The Contemporary

Science Series

The Germ-Plasm

®t|p i. 1. ItU SItbrarg

QH431
¥44

'^

^::=?L^.

N.C. STATE UNIVERSITY D.H. HILL LIBRARY

S001 42329 L

This book is due on the date indicated
below and is subject to an overdue
fine as posted at the circulation desk.

EXCEPTION: Date due will be
earlier if this item is RECALLED.

150M/01 -92— 941680

THE GERM-PLASM

/U<i. /O 9/.

THE GERM-PLASM

A THEORY OF HEREDITY

BY

AUGUST WEISMANN

Professor in the University of Freibiirg-in- Baden

TRANSLATED BY
\V. NEWTON PARKER Ph.D.

Professor in the University College of South Wales and Monmouthshire

AND

HARRIET RONNFELDT B.Sc

WI TH TWENTi^FV myiLL US TRA TIONS

NEW YORK
CHARLES SCRIBNER'S SONS

1893

COPYRIGHT, 1892, 1893, BY
CHARLES SCRIBNER'S SONS.

TO THE MEMORY

OF

CHARLES DARWIN

146931

NOTE BY TRANSLATOR

T N preparing the English edition of the present work, I have
-■- had the great advantage of being able to consiih Professor
Weismann personally with regard to many of the more ditificult
passarges." Only those who have attempted to make a translation
of an abstruse work from German manuscript, can appreciate
the difficulties of rendering such a work into good English, and
at the same time of keeping closely to the text. As the time in
which the translation had to be prepared was a comparatively
short one, I have been unable to revise the style as thoroughly
as I could have wished, but trust that the author's meaning has
been expressed wdth tolerable accuracy.

In the case of special technical terms which have no recog-
nised English equivalents, I have in all cases added the German
word in brackets the first time they are used. For the extremely
useful and untranslateable word ' Anlage/ the somewhat awkward
term ' primary constituent ' has been used when it refers to the
concrete vital units : in other cases, it has been rendered by
' rudiment ; ' or, when it has a more abstract meaning, by ' pre-
disposition.' The words ' Eigenschaft,' ' Charakter,' ' Merkmal,'
and ' Qualitat,' are often used synonymously by the author, and
have therefore been indiscriminately translated by ' character-
istic,' 'character,' 'peculiarity,' and ' quality.'

I must express my thanks to Dr. G. H. Parker, of Harvard
University, Cambridge, Mass., who kindly undertook a first
revision of Chapters XIII. and XIV., and thereby rendered an
earlier publication of the book possible ; as well as to my friend
and colleague Mr. Franck Arnold, for help in elucidating some
of the more complicated sentences, and for many suggestions.

W. N. PARKER.
Cardif. Nov. iZth, 1892.

vii

" Naturgeheimniss werde nachgestammelt." — GOETHE.

PREFACE

ANY attempt at the present time to work out a theory
of heredity in detail may appear to many premature,
and ahnost presumptuous : I confess there have been times
when it has seemed so even to myself I could not, how-
ever, resist the temptation to endeavour to penetrate the
mystery of this most marvellous and complex chapter of
life as far as my own ability and the present state of our
knowledge permitted.

Even though the present attempt may be very imperfect
and incomplete, I cannot regard it as premature. Our
knowledge has increased during the last twenty years to
such an extent, that it does not seem to be altogether a
hopeless task to inquire into the actual processes on which
the phenomena of heredity depend. It is, moreover, very
essential that we should possess a theory of heredity, worked
out in such a manner as to suggest new problems, which in
their turn will lead to new solutions.

Previous hypotheses have been insufficient in this respect,
owing to the fact that they have not been worked out in
detail. They are rather to be regarded as paving the way
to future theories, by merely formulating explanatory prin-
ciples without professing to apply them to all the different
groups of phenomena which come under the head of
heredity, by which means alone their true value can be
tested. Even Darwin's theory of * pangenesis' was inade-

ix

X PREFACE

quate in this respect : owing to the comparatively limited
number of facts then at his disposal, it could not but be
what we may call an ideal theor\- ; that is to say, it is
founded upon certain principles without inquiring how far
they are based upon actual facts. In themselves, such
theories can hardly be looked upon as suggestive, for if
once the assumed principle is accepted, all the phenomena are
thereby explained, and the matter is open to no further
doubt.

Let us assume that the germ contains millions of the
primary constituents ('Anlagen') of all the most minute
portions of the body ; moreover, that these constituents are
always present at the right place and in the right combina-
tion during the process of development ; and, further, that
they are capable of giving rise in their turn to the parts or
organs to which they severally correspond. Such a theory
explains everything, or nothing — the premises alone can be
attacked. No new problems can arise from it till it has
been placed upon a sound basis ; the premises must be
shown to be correct, and it must be proved that the germ is
actually composed of primary constituents, which by some
means or other become combined into groups and are
capable of giving rise to the various parts and organs in
question. Then, and then only, would the theory serve as
an incentive to further investigations into the phenomena of
heredity of all kinds, and experiments might be made which
would support or contradict it.

There is no doubt a natural tendency to base experiments
upon certain preconceived ideas ; but it is one thing to be
guided solely by such phenomena as may at the moment
appear of especial importance, and another to base opera-
tions upon the completed outline of a theory founded upon

PREFACE XI

the principal data bearing upon the question. I have myself
more than once abandoned a line of research undertaken in
connection with the problem of heredity, because I felt that
to proceed without the guidance of a theory more or less
complete in itself, and developed on a basis of ascertained
facts, would be little better than groping in the dark. The
importance of such a theory lies primarily in its suggestive-
ness, by which alone it becomes a step towards the ideal at
which we aim, viz., the formulation of the true and co?nplete
theory.

The growth of this book has been very gradual. What
first struck me when I began seriously to consider the
problem of heredity, some ten years ago, was the necessity
for assuming the existence of a special organised and living
hereditai-y substance, which in all multicellular organisms,
unlike the substance composing the perishable body of
the individual, is transmitted from generation to generation.
This is the theory of the contimtity of the germ-plasm. My
conclusions led me to doubt the usually accepted view
of the transmission of va7'iations acqiiired by the body
(soma) ; and further research, combined with experiments,
tended more and more to strengthen my conviction that in
point of fact no such transmission occurs. Meanwhile, the
investigations of several distinguished biologists — in which I
myself have had some share — on the process of fertilisation
and conjugation, brought about a complete revolution in
our previous ideas as to the meaning of this process, and
further led me to see that the germ-plasm is composed of
vital units, each of equal value, but differing in character,
containing all the primary constituents of an individual.
These ' ancestral germ-plasms ' (' Ahnenplasmen '), or ' ids,' as
I now prefer to call them, afforded additional matter where-

Xil PREFACE

with to construct a theory of heredity, though much was
wanting to render it complete.

In my last essay I certainly suggested the possibility of
solving one of the most difficult problems in heredity — viz.,
the co-operation of the hereditary substance of the parents
in sexual reproduction — by assuming the existence of these
' ids ' ; but I did not for a moment suppose that in doing
so I had propounded a complete and elaborated theory of
heredity, as some of my readers have thought to be the
case ; much still remained to be done first. I had as yet
not touched upon such phenomena of heredity as have no
direct bearing on the question of sexual reproduction, and
had also abstained from any mention of the fundamental
point of my theory of heredity — namely, the constitution of
the ids. Although I pointed out that they must possess
a complex structure which undergoes gradual and regular
changes during the development of the individual from the
egg-cell, I did not enter into any further details. This
question remained in abeyance, for I was by no means sure
whether the conception that I had formed on a priori
grounds of the minute structure of the ids would prove
tenable when viewed in the light of all the many phenomena
of heredity. No conclusion could be arrived at respecting
the structure of the ids till these phenomena had been
individually considered.

All my investigations on the problem of heredity were so
far only hnks, to be some day united into a chain which had
as yet no existence. The question of the ultimate elements
on which to base the theory was the very point on which I
remained longest in doubt. The ' pangenesis ' of Darwin,
as already mentioned, seemed to me to be far too inde-
pendent of facts, and even now I am of the opinion that the

PREFACE XUl

very hypothesis from which it derives its name is untenable.
There is now scarcely any doubt that the entire conception
of the production of the ' gemmules ' by the body-cells, their
separation from the latter, and their ' circulation,' is in real-
ity wholly imaginary. In this regard I am still quite as much
opposed to Darwin's views as formerly, for I beheve that
all parts of the body do not contribute to produce a germ
from which the new individual arises, but that, on the con-
trary, the offspring owes its origin to a peculiar substance
of extremely complicated structure, viz., the * germ-plasm.'
This substance can never be formed anew ; it can only
grow, multiply, and be transmitted from one generation to
another. My theory might therefore well be denominated
*" bias to -gene sis ' — or origin from a germ-plasm, in contradis-
tinction to Darwin's theory of ^pangenesis ' — or origin from
all parts of the body.

My doubts as to the validity of Darwin's theory were for a \,
long time not confined to this point alone : the assumption |
of the existence oi p7'eformed constituents of all parts of the
body seemed to me far too easy a solution of the difficulty, \
besides entailing an impossibility in the shape of an abso- |
lutely inconceivable aggregation of primary constituents. I ^
therefore endeavoured to see if it were not possible to
imagine that the germ-plasm, though of complex structure,
was not composed of such an immense number of particles,
and that its further complication arose subsequently in the
course of development. In other words, what I sought was
a substance from which the whole organism might arise by
epigefiesis, and not by evolution* After repeated attempts,

* The theory of * evolution ' or * preformation ' of the early physiolo-
gists supposed that all parts of the fully-formed animal or plant were
present, in a minute form, in the germ. The rival theory of * epigenesis '

XIV PREFACE

in which I more than once imagined myself successful, but
all of which broke down when further tested by facts, I
finally became convinced that an epigenetic development is
an impossibility. ^Moreover, I found an actual proof of the
reality of evolution, which will be explained in the chapter
on the structure of the germ-plasm. It is so simple and
obvious that. I can scarcely understand how it was possible
that it should have escaped my notice so long.

It is gratifying to me to find myself at one with the great
English naturalist Darwin, — as well as with de Vries and
Wiesner, — at all events in the main point at issue ; and this
agreement seems to me to point to the possibility of solving
in the end the problem of heredity, which might seem to be
open only to the wildest speculations : we may now perhaps
hope to succeed in recognising the probable explanations
among the vci'diwy possible ones, and in finally selecting from
among these the 7ral solution of the problem. This will
assuredly be the work of time, and our approach to the truth
will be a very gradual one. But our path is marked out ;
reasoning supported by observation will lead us to the goal.
We are led by the observation of facts to form an opinion
as to their bearing on each other. This gives rise to further
problems and fresh investigations, which in their turn lead
to a new interpretation. In this way light has before now
been thrown on many a problem that seemed to bafile
explanation. I need only mention the insight that we have
now gained into the phenomenon of sexual reproduction.

taught that there is no preformation of parts in the germ, but that the
fully-formed organism is produced by a gradual process of differentia-
tion. It will be seen that the word * evolution,' as here used, has no
connection with the doctrine of descent with which it is usually con-
nected. — VV. N. E

PREFACE XV

111 the same way we shall succeed in obtaming a firmer and
firmer grasp of the problem of heredity, which but a short
while ago appeared so utterly unapproachable.

What in this particular question appears to afford additional
promise of success is the fact that we can in a sense approach
it from two sides ; — namely, by observations, firstly, on the
plienomena of heredity, and secondly, on the hereditary sub-
stance itself, with which we are now of course acquainted.
We can now form an estimate as to whether an explanation
of any particular phenomenon of heredity is of a merely hypo-
thetical nature, or whether it may attain to the v^alue of an
established fact, inasmuch as we are in a position to judge,
within certain limits at all events, whether it is consistent
with the actual behaviour of the hereditary substance.
Hitherto this has not been possible, and hence all previous
theories, including both that of IJarwin's gemmules and of
Herbert Spencer's units, were up to a certain point purely
speculative. We are now better off in this respect ; and I
have no doubt that further research will enable us to pene-
trate far more deeply still into the complicated processes
connected with the idioplasm, if we are prepared to reason
on the results of our observations, and to utilise every
theoretical advance as an incentive to fresh questions
regarding the processes in connection with the distribution
of the mysterious nuclear substance.

We are still far from having attained a complete insight
into the matter, but I trust nevertheless that the present
attempt at a theory of heredity is no mere work of the
imagination ; and though it still be no more than an attempt,
which will be followed by better ones, I venture to believe
that time will prove it to contain more definite points,
forming the centre of numerous possibilities, than many will

XVI PREFACE

for the present be prepared to admit. Nevertheless I am
well aware that it is but the beginning of a theory, and for
this reason I have presented it in the form of an inquiry
rather than of an established system. My plan has been
not so much to advance doctrines as to propound questions,
and to answer them with a greater or lesser degree of cer-
tainty, or in some cases even to leave them to be decided by
future researches. I do not regard my theory as a complete
and perfect one, but trust that it is of such a nature as to be
capable of improvement and further development.

It has been my endeavour to write as simply and intel-
ligibly as possible ; not as a specialist writing for speciaHsts,
but as one who desires to make his case clear to all inter-
ested in biological problems. For this reason a number of
figures have been inserted, which, though perhaps super-
fluous for specialists, will, I trust, assist all who are less con-
versant with the subject, such as physiologists, medical men,
and indeed all interested in natural science, to a clearer
conception of the matters under discussion.

As a zoologist, I have naturally, in the first instance,
considered the phenomena in their relation to animals, for
every one must base his ideas on the facts most famihar to
him. I have, however, done my best to lay due weight on
the data afforded by the study of plants, and to take into
account the views of botanists. It will be seen that the
very facts which are furnished by certain hereditary phe-
nomena in plants afford a strong support to certain funda-
mental points in my theory, and that even those which are
at first sight in apparent contradiction, are in reality in per-
fect accordance with it.

It may perhaps be considered by medical men that I
ought to have brought forward more evidence with regard

PREFACE XVI 1

to diseases. We certainly possess a rich material on which
observations concerning the transmission of diseases might
be based, and this I have made use of so far as seemed
expedient. It must, however, not be forgotten that the
transmission of so-called hereditary diseases is not always
due to a true process of heredity, but in some cases, at any
rate, results from an infection of the germ. Unfortunately,
we are not always able to distinguish between these two
causes ; and as long as this is the case, the data furnished
by diseases can only be used with great caution, as will be
shown in Chapter XII.

The manuscript of this book was practically completed by
the end of April last, but as the translation had then to be
made, its publication was delayed for some months. This
will account for the fact that no mention, or only a brief
one, has been made of researches which have appeared in
the interval. My sincerest thanks are due to the translator
— Professor W. N. Parker, — whose task has been by no
means an easy one : apart from the mere knowledge of
the two languages, an intimate acquaintance with the facts
treated of and with the whole science of biology is essential
in order to render the meaning of this complicated subject
clear, and at the same time to reproduce the original text
with anything like accuracy. I am of course unable to
judge how far Mr. Parker has succeeded in clothing my
ideas in good English, but am glad to state that they have
been given very correctly, so far as I can judge from those
parts which we have discussed together.

In conclusion, I must express my warmest thanks to the
Government under which I have the good fortune to live,
for the efficient way in which they have seconded my
endeavours, by releasing me from my academical duties

XVlll PREFACE

during two winter sessions. My hearty thanks are also due
to my friends and colleagues Professors Baumann, Liiroth,
Wiedersheim, and Ziegler, as well as to Professor Goebel, of
Munich, for information of various kinds ; and I am no less
indebted to Miss Else Diestel, who, in addition to much
help of a technical nature, has also been at the great pains
of preparing an alphabetical index.

I thus venture to bring into the light of day a work which
is the fruit of many years labour and of many doubts ; and
even though but few of my results should remain unmodi-
fied, I hope nevertheless that my work has not been in
vain ; for even error, if it originate in correct deductions,
must become a step towards truth.

AUGUST WEISMANN.

Freiburg, i/Br.,
May igth 1892.

CONTENTS

INTRODUCTION

PAGE

A. — HISTORICAL PART i

B. — DESCRIPTIVE PART ... . . .20

PART I.— THE MATERIAL BASIS OF HEREDITY

CHAPTER I

The Germ-Plasm —

1. The fundamental units ....... 37

2. The control of the cell ....... 45

3. The determinants ........ 53

4. The id in ontogeny ........ 60

5. Summary of sections 1-4, relating to the structure of the

germ-plasm . . -75

6. The mechanism for the phyletic changes in the germ-

plasm . ......... 77

7. The magnitude of the constituents of the germ-plasm . 85

PART II. — HEREDITY IN ITS RELATION TO
MONOGONIC REPRODUCTION

CHAPTER II
Regeneration —

1. Its cause and origin in the idoplasm • • • • 93

2. The phylogeny of regeneration . . . . .114

3. Facultative or polygenetic regeneration . . . .126

4. Regeneration in plants . . . . . . .132

5. Regeneration in animal embryos, and the principles of

ontogeny . . , ' , . . . . -134

xix

XX CONTENTS

CHAPTER III

PAGE

Multiplication r,\ Fission —

1. Preliminary remarks ....... 14c

2. The process of fission in the Naidcp . . . .146

3. The process of fission in the i\Iic7-oslomid(T . . . 149

4. The phylogeny of the process of multiplication by fission 151

CHAPTER IV

Multiplication by Gemmation —

1. The process of gemmation in animals .... 154

a. Coelenterata . . . . . . . .154

d. Polyzoa . . . . . . . . .158

c. Tunicata ........ i6o

2. The process of gemmation in plants . . . .163

3. Comparison of the process of gemmation in animals and

plants 166

4. The phylogeny of the process of multiplication by gem-

mation ......... 168

CHAPTER V

Alternation of Generations in its Relation to the

Idioplasm 173

CHAPTER VI

The Formation of Germ-Cells —

1. The continuity of the germ-plasm ..... 183

2. The germ-tracks ........ 192

3. Historical account of the theory of the continuity of the

germ-plasm . . . . . . . . .198

4. Objections to the theory of the germ-plasm . . . 202

5. Galls 218

CHAPTER VII
Summary of Part II. (Chapters II. -VI.) .... 225

PART III. — THE PHENOMENA OF HERED-
ITY RESULTING FROM SEXUAL RE-
PRODUCTION

Introductory Remarks on the Nature of Sexual Repro-
duction 230

CONTENTS

XXI

CHAPTER VIII

Modifications of the Germ-Plasm caused by Amphi-
mixis—

1. The necessity of a halving of the germ-plasm .

2. Proof that the essential part in the process of ' reducing

division ' consists in the extrusion of ids

PAGE

235

240

CHAPTER IX

Ontogeny Resulting from the Union of the Germ-Plasm
OF TWO Parents —

1. The nature of the offspring as determined by the process

of fertilisation ........

2. The share taken by the ancestors in composition of the

germ -plasm .......

3. The struggle of the ids in ontogeny .

a. Plant-hybrids ....

b. Intercalary remarks on variation .

c. The struggle of individual characters

4. The force of heredity ....

5. Summary of Chapter IX.

CHAPTER X

The Phenomena of Reversion in their Relation to
Amphimixis —

1. Reversion to racial characters in plant-hybrids

2. Reversion to individual characters in man

3. Reversion to the characters of ancestors far removed in

animals and plants ....

4. Reversion to rudimentary characters

5. Preliminary summary of sections 1-5

6. Reversion in asexual reproduction .

a. Reversion in the process of gemmation
h. Reversion in parthenogenesis

7. Proof that the determinants become disintegrated into

biophors .........

CHAPTER XI

Dimorphism and Polymorphism —

1. Normal dimorphism .......

2. Pathological dimorphism : haemophilia ....

3. Polymorphism ........

4. Dichogeny in plants .......

253

256
260
260
271
274
290

293

299
308

316

335

338
344

348

352
370
374
380

PAGE

XXll CONTENTS

CHAPTER XII

Doubtful Phenomena of Heredity —

1. * Xenia ' and telegony ....... 383

2. The influence of temporary abnormal conditions of the

parents on the offspring ...... 386

3. The transmission of diseases ..... 387

PART IV. — THE TRANSFORMATION OF SPE-
CIES: ITS ORIGIN IN THE IDIOPLASM

* CHAPTER XIII

The Supposed Transmission of Acquired Characters —

1. Difficulties in the way of a theoretical basis for this

assumption ........ 392

2. The hypothesis tested by facts ..... 396

3. Climatic variation in butterflies ..... 399

CHAPTER XIV

Variation —

1. Normal individual variation . . . . . .410

2. Pathological variation ....... 428

3. Summary of the above two sections and conclusions . 431

4. Variations on a larger scale ...... 435

a. The origin of these variations .... 435

b. The transmission of these variations . . . 444

SUMMARY AND CONCLUSION 450

LIST OF FIGURES

Fig. I. Diagram of nuclear division.

2. Two idants of Ascaris megalocephala.

3. Diagram of the cell-generations in the fore-limb of a newt.

4. Multiplication by fission in a marine worm {A/yriaiiida).

5. Transverse section through the 'zone of gemmation' in a

fresh-water worm (^iVais).

6. An early stage in the formation of a bud in a Hydroid polype

(^Eitdendruini) .

7. Apex of a shoot of a fresh-water Alga (^Chara).

8. Female water-flea {Daphnia) , with summer eggs.

9. Nauplius larva of Leptodora hyalina.

10. Alternation of generations in Botigainvillea.

11. Strobilation of a Medusa.

12. Degeneration of a Medusa into a mere gonophore.

13. Early stages of development of the primitive germ-cells in

Sagitta.

14. Early stages in the development of the primitive germ-cells in

Moina.

15. Early stage of the embryo of Rhabditis nigrovenosa.

16. Diagram of the germ-track in Rhabditis nigrovenosa.

17. Volvox and Pandornia.

18. Process of fertilisation in Ascaris megalocephala .

19. Diagram to illustrate the composition of the idants out of

individually different ids.

20. Formation of spermatozoa in Ascaris viegalocephala.

21. Formation of ova in Ascaris megalocephala.

22. Diagram to illustrate the combination of idants in hybriils.

23. The two varieties of Cypris reptans.

24. Bonellia viridis, male and female.

xxiii

INTRODUCTION

H

A. — Historical Part

ERBERT SPENCER was practically the first in the
present generation to attempt a theoretical explanation
of heredity when he propounded his theory of ' physiolog-
ical units.' The regeneration of lost parts, e.g., of a leg or the
tail of a salamander, led him to the conception of these units,
' in all of which there dwells the intrinsic aptitude to aggregate
into the form of that species ; just as in the atoms of a salt
there dwells the intrinsic aptitude to crystallise in a particular
way.'' He calls this aptitude the ' polarity of the organic units,'
and defines the latter as being intermediate between the
' chemical units ' or molecules and the ' morphological units '
or cells. They must be 'immensely more complex than the
chemical units,' and must therefore correspond to groups of
molecules. It is very interesting at the present day, now that
we have advanced somewhat further towards a theory of
heredity, to summarise the various aptitudes and forces which
Herbert Spencer thought it necessary to ascribe to his ' physio-
logical units,' in order to arrive at an explanation of the
phenomena. Although the sections on Heredity and Regener-
ation constitute only a small portion of his great work on the
' Principles of Biology,' and cannot therefore contain a detailed
treatment of the phenomena of heredity, his opinions on this
subject are evident.

Spencer considers, on the one hand, that the whole organism
is composed of these units, which are all alike in kind, and on
the other, that the germ-cells also contain small groups of them.
The former supposition makes regeneration possible to each
sufficiently large portion of the body, while the latter gives the
germ-cell the power of reproducing the whole: inasmuch as the

- ^ I

2 THE (iERM-PI^-VSM

' polarity " of the ' units " leads to their arrangement in such a
way that the whole 'crystal' — the organism — is restored, or
even formed anew. The mere difference in the arrangement of
units alike in kind determines the diversity of \\\^ parts of the
body, while the distinction between different species and that
between different individitals is due to a diversity in the con-
stitution of the units.

The units of an individual are therefore to a certain extent pro-
tean. They are capable of arranging themselves in an immense
variety of ways, and so form the most diverse cells, tissues,
organs, and parts of the body. But they only do this under the
directing influence of the whole, in such a way that the whole
forces the units of one part to arrange themselves in just such
a way as is necessary for the perfection of that part, — a perfec-
tion required for the harmony of the w-hole. Spencer himself
says very rightly, ' It seems difficult to conceive that this can
be so, but we see that it is so.' As a matter of fact, groups of
units removed from an organism possess the power of construct-
ing the whole anew ; and we are thus obliged to admit that the
tendency to take a specific form is present in all parts of the
organism. The 'units' are physiologically variable quantities,
w'hich in every case act in such a manner as the whole demands.

The assumption of these ' physiological units ' does not suffice
as an explanation of heredity : it proves insufficient even as
interpreting the diflferentiatior). of organs in simple ontogeny,
quite apart from the question of amphigonic heredity. But it
has the merit of having utilised the smallest vital particles as
constituent elements of the organism, and of having made them
the basis of a theory of heredity.

Ernst Briicke w-as the first to admit the existence of small vital
particles of this kind, and to give cogent reasons for so doing.
Although he did not denote them by any special name in his
extremely important paper entitled • Elementar Organismen,' * he
was the first to oppose the old theory of the cell, especially with
regard to its fluid contents, and to show that its body must pos-
sess an organisation, quite distinct from the molecular structure
of the organic compounds.

Darwin's theory of ' pangenesis ' was stated in the final chapter
of his great work on ' The Variation of Animals and Plants

* ' Wiener Sitzungsberichte,' Oct. lo, 1861, Bd. 44, ii., p. 381,

INTRODUCTION 3

under Domestication,' which appeared only a few years after
Spencer's ' Principles of Biology.'' The enormous wealth of facts
bearing on heredity which is accumulated in this book is in itself
sufficient to show how the gifted author felt himself urged on all
sides to consider this extremely difficult and complicated prob-
lem. For although Darwin modestly described his theory as
a provisional hypothesis, his was, nevertheless, the first compre-
hensive attempt to explain all the known phenomena of heredity
by a common principle. The theory has so often been discussed i
and is so well known that a brief account of its substance will
suffice here.

A multicellular organism, whether animal or vegetable, is
gradually built up by cell-division: but it is assumed that this
method of multiplication is not the only one. Each cell pos-
sesses in addition, at each stage of its development, the power
of giving off invisible granules or atoms, which, at a later period
and under certain conditions, can develop again into cells simi-
lar to those from which they originated. Numbers of these
' gemmules ' are being given off continually from all cells of the
body and conveyed into the blood, and thus circulate through
the body, finally settling down in some part, principally in
those regions in which the development of offspring will take
place later on, /.<?., in buds or germ-cells. As gemmules from
all the cells of the body are aggregated in these cells, they invest
the latter with the power of developing into a new and complete
organism. This occurs as follows: — each gemmule reproduces
the cell from which it is derived, and the gemmules of the
different cells become active in the same order as that in which
the corresponding cells followed each other in the ontogeny of
the parent.

The germ is not by any means composed exclusively of\
gemmules which have been derived from the organism in which
they were formed, but consists, at the same time, of a \er\-
large number of gemmules which are derived from parents and
ancestors even of very remote generations ; and hence a great
many more gemmules take part in each case of ontogeny than
there are cells formed. Each cell and each part is represented
by a great variety of gemmules. A selection must therefore
take place, as only one gemmule can form the required cell, and
the rest must remain dormant. In this way a number of gem-
mules, which have been hitherto dormant, are transferred from

4 THE r.ERM-PIASM

one generation to the next : they may, under certain conditions,
l)ecome active, and thus again bring into existence ancestral
traits which had disappeared in the parents.

This is, in brief, the theory of pangenesis. It does not take
into account the physical nature of the gemmules. They are
capable of multiplying, and do so continually : but the question
as to whether thev have anv definite arrangement, and if so.
what the nature of that arrangement is, is not touched upon ;
nor is any mention made of the causes and mechanism bv which
it comes about that they are always in the right place and
develop into cells at the right time.

I do not say this by any means as a reproach, but only to
bring out clearly the speculative character of the whole hypothe-
sis. Darwin did not go on to inquire whether all these assump-
tions were possible : he only asked what it was necessary to
assume in order to explain this or that fact of heredity, without
troubling himself to consider whether the assumption were borne
out by facts or not. And he was right in doing so, for at the
time when he propounded his hypothesis it was not possible to
found any theory of heredity on the only sound basis, — that of
a knowledge of the most minute cell-structure. I have already
pointed out how extremely important and fruitful his theory
of pangenesis has been : it drew attention for the first time to
all the phenomena that needed explanation, and showed what
assumptions must be made in order to explain them.

It will be shown later on that, in spite of the fact that a con-
siderable number of these assumptions are untenable, a part of
the theory still remains which must be accepted as fundamental
and correct, — in principle at any rate, — not only now, but also
for all time to come. I refer to the most general portion of
these assumptions only, namely, that presupposing the existence
of material particles in the germ which possess the properties of
the living being, and each of which is to be regarded as the
primary constituent (' Anlage ') of one portion of the organism.
I must honestly confess to having mentally resisted this fun-
damental point of the Darwinian doctrine for a long time. It
appeared almost impossible to me that such an enormously large
number of individual primary constituents as we must suppose
to exist, according to Darwin's view, could be contained in the
minimum of substance which, as will be shown hereafter, w^e have
to regard as the actual bearer of heredity. 1 tried in several

INTRODUCTION

5

ways to arrive at a satisfactory epigenetic theory,* which, start-
ing from a germ-substance of comparatively simple structure,
should exhibit the various differentiations of the organism as due
to regular changes brought about by the division of this primary
structure. But the more I considered the prol^lem as time
went on, the more I was convinced that such a solution was
impossible. And in this book I trust that 1 shall be able to
give a satisfactory proof that only one theory of evolution in
Darwin's sense, i.e.^ the assumption of minute primary constitu-
ents in the germ, is in accordance with the facts ; and the objec-
tion which for a long time prevented me from accepting this
very simple assumption, disappears with the discovery that what
is apparently impossible does really occur.

I certainly consider even now that Darwin's theory must be
looked upon, and that he probably considered it, rather as an
inquiry into the problem of heredity than as a solution of the
problem. His assumptions do not, properly speaking, explain
the phenomena. They are to a certain extent a mere para-
phrase of the facts, an explanation of a purely formal nature,
based on speculative assumptions, which were made not because
they seemed possible, or even likely, but because they provided
a formal explanation of all the phenomena on one principle.
If we suppose that each cell arises from a special gemmule, and
that these gemmules are present wherever they are wanted, it is
easy to see how that structure, the origin of which we wish to
explain, may appear in any given position. Further, when a
large number of cells is to arise in regular succession from 07ii
egg-cell, the desired sequence of cells must of course result
if we assume that the gemmules present become active in the
required order. But this supposition does not really explain
the phenomena. Even at the present day our explanations are
imperfect enough, and are far from going to the bottom of the
matter, but they dilTer from Darwin's provisional hypothesis in
that they attempt to find out the actual facts concerned in the
processes, and to arrive at a real, and not merely a formal, solu-
tion of the problem. The great naturalist's merit in having at
once found the right foundation on which to base a real solution

* The indication of such a theory is given, e.g., in the essay entitled
' Die Continuitat des Keimplasma's,' Jena, 1885, p. 38 et seq. (pp. 207 et seq.
of the English translation).

6 THE GERM-PL.'VSM

is not diminished by the fact of his having been less startled by
the consequences of his ' gemmule '-hypothesis when seeking for
a purely formal explanation, than he would have been had he
tried to adapt his hypothesis to the facts. The hypothesis, as
stated by him, could not be regarded as a real solution of the
problem of heredity, if only because it leaves unexplained the
giving off of the gemmules into the blood, their circulation
through the body, and intrusion into the germ- and other cells.
All these are assumptions without a basis in fact. This is evi-
dently the reason why modifications of the theory of pangenesis
were repeatedly made very soon afterwards.

Before considering these modifications, I should like once
more to state clearly the relation of Spencers ' physiological
units' to Darwin's 'gemmules.' Darwin himself considered
the former to be closely related to his gemmules ; and, in
fact, he would have regarded Spencer's ideas as essentially
coinciding with his own, had he not noticed certain passages
in Spencer's book which seemed to point to something quite
different.*

It will be apparent, I think, from what has already been said,
that these two views are entirely different. What is common
to both is that they assume the existence of minute living units,
multiplying by fission : but the part taken by them in the con-
stitution of the body is quite differently conceived. Spencer's
units are the elements which exclusively compose the living
body ; while Darwin's gemmules only give rise to cells, i.e.,
they are elements which are present for the special purpose of
bringing about heredity, without anything being specified as
to their share in the composition of the living body. As will
be shown more clearly later on, Spencer's hypothesis is superior
in this respect to Darwin's. On the other hand, Spencer's simi-
lar units are the bearers of all the characters of the species,
owing to their complex molecular structure : while the Dar-
winian gemmules are primary constituents of individual cells,
which are to be considered as differing in a manner correspond-
ing to the difference of the individual cells. Spencer's theory
is epigenetic, Darwin's evolutionary ; in this respect the latter is,
in my opinion, superior to the former.

♦Charles Darwin, 'The Variation of Animals and Plants under Domes-
tication," 2nd ed., vol. ii., London, 1888; note on p. 371.

INTRODUCTION 7

Galton * was the first to make an attempt to improve on the
theory of pangenesis. In a short but suggestive essay he
accepted the hypothesis of the gemmules, but rejected the
doctrines of their circuhition through the blood, and of the
aggregation in the germ-cells of gemmules given off by the body-
cells. Now as the gemmules which have been converted into
body-cells are used up, it follows that the germ-cells can only
contain those gemmules which are left — those, out of the enor-
mous number contained in a germ-cell, which have not developed
further. For each germ-cell, as both Galton and Darwin assume,
contains each kind of gemmule in many modifications, originat-
ing from the different ancestors of the organism. The theory
of the origin of the germ-cells from the remains of the germ
mass not used up in ontogeny (' the residue of the stirp ') has
been compared to, and regarded as the precursor of, the con-
ception of the continuity of the germ-plasm which I originated
long afterwards. A certain resemblance .does, it is true, exist
between the two conceptions, but it will be shown in the section
on the continuity of the germ-plasm that the similarity is only a
superficial one.

Herbert Spencer defines heredity as the capacity of every
plant and animal to produce other individuals of a like kind,
and states expressly that in this fact, which is perfectly famil-
iar to us, and for this reason seems to be a matter of course,
lies the real essence and principle of heredity, ^ the phenom-
ena commonly referred to it being quite subordinate mani-
festations.' Thus the blending of the individual ^characters''
of the paretits in the children has, as a rule, been placed in
the foreground in considering questions of heredity, and it has
been overlooked that this is quite a secondary phenomenon, —
important no doubt in many respects, and interesting in a high
degree, but still only the result of a certain mode of multipli-
cation, i.e., sexual reproduction, and by no means an essential
phenomenon of heredity. Darwin recognised this distinctly,
and concerned himself primarily with the theoretical explana-
tion of individual development (ontogeny). But the majority
of writers on heredity, including Galton, have turned their
whole attention to the blending of the qualities of the parents

* Francis Galton, ' A Theory of Heredity,' Journal of the Anthropo-
logical Institute, 1875.

8 THE GERM-PI,AS^i

in the children, — a problem which is doubtless well worthy of
investigation, but which, at the same time, deals only with a
side issue of the processes of reproduction. How little I under-
rate the significance of amphigonic heredity, even in its theo-
retical relations, will be evident in a later part of this book, in
which I attempt to derive the existence of the germ-plasm from
the phenomena of this form of heredity : but to me it seems
dangerous to investigate heredity theoretically from the point
of view of amphigonic descent exclusively, because one has here
to deal with the most complex of all the phenomena, and the
main point may easily be overlooked in a mass of confusing
secondary considerations. Even Galton, in my opinion, allowed
himself to be too much influenced by this aspect of the ques-
tion. Excellent as are his later researches on the laws relating
to the blending of characters of the parents in the children, I
consider his theoretical deductions on the fundamental phe-
nomena of heredity unsatisfactory. The few hints that he gives
as to the cause of ontogeny seem to me by no means equal to
Darwin's simple but truly penetrating and accurate deductions.
It is quite conceivable that the phenomena of the blending of
the characters of the parents in the children would be the most
interesting to a statistician and anthropologist like Galton, but
they have kept him w'ithin the limited range of these phenomena,
and have prevented him from arriving at really general principles
and at a comprehensive theory of heredity.

Galton has, however, the merit of having been the first to
deny the circulation of the gemmules, and, in connection with
this, to cast doubt upon the general validity of the doctrine of
the transmission of acquired modifications. He certainly be-
lieves the latter to be ' faintly heritable,^ and assumes, in order
to explain this transmission, that no general ^circulation of the
gemmules ' takes place, but that each cell sets free some gem-
mules which get into the circulation and eventually penetrate
into the sexual elements.

Galton's essay was published only a few years after the ap-
pearance of Darwin's theory of pangenesis ; but it cannot be
said that it exercised any influence on the subsequent develop-
ment of the theory of heredity. Apparently it was not much
noticed even in England, and on the Continent it remained
unknown for a long time. This must be my excuse for being
ignorant of the existence of this paper, and consequently for

INTRODUCTiON 9

not referring to it in my essays which appeared nearly ten years
later.* In one of these essays 'On Heredity' (1883), I con-
tested at first in general terms not only the existence but also
the theoretical possibility of the transmission of acquired charac-
ters, and tried to release the theory from the necessity of an
explanation which deprived it of any further development. In
this essay I further assumed the existence in the germ-cell of a
reproductive substance, the germ-plasm^ which cannot be formed
spontaneously, but is always passed on from the germ-cell in
which an organism originates in direct coiititmity to the germ-
cells of the succeeding generations. The diiTerence between the
• boay ' in the narrower sense (soma) and the reproductive cells
was also emphasised, and it was maintained that the germ-cells
alone transmit the reproductive substance or germ-plasm in unin-
terrupted succession from one generation to the next, while the
body (soma) which bears and nourishes the germ-cells, is, in a
certain sense, onlv an outgrowth from one of them.

A second attempt to improve upon the theory of pangenesis
must be considered here. I have already referred elsewhere f to
the interesting and ingenious book on ' The Laws of Heredity,' %
by W. K. Brooks. The author retains the fundamental points
of this theory, viz., the formation of gemmules in all the cells
of the body, their circulation through the latter, and their aggre-
gation in the germ-cells or buds : he dilTers, however, from
Darwin principally in ascribing to the male germ-cell a particu-
larly strong power of attraction for the gemmules, so that it
collects a special mass of them and stores them up. As this
assumption is made chiefly for the purpose of explaining varia-

* These essays first appeared separately in the years 1881-91. The only
complete edition of the collected essays which has hitherto appeared is the
English translation, ' Essays upon Heredity and Kindred Biological Prob-
lems' (edited by Poulton, Schonland, and Shipley, Oxford, 1889), contain-
ing Essays I, — VIII. A second edition appeared in 1891 as Vol. I., and
Essays IX. — XII. follow this year as \'ol. II. A French translation of all
these essays, with the exception of the last on ' Amphimixis,' cS:c., has also
appeared with the title, ' Essais siir I'Heredite et la Selection Xaturelle,'
traduits par Henry de Varigny, Paris, 1892.

t ' The Significance of Sexual Reproduction in the Theory of Natural
Selection.' — ' Essays upon Heredity,' p. 326.

t W. K. Brooks, ' The Laws of Heredity, a Study of the Cause of
Variation and the Origin of Living Organisms,' Baltimore, 1883.

10 THE GERM-PLASM

tion, I shall postpone any further consideration of it to the
section which treats of this subject.

In the following year, Nageli's - Mechanico-physiological
Theory of Descent'* appeared. This book, which abounds
in ingenious deductions and important suggestions, doubtless
exercised a great influence on the views of that time. Its im-
portance cannot be denied, even if, as I believe to be the case,
only a small portion of its theoretical propositions can be
retained. Many as are the fruitful ideas and anticipation of
facts afterwards proved which we owe to Niigeli, his own theory
of heredity has already become untenable. For this reason,
and also because the theory is so well-known, I will not describe
it fully here, but will only refer to the remarks which I made
on the subject some years ago,t and to the recent detailed criti-
cism by Wiesner.:!: Although I do not consider that Nageli's
hypothesis leads us towards a tme theory of heredity, it never-
theless contains an important suggestion, that of the idioplasfn,
which gives us a further insight into the problem. I had already
assumed the existence of a special reproductive substance — the
germ-plasm — 0:1 the changes of which development depends,
while heredity rests on its continuity : and now Nageli inde-
pendently postulated a special reproductive substance, an • Anla-
genplasma' or 'idioplasm.' which although much smaller in bulk
than the rest of the living substance of the body — the tropho-
plasm (• Ernahrungsplasma ') — determines the detailed constiuc-
tion of the latter. The correctness of this conjecture has not as
yet, so far as I know, been disputed, although it was very soon
shown that Nageli was wrong as regards the form in which he
imagined the idioplasm to exist. He represented it as consist-
ing of very fine parallel fibres which, by uniting into bundles and
crossing each other so as to form a network, traverse the sub-
stance of the cell, and being continuous from cell to cell, pervade
the whole body as a connected network.

At the time when Nageli's book appeared, it was already
suspected that the reproductive substance is not contained in

* C. V. Nageli, ' Mechanisch-physiologische Theorie der Abstam-
mungslehre,' Miinchen and Leipzig, 1884.

t Vide 'The Continuity of the Germ-plasm,' 1885 (pp. 180 et seq.,
192, &c.).

X Julius Wiesner, ' Die Elementarstructur und das Wachsthum der
lebenden Substanz,' Wien, 1892.

INTRODUCTION I T

the body of the cell but in its nucleus, and several discoveries
were made shortly afterwards which rendered it certain that the
idioplasm is to be looked for in the ' chromosomes ' of the nucleus,
— those rod-like, coiled, or grain-like structures which are dis-
tinguished by their remarkable affinity for certain colouring mat-
ters. I shall return to the proof of this fact in the following
section.

From this time onwards each subsequent theory of heredity
was based on a firm foundation of fact. It was now not only
known that the phenomena of heredity among the higher organ-
isms are connected with a definite substance, but the seat of the
latter had also been ascertained. I now therefore adopted this
firm basis for my theory of the germ-plasm, if I may call the im-
perfect form in which it then existed by such a name : I localised
the germ-plasm in the nuclear substance of the germ-cell, and
supposed that ontogeny was due to a qualitative change in it,
which hands the idioplasm on from one generation to the next
bv means of nuclear- and cell-division. But I soon went further.
From the fact of sexual reproduction, which brings together
equal amounts of paternal and maternal germ-plasm at each fer-
tilisation, I inferred not only the composition of the germ-plasm
out of a number of units, th^ '■ ancestral germ-plasms'' ('Ahnen-
plasmen '), but also the necessity of a reduction of the germ-
plasm each time to one-half of its bulk, as well as a reduction of
the number of the ancestral germ-plasms contained in it.* The
hypothesis of the ' reducing divisions of the germ-cells ' has been
thoroughly substantiated by subsequent observations : in fact it
has even been proved that in many cases this reduction occurs
exactly as I had foretold and had represented in a diagrammatic
figure ; f that is to say, by the non-occurrence of the longitudinal
division of the chromosomes which occurs in the ordinary nuclear
division, and by the distribution of these in the daughter-nuclei.
This holds good for the ovum as well as for the sperm-cell in
animals, and, as far as is known, in plants also. The germ-cell
must in all cases by division get rid of half of its nuclear rods, —
that is to say, of its germ-plasm, — in order to become capable
of fertilisation. This fact supports the other assumption of the
construction of the germ-plasm from ancestral germ-plasms,
which are not minute vital particles — analogous to Spencer's

* ' On the Number of Polar Bodies,' &c., 1887. t Ibid.

12 THE GKRM-PLASM

physiological units — but rather bodies of a highly complex
constitution, each containing all the primary constituents which
are necessary to the formation of an organism. Each ancestral
germ-plasm seemed to me to be of a ' special kind/ and just as
many ' different kinds of idioplasm ' are removed by the reducing
division ' from the ovum as are afterwards introduced bv the
sperm-nucleus' on fertilisation. It will be seen that I retain this
essential basis of the theory of the germ-plasm in its further
development as presented here, and I trust that I may now suc-
ceed in refuting the objections which have been urged against
.the '■ ancestral germ-plasms,' or, as I now call them, the ' ids.''
In any case it cannot be denied that they help to throw an
important light on the subject.

De Vries has so far been my most powerful opponent as re-
gards the ancestral germ-plasms, but his opposition is founded
on the misunderstanding I have already referred to, for he looks
upon them as the ultimate vital particles — an idea which was
foreign to me from the beginning. Of this, however, I do not
complain, as at that time I had left the question as to the
construction of the ancestral germ-plasms unanswered.* This
omission is supplied in the present book, and it will be shown
that, although each ancestral germ-plasm is in my opinion a
bearer of all the primary constituents required for the construc-
tion of an organism, my assumption does not exclude the possi-
bility of its being composed of these constituents in the form of
minute vital particles. The 'ancestral germ-plasm' is indeed a
unit, but one of a higher order. For this reason alone it can-
not be compared with Spencer's ' physiological units,' because
the latter, as ultimate vital particles, compose the whole body ;
while the ancestral germ-plasms only form the nuclear matter,
and merely serve the mechanical purpose of the processes of
heredity.

De Vries has in a significant manner developed a theory of

* De Vries is also mistaken in ascribing to me the opinion that ' there is
only one hereditary substance — only one material bearer of the hereditary
tendencies in each individual." The sentence quoted by him (' On the
Number of Polar Bodies,' p. 355) does not deal with this question ; it runs
as follows : — From several reasons already stated ' at least one certain
result follows, viz., that there is an hereditary substance, a material bearer
of hereditary tendencies, and that this substance is contained in the nucleus
of the germ-cell,' &c.

INTRODUCTION 1 3

heredity in his essay on intracellular Pangenesis."* The
opinions there expressed really contradict the title of the paper ;
for pangenesis, in Darwin's sense, means the development of
gemmules throughout the body, — the composition of the heredi-
tary substance from gemmules which are derived from all the
cells of the body. This very point in Darwin's hypothesis is set
aside completely by de Vries : the most characteristic part of it
is removed, and what remains is of a more general nature, con-
sisting of principles which, in one form or another, must form
the basis of every theory of heredity, at the present day at any
rate. Some ideas of his own. however, are then added, and it
is these which give a characteristic stamp to his whole series of
conceptions. If we regard his hypothesis, as de Vries himself
does, as an alteration of the Darwinian theory of pangenesis, it
is certainly a radical one, and is of such a kind as at one stroke
to infuse new life into the latter, which had become untenable
in its original form.

De Vries distinguishes two parts in Darwin's theory of pan-
genesis, one of which he rejects, while he retains the other. He
calls the former portion the ' transport hypothesis,' meaning
thereby the assumption of the origin of the gemmules in all the
cells of the body, their separation from the cells, circulation in
the blood, and ultimate aggregation in the germ-cells. And
relying on my rejection of the heredity of "^ somatogenic' charac-
ters, he shows that the assumption of the transportation of
the gemmules from all the cells of the body to the germ-cells
is superfluous. He thus does away with that portion of the
hypothesis of pangenesis which makes it unacceptable to most
people, and places the theory on a new and firmer foundation
on which it is capable of further development.

De Vries nevertheless goes too far if he looks upon the
' transport hypothesis ' as necessary only for explaining the
transmission of somatogenic qualities. It must not be for-
gotten that the idea of the continuity of the germ-plasm did not
exist in Darwin^s time. How could the gemmules of all the
cells of an organism enter its germ-cells unless they are formed
irr'the'~body-ceirs7 migrate therefrom, circulate through the body,
and come together in the germ-cells? A direct connection
between the fertilised egg-cell and the germ-cells of the organism

* ' Die Intracelliilare Pangenesis,' Jena, 1889.

14 THE GERM-PLASM

arising from it was not supposed to exist by any one at that
time, nor does it do so except in isolated cases. The ' transport
hypothesis " was therefore also necessary in order to explain the
production of germ-cells of each kind, which must again contain
the gemmules of the parents. (ialton, who also rejected the
' transport hypothesis,' thus found himself in the peculiar posi-
tion of being obliged to suppose that the germ-cells which the
organism produces can only contain the unused remainder of
the gemmules and their successors, i.e., those gemmules which
had been unable to take part in the construction of the organism,
and which had 'remained dormant and were individually of ?
different nature from the other gemmules. He made use of this
supposition to explain the difference between children of the
same parents, but found himself obliged to resort to a very
artificial assumption to account for the main problem of the
resemblance between such children and their parents.

That part of Darwin's theory which de Vries retains is the
existence of an hereditary substance composed of 'gemmules,' or
minute vital particles which are capable of growth and multi-
plication by fission, and which become active consecutively in
ontogeny, and so build up the organism. The theory is thus
deprived of its merely speculative elements, and by transferring
the gemmules, in accordance with the most recently ascertained
facts, to the germ substance, which, as we know, is passed on
by division from cell to cell, the theory of pangenesis is placed
on a firm footing.

De Vries, however, was not content with simply niodifying
Darwin's theory of pangenesis in a negative manner, by doing
away with one — almost the greater — portion of it; he also
reformed it positively by giving a new meaning to the ' gem-
mules.' There is an essential difference between Darwin's
gemmules of cells., and de Vries's pangenes, which are gemmules
of elements much smaller than cells — that is to say, of the
smallest parts of which a single cell is composed. These pan-
genes are the bearers of the individual qualities or ' characters '
of the cell.

The train of thought which led de Vries to imagine the con-
struction of the hereditary substance from such ' bearers of the
qualities' (' Eigenschaftstrager ') of the cells is too interesting to
be passed over. He bases this idea on the assumption of ' a
mutual independence of the hereditary qualities.'' According to

INTRODUCTION 1 5

his view, all species consist of a sum of ' hereditary qualities";
very few, or none of these, are peculiar to any one species, the
character of which is determined by the way in which they are
combined. The sa7ne quality recurs in many species, but in
different combinations. ' We constantly see how one and the
same hereditary quality, or how a definite small group of such,
may be combined with all kinds of other hereditary qualities ;
and how the ditferent characters of individual species are due to
the extreme varietv of these combinations.'' The different organs
of a species stand in the same relation to one another in this
respect, as do the different species themselves. They exhibit
the same qualities, but in different combinations. The individual
qualities which constitute a species ' can almost all vary inde-
pendently of each other,' and can therefore be increased even by
artificial selection according to the fancy of the breeder, without
requiring a corresponding change in the remaining qualities of
the species. But the qualities too are ' miscible in almost any
proportion,' as experiments in hybridising are intended to show :
'in no other way can we so clearly demonstrate the secondary
importance of a specific type ('Bild'), regarded as a whole
as opposed to the independent factors which constitute it.'
The qualities, or rather their material substratum, are there-
fore independent of one another, and miscible to almost any
extent.

Those ultimate vital _particles or pangenes, which de Vries |
substitutes for Darwin's gemmules, are therefore the bearers of
constituent qualities of the species.

The fundamental idea of de Vries's whole deduction is doubt-
less perfectly correct. Some ten years ago, when I first began
to devote my attention to the problem of heredity, I fully believed
in the possibility of an epigenetic theory, but, as will be seen in
the course of this book, have long since given up this idea as
untenable. I too now believe that the hereditary substance is
composed of primary constituents, and even trust that I can prove
this assumption to be not only sound, but inevitable. But, at
the same time, I do not imagine that it suffices as an explanation
of the phenomena of heredity. According to de Vries, the germ-
substance is formed of a number of different kinds of pangenes,
of which as many are present as there are qualities in the species.
He does not consider these pangenes as arranged in any definite
grouping, but as freely miscible, in accordance with the assumed

1 6 THE GERM-PLASM

' free miscibility of the qualities.' He contests as superfluous
the assumption of higher units, such as might be formed by a
certain number of pangenes in a definite order ; and this view
seems to me to be the weak point in his argument.

In the section on the control of the cell by the nuclear sub-
stance. I shall adopt what seems to me to be a remarkably happy
idea on the part of de Vries, who supposes that material parti-
cles leave the nucleus, and take part in the construction of the
body of the cell. These particles correspond to the ' pangenes,'
they are the 'bearers of the qualities" of the cell, the specific
character of which I believe to be stamped upon it bv the
nature, the different varieties, and the proportional numbers of
these particles.

But does the character of a species depend only on these
primary qualities of the cell ? Are there not qualities of various
degrees — primary, secondary, and so on? The pangenes are
pri))iary • bearers of qualities " : their mere presence in the heredi-
tary substance gives no indication, or at most, only a very slight
one, as to the character of a species. If, for instance, ' chlorophyll-
pangenes ' are present in the egg-cell erf a plant, the only conclu-
sions we can draw as to the specific character of the latter are
that it will have green cells of some sort : but we cannot thereby
determine where they will be situated, or which portions of the
plant will be green, and which variegated ; or again, whether its
flowers will be green, white, or of some other colour. Not until
we were able to find groups of pangenes in the germ-substance,
some of which were destined to give rise to leaves, and others
to flowers, should we be able to say whether the latter will be
green or otherwise.

In the course of his remarks, de Vries mentions the stripes of
a zebra. How can these be hereditary if the different kinds of
pangenes merely lie close together in the germ without being
united into fixed groups, hereditary as such f There can be no
'■ zebra pangenes,' because the striping of a zebra is not a cell-
character. There may perhaps be black and white pangenes
whose presence causes the black or white colour of a cell : but
the striping of a zebra does not depend on the development of
these colours within a cell, but is due to the regular alternation
of thousands of black and white cells arranged in stripes.

De Vries, in another place, refers to the long-stalked variety
of the alpine Primula acaulis, which is due to reversion to a

INTRODUCTIOX 1 y

remote ancestral form of the species. In this case, again, the
special peculiarity cannot depend on ' long-stalk pangenes,"
because the possession of a long stalk is not an intracellular
character. The specific form of the leaves and other parts of a
plant is likewise not due to the character of the individual cells
composing them : the serrated margin of a leaf, for instance,
cannot depend on the presence of ' serration-pangenes,' but is
due to the peculiar arrangement of the cells. The same argu-
ment would apply to almost all the obvious 'characters' of the
species, genus, family, and so on. For instance, the size,
structure, veining, and shape of leaves, the characteristic and
often absolutely constant patches of colour on the petals of
flowers, such as orchids, may be referred to similar causes : these
qualities can only arise by the regular co-operation of many
cells. The characteristics of the human race may be taken as
another illustration. The peculiarities as regards the shape of
the skull, nose, &c., cannot depend on the mere presence in the
germ of pangenes, which are destined to form the hundreds and
thousands of different cells constituting the respective qualities ;
but they must be due to 2i fixed grouping of pangenes, or some
other primary elements of the germ, which is transferable from
generation to generation.

The character of a species cannot depend only upon the num-
ber and relation of the pangenes in the germ. It is quite pos-
sible to conceive of two ditferent species of totally different
stmcture in which the pangenes of the germ were alike in nature
and amount, the difference being solely due to the grouping of
the pangenes in the germ. De Vries, it is true, traces 'sys-
tematic difference to the possession of different kinds of pan-
genes,' and considers that ' the number of similar pangenes in
two species is the real measure of their affinity ; ' * but this
statement seems to me to be somewhat at variance with his
fundamental view, according to which ' a number of hereditary
qualities constitute the character of each individual species, though
by far the greater jnajority of them recur in iujiumerable other
species.' Does he not, in so many words, emphasise the lact
that the almost formidable number of different pangenes which
are required for ' the construction of a single species * does not
necessitate the existence of an inconceivably large multitude of

* Lot. cit., p. 73.

1 8 THE GERM-PI^SM

different pangenes in the entire organic world, because ''the
number of individual hereditary qualities required for the con-
struction of the latter is relatively small when compared with
the number of species ' ! Each species appears to us as an
extremely complicated structure : the whole organic world, how-
ever, seems to be the result of innumerable different combina-
tions and permutations oi i' datively few factors.

The idea which is here so clearly and decidedly expressed of
the construction of innumerable species by various combinations
of relatively few pangenes, shows that, even from de Vries's point
of view, it is not the ' pangene viateriaV as such, which is the
main factor in determining the character of the species, but
rather its arraftgement, or as I shall afterwards express it, the
architecture of the genn-plasjn.

De \''ries certainly speaks frequently of ' groups of pangenes,'
but he only just touches upon this idea, and postpones entering
into details until further discoveries are made with regard to the
mechanism of nuclear division. Important as his fundamental
view as regards the composition of the germ substance out of
primary constituents undoubtedly is, it may easily seem to ex-
plain more than it really does ; without assuming the formation
of groups of such primary constituents for a number of orders
each included in the other, even the simplest case of ontogeny
cannot be explained, quite apart from reversion or any other
complicated phenomenon of amphigonic heredity. Darwin's
theory of pangenesis accomplishes more in this respect than
does de Vries's modification of it, inasmuch as the former at
least deals with the primary constituents of ^^//-structures. The
mere presence of a certain collection of pangenes in the germ
does not necessitate the formation in the offspring of similar
cells to those which existed in the parent ; for the character of
the individual cell is determined by a definite selection of pan-
genes. If, indeed, it be assumed that the required pangenes
always lie close together, and are always ready at hand when-
ever they are wanted, an explanation of any particular phenom-
enon of heredity is no longer difficult, but it seems to me that
it would then be necessary to show how the nature of the germ
can determine that the right primary constituents are always at
the right spot.

As already stated, de Vries occasionally speaks oi groitps of
pangenes, but, at the same time, he looks upon the view o^

INTRODUCTION

19

there being any ' higher units ' in the germ as a superfluous one.
I can only explain this inconsistency by supposing that he re-
gards the ' quahties ' as independent and perfectly freely niisci-
ble, and, in fact, postulates a germ-mechanism which admits of
their separation in any manner required. If this were really the
case, and the primary constituents were not combined into fixed
groups in the germ, how could composite characters composed
of many different kinds of cells wuth a definite arrangement, —
e.g., the eye-like spot on a certain feather of a bird, — become a
fixed specific character? I am of opinion that the view which
entails an independence and uncontrolled miscibility of the qual-
ities is a fallacy, originating in the conception of amphigonic
reproduction as a necessary element in heredity. The chapters
on amphigonic heredity, reversion, &c., will show how I imagine
the idea of the uncontrolled miscibility of the separate qualities
to have arisen.

It will frequentl}- be apparent in the course of this book that
my point of view is identical with that of the Dutch botanist in
many of the most important particulars. I believe, however,
that his * pangenes ' or similar minute elements do not suffice
in themselves for the construction of a theory of heredity, but
that something more must be added to make the phenomena
comprehensible at any rate in principle.

The manuscript of the present book had already been written
for some time when Wiesner's work on the elementary structure
and growth of living substance * appeared. Although this mono-
graph does not contain, and is not intended to offer, a theory of
heredity, it is nevertheless of great importance in this respect,
for it treats of the fundamental points of such a theory, viz.,
the composition of living matter out of ve>y small units.
Wiesner remarks that theories of heredity have hitherto always
adopted units invented for the purpose, whereas the same units
which make life possible at all, and which control assimilation
and growth, must also be the agents in bringing about the
phenomena of heredity. Spencer's ' physiological units,^ Dar-
win's ' gemmules,' HaeckeFs ' plastidules,' and my ' ancestral
germ-plasms," are all, in fact, elements of this kind, assumed
for the explanation of the problem of heredity. De Vries stands

* y. Wiesner, ' Die Elementarstruktur und das Wachsthum der lehenden
Substanz,' W'ien, 1892.

20 THE GERM-PLASM

alone in considering all living matter to be actually composed
of his ' pangenes/ though I have already indicated that my
' ancestral germ-plasms ' are also composed of similar primary
units, which do not exist in them alone. The minute vital
particles or ' plasomes,' adopted by Wiesner from Briicke. cor-
respond in all essential points to the • biophors ' or bearers of
life assumed by myself.

B. — Descriptive Part

By the term heredity is simply meant the well-known fact
that living organisms are able to produce their like, and that the
resemblance between a child and its parent, although never
perfect, may nevertheless extend to the most minute details of
construction and functions.

The fundamental phenomena of heredity are familiar in all
existing organisms : the transmission of the character of the
species from parent to offspring results whether the multiplica-
tion takes place by the halving of a unicellular organism or by
the process which occurs in multicellular organisms, which
consists in a complex succession of continually increasing
groups of cells, i.e., in development. These fundamental phe-
nomena of heredity are, however, complicated in all the higher
organisms by the connection of reproduction with that process
which may be described as amphimixis.* This consists in the
mingling of two individuals or of their germs, and owing to its
constant connection with reproduction in multicellular organisms
it is usually spoken of as ^sexual reproduction.' As will be
shown in greater detail further on, the various phenomena, such
as the blending of parental characters in the offspring, and
reversion, depend exclusively on the hold which amphimixis has
taken on the life of the species. Similar phenomena must occur
amongst unicellular organisms, in which amphimixis is widely
spread if not universal, in the form of conjugation, and is,
therefore, not directly connected with reproduction. At present,
however, we are ignorant of the details of heredity in these
forms, and are therefore compelled to base our conclusions
entirely upon what we know to occur in multicellular organisms.

* August Weismann, 'Amphimixis, oder die Vermischung der In-
dividuen,' Jena, 1891. See ' Essays upon Heredity,' vol. ii,, 1892.

INTRODUCTION 2 1

These phenomena have only been observed in detail in the
higher plants and animals, more particularly in man. In the
case of the higher forms of life a large number of facts have
now been accumulated which can be used for the purpose of
theoretical analysis.

Although the study of heredity is greatly complicated by
amphimixis, this mingling of the hereditary tendencies of two
parents, and even the process of sexual reproduction which accom-
panies it, afford us a much deeper insight into the actual proc-
esses of heredity than we could ever have obtained in any other
way. We may thus hope in time to penetrate further into its
nature by carrying out more detailed investigations of the phe-
nomena.

In order to do so, however, we must not forget that this form
of reproduction is neither the only nor the original one, and that
even in multicellular organisms reproduction is not necessarily
connected with amphimixis ; it must also be borne in mind
that so-called asexual {jnonogonic) reproduction forms the basis
of the amphigonic method. The fundamental phenomena of
heredity had already shaped their course in the living world
before the introduction of amphimixis, and have, therefore, no
connection with amphigonic descent and the complications aris-
inof from it. This fact has often been overlooked or left out of
consideration, and thus the solution of the problem of heredity
has been rendered much more difficult. A whole series of the
phenomena of heredity can be investigated theoretically without
considering the complications arising from amphimixis, though,
in point of fact, it is always a factor, and thus the problem to
be solved is very considerably simplified.

The natural course of such an investigation would be to pass
from the simple to the complex, but it is not advisable at present
to begin the study of heredity by a consideration of the simplest
beings, and to ascend from the unicellular to the multicellular
organisms. For besides the fact that we know nothing of the
individual phenomena, — such as the transmission of the indi-
vidual characters, — in the lower forms, the principal reason
for not following the ordinary course in this case is the fact
that amphigonic reproduction, or the processes of fertilisation
and the complicated development of multicellular organisms,
affords us, as already stated, a deep insight into the processes
of hereditv. The same is true in this case as in almost all

22 THE GERM-PLASM

physiological processes, — investigation cannot proceed from
the simple to the more complex without taking into considera-
tion the objects and processes for which it was first undertaken.
It must, on the contrary, avoid the densely overgrown path
and skirt the hedge which surrounds the enchanted castle of
the secret of Nature, in order to see if there be not somewhere
a gap through which it is possible to enter and obtain a firm
foothold.

Such a gap in the hedge which encloses the secret of heredity
may be found in the processes of fertilisation, if we connect them
with the facts of heredity as observed in the organisms which
have adopted sexual reproduction.

As long as we were under the erroneous impression that
the fertilisation of the ovum by the spermatozoon depended
on an aura se//i2Jtalis which incited the egg to undergo devel-
opment, we could only partially explain the fact that the father
as well as the mother is able to transmit characters to the
children by assuming the existence of a spiritus rector, con-
tained in the aura seminah's which was transferred to the ovum
and united with that of the latter, and thus with it directed the
development. The discovery that development is effected by
material particles of the substance of the sperm, the sperm-cells,
entering the ovum, opened the way to a more correct interpre-
tation of this process. We now know that fertilisation is noth-
ing more than the partial or complete fusion of two cells, the
sperm-cell and the egg-cell, and that normally only one of the
former unites with one of the latter. Fertilisation thus depends
on the union of two protoplasmic substances. Moreover, although
the male germ-cell is always very much smaller relatively than
the female germ-cell, we know that the father's capacity for
transmission is as great as the mother's. The important con-
clusion is therefore arrived at that only a small portion of the
substance of the ovum can be the actual hereditary substance.
Pfliiger and Nageli were the first to follow out this idea to
its logical conclusion, and the latter observer stated definitely
that it is impossible to avoid the assumption that no more
hereditary substance is contained in the egg-cell than in the
male germ-cell, and that consequently the amount of the sub-
stance must be infinitesimal, for the sperm-cell is. in most cases,
many hundred times smaller than the ovum.

The numerous and important results of the investigations of

INTRODUCTION 2^

many excellent observers on the process of fertilisation have
now rendered it almost certain — in my opinion, absolutely so
— that by far the larger part of the egg-cell does not consist
of hereditary substance, and that the latter only constitutes a
small portion even of the sperm-cell. From his observations on
the egg of the star-fish, Oscar Hertwig had suspected that the
essential part of the process of fertilisation consists in the union
of the luiclei of the egg- and sperm-cells, and as it is now known
that the hereditary substance is undoubtedly contained in the
nucleus, this view has, in this respect at least, proved to be
the right one. It is true that the nucleus of the male cell is
always surrounded by a cell-body, and that Strasburger's opinion
to the contrary is incorrect. We now know, through the
researches of Guignard, that even in Phanerogams a small cell-
body surrounds the nucleus, and that a special structure, the
'- centrosome,' — which is absolutely essential for the commence-
ment of development, — is contained within it. This structure
will be treated of in further detail presently, but I must here lay
stress upon my view, that the '• centrosotne^ with its 'sphere of
attraction ' cannot in any case be the hereditary siibstatice, and
that it is jnerely an apparatus for the division of the cell and
nucleus.

Both in animals and plants, however, essentially the same
substance is contained in the nucleus both of the sperm-cell and
egg-cell : — this is the hereditary siibstance of the species. There
can now be no longer any doubt that the view which has been
held for years by Strasburger and myself is the correct one,
according to which the nuclei of the male and those of the female
germ-cells are essentially similar, i.e.., in any given species they
contain the same specific hereditary substance.

The splendid and important investigations carried out by
Auerbach, Biitschli, Flemming, and many others, on the detailed
processes of nuclear division in general, and those dealing more
particularly with the fertilisation of the egg in Ascaris by van
Beneden, Boveri, and others, have given us the means of ascer-
taining more definitely what portion of the nucleus is the
substance on which heredity depends. As already remarked,
this substance corresponds to the ' chromosomes,^ those rod-like,
looped, or granular bodies which are contained in the nucleus,
and which become deeply stained by colouring matters.

As soon as it had been undoubtedly proved that the nucleus^

24 THE fJERM-PLASM

and not the body of the cell, must contain the hereditary sub-
stance, the conclusion was drawn that neither the membrane of
the nucleus, nor its fluid contents, nor the nucleoli — which
latter had been the first to attract attention — could be regarded
as such, and that the ' chromatic granules ' alone were important
in this respect. As a matter of fact several investigators, —
Strasburger. Oscar Hertwig, Kollicker, and myself, — reasoning
from the same data, arrived at this conclusion independently,
within a short time of one another.

It will not be considered uninteresting or superfluous to reca-
pitulate the weighty reasons which force us to this conclusion,
for it is clear that it must be of fundamental importance in a theory
of heredity to know for certain what the substance is from which
the phenomena which are to be explained proceed.

The certaintv with which we can claim the ' chromatin gran-
ules " of the nucleus as the hereditary substance depends firstly,
on the process of amphimixis ; and secondly, on that of nuclear
division. We know that the process of fertilisation consists
essentially in the association of an equal number of chromatin
rods from the paternal and maternal germ-cells, and that these
give rise to a new' nucleus from which the formation of the
offspring proceeds. We also know that in order to become
capable of fertilisation each germ-cell must first get rid of half of
its nuclear rods, a process which is accomplished by very peculiar
divisions. W^ithout entering into further particulars here, am-
phimixis may be described as a process by means of which one-
half of the number of nuclear rods is removed from a cell and
replaced by an equal number from another germ-cell.

The manner, however, in which the chromatin substance is
divided in nuclear division strengthens the above view of its
fundamental nature. This method of division leaves no doubt
that it is a substance of the utmost importance. I need only
briefly recapitulate the main points of the wonderfully complicated
process of the so-called mitotic or karyokinetic cell-division,
which follows a definite law even as regards the most minute
details.

When the nucleus is going to divide, the chromatin granules,
which till then were scattered, become arranged in a row, and
form a long thread, which extends through the nucleus in an
irregular spiral, and then divides into portions {chro7)iosomes) of
fairly equal length. The chromosomes have at first the form

INTRODUCTlOlsr

25

of long bands or loops, but afterwards become shortened, thus
giving rise to short loops, or else to straight rods or rounded
granules. With certain exxeptions, to be mentioned later, the
number of chromosomes which arise in this way is constant for
each species of plant or animal, and also for successive series
of cells. By the time the process has reached this stage a special
mechanism appears, which has till now remained concealed in
the cell substance. This serves to divide the chromatin elements
into two equal parts, to separate the resulting halves from one
another, and to arrange them in a regular manner. At the
opposite poles of the longitudinal axis of the nucleus two clear
bodies — the ' centrosomes,' each surrounded by a clear zone,
the so-called 'sphere of attraction ' — now become visible. The
importance of these was first recognised by Fol, van Beneden,
and Boveri. They possess a great power of attraction over the
vital particles of the cell, so that these become arranged around
them like a series of rays. At a certain stage in the preparation
for division, the soft protoplasmic substance of the cell-body as
well as of the nucleus gives rise to delicate fibres or threads : these
fibres are motile, and, after the disappearance of the nuclear
membrane, seize the chromosomes — whether these have the
form of loops, rods, or globular bodies — with wonderful certainty
and regularity, and in such a way that each element is held on
either side by several threads from either pole. The chromatin
elements thus immediately become arranged in a fixed and
regular manner, so that they all come to lie in the equatorial
plane of the nucleus, which we may consider as a spherical body.
The chromatin elements then split longitudinally, and thus
become doubled, as Flemming first pointed out. It must be
mentioned that this splitting is not caused by a pull from the
pole threads (spindle threads), which attach themselves to the
chromatin rods on both sides ; the division arises rather from
forces acting in the rods themselves, as is proved by the fact
that they are often ready to divide, or indeed have already done
so, some time before their equatorial arrangement has taken
place by means of these threads.

The splitting is completed by the two halves being gradually
drawn further apart towards the opposite poles of the nuclear
spindle, until they finally approach the centre of attraction or
centrosome, which has now fulfilled its object for the present,
and retires into the obscurity of the cell-substance, only to

.2.6 THE GERM-PLASM

become active again at the next cell-division. Each separated
half of the nucleus now constitutes a daughter-nucleus, in which
it immediately breaks up, and becomes scattered in the form of
minute granules in the delicate nuclear network, so that finally
a nucleus is formed of exactly the same structure as that with
which we started. Similar stages to those which occur in the
aggregation of the chromatin substance in the mother-nucleus
preparatory to division are passed through during the separation
of the daughter-nuclei, but in the reverse order.

It is evident, as Wilhelm Roux was the first to point out,
that the whole complex but wonderfully exact apparatus for the
division of the nucleus exists for the purpose of dividing the
chromatin substance in a fixed and regular manner, not merely
quantitatively, but also in respect of the diffej-ent qualities which
must be contained in it. So complicated an apparatus would
have been unnecessary for the quantitative division only: if,
however, the chromatin substance is not uniform, but is made up
of several or many different qualities, each of which has to be
divided as nearly as possible into halves, or according to some
definite rule, a better apparatus could not be devised for the
purpose. On the strength of this argument, we may therefore
represent tJie hereditary substajice as consisting of different ' qiial-
ities.'' The same conclusion is arrived at on purely theoretical
grounds, as will be shown later on when we follow out the con-
sequences of the process of amphimixis.

For the present it is sufficient to show that the complex
mechanism for cell-division exists practically for the sole pur-
pose of dividing the chromatin, and that thus the latter is
w'ithout doubt the most important portion of the nucleus. Since,
therefore, the hereditary substance is contained within the
nucleus, the chrotnatin must be the hereditary substance.

De Vries's objection to this view is, in my opinion, only an
apparent one ; for it has not been asserted that ' the nucleus
alone is the bearer of the hereditary characters,' as de Vries
thinks, but that the nucleus alone contains the hereditary sub-
stance, or that substance which is capable of determining not
only the character of a particular cell, but also that of its descend-
ants. This is never contained in the cell-body, but always in
the nucleus in multicellular organisms, and doubtless the same
holds good for unicellular beings. It is quite possible that in
certain lower Algae a few of the structures in the cell — such

INTRODUCTION

27

ju:

Fig. I. — Diagram of Nuclear Division.

A. — Cell with nucleus — «, and centrosomes
— cs, preparatory to division. The chromatin
has become thickened so as to form a spiral
thread — chr.

B. — The nuclear membrane has disappeared.
Delicate threads radiate from the centrosomes,
and form the ' nuclear spindle,' in the equator
of which eight chromosomes or nuclear loops
{chr) are arranged: these have been formed by
the spiral thread of chromatin in A becoming
broken up.

C. — The chromosomes have each become
split longitudinally into two, and are about to
be drawn apart by means of the spindle-threads.
(For the sake of clearness only four of the
eight chromosomes are indicated.)

D. — The daughter-loops pass towards the
poles of the spindle.

E. — The body of the cell has undergone
division; each of the resultant cells contains a
centrosome and eight nuclear loops.

2$ THE GERM-PLASM

as vacuoles and chlorophyll bodies — pass directly from the
egg-cell into the daughter-cells, although this cannot by any
means be considered as proved. In any case such a direct
transmission plays only a very insignificant part in plants, and
practically none at all in animals, for specific structures are
not present in the egg-cells of animals : there may at most be
deposits of nutrient material. These, however, are not living
structures of the cell, but only passive chemical substances. So
far from denying that the nucleus contains the hereditary sub-
stance, de Vries bases his whole theory on this incontestable
fact. The last doubts on this point were dispelled by the ex-
periments of Boveri,* who, after artificially removing the nucleus
from the eggs of a species A of sea-urchin, and then pouring
over them the sperm of another species /?, found that these eggs
developed into larvae of the latter species. In this case there-
fore the substance of the maternal germ-cell acted as nutrient
material only, whilst the paternal germ-cell impressed the char-
acter of the species on the larva. None of the maternal specific
characteristics were transmitted, and in this case, at all events,
tlie question of any ' heredity apart from the nucleus ' is therefore
excluded.

Several objections have been recently raised to my view that
the nucleus is the seat of heredity. Verworn.f for instance,
repeats the opinion, previously expressed by Whitman, that the
cell-body, quite as much as the nucleus, must be looked upon as
the hereditary substance, because the nucleus cannot exist with-
out the cell-body ; and also because, in his opinion, w^hich is
undoubtedly a correct one, the life of a cell consists in a con-
tinual interchange of substance between the cell and the nucleus.
But is the question as to whether the closest physiological rela-
tions exist between the nucleus and the cell, so that neither can
exist apart from the other, synonymous with that as to whether
the hereditary substance is contained in the nucleus or in the
cell-body? We must at least be allowed to make the hypothesis
that the '■ store of the primary constituents ' (^ Anlagenmagazin ')
of the hereditary substance is contained and preserved in the

* Boveri, ' Ein geschlechtlich erzeugter Organismus ohne miitterliche
Eigenschaften.' Gesellsch. f. Morph. u. Physiol., Miinchen, i6 Juli 1883.

t Max Venvorn, ' Die physiologische Bedeutung des Zellkerns,' Bonn,
1891 (Archiv. f, ges. Physiol., Bd. 51).

INTRODUCTION 29

nucleus ; for, as has already been indicated, and will subsequently
be shown more clearly, this substance can hardly be stored up
in two different places, seeing that a very complicated apparatus
is required for its distribution : a double apparatus would cer-
tainly not have been formed by nature if a single one suffices
for the purpose. Only as long as the phenomena of heredity
and the meaning of these phenomena are still far from being
known, is it possible to hold such opinions as that which pre-
supposes the distribution of the hereditary substance amongst
both cell and nucleus. As soon as a further insight into these
processes is obtained, it will no longer be possible to doubt that
the structure of the hereditary substance must be so complex
that we can only wonder how it could ever have been developed
at all. We know that the nucleus contains a substance which,
even with the imperfect means of observation at our disposal, is
seen to be extremely complex, and that it becomes modified in
a very remarkable manner after every cell division, only to be
again transformed at the approach of the following division. We
can, moreover, observe that the cell is provided with a special
apparatus which evidently enables it to halve this substance very
accurately. The statement that this substance is the Jiereditiwy
substance can, therefore, hardly be considered as an hypothesis
any longer.

It has also been supposed that fresh evidence against the
view that the chromosomes are the hereditary substance, has
been furnished by the recent observations of Fol * and Guignard,f
w^hich prove that the centrosome and its ' sphere of attraction,' —
which belong to the cell-body, and constitute the apparatus for
division, — pass into the ovum along with the sperm-nucleus in
the process of fertilisation. Suppose I take two heaps of grain
from different places, and load them on two carts, harness a
horse to each cart, and drive them to the same place ; does this
prove that the horses consist of grain? They are merely the
means by which the grain is transferred from one place to
another, just as the centrosomes are the means whereby the
sperm-nucleus is transferred to the ovum : whether they are any-

* ' Le Quadrille des Centres,' Archiv. Sc. Phys. et Nat., Geneve, 15
Avril 1891.

t 'Sur I'Existence des Spheres Attractives dans les Cellules \'6g6tales,'
Compt. Rend. Sc, 9 Mars 1891.

30 THE GERM-PLASM

thing more than this, — /.<?., whether they contain hereditary sub-
stance, — still remains to be proved, and such a conjecture is
hardly more probable than that the horses, besides being the
means of transport of the corn, should actually consist of corn !

It might, however, be urged that the transference not only of
the hereditary substance or store of prii?iary constituents, but
also of the centrosome or means of transference of this substance,
implies the transference of the rate of cell-division, which is
regulated by the centrosome and essentially decides the cell-
sequence in the offspring, and which consequently also takes
part in heredity. But I consider this also to be an incorrect
deduction, because the periods of activity of the apparatus for
division must obviously be dependent on the conditions of the
cell itself; these conditions, however, apart from nutrition, de-
pend on the ultimate specific structure of the cell. As, accord-
ing to my view, this structure is impressed on the cell by the
nuclear substance, the periodicity of cell-division must also be
dependent on the nucleus. The law that only a certain part of
the unclear matter is to be regarded as the hereditary substance
appears to me to receive fresh support from all the more recent
obse?'vations*

Chromatin substance is not only contained in the nucleus of
the germ-cells and of the fertilised ovum, but also in all the cells

* Many seem inclined to regard the process observed by Fol, which he
described as ' Le quadrille des centres,' as a proof that the centrosomes
nevertheless must — or at any rate might — contain a kind of hereditary sub-
stance. I believe, however, that this process is quite similar to that which
occurs in every nuclear division, except that in fertilisation, — owing to the
fact that the first segmentation nucleus receives a centrosome from each of
the two conjugating cells, — it is a double, and not a single process. Each of
these centrosomes divides and passes to the region of the two poles of the
future spindle, just as would occur if only a single centrosome were present
in the cells. I should be surprised if this were not the case, and if the
centrosome of the egg-cell passed to one pole, and that of the spermatozoon
to the other! Guignard is of the opinion that even if the nucleus is of great
importance as regards the transference of transmissible qualities, we must
nevertheless attribute to the ' spheres directives," ' le r6ie primordial dans
I'accomplissement de la f^condation.' This is true if it only indicates that
the beginning of embryonic development depends — as does every nuclear
division — on the presence of the apparatus for division. But the view is
not thereby refuted that fertilisation consists in the union of two nuclear
substances.

INTRODUCTION 3 T

of the entire organism in each phase of its development, at any
rate as long as they are capable of multiplying, and are pos-
sessed of vitality. The chromatin in all the cells of the body is
derived from that in the fertilised ovum, while the development
of the body from the egg-cell is effected by a series of cell-
divisions, each of which includes a division of the nucleus in
the manner just described. In the process of ontogeny the
chromatin of the first nucleus undergoes repeated subdivisions
into two parts of equal volume, and it would very soon become
so small as to be invisible even under the highest powers of the
microscope, if it did not continue to grow, as does the cell-body.
This occurs just as much in the case of numerous animal eggs
to wdiich no nutrient material is supplied during the develop-
ment of the embryo, as in that of those which are nourished
from the beginning, or of plants which as a rule begin to obtain
their own nutriment at a very early stage. The chromatin, or
hereditary substance of the fertilised ovum, enters upon a long
and complex process of growth, which only ceases when no
new cells are produced either for the formation of new parts, or
to replace old ones, — that is to say, at the end of the life of the
individual. This growing hereditary substance may be com-
pared to a tree whose branches arise in strict dichotomy, except
for the fact that the chromatin does not consist of one continu-
ous mass, but of a number of separate particles not actually
contiguous with one another; for at each cell-division the two
halves of the chromatin rods separate never to unite again in
one nucleus. Each is finally contained in a special nucleus,
which is separated from the rest by being enclosed in a special
cell-body. The question now arises as to whether all these
fragments of the hereditary substance which compose the chro-
matin 'tree' of an organism are similar to, or different from,
one another, and it can easily be shown that the latter must
be the case.

In order to prove this, we take as our basis the well-grounded
assumption that the chromatin in the nucleus of the fertilised
egg is the substance on which heredity depends. Thus we
know that the possibility of the offspring resembling its father, for
example, in a thousand different physical and mental characters
depends on the minute mass of a few chromatin granules in
the nucleus of the sperm-cell, and that the characters of a fully
formed organism depend as a whole, as well as in detail, on the

32 THE GERM-PLASM

arrangement, number, and nature of the cells which compose
it. The influence therefore which the minute mass of paternal
chromatin in the nucleus of the fertilised egg-cell exerts on
the course of development, can only be such as to regulate the
nature and the rate of multiplication of the cells in the body of
the offspring in such a manner as to cause them to resemble
the cells of the paternal body. The chromatin is therefore in a
condition to impress the specific character on the cell in the
nucleus of which it is contained.

As the thousands of cells which constitute an organism pos-
sess very different properties, t]ie chromatin which controls them
cannot be unifonn ; it must be different in each kind of cell.

The chromatin, moreover, cannot become different in the cells
of the fully formed organism ; the differences in the chromatin
controlling the cells must begin with the development of the egg-
cell, and must increase as development proceeds ; for otherwise
the different products of the division of the ovum could not give
rise to entirely different hereditary tendencies. This is, how-
ever, the case. Even the two first daughter-cells which result
from the division of the egg-cell give rise in many animals to
totally different parts. One of them, by continued cell division,
forms the outer germinal layer, and eventually all the organs
which arise from it — e.g.., the epidermis, central nervous system,
and sensory cells ; the other gives rise to the injier germinal
layer and the organs derived from it, — the alimentary system,
certain glands, &c. The conclusion is inevitable that the
chromatin determining these hereditary tendencies is different in
the daughter-cells.

This holds good in all subsequent stages of ontogeny ; the
difference between the developmental tendencies of the cells
resulting from the division of the ovum is in exact proportion to
that between the chromatin substance of their nuclei. Ontogeny.,
or the developjnent of the individual, depends therefore on a
series of gradual qualitative changes in the nuclear substance of
the egg-cell.

The fundamental principle of the view which has just been
brieriy sketched was put forward by me some years ago, and I
then made use of the term idioplasm to represent the substance
which is contained in the chromatin bodies of the nucleus, and
which determines the nature of the whole cell. Oscar Hertwig
also independently adopted this term, which had first been intro-

INTRODUCTION 33

duced by Nageli with a somewhat different meaning. As stated
in the first section of this book, Nageh defined idioplasm as the
guiding and controlling substance of the body, in contrast to the
more passive and controllable trophoplasm. It is open to doubt
whether the latter term should be retained, but the former is
certainly happily chosen. It is true Nageli did not mean to
indicate any definite substance visible under the microscope when
he used the word idioplasm, for the facts of nuclear division and
fertilisation were then unknown. But these facts are so convincing
that no doubt as to what is to be regarded as the idioplasm is any
longer possible, and Nageli's conception of an idioplasm forming
a network, traversing and connecting the contents of all, the
cells in the organism, may be regarded as abandoned. We are
therefore justified in transferring the term introduced by him to
the nuclear substance which determines the nature of the cell.

We now therefore understand by the term idioplasm the
nuclear substance controlling any particular cell. This is at the
same time the hereditary substance, for it is never formed afresh,
but is always derived from the idioplasm of another cell; more-
over, it not only determines the actual characters of the partic-
ular cell, but also those of all of its descendants.

Hence we must assume a difference in the idioplasm not only
in dealing with two cells differing in structure and functions,
but also in all cases in which we know that different primary
constituents are contained in two cells. This has often been
overlooked in using the term ' embryonic cells ' merely in the
sense of equivalent elements ' which may give rise to any parts,^
simply because they frequently resemble one another, assuming
that they must therefore always be actually equivalent. It is
quite true that the idioplasm of such cells appears similar, at
least we can recognise no definable differences in the chromatin
rods of two cells in the same animal. But this is no argument,
against the assumption of an internal difference. The perfect
external resemblance between two eggs is not a sufficient reason
why two identical chickens should be hatched from them. The
eggs may have been produced by different mothers, or they
may have been fertilised by two different males. We cannot
perceive these slight differences in either case, and we could
not even do so by attempting to analyse the idioplasm concealed
in the nuclei of the two eggs by the aid of our most powerful
objectives. Theoretical considerations will show later on that

34

THE GERM-PLASM

it must be so, and that the units of the idioplasm on which the
nature of the latter depends are far too numerous, and therefore
far too small, to be visible.

If therefore the two halves into which the chromatin rods are
split in karyokinesis look exactly alike, and even if the divided
portions of the granules (microsomes), of which the rods often
visibly consist, resemble each other exactly, there is still no
reason why they should not be different in their nature ; in some
cases one, and in others another occur.

We shall consequently in this connection have to assume two
kinds of nuclear division which are externally indistinguishable
from one another, in one of which the two daughter-nuclei con-
tain similar idioplasm, while in the other they contain different
kinds of idioplasm. These kinds of division we may speak of
as hofncrokuiesis and heteroki/iesis, that is, as a division into
parts similar or dissimilar to each other with regard to the hered-
itary tendencies they contain (- erbgleich ' and ' erbungleich ') .
The former must depend on a perfectly uniform distribution of
the primary constituents in the two halves of the rods, and will
consequently have been preceded by a duplication in the process
of growth ; in the latter this growth will be connected with a
heterogeneous grouping of these constituents.

Although we cannot ascertain anything directly about the
forces which cause this splitting of the chromatin rods, it may at
any rate be asserted that they must be contained within the sub-
stance of the latter, and be connected with the actual develop-
ment of the qualities of the idioplasm : for otherwise it could
not be understood how the qualities, which are changed during
the division of the nucleus, become separated sharply from one
another and arranged in the two daughter-nuclei. And yet this
must be the case if different cells with different kinds of idioplasm
can all arise from 07ie mother-cell, which is an undoubted fact.

It appears to me, therefore, that the regular ontogenetic
changes of the idioplasm, as they begin with the division of
the egg-cell and cease with the natural death of the organism,
depend on purely internal causes, which lie in the physical nature
of the idioplasm. In obedience to these, a division of the nucleus
accompanies each qualitative change in the idioplasm, in which
process the different qualities are distributed between the two
resulting halves of the chromatin rods. I shall speak of the
different kinds of idioplasm arising in this way as the ontogenetic
stages of the idioplasm, or shortly, t/ie onto-idic stages.

INTRODUCTION 35

Hereditary sttbstance, in the full meaning of the term — i.e.,
that substance which contains all the primary constituents
of the whole organism, is merely the idioplasm of the germ-
cell, and it is advisable for practical purposes to denote
this first onto-idic stage by the short term germ-plasm, which I
suggested for it at a time when the idea of idioplasm had not
been introduced. At that time I meant by the term 'germ-
plasm ' the hereditary substance of a germ-cell capable of
development, without expressing any opinion as to its position
or nature. We can now state that the germ-plastn is tJie first
ontogenetic stage of the idioplasm of an animal or a plant,
whether the cell, in the nucleus of which it is contained, is
sexually differentiated or not.

We must next attempt to form an idea of the constitution and
nature of the gertn-piasm, and of the ontogenetic stages of the
idioplasm, or onto-idic stages.

PART I

THE MATERIAL BASIS OF HEREDITY

CHAPTER I

THE GERM-PLASM
I. The Fundamental Units

Now that the conception of the germ-plasm as the hereditary
substance contained in the germ-cells has been fully established,
and since it has been shown in general terms that this form
of the idioplasm must become changed during ontogeny and
converted into the idioplasm of the cells which constitute the
mature organism, we must attempt to form some idea of its
nature ; for it would otherwise be impossible to construct a
theory of heredity. In attempting this, w^e shall for the present
entirely neglect the complication due to sexual reproduction,
and take as our starting-point a germ-plasm which does not
contain the primary constituents of two parents, but those of
one only, — that is to say, one which is constituted just as it
would be in a species which had at all times multiplied
asexually.

Before venturing to express an opinion concerning the consti-
tution of the germ-plasm, and to derive therefrom the phenom-
ena of heredity, I should like to premise that it is not m\-
intention to attempt an explanation of life. It is necessary to
distinguish between a theorv of life and one of heredity. De
Vries has pointed out very clearly that the former is impossible
at present, but that it seems by no means impossible to arrive
at a satisfactory explanation of the phenomena of heredity if one

D. H. HILL LIBRARY ^'

7^8 THE GERM-PLASM

takes for granted the essential phenomena of life, — nutrition,
assimilation, and growth.

These functions, together with the associated ones of sensa-
tion and movement, are connected in all organisms with which
we are familiar, from the simplest unicellular forms to the
highest plants and animals, with at least two different sub-
stances, viz., the idioplasm of the nucleus, — i.t\. the hereditary
plasm in the more general sense, — and the protoplasm of the
cell-body. These two differ as regards their functions, though
they resemble each other in being composed of living substance :
that is to say, the primary vital forces, nutrition and growth, are
developed within them. As the term 'protoplasm' is used in a
far too indefinite sense. I shall follow Nageli's example, and
call the vital substance of the cell the * formative plasm ' or
morphoplastn (Nageli's " trophoplasm ■"). in contrast to the idio-
plas))i. The latter is the active element in the process of for-
mation, and the former the passive one. As we now know that
the idioplasm is situated in the nuclei only.Ve cannot regard
the cell-bodies which determine the form of all parts of the
organism as mere ' nutrient plasm.''

Both forms of the living substance are included in the term
' protoplasm,' and we have now to decide how we are to im-
agine its constitution in detail. '■ Protoplasm ' has often been
conceived as a ' modification of albumen ' ; till quite recently,
in fact, this w'as the general idea. Briicke, however, pointed
out a considerable time ago that albvmien does not possess
the power of assimilation, and has therefore no vitality ; it
has moreover been proved by the study of physiological chem-
istry that other substances besides albumen are also obtained
from protoplasm, and that these cannot be assumed to be
insignificant without further proof. Although compounds of
sulphur and phosphorus, for instance, only exist in protoplasm
in comparatively small quantities, we must not infer from this
fact that they are of slight importance. In any case, we cannot
say that protoplasm is a modification of albumen, because we
can only examine it chemically when dead, and in this condi-
tion it has lost its most important properties, and has become
chanored in a manner which we need not here consider further.
As de Vries expresses it, protoplasm is not a chemical, but a
morphological conception. That is, it does not consist of a
confused mass of certain chemical molecules, but of morpho-

THE GERM-PLASM 39

logical units, which are themselves composed of molecules, or,
as Briicke first expressed it, protoplasm is 'organised/ As I
have shown in the historical introduction to this book, Herbert
Spencer, and more recently de Vries and Wiesner, have assumed
the existence of such organic units.

De Vries, moreover, points out that protoplasm possesses
certain ' historical ' properties besides its physical and chemical
ones. It may certainly be doubted, as de Vries states, whether
it will ever be possible to produce ' living protoplasm otherwise
than in a phylogenetic manner,'' that is to say, to make it artifi-
cially in the laboratory ; but it cannot be admitted that this
is so improbable, merely because the conception of protoplasm
demands that it should be derived from pre-existing protoplasm.
This would exclude for ever not only the possibility of its pro-
duction in our laboratories, but also its logically inevitable
and indispensable primary formation in the great laboratory of
Nature. Most, in fact probably all, kinds of protoplasm with
which we are acquainted possess historical qualities, not hi
addition to, but luitliin their physico-chemical ones ; that is,
they contain special modifications of construction peculiar to
themselves which arose in adaptation to the conditions of life,
and have been transmitted for a long period of time. But pro-
toplasm which does not yet possess 'historical ■ i.e., inherited
qualities, does not seem to me to be inconceivable. It would
be the simplest form of living matter which, in virtue of its
constitution, possessed the primary vital forces, — assimilation,
metabolism, and so on. The historical qualities of the proto-
plasm, its special hereditary tendencies, are not connected with
these primary vital forces. The latter must exist independently
in all protoplasm.

All those writers * who have assumed tlie existence of units
on which the vital forces of protoplasm depend, have pointed
out that they are not chemical molecules, for the latter do not
possess the power of assimilation and reproduction. Hence it
follows that protoplasm is a complex substance which is not
homogeneous, but which consists of different kinds of molecules.
There is therefore no molecule of protoplasm, but we have to
imagine that even in its simplest modifications, protoplasm
invariably consists of groups of molecules, each of which is

* Briicke, Herbert Spencer, de Vries, and Wiesner,

40 THE GERM-PLASM

composed oi different kinds of chemical molecules. I shall call
these units the '■bearers of vitality^ (' Lebenstrager ") or ^ bio-
phors,' because they are the smallest units which exhibit the
primary vital forces, viz., assimilation a?id metabolism, growth,
a n d m u Itiplication by fission .

As living protoplasm cannot be subjected to chemical analysis,
we cannot describe its chemical constitution more precisely ; but
what has so far been determined by the analysis of dead proto-
plasm certainly indicates that the albuminoids are not the only
bearers of vitality, as has generally been assumed, but that other
substances play a no less important part in living protoplasm, —
a fact which has been insisted on by Hoppe-Seyler and Bau-
mann. Besides albuminoids, compounds containing phosphorus,
such as lecithin and nuclein, which are not related chemically to
albumen, but enter into combination with it. are known to occur
in dead protoplasm ; and besides these, protoplasm also con-
tains cholesterin, which is probably a product of destructive
metabolism, and carbohydrates, such as glycogen, starch, inulin.
and dextrine, as well as compounds of potassium.* Although
we cannot at present guess from what chemical compounds in
living protoplasm these bodies have been derived, there can
be no doubt that ' a relation exists between them and the vital
processes' (Hoppe-Seyler), and that albumen, or different kinds
of albumen, do not alone bring about the vital processes, but
that several other substances, such as salts, and compounds
containing phosphorus, and more particularly water, are just as
essential : in short, life depends simply on the interaction of
molecules, differing chemically from one another, but defined
within certain limits.

After long consideration, I have decided to designate such a
group of molecules on which the phenomena of life depend by
the special term ' biophor.' This seemed to be advisable, because
the various terms introduced previously by others were either
left too vague for these minute vital particles to be identified with
them, or if defined more exactly, were used with a different mean-
ing. It would certainly be a mistake to make use of a name
already introduced, in another sense from its original one. Her-
bert Spencer's f ' physiological units ' are similar to the biophors,

* Cf. Hoppe-Seyler, ' Allgemeine Biologic,' Berlin, 1877, p. 75 (Part I.
of the ' Lehrbuch der physiologischen Chemie").

t Herbert Spencer, * Principles of Biology,' vol. i., p. 183.

THE GERM-PLASM 4 1

and he looks upon them as being intermediate between the
chemical units (molecules) and the morphological units (cells).
But he supposes their function in heredity to ])e dilTerent from
that which I ascribe to my biophors. Haeckel * understands by
the term ' plastidule,' introduced by Elsberg,f the hypothetical
ultimate particles of w'hich ' protoplasm ' is composed ; he re-
gards them as equivalent to the ' molecules ' of inorganic matter,
but supposes them to possess 'vital qualities' as well. Of
course this definition is in itself insufficient proof, as de Vries
very correctly remarks, that Haeckel' s plastidules are not mole-
cules in the physical sense ; these very ' vital qualities ' are
the point in which they differ from them. I could not adopt
Nageli's term either, because a 'micella' differs essentially in
its construction and properties from a biophor. It is defined
as 'a minute crystal, microscopically invisible, consisting of a
larger or smaller number of molecules, and is, when turgid,
surrounded by a layer of water.* J As regards the absolute size
of the micella, Nageli calculates that it may consist of one hun-
dred molecules, or on the other hand, of only a single molecule
of albumen. As in the case of HaeckePs plastidules, we have
here therefore to deal with a unit the vital character of which
does not depend on a peculiar grouping of several or even
many different kinds of molecules. Indeed Nageli draws atten-
tion in another part of his book (p. 63) to the unstable chemi-
cal composition of the proteids so far as can be made out
by analyses, and very correctly considers it extremely proba-
ble 'that there are various molecules of albumen which differ
from one another in containing unequal quantities of hydrogen,
oxygen, &C.'' This leads him to the further assumption ' that
the micellae of the proteids consist of a mixture of two or
more different kinds of molecules of albumen. In each proteid
the different molecules of albumen would be mixed in special
proportions, and, further, each would contain special quantities
of phosphates, salts of magnesia, lime, and so on.* This con-
ception, however, hardly agrees with that of the * crystalline '

* Ernst Haeckel, ' Die Perigenesis der Plastidule," Berlin, 1876.

t I^ouis Elsberg, ' Regeneration ; or. The Preservation of Organic Mole-
cules; a Contribution to the Doctrine of Evolution.' — ProcccJ. Am. Assoc,
for the Advancement of Science, Hartford Meeting, Aug. 1874.

X Carl Nageli, ' Mechanisch-physiologische Theorie der Abstammungs-
ehre,' Miinchen u. Leipzig, 1884, p. 35.

42 THE GERM-PLASM

nature of the micella, for crystals are not ' mixtures,' but chemi-
cally pure substances. And apart from this, we should be wrong
in inferring from this passage that Nageli considers the vital
properties of a micella dependent on the co-operation oi different
molecules united into a single group ; for in the passage quoted
above he also states that one molecule of albumen is sufificient
for the constitution of a micella.

For this reason alone it will be seen that the conceptions of
the biophor and of the micella do not coincide. They differ also
as regards the mode of multiplication : the fundamental impor-
tance of this will become apparent later on. The biophors, as
bearers of vitality, possess the power of growth and of multiplica-
tion by fission, just as is the case in all orders of vital units on
which direct observations have been made, beginning with the
microsomata, which constitute the chromatin of the nucleus, and
passing through the chlorophyll granules, nuclei, and cells, up
to the simpler plants and animals. Nageli"s micella also mul-
tiply, but the multiplication occurs ' by the free interposition of
new micella, similar to, or identical with, those already present,'
in the same manner as he supposed the addition of new particles
to take place in a starch grain, or as crystals separate from the
mother liquor. These new micellae would certainly have to be
formed by an influence, exerted by those already present, which
cannot be further defined.

The ' pangenes ■" of de Vries correspond almost exactly to my
biophors, for they are also accredited with the functions of growth
and multiplication by division, and play a similar part in heredity.
The biophors, as will be explained in the following pages, only
differ from the pangenes in being constituents of higher units of
the hereditary .substance.

The minute vital particles or ' plasomes," recently assumed by
Wiesner, resemble both pangenes and biophors as regards their
properties. The part they take in heredity is, however, only
hinted at, and it is therefore better for me to use the special
term biophor than to press tlie plasomes into the service of my
theory of heredity.

The biophors play the same part with respect to heredity as
that which de Vries ascribes to his pangenes, i.e., they are the
' bearers of the qualities or ' characters ' of the cells ; ' or more
accurately, the bearers of the cell-qualities. As all living matter
consists of biophors, the differences in it can only depend on

THE GERM-PLASM 43

the differences in the biophors composing it ; an animal cell
containing, for example, transversely striped muscular substance,
or delicate nervous or glandular structures, or again, a vegetable
cell enclosing chlorophyll bodies, must contain several different
kinds of biophors of which these various cell-structures are com-
posed, and which constitute the germ-plasm of a species.

There must be a great number of different kinds of biophors,
for otherwise they could not give rise to so great a variety of
cells as exists in the organic world. Nor is it difficult to infer
the possibility of an almost unlimited number of different kinds
of biophors from their assumed composition.

As the biophors are not individual molecules, but groups of
molecules, nothing prevents us from tracing a large number of
variations in them to the widely varying number of their mole-
cules. But even the chemical constitution of the molecules is not
by any means necessarily the same in all cases, although the
possible fluctuations are certainly confined within certain limits.

Numerous facts show that at any rate in the two main divis-
ions of the organic world, the animal and vegetable kingdoms,
several of the molecules composing the biophors differ chemi-
cally from one another, so that substitutions occur. Whereas
glycogen is a constituent which is never absent from animal
protoplasm, provided that the latter possesses amoeboid move-
ment, this carbohydrate has not yet been discovered in plants,
in which, as Hoppe-Seyler suspects, it is probably replaced by
amylum, dextrine, or gum. Similarly, the crystalline proteids
in plants, which are known as aleurone grains, are chemically
different from the yolk-granules in animals.

A difference in the biophors can, moreover, be conceived
without a change in their atomic composition, by regarding as
possible a rearrangement of the atoms in the individual mole-
cules. The molecule of albumen in particular has, according to
the conclusions of modern chemistry, a molecular weight of at
least I, GOO, so that innumerable isomeric molecules of albumen
seem to be conceivable. It is, however, impossible to state how
many of them actually exist.

In order to give as complete an explanation as possible of the
phenomena of heredity with the aid of the biophors, the latter
must be invested with the capacity for a further change, namel>-,
a rearrangemejit of the molecules^ analogous to the isomeric
rearrangement of the atoms in a single molecule. This assump-

44 THE GERM-PLASM

tion is not unfounded, inasmuch as several instances of molecular
compounds are known in chemistry, e.g.. the double salts and
the water of crystallisation of salts, in which definite numbers
of molecules are always present : this number is even retained
in spite of substitution. Thus alum always contains twenty- four
molecules of water of crystallisation, and this evidently indicates
a deforce oi affinity between the molecules. We shall have to
assume this property for the biophor also, for without it the latter
would not be a real unit at all. We shall, moreover, be able
to conclude that these degrees of affinity are of various kinds,
and that the molecules can combine in many different ways
and form groups, so that isomeric molecular compounds are
formed. Such isomeric compounds, however, will possess other
properties, just as in the isomeric arrangement of atoms in the
individual molecule ; and thus we conclude that the special
properties of a biophor are to be considered dependent not only
on the physico-chemical constitution of the molecule, but also
very essentially on their position and relation to one another ;
so that one biophor can be changed into another by an altera-
tion in the arrano:ement of its molecules.

According to this statement there are several kinds of
biophors, the difference between which depends on either the
absolute relative number of molecules, their chemical constitu-
tion (isomerism included), or their grouping; in fact we may
say that the niDiiber of possible kinds of biopliors is jinlii)iited^
just as is the number of conceivable organic molecules. We
shall, at any rate, meet with no theoretical difficulties on this
score, however large the number of different kinds of biophors
may be which we require to explain the theory of heredity.

The biophors are not., I believe., by any nieans mere Jiypo-
thetical iniits ; they uiust exist., for the phenomena of life must
be connected with a material unit of some sort. But since the
primary vital forces — assimilation and growth — do not proceed
spontaneously from either atoms or molecules, there must be a
unit of a higher order from which these forces are developed,
and this can only consist of a group consisting of a combina-
tion of dissimilar molecules. I emphasise this particularly,
because a theory of heredity requires so many assumptions which
cannot be substantiated that the few fixed points on which we
can rely are doubly valuable.

These biophors constitute all protoplasm — the morphoplasm

THE GER]\I-rLASl\r 45

which is differentiated into the cell-substance, as well as the
idioplasm contained in the nucleus. It will be shown subse-
quently in what manner these two kinds of protoplasm differ as
regards their constitution, and I will only remark here that the
idioplasm must have a far more complex structure than the mor-
phoplasm. The latter, as the cell-substance of a muscle or
gland-cell shows, can assimilate, grow, and also divide, but it is
not able to change into anything different from itself. The
idioplasm, on the other hand, is capable of regular change dur-
ing growth ; and ontogeny, or the development of the individual
in multicellular organisms, depends upon this fact. The two
first embryonic cells of an animal arise from the division of the
ovum, and continually give rise to differently constituted cells
during the course of embryogeny. The diversity of these
cells must, as I have shown, depend on changes in the nuclear
substance.

It now remains to be considered how we are to imagine this
capacity on the part of the idioplasm for regular and spontane-
ous change. The fact in itself is beyond doubt, when once it is
established that the morphoplasm of each cell is controlled, and
its character decided, by the idioplasm of the nucleus. The reg-
ular changes occurring in the egg-cell and the products of its
division in each embryogeny must then be referred to the corre-
sponding changes of the idioplasm. But what is tJie nature of
these changes, and how are they brought about ■

2. The Control of the Cell

In order to answer the question which has just been asked, it
will be necessary to consider the manner in which the idioplasm
of the nucleus determines the characters of the cell. At present
we only know that the idioplasm consists of a large number of
different biophors of various kinds. To exert a determining
influence on the minute structure of the cell-body and on the
chemical composition of its different components, it must either
be capable of exerting an emitted influence (* Fernwirkung ") or
else material particles must pass out of the nucleus into the cell-
body.

Strasburger * has endeavoured to prove a dynamical effect of

* E. Strasburger, ' Neue Untersuchungen iiber den Befruchtungsvorgang
bei den Phanerogamen," 1884, p. iii.

46 THE GERM-PLASM

the nuclear matter. In his opinion ' molecular stimuli are trans-
mitted from the nucleus to the surrounding cytoplasm, and, on
the one hand, control the processes of metabolism in the cell,
and on the other, give a definite specific character to the growth
of the cytoplasm, this growth being caused by nutrition/ Al-
though transmission of the molecular stimuli, proceeding from
the nucleus to the rest of the cell, is certainly conceivable,
de Vries has rightly shown that this is not a sufficient expla-
nation of the phenomena, because it takes for granted the fun-
damental point of the matter requiring explanation. If the cell
of any plant is to acquire the hereditary property of forming
malic acid, those pangenes in the cell-body which can produce
this acid could, it is true, come into play by molecular stimuli
being transmitted to them from the nucleus ; but this hypoth-
esis takes their presence for granted, and the main question as
to how these producers of malic acid get into the cell remains
unanswered.

Haberlandt * has attempted to trace the control of the cell by
the nucleus to the enzymatic action of the latter, i.e., to the giv-
ing off from the nucleus of certain chemical compounds which
cause the cell-substance to become changed in a given manner;
but this explanation is regarded by de Vries as insufficient, be-
cause here again it is necessary to presuppose a definite diff'eren-
tiation of the cell-body.

De Vries himself gives a solution of the problem, and his
hypothesis has, at any rate, the advantage of great simplicity
and lucidity. He supposes that some of the pangenes which
constitute the nuclear matter pass into the body of the cell
through the nuclear membrane, and there form its parts and
structures, of the qualities of which they are the special
bearers.

Although I formerly inclined towards Strasburger's view, it
always appeared to me rather as a formal than as a real expla-
nation of the problem, and I regarded it more as a provisional
formulation than as a solution of the difficulty. In my opinion
de Vries's idea of the migration of minute, specific, vital parti-
cles from the nucleus into the cell-body affords an extremely '
happy solution of the apparently inexplicable manner in which

* G. Haberlandt, ' Uber die Beziehungen zwischen Funktionen und Lage
des Zellkerns,' 1877.

THE GERM-PLASM 47

the cell is controlled by the nucleus. It, moreover, fits in very
well with my other views.

As long as I was engaged in seeking for an epigenetic theory
of heredity, an explanation of this sort was naturally impossible,
but as soon as I assumed that the germ-plasm consisted of
biophors, the various kinds of which are required for the various
characters of the respective cells, it was not only possible to
suppose that the particles exerted an influence of this nature on
the cell, but such an explanation of the phenomena became the
most natural and satisfactory one. Much may of course be
urged against this fundamental assumption, and it is not in
itself a sufficient explanation ; but it is not only fruitless to
attempt a satisfactory explanation from the other point of view,
but as will appear later on, de Vries's conception alone agrees
with certain fundamental biological principles.

If the nuclear substance exerted an emitted influence on the
cell-body so as to give rise to the structures characteristic of this
particular kind of cell, they would be formed by a kind of
'• generatio egiiivoca ' ; they would have arisen by the operation
of an external influence on the given substance in the cell, just
as would be the case in primordial generation. Particularly
favourable influences would have operated on certain combina-
tions of inorganic substances in such a way as to give rise to a
vital particle.

We know nothing of such a primordial generation as far as
our experience extends, and even if it must be considered to be
logically necessary, we have every reason to suppose that it has
no share in the origin of those forms of life with which we are
acquainted, but that these always arise by division from others
similar to themselves. Moreover, what is true of the independ-
ent organisms familiar to us must also hold good for all the
different orders of vital units which have united to form higher
organisms, for each of the earliest and lowest organisms must
have been neither more nor less than the eqidvaletU of one
biophor. If, then, in order to explain the presence of life on
the earth, we must assume that such individual biophors arose
at one time by primordial generation, they must have been
capable of reproduction by division immediately after their
origin, for such multiplication is caused directly by the pri-
mary forces of life, — assimilation and growth. We can only
imagine the very simplest biophors as having been produced

48 THE GERM-PLASM

by primordial generation : all subsequent and more complex
kinds of hiophors can only Jiave arisen on tJie principle of
adaptation to new conditions of life; they must have been
developed gradually by the long -continued co-operation of
heredity and selection. All these biophors of a higher order,
which are adapted to the special conditions of existence and
which in endless varieties form organisms as we see them around
us, possess 'historical' qualities; they can. therefore, only arise
from others like themselves, and cannot be formed spontaneously-
This fact is confirmed by experience. Not only does a cell
always arise from a cell, and a nucleus from a nucleus, as de
Vries. and more recently Wiesner, have shown, but all the other
constituents which occur in the cell-body and determine its
structure never arise, so far as we know, by ^ generatio equivocal
or, as de Vries expresses it, ' neogenetically.' They are always
produced by the division of similar structures already present.
This is apparently true of the green chromatophores and the
H-acuoles"' of plant-cells, as well as of the ^sphere of attraction,'
or centrosome, which controls the division of the nucleus : the
same must also hold good for those invisible vital units, the
various kinds of biophors, which have arisen during the course
of the earth's history by gradual adaptation to continually new
conditions of life.

If then, each vital unit in all organisms, from the lowest to the
highest grade, can only arise by division from another like itself,
an answer is given to the question with which we started ; and
we see that the structures of a cell-body, which constitute the
specific character of the cell, cannot be produced by the emitted
influence of the nuclear substance, nor by its enzymatic action,
but can only arise owing to the migration of material particles
of the nucleus into the cell-body. Hence the nuclear niatter
must be in a sense a storehouse for the various kinds of biophors
which enter into the cell-body and are destined to transfor)n it.
Thus the development of the 'undifferentiated' embryonic cell
into a nerve-, gland-, or muscle-cell, as the case may be, is
determined in each case by the presence of the corresponding
biophors in the respective nuclei, and in due time these
biophors will pass out of the nuclei into the cell-bodies, and
transform them.

To me this reasoning is so convincing that any difficulties we
meet with in the process of determining the nature of the cell

-J

THE GERM-PLASM 49

hardly come into account. We are still far from being able to
describe in detail the entire histological process of the differen-
tiation of a cell. The passage of invisible ' biophors ' through
the pores of the nuclear membrane is probably just as admis-
sible an assumption as that of the independent power of motion
thereby necessitated in these bearers of vitality ; but the histo-
logical structure of a cell is not completed by the mere emission
into the cell-body of a few kinds of biophors with great powers
of multiplication. Numerous questions suggest themselves in
this connection, all pointing to the fact that forces are at work
of which w'e are at present ignorant. The immigrating biophors
are the mere material which forms the histological structure of
a cell, only when subjected to the guiding forces — presumably
those of attraction and repulsion — which must be located in the
biophor.

We can as yet form no more exact conception of this process
than we can of the manner in which the biophors already con-
tained in the cell-body behave in respect to those which have
migrated into it from the nucleus. Presumably a struggle of
the parts occurs, in which the weaker are suppressed and serve
as nutritive material for the stronger ones. But although much
remains to be decided by future investigation, the main point at
issue, at any rate, viz., that the nature of the cell is really decided
by the elements of the nucleus, is definitely established. By the
nature of the cell must be understood not only the histological
structure of the cell as a whole and^it^ mode of reacting to
external influences, but more particularly its mode of division in
respect of time and place. It is true that the ctW-body itself and
its apparatus for division (the centrosome) primarily determine
whether a cell is to divide sooner or later, and into equal or
unequal parts ; but these processes always depend finally on the
nucleus, which controls the cell-body and impresses on the latter
its definite nature.

The most plausible objection which can be urged against the
migration of the particles of the idioplasm into the cell-body is
that the substance of the latter is chemically quite different from
that of the nucleus. Their behaviour as regards taking up
colouring matters is certainly different, as the terms chromosome
and chromatin indicate ; but even if a difference in their chemical
composition could be inferred from this fact, it would still fail to
constitute a decisive proof against the hypothesis of migration :

50 THE GERM-PLASM

for it is well known that the affinity of the chromosomes for
colouring matter varies markedly at different periods, and this
indicates that slight changes, which are beyond our control, take
place in the constitution of this substance, and are sufficient
to cause its most striking reaction with regard to colouring
matters to disappear for a time. Chemical analysis of the
substance contained in the nucleus has certainly established
the presence of ' nuclein ' ; but although it is probable from
Miescher's * excellent observations on the sperm of the salmon
that nuclein is derived from the nuclei of the sperm-cells, it is
not by any means certain from what part of the nucleus it
originates: if one supposes that over 48 per cent, of the dried
sperm consists of nuclein, it is doubtful whether this is contained
in the small mass of chromatin which we see in the form of
chromosomes.

