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TRANSLATIONS

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FOREIGN BIOLOGICAL MEMOIRS

IV.

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HENRY FROWDE

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‘ESSAYS UPON HEREDITY

AND KINDRED

BIOLOGICAL PROBLEMS

BY

Dr. AUGUST WEISMANN

PROFESSOR IN THE UNIVERSITY OF FREIBURG IN BREISGAU

AUTHORISED TRANSLATION
EDITED BY

EDWARD B. POULTON, M.A., F.L.S., F.G.S.

- TUTOR OF KEBLE COLLEGE, OXFORD
LECTURER IN NATURAL SCIENCE, JESUS COLLEGE, OXFORD

SELMAR SCHONLAND, Pu.D.

SUB-CURATOR OF THE FIELDING HERBARIUM IN THE UNIVERSITY OF OXFORD

AND

ARTHUR E. SHIPLEY, M.A., F.L.S.

FELLOW AND LECTURER OF CHRIST'S COLLEGE, CAMBRIDGE
DEMONSTRATOR OF COMPARATIVE ANATOMY IN THE UNIVERSITY OF CAMBRIDGE

Orford
AT THE CLARENDON PRESS
1889

AUTHOR'S PREFACE.

oe oo

THE essays which now appear for the first time in the form
of a single volume were not written upon any prearranged plan,
but have been published separately at various intervals during
the course of the last seven years. Although when writing the
earlier essays I was not aware that the others would follow, the
whole series is, nevertheless, closely connected together. The
questions which each essay seeks to explain have all arisen
gradually out of the subjects treated in the first. Reflecting upon
the causes which regulate the duration of life in various forms,
I was drawn on to the consideration of fresh questions which
demanded further research. These considerations and the results
of such research form the subject-matter of all the subsequent
essays.

I am here making use of the word ‘research’ in a sense
somewhat different from that in which it is generally employed
in natural science; for it is commonly supposed to imply the
making of new observations. Some of these essays, especially
Nos. IV, V, and VI, essentially depend upon new discoveries.
But in most of the remaining essays the researches are of a
more abstract nature, and consist in bringing forward new
points of view, founded upon a variety of well-known facts.
I believe, however, that the history of science proves that
advance is not only due to the discovery of new facts, but
also to their correct interpretation: a true conception of natural
processes can only be arrived at in this way. It is chiefly in

b

vi AUTHOR'S PREFACE.

this sense that the contents of these essays are to be looked
upon as research.

The fact that they contain the record of research made it
impossible to introduce any essential alterations in the trans-
lation, even in those points about which my opinion has since
changed to some extent. I should to-day express some of the
points in Essays I, IV, and V, somewhat differently; but had
I made such alterations, the relation between the essays as a
whole would have been rendered less clear, for each of the
earlier ones formed the foundation of that which succeeded it.
Even certain errors of interpretation are on this account left
uncorrected. Thus, for instance, in Essay IV it is assumed that
the two polar bodies expelled by sexual eggs are identical;
for at that time there was no reason for doubting that they
were physiologically equivalent. The discovery of the numerical
law of the polar bodies described in Essay VI, led to what I
believe to be a truer knowledge of them. In this way the
causes’ of parthenogenesis, as developed in Essay V, received
an important addition in the fact published in Essay VI, that
only one polar body is expelled by parthenogenetic eggs. This
fact alone explains why sexual eggs cannot as a rule develope —
without fertilization.

Hence the reader must not take the individual essays as the
full and complete expression of my present opinion; but they
must rather be looked upon as stages in research, as steps
towards a more perfect knowledge.

I must therefore express the hope that the essays may be read
in the same order as that in which they appeared, and in which
they are arranged in the present volume. The reader will then
follow the same road which I traversed in the development of
the views here set forth; and even though he may be now and
then led away from the direct route, perhaps such deviations
may not be without interest. |

I should wish to express my warm thanks to Mr. Poulton
for the great trouble he has taken in editing the translation,
which in many places presented exceptional difficulties. The

AUTHOR’S PREFACE. vil

greater part of the text I have looked through in proof, and I
believe that it well expresses the sense of the original; although
naturally I cannot presume to judge concerning the niceties of
the English language. I am especially grateful to the three
gentlemen who have brought these essays before an English
public, because I believe that many English naturalists, even when.
thoroughly conversant with the German tongue, might possibly
misinterpret many points in the original; for the difficulty of
the questions treated of greatly increases the difficulty of the
language.

If the readers of this book only feel half as much pleasure in
its perusal as I experienced in writing it, I shall be more than
satisfied.

AUGUST WEISMANN.

FREIBURG I. BREISGAU,
January, 1889.

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EDITORY PREFACE,

—+4_—_

TaE attention of English biologists and men of science was
first called to Professor Weismann’s essays by an article entitled
‘Death’ in ‘The Nineteenth Century’ for May, 1885, by Mr.
A. E. Shipley. Since then the interest in the author's argu-
ments and conclusions has become very general; having been
especially increased by Professor Moseley’s two articles in
‘Nature’ (Vol. XXXIII, p. 154, and Vol. XXXIV, p. 629), and
by the discussion upon ‘The Transmission of Acquired Cha-
racters,’ introduced by Professor Lankester at the meeting of
the British Association at Manchester in 1887,—a discussion in
which Professor Weismann himself took part. The deep interest
which has everywhere been expressed in a subject which con-
cerns the very foundations of evolution, has encouraged the
Editors to hope that a volume containing a collection of all
Professor Weismann’s essays upon heredity and kindred problems
would supply a real want. At the present time, when scientific
periodicals contain frequent references to these essays, and when
the various issues which have been raised by them are dis-
cussed on every occasion at which biologists come together, it
is above all things necessary to know exactly what the author
himself has said. And there are many signs that discussion has
already suffered for want of this knowledge.

A translation of Essays I and IL was commenced by Mr. A. E.
Shipley during his residence at Freiburg in the winter of 1884.
His work was greatly aided by the kind assistance of Dr. van
Rees of Amsterdam, to whom we desire to express our most
sincere thanks. The translation was laid aside until the
summer of 1888, when Mr. Shipley was invited to co-operate

x EDITORS’ PREFACE.

with the other editors in the preparation of the present volume;
the Clarendon Press having consented to publish the complete
series of essays as one of their Foreign Biological Memoirs.

We think it probable that this work may interest many who
are not trained biologists, but who approach the subject from its
philosophical or social aspects. Such readers would do well to
first study Essays I, II, VII, and VIII, inasmuch as some pre-
paration for the more technical treatment pursued in the other
essays will thus be gained.

The notes signed A. W. and dated, were added by the author
during the progress of the translation. The notes included in
square brackets were added by the Editors ; the authorship being
indicated by initials in all cases.

In conclusion, it is our pleasant duty to thank those who have
kindly helped us by reading the proof-sheets and making valu-
able suggestions. Our warmest thanks are due to Mrs. Arthur
Lyttelton, Mr. W. Hatchett Jackson, Deputy Linacre Professor
in the University of Oxford, Mr. J. 8. Haldane, and Professor R.
Meldola. Important suggestions were also made by Professor
E. Ray Lankester, Mr. Francis Galton, and Dr. A. R. Wallace.
Professor W. N. Parker also greatly helped us by looking over
the proof-sheets with Professor Weismann.

OxrorD, February, 1889.

TI, A, E. SHIPLEY .

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Til.

TV. SELMAR ScHONLAND

Translator.

