may still be accounted
for even if the germ-cells of the hybrids of generation (F)
are not pure in regard to any pair of contrasted characters,
such as gray and white, but that both characters are present
in all the germ-cells. The two kinds of germ-cells that the
hypothesis calls for may be referred to the alternating
dominance in the germ-cells of each of the two contrasting
characters. The Mendelian proportion can be accounted for
on this assumption as well as on the accepted interpretation
of pure germ-cells, and the latency of the dominant char-
acter in the extracted recessive can also be explained on my
view, but not on the other. An example may make my
meaning clearer.

Suppose a white and a gray mouse are paired. The germ-
cells of the white mouse are white-producing, or briefly white,
those of the gray mouse are gray-producing, or briefly gray. The
fertilized egg will contain both characters, and since the gray
dominates the white, the symbol G(W) will represent the con-
dition in the mouse itself. In its germ-cells both the gray, G,
and the white, W, exist, presumably combined in some way.

78 Experimental Zoology

At some time in the history of these germ-cells one or the other
of these two characters comes to dominate in each cell, so that
half of the cells will be G(W) and half W(G). This will be
true both for eggs and sperm. Chance meetings of the two
kinds of sperm and the two kinds of eggs will give —

G(W) W(G)
G(W) W(G)
1 GW) +2 GW) W(G+1WG)

This is the characteristic Mendelian proportion. The first
term, G(W), is a gray mouse, the so-called extracted dominant,
i.e. it is a mouse gray in color, which, if bred to other gray
extracted dominants, will produce only gray mice. This means
that the latent white remains in the latent condition in its
germ-cells, all of which are G(W).

The second term, G(W) W(G), of the proportion represents also
a gray mouse, since the gray, G, dominates the white, W, when
both occur in the ‘‘free” condition in the same body-cell. It
will be noticed that the presence of the two “free” colors, G
and W, in the cells of this type indicates that the type is the same
as the first hybrid formed by crossing G with W; and it is im-
portant to find that when inbred this type gives exactly the
same results (z.e. the Mendelian proportion again) as do the
first hybrids, GW. By the “free” condition I mean to imply
that the two characters, G and W, have not been brought into
the intimate relation to each other that is assumed to occur in
the germ-cells at the time when the alternating dominance and
latency occurs.

The third term, W(G), of the proportion is the extracted reces-
sive. It represents a white mouse containing gray in a latent
condition. If inbred, these white mice produce only white
mice, but if crossed in certain ways the latent gray color can
be brought out again.

The preceding example will suffice to show how the Men-
delian proportion can be accounted for on the assumption of
alternation of the contrasted characters in the germ-cells.

Experimental Hybridizing 79

The question may be asked whether this method of account-
ing for the results can be referred to the changes that take place
in the chromosomes of the germ-cells. The hypothesis de-
mands that the contrasted characters come into relation with
each other, and the union of the chromosomes might suggest
such a possible combination. After uniting, the fused char-
acters must be halved again (quantitatively not qualitatively),
and the reduction division might suggest a possible method of
accomplishing this result. On the other hand, there is no
apparent need to assume such a complicated mechanism to
bring about the union of the characters in the same cell, nor
for their subsequent separation. Moreover, by referring the
process to the chromosomes, we introduce the further assump-
tion that the characters of the cell are contained only in those
bodies — an assumption that is not itself established. For the
present, therefore, it seems premature to connect these results
definitely with any known change in the germ-cells, and the
same statement holds also, as we have seen, for the alternative
assumption of pure germ-cells.

Until we know more of the way in which characters are
represented in the germ-cells, we can only offer purely specula-
tive views of what we suppose might take place in order to give
the Mendelian results. The formule that we use are merely
symbols for handling these results. The fact that the char-
acters that “‘Mendelize” are different types or permutations of
the same characters suggests that they may represent stereo-
metric relations of the material basis of the characters, i.e. of
the molecules representing them. Thus we might represent
the characters gray and white in the hybrids as right- and left-
handed forms of the same molecule, and indicate this by GW
and WG. Such germ-cells meeting each other would give —

cw, {cw {we
eee

and these might be taken as representing the three Mendelian
groups. Interesting as such a speculation might be, could we

80 Experimental Zoology

find evidence in its support, it is unprofitable as an interpreta-
tion until or unless we can show that it is at least probable.

The preceding discussion is based on the idea that there must
be two kinds of germ-cells in order to give the Mendelian re-
sults. This is perhaps the simplest way to formulate the prob-
lem, but it should not be overlooked that the results can also
be explained by assuming that all the germ-cells of the hybrid
are alike, containing both dominant and recessive characters,
but that after fertilization internal or external factors determine
whether the one or the other character dominates. The diffi-
culty of this view is to account for the middle term of the Men-
delian proportion, in which, although the dominant develops,
the germ-cells seem to return to the two original kinds. Pos-
sibly this difficulty could be met by assuming that the two con-
trasted characters are so nearly balanced in half of the progeny,
that while the dominating character dominates when differen-
tiation begins, it is not able to do so in the germ-cells. It is
evident, however, that this interpretation is more complicated
than that of two kinds of germ-cells, and fails to account for
the three sharply marked groups.

LITERATURE, CHAPTER VI

Bateson, W. Mendel’s Principles of Heredity. 1902.

Note on the Resolution of Compound Characters by Cross-Breeding.
Proc. of the Cambridge Philos. Soc. XII. 1902.

On Mendelian Heredity of Three Characters Allelomorphic to Each
Other. Proc. Cambridge Philos. Soc. XII. 1903.

CasTLzE, W. E. The Laws of Heredity of Galton and Mendel, and Some
Laws governing Race Improvement by Selection. Proc. of the
Amer. Acad. Arts and Sci. XXXIX. 1903.

Mendel’s Law of Heredity. Proc. Amer. Acad. Arts and Sci.
XXXVIII. No. 18. 1903.
Mendel’s Law of Heredity. Science, n.s. XVIII. 1903.

Correns, C. G. Mendel’s Regeln ueber das Verhalten der Nachkommen-
schaft der Rassenbastarde, Ber. deutsch. bot. Gesellsch. XX.
1900.

DarsisHtre, A. D. On the bearing of Mendelian Principles of Heredity
on Current Theories of the Origin of Species. Mem. and Proc. of
the Manchester Liter. and Philos. Soc. XLVIII. 1904.

Focxe, W. Die Pflanzen-Mischlinge. Berlin, Borntraeger. 1881.

MENDEL, G. Versuche ueber Pflanzenhybriden. Verh. naturf. Vereins in
Briinn, IV. 1866.

Experimental Hybridizing 81

Ueber einige aus kiinstlicher Befruchtung gewonnenen Hieracium-
Bastarde. Verh. naturf. Vereins in Briinn, VIII. 1870.

Mott, J. W. Die Mutationstheorie. Biol. Centralbl. XXIV. 1904.

Morean, T. H. The Assumed Purity of the Germ-Cells in Mendelian
Results. Science, XXII. 1905.

Are the Germ-Cells of Mendelian Hybrids “Pure”? Biol. Cen-
tralbl. XXVI. 1906. ;

Prentiss, C. W. Polydactylism in Man and the Domestic Animals, with
Especial Reference to Digital Variations in Swine. Bul. of the Mus.
of Comp. Zool. at Harvard Col. XL. 1903.

TSCHERMAK, E. Weitere Beitriige ueber Verschiedenwerthigkeit der Merk -
male bei Kreuzung von Erbsen und Bohnen. (Vorlaufige Mitthei-
lung.) Ber. deutsch. bot. Gesellsch. XIX. got.

Die Theorie der Kryptomere und des Kryptohybridismus. I. Mit-
theilung. ueber die Existenz kryptomerer Pflanzenformen. Bot.
Centralbl. Beihefte, XVI. 1903.

Ueber die gesetzmissige Gestaltungsweise der Mischlinge. (Fort-
gesetzte Studie an Erbsen und Bohnen.) Zeitschr. f. landwirtsch.
Versuchswesen in Oester. V. 1902.

Witson, E. B. Mendelian Inheritance and the Purity of the Gametes.
Science, XXIII. 1906.

De Vries, H. Sur la loi de disjonction des hybrides. Compt. Rend.
CXXX. Paris. 1g00.

Das Spaltungsgesetz der Bastarden. Ber. deutsch. bot. Gesellsch.
XVIII. 1900.

Anwendung der Mutationslehre auf die Bastardierungsgesetze. Be- .
rich. der Deutsch. Bot. Gesellsch. XXI. 1903.

CHAPTER VII
EXPERIMENTAL HYBRIDIZING (Continued)
Experiments with Mice

THE ease with which mice can be kept in captivity, their
rapid rate of multiplication, and the occurrence not only of an
albino race, but of other fancy breeds as well, have made these
animals a favorite subject for experimental work.

The house mouse (Mus musculs) is of a gray, sometimes
called cinnamon or agouti, color. The white mouse is an
albino race derived without much doubt directly from the wild
form. Albino mice have been recorded as occurring in situa-
tions where there was no reason to suppose that their origin
could be due to the escape of domesticated albinos. It is also
known that in many groups of the animal kingdom albino indi-
viduals often suddenly appear.

The color of the hair of the gray mouse is due, according to
Bateson and Durham, to three kinds of pigment often asso-
ciated in each hair: (1) opaque black, (2) less opaque brown,
(3) transparent yellow. In albinos all three colors have dis-
appeared as well as the pigment in the eyes. As many as a
dozen or more races of fancy mice are known. It appears that
the color of the race is determined according to which of the
three pigments found in the gray house mouse predominates or
exists alone. Thus the golden agouti, of a tawny color, con-
tains yellow and brown pigments, but not black; chocolate mice
contain only the brown pigment; yellow contain only the yellow;
black contain black and brown. Variegated mice are those

in which irregular small spots of black or chocolate occur on a
82 :

Experimental Hybridizing 83

white background; spotted or pied or piebald mice are those
in which these various colors appear in splotches or marks.
In addition to these kinds, a white mouse with dark eyes is
known, which is probably — to judge from other white animals
with black eyes—not derived from an albino, but from a
spotted animal in which the spots of dark pigment have disap-
peared except in the eyes. The so-called dancing mice that
whirl around at times are said to be of Japanese origin, and may
have originated from a different wild variety.

Experiments on mice have been carried out by a number of
investigators, the principal results being those of von Guaita,
Cuénot, Parsons, Darbishire, Castle, Allen, Davenport, Schuster,
Haacke, and others. Von Guaita’s results were not consid-
ered by him in the light of the Mendelian ratio, but Bateson and
Davenport have more recently examined them and have pointed
out that many of them appear to follow Mendel’s law.

Cuénot’s results conform closely to the Mendelian law. He
found that when a gray mouse is paired with an albino, the off-
spring in the first generation (F,) are always gray mice — the
gray dominating over albinism. The next generation (f,) from
the inbred dominant gray mice gave 198 gray and 72 albino
mice, 7.e. in the ratio of 2.75:1, a near approach to the expecta-
tion of 3:1. Later Cuénot reported that his ‘‘pure” gray* mice
of the third filial generation (/’3) when crossed with albinos gave
several black mice. "These, black mice when bred with certain
albinos gave black mice, which appeared to be B(W), 7.e. black
dominant, white recessive, for, when bred inter se, the offspring
were three blacks to one albino. Some of these blacks were
shown to be ‘‘pure” and produced a race of pure blacks. When
individuals of this black strain were bred to ordinary gray mice,
the black was recessive, giving in the second generation three
grays to one black.

Cuénot made a further discovery of great interest. He found
when he bred the black mice to albinos that the results were

1 These were the offspring of the second generation that had bred true and
shown themselves to be pure G’s.

84 Experimental Zoology

different according to the kind of albinos that he used. For
example, albinos that had been derived through a gray ancestor,
z.e. the so-called extracted albinos, when bred with the black
strain produced gray offspring. If, however, albinos that had
been derived through a previous cross with a black mouse were
bred with a black, the offspring were black. Again, albinos
derived from a previous cross with a yellow gave either mixed
yellow and gray, or mixed yellow and black. These results
show that although albinos may all appear alike and breed per-
fectly true with each other, they belong in reality to different
classes, whose latent characters are dependent on previous
influences. Results of this sort are difficult to account for on
the supposition that the germ-cells are pure. This property of
latency is not something peculiar to albinos as certain published
statements might have lead one to infer, but holds for the differ-
ent colors also. A black race that breeds true may carry
another latent color that can be brought out by crossing. Un-
less this fact is also taken into account the crosses with albinos
may be misinterpreted. |

Experiments with waltzing mice have been made by Haacke,
von Guaita, and Darbishire. These mice are black, or
white, or yellow and white; the mixed colors introducing
a complication into the results, so far as color inheritance
is concerned. As has been said, von Guaita’s results with
these mice were not considered from the point of view
of Mendel’s law, but Bateson, who has later analyzed the data,
finds in some cases what seems to be an approximation to
the expected proportions; in other cases this is not evident.
A few of von Guaita’s facts and their possible interpreta-
tions may be given. When the black-and-white waltzers were
bred to ordinary albinos, the first offspring (F',) were gray, like
the house mouse, and of the same size, which is larger than
that of the waltzing race. They also show the wild disposition.
These mice (F) when inbred produced albinos and four colored
types,—black, gray, black-and-white, and gray-and-white.
There were 14 albinos and 30 colored individuals. The relation

Experimental Hybridization 85

of the colored types to each other as regards their inheri-
tance is too obscure to make it profitable to discuss the result
here.

Darbishire has carried out experiments with pink-eyed,
spotted, waltzing mice crossed with albinos. In respect
to their coat color, he recognizes six groups forming a continu-
ous series, depending on the extent to which the pigment spots
cover the surface of the mice. The colors of the spots were
yellow, gray, black, lilac, or chocolate. When these mice were
crossed with albinos, supposed to be pure, spotted mice were
produced with dark eyes. None of the mice of this generation
exhibited the waltzing habit. These hybrid mice (F,) when bred
inter se gave the following kinds of mice (F,): —

AlbInG ve ee ee ee ee we ee 1B7
Colored or piebald with dark eyes . . . 287
Colored or piebald with pink eyes . . . 131

Of these mice 97 showed the waltzing habit and 458 did not.
The Mendelian expectation for waltzers is 138.75. The actual
results fall considerably below the expectation, nevertheless it
may be that some of the mice that did not waltz were poten-
tially waltzers and might have transmitted this habit as do
‘“‘pure”’ recessives. It is interesting to note that, whether the
Mendelian proportion is or is not given, the waltzing habit dis-
appears in the first generation of hybrids (F) and reappears in
the second generation (/,) as do other Mendelian characters.
The expectation for albinos is approximately realized as well
as the expectation for the other two types. Since neither grand-
parent had dark eyes, this character must have been latent in
one of them, because it appears in all of the offspring (F,). Dar-
bishire points out that his results do not conform in all respects
to the Mendelian rule; but some, at least, of these difficulties are
not insuperable, I think, if it be granted that the so-called “pure”
recessives and “pure” dominants are really impure with latent
characters that come out on crossing. Darbishire contends,
and I think justly, that the behavior in inheritance of extracted

86 Experimental Zoology

recessives and of extracted dominants cannot be accounted for,
if they are supposed to have been formed by the union of pure
germ-cells. On the contrary, he thinks that the results become
intelligible only when the ancestry of these forms is taken into
consideration. In other words, if ‘‘pure’’ recessives and “‘pure”’
dominants are really pure (as modern Mendelians have as-
sumed), the ancestry of such forms could be ignored; but since
the results are inexplicable on this assumption, the most rea-
sonable conclusion is that the germ-cells are not pure. So far
I am in agreement with Darbishire; but if this conclusion is
meant to imply that the Galtonian assumption in regard to
inheritance in these cases is the only alternative, I should dis-
sent. I have tried to show how the Mendelian results may
still apply without assuming that the gametes are pure, but by
assuming that they show alternate dominance and latency.

Darbishire also crossed some of his first hybrids (F,) with
albinos and obtained 368 albinos and 378 dark-eyed piebald
(or sometimes uniformly colored) mice. This gives a close ap-
proximation to the Mendelian expectation. He found, more-
over, in this generation that the gap between the albinos and the
least-colored individuals was greater than among the offspring
of hybrids (F,) when inbred. It is also interesting to note that
with this cross the pigmented eyes appeared in all the piebald
offspring. In other words, the latent color, eye-pigment, is
maintained in the cross, since it dominates the pink-eyed type.
Darbishire notes especially that these offspring (obtained from
the hybrid and the albino) have more white in their immediate
ancestry than have the offspring of the hybrids when inbred; yet
the offspring show actually less white, and show more often the
wild color and black (as compared with yellow and fawn color).
The meaning of this he finds obscure, but possibly the results
may be accounted for on the assumption of the latency of pig-
ment in one or both parent types, which is brought out by
crossing.

The subsequent history of the three classes of individuals ob-
tained by inbreeding the first hybrids (F,) is as follows: (1) The

Experimental Hybridizing 87

extracted albinos give always albinos. Only 2 out of 94
waltzed. (2) The dark-eyed mice with colored coats belonging
to the middle term of the Mendelian series should also give, if
inbred, the three types again in the proportion of 1:2:1. In
this connection Darbishire points out that on Galton’s law of
inheritance the farther the individuals of this middle class are
removed from the first class, the fewer the albinos that should
appear, since they are farther removed from the original ances-
tor that was white. On the Mendelian law the members of
this middle term should always continue to give the same pro-
portion, 1:2:1. Experiments that Darbishire made to test this
point seemed to show that the results follow more nearly the
expectation of Galton’s law; but the purity of the types used
may have seriously affected his results.

Especially interesting, it seems to me, are Darbishire’s experi-
ments with the extracted mice of the second generation having
pink eyes and colored coats. If these are really pure, they should,
if paired with pure albinos, produce animals similar to those of
the parent cross between colored mice with pink eyes and
albinos, z.e. there should be produced only spotted mice with
dark eyes. This, however, was not the result obtained. Of
98 young, 12 were albinos, while one pink-eyed (colored) indi-
vidual also appeared. These results are complicated by the
fact that the albinos used were also the extracted offspring
of hybrids paired with albinos. Nevertheless, even granting
this, it offers no explanation of why albinos should appear, and
the only explanation that seems reasonable is that the albino,
latent in the pink-eyed mice, has affected the result, pre-
sumably being brought out again by crossing. Sixty-three of
the 98 young were obtained from such contaminated albinos
(extracted recessives). Only seven of the unions were between
such pink-eyed mice and albinos which did not contain pink-
eyed, spotted waltzers in their immediate ancestry. From these
pairs 35 young were obtained, of which 10 were albinos —a
relatively higher number of albinos, and approximately one
fourth of the whole. The results seem to show that the extracted

88 Experimental Zoology

“pure” race was not pure, since it may produce some albinos
when paired with albinos.

Allen has also carried out a large number of experiments with
mice. Crossing the gray house mouse with the albino gave
gray offspring, as most other experimenters have found. The
second generation (F,) gave approximately the Mendelian ratio
of three grays to one white, the former being partly pure grays,
partly mixed. Crossing the dominant recessives (heterozygote)
with the extracted albinos, where equality of grays and whites
would be expected, gave 84 pigmented young and 64 whites.
While there is only an approximation to equality here (74 of
each expected), the deficiency in white may be due to insuf-
ficiency of numbers, but possibly to some other factor. Allen
found that when spotted mice were bred to albinos the off-
spring were spotted, — often with Jess white than the original
spotted parent, — and in some cases the white almost, or even
completely, disappeared. Thus, although we may look upon
the spotted condition as a unit-character that is dominant, its
extent appears to be variable. In fact, a latent character may
also come to light here that is not seen in either parent, but must
be potential in one or in both of them. While the white parent
might have been expected to add more white to the offspring, —
on Galton’s hypothesis, — the result is exactly the opposite.

When pigmented, heterozygote individuals (Ff) were inbred,
they produced 159 pigmented young and 55 albinos (53.5 being
the Mendelian expectation). In another experiment, pig-
mented, heterozygote individuals (F,) were bred to pure albi-
nos, giving 69 pigmented and 69 albinos, exactly the anticipated
ratio.

Allen carried out a number of experiments made to test the
inheritance of partial albinism, as he calls the condition when
white areas are present along with colored areas. When ‘‘par-
tial albinos” are bred to pure albinos, the young (F,) were more
nearly totally pigmented (as stated above), some showing no
trace of white, others had white toes, or a white tip to the tail
or even a few scattered white hairs. One only had a white spot

Experimental Hybridizing 89

on the belly. Haacke and von Guaita found similar results.
“The influence of the albinos in these cases seems to be to
upset the condition of localization of the pigment, so that the
pigment patches become more extensive, tending to cover the
entire body surface, as in totally pigmented animals. In expla-
nation of this observation it is suggested that the character
total pigmentation may be transmitted by albinos, and when
so transmitted dominates over the spotted condition.” Cuénot
has offered a suggestion to account for this possibility. The
pigment is assumed to be due to the action of a ferment upon a
chromogene substance. The albino may transmit the ferment
but not the substance. The germ-cell of the black-and-white
individual would then be assumed to convey the pigment, and,
when to it more ferment is added by the white gamete, more
pigment is produced in the offspring.

Cuénot has given a very clear and important analysis of his
results with mice in his third contribution to “The Heredity of the
Pigmentation in the Mouse.’”’* He uses the letter C to denote
any colored character, and A for the albino character; G for
the color gray; B for the black; and Y for the yellow. Thus
the wild gray mouse will be represented by CG, and the ex-
tracted albinos, having potentially the gray color, by AG. The
black mouse will be represented by the formula CB; extracted
albinos derived through black ancestors AB; the yellow
mouse by CY; the extracted albino through yellow ances-
tors by AY.

When a germ-cell containing the character CG unites with
one containing CB, the gray; G, dominates. When gray, CG,
meets yellow, CY, the latter dominates. When CB meets
CY, the yellow again dominates. When a colored germ-cell, C,
meets an albino, A, the individual that develops has black
eyes, but the color of the hair depends on which color accom-
panies C or A. As an example Cuénot gives this case: a black
mouse, CB, crossed with an albino, AY (yellow latent), gives a
dihybrid, CBAY. This hybrid has black eyes, because for

1 Archiv. Zool. Expér. et Gén. 1904. Sér. 4, T. z. Notes et Revue, p. xlv.

90 Experimental Zoology

the eyes the color, C, always dominates; but the color of the
hair is yellow, because yellow dominates over black.

The four types with which Cuénot has experimented give
eighteen different combinations, which he shows by means of
the following table: —

HOMOZYGOTES HETEROZYGOTES
COLOR
OR PURE RACES MoNnoHYBRIDS DIHyYBRIDS
CGCB CGAB
Gray CG (wild) CGAG
Black CB CBAB
Yellow cY CYCG CYAG
* CYCB CYAB
CYAY
Albinos AG ‘ AGAB
AB AGAY
AY ABAY

Of these eighteen types there are six that are pure races,’ 2.e.
they produce germ-cells that are all alike (homozygotes). In-
bred, they give always the original types in all successive gen-
erations, and this holds also for the three albino types, having
the latent characters gray, black, and yellow, AG, AB, and AY.
The other twelve types are heterozygotes, resulting from the
crossing of pure races. Of these, nine are monohybrids, hav-
ing only one pair of antagonistic determinants; and three are
dihybrids, having two pairs of antagonistic determinants.

It will be seen that Cuénot thinks that albinos are not nec-
essarily all alike, although they may breed true to the albino
type, but that they are different according to the latent char-
acter that each contains. The latent character may appear in

1 According to Cuénot’s nomenclature. The three albinos, homozygotes, be-
long in my opinion to a different category, for although they breed true, yet they
contain a latent color that may come out in crossing.

Experimental Hybridizing gl

some of the descendants, if these albinos are crossed with colored
types. Thus if an albino AG be crossed with an albino AB,
the albino offspring will be AGAB. Its germ-cells will separate
into AG and AB, but these are albinos. If, on the other hand,
a black-mouse, CB, be mated with an albino AY, containing
yellow as a latent character, the offspring will be CBAY (yel-
low), whose germ-cells will be of four kinds, CB, AY, CY, and
AB. Crossing this yellow mouse (with its four kinds of germ-
cells) with a white mouse, AGAB, obtained in the way just
described, eight possible combinations may follow. The whole
process is indicated in the following table: —

Parenis AG (albino) AB (albino) CB (black) AY (yellow)
1st generation AGAB (albino) CBAY (yellow)
*
L i \

AGCB gray (one)
ABCB black (one)
; AGAY
2d generation eee { albinos (four)

ABAB }
CYAG }

CYAB} yellow (two)

Cuénot performed this experiment and obtained in the sec-
ond generation 151 young, distributed as follows according to
color: —
81 albinos, 34 yellow, 20 black, 16 gray.
The probability according to the formula 4 +2 n+n-+n is:
76 albinos, 38 yellow, 19 black, 19 gray.
The agreement is so close that there can be little doubt that the
hypothesis is substantially correct.

Heredity oj Piebald or Spotted Varieties. — The piebald condi-
tion is regarded by Cuénot as a special mutation, and not one
due to crossing colored and white forms. The piebald charac-
ter appears in crossing to be dominated by the untjorm colora-
tion, whatever may be its tint. For example, a spotted gray-
and-white mouse crossed by a uniformly black mouse gives a
uniformly colored gray mouse, showing that the coupled

92 Experimental Zoology

characters, spotted versus color uniform, are independent of the
coupled characters gray-black. Cuénot adds, therefore, a new
character to the formula representing the uniform color, U,
viz. its antagonist the spotted color, S.

From the point of view of heredity the spotted condition is
peculiarly interesting, since it appears to be continually vary-
ing, and by suitable selection this spotted character may be
carried through a regular progression until the dark color may
almost disappear. The depigmentation begins at the tail,
toes, and ventral surface of the body; more rarely there is a
small spot on the top of the head. This is the condition found
not infrequently in the wild gray mouse. Through selection
the caudal and ventral white areas enlarge and the latter in-
vades the flanks, right and left, finally meeting dorsally, pro-
ducing a white girdle. The white invades the muzzle, then
the head, where it may unite below with the ventral spot. Fi-
nally there remain two pigmented regions, both dorsal, one
anterior and one posterior. The eyes always remain black.
Cuénot is not sure that the development of the white may be
carried so far that the black totally disappears from the hair,
his experiments on these points not being sufficiently complete.

Cuénot studied the problems connected with the heredity of
the spotted condition by making the following combinations : —

1. Cross between spotted and uniform coat.

2. Cross between a form much spotted (with white) and a

form bearing the least possible amount.

3. Cross between two forms much spotted.

These experiments may now be considered in turn.

I. Cross between Spotted and Unijorm Coat. — The couple
uniform, U, and spotted-pigmented, S, follow rigorously the
Mendelian rule of dominance with disjunction of the gametes.
The spotted character is dominated in the first generation by
uniform color. If a much-spotted mouse is crossed by one
uniformly colored, the offspring that result show the dominant
color without trace of spots. This result is all the more para-
doxical, because if we cross a much-spotted mouse with an

Experimental Hybridizing 93

-albino, descended (extracted) from an animal with a uniform
coat, one might be led to suppose that the white of the albino
might go to augment the white of the spotted parent, but on the
contrary these hybrids are uniformly colored. In the next gen-
eration the two characters, U and S, separate, i.e. disjunct,
and the offspring give the Mendelian proportions U+2US
+S, i.e. one spotted to three uniform.

Il. Cross between Two Forms unequally spotted with White. —
If a much-spotted individual is bred to one very little marked
with white, having for example only a little white at the end
of its tail, the offspring shows that the maximal dark mark-
ing is the dominating character. All the young have the tail par-
tially white, but no other white marks on the body. Yet these
young are not all exactly like the darker parent, since the degree
of tail marking, for example, may be quite variable. In the
next generation, when the young are inbred, the phenomenon
of disjunction appears. Two groups of offspring arise, one
oscillating around the least amount of white (one grandparent
type), the other around the most white-spotted type (the other
grandparent type).

III. Cross between Two Much-spotted Forms. — Without ex-
ception the young are spotted, but in variable degrees. The
cross may even produce albinos if the two parents are hybrids,
including the character A. These albinos, in turn, possess la-
tent the spotted character.

Progression of the Spotted Condition by Selection. — Beginning
with mice little marked with white and excluding in each gen-
eration the less-marked individuals, Cuénot found that the white
areas could be increased slowly but regularly, so that in two
and a half years mice were obtained that contained much white,
and differed to a large extent from the first forms used. The
details of the spotted type seem not to be represented in the
germ-plasm, because the young and the parents are not identi-
cal. Local factors appear to determine the limits of variation.

In regard to the characters of albinos, it has been ‘pointed out
that they carry in a latent form the characters of the race from

94 Experimental Zoology

which they have sprung.’ If, for example, the characters
spotted and uniform coloration be considered, the number of
possible latent characters contained in albinos is given by
Cuénot in the following table : —

HOMOZYGOTES HETEROZYGOTES

OR PURE RACES MOoNOHYBRIDS DIHYBRIDS
AGU AGUABU AGUABS
ABU AGUAYU AGUAYS
AYU ABUAYU ABUAYS
AGS AGSABS
ABS AGSAYS
AYS ABSAYVS

All these forms “prove to exist”? and may lead to diverse re-
sults when different albinos are bred. Only by hybridizing
can the latent characters of the albino races be brought to light.
Cuénot thinks that the results of a number of authors find their
correct interpretation in the latent character in the albinos
employed.

In a later communication Cuénot gives the results of some
further experiments with gray, G; black, B; brown, R; and
yellow, Y; and with the corresponding albinos, AG, AB, AR,
and AY. A most remarkable result was found in the case of
the behavior of the yellow race. It dominates all the other
colors, yet when a yellow mouse is crossed, for instance, with a
gray, half of the offspring only are yellow, the other half being
gray, or black, or brown (according to the recessive colors pres-
ent). In Mendelian terms this means that the yellow mouse
never produces pure yellow gametes alone, but some yellow
and some of another color (gray, black, or brown). The same
result follows when white mice, having recessive yellow, are
crossed with gray, black, or brown. There result not only yel-
low offspring, but the other colors as well. If the yellow is a

? Whether an albino mutant differs in this respect from an extracted albino
cannot be stated. Cuénot appears to deal with both types, regardless of their
origin.

Experimental Hybridizing 95

heterozygote, it should give, when crossed with a pure race, half
yellow and half the other color (as stated above), according
to the formula: CYCG x CGCG = CYCG (yellow) + CGCG
(gray). The gray offspring should be pure and never produce
any yellow. Such was found to be the case. Thus of 355
young there were 177 yellow and 178 gray or black. The
gray or black do not include the recessive yellow —at least
yellow mice never appear in their progeny.’

The remarkable fact, referred to above, is that it is impossible
to obtain a pure yellow race, CY. Theoretically one would
expect to obtain, Cuénot thinks, such a race by crossing two
similar, heterozygote, yellow mice. Thus —

CYCY+2CGCY CGCG

CYCG x CYCG=
3 yellow I gray

One third of the yellows should be pure yellow with gametes
all CY’s. They should breed true to their color. Of 81 yellow
mice obtained by such combinations, not one proved to be pure
CY’s. In connection with this result, Cuénot finds that there
are always fewer yellows than expected on the last formula,
which gives 75 per cent yellow to 25 per cent grays. If we
assume that this is due to the absence of CYCY, then we should
expect 66.6 per cent yellows to 33.3 per cent grays; but neither
does this occur,’ and the proportions are more nearly 75 per
cent to 25 percent. Therefore Cuénot offers the following ex-
planation: Since his yellow mice were never homozygotes, it
follows that the combination of CY with CY can never occur
in fertilization, although Cuénot thinks that other results
show that a disjunction of CY from the other colored charac-
ters takes place. This means that these gametes, CY, can
never meet to give the zygotes having the formule CYCY,

1 Nevertheless yellow must have been latent, as in the case of albinos that
contain a latent color and still breed true.

2 Professor E. B. Wilson has pointed out that even if the CY sperm never
fertilizes the CY eggs, the expectation would still be 75 :: 25, because the CY
eggs would be fertilized by other sperm.

96 Experimental Zoology

AYVAY, and CYAY. In other words, selective fertilization
occurs.

It seems to me that this is improbable and that a simpler
assumption may account for the results. Cuénot’s yellow mice
were obtained through an albino of unknown ancestry. He
crossed them with gray (or with black) mice, and obtained
dominant yellow mice, to which he assigns the formula CYCG.
These when inbred should give, according to Cuénot, on the
theory of disjunction of the gametes, CY- and CG-gametes.
The offspring would then give the Mendelian proportion—

1 CYCY +2 CYCG+1 CGCG.

But no mice represented by CYCY were obtained. It seems
to me more probable from the results that the yellow does not
separate from the other colors, and if so all the germ-cells
would be on my view CY(CG) or (CY)CG. Such forms
inbred would not give CYCY, as Cuénot assumes, but the
dominant heterozygote CY (CG).

This point of view assumes that the yellow is so slightly
prepotent in the extracted dominant, CY(CG), that the gray
may dominate in half the germ-cells, giving CY(CG) and
CG(CY). If this is true, gray mice would appear in one-fourth
of the offspring of these dominants. The yellows differ on
this point of view from all other extracted dominants in the
failure of the yellow to remain dominant in the germ-cells.-

Schuster has made a number of pairings between gray and
white mice. Seventy of such families (Fy) were gray;* two
families contained yellow mice and gray mice; one family con-
tained four chinchilla mice only; and one contained two chin-
chillas and one gray. The appearance of these yellows in the
first hybrids is ascribed by the author to the presence of yellow
in the white parents,—the yellow dominating the gray of the
first hybrids. Whether the same explanation will account for the
chinchillas is not known, because the dominance of the chin-
chilla has not been tested.

} Containing 342 mice.

Experimental Hybridizing 97

The gray (F,) hybrids were paired with albinos and 537
young produced, of which 261 were albinos and 276 colored —
a close approximation to the expected equality. The colors
were of various kinds, but gray greatly predominated. When
gray (F,) hybrids were inbred, they produced 119 albinos and
308 colored —a rough approximation to the expectation of
I to 3. .

Haacke has published the results of a large number of crosses
between white mice, colored mice, and Japanese waltzing mice.
Although his experiments were not carried out to test the Men-
delian law, yet, as the author points out, they closely conform
to this law. Haacke states his general conclusion in a some-
what involved way, namely, that the possible races of species
equal the number of possible kinds of germ-cells, z.e. equal
a sum that consists of as many factors as a particular species
has independently variable qualities or germinal portions. In
the sum each factor equals the number of all possible modifica-
tions of its properties.

LITERATURE, CHAPTER VII

ALLEN, G.M. The Heredity of Coat Color in Mice. Proc. of the Amer.
Acad. of Arts and Sci. XL. 1904.
Bateson, W. The Present State of Knowledge of Color-Heredity in Mice
and Rats. Proc. Zool. Soc. II. Londen. 1903.
Castte, W. E., and ALLEN, G. The Heredity of Albinism. Proc. Amer.
Acad. of Arts and Sci. XXXVIII. 1903.
Cutnot, L. La loi de Mendel et l’hérédité de la pigmentation chez les
souris. Arch. Zool. Expér. et Gén. Sér. 3, X. Notes et Revue.
1902.
La Toi de Mendel et 1’hérédité de la pigmentation chez les souris.
Compt. Rend. CXXXIV. Paris. 1902.
L’hérédité de la pigmentation chez les souris (2™° note). Arch. Zool.
Expér. et Gén. Sér. 4,1. Notes et Revue. 1903. :
L’hérédité de la pigmentation chez les souris (3"° note). Arch. Zool.
Expér. et Gén. Sér. 4, II. Notes et Revue. 1904.

Les Races Pures et leurs Combinaisons chez les souris. Arch. de
Zool. Expér. et Gén. Sér. 4, III. Notes et Revue. 1905.
DarsisHIrE, A. D. Note on the Results of crossing Japanese Waltz-
ing Mice with European Albino Races. Biometrika, II. 1903.
Second Report on the Result of crossing Japanese Watzing Mice with

European Albino Mice. Biometrika, II. 1903.

H

98 Experimental Zodblogy

Third Report on Hybrids between Waltzing Mice and Albino Races :
On the Result of crossing Japanese Waltzing Mice with “Extracted”
Recessive Albinos. Biometrika, II. 1903.
On the Result of crossing Japanese Waltzing Mice with Albino Mice.
Biometrika, III. 1904.
DaveENporT, C. Review of von Guaita’s experiments in breeding mice.
Biol. Bull. II. 1g00.
Mendel’s Law of Dichotomy in Hybrids. Biol. Bull. II. r1gor.
Color Inheritance in Mice, Wonder Horses and Mendelism. Sci-
ence, n. s. XIX. 1904.
Guaita, G. von. Versuche mit Kreuzungen von verschiedenen Rassen
der Hausmaus. Ber. Naturf. Ges. zu Freiburg, X. 1898.
Zweite Mittheilung ueber Versuche mit Kreuzungen von verschiede-
nen Hausmausrassen. Ber. Naturf. Ges. zu Freiburg, XI. 1900.
HAAcKE, W. Ueber Wesen, Ursachen und Vererbung von Albinsmus
und Scheckung, und ueber deren Bedeutung fur vererbungs theo-
retische und entwicklungs-mechanische Fragen. Biol. Centralbl.
XV. 1875
Die Geseze der Rassenmischung und die Konstitution des Keim-
plasmas zuchtanalytisch ermittelt. Arch. f. Entw. mech. der Or-
ganismen, XXI. 1906.

CHAPTER VIII

EXPERIMENTS WITH OTHER MAMMALS AND WITH BIRDS

Experiments with Guinea Pigs

THE hair of wild guinea pigs or cavies shows the same three
pigments present in mice, viz. black, chocolate, and yellow.
Domesticated cavies may show the following colors: the agouti
cavy with the hair containing the black, yellow, and chocolate
pigment. The yellow cavy with only yellow pigment. The
red cavy is a dark, yellowish red. The chocolate cavy has
chocolate pigment predominating. The black cavy has black
pigment predominating over the other two. The albino cavy
is principally white, although an entirely white individual seems
never to occur; for pigment is generally found in the extremities
of the body, on the feet, or nose, or ears, and sometimes in hairs
on the body also.

In addition to these there occur spotted or pied races. Any
one of the colors may be combined with white. A great variety
of markings result in this way, since the pigment areas may be
of different colors.

Brindled cavies have black and red hairs interposed in the
same patches. Roan animals have white hairs interspersed
with red ones. Silvered cavies have black and white hairs.

Castle has carried out through several years experiments
with differently colored and marked races of these animals.

Crossing Uniformly Colored Types. — The self-colored races,
as well as the albinos, breed true: thus Castle found albinos bred
inter se gave 156 young, all albinos; pure pigmented individ-
uals gave 261 young, all pigmented. On the other hand, al-
binos mated to pigmented animals gave in the first generation

99

100 Experimental Zoology

314 young, all pigmented ; and these last give in later generations
some albinos. The results agree fairly closely with Mendelian
expectations.

Castle has found, as had Allen and Cuénot for mice, that
albinos, although breeding true, may carry latent or suppressed

Fic. 7. Upper figure, short-haired, smooth coat red guinea pig. Lower figure,
long-haired, rough coat albino. (After Castle.)

colored characters. Thus one albino individual, a male, when
mated with red females produced offspring marked with black.
Another albino crossed with the same and with other red females
gave young marked with black and red in about half the cases,
the other half showing only red or yellow, but no black. A third
albino mated with red females produces only red or yellow
offspring, never black ones. In the light of these results, Castle

Experiments with other Mammals 101

makes a distinction between recessive and latent characters.
Recessive characters are those that disappear in the Mendelian
sense when brought into contact with a dominant character.
This recessive character is transmitted distinct from the domi-
nant character in half the gametes of the hybrid. A latent
character is a “condition of inactivity in which a normally domi-
nant character may exist in a recessive individual or gamete.”
This latter is illustrated by the latent pigment carried by ex-
tracted albinos. Cross-breeding is necessary to bring the latent
character to activity again.

When elementary pigmented types, or so-called pure races,
of different colors are crossed, neither color dominates perfectly
in the offspring. For example, when a black individual is
mated with a red one, the offspring are blackish, but not so black
as the black parent, red being also present, although masked
somewhat by the black. The agouti type carries the three pig-
ments black, chocolate, and red-yellow. When crossed with
pure black it gave in one case one agouti and two black indi-
viduals.

Agouti crossed with red gave four young, three of them agouti
spotted with red, one red spotted with agouti and white. Black
animals seem to contain always some red pigment, which may
appear when crossed with albinos. Red animals may, however,
be free from black. Red crossed with white gives results that
depend on the latent pigment characters borne by the albino,
so that black offspring may appear among the others. A case
of special interest is found in white animals with black eyes,
which are therefore not albinos.t They do not seem to be al-
binos but may contain recessive albinism. They arose from
spotted ancestors, and Castle regards them as spotted animals
themselves with the pigment spots obliterated except in the eyes.
These animals bred inter se or with albinos produce offspring
with colored patches of greater or less extent. Whether by selec-
tion the white animals with black eyes could be made into a fixed
race remains to be shown, and judging from what breeders of

1 Two such animals appeared in Castle’s experiments.

102 Experimental Zoology

cattle have been able to do it seems not improbable that this
might be accomplished.

Castle draws attention to the curious point that red and yel-
low cavies, having no black pigment in their coats, do not trans-
mit black coat-pigment to their offspring, although they do
transmit black eye-pigment. It would be erroneous, he thinks,
to conclude from this that the eye-pigment is something alto-
gether different in its inheritance from coat color, because when
mice with coat patches but devoid of eye-pigment are mated
with albinos, the offspring have pigmented eyes —a character
that neither parent possessed

Heredity of the Rough Coat. —Some races of domesticated
guinea pigs show the hair arranged in whirls or rosettes. When
best developed the rosettes are found around the following
paired centers: (1) the eye, (2) a point immediately behind the
ear, (3) the shoulder, (4) a point dorso-lateral on the body,
(5) the hip, (6) the groin, (7) each of the single pair of mamma;
and from two unpaired centers, viz. (8) the middle of the fore-
head, and (9) the navel. The direction of the hair is also re-
versed on the toes.

These rough-coated individuals breed true. When crossed
with smooth-haired individuals the rough character dominates.
The rough character of the offspring is usually as fully developed
as in the rough parent. However, certain smooth individuals
when crossed bring about a weakened condition of the rough
character, some of the rosettes being less developed or even
absent. These partially rough individuals may transmit to
their descendants the fully rough condition. The result is
important in that it shows that what we must regard as a new
character in the species, yiz. a rough coat, dominates when a
back cross is made.t On the other hand, Castle has also found
that repeated crossing of rough individuals with prepotent smooth
ones results in further weakening of the rough character until it
is almost eliminated — one after another of the rosettes disap-
pearing. The weakening does not follow a definite decline, but

* See Castle’s analysis, pp. 47-50.

Experiments with other Mammals 103

in some cases may be slow, in others sudden, so that the inter-
mediate steps are passed over at once.

Heredity of the Long Coat. — A race of long-haired cavies is
known, either smooth (Angoras) or rough (Peruvians). The
two sets of characters, long versus short, and smooth versus

La wR eg AN a

Fic. 8. Upper figure, long haired, smooth coat albino guinea pig. Lower
figure, short-haired, rough coat, black-red pigmented guinea pig. (After Castle.)

rough, are independent of each other in their behavior in heredity.
The long-haired is recessive in relation to the short-haired or
ordinary types. The long hair is not present at birth. The coat
of the short-haired guinea pigs reaches its maximum length
(about 4 cm.) not far from the age of one month, and is then
gradually shed and renewed. On the other hand, the hair of the
long-coated animals is apparently not shed at this period, but

104 Experimental Zoology

keeps on growing. At three months it is 6 tog cm. long. If
shedding now takes place the animal never acquires longer hair;
but animals that reach the age of four months without shedding
their longest hairs may ultimately acquire hair 14 to 16 cm. long.

It has been stated that long hair is recessive to short hair.
When two long-haired individuals are mated, having hair of
different lengths, the offspring have hair like that of the shorter-
haired individual. In the next generation both long- and
short-haired individuals occur, but the number of long-coated
individuals exceeds the Mendelian expectation.

A pure albino, long-haired, and rough-coated individual, was
crossed with nine diffcrent pure, pigmented, short-haired, and
smooth females. Twenty-nine young were produced, all pig-
mented, short-haired, and rough-coated. Thus while two of
the dominant characters come from the smooth-haired father,
the third comes from the rough mother — the rough coat dis-
sociating from the long hair to influence the arrangement of the
short hair of the progeny.

Castle and Forbes have recently extended Castle’s earlier
experiments with short- and long-haired guinea pigs. When
crossed the short hair dominates, but the hair of the hybrid is
nearer the upper limit of variation of the pure, short-haired type,
showing perhaps the influence of the other character. When
the hybrids were inbred not only the two grandparental types
reappeared, but a new intermediate type was present. This
type contained individuals of all intermediate grades between
the long- and short-haired types. It contained, first, individuals
whose hair grew continuously from the age of twenty days on,
but much more slowly than does the long hair, and shows a ten-
dency to break off at lengths much less than that of the long hair.
The long hairs were less numerous, as if some of the hairs only
were continuous in growth, while the other hairs ceased to grow
after a time. The alternative characters behaved somewhat
like a mosaic. In other individuals the hair ceased to grow,
z.e. was determinate in growth, but not until it had reached a
length of 60 to80mm. These cases seemed to be a blend, but no

Experiments with other Mammals 105

sharp line existed between the blend and the mosaic condition.
In all there were 29 short-haired, 12 intermediate, and 10 long-
haired individuals. The Mendelian expectation for these 51
individuals would be 38 short-haired and 13 long-haired. Thus
there are fewer long-haired and fewer short-haired individuals,
and the presumption is that each kind has contributed to the
intermediate group.

When a long-haired individual was mated with a short-
haired dominant-recessive (which gives in other Mendelian cases
equal numbers of dominant-recessives and recessives) there were
produced 14 short-haired, 17 intermediate, and 19 long-haired
offspring. The results seemed to show that many of the germ-
cells of the dominant-recessive had intermediate characters,
and were not segregated into the two groups of pure Mendelian
gametes. The authors conclude that the intermediate group is
probably a new hybrid combination, and that its germ-cells do
not split up into long- and short-haired types.

A few other experiments were also carried out that seemed to
show that cross-breeding produces a “contamination of the
gametes.’ Instead of pure types separating in the germ-cells
of the hybrids, some mixing occurs. Another cross increases the
amount of contamination, or at least it produces a larger number
of intermediate forms. The authors appear to look upon the
“contamination”? as a partial or incomplete separation of the
characters in the germ-cells of the hybrid. The evidence may be
equally well interpreted, I think, to mean that the results are
due to incomplete dominance of the dominant character. In
other words, it seems to me that these results may be more easily
“explained”? on my interpretation of the behavior of the
gametes in Mendelian cases than in the ‘“‘modified” Men-
delian explanation of the authors.

Castle and Forbes found in a family of short-haired guinea
pigs a few individuals with hair about twice as long as that of
their parents. Mated together they produced all long-haired off-
spring of the same kind. By selecting the best long-haired
individuals for two generations a race of imperfectly long-haired

106 Experimental Zoology

guinea pigs was produced, which compared favorably with the
intermediate groups described above. Crossed with long-haired
individuals two kinds of offspring were produced, with long and
intermediate hair, but with no definite line of separation.

Experiments with Rabbits

Hurst has carried out a series of experiments with rabbits that
have given results of unusual interest, especially in connection
with the inheritance of color and of length of hair. Two races
that were known to breed true were used, namely, white An-
goras and Belgian hares. The former is an albino breed with
pink eyes and silky hair. These animals have a peculiar habit
of swaying the head when at rest. The Belgian hare has a pig-
mented skin, dark eyes, and short yellow-gray fur. When crossed
the hybrids were pigmented like the Belgian hares, but the hair
had lost the yellow color and was gray, like that of the common
wild rabbit. When these first hybrids were inbred they produced
14 distinct types in the second generation, viz. : —

Hair short, pigmented, gray, uniformly colored.
Hair short, pigmented, gray, marked.

Hair short, pigmented, gray, Dutch marked.
Hair short, pigmented, black, uniform.

Hair short, pigmented, black, marked.

Hair short, pigmented, black, Dutch marked.
Hair short, albino, white.

Angora, pigmented, gray, uniform.

Angora, pigmented, gray, marked.

Angora, pigmented, gray, Dutch marked.

. Angora, pigmented, black, uniform.

12. Angora, pigmented, black, marked.

13. Angora, pigmented, black, Dutch marked.
14. Angora, albino, white.

DO MW AT AY vo

ee
H

This “ epidemic of variation” in the second generation of hybrids
has been the common experience of experimenters both in ani-
mals and plants, and before the Mendelian principles became

Experiments with other Mammals 107

known remained practically unexplained. By the aid of the
Mendelian principles we are able to see at once that there are at
least four pairs of distinct characters concerned in the offspring
of the second generation, each pair being inherited independently
of the other; namely, short hair versus long hair, pigmented coat
versus albino, gray versus black coat, uniform versus marked
coat. Hurst takes up these four pairs of contrasting characters
and deals with them separately.

Short Hair versus Long Hair.—The short hair of the Belgian
breed rarely exceeds one inch, while the long hair of the Angora
breed may exceed six inches. The first hybrids had short hair,
the influence of the Angora not being apparent. A careful ex-
amination, however, revealed what appeared to be faint traces
of the Angora influence in both length and texture. The hairs
of the hybrid coat were slightly longer, seemed softer to the touch,
and were apparently more densely distributed than in the pure
short coat. These traces of Angora influence are slight and
might easily be overlooked.

When these short-coated hybrids (7) were inbred, there were
produced 171 young, of which 70 reached the age of two months
or more, when the character of the hair becomes manifest. Of
these 53 were short- and 17 long-haired. This is a close ap-
proximation to the Mendelian expectation of 51 to 17. The
short hair was like that of the grandparent, the long hair like
that of the other grandparent. When the long-haired Angoras
were bred together they produced in the next generation only
Angoras. The short-coated individuals mated together gave
both short-haired and Angoras in Mendelian proportions.

Pigmented Coat versus Albino.—The cross gave in one case
26 totally pigmented individuals; in another case the fore-feet
showed some white markings, which Hurst thinks is not due
to the albino influence, but to Dutch marking latent in the
albinos. The hybrids, when inbred, produced 132 pigmented
and 39 albinos, the Mendelian expectation being 129 : 43.

1 In the marked coat there is white on the ends of the feet and the tip of the
nose. ‘The Dutch is an extreme form of this marking.

108 Experimental Zoology

The albinos subsequently bred true; the pigmented types were
of two kinds, — pure and hybrid.

Gray versus Black. — When the yellow-gray Belgians were
mated to the white Angoras, wild gray hybrids were produced.
These, as stated above, when inbred, gave both colored and white
offspring in Mendelian proportion, but some of the colored in-
dividuals were black instead of gray. There were 85 grays to
25 blacks. The grays, as stated, were like the wild gray instead
of the yellow gray of one grandparent, although a few appeared
to contain somewhat more yellow than the wild type. The blacks
had no gray, but it is interesting to note that after the first molt
a few white hairs appeared, which increased in number with each
molt until some of the individuals resembled the silver-gray
breeds (chinchilla). The blacks when inbred produced only
blacks, the grays were of two kinds, — pure and hybrid.

“The sudden appearance of the black character in the sec-
ond generation was quite unexpected as there had been no black
individuals in the ancestry of either of the original parents . .
for at least eight generations, and probably many more. The fact
that these black individuals appeared in about the proportion
of one quarter, and bred true at once, was very significant from
the Mendelian point of view. It suggested that the hybrid grays
of the first generation were giving off gametes, one half of which
contained the factor for black coat color. That it was not intro-
duced by both is clear from the absence of black in the first gen-
eration.”’ It could not have been introduced with the Belgians
because these mated to black gave only grays. The black
must, therefore, have been introduced with the albinos. Hurst
carried out some further experiments that seemed to substantiate
this view. One male, albino Angora mated with four black does
produced 16 black young; but another albino female mated
with black produced 5 black and 6 gray young. Hurst inter-
prets these results to mean that the first albino gave off gametes
that all carried black, while the second albino gave off gametes
some of which carried black, others gray. When the white male
and the white female used in these experiments were mated, only

Experiments with other Mammals 109

white offspring were produced; but when one of these offspring
was mated with a pure black, 5 blacks and 1 gray were produced.
The albino must have carried, therefore, both gray and black.

Unijorm versus Marked Coat. — It was found that the uniform
coat of the Belgians breeds true, but mated with albinos gave,
in two cases, somewhat different results: in one case the off-
spring showed no trace of markings (white); in the other case
15 of the young were marked with more or less white on the fore-
feet, shoulders, breast, nose, and forehead.

The subsequent history of these two sets of offspring was as
follows: (a) Three of the uniform or self-colored individuals
were bred together and with a pure self-colored individual, pro-
ducing 35 self-colored and 2 slightly marked individuals having
a few white hairs on. the tip of the right paw. The third gen-
eration of the uniform individuals gave only uniform offspring.
(6) Four of the marked individuals produced 67 young, of which
16 were uniform, 34 were slightly marked, and 17 had the maxi-
mum of white (Dutch markings). In the third generation
three of the uniform individuals produced 14,uniform and 1
slightly marked. Also in the third generation the Dutch marked
produced 10 young, all Dutch; and three of the marked indi-
viduals produced 3 uniform, 12 marked, and 2 Dutch. Hurst
interprets these results to mean that one of the Angoras, al-
though of pure stock, contained Dutch markings, latent, and
when crossed these appeared. When these gametes united
with the pure uniform gamete the slightly marked individuals
of the first generation were produced. Afterward segregation
of the gametes occurred, so that subsequently gametes for uni-
form and for Dutch markings appeared in about equal numbers
with the result that in the second generation there were one
quarter pure uniform, one half hybrid marked, and one quarter
pure Dutch. It is interesting to note that this analysis leads to
the conclusion hat “the coat-pattern characters — unlike the
previous characters dealt with —are neither dominant nor
recessive toward one another, but when crossed give intermediate
hybrids in the first generation. In the second and third gen-

110 Experimental Zovlogy

erations, however, these characters appear to follow the ordinary
Mendelian rules of segregation and gametic purity.” Hurst
calls attention to the similarity of these results to those of Cuénot
with spotted mice. The results are in agreement as far as the
marked coat is a unit-character following Mendelian lines, and in
so far as it may be carried by albinos in a latent state. The two
results differ in that the marked coat in mice is completely re-
cessive to uniform coat, while the Dutch markings in rabbits are
neither dominant nor recessive at first, but give variable hybrids.

Castle has made a few experiments with rabbits, but the re-
sults gave little that was new in principle. ‘A cross between
two different types of albino rabbits, Himalayan and pure white,
shows imperfect dominance of the Himalayan character in the
offspring, but complete segregation among their gametes.”
Long-haired rabbits bred to short-haired individuals give off-
spring with short hair. When rabbits with ears of different
length are mated, the offspring have ears intermediate in length.
In this character, blending appears to take place, and neither
dominance nor segregation.

Experiments with Rats

Rats have been used much less than mice, and the results seem
to be more complicated. Crampe has published the results of
a large number of experiments, extending over ten years; but as
ihe experiments were made before the importance of Mendel’s
theory was appreciated, it is difficult to interpret from this point
of view the data obtained. No more striking instance could be
given of the insight into cross-breeding experiments furnished
by Mendel’s law than a comparison of the work before and after
this period. Confused and irregular as the earlier results appear,
they arrange themselves into orderly groups in the light of this
law. It is, of course, difficult now to show in all cases that Men-
del’s law unravels Crampe’s results, since the records are often
incomplete on important points, where further tests are requi-
site to interpret the result. Nevertheless Bateson’s analysis
of Crampe’s data indicates that the outcome shows in many

Experiments with other Mammals III

cases the Mendelian expectation. Crampe found that when
wild gray rats, Mus decumanus, were bred to albinos, the off-
spring (#*) were of two kinds, viz. either (I) gray like the wild rat,
or (II) gray with white marks. If the former (I) were bred inter
se, the following types appeared : —

Self-gray.

Gray with white marks.
White and gray.

White (albino).
Black-and-white.

Black with white marks.
Black without marks.

OD eS Goa wl

If the other group of offspring (II) was used, z.e. inbred, all of
the preceding types except 3 (white and gray) and 5 (black-and-
white) were produced.

Bateson states that the great variety of types that appear here
is difficult to interpret, but that such occurrences are by no means
uncommon. He suggests that two classes of germ-cells may
be present either in the albinos or in the wild gray rat.

The albino is recessive to all the other six types, as shown by
crossing these with albinos. The extracted albinos bred inter
se, whatever their origin, gave only albinos. In this connection
Crampe makes another statement of interest. Albinos that
‘had been bred true for several generations behaved differently
from extracted albinos. The former albinos were simply re-
cessive on being crossed with colored rats, while the extracted
albinos gave a mixture of ancestral types when crossed with
colored types. The result appears to be similar to Cuénot’s
with mice where the ancestry of the albino appears as a factor
in the product.

On breeding inter se each of the seven types given above (F;),
Crampe found that the offspring (F,) belonged to the following

types : —
Type 1 might give types I 2 4
Type 2 might give typesI 2 3 4

112 Experimental Zoology

Type 3 might give types 3.4 5

Type 4 might give types 4

Type 5 might give types 4.5

Type 6 might give types 45 6 7
Type 7 might give types 4 6 7

Thus the wild type 1 is dominant to all the others, z.e. its
offspring may belong to any one of the other types which must
have been recessive in its germ-cells. The gray forms 1, 2, 3,
are also dominant, in the same sense, to the black forms. The
albinos give albinos only. “It appears that types 3 and 5 could
be ultimately bred true. As to 6 and 7, the evidence is not very
clear; but as I understand the account, neither was completely
freed from throwing the other. The breeding in these types
was the least successful and extensive. Possibly they are illus-
trations of the Mittel-rassen of de Vries. It is especially
noteworthy that the gray-and-white type 3 and the black-and-
white type 5 do not give rise to self-gray gametes or to self-
black gametes, a fact found again in mice. We see, therefore,
that there are gametes for black-and-white and for gray-and-
white, each of which may behave as a single character and domi-
nate over albino.” *

When pure black-and-white rats were crossed with the wild
gray rats, all the colored types might appear in generation (F,)
except albinos. In other words, the black-and-white do not
separate, they are not resolved in the germ-cells, as other experi-
ments also indicate. Crampe found further that black-and-
white individuals that gave albinos in the first generation when
bred inter se also gave albinos when bred to albinos. In this
case the black-and-white individuals had probably arisen from a
cross between black-and-white and albino, so that the albino
(and not the white of the black-and-white) gave the white mice
just mentioned. On the other hand, Crampe found that when
the black-and-white rats did not themselves throw albinos, they
did not do so in the first generation when bred to albinos.

1 Bateson, Proc. Zodl. Soc.

Experiments with other Mammals LL3

In the light of this analysis that Bateson has made of Crampe’s
data, there can be little doubt that Mendel’s law applies to many
at least of the phenomena of heredity in rats. This is made
highly probable by the recent results of Doncaster. He finds
that there are only two color types, brown (or gray) and black, and
only two color patterns in the colored-and-white individuals.
In addition there are albinos, but these may carry in a latent
condition either the uniform black or brown color, or the pie-
bald markings. Crosses between the black or the brown wild
rat with the albino may bring out the latent characters of the al-
binos. Doncaster points out that Crampe’s work shows that
brown (gray) dominates black, and both brown and _ black
dominate white. The self or uniformly colored races 1, 2, 6, 7
(and those having small white areas below) dominate the pie-
bald condition 3 and 5.

Doncaster states that Crampe’s brown forms 1, 2, 3,
correspond exactly with the similar black forms 7, 6, 5, ‘‘but are
less simple to work with since they may contain recessive black.”
He finds two varieties of type 6, one with much white and one
with very little. The latter belongs, in his opinion, to the uni-
form type. Crampe failed to make this deduction, so that one
of his forms was probably heterozygous. Evidence that the
inheritance in rats is Mendelian was found by Doncaster in a
number of the crosses made to test this question.

Experiments with Cats

Doncaster has brought together-a number of records, ob-
tained from owners of pedigree cats, that show the color in-
heritance of certain breeds. He examined more especially the
question as to why tortoiseshell cats are nearly always females.
His conclusions, as will be seen, have an important bearing on
the problem of dominance in relation to sex. Tortoiseshell kit- .
tens may be obtained in any of the following matings : —

(a) Tortoiseshell ? by tortoiseshell ¢
(b) Tortoiseshell ° by black or blue ¢

I

114 Experimental Zoology

(c) Tortoiseshell 9 by orange s
(d) Orange 9° by orange $
(e) Orange ? by black or blue ¢
(f) Black or blue 9 by orange 8

In all of these matings, in addition to tortoiseshell, kittens of
other colors may appear, viz. : —

(a) Tortoiseshell 2 bytortoiseshell $ gives tort., orange, black.

(b) Tortoiseshell 2 by black or blue ¢ gives tort. ?,
orange ¢, black ¢, @.

(c) Tortoiseshell 2 by orange ¢ gives tort., orange, black.

(d) Orange ° by orange ¢ gives either tort., orange (or
blue) or only orange.

(e) Orange ? by black ¢ gives tort., ?, orange ¢.

(f) Black @ by orange ¢ gives tort., black (and probably
orange).

(g) Black @ by black ¢ gives only black (or blue).

From these results it appears that tortoiseshell is a heterozy-
gous color produced by the meeting of orange and black gametes.
The explanation that tortoiseshell cats are nearly always females
and rarely males is owing to orange nearly always dominating in
the male over black, while in the female the dominance of the
orange is incomplete, so that tortoiseshell results. In other
words, in the female sex the orange and the black both exist to-
gether, while in the male sex the yellow usually dominates. A
few examples will make the conclusion clearer. For instance,
in mating (e) when an orange female is crossed with a black
male, only tortoiseshell and orange kittens are produced ; if both
the orange and the black breeds are “‘ pure,” the female offspring
are tortoiseshell and the males yellow. In the reverse mating (f),
where a black female is crossed with a male orange, the male may
be heterozygous (i.e. having both black and orange germ-cells),
hence black kittens may also be produced. The kittens will be
black males or females, tortoiseshell females, and orange males.

When a tortoiseshell female is mated with a black male, the
male offspring will be orange, because the tortoiseshell is hetero-

Experiments with other Mammals 115

zygous. It is also evident why orange females are very rare,
although orange males are common, since in all matings in
which one of the parents is black, orange can appear only in the
male offspring. ‘‘If, therefore, the great majority of orange
males contain recessive black, when they are paired with tor-
toiseshells, only a quarter of the kittens will be pure orange, and
only half of these females.”

The preceding statements show the relation of the colors orange
and black. The inheritance of two other colors was also ex-
amined; namely, cream and blue. Cream appears to be a dilute
form of orange, and blue of black. The blues breed true (when
derived from yellow ancestors) and are therefore recessives or
homozygous. A cream female and a blue male give blue tor-
toiseshell (blue and cream), cream males, but no blues, since
the cream dominates incompletely in the female, completely in
the males. On the other hand, a blue female and a cream male
give blue tortoiseshell females, blues of both sexes, and possibly
cream males. These and other results show that the dilute
forms behave in the same way as do the stronger colors. Thus
cream is dominant over blue in the male, but when blue and
cream meet in the female a tortoiseshell results.

It has been stated that male tortoiseshell cats are known,
although they are rare. It must be assumed that in such cases
the dominance of the yellow is incomplete as in the female. This
means that while complete dominance is usually associated
with the male character, it is not necessarily always associated
with this sex. It is interesting to find that when a male tor-
toiseshell is mated with a female of the same color, the kittens
are tortoiseshell, orange, and black. This is what is expected
on the assumption that the germ-cells of the tortoiseshell are
black and orange (with the alternate character latent on my view).
The prepotency of different tortoiseshell individuals (males)
seems, however, to vary.

It should also be pointed out that the colors described above
may be associated with a certain amount of white which reap-
pears in the offspring without, however, affecting the inheritance

116 Experimental Zoology

of the other colors. The piebald character stands as a unit con-
trasted with uniform coat, but is independent of any particular
color.

Data jor Other Mammals and Man

A few other cases in mammals, that seem to show discontinu-
ous inheritance, are known. Castle and Davenport have both
called attention to cases of so-called wonder-horses, 7.e. horses
with remarkably long mane and tail. In the case of “Linus I”
the mane was 18 feet long and the tail 21 feet. The parents
and grandparents of these horses also had unusually long hair,
which increased in successive generations. The data are insuffi-
cient to show the relation of dominance and recessiveness in this
case, but the persistence of the long hair seems to indicate its
dominance."

Harper and Hurst have recently examined certain data in
regard to the inheritance of coat color in horses. Harper deals
with the problem from the standpoint of prepotency of certain
colors in regard to ancestry, selection, age, and sex. Hurst shows
that bay and brown colors dominate completely chestnut, and
there are definite indications that these two colors follow Mendel’s
law.

Some statistics recently published (1904) by A. G. Bell have
furnished Davenport with material to study the relation of black
color to white color in sheep. The data show that when three
white individuals having as far as known white ancestors were
crossed with black sheep, the 13 lambs resulting (F,) were white,
showing the dominance of white. Of 20 offspring from black
parents all were black.” When a black (recessive) individual
was mated with a dominant white (one of whose parents was
white and one black), 26 lambs were white and 2s black, which
is the Mendelian expectation. When a dominant-recessive
white was mated to a dominant-recessive white, 40 were white
and 7 were black. The expectation is 25 per cent black. The

In guinea pigs the long hair is recessive.
? One uncertain case of white is given that is not above suspicion.

Experiments with other Mammals 117

number of black lambs is too small on the assumption that
chance meeting of equipotent “pure” germ-cells brings about
the results.

Poulton * has given some records of polydactyl cats that appear
to be explicable, so far as they go, along Mendelian lines. Three
young were produced from a polydactyl female by an unknown
father. They were all polydactyl. If polydactylism dominates
over the normal condition, this result is simple dominance. One
of these individuals (Ff) produced three litters (by unknown
fathers), in which four normal and six abnormal kittens ap-
peared. If the father was normal, five normal and five poly-
dactyl young would be expected. Thus: —

P+N
N+N
2NP+2NN

Only two kinds of discontinuous inheritance that may possibly
follow Mendel’s law have been shown for man. Albinism, ac-
cording to certain data collated by Castle, may perhaps follow
this rule. The cases referred to were albino negroes. Albinism is,
of course, different in this respect from white. In the latter case,
blending of the black and white occurs to produce mulattoes.

The other case is that of polydactylism. Fachenheim has
given some statistics,” that Davenport has examined from the
point of view of Mendelism. The accompanying table gives the
inheritance through three generations : —

Gen. NxP

| |
I, b ¥ Na Pen) Noa) aka AGN) | re

II. 6N3P4N3P 7N3N 2P8N 2P ee

2N 3P

Si sy N ie ae
wzaN3N2P 3N N

1 Nature, 1883. ? Jena Zeitschrift, XXII, 1888.

118 Experimental Zoology

If polydactylism is dominant and the normal condition is reces-
sive, the chances are that any polydactyi person has had one nor-
mal parent and the germ-cells are therefore P and N.’ Paired
with a normal individual (N + N), half the children should be
polydactyl and half normal. In the above case there were in
fact in the first generation four normal and four polydactyl chil-
dren. In the second generation when normal offspring paired
with normal, 5 polydactyl children and 21 normal were pro-
duced; and when the polydactyl descendants paired with nor-
mal, 7 polydactyl children and 12 normal were produced.
For the small number recorded, the latter result is not very
different from 1:1, the Mendelian ratio. Again in the third
generation when P was mated to N, 5 normal and 5 polydactyl
children were born.

Struthers gives the following case of polydactyl inheritance in
man : —

I. Px ?
1 P(xN) 1oP
II.
3P. rt P(xN)
II. 4N 4P

The result can only be explained on the Mendelian view byassum-
ing that both parents of the first generation were polydactyls, 7.e.
produced germ-celis bearing polydactylism. It is necessary to
make this assumption in order to account for the second genera-
tion that descended from one of the first filial generation. Here
a polydactyl parent married a normal individual and produced
only polydactyl children, showing that no normal germ-cells
were present in one parent. Had there been some normal germ-
cells, some normal children would be expected, provided the
numbers are really large enough to give this result a chance to
appear. In the third generation an equal number of the two kinds
of offspring are expected, and such are found. A third case is

1 Or P(N) and N(P) on my view.

Experiments with other Mammals 119

given by Struthers. A normal man married a woman who had
six fingers on the left hand. There were 18 children, only one
of whom was abnormal. In this case the polydactylism was
not dominant except in one case; but among the normal chil-
dren in the third generation one polydactyl individual is re-
corded, indicating that polydactylism was in the strain. The
failure of the polydactyl condition to dominate in this case,
except in one instance, shows how unsafe it is to argue from a
few cases to all others. The same character may be a dominant
one in certain strains and not in others.”

The preceding records and observations are made much clearer
by Castle’s recent experiments with polydactylous guinea pigs.
There was born of normal parents a male guinea pig with an
extra toeon the left hind foot. The toe bore a claw, which was not
connected to the foot by appropriate muscular and tendinous
connections.2 From the polydactylous male were obtained 15
individuals with extra toes out of a total of 77 offspring. In sub-
sequent generations, partly inbred, the number of extra-toed
offspring varied. When‘ the male was paired with females from
families in which polydactylism was not known, there were pro-
duced about 6.25 per cent extra-toed young. Females, de-
scended from the original father, that gave the first polydactylous
male, gave 25 per cent extra-toed offspring. Females, them-
selves polydactylous, gave 44 per cent polydactylous young.

Many of the young of the first male had extra toes on both
hind feet, and in several cases they were better developed than
in the original male. The extra toes were supplied with all the
muscles characteristic of functional toes. Castle has traced the
descent of this race through five generations, and has obtained
some important data regarding the inheritance of the anomaly.
He finds that the “potency” of certain individuals is a more im-
portant factor in the transmission of their characters than is their

1 Edinburgh New Philosophical Journal, 1863.

2 Several other cases of inherited polydactylism are given by Gregg Wilson.

3'The same father that produced this “sport” subsequently produced also
five others out of 147 offspring.

120 Experimental Zoology

ancestry. If the various sires are arranged ‘‘in the order of the
respective amounts of polydactylous ancestry which they possess,
we see at once that this is not the order of their potencies, for
those having the same amount of polydactylous ancestry often
differ much in the potency with which they transmit the poly-
dactyl character.”

When polydactylous individuals were mated with normal ones,
the results were far from being uniform. Some of the offspring
have the extra toes greatly weakened; in other cases there is no
toe at all, while in still other cases the extra toe may be fairly
well developed. ‘The inheritance is neither sharply alterna-
tive (Mendelian) nor completely blending.” It is clear that in
its inheritance the extra toe of these guinea pigs does not follow
Mendel’s law. Castle concludes that the extra toe is inherited
in a manner intermediate between blending and alternative in-
heritance. The gametes, he thinks, only partially blend in the
zygote, producing a variable result. “If the inheritance were
sharply alternative, we should expect to get, not a series of gradu-
ated forms, but two or at most three sharply distinct groups,
but this is not the result observed. If, on the other hand, the
inheritance were fully blending, all the offspring of two pure par-
ents, or of two cross-bred parents should be alike, but this is not
the result observed. We are forced to conclude, therefore, that
there occurs a partial blending of gametes [characters] in the
zygote, and a partial segregation as the zygote gives off
gametes.”

Castle points out, further, that partial blending is the more
common result of hybridizing, since both sharply alternative
inheritance and complete blending are rare. By selection the
breeder is able to produce an almost pure race by picking out the
more potent individuals in each generation. It is interesting to
note that the potency of the male is a germinal variation, tend-
ing toward determinate inheritance, and not simply an extreme
fluctuating variation due to external conditions. Hence, in
selecting prepotent individuals the process involves the choice of
certain individuals that transmit certain qualities in a high degree,

Experiments with Poultry 121

rather than involving the selection of extreme somatic fluctua-
tions. These two kinds of selection may be superficially similar,
but involve in reality an important difference in principle.

Experiments with Poultry

The different breeds of poultry have furnished Bateson and his
co-workers with excellent material for experimental study. The
domesticated breeds differ not only in color, but also in the char-
acter of the combs, in the feathered or unfeathered condition of
the shanks, in the number of toes, in crested and uncrested heads,
and the habit to sit or not to sit on the eggs. These characters
are inherited discontinuously, showing dominance and recessive-
ness, and also often giving the Mendelian ratio.

In his earlier work (begun in 1898 and published in 1902),
Bateson used principally Indian Game and white Leghorn,
but subsequently brown, and white Dorkings and one Wyan-
dotte were used. He found that as a rule pea comb, rose comb,
and extra toe are dominant characters, while single comb and
normal foot are recessive. Nevertheless, the first generation
sometimes shows blending in various degrees, and in consequence
the dominance may be considerably reduced. When the F’s
are inbred, some of their offspring show one character, and
others the other “‘in proportions following Mendel’s law with
some consistency,” but here again the results do not always
conform to the expectation. Other conflicting results are also
recorded that are difficult to explain.

In a recent communication (published in 1905) by Bateson and
Punnett further details are given; and in a supplementary paper
by C. C. Hurst, some experiments with Leghorns, Houdans,
black Hamburgs, and buff Cochins are described. As the re-
sults of Hurst are more easily presented in a less technical form,
I have relied on them mainly in the following account.

The leaf comb of the Houdan is dominant over the single comb
of the Leghorn and Cochin. In a few cases the dominance is
complete, but in the majority of cases it is incomplete — interme-
diate combs being produced.

eh.

Fic. 9. Showing single comb, Fig. 1; Pea comb, Fig. 2; Rose comb, Fig. 3.

122

Experiments with Poultry 123

The Hamburg
rose comb is domi-
nant over the single
comb of the Leg-
horn and Cochin,
the dominance be-
ing complete.

The Hamburg
rose comb is domi-
nant over the Hou-
dan leaf comb, al-
though the latter,
as stated above, is
itself dominant in
other combinations.
In the second gen-
eration (F,) the
Mendelian expecta-
tion is largely rela-
ized.

The white plum-
age of the Leg
horn dominates
over the black of
the Houdan and
Hamburg, and also
over the buff of the
Cochin. In onlya
few cases, however,
is this dominance
of white complete.
In the majority of
cases it is incom-

Psy ams
a
"ene ” Fed,
cormantto av TAL
FREUnALE Pouttiy Jou,

Fic. 10, A. Buff Cochins. (Aeliadble Poultry
Journal.)

plete, the white feathers being ticked with black, or there are
patches of buff or brown. A few exceptional cases were noted
where white did not appear to dominate.

124 Experimental Zoology

The black plumage of the Houdan and the Hamburg domi-
nates over the buff of the Cochin, but incompletely, the black
being marked and shaded with brown.

When the hybrid dominant whites (F;) were mated, the off-
spring (F,) were dominant whites and recessive blacks in the pro-
portion of 3.1 :1. When the hybrid dominant whites (F',) were
mated with pure recessive blacks, there were produced dominant
whites and recessive blacks in the proportion of 1:1. When the
hybrid dominant whites (F,) were mated with a pure buff, they
gave whites and blacks in the proportion of 1: 1.

The experiments in which animals with the normal number of
toes are crossed with races having an extra toe give results of
unusual interest. In general, the extra toe (of the Houdan) is
dominant over the normal foot (Leghorn, Hamburg, Cochin).
In some cases the dominance is complete, z.e. the extra toe is full
size; in other cases all gradations in the size of the extra toe were
found ‘‘down to the mere duplication of the nail.” The extra
toe was found in some cases only on one foot, the other appearing
as in the normal. There were some cases in which the normal
foot appeared to dominate, but whether such cases are real
dominance of the normal, or the failure of the extra toe to appear
in an individual that has it potentially present can only be deter-
mined by subsequent breeding.

When the dominant extra-toed hybrids (F,) were bred together,
they gave dominant extra toes and recessive (apparently normal)
individuals in the proportion of 3.8: 1 ; when the Fy’s were bred
to pure recessives without extra toes, they gave dominant extra
toes and recessives, apparently without extra toes, in the propor-
tion of 1: 1.5.

There were two exceptional cases of F,, in which the normal
foot seemed to dominate. These were a male and a female.
When mated they gave 22 chicks, of which 14 had an extra toe
and 8 had normal feet. The result shows that the parent birds
are really RD’s, since chicks with extra toes appeared when the
birds were bred together. This conclusion was confirmed by
breeding the cockerel to another pure individual with (reces-

Experiments with Poultry 125

sive) normal feet, which gave 11 individuals with and 1 3 without
extra toes. A similar experiment with the hen (F,) gave analo-
gous results. Evidently in this case a character usually domi-
nant has become recessive. It is clear that unless great pre-
cautions are taken, such cases might easily be put down, in other
experiments, amongst the recessives.

The shank feathering of the Cochin dominates over the clear
shank of the Leghorn, Houdan, and Hamburg, but the dominance
is always incomplete. When the F,’s were bred together, they
produced a large number of F, with feathered shanks, and a few
recessive clear shanks in the proportion of 28.7:1. The Mende-
lian expectation is 3:1. In other combinations the expectation
is much more nearly realized. Hurst concludes that ‘‘the Men-
delian principles are at work in these aberrant phenomena, but
are masked by something not yet perceived.”

Hurst’s general conclusions are as follows: Dominant char-
acters are rose comb, white plumage, extra toes, feathered shanks,

126 Experimental Zovblogy

white and blue shanks, crested head, brown egg color, and broodi-
ness; while leaf comb, single comb, black plumage, buff plum-
age, normal foot, clear shanks, uncrested head, white egg color,
and non-broodiness are all recessive to the dominants given
above. Some oj these recessives may, however, be dominant over
others. Thus leaf comb and black plumage are dominant over
single comb and buff plumage that remain recessive.