Another recent observation may be mentioned here, which
proves at any rate that matter is actually transferred from the
chromosomes of the nucleus into the cell-body just at the time
when the characteristic structure of the cell-body is being
formed. I refer to Riickert's observations on the remarkable
alteration in the size of the chroiuosonies of the nucleus during
the growth of the ovum of the dog-fish. f One of the youngest
ova observed in the ovary — which measured 2 mm. in diameter
— contained from 30 to 36 chromosomes, each of which was
12 microns:}: long, and 2 cubic microns in bulk: later on, in
nearly ripe eggs, the length of a chromosome reaches 100 /x,
and its cubic contents 7,850 cubic /x, or more accurately, since
it has meanwhile become doubled by division, 15,700 cubic /x.
Still later, just before the formation of the first polar body,
when the ovum is ripe and has attained its full size, the length
of the individual chromosome diminishes to 2 /x, and the cubic
contents of a double rod to 3 cubic /x. Ruckert infers from
these facts that the chromosomes give off a great amount of
substance to the ovum during the gradual ripening of the latter,
and we can only agree with him on this point. But the ques-
tion arises as to how this transference of substance takes place, —

* Miescher-Rusch, ' Statist, u. biolog. Beitrage zur Kentniss vom Leben
des Rheinsalm,' 1880; Schweiz. Literatursamml. z. internationalen Fisch-
ereianstell. in Berlin.

t j. Ruckert, ' Anat, Anzeiger,' loth March, 1892.

X A micron (m) is the lo'on of a millimetre.

THE GERM-PLASM 51

whether it occurs in the ordinary way, fluid nutrient material
being given off, and then assimilated by the cell-body, or in
some other manner. There seems to me to be no reason why
we should not assume that niitiute, specific, vital particles, and
not merely nutritive substances, are produced by the chromo-
somes during the growth of the ^gg-, and are then emitted
through the nuclear membrane into the cell-body. Further
facts must be ascertained before we can attempt to explain the
details of the curious morphological transformations which the
chromosomes undergo during this period. We are already,
however, in a position to state that the extremely interesting
processes described by Ruckert must have a wide significance,
and must occur in all cells which become histologically differen-
tiated as well as in all animal ova. But they cannot appear so
distinctly in these other cells, for no animal cell grows to such
an enormous size as does the egg-cell. I shall again refer to
the process in a later section, in order to emphasise one of the
consequences which results from it still more strongly.

Let us now suppose with de Vries that the nature of a cell
depends on the extrusion of minute vital particles of different
kinds from the nucleus into the cell-body, and that these subse-
quently multiply and become regularly distributed and arranged
in groups according to the forces of attraction and repulsion
situated within them. On this supposition, heredity could be
simply and easily accounted for in unicellular organisms, for in
them multiplication depends on a division of the whole body
and of the nucleus into two parts, and thus each product of the
division receives a similar supply of latent biophors which form
its nucleus, and from which it can then provide the necessary
material to the cell-body.

As the influence of amphimixis is not taken into account in
the present connection, I may here leave out of consideration
the fact that the nucleus may be differentiated into two different
kinds of nuclei. This arrangement is practically universal
amongst the highest unicellular forms — the Infusoria — and is
merely an adaptation for conjugation. In the unicellular forms
heredity will therefore depend, firstly, on the fact that all the
different kinds of biophors which are required for the construc-
tion of the body are present in the nucleus in a latent condition,
and in definite proportions — very probably they have also a
definite style of architecture; and secondly, on the periodi-

52

THE GERM -PLASM

cal or occasional migration of these biophors into the cell-body,
where they multiply, and become arranged in obedience to the
forces acting within them. The difficulty of ascertaining the
actual mode of arrangement is nowhere greater than in the case
of the higher unicellular forms. How is it possible that the
nucleus should always allow only those kinds of biophors to
migrate which are required to replace those structures lost by
division ? And why do these biophors always move either in the
direction of the missing oral region, or towards the posterior end
of the body, according to which parts are wanting in the two
daughter-animals? For the present these questions are un-
answerable ; and in the meantime we must be content with
having shown how the materials for the construction of the
cell-substance are transmitted from mother to daughter, and in
what way they are placed at the disposal of the forces acting in
the cell-body.

The experiments made by Nussbaum * and Gruber f on the
artificial division of Infusoria prove that the nucleus really
controls the cell-body. These observers found that only those
portions which contained a part of the nucleus were capable of
giving rise to a complete animal ; the other pieces lived for a
time, and then perished. One of Gruber's observations also
tends to show that when regeneration of missing parts occurs,
the nucleus sends out invisible material particles into the cell-
body. He cut a large Stentor which was preparing for division
transversely into two parts, so that the posterior portion con-
tained no trace of the nucleus, and then observed that regenera-
tion of the missing parts nevertheless took place, especially in
the oral region. If the control of the cell depended on the
emitted influence of the nucleus, this regeneration would be
totally inexplicable ; if, however, biophors proceed from the
nucleus into the cell-body when regeneration is to take place,
this might have already occurred in an animal preparing for
division, as this one was before it was artificially divided.

The descendants of unicellular animals are similar to their
ancestors : two daughter-cells are produced by the division of

* Nussbaum, ' Ueber die Theilbarkeit der lebenden Materie,' Archiv. f.
mikr. Anat., 1886.

t Gruber, ' Ueber kiinstliche Teilung bei Infusorien,' ' Biol. Centralblatt,'
Bd. iv. ; and ' Beitrage zur Kentniss der Physiologic und Biologic der Pro-
tozoen," Ber. d. naturf. Geseilsch. zu Freiburg i/Br., 1886.

THE GERM-PLASM 53

the mother-cell, and thus the nuclear substance is always com-
posed of different kinds of biophors. But how does this apply
to multicellular forms in which so large a number of different
kinds of cells, each presupposing a different structure of the
nuclear matter, arises from the germ-plasm of the ovum ? Thus
we find ourselves brought back to the question asked at the end
of the last section : — on what do the regular series of changes in
the germ-plasm during ontogeny depend ?

3. The Determinants

As has just been shown, the nuclear matter of an Infusorian
must be composed of a great number of different kinds of
biophors, each of wiiich corresponds to the primary constituent
of a definite portion of the unicellular organism. If the cells of
a multicellular animal were represented in the germ-plasm by all
the kinds of biophors occurring in them, such an enormous
aggregation of biophors would result that, even if they were ex-
tremely small, the minute quantity of matter in the germ-plasm
would not be able to contain them. It was this consideration
more than any other which for many years made me persevere
in my attempt to discover an epigenetic theory of heredity. I
thought that it must be possible to imagine a germ-plasm which,
although highly complex, nevertheless did not consist of such an
inconceivably large number of separate particles, but which was
of such a structure as to become changed in a regular manner
during its growth in the course of ontogeny, and, finally, to yield
a large number of different kinds of idioplasm for the control of
the cells of the body in a specific manner.

Hatschek,* too, has recently put forward the view that • the
egg-cell may be supposed to contain a relatively small number
of qualities,' and that this number is not larger than that which
is to be assumed in the case of any other histologically differen-
tiated cell of the body. The diversity in structure seen in
multicellular organisms is due, in his opinion, to the fact that
in spite of the limited diversity as regards the qualities contained
within a single cell (including the ovum), a far greater complica-
tion of the bodv as a whole is attained bv the variation of these
few qualities (' des einen Gmndthemas').

* B. Hatschek, ' Lehrbuch der Zoologie," 2te Lieferung. Jena, 1889.
p. 232.

54 THE GERM-PLASM

If in considering a theory of heredity we had only to deal with
an explanation of the transmission of an unalterable structure
from the parent to the offspring from generation to generation,
there would be theoretically no objection to the assumption of
such a structure of the germ-plasm. We have, however, to deal
with the transmission of parts which are variable, and this
necessitates the assumption that just as many independent and
variable parts exist in the germ-plasm as are present in the fully
formed organism. It is impossible that a portion of the body
should exhibit an independent variation capable of transmission
unless it were represented in the germ-plasm by a special par-
ticle, a variation in which is followed by one in the part under
consideration. If this were represented, together with other
parts of the body, by one particle of the germ-plasm, a change
in the latter would be followed by a variation in all the parts of
the body determined by it. The independently and hereditarily
variable parts of the body therefore serve as an exact measure for
determining the nnniber of ulti))iate particles of whicJi the ger?n-
piasm is composed: the latter must contain at least as great a
fiumber as would be arrived at by such a computation.

An example may make it clear that the independently variable
parts are not identical with those which are merely hereditary.

It is well known that butterflies pass through a metamor-
phosis in the course of development, the stages of which are
independently variable from the germ onwards : that is to say,
a variation in the caterpillar is not necessarily followed
by one in the butterfly, and vice versa. The caterpillars
of a species may be dimorphic, some being green, and
others brown, but both of these forms nevertheless give
rise to butterflies with a similar coloration. If, therefore, the
phyletic modifications depend on changes in the minute
structure of the germ-plasm, there must be at least two inde-
pendently variable units in the germ-plasm of such a butterfly ;
for if there were only one, the butterfly as well as the caterpillar
would be affected by a variation in it. But a comparison of
nearly related species shows us that the individual parts of the
caterpillar or butterfly must also be variable from the germ
onwards : the limbs, for instance, of two species may be very
similar, while their wings are different, and even the separate
parts of the wings may vary independently of one another. We
must therefore assume that the germ-plasm contains a large

THE GERM-PLASM 55

number of units, on the variation of which the independent
changes of certain parts of the body depend.

In all the higher animals the number of these units must be
very large, because the parts which are independently variable
from the germ onwards is large also.

A consideration of the individual and hereditary characters in
the human species will show most clearly how great this number
may be. I know of a family in which a depression of the size
of a pin's head in the skin in front of the left ear has been
transmitted through three generations. This slight abnormality
must therefore have been contained potentially in the germ-
plasm of the respective individuals, and their germ-plasm must
ditfer from that of other people in the slightly abnormal form of
the element which determines this peculiarity. We are logi-
cally compelled to assume a particular element of the germ-
plasm for each peculiarity of this sort, not because heredity may
be manifested in details so minute, but because the tra7istuission
of such details 7)iay be independent. If all people possessed such
a depression in front of one ear, we could not thereby conclude
that it must be represented by a special element in the germ-
plasm merely because it is hereditary. It might conceivably be
represented, together with the skin of half the face, by one
element or biophor, which in the course of ontogeny became
divided into a number of secondary ones of divers sorts, one of
which proved to be abnormal and came to be situated at that
particular spot in the skin. What compels us to accept the
above assumption is the fact that all people do not possess this
depression, and that two persons might conceivably resemble
one another in all other respects except in the possession of this
abnormality. The germ-plasm of both these persons would be
almost identical, but not perfectly so, for it would contain a
certain element which differed in the two cases. This simply
means that this particular character which is independently
variable from the germ ofiwards is also represented by a special
element in the gertn-plasm. It would not have been possible to
infer this from its transmissibility alone. A hundred different
characters might conceivably be determined by a single element
in the germ-plasm ; the whole hundred would then be trans-
mitted as soon as the determining element was present in the
latter, but not one of them would be independently variable
from the germ onwards ; but if the determining element varied.

56 THE GERM-PLASM

all the hundred characters would vary at the same time. The
capacity for transmission and that of independent variation from
the germ onwards are distinct from one another.

The germ-plasm must consequently be composed of as many
units as there are transmissible parts in the body which are in-
dependently variable from the germ onwards. Each of these
units cannot be smaller than a biophor, and they can therefore
not be simple molecules within a biophor; for variation is a
biological conception, and a biological element does not pre-
suppose a one that is merely physical.

What parts of the body of a multicellular organism are repre-
sented in the germ by special particles of the minimum value of
one biophor? Is each cell, or even each part of a cell? Darwin
adopted .the former, and de Vries the latter of these two alter-
natives. Darwin's gemmules are germs of cells, so that every
cell of the body would be represented in the ovum by these
units ; while de Vries's pangenes are in a sense germs of the
characters or structures (' Zellorganen ') of the cell. There is
no doubt that the hereditary variations in plants and animals
manifest themselves in alterations of the individual parts or
structures of the cell, and not only in the iiiunber, relative
arrangement, and the changes in the form, size, and nature
of the cells as a whole. The variegated varieties of our orna-
mental plants possess similar cells to those of their ancestral
forms, but the green colour of the leaf is absent in certain
of the cells : the red tint of the leaves of the copper beech,
and other varieties of plants, depends on the red colour of the
sap in a certain layer of cells, and this colour is transmissible.
The coloured pattern on a butterfly's wing or a bird's plumage
depends on cellular elements which were probably all alike
in remote ancestors, but which afterwards became gradually
changed by hereditary variations in the individual components
or in the structure of the cell. Although the entire phyletic
transformation of a species does not by any means alone depend
on its zV/Zr^^-cellular variation, the latter has, nevertheless, con-
stantly accompanied the other variations, and has shared to
a greater or less extent in the transformation of the species.
Hence it cannot be doubted that even in multicellular forms
not only the cells as a whole, but also their parts, are determined
from the germ onwards.

It seems therefore impossible to avoid the stupendous as-

THE GERM-PLASM

57

sumption that each of the millions of cells in a multicellular
organism is represented in the germ-plasm by several or manv
different kinds of biophors. There is, however, a simple and
natural way out of this dilemma, as soon as we inquire whether
every cell of a plant or an animal is independently variable at
all, and whether consequently it must be represented l)y special
elements in the germ-plasm.

I shall designate the cells or groups of cells which are inde-
pendently variable from the germ onwards as the ' hereditary
parts ' or ' deter ndnates,'' and the particles of the germ-plasm
corresponding to and determining them, as the ' deteriiiiniiig
parts'" or ^determinants.'' It is evident that many of the cells
in the higher animals are not represented individjially in the
germ-plasm by a determinant. The millions of blood-corpuscles
which are formed during the life of a Vertebrate might possibly
be coiitrolled in the germ-plasm by a single determinant. At
any rate no disadvantage to the species would result from this,
because the capacity for being independently determined on the
part of the individual blood-corpuscles, or even individual thou-
sands of them, would be of no value to the animal. They are
not localised : one of them has the same value as another, and
their variability therefore might well be controlled from a single
point. In conformity with the law of economy, Nature would
not have incorporated more determinants than was necessary
into the germ-plasm.

Thus there are probably many groups of cells in the higher
animals, the constituents of which are not represented individu-
ally in the germ-plasm. All the nerve-cells of the brain do. it
is true, possess their special determinants, as otherwise the trans-
mission of such fine shades of mental qualities in man would be
inexplicable; but it can matter little whether each fibre of a
muscle, or each cell of the epidermis or of the epithelial lining
of the alimentary canal, has its special determinant : in the last-
mentioned cases larger or ^w^tWqx groups of cells are presumably
controlled by a single determinant. The manner in which tTie
epithelium of the alimentary canal is renewed amongst insects
may perhaps be taken as pointing to this assumption. In flies
and butterflies, for instance, as I have proved long ago, the
alimentary canal of the larva undergoes disintegration, and that
of the imago, which has a very different structure, is developed
out of its remains. Kowalewski and van Rees have since

58 THE GERM -PLASM

shown that the process takes place as follows: — the formation
of portions of the new alimentary canal begins in certain cells
which are separated by fairly regular intervals ; these then
spread until they come into contact with one another. The
idioplasm of the new intestinal cells is consequently only con-
tained in these formative cells, and it is natural to suppose that
each of them contains only one kind of determinant.

The same appears to be the case with the hair of mammals.
Every hair does not possess a special determinant in the germ,
but more or less extensive regions of the hairy covering are
represented each by one determinant. These regions are not
large, as is shown by the stripes and spots on the coat of such
animals as the tiger and leopard. The recurrence in the son,
on exactly the same part of the head as in the parent, of an
abnormal tuft of white hair, has been observed in the human
subject.

- Similar hereditary parts or determinates may be observed in
butterflies, in which the colours on the wings often form very
complicated lines and spots of slight extent but of great con-
stancy. Such regions are often limited to quite a few scales
(cells) : Lyccrna argiis, for instance, possesses a black spot on
a particular part of the anterior wing consisting of only ten
scales, while the surrounding parts are blue. In this case we
may therefore conclude that the black cells are represented in
the germ-plasm by at least one determinant. The determination
may possibly be carried out in still further detail in this instance,
and each cell in the black spot may be determined from the
germ onwards ; and possibly it is only the constant inter-
mingling of two hereditary tendencies in sexual reproduction,
and the consequent variability in the number of scales, which
prevents us from recognising the fact. We can at any rate,
however, find instances of the determination of single cells
in other species of animals. For example, in many Crusta-
ceans a number of sensory organs are situated on the anterior
antennae : each of these corresponds to one cell. The number,
position, and form of these ' olfactory ' setae is determined
exactly for each species. The Ostracod Cypris possesses only
otie olfactory seta on each antennule, while in the common
fresh-water species of Ganitnarus^ there are about twenty of
these structures, each of which is separately attached to one of
the consecutive joints of the feeler. In many blind Crustaceans,

THE GERM-PLASM 59

which live in the dark, the number of these setae is greater than
in the case of related forms which possess the sense of sight.
And though in all these instances individual deviations occur,
we may nevertheless suppose them to be hereditary, for other-
wise the increase in the number of olfactory setae incident on
a life in darkness, could not have been established as a specific
character.

In smaller and simpler organisms each individual cell may
well have been determined from the germ onwards, and not
merely with the result that the number of cells is a definite one,
and the position of each definitely localised : the determination
may also have caused individual peculiarities of each cell, in so
far as they depend on changes in the germ-plasm at all — i.e.^
are * blastogenic,^ — to reappear in the corresponding cell in the
next generation, just as in the case of a birthmark in the human
subject which recurs in precisely the same place on the same
side of the body. This may also be true of animals as simple as
the DicyeinidcB or the Tardigrada, although it is not possible to
prove it positively.

In all the more highly differentiated animals there can be
little doubt that the number of determinants is always very much
less than that of the cells which are the factors in the process of
ontogeny. If we compare this statement with Darwin's assump-
tion of the presence of a gemmule — or rather of several gem-
mules — for each cell, it is evident that the germ-plasm is thus
to some extent relieved of a burden.

We must not forget, however, that a cell may vary as regards
transmission not only as a whole but also in its parts, so that
not one but several biophors must be assumed for each deter-
minant of a cell or group of cells ; we must, in fact, suppose just
as many to be present as there are structures in the cell which
are variable from the germ onwards. We ought, properly speak-
ing, to speak of these bearers of qualities, which correspond to
de Vries's pangenes, as determinants also, for they determine
the parts of a cell. As the name of biophor has been given to
them, however, it is better to retain this term, and to define a
deter 7niiiant as a priinary constituent of a cell or group of cells.
Thus a determinant is always a group of biophors, and never a
single one.

It may now, I believe, be proved without difficulty that the
biophors determining a cell not only lie close together in the

6o THE GERM-PLASM

germ-plasm so as to form a group, but that they also combine to
form a JiigJier utiit. The determinant is not a disconnected
mass of different biophors, but a vital unit of a higher order than
the biopJior, possessed of special qualities.

The fact that the determinants must possess the power of
multiplication is in itself a sufficient proof of this. We know
how greatly the nuclear matter contained in the fertilised egg-
cell increases in volume during development, and this can only
be due to the multiplication of its vital particles, the biophors.
Such a multiplication could never occur with as much precision
and regularity as is necessary for the preservation of the char-
acter of a certain cell, if the biophors which determine it were
scattered at random instead of being definitely separated from
those of other cells. Hence the multiplication of the biophors
must occur within the fixed limits of the determinant, and must
be preliminary to the division of the determinant itself. And
consequently the latter is also a vital unit.

In accordance with our assumption, which can scarcely be
refuted, a single determinant of the germ-plasm frequently con-
trols entire groups of cells : this is a further proof that the de-
terminants as such must multiply. This is only possible if
they do so in the process of ontogeny. It is very probable,
moreover, that the nucleoplasm of any cell in the body never
contains one specimen only of the determinant controlling it,
but several ; otherwise, how could such a cell be visible at all
under our microscopes? Biophors, at any rate, are far beyond
the limit of vision, and even determinants can hardly come
within it.

Thus the assumption made by the gifted propounder of the
theory of pangenesis is so far justified. ' Gemmules " of cells
really exist, and multiply by fission ; but they are not the ulti-
mate vital units, nor are special gemmules of all the cells of the
body already present in the germ-plasm.

We have next to deal with the question as to how these two
elements of the germ-plasm, which have now been formulated,
are instrumental in the process of ontogeny.

4. The Id in Ontogeny

We can now make an attempt to solve the problem stated at
the close of the last section concerning the way in which the

THE GERM-PLASM 6 I

germ-plasm is capable of giving rise to the various kinds of
idioplasm required in the construction of the organism.

As we have seen, the germ-plasm contains the primary con-
stituents of all the cells in the body in its determinants, and it
only remains to inquire how each kind of determinant reaches
the right part in the right number. Although we do not know
what forces are called into play for this purpose, the elements of
the germ-plasm now formulated, and the processes and course
of ontogeny, nevertheless enable us to draw certain conclusions
as to the structure of the germ-plasm and the nature of the
changes it undergoes ; and I trust that these conclusions will
not lead us too far from the truth.

We can, in the first place, state with certainty that the
germ-plasm possesses a fixed arcJiitectiire^ wJiich has been trans-
mitted historically. In working out the idea of determinates, it
was stated that probably not nearly all the cells of the higher
organisms are represented in the germ-plasm by special deter-
minants : possibly all the blood-corpuscles, or the thousands of
fibres in a particular muscle, for instance, are represented each
by one determinant. But it does not therefore follow that all
the cells of a similar kind which exist in the body can be repre-
sented by one common determinant : this would be equivalent
to abandoning the conception of determinants altogether. If,
for instance, all the transversely striped muscles of a Vertebrate
were represented in the germ-plasm by a single determinant,
each variation in the latter would also produce a corresponding
change in all the muscles, and the independent variation of
which each individual muscle is actually capable would then be
impossible.

Several, or even many, similar determinants must therefore
exist in the germ-plasm of an anijnal. Muscle-cells and nerve-
cells are repeatedly formed even in the fully developed organism,
and, in so far as they can vary individually at all from the germ
onwards, will be represented by identical or by very similar
determinants in the germ-plasm.

If such identical determinants represent a single fixed cell or
group of cells, they cannot be situated anywhere in the germ-
plasm, nor can they change their position according to varying
influences : the determinants must be definitely localised, for
otherwise, they would not be certain to reach the right cell and
tiie right position in the course of ontogeny. 1 have already

62 THE GERM-PLASM

mentioned the olfactory setae of Gammarus, which are situated
individually on particular segments of the feeler. Each of these
can vary hereditarily, and thus it is necessary to assume special
determinants for them in the germ-plasm ; these, however, will
all be similar to one another. This is also true of the black
spots on the wings of certain butterflies, already referred to. In
LyccBua Argus, for instance, there is a spot on that part of the
wing which is known to entomologists as ' cell i b,' and this
spot is independently variable : it may be larger or smaller, and
the variations in it can be transmitted quite independently of
the numerous other black marks on the wing. The particular
spot referred to may have disappeared entirely in another species
of Lyccena^ while a precisely similar spot in ' cell 4 ' has become
much larger. We have also decided indications that homologous
parts in the two halves of the body in bilaterally symmetrical
animals can vary independently of one another. The human
birthmark mentioned above was always inherited on the left side,
and never on the right.

If each detenninant occupies a fixed position in the germ-
plasm, // cannot have an indefinite or variable size and form,
but must form a complete unit by itself, from which nothing can
be removed, and to which nothing can be added. In other
words, we are led to the assumption of groups of determinants,
each of which represents a separate vital unit of the third degree,
since it is composed of determinants, which in their turn are
made up of biophors. These are the units which I formulated
on different lines long ago, and to which the name of ancestral
germ-plasms was then given. I shall now speak of them as
'■ids,''* a term which recalls the 'idioplasm'' of Nageli.

I assume that just as the individual biophor has other quali-
ties than those of the determinant, which is composed of
biophors, so also does the id possess qualities differing from
those of its component determinants. The fundamental vital
properties — growth and multiplication by division — must how-
ever be attributed to the id as to all vital units. Several reasons.

* I have already used this term in my essay on 'Amphimixis ' ('Amphi-
mixis, Oder die Vermischung der Individuen," Jena, 1891, p. 39). In my
earlier essays the ids were spoken of as ' ancestral germ-plasms,' the mean-
ing and derivation of which term will be explained in the chapter on
amphigonic heredity.

THE GEHM-PLASM 63

more especially those furnished by the phenomena of heredity
in sexual reproduction, lead us to assume that the germ-plasm
does not consist of a single id, but of several, or even many of
them, and this assumption must be made even in the case of
asexual reproduction.

I shall therefore assume that each idioplasui is conposed of
several or many ids, which are capable of growth and iniiltipli-
cation by division. If animals existed, in the whole series of
ancestors of which sexual reproduction had never occurred,
these ids would be exactly similar to one another. But in all
cases every id of the germ-plasm contains the whole of the ele-
ments which are necessary for the development of all subsequent
idic stages. Theoretically, therefore, one id would suffice for
ontogeny.

We assume that the cJianges in the id of gerni-plasni during
ontogeny consist merely in a regular disintegration of the deter-
minants into smaller and smaller groups, until finally only one
kind of determinant is contained in the cell, viz., that which has
to determine it. It is highly improbable that all the determi-
nants in the id of germ-plasm are carried along through all the
idic stages of the ontogeny. In discussing regeneration and
gemmation later on, I shall have to show that, under certain cir-
cumstances, groups of determinants are supplied to certain series
of cells, and that these are not actually required for determining
the cells; this arrangement, however, depends, I believe, on
special adaptations, and is not primitive, at any rate not in the
higher animals and plants. Why should Nature, who always
manages with economy, indulge in the luxury of providing all
the cells of the body with the whole of the determinants of the
germ-plasm if a single kind of them is sufficient? Such an
arrangement will presumably only have occurred in cases in
which it serves definite purposes. The enormous number of
determinants contained in the germ-plasm also stands in the
way of such an assumption, for in the higher animals they can
be reckoned by hundreds of thousands at the very least ; and
although we may assume that they all remain in a latent condi-
tion in every cell, and so need not interfere with the activity
of the determinants which control the cell, they nevertheless
deprive the active determinants — which we must also suppose
to exist in large numbers — of a considerable space.

If we wished to assume that the whole of the determinants ol

64 THE GERM-PLASM

the germ-plasm are supplied to all the cells of the ontogeny, we
should have to suppose that diflferentiation of the body is due
to all the determinants except one particular one remaining
dormant in a regular order, and that, apart from special adapta-
tions, only one determinant reaches the cell. viz.. that which has
to control it. This latter supposition is undoubtedly less likely
than the former.

If however we do make this assumption, the question then
arises as to what factors can cause tlie gradual disintegration of
the id of germ-plasm into smaller and smaller groups of deter-
minants, — that is to say, into ids which contain fewer and fewer
kinds of determinants.

This disintegration I believe to be due to the co-operation of
three factors : these are — the inherited architecture of the germ-
plasm, in which each determinant has its definite position ; the
tinequally vigorous multiplication of the various determinants ;
and possiblv also, the forces of attraction which are situated
within each determinant, and result from its specific nature as a
special and independent vital unit. The architecture of the
germ-plasm has already been discussed in general terms : for the
present, at any rate, we can hardly conjecture the actual details
of its structure. In order to do so, it would be necessary to sup-
pose that hundreds of thousands, or millions, of determinants,
which are all definitely localised, take part in the formation of
the higher organisms. The fact that the right and left halves
of the body can vary independently in bilaterally symmetrical
animals, points to the conclusion that all the determinants are
present in pairs in the germ-plasm. As, moreover, in many
of these animals, e.g. the frog, the division of the ovum into
the two first embryonic cells indicates a separation of the body
into right and left halves, it follows that the id of germ-plasm
itself possesses a bilateral stmcture, and that it also divides so
as to give rise to the determinants of the right and left halves of
the body. This illustration may be taken as a further proof of
our view of the constant architecture of the germ-plasm. An id
is evidently not constiuited like the sediment of a complicated
and well-shaken mixture, in which the heavier particles come to
lie at the bottom and the lighter ones at the top : nor is it con-
stituted in such a manner that the respective positions of the
particles are only determined independently by the forces acting
on them and between them momentarilv. Its structure may be

THE GERM-PL.\SM 65

compared to that of a complicated ancient building, the stones
of which we may suppose to be alive, so that they can grow and
increase, and thus cause displacements and fissures in the walls,
in which process the forces of attraction present within these
living stones take part. The historical transfuission of the
architecture of the i^erni-plas/zi forms the basis of the entire
ontogenetic developjnoit of the idioplasm.

If however the id has a right and left half in bilateral animals,
we must not thereby infer that it is merely a miniature of the
fully-formed animal, and tliat therefore we are once more deal-
ing with the old tlieor}- of preformation. Quite apart from all
coniectures as to the detailed architecture of the id of orerm-
plasm, it is at any rate certain that the arrangement of the deter-
minants in it is quite different from that of the corresponding
parts in the fully-formed organism. This is proved bv a study
of development, and need scarcely be treated of in detail here.
Any one with a knowledge of animal embryology knows how-
great a difference there is between the mode of development of
the parts from one another in the embryo and their respective
relation in the mature organism. The earlv stages of sejrmenta-
tion of the ovum show that groups of determinants have been
formed in the id of germ-plasm, and that these, moreover, cor-
respond to the parts of the body w^hich arise from one another
consecutively, though they can have no resemblance to them
either in form or in their degree of perfection.

In some worms the two first blastomeres do not give rise
respectively to the right and left sides of the body, but to the
entire ectoderm and endoderm. In these cases the id of orerm-
plasm must break up into two groups, one of wdiich contains all
the determinants of the ectodermal organs, and the other all
those of the endoderm : it is evident that this arrangement has
no analogy to that which obtains as regards the organs of the
fully-formed animal. If in any species we knew the 'value in
primary constituents ' (' Anlagenwerth ') — if I may use such a
term — of each cell in the ontogeny, w'e could give an approximate
representation of the architecture of the germ-plasm : for. begin-
ning with the last formed cells, w^e could infer the nature of the
determinants which must have been contained in each ]-)revious
mother-cell, passing gradually backwards to the ovum ; thus we
should reach the tw^o first blastomeres, and finally the egg-cell
itself. The groups of determinants which are present at each

66 THE GERM -PLASM

stage would tlius be known, and we might in imagination then
arrange them in such a way that it would be possible to picture
their disintegration into the respective series of smaller and
smaller groups.

Such a representation of the architecture of the id of germ-
plasm would, however, never be an accurate one, because its
parts must be subjected to incessant slow displacement during
the growth of the idioplasm and in the course of development.

This brings us to the second factor which takes part in the
ontogeny of the idioplasm, viz., the uneven rate of DiultipUcatioii
of the deteriniiiaiits. An id of germ-plasm composed entirely
of similar determinants, would have to retain its original archi-
tecture even daring vigorous growth and continued division ;
just as would be the case in one of the lowest forms of life — a
Moner — consisting of a number of identical biophors, which
must remain the same throughout all the divisions which it un-
dergoes. In a germ-plasm consisting of a number of different
determinants, a perfectly even rate of multiplication cannot be
assumed in the case of all of them. For the difference between
two determinants depends presumably on the differences in the
nature, number, or arrangement of their constituent biophors,
and the latter differ again in their molecular structure, i.e. in their
essentia] phvsico-chemical properties. Hence the determinants
will behave differently as regards their reaction to external influ-
ences, — more especially in respect of their rate of growth and
increase, — according to their constitution. The same conditions
of nutrition will therefore stimulate one to a faster, and another
to a slower, growth and corresponding multiplication, and thus
an alteration in the proportional numbers in which the indi-
vidual kinds of determinants are present in the germ-plasm must
occur continually in the course of embryogeny ; for the latter is
connected with a constant growth of the idioplasm, and therefore
also witli a continual increase of the determinants. This
must cause a disarrangement in the architecture of the germ-
plasm, in which process the third factor concerned in these
changes, viz., tJie forces of attraction in tJie determinants., may
take part.

The assumption of such forces can scarcely be avoided. For
it is very probable, a priori, that vital units do act upon one
another in different degrees, and this view is supported by a con-
sideration of the processes of nuclear division, together with the
distribution of the primary constituents in ontogeny.

THK CF.RM-PLASM

67

So far I have not touched upon the question as to whai
observable parts of the idioplasm are to be regarded as ids.
This point cannot be decided with certainty at present, but I
have elsewhere expressed the opinion that those rod-like, loop-
like, or granular masses of chromatin in the nucleus, — the
chromosomes, — are to be considered equivalent, not to single
ids, but to series or aggregations of ids. 1 have therefore pro-
posed to call the chromosomes idants} in order to keep uj) a
certain uniformity in the nomenclature. It is probal)le that the
ids correspond to the small granules hitherto called ' microso-
mata,"' which are known to form the individual idants in many
animals : we may mention as an example, Ascaris niegalocep/iala,
as in it the nuclear structure is best known.
These microsomata, although lying very
close together in one row, are nevertheless
separated by a thin layer of intermediate
substance ; the whole idant cannot there-
fore be equivalent to one id, for the latter
is a clearly defined vital unit possessing a
fixed architecture, and cannot consist of
completely separated parts.

The great variety as regards size, number,
and form of the chromosomes in different
species of animals, indicates that they pos-
sibly have not always a similar morpholog-
ical value. As however there is no reason for assuming that
the number of ids must always be the same in all species, and
as, on the contrary, it is much more probable that their number
varies greatly, it is impossible to make use of the above fact as
a decisive argument. We can onlv state that the individual
chromosome or idant in air probability represents a different
number of ids in different species.

Division of the nucleus depends on the longitudinal s])litting
of the idants, in which process each of the spherical ids — assum-
ing these to correspond to the microsomata — becomes halved.
Each half then becomes rounded off, and passes, together with
the idant to which it belongs, into one of the two daughter-nuclei.

In the ordinary process of cell-division in tissues, which
results in the formation of dauijhter-cells similar to those from

Fig. 2.

Two Idants with theii
contained Ids of As^
can's inegalocephala.
(After Boveri.)

1 ' Amphimixis,' pp. 39, 40.

68 THE GERM-PLASM

which they arose, the ids produced by the division naturally
contain precisely similar determinants ; in embryogeny, on the
other hand, divisions occur which ensure that the two daughter-
nuclei contain combinations of determinants which are usually
entirely different from one another. We have an example of
such a nuclear division in the segmentation of the ovum in the
case, for instance, of certain worms already referred to, in which
two cells are formed by the first division of the egg-cell, one of
which contains all the determinants of the internal, and the
other all those of the external germinal layer. A division of
this latter kind we may speak of as differetitial or dissimilar
as regards heredity (• erbungleich '), in contrast to the former,
which is integral or similar as regards heredity ('erbgleich ").
As in the case of the entire idants, the ids are split by an inter-
nal force, and are not pulled apart mechanically by the threads
of the • nuclear spindle ' which are attached to them. Flemming
has shown that this splitting often takes place long before the
spindle-threads become active. The forces of attraction in the
determinants must therefore take part in this process, just as they
must be assumed to act between the biophors which constitute
the body of a dividing cell.

It appears to me, therefore, that the inherited architecture of
the id of germ-plasm undergoes a gradual change, owing to the
uneven rate of multiplication of the determinants, and that it
is further regulated by the forces of attraction which we must
suppose to act between them. We might represent the archi-
tecture of the id by a very complicated geometrical figure, which
gradually becomes changed during the growth of the id ; this
change does not occur in the first division, the preparation for
which has been accurately made in the original figure, but in
the subsequent stages of ontogeny. As the greater number
of these divisions is connected with a diminution in the number
of kinds of determinants, the geometrical figure representing
the id gradually becomes simpler and simpler, until finally it
assumes the simplest conceivable form, and then each cell will
contain the single kind of determinant which controls it. The
disintegration of the germ-plasm is a wonderfully coniplicated
process ; it is a true ' development," in which the idic stages
necessarily follow one another in a regular order, and thus the
thousands and hundreds of thousands of hereditary parts are
gradually formed, each in its right place, and each provided with
the proper determinants.

THE GERM-PLASM 69

The construction of the whole body, as well as its differentia-
tion into parts, its segmentation, and the formation of its organs,
and even the size of these organs, — determined by the number
of cells composing them, — depends on this complicated disin-
tegration of the determinants in the id of germ-plasm. The
transmission of characters of the most s^ejieral kind — that is to
say, those which determine the structure of an animal as well as
those characterising the class, order, funily, and gouts to which
it belongs — are due exclusively to this process^. The slight
differences only, namely those which distinguish species from
species, and individual from individual, depend partly on the
characters of the individual cells. De Vries has overlooked
this in his attempt to explain all the facts of heredity by the
theory of ' /////'^'-cellular ' pangenesis. As was mentioned in
the * Historical Introduction' to this book, it must be borne in
mind that most of the ' characters ' of any of the higher forms
of life result not from the characters of the individual cells,
but from the way in which they are combined. On the other
hand, the construction of a living organism is not conceivable
unless we presuppose the determination of the characters of each
cell.

We have therefore to give an explanation of this concluding
part of the process of ontogeny ; this has already been done to
a great extent above, in the section treating of the control of the
cell by the idioplasm. I there assumed, as de Vries has also
done, that this determination depends on the migration of minute
vital particles from the nucleus into the cell-body. We have
now seen by what means the biophors characteristic of any
particular cell reach that cell in the requisite proportion. This
results from the fact that the biophors are held together in a
determinant which previously existed as such in the germ-plasm,
and which was passed on mechanically, owing to its ontogenetic
disintegration, to the right part of the body. In order that the
determinant may really control the cell, it is necessary that it
should break up into its constituent biophors. This is an inevi-
table consequence of the assumed mode of determination of the
cell. We must suppose that the determinants gradually break
up into biophors when they have reached their destination.
This assumption allows, at the same time, an explanation of the
otherwise enigmatical circumstance, that the rest ot the de-
terminants, which are contained in every id except in the last

70 THE GERM-PLASM

stages of development, exert no influence on the cell. As each
determinant consists of many biophors, it must be considerably
larger than a biophor, and is probably therefore unable to pass
out through the pores of the nuclear membrane, which we must
suppose to be very small and only adapted for the passage of
the biophors. Although it is impossible to make any definite
statement witli regard to the internal stmcture of the determi-
nants, it must be owing to this structure that each determinant
only breaks up into biophors when it reaches the cell to be
determined by it. We may suppose that, just as one fruit on a
tree ripens more quickly than another, even when the same
external influences act on both, so also one sort of determinant
may mature sooner than another, although similar nourishment
is supplied to both.

It must not, however, be overlooked, that a difference in the
time of maturation of the determinants in the embryogeny of
animals is chiefly to be assumed only in the case of the actual
embryonic cells ; for the histological differentiation of the cells
of the body, and the differentiation of the parts of the latter,
occur at about the same time ; that is, not until the organs
already exist as definite groups of cells. This is equivalent to
saying that the disintegration into biophors occurs when the
id only contains the single determinant which controls that par-
ticular kind of cell. It is well known how suddenlv the histolog-
ical differentiation of the cells occurs in the embryogeny of an
animal. For a long time the various parts and tissues are very
similar to one another, though not perfectly so, and then histo-
logical differentiation suddenly sets in. This is very markedly
the case as regards the transversely striped muscles of Arthropods
and Vertebrates, in which the contractile substance is first seen
as a mere narrow ring around the cell, and then gradually
becomes thicker, so as to replace the greater portion of the cell-
body, — just as one would expect if it were caused by muscle-
biophors which had migrated into the cell-substance and there
multiplied.

The assumption of a 'ripening'' of the determinants, which
though not simultaneous, is yet exactly regulated, nevertheless
remains indispensable ; or, to express it differently, we must
assume that the determinants pass througli a strictly regulated
period of inactivity, at the close of which the disintegration into
biophors sets in. The determinants certainlv continue to grow-

THE GF.R^r-PLASM 7 I

and multiply without interruption during this period, as may be
deduced from the fact that the amount of the nuclear substance
in the individual cell does not decrease during embryogeny.
although such an enormous increase in the number of cells
takes place. No accurate and methodical observations have at
present been made with regard to the comparative size of the
chromosomes in the various stages of development and in the
different organs of the body, but it may nevertheless be taken as
certain that the entire mass of the nuclear substance grows con-
siderably during embryonic development. It appears to me,
however, to follow from the observations of Ruckert 1 have
already referred to concerning the chromosomes of the ovum of
the dog-fish,* that the most marked growth of the determinants
takes place immediately before, and during, their activity. Dur-
ing the period of growth and histological differentiation of the
egg in this fish the idants grow enormouslv, and towards the
completion of these processes they gradually decrease in size,
until finally, when the ovum is ripe, they have become almost as
small as they were originally.

This may be expressed, in the terms we have adopted, as
follows : the determinants which control the histological struc-
ture of the egg\ multiply enormously during the growth of the
ovum, into the body of which they transmit their 7iumeroits
biophors. After this has occurred, only those determinants of
the germ-plasm are left which have in the meantime been in-
active, and which have only increased to a slight extent ; these
are thus contained in those idants which are not much larger
than they were in the young egg-cell. From the beginning of
ontogeny and onwards, one determinant after another becomes
active, and during their activity they also multiply. It has for
a long time appeared to me probable that the determination of
a cell does not take place, as one might suppose, by the agency
of a single determinant, but bv that of many determinants of

* Anat. Anzeiger, loth March 1892.

t These determinants of the ovum correspond to the ' oogenetic nucleo-
plasm ' of my earlier essays, and constitute the substance which deter-
mines the growth and histological differentiation of the egg. For a long
time I believed that this substance was extruded from the ovum at the
close of the period of maturation by means of the polar bodies. We now
see that such an extrusion is not required, as this substance is used up in
the differentiation of the tg'g.

72 THE GERM-PLASM

a similar kind ; and I imagine that that kind of determinant
which has to control a particular cell, multiplies considerably
by division before — and perhaps even during — the process or
determination. This view is completely borne out by Ruckert's
interesting observations.

Every cell during the whole period of ontogeny is, however,
controlled — not only as regards its structure, but also in respect
of its mode of division — by 2i single determinant only. The in-
active determinants remain without exerting any influence on
the cell-body ; they how^ever determine the architecture of the
id, and therefore the further formation of the embryo also. For,
indeed, the mode of disintegration of the id into smaller groups
of determinants is necessitated by its architecture.

I have above attributed to the determinants forces of attrac-
tion which take part in the configuration of the structure of the
ids. Such forces must be present, for otherwise the id could
not possess a definite architecture ; but I do not wish it to be
understood that these forces are the principal factors in the
arrangement of the determinants. They are concerned in con-
necting together the parts of which the determinants are com-
posed, and not in their continual rearrangement during the
course of ontogeny. It is primarily always the inherited definite
architecture of the id of germ-plasm which results mechanically
in the idic figure of the subsequent stages ; disarrangements in
this architecture are due to the unequally vigorous increase of
the various kinds of determinants, all of which naturally are
definitely determined beforehand. The arbitrary or accidental
action of the forces of attraction takes no part at all in this
process.

I must emphasise this view particularly, in contrast to that of
Galton, who speaks of ' repulsions and affinities ' of the gemmules
which compose the ' stirp."' He compares the masses of these
gemmules, which undergo active and incessant changes of their
mutual positions owing to attraction and repulsion, to a swarm
of flying insects, in which • the personal likings and dislik-
ings of an individual may be supposed to determine the posi-
tion that he occupies in it." Witli this view I can by no means
agree, for it rests on the assumption that the germ-substance is
composed of many homologous gemmules (• competing germs ')
wiiich struggle for the supremacy, onlv those which are success-
ful determining the cliaracter of the future organism. P'rom

THE GERM-PLASM 73

the very first Galton takes into consideration the compHcations
or the germ-substance caused by sexual reproduction, which, as
will be shown subsequently, are due essentially to the fact that
the germ-plasm contains majty, and not a single specimen of
each primary constituent, and that these are present in various
modifications. It is this struggle between the honiologoiis
primary constituents which Galton refers to in the passage just
quoted, which indicates that first one, and then another, reaches
the desired spot, without any definite order being observed.
This conception appears still more plainly in another passage,
in which he compares the germ-plasm (the 'stirp') to a nation,
and those gemmules 'that achieve development." — i.e., become
transformed into the corresponding parts of the body — ' to
the foremost men of that nation, who succeed in becoming its
representatives.^

Excellent as these similes are in themselves, I cannot help
thinking that they lead to error if intended as an explanation
of ontogeny. If we take up the position which Galton occupies
with regard to the essential part of the theory of pangenesis, we
must suppose that a large number of gemmules — many more
than are necessary for the construction of the body — are con-
tained in the stirp ; that is. in the germ-substance of the fer-
tilised ^gg. For only one gemmule is required for each cell of
the body, but nevertheless a large number are present ; and
these, so to speak, struggle for the precedence, the successful
gemmale alone becoming converted into the cell which is to be
formed. In this conception the fact is entirely overlooked
that ontogeny itself cannot possibly depend on this struggle,
but would take place just the same if only one gemmule were
present in the * stirp ' for each cell, and that the cause for the
progress of development must therefore be sought elsewhere
than in the rivalry between homologous gemmules : it must be
due to the right succession of the gemmules. Galton considers
that the • purely step-by-step-development ' assumed by Darwin
in his theory of pangenesis is insufficient, but I think, neverthe-
less, that Darwin's opinion is the more correct one.

Neither does Galton's simile of the swarm of insects seem to
me to be appropriate as an explanation of the struggle between
homologous oremmules derived from different ancestors. Even
if the gemmules in the 'stirp' were in perpetual motion, and if
on this depended the decision as to which of them obtained the

74 THE GERiM-PLASM

privilege of taking part in the formation of the organism, how
could one explain the existence of identical twins, about w^hich
we have received such valuable information from Galton him-
self ? How would it be possible for the exactly corresponding
gemmules in two individuals in the flying and ever-changing
swarm always to reach the most favourable position, even if the
' stirp ' contained precisely similar gemmules ?

In a subsequent section I shall attempt to show that this
struggle between homologous but individually different primary
constituents can be proved in quite another manner in connec-
tion with the idioplasm. It was necessary to mention Galton's
view here, in order to show that the forces of attraction and
repulsion, assumed by him, are introduced for an entirely
different purpose from that which I have stated with regard
to the similar forces in connection with the biophors of the
idio-plasm.

Two physiological conditions of the elements of the idio-
plasm exist, — an active and an inactive. In the former, these
elements become disintegrated into their constituent parts ;
while in the latter, they remain entire, although they are capable
of multiplication. When determinants are active, they become
disintegrated into biophors, and are then capable of controlling
the cell in the nucleus of which they are situated. The activity
of entire ids depends on a disintegration into determinants,
which, though certainly successive, is often very slow; it must
be contrasted with the inactive state, which in both elements of
the idioplasm depends on the fact that their constituent parts
do not become separated from one another, but remain in their
primarily entire condition. In the immature ovum, for instance,
only 07ie kind of determinant — the 'oogenetic' determinant — is
active, and this controls the growth and histological difterentia-
tion of the egg ; all the other kinds remain inactive, as do also
the ids which are formed from them. Only when fertilisation
has occurred do they become active, — that is to say, one kind of
determinant after another begins to separate itself from the
entire id. We shall see later on, however, that ids of the germ-
plasm also exist which remain inactive even after fertilisation
has occurred, and are passed on from cell to cell in what we
may call an unalterable (^ gebu7idene)n^) condition, so as to form
subsequently the germ-cells of the embryo. We know as little
about the cause of this condition as we do about that of the

THE GERM-PLASM 75

state of the brain during sleep, or of the latent period of certain
fertilised animal eggs, which, after beginning to undergo develop-
ment, remain inactive at a certain stage for months. The facts
with which we are acquainted, however, render the assumption
of an active and an inactive state of the ids and determinants
unavoidable, as will become more evident in the course of this
book. A similar assumption has been made by all those who
have formulated vital units : thus Darwin has assumed these
conditions in connection with his 'gemmules,' and de Vries with
regard to his * pangenes/

Two forms of heredity, which we call homotopic and hotno-
chronic, may be deduced from the theory given above. As
the individual determinants — from the germ-plasm onwards,
throughout all the stages of ontogeny — take up a definite posi-
tion in the id, they must reach the right place in the body, and
there cause the development of a structure corresponding to
that of the parent. As, moreover, the period of maturation of
each determinant is decided by the nature of the latter, the
determinant will become active in the individual and will cause
the formation of any particular part of the body at the same
stage of development as in the parent. Exceptions to this rule
occur in the case of abnormalities, and also in that of phylo-
genetic displacements.

5, Summary OF Sections 1-4, relating to the Structure

OF THE Germ-plasm

According to my view, the germ-plasm of multicellular organ-
isms is composed of ancestral germ-plasms or ids^ — the vital
units of the third order, — each nuclear rod or idant being
formed of a number of these. Each id in the germ-plasm is built
up of thousands or hundreds of thousands of determinants, — the
vital units of the second order, — which, in their turn, are com-
posed of the actual bearers of vitality (- Lebenstrager'), or bio-
phors, — the ultimate vital units. The biophors are of various
kinds, and each kind corresponds to a ditTerent part of a cell :
they are, therefore, the • bearers of the characters or qualities '
(•Eigenschaftstrager') of cells. Various but perfectly definite
numbers and combinations of these form the determinants, each
of which is the primary constituent ("Anlage") of a particular

76 THE GERiM-PLASM

cell, or of a small or even large group of cells {e.g., blood-cor-
puscles) .

These determinants control the cell by breaking up into bio-
phors, which migrate into the cell-body through the pores of the
nuclear membrane, multiply there, arrange themselves according
to the forces within them, and determine the histological struc-
ture of the cell. But they only do so after a certain definitely
prescribed period of development, during which they reach the
cell which they have to control.

The cause of each determinant reaching its proper place in
the body depends on the fact that it takes up a definite position
in the id of germ-plasm, and that the latter, therefore, exhibits
an inherited and perfectly definite architecture. Ontogeny
depends on a gradual process of disintegration of the id of germ-
plasm, which splits into smaller and smaller groups of determi-
nants in the development of each individual, so that in place of a
million difterent determinants, of which we may suppose the id
of germ-plasm to be composed, each daughter-cell in the next
ontogenetic stage would only possess half a million, and each
cell in the next following stage only a quarter of a million, and
so on. Finally, if we neglect possible complications, only one
kind of determinant remains in each cell, viz., that which has to
control that particular cell or group of cells. This gradual dis-
integration of the id of germ-plasm into smaller and smaller
groups of determinants in the subsequent idic stages does not
consist in a mere division of the id into portions, but — as occurs
in all disintegrations of vital units — is accompanied by dis-
placements in the groups of these units, brought about by the
unequally vigorous multiplication of the various individual deter-
minants, and regulated by the forces of attraction acting within
them. In spite of all the alterations in the arrangement of the
determinants which must occur, owing to the differential nuclear
divisions together with unequal growth of the various kinds of
these units of the second order, the original position of each
determinant in the extremely complex structure of the id of
germ-plasm renders it necessary that it should take up a definite
and fixed position in each idic stage ; and also that it should
traverse the precisely regulated course from the id of germ-
plasm., through perfectly definite series of cells, to the cell in
which it reaches maturity in the final stage of development. In
this cell it breaks u]) into its constituent biophors, and gives the

THE GERM -PLASM 77

cell its inherited specific character. Each id, m every sta^e, has
its dejiiiitely inherited architecture ; its structure is a complex
but perfectly definite one, which, originating in the id of germ-
piasni, is transferred by regular changes to the subsequent idic
stages. The structure exhibited in all these stages exists poten-
tially in the architecture of the id of germ-plasm : to this architec-
ture is due, not only the regular distribution of the determinants,
— that is to say, the entire construction of the body from its
primary form to that in which its parts attain their final arrange-
ment and relation, — but also the fact that the determinant, of a
small spot on a butterfly"'s wing, for example, reaches exactly the
right place ; and that, to take another instance, the determinant
of the fifth sefjment in the feeler of a Gannnarus reaches this
particular segment. The determination of the character of the
individual cell depends on the biophors which the correspond-
ing determinant contains, and which it transmits to the cell.

6. The Mechanism for the Phyletic Variations in

THE Germ-plasm

The causes of phyletic development will be treated of in the
chapter on Variation : the present section merely gives an
account of the mechanism existing in the idioplasm in connec-
tion with this process. I shall here attempt to show how the
phyletic changes in the idioplasm follow mechanically from its
assumed ultimate structure.

Since all parts of the organism are determined from the germ
onwards, permanent variations in these parts can only originate
from variations in the germ. Each phyletic variation must
therefore be due to a variation in the structure of the id of
germ-plasm. If we suppose, with Darwin, that the transforma-
tion of species is a gradual one, originating in individual varia-
tions which become increased and directed by selection, it
follows that the corresponding process in the idioplasm cannc>t
be due to a sudden and complete variation in the entire id,
but must begin with changes in the individual biophors or
in individual determinants and groups of determinants also,
and must then extend gradually to more numerous groups, until
finally the nature of the id becomes entirely, or to a great
extent, changed.

78 THE CERM- PLASM

The basis of the process must be sought in the variability of
the biophors. which is followed in turn by that of the units of a
higher order, — the determinants and ids. These variations are
not by any means confined to the structure of the individual
cell, but concern primarily the tuDnber of cells of which an
organism consists. A leaf of a plant, or a bird's feather, may
increase considerably in size during the course of phylogeny,
without a change necessarily occurring in the cells which form
these parts. The variation will depend primarily on a multipli-
cation of the respective determinants. If the primitive eye of a
lower animal consisted of a single cell, constituting a visual rod,
and the power of multiplication of its determinants gradually
increased in the course of phylogeny, the number of identical
determinants which would arise during development by the
multiplication of the single determinant in the germ-plasm
would gradually increase so as to suffice for two cells instead of
one. The eye would then possess two visual rods, and if the
power of multiplication increased still more, a whole group of
visual rods would be controlled by one determinant. We are
unable to conjecture on what internal variations in the determi-
nant such an increase in the power of multiplication depends ;
but the fact that every individual cell in the body does not
possess a special determinant, while large groups of cells are
controlled by a single one, proves that such variations must be
possible.

Such a very simple phyletic variation, resulting in the local
increase of the number of cells, will be followed by a further
variation as soon as the multiplication of the determinant of,
e.g., an undifferentiated sensory cell, is not confined to the later
stages of ontogeny, but occurs also in the germ-plasm itself;
that is, when the doubling of the determinant has already taken
place in the id of germ-plasm. For in this case the group of
sensory cells, which have become developed phyletically from
the originally single cell, will now be controlled by two deter-
minants, each of which can vary independently of the other, and
can transform the group of cells under its control. Thus one of
these groups might give rise to auditory cells, and the other to
gustatory or olfactory cells.

Thus the increase in the differentiation of the body depends
primarily upon the multiplication of the determinants in the id
of germ-plasm, but this differentiation is only rendered complete

THE GERM-PLASM 79

by variations in the determinants of similar origin taking place
in different directions. The mere addition of a new ontogenetic
stage can very easily be conceived without an increase occurring:
in the determinants of the id ; but as soon as the double number
of cells which are present in the new idic stage have to become
differentiated in various ways, the differentiation must be pre-
ceded by a doubling of the determinants in the id of germ-
plasm. A higher degree of differentiation will therefore be
primarily connected with an increase in the number of cells of
which the organism is constituted. It is known that the extreme
prolongation of development, due to the constant addition of
new generations of cells at the end of ontogeny, can be neutral-
ised by the abbreviation and reduction of the ontogenetic stages :
this process may be also to some extent understood if we trace
it to its origin in the structure of the idioplasm. The reduction
in the number of generations of cells from two or more to one,
depends on the fact that the process of multiplication and rear-
rangement of the determinants takes place more rapidly during
these particular stages, than does that of cell-division ; so that
several idic stages, each of which formerly characterised a
particular stage of the cell, pass into one another during the
same stage of the cell. The respective idic stages have not
here disappeared completely' : they only follow one another
more rapidly, and therefore disappear as recognisable stages in
development.

In lowly organised beings the differentiation of the body may
become increased by a simple reduction of the iiereditary parts
or deterf/iifiates, without an increase taking place in the cell-
generations. If a determinant which controls a region con-
sisting of a hundred cells divides into two, each of which only
controls fifty cells, the two resulting groups of cells can vary
independently of each other from this point onwards, and may
give rise to very different structures. In this way a continued
division of the determinants, and consequently also a constantly
increasing differentiation of the species, may occur, without
necessitating an increase in the total number of cells present in
ontogeny.

Each additional differentiation denotes an increase in the
degree of organisation. But the phyletic development of the
organism is by no means invariably connected with an increase,
or. in fact, with any other change in the degree of organisation.

So THE GERM -PLASM

The species of a genus, and often the genera of a family, cannot
be distinguished from one another by the number of cells com-
posing them, nor by an increase in the variety of these cells,
but only by qualitative differences in the structure of the various
parts. Hence the phyletic development of living beings cannot
simplv be due to the augmentation of the number of determinants
in the id of germ-plasm, but must also be attributed to a change
in the iiature of the determinants and in that of their com-
ponent biophors.

The structure of the idioplasm which we have here assumed,
also offers an explanation of the phenomena of paj-allelism
between ontogeny and phylogeny, which depend on the law of
biogenesis as well as on the relegation of the fijial characters to
earlier and earlier ontogenetic stages in the course of phylogeny.
Let us first consider the former of these phenomena. We have
assumed that each ontogenetic stage is characterised by a definite
'determinant figure.'' /.<?., a sort of geometrical structure com-
posed of the determinants. The nature of each individual cell
is certainly controlled by those determinants in the nucleus
which have reached maturitv, — that is to say, have arrived at
the stage in which they break up into biophors and migrate into
the cell-body. But the manner in which the embryonic develop-
ment of an animal occurs does not by any means depend only
on the histological structure of the individual cells in each stage,
— it rests to a much greater degree on the manner in which
these cells divide and on the rate of their division, and also
primarilv on the way in which the ' unripe ' determinants of the
nuclear substance, which are still latent, are grouped together
and distributed by means of the cell-divisions. This distri-
bution of the primary constituejits among the different cells is
of the first importance in determining the character of the
ontogeny ; and one could easily imagine a case of animal
embryogeny in which ten or twenty generations of similarly
constituted • embryonic cells ' followed each other, and in which
a perfectly definite distribution of the primary constituents
(determinants) had nevertheless occurred, although only now
apparent for the first time. It is well known how close a
resemblance exists between the cells of the embryo in various
stages in the case of the higher animals.

The regular distribution of the determinants which are still
latent or ' unripe " must therefore decide the course of ontogeny ;

THE f;KRI\I-PI,ASM 8 I

and the viainier of tJiis distribution Jiiids expression in tJie
architecture of each idic stage^ or, as I have expressed it, in each
' determinant-figure.''

It is obvious that tlie same geometrical figure may be con-
structed out of different elements, just as the same form of
crystal may be produced from molecules of a different nature.
Thus the resemblance between the ontogenetic stages of nearlv
allied species is to be explained by the degree of similarity
between their respective *■ determinant figures,' which persists
although the individual determinants constituting the figure
differ more or less from one another. As the study of develop-
ment shows, an explanation is thus offered of the fact that the
earlier ontogenetic stages are so very much alike in allied
species, and that thd differences only appear later on ; for in
the early idic stages, the differences as regards the nature or
power of multiplication of single determinants, or groups of de-
terminants, can exert no marked influence, because the entire
number of determinants is still very large, and thus the archi-
tecture of the id will be practically the same in corresponding
stages. But the further ontogeny advances, and the smaller
the groups become into which the determinants separate, the
greater also will be the diversity in the architecture of the id,
and in the further distribution of 'unripe'' determinants resulting
from this architecture. Thus a certain part will be longer or
shorter, a spot of colour larger or smaller, and the final stages
of ontogeny — in which the cells possess only one determinant —
will differ according to the degree of difference which obtains
in the respective determinants. This explains the fact that the
segmentation cells in allied species are frequently exactly alike,
and also that the resemblance between many mammalian em-
bryos in their earlier stages, though not complete, is nevertheless
a very close one.

The law of biogenesis, as far as it applies at all. depends on
the fact that phyletic development is partly due to new onto-
genetic stages bein<i added at the end of ontogenv. In order
that these new stages may be reached, the stages which were
previously the final ones must be passed through in each on-
togeny. This may be expressed in terms of the idioplasm as
follows: — the determinants of the id of germ-plasm become
endowed with a greater power of multijilication. so that each
one of them causes the addition of one or more cell-generations

82 THE GERM-PLASM

to the end of the ontogeny. At the same time, the determinants
in the germ-plasm increase in number, and each of them be-
comes differentiated in a fresh manner. As, however, every two
new determinants always follow the same course from the id
of germ-plasm to the final stage in ontogeny as was taken by
the single original determinant, they will pass through the same
determinant figures as before, and only lead to the formation of
new stRictures in the final stages, when they become separated
from one another.

The ontogenetic stages of the ancestors will be repeated less
accurately the nearer development approaches its termination.

The disappearance of a character or of a part which has
become useless, may also be traced to the mechanism of the
idioplasm. The group of determinants which gives rise to a
particular character, will have to be removed entirely from the
germ-plasm if the corresponding part is to disappear com-
pletely. But this is a very complicated process, and one of long
duration as regards more complex organs, such as, for instance,
the limbs of Vertebrates. For the determinants which take part
in the formation of an extremity are very numerous, and of many
different kinds ; and moreover, they cause the rudiment of the
limb to appear very early in ontogeny. Hence the determinants
will have to suffer successively many retrogressive and simplify-
ing changes before a noticeable reduction of the organ occurs.
The functionless and rudimentary wings of the Australian Kiwi
{Apteryx), w^hich are concealed by the plumage, possess all
the bones of the perfect wing, though these are very much re-
duced in size. This is to be explained by supposing that the
entire group of determinants for the wing still remain in the id
of germ-plasm, but that it has decreased in strength, — that is to
say, its elements no longer increase so rapidly, — and they there-
fore can only control smaller groups of cells. If the process of
degeneration continued, the organ would not only grow' smaller
and smaller, but its component parts would also disappear at
different rates, and, losing their characteristic form, would
appear as indistinguishable rudiments. Such a degeneration
has occurred in certain species of whales, in which the rudi-
ments of the posterior extremity lie concealed beneath the skin ;
while in other species, the form of the separate bones has been
to some extent preserved, and those of the thigh and shank can
still be plainly distinguished. In these cases, many of the

THE (;erm-plasm S^

determinants whicli were formerly present must have dis-
appeared entirely from the id of germ-plasm, and the remainder
must have lost the power of multiplication to a greater extent
than has occurred in the case of the wing oi Apte?yx.

We know, however, that even in such animals as snakes, in
which the extremities have in most cases disappeared com-
pletely in previous geological periods, the rudiments of the
limbs arise in the form of ' muscle-buds ' in the earlier stages of
development, and then disappear very shortly afterwards.*
This fact may be expressed in terms of the idioplasm as
follows: — the power of multiplication in the small remnant of
the group of determinants of the extremity which still exists in
the id of germ-plasm, has decreased so considerably that it only
suffices for these early embryonic stages. The youngest deter-
minants, and consequently the most recent hereditary structures,
are the first to disappear, the loss of the older ones taking place
gradually, until even the oldest of all are no longer present.
This must be due to the manner in which the deter-
minants increase, although the actual connection between the
two phenomena is not apparent. It may perhaps be traced to
the fact that those determinants which are the youngest phy-
letically are destined for the latest ontogenetic stages, in which
only therefore they become ' ripe,' and undergo disintegration
into biophors. If then, their power of multiplication decreases
considerably during the process of degeneration, the number of
determinants required for the control of any particular group of
cells will not be reached, nor will the determinants even become
ripe. Although still present, they are unable to exert any in-
fluence ; whereas the determinants of the older phyletic stages
which are still passed through, ripen in the earlier stages of
ontogeny.

The process of degeneration of an organ may be represented
as depending on the fact that the determinants first become
changed in such a manner as to cause a decrease in tlieir power
of multiplication, and this then leads to a very gradual reduction
affecting an increasing number of determinants belonging to
the group in question. At the same time, the power of mul-

* Cf. J. van Bemmelen, ' Over den oorsprong von de vorsle lede-
maten en de tongspieren bij Reptilen.' Kon. Akademie de Wetenschappen
te Amsterdam, 30th June 1888.

84 THE GERM-PLASM

tiplication in the remaining determinants also diminishes, so
that the groups which they constitute gradually extend a less
distance into the ontogeny, until finally they drop out of it
altogether.

It must not be understood that I have given a mechanico-
physiological explanation of the process of degeneration because
I have connected it with the theory of determinants. As long
as we know practically nothing about the forces which act
within and among the biophors, it will be impossible to offer an
explanation of this kind. I have only attempted to show that
this doctrine does not contradict the facts, but that, on the con-
trary, it agrees with them up to a certain point. The phenomena
of degeneration have not hitherto been considered from this
point of view. When a deeper insight into the actual phenomena
has been obtained, w^e may perhaps be able to make further
theoretical deductions, and it would then be possible to develop
the theory of determinants more fully.

A few words may now be said as regards correlated varia-
tions. Darwin has shown what an important part these varia-
tions play in the transformation of species, and how changes
which we must consider to be primary are followed by a number
of others in various parts of the organism. Thus an increase in
size in a stag's antler necessitates a thickening of the skull, and
a strengthening of other parts, viz., the muscles of the neck, the
spines of the cervical vertebrae, the ligamentum nuchas, and
even the thoracic skeleton and fore-limbs. Referring all these
variations to the processes which take place in the idioplasm,
they will be seen to depend on changes in the corresponding
groups of determinants in the id of germ-plasm, which cannot
be due directly to the change and increase in the group of
determinants of the antler : thev must have arisen secondarilv,
owing to the occurrence of variations in the determinants upon
which selection could act. There is also an entirely different
kind of correlation, in which the variation in one part is accom-
panied by that in another, the latter having no anatomical or
functional connection with the former. Thus Darwin states, for
instance, that cats with blue eyes are generally deaf, and that
pigeons with feathered legs have a web between the outer toes.

I do not think such correlations can be traced to a connection
of the parts by means of tlie nervous system : it is perhaps
more likely that they are due to the contiguity of the deter 7ninants

THE GERM-PLASM 85

/;/ the id of genn-piasin of those parts which vary correlatively.
It will be shown later on that local differences in nutrition occur
in the id, and that these may cause changes in the determinants
affected by them. If, now, the determinants controlling regions
of the body which are far apart, are situated close togetlier in
the id, they might easily be affected simultaneously by influences
producing variation. But the perfectly definite architecture of
the id of germ-plasm, on which we base our argument, does not
only permit of a vicinity of the determinants of parts of the
body far removed from one another, but actually requires it.
For, according to our assumption, the id of germ-plasm is not a
representation of the body in miniature, but a structure of a
special kind, in which the individual component parts are
arranged in the order in which they are passed on subsequently
in the process of ontogeny to their final destination, viz., to the
determinates or hereditary parts. This however requires that the
determinants of the ectoderm should be closely adjacent to
those of the endoderm in the id, if they are to be distributed to
a primary ectoderm and a primary endoderm cell in the first
division of the ovum. A cell-division which leads to the
separation of widely differing groups of determinants, admits
of a close aggregation of these different groups in the id of
the mother-cell. This may give some slight insight into the
above-mentioned phenomena of correlation.

7. The Magnitude of the Constituents of the

Germ-plasm

The assumption that the germ-plasm is composed of biophors,
determinants, and ids, implies the existence within a narrow
space of a large number of ultimate vital units (biophors)
in all the higher organisms. The question arises whether a
sufficient number of these units can be contained within an id.
Although I believe it is at present quite impossible to obtain
anything like a reliable answer to this question by calculating
the relative sizes of the elements of the germ-plasm, it may
perhaps not be uninteresting to attempt to make such a calcu-
lation.

In order to solve the problem with any approach to accuracy,
it would at least be necessary to know the sizes of a biophor and

86 THE GERM-PLASM

of an id, and also the number of determinants in a given species.
Unfortunately, however, we are completely ignorant on these
points, nor do we even know how many molecules take part in
the construction of a biophor : even the computed size of the
molecule is somewhat uncertain.

The diameter of a molecule has been estimated at betw'een
the i,(x/).iAijth and the KM^^.uuijth of a millimetre by four different
lines of reasoning, * founded respectively on the undulatory the-
ory of light, on the phenomena of contact electricity on capillary
attraction, and on the kinetic theory of gases.' * O. E. Meyer
has calculated the size of a molecule ' from the properties and
behaviour of vapours. From the constant of friction and the
comparison between the space occupied in the liquid and gas-
eous conditions, together with the deviations from Boyle and
Mariott's law, we can approximately calculate, firstly, the volume
of all the particles contained within a given space ; secondly,
that of a single particle ; thirdly, the number of particles ; and
finally, the weight of a single particle."' The result of .such a
calculation agrees with that given above.

If we take the average diameter of a molecule to be arooo^Kith
mm., and reckon that each biophor, which we will suppose to be
a cubical structure, is composed of i,ooo molecules, the biophor
would measure lo molecules in length. A row of 200 biophors
w^ould therefore measure i />i, and 8,000,000 biophors would oc-
cupy the space of i cubic /x. A human blood-corpuscle measures
'J .'] ix. in diameter ; if we imagine it to be enlarged so as to form
a cube of j .j m- in diagonal length, this space would contain
703,000,000 biophors. Let us further assume that those por-
tions of the cell which, according to the facts at our disposal,
must contain the idioplasm, viz.. the chromosomes, are mostly
a great deal smaller than the nucleus in which they are situ-
ated, and that the germ-plasm is composed not of ofie but of
several ids, each of which contains all the biophors required
for the construction of the entire body, it will then be evident
that only a limited number of biophors can be contained in
one id.

The chromosomes in the germ-plasm of Ascaris megaloce-
phala are the largest which are at present known to us. Each

* Sir William Thomson, 'Popular Lectures and Addresses,' Vol. I.,
1889, p. 148.

THE GERM-PLASM 87

nucleus in this animal contains two or four rod-like chromosomes
(see fig. 2), each of which is composed of ' six thickened granu-
lar or disc-shaped portions, which become deeply stained with
colouring matter, and which are separated by portions staining
less deeply ' (Boveri). If we connect this fact with the hypo-
thetical composition of the germ-plasm out of ids, it follows
that an id cannot in any case be larger, and is probably smaller,
than one of these granules or microsomata. It cannot be
larger, because the id is a unit which is capable of division into
two daughter-ids, but which cannot remain permanently sepa-
rated into several parts by a different kind of intermediate sul)-
stance. If we suppose the id to be as large as it can possibly
be, — that is to say, to correspond in size to a microsome, — it
will measure, according to Boveri's drawing and scale of enlarge-
ment, .0,008 mm., or not quite i m in diameter. Only the ter-
minal granules of the rods, however, are as large as this ; the
greatest diameter of those in the middle measures .0.006 mm.,
while their shorter diameter is about .0,003 — .0,004 mm. The
terminal granules, looked upon as spherical bodies, would be
capable of containing about two million biophors of the size
given above.

This number is certainly a very considerable one, and it would
apparently be sufficient to make up the number of determinants
in such a lowly organised animal as Ascaris. But even in
Arthropods the number of determinates, and therefore that of
the determinants also, is considerably greater. Each of the
olfactory setae on the feelers of Crustaceans, which were
mentioned above, must be capable of being determined from the
germ onwards ; and this is also true of the spots and lines on
a butterfly's wing, each of which represents at least one deter-
minant, and in case of all the large markings several, or even
many, of these units. If we consider that the pattern on the
wing is often very complicated, and frequently differs on the
under and upper surfaces, it is evident that hundreds of deter-
minants must exist for this pattern alone. But there are. again,
several peculiarities in the structure of the wing-scales, and thus
it is probable that almost every scale can vary independently
from the germ onwards. In some males of the family Lycaenidce,
e.g., Lycaena adonis, small guitar-shaped odoriferous scales (the
'androconia' of Scudder) are distributed regularly amongst
the colour-scales, while these are entirely absent in other nearly

8B THE GERM -PLASM

allied species, such as Lycaena agestis : hence we must conclude
that these androconia have arisen by the transformation of
ordinary scales. This, however, presupposes the independent
variability of the scales which are to become changed phyleti-
cally, and consequently also their capability of being determined
from the germ onwards. Were this not the case, a single
scale could never have varied from the others hereditay'ily. In
Lycaena adonis there are 30,830 scales on the upper surface
of the wing.* If each of these is to be looked upon as cor-
responding to a determinate, the enormous number of about
240,000 determinants of the germ-plasm would result merely
from the scales covering the wings, provided that the upper and
under surface of the four wings possess each about the same
number of scales.

I have endeavoured by direct experiment to ascertain the
lowest limit to the size of a determinate, — that is to say, the
size of the smallest determinates for a particular character of a
certain species. For this purpose I selected one of the Ostra-
coda, Cypris reptans, which multiplies parthenogenetically. and
in which it is easy to compare the different green spots on the
shell in the mother and daughter. It appears that the larger
spots are strictly transmitted, though this is not the case as
regards the very small ones, which consist of only one or two
pigment-cells. The form of these larger spots, which consist of
fifty or a hundred pigment-cells, also varies to some extent, so
that the number is here also somewhat inconstant. If partheno-
genetic reproduction could be looked upon as being purely
nniserial, it might be inferred that the determinates are not in
this case single cells, but groups of cells. Unfortunately, how-
ever, this experiment cannot be considered conclusive, for — as
will appear later on — the germ-plasm is here composed, just as
in the case of sexual reproduction, of dissimilar, and not oi simi-
lar ids, and consequently variations in heredity may thus arise.

We must conclude, even from the external coloration, that a
very considerable number of determinates exists in the case of
the higher Vertebrates. Thus most, if not all, of the contour-
feathers of a bird must be controlled by special determinants in
the germ-plasm, for they are independently variable hereditarily.

* My assistant, Dr. V. Hacker, was good enough to make this calcula-
tion for me.

THE GERM-PL.ASM 89

The number of wing- and tail-quills is nevertheless definitely
fixed for every species of bird, and each of these feathers pos-
sesses a definite form, size, and coloration. We must assume
that viore than o)ie determinant is necessary for an entire
feather, for a feather is formed from thousands of epidermic
cells, which are not by any means all similar to one another,
either as regards form, mode of combination, or colour. Many
feathers are striped, while others have a brilliant ornamental
spot at the tip ; as in the case, for instance, of the peacock,
many humming-birds, and certain birds of paradise. The cells
to which these stripes and spots owe their origin, must contain
determinants which differ from those of the rest of the cells
which take part in the construction of the feather. We must
therefore conclude that at least one^ and often several, determi-
nants of the germ exist for each of these two kinds of cells ; for,
as is well known, ornamental spots of this kind are often formed
of several colours, and are very complex.

It would be erroneous to suppose that the contour-feathers are
not determined individually in such birds as the raven, in which
the plumage is all of one colour ; but in such cases the qualita-
tive differences refer less to colour than to form and size. The
fact that each part of the feather is determined hereditarily,
even as regards its colour, is proved by the variation which
occurs, and which in individual species has resulted in certain
feathers being partially or entirely white, or being brilliantly
coloured, as in the case of the bird of paradise, which is allied
to the raven. One need only look through a collection of
humming-birds, and compare the females, which are so often
plainly coloured, with the wonderfully variegated males, in order
to become convinced that almost every contour-feather can vary
in almost any direction as regards coloration, form, size, and
minute structure.

As has already been remarked, the internal organs are ap-
parently by no means so specially determined from the germ
onwards as are the external parts : their determinants must
therefore control larger regions of cells, as in the case, e.g., of
blood-corpuscles and epithelial cells. The number of deter-
minants in the germ-plasm of the higher animals is nevertheless
an enormous one, and it might certainly be doubted whether
such a large number of biophors as must be required for the
construction of an id of the germ-plasm could be contained
within a single id.

90 THE GERM- PLASM

It is impossible, as we have already seen, to obtain a satis-
factory answer by means of a calculation. But let us assume
for the moment that we possess reliable data as to the number
of determinants and the size of an id in a particular species.
We will further assume that each determinant is composed of,
let us say, fifty biophors, and each biophor of a thousand mole-
cules, and that the average diameter of a molecule is 2;TioJ7i(5oth mm.
Supposing we found that all these units could not be contained
within an id of the size we have assumed, we should be forced
to conclude that one or more of these quantities had been over-
estimated. This result would not weaken the theory of deter-
minants, for minute particles iniist exist in the germ-plasm for
each hereditary and independently variable part of the body.
I therefore consider it fruitless to attempt a more accurate esti-
mation of the number of determinants in individual species, and
to endeavour to find a support for this fundamental theory by
means of such calculations. The theory is correct in any case,
although our conception of the structure of the germ-plasm may
be very incomplete.

The object of making the above calculation was simply to
arrive at this result. The germ-plasm is an extremely delicately-
formed organic structure. — a microcosm in the true sense of
the word, — in which each independently variable part present
throughout ontogeny is represented by a vital particle, each of
which again has its definite inherited position, structure, and
rate of increase. A theojy of evolution appears to me to be ojily
possible ill this sense. The constituents of the germ-plasm are
not miniatures of the fully-formed parts, or even particles exist-
ing solely for the formation of the corresponding parts in the
body. But each of these particles (the biophors and deter-
minants) has a definite and important share in the preceding
stages of development, for it takes part in determining the
architecture of each idic stage, and consequently also assists in
the further ontogenetic disintegration and distribution of the
determinants amongst the subsequent cell-stages. All the more
essential differences in the structure of organisms depend on
this fact. The determinants are particles on whose nature that
of the corresponding parts in the fully-formed body depends,
whether the latter consists of a single cell or of several or many
cells. The assumption of such particles is inevitable in a theory
of hereditv. and it alone necessitates an almost inconceivable

THE GERM-PLASM 9 1

complexity in the architecture of the germ-plasm. But if we
suppose that the number of ultimate particles of which the
germ-plasm is composed is less than the number of parts in the
body which are independently variable hereditarily, it would
then follow that several minute parts of the body must become
changed simultaneously with the variation of one of these parti-
cles,— that is to say, the number of determinants then become
too small theoretically.

PART II

HEREDITY IN ITS RELATION TO MONOGONIC

REPRODUCTION

Introductory Remarks

In the following part those phenomena of heredity will be
considered which do not result directly from the composition of
the germ-plasm as already described, but which would also
occur if there were no such thing as sexual reproduction. In
considering the phenomena of the regeneration of lost parts, of
multiplication by fission and genunation, of the production of
nnicellular germs, and of the continuity of the gerni-plasjn, it will
materially facilitate the attainment of clear results if the inves-
tigation is conducted throughout as though these phenomena
occurred in organisms in which the process of multiplication is,
and always has been, entirely an asexual one. The complica-
tions resulting from sexual reproduction can be considered after-
wards, and it will then be easy to connect them with all these
phenomena of heredity.
92

REGENEkATlON

93

CHAPTER II
REGENERATION

I. Its Cause and Origin in the Idioplasm

It does not follow directly from what has already been said
with regard to the structure of the germ-plasm, that lost parts
can be more or less completely replaced. The only deduction
that can be made so far is, that all the parts of which the entire
organism is composed are formed once during the development
of the organism from the ^g'g : no explanation is given of the
fact that individual parts can be produced a second time, when
they have been lost by the action of external influences. During
ontogeny, the determinants of the part in question pass from the
ovum into the segmentation-cells, from these into embryonic
cells of a later stage, and finally into those cells which constitute
the fully formed part. If tliis part is forcibly removed from the
organism to which it belongs, its determinants are removed
along with it : this follows from what has already been assumed
with regard to the ontogenetic stages of the idioplasm. We
must now' therefore attempt to explain the fact that a part of the
body can nevertheless be reconstructed.

If the capacity for regeneration were possible at all, it is
obvious that it would have to be introduced by Nature, for its
physiological importance is apparent. The power of replacing
larger or smaller parts of the body must in all cases be useful to
the organism, and is often indeed indispensable to its further
existence. Arnold Lang * is certainly right in considering the
faculty of regeneration in animals to be one of the arrangements
for protection which prevent the species from perishing. The
capability of completely restoring those parts of the body which
have become injured by the bite of an enemy, forms a more

* ' Ueberden Einfluss der fesfsitzenden Lebensweise auf die Thiere,' &c.
Jena, 1888, p. 108.

94

THE GERM-PLASM

efficient protection in many of the lower animals — more espe-
cially in polypes and worms — than would the possession of
shells, stings, poison-organs, and all other kinds of weapons, or
even protective coloration. For although all these arrange-
ments certainly serve as a protection from many enemies, and
from various dangers, they are not always effective, and there-
fore the capability of restoring losses of substance would cer-
tainly be extremely valuable in any case. This fact must not be
forgotten in any inquiry with regard to the question of regenera-
tion. If we consider how highly important regeneration is from
a physiological point of view, its wide and even general distribu-
tion in the animal kingdom need not surprise us, and we shall
be able to understand why it has been introduced even into the
course of normal life : for the functions of certain organs depend
on the fact that their parts continually undergo destruction, and
are then correspondingly renewed. In this case it is the process
of life itself, and not an external enemy, that destroys the life
of a cell. I refer, of course, to the process of physiological
regeneratioti .

Our knowledge of histology is not yet sufficient for us to be
able to determine what tissue-cells in the higher animals become
worn out by use during life, and have therefore to be continually
replaced ; but it has been proved in many cases that the wear-
ing away of the cells goes on incessantly, and that life could not
last if these cells were not constantly replaced. Such a constant
loss and renewal of the cells occurs in the cases of the epidermis of
the higher \^ertebrates, the human finger-nails, blood-corpuscles,
hairs and feathers, claws and hoofs, the epithelial lining of the
respiratory and other passages, and even in the antlers of stags.
In all these cases a continual or periodic wasting away or shed-
ding of groups of cells occurs normally, and a corresponding
replacement of these cells is one of the normal functions of the
body, and is therefore provided for.

It is not difficult to explain the simplest of these cases of
physiological regeneration theoretically. If a tissue such as the
human epidermis, for instance, consists of one kind of cell only,
it is only necessary, in order that regeneration may take place,
that all these cells should not be thrown off simultaneously, and
that the tissue should be composed of cells of various ages, the
vounjrest of which, under certain influences of nutrition and
pressure, always retain the power of reproduction, and so form

REGENERATION

95

a stock in which the necessary substitutes for the older cells can
constantly be produced. The whole supply of the corresponding^
determinants is not therefore removed from the bodv simul-
taneously by the loss of the worn-out cells, for the young cells
which remain contain determinants of the same kind. In the
human epidermis, this stock of young cells constitutes the so-
called ' rete Malpighii ' or ' mucous layer,' in which new cells are
constantly being formed by division ; these, in proportion as
they become older, are gradually pushed upwards mechanically
from the deeper into the superficial layers, while the deepest
layer of all consists entirely of young cells which are capable of
division.

No special theoretical assumption need be made to explain this
process. We must only suppose that the first formed epidermic
cells are endowed in advance with a capacity for reproduction
during many generations. It must be assumed that the repro-
ductive power of a cell is regulated by the idioplasm, because
the power and rate of multiplication are essential qualities of a
cell, and, as we have seen, are controlled by the nuclear substance.
But we cannot at present even form a conjecture as to which
qualities of the idioplasm the degree and rate of the capac-
ity for reproduction are due. We must be satisfied with
attributing to the cells which form the epidermis of the embryo
an idioplasm possessing a definite reproductive power, which
gradually decreases. We can further only suppose that the
idioplasm retains its constitution during life, or, in other
words, that the determinant of a particular part of the epidermis
is always retained in the permanent stock of young cells.
Regeneration depends simply on a i-egular increase of those cells
which contain epidermic idioplasm.

The nature of the epidermis is not the same in different parts
of the human skin : thus it differs on the volar and on the dorsal
surfaces of the fingers ; and, again, on the two basal and on the
unirual seo:ments. But this fact does not stand in the wav of
the theoretical explanation of regeneration, for the determinants
of different parts must differ somewhat from one another. Even
in places where two or more dissimilar parts are situated close
together, the retention of the limits between them, during their
continual regeneration, may be explained simply l)y the fiict that
the different regions of the tissue are regenerated by formative
cells possessing different determinants.

g6 THE GERM-PLASM

Many tissues, even in the highest animals, when they have
suffered an abnortnal loss of substance, are renewed in precisely
the same way as in the cases of physiological regeneration
already mentioned. Thus in mammals, for instance, portions of
muscular tissue, of epithelium covering an organ or lining the
duct of a gland, and of bone, can be replaced by cellular elements
of a similar kind ; and recent researches in pathological anatomy
render it almost certain that all these regenerative processes orig-
inate in the cells of the tissue which is to be replaced. Hence
these tissue-cells retain the power of multiplying by division, but
they only begin to exercise this power in response to certain
external stimuli, more particularly to that which is produced by
a loss of substance in their immediate vicinity. Thus epithelial
cells multiply around a defect in the epithelium ; and in an
injured muscle, the nuclei multiply and cause the surrounding
protoplasm to be transformed into cells, which become spindle-
shaped, and give rise to muscle-fibres. In both these cases we
must merely attribute to the idioplasm the capacity for multipli-
cation : the cells in question only begin to divide w'hen inHu-
enced by a stimulus due to the loss of substance, or, as it would
be expressed in the language of modern pathology,* *by the
removal of the resistances to growth.' Thus in these very simple
cases of the abnormal loss of parts, the rest of the tissue gives
rise to a stock of determinants from which replacement of the
part can occur.

In more complicated tissues, the process of regeneration is
less simple. Thus Fraisse has shown that in the Amphibia
the entire epider)nis, toi^ether with the slinie-glands and the
integumentary sense-organs, is regenerated by the epidermic
cells in the vicinity of the defect. In this case also, the new
material is furnished by the deeper uncornified layers of the
epidermis. But the newly-formed cells do not all develop into
the same kind of tissue. The main mass of them gives rise to
the stratified epidermis, while others 'unite to form pearl-shaped
masses in the deeper part of the epidermis, the cells becoming
grouped around an imaginary centre.' ' Connective tissue-cells
then migrate from the cutis, and these masses, each consisting of
from ten to twenty cells, thus become marked off from the epi-
dermis.'' 'At the same time ])igment-cells wander into the

* Cf. E. Ziegler, ' Lehrbuch der pathologischen Anatomic,' Jena, 1890.

REGENKRA'IIOX 97

epidermis, and finally the development of smooth muscle fibres
takes place.' * New integumentary sense-organs arise in a sim-
ilar way. A number of young cells become arranged so as to
form a rounded mass in the deeper portion of the newly-formed
epidermis : these then become elongated in a direction vertical to
the surface of the epidermis, the central element undergoing
differentiation into sensory cells, while the peripheral ones form
an investment around these.

It is evident that the process is rendered more complicated in
this case by the fact that the young epidermic cells, formed by
the proliferation of those already present, give rise to cells of
various kinds, viz., to ordinary epidermic cells, to gland cells,
and to sensory and * investing ' cells ; and the complication is
further increased by all these cells being arranged and localised
in a perfectly definite and more or less prescribed manner. We
certainly must not assume that the formative cells which undergo
these various differentiations are really identical, although they
may appear so. It cannot possibly depend on external influ-
ences alone whether one of these subsequently becomes trans-
formed into a horny, glandular, or sensory cell ; for we cannot
assume the existence of such a regular and localised difference
in the external influences. The various differentiations of the
formative cells must therefore depend on their own nature — that
is to say, on the determinants contained within them, which have
hitherto been latent but which have now become ripe, and have
impressed a specific character upon each cell. Tliese formative
cells f/mst have contained differoit sorts of determinants from
the first.

Fraisse compares the processes which can be observed in the
regeneration of the skin in Amphibia with those which occur in
the embryogeny of this class, and shows that they are essentially
similar. We shall therefore be justified in imagining these proc-
esses— which are invisible to us even under the highest powers
of the microscope — to be homologous with those which take
place during the development of the embryo.

We can thus further assume that the stratified cells in the
'mucous layer" of the epidermis, although apparently all alike,

* Cf. Fraisse, ' Die Regeneration von Geweben u. Organen bci den Wir-
belthieren, besonders bei Amphibien u. Rcptilien." Cassel and Berlin,
1885.

98 THE GERM-PLASM

— as are those cells which form the first rudiment of the embry-
onic integument, — must nevertheless possess several kinds of
determinants. We can hardly venture to say whether the three
kinds of determinants with which we are here concerned are all
present together in the formative cells, and only become distrib-
uted amongst special cells when regeneration sets in, or whether
they are distributed amongst special cells from the first. Either
arrangement is possible. Hence we may assume that some of
the young formative cells contain determinants for the glands,
and others those for horny or sensory cells, and that the propor-
tional numbers and topographical arrangement of these are defi-
nitely fixed from the first. A precisely similar assumption is
also necessary in the case of embryogeny.

If, for instance, the sensory organs of the lateral line in a
fish or amphibian occur only along the lateral lines and their
branches, we must suppose that the subdivision of the idioplasm
of the ectodermic cells occurs during the development of the
epidermis in such a way that the cells containing the determi-
nants of these sensory organs come to be situated only along the
lateral lines, and only in definite places on these lines. If now,
all the formative cells of the sensory organs do not undergo
further development at once, but some of them, on the contrary,
remain undeveloped in the immediate neighbourhood — i.e., in
the deep layer of young cells — until a necessity for regeneration
arises, we can understand in principle why a similar topographi-
cal arrangement and numerical relation of the sensory organs to
the remaining epidermic elements occurs in the case of regenera-
tion, as well as in that of the primary formation of the epidermis
in the embryo.

The idioplasm of the cells does not alone decide what will
happen in regenerative processes of this kind. This is shown
by the fact that the occurrence of regenerative cell-multiplication
depends on a loss of substance, and that the cells cease to pro-
liferate as soon as the defect is made good. The stimulus to
the further proliferation of the cells ceases at the same time.
These facts, however, only give us a very vague insight into the
causes of the limitation of the regenerative process ; and w'e
shall presently see that the above explanation is insufficient in
more complicated cases of regeneration, and that we must, in-
deed, assume in addition the existence of other regulating fac-
tors, which are situated within the active cells, and not outside
them.

REGENERATKW

99

It is well known tliat the limbs of a salamander <rro\v asrain
after they have been cut off, and we owe our accurate knowledj^e
of the regenerative processes concerned mainly to the researches
of Gotte * and Fraisse.f The investigations of these observers
show that the regeneration of the limbs and their formation in
embryogeny take place in a similar manner : the individual parts
and segments of the extremity become developed in the same
order, and are formed of similar cell-material in each instance.
Both here and in the case of the epidermis described above, the
regeneration is palnigenetic .

If we take as our basis the law, which holds good at any rate
as regards Vertebrates, that in regeneration each specific tissue
can only reproduce its own specific cells, we can test the theory
of regeneration by taking as an example a single tissue of an
extremity. It is certain as regards the bones, for instance, that
regeneration always proceeds from the injured bone, or rather
from its periosteum. If the extremity is disarticulated from the
shoulder-girdle, for example, and the bones are uninjured, these
latter do not become re-formed. Although it cannot be denied
that the various tissues which are required for the regeneration
of the entire limb have an influence upon one another, especially
when pressure is exerted by one part on another situated near
to it, it is clear that the formation of new bones depends entirely
on the bones present in the stump of the amputated limb, which
not only determine the quality of the tissue, but also regulate
the size and shape of the bone which is to be formed anew.
These last-mentioned facts are the most important of all in
explaining the phenomenon of regeneration of a limb. From
what has already been said, it is evident that the bony tissue,
including the periosteum, can be formed from the cells of the
corresponding pre-existing parts. All that is neces.sary in order
that the process may take place is a supply of cells, capable of
proliferation, which contain "bone-idioplasm,* and which are
incited to multiply by the stimulus due to the injury in the
tissue surrounding them. The regeneration of the epidermis
may be explained in a similar manner. But as regards these
bones, it is not merely the production of bony tissue of a definite

* Gotte, ' Uber Entwickelung u. Regeneration des Gliedmassen-Skeletts
der Molche,' Leipzig, 1879.
t Fraisse, loc. cit. on p. 07.

lOO THE GERM-PLASIM

structure which has to be considered, but the fonnation of
a definite number of bones of a definite shape and size, arranged
in a definite series, must also be taken into account. What
assumptions .must we make in order to explain such an ac-
curately prescribed and complex mode of construction ot
these parts? If the fore-limb of a newt {Triton) is cut off
between the shoulder and elbow\ not only does the lost portion
of the humerus become formed afresh, but the radius and
ulna, and all the bones of the wrist and hand, are regenerated
accurately, even as regards the number of segments. It
seems hardly possible that so complex a structure could be
produced merely by the co-operation of proliferating cells, and
it might be supposed that an invisible power — a spiritus rector
or a vis forinativa — must be present to direct their mode of
increase and arrangement. We are nevertheless probably right
in assuming that no such external direction takes place, and
that the complex structures in living beings are produced merely
by the agency of the forces which are present in the individual
cells.

We can understand these processes to some extent in the
case of embryogeny if we base our reasoning on the principle of
the gradual transformation of the idioplasm, which has already
been treated of in connection with ontogeny. This principle
may be roughly illustrated with respect to the skeleton of the
anterior extremity in the following manner.

When the fore-limb of a Triton begins to arise as a small
blunt elevation of the skin, it consists of cells of the external
and middle embryonic layers. The whole of the former, and
that portion of the latter which forms the cutis, may be left out
of consideration ; they together give rise to the integument.
The rest of the mesoderm now forms a mass of cells which have
not yet begun to undergo differentiation, and which individually
do not apparently differ essentially from one another. They
must, nevertheless, be very different as regards the primary
constituents which they contain, for some of them w'ill subse-
quently give rise, for instance, to muscles, others to connective
tissue or to blood-vessels, and others, again, to bones. These
cells, which are so differently predisposed, must therefore con-
tain various determinants, which, when they obtain control over
them in the course of further cell-divisions, impress on the sub-
sequent generations the character of muscle- or bone-cells.

REGENERATION lOI

Each of these kinds of cells must be present from the first in a
perfectly definite number, and must occupy a perfectly definite
position.

Let us follow out this line of reasoning with reijard to one
system of organs, namely, the bones, and assume for the sake of
simplicity that only a single bone-forming cell is present in the
first rudiment of the limb. This cell would virtually contain the
entire skeleton of the limb ; and we should have to attribute to
its idioplasm the power not only of giving the succeeding cells
of a certain number of generations the character of bone-forming
cells, but also of determining the entire sequence of these cells
as regards quantity, quality, and mutual arrangement, as well as
the rhythm in which the divisions will follow one another. For
the particular point at which an interruption occurs in the con-
tinuity of the bone, and consequently also the boundary line
between two segments of the bony chain, might essentiall\-
depend, indeed, on this rhythm.

We must therefore suppose that the composition of the idio-
plasm of the first primordial bone-cell of the limb causes all
these sequences to take place : in other words, tJie idioplasm
1)1 list contain the detenninants of all tJie succeeding bone-cells.
This may be illustrated by the following diagram (fig. 3), in
which the actual processes, which concern hundreds of thousands
of cells, are represented as greatly abbreviated, and the different
generations of cells are indicated arbitrarily by a genealogical
tree, which, however, does not by any means always represent
their actual connection.

Each primary cell of the individual bones is represented in
the figure by a circle, and is supposed to be so simple that it
can be controlled by one determinant. Thus the primary cell
of the entire series of bones is controlled by determinant i. but
also contains the determinants 2-35 in its ids. In the first cell-
division this cell divides into two, — the primary cells of the upper
arm (humerus), and of the fore-arm and hand. The former
contains determinant 2, and its further division is indicated by
the cells containing determinants 2a-ix. The latter contains
the remaining determinants 3-35, which become separated into
smaller and smaller groups in each cell-division, until finally
each cell only contains a single determinant. The diagram only
represents the main bones of the extremity. — the individual
carpals are omitted.

I02

THE GERjNI-PLASM

Let us now return to the question of regeneration. If each
cell in the fully-formed bone only contains that kind of idioplasm
which controls it, and which is therefore the molecular expres-
sion of its own particular nature, it would be impossible to
understand how the regeneration of the bone could be effected —
when, for instance, it had been cut through longitudinally.

■7)et.Wh-f-35

2r

Humerus Det.2l.

J?et3

JiadiusfUlna, f^Hand/
-35

2c

<2b

Itadius,

■tradiaJe,
Ha.

2x

Jla:

2a.i

Mei

Rod.:

radiaJe,

H-20

/j]eA6-2a

fDufiii

Meti

radA

vus

radutle^

Mtteih.i

Jkc.

2-20

J^ecfO,
MstaooTpJ

'ijgitusir

V6-20

dUna.

y ulnareJj.iZ.id

13 -3S
\uln>.Sand'

D^.22

JJet.t

Oee-ZT-SS

\uln.Metacarpiig
+Digitt

ulrv-CarpuS

'Jet. 33 -35

JDetX

DL

JJetJl^

Metaca/j?JJ

^hain^m

■^^^^rrr Metacarpus

J}iguiisI7

m+IV

rvL
IV

Ph

<19

PhaUJ

.-On

u

i+m

^I>ec29

'JUS

C.3S-

UetSf

Fig. 3. — Diagram of the Cell-Generations in the Fore-Limb of a Triton.

Supposing that a stimulus, produced by the injury, caused the
cells of the injured part to undergo multiplication : bony tissue
would then, indeed, be developed, but a bone of a definite shape
and size would not necessarily be formed. The formation of a
definite bone can only take place if the proliferating cells possess,
in addition to their active determinants, a supply of determinants

REGENERATION IO3

which control the missing parts which have to be renewed. If
therefore we wish to suppose that Bhnnenbach's ' tiisus forina-
tivus ' is situated in the idioplasm of the cell, it appears neces-
sary to assume that each cell capable of regeneration contains
an accessory idioplasm, consisting of the determinants of the
parts which can be regenerated by it, in addition to its primary
idioplasm. Thus, for instance, the cells in the bone of the upper-
arm must contain, in addition to their controlling determinant 2,
the determinants 3-35 as accessory idioplasm, which can cause
all the bony parts of the fore-arm to be formed anew ; the cells
of the radius, again, must contain the determinants 4-20 as acces-
sory idioplasm for the reconstruction of the radial portion of the
wrist and hand.

This theoretical illustration may be looked upon, indeed, as
representing the phenomena as they occur in reality. It is
very possible that the required accessory idioplasm becomes
separated from the disintegrating embryonic idioplasm in the
earliest stage of development of the entire organ. According
to our assumption, the individual determinants are present sitigly
in the germ-plasm, and their multiplication increases the further
ontogeny advances. As only those determinants which corre-
spond to parts to be formed subsequently are required in the
accessory idioplasm, the material for the latter is always present ;
and we need only assume that in each division of the primary
cell of any bone, a portion of the determinants required for the
formation of the subsequent parts becomes split off as secondary
idioplasm, and remains inactive within the cell until a cause for
regeneration arises.

I shall speak of this group of determinants as accessory idio-
plasm (' Neben-Idioplasma""), and its component determinants
as siippieinentary determinants (' Ersatz-Determinanten ") . We
may imagine that these form a special and minute group en-
closed within the id in the neighbourhood of the determinants
which control the cell in question. A similar assumption may
be made as regards the individual bones of the entire limb.
The regeneration of the bisected humerus can be explained by
supposing that each cell capable of regeneration possesses an
accessory idioplasm, containing the determinants of the cells
which will subsequently be formed in a distal direction ; this
formation will be possible because the necessary • determinant-
material ' is present. The process only depends on the tact that

104 '^"Hf" GERM-PLASM

in each differential cell-division a certain number of determinants,
which ripen later on, become split off from the rest, and are
retained in the cell as accessory idioplasm. The mechanism for
regeneration is certainly a very complicated one, for each separate
bone is controlled by a number of different determinants, and
not by a single one ; and all these special determinants are
contained in the accessory idioplasm. As far as we can judge
from the investigations made hitherto, the bones are at any rate
regenerated in detail fairly exactly. The complexity of the
mechanism accounts, in my opinion, for the fact that the fore-
limb, which has such a marked power of regeneration in the
salamander, has lost this power completely in the higher
Vertebrates, for in them the mechanism would have become
too complex.

A simpler mechanism than that which we have supposed to
exist can only be conceived, if, with Herbert Spencer,* we attri-
bute to each of the units composing the body the power com-
bining to form any necessary organ just when it is wanted. We
might then compare the entire animal to a large crystal, in the
individual parts of w'hich ' there dwells the intrinsic aptitude
to aggregate into the form of that species ; just as in the atoms
of a salt there dwells the intrinsic aptitude to crystallise in a
particular way.' The only difference between the particles of
the crystal and those of the organism w^ould be that the former
are all permanently alike ; and that the latter, in order that
regeneration may be possible, are arranged in many different
ways, according to whether an entire limb, a tail, a gill, or a
single toe, fore-arm, or finger is to be replaced. How are the
' units ' shown in each individual case wdiat part is missing, and
what form their arrangement is to take in order to produce the
part anew? We are thus once more brought back to Blumen-
])ach's ' iiisusforinativtis.'' Spencer himself says : — 'If in the
case of the crystal we say that the whole aggregate exerts over
its parts a force which constrains the newly-integrated atoms to
take a certain definite form, we must, in the case of the organism,
assume an analogous force.' This force would correspond to what
was formerly spoken of as the ' spiritus rector'' or ' nisus forma-
tivus ; ' and even supposing it to exist, it does not in the least
help us in the attempt to explain the mechanism of the phenom-

* Herbert Spencer, ' The Principles of Biology,' Vol. i, p. i8i.

REGENERATION 105

ena. Spencer adds that his view ' in truth is not a hypothesis/
but only 'a generaHsed expression of facts;' and remarks in
another passage, that although it is ' difficult " to imagine regene-
ration as a sort of process of crystallisation, ^we see that it is so.''
It is just this point that I must object to. We see that it is so,
or rather appears to be so, sometimes, but we also see that it is
often ;/^/ so. If the units of the body were capable of becom-
ing modified at will under the influence of the whole, and of
crystallising into the missing part, they must be able to do so
in all species and in all organisms. This, however, is not the
case. The limb of a salamander can be regenerated, and tiiat
of a lizard cannot. In a special section of this chapter I shall
be able to show in greater detail that regeneration depends
on special adaptation, and not on a general capacity of the
animal-body.

It will be unnecessary to give a special diagram illustrating
the regeneration of a single bone, such as that of the upper
arm, and showing the supplementary determinants of each of
the cells composing the bone which are necessary in order
that regeneration may set in at any point. The diagram given
for the entire limb is sufficient to make the general principle
clear : an approach to an explanation of the actual details is
out of the question, as is evident if we compare the numl^er of
cells given in the diagram with that of whicli tlie bones actually
consist. For this reason I have not attempted to enter into
minute histological details, or to define the quality of the cells
which are capable of regeneration, — that is, to state whether
they belong to the periosteum or to the bone itself, and whether
all or only certain cells take part in the process. We only re-
quire a diagram which can be adapted to the actual details of
the processes when these are known. It is sufficient at present
to show that regeneration may be understood by considering
the activity of the cells themselves, without having recourse
to the assumption of an unknown directive agency. The
^ jtisHS formativus' descends from its previous position as a
single force directing the whole, and breaks up into an un-
limited number of material particles which are situated in the
individual cells, and each of which prescribes the course of life
of the cell. These particles are determined as regards their
kind, and are distributed to their proper places so accurately, that
by their united effect they give rise to a composite whole, sucli

I06 THE GERM -PLASM

as, for instance, a series of l:)ones, together with their articular
capsules and ligaments, and the muscles, nerves, blood-vessels,
connective tissue, and integument which come into relation with
them. The diagram I have given to illustrate the regeneration
of a bone can obviously be adapted to represent any other part
or tissue. We must not look upon the bone as something quite
disconnected from the rest of the limb, as we may very likely be
inclined to do if we are specialists. The bone is in reality con-
nected most intimately along its entire surface with the surround-
ing tissues, — the periosteum and loose connective tissue external
to the latter, the numerous blood-vessels which penetrate into
the substance of the bone, the nerves, and so on. The first
rudiment of the limb consists, in fact, of a mass of mesodermic
cells, which give no indication of the various structures which
will later be developed from them. Nevertheless, their differen-
tiation does not, in my opinion, depend on their accidental posi-
tion within the limb, or in fact on any other external influences,
but is primarily due to their individual nature, that is. to tJie
constitution of tJieir idioplastn. The determinants composing
the id control the subsequent development of the cell and of its
successors. The further changes which the id undergoes in the
course of cell-division, and the manner in which the deter-
minants undergo disintegration in the ids of the daughter-cells
of all the subsequent generations, is decided by the composition
of the id.

We can thus understand, at least to some extent, how it is
possible that such a complicated part as a limb, the structure of
which is so accurately prescribed, can arise by degrees from a
mass of cells which are apparently all similar to one another.
The harmony of the whole is primarily brought about by the
variation and increase of the cells, the kind and rhythm of
which respectively, is prescribed by the idioplasm of each in-
dividual cell, rather than by the mutual influence of the cells
durinjr their sradual differentiation. A muscle becomes de-
veloped at any definite spot, because one particular cell amongst
all the apparently similar cells in the first rudiment of the limb
contained the determinants which are capable of giving to a
large number of the successors of this cell the special character
of muscle-cells ; and because, again, the id of this particular
cell caused a rhythm of multiplication to set in, which, on
mechanical grounds, rendered it necessary that certain succes-

REGENERATION IO7

sors of this cell which contained muscle-determinants should
take up their position in the precise region of the limb in which
this particular muscle is situated.

We must not, however, imply from what has been said above,
that external influences are of no importance whatever in ontog-
eny, but merely that they certainly only play a secondary part
in the process. A limb will certainly grow crooked if a corre-
sponding external pressure is brought to bear upon it. Growing
cells do not cease to multiply directly they are subjected to
abnormal external influences, for they can accommodate them-
selves to circumstances. It is such cases as the regeneration of
broken bones and the formation of new joints under abnormal
external conditions, which prove that the cells continue to
perform their functions of growing and of giving rise to organs
under circumstances which deviate very markedly from the
normal. These false joints also show what a considerable
power of adaptation is possessed by the cells, and how efficient
may be the parts which these cells are able to produce under
abnormal conditions. But although the principle formulated
by Roux * of the struggle of the parts, or as it might well be
called ' intrabiontic selection,^ is certainly a very important one,
I think it would be a great mistake to refer the normal process of
ontogeny mainly to this principle. The groups and masses
of cells must certainly press upon one another durin<; the process
of differentiation : in the process of the formation of a joint, for
instance, proliferating connective-tissue cells do actually force
themselves amongst the cartilage cells in one part of the rudi-
mentary bone, in order to separate them from one another. But
this proliferation and pressure are taken account of, just as much
as are the processes of dissolution or absorption that occur in
those cells in the primordial cartilage which are situated in the
region of the joint. It might be supposed that the existence
of so-called 'identical' human twins contradict mv conception
of ontogeny ; for although they are undoubtedly derived from
a single ovum and sperm-cell, and hence possess the same kind
of germ-plasm, they are never really identical, but only very
similar to one another. But apart from the fact that the abso-
lute identity of the germ-plasm has not been proved in these
cases, the very close resemblance between these twins shows

* W. Roux, ' Der Kampf cler Theile im Organismus,' Leipzig, 1881.

I08 THE GERM-PLASM

how slightly the diversity of external influences affects the
development of an organism. How wonderfully accurately the
course of ontogeny must be prescribed, if it can be kept to so
closelv. through thousands of generations of cells, that •• identi-
cal ' twins result ! We may compare the process of development
of such twins with the course taken by two ships, which, start-
ing from the same place, proceed along the same devious route
whiqh has been carefully mapped out beforehand in all its
thousands of definite changes in direction, until each finally
reaches the same distant shore independently, within a mile of
the other.

A careful consideration of such a case as this leaves no doubt
that a very exact and definite course is mapped out for the egg-
cell by its idioplasm, which, again, directs the special course to
be taken by each of the innumerable generations of cells, in the
direction of which course external influences can only play a very
subordinate part. If this consideration be borne in mind, it will
be less likely that the objection may be made that a much too
complicated stmcture has been attributed to the idioplasm. Its
structure must be far more complex than we can possibly imagine ;
and in this respect, its construction, as we have represented it
theoretically, must certainly be far simpler than is the case in
reality. For the same reason, it is less probable that similar
objections may be made to the theory of regeneration as here
stated. Complicated phenomena cannot possibly depend on a
simple mechanism. The machines in a cotton factory cannot
be constructed of a few simple levers, nor can a phonograph be
manufactured from two lucifer matches.

That form of regeneration which has been considered above
may be described as palingenetic^ for it pursues the course taken
by the primary or embryonic development ; but as soon as it
leaves this course and takes a shorter one, it may be distin-
guished as coenogenetic.

Coenogenetic variations of the primary process of development
probably always occur in cases of regeneration of complex struct-
ures ; and even the reconstruction of the extremities, which we
have chosen above as an example, will hardly take place in
exactly the same way as occurs in the primary development of
these parts, although it may resemble the latter in its principal
phases.

Even if mere abbreviation of the development of a part can

REGENERATION IO9

be easily conceived by supposing an aggregation and redistribu-
tion of the determinants to occur in the idioplasm, the process
of idic division becomes very complicated when the primary
and secondary development take place along ditlierent lines ;
for in the latter process the combinations of supplementary
determinants in the id of the cell-generations must be different
from those which occur in the former. But this difference is
evidently due merely to a greater complication of the process,
and it does not stand in the way of the theory. In all cases of
regeneration, the mode in which the supplementary determi-
nants become split off must be previously arranged for in the id.
The assumption of a mere increase in the power of multiplica-
tion of certain determinants might seem sufficient in the case
of palingenetic regeneration, for this would lead to one portion
of a certain group of determinants becoming separated off as
accessory idioplasm at a particular ontogenetic stage. In
ccenogenetic regeneration, however, we can only assume that a
double or still greater number of determinants are present in the
germ-plasm, one set of which are destined for embryonic devel-
opment and the others for regeneration ; and these are previ-
ously arranged with reference to their internal forces, particularly
that of multiplication, so that at a certain stage of development
they become split off as ' accessory idioplasm,' either alone or
together with the adjacent ' regenerative determinants.'

It seems to me, how^ever, that palingenetic regeneration
cannot be satisfactorily accounted for unless we assume the
existence of special regenerative determinants, for it would
otherwise be impossible to explain the phyletic origin of the
ccenogenetic variations in the process of regeneration. These
latter must, indeed, depend on variations in a determinant of
the germ-plasm. If however the latter contained only the one
determinant destined for embryogeny, variations must occur in
the latter process at the same time. But this is not the case,
and consequently a kind of double determinant must be con-
tained in the germ for those hereditary parts (determinants)
which are capable of becoming regenerated : — that is to say. two
originally identical determinants must be present, one of which
becomes functional in embryogeny and the other in regenera-
tion. This will be made apparent if we take some examples.

In most existing amphibians the caudal region of the vertebral
column may undergo regeneration, although its embryonic

no THE GERM-PLASM

foundation, the notochord, is never formed anew. The carti-
laginous sheath of the notochord has an important share in the
primary formation of the vertebral column, but it disappears
to a greater or less extent at a later stage. If it became pos-
sible for the vertebra to undergo regeneration after a portion
of the tail had been lost without a renewal of the notochord
taking place, the result would be a useful abbreviation of the
process of regeneration. Such an abbreviation has occurred,
and everything supports the assumption that at an earlier stage
of phyletic development the notochord was capable of ujidergoing
regeneration, and that it has only lost this capacity secondarily.
In the case of frog-tadpoles, the power has been retained of re-
generating the tail when it is cut off together ivith the notochord.
We must not assume that the notochord does not become
restored in other amphibians because it no longer persists in the
full-grown animal ; for it is entirely absent only in a few of
them {e.g., Sabnandrind), and the notochord of the larval sala-
mander cannot be regenerated any more than that of the adult.
Thus the capacity for regenerating the notochord has been lost
by most amphibians in the course of phylogeny. Such a process
of degeneration is certainly to be explained most easily by
assuming the existence of special regenerative determinants,
which may gradually disappear without in the least affecting
their embryogenetic partners.

The necessity of this assumption is shown still more conclu-
sively in the case, for instance, of the restoration of the solid
axis of the tail in reptiles. The tail of a lizard quickly becomes
restored after it has been cut off, but its stmcture is then different
from that of the original tail ; for, according to the statements
of Leydig and Fraisse, the spinal cord and vertebral column
are not renewed. The former is, however, represented by an
epithelial tube, but gives off no nerves ; and the latter is replaced
by an unsegmented cartilaginous tube. As Fraisse points out,
this tube does not correspond to the regenerated notochord, but
is a new structure which is substituted for it.

A phyletic de7>elopment, tending essentially towards a sim-
plification of the parts, has taken place in this case as re-
gards the processes of regetieration. A gradual degeneration
has occurred, just as may take place in the tail or any other
organ of an animal in the course of phylogeny. The caudal
region of the vertebral column has undergone a reduction,

RFXiENERATION T I T

which does not influence its primary (embnonicj ontoji^cny.
but only its secondary formation by regeneration. A vertebral
column is formed primarily ; but if the re-formation of a part of
it becomes necessary, in consequence of the loss of the tail, the
secondary reduced process for the development of the axis
comes into play, and a simple cartilaginous tube is formed.
This process recalls the phenomena of ' dichogeny ' which take
place so frequently in plants, and in which the same group of
cells may develop in either of two different ways, according to
the nature of the external stimulus which is applied to them.
Thus a shoot of ivy will produce roots on a certain side if it is
shaded, and leaves if it is exposed to light. The determination
of the sex of an animal may perhaps be referred to similar
causes, — if, at least, we may assume that the sex is not always
universally decided by the act of fertilisation, and that influences
exerted upon the organism subsequently may have an eiTect in
this determination. In the case of certain parasitic Crustaceans,
the Cyinathoidcp^ the male sexual organs are developed first :
and when the animal has fulfilled its function as a male, the
female organs become developed, and give the animal the char-
acter of a female. The two developmental tendencies here come
into operation temporarily, one after the other ; just as in the
case of the lizard's tail, in which the tendency to form the verte-
bral column first comes into play, and then that to form the sec-
ondary cartilaginous tube. The necessity for the formation of
this tube certainly need not arise at all ; just as that side of the
shoot of ivy from which the roots arise need not necessarily be
subsequently exposed to the light, and give rise to leaves ; the
possibility of such an occurrence is, however, foreseen by Nature.
It might be urged that there is an important difference between
the regeneration of a lizard's tail and the successive development
of the two kinds of sexual organs in the Cymathoids, since in
the latter case the rudiments of these organs are present in the
embryo, and it is only their final development which takes place
successively. This is 'certainly a difference, but it is just such
a one as to indicate to us how these cases of supplementary
substitution may be explained theoretically. The cells in the
tail of a lizard which give rise to the secondary cartilaginous
tube must contain determinants which differ from those of
the embryonic formative cells of the caudal vertebra?, just as
the idioplasm of the formative cells must contain ihfVerent

I I 2 THE GERM-PLASM

determinants for the testes and ovaries. The suppicmetitary
deteynii)ia}its with which the idioplasm of certain cells of the
vertebral colunui was provided for the purposes of regeneration^
must have become changed in the course of phytogeny.