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CONTENTS.

Title.
Tue DurAtTIon oF Lire, 1881

On Herepity, 1883
Lire AND DratH, 1883

Tur CONTINUITY OF THE GERM-PLASM AS THE
FounDATION oF A THEORY OF Herepity, 1885

THE SIGNIFICANCE OF SEXUAL REPRODUCTION IN
THE THEORY OF NATURAL SELECTION, 1886

On THE NUMBER OF PoLAR BODIES AND THEIR
SIGNIFICANCE IN HerReEpiry, 1887

On tHE SuprposeD BoTANIcaAL PROOFS OF THE
TRANSMISSION OF ACQUIRED CHARACTERS,"1888.

Tur Suppos—ED TRANSMISSION OF MUTILATIONS,
1888

(<\

161

251

333

385

419

Abstracts of Professor Weismann’s Essays on Heredity and Kindred
Problems, already Published in this Country.

I. A short abstract in ‘ Nature,’ Vol. XXXVII, pp. 541-542, by P. C. Mrrcwett.
II. A short abstract in ‘ Nature, Vol. XX XVIII, pp. 156-157, by P. C. Mrrcnxtn.

III. A short article on the subject of this Essay in ‘The Nineteenth Century’ for
May, 1885, by A. E. SHrPrey.

TV. Abstract in ‘Nature,’ Vol. XXXIII, pp. 154-157, by Professor MosEtay,
V. Abstract in ‘ Nature,’ Vol. XXXIV, pp. 629-632, by Professor MosELEY.
VI. Abstract in ‘ Nature,’ Vol. XXXVI, pp. 607-609, by Professor WEISMANN.

VII, VIII. The Essays being of so recent a date no abstract has yet appeared in this
country.

A criticism of Professor Weismann’s theories will be found in ‘The Physiology of
Plants,’ by Professor Vines, Lecture XXIII, pp. 660 et seqq.

THE DURATION OF LIFE,

THE DURATION OF LIFE.

PREFACE.

Tue following paper was read at the meeting of the Association
of German Naturalists at Salzburg, on September 21st, 1881 ;
and it is here printed in essentially the same form. A somewhat
longer discussion of a few points has been now intercalated ; these
were necessarily omitted from the lecture itself for the sake of
-brevity, and are, therefore, not contained in the account printed in
the Proceedings of the fifty-fourth meeting of the Association.

Further additions would not have been admissible without an
essential change of form, and therefore I have not put into the
text a note which ought otherwise to have been there, and which is
now to be found in the Appendix, as Note 8. It fills up a gap
which was left in the text, for the above-mentioned reason, by
attempting to give an explanation of the normal death of cells of
tissues—an explanation which is required if we are to maintain
that unicellular organisms are so constituted as to be potentially
immortal.

The other parts of the Appendix contain, partly further expan-
sions, partly proofs of the views brought forward in the text, and
above all a compilation of all the observations which are known to
me upon the duration of life in several groups of animals. I am
indebted to several eminent specialists for the communication of
many data, which are among the most exact that I have been able
to obtain. Thus Dr. Hagen of Cambridge (U.S.A.) was kind enough
to send me an account of his observations upon insects of different
orders: Mr. W. H. Edwards of West Virginia, and Dr. Speyer of
Rhoden—their experience with butterflies. Dr. Adler of Schleswig
sent me data upon the duration of life in Cynipidae, which have a
special value, as they are accompanied by very exact observations

B 2

4, THE DURATION OF LIFE.

upon the conditions of life in these animals; hence in this case we
can directly examine the factors upon which, as I believe, the dura-
tion of life is chiefly based. Sir John Lubbock in England, and
Dr. August Forel of Ziirich, have had the kindness to send me
an account of their observations upon ants, and 8. Clessin of

Ochsenfurth his researches upon our native land and fresh-water

Mollusca.

In publishing these valuable communications, together with all
facts which I have been able to collect from literature upon the
subject of the duration of life, and the little which I have myself
observed upon this subject, I hope to provide a stimulus for
further observation in this field, which has been hitherto much
neglected. .The views which I have brought forward in this paper
are based on a comparatively small number of facts, at least as far
as the duration of life in various species is concerned. The larger
the number of accurate data which are supplied, and the more
exactly the duration of life and its conditions are ascertained, the
more securely will it be possible to establish our views upon the
causes which determine the duration of life.

A. W;

Nap zs, Dec. 6, 1881.

:

THE DURATION OF LIFE.

Wirn your permission, I will bring before you to-day some
thoughts upon the subject of the duration of life. I can scarcely
do better than begin with the simple but significant words of
Johannes Miller: ‘Organic bodies are perishable ; while life main-
tains the appearance of immortality in the constant succession of
similar individuals, the individuals themselves pass away.’

Omitting, for the time being, any discussion as to the precise
accuracy of this statement, it is at any rate obvious that the life of
an individual has its natural limit, at least among those animals
and plants which are met with in every-day life. But it is equally
obvious that the limits are very differently placed in the various
species of animals and plants. These differences are so manifest
that they have given rise to popular sayings. Thus Jacob Grimm
mentions an old German saying, ‘A wren lives three years, a dog
three times as long as a wren, a horse three times as long as a dog,
and a man three times as long as a horse, that is eighty-one years.
A donkey attains three times the age of a man, a wild goose three
times that of a donkey, a crow three times that of a wild goose, a
deer three times that of a crow, and an oak three times the age of
a deer.’

If this be true a deer would live 6000 years, and an oak nearly
20,000 years. The saying is certainly not founded upon exact obser-
vation, but it becomes true if looked upon as a general statement
that the duration of life is very different in different organisms.

The question now arises as to the causes of these great differ-
ences. How is it that individuals are endowed with the power
of living long in such very various degrees ?

One is at first tempted to seek the answer by an appeal to the
differences in morphological and chemical structure which separate

6 THE DURATION OF LIFE.

species from one another. In fact all attempts to throw light upon
_ the subject which have been made up to the present time lie in
this direction.

All.these explanations are nevertheless insufficient. In a certain
sense it is true that the causes of the duration of life must be con-
tained in the organism itself, and cannot be. found in any of its
external conditions or circumstances. But structure and chemical
composition—in short the physiological constitution of the body in
the ordinary sense of the words—are not the only factors which
determine duration of life. This conclusion forces itself upon our
attention as soon as the attempt is made to explain existing facts
by these factors alone: there must be some other additional cause
contained in the organism as an unknown and invisible part of its _
constitution, a cause which determines the duration of life.

The size of the organism must in the first place be taken into
consideration. Of all organisms in the world, large trees have the
longest lives. The Adansonias of the Cape Verd Islands are said
to live for 6000 years. The largest animals also attain the greatest
age. Thus there is no doubt that whales live for some hundreds
of years. Elephants live 200 years, and it would not be difficult
to construct a descending series of animals in which the duration
of life diminishes in almost exact proportion to the decrease in the
size of the body. Thus a horse lives forty years, a blackbird
eighteen, a mouse six, and many insects only a few days or
weeks.

If however the facts are examined a little more closely it will be
observed that the great age (200 years) reached by an elephant
is also attained by many smaller animals, such as the pike and —
carp. The horse lives forty years, but so does a cat or a toad;
and a sea anemone has been known to live for over fifty years. The
duration of life in a pig (about twenty years) is the same as that in
a crayfish, although the latter does not nearly attain the hun-
dredth part of the weight of a pig.