OPYRICHT ED BY
Z°,RELIAGLE 4”

Fic. 10,C. Silver-spangled Hamburgs. (Reliable Poultry Journal.)

Hurst points out further that dominance may be complete, when
it is indistinguishable from pure dominance, or incomplete,
showing the influence of the recessive character in different de-
grees. For some characters the dominance is always complete;
in some it is always incomplete; and in others it is sometimes
complete, but more often incomplete. The incomplete domi-
nants appear to be about twice as numerous as the complete.

In the second generation F, the dominants are again complete
and incomplete.

It is to be remembered that all the preceding characters behave

Experiments with Poultry 127

independently of each other, there being no correlation between
the characters usually associated with a particular breed, so far
as their inheritance is concerned.

One of the most curious results in crossing black and white
fowls is the occasional appearance of a blue (or Andalusian)
color, which consists of a minute patchwork of black and white.
It has been known for some time that this color does not breed
true, but a pair of such blue fowls gives rise to 25 per cent black,
50 per cent blue, and 25 per cent white. The explanation of
this is that the germ-cells of the blue individuals are black and
white, hence the result; but the special condition that leads to
the occasional formation of blue by the combination of black
and white is not known. Possibly the presence of latent colors
determines the result.

The most recent experiments with poultry are those of Daven-
port. His work confirms many of the results already obtained
by Bateson and Hurst, and also establishes the relation of domi-
nance and recessiveness for some new characters. It is these
latter points that will be especially considered here.

A cross was made between the single comb, black Minorca and
white-crested, black Polish. These races and their hybrids
are shown in Fig. 11; 1-6. In the hybrid (5 and 6) the comb
is single anteriorly and bifurcated behind. There is much variety
in the extent to which the comb is split. In fact, it was single in
one case. Neither parent type can be said to dominate, the
Minorca having a large single comb, and the Polish a much-
reduced bifid comb. Davenport suggests that these two types
of comb may both be dominant types that combine to form the
Y-shaped comb. When the hybrids were inbred, there resulted,
in a total of ror offspring, 29.7 per cent showing single comb,
46.5 per cent Y-shaped comb, and 23.8 nocomb (oronly papillz).
Two interpretations are possible. On the assumption of two
dominant and two recessive types, viz. (1 a) median comb and
(1 b) no median, and (2 a) no splitting and (2 b) splitting, the re-
sults agree more or less with the expectation. But on another
assumption the results conform even more closely, viz. if we

128 Experimental Zoology

assume that single comb and V-shaped comb (that of the Polish)
are contrasted characters, and the Y-shaped comb is a combined

Fic. 11. Male and female black Polish fowls, Figs. 1 and 2; male and female
black Minorca, Figs. 5 and 6; and hybrids, male and female, Figs. 3 and 4.
(After Davenport. )

form constantly reproducing itself. Further work must decide
between these alternative views.

Experiments with Poultry 129

This idea of combined dominance was first suggested by Bate-
son and Punnett to explain the walnut comb of certain hybrids.
Thus when rose comb and pea comb are crossed, walnut comb
results. The gametes produced by these hybrids are of four
types and in equal numbers; namely, single, rose, pea, and wal-
nut, giving when inbred 9 walnut (rose-pea), 3 rose, 3 pea, 1 sin-
gle comb. The interpretation offered by Bateson and Punnett
for these facts is that the characters of the original parents with
rose and pea combs are rose and no pea, and pea and no rose.
The contrasted characters are rose and absence of rose, pea and
absence of pea. When rose and pea bearing gametes meet, the
walnut comb is produced. The results follow the rule for two
characters. In other words, rose and pea combs are not them-
selves contrasted characters, but the allelomorph of each is its
absence. The authors point out, however, the danger involved
of making general assumptions of this sort when two characters
meet.

The nostrils of the Minorca are slitlike, and this dominates
the wide-open nostril of the Polish, but the dominance is imper-
fect in the first hybrids. In the next generation, Fy, the split
nostril is present in 21 per cent, but even in this generation the
high or dominant nostril is frequently imperfect.

The Polish fowls have a large cerebral hernia on the top of the
head, covered by a hardened layer of outer brain coat or dura
mater, and by the skin. The Minorca breed has a normal head.
In the hybrids, F,, not a single case of hernia occurred, but
most of them showed evidence of their mixed ancestry in the pres-
ence of a frontal eminence. In the second hybrid generation,
F,, the hernia reappears again in 23.5 per cent of cases —
a close agreement with Mendelian expectation.

A crest is present in all the hybrids, but always reduced in size.
“The crest is dominant, but dominance is imperfect.” The
crest is larger in the female hybrids, as it is in the female Polish
breed. In the second hybrid generation the crest was absent in
about 30.7 per cent.

Davenport concludes that in this cross the dominance is incom-
K

130 Experimental Zoology

plete in all cases; the nearest approach to typical Mendelian
inheritance is exhibited in the crest, but in the first generation it
is always reduced. The white color of the crest is recessive in
the male hybrids, but is not entirely absent in the females. The
high nostril is recessive, yet it shows its influence in the first hy-
brids. The comb in the first hybrids is different from that of
either parent, yet in the second generation there is a partial
return to the two parent types.

An interesting cross was made between the Japanese long-
tailed fowl or Tosa fowl (Fig. 12; 2) and the white Cochin Ban-
tam (Fig. 12;1). In the Tosa fowl the feathers of the tail show
continuous growth, reaching in extreme cases 18 feet, and
generally 7 to 8 feet.

There is a marked sexual difference in the Tosa breed, but not
in the white Cochin. The male hybrids had the coloration of the
Tosa cock except that every feather was barred with white
(Fig.12; 3). Thefemalehybrids were like the Tosa hen, excepting
that the shafting was much broadened, and the saddle feathers
and the secondaries were black and buff barred. In the second
generation the two original types reappeared. There were 28.1
per cent white (Fig. 12: 4) and 71.9 percent pigmented individuals.
However, of the whites, only five were without reddish pigment,
showing that they were contaminated by the cross. ‘The 41 pig-
mented individuals showed a curiously mixed lot of coloration. Of
41 mature females 6 are like the female Tosa fowl, without barring,
but sometimes with wider shafting than the male Tosa fowl.
The remainder have feathers of the back and wing coverts barred
with lighter, even with white —a condition not found in the female
first hybrids. One of these shows a mixture of female Tosa
and female Partridge Cochin coloration. Asno Partridge Cochin
is involved in the immediate ancestry, this looks like a ‘rever-
sion’; the characteristic has probably lain latent in the White
Cochin. Of 10 males, 2 showed no trace of white, and may
consequently be considered as homozygous. The remainder are
more or less barred with white. One bird shows a remarkable
mixture of Tosa and male Partridge Cochin coloration.” The

Experiments with Poultry rat

statement shows besides the contamination of the second gen-
eration the complexity of the result due to latent characters other
than those patent in the breeds used.

In regard to tail length, it was found that the first hybrid

Fic. 12. White Cochin Bantam, Fig. 1; male Tosa fowl, Fig. 2; hybrid,
Fj, male, between last, Fig. 3; second generation hybrid extracted recessive,
Fig. 4. (After Davenport.)
males developed abnormally long middle-tail feathers, but the
tail was not so long as in the Tosa fowl.

It will be observed that in this cross white is apparently not
dominant, as it is as a rule in other combinations; but evidence of
the white is seen in the barred feathers of the first hybrids.

142 Experimental Zoology

“The white Cochin has no sexual dimorphism in plumage color,
while the Tosa fowl is strongly dimorphic. Every one of the
first hybrids is dimorphic in plumage coloration, the two sexes
resembling, except for the white, respectively the female and the
male Tosa fowl. It is striking to see how from a germ-cell of
the male Tosa fowl either a bird colored like the male Tosa or a
bird colored like the female Tosa may arise. The male germ-
cells contain the Anlagen not only of the male characteristic but
also of the female characteristic.”

In another case two races, both having sexual differences, were
crossed. These were the Tosa and the dark Brahmas. The
hybrids were also sexually different, showing the dominant color
of their respective sex. Thus the red wing-bar and white wing-
bar are found in the males, and the shafting and penciling of
females in the female hybrids.

There are two races of fowls that have aberrant feathers. In
the Frizzled fowl (Fig. 13;2) the contour feathers have a shaft con-
vex inward so that the feather is lifted up and even turned forward.
The primaries of the wings show groups of the barbs that are
twisted in corkscrew fashion. In the Silky fowl (Fig. 13; 1) the
contour feathers are like down feathers, with a weak shaft, and
the barbs are subdivided, producing a fluffy effect. The quill
feathers of the wing and tail are less modified. The Silky fowls
used in the experiments were white, the Frizzle were dark (black,
red, and buff).

Some of the hybrids were white (Fig. 13; 4) and some were dark
(77-4 per cent). Here the white is neither dominant nor reces-
sive, but the explanation of the result is not clear unless one or
both races are impure in their color inheritance. None of the
hybrids showed any silkiness, which is, therefore, seen to be reces-
sive to non-silkiness. Only 6 of the 10 hybrids were frizzled, the
other four having flat feathers. Davenport explains this ap-
proach to equal numbers on the ground that his frizzled animals
produced both frizzled and plain bearing germ-cells.

In another cross the breed of black-breasted red Game
(Fig. 14; 2), in which the tail and rump are entirely absent, was

Experiments with Poultry 133

crossed with the white Leghorn breed (Fig. 14; 1). Of 24 hytrids,
12 were white (Fig. 14; 3), or prevailingly so; black was usually
present, and more rarely some buff. The other 12 hybrids were

Fic. 13. Silky fowl, Fig. 13 Frizzle fowi, 2; extra toe, Fig. 3; hybrid, Fy,
between Silky and Frizzle, Fig. 4. (After Davenport.)

black and white barred, or black with reddish. The tail and
rump were normal. Rumplessness is recessive in the strain
used by Davenport.

134 Experimental Zoology

Aside from the details of Davenport’s work, his chief results
consist in showing that in nearly all characters examined the
influence of the recessive character is to be seen in the first hy-
brids, and even in the second hybrids, so far as obtained, the

Fic, 14. Single-comb White Leghorn, male, Fig 1; rumpless Game male,
Fig. 2; hybrid, F), between last two races, Fig. 3.

impurity of the offspring is often apparent. As yet in only
a few cases have the experiments been carried to the second
generation. The results are evidently complicated by the im-
purity of some of the strains that were used and by the presence

Experiments with Pigeons 135

of latent characters in these strains. Despite the fact that some
of the birds were bought as ‘‘pure’’ stock, i.e. stock that breeds
true as long as inbred, yet the presence of latent qualities in
them is admitted. Under these circumstances it does not seem
to me “contrary to experience” to admit that “pure” strains
carry latent characters.

Davenport contrasts the dominance versus the recessiveness
of new characters with the original characters of fowls as indi-
cated by their presence in the wild parent species of Indian game
and of Aseel. He finds that the new characters dominate as
often as they are recessive. Hence there is no reason to suppose
that new characters are at a disadvantage in respect to dominance
as contrasted with old characters.

Experiments with Pigeons

Darwin has given the results of several experiments in crossing
pigeons. There are also several other recorded results of hy-
bridizing races of pigeons. The hybrids (F,) appear to be
variable in color, but some of the markings peculiar to the wild
Rock Pigeon are apt to appear. For example: A male ‘ Nun”
that is white, with head, tail, and primary wing feathers black,
was crossed with a red “‘Tumbler.” Neither parent had any
blue in the plumage, nor bars on the tail or wings. Of the sev-
eral young, one was red over the back, but the tail was as blue
as that of the Rock Pigeon; two others were quite similar; a
fourth was brownish and the wings showed a trace of a double
bar; a fifth was pale blue over the back, breast, and tail, but
the neck and primary wing feathers were reddish; the wing
had two distinct red bars.

When a black ‘“‘Barb” was crossed with a red ‘‘Spot,” the
young were black, or dark, or pale brown, sometimes slightly
piebald with white. Six of these birds had double wing bars,
etc. When a black Barb was crossed with a snow-white Fan-
tail, some of the hybrids were black with a few white feathers,
others were dark, reddish brown, and others snow-white. None
of them had wing bars.

136 Experimental Zoology

These and other experiments with pigeons show that the results
are more complicated than in the case of fowls. It is not possi-
ble to bring the results under a single point of view at present.
Ewart’s experiments’ with pigeons indicate that while certain
kinds of crosses may give rise to offspring resembling the Rock
Pigeon, yet in other cases a more immediate ancestral color may
comeout. A dark blue Fantail, having all the characteristic bars
of the Rock Pigeon, was bred to a less pure blue Fantail. On
two occasions an absolutely pure white Fantail was produced.
This result, Ewart thinks, is due to a reversion to a white grand-
parent. Ewart also crossed a white Fantail with a white Pouter.
The offspring was white in color, but in form resembled the
Pouter. A hybrid between an “archangel” and an “owl” was
bred to a white Fantail. The two offspring were blue, one of
them being almost identical with the wild Rock Pigeon, more es-
pecially with the Indian variety. Not only was there reversion
in color, but in form as well.

Ewart seems to think that reversion amongst closely inbred
races of dogs, horses, and pigeons leads to a sort of rejuvenes-
cence of the stock. Those individuals showing the ancestral
characters prove to be stronger and more active.

The reversion to the type of the Rock Pigeon, that seems to play
often so conspicuous a réle in these experiments with pigeons,
recalls the return to the gray color in mice when fancy breeds
are crossed, but in the second generation of mice there is a re-
turn in some of the forms to the parent types. How far this
occurs in pigeons is not clear from the evidence at hand.

A brief but important note in alternate inheritance in pigeons
is given by Staples-Browne. Webfoot sometimes suddenly
appears in pigeons. A pigeon of this sort crossed with another
having normal feet produced six normal-footed offspring. These
individuals (F,), inbred, produced in one case nine with normal
feet and three with webbed feet. Another pair (Ff), however,
produced seventeen normal birds. Extracted web-footed
individuals produced six web-footed. It appears that the latter
condition is recessive to normal feet.

1 The Penycuik Experiments.

Experiments with other Mammals 137

LITERATURE, CHAPTER VIII

AHLFELD. Missbildung des Menschen. Leipzig. 1880.

Battowitz, E. Ueber hyperdactyle Familien und die Vererbung der
Vielfingerigkeit des Menschen. Arch. f. Rassen und Gesellschafts
Biologie, I. 1904.

BarRINGTON, A., and Pearson, K. On the Inheritance of Coat Colour in
Cattle, Part I. Biometrika, IV. 1906.

Bateson, W., and Saunpers, E. Experimental Studies in the Physiology
of Heredity. Reports to the Evolution Committee of the Roy. Soc.
Report I. London. 1902. Report II. rg04.

BATESON, W., and Punnett, R. C. A Suggestion as to the Nature of the
“Walnut” Comb in Fowls. Proc. Cambridge Phil. Soc, XIII. 1905.

BARDELEBEN, K.v. Hand und Fuss. Verhandl. Anat. Gesell. VIII. 1894.

CastLe, W. E. The Heredity of “Angora” Coat in Mammals. Science.
1903.

Note on Mr. Farabee’s Observations (on Negro Albinism). Science,
n.s. XVII. 1903.

Heredity of Coat Characters in Guinea Pigs and Rabbits. Carnegie
Institution of Washington. 1905.

Recent Discoveries in Heredity and their Bearing on Animal Breeding.
The Pop. Sci. Monthly. 1905. ”

The Origin of a Polydactylous Race of Guinea Pigs. Carnegie Insti-
tution, XLIX. 1906.

CasTLE, W. E., and Forses, A. Heredity of Hair-Length in Guinea Pigs
and its Bearing on the Theory of Pure Gametes. Carnegie Insti-
tution, XLIX. 1906.

Correns, C. Ueber Bastardirungsversuche mit Mirabilis-Sippen. Erste
Mittheilung. Ber. deutsch. bot. Gesellsch. XX. 1902.

Crampe, H. Kreuzungen zwischen Wanderratten verschiedener Farbe.
Landwirtsch. Jahrbucher, VI. 1877.

Die Gesetze der Vererbung der Farbe. Landwirtsch. Jahrbiicher,
XIV. 1885.

DoncasTER, L. On the Inheritance of Tortoiseshell and Related Colours

in Cats. Proc. Cambridge Phil. Soc. XIII. 1905.
On the Inheritance of Coat Color in Rats. Proc. Cambridge Phil.
Soc. XITI. 1906.
Ewart, J. C. Variation: General and Environmental. Sci. Trans.
Roy. Dublin Soc. VII. gor.

FarRABEE, W.C. Noteson Negro Albinism, Science, XVIT. No. 419. 1903.

Haacke, W. ‘Ueber Wesen, Ursachen und Vererbung von Albinismus
und Scheckung und ueber deren Bedeutung fiir vererbungstheoritische
und Entwickelungsmechanische Fragen.’ Biol. Centrbl. XV. 1895.
Die Gesetze der Rassenmischung und die Konstitution des Keim-
plasmas. Arch. Ent. mech. XXI. 1906.

Harper, E. Studies in the Inheritance of Color in Percheron Horses. Biol.
Bull. IX. 1905.

Hurst, C. On the Inheritance of Coat Color in Horses. Proc. Roy.
Soc. B. LXXVIT. 1906. —

Notes on the “Proceedings of the International Conference on Plant
Breeding and Hybridization, 1902.” Jour. of the Roy. Horticul.
Soc. XXIX. 1906.

138 Experimental Zoology

Experimental Studies on Heredity in Rabbits. Jour. of Linnean

Society, XXIX. I9g05.

KattMann. Handskelet und Hyperdactylie Verh. Anat. Gesell. 1888.

Raynor, G. H., and Doncaster, L. Experiments on Heredity and Sex-
determination in Abraxas grassulariata. Report 74, Meet. British
Assoc. Cambridge. 1904.

RykesuscH. Bijdrage tot de Kennis der Polydactylie. Archiv Neer-
landaises, XXII.

ScHusteR, M. A. Hereditary Deafness. Biometrika, IV. 1906.

StaPLes-BrowneE, R. Experiments on Heredity in Web-footed Pigeon.
Report 74, Meet. British Assoc. Adv. Sc. 1905.

STRUTHERS. On Variation in the Number of Fingers and Toes. Edin.
New Phil. Journ. 1863.

Witson, G. Hereditary Polydactylism. Journ. of Anat. and Phynol.
XXX. 1896.

Woops, F. A. Mendel’s Law and Some Records in Rabbit Breeding,
Biometrika, II. 1903.

ZANDER. Ist die Polydactylie als Theromorphe Varietat oder als Missbil-
dung anzusehen. Virchows Archiv, CXXV. 1891.

00000 @

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00000 0000 | 00000
00000 00000 12345 12345
4 3 i t
00000 00000 12345 7 12345
00000 123457 o0000d =—:12345

Fic. 15. Helix hortenses.

sives, as shown by inbreeding (fourth line).

(After Lang.)

First (top) line, a bandless and a handed shell.
these are inbred they give bandless and banded (third line) in the proportion of 6 to 2 (#.e. 3

When indivi:

; 4
12345 7 12345
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12645 12545 12345 | 12345

00000 “ C0000 12345 12345

i i t t
00000 00000 7 12345 7 12345

00000 =—«-:123457 00000d = 12345

als of these two kinds are paired the first generation (second line) are all bandless. When
1). Of these two are extracted dominants, four are dominant-recessives, and two extracted reces-

CHAPTER IX
EXPERIMENTS WITH SNAILS, MOTHS, AND BEETLES

TuHeE European snails, Helix horiensis and Helix nemoralis,
have been studied by Lang, whose experiments in breeding them
have extended over several years and, though still in progress,
have already yielded decisive results on a number of important
points. The young snails require at least two years, generally
three, and often four years to reach maturity. The animals,
although hermaphroditic, do not self-fertilize, as isolation experi-
ments have shown. The sperm, received during copulation,
may remain alive for several years in the receptaculum seminis,
therefore wild individuals cannot be used safely in breeding ex-
periments, but the young snails must be first reared and isolated
and paired in order that the parentage of their offspring can be
certainly known.

The bands of pigment on the shell are the chief characters
that Lang has studied in his experiments. Colonies of snails
are sometimes found in which only two kinds of individuals are
met with, — those with five bands and those without bands, no
intermediate types existing in such colonies. These colonies
furnish the best materials for breeding experiments. In such
colonies the banded individuals are “‘pure,”’ that is, they breed
true to their type. It is more difficult to obtain “true” indi-
viduals without bands. Those are most likely to breed true
that are found in colonies in which only bandless individuals
exist.

If virgin banded and bandless individuals are allowed to
pair and are then separated, each will make a nest in the ground
and deposit from 4o to 60 eggs, or more. The banded indi-

139

140 Experimental Zoology

viduals may be designated by 12345 and the bandless by 220°,
12345 ©0000

The young of the first generation are all without bands. In other
words, the banded condition is the recessive. The formula used
00000 d
123457

When the individuals of the first generation (F) are paired, the
offspring of the second generation are of two sorts (like the grand-
parents). Those having the dominating character (bandless)
are to those with bands as 3 to1. In other words, the Mendelian
ratio appears. That this is really the case is shown by further
12345

12345"
and produce only banded offspring. The bandless are of two
C0000

by Lang for these individuals is

experiments. The banded individuals are the recessives,

kinds, one third true or “pure” dominants, » and two

thirds dominant recessives, ie There are no external char-
acters that distinguish the pure dominants from the dominant
recessives, since both are bandless; but if they are separated in
pairs and their offspring obtained, it will be found that in some
cases all of the offspring will be bandless. They have arisen
©0000 | 00000

, or by
©0000 00000
the union of one extracted dominant and of one dominant reces-
OQ0000 x 00000

©0009 12345"
are bandless and one third banded, showing that two dominant
00000 00000
12345 12345

Lang has also carried out some experiments with snails in’
which two characters are involved. The individuals of Helix
nemoralis or of H. hortensis differ not only in their banding but
also in the color of the shell, which may be yellow or red. These
colors also form a pair of antagonists that follow the Mendelian
law. The double combination of banded and bandless, yellow
and red, gives the Mendelian expectation for two contrasted
characters.

by the union of two extracted dominants,

sive, In other cases two thirds of the offspring

recessives have paired,

Experiments with Snails, Moths, and Beetles 141

Individuals of these two species also show other antagonistic
characters. There are size differences, and these also Mendelize.
The form of the navel also differs and gives a fourth character.

The preceding breeding experiments relate to differences
within the species, but Lang has also carried out experiments
between the two species, H. hortensis and H. nemoralis. The
results will be described in the next chapter. One point of
interest must, however, be mentioned here. As stated above, each
species contains individuals that are banded or bandless, red
or yellow, etc. These are varietal differences. The point of
interest is that when the two species are crossed the offspring —
species hybrids — show that one of the antagonistic characters
dominates in the same way as when the varieties within the spe-
cies are crossed. For example, if a bandless individual of Helix
hortensis is paired with a five-banded individual of Helix nemo-
ralis, the hybrids are in some cases entirely bandless, in other
cases partly bandless and partly five-banded. ‘Lang accounts for
these results by assuming in the first case that a “ pure’ bandless
form was used, all of whose germ-cells were “‘ pure’ and hence
dominated; while in the other case a dominant recessive was used
which would produce both kinds of germ-cells. Ontheotherhand,
one should not lose sight of the fact that the hybrid-crossing may
itself set ‘“‘free”’ latent characters, as in mice, so that the results
may have arisen in this way.. Lang concludes that those varietal
characters that Mendelize within the species behave in the same
way when different species are crossed.

Experiments with Silkworms

Elaborate series of experiments with silkworms have been
carried out by Coutagne.’ The results of ten years’ work were .
published in 1903. Although ample evidence is furnished of
alternative inheritance, the results are not treated by the author
from the point of view of Mendel’s law, although there are indica-
tions in many places that some at least of the results might profit-

1 Bulletin Scientifique de la France et de la Belgique, XXXVII, 1903.

142 Experimental Zovblogy

ably have been thus considered. On the other hand, there are
also numerous instances where it seems probable that the inheri-
tance is of a different kind. To what the results are due is not
clear in all cases, but it seems not improbable that some of the
domesticated races of silkworms have originated from different
wild species, or even genera; while others have arisen under
domestication as sudden variations. Several instances of this
sort are given and invite special attention. Some of the races
may be crosses, i.e. hybrids, and even although breeding true
inter se the individuals may carry in a latent state the qualities
of other strains. In the light of these possible complications
we can do little more than examine Coutagne’s results as they
stand. It is to be hoped that this most promising field of inquiry
may be further investigated.

Coutagne distinguishes between (1) an “alliage homogéne,”
or fusion, in which the hybrid character is something new and
intermediate between the parental characters (which are united
or fused ‘“‘fondus’’); (2) a ‘‘mélange hétérogéne,” or mixture,
in which some of the hybrids are like one parent and some like
the other in respect to a particular character, and still others inter-
mediate; (3) a “‘liquation” or separation in which there is no
fusion of characters in any of the individuals, but they are strictly
like one or the other parent type.

The inheritance of the following characters was examined by
Coutagne : —

1° The caterpillars (“ worms”) :
White (with or without a masque) a, 1 mode albus
Black a2 “ niger
Zebra a,3 “ virgatus
2° The cocoons:
Yellow b,r “ flavus
White b,2 “ niveus

3° The moths:
White, with or without “cendre” markings c,r “ canus
Black c,2 “ casteneus

Experiments with Snails, Moths, and Beetles 143

Coutagne states that any one of the three kinds of larve, white,
black, or zebra, may be associated with either of the two kinds
of cocoons, yellow or white. This gives six combinations. Any
of these six may be associated with either of the two kinds of
moths, white or black, giving a total of twelve possible combi-
nations. The following results were obtained : —

(1) Crossing individuals with two different characters often
gives offspring in the first generation that are intermediate in
character. Thus the race Chang-hai has a white cocoon that is
small and spherical, while the race Jaune Var has a rose-yellow
cocoon that is large and ellipsoidal with a constriction. The
cocoons of the hybrid have a pale yellow tint, about intermediate
in color,’ the form also is intermediate — ellipsoidal, but less
elongated, and the constriction absent or scarcely marked.

(2) Crossing individuals of different races may give in the first
gencration a /usion of two characters, but subsequent generations
descended from the first show a mixture of the two characters
in question.

The statement is also illustrated by reference to cocoon-char-
acters, but the line of separation of the colors does not appear
very sharp. 7

(3) When the crossing has given a /usion in the first generation,
and in the second generation a mixture (“mélange hétérogéne’’),
it is possible to produce a homogenous race, by means of selec-
tion, that shows the characters that fused. In each successive
generation the individuals presenting the united characters, 7.e.
those ‘‘fondus,” must be selected. This statement Coutagne
puts in the form of a question, because he has not, he says,
indisputable facts in support of it.

(4) Crossing individuals with different characters often gives
a separation in equal parts of the two characters. For example,
an individual of a race having white worms, white cocoons, and
white moths was crossed with an individual of a race having

1 Two to three per cent, however, were pure white, small, and spherical (type
Chang-hai); and 4 to 5 per cent were yellow, nearly of the yellow type but less
yellow. These Coutagne suggests were due to accidental mixing.

144 Experimental Zoology

striped worms, yellow cocoons, and black moths. The results
are shown in the table, in which the horizontal lines represent the
stages of each individual.

No. or INDIVIDUALS Worms Cocoons MortHs
116 striped yellow black
124 white yellow black
IIL white white black
108 striped white black

Thus there are nearly equal numbers of white and striped worms
(235 and 224); the same holds nearly for the two characters
of the cocoons (240 and 219). No worms intermediate in color
were found nor were there intermediate conditions between the
cocoons.’ It will be seen that all the moths were black (mela-
nitic), yet Coutagne thinks some influence of the white was
present.

(5) Crossing individuals with different characters gives at times
offspring all like one parent, without the other character show-
ing any influence in the first generation. For example, an
individual of a race with white worms, cocoons, and moths was
crossed with an individual having white worms, but yellow co-
coons and black moths. All of the cocoons were white.

An individual of a race having black worms, white cocoons,
and moths was crossed with an individual having white worms,
yellow cocoons, and black moths. All of the cocoons were
white.

An individual of a race having black worms, white cocoons,
and white moths was crossed with an individual having white
worms, yellow cocoons, and black moths. All the worms were
black, about half the cocoons were yellow and half white (262 and
248), without any intermediates. All the moths were black or
blackish, but rarely one was almost white. .

In a third case an individual of a race having black worms,
white cocoons, and white moths was crossed with an individual

* Certain double cocoons that were rejected did not show intermediate colors.

Experiments with Snails, Moths, and Beetles 145

having white worms, yellow cocoons, and white moths. All of
the worms were black, half of the cocoons were yellow, and half
were white (180 and 188), without any intermediate ones.

In a fourth case an individual (female) with white worms,
cocoons, and moths was crossed with another (male) having
black worms and moths, but yellow cocoons. All the cocoons
were white. The worms were 265 black like the father and
253 white like the mother; some of the moths were white, others
black, others intermediate. In the group of 265 black worms
there were no more black moths than in the group of 253 white
worms. Thus the characters black or white worms and the
characters black or white moths have no mutual relation.

In a fifth case, the female of a variety ‘‘ Jaune Var” was used
that has the black character of the worm and of the moth almost
completely “‘fixed,’’ but fixed only recently, while the yellow of
the cocoon had been fixed for a long succession of generations.
The male was of the race “Blanc des Alpes” and had white
characters throughout. The result of the cross gave white co-
coons, the paternal type — the reverse of what occurred in other
cases where the white was maternal. The worms were 258
white like the father and 182 striped like the mother (3 to 1).
This predominance Coutagne attributes “to the ancestors of
the white worms of the mother; possibly if the striped char-
acter had been fixed for a greater number of generations there
would have been an equality between the white and striped
worms.’? The questionable character of this explanation is at
once apparent in the next experiment that Coutange gives, in
which the same types were crossed as before and all the worms
were striped.

(6) It has been found that white cocoons dominate over yellow,
but in another combination the reverse was found to be the case,
showing that there is no absolute rule for all races in regard to
the inheritance of white versus yellow.

(7) After crossing individuals of two races which give offspring
showing the character of one parent, it often happens that the

concealed character reappears in the following generations.
L

146 Experimental Zoology

When Blanc des Alpes are crossed with Jaune Var, all the off-
spring have yellow cocoons. The next generation gave yellow
and white cocoons with none intermediate; the proportion of
yellow averaged in fifteen lots 75.2. Here there seems to be
an approach to one fourth that strongly suggests the Mendelian
ratio. In other cases, however, an average of 49.3 per cent was
obtained. The evidence here is opposed to this interpretation,
unless one of the races itself had a latent character that did not
appear until the second generation.

(8) When two crossed races give a separation in equal parts
of the two characters, the following generations give equally
again the separation of the two characters without its being pos-
sible to realize their fusion in a single individual.

For example, two types were crossed, each having striped

worms and yellow cocoons.

The offspring gave four classes : —

Worms Cocoons
I 236 striped yellow
2 80 striped white
3 89 white yellow
4 34 white white

In another case the female belonged to a race with white worms

and cocoons, the male striped and yellow cocoons.

The crosses

were : —
Worms Cocoons
I 95 striped yellow
2 84 striped white
3 103 white yellow
4 109 white white

Seven other combinations are given, but until some principle
running through these cases can be formulated it is needless to
recount all the results here. It is true that Coutagne attempts
to show that the rule in subsequent generations is such that the
inheritance of the contrasting characters follow the sequence of

Experiments with Snails, Moths, and Beetles 147

3,+4,+ 4, +75, which is the same as Galton’s law, but so far as
I can interpret his data this is not strikingly apparent. Failing
to carry the results discussed above through subsequent genera-
tions leaves the matter in an unsatisfactory condition. In only
one experiment is the result of the next generation given, which
shows, if I interpret it correctly, that each of the four types
gives, when inbred (?), individuals amongst which most of
the same types reappear, but in very different proportions; the
most striking result being that each type gives a much higher
percentage of its own kind.

(9) In the course of separation of two characters which takes
place during a series of generations it happens at times that
when two similar individuals are paired, z.e. both having the same
character, the other contrasted character never appears again
in their descendants. It has not simply become latent, but has
gone entirely.

It is obvious that this would happen, according to Mendel’s
formula, whenever the individuals are pure dominants or pure
recessives.

(10) Crossing individuals with different characters often gives
in the first generation a mixture, ‘‘mélange hétérogéne,” of two
characters with marked predominance of one of them in the com-
bination, and in this case it is easy, by means of selection in suc-
cessive generations of the individuals having the most marked
character, to fix rapidly this character and even to exaggerate
its relation to the other.

In cases of this sort it is not clear that Mendel’s law holds at
all, and some other principle must be involved, especially if the
author means that the results are obtained by simply discarding
the individuals having the disappearing character and allowing
the rest to breed together.

(11) Crossing two individuals with a different character often
gives in the first generation a separation of these characters in a
part of the offspring and in the other part a union constituting
a new character. Subsequent generations show a separation or
liquidation of these characters.

148 Experimental Zoology

For example, a female of a race with black worms and white
cocoons was mated to a male with striped worms and yellow co-
coons. The offspring were : —

No. oF INDIVIDUALS Worms Cocoons
89 black and striped yellow
86 black and striped white
7 black yellow
7 black white

The mother had been a white moth, the father a black
moth. The offspring (of this table) consisted of a small number
of whites, and the majority of a “‘gris-marron ou moues foncé,”
in other words, a mélange hétérogéne of the white and the
black character, but without predominance of either.

“The fact of greatest interest in this cross is the fusion, or at
least the close juxtaposition, of the black and the striped charac-
ters in half of the caterpillars of this lot. These worms at the
same time black and striped are most curious and constitute a
new character without any intermediate.”

The further evolution of these new types was as follows:
The group with black-and-striped worms and yellow cocoons
gave :—

No. or Inpivipuats Worms Cocoons
a individuals black and striped yellow
7 37 black and striped white

29 black yellow
47 1 38 black white
13 white yellow

20 ; 3
i, white white
re 66 striped yellow
63 striped white

The group with black-and-striped worms and white moths
gave:—

Experiments with Snails, Moths, and Beetles 149
No. oF INDIVIDUALS Worms Cocoons
é 70 black and striped yellow
59 189 black and striped white
gg {27 black yellow
61 black white
11 white yellow
a 23 white white
— | 39 striped yellow
7 striped white

Coutagne calls attention to the reappearance of the white char-
acter-that is atavistic on his interpretation. It will be noticed
in each group the numbers are nearly exactly halves, thus 70
and 189, 27 and 61, 11 and 23, 39 and 77.

Although it is not apparent that an application of the Mende-
lian law is competent to explain all the results of these experi-
ments, it is probable that some such rule lies behind several of the
observed cases. Other cases clearly show blended inheritance,
and still others show in some characters one kind and in others
other kinds of inheritance. It is difficult in many cases to under-
stand just what really occurs, but the results show plainly how
complicated the problem of inheritance in a single group of
forms may be.

In a note published later, Coutagne compares his results with
the Mendelian formula and points out that certain classes of his
results conform to this law. It seems to me not improbable that
if the latent qualities of some of the races be taken into consid-
eration, the conformity may be greater than Coutagne admits.
On the other hand, it appears probable that some characters do
not dissociate according to the Mendelian expectation.

A short paper by Toyama on Mendel’s law as applied to silk-
worm crosses has very recently appeared, in which it is shown
that many of the same characters studied by Coutagne give the
Mendelian expectation. Unfortunately no reference is made to
Coutagne, although a comparison would have been valuable.

When moths of the Siamese breed, having either yellow or

150 Experimental Zoology

white cocoons, are crossed, the offspring (F',) produce only yel-
low cocoons. When these hybrids are inbred they give two
kind of individuals; namely, those producing white and those
producing yellow cocoons in the proportion of 25.03 per cent
whites to 74.96 per cent yellows. The whites are extracted re-
cessives, and are found to breed true. The yellows are of two
kinds, one kind giving only yellow-producing offspring (the ex-
tracted dominants) and the other kind producing yellow as well
as white in the proportion of 3 to 1 (the dominant reces-
sives). A more complex result was obtained when Japanese
‘whites’? and Siamese “yellows”? or when Japanese ‘“ whites”
and European “‘yellows” are paired. The first generation give
offspring that produce yellow cocoons. When these indi-
viduals are paired, they give in the second generation four
kinds of cocoons: (1) pure yellow, 70 cases; (2) pale pinkish
yellow, 21 cases; (3) greenish white, 24 cases; and (4) pure
white, 12 cases.

The results show apparently that we have to do here with three
characters, one of the races used being a monohybrid and the
other a dihybrid. In subsequent generations the pure white
type breeds true. Some of the yellows give only yellows; others
give white (25 per cent) and yellow (75 per cent); others give
yellow (75 per cent), flesh-colored (25 per cent), and still others
white and greenish white (25 per cent), yellow (56 per cent), and
flesh-colored (19 per cent).

The flesh-colored forms of the second generation gave white
(25 per cent) and flesh-colored (75 per cent). The greenish
white type of the same generation gave white and greenish white
offspring.

Toyama used two breeds of caterpillars, pale whites character-
ized by the absence of markings, and striped whites having dark
stripes. The former breeds true, while the latter produces some
pale whites and is therefore a cross breed form. The hybrids
produced by uniting these breeds were white and striped worms
in equal numbers. In subsequent generations the whites re-
mained true while the striped kind gave both whites and striped.

Experiments with Snails, Moths, and Beetles 151

Other crosses were made between a brood showing no markings
and yellow cocoons, and striped with white cocoons, and gave
the usual Mendelian results of splitting and recombination.

Experiments with Beetles

In the Californian beetle, Lina lapponica, two types exist, a
spotted type shown in Fig. 4 and a black type shown in Fig. 6.
The heredity of these two types has been studied by Miss Mc-
Cracken. When the beetle emerges from its pupal case the wings
are nearly pure white. The large median black spot and the
two lateral spots on the prothorax are present (Fig. 1). In the
course of ten minutes faint indications of spots appear on the
wings (Fig. 2), and after fifteen to twenty minutes these become
more distinct, as seen in Fig. 3. In the course of forty-five min-
utes the spotted type has reached its final condition (Fig. 4).
The black type passes through a stage like that of Fig. 3 to that
of Fig. 5, and finally after forty-five minutes to Fig. 6. Ina
sense the black type passes through the spotted type, the back-
ground becoming as dark as the spots themselves, so that the
spots can no longer be seen.

If these two’kinds of individuals are collected at random and
isolated in pairs, it isfound that the spotted pairs sometimes (in
half the cases) produce their own kind only, and in other cases pro-
duce both spotted and black types. There are no intermediates.

When the blacks are paired they produce in some cases broods
that are all black, and in other cases broods in which some of
the individuals are black and some are spotted, in the proportion
of 1 spotted to 1.7 black.

If a black and a spotted individual are paired, mixed broods
are produced with a preponderance of the spotted type.

The results for the second generation are especially interesting.
If those broods from black parents are picked out in which all
of the individuals are black and these are paired, all of their off-
spring, without a single exception (in 4985 cases), are black.
The third generation is also black. Hence there is a pure race
of blacks. The black is evidently the recessive form.

152 Experimental Zoology

The results of the second generation are different when the
spotted forms from the pure spotted brood produced by spotted
parents are paired. In this case both spotted (1021) and black

Fic. 16. Development of the two-color patterns of Lina Japonica. (After
McCracken.)

Experiments with Snails, Moths, and Beetles 153

(345) are produced in the proportion of 3:1. In two cases, how-
ever, all the offspring (265 in one case, 182 in the other) were
spotted. If these are paired, they produce only spotted forms.
Clearly we have to deal here with extracted dominants.

The results may be briefly summed up as follows: the two
types of beetle show alternate dominance and recession, the
spotted character being dominant, the black being recessive.
By isolation both types may be obtained “pure.” In nature
both are continually crossing and recrossing, so that the chances
are that most individuals are impure, but by selection pure
breeds can be quickly obtained from them.

Experiments with the Currant Moth

There is a curious case, reported by Raynor and Doncaster
for the currant moth (Abraxas grassulariata), in which there is
a rare variety, A. lacticolor, that had previously been found only
in the female sex. The variety is recessive in the first generation
when crossed with the parent form. The offspring, however
(F,), produce males, all of which are the ordinary variety grassu-
lariata, and females, half of which are like the males and the
other half are var. lacticolor. When, however, a lacticolor fe-
male is paired with a (F,) male hybrid (L 9 x G (L) @), some of
the male offspring are lacticolor (and others female). The ex-
planation of the“transference to the male of the female character
is not apparent.

Experimenis with Tephrosia

A series of hybridizing experiments between the moths
Tephrosia bistortata and Tephrosia crepuscularia have been
described by Tutt (based on records by Riding and Bacot).
These two species are sufficiently similar to have been put to-
gether as one by some entomologists because occasionally indi-
viduals have been found that could not be referred with cer-
tainty to either species, but Tutt describes a number of constant
differences between the two forms and regards them as distinct.
Both species have a melanitic variation, that of T. bistortata

154 Experimental Zoology

being exceedingly rare and almost confined to South Wales, but
that of T. crepuscularia is widely distributed and known as
aberratio delamerensis. The first generation of hybrids (F,)
is described as containing individuals that are like the one or
the other parent form, although each kind may show traces of
the other species. Tutt states that he has avoided the use of the
term ‘intermediate’? when describing these hybrids because the
term has been made to cover so many different things; but that
nevertheless all of the hybrids with few exceptions are inter-
mediate to varying degrees, 7.e. ‘almost every specimen appeals,
in some part of its facies, to a specialist, as resembling T. bis-
tortata, whilst the same specimen, in other particulars, strikes
one as resembling T. crepuscularia.’”’ When inbred the hybrids
produce a large percentage of individuals differing much from
either parent-form. The crossing of the hybrids, obtained
from original reciprocal crosses, tends to produce a mixed prog-
eny, some referable to known forms of the crossed species,
others quite unlike anything ever obtained in nature. . . . These
experiments support Eimer’s view that sexual combination can
lead to the production of new forms. I doubt, however, very
much whether they could be perpetrated without selection,”
for if crossed with one of the present species they would “in my
opinion”’ revert to the wild form.

Some interesting results were also obtained in regard to the
sex of the hybrids. “The greater vigor of the male results
largely in the production of female offspring. When the male
is of the dominant species, females are developed in fair propor-
tion; when the female is of the dominant species, males are
largely in excess.”

LITERATURE, CHAPTER IX

Couracne, G. Recherches experimentales sur ’hérédité chez les vers
4 soie.. Bull. Sci. France et Belg. XXXVII. 1902.
Sur les facteurs élémentires de Vheredite. Comp. Rend. Acad. Sci.
Paris, CXXXVII._ 1903.
Sur les croisements entre taxies differentes. Comp. Rend. Acad. Sci.
Paris, CXXXVII. 1903.

Experiments with Snatls, Moths, and Beetles 155

Lane, A. Kleine biologische Beobachtungen ueber die Weinbergschnecke
(Helix pomatia, L.). Viertelzahrschrift d. naturf. Gesell. Ziirich.
XLI. 1896.

Ueber Vorversuche zu Untersuchungen ueber die Varietaétenbildung
von Helix hortensis Miiller und Helix nemorales, L. Fest. von
Haeckel. 1904.

Ueber die Mendelschen Gesetze, Art und Varietditenbildung, Muta-
tion und Variation inbesordere bei unsern Hain- und Garten-
schnecken. Verh. d. Schweiz. Naturf. Gesell. 1905-1906.

McCracken, I. A Study of the Inheritance of Dichromatism in Lina
lapponica. Jour. Exp. Zool. II. 1905.

Toyama, K. Mendel’s Laws of Heredity as applied to the Silkworm
Crosses. Biologisches Centralblatt, XX VI. 1906.

Torr, J. W. Some Results of Recent Experiments in Hybridizing Te-
phrosia bistortata and Tephrosia crepuscularia. Trans. Entomol.
Soc. London. 1898.

CHAPTER X
OTHER KINDS OF HYBRIDIZING

Blended Inheritance

WE have seen in the cases that come under Mendel’s law that
the contrasted characters do not both develop at the same time, the
offspring in the first generation being often like one or the other
parent. Yet in some of these cases there is evidence that the
dominant character may be weakened by the recessive one. We
may now consider cases in which the contrasted characters of
the two parents fuse or blend completely in the offspring. Cases
of the sort are found not only between races, varieties, and elemen-
tary species, but this method of union has long been supposed to
be a characteristic feature of hybridization when Linnzan species
are crossed.

The most familiar and striking case of fusion or blending of
two characters is found in the mulatto — the result of union of a
white and a black individual. The mulatto breeds true in all
successive generations, neither the white nor the negro ever
appearing again in the pure form. If the mulatto again crosses
with the white stock, the dark color is again lessened, but even
after several generations of crossing with the white stock traces
of the dark pigment remain. Conversely crosses between the
mulatto and the black race produce ever increasing shades
of darkness in successive generations of offspring. Not only
the color, but the character of the hair also shows a tendency
to blend in the hybrid.

Flourens made crosses between the domestic dog and jackal,
the latter being, however, ‘“‘prepotent.” The horse and the ass

give the mule, that is intermediate in many respects, but the
156

Other Kinds of Hybridizing 157

characters of the ass are more prepotent. The lion has been
crossed with the tiger and an intermediate hybrid produced.
The brown bear, crossed with the polar bear, gave a mixing of
colors with the head and neck white. Darwin states that the
pheasant crossed with domestic fowls gives a hybrid, showing
the pheasant characters “‘prepotent.”” Darwin describes a cross
between the penguin variety of the common duck and the Egyp-
tian goose that is intermediate in character.

Mosaic Inheritance

A mosaic character sometimes appears when differently
colored individuals are mated. Each character appears in its
pure form over certain regions. Thus when a gray and a white
rat are crossed, individuals sometimes appear that are mosaics,
but it is questionable in this case, and perhaps in all such cases,
whether the results may not be due to a latent mosaic character
coming to light. As has been pointed out, offspring of the
same litter may be different, some being of a single color, others
mosaic. Whether spotted or piebald, domesticated races — horses,
cattle, dogs, cats, etc. — owe their origin to a cross between two
uniformly but differently colored parents, or are themselves
sports that breed true, or have been back-crossed, is an open
question. In pigeons, as we have seen, the mosaic character of
the offspring is apparent. The results are complicated, how-
ever, by the ancestral (latent) blue color appearing in parts of
the body. The inheritance of the mosaic pattern in mice and
guinea pigs has been already discussed.

Hybridization between Linnean Species

Most wild species of animals and plants differ from each other
in more than in a single character. In the great majority of
cases it is perhaps not going too far to state that species differ
in all their characters — in some parts more, in others less. Ag
already pointed out this is due, on the mutation theory, to the

158 Experimental Zoology

saltations having been in most cases more than a single one, and
often in different directions.

Now it is almost a universal rule— with exceptions, however —
that wild species are infertile when crossed with other wild species,
the degree of infertility differing enormously in different species
—from complete fecundity to complete sterility. It is also a
general rule — again with exceptions — that the more widely
different two species are the greater the difficulty in crossing
them. When the species are so different that they are put into
different genera, the chance of their crossing is small. If the
species belong to different families, the chance of crossing
is much smaller still; and if to different orders, there is scarcely
any chance at all of their crossing. Crosses between the do-
mestic horse and zebras of different species produce infertile
hybrids. A jack-ass crossed with a mare (horse) gives a mule,
which is sterile. Conversely, a she-ass crossed with a pony gives
a hinny, also stcrile. On the other hand, the American bison has
been crossed with the wild ox of Europe and has produced a
fertile hybrid. Similarly, the humped cattle of India crossed with
the domesticated ox produces fertile offspring. Crosses between
the common goose and the Chinese goose, which are very differ-
ent species, give fertile hybrids. Simliarly, for the common duck
and the pintail duck, and for different species of pheasants.’
A cross between a fowl (a langshan cock) and the common guinea
hen has been brought about, but the hybrid is sterile.’

In regard to the characters of these hybrids no general state-
ment can be made. Sometimes the hybrids appear to be inter-
mediate in one or more characters; sometimes the character of
one or of the other parent predominates, and in still other cases
the hybrid may have characters peculiar to itself. Latent char-
acters may also be brought to the front by hybridizing, and
these dormant characters seem sometimes to be characters that
the ancestors of the species may be supposed to have possessed.

+The preceding cases are quoted from Ewart’s “Penycuik Experiments,”

1899.
> Guyer, M. F., Science, XXI, June, 1905.

Other Kinds of Hybridizing 159

De Vries has found that some of the elementary species of the
evening primrose also show a certain degree of infertility when
crossed ; and there can be little doubt that infertility may begin
with the appearance of elementary species, and increase in pro-
portion to every new change in the germ that takes place.
Whether infertility is a general rule for elementary species, may
be questioned.

Many cases of crossing between wild species of animals and
plants have been recorded as occurring in nature, and many
more cases have been experimentally brought about, especially
in plants, with wild forms kept under domestication. The re-
sults are different in different cases, but it is a generally accepted
opinion that the species-cross is generally intermediate be-_
tween the parents. This conclusion needs, perhaps, careful
revision in the light of the results of recent years on crossing types
that differ in only one character, where in many cases discontinu-
ous inheritance is the rule. Darwin was so impressed with the
difference in the results of crossing Linnean species and sports
that he concluded that wild species could not have arisen as
sports, since the latter when crossed show discontinuous inheri-
tance, while wild species give intermediate forms. Since Dar-
win’s time our knowledge of the results of hybridizing has greatly
increased, and his argument seems less conclusive, because, in
the first place, a single mutation may show incipient infertility,
as in de Vries’s cenotheras; in the second place, because the
results of crossing elementary varieties and elementary species
with the parent forms or with each other do not always show
discontinuous inheritance; thirdly, because wild species have
undergone so many changes of different kinds that the results
are too complicated for an analysis of single characters; and,
fourthly, because discontinuous inheritance may sometimes oc-
cur between wild species, if unit characters rather than the
ensemble of characters is considered. It is the failure to recog-
nize this last point that has probably led to an exaggerated idea
of the difference between the inheritance of single variations and
of complex variations that characterize Linnean species.

160 Experimental Zoology

Lang has studied the hybrids between the closely related
species of Helix hortensis and Helix nemoralis. These species
are very similar, and it has often been disputed whether they
are separate “species.” They differ principally in size, in the
form of the peristome of the shell and in the color of the lip, in
the form of the ‘‘dart” and in the finger-formed gland. The
hybrids are infertile with each other. Within the limits of each
of these species there are the same kind of varietal differences,
and these, as pointed out in the previous chapter, dominate and
recede in the first species-cross. It is not possible to test their
further behavior, since the species-hybrid is infertile. In re-
gard to other characters, Lang states that these also dominate
and recede. The hybrids are not intermediate, but have the
form of the peristome of H. hortensis and the pigmentation of
the lip and of the throat of the shell of H. nemoralis. The
dart and the finger-shaped gland are exactly those of H. hor-
tensis. Here it is evident that the hybrids are mixed, but
that in some characters they are true to one species and in others
to the other species. If the numbers of such characters were
larger, the hybrid might appear to be a blend of the different
characters, while in reality it might be only a mixture of one and
the other parental characters. It is evident that the charac-
ters must be studied separately in such cases before we can con-
clude whether species-hybrids show blending of the parental
characters or whether they give mixtures (mosaics) in their char-
acters. Some characters may blend, others alternate in their
inheritance."

Dimor phism

The word “dimorphism” is sometimes used for cases in which
the male and female differ markedly in form, but it is also used
for those cases in which two forms of the same sex exist. I shall
use the term here in the latter sense. Few cases of this sort exist
amongst animals, and no experiments have been made to test
the inheritance. The male hercules beetles occur under two

* Correns has described similar results in plants.

Other Kinds of Hybridizing' 161

forms, one with the other without horns. There is a large and a
small form of earwig. Papilio glaucus has two forms of females,
one yellow, the other black. The latter is found over the south-
ern range of the species. “In this region both yellow and black
forms have been reared from eggs produced by a single female.”

Fic. 17.

A number of cases are known in the higher plants, and their
inheritance has been examined in a few instances. These are all
hermaphroditic forms in which two kinds of flowers — both pre-
ducing ova and pollen — are present on the same or on different
plants. The European cowslip, Primula veris,occurs in two
forms, the long-styled and the short-styled (Fig. 17). Each

plant bears flowers that belong to one type only; the two types
M

162 Experimental Zoology

never appearing on the same plant. The long-styled, as its
name implies, has a long style reaching to the top of the corolla;
the corolla is larger, the stigma globular, the papilla longer,
and the pollen grains larger than those in the short-styled
form. This long-styled form flowers first, on an average, but
the short-style averages more seeds. The stamens are in the
middle of the tube in the long-style form, and at the top of the
tube in the short-style form.

Darwin carried out a series of important experiments on these
plants. He calls illegitimate unions those in which long-styled
flowers are fertilized by pollen from the same flower or from a
similar flower of the same or of a different plant. Similarly
for the short-styled form: legitimate unions are those between
long-styled and short-styled flowers.

The behavior of the offspring from seeds of legitimate and ille-
gitimate unions is most surprising. In one case‘ an illegitimate
union between short-styled forms produced seeds that germinated
so badly that only 14 plants were obtained, of which 9 were short-
styled and 5 long-styled.

In another experiment the stigma of a long-styled flower was
fertilized by the pollen of a long-styled flower. Three long-
styled plants resulted. From these, in turn, self-fertilized, 53 long-
styled offspring were obtained; from their seed 4 long-styled
plants; from their seed 20 long-styled; and from their seed 8
long-styled and 2 short-styled.

In another plant, Lythrum salicaria, 3 forms occur: the
long-styled, the mid-style, and the short-styled type. The sta-
mens also occur under the same three lengths. Figure 17 shows
the conditions of the three kinds of flowers.

There are 6 possible legitimate unions and 12 illegitimate
ones. To test these two kinds of unions, legitimate and
illegitimate, 18 distinct kinds of crosses must be made.

The results of these experiments are shown in the following
table: —

1“ Different Forms of Flowers,”’ p. 217.

Other Kinds of Hybridizing 163

AVERAGE AVERAGE No. OF
jimall
The legitimate

unions 75 56 96.39 71.89
The illegitimate
unions 146 36 44.72 II.03

The fertility of the legitimate to the illegitimate was found to
be as 100 to 33, judged by the flowers that produced capsules,
and as 100 to 46, as judged by the average number of seeds.
Darwin concluded that only the pollen from the longest stamens
can fully fertilize the longest pistil; only pollen jrom the mid-
length stamens can fully fertilize the mid-length pistil, and only
the shortest stamens can fully fertilize the shortest pistils. The
meaning of this difference is entirely obscure. It is of much
interest to find a condition of this sort between individuals of
the same species. It suggests a comparison with the infertility
that exists between different species; but in point of fact the
results are just the reverse, for, in the present case, it is the same
kind of flowers that imperfectly fertilize each other, while the
flowers having a different form are more fertile.

In the case of dimorphic or trimorphic plants Darwin makes
a determined effort to show that selection of fluctuating variations
has brought about the two kinds of flowers. This argument is
so instructive that I shall give it in full.

Since heterostyled plants occur in fourteen different families
of plants, it is probable, Darwin thinks, that this condition has
been acquired independently in each family and “that it can be
acquired without any great difficulty.” The first step in the pro-
cess he imagines to have been due to great variability in the
length of the pistil and stamens. ‘As most plants are occasion-
ally cross-fertilized by the aid of insects, we may assume that
this was the case with our supposed varying plant but that it
would have been beneficial to it to have been more regularly
cross-fertilized.” ‘This would have been better accomplished
if the stigma and the stamens stood at the same line; but as the

164 Experimental Zoology

stamens and pistils are supposed to have varied much in length,
and to be still varying, it might well happen that they could be
reduced much more easily through natural selection into two sets
of different lengths in different individuals than all to the same
length and level in all individuals.”

Darwin points out that the mutual sterility of these plants could
not have resulted from natural selection, and although he thinks
that the difference in the length of the stamens and pistils has
resulted from a process of natural selection, yet he admits that
one of the most striking facts in the case is that the individuals
have in consequence become partly sterile to half the other indi-
viduals in one case and to three fourths in the other. This con-
clusion in itself shows, it seems to me, how futile it is to apply the
theory of selection of fluctuating variations to the process of
evolution of these forms. .

Bateson and Gregory have examined the inheritance of hetero-
stylism in Primula and have found that the Mendelian rule is
followed. In the case of P. sinensis, the short-styled is dominant
over the long-styled form. When long-styled was crossed with
long-styled —pure forms being used —all the offspring (F,) were
long-styled. When these were inbred again, only long-styled
forms (F,) were produced.

The short-styled plants that were obtained for experimental
work proved to be heterozygous (DR). When these short-styled
were crossed with short-styled forms, there were produced
26 short- and ro long-styled — the Mendelian expectation being
3:1. Of these 26 short-styled forms some were pure dominants
(DD) and others dominant-recessives (DR). The latter (DR)
inbred gave short (24) and long (4). It was found that other
combinations also conformed to the Mendelian expectation.

Bateson and Gregory also examined the inheritance of a
peculiar form of Primula sinensis known as equal-styled. The
anthers are at the same level as in long-styled flowers, but the
style is short and does not reach above the level of the anthers.
The corolla has a central yellow flush extending over half of each

* A few departures difficult to explain were also met with.

Other Kinds of Hybridizing 165

petal. “The flush is transmitted independently of the length
of the style or the size of the pollen grains, for it may be trans-
ferred to the true short-styled or ‘thrum’ type. But when the
flush is developed in plants which by gametic composition would
be long-styled, the style does not pass through the anthers and
the equal-styled condition is produced. Why the development
of the yellow flush in these flowers should entail the reduction of
the style, we cannot in any way suggest.”

The discovery, that Primula follows the same law of inheri-
tance as do other discontinuous variations, some of which are
known to have appeared suddenly, furnishes an argument in
favor of the view that the dimorphism in Primula owes its origin
to discontinuous variation. Compared with the involved argu-
ment by which Darwin attempts to show how natural selection
has brought about the result, the mutation theory offers a much
simpler and in my opinion a much more plausible interpretation.

Reversal of Symmeiry

In some species of snails the spiral of the shell turns to the
right, in other species to the left. Occasionally in a right-
handed species an individual that is left-handed is found, and
there can be little doubt that such a form may suddenly arise.
It is a discontinuous variation, but whether it is a mutation is
not so clear, since the result may be due to an accidental shift-
ing of the blastomeres on each other at the time when the
unsymmetrical mesoblast cell is laid down. Nevertheless, the
fact that entire species are characterized by the right- or the left-
handed condition indicates that the factor that produces the one
or the other result may at times be impressed on or arise in the
egg, and be inherited. There are also species in which some
individuals are right-handed, others left-handed. Here both
possibilities seem to exist in the egg; but whether this can be
referred to alternating dominance and recession, or to purely
local conditions that arise during segmentation, is unknown.

Somewhat similar conditions occur in the two kinds of chele
of crabs, prawns, and other decapods; and perhaps the right-

166 Experimental Zoology

handedness and left-handedness in man belongs to the same
category. It is also known that in man all of the viscera may be
in a reversed position, and the aortic arch also.

Lang has bred together some left-handed snails of the right-
handed species, Helix pomatia. They produced only right-
handed individuals. These were also inbred and produced only
right-handed young. It is evident that this new character is
not inherited in this instance and also that it does not behave
as a recessive to the right-handed condition, for, if it did so, it
would have reappeared in the grandchildren.

Physical difficulties seem to interfere with the union of the
sinestral individuals of Helix with dextral forms, so that as a rule
the sinestrals are precluded from breeding, unless by chance
they meet one of their own kind; but even if this occurs all the
offspring will be dextral again, and no opportunity exists to per-
petuate the new race. If, however, in other species the left-
handed condition should be heritable, such individuals might, if
once started, establish a variety alongside of the parent forms,
although the two forms might be prevented by their difference in
structure from pairing. That the left-handed condition must
sometimes be inherited is shown by the existence of species of
this sort.

Conclusions regarding Mendelian Inheritance

The examples of discontinuous inheritance given in the pre-
ceding chapters make it clear that Mendel’s law accounts in
many cases for the results, and is therefore an invaluable acqui-
sition to our method of interpretation; yet in some other cases
it is evident that the inheritance is not strictly Mendelian. Used
with discretion the law may still unlock many problems, but if
attempts are made to force it to interpret cases that do not
belong to its proper field of action, especially in regard to dissocia-
tion in the germ-cells, harm rather than good may temporarily
result.

The most striking exceptions are those recorded by Standfuss and by de Vries
(cenothera). Other cases involve blending, etc.

Other Kinds of Hybridizing 167

The Mendelian inheritance of coat color in mice and in some
other animals may give an exaggerated idea of the inheritance of
color in general. That the inheritance is not always of this sort
seems to be shown by anumber of cases, some of which have been
given. Pearson’s examination of the inheritance of color in the
coat of dogs, horses, and of the eye color in man, has led him to
conclude that in these cases there is no evidence of Mendelian
inheritance. He points out that when the whole range of the
ancestry is examined we get more nearly an idea of what the
color of the offspring will be. For example, in one case, where
the color of the eye of the mother and of the father was blue,
only two of the children had blue eyes; but in another case, where
the father and the mother and all four of the grandparents and
five recorded grandparents had blue eyes, four children had blue
eyes. Single cases of this kind in themselves do not show much,
but these are only samples of what is generally found in many
cases of the sort. Pearson thinks that the prediction of what
the color of the eyes is likely to be will be closer when we use the
ancestry and not the parents alone. If any of these eye colors
follow the Mendelian rule of dominance and recession, some evi-
dence of this would appear in the statistics, but nothing of the
sort has been found.’ It is evident, nevertheless, that these
cases require careful reéxamination, since there has probably
been great intermingling of different colors in the past.

De Vries and some other students of mutation have laid much
stress upon the immutability of unit characters. De Vries as-
sumes that transitions between unit characters exist as little as
between the molecules of chemistry. It cannot be maintained,
I think, from the evidence that we possess, that unit characters
are immutable, for there are some cases in which it appears that
the unit character may be halved by every crossing. It is true
that some of these cases may be explained by antagonistic char-
acters both developing and mutually influencing the result; but if
they do not subsequently separate, it is impossible to tell whether
or not a new unit character has been formed by combination.

1“ The Law of Ancestral Heredity,” Biometrica, II, 1903.

168 Experimental Zoology

Cases of blended inheritance especially seem to come under
this heading. But so long as we do not know definitely what
occurs in these cases, it seems to me arbitrary to speak of unit
characters as immutable and quite unnecessary to make this idea
a cardinal point of the mutation theory. The behavior of cer-
tain characters in heredity shows that they do act as units, and it
is a great convenience to deal with them as such, but unnecessary
to push the matter so far as to hold that they are immutable.
If unit characters can be halved, altered, added to, or changed in
any way, their immutability does not seem to be an essential
point of their characterization, and, as has been said, there is
some evidence to indicate that such changes may take place.
The idea of immutability is not likely to suggest itself when results
of this kind are considered, but in cases where complete domi-
nance of one form over the other occurs, the idea of unit char-
acters is more likely to arise. If, however, as J think probable,
we are dealing here with alternate or contrasted characters that
cannot both develop at the same time, the isolation of the char-
acters in question is due to their mutual exclusion rather than to
their immutability. Even if it be true that mutations take place
by definite steps without the presence of intermediate forms, it
does not necessarily follow that these steps may not subsequently
become subdivided by each crossing with the parent form. The
central idea of mutation remains, even if the unit-character that
marks the steps is capable of being changed.

The results of hybridizing of forms differing by a single unit-
character seem to show that when in the first generation the
hybrids (F;) are strictly like one of the two parents, the hybrids
of the new generation (fF) also show complete development! of
one of the two characters in the extracted dominants and of the
other in the extracted recessives. This may be called the law of
incompatibility. If, on the other hand, the dominance is incom-
plete, z.e. if both dominate in the first generation, there is in-
complete dominance in the second generation. This may be
called the law of compatibility. If further facts establish these

? In the sense of contrast, not necessarily of actual separation.

Other Kinds of Hybridizing 169

laws as general, the results suggest certain questions of great
theoretical importance.

For instance, the results seem to me to indicate that we are
dealing not with a question of partial purity of the germ-cells,
but with the question of the relation of dominance and recessive-
ness of contrasted characters. In the first generation there can
be no doubt that both characters are present. In the first
class of cases the activity of one character completely suppresses
the activity of the other — the characters are mutually exclusive,
z.e. the development of one suppresses the development of the
other. In the second class of cases both characters may become
active either at the same time in the same cell, producing blend-
ing; or in different parts of the soma, —different cells or groups
of cells, —producing a mosaic or a piebald condition.

If, then, the same condition holds in the second generation,
the most probable conclusion is, I think, that there has really
been no separation of the contrasted characters in the germ-cells,
but only a condition of relative dominance and latency estab-
lished that is akin to but not identical with the dominance and
recession in the first generation. Such a.conclusion seems to
me more in conformity with the results than that which tries to
explain the facts of the second class as due to imperfect separation
in the germ-cells of the two contrasted characters; for on my view
the results in the first and second generation are accounted for on
the same assumption; while in the current interpretation it is not
apparent why imperfect separation in the germ-cell of F, should
not occur as often in the first class of cases as in the second.

In the preceding pages the heredity of a large number of char-
acters of domesticated animals has been described. Relatively
few facts regarding wild species have been given. The objec-
tion has sometimes been raised that our domesticated animals
are contaminated to such an extent by crossing that they offer
questionable material for studies in heredity, and that the study
of wild forms is more profitable. This objection is misleading,
since it directs attention away from the point at issue and rests
on several false or questionable assumptions.

170 Experimental Zoology

In the first place, if we are to study those characters that be-
have, as a rule, as units, we can find for this purpose no better
material than some of the domesticated races that differ from
each other by single unit-characters. We are dealing here with
the simplest cases of discontinuous variation, and the simpler the
problem the better the opportunity to study it. In so far as the
contamination due to previous hybridization is concerned there is
introduced, it is true, a complication (but one that can be dealt
with, in most cases), for the unit-characters, as such, are not
necessarily affected by the latent characters, but can be studied
independently of them. Hence it is not a serious difficulty to
find that previous contamination has occurred. To ignore this
point shows a misconception of the problem of the heredity
of discontinuous variation.

It is important, in this connection, to bear in mind that thesame
rule of discontinuous heredity has been found to be true for wild
forms also that differ from each other by a single character.
Furthermore, how do we know that wild species are not also
hybrids? If evolution has taken place by mutation, then it is
possible that many wild species are complex hybrids, even if all
of them are not. There will also be recalled in this same connec-
tion de Vries’s conclusion that even his elementary species of
Oenothere must also have arisen by hybridization, since it is im-
probable that a mutating germ-cell should meet another of its
kind. Therefore, until it is more evident that wild species
are not hybrids, this side of the argument carries little weight.

It is sometimes said also that wild species that have bred true
for hundreds (or thousands) of generations are much purer than
our ‘“‘pure”’ domesticated races that have been bred for a rela-
tively small number of generations. This may be true, provided
it can be shown that a hybrid is purer the longer it is inbred.
There is little direct evidence that this is the case, but the state-
ment is likely to pass unchallenged because it seems so plausible !

Finally, if evolution has taken place by single steps, our first
problem is to study the heredity and results of crossing of these
single steps. Most wild species must differ from each other by

‘ Other Kinds of fTybridizing 171

a large number of steps, and the results of hybridization of such
forms may differ from the results of hybridization of single steps.
If new elementary species arise by single steps and are subjected
to crossing with the parent type, as would almost inevitably take
place in many cases, the problem of paramount importance,
from the point of view of evolution, is to study the results of such
hybridization. Crosses between already established species are
infrequent in nature and seem only in rare cases to give rise to
new species, hence their study is of secondary importance from
the standpoint of evolution, provided always that evolution
has taken place discontinuously. On the other hand, it is un-
fortunate in many ways to attempt to estimate the value of the
study of unit characters by the supposed bearing of the results
on the theory of evolution; for while those who believe that evolu-
tion has taken place by simple discontinuous steps will ascribe
the highest value to studies of this kind, those who believe that
evolution has taken place in some other way will be likely to
underrate the importance of the results as a contribution to the
theory of heredity. Already the bitter controversies that the
publication of these results has aroused are less concerned with
the results themselves, that are accepted by all, than with the
imagined bearing of the results on the theory of evolution. The
intolerance that each side sees-in the other is due not to the ac-
ceptance or denial of the results of the experimental work, but to
the arguments that pretend to show that evolution has or has
not taken place by discontinuous variation. Meanwhile there is
danger that we forget the importance of the experimental results
as a contribution to the study of heredity, whatever their bearing
on the theory of organic evolution may be.

LITERATURE, CHAPTER X

ACKERMANN. Tierbastarde. Kassel. 1898.

Bateson, W. Materials for the Study of Variation. 1894.

Bateson, W., and Grecory, R. P. On the Inheritance of Heterostylism
in Primula. Proceed. Roy. Soc. of London, LXXVI. 1905.

Conxiin, E. G. The Cause of Inverse Symmetry. Anat. Anz. XXIII.

1903.

172 Experimental Zoology

Crampton, H. E. Reversal of Cleavage in a Sinistral Gasteropod. Ann.

N. Y. Acad. Sci. 1894.
Darwin, C. The Effects of Cross- and Self-Fertilization in the Vegetable

Kingdom.

The Variation of Animals and Plants under Domestication. 2d ed.
1890.
Hotmes, S.J. The Early Development of Planorbis. Jour. Morph. XVI.
1900.
Rasi, C. Homologie und Eigenart. Verh. d. Deutsch. Pathol. Gesell.
II. 1900.

DE Vries, H. Species and Varieties. 1905.

CHAPTER XI
BEHAVIOR OF THE GERM-CELLS IN CROSS-FERTILIZATION

In the preceding chapters the characters of the hybrids re-
sulting from cross-fertilization have been considered. The
present chapter will deal with the behavior of the germ-cells
themselves when cross-fertilization is attempted. In certain
respects this topic covers a wider field than the preceding, since
there are many more species in which the eggs may be*entered
by the spermatozoa of other species (and the early development
take place) than of those that produce adult hybrids.

Experiments with Amphibia

Different species of European frogs have been frequently
utilized in crossing experiments. The most important results
are those of Rusconi (1840), Lataste (1878), Pfliiger (1882),
Born (1883), and Heron-Royer (1883).

It has been found in a number of different forms that the
spermatozoa of one species will enter the eggs of other species
and start the development. The egg may cleave, generally quite
irregularly, but later stages than this may not develop. In some
combinations the early, or even the later, gastrula stages may
develop, but the embryos perish without going farther. Finally,
in a few cases tadpoles, often having a weak constitution, may
be formed. Thus the two closely similar species, Rana fusca (¢)
and Rana arvalis (?), cross readily, and tadpoles have been reared
as far as the frog stage. The reverse cross gave no results.
Bufo variabilis and Bufo cinereus also cross, and toads may be
produced. The different races of Rana fusca intercross as
readily with each other, as each race fertilizes its own eggs.

173

174 Experimental Zoology

The other extreme is found where the eggs of Rana fusca are
fertilized by the eggs of the salamander, Triton, and divide
irregularly, but go no farther.

Pfliiger concluded from his results that cross-fertilization de-
pends less on the similarity of the adults than on the peculiarities
of the spermatozoa. Thus the spermatozoa of Rana fusca and
Rana arvalis are very different, and while cross-fertilization
takes place in one direction it does not in the other. On the other
hand the spermatozoa of Bufo cinereus and B. variabilis are much
alike, and reciprocal cross-fertilization is successful. In support
of his view Pfliiger points out that the spermatozoa are most suc-
cessful in crossing that have the thinnest or most pointed heads.
Furthermore those eggs are most easily crossed that belong to
species whose spermatozoa have the largest heads, because, being
as it were so constructed as to admit their own large-headed sper-
matozoa, they do not exclude spermatozoa of smaller size. This
view of the matter may explain the power of certain kinds of
spermatozoa to enter the eggs of other species, but it does not ex-
plain why, after entering, certain combinations develop normally
and others scarcely at all.

The conditions for normal development appear to be most
readily fulfilled when the two species are like each other in struc-
ture, which usually, though not invariably, means consanguinity.
Pfliiger found that the eggs of the frog have the greatest power
of being cross-fertilized at the height of the breeding season.
Certain experiments that I have made on other forms indicate
that this result may be due not so much to the eggs as to the
greater mobility of the spermatozoa at this time. Hertwig has
questioned Pfliiger’s conclusion, basing his objection on the evi-
dence derived from some experiments that he carried out on the
eggs of the sea urchin. He found that eggs could be more easily
crossed when overripe or stale, as when they have stood for some
hours in sea water, or after they have been injured by poisons.
It has been shown more recently by Vernon that while in a few
cases (7.e. in some species) more eggs can be cross-fertilized if
they have stood twelve to twenty-four hours in sea water, yet in

Behavior of the Germ-cells 175

most cases this does not hold. Eggs lose, as a rule, rather than
gain in their responsiveness to foreign sperm if kept too long.

Experiments with Echinoderms

A number of investigators have made crosses between different
species of sea urchins with varying success. In recent years
Vernon, Boveri, Seeliger, Morgan, Driesch, Herbst, Loeb, and
Godlewski have carried out experiments with these forms.
Vernon found that out of sixty-four possible combinations,
forty-nine gave the following results: twenty-nine developed
to the pluteus stage, nine to the segmentation, blastula, or
gastrula stages, and in eleven fertilization did not take place.

Vernon tried to show that the characters of the hybrid embryo
are dependent upon the relative ripeness of the eggs and of the
sperm in the two species that are crossed. He carried out his
experiments with several species of sea urchins found in the
Mediterranean. The breeding period of these animals extends
over several months, or even in one species throughout most of
the year. The height of the breeding scason may be different
for different species. During the time preceding and following
that of the full maturity of the eggs and sperm, the eggs may still
be fertilized, although fewer of them develop normally. For
example :—the eggs of Strongylocentrotus reach their optimum
in December or January, their minimum in July or August.
Spherechinus gives throughout the year mature eggsand sperm,
although in summer the percentage of larve that develop is
smaller. If the eggs of Spherechinus are fertilized by sperm
of Strongylocentrotus during the summer months, May, June,
July, the hybrid larve resemble the Spherechinus type (the
mother), although some of them (4 or less) show traces of the
paternal (Strongylocentrotus) type of larve. In November
the hybrids approach more nearly the type of Strongylocen-
trotus (the father), and in December are entirely of this pater-
nal type. In other words, as the sperm of Strongylocentrotus
becomes more and more mature, it transmits to the larvee its
own characters.

176 Experimental Zoology

The reciprocal cross, Strongylocentrotus ° and Spherechi-
nus 2 gave less striking results, because of the greater difficulty
in making the cross. In April, May, June, young stages were
obtained that died. In July and August 29 per cent reached
the pluteus stage. The hybrids showed no indication of their
double origin, but were pure Strongylocentrotus (maternal).
In November and December no eggs cross-fertilized.

Doncaster has carried out experiments in hybridizing sea
urchins that lead him to conclude that the different hybrids ob-
tained by Vernon at different times of year owe their peculiarities
to the temperature of the water in which they develop. Herbst
has recently carried out a more elaborate series of experiments
that lead him to a similar conclusion, although he thinks that
some other condition than temperature is also operative. What
the other condition is he did not determine; but he does not
think that it can be due to the relative condition of ripeness of
the male sex cells. In fact, his analysis of Vernon’s results, in
the light of his own observations, seems to show, for the sperm
at least, that Vernon’s evidence is most unsatisfactory.

The preceding results apply more especially to the later larve
or pluteus stages. In some respects the results seem to be in-
consistent with results that other observers have obtained with
the younger stages of these hybrids. Driesch has found, for
instance, that the method of cleavage, its tempo, the character
of the mesenchyme formation, and of gastrulation are char-
acteristic of the egg irrespective of the kind of sperm that is used.
In later stages, when the skeleton develops and the pigment ap-
pears, the larvee first begin to show their hybrid origin. On the
other hand, Boveri thinks that the hybrid characters appear
very early, but the principal difference between his view and that
of Driesch lies in the age at which each supposes the differences
to become apparent. There can be little doubt, however, that,
as a rule, in the early stages little or no trace of the paternal ele-
ments appear, and only later do they influence the characters of
the hybrid. This difference may be interpreted to mean that,
at first, the elements introduced by the sperm — the nucleus or

Behavior of the Germ-cells 179

the cytoplasm — have not had time to act or to increase suffi-
ciently in amount to affect the development. Most embryolo-
gists seem inclined to ascribe the effects entirely to the nucleus,
which they believe dominates all the changes in the protoplasm.
On the contrary, I am inclined to think that it has not been
shown conclusively that this influence is nuclear in origin, but
may possibly be due to the protoplasm introduced with the
sperm. The slow increase in amount of the introduced proto-
plasm might account for the insignificant part it plays during
the early development, when it is very small in amount com-
pared to that of the egg-protoplasm. If, as others suppose,
the chromatin of the nucleus is the all-controlling influence, it
is difficult to see why this is not apparent at once, since the
nucleus of the hybrid has equal amounts of paternal and ma-
ternal material. It may be fairly claimed, however, that the
introduced sperm-nucleus requires time to change the protoplasm
into its own sort of material.

The most striking case of the lack of influence of the sperm
nucleus on the egg is that recently given by Godlewski. By
following Loeb’s method and making the sea-water alkaline, he
has succeeded in fertilizing the egg of the sea urchin (Echinus
and others) with the sperm of the crinoid (Antedon). The hy-
brids were of the sea-urchin type in all respects observed, includ-
ing the pluteus stage.