A transmissible variation of this kind must, however, also have
had some effect on embryogeny, if only one and the same deter-
minant were present in the germ-plasm for the two modes of
development. Hence each determinant of these caudal vertebrce
must be doubled in the germ-plasm.

It would be premature to go beyond this assumption, and to
attempt to decide anything about the manner in which the
various supplementary determinants which are required for the
restoration of one of the larger parts — such as, for instance,
the caudal vertebrae — come together, and how and when they be-
come separated from the primary determinants. The processes
of regeneration have not as yet been examined from the point of
view which I have here suggested ; and in many cases it is not
even known for certain from what cells regeneration proceeds.

Hitherto we have not discussed in detail the question as to
the kind of cells which contain the supplementary determinants^
and from which regeneration thus takes place. May these
determinants be present in any kind of cell belonging to any
tissue, or is their distribution always limited to young and
apparently undifferentiated cells of the so-called ' embryonic
type ' ?

If w'e only consider Man and the higher Vertebrates, we shall
be disposed to look upon the latter of these two alternatives as
the one which is in general correct. Even recently, in fact,
many authors seemed to be in favour of this view : ' embryonic
cells ^ were supposed to be contained in all those tissues which
are capable of regeneration, and it was, indeed, believed by
many that the leucocytes are cells of this nature. The latest
investigations, however, lead us to the conclusion that this is
not the case, and that although the white blood corpuscles are
extremely important as conveyers of nutriment in the process of
regeneration, they do not serve as formative elements in the
construction of a tissue. In his text-book on Pathological
Anatomy, Ziegler speaks of a formal ' law of the specific
character of the tissues,' w^hich he explains as follows : — ' the
successors of the various germinal layers which separate from
one another at an earlv embrvonic stas^e, can onlv give rise to

re(;ener.yiion ii^

those tissues which belong to the germinal layer from which
they were developed.' But this statement can only be true in
the case of the highest Vertebrates, for, as the brothers Hertwig
have shown, the germinal layers of the Metazoa are not primi-
tive organs in the histological sense; and moreover, in the lower
animals, several, if not all of the tissues, can be formed from
each of the germinal layers. In lower animals, not only all the
varieties of tissue, but under certain circumstances even rows of
cells of one primary germinal layer and even indeed the entire
animal, may arise from young cells belonging to the other
germinal layer. In the chapters on multiplication by fission
and gemmation, this process will be traced to its origin in the
idioplasm. At present we have only to deal with the question
as to whether the determinants of the various kinds of cells
which are required for regeneration are contained within young
cells only, or whether they are also present in those which have
become differentiated histologically.

Although the supplementary determinants are certainlv in
many cases contained in young cells without any specially
marked histological character, their distribution can neverthe-
less hardly be limited to these cells exclusively. It may happen —
as will be shown in greater detail subsequently — that cells, which
are fully developed histologically, both in plants and in the
lower animals, contain all the determinants of the species ;
that is to say, they may contain germ-plasm as supplementary
idioplasm. Hence there is no reason to assume that smaller
groups of determinants may not have been supplied to specific
tissue cells wherever they were required, although I am unable
to give a definite example of such a case.

Although regeneration may originate in most cases in young,
or so-called • embryonic ' cells, it is nevertheless quite a mistake
to connect the idea of the undifferentiated state of these cells
with this fact, as is so often done. These ' embryonic cells' are
not 'capable of giving rise to anything and everything,' for each
of them can only develop into that kind of cell the determinant
of which it contains. Under certain circumstances such a cell
may contain several different determinants at the same time,
which are only distributed amongst the individual cells in sub-
sequent cell-generations ; but the structure which can and will
become developed from it always depends on the cell itself, and
its fate is determined by the idioplasm it contains, and can only

114 1HK GERM-PLASM

be affected secondarily l)y external intiuences. Cells moreover
exist, the idioplasm of which permanently retains the possi-
bility of development along one of two lines. "Dichogeny' in
plants, wliich has already been mentioned, is likewise deter-
mined by the idioplasm, inasmuch as the latter must contain two
kinds of determinants, one or the other of which either remains
inactive owing to the nature of the external intiuences acting
upon the cell, or else becomes active and determines the cell.

There are, however, no such things as 'embryonic cells' in
the sense in which this term is used by authors. In the fresh-
water polype {Hydra), for instance, cells which are young
and histologically undifferentiated — the so-called -interstitial
cells' — are present in the deeper part of the ectoderm: these
can certainly give rise to various structures, viz., to ordinary
ectoderm-cells, nettle-cells, muscle-cells, sexual-cells, and in all
probability to nerve-cells also. It would nevertheless be absurd
to suppose that any particular interstitial cell is capable of
developing into any one of these structures. It obviously con-
tains either germ-plasm, i.e., the whole of the determinants, — in
which case it can develop into a sexual cell, — or only the deter-
minants of a thread cell or of one of the other kinds of cells, and
then it can only give rise to one of the corresponding structures,
and can never develop into a sexual cell.

2. The Phvlogeny of Regeneration

It may, I believe, be deduced with certainty from those phe-
nomena of regeneration with which we are acquainted, that the
capacity for regeneration is not a pri)nary quality of the organ-
isnif but that it is a phefionienon of adaptation .
• The power of regeneration has hitherto been practically always
regarded as a primary quality of the organism, — that is to say,
as a direct result of its organisation : it has been looked upon
as a faculty for which no special arrangements are required,
but which naturally results as an unintentional secondary effect
of the organisation which exists independently of it.

This view is based on the idea, which is in general a correct
one, that the regenerative power of an animal is inversely
proportional to its degree of organisation.* If this were univer-

* Cf. Herbert Spencer Hoc. cU., p. 175), who, however, expresses him-
self very cautiously with regard to this ditftcult subject as follows : — 'so

REGENERA'IION I j :;

sally true, it would nevertheless not be a convincing arcjument
for the above view, although it would certainly support it. Hut
a closer examination into the facts shows that this statement is
not absolutely correct. Although the capacity for regeneration
is never so far-reaching in the highest animals as it is in the
case of the lower ones, — and tliis must be due to some cause,
— the regenerative power may nevertheless even vary widely in
animals of the same degree of organisation, and may in fact
be far greater in one of the higher than in one of the lower
forms. Thus fishes are unable to regenerate a lost pectoral or
pelvic fin, while the much more highly organised salamander
has been known to regenerate a limb six times in succession
(Spallanzani).

The regenerative power often varies in degree even within
the same group of animals. In Triton and Salajnandra the
entire limb grows again after amputation, but apparentlv, so far
as I have been able to observe, this does not occur in Proteus.
The tail, too, is only replaced slowly and imperfectly in the latter
animal, w^hereas it easily becomes restored in the salamander.
In the year 1878 I received a \\\\\\g Siren lacertina, the fore-limb
of which had been torn off, so that only the stump of the upper
arm remained, and the entire limb did not grow again in the
course of the ten years during which I kept the voracious animal,
and gave it abundant food. In this case again the power of
regenerating the extremities seems to be less than in that of
salamanders, which are much younger phyletically, and much
more highly organised.

It is well known also that the limbs of a frosf do not <jrow
again when they have been cut off, even when the animal is in
the larval condition. The fact that the regenerative power can
vary so considerably within the same genus is still more re-
markable. Schreiber observed that the power of regeneration
in Triton niar/noratns is relatively very slight as comi)ared with
that which is possessed by all other species of Triton which
have been examined for this purpose. 'In captivity, at any
rate, even slight injuries in such parts as the crest are never re-

that the power of reproducing lost parts is greatest where the organisa-
tion is lowest, and almost disappears where the organisation is highest.
And though we cannot say that between these extremes there is a constant
inverse relation between reparative power and degree of organisation, yet
we may say that there is some approach to such a relation."

Il6 THE GERM-PI ASM

])laced while tlie animal invariably succumbs to greater injuries.'
Fraisse gives similar instances. Thus 'an amputated extremity
never grew again to its normal size : merely a somewhat de-
formed protuberance was formed on the stump. The tail also
was only reproduced to a very slight extent.' *

With regard to reptiles, Fraisse points out that the regenera-
tive capacity obtains to a much slighter extent in some groups
than in others. Chelonians, crocodiles, and snakes are unable
to regenerate lost parts to any extent, while lizards and geckos
possess this capacity in a high degree.

The dissimilarity, moreover, as regards the power of regener-
ation which exists in various inembers of ihc same species, also
indicates that adaptation is an important factor in this process.
In Proteus^ which in other respects possesses so slight a capac-
ity for regeneration, the gills grow again rapidly when they
have been cut off. In lizards, again, this power is confined to
the tail, and the limbs cannot become restored : in these ani-
mals, however, the tail is obviously far more likely to become
mutilated than are the limbs, which as a matter of fact are
seldom lost, although individuals with stumps of limbs are occa-
sionallv met with. The physiological importance of the tail of a
lizard consists in the fact that it preserves the animal from total
destruction ; for pursuers will generally aim at the long trail-
ing tail, and thus the animal often escapes, as the tail breaks
off when it is firmly seized. It is, in fact, as Leydig was the first
to point out, specially adapted for breaking off, the bodies of the
caudal vertebra; from the seventh onward being provided with
a special plane of fracture, so that they easily break into two
transversely. Now if this capability of fracture is provided for
by a special arrangement and modification of the parts of the tail,
we shall not be making too daring an inference if we regard the
regenerative power of the tail as a special adaptation^ produced by
selection, of this particular part of the body, the frequent loss of
which is in a certain measure provided for, and not as the out-
come of an unknown 'regenerative power' possessed by the
entire animal. This arrangement would not have been pro-
vided if the part had been of no, or only of slight, physiological
importance, as is the case in snakes and chelonians, although
these animals are as hijjhlv orranised as lizards. The reason

* Loc. cit., p. 152.

REGENERATION I i 7

that the liml:)s of lizards arc not rephiced is, I believe, due to
the fact that these animals are seldom seized hv the le<r, o\vin«r
to their extremely rapid movements. Hut if a lizard does hap-
pen to be caught by one of its limbs, it must fall a prey to its
pursuer, and the capacity for regenerating the limb would be
useless. The case is very different with regard to such animals
as Tritons. Their movements are much less rapid, and their
assailants, being too small to swallow the whole animal, fre-
quently bite off a limb. They are often attacked by members
of the same species, which gnaw off a gill, limb, or the tail of a
weaker comrade, bit l)y l)it. If a considerable power of regen-
eration were possible at all, it would certainly be provided in this
case. This power is possessed in a much smaller degree by
Proteus ; but these animals are only found in the caves of Car-
niola, where enemies larger than themselves do not exist, and
in which there is no great competition for food, and therefore,
at least as far as my observations extend, they do not bite one
another.

Spallanzani has stated that nature does not reproduce every part
that is cut off; expressed in theoretical terms, this simply means
that tJie various organs of an animal possess the power of regener-
ation in different degrees. If we inquire further into the question,
we shall find that those parts which are most frequently exposed to
injury or loss must possess the power of regeneration in the high-
est degree. So far as I can judge from the facts with which we
are at present acquainted, this remark appears to me to be a per-
fectly correct one. Unfortunately Spallanzani gives no instance
in support of the above statement, so that we do not know what
parts he referred to. I have myself, however, made some investi-
gations in order to ascertain whether the degree of regeneration
of a part bears any relation to its liability to injury.

If regeneration is a phenomenon of adaptation, the internal
organs — which are not exposed to injury in the natural life of
the animal — cannot possess any regenerative power, even in
those animals in which the external parts — which are exposed
to the attacks of enemies — possess it in a high degree.

The following experiment bears upon this point : — I cut open
a large newt (^Triton cristatns), removed about half ot the right
lung, and sewed the skin together again. The animal soon
recovered from the effects of the chloroform, and its wounds
healed : it was then well cared for for fourteen months, and.

Il8 THE GERM-PLASM

afterwards killed. An examination showed that the right lung
had not become restored : it was only half as long as the left one,
and its end was blunt, and not pointed as in the normal lung.
Four other similar experiments yielded like results : in one of
these it was doubtful whether a growth of the lung had not taken
place, but even in this case it had not recovered its long, pointed
form.

These experiments are still being continued, but we may
already deduce from them that a striking disproportion exists
between the regenerative power of the external parts of a newt
and that of its lungs. This difference seems even more marked
if we bear in mind that in the case of a limb the process of
regeneration is a very complex one, for complicated parts, con-
sisting of many entirely different portions, have to be reproduced ;
whereas a lung is a simple hollow sac, which has no joints, and
the histological structure of which is relatively simple.

We therefore infer that the internal parts, which are not ex-
posed to injuries of an ordinary kind, do not possess a greater
capacity for regeneration in these species than they do in the
highest Vertebrates, which are so exceptionally inferior to them
as regards the regenerative power of the external parts. Hence
there is no such thing as a general powe7' of regeneration : in each
kind of animal this power is graduated according to the need of
regeneration in the part under consideration : that is to say, the
degree in which it is present is mainly in proportion to the
liability of the part to injury.

This conclusion is closely connected with the fact that the res-
toration of a part which possesses the power of regeneration in a
high degree, can only take place as the result of definite injuries
which are in a manner provided for, and not from a7iy kind of
injury. Philippeaux was the first to discover that the limb of a
Triton does not o;row again when it has been removed at the
joint, and that, in fact, it only does so when it is cut or torn off,
so that the bone is injured. This fact has been explained by
referring it to the law of the specific nature of the tissues, accord-
ing to which bone can onlv be formed from bone, and the bone
of the limb must be injured before it can become capable of
being formed anew. It seems to me that this explanation is
insufficient, although it is founded on a correct principle, accord-
ing to which the injury to the bone causes the stimulus by which
the cells of the stump are incited to proliferate. This is certainly

REGENERATION

119

correct, and may be expressed according to our theory l)y say-
ing that the supplementary determinants which are i)resent in a
passive condition in the cells, are prompted to become active by
the stimulus. But if an articular cavity is exposed, a stimulus
is likewise produced, which must aftect the cells of the articular
cartilage, and doubtless also those of the underlying bone or
periosteum. If. therefore, all the cells in this region weie
capable of reproducing the missing bones, and if the exposure
of the articulation were the ordinary form of injury, these cells
would certainly be just as much adapted for and capable of
responding to this stimulus, by a formative growth, as would
those situated at the broken ends of a bone. But the disarticu-
lation of a limb, or of a part of a limb, hardly ever takes place
in the natural conditions of life, and tJicrefore could Jiot have been
provided for by the organism ; the respective cells of the exposed
articular cavity could not consequently have been supplied with
the supplementary determinants iiecessary for rege?ieration.
Hence these cells are incapable of reacting in an adequate
manner to the stimulus due to the disarticulation.

In spite of all the facts already mentioned, it might still
appear doubtful whether regeneration really depends on a special
adaptation of the part in question, and whether it does not result
from the degree of organisation of the animal, or at any rate
from a ^^;/(?;'rt:/ regenerative force possessed by the entire organ-
ism. The following considerations must, however, I think, set
aside all doubts on the question. Physiological and pathologi-
cal regeneration obviously depend on the same causes, and often
pass one into the other, so that no real line of demarcation
can be drawn between them. We nevertheless find that in
those animals in which the power of regeneration is extremely
great physiologically, it is very slight pathologically. Tliis
proves that a slight power of pathological regeneration cannot
possibly depend on a general regenerative force present within
the organism, but rather that this power can l)e i)rovided in
those parts of the body which require a continual or periodic
regeneration : in other words, the regenerative pmver of a part
depends on adaptation. Let us take a few examples. It was
mentioned above that fishes are said to possess a very slight
' general regenerative power," because they are unal)le to replace
lost external parts, especially such stnictures as fins. Neverthe-
less many fishes are provided with teeth which are very liable to

120 THE GERM-PLASM

become worn out, and consequently they possess the power of
constantly producing new teeth to replace the old ones. In the
mouth of a ray or dog-fish the teeth are arranged in several
rows along the edges of the jaws, the outer rows containing
those which are worn out, and the inner the younger teeth
which take their place. Birds, again, possess a very slight
power of repairing defects which have arisen accidentally, and
hence they are considered to have a very sliglit capacity for re-
generation. But their power of physiological regeneration with
respect to certain parts is nevertheless extraordinarily great : —
all the feathers are cast off and renewed once a year. Patho-
logical regeneration occurs to a very slight extent in mammals ;
defects in the superficial epithelium, the epithelium of the ducts
of glands, the various supporting tissues, including bone, and in
nerve-fibres, can be repaired from the elements of the respective
tissues ; but in no mammal does a segment of a finger or an
eyelid grow again when once it has been cut off. In certain
mammals, however, the power of physiological regeneration
with respect to certain parts is unusually marked. Male stags
shed their antlers annually, and new ones are formed in four or
five months. If we take into consideration the mass of organic
tissue which is thus formed in such a short time, this feat out-
strips even the regenerative performances of the full-grown
salamander. For according to Spallanzani. it takes a salaman-
der more than a year to restore an amputated limb to its normal
size and strength. Young individuals can. however, certainly
reproduce a limb in a few days ; and this gifted experimenter
observed in the case of a young Triton that the four limbs and
tail when they were cut off grew again six times in the space of
three summer months !

In one respect, however, viz., as regards the cojuplexity of
the part replaced^ this remarkable regenerative power in stags
and birds is far inferior to that which obtains in the Triton.
Although a bird's feather is a very wonderful structure, it is
formed merely from epidermic cells, and a stag's antler is only a
dermal bone covered over by the epidermis. But the limb of a
Triton, on the other hand, consists of every kind of tissue with
the exception of endodermal epithelium, — viz., of skin, muscles,
a large number of skeletal parts, connective tissue, blood-
vessels, nerves, and so forth ; and all these have a very definite
arrangement, number, and form. There is no doubt therefore

regeni:ration i 2 1

that the regeneration of a limb is a greater feat than the renewal
of feathers or antlers ; and the fact has been long recognised,
that the more complex origans are regenerated less easily than
those which have a simpler structure. A series of carefully
performed experiments, made with the view of testing this
somewhat vague statement, would be of great value theoreti-
cally. We may predict that in one sense it would be confirmed,
and that we should find that under similar conditions the
simpler organs are on the whole regenerated much more easily
than the more complex ones in any particular species. Even
in the human race, many simple tissues — such as the connective
substances, epithelia and nerves — can be repaired, and it is
only the cells of the glands and ganglia, which are the most
highly differentiated histologically, which are not replaced at
all, or at most only to a very slight extent. We can see from a
theoretical point of view that a far less complex apparatus is
required in these cases than in those which concern a regenera-
tion of entire parts of the body, such as the tail or limbs ; for it
is only necessary that the respective tissues should contain cells
which are capable of multiplying, in response to the stimulus
produced by the loss of substance in their immediate neighbour-
hood, and w^hich continue to do so until the loss is made good.
When, however, several kinds of cells take part in the restora-
tion, and a strict regulation as to their arrangement in groups,
their direction of growth, and rate of reproduction is required,
it becomes necessary for the individual cells from which the
restoration takes place to be accurately provided with supple-
mentary determinants of various kinds ; and it is clear that this
w'ill gradually become more difficult and complex, the greater
the complexity of the part to be regenerated, and the more
accurately all the details of its structure have to be preserved.

If, however, we review the facts known to us concerning
regeneration in animals of various degrees of organisation, we
meet with such marked differences even as regards the regenera-
tive power of homologous parts, that we cannot help receiving
the impression, which has affected all writers on this subject,
that in general the regenerative power is greater in less highly
organised animals than in those of a more complex structure.
The question thus arises as to how this view is to be intcrjircted
and presented in a scientific form.

Even in Vertebrates, certain facts seem to indicate that the

122 THE GERM-PLASM

'lower'' forms, as such, always possess the power of replacing
lost parts in a greater degree than do the higher ones. It is
true that the capacity for regeneration is certainly much slighter
in fishes than in the more highly organised amphibians ; but
althousfh the limb of a Triton becomes restored, and the fin of
a fish does not, it must not be forgotten that the physiological
importance of the two organs is somewhat unequal. On the
other hand, the fore limb of a Triton and the arm of a man are
not only homologous structures, but are also of almost equal
physiological importance, and yet their power of regeneration is
very unequal. We must therefore inquire into the causes of this
dissimilarity. The power of regeneration in any particular part
cannot depend only on the conditions which exist as regards the
species under consideration : it must also be due to arrangements
for reg^eneration which have been transmitted bv the series of
ancestors of this species. Leaving this question aside, and re-
garding the power of regeneration as merely depending in each
individual case on adaptation, we should arrive at some such
conclusion as the following : — the provision of the cells of a
certain part with supplementary determinants for the purposes
of regeneration, depends primarily on the liability of this part to
frequent injury — that is to say, on the degree of probability of
injury. Precautions are not taken for injuries which seldom
occur, even though these may be very disadvantageous to the
individual ; for the loss thereby resulting to the species as regards
the number of individuals would be extremely small and unim-
portant, and therefore processes of selection would not take
place in order to counterbalance this loss.

In the second place, Xho: physiological or biological importance
of the organ itself must be taken into consideration. A useless
or almost useless rudimentary part may often be injured or torn
off without causing processes of selection to occur which would
produce in it a capacity for regeneration. Thus, so far as is
known, the minute limbs of Siren and Proteus, which are often
bitten off, or not replaced ; while the gills of these animals and
of the Axolotl, which are equally liable to similar injuries, become
regenerated : — in the latter case the organs are physiologically
valuable, while in the former they are not. The tail of a lizard,
again, which is very liable to injury, becomes regenerated, be-
cause, as we have seen, it is of great importance to the individual,
and if lost its owner is placed at a disadvantage.

REGENERATION 1 23

Finally, the complexity of the individual parts constitute tlie
third factor which is concerned in regulating the regenerative
power of the part in question ; for the more complex the struc-
ture is, the longer and more energetically the process of selection
must act in order to provide the mechanism for regeneration,
which consists in the equipment of a large number of ditTerent
kinds of cells with supplementary determinants, which are ac-
curately graduated, and regulated as regards their power of
multiplication. Thus w^e can understand, for instance, why the
fore-limb of a Triton becomes regenerated, while that of a bird
does not, although the wing is of far greater importance and is
much more indispensable to its owner than is the fore-limb in
the case of the Triton. Although there are fewer bones in a
bird's wing than in a Triton's limb, the former is by far the
more complicated structure ; for it is covered with feathers, and
as each quill has a special size, form, and coloration, the wing
must contain a large number of special determinants in its for-
mative cells. These determinants must all be contained and
arranged in the germ-plasm, so that they can be passed on
during embryonic development through a certain series of cells,
— first into the outer germinal layers, then into the epidermis
of the fore-limb, and finally, by the agency of further series of
cells arising in the course of growth, to the region to which they
specially belong. It is difficult enough to imagine how the
distribution of the determinants can possibly take place in so
accurate and certain a manner as must be the case in reality, so
that not only the shape of the feather but even every speck of
colour on it is accurately repeated in every individual of the
species ; and it might well, indeed, be considered impossible that
the whole of this complex mechanism should also be capable of
becoming modified in such a manner, that the entire wing, with
all its feathers and patches of colour, could be regenerated
from a cut surface in any part. Did this occur, the cells of any
section of the wing would, according to our theory, have to con-
tain the whole of the determinants of all the cells required for
the construction of the portion of the wing distal to the cut sur-
face as supplementary determinants, in addition to their own
special idioplasm ; and moreover, these determinants must then
be distributed proportionately among the cells of the radial and
ulnar, and of the upper and under surfaces of the wing, and the
power of multiplication of each cell and its successors would have

124 THE GERM-PLASM

to be accurately adjusted. Although we cannot easily judge as
to what is possible in nature, and are so often impressed by the
discouraging conviction that many vital processes are still incom-
prehensible to us, we may perhaps in this case feel justified in
inferj'ing the impossibility of such an occurrence from the fact
that it does not take place ; that is, to infer that the regeneration
of a bird's wing is impossible on account of the complexity of
the mechanism required for it. ])ecause it does not actually
occur.

We cannot, however, regard this as a formal proof of the fact
that regeneration does not take place in this case. This would
be inadmissible, if only because the first of the three factors which,
as we have assumed, produces the mechanism of regeneration —
that is, the probability of loss — is not present. In the state of
nature, at any rate, a bird's wing is seldom injured without loss
of life ensuing at the same time. For this reason alone, selec-
tive processes in connection with a regenerative mechanism
could not be introduced. I have not brought forward the
above example for the purpose of proving the case for this
instance in particular, but because it seemed to me to be spe-
cially fitted to show how extraordinarily the complexity of the
regenerative mechanism must increase along with the greater
complexity of the part. But this brings us back to the consid-
eration of the general power of regeneration possessed by the
lower ^ in contrast to the higher, animals.

The supposition that this power exists, may, I believe, be con-
ceded in a certain sense : that is to say, in consequence of the
slighter complexity in structure of all the parts in one of the
lower groups of animals, any particular part may also become
capable of regeneration more easily than in the case of the higher
groups. We must, however, always presuppose that the two
other factors — the probability of injury, and the physiological
importance of the organ — are present in the required degree;
so that in speaking of the greater power of regeneration pos-
sessed by animals of a lower type, we are only using another
expression for the third factor which takes part in the process,
viz., the comple.xity of the organ to be regenerated.

The question, however, arises as to whether the capacity of
each part for regeneration results from special processes of adap-
tation, or whether regeneration occurs as the mere outcome —
which is to some extent unforeseen — of the physical nature of

REGENERATION I 2 5

an animal. Some statements which have been made on this
subject seem hardly to admit of any but the latter explanation.
Thus, according to Spallanzani, the jaw of a Triton may become
regenerated along with its bones and teeth. Bonnet states that
even the eye of this animal is replaced after it has been extir-
pated. It has never come before my notice that in the natural
state Tritons frequently lose the lower jaw in combat : but some
of these animals which I had put for a short time in a small
vessel attacked each other vigorously, and several times one of
them seized another by the lower jaw, and tugged and bit at it
so violently that it would have been torn off if I had not sepa-
rated the animals. The loss of part of the jaw or eye may
therefore occur not infrequently in the natural state, and we may
thus perhaps assume that these parts are adapted for regenera-
tion. Kennel, moreover, gives an account of a stork, the upper
beak of which had accidentally been broken off in the middle,
the lower one then being sawn off to the same length, and both
were subsequently regenerated. Such cases, the accuracy of
which can scarcely be doubted, indicate that the capacity for
regeneration does not depend only on the special adaptation of
a particular organ, but that a general power also exists which
belongs to the whole organism, and to a certain extent affects
many, and perhaps even all, parts. By virtue of this power,
moreover, simpler organs can be replaced even when they are
not specially adapted for regeneration.

From our point of view, such cases are not incomprehensible
in principle. We need only assume that in all, or at any rate
in many, of the nuclear divisions in the embryo, some of the
earlier determinants remain associated with later generations
of cells as accessory idioplasm. It only remains to trace this
arrangement — which is a more or less universal one, and affects
the whole body — to its origin ; for no arrangement can be pro-
duced wiiich is not useful, especially when it concerns such a
complicated mechanism as that for supplying the idioplasm with
accessory determinants. We are therefore led to infer that tJtc
general capacity of all parts for regeneration may have been
acquired by selection in the lower and simpler forms, and that
it gradually decreased in the course of phytogeny in correspond-
ence with the increase in complexity of organisatio7i ; but that it
may, on the other hand, be increased by special selectii>e processes
in each stage of its degeneration, in the case of certain parts

126 THE GERM-PLASM

which are physiologically i}>iportant and are at the savte iifne
frequently exposed to loss. In all probability this view is the
correct one.

3. Facultative or Polygenetic Regeneration

The tail of a lizard or the limb of a Triton grows again when
it has been cut off, but the part amputated does not reproduce
the entire animal. In some segmented worms, on the other hand,
such as iVais and Lunibriculus, not only does the amputated tail-
end become restored, but this end itself reproduces the anterior
part of the body, so that two animals are formed from one.

This fact evidently cannot be deduced merely from the
assumption we have made with regard to supplementary deter-
minants ; for were this the case, determinants of one kind only
— viz., those which are necessary for the construction of the lost
part — would be present in the cells. But in the above instances
the same cells give rise to entirely different parts, according to
whether they are situated on the surface which is anterior or
posterior to the plane of amputation : in the former case they
reproduce the tail-end, and in the latter the head-end. The
fact that both parts grow again when the worm is cut into two
through any region of the body, proves that regeneration in
either direction may proceed from the sa7)ie cells ; it therefore
follows that the cells situated in any particular transverse plane
of the body are not merely provided wuth the supplementary
determinants for the formation of the head- or tail-end only, but
every cell can react in either way, according to whether it is
situated anteriorly or posteriorly to this plane. In order there-
fore to explain the twofold action of these cells in accordance
with our fundamental view, — which presupposes that the cells
taking part in regeneration are arranged and controlled by the
forces situated within them, and not by an external agency, — it
seems necessary to assume that each cell possesses two different
supplementary determinants, one for the construction of the
head-end, and one for that of the tail-end ; and that the one or
the other becomes active according to whether the stimulus, due
to the exposure of the cell, is applied to its anterior or to its
posterior surface.

Before attempting to verify this assumption, I must mention
certain cases in which the regenerative activity of the cells may
even be threefold.

REGENERATION 1 2 7

It appears to me that tlie regenerative processes which have
been observed in the fresh-water polype Hydra and in the sea-
anemones {Aciijiice) are instances of this kind of regeneration.
If a worm is cut through in the median or anv other lon<ritu-
dinal plane, neither part grows again, and each soon dies. The
case is different in Hydra. If this animal is cut through
longitudinally, the two parts grow again into entire individuals,
irrespective of the plane of section. As the transverse section
of the animal at any point is likewise followed by the restoration
of each part, it follows that Hydra, in every part of the body,
must be capable of a threefold regeneration, i.e., of regeneration
in the three directions of space. And as the body is differently
constructed in these three directions, we are compelled to assume
that each cell contains groups of determinants of three different
kinds, viz., those which are concerned in the formation of the
proximal and distal ends, and in the completion of the body-wall.
An individual cell * must therefore be capable of dividing in
three different planes, and of giving rise to a part of one of three
different regions of the body ; and, moreover, the plane in which
division actually occurs, and consequently the kind of deter-
minants whicn become active and control the cell, is decided
not by the quality, but by the kind of division resulting from the
stimulus produced by the injury.

The processes of regeneration in Hydra can, I think, to a
certain extent be understood on this assumption. If. for in-
stance, the group of supplementary determinants of the proxi-
mal end of the body becomes active, it will cause the develop-
ment of linear rows of cells extendinor in the direction of the axis
of the body and united laterally so as to give rise to a tube ; these
cells, moreover, will have the tendency to close in towards the
centre as soon as possible, so as to form the disc or foot, and will
also cause the differentiation of the ectoderm cells of the foot
into glandular cells which secrete slime : the determinants for
the formation of tentacles are wanting in this group. If, again,
the group of supplementary determinants of the distal end
becomes active, rows of cells arise which will tend to close in to
form the oral disc, leaving a large space in the centre tor the
mouth. Tentacles will then grow out from certain points around

* I shall not refer to the histological details with regard to the process
of regeneration in Hydra, as the necessary data appear to be too uncertain
and incomplete.

128 THE GERM-PLASM

the mouth, and it is certainly not easy to explain why the de-
terminants which cause their formation become active at these
points only. It will, however, be shown later on that the
cells of Hydra — and probably those of all animal tissues
— are in a certain sense polarised; that is to say, they are
dififerently constituted in the three directions of space. The
fact that the determinants of the tentacles — which we must sup-
pose to exist in all regions of the body — only become active in
certain cells around the margin of the mouth, may be due to the
polarisation of the cells as well as to the peculiar conditions of
pressure within the cellular dome of the oral disc.

What has just been said can certainly not be looked upon as
anything more than the merest provisional explanation of the
facts, but it appears to me to be impossible to give a better one
at present. It nevertheless, I think, penetrates somewhat further
into the problem than does Herbert Spencer's hypothesis, in
which regeneration is compared in general to crystallisation,
and the capacity of arranging itself on every occasion under
the influence of the w-hole aggregate in the manner required
for the renewal of the missing part, is attributed to every ulti-
mate particle. If we take the fresh-water polypes alone into
consideration, one of these explanations seems just as good as
the other ; but if other groups of animals are included, it is at
once apparent that this capacity is not by any means always
possessed by the particles, but that even the cell may give rise
by regeneration sometimes to various parts of the whole aggre-
gate, at other times only to one certain part, and at others again
only to those similar to itself, and that it must therefore contain
something which makes it specially capable of one or of the
other kind of regeneration. This something is the group of
supplementary determinants.

If a polype or worm is cut through transversely, or if a loss of
substance is caused artificially in any organism, the conditions
of pressure previously existing in the cell in the region of the
injury become changed, the pressure previously exerted by
the lost part suddenly ceasing. This induces a change in the
vital conditions of the cells thus affected, which must have a
definite morphological and physiological result. We are unable
at present to state more precisely what this change is ; but as we
know that such losses of substance are followed by the multi-
plication of the cells, we may safely assume that it exerts a stimu-

REGENERATION

129

lus on the cell, and more especially on its idioplasm, which
forces the latter to undergo multiplication. This view is main-
tained by those who have the greatest opportunity of investi-
gating the details of such processes, — I refer to the pathological
anatomists. The proliferation which ensues in the surrounding
tissue after a loss of substance, is not explained by them as
being due indeed to a stimulus — in the ordinary sense of the
word — exerted on the surrounding cells, but rather to a cessa-
tion of the ' resistance to growthj and this may in one sense also
be described as a ' stimulus,'' inasmuch as it is an ' incitement '
to growth.

If the cells were constituted alike in the three directions of
space, the etTect on the idioplasm would be the same whether
the stimulus due to the loss of substance acts from before, from
behind, or from the side. One of the three groups of deter-
minants could not possibly be alone effected by the stimulus and
thus rendered active in one case, the second only in another, and
the third only in a third instance. We have, however, every
reason to suppose that the structure of one of these tissue-cells
is not the same in the three directions of space, and that they
are, in fact, variously differentiated according to each of these,
and consequently respond to stimuli in different ways according
to the direction in which the latter act upon them. Vochting *
has proved that at any rate in higher plants, ' a different upper
and lower, anterior and posterior, and right and left half, can be
distinguished in each living cell in the root and stem."' Portions
of the root of the poplar transplanted on to the stem, or por-
tions of the stem transplanted on to the root, only grew and
flourished when they were fixed in a certain position ; in the
reverse position they sometimes indeed grew, but soon showed
phenomena of degeneration. Vochting infers from this obser-
vation that the cells are ' polarised,' this term being taken
merely in an analogous sense to that in which it is generally
used. The root and stem behave in a certain sense like a
cylindrical magnet, which is composed of sections equall\- mag-
netised in the radial and longitudinal directions. Such a mag-
net, like the stem and root, may be separated into pieces. If
the smooth adjoining surfaces of the portions of the magnet are

* H. Vochting, ' Uber Transplantation am Planzenkorper,' Tubingen,
1889, p. 400.

130 THE GERM-PLASM

placed with their opposite poles as close together as possible, the
entire magnet is once more formed. Similarly, if the root of a
poplar is cut in half transversely, each half produces buds and
roots at the corresponding poles ; but if, on the other hand, the
two portions are joined together in the same relative position as
that which they occupied originally, they become united together,
so that a single piece of root, with its two poles, results, quite
similar to the original piece.

These important results which Vochting has obtained by his
experiments on transplantation, are mentioned in this place
because they can be utilised in considering the phenomena of
regeneration in animals, which have just been discussed. We
may in this respect compare a fresh-water polype with a poplar
root. After a Hydra has been cut in half transversely, the dis-
tal portion gives rise to a new foot at its proximal end. and the
proximal portion produces an oral region at its distal end. We
might therefore in this case speak of pedal- and oral-poles, instead
of root- and stem-poles, as in the case of the poplar. And, in
fact, if a Hydra is cut transversely into three portions, the distal
part or oral pole of the middle piece develops a new oral region,
and its proximal part or pedal pole gives rise to a new foot. It
might not be impossible for a clever experimenter to cause this
middle piece to unite with the two terminal portions of the body
before the former had had time to develop into a complete ani-
mal, by joining the three portions together with bristles. This
would result in a union just as in the case of the poplar.

It would be a mistake to try to deduce that one of the poles of
the poplar root must grow shoots and the other roots merely
from the fact of its polarisation : one might as well try to deduce
it from the fact of the polarisation of a real magnet. Something
more is required before this can take place : — the cells of the
poplar root must contain tJie primary constituents for thefor7na-
tion of shoots and roots ; that state of the cells which Vochting
describes as polarisation only produces the conditions under
which one or other of the primary constituents becomes active,
and thus undergoes development. The hypothesis of the polar-
isation of the cells does not, therefore, relieve us in the least
from the necessity of making a theoretical assumption to explain
how it comes about that the primary constituents of different
kinds of structures are present in one and the same cell.
According to my view, we must assume in the case of the poplar

REGENERATION T T^ I

root that the cells are provided with two different kinds of idio-
plasm, which remain inactive until the adequate stimulus arises
and causes the idioplasm of either the root or of the stem to
become active. In both cases the loss of substance must be re-
garded as the stimulus, and the direction in which it acts must
decide the quality of the reaction.

If the idioplasm of the tissue-cells were capable in itself of
responding to the effect of this stimulus by causing a regenera-
tion of the missing parts of the body, worms possessing the
regenerative power in a high degree, such as A'a/s and Luinbri-
chIus, would be capable of regeneration in a lateral as well as in
the anterior and posterior directions. This, however, as Bonnet
has previously proved, is not the case : when cut in half longi-
tudinally, the missing right or left half is not reproduced, and
the cells of these animals must therefore be wanting in that sub-
stance— viz., in the aiitinieral supplementary determinants —
which renders this kind of reproduction possible.

From our point of view, it is not surprising that these deter-
minants are absent in worms ; for in the natural state these
animals are never torn in half longitudinally, and there was
therefore no need for Nature to provide for such a contin-
gency.

If we consider that the groups of supplementary determinants
must become more complicated in proportion as the organism
and the part to which they give rise increase in complexity, we
can understand \s\\\ facultative regeneration only occurs in rela-
tively simple organisms, and that it apparently takes place in
three dimensions in Polypes and Flat-worms only, in two dimen-
sions in Annelids and Starfishes, and merely in one dimension
in Arthropods, Molluscs, and Vertebrates.

It must not be supposed that other factors do not also take
part in limiting the capacity for regeneration, — such as, in
particular, the vulnerability of the higher organisms, and the
fact that they are dependent on the circulation and tem-
perature of the blood, even apart from tlie influence of the
nervous system, of which we are practically still very ignorant.
The relatively small quantity of substance in the part removed,
as compared with that of the rest of the body, would also pre-
vent the amputated limb of a salamander, for instance, from
becoming regenerated into an entire animal. All these con-
siderations help to e.xplain why bi-dimensional regeneration —

132 THE GF.RM-PLASM

that is, regeneration in two directions — cannot take place in the
higher animals.

If, then, regeneration depends on the distribution of supple-
mentary determinants to certain cells, which occurs whenever it
is necessary or possible, the process must be primarily traceable
in the case of the Metazoa to the doubling of the ids in a certain
ontogenetic stage. And since a division and doubling of the
idants takes place in every mitotic nuclear division, this hypothesis
is supported by actual fact, even although we are still far from
being acquainted even with the general details of the processes
of growth and doubling of the ids and determinants, not to
mention the systematic transference of such inactive determi-
nants to definite cells and series of cells. Here again, however,
Nature will have caused an advance from the simple to the more
complex ; and it therefore follows that, just as complicated
organisms could onlv arise in the course of innumerable series
of generations and species, so also the complex apparatus for
regeneration in the tail or limb of a newt could not have been
developed suddenly, but must have arisen in consequence of
similar modifications in innumerable ancestors.

It might be possible to picture to one's self approximately the
series of modifications which the apparatus for regeneration has
gradually undergone, beginning at the lowest multicellular
forms, and passing upwards to those animals in which the power
of regeneration is the most highly developed and complex. I
shall not, however, attempt to do so. At some future date it
may perhaps be found that differences occur as regards the
number of ids contained in the cells of those which have, and in
those which have not, a marked capacity for regeneration : it
will not be worth while to trace in detail the courses which the
development of the power of regeneration has taken, until our
knowledge of the idioplasm is sufficiently complete to furnish a
basis for the theory in fact.

4. Regeneration in Plants

The process which may be described as regeneration in the
case of the lower plants — the algae, fungi, and mosses — will be
treated of in greater detail subsequently. In this place, I merely
wish to point out that true regeneration only occurs in a very
slight degree in all the higher plants which are regarded as

REGENERATION

133

cormophytes or plant-stocks. If a piece is cut out of a
leaf of a tree or of any otiier Phanerogam, the leaf does not
become regenerated. If, again, an anther or a stigma is cut off
from a flower, the corresponding filament or style will not give
rise to a new anther or stigma. The cells of these organs are
therefore not adapted for regeneration, and do not contain
' supplementary determinants.'

Botanists might be inclined to explain this fact by supposing
it to be due to the cells having already reached their full size,
and having therefore lost their power of multiplication. This is
certainly the case, but it does not explain matters in the sense
I mean : the question still remains as to why these cells have
not been provided with supplementary determinants. The large
number of cases in which adult cells of leaves or other parts,
W'hich have reached their full size, may under certain circum-
stances begin to multiply, and form buds from which entire
plants arise {e.g., Begonia), proves that such a provision is
possible.

The solution of the above problem is to be sought for in the
fact that it would have been of far too slight importance to the
plant to be able to restore such defects in its leaves, as it pos-
sesses the power of producing new leaves. Buds can be
formed and undergo further development in many parts, and
thus the plant gains much more than it could possibly do by
mere regeneration. Regeneration can be dispe)ised wit/i, as
the far 7nore i)nporta7it power of budding is possessed by the
plant.

The fact that the higher plants are unable to restore such
parts as portions of leaves, furnishes an additional important
proof that regeneration is dependent on external circumstances,
and that it is a phenomenon of adaptation. True regeneration,
however, occurs in those cases in which the losses or injuries
would be harmful to the plant, and cannot be made good by the
development of buds. Thus a loss of substance in the bark of a
tree becomes replaced by the formation of callus, which arises
from the edges of the wound, and grows over it, and thus the
underlying wood is protected from injury. The cut or broken
surface of a branch, even in the case of many herbaceous
stems, becomes covered over in a similar manner by a mass of
proliferating callus, which may even give rise to new growing
points of shoots and roots, and thus become the place of origin of

134 THE GERM-PIJ^SM

new individuals.* The stimulus to proliferation, as in the case of
regeneration in animals, is due to the removal of the opposition to
growth ; the cells must, however, be adapted for this reaction, other-
wise the proliferation cannot take place ; the stems as well as the
roots and veins of herbaceous plants do not by any means always
respond to an injury by the formation of callus. This process is
therefore not a primary quality of the plant, but an adaptation,
due, in my opinion, to the association of certain supplementary
determinants with the active idioplasm of certain kinds of cells.
The formation of callus is probably the only process in plants
which can be regarded as an actual regeneration.

5. Regeneration in Animal Embryos, and the
Principles of Ontogeny

The theory of heredity which has now been formulated, — and
more especially that portion of it which concerns the composition
of the germ-plasm out of determinants, and the gradual disin-
tegration of the mass of determinants in the germ-plasm during
the course of ontogeny, — is based on the assumption that the
cells co)itrol the)nselves : that is to say, the fate of the cells is
determined by forces situated within them, and not by external
^influences. The primary cells of the ectoderm and of the endo-
"derm arise by the division of the fertilised egg-cell and its con-
tained germ-plasm, because the determinants of the ectoderm
are passed into one cell and those of the endoderm into the
other, and not because some external influence, such as the
force of gravity, aff"ects the cells in a diiTerent manner. Simi-
larly a certain cell in a subsequent embryonic stage does not
give rise to a nerve-, a muscle-, or an epithelial-cell because it
happens to be so situated as to be influenced by certain other
cells in one way or another, but because it contains special
determinants for nerve-, muscle-, or epithelial-cells.

This conception of the predestination of the individual cells,
the fate of which, together with that of their successors, is deter-
mined by the idioplasm they contain, was first imperfectly ex-
pressed in the theory formerly propounded by His,t in which he

* J. Sachs, ' Lectures on the Physiology of Plants,' Leipzig, 1882, p. 709.
(English edition, translated by H. Marshall Ward, Oxford, 1887.)

t Wilhelm His, ' Unsre Korperform u. das physiologische Problem ihrer
Entstehung,' Leipzig, 1874.

REGENERATION

135

formulated the existence of "special regions in the germ, which
give rise to special organs.' His imagined that the • primary
constituents of the organs of a chick were present in superficial
extension in the germinal disc,' i.e. in the cell-body of tlie ovum,
and that each organ is therefore represented by a definite part
of the body of the tgg. As has already been mentioned in the
historical introduction, subsequent investigations, made in the
course of the following ten years, proved that the ' primary con-
stituents ' of the various structures are to be found in the nuclear
substance. The special form in which His expressed his views
was thus certainly contradicted, although the fundamental prin-
ciple of his theory was not thereby aftected in its general sense,
which indicates that the differentiating principle of ontogeny is to
be looked for in the cells themselves, and not in external influ-
ences. Wilhelm Roux* was the first to prove definitely that the
differentiation of the ^g% into the embryo is certainly not caused
by influences existing apart from the egg, but that it is due to
causes originating in the egg itself. Pflugerf showed with regard
to the ovum of the frog, that whatever position the egg is forced to
take up the upper side always gives rise to the animal pole of the
embryo, and it was thought that this must be due to the force of
gravity. Roux, however, proved that frogs' eggs which are
rotated slowly in a vertical direction, develop just as well as
those on which the force of gravity is not interfered with. It
has further been proved by Born \ that, although when an ^g'g
undergoes development in a fixed position the substance of the
cell-body does not become displaced at first, the nucleus never-
theless changes its position, for it very soon passes to the upper
pole of the ^gg^ at Avhich point development then begins.
These observations undoubtedly proved that the formative
forces are situated in the egg itself; but they still left it unde-
cided whether the differentiation of the ovum is due essentially
to the action of the individual cells alone, — tliat is to say.
whether differentiation occurs independently in each individual
cell, so that it would, if necessary, be capable of passing through

* Wilhelm Roux, ' Beitrage zur Entwicklungsmechanik des Embryo,'
Miinchen, 1885.

t Pfluger, ' Ueber den Einfluss der Schwerkraft auf die Thielung der
Zellen u. auf die Entwicklung des Embryo,' Arch. f. Physiol., Bd. xxxii..
1883, p. 68.

X Born, ' Biologische Untersuchungen,' (I.) Arch. f. mikr. Anat. Bd. 24.

136 THE gp:rm-plasm

its prescribed course of development apart from the rest of the
embryonic cells, — or whether the various cells of the embryo
become differentiated by their mutual interaction : or, in other
words, whether a determinating influence is to a certain extent
exerted by t)ie luhole on its parts and thus prescribes the fate of
the various cells.

The experimental proof of the self-differentiation or predis-
position of the individual cells was, I believe, furnished by
Roux,* whose ingenious experiments are always accompanied
by keen deductions. Roux destroyed a single segmentation-
cell in each of a series of frogs' eggs by means of a hot needle,
and then observed that eggs treated in this manner developed
into • half or three-quarter embryos,' that part being absent
which corresponded to the cell thus destroyed. When one
of the first two segmentation-cells was demolished, half of the
embryo was formed, and this corresponded either to a lateral
or to the anterior or posterior half, according to whether the
first segmentation had resulted in a division of the hereditary
substance into portions belonging to the right and left, or to the
anterior and posterior halves. The process of segmentation in
the frog is known to vary in this respect. When one of the first
four segmentation-cells was destroyed, three-quarters of the
embryo was formed.

These experiments must be regarded as affording a proof of
the self-differentiation of the cells. Observations have since been
made which seem to contradict this deduction ; and although
these are still incomplete, and can only be regarded as the
preliminaries to more detailed investigations, they must not be
passed over in silence, especially as I am convinced that they
do not really contradict the hypothesis of the self-determination
of the cells.

Chabry's f experiments on the eggs of Ascidians must be
mentioned first. By means of a special apparatus, he destroyed
one of the first two segmentation-cells, and then observed that
the remaining cell continued to develop, and eventually gave
rise, not indeed to half an embryo, but to an entire one of
half the normal size. Such embryos were certainly not quite

* Wilhelm Roux, ' Beitrage zur Entwicklungsmechanik des Embryo,*
(V.) Virch. Arch. Bd. 94.

t L. Chabry, ' Embryologie normale et t^ratologique des Ascidies,' Paris,
1887.

RF.GEN ERATION I 3 7

perfect, but only organs of slight importance were wanting in
them. Chabry himself has drawn no theoretical conclusions
from his observations ; Driesch,* however, has made certain
deductions from a series of similar experiments on the eggs of
Sea-urchins. By continued shaking, Driesch effected a mechani-
cal separation of the two first segmentation-cells, and observed
that at first each of them continued to undergo further segmen-
tation just as would occur in the entire egg, but that later on the
resulting //£?;;//-blastula became completed to form an entire one.
In some of these hemi-blastulas development proceeded still
further, the invagination taking place to form the primary diges-
tive cavity of the gastmla, so that eventually a rudimentary
pluteus-larva — which, though small, was in other respects nor-
mal— could be recognised.

Driesch sums up his results in the following words : — ' These
experiments therefore show that under certain circumstances
each of the two first segmentation-cells of EcJiinus viicro-tiiber-
culatiis can give rise to a larva of the normal form, which is
entire as regards its shape ; and that a partial forviation, and
not a semi-formation^ occurs in this case.' The author con-
cludes that his results '- fundamentally disprove the existence of
special regions in the germ which give rise to special organs,'
and adopts the following view stated by Hallez f : — 'II n'est pas
des lors permis de croire que chaque sphere de segmentation doit
occuper une place et jouer un role, qui lui sont assignes a Tavance.'

Although I am far from wishing to assert that we are at
present in a position to give a perfectly reliable and detailed
explanation of the extremely interesting and important results
of the experiments just described, I nevertheless cannot help
thinking tliat they do not in the least necessitate the giving
up of the view which entails a predestination of the individual
segmentation-cells, and, in fact, of cells in general. Other than
experimental methods may lead us to fundamental views, and an
experiment may not always be the safest guide, although it may
at first appear perfectly conclusive. Even Driesch himself
doubts whether the above-mentioned experiments made by
Roux are really conclusive, though, in my opinion, he is wrong

* H. Driesch, ' Entwicklungsmechanische Studien, Zeitschrift f. wiss.
Zoologie,' Bd. 53, 1891.

t Hallez, ' Recherches sur rembryologie des Nematodes,' Paris, 1885.

138 THE GERM-PLASM

in doing so : he asks, in fact, whether the uninjured segmenta-
tion-sphere of the frog would not behave exactly in the same
manner as that of the sea-urchin if it could be actually isolated,
instead of remaining in close connection with the other injured
sphere. Thus even the apparently incontrovertible result of this
experiment may be doubted.

It seems to me that careful conclusions, drawn from the
general facts of heredity, are far more reliable in this case than
; j are the results of experiments, which, though extremely valuable
and worthy of careful consideration, are never perfectly definite
and unquestionable. If what was said in support of the theory
of determinants in the first chapter of this book be borne in
mind, the conviction that ontogeny can only be explained by
evolution, and not by epigenesis, seems to force itself upon us.
It would be impossible for any small portion of the skin of a
human being to undergo a hereditary and independent change
from the germ onwards, unless a small vital element correspond-
ing to this particular part of the skin existed in the germ-sub-
stance, a variation in this element causing a corresponding
variation in the part concerned. Were this not the case, '■ birth-
marks ' would not exist. If, however, determinants are contained
in the germ-plasm, these can only take part in controlling the
formation of the body if, in the course of embryogeny, they
reach those particular cells which they have to control, — that
is to say, if the differentiation of a cell depends primarily on
itself^ and not on any external factor.

If therefore ontogeny is not, as Roux aptly expresses it, a
< new formation ' of multiplicity, or an epigenesis, but is merely
the unfoldi7ig of multiplicity, i.e. an evolution, — or, as it might
also be called, the appeaj-ance of a previously invisible multi-
plicity.,— the principle of self-determination is certainly only
established with regard to the egg as a whole: the self-deter-
mination of each cell, and its control of ontogeny, do not neces-
sarily follow from this conclusion. We can only thereby arrive
at the very simple assumptions, that the primary constituents of
the germ-plasm are distributed by means of the processes which
can actually be observed in the nuclear divisions, so that they
come to be situated in those regions which correspond to the
various parts of the body, and that those primary constituents
are present in each cell which correspond to the parts arising
from it.

REGENERATION I 39

As has just been shown, it is also possible to make the reverse
hypothesis, and to suppose that ahhough the whole of the idio-
plasm is contained in each cell, only that particular primary
constituent which properly concerns the individual cell has any
effect upon it. The activity of a primary constituent would thus
depend not on the idioplasm of the cell, but on the influences aris-
ing from all the cells of the organism as a whole. We should
thus have to suppose that each region of the body is controlled
by all the other regions, and should therefore practically be
brought back to Spencer's conception of the organism as a
complex crystal. This simply means giving up the attempt to
explain the problem at all, for we cannot form any conception
of such a controlling influence exerted by the whole on the
millions of different parts of which it consists, nor can we bring
forward any analogy to support such a view, the acceptance of
which would render a great number of observations on the
phenomena of heredity totally incomprehensible. What ex-
planation, for instance, could be given of the fact that a certain
human birthmark is always inherited on the left side only ?
According to this hypothesis, the germ-plasm contained in the
cells of this region would be present on the right side just as
much as on the left : as the two halves of the body are alike
in other respects, we cannot suppose that the whole aggregate
exerts different influences as regards this region on the left and
on the right sides.

It seems to me, therefore, that we must not give up the hy-
pothesis of the self-determination of the cells, in spite of its
apparent refutation by the facts described by Chabry and
Driesch. Moreover, I think these facts can be explained — in
principle at any rate — in another manner, viz., by attributing the
processes observed to regeneration^ the arrangement for which,
however, has not been provided for the first stages of segmenta-
tion, but for a later period of ontogeny.

It is hardly to be expected that the first stages of segmenta-
tion should be in a sense purposely arranged for regenera-
tion. Both in Ascidians and sea-urchins the number of eggs
produced is so large, that it probably matters very little whether
a segmenting ovum perishes or becomes regenerated when one
half of it has been eaten by a small enemy. 1 do not, however,
wish to do away entirely with the idea that the eggs of certain
animals may conceivably be protected in this manner trom

140 THE GERM-PLASM

numerous enemies, but in this place I must refrain from includ-
ing such a possible occurrence in the argument.

The following explanation of the phenomena, however, still
remains. The first division of the ovum separates the group of
determinants into two, viz., that for the right and that for the
left half of the body ; each of these groups does not constitute
a perfect germ-plasm, as each determinant it contains is not
doubled : but it is very probable that the ids are capable under
certain circumstances of dividing in such a way that each be-
comes doubled. Such a germ-plasm could not contain /;/
potentia a birthmark, or any other asymmetrical peculiarity of
the other side of the body, but it would be able to give rise to
a complete animal. The destruction or mechanical removal of
one segmentation-cell in the first stage of segmentation may be
the primary cause of the doubling of the ids in the other cell.

The capability of becoming doubled, which the undivided
germ-plasm possesses in certain cases, may be mentioned in
support of this view of the regeneration of an isolated cell in the
first stage of segmentation. The fact that in each integral
division of the cell and nucleus, a longitudinal splitting of the
nuclear rods and their contained macrosomes occurs, shows
that the ids are as a rule capable of growth and of doubling
their number by division. The assumption of a doubling of the
ids of germ-plasm must be made in dealing with the origin
of identical twins, i.e. those twins in which we must suppose
that the division of the nucleus of the ovum from which they
arise occurs after and not before fertilisation ; for otherwise the
embryos could not be identical, as two spermatozoa would then
take part in the process. In the case of facultative partheno-
genesis, a doubling probably also occurs in the ids and idants
of the ovum, half of them having previously been removed
by the 'reducing divisions.'

The formation of an entire embryo by the regeneration of one
of the two first blastomeres admits, however, of another inter-
pretation. Ascidians multiply very freely by budding, and not
only by sexual reproduction. It is true that this is not the case
with sea-urchins, but the power of regeneration which these
animals possess is unusually great. This fact was explained in
the present chapter by assuming that certain idic stages of
ontogeny are provided with an ' accessory idioplasm,' consisting
of the determinants required for regeneration. In a subsequent

REGENERATION 1 4 r

chapter I shall have occasion to show that we must make a
similar assumption in the case of budding. Such assumptions
are indispensable if we accept the hypothesis of the germ-plasm
and determinants. The accessory idioplasm required for
budding causes the reproduction of the entire animal, and must
therefore contain all the determinants of the germ-plasm, and
must exist in the ovum before segmentation, remaining in a
latent condition in a definite series of cells during all the stages
of development. If now this accessory idioplasm were capable
of becoming active under certain abnormal influences, — such as
that produced by the destruction of the other blastomere, — a
regeneration of the whole embryo might thus result.

These explanations are, however, only possible ones, and I
should not have been sorry to leave them out of consideration
altogether, for I am fully aware of their incompleteness and
unreliability : I merely wish to show that the observations men-
tioned above do not render an explanation impossible, even
although we are not able at present to state that any particular
interpretation of the phenomena is the correct one, because the
observations themselves are far too incomplete and deficient.
For this reason I shall not attempt to give a more precise
explanation of the peculiar development of these embryos.

I must, however, draw^ attention to the different behaviour of
the eggs in the case of the frog and in that of Ascidians and
sea-urchins. Leaving aside the question of 'post-generation,'
w'e have seen that only half an embryo arises from o)ie blasto-
mere of a frog's ovum, while an entire animal becomes developed
from one blastomere in the case of either of the other two ani-
mals mentioned. However imperfect the explanation I have
offered may be, the fundamental assumption on which it is
based must in general be a correct one, — viz., that the first
blastomeres of the ^^^^ of an Ascidian or sea-urchin must possess
a capacity which is absent in the case of the frog's ^g%- As,
however, forces are dependent on substances, it is probable
that the blastomeres of an Ascidian and of a sea-urchin contain
an excess of substance — the accessory idiopiasjii — which gives
them the power of regeneration, and that this substance is want-
ing in the blastomeres of the frog. Driesch, as already mentioned,
expresses a doubt as to whether the blastomere of a frog would
not behave in a similar manner to that of a sea-urchin, if, like
the latter, it could be completely separated and isolated from its

1^2 THE GERM-PLASM

injured fellow blastomere. This doubt seems, however, to be
hardly justified, as such an isolation was not effected in Chabry's
experiments on the ascidian ovum, but nevertheless the develop-
ment into a complete animal ensued just as in the case of the
egg of the sea-urchin.

Although the half of a frog's egg develops into half an embryo
only in the first place, the latter may subsequently become com-
pleted by a very peculiar regenerative process, which was first
observed by Roux in 'half and 'three-quarter embryos/ and
which he designated as 'post-generation/

Roux observed that a segmentation-cell of a frog's egg may be
' re-animated ' after it has been deprived of its capacity for devel-
opment. A considerable number of nuclei pass into the vitellus
of the injured part from the normally developed half of the egg,
and there increase and give rise to cells. ' The post-generative
formation of the germinal layers takes place from the cell-material
subsequently formed, while the process of differentiation contin-
ues to advance in the quiescent cell-material.' Roux thought he
observed that a complete restoration of the embryo may take
place in this manner, so that it can continue to live ; and, in
fact, he actually succeeded in keeping such an embryo alive for
some time.

Considerable attention has naturally been drawn to these
observations, which are certainly of the greatest interest ; but
I doubt whether in their present state they are sufficiently com-
plete to form the basis of fundamental theoretical conclusions.
With all respect for Roux's accuracy of observation and skill
in research, I cannot help thinking that the half embryos which
were subsequently ' post-generated ' to entire animals, were pos-
sibly those in which the thrust with the hot needle had not
affected the nucleus of the segmentation-cell.

In any case, it was only possible to observe the actual effect
of the operation and its result on the whole series of processes
which followed, and which led to the restoration of individuals
ot/ier than those which ultimately became complete. To pierce
a segmentation-cell with a hot needle must be a tolerably rough
operation, and something different may be destroyed each time
it is performed : not only the nuclear matter as a whole, but
also the individual idants, might possibly remain uninjured.
The idants, again, might subsequently increase to the normal
number by doubling, and so bring about the development of the

REGENERATION 1 43

half of the egg. Roux certainly states that * post-generation '
does not occur in the same manner as does the normal develop-
ment of the two primary halves, — that is to say, the germinal
layers are not formed independently in each ; but the processes
which take place in the interior of the ovum can only be followed
out by means of sections, the preparation of which necessitates
the killing^ of the embrvo.

In such experiments, moreover, no two cases are alike, and it
would be necessary to examine a very large amount of material
before stating with any degree of certainty that the egg which
has been cut into sections, and that in which the development
and post-generation have been followed out, have a precisely
similar internal structure.

Roux observed a ' re-animation ' of three different kinds in the
halves of the eggs operated upon, one of which consisted in a
growth of the cells in the external layer of the living half around
the dead half. In this instance, however, post-generation did
not result : it only occurred in certain, but not all, of those cases
mentioned above in which nuclei passed from the living half
into the part which had been operated upon, and in which only
sligJit pathological cJianges Jiad occurred in the yolk. It is there-
fore natural to suppose that post-generation only occurred when
the injury was a slight one, and when some nuclear matter
remained and subsequently caused a formation of cells. This,
however, does not imply that living 'nuclei' did not penetrate
into the injured half of the tgg ; the segmentation-cells, even in
normal development, have to undergo an enormous increase,
and it is therefore not surprising that after the opposition to
growth has been removed by the operation on the other half of
the tgg, they should increase at the expense of the latter. In
those cases in which the other half of the embryo was subse-
quently completed, this completion must have resulted from a
kind of infection of the cell, of such a nature that mere contact
with ectoderm or mesoderm cells, for example, caused the undif-
ferentiated cells of the injured half of the egg to become corre-
spondingly differentiated into ectoderm and mesoderm cells.
But I could only accept such a revolutionary hypothesis as this
if it could be proved by incontestable facts. -

Roux himself has, however, only looked upon his contribu-
tions to this subject as 'a first instalment of a large work,' and
has led us to expect a continuation of his experiments. But as

144 'J'HE GERM-PLASM

long as the processes which he describes admit of more than
one interpretation, we cannot reject the hypothesis of the pre-
destination of the cells by means of the distribution of certain
determinants and groups of determinants to them, for this view
is supported by so large a number of facts, and even by the
earlier experiments of Roux himself. It would certainly, how-
ever, have to be rejected if we could prove that the cells of the
germinal layers were really capable of being determined in their
nature by the region which they accidentally reach, or by their
accidental surroundings.

Further research along the line opened up by Roux will, I am
convinced, show us the facts in another light, and will enable us
to reconcile them to the rest of our conceptions as to the causes
of ontogeny. But I do not consider it worth while at present
to enumerate all the possible causes which must be taken into
account in an attempt to explain 'post-generation.'

MULTIPLICATION BY FISSION 1 45

CHAPTER III
MULTIPLICATION BY FISSION
I. Preliminary Remarks

Until a short time ago the process of muhiplication by gem-
mation was looked upon as having been derived phyletically from
the corresponding process by fission, and the two were thought
to be closely related, and connected by gradual transitions. Von
Wagner * has, however, recently attempted to contest this
opinion, and to show that the two processes should be more dis-
tinctly separated from one another than they have hitherto been,
and that they are, in fact, genetically distinct. By the term
(fission), Wagner means to indicate a process of multiplication
which is preceded by a symmetrical growth of the parent
organism, by means of which the individuality of the latter be-
comes changed and to a certain extent abolished : the term
(gemmation), on the other hand, he takes to mean a process of
multiplication which is preceded by an unsymmetrical (differ-
ential) growth of the parent organism, in which the individuality
of the latter is not abolished and its place taken by a new
individual.

This view I agree with in so far as I am convinced that in
multicellular organisms the processes of multiplication by fission
and budding have not arisen genetically from one another : these
processes differ so essentially that it will be advisable to discuss
them separately.

Following von Wagner's example, I shall include under this
head of fission all the processes of asexual multiplication which
occur in the flat-worms {Turbellaria, Cestodd), the annelids
proper {Syllidce, NaidcE, TubificidcE^ &c.), and also in the higher
Medusae (strobilation) . In all these cases multiplication is
effected by the division of the parent animal into two or more

* Franz von Wagner, 'Zur Kentniss derungeschlechtlichen Fortpflanzung
uon Microstoma nebst allegemeinen Bemerkungen iiber Tliielung u.
Knospung im Thierreich," Zool. Jahrbiicher, Abth. f. Anat. u. Ontogenie.
Bd. iv., Jena, 1890.

T46

THE GERM-PLASM

Fig. 4. — Myriatiida, a marine worm which
multiplies by fission (after Milne-Edwards,
from Hatschek's ' Lehrbuch der Zoolo-
gie'). The letters a— ^ indicate the rel-
ative ages of the daughter-individuals
resulting from the division of the parent.

parts : this therefore neces-
sitates a regeneration of one
or of the other end of the
body, or even of both ends.
This process may begin to
take place either after {Lum-
brkulus^ or before the divis-
ion has taken place, and in
the latter case is more or
less complete before the fis-
sion begins. The actual
process of the formation of
the new organism is essen-
tially the same in both cases,
and important differences
only occur as regards the
various groups of animals.

We are particularly well
acquainted with these pro-
cesses of regeneration —
which may either precede

or succeed fission — in the
case of various kinds of worms, and we will therefore first illus-
trate them in their main features by reference to these animals.

2. The Process of Fission in the Naid^

The process of fission in these small fresh-water segmented
worms has been very accurately followed out by Semper. An
individual undergoes division into two, or usually into several,
daughter-individuals at the same time, the fission being regularly
preceded by a circular growth of cells taking place around the
circumference of the body at one or more definite regions, each
of such ring-like thickenings eventually giving rise to a new
head- and tail-end respectively. These rings of cells have
hitherto been spoken of as ' zones of gemmation ; ' but it would
be better to call them ' zones of regeneration,' as they are not
concerned with budding in the true sense of the word. Two of
these rings are as a general rule formed in each animal, and
when the anterior and posterior ends of each of the resulting
three sections are fully developed, the separation into the corre-

MULTIPLICATION BY FISSION

147

spending three daughter-individuals takes place by a construction
in the middle of each zone of regeneration.

In Nats the zones of regeneration are always formed at the
boundary line of two segments : that is to say, they arise from
the contiguous margins of two segments, in the following way.
Cells of the epidermis first begin to multiply, and give rise to a
circular layer of small stratified cells, which is thickest on the
ventral side. The cells have at first no definite histological
character. At the same time an increase in the length of the
internal organs takes place : this is rendered necessary by the
growing zone penetrating between the segments from which it
arises and thus forcing them apart. The alimentary canal, how-
ever, is the only internal organ which becomes regenerated from
its own cells : all the other new formations, including the ventral
nerve cord, muscles, blood-vessels, 'liver '-cells, and excretory-
organs, are developed from the ring of proliferating epidermic
cells.

As Semper has pointed out, the process of the reconstruction
of the anterior and posterior ends which prepares the way for
fission, may in a sense be compared with the embryonic develop-
ment of the animal subsequent to the gastrula stage, in which
the two primary germinal layers are already distinct.

In these regenerative processes two layers of formative cells
are likewise produced, owing to the proliferation of the ectoderm
cells on the one hand, and those of the alimentary canal on the
other ; the epithelial lining of the latter only is formed from the
internal layer, the outer layer giving rise to all the other organs,
including the mesodermic structures as well as those which
belong to the ectodermic part of the integument. In fact the
resemblance between the processes which take place in embry-
ogeny and in regeneration is so close, that in both cases the
mesoderm becomes split oiT from the mass of formative ectoderm
cells in the form of two longitudinal bands, from which the
blood-vessels, muscles, Slc, are then dilTerentiated.

In order to explain these processes theoretically from our
point of view, we must suppose that those cells of the epidermis
from which the formative cells arise possess an *• accessory idio-
plasjtu^ containing the determinants of those organs which are
formed from them in regeneration in addition to their own
specific idioplasm. The rate of division of each of these cells,
as well as the manner in which the groups of determinants con-

148

THE GERM-PLASM

tained in them becomes disintegrated in the course of the sub-
sequent divisions, is strictly definite, and determines the number
of successors which each cell produces, as well as the relative
position and combination into organs, and histological differen-
tiation of the cells. When the process of proliferation begins,
the newly-formed cells no longer retain the specific epidermic
character, and their successors may, indeed, be said to possess
an 'embryonic character,' in the sense in which that term has
usually been used, — if it is not thereby understood that they
must all contain similar primary constituents. Their further
development shows that this cannot be so : the cells of one
particular region give rise, for instance, to the dorsal vessel ;
those of another to the nerve-cord ; those of a third region to
certain muscles, and so on.

We must therefore suppose that the various epidermic cells

of the parent animal are pro-
^ vided with active accessory

idioplasm, somewhat in the
manner I have indicated in
the accompanying diagram.
The cells marked «, for in-
stance, would contain the
groups of inactive determi-
nants for the formation of the
ventral nerve-cord ; those of
the epidermis marked m^
groups of determinants for
the mesoderm structures, in
addition to their own proper
idioplasm, — )n^ containing
those for the lateral muscles,
in- those for the ventral
blood-vessel, nt^ those for the
'liver-cells' and mesodermic
part of the intestine, ;//■* those
for the segmental organs and the adjacent system of muscles,
ifv" those for the dorsal vessel, in^ those for the dorsal muscles,
and so on. For the sake of simplicity I have supposed that the
epidermis consists of only one layer of cells, although in reality
there are two layers in many parts : the diagram, in fact, is not
by any means intended to represent the actual structure of the

IFtrtT-'

Enc

*n»-'. J

Fig. 5. — Transverse section through a Nais
in the region of the zone of regeneration
(modified from Semper) : Ekt, Integu-
ment; Ent, Epithelium of the ali-
mentary canal; N, Nerve-cord; Ms,
Visceral mesoderm; Vd, Dorsal blood-
vessel; in. Cells with accessory deter-
minants for the mesoderm; n. Cells
with accessory determinants for the
nerve-cord.

MULTIPLICATION BY FISSION 1 49

animal in detail, or even to indicate accurately the part which
the individual cells take in the process of budding.

The question of the origin of t/ie supple))ie7itary deterininaiits
which we have assumed to exist in the cells of the epidermis,
does not stand in the way of this explanation of the regenerative
process ; for, as already stated, a similar course is in the main
followed both in embryogeny and regeneration. In both pro-
cesses the primary mesoderm arises from the primary ectoderm.
The definitive ectoderm cells have therefore an opportunity dur-
ing their embryonic development of taking over certain primary
mesoderm determinants as accessory idioplasm from the primary
ectoderm cells : these can then become separated into several
groups during the multiplication of the ectoderm cells, so that
the epidermic cells around the circumference of the body are
provided with an accessory idioplasm consisting of various meso-
derm determinants.

It must, however, also be borne in mind that the growth in
length of the worm only takes place at the posterior end of the
body, just as occurs in the regeneration which prepares the way
for division. In both cases a new body-segment is formed be-
tween the last segment and the last but one, in which process
the epithelium of the intestine alone arises from the endoderm,
the integument and all the mesodermic structures being formed
from the ectoderm. Thus the accessory determinants which we
have assumed to exist in the epidermic cells, and which render
the subsequent regeneration possible, are not derived from the
embryo directly, but from the zone of growth in the tail-end,
into which again they have passed during embryogeny.

3. The Process of Fission in the.Microstomid^

It is not, then, in the nature of every ectoderm cell to give rise
to all the possible kinds of cells and organs with the exception
of the epithelium of the alimentary canal : each one must be
specially equipped for the purpose. This is proved by the fact
that the ectoderm by no means always performs this function in
animals which multiply by fission : even in some worms this is
not the case.

According to von Wagner's * excellent researches, the cells
of the epidermis in a certain flat-worm, Microstoma linearc, take

* Loc. cit.

150 THE GERM-PLASM

a very small share in the reconstruction of the anterior and
posterior ends of the animal during the process of fission, the
restoration being effected in this case chiefly by the mesoderm,
or so-called ' connective tissue ' cells, which ' are suspended in
large numbers in the perivisceral fluid between the supporting
trabeculae/ These cells begin to increase in number when the
animal is preparing for division, and by their multiplication
they form a ventral mass of so-called ' embryonic ' cells, which
gives rise to the pharynx, the pharyngeal and prostomial glands,
all the parts generally known as " parenchymatous ' or ' meso-
dermic structures,' and also apparently to certain parts of the
nervous system. Kennel * found that a similar mode of develop-
ment of these parts occurs in a Planarian. In such cases we
must therefore suppose that the accessory determinants required
for regeneration are supplied to the mesoderm cells, instead of
to those of the ectoderm. We cannot at present determine
whether this is effected by each of these cells being provided
with all the supplementary determinants for the mesoderm,
which only become disintegrated and distributed amongst the
other cells when these begin to multiply, or — as we assumed in
the case of the ectoderm cells of A^a/s — by the distribution of
the different determinants to a number of these cells before pro-
liferation occurs.

The regularity with which all organs are formed in the proper
position and mutual relation, may perhaps be taken as a proof
of the assumption that they contain latent determinants which
are from the first separate, and which differ according to the
topographical position of the organ. It is hardly possible that
the contrary assumption can be the correct one, for this would
render it necessary to suppose that although all the determinants
are certainly present in every formative cell, only that one can
undergo development which corresponds to the region in which
the cell happened to be situated.

Here, again, we meet with no serious difficulty as regards
the derivation of the required supplementary determinants in
ontogeny : in fact there is less difficulty in this case than in that
of Nais, for the cells of the different layers of the body contain
the determinants for the corresponding org2ins.

* J. Kennel, ' Untersuchungen an neuen Turbellarien,' Zool. Jahrbiicher,
Bd. 3. Abth. f. Anat, u. Ontog. d. Thiere, p. 447.

multiplication by fission 151

4. The Phylogeny of the Process of Multiplication

BY Fission

There can be little doubt that the process of spontaneous
division which occurs in flat-worms and in annelids is to be
derived phylogenetically from regeneration, as Kennel * has
recently attempted to prove. He has rightly, I believe, shown
that multiplication by a spontaneous separation into parts, such
as occurs regularly in the freshwater worm Liinibriculiis, must
be looked upon as a preliminary stage of that kind of fission,
accompanied by regeneration, which occurs in the Naidcr, for
instance. The difference between the two processes consists
essentially in the fact that in A'ais the separation into parts is
preceded and prepared for by the formation of new head- and
tail-ends, which appear between the old segments at the point
at which the separation is to take place. Such a preparatory
process does not occur in Lnmbriciiliis : the region in which
division will take place in this worm cannot previously be dis-
tinguished, and the new head- and tail-ends are formed subse-
quently, after the division has occurred.

The capacity for division of an individual into parts must
naturally be looked upon as an adaptation, and it presupposes
some kind of histological and physiological arrangement of
which we are at present ignorant. It is, however, quite conceiv-
able that when fission had once occurred in a species, it may
have been advantageous for a more thorough preparation for the
process to take place, and for the structures necessary for the
completion of the individuals thus formed to become developed
beforehand. Such a capacity for multiplication by spontaneous
division necessitates, moreover, the previous possession of the
power of regeneration. Hence the latter must have existed in
the animal before spontaneous division could take place regularly
in the species, and we must thus conclude that the capacity for
regenerating portions of the body which had been accidentally
torn asunder was first acquired very early in the philogeny of
multicellular animals; and that the special arrangements for
multiplication by fission subsequently originated from this
capacity for regeneration, and was followed by the formation of
new head- and tail-ends. The formation of the new parts pre-

* J. Kennel, ' Uber Theilung u. Knospung der Thiere,' Dorpat, 1888.

152 THE GERM-PLASM

viously to the division must be looked upon as a still later
modification of the process.

This conclusion receives further support from the fact that, as
already shown, the capacity of regeneration is not by any means
an inherent quality of the organism : that is to say, it is not a
direct and inevitable result of a particular degree of organisa-
tion, but is due to an adaptation produced by natural selection,
and constitutes a special arrangement which may exist in
different degrees of perfection, or which may, again, be entirely
absent. If an earthworm is cut into two, the anterior portion
develops a new tail-end, but the posterior portion does not
give rise to a new head-end : the arrangement existing in
Limibriculus and Nais is therefore absent in this case. This
fact I should explain by assuming that in the last-named animals
the determinants required for the formation of the head-end
are supplied to the cells of the integument and alimentary canal
as accessory idioplasm, while in the earthworm these cells
only possess the determinants required for the formation of the
tail-end.

It is very possible that the arrangement for the regeneration
of the tail-end may have taken place more easily than that
for the restoration of the head in the case of segmented
worms, owing to the fact that the last segment possessed the
power of giving rise to entire new segments. The growth of
the animal is effected by the formation of new segments at the
posterior end of the body, which would therefore be already
provided with the requisite accessory determinants, and it would
then only be necessary that these should be transferred to the
corresponding cells of the other body-segments. This might
have taken place in a relatively simple manner in the course of
philogeny, by a portion of the accessory determinants being
left in the cells of each new body-segment as it became formed.
The determinants of the head-end, on the other hand, can only
have been supplied to the respective cells as accessory idioplasm
before or during embryonic development ; we can therefore
understand why the capacity for forming a new head-end was
only acquired later, and that some worms are able to regenerate
the posterior, but not the anterior end by the body when it is cut
in half.

We can therefore trace a series of stages of gradually increas-
ing complexity in the development of the process of regenera-

MULTIPLICATION BY FISSION I 53

tion in worms, beginning -with the formation of segments at the
growing tail-end, and then passing from the regeneration — first
of the tail, and then of the head-end, to the actual fission of
Lujubriculus, and finally to that of Nats. And accordino; to
our view, this course of development depends on the regular
distribution of certain accessory determinants to particular
tissue-cells, and the gradual increase in the complexity of this
distribution.

The regenerative process which renders fission possible must
be traced to the doubling of certain groups of determinants in
the idioplasm, so that the half of them remain latent. I imagine
that this doubling need not necessarily take place and be fol-
lowed by the subsequent multiplication of the inactive groups
of determinants in the germ-plasm itself: such a multiplication
would, in fact, be a useless encumbrance to the germ-plasm.
The latter need only contain the determinants for fission when
this process leads to an alternation of generations ; that is to
say, when the animals formed by division have a different struc-
ture from those which arise directly from the ovum : for in the
latter case the forms which arise by fission are independently
variable hereditarily. This must be the case in the alternation
of generations in certain marine annelids, such as the Syllidce,
and also in the strobilation of polypes, which will be discussed
later on. In all such cases, two kinds of ids must be assumed
to exist in the germ-plasm. In the ordinary kind of fission
which occurs in the fresh-water annelids, on the other hand, the
separation of the groups of determinants necessary for the
regeneration of a part may take place during embryogeny :
nothing definite, however, can be said on this point at present,
but in any case the process of splitting off of the accessory deter-
minants may conceivably be thrown back from the later to the
earlier stages of ontogeny, until it finally takes place in the
fertilised ovum, so that double ids are present in the germ-
plasm. It will be assumed in the next section that certain forms
of budding owe their origin to the presence of these double ids.

154 THE GERM-PLASM

CHAPTER IV

MULTIPLICATION BY GEMMATION
I. The Process of Gemmation in Animals

If, with von Wagner, we look upon gemmation as 'a process in
which entire individuals are formed anew/ and which depends 'ex-
clusively on a special (differential) growth differing from the nor-
mal one,' we must include under this term the processes of asexual
multiplication which occur in most of the Caloiterata, the Polyzoa^
and the Tunicata.

A . — Ccelenterata

Hitherto it has been considered that we were fully acquainted
with the process of gemmation in the Cceletitarata. especially in
the case of the Hydrozoa. It had been observed that the two
layers of cells which form the body-wall of these animals are
present even in very young buds of Medusas and Hydroid-
polypes. These layers surround the digestive-cavity just as they
do in the parent animal, and since the body-wall as well as the
cavity it encloses are in direct connection with those of the
parent, nothing was more natural than to suppose that the bud
arises as an evagi7iation of the body-wall of the parent, both layers
of the latter taking part in its for7)iation from the first. A doubt
as to the correctness of this statement was less likely to arise
owing to the fact that even in the youngest buds of a Hydroid-
polype, before they become hollow, the ectoderm and endoderm
were seen to consist of a number of cells engaged in active
multiplication. I myself made such a statement in connection
with my investigations on the formation of the sexual cells in
Hydroids,* and no doubt has yet been raised as to its correct-
ness, or rather as to its interpretation.

The assumption that both germinal layers of the parent take
part in the formation of the bud is nevertheless an incorrect one ;
for the bud arises from the ectoderm only., and the young cells

* ' Die Enstehung der Sexualzellen bei den Hydromedusen ' (with 25
plates), Jena, 1883.

MULTIPLICATION BY GEMMATION 155

which form the endoderm in these buds are not derived from the
endoderm of the parent, but have migrated from the ectoderm.

Purely theoretical considerations first led me to suppose that
this must be so. The origin of the process of gemmation in the
idioplasm can only be brought into agreement with the theory
of the continuity of the germ-plasm, if the cells of the parent-
orgfanism from which buds arise collectivelv contain all the
determinants of the species as accessory idioplasm. If this were
not the case, an entire animal, capable of reproduction, could
never arise from the bud. If, now, a certain cell of the ectoderm
contained all the determinants for the outer, and one of the endo-
dermal cells all those for the inner layer, a bud could only be
formed when these two cells happened to lie exactly opposite to
one another in the body-wall. As, however, the endoderm cells
form a definite and continuous epithelial layer, and have a fixed
relative position, and, moreover, the position of the ectoderm
cells, although not quite so definite, is still on the whole a fixed
one, I found it difficult to imagine how budding could take place
at perfectly definite parts of the polype and of the stock in such
a regular manner as actually occurs in many cases. The assump-
tion that all the cells of the ectoderm and endoderm are equally
provided with the necessary accessory idioplasm is excluded by
the fact that budding occurs in such a regular manner. I am
therefore led to suppose that the distribution of the ' blastogenic *
germ-plasm ' might possibly be confined to 07ie germinal layer
only; and since it is known that in Hydroids the germ-cells are
always developed from the ectoderm, it is natural to conclude that
the blastogenic idioplasm is contained in the cells of this layer.

This conclusion has now been confirmed by investigations
carried out by Mr. Albert Lang, in the Zoological Institute at
Freiburg. In various Hydroid-polypes (^Eudendrium^ Pliinnt-
laria, and Hydra) the bud arises in the following way. The
cells in a certain small circumscribed region of the ectoderm
first begin to multiply, the ' supporting lamella,' which separates
the two layers of the body-wall, gradually becoming thinner and
softer at the same time, and then a few of the newly-formed cells
penetrate into the endoderm through this membrane. Here they

* The term ' blastogenic idioplasm ' is here used in the special sense of
' Knospungs-Idioplasma,' and not in the more general sense in which
it is usually used by the author (cf., e.g., the chapter on ' The Supposed
Transmission of Acquired Characters '). — W. N. P.

»56

THE GERM-PIJ^SM

form a layer of young actively dividing cells, such as I had for-
merly observed in very young buds : this layer forces the older
endoderm cells away from the supporting membrane, in conse-
quence of which they loosen their connection with the rest of
the endoderm. undergo disintegration, and gradually become
absorbed. The cells, however, which have migrated from the
ectoderm then give rise to the endoderm of the bud.

Now that these facts have been proved by Lang's investiga-
tions,* it is easier to give a theoretical explanation of the process

-EkP

Fig. 6. — Diagrammatic section through the rudiment
of a bud of EndendriHin. (Modified from a figure
by Albert Lang.) Ps, the horny perisarc; Ps ,
portion of the perisarc which has become very
thin owing to the proliferation of the underlying
ectoderm (Eki'); Ent' , the region of the endoderm,
in which a number of proliferating ectoderm cells
have broken through the supporting lamella {st),
migrated into the endoderm, and caused the latter
to project into the gastric cavity.

of gemmation in Hvdroids. We must, however, still assume
that certain cells and series of cells in the ectoderm are provided
w'ith an accessory idioplasm, which contains all the determinants
of the species, and which is therefore a kind of germ-plasm,
though perhaps not quite identical with the germ-plasm proper :

* Albert Lang, ' Ueb. die Knospung bei Hydra u. einigen Hydro-
polypen," Zeitschr. f. wiss. ZooL, Bd. 54, 1892, p. 365.

MULTIPLICATION P,Y GEMMATION T57

I therefore speak of it as ^ blastogenic idioplasm.' It cannot be
stated with certainty luhich cells of the ectoderm contain this
idioplasm ; it seems, however, that the growth of the bud
originates in the deeper layer, i.e., in the 'interstitial' cells.
We may therefore suppose that some of these interstitial cells
contain inactive blastogenic idioplasm, which, after a certain
series of cell-divisions necessary for the growth of the polype,
obtains the control of one of the offspring of these cells, and so
causes budding to take place. Eacli bud must originally arise
from one cell only, although this fact has not as yet been actually
proved ; and in the first division, or at any rate in the early divis-
ions of this cell, the group of determinants of the ectoderm must
become separated from that of the endoderm, the 'bearers' of
the latter group migrating into the old endoderm through the
disintegrating supporting lamella. The remaining details of the
process require no further explanation.

/;/ tJie Hydromedusce., then, each bud origiiiates in a sinj^le
cell, and the process of multiplication by gemmation therefore
differs essentially from that of reproduction by fission. For
gemmation owes its origin to the entire mass of the determinants
of the species, which only undergo disintegration at a later stage ;
while the new structures which arise by fission originate simul-
taneously from numerous smaller groups of determinants, corre-
sponding with those of the later stages of ontogeny.

We should nevertheless be mistaken in supposing that the
essential difference between fission and gemmation is due
altogether to this difference as regards the group of determinants
concerned in the two processes. This is rendered evident by a
comparison with the processes of budding in other groups of
animals. It still remains to be shewn whether the process of
gemmation in other Ccelenterates, viz., in the Actinozoa, the
higher Medusa., and the Ctenophora, also only apparently origi-
nates from both layers of the body-wall, or whether it actually
arises from one layer only. As the possibility of the latter mode
of origin has not till now been considered, it is very possible that
the migration of cells may have been overlooked in this case also.

If we now turn our attention to the other groups of the animal
kingdom in which gemmation occurs, viz., to the Folysoa and
Tunicata., we shall find that we possess the results of very excel-
lent investigations on which our arguments can be based ; and
the histological structure of these animals is such as to render

158 THE GERM-PLASM

it unlikely that any oversight as regards the migration of cells
can have occurred.

B. — Polyzoa

The small stocks or colonies formed in the Polyzoa arise by
a process of gemmation ; and even the small number of species
■which do not form stocks multiply vigorously by budding, but
in these cases the buds become detached from the parent sooner
or later.

The process of gemmation seems to be essentially similar in
all Polyzoa. A proliferation, which primarily originates in one
cell, takes place in a certain region of the epidermis ; the masses
of cells which are thus produced form a hollow invagination,
which extends into the body-cavity of the animal and gives rise
to the entire alimentary canal — including the fore-, mid-, and
hind-guts, as well as to the preoral 'atrium' with the tentacles
('lophophore'). Certain 'free mesoderm cells' are then said
to migrate from the body-cavity of the parent into the bud, in
which they give rise to the muscles and sexual organs, and also
in certain groups of Polyzoa to the outer (serous) layer of the
intestine ; while in others again they form a subcutaneous layer
of cells. This is at any rate the case according to the recent
observations made by Seeliger,* which are undoubtedly very
accurate and trustworthy. But one point, however, still seems to
be doubtful, viz., whether the sexual organs may not perhaps, after
all, arise from the primary proliferation of the epidermic cells.

These processes of gemmation interfere very considerably
with the ordinarily accepted and extremely conventional ideas
of the germinal membranes ; for the epithelium of the alimentary
canal, which characteristically belongs to the inner germinal
laver, here arises from the ectoderm. This, however, causes no
difficulty from the point of view of the theory of the germ-plasm :
we need only assume that the group of determinants for the
endoderm is passed on to certain cells of the epidermis as acces-
sory idioplasm. This transference must take place at an early
stage in embryogeny, before the separation of the primary endo-
derm and ectoderm occurs.

Nitsche, whose means of observation were comparatively im-

* O. Seeliger, ' Die ungeschlechtliche Vermehrung der endoprokten
Bryozoen,' and ' Bemerkungen zur Knospenentwicklung der Bryozoen,'
Zeitschr. f. wiss. Zool., Bd. 49 and 50, 1889 and 1890.

MULTIPLICATION BY GEMMATION

159

perfect, but whose researches are nevertheless of great vahie,
conckided that the whole bud was derived from the proHfera-
tion of the ectoderm. Had his statement proved correct, the
explanation of the process of budding in the Polyzoa, based on
the idioplasm, would be just as simple as in the case of Hydroids :
it would then only have been necessary to suppose that the cell
in which proliferation first began contained accessory idioplasm
in the form of ' blastogenic idioplasm.' Seeliger was, however,
unable to support Nitsche's statements, and the most recent
observations of Oka,* Davenport,f and Braem,J prove beyond
doubt that the • mesoderm cells ' of the parent take part in the
formation of the buds. We must therefore suppose that certain
mesoderm cells, provided with definite groups of determinants
for muscles, endothelia, and sexual organs, migrate into the bud.
It is quite conceivable that muscles, and more especially endo-
thelia, should be developed in this manner, but it would be
difficult to understand how free cells from the body-cavity of the
parent could migrate into the bud, and there give rise to sexual
organs at perfectly definite regions : were this so, we must sup-
pose that in reality certain of the cells only, and not any of
them, are concerned in the migration. Such an assumption is,
however, contradicted by the abnormal processes of budding
which occur, for instance, in Pedicillina. I therefore do not
consider that the question of the origin of the sexual organs is
yet decided, but I suspect, nevertheless, that one or two of the
mesoderm cells of the bud are derived from the primary prolif-
eration of the ectoderm. This view is supported by Seeliger's
statement that he considers such a derivation of individual
mesoderm cells of the bud possible, at any rate, in the case of
Loxoso7na.%

As, however, we are not specially concerned with the process
of budding in the Polyzoa in particular, but are only making

* A. Oka, ' Observations on Freshwater Polyzoa,' Journ, of College of
Science, Imperial University, Japan, Vol. iv., Pt. i, 1890.

t C. B. Davenport, ' Observations on Budding in Paludicilla and some
other Bryozoa," Bull, of the Museum of Comp. Zool. at Harvard College,
Vol. xxii.. No. I, 1891.

t F. Braem, ' Untersuch. iiber d. Bryozoen des sussen Wassers,* Bibl.
Zool., Cassel, 1890.

^S Seeliger, ' Bemerkungen zur Knospenentwicklung der Bryozoen,'
Zeitschr. f. wiss. Zool., Bd. 50, p. 564.

l6o THE GERM-PLASM

use of it as an example of gemmation in which two germinal
layers are primarily concerned, we can leave this question aside.
It is at any rate true that in the Polyzoa parenchymatous cells,
as well as a certain ectoderm cell, take part in each process of
budding. We must therefore assume that the determinants of
the species cannot be contained altogether in one cell as blasto-
genic idioplasm, as in the case of the Hydrozoa, but that a
number of them — including those for the muscles, endothelia,
blood-corpuscles, and perhaps those for the sexual organs also
— are supplied to certain mesoderm cells of the parent. The
development of sexual cells renders it necessary that those cells
from which they arise shall also contain germ-plasm ; and the for-
mation of the epidermis of the bud, which results to some extent
on purely mechanical grounds, presupposes the existence of deter-
minants for the ectoderm in the epidermic cells of the parents.

The disintegration of the determinants, which is necessary
before budding can take place, is obviously, however, of a very
different kind from that which occurs in embryonic develop-
ment. Seeliger, indeed, has called attention to the fact that the
ontogeny which results from gemmation is a much shorter
process than that which occurs when an embryo and larva are
formed. In the former case, not only are the whole series of
stages of segmentation and development of a free-swimming
larva absent, but even in the later periods of development none
of the stages in embryogeny and in gemmation exactly corre-
spond to one another. Without following out these two pro-
cesses in detail, I should be inclined to explain them in general
by assuming that the groups of latent supplementary deter-
minants, with which certain cells are provided in the course of
embryogeny, contain combinations of determinants different
from those which lead to the development of the embryo.

C. — Timicata

The fixed Ascidians usually multiply very extensively by gem-
mation, and thus give rise to stocks, the individual persons of
which are more or less closely connected with one another.

We owe our detailed knowledge of the process of budding
in the genus Clavelina to the researches of Seeliger.* The

* O. Seeliger, ' Zur Entwickliingsgeschichte der Ascidien ; Eibildung u.
Knospung von Clavelina lepadi for mis', Sitzungsber. d. Wien. Akademie,
Bd. 8;, 1882.

MULTIPLICATION RY TIEMMATIOX l6l

liarent, which was developed from an ovum. i)roduccs Ions; stalk-
like processes or stolons, on which new animals are produced
by budding. Each of these stolons is made up of three lavers
of cells — an outer ectoderm, an inner endoderm, and an inter-
mediate layer of motile 'mesoderm cells/ The ectoderm layer
gives rise merely to the epidermis of the bud : the epithelium of
the alimentary canal and its accessory organs, the branchial sac
("peribranchial tube'), and the pericardial tube, being developed
from the endoderm; and the muscles, ganglion (?), and sexual
glands from the ' free mesoderm cells.'

The endodermal tube mainly determines the form of the
animal in these processes : it becomes definitely segmented,
and on it the growing ectodermal tube is moulded, so to speak.
We may thus conclude that a series of homologous formative
zones of structure are to be found in the endodermal tube of
the stolon, each of which may consist originally of a single
circular layer of cells. At the point where a bud will arise, the
corresponding zone of cells grows out to form a bladder-shaped
enlargement, which becomes detached from its point of origin
on the endodermal tube of the stolon and regularlv differenti-
ated, so as to give rise to the peribranchial tube, the intestine,
and so on. The cells of this endodermic vesicle cannot all be
equivalent, nor can they contain exactly similar determinants :
were that the case, such a differentiation could not occur, and
the walls of the peribranchial chamber could not arise from one
part, and the intestine from another. But even as regards the
primitive intestinal vesicle itself, one cell must contain the deter-
minants of the stomach, another those of the hind-gut, and so
on. In short, we must assume that — just as occurs in principle,
if not as regards actual details in the case of embryogeny — a dis-
integration of the idioplasm and a distribjttion of the groups of
detenninafits among the different cells takes place during devel-
opment. The determinants of all parts arising in connection
with each endodermal vesicle, must be collectively contained in
each zone of cells of the endodermal tube from which such a
vesicle is developed.

The formation of those organs which arise from the ' free
mesoderm cells' of the stolon is the most difficult to understand.
There is certainly no reason why we should not suppose that
these cells contain very different kinds of idioplasm : one, for
instance, might contain ' muscle-determinant.s/ another ' nerve-

1 62 THE GERM-PLASM

determinants," and a third ' blood - corpuscle determinants.''
Various kinds of these cells may easily be distinguished while
they still float freely in the blood of the stolon . The difficulty only
consists in ascertaining the exact part they play in the formation
of the developing bud. Those which are to give rise to the
longitudinal muscles become arranged in rows, which, diverging
obliquely from one or two definite points, extend over the animal
from jjehind forwards, and are attached at more or less definitely
fixed points anteriorly. The ganglion and the sexual glands
have also perfectly definite positions in the animal. \w eni-
bryogenv, as well as in the development of the endodermal
vesicle of the bud, the position of every cell is assigned to it
mechanically, in consequence of its origin from previous cell-
generations. — that is, bv the rhvthm of the cell-divisions. In
the case, however, of the ganglion for instance, the cells of which
it is composed must come together at the right place by means of
their power of locomotion. A similar process is known to occur
in embryogeny in the case of several groups of animals, such as
the Echitioderniata^ for instance ; and until we know more of
the actual facts concerned, we can only — however unsatisfactory
such an assumption may be — attribute to the cells a tendency
to become attached at definite points according to the manner
in which they have previously been determined. The reverse
assumption — that these cells develop into muscle-, nerve-, or
sexual-cells according to their point of attachment — seems to me
at any rate a less likely one.

If we compare the processes of gemmation and embryogeny
in Ascidians, important differences are seen to exist between
them. In the former, all the stages of segmentation of
the egg and gastrulation, together with the formation of the
mesoderm, are omitted ; and many parts, again, arise from
the ectoderm in the embryo and from the mesoderm in the
bud. These differences are perhaps still more marked in the
free-swimming SalpcE. These animals also multiply by buds
produced on a kind of stolon ; and, as in the other Ascidians
referred to, the ectoderm forms practically nothing except the
epidermis, and the endoderm gives rise to only a few structures,
by far the greater number of parts arising from the ' mesoderm-
cells.' Seeliger* explains this by supposing that ' the mesoderm

* Seeliger, ' Die Knospung der Salpen,' Jena, 1885.

MULTIPLICATION P,Y GEMMATION 1 63

of the mother-animal which passes into the buds, practical! v
corresponds only to the future sexual apparatus.' This, however,
can merely be taken as an explanation of the fact in so far as it
indicates the possibility of the ' mesoderm cells ' of the stolon
and bud containing the groups of determinants required for these
different structures. For all the determinants must be present
in the sexual cells, and, owing to their disintegration during cell-
division, they may become arranged in very varied groups, so
that certain mesoderm cells may become furnished with one
group and others with another. This certainly presupposes
that the process of the distribution of the determinants in this
case is entirely different from that w^hich takes place during
embryogen}', and this difference, again, can only depend on
a difference in the original architecture of the idioplasm. In
discussing the process of alternation of generations I shall
once more return to this point, which, from a theoretical point
of view, is a very fundamental one.

2. The Process of Gemmation in Plants

Our conception of the process of gemmation has been in the
first instance derived from the vegetable kingdom : all the higher
plants correspond to stocks or corms which arise by copious and
regular budding, much as occurs in the case of such animal-
stocks as those of the Hydrozoa, for instance. Although the
physiological individuality of separate ' persons ' in a plant is
often less defined than in the case of many animal colonies, there
can nevertheless be no doubt as to the morphological value of a
shoot as a 'person,' in the sense in which Haeckel uses the term.

Although as regards animal colonies, it has not yet in all
instances been possible to ascertain with absolute certainty the
actual origin of the processes of budding in connection with the
cell-generations of the first person of the colony, this has been
done very accurately in the case of plants ; a theory of heredity
can therefore be much more safely applied to the process of
gemmation in plants than to that in animals.

In many i)lants, at any rate, budding originates from a si>ii!:;le
cell, situated at the apex of the growing shoot, and known as the
' apical cell.' This cell grows and undergoes a series of divisions,
much as occurs in the development of the ovum, and thus gives
rise to a group of cells, the number, form, and arrangement of

164

THE GERM-PLASM

which is perfectly definite. The primary constituents of the
entire new shoot are contained in this group, and it is possible to
predict what parts of the shoot will be formed from each of its
cells. The successors of this group of cells continue to multiply
up to a certain limit, and have then only to become elongated in
one or more directions, and more highly differentiated, in order
to give rise to a fully developed 'person' of the stock. This
person does not undergo any further essential changes, but it
is capable of giving rise to a new person from its apical cell ;

Fig. 7. — The apex of a shoot of Chara, in longitudinal section. (From Sachs'
' Lectures on the Physiology of Plants.')

for the latter is always being renewed, or, in other words,
it always remains the same.

Let us take as an example the alga C/iara. A glance at Fig. 7
will at once make it apparent that the idioplasm of the apical
cell (v) cannot undergo separation into different groups of deter-
minants in the first division, because one of the resulting two
cells remains as the apical cell, while the other, or *• segmental
cell,' gives rise to an entire shoot, — that is to say, to that very
structure which the apical cell is capable of producing. The

MULTIPLICATION BY (;EMMATI0N 1 65

next division of tlie lower of the two daughter-cells, however,
separates the determinants into two dissimilar groups, for it re-
sults in the production of an upper biconcave ' nodal cell,' from
which the leaves {b\ b^\ //", <^'^), the lateral shoot (k), and the
sexual organs {a and ^), will subsequently arise ; and of a lower
biconvex cell, which does not undergo further division, but onlv
grows considerably in length, so as to form a segment of the
main axis (/', /", /"'. /"'). The idioplasm of this * internodal
cell ' does not therefore undergo further disintegration ; the nodal
cell, however, divides vertically, so as to form cells which, since
they give rise to other parts of the shoot, must contain various
groups of determinants. Thus a comparison of the younger
with the older segments of the shoot, shows that the outer of the
five nodal cells in the figure gives rise to a whole leaf, together
with the sexual organs, the inner ones forming the actual node.
The division of the outer cell is accompanied by constant though
usually unimportant changes as regards its idioplasm : a glance
at the structure of the leaf, in which similar segments are
repeated many times over, will make this evident. If we now
leave out of consideration the accessory idioplasm which is
present in the cells along with the primary idioplasm, it will be
seen that the distribution of the group of determinants derived
from the apical cell must simply take place so as to result in each
cell, as it is formed, receiving that group of determinants only,
the individual constituents of which are required by its successors
for the control of the individual cells. We must therefore sup-
pose that the internodal cells of the stem only contain their own
specific idioplasm, composed of • internodal determinants/ for
they do not give rise to any other structures. The primary
nodal cell, on the other hand, must contain an entire group of
determinants, as it gives rise to a number of cells which have
various forms and perform various functions.

Although the cells of plants are often apparently very much
alike, and no essential difference can be observed between them,
such a difference must exist if the origin of the specific leif,
stem, and reproductive organs can be proved theoretically at
all. For the origin of these structures can only be explained, at
any rate in principle, by supposing that each of these centres of
vitality is controlled by a specific idioplasm ; that is, by a deter-
minant which differs in some way or other from those in the
other cells.

r66 THE GERM-PLASM

3. Comparison of the Process of Gemmation in
Animals and Plants

Various stages may be recognised in the different kinds of
gemmation with regard to the kind of idioplasm concerned in
the process. The simplest form of budding is seen in those
plants in which the production of a new ' person ' by budding
always originates from a single cell. We must therefore
assume that the idioplasm of this cell contains all the deter-
minants of the shoot, and very probably those of the root
also. For most of the shoots of a plant, when they have been
cut off from the stem, are capable of giving rise to roots under
favourable circumstances. This does not as a rule occur under
normal conditions, — that is to say, while the shoot is still con-
nected with the parent-plant. The ' blastogenic idioplasm'
cannot be quite identical with germ-plasm proper ; for although
precisely the same parts may arise from it as from the fertilised
egg-cell, the different succession of cells which results in embrv-
ogeny and in gemmation indicates that the determinants must at
any rate be differently arranged in the idioplasm, and that possibly
their proportional number is also different. '• Blastogenic idio-
plasm ' and geriii-plasni Diay in a sense be 7'egarded as ' isomeric '
idioplasms, using the term in an analogous sense to that of
isomeric chemical compounds.

The same would be true as regards such animals as Hydroids,
in which the formation of a bud originates from a single cell.
In this case, again, the resemblance between embryonic develop-
ment and the process of gemmation, although to a certain extent
approximate, is not a complete one ; and it must again be
assumed that the wJiole of the determinants of the species are
contained in the blastogenic idioplasm, — not only those which
as a rule undergo development, but also those required for the
formation of the attached ends in the case of Hydroids, or of
roots in the case of plants. This conclusion is supported by the
phenomena of budding in polypes like Hydra, in which the buds
regularly become detached, and carry on an independent exist-
ence. In such cases the daughter-polypes do not develop a
'foot' until they become detached from the parent.

The next stage in the process of budding is seen in the
Polyzoa. All the determinants of the species from which tlie
bud is formed are no longer contained in a single cell, but are

MULTIPLICATIOX V.Y GEMMATION 167

arranged into main groups, one of which is supplied as accessorv
idioplasm to one cell of the ectoderm, and the other to one or
more cells of the mesoderm. The siiigle ectoderm cell gives
rise to the entire endoderm, but it must nevertheless not be
considered equivalent to those cells of the embryo which give
rise to the endoderm by invagination, for it forms parts which
are either not developed at all in the embryo, or else arise from
other ectoderm cells. Without entering into details here, the
facts may be expressed in terms of the idioplasm by supposing
that the ectoderm-cell from which the bud arises is provided
with an idioplasm which contains the whole of the determinants
for the endoderm, as well as a number of others, and that this
combination of determinants does not occur in embryogeny.
The mesoderm-cells of the parent which gives rise to the endo-
thelia, muscles, &c., of the bud, must also contain a peculiar
combination of determinants which is not exactlv similar to
that which occurs in embryogeny. The gemmation must there-
fore be prepared for in embryogeny by certain series of cells in
the ectoderm and mesoderm being provided with these groups
of determinants in the form of accessory idioplasm.

A third stage is represented by the gemmation of fixed Asci-
dians and Salps. In these the bud originates — in the fully-
formed animals, or in those which are still undergoing develop-
ment— from three kinds of cells, viz., those of the ectoderm,
mesoderm, and endoderm. And here again those groups of
determinants which must be supposed to exist in the three
kinds of cells do not correspond exactly to those which must be
contained in the prmiary ectoderm, endoderm, and mesoderm
cells. In fact, no group of cells which occurs in embryogeny
can contain precisely the same group of determinants as does
the endoderm cell of the bud. A collection of determinants
especially adapted for budding must therefore be provided on a
large scale during embryogeny in this case, so that eventually
certain cells may receive their supply from it in the form of
accessory idioplasm.

This last kind of budding resembles reijeneration verv closelv
as regards the idioplasm concerned in the process. It must
not, however, be therefore implied that the former process
has been derived phylogenetically from the latter. The re-
semblance only consists in the formation of a new ' Derson,'
which in both cases originates in several cells provided with

1 68 THE GERM-PLASM

different groups of determinants, these eventually completing
one another, and interacting in such a manner that a fully-
formed person must result.

4. The Phylogenv of the Process of Multiplication

BY Gemmation

In all probability the phylogeny of gemmation has taken place
along different lines. The process most likely arose independ-
ently in animals and in plants, and perhaps even in different
groups of animals it has had a different origin.

In many of the lower plants, the cells and organs of which
are only slightly differentiated, all, or at any rate many, of the
cells can individually give rise to a new plant under certain cir-
cumstances. In such cases we might be inclined to suppose that
each cell contained originally, i.e. at the time of its phyletic
origin, the entire mass of determinants of the species, or, in other
words, contained germ-plasm. The various differentiations of
the cells on the upper and lower surfaces, for instance, would
consequently depend on the different determinants becoming
active in response to different external stimuli : some, for in-
stance, might be stimulated by a bright light, and others by a
dim light.

This explanation would hardly suffice in the case of the higher
plants, the differentiation of which is far too complicated to be
due to the effect of external causes. A large number of the
cells must nevertheless contain germ-plasm, which, however, is
in the unalterable (* gebundenen ') state, — that is to say, it is
not merely inactive, but is incapable at the time of undergoing
disintegration. This stage in the phylogeny of gemmation may
be derived from the earliest stage. As the plant underwent an
increasing differentiation, cells appeared which only contained
special determinants, in addition to those with germ-plasm
proper ; and this may have led to the condition which we now
find in the highest plants, and which is distinguished by the
fact that many cells only contain specific determinants, while a
large number of others possess in addition germ-plasm in the
unalterable condition, which only becomes active under certain
influences. I shall have occasion to return to this subject
later on.

In the case, again, of the various groups of the lower animals

MULTIPLICATION RV GEMMATION 1 69

which multiply by gemmation, we cannot assume that this proc-
ess has a common origin. But although it may have arisen
independently in the various subdivisions of the animal king-
dom, the history of its origin will have been essentiallv the
same in all cases, for ' blastogenic ' idioplasm must have become
dilTerentiated from the germ-plasm even in the egg-cell, as all
the determinants of the species are contained only in the latter.
Even at the present day the blastogenic idioplasm must be
present as such in the germ-plasm, for otherwise it could not
have undergone independent and hereditary variation : the for-
mation of medusae from polypes by gemmation, and many other
cases of alternation of generations, prove that this has actually
occurred.

Balfour attempted to derive the process of budding from a
division of the fertilised ovum into two separate parts, such as
has been observed in certain animal forms, and which leads to
the formation of two individuals. He imagined that if this
process of doubling were transferred to a later ontogenetic stage,
budding would result, and expressed his views as follows : —
' While it is next to impossible to understand how production
of a bud could commence for the first time in the adult of a
highly organised form, it is not difficult to form a picture of the
steps by which the fission of the germ might eventually lead to
the formation of buds in the adult state.''* Unfortunately this
gifted observer did not work out this idea in detail : it seems to
me, however, that the derivation of budding from the doubling
of the fertilised ovum by division is not so simple or self-evident
as we might expect at first sight.

Let us suppose that a fertilised ovum became capable of
dividing into two parts: these two first segmentation -cells
would not then be blastomeres, but would correspond to egg-
cells, each of which could give rise to an entire animal. But this
could not be called gemmation, nor would the latter process
occur if the doubling were transferred to a later stage : — this
would only cause a multiplication of the egg-cell, which would
result in the formation of four, eight, sixteen, (S:c.. ova. instead
of two.

If, however, we suppose that the division of the egg is of such
a kind that the two halves at first remain together so as to form

* F. M. Balfour, ' Comparative Embryology,' Vol. i., Introduction, p. 13.

lyO THE GERM-PL.\SM

only one embryo, the condition observed in a certain earth-
worm i^Lunibricus trapezoides^ by Kleinenberg would result. In
this animal the development is apparently single up to the gas-
trula stage, at which the separation of the two embryos first
occurs. Did this separation take place at a much later stage,
perhaps not until the two individuals are fully developed, the
process would not be one of budding, but only of a doubling of
the embrvo.

An essential modification of this process is indispensable if
gemmation is to result from it, and this consists in the postpone-
ment of the developinent of one half of the egg. Let us suppose
that one of the two equivalent blastomeres of an ovum did not at
once undergo development at the same time as the other, but
remained in a unicellular condition enclosed within the embryo
formed from the active blastomere. and subsequently began to
develop when the latter had already given rise to a full-grown
animal: this would be a true process of gemmation. I do not
wish to assert definitely that the phylogeny of budding might
not have taken place on similar lines. A postponement and
subsequent transference to a later stage of ontogeny of the de-
velopment of one of the blastomeres is not actually inconceiv-
able. But such a transference must have undergone a still
further modification, before even the simplest form of budding
with which we are acquainted could arise. The shifting must
have occurred in a backward as well as in a forward direction ;
that is to sa}-, tJie division of the egg into two separate ones must
have been suppressed, and represented by the mere division of the
gerjn-plasm.

Thus in Hydroids and other animals which multiply by bud-
dinof, we see, in fact, that one of the two blastomeres into which
the egg-cell divides does not serve, so to speak, as a reserve cell
for subsequent gemmation ; both blastomeres, on the contrary,
continue to divide, and together give rise to the embryo : and even
in the latter none of the cells can be distinguished as 'blastogenic-
cells': the cells which take part in the formation of the buds
only appear at a much later stage, when the polype is fully
formed. If therefore gemmation has in this case originated
from the doubling of the egg, the latter process must itself have
become degenerated, only the essential part of it remaining: the
germ-plasm concerned in it must have remained associated with
that of the egg-cell in the form of ' unalterable ' germ-plasm, and

MULTIPLICATION BY GEMMATION I 7 I

must then have been passed into certain series of cells in the
course of ontogeny.

Whether the process of budding has actually been derived
from that of the doubling of the egg or not, it seems to me to be
certain at any rate that the first process undergone by the idio-
plasm ;//us/ Jiave been that of the doubling of the ids of the
^erm-plas»i in the fertilised egg-cell, and that this was not con-
nected with the division of the egg-cell ; one half of the germ-
plasm consequently remained in an unalterable and inactive
condition, in which, however, it was capable of development.
This blastogenic germ-plasm was then supplied to one of the
first segmentation-cells in the form of accessory idioplasm ; and
from these it was passed on through certain series of cells in an
unalterable condition, only becoming active when it had reached
certain parts in the fully formed animal, in which it then caused
gemmation to occur.

It does not seem to be inconceivable that the process of bud-
ding owes its origin phyletically to such a spontaneous division
and doubling of the germ-plasm, and that this was originally
connected with the inactivity of half the germ-plasm : its con-
nection with the doubling of the ovum was consequently not
such as was indicated above, — that is to say, gemmation did not
owe its origin to the doubling of the egg, but both processes orig-
inated priniarily i?t the division and doubling of the gerni-plasni
of the egg-cell, to which in any case the doubling of the egg must
be due. The difference between the two processes would then
consist in the fact that in budding one-half of the germ-plasm
would pass into the inactive condition, while in the doubling of
the egg both halves would at once become active.

The modifications of the idioplasm which result in gemmation
must become more complex as soon as two, or all three, of the
germinal layers take part in the process, instead of one only.
In such cases the blastogenic germ-plasm must undergo disin-
tegration at certain ontogenetic stages, e.g., at the separation of
the ectoderm from the endoderm, and again at the separation of
the mesoderm from one of the two primary germinal layers.
Precisely the same combination of determinants need not
necessarily be produced by the disintegration of the accessory
germ-plasm into two or three groups of unalterable accessory
idioplasm, such as are formed in embryogeny. We can thus
explain the origin of endodermal organs from the ectoderm cells

172 THE GERM-Pr,ASM

of the bud, as occurs in the Polyzoa for instance, and also even
the co-operation of tliree germinal layers in the formation of the
bud.

It seems to me to be improbable that the phylogeny of gem-
mation in animals has taken place in the reverse manner. We
mighr assume that in the lowest Metazoa, which no longer exist
at the present day, all or many of the cells also contained germ-
plasm proper, just as in the case of the lower multicellular
plants. Under certain circumstances a perfect animal might
have been produced from each of these cells. But this assump-
tion would only suffice as long as the individual formed by bud-
ding was exactly similar to that arising from the egg. Even the
slightest difference between these would necessitate the presence
of special ids in the germ-plasm . For such a difference can only
depend on the fact that the two kinds of individuals are capable
of independent variation from the germ onwards. We should
therefore have to assume further, that in the course of phylogeny
the germ-plasm of these somatic cells from which the buds orig-
inated became doubled in the earlier stages of ontogeny, and that
it was consequently present in the germ-plasm of the egg-cell in
the form of a special group of ids. But this, to say the least, is
a very involved assumption, and can hardly be considered very
probable : that which presupposes a primary doubling of the ids
of germ-plasm is certainly far preferable to it. The following
chapter will make this point still more evident.