_ It is therefore evident that length of life cannot be determined
by the size of the body alone. There is, however, some relation
between these two attributes. A large animal lives longer than a
small one because it is larger; it would not be able to become even
comparatively large unless endowed with a comparatively long dura-
tion of life.

THE DURATION OF LIFE. ; 7

Apart from all other reasons, no one could imagine that the
gigantic body of an elephant could be built up like that of a mouse
in three weeks, or ‘in a single day like that of the larva of certain
flies. The gestation of an elephant lasts for nearly two years, and
maturity is only reached after a lapse of about twenty-four years.

Furthermore, to ensure the preservation of the species, a longer
_ time is required by a large animal than by a small one, when both |
have reached maturity. Thus Leuckart and later Herbert Spencer
have pointed out that the absorbing surface of an animal only in-
creases as the square of its length, while its size increases as the
cube; and it therefore follows that the larger an animal becomes,
the greater will be the difficulty experienced in assimilating any
nourishment over and above that which it requires for its own
needs, and therefore the more slowly will it reproduce itself.

But although it may be stated generally that the duration of
the period of growth and length of life are longest in the largest
animals, it is nevertheless impossible to maintain that there is any
fixed relation between the two; and Flourens was mistaken when
he considered that the length of life-was always equivalent to five
. times the duration of the period of growth. Such a conclusion
might be accepted in the case of man if we set his period of growth
at twenty years and his length of life at a hundred; but it
cannot be accepted for the majority of other Mammalia. Thus
the horse lives from forty to fifty years, and the latter age is at
least as frequently reached among horses as a hundred years among
men ; but the horse becomes mature in four years, and the length
of its life is thus ten or twelve times as long as its period of
growth.

The second factor which influences the duration of life is purely
physiological : it is the rate at which the animal lives, the rapidity
with which assimilation and the other vital processes take place.
Upon this point Lotze remarks in his Microcosmus— Active and
restless mobility destroys the organized body: the swift-footed animals
hunted by man, as also dogs, and even apes, are inferior in length
of life to man and the larger beasts of prey, which satisfy their needs
by a few vigorous efforts.’ ‘The inertness of the Amphibia is, on
the other hand, accompanied by relatively great length of life.’

There is certainly some truth in these observations, and yet it
would be a great mistake to assume that activity necessarily implies

8 THE DURATION OF LIFE.

a short life. The most active birds have very long lives, as will
be shown later on: they live as long as and sometimes longer than
the majority of Amphibia which reach the same size. The organism
must not be looked upon as a heap of combustible material, which
is completely reduced to ashes in a certain time the length of which
is determined by size, and by the rate at which it burns; but it
should be rather compared to a fire, to which fresh fuel can be
continually added, and which, whether it burns quickly or slowly,
can be kept burning as long as necessity demands.

The connection between activity and shortness of life cannot be
explained by supposing that a more rapid consumption of the
body occurs, but it is explicable because the increased rate at which
the vital processes take place permit the more rapid achievement
of the aim and purpose of life, viz. the attainment of maturity
and the reproduction of the species.

When I speak of the aim and purpose of life, I am only using
figures of speech, and I do not mean to imply that nature is in any
way working: consciously.

When I was speaking of the relation between duration of life
and the size of the body, I might have added another factor
which also exerts some influence, viz. the complexity of the struc-
ture. ‘I'wo organisms of the same size, but belonging to different
grades of organization, will require different periods of time for
their development. Certain animals of a very lowly organization,
such as the Rhizopoda, may attain a diameter of -5 mm. and may
thus become larger than many insects’ eggs. Yet under favourable

- eireumstances an Amoeba can divide into two animals in ten
minutes, while no insect’s egg can develope into the young animal
in a less period than twenty-four hours. Time is required for the
development of the immense number of cells which must in the
latter case arise from the single egg-cell.

Hence we may say that the peculiar constitution of an animal
does in part determine the length of time which must elapse before
reproduction begins. The period before reproduction is however
only part of the whole life of an animal, which of course extends
over the total period during which the animal exists.

Hitherto it has always been assumed that the duration of this
total period is solely determined by the constitution of the ani-
mal’s body. But the assumption is erroneous. The strength of

THE DURATION OF LIFE. 9

the spring which drives the wheel of life does not solely depend
upon the size of the wheel itself or upon the material of which it
is made; and, leaving the metaphor, duration of life is not ex-
clusively determined by the size of the animal, the complexity
of its structure, and the rate of its metabolism. The facts are
plainly and clearly opposed to such a supposition.

How, for instance, can we explain from this point of view the
fact that the queen-ant and the workers live for many years, while
the males live for a few weeks at most? ‘The sexes are not dis-
tinguished by any great difference in size or complexity of body,
or in the rate of metabolism. In all these three particulars they
must be looked upon as precisely the same, and yet there is this
immense difference between the lengths of their lives.

I shall return later on to this and other similar cases, and for
the present I assume it to be proved that physiological con-
siderations alone cannot determine the duration of life. It is not
these which alone determine the strength of the spring which
moves the machinery of life; we know that springs of different
strengths may be fixed in machines of the same kind and quality.
This metaphor is however imperfect, because we cannot imagine
the existence of any special force in an organism which deter-
mines the duration of its life ; but it is nevertheless useful because

it emphasises the fact that the duration of life is foreed upon |

the organism by causes outside itself, just as the spring is fixed in
its place by forces outside the machine, and not only fixed in its
place, but chosen of a certain strength so that it will run down
after a certain time.

‘To put it briefly, I consider that duration of life is really de- —
pendent upon adaptation to external conditions, that its length,
whether longer or shorter, is governed by the needs of the species,
and that it is determined by precisely the same mechanical process
of regulation as that by which the structure and functions of an
organism are adapted to its environment.

Assuming for the moment that these conclusions are valid, let
us ask how the duration of life of any given species can have
been determined by their means. In the first place, in regulating
duration of life, the advantage to the species, and not to the
individual, is alone of any importance. This must be obvious
to any one-who has once thoroughly thought out the process of

10 THE DURATION OF LIFE.

natural selection. It is of no importance to the species whether
the individual lives longer or shorter, but it is of importance
that the individual should be enabled to do its work towards the
maintenance of the species. This work is reproduction, or the
formation of a sufficient number of new individuals to compensate
the species for those which die. As soon as the individual has
performed its share in this work of compensation, it ceases to be
of any value to the species, it has fulfilled its duty and may die.
But the individual may be of advantage to the species for a longer
' period if it not only produces offspring, but tends them for a
longer or shorter time, either by protecting, feeding, cr instructing
them. This last duty is not only undertaken by man, but also
by animals, although to a smaller extent; for instance, birds teach
their young to fly, and so on.

We should therefore expect to find that, as a rule, life does not
greatly outlast the period of reproduction except in those species
which tend their young; and as a matter of fact we find that this
is the case. ;

All mammals and birds outlive the period of reproduction, but
this never occurs among insects except in those species which
tend their young. Furthermore, the life of all the lower animals
ceases also with the end of the reproductive period, as far as we
can judge.

Duration of life is not however determined in this way, but
only the point at which its termination occurs relatively to the
cessation of reproduction. The duration itself depends first upon
the length of time which is required for the animal to reach
maturity—that is, the duration of its youth, and, secondly, upon
the length of the period of fertility—that is the time which is
necessary for the individual to produce a sufficient number of de-
scendants to ensure the perpetuation of the species. It is precisely
this latter point which is determined by external conditions.