Boveri carried out the ingenious experiment of fertilizing a
non-nucleated piece of the egg of one species of sea urchin with a
spermatozoén of another species." The pluteus obtained was
purely paternal. He concluded that the result was due to the
introduced nucleus. Both Seeliger and I have taken exception
to Boveri’s evidence on the ground that the hybrid pluteus that
can be obtained from nucleated pieces or from entire eggs is too
variable in its characters to give support to Boveri’s conclusion.
In fact, some of these hybrids are so similar to the paternal
type that they cannot be distinguished from it. Godlewski’s

1©. and R. Hertwig had previously shown that pieces of the sea urchin’s egg
without a nucleus may be entered by spermatozoa of the same species.
N

178 Experimental Zoology

experiment, described above, in which he used a non-nucleated
piece of the sea urchin’s egg and the spermatozoa of the crinoid
produced larve entirely of the maternal type, which is the con-
verse of Boveri’s result.

Factors involved in the Entrance of the Spermatozoon

The entrance of a foreign spermatozo6n into an egg is closely
connected with the question of normal fertilization. What brings
the sperm and egg together? How does the sperm enter the
normal egg, and what delays or prevents its entrance into eggs of
another specics? The immense collections of sperm around the
egg in normal fertilization has led to the idea that the egg at-
tracts the sperm. Certain experiments seemed to support this
view. Pfeffer’s experiments with the antherozoids of ferns have
often been cited as an instance of such an attraction. He found
that when a dilute solution of malic acid was inclosed in open
capillary tubes, and these tubes were immersed in a drop of
water containing antherozoids, the latter collected around the
open ends of the tubes, as though attracted by the malic acid.
The evidence in favor of this interpretation has recently been
considerably weakened by Jenning’s study of the behavior of
protozoa. These also will collect in a drop of acid, not, how-
ever, because they are attracted to the drop, but because no re-
action takes place when they pass from water into a more acid
solution. A reaction does occur, however, in passing from an
acid region into water. The reaction involves a backing of the
individual into the drop followed by a movement forward again
in a new direction. On coming a second time to the edge of the
acid area the reaction is again repeated. All individuals that
pass by chance into the acid remain there — caught like rats in
a trap —so that in time an accumulation occurs that might read-
ily suggest that the animals had been attracted to this region.

Strasburger claimed that the eggs of the seaweed, Fucus,
excrete a substance that attracts the spermatozoén from a dis-
tance of two diameters of the egg, but Bordet and Buller have
failed to corroborate this observation.

Behavior of the Germ-cells 179

Buller’s experiments with the eggs of sea urchins and starfish
have given important results. He pointed out that spermatozoa
accumulate around immature eggs, and also around mature eggs
that had been killed in osmic acid and then thoroughly washed
in sea water. In this case it is highly improbable that any attrac-
tion could exist. The results are due to those spermatozoa that
accidentally run into the membrane of the egg, remaining stick-
ing there as a result of some physical property of the jelly or of
some reaction on the part of the spermatozoédn. Buller showed
by means of the following experiment that the eggs do not se-
crete an attracting substance. Eggs were allowed to stand in a
little water from two to twelve hours. Capillary tubes were then
filled with this water and placed in a drop of sea water containing
the sperm. No collecting of spermatozoa around the ends of the
tube was observed. He alse tried other substances in the tubes,
viz. salts, sugars, ferments, and alcohol, etc., but no evidence
of their action in attracting the spermatozoa was observed.

Buller thinks that the spermatozoa are sensitive to contact,
hence on coming into contact with the membrane bore into it.
The spermatozoa swim in spirals in the water, but on entering
the jelly they take a straight course, which in most cases will
bring them into contact with the egg, although, if they should
enter quite obliquely they may bore through the periphery of the
membrane and pass out again on the other side. Thus even
after entering the membrane, there is no evidence that the egg
attracts the spermatozoa toward itself.

On the other hand it has been shown by von Dungern that the
egg and its membranes may contain and even give off substances
that act in some cases injuriously on the spermatozoa of other
species. The egg of the starfish, Asterias glacialis, contains a
substance that acts as a poison on-the spermatozoa of the sea
urchins — Echinus or Spherechinus. The minimal lethal dose
for sperm mixed with two cubic centimeters of sea water varies
with different individuals between the limits of g35 to ggoq
parts forhalfan hour. The same poison is found also in the skin
of the starfish. Cross-fertilization of the eggs of this starfish by

180 Experimental Zoology

sea urchin’s sperm is thus prevented by the action of the poison,
but it is going beyond the evidence to extend this conclusion, as
has been done, to other starfish and other sea urchins without
further examination.

In the sea urchin, Spherechinus, there is a poison in the pedicel-
lariz that is injurious to the spermatozoa of the starfish, but if
this poison exists also in the egg it is not strong enough to prevent
the spermatozoon of the starfish from entering, at least in those
species of sea urchins in which such a combination has been
artificially brought about. The spermatozoa of Spherechinus
itself are killed by this same poison from its own pedicellariz, but
a much stronger dose is required.

It has been also found by von Dungern that extracts of the
eggs of Echinus, Sphzrechinus, Strongylocentrotus, or Arbacia
do not kill the spermatozoa of the starfish even in the strongest
solutions. Why, then, do not the spermatozoa of the starfish
readily enter the eggs of these sea urchins? ‘Their inability to
enter appears to be due to another factor. The egg membrane
of these sea urchins agglutinizes the spermatozoa of the starfish,
z.e. they stick to it, and this interferes with their penetration.
In Spherechinus, however, agglutinization does not take place.
Why, then, does not cross-fertilization occur here? Von Dun-
gern claims to have found still another substance in this sea
urchin’s egg that excites even immature sperm to greater activity.’
He believes that these exciting substances may in some cases
prevent cross-fertilization because they change the kind of ac-
tivity shown by the spermatozoa. He observed that spermato-
zoa that do not show rotational movement on meeting a solid,
or semi-solid, body may do so when excited in this way and fail
in consequence to penetrate.

Von Dungern also tries to show that eggs or egg membranes
may contain substances that favor fertilization by its own sperm.
Eggs of Echinus were rubbed up and mixed with pieces of jelly
that had first been carefully washed. Spermatozoa were then

‘Immature sperm excited in this way may then even fertilize their own kind
of egg, 7.e. Starfish eggs.

Behavior of the Germ-cells 181

added and observed to stand vertically to the surface of the jelly
as they penetrated. If the piece of jelly was not’ first mixed
with the extract, the spermatozoa simply rotated on its surface.
On the other hand, starfish spermatozoa on coming into contact
with Arbacia jelly behave as with the simple jelly alone, i.e.
they do not stand vertically to it. The vertical position may be
due, von Dungern thinks, to the presence of some substance in
the extract that lowers the excitability of the spermatozoa to con-
tact. Much still remains obscure, but these results show clearly
some of the factors involved in fertilization and cross-fertiliza-
tion.

Artificial Helps to Fertilization and to Cross-jertilization

A number of embryologists have found that normal fertiliza-
tion may be assisted by adding certain substances to the water
in which the sperm are placed. These substances excite the
spermatozoa to greater activity, and in this or in other ways
promote fertilization. It has also been known for a long time
that the glands connected with the ducts of the male may secrete
substances that make the spermatozoa active. For instance,
KGlliker discovered that the secretion of the prostate glands of
the male greatly excites the spermatozoa. Ordinarily the sper-
matozoa are quiescent as long as they remain in the testes or
even in the ducts leading from them, but become active when the
secretions are added or when set free in water. Roux states that
the fertilization of the frog’s egg is helped by the addition of one
fourth per cent sodium chloride to the water, and Wilson has
found that the spermatozoa of Patella can fertilize a much larger
proportion of the eggs if a little potassium hydroxide is added
to the sea water. Torelle and I have found that somewhat
immature sperm of the starfish can be made active by ether, am-
monia, salt solutions, etc., and will then fertilize the eggs. Von
Dungern had previously observed that extracts of the eggs of
the sea urchin excite immature spermatozoa to activity.

The most striking case of this sort is that of the fertilization of
the sea-urchin egg by sperm of the starfish, recently described

182 Experimental Zoology

by Loeb. If to 100 parts of an artificial sea water containing
0.3 cubic centimeter sodium hydroxide the eggs of Strongylo-
centrotus and the sperm of the starfish be added, as many as 50
to 80 per cent of the eggs will be fertilized. The eggs segment
and it appears that even the early stage of development — more
or less abnormal — may be reached.

Polys permy

In the great majority of known casesonly a singlespermatozoén
normally enters the egg to fertilize it. The moment one sperma-
tozo6n penetrates, some reaction takes place in the egg that pre-
vents other spermatozoa from entering. The reaction involves,
in some cases, contraction of the egg from the surrounding mem-
brane; in other cases the formation of amembrane; and we may
infer that other kinds of changes in the protoplasm may in other
cases interfere or prevent the entrance of more than one sperma-
tozodn. If the egg is injured or narcotized or is stale, more than
one spermatozoén may enter. The result is generally injurious,
for, while a regular or more often an irregular cleavage may fol-
low, the embryo fails to develop properly. This failure seems
to be due, in the main, to the unequal distribution of the chroma-
tin or other materials to the different parts of the egg.

It has been found in hybridizing that in certain cases more
than a single sperm enters the egg, and the irregularities in
cleavage that follow seem to be due in part to this condition, but
in other cases where one spermatozoén enters, the failure to
develop must be ascribed to different causes. In these cases
the results may be due sometimes to irregularities in the mechan-
ism of division, but at other times to some incompatibility or
failure of the two uniting elements to work together to a com-
mon end.

According to the degree of perfection to which the distribution
of the materials of the egg is carried out, the subsequent develop-
ment is more or less perfect. Even in cases where the hybrid
reaches the adult condition, it has been observed that it may have
a weak constitution, and even when it is strong the hybrid is

Behavior of the Germ-cells 183

sometimes sterile. This sterility is due apparently, in some cases
at least, to irregularities in the division of the germ-cells. When
we recall that at one stage in the development of the germ-cells
there may be a pairing and subsequent fusion of the ma-
ternal and paternal chromosomes, we can readily imagine that
any differences in their behavior at this time might lead to
disastrous results.

LITERATURE, CHAPTER XI

BorDeEt. Contribution 4 l’Etude de I’Irritabilité des Spermatozoides chez
les Fuccacées. Bull. de l’Acad. Belgique. 3° sér. XXVII. 1894.
Boveri, Ta. Ein Geschlechtlich erzeugter Organismus ohne miitterliche
Eigenschaften. Sitz.-ber. d. Ges. f. Morph. u. Phys. Miinchen, V.
1889.
Ueber die Befruchtungs- und Entwicklungsfihigkeit kernloser Seeigel-
eier. Arch. f. Entw.-Mech. II. 1895.
Ueber den Einfluss der Samenzelle auf die Larvencharaktere der
Echiniden. Arch. f. Entw.-Mech. XVI. 1903.
Noch ein Wort ueber Seeigelbastarde. Arch. f. Entw.- Mech. XVII.
1903.

Buiter, A.H. The Fertilization Process in Echinoidea. Report 70, Meet.
Brit. Assoc. got.

Is Chemotaxis a Factor in the Fertilization of the Eggs of Animals?
Quart. Jour. Micro. Sc. XLVI. 1903.

Contributions to our Knowledge of the Physiology of the Spermatozoa
of Ferns. Ann. of Botany, XIV. 1900.

CaLKins, G. Studies on the Life-History of Protozoa, I. The Life-Cycle
of Paramcecium Caudatum. Arch. f. Entw.-Mech. d. Organismen.
XV. 1902.

Studies on the Life-History of Protozoa, III. The six hundred and
twentieth generation of Paramcecium Caudatum. Biol. Bull. III.
1902.

Duke Y. Embryons sans noyau maternel. C. R. Acad. des Sc. V.

, 1898.

Etudes sur la Merogonie. Arch. de zool. expérim. et gén. V. 1899.

Sur interpretation de la fecondation merogonique et sur une théorie
de la fécondation normale. Arch. de zool. expérim. et gén. VII.
1899.

Etudes expérimentales sur la maturation cytoplasmique et sur la
parthénogénése expérimentale. Arch. de zool. expérim. et. gén. IX.
1901.

Dine J. Ueber Gesetzmassigkeit in der Ortsverinderung der Sperma-
tozoen und in der Vereinigung derselben mit dem Ei. Arch. f. die
gesammte Physiol. XXXVIII. 1886.

DriescH, H. Ueber rein miitterliche Charaktere an Bastardlarven der
Echiniden. Arch. f. Entw.-Mech. VII. 1898.

Ueber Seeigelbastarde. Arch. f. Entw.-Mech. XVI. 1903.

184 Experimental Zoology

v. DuNGERN, E. Die Ursachen der specifitat bei der Befruchtung. Centrbl.
f. Phys. XIII. gor.

Neue Versuche zur Physiologie der Befruchtung. Zeitschr. f. allgem.
Phys. I. rgo2.

Giarp, A. Développement des ceufs d’Echinodermes sous l’influence
d’actions kinétiques anormales (solution salines et hybridization).
Compt. Rend. des Sc. et Mém. de la Soc. de Biologie. Paris.
1900.

Ciitevcce E., Jun. Die Hybridisation der Echinideen- und Crinoideen-
familie. Bull. de. Acad. des Sc. de Cracovie. 1905.

Untersuchungen ueber die Bastardierung der Echiniden- und Crinoi-
den familie. Arch. f. Entw.-Mech. XX. 1906.

GurwitscH, A. Zerstérbarkeit und Restitutionsfahigkeit des Protoplas-
mas des Amphibieneies. Verh. Anat. Ges. 18 Vers. Jena. 1904.

Guyer, M.F. Spermatogenesis of Normal and of Hybrid Pigeons. Univ.
of Cincinnati. Bull. LXII. Ser. II. Vol. II. r1go0.

HAcKER, V. Ueber das Schicksal der Elterlichen und Grosselterlichen
Kernanteile. Jen. Zeitschr. f. Naturw. XXVII. 1902.

Bastardirung und Geschlechtszellenbildung. Zool. Jahrb. Suppl. VII.

1904.

Keuerern, A. Physiologische Studien ueber die Samenflussigkeit. Zeitschr.
f. wiss. Zool. VII. 1856.

Liprorss. Ueber den Chemotropismus der Pollenschlauche. Ber. d.
Deutsch. Bot. Gesell. XVII. 1895.

Logs, J. On the Artificial Production of Normal Larve from the unfer-
tilized Eggs of the Sea Urchin. Amer. Jour. of Phys. III. rgoo.

Further Experiments of Artificial Parthenogenesis. Amer. Jour. of
Phys. V. 1900.

On a Method by which the Eggs of a Sea Urchin (Strongylocentrotus
purpuratus) can be fertilized with the Sperm of a Starfish (Asterias
ochracea). Univ. of California Pub. Phys. I. 1903.

The Fertilization of the Egg of Sea Urchin by the Sperm of Starfish
Univ. of California Pub. Phys. I. 1983.

Ueber die Befruchtung von Seeigeleiern durch Seesternsamen.
Pfliigers Arch. CXC. 1903.

Further Experiments on the Fertilization of the Egg of the Sea Urchin
with Sperm of Various Species of Starfish anda Holothurian. Univ.
of California Pub. Phys. I. 1904.

Further Experiments on Heterogeneous Hybridization in Echinoderms.
Univ. of California Pub. Phys. II. 1904.

Weitere Versuche ueber die heterogene Hybridization bei Echinoder-
men, Pfliigers Arch. CIV. 1904.

On an Improved Method of artificial Parthenogenesis (I, II, III, Com-
munications). Univ. of California Pub. Physiol. II. 1905.
Loew, O. Die Chemotaxis der Spermatozoen in weiblichen Genitaltract.

Sitzungsber. d. Wienner Akad.; Math.-naturw. Cl. CXI. 1903.
Massart, J. Sur I’Irritabilité des Spermatozoides de la Grenouille. Bull.
d. Acad. Roy. de Belgique. 3° sér. XV. 1888.

Sur la Penetration des Spermatozoides dans |’uf de la Grenouille.
Bull. de l’Acad. Roy. de Belgique. 2™? sér. XVIII. 1889.
Mivyosat, C. Ueber Reizbewegungen der Pollenschlatiche. Flora.

LXXVIII. 1894.

Behavior of the Germ-cells 185,

Motiscu, H. Ueber die Ursachen der Wachtumsrichtungen bei Pollen-
ea Sitzungsber. d. k. Acad. d. Wiss. in Wien. 18809,
1893.

Morean, T. H. The Fertilization of Non-nucleated Fragments of Echino-
derm Eggs. Arch. f. Entw.-Mech. II. 1895.

Some Further Experiments on Self-Fertilization in Ciona. Biol. Bull.
VIII. 1905.
“Polarity” considered as a Phenomenon of Gradation of Materials.
2 Jour. of Exp. Zool. II. r1g05.

HERON Rover Et Cx. vAN BaMBEKE. Hybridation des batraciens anoures
et ses produits congénérés et biogénérés. 1883.

Observations relatives 4 la ponte du Bufo vulgaris et aux couches pro-
tectrices de l’ceuf des Batraciens. Bruxelles. 188s.

Nouveaux Facts d’Hybridation Observe chez les Batraciens anoures.
Mem. d. 1. Soc. Zool. d. France, XLIX. 18g.

Dev'Iste, A. Del’hybridation chez les amphibies. Ann. d. sc. naturelles,
XVII. 1873.

PrerFER, W. Locomstorische Richtungsbewegungen durch chemische
Reize. Untersuchungen aus. d. Bot. Inst. zu Tiibingen. I. 1884.

Priucer, E., und Smrra, W. Untersuchungen ueber Bastardierung der
anuren Batrachier und die Prinzipien der Zeugung. Pfliigers Arch.
XXXII. 1883.

Rossi, U. Sull ’azione dell’ electtitria nello sviluppo delle uova degli
Amfibi. Arch. f. Entw.-Mech. IV. 1896-1897.

Ruscont, M. Ueber kiinstliche Befruchtung von Fischen und ueber einige
neue Versuche in Betreff kiinstlicher Befruchtung an Frdéschen.
Miillers Archiv. 18 40.

SEELIGER, O. Gibt es geschlechtlich erzeugte Organismen ohne miitterliche
Eigenschaften? Arch. f. Entw.-Mech. I. 1895.

Bemerkungen ueber Bastardlarven der Seeigel. Arch. f. Entw.-Mech.
III. 1896.

StemnacH, E. Untersuchungen zur Vergleichenden Physiologie der mann-
lichen Geschlechtsorgane insbesondere der accessorischen Ge-
schlechtsdriisen. Arch. f. d. gesammte Phys. LVI. 1894.

Stevens, N. Experimental Studies on Eggs of Echinus Microtuberculatus.
Arch. f. Entw.-Mech. XV. 1902.

STRASBURGER, A. Das botan-Prakticum. 2 Auf. 1887.

TrIcHMANN, E. Ueber Furchung befruchteter Seeigeleier ohne Beteili-
gung des Spermakerns. Jen. Zeitschr. f. Naturw. XXXVII. 1902.

VeRNoN, H. M. The Relation between the Hybrid or Parent Forms of
Echinoid Larve. Philos. Trans. Roy. Soc. B. CXC. 1898.

Cross-Fertilization among Echinoids. Arch. f. Entw.-Mech. XIX.
1900.

The Effect of Environment on the Development of Echinoderm Larve.
Trans. Roy. Soc. 1895.

The Relations between the Hybrid and the Parent Forms of Echinoid
Larve. Proc. Roy. Soc. LXIII. 1898.

Witson, E. B. Experimental Studies in Germinal Localization, II. Jour.
Exp. Zool. I. 1go04.

ra

CHAPTER XII
INBREEDING

For species with separate sexes the term “‘inbreeding”’ is
used to express either the union between brothers and sisters
or between offspring and parent, in one or more gen-
erations. It would have been convenient to apply the word
“interbreeding”? to unions between individuals of the same
species — individuals not standing in close filial relationship
—and to contrast this process with “cross-breeding ” (or cross-
ing or hybridizing) where the union is between different races,
varieties, or species. Convenient as these distinctions might
be, they have not been adhered to by writers, and the term
“interbreeding ” is sometimes used where inbreeding seems the
preferable term.*

It is a general belief amongst breeders that inbreeding leads to
injurious results. If this statement is intended to apply to all
living species of animals and plants it is probably not true, but
that it may be true in certain cases seems fairly well authenticated.
Even in these latter cases the question may fairly be raised
whether the results may not be due, in part at least, to the similar-
ity of the external conditions under which the individuals have
been kept. In other words, there is some evidence to show that
if the external conditions are different for different individuals,
the injurious effects of inbreeding are lessened or disappear in
some cases; and conversely the breeding of individuals of dif-
ferent descent if subjected to identical external conditions
might show the same deleterious effects as those associated with

1 Darwin often used interbreeding where inbreeding in the above sense is
meant.

186

Inbreeding 187

inbreeding. It must always be remembered that some hermaph-
roditic plants are nearly always self-fertilized without apparent
injury. They seem to be adapted to this process, while other
plants that are adapted for cross-fertilization may show injurious
effects if too closely inbred.

Darwin has paid much attention to this question of inbreeding.
He has dealt with it extensively in his book on ‘‘Animals and
Plants under Domestication.” His general position may be
summed up in the following quotations: ‘‘The evil effects from
close interbreeding are difficult to detect, for they accumulate
slowly and differ much in degree in different species, whilst the
good effects which almost invariably follow a cross are from the
first manifest. It should, however, be clearly understood that
the advantage of close interbreeding, as far as the retention of
character is concerned, is indisputable, and often outweighs the
evil of a slight loss of constitutional vigor.”

“The consequences of close interbreeding carried on for too
long a time are, as is generally believed, loss of size, constitu-
tional vigor, and fertility, sometimes accompanied by a ten-
dency to malformation.” It is generally supposed that the
evil effects do not appear for several generations. ‘‘On the
other hand, the benefit from a cross, even when there has not
been any very close interbreeding, is almost invariably at once
conspicuous.”

Darwin says ‘That any evil directly follows from the closest
interbreeding has been denied by many persons; but rarely, by
any practical breeder; and never, as far as I know, by one who
has largely bred animals which propagate their kind quickly.”
Despite the fact that crossing interferes seriously with the results
of the breeder who is trying to keep his strain pure, all breeders
practically without exception make use of crossing from time
to time to strengthen their stock. Darwin quotes numerous
cases amongst domesticated animals to show that inbreeding is
injurious. In several cases where the pedigree has been kept
with care there is distinct evidence of injury.’

1 For details see ‘Animals and Plants,” Chap. XVII.

188 Experimental Zoology

Experiments with Mice and Rats

Von Guaita! has made some experiments with mice that had
been already inbred by Weismann for 29 generations. He in-
bred the animals through seven generations and found clear
evidence of diminishing fertility. In the following table the
total number of young, the number of litters, and the average
young in a litter are given.

WEISMANN’S DATA

1 to ro generation: 1345 young; 219 litters; aveg. per litter 6.1
Ir to 20 generation: 252 young; 62 litters; aveg. per litter 5.6
21 to 29 generation: 124 young; 2g litters; aveg. per litter 4.2

Von Guaita’s data, beginning with the mice of the last pemeration of
Weismann’s stock, are as follows: —

ist and 2d generations . : : . : . . : - 3.5
gdand 4th. : Sos . . : : ‘ c . 3.6
sth and 6th . : ia ol : : . . : ‘ + 29

Comparing the first and the last averages, it is seen that there
has been a reduction in fertility of about 30 per cent.

Ritzema Bos inbred rats for 30 generations. The family
started with a female albino white rat, that was paired with a
wild rat, and gave twelve young. A white male of different
parentage was bred to seven of these offspring, but later no
other foreign blood was introduced during the six years of the
experiment. Parents were bred to offspring, and sisters to
brothers. The average number of young per litter is shown
in the following table: —

1887 1888 1889 1890 1891 1892
74 14 14 633 age 33
During the first 20 generations (in the first four years) there was
scarcely any decline in the productiveness, as the table shows,
but in the following ten generations there was a marked and
sudden decline. The number of pairings that were sterile in-
creased steadily as shown in the next table: —

' Berichte der Naturfarschenden Gesellschaft zu Freiburg. 1900.

Lnbreeding 189

1887 1888 1889 1890 1891 1892
0 ~ 2.63 5.55 17.39 50 41.18

The young of later generations also died at amuch greater rate,
as shown in the following table: —

1887 1888 1889 1890 1891 1892
3.9 4.4 5.0 36.7 36.4 45.5

It is noticeable that the mortality rose suddenly at the same time
(1891) that the number of young per litter decreased. Pairing
between brother and sister was found to give poorer results than
pairing of father and daughter, or mother and son. Thirty-
six per cent of the pairs of siblings were infertile, but only
21.4 per cent of the unions between parent and child. Pair-
ing of young of the same litter gave essentially the same result as
pairing of young of different litters of the same parents.

The weights of the rats decreased during the time of the experi-
ment. The maximum size of a full-grown male was 300 grams
in 1891; in 1892 few individuals reached the weight of 275
grams; and after six years the weight had declined to 240 grams.
Crampe, who inbred rats, found that they became diseased and
abnormal after a time, but Ritzema Bos did not find this in his
experiments. He thinks that Crampe must have begun with a
diseased stock, which became more diseased as a result of the
weakening induced by inbreeding.

In the case of man there are many customs, beliefs, and,super-
stitions regarding the effects of close inbreeding. Races in
widely different parts of the world have rules prohibiting mar-
riages between relations. It would be interesting to know how
such regulations have arisen, since, as Darwin points out, savages
have no finer moral or social feelings to consider, and are not
likely to have taken into account the effects on distant progeny ;
for it is admitted that if evil effects are produced they are not
likely to appear before several generations have closely inter-
married. The abhorrence of incest, that is so pronounced in
most races, can scarcely be claimed to be an inherent instinct, as
Darwin points out. The Hindus have developed to an equal

190 Experimental Zoology

degree the abhorrence of defilement by contact with other castes
or with certain animals.

George Darwin, who examined the question, as far as the sta-
tistics were known, found no clear evidence of any evil effects
caused by close intermarriage, or at most the evil was very small
in amount, and out of all proportion to the prejudice against
consanguineous unions. One might easily offer a number of
possible explanations of the basis for this feeling, but such specu-
lations would have little scientific value in the present state of
our knowledge.

Experiments with the Pomace Fly

The most extensive series of experiments on inbreeding that
have been made in recent years are those of Castle and his pupils.
They have used the fruit or pomace fly, Drosophila ampelophila,
which can be easily kept in confinement, and will breed through-
out the year in a warm place. The whole life-cycle may
be completed in eleven or twelve days (egg three, larva three,
pupa three days, and two days before the imago begins to lay
eggs).

Brothers and sisters were paired in each generation. A pair
of pupz of the same parentage was put into each jar. If the
flies proved to be of different sexes, they were left together; if of
the same sex, they were rearranged. The larve left the fruit
on which they had fed and pupated on the top or sides of the
glass. They were collected and counted, and the productive-
ness of the pair calculated on this basis. Sometimes no eggs
were produced, which was at times due to the sterility of the
male, at other times of the female, as shown by pairing them with
other individuals.

The longest, or A-series, was carried through 59 generations
extending over three years. We can appreciate the extent of the
series if we compare it with the time required to produce the same
number of generations of mankind. If we allow three genera-
tions to a century, it would take 20 centuries to give 60 genera-
tions. In other words, a corresponding experiment on man would

Lnbreeding IgI

have extended from the beginning of the Christian era to the
present time. The productiveness of the fertile pairs is shown
in the accompanying curve. It will be seen that the productive-
ness fell during the winter of 1903, rose during the spring, and cul-
minated in the late summer. It fell again during the next au-
tumn and winter, but rose again during the next summer. It
fell again sharply in the next autumn, but rose rapidly during

/Avernge

No. Young
350

150

100

50

NIA

TOA ava y
el

Generation 6 10 1 15

[20] 530) a5 ~~ 40~—S«”y:~« SSCS Tb

' Ul
Oct.1002 Jan.1908 Apr.1909Tune 1908 Oct/1903Jan'1904 Apr.1904 June 1904 Oot.1004Jan.i906 Apr.1005

Fic. 18. Curve of productiveness of fertile pairs of A series of Pomace flies.
(After Castle.)

the winter and spring, reaching a point much higher than at any
previous time, but fell again during the spring. The last rise
was connected with a transfer of the flies to a warm chamber.
This A-series is characterized by low productiveness, and an
inclination to sterility, when compared with otherseries. Theaver-
age number of young was never as high as 200 until the fifty-third
generation, usually it was under 100, and about one in five pairs
was sterile. ‘Control cultures made under identical conditions,
but from stock not inbred, had a productiveness two or three

.

192 Experimental Zoology ‘

times as great and showed no signsof sterility.” The productive-
ness of the A-series was not due to inbreeding, but was inherent
in this stock from the beginning. ‘That this is the explanation of
the results is shown by comparing the A-series with other inbred
series; and also by the sudden rise in productiveness of the
A-series at the end of the third year.

Two other inbred series, M and N, were from the beginning
more productive than the A-series. The average of eggs never
fell below 200, except in a single generation, and was usually
nearer 300, occasionally rising to 400 or more. The productive-
ness of this series, carried through 28 generations, is shown in the
next diagram. No sterile individuals were found in the N-series
and only one in the seventh, one in the tenth, and one in the four-
teenth of the M-series. In the M-series “‘the average brood of
the first inbred generation was 213, while the average for the
series as a whole is 280, an apparent increase of fertility under
inbreeding. In the N-series the first generation gave an aver-
age of 231 young, while the series as a whole gave 278.5 young to
a brood.”” In both cases the first inbred generation may have
happened to be lower than the average because of less skillful
treatment. Hence a fairer comparison is between the first and
the second half of the series, thus: —

| GENERATION 1-7 GENERATION 8-14
M-series 263 296
N-series 317 240

This comparison shows an increase in the productiveness in the
M-series and a decrease in the N-series.

An interesting experiment consisted in crossing the M- and
the N-series with the A-series. After crossing the two broods
the offspring were mated inter se (brother and sister) for several
generations in order to observe the effects on subsequent genera-
tions. ‘The results showed that when a female of the A-series is
mated with a male of the M- or N-series, her productiveness is not
increased. The daughters, however, produce more young than

Lnbreeding 193

the mother and more than other A-mothers. Their productive-
ness is nearly the same as that of the M- and N-series. In other
words, the male has transmitted the greater productiveness of his

Average
Ne oO Young

450

——
os
q

=

_——

_

=
ad
ae
=
eucaces or

Genemtiont 3 6 7 9 M1 13 16 17 19 2] 23 O57 28
Qcti1908 ! Tune 1904 Oct 1904 Jan.1905 Apr. 1005

Fic. 19. Curve of productiveness of fertile pairs of M- and N-series of Pomace
fly. (After Castle.)

race to his offspring. Complete sterility, however, appeared

more often among the daughters than in the M- and N-series;

and in this respect the influence of the A-series is observed.
Conversely, if a female of the M- or N-series is mated with a

ce)

194 Experimental Zoology

male of the A-series, her productiveness is not decreased. In
other words, the A-male produces enough sperm to fertilize more
eggs than produced by the females of the A-series, and as much
as needed for the M- or N-series. The difference in productive-
ness in the A- and the M- or N-series is due to a difference in the
number of fertile eggs.

The daughters of this union showed the same productiveness

as the mothers of the M- and N-series; but none were sterile.
Productiveness, therefore, in both cases dominates less produc-
tiveness, unless, of course, the results are due to the cross itself
bringing up the productiveness to the normal.
, The grandchildren of both cross-series showed much varia-
tion in their productiveness. Some were as unproductive as the
A-series and contained sterile individuals. Others are as pro-
ductive as the M- or N-series. It appears that low productive-
ness may skip a generation and then reappear, as do characters
that Mendelize.

Selection of individuals of the same brood, that show high
or low productiveness, gives positive results. The pairs taken
from more productive broods are invariably more productive, as
shown by the following figures : —

PARENTAL Broop FILiaL Broop
Lower productiveness 196 197
Higher productiveness 232 239

The annual cyclical rise and fall of productiveness, shown by
the A-series in particular, but also by others, suggests the influ-
ence of external factors, especially when transference to a warm
chamber increased at once the productiveness. The experi-
ments show, in fact, that temperature, upon which proper fer-
mentation of the food depends, has a distinct influence; but
independently of this, some families are characterized by high pro-
ductiveness, others by low. ‘Improved conditions increase the
productiveness of all . . . but the response is more prompt and
vigorous on the part of a race normally high in productiveness.”

Inbreeding 195

Castle’s general conclusion is that ‘inbreeding probably re-
duces very slightly the productiveness of Drosophila, but the
productiveness may be fully maintained under constant in-
breeding (brother and sister) if selection is made from the more
productive families. There are also indications that cross-
breeding increases the productiveness of closely inbred families.

Behavior of Germ-cells in Inbreeding

Whether the decrease in fertility observed in some cases of
inbreeding is due to the failure of the spermatozoa to enter the
eggs, or to the failure of the fertilized eggs to develop, or to both
factors, cannot be stated, but there are a few observations that
show, indirectly, that the germ-cells themselves may be affected.
These cases unfortunately apply only to hermaphroditic species.

It has been known to botanists for a long time that in certain
flowers pollen will not fertilize the ovary of the same plant. In
other cases where self-fertilization will occur, the pollen of other
plants is prepotent over that of the same plant. It seems not
improbable that the failure of the pollen to fertilize its own ovary
is due to the failure of the pollen tube to grow down into its own
stigma or style sufficiently far to reach the ovules. The pre-
potency of foreign pollen would be due on this view to more
rapid growth than that of the pollen of the same plant.

Only one case of this sort is known in animals. Castle dis-
covered that the spermatozoa of the hermaphrodite ascidian,
Ciona intestinalis, fail, as a rule, to fertilize the eggs of the same
individual, although they will fertilize the eggs of any other indi-
vidual. I have carried out a series of experiments on this and
other species in the hope of discovering to what this action is due.
The results are complicated and difficult to interpret.

In the first place the sperm of an individual (Ciona intestina-
lis) does not fertilize equally well the eggs of all other individuals.
In some cases 100 per cent of the eggs are fertilized; in others
25 per cent or less; but I have seen no clear case where the sperm
in good condition is as sterile with eggs of any other individual
as with its ‘“‘own” eggs. The failure to fertilize properly is due

196 Experimental Zoology

to inability to enter the egg, and not to failure of the develop-
mental process after fertilization.

If the spermatozoén succeeds in entering, as it does in excep-
tional cases, the egg develops normally. It is possible by exciting
the spermatozoa to unusual activity to cause them to enter the
eggs of the same individual, and these develop. The failure,
then, is due to the spermatozoa being unable to overcome
some resistance met with in attempting to enter their “own”
eggs. There is some evidence that the failure is due to some-
thing in the egg or its membrane that lessens the activity of its
“own” spermatozoa that come into contact with it, or pos-
sibly to a failure of the egg to receive the proper stimulus to
take in the spermatozoén; but if the latter, it is difficult to
understand how exciting the sperm to greater activity should
cause it to enter.

It is not possible to make the egg receive its own sperm by
immersing it in the blood or extracts of the ovary of another indi-
vidual. Conversely, it is not possible to make the sperm enter
its “own” eggs by soaking it first in the blood or testis-extract
of another individual. Allowing the eggs of an individual to
stand for some time in sea water, so that any substance they
excrete may be set free, and then placing the eggs and sperm of
another individual in the same water, does not lead to self-
fertilization. This experiment shows that the results are not
due to soluble substances set free by the egg causing the egg to
become fertilized by the sperm of another individual. Con-
versely, if the eggs of an individual are soaked for some time
in sea water, and then sperm of the same individual is added in
the same sea water and later the eggs of another individual are
added, they will be fertilized. This result shows that the eggs
do not set free a substance that brings to rest their own sperm,
for, as shown in the experiment, the sperm will fertilize other
eggs, if they are added to the solution. These and other ex-
periments show that whatever the nature of the result, it is
brought about by some insoluble substance in the egg, or is due
to the failure of the egg to respond to the stimulus of its own

Lnbreeding 197

sperm, owing, perhaps, to the absence of some substance in the
sperm. If, however, the sperm is excited to greater activity by
alcohol, ammonia, ether, or by certain salts, it may force an
entrance despite the resistance.

In another ascidian, Cynthia partita, I have found that self-
fertilization often occurs, but the sperm generally fertilizes
its.own eggs less often. than those of another individual. In
the ascidian, Molgula manhattensis, self-fertilization occurs as
readily as fertilization by sperm of another individual.

The failure to self-fertilize in some hermaphroditic or-
ganisms suggests that factors similar to those in Ciona may be
at work in forms closely inbred for several generations, where the
closely related males and females would seem to stand in some-
what the same relation to each other as the male and female gen-
erative organs in the same individual.

LITERATURE, CHAPTER XII

Bos, Ritzema. Untersuchungen ueber die Folgen der Zucht in engster
Blutverwandtschaft. Biol. Centralb. XIV. 1894.

Caste, W.E. The Early Embryology of Ciona Intestinalis. Bull. Mus.
Comp. Zool. XXVII. 1896.

Castle, W. E., CARPENTER, F. W., Clark, A. H., Mast, S. O., and Bar-
rows, W. M. The Effects of Inbreeding, Cross-Breeding, and
Selection upon the Fertility and Variability of Drosophila. Proc.
Amer. Acad. Arts and Sciences, XLI. 1906.

Darwin, C. Animals and Plants under Domestication.

FABRE-DoMENGUE, P. Unions consanguines chez les Colombins. L’Inter-'
médiaire des Biologistes, I. 1898.

Guaita, C. von. Versuche mit Kreuzungen von verschiedenen Rassen der
Hausmaus. Ber. Naturf. Gesell. Freiburg, X. 1898.

Morcan, T. H. Self-Fertilization induced by Artificial Means. Jour.
Exp. Zool. I. 1904.

Some Further Experiments on Self-Fertilization in Ciona. Biol. Bull.
- VIII. 1905.

CHAPTER XIII
INFLUENCE OF SELECTION

ARTIFICIAL selection is par excellence an experimental pro-
cess, and has been applied with success to all of the known forms
of variation. It has been used in the case of elementary varieties,
that follow the law of discontinuous variation and inheritance
as described in the preceding chapters. Whenever, in these
cases, Mendel’s law holds, the formation of “pure” races can
be quickly brought about by selecting the extracted dominants
and extracted recessives. Where more than a single character
is involved, the formation of a new race by selecting individuals
containing two, three, or more desirable characters is more in-
volved, and yet can be carried out with certainty. Selection
produces nothing new in such cases, except in forming new com-
binations of already existing characters.

Selection may be applied to elementary species, which are new
forms or sports that present one or more new characters. In
this case isolation is the conserving principle of most impor-
tance; but in cases where a single individual alone comes into
existence, it must be crossed with the parent type, and from the
offspring obtained selection of those that have the new character
must be made. If the new character is transmitted to all the
offspring, no selection amongst them may be necessary; but if
the new character follows the Mendelian law of splitting in the
second generation, selection and isolation may be necessary to
obtain as quickly as possible a new race. In this form of selec-
tion, also, nothing new is created, but what has appeared ‘‘spon-
taneously” is preserved.

The third form of selection is that applied to fluctuating

variations. In the present chapter this topic especially will be
198

Influence of Selection 199

considered. It is a vital question for the theory of’ evolution
whether new forms, new species, can be created by the selection of
fluctuating or individual variations as the Darwinian school has
claimed for nearly fifty years. Experiment alone can decide
whether this claim is justified. That new forms have appeared
as a result of selection by man no one will deny; for most of
our domesticated animals and cultivated plants bear direct testi-
mony in favor of this view; but the cardinal question remains
whether elementary varieties and species, or fluctuating varia-
tions, have been intentionally or unintentionally picked out in
the formation of new races.

Fluctuating or Individual Differences

Whatever systematic definition of species may prove most
satisfactory, the fact that no two individuals are alike is what
concerns us at present. I shall first give an account of the law
followed by these fluctuations, then an account of their inheri-
tance, and finally consider what man can accomplish by their
selection and breeding.

Quetelet first drew attention to the law followed by fluctuating
variations, and this law is spoken of as Quetelet’s law. The
fluctuations of animals and plants appear as though they were
the outcome of chance, or, expressed differently, ‘“‘the deviations
from the average obey the law of probability.” Let us take
an example.’ If we examine a group of men as to their height,
we find by arranging them in a row that an almost continuous.
straight line will connect the tops of their heads. The line
slopes from the tallest to the shortest man, but the slope is less:
inclined in the middle than at the ends. This means that there
are in the middle region more men that are nearly the same
height. In another way this important fact can be brought out
more clearly. If we place in one column all the men between
64 and 65.9 inches; in another column to one side of the last all
the taller men between 66 and 67.9 inches; in another column

1 This example is taken from Davenport’s paper, Popular Science Monthly,
1902.

200 Experimental Zoology

on the other side all the shorter men between 62 and 63.9, and
continue in this way until all the men are classified, we find a
triangle-like group (Fig. 20, B). If we join the tops of the
columns, we get a curve that corresponds in form to the mathe-
matical curve of probability (Fig. 20, A).

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O
OO
| \ eee®
i \ eee
COO
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fi \ OOOO
q OMOOOIO
a Sy eeeee®
; OOOO COOS
B
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i
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Fic. 20.

The same result may be shown graphically in another way.
If we take a lot of peas and put all of a size (i.e. within certain
limits) in’ similar cylindrical jars, and then arrange the jars so
that the one with the medium-sized peas is in the middle, and
those with the next larger peas on one side according to their
size; and those with next smaller peas on the other side accord-
ing to their size, etc., it will be found that the tops of the piles
will give roughly the curve of probability. In both of. these cases

Lnfluence of Selection 201

the curve is given by the individuals themselves, but in practice
this method is seldom employed. The measurements are
used directly to construct the curve.

The causes of the differences in individuals are to be found
partly in the various external conditions under which the indi-
viduals have developed, and partly in initial differences at the
beginning of development in the individuals themselves. How
far these latter may be traced to still earlier effects of external
conditions, and how far to the possibilities of combinations of
inherited differences within the organism itself is difficult to say.

It is clear that a knowledge of the curve of variability does not
contribute, in any large measure, to an elucidation of the causes
that bring about the fluctuations. These causes must be studied
by examining the effects of the external factors that influence the
organism.

If the curve of variability is high and does not extend out far
along the base line, this indicates that the form is little variable.
If the curve is low and extends far out at the sides, the vari-
ability is large in amount. One side of the curve is sometimes dif-
ferent from the other. Such a curve is asymmetrical (Fig. 20, C).
Thus the curve may rise abruptly on one side and rise very slowly
on the other side. The meaning of this is obscure. Equally
important are the cases in which the curves have more than one
summit. As an example, I may cite the case of the rhinoceros
beetle (Fig. 20, D). -Its curve hastwosummits. Suchacurve is
said to be bimodal. This means that there are many long-
horned beetles and many short-horned ones, but few of the in-
termediate size. Here we see variations about two means, as
though the group were about to split into two groups; but
whether anything of the sort is really occurring is entirely un-
known. It has been observed in certain plants that the secon-
dary summits coincide with the points on which in allied species
the main summit lies, and this appears to give us a suggestion
concerning the factors producing the change. If, for example,
we look upon the characters seen in the allied species as dormant
in the one with two modes, but which may, under certain

202 Experimental Zoology

conditions, be awakened to activity, we seem to get further insight
into the conditions. That alternate or contrasted characters
may exist in a single germ is well recognized. A further exam-
ple will show the probability of this explanation. If individ-
uals of two races, having different ‘‘modes,”’ are crossed, the
offspring will often show a tendency to give a curve with two
summits — one for each parent mode. The characters of both
parents are present in the offspring and sometimes one, some-
times the other, dominates. These, however, are special cases
and are by no means to be taken to illustrate the usual effects
produced by crossing two races.

Selection of Fluctuating Differences

We come now to the question of inheritance in regard to fluc-
tuating variation. Ordinarily, succeeding generations show
the same variations. This applies when the pairing of the indi-
vidual is left to chance alone, and when the external conditions
are similar. But the outcome is different if individuals show-
ing the same kind of fluctuation are picked out or selected and
inbred. The most striking results are produced, of course, when
individuals showing one extreme or the other are utilized. Sup-
pose, for example, we select two individuals, standing near the
same outer limit of the curve —two individuals, let us say,
showing the fluctuation in question developed almost to the
greatest degree ever found. If the offspring of this pair be
measured, it will be found, in most cases, that the average has
been raised in the direction selected. The curve has moved
toward the variation selected in the parents. In other words,
and this is the important point, the average of the individuals
is now higher than before, i.e. there are now more individuals
than in the preceding generation with the character in question
better developed. By selecting again individuals from the same
extreme, the average can again be transferred in the same direc-
tion. As the process is continued, however, the average gain
soon decreases, and finally stops without ever having transgressed
the outer limit of fluctuation found in the first instance, provided

Influence of Selection 203

a sufficient number of individuals had been examined. It is
true that these extreme individuals may be more common after
careful selection through many generations, but the species can-
not be carried as a whole beyond or much beyond the upper limit
to which it sometimes attains without selection. Thus it appears
impossible by the process of selection of fluctuating variations
to transform the species into anything new or different from what
it was.

There is a further point of especial importance in this connec-
tion. If vigorous selection of the extreme individuals is not
carried out in every generation, there is a quick return to medi-
ocrity, and what has been laboriously gained is quickly lost, —
the species left to itself swings back to its more stable condition.
The effects of selection are only temporary, and nothing perma-
nent can be acquired in this way. Consequently we are justified,
I think, in denying that through natural selection of indi-
vidual differences the process of evolution could have taken
place.

We cannot leave the subject without referring to Galton’s im-
portant law of ancestral inheritance. This law states that each
individual inherits on the average from his two parents together
50 per cent, or one half of the whole inheritance; from the four
grandparents 25 per cent, or one fourth; from the eight great
grand-parents one eighth of the whole, etc., the total inheritance
being 100 per cent. It is probable that this rule* applies only
to fluctuating variations, and not to cases of sudden or discon-
tinuous variation to be discerned later.

Finally, correlated variation is often observed between differ-
ent parts of the same individual, which vary together. For
example, height in man is correlated with the length of the
humerus. There are, however, other organs in man which do
not seem to be correlated with height; for instance, the length
of the clavicle.

' Pearson’s law of inheritance is different (see Darbeshire), the correlation
between successive generations being expressed by the fractions .3, .15, .075,

-0375, etc.

204 Experimental Zoology

Elimination within the Species

Although elaborate data have been published in respect to
fluctuating variation and elimination within a given generation,
comparatively few data have been gathered to show what occurs
in successive generations under identical, as well as under differ-
ent, conditions. Haphazard mating seems to produce the same
variation in successive generations, despite the accidental elimina-
tion in each, hence in many cases nothing appears to be gained
permanently by the elimination of the unsuccessful or of the
unfortunate individuals. In order to discover to what extent
the causes of fluctuating variation are internal and to what extent
external, we need experiments carried out through several gen-
erations on pedigree animals, selectively paired, and kept under
constant conditions. While such results may or may not throw
light on the formation of new species, they will be of much
importance as a contribution to the study of variation within
the limits of the species.

The elimination of individuals within each generation may be
due to catastrophes that overwhelm all the individuals involved,
without regard to their individual differences, and the great
destruction of immature individuals appears to belong largely to
this class; or the elimination may follow the lines of individual
differences which gives a selective value to the survivors. Which
of these, or of other kinds of elimination, occur even for those
forms that have been studied statistically, is known only incom-
pletely in a few cases.

Weldon has measured the frontal breadth of Carcinus menas,
found living in the harbor of Plymouth. The measurements
were made in the years 1893, 1895, 1898, and show, during this
time, a decrease, which he believes to be due to elimination
brought about by a change in the water of the sound. The mean
of the frontal breadth for 1895 was less than for 1893, and a
further change was found in 1898. These results are for male
crabs. The changes in the female were less in amount. The
elimination is due, Weldon thinks, to the fine mud, brought

Influence of Selection 205

down by the rivers that became greater in amouat during these
years. He tested this view by keeping crabs in water contain-
ing the same mud in suspension. The crabs that died were on
the whole broader than those that survived. Weldon suggests
that the frontal breadth is correlated with the breadth of the
opening into the gill chamber, and the latter determines the
amount of mud that enters, and in this way the elimination is
produced.

Should these conclusions of Weldon be established, they show
that a change in the external conditions may cause the elimina-
tion of certain individuals of the species, and in this way affect
the mean of the survivors. In other words, natural selection
occurs within the limits of the species. But it does not follow
from this, as Weldon takes for granted, that if the same process
of elimination were continued, a new species would be evolved.
Raising the mean to the highest point attainable within the limits
of the species does not necessarily lead to the formation of a new
species. This point has already been discussed in other connec-
tions.

Crampton has studied variation and elimination in the moths
of Philosamia Cynthia. In the first generation that he studied
there was a high rate of mortality, and since the pupe were col-
lected in their normal environment, before they had been sub-
jected to the winter frost, the death rate must have been con-
nected with some inherent weakness or with conditions that they
met with during their normal life. Out of 942 pupe collected,
there were 628 that were dead, so that only 329 ‘‘selected”’
individuals remained. The causes of the death of the 628
pupe are not known. One might suspect that they had become
parasitized ; but Professor Crampton tells me that he examined
them to see if this was the case, and found no evidence of the
sort. It does not seem probable that the external conditions
to which the pupe were exposed had anything to do with the
high death rate, and its causes are probably to be found in the
organization of the animal. Whether bacterial disease, or dis-
ease of some other kind, is responsible for the results, was not

206 Experimental Zoology

determined, but Crampton found no evidence that the death
could be assigned to such causes.

Only 181 perfect moths (97 males, 84 females) emerged; 75
were imperfect to a slight degree, 38 were malformed, and 16
failed to metamorphose at all. It will be seen that only 16.6
per cent of the individuals that pupated became perfect adults.

Measurements of the pupz show that the survivors are longer,
narrower, and deeper than the eliminated pupe. It also appears
that survival does not necessarily or invariably accompany a
condition of lower variability, although such a relation is gen-
erally observed. Crampton’s general conclusion is that the
elimination is based on the general and total efficiency of the
individual, and ‘‘this is determined by the proper codrdination
of functional and structural elements. The actual basis for
elimination is, in a word, ‘correlation.’ If this correlation is
slight, the individual ranks as unfit; if it is higher, the individual
is fit and more likely to survive.” Whether the correlation itself
determines the outcome, as Crampton appears to mean, or
whether the lack of correlation is due to some deeper-lying im-
perfection, such, for example, as the presence of disease in the
caterpillar which determines both the lack of correlation and the
failure to develop, cannot be stated. If the latter is the case, the
lack of correlation is only an index of a diseased or imperfect
organization, and therefore not in itself responsible for the
elimination. Crampton’s idea, however, is that the imperfect
correlation is due to formative factors in the insects themselves
which leads directly to their death, and not to elimination due to
external conditions.

Variation and Parthenogenesis

The problems of variation and of inheritance have generally
been studied in animals that reproduce sexually. In only a few
parthenogenetic forms, namely, in some insects, in aphids
and in the honey bee, and in the crustacean, Daphnia,
has the problem of variation been examined. Weismann ad-
vanced the idea that the purpose of sexual reproduction is to

Influence of Selection 207

induce variability. It is interesting, therefore, to find that
variability may be as marked in non-sexual forms produced
parthenogenetically as in sexually produced forms.

Warren has studied the variation of one of the aphids, Hyalop-
terus trirhodus, and of Daphnia. The aphids, to consider only
this case, were inclosed in small bags on the leaves of their na-
tive plant, and the parents and their full-grown progeny were
later killed and measured, the distance between the eyes and
the antenna length being the two measurements taken. It was
found that the external conditions, including also the natural
changes in the food plant, produced very decided effects, espe-
cially in size, so that it was not possible to rear successive genera-
tions under identical conditions. Of 522 offspring registered,
455 grew up. The death rate for the second generation was 12.8.
The larger mothers tended to produce healthier offspring. The
variability of the second generation was found to be greater than
that of the parents, and this is the rule also for sexually produced
offspring — in man, for example. In the third generation the
variability was diminished, attributed by Warren to the poorer
external conditions, which, affecting the size, reduce the dis-
tance between the eyes more than they reduce the length of the
antenne. In general, the results show that the variability of a
parthenogenetic race is not smaller than that of sexually pro-
duced forms. Casteel and Phillips, have found in the hive bee
that the males or drones that develop from unfertilized eggs are
more variable than the females that come from the fertilized

eggs.

The Results of Selection and Hybridization of Wild Elementary
Species

Domesticated animals and plants in different countries often
show differences other than those that can be attributed to differ-
ences in climate, etc. It has been stated that in some cases the
domesticated forms bear a close resemblance to the local wild
races of the country, or else show traces of such a similarity.
The results may be due to man having brought under domestica-

208 Experimental Zoology

tion the particular wild forms found in his vicinity. More often,
however, the results may be due to the wild forms having been
crossed with the domesticated forms so that the characters of
the former have become impressed on the domesticated animals
and plants.

There is satisfactory evidence in the case of plants to show that
wild “varieties” and elementary species have often been crossed
with domesticated plants in order to produce hybrids having one
or more of the characters of the wild forms. Not only have wild
varieties of the surrounding country been used, but wild varieties
from all parts of the world, especially in modern times. Thus
many of our domesticated animals and plants are hybrids, and
the process of selection has been employed to pick out, amongst
the great diversity of types produced in this way, those forms
that have a practical value or that appeal to man’s esthetic or
commercial taste, or to his fancy for novelties.

It is probable in many of these cases that the wild “varieties”
that have yielded valuable results are true elementary species
having fixed characters that have come from germinal variation,
and not local races whose peculiarities are due solely to the exter-
nal conditions to which they have been subjected.

Selection under Domestication of Mutations, Saltations, Sports,
and Discontinuous Varieties in General

It is probable that the sudden occurrence of sports or saltations
has furnished breeders with much of the material for the produc-
tion of new forms. It has long been known that new types ap-
pear in domesticated animals and plants. If these are picked
out — selected — and progeny raised from them, it is possible
to establish a new race. In the case of hermaphroditic plants,
self-fertilization of the new individual will give at once a large
number of individuals like the parent. In unisexual animals
and plants the new form must be crossed with the original
stock from which it sprang. In some cases all of the offspring
may be like the new form, and from some of them a new, fixed
type may be raised. In other cases, all the offspring may be

Lnfluence of Selection 209

like the old form, but, if interbred, some at least of their descend-
ants will give the new race. Occasionally the new type bred
back to the parent type may give a hybrid intermediate between
the two parents, and this hybrid may become the parent of a new
race that does not revert to either parent type. These different
cases will later be considered more fully, For the present it will
suffice to call attention to the fact that it is generally possible
in one of these different ways to produce a new race by selection
and isolation, and it appears probable that sudden variations
of this kind have furnished the breeder with his most valuable
material.

A point of great importance is that the new type may at first
not differ very much from the original stock; and hence may
often appear to be only a fluctuation. If, however, the new type
reappears in the same strength in its offspring, it shows istelf to
be a mutation and not a fluctuation. Until within quite recent
years this distinction has not been fully appreciated, and all
small differences were assumed to be fluctuations.

The origin of most of our domesticated forms is unknown;
their history goes back to a remote time. Nevertheless within
comparatively recent years a number of new types — sports or
mutations — have arisen and their history is a matter of record.
A relatively large number of such instances are known to bota-
nists, and de Vries has recently given a careful analysis of such
cases that appear to be authentic. Fewer cases are known
to zodlogists; the more important of these may be briefly
given.

The most remarkable instance is that of the ancon ram.
There appeared in 1791 in a flock of ordinary sheep a ram with
short legs and a long body. This ram, bred to a ewe of the com-
mon type, produced descendants having the same characteristics
as the ancon father. In this way the new race originated which
breeds true to the new type. The turnspit dog has short and
crooked legs, recalling the condition of the ancon sheep. Darwin
states that this kind of monstrosity is not uncommon in various

animals, and cites the case of jaguars in Paraguay, and of a
P

210 Experimental Zoology

pariah dog in India. It would seem that the result must be due to
a single change of some sort in the germ that involves a correlated
change in many parts at once.

The Mauchamp breed of sheep originated also from a single
individual ram that appeared in 1828. The wool is long and
straight, not frizzled as in the merino. The ram and his imme-
diate descendants showed also other new structural characters.
The individuals of this breed are “‘of small size with large heads,
long necks, narrow chests, and long flanks.” These peculiarities
are removable by ‘‘judicious crosses and selection.”

A monstrous breed of cattle exists in South America, the
niata cattle, that must have originated from the introduced
cattle; and to judge from its peculiarities, the breed must have
appeared as a sport, although its actual origin is unknown.
The upper lip is drawn back and the nostrils are situated high
up. The exposed incisors give a peculiar appearance to these
animals. The skull is much modified, the hind legs are shorter
than usual, etc. The animals breed perfectly true to their kind.
The short-horn cattle are also said to have originated from a
sudden variation.

A most interesting case is that of the Japan peacock, or
black-shouldered kind. This form has appeared several times
in England and differs so much from the ordinary forms that
have given birth to it that it would be ranked as a different
species were not its parentage known. ‘The birds breed true
and have been known to replace in time the flock from which
they arose.

Several other cases are on record. Sports have been recorded
among domesticated fowls, pigeons, horses, mice (rhinoceros
mice), and frequently in plants of many kinds. They are known
to have furnished the basis for several domesticated races. Un-
fortunately exact information of the origin of races in this way
would only be recorded when the saltation had been large in
amount. When small, the change would be put down to fluctu-
ating variations, especially by breeders who have had heretofore
little interest in the difference between these two kinds of varia-

Influence of Selection 211

tion. The fact that so many of the domesticated races breed true
furnishes the strongest evidence in favor of the view that they
have originated as sports and not as the result of the selection of
fluctuating individuals. In fact, as has been said, it appears
that a race produced by selection of fluctuations can only be
kept up by a rigorous process of selection, as shown in the cases
of some cultivated grains, flowers, and beets. Most domesti-
cated races, hawever, differ from these in one all-important re-
spect. They do not need a process of selection to maintain
them after they have appeared, and, as I have said, this is very
strong, indirect evidence in favor of the view that they have
arisen by the selection of sports or discontinuous varieties, whose
chief peculiarity is that they transmit from their inception the
new characters to their offspring.

LITERATURE, CHAPTER XIII

Bumeus, H.C. The Variations and Mutations of the Introduced Sparrow.
Biol. Lect. Woods Holl. 1897.

The Variations and Mutations of the Introduced Littorina. Zool.
Bull. I. 1898.

CastEeEL, D. B., and E. F. Pairirps. Comparative Variability of Drones
and Workers of the Honey Bee. Biol. Bull. VI. 1903.

Crampton, H. E. On a General Theory of Adaptation and Selection.
Jour. Exp. Zool. IT. 1905.

Davenport, C., and BLanxinsuip, J. A Precise Criterion of Species.
Science, VII. 1898.

DAVENPORT C., and BuLtarp, C. Studies in Morphogenesis, VI. A
Contribution to the Quantitative Study of Correlated Variation and
the Comparative Variability of the Sexes. Proc. Am. Acad. Arts
and Science, XXXII. 1896.

Davenport, C.B. The Statistical Study of Evolution. Pop. Sci. Monthly.
190l.

On ce Variation of the Shell of Pecten, etc. Amer. Natural, XXXIV.
1900.

Gusntitative Studies in the Evolution of Pecten, III. Proc. Am.
Acad. Arts and Science, XXXIX. 1903.

Duncxer, G. Die Methode der Variations-Statistik. Arch. f. Entw.-
Mech. d. Organismen, VIII. 1899.

Fretp, W. A Contribution to the Study of Individual Variation in the
Wings of Lepidoptera. Proc. Am. Acad. Arts and Science,
XXXII. 1898.

Gatton, F. Correlations and their Measurement, chiefly from Anthro-
pometric Data. Proc. Roy. Soc. London. XLV. 1888.

Natural Inheritance. London 1889.

212 Experimental Zoology

The Average Contribution of Each Several Ancestor to the Total
Heritage of the Offspring. Proc. Roy. Soc. London, LXI. 1897.
Pearson, K. Contributions to the Mathematical Theory of Evolution.
[I. On the Dissection of Frequency Curves.] Phil. Trans. Roy. Soc.
London, CLXXXV._ 1894.

Contributions to the Mathematical Theory of Evolution, II. Skew
Variation in Homogeneous Material. Phil. Trans. Roy. Soc. Lon-
don, CLXXXVI. 1895. ;

Mathematical Contributions to the Theory of Evolution, ITI. Regres-
sion, Heredity, and Panmixia. Phil. Trans. Roy. Soc. London,
CLXXXVII._ 1896.

Mathematical Contributions to the Theory of Evolution. Ona Form
of Spurious Correlation, which may arise when Indices are used
in the Measurement of Organs. Proc. Roy. Soc. London, LX.
1897.

Wathenatical Contributions to the Theory of Evolution. On the Law
of Ancestral Heredity. Proc. Roy. Soc. London, LXII. 1898.

The Chances of Death, and Other Studies in Evolution. 2 vols.
London. 1897.

The Grammar of Science, 2d ed. 1900.

Pearson, K, and Fiton, L. Mathematical Contributions to the Theory of
Evolution, IV. On the Probable Errors of Frequency Constants
and on the Influence of Random Selection on Variation and Corre-
lation. Phil. Trans. Roy. Soc. London, CXCI. 1808.

SHULL, G.H. Galtonian Regression in the ‘‘Pure Line.” Torreya, Vol. 5,
No. 2. 1905.

Tuompson, H. On Correlations of Certain External Parts of Palemon
Serratus. Proc. Roy. Soc. London, LV. 1894.

Varicny, H. pe. Experimental Evolution. 1892.

DE Vries. Ueber halbe Galton-Kurven als Zeichnen diskontinuirlichen
Variation. Ber. deutsch. Bot. Ges. XII. 1894.

Eine zweigipfelige Variations-Kurve. Arch. f. Entw.-Mech. II.

1895.
Warren, E. Variation in Portunusdepurator. Proc. Roy. Soc. London,
LX. 1896.

An Investigation on the Variability of the Human Skeleton with Espe-
cial Reference to the Naquada Race. Phil. Trans. Roy. Soc. Lon-
don, CLXXXIX. 1897.

WELLDON, W. The Variations occurring in Certain Decapod Crustacea, I.
Crangon vulgaris. Proc. Roy. Soc. London, XLVII. 18g.

Certain Correlated Variations in Crangon vulgaris. Proc. Roy. Soc.
London, LI. 1892.

On Certain Correlated Variations in Carcinus manas. Proc. Roy.
Soc. London, LIV. 1893.

Report of the Committee for Conducting Statistical Inquiries into the
Measurable Characteristics of Plants and Animals. Part I: An
Attempt to Measure the Death-rate due to Selective Destruction of
Carcinus mznas with Respect to a Particular Dimension. Proc.
Roy. Soc. London, LVII. 1895.

CHAPTER XIV
THE THEORY OF EVOLUTION

In the preceding chapters we have examined the evidence
concerning the influence of external conditions in bringing about
changes in the structure of animals; we have examined the
claim that acquired characters are inherited; we have also
studied the results from hybridizing in Mendelian and other
cases, and we have considered the problems connected with the
differences between the inheritance of fluctuating (or continu-
ous) and of definite (or discontinuous) variations. The evi-
dence from these different sources has unquestionably a very
direct bearing on the problem of organic evolution, yet so many
questions are still unsettled that any conclusion drawn from the
evidence that we now possess must be provisional and perhaps
premature. It is with this understanding that I venture on the
following analysis of the bearing of the evidence on the theory of
evolution. That the process of evolution is complex, and that
many conditions and several kinds of variation may have con-
tributed to it, most zodlogists seem at the present time willing to
admit. Ifin the following pages I have laid especially emphasis
on the results of recent experimental work on discontinuous
variation and inheritance, it is because we have here, I think, the
most satisfactory evidence in regard to certain factors. that may
have played an important réle in the process of evolution. Other
factors may also have been involved, but we have insufficient
experimental evidence to prove that this is the case.

The formulation of the modern theory of evolution we owe
chiefly to the French naturalists, Buffon, Lamarck, and Geoffroy
St. Hilaire; to some extent also to Erasmus Darwin. Their

theories were based largely on evidence from comparative
213

214 Experimental Zoology

anatomy, embryology, and paleontology. Charles Darwin
occupies a unique position. Like his predecessors, he argued
for the theory of evolution on the basis of comparative anatomy
and paleontology, but in addition he brought forward his theory
of natural selection to account for the ‘‘origin of species,” as
the result of the survival of the individuals that are better
adapted. In support of his theory of natural selection he
introduced the evidence from artificial selection as carried out
by man. Here almost for the first time the experimental study
of evolution was begun. Darwin’s more immediate followers,
however, did not carry on the experimental work much beyond
the stage at which he left it. In the last decade, however, great
activity has begun along experimental lines with most promising
results, and at the present time the study of evolution has passed
into an experimental stage.

The results that de Vries has obtained with plants in his experi-
mental garden have opened a new era in the study of evolution,
for he has shown that the process may not be so slow that it can
only be detected by elaborate mathematical calculations, or at
least that it may at times be rapid and easily detected; and,
what is more important, he has shown that this process is going
onat the present time. De Vries’s results will be described later,
and his evidence compared with that obtained by zodlogists for
animal forms; but before taking up de Vries’s work, certain
other considerations must first be dealt with.

The Analogy with Artificial Selection

If, as Darwin supposed, the domesticated races have been
formed largely by the selection of fluctuating variations, a strong
argument, at least, would have been established in favor of an
analogous method in nature having produced wild species; but
it is probable that Darwin relied too extensively on the statements
of breeders and amateurs as to the process by which their results
were obtained. It is clear that many of these breeders lack the
scientific training and appreciation of the value of evidence to
make their conclusions trustworthy, and however interesting

The Theory of Evolution 215

and suggestive their work may be, it needs to be carefully revised
and verified before it is safe to build on it a theory of evolution.

In the case of artificial selection it now seems probable that
selection of definite variations, the introduction of wild races,
and the results of hybridizing have all played a far more im-
portant réle than the selection of fluctuating variations. ‘The
evidence is clearer in the case of plants than in that of animals.
In animals the domesticated races have been, in most cases,
longer in confinement, and the history of the origin of the different
forms is lost in the remote past. Nevertheless it is not probable
that the method employed in producing domestic races of ani-
mals has been essentially different from that of plants; and ina
few cases, at least, there is evidence that the same method has
been employed for both.

The Influence of the Environment

Zodlogists and botanists alike have been impressed by the fact
that under different external conditions different species are found.
While this statement does not hold for all species, yet it holds
for many. Natural barriers also are likely to separate different
forms (races) of the same species, and isolation of any kind seems
to be connected in some way with the occurrence of different
forms. It is not my purpose here to consider the different
hypotheses that have been built up on this evidence, but the
fact itself is so patent and so important for our present purpose
that it must receive here somewhat fuller treatment.’

A not unnatural inference from the facts of distribution is that

1 Gulick has recently (1905) gone over this ground exhaustively, and the in-
fluence of isolation has been discussed by a number of specialists, Jordan, Allen,
Merriam, and others in recent numbers of Science (1905-1906).

2 Isolation is often assumed to produce its results by preventing the new forms
(however they may arise) from crossing back with the parent species, yet it is
by no means true for all species and varieties that they are isolated from each
other except in so far as their special conditions of life are different. Internal dif-
ferences may produce physiological isolation which appears as potent a factor as
geographical isolation. It might be confusing cause and effect to assume in
the former case that the isolation produced the new forms. A new mutation
may itself cause its own isolation.

216 Experimental Zoology

external conditions have slowly modified the forms, and the evi-
dence for this view seems almost irresistible when we find that
many animals and plants are distinctly modified when placed
in a different environment, without, in most cases, however,
transcending the limits of the variety or race. On the other
hand, it is equally true that when brought back to the original
environment the changed forms return in most cases to their
former condition. In this respect they differ from wild species,
from fixed varieties, and from elementary species, which remain
true to their type under varying conditions, except for the tem-
porary changes just mentioned. Therefore it does not seem
probable that changes of this kind directly transformed one
species into another. Suppose, however, that a new environ-
ment may sometimes call forth in the germ-cells effects that are
definite and inherited, — fixed, in other words, in the sense that
the germ-cells are true to their kind, —then we can perhaps
harmonize the apparently contradictory evidence. The new mu-
tation thus induced might be, in some forms and under certain
conditions, similar in kind to the effects produced on the body-
cells. The results, however, would not be reached through
the body-cells, but by independent, definite variation of the
germ-cells, as a result of a new environment. Offspring from
such germ-cells should remain true to their new kind when
returned to the original environment unless the germ-cells are
again affected; but whether a return to the original type would
even then occur cannot be stated. Macdougal has recently
made the important discovery that mutations may be induced
by treating the germ-cells of the evening primrose with salt solu-
tions.

In other cases the new mutation or mutations may not be in
the direction in which the body-cells are temporarily affected
(if affected at all), but in some other direction or directions, and
new types are formed bearing no relation to the former modifi-
cations affected in the body-cells. That the new type may also

1 This statement is somewhat arbitrary. It is intended to mean that with a
return of the former conditions the new type returns to its original condition.

The Theory of Evolution 217

have temporary effects produced in its somatic cells by the en-
vironment must be granted, but such effects should not be con-
fused with these permanent mutations brought about in the germ-
cells. On some such hypothesis as this it may be possible to
harmonize current views. ,

De Vries’s Results with GEnothera

Although de Vries’s experiments have been made entirely with
plants, the results are of so great importance that they must be
stated here briefly; for there can be little doubt that if his con-
clusions are established they will apply equally well to animals.
His principal work has been with Lamarck’s evening primrose,
(nothera Lamarckiana. This American species has been intro-
duced into Europe, where it has been cultivated in gardens for a
long time. It has also escaped and established itself in a wild
condition. De Vries found a field at Hilversam near Amster-
dam in which the escaped evening primrose grew, but also several
other closely related forms which as subsequent experiment
showed had sprung from Lamarck’s primrose. ‘These new
forms bred true, when self-fertilized, showing that they were
fixed types and not local races depending on special conditions.
They are new elementary species.

When the seeds of Lamarck’s primrose were sown in de Vries’s
experimental garden, a small percentage of them gave rise to
some of the same types that had been found growing wild.
Other types also appeared. In all, de Vries has described seven
such elementary forms. The chief character of these mutations
is that they breed perfectly true to their type when self-fertilized.
Occasionally another new type may appear from the seeds of a
mutation itself. These also in most cases breed true to their kind
if self-fertilized. The parent type, O. Lamarckiana, has been
found to produce year after year its quota of mutations. The
percentage of new forms, although small, — not much more than
about 1.5 per cent, — seems to be constant, and the mutants

continue to appear year after year.
De Vries has formulated a theory or rather several theories

218 Experimental Zovlogy

based on these results. He lays much emphasis on what he
calls physiological units or unit-characters, but disclaims any
desire to locate these in any special part of the cell, and believes
the evidence justifies him in supposing that each new step or
mutation involves a change in the organization of the germ-cell of
such a kind that a new unit-character appears. The change
may be slight or great compared with the parent form, but its
unity is shown by its behavior in heredity. New elementary
species are characterized by having at least one new physiological
unit. By way of example I may cite the following cases of
mutations amongst animals. The appearance of the merino
ram was probably due to some physiological change in a germ-
cell of one of its parents of such a sort that one character espe-
cially, viz. the wool, was changed. Other less striking characters
were also present, and inasmuch as these are always associated
with the merino type of hair, they all belong together. The
ancon ram showed differences in the proportion of nearly the
whole body; but the entire change must probably be referred
to a single, although profound, unit change in the germ-
cells. The occasional appearance of the turnspit type of dog,
which resembles the ancon ram in the form of its body, indicates,
if it does not prove, that this kind of change, involving nearly
all the parts of the body, may readily occur. So numerous are
the parts affected in these cases that it is impossible to ascribe
the results to ‘‘accidental” combinations of the different organ
elements, for it is inconceivable that just these combinations
could ever appear more than once, but must be due to some
definite change in the germ-cells which can appear in this
particular way. The same statement must also hold for
Lamarck’s primrose, where, however, instead of one possible
combination there are several possible ones that become realized.
If we use Galton’s oft-quoted comparison of a polyhedron, we can
see that resting on its most stable face it may be rocked back and
forth, but always returns to its same resting-place. Such oscil-
lations on a single face would correspond to the fluctuating varia-
tions of a species. Should a greater movement take place, the

The Theory of Evolution 219

polyhedron may roll over on to another face, which would corre-
spond to the change of a species into a new one. Should the
polyhedron be so constructed that one of the new faces is much
more stable, or easily reached, we can understand why in some
cases only a single mutation is likely to occur when the balance is
upset. Such would be the case with the turnspit type of dog,
the Japanese peacock, the merino sheep, the short-horned cattle.
If, on the other hand, one new face is as likely as the other to
give a stable condition, results like those in the primroses would
seem to be produced. We might even go farther and conclude
that this has been the case with the two hundred elementary
species of Draba verna, with the many elementary species of the
wild apple, and possibly with the races of domesticated pigeons.
The analogy need not be pushed too far, nor be taken for more
than it is worth.

De Vries attempts to draw a sharp line between elementary
species and varieties, and while the distinction is useful, yct, on
the whole, it seems rather arbitrary. A variety is a type in
which one of the parent-species characters has disappeared —
not necessarily absolutely, for it may remain only latent in the
germ-cells, but it never develops or only under unusual condi-
tions. Asan example, an albino mouse is a variety of the house
mouse. It is characterized by the loss of a character — the
pigment. Nevertheless as we have seen in the preceding chap-
ters the character that disappears may really be only latent in
the germ-cells.

It is evident that it may often be convenient to distinguish
between new types in which a new character or combination of
characters has appeared, and new types distinguished only
by the loss of a character. The distinction may seem to be one
of only minor importance were it not that it has been held to
separate new types that obey quite different laws of heredity.
It is sometimes said that varieties, in de Vries’s sense, follow
Mendel’s law, while elementary species do not. De Vries has
indicated how he supposes this difference might come about.

1 Hybridization often brings such latent characters once more to the front.

220 Experimental Zoology

A variety has the same number of unit-characters as has the
parent species, and when the two are crossed the unit-characters
pair, each with its kind, subsequently in the second generation,
separating again to give the Mendelian ratio. On the other hand,
when a new species arises, a new unit-character is formed that
is assumed to have no pair in the parent form, hence de Vries
assumes that it must divide and go to all the cells when the Men-
delian characters simply separate. It is not evident why a new
unit-character should be assumed to divide and an old one that
has lost its pair should not divide. It seems simpler to assume
that both kinds always divide, but that in the variety the con-
dition of latency has appeared, and that this condition alter-
nates in the hybrid’s germ-cells with the old character, while in
the “progressive” mutant the new character always dominates.’

Linnean species are, according to de Vries, large groups com-
posed often of many elementary species, some of which at least
— if we exclude local races due to environment — take the rank
of varieties of ordinary systematic writers. Linnean species are
separated from each other partly by progressive, partly by retro-
gressive and degressive, characters,” i.e. they may differ both by
true specific differences (progressive) and also by varietal char-
acters (retrogressive and degressive).

In this connection it is worth while to call attention to a point
that de Vries has considered in regard to the origin of the muta-
tion itself. The new type originates by the union of a male
and a female cell — spermatozoén and egg. In which of these
did the mutation first appear? If only in one, the mutation is a

1 As stated above, de Vries ascribes the behavior in inheritance of a progres-
sive character to the absence of a fellow-unit with which to pair. Is it not more
probable that the new character is a modification of some original one which is
its pair? From this point of view the behavior ofa progressive character is not
due to its pairing or lack of pairing, but to some inherent quality that makes it
dominate in the pair. There is some evidence that all new characters do not
behave in this way, and that some of them are recessive to the original characters
as seen in the long hair of the guinea pigs.

? Progressive characters are mew characters. Retrogressive characters are

those that have disappeared, z.e. become latent. Degressive characters are la-
tent characters that have become active.

The Theory of Evolution Bat

hybrid. If all hybrids followed Mendel’s law, which they do
not in fact, we should expect a resolution of the parent types in
the second generation. Since this does not occur in most of the
mutations of cenothera, we must infer that if the mutation is a
hybrid it belongs to the class of stable hybrids. This is in sub-
stance de Vries’s interpretation. If, as de Vries assumes, the
mutant is the result of a mutating germ-cell meeting one of the
ordinary kind and producing a stable hybrid, it might appear
that if the hybrid were back-crossed with the parent, the off-
spring should be again like the hybrid, but this was not the
result found. These and other considerations show that there are
some obscurities concerning the origin of the cenothera mutants.
Their behavior when hybridized is also difficult to harmonize
with other cases of discontinuous inheritance.

Variation and Mutation in Helix

In connection with his experiments on snails Lang has made
some important observations on the relation of continuous
(or fluctuating) and discontinuous variation. He has found that
in certain colonies of Helix hortensis, in the neighborhood of
Zurich, only two kinds of individuals exist: yellow bandless
and the yellow five-banded. When inbreeding they produce
only these two types again, the bandless condition dominating
in the first generation as already explained. No intermediate
forms appear. This is an example of strictly discontinuous
inheritance.

If the banded individuals in other colonies are closely
studied, it is found that the number and character of the bands
vary; they are broader or narrower, they may remain separate or
become united, especially the fourth and fifth bands. Their
color may be dark or light. An examination of colonies from
other localities shows that all transitions exist from the most
highly developed five-banded condition to a bandless individual.
Experiments establish that the offspring of the same parent may
show these extremes. Here there seems to exist a fluctuating
variability whose extremes include the two types of discontinuous

222 Experimental Zoology

forms, or mutations mentioned above. This comparison raises
the question whether the two forms of variation, fluctuating
and discontinuous, are fundamentally different, or whether they
are only extremes of the same process. Let us examine this
question more closely.