ALTERNATION OF GENERATIONS I 73

CHAPTER V

ALTERNATION OP^ GENERATIONS IN ITS RELATION TO

THE IDIOPLASM

Starting from the germs specially adapted for amphimixis
(sexual intermingling), we liave designated as ge^'Jii-plasni the
definitely arranged group of determinants which must be con-
tained in the sexual cells. By this term is meant an idioplasm
which contains all the determinants of the species. At the
same time a large number of species exist in which the sexual
cells are not the only ones which contain all the determinants,
and in which the development takes place, for tJie second time
during the course of the life-cycle, from a single cell ; the idio-
plasm of this cell must therefore also be composed of all the
determinants of the species. This is the case in most of the
lower plants, such as mosses, horse-tails, and ferns, — in all of
which sexual reproduction alternates with the formation of
asexual ' spores,' — as well as in those groups of animals in which
that form of alternation of generations which is known as
heterogeny occurs. But even in the case of alternation of gen-
erations in the more restricted sense, — i.e., the alternation of
sexual reproduction and gemmation, — the development of an
individual may take place twice successively from a single cell,
as was mentioned above with regard to the stocks or colonies
of plants and of Hydroid-polypes. In all these a cell, the idio-
plasm of which contains all the specific determinants, occurs
twice in the course of the life-cycle from one fertilised ovum to
the next one. The question therefore arises as to whether the
idioplasm in each case is to be considered identical, and may
merely be described as germ-plasm.

This question has already been discussed in the section on
the process of gemmation in plants ; and it was there concluded
that the idioplasm of the apical cell and that of the fertilised
ovum cannot be assumed to be perfectly identical, owing to the
fact that the course taken by embryogeny — in which process the
first shoot and roots are formed — is different from that followed

I 74 THE GERM-PLASM

by the cell-divisions which result in the apical cell producing a
new shoot. The same is true as regards the formation of a new
polype from a blastogenic cell and from an ovum. In both
cases the final result is the same, or at any rate very similar,
though the method bv which it is attained is different. Although
a precisely similar organism might be produced by either of
the two methods of development, and the primary cells would
therefore contain the same determinants in both cases, the
grouping of the latter in the two idioplasms at any rate must
be different, for they must pass through different groups during
ontogeny before their ultimate disintegration into single deter-
minants. Even in this very simple case it is necessary to dis-
tinguish the 'germ-plasm proper' of the egg- and sperm-cells
from the 'apical-plasm' or 'blastogenic germ-plasm.' It is
convenient, however, to speak of every kind of idioplasm which
contains all the determinants of the species as ger)n-plas7}i in
the wider sense, and to distinguish its various subdivisions as
'blastogenic' and 'sporogenic' germ-plasm, and so on; these
latter may all be included under the term accessory genn-piastns
or para-ger7n-plasms, in contrast to the primary or ancestral
gerjn-piasin.

Wherever two kinds of germ-plasm occur in the life-cycle of
a species, \ve might be inclined to assume that they change into
one another in the course of life. But this view is untenable, as
has been shown above, and we are on the contrary forced to
assume that both kinds of genn-piasni contituially pass siuiul-
taneonsly along tJie germ-tracks, and that each of them becomes
active in turn.

This assumption is unavoidable, for the phyletic development
of the species shows that the individual generations in cases of
alternation of generations can vary independently and heredi-
tarily. It. however, presupposes that special determinants are
present in the germ-plasm in each generation, for otherwise both
generations would be affected at the same time by a variation
in the germ. A similar assumption must be made in the case of
metamorphosis. The wings of a butterfly must be represented
in the germ-plasm by a group of determinants. If the wings
were formed by the transformation of some of the determinants
of the caterpillar, they could never vary without at the same
time producing a variation in some parts of the caterpillar, and
vice vers A.

ALTERNATION OF GENERATIONS

ns

It will not be unin-
teresting to give some
examples by way of
illustration.

We will first take a
case of Jieterogefiy, or
alternation of gener-
ations in which the
two generations do not
differ at all from one
another in the full-
grown condition. As
a rule, the difference
between the two suc-
cessive generations in
the Daplinidce or water-
fleas, for example, con-
sists in the fact that
one generation is de-
veloped from summer-
eggs, which contain a
small amount of yolk,
while the other arises
from winter-eggs, in
which the yolk is very
abundant. From both
of these two kinds of
eggs similar females
are developed : — the
complication arising
from the periodic ap-
pearance of the male
may be neglected for
the present. The sum-
mer-eggs are nour-
ished by the blood of
the mother, while the
winter-eggs are not ; for
the amount of yolk in
the latter necessitates
a different kind of

Fig. 8. — a female of DapJntia pulcx,
with two parthenogenetic eggs in
the brood-chamber {b). (After R.
Hertwig.)

176

THE GERM- PLASM

ontogeny, and this presupposes not only a difference in
the arrangement of the determinants in the germ-plasm as
compared with the meta-germ-plasm (' Nach-Keimplasma'), but
also the presence of different determinants for some of the
embryonic stages. The case becomes still clearer if we take one
particular species of Daphnid {Leptodora hyalind) into con-
sideration. In this animal the embryogeny of the winter-eggs
only extends as far as to the formation of the primitive crustacean
larva, or nauplius, which possesses three pairs of limbs : the
summer-eggs, on the other hand, develop at once into the adult
form of the animal, in which all the limbs are present. The
summer-eggs certainly also pass through the stages from the
ripe ovum to the nauplius, but these are abbreviated, and

Fig. 9. — Nauplius larva of Leptodora hyalina. (After Sars, from Korschelt and
Heider's ' Lehrbuch der vergleichenden Entwicklungsgeschichte.')

though this nauplius also possesses three pairs of limbs, these
are only rudimentary, and are useless as swimming organs.
There must therefore be two kinds of germ-plasm in Leptodora.,
one of which still contains all the determinants of the nauplius.
while the other only contains a portion of them, and even these
have probably undergone some change. The two kinds of
germ-plasm must be passed separately along the germ-tracks
from one generation to another, so that each must always con-
tain the other, which is, so to speak, stored away in it in an
inactive condition. It seems to me impossible to explain the
facts in any other way, for it is inconceivable that the germ-
plasm of the summer-eggs, which has undergone reduction,
and possesses comparatively few determinants, should be able to
develop the lost determinants ojit of its own snbstajice.

ALTERNATION OF GENERATIONS 1 7 7

The phyletic development of these two kinds of germ-plasm
would be very enigmatical if we were compelled to assume that
only a single unit of the germ-plasm is present in the nucleus of
the germ-cell. We have, however, made the reverse assump-
tion from the first, and it will be shown later on that a con-
sideration of sexual reproduction, or amphimixis, leads us to
assume that several, and in fact probably a large number of
units or ids must be contained in the germ-plasm of every
species which multiplies sexually. If now a reduction of the
determinants for the nauplius in the summer generation of
Leptodora were advantageous, it would have appeared, increased,
and become fixed in the course of generations by selection, and
an abbreviation of embryogeny would thus have resulted. This
would only have occurred gradually, so that at first the summer-
eggs would contain more reduced than unreduced ids only in the
case of a few individuals ; and if the original unabbreviated form
of embryogeny were of greater advantage to the winter genera-
tion, the determinants for the nauplius would not become lost or
modified in all the ids, but only in certain of them. A balance
of the two kinds of ids would finally take place from the struggle
between the modified ids. which were more advantageous in
summer, and the unmodified ones, which were of greater advan-
tage in winter, and this would result in the germ-plasm of the
species being composed of an equal number of modified and
unmodified ids ; these would alternately control the cell, so that
each would remain inactive and unalterable during a certain
number of generations, and become active during certain others.

This regular alternation between definite periods of activity
and inactivity in the two kinds of germ-plasm can be directly
observed, for we can determine how many generations occur
which give rise to summer-eggs before one again appears in
which winter-eggs are produced. As I was able to prove a con-
siderable time ago, this regularity varies very much in difierent
species of Daphnida^, and stands in close relation to the mode
of life of the species. In those species which live in very small
bodies of water which are liable to become rapidly dried up, the
formation of the two kinds of eggs alternates very frequently;
this is due to the fact that the extermination of the animals by the
sudden drying up of the pond is only prevented by the thick
shells by which the winter-eggs are surrounded. On the other
hand, all the species which live in large bodies of water, such as

lyS THE GERM-PLASM

pools and lakes which never become dried up, produce summer-
eggs alone for a large number of generations, and only give
rise to eggs of the other kind on the approach of winter ; and
these, on the death of the animals which produced them, ensure
the continuance of the species in the following spring.

The occurrence of changes in \.\\tjinal stages of ontogeny must
be accounted for in a similar way.

In plant-lice belonging to the genus Aphis, the fertilised ^gg
gives rise to females, which are, however, incapable of being
fertilised, for the receptaculum seminis is wanting, and this is
essential in the process. Their eggs are, however, capable of
undergoing development in the ovary parthenogenetically. The
resulting offspring give rise to similar females possessing no
arrangements for fertilisation, and these again produce others of
the same kind. Ultimately, however, one of these gives rise
parthenogenetically to females which are capable of fertilisation,
as well as to males. The two sexes as a rule differ as regards
the shape and colour of the body, apart from the structure of the
reproductive organs and sexual products, but the embryogeny of
these sexual animals is similar to that of the others.

In this case, therefore, the determinants of the mature animal
become modified in the parthenogenetic generations, for sexual
reproduction is the more primitive of the two forms of the proc-
ess. If therefore we make the assumption, which, however,
is not a strictly correct one, that the sexual generations have
remained quite unaltered since the introduction of alternation of
generations in these animals, we should have to represent the
phyletic change in the germ-plasm as being of such a nature as
to cause the degeneration of the determinants of the seminal
vesicle in one half of the ids, and to produce a change in other
determinants, such as those w'hich control the colour of the in-
tegument, for instance. The modified, as well as the unmodified
ids, must be contained within the same germ-plasm, but they
control the egg alternately, and never become active at the same
time.

In this instance the generations which have been interpolated
have only suffered a slight change as regards the structure of the
whole body. But in many cases of alternation of generations
very important differences of structure occur, so that not infre-
quently one might easily believe that the different generations
belong to two entirely different groups of animals.

ALTERNATION OF GENERATIONS

179

This is the case in the alternation of generations in medusa;.
The polype is the original form, and even at the present day the
fertilised ovum of the medusa gives rise to a polype in most
species. By the budding of this polype, or at any rate of the
offspring which have been produced by gemmation, medusae are
again developed. If, for the sake of simplicity, we neglect the
slight differences Avhich may exist between the germ-plasm of

Fig. 10. — Bougaifivillea ramosa. (After Allman.) Polype slock with gastro-
zooids (A) and medusoid buds {mk)\ >ii, young Medusa {Margelis ramosa),
which has beconae free. (From A. Lang's ' Lehrbuch der vergieichenden

Anatomie.')

the egg and that of the bud, it is evident that two germ-plasms
take part in the cycle of development of the species, and these
must differ as regards very many, if not almost all, of their deter-
minants, for the medusa is provided with a number of parts and
organs which the simple polype does not possess. Thus we
must assume that there are two different kinds of ids of wiiich

l8o THE GERM-PLASM

the germ-plasm is composed in equal numbers, the periods of
activity of which alternate with one another. The ids of the
accessory germ-plasm, which arose subsequently, must be larger
than those of the ancestral germ-plasm, because they contain
more numerous determinants. If at some future time it should
be definitely ascertained that those granules or microsomes,
which are arranged like beads in a necklace in the nuclear rods,
really correspond to ids, we may possibly, perhaps, be able to
prove by the aid of the microscope that such differences in size
actually exist. A knowledge of the entire number of nuclear
rods or idants may also possibly help to confirm the theory,
for it is probable that in species in which alternation of genera-
tions occurs, the ids, and therefore the idants also, have been
doubled during the development of the species. For if my view
is correct that a definite amount of germ-plasm is necessary for
the normal development of a certain kind of egg, the periodical
inactivity of half the ids must have been accompanied by a cor-
responding doubling of these structures.

The mechanism of the idioplasm in alternation of generations
becomes somewhat different, and rather more complicated, as
soon as the second generation arises, not from a single cell, but
from several cells at the same time, derived from different layers
of the body. This is the case as regards ^/le strobilation of the
higher inednsce and that of tape-ivo}'ms, and an intermediate
stage is seen in \\\e gemmation of the SalpcE.

In the last-named animals, two generations differing as regards
the form of the body and mode of reproduction follow one
another. A number of individuals are united into a ' chain ' in
the first generation, in which sexual multiplication occurs ; and
in the second generation the individuals are separate, and mul-
tiply by budding. It has already been pointed out in the chapter
on gemmation that this budding is produced by a co-operation
of the ectoderm and mesoderm cells. We must imagine that in
this case, again, the germ-plasm of the egg- and sperm-cells is
composed of two kinds of ids, which alternately become active,
one of which contains the determinants for gemmation, and the
other those for embryogeny. In the case of the Hydroid-polypes
and medusae, the determinants of the ' blastogenic ' ids remain
together in one cell, but in the single form of Salpa they must
be separated into groups during embryogeny, and these groups
would be supplied — in part to the ectoderm, and in part to the

ALTERNATION OF GENERATIONS

8l

mesoderm and endoderm — as inactive and 'unalterable' acces-
sory idioplasm. These only become active, and cause budding,
when they have reached some definite part, such as the ovarv or
proliferating stolon.

The development of the higher medusas or AcalephcE by
strobilation can easily be traced from the above processes. In
these animals the sexual forms arise asexually : the polype
becomes divided into disc-shaped portions, and so comes to
resemble a pile of saucers, each disc eventually being trans-

FiG. \\. — Developmeiit of MednscE by strobilation — i, the young larva; 2-5, its
development into a polype; 7, a polype viewed from the oral pole; 6, 8,
and 9, transverse division of a polype into disc-shaped portions; 10, the con-
striction of these portions into young Medusae; 11 and 12, a young Medusa.
(From Hatschek's ' Lehrbuch der Zoologie.')

formed into a medusa. If the medusa undersvent division, the
process would be one of simple regeneration : the differentiation
of one of these discs into a medusa depends on a mechanism in
the idioplasm exactly similar to that which gives rise to the process
of regeneration in a worm the anterior end of which has been
cut off, or which has undergone spontaneous division. The

I 82 THE GERM-PLASM

various cells in the body of the polype must be provided with
different groups of determinants of the medusae in the form of
inactive accessory idioplasm, and these must become active in
the process of strobilation, and cause the development of highly
complex medusie with eight or more radii, and provided with
eyes, auditory organs, and olfactory pits. The difference between
this process and that of simple division followed by regeneration,
consists in the fact that in the latter the supplementary deter-
minants of the cells of the body are of the same kind as those
from which the body was constructed : in strobilation, on the
other hand, the germ-plasm of the egg- and sperm-cells, which
gives rise to the sexual generation or medusa, must contain not
one, but two kinds of ids, viz., those of the polype and those of
medusa ; the latter, although they remain inactive during the
ontogeny of the polype, and take no part in the control of the
cell, are nevertheless not absolutely unalterable, for they break
up during ontogeny into many different groups of determi-
nants, and at the same time become distributed among different
cells in a regular and perfectly definite manner. It is very
probable, however, that a// the cells of the polype — those of the
ectoderm as well as of the endoderm — are provided with acces-
sory determinants, so that each cell of the polype contains in
addition the primary constituents of some cell of the medusa.
We have, however, no positive knowledge on this point, for no
investigations have as yet been made with regard to the succes-
sion of the cells which lead to the formation of the medusa from
the polype.

T/ie basis of the alternation of generations as regards the idio-
plasm must therefore in all cases consist of a germ-plasm co?n-
posed of ids of at least two different kinds, which ultimately take
over the control of the organism to which they give rise.

THE FORMA'l'ION OK CERM-CEI.T.S 1 83

CHAPTER VI

THE FORMATION OF GERM-CELLS
I. The Continuity of the Germ-plasm

If heredity depends on the presence of a substance, the
germ-plasm, which causes the production of the new individual
by directing the process of division in ontogeny, in the course of
which it becomes changed in a regular manner, the question
arises as to how unaltered germ-plasm can nevertheless reappear
in the orerm-cells of the new individual. The transmission of
characters from parent to child can only depend on the germ-
cell from which the offspring arises containing ids of germ-plasm
precisely similar to those of the germ-cell from which the parent
was developed. The germ-plasm, however, undergoes an enor-
mous number of changes during the development of the ovum
into the parent : how is it possible therefore that this substance
can reappear in the germ-cells of this parent?

There are obviously two possible solutions of this problem.
The changes which the germ-plasm undergoes during the con-
struction of the body must either be of such a kind that they
can take place in the reverse order when the idioplasm of all, or
at least of a portion of, the somatic cells is re-transformed into the
germ-plasm from which it was, in fact, indirectly derived ; or, if
such a reversal is impossible, the germ-plasm of the germ-cells
must be handed on directly from parent to offspring. This
latter hypothesis was suggested by me some years ago under the
name of the contimiity of the germ-plasm.* A third solution of
the problem is impossible, for it is quite out of the question that
the germ-plasm can be entirely formed anew.

The hypothesis of the continuity of the germ-plasm depends
on the assumption of a contrast between the somatic and the
reproductive cells, such as can be observed, in fact, in all multi-
cellular plants and animals, from the most highly differentiated
forms to the low-est heteroplastids amongst the colonial Algae.

* ' Die Continuitat des Keimplasma's als Grundlage einer Theorie der
Vererbung; Jena, 1885 (English translation, 2nd ed., p. 163).

184 THE GERM-PLASM

I assume that germ-cells can only be formed in those parts of
the body in which germ-plasm is present, and that the latter is
derived directly, without undergoing any change, from that
which existed in the parental germ-cell. Hence, according to
my view, a portion of the germ-plasm contained in the nucleus
of the egg-cell must remain unchanged during each ontogeny,
and be supplied, as such, to certain series of cells in the develop-
ing body. This germ-plasm is in an inactive condition, so that
it does not prevent the active idioplasm of each cell from im-
pressing a specific character on the latter in a greater or less
degree. It must, moreover, differ from ordinary idioplasm,
inasmuch as the determinants it contains are kept closely to-
gether, and are not distributed in groups among the daughter-
cells. This accessory germ-plasm is thus passed on in an
unalterable condition through longer or shorter series of cells,
until it ceases to be inactive in a certain group of cells, more or
less remote from the egg-cell, and then impresses upon the par-
ticular cell the character of a germ-cell. The transmission of
the germ-plasm from the ovum to the place of origin of the re-
productive cells (' Keimstatte ") takes place in a regular manner,
through perfectly definite series of cells which I C2i\\ germ-tracks.
These are not actually recognisable, but if the pedigree of the
cells in the embryogeny is known, they may be traced from their
termination in the germ-cells backwards to the ovum.

This assumption is supported by the fact that a direct, or at
any rate a very close, connection can be proved to exist, although
only in rare instances, between the germ-cells of two consecu-
tive generations. In the Dipt era the first division of the egg-
cell separates the nuclear material of the subsequent germ-cells
of the embryo from that of the somatic-cells, so that in this
case a direct continuity can be traced between the germ-plasm
in the germ-cells of the parent and offspring.

The process in this case must certainly, however, be looked
upon, not as a primary one which has been passed on unchanged
from very ancient times, but as a special arrangement peculiar
to this order of insects. It nevertheless proves the possibility
of each generation of germ-cells being derived directly from the
preceding one, and also that the germ-plasm which has been
prevented from taking part in the construction of the somatic
portion of the embryo is not required in this process.

We may next take the case of the embryogeny of the

THE FORMATION OF GERM-CELLS 185

DaphnidcF. In these animals the primary germ-cells become
separated from the somatic cells in the first stages of the seg-
mentation of the Q.gg \ and in Sagitta again, this separation
takes place at the gastrula-stage. In Vertebrates this process
occurs much later, although it always takes place within the
first half of embryogeny ; while in Hydroids — both in colonial
and solitary forms — the germ-cells do not appear in the * per-
son ' which is developed from the ovum at all, and only arise in a
much later generation, which is produced from the first by con-
tinued budding. The same is true as regards the higher plants, in
which the first shoot arising from the seed never contains germ-
cells, or even cells which subsequently become differentiated
into germ-cells. In all these last-mentioned cases the germ-
cells are not present in the first person arising by embryogenv
as special cells, but are only formed in much later cell-genera-
tions from the offspring of certain cells of which this first person
was composed. These ancestors of the germ-cells cannot be
recognised as such : they are somatic cells, — that is to say, tkey,
like the numerous other somatic cells, take part in the construc-
tion of the body, and may be histologically differentiated in
various degrees.

A series of organic species might therefore be formed in
which the formation of the germ-cells begins at very different
degrees of remoteness from the egg-cell. This would admit of
the interpretation that the fertilised egg-cell of the earliest
Metazoa first divided into two cells, one serving. for the forma-
tion of the body (soma), and the other for that of the germ-
cells ; and that a shifting had occurred subsequently, owing
to a separation of the material for both parts in the germ-plasm,
so that the portion of the germ-plasm which remained un-
changed was supplied in an inactive condition, in the form of
accessory idioplasm, to one of the somatic halves of the egg-
cell, and was transmitted by the latter to a somatic cell of
the second, third, or fourth generation. The shifting of the
process of separation into germ-cells and somatic cells finallv
reached its extreme limit, as in the case of the Hydroids, and
the unchanged germ-plasm of the fertilised ovum then only led
to the formation of germ-cells after passing through a long series
of somatic cells.

These facts do not, however, as yet constitute an actual proof
of the correctness of this interpretation : they might be taken

I 86 THE GERM-PLASM

as indicating that the series has been developed in the reverse
direction, the late differentiation of the germ-cells being the
primary condition and the earlier separation of the two parts
then having arisen gradually in individual cases. There can
hardly be any doubt, indeed, that the early differentiation of the
germ-cells of the Diptera and Daphnidae is of a secondary
nature ; and it will presently be shown that in the case of
Hydroids such a shifting of the formative areas (' Bildungs-
statte") of the germ-cells — /.6'., the fact of their earlier differen-
tiation — can be actually proved. But the facts which have been
stated still support the interpretation of them given above, in so
far as they show that the germ-cells are by no means formed at
the time and in the place where they are actually wanted, and
that the time of their formation, in fact, varies very much, and
must have been changed in the course of phylogeny. The
direction in which this shifting originally took place — that
is to say, whether it proceeded from the egg to the close of on-
togeny or in the reverse direction — must be decided when our
knowledge of the facts is more complete.

We might here lay stress on the fact that the destruction of
the sexual glands in an animal, however low in the scale, is not
followed by the formation of sexual cells in any other part of the
body. Castration might be expected to have this effect if germ-
cells could be formed from any young cells of the body. But
just as in the case of any other highly specialised organs, such
as the liver, kidneys, and central portions of the nervous system
in Vertebrates, such a replacement never occurs. This fact is
to be explained according to our present view by supposing that
the formation of these latter organs anew is impossible, because
the determinants necessary for such a development are not
present in any other cells of the body. The same conclusion
will, it seems to me, be inevitable in the case of the germ-cells ;
the idioplasm necessary for the format ion of germ-cells — i.e.,
germ-plasm — must be absent in these cases, and germ-plasm at
any rate caiuiot be formed from somatic idioplasm.

The case of the Hydroids * is probably still more convincing,
for here a natural shifting of the place of origin of the germ-cells
has actually taken place. As has already been mentioned, the
germ-cells of Hydroids first arise very late in the life-cycle,

* Weismann, ' Die Enstehung der Sexualzellen bei den Hydromedusen,'
Jena, 1883.

THE FORMATION OF GERM -CELLS

187

and hundreds or even thousands of cell-generations are passed
through from the fertilised egg-cell onwards before they appear.
In species which exhibit a complete alternation of generations,
they are first formed in particular parts of the medusae which
have arisen from the polype-stock by budding — usually in the
ectoderm of the manubrium. No trace of them is to be seen in

D

Fig. 12. — Diagram of the degeneration of the Medusa into a mere gonophore.
A, Medusoid bud; B, a Medusa shortly before it is set free; C, degenerated
Medusa, in which the manubrium is present, but the mouth and tentacles are
wanting; D and E, further stages in degeneration. (From Hatschek's ' Lehrbuch
der Zoologie.')

the young bud, and in many cases thev only become differentiated
from the other ectoderm-cells after the medusa has become de-
tached from the stock, and has developed into an independent.

1 88 THE GERM-PLASM

free-swimming animal. Some of the ectoderm cells of the part
in question then become transformed into egg- or sperm-cells.

In the case of certain species of polypes, free sexual medusae
were produced in the earlier period of the development of the
species, but at the present day these do not become detached,
but always remain attached to the stock : they thus no longer
serve for the dispersal and ripening of the sexual products but
only for their production and ripening. These species illustrate
the different stages in the process of degeneration of the medusae
to mere gonophores, or sexual sacs. In some species the form
of the medusa is completely retained in the sexual persons of the
stock, only the eyes and marginal tentacles being absent ; in
others, the bell has become degenerated into a closed sac, the
walls of which still retain the circular and radial canals ; and in
other species again, these canals have also disappeared, only the
three characteristic layers of the medusa remaining, and even
these have become so thin that their presence can only be de-
tected in microscopic sections. Finally, these three layers also
undergo degeneration, the w^all then consisting of a single layer,
so that the derivation of the sac from the bell of the medusa can
only be proved indirectly. Throughout all these stages of de-
generation, however, the ova or spermatozoa always ripen in the
gonophores.

The behaviour of the germ-cells is the chief point of interest
to us in the course of this process of degeneration. For the
entire degeneration of the medusa proceeds from its germ-cells,
and 'is due to the fact that t/ie development of the latter has
gradually to be thrown back to earlier stages, so that the sexual
elements are ripened more quickly.

It will not be necessary to enter into the reasons for this
hastening of the sexual maturity ; it is sufficient to know that in
some species in which the medusae become detached, e.g., Podo-
coryne cornea., the egg-cells are developed earlier than the
medusae in which they subsequently ripen, and in proportion as
the degeneration of the medusa advances the place of origin
of the germ-cells recedes more and more into the older and
earlier formed parts of the stock. The advantage of this is,
that the germ-cells develop earlier, and afterwards enter the
germ-sacs in a riper stage : they thus reach maturity much more
quickly.

The remarkable thing about this process is the fact that

THE FORMATION OF GERM-CELLS 1 89

active migrations of the germ-cells take part in it. Originating
in the ectoderm, these cells wander into the endoderm, and
subsequently back again into the ectoderm ; and this remark-
able process occurs in a definitely prescribed and regular
manner. In spite of the relegation of their place of origin to
earlier persons of the stock, the germ-cells always originate from
the same layer of cells as that from which they arose in the
ancestors of the species. It may thus be said that they are
developed ontogenetically from the ancestors of those cells from
which they would have arisen if the polype stock still produced
free medusae ; or, in other words, they arise lower down on
the germ-track at present than they did formerly. Thus in
Hydractinia echinata, for instance, the youngest egg-cells first
become visible in the endoderm of certain polypes in the same
regions from which gonophores (degenerate medusas) subse-
quently bud out. The egg-cells then migrate into the latter, and
enter the ectoderm of the manubrium as soon as it is formed ;
and in this way they return to the old place of ripening, which in
earlier ti?nes was also the place in which they were formed. At
the present day, however, the egg-cells only apparently originate
in the endoderm of the polype : it can indeed be proved that
they are derived from the ectoderm, but migrate into the endo-
derm while still in a very young condition, before they exhibit
the definite character of egg-cells. They therefore originate in
the same region in which at an earlier phyletic period the
ectodermal layer of the manubrium of the medusa was devel-
oped ; or, in other words, the same ontogenetic series of cells
which produce the egg-cell at the present day did so in former
times. This fact probably only admits of one interpretation,
and this is. that 07dy certain series of cells contaijt the primary
constituents of the gerjn-cells, and wherever it became useful in the
course of phylogeny for the germ-cells to be situated in another
position and in another layer of the body-wall, this change of
position could only be effected by the ceils of the germ-track
becoming transformed into germ-cells at an earlier stage, and at
the same time migrating into the other layer of the body-wall.
If any — I will not say all — of the cells could give rise to germ-
cells, this complicated mode of procedure would be quite inex-
plicable, for Nature always takes the shortest possible course.

If this reasoning is correct, the hypothesis of the germ-tracks.
as I have formerlv stated it, is inevitable ; and the fact that the

ICjO THE GERM-PLASM

cells lying in these tracks are alone capable of giving rise to
germ-cells, can hardly be explained otherwise than by assuming
that these cells alone contain germ-plasm along with their
special idioplasm. If germ-plasm could be produced from the
idioplasm of ordinary somatic cells, it would be impossible to
see why germ-cells should not be formed in Hydroids in case of
need by the transformation of young ectoderm cells : but this
never happens. And even if we wished to assume that the
endoderm cells, as such, possessed an idioplasm which could not
be transformed into germ-plasm, while the nature of the ecto-
derm cells rendered such a transformation possible, this assump-
tion would be contradicted by other facts : for, as far as we
know, the germ-cells arise exclusively from the endoderm cells
in the higher medusae, and in the polypes nearly related to
them. In this case therefore the germ-tracks are situated in
the endoderm, — that is to say, the germ-plasm is only passed
into certain series of cells in the endoderm, and the reserve
material of unalterable germ-plasm, which will serve for the
formation of the germ-cells, is passed into the primary endo-
derm cell only in the process of segmentation of the ovum, and
is handed on by it. In the Vertebrata the germ-cells become
differentiated from certain groups of mesoderm cells, and they
are never found in any other part of the body. In this case
the germ-track passes from the fertilised egg-cell into those
segmentation cells from which the primary cells of the whole
mesoderm are formed, in which latter it follows a closely con-
fined course.

All these facts support the assumption that somatic idioplasm
is never transformed into germ-plasm, and this conclusion forms
the basis of the theory of the composition of the germ-plasm as
propounded here. It is obvious that its composition out of deter-
minants which gradually split up into smaller and smaller
groups in the course of ontogeny, cannot be brought into
agreement with the conception of the re-transformation of
somatic idioplasm into germ-plasm. If, as we have assumed,
each cell in the body only contains o;ie determinant, the
germ-plasm — which is composed of hundreds of thousands
of determinants — could only be produced from somatic idio-
plasm if cells containing all the different kinds of determinants
which are present in the body were to become fused together
into o/ie cell, their contained idioplasm likewise coml^ining to

THK FORMAl I(3x\ OK GERM-CEU.S IQI

form one nucleus. And, strictly speaking, even this assumption
would be by no means sufficient, for it does not account for the
architecture of the germ-plasm : the material only would be
provided. Such a complex structure can obviously only arise
historically.

The fact that somatic idioplasm cannot again give rise to
germ-plasm serves as an additional support for the theory of the
germ-plasm as here developed. Invisible, or at any rate un-
recognisable, masses of unalterable germ-plasm must ha\e been
contained in the body-cells in all cases in which such a trans-
formation has apparently occurred.

These masses need not necessarily be invisible, for they can-
not be smaller than ids ; and if it should subsequently be proved
that the microsomes of the nuclear rods do actually correspond
to ids, we may hope to ascertain the exact number of these ids
in the individual species. An extensive field would then be
opened out for further investigation, for it would be possible to
decide by direct investigation whether the cell-series of the
germ-tracks carry along with them a larger or a smaller number
of ids than is contained in the fertilised egg-cell, and also the
relative proportion of the number of ids in the somatic cells in
the germ-track. We may thus hope that facts will come to light
which can be utilised in connection with this theory.

Observations of this kind have already been made which indi-
cate an actual continuity of the germ-plasm. Boveri * observed
that the differentiation of the somatic cells from the primary
sexual cell in the segmenting &gg of Ascaris tnegaloccp]iala is
accompanied by a peculiar diversity in the nuclear structure.
The nuclei of the somatic cells throw off a large part of their
chromatin, in which process each idant loses a similar amount
of substance. Further facts and illustrations of the process are
still wanting, but even did we possess them it would be neces-
sary to postpone the detailed theoretical explanation of such
observations until we were able to judge as to the universal
occurrence of the process. Observations which my assistant.
Dr. V. Hacker,t has made on the segmenting ovum of a crus-

* Theodor Boveri. 'Anatom. Anzeiger II. Jahrgang," No. 22, 1887; and
' Zellen-Studien,' Heft 3, Jena, 1890, p. 70.

t Valentin Hacker, Zool. Jahrbuclier, Abtli. f. Anat. und Ontog.. Bd. v.,
1892; and Archiv. f. mik. Anat., Bd. 40, 1892.

192 THE (lER.M-PI.ASM

tacean {Cyclops), have indeed also proved that the behaviour of
the somatic segmentation cells is different from that of the
primary sexual cell, but the process differs essentially from that
which occurs in Ascaris. When we are in possession of similar
observations on various types of animals, we shall be able to
recognise the essential parts of the process, and shall then be
in a position to offer an explanation of them.

From a theoretical point of view, we must expect that the ids
of germ-plasm become doubled in the nucleus of the fertilised
egg-cell or even previously, one half of such a double id being
in the active condition in which it can undergo disintegration,
and the other being in the inactive and unalterable condition.
The former direct ontogeny, and the latter are passed on in a
passive condition to the primary sexual cells. As these, how-
ever, behave at first like somatic cells, — that is to say, they
multiply in a regular manner, and are distributed amongst defi-
nite series of cells to definite parts of the body, — they must
possess active idioplasm in addition to unalterable germ-plasm.
They must therefore contain 7nore ids in their nuclear matter
than do the somatic cells. The above-mentioned observations
on Ascaris can thus be explained in accordance with our theory
up to this point, but more than this cannot be stated at present.

2. The Germ-tracks

Taking sexual reproduction only into consideration for the
present, the course of the germ-tracks in existing Metazoa ap-
parently varies both as regards its length and the direction it
takes. The germ-track is shortest in the Diptera, in which the
primary germ-cell becomes separated off in the first division of
the ovum, so that in this case we might speak of a division of the
ovum into a primary germ-cell and a primary somatic-cell. In
the Daphnidce the germ-track is longer ; for, counting from the
ovum, five successive divisions occur before the primary germ-
cell is formed. In the free-swimming marine worm Sagitia it is
longer still, two primary germ-cells only appearing after ten or
more successive divisions have occurred, and the mass of
embryonic cells has already given rise to a gastrula-larva. In
other worms, such as the Nematodes, the primary germ-cells
become separated from the somatic cells in a still later genera-
tion of cells, wliicli lias so far not been actually determined ; and

THE KORMATION OF (JKk.M-CKLLS

193

in most of the higher Metazoa this only occurs after the forma-
tion of hundreds or thousands of cell-generations.

The position of the germ-track may also vary. In the Diptera
it is quite distinct from the somatic cell-tracks, and the <renea-
logical trees of these two kinds of cells separate at the root. In
the Dap/inidcE the germ-track passes through each of the first four
segmentation-cells, and then branches ofl" from the somatic

Fig. 13. — Three early stages in the development of Sagitta. — (After ( ). Heriwiy.;
In A the differentiation of two primitive germ-cells (g) from the endoderm, and
in B and C the multiplication and separation of these cells is shown. (From
Lang's ' Lehrbuch der vergleichenden Anatomic.')

tracks. In Sagitta it passes through the primary endoderm
cell, and the primary germ-cells separate from the primary
endoderm before the definitive endoderm of the alimentary canal
has been formed from the latter. In RJiabditis )ii^rove}iosa the
germ-track extends through three generations of endoderm cells,
passes into the primary mesoderm, and after several generations
branches off from two of the mesoderm cells. In most of the
Metazoa, however, the formation of the primary germ-cells is
postponed to a still later period, so that the separation of the
germ-branch from the somatic branch takes place at a much
higher level on the genealogical tree of the cells, and often first
occursin the younger and smaller lateral branches. The primary
germ-cells do not always branch off from the track of the endo-
derm, but may just as often diverge from that of the ectoderm.
In the lower Medusae, for instance, in which the development is
a direct one, the germ-cells become differentiated at a very late
stage from the ectoderm cells of the body, which is already
fully formed and often independent and self-supporting ; while
in the higher Medusae and Ctenophora the primary germ-cells
are derived from the endoderm. We thus see that the germ-
tracks or series of cells which lead from the egg-cell to the

194

THE GF.RM-PLASM

primary germ-cells frequently take very different courses : they
are in some cases very short, and in others Ioniser — sometimes
so long that they pass through very different embryonic cells ;
in some instances they branch off from the primary endoderm-
cells. and in others from those of the mesoderm, and they mav
even arise from later generations of the mesoderm, ectoderm, or
endoderm-cells.

ms

Fig. 14. — Three Stages in the development of the siimtner eggs of Moina. —
(After Grobben.) A, Stage of segmentation viewed from the vegetative pole,
in which thirty-two cells are present; B, Blastula stage, from the vegetative pole;
C", Gastrula stage, in longitudinal section; g, the primitive germ-cells. (From
Korshelt and Heider's ' Lehrbuch der vergleichenden Entwickelungsgeschichte.')

The same germ-track is always strictly followed in each of
these cases respectively, no deviation ever taking place : thus
the primary germ-cells never arise from endoderm-cells in a
group in which the normal germ-track lies in the ectoderm,
and vice versa. We consequently cannot help arriving at the
conclusion that the cells in the germ-track fnust have some
advantage over the rest of the cell-tracks in ontogeny, for they

THE FORMATION OF GERM-CELI.S

195

alone are capable ofghnng rise to the pri)nary gerjns-cells . M ore-
over, if we remember that in the case of the Hydroid polypes
the period of the separation of the primary germ-cells can be
relegated to earlier or later stages, it will be clear that not only
the cells at the terminations of the germ-track in which this
separation actually occurs in indi-
vidual cases, but also the entire
preceding series of cells, possess
qualities which are absent in the
other cells of the organism, and
which, sooner or later, render the
cells of the germ-track capable of
giving rise to primary germ-cells.

The cells of the germ-track do not
themselves correspond to primary
germ-cells, the character of which
latter is not as yet apparent ; they
are cells of a mixed character, — that
is to say, they contain different
primary constituents, which are
gradually separated off until event-
ually only two of them remain,
and these then separate from one
another by means of a final cell-
division.

The embryogeny of a parasitic
worm {Rhabditis nigrovenosa^ from
the frog's lung may serve to illus-
trate this point. In fig. 15, A to D
represent the first four stages in >^/y^'
segmentation up to the differentia-
tion of the primary mesoderm-cell
{mes) . This and the following stages
are represented diagrammatically
in fig. 16, which shows the 2:enea-

uvu*.

Fig. 15. — Stages tu the segmen-
tation of the ovum and forma-
tion of the germinal layers in
Rhabditis nigrovenosa. — (After
Gotte.) ect. Ectoderm; ent, En-
doderm; ;//<'j, Mesoderm.

logical tree of the cells and the
germ-track. The ovum (^Kiz) must of course be considered as
containing the whole of the primary constituents of the organism
before its first division into a primary ectoderm {itrEkt) and a
primary endoderm cell (i/rEtit). The latter retains all the
primary constituents of the mesoderm and primary germ-cells.

196

THE GERM-PIASM

in addition to those of the endoderm. and is therefore not
merely a primary endoderm cell. This then divides again and
forms two cells, of which the one marked 3 on the left side of
the figure only contains primary constituents of the endoderm,
and is therefore an endoderm cell proper ; while that marked 3'
represents the first rudiment of the mesoderm and of certain
portions of the endoderm, and contains in addition the primary
constituents of the primary germ-cells. This cell (3') divides into
two (4' and 4"). thus separating the above-named rudiments into
those for the right and the left sides of the body ; and finally, the

y^^<E/r.

Fig. 16. — Diagram of the germ-track of Rhabditis tn'grovenosa. — The various
generations of cells are indicated by Arabic numbers, the cells of the germ-track
are connected by thick lines, and the chief kinds of cells are distinguished by vari-
ous markings: — the cells of the germ-track by black nuclei, those of the meso-
blast (Mes) by a dot in each, those of the ectoderm (Eki) are white, those of the
endoderm (Efii) black; in the primitive germ-cells (ur Kz) the nuclei are white.
The cells are only indicated up to the twelfth generation.

complete separation of the rudiments of the mesoderm and endo-
derm occurs, and results in one daughter-cell (5")- containing the
primary constituents of the mesoderm and primary germ-cells,
while the other gives rise to a cell of the endoderm proper.
The primary constituents of the primary germ-cells remain con-

THE FORMATION OF GKRM-CELLS I97

nected with certain of those of the mesoderm during several
generations of cells, and in each subsequent division certain of
the latter pass out alone into one of the daughter-cells, the
other retaining the primary constituents of the primary germ-
cells in addition to those of the mesoderm. Finally, in the
ninth series of cells in the diagram — in which the processes are
represented as greatly abbreviated — the separation of these two
sets of primary constituents occurs, and the first primary germ-
cell {itr Kz) is formed.

So much is certain, and does not depend on any hypothesis.
Opinions may dififer as to whether the cells situated in the germ-
track are to be described as real somatic cells. I have called
them so, because in many cases the germ-tracks extend far
beyond the period of embryogeny into the fully-developed func-
tional tissues, and because it can be proved that eiien cells which
are histologically differetitiated may produce germ-cells under
certain circu?nstances. This occurs amongst plants — in the
prothallus of ferns, for instance — and also in the cells of certain
Polyzoa from which gemmation may take place, and which must
therefore contain inactive germ-plasm. In these cases it is
certain that real somatic cells are situated along the germ-
tracks ; in all cases the cells of the germ-track are not
germ-cells from the firsts and they always take part in the con-
struction of the body. And if we further consider that a large
number of somatic cells must contain accessory idioplasm of
.some kind, — either that which will serve for simple regeneration,
or for the regeneration of more complex parts, or again, for the
formation of buds, — we can hardly assume that the character of
a somatic cell is thereby abolished : I can see no advantage ///
objecting to describe a cell of the germ-track as a somatic cell.

The change which the idioplasm of the cells constituting the
germ-track undergoes, can obviously only consist in its active
portion gradually becoming separated off in the course of the
ontogenetic cell-divisions, so that ultimatelv the cell contains
germ-plasm only, which then stamps it as a germ-cell. Even
then the germ-plasm remains unalterable as long as this first
or primary germ-cell continues to produce others similar to
itself. The cells only become differentiated into spermatozoa
and ova when this multiplication ceases, and this presupposes
the splitting off of special spermatogenetic or ontogenetic de-
terminants. The disintegration of the germ-plasm which results

198 THE GERM-PLASM

in a new embryogeny — provided that tlie necessary conditions
have been fultilled — can only begin when this has occurred.

3. Historical Account of the Theory of the Conti-
nuity OF THE GeRxM-PLASM

When my essay on the ' Continuity of the Germ-plasm '
appeared seven years ago,* I was under the impression that I
was the first to give utterance to this conception. Since then,
however, I have found that similar ideas had arisen, in a more
or less distinct form, in other brains ; and I gradually discovered
that a number of authors had independently recognised more or
less clearlv the distinction between the bodv-cells and germ-
cells and the direct connection between the germ-cells of dif-
ferent generations : some had merely made the assertion, and
others had attempted to support it by facts. I shall here give
an account of those theories which preceded mine, taking them
in chronological order.

As early as 1849, Sir Richard Owen had indicated that
differences may arise in the developing germ-cells between
those which become greatly changed and form the body, and
those which only undergo a slight change and form the repro-
ductive organs, t

Francis Galton was the first to express certain ideas which
bore some resemblance to the conception of the continuity of
the germ-plasm. In a paper which appeared as early as 1872.
the individual is conceived ' as consisting of two parts, one of
which is latent, and only known to us by its effects on his pos-
terity, while the other is patent, and constitutes the person
manifest to our senses. The adjacent and, in a broad sense,
separate lines of growth in which the patent and latent elements
are situated, diverge from a common group, and converge to a
common contribution, because they were both evolved out of
elements contained in a structureless ovum, and they jointly
contribute the elements which form the structureless ova of their
offspring.'' J

* ' Der Continuitiit des Keimplasma's,' Jena, 1855 (Essay iv., p. 163, in
the second English edition).

1 1 quote this statement from Geddes and Thomson's ' Evolution of
Sex ' (London, 1889), p. 93, in which the original authority is not given.

X Proc. Roy. Soc, No. 136, p. 394.

THE FORMATION OF GERM -CELLS I 99

A few years later, Galton changed his opinion and adoi)ted
Darwin's theory of pangenesis, which he modified considerably,
and only used ' as a supplementary and subordinate part of a
complete theory of heredity.' This theory has already been
discussed in the Historical Introduction to this book. The
• cremmules ' which are contained in the fertilised ovum together
constitute the ' stirp ' or stock, which by means of the egg-cell
gives rise to a new individual. Each 'sort of gemmule " is repre-
sented by a number of gemmules which differ somewhat from and
compete with one another ; and since the successful ones in the
competition for taking part in the construction of the body form
the various parts of the body and are therefore contained in
them, the rest remain unused, thus constituting the • residual
germs.' These, then, are ' the parents of the sexual elements and
buds.* The 'dominant' gemmules may also take a part, though
only a slight one, in the formation of the germ-cells, ' as they
are the least fertile in the production of gemmules." The germ-
cells are therefore mostly formed from gemmules which have
remained latent, and this accounts for the fact that the offspring
usually do not exhibit the most marked peculiarities of the
parent. As this hypothesis only accounts for the dissimilarity
between parent and child, so far as it exists, and not for the far
commoner resemblance between them. Galton assumes that the
parts of the body can also give off gemmules which become dis-
tributed and extend beyond the boundaries of the cells in which
they arose, and so may even penetrate into the sexual elements.
He thus substitutes the idea of a locally restricted distribution of
the o;emmules for Darwin's view of their • free circulation.' If we
attempt to make this somewhat vague and unrealistic idea rather
more comprehensible, by considering the • residue of the stirp '
as equivalent to the ' unalterable ' reserve germ-plasm, Galton's
hypothesis will be found to bear some resemblance to the theory
of the continuity of the germ-plasm. But there is still a
fundamental difference between them, for Galton's idea is only
conceivable on the presupposition of the occurrence of sexual
reproduction, while the theory of the continuity of the germ-
plasm is entirely independent of any assumption as to whether
each primary constituent is present in the germ sini:^lv or in
numbers. According to my idea, the active and the reserve
germ-plasm contain precisely similar primary constituents, gem-
mules, or determinants : and on this the resemblance of a child

200 THE GERM-PLASM

to its parent depends. The theory of the continuity of the germ-
plasm, as I understand it. is not based on the fact that each
' gemmule ' necessary for the construction of the soma is present
many times over, so that a residue remains from which the germ-
cells of the next generation may be formed : it is founded on the
view of the existence of a special adaptation, which is inevitable
in the case of multicellular organisms, and which consists in the
germ-plasm of the fertilised egg-cell becoming doubled primarily,
one of the resulting portions being reserved for the formation of
germ-cells.

Gustav Jager * was the first to express the idea that in the
higher organisms the body consists of two kinds of cells, which
he calls respectively • ontogentic ' and ' phylogentic : ' and that
the latter, or reproductive cells, are not a product of the former,
or body-cells, but are derived directly from the germ-cell of the
parent. f He took it for granted that the 'formation of repro-
ductive substances occurs in an animal during the early em-
bryonic stages,' and imagined that he had thus proved the
existence of a connection betw^een the germ-cells of the parent
and those of the child. Although these opinions were not
founded on fact or followed out in detail, they ought to have led
to further ideas on the subject. They, however, together with
the book in which they were contained, remained unnoticed.

A few casual remarks made by Rauber,:}: in a paper on " Form-
bildung und Formstorung in der Entwicklung von Wirbelthieren,'

* Gustav Jager, ' Lehrbuch der allgemeinen Zoologie,' Leipzig, 1878,
II. Abtheilung.

t The praiseworthy attempt to do justice to my predecessors in this par-
ticular subject has perhaps Vjeen carried too far. In Geddes and Thomson's
' Evolution of Sex ' (p. 93), for instance, a quotation is given from Jager
which seems to prove that he anticipated me with regard to the theory
under consideration. The quotation in which this idea is expressed is,
however, not taken from the book published in 1878, but from an essay
written ten years later, and it concludes with the following words : — ' This
reservation of the phylogenetic material I described as t/ie couthiuity of the
germ-plasm! But no mention is made by Jager of the continuity of the
germ-plasm in his book which appeared in 1878, in which a connection
between the ^ftxxn-cells of different generations is supposed to exist : — and
this is not the case. The entirely new statement of his ideas has been
influenced by those contained in my essays which had appeared in the
meanwhile.

+ ' Morphol. Jahrbuch,' Bd. 6, 1880.

THE FORMATION OF GERM-CELI.S 201

suffered the same fate. This author states that 'as regards the
effect of fertilisation, it can only convert a 'portion of the eirir,
viz., the personal part, into the form of a person : the other
portion does not experience this effect, for it has a stronger
power of persistence."

Finally, Nussbaum * was likewise led to the idea of the con-
tinuity of the germ-^<?/A\ He, too, assumed that • the segmented
ovum divides into the cell-material of the individual and the
cells for the preservation of the species,' and he supports this
statement by quoting the cases already mentioned of the very
early differentiation of the sexual cells.

I will conclude this section with the words which appeared in
the preface to a short paper intended as a defence against the
accusation of plagiarism which had been made against me.
'A fertile scientific idea has rarely appeared without having
been contested on the one hand, and set down as already known
on the other. The former is certainly a perfectly justifiable and
even necessary course of proceeding, for a clear and definite knowl-
edge of the truth can only result from the contest of opinions :
and even the latter is to some extent justifiable, for an idea of
this kind probably very rarely arises without having been pre-
ceded by similar attempts directed towards the same object ;
and it is only natural that those who first made such attempts
should overlook the difference between these struggles towards
the desired object and its attainment."

Others may decide the reason why no attention had been
drawn to the suggestions mentioned above as having been made
previously to my theory of the continuity of the germ-plasm,
and why these did not exert any influence on scientific thought.
This is certainly the case : and it practically follows from the
fact, that all the objections which have been made have been
directed against tne. Some of these objections will be discussed
in the following chapter. That I am far from desirous of de-
tracting from the merit of others, has, I trust, been shown by
the fact that as soon as I became aware of previous views on
the subject I brought them forward. Jager's ideas, for instance,
might have long remained unnoticed, had not I brought them
to light. But an historical account of the various previous views

* M. Nussbaum, ' Die Differenzirung des Geschlechts im Thierreich,'
Archiv. f. mikr. Anatomie, Bd. xviii., 1880.

202 THE GERM-PL.\SM

on this subject * cannot be considered to be an impartial one,
if no 7nentio)i is at the same time fnade pf the fact that all these
suggestions remained iDinoticed^ and had no effect on the prog-
ress of scientific thougJit. That this is the case there can be
no doubt. And although it may be a satisfaction to every one
to have expressed a correct idea, no such idea can be con-
sidered as fertile, and as having an important influence on the
progress of scientific thought, unless its meaning is so obvious
that it results in further progress. Such a result, however, only
followed after my essays had appeared.

4. Objections to the Theory of the Germ-plasm

Important objections to this theory have been raised by
several botanists ; and at first sight the facts on which these
are based may easily give rise to the impression that the
theory cannot be carried out in the case of plants. If this were
so, however, its correctness would be altogether doubtful, for the
hereditary mechanism cannot be totally different in plants and
animals. We must therefore make a closer examination into the
facts as they concern plants, and I hope to be able to show that
the fundamental ideas which I have assumed are applicable to
plants as well as to animals, although they did not originate
from the botanical point of view.

Certain misconceptions and inaccurate representations must
first be put on one side. Many botanists deny the existence of
the germ-plasm entirely.

Vines f considers the assumption of a special ' reproductive
substance ' unnecessary, as the capacity for reproduction is a
fundamental property of protoplasm. A cutting gives rise to a
complete plant, just as a broken crystal becomes complete when
immersed in the mother-liquor, for it produces the missing
parts, viz., roots. It is not necessary to assume the existence of
a special • reproductive substance ' in either case.

I need not especially emphasise the fact that this stimulus
which results in the completion of a part is not by any means a
universal phenomenon, and that, for instance, some parts of
plants cannot be reproduced from cuttings. I shall simply

* Cf. the account given in Geddes and Thomson's ' Evolution of Sex,'
pp. 93 and 94.

\ Cf. 'Nature,' Oct. 24, 1889; and 'Lectures on the Physiology of
Plants," Cambridge, 1886.

THE FORMATION OF GERM-CELLS 203

confine myself to calling attention to the fact that even if a uni-
versal reproductive power existed in protoplasm, it certainlv
would not explain matters, /v^r this power is just what has to
be explained.

If we know, for instance, that Infusoria are able to replace
great losses of substance, — so that when the oral region is cut
off, it, together with all the cilia and other minute structures, can
be formed anew, — a proof is thereby obtained that these uni-
cellular organisms actually possess the universal reproductive
power which Vines wishes to ascribe to vegetable protoplasm.
But does this help us in the slightest degree to understand the
fact, or to explain why the ultimate particles of the cell-body
become rearranged and transformed after a loss of substance
has occurred, just as is necessary for the reappearance of the
species ? Do we thereby gain the faintest idea as to how and
why the residual particles of the cell-body are compelled to give
up their previous form and connection, and to reconstruct
exactly that part which is required in order to render the whole
complete ? The assumption of a * reproductive power ' simply
amounts to the statement of the fact that regeneration occurs ;
and this, it seems to me, is equivalent to saying that the
reproductive power is a fundamental property of vegetable
protoplasm.

In the case of the unicellular Infusorian we can. however,
hardly venture at present to attempt an explanation of this
problem, as we know very little of the vital units of which the
cell-body is composed, and of the forces situated within them.
But the case is different with regard to those organisms which
consist of many physiologically differentiated cells : in these we
are at any rate acquainted with the form and function of one
arrangement of the vital units of which the whole aggregate is
composed, and so we can attempt to deduce the functions of
the whole body from those of the units, and conversely to refer
the latter processes to a distribution of the forces amongst the
units composing the whole. We need not therefore confine
ourselves to the mere statement of the fact that a process
occurs by means of which the whole is completed, but we may
further inquire as to when this occurs, from what elements it
proceeds, how the whole body arises at all, and how so complex
a structure can be formed from the apparently simple substance
of the germ.

204 THE GERM-PLASM

In order to give a satisfactory answer to these questions, I
have assumed the existence of a germ-plasm, but have not
primarily regarded this as a 'special reproductive substance'
which is very different from all other substances in the body ;
I have looked upon it, on the contrary, as the substance which
gives rise to all the other formative substances of the entire
individual. Every part of the body contains a portion of this
substance, and the whole organism can only be formed anew
when all the portions of this controlling substance (the idio-
plasm) are combined ; that is to say, when germ-plasm is
present. The assumption of germ-idioplasm or germ-plasm
is, I consider, quite unavoidable, for we must at the present
day take it as proved that the hereditary tendencies are con-
nected with a substance. In my opinion, it is also an irrefutable
fact that this germ-plasm undergoes regular changes from the
ovum onwards : it must, indeed, undergo change from cell to
cell, for we know that the individual cell is the seat of the forces
which give rise collectively to the functions of the whole. The
forces which are virtually contained in the germ-plasm can
therefore only become apparent when its substance undergoes
disintegration, and its component parts, the determinants, be-
come rearranged. The difference in function seen in the various
groups of cells in the body compels us to suppose that these
contain a substance which acts in various ways. The cells are
therefore centres of force of different worth, and the substance
{idioplasm) which controls thetn must be just as dissimilar as
are the forces developed by tJiem.

The apparent similarity of many young plant-cells may account
for the vagueness with which Vines, following Sachs's example,
speaks of an ^embryonic substance'' {xova which reproduction is sup-
posed to proceed in all cases, and which is assumed to be present
in all ^ young ' cells. In my opinion the hereditary value of a cell
can be estimated as little by its age as by its appearance. The
mass of cells resulting from the seijmentation of an animal egg:
certainly possess the character of youth, and in a certain stage of
development these cells are all of the same age and all look
alike. They have, however, entirely different hereditary values ;
and if we are accurately acquainted with the ontogeny of the
animal in question, we can tell what hereditary tendencies lie
hidden in each cell. The primary constituents of the entire endo-
derm, for instance, mav be contained in one cell, and that of the

THE FORMATION OF GERM-CELLS 2O5

ectoderm or mesoderm in another : or, again, in a later stage,
only a rudiment of a particular part, organ, or portion of an or-
gan belonging to the germinal layer in question, may be present
in an individual cell. But if we inquire whether the entire body
could arise from each of these cells, known facts give a verv
decided answer in the negative. Only one, or a few perfectlv
definite cells amongst them, which we speak of as germ-cells,
can reproduce the whole animal under favourable circumstances.
This is true of all the higher Metazoa : the cells of the segment-
ing oviivi are coinpietely dissimilar as regards their hereditary
value, although they are all 'young'' and ^ embryonic^ and are
not iftfrequently quite similar in appearance. It therefore
seems to me to follow from this, as a logical necessity, that the
hereditary substance of the egg-cell, which contains all the
hereditary tendencies of the species, does not transmit them ///
ioto to the segmentation-cells, but separates them into various
combinations, and transmits these in groups to the cells. I have
taken account of these facts in considering the regular distribu-
tion of the determinants of the germ-plasm and the conversion
of the latter into the idioplasm of the cells in the different stages
of ontogeny. All these cells contain ' embryonic substance.' but
the determinants contained in one set differ from those in
another, and therefore contain different hereditary tendencies.
Hence it is comparatively meaningless to speak merely of an
' embryonic substance.'

De Vries regards some of mv views in a verv different wa\ .
and from an entirely different aspect. In an extremely able
manner he brings forward a number of facts concerning hered-
ity in plants, and finds that they usually do not fit in with my
views. I have followed his deductions with great interest,
and have gratefully made use of the facts which he has brought
forward ; but I nevertheless believe that the chasm which sepa-
rates his views from mine can be bridged over.

In the first place, de Vries accuses me of having taken a one-
sided view of the question by considering the processes as they
occur in animals only : in these it may be possible, as I have
assumed, to draw a sharp line of distinction between somatic-
and germ-cells, but this cannot be done in the case of plants.
In the latter, those series of cells which I have called germ-
tracks may give rise to many other cells besides germ-cells,
although this as a general rule is only exceptionally the case :

206 THE GKRM-PLASM

that is to say, it occurs in response to definite external influences.
It would not, however, only take place in those parts of the
plant which might be assumed to be specially adapted for this
capacity, but might also occur in those in which adaptation is
out of the question. We are therefore compelled to assume that
))iost, if not all, of the cells contain all the prhnary constituents
of the species in a latent condition.

I will first discuss the manner in which de Vries applies mv
hypothesis of the germ-tracks to the case of plants, and the con-
clusions at which he has arrived and has illustrated by describing
a number of genealogical trees representing the various series of
cells in plants.

De Vries draws a distinction between ' primary ' and ' accessory '
germ-tracks. The former correspond to the germ-tracks I have
already assumed : that is to say. to those cell-series which nor-
mally lead from the fertilised egg-cell to the new germ-cells (ova,
spermatozoa, pollen-grains). By 'accessory germ-tracks' are
meant those cell-series which lead to germ-cells ' through ad-
ventitious buds.' These accessory germ-tracks are, accordinsf
to de Vries, absent in the higher animals, but are of common
occurrence amongst plants, and I am accused of not having
taken them sufficiently into account. The 'accessory germ-
tracks." if I understand the term aright, are regular germ-tracks,
which do not, however, always come into use. In many of the
lower plants, such as mosses and fungi, 'almost all of the cells
may develop into new individuals ; " and in the higher plants,
buds, from which entire plants possessing germ-cells may arise,
can. under certain circumstances, be formed at any rate from
certain kinds of tissue, which may consist of young (meristematic)
cells or indeed even of full-grown cells.

Let us first consider the ' primary germ-tracks.' De Vries
thinks that their behaviour is essentiallv different in the hisher
animals and in plants : in the former, the genealogical tree of the
cells of the germ-track ' is straight, and only slightly branched at the
apex,' while in the higher plants • the branches are so numerous
and subdivided from the base upwards that they frequently over-
top the main stem ; or, more accurately, the main stem is hardly
recognisable.' No objection can certainly be raised to this state-
ment, which we may illustrate by a blossoming apple tree, in
which the blossoms which crown the top may be taken as cor-
responding to the germ-cells. But how is this difference to be

THE FORMA riON OF (iFRM-CELI^ 207

proved, and on what does it depend? It is not based on the
animal or vegetable nature of the organisms, for as de \'ries
himself incidentally acknowledges, we find a similar kind of
branching of the primary germ-tracks in the Hydroid-polvpes.
It simply depends on the fact that a higher individuality of the
stock exists in these animals, just as in the case of the higher
plants. In both cases we have to deal not with a single person
and the formation of its germ-cells, but with a number of per-
sons w^hich arose from the primary one by budding, each of
which has a bodv of its own. and gives rise to its own jrerm-
cells. The germ-track is concealed within the first person of
the stock produced from an egg, and gives off a lateral branch
as soon as this first polype develops a bud. Shortly afterwards,
the polype gives rise to a second bud. into whicli a lateral germ-
track likewise extends ; and when these two buds have developed
into complete polypes, they again give rise to buds, into which
germ-tracks are once more given off, and so on. The copious
branching of the germ-track is thus accounted for, and it is quite
immaterial whether the separate persons of the stock are more
or less independent and perfect, and to what extent they may be
regarded as • individuals.'' In those cases in which a periodic
segmentation of the body into serially homologous segments or
metameres takes place, — each of which has almost a similar
origin and is able to produce germ-cells. — the type of the genea-
logical tree of the germ-track, as described above, results.

If. however, we inquire as to the conclusions which can be
drawn from the course taken by the germ-tracks in animals and
plants, we receive a reply from de Vries which is very significant
of the way in which this problem is at present regarded by
many botanists : — the whole question wJiich I Jiave raised luith
regard to the continuity of the genn-plasni is an idle one. In
his opinion, 'the whole question as to whether somatoplasm can
become transformed into germ-plasm has no basis in fact.' ' A
germ-track,' says de Vries, 'never arises from a somatic-track."
and ' a continuity of the germ-cells exists, not merely in the
very rarest cases, but universally and without exception, although
it often takes place through a long series by means of the germ-
tracks.''

"With the exception of the last one, these sentences merely
repeat my own views, and the apparent contradiction of the latter
is simply due to the fact that de Vries adopts expressions which

208 THE GERM-PL.ASM

I have used in another sense. In stating that germ-cells arise
from somatic cells in innumerable cases, I referred to the so-
matic cells which are situated along the germ-track, the existence
of the latter being assumed for this special purpose. De Vries,
however, disputes the somatic character of these cells, because
he considers that they also contain ' germ-substance.' I should
attach slight importance to a mere name, if a very definite idea
did not depend on this name, the abandonment of which would
lead to confusion. It appears to me to be dangerous to intro-
duce a third category of cells — viz., those of the germ-track —
between the somatic and the germ-cells. In the first place, it is
unpractical to do so, for the appearance of a cell does not reveal
to us whether it is situated in the germ-track or not : and
secondly, it would lead to a total confusion of the ideas of
somatic and germ-cells ; for, as has been shown in the previous
chapters, there are a number of cells in plant- and animal-stocks
which are undeniably somatic, and which must therefore con-
tain germ-plasm. Since we regard the ' blastogenic ' idioplasm
of plants and Hydroids as a modification of germ-plasm, we
must also look upon a very considerable number of the cells
which constitute these organisms as cells of the germ-track,
and we should therefore arrive at the absurd conclusion that a
soma (body) is not present at all in these cases. The soma
nevertheless is present, and a contrast also exists between it and
the germ-cells in plants as much as in animals.

De Vries contradicts himself when he states that a universal
' continuity of the g^rra-cells ' exists through the germ-track ; for
in other passages he emphasises the fact that germ-cells do not
as a rule arise directly from one another (p. 84), and that a dis-
tinction must be made between germ-cells and cells of the germ-
track. The somatic character of the cells of a fern-prothallus,
for instance, cannot be denied, for they function as somatic cells,
and at first are all similar in appearance, so far as we are able
to observe. But nevertheless some of them are situated on the
germ-track, and give rise to male and female germ-cells.

If de Vries puts aside the whole question of the continuity of
the germ-plasm because he is able to prove that germ-cells
always arise from cells of the germ-track, it is evident that he
must be labouring under a similar delusion to that which induced
Sachs to claim precedence as regards the theory of the con-
tinuity of the germ-plasm. Both these observers consider it

THE FORMATION OF GERM-CELl-S 209

self-evident that each apical cell contains the germ-substance of
the ovum, because in plants all growth takes place from the
growing point and originates in the apical cells, which are
derived directly from the egg-cell.

This, however, is at any rate only self-evident in the case of
the first apical cell of the main shoot, and is certainly not so in
that of the lateral shoots, which are, indeed, only derived indi-
rectly from the former. All the cells of a plant are undoubtedly
descended in a direct line from the ovum ; but this fact does not
imply that they must all give rise to apical cells or must all con-
tain germ-plasm, nor does it in any way explain the fact that
only relatively few of them can become germ-cells and the rest
cannot. These were the very facts which the hypothesis of the
continuity of the germ-plasm was intended to make compre-
hensible, to some slight extent at any rate. The origin of a cell
from the ovum gives no clue to its nature : and, as de Vries him-
self says, the entire description in detail of the cell-series leading
from the ovum to the first apical cell, although very interesting
in itself, gives us no information as to the origin of the germ-
substance present in certain parts of the plant-body. I do not
understand therefore how de Vries can look upon the fact of the
existence of these cell-series as constituting in itself an important
explanation of the problem, without attempting to explain it
further. It seems to me that the cell-series can only be of any
explanatory value when they are regarded as germ-tracks in the
sense in which I use the term, — that is to say, as those series
through which the germ-substance is transmitted from the egg-
cell to the remotest parts of the plant.

I must say that it seemed to me to be a somewhat crude idea
to suppose that the same kind of idioplasm is contained in all
the cells of the germ-track, including the apical cell, and that it
is equivalent to 'germ-substance.'' Why do not the apical cells
in the sterile shoots of the horse-tail give rise to germ-cells,
while those of the fertile shoots do so? This must be due to a
difference in the idioplasm of the apical cells of these shoots. And
although structures bearing germ-cells may become developed
from the apical cell of a fertile shoot of the plant, all the cells of
the latter do not nevertheless give rise to germ-cells : only certain
cell-series lead from the apical cell to the new germ-cell, and
these are the cells of the germ-track which contain germ-plasm I
The process of the formation of a shoot from an apical cell is

2IO THE GERM-PLASM

analogous to that of the production of a single polype by budding
from a polype-stock. But both these processes are essentially
the same as that of the development from the ovum in a higher
animal. In all three cases the formation of the new animal
originates in one cell. The latter must therefore possess an
idioplasm which contains all the primary constituents of the or-
ganism : and. moreover, if the organism is to be ' fertile/ — in the
sense in which this term is used by botanists. — the original cell
from which it is derived must contain the primary constituents
of all the structures characteristic of the species in its idioplasm :
that is to say, it must contain germ-plasm. If we trace the
development of such a shoot or organism, we shall find that it
follows a precisely similar course to that which we have already
described in the case of embryogeny ; and that at each cell-
division the primary constituents break up into smaller and
smaller groups, until at last each cell only contains one such
element. And yet all these very different kinds of cells are
descended in a direct line from the original cells. How, then,
can we account for the fact that one or several of them contain
all the primary constituents of the species in a latent condition,
in addition to one specific primary constituent of a particular
kind of somatic cell, as must be the case in those which give rise
to germ-cells? It would, indeed, be a very simple matter if a
continuous series of cells which contain ' germ-substance ' only,
led from the original cell to the new germ-cells. But as simple
a case as this only occurs in the Diptera : in all other instances
the i}iterniediate cells which constitute the germ-track can be
proved to contain perfectly definite somatic elements in addition
to the germ-plasm ; and this is the case in plants as well as i)i
ajiimals.

To make this clear, it is only necessary to glance at the genea-
logical tree representing the ontogeny of Rhabditis nigrovenosa
(fig. 1 6). How does it come to pass, for example, after the
division of the primary endoderm cell into the first endoderm
and first mesoderm cell, that the latter is nevertheless capable
of producing cells subsequently which contain ' germ-substance,'
i.e., germ-cells? At its origin this cell gave up the primary con-
stituents of the endoderm to the sister-cell ; by what means do
these primary constituents — even those of the ectoderm which
were previously given up — reach the germ-cells which eventually
arise from this cell? My answer to these questions has already

THE FORMATION OF GERM-CELLS 211

been given, and is as follows : — in addition to their active meso-
derm-idioplasm, the cells which will give rise to germ-cells carry
along with them a certain amount of germ-plasm in an unalterable
condition. De Vries, and those botanists who agree with him,
consider my answer superfluous. Every one. of course, is at
liberty to reject the solution of a problem, but in that case he
must not claim to have explained it.

I now come to the consideration of de Vries's ' accessory
germ-tracks.'' As has already been stated, this term is used to
describe those series of cells which give rise to germ-cells
through the agency of 'adventitious buds.'' According to
Sachs,* adventitious buds correspond to those growing points
which are not derived from those already present, or ' from
embryonic tissue already present,' but which ' originate at places
where the tissue has already passed over into the permanent
condition, — in fully-developed roots, in the interfoliar parts of
shoot-axes, and more particularly in foliage leaves, the tissues of
which are already completely differentiated and developed.''

In my former essays I have endeavoured to account for these
' adventitious ' buds, — such as are formed, for instance, in a
Begonia leaf when it is placed on damp soil, — by supposing them
to be adaptations of particular species of plants to this peculiar
method of reproduction : I assumed that certain series of cells
which in these species take part in forming the leaves contain
unalterable and inactive germ-plasm in addition to their own
active idioplasm.

In opposition to this interpretation much may be, and in fact
has already been, said, and the principal objections must now
be considered.

It has, in the first instance, been urged that the capacity pos-
sessed by leaves, roots, and so on, for producing adventitious buds,
cannot be regarded as an adaptation, because so many cases
are known in which this process only occurs exceptionally, and is
of no advantage to the plant. There can be no doubt, however,
that the power possessed by Begonia, Bryophylluni, Car da mine
pratensis, and Nasturtmm officinale, of giving rise to buds in
those parts in which they are not formed in most plants, is due
to an arrangement peculiar to these plants. Neither in Begonia
nor in Bryophyllmn can the buds and young plants arise from

* ' Lectures on the Physiology of Plants," p. 477.

212 THE GERM-PLASM

all parts of the leaf; they are only formed in perfectly definite
regions, e.g., on the margins of the leaves in Bryophylhun and
in the angles between the points of origin of the large veins in
Begonia. All the cells of the leaf do not, therefore, as de Vries
supposes, possess this capacity, which is limited to perfectly
definite though numerous cells. These therefore correspond to
somatic cells, quite as much as do those which produce the
several cells in the prothallus of a fern, which contain unalter-
able germ-plasm in addition to the active somatic idioplasm,
the former only becoming active by the influence of particular
external influences.

These conditions may be fulfilled in thousands of other leaves
without resulting in the production of young plants. There are
indeed a whole series of observations which apparently prove
that ' every small fragment of the members of a plant contains
the elements from which the whole complex body can be built
up, w^hen this fragment is isolated under suitable external condi-
tions.' Phenomena of this kind are exhibited by cuttings and
adventitious buds which arise on a twig the apex of which has
been cut off. In the higher plants, the development of roots on
a cutting, or the formation of adventitious buds, does not take
place in all parts of the plant, but only in those which contain
'a number of cambium cells.' These cells alone therefore con-
tain accessory idioplasm, which, according to the nature of the
stimulus acting on them, renders them capable of growing in a
manner which is very different from the normal. There can be
very little doubt that the whole of the cambium layer of these
plants is endowed with the capacity for reproduction. The only
question is, whether this is a result of special adaptation, or
only the outcome of the normal constitution of each plant-cell.

I should still, however, be inclined to consider it as a special
adaptation, and will endeavour to state my reasons for this
view ; not being a specialist in botany, however, I am unable to
deal with the various groups of the vegetable kingdom in such
detail as I could wish.

The question to be decided is, whether each cell was pro-
vided with all the specific primary constituents in a latent con-
dition at the time when the multicellular plant arose from the
unicellular form ; or whether, owing to the diversity of the
differentiation of the idioplasm, a sharp distinction first arose
between the somatic cells and the germ-cells, and the idioplasm

THE FORMATION OF GERM-CELLS 213

of the somatic cells was only subsequently provided with germ-
plasm in a latent condition in those cases in which this arrange-
ment was a useful one. I hold the latter view, and de Vries the
former one. It is important for the theory of the germ-plasm
to decide between these two opinions ; for it would be incom-
patible with this theory for germ-substance to be present as the
idioplasm of the somatic cells at the phyletic origin of the
soma. According to my conception of the germ-plasm, the
phyletic origin of the somatic cells depends on the determinants
contained in the germ-plasm being separated into groups. It
would entirely contradict this assumption if those somatic cells
which were phyletically the first to be formed, had contained all
the other characters of the species in a latent condition in addi-
tion to their manifest specific characters. De Vries thinks that
the marked distinction which actually exists between the somatic
and germ-cells of the higher animals has led me to assume
the universal existence of this contrast, which is not nearly
of such a decided nature in the case of plants in which gradual
transitions from somatic-cells to germ-cells can be proved to
exist.

This, however, I believe is not the case : transitions between
somatic and germ-cells never occur, and de Vries's opinion
simply rests on the fact that he confuses germ-cells with the
cells of the germ-track. That the latter must be regarded as
somatic cells has already been shown.

In my opinion, germ-cells were sharply distinguished from
somatic cells on their first appearance in phylogeny, and this
distinction has since persisted. In no species, whether animal
or vegetable, can there be any doubt as to the cells which are to
be looked upon as germ-cells ; and as regards the somatic cells,
such a doubt can only arise when cells in the germ-track are
regarded as germ-cells.

I know of no more convincing proof of my view than that
which is furnished by the VolvocincB. These organisms consist
of communities of cells which may or may not exhibit a division
of labour, and in which a contrast between the somatic and
germ-cells may or may not exist. In Pandorina all the cells of
the colony are similar to one another, and each performs all the
vital functions. In Volvox the cells are differentiated : some
of them have the function of maintaining the individual, and
others that of preserving the species : that is to say. they are

214

THE GERM-PLASM

differentiated into somatic cells and germ-cells. The hetero-
plastid genus / 'ok'ox must have arisen phyletically from a
homoplastid form : but we can hardly imagine that there can be

-^^.

tr'K^

Fig. 17. — I. Pandorina tnoni'.n. — A colony of swarming cells. II. A colony
which has given rise to daughter-colonies: — all the ceils are similar to one
another. (After Pringsheim.) III. A young individual of Volvox itiinor, still
enclosed within the parent (after Stein) : the cells are diflerentiated into somatic-
and germ-cells.

THE FORMATION OF nERM-CFI.I.S 215

many intermediate stages between these two. for at the present
day the two kinds of cells in Fo/tox ha.rd\\ differ as much as do
the somatic- and germ-cells in the case of the higher organisms.
The somatic cells have nevertheless entirely lost the capacity of
reproducing the entire organism.

Transitions between these two kinds of cells could naturally
only arise by the germ-cells first becoming only slightlv differ-
entiated from the somatic cells, and could not have been pro-
duced, as de Vries thinks, owing to all the cells containing
germ-substance in a more or less latent condition from the firsi.
There is no germ-substance in the somatic cells of I'o/vox,
which, figuratively speaking, have only just become differentiated
from the germ-cells, if the latter are artificially removed from
a colony, the somatic cells continue to exist for a long time, but
they do not give rise either to new germ-cells or to a new colony.

And why should they do so? Of what advantage would this
be to the species, since millions of individuals, each of which
again produces daughter-individuals, exist in the same pond?
The ordinary process of multiplication is so vigorous that special
means for ensuring the existence of the species are unnecessary.

Such means have, however^ beeji adopted in very juany, if tioi
by far the greater nunibet , of the more highly organised phints.

The power possessed by fungi and mosses of reproducing a
new individual from any bit of the plant under favourable condi-
tions, has been supposed to contradict my view. But I do not
see what prevents us from regarding this power as an adaptation
for ensuring the existence of a species surrounded by dangers
of all kinds. When the top of a toadstool is knocked off, a new
one is formed (Brefeld) ; and this arrangement is obviously of
great use in the preservation of the species. An entire liverwort
can be regrown from the smallest fragments of the plant (Vbch-
ting). Why therefore should the assumption be improbable that
this power has been acquired in order to insure the persistence
of a species the existence of which is threatened by every sudden
drought? My knowledge of plant life is not sufficient for me to
be able to support this statement in detail, but other facts will, I
think, to some extent confirm my opinion from the opposite point
of view.

Why is this power of regeneration not possessed by adult
ferns and horse-tails? If a frond of a fern is cut off, it is not
reproduced from the stalk, and even the individual pinna; cannot

2t6 the germ-plasm

be formed anew. In answer to this it might be urged that the
somatic cells of ferns have become too highly differentiated : but
this is contradicted by the fact that many, although by no
means all, ferns can produce bulbils on their fronds. I must
leave botanists to decide why this occurs ; but were I asked
^vhether the power of producing entire plants from somatic cells
would not have been of advantage to the other ferns, and would
therefore be expected to be possessed by them, I should be in-
clined to reply that all ferns are able to replace lost fronds by
forming new ones, — not by the regeneration of the injured leaf,
but by budding from the stem. This suffices to restore the plant
when it has been injured.

We must now consider the Phanerogams in this connection.
In these plants, again, ^accessory germ-tracks ' are usually absent
in the leaves : that is to say, the cells of the leaves are not
capable of producing buds or even of restoring a piece which has
been cut out The axes of the shoots, on the other hand, possess
this power in a high degree, and it depends on the presence of
cambium cells, all of which are apparently capable of giving rise
to new growing points, which produce new shoots with leaves
and blossoms, and consequently also germ-cells. The power of
regenerating the leaves is, as a rule, useless ; for the formation
of new persons of the plant stock can take place to an imliniited
extent by means of the cambial layer : and this mode of compen-
sation for losses sustained is more effectual than the restoration
of defects in the leaves would be. The power of growing adven-
titious buds is probably unnecessary in the case of most leaves,
on account of the enormous number and certain dispersion of
the seeds produced by the plant. Amongst animals the same is
true as regards polype-stocks. Cells are distributed through-
out the stock which have the appearance and functions of
ordinary somatic cells, but which can give rise to new^ persons
under certain circumstances, such as, for instance, when the
stock has become injured. In a living stock of Tubularia
mesonbryanthemnm, which I once brought from Marseilles to
Freiburg, the crowns of the polype died one after the other
within a week, probably on account of want of nourishment ;
but within a few days afterwards all the stalks had given rise to
new crowns ; and though these were very small, they would un-
doubtedly have grown to the full size had it been possible to sup-
ply them with food. The capacity for regeneration is apparently

THE FORMATION OF GERM-CELLS 217

provided for in this case ; it is at any rate stated by Loeb * in a
recent paper that a shedding and new formation of the crowns
occurs periodically. The same writer also showed by experi-
ment that, under favourable circumstances, crowns may bud out
at any point of the stem, either at the distal end or at the base.
On the other hand, he never succeeded in getting the root-like
organs of attachment to be produced at the apical end.

No one will be surprised that such a growth did not occur who
agrees with me in looking upon all these processes of regenera-
tion and budding as resulting from adaptation. Under natural
conditions the apex of a stem could hardly be situated in such a
position as to render the formation of roots necessarv, for it
never comes to lie upside down in the earth, and consequentlv
none of the cells in the apex contain ' rhizogenic idioplasm '
(^ Wurzel-Idioplasma'). On the other hand, however, it is easy
to understand why the power of budding exists in such a marked
degree in polypes, if one considers how liable the soft body is to
be attacked by crabs, worms, gasteropods, Pycnogonids, and
other small enemies. If these polype-stocks did not possess the
power of continually producing new crowns, — i.e. new individuals,
— when the old ones have been bitten off, the whole colony
w^ould soon perish owing to the absence of 'nutritive persons.'
The fact that regeneration is possible to such an enormous ex-
tent results, at any rate /// part, from the aggregation of persons
to form the higher stage of individuality of the stock. For such
a combination of individuals procures the advantage of perma-
nent nutrition as long as all the individuals of the stock have not
fallen victims to their enemies, and thus it is favourable to the
production of new buds.

Amongst the Polyzoa the case is very similar. In many of
these animals the normal form of gemmation takes place with
great regularity, and the region at which the next bud will arise
can be predicted beforehand : on this fact depends, as in the
case of the Hydroid polypes, the characteristic form of the
stock in the different species, which is sometimes branched like
a foliage tree, and sometimes like a fir-tree or a feather. In these
animals, therefore, definite cells must be provided with 'blasto-
genic' idioplasm in advance, and merely the stimulus due to ordi-

* Jacques Loeb, ' Untersuchungen zur pliysiolog schen Morphologie der
Thiere,' I. Uber Hetermorphose," Wiirzburg, 1891.

2l8 THE GERM-PLASM

nary nutrition is required in order to incite them to form buds.
The series of cells which lead directly to the cells of these buds
must be looked upon as the main germ-tracks, using the term
in de Vries's sense. According to Seeliger's researches, how-
ever, budding takes place from other regions as well as the
ordinary ones in certain Polyzoa, e.g.^ Pedicelliiia. If the crown
is lost in this animal, so that only a stump of the stalk remains,
new crowns are produced on the end of the stalk ; and in this
case, therefore, budding origitiates in the fiat epithelial cells
characteristic of the ectodenn, whicli did not previously appear
to be capable of proliferating at all. This is another instance of
the presence of accessory germ-tracks. The cells of the epi-
dermis are provided with blastogenic germ-plasm, although they
do not as a rule take part in the formation of buds, but only
give rise to them in response to unusual stimuli. These cells are
just as much exposed to destruction as are those of the Hydroid
polypes, and we need therefore not be surprised that arrangements
for budding should have been made in the stalk, even were we
not aware of the fact that in Fedicellina, under normal condi-
tions, the crowns drop off periodically from the stalk, and are
replaced by others which bud out afresh. This process cer-
tainly occurs at the upper end of the stalk, but it is quite com-
prehensible that it would be advantageous for the lower end of
the stalk also to be provided with blastogenic idioplasm.

The arrangement which exists universally in the higher plants
for the production of adventitious buds is, in my opinion, to be
explained in a similar manner. In this group of organisms the
cambium layer in particular is provided with the means of re-
placing lost leaves and entire shoots. This obviously affords an
important protection against numerous enemies — such as insects
more especially — the number of which is often incalculable. It
is therefore not surprising that such an arrangement — viz., the
addition of unalterable germ-plasm to the cambium cells — is
here met with.

5. Galls

De Vries has also brought forward the question of the formation
of galls as furnishing an additional argument against my views.
In his opinion the production of galls proves that a vegetable
cell, even when it exhibits a specific histological differentiation,
contains the primary constituents of every other kind of cell in

THE FORMATION OF OERM-CEFJ^S 219

a latent condition, and these are ready to become active as soon
as a suitable stimulus is brought to bear upon them. This
proof he considers to be incompatible with the assumption of
the existence and continuity of a germ-plasm.

The development of galls is undoubtedly a highly interesting
problem, which, in my opinion, has not yet been fully explained,
in spite of the numerous and excellent researches on the subject
which have been made within the last ten vears. Amongst
these, the contributions of Adler * and Beyerinck f in particular
have materially helped to throw light on the problem.

The most important point in the consideration of this question
is the fact that galls are not by any means exclusively composed
of those kinds of cells which occur in the organs of the plants
upon which they arise, but may also contain cells of other kinds.
' Cells which are usually only developed in the bark of a plant
may also frequently be found in the galls produced by those
CynipidcE and Diptera which infest leaves.' It is therefore cer-
tain that the power of producing forms of cells which do not
usually occur in the leaf, for instance, ' is not confined to those
organs in which they are developed normally,' but is present
also in certain cells of the leaf, and even indeed, de Vries thinks,
' in all other parts of the plant."

This is not surprising if we look upon the formation of the gall
as due to an adaptation of the plant to its parasites, such as we
may assume to have occurred with regard to the peculiar
arrangements exhibited by certain tropical plants for the pro-
tection of ants, which in their turn again protect the plant.
Reciprocal adaptation has taken place in this case ; the animal
has become adapted to the plant, and the plant to the animal,
because a joint existence is advantageous to both of them. \\\
the case of the galls of the CyuipidcE and TentJiredinidcr, the
advantage which might result to the plant from the presence of
the parasite is not apparent, and we may therefore be inclined

* Adler, ' Beitrage zur Xatuigeschichte der Cynipiden,' Deutsche
entomolog. Zeitschr. xxi., 1877, p. 209 ; and ' Uber den Gcnerations-
wechsel der Eichengallenwespen,' Zeitschr. f. wiss. Zool., Bd. x.vxv., 1880,
p. 151.

t M. N. Beyerinck, ' Beobachtungen iiber die ersten Entwicklungs-
phasen einiger Cyidipidengallen,' Akadeniie d. Wiss. zu Amsterdam, 1882;
' Die Galle von Cecidomyia pocie,' Bot. Zeitung, 1885 ; ' Uber das Cecidium
von Nematus caprese,' Bot. Zeitung, 1888, No. i.

2 20 THE GERM-PLASM

to explain their formation as due to a reaction of the plant in
response to the stimulus exerted by the animal. If, as was
formerly supposed, the gall resulted from the action of a
poison which is inserted into the tissues of the plant by the
female during oviposition, this explanation would be totally
inadequate ; for it is not conceivable that the infusion of a
poison, which happens only once, could with such regularity
produce a gall which grows slowly, and only gradually attains its
definitive and often complex structure. Moreover, several kinds
of galls, differing very much from one another, may be produced
from the saine substratum, such as an oak-leaf, for instance. We
know, however, from the researches of Adler and Beyerinck.
that the formation of the gall is not due to the sting of the
parent animal, but to the activity of the larva after it has been
hatched. We must therefore suppose that this peculiar specific
proliferation of the tissues of the plant is due, in the first in-
stance, to the stimulus produced by the bodies of the larvae
when they begin to move about and to feed, the specific secre-
tion of their salivary glands then also having an effect. The
diversity of the galls arising from the same substratum must
therefore be due to differences in these factors ; and the con-
spicuous adaptations of the galls, which serve to protect, sup-
port, and nourish the parasite, must depend on adaptations of
the latter as regards its mode of feeding and movement, and
the chemical composition of its salivary secretion. We cannot
help accepting this interpretation of the facts since no other is
forthcoming ; and we must therefore suppose that natural selec-
tion has operated so long on these factors, and has gradually
effected such an improvement, that the kind of gall which pro-
vided the best protection and nourishment for the species was
ultimately produced by the larva.

Beyerinck has, in fact, proved that cells and tissues often
occur in galls which very closely resemble those in different
parts of the plant, but which do not exist in the substratum {e.^.
a leaf) on which the gall is produced. De Vries infers from
this fact that the primary constituents of such tissues must have
been contained in the cells of the leaf, although they could not
previously be recognised as such. This inference does not
seem to me to follow of necessity, for the stimulus produced by
the parasite might conceivably have modified the idioplasm of
the cells of the leaf so as to result in the formation of cells

THE FORMATION OF GERM-CELLS 22 1

differin<i^ from those ordinarily present in the leaf. It will he
shown in the chapter on Variation that changes of this kind
do occur, and that somatic idioplasm may at times, owing to
known or unknown causes, become so modified as to produce a
deviation from the inherited form of the cells of the series. The
sudden appearance of such peculiarities as those exhibited bv
the moss-rose may be taken as an instance. It is ver\- possible,
therefore, that owing to the specific stimulus produced bv the
larva, and more especially by its secretion, the idioplasm of
certain layers of cells in the gall becomes modified and causes
the cells to assume another character, such as that of woody
fibres.

This view receives decided support from the circumstance
that the gall is by no fneans only cojnposed of those kinds of cells
ivJiich occur in other parts of the plant. A similar statement to
this is, indeed, made by de Vries, who, however, makes an
exception in the case of ' the peculiar layer of sclerenchyma
in some Cynipid galls, which afterwards becomes modified into
thin-walled, nutritive tissue.' I cannot look upon this as being
'only an apparent exception' to the rule, for it seems to me to
be a very valuable proof that no such rule exists, and that the
above instance is to be explained as an apparent reversion to
inherited forms of cells, such as were already contained in a latent
condition in the cells of the leaf. I should rather be inclined to
regard these ' exceptions ' as a proof that definite new forma-
tions occur in galls, and that these are due to modifications of the
cells from which they arise in response to the stimulus produced
by the larva. It can hardly be a matter of surprise that a marked
resemblance exists between these cells and those occurring in
other parts of the plant, for the changes produced by the larva
take place in an idioplasm consisting of determinants of the
species in question ; these changes would not therefore at first
result in combinations of biophors (determinants) very dififerent
from those which ordinarily occur in the plant. The new cotn-
bination of the biophors in different ways results, I believe, from
the action of the larva, and thus modifications of the determinants
are produced.

The galls of Cecidomyia poce, which de Vries mentions when
contesting my views, are probably to be accounted for in the same
manner. In response to the stimulus produced by the larva,
these stalk-galls become covered with a thick felt of rootlike

2 22 THE GERM-PLASM

outgrowths, which doubtless serve as a protection : these, when
they gain access to the soil, become branched like ordinary
roots. The assumption that under certain circumstances the
idioplasm of certain somatic cells becomes modified in response
to the stimulus produced by the parasite, so as to give rise to a
structure similar to that of another tissue or even organ of the
same plant, seems to me by no means to prove that the primary
constituent of this tissue must previously have been contained
in these cells. In animal tissues transformations of this kind
are certainly not known to occur. Pathological anatomists are
now of the opinion that only those kinds of cells occur in
tumours which actually belong to the sort of tissue from which
the tumour arises. This is not surprising, for tissues of animals
are far more highly diflferentiated than those of plants, and
corresponding elements in the idioplasm must also differ in a
corresponding degree ; and consequently, in spite of the displace-
ments and re-arrangements which may be produced by stimuli,
they can never form precisely the same combinations as those
which occur in the various other tissues of the body.

I shall not discuss the case of the gall of Neinatus, as Beyer-
inck's observations with regard to it are not yet complete. If it
should be shown that a complete willow can be produced from
the leaf-gall of the plant, as de Vries considers probable, it will
then certainly have been proved that the cells in the leaf contain
germ-plasm, just as in the case of the leaves of Begonia. At
present, however, it is only known that the gall can give rise to
roots, and although normal roots are always capable of forming
adventitious buds, it cannot be said at present whether these
abnormal roots are able to do so. In the willow, in any case,
the primary constituents of roots are distributed throughout the
stem in the form of invisible determinants, contained within
visible cells, and this accounts for the fact that the production
of new individuals by means of cuttings takes place exceptionally
easily in this plant. This may perhaps be accounted for by a
wider distribution than usual of the merely • unalterable ' group
of determinants for roots taking place in the plant, in connec-
tion with the wide distribution of the corresponding primary
constituents. But I do not by any means imagine that in all
these cases in which the cells of a plant possess inactive germ-
plasm, its presence is actually useful at the present day. If the
distribution of unalterable germ-plasm once took place in an

THE FORMATION' OF GERM-CELLS 223

organism to such a considerable extent as has occurred in most
plants, it would be a matter of slight importance in the economy
of the plant whether the cells of those organs which at the
present day are no longer in a condition to make use of this
substance were provided with a minimum of germ-plasm or
not. Such a provision might have been of advantage to the
ancestors of the species ; and if this were not the case, we know
so little of the processes by means of which the various qualities
of the idioplasm become separated mechanically in nuclear
division, that we cannot altogether reject the assumption of an
occasional chance admixture of germ-plasm to somatic idio-
plasm, especially in the case of the higher cormophytes, which
must in any case possess a number of cells containing germ-
plasm throughout the entire plant. Time will show whether we
require this assumption.

The difference between my view and that of de Varies does
not consist in the fact that I am compelled to deny the admix-
ture of germ-plasm in the case of a large number of cells in the
body on principle, but in my assumption that each somatic cell
contains a definite somatic idioplasm consisting of a limited
number of definite determinants, to which any other • unalter-
able ' accessory idioplasm may be added if required. De Vries,
on the other hand, considers that the whole of the primary con-
stituents of the species are contained in the idioplasm of every,
or nearly every, cell of the organism. But he does not explain
how it is that each cell nevertheless possesses a specific histolog-
ical character. A new- assumption, w^hich would not be easy to
formulate, would therefore be required to explain why only a
certain very small portion of the total amount of idioplasm —
which is similar in all parts of the plant — becomes active in
each cell. This theory explains the differentiation of the body
as being due to the disintegration of the determinants accumu-
lated in the germ-plasm, and requires a special assumption, —
viz., that of the addition of accessory idioplasm when necessary,
— in order to account for the formation of germ-cells, and the
processes of gemmation and regeneration. The reconstruction of
entire plants or of parts from any point can be easily accounted
for by de Vries's hypothesis, just as it can by Darwin's theory of
pangenesis, for the pangenes or gemmules are present wher-
ever they are wanted. But de Vries is unable, on the basis of
his hypothesis, to oifer even an attempt at an explanation of the

2 24 THE GERM -PLASM

diversity of the cells in kind and of the differentiation of the
body.

These two assumptions appear to me to be of equal value in
explaining the fact that in many of the lower plants each cell,
under certain circumstances, can apparently reproduce an entire
individual. The dififerences between the somatic cells are here
only slight ones, and are so few in number, that we might be
inclined to consider them as due to reactions of the same idio-
plasm to different kinds of influences exerted by the environ-
ment. Such is the case, for instance, in liverworts. But this
assumption ceases to be tenable as soon as the soma can be-
come variously differentiated, and any explanation must in the
first place account for this differentiation : that is to say. the
diversity which always exists amongst these cells and groups of
cells arising from the ovum must be referred to some definite
principle. De Vries's principle is of no use at all in this case,
for it only accounts for the fact that entire plants may. under
certain circumstances, arise from individual cells, and does not
even touch the main point. In fact, no one could even look
upon it as giving a partial solution of the problem, if differentia-
tion is supposed to be due to that part alone of the germ-plasm
always becoming active, which is required for the production of
the cell or organ under consideration. But the higher we ascend
in the organic world, the more limited does the power of pro-
ducing the whole from separate cells become, and the more do
the numerous and varied differentiations of the soma claim our
attention and require an explanation in the first instance.

The presence of idioplasm in all parts containing all the
primary constituents does not help us in this respect : and even
in attempting to explain the formation of germ-cells, it is of
very little use to assume that they arise from cells which, like
the rest, contain all the primary constituents of the species.
How is it that these cells, and these alone, in the entire soma of
the animal, give rise to germ-cells? In the lower plants the
fact of the differentiation of the soma is liable to be overlooked
or underrated, but this cannot possibly be the case as regards
the higher animals.

SUMMARY OK TART II 225

CHAPTER VII

Summary of Part II

We have now seen that the idioplasm of the fully-formed in-
dividual animal- or plant-cell may exhibit a considerable amount
of difference as regards its degree of complexity ; and before
going further, it may be as well to state clearly in what this
difference consists.

We suppose that the process in the idioplasm which brings
about the ontogeny of a multicellular organism is due to the
thousands of determinants, which constitute the germ-plasm ot
the fertilised ovum, becoming systematically separated into
groups, and distributed among the successors of the egg-cell.
This separation into smaller and smaller groups of determinants
continues to take place, until each cell contains determinants
of one sort only, and these then either control a single cell, or,
in case the hereditary character (* determinate ') is constituted
by a group of cells with a common origin, the control is exerted
over this whole group.

All the determinants are not active at the same time ; every
cell, in fact, which appears in the entire course of ontogeny is
controlled by one determinant. This is effected by the disin-
tegration of the detcminant into its constituent biophors, which
migrate into the cell-body. Even in the earlier stages of ontog-
eny, in which the idioplasm of a cell consists of a still larger
number of different kinds of determinants, the cell is also con-
trolled by only one of them in this manner. The rest of the
determinants have in any case an important function with regard
to the course taken by ontogeny, for each of them has its own
rate of increase, and thus an alteration is produced in the pro-
portion of the various determinants originally present in the
germ-plasm, and consequently its definite architecture also
undergoes alteration, the subsequent disintegration being con-
trolled by the determinants which have thus been rearranged.
These determinants are therefore only inactive with respect to

2 26 THE GERM-PLASM

the cell ill which they are situated, and not as regards develop-
ment as a whole.

\'arious circumstances may, however, produce complications
in this simple course of development of the idioplasm.

In the first place, it is necessary that the organism should be
able to replace losses of substance. In order that this may be
possible, the final cells of ontogeny, at any rate. — i.e., those of
the various tissues. — must be rendered capable of producing
others similar to themselves. The po.ssession of this power
necessitates that each cell shall be capable of unlimited multi-
plication, so that the single determinant which controls it can
likewise grow and multiply. This would result in the cells being
able to produce others of a like character by cell-division.

We find, how'ever, that regeneration is not limited to this very
simple form of restoration : more complex tissues can also be
reproduced ; and even entire organs, such as limbs, and still
larger parts of the body, such as the head and tail, may, in
certain groups of animals, undergo restoration when they have
been lost. These facts are to be explained in terms of the idio-
plasm by supposing that in these cases also the determinants for
the groups of cells which are to be capable of regeneration have
undergone an increase, and have been supplied to certain cells
in the course of ontogeny in the form of inactive accessory idio-
plasm. The equipment of such cells with determinants for
regeneration is due to adaptation, and is only connected with
the degree of organisation of the animal in so far as the difficulty
of providing a large number of cells with accurately graduated
determinants for regeneration increased as the number of cells
from which regeneration had to proceed became larger, and as
the number and degree of differentiation of the organs to be
restored also increased. An increase, both as regards cells and
organs, takes place in correspondence with the complexity of
structure, and the • regenerative power,' therefore, as a rule, grad-
ually undergoes a proportionate decrease.

Cells which possess the regenerative pow^r are tlierefore those
which contain, in addition to their own active determinants, a
larger or smaller group of inactive determinants : these latter
belong to those cells and cell-series which are capable of taking
part in the reconstruction of that part of the body which has
been lost, and which is situated distally to them when the
determinants become active. The occurrence of regeneration in

SUMMARY OF PART II 227

a high degree is only rendered possible by the cells in a definite
transverse plane of the body being regularly equipped with
various groups of suitable supplementary determinants which are
capable of acting together as a whole.

This form of regeneration leads directly to the process of
reproduction by fission, which simply consists in the employment
of a marked power of regeneration for the purpose of increasing
the number of individuals.

While the possession of the regenerative power in a low, as
well as in a high degree, is due to the equipment of cells with
certain larger or smaller groups of determinants in the form of
' unalterable " accessory idioplasm, the developnioit of new per-
sons by gemmation depends either on the fact that a single cell
contains all the determinants of the species in an inactive and
unalterable condition, in the form of accessory idioplasm ; or
else is due to two or three cells in the different layers of the
body containing large groups of determinants as accessory idio-
plasm, which together constitute all the determinants possessed
by the species — i.e., germ-plasm.

In cases where budding originates in a single cell, as in the
Hydroid-polypes, the blastogenic idioplasm concerned in the
process must be regarded as a modification of the germ-plasm,
which consists of all the determinants of the species, though
these have a different arrangement to that which obtains in the
germ-plasm proper of the fertilised egg-cell. The fact that a
bud may originate in two or three cells does not show^ that these
cells contain exactly those groups of determinants which corre-
spond to those of the two or three germinal layers of the Meta-
zoon in question. In fact, the combination of the determinants
differs more or less in all known cases, and is adapted to the
circumstances under which budding occurs. This proves that
in the embryogeny of the species, divisions of the accessory
idioplasm occur quite independently of the ordinary divisions in
the mass of determinants, and these result in certain cells being
provided with a definitely constituted accessory idioplasm.

The blastogenic germ-plasm must be contained in the germ-
plasm of the sexual cells in the form of special ids, for buds can
vary independently of the persons which produce them. On
the other hand, primary idioplasm must also be supplied to the
bud during its development, and this may be effected by means
either of special cells which contain the idioplasm in an unal-

2 28 THE GERM -PLASM

terable condition, or else of certain ids of the primary idioplasm
being added in an unalterable condition to the blastogenic
germ-plasm when it becomes separated from the primary idio-
plasm.

As, apart from plants, gemmation only occurs in compara-
tively low forms of animals, viz.. in the Coelenterata, Polyzoa,
and Tunicata, we may infer that the addition of this blastogenic
idioplasm, consisting of accurately graduated groups of deter-
minants, eventually reaches a limit, owing to the increasing
complexity of the structure of the animal and to the surprising
extent to which the number of determinants increases.

According to our view, the cells of the Metazoa and Metaphyta
may not only be provided with the above-mentioned accessory
idioplasm, but may in addition contain primary gerni-plasjii, and
these cells are to be found along the 'germ-tracks^^ — that is to
say, they are situated in the direct line of development which
leads from the ovum to the germ-cells which are eventually
formed from it. As each somatic cell is only controlled by one
of the large number of determinants belonging to the germ-
plasm, -and as determinants cannot be produced spontaneously,
those cells which are to give rise to germ-cells must contain
unalterable germ-plasm in addition to the active determinants
which control them ; and the former can only be derived from
the cell to which the whole organism owes its origin, for this
alone contains the whole of the determinants organically united
to form germ-plasm. A series of cells containing germ-plasm
in the form of unalterable accessory idioplasm, must therefore be
traceable from the egg-cell to that region of the body which
sooner or later gives rise to germ-cells ; that is, there must be
a continuity of the germ-plasm.

The number of germ-tracks in the lower plants and animals
is a very large one ; under normal circumstances, germ-cells are
not only found in very many parts, especially in the case of
animal- and plant-stocks, but new persons may in exceptional
cases be formed in many regions by budding, especially when
injuries to the stock have occurred ; and these persons can again
produce germ-cells. In the case of the Hydroid-polypes and
Polyzoa, a large number of cells of the stock must be provided
with germ-plasm, although it is impossible to say whether these
are the same as those which contain blastogenic idioplasm, or
whether the latter is situated in other adjacent cells which also

SUMMARY OF PART II 229

take part in the formation of the bud. In any case blastogenic
idioplasm and the germ-plasm of the ovum are not identical,
even if, as in plants and Hydroid-polypes, the former contains
the whole of the determinants of the species. The determinants
must at any rate have another arrangement : not infrequentlv,
indeed, the blastogenic idioplasm must consist of entirelv dif-
ferent kinds of determinants, and in the case of the alternation
of generations of the Medusae of far more numerous ones.

The cells of the germ-tracks are somatic cells ; that is to say,
each of them is controlled by a special determinant, and con-
tains germ-plasm in an inactive as well as in an unalterable
condition. In the latter state, it only again becomes capable of
disintegration when the cells in which it is situated give rise to
germ-cells, and begin to undergo development into embryos.
This germ-plasm, like the unalterable blastogenic idioplasm,
may be contained in young cells with only a slight amount of
histological differentiation, as well as in cells with a sharply
defined histological character.

We thus see that in many cases the cells of the adult organism
contain an accessory idioplasm in addition to the determinants
which control their special character, structure, and physiologi-
cal activity for the moment : the former may become active in
the ordinary course of development, — as occurs in the normal
formation of germ-cells and in multiplication by fission and
gemmation ; or its activity may be due to abnormal causes only,
— such as those resulting from injuries and mutilations, — from
which the processes of regeneration or gemmation in the first
place originated.

PART 111

THE PHENOMENA OF HEREDITY RESULTING
FROM SEXUAL REPRODUCTION

Introductory Remarks on the Nature of Sexual

Reproduction.

The phenomena of heredity have so far been considered in
connection with a purely asexual form of reproduction only : the
complications of the germ-plasm arising from the intermingling
of the hereditary parts of two parents have been left aside, and
the composition of the germ-plasm has been assumed to be of
such a nature as would result if jnonogoiiic reproduction were
the only form in which the process existed. The advantage of
this method of procedure is seen in the fact that it has only been
necessary for us to bear the essential part of the processes in
mind when analysing the fundamental phenomena of heredity,
and this essential part has therefore not been lost sight of in the
confusing and ever changing intermixture of individual variations
which result from amphigonic reproduction. The course we
have followed is justified by the fact that fundamental processes
— such as ontogeny, regeneration, and multiplication by fission
and gemmation — cannot owe their origin to amphigonic repro-
duction, but w'ould take place even if this form of multiplication
did not exist at all.

Bearing this fact in mind when considering the complications
arising from sexual reproduction with which we have to deal
in analysing the phenomena of heredity and their material sub-
stratum, it will now be profitable to consider the facts concerning
this form of reproduction, and to see how they can be explained.
I will now therefore give a short account of the processes in
230

AMPHIGONIC HEREDITY 23 1

question as far as is necessary in order to render comprehensible
the complications in the phenomena of heredity resulting from
them.

Until far into the present century ' sexual reproduction ' was
considered to be the essential and primary form of the process ;
and although it gradually became more and more evident that
several kinds of * asexual reproduction ' may also occur, these
nevertheless only take place in the lower forms of animals and
plants. As the details of the phenomena of reproduction were
for a long time known almost exclusively with regard to the
higher animals, and as in them sexual reproduction alone occurs,
the special peculiarities of the latter were naturally considered to
be necessary and indispensable in the process. Fertilisation
was looked upon as an essential part of this process, and it alone
was supposed to render life from one generation to another pos-
sible at all ; in short, fertilisation was regarded as a ' process of
rejuvenescence,' and sexual reproduction was considered to form
the foundation from which all forms of reproduction have arisen.
The existence of different forms of asexual reproduction was ex-
plained as an ' after-effect ' of the process of fertilisation or reju-
venescence occurring in sexual reproduction.

This view appeared to receive support from the fact that sex-
ual reproduction is of universal occurrence, from the lowest to
the highest forms of animals and plants ; and also that asexual
reproduction never takes place in the higher organic forms, and
even in the lower ones it only alternates with the sexual form of
the process.

The present state of our knowledge of the process of fertilisa-
tion, however, justifies us in considering these earlier views to be
totally erroneous. In no case does fertilisation correspond to a
rejuvenescence or renewal of life, nor is its occurrence necessary
in order that life may endure : it is merely an arrangement which
renders possible the intermingling of tiuo different hereditary
tendencies. We shall deal later on with the question as to why
such a mingling has been introduced and so extensively adopted
by Nature ; at present it is only necessary to prove that this is
the case. P'ertilisation consists in the union of two hereditary
substances, i.e.^ of the germ-plasms of two individuals : all the
complicated and varied phenomena of differentiation — beginning
with that of the two different kinds of reproductive cells, usually
known as male and female, up to that of the individuals them-

232 THE GKRM -PLASM

selves into males and females, and including the innumerable
other resulting adaptations and phenomena — take place solely
for the purpose of rendering possible the union of the primary
constituents of two individuals.

This process of the fusion of two germ-plasms, which consti-
tutes the essential part of fertilisation and is as a rule connected
with the fusion of two cell-bodies, I have designated as afup/ii-
mixis. It is not always connected with reproduction, for these
two processes take place independently of one another in all
unicellular organisms. In the Infusoria, for instance, two indi-
viduals in the course of their life-history come into contact with
one another, and then either fuse completely into one, or else
undergo a partial or temporary fusion ; in both of which cases
half the hereditary substance is transferred from one individual
into the other, and thus amphimixis is brought about. The
latter process is only invariably connected with reproduction in
the case of multicellular forms : this is necessitated by the fact
that the union of two different germ-plasms cannot take place by
the fusion of entire individuals, as the germ-plasm is enclosed in
separate cells, a male and a female, the fusion of which takes
place in a similar manner to that which occurs in the process of
conjugation in unicellular organisms. This act of amphimixis
must then be followed by the multiplication of the fertilised egg-
cell, accompanied by the differentiation of its successors, — or,
in other words, by the outogenv of a new individual ; for did this
not result, the process of amphimixis would be useless. Amphi-
mixis is therefore always connected with reproduction in all jhuI-
ticellular forms, and these tw^o processes together constitute
' sexual reproduction ' or ' amphigony ' (Haeckel).

The process of amphimixis, as it occurs in amphigonic repro-
duction, is briefly as follows. The two kinds of germ-cells
mutually attract one another, and then fuse together, the smaller
male element always entering the larger female one. The nuclei
of the two cells then approach each other, and so come to be
situated close together, each being accompanied by its ' centro-
some,' — i.e., that remarkable body, enclosed in a clear sphere,
which, as already stated, constitutes the apparatus for division.
The germ-plasm in both nuclei is at first distributed in the form
of fine threads, as is represented in the case of the female nucleus
in Fig. 18, I. : it subsequently, however, becomes contracted, so
as to give rise to nuclear rods or idants (Fig. 18, II.). Edouard

AMI'HIGONIC HERKDITY

233

van Beneden was the first to prove that the number of these
idants is the same in both of the conjugating-cells, and this dis-

'■\.TU>9

'Rkl

KkT

noo

W

Fig. 18. — Diagram of the fertilisation of the egg in Ascaris megalocephala. —
(Compounded from the figures and descriptions of Boveri and others.)

I. — The sperm-cell (i/) is about to enter the ovum, which contains a nucleus {noz>')

and centrosome {cs). Rk I and Rk 11 — the two primary polar-bodies, the first
of which has divided into two; each contains two idants.

II. — The sperm-nucleus {nsp) has passed into the egg. near the nucleus of which
it is situated. Each of these nuclei contains two idants, and also a centrosome,
which has divided into two.

III. — The two nuclei are now close together: the centrosomes, with their ' spheres
of attraction,' are connected together in pairs, and are situated at the poles of
the spindle, which is already visible.

IV. — The nuclear membrane has disappeared, and the first embryonic nuclear
division is now taking place.

coverv — which has since been confirmed in the case of a large
number of species of animals, and has been proved quite recently

234 I'HE GERM-PLASM

by Guignard to apply to plants also — is of decided importance
in connection with the conception that idants constitute the
hereditary substance. As the two nuclei are approaching one
another, their centrosomes become doubled, and the correspond-
ing pairs unite to form the two poles of a nuclear spindle (Fig.
1 8, III.), which direct the first cell-division leading to the for-
mation of the embryo ; this usually only occurs after the nuclear
membrane has cortipletely disappeared (Fig. i8, IV.).

The process of fertilisation therefore consists in the union of
the nuclei of the two sexual cells within the maternal germ-cell,
and also of the bodies of the cells, together with their apparatus
for division. One half of the germ-plasm of the 'combination-
nucleus ' (' Copulationskern ") thus formed by the union of the
sexual nuclei consists of idants derived from the mother, and
the other half of those derived from the father, and the resulting
combination of two hereditary substances directs the ontogeny
and controls the building-up of the new individual. The entire
number of idants nevertheless always remains the same in all
the cells of the body : thus, for instance, if eight paternal and
eight maternal idants were brought together in the process of
amphimixis, there would be sixteen idants in every * cell in the
body of the individual arising from the fertilised ovum ; and if,
again, as represented in Fig. i8, there are only two idants in
each germ-cell, each somatic cell will contain four idants.

The nature of sexual reproduction depends therefore on the
interminglino; of two hereditarv tendencies which are individuallv
different from one another ; or, to pass from the abstract to the
concrete, it depends on the union of two hereditary substances in
the first rudiment of the individual. We must next investigate
the manner in which this combination of hereditary substances
affects the composition of the germ-plasm.

* A recent observation renders it doubtful whether ' every ' cell contains
the same number of idants ; but this need not here be taken into considera-
tion, as its importance cannot at present be estimated.

EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM 235

CHAPTER VIII

MODIFICATIONS OF THE GERM-PLASM CAUSED BY

AMPHIMIXIS

I. The Necessity of a Halving of the Germ-Plasm

By the process of amphimixis the hereditary substances of two
individuals become united into one substance in the offspring.
If the process is repeated in every generation, a doubHng of
these individually different hereditary substances must take place
each time, and the mass of germ-plasm and the number of idants
must likewise be doubled. As a matter of fact this cannot and
does not occur, for in every species the number of idants remains
the same throughout all generations. The unlimited increase
of the germ-plasm must therefore be prevented in some way or
other.

The mass of germ-plasm might possibly remain constant if
its growth stopped in the young germ-cells when only half the
normal quantity had been formed. It is quite conceivable that
a continual increase in mass might in this way be prevented, if,
contrary to the theory of the germ-plasm here propounded, we
were to imagine that the idioplasm merely consists of ultimate
vital particles — 'pangenes,' 'primary constituents,' or whatever
else we choose to call them — which are not combined into units
of a higher order.

If, however, we assume the existence of a germ-plasm in the
sense in which I use the word, — /.<?., an idioplasm in which the
ultimate bearers of vitality (biophors) are combined to form
units of a higher order, the determinants and ids, having a
definite structure and size, — it is evident that the amount of
germ-plasm would not remain constant, or at most it would only
remain so for a few generations, as long, that is, as each kind of
germ-plasm is represented by several ids. As soon as this stage
was reached, a decrease in growth could no longer prevent a
doubHng of the mass ; this could, in fact, only be prevented by
the removal of half of the nuniber of ids present in the cell.

236 THE (iERM-PLASM

This actually occurs before the germ-cells unite in the process
of 'reducing division' of the nuclear matter of the germ-cells.
This fact may probably be taken as indicating the correctness
at any rate of the fundamental idea on which the theory of the
germ-plasm is based, viz., that the hereditary substance is com-
posed of ids. These parts of this substance, the existence of
which I formerly concluded from purely theoretical considera-
tions, and which I have called • ancestral germ-plasms,' must
exist in reality. I venture to make this assertion with all the
more assurance, owing to the fact that at the time when I postu-
lated the 'reducing division' merely on theoretical grounds, the
existence of such a process could not be gleaned from recorded
observations even in the case of the female germ-cells of animals,
in w^hich it can be observed comparatively easily, quite apart from
that of the male cells of animals, or of the germ-cells of both
sexes in plants.

We now know that this reduction of the number of ids by
one half is of general occurrence, and is effected by means of the
nuclear divisions which accompany cell-division. The divisions
which result in the formation of the. polar bodies perform the
function of the 'reducing divisions' as regards the ovum, and
the final divisions of the sperm mother-cells have this function
in the case of the spermatozoa. In both cases the reducing
division does not consist in the idants becoming split longitudi-
nally, and in their resulting halves being distributed equally
amongst the two daughter-nuclei as in ordinary nuclear division,
but in one half of the entire number of rods passing into one
daughter-nucleus, and the other half into the other. The process
is somewhat more complicated than would appear from this
statement, and it will be discussed more fully later on ; but the
final result is the same.

The following considerations may perhaps help to explain
why the constant doubling of the germ-plasm could only be
prevented by this method of removing entire nuclear rods, and
will at the same time indicate what are the primary causes of
the changes in the stiaicture of the germ-plasm caused by
amphimixis.