'There is no species of animal which is not exposed to de-
struction through various accidental agencies—by hunger or
cold, by drought or flood, by epidemics, or by enemies, whether
beasts of prey or parasites. We also know that these causes of
death are only apparently accidental, or at least that they can
only be called accidental as far as a single individual is concerned. —
As a matter of fact a far greater number of individuals perish

THE DURATION OF LIFE. 1l

through the operation of these agencies than by natural death.
There are thousands of species of which the existence depends upon
the destruction of other species; as, for example, the various kinds
of fish which feed on the countless minute Crustacea inhabiting
our lakes.

It is easy to see that an individual is, ceteris paribus, more ex-
posed to accidental death when the natural term of its life becomes
longer; and therefore the longer the time required by an in-
dividual for the production of a sufficient number of descendants to
ensure-the existence of the species, the greater will be the number
of individuals which perish accidentally before they have fulfilled
this important duty. Hence it follows, first, that the number of
déscendants produced by any individual must be greater as the
duration of its reproductive period becomes longer ; and, secondly,
the surprising result that nature does not tend to secure the
longest possible life to the adult individual, but, on the contrary,
tends to shorten the period of reproductive activity as far as
possible, and-with this the duration of life; but these conclusions
only refer to the animal and not to the vegetable world.

All this sounds very paradoxical, but the facts show that it is
true. At first sight numerous instances of remarkably long life .
seem to refute the argument, but the contradictions are only
apparent and disappear on closer investigation.

Birds as a rule live to a surprisingly great age. Even the
smallest of our native singing birds lives for ten years, while the
nightingale and blackbird live from twelve to eighteen years.
A pair of eider ducks were observed to make their nest in the
same place for twenty years, and it is believed that these birds
sometimes reach the age of nearly one hundred years. A cuckoo,
which was recognised by a peculiar note in its call, was heard in
the same forest for thirty-two consecutive years. Birds of prey,
and birds which live in marshy districts, become much older, for
they outlive more than one generation of men.

Schinz mentions a bearded vulture which was seen sitting on
a rock upon a glacier near Grindelwald, and the oldest men in
Grindelwald had, when boys, seen the same bird sitting on the
same rock. A white-headed vulture in the Schénbrunn Zoo-
logical Gardens had been in captivity for 118 years, and many
examples are known of eagles and falcons reaching an age

12 THE DURATION OF LIFE.

of over 100 years. Finally, we must not forget Humboldt’s!
Atur parrot from the Orinoco, concerning which the Indians said
that it could not be understood because it spoke the language of
an extinct tribe.

It is therefore necessary to ask how far we can show that such
long lives are really the shortest which are possible under the
circumstances.

Two factors must here be taken into consideration ; first, that
the young of birds are greatly exposed to destructive agencies ;
and, secondly, that the structure of a bird is adapted for flight and
therefore excludes the possibility of any great degree of fertility.

Many birds, like the stormy petrel, the diver, guillemot, and
other sea-birds, lay only a single egg, and breed (as is usually the
ease with birds) only once a year. Others, such as birds of prey,
pigeons, and humming-birds, lay two eggs, and it is only those
which fly badly, such as jungle fowls and pheasants, which produce
a number of eggs (about twenty), and the young of these very
species are especially exposed to those dangers which more or less
affect the offspring of all birds. Even the eggs of our most
powerful native bird of prey, the golden eagle, which all animals
fear, and of which the eyrie, perched on a rocky height, is beyond
the reach of any enemies, are very frequently destroyed by late
frosts or snow in spring, and, at the end of the year in winter, the
young birds encounter the fiercest of foes, viz. hunger. In the
majority of birds, the egg, as soon as it is laid, becomes exposed to
the attacks of enemies ; martens and weasels, cats and owls, buzzards
and crows are all on the look out for it. At a later period the
same enemies destroy numbers of the helpless young, and in winter
many succumb in the struggle against cold and hunger, or to the
numerous dangers which attend migration over land and sea,
dangers which decimate the young birds.

It is impossible directly to ascertain the exact number which
are thus destroyed; but we can arrive at an estimate «by an
indirect method. If we agree with Darwin and Wallace in
believing that in most species a certain degree of constancy
is maintained in the number of individuals of successive gene-
rations, and that therefore the number of individuals within
the same area remains tolerably uniform for a certain period of

' Humboldt’s ‘ Ansichten der Natur.’ '

THE DURATION OF LIFE. 19° *

time; it follows that, if we know the fertility and the average
duration of life of a species, we can calculate the number of those
which perish before reaching maturity. Unfortunately the average
length of life is hardly known with certainty in the case of any
species of bird. Let us however assume, for the sake of argument,
that the individuals of a certain species live for ten years, and that
they lay twenty eggs in each year; then of the 200 eggs which
are laid during the ten years, which constitute the lifetime of an
individual, 198 must be destroyed, and only two will reach maturity,

if the number of individuals in the species is to remain constant.
~ Or to take a concrete example ; let us fix the duration of life in
the golden eagle at 60 years, and its period of immaturity (of which
the length is not exactly known) at ten years, and let us assume
that it lays two eggs a year ;—then a pair will produce 100 eggs ~
in 50 years, and of these only two will develope into adult birds ;
and thus on an average a pair of eagles will only succeed in bring-
ing a pair of young to maturity once in fifty years. And so far
from being an exaggeration, this calculation rather under-estimates
the proportion of mortality among the young; it is sufficient how- -
ever to enforce the fact that the number of young destroyed must
reach in birds a een high figure as compared with the number of
those which survive?

If this argument Ate and at the same time the fertility from
physical and other grounds cannot be increased, it follows that
a relatively long life is the only means by which the maintenance
of the species of birds can be secured. Hence a great length
of life is proved to be an absolute necessity for birds.

- I have already mentioned that these animals demonstrate most
clearly that physiological considerations do not by any means suffice”
to explain the duration of life. Although all vital processes take
place with greater rapidity and the temperature of the blood is
higher in birds than in mammals, yet the former greatly surpass
the latter in length of life. Only in the largest Mammalia,—the
whales and the elephants—is the duration of life equal to or
perhaps greater than that of the longest lived birds. If we com-
pare the relative weights of these animals, the Mammalia are
everywhere at a disadvantage. Even such large animals as the horse
and bear only attain an age of fifty years at the outside; the lion

1 See Appendix, note 1, p. 36.

14 THE DURATION OF LIFE.

lives about thirty-five years, the wild boar twenty-five, the sheep
fifteen, the fox fourteen, the hare ten, the squirrel and the mouse six
years!; .but the golden eagle, though it does not weigh more
than from 9-12 pounds, and is thus intermediate as regards weight
between the hare and the fox, attains nevertheless an age which is
ten times as long. The explanation of this difference is to be found ©
first in the much greater fertility of the smaller Mammalia, such
as the rabbit or mouse, and secondly in the much lower mortality
among the young of the larger Mammalia. The minimum duration
of life necessary for the maintenance of the species is therefore
much lower than it is among birds. Even here, however, we are
not yet in possession of exact statistics indicating the number of
young destroyed; but it is obvious that Mammalia possess over
birds a great advantage in their intra-uterine development. In
Mammalia the destruction of young only begins after birth, while
in birds it begins during the development of the embryo. This
distinction is in fact carried even further, for many mammals
protect their young against enemies for a long time after birth.