Lang gives the following imaginary case that bears on our
problem. Suppose there existed a species having three varieties,
differing according to size, being 4, 6, and 8 centimeters in
length. On crossing they Mendelize and do not give intermedi-
ate types. Differences in nourishment, however, may cause the
size of any one of the three types to be a little larger or smaller,
2 millimeters being the extreme in either direction, but this
will cause no overlapping. Suppose, however, that the fluctua-
tions, due to differences in food, for example, are so great that
the largest individual of the 4-centimeter variety is as large or
larger than the smallest individual of the 6-centimeter variety.
In other words, the individuals éransgress the limits of each
variety, z.e. the breadth of the fluctuation of the variety is so
great that it overlaps the limits of the nearest related variety.
Then we should have a population in which a continuous series
could be traced from one extreme to the other. Apparently we
should be dealing with a continuous variation, but in reality
with three pure lines that overlap. Breeding experiments alone
could determine that these three types actually exist. Such
experiments would be comparatively simple where self-fertiliza-
tion is possible; but where cross-fertilization is necessary, the
separation of the population into its three true races becomes
more difficult.

Lang has found in Helix hortensis somewhat similar conditions
to those given in this imaginary case. It has been pointed out
that while in some colonies only two sharply separated types
appear, in other colonies numerous variations are found whose
extremes run into each other. Within the species there exist the
possibilities of eighty-nine variations in the banding, and the color
may be white, yellow, orange and brown, or ash-gray. There
are size varieties also, and the bands themselves may be continu-

The Theory of Evolution 223

ous or regularly broken, and these latter by fusing produce even
cross-bands. A band may be absent or two bands may unite
into one, etc. It is possible, as has been said, to arrange these
variations so that a continuous series is formed. Lang con-
cludes for these snails that “‘there exist colonies in which two
forms are found that behave like well-defined mutations, but
which are connected in other colonies by continuous series of
intermediate forms that appear to be variations.” Lang has
begun an experimental examination as to whether these “varia-
tions”? are themselves constant, 7.e. whether true races exist
within the population, or whether there is simply a continuous
fluctuation between the extremes of the series. He has already
determined that a large number of these varietal characters are
inherited to a high degree. Many forms of banding exist, such
AS 12345, 10305, 00300, 00345, 00045, with the colors white, green-
ish yellow, orange-yellow, red; also the intensity of the coloring,
the opacity, and the dotted condition of the bands. Even
the breadth of the bands and different forms of fusion of the

oo Re

bands, such as 12345, 12345, 12 345, are inherited. Lang states
his conviction that further research will reveal that there is
scarcely a single, or at least very few characters that may not
be hereditary. In such cases it is possible to separate the pure
lines from a complex population. Lang also states “that almost
every character may appear at one time with the heritable char-
acter of a mutation, and at another time with the not heritable
character of a fluctuating variation.” The difference between
a mutation and a variation must rest not on the criterion of dis-
continuity or of saltation on which “‘so much stress has been
laid, but on the character of the heredity.” Fluctuating varia-
tions differ quantitatively, mutations qualitatively; but Lang
thinks it advisable not to draw too sharp a line between the quan-
titative and qualitative differences.

These studies of Lang recall the observations of Gulick on
the land snails of the Sandwich Islands, where almost each valley
has its peculiar variety. There seems to be in this case a highly
mutable species. Experiment alone can fully settle this point.

224 Experimental Zoology

Evolution by Means of Definite Variation

The evidence which we now possess indicates, with some
probability, I think, that in some cases, at least, the process of
evolution may have been by definite or discontinuous changes
from one fixed form to another fixed form. The essence of the
process is not that the change has been marked or great, but
rather that the new type is, from its first appearance, a definite
step in a new direction. What has been gained does not need
to be maintained by any process of selection, but becomes a
part of the permanent inheritance. It is probable that it may
be found convenient to distinguish between different kinds of
mutations. In fact, de Vries has already distinguished between
mutations that involve only a loss of a character (leading to the
formation of a “‘variety’’ —a varietal or retrogressive mutation),
and mutations that are new and form new elementary species —
progressive mutations; and mutations that are due to a latent
character reappearing —degressive mutations. Of greater im-
portance for the theory of evolution would be the distinction
between those progressive mutations that become recessive in the
first generation, when crossed with the parent forms, and subse-
quently split according to the Mendelian proportion, recessive
mutations ; those mutations that dominate in the first generation
after a back-cross, and may or may not split subsequently,
dominant mutations; and those that produce a new permanent
form or fixed hybrid, when crossed with the parent hybrid
mutations. Any one of these three kinds of mutations may give
the starting point for a new group of organisms, a new species,
or a new variety, but the chance that they may do so will be dif-
ferent in different cases, and this important consideration must
next be examined. .

It is recognized that one of the greatest difficulties that Dar-
win met with in his theory of natural selection was the swamp-
ing effects of crossing. If a new and better fluctuation were
to appear, it would, in most cases, in order to perpetuate itself,
have to unite with the parent form. It will be recalled that

The Theory of Evolution 225

Darwin was obliged in the Jater editions of the “Origin of
Species” to assume that a beneficial variation, in order to get a
foothold, must appear in a relatively large number of individuals.
His critics were not-slow in pointing out that if we suppose a
variation to be so common that it occurred in many individuals
at the same time, it is probably the result of some definite process
taking place, and the change into a new species might be brought
about in time without selection of any kind being necessary.
Hence one assumption will do instead of two, and the explanation
is correspondingly simplified. If we still try to save Darwin’s
theory by assuming that we need only suppose that half of the
fluctuations in a given direction, z.e. those on one side of the
mean, survive and furnish the basis for the next generation, and
that this process repeats itself in every generation, we meet with
a fact, apparently well established, that shows how futile it is to
attempt to save the theory in this way. It has been shown by
actual experiment that all that such a process accomplishes is
to raise the average in the direction of selection, and that how-
ever long the process is continued, and however severe the selec-
tion, nothing new, no new species, can be created in this way.
As soon as the selection ceases, the form quickly slips back to its
original condition.

Let us see if the mutation theory can surmount this difficulty,
for if mutations appear in the small numbers shown by de Vries’s
experiment, it may seem that they, too, might quickly become
swamped by back-crossing with the parent type.

In the first place the new type is at the start different from the
parent, and may be capable of living in a different locality, 7.e.
under somewhat different external conditions. Hence the chance
of becoming swamped by back-crossing is lessened. If more
than one individual occurs in the proper locality, the chance for
propagation is given. Since the mutations in the evening prim-
rose are produced year after year, this condition may at some
time be realized. In hermaphroditic forms, moreover, the oppor-
tunity of self-fertilization exists, and this increases the chance of
the mutation establishing itself.

Q

226 Experimental Zoology

Even if the new form is adapted to the same locality as the
parent species, it may ripen its germ-cells at a different time, and
this will again increase its chance of isolation from the parent
species.

If the new form is only a variety in de Vries’s sense, it will get
an opportunity of surviving in another way. Suppose it does
back-cross in the first generation, it will reappear in every succeed-
ing generation, so that it may go on increasing in numbers every
year. If it should be a form with better chances of survival
than the parent, it may subsequently become the more common
type.

It has been shown, especially in some animals, that the new
mutation may dominate in crosses with the parent form. Con-
sequently, in the next generation it may appear in many indi-
viduals, and the number of new individuals will increase in every
generation. Should the new character follow Mendel’s law,
some of the hybrids will be pure, others mixed ; but in either case
the opportunity of surviving is given, and if the dominating type
is at all capable of existing, it will always remain in existence,
and, under certain conditions, as when, for instance, it is better
suited than the parent form to another locality, it may establish
itself there.

The important experimental results of Standfuss bear directly
on the problem of the crossing of new mutations with the
parent species. He describes a number of cases that he has
himself observed, and records anumber of aberrations recorded
by other entomologists, in which a new type was crossed with
the parent stock. A few cases are also given in which two
aberrations were inbred. In both groups the offspring were
either like the parent species or like the aberration, and no inter-
mediate forms were produced. These results were obtained in
the first generation of hybrids. In a few cases a new genera-
tion was also reared and the same phenomenon of splitting was
observed. These results show that a new type that appears may,
even if crossed with the parent species, reproduce itself in its pure
form without blending. The process continuing in succeeding

The Theory of Evolution 229

generations would soon lead to the production of a large number
of individuals of the new type. The possibility of the forma-
tion of a new species in this way is clearly shown. . The pro-
portion of individuals belonging to the new and to the old type
varies greatly in different forms, and there appears to be a curi-
ous relation between the type produced and the sex of the indi-
vidual, but with this question we are not here concerned. A
single example will suffice to illustrate the character of the result.
The moth Aglia tau produces an aberrant form, A.lugens. A
cross between the two types produced 14 males and 28 females
of A. tau, and 31 males and 13 females of A. lugens. Two indi-
viduals (F;) of the type A. lugens belonging to the second genera-
tion (F,) were mated. They produced 3 males and 8 females
of A. tau, and 49 males and 42 females of A. lugens. The results
do not appear to conform to the Mendelian law, but it is not
improbable that the aberration was not a pure form but a domi-
nant-recessive. However this may be, the appearance of both
types and the absence of intermediate grades is of the highest
interest.

Standfuss’s general conclusion is given in the following state-
ment: A union between the parent species and an aberration
that has arisen suddenly and discontinuously (‘‘sprungweise’’)
— a variety in the technical sense — produces in many cases not
any intermediate forms, but the parental species type and that of
the aberration. On the other hand, a union between the parent
species and an individual of a local race showing a series of inter-
mediate gradations produces a series of intermediate forms.’

If the new character is transmitted to all of its hybrid offspring
and to their descendants, it will become sooner or later trans-
ferred to all of the individuals that are met with, and in time may
become a part of the common inheritance.

It remains only to consider another point — one on which
de Vries has recently laid some emphasis. He finds in the even-
ing primrose that the new progressive mutations are to some

1 Of another combination, however.
2 This is the case apparently in the crosses described by Tutt.

228 Experimental Zoology

extent sterile with the parent type, in the same sense that even
closely related species are often sterile. De Vries’s statement
gives the impression that he regards this property of sterility as
a general property of mutations. But there are other cases of
mutations where nothing of the sort is evident. Such types as
the ancon ram, the merino sheep, the turnspit dogs, the Japa-
nese peacock, the guinea pig with whorled hair, certain breeds of
fowls with different combs, show no evidence of infertility; when
crossed with the parent type. Furthermore if many of our do-
mesticated races owe their origin to the appearance of mutations,
and to the introduction of wild elementary species (which in
principle may amount to the same thing), it is surprising to find
no trace of sterility between the different breeds, as we should ex-
pect if sterility is one of the general peculiarities of mutations.
If many Linnean varieties are elementary species’ that have
arisen as mutations, it is well known that they are, as a rule, per-
fectly fertile when crossed.

It seems likely, therefore, that the sterility of some of the
cenotheras may be the exception and not the rule for elementary
species, and that the same principle holds here that Darwin
has so ably developed, viz. that fertility is a graded process,
not strictly following the lines of species and varieties, but
dependent on the extent of physiological differences between
two forms.

Our examination of the possibility of new mutations becoming
swamped by intercrossing shows that there is no real difficulty
to be met in this direction, although it is also apparent that some
kinds of mutations may, of course, become lost by crossing.’
The opportunity exists, however, for some of them, especially
those that dominate, to find a place in the economy of nature.
No one will pretend that all new forms that appear will survive.

1 Some of them may be ‘elementary varieties,’ others local varieties due to
external conditions, and others hybrids in which there is a blending of the new
and old characters.

? Those, for example, in which blending occurs, for this, too, is one of the re-
sults of hybridizing even where we have reason to think that the new character
arose as a discontinuous variation.

The Theory of Evolution 229

Many will be incapable of surviving, however often they appear,
and even some kinds that might survive may be swallowed up
again, so to speak, by the parent species.

Many further questions concerning the factors of evolution
suggest themselves which cannot be answered at present. For
example: if a mutation arises and survives, is another mutation
in the same direction more likely than in a different direction ?
If a mutation is traceable directly to external factors acting on an
internal condition or at a particular stage of the germ-cells, will
it recur again and again in all individuals, subjected to the same
conditions, or will it be confined to the descendants of those strains
in which they have first “accidentally” (?) arisen? What pro-
portion of mutations survive autonomously and what proportion
by crossing with the parent stock? How many are lost by being
recessive, and how often do these recessive individuals form ele-
mentary varieties? Do progressive changes take place that are
definite (i.e. not fluctuating) in character and are too small
for us to recognize them as mutations because the steps fall
within the range of fluctuating variations?

These and many other questions demand to be answered
by any one who attempts to apply the observed facts of muta-
tion and discontinuous inheritance to the theory of evolution.
It is obvious that until we can answer them we must remain
in the dark concerning the influence on evolution, and can
only suggest, but not prove, that mutations have furnished
some of the materials for organic evolution.

Adaptation

The problem of evolution of organisms has become so closely
associated with the question of adaptation that we must briefly
refer to this question in connection with the mutation theory.
If a species could be changed (so that it became a new species
adapted to a new environment) by picking out those fluctuating
variations of an adaptive kind, the problem of adaptation would
occupy an important place in experimental zoédlogy. But if
this is not the case, the question of adaptation occupies a secon-

230 Experimental Zoology

dary place, for adaptive mutations, like all others, are given;
not made by selection.

If it were possible to change each character of a species, so that
first in one respect, then in another, the organism would become
better suited to new conditions, — molded to them, as it were, —
we could imagine that the evolution of organisms has taken
place in this way. The process of adapting would be the same
as the process of evolving. This view assumes not only that
each character may, in turn, be changed without the rest of the
organism becoming seriously affected, but also that new species
may be created in this way. Itis this process that the Darwinian
school has assumed to take place, and hence, for them, evolution
and adaptation are closely similar processes.

On the other hand, the mutation theory assumes that new
species appear without regard to whether the change will be an
adaptive one or not. If the new form should be one suited
to the old, or to some new locality, it has a chance of surviving,
i.e. it is sufficiently adapted to exist. From this point of view the
problem of evolution is a different one from that of adaptation.
Moreover, it will be seen that while the process of evolution is one
that can be studied by scientific methods, the adaptation of an
organism is not a causal problem at all. If anew form is adapted,
that is the end of the matter; if it is not, it perishes. The scien-
tific problem deals with the origin of mutations and their causes.
Their adaptation is an independent question, and depends on
whether the proper external conditions exist at the time when
the mutation appears. Inasmuch as only those mutations sur-
vive that can survive, we find that organisms are always adapted
to the environment in which they exist, and this condition of
living things gives the appearance of a fundamental problem
where in reality no such problem exists. The causal prob-
lem is the problem of the origin of new forms; the question
of their survival is only an historical question for all living
species.

Under certain conditions, and ‘in certain cases, the two prob-
lems appear, at first sight, to merge into each other. For if,

The Theory of Evolution 231

as we have supposed, external conditions may sometimes cause
adaptive changes in adult organisms, i.e. in their body-cells,
and if the egg is at times similarly affected so that the next genera-
tion shows from birth the same changes; and further if these
general changes are mutations, i.e. fixed in character, it will
appear as though the process of adaptation and of evolution has
taken place at the same time. It appears, however, when the
whole field of variation is examined that this is only a special
case. In other cases the changes affected by external conditions
may be different from those brought about in the germ-cells,
and some of the new mutations may be adapted to a different
environment, or to the old one in a different way. In both cases
the problem is fundamentally the same, and it is the process of
variation that is our real problem.

Lloyd Morgan, Osborn, and Baldwin have suggested that adult
animals may at times become adapted to a new environment
by a direct response, as by the use and disuse of certain organs
of the body, and maintain themselves in this way until the proper
germinal variations occur that fix, as it were, the new characters
so that they become a part of the permanent inheritance of the
species. Thus the organism, owing to its power of responsive
adaptation, adjusts itself to a suitable environment and awaits
the time when the fixation of the new characters may success-
fully be accomplished as the result of a germinal variation of
the right sort. It is assumed that fluctuating variations bring
about the permanent change, but obviously a mutation would
give the same result. To what extent the advent of the new
variation, whatever its origin, is anticipated in the way assumed,
is not known. On the mutation theory it is doubtful whether
this subsidiary assumption is needed to explain how new species
arise. On the theory of the selection of fluctuating variations
the assumption of “organic adaptation” seems to cover an ad-
mitted weakness of Darwin’s theory, but whether the selection of
fluctuating variations could ever fix permanently a character is
a question that seems from the experimental evidence to be
answered in the negative.

232 Experimental Zovlogy

The Selection Theory and the Theory of the Survival of Mutations

The differences between the Darwinian theory of natural selec-
tion and the theory of the survival of mutations have been already
indicated in their main features. It remains only to clear up
certain minor points.

Darwin has stated his theory in such general terms that it
may easily be supposed to cover also the theory of the survival of
mutations. In fact, he does at times include the latter view in
his theory of survival; for he believed that competition and sur-
vival take place not only between the individuals of a species
(fluctuating or individual variations), but also between varieties
and species themselves. The latter is, in principle, nothing
more than the theory of the survival of the better-adapted ele-
mentary species. In this regard we are at one with Darwin’s
view. But in the application of the Darwinian theory both by
Darwin himself, and especially by his followers, the whole weight
of the argument has been thrown in favor of the selection of
fluctuating variations. Moreover, although Darwin was perfectly
familiar with the occurrence of sports, mutations, and saltations,
he has argued at times that the latter cannot have given the basis
for the evolution of wild species, because the laws of hybridiza-
tion that govern the crosses between wild species and varieties
differ from the laws of hybridization in the case of sports.

Darwin’s theory dealt with the ‘‘origin of species,” and the
theory of natural selection was offered to account for the origin
of species through the selection of fluctuation variations. Many
cases are given in which it is attempted to show how individual
differences become built up into varietal differences, and the
latter into specific differences. In sharp contrast to this view the
other theory, the theory of the survival of mutations, affirms that
species do not originate in this way. Individual differences do
not slowly change into specific differences. Specific differences
appear suddenly as mutations. The origin of species thus be-
comes a very different question from that imagined on Darwin’s
theory of selection. On the mutation theory selection destroys

The Theory of Evolution 233

species; it does not originate them. The same conclusion fol-
lows if we suppose that species have been formed by the direct
action of the environment, or by progressive stages resulting from
internal factors (orthogenesis); the survival of the adapted
new forms accounts for the general condition of adaptation of
living things, but not for the origin of the adaptations, or for
the origin of species. The origin of species and the adaptation
of living things may, after all, be different problems. In fact,
the question of what constitutes a species has given rise to
widely different expressions of opinion, and the entire problem
of evolution may have been prejudiced by too much emphasis
having been laid on the origin of species. If we admit that
species are arbitrary scholastic conventions, their origin is of
secondary importance for the theory of evolution compared with
the problem of adaptation of living things. It is only from the
point of view of classification that the origin of specific differ-
ences is of value. If their origin is the same as that of adap-
tive differences between individuals, it may be that the conclu-
sions derived from the study of specific differences may throw
light on the origin of adaptive differences, but if the origin of
specific differences is different from the origin of adaptive dif-
ferences, the two problems should be studied separately.

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1896.
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Burron, G. L. Historie Naturelle. 1755.
Crampton, H. E. On a General Theory of Adaptation and Selection.
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DarRBESHIRE, A.D. On the Difference between Physiological and Statis-
tical Laws of Heredity. Mem. and Proc. Manchester Lit. and Phil.
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Darwin, C. The Origin of Species. 1859.
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Darwin, E. Zoédnomia. 1794.
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Duccescat, V. Les Problémes biochimiques dans la Doctrine de l’Evo-
lution. Arch. ital. de Biol. XLIIT. 1905.

234 Experimental Zovlogy

Emer, TH. Die Entstehung der Arten, I Theil. Jena. 1888.
Orthogenesis der Schmetterlinge. Leipzig Engelmann. (II Theil
der Entstehung der Arten.) 1897.
Artbildung und Verwandtschaft bei den Schmetterlingen. Jena.
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Emery, cC. Einige Bemerkungen zu Herrn Dr. G. Wolff’s Aufsétze zur
kritik der Darwin’schen Lehre. Biol. Centrbl. X. 1890-1891.
EntTeMAN, W.M. Coloration in Polistes. Carnegie Institution of Wash-
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Ewart, J. The Experimental Study of Variation. Brit. Ass. Advance.
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Evolution, Racial and Habitudinal. Carnegie Institution. 1905.
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Rassen und Gesell. Biol. I. 1904.
Haase, E. Untersuchungen ueber die Mimicry auf Grundlage eines
natiirlichen Systems der Papilioniden. Biblioth. Zool. VIII.
1891.
Hartoc, M. The Fundamental Principles of Heredity. Natural Sci. XI.
1897.
Some Problems of Reproduction. Mic. Sc. Jour. Quart. XLVIL
KOxiIkEr, A. Ueber die Darwin’sche Schépfungstheorie. Z. wiss. Zool.
XIV. 1864.
Morphologie und Entwicklung des Pennatulidenstammes. Abh.
Senckenberg. Ges. Frankfurt. 1872.
Lamarck, J. B. A. pe. Recherches sur l’Origination des Corps Vivants.
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Philosophie Zoologique. 1809.
Macpoveal, D. T. Mutants and Hybrids of the Enotheras. Carnegie
Institution of Washington. 1905.
Heredity and the Origin of Species. Chicago. 1905.
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Color-Patterns of Lepidoptera. Brooklyn Instit. Mus. Sci. Bull. I.
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Minor, C. S. Vererbung und Verjiingung. Biol. Centrbl. XV. 1895.
Morean, Lioyp. Animal Life and Intelligence. 1890.
Morcan, T. H. Evolution and Adaptation. 1903.
The Origin of Species through Selection contrasted with their Origin
through the Appearance of Definite Variations. Pop. Sci. Monthly.
1905.
Osporn, H. The Paleontological Evidence for the Transmission of
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Are Acquired Variations Inherited? Amer. Natural. XXV. 1891.
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The Limits of Organic Selection. Amer. Natural, XXXI. 1897.
The Law of Adaptive Radiation. Amer. Natural, XXXVI. 1902.
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The Theory of Evolution 235

Romanes, G. Darwin and After Darwin, II. 1895.

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SPAULDING, E. A. Species and Varieties: their Origin by Mutation.
Philos. Review, XIV. 1905.

TscHERMAK, E. Die theorie der Kryptomerie und des Kryptohybridismus.
Berich. z. bot. Centrbl. XVI. 1903.

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EXPERIMENTAL STUDY OF
GROWTH

CHAPTER XV

EXPERIMENTAL STUDY OF GROWTH

INTRODUCTORY

Normal Growth

ALTHOUGH growth is recognized as one of the fundamental
properties of living things, comparatively little zodlogical work
has been done in this field. Botanists have paid more attention
to the phenomena of growth and with marked success. In al-
most every field of biological investigation the process of growth
is directly or indirectly involved in the changes that take place;
yet the connection between these changes and growth is often
obscure, for as yet we know almost nothing in regard to what
takes place in the protoplasm during growth, and very little in
regard to the causes that incite growth or inhibit it.

In this and in the following chapters I shall attempt to give
some of the most suggestive results that have been obtained
regarding the growth of animals, although the purely physio-
logical side of the question, where most, in fact, has been
accomplished, will occupy a secondary place. Attention will be
directed more particularly to the gross phenomena of normal
growth and to the external factors that influence the growth of
animals.

The process of growth may be said to begin with the egg and
to end with the adult. While in some animals the adult condi-
tion coincides in a very general way with the condition of sexual
maturity, in other animals growth may continue throughout the
length of life of the animal, becoming smaller in amount as the
size increases beyond what we may speak of as the adult condi-
tion. Ina general way we may class these two kinds of growth

239

240 Experimental Zoology

as determinate and indeterminate, although the distinction is of
only secondary value.

Growth and differentiation are often spoken of as the two
processes by which the embryo is transformed into the adult.
The distinction is difficult to apply, for differentiation is often
accompanied by growth and growth by differentiation.

The most general definition of organic growth is that of in-
crease in volume. Sachs has pointed out that an increase in
volume alone does not necessarily mean growth, because it may
be due simply to swelling, as when a piece of dead wood im-
bibes water and becomes larger. He defines growth as an in-
crease in volume accompanied by a change of form; yet a
change in form is not always apparent when we have reason to
think that growth has really occurred. If, however, we include
in our definition of growth the idea of an increase in the volume
of the living material, we arrive at a more satisfactory definition.
Some examples of normal growth may serve to bring the phe-
nomena before us in a more concrete form.

Davenport has measured the amount of water and of dry sub-
stance in the tadpoles of frogs at different stages of their develop-
ment. His results are summarized in the following table: —

aes Days AFTER | AVERAGE WEIGHT piso WEIGHT OF PER CENT
HatcHInG tn Mo’s SaRerA NCE WATER oF WATER
May 2 I 1.83 80 1.03 56
3 2 2.00 83 1.17 59
6 5 3-43 80 2.63 a7
8 7 5.05 54 4.51 89
10 9 10.40 72 9.68 93
15 14 23.52 1.16 22.36 096
June 10 41 IOI.0O 9.90 QI.10 go
July 23 84 1989.90 247.99 1742.00 88

During the segmentation stages of the egg and during the
early period of formation of the embryo (not included in the
table) the increase in size of the embryo is not very great. After

Experimental Study of Growth 241

hatching (when the records begin in the table) there is a steady
and rapid growth due to the inbibition of water. The dry sub-
stance even decreases during this time. Then follows a period
when the dry substance increases enormously, so that the percent-
age of water falls, but nevertheless a great amount of water
continues to be absorbed, and is mainly responsible for the in-
crease in size.

The next table gives results obtained by Potts for the growth
of the chick: —

Hours or BRoopInG ABSOLUTE WEIGHT IN GRAMS Per CENT OF WATER
48 0.06 83
54 0.20 go
58 0.33 88
gr 1.20 83
96 1.30 68
124 2.03 69
264 6.72 59

Here we see that the percentage of water falls in later stages, and
the increase in weight must be due in a greater degree to the
assimilation of the materials of the yolk. Nevertheless, there
must be a continuous absorption of water from the albumen or
white, for, although the percentage sinks, the amount of water
is continually greater.

Similar results have been found by Fehling for the human
embryo as shown in the following table: —

AGE IN WEEKS ABSOLUTE WEIGHT IN GRAMS PER CENT OF WATER

6 0.975 97-5
17 36.5 91.8
22 100. 92.0
24 242. 89.9
26 569. 86.4
30 924. 83.7
35 928. 82.9
39 1640. 94.2

R

242 Experimental Zoology

These results are for higher animals. In the lower animals,
where the percentage of water is often very great, the results
may be somewhat different, but we lack data at present on this
point. The amount of water in adult medusz is about 95 per
cent; in sea anemones, 87; in sponges, 74 to 84; in the earth-
worm, 87; in the slug, 87; in one of the ascidians, Botryllus,
93; inthe crayfish, 71 percent. Ina chick twenty-one days old,
i.e. at the time of hatching, the water present is 80 percent. In
man there is 66 per cent of water; but this varies greatly in
different parts of the body: thus, in the enamel, 2 per cent; in the
bones, 22; in the muscles, 75; in the blood, 79 per cent.

The most complete account that we have at present of the
growth of an animal from birth to maturity is that by Minot
in his paper entitled ‘“Senescence and Rejuvenation.”’ I shall
give, therefore, a detailed account of his results. Minot used
guinea pigs for his work. The number of young born in differ-
ent litters is given in the following table: —

No. in a litter. : : : . ‘ 5 I2 345678
No. of litters observed ‘ . : ; 23 58 37 182 221

It will be seen that litters of two are most frequent, correspond-
ing with the number of mamme of the mother. Yet there is no
close correspondence, since over half the litters contain more than
two young. What the conditions are that determine the num-
ber of young in a litter is not entirely clear; one fact at least was
made out, viz. that older mothers had larger litters. This is
shown in the following table: ? —

No. inalitter . . . . I 2 33 4 56 7 8
Average age of mothers 200.9 286 289.7 464.8 104 ? 200 433 days
No. of observations . . 18 5I 27 15 iro. © 2

Minot thinks that there is also an individual tendency for
certain individuals to produce litters of a definite size, which is
probably due to a tendency to set free from the ovary fewer or
more eggs. For instance, two cases of successive litters gave:

" Litters of 5, 7,and 8 are based on a single observation, and Minot says have
little value.

Experimental Study of Growth 243

3) I, 4, 4 young, and 5, 2, 7, 6 young. Larger litters appear
on an average during warm weather. Gestation takes from
9 to 10 weeks, so that young born in July are influenced by
conditions existing in May, and those in November by August
conditions, etc. At birth male guinea pigs average 70.8 grams
and females 70.1. The individual variation is very great.
For instance : —

Largest male weighed - , 7 , . 128 grams?
Smallest male weighed . i . . 35 grams
Largest female weighed . é j 3 - III grams
Smallest female weighed . ‘ : F 35 grams

The causes of this variability were in part detected. In the first
place it was found that the larger the litters the smaller the pigs.
“At first sight this seems easily explained as a mere ratio of food
supply and demand; this would accord with certain views of
Herbert Spencer, but not with the facts of nature.” There
must be also some other cause for the variability than the num-
ber of young, otherwise the heaviest individuals would occur
only in litters of two or of one, but this is not the case. The
explanation of this fact is found in the length of the gesta-
tion period, which is shorter the larger the litter. ‘The indi-
viduals of larger litters weigh less at birth than the members of
smaller litters,’ because they are born sooner or, in other words,
have not been growing so long. This is shown in the following
table. The average time of gestation is 67 days.

No. or Younc AVERAGE GESTATION IN Days

I 68.7

2 67.6

3 66.7

4 67

5

“ 66

7

8

1 An exceptional case, the next highest being 113 grams.

244 Experimental Zoology

The weight varies also with the season of the year, probably
in consequence of more favorable nutrition at certain times.

The individuals of the same litter vary in weight, as shown in
the following table: —

The young differed in weight between

o- 3 grams in 12 cases

4- 7 grams in 19 cases

8-11 grams in Io cases

12-15 grams in 6 cases

16-19 grams in 6 cases

20-23 grams in 4 cases

23-27 grams in I case

35 grams in 1 case

This difference in weight between members of the same litter is
probably due to differences in position of the embryo in the uterus,
or to the number of young in each uterus, three young being in
one uterus and only one in the other. If this is true, odd num-
bers should show greater differences, and this is often the case.

For a few days after birth male guinea pigs lose weight;
jemales gain weight. The females average, when born, a little
less than the males, but they gain during the first days, so that
they may actually then weigh more, and this ‘‘advantage is long
maintainec.” A more minute examination shows that both sexes
really lose after birth, but the males more; the females begin very
quickly to gain, and obscure the initial loss. In both males and
females the growth during the first few days is less than that from
the fifth day onward. It is not until the twenty-ninth day that
the male catches up, weighing 203 grams as against 203.7 for
the female. When the adult condition is reached, the males
weigh much less than the females. After the first month to the
end of the first year the males average more. During the second
year the averages are too variable for generalization, although
the males weigh somewhat less on the average. During the
youth of the guinea pigs one point comes out clearly, viz. that
“each individual appears to be striving to reach a particular size.”
Thus if an individual grows for a period excessively fast, there

Experimental Study of Growth 245

follows a period of slower growth; and, vice versa, those that fall
behind make it up later, if they remain in good health. A young
guinea pig may lose one third its weight from intestinal catarrh,
and make the loss good later. ‘It is probable that the same is
true of man, and that the usual and even the severer illnesses of
childhood and youth do not greatly affect the ultimate size of
the adult.” Pagliani shows that children brought up in poverty
and undersized will recover in the most surprising manner if
placed under favorable circumstances.

“Tt has been asserted by Carpenter, Spencer, and others that
the functions of nutrition and reproduction are in principle
opposed to one another, because reproduction makes such a de-
mand upon the parent for material that the supply of nutrition
and growth of the parent is lessened.” Unfortunately for this
philosophic generalization the premises are wrong — the grow-
ing animal is not growing at its maximum of assimilative power.
It has been shown that young female guinea pigs grow about the
same whether they are carrying young or not. Minot concludes
that “gestation does not represent a tax upon the parent but a
stimulus — it does not impede growth, but on the contrary
favors it.” Spencer’s “dogmatic assertions’? concerning the
opposition of growth and reproduction are open to justly severe
criticism.

A guinea pig reaches its full size by the end of the first year,
when it weighs about 775 grams. A rabbit is also full-sized a
year after birth, and weighs 25000 grams. Man may be said to
be full grown at the end of twenty-five years, and has then an
average weight of 63000 grams. If we add to these times the
length of the period of gestation, and divide the weights by these
numbers, we obtain the average rate of growth a day.

Guinea pig . : ; : 775+ 365+ 67 days=1.82 per diem
Rabbit : : : 2 2500+ 365+ 30 days= 6.30 per diem
Man . é : ‘ . 63000 + 9139 + 280 days = 6.69 per diem

The calculation shows that man is larger than the rabbit because
he grows for a longer period; but the daily increments are nearly

246 Experimental Zoology

the same for both, On the other hand, rabbits attain a larger size
than guinea pigs, not because they grow for a longer time, but
because they grow faster. Thus there are two different ways

of attaining larger size.

Senescence

After reaching a certain stage in their growth some organisms
begin to “grow old.” There is no very fixed period at which the
decline may be said to begin, for after reaching full growth
there may follow a relatively long period before any evidence of
growing old can be detected. Thus in man the full growth is
reached about the twenty-fifth year, but for the following ten
years or more there may be little change indicating a decline. In
many insects, on the other hand, the complete growth coincides
very nearly with sexual maturity, and after the eggs of the
female are laid the decline may follow very. quickly; in fact, in
some cases death follows at once, so that there is no period
of senescence at all.

In some of the crustaceans, there are species in which,
apparently, the individuals grow larger as long as they live, as
seen in the lobster and to a less extent in crabs and crayfishes.
Some mollusca also seem to continue to grow for many years,
adding each year a new and larger edge to the shell. In most
of these forms the period of egg laying occurs oncea year. Inthe
vertebrates we find that fishes and some species of amphibians
and reptiles grow continuously, although very slowly, after a
certain size has been reached —so slowly, indeed, that they may
be said to have an upper limit of growth. On the other hand,
birds and mammals cease to grow after a certain size has been
reached that may be quickly attained.

Minot looks upon senescence as due to the loss of power to
grow. He thinks that previous writers have given an incorrect
interpretation of the rate of growth. They count the absolute
increments of equal successive periods, but during each period
the size of the body increases, and this should be considered.
Under these conditions if the rate of growth were constant,

Experimental Study of Growth 247

the proportionate increment would remain the same, but
the absolute increments would become steadily larger. In other
words, Minot believes that the rate of growth at any particular
time must be measured in terms of the weight of the body at that
time. He finds when measured in this way that, “first, the rate
of growth diminishes almost uninterruptedly from the time on-
ward when the animal recovers from the post-natal loss of
weight; second, that diminution is rapid at first but slower
afterward.” .

Minot shows for guinea pigs that after the post-natal retarda-
tion, the increments of growth increase from the second to the
fifth day at about 5.5 per cent in terms of body weight. From
this time onward the increment decreases very rapidly at first,
and then more slowly. Thus from the fortieth to the fiftieth day
it is 1.2; from the one hundred and ninetieth to the two hun-
dredth day it is .2; and after 22 to 24 months about .o2. Con-
versely, if equivalent amounts of growth are taken and compared
with the time required to acquire them, we find that to increase
in weight from 200 to 222 grams takes 4.9 days; to increase from
470 to 523 takes 20 days; from 697 to 766 takes 4o days. In
each of these cases the weight is increased about ro per cent.
The results show a progressive loss which after a time, as in man,
may come practically to a standstill. In one sense, therefore,
the animal may be said to begin to grow old almost from the
moment that it is born. This, however, is not what is usually
meant by growing old, although the phrase has been employed in
various ways. Generally we refer to the decline that occurs after
growth has come to an end; but there may be a considerable
interval in an animal’s life after it has ceased to grow larger,
during which time it has not begun to “grow old.” On the
other hand, animals that continue to grow, however slowly, as
long as they live, can scarcely be said ever to grow old,
although they may be very old in point of time. This brings
us to the question of the length of life of different animals,
and whether it can be artificially prolonged by altering the
conditions of life.

248 Experimental Zoology

Length of Life in Different Species

In contrast to the method of studying growth and senescence
described in the preceding pages another method has been fol-
lowed by Weismann, viz. the method of comparison of the
length of life in different species.

In his essay on “‘The Duration of Life,” Weismann has
brought together some interesting data relating to the length of
life of animals. He shows how meager our information is con-
cerning the length of life of lower organisms, except in the-case of
those that live only through a certain season of the year, as many
of the insects do. Weismann points out that it is often stated
that both the duration of the period of growth and the length.
of life are longer for larger animals and shorter for smaller ones..
An elephant may live for 200 years, a horse for 40 years, and
since they both require a relatively long time to grow up, it fol-
lows that part, at least, of their life must be longer than that of a
smaller animal that may reach its full age in a few months.
On the other hand, other smaller vertebrates, such as certain
fish —the pike and carp — may live as long as an elephant;
and a cat or a toad may live as long as a horse.

Flourens thought that the length of life of an animal was equiv-
alent to five times its period of growth. Thus if man grows
for twenty years, he lives to 100; but on the other hand a horse
becomes mature in four years and may live to 40 years, or ten
times as long as its growth period. From this and from other
cases it is evident that no such ratio as that of Flourens will hold.
Neither does greater activity mean necessarily a shorter life, for
some of the most active birds are the longest lived, and may
live as long as do some of the sluggish amphibians.

Weismann’s conclusion is that neither size, activity, complexity
of structure, nor ‘‘constitution”’ can account for duration of life,
but that it ‘‘is dependent upon adaptation to external conditions;
that its length, whether longer or shorter, is governed by the
needs of the species, and that it is determined by precisely
the same ‘mechanical process’ of regulation as that by which

Experimental Study of Growth 249

the structure and functions of an organism are adapted to its en-
vironment,”’ i.e. by natural selection of individual differences.

Weismann believes that “in regulating the duration of life,
the advantage to the species, and not alone to the individual, is
of any importance.” The purpose of the individual is the per-
petuation of the species, and its length of life has been regulated
accordingly. As soon as the individual has performed this
purpose, it “has fulfilled its duty” and may die.?

Weismann believes that death is essential to the species, be-
cause the individuals become injured and must be replaced by
new and more perfect forms. From this, he says, follows the
necessity of reproduction and the utility of death. He points
out that worn-out individuals are even harmful to the species,
for they consume the food that sound and reproducing indi-
viduals might make use of. Death ‘is not a primary necessity
but . . . has been secondarily acquired as an adaptation.”
Plausible as this conclusion may be made to appear when
stated in this abstract and general way, it will not, I believe, bear
critical examination. Without attempting an elaborate refuta-
tion of Weismann’s conclusion the following objections may be
briefly stated :—

(1) To put the problem of senescence and death in a different
category from other physiological processes, seems entirely arbi-
trary. Weismann’s contention that death has been imposed
upon each species not by internal physiological changes, but
from without by natural selection, is not only paradoxical, as he
states, but it seems to me a confused point of view.

(2) Is it not putting the cart in front of the horse to argue that
the length of life is adjusted to the power of reproduction?
It seems more reasonable to assume, if we must make some as-
sumption, that if there is any connection between the two things,
the length of life determines in a general way the cessation of re-
production; for the general decline that leads finally to a “‘natu-
ral death”? may at its inception affect the reproductive organs.

1 This includes the idea not only of giving birth to the new individuals, but
in some cases caring for them or in protecting them during their immaturity.

250 Experimental Zoology

(3) It seems to me entirely wrong in principle to suppose that
any one function of a species can be thought of as separated from
all the others and increased or decreased to any extent demanded
by the theory of natural selection without altering the other
functions of the organism. Even if this could be done to some
extent by selection of individual differences, the change would
only be temporary and disappear when the selection ceased.
No one will suppose, I imagine, that in every generation the
length of life of the species in relation to its power of reproduc-
tion is being regulated in this way, and, unless selection is in-
cessant, little or nothing can be gained, because the function or
structure will return to its natural condition if let alone. The
length of life of each species is as characteristic of it as any
other of its functions, and must have appeared when the species
came into existence.

(4). In regard to Weismann’s view as to the origin of death in
consequence of the advantage conferred thereby on the species,
it is only necessary to point out that if matters of this sort were
decided solely on the grounds of the advantage to the species, it:
might often be of much greater advantage to improve the power
to repair the injuries of the adult than to bring in death as a solu-
tion of the difficulty. Those animals that have reached maturity
after overcoming the dangers of development and growth might
be of greater use by living in maintaining the species than by dy-
ing, and if death can be determined by selection, so could no
doubt the prolongation of the reproductive power (to replace
accidental losses in numbers) and the power to repair.

This whole method of arguing seems to me to be so uncertain
that it is unprofitable. What we need is obvious, namely, to
study the physiology of the process of senescence in different
groups.

_ If growth and length of life are simply physiological changes,
why cannot the process be maintained artificially at any desired
point, or why not reversed and old age grow young again? We
know too little at present regarding the process of growth to
make it worth while even to hazard a guess whether the growth

Experimental Study of Growth 251

process could ever be maintained in equilibrium or reversed.
Optimistic enthusiasts may claim that we are on the verge of
this discovery, and that the day after to-morrow we may hope
to begin to grow young; but we might well hesitate to take
the prescription until we were certain that its action could be
stopped, for it would be as awkward to disappear in an egg as to
end one’s life in the other direction by growing old. Never-
theless there is urgent need that the phenomena of growth
be more thoroughly studied, and the work of such serious in-
vestigators as Metschinkoff shows at least that there is a wide
field for future investigation in the phenomena of senescence.

Absorption of Parts by Larve

We ordinarily think of all parts of the body growing old at the
same time, but there are some apparent exceptions to this rule.
An animal that is still growing may absorb certain parts of itself,
and if the absorption of the part is not strictly comparable to the
growing old of the whole organism, the two processes are so similar
that they invite comparison. The most familiar example is the
absorption of the tail of the tadpole, although similar cases are
known to every embryologist. At the time of metamorphosis
the large tail of the tadpole is rapidly absorbed, and although we
have many descriptions of the way in which the breaking down
and absorption of the tissues occur, we have not the slightest clew
as to what initiates the process. We might imagine that with the
changes in the gill region and with the imperfect beginning of
lung respiration the amount of oxygen absorbed is decreased,
so that the blood that reaches the tail can no longer supply that
part with the necessary oxgyen, hence the tissues die and are
eaten up by the phagocytes. The skin of the tail might absorb
enough oxygen directly from the water or air to maintain itself,
in part, during this time, hence is not so seriously affected.

That the result is not due to this cause is shown by the fact
that the gill respiration is maintained during the period of ab-
sorption of the tail. It might be supposed that the beginning of
lung respiration that takes place at this time might be connected

252 Experimental Zoology

with the result. Some experiments that I have carried out
disprove this suggestion also. The lungs of tadpoles about to
transform were removed at the base. The animals underwent
their normal metamorphosis, as when the lungs are present. Not
only was the tail absorbed, but the gills also. If the operculum
that covers the gills is removed so that the gills are exposed to
the water, they become absorbed at the proper time. How far
these changes are comparable with senescence remains to be

shown.

LITERATURE, CHAPTER XV

DAVENPORT, C. B. The Role of Water in Growth. Proc. of the Boston

Soc. of Natur. Hist. XXVIII. 1897.
Experimental Morphology, I. 1897. II. 1899.

FEHLING, H. Arch. f. Gynakologie, XI. 1877.

Minor, C. S. Senescence and Rejuvenation. Jour. of Physiol. XII.
189g1. ‘

Morcan, T. H. The Origin of Species through Selection contrasted with
their Origin through the Appearance of Definite Variations. Pop.
Sci. Monthly. 1905.

Ports, R. Landwirth. Versuch-Stat. XXIII. 1870.

WEISMANN, A. Duration of Life, 1881. (Essays upon Heredity.)

CHAPTER XVI
EXTERNAL FACTORS THAT INFLUENCE GROWTH

In the last chapter we studied some of the characteristics of
normal growth. In this and in the following chapter the exter-
nal and internal conditions that modify growth will be considered.

In anticipation of what is to follow I may state that while the
rate of growth may easily be accelerated or retarded, the char-
acter of the growth is more difficult to modify. The following
external agents have been found to affect the rate, and in
some cases the character of growth: (1) Food; (2) Stimuli;
(3) Salts; (4) Heat; (5) Light; (6) Gravity; (7) Electricity;
(8) Pressure and Contact.

Influence of Food

To a certain extent the rate of growth depends on the amount
of food. It is clear that if less food is obtained than needed to
make new tissues, the rate of growth must slacken. On the
other hand, if more food is given than can be assimilated, the
rate of growth is not thereby increased. This is, however,
by no means the whole question; for animals show a curious
regulative power, and store up reserve materials usually in the
form of fats when an excess of food is taken which may be drawn
upon later if the food supply temporarily diminishes. Let us
take an imaginary case. Suppose an individual to be growing
at its normal rate with an abundance of food. Ifa certain small
amount of this food is taken away, it does not follow that the rate
of growth will correspondingly decrease. Take more away and
the rate will decrease somewhat, but again not in proportion to
the amount removed. The same fact is brought out in another

253

254 Experimental Zoology

way in Minot’s experiments with pregnant guinea pigs not fully
grown. Not only did the mother grow at the normal rate, but
she supplied the embryo also with a large amount of nourish-
ment.

We can account for the facts on the assumption that when less
than the optimum is given more substance is digested from the
food, and when more than the optimum is taken proportionately
less is absorbed or assimilated. The appetite of the animal fur-
nishes in a measure an index of the amount needed for growth
and repair, but one that cannot be entirely relied upon.

The size of the adult stage of some of the lower animals is
much affected by the amount of food that can be obtained.
There is, however, an upper limit that is quickly reached when
food is abundant that cannot be surpassed. Most of the varia-
tion in size lies below this condition. For example, fresh-water
planarians if kept without food for several months decrease in
size, and finally may be not more than about 1/15 of the original
volume. If such a starving worm is cut in two, each piece still
has the power to regenerate the missing part, drawing on or
making use of the starving tissues in order to make the new
growth. Thus, although the worm is starving it will grow rela-
tively rapidly at the cut surface and produce a new part. If
a starving earthworm is cut in two in the middle, the anterior
piece makes a new tail, and the posterior piece also regenerates
a tail (reversed in direction) at the anterior end of the piece.
In the latter case the old part continues to waste away, while
the new reversed tail continues to add new segments to its grow-
ing end. Thus while starvation is taking place in the old piece,
growth goes on in the new, and the latter must derive all of its
material from the starving portion. In the “winter” salmon of
the Rhine the tissues of the body are used as food after the fish
have entered fresh water, when they cease to feed. These sal-
mon may live for months (from 8 to 15, according to Meischer)
without food. At the end of this time the fish are much emaci-
ated. During the period of starvation the eggs develop. There
can be little doubt that a starving mammal, if pregnant, would

External Factors that Influence Growth 255

continue to supply nourishment to the young in the uterus; and
while the embryos might not grow as fast as do those in a well-
nourished individual, they would probably grow at least as long
as reserve material was present in the mother, and perhapg even
while the formed tissues of the mother were slowly being used up.
It would be interesting to carry out a series of exact experiments
of this kind on such animals as rats, mice, or dogs — especially
on forms that can go a long time without food.

These conditions show plainly that growth depends on other
factors as well as on the amount of food that an animal obtains,
or has in reserve.

Not only the amount of food but the kind of food also has
an important bearing on the rate of growth. Yung fed tadpoles
on several kinds of food, including aquatic plants, the jelly of
frogs’ eggs, yolk of hens’ eggs, white of hens’ eggs, fish, and beef.
The results are shown in the following table: —

Aquatic | JELLY oF YELLow oF} WHITE oF
Prants | Frocs’ Eccs| Hens’ Eccs| Hens’ Ecos| FISH | Beer

Length of tadpole 18.3 23.2 26.0 33.0 38.0] 43.5
Breadth of tadpole 4.2 5.0 5.8 6.6 8.8] 9.2

Kinp oF Foop

Fed on beef the tadpoles grew nearly three times as fast as on
plant food. The experiments were continued, in some cases,
to the time of metamorphosis. The large tadpoles transformed
first, but the smaller ones not until they had grown large.
Cuénot has also reared tadpoles under different conditions, and
has recorded the time of their metamorphosis. He also found
that the well-fed animals transformed first. Barfurth has shown,
on the other hand, that starvation just prior to the time of meta-
morphosis hastens the change.

Experiments made by Lawes and Gilbert have shown that
‘“‘srowth tends to increase with the quantity of nitrogeneous
food.” A curious instance confirmatory of this conclusion is
given by Présher. He compared the times required by different
mammals to double their birth rate with the composition of the
milk of the mother. His results are given in the following table.

256 Experimental Zoology

Man is taken as the standard, and the time required by the other
forms to double their birth rate is given in fractions of that of
man.

pauls ae Fat SUGAR ALBUMEN CaO . P.O5
Man I 3.5 6.6 1.9 I I
Horse $ I.I 6.1 2.3 4 3
Ox q 4.5 45 4.0 — 5 4
Pig io 6.9 2.0 6.9 - -
Sheep re 10.4 4.2 7.0 8 9
Dog aE 10.6 3.1 8.3 I4 Io
Cat v5 3:3 4.9 9.5 = =

It will be seen that the rate of growth is proportional to the
amount of albumen in the milk. Thus the kitten doubles its
weight in 1/33 of the time required by the human infant, and the
milk contains five times as much albumen. It is improbable
that the results are due solely to this factor, but that an internal
factor largely regulates the growth of the young. Were this not
the case, we should find that a baby fed on sheep’s milk would
doubt its weight eighteen times as fast as a baby fed on human
milk.
Stimulants affecting Growth

I have already stated that the rate of growth is not simply a
question of the amount or of the kind of food. In fact, growth
depends in some cases on a response to a stimulus, and the amount
of food stuffs converted into tissues sometimes depends on the
presence of a stimulus. For example, the embryo in the uterus
of the mammal appears to supply such a stimulus. Many
chemical substances ‘‘that are not themselves food may stimulate
the growth processes.’’ Just as certain poisons accelerate the
movements of animals, so may they also accelerate metabolic
processes and lead to increased growth. ‘Schultz found that
various poisons, such as corrosive sublimate, iodine, bromine,
arsenious acid, increase the activities of yeast in fermentation.’

* Davenport, “ Experimental Morphology.”

External Factors that Influence Growth 257

It has also been shown for other molds, Aspergillus, Penicillium,
and Botrytis, that alkaloids and other poisonous substances if
present in small amounts accelerate growth.

Few cases of this sort can be given for animals. The best-
known case perhaps is that of lecithin. Danilewsky has found
that lecithin has a marked influence on the rate of growth. Tad-
poles were placed in a solution containing one part of lecithin
to 15000 parts of water. The rate of growth of the tadpoles was
compared with that of similar tadpoles kept in water alone.
The results are given in the next table: —

WATER LecitHin
June 12 Irmm. 18 mm.
June 21 12 mm. 18 mm.
July 18 13 mm. 2I mm.
Aug. 5 15 mm. 27 mm.

The lecithin tadpoles are three times heavier and nearly twice
as long as those in water alone. The results are ascribed to
the stimulating effects of the lecithin. Ido not feel convinced
that this is the case, because any one who has reared tadpoles
in confinement must have found that great variations in size
are found in different dishes in which the conditions seemed to
be the same. Even in the same dish very great differences in
size are also found.

Danilewsky claims to have obtained similar results with
young rabbits and dogs by injecting solutions of lecithin into
them at intervals. Desgrey and Zaky have also studied the in-
fluence of lecithin on growth, and reach the conclusion that the
beneficial effect is due to the stimulating effect of the substance.
Hatai also has found that in young rats lecithin causes an
increase in weight whether given as injections or with the food.

Effects of Salt on Growth

The eggs of many animals, especially those laid on the land,
contain a sufficient quantity of materials to carry them through
s

258 Experimental Zoology

their early development until the embryo hatches and begins to
feed for itself. It appears, however, in the case of some water
animals that the egg or the early embryo derives some of the
materials necessary for development from the surrounding me-
dium. In the development of the eggs of the lower marine
animals it has been shown that certain inorganic constituents of
the sea water enter directly into the embryo and take part in its
growth. The most thorough study of this sort that has been
carried out is that of Herbst. His final conclusion is that for
normal growth all of the common constituents of sea water are
necessary. For a few of them other closely related chemical
compounds may be substituted, but it is surprising how little
substitution of this sort is possible without altering the normal
processes of growth. In such cases it must not be overlooked
that the egg itself is a storehouse of food materials, and that the
growth consists mainly in adding water to the materials already
present; but there can be no doubt that extraneous salts are
also fixed by the tissues of these organisms and take a part
in the growth. For example, the young embryo of the sea
urchin produces a calcareous skeleton. Unless calcareous salts
are in the water, the skeleton is not formed; even other closely
related salts cannot be substituted. Quite recently Maas has
carried out an experiment with sponges. He finds in them also
that unless the calcareous constituents of the sea water are pres-
ent, no spicules are formed, and the development does not extend
beyond a certain point.

By adding salt to fresh water the density may be increased.
By diluting sea water its density may be decreased. Both
changes may affect the rate of growth, as shown by the following
experiments. Yung (1885) added 2.0, 4.0, 6.0, 8.0 grams of sea
salt to 1000 grams of fresh water, and placed frog embryos in
the solutions. In the .2 per cent solution the rate was nearly
the same as in pure water. Retarded development occurred in
all the others. In the .8 per cent solutions the tadpoles hatched
17 days after the normal time.

It is not altogether clear in this case whether the effects are

External Factors that Influence Growth 259

due to a difference in density or to an injurious effect of the salt.
Control experiments should have been made with different solu-
tions of such substances as sugar or urea.*

Frazeur tried the effects of solutions of sodium chloride on the
rate of regeneration of Nais. The twelve anterior segments were
first cut off and placed in solutions of different strengths. After
ten days the number of new segments that regenerated was
counted. The results are shown in the table: —

SoLUTION No. oF InpIvipuaLs a cn re
Water I5 2.13
0.125 NaCl 5 1.72
0.188 “ 16 1.42
0.250 6 4 1.19
0.375 “ 5 1.18
0.500. iy 1.14

Sargent has studied the rate at which the process of fission takes
place in Dero vago. Ordinarily Dero doubles its numbers every
ten days. The worms were kept in solutions of sodium chloride,
magnesium sulphate, calcium chloride, magnesium chloride,
and potassium chloride. The results seem to show that the
rate of multiplication falls off rapidly with an increase in the
strengths of the solutions employed.? As the results stand, it
is not clear that they are due, solely, to the increased osmotic
pressure, but the salts themselves or their ions may have been
directly injurious.

Loeb has shown that the new head of the marine hydroid
Tubularia grows longer in diluted sea-water than in pure sea
water, and he thinks the results must be due to a change in the
osmotic pressure of the solution. This might be tested by first
diluting the sea water and then bringing up its osmotic pressure

1 In some recent experiment that I have carried out I found that the eggs of
the frog are affected both by the osmotic pressure and by the chemical substances.

2 These results of Frazeur and of Sargent are taken from Davenport’s “Ex-
perimental Morphology.”

260 Experimental Zoology

by a sugar solution to that of normal sea water. If the increase
in length still occurs, the results must be ascribed to the chemical
effects of the salts rather than to the osmotic pressure.

Effects of Heat on Growth

Of the external factors that influence the rapidity of growth,
temperature has long been known to have the most influence;
and since heat facilitates chemical reactions, it is generally as-
sumed that it acts in the same way in the organism.

Higgenbotham kept frogs’ eggs at 60°, 56°, 53°, 51° F., and
found that they hatched respectively in 9, 14, 20, and 20 days,
and changed into frogs in 73, 161, 171, and 235 days.

The rate for tadpoles has been measured by Lillie and Knowl-
ton (1898) and by O. Hertwig (1898). For the tadpoles of Rana
virescens, and of the toad, Bufo lentiginosus, Lillie and Knowl-
ton obtained the following results : —

‘TEMPERATURE FRoc Toap
9-I0.9 45 3.0
II-12.9 5:3 5.3
13-14.9 4-3 15-5
15-16.9 16.3
17-18.9 9-5
I9-20.9 19.8 21.2
21-22.9
23-24.8 41.3
25-26.9 31.5 39:0
27-28.9 40.0
29-30.9 47 36.8
31-32.9 40.2 55:3
33734-9 43-5

The rate of growth for the frog increases up to nearly 30° C. and
then decreases. The upper limit for the toad is about the
same or a little higher.

Hertwig has compared the rate of growth of tadpoles of
Rana fusca at different temperatures. Their maximum nor-

External Factors that Lnfiuence Growth 261

mal growth takes place at 25° C. At this temperature tadpoles
develop as far in 24 hours as do those at 16° in two days.
I have found that the young tadpoles of Rana palustris kept at
a temperature of 2 to 2.5 C. develop scarcely at all, and may be
kept for at least a month in practically the same condition.

Rauber states that the eggs of minnows and salmon, which
develop during the winter season, will not grow above 12-15%,
but do grow at o°. On the other hand, it is known that those
fish that deposit their eggs in summer develop faster at tempera-
tures higher than these.

The optimum temperature for development of the hen’s egg
is about 38° C., but it will slowly develop at 25° C. The maxi-
mum point at which development takes place normally is 42°.
The following table shows the rate for the chick : —-

Temperature . . 347% 35° 36° 37° 38° 39° 40° 41°

Index of development 0.65 0.80 0.72 ? (1.00) 1.06 1.25 1.51
Most other animals have a much greater range at which normal
development occurs. It is evident that for each species of ani-
mal there is an optimum temperature for growth, which is very
different in different species. Somewhat above the optimum
temperature, growth may be more rapid although less normal;
but as higher temperatures are reached, the development is inter-
fered with very greatly. A much wider range of possibilities exists
for lower temperatures. This difference is probably due to the
coagulation of the protoplasm. Slightly above the optimum
temperature the colloidal substances of the protoplasm become
coagulated. It requires much greater change in the other direc-
tion, z.e. for lower temperatures to coagulate the colloids. In
many cases a freezing temperature is necessary to produce this
effect, but even this temperature is not injurious to many animals.

Effects of Light on Growth

In studying the effects of light on growth one must be careful
to exclude the heat rays. This can be done by interposing a
screen containing solutions that absorb the heat rays and allow

262 Experimental Zoology

the light rays to pass through. Furthermore by light we usually
mean sunlight or “white light,” which is a composite light made
up of vibrations of different lengths that we recognize as colors.
One of the most unexpected facts in regard to the influence of
light is that in plants light is antagonistic to growth, although
light is necessary for the normal existence of all green plants.
The paradox finds its explanation in that most of the actual
growth takes place at night; while light is a form of energy
necessary to fix the carbon of carbon dioxide in the chlorophyll
body. Out of this carbon and other things the starch is formed,
and starch when transformed into sugar is the food of the plant
supplying the material for growth. Davenport has pointed
out “that it is noteworthy that embryonic tissue, and indeed the
entire embryonic individual, is usually sheltered from sunlight.
In animals the embryo is sheltered in the darkness of the maternal
body; in birds and reptiles the egg shells are more or less opaque,
and, moreover, the whole egg is usually hidden from light.”
Blanc (1892) has shown that the development of the hen’s egg
is much retarded if subjected to light.

Many animals hide their eggs, but whether this has only the
advantage of concealment from enemies, or of placing them out
of the light, we donot know. On the other hand, pelagic fish eggs
are exposed during the day to the full light, and yet develop nor-
mally. Maupas found that ciliate infusoria multiply equally
fast in the light and in the dark. Experiments have also been
made with tadpoles. Edwards in 1821 stated that tadpoles
would not develop at all in the dark. Later Higgenbotham
(1850 and 1863) and Macdonnell showed that they grow at the
same rate in light and in the dark. Yung also made a study
of this problem. His results are given in the following table.
In each case ‘‘typical” tadpoles were measured in each lot.

Licut Dark RELATION SIZE

Lot 1, 30 days’ length 23.1 19.6 117
Lot 1, 60 days’ length 32.1 30.3 106

External Factors that Influence Growth 263

The table appears to show that tadpoles grow faster in light,
but it may be doubted whether this is due to the influence of the
light on the tadpole itself rather than indirectly to the influence
of light on the plants that grow in the water, which are used by
the tadpole as a part of its food. Possibly also the setting free
of oxygen by these plants during the daytime may make the
conditions more favorable for growth. Too much caution cannot
be used in considering all possible sides of the experiment.

Yung also made experiments with other animals. Two hun-
dred fertilized eggs of the trout were put into four liter vessels,
and kept in running water. Those exposed to daylight hatched
a day sooner than those in the dark. In this case the question of
food does not enter into the problem, and the running water
would seem to equalize other conditions. The eggs of the pond
snail, Lymnza, hatched in the light in 27 days; in the dark in
33 days.

It has been known for some time that for plants the blue rays
act more nearly like white light, while the yellow rays give more
nearly the effect of darkness. Some work has been done with
animals to test the effects of light of different colors. Yung
finds that Hydra viridis dies if kept in the dark. It grows more
rapidly in violet light than in green, and in green than in white,
and in white than in red light. Since hydra contains green bodies
supposed to be green algz, the influence of light is probably on
the alge, and it is significant to find that the order of colors given
is, in a general way, found to have the same effect on green plants;
but it is doubtful whether these results can be accepted as final,
because it is not evident that sufficient precautions were taken
to have the intensity of the light the same in all cases and the
food conditions also identical.

Yung also carried out some experiments with tadpoles reared
behind screens of nearly monochromatic colors (solutions).
At the end of a month three tadpoles taken at random gave the

following results: —

264 Experimental Zoology

AvERAGE LENGTH LENGTH COMPARED WITH
WuiteE Licut
White 24.4 100
Violet 28.5 117
Blue 25.6 105
; Yellow 24.3 99
Red 20.3 83
Green 16.9 70

Here also I question whether the results have any certain value
because of the variability shown by tadpoles reared in dishes, and
on account of the difficulty of keeping the other conditions the
same.

Vernon’s results with sea-urchin larve are more significant,
because the growth, in this case, depends little, if at all, on the
food supply.

' LenctTu
Semi-darkness tas
Complete darkness ~1.3
Blue (copper sulphate) —4.5
Green — 48
Blue (Lyons blue) — 7.4
Red —6.9
Yellow —8.9

In violet light all the larvae died, owing to bacteria. The order
of growth for the other colors was: white, blue (copper sul-
phate), green, red, blue (Lyons blue), yellow. The order is so
different from that given by Yung that, although done on
different animals, the interpretation of the real influence of the
light is probably open to question.

Beclard found that the eggs of the fly, Musca carnivora,
when reared under different colors developed fastest in violet
light and most slowly in green. The sequence for all the colors
used is violet, blue, red, yellow, green. This order agrees more

External Factors that Influence Growth 265

nearly with that given by Yung for tadpoles than with that of
Vernon for the sea-urchin larve. It is noticeable that in all
cases the violet stands near the top of the list, but the order of
red, green, and yellow is not the same in any two cases.’

Growth toward the Light; Phototropism

The turning of plants toward the light is a familiar phenom-
enon. It is brought about by more rapid growth on the
shaded side.

Most animals are free to move, and some of them move toward
or away from the light. Such turing cannot be called a pro-
cess of growth, but is due to contraction of the muscles of the
body. Sense organs, muscles, and nerves are the physiological
agents in the process. But some animals that are fixed turn
toward the light, and in these the process seems more nearly to
approach the condition in plants, although it remains still to
be determined whether, in reality, the method of turning is the
same in the two cases. One of the serpulid worms, Spiro-
graphis, lives in a tough tube formed as a secretion of its
body. If illuminated from one side the worm turns toward
the light, causing the tube to bend in this direction. As
additions are made to the tube, the new part is made in
the direction of the source of light. Here there cannot be
said to be a growth of the animal, but only a growth of the
tube.

The stolons of the hydroid, Sertularella polyzonias, grow away
from the light, while the hydranths grow toward the light. In
another hydroid, Eudendrium, the hydranths also grow toward
the light.

In these cases the bending appears to be more nearly like that
of plants, but, as I have said, the way in which the bending
occurs has not yet been sufficiently examined.

‘Yung found the development of Sepia officinalis to be affected by light
in the following order: violet, blue, yellow and red, green. Fatigat (Compt.
Rendus, LXXXXIX, December 1879) found for infusoria that violet light
accelerated and green light retarded the development.

266 Experimental Zoology

Influence of Gravity on Growth

It is well known that gravity has an important influence in
determining the direction of growth in plants: roots turn and
grow downward, stems upward. In most animals, on the con-
trary, gravity appears to have no determining influence on
growth, although an important influence in the orientation of
some freely moving forms. In animals that are fixed we might
expect to meet with a response to gravity similar to that in plants,
and this has been found to occur in a few cases. It is a matter
of general observation that most fixed forms grow at right an-
gles to the surface to which they are attached, but in many cases
the direction of their growth cannot be due to gravity, for the
surface of attachment may be oblique or even vertical. In such
cases contact reaction, i.e. stereotropism, or some tropism other
than geotropism, must determine the direction of growth. In
several species of hydroids, however, it has been shown that
gravity determines the direction of growth. Loeb has shown
that stolons of Aglaophenia, if they do not come in contact with
a solid body, grow out at first horizontally and then downward.
Another hydroid, Antennularia antennina, also responds to
gravity. Pieces of this hydroid produce new stems that grow
upward, and stolons that turn downward. This is strikingly
seen when a piece is put into an oblique position. New stems
arise from the upper parts of the old one and stolons from be-
neath. Even an inverted piece was found to produce a root
from its lower or distal end, and a stem from its upper or basal
end. Stevens has shown for A. ramosum that the level at which
the piece is cut off is a more potent factor in the result than grav-
ity. Driesch observed in a species of Sertularia that whenever
he altered the position of the piece the new growth changed its
position so that the new part turned away from the center of
the earth.

These are the only cases in which a response to gravity has been
recorded. How the response is affected is not known, but it isnot
improbable that the result may be caused by the rearrangement

External Factors that Infiuence Growth 267

of the content of the cells, so that the heavier parts sink down-
ward and the lighter rise upward. ‘This interpretation finds its
chief support in the fact that in plants when a centrifugal force
is substituted for gravity the direction of growth is thereby
determined ; and also in the fact that in the frog’s egg an actual
rotation of the protoplasm has been observed.

Effects of Electricity on Growth

There is even less to be said on this subject than in the case of
gravity. Certain writers have found that a current of electricity
running at right angles to the axis of an embryo chick either
brings the development to an end or causes abnormal develop-
ment.

Roux placed the unsegmented eggs of the frog between two
electrodes and found that the pigment arranged itself around two
centers corresponding to the magnetic poles. After segmenta-
tion, each cell showed a similar arrangement of its pigment.
No influence was found on the direction of the cleavage planes,
and normal development took place. The cause of the arrange-
ment of the pigment granules is not clear, nor is it understood
why, in each cell, a separate center was formed. Whether the
current actually passed through the egg or only over its surface
is not certain.’

Pressure and Contact

That the growth of parts of an animal may be changed by
pressure is a familiar experience. Our own epidermis responds
to pressure, and even such resisting structures as bones respond
most surprisingly to continued pressure, as when, for example,
a new socket can be made in the pelvis for the head of the femur,
or when the shape of the head of the children of savage races
is altered by pressure, or when the ribs of women become
deformed as a result of lacing.

A direct response to contact is best shown in fixed animals.

1 Rossi repeated this experiment and found abnormal cleavage and abnormal
development.

268 Experimental Zoology

Thus the stolons of “hydroids and of bryozoans of some of the
compound Ascidians” cling to a substratum with which they
have come into contact and refuse to leave it, following all of
its irregularities. Even the under surface of the film on the top
of water will call forth this response.

In the development and regeneration of new parts from a
piece of the stem of Tubularia a response to contact can easily
be demonstrated. If anew hydranth of Tubularia on emerging
from the stem comes into contact with a solid body, it turns away
from it and grows at right angles to the surface of contact. If,
again, the basal end of a piece of the stem of Tubularia comes in
contact with a solid, it develops a stolon (and not a hydranth).
On the other hand, the oral: end if brought into contact with a
solid develops a hydranth, 7.e. it does not respond to contact.
Other hydroids, however, will develop a stolon at the oral end
if this touches a hard surface. I have suggested that the con-
tact reaction between the cells in different parts of the body may
be one of the important factors in determining not only the
molding of the form of each organ during the development, but
that the pressure relations of the parts may be an important
factor in their growth.

The Formation of Galls

The formation of galls isa remarkable phenomenon of growth,
for galls are well-defined structures, differing from anything else
that the plant normally produces. The best-known galls are
those found on the higher plants and are caused mainly by in-
sects. A few gall-like growths also occur on animals, as in the
case where certain crustacea infesting corals cause gall-like
swellings toappear. Perhaps the cyst on fish caused by the para-
sitic larvee of the fresh-water mussel, Anodonta, may also be con-
sidered a gall, as well as the cysts found about parasitic trichine,
chigoes, etc.

The galls that have been most studied are those on plants,

1See Loeb (1892).

External Factors that Influence Growth 269

particularly those on the oak, willow, and rose. The most com-
mon of these contain the larve of gallflies. Adler and Beyer-
inck have studied experimentally the process of gall formation
by gallflies, and their results have thrown much light on the
processes involved. Previously it had been generally held that
the growth leading to the formation of the gall is caused by a
poison injected by the insect at the time of deposition of the egg.
The swelling caused in animals by the sting of bees, for instance,
may have led to this idea; but it has been shown that, in most
cases examined, the secretion poured over the egg at the time of
deposition only serves to fix the egg in place. It has also been
shown that the poison of the bee does not produce a swelling or
a gall when injected into the young tissue of a plant. Most galls
do not begin to develop until the larva hatches and fastens its
jaws in the surrounding cells. In only two forms has it been
shown that a secretion may be responsible for the gall formation,
which begins at once and is far along before the egg hatches.
It has also been supposed that the wounding of the tissue caused
by the puncture of the ovipositor is responsible for the growth,
but this has been entirely disproven, because some forms, the
Cecidomyide, do not pierce the tissue, but push the ovipositor
into the bud without wounding it; also because the wounded
part does not, as a rule, produce the gall, but only the region
around the larva; the egg itself may be placed on a free surface
not pierced by the sting. Furthermore, in some cases, the plant
is pierced a long time before the gall develops, the latter occur-
ring only when the larva emerges; thus Trigonaspes crustalis
pierces the young leaf in May, but the larva does not hatch until
September, and then the galls begin to develop.

The galls become the abode of other species of gallflies and
of other insects, parasites, and inquilines. When the larva that
makes the gall is parasitized by the invading insects, the growth
of the gall stops, as a rule, when the larva that made it is
killed; but there are a few instances known in which the
presence of the parasite seems to suffice to cause the continued
growth of the gall, although it does not appear that the parasite

270 Experimental Zoology

can itself start the gall. Inquilines, or guests, also frequently
occur in galls. They appear to have, in most cases, no effects
on the gall-growth so long as they do not injure the gall maker.

Most of the species of gallflies show an alternation of sexual
and parthenogenetic generations, both generations producing
galls, generally on the same species of plant, but the gall may
differ somewhat in character. Adler has studied especially the
galls of the oak. He collected the galls and kept them under
proper conditions until the gallflies emerged. He placed these
on the leaves of young twigs of little oak trees grown in pots,
covering the branches with gauze to confine the flies. The buds
that were pierced were marked with threads and their history
followed. ‘The life history of one of the species of gallflies will
serve to illustrate the details of gall formation. Neuroterus
lenticularis produces galls on the under surface of oak leaves,
sometimes forty to fifty on one leaf. The galls are 4 to 6
millimeters in diameter and of a yellowish red color. They
appear in June, and, maturing in September, fall from the
leaves to the ground about the beginning of October. At
this time the larva is still minute and requires much moisture
for its subsequent development. If the galls are laid on
damp soil, the larva will develop, at house temperature, in
about four weeks; but out of doors, under natural conditions,
the flies do not emerge until April. Adler put the galls in
pots filled with earth which were then sunk in the soil. Each
pot was covered with gauze to confine the gallflies when they
emerged. He placed the flies on his saplings and saw them
pierce the buds. The ovipositor is pushed under one of the
bud scales as far as the base of the bud, which is then pene-
trated from without inward, and the egg is then deposited in
the bud. When the young leaves develop the gall is very small
and difficult to detect, but soon grows rapidly. Relatively few
of the buds pierced produce galls, because apparently of the
difficulty in placing the egg in exactly the right place. The
fly that emerges from the gall in June is known as Spathegaster
baccarum, and until its connection with Neuroterus lenticularis

External Factors that Influence Growth 271

was shown by Adler’s experiments it had been supposed to
belong to a different genus. The gallflies of Spathegaster are
both males and females. The males emerge first, and as soon
as the female appears copulation ensues. If the fertilized fe-
males are placed on oak saplings having tender young leaves in
actual growth, these are pricked on the under surface. Galls be-
gin to develop after two weeks, and remain on the tree until the
autumn. The fly that emerges from these is Neuroterus len-
ticularis, which completes the cycle.

Other species of gallflies have only one generation a year,
which is parthenogenetic. Others still may have two partheno-
genetic generations a year, and consequently no sexual reproduc-
tion at all. There is great diversity in the kinds of galls. In
fact, the galls often differ more from each other than do the species
of gallflies that produce them, and are more easily identified.

Three points in the formation of the galls are especially inter-
esting. First, the gall develops always from unformed tissue,
particularly from the meristem of the plant. Second, the kind
of cells and tissues that form the galls are those peculiar to the
plant on which the gall develops; but while some of the cells
may retain to a large extent their original structure, others be-
come changed. Spiral vessels grow into the gall and ramify
through its walls in definite courses. Third, while the growth
begins in the vicinity of the larva, the principal changes may not
be in this part, but removed some distance from the larva, espe-
cially in the more complicated forms of galls. There can be
little doubt that something set free from the larva affects the
cells and causes their remarkable growth. Furthermore, the
stimulus must be kept up if the gall is to continue to grow. We
can easily imagine that very small quantities of the stimulating
substance, continually applied, are effective in producing the
change, and we can see, if this is the case, how difficult it may be
to imitate artificially such a process; yet experiments along these
lines should be undertaken, for, if gall-like growths could be
artificially induced, we would have a better means for studying
the processes involved.

272 Experimental Zoology

Beyerinck made a careful study of the galls of the willow
(Salix amygdalina) produced by the gallfly, Nematus capree.
This case is especially interesting, since the galls begin almost at
once to develop, and may be full sized before the larva hatches.
The cause of the growth in this case is the albuminous secretion
that the gallfly injects along with the egg into the leaf. That
the secretion and not the egg is the cause of growth has been
shown by puncturing, and thus killing the egg with a fine
needle. The gall continued to develop. This gallfly deposits
its egg in the young leaves of the willow in the early spring.
The gall can be detected within two days and has finished its
growth in three weeks. The larva feeds on the inner wall of
the gall, and finally bites a hole in one side through which it
escapes after the gall falls to the ground. It then spins a cocoon
and in August the adult fly, the second generation, emerges.
It seeks young growing buds of the willow and pierces them.
The gall develops in the autumn and falls to the ground with the
leaves., The larva spins its cocoon, inside or outside the gall,
and overwinters in this condition. In the first generation, in
the spring, there are no males; in the second generation occa-
sionally males may be found, but nevertheless Beyerinck thinks
that this generation also reproduces by parthenogenesis.

Beyerinck has discussed the question of the kind of changes
that occur when a gall is produced, and he has carried out some
experiments that bear on this important question. He tried
to determine whether the cells of the gall are permanently
changed, 7.e. whether their structure has been so affected that
whatever they produce will be different from the tissues of the
parent plant from which they arise; or whether the change is
only temporary, depending on the presence of some substance
exciting them to a peculiar mode of growth. The latter view
seems to him the more probable as shown by the following facts:
If the leaves are removed from a twig bearing the so-called
“willow-rose’’ gall, new buds grow out of the axils of the leaves
of which the gall is made. ‘The first leaves are somewhat modi-
fied, like those of the “‘willow-rose,’’ but the later leaves are like

External Factors that Influence Growth 273

the leaves of thenormal plant. Thesame result was obtained with
the witches broom (Hexenbesen) of the birch (caused by Phy-
toptus betuli). Similarly for the Bedegar of the rose caused by
Rhoditis rose. Again, the galls produced on the grass, Poa
nemorales, by Cecidomyia po, send out rootlike processes, and
if these are covered with earth, they produce true roots that re-
semble histologically those of the grass in every respect. Thus
the gall itself stimulates a part of the plant to produce roots
that never does so normally, and these roots are similar to the
normal roots of the same plant, although arising from the gall.
The galls of Nematus viminalis produccd on Salix purpurea
fall to the ground in the autumn and may remain alive through
the winter. In the spring they ‘may increase in size, develop
more chlorophyll, and produce lenticels over the surface.
Beyerinck succeeded, by keeping these galls on moist sand, in
causing them to produce roots from the inner surface, and these
may even protrude through the opening in the gall. The roots
are like the normal roots of the plant that produced the gall.

The nature of the substance injected along with the egg into
the leaf by Nematus caprez is unknown, except in so far that it
is an albuminous matter secreted by glands connected with the
ovipositor. Its amount is exceedingly small, both the egg and
the surrounding secretion measuring not more than 0.06 milli-
meter, and more than half of this mass is taken up by the egg.
The gall produced is about 10 millimeters in diameter. The
disproportion in size is so great that Beyerinck suggests that the
substance injected contains an enzyme that acts on the cells of
the plant and excites them to the growth that leads to thg
formation of the gall.