As already remarked, the nuclear rods must, before the intro-
duction of the process of amphimixis into the organic world,
have consisted of a number of identical ids, each corresponding
exactly to the individuality of the organism in question. These

EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM 237

ids must have been united into idants, which were all equal in
value, their number, as well as that of the ids, remaining the
same in subsequent generations. When sexual reproduction
first arose, the same number of idants from both parents became
enclosed in one nucleus, the total number of idants and mass
of germ-plasm of which w^re thereby doubled. This may have
been of no disadvantage if it occurred once only, but as the
process was repeated, an arrangement for preventing the germ-
plasm from increasing to an unlimited extent became necessary
each time amphimixis took place.

Were the germ-plasm an unorganised, or even a perfectly
homogeneous substance with no internal differentiation, — /.^.,
were it not composed of units of different orders, — its doubling
every time amphimixis occurred might have been prevented
simply by a limitation of its growth in each germ-cell, so that the
latter would contain only half the mass of germ-plasm formerly
present. But as soon as the germ-plasm came to consist of a
definite number of units, a diminution of the latter could not
result from a mere limitation as regards growth, for their num-
ber would nevertheless remain the same. This result could
only be attained by the appearance of a process by means of
which the juimber of units was reduced to half, and we have
seen that such a process occurs in the form of the remarkable
' reducing divisions' already described.

It is not difficult to ascertain what changes must result in the
composition of the germ-plasm by the combination of this proc-
ess with continued amphimixis.

Let us suppose that before the introduction of the latter process
the germ-plasm of a species consisted of sixteen idants. When
amphimixis, accompanied by the ' reducing division,' occurred
for the first time, eight paternal idants A would unite with eight
maternal idants B in the fertilised egg-cell to form the segmen-
tation nucleus. In consequence of the reducing division, each
of the germ-cells of the next generation would contain a com-
bination of the idants A and B, — e.^^., 4 A + 4 B. These
would again unite in the next amphimixis with eight idants —
e.g., 4 C + 4 D — in the germ-cell of another individual with
different hereditary tendencies ; and the ontogenv of the third
sexual generation would therefore be controlled by a germ-
plasm composed of the idants 4A + 4B +4C-I-4D. Let
us assume, for the sake of simplicity, that the reduction always

238 THE GERM-ri.\SM

affected everv kind of idant to the same extent ; the serm-
plasm of the fourth generation would then consist of the idants
2A + 2B + 2C+2D + 2E + 2F + 2G + 2H, and that of
the fifth, of a number of individually different idants, — provided,
of course, that interbreeding had not occurred. The germ-plasm
of this fifth generation would therefore consist of the idants A — Q.

This naturally does not imply that the process would really
take place in such an even and systematic manner; it must, on
the contrary, be a very irregular one. But although it may not
in five generations have resulted in the germ-plasm being com-
posed of a number of different ids, this result must certainly fol-
low in the course'of a greater number of generations.

The modification of the germ-plasm will not, however, then
have reached its limit. If my view of the composition of idants
out of ids is a correct one, and the id is really a unit which con-
tains all the primary constituents of the species, — that is to say,
if it contains all the determinants required for the construction
of a single individual, — it follows that the conipositioji of tJie
individual idants must gradually have become changed, so tJiat
each idant, instead of being made up of similar ids, comes to be
constituted by dissimilar and individually different ids.

The idants are not, in my opinion, perfectly invariable quan-
tities ; certain phenomena of heredity have led me to conclude
that they are in any case only relatively constant, and that their
composition becomes modified from time to time, so that the
ids which previously belonged to the idant A may later take
part in the composition of the idant B or C. Our present knowl-
edge of the processes of the division of the nuclear substance
does not enable us to say how frequently and regularly this
occurs ; but even if it only takes place at irregular intervals, during
long periods of time, it must nevertheless have resulted in a very
varied composition of the idants in the course of the enormous
number of generations which have ensued since the introduction
of the process of amphimixis into the organic world. As new
idants are always added to those already present in one of the
parents each time amphimixis occurs, a continual interpolation
of new ids can take place in the idants ; and as this process is
repeated an indefinite number of times, a single idant must
ultimately — if we neglect the repetition of similar ids which
results from interbreeding — come to consist of a number of
individually different ids.

EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM

■39

The process of mingling the ids would proceed most rapidly
if the paternal and maternal ids regularly combined each time
amphimixis took place, so as to bring together the half of the
different kinds of ids in the idants of both parents. If, for in-

Father.

Mother.

Offspriiig.

GENERATION I.

GENERATION II.

GENERATION III.

GENERATION IV.

Fig. 19. — Diagram illustrating the composition of the idants out of individually
different ids. (From Weismann's ' Essays,' Vol. I., p. 369.)

stance, each idant in an individual consisted of sixteen ids which
were all similar to one another on the first appearance of sexual
reproduction, the first occurrence of amphimixis would result in
idants consisting of eight paternal and eight maternal ids, which
are represented respectively by the black and white parts in the

240 THE GERM-PLASM

accompanying diagram (Fig. 19). (The boundaries between the
single ids are only indicated in generation IV. in the figure.) In
the second generation four groups, each consisting of four
similar ids, would be combined ; in the third, eight groups of
two ids : and in the fourth, sixteen groups, each consisting of
only one id. The accompanying diagram illustrates this proc-
ess : the two parental idants are shown on the left, and their
fusion to one idant in the offspring on the right. The different
kinds of shading and dotting indicate the individual differences
between the ids.

The mingling of the ids in the individual idants, just as in the
case of the mingling of the idants themselves, will not have
occurred so quickly and regularly as is indicated in the diagram ;
but the final result is the same, whether the process takes place
more quickly or more slowly.

The introduction of sexual reproduction will thus have gradu-
ally resulted in a greater degree of complication of the germ-
plasm, so that it is no longer composed of si?nilar ids, but is
mainly made up of ids which are individually different from one
another. All those p/ieno;ne?ia of heredity which are spoken of
as the intermiiigling of the characters of ancestors^ such as
degeneratioji or atavism of all kinds and degrees^ depend^ I
believe^ on this complicated strnctnre of the germ-plasm.

In the following chapter an attempt will be made to explain
these phenomena theoretically. It will, however, first be neces-
sary to glance for a moment at the process of the reduction of
the ids, as far as we are acquainted with it.

2. Proof that the Essential Part in the Process of
'Reducing Division' consists in the Extrusion of Ids

In the chapter on the architecture of the germ-plasm, it was
pointed out that the ids are probably identical with the ' micro-
somes "" which are known to exist in many cases in the nuclear
rods, and not with the entire rods, or idants. This conjecture
was based on the fact that the rod-like chromosomes, the struc-
ture of w'hich we are best acquainted with, consist of a series of
granules, or microsomes, which are separate and independent
structures. The composition of these rods evidently excludes
the possibility of considering each of them to be equivalent to a
single id. For an id is a vital unit, with a definite structure, and

EFFECTS OF AMPHIMIXIS ON THE (JERM-PLASM 24 1

cannot be composed of a row of loosely-connected spherical
bodies, each containing only a portion of its determinants.
Moreover, the fact that the number of idants is on the whole
a small one, speaks against their being regarded as ids : the
phenomena of reversion alone, it seems to me, require the
assumption of a larger number of ids.

The chromosomes are not, it is true, in all cases rod-like, and
may have a more spheroidal form ; the existence of microsomes
has, moreover, not been definitely proved in all cases. We might
therefore be inclined to look upon the chromosomes as structures
which are not always and absolutely equivalent, and to regard
some of them as single ids, and others as rows of ids. This
conception receives support from the fact that a considerable
variation as regards the number of chromosomes is seen in
nearly allied species, in which we might expect the processes of
heredity to occur in almost the same way. Thus, for instance,
the usual number of nuclear rods in Ascaris Imiibricoides is
twelve, and in Ascaris inegalocepJiala two or four ; in other
worms belonging to the same order the normal number of rods
may be eight, twelve, or sixteen. I should not, however, con-
sider these differences sufficiently great to warrant the assump-
tion that these rods have a different value in different cases ;
and this view receives support from the observations of Boveri
and Oscar Hertwig, which prove that in the same species
(^Ascaris jjiegalocepJiala) two varieties occur, in one of which two,
and in the other four, nuclear rods are present in the cells. In
this case, then, the one variety likewise possesses twice as many
microsomes as the other ; and although it is not always easy to
determine the number of microsomes in the case of other
Nematodes, we may infer their existence from the form of the
idants. For these reasons I am inclined to regard the inicro-
sojiies as corresponding individually to ids, a)id the 7inclear rods
as representing groups of ids ', for this reason I have called them
idajds.

The number of idants, and even that of the ids contained in
each of them, is a definite one for each individual species, but it
varies considerably in different species. Each id of any particular
germ-plasm could direct the entire ontogeny if it were present
in sufficient numbers ; that is to say, every id contains all the
determinants required for one individual : but, as has already
been remarked, the ids contained in the idants of a species

242 THE GERM-PLASM

which muhipHes sexually do not contain precisely identical
determinants, but these differ more or less from one another,
at any rate to such an extent that they correspond to the
individual differences existing in the species at the present day.
It results from the mechanism for nuclear division \\\^X all the
different kijuis of ids pass into all the cells througJwiit ontogeny,
and therefore the character of every individual cell occurring in
ontogeny must be deter nmied by an aggregate of ids ; so that all,
or at any rate the greater portion of the ids of which the idants
are made Jip, determine the constitution of the cell in question,
this determination resulting from the forces luithin the cell.
These preliminary remarks will serve as a general basis for the
following considerations on the effects of sexual reproduction.

We can now consider the process of the ' reducing divisions '
somewhat more closely. We require to know ivhat influence the
?' educing division exerts on the composition of the germ-plasm,
ajid of what kind are the ids which are consequently respectively
removed fro)u, and retaijied in, the ger?n-plasm.

Direct observation of the process is not alone sufficient to
explain it ; for not only do the ids and idants appear alike to our
eyes, but we cannot even determine whether the idants of the
young germ-cells of a new individual are the same as those of
the fertilised egg-cell from which this organism arose ; that
is to say, whether an idant is 2l permanent structure, and whether
a particular idant remains the same from one generation to
another.

We know that during the process of amphimixis the paternal
and maternal idants are situated close together, and are en-
closed within a common nuclear membrane. There is often a
small, though distinct space between the two groups of rods ;
and did this remain distinct during the whole period of ontog-
eny until new germ-cells were formed and underwent reducing
divisions, we might be able to determine directly whether the
paternal and maternal groups became separated, or whether half
the number of the paternal rods remained in connection with the
maternal ones, or also whether different combinations of rods
are removed by the reduction.

The matter is, however, not so simple as this : the idants of
the fertilised ovum only retain this form during the first division
of the egg-cell, and then become broken up into a number of
minute granules, which are distributed throughout the nuclear

EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM 243

substance, and only recombine to form nuclear rods when the
second division begins to take place. This process of disin-
tegration and subsequent recombination of the idants is repeated
every time a cell is formed by division during ontogeny, and
thus it is impossible to decide whether a certain idant of any
particular cell is derived from the father or from the mother. And
further, we cannot even ascertain with any degree of certainty
by mere observation, whether the idants of the subsequent cells
are the same as those of the fertilised egg-cell, — that is to say,
whether they contain the same kinds of ids in the same order.
It is very possible that the ids may become entirely separated
from each other whenever the idants undergo disintegration, and
then become arranged in some other order subsequently. The
number and nature of the ids contained in the entire idioplasm
would then certainly remain the same as before, but the indi-
vidual idants would diifer, because the combination of ids would
be different. It would then be immaterial whether the idants
on the right were separated from those on the left in the reducing
division, or whether the halving of the number of idants were
effected in some other way ; all the idants would consist of new
combinations of ids already present, and their combination
would necessarily differ completely from that of the idants of the
fertilised egg-cell, which is almost always separated by a number
of cell-generations from the new germ-cells, in each of which a
rearrangement of the ids must have taken place. The removal
of ^////r^ idants in the reducing division would obviously there-
fore be unnecessary, for the mere qualitative division of the
whole of the idioplasm into two halves would be sufficient for
the purpose.

As, however, the reducing division actually consists in the
removal of half the number of idants, and as, moreover, this
division is, as we shall see, a double one, I conclude that the
disintegratwn of the idants after every nuclear division is 07ily an
apparent one, and that the separate ids of the idant, on the con-
trary, remain connected together by fine threads of the cementing
substance, or * linin " ; and at the approach of nuclear division,
they become rearranged in the same order as before.

That this is the case may be concluded from certain phenom-
ena of heredity ; a child, for instance, not unfrequentlv takes
after one parent, e.g., the father only, or at any rate to such an
extent that the resemblance to the mother is unnoticeable. We

244 THE GERM-PIASM

must therefore suppose that the fertilised ovum from which the
child arose contained a very similar combination of ids and
idants to that which controlled the ontogeny of the father. It
must therefore be possible, and cannot be altogether a matter of
chance, that the germ-cell of the father contains these paternal
or maternal idants, — or, in other words, almost precisely the
same ids as those which directed the development of the father
or mother, arranged in almost the same order. This is onlv
conceivable, it seems to me. if the combination of ids into idants
usually, at any rate, persists even during the disintegration of the
latter in the nucleus.

Many recent observations support this conclusion, inasmuch
as thev show that fine threads of ' linin ' connect the individual
microsomata (ids) , even when the idant has apparently undergone
disintegration. In fact. Dr. Otto vom Rath * has just shown that
such connecting threads even extend between the idants. It is
therefore probably not too bold an hypothesis to assume the
existence of such an arrangement for connecting the ids together.

I am therefore of the opinion that the idants only apparently
undergo disintegration into granules during the ' resting-stage '
of the nucleus, and I agree with van Beneden and Boveri in
considering the idants to be essentially per)na)ie)it structures.
I do not, however, as already mentioned, wish this statement to
be taken too literally : it must not be supposed that the structure
of an idant must alwavs remain the same throughout all genera-
tions, or that the reconstruction of an idant after its disintegra-
tion must /';/ all cases result in the ids being rearranged in the
same order. I imagine, on the contrary, that deviations from
the original serial arrangement frequently occur in the ids. The
fact of the constant change of individuality and non-recurrence
of the same individual which can actually be observed in the
human race in the course of generations, indicates, in my
opinion, that an occasional change of the ids within the idants
can take place in the course of generations, although this does
not occur every time the idants are reconstmcted.

If this is the case, and essentially the same idants persist
during ontogeny from the fertilised ovum to the germ-cells of

* ' Zur Kentniss des Spermatogenese von Grylloptalpa vulgaris.' — Arch.
f. Mikr. Anat., Bd. 40, p. 120.

EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM 245

the new organism,* we may conclude from certain phenomena
of heredity that the reduction of the number of ids to one half
does not result in the separation of groups of ids which are
alivays the same, and are dejinitely determined beforehand, but
in the removal of different groups on differ ejit occasiojis. The
germ-cells of one and the same organism must consequently con-
tain very different combinations of ids, and consequently also
of primary constituents, than those which were present in the
parents of this organism. The reduction affects the paternal
and maternal idants in a precisely similar and equal way ; it
takes place in such a manner that any combinations may result
from the halving of the number of idants. Let us take, for in-
stance, four idants a + b and c + d ; not only may the paternal
group a + b and the maternal group c + d, as well as combina-
tions of a + b and c + d. be present in the fully-formed germ-cell,
but also the combinations a + c and b + d, or a + d and b + c, —
that is to say, combinations each of which consist of one paternal
and one maternal element.

A moderate amount of difference between the germ-cells of
an organism as regards their contained primary hereditary con-
stituents will thus result. In the case of the four idants taken
above as an example, only six combinations would be possible,
and consequently there could only be six kinds of germ-cells
differing from one another in respect of their primary constit-
uents. The number of possible combinations, however, in-
creases very considerably with the increase in the number of
idants ; for example, 70 combinations are possible with eight
idants, 12,870 combinations with sixteen idants.

* Appearances certainly seem to contradict this assumption, and I am
fully aware of the fact that Oscar Hertwig, and more recently Guignard,
have stated their opinion to the contrary. In many conditions of the
nucleus it is, in fact, impossible to recognise the idants, and tliey certainly
do not exist as such, — that is, in the form of compact rods. But it is quite
conceivable that the connection of the ids in an idant may nevertheless
persist, and that the individual ids are connected together by fine threads
of ' linin.' An observation made by my assistant. Dr. Hacker, supports
this view. He noticed that the microsomes of the rod-like idants of the
growing G^%g in Copepods become separated from one another, but always
remain connected by a delicate thread of linin, which in this instance can
be stained: the linear arrangement of the microsomes certainly persists in
this case. {Cf. Hacker, ' Die Eibildung bei Cyclops und Canthocamptus,'
Zool. Jahrbiicher, Abth. f. Anat. und Ontog., Bd. v.. p. 237.)

246 THE GERM-PLASM

In Ascan's jncgalocephala the number of idants is only two or
four ; but as far as we know, a greater number is present in the
case of all other animals, and also in that of plants : thus there
may be eight, sixteen, thirty-two, and even a hundred or more.*
A simple and single reduction, such as we have hitherto assumed,
will therefore in general secure a very considerable amount of
variety as regards the combinations of primary constituents
caused by the reducing division. Nature seems, however, to
have aimed at a far greater degree of variety, at any rate in the
case of animals, in which a double instead of a single reduction
of the niunber of idants to one half always occurs ; and this, as
I have recently attempted to show, must have the eiTect of in-
creasing the number of possible combinations of idants very
considerably.!

The facts as they concern the Metazoa may be briefly sum-
marised as follows. In all those species which have been in-
vestigated for this purpose, the germ-cells are formed by the
mother-cell undergoing two consecutive divisions, each of which
results in a halving of the number of idants, one half passing into
the one daughter-cell, and the other half into the other. In the
second division this would lead to the presence of only a quarter
of the original number of idants, if the number in the mother-cell
were not doubled by each idant becoming split into two before
the first division takes place. Thus there is first a doubling, and
then a halving, of the number of idants. It is a matter of
secondary consideration in the question of heredity that in the
formation of the female germ-cell or ovum three of the cells pro-
duced by the division of the mother-cell give rise to the evanes-
cent '■ polar-bodies,' one cell alone becoming an ovum capable
of development, while all four of the male germ-cells become
functional. The chief point which now concerns us is the proc-
cess of doubling, and the two subsequent halvings of the number
of idants : this is known to occur in all classes of the Metazoa
from the low-est to the highest forms, and, as far as we know^ is
only wanting in those eggs which are adapted for partheno-
genesis. Even in these cases the doubling also occurs, but it is
followed by only a single halving of the number of idants, in

* Dr. vom Rath informs me that in the crayfish {Astacus fiiiviatilis) the
number of idants reaches 108-125.

t Cf. 'Amphimixis,' Jena, 1891 (Essay xii. in the Enghsh Translation,
Vol. ii., p. 105).

EFFECTS OF AMPHIMIXIS UN THE UEKM-PLASM 247

correspondence with the absence of amphimixis. For the full
number of idants only appears a second time in an ovum adapted
for fertilisation, by the union of the nucleus of the sperm-cell with
that of the ovum.

RedU

Fig. 20. — Diagram of the formation of spermatozoa in Ascnrz's viegalocephala, var.
bivalens. (Modified from O. Hertwig.) — A, primitive sperm-cells; B, sperm-
mother-cells; C, first ' reducing division'; D, the two daughter-cells; E, second
' reducing divisions '; F, the four granddaughter-cells (the sperm-cells).

I consider this remarkable and apparently useless * process
of the doubling and two subsequent halvings of the idants as a
method of still further increasing the ttumher of possible com-
binations of idants in the germ-cell of one and the same individ-
ual^ and have given reasons for this opinion in the above-named

* It might be supposed from the observations of Riickert on the ovum of
the dog-fish, which were described in Chapters I. and II., that this doubling
is simply concerned with a doubling as regards mass, and consequently
with the activity of the idants: their activity must be very considerable in
this case, for the egg of the dog-fish is very large, and requires a consider-
able amount of multiplication of the ' oogenetic ' determinants. But a
douh)liiig of the idants occurs also in all other animal eggs, even in the

248

THE GERM-PLASM

essay. As already stated, a single halving of four idants can
only result in six combinations. But if, as actually occurs, each

Red.][

Fig. 21. — Formation of ova in Ascnris vtegalocephala, var. bi7'alens. — A, primi-
tive germ-cell ; B, fully-developed egg-cell, the number of the idants in which have
increased from four to eight ; C, first ' reducing division ' ; D, the egg with the first
polar-body, immediately succeeding the stage represented in C; E, the first polar-
body has divided into two daughter-cells (2 and 3), the four idants which remain
in the egg giving rise to the second ' reducing spindle '; F, stage immediately suc-
ceeding the second 'reducing division' — i, the ripe egg-cell; 2, 3, and 4, the
three polar-cells; each of the four cells only containing two idants.

smallest — in which a very small amount of yolk is contained — as well as in
the sperm-mother-cells, which never attain to such a size or structural dif-
ferentiation as do the ova. The process cannot be concerned with an in-
crease of the germ-plasm contained in the idants, for in the formation of the
ova three-quarters of the mass of germ-plasm passes into the polar bodies
and is again lost. The explanation of the process here given seems there-
fore to be the only possible one.

EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM 249

iclant were doubled before the division, ten combinations would
be possible. This means that one individual of any species
possessing four idants in each of its cells can produce ten kinds
of ova and spermatozoa ditTering from one another as regards
individual hereditary tendencies. Two new idants are added to
such an ovum when one of them is fertilised by the spermatozoon
of another individual ; and since each parent produces ten
different kinds of germ-cells, as many offspring differing in
character from one another may arise from these two parents as
there are possible combinations of the ten kinds of spermatozoa
of the father with the ten kinds of egg-cells of the mother, i.e.,
10 X 10 = 100.

With eight idants, 70 combinations are possible without, and
266 with, doubling ; and following this up, twelve idants will
thus give 924, or 8,074 combinations ; sixteen, 12,870, or 258,570 ;
twenty idants, 184.756, or 8.533,660; and with thirty-two idants,
about five hundred times as many combinations would be
obtained with doubling as without it.

Since the same number of idants from each of the conjugating
cells come together in the process of fertilisation, and each of
the parental germ-cells only contains one of the many possible
combinations of idants, the number of variations in the germ-
plasm which it is possible for two parents to produce must be an
enormous one. It can be calculated by multiplying the number
of possible combinations in the two conjugating cells together :
thus in the case of twelve idants only, it would amount to 8,074
X 8,074. Unfortunately we are unacquainted with the number
of idants in the human subject, in which we are best able to
recognise individual differences in most minute detail. We may,
however, suppose that this number is more than four. If, for
instance, it were as high as twelve, we need not wonder that
two children born consecutively are never identical, as must be
the case if they had originated from the same combination of ids
of the germ-plasm. Approximately identical children only occur
in the case of twins, and we have every reason to believe that
these originate from one sperm-cell and one ovum.

We cannot as yet judge with certainty as to how far the entire
idants pass unchanged as regards their constituent ids from the
germ-cells of one generation into those of the next. The phenom-
ena of the reduction in the germ-cells which have recentlv been

&

made known to us in the case of various Arthropods by the re-

250 THE GERM-PLASM

searches of Henking, vom Rath, and Hacker, indicate that even
the idants may become changed during the process. If we sup-
pose that in the mother germ-cell, when it is preparing for the
first reducing division, the ids become arranged in their original
order so as to form a long thread which doubles back on itself,
and thus gives rise to a ring, the latter would become trans-
versely divided in certain places. If the transverse divisions
could take place at different points, it would be possible either
for the old idants to be accurately restored, or for the new ones
to differ from them to a greater or lesser extent.

This assumption is not, however, essential for a theory of
amphigonic heredity, and we may here disregard it, although it
will doubtless be found to apply to some extent, as was indicated
above with regard to such a slow and slight change of the idants
due to the disarrangement in the combinations of ids contained
in them. It must be left to future researches to follow out this
process in detail, and to show whether the differences in the
combination of ids is merely due to the halving and rearrange-
ment of the idants, or whether regular, or at any rate frequent,
changes in the composition of the idants out of ids also occur.
For the present we must be content with knowing that \\\tger7n-
cells of an indiindiial contain very inany different combinations
of idants^ and that a frequent repetition of amphimixis never
ijideed results in the germ-cells of the same parents containing the
same combinatio?is. It therefore follows that the combination ot
parental and ancestral characters continually varies, and this
variation is characteristic of amphigonic heredity.

This statement also holds good for plants, in which we know
that a reduction to half the number of idants takes place in the
germ-cells. According to the researches of Guignard,* the

* L. Guignard, Compt. rend. May 11, 1891, and ' Nouv. etudes sur la
fecondation,' Ann. scienc. nat. Bot. Vol. xiv., 1891, p. 163.

Particulars regarding Giiignard's valuable researches cannot be entered
into here. They have not only proved that in plants the mature germ-
cells likewise contain only half as many ids as do the somatic cells, and
that the normal number is again produced by the union of the nuclei of the
male and female cells, but have also shown that the centrosome passes on
from one generation to the next. In spite of the fact that these observa-
tions are obviously perfectly accurate, I cannot help doubting whether the
reduction in the number of idants actually occurs without a nuclear divis-
ion, as Guignard states is the case, I have arrived at this conclusion not

EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM 25 I

somatic cells, as well as the mother-cells of both kinds of germ-
cells in Liliuni niartago)i^ contain twenty-four idants, while the
mature germ-cells contain twelve only. We do not as yet know
whether this reduction is effected by a single reducing division,
or by two such divisions preceded by a doubling, as in the case
of animals. P^or evident theoretical reasons I consider it ex-
tremely unlikely that the reduction occurs in the mother-cell
while it is preparing for division, as Guignard thinks is the case.
It is very possible, however, that only one reducing division
takes place in this instance.

The details of these processes in the lower plants are quite
unknown, probably owing to the minute size of the idants, which
till now has rendered the difficulties of such investigations insur-
mountable. It has, however, at any rate been ascertained that
in many marine algae (^Fucoidea) the development of the egg-
cells is accompanied by the formation of ' polar bodies,' which
certainly correspond to stunted and phyletically degenerated
ova : this was first shown to be the case by Butschli and Giard,
and the fact has long been recognised by other zoologists besides
myself. In the genus Fhcjcs these polar bodies do not occur,
and eight eggs are formed from the primary ovum, — if I may
venture to apply this term to the original cell of the so-called
oogonium or ovary ; in an allied species of wrack, AscopJiylliDn
7iodosu?;i, only four eggs are formed from the primary ovum, but
four polar bodies are also produced ; in Pelvetia canaliculata
the primary ovum gives rise to two eggs and six polar bodies ;

merely from a comparison of the analogous process in animals, but also
because I cannot help thinking that it is possible, and even probable, that
in this respect these otherwise admirable observations are not quite com-
plete. In the formation of the male germ-cells, a reducing division may
perhaps take place between the 'cellules m6res primordiales ' and the
'cellules meres definitives;' and as regards the female germ-cell, it will
occur in the division which gives rise to the ' cellule mere du sac embry-
onnaire.' In both cases even the most acute observer might fail to notice
the reducing division if his attention were not specially directed to this
point. Why should an arrangement for a ' reducing division ' have been
made in the case of animals if the reduction could take place without
nuclear division, and could produce the same result ? Of all the other
numerous observations which have been made on the process of karyo-
kinesis, not a single one supports the view that the single (?) chromatin band
of the ' skein ' stage can become disintegrated into half as many idants as
were previously present in the nucleus.

252 THE GERM-PLASM

and in Hi)nanihalia lorea only a single egg and seven polar
bodies are formed.*

We have here, however, no information as regards the reducing
divisions, which, as I pointed out long ago, need not by any
means be connected with the degeneration of several germ-cells.
We can only state that the three successive divisions of the
primary ovum, which occur in all the above-mentioned cases,
affords more than sufficient opportunity for one or even two
reducing divisions, and that it is extremely probable that one, at
least, actually occurs.

We may therefore assume that in plants very varied com-
binations of the germ-plasm derived from each parent usually
take place in the germ-cells of the offspring, and that perfectly
' identical " germ-cells can very rarely occur either in plants or
in animals.

* Cf. Oilman's ' Beitrage zur Kentniss der Fucaceen,' Cassel, 1889.

EFFECTS OF AMPHIMIXIS ON ONTOGENY 253

CHAPTER IX

ONTOGENY RESULTING FROM THE UNION OF
THE GERM-PLASM OF TWO PARENTS

I. The Nature of the Offspring Determined by the

Process of Fertilisation

The first question which presents itself in the consideration of
' amphigonic heredity ' is concerned with the relative share taken
by the germ-plasm of each parent in the control of ontogeny : —
whether the paternal and maternal ids always co-operate simul-
taneously, and the forces contained within them together form a
single resultant, or whether one group only is active while the
other remains passive. This question cannot at present be de-
cided from the results of observations on the nuclear substances
themselves ; the phenomena of heredity, together with what we
know concerning the composition of the idioplasm resulting
from amphimixis, can alone help to elucidate this problem.
These phenomena must therefore be analysed as accurately and
minutely as possible.

We must base our analysis on the fact which we have already
proved, that the germ-cells of an individual differ from one
another as regards the hereditary substance they contain, and
that the proportion of paternal and maternal ids in a germ-cell
varies between wide limits, the degree of variability being greatly
increased by the union of the germ-cells of two individuals in
the process of amphimixis. This fact is sufficient to account for
the difference existing in the human race between children of the
same parents. The fundamental law of amphigonic heredity
enunciated by Victor Hensen follows directly from this fact : —
'•the individual is deter )nined at the tivie of fertilisation ; ' or, in
other words, the individuality of an organism results from the
fact that the germ-plasm is composed of the paternal and maternal
ids which are brought together in the egg-cell.

This law is not self-evident, for we might have believed, a priori^
that the development and mingling of parental characters in
the offspring is due entirely, or at any rate to a great extent, to

254

THE GERM -PLASM

external influences of nutrition, &c., to which the germ is sub-
ject after fertiHsation. The existence of 'identical' human
twins, however, proves the contrary. Some twins do not re-
semble one another more closely than do children of the same
parents which are born consecutively ; and, in fact, this is
apparently true of the greater number of twins, in which the
dissimilarity may even be very considerable. We have every
reason to suppose that such ' dissimilar' twins are usually derived
from two ova, which must of course have been fertilised by two
different spermatozoa. On the other hand, in the case of those
twins which I speak of as • identical,' the resemblance, although
not perfect, is much closer than has ever been observed in
children born successively. There is every reason to suppose
that such identical twins are derived from a single ovum and
spermatozoon. If this is actually the case, it furnishes a proof
of the above statement that heredity is potentially decided at the
time of fertilisation, or, expressed in terms of the idioplasm, that
the nature of the combination of the parental ids which takes
place during fertilisation predetermines the whole subsequent
ontogoiy. The slight differences which exist between identical
twins would therefore probably indicate to what extent the course
of development may be affected by external influences. These
differences are generally so slight that it is difficult to observe
them at all, unless they are specially sought for ; as a rule such
twins can only be individually recognised by the parents or
brothers and sisters, and cannot be distinguished from one
another by strangers.

These slight differences might, however, be due to an imper-
fect predetermination of the influence which is exerted at every
ontogenetic stage by the idioplasm of each of the parents. We
can hardly decide between these views from the consideration of
identical twins only. Mr. Otto Ammon, of Karlsruhe, has kindly
furnished me with two photographs of identical twins taken in
consecutive years at the ages of seventeen and eighteen, as well
as with exact measurements of all parts of their bodies. In
spite of a striking resemblance, not only in face but in all parts
of their bodies, certain differences are nevertheless recognisable
between them. For instance, the height of the one marked
No. 507 on Mr. Ammon's list, measured, when lying down, 172
cm., and that of No. 508 only 170 cm.; and again, although
the length of the hand and of the left arm is the same in both, —

EFFECTS OF AMPHIMIXIS ON ONTOGENY 255

the latter measuring 74 cm., — the right arm of No. 507 is only
71 cm. long, while that of No. 508 reaches 74 cm. The relative
lengths of the upper arm and fore-arm are also different, that
of the left upper arm of No. 507 being 27 cm., while in No. 508
it is 27.5 cm. ; and consequently the length of the fore-arm in No.
507 is also 27 cm., while in No. 508 it is only 26 cm. Even if
we possessed the measurements of the parents at the same age,
we should probably be unable to draw any definite conclusions
as to whether these slight differences in size are due to a corre-
sponding difference in the combination of the germ-plasm, such
as might arise from a slightly inexact division of the nucleus of
the fertilised ovum in the process of doubling or at a later stage,
or whether they simply owe their origin to slight general or
local differences of nutrition taking effect during ontogeny.

Other facts are, however, known, which prove that although
the nature of the combination of the parental idioplasms during
ontogeny is in general, as a matter of fact, determined at the
time of fertilisation, it is nevertheless liable to slight individual
fluctuations. Instances of this kind are furnished by the hybrids
of certain species of plants^ many parts of which exhibit a con-
siderable degree of variability, and fluctuate between the specific
characters of the two parents. The blossoms of the hybrid
plants obtained by crossing Digitalis lutea and D. purpurea^ for
instance, ' vary in colour ; in some instances they are pale, with
a slight pink tinge, which latter, again, may be entirely absent ;
and in others they have a more or less bright purple colour.' *
These observations appear to me to be particularly important,
owing to the fact that we may assume with certainty in this case,
in which two distinct and sharply defined species were crossed,
that both parents possessed the specific characters in the same
degree of purity and strength, and that consequently the relative
proportion of the parental idioplasms does not remain quite con-
stant during ontogeny, owing either to slight irregularities in the
nuclear division, or — and this is less probable — to inequalities
in nutrition and in the growth of the idants derived from the
two parents. Owing to the kindness of Professor Hildebrandt
of Freiburg i. Br., I have had an opportunity, in the case of
hybrids of two species of Oxalis, of observing how extremely
detailed the process of predetermination is. The flowers of

* Focke, ' Die Pflanzen-Mischlinge,' Berlin, 1881, p. 316.

256 THE GERM-PLASM

one of the parent-species were large, and of a pale lilac colour,
while those of the other were smaller, and their colour was red,
with a dark crimson ground. The flowers of the different
hybrids were by no means quite similar, but three principal forms
could be distinguished according to the combination of colours
in the flowers, which I shall not describe in detail: the flowers
of the same hybrid^ Iiowcver, resejiibled each other in their /nost
minute details. One plant, for instance, had violet petals of a
rather pinker tint than those of one of the parent-species, and
all the petals were strongly tinged with red on one and the same
lateral margin. As far as I could observe, all the flowers were
similarly coloured on this stock. On another stock, all the sepals
had brown rims, and on a third there was a narrow dark orange-
coloured band in the centre of each flower. In these cases, there-
fore, the combination of the colours of the parents which appeared
in the petals of the hybrids must have been decided at the time of
fertilisation. It will be shown later on how this combination
may vary somewhat in different plants.

Even although the slight differences in identical human twins,
which can be proved to exist, are certainly due in part to minute
differences in the idioplasm itself, some of them must neverthe-
less with equal certainty be attributed to the effect of various
external influences. My photographs of the above-mentioned
identical twins show that No. 507 has particularly white hands,
while those of No. 508 are much browner. No one would
attribute this fact to dissimilarity in the respective germ-plasms,
or to an alteration in the proportion of paternal and maternal
idioplasm which occurred during ontogeny : it must be due to
the fact that the hands of No 508 had been more exposed to
the sun than those of No. 507 ; and, as it happens, the former
of the two had been more employed in the open air than the
latter before the photograph was taken. Several differences
in the proportional sizes of parts of the body may possibly have
been brought about in a similar wav.

2. The Share taken by the Ancestors in the
Composition of the Germ-plasm

If, then, it is certain that the characters of the developing
offspring are essentially decided by the mingling of parental
idioplasms which takes place in the process of fertilisation, we
must next try to ascertain whether the entire parental idioplasjn,

^•.

EFFECTS OF AMPHIMIXIS ON ONTOGENY 25/

with all its constituent determinants^ or only a portion of it, is
passed into the germ-cell which will give rise to a new individual ;
and also what proportion of it is constituted by the gerni-piasni
of the grandparents, great grandparents, and more re/note ances-
tors. The fact that the reducing division, which takes place
both in the male and female germ-ceils before fertilisation,
removes half of the idants from each, leads us to conclude
that only half the normal nu tuber of ids can be contained in
each germ-cell •, and this could only be the case if two of each
kind of parental idant were present in the germ-plasm, and
if the reduction resulted in each individual germ-cell containing
a similar group of idants. But this cannot be so, for the germ-
plasm must consist of a number of entirely different idants,
unless, in consequence of interbreeding, two of the same kind
are present in certain of the groups. The whole of the idants
of both parents evidently cannot possibly be contained in any one
germ-cell, because the total number would be twice as great as
that which actually occurs in a ripe germ-cell. If in Man, for
example, there were thirty-two idants in the fertilised ovum,
sixteen of them would be derived from each parent. Of this
latter number, sixteen at most could be derived from one grand-
parent, and this could only occur if no idants at all from the
other grandparent had passed into the germ-cell in question.
It is evidently more than inaccurate to lix the limit of the he-
reditary power — as is done by animal-breeders — of a parent at
\, of a grandparent at \, of a great-grandparent at \, and so
on.* These numbers do not even represent the maximum or
minimum share in heredity which may be taken by the respec-
tive ancestor in the constitution of the fertilised ^g^- The

* Galton has also emphasised this fact in the concluding chapter of his
book on 'Natural Inheritance' (p. 187 et seq.). According to his view,
the ' personal heritage ' of each parent = \, and the heritage of ' latent
elements ' of the parent likewise = 4, the two together thus making up \.
Naturally I cannot agree with this calculation, for in my opinion the
latency of the characters of a parent does not result from the ' primary
constituents ' of these qualities, but from the struggle between the primary
constituents of both parents ; and I do not in the least suppose that the
primary constituents which practically give rise to the individual become
separated from those which form the latent germ for the germ-cells of the
next generation. But I fully agree with Galton that all the 'characters'
of the ancestor — the grandparent, for instance — are never present in every
germ-cell from which a grandchild may arise.

258 THE GERM-riJ^SM

parent is certainly always represented by one-half, but the share
varies even in the case of the grandparent ; in the instance just
given it would varv between o and 16. For the reducing di-
vision may, for instance, cause the sixteen paternal idants result-
ing from the reduction of the thirty-two originally present in
the sperm-cell, to contain idants derived from the grandfather
only, and none from the grandmother ; or, again, there might
be fifteen from the grandfather, and one from the grandmother,
or fourteen pp and two //////. or thirteen pp and three ww, and
so on.* This would be so, at least, if any kind of combination
of the idants may result from the reducing division. It may
perhaps not be the case absolutely, but the capriciousness with
which reversion to a grandparent may occur nevertheless in-
dicates that a considerable latitude exists with regard to this
combination.

In passing back to the third, fourth, and fifth generations, we
cannot in the least determine, a priori^ to what extent an indi-
vidual ancestor of the animal in question is still represented in
the germ-plasm of a germ-cell ; we can only state the maximum
which might be possible in the most favourable case. In the
above-mentioned instance, an ancestor of the third generation
jjiig/it still be represented by sixteen idants, for the sixteen
idants which this ancestor furnished for the purposes of amphi-
mixis in the second generation might, all in fact, possibly have
passed into one germ-cell in the process of reducing division in
this generation, and the same, again, might have occurred in the
first generation. Such a case can only be of rare occurrence,
but it apparently accounts for the instances of reversion in Man
to ancestors more remote than grandparents, which, though
rare, certainly occur occasionally. The more remote the gen-
eration, the greater are the chances against the entire half of
the total number of idants remaining together through several
generations in individual germ-cells, and the probability of such
an occurrence will very soon be reduced to zero.

We may suppose that, as a general rule, the number of ances-
tral idants contained in a fertilised egg-cell becomes less in pro-

* I shall now denote the paternal ids or idants by the letter p, the
maternal ones by /«, and those derived from the grandfather hy pp or
ptn, and from the grandmother by mp or mm, &c. The first letter in each
case signifies the parent, the second grandparent, the third the great-
grandparent, and so on.

EFFECTS OF AMPllIiMIXIS ON ONTOGENY 259

portion as the ascendancy of the ancestor concerned decreases.
Any more exact calculation of the share taken by a certain
remote ancestor in the composition of the germ-plasm of its
descendant would be erroneous. Hitherto the customary
method of making such a calculation has been to assume that
the following shares are taken by the various ancestors in the
predisposition of the offspring: — parents, 2 x '. ; grandparents,
4x\, and so on ; that of the sixth generation of ancestors being
32x^1^. Thus, in the last-mentioned generation, <^w^ idant out
of the thirty-two assumed to be present in Man would, according
to the theory of the germ-plasm, still remain. This does not by
any means imply that each of the thirty-two ancestors of the
sixth generation is still represented by one idant in the germ-
plasm of the descendant ; it is quite as probable that thirty or even
twenty of these ancestors take part in its constitution, and the
number may possibly, though improbably, be still less than this.
In treating of the phenomena of reversion, I shall have occasion
to refer to this subject again.

It is at any rate certain that m no case can 7nore than the half
of the idant s of one parent be present in the gerjn-piasm of the
fertilised egg-cell.

This statement is, how^ever, apparently contradicted by certain
facts.

Plant-hybrids frequently keep to the mean between the two
ancestral species : that is to say, they contain all the characters
of these two species in equal proportions. Thus all the primary
constituents of each parent would be contained in the fertilised
egg-cell, although, according to our theory, only half of the
parental idants are concerned in its constitution. This contra-
diction is easily accounted for, if it be borne in mind that we are
here concerned with the mingling of the characters of two species,
and not of those of two individuals of the same species. The
characters of the species must be contained in the majority of ids
in each idant., if not in every id, and half the idants may in this
case produce the sanie effect as would result if all the idants were
present: that is to say, they contain e^/ery specific character.
In cross-breeding, specific characters are opposed to specific
characters, and in comparison with the greater differences
between these, the lesser individual ones disappear.

The reverse is true in the case of reproduction in Man. espe-
cially within one and the same race. The specific characters

26o THE (;ERM-PLAS]M

are probably contained in all the ids of the father as well as of
the mother, and the differences between the parents refer to
individual characters only. Our theoretical conception of the
idants as a collection of ids seems incompatible with the above-
mentioned statement that the child can only closely resemble
one parent, for only half of the idants of this parent take part in
the construction of the child. We shall, however, be able to
e.xplain this apparent contradiction later on.

The facts of the case may be stated in s^eneral terms as
follows. Half the number of parental idants always reach the
germ-cells of the offspring, but this half may consist of all
possible combinations of the parental idants : that is to say,
either of idants derived from the grandfather or grandmother
only, or of a combination derived from both, in which one or the
other may predominate. Nothing will be gained by taking the
ancestors of the third or fourth generation into consideration
until we come to consider the phenomena of reversion.

3. The Struggle of the Ids in Ontogeny
a. Plant-Hybrids

The structure of the offspring results from the struggle of all
the ids contained in the gerin-plas)n.

That this statement must be in general correct is to some
extent indicated by the fact that all parts of hybrid plants, pro-
duced by crossing two species or varieties, usually possess the
characters of both parents. The details concerning hybrids are
of far greater value for theoretical purposes than are those re-
lating to the normal offspring of any particular species, as we
know for certain that the characters which compete with one
another or combine, so as to result in the production of a hybrid,
must be contained /;/ every idant of one or other of the parents ;
for these characters are those of the species.

The difference as regards the idioplasm between individual
and specific characters, seems to me to be due to the determi-
nants of the latter being present in an overwhelming majority in
all the ids of every idant of the germ-plasm, while the deter-
minants controlling the structure of individual characteristics
are only contained in a portion of the idants of which the germ-
plasm consists : at most they can only be present in all the
idants of one of the parents, — that is, in half the entire number

EFFECTS OF AMPHIMIXIS ON ONTOGENY 26 1

of idants. The extent to which the determinants of any indi-
vidual character are represented — whether they are contained in
all the ids, or only in a small portion of them — could only be
ascertained from the phenomena of heredity if we knew tl.e
cause of the predominance of any particular ' successful ' charac-
ter. This, however, can only be inferred from crosses between
species, the great constancy of the specific characters in which
leads us to presuppose that their determinants predominate in all
the idants of the parental germ-cell.

In plant-hybrids, paternal and maternal idants come together
in the process of amphimixis, and each group of them may be
assumed to consist of s//////ar idants. The effect this arrange-
ment will produce on the phenomena of heredity must now be
considered, and conclusions drawn from these considerations.

From a very large number of observations on hybrid-plants,
we find that, in the first place, parental characters may be
variously intermingled. From a comparison of all the cases
observed up to the year 1881, Focke * concludes that these com-
binations of characters may be divided into three principal
groups, viz . : — ( i ) a mean between both parents is maintained
in all parts of the plant ; (2) the characteristics of the father or
7)iotJier predomijiate ; and (3) certain parts of the hybrid exhibit
the maternal, and others the paternal., characters.

The first-mentioned case is by far the most frequent : we
may take as an example the hybrid obtained by Kollreutter from
two species of the tobacco plant, — Nicotiana rnstica 9 and N.
Paniculata Z- Kollreutter f himself stated that this plant was
exactly intermediate between its parents, while it was considered
by Gartner to bear a slightly greater resemblance to N. panicu-
lata., and by Focke to N. rnstica. Kollreutter's opinion is probably
therefore a fairly accurate one, for in any case the point can
only be estimated, and cannot be decided with mathematical
precision. According to Focke, the corolla-tube of N. rnstica
is 14 mm. long, that oi N. paniculata 26 mm., and that of the
hybrid 19 mm. The exact mathematical mean between these is
20 mm. ; and hence in this character the hybrid approaches N.
paniculata. As regards the diameter of the widest part of the

*'Die Pflanzen-Mischlingen,' Berlin, 1881.

t Joseph Gottlieb Kollreutter, ' Vorlaufige Nachricht von einigen das
Geschlecht der Fflanzen betreffenden V'ersuchen u. Beobachtungen," 1716.

262 THE GERM-PLASM

corolla-tube, on the other hand, the hybrid inclines more to iV.
rustica^ while in the narrowest part it exhibits the exact mathe-
matical mean. This case is instructive, for we cannot recognise
the true physiological mean, because the length, as well as the
diameter, of the corolla-tube is determined by the same cells.
Any estimation of the mean between the colours must be still
less precise, for the different shades depend on entirely dif-
ferent morphological constituents. If the yellow and red of two
different species were blended in the flower of a hybrid, the
intensity of both these colours might conceivably be as great as
in the parent plant, and yet one of them might predominate
because it happened to cover the other. F'or the yellow is due
to special pigment-granules, while the red occurs onlv in the
cell-sap, which might possibly be nearly hidden by a superficial
layer of chromatophores.

In any inquiry with regard to the factors which control the
struggle of the parental characters, it must, above all, be
borne in mind that tJie cells are always the deter)ni}ii}ig agents.
The determinants of the father and mother come together in the
cell, and in the cell only : and all characters, whether relating to
a large part of the organism, or merely to a single cell, can only
be determined by processes taking place within the substance of
one or of many cells.

This does not mean that the visible differentiation of every
individual cell always constitutes a 'character' of the organism.
The nature of the histological differentiation of the cells — that is
to say, whether muscle- or nerve-substance, or chlorophyll
granules, for instance, are produced in the cell-body or not —
only comes into consideration in connection with the lumiber of
cells in the definitive cell-aggregate constituting the organism.
Very many characteristic qualities cannot be due to this fact,
but must chiefly depend on the number and arrangement of the
cells in an organ, which again must be due to qualities of the
embryonic cell which are invisible to us — principally those which
relate to its method of tiivision^ and its vigour and rate of multi-
plication.

We must suppose that these factors are wholly determined by
the idioplasm of the cells quite as much as is the visible differen-
tiation of the latter. The division of a cell certainly originates
in its apparatus for division, and primarily in the ' sphere of
attraction ' and its contained centrosome ; but we should ha\e to

EFFECTS OF AMPHIMIXIS ON ONTOGENY 263

give up the whole idea of the controlHng influence of the nuclear
substance were we to suppose that this apparatus actually directs
the process of division. The entire process of development of
the animal from the ovum depends so essentially on the rate of
division of the cells, that the nuclear substance could no longer
be considered to correspond to the hereditary substance if it
merely caused the visible ditTerentiation of the cell. But in
the Introduction I have already stated the reasons which
prove beyond a doubt that the nuclear matter actually contains
the primary hereditary constituents, and it follows from this fact
that there can be no question of a self-determination of the
apparatus for division. We must, in fact, suppose that the ulti-
mate structure of the cell-body, which is invisible to our eyes,
controls its entire growth as well as its method and rate of
division, this structure itself being controlled by the nuclear sub-
stance or idioplasm. Ultimately, therefore, everything depends
on the determinant of the cell ; and in the case of sexual
reproduction, the co-operation of the paternal and maternal de-
terminants determines the character of the cell, whether this char-
acter is visible or invisible. As every cell in the entire ontogeny
is, according to our view, controlled by one kind of determinant
only, irrespective of the fact whether it also contains other
determinants in a latent condition or not, the co-operation of
maternal and paternal determinants always determines the
character of the cell, and controls the development of the
individual as far as the influence of this cell extends. In spite
of its greater histological differentiation, a slighter influence is
therefore obviously exerted by one of the final cells in ontogeny,
— that is, one of the tissue-cells, — than by one of the first
four segmentation-cells, or by the primary cell of the entire
germinal layer, or, again, by any cell from which many and
various kinds of cells may subsequently arise. On the other
hand, w^e must not forget that each embryonic cell only
determines its oivii method of division, and not necessarily
that of its daughter-cells, and that consequently in these a new
counter-balance or co-operation of the paternal and maternal
determinants again takes place.

The determinants which control the daughter-cells are, how-
ever, derived from the latent ids of the mother-cell, and it there-
fore essentially depends on the methods of division and on the
architecture of these ids as to which determinants are to control

264 THE GERM-PLASM

the daughter-cells. TJiat portion of the ids of the inothej'-cell^
tJierefore, luhich is for the time latent^ exerts a defnite influence
on the determination of tJie subsequent developjuent ; on it alone,
indeed, depends the number and order of succession of the
determinants which will subsequently become active, and all
those characters which result from the number and grouping of
the cells, as well as from the histological nature of the individual
cell, are also determined in the first instance by their ids.

We can thus explain why cross-breeding is only successful in
the case of nearly-related species, and not in that of members of
entirely different families. If it were possible, for instance, for
the ovum of a sea-urchin to be fertilised by the spermatozoon of
a worm, such as Rhabditis nigrovenosa^ the disintegration of
the ids of germ-plasm derived from the father and mother
respectively would take place in an entirely different manner
even in the first stage of segmentation ; the maternal ids might
become disintegrated into the groups of determinants for the
right and left halves of the body^ while the paternal ids became
separated into the groups for the external and internal germinal
layers. But dissimilar groups of determinants of this kind could
not co-operate and give rise to intermediate structures ; and
even if the ontogeny advanced a few more stages, embryonic
structures could never be produced which would work together
harmoniously.

The term homologous determinants and ids may be applied to
those which control homologous cells and groups of cells. It
may therefore be stated that structures which are intermediate
between those of the two parents may arise wJienever homologous
detern?inants and ids come together. If a small spot is present
on a certain region of the wing in two allied species of butter-
flies, for instance, and this is represented in the germ-plasm by
a single determinant, the homologous determinants of the two
cells in which this spot was first contained will be brought to-
gether when these two species are crossed, and will eventually
be able to share in the control of the cell in question. They
need not, however, necessarily be exactly alike ; the spot may
be brown in species A, and red in species B. In this case the
determinants would be homologous but not homodynajnous, and
might possibly combine to form a reddish-brown spot. The
essential point in amphigonic heredity is, that the idioplasm in
each cell in the entire ontogeny contains ids which are individually

EFFECTS OF AMPHIMIXIS ON ONTOGENY 265

different from one another^ and does not co7isist of a niDtiber of
identical ids ; and that the cell may take on an intermediate
character in consequence of their co-operation. In normal repro-
duction the active ids of the idioplasm are all ho/nologous, — that
is to say, their function is to control the same part of the body,
— but they differ among themselves, i.e., are heterodynamons,
or, in other words, they tend to impress a somewhat different
character on the same part of the body. In the crosses between
different species, the idioplasm of a cell in many stages will be
composed of homologous as w'ell as of heterologous ids, and,
as has just been shown, it then remains to be decided whether
a common control of the cell is possible at all, and if so, over
how many generations of cells it can extend.

The scales of butterflies are cells belonging to the final stage of
ontogeny, and their ids are made up of determinants of one kind
only. The further back we pass towards the beginning of on-
togeny, the more numerous are the determinants composing the
ids ; only one of them, however, breaks up into biophors and
controls the cell. If the determinant which actually controls the
cell is homologous in both parents, an intermediate form of cell
may result ; but as soon as the rest of the determinants in the id
no longer correspond to one another, the further development
gradually becomes checked, and will ultimately be brought to a
stand-still. The processes which must be supposed to take
place from the beginning of ontogeny onwards in the case of the
supposed cross between a sea-urchin and a worm may very
possibly be only partial i-e., they may refer merely to special
parts. Let us suppose that a certain species of insect, which is
normal in other respects, possesses limbs on the dorsal side
instead of wings, and that this is crossed with another insect
possessing normal wings, development of the fertilised ^g^
taking place up to the stage in which the wings arise. The
idioplasm of the primary cell of the wings and ' dorsal limbs '
respectively would then contain two perfectly Jieterologous ids,
one derived from the father and the other from the mother, and
none of the determinants in these two ids would be respectively
homologous. In the first stage of the development of the wings
or ' dorsal limbs,' as the case may be, antagonistic determinants
would be opposed to one another in each cell-generation, and
would prevent a common determination of the cell.

Such extreme cases do not actually occur, as very different

2 66 THE HERM-PLASM

species do not interbreed ; but the principles which may be
deduced from these imaginary cases may be appHed to the
production of hybrids. Homologous determinants and ids co-
operate, while heterologous ones do not ; and the larger the num-
ber of heterologous determinants present in the homologous
ids of the parents, the more do the hereditary tendencies of the
latter tend to nullify one another.

We must now attempt to give a theoretical explanation of the
/i?'st of the three kinds of cofjibination of parental characters in
the offspring mentioned above^ viz., that in which 'a mean betiveen
both parents is maintained in all parts of the plant.'

If we assume that the two parental species are so closely allied
that each determinant in the one corresponds to a homologous
determinant in the other, a form exactly intermediate between
the two species must result, supposing that the number of ids in
either parent is the same, and that a similar controlling force
acts on the homologous determinants.

If the idioplasm of one parent is represented by a larger
number of ids, its controlling force must obviously be greater ;
and as regards the controlling force of the individual determi-
nants, we may state as follows : — the control of the cells is in
our opinion effected by the disintegration of the determinant
into biophors, which, like the pangenes of de Vries, migrate
into the cell-body, multiply at its expense, and give rise to
definite cell-structures. This multiplication must take place with
a certain amount of energy, the degree of which varies in the
different kinds of biophors. Thus, whenever such controlling
biophors, possessing the energy of growth in different degree.s,
migrate into the same cell-body, a struggle of the parts (Roux)
must ensue, in which the stronger part will be successful,
and the weaker will be more or less, or even completely,
suppressed .

The formation of structures which are strictly intermediate
between those of the two parents, implies that the homologous
determinants possess a similar controlling force. All the deter-
minants of any two species are, however, never homologous :
this follows from the fact that the ttumber of cells in homologous
parts is often very differen-t. The characters of the species do
not by any means depend only on the histological nature of the
individual cells, but, as already stated, are due in almost a
greater degree to the number and arrangement of the cells, to

EFFECTS OF AMPHIMIXIS ON ONTOGEXY 267

the repetition and position of certain organs, and so on. Thus
the flower of Nicotiana paiiiculata is decidedly longer than
that of N. rustica^ and the former species is more extensively
branched and possesses a larger number of glands than the
latter. The above assumption that the individual determinants
correspond to one another in the two species cannot therefore
be quite an accurate one ; the germ-plasm of N. paniculata
must, on the contrary, contain a larger number of determinants
than that of IV. rnstka, and the process of disintegration of the
two species must differ in many ways.

If an equal number of ids from each of these species occurs
in the hybrid, the two kinds of ids will only be able to co-
operate in ontogeny as long as their determinants still corre-
spond to one another. As soon as a point is reached in
which the ids of JV. rustica begin to decline in number, and
their last determinants have become disintegrated, the ids of
N. paniculata will alone be able to produce series of cells ; but
as only half the normal number of these ids are present, the
structures arising from them cannot be as complete as they are
in the pure ancestral form : and, apart from this, the ids of
N. 7'ustica may possibly not have disappeared entirely, but the
succession of the determinants, which are, properly speaking, the
final ones, may persist beyond the normal period, and may in
this way interfere with the development of the pure characters of
N. paniculata. This would, at any rate, render the fact com-
prehensible in principle that intermediate forms may also arise
in cases in which the struggle of the parental ids does not
extend into the final cells of ontogeny, and the characters of
the species come into contact at an earlier stage, as in the case
of the greater or less degree of ramification in plants. The
following considerations will make this more evident.

In the second kind of combination., either the paternal or the
maternal characters predominate in all parts of the hybrid, so
that the latter bears a closer resemblance to one parent than
to the other : in this case, therefore, the transmission is appar-
ently monogonic.

Several cases of this kind have been recorded, in some
of which the paternal, and in others the maternal characters
predominate. Instances of both kinds occur in the genus
Nicociana. 'The hybrid Nicotiana paniculata 9 x 7'incce/lora $
bears so close a resemblance to the last-named species that the

268 THE GERM-PLASM

character oi A", paniculata can hardly be recognised at all/ * In
this case, therefore, the paternal plant predominates, while the
cross between N. suai'eolens 9 and N. langsdorffii $ - bears little
resemblance"' to the latter species. Here, then, the maternal
characters predominate, and the hybrid plant?, are ' extremely
similar ' to A^. sua7>eole)is, and are only to be distinguished from
it by a partial separation of the stamens from the corolla-tube, a
slight difference in the colour and size of the flowers, the violet
or bluish coloration of the anthers, and by complete sterility/

On p. 474 of his book, Focke gives further instances. 'In
many cases the resemblance of the hybrid to one of the ancestral
forms is so close, that it might easily be taken for a minor
variety of this form.' Thus in the following instances the hybrid
resembles much more closely the parent form mentioned after
each : — Dimithus armeria x deltouies, D. deltoides ; Diant/ucs
ca?yop/iylli(s x chinensis^ D. caryophylliis ; Melaiidryuni rubruni
X 7ioctiflorunu M. rubniin ; Verbascum blattaria x nigniui, V.
7iigru)ii ; Digitalis purpurea x I idea, D. lutea.

These cases seem to me to be of special importance on account
of the conclusions which can be drawn from them respecting
perfectly similar cases of individual transmission in Man. This
^ pseudo-monogonic transmission,*' as I shall call it, must be
explained in terms of the idioplasm somewhat as follows.

The predominance of one parent — e.g., the mother — might
be due to the presence of a larger iiuiiiber of idants and ids of the
species in question. If, for instance. Digitalis lutea possessed
thirty-two idants and D. purpurea sixteen only, the idants in
both cases consisting of the same number of ids, and if the
controlling force of the ids were the same in both species, the
ids of D. lutea would then predominate over the others in
every cell during the entire ontogeny ; that is to say, the char-
acter of D. lutea would be impressed more strongly than that
oi D. purpurea on the cell. The objection might be raised that
the cell which is thus produced must possess an intermediate
character, and cannot be a pure cell of D. lutea, even though it
resembles the latter most closely. As yet, however, we are
unable to determine to what extent intermediate forms of indi-
vidual cells may occur in individual cases ; and the expression
' resultant," in connection with the unknown forces of the

* Focke, loc. cit. p. 289.

EFFECTS OF AMPHIMIXIS ON ONTOGENY 269

biophors, does not indicate anything beyond a mere abstract
idea, whicli is in any case totally insufficient as an explanation
of the phenomena. We must be satisfied with the statement
that, when the co7itrolling forces of the two detenninants acting
together in the cell are very unequal, the effect of the weaker
will be extremely small under certain circumstances. A
^struggle between the biophors'' takes place, in which we may
suppose that the stronger assimilates, grows, and multiplies
more quickly, and thus deprives the weaker of room and nourish-
ment, prevents its multiplication, entirely destroys it, and even
makes use of it as nourishment. Without a very considerable
multiplication, the crowd of biophors which migrate from the
nucleus into the cell-body cannot, indeed, exert any determining
influence on the latter. It therefore seems to me to be conceiv-
able that an apparent pseudo-monogonic transmission — i.e.,
a complete suppression of the elements of one parent — may take
place even when exactly the same number of ids are derived
from both parents.

This is still more likely to be the case if the number of ids
derived from one parent is greater than that from the other.
We know that the number of idants may vary considerably,
even in allied species, and it is therefore not improbable that
'pseudo-monogonic' heredity is sometimes due to this fact. In
many plant-hybrids this assumption can be tested directly by
ascertaining the number of idants present.

The third of the above-mentioned kinds of combination of
parental characters seems to me to be theoretically almost the
most important of all, for it is most intimately concerned with
the ultimate processes which take place in the idioplasm. /;/
this case the parts of the hybrid plant most nearly resemble those
either of the paternal or the maternal form.

Such cases are apparently not often very pronounced, but
fluctuations from the paternal to the maternal side occur to a
slight extent in almost all those hybrids which are usuallv
described as intermediate forms. The hybrid between A'.
paniculata $ and A', rustica 9 , which Kollreutter considered to be
a pure intermediate form, bears, as mentioned above, a somewhat
closer resemblance to the former species as regards the length,
and to the latter as regards the diameter, of the corolla-tube.
Such slight fluctuations to one side or the other from the pure
intermediate form seem to be of frequent occurrence. Cases

270 THE GERM-PLASM

are, however, also known in which these fluctuations are so
considerable as to become conspicuous. Sometimes, for in-
stance, the hybrid resembles one species as regards its leaves,
and the other with regard to its blossoms. Brandza* has
recently e.xamined the general, as well as the microscopic
structure of certain hybrids, in order to ascertain the parental
characteristics exhibited by them, and has succeeded in proving
that a fluctuation of this kind exists. Mar7'iibi7U}t vaillantii, a
cross between Leomirjis cardiaca and Marrubiittii vulgare, for
instance, exhibited the winged leaf-stalk characteristic of M.
I'ula^are, while the arrangement of the vascular bundles of the
petiole resenTl)led that existing in Leotiurus. The upper surface
of tlie petiole bore branched hairs like those of Leonunis^ while
on the lower surface the hairs resembled those of Marj'ubiuin.

I shall speak of this fluctuation in the resemblance of the
organs to those of the parents as ' the shifting of the hereditary
residtants in ontogeny.''

We might imagine, a priori, that such a shifting cannot
possibly occur. If, as has been proved, transmission is virtually
completed at the time of fertilisation, and if therefore the
relative proportion of the two parental idioplasms is also fixed
for all the subseqneiit ontogenetic stages when they have once
been combined, we might expect a similar combination of the
characters of the parents to appear in all parts of the young
plant, so that these would either be exactly intermediate
between those of the two parents, or else that the paternal or
maternal characters would everywhere predominate in a similar
manner. The fact that this may not be, and in fact is usually
not the case, may be accounted for in several ways.

In the first place, we must bear in mind the statement made
above, that a combination of two characters in equal proportions
need not always give rise to one which is apparently inter-
mediate, and that it is impossible to give an exact definition of
what is meant bv such a character, because we onlv observe the
final eff"ect of forces acting within the cell, and not the actual
processes by w'hich this eff"ect is produced. This, however,
would not be a true shifting of the hereditary resultants, but
only an apparent one.

Genuine cases of such a shifting, however, undoubtedly occur;

* Brandza, ' Compt. rend.' 1890, T. Ill, p. 317.

EFFECTS OF AMPHIMIXIS OX ON"rO(;ENV 271

and I believe that they are principally due to the fact that the
number of hoinodynanwiis determinants in the idioplasm of a
cell may vary in the course of ontogeny, and that, in fact, it must
always do so. In one stage or in one organ the paternal, and
in another the maternal ids will contain the majority of homo-
dynamous determinants, and the fluctuations in the predominance
of the maternal or paternal characters, which is definitely deter-
mined in advance, must depend on this fact.

To make this clear. I must intrench somewhat upon the
chapter on Variation.

b. Intercalary Remarks 071 Variation

Hitherto we have assumed that the germ-plasm of a species is
composed of ids, each of which contains all the characters of
the species. A brief consideration, however, shows that this
cannot be the case. Not only is it conceivable, but it is even
necessary to assume that the development of the characters of
the species, as well as those of the individual, is only the
expression of the forces to which these processes are due, the
ids in which the forces are situated being by no means perfectly
similar to one another. A great majority of the ids certainly
contain all the determinants of the species, — that is, they are
capable of giving rise to all the specific characters ; but in a
minority of the determinants, to which, indeed, the origin and
development of this species w-as due. will not yet have begun to
undergo a phyletic transformation. All the determinants in this
minority need not necessarily be similar to one another ; one
id, for instance, may contain unmodified determinants of the
ancestral species ; while another, although still retaining some of
the old determinants, may exhibit a greater resemblance to the
pure ids of the existing species, and so on. Such a gradual
transformation of the ids affecting the majority of their deter-
minants must constitute, indeed, the process of the formation of
the species ; and it will be quite in accordance with the principle
of variation to assume that modifications must take place in
the invisible vital units of a lower order, viz., the ids and
biophors, in exactly the same way as they occur in different
degrees and directions in different individuals in the visible
vital units, — the unicellular organisms and persons of stocks.

Thus the proi'isional hypothesis put forward aboz'e, that the
germ-plasm of a species — as far as it concerns the specific char-

272 THE GERM-PLASM

aders — consists of a number of identical ids, cannot be a strictly
correct one : the germ-plasm imist^ on the contrary^ contain a
majority of completely modi/ied ids provided with new deter-
mina)its of the species, and of a ininority of ids of the aftcestral
species which are only slightly or not at all juodified. The
number of the latter will diminish in the course of time in
consequence of selection of the individuals, and the new specific
characters will gradually lose their original variability. Owing
to natural selection, the germ-plasm will gradually be relieved of
those of its ids which have become only slightly or not at all
modified in the new direction, for those individuals in which it
still contains a large number of unmodified ids are as well
adapted as the others for the conditions of existence. As these
individuals are gradually eliminated in the struggle for existence,
the number of unmodified ids in the subsequent generations
must gradually become reduced; ?iX\(}i. this process of selection in
the germ-plasm will only reach a limit when the number of
unmodifed or incoinpietely niodificd ids has become so small that
their influence on the development of the essential characters of
the species is inappreciable.

This process of transformation of the ids of the germ-plasm
will, however, reach a limit, as do all processes of selection, when
its continuance is no longer of any advantage. All adaptations
in an animal remain stationary and are not further perfected,
directly further improvement becomes useless ; and in the
same way the process of modification and elimination of the
ids, which forms the basis of other adaptations, will cease as soon
as the completely modified ids are present in such a majority
that the others can only exert an inappreciable influence on the
nature of the offspring. The useful and adaptable characters
will always undergo complete development in the normal process
of reproduction, — that is to say, in the intermingling of
individuals of the species, — even though some unmodified ids
may be present in the germ-plasm. Let us take as an example
the well-known case of the butterfly Kallima parallecta, which
resembles a leaf. The resemblance is very marked, although
not a complete one. The form of a leaf, with its midrib
and veins, — some regions appearing more or less faded, and
some dry or wet, and even of the appearance of a dewdrop, — is
indicated on the folded wings, but only certain of the secondary
veins on the right and left of the middle line are represented.

EFFECTS OF AMPFilMIXIS OX ONTOGKNY 273

This is quite sufficient to deceive the birds which pursue the
butterfly, and a more accurate copy of the markings on a leaf
would not increase the deception ; for it only needs to be effec-
tive at a certain distance, and therefore has not increased in per-
fection, but has reached its limit at this stage. A precisely
similar occurrence must take place as regards the modification
of the ids of the germ-plasm if a new adaptation of the species is
concerned. In such a case, again, only a relative and not an
absolute perfection will be attained. The majority, but not all^
of the ids may become modified by selection, while a minority
must accompany these through long periods and generations in
an unmodified or slightly modified condition.

I have had to make this digression on the transformation of
species in its relation to the germ-plasm, in order that my ex-
planation of ' tJie 07itoge7ietic shifting of the hereditary resultant ''
may be comprehensible. It follows from the process of the
gradual transformation of the ids of germ-plasm and the trans-
ference of unmodified ids from one cell-generation to another,
that the ger77i-plasjn of every species must consist of a cojn-
binatioji of ids of a soinewhat different nature. For although
specific characters must have appeared simultaneously, and
many others must have arisen successively, in the course of
many generations, all the characters of the species will not be
represented by determinants in the same number of ids. The
oldest character, in fact, will be contained in almost all the
ids ; those which are somewhat younger, in a considerable
majority ; and still younger ones, perhaps also in a slight ma-
jority ; while those which have only just begun to be of use
to the species will only be present in a minority of the ids of
each individual.

This circumstance is evidentlv connected with the degree of
variability of characters, which may, in fact, be a very different
one as regards the different characters of a species. Characters
on which selection is only beginning to act, can only be
represented by a majority of ids in a minority of individuals,
and the less variable characters are those which have been
selected for a greater number of generations, and are there-
fore present in a large number of ids of numerous individuals ;
and again, those characters which have long become definitely
fixed in all or almost all the individuals of a species, must be

2 74 THE GERiM-PLASM

also represented in the majority of the ids in ahnost all these
individuals. Conversely, those characters which are beginning to
become useless to the species must be contained only in a gradually
increasing mi?iority of the ids ; the number of the latter must
gradually decrease, until it finally becomes so small that it can
no longer give rise to the character in question in consequence
of an oyerwhelming majority of the other ids.

The above conception of the germ-plasm enables us to un-
derstand why the force of heredity may vary /;/ the course of
ontogeny in the case of crosses bet\veen two species, and why
the paternal tendency may dominate in one character and the
maternal in another. For, quite apart from the possible degree
in which the force of heredity may be present in the individ-
ual determinants, and assuming it to be equal in the two
species, the nu))iber of ids which contain homodynamous
determinants will nevertheless vary according to the age of
the character in question. A greater number of ids containing
homodynamous determinants will indicate a greater force of
heredity. If the form of the flower in a species A was acquired
long ago, while that of the leaf is a new acquisition, and if the
reverse is tRie in the case of a species B, the hybrid which
would be obtained by crossing these two species would resemble
species A in the form of its flowers and species B in that of its
leaves. A larger number of homodynamous determinants of
species A, for the rudiments of the flowers, will be opposed to
a smaller number of homodynamous determinants of the species
B, which, on the other hand, will contain a larger number of
homodynamous determinants for the rudiments of the leaves.
The importance of the whole principle will be made still more
evident in the following section.

c. The Struggle of Individual Characters

The question of individual characters in the two parents has
not been taken into account in our previous investigations : in
crosses between different species they may be looked upon as
insignificant when compared with the specific characters. We
must now consider those cases in which the two parents diff"er in
respect of slight individual characters only, confining ourselves
to the human race, the individual characters of which we can
recognise most clearly.

EFFECTS OF AMPHIMIXIS ON ONTOGENY 275

What strikes us most forcibly in connection with the process
of reproduction and transmission in man, when compared with
that of the formation of hybrids in the vegetable kingdom, is the
dissimilarity between children born of the same parents. In
the case of plant-hybrids, a striking constancy is observable in
the offspring of a cross, and this is true not only of the offspring
of the same parents, but also of all the hybrids produced by
crossing any individuals of the two species in question, if the
latter also are constant.

The dissimilarity between the children of the same parents has
been already mentioned, and was explained as resulting from
the halving of the germ-plasm in the process of ' reducing
division,' which takes place in a different manner each time,
and, w^hen a larger number of idants are present, gives rise to
a surprising number of combinations. As the idants are very
different with regard to the individual primary constituents they
contain, new combinations of the latter are thus continually
being formed without affecting the characters of the species.

Three principal kinds of combination have to be considered
in any attempt to explain the blending of parental characters in
the child, — that is to say, to refer it to processes which take
place in the idioplasm : these are (i) the characters of the child
are intermediate between those of the parents ; (2) the child
exclusively or prijicipally resembles one parent; and (3) the
child resembles the father as regards some characters and the
mother in respect of others.

The first case, if it ever strictly occurs, must be attributed to
the presence of a precisely similar controlling force in all the
determinants.

The number of idants derived from each parent must be the
same in this case, as the parents belong to the same species,
and there will certainly also be very little difference as regards
the number of ids ; the number of determinants, moreover, will
be the same, or almost the same, in the germ-plasm of the two
parental germ-cells. Theoretically, an exactly intermediate form
would therefore result if each determinant of the father and
mother were homologous to one another, and if the homolo-
gous determinants were controlled by precisely similar forces,
— i.e., if they contained the same number of biophors, and the
homologous biophors of either side possessed the power of as-
similation and reproduction to the same extent. These condi-

276 THE GERM-rLASM

tions which we have presupposed will, however, hardly exist all
together, but they may concur approximately with regard to a
certain number of characters.

It can hardly be doubted that the second kind of combination
occurs, and that in some cases the offspring take after one or
other of the parents only, — not only in respect of those charac-
teristics with regard to which the term * resemblance ' is generally
ajjplied, such as those which concern the form and expression
of the face, but equally as regards stature, form of the body,
proportions of the limbs, nature of the skin and hair, character,
and temperament.

We are met with a two-fold difficulty in attempting to explain
these facts : in the first place, how is it possible for all the pri-
mary constituents of one parent — e.g., the father — to be pres-
ent in one of the germ-cells of this parent, as the germ-plasm
was halved by means of the reducing division before these germ-
cells become ripe? — and secondly, how can it happen that the
maternal germ-plasm exerts no influence on the formation of
the child?

Let us first consider the former of these two difficulties : how
is it possible for all the characters of the father to be con-
tained in one paternal germ-cell in spite of the reducing divis-
ion? If the latter process resulted in a quantitative halving
of the germ-plasm, no further explanation would be necessary,
for the quality of half the mass might be exactly similar to that
of the whole. But a reduction of the units of the germ-plasm
to half their number occurs in this process ; the number of ids
is reduced by one half, and the structure of the offspring results
from the combination in the germ-plasm of the ids of both par-
ents, as was shown in the case of plant-hybrids ; it is difficult,
therefore, to understand why half the number of ids can never-
theless give rise to all the characters of the parent. Strictly
speaking, it is immaterial whether we concern ourselves with all
the characters, or with only a single one ; for many characters, in
fact, depend on the co-operation of all the ids of the ontogenetic
stages in question.

There is only one way out of this difficulty, — we must accept
the assumption, which has been confirmed by fact, that the
controlling power of the ids of one of the paretits may become
)uillified at every ontogenetic stage. Observations on plant-
hybrids are invaluable in this connection, for in them we know

EFFECTS OF AMPHIMIXIS ON ONTOGENY 277

for certain that the ids of another species are present, even
although they may not produce a perceptible effect. We may
conclude from the phenomena of ' pseudo-monogonic ' heredit}'
exhibited by these hybrids, that the ids of one parent may, as
has already been explained, prevent those of the other from
taking part in the control of the cell, group of cells, or part of
the body.

We may therefore assume that if by means of the reducing
division, all the idants luJiicJi controlled or dominated the
ontogeny of the motlier^ for instance, reach one of the germ-cells
produced by this parent, this germ-cell will be capable under
certain circumstances of reproducing the maternal 'type'* in
the child. But in order that this may happen, it is necessary for
this cell to unite with a sperm-cell, the germ-plasm of which
possesses on the whole a weaker controlling power than its own,
so that the germ-plasm of the father is dominated by that of
the mother.

As in the case of crosses between species, the controlling
power of the idioplasm will not always be dependent on the
same cause.

We cannot enter more deeply into those cases in which a more
marked force of heredity of individual characters occurs. Darwin
mentions, for instance, that the ' white ' colour, in flowers as well
as in animals, is very commonly transmitted to the offspring
when white individuals are crossed with dark-coloured ones, the
majority of the descendants inheriting the white colour. We
can in this case only assume that those biophors which domi-
nate the cell, and give rise to the white colour, must be
'stronger' than those which cause the formation of pigment,
and this ' strength ' must, in fact, be attributed to the possession
of a greater power of assimilation.

The case is diff'erent in many other instances in which the
greater hereditary power is attributable to quantitative differ-
ences in the constitution of the paternal and maternal groups of
idants.

The number of ids contained in each idant is certainly
constant, or nearly so, in all individuals of the human spe-

* I shall make use of the term ' type ' (' Bild '), to express the whole aggre-
gate of essentia] characteristics which together constitute the individuality
of a human being.

278 THE CJERM-PLASM

cies ; * but a predominance of paternal or maternal ids may
nevertheless exist, so that there may be a greater 7iuniber of
Jio))iod\')iai)ioHs determinants in one line of ancestors than in the
other, as has already been shown to be the case in hybrids.

By the term homologoits determinants, we understand those
elements of the idioplasm which are capable of so controlling
homologous parts of the body or determinates ; Jionwdynanious
determinants, on the other hand, are those of the homologous
determinants which have the special function of impressing a
like character on any part of the body, and which, taking an
example we have already made use of, serve to produce a
particular form and colour in a certain region of a butterfly's
wing. That parent in which the ids contain numerous homo-
dynamous determinants, or at any rate a larger number than
are contained in the ids of the other parent, must undoubtedly
exert the greater controlling power. The power of homo-
dynamous determinants is simply cumulative, whereas dissimilar
or heterodynamous determinants may, in the most favourable
cases, co-operate to form a single resultant, but may, under
certain circumstances, counteract or even neutralise one another.
The larger the number of homodynamous determinants which
the entire aggregate of ids of a parent contains at any ontogenetic
stage, the greater will be the likelihood that these will pre-
dominate in the struggle of the parts which takes place in tlie
cell, and will therefore stamp the latter more or less distinctly.

The colour of the eyes may be taken as an example. Let us
take a case in which those of the mother are blue, and those of
the father brown. The number of ids in the idioplasm of the
pigment cells of the iris will be the same in both parents, but
in the case of the father nine-tenths of them, let us say. are com-
posed of ' brown ' t determinants, and only one-tenth of deter-

* It is conceivable that individual fluctuations in the number of ids may
occur, although the number of idants always, indeed, remains the same — if
one can judge by its constancy in many animals and plants. Unfortunately
we do not yet know the number of idants in the case of man ; I have, at
least, been unable to obtain any information on this point.

t ' Brown ' and ' blue ' determinants are spoken of in this and subsequent
passages simply for the sake of brevity. I am not ignorant of the fact that
the blue colour of the iris is not due to blue pigment. The above terms
merely indicate that the determinant produces a structure in the iris which
causes it to appear blue or brown, as the case may be, quite apart from the
histological details on which this depends.

EFFECTS OF AMPHIMIXIS 0\ ONTOGENY 279

minants of other colours ; two-thirds of the ids of the mother
on the other hand consist of bhie, and one-third of brown deter-
minants. The iris of the child will therefore most probably
be brown, for in its formation nine-tenths of the determinants
of the father will co-operate with one-third of the determinants
of the mother, which are homodynamous with these. The pre-
dominance of the brown determinants would also, however, still
be assured if the maternal ids contain no determinants at all of
this colour, but only red or green ones, — supposing that red or
green pigment cells occurred in the human iris. For in this case
nine-tenths of the paternal determinants would be opposed to
various small groups of heterodynamous determinants of the
mother. The latter might possibly modify the brown colour
which would be produced by groups of paternal determinants
alone^ for they also control part of the cell-body ; but it is quite
as conceivable that they might be completely overcome by the
paternal determinants, and thus excluded from the control of
the cell. We cannot, as already stated, at present judge as to
the result of the struggle of the parts in individual cases, but
there is no doubt that, provided the controlling power of the
determinants is similar, the ]iu7}iber of the latter is of the first
importance.

Since, therefore, the nature of the combination in the orerm-
plasm is diflferent in every individual in consequence of sexual
reproduction, the number of homodynamous determinants of
any particular characteristic must also be different in every case.
It will be shown in the chapter on variation that processes of
selection may even bring about an increase or decrease in the
number of the homodynamous ids of individual characters,
although these are never of sufficient biological importance to
give rise to specific characters.

The same competition of forces must take place in the case of
individual as in that of specific characters. A child may inherit
the colour of the eyes from its father, and the shape of the
mouth from its mother, just as in a plant-hybrid the form of the
leaf may resemble that in the paternal, and the flower that in
the maternal plant. In both instances the character is inherited
from that parent in which the group of idants contains a prc-
ponderatntg Diajority of Jioniodyiiamous deter uiinants of this
character. When this is the case, this majority preponderates
over the scattered minority derived from the other parent.

28o THE GERM-PI.ASM

In neither instance, however, does the preponderance of the
father, as regards a siugle character, necessitate that all the
other characteristics will be controlled to the same degree
by this parent. The germ-plasm consists of an equal number
of paternal and maternal idants and ids, which remains
constant throughout ontogeny. We suppose that every id of
the germ-pJasm contains all the determinants of the species, e.g.,
the determinant a for the character A, the determinant b for the
character B, and so on. But all ids of the germ-plasm need
not contain a number of homodynamous determinants only ; for
if id i, for instance, contains the determinant a'^, id ii may per-
haps contain the determinant a'^ for the homologous character,
and id iii the determinant a^, id iv the determinant rt^ and so
on. There is no reason why id i, in which the determinant a^
represents the character A, should not contain the determinant
^2 instead of ^ for the character homologous to B. If we denote
the determinants of corresponding characters — i.e., those which
may become substituted for one another — by the same letters, a
certain id of the germ-plasm — e.g., id i — might contain the de-
terminants rt\ b'^, c^, d% e^,/^,g^, h^, i^, &c. If in the germ-plasm
of the father id i, id ii, id iii, id iv, and so on to the last, which
I will call id xx, all contain the determinant a^ for the character
A, and none of the variants a-, a^, &c., are present in them,
this determinant a> will be more powerful than any other
variant of a derived from the mother, which may exist in the
idioplasm of the cell in question, provided that the total number
of each of these variants is less than twenty. Hence the char-
acter A^ will be impressed on that particular cell, or group of
cells, and not A~ or A^. The case may be entirely different as
regards the character B ; the determinant b^ or b^, for instance,
may be the dominant one in most of the ids and idants : and in
this case, one of the other variants of i5, such as B^ or B^, will
be produced.

If we may compare the groups of paternal and maternal
idants to two centres of force each of which attempts to obtain
control over the cell, each of these two forces will be determined
by the individual forces of the idants in which they are con-
tained, while the force in each idant will be controlled by the
individual forces of the ids of which it is composed. If, for
instance, there are two idants derived from each parent, each
consisting of ten ids, the following considerations would enable

EFFECTS OF AMPHIMIXIS ON ONTOGENY 28 1

US to ascertain which variant of the character must actually be
produced. The paternal idants P^ and P'^ might each consist
of ten ids, the same determinant a^ being contained in six ids in
/*!, and in eight ids in P'^. These two idants would then com-
bine in the attempt to give rise to the character A^^ with a con-
trolling force of 6 + 8 = 14 ids. Each of the maternal idants M^
and AP might also contain ten ids, M^ being composed of two
determinants a}, four a^^ three a"'^ and one a}'^ ; this idant will
therefore tend to produce the character A^ with a power of four
ids only. If now, the other maternal idant M'^, with all its con-
tained ids, tended to give rise to the character A^^ with a power,
that is, of ten ids, the group of paternal idants would neverthe-
less predominate over that derived from the mother, as fourteen
paternal are opposed to ten maternal homodynamous deter-
minants. In this case both parental groups might possibly
control the character of the child together, but the paternal
group would be the stronger of the two. If, however, eighteen
homodynamous ids of the father were opposed to four homo-
dynamous ids of the mother, the influence of the latter would be
entirely suppressed as regards the character A. We must at
any rate conclude from the facts of the case, that the characters
of one parent may be strictly inherited without any apparent
intermingling of the corresponding characters of the other
parent. As already mentioned, this very point in the theory
seems to me to be the most reliable one, and known facts
concerning plant-hybrids compel us to accept this assumption.
The controlling force of the groups of paternal or maternal
idants may be entirely different in respect of individual char-
acters and groups of characters, according to the number of
homodynamous determinants by which these are respectively
represented. Moreover, this depends not only on the fact as
to whether the individual character is derived from the father
or the mother, or is a mixed one, but also on the entire number
of homodynamous determinants present from each parent.

Although I have all along spoken of idants, I do not wish
to imply that every idant behaves as an indepoidoit whole.
Neither do I suppose that the resultant of the forces of the
whole aggregate of paternal, is opposed to that of the maternal
ids. It is quite conceivable that the same homodynamous
determinants occur in the ids of both parents, and that their
forces are cumulative, just as they would be if present in one or

282 THE GERM-PLASM

the other parent onl\-. Homodynamous ids ;nust^ indeed, fre-
quently occur in both parents in cases of close interbreeding, as
well as in species which exist in comparatively small numbers
in small isolated districts.

It is obvious that this stmggle of the parental ids takes place
at every stage in ontogeny, and that its result is different
according to the state in which the forces exist at this stage.
This accounts for the frequent changes as regards the resem-
blance to one or other of the parents, and for the combination
of parental characters which occurs in different parts of the
body.

The facts, so far as they are known to me, apparently agree
very satisfactorily with* the above explanation. I have attempted
to collect fresh evidence bearing upon this question, which till
now has not been closely kept in view. This, however, has
unfortunately not been so easy a matter as might have been
expected, but I will mention some of the facts relating to this
point.

In some cases a child resembles its father in most respects,
and takes after its mother as regards a few minor parts. This
would be inexplicable if the whole part were not controlled by
the resultant of a determinant other than that for the individual
portions of this part. The single determinant and its immediate
successors, which control the primary cell of the whole part,
determine, in the first place, the rate of the cell-division and the
primary form of the entire organ ; but in each subsequent stage,
one of the succeeding determinants takes on a controlling
function, and as its influence is always the resultant of the
homologous determinants of all the ids of the cell, the successors
of this cell may at any time differ from one another with regard
to their resemblance to those of the father or mother.

A person of my acquaintance resembles his father very closely
in respect of that portion of the skin which is derived from the
external germinal layer. In both the father and son the epi-
dermis is thick and inclined to be horny. The nails of the
hands are much thickened, and the skin on the soles of the feet
is especially hard. In this case, therefore, the force-resultant of
the paternal determinants must continually have been stronger
than that of the maternal ones from the primary ectoderm
onwards, through a long series of cells. On the other hand,
the resultant of the maternal determinants seems to have pre-

EFFECTS OF AMPHIMIXIS ON ONTOGENY 283

dominated at the formation of the brain, which Hkewise arises
from the external germinal layer, for the person in question
resembles his mother as regards most of the mental qualities,
such as intelligence, talents, and strength of will. This can
only be accounted for on the supposition that the determinants
of the subsequent descendants of the primary ectoderm-cell were
unable to exert any influence upon this cell itself; in it they
were still latent, and in this condition were merely passed on to
succeeding cells. If the ids of subsequent generations of these
primary ectoderm-cells which formed the rudiments of the brain,
contained more maternal than paternal homodynamous deter-
minants, a resemblance to the mother instead of to the father
would arise at this point.

In the above case, the entire brain does not seem to have
taken after that of the mother, for marked paternal traits also
exist in it. According to our theory, it may well be conceived
that even such a repeated alternation of hereditary tendencies as
has occurred in this instance is predetermined in the germ, for
the power of the paternal, and naturally also of the maternal ids,
varies throughout ontogeny at every further stage in the division
of the ids, and the relation between the controlling forces of the
father and mother may be transposed. In general we might
even expect that one or other group would predominate in most
cases, and that the child is consequently composed of a com-
bination of parental characters which varies in different parts.
The parts or organs which resemble those of the same parent
may also vary very considerably as regards size : this is
possibly true of a single cell, as well as of a whole organ or an
entire germinal layer, or even of the entire organism.

This theoretical deduction is in general supported by facts,
for a child is rarely or never an exact repetition of either parent.
It is by no means easy, indeed, to form a correct estimate with
regard to the resemblance between parent and child, for in order
to do so, an exact knowledge of both at the same ages would be
necessary, and a detailed comparison is only possible between
father and son, or mother and daughter. It would therefore
be essential to compare photographs of the father and son, at
the same ages ; and this, as far as my knowledge of the observa-
tions which have been made on heredity extend, has never yet
been done. It would, moreover, be necessary to photograph
the whole bodv, and not merelv the face.

284 THE GERM-PLASM

As far as we can at present gather from the facts, even
in those cases in which the resemblance is a close one,
the child always differs from the parent either as regards
individual characters, or, as is more frequently the case,
in undefinable slightly characteristic details, such as the length
of the limbs, colour of the hair or eyes, or the quantity of hair.
These parts cannot be said to resemble those of the other
parent, but they give the impression that the main direction
in which heredity has tended has been slightly changed in
an undefinable way. A daughter may resemble her mother,
for instance, so closely that she is universally said to be an
exact image of her mother ; and yet a close comparison will show
that the likeness is by no means an exact one, and although the
child may not display a single paternal character, there are
nevertheless a number of parts which respectively differ from
those of the mother. In the case of identical twins, there can
be no doubt that manv of the minor differences existinof
between them are due to differences in the germ-plasm, and not
to the diversity of external influences. The germ-plasm of both
parents, that is to say, has taken part in determining the differ-
ent parts of the child, although in the case of one parent this
determination is slight and little marked, and has caused a
slight deviation from the maternal characters rather than the
development of specifically paternal ones.

If this view is correct, and the germ-plasm of one of the
parents alone never determines the formation of the child, it
becomes more obvious than ever that even when heredity tends
to follow in the direction of one of the parents in the greatest
possible degree, the mother and daughter can never resemble
one another so closelv as do identical twins. Owingr to the slight
influence of the germ-plasm of the father, the type of the
daughter deviates somewhat from that which would have been
produced from the maternal germ-plasm alone ; and similarly,
if the mother owes her nature to the predominance of the germ-
plasm of one parent, a slight deviation must have occurred
owing to the weaker influence of the other parent. But the
whole of the germ-plasm of do//i grandparents cannot possibly
have been contained in the maternal germ-cell from which the
daughter arose, for the reducing division causes the removal of
half the germ-plasm from the egg-cell before fertilisation takes
place. Even if the idants which materially determine the type

EFFECTS OF AMPHIMIXIS ON ONTOGENY 285

of the mother remained in the ovum from which the daughter
was developed, the group of idants of the other grandparent,
which would modify this type somewhat, must necessarily be
absent ; the types of the mother and daughter consequently
cannot exactly correspond, for the double reason that the influ-
ence of one grandparent was wanting at the development of the
daughter, while that of the father w^as present in addition.

The following examples may serve to show i)i how viany
different ways tJie hereditary te/idencies of the parents may, in
accordance with our theory, be interchanged in the course of
ontogeny. In the bilaterally symmetrical human being, all those
parts which are not situated in the median line are paired, and
the corresponding organs generally behave nearly, if not quite,
similarly as regards heredity. If one hand bears a decided re-
semblance to that of the mother, the same will, as a rule, be
true of the other also ; and if the left leg is intermediate be-
tw^een the character of both parents, the right one will also be
so in exactly the same degree. Even such a subtle characteris-
tic as the colour of the eyes usually corresponds in both organs ;
and even in those cases in which it is intermediate between that
of the two parents, the colour only varies slightly in shade in
the two eyes. One might be disposed to conclude from this
fact that paired organs are represented by a single primary con-
stituent in the germ-plasm. This, however, would be an erro-
neous conclusion ; for, apart from the facts already mentioned
which contradict such a view, there are exceptions to the rule
thdt paired organs are similar to one another. One brown and
one blue eye sometimes occur in dogs, especially in boar-hounds,
and I know of a similar instance in the human subject : the
father, a brewer in a small Suabian town, has blue eyes, and the
mother brown ones, while a daughter of twelve years of age has
one blue and one brown eye.

In addition to these cases, the frequent inherita)ice of birtJi-
7narks and other minor characters on one side of the body only,
necessitates the assumption of double determinants for the cor-
responding parts of each half of the body. We must therefore
imagine that each id of the germ-plasm of bilateral organisms is
primarily bilateral, and that all the determinants, indeed, of the
whole body are double, even of course including those for the
organs which are apparently situated in the median plane, but
which actually consist of corresponding halves. In a large num-

286 THE GERiM-PLASM

ber of animals the two first segmentation-cells of the egg, or
blastomeres, correspond to the future right and left halves of the
body. The first division of the nucleus of the fertilised ovum
must therefore separate the determinants for the right and left
halves of the body ; and the occurrence of this process is ren-
dered all the more probable from the fact that a longitudinal
division of the idants actually occurs, and results in each of the
spherical ids being halved.

The explanation of the striking correspondence of the homolo-
gous parts of aiitimerous or paired organs must be referred to
the history of the transformation of species. The permanence
of this resemblance during the continued crossing of individuals,
is naturally due to this approximate similarity of the correspond-
ing determinants for the right and left sides in both parents.
It is evident, according to our conception of the structure of the
idioplasm, that the resemblance of the corresponding parts of
the child originates in the antimerous determinants of both par-
ents, for the latter determine the comparative number of homo-
dynamous ids on the paternal and maternal sides, together with
the relative ' controlling power ' of the idioplasm of the father
and mother in the organ in question. And since this must
be the same on both the right and left sides, the organ itself
must display the same combination of paternal and maternal
characters on either side : that is to say, its two sides must be
alike.

I do not think that these facts can be understood by the aid
of any other theoretical assumption with regard to the structure
of the hereditary substance. The assumption of the existence of
pangenes, for example, might certainly explain the circumstance
that a combination of the paternal and maternal characteristics
does indeed occur in the organ in question, — in the external
ear, for instance, — but it cannot account for the fact that this
combination is similar in the right and left ears.

These very facts seem to m.e to furnish a further welcome
proof of the correctness of the view arrived at by other methods,
that the hereditary combination in each part is predetermined
from the germ onwards. The right and left ears could not
possibly resemble each other, if the relative strength of the
hereditary tendencies on both sides were not predetermined for
all parts of the child by the nature of the paternal and maternal
idants.

EFFECT'S OF AMPHIMIXIS ON ONTOGENY 287

There are, however, exceptions to this rule. As already
stated, the homologous parts of the two antimeres may differ
from one another ; and such a difference is even frequent in
certain animals, though it only affects characters which are of
minor biological importance. Little attention has hitherto been
paid to the fact that many of our domestic animals have lost the
original sy)}U)ietry of the marking of their coats. Piebald cats,
dogs, horses, cows, and guinea-pigs are not uncommon, and
show that the symmetry of the markings may become com-
pletely lost by domestication. This must be owing to the fact
that these originally symmetrical patches of colour are, in conse-
quence of domestication, no longer of biological importance. If
the determinants for these characters varied in different ways on
the right and left sides, and the animal in question thus became
spotted, no disadvantage would thereby result, and the animal
would nevertheless be able to exist and produce offspring. If
two individuals with different piebald markings then paired, the
asymmetry in the coloration and markings would be increased ;
and as a matter of fact in many of our races of cattle no parts
of any two animals are alike in this respect, and the same is
true with regard to many dogs and guinea-pigs. We know how
important these markings and coloured patterns may be for the
preservation of individuals and species /;/ the natural condition^
and are therefore justified in attributing the retention of the
symmetry to natural selection, and its loss to panmixia.

Certain facts with regard to the metameres, or successive parts
of which the body of a segmented animal is composed, show
that the maternal or paternal characters may preponderate in
different segments. This naturally cannot be proved in the
case of Man, as the metameric segmentation only affects the
bones, muscles, and nerves, which are not externally visible.
But I think I have observed that consecutive parts, even when
they are homologous, may occasionally exhibit different heredi-
tary types. A child may closely resemble its mother as regards
the arms and hands, and nevertheless may take after its father
in respect of the legs and feet. I have endeavoured to ascer-
tain whether any definite rules are followed with regard to
certain organs which are closely related to one another in
ontogeny, according to which these organs would exhibit
a similar combination of parental characters ; but the only
rule I could discover is that which relates to the symmetry

2 88 THE GERM-PLAbM

of the two halves of the body. Apparently all possible combina-
tions may actually occur. The form of the skull may resemble
that of the father, and the face that of the mother ; or the form
of the entire head and face may be like the mother's, while the
eyes may be similar to the father's in every detail. The son
may, like his father, possess a dimple on the chin, although he
takes much more closely after his mother as regards the shape of
the face and nose. That the combination of parental character-
istics may even extend into far greater details, is shown espe-
cially by the remarkable amalgamation of the mental qualities
of the parents w^hich often occurs. The intellect and practical
talent may be inherited from the mother, and strength of will
and unselfishness from the father ; and all these qualities may
be contained in one skull, the form of which essentially re-
sembles that oi one of the parents only. These combinations of
mental characteristics of the parents cannot, however, always be
definitely analysed, owing in the first instance to the fact that
they are not always sharply contrasted in the parents, but more
frequently are only different in degree. We may, however, at
any rate consider it certain that the brain rarely resembles thai
of one parent only in all its parts and as regards the most mi-
7iute details of its strnctnre ; it usually, on the contrary, exhibits
a combination or alternation between that of the two parents,
and this combination is of the most varied kind.

In connection with this statement it may be mentioned that
no part of the human body is so important as the brain in the
stRiggle for existence, and its importance extends even to the
minutest details ; its parts must therefore be subjected to in-
cessant processes of selection. In other words, the number of
homodynamous determinants in the various parts of the brain
must be extremely different in the individual, and must vary
extremely in different individuals.

These statements with regard to the stmggle of the individual
characters may perhaps be objected to on the ground that they
contradict the suppositions with which we started. It may be
contended that an alternation of paternal and maternal hereditary
parts is rendered possible on the basis of my theory, because
the transmission of a paternal character implies that the whole
dominant grouj) of idants of the father passed into the germ-
plasm of the child at the reducing division, and the transmission
of a maternal character necessitates the presence of the whole

EFFECTS OF AMPHIMIXIS ON ONTOGENY 289

of the dominant group of the mother. It might be considered
improbable that both of these groups should come together in
one germ-cell, or that this can occur as often as must be the
case in reality, considering the frequency with which a combina-
tion of the two parental characters exists.

In reply to this it may be stated that the meeting of two
germ-cells in the process of fertilisation, one of which contains
the dominant group of idants of the mother and the other that
of the father, must take place from time to time, for every pos-
sible combination will occur at one time or another. It must,
moreover, not be forgotten that it is extremely difficult to dis-
tinguish between the pure individual characters of a parent and
those of one of the immediate ancestors of this parent ; but the
production of these characters in the latter does not necessitate
the presence of the whole of the dominant group of parental
idants, for a portion of them will undoubtedly suffice if the
character in question is represented by a majority of homody-
namous determinants. The characters which alternate are very
often not the specifically paternal or maternal ones, but those
which in general characterise the mother's or the father's
family. These, however, must be represented in most of the
ids of the dominant group of idants. and may therefore become
apparent even when the reducing division only causes a certain
number of idants, instead of the entire dominant parental group,
to pass into the germ-cell of the child.

On the other hand, the frequency with which a child bears a
closer resemblance either to its father or mother — and this in
many families is actually the rule — can also be easily explained
by our theory. In such cases, not only can the whole number
of dominant idants produce the type of the parent in question
with approximate accuracy, but even a majority of them will be
sufficient to do so, provided that a large number of ids with ho-
modynamous determinants are contained in them. Many germ-
cells will, therefore, contain a sufficient number of the dominant
idants of the parent, while others, although perhaps containing
an equally large number of these idants, will consist ot various
combinations of ids enclosing relatively few homodynamous de-
terminants. If, then, an idant of the former kind, derived from
the father, should, in the process of amphimixis, meet with one
of the latter kind, derived from the mother, the type of the father
would predominate in the child, and vice versa. If two germ-

290 THE GERM- PLASM

cells of the first kind, derived from the father and mother respec-
tively, came together, a mixture of the characters of the two
parents would result ; and if two of tlie latter kind came into
connection, none of the special characteristics either of the father
or mother, would be recognisable in the child, which would only
possess such characters as are common to the two families.

4. The Force of Heredity

We have now seen that ' pseudo-monogonic ' heredity is to be
explained in terms of the idioplasm as follows : — the dominating
group of idants from one parent — the mother, let us say —
passes over completely into the germ-cell of the offspring, and
there meets with a weaker group of idants from the father in
the process of amphimixis. Although not all, but only a large
number of the determinants in the maternal group of idants
predominate, a marked resemblance between the mother and
child will result.

The fact that such cases as this occur at all, was taken above
as a proof that the combination of ids in the idants persists
during ontogeny, — i.e.^ from germ-cell to germ-cell, — and that
the idants often, or even generally, remain unchanged in the
reducing division. It naturally does not follow, however, that
precisely that combination of idants which predominated in the
ontogeny of the parent must remain unaltered in the germ-cell
of the offspring : it may do so, and such cases will frequently
occur amongst the thousands of ova or still more numerous
sperm-cells which are produced by a single individual in the
course of its life.

In some families it certainly appears as if the perfect type
(' Habitus ') of an ancestor had been transmitted with great con-
stancy to the children through a great number of generations,
and we must therefore assume that the dominant group of
idants in the ancestor reappears very frequently in the germ-
cells of the offspring. Thus the high forehead, widely-separated
eyes, and small mouth of the imperial family of the Caesars,
the large and peculiarly hooked nose of the Bourbons, and the
projecting lower lip of the Hapsburgs, can all be traced through
several generations. It is, however, difficult to say whether
such similarities are not accidental, or whether our recognition
of them is merely due to an incomplete knowledge of the facts,
only those descendants being taken into consideration in whom

EFFECTS OF AMPHTMIXIS ON ONTOGENY 29 1

these family characters were prominent ; but similar observa-
tions with regard to animals lead to the conclusion that they are
not all due to chance.

The phenomenon which breeders describe as ^individual
Prepoteiicy'' comes under this category. It seems that a marked
tendency occasionally exists in certain individuals to transmit
their special individual characters to the majority of the off-
spring. It has often been observed that individual hoises,
cattle, sheep, and other domestic animals possess this capacity
in a high degree ; and breeders pay enormous prices for such
individuals, which must, it is true, excel not only as regards this
supposed special power of transmission, but also in respect of
certain particular and desirable characters. It is, however,
believed that similar observations have also been made with
regard to plants. Vilmorin,* one of the most eminent raisers
of plants, at any rate distinguished in his experiments between
individuals possessing the capacity of transmitting their own
characters to the oiTspring in a greater and in a lesser degree.
The former he called " botis etalons,' and made use of them alone
for purposes of propagation. He could not, however, find out
by a mere examination of the plant whether it belonged to this
preferred group. This could only be ascertained by examining
the offspring, which therefore served as the guides in the selec-
tion of the plants for purposes of propagation.

Darwin,! Prosper Lucas, J and Settegast§ give many instances
of this kind, one of the best known of which is that of the * otter
sheep.' This race was descended from a ram w^hich was remark-
able for having short, crooked legs and a long body. It trans-
mitted this peculiarity to many of its descendants, and so
enabled the owner to breed a special race of sheep with crooked
legs, the advantage of this peculiarity being that they were
unable to leap over fences. Similarly, English thoroughbred

* Quoted from de Vries, loc. cit. p. 88. — L. Leveque de Vilmorin,
' Notices sur I'amelioration des plantes par le semis,' Nouvelle edition,
1886, p. 44.

t Darwin, 'Animals and Plants under Domestication/ Vol. II., p. 40,
et seq., London, 1888.

X Prosper Lucas, ' Traite philosophique et physiologique de I'heredit^
naturelle dans les etats de sante et de maladie du syst^me nerveux,' Paris,
1850.

\ Settegast, ' Die Thierzucht," Breslau, 1878, p. 197.

292 THE GERM-PLASM

horses owe their superiority to three individuals, — viz., to the
Turkish horse ' B yerley,' and the Arabs ' Barley ^ and ' Godol-
phin ' ; and the celebrated race of Orlow trotters can be traced
back to the stallion " Bars the First/

If these animals really possessed a stronger ' force of heredity '
in the sense indicated, it must not be confounded with the
property oi faithful transmission in a race. This property of
' breeding true ' must be due to the presence of a large majority
of homodynamous determinants in the germ-plasm, or, what
amounts to the same thing, to the existence of similar, i.e.^ of
'racial,' determinants for every character in most of the ids.
The longer a pure race has been kept up, all the individuals
which exhibit variations being carefully eliminated, the greater
will be the number of ids containing ' racial ' determinants, and
the more rarely will variations appear in individuals.

At present, however, we are concerned with individual, and
not with racial characters. These cannot possibly have been
contained in a preponderating majority of the ids of the germ-
plasm from which the individual arose, for the germ-plasm is
composed of paternal and maternal ids. The transmission of
the proper ' type ' can in this case therefore only be due to the
fact that the group of idants which preponderated in the devel-
opment of the parent is once more present in the germ-cell. I
should consequently prefer to account for the so-called pre-
potoicy in transmission by assuming that in some individuals
the reducing division simply occurs in such a manner as to
separate the paternal and maternal groups of idants, while
ordinarily it results in combinations of idants of all kinds.
It is impossible to say at present on what peculiarity of the
idants themselves or of the apparatus for nuclear division,
this must depend ; but it can at any rate be stated that the
dominant group of idants cannot possibly be contained in every
germ-cell of such an individual, even in the most favourable
case. On the contrary, it can only be present in half of them ;
for, according to our assumption, the reducing division always
causes the dominant group of idants to pass into ojie of two germ-
cells only, the subsidiary group passing into the other. This
supposition is in accordance with the facts ; for, so far as I
know, it has never been observed that all the offspring resemble
the parent which exhibited 'individual prepotency,' but this, on
the contrary, was only the case as regards some of them. In

EFFECTS OF AMPHIMIXIS ON ONTOGENY 293

fact, it is expressly stated as regards the ^ otter sheep,' that
the offspring of the first ram ' closely resembled either the
mother sheep of the ordinary breed, or the ram ' ; and this
statement is in correspondence with the theory. Those sperma-
tozoa of the ram which contained the dominant group of idants
preponderated over the group of idants of the egg-cell, and an
'otter sheep' thus resulted; while those which contained the
subsidiary group of idants could only tend to produce an
ordinary sheep of the ancestral breed.

A greater ' force of heredity' is also spoken of in the sense of
the prepotency of one race over another. According to Darwin,
the short-horn race of cattle seems to possess a particularly
marked power of transmission in contrast to other races ; and
this power is more marked in the pouter pigeon than in the fan-
tail, so that when these two races are crossed, the characters of
the pouter preponderate in the offspring. This preponderance of
one race over the other must be due to the same causes as those
which produce a much greater resemblance to one of the parents
in the case of plant-hybrids, which were discussed in another
section of this chapter. In both cases the preponderance may
be due to the presence of a larger number of idants, of ids, or
possibly even of biophors only, in the individual determinants.

5. Summary of Chapter IX

It may be advisable before proceeding further to give a short
summary of the results arrived at in the presenC chapter, and to
test the soundness of the assumption on which they are based.

According to my view, the co-operation of the hereditary
substances of the two parents in the fertilised egg depends on
the presence in each parental germ-plasm of a large number
of units, and not of a single one. These units or ids are,
moreover, not all similar to one another in the case of each
parent, and although in iiorinal sexual reproduction they all
contain homologous determinants, they exhibit slight individual
differences. The differences between the ids of the two parents
need not in anv case be greater than those existinij between the
ids of the father or the mother alone ; it may, indeed, happen
that individual ids derived from both parents may be similar to
one another, and this is more likely to be the case the oftener
inter-breeding has taken place in previous generations.

294 THE GERM-PLASM

Each id of the germ-plasm passes through all the ontogenetic
stages ; that is to say, the number of paternal and maternal ids
remains constant throughout ontogeny, and every cell is there-
fore controlled by an equal number of ids from both parents ;
and, moreover, in normal reproduction between individuals of
the same species, the same number of paternal and maternal ids
take part in the process.

The fact that the structure of each cell, organ, or part of the
body of the offspring is nevertheless not exactly intermediate
between that of the corresponding parts of the parents, must be
due to the following causes.

In the first place, an exactly intermediate structure need
not necessarily arise even if all the active paternal and
maternal determinants of a cell were exactly alike, or homo-
dynamous ; for even in such a case, the ' controlling forces ■ of
the maternal determinants might preponderate over those of the
paternal ones, owing to a superiority as regards the rate of assim-
ilation and multiplication, or in some other respect. The cell-
body would then become filled more rapidly by the biophors of
the maternal determinants which pass into it from the nucleus,
and the multiplication and distribution of the paternal deter-
minants would thus be checked. Hence the controlling force
itself may be different with regard to the homologous paternal
and maternal determinants.

The differences are, however, still further increased, owing to
the fact that the ids, and more especially the homologous
determinants derived from either parent, are not always homo-
dynamous, but are, in part at any rate, almost always heter-
odynamous. The controlling force of the homodynamous
determinants must, however, necessarily be cumulative, and tlie
inequality in the force of heredity of the father and mother at
any particular stage in ontogeny, is essentially due to the fact
that although the number of ids is the same, the number of
homodynamous determinants — i.e., those which have a cumula-
tive effect — is different.

It was deduced from the consideration of the phylogeny of
variation, that the determinants must have varied independently
of each other in the different ids of a germ-plasm, so that the
homologous determinants may be present in very different
variants in their ids ; and that, moreover, these variants of the
determinants in the different onto-idic stages may be combined

EFFECTS OF AMPHIMIXIS ON ONTOGENY 295

in a special way in each id. It therefore results that the
number of homodynamous determinants may vary in the differ-
ent stages of ontogeny, and consequently the paternal and
maternal hereditary tendencies may preponderate according to
the stage or organ under consideration.

The fact that the offspring may resemble one parent much
more closely than the other, is not incompatible with the fact
that only half the total number of ids of this parent are present
in each of its germ-cells, for all the ids only co-operate when
strictly intermediate structures are formed ; but in those cases
in which the ids of one parent are overpowered and rendered
inactive by a preponderating majority of homodynamous ids of
the other parent, the control of the cell is effected by the successful
or'- dominant' ids, and the others exert no influence. If, there-
fore, the determinants of one parent, in very numerous stages
of development, preponderate in this manner over those of
the other, an instance of apparently vionogonic heredity will
result, and the offspring will bear a closer resemblance to this
parent ; and if, moreover, the • reducing division ' should happen
to take place in one of the germ-cells of this offspring in such
a way as to result in those ids which were ' dominant,' and con-
trolled the development of this offspring remaining together in
the germ-cell, they might possibly preponderate in the next
generation over the ids introduced by another parent in the
process of amphimixis.

The problem concerning the possibility of the offspring bear-
ing a much closer resemblance to one parent than to the other
in spite of the fact that the hereditary substance of both parents
is contained in the fertilised ovum, has already been stated in
my essay on ' Amphimixis,' and its solution is to be sought in
the struggle of the ids which takes place in every cell in the
entire course of ontogeny. This struggle, however, only occurs
when the determinants become active, and presumably concerns
the biophors which pass into the cell-body, the stronger ones
annihilating those with a lesser power of assimilation. It does
not concern the determinants which are still ' unalterable,' and
are inactive as regards the control oi the cell. Moreover, the
struo:orle does not occur between the elements of the ' reserve
germ-plasm^ which brings about the formation of the germ-cells
of the offspring ; and we can therefore understand that the off-
spring does not by any means only produce germ-cells con-

296 The germ-plasm

taining the group of ids which controlled, or was * dominant ^
at, its ontogeny, but man}- other combinations of ids may be
contained in its germ-cells.

In this connection I should like to call attention to an inter-
esting essay which appeared when I had almost tinished putting
the final touches to my manuscript. It bears the pseudonym
'Josef Miiller'* on the title-page, and contains in particular an
attempt to solve the problem discussed above. The ingenious
author, who is accurately acquainted with the subject he treats
of, doubts my hypothesis of the ids, but endeavours to account
for the very remarkable disappearance of the hereditary ten-
dencies of o)ic of the parents in ' pseudo-monogonic ' heredity
by supposing that the two homologous primary constituents
(^ Anlage ') of the father and mother respectively take part in a
struggle (• gamomachia '), which results in the destruction and
complete consumption (' gamophagia ') of one of them. In
principle this explanation of the problem approaches very
closely to the solution I have attempted to give, and though I
consider the fundamental idea it contains to be correct, I do
not think that we may suppose, as the author does, that this
struggle occurs at the beghuiing of ontogeny. Basing this con-
clusion on a statement made by Oscar Hertwig, from which it
is conceivable that the homologous ' primary constituents ' of
the parents unite in the process of fertilisation, he further con-
cludes that the struggle takes place during this union, and leads
to the destruction of one of them. Apart from the fact that the
paternal and maternal idants remain separate during fertilisation,
it seems to me that a large number of the phenomena of heredity
contradict the idea of such a union and subsequent struggle.
The reappearance of the 'destroyed' primary constituent in the
germ-cells — and consequently in the next generation, the phe-
nomena of reversion — which show that every primary constitu-
ent must be present in more than two variants in the germ-plasm,
and, finally, sexual dismorphism, the occurrence of a large
number of very different hermaphrodite structures in certain
cases, and sexual reversion, — all tend to disprove such a
hypothesis. Moreover, apart from my theory of the ids, I
believe that this struggle of the homologous primary constituents

* Josef Miiller, ' Uber Gamophagie, ein Versiich zum weiteren Aufbau
der Theorie der Befruchtung u. Vererbung," Stuttg.ut, 1892.

EFFECTS OF AMPHIMIXIS ON ONTOGKNY 297

must occur in the individual cells, in which the decision both
as regards the preponderance or suppression of certain of the
primary constituents, and the niunber derived from each parent
which are to become effective, takes place. For it does not
appear to me to be essential that any one of them 7nust be
entirely suppressed, although this will probably occur in most
cases.

If I am not mistaken in my interpretation of a statement
made by de Vries, there is no doubt that the primary constitu-
ents from both parents may undergo development in one and the
same cell. By crossing a red-flowered with a white-flowered
species of bean, this observer obtained a hybrid with pale red
blossoms, on which the red colouring matter could be recog-
nised in solution in the vacuoles of the cells.* If parts only of
the cells were coloured, while other parts were colourless, it
proves that at least two different (heterodynamous) kinds of
biophors, derived from both parents, may control the same cell.
There is here, however, an extensive field for further investiga-
tion.

My explanation of the process of mingling of the parental
characters is based on the assumption of hereditary units or ids,
each of which contains the whole of the ' primary constituents '
of the species, which are, however, modified in the individual.
In this connection it may therefore be as w^ell once more to
summarise the reasons which lead to this assumption.