It is unnecessary to go further into the details of these cases, or
to consider whether and to what extent every class of the animal
kingdom conforms to these principles. Thus to consider all or
even most of the classes of the animal kingdom would be quite
impossible at the present time, because our knowledge of the
duration of life among animals is very incomplete. Biological
problems have for a long time excited less interest than morpho-
logical ones. There is nothing or almost nothing to be found in
existing zoological text books upon the duration of life in animals ;
and even monographs upon single classes, such as the Amphibia,
reptiles, or even birds, contain very little on this subject. When
we come to the lower animals, knowledge on this point is almost
entirely wanting. I have not been able to find a single reference ~
to the age in Echinodermata, and very little about that of worms,
Crustacea, and Coelenterata?. The length of life in many mol-
luscan species is very well known, because the age can be deter-
mined by markings on the shell*. But even in this group, any
exact knowledge, such as would be available for our purpose, is still

1 See Appendix, note 2, p. 38.
2 See Appendix, note 4, p. 54.
® See Appendix, note 5, p. 55.

THE DURATION OF LIFE. 15

wanting concerning such necessary points as the degree of fertility,
the relation to other animals, and many other factors.

Data the most exact in all respects are found among the insects’,
and to this class I will for a short time direct your special atten-
tion. We will first consider the duration of larval life. This
varies very greatly, and chiefly depends upon the nature of the
- food, and the ease or difficulty with which it can be procured. The
larvae of bees reach the pupal stage in five to six days; but it is
well known that they are fed with substances of high nutritive
value (honey and pollen), and that they require no great effort to
obtain the food, which lies heaped up around them. The larval
life in many Ichnewmonidae is but little longer, being passed in
a parasitic condition within other insects ; abundance of accessible
food is thus supplied by the tissues and juices of the host. Again,
the larvae of the blow-fly become pupae in eight to ten days,
although they move actively'in boring their way under the skin
and into the tissues of the dead animals upon which they live.
The life of the leaf-eating caterpillars of butterflies and moths: lasts
for six weeks or longer, corresponding to the lower nutritive value
of their food and the greater expenditure of muscular energy in
obtaining it. Those caterpillars which live upon wood, such as
Cossus ligniperda, have a larval life of two to three years, and the
same is true of hymenopterous insects with similar habits, such as
Siren.

Furthermore, predaceous larvae require a long period for attaining
their full size, for they can only obtain their prey at rare intervals
and by the expenditure of considerable energy. Thus among the
dragon-flies larval life lasts for a year, and among many may-flies
even two or three years. 7

All these results can be easily understood from well-known physio-
logical principles, and they indicate that the length of larval life is
very elastic, and can be extended as circumstances demand ; for
otherwise carnivorous and wood-eating larvae could not have sur-
vived in the phyletic development of insects. Now it would be
a great mistake to suppose that there is any reciprocal relation
between duration of life in the larva and in the mature insect,
_ or imago; or, to put it differently, to suppose that the total

duration of life is the same in insects of the same size and activity,
1 See Appendix, note 3, p. 38.

TL

16 THE DURATION OF LIFE.

so that the time which is spent in the larval state is, as it were,
deducted from the life of the imago, and vice versa. That this
cannot be the case is shown by the fact already alluded to, that
among bees and ants larval life is of the same length in males and
females, while there is a difference of some years between the lengths
of their lives as imagos.

The life of the imago is generally very short, and not only ends
with the close of the period of reproduction, as was mentioned
above, but this latter period is also itself extremely short}.

The larva of the cockchafer devours the roots of plants for a
period of four years, but the mature insect with its more complex
structure endures for a comparatively short time ; for the beetle itself
dies in about a month after completing its metamorphosis. And
this is by no means an extreme case. Most butterflies have an
even shorter life, and among the moths there are many species (as
in the Psychidae) which only live for a few days, while others
again, which reproduce by the parthenogenetic method, only live for
twenty-four hours. The shortest life is found in the imagos of
certain may-flies, which only live four to five hours. They emerge
from the pupa-case towards the evening, and as soon as their
wings have hardened, they begin to fly, and pair with one another.
Then they hover over the water; their eggs are extruded all at
once, and death follows almost immediately.

The short life of the imago in insects is easily explained by the
principles set forth above. Insects belong to the number of those
animals which, even in their mature state, are very liable to be
destroyed by others which are dependent upon them for food; but
they are at the same time among the most fertile of animals, and
often produce an astonishing number of eggs in a very short time.
And no ‘better arrangement for the maintenance of the species
under such circumstances can be imagined than that supplied by
diminishing the duration of life, and simultaneously increasing the
rapidity of reproduction.

This general tendency is developed to very different degrees
according to conditions peculiar to each species. The shorten-
ing of the period of reproduction, and the duration of life to the
greatest extent which is possible, depends upon a number of co-
operating circumstances, which it is impossible to enumerate

1 See Appendix, note 3, p. 38.

THE DURATION OF LIFE. 17

completely. Even the manner in which the eggs are laid may
have an important effect. If the larva of the may-fly lived upon
some rare and widely distributed food-plant instead of at the
bottom of streams, the imagos would be compelled to live longer,
for they would be obliged—like many moths and butterflies—to
lay their eggs singly or in small clusters, over a large area, This
would require both time and strength, and they could not retain
the rudimentary mouth which they now possess, for they would
have to feed in order to acquire sufficient strength for long flights ;
and—whether they were carnivorous like dragon-flies, or honey-
eating like butterflies—their feeding would itself cause a further
expenditure of both time and strength, which would necessitate a
still further increase in the duration of life. And as a matter of
fact we find that dragon-flies and swift-flying hawk-moths often
live for six or eight weeks and sometimes longer.

We must also remember that in many species the eggs are not
mature immediately after the close of the pupal stage, but that
they only gradually ripen during the life of the imago, and
frequently, as in many beetles and butterflies, do not ripen simul-
taneously, but only a certain number at a time. This depends,
first, upon the amount of reserve nutriment accumulated in the body
of the insect during larval life ; secondly, upon various but entirely
different circumstances, such as the power of flight. Insects which
fly swiftly and are continually on the wing, like hawk-moths and
dragon-flies, cannot be burdened with a very large number of ripe
eggs. In these cases the gradual ripening of the eggs becomes .
necessary, and involves an increase in the duration of life. In
Lepidoptera, we see how the power of flight diminishes step by
step as soon as other circumstances permit, and simultaneously how
the eggs ripen more and more rapidly, while the length of life
becomes shorter, until a minimum is reached. Only two stages
in the process of transformation can be mentioned here.

The strongest flyers—the hawk-moths and butterflies—must be
looked upon as the most specialised and highest types among the
Lepidoptera. Not only do they possess organs for flight in their
most perfect form, but also organs for feeding—the characteristic
spiral proboscis or ‘ tongue.’

There are certain moths (among the Bombyces) of which the
males fly as well as the hawk-moths, while the females are unable

C

18 THE DURATION OF LIFE.

to use their large wings for flight, because the body is too heavily
weighted by a mass of eggs, all of which reach maturity at the same
time. Such species, as for instance Aglia tau, are unable to dis-
tribute their eggs over a wide area, but are obliged to lay them all
in a single spot. They can however do this without harm to the
species, because their caterpillars live upon forest trees, which pro-
vide abundant food for a larger number of larvae than can be pro-
duced by the eggs of a single female. The eggs of Agha tau are
deposited directly after pairing, and shortly afterwards the insect
dies at the foot of the tree among the moss-covered roots of which
it has passed the winter in the pupal state. The female moth seldom
lives for more than three or four days; but the males which fly
swiftly in the forests, seeking for’ the less abundant females, live
for a much longer period, certainly from eight to fourteen days 1.