Some species of Aphids and of Phylloxerans also produce galls,
many of them of remarkable size and beauty. The stem-mother
after emerging froma winter egg crawls out on to the young leaves
and affixes herself at one spot on the under surface, and begins
to suck the juice of the plant. Her presence, or more probably
some secretion that flows from her proboscis into the wound, ex-

cites growth on one or on both sides of the leaf. A hollow gall
rT

274 Experimental Zoology

begins to develop, in the middle of which sits the stem-mother.
She soon begins to give birth, parthenogenetically, to offspring
which also remain in the gall, and in some cases hundreds of
individuals almost filling the interior are produced in one or
more generations. These also suck the juices of the plant by
thrusting their proboscides into the inner walls of the gall.
The gall may continue to grow during the whole summer, but
generally its full size is soon attained. The winged insects
emerge from a preéxisting opening in the gall, or in some cases
the gall bursts when the inmates are mature. The phylloxerans
produce galls on the leaves of hickories and other trees, and
on the roots of the grape vine. Large bottle-shaped galls are
produced by aphides on the elm; the coxcomb galls of the elm
also owe their origin to them. The cause of the development of
the galls in these cases is entirely unknown. Possibly the
stem-mother and her offspring that suck the juices of the plant
by thrusting their beaks into the inner walls of the gall may
secrete some fluid that acts on the tissues of the plant.

LITERATURE, CHAPTER XVI

ADLER, H. Ueber den Generationswechsel der Eichengallen. Zeitschr.
f. wissen. Zoologie, XXXV. 1881. (Translated by C. R. Stratton.
Clarendon Press, Oxford, 1894.)

BarFurTH, D. Versuche ueber die Verwandlung der Froschlarven.
Archiv. f. mikr. Anat. XXIX. 1887.

Bepiarp, A. DE. Influence de la lumiére sur les animaux. Compt.
Rendus, XLVI. 1858.

BEYERINCK, M. W. Beobachtungen ueber die ersten Entwickelungsphasen
einiger Cynipidengallen. Verhandl. d. k. Akad. d. Wetenschappen
Amersterdam, XXII. 1883.

Die Galle von Cecidomya an Poa nemoralis. Botan. Zeit. XLIII.
1885.

Ueber das Cecidium von Nematus Cupreae auf Salix amygdalina.
Botan. Zeit. XLVI. 1888.

Bitton, M. M. F., et Stassanp, F. Action de quelques composés phos-
phores sur la nutrition. Comp. Rend. de la Soc. Biol. LV. 1903.

Branc, L. Note sur les effets teratogéniques de la lumiére blanche sur
l’ceuf de poule. Compt. Rendus de Biol. Paris, XLIV. 1892.

Boscuer. Die Beziehung der Wachthumsgeschwindigkeit des Siug-
lings zur Zusammensetzung der Milch bei verscheidenen Saugethie-
ren. Zeitschr. f. physiol. Chemie, XXIV.

External Factors that Influence Growth 275

DANILEWSKI, B. Del’influence de la lecithine sur la croissance et la multi-
plication des organismen. Compt. Rendus, CXXI. 1895.

De Vinfluence de la lécithine sur la croissance des animaux 4 sang
chaud. Compt. Rendus, CXXIII. 1896.

Davenport, C., and CAsTLe, W. On the Acclimatization of Organisms
to High Temperatures. Arch. f. Entwickelungsm. d. Organismen,
IT. 1895.

DAVENPORT, C., and NEAL, H. On the Acclimatization of Organisms to
Poisonous Chemical Substances. Arch. f. Entwick. d. Organ. II.
1896.

Descrey et ZAky, Analyse de mode d’action de lecithines sur l’organisme

,animale. Compt. Rendus, XIV. 1902.

Etude de Vinfluence des lecithines sur l’organisme animale. Jour.
physiol. et de pathologie générale, IV. 1902.

Drisescu, H. Studien ueber das Regulations vermégen der Organismen,
I Arch. f. Entw.-mech. V. 1897.

Epwarps, W. F. De Vinfluence des agens physiques sur la vie. 1824.

Hatat,S. The Effect of Lecithin on the Growth of the White Rat. Amer.
Jour. Physiol. X. 1904.

HEIDEL, R. L. Ecology of the Willow-Gall. Amer. Natural. XXXIX.
1905.

Hersst, C. Experimentalle Untersuchungengen ueber den Einfluss der
verénderten chemischen Zusammensetzung des umgebenden Me-
dium auf die Entwickelung der Thiere. I. Zeit. wiss. Zool. LV.
1892. II. Mittheil. Zool. Stat. Neapel, XI. 1893. IIDI-VI. Ar-
chiv Entw.-mech. II. 1896.

Ueber die zur Entwickelung der Seeigellarven nothwendigen anor-
ganischen Stoffe. I. Archiv Entw.-mech. V. 1897. II. Archiv
Entw.-mech. XVII. 1904.

Formative Reize in der Tierischen Ontogenese. gor.

Hertwic, O. Ueber den Einfluss verschiedener temperaturen auf die
Entwickelung der Froscheier. Sitz. Berlin Akad. 1896.

Ueber den Einfluss der Temperatur auf die Entwickelung von Rana
fusca und Rana esculenta. Arch. f. mikr. Anat. LI. 1808.
HiIGGENBOTHAM, J. Influence of Physica! Agents on the Development of
the Tadpole of the Triton and of the Frog. Phil. Trans. Roy. Soc.

London. 1850. ;

Influence des agents physiques sur le développement du Tétard de la
grenouille. Jour. de la Physiol. VI. 1863.

Kocu, W. Die Lecithaire und ihre Bedeutung fiir die Icbende Zelle.
Zeit. f. physiol. chemie. XXXVII. 1902.

Lawes, J. B., and J. H. Girserr. On the Composition of Foods, in Rela-
tion to Respiration and the Feeding of Animals. Rept. 22 Meeting
British Ass. Adv. Sci. 1852.

Logs, J. Ueber Geotropismus bei Thieren. Arch, f. d. ges. Physiol.

XLIX. 1801.
Untersuchungen zur physiologischen Morphologie der Thiere. II.
1892.

Maas, O. Ueber den Aufbau des Kalkskeletts der Spongien in normalem
und in CaCOx frei Seewasser. Verhn. d. Deutsch. Zool.. Gesell.
Ig04. 7
Minor, C.S. Senescence and Rejuvenation. Jour. of Physiol. XII. 1891.

276 Experimental Zoology

Morcan, T. H. Regeneration. t1gor.
Regeneration in Antennularia. Biol. Bull. IJ. 1gor.
Rauper, A. Ueber den Einfluss der Temperatur, des atmosphiarischen-
drucks und verschiedener Stoffe auf die Entwickelung Thierischen
Eier. Sitzb. Natur f. Ges. Leipzig, X. 1884.
Rossi, U. Sul? azione dell’ elettricita nello sviluppo delle uova degli
Amphibi. Arch. Entw.-mech. IV. 1896-1897.
Stevens, N. M. Regeneration in Antennularia ramosa. Arch. f.
Entw.-mech. XV. 1902.
Vernon, H. M. The Effects of the Environment on the Development of
Echinoderm Larve. Phil. Trans. Roy. Soc. London, CLXXXVI.
1895.
Wats, B. D. Galls and their Architects. Amer. Entomol. IT.
On the Insects Coleopterus, Hymenopterus, and Dipterus inhabiting
the Galls of Certain Species of Willow. Proc. Phil. Entomol. Soc.
III.
Yunc, E. Contributions 4 histoire de l’influence des milieux physiques
sur les étres vivants. Arch. Zool. Expér. et Gén. XV. 1878.
De l’influence des lumiére colorées sur le développement des animaux.
Mittheil. a. d. Zool. Station zu Neapel, II. 1880.
Contributions 4 Vhistoire d l’influence des milieux physico-chimique sur
les étres vivants. Arch. de Zool. (2), I. 1883.
De V’influence des lumiére colorées sur le développement des animaux.
Compt. Rendus, CXV. 1892.

CHAPTER XVII
GROWTH AND REGENERATION

WHEN an animal reaches a size that is characteristic for the
species it ceases to grow, and it may appear that this happens
because the cells of the body have lost the power of further
growth. That the cessation of growth is not due to such a loss
of power is shown by the ability of many animals to regenerate
a lost part.

In some animals practically all the organs of the body show
this remarkable regenerative power, so that there can be no
doubt that the cessation of growth is not due to the loss of
power of the cells to grow, but rather to something that inhibits
their growth.

One of the most curious facts connected with the regeneration
of a part is its rate of growth at different levels. If the tail of a
fish or of a salamander is cut off near its base, the new part grows
faster than when the tail is cut off nearer to the tip. Moreover,
the new part that arises from the basal cut grows more rapidly
at first and more slowly later. What is the meaning of this
difference in rate? The result cannot be explained as due to
differences in the food supply at different levels, for the rate of
growth is very little affected whether the animal is fed or starved.
The difference in rate cannot be explained as due to a difference
in the material at the two levels, because the material derived
from the base, that grows more rapidly for a time, begins also to
grow less rapidly as the new part gets longer, and behaves in this
respect in the same way as the material derived from nearer the
tip. The difference does not seem to be due to the difference in
size of the two surfaces, except perhaps at the very beginning,
for in other forms, in the earthworm, for example, the same

277

278 Experimental Zoology

difference of rate is found even in a greater degree, and the two
surfaces are practically of the same size. It also seems probable
that the difference in rate is not due to any difference in the initial
stimulus of the operation, for the new growth at the two levels
is more nearly the same at first than it is later. So much for
what the difference is not; and in this list we may seem to have
exhausted nearly all of the physiological possibilities in the ordi-
nary sense of that term. The results seem comparable in many
ways with the results of normal growth. A young animal grows
rapidly at first, later more slowly as the adult form is approached.
The same thing happens with the regenerated part. In both
cases some inhibition takes place when a certain form or size
is attained. Before discussing this point further let us consider
more in detail some further facts about the earthworm which
give a better basis for the discussion that will follow.

When the posterior end is cut off near the tip of the tail, re-
generation of a new tip goes on with great slowness. When the
worm is cut in two near the middle of the body, the regeneration
of the posterior end takes place much more rapidly than in the
last case. If even more of the posterior part is removed, the
missing part is regenerated somewhat faster than when cut in the
middle. On the other hand, when the anterior end of a worm is
cut off, the results appear to be different, but in reality the differ-
ence is more apparent than real. When one, two, three, four,
or five anterior segments are cut off, the same number that was
removed comes back, as a rule; but when more than five are cut
off, only five at most come back. In these cases all the new seg-
ments are laid down at once, and no more are formed later. In|
the tail region the new terminal part is also laid down for all
levels at about the same time, but a growing region is formed
near the end, and from this the new segments are added. It is
in this terminal growth, characteristic of the posterior end alone,
that the difference in the rate of growth is found.

It can be shown, I think, with some probability, that we have to
deal in these cases with more than a single factor. In the first
place, the kind of differentiation at each level may determine not

Growth and Regeneration 279

only the kind of new organ that is produced, but tosome extent the
rate with which the new part makes its first appearance. For
example, in the earthworm a new head will develop only as far
back as the 15™ to the 18 segment. Behind that level a reversed
tail develops from the anterior end of the piece. The simplest
explanation of this is, I think, that the differentiated material
of the head has so far diminished when this level is reached,
and the differentiated material of a tail is so much in excess,
that a tail and not a head is formed. In the region of the
15"-to the 18" segment a head sometimesdevelops, but it is always
imperfect. Conversely, a posterior end will not develop from a
posterior cut surface farther forward than about the 12 to the
15" segment. Here, also, we may assume that the head
material has so much increased that a new tail no longer de-
velops. But this explanation will not account for the different
rates of development of a tail at different levels, unless we as-
sume that there is more of the tail-differentiated material in the
region anterior to the middle of the worm, which is inconsistent
with the preceding assumption of the distribution of the materials.
Moreover, even if this were the case the assumption would not
explain the facts, for a new tail, that is derived from the material
at the middle of the worm, also grows more slowly as it reaches
the termination of its length.

Other factors, therefore, must be postulated to account for
the difference in the rate of growth at different levels. Two such
factors may possibly be recognized. In the first place when a
new part is first laid down in the newly proliferated material
the terminal part is the first formed, and as much of it is pro-
duced as the proportions of the new part allow. For instance,
when more than five segments are removed from the anterior
end of the earthworm only the five distal segments at most
are formed; and since no growing region is produced between
the old and the new part, or in the new part itself (since a
growing region is not characteristic of the anterior end), no
more than five segments are ever produced. When the pos-
terior end of the worm is cut off there is also formed at first the

+

280 Experimental Zoology

distal end, but this is characterized by the presence of a grow-
ing region, which continues to add new segments to the new
part.

What factor determines that the éerminal organs are those that
are first laid down in the new part? It cannot be the shape
of the new part as such, for this is practically a dome-shaped
knob for all new parts. The bounding surface seems certainly
to be a factor in this relation of the parts, as does also the relation
of the old organs or layers at the cut surface. Between these two
boundaries the relation of the parts to each other determines the
result. A number of considerations, that I cannot enter into
more fully here, have led me to suspect that this relation of the
parts can be accounted for as due to a condition of stratifi-
cation or polarity, due to the mutual pressure of the parts on
each other, which acts as the stimulus for the differentiation of
the cells. By these same assumptions we can, I think, also give
a fairly consistent explanation of the difference in the rate of
growth at different levels.

Let us take, by way of illustration, the results that have been
obtained in another worm, lumbriculus. If the worm is cut in
two at almost any level, there develops from the posterior end of
the anterior piece a new tail, and from the anterior end of the pos-
terior piece anew head. The material out of which these two new
parts develop must be identical. What determines, then, that
the new material forms in one case a head and in the other a tail ?
Since the development of these new parts seems to be largely a
centripetal phenomenon, we cannot assume that the influence of
the old part on the new, a centrifugal influence, determines the
result; but since the order or sequence of the differentiation in the
new part is the same as that in the old part, this may determine
whether a head or a tail develops. In other words, the polarity
of the new part is in each case the same as that of the old. This
polarity is an expression of the stratification of the differentiation;
at least, this is the most probable view of polarity, I think, that
we can find at present. The centripetal influence acting on
the new material at the anterior end determines therefore that

Growth and Kegeneration 281

this is a head, and acting on the new material at the posterior
end determines that this is a tail. The centripetal influence is,
according to my interpretation, nothing more than the tension
of the outer layer of cells, and the pressure relations in general,
in the rounded dome-shaped mass of new materials. In this
way we can give a formal solution of the development of a head
in one case and of a tail in the other.

Let us see whether the same hypothesis will explain the differ-
ent rates of growth of the posterior end according to the level
of the cut, as seen in the earthworm, salamander, and fish. A
growing region is present near, but not quite at, the tip of the tail.
From this region new material is continually being produced, out
of which the new part is differentiated. The way in which this
new part differentiates is determined by the pressure relation of
the neighboring parts. This pressure relation is the result of the
differentiation, with its concomitant pressure relations, that has
already taken place in the old part on the one side, and of the
tension of the new material of the tip on the other side. The
new part differentiates therefore into something that is less than
the former and more than the latter. In consequence there
will be an ever decreasing stimulus and differentiation as the
new parts are formed, until finally no further stimulus for
growth and differentiation is present or is strong enough to act,
and the growth comes to an end.

In some such way as this we can, provisionally at least, ac-
count for the difference in rate at different levels, since the rate
is determined by the pressure relations of the different parts,
and this pressure decreases as a stimulus from the middle
toward the posterior end. In principle this assumption refers the
changes that take place to a formative factor or factors. It as-
sumes that the differentiation for a given material is a response
to pressure relations. The nature of response is unknown, as
in all other cases where living material responds to external con-
ditions, but that living material possesses a power to respond
by differentiating to external agents, and even to pressure rela-
tions, is too well known to require demonstration. We are deal-

282 Experimental Zoowgy

ing here, no doubt, with an extremely elusive and difficult prob-
lem, but one that is of fundamental importance in all
questions in which the problem of organic form appears. It
may be that there are factors at work here of which as yet we
have little real conception, and any such attempt as this that I
have made to give the facts an explanation on more or less famil-
iar assumptions may be premature. I hope, nevertheless, that
I may have succeeded in calling attention to certain important
phenomena of growth, and even if my attempt to bring the re-
sults under one point of view should prove unsatisfactory, the
attempt may at least serve as a sHEeeuOn for further work
along these lines.

To sum up: I have attempted to account for certain phe-
nomena of regeneration by a process of growth in which the
following factors appear to enter: (1) the differentiated material
as a factor in limiting the character of new parts; (2) the relation
of the cells to each other as a factor in their differentiation, and
assume that this relation is due to the mutual pressures or
tensions of the cells on each other; (3) the differentiated cells
also determine the existing tension in that part, and this may
in turn react on the new cells with which they are in contact.
Remove a part and the pressure relations are upset, but this
leads ultimately to the reéstablishment again of the same rela-
tions of pressure.

LITERATURE, CHAPTER XVII

Kine, H. D. Further Studies on the Regeneration of Asterias vulgaris.
Arch. f. Entw.-mech, IX. 1900.
Morean, T. H. Regeneration. Igor.
The Physiology of Regeneration. Jour. Exp. Zool. III. 1906.
pie ea to L. Prodromo di un’ opera sopra le Riproduzioni animali.
1826. ~
WEIsMANN, A. Essays upon Heredity. Trans. 1891-1892.
ZELENY, C. A Study of the Regeneration of the Arms of the Brittle-Star.
Biol. Bull. VI. 1903.
Compensatory Regulation. Jour. Exp. Zool. II. 1905.

EXPERIMENTAL STUDIES IN
GRAFTING

CHAPTER XVIII
EXPERIMENTS IN GRAFTING

ALTHOUGH the process of grafting has long been carried out
by horticulturists for practical purposes, it is only within recent
years that the process has been extensively used with animals
in order to study certain scientific problems.

Trembley’s experiments in grafting hydra, carried out in 1744,
are the first, so far as I know, recorded for animals. Later
Hunter and Duhamel grafted the spur of a cock on the comb,
where it continued to grow. The experiment is interesting only
on account of the bizarre nature of the combination.

The method of grafting carried out with plants is different
from that practiced with animals in one essential respect;
namely, that while in plants small buds are inserted in the stock,
in animals the cut ends of fully formed structures are united,
and this may sometimes involve the union of cross-sections of
the entire animal.

A number of important questions are involved in the results
of grafting. The main topics to be considered here fall under
four headings: (x) the facility with which different regions
can be united; (2) the influence of united parts on each other;
(3) the possibility of hybridizing by grafting; (4) special prob-
lems of embryonic development as studied by means of grafting.

The Union of Different Regions

The simplest kind of union is that where a part of an animal
is cut off and put back in place, as when a limb is cut off and
grafted again in the same position, or as when a worm is cut

in two and the two parts (complementary parts) are reunited.
285

286 Exxperimenwuce “Luvewsg y

A step further involves the union of different regions, and many
combinations of this sort have been made. It is a point of some
interest to find that complementary parts unite with no greater
facility than unlike parts, and even parts with reversed orienta-
tion unite as readily as do those having the same orientation.

A series of important experiments in grafting earthworms have
been made by Joest. The cut surfaces are held together by
means of two or three ligatures through the skin. In the course
of a few days the contact surfaces unite with each other, and
the threads are sloughed off. Parts of different individuals can
be united as readily as parts of the same worm. Not only com-
plementary regions can be united, but cut surfaces of different
levels, and in this way short or long combinations can be made.
For instance, if the middle region of a worm is cut out, and the
end pieces grafted together, a ‘“‘short” worm is produced. If
the union is perfect, no regeneration takes place where the pieces
have grown together. It also appears that the tail end of the
short worm does not continue to grow to make the worm
longer. A “‘long’’ worm may be made by inserting a middle
piece between the anterior and posterior halves of another
worm. This combination is also permanent, if the parts neve
been perfectly united.

In the preceding cases a posterior cut surface is united to an
anterior cut surface, 7.e. the pieces have the same orientation.
It is possible also to unite two anterior cut surfaces or two poste-
rior cut surfaces. For example, if two tail ends of two worms
are sewed together by their anterior cut surfaces, a permanent
union may be effected without subsequent regeneration, al-
though the combination has two tails and no head, and must
slowly starve todeath. It is more difficult to unite two posterior
cut surfaces, not because of any inherent difficulty in the growing
together of the parts, but because the pieces tend to crawl away
from each other, and break the ligatures before union of the
tissues has been effected.

The results are somewhat different if the cut surfaces are
brought together so that the median plane of one piece does not

Experiments in Grafting 287

correspond with that of the other. If, for instance, the nervous
system of one piece is not opposite the nervous system of the
other piece, regeneration at the place of union may subsequently
occur. The nervous system is the most important factor in the
result; for it acts as a center for the formation of the new part,
which may be a head or a tail, or two heads, etc., according to the
region that regenerates at the union. In some cases where the
cut ends of the nervous system are not exactly opposite they find
each other, and become united by a connective of nerve tissue,
derived from the ectoderm of the region of union. The further
the cut ends of'the nervous cord are apart, the less likely are they
to unite, and regeneration from one or from both cut ends is
more likely to take place.

Born has shown that cut surfaces of very young tadpoles may
be united, and he has made a great variety of combinations of
these embryos. The method of grafting is very simple. The
tadpoles are taken from the jelly capsules, cut in two with a
sharp knife, and the cut ends quickly brought in contact with
each other. The pieces are held in place by means of small
blocks of silver." The combinations are permanent, and in no
case does regeneration take place from the cut surface even
when like organs are not united. The tail region of one animal
has been united to various parts of the body of another animal,
where it remains attached and continues to develop, producing
its normal structures. In one case, the tail grafted upon the
ventral surface of another individual showed signs of being
absorbed at the time of metamorphosis, when the normal tail
was absorbed.

These results of grafting pieces upon parts of the body, differ-
ent from those with which the part in question is continuous
under normal conditions, show that the development of the part
is due to self-differentiation, and that its development is not
dependent on relation of the part to the rest of the organism.

Parts of planarians have been grafted together by Mrs. Mor-

1 T have found that bent pieces of aluminium wire and short straight pieces of
the same wire cut off very obliquely can be used to hold the pieces together.

288 ' Experimental Zoology

gan. The principal results are those with Phagocata gracilis
and Planaria maculata. Owing to the delicacy of the tissues
and the mobility of the pieces a special method of holding the
parts together has to be used. By placing the pieces to be united
between wet sheets of very thin paper, they can be brought into
close contact and kept in place by means of pieces of glass at the
sides and needles at the ends. After several hours the pieces
grow together. It has been found possible to unite cut surface
from all different levels of the body. If the union is perfect and
if the pieces are of the same size so that no free edges are left,
the combination is permanent and regeneration does not take
place at the cut surface. If the union is not perfect, regeneration
may take place at the line of union. In this instance also it
appears that when the cut ends of the nerve cords unite, re-
generation at the line of graft does not occur; but if one or both
of the nerve cords is free in either piece, one or two heads may
appear. Especially interesting in this connection are the cases
in which one of the pieces is turned upside down. This sort of
union was made in several cases where the pieces were united
by their anterior ends. Two heads regenerated at the line of
graft, one above, the other below. Each was connected with the
ventral nerve cord of one piece, and the distribution of pigment
in the new heads showed that each head was made up on one
surface of the material derived from one worm and on the other
surface of material from the other worm.

Trembley first discovered that pieces of hydra could readily be
grafted by simply bringing the cut surfaces together for a few
minutes. It has been possible to make practically all kinds of
unions, including pieces grafted in the side of another individual.
As these results will be described in some detail in the next sec-
tion, further description may be omitted here.

Another hydroid, Tubularia, has also been used for grafting
experiments. Miss Peebles has shown that cut ends of the stalk
readily unite if simply held together for a minute or two. In
some cases no regeneration takes place at the line of graft, espe-
cially if regeneration takes place elsewhere, or in the vicinity

Experiments in Grafting 289

from a free end, but at other times one or two hydranths may
develop near the region of union.

Crampton has carried out the ingenious experiment of uniting
halves and portions of the pupe of moths. After bringing the
cut surfaces together they are held in place by melted paraffin
placed around the line of union. The parts stick together and
moths may emerge united in different ways according to the
combinations formed at the time of union. The internal organs
do not unite, and the union is incomplete in this respect, since,
although sticking together, there is no continuity between the
internal organs, but the skin and the integumentary organs in
general do unite.

The preceding cases relate to combinations where large parts
of the animals have been united. It has been long known in
surgery that small pieces of different organs can be transplanted,
and it is a regular practice to graft pieces of skin over exposed
surfaces. These pieces of skin become attached, and their
epithelial cells may spread over the exposed parts. Leo Loeb
has found in the case of guinea pigs that black skin can be per-
manently established in the midst of a region of white skin, but
white skin grafted upon a surface formerly occupied by black
skin is slowly thrown off and replaced by black skin.

Ribbert has carried out many experiments in which pieces of
different organs were grafted in foreign parts of the body. If
these pieces are very small, they may remain alive for a long time,
and even begin to proliferate new material. Subsequently, how-
ever, they become absorbed.

The Influence of the United Parts on Each Other: Formative
Factors

In the cases given in the preceding section, with the exception
of hydra and of the planarians, there is no attempt on the part
of the united pieces to produce a new whole organism by a re-
modeling of the old parts. The case is different with hydra
and planarians, especially the former, in which a process of

remodeling or morphallaxis takes place in case the union does
U

290 Experimental Zoology

not produce the typical form, so that a new whole of the charac-
teristic type results. The difference in these two classes of cases
is due to the different kinds of regenerative changes of which
the animals are capable. Pieces of hydra and of planarians
can transform themselves into the typical form with very little
production of new tissue, and it is owing to this power that the
changes about to be described are due. In the other animals,
the tissues, when once formed, cannot remodel themselves into
a new whole. At most they can only replace lost parts by first
proliferating new material at the cut surface. The experiments
to be described are the results of the work of Trembley, Wetzel,
King, Rand, Peebles, and others.

When the anterior half of a hydra is grafted to the posterior
half of a hydra, a single individual of normal proportions results,
and no further changes take place (Fig. 21); but the result is dif-
ferent if the united pieces are shorter or longer than a normal
hydra. If the anterior piece is less than half and the posterior
piece is less than half, a short hydra is produced (Fig. 2). In
this case the combination grows longer from day to day until
the normal proportions are reached and a hydra of typical form
results. If, on the other hand, both the anterior and the pos-
terior pieces are each more than a half, a ‘‘long”’ hydra results
(Fig. 3). The only way in which such an animal could become
normal in length would be by the absorption of some of its
parts, but it appears that hydra does not follow this method of
remodeling. Instead it regenerates at the line of union a new
foot that belongs to the anterior part, and new tentacles at the
line of union that belong to the posterior part (Fig. 4). The two
parts pinch in two between the new foot and the new tentacles,
and two hydras result. Evidently the conditions in a “‘short”’
hydra and in a “long” hydra are different, since different results
follow. In both cases we must suppose that the union between
the cut ends is perfect, so that the result depends on some other
relation in this region than imperfect union. The most plausible
explanation is found, I think, in the adjustments of the pressure
or tension relations of the united pieces. In a “short” hydra

11

~ CU

Fic, 21.

A
ve
p

Grafting in Hydra.

138

18

291

—s

For details, see text.

292 Experimental Zoology

the united parts continue to change their form, in the same way
as do separate pieces, until the relation of the parts of the cut
surfaces is the same as that for the normal in this region, when
further adjustment ceases. In the “long” hydra the same kind
of changes take place, but the only way in which the typical
relations could be adjusted would be either by absorbing the
excessive parts, or by each piece, acting independently, assuming
the typical form. The latter method is the one followed. This
attempt to account for the different behavior of the two grafts
may appear to be little more than a restatement of the facts;
but if the restatement is correct, it has at least the advantage
of referring the results to the factors that are operating. The
hypothesis also involves a principle that will account for the
other formative changes to be described.

Tf two pieces of hydra are united to each other by their anterior
cut surfaces (Fig. 5), tentacles soon appear around one or both
pieces near the line of union. If each half develops tentacles,
the halves may pinch apart in the intermediate region (Fig. 6);
if only one set of tentacles develops, a mouth may form, and the
two pieces have a single crown of tentacles. Little by little the
two pieces fuse together lengthwise into a single body as shown
in Figs. 7 and 8, until finally one hydra results. The develop-
ment of the tentacles at the line of union may possibly be ac-
counted for by the inability of each piece to adjust its tension
relations to the other; for although when halves are united in
opposite directions the tension at the place of contact is the same
as in the normal, since like regions are united, the reversal of the
direction of the tensions in the opposed pieces makes it impossi-
ble for further relations to become established except by each
part behaving as a separate individual. Here we meet with the
conception of polarity as involved in the pressure relations. The
polarity from this point of view is an expression of the graded
pressure relations from one end of an organism to the other,
which in turn may be an expression of the gradation of the
tissues, and in turn may itself, under certain conditions, be
the cause of the differentiation.

Experiments wn Grafting 293

When two cut aboral ends are united (Fig. 9), two new foot
processes are formed (Fig. 10), and the two pieces subsequently
pinch apart to form each a separate individual. The explana-
tion is the same as in the last case.

These results can be somewhat better understood when taken
in connection with the experiments of hydras partiaily split length-
wise. If the anterior end is split for only a short distance, each
half will round up and produce a separate head (Fig. 11).
Subsequently the two heads may slowly fuse into a single one.
The result is similar to the fusion of two bodies in the grafted
hydra with one head. If the split extends farther into the
hydra, as in Fig. 12, the two parts slowly draw apart until
they finally pinch off at the foot region and produce two
hydras. Why in one case the two separated parts unite and
in the other case separate farther can possibly be ex-
plained on the view that I have tentatively suggested above.
It will be noted that in the case of slightly separated heads
the influence of the single body prevails, and reunites the
parts from behind forward by drawing them together and es-
tablishing the normal tension relations; and in the case of the
more separated anterior ends, these ends prevail and cause
the single trunk to pinch farther apart, causing, as it were, the
halves of the single trunk to draw up into their respective
anterior ends.

The results of uniting the squarely cut posterior end of one
individual with a cut surface in the side of another individual
(Fig. 13) are similar in many ways to the last. The united
pieces usually adjust themselves in such a way as to share the
common trunk (Fig. 14). Subsequently the two pieces split
apart farther and separate in the region of the foot. If the
grafted piece does not succeed in halving the trunk, z.e. if it
does not turn anteriorly, a foot develops at the line of union
(Fig. 15), and the graft pinches off without passing down to the
base of the stock. What conditions lead to this difference in
behavior have not been sufficiently made out, but it is probable
that the kind of union, or the relative sizes of the graft and

294 Experimental Zoovlogy

stock, or the likeness or unlikeness of the united levels, may
account for the results.

A third possibility also exists in grafts made in this way. The
graft itself may swing around into line with the trunk of the stock,
and become the head of the new hydra (Fig. 16). The head of
the stock shifts down to the base and there pinches off to pro-
duce a new hydra, leaving its original posterior end in the posses-
sion of the new head.

If a very short piece of the anterior end is grafted near the
head of another hydra (Fig. 17), the two heads fuse into one.
At first there are too many tentacles, but some of these are ab-
sorbed or even two tentacles fuse into a single one — a process
not uncommon in hydra, producing, while in process of comple-
tion, the forked tentacles not infrequently found.

If the cut end of a hydra is grafted into the side of another
hydra, and then, after union, the graft be cut off close to the
stock (Fig. 18), the small ring (whose outer end closes) will be
slowly absorbed into the stock. The result may be expressed
in terms of my hypothesis as follows: The resistance in the cells
of the small piece is insufficient to allow the piece to pinch off,
and it is too different to permit it at first to share the common
trunk. Not being able to free itself, and unable to maintain
itself under unfavorable conditions of tension, it is absorbed,
or changed over into a part of the body wall, the process
being very slow as a rule.

It has been found’ that different species of hydra behave
somewhat differently. In Hydra fusca, lateral grafts tend to
move forward until the head end is of the same length as that
of the stock when fusion begins and unites the two parts. If,
however, the lateral graft is inserted less than one fifth the dis-
tance from the lower end, it moves downward and constricts
off at the base. In Hydra viridis, lateral grafts tend to move
downward wherever united to the stock, and separate at the
foot, unless inserted very near the head end of the stock, when
the two heads fuse.

1 Hefferan (1902).

Experiments in Grafting 295

In grafting or in splitting individuals, tentacles are sometimes
displaced from their normal position, and may come to lie at
some distance from the oral end. This result may easily be
obtained by cutting off the head just below the line of the ten-
tacles. When the new trunk is formed individual tentacles often
become drawn out of place (Fig. 19). In all these cases the
tentacles become absorbed after a time. The result is impor-
tant, because the special conditions in the case of hydra preclude,
with much probability, the explanation of the absorption as due
to food relations. The tentacles are hollow and open directly
into the central digestive space. A tentacle attached at the side
is situated in as favorable a place for receiving food as are those
of the ring, yet it is absorbed. The result is similar to the
absorption of small grafts in the same position, and the expla-
nation of the absorption is probably the same in both cases.

One of the most important results obtained by grafting hydra
is the reversal of polarity that can be brought about in the fol-
lowing way: If two pieces are united by their anterior or oral
ends, and then one of them is cut off near the line of union
(Fig. 20), it will often produce, if very short, a mouth and ten-
tacles (head) from its exposed aboral or posterior end (Fig. 21).
Thus instead of a foot a head develops. This result occurs only
when the piece is very short. It seems that the polarity has been
reversed, owing to the union with a larger piece. The result is
difficult to explain; possibly the conditions of tension may actu-
ally be changed in the smaller piece through the influence of
the larger piece, so that the orientation is reversed. The ex-
periment needs very careful reéxamination before we can safely
offer a probable explanation of the result.

The planarians are also capable of remodeling the proportions
of the old parts to produce a new worm of typical proportions.
In this case, however, the new head and tail are formed in pro-
liferated material, but the rest of the body is remodeled. Ab-
normally placed parts, resulting from grafting or from cutting,
do not show the power of pinching apart exhibited by hydra, but
may after a time be absorbed, especially if small and not sup-

296 Experimental Zoology

plied with a mouth opening. The reversal of polarity described
for reversed grafts of hydra has been discovered by Mrs. Mor-
gan to take place in planarians also when the piece is very short.
If, for instance, anterior cut surfaces of two pieces are united,
and one of the pieces is subsequently cut off near the line of
union, that piece if very short produces a head at its posterior
exposed end. Here also it appears that the results may be due
to the influence of the old part. In this case there is no question
of absorption of the smaller piece, but there is on the otherhand
ample opportunity for inwandering cells to pass from the large
part through the small piece into the new material. The result
is further complicated by the fact that very short pieces of other
species of planarians, that are not grafted, may produce a head
at the anterior end and another at the posterior end, and while
at present this result has not been obtained with Plagocata, the
possibility still exists that the results may be due to the cut-off
pieces that have been tested for double heads being somewhat
longer than the small grafted piece in the reversed position. If
the grafted piece is longer, it produces a tail at its posterior end
and not a head.

Miss Peebles has shown in Tubularia that small pieces grafted
on to the ends of large pieces may sometimes take part in produc-
ing the single head that develops. This may occur also when
the small piece is reversed in direction; but it cannot be shown
in this case that the reversed head is due to its union with the
larger piece, because Tubularia produces a new head so readily
from either the aboral or oral end that even if some influence
of the larger piece exists it would be difficult to prove.

The peculiar power of pieces of hydra, planarians, etc., to
mold themselves into a new form of typical proportions is clearly
similar to the molding that takes place in embryonic develop-
ment. The older writers used the term “formative force”
to account for this power to undergo changes in form, but
modern investigators avoid the use of this term, because no such
form of energy is known in the physical world; and because we
should have to postulate as many kinds of formative forces to

Experiments in Grafting 297

explain the results as there are different species of animals hav-
ing this power. In other words, the use of the term “formative
force” is only a restatement of the problem, not a causal ex-
planation of it. Perhaps the main objection to this, and to
similar terms, is that they imply the existence in living matter of
forces, or energies, or factors that are not exhibited by non-living
things. In so far as there is postulated only a different kind of
physical action from any so far described by physicists, little
objection can be raised; but until the nature of this new force can
be demonstrated, very little if anything is gained by assuming
its existence. If, on the other hand, the postulated principle
is supposed to be different in character from all other physical
events, then the matter becomes more serious; for the assump-
tion is either metaphysical, and therefore outside of the proper
field of science, or if not metaphysical, the assumption attempts
to account for known events by a principle entirely unknown.
In the latter case nothing is gained, and since the nature of the
question itself is prejudged, harm may be‘done. In the preceding
pages, in attempting to account for the changes in hydra, etc.,
I have assumed that the formative changes are the outcome of
a relation of tension in the parts. From this point of view the
condition of tension is the stimulus to which the material basis
of the organism responds.

The postulated factor is a physical one, and the response of the
cells that determines the result is supposed to be, in most cases,
the familiar response of contraction shown by all animals. In
this respect the nature of the process is assumed to be the same as
that seen in other phenomena involving contact and response by
contraction. There is nothing in the nature of this reaction that
seems to preclude a purely physical process. In addition, how-
ever, it is necessary to assume that the differentiation of the cells
also takes place as a result of the mutual pressure of the parts
on each other. This assumption is more arbitrary and more
difficult to bring into accord with our present knowledge; yet the
facts seem to demand, I think, some such view, although at pres-
ent it can only be offered as a provisional or working hypothesis.

298 Experimental Zoology

The Union of Paris of Different Species

As Darwin long ago pointed out, the union of parts of different
species has many points in common with the fertilization of the
egg of one species with the sperm of the other. In both cases the
combination can sometimes be successfully made, while in others
it cannot. Furthermore, the general statement may be made
for both cases that closely related species combine more readily
than those far apart, i.e. the results are more successful for unions
between closely “related” forms than between distantly ‘“re-
lated” forms. Certain exceptions exist, however, in both direc-
tions.

The principal experiments with animals are those of Joest
with earthworms; of Born with tadpoles; and of Wetzel with
hydra.

It has been found to be more difficult to unite pieces of differ-
ent species of earthworms than of the same species; nevertheless
Joest has succeeded in making a number of combinations of
different species. The most successful union, 7.e. the one that
was most easily made, is that between Lumbricus rubellus and
Allolobophora terrestris. A compound worm of this sort lived
for eight months. Neither part showed any material influence
of the other half, although the blood circulating through the
worm went from one to the other. It may appear, in this case,
that since both parts were completely formed at the time of
union, there is given no chance for the influence of the parts on
each other to manifest itself. If, however, a part of one of the
components is removed and a new part regenerates, the new
part must derive its nourishment from the materials in the blood
that come from both components. The possibility of such an
influence was tested in a compound made up of Lumbricus ru-
bellus and Allolobophora foetida. The new head regenerated
from Lumbricus rubellus, that is the lighter in color. It was of
the same color as L. rubellus, and showed no influence of the
darker color of the other component, A. terrestris.

Born has also found that it is more difficult to get pieces of

Experiments in Grafting 299

different species of tadpoles to unite than pieces of the same
species, although Rana esculenta can be united with ease to
Rana fusca or to R. arvalis. The facility with which the pieces
unite is owing to the rapidity with which the ectoderm of Rana
esculenta covers exposed surfaces. Rana fusca and R. arvalis
do not combine so readily, because the ectoderm unites very
slowly. On the other hand, it is very easy to unite Rana escu-
lenta with Bufo igneus, but not permanently, as the parts die
or separate. In one case, however, the union lasted for three
weeks. Partial union, but of short duration, was effected be-
tween the tadpole of Rana esculenta and a larval Triton.

Although some of these combinations between different
species lasted for some time, and seemed to be permanent in
several cases, each part developed only its own specific peculiari-
ties, as seen especially in the color. No hybridizing effects were
apparent.

Harrison has united the anterior half of Rana virescens to the
posterior half of Rana palustris, and has reared young frogs
from the combination. The anterior half of the body of these
frogs showed the characters of one species and the posterior part
that of the other. There was no trace of mutual influence
between the two halves.

I have examined a case in which the relation of the parts is so
intimate that were there any influence exerted we should expect
to get evidence of it. The tip of the tail of a young tadpole of
Rana sylvatica was united to the tail of Rana palustris (from
which previously an equivalent piece had been removed) (Fig.
22, B). As the new tail grows out, the dark ectoderm of R.
sylvatica is carried out to the tip (Fig. C), while the underlying
cells remain behind, and come to be covered over by the light
yellow ectoderm of R. palustris. If now the tail is cut off at
the line indicated by the vertical line a—-a, this cut end will be
made up of the inner organs of R. sylvatica, and, externally, of
the skin of R. palustris. When the new tail regenerates (Fig.
E) the opportunity for mutual influence of the parts is afforded,
yet no such influence was apparent. Other experiments of a

300 Experimental Zoology

similar kind gave the same result. Especially instructive is
the case where the new ectoderm is derived partly from one
component, partly from the other species (as when the tail is
cut off as shown at 0-b in Fig. C). No influence of the ecto-

E
Fic. 22. Grafted tail of frog. Fig. A, Rana sylvatica with grafted tail of
Rana palustris. Fig. B, reciprocal graft. Fig. C, late stage of last. Fig. D,
newly regenerated tail of Fig. A cut off at a-a. Fig. E, newly regenerated tail of
Fig. C cut off at a-a.

derm cells on each other could be detected, although they are
in intimate contact.

The attempt to unite pieces of brown hydra, H. fusca, to
pieces of green hydra, H. viridis, has not succeeded. Wetzel
has shown that although the two pieces will stick together for
a few hours, or even days, they subsequently separate; but the
union of Hydra fusca and H. grisca was successfully carried out.

Experiments in Grafting 301

Crampton has studied the effects of uniting parts of different
species of moths. The pupe of the species to be united were cut
across, and the exposed surfaces brought into contact and held
in place by a coating of paraffin. In successful cases the parts
united, at least so far as the integumentary organs are concerned,
and the moths that subsequently emerged were made up of parts
of two species. As a rule, each part developed its own specific
coloring, showing no mutual influence of the parts on each other.
In two cases, however, evidence of some influence was found.
The posterior part of the abdomen of a female pupa of Callosa-
mia promethea was united to the rest of the body of Samia
cecropia. In the moth the promethea part assumed the color
of the cecropia. In another union between Telea polyphemus
and Samia cecropia the small piece of the latter assumed the
color of the former. According to Mayer, the color in these
moths is largely due to the drying of the hemalymph. The
explanation of the two cases just given may be that some
of the hemalymph of the larger component got into the smaller
part. If the result is due to this, the outcome is only remotely
connected with the “influence” of one part on another.

In conclusion, it is apparent that grafted pieces of different
species have no mutual influence on each other of the kind that
characterizes species. In this respect the results are quite differ-
ent from the effects of cross-fertilization; the difference is prob-
ably due to the actual union within the same cell of the characters
of the two parents in cross-fertilization, and to the absence of
any such intimate fusion in the case of grafting. In other words
the cells retain their specific characters in the unions by graft-
ing, and although cells of different species may live side by
side and form a part of a new individual, they have no influ-
ence of a specific nature on each other.

Special Problems of Development

It has been pointed out that by means of grafting it has been
found possible to study the question of self-development versus
correlated development of parts of the embryo. In most cases

302 Experimental Zoology

it has been found that after the development and differentiation
of a part have begun it continues to self-differentiate even if re-
moved to another part of the embryo. As an example of corre-
lated development, an experiment carried out by Lewis may be
cited.

Lewis has shown that any part of the ectoderm of the embryo
of the frog can produce the lens of the eye, provided a piece of
the optic vesicle is transplanted beneath it. The result shows
that the optic vesicle exerts a formative stimulus on the ecto-
derm, calling forth the lens formation. ‘This case is the best-
ascertained example of the so-called formative stimulus and
one of the few indisputable cases of the sort. Although em-
bryologists have some reason to conclude that similar stimuli
may play an important réle in development, it has been found
difficult to obtain evidence of this kind of action. On the other
hand, the power of independent self-differentiation of the parts
has been demonstrated in a large number of cases. Both
principles appear to play a réle in the development of the
embryo.

In a somewhat different way Lewis has examined the same
problem. He removed the ectoderm lying over the eye vesicle
and transplanted there a piece from another animal — a differ-
ent species, in fact, whose skin was differently colored. The
lens developed at the proper time from the grafted piece.

In contrast to these results Lewis found that pieces of the eye
vesicle transplanted in different parts of the body underwent
there self-development and differentiation.

Harrison has studied the perplexing problem of the normal
growth of the nerves in the embryo, by uniting parts of the bod-
ies of young tadpoles in such a way that the nerve must extend
into a territory that is normally foreign to that nerve. Since
the nerve extends into the new region, Harrison concluded that
its growth takes place by its own substance extending outward,
and not by the addition of cells in the new part.

Braus has recently studied the same problem in another way.
The limb bud of the tadpole, at a very early stage in its develop-

Experiments in Grafting 303

ment, was extirpated and grafted on another part of the body.
At the time of removal he believed that the nerves had not ex-
tended into the region of the limb. The bud developed in its
new location into a complete structure possessing the normal
nerves, which were larger than the body nerves with which
they were found to be connected. Braus concluded that the
nerves of the limbs do not grow out from the spinal cord, as His
and his followers claim, but differentiate in the limb itself.

In another experiment the problem has been examined in a
different way. Ithad been shown by Harrison that if the dorsal
part of the young embryo of the frog, including the nerve cord,
be removed, the embryo may continue for a time to develop.
Braus carried out this operation, and after the bud of the hind
limb had appeared he removed it, and transplanted it upon an-
other normal embryo. The bud developed into a normal limb,
except that it entirely lacked nerves. In this respect it differed
from the preceding case. It might appear that in the first case
the nerves had already grown into the limb, hence their develop-
ment when the limb was transplanted. This interpretation,
Braus states, is negatived by two considerations: first, he could
not find that the nerves had grown into the limb at the time of its
removal; second, even if they had we should expect them to de-
generate when removed from their central connections. More-
over, if nerves grew out from the central nervous system into the
grafted limb, it is not evident why they should grow out in the
first case given above and not in the present case. Braus inter-
prets his results as follows: The peripheral nerves are formed out
of the intercellular protoplasmic connections between the cells of
the embryo. The stimulus that leads to their development into
nerve bundles originates from the nerves that start from the
central nervous system; hence the nerves themselves appear as
though extensions of the nerve processes of the cells coming
from the nerve cord. If the central nervous system is destroyed
at an early stage, the time may pass by when the intercellular con-
nections in the limb bud can respond to the central influence,
hence the failure of such a limb bud to develop its nerves when

304 Experimental Zoology

grafted upon another individual. On the other hand, the
limb bud of a normal individual has already received its proper
stimulus before its removal, hence the independent development
in it of the peripheral nerves. While the argument is far from
convincing,’ yet the method gives promise of throwing light on a
very difficult and obscure point.

There is another result of interest connected with these graft-
ing experiments of Braus. He finds when the bud of the fore leg
is grafted in another part of the body — near the hind leg, for
example — that in the majority of cases two legs develop, — one
at first more advanced than the other, and the more advanced leg
alone contains nerves. In the light of some recent experiments
of Tornier, in which four complete hind limbs of the frog are ar-
tificially induced by splitting the limb buds, there can be no doubt
that the less advanced leg, in Braus’s experiment, is a regenera-
tive product. It fails to receive nerves either from its twin leg,
of which it is a mirror figure, or from the main body of the tad-
pole, or at least no medullated nerves.

Aside from the interesting questions concerned in the forma-
tion of the nerves the experiments of Harrison and of Braus are
important in showing that as complicated an organ as the leg
of the frog may develop in the complete absence of peripheral
nerves. The result shows, if true, that the self-differentiation of
all the tissues of the leg may take place in the entire absence of
connection by means of nerves with the central nervous system.

' The experiments of Braus have been repeated by Harrison upon frog and
toad embryos, the result having been briefly reported at the Toronto meeting of
the British Medical Association, August, 1906. Contrary to Braus, Harrison
finds that nerves are present, after a time, in limbs developed from buds which are
transplanted from ‘‘nerveless’” embryos. These nerves have normal relation
to the other structures in the limb, at least as far as the main features are con-
cerned, though the finer details have not yet been studied. Furthermore, in
cases where two limbs have developed out of a single transplanted bud, Harrison
has found that nerves are present in both. Braus’s results are probably to be
explained by the fact that he did not keep his specimens alive for a sufficient
length of time after the operation. Be this as it may, Harrison’s experiments show
beyond doubt that the nerves grow from the body of the host into the trans-

planted appendage, and also that it is the configuration of the organs and tissues
in the appendage that determines the course taken by the outgrowing fibers.

Experiments in Grafting 395

LITERATURE, CHAPTER XVIII

Banca, A. Sviluppo deglei arti addominali del ‘‘ Bufo vulgaris” innestati
in sede anomala. Accad. med. fisica Fiorentina. XV. 1904.
Monitore Zool. ital. Anno 15. 1904.

Born, G. Ueber Verwachsungsversuche mit Amphibienlarven. Arch. f.
Entw.-mech. IV. 1897.

Braus, H. Versuch einer experimentellen Morphologie. Natur. hist.
med. Verein Heidelberg. Sitz.17. 1903.

Einige Ergebnisse der Transplantation von Organanlagen bei Bombi-
natorlarven. Verh. Anat. Gesell. 18 Vers. Jena. 1904.

Experimentelle Beitrage zur Frage nach der Entwickelung peripherer
Nerven. Anat. Anz. XXVI. 1905.

Crampton, H. E. An Experimental Study upon Lepidoptera. Arch. f.
Entw.-mech. VII. 1808.

Harrison, R.G. The Growth and Regeneration of the Tail of the Frog
Larva. Arch. f. Entw.-mech. VII. 1898.

Experimentelle Untersuchungen ueber die Entwickelung der Sinnesor-
gane der Seitenlinie bei den Amphibien. Arch. f. mikros. Anat.

“LXIII. , 1903.
HEFFERAN, M. Experiments in Grafting Hydra. Arch. f. Entw.-mech.
XIII. 1902.

Jorst, E. Transplantationsversuche an Regenwiirmem. Sitz.-ber. d.
Gesell. z. Berf. d. ges. Naturwiss. Marburg. 1895.
Transplantationsversuche an Lumbriciden. Arch. f. Entw.-mech. V.
1897.
Kine, H. D. Observations and Experiments on Regeneration in Hydra |
viridis. Arch. f. Entw.-mech. XIII.
Lewis, H. W. Experimental Studies on the Development of the Eye in
Amphibia. Jour. Exp. Zool. II. 1905.
Morean, L. V. Regeneration of Grafted Pieces of Planarians. Jour.
Exp. Zool. III. 1906.
Morcan, T. H. Regeneration of Tissue composed of Parts of Two Spe-
cies. Biol. Bull. I. 1899.
Further Experiments on the Regeneration of Tissue composed of Parts
of Two Species. Biol. Bull. I. 1899.
Regeneration in Bipalium. Arch. f. Entw.-mech. IX. 1900.
Parke, H. H. Variation and Regulation of Abnormalities in Hydra.
Arch. f. Entw.-mech. X. 1go0.
PEEBLES, FLORENCE. Experimental Studies on Hydra. Arch. f. Entw.-
mech. V. 1897.
Experiments in Regeneration and in Grafting of Hydrozoa. Arch.
f. Entw.-mech. X. 1g00.
Ranp, H. W. Regeneration and Regulation in Hydra viridis. Arch.
f. Entw.-mech. VIII. 1899.
The Regulation of Graft-Abnormalities in Hydra. Arch. f. Entw.-
mech. IX. 1899.
Rissert, H. Ueber Veranderungen transplantierter Gewebe. ‘Arch. f.
Entw.-mech. VI. 1897.
Ueber Riickbildung an Zellen und Geweben und ueber die Entstehung
der Geschwiilste. Bibl. med. 1897.
x

306 Experimental Zovblogy

Ueber Transplantation von Ovarium, Hoden, und Mamma. Arch. f.
Entw.-mech. VII. 1898.
Tornier, G. An Knoblauchskréten experimentell enstandene ueber-
zahlinge Hintergliedmassen. Arch. f. Entw.-mech. XX. 1905.
TremBLey, A. Mémoires pour servir & histoire d’un genre de Polypes
d’eau douce. Leide. 1774.
WETZEL, G. Transplantationsversuche mit Hydra.. Arch. f. mikr.
Anat. XLV. 1895.
Transplantationsversuche mit Hydra. Arch. f. mikr. Anat. LIT. 1898,

EXPERIMENTAL STUDIES OF THE
INFLUENCE OF THE ENVIRON-
MENT ON THE LIFE-CYCLE

CHAPTER XIX

CHANGES IN THE LIFE-CYCLE AND CHANGES IN THE
ENVIRONMENT

The Life Histories of Some Animals

In the life histories of many animals and plants a series of
stages succeed each other until, with the completion of the cycle,
the starting point is reached again. In some species the series
of forms that make up the life-cycle seem to change without any
change taking place in the environment, that remains the same
throughout, but in other cases the change in the cycle is asso-
ciated with a change in the environment. It is this latter class
that we may now examine, although it will also be profitable not
to neglect the other.

The life-cycle of most animals is made up of a single adult
individual and a series of embryonic stages through which the
embryo passes to become an adult. The cycle consists, therefore,
of adult, egg (and sperm), embryo, larva, adult. In other cases
the life-cycle is more complex, as it may contain more than
one adult form and include several modes of reproduction.

In some species the cycle seems to complete itself under con-
stant external conditions, as stated above, while in closely related
groups the change appears to be connected with a change in the
environment. For instance, many of the marine hydroids show
an alternation of generations that does not appear to be directly
connected with changes in their surroundings. The egg develops
into a swimming embryo that settles down and becomes a polyp.
The polyp produces buds, all of which are also polyps. This
process continues until the colony has reached a certain stage,
when a new kind of bud arises that becomes a jellyfish. The
jellyfish detaches itself, and comes to lead an independent exist-

309

310 Experimental Z oology

ence in the sea. This jellyfish produces eggs, and the egg after
fertilization develops into the primary polyp, completing the
cycle. The cycle is passed through during the summer, as a
rule, in our northern waters, and there is nothing to indicate
that external factors determine that a bud, developing later
in the season, must become a jellyfish and not a polyp. We may
suppose that differences in temperature or in the food supply
bring about the change, but as yet there is no proof that this
is the case. It would certainly be worth while to attempt to
find out whether external conditions are factors in the result;
but so far as we can see at present the jellyfish bud appears
when the colony reaches a certain size. It should not be over-
looked that this size is far from being fixed, and may vary in
different colonies when the jellyfish are formed. The impor-
tant consideration is that as the colony grows larger its relation
to the environment must become different from what it was at
first, so that even if the environment remains the same the animal
enters into a different relation with it.

This point is well illustrated by certain recent experiments of
Klebs. The flowering plants produce at first only leaves and
branches. When they reach a certain size they produce flowers.
Klebs points out that most botanists look upon the flowering
of the plant as the culmination of its form. The form is some-
thing that perfects itself under favorable conditions without re-
spect to the environment. Klebs, on the other hand, thinks
that the development of the flowers is simply due to a relation
that becomes established between the plant (when it attains a
certain size or stage) and the external conditions, and he brings
forward in support of his idea a number of confirmatory ex-
periments. These show how by altering the conditions a
shoot that would ordinarily develop into a flowering branch
continues to grow vegetatively, producing only leaves.

In the green hydra the change from the non-sexual to the
sexual mode of reproduction seems to be connected with defi-
nite seasons of the year, and possibly, therefore, with a change
in the environment. Throughout the summer, the autumn, and

Changes in the Life-cycle 311

the winter the hydra produces buds that pinch off and become
new individuals instead of remaining a part of the parent to form
a colony. The size of hydra remains more or less constant. In
the spring, sperm and eggs are produced. If the hydras are
brought into a warm place in the autumn or in the winter, they
will begin to produce sperm and eggs in a few weeks. This fact
seems to indicate that the change produces the result; but whether
this change is the effect of the cold, or of the warmth after the
cold, or of both combined, or of some food relation, has not yet

‘been made out.

Passing now to an examination of those cases where the
changes in the life-cycle are known to be definitely connected with
changes in the environment, I shall mention first the well-known
case of the rose aphids or plant lice. These insects produce
generation after generation of wingless,’ parthenogenetic indi-
viduals throughout the summer. In the autumn a generation
of winged males and wingless females appears. These pair
and afterward the females deposit each a few “winter eggs”
on the food plant. From these eggs the young parthenogenetic
females hatch in the following spring and start the summer
broods.

It has been shown for the rose aphid that if the parthenogenetic
summer forms are brought into the greenhouse and put on roses,
they will continue to produce parthenogenetic young, and the
sexual forms never appear. Bonnet saw nine generations of
parthenogenetic forms; Duval counted eleven in the course of
seven months; Kyber kept aphids in a hothouse for four years
and observed only parthenogenetic reproduction. It is evident,
therefore, that if the external conditions are favorable, the non-
sexual mode of reproduction may continue forever, as far as we
can see. What, then, are the external conditions that determine
the production of the sexual forms? The change occurs in the
autumn when the cold weather begins to set in, and it may
appear that the cold is the cause of the change. I have

1A few winged individuals appear in almost every generation, but these are
not the sexual forms as has been sometimes supposed.

ele Experimental Zoology

subjected roses with aphids on them to the cold of an ice
box without producing any effect. Moreover, the sexual form
may appear in the autumn before the cold weather has come.
Even under ordinary conditions all of the aphids do not become
sexual forms and hundreds of them perish by the frost. I have
kept a potted rose out of doors for three months after the sexual
forms had appeared, and yet those individuals that remained
alive after this time continued to reproduce parthenogenetically.
Finally, the individuals that seem to be less affected are those
that are found on the growing tips of the branches, where the
leaves are still young and succulent. These observations sug-
gest that the change is not due to the cold, but to some changes
in the food plant that take place in the autumn. Whether the
_result is due, as seems probable, to a lack of food, or to a reduc-
tion in the amount of water, or to both combined, remains to be
shown.

A case apparently similar to that of the aphids is found in the
Daphnians. These crustacea also produce parthenogenetically
during the summer, and in the autumn the sexual forms appear.
It has recently been shown by Issakowitsch that the change is
probably due to a change in the food supply, and that the trans-
formation can be quickly induced artificially by altering the
external conditions. This and other cases will be given more
fully in later chapters.

In the life histories of the ciliate infusoria, periods of division
are succeeded by periods of conjugation. Maupas thought that
after a long succession of divisions this mode of propagation
comes slowly to an end, and that unless conjugation occurs
the individuals will die. Immediately after conjugation, when
division is very active, there is little tendency to conjugate
again; but the longer the time elapsing after this process, the
more prone are the individuals to unite in pairs, especially those
of a different parentage. It would seem that an internal factor
is here involved; but there are some indications that external
factors may also enter into the result. It has been shown by
Calkins that when parameecium has undergone many divisions,

Changes in the Life-cycle 313

and the process appears to be reaching its limit, the individuals
may be rejuvenated by altering the composition of the solution
in which they live. They will then once more begin to divide
actively and start a new cycle or a new phase of the old cycle.
After a time under a constant environment their activity may
again begin to fall off, when a new change may rejuvenate them
once more. The precise effects produced are not known; for
it has been found that a change that affects rejuvenescence at
one time will not do so at another, and a different solution must
be used. The conclusion seems near at hand that conjugation
between individuals that have lived under different environments
may effect the same result in the same way as a change in the
environment itself. The problem may be, however, more com-
plicated than this.

Influence of Food on the Life-cycle of Lepidoptera

The caterpillars of moths and butterflies undergo a series of
moults as they grow larger, and during this time they consume
a very large amount of food. The length of the larval life
varies enormously. Thus according to Pictet the caterpillar
of Argynnis paphia takes 15 to 20 days from egg to chrysalis.
The caterpillars of the Vanessas take three to four weeks;
Ocneria dispar and Saturnia pavonia two and a half to three
months; Lasiocampa quercifolia eleven months (of which five
are in hibernation); Cossus cossus holds the record with a
larval life of two to three years.

The pupa or chrysalid stages also have various periods.
Certain Vanessas take 12 to 18 days, and Ocneria dispar, Lasio-
campa quercifolia, and Cossus cossus take 18 to 28 days. Satur-
nia pavonia and S. pyri that become inclosed in the spring
generally pass two winters as chrysalids or pupe. The length of
life of these stages is, however, subject to great variation. ‘The
amount of food obtainable is an important element in the result.

Lasiocampa quercus hatches from the egg in the summer.
The larva remains active till about the first of November, hiber-
nates in the caterpillar stage until the end of April, when it

314 Experimental Zoology

becomes active again with the return of vegetation, and becomes
a chrysalid, emerging after a month in this condition in July or
August, when the eggs are laid. If the caterpillars are kept in a
warm place in the autumn and during the winter, they hiber-
nate nevertheless, although the resting stage, the diapause, may
be much affected. If the caterpillars are left outside in the
cold for a while and then brought into the warmth, they awake
a month sooner than normal, 7.e. at the beginning of March, but
nevertheless produce their typical moths at the normal time.
Caterpillars that are not subjected to cold at all, but are brought
into a warm room in the autumn, hibernate only a month, and
begin in December their second period of activity preparatory
to becoming a chrysalis, which occurs in March or April; but
they do not emerge as moths until after an entire year, thus re-
maining chrysalids 13 months instead of only 28 days. The
moth is identical with the normal. Thus by shortening the
larval life the pupal life is prolonged.

Changes in food also affect the length of the different stages
of development. For example, Ocneria dispar lays its eggs
in July, and these do not hatch until the following April; the
larval life extends to the middle of June; the pupa condition
lasts until the middle of July. This is the normal course. Fed
on the leaves of the walnut, the young caterpillars do not reach
their complete development until the beginning of July, when
they pupate, but the pupal life is correspondingly shortened and
lasts only 20 days. Similar changes are affected by other plants
that also supply insufficient nourishment. In all of these cases
it was found that the larval life is prolonged and the pupal
life shortened. The reverse effect is produced by those plants
that furnish a better nutriment than the normal. The cater-
pillars grow rapidly and pupate 15 days sooner than do
those fed on their habitual food plant, but the pupal life is
sensibly prolonged. It lasts after Pimprenelle and Esparcette
40 days, after dandylion 43 days, instead of the ordinary 28
days. ‘‘ What the insect gains in the larval stage it loses in
the pupa stage.”

Changes in the Life-cycle 315

Influence of the Environment on the Time of Ripening of the
Sexual Organs

In most animals the sexual organs ripen their products when
the adult form is reached, but in a few cases it has been shown
that sexual maturity may obtain while the form of the body
remains in the young or larval state. This phenomenon in
its various forms has been called neotenia. The classical case
is that of the axolotl. This salamander becomes sexually ma-
ture while still leading an aquatic life. It is a large, newt-like
amphibian, eight to nine or even twelve inches in length. It
has three pairs of branched external gills, a long tail with a dor-
sal and ventral fin. The animals were first found in the lakes
near the City of Mexico. ‘For many years these creatures were
looked upon as a species of Perennibranchiate, under the generic
name of Siredon (S. axolotl, S. pisciformis, S. mexicanus, etc.),
although Cuvier suspected that they were but the larve of an
otherwise unknown terrestrial urodele. The mystery was not
cleared up until the year 1865, when some axolotls, which had
been kept for a year in the Jardin des Plantes at Paris, suddenly
began to pair and laid eggs which within six months developed
into full-sized axolotls. This certainly looked as if these crea-
tures were not larval, but a true Perennibranchiate species.
But to the general surprise several of these young Axolotls gradu-
ally lost their gills, the clefts closed up, the fins of the back and
tail disappeared, the head became broader, the creatures left
the water permanently, and in fact turned into the already
well-known terrestrial Amblystoma tigrinum.” *

It was this discovery that first gave the hint that the axolotl
is a larval form. At Weismann’s suggestion Mlle. Marie v.
Chauvin tried to bring about this change artificially, and suc-
ceeded in discovering the necessary conditions. The axolotls

1 Gadow, H., “Amphibia and Reptiles,’ The Cambridge Natural History.

2It appears that other observers had already recorded similar conditions.
Filippi in 1861 found tritons sexually mature but without the adult form, and

Jullien found four female larve of Lissotriton punctatus with mature eggs, two
of which were laid.

316 Experimental Zoology

were first well fed so that they might be strong enough to pass
through the period of metamorphosis during which they take
no food; for if in poor condition they die before the transforma-
tion is completed. They were then put into shallow water, or on
a moist surface, and in the course of one to three weeks, or
longer, the change took place. The skin loses its sliminess,
the dorsal fin disappears, the tail loses its broad border, and the
gill slits close. The change appears to be connected with the
enforced respiration by means of lungs instead of gills; possibly
the drying of the skin may be a factor in the result.

Shufeldt’ has given a very complete account of the metamor-
phosis of Amblystoma tigrinum in New Mexico. He finds that
axolotls change into Amblystomas more readily if kept in water
containing little air. If the transformation is induced to a cer-
tain point, the animal will complete it without further stimulus.
Young axolotls are more easily made to transform than are older
individuals. The metamorphosis is hastened by regularly sup-
plying the animals with an abundance of food, “and what is
still more interesting, when they are thus treated it markedly
affects the appearance of the transformed Amblystomas.” If
well fed on meat the Amblystomas are not only larger, but of a
deep black color without spots, while those not well fed are
mottled with bright yellow and pale brown. The depth of the
water seems to have ‘‘a wonderful influence”? upon the meta-
morphosis; the deeper the water, the slower the metamorphosis.”
A moderate increase in temperature seems to hasten the trans-
formation. In the same pond in process of drying up all stages
in the metamorphosis may be found,—animals caught in
shallower parts undergoing a rapid transformation; those in
deeper parts being unchanged.

Neotenia has also been observed in the tadpoles of frogs, but
to a less degree, since they do not reach sexual maturity in this
form. Certain organs, only, advance in their development, while
others remain at an earlier stage. If the transformation of the

' Science, September, 1885, p. 263.
* This may possibly be due to the diminished chance of using the lungs.

Changes in the Life-cycle B17
‘tadpole is prevented by keeping the animals in cold water, the
hind legs may continue to grow larger and their bones become
harder. The long spiral intestine shortens, as it does when the
tadpole changes into the frog, but the sexual organs do not
mature.

How extensive the phenomenon of neotenia occurs in other
-amphibia is not known. It has been suggested that forms like
Necturus, that always have external gills, do not representmore
primitive members of the group, but are neotenic species that
never undergo a metamorphosis, and become sexually mature
in the larval stage. Jn other groups of the animal kingdom the
same phenomenon occurs, according to some zodlogists. Thus
it has been suggested that the rotifers represent the sexually
mature trochophore larval stage of the annelids, which has lost
its power to transform. Similarly, Appendicularia has been
supposed to be a sexually mature larval form of an ascidian,
. and it has been suspected that even Amphioxus may be a neo-
tenic form.

Boas has brought together many cases in which special or-
gans remain in a lower stage of development, while other organs
in the same animal undergo the characteristic transformation.
Thus we seem to have three forms of neotenia: one in which the
body remains at a lower stage and the sexual organs ripen;
another in which some of the organs advance (as in the tadpole),
but sexual maturity does not supervene; and third, those in
which some organs fail to develop at the time of sexual maturity.

A case that appears to be a parallel one in some aspects exists
in the white ants according to Grassi. The members of the
colony consist of a royal couple, workers, soldiers, and young.
If one of the royal pair is removed, a royal substitute form is
reared, either male or female, according to which is needed.
If both royal individuals are removed, two royal substitutes are
reared. Grassi has shown experimentally that this occurs, but
the way in which the transformation is effected was not deter-
mined, although he thinks that the amount or the kind of food,
given to the young determines the result. The young appear

318 Experimental Zoology

at first to be all alike, and potentially have the power to become
workers, soldiers, or substitute royal forms. The workers and
the soldiers have rudimentary reproductive organs and are of
both sexes. Their development is, in a sense, arrested, although
they are something more than simply undeveloped individuals,
since they have peculiarities that belong to their caste. The
substitute royal forms are, however, according to Grassi, neotenic.
individuals in which the sexual organs develop, but the wings do
not. In this and in some other respects they represent sexually
mature larval forms. Although attempts have been made to
discover the nature of the food that determines the fate of
individuals of the colony, nothing definite has as yet been
ascertained.

Grassi succeeded in establishing colonies in small glass tubes
that could be carried about in his waistcoat pocket and studied
through the walls of the tube. Colonies of from 15 to 40 indi-
viduals of different ages were thus established, made up of
workers, soldiers, and young, but without sexual forms. After
a few days, from two to six incipient substitute pairs appeared,
characterized by pigmented eyes. In fact, Grassi says, even
after 30 or 40 hours in summer he could tell which individu-
als would acquire the ocular pigment. The formation of these
incipient royal substitutes does not take place if a royal pair is
present; but if the king or the queen is removed for 24 or 48
hours, a few substitute forms begin to appear. In one case a
nest was made up of only three large larve, and in two weeks one
of these exhibited pigmented eyes, and was in process of be-
coming a substitute form. If a nest contains adults ready to
fly, z.e. royal forms that have not yet paired, the formation of
royal substitutes takes place nevertheless, as described above.
In other words, the winged sexual forms if present do not be-
come kings and queens, when these are needed in the nest, but
substitute forms are made. Only after the nuptial flight do the
royal forms become the heads of new colonies.

The food of Calotermes consists of wood, matter excreted
or disgorged by other individuals, the exuvie, the corpses of

Changes in the Life-cycle 319

other individuals, their own saliva and that of others, and water.
The feces form an important element in the diet. ‘When a
calotermite wishes to feed, he accosts one of his fellows and ca-
resses the abdomen with the antenne and palpi. If the one thus
accosted is prepared to eliminate, he at once extrudes the scyba-
lum from the anal aperture. The other then removes it, chiefly
by aid of the auxiliary palpi, and usually in two operations
separated by a short interval, drawing it at first half, and then
completely out. He then rapidly seizes it with the mandibles,
suspending his caresses for this purpose, and when he has
possessed himself of it nibbles at and ingests it little by little.” 1
Pellets can sometimes be seen in the mouth composed of re-
gurgitated food, which is at times used for building, but may be
seized and eaten by another individual. The saliva may be
used either as cement in building or as food for others. It is
given in abundance to larve that are too young to eat wood, and
to those that are in course of becoming royal substitutes. These
individuals, fed with saliva, “exhibit a great transparency of the
abdomen, an indication that they are in process of becoming
royal substitutes.” Their digestive tract contains no proto-
zoans that are usually present in other forms. Possibly they
have been killed by the saliva. These and other facts suggest
that the saliva is the cause of the transformation of young ter-
mites into substitute forms. What determines whether a young
termite is to become a soldier or a worker is not known; but
Grassi thinks that this difference is also determined by the food.

These experiments and observations leave little doubt that
the different castes of termites arise from the same kind of egg,
and are not predetermined in the egg. Each egg has the possi-
bility of becoming a royal form, a royal substitute, a worker, or a
soldier. It is probable that the results depend on the amount
or the kind of nutrition that is given to the young by the older
members of the community.

In the honey bee the relation between the food and the develop-
ment of the worker and queen is better understood. It appears

1 Translated by W. F. Blandford.

220 Experimental Zoology

that any fertilized egg may produce a fertile queen, and the
amount of food supplied to the young larve determines the re-
sult. If the old queen dies and young larve are present that are
not more than a day or two old, the workers can produce one or
more new queens by feeding the young grubs on the royal jelly.

Leuckart found that the food of the young larve of queens and
workers is the same for the first three days and then is changed
for the worker while the queen grub continues to be fed with
the same food rich in honey. At this time the female genital
organs appear, so that if the weaning of the worker grub does
not take place promptly, any excess of the royal jelly may cause
the ovaries to continue their development and produce a fertile
worker.

In the bees all the workers are females with undeveloped
sexual organs. They have all the marks of perfect insects in
other respects, and cannot, therefore, be said to be neotenic.*
The males or drones develop from unfertilized eggs, and their
characters are not affected by the kind of food they receive,
except that a drone fed on royal food may become unusually
large.

LITERATURE, CHAPTER XIX

Barpiani, G. Sur la reproduction et l’embryonogie des Pucerons. Comp.
Rendus, LXII. 1866.
Mémoire sur la génération des Aphides. Ann. Sci. Nat. (5th), XI,
XIV, XV. 1869-1872.
BaARFuRTH, D. Versuche ueber die Verwandlung der Froschlarven. Arch.
mikr. Anat. XXIX. 1887.
Boas, J. E. V. Ueber Neotennie. -Festscher. f. Carl Gegenbaur. 1896.
Bonnet, C. Traité d’Insectologie. 1745.
Born, G. Ueber Versuche von Salamandra maculata und Anguis fragilis
ausserhalb des Leibes der Mutter aufzuziehen. Zool. Anz. II.
1879
CALKINS, % Studies on the Life-History of Protozoa, IV. Death of the
A’ series. Conclusions. Jour. of Exper. Zool. I. 1904.
Cakins, C., and Lies, C. Studies on the Life-History of Protozoa, II.
The Effect of Stimuli on the Life-Cycle of Paramcoecium caudatum.
Arch. f. Protistenkunde, I. 1902.

‘In this case it is the germ-cells that fail to develop, while the body tissues
pass over into the adult condition. In a sense this is the converse of neotenia.

Changes in the Life-cycle 321

CHavuvIN, Marie v. Ueber die Verwandlung der mexicanischen axolotl
in Amblystoma. Zeit. f. wiss. Zool. XXVII. 1876.

Gace, S. H. Life-History of the Vermilion Spotted Newt. Am. Nat.
XXV. 1891.

Grassi and Sanpias. Constituzione e Sviluppo della Societa dei Termitidi.
Atti dei Accademia gloenia di Scienze Naturaliin Catania. Vols. VI
and VIII. 1893-1894. Trans. by Blandford. Q. J. Micro. Sc.
XXXIX and XL. 1897-1898.

Heat, H. The Habits of California Termites. Biol. Bull. IV. 1902.

IssakowitscH, A. Geschlechtsbestimmende Ursachen bei den Daphni-
den. Biol. Centralb. XXV. 1905.

JuttiEn, J. Observation de Tétards de Lissotriton punctatus reproduisant
Vespéce. Compt. Rend. LXVIII. 1869. .

Kammerer, P. Beitrag zur Kenntnis der Verwandschaft, Verhaltnisse von
Salamandra atra und maculosa. Arch. f. Entw.-mech. XVII.
1904.”

Ueber die Abhangigkeit des Regenerationsvermégen der Amphibien.
larven von Alter, Entwickelungstadien und specifischer Grdsse.
Arch. f. Entw.-mech. XIX. 1905.

Kress, G. Willkiirliche Entwickelungsinderungen bei Pflanzen. Ein
Beitrag zur Physiologie der Entwickelung. 1903.

Leucxart, R. Bericht ii. Zergliederung einer unbefruchtet ein-und durch
gewinterten Bienenkénigin. Fichstédt. Bienen Ztg. XI. 1855.

Sur l’Arrenotokie et la Parthénogenése des Abeilles et des autres
Hymenoptéres qui vivent en Société. Bull. Acad. Brux. (2), III;
Trans. Eichstédt. Bienen Ztg. 1857.

Die Fortpflanzung der Rindenlause. Fin weiterer Beitrag zur Kennt-
niss der Parthenogenesis. Arch. Naturgesch. XXV. 1859.

La sexualité des abeilles. L’Apiculteur. 1862.

Mavpas, E. Recherches experimentales sur la multiplication des Infu-
soires ciliés. Arch. Zool. Exp. et Gen. (2), VI. 1888.

Le regennissement karyogamique chez les ciliés. Ibid. VII. 1889

Nusspaum, M. Geschlechtsentwickelung bei Polypen. Verh. Nat. Ver.
Bonn, 49 Jahrg. Sitz. Med. Sect. 1893.

Pictet, A. Influence d Alimentation et de !’Humidité sur la variation
des Papillons. Mem. Soc. Phys. et Hist. Nat. Genéve, XXXV.
1905.

SHUFELDT, R. W. Science. 1885.

Wooprurr, L. L. An Experimental Study of the Life-History of Hypo-
trichous Infusoria. Jour. Exp. Zool. IT. 1905.

t

CHAPTER XX
ALTERNATION OF SEXUAL AND PARTHENOGENETIC FORMS

PARTHENOGENES!S plays a double réle in the animal kingdom ;
in some species parthenogenetic generations alternate with sexual
ones, and in such cases both the male and the female individuals
develop from unfertilized eggs; in other species, where no such
alternation of generations exists, the fertilized eggs produce indi-
viduals of one sex, the female as a rule, and the unfertilized eggs,
produce the other sex. It may be well to keep these two cate-
gories apart, although intermediate conditions are not un-
known, as where unfertilized eggs produce both males and
females. The cases in which external conditions influence the
kind of reproduction belong, for the most part, to the first cate-
gory, where parthenogenetic and sexual generations alternate.

Alternation of Generations in the Aphids and Phylloxerans

It was pointed out in the last chapter that in the group of
Aphids an alternation of parthenogenetic and sexual generations
occurs. In some species the change from the one to the other
method depends on a change in external conditions; but in other
species a definite succession of generations exists, and the change
is less obviously connected with the effects of the environment.

The discovery that many of the aphids give rise during the
summer to young without the presence of males appears to have
been first made known by Leeunwenhoek in 1695, and later
by Réaumur in 1737, but the series of experiments carried out
by Bonnet in 1745 attracted more widespread interest in this
mode of development, because of his philosophical discussion of

the phenomenon.
322

Sexual and Parthenogenetic Forms 335

The first experiment showing that external influences have
an effect on the mode of reproduction was that of Dageer in
1773, who kept plants containing aphids in a warm place,
and found that the sexual forms did not appear. The cycle of
the species with which he worked was shown, therefore, to be
an open one, its completion depending on external conditions.
Kyber in 1815 raised fifty successive generations of partheno-
genetic individuals during four years by keeping the animals
and their food plants in the warmth during the winter. It
would be interesting to know whether, in warm climates where
the plants — the rose, for instance — grow throughout the year,
the sexual form ever appears.