In the first place, such an assumption naturally follows from
the view that the germ-plasm is made up of ' determining parts '
or determinants, for the latter necessitate a definite architecture
of the germ-plasm. There must therefore be at least one limited
unit of the germ-plasm, to which nothing can be added and from
which nothing can be removed without producing an alteration
in its capacity for directing ontogeny. But since the process of
amphimixis unites the paternal and maternal germ-plasms, each
of which contains all the primary constituents of the species, each
being which is produced sexually must contain at least two ids
in its germ-plasm.

The phenomena of reversion, w^hich will be treated of in
greater detail in the following chapter, show that there must be

* QC de Vries, I.e., pp. 177, 178. The two species referred to are Pha-
seoltis multifiorus and Phaseolus vulgaris nana.

298 THE GERM-PLASM

several, and indeed many, ids in the germ-plasm of each indi-
vidual. We know that the personal characters of the grand-
parents, as well as those of the parents, may reappear in the
offspring, and we may therefore conclude that hereditary units
or ids derived from the grandparents must be present in the
germ-plasm of this offspring, and that it must therefore be com-
posed of more than two ids.

A similar conclusion is arrived at on other grounds. If the
assumption of hereditary units in the form of ids is once made,
it follows as a matter of course that their number must be
doubled in each process of amphimixis ; and it becomes evident
that this number must have increased enormously, in arithmet-
ical progression, if the ' reducing division ' had not intervened
and reduced it to the half before each occurrence of amphimixis.
This 'reducing division' must have appeared at a certain stage
in the phylogeny of amphimixis. If it arose in the germ-cells
of the first animal which was produced sexually, — supposing
that the germ-plasm of each of the parents previously consisted
of only one id, — it would always have caused the removal of
the id of one parent from each germ-cell of the offspring, and
thus no grandchild could ever have inherited characters from
both grandparents. According to our theory of the presence of
a large number of ids, such a case would seldom occur, although
it is apparently not impossible. A further consequence, how-
ever, would be seen in an unusually great uniformity in the
structure of consecutive generations ; for if only two ids were
present, one of which was always removed in the next genera-
tion, the same individual ids would pass through a great num-
ber of generations, and the diversity of the individual, such as
occurs to so great an extent in the human race, would be ex-
tremely limited. It is, however, just this extraordinary individ-
ual diversity which seems to me to be due to the multiplicity
of the ids ; it could not have been produced by only two ids
taking part in the process of amphimixis.

Finally, as soon as we have recognised, on theoretical grounds,
the existence of ids at all, the fact that a number of them exist
in the cell is supported by direct observation. For whether
they correspond to the 'chromosomes' of other writers, which
I speak of as idants, or to the ' microsomes,' of which the
chromosomes are composed, as I assume to be the case, a large
number of ids may always be observed to be present in the cell.

THE PHENOMENA OF REVERSION 299

CHAPTER X

THE PHENOMENA OF REVERSION IN THEIR RELATION

TO AMPHIMIXIS

1. Reversion to Racial Characters in Plant-Hybrids

By the term reversion, is meant the appearance of characteris-
tics which existed in the more remote ancestors, but were absent
in the immediate ancestors — i.e., the parents.

The facts relating to these phenomena are familiar enough,
and I shall therefore only refer to as many of them as are nec-
essary for the further development of my theory.

The simplest case of reversion occurs in hybrids. It occa-
sionally happens that hybrids which have been fertilised by
their own pollen, produce offspring some of which more or less
resemble only one of the two ancestral species. In such cases,
therefore, a simple reversion to a grandparent takes place. In-
stances of this kind certainly occur, though not in all hybrid-
plants ; nor are they often met with even in those species in
which they do occur. On this point Darwin quotes two contra-
dictory statements made by Wichura and Naudin respectively,
the former of whom never observed instances of reversion in his
specimens of willow-hybrids, while the latter insisted strongly
on the frequent occurrence of reversion in the Cuciirbitacece.
Darwin thought that this contradiction is explained by Gartner's
statement that reversions seldom occur in hybrid-plants raised
from wild species, but are of frequent occurrence in those pro-
duced from cultivated species. Opinions on this point have
since undergone some modification, for Focke states that ' with-
out the influence of the pollen of the parent-species, complete
reversions to the ancestral form occur practically only in hybrids
of closely allied races.' Instances of reversion of this kind do
therefore at any rate occur.

Such cases can easily be explained on the basis of our theory.
The germ-mother-cells of the hybrid contain a group of idants
derived from the paternal, and another from the maternal an-

300 THE GERM-PLASM

cestral species. If therefore the • reducing division ' halves the
germ-plasm of these mother-cells in such a manner that idants
of the mother alone reach one ripe germ-cell, and those of the
father alone ar& contained in another, it is possible that two such
germ-cells may unite when fertilisation takes place between
these hybrids. In such a case a plant completely resembling
one of the ancestral forms would arise, for it would have been
produced from a germ-plasm which contains idants of this species
only. As however such cases do not often occur, we may con-
clude that the reducing division only rarely effects such a com-
plete separation of the paternal and maternal groups of idants,
and that, in fact, as a rule, both paternal and maternal idants
are distributed to each of the four germ-cells produced by the
mother-germ-cell. As this halving of the germ-plasm occurs, as
we have seen, in a different manner in different instances, we
may presuppose that it will also exhibit differences with regard
to the proportion of paternal and maternal idants which come
together in each germ-cell in consequence of the reducing
division ; and this supposition is most satisfactorily borne out
by the facts, for it is well-known that the offspring of hybrid-
pia7its, produced by fertilisatioji with their ow7i pollen^ become
very variable in the following generation. It is evident, indeed,
that they vmst vary greatly, according to whether each one has
received a greater number of maternal or paternal ids, or an equal
number of both, from the two germ-cells which combined in
the process of fertilisation to produce this particular individual.
Thus Focke describes the offspring of hybrid-plants of the first
or second year as being ' as a rule unusually diverse and rich
in forms,' and gives as examples the genera Pisiim, Phaseolns,
Lactuca, Tragopagon, and Datnra, mentioning especially in this
connection the hybrid of Nicotiana alata and N. langsdorffii.
De Vries * also refers to these facts, and describes them very
aptly in the following passage : — ' The hybrids of the first gen-
eration have perfectly distinct characteristics in the case of
every pair of species. If a hybrid is produced from two species
which have already been crossed successfully by previous ex-
perimenters, we may be sure that the description given by
them will as a rule apply exactly to the intermediate form in
question. If the hybrid is fertile without the help of the parent-

* Hugo de Vries, ' Intracellulare Pangenesis,' Jena, 1889, p. 25.

THE PHENOMENA OF REVERSION 3OI

forms, and if in a few generations thousands of specimens of its
offspring are raised, it will always be found that scarcely two of
them are alike. Some revert to the paternal and some to the
maternal form, while others, again, are intermediate between
the two. The remainder present the most varied alternation of
paternal and maternal characteristics, and show almost every
degree of mutual intermingling.'

De Vries states this merely as a proof of what he calls the
' free miscibility of the characters,' without attaching importance
to the fact that the hybrids of the first generation behave quite
differently from those of the second, or attempting to account
for this fact theoretically.

Professor Liebscher * has recently brought forward the fol-
lowing interesting instance, in which the details were very
accurately investigated. He crossed two species of barley,
Hordeimi steudelii 9 and Hordetun trifurcatwn $ , in the for-
mer of which the spikelets are arranged in two rows and are
black, while in the latter they are arranged in four rows and are
white. The hybrid is as nearly as possible intermediate between
the two forms, ' all the ears,'' moreover, ^ being strikingly nniforyn^
as one would be led to expect theoretically. In all the hybrids
the spikelets are arranged in two rows, and in the main spike-
lets the tips are black, while in the lateral ones they are white,
and the ' Loffel ' — which are peculiar to Hordeum trifurcatiwi —
are black and white. The offspring produced from these hybrids
were exceedingly variable in the first, as well as in the second
generatioji .

Liebscher has attempted to account for this variability by
assuming that a 'loosening (• Lockerung ') of the structure of
the germ-plasm,' as well as ' re-combination of the individual
characters,' is produced by the process of reproduction. The
former results in 'a weakening of the power of faithful trans-
mission in the generative products,' i.e., 'an inclination to
individual variation in the descendants.' This statement indi-
cates that Liebscher certainly had some kind of idea of the
actual process which occurs in the idioplasm, although it is not
made clear in what this ' loosening ' consists".

A definite meaning, however, underlies this expression as
soon as it is recognised that the germ-plasm consists of a large

* Liebscher, ' Vererbung," etc., Jen. Zeitschrift, Bd. 23, 1888.

302 THE GERM-PLASM

number of ids. The 'loosening' depends on the repeated
removal of half of the ids, which occurs every time the germ-
cells are formed. Half the germ-plasm of the hybrid consists of
maternal ids of the species A, and the other half of paternal ids
of the species B ; in the formation of the germ-cells, this per-
fectly uniform composition becomes extremely diversified, owing
to the fact that the ' reducing division ' halves the germ-plasm
in different ways. If we suppose that the ids, or even idants,
are arranged in a circle, the plane of division will sometimes
cut the circle across one diameter, and sometimes across
another, and the combination of the germ-plasm in the germ-
cells containing ids of A and B will thus be very diversified.
If the hybrid is then fertilised by its own pollen, so that amphi-
mixis occurs between two of the differentlv constituted germ-
cells, a still greater diversity in the idic combinations will result,
and a high degree of variability in the offspring must inevitably
ensue.

The offspring of hybrids are also very variable, even in those
cases in which they are produced by a recrossi?ig with one of the
ancestral species. ' Fairly dissimilar offspring generally result
from the fertilisation of a hybrid with the pollen of an ancestral
form ; intermediate forms between the hybrid and the ancestral
species in question are as a rule the most numerous ; while a
smaller number of examples resemble either the original hybrid
or the ancestral species.' * This statement agrees perfectly with
our theory, for a consideration of the reducing division renders
it evident that in recrossing with one of the ancestral species,
a very dissimilar numerical proportion of idants of the two
ancestral species must prevail in the fertilised egg-cell of the
following generation. Such hybrids are generally spoken of as
' I hybrids,' because in them the force of transmission of one of
the ancestral species is assumed to be one-quarter only, and that
of the other three-quarters. This term is probably indispen-
sable in practice, but is obviously totally inaccurate. ' Three-
quarter hybrids ' do not by any means contain all the hereditary
substances of the two ancestral species in the proportion of three
to one ; the proportion is in fact a very variable one. A ' f
hybrid ' is produced, for instance, from two species of pink,
Dianthus chinejisis and D. barbatus, by fertilising the hybrid

* Focke, p. 485.

THE PHENOMENA OF REVERSION

3^3

D. chi'icnsis '^ x barbatus $ hy the. ^^oWtn o{ D. bar bat hs. The
formula of this hybrid would therefore be — Dia7ithiis {chineiisis y,
bar bat us) 9 x bar bat us $ , or in more general terms by (A + B)
9 X A J.

For the sake of simplicity let us assume that the number of
idants and force of transmission is similar in the case of both
ancestral species. We will suppose that sixteen idants * are
arranged in a circle, as represented in Fig. 22, I. In the
production of the h hybrid, eight idants of A combine with

Fig. 22. — Diagram of the composition of the germ-/>last7i in hybrids. — I. The
germ-plasm of the parental species, each composed of sixteen idants; A, Dianthns
chinensis; B, Diatithns barbatus. II. Germ-plasm of the hybrid, composed of
eight idants from A and eight from B: z, the plasm of section, which may be
rotated. III., IV., V. Three of the possible combinations of the germ-plasm which
might arise by crossing hybrid II. with the parental species B. III. A true three-
quarter hybrid. IV. Contains idants of the parental species B only. V. Contains
nine idants from B and seven from A.

eight idants of B ; the mother germ-cells of this hybrid will
therefore always contain 8A x 8B idants (Fig. 22, II.). The
reducing division then occurs, and bisects the circle at some
point. Fig. 22, II., will make it apparent that by rotating the

* We might just as well speak of ' ids ' as ' idants ' in this illustration.
I have referred to the idants simply because they are visible units, and not
merely hypothetical structures, and also because the number of idants may
be assumed to be less than that of the ids, and is thus more easily con-
trollable.

304 THE GERM-PLASM

plane of section, the circle may be separated into nine dif-
ferent combinations of the black idants A of D. chinensis and
the white ones B of D. barbatus, viz. : 8A ; 7A + iB ; 6A + 2B ;
5A + 3B; 4A + 4B; 3A + 5B; 2A + 6B; 1A + 7B; and 8B.
Nine kinds of germ-cells, differing in quality, may therefore be
formed, the egg-cells only being taken into consideration in this
connection.

In the formation of the so-called | hybrid, one of these egg-
cells unites with a germ-cell of the pure ancestral species B.
The following combinations of idants may then result : —
8B X SB, which would produce the pure ancestral species B
(7A+1B) 9 xSB ^ ; (6A + 2B) 9 x8B ^ ; (SA + 3B) 9 x8B^
(4A + 4B) 9 x8B^ ; (3A + 5B) 9 x8B $ ; (2A + 6B) 9 x8B J
(1A + 7B) 9 X 8B ^ ; and 8A 9 x 8B ^ . Theoretically, therefore,
all stages from the pure ancestral form B (Fig. 22, III.) to
the form which is intermediate between the ancestral species
may occur, but no individual can arise which inclines more
strongly towards A than does the exactly intermediate form.
Whether all these are actually produced, and in what relative
frequency they occur, can only be decided by further researches
carried out from this point of view. Those which have till now
been made are insufficient, as the number of seedlings raised
has always been too small. We may, however, infer from the
facts we already possess, that the different combinations of the
two kinds of idants do not ocair with equal freqiieiicy, and that
the intermediate combinations are the most usual. Were this
not the case, ' the form which is intermediate between the \
hybrid and the ancestral species ' could not be ' the most fre-
quent.' This is also most probable theoretically, and becomes
more so as the number of idants is greater. The cells of many
Phanerogams possess far more than sixteen idants, and even if
we also assume that the position of the line of section is entirely
a matter of chance, the rarest case will always be that in which
it accurately separates the idants of A from those of B, and it
will happen much more frequently that it divides them in some
other plane. This is equivalent to saying that germ-cells of
\ hybrids rarely contain idants of A or of B only, and that in
most cases there is a combination of the two.

If the so-called f hybrids are again crossed with the ancestral
species B, I hybrids (the third hybridised generation) are
obtained, and these ' as a rule bear a close resemblance to that

THE PHENOMENA OF REVERSION 305

species of which Zths is represented, although the individual
specimens still display considerable differences in form.'* The
result of these observations also agrees with our theory, for even
in the second generation of hybrids, eight was the highest pos-
sible number of ids of the ancestral species A in the mature
germ-cells of the hybrid, and in the process of fertilisation these
met with the same number of idants of the ancestral species B,
which was used for recrossing. The germ-mother-cells of the
f hybrid cannot contain more than 8A X 8B, and, as a rule, a
smaller number of the idants of A will be present. The reducing
division then again halves these sixteen idants in various ways ;
and in the case which is most favourable for the preservation of
the hereditary substance of A, the halving of one of these germ-
mother-cells, which was most favourable for A, might again lead
to the formation of germ-cells with eight idants of A and eight
of B. But in all other 'l hybrids the germ-mother-cells already
contain more idants of B than of A, as is shown in Fig. 22, III.,
and the reduction therefore results in the germ-cells containing
either idants of B only, or, at any rate, some of B in addition
to those of A. If these egg-cells are then fertilised by germ-
cells of the pure ancestral form B, the largest conceivable
number of idants of A which can be contained in the fertilised
egg-cell will again be eight. Hence the form exactly inter-
mediate between the two ancestors may possibly also appear in
the third generation, but this will occur m^^c/i 7iwre rarely than
in the second generation. On the other hand, those individ-
uals the idioplasm of which contains only a few idants of A
will preponderate ; and in others, even idants of B only may
be present, for the idants of B were in the majority in most of
the mother-germ-cells of the second generation : the halving
due to the reduction must therefore have frequently resulted in
the distribution of idants of B only to one of the germ-cells, and
these then once more combined in the third generation with
idants of the pure species B. Theoretically, therefore, the
\ hybrids must in part revert to the ancestral form B. This
conclusion is supported by facts, inasmuch as Kolreutter and
Gartner, who made a large number of experiments on recross-
ing, found ' that three to six generations (four to five as a
rule) were required for the complete transformation of the

* Focke, p. 485.

306 THE GERM-PLASM

hybrid into one of the ancestral forms." As these investigators
were only concerned with obtaining a complete return to the
ancestral form, they always selected for purposes of propagation
those plants which bore the closest resemblance to the desired
parental type. They, nevertheless, still obtained individual
plants which bore traces of the other ancestral species, up to
the fifth generation, just as would be expected on theoretical
grounds.

The agreement with our theorv extends even further than
this : — i)idi7>idual cases of complete reversion to the ancestral
form must theoretically occur ifi the second generation^ a?id such
cases have actually been observed. Godron, indeed, found that
' Melandrynm album x rubru?n., even when fertilised by its
own pollen, reverted in the second generation to the ancestral
species.' * This is easily explained on the basis of our theory.
If the reducing division took place in certain of the mother-
oferm-cells of the hvbrid in such a manner that each of the two
mature germ-cells contained \ idants f of J/, album or M. rubrum
only, two possible combinations might occur in the fertilisation
of these germ-cells : — either a germ-cell with idants of J/.
rubrum would meet with another containing idants of M. album,
or else both the conjugating cells would contain idants of the
same species. In the former case, the hybrid form would again
be produced, while in the latter, the pure ancestral form would
result. Such reversions seem to be of rare occurrence, and this
may indicate that when they do occur, some unknown conditions
favour the halving of the germ-plasm in the mother-germ-cells
along a defi)iite plane of section, viz., that which passes between
the groups of paternal and maternal idants.

Although not proved by actual observation, it is apparent
from our theory that the hybrid may revert co7)ipletely to the
ancestral form. This would be impossible if the reducing
division did not occur, or if the germ-plasm consisted of a
homogeneous mass which became completely mixed with that
of the other species in the process of hybridisation. The germ-
plasm of the one species would then, moreover, never be removed
by means of the reducing division : — the reduction would be
quantitative, and not a qualitative one. Reversion to the an-
cestral form could then in no case occur on fertilisation taking

* Focke, p. 485.

t n signifies the normal number of idants in the species in question.

THE PHENOMENA OF REVERSION 307

place with the pollen of the hybrid ; and even by continued
recrossing with one of the ancestral species, the combination of
the germ-plasms of the first generation would gradually become
weakened, and perfectly pure germ-plasm of the one ancestral
species could never be produced. But as soon as we assume
that the germ-plasm consists of units which remain separate in
the form of idants or ids, it is evident that the whole of the units
of one species may be removed from the germ-plasm of the
hybrid-offspring, either with or without the occurrence of re-
crossing ; and even if recrossing does take place, reversion must
occur sooner or later in individual descendants.

Our theory also enables us to make certain predictions, which,
as far as my knowledge extends, have not up to the present been
confirmed by facts. If sometimes, though rarely, all the idants
of A pass into one germ-cell, and all those of B into the other,
in the process of halving of the germ-plasm of the primary
hybrid A x B a reversion to both the ancestral forms will occur
when the second generation is fertilised with the pollen of the
hybrid, as has just been shown : that is to say, such a reversion
will take place when a pollen-grain containing idants of A only
comes in contact with an egg-cell which also contains idants of
A only, or when idants of B meet with others of similar origin.
The same must also be possible in the hybrids of the third gen-
eration, even when all the complete reversions of the second
generation are left out of consideration, simply for the reason
that certain individuals of the second generation are produced
from the combination of //A with ;/B idants, and are therefore
exactlv intermediate forms like all the hvbrids of the first iren-
eration. The occurrence, even if only in individual cases, of
such intermediate forms, renders it possible that germ-cells may
again arise which contain idants of A or B only, so that ultimately
reversion to one or other of the two ancestral forms becomes
possible. These reversions will certainly be of rare occurrence;
and as it is entirely a matter of chance that germ-cells which
contain the most infrequent combinations of idioplasm should
come together in the process of fertilisation, it would be neces-
sary to examine a very large number of seedlings before such
cases w^ould be found.

Before passing on to the consideration of reversion to mdi-
vidual characters, it should be remarked that the same law of
reversion which can be recognised in the case of plant-hvbrids.

308 THE GERM-PLASM

holds good as regards the crossing of the different human races.
According to this law, the intermingling of a racial character
is uniform in the first generation, but subsequently becomes
quite irregular when recrossing with one of the original races
occurs. The skin of mulattoes, which are a cross between
white races and negroes, is never quite white, its colour being
as a rule an approximate mean between that of the two parents.
When mulattoes and white races are recrossed, the skin does
not regularly become less black, but the descendants of the
third and fourth generations are sometimes white and sometimes
fairly dark : this fact indicates the absence of uniformity in the
process of 'reducing division.'

2. Reversion to Individual Characters in Man.

The essential difference between the process of reversion in
plant-hybrids and in Man, consists in the fact that the former is
concerned with the intermingling and subsequent separation of
specific or racial characters, while in the case of the reproduc-
tion of human beings of the same race individual characters
only are intermingled. As regards hybrid-plants, the idants of
each parent might be assumed to be similar, — although this may
not be strictly true in all instances, as will be seen later on, — but
in the case of individual differences the idants of each parent
cannot be regarded as similar. Each of these idants consists
of a number of separate ids, which may differ in many respects.
In all of them the determinants are as similar as the retention
of the specific character renders necessary : that is to say, all
the determinants of the same ontogenetic stage are homologous,
though at the same time they are never all homodynamous,
but differ in many respects owing to slight individual devia-
tions. Hence different ids may contain different variations
of any particular homologous determinant. In the following
examples, each homologous determinant is indicated by a letter,
and variants of a determinant are distinguished by dashes after
each letter. Thus id i. of the germ-plasm might, for instance,
contain the determinants a, b, c, d, e, n ; id ii., the deter-
minants a, b', c, d', e, n' ; id iii., a', b'^ c', d'", e', n,

and so on. The total effect of the idants is decided by the
struggle of the ids ; the laws which regulate this struggle
cannot at present be determined more precisely, and until we
know more about them, we may suppose that those variants

THE PHENOMENA OF REVERSION 309

which are present in the largest numbers always have the best
prospect of controlhng the cell entirely, or at any rate chiefly ;
their elTect must be cumulative, and a small minority of homo-
dynamous determinants will not be able to take effect against a
large majority of some other variant. The control of the cell
therefore results from this struggle of the determinants, which
must naturally not be conceived to take place in such a manner
that the group of paternal ids struggles with the maternal group,
but so that all the active determinants which are contained in
the idioplasm migrate into the cell body and there strive to
obtain the control. If the parents of the organism in question
are closely related, the same homodynamous determinants may
very possibly be contained in the idants of both parents, and
the forces of these will then combine just as would be the case
if they had been contained in one idant of the father or mother.
It will, however, happen more frequently that homodynamous
determinants are present in the ids of each parent respectively,
and a majority of homodynamous maternal determinants will
then compete with a majority of paternal ones ; they will then
either control the cell together, or the prepotency of one of
the parents will be so great as to suppress the influence of the
other entirely.

In order to test to what extent these ideas of the co-operation
of parental idioplasms may be applied to the phenomena of
reversion in Man, we must again consider the simplest of these
phenomena, viz., that oi 7-671 ersion to a grajidparent.

It is well known that a child not infrequently resembles its
father or mother in a high degree, and it is also assumed that a
father may beget a child which does not resemble him, but it
and its fathers mother are as 'like as two peas.' This pre-
supposes that the father himself bore no resemblance to his
mother, for otherwise it would not be an instance of reversion
of the child to the grandmother.

This case may be explained theoretically by assuming that
the ' reducing division ' of the respective germ-cells of the two
generations accurately separated the paternal from the maternal
idants, and that, as was shown above, the group of idants of
one of the parents may possibly have had no influence on the
formation of the child, the other group being the dominant one.
The fertilised egg-cell from which the father was developed
must therefore have contained the two groups of idants A and

3IO THE GERM-PLASM

C, A being derived from the grandmother, in the development
of whom this group dominated. C was derived from the grand-
father, and preponderated to such an extent over A that it alone
determined the type of the son. Hence A remained latent
in the idioplasm of the father, who therefore resembled the
grandfather and not the grandmother. If now, that germ-cell
of the father, from which the child of the third generation was
developed, just happened in the reducing division to receive
the group of idants of A only, which remained without influence
on the development of the father, but which was dominant in
the grandmother, a reversion to the grandmother would then
become possible. It would occur when this sperm-cell unites
with an egg-cell in which the group of idants D has a much
weaker controlling force than A. The child would then resemble
neither the paternal nor the maternal type, but would be like his
grandmother, for the group of idants A determined the type of
the grandmother.

I nevertheless consider it doubtful w^hether reversion of this
kind ever occurs so thoroughly and completely as is conceivable
theoretically. The facts are unfortunately by no means so
decisive as one could wish. No one, so far as I know, has
hitherto attempted to ascertain whether complete reversion to a
grandparent ever occurs, and from a theoretical point of view I
should consider this to be improbable. Similarity to a grand-
parent may undoubtedly often be observed, but it does not
therefore by any means follow that a correspondence exists
between all, or at any rate the greater majority of individual
characters, such as is seen in identical' twins. All the parts
of the child and grandparent have never been carefully compared,
not to mention the fact that such a comparison has never been
made at corresponding ages. Moreover, to use the same ex-
ample, only those cases could be utilised in questions of re-
version in which the father bears no resemblance to the type
of the grandmother. Cases of this kind are, however, not quite
reliable, and are certainly not common.

I am therefore inclined to think that in all these cases we are
only concerned with a partial and not a complete reversion
to the grandparent, — that is, with the reappearance of a more
or less extensive aggregate of characters of the grandparent ;
and this is certainly sufficient in many cases to produce what
appears to be the type of the latter. More or less extensive

THE PHENOMENA OF REVERSION 3II

aggregates of the characters of the grandparents undoubtedly
reappear in the child, and these may be more easily explained
theoretically, without the necessity of supposing that chance
plays so great a part. For in order that perfect reversion may
be possible, numerous most infrequent occurrences must take
place together. To use the same example, in each individual of
three consecutive generations, only one of the two groups of
parental idants could determine the type of the child, and the
reducing division, moreover, must affect one of the mother-
germ-cells of each of four individuals — />., the two grand-
parents and the two parents — in such a manner as to separate
the two groups of parental ids. All these possibilities may
sometimes concur, but we can only imagine this to be so if
it is proved that an exact resemblance exists between the child
and grandparent.

K partial resemblance is theoretically far more likely to occur.
Let us suppose, for instance, that the germ-plasm consists of
sixteen idants, eight of which are derived from the mother and
eight from the father, and that the reducing division causes six
idants of one parent and two of the other to be present in one
germ-cell : the former might possibly contain many of those
determinants to which the type of this particular parent was due.
This need not necessarily be the case, for all the sixteen idants
may have had a farely equal share in the production of this type,
and six out of the sixteen idants could not then possibly give
rise to the same type even approximately.

It might, on the other hand, happen that the type of the
parent was essentially determined by the group of idants derived
from the father or mother of this parent only, and a resemblance
of the child to its father could then only occur when the whole
or a great part of that group which controlled the development
of the father was also present in the germ-cell from which the
child was developed. Even then, the production of the type of
the father would not absolutely be ensured, for in the process of
fertilisation by the other parent a group of idants might be
added, in w'hich the controlling force preponderates over those
already present, either entirely, or at any rate as regards many
or most of the determinants.

We may, however, conclude from all these considerations,
which are unfortunately still very vague, that a large number of
idants of the grandfather, for example, may be present in the

312

THE GERM- PLASM

germ-cell which will give rise to the father, and still may not
result in the production of the type of the grandfather ; and that
these may then pass into a germ-cell of the next generation,
and partially determine the type of the son, provided that they
are opposed to a group of idants which has a weaker controlling
force. At every ontogenetic stage, then, the struggle of the ids
and idants decides which group of the latter is to control the
cell. According to the hypothetical principle on which we have
supposed this struggle to take place, the majority of homodyna-
nious determinants would always represent the greatest con-
trolling force, so that certain of the more comprehensive or
special characters of the grandparent might very well reappear
in the grandchild, even if only six or eight idants which con-
trolled the development of the grandparent were present in the
germ-plasm from which the grandchild arises.

We may even assume that, as regards Man, oiitogoiy is hardly
ever passed through unthout reversions occurring to one or other
of the grandparents ; for determinants of one of the grand-
parents will almost invariably have been suppressed in the
development of the parent by stronger ones derived from the
germ-cell of the other grandparent, and will predominate in
the formation of the grandchild, because they are here again
opposed to other combinations of ids over which they may pre-
ponderate under certain circumstances. It may therefore happen
that some of the individual characters of the grandparent may
reappear in the grandchild, although there may be no general
resemblance between the two.

The answer to the above question relating to the causes on
which reversion to a grandparent depend, may be briefly stated
as follows : — such a rei'ersion is due to the fact that the whole or
part of the group of idants which determine the type of the
grandparent were present in that germ-cell of the parent from
which the grandchild was developed, and that it was there
opposed to a weaker group derived from the other parent.

We do not know what number of the controlling idants of the
grandparent must be present in the germ-plasm of the grand-
child in order that reversion to the grandparent may occur.
Complete reversion can only take place if none of the deter-
mining idants are absent ; but, as we have seen, such complete
reversion has not by any means been proved to occur in Man.
Still less is this the case in the next following generation, viz.,

THE PHENOMENA OF REVERSION 313

as regards tJie great-grandparoit . Instances of a descendant
of an ancient family reverting to a great-grandparent whose
characteristics were not present in the intermediate generations,
are certainly occasionally mentioned in novels, but it is only
natural to doubt the accuracy of such cases, even when they
claim to be true. A great-grandchild certainly often resembles
its great-grandparent, but when this is so, this rese?nblance had
not disappeared entirely in the i?iter mediate generatiojis. As has
been shown above, it would certainly not be incredible, from
a theoretical point of view, that the group of idants which con-
trolled the development of the great-grandparent should remain
intact in certain germ-cells of two generations, and should be
suppressed by more powerful groups derived from germ-cells of
other parents, once more to predominate in the third generation.
If reversion of this kind could be proved to occur, it might be
explained in this manner, and we should be justified in assuming
that in many cases the idants of the two parents may again be
separated into their original groups by the reducing division.
The great variabilitv of hvbrids of the second generation
proves that this only occurs very rarely, and in most instances
not at all.

We have seen that the difference between reversions in
hybrids and in human beings of the same race is simply due to
every idant of one parent, in the case of hybrids, containing
specific characters, so that all the idants of the one parent are
similar, and all the homologous determinants may also be
considered homodynamous. When therefore an ontogeny is
directed by the ids of ojie of the parent species only, the type
of this species is produced. The type of a human being, on the
other hand, is constituted by ids of very many different kinds,
no two of which are exactly alike, for each of them contains
determinants of a somewhat different kind from the rest ; and,
speaking generally, the type is in all cases only the resultant
of all these different components. The same type, or combi-
nation of characters, can only appear for a second time if the
same components are again brought together. Except in the
doubling of the fertilised egg in the case of ' identical ' twins,
this can, however, never occur, for a new combination of ids and
idants which never existed before arises every time fertilisation
takes place. Hence cases of complete reversion as regards
individual characters to a previously existing form, can never

314 THE GERM-PL.4SM

occur ; and only those instances are possible in which the rever-
sion concerns more or less extensive groups of characters which
already existed in an individual of a former generation.

We have now discussed the question of transmission from the
parents, grandparents, and great-grandparents, to the child ; and
it will be as well, before going further, to summarise the various
possible cases, and to see whether observation and theory con-
tradict one another in any respect.

The most frequent case seems to be that in which the child is
a mixture of the types of the two parents, the characters of both
of whicli are either completely united, or else they alternate with
one another in the different cells, parts, and organs, and even in
the systems of organs themselves. In all such instances, the
group of idants which predominated in the ontogeny of the
parent, or rather of both parents, must have been contained in
the germ-cell from which the child arose.

The question might here be asked as to how it can so fre-
quently happen that only the dominant group of idants is
present in the germ-cell destined to undergo amphimixis. We
must, however, remember that a perfectly equal blending of the
types of the two parents in the child occurs much more rarely
than is generally supposed ; and that it is difficult, and frequently
in fact impossible, to say whether the maternal portion of a
character is really derived from the type of the mother, and not
from that of her father or mother. General characters only, of
the mother's family are most usually combined with others
derived from the family of the father. But those characters
which have been peculiar to many members of a family for
several generations are the most likely to occur in numerous ids
and idants, and they therefore reach the germ-cells in larger
numbers in many of the modes of ' reducing division.' Theo-
retically the dominant group of idants of the parent would not
be necessary for such a transmission of a general family resem-
blance, but only a majority of the idants of this group.

The case, however, is different when the resemblance refers
mainly to the type of one parent ; and w^e must then assume that
the dominant group of idants of this parent is present, and is
opposed to another weaker group derived from the other parent,
and provided with a smaller number of homodynamous deter-
minants.

THE PHENOMENA OF REVERSION 315

A third case may also occur in which the type of an uncle or
aunt is more or less accurately reproduced in the child, or in
which this type is blended with the characters of the parent on
the other side. I know a man w'ho closely resembles a maternal
aunt, but nevertheless possesses many general characters of his
father's family. This fact may in all probability be explained in
terms of the idioplasm as follows : — the egg-cell from which
this person was developed contained the group of idants which
was dominant in the ontogeny of the mother's sister, and not
that which predominated in the case of the mother. Theo-
retically this might very well be the case. Let us suppose that
the primary germ-cells of the maternal grandfather (m p)
contained the idants a, b, c, d, e, f, g, h, and those of the grand-
mother the idants i, k, 1, m, n, o. p, q ; and also that the fertilised
egg-cell from which the mother was developed contained the
idants a, b, c, dxi, k, 1, m, and that from which the aunt was
developed the idants «, b^ c, i x I, n, o, p. We will further
assume that the group of idants which was dominant in the
ontogeny of the aunt were those indicated by the letters a, b, c,
and /, printed in italics. It will then be seen that the same
combination a, b, c, I can also be formed from the germ-plasm
of the mother by means of the reducing division, for all four of
them are present in this germ-plasm {a, b, c, dxi, k, /. m). It
is doubtful whether such a case ever occurs so accurately, and
I know of no instance which renders this hypothesis necessary :
the resemblance is always an imperfect one.

We must also consider a fourth case, in which the child
neither specially resembles its father or mother, presents a
recognisable combination of the characters of both, nor bears a
striking resemblance to one of the four grandparents, but dis-
plaNs an entirely new combination of characters. Such a child
would probably always bear some resemblance to both, or at
any rate to one, of the parental families, but it would not exhibit
certain marked characters of the respective fore-fathers.

These cases do not contradict our theory, for by means of the
reducing division it may possibly happen that none, or only
certain of those very idants which were dominant in both
parents are present in the germ-cells of the parents which
undergo amphimixis.

If, for instance, the germ-plasm in the ontogeny of the father
had the composition a, b, c, d, e,f, g, h, of which the idants a, b,

3l6 THE GERM-PLASM

d, f, were dominant, and the germ-plasm in the ontogeny
of the mother consisted of the idants /, k. /, m. //, o. p, q, of
which i, 1. n, o essentially determined her type. If the reduc-
ing division then resulted in an egg-cell containing the idants
k, m, p. q, and a spermatozoon was formed which contained
the idants c, e, g, h, and these came together in the process of
amphimixis, the resulting gernvplasm would certainly contain
characters of both families, but it could not possibly produce
the type either of the father or mother, or a mixture of both.

I have already discussed the question of reversion of the
child to one of its four grandparents in detail, and need not
return again to this point. I should, however, like to call
attention to the fact that, theoretically, a child can never exhibit
a cojnbifiation of the types of two of its grandparents, simply
for the reason that at least half of the idants of a germ-plasm
determine the type of the child, the germ-plasm of which can
only contain one quarter of the idants of two grandparents.
An exception to this rule could only occur in cases of close
interbreeding, in which both grandparents in part contained
the same idants.

No case has, however, ever come before my notice in which
a child bore no resemblance to its parents and yet exhibited a
combination of the types of that of two of the grandparents.

3 . Reversion to the characters of ancestors far removed in

animals and plants.

We will now consider the question of reversion to remote
ancestors. Cases of such reversion are so well known through
Darwin's researches, that a more detailed description of the
mere facts might almost be considered superfluous. But
.such a description is nevertheless necessary, and I must even
express the opinion that the facts have not been so accurately
investigated from all points of view as is desirable in order that
a theoretical explanation of them may be arrived at. Darwin
was the first to point out that in cross-breeding, either of species
or of mere varieties, characters not infrequently appear in
descendants which were not present in the parents, in some of
which cases, indeed, it can be proved, and in others shown to
be very probable, that they have been derived from remote
ancestors. Mules, for instance, sometimes exhibit distinct
stripes, like those of the zebra, on the fore-legs and shoulders,

THK PHENOMENA OF REVERSION 317

while such stripes are very seldom present in the horse and ass,
and are even then only very faint ; but we must nevertheless
suppose them to be derived from the ancestral form of the two
species. And, again, when certain races of pigeons are crossed,
offspring are produced in which the plumage has the slaty-blue
colour of the wild rock pigeon, although the races used for
crossing were of quite another colour ; in this case, moreover, the
descent from the wild rock pigeon is certain. Similar instances
also occur in plants. The hybrids of Datura ferox and D.
Icpvis, in both of which the flowers are white, regularly bear
blue (purple ?) flowers, and Darwin * has shown that this is to
be looked upon as a reversion to ancestors which possessed blue
flowers, as, in fact, is the case at the present day in an entire
group or species oi Datura.

I will now attempt to explain these three instances in accord-
ance with my theory. In them, as is universally the case, rever-
sion must be attributed to the presence of old unmodified
determinants in the germ-plasm, which take the place of the
younger homologous determinants as regards obtaining the
control of the cell or region of cells in question. Similar
assumptions must be made in every theory of heredity. In his
theory of pangenesis, Darwin makes use of old gemmules for
this purpose, while de Vries assumes that reversion is due to the
presence of old pangenes. Some unmodified portion or other
of the hereditary substance must always form the starting-point
in attempting to explain the problem ; and the only question is,
whether we are to remain satisfied with such a statement, and
leave everything else in obscurity, or whether it is possible to
obtain a certain insight, in principle at any rate, into the question
as to why these minute parts can remain unmodified, and why
and under what circumstances they can suddenly obtain control
precisely in the region of transformed parts which are homol-
ogous to them.

A solution of the first of these problems has already been
given above. It has been shown that, according to the principle
of selection which controls the whole, a transformation such as is
required by the vital conditions of a species never necessitates
the transformation of all the determinants relating to the parts
to be transformed, but that this process is only necessary in the

* Darwin, ' Animals and Plants under Domestication,' Vol. II, p. 254.

3l8 THE GERM-PLASM

case of a majority of them, which is sufficient to ensure the occur-
rence of the modification in question in every ontogeny, — i.e.,
in every individual of the species. This is all that is required,
and consequently tJie processes of selection cannot accomplish
jnore. After every transformation of the body in the process
of development of the species, the germ-plasm will thus con-
tain some unmodified determinants in addition to those which
have been transformed in any part, and these will only disappear
very gradually in the course of the further history of the species.

The existence of the material by means of which reversions
to all specific characters operate, can therefore be proved
on the basis of the theory of selection : every germ-plasm mnst
contain a larger or smaller nufnber of old deter 7ninants corre-
sponding to the characters of the ancestral species.

The solution of the second problem, as to why these ' ancestral
determinants " always exert their influence at the right spot in
the body, naturally follows from our theory, in which the
mechanism of ontogeny is referred to the gradual disintegration
of the germ-plasm.

The third problem then only remains : how can the ancestral
determinants J which are present in a 7ninority, gain control over
the majority of yoimger ones ?

We have seen that reversions to the ancestral form occur in
the offspring of hybrid plants, — even if they are fertilised by
their own pollen. — when those cells to which the entire group
of idants of the ancestral species of the sa7ne parent has been
distributed at the reducing division happen to come together in
the process of fertilisation. Whenever the germ-plasm of the
fertilised egg-cell contains idants of the species A alone., an
organism of the species A can alone arise. But this occurrence
is out of the question in cases of reversion to the characters of
remote ancestors. The germ-plasm can then never consist
entirely of idants of the ancestral species ; and. in fact, we may
doubt whether entire ancestral idants exist at all in the germ-
plasm of any individual of a long-established species. For the
number of idants (nuclear rods) is not extremely large in any
species ; and if, as in the instance given above, we assume this
number to average sixteen, even the proportion of one unmodi-
fied to fifteen modified idants would be rather large, and would
render an occasional reversion to the ancestral form possible
in the ordinary reproduction of the species. Fertilisation need

THE PHENOMENA OF REVERSION 319

only occur eis^ht times successivelv, two of the unmodified
ancestral idants being added each time to those already pres
ent, in order to produce a germ-plasm consisting of pure ances-
tral idants, and the organism developed from it would then
agree with the ancestor in question in all its characters. Under
particularly favourable circumstances the process might even
be completed in four generations, as is shown in the following
table : —

^ .-if a', b, c, d e, f, g, h = ^ germ-cell.

Generation I. -' , . , , & ^^ ,,

\ a', i, k, 1

^ ^. TT ^ a', b, a', d

Generation II. - , '

i a', q, a', s

Generation III

m, n, o, p = 9 "

c, i, k. 1 = ^ "

t, u, V, w = 9 "

b, d, q, s = Z '^

a, /?. 8, e = 9

Generation IV. - , , ^' ^ ^ / / / _ o
(a^ a^ a^a<a^a,a^a — q;

u

The letters in the above table indicate the idants, which —
with the exception of a', which represents the unmodified ances-
tral idant, and only occurs once in the first generation — are
only individually different from one another. The vertical line
represents the reducing division of the mother-germ-cell, which
in the most favourable case we are now^ considering, always
causes this ancestral idant a' to pass into the germ-cell about
to undergo amphimixis. For the sake of simplicity, the con-
jugating germ-cells are assumed to be similar in respect to
their contained ancestral idants. It would thus follow that
germ-plasm consisting of ancestral idants only, would be pro-
duced in the fourth generation.

Although we must certainly admit that so favourable a com-
bination of events can hardly ever occur, it cannot be doubted
that an accumulation of ancestral idants may take place in one
germ-cell in the course of a large number of generations, and
that a jfiajority of these ids may consequently come together
in fertilisation. In this case a more or less complete reversion
to the ancestral form must take place. As such reversions to the
complete type of the ancestral species cannot be observed to
occur in the normal reproduction of pure and long-established
species, we must conclude that all the idants Juwc become
niodijied in such old and pure species, each of which produces

320 THE GERM-PLASM

the type of the species, and not that of an ancestor, in case it
alone has to control the ontogeny.

The foregoing considerations seem to me to afford a good
explanation of the frequent occurrence of reversion in younf^
species w/iich are not yet definitebj established. Cultivated
varieties of flowers, such as the Heartsease {Viola tricolor),
always produce, among a large number of seedlings, certain
plants which bear flowers more or less resembling those of the
wild species. Evidently only a portion of the idants have become
transformed in this modern form, another and smaller portion
having remained unmodified. Since the reducing division sepa-
rates the idants into two groups of all possible combinations,
germ-cells, both male and female, containing a' preponderance
of unmodified ids, must also occur ; and if two such germ-cells
come together in fertilisation, reversion must result.

Let us suppose that, of the sixteen idants in the germ-plasm,
ten were modified and six unmodified. Under favourable cir-
cumstances a case might then occur in which the majoritv of
idants would remain unmodified, without further accumulation
taking place in the course of generations — i.e., at every fertilisa-
tion. This may be illustrated by the following table, in which
the letters with a dash represent the unmodified, and those
without one the modified idants, the line dividing the letters
indicating the reducing division :

i,k,l',m,n, o,p,q.
y, z, a, b', c, d, e, g.

Maternal germ-plasm ^ a', b', c, d', e, f, g', h
Paternal " \ f, r', s, t', u', v', w', x

Germ-plasm of offspring I. : — a', b', c, d', e, f, g', h x f, r', s,
t', u', v', w', X.

Owing to the manner in which the reducing division takes
place, the maternal and paternal germ-cells each contain five
unmodified idants, so that in the germ-plasm of the offspring
ten unmodified and six modified idants are present. If. on the
other hand, we suppose that the other two cells produced by the
reducing division from the same mother-germ-cell undergo ferti-
lisation, the germ-plasm of the offspring II. will have the follow-
ing composition : —

i, k, 1', m, n, o, p. q, y, z, a, b'. c, d, e, g.

It therefore only contains two unmodified idants, and must
consequently give rise to the modified form of the plant.

THE PHKNOMENA OF REVERSION 32 1

A large number of cases of heredity which have not hitherto
ijeen understood can thus be explained in a very simple way.
Take, for example, the varying degrees of certainty with wliicJi
varieties of cultivated plants transmit their peculiarities. The
extent to which such peculiarities are transmitted must depend
on the number of idants in the germ-plasm which have remained
unmodified, and the greater this number, the more easily will
reversion occur.

These theoretical considerations will probably account for the
first of the three cases which were mentioned above as types of
reversion to more remote ancestors, viz., that of Datura ferox
X lcB7'is. The two species of Datura have white flowers, but
produce hybrids in which the flowers are blue ; and this is not
only occasionally the case, but occurred in every one of the 205
plants which Naudin raised from this cross,* and also in all
those cases which had been observed previously by Kolreutter
and Gartner. t If we assume that, in addition to their own
specific idants, the two species of Datura contain a certain
number of ancestral idants, the latter might be relatively in-
creased in individual germ-cells in consequence of the reducing
division ; and when these met with germ-cells of the other
species, which also contain numerous ancestral idants, a germ-
plasm possessing a larger number, and perhaps even a majority,
of these idants would be constituted. Individual cases of re-
version to the common ancestral form might thus occur. But
this assumption is evidently insufficient to account for the facts,
for the blue colour of the flower appears in all the hybrids.
The reversion in these cases must therefore be independent of
a greater accumulation of ancestral idants which may possibly
occur in individual germ-cells in consequence of the reducing
division. These idants must, on the contrary, come together in
each fertilised egg-cell in a sufficient number to preponderate over
the modifed idatits, and to control the ontogeny. They cannot
possibly, however, be stronger jiumerically in every case, and
another factor must therefore take part in the process, which
causes the ancestral primary constituents to preponderate in
every case; and this is in all probability the specif c diversity of
the modified idants. We have assumed from the first that

* Darwin, 'Animals and Plants,' Vol. II., p. 254.
t Cf. Focke, ' Pflanzenmischlingc,' p. 269.

32 2 THE GERM-PLASM

homodynamous determinants liave a cumulative effect, while
heterodynamous determinants tend to counteract one another.
The same must be true of the groups of determinants, — the
ids and idants ; similar idants must increase the effect produced,
while dissimilar ones will interfere with one another to a greater
extent the more diversified their composition out of ids and
determinants is. Let us confine our attention for the present
to the groups of determinants for the flowers. The two species
of Datura certainly contain determinants which tend to form
white flowers, and we might therefore be disposed to infer that
these are homodynamous, and that their effect must be cumu-
lative. But this conclusion would be erroneous, for it is quite
possible that these determinants only correspond as regards the
production of a ' white ' colour^ and differ widely in respect of
manv other characteristics of the cells, such as those relating to
their size and minute structure. On the other hand, the 'blue '
determinants are actually homodynamous. and correspond not
only with regard to the colour to which they give rise, but also
in respect of all the other characters of the cells of the flowers,
for they are derived from the common ancestral species. When
the hybrid begins to develop flowers, the structure of the cells
in the latter will depend on the determinants of the two white
species and of the blue ancestral form. Although the 'blue'
determinants are in the minority in the idioplasms of either
parent, they may nevertheless, if they all combine, possess a
greater power of transmission than the ' white " ones, if the
latter are not homodynamous. i.e., do not possess an exactly
similar force of heredity, and consequently cannot produce an
intermediate effect. They interfere with one another in their
action, as they act in different directions to a greater or less
extent. Many cases of reversion can be understood — even
though only in principle — by means of this law. They can,
moreover, easily be rendered comprehensible in individual cases
if we have recourse to figures.

Let us suppose that the 'blue' ancestral determinants are not
only contained in individual ids, but are present in all the ids of
entire idants : a minority of old ancestral idants would then be
opposed to a majority of modern ones, half of which, however,
would correspond to the type of D. Icrvis and half to that of D.
ferox. VVe assume that there are in all sixteen idants, six of
which are ' blue " and ancestral, and ten • white.' Since, therefore,

THE PHENOMENA OF REVERSION 323

five of the latter belong to D. Icevis and five to D. ferox, and
these are consequently dissimilar, the six similar ancestral idants,
which have a cumulative effect, will preponderate, because the
2x5 different idants of D. IcEvis and D.ferox do not produce
the cumulative effect of 10.

Hitherto I have assumed that we are in this case dealing
with a complete reversion to the ancestral form, and not merely
with reversion to i7idividual ancestral cJiaracters. I cannot,
however, judge with certainty in this respect from the facts
known to me concerning these hybrids ; and as this instance
was not chosen for its own sake, but merely as an example in
which complete reversion to more remote ancestors might be
accounted for, it must remain undecided whether it really belongs
to the above category, and whether such instances of complete
reversion actually occur. The blue colour of the flowers is at
any rate not the only apparently new character in these hybrids ;
the stem, for instance, is brown, while it is green in the two
pure species. In only one of these species {D.ferox) is the stem
brown at first, and this colour is onlv retained as a brown ring
around the base of the stem. I must leave botanists to decide
whether the shape of the leaves, and the structure of the stem or
fruit, afford any reason for considering these parts as intermediate
between those of the two parental species, or whether they are
to be regarded as deviations from both, and presumably, there-
fore, as reversions to the ancestral species.

It may in all probability be assumed that the process is not
a complete one in the above-mentioned cases in which reversion
to the wild ancestral form in different races often occurs when
the latter are crossed. Darwin certainly gives one instance of a
pigeon which ' was hardly distinguishable from the wild Shetland
species,' but which was, nevertheless, descended from four
grandparents which differed very considerably from the wild
species {Coliimba livia).* This bird, which was blue, and pos-
sessed the typical black bars on the wings and tail, was descended
from a red spot, a white fantail. and two black barbs. These
breeds, as is well known, differ from the wild pigeon in colour
as well as in many other details, such as the length of the beak
and number of tail feathers ; and it would therefore be interest-
ing to ascertain whether these racial characters had all disap-

* 'Animals and Plants under Domestication,' p. 14.

324 THE GERM-PLASM

peared in the grandchild, and liad become retransformed into
the corresponding characters of the wild species. Were this
so, the reversion might be considered complete, and the same
theoretical explanation could be given for it as in the case of
the Datura hybrid. Unfortunately Darwin leaves this point
untouched, as he devoted his attention chiefly to the coloration
so characteristic of the species. It seems to me to be very
probable, however, from several of his statements, that this was
also essentially a m.ere case of reversion as regards the colora-
tion of the plumage. I conclude this principally from the fact
that the blue or original coloration of Columba livia occurs in
all the principal breeds of pigeons, although these blue sub-
varieties are rare in some of them. The other racial charac-
teristics do not at any rate exclude the possibility of a blue
coloration ; and thus, on the other hand, reversion to the blue
colour is not necessarily accompanied by a reversion to all the
other characters of the ancestral form.

It is perfectly certain that in most cases the reversion pro-
duced by cross-breeding is not complete, even as regards the
coloration, but gradually becomes more marked, so that at
first very faint and hardly perceptible indications of the wing-
and tail-bars are seen, and these become more pronounced by
degrees, so that a partial blue coloration with perfect bars, and
finally the perfect slate-colour and complete bars of the ances-
tral form are produced. The greater number of reversions in
pigeons must consequently be inco))iplete, — i.e., they must refer
to individual characters or groups of characters only, and we
are here concerned with the theoretical explanation of such cases
as these.

I take for granted the facts that all valuable races of pigeons
breed true, that all the main breeds are characterised by diflfer-
ences in fornix and that sub-breeds differ merely as regards
colour. In my opinion this implies, in the first place, that the
germ-plasm of the main breeds has become essentially modified
from that of the rock-pigeon, and that only lesser portions of it
correspond to that of the ancestral form ; and also, that all the
deterviinaiits have not become modified to an equal extent, — those
for the coloration having undergone the least, and those for
the whole body the most, alteration. I therefore suppose that
the germ-plasm of one of the main breeds consists of a number
of modified idants, none of which any longer correspond exactly

THE PHENOMENA OF REVERSION • 325

to those of the ancestral form, so that no one of them, did it
control the ontogeny, could resuh in the development of a
rock-pigeon. This conclusion depends entirely upon the race
breeding true, for a germ-plasm which still contained individual
unmodified idants of the ancestral form would also necessarily,
or at any rate probably, once in a way, contain a majority of
ancestral idants at the same time, in consequence of the incessant
recombination of the idants in the reducing division and in fer-
tilisation ; and this must result in reversion to the ancestral form.
Such a reversion, however, never occurs in purely bred races,
but only when crossing takes place.

The germ-plasm of a race of pigeons therefore consists,
according to my view, of a certain number of idants, each of
which represents the type of the race. The majority of the ids
of which each idant is composed must consequently virtually
contain this type ; or, expressed more accurately, the whole of
the racial determmants — as compared with those unmodified
determinants which may still be present — are in the 7najority in
all the ids of every idant.

The fact that races breed true can in this way be thoroughly
explained.

As regards reversion to the coloration and markings of the
wild pigeon, we must suppose that, in the process of artificial
selection to which the different races owe their origin, only just
as many ids have become completely transformed into racial ids
as were required to ensure the desired object, viz., the preserva-
tion of the racial characters. A larger or smaller number of
determinants in many or perhaps all the ids must have remained
unmodified in all, or at any rate in many, of the idants. The
determinants on which the coloration depends, must have
remained unmodified in larger numbers than did those relating
to any other characters, for the coloration is the most liable
to revert.

Reversion in the coloration will therefore occur when the
ancestral determinants for any particular region of the bird's
plumage gain a predominance over the racial determinants in
the course of development ; and in cross-breeds it will take
place when the racial determinants are so differetit that their
forces counteract one another instead of being cumulative.
Although the ancestral determinants are in the minority in the
grerm-cells of the two different breeds which unite at fertilisation.

326 THE GERM-PLASM

their controlling forces will nevertheless be cumulative, and if
they are sufficiently numerous, they will determine the colora-
tion in question, and thus produce reversion.

In this way we may, I think, account not only for the phenom-
enon of reversion in general, but also for many special details,
and more especially for the different degrees of reversion in
different races of pigeons . A sufficient number of experiments
has certainly not been made with regard to this point, but it is
nevertheless recognised that reversion occurs more easily and
more markedly in some races than in others. Darwin, for
instance, by crossing two black barbs with tw^o red spots
obtained dark hybrids, of which ' no less than six presented
double-wing bars," * On the other hand, the mongrels derived
from two black barbs and two snow-white fantails showed no
trace of reversion. This must have been due to the retention
of a different number of unmodified specific determinants in
the germ-plasm of the two races, as well as to the diiference of
the modified racial determinants ; for the more marked the
difference between the two crossed races, the more easily will
the ancestral determinants gain the predominance over them.

The last-named experiment by Uarwin was continued by
pairing two of these hybrids, one of which was brown and the
other black. The first brood (it is not stated how many there
were) displayed wing-bars ' of a darker brown than the rest of
the body."" This must have been due to the accumulation of a
larger number of unmodified determinants in individual germ-
cells in consequence of the reducing division, and to the subse-
quent union of two of these cells in the process of fertilisation.
We should therefore expect that reversion would not occur in
all the offspring of this pair, for the reducing division must also
result in certain germ-cells containing a majority of the modified
determinants. In the second brood of the same parents, in
fact, a brown bird was produced which possessed no trace of
wing-bars.

It is easy on the basis of our theory to account for the fact
that a simple crossing of two species did not in many instances
produce any traces of reversion, although reversion resulted
from the subsequent double crossing. The most complete case
of reversion obtained by Darwin was produced as follows. A

*Danvin, I.e., \^ol. I., p. 208.

THE PHENOMENA OF REVERSION 327

mongrel female barb-fantail and a mongrel male barb-spot were
paired. Neither of these birds 'had the least blue about them,
nevertheless the offspring ' (the number is not stated) ' from
the above two mongrels was of exactly the same blue tint as
that of the wild rock-pigeon from the Shetland Islands over
the whole back and wings ; the double black wing-bars were
equally conspicuous.' * In this case the ancestral determinants
evidently prevailed all the more strongly because they were
opposed by racial determinants of three or four different kinds,
the controlling forces of which could not simply be cumulative,
like those of the ancestral determinants, but could only partially
weaken and neutralise one another.

The fact which may be deduced from Darwin's observations,
viz. that the offspring of simple crosses display practically a
very similar tendency to reversion, can also easily be explained
theoretically. For the germ-plasm of a well-established race
will contain a certain percentage of ancestral determinants, and
the o^erm-cells of an individual will be liable to but few fluct-
uations in this respect. In such simple cases of crossing, an
almost similar number of ancestral determinants, as well as of
racial determinants, must therefore come together at each fer-
tilisation ; and the struggle between these different kinds of
determinants must always produce approximately the same
result. A perfect uniformity in the offspring of the same cross
cannot be expected ; but if reversion does occur in some cases,
it will be absent in others, and if it occur partially in some,
certain parts in others will revert. An instance of the former
kind is seen in the above-mentioned cross between a black barb
and a white fantail, and of the latter in the case described by Dar-
win on p. 207 of his 'Animals and Plants under Domestication/
Vol. I. He crossed a white nun with a red tumbler, and reared
five young, all of which presented traces of reversion. One
possessed a blue tail ; the second and third ' presented a trace of
the bar at the end ' of the blue tail : the fourtli • was brownish,
and the wings showed a trace of the double bar" ; and the fifth
' was pale blue over the whole breast, back, croup, and tail, but
the neck and primary wing-feathers were reddish, and the wing-
bars presented two distinct bars of a red colour.' Thus all the
five young had reverted — some more and some less markedly —

* Loc. cit., Vol. I., p. 209.

328 THE GERM-PLASM

in one part or another. The differences between the young
support the theoretical assumption that the number of ancestral
determinants, which determined the type of the five young ones,
was by no means the same in the different germ-cells of the
two parents. Their dissimilarity, moreover, admits of a further
conclusion, which could likewise be foreseen theoretically, viz.,
tJiat the deterDiinaiits for the coloration of the different regions of
the body are represented in the different germ-cells in fluctuating
numbers. Not only should we naturally suppose this to be the
case from the structure of the germ-plasm as here assumed, but
we can hardly even imagine it to be otherwise. If the various
idants in the germ-plasm of a hybrid together contain twenty
ids with ancestral determinants, it does not follow that the latter
only are present in each id. There is no reason why in any one
id the determinants for the colours of the wings, for instance,
should not be ancestral, while those for the colours of the tail
are racial, or vice versa. The bird would display reversion in
the tail or wings, according to whether the former or the latter
kind of determinants is in the majority in the germ-cell : for
our theory shows that the struggle of the determinants takes
place independently in every cell, and consequently also in
every part of the organism ; and the result will therefore be
independent in each part, and will depend solely on the com-
bination of homologous determinants which contend with each
other in the cell or group of cells under consideration. It can
thus easily be explained how it comes about that though all the
offspring of a certain cross show reversions, the latter are never-
theless combined in very different ways with the racial characters
of the parents.

We must now consider the third type of reversion mentioned
at the beginning of this section, viz., that which concerns in-
dividual characters of very remote ancestors., and is seen, for
instance, in the stripes on the legs of horses and asses, and
more especially of mules.

It will not be necessary to repeat the arguments which Darwin
used to show that the horse was primitively dun-coloured and
striped. Indications of such a coloration are seen at the present
day in horses of different colours in all parts of the world, in the
form of a dark stripe down the back, and of transverse markings
on the legs and shoulders ; such markings, however, are not
common, and most frequently occur in dun-coloured horses.

THE PHENOMENA OF REVERSION 329

The ass has been proved to be descended from a species pos-
sessing stripes on the legs, and corresponding to the existing
wild species {Ashius tcvniopits^ of Abyssinia. The domesticated
ass has, as a mle, only retained the crossed stripe on the back
and shoulders, but occasionally transverse markings occur on
the legs, and this is most often the case in animals of a pure
grey ground colour. The stripes on the legs are very rare both
in horses and asses, as I know from personal observation, and
even when present they are very faint.

In mules these stripes occur much oftener and much more
distinctly — particularly in light grey animals, and are some-
times seen on the hind-legs as well as on the fore-legs and
shoulders. Thus reversion occurs to the ancestral form of both
sides.

The theoretical explanation of this fact must be based on
the assumption that a certain number of unmodified ancestral
determinants for the hairy coat have been retained in the germ-
cells of the two species ; and that these, when they are brought
together in the germ-cells of the two parents, may predominate
over the modified determinants of the parents. The fact that
such reversion does not by any means always occur in mules,
points to the conclusion that the number of ancestral deter-
minants varies very considerably in the germ-cells of individuals,
and that these may even be entirely absent, or only sparsely
represented, or, on the other hand, may be present in large
numbers. In the latter case, when two of these germ-cells come
together in fertilisation, partial reversion will occur, and will be
more marked and extensive the greater the number both of the
ancestral determinants which meet together and the parts of
the body to which these belong. The determinants for the
stripes on the hind-legs are evidently much less numerous in
the combined germ-plasm of the two existing species than are
those for the marks on the fore-legs as is shown by the manner
in which these stripes have been relatively retained by several
existing wild species of Equus. All these determinants must,
however, be present in very varying numbers in the germ-plasms
of different individual horses and asses, for, as already stated,
reversion does not by any means always occur in mules ; in Italy,
where these animals are used in large numbers, it is not common,
and, as far as my experience extends, is perhaps only exhibited
in one or two animals out of a hundred. Gosse, however, states

S3^ THE GERM-PLASM

that in the United States of America nine mules out of every
ten are striped.

Before concluding this section, I will analyse a case of
reversion to remote ancestors in plants in greater detail. Mv
reason for so doing is not because different principles are
necessary for its explanation, but because we possess the results
of experiments which render a closer examination of the theory
possible.

I will select as an example the case of the reversion of
irregular or unsy7n}jietrical flowers to a regular or peloric form.
Many instances of reversion of this kind have been described,
but they are very exceptional : their origin cannot be connected
with any external causes, and must evidently be due to purely
internal ones, viz., to the composition of the germ-plasm.

If my opinion concerning the transformation of the germ-
plasm in the course of phylogeny is correct, it follows, as has
already been shown, that individual unmodified determinants of
an old character must always appear here and there in the
germ-plasms of the modern species, even after an enormous
number of generations. Such ancestral determinants — those
of the original regular flowers, for instance — need not by any
means be contained in the germ-plasm of every individual
plant ; and the older the modern species is, the fewer will be
the number of these determinants, wiiich we will call ' peloric.'
They must gradually be displaced by the ' asymmetrical ' deter-
minants ; for the latter, being better adapted to the existing
conditions, have a better chance in the struggle for existence.
A large number of plants, such as Corydalis tuber osa, for
instance, will thus no longer contain any ' peloric ' determinants,
and this makes it apparent why reversion so seldom occurs in
these cases. The fact that it can occur at all, is to be accounted
for by the processes of reducing division and amphimixis in two
germ-cells, the latter of which always follows on after the
former. For if a small number of ' peloric ' determinants still
remained in various idants of individual plants of the species in
question, they might occasionally come together in one germ-
cell owing to the reducing division ; and if two such germ-cells
meet one another in the process of fertilisation, this group of
determinants may predominate, and reversion will then occur
— provided that the combined power of these ' peloric " deter-

THE PHENOMENA OF REVERSION 33 1

minants is sufificient to predominate over the ' asymmetrical '
determinants.

The experiments which have been made with respect to the
transmission of characters in such abnormally peloric flowers,
prove that this explanation must be correct in principle. Dar-
win crossed the peloric snapdragon (^Antirrhimnn viajus)
with its own pollen, and from the seeds thus obtained raised
sixteen plants, which ' were all as perfectly peloric as the parent
plant.'* We need not be surprised that the pelorism was
inherited, for the 'peloric' determinants were in the majority in
the parental germ-plasm on either side ; it may, however, have
been due to chance that all the sixteen plants which were reared
proved to be peloric. If a larger number of offspring had
been raised, some of them would certainly have produced
asymmetrical flowers, for the reducing division would in most
cases divide the ' peloric ' determinants unequally amongst the
two resulting cells, and consequently two germ-cells containing
no, or only a minority of, 'peloric' determinants might meet
together in fertilisation. Reversion to the ordinary form of the
flower must then occur.

The result of Darwin's counter-experiment is particularly
interesting. The peloric snapdragon was crossed with the
common form, and ' two great beds of seedlings ' f raised, ' ?iot
one of which was peloric." And in ninety plants which were
carefully examined there was not a trace of pelorism, 'except
that in a few instances the minute rudiment of the fifth stamen,
which is always present, was more fully or even completely
developed.' Darwin attempted to explain this fact by the
assumption that in this case the common form of the flower
possessed a ' prepotent force of transmission ; ' but apart from
the fact that this statement is merely another formulation of an
observed fact, and can hardly be looked upon as a real expla-
nation, it does not hold in the case of Darwin's subsequent
experiments. For the plants obtained by crossing the common
snapdragon with the peloric form, ^ which perfectly resembled
the common snapdragon, were allowed to sow themselves ; and
out of a hundred and twenty-seven seedlings, eighty-eight
proved to be common snapdragons, two were in an intermediate

* 'Animals and Plants,' &c., Vol. II., i888, p. 46.
t Loc. cit.. Vol. II., 1888, p. 46.

332 THE GERM-PLASM

condition between the peloric and normal state, and thirty-seven
were perfectly peloric/ * If now, the character of asymmetry
possessed a ' prepotent force of transmission,' we should expect
it to become more apparent when the flowers of both parents
were asymmetrical, than when this was the case as regards one
parent only. In the chapter of the same book which treats of
pangenesis, Darwin has attempted to give a special explanation
of the fact ' that a character gains strength by the intermission
of a generation' in which this character is not present.

My own explanation of the above-mentioned facts follows
almost directly from what has already been said. The pro-
duction of so many individuals of the ordinary kind by the
crossing of the common snapdragon with the peloric form, and
the fact that a large number of 'perfectly peloric' offspring,
as well as of the common form, were produced when those
obtained by the first cross were fertilised with their own pollen,
is due simply to the reducing division of the germ-mother-cells
occurring in different ways, so that sometimes only 'ordinary'
and sometimes only ' peloric ' determinants reach the germ-
cell, and sometimes, again, a combination of both, in which
either the 'peloric' or the 'ordinary' determinants preponderate.
The daughter-plant will produce flowers which are either quite of
the common form or more or less peloric, according to whether
the determinants which are brought together by the two germ-
cells in the process of fertilisation are chiefly ' peloric' or chiefly
of the 'ordinary* kind. It is evident, however, that most of the
parents of this generation, which possessed flowers of the ordi-
nary form, must have contained ' peloric ' as well as ' ordinary '
determinants in their germ-plasm before the reducing division,
for they were all derived from a peloric father or mother. The
predominance of the ordinary form in the next generation may
be explained as being due to the peloric grandparents possess-
ing only a slight majority of 'peloric' determinants in their
germ-plasm, in addition to a considerable number of 'ordinary'
determinants. The whole of the germ-plasm of the germ-cells
from which the parent generation arose must have contained
very many more 'ordinary' than 'peloric' determinants.

* Loc. cit, Vol. II., p. 46.

THE PHENOMENA OF REVERSION ^;^;^

4. Reversion to Rudintentary Characters.

It is well known that organs which have lost their value for
the preservation of the species become rudimentary in the course
of generations : they diminish in size, become stunted, and ulti-
mately disappear altogether.

This may be expressed in terms of the idioplasm as follows.
The group of determinants in the germ-plasm for the organ in
question becomes reduced in one id after the other, first in one
determinant and then in another, until finally it disappears com-
pletely ; and the process is repeated in an increasing number of
ids, until eventually this group of determinants is no longer
contained in any of them. It cannot be stated how long a time
and how many generations are required for this process, but it
may at any rate be asserted, and even proved, that individual ids
still contain determinants for the organ in question long after its
disappearance from the mature individual. The fact that the
organ occasionally reappears, and that consequently reversion
may take place^ proves that this is the case.

The siipeyjiiLnierary nipples which occasionally occnr in human
beings are an interesting example of this kind. The two normal
nipples generally occur in a rudimentary form in men, but, in
addition to these, very diminutive ones are occasionally met with
in parts in which they are normally present only in the lower
orders of mammals, such as carnivores, rodents, and lemurs : —
a pair may be present above the normal teats near the axillary
region, and two or three others lower dow^n on the abdomen.
All of them certainly never occur in the same individual, but
usually only a single one or a pair are present : these, however,
are found both in men and women. They are undoubtedly to
be looked upon as reversions to extremely remote characters
possessed by our lower mammalian forefathers. We owe an
accurate account of their occurrence in the male sex to the
numerous and detailed investigations of Otto Ammon,* who
found them in three per cent, of our recruits.

As Amnion's researches only extend over two, or at most
three generations, and only refer to individual cases, we cannot
form a precise estimate as to the degree and extent to which
these structures are transmitted. We may, however, attribute the

* I am indebted to Mr. Ammon for these details, which are not yet
published.

334 THE GERM-PLASM

sti)nulus to reversion in these instances to interbreedins: : that
is to say, to the amphimixis of such germ-cells as contain a
certain number of ancestral determinants in their germ-plasm
on either side for those regions of the skin in which the nipples
were situated in remote ancestors. Merely in consequence of
the reducing division, these determmants may possibly accumu-
late in one germ-cell in sufficient numbers to produce the
character in question. The same may be said with regard to
many small individual marks, and such very ancient hereditary
parts as supernumerary nipples usually follow the same rule as
do individual characters ; they have to a certain extent degener-
ated so as to come under the same category, and for a long time
past have not been contained in the germ-plasm of every indi-
vidual ; their determinants, on the contrary, are entirely absent
in most cases, and are only found in a certain number of ids in
certain individuals. These determinants, like those of individual
characters, may be transmitted for several generations without
attaining development, and may then suddenly become manifest
in consequence of a favorable combination of two conjugating
germ-cells. The only difference between these ancestral charac-
ters and ordinary individual peculiarities is, that the determi-
nants of the latter are contained in a larger number of ids, and
we must therefore conclude that they become developed far
more frequently and regularly.

Although in the case of the supernumerary nipples in the
human race we cannot definitely indicate the ancestor from which
they were derived, or the length of time during which they have
been transmitted, it is at any rate possible to do so approxi-
mately in the case of the superniDnerary toes of horses. For,
thanks to the excellent researches of Kowalewsky, followed by
those of Marsh, we are well acquainted with the phyletic devel-
opment of the horse : we now know that horses belonging to
the genera Mesohippus, MioJiippiis, and Protohippus or Hip-
parion, which possessed two smaller lateral toes in addition to
the large median ones, existed in the middle Tertiary period.
When horses are occasionally born at the present day in which
one or two such accessory toes are present on two or even
on all four feet, we are perfectly right in considering the devel-
opment of these toes to be due to reversion to an ancestor of
the Miocene period. We must therefore assume that in certain
series of generations of the existing horse, some idants have

THE PHENOMENA OF REVERSION 335

been retained in which ancestral determinants of the fore and
hind-feet are present, but that these are in the minority, and a
large number of them can only accumulate in one germ-cell
when a particularly favourable reducing division occurs. This,
however, would not in itself be sufficient to produce the character
in question : chance must also play a part, in order that such an
egg-cell, containing an abundance of ancestral determinants,
may be fertilised by a spermatozoon in which a certain number
of these determinants are also contained. Then, and then only,
will there be a likelihood that the entire number of the latter
will be sufficiently large to preponderate over the modern deter-
minants of the foot in the process of ontogeny.

Cases of reversion of this kind occur very rarely. Marsh has,
however, brought forward a small series of such instances, the
oldest of which relates to a horse belonging to Julius Caesar, and
the most recent w-as observed by him in a living animal.*

5. Preliminary Siiuwiary of Sections 1-5.

All the phenomena of reversion which have so far been
considered can be explained on the supposition that every
germ-plasm is composed of a large number of equivalent units
or ids, each of which possesses all the determinants required for
the development of an organism ; every character is therefore
produced by the co-operation of many determinants of the
same region (homologous determinants). The transformation
of a species or race into a new one, moreover, never depends
primarily on a simultaneous modification of all the ids and
determinants : when the modification begins to take place, even
entire groups of ids (idants) may remain unmodified, and this
will subsequently be the case as regards a minority of ids, and
still later at least with regard to certain of the determinants in
individual ids. The characteristic form of every individual cell
which takes part in the process of ontogeny is the result of the
struggle of the ids which occurs in this cell ; and as some
amongst the mass of determinants which constitute every id in
the germ-plasm may become modified and others not, and the
proportion of modified to unmodified determinants may varv
from id to id, we can understand that in crosses between species

* O. C. Marsh, ' Recent Polydactyle Horses,' American Journal of
Science, Vol. xliii., April 1892.

336 THE GERM -PL-ASM

or races, reversion is sometimes more and sometimes less marked
— in some cases restricted and in others very extensive. In
those cases, therefore, /;/ which rudi)nentary and insigtiijicant
characters only are concerned, such as stripes in nudes ^ pelorism
in flowers, and rudimentary nipples in the Jiufuan race, indi-
vidual variations must frequently occur ifi all degrees, from the
complete absence of reversion to the most ?narked form of it.

Conversely, it is just as easy to explain why reversion occurs
without exception in other cases, such as in that of Datura-
hybrids, if we assume that entire groups of ids or idants, as well
as certain determinants in individual ids, have remained unmod-
ified. For if the former are present in such numbers as to pre-
ponderate in a hybrid over the two kinds of modified ids, which
counteract one another, they must be contained in nearly equal
numbers in the germ-plasm of every individual.

We can in principle understand the whole series of the
phenomena of reversion, if we represent the transformation of
the germ-plasm as being due to the majority of the ids in most
of the idants becoming modified first, while a minority of them
remain unmodified : in the course of generations the number of
these unmodified ids then gradually becomes smaller, owing to
the action of natural selection, until the ancestral species is only
represented bv a number of scattered ids ; and finally, by con-
tinued selection, these ids also become modified in the same
direction, to such an extent that eventually only the determinants
of those individual characters escape modification which are less
important or quite meaningless for the life of the organism. We
know that complete reversion to the ancestral form may occur
in young species in the case of favourable crosses between
allied species {Datura^ ; and that in the further course of
phvlogeny, that is, in older species, total reversions can no
longer occur, although reversion to single characters, or even to
entire groups of characters, may still take place, and does so
with certainty under certain conditions — as is exempHfied by the
reversion of certain hybrids between different races of pigeons.
In its last stage, then, reversion occurs as an entirely uncertain
and apparently capricious re-appearance of an individual an-
cestral character, such as, for instance, that of the occasional
striping in mules.

By means of our theory much may also be deduced and ren-
dered comprehensible in principle concerning the external causes

THE PHENOMENA OF REVERSION 337

of reversion. It can easily be understood that the crossing of
different species and races is liable to lead to a preponderance of
those ancestral idants, ids, or determinants which are common
to the two parents. We might, however, also infer from the
theory that certain of the offspring of hybrids are very liable to
revert in the next generation to one of the ancestral species ;
for the germ-cells of the hybrids are very dissimilar as regards
the germ-plasm they contain, in consequence of the reducing
division which every germ-cell undergoes at its formation : most
of them will contain idants of both parents in every conceivable
proportion, while in others, idants of only one of the parents
will be present. The " reducing division ^ is tJierefore one of the
most efficacious of the primary causes of reversion^ for it renders
possible the uneven distribution of the different qualities of the
idants which were contained in the germ-plasm of the germ-cells
of this parent. This principle applies equally to the cases in
which not a single entire idant remains untransformed in the
germ-plasm, which then only contains a minority of unmodified
ids or even merelv of determinants, scattered amongst several
idants.

Reversion in any degree therefore depends^ in the first instance^
on the process of amphimixis^ for without sexual reproduction the
reducing division would not have been introduced into the organic
world, and the second extremely important factor in reversion
— viz., the crossing of different germ-cells — would also not exist.
Reversion is, however, not of necessity connected with actual
amphimixis, but may, as I shall show later on, occur in con-
nection with parthenogenesis and gemmation ; although this is
only true in the case of those organisms in wOiich amphimixis
formerly occurred, and the germ-plasm of which may therefore
contain ancestral ids or ancestral determinants. It is evident,
however, that the chance of reversion occurring must be much
greater when amphimixis takes part directly in the process of
reproduction ; for the relative proportion of modified and un-
modified units of the idioplasm of this order may be quickly
or suddenly altered in favour of the unmodified units, and thus
the ancestral units which were originally present may undergo
accumulation, and predominate over the somewhat dissimilar
homologous units of the modified kind.

Our comprehension of the problem just discussed is certainly
increased by the above considerations, and the following passage

;^^S THE GERM-PLASM

from Darwin's * Animals and Plants under Domestication ' * will
best show how much our insight has since then advanced : —
• In purely-bred races, occasional reversion to long lost characters
of the ancestors often occurs without our being able to assign
any proximate cause.'

6. Reversion i\ Asexual Reproduction
(a) Reversion in the Process of Gemmation

As already stated, reversion does not entirely depend on am-
phimixis, and may in fact, also occur apart from the crossing
of two individuals. The bud-variations of plants form a well
known instance of this kind.

For many years I possessed in my garden a maple {Acer
nee^nndo) with variegated leaves Avhich were almost entirely
white, and one branch of this tree bore ordinarv green leaves,
flowers, and seeds. Owing to the greater amount of chlorophyll,
this branch grew and bore flowers and seeds far more luxuriantly
than the main branch from which it arose. If we look upon the
oftshoots of a tree as persons, this would be an instance of a
person of the plant-stock produced asexually. which reverted
to the ancestral form.

We must go back to the origin of variegated species in
order to find a theoretical explanation of this phenomenon.
Like most similar varieties, indeed, of our trees and shrubs,
this form must have arisen by bud-variation ; in other words,
a normal maple, from certain unknown causes, gave rise to a
branch bearing variegated leaves. The cause of this modification,
traced to the idioplasm, must have been due to the determinants
of the leaves and other green parts of the shoot becoming
modified in such a way as to result in the production of organs
deficient in chlorophyll. If. however, only the majority, and not
all the ids in the apical cells of the first variegated shoot became
modified in this manner, a reversion of the variegated variety
to the green ancestral species would become possible.

Another assumption, however, which cannot yet be proved to
be tme. is required in order to account for the appearance of a
green branch upon a variegated tree : we must suppose that
even in ordinary cell and nuclear divisions, the division of the
idioplasm may take place in an irregular manner, so that all the

* Vol. IL, p. 25.

THE PHENOMENA OF REVERSION 339

ids do not reach both the daughter-nuclei ; but that both the
halves of some of them w hich result from the division pass into
the same daughter-nucleus. On the other hand, it might even
perhaps be possible for an entire idant to be transmitted to only
one of the daughter-nuclei. It must be possible for some such
irregularity to occur in the distribution of ids during nuclear
division, for otherwise the occurrence of a different combination
of the primary constituents in the course of growth — such as,
indeed, actually occurs in cases of bud-reversion — would be
inexplicable.

Let us distinguish between those ids which correspond to
the original form, and those which have become modified, by
describing them respectively as 'green' and 'variegated' ids.
The reversion of a shoot must then be due to the unequal
distribution of the ids amongst the daughter-nuclei during the
divisions of the apical cells of the shoot, so that a majority of
'green' ids were distributed to a cambium cell containing
' blastogenic ' germ-plasm, or even also to an apical cell of
a young lateral bud. Conversely, a variegated shoot might
subsequently originate from a green one, as in fact actually
occurs.

I should be inclined to offer a similar explanation of the re-
versions to the ancestral form which so frequently occur in all
the numerous varieties of our trees and shrubs, such as, for
example, the oak-leaved heterophyllous hornbeam, the fern-
leaved oak, the varieties of the maple and birch which possess
greatly subdivided leaves, and the copper-beech and copper-
hazel. The tendency to revert is very varied even in the
different varieties of the same species. The golden-striped
variety of Eiionynms japonica, for instance, is very liable to
revert, while in the silver-striped variety of this plant reversion
rarely occurs.

This difference would simply depend on the relative minority
of ancestral ids in the variety in question. Reversion will never,
or only rarely, occur when the idioplasm only contains a few
ancestral ids ; if, however, there are so many of these that the
unmodified ids only form a small majority, reversion can easily
take place.

In this connection I must mention another instance, which
has very frequently been discussed since Darwin's time, viz..
that of the peculiar ' graft-hybrid ' Cytisus adafni. The com-

340

THE GERM-PLASM

bination of the characters of the two ancestral species con-
tinually varies in this plant, which sometimes bears the yellow
clusters of flowers characteristic of the common laburnum
{CylisKS laburniun'), and sometimes purple flowers like those
of Cytisus purpurea^ or, again, both these colours may be
present in the same flower. I will here quote Darwin's descrip-
tion of the plant*: — 'To behold mingled on the same tree
tufts of dingy-red, bright yellow, and purple flowers, borne on
branches having widely different leaves and manner of growth,
is a surprising sight. The same raceme sometimes bears two
kinds of flowers, and I have seen a single flower exactly divided
into halves, one side being bright yellow and the other purple,
so that one half of the standard-petal was yellow and of larger
size, and the other half purple and smaller. In another flower
the whole corolla was bright yellow, but exactly half the calyx
was purple. In another, one of the dingy-red wing-petals had
a narrow bright yellow stripe on it ; and lastly, in another
flower, one of the stamens, which had become slightly foliaceous,
was half yellow and half purple.'

The result of the struggle of the parental idants evidently
cannot depend in the case of Cytisus adami, as it does in that
of the individual characters of the human race, upon the fact
that the number of homodynamous determinants varies in the
parental idioplasm according to the part concerned : for were
this the case the same parts of the flowtr could not sometimes
be yellow in some instances and red in others, — all the flowers,
on the contrary, would display the same composition as regards
the parental hereditary parts, even though slight variations might
occur, such as would be produced by a dissimilarity in the con-
ditions of nutrition. As in the case of the hybrids of Oxalis
already mentioned, the flowers would at least display a certain
combination of parental characteristics which would be uniform
in one and the same plant. The fact that this is not the case,
seems to me to aff"ord a decisive proof that Cytisus adami is a
real graft-hybrid^ and not an ordinary seminal hybrid^ as in
fact was stated to be the case by the nurseryman Adams, who
first produced it. I therefore consider the controversy ended
as to whether graft-hybrids exist at all, and offer the following
explanation of the hereditary phenomena concerned.

* Loc. cit., Vol. I., p. 414.

THE PHENOMENA OE REVERSION 34 1

Cytisus adami was obtained from a young bud on a portion
of the bark of Cytisiis purpurea, which was grafted into the
stem of C. laburnum. This bud developed into a shoot which
exhibited an intimate combination of the parental characters.
The shoot was afterwards propagated, and the plants raised
from it exhibited •• reversions to both of the parental forms." as
well as dingy-red, i.e.. mixed blossoms, so that the pure characters
of the parents were displayed in more or less extensive regions
of the hybrid.

From a theoretical point of view, it can obviously be granted
that a mongrel-plant may originate by contact of the living
tissues of the parents, only if the transformation of the rudiment
of an existing shoot is out of the question, and if the rudiment
then appears for the first time. An existing dormant bud, which
contains all parts of the shoot, cannot become modified as
regards the idioplasm which it contains by the stock of another
species which nourishes it : its apical cells, from which further
growth proceeds, cannot receive a supply of extraneous idioplasm
from the supporting stock ; for the nuclear rods alone contain
idioplasm, and this is a solid substance, and can only undergo
combination by the fusion of two cells and their nuclei. It is
therefore also worthy of notice that Adams did not observe that a
hybrid was developed from the single dormant bud which was from
the first present on the graft, but that the hybrid arose from one
of the later buds which were formed in the second year ; moreover,
only onCj and not all, of these buds produced bud-hybrids. The
formation of this single hybrid-bud must have been regulated bv
an unusual and accidental occurrence, for all efforts to produce the
hybrid a second time have so far been in vain. This accidental
occurrence must have been that the cambium-cells of the two
species came to lie close together, so that they could both enter
the same bud arising from the cambium. Botanists must
decide whether two cambium-cells, belonging to different species,
can conceivably become united into one by a process of con-
jugation similar to that which occurs in the union of the male
and female cells in fertilisation, and whether the foundation may
in this way be laid for a new growing point.* If such a process

* K fusion of nuclei, apart from that seen in the process of fertilisation,
actually occurs in plants in the case of the embryo-sac. Guignard {^loc.
cit.) describes this process in detail as follows: — The 'upper and lower
pole-nuclei ' of the embryo-sac approach one another, each accompanied by

342 THE GERM-PL.\SM

occurred in this case, the niDubcr of ida^its of Cytisiis adajni must
be as large as that of the two ancestral species taken together ;
for, as far as we know, a process of reducing division only
occurs in the formation of sexual cells. The correctness of my
?fssumption can therefore be controlled by observation.

It is hardly conceivable that two young plant-cells could,
without fusing, have formed the growing point of the hybrid-
shoot ; for probably only one of these cells could have performed
the function of an apical cell, and consequently tlie hereditary
influence of the other could not have extended to countless
daughter-shoots, as was actually the case. In the course of
growth, every cell below the apical cell must necessarily have
gradually come to be situated further away from the growing-
point. Such an intimate combination of characters as actually
occurred could not have been effected in this way.

I am therefore inclined to suppose that the unusual phe-
nomena exhibited by Cytisiis adanii were due to an abnormal
kind of amphimixis, so that the idants of both species were
combined in the apical cell of the first shoot ; but that in the
subsequent cell-divisions an unequal distribution of the two
kinds of parental idants amongst the daughter-nuclei took
place, thus producing the variations in the combination of the
characters.

Such an unequal distribution of the superabundant idants
might also occasionally occur in an apical cell itself. This proc-
ess may, moreover, be connected with the frequent complete
reversion of an entire branch to one of the ancestral species, as
well as with the fact that a modification tending to make
the parental characters more and more distinct has occurred in the
course of time in many examples of hybrids. Shortly after the
first appearance of the Cytisus-hyhndj the colour of all the flowers
was a dingy-red, — that is, an intimate mixture of the two ancestral
colours, yellow and purple ; but by degrees this mixture became
less perfect, until eventually pure yellow and pure purple flowers,
and even entire clusters of flowers and entire branches possessing
almost the pure character of one or other of the parent-species,

its centrosome, which has already become doubled by division. The
nuclei then come to be situated close together, the centrosomes uniting in
pairs, just as in the ordinary process of amphimixis; and finally the two
nuclei fuse together completely. A spindle of division is then formed, and
several divisions follow close after one another.

THE PHENOMENA OF REVERSION 343

were formed. It seems only possible to explain this circumstance
by supposing that the connection of the two idioplasms was
easily severed, and that differential nuclear divisions occurred
in such a manner as to cause a larger number of idants of
C. purpiu'CHs to pass into one daughter-nucleus, and a larger
number of those of C. laburmini into the other ; or, at any rate,
to cause the one or the other idant to pass completely into one
of the daughter-nuclei, instead of dividing longitudinally and
one of the resultant halves entering each daughter-nucleus.
Though this is certainly only a conjecture, it is, however, not
altogether an unjustifiable one, for the apparatus for division in
each of the two species is certainly concerned with a smaller
number of idants than that which must be present after the
fusion of two nuclei ; irregularity might therefore easily occur at
the division. It is possible that unknown forces of attraction
may also play a part in the process : the idioplasms of the two
species do not at any rate exhibit a marked attraction to one
another, as might perhaps be conjectured from the negative
results obtained by Darwin, Reisseck, and Caspary in ordinary
experiments on hybridising. By fertilising C. labiirmiin with
the pollen of C. purpiireiis^ Darwin obtained pods which dropped
off ' in sixteen davs after the withering of the flowers,'' and the
reverse cross resulted even less successfully.

However this may be, — and the point can be settled by deter-
mining the number of idants, — the phenomena of heredity at
any rate indicate that the idioplasms of the two parents . can
easily again become separated in the course of cell-divisions.
This separation might perhaps begin with the passing over of
one idant only from one side, which would result in the pre-
ponderance of one parent in many of the blossoms, &c. This
preponderance would have increased considerably in the course
of growth, so that far larger groups of cells now frequently con-
tain pure idioplasm of C. laburtinin or C. pnrpureus, and new
shoots are formed which apparently contain idioplasm of one
or the other species only. The fact that the plants raised by
Herbert from seeds of pure yellow flowers oiCytisiis adanii. which,
although they bore yellow flowers, showed a purple tinge on the
flower stalks, proves that even these shoots may still contain
some individual idants of the other ancestral species. The
idants of C. pm-pureus seem, however, to have disappeared
entirely from certain shoots ; for Darwin raised plants from

344 THE GERM-PLASM

the seeds of yellow flowers, which resembled C. laburnum 'in
every character, with the exception that some of them had
remarkably long racemes.'

My explanation of the fluctuation in the combination of
parental characters in Cytisiis adanii rests on the fact that we
are not here concerned with the subtle differences in the forces
which, in the reproduction of the human race, cause either the
maternal or the paternal idants to predominate, and which we
have attributed to the number of homodynamous determinants,
which differs according to the characters concerned : this fluct-
uation depends on the grosser differences, and on the number
of idants on the two sides. Sometimes the idants of C. pur-
piD'eus^ and at others those of C. labienut^n, predominate in the
idioplasm of a cell ; and, under certain circumstances, even only
the o)ie kind of idant may be represented in a cell, in which
case it alone will also be present in all the offspring of this cell.

My theory is not therefore rendered less likely by the case of
Cytisus adami ; we can, on the contrary, even explain the most
minute details of such phenomena by its means, although it was
not formulated for this purpose.

6. Reversion in Parthenogenesis.

Reproduction by means of unfertilised ova occurs regularly in
some plants and in many animals — more especially in crustaceans
and insects. We might be inclined to expect, a priori, that no
great degree of deviation between the mother and daughter
could occur at all in this monogonic form of reproduction, and
that at most a reversion to more remote ancestors could take
place.

Such a supposition is not, however, borne out by experience.
A series of experiments with parthenogenetic species, which I
have made during the course of the last eight years, have shown
that although the expected uniformity between parent and ofT-
spring generally results in a very liigli degree, exceptions occur
occasionally, and that these must be regarded as exhibiting
reversions to an ancestral form many generations removed.

The facts are briefly as follows. Two varieties of a small
Ostracod {Cypris repfans), possessing a very marked coloration,
occur in certain ponds in the neighbourhood of Freiburg. The
colour of one variety A, is light yellow ochre, five green spots
being present on either side of the shell : the other variety B,

THE PHENOMKNA OF RF^VERSIOI^

345

appears dark green, owing to the yellow-ochre ground colour
being reduced in extent by the presence of six large green
patches. These patches exactly correspond as regards position
in the two varieties, and are merely much smaller in A than in
B, the sixth spot being wanting in A. Both varieties are repro-
duced parthenogenetically in the neighbourhood of Freiburg, and
males never appear.

Fig. 23. — The two varieties of Cv/>r/s reptans. 1-6, indicate the six main patches

of colour on the shell.

My experiments were made by isolating one female of each
variety in a small aquarium, feeding them well, and allowing
them to multiply until each vessel was filled with their mature
descendants, which in their turn produced eggs. The individuals
of the colony were then examined, and the greater number killed
and preserved, one or more of them being kept alive and placed
separately in fresh aquaria for breeding purposes.

These animals breed very rapidly throughout the year, and
thus in the course of the last eight years many thousands of
individuals have passed through my hands.

The first result obtained by these experiments confirmed

346 THE GERM-PL-\SM

my expectations : the descendants of the same mother resembled
one another as well as the parent with which the experiments
began J even as regards minute details of the markings. The
differences were mostly as small as those which may be observed
in identical human twins : it cannot be stated whether these
were due to a want of similarity in the germ-plasm, or whether
they were to be accounted for by accidental differences in
nutrition.

Apart from the exceptions about to be mentioned, no modifi-
cations occurred even in the course of many generations. I
now possess colonies of A and B which cannot be distinguished
from their ancestors in the year 1884. Reckoning that from
five to six generations were produced each year, about forty
generations have been passed through since then.

In 1887, some individuals of the dark-green variety B appeared
for the first time in an aquarium containing the typical yellow-
ochre-coloured variety A ; and since then I have twice observed
a similar occurrence in other broods of A. In the last of these
cases, examined in May 1891, it could be proved that only a
single Cypris out of 540 adults contained in one aquarium had
changed into the dark variety suddenly, and without apparent
cause. In another case, intermediate forms between the two
varieties were found, as well as ordinary individuals of the
variety B ; and this fact is not only of interest theoretically,
but it also removes all doubt as to the trustworthiness of the
experiments.

For a long: time I waited in vain for the occurrence of the
reverse transformation from the dark variety to the light-coloured
one, and was inclined to consider that the former was the ances-
tral form. But in the winter of 1890-91, a colony of B appeared
in which a few typical individuals of the variety A were found,
together with typical specimens of B which had been bred in this
aquarium for many years.

It is out of the question that these sudden transformations
were due to external influences, for both forms made their
appearance together in the same small aquarium, under precisely
the same conditions. We can only. suppose that modijicatiojis
in the composition of the germ-plasm must have taken place,
and I think it is possible to prove that this was the case.

Parthenogenesis, using this term in the strict sense, has in all
cases been derived from sexual reproduction, as is proved in

THE PHENOMENA OF REVERSION 347

this case by the fact that the unmatecl females retain the recep-
taculum seminis, which is unused and always empty.

The two varieties must have originated at a time when sexual
reproduction occurred — at any rate periodically : were this not
so, primary constituents of A could not be present in the germ-
plasm of B, and vice versa. The co-existence of both kinds of
primary constituents in the same animal, can only be understood
if we suppose that sexual reproduction had occurred at no very
distant period.

The explanation of the process of reversion naturally follows
from the fact that /// species in which parthenogenesis regularly
occurs., a reducing division nevertheless takes place, but only a
single one : one polar body is separated off from the ^g%, and
not two, as in sexual reproduction. This single halving of the
idants in the ovum must undoubtedly be preceded by a doubling,
just as occurs in sexual reproduction ; for a reduction of the
number of idants to one half would otherwise take place from
generation to generation, so that ultimately only a single one
would be left. If, however, a reducing division preceded by a
doubling of the number of idants takes place, reversion becomes
possible.

Let us take a case of the simplest possible kind, and suppose
that there are four idants in the germ-plasm, three of which are
entirely composed of ids of the type A, and the fourth of ids of
the type B. The four idants a, a. a, b, first become doubled, so
that eight idants, a, a, a, a, a, a, b, b, result. Let us assume the
most favourable case for reversion towards the variety B to
occur, the reducing division resulting in the separation of these
idants into the groups a, a, a, a, and a, a, b, b, the latter of which
forms the nucleus of the egg-cell : the daughter individual arising
from this ^gg would then produce primary ova containing the
group of idants a, a, b, b. After the process of doubling in the
ripe ovum, this group would then have the composition a, a, a, a,
b, b, b, b ; and if the reducing division occurred in such a manner
as to result in the combination a, a, a, a, passing into the polar
body, and the combination b, b, b, b, remaining in the nucleus
of the ovum, an individual of the variety B would untloubtedly
arise from the egg, and reversion would ensue.

These processes would in reality be much less simple, and
would take place much more slowly. For the number of idants
is doubtless in most cases much larger, and the addition of a

348 THE GERM-PLASM

single idant of the other form would result in a much smaller
percentage. I have been able to prove the existence of 24-26
idants in the case of Arteniia salijia^ a crustacean which multi-
plies parthenogenetically. If even several of these belonging to
one of the two ancestral varieties were present in a minority,
it is questionable whether they would ever become accumulated
in one and the same germ-plasm in the course of generations
and the corresponding reducing divisions, so as to form a pre-
ponderating majority : such an accumulation is, however, con-
ceivable. This would partly depend on chance, for the majority
of the ova produced by an individual always perish, and rare
combinations which would produce reversions are therefore also
usually lost.

We may thus account for the fact that reversion only occurred
rarely in my experiments and only in certain individuals of a
colony at the same time, as well as for its appearing either
suddenly, or else after the production of intermediate forms.
The latter case is to be explained theoretically by supposing
that a balance of the two kinds of idants was first produced, and
that this then in the offspring partially led to the preponderance
of the idants on which reversion depended.

The possibility of reversion in parthenogenesis therefore de-
pends upon two factors : firstly, upon the composition of the
germ-plasm out of different kinds of ids and idants, — i.e., upon
the occurrence of sexual reproduction in a previous generation ;
and, secondly, upon the 'reducing division' which always takes
place at the formation of germ-cells.

7. Proof that the Deter miiiants becoine Disintegrated into

Biophors

I will conclude this chapter with some remarks which might
have been more suitably inserted in the chapter on the control
of the cell by the idioplasm^ but in that place they would have
been unintelligible, as they depend upon a knowledge of the
phenomena of maturation of the ova and spermatozoa.

The fact that actual germ -plasm is removed from the animal-
ovum by means of the cell-divisions occurring before maturation,
— that is, in the process of the separation of the two polar
bodies, — was not by any means easily determined, and was only
proved after at least ten years of prolonged and difficult investi-
gation and reasoning. I have not thought it advisable to give

THE PHENOMENA OF REVERSION 349

an account of the whole history of the development of our
knowledge of this subject,* as this is unnecessary for under-
standing the phenomena of heredity. I must, however, refer
to a period of this history, which is instructive as regards the
method by which the cell is controlled by the emigration of
nuclear matter into the cell-body as enunciated by de Vries and
accepted by myself.

After the so-called ' extrusion ' of the polar bodies of the egg
had been shown to correspond merely to a very unequal cell-
division, and convincing proof had been given that the controlling
substance must be situated in the chromatin of the nucleus, it
followed as a logical postulate that the ovum, like every other
cell, must be supposed to contain a special controlling substance,
or specific idioplas)n^ the function of which is to produce the spe-
cial histological nature of the cell in question. This conclusion
I arrived at, and assumed that the ovum, from its earliest stage
to the attainment of its full size and specific nature, is controlled
by a special idioplasm, which differs entirely from the idio-
plasm which becomes active after the completion of maturation.
If the nature of the cell is determined at all by its idioplasm,
the ovum, while still growing and undergoing histological de-
velopment, cannot possibly be controlled by the same idioplasm
as that which serves for embryonic development. I consequently
assumed the existence of an '■oogenetic^ idioplasm in the ^^'g
during the period of its histological differentiation, and also that
after maturation, this substance gives up the control of the cell
to the germ-plasm.

The question then arises as to what becomes of the oogenetic
idioplasm when this change in the control takes place.

My answer to this question was, that the oogenetic idioplasm
is removed from the ovum by means of the polar divisions, and
that it was thus rendered possible for the germ-plasm — which
was already present in the nucleus of the ovum, and had in the
meantime increased considerably in bulk — to obtain control of
the cell.

This conjecture has since turned out to be erroneous. Investi-
gations which I subsequently made soon showed that at least
one of the two polar divisions has a totally different significance.

* Such an account is given in my essay on ' Amphimixis,' and in still
greater detail in the previous ' Essays upon Heredity.'

350 THE GERM-PLASM

inasmuch as it results in the halving of the germ-plasm itself.
It became apparent subsequently that both divisions serve this
purpose, and each of them causes the removal of germ-plasm,
and not of oogenetic idioplasm from the ovum.

My hypothesis must therefore be given up, but I nevertheless
believe that the conclusion on which it was based was a correct
one, and that it may be further utilised in the light of the theory
of heredity here developed. The oogenetic idioplasm must
exist, and, using the terminology I have now adopted, it may
be spoken of as the oogenetic ' determinant.' This determinant
will consequently be the first to become separated from the
mass of germ-plasm of the young egg-cell, to disintegrate into
its constituent biophors, and to migrate through the nuclear
membrane into the cell-body. In this way alone can we account
for no trace of it remaining in the nucleus, and for embryonic
development not being subsequently impeded by its presence.
This determinafit is used up, and disappears as sitch ; and the
fact that it is not expelled from the egg strongly indicates, if it
does not prove, that the control of a cell by a determinant is
accompanied by the absorption of the latter, and a further sup-
port may thereby be obtained for the hypothesis of emigration.

A precisely corresponding process must be assumed to occur
in the formation of the sperm-cells, in which also the function
of the idioplasm during the histological differentiation of the cell
differs widely from that of the germ-plasm of the mature sperma-
tozoon. The necessity for assuming the existence of 'histoge-
netic ' determinants is perhaps rendered still more evident in the
case of the egg-cell, as in some animals two kinds of eggs are
produced which are very different as regards size, the relative
quantity of food-yolk, colour, and nature of the shell. The
assumption of iTvo kinds of oogenetic determinants cannot be
avoided in this case, for we cannot suppose that the same germ-
plasm can have such different effects on the cell. In the section
on alternation of generations, it was shown to be necessary to
assume that such species contain two kinds of germ-plasm,
containing determinants which are in part similar and in part
dissimilar. Thus the germ-plasm from which the winter-eggs
of the Daphnido' are developed must contain an oogenetic
determinant which is quite different from that of the germ-
plasm in the summer-eggs, for these two kinds of ova are
entirely dissimilar.

THE PHENOMENA OF REVERSION 35 I

I know of no instance in which there is such a wide difference
as regards the activity of the idioplasm in successive cell-
generations as is the case in the germ-mother-cells and the
mature germ-cells arising from them. If, however, even in this
very striking instance of a sudden change of function of the
idioplasm, the idioplasm which was active at first is not removed
from the cell, such a process cannot occur in any other case ;
and we are consequently justified in applying to all other cells
the conclusion derived from the behaviour of the germ-cells,
and in considering it as proved that the active idioplasvi of a
cell becomes used up in consequence of its activity.

352 THE GERM-PLASM

CHAPTER XI

DIMORPHISM AND POLYMORPHISM

I. Normal Dimorphism

The phenomena of reversion discussed in the last chapter
depend on the capacity possessed by organisms of conveying
in their idioplasm characters which they do not themselves
possess in the form of ' latent ^ primary constituents, and of
transmitting these to their descendants, in which they may,
under favourable circumstances, undergo development.

It has hitherto been supposed that all the individuals of a
species possess these latent primary constituents in the same
degree ; and that, consequently, characters which are capable
of becoming occasionally manifest at all in any organic form
exist in a latent condition in all individuals of the species, their
subsequent appearance or non-appearance depending solely on
certain developmental conditions. Even Darwin was of this
opinion, as is shown by many passages in his works. He
imagined that latent ' gemmules ■ existed, for instance, of stripes
like those of the zebra in every horse, of the slate-blue colora-
tion of the rock-pigeon in every domesticated pigeon, and of
the two parental species in every hybrid. In the last chapter
I attempted to show that this may be true in many cases, —
such as, for instance, in the races of pigeons, — but that it is by
no means necessarily so ahvays, and that in numerous instances
certain latent ancestral characters are not present in all, but
only in a larger or smaller number of individuals of the form
in question. We have seen that the reducing division may,
indeed, from one generation to another, divide the germ-plasm
of the parent in such a manner that the germ-plasm of some of
the offspring receives no portions of the idioplasm of one of the
grandparents at all. The most striking example is seen in
plant-hybrids, in which reversion to one of the parental forms
may even occur amongst the oifspring of the hybrid. In spite
of the nearness of the generations between such descendants

DliMORi'HISiM AND POLYMOKFHISM 353

and the parental forms, none of the characters of one of the
parents are present in a latent condition.

Such a rapid removal of characters from the germ of indi-
vidual descendants is, hov^'ever, only possible if — as in the case
of these hybrids — they belong to one parent only. Thus in
the human race, any individual traits of the mother or father
may not only be absent in certain of the children, but the
corresponding determinants may even be wanting in their
germ-plasm, so that these traits cannot reappear in the grand-
children or great-grandchildren. The case, however, is very
different with regard to those characters which are possessed
by both parents. These cannot disappear from the germ-
plasm of individual descendants from one generation to the
next, for their determinants constitute a majority in the maternal
as well as in the paternal half of the germ-plasm. As long as
these characters are present in all the individuals of the group
of forms in question, the determinants for these characters will
predominate in most of the idants, and it will then hardly be
possible for them to dwindle into a minority in consequence of
successive reducing divisions. But let us take the case of a
gradually disappearing character, and select as an example the
wing-bars characteristic of the rock-pigeon which are present in
the domesticated breeds. The absolute number of the 'wing-
bar' determinants in the germ-plasm must have been diminished
gradually in the course of the processes of selection which led
to the formation of the various races, owing to many of them
becoming transformed into differently constituted or 'modern'
determinants. The smaller the number of * ancestral' deter-
minants, the more liable are they to be totally eliminated from
07ie half of the germ-plasm in the reducing division. When,
however, they are entirely wanting in individual germ-cells, two
of these cells may possibly come together in conjugation ; and
an animal would then be produced which possessed no ' wing-
bar' determinants, or, in other words, which no longer contained
this character in a late7it condition. In proportion as such
individuals became more frequent, the average number of ' bar '
determinants in the germ-plasm of the race must still further
decrease, owing to the constant interbreeding of these indi-
viduals with others, until finally only a small percentage of
individuals would contain such determinants at all.

I do not think that the above argument can be strictly applied

354 'i'HE GERM-PLASM

in the case of this particular character of the pigeon. The
frequent appearance of bars in crosses of different races of
pigeons indicates, on the contrary, that most individuals still
contain a number of these • bar "-determinants. We may,
however, conclude with regard to the occurrence of stripes like
those of the zebra on horses and asses, that the infrequency with
which reversion occurs, even in the case of crosses, is probably
due to the fact that the character in question has long ceased
to be present in a ' latent ' condition in all individuals of both
species, and that the germ-plasm usually no longer contains
' zebra ^-determinants.

We certainly cannot ascertain whether some few ancestral
determinants may not nevertheless still be present occasionally,
for even in crosses their cumulative effect might no longer be
apparent ; but general considerations compel us to assume that
even these sporadic old determinants must ultimately disappear.
They will be contained in the germ-plasm of a gradually de-
creasing number of individuals, and as these derive no benefit
from their presence, they will presumably become less and less
frequent.

We must therefore take into consideration, on the one hand,
the dwindling specific or racial characters, which may be pres-
ent in a latent condition, though all of them need not by any
means necessarily occur in all individuals ; and on the other,
the individual characters, which may exist in a latent condition
in a varying number of individual descendants.

The actual specific characters are, however, transmitted to every
individual, though they also do not always become manifest, for
some regularly remain latent when another opposing group of
characters becomes apparent. I am here alluding to primary
and secondary sexual characters : but all characters — even the
non-sexual ones due to dimorphism and polyjtiorphism — come
under this category. I must now attempt to explain these
phenomena in accordance with my theory.

In considering this problem, we must naturally begin with
the simplest case of sexual differentiation, in which the origi-
nally monomorphic germ-cells become differentiated into male
and female. The question then arises as to the origiti of this
differentiation in the idioplasm, and how it has arisen phyleti-
cally.

Let us take a definite example. The VolvocinecB are lowly

DIMORPHISM AND POLYMORPHISM 355

organised multicellular algae of a globular form, which rotate
and are propelled through the water by the movement of their
cilia. In addition to multiplication by means of asexual germ-
cells, several genera of this order, such as Eadorijia and Pan-
dorina, exhibit a sexual mode of reproduction, consisting in the
conjugation — i.e. complete fusion — of two germ-cells which are
apparently quite similar to one another.

As long as this is the case in all individuals, we might sup-
pose that the reproductive-cells are controlled by the idioplasm
which directs the development of the species in general, — that
is, by the germ-plasm, which is composed of a varied number of
similar determinants. This will, however, no longer be the case
as soon as the conjugating germ-cells become differentiated
into male and female, as has occurred in the allied genus Vol-
vox. The utilitarian motive for this differentiation is not far to
seek, for it must be advantageous for the germ-cells to contain
the greatest possible accumulation of nutritive material ; and
this could only occur to a very slight extent as long as the two
germ-cells destined to undergo amphimixis retained a marked
power of movement, as is the case in Paiidorina. They con-
sequently become differentiated into the stationary egg-cell,
containing a large quantity of nutritive material, and the small
motile sperm-cell, in which very little food-material is present.
On what process in the idioplasm, then, does this differentiation
depend?

The idioplasms of the ^gg- and sperm-cells evidently cannot
be precisely similar. They cannot simply consist of germ-
plasm ; but the egg-cell must contain a determinant which gives
it the histological character by which it is distinguished from
the sperm-cell, which latter must also contain a determinant
controlling its histological development. The germ-plasm of
Volvox must therefore contain sperniatogenetic and oogenetic
determinants besides those for the ciliated somatic cells, only one
or the other of which, however, becomes active, and impresses
the male or female character on the germ-cell. I imagine that
these sexual determinants are double, each of the two parts always
occurring together and being closely united, but so regulated that
only one of them becomes active at a time. We might repre-
sent this figuratively by supposing that each id of the germ-cells
consists of a central globular mass of germ-plasm, surrounded
by a layer composed of this sexual double-determinant, either

356 THE GKRM-PLaSM

the male or female part of which may be external, and conse-
quently dominant.

This is only a metaphor, and is not intended to represent the
actual occurrences. We are ignorant of the forces and sub-
stances which are here concerned, but we at any rate know that
the idioplasm of the primary germ-cells in the higher animals is
still capable, in by far the most cases, of giving rise to either
kind of orerm-ceils, and that the decision as to whether the
germ-cells will develop into ova or spermatozoa occurs at some
early stage in embryogeny : in the eggs of the bee it takes place
at the beginning of embryonic development, long before the
first primary germ-cell is differentiated, while in other animals
it perhaps occurs at a later stage. The well-known researches
of Siebold and Leuckart, prove that at any rate in the case of
bees, this decision undoubtedly rests upon the occurrence or
omission of fertilisation, — it occurs, that is, at the time when the
germ-plasm which controls the new organism is constituted ; and
this fact seems to me to be of great importance. If fertilisation
takes place, the organism develops into a female, and if not,
a male will result. This at least proves that the decision jnay
take place at such an early stage, and I doubt whether it can in
any case occur later : in some animals, at any rate, it occurs
still earlier, during the period of maturation of the egg. The
Phylloxera lays large eggs which produce females, and small
ones from which males arise. Both become fertilised, so that
in this case fertilisation takes no part in the determination
of sex.

These questions cannot be discussed more fully here. We
are only concerned in making it clear that sexual determinants
in the sense indicated must be assumed to exist, and that both
kinds are contained in the primary germ-cells. The following
considerations will render it apparent why we have assumed
these determinants to be double, — i.e., to consist of two groups
of biophors of a common origin lying close to one another. Even
apart from such low organisms as Volvox, it is well known that
the male and female individuals only differ from one another as
regards the kind of sexual cells they produce in many of the
lower Metazoa — viz., the sponges and Hydromedusae. In these
forms the sexual determinants alone are double. In most other
animals, however, the difference of sex is not confined to the germ-
cells, but affects the soma itself to a greater or less extent. Hence

DIMORPHISM AND POLYMORPHISM 357

in all sexually dinuvp/iic organisms the germ-plasm must contain
a varied number of double jirimary constituents of somatic char-
acters, — namely, those which represent characters which differ
in male and female individuals. These in the first instance con-
cern the organs in which the sexual cells are developed, nourished,
stored, and removed, — that is to say, the so-called sexual glands
and their ducts ; then follow the active and passive copulatory
organs, and the stmctures connected with oviposition ; and,
finally, special sexual characters arise with regard to the organs
for supporting and nourishing the offspring — such as mammary
glands, teats, and uterus, — or they may refer to the instinct
of carrying the eggs in the mouth, as in the male of a tropical
species of frog, or to that in the female butterfly, which deposits
its eggs in a definite manner on a certain plant. In the last two
instances, the structure of the body and the nerve-centres must
also be different in the two sexes, and the male and female type
of these parts must exist in a latent condition in every germ-
plasm. Under this head, moreover, ^secondary'' sexual cha?'ac-
te?'s must be included, such as the various tracking and alluring
organs of the males, the gorgeous colours of male birds and
butterflies, the scent-producing organs of the latter, which exhale
perfume, and the song of male birds and insects.

In the human race we know that all the secondary sexual
characters are transmitted by individuals of the opposite, as well
as of the corresponding sex. A fine soprano voice, for instance,
may be transmitted from mother to granddaughter through a
son, and a black beard from the father to the grandson through
a daughter. And in other animals, the sexual characters of both
sides must be present in every sexually differentiated organism,
some of them becoming manifest and others remaining latent.
This fact can only be proved in certain cases, for we seldom
notice the individual differences of these characters with suffi-
cient accuracy ; it can, however, be shown to be true, even in
tolerably simply organised species, and we must therefore sup-
pose that latent characters belonging to the other sex are always
present /;/ each sexually differentiated organistn. In bees, the
males developed from unfertilised eggs possess the secondary
sexual characters of the grandfather ; and in the water-fleas, in
which several orenerations of females arise from one another,
the last of these generations produces males with the secondary
sexual characters of the species, which must consequently have

358 THE GERM-PLASxM

been present in a latent condition in an entire series of female
generations.

The germ-plasm of the fertilised egg-cell must therefore
contain the primary constituents of all the secondary male and
female sexual characters, as well as those of the male and female
germ-cells. We might, then, suppose that this would be
accounted for by the assumption that the determinants of both
kinds of characters are contained in the germ-plasm, and that
the decision as to sex is not only determined with regard to the
sexual determinants, — so that the germ-cells take on a male or a
female character, — but also as regards the somatic determinants,
in order that the secondary sexual character of the male or of
the female may take the lead in the building up of the body.

This assumption is certainly indispensable, and it suffices as
far as the latent transference of secondary characters of both
kinds from the germ-plasm of one generation to that of the
next is concerned, but it still requires an essential addition.
A number of facts indicate that the latent primary constituents
of secondary characters of both kinds are not only present in
the germ-plasm from which the organism arises, but also in the
fully developed body of the organism. The fact that the char-
acters of both sexes can be transmitted to descendants, proves
that the germ-plasm must contain the corresponding primary
constituents of both ; and in the sixth chapter it has already
been shown how this transmission may be accounted for by a
continuity of the germ-plasm from one generation of germ-cells
to the next. We are now concerned with the fact that mature
individuals may also contain the primary constituents of sec-
ondary sexual characters of both kinds, and that those of the
female may be present in the body as well as in the germ-cells
of a male organism, and conversely those of the male may exist
in the female. The well-known facts to which I refer have
been carefully collected and fully discussed by Darwin, and are
briefly as follows : — the secondary sexual characters of one
sex may, under special circumstances, become developed sub-
sequently in fully-developed individuals. This results in both
sexes, especially in the case of castration. The removal of the
sexual ' glands ' from young mammals and birds prevents the
development of secondary male sexual characters. Castrated
cocks, for example, retain the appearance of hens, and do not
develop the beautiful tail or the large comb and spurs of the

DIMORPHISM AND POLYMORPHISM 359

male bird, nor do they crow ; and conversely, when hens
become sterile from age, or if their ovaries become degenerated,
they take on the external sexual characters of cocks. I possess
a duck which no longer lays eggs, and has assumed the colora-
tion of the drake. Men who have been castrated in their youth
retain a high voice like that of the other sex, and the beard does
not become developed.