The females of the Psychidae also deposit all their eggs in one
place. The grasses and lichens upon which their caterpillars live
grow close at hand upon the surface of the earth and stones, and
hence the female moth does not leave the ground, and generally
does not even quit the pupa-case, within which it lays its eggs;
as soon as this duty is finished, it dies. In relation to these habits
the wings and mouth of the female are rudimentary, while the
male possesses perfectly developed wings.

The causes which have regulated the length of life in these cases
are obvious enough, yet still more striking illustrations are to be
found among: insects which live in colonies.

The duration of life varies with the sex in bees, wasps, ants, and
Termites: the females have a long life, the males a short one; and
there can be no doubt that the explanation of this fact is to be found
in adaptation to external conditions of life.

The queen-bee—the only perfect female in the hive—lives two
to three years, and often as long as five years, while the male bees
or drones only live four to five months. Sir John Lubbock has
succeeded in keeping female and working ants alive for seven
years—a great age for insects ,—while the males only lived a few
weeks,

1 This estimate is derived from observation of the time during which these insects
are to be seen upon the wing. Direct observations upon the duration of life in this
species are unknown to me.

(? Sir John Lubbock has now kept a queen ant alive for nearly 15 years. See note
2 on p. 51.—E. B. P.]

THE DURATION OF LIFE. 19

These last examples become readily intelligible when we remember
that the males neither collect food nor help in building the hive.
Their value to the colony ceases with the nuptial flight, and from
the point of view of utility it is easy to understand why their lives
should be so short 1. But the case is very different with the female.
The longest period of reproduction possible, when accompanied by
very great fertility, is, as a rule, advantageous for the mainten-
ance of the species. It cannot however be attained in most
insects, for the capability of living long would be injurious if all
individuals fell a prey to their enemies before they had completed
the full period of life. Here it is otherwise: when the queen-bee
returns from her nuptial flight, she remains within the hive until
her death, and never leaves it. There she is almost. completely
secure from enemies and from dangers of all kinds; thousands of
workers armed with stings protect, feed, and warm her; and in
short there is every chance of her living through the full period of —
a life of normal length. And the case is entirely similar with the
female ant. In neither of these insects is there any reason why
the advantages which follow from a lengthened period of repro-
ductive activity should be abandoned ”.

That an increase in the length of life has actually taken place in
such cases seems to be indicated by the fact that both sexes of the
saw-flies—the probable ancestors of bees and ants—have but a
short life. On the other hand, the may-flies afford an undoubted
instance of the shortening of life. Only in certain species is life as
short as I have indicated above ; in the majority it lasts for one or
more days. The extreme cases, with a life of only a few hours,
form the end of a line of development tending in the direction of a
shortened life. This is made clear by the fact that one of these
may-flies (Padingenia) does not even leave its pupa-skin, but repro-
duces in the so-called sub-imago stage.

It is therefore obvious that the duration of life is extremely
variable, and not only depends upon physiological considerations,
but also upon the external conditions of life. With every change
in the structure of a species, and with the acquisition of new
habits, the length of its life may, and in most cases must, be
. altered. .

1 See Appendix, notes 7 and 9, pp. 59 and 63.
? See Appendix, note 6, p. 58.
C2

20 THE DURATION OF LIFE.

In answering the question as to the means by which the lengthen-
ing or shortening of life is brought about, our first appeal must
be to the process of natural selection. Duration of life, like every
other characteristic of an organism, is subject to individual flue-
tuations. . From our experience with the human species we know
that long life is hereditary. As soon as the long-lived individuals
in a species obtain some advantage in the struggle for existence,
they will gradually become dominant, and those with the shortest
lives will be exterminated.

So far everything is quite simple; but hitherto we have only
considered the external mechanism, and we must now further in-
quire as to the concomitant internal means by which such processes
are rendered possible.

This brings us face to face with one of the most difficult problems
in the whole range of physiology,—the question of the origin of
death. As soon as we thoroughly understand the circumstances
upon which normal death depends in general, we shall be able to
make a further inquiry as to the cireumstances which influence its
earlier or later appearance, aswell as to any functional changes in
the organism which may produce such a result.

The changes in the organism which result in normal death,—
senility so-called,—have been most accurately studied among men.
We know that with advancing age certain alterations take place
in the tissues, by which their functional activity is diminished; that
these changes gradually increase, and finally either lead to direet or
so-called normal death, or produce indirect death by rendering the
organism incapable of resisting injuries due to external influences.
These senile changes have been so well described from the time of
Burdach and Bichat to that of Kussmaul, and are so well known,
that I need not enter into further details here.

In answer to an inquiry as to the causes which induce these
changes in the tissues, I can only suggest that the cells which
form the vital constituents of tissues are worn out by prolonged.
use and activity. It is conceivable that the cells might be thus
worn out in two ways; either the cells of a tissue remain the
same throughout life, or else they are being continually replaced
by younger generations of cells, which are themselves cast off in
their turn. .

In the present state of our knowledge the former alternative can

THE DURATION OF LIFE. 21

hardly be maintained. Millions of blood corpuscles are continually
dying and being replaced by new ones. On both the internal and
external surfaces of the body countless epithelial cells are being
incessantly removed, while new ones arise in their place ; the activity
of many and probably of all glands is accompanied by a change in
their cells, for their secretions consist partly of detached and partly
of dissolved cells ; it is stated that even the cells of bone, connective
tissue, and muscle undergo the same changes, and nervous tissue
alone remains, in which it is doubtful whether such a renewal of
cells takes place. And yet as regards even this tissue, certain facts
are known which indicate a normal, though probably a slow renewal
of the histological elements. I believe that one might reasonably
defend the statement,—in fact, it has already found advocates,—
that the vital processes of the higher (i.e. multicellular) animals
are accompanied by a renewal of the morphological elements in
most tissues.

This statement leads us to seek the origin of death, not in the
waste of single cells, but in the limitation of their powers of repro-
duction. Death takes place because a worn-out tissue cannot for
ever renew itself, and because a capacity for increase by means of
cell-division is not everlasting, but finite’, This does not however
imply that the immediate cause of death lies in the imperfect re-
-newal of cells, for death would in all cases occur long before the
reproductive power of the cells had been completely exhausted.
Functional disturbances will appear as soon as the rate at which the
worn-out cells are renewed becomes slow and insufficient.

But it must not be forgotten that death is not always preceded.
by senility, or a period of old age. For instance, in many of
the lower animals death immediately follows the most important
deed of the organism, viz. reproduction. Many Lepidoptera, all
may-flies, and many other insects die of exhaustion immediately
after depositing their eggs. Men have been known to die from
the shock of a strong passion. Sulla is said to have died as
the result of rage, whilst Leo X succumbed to an excess of joy.
Here the psychical shock caused too intense an excitement of the
nervous system. In the same manner the exercise of intense effort
may also produce a similarly fatal excitement in the above-
mentioned insects. At any rate it is certain that when, for some

, 1 See Appendix, note 8, p. 59.

22 THE DURATION OF LIFE.

reason, this effort is not made, the insect lives for a somewhat
longer period.

It is clear that in such animals as insects we can only speak
figuratively of normal death, if we mean by this an end which is
not due to accident. In these animals an accidental end is the rule,
and is therefore, strictly speaking, normal 1.