The earlier students of parthenogenetic development of the
aphids were much puzzled over the viviparous production of
young in the absence of males, and it was thought by some ob-
servers that the development could not be by means of eggs, but
was the result of a process of internal budding. It was shown
later, however, that the embryo arises from an egg produced in
an ovary. The ovary and the way in which the egg appears in
it are nearly the same as in other related species in which eggs
are deposited; but a remarkable condition exists in the vivipa-
rous aphids: the egg begins to develop almost immediately after
it has left the ovary, when it is very small and when from com-
parison with other species it appears to be in a very immature
condition. However, since the ripening process of the nucleus
is the same as in other parthenogenetic eggs that are larger, and
are regularly deposited,’ there is little real basis for calling the
aphid egg immature, because it is small when it begins to de-
velop. The commoner forms of aphids, the rose aphid, for
example, pass through the following series of forms, with some
variations. In the spring, the eggs (that had been deposited on
the stems of the food plant) complete their development, and
parthenogenetic wingless females emerge. These grow rapidly
to the full size, and produce a new generation of wingless forms
amongst which a few winged individuals often occur. This

1 The phylloxerans, for example.

324 Experimental Zoology

process continues throughout the summer, but even during this
time, especially if the food plant becomes crowded, winged indi-
viduals appear, and these are also parthenogenetic. They
usually fly away when the sunlight falls on them, and if they are
fortunate enough to find a new plant of the right kind they start
there a new generation of parthenogenetic forms. In the au-
tumn the males and females appear. The males are winged, the
females are wingless. The parents of the males are also wing-
less, but the parents of the sexual females are winged, these
conditions varying, however, in different species of aphids. In
some both the males and sexual females are winged. More-
over, according to Lichtenstein, while it is usual for the mother
of the sexual forms to produce only males or only females, in
certain species both sexes are produced by the same parent.
The males and females pair, and the sexual eggs are deposited
on the food plant, where they remain over winter. In the spring
only parthenogenetic females hatch from these eggs. Even
if kept in a warm place, the winter eggs will not develop until
several months have passed.

In cases like this one of the rose aphid, the whole cycle com-
pletes itself on the same plant. The rose aphid will also, under
compulsion, live on other plants and even multiply there with
great rapidity, as on the dock, for example. I have tried in a
few cases to produce the sexual forms by changing the food plant,
but so far without success.

Balbiani has discovered the important fact that the same
female may give birth to parthenogenetic individuals, as well as
to sexual females and males. This result removes all grounds
for the assumption that there are two lines of parthenogenetic
individuals, — one culminating with males, the other with sexual
females. Balbiani isolated the females of Centaurea jacea at
the time of year when the transition from the parthenogenetic
to the sexual mode of reproduction was taking place. Each day
the number and the kind of young produced was noted. The
number varied from one to seven per diem. After a day of great
productiveness there followed one or more days when no young

Sexual and Parthenogenetic Forms 325

or very few were produced. Some females gave birth only to
parthenogenetic young, others first to parthenogenetic and after-
ward to a mixture of parthenogenetic and sexual young. The
following records give the kind and the number of offspring for
ten isolated females.

The parthenogenetic females are indicated by P, the males
by M, and the sexual females by F.

First female, Aug. 21 to Sept 5.
3P, 3P, 2P, 4P, 5P, 3P, 4P, 6P, 3P, 3P, 2P, 3P, 3P,1P,1P. Total, 46P.

Second female, Aug. 21 to 25.

13F 3P, aM 2P, 3P. Total, 14P, 3M, and 2F.
? ?

Third female, Aug. 23 to 28.
2F, 2F, 3F, 6F, 3F, 2F, 1F. Total, oF.
Fourth female, Aug. 26 to Sept. 20.

if M
6P, sP, sP, 4P, 3F, 2F, 0, ie so 2 2M, 4F, { th. SP, 3F, 4F, oF,
0, 0, 0, 2F, 1F, 1F, 2F, 0, 0, 0. Total, 20P, 6M, 44F.

Fifth female, Aug. 3 to Sept. er

(eM M M
a {Se (ie, (2h (oe

4F, IF, 6F, 4F. Total, 11M, 56F.
Sixth female, Aug. 31 to Sept. 20.
ea 5F, { _ ea \on {ie 3E, 7F, 2F, 4F, sF, 3F, 6F, rF,
2F, 1F, 5F, 0, 0, 1F, 1F, 0,0. Total, 14M, 56F.
Seventh female, Ria 16 to ee ca
aF, 0, 2F, ae 7 {2M [2h SE, ak, aF, SF, aF, 4F, aF, 4F, sF.
Total, oMt Oa

Eighth female, Sept. 16 to Oct. 7. 5 aR EEE
2M f
{ a 1M, a ay 2F, 4M, ea es ie IF, 2F, 2F, 3F, 3F,
o, 3F, 2F, 1F, 4F, 0, 2F, 2F, 0, 1F, 3F, 0, oF, IF, 2F, 0, 1F, 1F.
Total, 12M, 55F.
Ninth female, Sept. 16 to Sept.’ 26.

4F, 1M, «F, {3m IM, 0, 3F, 0, 4F, 4F, 1F. Total, 5M, 18F.

3F, 6F, 3F, 1F, 4F, 2F, 1F,

Tenth female, Sept. 16 to Oct. 4.

3M, i {ane IF, 4F, 3F, 3F, xF, 3F, aF, 2F, 1F, 2F, 2F, 1F, 0,
o, IF, 1F. Total, 6M, 32F.

326 Experimental Zoology

The results are summed up as follows: Toward the end of the
season an individual may first give birth for a few days to a mix-
ture of parthenogenetic and sexual individuals, but at this time
the males predominate as a rule. The parthenogenetic young
now cease to appear, and only sexual individuals are born.
After two or three days of mixed sexual broods, the males cease
to appear, and there then follows a long series of sexual
females, that terminates only with the death of the mother.
Not more than 20 per cent of the sexual forms were males
in one of the species studied by Balbiani, and many of these
died in the larval stages. Certain females begin at once to
produce sexual forms without having first produced partheno-
genetic young.

Balbiani isolated young females about to give birth to sexual
young, and placed them on young branches of the same plant
in order to see whether the kind of young produced is affected
by the condition of the food plant. They continued, despite
the change, to produce sexual individuals throughout the rest
of their lives. Conversely, a female producing parthenogenetic
young was placed on a branch cut from the plant. She con-
tinued to produce parthenogenetic young. As the branch be-
came dry she acquired wings and flew away. Balbiani concludes
that food does not produce any effect on the mode of reproduc-
tion unless the organism is ‘‘predisposed to submit to its influ-
ence.” This predisposition appears at certain times of year
characteristic for each species. Possibly the results may be
interpreted, I think, to mean that external conditions require
more than one generation to produce their effects, and when once
effected the mode of reproduction for that individual cannot be
altered. It remains to be shown, I think, that there is an in-
born predisposition that is independent of external conditions to
produce sexual forms. On the contrary, the possibility of pro-
longing the parthenogenetic series indefinitely by proper condi-
tions shows that the predisposition is connected with external
changes.

Stevens has recently confirmed Balbiani’s discovery that the

Sexual and Parthenogenetic Forms 327

same female may produce both parthenogenetic and sexual
young, or males and females, or only one sex alone.

In contrast to species like the aphids with an open cycle,
there are other species with closed cycles, and in some of these
there is an alternation of plants closely associated with an alterna-
tion in the successive forms; but it is important to note that the
change in the aphid has already been initiated on the old plant
before the migration to the new one takes place. This is also
true in some cases for the sexual forms. Some examples will
make my meaning clearer.

There are two American species of aphid that alternate be-
tween the red birch and witch-hazel, producing galls on the
latter and crinkling of the leaves of the former. Pergande has
made out the following life-cycle for these species: One spe-
cies, Hormaphis hamamelidis, passes through seven generations»
in its life-cycle, and the forms that appear in this cycle show
great differences in structure. The eggs that have wintered
over on the witch-hazel hatch about a week before the
young leaves appear, and the young animals congregate on the
still-closed buds. They settle finally on the under side of the un-
folding leaves, and insert their bristles in the substance of the
leaf. Within a few days the gall begins to develop, a swelling
appearing above and a cuplike depression beneath. The gall
develops rapidly and assumes the form shown in Fig. 23, 1a, 1b.
The stem-mother, as this first inhabitant of the gall is called,
moults three times before assuming the full size (Fig. 2). She
gives birth to many young, which cover the inner walls of the gall ;
they moult four times and become winged (Fig. 3). These mi-
grants gradually leave the galls during June. Each contains
about 50 embryos. They fly to the birches, and deposit their
young on the under sides of the leaves. These young form the
third generation. They also moult four times and produce the
aleurodiform stage (Fig. 4). Each is flat and surrounded by a
circle of waxy rods, and gives birth to young of the fourth genera-
tion that are like their parents. The fourth generation produces
a fifth similar to the two last. A curious fact regarding these

Fic. 23. Hormaphs hamamelistes, showing life-cycle of non-sexual, winged and wingless; and
sexual, wingless forms. (After Pergande.)

328

Sexual and Parthenogenetic Forms 329

aleurodiform stages is that many of the females are devoid of
eggs and die after reaching maturity, others give birth to only a
few young, while the most prolific produce only about. 10 to
15 young.

The individuals of the fifth generation give birth to young
while still on the birch, which become the return migrants
(Fig. 5). They reach the adult condition toward the end of
August. These return migrants are like the earlier migrants,
except that they are smaller. They fly to the leaves of the
witch-hazel, and give birth there to the sexual forms, male (Fig. 7)
and female (Fig. 6). Pairing occurs, and 5 to 10 winter eggs
are laid by each female around the bases of the leaf buds.
From these eggs the young hatch in the spring.

The other aphid, Hamamelistes spinosus, described by Per-
gande, also alternates between the witch-hazel and the birch,
producing on the former large spiny galls (Fig. 24, 1), and
on the birch a peculiar crinkling or gall-like formation of the
leaves (Fig. 5). The winter egg is laid in June or July, but does
not hatch until the spring of the following year. Two years are
required to complete the life-cycle. The young that emerge
from the egg on the witch-hazel find a young flower bud, and
settling on it begin to suck its juices. The bud becomes a gall
inclosing the insect. Each gall has an opening at the bottom
(Fig. 1). The stem-mother when full grown is shown in Fig. 2.
She gives birth to a large number of young, possibly 300 in
all, and the young fill the gall. These young become, early in
July, the migrants (Fig. 3). Each contains 30 to 4o young.
The migrants alight on the birch leaves where the young are
deposited. The young feed on the leaves, then move to the
twigs, and settle down close to a bud. After three moults they
become degenerate, scale-like insects — they are the coccidiform
or hibernating females (Fig. 4). In the following spring they
give birth to young that migrate to the new leaves, where they
settle down between the folds. Their presence causes the edges
of the leaves to turn under while the upper surface of the leaves
bulges out into ridges and corrugations (Fig. 5). In about

Fic. 24. Hamamelistes spinosus, showing life-cycle of non-sexual, winged and
wingless; and sexual, wingless forms. (After Pergande. )

330

Sexual and Parthenogenetic Forms 331

20 days they become mature and produce numerous young of
the fourth generation (Fig. 6). These produce in turn the fifth
generation, that is, the winged migrant (Fig. 7). Some of the
individuals of the fifth generation do not become winged — pos-
sibly these vhort-circuit the life-cycle. In June the migrants
are ready to leave the birch and migrate to the witch-hazel,
where they give birth to the sexual males (Fig. 9) and females
(Fig. 8). They pair and the females deposit one to five large
eggs on the twigs near the flower buds. These eggs remain un-
developed through the whole summer and the next winter, and
hatch only in the following spring, when they move to the young
flower buds and produce there the gall of the witch-hazel.

A number of other aphids also alternate between two plants,
although the entire life history is not so well known as in the
last cases. In the aphid of the apple tree, Aphis mali, several
apterous generations occur in the spring. Winged forms then
appear that have been supposed to migrate to the stems of wheat
or grass, and there produce, parthenogenetically, wingless forms.
In time, winged individuals again appear which migrate to the
apple tree, producing the sexual forms that unite and produce
the winter eggs from which the new generation appears in the
following year.

The aphid of the elm, Tetraneura ulmi, of Europe has, ac-
cording to Blochmann, the following life-cycle: The winter eggs
hatch about the end of April. The aphids produce galls on the
leaves within which viviparous young are born. These are
winged and leave the galls. They disappear for a month, and
were supposed to live on some other plant, but according to
Riley for an American species they live on the bark of the elm
tree, where they produce one or more generations. Winged
forms appear in August, returning to the branches, where they
deposit their eggs, from which sexual males and females appear.
These pair, and one fertilized egg is laid by each female on the
bark of the tree, where it remains over winter and produces in
April the first generation of summer forms.

The species of the genus Chermes have a very complicated life

ere Experimental Zoology

history, as shown by Blochmann, Dreyfus, and others. The
young appear in the spring on the fir. They become mature
and deposit eggs. The young developing from these eggs pro-
duce galls, within which they mature as winged individuals.
Most of these leave the fir and go to the larch, where each female
deposits about 4o eggs, which, hatching in 10 to 14 days, pro-
duce young that feed on the leaves. Later these wander to the
stem, where they hide in the crevices throughout the winter. In
the spring they grow larger and begin to lay eggs about the
middle of April. The eggs soon hatch and the young wander up-
ward to the young needles, on which they feed and grow to winged
parthenogenetic females. These leave the larch about the end
of May and fly to the fir, settling down on the under sides of
the old leaves, where they deposit 8 to 10 stalked eggs from which
sexual males and females develop. These pair and the female
lays one egg on the stem. The eggs develop slowly, the young
emerging in October. They wander to the bases of the buds,
stick the proboscis into them, and winter in this condition. In
the spring the female deposits many eggs, from which the young
that emerge are those that produce the galls first mentioned.
This brings the cycle back to the starting point.

There is also in Chermes what is known as a parallel or col-
lateral series. Part of the parthenogenetic females on the fir
that are late in leaving the galls remain on the fir, and deposit their
eggs there (instead of on the larch). The young wander to the
ends of the branches, where they remain until the next spring with
the proboscis stuck into the tissues of the buds. In the spring
they produce galls. Presumably the cycle may begin again from
the individuals that emerge from these galls, which have, as it
were, short-circuited the life-cycle.

It is of great interest to observe that the sexual forms do not
appear in this short-circuited generation that remains on the fir,
and the inference is plausible that the conditions existing on the
larch are those that call forth the sexual forms.

In another species, the phylloxera of the grape, the alterna-
tion of generations takes place between the roots and the leaves.

Sexual and Parthenogenetic Forms a34

Wingless parthenogenetic individuals are found during the
winter on the roots, where they cause swellings, or gall-like
thickenings. Immense damage has been done to the vineyards
of Europe by these root-inhibiting forms. At the end of June,
winged parthenogenetic individuals develop, and wander up-
ward to the leaves. They deposit eggs of two kinds, — large
ones that produce the sexual females, and smaller ones that
produce the males. The males and females pair, and the fe-
male deposits on the bark a single large winter egg. In the
spring this egg produces a wingless female that goes to the
leaves and produces there gall-like growths. Offspring are
produced, some or all of which find their way in time to the
roots, where they remain over winter.

In the phylloxerans of the hickories the life history is somewhat ,
thesame. Following Pergande the cycle for most of the Ameri-
can species is as follows: From the winter egg a female — the
stem-mother —is produced. She wanders to the young leaves
and fixes her proboscis in the under surface. A gall develops
around her, within which she lays eggs that give rise to winged
migrants. ‘These leave the gall by the opening on its under
surface, and fly away. Those that alight on the bark of the
hickory deposit their eggs in its crevices. These eggs are large
or small, and produce respectively female or male individuals.
These pair, and the female lays a single large egg that over-
winters and gives rise in the following spring to the stem-mother.
In a few species the stem-mother produces forms that are, as
a rule, without wings. These deposit the large and small eggs
within the gall. Presumably the sexual forms that hatch from
these eggs leave the galls and deposit the fertilized egg on the
stem.

The life histories of these different forms offer many points
for consideration, and what is more important would seem to
give unusual opportunities to test the factors that lead to the
transformations of one form into another. What would happen,
for example, if the life-cycle were to be artificially altered at dif-
ferent points in its course by transferring the individuals prema-

334 Experimental Zoology

turely to other plants or by preventing the normal migration ?
A few attempts that I have made in this direction show that the
experiment is not always easily carried out, at least in the forms
that I have used, since if transferred too soon to the alternative
plant host the individual often dies. It would no doubt be
easier to obtain results by preventing the normal migration. In
the case of the fir, where migration is sometimes deferred natu-
rally, at least for a percentage of the individuals, the cycle goes
back to its starting point. What nature has done in this instance
we should be able to imitate in other species by inclosing in
gauze bags on their plants the individuals about to migrate.

It is important to note that the different parthenogenetic indi-
viduals of the same species that occur on the two plants may
differ enormously in structure in some cases, as shown especially
in the case on Hormaphis hamamelidis. The change must
be effected not by the new plant but by the original one; for
the parthenogenetic young may be fully developed before the
migration takes place, but whether their subsequent changes on
the new plant, that are sometimes very great, are brought about
by the new plant remains to be shown experimentally.

LITERATURE, CHAPTER XX

Baxsiant, M. Le Phylloxera du Chéne et le Phylloxera de la Vigne, études
d’entomologie agricole. 1884. Mém. a l’Acad. des Sci. XXVIII.
1884.
BLocHMANN, F. Ueber die Geschlechtsgeneration von Chermes abjetes L.
Biol. Centralbl. VII. 1887.
Ueber die regelmassigen Wanderungen der Blattlatisen speziell ueber den
Generationscyklus von Chermes abietes L. Biol. Centralbl. IX.
1889.
Bonnet, Co. Traité d’Insectologie ou observations sur quelques espéce
de Vers d’eau douce et sur les Pucerons. Paris. 1745.
Burnett, W. I. Researches on the Development of viviparous Aphides.
Proc. Am. Assn. Adv. Sci. 7th Meeting. 1853.
CastLE, W. E. The Heredity of Sex. Bull. of the Mus. of Compar. Zool.
at Harvard Col. XL. 1903.
Cuotopovsky, N. Noch einiges zur Biologie der Gattung Chermes L.
Zool. Anz. XII. 18809.
Weiteres zur Kenntniss der Chermes-Artern. Zool. Anz. XII. 1889.
Neuve Mittheilungen zur Lebensgeschichte der Gattung Chermes.
Ibid. XII. , 1889.
Cornu, M. M. Etudes sur le Phylloxera Vastatrix. Mém. a ]’Acad. de.
Sci. Instit. de France, XXVI.

Sexual and Parthenogenetic Forms 335

>

DecEER. Mémoires pour servir 4 histoire des Insectes. Stockholm.
17471.
Dreyrus, Lupwic. Ueber neue Beobachtungen bei den Gattungen

Chermes L., u. Phylloxera, Boyer de Fonsc. Tagebl. d. 60 vers.
deutsch. Naturf. u. Artze. 61 vers. 1888.

Ueber Phylloxerinen. Weisbaden. 1889.

Zu Prof. Blochmann’s Aufsatz, ‘“‘Ueber die regelméssigen Wander-
ungen der Blattliuse, speziell u. den Generationssyklus von Chermes
abietes.” Biol. Centralbl. IX. 1889.

Neue Beobachtungen bei den Gattungen Chermes u. Phylloxera
Zool. Anz. XII. 1889.

KRASSILSTECHIK, J. Zur Entwickelungsgeschichte der Phytophthires, ueber
viviparitat mit geschlechtlicher Fortpflanzung bei den Cocciden.
Zool. Anz. XVI. 1893.

Kyser, Jou. Friepr. Einige Erfahrungen ueber Bemerkungen ueber.
Blattlause. Germar’s Mag. f. Entom. 1815.

PERGANDE, TH. The Life-History of Two Species of Plant Lice inhabiting
both the Witch Hazel and the Birch. U.S. Dept. Agricult. 1gor.

North American Phylloxerine affecting Hickoria (Carya) and Other
Trees. Proc. Davenport Society. Davenport, Iowa, IX. 1904.

Puitiips, E. F. A Review of Parthenogenesis. Proc. Am. Philos. Soc.

XLII. 1903.
Réaumur, RENE DE. Mémoires pour servir 4 Vhistoire des Insectes.
Paris. 1737.

Observations sur les Insectes qui se multiplient sans accouplement et
par la seule fécondité de chaque individu. Mém. Acad. Sc. Paris
Histoires. 1741.

Ritey, C. V., and Monety. Notes onthe Aphidide of the United States.
Bull. U. S. Geolog. Survey, V. 18709.

STEVENS, N. M. Studies on the Germ-Cells of Aphids. Carnegie Institu-
tion, No. 51. 1906.

CHAPTER XxXI

INFLUENCE OF EXTERNAL CONDITIONS ON THE LIFE-
CYCLE OF THE LOWER CRUSTACEANS

In the lower subdivisions of the group of Crustacea there
are a number of forms in which parthenogenetic and sexual
modes of reproduction alternate with each other. Experiments
have shown in some cases, at least, that the occurrence of these
two modes of reproduction is dependent on external conditions.
Here, again, we meet with the same fact, referred to in the case
of the aphids, that the external conditions determine, in the
main, whether the reproduction is to be parthenogenetic or sex-
ual, not whether individuals of one or the other sex appear. In
other words, the effect is not to produce males or females, but
a different mode of reproduction, involving only females in the
case of parthenogenetic, and both males and females in the case
of sexualforms. It might be claimed that the change only brings
about the appearance of males when the sexual generation ap-
pears, while females develop under both conditions. This view
of the matter does not seem to hold in those cases where the
sexual females are quite different forms from the parthenogenetic
females, as in the aphids, but in the case of the crustaceans to
be described this claim might be maintained. It is also true
that the proportion of sexual females and males may be to some
extent affected by the character of the external conditions, but
this may be a different side of the question, and will be discussed
later.

Artemia salina reproduces both by summer eggs that develop
in the embryo sac, the young being born alive (viviparous) or
by means of resting eggs that are deposited (oviparous) ; but the
time of year at which the one or the other method is followed

336

Life-cycle of the Lower Crustaceans 257

seems to differ in different localities. In some local varieties
of this species the males are always present and abundant (as
in England, Odessa, Utah, and Cagliari), while in other races
the males are rare and not more than one or two males to a
thousand females are found. In the latter case all the summer
eggs develop by parthenogenesis. Whether the winter eggs
must be fertilized in these forms is not known. In the other
cases both the summer and the winter eggs are fertilized, and
fail to develop if they are not, — at least this has been definitely
shown to be the case at Cagliari by Artom.

Most of the daphnias, or water fleas, show an alternation of
parthenogenetic and sexual forms. Many of the more familiar
species pass through the following cycle. From the winter egg
there emerges in the spring a female parthenogenetic individual.
Her offspring are also parthenogenetic, and throughout the
summer a series of these forms succeed one another in rapid
succession. In the autumn, males and sexual females appear, and
the latter, after fertilization, produce the winter eggs from which
the new brood of the following year emerges.

This cycle differs somewhat in different species, and it is
known that sometimes males and sexual females are found dur-
ing the summer, and that in other species they appear under
special conditions. Kurz made some experiments which he
thought showed that the sexual forms appear when the water
becomes stale or begins to dry up. By hanging pieces of raw
cotton over the edges of the jar in which the daphnias lived,
the water was slowly drawn off, and after 14 days it was
reduced to about one sixth of its original volume. The experi-
ment was carried out in May, and many males and sexual
females appeared in the four different species present. In
another experiment he failed to obtain the same result, although
in foul water the sexual forms appeared.

Weismann has made a most elaborate study of the life history
of the Daphnide. He believed that his observations and ex-
periments show that external factors do not determine the ap-

pearance of the sexual generation, but that the change is due
Zz

338 Experimental Zoology

solely to an internal factor. The latter, he believes, is the out-
come of natural selection which has acted in such a way that the
internal mechanism has been adjusted to meet the changes in the
environment, without, however, being affected directly by them.
Inother words, natural selection has changed the life-cycle so that
it undergoes a parallel series of changes to the cyclical changes
in the environment, or as Weismann has put it, ‘“‘The organism
is so constituted that it reproduces sexually at the proper time,
and it is to a certain degree a matter of no consequence what
external conditions exist at that time, so long as they are not of
a sort to very materially interfere with the process of assimila-
tion or to threaten the life of the individual.”’ In other words,
natural selection has wound up the individual to go in the same
time as the environment.

Weismann points out that the usual appearance of the sexual
forms in the autumn has suggested that the cold brings about
the result, but this is disproven by the fact that in the early spring
the water may be as cold asin the autumn, yet in the spring the
reproduction is only by parthenogenesis. He made an experi-
ment that confirmed this conclusion. During the summer he
kept pieces of ice in an aquarium in which a large number of
individuals lived, so that the temperature was reduced to from
5’ to 10° C. The daphnias continued to reproduce partheno-
genetically, although more slowly.

In another way, Weismann thinks, the insufficiency of the
temperate hypothesis is shown. Daphnias that live in swamps
(Polyphemus, for example) have a double sexual period: the
first at the beginning of July, when the water is at its warmest;
and the second at the end of October, or the beginning of No-
vember, when the temperature is near the winter minimum.

Weismann does not think that the condition of the food supply
can account for the result, because he believes that food is always
abundant. Most species of daphnias live on fine slime or par-
ticles of organic matter, sticking to thé bottom or suspended in
the water. Weismann thinks there is a superfluity of this ma-
terial at all times. He states, moreover, that the sexual genera-

Life-cycle of the Lower Crustaceans 339

tion appears when food must be at its time of greatest abundance.
That Weismann is probably mistaken in this regard will be
shown presently.

Weismann also rejects Kurz’s idea that the drying up of the
water causes the sexual forms to appear, and bases his objection
most justly on the failure on Kurz’s part to carry out control
experiments in which the water was not diminished. Weismann
points out that the sexual forms often appear in confinement
when the level of the water remains the same. Schmanke-
witsch’s suggestion, that increasing the amount of salt contained
in the water causes the parthenogenetic forms to give rise to the
sexual generation, is also rejected by Weismann. Schmanke-
witsch pointed out that Moina rectirostris lives in small pools in
the spring and autumn; it disappears in the summer when the
salt becomes too strong, but before this occurs the sexual genera-
tion appears. Weismann objects that while the facts may be
correct it does not follow that the salt is the cause of the change;
for in other localities the same species of Moina lives in fresh
water and at all times of the year, and in every generation
except the first (that hatches from the winter eggs), brings
forth males and sexual females.

‘Weismann carried out some experiments which show, he
thinks, that external conditions do not regulate the alternation
of generations. He found when two lots of daphnias are kept
in separate dishes under conditions as similar as possible that
one set may produce males and sexual females, while the other
set continues to reproduce parthenogenetically. It seems to
me hazardous to claim that the conditions are the same in
any two such dishes, for while light, temperature, and the amount
of water may be the same in both, the food supply may be very
different, depending in large part on the number and kind of
animals that are devouring the food. Experience will show that
it is well-nigh impossible to keep the relation between the plants
(that serve as food), the bacteria, and the animals in a constant
relation in any twodishes. Any initial difference at the beginning
(and such must nearly always exist) may lead to very different

340 Experimental Zoology

results. Weismann cites the following experiments in support
of his statement. Six young daphnias, born parthenogenetically
from the same mother, were isolated each in a separate glass.
Fourteen days later No. 1 had produced three parthenogenetic
young; No. 2, eight of the same; No. 3, six; No. 4, five;
No. 5, seven sexual females and three males; No. 6, five
parthenogenetic females.

In another experiment a male and a female were placed in the
same small glass on March 5. They belonged to the fourth
generation from the winter egg. They were not observed to
pair. Three weeks later 8 parthenogenetic females were pres-
ent. After another 3 weeks 50 young were counted, of which
many were males. Ten days later there were 18 sexual females
and 5 males. A week later numerous young were found, and 9
days later still, when they were counted, there were found 3
females with winter eggs, 20 parthenogenetic females, 1 male,
and 79 young. In this experiment the sexual forms did not ap-
pear for some time; but in a parallel experiment beginning with
25 males and 8 females (March 24) the sexual forms appeared
at once. It is difficult to understand why Weismann should con-
clude from these experiments that the life-cycle is independent
of the environment. It is more probable that there was present
some unknown condition that caused the difference in the results.
One such condition may have been the crowding with the conse-
quent decrease in the available food.

Weismann has laid a good deal of emphasis on the differences
in the life history of species of Daphnide that live under different
conditions. Thus monocyclic species in which the sexual genera-
tion appears only once in the course of the year are found in
large bodies of water that never dry up, —in lakes, ponds,
swamps, and in the ocean. Sida crystallina is a species of this
kind. Throughout the summer only parthenogenetic indi-
viduals appear, and in the autumn males and sexual females.
Polycyclic species inhabit puddles and small swamps that are
often dried up. The winter eggs can withstand the drying, but
not the parthenogenetic adults; therefore in order to exist in

Life-cycle of the Lower Crustaceans 341

such localities the winter eggs, more appropriately called the
resting eggs, must be often produced. Such, in fact, is the case,
and males and females may appear more than once a year.
For example, Moina rectirostris and M. paradoxa usually pro-
duce three to five successive parthenogenetic broods, but amongst
these, sexual forms are always to be found, even at times in the
first parthenogenetic generation, but always in the second and
third. Again in Daphnia pulex, which lives in swamps, the
first generation consists of parthenogenetic individuals. Males
and females may appear in the second; they are not infrequent
in the third, and almost always present in the fourth, fifth, and
sixth generations.

There are a few species that seem to be acyclic, z.e. produce no
sexual forms at all, but throughout the winter and the summer
reproduce by parthenogenesis. Weismann believes that this is
a local condition, and that in other localities the same species
may have a sexual generation. Chydorus spherius produces
in Freiburg only parthenogenetic females, but in other places
sexual forms have been found.

From the conditions under which these forms live Weismann
thinks that through natural selection the life-cycle has been made
what we find it to-day, so that each species has become adapted
to the special conditions under which it is found. It may be
pointed out that the results can just as readily be explained by
assuming that, the life-cycle being what we find it, certain species
can live only in certain localities and others in other localities,
How the species having these cycles originated, is a question
that we need not discuss at present.

There are certain facts connected with the reproduction of
these crustaceans that are of general biological interest. The
same individual may alternately produce winter and partheno-
genetic eggs. If, for example, the winter egg is not fertilized, it
goes to pieces, and this “acts as a stimulus” to further partheno-
genetic reproduction. Four cells go to produce a partheno-
genetic egg, three of them serving as food for the fourth, but a
large number of cells contribute to the formation of the winter

342 Experimental Zoology

egg. It is interesting to note that if the winter egg is not fer-
tilized it does not develop parthenogenetically, although it is
better supplied with food than the parthenogenetic egg. Thus
while there is a sharp distinction between the two kinds of eggs,
there is no such distinction between the females that produce
them; for the same female may first produce one sort and then
the other. If external conditions determine which kind of re-
production is to take place, they do so by affecting the kind of egg
rather than the kind of individual; but it is interesting, even here,
to notice that the conditions that call forth the males are those
that call forth the production of the winter eggs in the females.

The winter egg is also a resting egg, 7.e. although after fer-
tilization it passes through the cleavage stages its further de-
velopment is there arrested, even if placed in water at a favorable
temperature. In some species it has been found that the rest-
ing eggs will not develop further unless they are first dried *—
an excellent adaptation for preserving the race, for the eggs re-
main undeveloped until the pool becomes dry, and later may
stock the pool again with a new brood. In other species, in
Moina and in Daphnia, according to Weismann, the resting eggs
will develop after a time if kept continuously in water. The
resting eggs of Moina paradoxa took from 30 to 4o days to
develop in June, but in September only 10 days. For Daphnia
pulex, the egg took from 18 to 31 days in the spring, and from
47 to 60, or even to 80, days in September.

A series of experiments have been recently carried out by
Issakowitsch, under the direction of R. Hertwig, that go far
toward showing that external conditions regulate the life-cycle
of the daphnias.

Issakowitsch kept a species of Daphnide, Simocephalus
vetulus, at different temperatures and obtained different kinds
of individuals according to the temperature employed. For
instance: a parthenogenetic female was isolated and kept at
24°C. Between February 6 and April 5, 6 generations appeared

? Brauer found that the eggs of Apus will not develop unless first dried, and
Spangenberg found that this is also true for Branchipus.

Life-cycle of the Lower Crustaceans 343

composed of 70 broods. There were about 500 individuals
produced. All were parthenogenetic, and no males and no
sexual females appeared. In another similar series a few males
and sexual females developed, but the tendency seemed to be to
produce purely parthenogenetic forms. When kept at room
temperature, 16° C., an isolated parthenogenetic female pro-
duced from February 6 to April 15 two generations, and 13
broods in all, composed of about 100 individuals. The first
brood consisted of females and 5 males. The sixth brood was
composed of parthenogenetic females which brought forth males,
etc. Another culture under the same conditions gave 30 broods
of 250 individuals, Again there was observed a transition from
females to males through an intermediate mixed brood. In both
cultures the females of the first generation produced the shell
for the winter egg —the so-called ephippium. This was then
thrown off because the egg was not fertilized. The next brood
of the same individuals consisted only of parthenogenetic fe-
males, but the next brood after this was made up either entirely
of males or else another ephippium was formed. When a
female produces the ephippium with the winter egg in it, the
sexual condition is present, and therefore it will be observed
that the same individual may function at one time as a partheno-
genetic and at another time as a sexual female. In the case last
mentioned where another ephippium is produced this may be
thrown off and the following brood will be again males or an-
other ephippium develops. When kept in the cold, 6° C., there
is a still stronger tendency to produce sexual forms. This is
shown in the two following experiments. All the offspring of
one female were taken from the warm culture and separated
into two groups. One set was kept at 24° and the other at 8°.
The results for the two cases are shown below :—

24° 8° 24° g°
2 2 2 winter egg
2 $ 9 winter egg
fe) winter egg 2 died

died winter egg died

344 Experimental Zoology

At the higher temperature three successive parthenogenetic
female broods appeared. In the cold the females brought forth,
in the one case, first parthenogenetic forms, then males twice,
and then winter eggs twice. In the other case (the right-hand
table) the offspring of five sisters kept in the cold produced win-
ter eggs twice and then died. These results show that at high
temperatures only or largely parthenogenetic females appear,
while at a lower temperature the sexual method of reproduc-
tion for the females and the appearance of males occurs.

Issakowitsch attempted to determine whether temperature
acts directly or only indirectly by affecting the condition of
nourishment. Females containing eggs in the brood pouch were
selected in order to be certain that from the beginning of devel-
opment the young themselves were subjected to a condition in
which food was insufficient. These animals were put into pure
filtered spring water and left there foreight days. This brood after
hatching produced always sexual animals, males, and females
bearing winter eggs. The results show that even at the high
temperature the sexual part of the cycle appears if food is lack-
ing. Issakowitsch concludes, therefore, that the animals kept
in the cold produce sexual forms, not because of the cold, but on
account of the influence of the cold on certain conditions of nour-
ishment in the ovary, and he points out the following facts that
seem to support his point of view. In the first place, even in the
cold if an ephippium is first produced and then thrown off (fer-
tilization not having occurred) the next brood consists of par-
thenogenetic females, whatsoever the other external conditions
may be. Issakowitsch offers the following interpretation of the
change. If the winter egg in the ephippium is not fertilized,
it is absorbed, and furnishes the ovary with food materials.
Owing to this better condition of nourishment the next genera-
tion is parthenogenetic. After this brood has been set free the
equilibrium between temperature and food is again established,
and external conditions will determine what the subsequent
events will be.

There are certain peculiarities in regard to the origin and

Life-cycle of the Lower Crustaceans 345

development of the parthenogenetic and the ‘‘winter” egg that
have a bearing on the present question. The wall of the ovary
swells up, and its cells become filled with a fluid when the par-
thenogenetic egg is about to develop. The egg seems to get its
nourishment from these cells. On the other hand, when the
winter egg develops the walls of the ovary do not enlarge, but the
egg develops at the expense of the surrounding cells that afford
the nourishment for its growth. The conditions in the ovary
itself would seem, therefore, to be determinative as to which
kind of egg will develop. It should be observed, however, that
we do not know whether it is the same cell that becomes in one
case a winter egg and in the other a parthenogenetic egg. In
fact, Weismann has stated that the two kinds of eggs come from
different parts of the ovary. The special conditions that lead
to formation of the male egg are unknown.

LITERATURE, CHAPTER XXI

Artom, C. Ricerche sperimentali sul modo di riprodussi dell’ Artemia
salina Lin. di Cagliari. Biologisches Centralblatt. XXVI. 1906.

Braver, Aucust. Zur Kenntniss der Reifung des parthenogenetische sich
entwickelnden Eies von Artemia salina. Arch. f. mikr. Anat.
XLII. 1893.

HeErtwic, R. Ueber das Problem der Sexuellen differenzierung. Ver-
hand. deutsch. Zool. Gesell. XV. 1905.

IssakowirscH, A. Geschlechtsbestimmende Ursachen bei den Daphni-
den. Biolog. Centralbl. XXV. 1905.

KERHERVE, L. B. DE. De l’apparition provoquée des males chez les Daphi-
nies (Daphnia psittacea). Meém. soc. zool. France, VIII. 1895.

Kurz, Witg. Dodekas neuer Cladoceren nebst einer kurzen uebersicht
der Cladocerenfauna Béhmens. S. B. math. naturw. Wien. 1875.

ScHMANKEwITscH, W. J. Zur Kenntniss des Einflusses der aeusseren
Lebensbedingungen auf die Organisation der Thiere. Zeit. wiss.
Zool. XXIX. 1877.

SIEBOLD, C.V. Sitzungsber.d. Kgl. Akad.d. Wissen. Miinchen, III. 1873.

Verhandl. d. 59, Jahresversammlung der Schweiz naturf. Gesell. in

Basal. 1877.

Warren, E. On the Reaction of Daphnia magna to Certain Changes in its
Environment. Quart. Jour. Micro. Sci. XLIII. 1900.

WEISMANN, A., and IscHikAwa. Ueber die Paracopulation im Daphnidenei
Zool. Jahrbucher. 1889.

WEISMANN, AuG. Beitrige zur Naturgeschichte de Daphniden Theil I.
Z. wiss. Zool. XXVII. 1876. Theil II-V, Ibid. XXVIII. 1877;
Ibid. XXX. Supplement (1878); Theil VI-VII, Ibid. XXXIII
(1879). Separate, Leipzig. 1876-1879.

CHAPTER XXII

INFLUENCE OF EXTERNAL CONDITIONS ON THE LIFE-CYCLE
OF THE ROTIFER; HYDATINA SENTA

IN the rotifer Hydatina senta, both parthenogenetic and sexual
reproduction takes place, and appears to be directly determined
in part by external conditions; but also in part by an internal
factor, viz. by the early fertilization of certain females. Maupas
and later Nussbaum have carried out experiments with this
form, and while they agree on certain essential points, especially
in regard to the mode of reproduction depending on an external
factor, yet Maupas believes the effect is produced by the tem-
perature, while Nussbaum thinks that the cause of the result
is the food supply, which may be affected by the temperature
in the sense that at a higher temperature more food is required
than at a lower, because the life processes go on faster.

The males of Hydatina senta are smaller than the females,
measuring .25 millimeter and the females .75 millimeter. The
males take no food, and have, in fact, no digestive tract. They
are not sexually mature when they leave the egg capsule, but the
sperm ripens during the first day. They live two or three days
only, but may fertilize several females during that time. The fe-
males require a great deal of food and will die in two days if
starved. In five minutes a hungry female will fill her digestive
tract, and begin to empty it in 20 minutes. Microscopic organ-
isms are eaten, Euglena being the chief food.

The females may produce the soft-shelled summer eggs or
the hard-shelled winter (or resting) eggs. The summer eggs
develop without fertilization, and may produce either males or
females, one sex or the other being produced by an individual,
not both by the same individual. The winter eggs are fertilized ;

346

Life-cycle of the Rotifer; Hydatina Senta 347

in fact, they do not appear at all unless previous union with
a male has occurred.

Maupas found that of 796 females isolated from males only
parthenogenetic eggs were produced (male or female). Of 172
females to which males had been added at the right time (about
6 hours after hatching), 84 produced winter eggs and 88
parthenogenetic eggs (male or female). In another experiment
822 females were used, and 342 of these were observed to unite
with males. Of these (342) there were 252 that laid winter eggs,
the other go produced summer eggs. The ratio of individuals
laying winter eggs to those laying summer eggs is, therefore, 74
to 26. The remainder of the 822 females that were not seen to
pair, 480 in number, gave the following results: 361 produced
males and 119 produced females. The ratio is 75 to 25. This
is nearly the same ratio as the ratioof winter eggs to summer
eggs given above. From this similarity in proportion Mau-
pas concludes that only male eggs can be fertilized, and these
become winter eggs. The winter eggs give rise solely to females.
If this conclusion is established, it follows that fertilization
changes the sex of the egg, asin the bee. Nussbaum believes the
similarity in the ratios may be purely accidental, but will not
deny that the conclusion may be correct.

Maupas tried to show that temperature is the factor that
determines the fate of the eggs. If the temperature is lowered,
females are produced. If the temperature is raised, males are
produced. He gives the following experiment to establish this
conclusion. Five immature females were kept at a temperature
of 26° to 28° C. They laid 104 eggs, of which 97 per cent pro-
duced individuals that laid male eggs and only 3 per cent
produced individuals that laid female eggs.

In the control experiment five immature individuals from the
same culture were kept at a temperature of 14° to16°C. They
laid 260 eggs, of which 5 per cent became male and 95 per cent
became female producers. It will be noticed that in these two
experiments both the mother and the eggs she produced were
under the same conditions of temperature, hence the effect

348 Experimental Zoology

may be produced either on the eggs in the ovary or on the eggs
after they are laid, or on the embryo itself. Another experiment
described below is required to settle this question. The main
point at present is that the sex changes with the temperature.

Nussbaum has objected to this conclusion on the grounds that
it isnot clear whether Maupas’s calculations rest only on the rela-
tive sizes of the eggs produced, or on the kind of individuals that
emerged. If on the former, the result must be inconclusive be-
cause many eggs of intermediate size are usually present that
cannot be accurately referred to one rather than to the other
sex. It is improbable, I think, that Maupas has made such a
mistake. Nussbaum also points out that at 26° to 28° C. the con-
ditions were not favorable, as shown by the relatively small num-
ber (104) of eggs produced. The maximum number for this many
individuals would be 250 eggs. The results may, therefore,
have been due to insufficient nourishment rather than to tem-
perature.

In another similar experiment Maupas got the same results.
Thus, five young Hydratina females laid their first eggs at a
temperature of 14° to 28° C. Twenty-four per cent of these
eggs produced individuals that laid male eggs, and 76 per cent
produced individuals that laid female eggs. Five other indi-
viduals were kept at a higher temperature, 26° to 28° C., and laid
118 eggs, of which 81 per cent gave rise to individuals that pro-
duced male eggs and only 19 per cent produced individuals that
laid female eggs.

Maupas found that under different conditions of temperature
the same individual may at one time produce eggs that give rise
to females that lay only female eggs, and at another time may
produce eggs that give rise to individuals that lay only male eggs.
The result shows that whether the egg becomes a male-producing
or a female-producing individual is determined after it has been
laid and not while it is in the ovary. Nussbaum’s results show,
in fact, that this is determined during the course of the first few
hours after the individual hatches.

Maupas isolated six young females. They were kept in the

Life-cycle of the Rotifer; Hydatina Senta 349

cold and laid there 34 eggs, of which 12 per cent only became
individuals that laid male eggs and 88 per cent individuals that
laid female eggs. The same parents were then transferred to
a warmer temperature and laid 95 eggs that were male pro-
ducersand 5 eggs that were female producers. They were changed
back and forth from warmth to cold, and each time a correspond-
ing change occurred in the individual that came from the eggs.
During the last four days of their life they produced 50 eggs,
of which 17 per cent were male-producing and 83 per cent
female-producing.

Nussbaum has tried to show that the sex of the offspring is de-
termined by the environment and not by influences on the egg.
If the young parthenogenetic female after she has emerged is
well fed, she produces large eggs that develop, without fertiliza-
tion, into female individuals. If on the other hand an individual
is at first poorly fed, she produces small eggs, and these develop
without fertilization into males. Before and after this period
the sex of the egg is not determined by the food supply, 7.e. after
the first egg has been laid it is no longer possible to change the
sex of the other eggs.

Nussbaum thinks that Maupas was mistaken in regard to the
influence of temperature, because he used too many individuals,
whose numbers in proportion to the food supply determined
the conditions of nutrition that exist. Maupas may also
have judged the sex of the offspring from the size of the egg.
This is not always possible, according to Nussbaum; for while
it is true that the biggest eggs become females, and the smallest
males, there are many of each kind whose sizes overlap.

Temperature favors the appearance of the males in so far as
each animal lives at a higher rate at a higher temperature and
needs more food, also at the higher temperature the output of eggs
is faster, hence the food conditions poorer. Ata lower tempera-
ture the output is slower and food conditions are relatively better ;
but since the kind of egg produced is determined before the first
egg is laid, it is not obvious that this criticism is apropos.

Plenty of food during the development of the ovary leads to

350 Experimental Zoology

female summer eggs. Early fecundation and scarcity of food
during the time of development of the ovary lead to winter eggs,
from which only females emerge.

Lauterborn has found in another species of rotifer, As-
‘planchna, that the same individual may contain a winter egg
and male embryos. This is strong evidence in favor of the view
that the male eggs and the winter eggs are the same, as Maupas
inferred for Hydatina. The result recalls the somewhat similar
observations of Issakowitsch on daphnia, where, however, the
winter egg and the males appear consecutively. Lauterborn,
following Weismann’s idea for the daphnias, thinks that in roti-
fers, other than Hydatina, external conditions do not determine
the sequence of parthenogenetic and sexual generations. Each
species has a more or less definite number of generations in the
parthenogenetic part of its cycle, but he brings forward no
experimental evidence in support of his conclusion.

LITERATURE, CHAPTER XXII

LAUTERBORN. Ueber die zyklische Fortpflanzung limnetischer Rotatorien.
Biol. Centrbl. XVIII. 1898.
LENSSEN. Contribution 4 1’étude du Developpement et de la Maturation
des ceufs chez Hydatina Senta. La Cellule, XIV. 1898.
Mavpas, M. Sur la multiplication et la fécondation de l’Hydatina senta,
Ehr. C.R. Ac. Sc. Paris,,CXI. 1890.
‘Sur la fécondation de l’Hydatina senta, Ehr. C. R. Ac. Sc. Paris,
CXI. 1890.
Sur le déterminisme de la sexualité chez )’Hydatina senta. C. R. Ac.
Sc. Paris, CXTII. 1891.
Nusspaum, M. Zur Differenzirung des Geschlechts im Thierreich.
Arch, f. mikr. Anat. XVIII. 1880.
Prate, L. Zur Kenntniss der Rotatorien. Zool. Anz. VII.
pe zur Naturgeschichte der Rotatorien. Jena. Z. naturw. XIX.
1885.

CHAPTER XXIII
THE LIFE-CYCLE OF SOME HYMENOPTEROUS INSECTS

It has been pointed out that in some cases parthenogenetic
generations alternate with sexual ones, the last parthenogenetic
generation giving rise both to male and female sexual forms.
In other cases, unfertilized eggs may produce only one kind of
individual, usually the male, and in a few cases this may be the
regular method of production of the male sex. The best-known
case of this sort is that of the honey bee.

The queen bee leaves the hive, a few days after emerging from
the royal cell, for her nuptial flight. She is followed by one or
more drones, and union takes place in the air. Her receptacle
becomes filled with sperm, and this supply lasts her for the rest
of her life, that may extend over three or four years. The
spermatozoa must remain alive during all this time, yet the
oldest produce the same effect, so far as sex is concerned, as do
the youngest. The receptacle, or spermatheca as it is called,
opens by. a short tube into the oviduct. It has a muscular
wall, and a few of the spermatozoa are supposed to be squeezed
out as the egg passes the opening — at least in the case of eggs
that are to become fertilized.

A queen may produce half a million eggs in the course of her
life, and must receive, therefore, at least as many spermatozoa,
and probably more, since it is not improbable that several or
many are set free for each egg. ‘The eggs are deposited in the
brood cells or chambers of the comb that have been prepared by
the workers. The queen thrusts her abdomen to tlie bottom of
each cell, and deposits there an egg, that is attached at the end
opposite to the micropyle, or opening through which the sper-
matozoon enters. There are three kinds of brood chambers:

351

352 Experimental Zoology

the worker cells that are very numerous; the drone cells that
are much fewer in number; and the royal cells, of which there
are, as a rule, only a very few. It has been shown both by di-
rect observation and by experiment that the eggs laid in worker
cells and in the royal cells are fertilized. These eggs produce
females only. In the worker, the female sexual organs never fully
develop. The eggs laid in the drone cells are unfertilized, and
give rise to the drones or males. Rarely a queen may make
a mistake and lay a fertilized egg in a drone cell, where it be-
comes a worker; or lay a drone egg in a worker cell, where it
becomes a drone. Evidently, then, the sex is not determined by
the character of the cell, but by the presence or absence of the
spermatozo6n.

A worker egg can be changed into a queen if the young larva
is supplied with the food given to the young queens. This can
be accomplished artificially by placing some of the royal jelly
taken from a royal cell (where it exists in excess) in a worker cell,
containing a young maggot not more thanaday ortwoold. The
bees will then continue to supply this cell with the royal jelly, and
will break down the neighboring cells, destroying their inmates
and thus make a larger, royal cell around this nouveau riche
member of royalty. A drone egg cannot be changed into a
worker or into a queen, although an exceptionally large male
may be produced by feeding on the royal nutriment.

When a queen has become old and has used up all of the sperm
in the spermatheca, she can produce only drone bees. Also
when a young queen is prevented from leaving the hive she
produces only drones; for it appears that union with the males
cannot take place within the hive. Workers also have been
known at times to lay eggs which produce only drones. These
and other facts led Dzierzon, in 1845, to formulate his famous
theory. The substance of his view has already been given. He
stated that a queen must be fecundated if she is to produce
new queens and workers; that drone eggs are not fertilized; but
that worker eggs and queen eggs are always fertilized. Dzier-
zon also discovered that during copulation the eggs in the ovary

Life-cycle of Some FHymenopterous Insects 353

are not directly fertilized, but the seminal fluid is stored up in the
receptacle, and this supply, once received, must serve for all sub-
sequent fertilizations. Dzierzon thought that the queen has
the power of determining at will whether an egg is to become
fertilized or not, and determines thereby the sex of the individual.
He thought that all the eggs in the ovary are alike, and the sex
of the individual is determined by the occurrence or absence of
fertilization.

The theory of Dzierzon has been much discussed, and while
its main facts have been confirmed there have not been lacking
those who have disputed one or another of its postulates.

It was soon appreciated that the theory could be tested if the
queen of one race is fecundated by the male of another race;
for according to the theory the queens and workers that are
produced by such a queen should be hybrids, but the males
should belong to the pure maternal race. Thus ifa virgin Italian
queen is put into a hive of German bees, lacking their own queen,
the new queen may be fecundated bya German male. Her female
offspring, queens and workers, will be hybrids; but her male
offspring, the drones, will be pure Italians. Dzierzon himself
found one instance that was an apparent exception, and so great
was his confidence in this test that he gave up, for a time, his
own theory, although later he appears to have returned to it
from the confirmatory results of v. Berlepsch and v. Siebold.

This hybridizing experiment has been a source of contention
down to the present day, and well serves to illustrate how many
dangers may lurk in even a comparatively simple experiment.
Some of the objections to the theory may now be briefly con-
sidered. It appears that in some cases the drones are like the
hybrids, or even like the male, presumptively not their father,
that fertilized the queen. Thus Perez found that after intro-
ducing an Italian queen fertilized by a French drone into a
French hive, she produced 300 drones of which 5 were pure Ital-
ians, 83 pure French, and 66 were hybrids. The result proved,
he thought, that the drones were hybrids. Similar cases have
been described by other observers. The results seem to be

2A

354 Experimental Zoology

incompatible with Dzierzon’s theorv; yet several explanations
have been offered to account for these exceptional cases, for
it is generally admitted that they are exceptional and do not
represent what usually takes place.

In the first place, the pedigree of the queen may not have been
above suspicion, and consequently some of her offspring may
show traces of their mixed origin. It is said that at the present
time the domesticated races of bees have been so often crossed
that it is difficult to obtain a pure individual. In any particular
instance the matter could be easily tested by rearing sister queens
of the same brood, crossing one and breeding the other true.
This simple test does not appear to have been made.

In the second place, the hybrid drones may have come from
the hybrid workers that sometimes lay eggs, rarely in the com-
mon hive bees, but more often in the Syrian and Palestine races.
In the third place, it has been found that Italian bees of presum-
ably pure stock often produce drones that are very variable in
their markings. In a recent paper by Casteel and Phillips it
has been pointed out that the males are often more variable than
the workers — a fact of much biological interest in itself, aside
from the present question, since it shows that variation is not
necessarily connected with the union of two germ-cells from
different individuals, as Weismann and his school have assumed.

In another way Dzierzon’s theory was tested, and apparently
found wanting. Landois (1867) put worker eggs into drone
cells, and drones were produced. Conversely he put drone eggs
into worker cells and workers were produced. The real expla-
nation of this result is, however, not that the kind of cell de-
termines the sex of the individual, as the experiment seemed to
show, but that the bees themselves often clean out a cell into
which an egg has been introduced, perhaps because of the
imperfect method of fixation of the egg, and subsequently the
queen will deposit the proper kind of egg in the cell. Thus
the bees themselves easily defeated the purpose of the experi-
ment. The experiment could be properly carried out by
removing the queen when the transfer is made, so that no

Life-cycle of Some Hymenopterous Insects 355

opportunity is left to deposit new eggs in the cells, if the workers
remove them.

In recent years an apiarist, Dickel of Darmstadt, has claimed
to have discovered facts that contradict the Dzierzon theory, and
his contention, although it appears ill-founded, has led to a re-
examination of the theory. He has tried to show that drone eggs
as well as queen and worker eggs are fertilized, and that the
sex is determined by the treatment to which the eggs are sub-
jected by the workers. He admits that males may come from
unfertilized eggs of queens and of workers, but he calls these
false drones that are incapable of procreation. He believes that
before fertilization the egg contains only the male elements.
The sperm cell contains only female elements. After union
these sexual elements are equally balanced, and the fate of the
embryo is determined by the following external factors. In
making the brood cells the wax is kneaded in the mouths of the
workers, and impregnated with the saliva. The kind of saliva
used is different for the drone and for the worker cells, and its
presence in the walls of these cells determines the kind of secre-
tion that is poured over the egg by the workers when it is laid.
This secretion determines the sex of the egg. One kind of
saliva is used for the queen larva, and causes a fertile female to
develop; another kind causes the male to develop. If both of
these are put into the same cell, they retard the development of
the sexual organs and a worker is produced. There is also an-
other nourishing secretion that is used for all kinds of larve,
and does not affect the sex. Dickel tried to show by transferring
drone eggs to worker cells, and vice versa, that the sex of the
young larva can be changed by the change in food that the bees
give to it in a foreign cell. The experiment is open to the criti-
cism mentioned above, namely, that the workers may remove the
egg, although the possibility might be very simply tested by
removing the queen.

This ingenious and highly speculative, not to say fanciful,
idea of Dickel has so little in its favor in the way of scientific:
evidence, and so much opposed to it that seems fairly well

356 Experimental Zoology

established, that it need scarcely be discussed seriously. Pe-
trunkewitsch has severely criticised Dickel’s views, and has
pointed out that the supposed experimental proofs are all open
to question; that his ideas in regard to fertilization are hazy;
and that a renewed investigation by means of the more perfect
moder technique has confirmed v. Siebold’s and Leuckart’s
statements that spermatozoa are not present in drone eggs.
For instance, Petrunkewitsch found the sperm nucleus in 23 out
of 29 eggs from worker cells. On the other hand, of 94 eggs
from drone cells not one contained a spermatozoén. These
eggs were in the stage when the first polar body was present.
In the following cases the second polar body was forming :—
62 eggs from worker cells all contained sperm asters, while 272
from drone cells contained no asters, except inonecase. It may
be that in this one case the queen had erred and placed a fer-
tilized egg in a drone-cell, which is supposed to occasionally
occur. These results furnish a further confirmation of Dzier-
zon’s theory.

Other Hymenoptera

The ants are closely related to the bees and show apparently
similar conditions in regard to the determination of sex. It has
long been claimed that the unfertilized eggs of the queen and
the unfertilized eggs of the workers produce males. Hence
Dierzon’s theory has been extended to this group also.

In the last few years, however, some cases have been recorded
that seem to show that workers may appear in nests that have
no queen, and which, therefore, have come from workers’ eggs,
which are never, as far as known, fertilized. The seminal recep-
tacle is absent in the workers, hence there would be no oppor-
tunity for the storage of sperm, and since in one case workers
appeared during three years, they must have come from par-
thenogenetic eggs.

Wheeler has recently brought together the best authenticated
cases of this kind. Tanner records for Atta cepholotes that a
group of worker ants in an artificial colony produced eggs most

Life-cycle of Some Hymenopterous Insects 357

of which were males, but two queens also appeared. Reichen-
bach started with eleven workers of Lasius niger. They laid
eggs that became typical workers. In August, when the sexual
forms appeared in the out-of-door nests, about a dozen males also
developed in the artificial nest, although none had developed
before. After a few weeks the ants continued to increase in
numbers, and in the following year produced two to three dozen
males and as many as 300 workers. The same appearance of
males took place in the third year.

Another case has been given by Mrs. Comstock for Lasius
niger in which. workers appeared in an artificial nest containing
only workers. The queen of this species is so much larger
than the workers that a mistake as to the character of the
individuals was not possible.

In another group of the Hymenopterous insects, the Tenthre-
dinidz, or sawflies, some curious cases of parthenogenesis are
known. Asarule, in this group the males are much less numer-
ous than the females. The common currant sawfly, Nematus
ribesii, produces parthenogenetic progeny which is nearly always
of the male sex. It is said that on rare occasions an unfertilized
egg becomes a female, a point of some interest in connection with
the case of the bees.

Doncaster has recently given some observations of his own,
and reviewed the results of others in regard to the sex of saw-
flies from parthenogenetic eggs. He points out that according
to all observers females alone arise from virgin eggs of Croesus
varus and Pecilosoma luteolum, only one observer ever having
seena male. He quotes Mrs. Chawner to the effect that she has
bred thousands of the latter species for several years in succession
without obtaining males, and without finding any diminution
in the fertility of the females. Hemichroa rufa “‘is known to
give chiefly females from virgin eggs, but occasionally males are
produced.” On the other hand, certain species produce males,
as a rule, from parthenogenetic eggs. Cameron states that Ne-
matus pavidus yields only males from unfertilized eggs. In
Nematus ribesii (N. ventricosus of von Siebold) it has generally

358 Experimental Zoology

been found that virgin eggs produce males, but von Siebold ob-
tained 13 females among 1700 males; he thinks these may have
been introduced with the food. ‘Other observers have obtained
only males.”

In another group, the gallflies, Cynipide, females appear
in preponderating numbers and reproduce by parthenogenesis
in one generation at least. Males in some forms are of the rar-
est occurrence, and have never been found in some species.
It has been observed, however, that in some species the eggs
produced by the parthenogenetic females give rise to insects,
both male and female, quite different in appearance from the
parthenogenetic generation. The complete life histories of
some of these species have been given on page 268 in describ-
ing the formation of galls. According to Adler, not more than
2 percent of males appear in the sexual generation of Rhodites
rose and R. eglantene. The female of Phromali lays her egg
in the caterpillar of Vanessa urticee. In several cases caterpillars
were reared, which were kept uncontaminated, and an unfer-
tilized female of Phromali deposited eggs in them. When the
flies emerged from the pupa there were found in one case 124
males, in another case 62 males, in a third case 75 males and 5
females, and in a fourth case 45 males and 4 females. Thus
while the unfertilized eggs of this species produce males, occa-
sionally females also appear.

These cases serve as a warning not to insist with too much
positiveness on the view that in the bee unfertilized eggs always
produce males, since they show that exceptions to such a rule
may sometimes occur.

LITERATURE, CHAPTER XXIII

Cameron. Monograph of Phytophagus Hymenoptera. 1882-1884.
CasTEEL, D., and Puitiips, E. Comparative Variability of Drones and
Workers of the Honey Bee. Biol. Bull. VI. 1903.
Dicxet, F. Das Prinzip der Geschlechtsbildung. Darmstadt. 1898.
Meine Ansicht iiber die Freiburger Untersuchungsergebnisse von Bien-
eneiern. Anat. Anz. XIX. gor.
Ueber Petrunkewitsch’s Untersuchungsergebnisse von Bieneneiern.
Zool. Anz. XXV. got.

«

Life-cycle of Some Hymenopterous Lusects 359

Der gegenwdrtige Standpunkt meiner Entwickelungstheorie der
Honigbiene. Nat. Woch. XVI. 1901.
Die Ursachen der geschlechtsliche Differenzerung im Bienenstaat.
Pfliigers Archiv, XCV. 1903.
Dominique, J. Parthénogenése et thelytokie chez les Phasmides. Bull.
Soc. Ac. méd. Ouest, IX. 1899.
Doncaster, L. On the Maturation of the Unfertilized Egg and the Fate
of the Polar Bodies in the Tenthredinide. Quart. Journ. Micros.
Sci. XLIX. 1906.
DzrERzon, JoHANNES. Eichstidt. Bienen-Ztg. I. 1845.
Noch etwas uber die Befruchtung der Kénigin. Ibid. I. 1845.
Bestimmung u. Bestimmungslosigkeit d. Drohnen. Ibid. II. 1846.
Theorie u. Praxis des neuen Bienenfreundes. Brieg. 1848.
Wer legt die Drohneneier? Eichstadt. Bienen-Ztg. VII. 18571.
Noch etwas sur Befruchtungsgeschichte d. Kénigin. Ibid. VII. 1851.
Nachtrag zur Theorie u. Praxis des neues Bienenfreundes. Brieg.

1852.
Meine Theorie auch durch ihre Gegner bestitigt. Eichstadt. Bienen
Ztg. IX. 1853.

Ueber Fruchtbarkeit der Kénigin. Ibid. X. 1854.
Beitrag zur meiner Drohnentheorie. Ibid. X. 1854. XI. 1855.
Drohnenzellenbau, Drohneneierlage d. Kénigin. Ibid. X. 1854.
Meine Drohnentheorie vor d. wissenschaftlichen Kritik. Ibid. XII.
1856.
Beitrag zur Befruchtungsgeschichte der Kénigin. Ibid. XVII. 186r.
Zur Parthenogenesis. Ibid. XVIII. 1862.
Zur Theorie d. Drohnenerzeugung. Ibid. XXXII. 1876.
Fietpe, A. A Study ofan Ant. Proc. Acad. Nat. Sci. Phila. 1901.
Lanpois, H. Ueber das Gesetz des Entwickelung der Geschlechtes bei den
Insecten. Zeit. f. wiss. Zool. XVII. 1867.
Kenntniss der Alten ueber die Entstehungsweise der Bienen. Natur.
u. Offenbarung, VII. 186.
Kenntniss d. Neuern, etc. Ibid. VII. 1861.
Ueber das Gesetz der Entwickelung der Geschlechter bei den Insekten.
Z. wiss. Zool. XVII. 1867.
Pautcke. Zur Frage der parthenogenetischen Entstehung der Drohnen.
Arch. f. d. Ges. Physiol. XCIX. 1903.
PEREZ, J. Sur la ponte de l’abeille reine et la théorie de Dzierzon. Compt.
Rendu. LXXXVII._ 1878.
Observations sur la parthénogenése de l’abeille. Act. Soc. Linn.
Bord. XXXII. 1878.
Mémoire sur la ponte de l’abeille reine et la théorie de Dzierzon. Ann.
Sc. Nat. (6), Zool. VII. 1878.
The Theory of Dzierzon. Am. Bee Jr. XV. 1879.
Reflexion sur les observations de M. Matter 4 propos de la théorie de
Dzierzon. Act. Soc. Linn. Bord. (4), III. 1879.
Sur la prétendu parthénogenése des Halictes. Ibid. 68. 1895.
PETRUNKEWITSCH, ALEX. Die Richtungskérper und ihr Schicksal im
befruchteten und unbefruchteten Bienenei. Zool. Jahrb. XIV.
IgOI.
Das Schicksal der Richtungskérper im Drohnenei. Ibid. XVII.
1902.

360 Experimental Zoology

REICHENBACH, H. Ueber Parthenogenese bei Ameisen, etc. Biol. Cen-
tralbl. XXII. 1g02.

Srepotp, C. TH. E. v. Bemerkungen ii. die Lebensweise und den
Haushalt der Bienen. Arbeit. schles. Ges. vaterl. Cultur. XXIX.
1851.

Pars adetunie einer vom Begallungsausfluge heimgekehrten Bienen-
kénigin. Eichstadt. Bienen Ztg. X. 1854.

Wahre Parthenogenesis bei Schmetterlingen und Bienen. Leipzig.
1856.

Ueber Parthenogenesis. Vortrag gehalten k. Akad. Wiss. 28, Mar.
1862.

Ueber Zwitterbienen. Z. wiss. Zool. XIV. 1864.

Ein wort iiber die agyptischen wahren Drohnenmutter. LEichstadt.’
Bienen Ztg. XXII. 1866. _

Zusatz zur Landois’ vorliufiger Mittheilung. Zeit. fiir wiss. Zool.
XVII. 1867.

Ueber Pedogenesis bei Strepsipteren. Z. wiss Zool. XIX. 1869.

Ueber Parthenogenesis bei Polistes gallica. Z. wiss. Zool. XX. 1870.

Ueber Parthenogenesis. S. B. Akad. Wiss. Miinchen. 1871.

Sulla partenogenesi del Bombyx mori. Bull. soc. ant. Ital. IIT. 1871.

Beitrige zur Parthenogenesis der Arthropoden. Leipzig. 1871.

Intorno alla partenogenesi riconosciuta nella farfalle da antichi Italiana.
Bull. soc. ent. Ital. IV. 1872.

Nuove ossewazioni sulla partenogenesi del Bombyx mori. Ibid. V.
1873.

Ueber Parthenogenesis bei Artemia salina. S. B. math. phys. Cl. bayer.
Ak. Miinchen, IIT. 1873.

Ueber die in Miinchen geziichtete Artemia fertilis aus dem grossen
Salzsee von Utah. Verh. Schweiz. naturf. Ges. Basel, LIX. 1876.

Vorlaufige Mittheilung ii. Parthenogeneses bei Tenthredinide u. bei
einer Ichneumonidenspecies. Entom. Nachricht. X. 1884.

TANNER. (Ecodoma cephalotes. Trinidad Field Naturalist’s Club, I.
1892.

Datta Torre, K. W., and H. Friese. Die hermaphroditen und gynan-
dromorphen Hymenopteren. Ber. naturwessen-med. Ver. Innsbruch.
XXIV. 1898.

WHEELER, W. M. The Origin of Female and Worker Ants from the Eggs
of Parthenogenetic Workers. Science, XVIII. 1903.

Some New Gynandromorphous Ants, with a Review of the previously
Recorded Cases. Bull. Am. Museum Nat. History, XIX. 1903.

EXPERIMENTAL STUDY OF THE
DETERMINATION OF SEX ,

CHAPTER XXIV
THE DETERMINATION OF SEX

Introduction: the Different Kinds of Sexual Individuals

FEw questions in biology have attracted more interest than
that of the determination of sex. Blumenbach, in his fascinating
brochure, ‘‘Ueber den Bildungstrieb,” points out that Drelin-
court brought together two hundred and sixty-two groundless
hypotheses of sex that had been proposed, and Blumenbach
remarks quaintly that nothing is more certain than that Drelin-
court’s own theory made the two hundred and sixty-third. In
recent years a large number of facts bearing on the determina-
tion of sex have been brought together, and yet the question
remains as much a puzzle as it was in the time of Drelincourt
and Blumenbach.

Before attacking the main question of the determination of
sex let us consider the different kinds of sexual individuals found
in the animal kingdom. By sexual individuals we mean those
forms that reproduce themselves by eggs and sperm in contrast
to reproduction by budding, or by division, or by spores; but
while this distinction will hold in most cases, it is an arbitrary
distinction, since we recognize sexual reproduction in the pro-
tozoa, where entire individuals unite or even fuse, and the dis-
tinction between a spore and a parthenogenetic egg depends
largely upon what we suppose to have been the historic origin
of these two kinds of reproductive cells, rather than on any
inherent difference in the cells themselves.’

1 One or even two polar bodies are given off from parthenogenetic eggs, but
not from spores. The bearing of this difference on the sexual problem is un-
known. It is not necessarily connected with a reduction on the number of chromo-

somes.
363

364 Experimental Zoology

Individuals of two kinds exist in many species of animals, —
males and females. Such individuals are said to be uni-
sexual, and the species dicecious (or of two households).
In other species there is only a single kind of individual
that produces both eggs and spermatozoa. Such individ-
uals are bisexual or hermaphroditic; and the species is said
to be moneecious (or of one household). Closely related species
may belong to the one or to the other of these two groups, so
that the distinction does not appear to be of fundamental im-
portance. Moreover, individuals sometimes appear in dicecious
species that are hermaphroditic; and, conversely, individuals of
separate sexes sometimes appear in moncecious species. The
prevailing view is probably correct, that male individuals carry
in a latent or potential condition the female characters, and the
female those of the male. If the latent character develops, an
hermaphrodite appears; and if, in a moncecious species, one
set of characters fails to develop; a male or a female appears.
From this point of view we can readily understand how easily
the transition from one to the other kind of sexual individual
may take place.

A third kind of individual is also recognized, namely, the par-
thenogenetic. Such an individual is looked upon as a female,
in which the eggs have the power to develop without fertilization.
This may sometimes happen in ordinary females, so that par-
thenogenetic reproduction is not sharply separated from sexual
reproduction. Even in the same individual, as we have seen in
the case of the bee, the eggs may develop with or without fertiliza-
tion. Since parthenogenetic individuals may produce males as
well as sexual females, we must conclude that the male charac-
ters are carried in a latent condition by these parthenogenetic
females, often through a long series of purely parthenogenetic
generations. The possibility that there might be male and fe-
male lines of such parthenogenetic females is excluded by find-
ing that the same individual may produce both males and
females, as seen in aphids and daphnia.

The Determination of Sex 365

Diecious Species, composed of Unisexual Individuals

When the species consists of male and female individuals, we
find in some cases that the sexes are so much alike externally, that
without a knowledge of their internal structure we could not tell
them apart. Some birds, pigeons, for example, belong to this
class, and there are even some mammals, mice for instance,
in which the sexes are so closely similar that an inspection of the
external organs of reproduction is necessary to reveal the sex.
In many other cases the external differences involve mainly
the external organs of reproduction, while in still other groups
the whole form of the body may be different, and in extreme
cases this difference is so great that were not the relation of
the individuals known they would be placed in different
species.

Where the sexes are separate it may be said to be the rule
for equal numbers of males and females to exist, but this
tule is far from invariable.

In regard to the human race, statistics from the whole of
Europe, including records of ‘“‘millions” of births, show that
for every 100 female children born there are 106 males. If a
more limited range is taken, or a shorter period, some devia-
tions from this proportion are found.

Thus between the years 1887-1895 the following ratio of the
sexes to each other is given by Bodio: —

MALES
Italy. , : 5 ‘ ‘ : 105.8
France P 5 : - ‘ . . 104.6
England. . . ; . : ‘ 103.6
Germany . ‘ : : 3 i ; 105.2
Austria j ‘ ; s ‘é : : 105.8
Hungary. ‘ ; : : ‘ . 105.0 to 100 females
Switzerland . : é 2 : ‘ é 104.5
Belgium ; : F ‘ 2 : 104.5
Holland 3 é . 7 : 3 i 105 5
Spain . F é . : 5 é 108 3
Russia : é F 3 5 é ‘ 105.4 J

366 Experimental Zoology

Thus, despite the different conditions, social and_politi-
cal, in different countries, the proportion remains nearly the
same.

These figures give the birth rate. The proportion differs con-
siderably from the proportion of adult males to females; for
in Europe, for example, there are 1000 men to every 1024
women. Thus the adult proportion is the reverse of the pro-
portion at birth; or, in other words, the excess of males at birth
is more than made up by the greater death rate of boys as
compared with that of girls.

Local conditions also affect the proportion, for in Italy and
in the Balkan Peninsula there are more men than women.
As stated, the death rate of male children in Europe is known to
be greater than that of female children. At about the twelfth
to the fifteenth year the numbers become equal. After the
thirty-fifth year the men die oftener than the women, which finally
brings about the conditions mentioned above. Male children
appear less resistant than female, and adult males die more
often from greater exposure to danger, from alcoholism, and
from crime, etc.

For animals the following records have been collected by
Lenhossek : —

Mates FEMALES
Horse . 2 . é Z - : i 98.31 100 (Dusing)
Cattle . 3 : r : ‘ A 107.3 too (Wilchens)
Sheep . ‘ 3 ‘ ‘ : F ‘ 97.7 too (Irwin)
Pig : 4 : : ‘ : : ‘ 111.8 zoo (Wilchens)
Rat. F : z : F : F 105.0 roo (Cuénot)
Dove . 3 . ‘ é : 4 , 105.0 too (Cuénot)
Hen . ; ‘ : . : : : 94.7 too (Darwin)
Grassfrog . F 3 ‘ - F 2 82.0 too (Cuénot)
Fly. : % : , : g : 96.0 too (Cuénot)

There are some other remarkable cases in the vertebrates.
Pfliiger found the disproportion between male and female
frogs reared from eggs to be astonishingly great. This led him
to examine the proportion of adult males and females in the

The Determination of Sex 367

localities from which he obtained the eggs, with the following
results : —

Prop. 1n NATURE OBTAINED ARTIFICI ALLY

Male Female Male Female
Utrecht 13.2 100 18.1 100
Bonn 36.6 100 35-7 roo
Konigsberg 467 100 48.5 100

The meaning of the enormous difference is by no means clear.
Lenhossek has suggested that if the Utrecht egg were to be fer-
tilized with sperm from the Kénigsberg race, and vice versa,
interesting results in regard to whether the egg or the spermato-
zoon determines the sex might be obtained.

Geddes and Thomson state that in fishes the females are usu-
ally more numerous than the males, never less so, except in
anglers and catfish. They give the following data: —

FEMALE Mae
Flounder a r : : : é 2 P I to I
Roughead . : ; ‘ ‘ : : 2 12 to I
Cod. . 3 = ‘ . ‘ a : 3 to 2
Gurnard ‘ : j ; : : ‘ é 9 to 2

There is a further point of some importance in regard to man,
namely, the proportion of males and females that occur in abor-
tive births and of still-born infants. The statistics show that a
much larger number of these are males. Thus Rauber found
the proportion for 57 embryos was 159 males to 100 females.
Lenhossek examined 156 embryos (between the third and sixth
month), and found the proportion to be 160 males and 100
females.

For still-born infants, fully formed, but not alive, the follow-
ing figures have been given: —

368 Experimental Zoology

Quetelet found 133.5 males to 100 females; Bodio gives: —

MALES
Italy . 3 ; ¢ < a : : I3I.1
France : : : : : 5 F 142.2
Germany . j : ‘ ‘ . : 128 3
Austria é 3 ‘ ‘ ‘ : : 132.1
Hungary. 4 ‘ ‘ 3 . : 130.0
Switzerland . wy ie i a Ge! Se 135.0 } to 100 females
Belgium 5 ‘ ‘ 3 ‘ : : 132.0
Holland . ‘ : . : ; ‘ 127.1
Sweden . 3 : ; : ; ; 135.0
Norway ‘ : ‘ 3 : : ‘ 124.6
Denmark. : : b) #3 . : 132.0

From these results it is clear that the proportion of male to
female embryos is distinctly in favor of the males. The mean-
ing of this is unknown.

Monecious Species, composed of Bisexual or Hermaphrodite
Individuals

Since the groups of animals with which we are most familiar,
the vertebrates, insects, and higher crustaceans, are composed
of individuals with separate sexes, we are apt to forget how often
in other groups the sexes, or more accurately the sex-cells, are
united in the same individual. In the following list some
of the groups are given in which hermaphrodite individuals
exist : —

Sponges, sexes separate or united.

Ccelenterates, sexes separate or united.

Flatworms, sexes united, a few cases of separate sexes,

Nemerteans, sexes rarely united. Separate sexes the rule.

Nematodes, sexes separate as a rule, but united in some cases.

Annelids, sexes united in earthworms, separate in marine worms (except
Protodrilus).

Crustacea, sexes separate, but united in some barnacles.

Mollusca, sexes separate in some, united in other groups.

Echinoderms, sexes separate, united in a few cases.

Thus in the greater number of the great groups, hermaphro-

The Determination of Sex 369

ditism is found, although in only one of these, the flatworms, is
it the almost exclusive condition.

It would be interesting to take up these groups in turn and
examine the relation of the two kinds of sexual individuals, but
I shall select only one group in which the relations are especially
instructive, although exceptional.