These facts obviously compel us to assume that the capacity
for the development of secondary female characters exists in a
latent condition in the body of the male, and vice versa, and
that these are ready to undergo development under certain con-
ditions. Darwin also arrived at the same conclusion. The only
argument which might be advanced against its correctness is
that a change of secondary sexual characters in any particular
individual has only been observed in rare instances, and in very
few species of the higher animals, such as birds and mammals.
It might, therefore, be doubted whether it is possible to draw
a general conclusion from such isolated observations. These
instances nevertheless remain to be accounted for, and we must
attempt to explain them in accordance with our theory.

Let us take a very simple example. In many butterflies of
the family Lyccenidce, the upper surface of the wings is brown in
the females, and blue in the males ; and it seems very probable
that brown was the original colour, as species of LyccEua exist
at the present day in which both sexes are of this colour.
The process which took place in the idioplasm in order that
the change of colour may have occurred, must have consisted
in the primary determinants of those cells which decide the
colour of the wings — which we will speak of as ' brown ' deter-
minants — becoming transformed into ' blue ' determinants in the
germ-plasm, this change only occurring after they had become
doubled and in such a manner that only one of the twin-deter-
minants in each case remained brown, an arrangement also taking
place which only allowed each to become active alternately.
We thus arrive at the assumption of double detertninaiits, just
as in the case of the determination of sex in the germ-cells.
I at first believed that it was indispensable to assume the
existence of determinants with different halves, merely because
the presence of inactive determinants in the corresponding
part of the body could not be otherwise explained. As the
adoption of the characters of the opposite sex cannot possibly

360 THE GERM-PLASM

be of any use to the species, natural selection can have taken no
part in the addition of male determinants to the somatic cells
of the female body, or 7>ice versa. Such a transference must
therefore depend on an unintentional secondary effect of existing
arrangements and forces. I soon, however, recognised that
such arrangements actually exist, and that the assumption of
mechanically unseparable double-determinants is not necessary
in order to explain the presence of the two sexual determinants
in the region where one of them undergoes development. I
therefore attach no value to the idea of the material connection of
the two dimorphic halves of the primary determinants in question.
In fact I shall have occasion to show presently that these halves
must, in any case, sooner or later become separated as inde-
pendent determinants in the course of phyletic development.

The reason that such double determinants must, how^ever,
ahvays remain close together, even after their separation, results
simply from the mechanism for the ontogeny of the idioplasm,
which we suppose to consist in the gradual disintegration of
the mass of determinants of the germ-plasm into smaller groups.
They divide according to a definite law into smaller and smaller
groups in the course of the embryonic cell-divisions. None of
them remain unused or undergo destruction, but each passes
through a definitely prescribed course, and the determinants
for any particular part or region of the body must necessarily
remain together, even when they are not inseparably connected
mechanically. In 3. p/iysiological ?,&\\'f,e. therefore, they are still
double determinants, — i.e., each half controls the same region, —
and in this sense I shall now use the term.

A transfer of the secondary sexual cliaracters, such as occurs
in birds, cannot take place in the LycanidcE., because the wing-
scales are never formed more than once in the course of life,
and consequently we have no means of proving the presence
of double determinants in the cells of the wing. Other ob-
servations on insects indicate, however, that their idioplasm is
nevertheless capable of producing such a sexual transference.

This is especially shown in the case of the occasional herma-
phrodite forms of insects, the most instructive instance — which
has been very accurately investigated by Leuckart * and von

* R. Leuckart, ' Sitzungsberichte der deutschen Naturforscherver-
sammlung,' 1864.

DIMORPHISM AND POLYMORPHISM 36 1

Siebold,* and more recently by Kraepelin — being furnished
by bees. Tlie male and female secondary sexual characters are
combined in the most wonderful manner in these hermaphrodite
forms : in some bees, the 7'ight side was fejuale and the left male ;
in others, the anterior half of the body was male and the poste-
rior female ; while in others, again, the entire trunk was male
and one side of the head. female. As Leuckart remarks, 'the
male and female characters ' in these hermaphrodite bees " have
been intermingled in the most varied and unsystematic manner,
so that it is difficult to discover two individuals with perfectly
similar characteristics."

We are indebted to Kraepelin for an excellent account of
the external sexual parts in these hermaphrodite bees.f in-
cluding the copulatory apparatus. His description shows that
the blending of the male and female characters even affects
very small parts. It not only often happens, for instance,
that half the entire stinging apparatus of the left side is
female, while an intromittent organ is developed on the right,
but certain chitinous plates on the ventral side of the last
abdominal segment, which is almost male in character, also
display a distinct tendency to take on the form of the corre-
sponding plates of the female stinging apparatus ; in other
words, these chithioies plates are intermediate in form between
those of the male and female. Their formation must therefore
have been controlled by a combination of ' male ' and ' female '
determinants. It would be incredible that these harmonising
determinants could have met together at the right point in the
extremely complex combination of determinants in the germ-
plasm if they had not been arranged together from the first, and
if an arrangement of male and female double determinants had
not previously existed in every such region of the germ-plasm,
so that they reached the corresponding part of the body together
in the course of ontogeny, either the male or the female half
then becoming active.

In the determination of sex in the normal development of bees,
all these somatic double determinants must be correspondingly
determined. We do not know to what factor the prevention of
the determination of sex in a similar manner in the formation

* C. Th. von Siebold, ' Zeitschrift fiir wissenchaftliche Zoologie," Bd.
xiv,, 1864, p. 73.

t Kraepelin, ' Zeitschr. f. wiss. Zoologie,' Bd. xxiii., 1873, P- 326.

362 THE GERM-PLASM

of the hermaphrodites is due. Siebold attempted to explain
this difficuhy by supposing that the eggs which produced
hermaphrodites were imperfectly fertilised. He found that in
the queen, after producing hermaphrodites, the receptaclum
was almost empty ; and as drones arise from unfertilised, and
females from fertilised eggs, the view that imperfect fertilisa-
tion must produce hermaphrodites appeared to be a plausible
one. It was not known at that time that a single spermatozoon
suffices for fertilisation, and at the present day we are no longer
justified in using such an expression as 'imperfect fertilisation.''
Whenever a living spermatozoon enters an egg, the latter
becomes fertilised ; and an imperfect fertilisation could only be
supposed to occur if the spermatozoon is abnormal — if, for
instance, it contains too few idants. But even if such a case
occurred, it would be of verv little value theoreticallv. and we
could only state that the determination of sex did not take place
in all the double determinants at once, but only in certain larger
or smaller groups, — in the case of the germ-cells as well as of the
dimorphic parts of the body. Besides the ordinary case, in
which the sexual gland of an individual was developed on the
female type on the right side and the male type on the left,
other instances occurred in which female and male germ-follicles
were formed on the same side, seminal tubules and ovarioles
being present close together. With reference to this fact, von
Siebold remarks that ' the hermaphroditism of the sexual appa-
ratus hardly ever corresponded to that observable in the external
form ; ' and this seems to me to be of special theoretical interest,
as it allows us to conclude with certainty that the harmony
of the 7ior?>ial condition is due to a sinucltatieous decision
respecting the double determinants of the germ-cells and those
of the body, and not to a primary determination of sex in the
sexual glands, from which the somatic male or female sexual
characters would only be determined secondarily. The existence
of double determinants in the germ-plasm can be actually proved
in the case of bees. For if any egg can develop into a male
or female according to whether it is fertilised or not, it must
contain both kinds of determinants.

Although this assumption is undoubtedly a correct one, it
alone is insufficient, because the secondary differences of sex
do not always only concern single cells or groups of cells which
correspond exactly in the two sexes, such as, for example, the

DIMORPHISM AND POLYMORPHISM 363

brown and blue scales of the LyccenidcE ; and because in ?nany,
and perJiaps in most cases, the dimorphic parts correspond only
partially or not at all.

The degree of sexual differentiation is very different in the
various groups of the animal kingdom. In the lower and
higher Crustacea the males commonly possess more ' olfactory
set£e' on their antennae than do the females. In the large
water-flea, Leptodora hyalina, for instance, the anterior antennae
are represented in the female by short stumps provided with
five olfactory setae, while the male has long rod-like feelers,
which bear about eighty set^e. This difference evidently cannot
be referred to one double determinant. Each olfactory seta
must be represented by a special determinant in the germ-
plasm ; even if the first five corresponded, and the differences
between them could be attributed to the double determinants,
more than seventy determinants for these setae — which occur in
the male sex only — would still be left, quite apart from those for
the feeler itself. These seventv determinants are not double,
because the corresponding parts are absent in the female, and
consequently two groups of determinants must exist side by side
in the germ-plasm, — those, namely, for the antennae in the male
and in the female. — only o>ie of which becomes active in each
case. We might imagine that the two groups pass, in close
proximity to one another, through the cell-series of embryogeny
up to the formation of the rudiments of the antennae, and then
become separated, the inactive group remaining in an * undif-
ferentiated ' cell at the base of the feeler, while the other causes
the development of the antennae of the sex in question by
continued cell-division.

The same must be true in numerous other and much more
complicated cases. If a tail feather in a male humming-
bird, for instance, is six times as long as the corresponding
feather in the female, the colour of the former being of a
brilliant ultramarine, and that of the latter greyish, we must
assume that two groups of determinants are present, which
differ in number and in nature. The two groups are situated
close together in the germ-plasm, pass through the same cell-
series in ontogeny, and ultimately reach the same part of the
skin covering the last caudal vertebra. Here, however, one of
them remains inactive, the other alone causing further cell-divi-
sions and the consequent development of a feather to take place.

3^4 THE GERM-PLASM

The greater the extent to which sexual dimorphism occurs,
and the larger the parts which it affects, the larger the two
groups of determinants must be, and the earlier in ontogeny
will one of them remain inactive in a cell and cease to undergo
further division, while the other gives rise to further cell-
divisions.

Sexual dimorphism largely consists in the arrest of develop-
ment in an organ in one sex. In many female butterflies, for
instance, wings are wanting. This must be due to the group of
determinants for the wings, which existed in a double condition,

— i.e.^ were male and female in the earlier phyletic stages, — be-
coming arrested as regards their male portion. Such females
commonly possess rudiments of the wings, and in this case the
two groups of wing-determinants must pass through ontogeny
together up to the stage in the caterpillar in which the forma-
tion of the imaginal disc of the wing arises from a cell of the
hypodermis. If the animal is a female, the arrested group,
and if a male, the perfect group of wing-determinants will
then become active. It is, however, also conceivable that the
development of the group of determinants in the female might
continue to be arrested until they disappeared completely, as in
the case of female PsychidcE, in wiiich the wings are altogether
wanting.

But the highest degree of sexual dimorphism is not reached
even when certain parts disappear completely. In various
groups of the animal kingdom species exist in which the males
differ front the females in nearly all their characters. In many
Rotifers the males are very much smaller than the females, and
exhibit an entirely different structure ; the alimentary canal,
moreover, is entirely wanting. In Bonellia viridis, a marine
gephyrean worm, the male differs so much from the female that
one might be tempted to class it with an entirely different group

— the Turbellaria. The difference in the sizes also of the two
sexes is still more marked in this instance, the length of the male
being 1-2 mm., and that of the female 150 mm. ; the former,
moreover, lives as a parasite within the latter. In this case the
eggs which give rise to males cannot be distinguished from
those which develop into females, even in size ; and the rela-
tively enormous bulk of the female simply results from subse-
quent growth. Even the young male and female larvae cannot
be distinguished from one another, and their development only

DIMORPHISM AND POLYMORPHISM

365

begins to dififer when metamorphosis of tlie larva into the sexu-
ally mature animal takes place. The larva is of an elongated
oval shape, is furnished with two bands of cilia by means of
which it swims about, and possesses an alimentary canal with
a mouth and anus. The transformation into the definitive form
of the species begins with the loss of the posterior band of cilia,

'TTt

Fig. 24. — Boiiellia viridis. (After Huxley, from R. Hertwig's "Zoology.") On
the left hand the male is shown, considerably magnified; and on the right the
female, about natural size, d. Rudimentary intestine of the male; z'd. Sperm
duct; i, Intestine of the female.

and from this point onwards the mode of development is different
in the two sexes. The females grow rapidly, develop a ' pro-
boscis ' at the anterior end, and the intestine increases in
length ; the males, on the other hand, become entirely covered
with cilia, and the mouth and anus, as well as the fore- and
hind-gut disappear, the mid-gut, filled with yoke-granules, alone
being retained. Notwithstanding their great diversity, the two
sexes are formed on the same plan ; the male, however, may be
said in general to remain stationary at a certain stage of organi-

366 THE GERM-PLASM

sation, while the female continues to develop, and reaches a
much higher stage of organisation, at any rate as regards many
organs, such as the nervous and vascular systems, which are
altogether wanting in the male. But neither is this the full
extent of the difference between them, for the testes, as well as
the skin and certain hooked organs of attachment are only
developed in the male at a later stage in a peculiar manner.
A certain correspondence still remains in the most essential
points of structure of the body, in spite of the great differ-
ence between the adult sexual animals ; so that, as Sprengel
says, the male is also ' a Gephyrean possessing all the known
structural characteristics of the group.'

In terms of the idioplasm, this course of development may be
described somewhat as follows : the determinants which direct
the development of the larvae are single, and consequently
monoworphic larvae are produced. The idioplasm of all or most
of the cells, however, which constitute the organs of these larvae,
contain double determinants or double groups of determinants,
of which those for the female are, in most cases, far the larger:
in fact, the group for the female will usuallv not be opposed
by any for the male at all, — in the case, that is, of such organs
as the long proboscis, of w^hich there is no homologue in the
male. It is certainly very remarkable that these groups of
determinants, although present in the male and unopposed by
others, do not become active ; but, even although we do not in
the least understand how it comes about that this inactivity is
enforced, the case is not more surprising than that of the deter-
mination of sex as a whole, and the inactivity of existing indi-
vidual primary constituents. Why do the wings first appear at
the pupal stage of the caterpillar, and not long previously, since
they must be present all along in rudiment — i.e.^ in the form of a
group of determinants in certain cells of the hypodermis? Or
why does a boy not grow a beard, seeing that the necessary
determinants must be contained in certain cells of the skin?
We can no more account for all these cases than for the in-
activity of sexually differentiated determinants. All that can be
said is, that these determinants have the peculiarity of only
becoming active at a certain ontogenetic stage ; but this scarcely
gives any further insight into the matter than does the statement
that sexually differentiated determinants become active or remain
inactive, according to the sex of the organism in question.

DIMORPHISM AND POLYMORPHISM 367

The ids of the germ-plasm must contain more determinants
in sexually dimorphic than in monomorphic species, the number
increasing in proportion to the increase of difference between
the sexes. They must also increase in size; and the question
therefore arises as to whether dimorphism may not perhaps
increase to such an extent and finally involve all parts of the
body, that double ids arise ; that is to say, each id of the germ-
plasm comes to consist of a male and a female half, in which
all the determinants are different. This seems to be practically
the case in some animals : in the Rotifera, for instance, the
males commonly differ so much from the females that they,
like the sexual form of Phylloxera, arise from special eggs,
smaller than those which develop into females. But it would
nevertheless be incorrect to suppose that each of these two
kinds of eggs contained either male or female ids only. The
number of common determinants present must certainly be
small, but even here the germ-plasm of each tgg must never-
theless contain all the male and female determinants. This is
proved by the interpolation of generations consisting of females
only in the cycle of generations which is passed through each
year, the parthenogenetic females eventually producing males.

We must now return to the question of sexual reversion — if I
may so call it — which has already been referred to ; that is to
say, the appearance of characters of the opposite sex after cas-
tration or degeneration of the sexual glands. Hitherto this has
always been considered a universal phenomenon, but I do not
think that such a conclusion is justified. As already mentioned,
observations with regard to such cases of ' sexual reversion '
have practically been confined to birds and mammals, and even
in these do not always refer to all the parts which are sexually
dimorphic in the species in question. Cases have certainly been
observed in which, for instance, an old ' hen which had ceased
laying, assumed the plumage, voice, spurs, and warlike dis-
position of the cock.' * This proves that in these birds all the
secondary sexual characters of the male are present in a latent con-
dition in the soma of the female. We might, however, suppose
that this form of reversion only takes place when the characters
concerned are completely homologous in the two sexes ; that is

* Darwin, ' Animals and Plants under Domestication," \'ol. II., p. 26.

36S THE GERM-PLASM

to say, when they exactly correspond with regard to the time
and place of their ontogenetic origin and the number of their
determinants : but if this is not the case, such a ' reversion ' to
the characters of the other sex can hardly be possible, because
the foundation is wanting from which the reverted organ could
arise. Let us suppose that the same conditions as apply to
fowls hold good in the case of butterflies : — that is to say, that the
secondary sexual characters are present in a latent condition in
the soma of the other sex, and undergo development on removal
of the sexual glands. The male of LyccEua alexis, for example,
which has blue wings, would then develop brown ones on
being castrated. This w'ould take place by the shedding of old
scales and the growth of new ones ; or if castration had been
effected in the caterpillar stage, the scales in the growing wing
would be brown from the first. The scales are homologous
structures in the male and female, and each of them is controlled
by a single determinant. If, therefore, a cell containing the
determinant for a brown scale of the female is situated at the
base of the already developed scale of the male, a reversion
to the colour of the scales in the female might occur under such
circumstances, which are of course purely imaginary.

The matter would, however, be entirely different if the female
LyccE)ia had no wings at all, as is the case in females of some
Bombycidce. The character of the blue scales in the male
would then have no homologue in the female, and an inactive
cell, with the determinant of a brown scale, could not possibly
be situated at the base of the blue scales in the male. This
may be expressed in general terms as follows : — doiible-detcr-
7nina)its, possessing a defi)iite ;nale and female character, can
only be present up to the phase and point in ontogeny in which
the development of the two sexes is exactly homologous. We can
therefore only expect a reversion to the secondary characters of
the other sex to occur when this point remains permanent. In
Lyccena the divergence would occur at the formation of the wing-
scales ; in Psyche (the female of which is wingless), in a certain
group of cells in the hypodermis of the thorax ; in Bonellia, in
all the cells of the larva ; and in the Rotifera, in the egg itself.
If our view is a correct one. a female Bonellia would conse-
quently be incapable of developing male characters in conse-
quence of castration, because it has long since passed that stage
of ontogeny in which the divergence into a male or female

DIMORPHISM AND POLYMORPHISM 369

occurs ; and in the case of the Rotifera. it is not to be expected
that any influence could cause a male to produce female char-
acters.

In the part treating of sexual reproduction — including the
section on the struggle of the paternal and maternal hereditary
tendencies which takes place during the development of the
offspring — no mention was made of a very general, and, in my
opinion, erroneous conception of sex ; and it will be as well to
explain this omission before proceeding further.

The transference of sex has hitherto usually been looked upon
as an act of transmission. This cannot be the case, inasmuch
as every germ-plasm contains the primary constituents for both
sexes, and the process of transmission itself has evidently nothing
to do with the determination of sex. As already mentioned,
it does not by any means follow that because a child is a female,
its secondary or primary sexual characters will resemble those
of its mother. This, indeed, has long been known, but has not
led to a general recognition of the fact that sex is not trans-
mitted at all ; and that, on the contrary, the primary consti-
tuents of both sexes are always passed on from both sides : the
decision as to which of them are to become active depends on
secondary factors, which have not yet been clearly recognised
in any case. The male halves of the sexual double-determinants
of the mother are just as capable of undergoing development as
are the female halves, and vice versa : — the ' law of sexual trans-
mission,'* which was propounded by Haeckel some time ago, is
not tenable. Expressed in a purely empirical manner, the facts
have been more correctly formulated by Dejerine f according
to Darwin's ( ?) views, in his valuable work on the heredity of
nervous complaints : — ' the prepotency of one of the parents in
transmission may be direct, and follow the sex, or may cross
over, and become manifest in the opposite sex.'

For this reason the so-called 'transmission of sex' was entirely
left aside in the section on the struggle of the parental char-
acters during the formation of the child : transmission of the
primary and secondary sexual characters occurs, but sex itself
cannot be transmitted.

* Ernst Haeckel, ' Generelle Morphologic der Organismen,' Bd. II.,
Berlin, 1866, p. 183.

t J. Dejerine, ' L'Heredite dans les maladies du syst^me nerveux,' Paris,
1886, p. 17.

370 the germ-plasm

2. Pathological Dlmorphism : H/Kmophilia.

In connection with the attempt to trace sexual dimorphism
to its origin in the idioplasm. 1 will now make a few remarks
with regard to a certain disease — or rather structural anomaly
— which affects the human race, and which. I believe, will be
better understood from this point of view ; the analvsis of it
may. moreover, possibly throw a new light upon the causes
of sexual dimorphism.

This anomaly is known as hdinopJiilia^ and although of rare
occurrence, it has been very accurately observed in a number of
cases, and is known to be transmitted in a marked degree and
in a very peculiar manner. It only occurs in the tnale members
of a family, but is transmitted by the female members, and in
this respect resembles a secondary sexual character.

The disease, however, is apparently not connected with the
sexual organs, or with those parts which differ in the two
sexes. It consists in an abnormal liaccidity of the walls of the
blood-vessels, in consequence of which slight injuries cause
serious hajmorhage, which cannot be easily stopped. As the
blood-vessels are developed from certain cells of the so-called
mesoblast or • parablast,' haemophilia is described in many
text-books on pathological anatomy as an anomaly of the
* parablast " or ■ Bindegewebekeim.' It must certainly be assumed
that this disastrous variation in the blood-vessels — or rather of
the cells from which the blood-vessels are formed — is due to
some variation in certain mesoblast cells which cannot be more
definitely defined at present. The determinants for the cells of
the blood-vessels must in this case have varied in some way or
other in the germ-plasm of the individuals affected, and the anom-
aly must originally have occurred in a male individual. There
is apparently no reason why a similar variation of the deter-
minants should not take place in a female, and cases of women
exhibiting the haemorrhagic diathesis may perhaps still be ascer-
tained. The exclusive transmission of this anomalous condition
to the male sex seems to me, however, to indicate that the deter-
minants for the blood-vessels differ in men and women, in spite
of the apparent similarity of the vessels themselves : — these
determinants must be double.

On this assumption, we can easily account for the otherwise
mysterious phenomena of heredity observed in these cases. As

DLMORPHISM AND POLYMORPHISM 37 I

this disease only appears in men, the pathological variation
must aftect the ' male ' half of the determinants for the cells of
the blood-vessels of the person aft'ected, and we may compare
it with the variation which occurs in the cells constituting
the larynx, the determinants of which must certainly be re-
garded as being double, and as undergoing variation in the
•male' half. The decision as to which halves of the double-
determinants in the idioplasm are to be active during embryogeny
and which passive, takes place simultaneously with that as to the
sex of the embryo, as is proved by the case of hermaphrodite
bees. It is therefore self-evident that this disease must remain
latent — i.e.. no diseased formation of the tissues whatever can
be produced — in the case of every female descendant of a
' bleeder,' for in them the ' female ' untransformed halves of the
determinants for the cells of the blood-vessels become active.
If, however, the offspring develops into a male, the pathologi-
cally transformed ' male " halves of these determinants become
active, and the disease can develop, provided that a stronger
hereditary influence is not exerted in the formation of the blood-
vessels of the healthy maternal side, so that the tendency to
disease, which has been derived from the father, is overcome by
the healthy tendency inherited from the mother. This was the
case in four generations of a familv of * bleeders,' observed
by Chelius, Mutzenbecher, and Lossen, — * the sons were not
affected. In another case described by Thulasius-Grandidier, on
the other hand, the disease was transmitted from the father to the
male members of three generations. We can understand both
cases from our point of view, for in no instance is an individual
variation due to the variation of the corresponding determinants
in all the ids of the germ-plasm, but only in the majority of them.
But this majority may become reduced to a minority at every
' reducing division ' and every time amphimixis occurs, the varia-
tion thus ceasing to manifest itself. As soon therefore as only
a small majority of the ids contain • ha^morrhagic determinants,'
a considerable number and hereditary force of the healthy
maternal determinants for the blood-vessels would preponderate
over the morbid paternal ones, and consequently the male
descendants would not inherit the disease. If, however, a con-
siderable majority of ' haemorrhagic determinants' were present

* Klebs, ' Lehrbuch der pathologichen Anatomic.'

372 THE GERM-PLASM

in the germ-plasm of the father, a favourable reducing division
is necessary if the son is to remain free from the disease. We
can, moreover, even account for those cases in which several
female members of a haemorrhagic family in which the fathers
were healthy, bear sons all of whom suffer from the disease.
For the ' male ' halves of the double-determinants in almost all
the ids might have undergone a pathological change in the
germ-plasm of the mothers, without producing any apparent
result in them : in the sons, on the other hand, this would
lead to the development of the disease, unless an unusually
favourable reducing division had counteracted the marked pre-
ponderance of the morbid determinants. Haemophilia, which
remains latent in the mother, is just as liable to be transmitted
by the mother to her male descendants as is any other mascu-
line characteristic of the grandfather, such as the colour of his
beard, or the quality of voice.

It seems to me that we cannot overlook the indication afforded
by this agreement between the mode of transmission of ordinary
sexual characters and of haemophilia that all, or nearly all, the
determinants in the hiiniaji germ are double^ half being 'male'
and half ' female,' so that a determinant for any particular part
may cause the development of the male or female type of the
corresponding character.

The facts which Prosper Lucas * brought forw-ard, and illus-
trated by numerous examples, concerning the occasional trans-
mission of new characters to one sex only, — even when they
have nothing to do with secondary sexual characters in the strict
sense, — may be understood by this assumption of a wide distri-
bution of double-determinants in the germ. The modification
affects only the ' male ' or the * female ' halves of these deter-
minants of the germ-plasm in such instances. This is true
as regards the disease we have just considered, inasmuch
as it must have made its first appearance at some time or
other, — as well as in numerous instances of colour-blindness, of
the possession of supernumerary fingers, of the absence of certain
fingers or of segments of fingers, and so on. Even the peculiar
nature of the epidermis in the well-known case of Lambert, the
'porcupine-man,' was only transmitted to the male descendants.

* ' Traite philosophique et physiologique de I'heredite naturelle,' Tom.
IL, Paris, 1850, p. 137.

DIMORPHISM AND POLYMORPHISM 373

In certain instances, polydactylism is known to be transmitted
to the male members of a family only, while in others it passes
from the mother to the daughters exclusively.

It appears, however, that such modifications of the one half
in the double-determinants may in the course of time be trans-
ferred to the other half, even though, in the first instance, this
onlv occuis to a slight extent; for cases are known in which
an abnormality lirst arose ( ?) in the male sex, and afterwards
passed over to certain individual female descendants. These
cases have certainly not been followed out with sufficient accu-
racy to enable us definitely to deny that a modification of both
halves of the double-determinants in question might possibly
have taken place from the first, and that it only affected those
in the one half (the homologous determinants) in a smaller
number of ids.

Numerous instances in which an abnormality appears, some-
tim.es in the male, and sometimes in the female members of a
family, prove that both halves may become modified at the same
time. Prosper Lucas mentions several instances of this kind, such
as that of the family Ruhe : — in the first generation observed,
the mother transmitted her polydactylism to the daughter, and
in the second this peculiarity was passed on from the mother to
the son, while in the third it was transmitted from the father to
the son. Numerous facts in zoology indicate the correctness of
the assumption that modifications in one half of a double-deter-
minant exert an influence on the other half, so as to result in a
similar transformation. It is well known that manv secondarv
sexual characters of the male in birds and insects appear in a
slighter degree in the female. Amongst the Lyccrnidcp — which
are called ' blues,' on account of the preponderance of this
colour in the members of the family — some species exist in
which both sexes are brown, while in most the male is blue and
the female brown, and in a small number of southern species,
again, both sexes are blue. There can be no doubt that brown
w'as the original colour of these species, and that the blue tint first
appeared in the males, while the females remained brown ; and
that, finally, in certain species, the females also became blue,
although not so markedly so as the corresponding males. The
males therefore preceded the females as regards the change of
colour; and if, with Darwin, we attribute the impulse to this
change to sexual selection, it follows that the blue colour of the

374 THE GERM-PLASM

females must have been introduced mechanically, owing to its
previous existence in the males, and that this secondary sexual
character affected the other sex in the course of a great number
of generations. According to our theory, this must have
been due to the modification of the ' male *' halves of the
determinants having gradually caused a similar, if less marked,
modification of the 'female' halves. We can thus also to some
extent understand how it has been possible for certain females
to precede the others as regards this modification, as the
influence which produced it would not take effect to the
same extent and at a similar rate in all individuals. In
many species of Lyccenida in which the females are brown,
individuals of this sex occur more or less frequently which are
clouded with blue, or even exhibit this colour in a marked
degree.

3. Polymorphism

Sexual tri7)i07-pJiis))i is of frequent occurrence in the animal
kingdom. I will here refer to certain instances amongst butter-
flies, which were first discovered by Alfred Russel Wallace, and
will begin with a case in which apparently the first step towards
polymorphism has been taken.

The male of a common North American butterfly, PapUio
iurnus, resembles the ordinarv ' swallow-tail,' having vellow
wings ornamented with black transverse stripes ; while the
females sometimes resemble the males, and are sometimes quite
black, and may thus differ markedly from one another. The
yellow females occur in the eastern and northern parts of the
United States, and the black ones in the west and south ; we
must therefore suppose that two local varieties of this butterfly
exist, in the northern of which the two sexes have a similar
coloration, while the southern form is dimorphic. This indi-
cates, in terms of the idioplasm, that the determinants for the
wing-scales are single in the northern, and double in the southern
variety. Describing these determinants according to the colour
which they produce, we may say that the last-named variety
possesses double-determinants, the ' male ' half of which is
'yellow' and the 'female' half 'black' ; while the single-deter-
minants of the northern form are ' yellow.' This species is
properly speaking, therefore, not trimorphic, but includes two
local varieties, one of which is dimorphic. If the two varieties

DIMORPHISM AND POLYMORPHISM 375

interbreed, — as is actually the case at the junction of the dis-
tricts in which the two forms are respectively distributed, — the
double-determinants of the southern form will meet with single-
determinants of the northern form in the germ-plasm of the
offspring. The male descendants of such a cross would remain
unmodified, but the females would be either black or yellow
according to the power of transmission of the 'female' halves of
the determinants, or — as was observed by Edwards* — a com-
bination of both these colours. Such combinations might either
arise from the cross between a yellow female and a yellow male
of the dimorphic variety, or from that between a black female
and a yellow male of the monomorphic form ; for in the
dimorphic variety the germ-plasm contains double-determinants
in the males as well as in the females. If we suppose that
these crosses occur frequently, the number of females of an
intermediate form in the borderland of the two districts would
gradually increase, and might ultimately result in the production
of a constant intermediate female form. But if the males ex-
hibited a preference for the females corresponding to them, the
female forms would remain essentially distinct from one
another. This seems to be the case in PapUio turniis, at least
Edwards states that the intermediate forms are rare.

The dimorphic and polymorphic females in many butterflies
may perhaps be looked upon as belonging to sexually diniorpJiic
local forms which have siibsequoitly spread and occasionally
crossed. In places where the varieties merely exist side by side
without interbreeding, each of them also contains either single-
or double-determinants, according to whether it is sexually
monomorphic or dimorphic ; but when interbreeding occurs,
the determinants of the two races come together, and then
several homologous double-determinants may even meet in the
same germ-plasm, — some in certain ids and some in others.

In Papilio tnrnns the case is not quite so simple as I have
stated it : as a matter of fact, this species exhibits a double
di/norphisni, for the yellow females do not exactly resemble the
males, but differ considerably from them as regards the shade
of yellow and the pattern on the wings : — thus they possess an
orange-coloured eye-spot on the posterior wings, which is absent
in the male. We must therefore assume that double-deter-

* W. H. Edwards, ' Butterflies of North America.'

376 THE (JERM-PLASM

minants are present in the yellow variety also. If we imagine
that the two sexes were identical as regards the wing-marking
in the immediate ancestors of Papilio tiirnus, as they are in the
closely allied European species Papilio inachao}u and that this
monomorphic ancestral form had persisted — in California, let
us say, — we should have an instance of that kind of poly-
morphism which. Wallace has described in the case of Papilio
jiiennioii^ in which there are one male and three female forms.
In this case w'e must suppose that the first and oldest form
possessed j/z/if/^-determinants for the wings ; while in the
second and third forms the determinants were double, and their
' male ' halves retained their original nature, the female halves
becoming modified in two different directions.

It is therefore not necessary, as might have been supposed,
to assume the existence of triple determinants from the fact of
the trimorphism of a species alone, or of quadruple or quintuple
ones in the case of polymorphism.

The polymorpJiisin of anitual and plant stocks rests on a
different basis, as it concerns the physiologically dissimilar
members of a higher stage of individuality — that of the stock.
This kind of polymorphism has already been treated of as a
phenomenon of development in connection with alternation of
generations. We must, however, take the closely allied poly-
vwrpJiis)?! of animal cotnniunities into consideration.

The differences between the male and female individuals in
bees has already been referred to the existence of double-
determinants. But a third form of individual, the worker,
occurs in the honev-bees. These workers differ from the
females in the slight development of the ovaries, the ovarioles
not only being fewer in number than in the ^ queen.' but even
frequently containing no eggs at all, and at most only very few.
The receptaculum seminis is also more or less reduced, and the
abdomen is much shorter and thinner than that of the queen
bee. If these were .the only differences between the two forms,
there would scarcely be any need to assume the existence of
special determinants for these parts in the germ-plasm of the
workers : we might imagine that the determinants of the
ovarioles, for example, were so constituted as to become active
in consequence of abundant nourishment, and to cause the
development of ovarioles ; while a smaller su]3ply of nutriment
would not always be sufficient for this purpose, and thus the

DLMORPHISNf AND POLYMORPHISM 377

complete formation: of the sexual organs would be prevented.
We know, indeed, that a fertilised egg may develop into a queen
or a worker, according to whether the larva arising from it is
fed on royal diet or with the less nutritious food supplied to
the workers.

This explanation, however, even if correct as regards the
degenerated parts of the workers, does not sufficiently account
for the other ditferences between the two kinds of females.
f\ir the workers are not inferior to the queen bee in all respects :
on the contrary, the worker's sting is straighten longer, and
stronger, and is provided with more teeth, than the queen's ;
the wings, moreover, are longer, the tarsal segment of the
hind-limb is provided with the well-known brush, and the tibia
has a depression known as the pocket, for carrying the masses
of pollen which the insect collects. These two characteristic
parts are wanting in the queen. Important differences must
also exist as regards the minute structure of the brain, for the
instincts of the queen are very different from those of the
workers. After fertilisation has taken place, the queen lays
eggs, but she neither gathers honey from flowers, excretes wax,
nor makes the honeycomb. It is therefore incredible that the
queen and workers should be formed by the agency of similar
determinants. The germ-plasm must contain double-deter-
minants for certain parts of the body of the queen and workers
respectively. But as we have already assumed the existence
of double-determinants for the formation of male and female
bees, or at any rate for the development of those parts which
diiTer in the two sexes, we can only make the further assumption
that the ^ fe)nale ' halves of the double-determinants may the?n-
selves cojisist of two Jiah'es, corresponding to the queen and
worker respectively, and that each of these halves must naturally
be looked upon as a complete determinant as regards size
and structure. It is of no consequence whether they are re-
garded as being closely connected together, or as independent
structures in close proximity with one another : in either case
they must have arisen by the doubling or tripling of a single
ancestral determinant. The terms ' double-determinant ' and
^half-determinant^ are simply used for the sake of simplicity.
Their relation to one another is similar to that existing between
the homologous but heterodynamous determinants of different
ids.

378 THE GERM-PLASM

In the case of bees, the factor that determines which of the
two halves of the ' female ' determinants is to become active,
seems to be the quality of the food supplied to the larva,
so that the critical moment only arrives long after the termina-
tion of embryogeny, and before the chrysalis stage is reached.
It is well known that when the queen is lost, another one is
produced by feeding a larval worker with royal diet. Thus the
sex depends on the occurrence or omission of fertilisation, but
the modification into a queen or worker takes place much later,
when the animal has reached the larval stage. The idea of the
trimorphism of certain determinants thus becomes much less
difficult to realise. We must look upon them as double-deter-
minants contained in the ids of the germ-plasm, the female
hemisphere of which is again composed of tw'o dissimilar
quarter-spheres. If the egg becomes fertilised the male half
becomes inactive, and we have already represented this figura-
tively as taking place by the ' female ' hemisphere extending
over the 'male' hemisphere, and enveloping it like a mantle.
This ' female ' ' determinant mantle ' consists of tw^o halves,
representing the queen and worker respectively, and we may
suppose that the subsequent determination during the larval
stage as to which half is to control the cell, takes place in such
a manner that the 'worker' half extends over the other when
the nourishment is poor, while with more abundant food the
* queen ' half grows more rapidly, and prevents the ' worker '
half from exerting any influence on the cell. I naturally do not
in the least suppose that this figurative representation of the
process represents the actual facts of the case, but it at any rate
shows that the existence of trimorphic determinants — or more
accurately, of double-determinants each possessing a dimorphic
half — is conceivable.

We might, however, also assume the existence of three in-
dependent determinants side by side, so arranged that they
become active under other definite influences ; and this con-
ception would better agree with the unavoidable assumption
that the three determinants which act vicariously are of a
similar size.

The differentiation of the determinants into several equivalent
parts, each of which prevents the others from becoming active,
may take place to a still further degree by the ' ))iale ' half of the
doiible-deteniiiiuDits becoiuing differentiated into two dissimilar

DIMORPHISM AND POLYMORPHISM 379

halves. The Termites, in addition to the workers or stunted
females, possess 'soldiers' or males, in which the sexual organs
are stunted, which possess very strong mandibles, and differ in
other important structural details from the ordinary males. In
this case, therefore, four determinants must be present, each
capable of being substituted for another, and only one of which
can be active at a time.

Apart from local dimorphism, a temporary dimorphism occa-
sionally occurs, and is especially well known amongst butterflies
as seasonal diinorpJiisui. In this case the individuals of the same
generation, hatched nt the same time of year, are alike, but the
summer and spring generations differ from one another.

In the European species Vanessa levana-prorsa, the indi-
viduals of the spring generation are characterised by a yellow
and black pattern on the upper side of the wings ; while the
summer form {prorsa) has black wings, with a broad white
transverse band, and delicate yellow lines running parallel to the
margins. Were we to superpose these two patterns, it would
be seen that the black parts in prorsa do not correspond to
the yellow ones in levana, and that the white band in the
former does not correspond to a yellow or black part in the
latter. This band is, on the contrary, entirely wanting in
levana^ and is represented by both black and yellow regions.

These cases of dimorphism can also, it seems to me, only
be accounted for in terms of the idioplasm by the assumption
of double-determinants, which, however, are concerned in this
case merely with the wing-scales, and essentially with those
on the upper side of the wings only ; for the lower surface,
though not precisely similar, differs far less in the two forms
than does the upper side. We will speak of the halves of these
double-determinants as ' winter ' and ' summer ' determinants,
and may suppose that the influences of temperature which affect
them at the beginning of the pupal stage determine which of
the two halves is to predominate over the other. Nearly twenty
years ago I showed that it is possible to compel the pupas of the
summer-generation to assume the winter form by exposing them
to a low temperature, so that the butterfly emerged as a le7>ana
instead of a prorsa. The converse experiment was also occa-
sionally successful, the pupae of the winter-generation being
forced to assume the summer form by the influence of a higher
temperature during, or shortly after, pupation. We may per-

380 THE GERM-PLASM

haps therefore suppose that an increase of temperature prevents
the 'winter' halves of the double-determinants in question from
developing, while it is beneficial to the 'summer' halves: and
that conversely, the development of the 'summer' halves
remains stationary when the temperature is lowered, while the
•winter 'halves continue to develop. This maybe illustrated,
as in tlie case of ordinary sexual dimorphism, by supposing a
determinant of the germ-plasm to be spherical, and to consist
of a ' summer ' and a ' winter ' half. This determinant would
remain unchanged throughout embryogeny. and even during
the entire caterpillar stage ; and the increased or diminished
temperature would only determine which half should outgrow
the other and prevent it from controlling the cell, at the
beginning of pupation, when the wings are formed.

In Va?iessa levana the males and females resemble each
other so closely in the pattern of the wings that they cannot be
distinguished from one another with any degree of certainty :
but in many other seasonally dimorphic butterflies, sexual dimor-
phism is exhibited in the pattern and coloration of the wings,
and we must therefore assume that they possess double-deter-
minants consisting of ' male ' and ' female ' halves, each of
which is again subdivided into a ' summer ' and a ' winter * half.
We do not know what factors determine the sex of butterflies,
but in many cases the determination is effected early, for ovaries
and spermaries can be distinguished from one another in the
full-grown caterpillar. Thus, as in the case of Termites, the
decision concerning the subdivisions of the double-determinants
takes place subsequently to that with regard to the primary

halves.

4. DiCHOGEXY IN Plants

De Vries has made use of the term ' dichoo:env' to describe
that form of dimorphism which becomes manifest when a
young vegetable tissue, under normal conditions, is capable of
developing in different ways according to the external influences
to W'hich it is exposed. Shoots of ivy bear leaves on the side
which is exposed to the light, and roots on the opposite side ;
but if the plant is rotated, the same shoot will grow leaves on
the side which previously bore roots, and vice versa. The
stimulus due to light therefore apparently causes a group of
cells, which would have formed roots if they had been in the
shade, to give rise to leaves.

DIMORPHISM AND POLYMORPHISM 38 I

The assumption, which is in accordance with de Vries's ideas,
that all the hereditary tendencies of the species are contained
in every cell of the ivy-shoot, and that those which concern the
leaves only undergo development in response to the stimulus
caused by the light, and those corresponding to roots only to
that produced by the shade, does not materially help us in the
solution of the problem. In point of fact, the smiie cells are
not capable of forming roots and leaves ; the leaves are much
less numerous than the short and closely aggregated roots, and
therefore a large number of cells, or groups of cells, which give
rise to roots when shaded, do not develop leaves when exposed
to the light ; they consequently contain no ' leaf-determinants.'
Hence the same idioplasm cannot produce roots when shaded
from, and leaves when exposed to, the light ; but the deter-
minants for the roots or leaves respectively must be distributed
very differently in the cells.

The predisposition to form either of these two structures is
obviously determined in the growing point or apex of the shoot.
The cells which are continually being produced by the apical
cells are destined at a very early stage to form the rudiments of
roots or leaves ; only a certain number of cells on the illumi-
nated side are provided with leaf-determinants, while much
more numerous cells on the shaded side are furnished with
root-determinants. The determination therefore takes place at
a very early stage, when the shoot is actually in an embryonic
condition ; and the degree of illumination determines which side
is to be provided with leaf- and which with root-determinants,
— each kind being distributed in accordance with a different
law, — as well as the side to which the groups of each kind of
determinants are to pass during the nuclear divisions in the
cells arising from the apical cells. This case is analogous to
that of the inversion of the viscera in man, except that we do
not know the cause of this change of position. Some influence,
however, must in this case also be exerted during the early
embryonic stages, and cause the liver to take up a position on
the left side, and the spleen and heart on the right, long before
these parts are actually formed. No subsequent influences
could cause the liver to become shifted from the right to the
left side, or could transform the liver into the spleen ; just
as in the ivy-shoot no influences can lead to the formation of
leaves on the shaded side when it is once covered with roots.

J

82 THE GERM-PLASM

It therefore seems to me that the sa/^ie cells cannot give rise
to two kinds of structures in this case, but that some groups
form roots, and others leaves ; and that, moreover, the idio-
plasms of these groups of cells dift'er from, and cannot be trans-
formed into, one another, but that the sides of the shoot which
produce roots and leaves respectively, are determined by the
influence of the light as soon as the first embryonic rudiments of
the shoot is formed.

The fact that rudiments of roots can actually be proved by
the aid of the microscope to exist in the tissue of the stem in some
plants, seems to me substantially to support this view. This is
the case in the willow, in which the power of producing roots
from cuttings exists in a high degree. But numerous other
plantc are also capable of multiplying by means of cuttings, and
we may suppose that in them, in spite of the absence of visible
root-germs, the determinants for the tissue of the roots are
present, and are ready to develop into germs whenever external
influences leading to the formation of roots come into play.

All cases of dichogeny, however, cannot be explained in this
manner. In some instances the sa;//e cells may actually develop
in different ways, and the idioplasm may therefore become
transformed by external influences. A transformation as re-
gards the dorso-ventral arrangement of the parts in a young
shoot of Thuja takes place when it is turned round so that the
relative positions of the upper and under surfaces are reversed.*
The cells which, under ordinary conditions, take on the form
of palisade cells, then assume the structure of the cells on the
lower surface, and vice versa.

It appears to me that this fact is to be explained by suppos-
ing that the determinants of these two forms of cells are both
present in each cell, but that only one or the other of them
becomes active, according to the degree of illumination. I can,
however, form no idea as to why such an arrangement is met
with in this case.

* Compare the statements made on this subject by Detmer, Biolog.
Centralblatt, Vol. VII., No. 23.

DOUDIFUI. rHHXUMENA OK HERF:1)ITY 383

CHAPTER XII
DOUBTFUL PHENOMENA OF HEREDITY

I. 'Xenia' and Telegony

Although it is certainly unnecessary in a theory of heredity
to discuss all the possible kinds of phenomena which are with
doubtful justice included under this head, I should not like to
pass over in silence the consideration of certain presumptive
observations, as they have so often been discussed, and were
considered worthy of notice by so eminent an authority as
Darwin. These refer in the first place to the so-called ' xoiia^
and to the phenomenon generally known as 'infection of the
gerni,' — which, in case it really exists, I should prefer to speak
of as telegony *

Focke has used the term 'xenia' to describe those cases
in which ' hereditary characters are supposed to have been
transmitted by the pollen to the tissues of the fruit as well as
to the fertilised egg-cell and the embryo arising from it.'

Darwin mentioned many instances of this kind, and attempted
to account for them by supposing that an emigration of 'gem-
mules' takes place from the sperm-cells (pollen-tubes) to the
surrounding tissue of the fruit. Focke has collected all the known
cases, and on reading them, one receives the impression that
they may very likely be deceptive. Blue grains occasionallv
occur amongst the vellow ones in cobs of the \ellow-grained
maize {Zea) after fertilisation with the pollen of a blue-
grained species. It is possible that previous crossings of the
two species may have produced this result, which might wrongly
be ascribed to the immediate influence of the pollen of another
species on the fruit. J. Anderson Henry even thought he had
observed that all the flowers in an inflorescence of a white
Calceolaria were reddened by the influence of the pollen from
a red kind on a single flower of this inflorescence!

* From T^Xe — at a distance, and 761*0$ — offspring.

384 THE GERM-PLASM

As such eminent botanists as Focke,* and more recently cle
Vries,t have expressed much doubt with regard to these obser-
vations— or rather interpretations, — we must wait until these
cases have been critically reinvestigated before attempting to
account for them theoretically. The chief difficulty we should
meet with in any such explanation would be due to the fact that
we are here concerned with the influence of the ger))i-pias}n of
the sperm-cell on a tissue of another plant which only con-
stitutes a part of this plant. It would thus be necessary to
assume that all the determinants of this germ-plasm are not
active, and that only those take effect which determine the
nature of the fruit.

The uncertainty of the observations is still greater in the in-
stances of so-called i)ifection of the germ. If the case recorded bv
Darwin — but not observed by him personally — is reliable, and
has been accurately described, all doubts must be set aside. A
mare belonging to Lord Morton ' bore a hybrid to a quagga,'
and subsequently * produced two colts by a black Arabian horse ;
these colts were partially dun-coloured, and were striped on
the legs more plainly than the real hybrid, or even than the
quagga. One of the two colts had its neck and some other
parts of the body plainly marked with stripes,' and the hair of
the mane is said to have resembled the short, stiff and upright
mane of the quagga, instead of that of the horse. J Similar cases
of the influence of a previous fertilisation on the structure of
subsequent offspring are related of several domestic animals, —
viz., of cows, sheep, pigs, dogs, and pigeons, as well as of human
beings when crosses occur between white races and negroes.

Up to the present time no experiments have been made with
this special object, and it would be necessary to use every con-
ceivable precaution in conducting such experiments, or they
would be valueless ; they could, therefore, be best made in
zoological gardens, not only because of the undoubtedly pure
material which might be used for the purpose, but also because

* Focke, ' Die Pflanzen-Mischlinge,' Berlin, 1881, p. 510, et seq.

t Hugo de Vries, ' Intracellulare Pangenesis," Jena, 1889, p. 206.

X I have quoted this case from Darwin's ' Variation of Animals and
Plants under Domestication,' 2nd ed.. Vol. I., p. 435. Darwin does not
seem to have known of the drawings of these colts which will be mentioned
in a subsequent paragraph. I have not seen them, and only learnt of
their existence from Settegast's book-

DOUBTFUL PHENOMENA OF HEREDITY 3S5

it would be possible to isolate the animals, and for the keepers
to exert a strict control over them for considerable periods of
time.

The philosopher Carneri mentions a case which came under
his own notice. He kept a herd of cattle of the dark grey
Miirzthal breed. On one occasion he put one of the cows to a
'light-coloured Pinzgau' bull instead of to one of the same
breed. The cow threw a calf with the characteristic brown and
white patches of the Pinzgau breed, as well as with distinct
traces of the • dark grey Miirzthal cross.' The cow was sub-
sequently covered by a Miirzthal bull, and, contrary to expecta-
tion, the second calf was also a ' hybrid/ being for the most
part grey, ' but possessing large brown spots like those of the
Pinzgau breed.'

Both the above mentioned cases are not so conclusive as they
appear to be at first sight. A drawing by Agasse of the foal
possessing the characters of the quagga is to be seen at the
Royal College of Surgeons in London, and shows indistinct
dark stripes on the neck, withers, and legs. Similar stripes are,
however, not very uncommon on purely bred foals, and ordinarily
disappear as the animal grows older. No further resemblance
to the quagga can, however, be detected in these pictures.*

I must not omit to mention that before having heard of the
hypothesis of * infection,' Carneri accounted for the case of the
two breeds of cattle described above by supposing that - a drop
of Pinzgau blood' must have previously got into the Miirzthal
herd without his being aware of it.

Thus even the best of these ' cases ' are not reliable and
actually convincing. We may, however, at any rate suppose
that this so-called '■ infection,' if not altogether deceptive, only
occurs in rare instances, and by no means regularly, or at most
only in some cases. Experienced breeders, like Settegast and
Kiihn of Halle, do not believe in it ; for although they have
frequently crossed various domestic animals, they have never
observed an instance of it. Such cases could only be accounted
for from our point of view by supposing that spermatozoa had
reached the ovary after the first sexual union had occurred, and
had penetrated into certain ova which were still immature. The

* According to Settegast (' Thierzucht,' Breslau, Bd. I., 1878, pp.
223-234).

386 THE GERM-PLASM

immediate fertilisation of the latter is rendered inconceivable
by the fact of their immaturity, and the sperm-cell must have
remained in the body of the ovum until the maturation of the
latter, with the nucleus of which it then united in the process of
amphimixis. If this occurred sometime after the first of the
offspring was born, it might easily have coincided approximately
with tlie second coitus^ from which the fertilisation would then
apparently be due. If the 'infection' were proved beyond a
doubt, a supplementary fertilisation of an egg-cell in this
manner must be considered possible ; we certainly might then
reasonably ask why mares, cows, or sheep, should not occa-
sionally become pregnant without being covered a second time.
But this has never yet been known to occur, and I incline to
Settegast's view that there is no such thing as an ' infection ' of
this kind, and that all the instances which have been recorded
and discussed critically by him are based upon a misconception.

2. The Influence of Temporary Abnormal Conditions
OF THE Parents on the Child

Although I do not consider that the cases which come under
the above heading have anything to do with heredity, I should
not like to leave them entirely on one side.

It has often been supposed that drunkenness of the parents at
the time of conception may have harmful effect on the nature of
the offspring. The child is said to be born in a weak bodily
and mental condition, and inclined to idiocy, or even to mad-
ness, &c., although the parents may be quite normal both
physically and mentally.

Cases certainly exist in which drunken parents have given
rise to a completely normal child, although this is not a con-
vincing proof against the above-named view ; and in spite of
the fact that most, or perhaps even all, the statements with
regard to the injurious effects on the offspring will not bear a
very close criticism, I am unwilling to entirely deny the possi-
bility that a harmful influence may be exerted in such cases.
These, however, have nothing to do with heredity, but are
concerned with an affection of the germ by means of an external
influence.

The experiments of the brothers Hertwig show that the develop-
ment of the fertilised o.^^ in lower animals may be considerably

DOUBTFUL PHENOMENA OF HEREDITY 38'/

retarded by the action of various chemical substances, such as
chloral, quinine, and morphia ; and we also know that the ova of
sea-urchins, if kept too long in the sea-water before being fer-
tilised, tend to lose their vital energy, and consequently many
spermatozoa, instead of a single one, are likely to enter each of
them. A similar result may follow from the effects of the above-
mentioned chemical reagents^ and in both cases an abnormal
development of the egg, such as a duplication of parts, may be
the consequence.

It does not appear to me impossible that an intermixture of
alcohol with the blood of the parents may produce similar
effects on the ovum and sperm-cell. According to the relative
quantity of alcohol, either an exciting or a depressing influence
might be exerted, either of w'hich would lead to abnormal de-
velopment. A depressing influence exerted on dot/i germ-cells
would certainly retard, or even quite prevent, the process of
fertilisation ; while if the egg-cell were a/one affected, super-
fertilisation (polyspermy) might result ; and the same might
occur by an excitation of the sperm-cells alone. The entrance
of several spermatozoa into the small human ovum^ which con-
tains only a small amount of yolk, might produce an abnormal
development just as much as in the case of the eggs of the star-
fish or sea-urchin. A high degree of excitation in both germ-
cells might, on the other hand, cause the complicated processes
of the increase of the germ-plasm in the ovum and the subse-
quent conjugation of the two germ-plasms to take place in an
inexact manner, owing to their being passed through too quickly,
and would then produce an irregular development.

A^eiu predispositions can certainly never arise owing to such
deviations from the normal course of development, and therefore
a modification of the process of heredity itself is out of the
question. It is, however, conceivable that more or less con-
siderable abnormalities may aff"ect the course of development,
and either cause the death of the embryo, or else produce more
or less marked deformities. The question as to whether such
deformities really result in consequence of the drunken con-
dition of the parents can only be decided by observation.

3. The Supposed Transmission of Diseases

There is no doubt that some diseases are passed on from one
generation to another. All such cases are not, however, con-

.^88 THE GERM-PLASM

J

nected with lieredity, and many of them are in all probability
to be explained as the result of infection of the parental
germ-cell with microscopic parasites, and ought consequently
to be described as infections of the genn.

In man such a transference of disease has only definitely
been proved to occur in the case of syphilis.* The father, as
well as the mother, is capable of transmitting this disease to the
embryo, and the only possible explanation of this fact is. there-
fore, that the specific bacteria of syphilis can be transmitted
by the spermatozoon. Amongst the lower animals the 'peb-
rine ' of the silkworm is an example, which has been well
known for several decades, of the transference of a fatal
disease from one generation to another through the ^^g : the
germs of the fungus which produces the disease penetrate into
the yolk. It is not known why these germs do not develop
and multiply within the egg, and thus destroy it, but this is,
however, the case. The fungi only begin to multiply in the
young caterpillar f when it is half- or full-grown, or the disease
may, again, only be fatal in the butterfly stage.

As we now know that many diseases of man and other
mammals are due to such low forms of parasites, it is natural
to suppose that the transmission of such diseases results from
infection of the germ-cell with microbes, and not from inheri-
tance in the true sense of the word — that is, from the trans-
mission of an anomalous state of the germ-plasm itself.

I have elsewhere attempted to trace the ^ heredity ' of ' epilepsy,'
produced artificially in guinea-pigs, by supposing that in this
case a similiar process occurs. The slow development of this
form of ' epilepsy,' resulting from an injury to the spinal cord
or one of the larger nerves, seems to me, indeed, to support the
conclusion that its symptoms, which resemble those of true
epilepsy, are due to the migration of microbes, which advance
from the injured part along the nerves in a centripetal direction
until they reach the brain, where they set up the state of irrita-
tion characteristic of the disease. The great inconstancy of
the symptoms, and the variety of forms of nervous diseases
which the offspring exhibit, also indicate that a true heredity is

* Cf., e.g., Dohrn, 'Zur Frage der hereditaren Infection,' Deutsche
med. Wochenschrift, Sept. 15, 1892.

t Cf., F, Haberlandt, ' Der Seidenspinner des Maulbeerbaums u. seine
Krankheiten W'ien, 1871.

DOUBTFUL PHENOiMENA OF HEREDITY 3S9

not concerned in the process, and that the transmission is in this
case due to infection of the germ with the microbes by which
the disease is induced.*

The • transmission ' of carcinoma might be accounted for in
a similar way. — if, as has recently been supposed, this disease
is really due to microbes.

It is, however, also conceivable that both causes — the trans-
mission of abnormal predispositions, and infection of the
germ — might combine to bring about the transference of a
disease from one generation to another. Without desiring to
encroach upon the domain of pathology, I am inclined to sup-
pose that this is the case as regards ' hereditary' tuberculosis :
there is no doubt about the occurrence of a 'tuberculous
habit," — that is, a certain complication of structural peculiarities
which is commonly connected with the disease, such as a
narrowness of the chest, for instance. These peculiarities
must result from the structure of the germ-plasm, in which a
definite variation of certain determinants and groups of deter-
minants must have taken place, and they are therefore certainly
transmissible. The disease itself, however, is not due to this
^ habit/ but is caused by the presence of specific parasites, the
tubercle-bacilli, which have a harmful effect upon the various
living tissues. They may be introduced artificially into the blood,
and then produce the disease even in perfectly normal individuals.
They may, moreover, enter the body ' spontaneously,' ^-.^-^ by
some natural means, and will then also give rise to the disease.
But in the latter case the probability of infection seems largely
to depend upon the susceptibility or power of resistance of the
individual, and at the present day pathologists are of opinion
that persons exhibiting the * tuberculous habit ' already referred
to have a much slighter power of resistance to the parasites
which have passed into the body than strongly-built people.
The inheritance of the disease w^ould accordingly depend on the
transmission of a constitution very liable to infection.

Without wishing to deny the e.xistence of such a predispo-
sition to infection. I do not believe that the transmission of
tuberculosis is due merely to the inheritance of a greater degree

* A more detailed account and proof of this view concerning the infec-
tious nature of traumatic epilepsy in guinea-pigs is contained in my essay
on ' The Significance of Sexual Reproduction,' Appendi.x iv.

390 THE GERM-PLASM

of susceptibility. A large number of facts seem to me, on the
contrary', to support the view that infection of the germ plays
the chief part in tlie process. It would be out of place to enter
into particulars and attempt to prove this view here — the question
belongs to the province of the pathologist : 1 merely wished to
point out in this connection that a combination of hereditary
transmission and infection of the germ is perfectly conceivable.
The phyletic origin of such constitutional diseases is presumably
to be explained as being due to the occurrence of certain in-
dividuals possessing constitutions which were abnormally sus-
ceptible to a certain kind of microbe. Such persons would be
more readily attacked from without by this particular disease.
If, however, it once attacked them, and were it of such a kind
as to cause death only after some time, a further and much
surer opportunity was offered to the microbes for transferring
themselves to other hosts than was previously the case when
they passed into the body from without : — they settled in the
germ-cells of the individual affected, and were thus transferred
to the descendants of this individual. Although the presence
of parasites in the germ-cells has not yet actually been proved
in the case of tuberculosis, in my opinion it by no means follows
that such an infection does not nevertheless take place : we do
not even know whether such microbes are of the ordinary
form and size. In any case they must possess different vital
qualities ; for did they multiply in the tgg- or sperm-cell in
the same manner as in the tissues in which they are known
to occur, the germ-cells would soon be destroyed. Numerous
adaptations to the host may have occurred in this case as in
that of other parasites ; and, moreover, latent periods of develop-
ment may have arisen during which the parasite does not
undergo multiplication. It seems improbable that such arrange-
ments should not be met with, and that the parasite should not
make use of the favorable opportunity of becoming distributed
with the greatest certainty. Latent periods very commonly
occur in the germs of animals and plants whenever they are
useful, and hence this arrangement must come about without any
great difficulty.

Even although our most eminent pathologists, such as Ernst
Ziegler, are now of opinion that tuberculosis is not transmitted
by infection of the germ, because such a transmission has not
been directly proved, and because, on the other hand, an in-

DOUBTFUL PHENOMENA OF HEREDITY 39 1

fection from without cannot be conclusively disproved in any
individual instance, I am inclined to believe that they have been
too cautious in their conclusions, of which only a negative proof
is furnished by either factor. For neither of these in the least
proves that infection- of the germ does not take place : from a
more general, biological point of view, indeed, it seems to be far
more probable that it does.

It will, I think, at any rate be conceded that a '■ constitu-
tional ' disease cannot be taken as a proof that the processes of
heredity are therein concerned until we can determine whether
we are actually dealing with heredity, — i-e., the transmission of
a constitution, — and not only with a transference of microbes;
and the main object of this section was to make this clear.
But at the same time I have stated my reasons for using so
few facts from the domain of pathology in support of my
theory.

PART IV

THE TRANSFORMATION OF SPECIES: ITS
ORIGIN IN THE IDIOPLASM

CHAPTER XIII

THE SUPPOSED TRANSMISSION OF ACQUIRED

CHARACTERS

I. Difficulties in the Way of a Theoretical Basis

FOR THIS Assumption

By acquired characters I mean those which are not performed
in the germ, but which arise only through special influences
affecting the body or individual parts of it. They are due to
the reaction of these parts to any external influences apart from
the necessary conditions for development. I have called them
'■somatogenic'' characters, because they are produced by the
reaction of the body or soma, and I contrast them with the
' blastogenic ' characters of an individual, or those which origi-
nate solely in the primary constituents of the germ (' Keimesan-
lagen'). It is an inevitable consequence of the theory of the
germ-plasm, and of its present elaboration and extension so
as to include the doctrine of determinants, that somatogenic
variations are not transmissible, and that consequently every
permanent variation proceeds from the germ, in which it must
be represented by a modification of the primary constituents.

I will first attempt to show how this conclusion is arrived at
theoretically, and will then proceed to test it by ascertaining
how far it is in agreement with actual observation, and whether
the theory can be justified by facts.
392

SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS 393

Somatogenic variations may be classified according to their
origin into three categories, — viz., injuries, functional varia-
tions, "AXidi variations depending on the ?,o-c2i\\Q:6.' influences of
envi?'onment,' — which include mainly climatic variations.

The hereditary transmission of any of these three kinds of
somatogenic variations could be accounted for theoretically only
by the assumption that that part of the soma which had been
changed by external influences, could modify the germ-plasm
contained in the germ-cells of the same individual, so that its
offspring would, from the germ onwards, undergo similar varia-
tions to those which had been acquired by the action of external
influences on the parental part in question.

As far as I can see, there are only two ways in which such a
variation could conceivably occur in the germ-plasm in con-
sequence of a corresponding somatic variation. We should
either have to assume the presence in all parts of the body of
definite tracks along which each somatogenic variation might
be transferred to the germ-cells, in the germ-plasm of which it
would produce a corresponding change ; or else that gemmules.
such as Darwin supposed to exist, are given off from every
somatic cell and are conveyed to the germ-cells, — either
through the vascular system, when one exists, — or by some
other means, and that they must then penetrate into these
cells, and become incorporated in their germ-plasm. Thus
either the presence of hypothetical tracks along which a
modifying, though totally inconceivable, influence might be
transferred to the germ-cells, or else the discharge of material
particles from the modified organ, must take part in the for-
mation of the germ-plasm : there is no third way out of the
difficulty.

Both these hypotheses have already been used to explain
the supposed transmission of somatogenic variations, — the
former, it is true, only in vague allusions hinting at ' nerve-
influences/ which are supposed to start from the modified part,
and to produce a corresponding alteration in the hereditary sub-
stance of the germ-cells. But no one has yet ventured to state
more precisely how nerve-excitation can modify the germ-plasm
materially, and /;/ accordance with the somatic variation. It
would probably be useless even to expect an answer to the
. question as to how a part, such as a muscle, enlarged by functional
hvpertrophy, is capable of producing a specific nervous current

394 1HE GERM-PLASM

signifying 'enlargement/ If such an explanation were at-
tempted, we should be compelled to imagine that every cell in
the body was placed in communication with every germ-cell of
the ovary or spermarium by means of a large number of nerve-
tracks, and was capable of continually sending information to
the germ-cells of what was occurring in its own substance, and
of the manner in which it was influenced, and also of giving
instructions how each of the millions of units in the germ-plasm
should behave. I believe that it would be impossible to avoid
absurdities in explanations of this kind, and consider the whole
idea inadmissible.

The second possible explanation appears to me to be less
acceptable at the present day tlian when it was put forward
by Darwin in the form of a hypothesis of pangenesis. And. as
already stated in earlier essays, I believe that the talented
author of this hypothesis of heredity did not look upon it as
a well-grounded assumption, but considered it merely as a
working hypothesis, only intended to lead to a better insight.
Meanwhile, many changes have been made, and we have
become acquainted with facts which compel us to reject the
idea of a ' circulation of gemmules," and I am surprised that
this has not hitherto been done. This hypothesis is rendered
inadmissible, not merely because we must imagine that the
gemmules are gh'e?i off, and then circulate through the body,
but principally on account of the implied addition of gemmules
— i.e., of p?'i>nary constituents — to the ger/n-plasni of the
germ-cells !

According to Darwin's idea, there must be a constant addition
of * primary constituents " or gemmules to the germ-plasm
already present in the germ-cells, unless, indeed, it is assumed
that the entire nuclear substance in the germ-cells is formed by
gemmules which migrate into them. Such an assumption is,
however, contradicted by the fact that the hereditary substance
of the germ-cells, which we obser^'e in the for )n of nuclear rods or
idants, receives 7io addition to its organised bodies, the primary
constitue?its. I have come to this conclusion, not from the fact
that we have never observed an addition of this kind, but from
the way in which the hereditary substance has been shown to
behave during its multiplication. We know that the cell contains
a most wonderful mechanism which apparently has the sole
function of distributing the idants quantitatively and qualitatively,

SUPPOSED TRANSMISSION OF ACQUIRED CHARACi ERS 395

according to the primary constituents which they contain ; and
this is done as equally as possible, or, at any rate, in a definitely
prescribed manner. Why should the centrosomes and spindle-
threads be present, and the longitudinal fission of the idants
occur, if myriads of primary constituents of the germ-plasm
circulate separately through the body, and are capable of enter-
ing the germ- and other cells from without, as well as of
becoming properly arranged in them in the order in which thev
subsequently undergo development? Why should nature be
so scrupulously careful to divide the idants as accurately as
possible, if their composition were open to alteration at any
moment by the entrance of new primary constituents or gem-
mules? T/ie process of the fission of the idioplasm in nuclear
and cell-division seems to nie directly and conclusively to refute
the whole idea of the circulation of gennnules. For the very
reason that these nuclear rods or idants can never receive an
addition of new primary constituents from without, the most
extreme care is required, during their multiplication by division,
to prevent the different qualities of the mother-cell from being
distributed improperly amongst the daughter-cells, and causing
an irreparable loss of certain primary constituents to one of the
latter and its descendants.

It is impossible to assume the transmission of somatogenic
variations in any theory which accepts the nuclear substance
of the germ-cells as germ -plasm or ' hereditary substance " ; for
it is theoretically impossible to account for these variations, no
matter how ingeniously the theory is constructed.

At the present day I can therefore state my conviction still
more decidedly than formerly, that all permanoit — i.e., heredi-
tary — variations of the body proceed from primary modifications
of the primary constituents of the germ ; and that neither injuries,
functional hypertrophy and atrophy, structural variations due to
the effect of temperature or nutrition, nor any other influence of
environment on the body, can be communicated to the germ-
cells, and so become transmissible.

This statement naturally implies the rejection of Lamarck's
principle of variation ; for those factors which this talented
philosopher and investigator believed to be all-important in the
modification of species, — viz., the use and disuse of part.s, — can
have had no direct share in the process. I am by no means the
only one to hold this view at the present day ; and although

96

THE GERM -PLASM

the truth cannot be decided by a consensus of opinion, it is
nevertheless a significant fact that the views of such naturahsts
as Rav Lankester.* Thisehon Dver, Brooks, Mevnert.t van
Bemmelen.l and others, coincide with my own.

The fact, however, that we deny the transmission of the effects
of use and disuse, does not imply that these factors are of no im-
])ortance : and I have already attempted to show in former essays
that both use and disuse may lead indirectly to variations, — the
former wherever an increase as regards the character concerned
is useful, and the latter in all cases in which an organ is no
longer of any importance in the preservation of the species, and
in which, so far as the disused organ is concerned, 'panmixia'
occurs.

Want of space prevents me from discussing these questions
in detail ; their consideration belongs rather to a work on the
theory of descent than to one on that of heredity, and I need
only refer to my former essays,§ in which. I think, sufficient
proof is given to show that the gradual degeneration of organs
which are no longer of use does not require the assumption of
the transmission of somatogenic variations, and that conse-
quently the facts do not compel us to adopt a hypothesis which
we seem unable to accept theoretically.

It therefore remains to be seen whether w^e are not acquainted
with other facts which are explicable only on such a hypothesis :
one side of this question will now be treated of in particular.

2. The Hypothesis Tested by Facts

A few words will suffice concerning the hypothesis of the
transmission of injuries and vuiiilations, which has been accepted
for so long a time, and is obstinately defended even at the
present day ; for since the appearance of my essay on ' The

* Ray Lankester, 'The History and Scope of Zoology,' ' Enc. Brit.,'
Vol. xxiv.

t Meynert, ' Mechanik der Pliysionomik,' a Lecture held at the Meeting
of German Naturalists at Wiesbaden, 1887.

X J. F. van Bemmelen, ' De Erfelijkheid van verwooven Eigenschappen,'
s'Gravenhage, 1890.

\J Cf., ' Uber die Verebung,' Jena, 1883, and ' Die Continuitat des
Keimplasma's als Grundlage einer Theorie der Verebung,' Jena, 1885.
English translation, ' Essays upon Heredity,' pp. 71 and 165.

SUPPOSED TRANSMISSION OF ACQUIRED CHAR.\CTERS 397

Supposed Transmission of Mutilations/ * no new observations
on this point have appeared. f The old arguments, on the
scientific worthlessness of which I then expressed my opinion,
are constantly being brought forward, — in part altered, and in
part with a new interpretation. It is now even less necessary
than ever to return to the matter, as even among those observers
who supported the view of the transmission of functional varia-
tions, a few agree with me in denying the transmission of
mutilations. As an instance, I may mention Osborn. who,
however, goes a little too far when he compares the contest of
the old view of the transmission of mutilations with Don
Quixote's celebrated fight with the windmills. :|: Only a few-
years ago, at a meeting of the German association of naturalists,§
two ' tailless ' cats were exhibited, in which the absence of the
tail was supposed to be due to their mother having accidentally
lost hers ; and biologists of such eminence as Ernst Haeckel j|
have accounted for similar cases in the same wav. Since such
men still regard the inheritance of mutilations as possible, the
exposition of the subject has not been a superfluous task.^

There is, however, a third kind of somatogenic variation^
produced by the influence of enviroiDnent, the mode of nntrition,
the climate, and so on ; this often appears to be transmissible,
and consequently capable of becoming increased in the course
of generations. I myself called attention to this fact a number
of years ago in the following words : — 'I only know of otie class

* Jena, 1889, ' Essays upon Heredity,' Oxford, 1889, p. 421.

t None, that is to say, which are in opposition to my views. My experi-
ments with mice have been confirmed by Ritzema Bos and by Rosenthal.
I have now continued these experiments to the nineteenth generation —
always with the same negative results; cutting off the tails has no influence
on the tails of the descendants. A similar result was obtained by both the
above-mentioned observers from experiments on rats {cf. ' Biolog. Central-
blatt,' Vol. xi., 1891, p. 734, &c.).

X Osborn, ' Are Acquired Variations Inherited? ' Boston, 1890, p. 3.

\J At the meeting at Wiesbaden, 20th September 1887.

Jl Haeckel, ' Xatiirliche Schopfungsgeschichte,' 3rd ed., 1889, p. 194.
English edition, ' The History of Creation,' London, 1876, Vol. i., p. 214.

T[ See the Section on the Transmission of Mutilations in Eimer's book
entitled 'Die Entstehung der Arten,' <S:c., Jena, 1888. (Cf. 'Organic
Evolution as the Result of the Inheritance of Acquired Characters,' Sec,
translated by J. T. Cunningham, London, 1890.) A whole collection of
' proofs ' are there given.

39^ THE (lERM-PLASM

of changes in the organism which is with difficulty explained
by the supposition of changes in the germ ; these consist
in modifications which appear as the direct consequence of
sovte alteration in the surroundings. But our knowledge on
this subject is still very defective, and we do not know the
facts with sufficient precision to enable us to pronounce a final
verdict as to the cause of such changes/ Mention was then
made of a few of the large number of cases which have been
repeatedly quoted, and I attempted to show that none of them
stood criticism, that they could not be explained in the way some
investigators supposed, and that somatogenic variations are
only apparently hereditary ; for in reality a change must first be
brought about in the germ-plasm by the influence of the surround-
ings before such a variation can be produced. I then continued :
— ' It must be admitted that there are cases, such as the climatic
varieties of certain butterflies, which raise some difficulties against
this explanation. I myself, some years ago, experimentally in-
vestigated one such case, and even now I cannot explam the
facts otherwise than by supposing,' as I did then, that somatic
variations were transmissible. ' It must be remembered, how-
ever, that my experiments,' which have been repeated upon
several American species by H. W. Edwards, 'were not under-
taken with the object of investigating the question from this
point of view alone. New experiments, under varying con-
ditions, will be necessary to aff'ord a true explanation of this
aspect of the question.' *

Since 1883 I have waited in vain for some skilled entomolo-
gist or for one of the numerous advocates of the transmission
of acquired characters, to carry out the proposed experiments.
In the meantime, as far as the time and material at my disposal
permitted, I have myself made a start on this line of research,
and now possess the results of a series of new experiments,
which, though not so numerous, complete, or exhaustive as I
could have wished, are nevertheless sufficient to form a more
trustw^orthy basis for a theory dealing with variations of this
kind. Some of these are described in the following section, a
more detailed account of them being left for another occasion.

* Cf. my essay, 'Uber Vererbung," Jena, 1883. English edition, Oxford,
1889, pp. 98-99.

supposed transmission of acquired characters 399

3. Climatic Variation in Butterflies
Polyommatus phlceas, a butterfly belonging to the family
Lyccpuidce. is distributed over the whole of the temperate and
colder parts of Europe and Asia. It also occurs on the shores
of the Mediterranean, in Maderia, the Canaries, and in part of
North America. Before the glacial epoch this species must
have inhabited the more northern circumpolar regions, and
have been driven southward during that epoch ; subsequently
it must have again migrated towards the north. In our lati-
tudes the upper surface of the wings of this form is of a beautiful
reddish-gold colour, and hence it has received the popular name
' Feuerfalter ' (fire butterfly). Further south, the reddish-gold
colour is more or less thickly dusted with black, and specimens
from Sicily, Greece, or Japan often display only a few reddish-
gold scales, the general appearance being almost black. In
Germany this butterfly is double-brooded, and the two genera-
tions are similar ; but in certain districts of Southern Europe,
such as the Riviera di Levante, the first generation is
reddish-gold, — the second, which flies in midsummer, and is
known as the variety eleus, having the wings well dusted with
black. As in Germany, during exceptionally hot summers,
individuals with a blackish tint have repeatedly been caught
together with the ordinary form, and as, moreover, in the extreme
southern limit of their range — so far as my experience extends
— both generations have a blackish colour, it would appear at
first sight that the modifications are merely due to the effect of
heat ; — the butterfly becomes red when exposed to a moderate
temperature, and black when the heat is greater.

The following experiments, however, prove that this conclusion
cannot be a correct one. Caterpillars were raised from the eggs
of the German form of P. phlcsas, and the pupae were then
exposed to a much higher temperature till the emergence of the
butterfly. The result was that many of the butterflies were
slightly dusted with black, but none of them resembled the
darkest forms of the southern variety eleiis. I then made the
reverse experiment, subjecting caterpillars which had just en-
tered the pupal stage, and had been raised from the spring
generation of the Neapolitan form, to a very low temperature.*

* I must take this opportunity of expressing my warmest thanks to Dr.
Schiemenz, of the Zoological Station at Naples, for the kind and generous

400 THE GERM-PLASM

Many butterflies were thus obtained which were not so black as
those which had emerged from pupae kept at a higher tempera-
ture, but iio7ie were so light-coloured as the ordinary Gertnan
form. The difference between the Neapolitan specimens which
had become ligJit -coloured from exposure to cold, and the normal
Gcnnan form, on the one hand ; and that between the German
specimens artificially darkened by warmth, and the normal
Neapolitan form, on the other, is too great to be attributable to
the ificompleteness of the experiments. The German and the
Neapohtan forms are therefore constitutionally distinct., the
former tending much more strongly towards a pure reddish-gold,
and the latter towards a black coloration.

Both experiments, however, prove the correctness of the old
assumption of Lepidopterists that the action of heat on a single
generation is capable of giving the German form of a blackish tint ;
and since, moreover, it is clear that the development a single
generation at a lower temperature can render the colour of the
Neapolitan butterfly less black, it appears that the two varieties
may have originated owing to a gradual cumulative influence of
the climate, the slight effects of one summer or winter having
been transmitted and added to from generation to generation.
This would then seem to be an instance of the transmission of
acquired characters.

I do not believe, however, that this is the correct interpreta-
tion of the facts. If it were, there could be no region in which
the species is seasonally dimorphic, as I have myself ascertained
it to be on the Ligurian coast. The germ-plasm would then
contain either the primary constituents of the red variety, if
the colony had been exposed for many generations to a low
temperature ; or those of the black one, if a high temperature
had influenced it for the same length of time. It would then
make no difference to what degree of temperature a single
generation were exposed at the present day in artificial breeding,
for the colour would have already been determined in the
germ-plasm, which would contain, to use my own phraseology,
either ' reddish-gold ' or • black " determinants for the wing-
scales in question. Hence it would be quite impossible for the
.spring generation to develop reddish-gold, and the summer one

assistance he has given me in my efforts. Without his help I should have
been unable to obtain the necessary living specimens.

SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS 4OI

black scales, for the germ-plasm would only contain either ' red '
or ' black ' determinants for a certain spot on the wing.

The theory of determinants will, I believe, supply a very
simple explanation of this apparently complicated case, which
I consider of great value, because it confirms this theory.
Instead of supporting the doctrine of the transmission of
somatogenic characters, this example shows how suc/i a process
may appa7-ently be brought about, and on what it depends.
A somatogenic character is not in this case inherited, but
the modifying influence — the temperature — affects the primary
constituents of the wings in each ijidi^'idual, — i.e., a part of
the soma, — as well as the geri}i-plasni contained in the germ-
cells of the animal. It modifies the same determinants in
the rudiments of tlie wings of the young chrysalis as in the
germ-cells, — namely, those of the wing-scales. The variation
cannot be transmitted from the wings to the germ-cells, but only
affects the coloration of these organs of the individual in ques-
tion ; whereas it is transmitted from the germ-cells to successive
generations, and consequently controls the coloration of their
wings in so far as this is not again modified by subsequent influ-
ences of temperature ; for the same determinants which are now
present in the germ-cells of generation I are afterwards passed
into the rudiments of the wings in the caterpillar and chrysalis
of generation II, and the change which they underwent while
lying in generation I may be increased or weakened by the
influence of the temperature to which they are exposed after
entering into generation II.

Since warmth aff"ects the whole body, it is not surprising that
the determinants which are modified by it should undergo these
modifications, whether they are contained in the germ-plasm of a
young egg or sperm-cell of the caterpillar, chrysalis, or butterfly,
or in certain cells in the rudiments of the wings in the chrysalis
or caterpillar. This, however, does not imply that they must
undergo the sa7ne amount of variation in both i)laces, for they
have not by any means the same environment in the two situa-
tions. In the germ-plasm they are grouped amongst thousands
of determinants of the species, all of which constitute the germ-
plasm ; while in the rudiments of the wings, they are associated
with only a few other kinds of determinants, and a time must
come when each of them controls a cell by itself and transforms
it into a red or a black wing-scale.

402 THE GERM -PLASM

We know, however, of a fact which definitely proves that the
susceptibility of the scale-determinants to the influence of
temperature is greatest at a certain stage in the development
of the butterfly — much greater than either before or afterwards.
I have frequently noticed in seasonally dimorphic species like
Vanessa prorsa-levana^ that the modifying influence of heat
or cold only acts at the beginning of the pupal stage.
Although I have not yet been able to ascertain the time at
which this occurs more precisely, it can be definitely stated that
the winter pupae of Vaiiessa levatia, for instance, which have
been exposed to a high temperature even only a month after
entering the pupal stage, are never transformed into the prorsa
form ; they all emerge as V. levana.

This is not due to the fact that the colour of the wings is
already deposited a month after the insect has entered the pupal
stage, for at this time there is no trace of colour w^hatever.
There must, consequently, be a period in the disi?itegration of
the determinants when they are most susceptible to the influences
of temperature : subsequently this is no longer the case, and
although they are susceptible before this period, I nevertheless
venture to suppose that they were so to a far slighter extent.
This may be due to their connection with other determinants,
or to other causes which we are not yet able to discover.

If, then, the determinants for the scales are only influenced
very slightly by the temperature as long as they are situated in
the germ-plasm, and are subsequently greatly affected by it at
a certain period in the development of the w'ings, the above-
mentioned phenomena admit of a simple explanation. The
germ-plasm of the southern colony of F. pJdceas must contain
many determinants among those for the wings, which, in con-
sequence of the exposure of thousands of generations to heat,
have been adapted for the production of black scales, together
with a large number of others which only require a small
increase of temperature during pupation in order to give rise
to a black colour. These latter kind cause such fluctuations
in the coloration as occurred in my experiments ; while the
former produce the black coloration of the wings, which has
become fixed in the constitution of the southern colony, and
can no longer be removed by the action of cold on the young
chrysalis.

In this case it is taken for granted that the ancestral form

SUPPOSED TRANSMISSION OF ACQUIRED CHAR.\CTERS 403

possessed pure reddish-gold wings, and that it inhabited high
northern latitudes, — an assumption which alone enables us to
understand the present distribution of the species, and which
has been adopted by Hofmann * in his splendid researches on
the origin of European butterflies. This, however, is of no
great importance in the present question, but we must assume
that either the reddish-golden tint, or the deep black dusting is
the primary colour. The seasonal dimorphism and the occur-
rence of blackish specimens in Germany in hot summers are
easily acounted for on the former assumption.

In consequence of the increase in temperature of the habitat
of the species, many scale-determinants in the germ-plasm
would gradually become so modified that the action of only a
slight further increase on the rudiments of the pupal wings
would lead to the production of black scales. In Germany the
species has attained this point in its phyletic modification ; and
if the weather happens to be hot when the second annual brood
enter upon the pupal stage, some butterflies of a blackish tint
will be produced. This will be more likely to happen as the
internal transformation of the determinants in question advances
further, and the blackish tint will become more conspicuous as
the scale-determinants which have reached this stage of modi-
fication in the germ-plasm become more numerous. These
two conditions will obtain most often in districts where the
summer is usually tolerably warm ; and the fact is thus ac-
counted for that dark specimens of P. phlceas are rarely caught
in northern Germany, and in the far north not at all, although
very dark forms occur comparatively often in the warm valleys
of Valais.

In still warmer districts, like the Riviera, the summer brood
of P. phlcnas is almost always exposed to a high temperature,
and hence the transformation of the determinants for the scales
has become so great, that with the help of the usual summer
heat at the time when the caterpillar enters the pupal stage, the
variety eleits has been produced. This variety does not appear
in the spring brood, because the additional heat required for the
complete transformation of the determinants for the scales is
absent during the pupal stage.

* Ernst Hofmann, ' Isoporien der europaischen Tagfalter, ' Stuttgart,
1873.

404 THE GERM-PLASM

If the area of distribution of the species extended unin-
terruptedly from the Polar regions to South Italy or North
Africa, all the intermediate forms would occur, from the pure
reddish-gold single-brooded form in Lapland to the black double-
brooded variety eleus : there would thus be — first, two similar
reddish-gold broods ; then similar ones, — those butterflies which
are exposed to a higher temperature during the pupal stage
having a tendency to develop a black tint ; and then seasonal
dimorphic forms, the butterflies being black in the summer and
reddish-gold in spring, as is actually the case in Genoa. A
still longer action of a higher temperature would at first change
a small and then an increasing number of determinants for the
scales into the ' black' variety, so that finally two broods, both
consisting of the black form elens, would occur. The case of the
Neapolitan colony is somewhat similar to the last-mentioned
one, for many black specimens certainly occur amongst the
spring brood, although there are also many light-coloured ones ;
none, however, are as light as the northern reddish-gold form of
P. phlceas. I do not know whether a complete change of colour
has been attained by both annual broods in any locality ; but if
it has, I should expect it to be seen in Southern Japan rather
than elsewhere, for the butterflies which I possess from the
neighbourhood of Tokio display an unusually dark colour.

This case has been discussed at length because it appears to
me to be especially significant, not only in the explanation of the
climatic varieties of butterflies, but also as regards the theory of
heredity, and the assumption of material determinants which
exist in the gerin-piasm and are passed on from one genera-
tion to another. The facts are so evidently in favour of this
assumption that no other explanation seems possible. It must,
however, be remembered that the artificial modification of the
colour on the wings does not take place if the change in tem-
perature occurs only when the scales begin to become coloured.
The colouring matter is consequently not produced by the direct
influence of chemical transformations, but by an indirect influ-
ence, which we may suppose to be due to a mutual disarrange-
ment and rearrangement of the ' biophor-material ' of which the
determinants consist, by the co-operation of which latter the
chemical process forming the colour is derived.

The occurrence of seasonal dimorphism alone, shows with
certainty that the determinants for the scales are influenced by

SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS 405

temperature in the germ-plasm to a much slighter extent than
in the rudiments of the wings. If the modifying influence had the
same effect in both cases, the germ-plasm in the germ-cells of
a butterfly of the summer generation would be modified as much
as the wings of the same individual ; and consequently the off-
spring, even if exposed to a low temperature, would necessarily
display a greater tendency towards the summer coloration,
because the latter was already potentially contained in the germ.
This, then, would only be the case if the influence of the cold
were stronger than that of the heat. In any case, however, a
coloration intermediate between that produced by cold and by
heat respectively would result, and would be transmitted to both
generations, even when the two influences were equally strong.
If we indicate the winter and summer colorations respectively
by A and B, the coloration of each generation would then be
I A + I B. It is only when the germ-plasm is modified to a much
smaller extent than the determinants which have already entered
the rudiments of the wings, that an alteration of coloration can
become permanent.

In many other animals and plants influences of temperature
and environment may very possibly produce permanent heredi-
tary variations in a similar manner ; but it is difficult — in fact
almost impossible — to identify such cases with anything like
certainty from the observations which have hitherto been made.
Thus we find it stated that ' in Cashmere dogs soon become
covered with a woolly hair ; ' * but we do not know who observed
this, or who ascertained that such a change — if it really does
occur — is transmitted. ^Merino sheep lose their fine wool when
they are transported to a tropical climate ; ' but I have not been
able to discover whether this loss occurs in the first, or in the
course of several, generations. We are thus left in uncertainty
as to the possibility of a direct climatic variation of a somatic
part having taken place in these instances, which would again
disappear in the next generation provided that the descendants
were placed under the original climatic conditions. The same
applies to the races of naked dogs from the tropics, such as the

* These statements are quoted from an essay by Giard, who takes
them as a proof of the transmission of somatogenic modifications. Cf.
' L'Heredite des modifications somatiques," Revue Scientifique, December
6th, 1890.

406 THE GERM-PLASM

Guinea dog, for ' they do not become covered with hair when
transported to a temperate climate.'' *

Many cHmatic varieties of plants may also be due wholly
or in part to the simultaneous variation of corresponding de-
terminants in some part of the soma and in the germ-plasm of
the reproductive cells, and these variations must of necessity
be hereditary. Temperature, and nutrition in its widest sense,
affect the whole body of the plant, — the somatic-cells as well as
the germ-cells. It cannot, however, at present be stated whether
the determinants in the soma are in this case influenced more
strongly than those which are still in the germ-plasm. It is
conceivable, and, I am inclined to think, even most usual, that
certain determinants are affected to the same extent whether
the influence of the environment happens to act on them in the
germ-plasm, or in any stage of somatic transformation. In this
case the change may have been perhaps scarcely or not at all
noticeable in the first generation, and may gradually have become
apparent, and also transmissible, in the course of subsequent
generations. On the other hand, there are probably many
influences of environment which produce a considerable change
in the body of the plant, without, however, modifying the cor-
responding determinants in the germ-plasm. The experiments
made by Nageli and many others on the genus Hieraciui}i at
any rate support this view, though they have hardly been carried
on long enough to exclude the possibility of a very faint and
gradual alteration occurring in the germ-plasm.

The question as to which influences are capable of simul-
taneously modifying the developing and growing soma and the
corresponding determinants in the germ-plasm, even in a very
different degree, can only be solved by future experiments. The
cases of an apparent inheritance of somatogenic variations are
due to this coincidence ; — no others are, it seems to me, con-
ceivable. All those influences, however, such as the use and
disuse of a part, which can only affect this part itself in a specific
manner, are incapable of producing a correspondijig change in
the respective determinants of the germ-cells, and consequently
cannot lead to hereditary modifications. In such cases the

* Compare also the cases of degeneration in the descendants of the
European dog in India, which have been carefully collected by Darwin in
his ' Variation of Animals and Plants under Domestication,' Vol. I., p. 45.

SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS 407

external influence affects only the fully-formed organ, — such as
a muscle which has become enlarged by exercise ; for the influ-
ence consists in the increased activity of the organ, which takes
place within it alone ; the germ-plasm of the germ-cells, and
even the determinant in the germ-plasm for the muscle in ques-
tion, are not thereby affected. In all cases of functional hyper-
trophy or atrophy, the external influence affects none of the
determinants, but only the fully-formed organs, — i.c.^ groups of
specific cells produced from determinants. In my opinion it is
very probable that such twofold modifying influences of environ-
ment as we meet with in F. pJilaas, can only occur when the
determinants which have not yet been transformed into the
organ, as well as the germ-cells, have been affected by the modi-
fying influence. And this will be most likely to happen in those
structures which, like the scales on the wings of butterflies,
are formed at a later stage of the animal's existence, and the
deten)iiiunits of which are consequently stored in an undeveloped
condition in the idants of cert ai 71 somatic cells during a threat part
of the ontogeny. The w'ings of the butterfly arise as outgrowths
from the hypodermis of the caterpillar. Before these outgrowths
can be formed, the determinants for the wing-scales must be
contained in the idioplasm of some of the cells of the hypo-
dermis, but after their appearance they would be found in some
of the cells in the rudiments of the wings. The wings at first
are small, and contain by no means so large a number of cells
as when full-grown, so that inactive determinants for several
wing-scales must be contained in the idants oi one nucleus. At
a certain period in the course of further growth, however, the
number of cells increases to such an extent that each determinant
constitutes the idioplasm of a particular cell, and the modifying
external influences seem then to have the greatest effect on these
determinants. By means of experiments it may be possible to
ascertain exactly when this occurs.

It might have been expected that in this section I should enter
into the whole question of the possibility of the transmission of
acquired variations, about which there has been ,0 much dispute
of late years, and that all the arguments and facts which have
been put forward in favour of the theory should be discussed.
But, as I have already remarked, this seems to be out of place
in a theory of heredity, the object of which is to show whether
this form of transmission is or is not possible from a theoretical

408 THE GERM-PI^SM

point of view, and to ascertain further whether in the latter case
such an apparent transmission might not possibly occur under
certain circumstances, and to account for this theoretically. I
have always emphasised the fact that it is easier to explain the
transformation of species on Lamarck's principle ; but this is no
reason for the retention of a theory which cannot be accepted
on theoretical grounds, unless no other explanation can be
given for the facts. So far, my opponents have been unable to
prove that this is the case.

The above explanation of the causes of the climatic variations
of butterflies may perhaps convince some of those who have
till now opposed my views that we are here not dealing blindly
w'ith mere principles, but with inductive methods. The view of
the non-inheritance of acquired modifications has been espe-
cially opposed in America, principally by the palaeontologists.
It can certainly not be denied that certain facts in palaeontology,
such as the development of the feet and teeth in Ungulates,
furnish us with an extremely fine and uninterrupted series of
forms which may apparently be very easily explained on the
assumption of the inheritance of acquired modifications. But is
not this exactly what would be expected, in case phylogeny essen-
tially depends on selection — that is, on an increasingly complete
adaptation to certain external conditions of life of a purely
general nature? It appears to me that neither the complete-
ness of the developmental series, nor the close relation of the
nature of the modifications to function, give any clue to the
causes which have produced these series. They may quite as
well have originated by continued selection alone, as by continued
transmission of functional variations.

The eminent American naturalist, Lester Ward,* is, how-ever,
in error if he supposes that the proof that climatic influences
are capable of modifying the germ-plasm, contains all that is
required by the neo-Lamarckian school. Further details will be
given in the next chapter as to the manner in which I now
suppose variation has originated : but quite apart from this, the
supposition that climatic influences can produce modifications of
the germ-plasm, has certainly nothing to do with the view that

* Cf. the essay directed against my views by this author (' Neo-Darwinism
and Neo-Lamarckism,' Washington, 1891), which is written from the thor-
oughly objective and truly scientific point of view.

SUPPOSED TRANSMISSION OF ACQUIRED CHAR.\CTERS 409

functional modifications of any particular organ can cause a
corresponding change in the germ-plasm. I believe I have
here furnished a proof that the former supposition is a correct
one : the onus of proof of the latter lies with the neo-
Lamarckians.

4IO THE GERM-PL.\SM

CHAPTER XIV
VARIATION

I. Normal Individual Variation

Heredity is the transmission of the physical nature of the
parent to the offspring. We have seen that this transmission
affects the whole organism, and extends to the most trifling
details ; and we also know that it is never complete, and that
the offspring and parent are never identical, but that the former
always differs more or less from the latter. These differences
give rise to the phenomenon of variation, which thus forms
an integral part of heredity^ for the latter always includes the
former.

A theory of heredity must therefore include a theoretical
substantiation of variation, such as I will now attempt. Why
is the offspring never an exact duplicate of its parent, even
when it possesses but one parent, as is the case in partheno-
genesis and reproduction by budding? And what forms the
basis of the constant ' individual variations "■ which, after the
precedent of Darwin and Wallace, we regard as the foundation
of all processes of natural selection, and as the means which
rendered possible such a rich development of organic forms of
the most diverse kinds on the face of the earth?

Darwin himself considered the difference between parent
and offspring as due to the diversity of external influences,
and I was essentially of the same opinion formerly, and stated
that ' all dissimilarities of organisms must depend upon the
individuals having been affected by dissimilar external influences
during the course of the development of organic nature.' * At
that time I attributed to the organism the virtual ' power of

*'Studien zur Descendenztheorie," II., Leipzig, 1876, p. 304. English
edition, ' Studies in the Theory of Descent,' translated and edited by R.
Meldola, with a prefatory note by Charles Darwin, London, 1882, Vol. II.,
p. 677.

VARIATION 4 I I

giving rise by multiplication only to exact copies of itself ; but
this capacity, in my opinion, did not produce accurate results,
because the organism is also capable of reacting to external
influences, and may therefore deviate from the inherited ten-
dency in one or another direction, according to the nature of
these influences.

Variation would consequently not depend upon a special
force existing in the organism, but would simply be the result
of external influences, which, either directly or indirectly, are
capable of preventing the organism from keeping strictly to the
inherited course of development.

Although I still consider this view to be in general correct,
the origin of individual variation, on which the transformation
of species is based, cannot be deduced so easily from the action
of dissimilar external influences as seemed possible when I wrote
the passage quoted above. I have discussed this question in
full on a previous occasion, and will here only briefly refer
to it.*

At that time, no one supposed that any difference existed
between the modifications which may be brought about in the
soma and those which proceed from the germ-plasm. Since
then, how'ever. we have been compelled — at least in my opinion
— to consider that only those variations which are ' blastogenic,''
and not those which are ' somatogenic,'^ can be transmitted. We
can no longer regard the direct influence of external impressions
on the soma as a means of producing hereditary individual
variations. It therefore remains to be seen what is the origin
of these variations, upon the existence of which we imagine the
entire development of organic nature to depend.

This development could be accounted for most easily on
Nageli's hypothesis, according to which the idioplasm is so
constituted, that in the course of generations it could exert a
definite and regular transforming influence upon itself, and by
this means could convert one species into another. Many
reasons may, however, be urged against this hypothesis. In
the first place, development by internal forces only is contra-
dictory to the close adaptations of organisms to their conditions
of life ; and secondly, we should not make use of unknown

*Cf. ' Die Bedeutung der sexuellen Fortpflanzung fiir die Selections-
theorie," Jena, 1886. English Edition, Oxford, 1889, p. 255.

412 THE GERM-PLASM

forces for the explanation of natural phenomena until we have
proved that the known ones are insufficient.

Such a phyletic principle of development would naturally
not produce ordinary individual variations, but would lead to
modifications lu/iich affect all the individuals of a species in the
same 7nanner. All the individual modifications which actually
exist would, in that case, have to be considered insignificant
in phyletic development ; and in plants Nageli actually
looked upon them to be transitory, and not hereditary habitat-
modifications ( • Standorts-Modificationen ') . But this view is
opposed by the transmission of an immense number of indi-
vidual differences in man and animals.

If we recognise that the processes of selection are the chief
factors in organic development, we must attach the greatest
importance to these hereditary differences in individuals, and
must try to discover their origin.

Brooks accepts this view, and has propounded a theory of
heredity which is based on Darwin's hypothesis of pangenesis,
and according to which variation is in the main dependent on
sexual reproduction.* V'ariability, in his opinion, is caused by
every ' gemmule ' of the spermatozoon uniting with ' that particle
of the ovum which is destined to give rise in the offspring to the
cell which corresponds to the one which produced the germ or
gemmule. . . . When this cell becomes developed in the
body of the offspring // will be a hybrid, and it will therefore
tend to vary.'' Brooks, moreover, assigns different functions
to the two kinds of germ-cells, and represents them as being
charged or filled to different degrees with ' gemmules,' the
egg-cell containing a much smaller number than the sperm-cell.
In his opinion the egg-cell is the conservative principle which
controls the transmission of purely racial or specific characters,
whereas the sperm-cell is the progressive element which causes
variation.

Brooks has ingeniously advanced every argument which could
be brought forward in favour of his theory, but I doubt whether
he still holds to it, for many new discoveries have since been
made which contradict it. Although the view that 'acquired'
characters are not hereditary is not universally admitted, it has

* W. A. Brooks, ' The Law of Heredity : A Study of the Cause of Varia-
tion and the Origin of Living Organisms,' Baltimore, 1883.

VARIATION

413

been accepted by Brooks ; and the assumption of a diversity of
the hereditary substances contained in the male and female
germ-cells, on which his theory is based, is rendered untenable
if it is recognised that the idants of both cells remain precisely
the same as regards number and quality during the process of
amphimixis. It will not, therefore, be rash to conclude that the
few observations which seem to be in favour of the dissimilar
effect of male and female germ-cells are not convincing,
although we may not be able at present to explain them fully.
To these belong certain rare cases, which have, perhaps, not
been very accurately observed, and possibly form exceptions to
the rule that hybrids of two species are identical, whether the
father or mother belonged to the species A or B.

If we are forced to reject the assumption of the dissimilar
action of the two germ-cells, together with that of an internal
force of transformation, we can only refer hereditary individual
variability to the inequality of external influences ; and it then
remains to be seen how such influences can produce hereditary
differences if soinatogenic modifications are not hereditary : for
external influences act directly, and often exclusively, upon the
body^ and not on the germ-cells.

In a former essay I have already attempted to show that the
constant occurrence of individual variability and the continual
transformation in the intermixture required by selection is
brought about by amphimixis, although it is not the primary
cause of this variability ; and that the accomplishment of
sexual reproduction is even based in almost all the known
organic forms on this necessity of preserving and continually
remodelling the hereditary variability of individuals. I am
convinced that the two forms of amphimi.ris — namely, the
conjugation of i(?iicellHlar, a7id the sexual reproduction of
iiiulticellular organisms — are means of producing variation.
The process furnishes an inexhaustible supply of fresh com-
binations of individual variations which are indispensable to the
process of selection.

Hatschek* has contested this view of the significance of sexual
reproduction, and states that ' transformations of species occur
far too rarely to admit of their explaining such an uninterruptedly
active process as sexual reproduction.' It seems to me, how-

* B. Hatschek, ' Lehrbuch der Zoologie," Jena, 1888, p. 10,

414 THE GERM-PLASM

ever, that he has overlooked the fact that the transforviation of
a species, as well as the preservation of its constancy, are based
upon natural selection, and that this is incessantly at work,
never ceasing for a moment.

From what was said in the chapters on the struggle of the
determinants of the two parents in ontogeny, and on the ' re-
ducing-division ' of the germ-plasm which is indispensable in
amphimixis, it follows that by means of the latter process fresh
combinations of the possible variations in a species must con-
stantly be produced. On the one hand, the germ-plasm of a
new individual produced by amphimixis never receives more
than half the ids of each parent — and these are diiferently
selected and arranged in each case ; and on the other, the co-
operation of the ids of both sides w^ould not always strike an
average in all parts of the new organism, but each part would
resemble that either of the father or the mother in proportion to
the number and controlling force of the individual homologous
determinants ; the resultant of the co-operating forces may be
different in different parts.

Although the process of amphimixis is an essential condition
for the further development of the species, and for its adaptation
to new conditions of existence amongst the higher and more
complicated organisms, // is not the primary cause of hereditary
variation. By its means those specific variations which already
exist in a species may continually be blended in a fresh manner,
but it is incapable of giving rise to new variations, even though
it often appears to do so.

When it first occurred to me that sexual reproduction was
necessary to produce the variations required for the action of
selection, I imagined that its influence upon the germ-plasm was
still more powerful. Since all differences — even the qualitative
ones — are ultimately of a quantitative nature, and as the union
of the primary constituents of the parents may either strengthen
or weaken a ' character,' I imagined that the combination of very
strong primary constituents for the same part in both parents
would not only cause the part to reappear especially markedly in
the child, but would also double the strength of the primary
constituents of the part in the germ-cells of the child ; and thus
the continued intercrossing of offspring in which this part is
strongly developed, might cause it to be increased more and
more, so that it exhibited far more than an ordinary individual

VARIATION

415

diiference. If we suppose this process to go on in the various
parts of the body, the transformation of a species would thus be
accounted for.*

The production of races by artificial selection appears, in fact,
to be in part due to such an accumulation of parental ' charac-
ters ' ; but I shall show later on that it is not accompanied by
an actual variation of the determinants, which alone could
gradually lead to a transformation of the species. We know that
the paternal and maternal idants do not fuse in the process of
amphimixis, and the immense number of cases of perfect trans-
mission proves that the determinants of both sides undergo no
alteration by being brought together. The modification of the
determinants is a process which is not directly connected with
sexual intermingling, but follows its own course, and must be
due to special causes.

This is made still clearer if we reflect that the lower organisms
— e.g., sponges and polypes — must possess a very small number
of determinants compared to those in higher forms, such as
birds and mammals. The number of determinants in an id
of germ-plasjn has therefore increased considerably, and even
enormously, in the course of phyletic developnient. A single
peacock's feather may possibly be controlled by as many de-
terminants as an entire polype. Amphimixis alone, however,
could never produce a multiplication of the determinants.

The cause of Jiereditary variation must lie deeper than this ;
it must be due to the direct effect of external influences on the
biophors and determinants, which I imagine to take place in the
following way.

The entire substance of the earliest organisms must have
consisted of equivalent biophors, the nucleus and cell-body not
having yet become diiferentiated. In these lowest forms, whether
they exist or not at the present day, the perfect constancy of the
composition of the body may occasionally have been disturbed
by external influences of different kinds, and these modifications
must have been preserved, as they persisted in the two parts
resulting from reproduction by binary fission.

When the morphoplasm and idioplasm subsequently became
differentiated, and the latter was enclosed in the nucleus as the

* Compare my essay ' Die Bedeutung der sexuellen Fortpflanzung," Jena,
1866, p. 40. English edition, Oxford, 1889, p. 279.

4l6 THE GERM-PLASM

hereditary substance and controlled the body of the cell, varia-
tions which had been produced solely by the direct action of
external influences on the body of the cell, could no longer be
transmitted to the offspring as an inheritance, for they were
dependent on the part of the morphoplasm in which they origi-
nated, and did not pass into the idioplasm, which comprises
all the primary constituents of the species. Every hereditary
variation must therefore have originated in the nucleus, even in
unicellular organisms — which in this respect therefore resemble
the Metazoa and Metaphyta, the sole difference being that in the
unicellular form we are concerned with the characters of one
cell and not of many. In many unicellular organisms — e.g., the
higher Infusoria — the cell-body is very highly differentiated: it
may possess a complex arrangement of cilia, undulating mem-
branes, trichocysts, and flagella, each having a definite position
in the body, — as well as a protective case of a definite form,
with a lid capable of being closed ; the animal, moreover, is
capable of reconstructing all these parts should any of them
become injured. We are therefore forced to admit that this
minute organism must have a centre in which the latent germs
for all these structures slumber, and from which reproduction
may take place. This centre is the nucleus, and modifications
of the nuclear matter can alone give rise to transformations of
a hereditary nature in the cell-body.

The fact that species have remained sharply defined as long
as we have known them, proves that these transformations occur
neither easily nor rapidly.

The germ-plasm of multicellular forms is obviously also very
constant, and the biophors constituting it are capable of nourish-
ing themselves and of growing, so as to furnish exact duplicates
of themselves when they divide. We could not otherwise under-
stand how it would be possible, in spite of the enormous growth
of the germ-plasm from one generation to another, for the specific
characters, and even the most minute individual characteristics,
to be preserved through so many generations.

The difficulty with which the germ-plasm becomes changed
is not so clearly proved by the instances of constancy
displayed by some species of ancient Egyptian animals — the
ibis and crocodile — through thousands of years, which were
formerly advanced by Nageli and myself for this purpose. It
may be objected that these species were always subject to the

VARIATION 417

control of natural selection, which eliminated every case of
deviation from the perfectly adapted form. If insignificant
individual characteristics which are of no use to the species can,
however, be preserved in the human race for several generations,
this must be owing to the fact that the corresponding de-
terminants have very little tendency during their growth and
multiplication to suffer any marked variation, and that, on the
contrary, they reproduce exact duplicates of themselves. I was
therefore quite justified in supposing that the germ-plasm pos-
sessed a great power of remaining constant.*

We can none the less avoid assuming that the elements of the
germ-plas?n — /.^., the biophors and determinants — are subject
to continual changes of composition Awxixig their almost uninter-
rupted growth, and that these ve?y minute fluctuations, which
are imperceptible to us, are the primary cause of the greater
deviations in the determinants, which we finally observe in the
forin of individual variations.

The assumption that such very minute fluctuations occur,
naturally follows from the impossibility of a complete uniformity
as regards nutrition existing during growth ; and in fact, though
underestimating its importance,! I formerly made this assump-
tion, in correctly supposing that the influences producing these
fluctuations ' are mostly changeable, and occur sometimes in
one and sometimes in another direction.' I had not then taken
into consideration the fact that the fluctuations accumulate
in consequence of the process of amphimixis. If a single
determinant increases to a hundred thousand during the multi-
plication of the germ-cells of an individual, it is not likely that
the nutrition of all these determinants during the process will
be absolutely the same in strength and kind. If this is not the
case, minute differences could not fail to appear in the subsequent
determinants. These minute fluctuations may undoubtedly
again disappear, as I formerly assumed, provided that the modi-
fied determinant is exposed to counteracting influences, and alone
they are quite incapable of producing an individual variation of
any perceptible character; but they may become cumulative.
For the germ-plasm always consists of a large number of ids,
each of which contains one of the homologous determinants in

'"'Cf. my essay 'Die Bedeutung der sexuellen Fortpflanzung, ' Jena,
1886, p. 28. English edition, Oxford, 1889, p. 271.
t Loc. cU., p. 272.

4l8 THE GERM-rLASxM

question, and the co-operation of all of these determines the
character. A hereditiDy individual variation will therefore
arise if many of the homologous determinants vary in the same
way.

We can thus understand the process of doubling of the
determinants, which must have occurred repeatedly, as more
complicated structures arose in phylogeny. More abundant
nourishment would cause a determinant to grow and multiply
more rapidly, and if the first multiplication occurs before the
* reserve germ-plasm ' for the next generation has become
separated off, this double determinant will be permanently
retained by the species. The corresponding part of the body,
however, will only display a perceptible variation wdien the
majority of the homologous determinants have become doubled.

Minute fluctuations must thus continually occur in the com-
position of the biophors and determinants. Their variability
depends on the same principle as the systematic disintegration
of the determinants in the germ-plasm, and is due to the
dissimilar composition of the elements of the growing substance.
If the determinants consisted of masses which were all exactly
alike, inequality of nutrition could never transform determinant
A into Aj : — it could only alter its rate of growth. They are,
however, composed of biophors of diiTerent kinds, which react
dissimilarly under different conditions of growth. This renders
possible a disarrangement of the proportional numbers of the
dilTerent biophors in a determinant, and consequently also the
variation of the latter. It is therefore quite conceivable that all
the qualities of a cell are not affected by these influences, but
that onlv certain of them varv, and that onlv a few amongst a
large number of similar determinants need become modified.

The facts already mentioned with regard to the climatic
variation of butterflies, prove that such an alteration can actually
occur. -In these insects the determinants of certain coloured
wing-scales are slowly changed in the course of generations, in
consequence of the rise in temperature of the climate, and thus
the colour of the scales becomes considerably modified. Such
cases of conspicuous variation are not often met with ; all species
of butterflies, at any rate, are not affected in this way by changes
of temperature, and those that are thus influenced do not display
the variation in all the different kinds of scales. This indicates
that the determinants have a strong tendency to remain constant.

VARIATION 419

and that the deviations to which they are subject on account of
inequalities of nutrition are as a rule so infinitesimal that their
effect is unnoticeable.

These deviations are nevertheless of great significance, for
they form the material from which the insible individual varia-
tions are produced by means of amphimixis together with selec-
tion ; and new species arise by the increase and combination of
these variations.

The hypothesis that the germ-plasm consists of ids is quite
indispensable in this case Every determinant is represented
in the germ-plasm as many times as there are ids in the latter,
for every id contains all the kinds of determinants. The deter-
minant N, for example, is represented a hundred times if the
germ-plasm consists of a hundred ids. Most of these ids must
differ slightlv from one another, for in the course of generations
they are continually brought together in new combinations by
the process of amphimixis. On the occurrence of this process,
however, the diversity of the ids persists, even if we go back to
the origin of the multicellular forms, or of the unicellular ones
from the primordial organism. A condition in which all the ids
could be considered similar is never found ; in fact, as already
stated, the dissimilarity of individuals must be traced to the
primordial orgatiism, at a period when neither the process of
amphimixis nor idioplasm had come into existence, and in which
every individual organism derived its individuality directly from
the dissimilarity of external influences. From these organisms
the dissimilarity was transmitted to the unicellular forms, which
cannot all have originated from one primordial organism, but
each species must have arisen polyphyletically from a large
number of similarly modified organisms. This point has often
been misunderstood, and I have been asked to explain, for
example, how the adaptations of flowers, fruits, and seeds in
Phanerogams, could have been derived from a combination of
characters acquired by the shapeless primordial ancestors. The
characters were not inherited from the pritnordial beings, but
variability, or the dissimilarity of individuals.

We might, however, be inclined to believe that external influ-
ences must affect all homologous determinants of a germ-plasm
in the same way, and must cause them to vary ; but this would
be erroneous.

Since reproduction is connected with amphimixis in the

420 THE GERM-PLASM

multicellular organisms, and the latter process is never entirely
wanting in any species, the germ-plasm of these forms consists
of many ids from different sources, — one half of them being
derived from the father, and one half from the mother, each
half, again, containing ids from the grandparents in varying
proportions ; the ids of the grandparents, moreover, are derived
from one, two, or even three great-grandparents, and so on.
As I have shown above, the proportions in which the individ-
ual ancestors may be represented by ids may vary very much ;
and consequently the germ-plasm of different individuals, even
when they are closely related, must always differ.

Each id contains all the determining elements of the species,
but in a manner peculiar to the individual. If a determinant N
differs slightly in every id, it will also vary a little during growth
if exposed to modifying influences ; so that, for example, the
determinant N may remain unaltered in id A, while N^ varies
in B. On the other hand, the modifying influence of nutrition
may very likely be slightly different in A and in B, and may
produce a variation in N, while N^ remains unaltered. Sufficient
factors would thus be present to cause a variation of one or
more homologous determinants in certain, but not in all ids. In
the only carefully observed cases of blastogenic variation, due
to the direct influence of external conditions, — viz., those of the
climatic variations of Polyoiiunatus piilcBas^ — it is evident that
the influence of temperature was not quite uniform. Some of
the captive Neapolitan specimens of this species are darker
and others lighter, although they were raised in a room under
conditions as nearly the same as possible ; and in those
caught in the neighbourhood of Naples, the amount of black-
dusting — the character modified by the climate — varied con-
siderably. The same influences, even when they act during
many successive generations, do not necessarily cause the indi-
viduals of a species to vary to the same extent ; and I account for
this by the fact that the ids of every animal contain different
variants of a particular determinant N, some of which are more
susceptible to heat than others. The germ-plasm, as a whole,
will vary to a greater or less extent in proportion to the number
of the determinants which are more or less variable.

Many enigmatical points, however, still remain. Thus the
transformation by heat of many of the original reddish-gold
wing-scales of P. phlceas into black ones does not occur evenly,

VARIATION 421

SO as gradually to change the entire reddish-gold upper surface
of the wing into black ; but certain parts first become darker,
and then other adjacent ones, the whole surface being blackened
only in the very darkest specimens. The margin and base of the
wing first turn black, and the change then spreads slowly towards
the middle, which, however, remains unaltered in most individ-
uals. Since we must suppose that scales of the same colour
arise from similar determinants, why are they affected to such an
unequal extent by the modifying influence of heat?