Assuming the truth of the above-mentioned hypothesis as to the
causes of normal death, it follows that the number of cell-genera-
tions which can proceed from the egg-cell is fixed for every
species, at least within certain limits; and this number of cell-
generations, if attained, corresponds to the maximum duration of
life in the individuals of the species concerned. Shortening of life
in any species must depend upon a decrease in the number of
successive cell-generations, while conversely, the lengthening of
life depends upon an increase in the number of cell-generations over
those which were previously possible.

Such changes actually take place in plants. When an annual
plant becomes perennial, the change—one in every way possible
—can only happen by the production of new shoots, ie. by an.
increase in the number of cell-generations. The process is not so
obvious in animals, because in them the formation of young cells
does not lead to the production of new and visible parts, for the
new material is merely deposited in the place of that which is worn
out and disappears. Among plants, on the other hand, the old
material persists, its cells become lignified, and it is built over by
new cells which assume the functions of life.

It is certainly true that the question as to the necessity of death
in general does not seem much clearer from this point of view than
from the purely physiological one. This is because we do not know
why a cell must divide 10,000 or 100,000 times and then suddenly
stop. It must be admitted that we can see no reason why the
power of cell-multiplication should not be unlimited, and why the
organism should not therefore be endowed with everlasting life.
In the same manner, from a physiological point of view, we might
admit that we can see no reason why the functions of the organism
should ever cease.

It is only from the point of view of utility that we can under-

1 See Appendix, note 9, p. 63.

THE DURATION OF LIFE. 23

stand the necessity of death. The same arguments which were
employed to explain the necessity for as short a life as possible, will
with but slight modification serve to explain the common necessity
of death 1. ,

Let us imagine that one of the higher animals became immortal;
it then becomes perfectly obvious that it would cease to be of
value to the species to which it belonged. Suppose that such an
immortal individual could escape all fatal accidents, through infinite

[* After reading these proofs Dr. A. R. Wallace kindly sent me an unpublished
note upon the production of death by means of natural selection, written by him
some time between 1865 and 1870. The note contains some ideas on the subject,
which were jotted down for further elaboration, and were then forgotten until
recalled by the argument of this Essay. The note is of great interest in relation to
Dr. Weismann’s suggestions, and with Dr. Wallace’s permission I print it in full

below.

‘THE AcTION oF NATURAL SELECTION IN PRropucING OLD AGE,
Decay, AND DEATH.

‘Supposing organisms ever existed that had not the power of natural reproduc-
tion, then since the absorptive surface would only increase as the square of the
dimensions while the bulk to be nourished and renewed would increase as the cube,
there must soon arrive a limit of growth. Now if such an organism did not produce
its like, accidental destruction would put an end to the species. Any organism
therefore that, by accidental or spontaneous fission, could become two organisms,
and thus multiply itself indefinitely without increasing in size beyond the limits
most favourable for nourishment and existence, could not be thus exterminated:
since the individual only could be accidentally destroyed,—the race would survive.
But ifindividuals did not die they would soon multiply inordinately and would inter-
fere with each other’s healthy existence. Food would become scarce, and hence the
larger individuals would probably decompose or diminish in size. The deficiency of
nourishment would lead to parts of the organism not being renewed; they would
become fixed, and liable to more or less slow decomposition as dead parts within a
living body. The smaller organisms would have a better chance of finding food, the
larger ones less chance. That one which gave off several small portions to form
each a new organism would have a better chance of leaving descendants like
itself than one which divided equally or gave off a large part of itself. Hence it
would happen that those which gave off very small portions would probably soon
after cease to maintain their own existence while they would leave a numerous
offspring. This state of things would be in any case for the advantage of the race,
and would therefore, by natural selection, soon become established as the regular
course of things, and thus we have the origin of old age, decay, and death ; for it is
‘evident that when one or more individuals have provided a sufficient number of
successors they themselves, as consumers of nourishment in a constantly increasing
degree, are an injury to those successors. Natural selection therefore weeds them
out, and in many cases favours such races as die almost immediately after they have
left successors. Many moths and other insects are in this condition, living only to
propagate their kind and then immediately dying, some not even taking any food
in the perfect and reproductive state. —E. B. P.]

24 THE DURATION OF LIFE.

time,—a supposition which is of course hardly conceivable. The
individual would nevertheless be unable to avoid, from time to
time, slight injuries to one or another part of its body. The
injured parts could not regain their former integrity, and thus the
longer the individual lived, the more defective and crippled it
would become, and the less perfectly would it fulfil the purpose of
its species. Individuals are injured by the operation of external
forces, and for this reason alone it is necessary that new and perfect
individuals should continually arise and take their place, and this
necessity would remain even if the individuals possessed the power
of living eternally.

From this follows, on the one hand, the necessity of reproduction,
and, on the other, the utility of death. Worn-out individuals are
not only valueless to the species, but they are even harmful, for
they take the place of those which are sound. Hence by the —
operation of natural selection, the life of our hypothetically im-
mortal individual would be shortened by the amount which was
useless to the species. It would be reduced to a length which
would afford the most favourable conditions for the existence of as
large a number as possible of vigorous individuals, at the same
time.

If by these considerations death is shown to be a beneficial
occurrence, it by no means follows that it is to be solely accounted
for on grounds of utility. Death might also depend upon causes
which lie in the nature of life itself. The floating of ice upon
water seems to us to be a useful arrangement, although the fact
that it does float depends upon its molecular structure and not
upon the fact that its doing so is of any advantage to us. In like
manner the necessity of death has been hitherto explained as due to
causes which are inherent in organic nature, and not to the fact
that it may be advantageous. .

I do not however believe in the validity of this explanation ;
I consider that death is not a primary necessity, but that it has
been secondarily acquired as an adaptation. I believe that life is
endowed with a fixed duration, not because it is contrary to its
nature to be unlimited, but because the unlimited existence of
individuals would be a luxury without any corresponding advantage.
The above-mentioned hypothesis upon the origin and necessity of
death leads me to believe that the organism did not finally cease

~

THE DURATION OF LIFE. 25

to renew the worn-out cell material because the nature of the cells
did not permit them to multiply indefinitely, but because the power
of multiplying indefinitely was lost when it ceased to be of use.

I consider that this view, if not exactly proved, can at any rate
be rendered extremely probable.

It is useless to object that man (or any of the higher animals)
dies from the physical necessity of his nature, just as the specific
gravity of ice results from its physical nature. I am quite ready to
admit that this is the case. John Hunter, supported by his ex-
periments on azabiosis, hoped to prolong the life of man indefinitely
by alternate freezing and thawing; and the Veronese Colonel
Aless. Guaguino made his contemporaries believe that a race
of men existed in Russia, of which the individuals died regularly
every year on the 27th of November, and returned to life on
the 24th of the following April. There cannot however be the
least doubt, that the higher organisms, as they are now con-
structed, contain within themselves the germs of death. The
_question however arises as to how this has come to pass; and
I reply that death is to be looked upon as an occurrence which
is advantageous to the species as a concession to the outer con-
ditions of life, and not as an absolute necessity, essentially inherent
in life itself.

Death, that is the end of life, is by no means, as is usually
assumed, an attribute of all organisms. An immense number of
low organisms do not die, although they are easily destroyed, being
killed by heat, poisons, &e. As long, however, as those conditions
which are necessary for their life are fulfilled, they continue to live,
and they thus carry the potentiality of unending life in them-
selves. I am speaking not only of the Amoebae and the low
unicellular Algae, but also of far more highly organized unicellular
animals, such as the Infusoria.