In the group of Nematodes, or round worms, we find a curious
case of alternation of two kinds of sexual individuals. Ascaris
nigrovenosa is an hermaphrodite form found in the lungs of
frogs. The eggs pass through the digestive tract of the frog and
hatch as the Rhabditis form that has separate sexes. Equally
remarkable are the cases in which there exists an hermaph-
rodite form that appears to represent the female individual in
which secondarily spermatozoa as well as eggs have developed.
Maupas has studied these species with great care, and I follow
his description.

Maupas points out that there have been described 34 specics
of Nematodes in which reproduction without males has been
established; of these 34 species 25 are hermaphroditic and 9
parthenogenetic.’ Of the 34 species, 16 belong to one genus
Rhabditis. We are concerned here only with the hermaphro-
ditic species. These forms differ in no bodily character from
the females of other species with separate sexes, and differ only
in the ordinary specific differences. A minute study of their
sexual organs at the time of maturity is necessary to detect their
__ hermaphroditic character. It is found that when the reproduc-
tive organ is fully formed, it functions first as a testis. The germ-
cells at the anterior end of the ovary begin to divide rapidly and
become small spermatozoa. These are stored up in a receptacle
of the uterus. Later other cells, also situated at the anterior
end of the ovary, begin to grow larger; they store up yolk,
and become large eggs. They enter the uterus and become fer-
tilized by their ““own” spermatozoa.

1 This list Maupas says will probably be augmented, for of the 206 species
that are known from fresh water and from land, 85 are known only by the
females.

2B

370 Experimental Zoology

In some species the females have neither the “disposition”’ to
unite with male individuals nor the apparatus for storing the
sperm. Males occur in some of these cases, but they also often
lack the sexual instinct, and their spermatozoa are condemned
to perish.

Another point of great interest has been made out by Maupas,
viz., the imperfection or insufficiency of the hermaphroditism.
More eggs are produced than there are spermatozoa present to
fertilize them. Those that are first set free from the ovary
become fertilized, the rest become rapidly disorganized. The
number of fertilized eggs produced by a well-nourished individ-
cual is only about 200 to 250.‘ This condition is far from being
an advantage to the species, for if separate sexes existed, at least
800 eggs might be fertilized. If only half of these eggs produced
females, still the number would be 400, which is larger than the
number of individuals produced by the hermaphroditic species.
In the second generation there would be 160,000 individuals
from the unisexual females, but only 60,c00 hermaphroditic
females. Thus there is no reason to suppose that this herma-
phroditic condition is the result of an adaptation of the species.
Instead of an advantage it is a process injurious to the species,
but suffices, nevertheless, to keep it in existence.

Males appear in these hermaphroditic species, but in relatively
small numbers. Maupas has given the proportions observed
in the following table: —

Diplogaster robustus . 2 6 . ©.13males }

Rhabditis guignardi . ‘ . . 0.15 males

Rhabditis dulichura. F ‘ . 0.7 males

Rhabditis caussaneli . : : . 1.4 males

Rhabditis elyaus 2 F : . 1.5 males

Rhabditis coronata 3 2 . . 5.0 males Wao tewtalks
Rhabditis perrieri ‘ : . . 7.0 males

Rhabditis marionii ‘ : : . 7.6 males

Rhabditis duthiersi F P , . 20.0 males

Rhabditis viguieri - ‘ . - 45.0 males

‘In one species only, Rhabditis guignardi, there are more, namely, from 520
to 530.

The Determination of Sex 271

This means that in Diplogaster robustus there is 1 male to
10,000 females, and in Rhibiditis viguieri 1 male to 450 females.

There is nothing in the structure or in the organization of
these superfluous males that would lead us to consider them im-
perfect. In every point of their organization they correspond
to the ordinary type of the males of unisexual species. Even
the testis is normal and produces spermatozoa which are identical
in form, size, and structure with those in the hermaphroditic
glands of the corresponding females. On the other hand, these
males have nearly lost their sexual instincts, and are rarely seen
to pair with the hermaphroditic females.

Maupas carried out the following experiments. He placed
males and females together for a number of days and recorded
the number of times fecundation took place. It will be observed
that more that half of the cases occurred in Rhabditis marionis,
in which it is known that the hermaphroditism is incomplete.

FEMALES MALES FECUNDATIONS Days
Rhoditis elegans 139 112 6 4to 9
Rhoditis caussaneli 42 35 ° 5 to 10
Rhoditis marionis 28 42 13 sto 7
Rhoditis duthiersi 62 41 I 4to 6
Rhoditis perrieri 26 35 ° 5 to 7
Rhoditis duthiersi 12 5 ° 6to 7
Diplogaster robustus 4 2 O° 4to 5
: 313 272 20

Thus 313 females put with 272 males gave only 20 fecundated
females. In all cases except R. marionis the sexual indifference
of the males is almost absolute. This is the more surprising,
since in other species of Nematodes with separate sexes the
copulation is readily observed and lasts for some time. Here,
then, we have a case of a psychical “decadence,” unaccom-
panied by structural degeneration. The sexual process is being
replaced by hermaphrodite fecundation.

This elimination of the male type in the Nematodes is shown

372 Experimental Zoology

by the series of species showing intermediate stages. Incomplete
females are found in which one ovary alone produces both kinds
of germ-cells, the other only the female germ-cells, and still
other individuals are hermaphroditic in both ovaries.

Parthenogenetic Species

Parthenogenesis is also of not infrequent occurrence in the
animal kingdom. It is found most often in insects and in the
lower crustaceans and rotifers. It is found in some Nematodes '
and in a few other groups. In many cases the eggs of unisexual
forms may begin their development if not fertilized, and in recent
years it has been shown that parthenogenetic development may
be induced artificially in many species of animals. Thus we
have come to look upon the egg as capable of producing an em-
bryo if the proper external stimulus to development is present.
In nature this process has been carried out quite often with
far greater success than has been as yet accomplished by arti-
ficial means.

In some cases, as in the aphids, the parthenogenetic females
(wingless or winged) can be distinguished structurally from the
egg-laying females. The latter alone have the receptaculum
seminalis for storing the sperm. In other cases the two
kinds of females may be, externally at least, almost identical.
The ova are, however, sometimes different, as in daphnia. .

Since the eggs of so many species show incipient parthenogene-
sis, we have come to regard natural parthenogenesis as having
arisen by the disappearance of the males in certain generations.
The rareness of the males in certain groups in which partheno-
genesis has replaced the sexual modes of reproduction — in some
rotifers, sawflies, etc. —seems to support this point of view.
The question is, however, not quite so simple as this if we exam-
ine it more closely. What has become of the male eggs if this
view is correct? Do they simply fail to. develop, or have they
become female eggs? There is no evidence to show that half

* Maupas records seven parthenogenetic species in the Nematodes.

The Determination of Sex 373

of the eggs (those that formerly produced males, let us say)
are present and fail to develop. The alternative view seems
more plausible, that all the eggs produce females. If this view
is the correct one, then we find that in parthenogenetic species
two changes have occurred, all the eggs produce females, and
at the same time these female eggs have acquired the power to
develop without fertilization. It may be that both processes
have gone on hand in hand, and the same tendency to develop
parthenogenetically is associated with a tendency to produce
only females. In not a single case has the sex of the embryo,
produced by artificial means, been determined, therefore we
lack experimental evidence to form any opinion on this point.
In species where occasional parthenogenesis occurs both male
and female individuals may develop from unfertilized eggs, as
in the silkworm moth. In other groups apparently more females
develop, as in certain other moths, but in these the process of
parthenogenesis is sometimes quite regular rather than occa-
sional. In the honey bee and in some other hymenoptera,
males as a rule develop from parthenogenetic eggs.

Whatever conclusion we reach finally in regard to the origin
of parthenogenetic species, one point must be always borne in
mind: parthenogenetic eggs carry latent, or in a potential con-
dition, the male characters which may at any time become domi-
nant. There are certain species that produce only partheno-
genetic females in one generation, but males and females in
approximately equal numbers may appear in the next generation,
as in the gallflies; and in other species that have a long succes-
sion of parthenogenetic females, males and sexual females may
appear in at least equal numbers. These cases seem to indicate
with some probability that parthenogenesis has not arisen by the
suppression of the male eggs, but by the dominance of the female
characters in all of the eggs. What is true for the so-called female
eggs is probably also true for the male-producing eggs in uni-
sexual forms. The male egg carries latent the characters of
the female, and the hereditary transmission by means of the male
of the maternal characters shows that this must be the true view.

374 Experimental Zoology

In certain moths, Solenobia triquelrella and S. lichenella
and Psyche helix, a succession of parthenogenetic females has
been known to occur. In the first two species males may not
appear for years, and then suddenly appear, and even excede the
females in numbers. In one group of Rotifers, Philodinide,
males have never been found. In other families small, semi-
parasitic males may be present, but it is believed that in some
of these cases these minute males do not fertilize the eggs.
Whether the males have thus slowly disappeared or have sud-
denly ceased to appear in the Philodinidz is not known or even
surmised.

In still other insects parthenogenesis occurs. The larva of
the fly, Miastor, produces eggs that develop within its body and
produce there young maggots. This method may go on for
several generations, but ultimately some of the larvee pupate and
the sexually perfect flies emerge. The conditions that determine
which mode of production takes place have not been determined.
In the gnats of the genus Chironomus, the pupa deposits eggs,
but the pupa is in reality an imago that does not ordinarily
leave its pupa skin. Thrips seem to reproduce by partheno-
genesis throughout a part of the year. Some caddis flies are
said to be parthenogenetic. The walking sticks of the genus
Bacillus also produce as a rule by parthenogenesis. In Bacillus
Rosii only 1 male and 20 females were found to emerge in
one case from parthenogenetic eggs. In some moths and butter-
flies, parthenogenesis occurs either as a regular or as anoccasional
process. Parthenogenesis has already been described in the
gallflies, and other cases in the hymenoptera will be described
later. In other groups of animals, parthenogenesis is also’
known, as in the Trematodes, for instance; but the group of
insects furnishes the most striking cases of this method of repro-
duction.

The Determination of Sex 375

LITERATURE, CHAPTER XXIV

BERNER, H. Ueber die Ursachen der Geschlechtsbildung. Biologisches
Centralblatt. 1883.

Bopio, L. Movimento della Populazione. Confronti Internazionali. 1895.

BLUMENBACH, J. F. Ueber den Bildungstrieb. Géttingen. 1791.

Diistnc, C. Das Geschlechtsverhiltnis der Geburten in Preussen. Ber-
lin. 18g0.

GEDDES, P., and J. A. Taomson. The Evolution of Sex. 2ded. _1gor.

Maupas, E. Modes et Formes de Reproduction des Nematodes. Arch.
Zool. Exp. et Gen. 1900.

RavBer, A. Der Ueberschuss an Knabengeburten und seine biologische
Bedeutung. 1900.

SADLER, M. The Law of Population. London. 1830.

ScHumaNN, A. Die Sexualproportion der Geborenen. Oldenburg. 1883.

Strepa, W. Das Sexualverhiltnis der Geborenen. Strassburg. 1875.

Turguan, V. Durée de la génération humaine. Rev. Sci. V. 1896.

WILCKENS. Untersuchung ueber das Geschlechtsverhiltniss und die
Ursachen der Geschlechtsbildung bei Haustieren. Landw. Jahr-
bucher, XV. 1886.

CHAPTER XXV
EXTERNAL FACTORS OF SEX DETERMINATION

WHETHER the sex of an individual is determined by external
factors during the course of development, or whether the sex is
already determined in the egg, has been the subject of much dis-
cussion. I shall first consider the experimental evidence that
has seemed to show that external conditions may determine sex,
and then examine the evidence in favor of the view that sex is
predetermined in the egg, either before or after fertilization.

Influence of Food

Of the different external factors that have been supposed to
determine the sex of the individual, nutrition occupies the first
place. Nutrition is supposed in some cases to influence the sex
of the young animal, while in other cases the egg itself is supposed
to have been affected by the condition of nutrition of the mother.

The work of Landois in 1867 was the first in the field. He
stated that he could produce, at will, males or females, of the
butterfly Vanessa urtica by regulating the amount of food. Six
years later Mrs. Treat claimed to have obtained similar results,
and in the same year, 1873, Gentry made a statement to the
same effect. Mrs. Treat made use of the caterpillars of Papilio
asterias. One lot deprived of food before the last moult gave
34 males and 1 female. Another lot, well fed, gave 68 females
and 4 males.

Riley carried out, in 1873, a more thorough series of experi-
ments on the same species, as well as on others. He tried espe-
cially the effects of starving the caterpillars, for he says that it
is not possible to make caterpillars take more food than they do
naturally, and under this condition they produce both males

376

1

External Factors of Sex Determination (377

and females. The starved caterpillars gave no special excess of
males, although there were, it is true, a few more males; but
Riley pointed out that under natural conditions the females in
several species take a longer time than do the males to reach the
pupa stage, undergoing in some cases one more moult, so that,
if starved, the mortality would therefore probably be greater
amongst the females, and hence more male than female moths
might be found. Other observers, Bessels 1868, Briggs 1871,
Andrews 1873, Fletcher 1874, have also found that an excess or
diminution of the food does not alter the proportion of the sexes.

The futility of many of these experiments has now become
apparent, since it has been shown that the sex of the caterpillar
is already determined when it leaves the egg. Under these cir-
cumstances it is not probable that feeding could produce a change
in the sex. It is much more probable that starvation or over-
feeding could only effect the proportion of males and females
by bringing about a greater mortality of the individuals of one
Sex.

Kellogg and Bell have studied food conditions in relation to
sex determination in silkworms (Bombyx mori). They have
guarded against the possibility of greater mortality of one sex.
The chief interest of their work is their examination of the pos-
sible effects of nourishment on the second generation. The
effects of feeding the caterpillars themselves different amounts
of food were as follows. Twenty larve were present at first in
each lot. .

“Lot I. Fed optimum food; no deaths before emergence of
moths; produced 8 males, 12 females.

“Lot II. Fed optimum food; 2 deaths before maturity;
produced 7 males, 11 females.

“LotIII. Fed one half (approximately) of optimum of food ;
4 deaths before maturity; produced 1o males, 6 females.

“Lot IV. Fed living minimum of food; 3 deaths before ma-
turity; produced 10 males, 7 females.

“Lot V. Fed living minimum of food; 6 deaths; produced 9.
males, 5 females.”

378 Experimental Zoology

The results seem to show a slightly larger number of females
in the well-fed lots, and a larger number of males in the starved
sets; but the numbers are too small to justify any conclusions
from them, and those that die may have altered the proportions.
In other experiments the authors found that the results do not
lead to any conclusions in regard to the influence of nourishment.

The possible influence of food in determining the sex of the egg
(or sperm) was also examined. In the following table those larve
that had the optimum amount of food are indicated byo. Those
that had a minimum of food are indicated by M. The amount
of food given to the grandparents, to the parents (third column),
and to the larva themselves (second column) is indicated by the
letters.

Lots FED PARENTS eee oe ee MALES FEMALES
I ° ° ° 2 13 Io
2 M ° ° 2 14 9
3 ° M ° 3 8 14
4 M M ° 6 8 II
5 M ° M ° 15 10
6 M ° M ° II I4
i] ° M M 20 2 2
8 M M M 21 2 2

It is not evident from these data that the sex of the egg is
influenced by the condition of the parents or grandparents.

Cuénot has carried out in recent years some important experi-
ments with moths and flies. When the caterpillars of Ocneria
dispar were given an abundant nourishment, they produced 52
females and 4 males. Caterpillars of Bombyx rubi were given
so small an amount of food that great mortality resulted. Of
the 27 survivors there were 13 males and 14 females.

Cuénot states that in the maggots of flies, the gonads, although
present in young larve, do not undergo their histological differ-
entiation until a relatively late period of larval life, hence
there might appear a better opportunity to influence the sex,

Lixternal actors of vex Determination 379

provided, of course, that it is not in reality already determined,
since the absence of differentiation of the sex cells does not neces-
sarily mean that the sex may not have been previously settled.
Lowne had found that flies from large larve are almost always
females, and those from small larve males; but Lowne himself
doubted the value of his observations, and Cuénot says his con-
clusion from them is erroneous. Weismann fed one lot of Cal-
liphora vomitoria abundantly, and starved another lot from time
to time. Both lots produced males and females. Cuénot’s ex-
periments were as follows: —

Experiment I. The larve after hatching were fed abundantly
on putrefying flesh, and produced large flies. ‘The proportion of

females to males is given in the next table. pea CeweGe
FEMALES
Lucilia cesar... . . . . 293 females 281 males 49.0
Calliphora vomitoria . . . . 224females 215 males 51.0
Sarcophaga carnaria . . . . . g6females go males 51.6

Experiment II. The larve were given as little nourishment
as possible; they. often were entirely without food, and the mor-
tality was very great. The flies were small. The proportion

of females to males was as follows: — ben Genrer
FEMALES
Lucilia cesar. . . . . - - + 95 females 69 males 57-92
Calliphora vomitoria . . . . . 93 females 91 males 50.05
Curtonevra pabulorum. . . . . 26 females 17 males 50.50

Experiment III. The larve were kept under varied condi-
tions, being fed on brain, suet, and a little meat. They were
in a starved condition at first and were then abundantly fed or
were fed at first and then starved. Several species were reared
together.

Calliphora vomitoria (April19) . . . . 185 females 166 males
Lucilia cesar (Aug.Sept.). . . . . . 143 females 141 males

The normal proportion of males to females in these species
of flies is approximately equal. This relation is also found in
those kept under the different conditions of the other experi-

380 Laperimental Zoology

ments. Cuénot concludes, therefore, that sex is not determined
by external conditions, but is predetermined in the egg.

The question turns, therefore, on what condition determines
the sex of the egg. Can the nutrition of the parents during the
larval stages affect the proportion of male and female eggs?
Cuénot carried out some experiments to test this possibility.

Experiment I. Larve of Calliphora vomitoria, that had been
poorly nourished from the time of hatching, produced 12 males
and 5 females that were not more than half the normal size.
The flies were inclosed in a cage and fed on sugar and meat.
They laid 20 lots of eggs. The larve that hatched were fed
and produced 359 females and 353 males. No influence due
to the starved condition of their parents was observed.

Experiment II. Adult flies of Calliphora vomitoria that had
hibernated in the laboratory all winter without food were cap-
tured in March and April. They were fed on sugar and meat,
and then laid 7 lots of eggs. The larve were well fed and
produced 306 females and 308 males. ‘This result also shows
that the condition of the parents is without effect on the sex of
the offspring.

Pictet has recorded the proportion of males and females that
appear when the food of caterpillars is different from the normal.
The general effects of the change of diet was in some cases to give
insufficient nourishment, in other cases more nourishment than
supplied by the normal food. The results seem to show that
when the food is insufficient the proportion of males is greater.
Thus Ocneria dispar fed on walnut leaves gave in the first genera-
tion 54 males to 46 females, and in the second generation 65
males to 35 females. When the first generation was fed on wal-
nut leaves and the second on oak leaves (the normal food),
there were 61 males to 39 females. When the first generation
was fed on walnut, the second on oak, and the third on walnut
again, there were 65 males to 35 females.

Esparcette and dandylion furnish an ample supply of nour-
ishment; the former gave in the first generation 51 males to 49
females, and the latter 52 males to 48 females.

External Factors of Sex Determination 381

Thus while poor nourishment increases the percentage of
males, good nourishment does not increase the percentage of
females above the normal. The results may be due, Pictet
admits, to mortality rather than to a change in sex affected by
the food.

A number of experiments have been carried out with tadpoles
of frogs. An observation made by Born seemed to him to
show that the sex of the tadpole of the frog is determined by the
amount or by the kind of food eaten by the young animal. He
found that when the tadpoles of Rana temporaria (from
eggs artificially fertilized) were fed on a mixed vegetable and
meat diet, that 95 per cent of them were females and 5 per cent
were males. This was a general result obtained from several
lots of 1443 tadpoles in all. In some aquaria all the indi-
viduals were females.

The experiments of Yung (1883) are more discriminating.
He fed tadpoles of Rana esculenta on different kinds of food.
In one lot (A) the flesh of fish was given. These tadpoles were
large and well developed. In another lot (B) the flesh of beef
and of fish was used. These tadpoles were also large. Ina
third lot (C) the white of eggs was used. These tadpoles were
smaller. In a fourth lot (D) the yolk of hen’s egg was given.
These tadpoles were even smaller than the last. The number
of males and females that developed is shown in the following
table: —

ae Matzs |Femates| P°V2™| Losr see

Frocs nue 3 FEMALES
Lot A (fish) 24 4 7 2 I 70 %
Lot B (fish and beef) 33 6 25 2 - 15%
Lot C (white of egg) Io 3 7 - I 470 %
Lot D (yolk of egg) 7 fe) 5 2 - 41%

There are also a few other statements by other authors in
regard to the influence of food. Thus Balbiani and Henneguy
state that an excess of females is found in tadpoles fed on yolk

382 Experimental Zoology

of egg* as compared with those that obtain only a vegetable
diet.

These results on tadpoles have been shown to be inconclusive
for the following reasons: —The method followed by Born of
determining the sex of the tadpoles at their time of metamor-
phosis was very inexact. He relied on the size of the gonads
(testis or ovary), but histological examination has shown that the
female gonad is not always larger than the male, and there are
always so many cases intermediate in size as to render the con-
clusions invalid. Even the method of teasing out the gonads,
a method also used by Born, cannot be relied upon. Further-
more, unless the number of tadpoles that die is taken into ac-
count (and the number may be considerable), we cannot be cer-
tain that the results may not be due to greater mortality of one
or the other sex under certain conditions. If further evidence
were needed to invalidate these results, they are to be found in
Pfliiger’s observations on the proportion of males and females
in certain species of frogs. As already pointed out, the eggs
from certain localities give a high percentage of females, and the
same disproportion of adult frogs is found under natural condi-
tions. It is true that this does not in itself show that the sex
may not be determined by the external conditions; but if the
natural disproportion of males to females is very great, error
may easily creep into the experimental results.

The most important objections to the results of Born and of
Yung are found in the more recent experiments of Cuénot. His
results are as follows: The eggs of Rana temporaria were used.
The first lot lived on a vegetable diet. They suffered from the
confined space, and the tail was often malformed. Their de-
velopment was retarded and the genital organs (gonads) were
small. The 26 young that underwent metamorphosis were all
females. In the second lot the conditions were the same as the
last, and 3 females and 4 males were found. The third lot was
kept in a large aquarium with cool, running water. The tails
were cut off several times with the intention of prolonging thereby

‘It is not clear that yolk of egg isa good food. See Yung’s results.

External Factors of Sex Determination 383

the metamorphosis, which took place, in fact, — owing probably
to the temperature, — two and a half to three months later than
the others. Sections of the gonads showed 30 females, 26 males,
and 1 hermaphrodite.

The fourth lot, fed for some days on the jelly of their own eggs,
were separated into three groups. The results were as follows : —

Mates FEMALES Not DIFFERENTIATED
Group I (vegetable food) 57 51 8
Group II (animal food) 22 14 °
Group III (mud and their own 12. 23 )
dead)

Groups I and II developed at the same rate. They were small,
owing to the size of the aquarium, and underwent their meta-
morphosis at the beginning of June. Group III was retarded.
The tadpoles were small and had begun to die at the beginning of
June. They were then fed on an animal diet, when they began
to grow rapidly, and metamorphosed at the end of July. The
frogs were small, and yet as the table shows there were more
females than males, while in the case of those of Group IT that
had animal food there are more males than females. In the case
of Group I the two sexes are nearly equal in number.

Cuénot points out that his own results as well as those of
others are open to the serious objection that the sex of those that
died is unknown, nevertheless he thinks that his experiments show
that food is not a factor in determining the sex of tadpoles.

The best-authenticated cases that seem to show the influence
of food on the determination of sex are those of Hydatina senta
and the daphnid, Simocephalus. The experiments that seem
to establish this relation have been given in previous chapters.
According to Nussbaum, the amount of food taken by Hyda-
tina during the first few hours after hatching determines whether
the kind of eggs laid will give rise to males or to females. This
may mean either that all the eggs have their sex determined at
this time, one way or the other, or that the food determines

384 Experimental Zoology

whether female or male eggs develop, assuming both kinds
to be present. As yet this point is unsettled. If Maupas’s con-
clusion is correct, that the male eggs and the winter eggs are
the same, fertilization or its absence determining their fate,
it would seem that the same external conditions that produce
a winter egg produce its potentially equivalent male egg. In
Simocephalus lack of food causes either the male or the winter
egg to develop. If Weismann’s statement is correct, that the
parthenogenetic egg and the winter egg come from different
parts of the ovary, it may prove that the male egg and the winter
egg are here also the same. This question needs, however, fur-
ther examination.

Supposed Influence of Nourishment in Determining Sex in
Man and Other Mammals

In the cases so far examined the eggs are laid immediately
after fertilization, so that if their sex were not already determined
by the parent, no further chance for such an influence exists.
In man and in other mammals the embryo develops in the uterus of
the parent, and the opportunity is afforded of influencing the sex,
if such were possible, by the condition of nutrition of the parent.
It has often been claimed that the sex of the child is determined
in this way, and as often denied. The method employed in this
case is to examine the statistics giving the proportion of males
and females born of parents living under supposed favorable
and unfavorable conditions of nutrition. It has been claimed
that more boys are born in the poorer classes and more girls in
the richer classes; but at best the differences on which these state-
ments rest are small, and other statistics seem to give a contra-
dictory result.

In support of the view that the conditions of nourishment
affect the proportion of the sexes, the following data have been
appealed to. In France the proportion of male to female births
is for the upper classes as 104.5 to 100, and for the lower classes
as 115 to 1co. In the Almanach of Gotha there are recorded
105 males to 100 females; but amongst Russian peasants there

External Factors of Sex Determination 385

are 114 male births to 100 female births. The statistics for the
nobility of Sweden show 98 male births to 100 females, while for
the clergy of the same country 108.6 male to 100 female births.

Diising, in particular, has developed this point of view. It
is not evident, however, that because more food is supplied, the
individual is necessarily better nourished, for this will depend
on other conditions as well. The amount of food taken, or
accessible, does not necessarily mean that more is digested.

It has also been pointed out in man that when twins develop,
each gets less nourishment than when only a single embryo is
present. The smaller size of twins shows that less food is
available, yet the proportion of male and female twins is the
same as that for single births.

Punnett has examined some statistics based on the census
of London for 1901. He finds more females born in the poor-
est classes, and more males in the highest classes. Intermediate
classes give intermediate results, 7.e. more nearly an equal pro-
portion. Even after taking into account the differential birth
rate, the results are still the same; for this and other factors
diminish the proportion of males in the poorer classes and make
even more probable the conclusion that more males are born
under favorable conditions. Punnett draws the conclusion
that the determination of sex is independent of parental nutri-
tion.

If nutrition were in reality a factor of any importance in sex
determination, it is surprising to find so little difference under
apparently very favorable and unfavorable conditions. It seems
much more probable that if the nutrition affects in any way the
proportion of the sexes, it does so indirectly by elimination, and
not by determining either the sex of the embryo or of the egg.

This conclusion is borne out by the results of some experi-
ments on other mammals recently carried out by Cuénot and
by Schultze.

Cuénot reared two sets of albino rats: one'set was well fed
with a variety of food; the other set was poorly nourished, being

fed mainly on bread. The well-nourished individuals produced
2C

386 Experimental Zoology

more young in a litter than the poorly nourished. Putting to-
gether all those cases where more than nine young were present
in a litter, which indicates favorable parental conditions, there
were produced 49 females and 43 males. Where fewer than
g were born there were 62 females and 71 males. Although
there is a slight excess of females in the former and of males in
the latter cases, the numbers are so nearly equal that the influ-
ence of nutrition cannot be said to be apparent. It may be
noted that when more young are present the conditions for
each are poorer, yet, in fact, more females were born.

Schultze has carried out some experiments with white mice.
One lot was starved. They were long in reaching maturity,
and weighed only a half to a third as much as well-fed mice.
This lot produced 4 males and 4 females. Another lot pro-
duced 42 males and 48 females. Clearly there was no effect of
nourishment to be seen. Schultze also examined the product
of the second generation of starved mice, some of which were
also starved and others fed. He obtained 36 males and 43
females for the starved ones, and 20 males and 28 females for
the well-fed individuals. He also paired poorly fed males with
well-fed females in order to see if, as has been claimed, the better-
fed parent would determine the sex. Well-fed females paired
with well-nourished males gave 46 males and 36 females. The
same females were then paired with poorly nourished males and
gave 38 males and 37 females. Again there is no evidence of the
sex being determined by the condition of the parent.

Schultze also tried the effects of feeding mice on foods that
were rich or poor in albumen. Potatoes and bacon were used
as having little albumen, but on this diet the animals did not
reach sexual maturity. Fed on white bread and water, they gave
18 males and 22 females. For a food rich in albumen, tropon *
was used, also other food rich in albumen, along with salt and
milk. There were produced 29 males and 26 females.

These experiments go far toward establishing the view that

1 Tropon contains 21 per cent albumen, 60 per cent carbohydrates, and 15
per cent fat.

External Factors of Sex Determination 387

the sex of the egg or embryo of mammals is not determined by
nutrition.

Geddes’s and Thomson’s Theory of Sex

The theory of sex that Geddes and Thomson elaborated in
their book on “The Evolution of Sex” is largely based on evi-
dence of the sort just examined. We have seen that all of the
earlier experiments that seemed to show that the sex of the em-
bryo is determined by the amount of food supplied are open
to grave suspicion. Hence the strongest evidence brought for-
ward by Geddes and Thomson in favor of their theory is no
longer valid; but since the theory is also based on certain sup-
posed analogies between the male and female condition in
general, we may briefly consider it here.

Geddes and Thomson think that the greater activity or
“vitality”’ of the male is due to a greater breaking down or
katabolism of the tissues; while the less active female condition
is more one of building up, or of anabolism. “ Deficient or
abnormal food, high temperature, deficient light or moisture,
and the like are such as tend to induce a preponderance of
waste over repair —a relatively katabolic habit of the body —
and these conditions tend to result in the production of males.
Similarly the opposite set of factors, such as abundant and rich
nutrition, abundant light and moisture, favor constructive pro-
cesses, 7.e. make for a relative anabolic habit, and these conditions
tend to result in the production of females.” It is pointed out
that the males themselves are often smaller, more active, have
a higher temperature and a shorter life; while the females are
larger, more passive, vegetative, and conservative forms. The
authors handle the entire problem of sex under the quasi-sym-
bolic terms of anabolism and katabolism without stating definitely
how such a condition determines sex. In fact, so vague and gen-
eral are most of the statements regarding sex determination that
their interpretation belongs rather to that class of hypothesis,
so common in much of our biological speculation, in which the
issue is obscured by the appeal to phenomena as uncertain

388 Experimental Zoology

and little understood as the problems that they pretend to ex-
plain. If we inquire whether the greater katabolism or anabo-
lism of the male causes it to develop male or female produc-
ing spermatozoa, we shall find no definite answer to our question.
The idea is, however, expressed that the condition of the mother
determines the kind of egg she produces, — male or female, — ,
also that the effects of nutrition in the embryo itself determine its
sex, which is assumed to be formed at some early stage in the
embryonic or larval development.

If the anabolic state is the one characteristic of the female,
how can she be supposed to produce male eggs, for however
much starved her tissues may become, she is not thereby changed.
into a male, and until this occurred it is not clear why male eggs
should appear; unless the eggs are supposed to be more easily
affected than the formed tissues of the parent. But the most seri-
ous objections to the theory are the two following: Statistics
show that even under extremes of nourishment of the parent both
males and females appear, and if any difference of ratio is found,
it is so slight that one can only be surprised that the data give
such meager results in favor of the theory. If, on the other hand,
the determination of sex is supposed to be due to the nourish-
ment of the embryo, the best-ascertained facts, both experimental
and statistical, are diametrically opposed to the hypothesis. The
results show rather that differences in nourishment may cause
a greater mortality of one sex, or possibly that they favor the
development of one kind of egg rather than the other. There- ‘
fore unless Geddes and Thomson can give their view, either a
clearer interpretation, or bring it into accord with the best-ascer-
tained facts, I do not think it can be looked upon as having in
any degree given even a proximate solution of the problem of the
determination of sex.

LITERATURE, CHAPTER XXV

AnprEws. Controlling of Sex in Butterflies. The Entomologist, VI.
1873.

BErTILLON. De influence de la primogéniture sur la sexualité. Bull.
Soc. Anthrop. Paris, XI. 1876.

External Factors of Sex Determination 389

BEssELs. Studien ueber die Entwickelung der Sexualdriisen bei den Lepi-
dopteren. Zeit. fiir wiss. Zool. XVII. 1867.

Die Landois’sche Theorie widerlegt durch das Experiment. Zeit. fiir
wiss. Zool. XVIII. 1868.

Born. Experimentelle Untersuchungen ueber die Entstehung der Ge-
schlechtsunterschiede. Breslauer drtzlichen Zeit. 1881.

Briccs. Notes on the Influence of Food in Determining the Sexes of In-
sects. Trans. of the Entom. Soc. of London, I. 1871.

Coun, L. Die willkiirliche Bestimmung des Geschlechts. Die bisherigen
Forschungen einschliesslich der Schenk’schen Theorie kritisch be-
leuchtet. Wiirzburg. 1898.

CornEvIN. Traité de Zootechnie générale. Paris. 1891.

Coste. Production des sexes. C. R. Ac. Sc. Paris, LVIII. 1864.

Cuénot, L. Sur la détermination du sexe chez les animaux. Bull. Sci.
de la France et Belg. XXXII. 1899.

Disinc, C. Die Regulierung des Geschlechtsverhiltnisses bei der Vermeh-
rung der Menschen, Tiere und Pflanzen. Jenaischen Zeit. f.
Naturw. XVII. 1884.

Die experimentelle Priifung der Theorie von der Regulierung des
Geschlechtsverhdltnisses. Jenaische Zeit. f. Naturw. XIX. N. F.
XII. 1886.

Die Regulierung des Geschlechtsverhdltnisses bei Pferden. Landw.
Jahrbiicher, XVI. 1887; XVII. 1888; und XXI. 1892.

FLeTcHER. Controlling Sex in Lepidoptera. The Entomologist, VII.
1874.

Geppes, P., and THomson, J. The Evolution of Sex. London. 1889.

GENTRY. Influence of Nutrition on Sex among the Lepidoptera. Proc. of
the Acad. of Nat. Sc. of Philadelphia. 1873.

GRIESHEIM. Ueber die Zahlenverhiltnisse der Geschlechter bei Rana
fusca. Arch. fiir die ges. Phys. XXVI. 1881.

HENNEBERG. Wodurch wird das Geschlechtsverhdltniss beim Menschen
und den héheren Tieren beeinflusst? Ergebn. d. Anat. und Ent-
wickl. VII. 1897.

Herri. Ueber die Stabilitat des Geschlechtsverhaltnisses bei Mehrlings-
geburten. Inaug.-diss. Freiburg. 1884.

HoFACKER, J. Ueber Eigenschaften, welche sich bei Menschen und Tieren
vererben. Tiibingen. 1828.

Janke, H. Die willkiirliche Hervorbringung des Geschlechts bei Menschen
und Haustieren. 2 Auflage. Berlin. 1888.

JHERING. Ueber Generationwechsel bei Sidugethieren. Arch. f. Anat.
und Phys. 1886. .

JoserH, G. Ueber die Zeit der Geschlechtsdifferenzirung in den Eier
einiger Lipariden. 48 Jahresber. der schles. Ges. f. vaterl. Cultur
(1870). 1871. ; :

Ketioce, V. L., and Bett, R. G. Notes on Insect Bionomics. Journ.
Exp. Zool. I. 1904. —- :

Keruervi:. De Vapparition provoquée des éphippies chez les Daphnies
(Daphnia magna). Mém. Soc. Zool. de France, V. 1892. et

Sur apparition provoquée des males chez les Daphnies (Daphnia
psittacea). Mém. Soc. Zool. de France, VIII. 1895.

Kuess. Ueber das Verhiltniss des mannlichen und weiblichen Geschlechts
in der Natur. Jena. 1894.

390 Experimental Zoology

Montcomery, T. H. The Morphological Superiority of the Female Sex.
Proc. Amer. Philos. Soc. XLIII. 1904.

Nussspaum, M. Die Entstehung des Geschlechts bei Hydatina senta.
Arch. f. mikr. Anat. XLIX. 1899.

PacKARD. When is Sex Determined? Amer. Natur. VII. 1873.

Priiticer, E. Einige Beobachtungen zur Frage ueber die das Geschlecht
bestimmenden Ursachen. Arch. fiir die ges. Phys. XXVI. 1881.

Hat die Concentration des Samens einen Einfluss auf das Geschlecht ?
Arch. f. die ges. Phys. XXIX. 1882.

Ueber die das Geschlecht bestimmende Ursachen und die Geschlechts-
verhaltnisse der Frésche. Arch. fiir die ges. Phys. XXIX. 1882.

Picret, A. Influence de |’Alimentation et de l’Humidité sur la variation
des Papillons. Mém. Soc. Phys. et Hist. Nat. Genéve. XXXV.
1905.

Punnett, R. On Nutrition and Sex-Determination in Man. Proc. of
the Cambr. Philos. Soc. XII. 1903.

Ritey. Controlling Sex in Butterflies. Amer. Natur. VII. 1873.

SADLER. The Laws of Population. 1830.

ScHENK, L. Meine Methode der Geschlechtsbestimmung. Verh. v.
Internat. Zool. Congresses zu Berlin. 1902.

Tuury. Ueber das Gesetz der Erzeugung der Geschlechter. Leipzig.

Treat, M. De la production des sexes dans les Lépidoptéres. Petites
nouvelles entomologiques, V. 1873.

VaRicNy. Unions consanguines des Colombins. Interm édiaire des Biolo-
gistes, X. 18098.

WEIsMANN, A. Die nachembryonale Entwicklung der Musciden nach
Beobachtungen an Musca (Calliphora) vomitoria und Sarcophaga
camaria. Zeit. f. wiss. Zool. XIV. 1864.

Younc, E. Contributions 4 Vhistoire de l’influence des milieux physico-
chimiques sur les étres vivants. Arch. Zool. ‘ixp. I. 1883.

De l’influence des variations du milieu physicochimique sur le dévelop-
nee des animaux. Arch. des Sc. physiques et naturelles, XIV.
1885.

Wappaus, J. Allgemeine Bevélkerungsstatistik. Leipzig. 1861.

Witckens, M. Untersuchung ueber das Geschlechtsverhaltniss und die
Ursachen der Geschlechtsbildung bei Haustieren. Landw. Jahr-
biicher, XV. 1886.

CHAPTER XXVI
INTERNAL FACTORS OF SEX DETERMINATION

MANvy suggestions have been made concerning the internal
factors that determine sex. We may roughly classify these views
under two headings; namely, those in which the condition of the
parents when the germ-cells are liberated is supposed to deter-
mine the sex of the individual,’ and second, those in which some
internal change that determines the sex is supposed to take
place in the germ-cells themselves irrespective of the condition
of the parents. The different hypotheses when brought to-
gether seem to include nearly all of the possible conditions that
might be imagined to determine the sex of the offspring. We
may group the different views under the following headings:
(1) the age of the parents; (2) the condition of germ-cells at the
time of fertilization; (3) the vigor of the parents; (4) the effects
of inbreeding; (5) the size of the egg; (6) the ratio of nucleus to
cytoplasm; (7) the extrusion of the polar bodies; (8) the forma-
tion of male and female spermatozoa and eggs; (9) the influence
of fertilization; (10) the influence of the cytoplasm.

Age of Parents

The only data bearing on this question are those for man and
for some of the domesticated animals. Hofacker (1823) and.
later Sadler (1830) brought together some statistics that seem
to show that when the father is older than the mother more
boys are born; and when the mother is older than the father

1 This influence, being in part external to the germ-cells themselves, might be
classified as an external influence in the same sense that food is an external factor.
There is no sharp line to be drawn in such cases between internal and external

factors.
391

392 Experimental Zoology

more girls are born. Other writers have published statistics
that give exactly contrary results. Geddes and Thomson give
such data in the following table : —

FATHER
FATHER YOuNGER
AVERAGE
OLDER THAN
FATHER PROPORTION
No. oF Propr, ‘i MoTHER
OBSERVER Biers Loca.ity Mins SAME AGE Propr. or MALES
AS MOTHER TO 100
TO 100 MALEs F
EMALES
FEMALES TO 100
FEMALES
Hofacker 1,996 | Tiibingen 117.80 92.00 90.60 | 107.50
Sadler 2,008 | England 121.40 94.80 86.50 | 114.70
Gohlert 4,584 108.02 93-30 82.60 | 105.30
Legoyt 52,311 | Paris 104.49 | 102.14 | 97.50 | 102.97
Boulanger 6,006 | Calais 109.98 | 107.92 | 101.63 | 107.90
Noivot 4,000 | Dijon 99.70 116,00 | 103.50
Breslau 8,084 | Zurich 103.90 | 103.10 | 117.60 | 106,60
Stieda 100,590 | Alsace-Lor- | 105.03 108.39 | 106.27
raine
Berner 267,946 | Sweden 104.61 | 106.23 | 107.45 | 106.00

It will be seen that while three other statisticians obtained the
same kind of results as did Hofacker and Sadler, two others
give exactly opposite results. Contradictory evidence has also .
been given for horses and sheep. As Schultze points out, the
results may mean, if they mean anything, that the conditions
do not determine the production of one sex or the other, but the
survival of an excess of one or the other kind of egg. Bidder
has shown in the case of women under 18 years of age bearing
children that there is an excess of boys in the proportion of 133.9
‘boys to 100 girls.

Schultze tried to test this problem experimentally. Sexually
immature female mice were placed with males of the same age,
so that at the first period of heat conception might occur. The
females at this time were between 10 and 15 weeks old. There
were produced in all 60 males and 65 females. In other cases
the females were isolated and first allowed to breed after the

Internal Factors of Sex-Determination 393

toth, rath, 14th, 15th, 16th, 21st, and 28th weeks. In none of
these cases was there found any evidence in favor of the view that
the sex of the first-born young is affected by the age of the parent.

Condition of the Germ-cells

It has been claimed in the case of certain mammals (Thury,
1863, Diising, 1838) that if the egg is fertilized soon after leaving
the ovary it zends to produce a female; if not fertilized till later,
it tends to produce a male. Thury based his conclusions on 29
experiments with cows, in which union took place at the begin-
ning or at the end of the period of heat. Others have failed
to confirm his conclusions, and contradictory results have been
obtained with rabbits and hens. The same argument has been
applied to the condition of the sperm, but there is no clear evi-
dence in its favor. It has been pointed out that such a view is
contradicted in the case of bats, in which the spermatozoa, re-
ceived by the females, may remain alive for months before the
eggs are set free; and yet the same proportion of males and
females found in other animals are produced. In the case of
the bee the sperm is stored up in the seminal receptacle of the
queen for several years; yet in all cases the fertilized eggs pro-
duce only females.

Vigor of the Parents

Schultze has carried out an experiment that goes to show that
continued or strained reproduction on the part of the female
does not affect the proportion of males and females. As soon
as the young mice were born they were removed and the mother
again produced a new litter. One female produced in a year
and 52 days 14 litters, containing 52 males and 53 females. An-
other female produced in 11 months 12 litters, containing 42
males and 34 females. A third produced in 3} months 6 litters,
containing 20 males and 11 females. A fourth produced in 43
months § litters, containing 19 males and 22 females. The total
is 133 males and 120 females, which for the numbers involved

is a fair approximation to equality.

394 Experimental Zoology

Effects of Inbreeding

It is a widespread belief that deterioration in strength or fer-
tility or in both follows close inbreeding, and while there is some
evidence to the contrary in many species, still, in a few, the evi-
dence that we have at present indicates injurious effects. Such
weakening influence has been supposed to affect the determina-
tion of sex. Schultze has carried out a series of experiments
to test this view by closely inbreeding white mice. His results
show that while for a limited number of births one or the other
sex may predominate, yet the general averages show approxi-
mately an equality.

Size of the Egg

In three species belonging to widely different groups it has been
discovered that the size of the egg is correlated in some way with
sex. In Phylloxera the large eggs develop without fertilization
into females, and the small eggs into males. In Hydatina senta
the same rule holds. In Dinophilus apatris, also, there are large
and small eggs, both of which are supposed to be fertilized.
The large eggs produce females and the small ones males. One
of the most striking instances in which sex seemed to ‘be deter-
mined by, or connected with, the size of the egg is that of the
silkworm. Joseph, in 1871, and Mme. Brocadello, in 1896,
stated that the large eggs became females and the small ones
males. As the following table shows, the small eggs gave from
88 to 95 per cent of males and the large eggs 82 to 92 per cent
of females : —

BROCADELLO’S TABLE FOR THE SILKWORM MOTH

Rice ies tr a PER pe piesa
Vartansi dihhorassan 88 82
Giallo Perugia 95 92
Chiacian di Chorassan go 92
Giallo Pirenei 88 : 92
Giapponese verde 88 88
89.8 90.6

Internal Factors of Sex Determination 395

Unfortunately for this case a reéxamination of the problem by
Cuénot has lead to a contradictory result. Cuénot separated
the large eggs from the small ones by passing them through the
meshes of sieves of different sizes. The results appear in the
next table: —

CUENOT’S TABLE FOR BOMBYX MORI

SMA.L Eccs Larce Eccs _
Male Female Male Female
I set 23 16 19 82
2 set 43 36 31 26
3 set 46 53 9 9
4 set 24 24 6 35
138 129 125 132

There were no deaths in this set. It will be noticed that there
is a slight excess of males for the small eggs and of females for
the large eggs, which is in accord with Brocadello’s hypothe-
sis, but the difference is too small perhaps to warrant one in
concluding that there is a definite relation between the size of
the egg and the sex of the moth.

In another experiment, in which, however, there were some
deaths, the small eggs produced 119 males and 133 females, and _
the large eggs 65 males and 108 females.

Cuénot also found in the moth Ocneria dispar that one lot of
large eggs gave 14 males and 18 females. These results lend,
however, little or no support to Brocadello’s view. Wherein the
difference in the results lies is difficult to explain. It is hardly
possible that Brocadello’s results can be accidental or erroneous.
Possibly the difference may lie in the fact that in the one case the
largest and smallest eggs laid by different individuals were com-
pared, and in the other case the largest and the smallest eggs of
the same batch were compared. If the eggs laid by different in-
dividuals differ in size, it is possible that the same difference
may extend both to male and to female eggs, hence accurate
results could only be hoped for by comparing the eggs of the

396 Experimental Zoology

same batch. In any case it is not clear, even if large eggs pro-
duce fewer males and small eggs more males, whether the result
is due simply to the size determining the sex, or whether
the female eggs tend to become larger than the male eggs.
If analogy has any value in this instance, it seems more
probable, from the cases of Phylloxera, Dinophilus, and
Hydatina, that sex may be sometimes predetermined in the
egg and this determines its size. The question needs, however,
a reéxamination.

Ratio of Nucleus to Cytoplasm

In a recent contribution Richard Hertwig has argued that
sex is determined by the relation of nuclear size to cell size. He
bases his argument on a supposed analogy with certain condi-
tions found in the protozoans, and on some experiments of
his own and of his pupils on Dinophilus, Daphnia, and Frogs.
The experiments on Daphnia have been already given. Hert-
wig thinks that after a succession of parthenogenetic generations
the nucleus’ in the egg-cell slowly increases —to judge by
analogy with the protozoa —and this leads toward the pro-
duction of males. This tendency may be held in check by
a high temperature, but if the temperature is lowered, males
appear. The argument is clearly based on a highly hypothetical
comparison, and lacks direct observation in its support. Issa-
kowitsch’s experiments seem to show that lack of food and not
temperature determines the turning point in the life-cycle of
Daphnia.

Hertwig’s experiments with frogs are very inconclusive, and
so far as they go lend little or no support in favor of his view.
Eggs of Rana esculenta, that had been forced to premature ripen-
ing, fertilized by a male already at its full sexual ripeness, gave
the following results. Of 40 individuals that reached the frog
stage, all were males. There was, however, a high rate of
mortality, and the results may only mean that all the females
died. In another experiment some eggs were fertilized at once,
others after 8 hours. The latter Hertwig thinks must have been

LN HLL LULLUTS UL DEA Determination 397

“overripe.”” Only a small number of them segmented. The
next table gives the results : —

Normally laid eggs, Culture B go females 78 males
Normally laid eggs, Culture C 21 females 89 males
Normally laid eggs, Culture D 84 females 189 males
Overripe egg, Culture E 13 females 317 males

The excess of males is much greater in the overripe set. Hert-
wig concludes from these data that the overripe condition favors
male production, because the nucleus undergoes an increase
in size during the period of overripening. The improbability
of this assumption is manifest when we recall that when the eggs
have reached the uterus, the nucleus has disappeared as such,
and the second polar spindle has formed in the egg. In regard
to the results from the eggs prematurely fertilized, Hertwig thinks
that they can be accounted for by assuming that the cytoplasm
has not fully developed at this time; but this assumption also
seems improbable, since the eggs of the frog reach their full
growth in the autumn of the year preceding their deposition.
Hertwig’s attempt to bring these results —as well as others
relating to the influence of temperature as a sex-determining
factor in the frog’s egg — seems forced, and his general theory
finds little support, I think, in the outcome of his experiments.

The Extrusion of the Polar Bodies, and the Analogous Process
in the Sperm-cells

The fact that the egg throws off two polar bodies before it
begins its development has led to much ingenious speculation
in modern times. Amongst other views that have been suggested
as to the meaning of this “‘maturation”’ process it has been urged
that in one or both of the polar bodies the male or the female
element is ejected. This view was first proposed by Minot in
1877, and adopted later by Balfour, 1879, and by van Beneden,
1883. More recently Castle has offered a more elaborate hy-
pothesis that rests on the same foundation.

The discovery that most eggs extrude two polar bodies was

398 Experimental Zoology

followed much later by the discovery that a process similar to the
formation of the polar bodies takes place in the male germ-cells.
This seemed to be in contradiction to the hypothesis that the
elements of the one or of the other sex are ejected from the egg,
but the difficulty has not been found theoretically insuperable.
An examination of what takes place during the “maturation divi-
sions” of the egg and the sperm-cell will make this clearer.

When the egg nucleus breaks down, preparatory to the forma-
tion of the first polar body (Fig. 25, 3), it is found that the chromo-
somes are only half as many as were present in the earlier or odgo-
nial divisions of the same germ-cells (Figs. 1 and 2). It is generally
admitted that this reduction in number is due to the chromosomes
having united in pairs. It is also supposed that, during this
pairing of the chromosomes, one derived from the male parent
(the paternal chromosome) unites with its homologous maternal
chromosome. In cases where the chromosomes are of very un-
equal sizes, there are, with rare exceptions, two of each kind in the
early germ-cells, one derived from each parent, and those of the
same size are supposed, as stated, to pair with each other (Fig. 3).
Therefore when the polar spindle is formed, the chromosomes,
half in number, really represent double chromosomes. These
chromosomes separate again when the first polar body is given
off, half going to one pole and half to the other pole of the spindle
(Fig. 4), so that some of the chromosomes in the first polar body
are maternal and the others paternal chromosomes. If all of
the paternal chromosomes were turned toward one pole of the ©
spindle, and all of the maternal toward the other pole, there would
be a complete separation of the chromosomes derived from the
mother from those derived from the father. But for certain
theoretical reasons it is supposed that this does not take place,
but that there is generally a haphazard separation leading to a
mixture of maternal and paternal chromosomes at each pole,
although one of each kind is present at each pole.

The chromosomes that remain in the egg become quickly
arranged on a new spindle (Fig. 5). Each then splits length-
wise, as in ordinary cell divisions, and a half of each goes to one

5 6

Fig. 25. — Diagram to show formation of polar bodies in egg: 1, early germ-
cell, o6gonium, wilh whole number of chromosomes — paternal, black dots ;
maternal, clear rings; 2, division of odgonial cell; 3, first polar spindle ; 4, first
polar body; 5, second polar spindle and division of second polar body; 6, egg .
after extension of polar bodies.

399

400 Experimental Zovlogy

pole and the other half to the other pole, and the second polar
body is formed (Fig. 6). Thus while the first division is a déj-
jerential division, the second is an equation division. It is
supposed, also, that in some eggs the above order is reversed,
and the first division is an equation division, and the second
a differential one. The end result is the same in both
cases.

An exactly parallel series of events takes place in the formation
of the spermatozoén. The early sperm-cells, the spermato-
gonia, contain the full number of chromosomes. These pair, as
in the egg, and two cell divisions take place later, — one a differ-
ential, and the other an equation division. In the case of the
sperm-cells, however, all four cells become functional sperma-
tozoa, while-in the egg the polar bodies do not develop. The
first polar body often divides. The three polar bodies and the
egg are equivalent to the four spermatozoa.

What support do these results give to the hypothesis that sex
is determined at one or at both of these divisions? The facts
so far stated furnish no real support for this idea, since the
differential division merely separates a paternal from a maternal
chromosome; and if we do not confuse paternal with male pro-
ducing, and maternal with female producing, there is nothing in
the facts, as I have said, that supports the idea that this division
is a sex-determining one. In regard to the equation division,
the observations show that the chromosomes split into exactly
like halves, and there is also nothing here to support the idca
that one half is male and the other half female determining,
for this division resembles, outwardly at least, all other equation
divisions of the body-cells that are supposed to produce equiva-
lent parts.

If this were the only evidence we possessed in regard to the
maturation divisions, it would lend no support to the hypothesis
that the sex of the egg or of the sperm is determined at this time;
but there are some recent observations showing that in a few

_groups two kinds of spermatozoa are formed, so far as their
chromosomal contents is concerned, and we must now examine

Internal Factors of Sex Determination 401

the evidence to see whether, in these cases, sex is determined by
the kind of division that takes place in the sperm.

The Formation of Male and Female Producing Spermatozoa

It has been known since 1836 that two kinds of spermatozoa*
are present in the snail Paludina, and both kinds have been found
in the upper part of the oviduct where the egg is fertilized, but
whether both kinds are functional is not known. Meves has
studied recently the method of formation of these two kinds of
spermatozoa. The hair-like spermatozoa resemble the ordinary
forms of spermatozoa, the worm-like spermatozoa have a long,
rounded, worm-like shape, as the name implies. In the forma-
tion of the worm-like form, a peculiar and perhaps a degener-
ate process occurs. Instead of containing the reduced number
(seven) of chromosomes, it gets only a single chromosome.

Two kinds of spermatozoa are also described by Meves in the
butterfly Pygera. One of the two kinds contains no chromatin
material whatever. It is headless and perhaps functionless also.

In addition to these cases giant spermatozoa, double the size
of the normal ones, have been described in several groups, espe-
cially in amphibians and birds. They are due to the incom-
pleteness of one or of both of the spermatocyte divisions, and
may be only abnormal forms incapable of fertilizing the egg.

The most remarkable spermatozoa, however, are those in
which half of the spermatozoa contain one more chromosome
than the other half.

Henking discovered in 1890 the presence of a peculiar chro-
matin-like body in the spermatogenesis of the bug Pyrrhocoris.
It has since been shown that this body is a true chromosome,
although it behaves differently from the other chromosomes in
certain respects. The more recent work of McClung, Mont-
gomery, Paulmeier, Sutton, Stevens, and especially of Wilson,
has cleared up some of the most important questions connected
with the behavior of the accessory chromosome.

' Discovered by von Siebold.

2D

402 Experimental Zoology

During the early divisions of the sperm-cells, the accessory
chromosome behaves like an ordinary chromosome, 7.e. it divides
into equal parts at each division. But in the two final divisions
of the male cells, — the spermatocyte divisions, — the accessory
chromosome shows certain peculiarities. It can sometimes be
distinguished from the other chromosomes by its slightly differ-
ent affinity for stains, by its inertness in one of the divisions, by
its forming at times a separate vesicle, and especially by the
fact that at one of the spermatocyte divisions it fails to divide
when the other chromosomes divide. It passes bodily to one
pole of the spindle; hence only half of the spermatozoa contain
the accessory.

It has been shown by Wilson with great probability that the
failure of the accessory to divide, when the others do so, is due
to the fact that it has no mate, as have all the other chromosomes,
so that when the other pairs separate the accessory remains
single. It has been suggested by other observers that the acces-
sory is a double chromosome, like the other pairs, but that it
fails to separate into its components when the others so separate.
This view seems improbable, because Wilson has found a con-
tinuous series of species in some of which the accessory has a
partner of equal or of slightly smaller size, while in others the
difference in size is very unequal — one being almost a vanish-
ing chromosome. We can imagine that one step farther would
lead to the complete disappearance of the mate of the acces-.
sory, so that it would have no pair. Whether the disappear-
ance is really due to its vanishing away, or to the absorption
by its mate, we cannot even surmise with any degree of
probability. If the latter view should prove correct, the
accessory is a double or fused chromosome, but not formed
by temporary union as are the others at one time, but by a
permanent fusion.

McClung offered the suggestion that the difference between
the two kinds of spermatozoa, with and without the accessory,
is connected with sex determination. His argument was that
since there are two kinds of sexual individuals, and two kinds

Lnternal Factors of Sex Determination 403

of spermatozoa, presumably both functional, there may be a
connection between the two conditions. This tacitly assumes
that all of the eggs are alike, which was apparently taken for
granted by McClung, but not examined. Since the accessory
is found in one half of the spermatozoa, McClung conjectured
that those spermatozoa that contained the accessory produce
males, presumably because of the seeming necessity for the new
males to have an additional chromosome, the accessory. This
argument proved invalid in the light of the observation of
Wilson, that the female and not the male contains an additional
chromosome, and of Stevens that the female of Tenebrio molitor
contains the large chromosome (the homologue of the acces-
sory) and the male the small chromosome.

In order to show how this process works itself out, let us take
the case of Anasa tristis, in which there are 21 chromosomes in
the male somatic cells and 22 in the female. There will be
rr chromosomes in half of the spermatozoa and ro in the other
half. All of the eggs may be supposed to contain 11 chromo-
somes. When a 1o-chromosome spermatozoén fertilizes an egg,
there is formed a male with 21 chromosomes, whose sperm will
again contain 10 and 11 chromosomes; when an 11-chromosome
spermatozoén fertilizes an egg, there will be 22 chromosomes —
the new individual will contain 22, and its eggs will later all
contain 11 chromosomes. The accompanying scheme shows
this result graphically : —

Spermatozoén Egg Male Germ-cells
Io ————> II = 21 ———>_ roorrr
or or Female
II ————-, III = 22 —_+> rrorir

The evidence that the eggs are all alike and contain the larger
number (z.e. 11 in the case just cited) is indirect, but apparently
conclusive. The chromosomes of the polar spindles have not
been counted, but the somatic cells contain 24 chromosomes,
as well as the early germ-cells before the pairing of the chromo-
somes. Moreover, in those cases in which the accessory of the

404 Experimental Zoology

male can be identified by its size, two such chromosomes have
been shown by Wilson to exist in the egg.

In these cases we have conclusive evidence that sex is asso-
ciated with certain kinds of spermatozoa. Two important
considerations arise: first, in what way does this take place;
and second, can we extend the same conclusion to other forms?
Let us examine in turn these two questions.

How does the spermatozoén with the accessory produce the
female? Two possibilities immediately suggest themselves,—
either the accessory chromosome contains the elementary char-
acters of the female sex, or it produces its results quantitatively.
The former alternative rests on the assumption that the chro-
mosomes contain the unit-characters of the germ-cells. It is
this view of the chromosomes that in general meets with wide
favor at the present time. Historically it goes back to the
gemmule hypothesis of Darwin, the pangenes of de Vries, the
biophors of Weismann, etc.—not to give other examples.
Those who believe that the characters of the species are con-
tained in living units of the germ that reside in the chromosomes,
and wander outward into the cell to bring about the cytoplas-
mic differentiation, will probably accept this first alternative in
regard to the influence of the accessory; but those who find this
view improbable will be more inclined to the alternative view.

There is one objection to the idea that the accessory carries
the female pangenes, that Wilson has pointed out, and which
may bea serious difficulty for this view. If we follow the history
of the accessory, we find that it passes from the female-producing
sperm into the egg to form there one of the pair of chromosomes
homologous with the accessory. In the next generation the
homologous chromosomes in the egg are supposed to separate
again, one going into the polar body and one remaining in the
egg. Now on the theory of chance, either one of the pair might
remain in the egg, and should it happen to be the ‘‘male-deter-
mining” chromosome, the egg should give rise to a male if en-
tered by a male-determining sperm; but if entered by a female-
determining sperm (i.e. with the accessory), the two chromosomes

Internal Factors of Sex Determination 405

would have exactly opposing influences. Apparently the only
way out of the dilemma is to assume selective fertilization on the
part of the sperm, so that only male-determining spermatozoa
(i.e. those without the accessory) can enter eggs containing
the male chromosome, and only female-determining (i.e.
those with the accessory) can enter eggs containing the fe-
male chromosome. Thus the primary assumption forces us to
make another one that lacks at present evidence in its support.
Moreover, instead of offering us a simple explanation of sex-
determination, the hypothesis seems unnecessarily complex,
since it postulates that both eggs and sperm are male or female
producing, while theoretically the result could be as readily accom-
plished by one of them only being male or female producing.

The complication is primarily due to the assumption that the
accessory chromosome is a carrier of female pangenes or bio-
phors. If we reject this assumption, we find, I think, a simpler
explanation of the influence of the spermatozoén carrying the
accessory in determining sex. Suppose we assume that the
result is due to the greater amount of chromatin brought
into the egg by the sperm with the accessory. It will make
no difference on this assumption which one of the homologous
accessory chromosomes happens to be present in the egg. The
result is parallel to that of the bee, in the sense that the fertilized
egg contains more chromosomes than the unfertilized, and pro-
duces in consequence the female. In the absence of all knowledge
as to how the greater quantity of chromatin produces a female,
one is tempted to assume that the result is reached through as-
similative changes that take place in the early cells, and there is
some evidence in favor of the view that one of the main func-
tions of the chromatin is to carry on the assimilative processes
in the cells.

The egg of the bee if fertilized produces a female, if unfer-
tilized a male. This relation has led a number of zodlogists
to conclude that all the eggs are male, but become female on
fertilization, since the female element introduced by the sperm
dominates the male elements of the egg. The spermatozoa con-

406 Experimental Zoology

tain from this point of view the female elements. The conclu-
sion seems plausible, if we believe that the male and female ele-
ments as such become separated in the spermatozoa and eggs.
It seems to me, however, that a simpler hypothesis may be for-
mulated. The sex of the embryo is not laid down as such in
the egg or sperm, but may be determined later by the quantitative
relation resulting from the activity of the chromatin in the cells
of the embryo: not in the sense in which Richard Hertwig has
expressed this relation, viz. as depending on the size of the
nucleus, but rather as the result of the increase in the assimilative
function of cells containing more chromatin material. On the
other hand, if the number of chromosomes in the unfertilized
egg of the bee become doubled before segmentation, the fer-
tilized and unfertilized egg will both produce cells having the
same number of chromosomes, and it is difficult to see how a
purely quantitative difference exists. It remains, however, to
be shown what really takes place in the drone eggs.

Since all intermediate stages have been found by Wilson and
by Stevens between cases where the accessory is single and where
it hasa partner of unequal or of equal size, it may seem probable
that even when we cannot recognize it by its singleness or by
its size difference, it is still a sex determinant. This argument
would be valid if it could be established that unit-characters are
carried by individual chromosomes; but if the influence of the
chromatin as a sex determinant is purely quantitative, the argu-
ment based on a continuous series loses its force; for as soon
as the difference in size ceases, the quantitative factor disappears.
However this may be, the importance of the discovery of the
accessory in sex determination should not be minimized; for
it is the one clear case in which an internal factor has been
found that is associated with sex production."

LITERATURE, CHAPTER XXVI
(See Chapter XXVII.)

+ The case of the bee and the ant that have long been known furnish another
and perhaps a parallel case.

CHAPTER XXVII
THE INTERNAL FACTORS OF SEX DETERMINATION (Continued)

The Origin of Gynandromor phs

THERE are occasionally found, especially in certain groups of
insects, individuals that have the characters of the male on one
side of the body and of the female on the other. Such cases
occur most often in bees, ants, and butterflies; and since in the
first, at least, it has been shown that the female sex is determined
by fertilization, and the male by the lack of fertilization, it seems
probable that when both sexes appear in the same individual
that this may be brought into connection with the fertilization.
These mixed sexual individuals are known as gynandromorphs.
The most celebrated case of this sort is that of the Eugster hive
of honey bees, studied by von Siebold. Similar instances had
been seen before, but von Siebold studied the forms more thor-
oughly, and brought his results into connection with Dzierzon’s
theory in regard to sex determination in bees.

The hive in which these gynandromorphous bees occurred
contained an Italian queen bee and German drones. The
workers that were produced were therefore hybrids. It is not
known whether the hybrid character of the workers had any-
thing to do with the frequent occurrence of gynandromorphism
amongst them. Purely bred colonies have also produced these
abnormal forms. The gynandromorphs showed the male
characters sometimes on the right side of the body and the fe-
male characters on the left, or vice versa; sometimes the ante-
rior end was male and the posterior female. Sometimes the
male and female character occurred in different parts of the
same organ, as in an eye or in an antenna. The normal worker

407 .

408 Experimental Zoology

has a sting, the drones are without this organ. When the ab-
domen of the gynandromorph was like that of a worker, the
sting was perfectly developed; but if the abdomen was more
or less like that of a drone, the sting was deformed and soft;
and when the abdomen was entirely male in character, the com-
plicated capulatory organs resembled completely those of a nor-
mal drone, and the sting was absent.

In regard to the internal reproductive organs, great irregularity
was found to exist. No definite relation was observable between
the kind of the somatic part and its contained reproductive
organ. Male and female parts were often combined in the re-
productive organs themselves, ovarian and sperm chambers
being united in the same organ. Ina few cases, where externally
a normal male genital apparatus was present, ovaries and ovi-
ducts occurred.

Von Siebold thought that these results could be explained in
accordance with the Dzierzon theory on the grounds that an
insufficient number of spermatozoa entered the egg so that parts
of it lacked sufficient quantities of the male element. This
view is, in principle, the same that has been used in more recent
times to explain the same result; although, owing to our com-
pleter knowledge of what takes place in fertilization, we can
bring the interpretation into better accord with modern views.

Gynandromorphs have also been found in other groups of
Hymenoptera. There are some eighty cases recorded by Dalle
Torre and Friese in 1899. Wheeler has recently reviewed all
previously recorded cases in ants and described some new ones.
In the group of butterflies and moths as many as 1074 cases are
recorded by O. Schultz.

Boveri has offered the following theory to account for the ap-
pearance of gynandromorphous forms. He assumes that the
spermatozoon fails to unite with the nucleus of the egg, but after
the female nucleus has divided, the male nucleus conjugates
with one of the two resulting nuclei. Consequently there will be
present in the embryo two kinds cf nuclei, — one kind derived
from the single nucleus resulting from the first division, and the

Internal Factors of Sex Determination 409

other kind resulting from the united nuclei. The former should
produce only male parts, because this is what happens when the
egg is not fertilized; the latter should produce female parts,
as does a fertilized egg. In other words, the results in the differ-
ent parts of the single, imperfectly fertilized egg are the same as
those formed in unfertilized and fertilized eggs.

I have pointed out that the results are capable of being equally
well explained in another way. It has frequently been noted
that two spermatozoa occasionally enter the egg of the bee. If
we suppose one of these only conjugates with the female nucleus,
its products should produce female parts, while the other sperm
nucleus that fails to conjugate will, if it divides and produces
nuclei, give rise to male parts. Boveri’s view and my own are
not mutually exclusive: either process may at times take place
and produce gynandromorphs. Ihave pointed out how it might
be possible under certain conditions to determine whether the one
or the other process has taken place. In a case like that de-
scribed by von Siebold, in which the mother belongs to one race
and the father to another, the parts of the embryo that contain
the single nucleus should be like the mother on Boveri’s view,
and like the father on my own; in both, the parts supplied by
the conjugated nuclei produce a hybrid result. An example
may make this clearer. Let us suppose, as in von Siebold’s
case, that the queen is Italian and the male a German drone.
If one spermatozoén enters the egg and conjugates with one
of the first two segmentation nuclei, as on Boveri’s view, the
male parts of the embryo that come from the single segmentation
nucleus must be like the mother, z.e. they should have the char-
acter of the Italian bees; the parts that come from the conjugat-
ing nuclei should be hybrid. On my view two spermatozoa enter
the egg, one only conjugating with the egg nucleus. The male
parts will come from the single sperm nucleus, and will, there-
fore, be like the father, z.e. they will have the character of the
German bees; the parts that come from the conjugating nuclei
will be hybrid in character. Thus on my view the male parts of
the gynandromorphous hybrid will be paternal; on Boveri’s

410 Experimental Zoology

view, they will be maternal. At present we lack the data to
decide between these alternatives.

In other groups of animals, gynandromorphous forms are
great rarities. Weber described a finch that had feathers like
the female on the left side and like the male on the right. There
was also an ovary on the left side and a testis on the right. Un-
til we know more of the conditions that determine the sex of
birds, it is useless to speculate about this case.

The Sex of Multiple Embryos

Another group of facts discovered in other hymenoptera seems
to show that the sex of the embryo is already determined in the
egg. The chalcid fly (Ageniaspis fuscicollis) lays its eggs in the
egg of amoth. Both eggs develop, the latter into a caterpillar
and the former into its parasite. The parasite develops in a
most remarkable manner. Marchal (1904) found that from a
single egg a chain of embryos develops, and these embryos are
all of one sex. Bugnion had already (1891) observed that the
individuals that emerge from a single caterpillar are generally of
one sex, and both authors have interpreted the result to mean that
if the egg has been fertilized, a chain of female individuals is
formed ; butif the egg is not fertilized, males are produced. More
recently Silvestri has studied the early development of another
insect (Litomastix truncatellus), and finds, in fact, evidence
showing that the eggs may or may not be fertilized, and that it con-
tinues to develop in either case. Silvestri finds that a single egg
may produce a thousand sexual individuals, and also several hun-
dred sexless larve that lack the circulatory and respiratory system.
The details of the early development of these forms are remark-
able, but cannot be considered at present. The point of espe-
cial importance is the conclusion that the sex of the embryo
must be determined at an early stage, and is not later altered,
since all the sexual embryos of a chain are of the same sex.

It is true that there may sometimes emerge from a caterpillar
both males and females, but this is supposed to be due to two eggs
—one fertilized, the other not —having been laid in the same egg

Internal Factors of Sex Determination 4II

of the moth. Silvestri has seen the same egg pierced more than
once by the same fly, and we may readily suppose that two
individuals often deposit their eggs in the same egg of the
moth.

A closely similar mode of development occurs in another
species, Polynotus minutus, which deposits its eggs in the larva
of Cecidomia destructor, the Hessian fly. Marchal found in
fourteen cases that the flies that emerge from the grub are all
of one or of the other sex, while in two cases there was a mixture
of both sexes.

The Sex of Human Twins and Double Monsters

There remains to be considered another class of facts, not dis-
similar from the last, that appear, if certain modern assumptions
are correct, also to mean that the sex of the embryo is already
determined in the fertilized egg, and is not affected by subsequent
events. I refer to the case of human twins. It is said that two
kinds of twins occur: in one kind the individuals are not more
alike than any two children born at different times: these are
fraternal twins. They may be of the same sex or different sexes,
and not more often of the same sex than of different sexes. They
are supposed to arise from two eggs simultaneously set free from
the ovary. In the other kind of twins, the two individuals
closely resemble each other —so closely, in fact, that they may
scarcely be distinguishable apart bytheir own parents. These
“identical”? twins are said to be always of the same sex, and it
has been suggested that they arise from the same egg that has
become separated into two parts at some stage in its develop-
ment. ‘This view seemed all the more plausible because in the
last decade it has been shown experimentally for some other eggs
that when the first two cells are separated each gives rise to an
entire embryo. This is true for the eggs of sea urchin and star-
fish, and in the vertebrates for the egg of amphioxus, the sala-
mander, and the fish. We do not know in these cases that the
isolated cells would produce individuals of the same sex, nor
do we know in man that the cells are really separated; but

412 Experimental Zoology

putting the different facts together it seemed plausible, as I
have said, that identical twins arise in this way.

There is a further fact that supports this hypothesis. Abnor-
mal embryos are sometimes born, composed of two individuals
united in various ways.’ These are the so-called double mon-
sters, and in the cases in which the sexual organs are separate
they are always of the same sex.

In the case where there are three offspring at the same birth,
triplets, all three may be identical, or two may be identical and
the third only fraternal, or they may be all fraternal. The iden-
tical twins are of the same sex, and on the hypothesis they come
from one egg, while when two only are identical the third may
be of the same or of the other sex. There is at least one authen-
tic case in which all three individuals were of the same sex,
and when they grew up were closely similar.

Jehring has confirmed a statement made on hearsay by Azara,
to the effect that in the armadillo (Tatusa hybrida) of Paraguay
the offspring of one birth are all of the same sex. In two preg-
nant females examined by Jehring all eight young were males.
Each embryo had its own aminon, but all were inclosed in a
common chorion. The latter fact is supposed to furnish con-
clusive evidence that the embryos must have arisen from the same
egg; but while this is probable, it has not been shown that the
chorions of different individuals might not unite into a common
envelope. The same reasoning has been applied to the case
of identical human twins that are supposed to be always in-
closed in a common chorion.

The idea that the two sorts of twins differ in the respects
noted above seems to have been noticed by Dareste in 1874,
and by Fisher in 1866; but Galton (1891) has especially drawn
attention to this difference. Recently Thorndike has disputed
the assumption that there is a sharp distinction between identical
twins and ordinary or fraternal twins. Thorndike finds as the
result of an examination of a large number of cases that there is
a perfect gradation between identical and fraternal twins. He

? Wilder (1904) has brought together the records of such cases.

Luternal Factors of Sex Determination 413

finds, moreover, that “identical” twins may be of opposite sexes.
He believes, therefore, that the closer resemblance sometimes
observed in children born at the same time is due to the similar
conditions of the germ-cells of the parent at the time of concep-
tion, or to the more nearly similar environment under which the
twins are reared. These two conditions he includes in what
he calls the “heredity” of the offspring. In general, the term
“heredity”’ has a different meaning to biologists, viz. to express
the idea of the inheritance derived from the germ-cells apart
from the special environment to which they or the offspring
that they produce have been subjected. Disregarding, however,
this difference of definition, the facts recorded by Thorndike
cast serious doubts on the assumption that there is a sharp dis-
tinction between the two kinds of twins. If the conclusion is
substantiated, we have little evidence left on which to base the
assumption that identical twins owe their resemblance and their
sex to a common origin. If then, as seems probable, identical
twins, double monsters, and the like are more often of the same
sex, it would appear that special external conditions existing at
the time in one or in both parents determine the sex of the
embryo. Improbable as this may seem, a careful reéxamina-
tion of the evidence should be made.

Two Recent Theories of Sex Determination based on the
Assumption of Male and Female Eggs

Beard has proposed an hypothesis of sex determination that
rests on the assumption of male and female eggs. He suggests
that the sex of the individual is determined by the egg alone.
According to his view, the egg and its two polar bodies are of the
same sex, male or female. Just as there are two kinds of eggs,
there are assumed to be also two kinds of spermatozoa; but these
have no function in respect to sex determination, and Beard as-
sumes, in fact, that one kind has even lost its power to fertilize
the eggs. When the female egg is fertilized it gives rise to a
female, and the male egg toa male. The female produces eggs
again of two kinds, male and female eggs; the male two kinds of

414 Experimental Zoology

sperm, one of them functionless, etc. It is apparent, therefore,
that Beard’s theory resolves itself into two assumptions: first, that
sex is determined by the egg alone; and secondly, that two kinds
of eggs are produced. The solution of the problem is merely
shifted to a new field, since it is assumed that the male and female
eggs are produced, and no explanation of how or when this occurs
is forthcoming.

Beard’s view encounters, moreover, special difficulties that
can only be explained away by further assumptions. For ex-
ample, in the aphids there is a long succession of female partheno-
genetic eggs, ending finally in the production of male and female
parthenogenetic eggs. It would seem that external conditions
must determine whether the eggs are to be all female eggs or
some male and some female, but there is nothing in Beard’s
theory that indicates how such a thing is possible.

In the case of the bee, Beard is forced into the position of as-
suming that the sex of the individual is not dependent on whether
it is or is not fertilized, although the clearest evidence that we
have points unmistakably in that direction. He argues that
only female eggs can be fertilized, while male eggs cannot be
fertilized; but if this were the case, it is not evident how the
queen could distinguish between the two kinds of eggs and lay
each in its appropriate cell.

Castle has also proposed a hypothesis of ‘The Heredity of
Sex.” His hypothesis is avowedly an attempt to account for
the determination of sex by means of the Mendelian method. -
It had already been suggested by Strasburger and by Bateson
that the sexual forms might bear the same relation to each
other as do the offspring of Mendelian hybrids. Castle has
elaborated this idea into an ingenious hypothesis. He assumes
that there are two kinds of spermatozoa, male and female;
and two kinds of eggs, male and female. If we assume, as
in the case of Mendelian hybrids, that all possible chance
combinations occur between the germ-cells, we should expect
three kinds of individuals (as in the Mendelian proportion), —
males, females, and hermaphrodites. There would be as many

Lnternal Factors of Sex Determination 415

of the latter as of the other two combined, as shown in the
formula : —
Spermatozoa M+F

Eggs M+F
IMM + 2MF + 1FF

Since we get no such results in animals, the Mendelian scheme
must evidently be altered if it is to be made use of as an hypothe-
sis of sex. Therefore Castle assumes that male spermatozoa
can fertilize only female eggs, and that female spermatozoa can
fertilize male eggs. Hence every individual is, as it were, a sexual
hybrid, since it contains both male and female elements. But
what determines the sex of the individual? Castle’s hypoth-
esis gives usno answer. It seems to me, therefore, that despite
the assumptions of male and female spermatozoa, and of male
and female eggs, and the further assumption of selective fertili-
zation, the hypothesis does not advance us toward an explana-
tion of sex determination. If it be claimed that the hypothesis
deals only with the heredity of sex rather than with its determi-
nation (although Castle places no such limitation on his view),
it is still not apparent what the hypothesis accomplishes, since
it assumes a separation of the sex-elements (which is entirely
hypothetical) only in order to bring them together again by means
of selective fertilization (likewise an assumption). Since we
have some evidence in favor of the view that both sex elements
are carried by the sperm as well as by the egg, the heredity of
sex is accounted for without any additional hypotheses.