The explanation previously used can. however, be applied in
this case also.

It was shown in the chapter on reversion that although new-
specific characters are produced by the modification of certain
determinants or groups of determinants, this modification never
affects the homologous determinants in all the ids of the germ-
plasm simultaneously. It must, on the contrary, be assumed that
variation affects only a small majority of modified determinants
at first, but increases by the selection and preference of the
most modified individuals, until finally a predominant majority
of all the ids contain modified determinants.

This evidently implies that new a7id old specific characters
are respectively represented by a small and a large majority of
7Jiodified determinants. If this statement be applied to the case
of P. p/dceas, we obtain a principle by the aid of which the
dissimilar effect of heat on the determinants for the middle and
the margin of the wing respectively may be understood. It is
very improbable that the surface of the wing of the brown ances-
tral form of P. p/dceas has changed uniformly into reddish-gold,
and it is much more likely that a lighter spot in the middle
became modified first and took on a reddish-gold colour, in
consequence of sexual selection, and that this then graduallv
extended towards the margin. If this were so, the reddish-gold
scales of the centre must be represented in the germ-plasm by a
greater majority of homodynamous determinants than are those
at the margin : and in this way we can understand why the
black-dusting of the wings affects the sides first, and the middle
last of all. This must be so if the old * brown' determinants
under the influence of heat give rise to black scales more easily
than do the ' reddish-gokP ones. Whether this explanation be
correct in this particular case or not, it is nevertheless true that
the diversity of the extent to which the determinants for the same

42 2 THE GERM-PLASM

character are represented in different regions of the body, affords
us a principle by which we can understand the unequal effect
produced by similar modifying influences upon various regions
of the body.

Even though it can no longer be doubted that climatic and
other external influences are capable of producing permanent
variations in a species, owing to the fact that, after acting uni-
formly for a long period, they cause the first slight modifications
of certain determinants to increase, and gradually affect the less
changeable variants of the determinants also, the countless
majority of modifications is not due to this cause, but to the
processes of selection. The question then arises as to the origin
of variations which are sufficiently considerable for natural selec-
tion to act upon them. External influences at first produce only
very slight fluctuations in the determinants — presumably not
only in some, but in all ; a continual supply of the most mi^iute
variations of the differe7it deter ininajits will thus always be
Presoit.

I do not, however, believe that variations, as we perceive
them, are the direct result of these minute fluctuations on the
part of individual determinants : they can only be produced by
the accumulation of a large number of fluctuatiotis of this kind.
This is an immediate consequence of the theory. Since the germ-
plasm consists of many ids, each of which contains the same
number of homologous determinants, and as, moreover, any
character is the result of the interaction of all its homologous
determinants, the variation of a single determinant would be
imperceptible ; a character can only be modified to an appreci-
able extent when a majority of the determinants are equivalent,
or at any rate are similarly modified.

In my opinion, a variation of this kind is produced by
solitary homodynamous determinants in different ids and in-
dividuals being brought together in one germ-plasm by means
of the processes of ' reducing division ^ and amphimixis, so that
they can thus form a majority.

I will illustrate this by a simple example. A small brown
European butterfly, Lyccena agestis, Hb., has a small black
spot formed by a few scales in the centre of the wing. Let us
suppose that this spot is controlled by a single determinant F,
and that the germ-plasm in this species contains a hundred ids,
and consequently a hundred determinants F ; and that, in

VAklAl'ION 423

consequence of a change in nutrition, some of these deter-
minants F in different individuals constantly vary in such
a manner that if they were in the majority in the germ-
plasm they would give rise to a white spot instead of a
black one. It will sometimes happen tliat they are in the
majority ; for, in consequence of the process of amphimixis,
the modified determinants F may become accumulated in the
course of generations, so as to exceed fifty in one or more
individuals. The black spot will then turn white, and among
thousands of individuals belonging to this species some will
exhibit this variation.

The subsequent course of the phyletic development of this
white spot depends upon its physiological value to the species.
Even if it is only of slight importance, it will gradually come to
be possessed by an increasing number of individuals, and will
ultimately be transmitted to all of them : i.e., it will become a
specific character. This extension could hardly occur without
amphimixis ; for by its means any minority of determinants
F^, wherever produced, may accumulate so as to constitute a
majority ; and otherwise they could not have been effective,
since a minority could never have produced a white spot.

This example was selected because it is based on facts. A
variety of Lyc<xna agestis actually exists which possesses a
milky-white spot on each of the four wings in the place of the
black one ; it is known as the variety artaxe?'xes, and occurs
in the north of England. It is immaterial whether sexual selec-
tion or a protective resemblance has caused the dominance of
this modification.

Many variations of one species or another are merely due to
the modification of a few or a large number of determinants ;
changes in the colour of individual parts or of the whole body
may occur without an accompanying increase in the total number
of determinants of the germ-plasm, but, on the other hand, as
was shown above, many modifications do result from an increase
in their total number. We have already seen that the doubling of
a determinant of the germ-plasm may be referred to the infiuences
of nutrition, and no difiiculties therefore arise in the application
of the principle just propounded to the multiplication of the
determinants. The important modifications in species, includ-
ing all enlargement of parts and higher differentiation of organs,
must be connected with this increase : and the accumulation

424 THE GERM-PLASM

of double determinants of single ids, as well as their purely
qualitative modifications, may be increased by means of the
' reducing division ' and amphimixis, till the variation becomes
perceptible, and natural selection comes into play.

The ' increase ' of a character derived from two parents which
merely possessed it to a slight extent, is an entirely different
matter. A fusioti of the primary constituents of a character
common to both parents, and a consequent increase of these
primary constituents, such as was hitherto supposed to take
place, evidently do not occur : such an assumption is contra-
dicted by ordinary experience ; for if the primary constituents of
the parents could accumulate in this way, all parts of the offspring
must be twice as large, or at any rate larger than, those of the
parent, and this is not the case. If, however, it were main-
tained that it is merely a question of the differences in the
primary constituents, and that the offspring receives half the
sum of the characters of the two parents, it could be replied that
this might be approximately true in some cases, but no expla-
nation would be given as to how an increase of a characteristic
can occur, and may even be produced artificially by pairing
animals which exhibit a certain tendency to vary in the desired
direction. If two animals are paired which possess the char-

acter a in the degree 2a, the offspring would contain — — —

— i.e., ia, and no increase would be produced. Moreover, the
real meaning of the term ' halving' would then remain perfectly
vague. Hensen* thought that 'the effect of transmission was to
halve the characters of the parents,' since ' a similar wdiole can
only result from two equal half-transmissions.' I believe that
this is to a certain extent true, but not in the sense which would
entail the halving of an indivisible primary constituent.

The solution of the problem is to be found in the multiplicity
of the ills and deterniifiaiits, and in distinguishing carefully
between the vague idea of the ' cJiaracte)' ' and the definite one of
the determinate or hereditary part. Each determinate is con-
trolled by as many determinants as there are ids in the germ-
plasm ; but half the ids of each parent, together with half the
determinants of every kind, are removed from the germ-plasm by
the reducing division of the germ-cell. Half the primary constit-

* V. Hensen, ' Pliysiologie der Zeugung.'

VARIATION 425

uents are, however, not removed in this process ; on tlie con-
trary, the occurrence of ' pseudo-monogonic heredity' proves
that every parent-organism transmits the whole of them to the
offspring ; evoy primajy constituent is, however, represented by
only Jialf the number of determinants.

From this theoretical basis, the results arrived at by breeders
are easy to understand. ^ Like begets like.' is their chief maxim,
and this is true whenever two individuals are coupled which
possess a certain character as an inheritance transmitted
through a long series of generations. For such a character
must be represented by homodynamous determinants in the
great majority of ids in the germ-plasm ; and as it consequentlv
will not be entirely removed from either half of the latter bv
the reducing division, it will in most cases be represented in a
majority of ids in the offspring.

The idea that the ' increase ' of a ' character ' can be produced
simply by crossing, is due to an inaccuracy of expression. ' Like
begets like,' but not something else ; and in this respect theory
and practice agree. It is theoretically inconceivable that pre-
cisely the same part — that is to say, the same determinate or
group of determinates — may be increased merely by the pairing
of parents possessing it. If, for instance, two individuals of
Lyccena agestis pair, and each of them possesses a white spot
instead of a black one, in the centre of the wing, none of the
offspring could exhibit a spot twice as large as, or even any
larger than, that of the parents. For the spot is controlled by
one or more homologous determinants, and if those of the white
variety are in the majority, the spot will be of this colour ; but
the adjacent determinants cannot thereby become modified.
The spot can at any rate only become pure white in case it was
previously merely grey, owing to the relatively larger number ot
' black ' determinants which took part in the control of the spot
in the parents having now been completely excluded from the
control of the cells by a preponderating majority of ' white '
determinants.

The term ' quality ' or 'character* (' Eigenschaft ") is the real
cause of confusion in this instance. In the Introduction to this
book I pointed out that this term may have several different
meanings with regard to heredity. Mliiteness in i)lumage, for
example, is a '• quality ' which a breeder tries to obtain by always
selecting the w'hitest bird for breeding purposes. From a breed

426 THE GERM-PLASM

of blue pigeons, for instance, a bird with a white head is paired
with another possessing a white tail, and in this way some
young may be obtained with white heads and tails. The
' qualtities " of the white head and white tail are here, therefore,
combined. The feathers of both parts of the body, however,
have their own determinants ; and in this cross the process
which took place in the idioplasm was not the accumulation of
homologous determinants, but the acquisition of a majority by
the ' white ' determinants for the feathers of the head and tail
over the 'blue' ones. This is, therefore, not a case of the
summation of like with like, but a victory of similar deter-
minants in different parts of the body.

Surprise has often been expressed at the case, mentioned by
Darwin, of two crested canaries which produced young none of
which possessed larger crests, while many, at any rate, w^ere
bald. As Darwin himself pointed out, the crest in birds is due
to a sparser covering of feathers on the head, and this peculiarity
may be increased so as to result in baldness in the offspring.
This is also an instance of the increase of a ' quality,' but only
that of baldness, and not of the crest which is valued by the
breeder. In terms of the idioplasm, this may be explained in
the same wav as was done with regard to the increase of the
character of whiteness in pigeons. It is due to the arrangement
of ' bald' determinants, — if I may use such an expression, —
some of which are derived from the father and control the
region a, the others from the mother controlling the region b.
In the father the region b^ and in the mother the region a,
still possessed a ' feather ' determinant ; in the ofTspring the
* bald ' determinants for both regions were accidently brought
together owing to the reducing division and the subsequent
amphimixis. It is shown by our theory that bald-headed
ofTspring need not invariably result in such a case ; and, as
a matter of fact, all the young birds did not exhibit this pecul-
iarity.

The increase in the number of feathers in any particular part,
such as has occurred, for instance, in a fantail pigeon, is another
case of this kind. The tail of this bird consists of about forty
quill-feathers, instead of twelve, as in the original form ; and the
breed has undoubtedly been produced by artificial selection,
those pigeons always being chosen for breeding purposes which
possessed an extra feather in the tail. The young frequently,

VARIATION 427

or at any rate in some cases, would thus exhibit more tail-
feathers than either of the parents. But this would not by any
means be an ' increase in the force ' of a ' character,' but simply
an increase in the ninnber of new feathers in the individual.
Let us suppose that the male bird possessed two extra feathers
which were situated at a^ and d^ between the normal feathers a
and b and c and d, and that the mother had two supernumerary
feathers in the positions f^ and //^ : a majority of the deter-
minants for the extra feathers of tlie two parents might then be
united in the germ-plasm of one of the offspring, so that the
latter would possess all the four new feathers. This * increase '
in the character of additional feathers in the tail, therefore,
depends upon the constitution of the germ-plasm, and can
consequently be transmitted to the next generation.

This example clearly shows that all really new structures are
not merely the result of transmission, but are due to the variation
and frequent i)iultiplicatio7i of the determinants. The mere
extension of a ' character ' over larger regions or the whole of
the body, even if we choose to speak of it as an ' increase, ' may
be produced by pairing individuals which possess the desired
'character' in ditTerent parts. But -an increase which is con-
nected with the formation of new structures, and consequently
with the multiplication of the determinants in the germ-plasm,
can never be produced by such means alone. When this
results, the cause of the modification must be the variation of
the determinants themselves.

Thus in the case of the fantail, a new feather can never be
produced by transmission alone : the otTspring can merely
possess new combinations of such feathers as were present in
the parents. All really new structures can only originate in a
previous modification of the germ-plasm.

Let us now take an example from among sexually dimorphic
forms, in which we are quite certain of the phyletic modification,
apart from the complication arising from the existence of
sexual double-determinants. TJie long tail-feathers of male
humming birds have arisen by a gradual lengthening of the
ordinary tail-feathers, such as are possessed l)y the females
at the present day. As already remarked, this lengthening is
the result of a considerable multiplication of the determinants
which give rise to the feather : the process of lengthening implies
that variations in the ids which possessed a larger number of

428 THE GERM-PLASM

determinants than those of the original feather were exposed to
selection. It was stated above that fluctuations in the structure
of the determinants, caused by inequalities of nutrition, may
also be produced in consequence of their more rapid growth and
earlier division. If, then, certain determinants for the feather
of the original form underwent division at an early stage in
individual males, so that their number became doubled, a
lengthening of the feather must have resulted as soon as the
doubling occurred in the majority of the ids. The majority
need not have appeared in one individual from the first, but,
like every other simple variation of a determinant, it may have
originated sporadically, in separate ids of different individuals,
and have become increased by the repetition of the process
of amphimixis in every generation. The modification would
become apparent and the processes of selection could take place,
as soon as these ids had attained a majority in any individual.

2. Pathological Variation

The above theoretical explanation of the 'increase' of a
character possesses the important advantage of accounting for
the sudden appearance of .more extensive variations. If more
abundant nourishment can cause the doubling of a determinant
in the germ-plasm, it is possible, and even probable, that many
or all the contiguous determinants for the same feather will
become doubled. The feather must consequently at once in-
crease to twice the size. Doubts have often been justly raised
as to whether the process of sexual selection at first produces
very slight variations, which would scarcely be noticed and
preferred by the selecting sex. We learn from the doctrine of
determijiants that it is unnecessary to take such minute varia-
tions into account, and that more extensive ones may suddenly
arise directly from the germ-plasm.

The doubling by division, not only of biophors and deter-
minants, ids and idants, but even of individual groups of
congncent determinants^ such as must be contained in the
germ-plasm in the form of primary constituents of any organ —
e.g., a feather — is theoretically possible. But this is one of the
more special questions, the details of which may be reserved
for subsequent investigations. I think it is highly probable
that many congenital deforfnities, such as the occasional
doubling of the tarsus in the hind-limbs of beetles and other

VARIATION 429

insects, are due to the doubling of a group of determinants,
and perhaps the much-discussed and debated problem con-
cerning suptniutnerary finders and toes in human beings may
be explained in a similar way. There is nothing impossible in
the assumption that the latter phenomenon is due to reversion
' to an extremely remote, lowly-organised, and many-fingered
ancestor,' for we know of other cases of reversion to very distant
ancestral forms. But in none of the reliable instances does rever-
sion to ancestral characters extend through such an enormous
lapse of time or immense number of generations as must be
assumed in this case. The striping on mules points back to an
early equine ancestor, and we are led to the conclusion that at
the present day the germ-plasm of horses and asses still contains
solitary ' zebra "• determinants. Reversion may occur to yet more
remote ancestors of the modern horse, — even to those possessing
three toes ; but cases of reversion to still earlier ancestors can
hardly be proved with any degree of certainty ; nor is it probable,
from a theoretical point of view, that any groups of determinants
of such extremely remote primitive mammals should have been
preserved in the germ-plasm of human beings. Moreover, it is
not at all certain that the primitive mammals possessed more
than five fingers and toes ; and it would be necessary to go back
to much more remote ancestors before obtaining any support for
the explanation of human polydactylism given by Darwin, and
formerly accepted by Bardeleben, Wiedersheim, and others.

Not only is there no firm foundation for this latter assumption,
but there appear to me to be very weighty reasons against it.
We must not, in the first place, overlook the fact that these primi-
tive ancestors did not possess ' human ' fingers ; supernumerary
fine^ers are, nevertheless, real fingers, and though thev are not
always perfect, they are furnished with the form of nail typical
of the human finger, and not with claws. In my opinion, we are
not justified in assuming that such a supernumerary finger is
represented in the germ-plasm by a group of determinants de-
rived from the primitive ancestor, and that this group has in the
meantime become transformed into the type of the human finger.

Apart from polydactylism, cases of the doubling of the limbs
are known, which, from their nature, cannot be looked upon as
atavistic : insects, for example, have never possessed a double
tarsus. There must, therefore, be another way in which this
doubling might originate.

430 THE GERM-PLAiiM

The sudden appearance of polydactylism and its great ten-
dency to transmissibility can, moreover, be easily understood if
we suppose that excessive local nutrition has caused the group
of determinants in question to become doubled.* For when
this doubling has once occurred in several ids in the germ-plasm,
it must be capable of being transmitted on account of the con-
tinuity of the germ-plasm; and the degree of certainty with
which this will take place will increase as the number of ids in
which it has occurred becomes greater. I am entirely of Ernst
Ziegler's opinion that polydactylsm is due to a germ variation :
this ?nust be so whenever it is hereditary, for it would not other-
wise be transmissible.

In this way we can also understand why polydactylism, after
it has once arisen and has been transmitted through several
generations, may finally disappear ; for at every fresh ' reducing
division' the number of abnormal ids is increased or decreased,
and in the latter case their effect may be entirely obliterated, in
consequence of their meeting with perfectly normal ids during
the process of amphimixis; and in the next generation they
may be for ever eliminated. Like all individual variations, they
may be absent from one generation and appear again in the
next ; but in case of continual crossing in normal human beings,
theoretically they may be expected once more to disappear com-
pletely. This agrees with fact, for supernumerary fingers have
never been observed in more than five consecutive generations.

We know that variation consists not only in the addition of
parts, but also in their disappearance. The process of degenera-
tion of parts must be attributed to the disappearance of the
respective determinants from the germ-plasm. In the chapter
on reversion, I have already attempted to show that regressive
transformations need not occur in all the ids of the germ-plasm
at the same time, and that reversion to long-lost ancestral
characters may be ascribed to the preservation of a minority

* Dr. R. Zander has recently declared himself in favour of the view that
supernumerary fingers, &c., are formed by the mechanical constriction of
the rudiments of the embryonic fingers by amniotic threads. But if this
is true, they would not be transmissible, and another explanation must be
made for the doubling of the tarsus in beetles. (C/C ' 1st die Polydactylie
als theromorphe Varietat oder als Missbildunganzusehen? ' Virch, Arch.,
Bd. 125, 1891, p. 453.)

VARIATION

431

of the determinants in question, which gradually decreases in
course of time. The cause of the regression of a determinant
is to be looked for in insufficient nutrition, — which condition
may occur in a determinant quite as likely as that of more
abundant nutrition. If this occurs in the majority of the ids
either directly, or in consequence of the accumulation produced
by amphimixis, the character controlled by these determinants
becomes regressive in that particular individual. If, however, it
no longer has a physiological value, it becomes slowly but surely
suppressed by panmixia in an ever-increasing number of indi-
viduals until it disappears. Specific characters which have long
been unrecognisable externally, instead of disappearing entirely,
may still be retained in individual ids in the form of incompletely-
degenerated determinants ; and, as already mentioned, these
may cause the reappearance of a character under particularly
favourable circumstances.

3. Siinufiajy of Sections i and 2, and Conclusions

The above remarks may be briefly summarised as follows : —
The origin of a variation is equally independent of selection
and of amphimixis, and is due to the constant recurrence of
slight inequalities of nutrition in the germ-plasm which aftect
every determinant in one way or another, and differ even in
the same germ-plasm, — not only in different individuals but
also in different regions. These variations are at first infini-
tesimal, but may accumulate ; and, in fact, they must do so
when the modified conditions of nutrition which gave rise to
them have lasted for several generations. In this way deviations
may occur in the structure of single determinants or of groups of
them, — never, perhaps, in all ids at once, but at any rate in sev-
eral or even many of them. A doubling of certain determinants
of the germ-plasm may originate in the same way. The proc-
ess of amphimixis has an important share in the accumulation
of these modified determinants, for it may raise the minority pre-
viously existing in the two parents to a majority by combining
their halved germ-plasms. Then, and then only, does selection
begin to take place.

The extreme importance of sexual reproduction in processes of
transformation only becomes evident, however, when we realise
that adaptations are usually concerned with several variations
at the same time, and rarely or never arise in connection

432 THE GERM-PLASM

with a single one. The process of a}npJii))iixis alone rendered
it possible for such ^nanifold combinations of characters to be
offered to selection, so that the proper choice could be made. If
the view which I have long held is correct, a single character
is never alone acted upon by natural selection, but the whole
aggregate of specific characters is incessantly exposed to this
process. The constancy as well as the transformation of the
existing specific characters, the removal of superfluous ones,
and the development of new characters, is due to the incessant
and uninterrupted control of selection. This is only rendered
possible by the continual intermingling of all the existing
varieties of characters, which can only be effected by amphi-
mixis. Hence, although the latter process is not the primary
cause of individual variation, it is nevertheless an indispensable
factor in selection, for by its means alone can the material from
which variations arise be so arranged that selection can operate.

The theory of variation here propounded also affords a more
satisfactory explanation of a further difficulty than can be
obtained by any other. In considering the unlimited number of
adaptations of organisms to the conditions of existence, we
must be surprised at the wonderful plasticity of the species. It
gives us the inpression that every variation, however unexpected,
might be produced by a species as soon as the species has
use for it. On reflecting how certain animals and plants, or
parts of plants, are imitated in colour, form, and marking by
other animals, we might be inclined to suppose that every part
of an animal may assume any required form, colour, or marking,
according to requirement.

This must not. however, be taken literally ; an organism
cannot assume every form, though it may become adapted in so
many ways that we cannot possibly attribute its immense
number of adaptations to rare, fortuitous I'ariations, occurriui^
only once. The necessary variations from which transformations
arise by means of selection, must in all cases be exhibited over
a)id over again by many individuals.

The presence of such an ever active material for primary varia-
tions is a direct consequence of the theory here propounded,
according to which every part or 'determinate^ of a spscies must
present every possible variant in different individuals in the course
of generations, and will sometimes be represented in a larger,
and sometimes in a smaller majority of modified ids. Since an

VARIATION 433

absolute equality of nutrition in homologous determinants, either
in different individuals or in the different ids of the same iierm-
plasm, is inconceivable, and as every minute variation of a
determinant does not disappear of itself witli the individual in
which it is present, but is transmitted directly to the germ-plasm
of the next generation, there can be no scarcity of variations of
every determinant, and the presence of the material required
for all the possible variations of all parts seems to be supported
theoretically.

Before considering the modifications which the germ-plasm
must undergo as a zuJiole during the transformation of species,
I should like to meet an objection which might be raised. If
all determinants are incessantly exposed to slight inequalities of
nutrition and consequently suffer slight variations, what is the
cause of the extraordinary pertinacity by means of which the
species is preserved without the type undergoing variation, —
luhat is the cause of the constajicy of species ? We might expect
that all organic forms must be in a constant state of transition,
and that no form, and no organ, could be retained for any length
of time.

Several points are disregarded in such a question. In the
first place, every species is under the uninterrupted control of
natural selection, as is clearly shown by the degeneration of
parts which have become useless. And since the old hypothesis
of the transmission of somatic variations must, it appears to me,
be definitely rejected, this process of degeneration can only be
explained as the result of panmixia, i.e.^ the cessation of the
control of natural selection over that part which is no longer
of use. We may, however, conclude from the fact that such
degeneration is universal, that the determinants are in a con-
stant state of fluctuation ; and as degeneration takes place very
slowly in all cases, I further infer tliat, in spite of the frequency
with which these fluctuations occur, tJiey only increase very
gradually so as to give rise to perceptible variations.

As at first stated, the individual fluctuations of the deter-
minants must be regarded as excessively small. Natural
selection could produce no direct result from an individual
variation, for it could not produce a cumulative effect ; an
accumulation can only be produced by amj^himixis, and I am
inclined to assume that much of the importance of the latter
process is due to this fact. It can cause minorities of modified

434 THE GERM-PLASM

determinants to accumulate into majorities by mingling the
halved germ-plasms of two individuals. By the aid of the
' reducing division " it can also level and equalise matters by
fortuitously dispersing the homologously modified determinants
of an individual.

It must not be forgotten that slight primary variations of a
determinant need not ahvavs continue in the same direction :
influences of nutrition in the reverse direction will frequently
cause them once more to disappear. Only after a determinant has
been modified to a considerable degree by the action of a uniform
influence during a long period, and the determinants of many
ids have become similarly and simultaneously modified, can a
variation become visible — after being first accumulated by
amphimixis. And even then it by no means forms a permanent
specific character, for the question as to whether it will or will
not give rise to one is decided by natural selection.

Thus several powerful influences prevent the constant varia-
tion of the specific type.

An answer to the question as to wJiat variations the idio-
plasm undergoes in the transfortnation of species will be found
in the chapter on reversion, and it will here only be necessary to
summarise what I have already said.

The transformation of species is due to the variation of some,
and frequently even of most, of the determinants. Many species
do not possess a single character which resembles that of an
allied species, and in this case all the determinants must be
diflferent. But this only implies that all the determinants a — x
have been modified /;/ tJie majority of ids ; a minority of the
latter will contain unmodified ancestral determinants. As the
transformation of a species proceeds, the number of modified
determinants increases together with the number of ids in
•which they occur. Nevertheless the dominating principle of
selection only permits the transformation of all the ids to occur
very gradually, so that the germ-plasm of a young species may
often contain completely unmodified ids of the ancestral species;
and even older species may contain solitary groups of unmodi-
fied determinants in many of their ids. This, and this alone,
renders reversion possible.

It has been recently maintained that, as a consequence of
my theory, I must adopt one of two alternatives, and assume
cither that the germ-plasm of the higher animals consists of ids

VARIATION

435

of the primitive protozoan ancestors,* or that every id is con-
structed in accordance with the existing character of the species :
my real view, however, is intermediate between these two. I
beheve that the germ-plasm of a species always consists to a
great extent of specific ids, amongst which, however, some more
or less unmodified ancestral ones are present, the number being
largest when the species is young. The germ-plasm varies in
different species, and must differ very much in the higher and
lower forms ; but its transformation does not go on at the same
rate in all the ids, for some are modified only gradually, or are
transmitted unaltered through long lapses of time till they are
at last casually removed by a ' reducing division.''

This might be regarded as a defect in the process of the
transformation of species, for the possibility of reversion, as well
as the retention of inactive ancestral ids in the germ-plasm, can
scarcely be considered useful to the species. But in nature no
contrivance is absolutely perfect, — not even the marvellously
developed human eye : all structures are only as perfect as
possible^ — that is to say, as perfect as they need be in order to
perform their required functions. This statement also applies
to the mechanism for the transformation of species : — it
approaches perfection as nearly as is necessary for the per-
formance of its function.

4. Variations on a Larger Scale

a. The Origin of these Vai'iations

We have till now chiefly confined our attention to the question
of general individual variation, as exemplified by those minor
hereditary differences which distinguish individuals from one
another. But there can be no question that variations some-
times occur on a larger scale, and these usually appear
suddenly, are met with only in single individuals, and are as a
rule hereditary. Darwin has given a large number of instances
of this kind. Although the special peculiarity of the black-
shouldered peacock, for example, may be due to reversion to an
unknown ancestral form, there are many well attested cases in

* Compare Marcus Hartog, ' Nature,' Vol. 44, December 1891. The
deductions made by this autlior from my former views are logically correct,
but are no longer justifiable, since in the meantime I myself have gained
further insight into the problems concerned.

436 THE GERM- PLASM

which a given structure in a species has suddenly become
considerably modified. How far such modifications have to
do with the formation of new species need not concern us at
present : we have only to consider the causes and method of
origin of these modifications.

A large number of cases of this kind have been observed,
especially amongst plants. Not only fruits^ but leaves, blossoms,
and entire shoots have been found to vary suddenly, and in
a striking manner. Several varieties of fruits must be included
in this category — such as the variety of peach known as the
nectarine, as well as the moss-rose, copper-beech, copper-hazel,
the varieties of the beech, hornbeam, and oak, the fern-leaved
variety of the maple, and numerous other plants cultivated in
our gardens.

These varieties in some cases first appeared as seedlings, i.e.,
as entire plants, and in others as simple branches or shoots, in
which latter case they are usually known as bud variations.

Let us first take into consideration the varieties which
have orii^inated from seeds. These occur most frequently in
cultivated plants, i.e., in species which have existed for some
length of time under conditions which differ more or less from
the natural ones. We are therefore undoubtedly justified in
attributing the cause of the variation to the influence of changed
external surroundings. But a wild plant, which is transplanted
into a garden soil, does not always begin to vary at once : it
has. in fact, been shown by Hoffmann's experiments, of which
w'e have already given an account, that many generations often
elapse before conspicuous variations occur. And even then they
do not appear in all the seedlings, occurring perhaps only in one
among several hundreds or thousands.

As in the cnse of ordinary individual variations of a minor kind,
the modification begins a long time before it becomes apparent.
A few determinants are first changed, and then a gradually
increasing number, until at last, by means of the ' reducing divi-
sion' and amphimixis, they occur in such numbers in certain
germ-cells that they form the majority. The fact that these
variations occur on a larger scale than the ordinary ones is due
to the permanent action of uniform changes in nutrition, which
give a constant direction to the modification of susceptible
determinants, so that an increase is effected. This is wanting
in the case of the ordinary and constantly varying nutrition.

VARIATION 437

* The acciumilative action of c/ia?i(^ed conditions of life,'' sug-
gested by Darwin,* is consequently theoretically supported to
a certain extent bv the theory of the continuity of the <rerm-
plasm : those determinants which varied in the first generation
continue to do so in a similar direction in the second and third.

Professor Hoffmann has for many years been makinjj: yerv
interesting experiments in the Botanical Garden at Ciiessen
which bear on this point, some of which I will now describe.

Various plants, bearing flowers of the normal structure, were
exposed during a number of generations to greatly modified
conditions of life ; they \vere, for example, grown crowded
together in small pots, so that each plant restricted the amount
of food obtainable by the others, and were thus scantily
nourished. Under this treatment some species — such as Nigella
danmscena, Papaver aipinii?n, and Tagetes patula — bore a
number of non-typical double flowers more or less frequently.
The fact that these deviations from the ordinary type in no case
appeared in the first generation, proves that they were due to an
influence exerted upon the germ-plasm, and not to the direct
influence of the abnormal conditions of nutrition upon the
soma of the plant. Seeds of normal wild flowering plants of
different species, when grown in cultivated soil or even when
thickly sown in pots, never produced plants possessing even
OTie double flower. Only in the course of several, and often
many generations, did any of these wild plants exhibit a greater
or less number of double flowers, or occasional modifications
in the leaves or in the colour of the flowers. It seems to
me that only one explanation can be given of this fact, viz.,
that the altered conditions at first only produced imperceptible
variations in the germ-plasm of an individual plant, — such,
for instance, as alterations in the determinants for the leaves
or flowers in individual ids, but not in all of them at once.
These modified determinants were transmitted to the next
generation in consequence of the continuity of the germ-
plasm ; but since the causes of variation continued to operate,
the homologous determinants in several other ids also became
modified, and thus the number of modified determinants for
the leaves and flowers continued slowly to increase, until finally
they exceeded the normal determinants in number, and the

♦Darwin, 'Animals and Plants under Domestication,' \'ol. II.. p. 249.

438 THE GERM-PLASM

abnormality became apparent as a variation in the flower or in
the leaf.

The reason why 1 lay special stress on these cases is, that
natural selection pla\s no part in them, for we are here dealing
with artificial, and not with the natural conditions of life. The
imperceptible variations of the germ-plasm naturally did not
increase continuously, for the processes of " reducing division '
and amphimixis exerted their influence on every fresh generation,
and helped to bring about either a marked decrease of the modi-
fied determinants, or their sudden increase — a doubling, or a
still greater augmentation. In this way it is easy to understand
all the particulars connected with the occurrence of the abnormal
flowers and leaves. As a rule, the number of these abnormali-
ties increased in the course of generations with a fairly con-
stant regularity. The following results, for example, w'ere
obtained from four generations oi Nigella damascena^ when sown
close together : —

1883. No double flowers.

1884. " "

1885. Six double and twenty-three typical flowers, or 24 per
cent.

1886. One double and ten typical flowers, or 10 per cent.
The number of double flowers was, however, not always

constant, and in some cases they again disappeared entirely.
Papaver alpinmn, for instance, which Hoffmann had cultivated
uninterruptedly since 1862, displayed 'a trace of variability in
the form of the leaves, and a more decided variation in the
colour of the flowers in 1882.'' These experiments were con-
tinued from 1882 to 1886, and yielded the following proportion of
double and normal flowers : —

In 1 88 1 the proportion was 40*0 per cent.

In 1882 " " 4-0

In 1883 " « 5-3

In 1884 " " 13-0

In 1885 " « o

In 1886 " " o

The complete disappearance of abnormal flowers in the two
last of these years seems inexplicable at first sight ; but it is
easily accounted for if we bear in mind the fact that natural,
and not artificial fertilisation occurred, — i.e., the plants were
crossed at random, — and that every reducing division and subse-

u
a
u
u
u

VARIATION 439

quent amphimixis were capable of reducing the modified deter-
minants to a minority, and even of removing them completely
from the germ-plasm. The modification of the determinants
manifestly occurs very gradually, as the small number of modi-
fied flowers in most of the years shows ; and the chances must
have been greatly in favour of the union in amphimixis of
oferm-cells which contained few or no modified determinants.

Had fertilisation been produced artificially, and abnormal
flowers always employed for the purpose, it would have been
easy in the course of a moderate number of generations to
modify the species entirely, and these cases would then have
served as an illustration of the process of natural selection, it
would have been still easier to produce the contrary effect by
selection, — that is to say, to keep the species constant, and
suppress any subsequent variation in this direction. P^or in the
present instance, variation evidently occurred slowly, and most
of the determinants were not easily affected by it ; and these
experiments furnish additional proof of the truth of the statement
propounded above that the elements of the germ-plasm only
change slowly, and with difficulty : they merely fluctuate to a very
slight degree, and only undergo an important change of any
duratio)i when uniform influences continue to act on them in one
direction for a longer time.

It by no means follows from what has just been said that
influences of environment and nutrition exist, which, when they
have acted for a long time, are able to modify the majority
of the determinants for certain parts of the body, and thus to
produce purely climatic variations, in the origin of which natural
selection has no share. Many — perhaps even most — of the
^climatic' varieties are rightly so called.

Such sudden variants, however, appear not only in entire plants
which have been produced sexually, but also in the individual
shoots of a plant. These bitd-variations are rarely met with ;
but in cases in which they occur, they can be propagated by
cuttings or grafting, and often even by seeds.

When I ventured some years ago to suggest that sexual
reproduction has come into force in organic nature in order to
preserve the variability which had existed since the time of
the primordial beings, facts concerning bud-variation were i)ut
forward by several persons to prove that variability may occur
in the absence of sexual reproduction. At that time I certainly

440 THE GERM- PLASM

did not attach sufficient importance to the variation of the
germ-plasm in consequence of influences acting directly ; but
the existence of bud-variations does not prove that variation
occurs without amphimixis. For all those plants in which bud-
variation has been observed are reproduced sexually, and their
ithoplasm therefore contains ids and determinants which differ
individually : the different intermingling and behaviour of
these in the process of growth would alone form a basis for
variations.

In my opinion, indeed, this heterogeneous composition of the
germ-plasm produced by amphimixis is an essential factor in
bud-variation, notwithstanding the fact that it may not in this
case give the first stimulus to variation, any more than it does
in ordinary individual variation. Plants which have for a long
time been propagated by means of buds and shoots, like the
potato and sugar-cane, must possess a germ-plasm consisting
of different kinds of ids ; for they were formerly reproduced
sexually, and the complex intermingling of their determinants
thus produced, cannot have undergone an appreciable modifica-
tion during the period in which they have multiplied asexually.
Their germ-plasm must therefore present far more favourable
conditions for variation than would one composed of identical
ids or of one kind of id only, — did such a germ-plasm exist.

The primary cause of bud-variation must be the same as
that of variation from seeds, and must be due to inequality of
mitritio7i in the gerni-piasm ; — the term ' nutrition ' being used
in its widest sense, so as to include differences in tempera-
ture, &c. This view not only receives support from theoretical
considerations, — for theoretically it could only be replaced by
the assumption of an internal phyletic developmental force, —
but it is also supported by observation. For all recorded ob-
servations go to prove that bud-variations are most likely to
occur when the plant is placed under abnormal conditions, and
especially when it is cultivated. As the direct modification of
the soma caused by these conditions is not hereditary {Nageli)^
and cannot be so, — for somatic variations are only hereditary
when they proceed from the germ. — we are obliged to assume
that the modification of several or many determinants in the
germ-plasm is due to inequality of nutrition.

That bud-variations are produced by the same causes as
those which occur in reproduction by seeds, is borne out by

VARIATION 44 1

the fact that the former occur most frequently in those sjDecies
which have ah-eady varied greatly in multiplying by seeds.*
Expressing this in terms of the idioplasm, we may say that
it occurs oftenest /;/ those species in wJiicJi the Jiojiiolos^ous
deteniiinaiits already exhibit considerable differences.

We attribute these variations to influences of nutrition, whicli
at first bring about slight, and then more marked deviations in
certain determinants of the germ-plasm during the course of their
growth and multiplication, if these influences continue ; but this
alone does not fully account for the process. The question then
arises as to how the modifying influences can cause a particular
bud to undergo variation while all the rest remain unchanged,
although they are exposed to the same influence. Some other
influence is therefore required before a modification of this
kind can appear.

If we remember that bud-variations sometimes occur in wild
plants, or in those which, like the forest-trees of our parks,
exist under practically the same conditions as many wild ones,
it will appear still more probable that the inequalities of nutri-
tion, while constituting the primary cause of bud-variation,
cannot alone bring it about.

My own conception of the process is as follows. Just as
in the case of ordinarv individual variation, bud-variation is
primarily due to those slight, fluctuating, structural changes
which all determinants undergo in consequence of minute and
inevitable fluctuations in nutrition. As in the former case, the
homologous determinants of the various ids are not all afiected
to the same extent ; some become greatly modified, and others
little or not at all. A difference is, however, seen in the fact
that in this case the sa))ie influence of change — e.i^., generally
improved nutrition — occurs for a considerable time, and through-
out several generations. As in the case of variations in plants
raised from seeds, a modification of greater extent can thus be
produced in these determinants.

Up to this point the process is quite similar to that of the
spasmodic variation of seedlings. A difl"erence, however, results
owing to the non-occurrence of amphimixis in the case of gem-
mation, for the variable bud does not arise from the germ-iilasm
of a seed, but from ^ blastogenic'' germ-plasm (' Knospcn-Keim-

* Darwin, loc. cit.

442 THE GERM-PLASM

plasma ') . The latter is derived directly from the former, from
which the plant in question — or one of its ancestors, if the
plant itself was raised from a cutting — arose. If, during the
growth of a tree, one of the determinants, N, varied in the same
way in certain ids. and tended, for example, to produce red
leaves instead of green ones, red leaves would nevertheless not
be produced till all, or at any rate a majority, of the determi-
nants N had become transformed into the red variety. If a
'■ reducing division ' intervened between the germ-plasm of the
bud and the growth of the latter into a shoot, a minority of ' red '
determinants might give rise to a majority in one of the two
daughter-cells ; but this reducing division is exactly what does
not take place in ordinary cell-multiplication.

Since bud-variations are of very rare occurrence, and only
one of many thousand buds on the sa?ne plant varies so as to
produce red leaves, for instance, while the altered conditions
simultaneously affect all the buds, I conclude that the modified
determinants which the tree contains 7nay so)neti))ies attain a
majority i?i consequence of an abnormal differential nuclear
division. Should this take place in the apical cell or cells
of a bud, the resulting shoot might, to return to our former
example, bear both green and red leaves ; for, according to our
pre-supposition, the 'green' and 'red' determinants of the
apical cells would have been separated during the cell-divisions
in such a manner that in some leaves the green determi-
nants, and in others the red ones, might be in the majority.
If the separation occurs at an earlier stage, before the apical
cells are formed, — that is to say, in the cambium, — a shoot
bearing red leaves alone might arise from the cell which
receives the ' red ' group at the ditTerential division of the
nucleus.

There is nothing impossible about this assumption ; for during
the process of mitotic nuclear division, irregularities might occur
in the complex apparatus by which this process is effected,
and individual cases of such irregularity have actually been
observed : — even the possibility of a direct nuclear division
cannot be entirely overlooked. I am, however, far from con-
sidering this hypothesis as established, and merely offer it as a
suggestion.

Nageli was of the opinion that all variations are slowly
prepared in the idioplasm in the course of generations before

VARIATION 443

they become apparent, and in stating this view he also specially
referred to bud-variations. I fully agree with him in this respect,
and in the course of this book have repeatedly shown how
these gradual modifications (• Umstimmungen *) of the germ-
plasm, or of individual parts of it, are to a certain extent the
natural result of its assumed structure. In the case of bud-varia-
tions these invisible modifications may occur in a much earlier
generation than that in which they appear ; and hence it is easy
to understand why this form of variation mostly occurs in those
plants which, like the rose and Azalea, have already varied in
reproduction by seed. For modified determinants are more
readily accumulated by amphimixis ; and a germ-plasm which
has inherited such determinants from its ancestors may, after
these have been still further modified, give rise to a bud in which,
by a fortuitous differential nuclear division, the modified deter-
minants are in the majority, and can thus become efTective.

Unfortunately it has not been observed whether complicated
modifications, like that of the moss-rose, owe their origin to bud-
variation. This is quite possible theoretically, for the invisible
preliminary to variation — the modification of certain determi-
nants— is just as likely to affect a single determinant as a whole
group ; even the formation of two or more new determinants,
by the multiplication of a primary one, is quite as possible here
as in the case of the ordinary transformation of species. The
sudden appearance of such modified groups of determinants is
due therefore to fortuitous differential nuclear division. Such
cases prove that the preparation for the modification is a slow
process, for it appears impossible to conceive of any cause pro-
ducing a sudden variation of an entire group of determinants by
any method.

The extent of any spasmodic variation will depend on the
extent to which the various groups of determinants have been
permanently exposed to abnormal nutrition. It would doubt-
less be as yet premature to bring their qualitative characteristics
into any causal relation with definite influences producing
variation. We can only state the necessity of assuming, a
priori, that the extraordinarily complex germ-plasm is provided
with special means for the transmission of nutrient fiuid, the
increase or diminution of which must produce purely local
differences of nutrition; and that, on the other hand, the vital
units must, owing to slight changes in their structure, modify

444 THE GERM-PLASM

the characteristics of the part of the body which they represent,
in some inexplicable way. It is not yet possible, for instance, to
give an exact account of the changes in the group of determinants
for the legs which led to the sudden production of a bow-legged
* otter '-sheep, or of the modifications of certain determinants in
the rudiments of the leaves which caused the formation of the
notches characteristic of a certain variety of birch.

b. The Transmissio7i of these Variations

The transmission of 'sports' in plants has hitherto been a
very obscure problem. Seed- variations are often propagated by
means of seeds, but cannot always be thus reproduced, or at
least only in individual cases. Bud-variations can usually only
be propagated by cuttings or grafting, but some instances are
known in which they have also been reproduced by seeds,
though only in a certain percentage of cases. The origin of
these irregularities was unknown, and no previous theory of
heredity could offer a reason for them ; all that could be said
was, that in these instances transmission was very capricious.
The theory of the germ-plasm, however, offers a very simple
explanation of them. We will first consider certain facts
recorded by Darwin, which are very valuable in respect of this
theory.

Darwin states that ' when a new peculiarity first appears,
we can never predict whether it will be inherited.'' * If both
parents exhibit the variation, 'the probability is strong that it
will be transmitted to at least some of their offspring.' Bud-
variations are propagated to a much smaller extent than seed-
variations, but the power of transmission often appears to be
very capricious, inasmuch as one and the same modification
on a single plant is transmitted by seed in one case and not in
another. Thus vain attempts were for a long time made to prop-
agate the weeping ash by seeds. Over twenty thousand seeds
developed into trees of the ordinary form, but offspring with
hanging boughs were finally raised from the seeds of another
specimen of this variety. The same ash-tree, however, did not
transmit this character to all its offspring, but only to a certain

*' The Variation of Animals and Plants under Domestication,' Vol. I.,
London, 1888, p. 460.

VARIATION 445

percentage of them ; and Darwin mentions a famous weeping
oak at Moccas Court which transmitted its special character to
all its seedlings, though in varying degrees.

According to our theory, the transmission of a variation by
seeds depends on the presence of a corresponding moditication
in the majority of the determinants in the germ-plasm of the
seeds. If the germ-plasm contains a hundred ids, the con-
trolling forces of which are equal, more than fifty determinants
N must be transformed into N^ before the modification would
be perceptible in the seedling. Since therefore, as was re-
marked above, new variations probably never appear simultane-
ously in all the determinants, but only in a varying percentage
of the ids, the chances are greatly against all the seedlings
produced by the transformed plant exhibiting this modification.
For every germ-cell has undergone a ' reducing division,' and
hence many of them will always only possess a minority of the
modified determinants N^, and this may even be a very small
one if the majority i/i the germ-plasm of the parent-plant was
small. When two Aich cells unite in the process of amphimixis,
the resulting germ-plasm contains only a small minority of
modified determinants N\ and the modification is inappreciable.
This accounts for the fact that the seedlings of a variety hardly
ever reproduce the variety in all cases ; and that in rarer in-
stances, such as that of the weeping oak already mentioned,
all the seedlings may exhibit the corresponding variation, al-
though in varying degrees. For the composition of the germ-
plasm must differ in each of them in consequence of the
processes of ' reducing division ' and amphimixis, even when the
parent-plant contains only a small but ever varying minority of
ancestral determinants.

We can, moreover, easily account for the fact that seedlings
of a variety, such as the balsamine, may resemble the parent-
plant, without transmitting the character of the variety to their
offspring. In this case all the daughter-plants of the variety
in question must retain a majority of the modified determinants,
but in very varying degrees. In those which contain a very
large majority in their germ-plasm, it necessarily follows that the
larger number of the germ-cells produced must contain a major-
ity of modified determinants ; but in the case of those in which
the proportions are more equal, the chances are in favour of the
seedlings containing only minorities of these determinants.

446 THE GERM-PLASM

Darwin mentions that only thirty per cent, of the seedHro-.s

* , J. ^

of the wild yariegated yariety of Ballota nis^ra possessed the
variegated leaves of the parent-plant ; but sixty per cent, in
the second generation were variegated. This is also quite in
accordance with our theory ; for the determinants for the leaves
in the mother-plant cannot all have been modified, but only the
majority ; these would then become differently grouped by means
of the reducing divisions of the germ-cells. A seed reproduces
either the variety or original form according to whether modified
determinants, which were brought together in fertilisation, con-
stituted the majority or minority. As in the instance first quoted,
variegated plants were alone retained for cultivation in the
second generation, and consequently a greater number of the
determinants of the variety were brought together at fertilisation :
the percentage of the variety was therefore bound to rise in the
third generation. This proportion would have increased still
- further if a fourth and fifth generation had been raised in the
same way ; for in those seeds which yield variegated plants
the germ-plasm must of necessity contain more modified deter-
minants than in the case of those producing the original form.
Hence with every generation the chances of an increased
majority of modified determinants become greater ; and I have
no doubt that in this instance, by the constant selection of the
most variegated plants for further cultivation, a ' pure ' variegated
race might eventually have been obtained, w'hich would have
transmitted its character to the great majority of its offspring —
or as we usually express it — 'to all its oflfspring.''

The fact that many weeping ashes transmit their special
character to many, but not all, the seedlings is also attributable
to the diverse eiTects of the ' reducing division ' and amphi-
mixis, the former of which causes the introduction into the
germ-plasm of a large majority of modified determinants, and
the latter of only a slight one. The manner in which the
variety arises must here, however, be taken into account. Bnd-
variations are much more rarely reproduced by seeds than are
variatio7is arisitig frojn seeds ; but, on the other hand, they can
almost always be propagated by grafting, budding, or by cuttings.
Hence the capacity of a certain individual for transmitting the
variation to its seeds may be owing to its having arisen from
a seed, while in another case the same variation arose in a bud,
and it could therefore rarely or never be transmitted by seeds.

VARIATION 44 7

The most difficult point to explain theoretically, is why bud-
variation is not generally, but only occasionall)-, transmitted by
seeds. Even this may to a certain extent be accounted for by
the present theory.

A bud is a growing point, enveloped by scales. It arises
from the apical cells, which produce the other cells of the shoot
by means of continual division, and these form the interfoliar
parts, leaves, and flower-stocks. In accordance with the
principle of the continuity of the germ-plasm, a part of the
' blastogenic " germ-plasm of the apical cells must be transmitted
in an ' unalterable ^ (' gebundenen ") condition to certain cells
of the shoot as accessory idioplasm (• Neben-Idioplasma") or
'■ reserve gerjn-piasDi.'' From these cells it is passed on to the
sexual organs, where it is used for the formation of germ-cells.
This reserve germ-plasm remains undisintegrated, and is
perfectly distinct from, and independent of, the ' blastogenic '
germ-plasm, which is gradually distributed during the ontogeny
of the shoot into groups of determinants.

The fact that bud-variations are so rarely transmitted by
seeds seems to me to be owing to the cause just mentioned ;
for the majority of modified determinants required to make a
modification apparent may evidently be present in the * blasto-
genic' germ-plasm, though absent in the reserve germ-plasm.
If we remember that these variations have been prepared long
beforehand in the germ-plasm, and that at first a few, and then
gradually a larger number of determinants become modified in
a similar way, and that finally a fortuitous differential nuclear
division must, on our assumption, intervene before the ' blasto-
genic' germ-plasm of a certain apical cell can contain a majority
of modified determinants, it becomes comprehensible why the
reserve germ-plasm contained in this cell may behave differ-
ently, and contain only a few or none of the determinants
in question in a modified condition. We must not forget that
the influences which produce the variation are not those of the
nutrition of the bud in which the modification appears, but
those which aff"ected the determinants during their long course
from the ancestral plant of a past generation to the bud in
which they now appear.

This explanation seems to me to sufficiently account for the
fact that the seeds which give rise to ' sports ' indirectly, need
not necessarily transmit the modification.

44^ THE GERM-PLASM

The question as to why this transmission is of such r-are
occurrence must, however, be considered more closely. I am
inclined to seek the causes of this fact in the processes of
' reducing division ' and amphimixis, to which the reserve germ-
plasm, but not the ' blastogenic ' germ-plasm, is subjected. Let
us suppose that the two were precisely similar at first with
respect to their contained modified determinants N^, each pos-
sessing a small majority of them : the shoot must then exhibit
the variation, l3ut only certain germ-cells would contain a
majority of N^, the rest containing a minority of these deter-
minants in consequence of the diversity resulting from the
reducing division. It is true that two germ-cells containing
majorities of N^ might unite in fertilisation ; but this extremely
favourable case would only occur very rarely, — when, as assumed,
the majority in the reserve germ-plasm w^as only a slight one ;
while the other cases, in which amphimixis leads to N^ being in
the minority, would take place much more frequently.

It is, however, by no means certain that reserve germ-plasm
and ' blastogenic ^ germ-plasm must contain a similar percentage
of N^ The two may very well differ in this respect, and it is
extremely probable that a larger percentage of N^ in the reserve
germ-plasm leads to the formation of seeds which give rise to
seedlings exhibiting variation. We may therefore suppose that
in the one case, when the •blastogenic' germ-plasm contains
even a small majority of N^ the modification of the shoot will
actually occur: and that in the alternative case, in which the
reserve germ-plasm alone contains a majority of N^ variation
will take place in a larger or smaller number of seedlings in the
following generations. When both contain a majority of N^
the shoot must vary, and some of the seeds arising from it
must transmit the variation.

This explanation is not so hypothetical as it might appear.
Certain facts point with certainty to the conclusion that these
'sports' in many cases really contain only a very slight majority
of modified determinants. In most bud-variations reversions
to the original form are of frequent occurrence, both in the
buds themselves and in the generation derived from them either
by seeds or buds. This has already been mentioned in the
chapter on reversion, and is so well known that a brief state-
ment of a few cases will suffice. In the botanical gardens at
Bonn, Professor Strasburger showed nae ^n immense horn-

VARIATION 449

beam with deeply notched leaves, like those of the oak, a large
branch of which had reverted completely, and bore leaves of
the ordinary form. I have in my garden a 'fern-leaved' beech,
some of the leaves of which have the usual form ; Darwin
states, in fact, that some shoots of this variety may produce
fern-like leaves, normal leaves, and various intermediate forms.
Slight inequalities in the nuclear divisions may in this case
displace the controlling resultant of the determinants N and
N\ supposing that the majority of N^ is only a small one.

SUMMARY AND CONCLUSION

All the phenomena of heredity depend on minute vital units
which we have called • biophors/ and of which living matter is
composed : these are capable of assimilation, growth, and mul-
tiplication by division. We are unacquainted with the lowest
conceivable organisms, and do not even know if they still exist.
But they must at any rate have done so at some time or other, in
the form of single biophors, in which multiplication and trans-
mission occurred together, no special mechanism for the purposes
of heredity being present. A higher order of beings would then
have been constituted by those organisms which were composed
of a large number of similar biophors. Of these also we have no
actual knowledge based on observation, but must suppose that
they too required no special apparatus for the processes of
transmission ; for a reproduction by binary fission must result
in two perfectly corresponding halves, each containing similar
biophors, and each of which, simply by the multiplication of
these units, is able to give rise to a complete organism exactly
like the parent.

This simple form of transmission must have become modified
when the biophors underwent differentiation in connection with
a division of labour, and became combined in various ways to
form the body of the organism. These two kinds of hypothetical
beings might be respectively distinguished as homo-biophorids
and hetero-biophorids. Not only might a firmer cortex and
softer internal substance be present in the latter, but a differen-
tiation into anterior, posterior, dorsal, and ventral regions miglit
occur ; several layers of the body substance, differing structurally
and functionally from one another, might also be developed, to-
gether with motile and non-motile processes — such as flagella,
cilia, spines, and hooks, — like those present amongst the Infu-
450

SUMMARY AND CONCLUSION

451

soria; and there might, moreover, be a permanent aperture in
the firmer outer layer through which soHcl food passed into the
interior, and so on.

When the body thus became constnicted, in a more or less
complex manner, of various kinds of biophors arranged in a
definite manner, simple binary fission no longer sufficed for the
transmission of the characters of the parent to the offspring.
If the parts situated in the anterior, posterior, right, left,
dorsal, and ventral regions differed from one another, all the
elements — i.e.^ all the kinds and groups of biophors — could not,
by any method of halving, be transmitted to both the offspring
resulting by division so that they could develop by mere growth
into an organism resembling the parent. Special means must
then have been adopted to render such a completion and con-
sequent perfect transmission possible ; and this ivas attained
by tJie fonnation of a nncleiis.

We may, with de V^ies, regard the cell-nucleus as having
originally served merely for the storage of reserve biophors,
which were destined to become doubled on the division of the
organism, each half rendering the completion to an entire
individual possible when those kinds of biophors which were
wanting were transferred to it. Subsequently — that is, in the
multicellular organs possessing highly differentiated cells — the
nucleus took on other functions, which regulated the specific
activity of the cell, though it still retained biophors capable of
supplying the cliaracters of the cells which were still wanting,
and therefore still served as the bearer of the biophors con-
trolling the character of the cell.

If, therefore, a special apparatus for transmission became
necessary in the hetero-biophorids or unicellular organisms, and
appeared in the ' cell ■ in the form of a ' nucleus,'' it must
have become still more complex on the introduction of the
remarkable process of amphimixis, which, in its simplest and
original form, consists in the complete fusion of two organ-
isms in such a manner that nucleus unites with nucleus and
cell-body with cell-body. In the higher unicellular organisms
this process is in most cases restricted to the fusion of the
nuclei, half the nucleus of one animal uniting with half that of
another. The process of division shows that the nucleus has a
structure precisely analogous to that of the nucleus in multi-
cellular organisms ; we may therefore assume that the hereditary

452

THE GERM-PL.A.SM

substance here likewise consists of several equivalent groups of
biophors, constituting ^ nuclear rods "• or ' idants/ each of which
contains all the kinds of biophors of the organism, though they
deviate slightly from one another in their composition, as they
correspond to individual variations. Half the idants of two
individuals become united in the process of amphimixis, and
thus a fresh intermixture of individual characters results.

The apparatus for transmission in those multicellular organ-
isms in which the cells have undergone a division of labour, is
essentially similar to that seen in unicellular beings ; although,
in correspondence with the greater complexity of their structure,
it is more complicated. As the process of amphimixis occurs
in them also, and the fusion of highly-differentiated multicellular
individuals seems only to be possible by a temporary return to
the unicellular condition, we find that the so-called ' sexual
reproduction,' which is of general occurrence amongst them,
consists in all the primary constituents (' Anlagen') of the entire
organism being collected together in the nuclear matter of a
single reproductive cell. Two kinds of such cells, which are
differently equipped, and mutually attract one another, then
unite in the process of amphimixis, and constitute what we are
accustomed to call the 'fertilised egg-cell,' which contains the
combined hereditary substances of two individuals.

According to our view, this hereditary substance of the multi-
cellular organisms consists of three orders of vital units, the
lowest of which is constituted by the biophors. In the uni-
cellular forms a more or less polymorphic mass of biophors,
having a definite arrangement, constitutes the individual nuclear
rods or idants, several of these making up the hereditary sub-
stance of the nucleus which controls the cells ; and similarly
in these higher forms, groups of biophors, arranged in a certain
order, constitute the primary constituents of the individual cells
of the body, and together form the second order of vital units, —
the cell-determinants, — or simply, the '• detenninaiits.''

The histological character of every cell in a multicellular
organism, including its rate and mode of division, is controlled
by such a determinant. The germ-plasm does not, however,
contain a special determinant for every cell ; but cells of a simi-
lar kind, when, like the blood-cells, they are not localised, may
be represented by a single determinant in the germ-plasm. On
the other hand, every cell, or group of cells, which is to remain

SUMMARY AND CONCLUSION 453

indepe}idently variable^ must be represented in the germ-plasm
by a special determinant. Were this not the case, the cell in
question could only vary in common with other cells which are
controlled by the same determinant.

Hie germ-cell of a species must contain as many determinants
as the organism has cells or groups of cells which are inde-
pendently variable from the germ omuards, and these de-
terminants must have a definite mutual arrangement in the
germ-plasm, and must therefore constitute a definitely limited
aggregate, or higher vital unit, the ^id.''

From the facts of sexual reproduction and heredity we must
conclude that the germ-plasm contains many ids, and not a
single one only. The formation of hybrids proves that the two
parents together transmit all their specific characters, so that in
the process of fertilisation each contributes a hereditary sub-
stance which contains the primary constituents of all parts of
the organism, — that is, all the determinants required for build-
ing up a new individual. The hereditary substance becomes
halved at the final stage of development of the germ-cells, and
consequently all the determinants must previously have been
grouped into at least two ids. But it is very probable that
many more ids are usually present, and that in many cases their
number far exceeds a hundred.

It cannot be stated with certainty which portions of the ele-
ments of the germ-plasm observable in the nucleus of the ovum
correspond to ids, though it is probable that only parts of, and
not the entire 'chromosomes,' are to be regarded as such.
Until this point can be definitely decided, our further detailed
deductions will be based on the view that the nuclear rods
(chromosomes) are aggregates of ids, which we speak of as
' idants." In a certain sense, the latter are also vital units, for
they grow and multiply by division : and the combination of itls
contained in them, although not a permanent one, persists for
some time.

The ' gerjn-plasm ' or hereditary substance of the Metazoa
and Metaphyta, therefore, consists of a larger or smaller number
of idants, which in turn are composed of ids ; each id has
a definite and special architecture, as it is composed of de-
terminants, each of which plays a perfectly definite part in
development.

The development of the primary constituents, contained

454 THE GERM-PLASM

in the germ-plasm of the reproductive cell, takes place in
the course of the cell-divisions to which the ontogeny of a
multicellular organism is due, in which process all the ids
behave in an exactly similar manner. In the first cell-division
every id divides into two halves, each of which contains only
half the entire number of determinants ; and this process of
disintegration is repeated at every subsequent cell-division, so
that the ids of the following ontogenetic stages gradually become
poorer as regards the diversity of their determinants, until they
finally contain only a single kind.

Each cell in every stage is in all cases controlled by only one
kind of determinant, but several of the same kind may be con-
tained in the id ; and the ' control ' of the cell is effected by the
disintegration of the determinants into biophors, which pene-
trate through the nuclear membrane into the cell-body ; and
there, according to definite forces and laws of which we are
ignorant, bring about the histological differentiation of the cell,
by multiplying more rapidly at the expense of those biophors
already forming the cell-body. Each determinant must become
'■ ripe,' and undergo disintegration into its biophors, at a definite
time or at a certain stage of ontogeny. The rest of the deter-
minants in the id of a cell, which are destined for subsequent
stages, remain intact, and have therefore no effect on the control
of the cell ; but the mode of their arrangement in the id, and the
special rate of multiplication of each kind, determine the nature
of the next nuclear division — that is, as to which determinants
are to be distributed to one daughter-cell, and which to the
other. The histological nature of these two cells, as well as the
control of their successors, is determined by this division ; and
thus the distribution of the primary constituents contained in the
germ-plasm is effected by the architecture of the id, which is at
first of a definite kind, but afterwards undergoes continual and
systematic changes in consequence of the uneven rate of multi-
plication and gradual disintegration of the ids.

The apparatus for cell-division is only of secondary impor-
tance in the process ; its chief part, the ' centrosome,' like the
hereditary substance, is derived from the parental germ-cell or
cells, but only constitutes the mechanism for the division of the
nucleus and cell, and contains no ' primary constituents. "" The
rate of the cell-divisions cannot, moreover, be determined by
the centrosome, although it produces the required stimulus : the

SUMMARY AND CONCLUSION 455

apparatus for division is set in motion by the cell, which is con-
trolled by the idioplasm. Were this not the case, the nuclear
matter could not be the hereditary substance, for most of the
hereditary characters of a species are due in a less degree to the
differentiation of individual cells than to the number and group-
ing of the cells of which a certain organ or entire part of the
body consists ; these, however, again depend on the mode and
rate of cell-division.

The processes occurring in the idioplasm which direct the
development of the organism from the ovum — or to speak in
more general terms, from one cell, the germ-cell, — do not
in themselves furnish an explanation of a series of phenomena
which are in part directly connected with the ontogeny, or else
result from it sooner or later : the phenomena of regeneration,
genifuation, and fissioi, and the fonnation of new genn-cells,
all require special supplementary hypotheses.

The simplest cases of regeneration are due to the fully
formed tissue, consisting of similar cells, always containing a
reserve of young cells, which are capable of replacing a nor-
mal or abnormal loss. This, however, is insufficient in the
more complex cases, in which entire parts of the body, such
as the tail or the limbs, are regenerated when they have
been forcibly removed. We must here assume that the
cells of the parts which are capable of regeneration contain
' supplementary determinants ' in addition to those which
control them, and that these are the primary constituents
of the parts which are to be formed anew in the process
of regeneration. They are supplied to certain parts of the
body at an earlier ontogenetic stage in the form of * inactive
accessory idioplasm,' and only become active when the opposi-
tion to growth has been removed in consequence of the loss
of the part in question. The equipment of a cell of any
part with supplementary determinants presupposes a greater
complexity in their distribution, in correspondence with the
greater complexity in stmcture of the part ; and thus the
capacity for regeneration is limited, for a part can no longer
be provided with an apparatus for regeneration when its
structure is too complicated. The ordinary assumption that
the regenerative 'force' decreases as the complexity in structure
increases, is therefore to a certain extent true, but not if it
implies the existence of a special force which provides for

456 THE GERM-PLASM

regeneration, and which always diminishes in correspondence
with the degree of organisation. Even if we imagine this
* force ' to be a mechanico-physiological one, it could not be
considered as a pyiiiiary quality of the organism, and to
some extent the inevitable result of life itself, but must be
looked upon as an adaptation.

Reproduction by Jission is closely connected with regeneration :
it presupposes the existence of a similar apparatus in the idio-
plasm, which, however, has in most cases reached a higher
stage of development ; fission must have arisen phyletically from
regeneration.

The origin of multiplication by gemination, and the phe-
nomena exhibited by this form of reproduction, are different
from those concerned in fission. In plants and Coelenterates,
gemmation originates in one cell, which must consequently
contain a combination of all the determinants of the species
closely resembling that existing in the fertilised ovum. In
the Polyzoa, however, this process does not originate in one
cell, but in at least two, and probably more, belonging to
two different layers of cells (germinal layers) of the body ;
and in Tunicata, again, the material for the bud is produced
from all three germinal layers.

The first of these forms of budding must be primarily due to
the admixture of ' unalterable ' (' gebundenem ') germ-plasm to
certain series of cells in ontogeny in the form of inactive ' acces-
sory idioplasm j"" or " blastogenic'' idioplasm (' Knospungs-Idio-
plasma"). In plants this is contained in the apical cells ; and in
hydroid polypes, in the cells of the ectoderm.

In the second group of animals mentioned above, we must
assume that the ' blastogenic ' germ-plasm becomes disintegrated
into two groups of determinants at an early ontogenetic stage,
and that each of these is passed on in an ' unalterable ' condition,
through various generations of cells, until the time and place of
its activity are readied.

In the third group, tlie inactive 'blastogenic' idioplasm
divides into three groups of determinants, one of which passes
into the ectoderm, the second into certain cell-series of the
mesoderm, and the third into others in the endoderm, until
they reach the part in which they have to become active.

Gemmation must have originated phyletically by a doubling
of the germ-plasm taking place in the fertilised ^gg^ so that

SUMMARY AND CONCLUSION 457

one half remained inactive, and was then either passed on
as inactive ' blastogenic ' germ-plasm, or else became divided
up in the course of ontogeny into groups, which were passed
separately to the same region, viz., that of the bud.

In all cases in which the power of budding was permanently
retained by the species, the occurrence of this process of
doubling of the germ-plasm seems to have persisted through
the ontogenetic stages from an early period ; for we find that
the individuals arising by gemmation very frequently vary in-
dependently of one another, and often even to a great extent.
But independent variation from the germ onwards implies
the existence of special determinants in the ' blastogenic '
germ-plasm. Medusae could never have been produced from
polypes by budding if independently variable determinants
of the buds had not been present in the germ of the fertilised
ovum. We therefore assume that ^luo kinds of genn-plasjn
exist in those species in which alternation of goierations occurs,
both of which are present in the egg-cell as well as in the,
bud, though only one of them is active at a time and controls
ontogeny, while the other remains inactive. The alternating
activity of these two germ-plasms causes the alternation of
generations.

The. formation of germ-cells is brought about by the occur-
rence of similar processes in the idioplasm to those which cause
gemmation. One part of the germ-plasm contained in the ferti-
lised egg-cell remains inactive and * unalterable,' — that is, it does
not immediately become disintegrated into groups, but is passed
on in the form of accessory idioplasm to certain series of cells
in ontogeny, and thus reaches the parts in wliich germ-cells are
to be formed. Thus the whole of the parental germ-plasm,
with all its determinants, forms the foundation of the germ-cells
which will give rise to the next generation, and the extremely
accurate and detailed transmission of j^arental characters to the
offspring is thereby rendered comprehensible.

In multicellular plants and animals, the germ-plasm becomes
more complex in consequence of sexual reproduction, in which
process the ids of two different individuals, the parents, are
accumulated in the fertilised egg-cell every time amphimixis oc-
curs. This has caused the occurrence of the ' reducing division,'
which accompanies the formation of male and female germ-cells,
and results in the number of ids and idants being reduced to

458 THE GERM-PLASM

the half. The reduction is important in elucidating the phenom-
ena of heredity in forms which are reproduced sexually, for the
ids of a germ-plasm are not by any means all alike, but dilTer
to the same relative extent as do the corresponding individuals.
As the reduction does not always occur in the same way, and
the resulting halves contain different idants on different occa-
sions, and these fall to the share of individual germ-cells, it is
possible for the germ-cells of one individual to contain very
different combinations of idants. This results in the dissimi-
larity between the offspring of the same parents, — or, to express
it in more general terms, in the extreme diversity as regards the
intermixture of individual differences.

During the development of a new individual from the fertilised
egg-cell, the ontogeny is directed by the ids of the two parents
which constitute the germ-plasm. Structures intermediate
between those of the parents thus frequently arise — but only
when perfectly homologous ids are opposed to one another,
and have a similar 'controlling force.'' This force depends
not only on the similar rate of multiplication of the biophors
transmitted by the controlling determinants into the cell-body,
and on the suppression of those already present, but also on the
number of precisely similar determinants derived from each
parent. The larger the number of ' homodynamous' deter-
minants, the greater is their controlling effect on the cell ; and
if a larger number of homodynamous determinants are opposed
to fewer heterodynamous determinants of the other parent, the
former gain the victory. The preponderance of one of the
parents in transmission is thus rendered comprehensible, whetlier
it concerns individual parts or the entire organism.

The type of the child is determined by the paternal and
maternal ids contained in the corresponding germ-cells meeting
together in the process of fertilisation, and the blending of
parental and ancestral characters is thus predetermined, and can-
not become essentially modified by subsequent influences. The
facts relating to identical twins and to plant-hybrids prove that
this is so. In the latter, the individuals produced by crossing two
constant species display as constant an intermixture of charac-
ters as would be the case if they constituted a natural species.
The ids of each species must be looked upon as perfectly
homodynamous as regards the specific characters ; two distinct
groups of homodynamous ids are opposed to one another, and

SUMMARY AND CONCLUSION 459

the preponderance of one or other parental group in any par-
ticular part of the plant depends on the presence of a larger
number of homodynamous determinants representing the part in
question, and on their possession of a greater controlling force.

Revej-sioii to grandparents and great-grandparents, or to
uncles and aunts, may be accounted for by the fact that, in the
first place, the idants and ids are not formed anew in the germ-
plasm of the parents, but are derived from the grandparents ;
and, secondly, that the combination of ids contained in the
individual germ-cells of the parent becomes very diversified in
consequence of the ' reducing division.' The usually accepted
assumption of breeders that one-fourth of the ' blood ' of the
grandchild is derived from each of the four grandparents, and
one-eighth from each of the eight great-grandparents, is there-
fore inaccurate. The number of ids of any particular ancestor
which are contained in the germ-plasm of a ripe germ-cell
depends entirely on the manner in which the reducing division
occurs ; and, under certain circumstances, a germ-cell might
presumably contain half the entire number of ids of one grand-
parent, and none of those of the other three. The larger the
number of ids derived from an ancestor, the greater is the prob-
ability that some of the characters of this ancestor will appear
in the descendant ; but this depends on the force of the ids of
the other parent, which comes into play when amphimixis takes
place, and also on whether the ids derived from this ancestor are
the dominant ones which determined his ' type ' (• Bild *).

Reversion to an ancestor must consequently always occur
when, in consequence of the ' reducing division,' the ids deter-
mining the type of this ancestor reach a particular germ-cell
of the individual in question, — if they are not opposed by a
stronger group of ids derived from the other parent in the
process of amphimixis. This holds good for each individual
part of the offspring, as well as for the entire organism, for the
number of homodynamous determinants may be, and generally
is, different in the various parts, — at any rate in the case of the
individual differences between human beings.

From this theory, it could be predicted that hybrid-plants
fertilised with their own pollen must produce very variable
offspring, and that individuals of these hybrids must. nu»reo\er,
revert to one or other of the ancestral species : botii these state-
ments are borne out by fact.

460 THE GERM-PLASM

Although reversion to ?nore rejiwte ancestors is also brought
about by the same factors, — viz., the 'reducing division' and
amphimixis, — it recjuires further elucidation. The theory of
selection requires that only a majority, and not all, the deter-
minants of a part which is to be modified shall undergo a
corresponding change. Old unmodified determinants of various
parts are therefore retained in the germ-plasm of a species, and
can only be removed from it very gradually by fortuitous
'reducing divisions.'' This renders reversion to the characters
of very remote ancestors possible; its occurrence, however,
depends upon the reducing division and amphimixis taking
place in a favourable manner. If the reduction causes similar
groups of ancestral determinants to be brought together in
several ids, and this germ-plasm, in the process of amphimixis,
unites with that of another germ-cell, which also contains
similar ancestral determinants in several ids, these may gain
the victory over the modern determinants in the struggle of
the ids during ontogeny. This, however, will chiefly depend
on the kind and strength of the modern determinants which are
opposed to them ; and thus reversion to ancestral characters
occurs very frequently in crosses between races (pigeons) and
species (mules), in which the modern determinants are heterody-
namous ; — they do not co-operate, and their forces counteract
one another, while the ancestral determinants are similar and
their forces cumulative.

Numerous phenomena of reversion in plants and animals
may be explained in a very simple manner on these principles,
and from this point of view it is also possible to understand
that form of reversion which occurs in gemmation and partheno-
genesis. The more remote the ancestors to the characters of
which reversion occurs, the more rarely will it take place.
Reversion to the three-toed ancestors of the horse, for instance,
is of extremely rare occurrence, for it is due to a retention of
the ancestral determinants in question — which have certainly
disappeared from all the ids in the germ-plasm of most existing
horses — in single individuals of certain series of generations,
and to the chance of the coming together of two germ-cells
containing such ancestral determinants.

The remarkable phenomenon of dir/iorphisni, which has been
introduced so extensively — more especially into the animal king-
dom— by means of sexual reproduction, must be due to the

SUMMARY AND CONCLUSION 461

presence in the idioplasm of double deter/fiinants for all those
cells, groups of cells, and entire organisms, which are capable
of taking on a male and female form. But only one half of such
a double determinant remains inactive, while the other becomes
active. The sexual differentiation of the germ-cells must thus be
due to the presence of spermatogenetic and oogenetic double
determinants ; and even all the secondary sexual characters must
be traced to a similar origin in the idioplasm. The corresponding
double determinants are contained not only in the germ-plasm,
but are passed on through the cell-stages of ontogeny to that
part of the body in which the two characters become separated
from one another. One of the determinants then becomes
active, its twin half remaining in an inactive condition in the
nucleus of a somatic cell, and under certain circumstances
becoming active subsequently. This, however, only occurs
exceptionally, in such cases as that in which a female animal
((?.^.,a hen or duck) develops male characters in consequence
of castration. Hermaphrodite bees, in which the whole body
consists of the most wonderful intermixture of male and female
parts, furnishes an instructive proof of the presence of both
kinds of characters in all parts of the body, and consequently
of the truth of the assumption of double determinants.

Double determinants not only occur individually, but entire
groups of male and female determinants are opposed to one
another, and these are just as dependent on one another as are
the two halves of the individual double determinant, one of
which always remains inactive when the other becomes active.
These groups may be very dissimilar; in many cases {e.g., the
olfactory organs of male crustaceans and the ornamental
feathers of male birds) the male group contains many more
individual determinants than the female. One half of the
double group may also become degenerated, so that the corre-
sponding organ {e.g., the wing in many female butterflies)
disappears in one sex.

The number of double determinants reaches its highest limit
when the two sexes differ completely from one another in all
their parts, as is the case in Bonellia viridis; even then, how-
ever, a number of single determinants may still be present, if, as
in this case, the larval stage is similar in both sexes.

The assumption of double determinants is also able to throw
some light upon certain enigmatical phenomena of heredity

462 THE GERM-1'LASM

exhibited by human beings. It has long been known that
hcBmophilia occurs in men only, but is transmitted by women.
If we assume that the visible sexual diiTerences, as well as those
existing in the system which we are unable actually to recognise,
are due to the presence of double determinants, the peculiar
limitation of this uncommon disease to one sex is explained.
The disease, like a secondary sexual character, is only trans-
mitted to the sex in which it first appeared, for this half of
the double detenninajits of the * mesoblast genn ' has alone been
niodified by the disease.

The sexual polymorphism exhibited by certain butterflies may
also be explained by assuming the presence of double deter-
minants of several local varieties of the same species which
interbreed with one another. The polymorphism of bees and
other animals which form communities, requires, however, the
assumption of triple or quadruple determinants. In these
animals the female half of the double determinant again becomes
doubled, and this may also be the case as regards the male half
(Termites).

Lastly, the assumption of double determinants in the idio-
plasm accounts for temporary dimorphism, such as seasonal
dimorphism.

The occurrence of dimorphism is in all cases attributable to
the presence of two kinds of determinants ; but the causes which
determine which of the two is to become active, are extremely
varied, and cannot in many cases be accurately indicated.
The determining influences, however, are always external ones
— such as fertilisation, nutrition, and the effect of light in cases
oi dichogeny in plants.

It is self-evident from the theory of heredity here propounded,
that only those characters are transmissible which have been
controlled — i.e., produced — by determinants of the germ, and
that consequently only those variations are hereditary which
result from the modification of several or many determinants
in the germ-plasm, and not those which have arisen subse-
quently in consequence of some influence exerted upon the cells
of the body. In other words, it follows from this theory that
somatogenic or acquired characters cannot be transmitted.

This, however, does not imply that external influences are
incapable of producing hereditary variations ; on the contrary,
they always give rise to such variations when they are capable

SUMMARY AND CONCLUSION 463

of modifying the determinants of the germ-plasm. Climatic
influences, for example, may very well produce permanent
variations, by slowly causing gradually increasing alterations
to occur in certain determinants in the course of generations.
An apparent transmission of somatogenic modifications may
even take place under certain circumstances, by the climatic
influence atfecting certain determinants of the germ-plasm at
the same time, and wlien they are about to pass to that part of
the body which they have to control. This is indicated by the
climatic variations of the butterfly Polyonuuatiis phlcras.

The primary causes of variation is always the effect of
external influences. Were it possible for growth to take
place under absolutely constant external influences, variation
would not occur; but as this is impossible, all growth is con-
nected with smaller or greater deviations from the inherited
developmental tendency.

When these deviations only aiTect the soma, they give rise to
temporary non-hereditary variations ; but when they occur in
the germ-plasm, they are transmitted to the next generation and
cause corresponding hereditary "iiariations in the body.

Since the germ-plasm undergoes a very considerable growth
from the fertilised egg-cell to the germ-cells of the off"spring,
minute fluctuations continually take place in the composition
of its vital units, the biophors and determinants. If permanent
and constant influences, such as those of climate, act upon them.
these minute fluctuations will become accumulated in the course'
of time and generations, and may thus give rise to appreciable
individual variations, and then gradually to racial, and even
perhaps to specific characters. If an influence acts in a certain
direction for a short time only, it alone may or may not give
rise to an individual variation in the soma, according to the
number of ids of the germ-plasm aftected by it. Whenever
a majority of ids become modified, a corresponding variation
must appear in the soma. As, however, an intermingling of
the ids takes place twice, owing to the successive proces.ses
of ' reducing division ' and amphimixis, minorities of modified
ids may be increased to majorities ; and sexual reproduction
may then cause the fluctuating material for invisible variations
in the determinants to give rise to perceptible somatic variations,
and these are made use of by natural selection, aided by con-
stantly recurring amphimixis. The latter process gives natural

464 THE GERM-PLASM

selection a choice of innumerable combinations of the most
diversified variations, resulting from the constant minute fluctua-
tions of all the units in the germ-plasm.

Strictly speaking, the process of amphi?nixis alone cannot
bring about an increase or decrease in the develop))ient of a
character ; though it may, indeed, establish it more firmly in
the germ-plasm by causing an increase in the number of ids,
the determinants of which produce the character. An increased
development, in the ordinary sense of the term, may, it is true,
take place by the extension of a variation over larger areas of
the body ; the multiplicity of the ids, and the possibility of the
constant production of new idic combinations by the process
of amphimixis, accounts for the statement of breeders that the
constancy of a character, as well as its increased development,
may be affected by selection. The so-called ' individual potency '
must, moreover, be due to the presence of a large number of
homodynamous determinants for all the more important char-
acters, and it probably results, not only from breeding a race
true for a long time, — although this is of course necessary, — but
also from favourable combinations of ids being produced by the
processes of reducing division and amphimixis.

Variations do not, however, depend merely on modifications
in the composition of a determinant or group of determinants,
but frequently result from a doubling or further multiplication
of the latter ; and this must also depend primarily on modified
external influences, such as those produced by changes in the
nutrition of a part of the germ-plasm. The apparently sudden
appearance of parts — such as feathers and other epidermic
structures, as well as of certain pathological structures, such
as the supernumerary fingers and toes of human beings — may
be explained in this manner. All such variations do not, indeed,
act?ially arise suddenly, but take place gradually in some of
the ids, and only suddenly become apparent when they have
accumulated to form a majority.

The suddenness with which variations appear is, in all proba-
bility, only apparent in most cases, as is well shown by
Hoffmann's experiments on wild plants, in which variations
were produced by abnormal conditions of life. The degenera-
tion of parts which are no longer required, or have simply
become useless, is due to the reduction and final disappearance
of the corresponding determinants from the germ-plasm. But

SUMMARY AND CONCLUSION 465

as this again depends on the fluctuating variations of these
determinants in the different ids. it will not occur to the same
extent in all the ids at the same time ; and thus the remains of
these reduced determinants are often preserved in individual
ids through countless generations, and may occasionally cause
reversion to take place when they have accumulated in conse-
quence of the ' reducing division * and amphimixis.

Sudden variations of bnds have only been observed in plants
which also are, or have been, propagated sexually, and in
which the structure of the germ-plasm is therefore just as
complex as in species in which sexual reproduction alone
occurs. These variations are also due to the effects of dis-
similar modifying external influences on the determinants
contained in the ' blastogenic' germ-plasm, which is jjassed on
from cell to cell during the process of growth. It would, how-
ever, be impossible to understand why only a single bud out
of millions should undergo transformation unless some other
cause were also at work. This may be due to occasional
irregular nuclear divisions, which would give rise to a similar
result to that produced by amphimixis in reproduction by
seeds, the modified determinants of individual ids occasionally
becoming accumulated and then taking effect.

The power of transmission of * sudden ' variations in plants,
which is apparently very capricious, may be easily understood
in principle. As the modification never occurs in all the ids
of the germ-plasm, but only in many of them, and as this
majority may be a slight or a considerable one. the tran.s-
mission of the variation will depend on whether the majority
is often obtained and even increased, or whether it becomes
diminished, or even entirely lost, during the reducing divisions
and amphimixis, when the plant is reproduced by seeds. In
the case of the first alternative, the ' sport ' will be transmitted :
while in that of the second, transmission will only occur rarely
or not at all. Even details of the apparently enigmatical
phenomena of heredity of known ' sport ' varieties — such as
those of the balsamines, weeping ashes, the variegated variety
of Ballota nigra, and others — thus receive a very simple
explanation.

The capricious transmission of bud-variations by seeds,
which only occurs in the smaller proportion of cases, cannot
be explained so easily. It is due to the * blastogenic' germ-

466 THE GERM-PLASM

plasm and the • reserve germ-plasm ' — destined to form germ-
cells — following different courses, and consequently they will not
always contain the same number of modified ids. The in-
frequency of the transmission of bud-variations by seeds may
depend on the production of a new combination of ids of the
' reserve germ-plasm ' from which the germ-cells are formed,
whenever this formation occurs, owing to the ' reducing divi-
sion ' and amphimixis. No such occurrence takes place in
the ^ blastogenic' germ-plasm as long as merely asexual repro-
duction continues.

According to our view, the power of transmission — which is
possessed by all organisms, and on which the development of
the higher organic forms is based — therefore depends on simple
growth merely in the case of the very lowest conceivable
organisms with which we are not acquainted ; while in all forms
which have already undergone differentiation, it results from
the possession of a special appa7-atiis for U'aiisinissioji.

This apparatus first occurs in the unicellular organisms, in
which it consists of a substance composed of the different kinds
of vital units or biophors, which occur in the substance of the
organism, and presumably in a similar proportion ; there are,
at any rate, numerous individual biophors of every kind, all of
which are arranged together on a definite plan. This substance
is surrounded by a membrane, — the nuclear membrane, — pro-
vided with pores, through which the biophors of the nucleus
can pass into the cell-body, there to multiply at the expense
of the nutritive materials — to which the vital particles of the
cell-body themselves may become reduced under certain cir-
cumstances — and to become arranged in virtue of the forces
dwelling within them.

To these processes is due the power possessed by the
organism of giving rise by division to two complete individuals
of a similar nature.

Even at this stage of differentiation, the hereditary substance
is rendered more complicated by the process of amphimixis,
or mingling of individual differences, in which this substance
periodically becomes halved, and is then again completed by the
hereditary substance of another individual. The result is, that
every part of the organism is represented in the hereditary
substance by different varieties of the same kind of biophor.

SUMMARY AND CONCLUSION 467

and that, consequently, the indivickials subsequently arising by
division will be approximately intermediate in structure between
the two parents.

In the multicellular forms exhibiting cell-differentiation, the
apparatus for transmission becomes more complicated the more
numerous and diversified are the kinds of cells composing the
organism. Multiplication can only take place if each individual
originates in, and again returns to, the unicellular condition.
The division of the entire organism would only result in the
production of two unequal halves, which could not of themselves
undergo completion, and would require a special apparatus for
the purpose. In order that this may arise, the previous produc-
tio7i of an apparatus for transmission, adapted for reproduction
by unicellular germs, is indispensable.

The production of this apparatus results from the formation
of a gerin-plasni, i.e., a nuclear substance which contains
reserv-e biophors for the construction of the corresponding
cell-body, as well as for the formation of all the cell-bodies
of the entire organism ; and all of which are connected together
into a definitely arranged structure, in such a manner that the
constituent parts share regularly and successively, and not
simultaneously, in the control of the cell-body. In order that
this result may be produced, the smaller vital units or biojDhors
are combined to form those of the next higher order — the deter-
minants, — each of which controls one kind of cell, and conse-
quently includes all the biophors required for the determination
of this particular kind of cell. The germ-cell contains at least
as inajiy determinants as there are different cells or groups of
cells z« the fdly-formed organism which are capable of being
individually determined fron the germ onwards.

Since the process of amphimixis or '•intermingling of indi-
vidual differences ' is also retained in multicellular forms, the
individual germ-cells must for this reason alone contain a mass
of germ-plasm, each unit of which contains all the kinds of
determinants of the species in close combination. The heredi-
tary substance of the germ-cell thus came to be composed of
ids and idants.

The multiplication of jnulticellular organisms by fission and
gemmation results from a considerable increase in the com-
plexity of the apparatus for transmission, in which process not
only the determinants required for the control of their own

468 THE GERM-PLASM

nature, but also ids of germ-plasm in an unalterable condition
— in which they are at the time incapable of undergoing
disintegration — were distributed to certain cells of the body.
This addition of latent germ-plasm to certain series of somatic
cells also results in the formation of germ-cells in most multi-
cellular forms ; while the power of regeneration depends on a
systematic addition of certain inactive determinants, or groups
of determinants, to certain cells of the body.

Further complications of the germ-plasm produce the phenom-
ena of alternation of generations and the polymorphism often
connected with it, as well as the sexual dimorphism always oc-
curring in a greater or a less degree in connection with '■ sexual
reproduction.' All determinants and groups of determinants
which exist in two or more forms, must be present in a double
or multiple condition in the germ-plasm, and be so arranged
that each constituent part only becomes active in turn. This
assumption is, however, insufficient in the case of alternation
of generations, in which several kinds of germ-plasm must be
present and become active in turn.

Thus an ever increasing complexity of the substance which
renders the repetition of the organism possible is gradually
produced in the phylogeny of living beings, and eventually
reaches so high a degree that it is difficult to believe that
such an infinite complexity of structure can actually exist in
particles so minute. The more deeply, however, we penetrate
into the phenomena of heredity, the more firmly are w^e con-
vinced that something of the kind does exist, for it is impossible
to explain the observed phenomena by means of much simpler
assumptions. We are thus reminded afresh that we have to
deal not only with the infinitely great, but also with the infinitely
small ; the idea of size is a purely relative one, and on either
hand extends infinity.

INDEX

Acer negundo, reversion in, 338

Actiniae, regeneration in, 127

Adler, on galls, 219

Adventitious buds, Sachs's view of,
211; formation by means of ac-
cessory idioplasm, 212, 218

Alternation of generations, in its
relation to the idioplasm, 173;
hereditary variability of individual
generations, 174; in Daphnidae,
175; in Aphis, 178; in Medusae,
1 79 ; caused by two kinds of
germ-plasm, 182, 457

Amphimixis, or the intermingling
of individuals, 20, 24; its relaticm
to reproduction, 232; resulting in
complications in the composition
of the germ-plasm, 235; the cause
of all reversion, 337; the means
of producing variation, 413; its
relation to bud-variations, 439;
the cause of an increase or de-
crease in the development of a
character, 464

Ancestral germ-plasms — not anal-
ogous to Spencer's ' physiological
units,' 1 1

Antirrhinum majus, Darwin's ex-
periments with the peloric form

of, 33^

Aphis, alternation of generations,
178

Ascaris megalocephala, the chromo-
somes of, 86; diagram, fig. 18,
page 233

Ascidians, regeneration, 139; eggs
contrasted with those of frogs,
141 ; gemmation, 160; compari-
son of gemmation with embry-
ogeny, 162

Auerbach, on nuclear division, 23

Balfour, phylogeny of gemmation,
169

Balsamine, transmission of varia-
tions of, 445

Baumann, chemical constitution of
protoplasm, 40

Bees, polymorphism in, 376

Begonia, adventitious buds, 211

Beneden, Ed. van, observations on
fertilisation of egg in Asmris, 23;
function of the centrosomes, 25;
view concerning idants. 244

Biophors, or bearers of vitality, the
smallest vital units, 40; a group of
molecules on which the phenom-
ena of life depend. 40; resem-
blance to Spencer's ' physiological
units,'4o; difference between them
and Nageli's 'micella?,' 41; de
Vries's ' pangcnes,' and Wiesner's
' plasomes,' 42; bearers of cell-
qualities, 42; number of kinds
unlimited, 44; all prutojilasm
constituted by, 45 ; their si/.e and
number, 85; combined anew in
the formation of galls, 221, 448

Beyerinck on galls, 219; gall of
iVef/iaius, 222

Blending, of parental characters nut
a fundamental phenomenon of
heredity, 8; of paternal and ma-
ternal idioplasm in ontogeny, 253 ;
of parental characters in the oft-
spring, 261, 266, 297

Blood-cori^uscles controlled by de-
terminants, 57

Blumenbach's ' nisus formativus,'
103. 104

Bonellia viridis, its dimorphism, 364

Bonnet, on the regeneration of eye
of Triton, 125; on worms, 131

469

470

INDEX

Born, development of eggs in a
fixed position, 135

Boveri, observations on fertilisation
of egg in Ascai'is, 23; function
of the centrosomes, 25 ; observa-
tions on the egg of the sea-urchin
artificially deprived of its nucleus,
28; chromosomes of Ascafis
niegalocephala, 86; differentia-
tion of somatic cells of Ascaris
niegalocephala, 191 ; number of
nuclear rods in two species of
Ascaris, 241; view concerning
idants, 242

Brandza, examination of hybrids
with regard to parental charac-
ters, 270

Brooks, W. K., on the laws of
heredity, 9; on variation, 412;
on the non-transmission of ac-
quired characters, 396

Briicke, Ernst, on ' Elementar
organismen,' 2; on ultimate vital
particles, 20; on albumen, 38;
on the organic nature of proto-
plasm, 39

Bryophyllum, budding in, 211

Bud-variation, 436; theoretical ex-
planation of, 441

BUtschli, investigations on the proc-
esses of nuclear division, 23;
the polar bodies correspond to
aborted ova, 251

Butterflies, the markings on the
wings of, as confirming the theory
of determinants, 54, 174, 264,
265; Kallima pa^-allecta as an
example of relative perfection,
272; the origin of climatic varia-
tions in the idioplasm, 379, 399,
407, 418

Cardamine pratensis, budding in,

211
Carneri, observations on telegony,

Caspary, experiments in crossing
Cytisiis laburnum and C. pur-
pureus, 343

Castration, as a cause of sexual re-
version, 358

Cecidomyia poae, formation of galls,
221

Centrosome, with its sphere of at-
traction, is an apparatus for the
division of the cell and nucleus,
23; continuity of the centrosome,
48

Chabry, experiments on eggs of
Ascidians, 136

Chromatin, the hereditary substance,
24, 26; must be different in each
kind of cell, 32

Chromosomes correspond to the
idioplasm, 10 ; their form, 24;
spoken of as idants, 67

Continuity of the germ-plasm, his-
torical account of, 198

Control of the cell, 45

Cuttings, development and origin
in the idioplasm of, 212

Cyclops, observations on segment-
ing ova, 191

Cymathoidce, temporary sexual di-
morphism, 1 1 1

Cypris reptans, markings on the
shell in their relation to the theory
of determinants, 88; reversion
in, 344; theoretical explanation
of reversion in, 347

Cytisus adami, 339; theoretical ex-
planation of the variation in, 344

Daphnia pulex, alternation of gen-
erations in, 175

Daphnidce, embryogeny of, 184;
secondary nature of the differen-
tiation of germ-cells of, 186;
germ-track of, 192

Darwin, Charles, 'pangenesis,' 2;
' gemmules,' 3; comparison of
the * gemmules ' with Spencer's
' physiological units,' 6; on cor-
related variations, 84; on the white
coloration in animals and plants,
277; on breeding, 291; on the
force of heredity in cattle, 293;
on reversion in plant-hybrids, 299;
on reversion to remote ancestors,
316; on reversion in pigeons,
323; in mules, 328; in Antir-
rhinuiii ?iiajiis, t^^i; experiments
on crossing Cytisus laburnum-

INDEX

471

and C. purpurea, 340; on latent
primary constituents, 352; on
transmission of acquired char-
acters, 384, 394; on variation,
406, 410; on baldness in birds,
426; on the transmission of bud-
variations, 441 ; on the trans-
mission of a variation in Ballota
nigra, 446

Datura ferox and D. loevis, reversion
of hyl)rids to an unknown an-
cestral form, 317

Davenport, on budding in Poly/oa,

159
Degeneration in its relation to the

theory of determinants, ^t,
Dejerine, on the transmission of

nervous complaints, 369
Determinants or determining parts,
57; control groups of cells, 57;
are a group of biophors, 59;
possess special qualities, 60; are
definitely localised in the idio-
plasm, 61; constitute the ids, 62;
their forces of attraction, 66;
their behaviour in the course of
ontogeny, 69; their disintegration
into biophors, 69; their growth
and multiplication, 71; their
number, 89; supplementary de-
terminants, 103, 112, 127, 131,
149; the separation of a group
of, 153; their disintegration in
gemmation, 160; their separation
into groups in the development of
somatic cells, 209; their modi-
fication in the formation of galls,
222; control of the cell by the
combined influence of paternal
and maternal determinants, 261 ;
homologous and heterologous
determinants, 264; their con-
trolling forces, 269 ; number
of homodynamous determinants
varies in ontogeny, 271; homo-
dynamous and heterodynamous
determinants, 264, 278; proof of
their disintegration into biophors,
348; in cases of dimorphism,
355; variations due to changes
in their composition, 406, 418,
448

Determinants, the theory of, 53;
summary, 225; reversion cx-
jilained by, 335; applied to sex-
ual dimorphism, 366

Determinates or herecHtary parts, 57

Diantliuschinensis and D. barbatus,
crosses between, 302

Dichogeny in plants, 114, 380, 462

Dicyemidit, distribution of heredi-
tary parts, 59

Digitalis lutea and D. purpurea,
crosses between, 255

Dimorphism, normal, 352; its jjasis
in the idioplasm, 354; patho-
logical, 370; dtmble, in J'apilio
turnns, 375; seastjnal, 379, 462

Diptera, course of germ-track, 192;
renewal of the alimentary epi-
thelium, 57

Disarticulation of the limb of Triton,
in connection with the hypothesis
of supplementary determinants,

Diseases, transmission of, 3S7;

haemophilia, 370
Doubling of limbs in insects, 429;

of whole groups of determinants,

428
D*iesch, experiments with eggs of

sea-urchins, 137; on the blasto-

meres of the frog, 141

Elsberg, Louis, 41
Embryonic cells, ^t,
Eudendrium, budding of, 155, 156,
fig. 6

Ferns, regeneration and gemma-
tion, 215

Fertilisation, essential part of the
process, 232

Fission in Naidiv, 146; in Micro-
stomidiv. 149, 456

F'lemming, on nuclear division. 23;
on tlie splitting of the chromo-
somes in nuclear division, 25, 68

Focke, on plant-hybrids, 261; on
hybritls of Xicotiana, 267; on
reversion in hybrids, 299; on re-
crossing, 302; on ' Nenia,' 383

472

INDEX

Fol, on the transference of the
centrosome into the ovum during
fertilisation, 29

Fraisse, on regeneration in Am-
phibia, 96 ; in Sala7nandi'a, 99;
in lizards, no, 116

Fucoidae, polar bodies of, 251

Galls, 218; definite new forma-
tions composed of modified cells,
221

Galton, Francis, acceptation of the
' gemmule ' hypothesis, but rejec-
tion of that of their free circula-
tion, 7; theory of the blending of
characters of parents in the chil-
dren, 7, 257 ; germ-substance com-
posed of homologous gemmules,
72; supposed priority as regards
the assumption of the continuity
of the germ-plasm, 198

Gartner, on plant- hybrids, 299; re-
crossing of hybrids, 305

Gemmation in Coelerentata, 154;
' blastogenic ' idioplasm, 157; in
Polyzoa, 158; in Tunicata, 160;
in plants, 163; comparison of the
process in plants and animals,
166; its phylogeny, 168; result-
ing from adaptation in polypes,
217; brief summary, 225, 456

Germ-cells, their formation, 183;
shifting of place of origin in the
course of phylogeny, 186; only
certain series of cells contain the
primary constituents of germ-cells,
189; distinction between germ-
and somatic-cells, 208; the com-
bination of ids in germ-cells, 245,
247; only half the number of
parental ids contained in them,

257. 457
Germ-plasm constituting the im-
mortal reproductive substance, 9;
is the first ontogenetic stage of
the idioplasm, 35; its funda-
mental units, 37; its composition
out of biophors, 40, 48; its fixed
architecture, 61 ; it forms a com-
plete unit by itself, 62; summary
relating to its structure, 75;
magnitude of its constituents, 85;

'blastogenic' idioplasm, 166; its
regular division, 171 ; accessory
germ- plasm, 174; its continuity,
183; its composition, 37, 77, 186;
its modification caused by am-
phimixis, 235; its composition
out of paternal and maternal
idants, 254; its partial variations,
249, 271; reversion due to its
ancestral determinants, 336 ;
transformation and the gradual
production of reversion, 336 ;
* blastogenic ' germ-plasm, 441 ;
reserve germ-plasm, 447, 453

Germ-tracks, course taken by the
germ-plasm from the ovum to
the reproductive cell, 184; their
course in the Metazoa, 192; their
cells alone capable of giving rise
to primary germ-cells, 194; each
cell in them contains perfectly
definite somatic elements, 210;
brief summary, 228

Giard, view that the polar bodies
correspond to aborted ova, 251

Godron, reversion in Alelandrywn
album and M. rtibrum, 306

Gotte, regeneration in Salaniandr-a,

Grul)er, August, artificial division

of Infusoria, 52
Guignard, on the reduction of idants

in the germ-cells of plants, 250;

on the union of nuclei, 30

Haberlandt, on the relations be-
tween the functions and position
of the nucleus, 46

Hackel, Ernst, on the * perigenesis
of the plastidule,' 41; on the law
of sexual transmission, 369

Hacker, Valentin, on the phenom-
ena of reduction in the germ-cells
of Arthropods, 250

Haemophilia, 370

Hallez, on the embryology of
Nematodes, 137

Hatschek, 53; on variation and
sexual reproduction, 413

Henking, on the phenomena of
reduction in the germ-cells of
Arthropods, 250

INDEX

473

Hensen, Victor, on amphigonic
heredity, 253; on the increase of
a character, 424

Herbert, experiments with Cytisus
adami, 343

Hereditary substance, not contained
in the bodv of the cell, but in its
nucleus, 10; composed of primary
constituents, 15; forms but a
small part of the substance of the
egg, 22; composed of different
qualities, 26; the extreme com-
plexity of its structure, 29; its
growth, 31; of unicellular organ-
isms, 45 1 ; of multicellular organ-
isms, 452

Heredity, its fundamental phenom-
ena, 20; in unicellular organ-
isms, 52; in reference to the
structure of the germ-plasm, 69;
homotopic and homochronic
forms, 75; in monogonic repro-
duction, 92; in sexual reproduc-
tion, 230, 253; explanation of
apparently monogonic heredity in
plant-hybrids, 259; in Man, 259;
as regards the colour of the eyes,
278; example of apparently mono-
gonic heredity, 267, 280; force
of, 290; as regards acquired
characters, 392; mutilations, 396;
'sports,' 444; the apparatus for
transmission, 466

Hertwig, Oscar, view of fertilisation
as a conjugation of nuclei, 23;
on the germinal layers of the
Metazoa, 113; on the nuclear
rods in two varieties of Ascaris
megalocephala, 241

Heterobiophorids, hypothetical pri-
mitive organisms, 450

Heterokinesis, nuclear division re-
sulting in parts which are dis-
similar as regards their hereditary
tendencies, 34

Hildebrandt, hybrids of two species
of Oxalis, 255

His, theory of special regions in the
germ giving rise to special organs,

134

Hoffmann, experiments with Papa-

ver alpi)iw?i, 437

Homobiophorids, hypf)thetical j.ri-
mitive organisms, 450

Homoeokinesis, nuclear division re-
sulting in parts which are similar
as regards' their hereditary ten-
dencies, 34

Hoppe-Seyler, chemical constitu-
tion of protoplasm, 40

Humming-birds, sexual dimorphism
of, 427

Hybrids of various species of plants,
255; the theory of idants, 259;
plant - hyljrids, 260 ; ai)parent
monogonic transmission in, 302

Hydra, regeneration, 127; gemma-
tion, 155

Hydractinia echinata, formatii)n of
germ-cells, 189

Hydroids, formation of the bud
from a single cell, 166; embryo-
geny, 184; shifting of place of
origin of germ-cells, 186; germ-
tracks of, 207

Id, the, in ontogeny, 60; the num-
ber contained in the individual
idant, 241

Idants, constituting the hereditary
substance in sexual reproduction,
234; composed of dissimilar ids,
238; constituting groups of ids,
241; their number, 245; become
doubled by division, 246; com-
bination of, 247; of ancestors
contained in thegcrm-])lasm, 257;
their combination in plant-hybrids,
297, 302 ; intermingling of, in
transmission, 312

Idioplasm, Nageli's conception of
the, 10; my view of the, 33; con-
trasted with morphoplasm, 38;
composed ot ids, 63 ; phylctic
variation oi, in the difTerentiation
of species, 79, 434

Ids or ancestral germ- plasms com-
posed of ckterniinants, 62; their
intlividual difference, 236; homo-
logous and heterologous, 264;
gradual transformation of, 27I;
number variable according to age
of characters, 273 ; struggle of
parental ids, 285

474

INDEX

Increase of a character by crossing,

424
Individual jirepotency, 291
Ivy, III; dichogeny in, 380

Jager, Gustav, supposed priority
as regards the hypothesis of the
continuity of the germ-plasm, 200

Kennel, regeneration of beak of
stork, 125; division, 150

Kiwi {Apteryx), disappearance of
wings, 82

Kleinenberg, development of Lum-
brictis trapezoiiies, I 70

Kolreutter, crossing of AHcotiana
rustica and A^ paniculata, 261,
269; on recrossing, 305

Kowalewsky, on supernumerary toes
of horses, 334

Kraepelin, investigations on her-
maphrodite bees, 361

Lamarck, on the transmission of
acquired characters, 395

Lang, A., the regenerative faculty
as an arrangement for protec-
tion, 93 ; budding in hydroids,

155
Leptodora hyalina, development of,

176; sexual characters of, and the

theory of determinants, 363

Leuckart, Rutlolph, discovery of
parthenogenesis in bees, 356; on
the combination of secondary
sexual characters in hermaphro-
dite bees, 360

Leydig, regeneration in lizards, no,
116

Liebscher, crossing of Ilordeiim
steiidelii and //. trifiircatum, 301

Loeb, gemmation in Tiibidaria
mesenibryantheniiivi, 216

Lumbriculus, regeneration in, 126,

Lucas, Prosper, on transmission of
polydactylism, 373

Lycaena, as illustration of the theory
of determinants, 58, 62; L. adonis
and Z. agestis, 87; colouring and
dimorphism of, 359, 373; varia-
tion in, 422

Marrubium vaillantii, 270

Marsh, on supernumerary toes in
horses, 334

Medusae, formation of germ-cells,
187 ; degeneration into a mere
gonophore, 187

Mesohippus, 334

Meyer, O. E., on molecules, 86

Micellar theory, 41

Miescher-Rusch, on nuclein, 50

Morphoplasm, 38

Mules, transverse stripes of, indi-
cating reversion, 316; theoretical
explanation of stripes, 329; patho-
logical variation, 429

Miiller, Josef, ' gamomachia ' and
' gamophagia,' 296

Nageli, Carl von, physiological
theory of descent, 9; the heredi-
tary substance constitutes only
an inhnitesimal part of the egg-
cell, 22; the 'micella,' 41; on
variation, 41 1 ; the ibis and croco-
dile in connection with variation,
416; bud-variation, 442

Nais, regeneration of, 126

Naudin, experiments with Datura,
321

Nicotiana rustica and N. paniculata,
crosses between, 261 ; difference
in the flowers of, 267; A^. alata
and N. langsdorffii, 300

Nipples, supernumerary, in human
beings as examples of reversion,

333
Nitsche, O., budding in Polyzoa,

Nussbaum, artificial division in In-
fusoria, 52; idea of the continuity
of germ-cells, 201

Oka, budding in Polyzoa, 159
Ontogeny, or development of the
individual, 32 ; depends upon
gradual changes in the germ-
plasm, 32; part played by deter-
minants in the course of, 80; can
only be explained by ' evolution,'
and not by epigenesis, 138; re-
sulting from the union of the
germ-plasm of two parents, 253

INDEX

475

Otter-sheep, 291 :

Owen, Sir Richard, on cells, 198 j
Oxalis, crosses between different '•
species of, 255

Pangenes, the, of de Vries as
bearers of constituent qualities,
15 ; their miscibility, 15 ; com- '■
pared with biophors, 42 1

Papilio turnus, 375 |

Pfjiiger, the hereditary substance
only an infinitesimal part of the
egg-cell, 22 ; influence of gravity
on the development of frogs' eggs,

Phanerogams, budding in, 216
Philippeaux on regeneration in

Tritons, 118
Phylogeny, 77 ; parallelism be-
tween phylogeny and ontogeny, '
80; of the process of fission in
the Metazoa, 151 ; of regenera-
tion, 114; of gemmation, 168;
of the apparatus for transmission,

.451
Pigeons, reversion to the rock-
pigeon, 323, 352; increase of a
character by selection, 426, 427
Plasomes, the, of Wiesner, 20
Plastidules, the, of Haeckel, 41
Plumularia, budding in, 155
Podocoryne cornea, 158
Polydactylism, the transmission of,

373. 429.
Polymorphism, 374; of animal and

plant stocks, 376, 462
Polyommatus phl^eas, 399, 420
Polyzoa, budding in, 158; forma-
tion of buds from difterent cells,
165; ' blastogenic ' germ-plasm,
218
Primula acaulis, reversion to long-
stalked variety in, 16
Protohippus, 334

Rath, O. vom, on threads of ' linin '
between the idants, 244; on the
phenomena of reduction in the
germ-cells of Arthropods, 250

Rauber, supposed priority as re- .
gards the view of the continuity ;
of the germ-plasm, 200 ,

Ray Lankcster on the nun-transmis-
sion of acquired characters, 396

Reducing division of the germ-
plasm, II, 22, 235; consists in
the extrusion of ids, 240 ; influ-
ence on the composition of the
germ-plasm, 242; cause of dif-
ference between children of the
same parents, 257; in its relation
to reversion, 306

Regeneration, its cause and origin
in the idioplasm, 93; constitutes
an arrangement fur protection,
93; physiological, 94; of the
epidermis, 95; jialingenetic, 105;
ccenogenetic, 108; of the cauilal
region of the vertebral column,
109; phylogeny of, 114; jjhysio-
logical and pathological, 119;
depends on adaptation, 119; in
fishes, birds, and mammals, 120;
difference of the capacity for in
lower and higher animals, 124;
acquired by selection, 125; facul-
tative or polygenetic, 126; in
plants, 132; formation of callus,
133; in animal embryos, 134; of
segmentation - cells, 139; sum-
mary, 225, 455

Reisseck, on crosses between Cytisus
laburtium and C. piirpHrcus, 343

Reproduction, sexual, 230, 457; its
effect, 240; in Man, ancl trans-
mission of parental characters>
257; by fission, 45O

Reversion in plant-hybrids, 299; to
the pure ancestral form, 305; in
Man, and the crossing of clifVerent
races, 308; tua grantiparent, 309;
to characters of remote ancestors,
316; in Datura, 317, 321 ; to an-
cestral forms of flowers in j^lants,
305; to rudimentary characters,
Z2>2)'> '" gemmation, },2y!>\ in par-
thenogenesis, 344; sexual, 307;
in bud-variations, 447, 458

Rhabiiitis nigrovenosa, segmenta-
tion of ovum, 195, fig. 15; germ-
track of, 196

Roux, W., on the purpt)se of the
apparatus for division of tlie nu-
cleus, 26; on the struggle of

476

INDEX

parts, 107; on gravity producing
no eftect in the differentiation of
the egg into the embryo, 135;
experiments on frogs' eggs, 136;
on post-generation, 142
Ruckert, J., behaviour of chromo-
somes during maturation in the
ovum of the dog-fish, 50, 71,

247

Salhs, on the formation of callus,
133; on the growth of Chara,
164; on adventitious buds, 211

Sagitta, embryogeny of, 185; course
of the germ-track of, 192

Salamandra, regeneration of limbs,

99 . .

Salp^e, budding in, 162; alternation

of generations in, 180
Schreiber, on the regenerative power

in Triton martnoratiis, 1 1 5
Seeliger, O., on budding — in Poly-

zoa, 158; in Clavelina, 160; in

Salpae, 162; in Pedicellina, 218
Self-differentiation of cells, 136
Semper, Carl, on the process of

fission in A^ais, 147
Settegast, on infection of the germ,

.385
Siebold, von, on the determination

of sex in bees, 356; on hermaph-
rodite bees, 361

Siren lacertina, regenerative power
in, 115

Spallanzani, on the capacity for re-
generation in different organs,
117; in Triton, 120; in the jaw
of Triton, 125

Spencer, Herbert, ' physiological
units,' 51 ; their relation to Dar-
win's ' gemmules,' 6; on heredity,
7; on regeneration, 104; regen-
eration compared with crystallisa-
tion, 128

Stimuli considered as causing an
incitement to growth, 129

Strasburger, on fertilisation in
Phanerogams, 23; on the essen-
tial similarity of male and female
nuclei, 23; on the dynamical
effect of nuclear matter, 45

Struggle of individual characters,
274; of the ids in ontogeny, 260

Supplementary determinants, 103;
their change in the course of
phylogeny, 112; in processes of
regeneration in Hydra, 127; an-
timeral, 131 ; the origin of, 149

Syphilis, the transmission of, 388

Tardigrada, extent of the heredi-
tary parts, 59
Telegony or infection of the germ,

383
Termites, polymorphism in, 379

Thuja, dichogeny in, 382

Transmission (^see Heredity).

Triton, lOO; diagram of regenera-
tion of fore-limb in, 102, 115;
experiment on regeneration of
lung in, 1 17

Tuberculosis, and infection of the
germ, 389

Tubularia mesembryanthemum, bud-
ding in, 216

Tunicata, budding in, 160

Twins, 140; identical and dissimi-
lar, 254

Type in a technical sense, 313

Vanessa, levana and prorsa, 379

Variation, intercalary remarks on,
271; theoretical demonstration
of, 410; normal individual varia-
tion, 410; cause of hereditary
variation, 415; pathological va-
riation, 428; summary of the
present view with regard to
variation, 431; variations on a
larger scale in plants, 435

Verworn, M., view that the heredi-
tary substance is also contained
in the cell-body, 28

Vilmorin, on individual potency in
plants, 291

Vines, view that the assumption of
a special reproductive substance
is unnecessary, 202; on embry-
onic substance, 204

Viola tricolor, reversion to the an-
cestral form, 320

Vochting, H., on 'transplantation
in the plant-body,' 129

INDEX

477

Volvox, proof of that a difference
exists between the somatic- and
the germ-cells, 213; as an exam-
ple of sexual dimorphism, 355

Vries, Hugo de, objection to the '
theory of ancestral germ-plasms,
12 ; ' intracellular pangenesis '
compared with Darwin's * pan-
genesis,' 12; objection to the view
that the hereditary substance is
contained in the nucleus only,
26; on the structure of proto-
plasm, 38; on Haeckel's * plas-
tidules,' 41 ; objection to Stras-
burger's and Ilaberlandt's views,
46; the ' pangenes ' as bearers of
cell-qualities, 42; on the con-
tinuity of cellular organisms, 56;
on primary and accessory germ-
tracks, 206, 211; on the con-
tinuity of the germ-plasm, 207;
on galls as contradicting the lat-
ter, 218; galls of Cecidotnyia po(F,
221 ; on the crosses between two
species of beans, 297; on plant-
hybrids, 300; on dichogeny, 380,

451

WAf.NKK, Y. von, on gemmation
and fission, 145; on fission in
Microstoma itneare, 149; on
gemmation, 154

Wallace, Alfred Kussel, on Papilio
7iu'}iinoH, 376 ; on variation,
410

Ward, Lester, Xeo-I)arwinism and
Neo-Lamarckism, 408

Weeping Ash, 444, 446

Wiesner, Julius, on the elementary
structure and growth of living
substance, 19; on the constitu-
tion of protoplasm, 39; on the
continuity of the constituent parts
of the cell, 48

Willow, rudiments of roots in the
shoots of the, 382

Zander, R., on polydactylism,

_430

Ziegler, Ernst, on the law of the
specific character of the tissues,
112; on tuberculosis, 390; on
polydactylism regarded as a germ-
variation, 430

Xortoooti ^DrfSB :

J. S. Gushing & Co. — Berwick & Smith.

Boston, Mass , U.S.A.

I

i

J