The Reduction Process in Parthenogenetic Eggs

The question of the number of the polar bodies extruded in
parthenogenetic eggs has also played a conspicuous réle in
modern speculation concerning sex determination. Weismann
was led by certain theoretical considerations to examine the eggs
of parthenogenetic species. He found only one polar body given
off in the parthenogenetic eggs of daphnia. This led to the
idea that the second polar body may act like the spermatozoén,

416 Experimental Zoology

and, being retained, brings about self-fertilization. Blochmann
showed, however, in the honey bee, that two polar bodies are
thrown out from the drone eggs, yet they develop without fer-
tilization. In the rotifer Asplanchna, it has been shown by
Erlanger and Lauterborn that while the female parthenogenetic
egg gives off only one polar body, the male egg gives off two.
These facts show that there is no universal rule in regard to the
number of polar bodies that are extruded by parthenogenetic
eggs. Furthermore, the fact that in hermaphrodite animals two
polar bodies are always extruded also shows that the problem of
sex determination is not necessarily connected with this process.
It would be erroneous, however, to conclude from these cases that
the retention of one polar body may not in certain species be a
possible factor in sex determination, although not necessarily
because of the retention of male or of female elements.

Since half the inherited chromosomes are supposed to be given
off in one or the other polar body, the question arises whether
this happens in parthenogenetic eggs in those cases where a
single polar body is produced. In the case of the aphids, Stevens
has shown that in the division that leads to the formation of the
single polar body of the parthenogenetic cycle, the full number of
chromosomes is present, and the division of the chromatin is an
equation division. ‘There has been no pairing of the chromo-
somes to give the reduced number, preparatory to the polar body
division, as in sexual eggs. Hence the full number of chromo-
somes is present in the polar spindle. I have obtained the same
result in phylloxera; both in the eggs that do not make the sexual
forms, and in the male and female producing eggs also. Since
the latter produce either male or female individuals, it seems
unlikely in these cases, and probably by inference in fertilized
eggs also, that the determination of sex is necessarily connected
with this division.

At the second or differential division when the other polar body
is formed, it is supposed that only the maternal and paternal
(united) chromosomes separate. There is nothing, therefore,
in this process to suggest that it is connected with sex differentia-

Internal Factors of Sex Determination 417

tion. In the case of the unfertilized egg of the bee it has been
shown by Petrunkewitsch that the half (reduced) number of
chromosomes is left in the egg. He states that there occurs a
doubling of the number of chromosomes before the first seg-
mentation nucleus is formed, so that the full number is restored.
This result is not in harmony with Meves’s study of the sperma-
togenesis, and as Giglio-Tos has pointed out the whole question is
in such a confused state that we must await further observations.

Influence oj the Cytoplasm

Most modern theories of sex determination have been based
on changes in the nucleus rather than in the cytoplasm. Yet
since the differentiation of the cell takes place in the cytoplasm,
it may seem that the initial differences might be traced to that
part; but owing to the fact that the different parts of the un-
differentiated cytoplasm show little differences in staining capaci-
ties, it has not been possible to gather many facts that can he
utilized in the formation of a cytoplasm theory. One considera-
tion above all others has, I think, led modern cytologists to re-
gard the nucleus rather than the cytoplasm as the initiator in the
formation of the characters of the embryo. It has become an
unchallenged or seldom questioned dictum that the nucleus
must transmit the qualities of the germ-cells, because the quali-
ties of the father are brought into the egg by the spermatozoén,
and it is assumed that the nucleus of the spermatozo6n is the
only part of it that contributes anything worth considering to
the fertilized egg. Furthermore, since the nucleus of the sperma-
tozoén consists of almost solid chromatin, it has been inferred
that the chromosomes alone carry the hereditary qualities of
the germ-cells. But a consideration of all the facts will show,
I think, that there is at least a possibility that the protoplasm of
the sperm-cell may have something to do with the transmission
of the hereditary qualities of the father. If we trace the history
of the spermatozoén, we find that at no stage is all of the cyto-
plasm of the germ-cell from which it arises totally eliminated from

the cell. It is true that a part of the cytoplasm —it may bea large
2E

418 Experimental Zovlogy

part — is used in the formation of the tail of the spermatozoén,
which may or may not enter the egg; but some of the cyto-
plasm forms at least the centrosome and is carried into the egg.
Owing to our inability to follow the history of this cytoplasm
in the egg, we have come to ignore its existence, while the nucleus
that can be easily traced has occupied exclusively the attention of
modern embryologists with rare exceptions.

If we admit that the cytoplasm of the spermatozoGn is not only
brought into the egg, but slowly increases in quantity there, can
we find any clew in such a condition that will help in the solu-
tion of the problem of sex determination? There are several
possibilities that should be considered. If we suppose that there
is predetermined male and female cytoplasm in the egg or
the spermatozo6n, or in both, we encounter precisely the same
difficulties that have been met with by assuming that predeter-
mined elements of these two kinds exist in the nucleus. From
this point of view the two assumptions stand on the same footing.
If we assume that the determination of sex lies in the cytoplasm,
not in the form of predetermined elements male and female,
but as one of the alternative conditions of differentiation of the
cytoplasm, we must still explain what factors — external or in-
ternal — determine whether the one or the other condition shall
dominate. Whether this is due at times to a relation between
the chromatin and the cytoplasm, or at other times to a relation
depending on the condition of nourishment of the cytoplasm,
etc., cannot be stated. However probable it might be made to
appear that the differentiation that appears in the cytoplasm
really originates there, and not in the nucleus (except in so far
as the latter induces important changes in the cytoplasm through
its assimilative changes), the fact remains that we cannot ex-
plain the mechanism in the cytoplasm through which the one or
the other condition comes to be the dominating one. Here, as in
the case of the nucleus, we are too ignorant at present of the real
chemical changes that take place to permit of more than purely
speculative views.

Ziegler has recently proposed an hypothesis of sex determina-

Lnternal Factors of Sex Determination 419

tion that rests on the assumption that the chromosomes that
arise from a female individual have a greater tendency to pro-
duce a female; and those that originate from a male individual
have a greater tendency to produce a male. Since the child
gets as many chromosomes from the father as from the mother,
the parental chromosomes as such cannot determine the sex.
But it is to be recalled that amongst the parental chromosomes
some have come from the grandfather and some from the
grandmother. The relative number of chromosomes from the
maternal and paternal lines will be variable in number on the
current assumption that at the reduction division it is merely a
question of chance which member of a pair of homologous chro-
mosomes goes to one pole of the spindle and which to the other.
If the chromosomes of the grandfather predominate in the
offspring, it will be a male; if the grandmother chromosomes
predominate, a female develops.

To take an example. If the somatic number of chromosomes
for the human species be assumed to be 24, the child gets 12
from the father and 12 from the mother. If amongst the former
there are 8 grandmother chromosomes and amongst the latter
7 grandmother chromosomes, the child will be a girl, for there
are at least 15 of the 24 derived from the grandmother’s
side. °

On Ziegler’s theory of sex it is evident that whenever the re-
duced number of chromosomes is even, there may occur an exact
balance of grandmother and grandfather chromosomes, hence
the child can have no sex at all. In the list of cases given by
Ziegler himself, we find that the reduced number of the chro-
mosomes is an even one in 29 species and odd only in ro.

It seems improbable that the equal balance of the maternal
and paternal chromosomes could be counterbalanced by the
presence of chromosomes derived from the grandparents, espe-
cially since these have also been contained in one or the other
parent whose sex, on the theory, should have influenced them to
acquire the character of that parent. These and other diffi-
culties make Ziegler’s hypothesis very improbable.

420 Experimental Zoology

Conclusions

We have examined the principal data that bear on the problem
of sex determination and have found that we still lack the clew
that solves the riddle. Nevertheless many important facts that
bear directly on this question have been made known, and the
ground has been pretty well broken for future investigation.
Concerning the various hypotheses that have been proposed, we
may place them under two principal headings, which I shall call
the morphological and the physiological points of view. Let us
examine these in turn.

The Morphological Conception of Sex Determination

According to this view, the characters that stand for the male
or the female condition are represented in the germ-cells as pre-
existing elements. If we assume that the male characters are
eliminated from the egg or spermatozoén, the female characters
remaining, the egg produces a female; if the female elements are
ejected, the egg becomes a male. This method of treating the
problem has the advantage of simplicity, and is capable of dia-
grammatic treatment. It is the method that has been largely
followed by modern theorists. The same principle has been
followed in recent years in dealing with the entire problem of
heredity. The most serious objections to this procedure are
that it is based on an assumption that cannot be verified, and in
practice it has not solved, except in a purely formal way, any
of the problems of heredity. The problem instead of being
solved is merely shifted into an unknown field where the imagi-
nation has full sway and where the conclusions cannot be
tested.

A modified form of this same method of treatment is to regard
heredity as the result of the dominance of one or of the other sex-

characters, both unit-characters being assumed to be present in
the same egg and sperm. There are certain facts that seem to
indicate that this is an advance over the other point of view that
tegards the germ-cell as either purely male or female; but here

Internal Factors of Sex Determination 421

also we lack the clew that tells us in what condition the charac-
ters are represented in the egg.

The Physiological Conception of Sex Determination

From this point of view we may consider the protoplasm as
a substance capable of assuming one or another condition that
is determined, for the time being, by the environment or by inter-
nal conditions. We may suppose that the same end-result
(sex, for example) may be determined by different factors in
different species. JI mean that while in certain species one kind
of factor commonly determines the result, in other species other
factors may determine which of the possible alternatives is fol-
lowed. The protoplasm may be looked upon as being in a con-
dition of equilibrium as far as the adoption of either alternative
is concerned, and which one is followed is determined by those
conditions that bring to the front the one or the other possible
state.

An illustration may make this clearer. In certain male crabs
the right and left chele are different in structure. In some
species it is always the right-hand claw that is the larger, in
others the left, while in some cases either the right or the left is
the larger. It has been shown by Przibram for some species of
the latter class that if the large chela is removed the small one
becomes the larger at the next molt, and in place of the large one
removed a small one regenerates in its place. Both claws have
potentially the same possibilities of becoming large or small.
External or internal conditions determine which kind of develop-
ment takes place; but the selection once made, the differentiation
proceeds strictly along one line.

Now in this case we might assume that when the big claw of one
side is removed, all the preformed elements (primoidia) of the
big claw contained in all the cells of the small claw begin to
develop and dominate and replace the already-controlling-small-
claw-forming elements. That is the morphological way of ex-
plaining the result. On the other hand, the other — the physio-
logical — idea makes use of no such mechanism, but assumes

422 Experimental Zoology

that the protoplasm has two possible ways of differentiating
which are determined by local conditions, internal or external.
The same ways of conceiving the problem may be applied to the
case of sex determination.

Which of these alternative interpretations we may think more
probable, or more profitable as a working hypothesis, is perhaps,
at present, a question of personal choice; for we lack, as I have
said, the data to decide between them. For myself, the physio-
logical conception seems more in accordance with our general
ideas concerning development, and above all to be a conception
that is more stimulating and suggestive as a working hypothesis
than the morphological idea, which seems to be quite sterile as a
point of view leading to further investigation.

Analysis of the Results

An analysis of the evidence that has been given in the preced-
ing pages concerning sex determination suggests the following
considerations.

The average equality of males and females indicates, I think,
that external conditions do not regulate the result, but that some
internal physiological mechanism exists that determines the sex.’
This physiological mechanism does not involve the separation
of male and female elements or units in the egg and sperm, but
only involves the production of those conditions that determine
whether one or the other sex will develop. In some cases the
initiatory processes may exist in the egg, in others in the sperm,
and in still others after the union of egg and sperm. In other
words, in all species with separate sexes the potentialities of pro-
ducing both sexes is present in all eggs and in all sperm; but the
development of the one or the other sex is determined by some
unknown internal relation. From this point of view it is mis-
leading to speak of male and female eggs or sperm in the sense
that such eggs contain only male or female potentialities, for all

1 Of course this statement does not exclude the possibility that external in-

fluences may not determine that the internal mechanism shall become active
in one way or another as seen in the cyclical modes of reproduction.

Lnternal Factors of Sex Determination 423

eggs and sperm contain both potentialities, and which is realized
is determined by some internal relation that regulates the domi-
nance of one or the other sex.

This idea leads to the following suggestion: — In Mendelian
hybrids of the first generation, in which two contrasted charac-
ters are present, it is assumed that when the germ-cells are pro-
duced half of them show already the dominance of one character
and the other half of the other character. This leads, after hap-
hazard matings, necessarily to the three types that appear in the
second generation. In the case of sex only two types are pro-
duced, hence it is not possible to imagine that two kinds of eggs
and two kinds of spermatozoa are produced.! It seems, there-
fore, logical to conclude that the condition that leads to the de-
velopment of the alternative characters may exist in the egg
alone (as for the male bee), or in the sperm alone (as for certain
hemiptera), or by the combination of egg and sperm, as for the
female bee. In the last case the conditions that will lead to the
development of the male are found in the unfertilized eggs; but
the addition of the sperm brings in a new condition that leads
to the development of the female — not that the spermatozoa of
the bee are female or even female-producing, but that the com-
bination of egg and sperm is female-producing. It is conceiv-
able that a non-nucleated egg of the bee fertilized by a sperm
would produce a male, the conditions being the same as in the
unfertilized egg.

Admitting that all eggs and all sperm carry the material basis
that can produce both the male and female, the two conditions
being mutually exclusive when development occurs, the imme-
diate problem of sex determination resolves itself into a study
of the conditions that in each species regulate the development
of one or the other sex. It seems not improbable that this regu-
lation is different in different species, and that, therefore, it is
futile to search for any principle of sex determination that is
universal for all species with separate sexes; for while the

1The assumption of reciprocal fertilization being rejected provisionally for
reasons already given.

424 Experimental Zoology

fundamental internal change that stands for the male or the
female condition may be the same in all unisexual forms, the
factor that determines which of the alternate states is realized
may be very different in different species.

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EXPERIMENTAL STUDY OF SEC-~-
ONDARY SEXUAL CHARACTERS

CHAPTER XXVIII
SECONDARY SEXUAL CHARACTERS

Introduction

HUNTER was the first, I believe, to use the term secondary
sexual characters for those differences in structure between the
males and females that are not directly connected with the organs
of reproduction. One of the most important facts connected
with the occurrence of secondary sexual characters is that they
are found almost exclusively in the higher groups of animals.
Moreover, they are, on the whole, characteristic of certain large
groups, so that it might appear that in these groups there is
some inherent tendency for such differences to develop. Thus
in the groups of mammals, birds, and insects secondary sexual
differences are quite common, while in the worms, mollusks,
echinoderms, and ccelenterates, the male and the female are
closely similar except for the differences in the organs of repro-
duction. Why the higher and the lower groups should differ
in these respects is difficult to explain. Two suggestions have
been proposed that may appear to account for the facts. The
groups in which these differences are found contain forms that
are extremely active, and the males are as a rule more active than
the females. Correlated with this difference in activity or “vi-
tality’? we may imagine that differences in the development of
some of the bodily structures may occur. This is the idea that
Wallace has suggested.

The other view is the one proposed by Darwin. He ascribes
the differences to the selection by the females of those males
that are more highly ornamented, or have finer voices, or that
develop certain odors, etc. This selection by the female implies
a highly developed condition of the sense organs, and perhaps of

429

430 Experimental Zoology

the central nervous system also. We find these organs most per-
fectly developed in the higher animals, hence it may appear that
we can explain on this basis the occurrence of secondary sexual
organs in these groups, and the absence of such differences be-
tween the sexes in the lower groups with less developed sense
organs.

Before we examine critically these and other hypotheses it
may be well first to pass in review some examples of secondary
characters, and then examine the experimental evidence that
shows how far the development of these characters is dependent
on the presence of the essential organs of reproduction."

In the mammals there are many excellent examples of differ-
ences between the male and the female. In the mandrill the
naked skin of the face of the male is bright blue, with the tip
and sides of the nose a brilliant red. The face is also marked
with white stripes. On the forehead there is a crest of hair and
on the chin a yellow beard. In the monkey Cercopithecus diana
the head of the male is intensely black, while that of the female
is gray. The enormous development of antlers in the stag is a
striking example of a secondary sexual difference.

In birds there are many species in which the males are entirely
differently colored from the females. The scarlet tanager is
a familiar example. The male during the pairing season is
a brilliant scarlet, with jet-black wings, while the female is a
modest yellow-green. The Baltimore oriole and the orchard
orioles show almost as great differences in the coloration of the
two sexes. Many of the humming birds show extraordinary
differences between the male and female, and the birds of para-
dise and the peacock excel in this respect.

* Darwin brought together in his book on “The Descent of Man” a great num-
ber of cases in which secondary sexual differences occur. Wallace also, on several
occasions, has given examples of the same sort. Scattered through the system-
atic literature of the groups of mammals, birds, and insects a vast number of
cases are described. Recently Cunningham has also brought together a large
number of cases of secondary differences and has suggested another way for
accounting for them, as will be stated later. Lameere has also given a number
of examples, especially among insects.

Secondary Sexual Characters 431

Differences exist not only in color and ornamentation, but in
other characters also. The male gorilla has a powerful voice,
the effect of which is enhanced by the presence of a laryngeal sac.
One kind of gibbon can produce a correct octave of musical notes.
The vocal organs of the monkey Mycetes are a third larger than
those of the female and produce an ‘“‘overwhelming” sound.
In birds also it is generally the male that sings; and in the case
of the thrushes and several other groups the voice of the male
is highly developed.

The scent glands or odoriferous glands of some of the mam-
mals are also secondary sexual characters. In many cases the
glands are present and equally developed in both sexes, but in
others the glands become enlarged in the male during the breed-
ing season, while in still other species the glands are more
developed in the males or even confined to them. At the rut-
ting season the glands on the sides of the face of the male ele-
phant emit a secretion that has a muskyodor. The males of
some of the bats have glands and protrudable sacs in various
parts of the body. The strong odor of the male goat and of
certain deer is well known. In the male of the musk deer
there is a region of the tail that is ‘‘bedewed with an odorif-
erous fluid,” while in the female this space is covered with
hair.

The dances accompanying courtship in certain male birds,
and the peculiar antics and apparent displays of plumage
shown by these, belong also in the class of secondary sexual
differences.

In the group of lizards, amphibians, and fishes, numerous
cases of secondary sexual differences are known, and in some
of these the differences are almost as marked as in the highest
members of the vertebrates.

In the insects many species show secondary sexual dif-
ferences. In the flies of the genus Elaphomia the males are
furnished with horns, recalling those of the stag. They are
branched or palmated, and in one species are of a beautiful
pink color edged with black. In the butterflies there are many

432 Experimental Zoology

cases in which both sexes are brilliantly colored, but the num-
ber of cases in which the males greatly excel the females is com-
paratively small. This statement holds also for other groups of
insects, where, despite the highly developed coloration, the males
are not, in most cases, more ornamented than the females.

In the crustaceans, where highly colored forms are not so com-
mon, there are only a few cases of secondary sexual differences,
yet in the group of copepods there are some species where the
male excels the female in ornamentation even more than does the
peacock excel the peahen.

These examples will serve to illustrate some of the more strik-
ing differences between the male and the female individuals.
Before discussing further the possible interpretations of these
facts, let us examine the evidence showing that a correlation
exists between the development of the secondary sexual char-
acters and the presence of the essential organs of reproduction.

Correlation between the Secondary Sexual Characters and the
Essential Organs of Reproduction

It has been long recognized that in vertebrates there exists
an intimate connection between the presence of the essential or-
gans of reproduction and the full development of the secondary
sexual characters. ‘The removal of the testes of the male has
been sometimes performed artificially, and sometimes has re-
sulted from disease or abnormal development of these organs.
The results are well shown in the case of the stag. If very young
males are castrated, before the knobs of the antlers have ap-
peared, the antlers never develop. If the operation is performed
at the time when the antlers have already begun to develop,
complete development does not take place; they remain covered
with skin and are not moulted or renewed. They form the
so-called peruke antlers. If the stag is castrated when the
horns have fully developed, they are precociously thrown off,
and are replaced by new antlers, showing a tendency toward per-
uke-formation, and these are not again renewed. If the castra-
tion of the young animal is on one side only, the horns show a

Secondary Sexual Characters 433

weaker development, but are no better developed on the side
opposite to that at which the removal took place than on the
same side. The influence, therefore, is a general one, and not
one-sided, as has been believed.

If castration in man takes place in youth, it causes several
important changes. The growth of hair on the face and other
parts of the body is scanty or suppressed. The larynx remains
undeveloped, so that the voice retains the high pitch of boyhood.
Tt is also said that the shape of the pelvis is different from that of
men, and even that parts of the brain are smaller. The large
body size of eunuchs is likewise supposed to be due to the effects
of castration. .

In oxen and horses, in which castration has been carried out
at an early age, the shape of the pelvis is altered, and it is said to
approach in form that of the female. It also has been shown,
in the case of cattle, horses, rabbits, dogs, and fowls, that one
result of castration is to retain to a later stage of development
all of the cartilaginous unions between the parts of the bones.
This sometimes leads to an increase of growth in length of the
bones, especially in those of the extremities, which may account
for the immense size of eunuchs. Associated with the increase
in the length of the bones is a certain degree of thinness in them.

The statement is often made that the effect of castration of
the male is to change his characters into those of the female, and
it is not difficult to cite cases that seem to favor this view. The
failure of the stag to develop horns reduces him to the condition
of the female; the failure of the male fowl to develop the plum-
age of the cock and the incomplete development of the comb,
wattles, and spurs, as well as his inability to crow makes him
more like the female bird; the absence of the beard in eunuchs
and the retention of the high voice are characters associated with
women. ‘These results are so evident that it is not surprising
that they have been interpreted as showing that the castrated
male has assumed female characters. On the other hand, it has
been pointed out that the results can also be interpreted in

another way. The castrated male may retain the immature
2F

434 Experimental Zoology

form and characteristics of the male, and since in the immature
condition the male has characters in common with the female,
the resemblance may be due to this rather than to the develop-
ment of the female characters by the male body. For instance,
the beard is undeveloped in boyhood and the voice is pitched
higher thanin man. In these respects the boy resembles the girl,
and should he fail to develop a beard and retain his high voice
when he is full grown, it may seem that he has assumed the
female characters, while in reality he has only retained the
immature conditions of his own sex.

There is a further possibility to be considered. Castration
may affect changes that are associated with this condition itself
and have no relation to the differences between the sexes.
These possibilities will show that great caution must be exer-
cised in interpreting the results of castration of the male.

A side light is thrown on the problem from another direction.
The removal of the ovaries of the female is supposed to induce
the development of male characters, and if this is the case the
results would seem to support the converse proposition discussed
above. Let us, therefore, next examine this question.

It has been shown experimentally that the removal of the
ovaries in young rabbits, guinea pigs, and dogs causes a lack
of development of the mammary glands. The uterus also fails
to reach its full development. Other effects than these are not
recorded. It is improbable that any one will claim that the
failure of the mammary gland to develop is due to the assump-
tion of a male character, and yet the argument is not dissimilar
from the failure of the larynx to develop in the castrated male,
and this has been interpreted, as we have seen, as due to the
development of a female character.

There seem to be no cases on record of artificial castration
of female deer, but there are a few cases recorded in which the
ovaries were degenerate and horns had developed. There are
also three other cases, cited by Rorig, in which the young doe
showed horns, and it was found upon examination that slight
abnormalities were present in the ovaries, but they were so slight

Secondary Sexual Characters 435

as to make it doubtful if the development of horns was due to
this defect. Cases of this sort are not of much value, since it is
possible that the abnormalities observed may both be due to
some abnormal condition of the whole and are not connected
as cause and effect.

The best-known cases in which the castrated female assumes
the characters of the male are found in poultry. Many in-
stances of this sort have been recorded.’ It has also been
observed that when female birds, such as pheasants, fowls, par-
tridges, peacocks, and ducks, become old, they may assume the
secondary sexual characters of the male. Darwin cites the case
of a duck ten years old that assumed the perfect winter and sum-
mer plumage of the drake. Waterton gives the case of a hen
that had ceased laying and had assumed the plumage, spurs,
voice, and disposition of the cock. This evidence points clearly
to the possibility of the castrated female assuming the charac-
ters of the male. Whether the converse is true for the male is,
as we have seen, more doubtful, but the possibility of its being
true must be admitted.

If the characters of the female are latent in the male, and those
of the male are latent in the female, as these and other facts seem
to show, the experiments with castration have an interesting
bearing on sex determination; for if, as seems probable, the
male characters may develop in a castrated female despite the
fact that the female characters had already been developed,
it shows that the question of sex determination, even if deter-
mined in the egg, or even in the embryo, is not final and the con-
verse change may occur even ata late stage. If it should prove
true that in the vertebrates the castrated female develops male
characters, but the castrated male does not develop female char-
acters, but simply remains at a lower stage of development, we
might perhaps assume that the male condition in this group
is a further stage of the female condition. Internal or external
factors may determine whether the egg or embryo remains at a
given stage or undergoes a further change. If the former, a

1 Darwin, “Animals and Plants,” Chap. XIII.

436 Experimental Zoology

female develops; if the latter, a male. We lack the evidence
to establish such a view at present, and our more immediate
problem is first to discover what factors determine the sex of
the individual.

So far we have considered only the effects of removal of the
essential organs of reproduction, the ovaries and testes, on the
general characters. It has been shown for the males that
the removal of these organs in youth also affects the acces-
sory parts of the reproductive organs, — the glands and ducts. If
the testes are removed from the adult mammal, there is a decrease
in the size of the prostate gland that may be due to functional
atrophy, or may be due to more direct influences. If the testes
are removed from a young animal, the prostate does not develop
further, and remains quite small. One-sided castration pro-
duces no effect, either on the same or on the opposite side. The
development of Cowper’s gland seems to be correlated with the
development of the prostate, and after castration remains un-
developed. Other parts of the accessory system also fail to
develop completely after castration.

It is generally assumed that the influence arising from the
testes must be in the nature of an internal secretion, which, set-
ting free certain materials in the blood, affects the development
of remote parts of the body. In support of this view is the fact
that the influence is not unilateral, but general. There is a
further result that shows how important a réle the internal secre-
tions of the reproductive organs may play in certain changes in
other parts of the body. It has been found by Frankel and
Cohen that the presence of the cup-shaped corpora lutea, left
on the surface of the ovary after the egg has escaped, is necessary
for the fixation of the egg to the wall of the uterus. The egg or
young embryo fixes itself six days after it has been set free from
the ovary. Ifthe ovary is removed at this time, the fixation does
not occur. Even if the ovary is left, but the corpora lutea are
destroyed by a galvanic needle, the fixation fails to take place.
It is probable that the corpus luteum is a gland, as its structure
suggests, and the substance produced influences the wall of

Secondary Sexual Characters 437

the uterus in such a way that it responds to the presence of the
embryo and grows up around it.

It has been shown in the vertebrates that the development
of the secondary sexual organs is intimately connected with the
presence of the testes. In the insects it appears that this con-
nection does not exist. Oudemanns has shown for Ocneria dispar
that when the testes are removed from the male caterpillar, the
secondary sexual characters of the male are unaffected. Kellogg
has confirmed this result for the silkworm moth. Oudemanns
also removed the ovaries from the female caterpillar and found
that the female moth showed the normal markings.

Pictet points out that in a few cases there exist a male form
and a female form of caterpillar, although as a rule in most ani-
mals the secondary sexual differences do not appear until the
adult stage. Often the size of the caterpillar is the only external
difference that is noticeable between the sexes, the males being
smaller than the females. In the case of Orgya antiqua and
Orgya gonostigma, however, the female caterpillars are superbly
colored, and covered with spines of different shades; the male
caterpillars are not only smaller, but simpler and without the
spines. In Ocneria dispar the larval sexual dimorphism is
marked, but only in the fully formed stage of the caterpillar.
Pictet found that when caterpillars of this species are fed on es-
parcette and dandelion, that furnish ample nutrition, the larve
develop with great rapidity, and all acquire the characters of
the female caterpillar. With pimprenelle, which also gives an
abundant nourishment, but not so well as the preceding, the
caterpillars that showed the female type of marking were in
excess. On the other hand, when fed on walnut leaves, giving
insufficient nourishment, the caterpillars all assumed the char-
acters of the male. In the case of other plants that gave insufh-
cient nourishment this effect could not'be seen, because the food
introduced other effects that changed the aspect of the caterpillar,

The caterpillars of Ocneria dispar normally transform into
chrysalids after the fifth moult. The males reach the stage first
and the females may still be in the fourth period when the males

438 Experimental Zoology

have transformed. At this time the females resemble the males,
and the secondary sexual differences come afterward, while the
females are getting still more food. This difference in the
amount of food may be the indirect cause of the secondary
sexual differences in these species.

In the crustaceans, Giard has found that when Stenorhyn-
chus is attacked by a parasitic copepod, Sacculina fraissei, the
reproductive organs are almost completely destroyed. When this
occurs in the male the female characters appear — the smaller
claws and broader tail. In the female there is a reduction of the
abdominal feet, a condition characteristic of the male. If these
effects are directly due to the removal of the sexual organs, as
experiment might show, and not due to the effects produced by
the parasite, whose roots penetrate to all parts of the body, then
they differ from the cases in the insects and are like those of the
vertebrates. As yet it is not possible to decide which is the true
view until castration has been artificially induced.

We have now passed in review the principal facts connected
with the occurrence and development of the secondary sexual
characters. We have seen that in the vertebrates their develop-
ment is connected with the presence of the testes, but that this
is not true for the insects so far examined. ‘This difference may
be interpreted to mean that in the vertebrates, the stimulus for
the development of these characters in the male comes from the
reproductive organs. The ability to develop these parts must,
nevertheless, be supposed to be latent in the body cells. In the
insects the body does not require the stimulus of the reproductive
organs to develop the characters of the male. This distinction
raises the question as to whether the secondary sexual characters
of the vertebrates belong to the same class as the differences
observed in insects, and this suggests the further question whether
if they are different they may not owe their origin to different
factors of evolution.

LITERATURE, CHAPTER XXVIII
(See Chapter XXIX.)

CHAPTER XXIX
SECONDARY SEXUAL CHARACTERS (continued)

Theories of the Origin of Secondary Sexual Characters

WuEN Darwin proposed his theory of natural selection in
1859 he brought forward also at the same time his theory of
sexual selection to account for the origin of secondary sexual
characters. ‘‘This form of selection depends not on a struggle
for existence in relation to other organic beings or to external
conditions, but 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.”’ Darwin believed that the secondary sexual char-
acters have arisen in either of two ways: either as a result of a
contest between the males that has led to the development of such
structures as the horns of the stag and the spur of the cock;
or as the result of selection by the females of the most adorned,
or brilliantly colored, or highly scented male individuals. This
latter form of selection has brought about the development of
such characters as the color, the ornaments, the song, and the
scent glands of the males.

Of these two forms of sexual selection the first is merely an
extension of the principle of natural selection applied to the male
individuals of the same species; while the second form of selec-
tion depends on the esthetic taste of the females.

In 1871 Darwin developed his theory of sexual selection much
more fully in his book on “‘The Descent of Man.” Here he
appears at times to give the theory precedence over the theory
of natural selection itself, and attempts even to account for the
development of color and ornamentation, when it exists in both
sexes, by assuming that through the selection of the female it

439

440 Experimental Zoblogy

was first developed in the male and was then transferred to
the individuais of the other sex.

There are serious objections to the theory of sexual selection,
some of which Darwin himself has considered. He points
out, for instance, that his theory demands “powers of dis-
crimination and taste on the part of the females which will
at first appear extremely improbable.”” The few instances that
Darwin gives to show that such discrimination may really exist
are far from convincing. Amongst domesticated animals there
is very little evidence that can be brought forward to support this
view, and much that is opposed to it. It must be borne in mind
that even although the female may be excited by the presence of
the male or by his antics and demonstrations, it is essential
that she consistently select the most adorned in order that any
result may follow. ‘Tegetmeier observed that a game cock, dis-
figured by being dubbed and with hackles trimmed, was accepted
by the females.

Mayer has recently carried out some experiments with the
moth Callosamia promethia that show very clearly that the
adornment of the male or of the female is not a factor in the
selection by the other sex Five females were placed in an open-
mouthed glass jar covered by netting. Five males, liberated one
hundred feet away, flew to the jar. If the jar was inverted so
that its opening was closed, the males did not approach, although
the females could be seen through the glass. It appears, there-
fore, that the males find the female by the sense of smell, not by
sight.

Some females were inclosed in a box with an open chimney.
The males flew to the chimney, although the females were not
visible. When the abdomen of the females were cut off the
males flew to these, and paid no attention to the winged body.
Wherever a female has stood for some time, the males are at-
tracted to that spot. If the antenne of a male are cut off, he
does not go to the female. These experiments also show that
the male finds the female by a sense of smell, and not by
vision, although the female is highly ornamented.

Secondary Sexual Characters 441

Mayer also made some experiments to test whether the females
show a choice in selecting the males. The male is dark or black-
ish; the female reddish brown. The wings of a female were cut
off and glued on to the wings of a male, so that he appeared like
a female; nevertheless he was accepted by a female, no aversion
was manifested, and the mating was normal. The converse
experiment was also made. The wings of a female were cut off
near the base and the wings of a male were glued on in their
place. Such females looked like males, yet the males mated
with them as with normal individuals. Clearly, then, neither the
coloration of the male nor of the female can be supposed to
have a selective value.

Males pay no attention to males with female wings; but will
pair readily with a female both of whose wings have been cut off.
Even when the wings of both the male and the female have
been removed, pairing occurs if they are brought together.

One series of experiments of this kind is not perhaps conclu-
sive, and there is great need for more experimental work along
these lines.

There are many other serious objections to be urged against
Darwin’s theory. I shall give here only a few of the more
important ones."

In most groups of unisexual animals — and it is only such
that come under the theory — the number of males and fe-
males is approximately equal. Therefore even if we admit
that the most ornamented individuals are first selected, the
remainder of the females must pair with the less ornamented
males. In consequence nothing would be gained in successive
generations.

It is assumed that by the selection of individual or fluctuating
differences the character of the males can be greatly changed.
It may be questioned if this is possible, even if rigorous selection
be admitted to occur. It is doubtful if anything more could be
accomplished in this way than to maintain the standard at a

1In my book on “Evolution and Adaptation” I have discussed this topic
more fully.

442 Experimental Zoology

somewhat higher level, which would be lost as soon as the
selection stopped.

The process as imagined by Darwin is highly wasteful, for
in order to build up the ornamentation of the males, countless
individuals must have been sacrificed, yet the end result is of no
advantage to the species. If competition is as keen in nature as
Darwin supposes, one would think that natural selection itself
would soon have put an end to such a wasteful and useless pro-
cedure. There cannot be much doubt that the horns of the stag
and brilliant coloration of many male birds must expose them to
greater danger, so that natural selection ought, from the Dar-
winian standpoint, to bring about their disappearance.

There is a considerable number of sexual characters, suchas the
loud voice of the stag, that Darwin believes have been developed
through use. In fact, he appeals not infrequently to Lamarck’s
theory when the evidence is unfavorable to the selection theory.
Since the theory of inheritance of acquired characters has itself
been brought into question in recent years, it is doubtful if Dar-
win’s position is strengthened by an appeal to such a principle.

The application of the theory to man shows how involved the
argument becomes. Darwin thinks that the beard in man has
been developed by women selecting those males that had this
outgrowth most developed. The absence of the beard in women
is explained as the result of men selecting those women that
had less beard until it was eliminated. These two conflicting
processes are supposed to go on at the same time, or alternately.
The greater energy, size, and pugnacity of men have come from
their competition with each other, while the standard of beauty
has been maintained for both sexes by men selecting the more
beautiful women, who have then transmitted this quality to the
male. The deeper voice in man has been developed by its long-
continued use by the male “under the excitement of love, rage,

? Natural selection also is wasteful, since the waste is unavoidable. If
amongst individuals of the same species it leads to the survival of the strongest,
this may be an advantage, so that in either case the outcome is different from
that of sexual selection depending on female choice.

Secondary Sexual Characters 443

and jealousy.” Human song has also been the result of court-
ship and rivalry.

The theory demands that the females select the most beautiful
males, and this implies either that the taste or appreciation of the
female must always improve in advance of the improvement in
the secondary sexual characters of the male, or that the female
is gifted with this appreciation from the beginning. If the for-
mer view is held, one may ask how has her taste been improved;
if the latter is held, one may ask how she has come to be so gifted.
The theory of sexual selection can give no answer to either alter-
native, yet cannot justify itself without some such assumptions.
Other objections might be urged, but these will suffice, I think,
to show how improbable it is that the secondary sexual char-
acters can be accounted for on the principle of female selection.

Wallace, who suggested the theory of natural selection si-
multaneously with Darwin, has never accepted the theory of
female selection, and has attempted to explain the facts in a
different way. In his essay on ‘‘Tropical Nature,” published
in 1878, he first developed his view, but has given the subject
much fuller treatment in his recent book on “Darwinism.” He
assumes that there is a tendency for the males of most animals,
especially birds and insects, to develop a greater intensity of
color, and this “‘tendency”’ is ascribed to the greater “vitality”
of the male. In the female, especially amongst birds, the de-
velopment of color is rigorously kept down by natural selec-
tion, especially in those species that build open or exposed nests.
In such cases the female sitting on the nest would be exposed to
the attacks of enemies were she brightly colored. In support
of this latter view, Wallace points out, the species in which both
sexes are equally colored, the kingfishes, woodpeckers, toucans,
parrots, turacos, hangnests, and starlings, make their nests in
holes in trees, or in the ground, or build a dome over them. The
female is in consequence not exposed on the nest. Conversely,
when the male is brilliantly colored and the female plain the nest
is open. The color, Wallace states, must have adapted itself to
the instinct, and not the instinct to the color, because habits

444 Experimentat Zoology

are more difficult to change than color; but Wallace brings
forward no evidence to establish this view.

In other vertebrates this hypothesis will not explain the plainer
coloration of the females, as Wallace points out, since they do not
incubate their eggs. Secondary sexual differences are known in
fishes, lizards, and mammals that appear to be similar to those
in birds. Wallace has recourse, therefore, to the theory of
natural selection amongst the males which has led to the sur-
vival of the stronger males. In polygamous races this leads to the
successful males becoming the fathers of the next generation.
The stronger animals are those whose colors and other such char-
acters will be more highly developed. In monogamous species,
and these are by far the most common, the result of competition
amongst the males will not produce any result, since the van-
quished males will also find mates. Wallace tries to meet this
difficulty by pointing out that the males of many insects emerge
before the females, and that the males of migratory birds are the
first to arrive at the nesting grounds. The most vigorous males
will be the first to pair and leave offspring. This precocity is
assumed to be an advantage, but it is obvious that it might be
also a disadvantage, and we have no means of weighing the rela-
tive merits of the individuals that pair first with those that pair
at the height of the breeding season.

Wallace thinks that the greater vigor of the males will lead
in itself to the higher coloration of that sex, and furthermore he
tries to show that the ornaments are more likely to occur over
those parts of the animal where the muscular and nervous de-
velopment is greatest; but the location of the tail of the peacock,
the crest of the herons, and many other characters that might be
cited would seem opposed to such an assumption.

Darwin has considered Wallace’s view in his ‘Descent of
Man” and has pointed out that while the theory might appear
to apply to sexual differences in color and ornamentation, it is
not obvious how other secondary sexual characters, such as the
vocal organs of birds and the scent glands of mammals, could be
thus explained. Wallace has replied to this criticism by assuming

Secondary Sexual Characters 445

that natural selection has first developed these organs as recog-
nition marks by means of which the individuals find each other,
and later the greater vitality of the male causes them to develop
more fully in that sex.

Cunningham has attempted to account for secondary sexual
characters by means of the theory of the inheritance of acquired
characters. His effort is to show how the theory will account for
these structures, the theory being assumed to be valid, for to
the author it is “obvious that if the removal of the testes can affect
the development of tissues in the head, the development of the
latter may affect the properties of the testes.”’ Such statements
only confuse the issue, which, after all, must rest on the experi-
mental proof as to whether or not the special development of
parts of the body does bring about corresponding changes in the
germ-cells. There has been no difficulty in showing that the
removal of the reproductive organs affects the body; but as yet
little or no evidence that is satisfactory has been obtained to
show the converse to be true.

Cunningham assumes that the use of the secondary sexual
organs often subjects them to special mechanical irritation when
other organs of the body are not affected. These mechanical]
strains and pressures affect the development of the organs, and the
results are supposed to be inherited. He asks, why are the secon-
dary sexual characters of the male restricted to the male off-
spring and those of the female to the female offspring? His
answer is that “heredity causes the development of acquired
characters for the most part only in that period of life and in
that class of individuals in which they were originally acquired.”
The same idea is more fully expressed in the statement ‘that
the direct effects of regularly recurrent stimulations are sooner
or later developed by heredity, but only in association with

1 Lameere also accepts that part of Darwin’s theory of sexual selection that
assumes the secondary sexual differences to be due to natural selection between
the individuals of one sex; but rejects Darwin’s idea that, the differences can be
accounted for by selection of the males by the females. In these respects he
agrees with Wallace.

446 Experimental Zoology

the physiological conditions under which they were originally
produced,” etc. Thus the secondary sexual characters ac-
quired by the adult male became inherited only by the males
and at the same time of life at which they first appeared in the
male. Asa purely formal assumption no objection can be made
to this kind of hypotheses. Our opinion as to whether it is
probable or not must be guided by the experimental evidence
that can be brought forward in its support. At present this evi-
dence is lacking.

It seems to me that Wallace’s view, that the secondary sexual
differences may be due to the greater “vitality” of the male
(which has resulted from the natural selection of the stronger
males), may be given a much simpler interpretation without neces-
sitating the assumption of any process of selection. When we
see the wonderful development in all groups of the animal king-
dom of pigmentation, and when we realize that there are inherent
differences between the males and females of each species, it is
not surprising that differences in color might also appear asso-
ciated with the male and female condition. So long as these
differences do not seriously interfere with the existence of the
species, they will perpetuate themselves. Only if carried too
far might the species exterminate itself. There is no need of
selection to account for their origin. At most “selection” might
account for their destruction if they develop beyond a certain
point.

This mode of explanation will not, I am aware, appeal to
those naturalists of the Darwinian school who attempt to explain
all organic conditions as the result of usefulness. They seek
to find a purpose in every existing condition, and therefore try to
find some purpose for the secondary sexual differences between
the male and the female. That a great number of the charac-
teristics of organisms have a purpose there can be little question.
This purpose is the maintenance of the species; but it is not logi-
cal to assume that because a great number of adaptations exist,
all characters must therefore be adaptations to something or
other, at least in the sense that they have a survival value. The

Secondary Sexual Characters 447

very argument that secondary sexual characters have arisen by
sexual selection implies an injurious process that involves a serious
loss to the species of the less ornamented individuals, and therefore
a lack of adaptation. Thus in the effort to show the purposeful-
ness of the secondary sexual characters, the.injury caused to the
species has been overlooked.

If it be granted that neither natural selection nor sexual
selection can explain the origin of the secondary sexual char-
acters, there opens a wide field for future thought and study.
If variations can occur that ultimately culminate in the gorgeous
tail of the peacock, the wonderful colors of the humming birds,
the splendid antlers of the stags, and the musical, mathematical,
and esthetic development in man, do we not seem to catch a
glimpse of a power of progressive development in organisms that,
given a suitable environment, may produce extraordinary re-
sults? It is improbable that these highly complex structures
have been the result ofa single variation, and therefore, if the
result of several or many successive variations, these must have
gone on building up in the same direction. The power of
advancing in one direction, unguided by selection, is one of the
least appreciated biological phenomena, and yet may be one of
profound significance.

LITERATURE, CHAPTERS XXVIII AND XXIX

Cunnincoam, J. T. Sexual Dimorphism in the Animal Kingdom. A
Theory of the Evolution of Secondary Sexual Characters. 1goo.

DALLE TorRE and Friese. Die Hermaphroditen und Gynandromorphen
Hymenopteran. Innsbruck. 1899.

Hann, H. Anatomischer und Physiologische Folgeerscheeinungen der
Kastration. Sitz. Gesell. f. harp u. Phynol. XVIII. 1902-1903.

Hecar, A. Korrelationen der Keimdriisen und Geschlechtsbestimmung.
1893.
Rattooe Wi L. Influence of the Primary Reproductive Organs on the
Secondary Sexual Characters. Journ. Exp. Zool. I. 1904.
Lamerrr, A. L’Evolution des ornements sexuels. Bull. PAcad. Roy.
de Belgique. 1904.

Maver, A.G. On the Mating Instinct in Moths. Annal. and Mag. Nat.
History, V. 1900.

Ovupemanns, J. Tx. Falter aus Castrirten Raupen. Zool. Jahrbiicher.
Abt. Syst. XII. 1899.

448 Experimental Zoology

PEcKHAM, G., and Euizasetu,G. Additional Observations on Sexual Selec-
tion in Spiders of the Family Attide, with Some Remarks on Mr.
Wallace’s Theory of Sexual Ornamentation. Milwaukee. 1890.

Occasional Papers. Nat. Hist. Soc. Wisconsin, I. 1890.

Roric, A. Ueber Geweihentwickelung. Arch. f. Entw.-mech. X. 1900.

Scuuttz, O. Allg. Zeitsch. f. Entomologie, IX. 1904.

SteinacH, E. Untersuchungen zur vergl. Physiologie der minnlichen
Geschlechtsorgane, insbesondere der accessorischen Geschlechts-
driisen. Arch. ges. Physiol. LVI. 1894.

STOLZMANN, JEAN. Quelques remarques sur le dimorphisme sexuel.
Proc. Zool. Soc. London. 1885.

THORNDIKE, E. Measurements of Twins. Arch. of Philos. Psychol.
and Scientific Methods. 1905.

INDEX

Abraxas, 153.

Accessory chromosome, 402-406.

Acherontia atropos, 36.

Adaptation, 229-232.

Adler, 269-271, 358.

Age, 391.

Ageniaspis, 410.

Aglia, 227.

Albino rabbits, 106-108; rats, 111-113;
man, 117.

Allen, 83, 88, 215, 223.

Allolobophora, 298.

Amblystoma, 315, 316.

Amphibians, 246.

Amphioxus, 317.

Ancon ram, 209.

Andalusian fowl, 127.

Andrews, 377.

Anikin, 41.

Annelids, 368.

Anodonta, 268.

Anolis, 13, 14.

Antedon, 177.

Antennularia, 266.

Antlers, 430.

Ants, white, 317-319.

Aphids, 207, 219, 273, 311-312, 322, 324-
334-

Arctiidae, 29.

Argynnis, 313.

Artemia, 18, 40, 41, 336-337.

Artom, 337-

Aseel, 135

Aspergillus, 257.

Asplanchna, 350.

Asterias, 179.

Axolotl, 315, 316.

Azora, 412.

Bacillus, 374.

Bacot, 153.

Balbiani, 320, 324-326, 381.

Baldwin, 231.

Balfour, 397.

Bantam, 130-132.

Barb, 135.

Barfurth, 255.

Bateson, 49, 82, 83, 84, I1I-113, 121,
127, 164, 414.

2G

Bats, 431.

Beard, 413, 414.

Beclard, 264.

Bee, 269, 320, 351-356, 358.
Beetles, 151-153.

Bell, 60, 116, 377-378.
van Beneden, 397.

v. Berlepsch, 353.
Berner, 392.

Bessels, 377.

Beyerinck, 272-274.
Bidder, 392.

Birds, 248.

Birth rate, 365.

Bison, 158.

Blanc, 262.

Blending, 156.
Blochmann, 331-332, 416.
Blumenbach, 363.

Boas, 317.

Bodio, 365.

Bombyx, 378, 395-
Bombyx neustria, 29.
Bonnet, 311, 322.
Bordet, 178.

Born, 173, 287, 298, 382.
Botrytis, 257.

Boulanger, 392.

Boveri, 175, 176, 177, 408-409.
Branchippus, 40, 41.
Brauer, 342.

Braus, 302-304.

Breslau, 392.

Briggs, 377-

Brocadello, 394, 395-
Brown-Séquard, 52, 53, 54, 55
Bufo, 173-174, 260, 299.
Buffon, 213.

Buller, 178, 179.
Bumpus, 211.
Butschinsky, 41.

Caddis flies, 374.

Cagliari, 337.

Calkins, 312-313, 320.
Callema, 37.

Calliphora, 379-380.
Callosamia, 301, 440-441.
Calotermes, 318.

449

450

Cameron, 357.

Carcinus, 204.

Carlton, 25.

Carpenter, 245.

Casteel, 211, 354.

Castle, 63, 83, 99-105, 110, 116-120, 190-
195, 397) 414-415.

Castration, 433.

Cats, 113-116, 117, 240.

Cattle, 158.

Cavies, 99-106.

Cecidomyia, 273.

Cecidomyide, 269.

Centaurea, 324.

Cercopithecus, 430.

Chalcid fly, 410.

Chameleon, 13.

Charrin, 54, 55-

von Chauvin, 315, 357-

Chermes, 331.

Chigoes, 268.

Chinese women, 57.

Chironomus, 374.

Cholodovsky, 334.

Chromosomes, 398-400, 416.

Chydorus, 341.

Cidaria variata, 30.

Ciona, 195-197-

Cochin fowls, 121, 123, 124, 125, 130.

Ceelenterates, 368.

Cohen, 436.

Cold, 12.

Comb, 121, 127-129, 285.

Comstock, 357.

Contact, 267-268.

Corals, 268.

Corpus luteum, 436.

Correns, 67, 100.

Cossus cossus, 29, 313-

Coutagne, 141-149.

Cowper’s gland, 436.

Crabs, 165, 204, 205, 421.

Crampe 110-113.

Crampton, 189, 205-233, 289, 301.

Crest, 129-130.

Croesus, 357.

Crustacea, 330, 368.

Crustacean, 246, 432.

Cuénot, 83, 89, 99, 91, 92; 93, 94, 95, 96,
100, 110, 255, 378-380, 382-383, 385,
395- '

Cunningham, 40, 430, 445.

Currant moth, 153.

Curtonevra, 379.

Cynipide, 358.

Cynthia, 197.

Cytoplasm, 417-419.

Lndex

Dageer, 323.

Dalle Torre, 408.

Danilewsky, 257.

Daphnia, 40, 207, 312, 337-342, 396.

Darbishire, 83, 84, 85, 86, 87.

Dareste, 412.

Darwin, C., 41, 42, 51, 60, 62. 63, 82,
162-164, 165, 187, 189, 209, 214, 224,
225, 232, 200, 404, 429, 430, 439, 440,
442, 443, 444.

Darwin, E., 213.

Darwin, G., 190.

Davenport, 83, 116, 117, 127-135, 199,
240, 256, 259, 262.

Davis, 61.

Death, 249.

Deer, 431.

Delamare, 54, 55.

Dero, 259.

Desgrey, 257.

Dickel, 355.

Dimorphism, 160-165.

Dinophilus, 394, 396.

Dicecious species, 364-368.

Diplogaster, 370, 371.

Dog, 12, 156, 167, 210, 257.

Doncaster, 113-115, 153, 176, 357-358.

Dorfmeister, 15.

Dorking fowls, 121.

Draba, 219.

Drelincourt, 363.

Dreyfus, 332.

Driesch, 175, 176, 266.

Drosophila, 190.

Ducks, 42, 157-158.

Duhamel, 285.

von Dungern, 179, 180, 181.

Diising, 385, 393.

Duval, 311.

Dzierzon, 352-353, 354, 356, 407.

Earthworm, 254, 258, 277-280, 286-287.
Echinoderms, 175, 176, 368. -
Echinus, 177, 179.

Edwards, 262.

Eigenmann, 424.

Eimer, 154.

Elaphomia, 431.

Electricity, 267.

Elephant, 248.

Ellopia prosapiaria, 30.

Epilepsy, 52.
Erlanger, 416.
Eudendrium, 265.
Eugster hive, 407.
Eunuchs, 433.
Eupithecia absinthiata, 36.
Evolution, 213-233.

Evonymus europaeus, 30.
Ewart, 60, 135.
Eye color, 167.

Fachenheim, 117.
Fantail, 135, 136.
Fatigat, 265.
Fehling, 241.

Fielde, 359.

Filippi, 315.

Fischer, E., 16, 17, 21, 22, 23, 43.
Fischer, G. H., 412
Fish, 277, 28x.
Flatfish, 4o.
Fletcher, 377.
Flounders, 13.
Flourens, 156, 248.
Food, 376.

Forbes, 104, 105.
Formative factor, 281, 289.
Formative force, 297.
Fowl, 158, 433-
Frankel, 436.
Frazeur, 259.

Friese, 408.

Frizzle fowl, 132.
Frogs, 396.

Frog’s egg, 267.

Gadow, 315.

Galls, 268-274.

Galton, 46, 62, 203, 412.
Galton’s Law, 87. 88.
Game fowl, 132.
Geddes, 367, 387-388.
Gentry, 376.

Geoffroy St. Hilaire, 213.
Germ-cells, 72-80, 173, 195-197.
Giard, 438.

Gibbon, 431.

Giglio-Tos, 417.

Gilbert, 255.

Giltay, 61.

Goat, 431.

Godlewski, 175, 177.
Gohlert, 392.

Goose, 157, 158.

Gorilla, 431.

Grafting, 285-305.
Grassi, 317-319-
Gravity, 266.

Gregory, 164.

Growth, 239-282.

von Guaita, 83, 84, 89, 188-189.
Guinea fowl, 158.

Guinea pigs, 99-106, 242-247, 289.

Gulick, 215.
Guyer, 158.

’

Lndex

451
Gynandromorphs, 407-410.

Haacke, 83, 84, 89, 97.

Hacker, 234.

Haeckel, 62.

Hamamelistes, 329-331.

Hamburg fowls, 121, 123, 124, 125.
Harper, 116.

Harrison, 299, 302-304.

Hatai, 257.

Heat, 200-261.

Hefferan, 294.

Helix, 139-141, 160, 166, 221-222.
Hemichroa, 357.

Henking, qor.

Henneguy, 381.

Hen’s egg, 261, 262.

Herbst, 175, 176, 258.

Hering, 58.

Heron-Royer, 173.

Hertwig, O., 200.

Hertwig, O., and R., 177.
Hertwig, R., 174, 342, 396-397.
Hessian fly, 411.

Heymons, 41.

Higgenbotham, 260, 262.
Himalayan rabbits, rro.

Hinds, 425.

Hofacker, 391-392.

Hormaphis, 327-329, 334-

Horses, 12, 42, 116, 158, 167, 248, 433.
Houdans, 121, 123, 124, 125.
Humming birds, 430, 447.
Hunter, 285, 429.

Hurst, 106-110, 116, 121-127.
Hyalopterus, 207.

Hybernia defoliaria, 39.

Hydatina, 346-350, 383, 394, 396.
Hydra, 213, 288, 289-297, 298, 300, 310.
Hydroid, 309, 310.

Hygiza, 18.

Hymenoptera, 351-365.

Inbreeding, 186-197.
Indian game fowls, rar.
Indians, 57.

Infusoria, 265.
Issakowitsch, 342-345.

Jackal, 156.

Jackass, 158.

Jaguar, 209.

Japanese fowl, 130-132.
Japonicus, 30.

Jehring, 412.

Joest, 286, 298.

Jordan, 215.

Joseph, 394.

452
Jullien, 315.

Kellogg, 377, 378, 437
King, 282, 290.

Klebs, 310.

Knowlton, 260.

Kurz, 337, 339-
Kyber, 311, 323.

Lamarck, 57, 213.

Lameere, 445.

Landois, 354, 376.

Lang, 139-141, 160, 166, 221-222,
Lasiocampa, 313.
Lasiocampa quercus, 29, 30.
Lasius, 357.

Lataste, 173.

Lauterborn, 350, 416.

Lawes, 255.

Lecithin, 257.
Leeunwenhoek, 322.

Leghorn fowls, 121, 124, 125.
Legoyt, 392.

Lenhossek, 366, 367.
Lepidoptera, 313-314.
Leuckart, 320, 356.

Lewis, 301.

Lichtenstein, 324.

Light, 261-265.

Lillie, 260.

von Linden, 16, 22, 23, 37.
Linnean species, 66, 156, 157.
Linnzus, 29.

Lion, 157.

Litomastix, 410.

Lizards, 13.

Loeb, J., 175, 177, 182, 259, 268.
Loeb, L., 289.

Lowne, 379.

Lucilia, 379.

Lumbricus, 298.

Lymnea, 263.

Lythria rotaria, 24.

Maas, 258.

McClung, 4or, 403.
McCracken, 151-153.
Macdonnell, 262.
Macdougal, 215.
Mallard, 42.

Man, 116, 167, 245, 256, 384, 442.
Mandrill, 430.
Marchal, 410, 411.
Marshall, 38.
Mauchamp sheep, 210.

Maupas, 262, 312, 346-349, 369-372, 384.

Mayer, 234, 301, 440-441.
Meischer, 254.

Lndex

Mendel, 67.

Mendelian inheritance, 166-171.

Mendelism and sex, 423.

Mendel’s law, 66-72, 84.

Merriam, 215.

Merrifield, 16, 24.

Metschinkoff, 251.

Meves, 401, 417.

Miastor, 374.

Mice, 74, 77, 78, 82-97, 188, 386.

Minorca, 127-129.

Minot, 61, 242-247, 254, 397.

Moina rectirostris, 40, 339, 341, 342.

Molgula, 197.

Mollusca, 368.

Mollusks, 246.

Moncecious species, 364, 368-372.

Montgomery, 386, 401.

Morgan, Lloyd, 231.

Morgan, L. V., 287-288.

Morgan, T. H., 77, 78, 175, 177, 181,
195-197, 299, 409, 416.

Moussu, 54, 55-

Mulatto, 156.

Musca, 264.

Mus musculus, 82.

Mycetes, 431.

Nageli, 44, 62.
Necturus, 317.
Nematodes, 368-372.
Nematus, 273, 357.
Nemerteans, 368.
Neotenia, 316-320.
Neuroterus, 270.
Niata cattle, 210.
Noctuidae, 29.
Noivot, 392.

Nun pigeon, 135.
Nussbaum, 56, 346-349, 383.

Ocneria dispar, 29, 31, 32, 33, 36, 39,
313, 314, 378, 380, 395, 437-

CEnothera, 159, 170, 217-221.

Orgya, 437.

Oriole, 430.

Osborn, H. F., 231.

Oudemanns, 437.

Ovary, 436.

Pecilosoma, 357.
Pagliani, 245.
Paludina, gor.
Pangenesis, 62.
Papaver, 47.

Papilio, 161, 376.
Papilio macchaon, 29.
Parnassus apollo, 23.

Index

Parrot, 42.

Parsons, 83.

Parthenogenesis, 206-207, 322.

Parthenogenetic species, 372-374.

Patella, 181. ~

Paulmeier, 4o1.

Peacock, 210, 432, 447.

Peahen, 432.

Pearson, 167.

Pearson's law, 203.

Peebles, 288, 290, 296.

Penicillium, 257.

Penycuik experiments, 60.

Perez, 353-

Pergande, 327, 333.

Peruke antlers, 432.

Petrunkewitsch, 356, 417.

Pfeffer, 178.

Pfliiger, 173, 174, 366, 382.

Phagocata, 288.

Phillips, 211, 335, 354.

Philodenide, 374.

Philosamia, 205.

Phototropism, 265.

Phromali, 358.

Phylloxera, 273, 274, 322, 332-333) 394
396.

Phytopterus, 273.

Pictet, 27, 29, 30, 32, 341 35) 45, 313-314,
380-381, 437. :

Pieris crategi, 34.

Pigeons, 135-136.

Pike, 248.

Planaria, 288, 289.

Planarians, 254, 295.

Polar bodies, 397-400.

Polish, 127-129.

Polydactylism, 117-121.

Polygnotus, 411.

Polyphemus, 338.

Polyspermy, 182.

Pomace fly, 190-195.

Porthesia chrysorrheza, 29.

Porto Santo rabbits, 41, 42.

Potts, 241.

Poulton, 13, 116.

Poultry, 121-135.

Pressure, 267-268.

Primula, 161, 162, 164-165.

Préscher, 255.

Prostate, 436.

Przibram, 421.

Psyche, 374.

Punnett, 121, 129, 385.

Pygera, 401.

Pyrrhocoris, 401.

Quetelet, 199.

453

Rabbits, 106-110, 245, 257.

Rana, 173-174, 200, 290, 299, 300, 381~
383, 396.

Rats, 110-113, 188-189, 257, 385.

Rauber, 261, 367.

Raynor, 153.

Réaumur, 322.

Regeneration, 277-282.

Reichenbach, 357.

Reptiles, 246.

Rhabditis, 369-371.

Rhodites, 273, 358.

Rhodocera rhammi, 23.

Ribbert, 289.

Riley, 331, 376-377.

Ritzema Bos, 188, 189.

Romanes, 42, 52, 53, 54, 63.

Rorig, 448.

Rossi, 267.

Rotifer, 317, 346-350, 374.

Rough coat, 102-104.

Roux, 267.

Rumplessness, 133, 134.

Rusconi, 173.

Sacculina, 438.

Sachs, 240.

Sadler, 391-392.

St. Hilaire, G., 42.
Salamander, 277, 281.
Salmon, 15, 254, 261.
Salts, 257-260.

Samia, 301.

Samter, 41.

Sarcophaga, 379.
Sargent, 259.

Saturnia, 313.

Scent glands, 431.
Schmankewitsch, 41, 339.
Schultz, O., 256, 408.
Schultze, 385-386, 392, 393, 394-
Schuster, 83, 96.

Scott, 234.

Secondary sexual characters, 429-447.
Seeliger, 175, 177.
Selection, 198-211.
Semon, 58, 59.
Senescence, 246-247.
Sepia, 265.

Serturella, 265.

Shad, 15.

Shank, 125.

Sheep, 116.

Shufeldt, 316.

Shull, 212.

Sida, 340.

v. Siebold, 353, 356, 357, 407, 408.
Silkworms, 141-149.

454

Silky fowl, 132.

Silvestri, 410, 411.

Simocephalus, 342-345, 383, 384.

Siredon, 315.

Skin, 12.

Snails, 139-141, 160, 165.

Solenobia, 374.

Spathegaster, 270, 271.

Spencer, H., $1, §2, 62, 24%, 245.

Spermatozo6n, 178-183, 417.

Spherechinus, 175, 176, 179, 180.

Spirographis, 265.

Sponge, 258, 368..

Stag, 432.

Standfuss, 16, 21, 22, 23, 24, 44, 166, 226-
227.

Staples-Browne, 136.

Starratt, 13, 14.

Stenorynchus, 438.

Stereotropism, 266.

Stevens, 266, 326, 335, 401, 403, 406, 416.

Stieda, 392.

Stimulants, 256-257.

Strasburger, 178, 414.

Strongylocentrotus, 175, 176, 180.

Struthers, 118.

Sutton, gor.

Tadpole, 251, 255, 257, 258, 260, 263,
287, 298, 301.

Tanager, 430.

Tanner, 356.

Tatusa, 412.

Tegetmeier, 440.

Telea, 301.

Telegony, 59.

Tenebrio, 403.

Tenthredinide, 357.

Tephrosia, 153-154.

Tetraneura, 331.

Thomson, 367, 387-388.

Thorndike, 412-413.

Thury, 393.

Tiger, 157.

Toad, 248.

Toe, 124.

Torelle, 18x.

Tornier, 304.

Tortoiseshell cats, 113-115.

Tosa fowl, 130-132.

Toyama, 149-151.

Transvaal, 38.

Treat, 376.

Trematodes, 374.

Lndex

Trembley, 285, 288, 290.
Trichine, 268.
Trigonaspes, 269.
Triton, 174, 299.

Trout, 263.

Tschermak, 67.
Tubularia, 259, 268, 288.
Tumbler pigeons, 135.
Turnspit dog, 209.

Tutt, 153-154.

Twins, 411-413.

Uniform coat rabbits, 109.

Vanessa, 313, 376.

Vanessa antiopa, 17, 18.

Vanessa atalanta, 17.

Vanessa cardui, 16.

Vanessa io, 37.

Vanessa levana-prorsa, 16, 24.

Vanessa polychloros, 39.

Vanessa urtice, 34, 358.

Variation, fluctuating, 198-203.

Vernon, 174, 264.

Verworn, 62.

de Vries, 45, 46, 47, 62, 64, 112, 159, 160,
167, 170, 209, 214, 217-221, 404.

Wallace, 430, 443, 444, 446.

Warren, 207, 345.

Watasé, 426.

Weber, 410.

Weismann, 51, 52, 55, 59, 62, 188, 248,
250, 281, 315, 337-341, 350, 384, 404,
415.

Weldon, 204-205.

Westphal, 54.

Wetzel, 300.

Wheeler, 235, 408.

Wiesner, 62.

Wilson, E. B., 401, 402, 403, 406.

Woodruff, 321.

Wyandotte fowls, rar.

Xenia, 61.

Yung, 255, 258, 262, 263, 264, 265, 381,
382.

Zaky, 257.
Zebra, 158.
Zeleny, 282.
Ziegler, 418-419

COLUMBIA UNIVERSITY BIOLOGICAL SERIES.

DESIGNED FOR INDEPENDENT READING AND AS TEXT- BOOKS —
FOR LECTURE AND LABORATORY COURSES
OF INSTRUCTION.

EDITED BY

HENRY FAIRFIELD OSBORN, Sc.D., LL.D.,
De Costa Professor of Zovlogy, Columbia University,

AND

EDMUND B. WILSON, Ph.D., LL.D.,
Professor of Zoslogy, Columbia University.

VOL. I.

FROM THE GREEKS TO DARWIN.

THE DEVELOPMENT OF THE EVOLUTION IDEA.

By HENRY FAIRFIELD OSBORN, Sc.D., LL.D.

Cloth. 8vo. 259 pages.

OPINIONS OF THE PRESS.

“ Professor Henry Fairfield Osborn has rendered
an important service by the preparation of a concise
history of the growth of the idea of Evolution. The
‘chief contributions of the different thinkers from
Thales to Darwin are brought into clear perspective,
and a just estimate of the methods and results of each
one is reached. The work is extremely well done,
and it has an added value of great importance in the
fact that the author is a trained biologist. Dr. Os-
born is himself one of the authorities in the science
of Evolution, to which he has made important con-
tributions. He is therefore in a position to estimate
the value of scientific theories more justly than would
be possible to one who approached the subject from
the standpoint of metaphysics dr that of literature.”

— President Davip STarR JORDAN,
in The Dial, Chicago.

«« A somewhat new and very interesting field of in-
quiry is opened in this work, which is devoted to
demonstrating that the doctrine of Evolution, far
from being a child of the middle of the nineteenth
century, of sudden birth and phenomenally rapid
growth, as it is by many supposed to be, has really
been in men’s minds for ages. It appears in the
germ in the earliest Greek philosophy; in vigorous
childhood in the works of Aristotle ; in adolescence
at the closing period of the last century; and reaches
full-grown manhood in our own age of scientific

thought and indefatigable research.”
— New Science Review.

“This is a timely book. For it is time that both
the special student and general public should know
that the doctrine of Evolution has cropped out of the
surface of human thought from the period of the
Greek philosophers, and that it did not originate
with Darwin, and that natural selection is not a
synonym of Evolution,... The book should be

Illustrated. Price, $2.00 net.

a

widely read, not only by science teachers, by biologi-
cal students, but we hope that historians, students of
social science, and theologians will acquaint them-
selves with this clear, candid, and catholic statement
of the origin and early history of a theory, which not
only explains the origin of life-forms, but has trans-
formed the methods of the historian, placed philoso-
phy on a higher plane, and immeasurably widened
our views of nature and of the Infinite Power work-
ing in and through the universe.”
— Professor A. S. PAcKARD,
in Sczence, New York.

“This is an attempt to determine the history of
Evolution, its development and that of its elements,
and the indebtedness of modern to earlier investi-
gators. The book is a valuable contribution; it will
do a great deal of good in disseminating more accu-
rate ideas of the accomplishments of the present as
compared with the past, and in broadening the views
of such as have confined themselves too closely to
the recent or to specialties. ... As a whole the
book is admirable. The author has been more im-
partial than any of those who have in part anticipated
him in the same line of work.” — The Nation.

“But whether the thread be broken or continuous,
the history of thought upon this all-important subject
is of the deepest interest, and Professor Osborn’s
work will be welcomed by all who take an intelligent
interest in Evolution. Up to the present, the pre-
Darwinian evolutionists have been for the most part
considered singly, the claims of particular naturalists
being urged often with too warm an enthusiasm.
Professor Osborn has undertaken a more compre-
hensive work, and with well-balanced judgment
assigns a place to each writer.”

— Professor Epwarp B. Poutton,
in Nature, London,

FIRST EDITION PUBLISHED OCTOBER, 1894.

VOL. I. AMPHIOXUS AND THE ANCESTRY OF
THE VERTEBRATES.

By ARTHUR WILLEY, Sc.D., Balfour Student of the University of Cambridge.

316 pages.

“This important monograph will be welcomed by
all students of zodlogy as a valuable accession to the
literature of the theory of descent. More than this,
the volume bears internal evidence throughout of
painstaking care in bringing together, in exceedingly
readable form, all the essential details of the structure
and metamorphosis of Amphioxus as worked out by
anatomists and embryologists since the time of Pallas,
its discoverer. The interesting history of the changes
it undergoes during metamorphosis, especially its sin-
gular symmetry, is clearly described and ingenious
explanations of the phenomena are suggested. Most
important, perhaps, are the carefully suggested homol-
ogies of the organs of Amphzoxus with those of the
embryos of the Vertebrates above it in rank, especiall
those of the Marsipobranchs and Selachians. ‘Though
the comparisons with the organisms next below Am-
phioxus, such as Asczdians, Balanoglossus, Cepha-
lodiscus, Rhabdopleura, and the Echinoderms,
will be-found no less interesting. In short, the book
may be commended to students already somewhat
familiar with zodlogical facts and principles, as an
important one to read, They may thus be brought
to appreciate to what an extent the theory of descent
is indebted to the patient labors of the zodlogists of

135 Illustrations.

Price, $2.50 net.

the last forty years for a secure foundation in observed
facts, seen in their correlations, according to the com-
parative method. ... The present work contains
everything that should be known about Amphioxus,
besides a great deal that is advantageous to know
about the Tunicata, Balanoglossus, and some other
types which come into structural relations with Am-
phioxus.”
— Professor Joun A. Ryper,
in The American Naturalist, Philadelphia.

“The observations on Amphioxus made before the
second half of the present century, amongst which
those of Johannes Miiller take a foremost place, showed
that this remarkable animal bears certain resemblances
to Vertebrates ; and since then its interest in this re-
spect has gradually become more apparent.... A
consecutive history of the more recent observations
was, therefore, greatly needed by those whose oppor-
tunities did not permit them to follow out the matter
for themselves, and who will welcome a book written
in an extremely lucid style by a naturalist who can
speak with authority on the subject.”

— Professor W. NEwTon Parker,
in Mature, London.

VOL. Ill. FISHES, LIVING AND FOSSIL.

AN INTRODUCTORY STUDY.
By BASHFORD DEAN, Ph.D., Adjunct Professor of Zoblogy, Columbia University.

300 pages.

344 Illustrations.

Price, $2.50 net.

This work has been prepared to meet the need of tne general student for a concise knowledge of the living
and extinct Fishes. It covers the recent advances in the comparative anatomy, embryology, and palzontology
of the five larger groups of Lampreys, Sharks, Chimzroids, Teleostomes, and Dipnoans— the aim being to
furnish a well-marked ground plan of Ichthyology. The figures are mainly original and designed to aid in prac-
tical work as well as to illustrate the contrasts in the development of the principal organs through the five groups,

“The intense specialization which prevails in
zodlogy at the present day can lead to no other result
than this, that a well-educated zodlogist who becomes
astudent of one group is in a few years quite left
behind by the student of other groups. Books,
therefore, like those of Mr. Dean are necessary for
zoologists at large.”

— The Atheneum, London.

““Dr. Bashford Dean is known to zodlogists, first,
as the author of exhaustive and critical articles in the
publications of the United States Fish Commission,
on the systems of oyster culture pursued in Europe,
and, secondly, as an embryologist who has lately been
doing good work on the development of various Ga-
noid fishes and the comparison that may be instituted
with Teleostei. His recent addition to the well-known
‘Columbia University Biological Series,’ now being
brought out by The Macmillan Company, under the
editorship of Professor H. F. Osborn, is an interesting
volume upon fishes, in which considerable prominence
is given to the fossil forms, and the whole subject is
presented to us from the point of view of the evolu-
tionist. This is the characteristic feature of the book.
From the very first page of the introduction to the
last page in the volume, preceding the index, which
is a table of the supposed descent of the groups of
fishes, the book is full of the spirit and the language
of evolution.” |©— Professor W. A. HERDMAN,

in Nature, London.

“The length to which this review has extended
must be evidence of the importance of Dr. Dean's

work. The suggestions here offered may be of use
for another edition. That another may be called for,
we may hope. For the work as it is, and for the care
and thought bestowed on it, our thanks are due.”
— THEODORE GILL,
in Sczence, New York.

‘*Louvrage de M. Bashford Dean nous parait fait
avec soin; les illustrations sont excellentes et trés
nombreuses, et il mérite le meilleur accueil de Ja part
des zoologistes.”

— Cu. BRONGNIART,
in Le Revue Scientifique, Paris.

“ For the first time in the history of Ichthyology,
students are now provided with an elementary hand-
book affording a general view of the whole subject. ...
The last sixty pages of the volume are devoted to
a list of derivations of proper names, a copious bibli-
ography, and a series of illustrated tabular statements
of the anatomical characters of the great groups of
fishes. These sections bear signs of having been
prepared most carefully and laboriously, and form an
admirable appendix for purposes of reference. There
will be much difference of opini peciali
as to the value of some of the tables and the judgment
pronounced by the author; but we have detected a
very small proportion of errors for so bold an enter-
prise, and students of the lower Vertebrata are much
indebted to Dr. Dean for an invaluable compendium.”

— ARTHUR SmiTH Woopwarp,
in Natural Science, Londoa-

DEVELOPMENT AND
INHERITANCE.

By EDMUND B. WILSON, Ph.D., LL.D.,
Professor of Zoslogy, Columbia University.

371 pages. 142 Illustrations. Price, $3.50 net.

“In the highest degree fascinating... .. Itisa tion to read the larger treatises on botany or histology
matter for. congratulation that Professor Wilson has in which the modern views on the structure and funce
given us in our own speech a book which is second | tions of the cell are to be found in detail... In
to none in the clear and comprehensive manner in the present volume they will find an admirable expo-
which the facts of cell-structure and division are set sition of the knowledge that has been acquired during
forth, and the masterly way in which the principal the last twenty years.” — London Lancet.
theories are stated and criticised.” — Nature.

“Tt certainly takes rank at once among the most | . “One of the very best scientific manuals publishea
important biological works of the period.” in America. . . . A noteworthy characteristic of the

book is its thoroughness. . . . Students and inves-

— Sctence. ‘ : .
tigators of biology, in whatever department they may

“We heartily recommend this book. There are be working, ought to be familiar with this important
many practitioners who have neither time nor disposi- | work.” — New York Nation.

VOL. V. THE FOUNDATIONS OF ZOOLOGY.

By WILLIAM KEITH BROOKS,
Professor of LZoslogy, Johns Hopkins University.

8vo. Cloth. viii + 339 pages. Price, $2.50 net.

“A book that will live as a permanent addition to much as to either, for it is full of that fundamental
the common sense of science. It belongs to literature wisdom about realities which alone is worthy of the
as well as to science. It belongs to philosophy as name of philosophy.” — Sczence.

VOL. VI. THE PROTOZOA.
By GARY N. CALKINS, Ph.D.,

Instructor in Loblogy, Columbia University
8vo. Cloth. 365 pages. Price, $3.00 net.

The object of this volume is to set forth the main characteristics of the Protozoa without undertaking an
exhaustive description. It is intended for students and for general readers who wish to know what the Pro-
tozoa are, and what their relations are to current biological problems. In the first few chapters of the book
the Protozoa are treated as a phylum of the animal kingdom. A short historical sketch leading up to the
present systems of classification is followed by a general description of the group, touching upon some of the
more special subjects, such as mode of life, motion, excretion, respiration, reproduction, colony-formation,
encystment, etc., and this is followed by more general subjects dealing with the Protozoa in relation to man
and other animals; e.g. their sanitary aspects, parasitism, symbiosis, etc. 2

In the final chapter the Protozoa are dealt with from the standpoint of phylogeny. Theories as to the
origin of life, spontaneous generation, and the relations of the classes of Protozoa to one another are con-
sidered, and the volume ends with a discussion of the various views regarding the origin of the Metazoa from

the Protozoa.

VOL. VII. REGENERATION.

By THOMAS HUNT MORGAN,
Professor of Biology, Bryn Mawr College,
Author of ‘‘THE DEVELOPMENT OF THE FROG’S EGG.”

VOL. VII. AN INTRODUCTION TO COMPARA-
TIVE NEUROLOGY.

By OLIVER S. STRONG, Ph.D.

THE MACMILLAN COMPANY, 66 Fifth Avenue, New York.

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