The process of fission in the Amoeba has been recently much
discussed, and I am well aware that the life of the individual is
generally believed to come to an end with the division which gives
rise to’two new individuals, as if death and reproduction were the
same thing. But this process cannot be truly called death. Where
is the dead body? what is it that dies? Nothing dies; the body
of the animal only divides into two similar parts, possessing the
same constitution. Each of these parts is exactly like its parent,

26 THE DURATION OF LIFE.

lives in the same manner, and finally also divides into two halves.
As far as these organisms are concerned, death can only be spoken
of in the most figurative sense.

There are no grounds for the assumption that the two halves of
an Amoeba are differently constituted internally, so that after a
time one of them will die while the other continues to live. Such
an idea is disproved by a recently discovered fact. It has been
noticed in Luglypha (one of the Foraminifera) and in other low
animals of the same group, that when division is almost complete,
and the two halves are only connected by a short strand, the proto-
plasm of both parts begins to circulate, and for some time passes
backwards and forwards between the two halves. A complete
_ mingling of the whole substance of the animal and a resulting
identity in the constitution of each half is thus brought about
before the final separation ?.

The objection might perhaps be raised that, if the parent animal
does not exactly die, it nevertheless disappears as an individual. I
cannot however let this pass unless it is also maintained that the
man of to-day is no longer the same individual as the boy of twenty
years ago. In the growth of man, neither structure nor the com-
ponents of structure remain precisely the same; the material is
continually changing. If we can imagine an Amoeba endowed
with self-consciousness, it might think before dividing ‘I will give
birth to a daughter, and I have no doubt that each half would
regard the other as the daughter, and would consider itself to be
the original parent. We cannot however appeal to this criterion of
personality in the Amoeba, but there is nevertheless a criterion
which seems to me to decide the matter: I refer to the continuity
of life in the same form.

Now if numerous organisms, endowed with the potentiality of
never-ending life, have real existence, the question arises as to
whether the fact can be understood from the point of view of
utility. If death has been shown to be a necessary adaptation for
the higher organisms, why should it not be so for the lower also?
Are they not decimated by enemies? are they not often imperfect ?
are they not worn out by contact with the external world?
Although they are certainly destroyed by other animals, there is

1 See Appendix, note Io, p. 64.

THE DURATION OF LIFE. 27

nothing comparable to that deterioration of the body which takes
place in the higher organisms. Unicellular animals are too simply
constructed for this to be possible. If an infusorian is injured - by
the loss of some part of its body, it may often recover its former
integrity, but if the injury is too great it dies. The alternative is
always perfect integrity or complete destruction. ;

We may now leave this part of the subject, for it is obvious that
normal death, that is to say, death which arises from internal
causes, is an impossibility among these lower organisms. In those
species at any rate in which fission is accompanied by a circulation
of the protoplasm of the parent, the two halves must possess the
same qualities. Since one of them is endowed with a potentiality
for unending life, and must be so endowed if the species is to persist,
it is clear that the other exactly similar half must be endowed
with equal potentiality.

Let us now consider how it happened that the multicellular
animals and plants, which arose from unicellular forms of life, came
to lose this power of living for ever.

The answer to this question is closely bound up with the principle
of division of labour which appeared among multicellular organisms
at a very early stage, and which has gradually led to the production
of greater and greater complexity in their structure.

The first multicellular organism was probably a cluster of similar
cells, but these units soon lost their original homogeneity. As the
result of mere relative position, some of the cells were especially
fitted to provide for the nutrition of the colony, while others
undertook the work of reproduction. Hence the single group
would come to be divided into two groups of cells, which may
be called somatic and reproductive—the cells of the body as op-
posed to those which are concerned with reproduction. This
differentiation was not at first absolute, and indeed it is not always
so to-day. Among the lower Metazoa, such as the polypes, the
capacity for reproduction still exists to such a degree in the somatic
cells, that a small number of them are able to give rise to a new
organism,—in fact new individuals are normally produced by means
of so-called buds. Furthermore, it is well known that many of the
higher animals have retained considerable powers of regeneration ;
the salamander can replace its lost tail or foot, and the snail can
reproduce its horns, eyes, etc. :

28 THE DURATION OF LIFE.

As the complexity of the Metazoan body increased, the two
groups of cells became more sharply separated from each other.
Very soon the somatic cells surpassed the reproductive in number,
and during this increase they became more and more broken up
by the principle of the division of labour into sharply separated
systems of tissues. As these changes took place, the power of
reproducing large parts of the organism was lost, while the power
of reproducing the whole individual became concentrated in the
reproductive cells alone.

But it does not therefore follow that the somatic cells were
compelled to lose the power of unlimited cell-production, although
in accordance with the law of heredity, they could only give
rise to cells which resembled themselves, and belonged to the same
differentiated histological system. But as the fact of normal
_ death seems to teach us that they have lost even this power, the
causes of the loss must be sought outside the organism, that is
to say, in the external conditions of life; and we have already
seen that death can be very well explained as a secondarily ac-
quired adaptation. The reproductive cells cannot lose the capacity
for unlimited reproduction, or the species to which they belong
would suffer extinction. But the somatic cells have lost this
power to a gradually increasing extent, so that at length they
became restricted to a fixed, though perhaps very large number of
cell-generations. This restriction, which implies the continual influx
of new individuals, has been explained above as a result of the
impossibility of entirely protecting the individual from accidents,
and from the deterioration which follows them. Normal death
could not take place among: unicellular organisms, because the indi-
vidual and the reproductive cell are one and the same: on the
other hand, normal death is possible, and as we see, has made its
appearance, among multicellular organisms in which the somatic
and reproductive cells are distinct.

I have endeavoured to explain death as the result of restriction
in the powers of reproduction possessed by the somatic cells, and I
have suggested that such restriction may conceivably follow from a
limitation in the number of cell-generations possible for the cells
of each organ and tissue. I am unable to indicate the molecular
and chemical properties of the cell upon which the duration of
its power of reproduction depends: to ask this is to demand an

THE DURATION OF LIFE. 29

explanation of the nature of heredity—a problem the solution of
which may still occupy many generations of scientists. At present
we can hardly venture to propose any explanation of the real nature
of heredity.

But the question must be answered as to whether the kind and
degree of reproductive power resides in the nature of the cell itself,
or in any way depends upon the quality of its nutriment.

Virchow, in his ‘ Cellular Pathology,’ has remarked that the cells
are not only nourished, but that they actively supply themselves
with food. If therefore the internal condition of the cell decides
whether it shall accept or reject the nutriment which is offered, it
becomes conceivable that all cells may possess the power of refusing
to absorb nutriment, and therefore of ceasing to undergo further
division. .

~ Modern embryology affords us many proofs, in the segmentation
of the ovum, and in the subsequent developmental changes, that
the causes of the different forms of reproductive activity witnessed
in cells lie in the essential nature of the cells themselves. Why
does the segmentation of one half of certain eggs proceed twice as
rapidly as that of the other half? why do the cells of the ectoderm
divide so much more quickly than those of the endoderm? Why
does not only the rate, but also the number of cells produced (so
far as we can follow them) always rémain the same? Why does
the multiplication of cells in every part of the blastoderm take
place with the exact amount of energy and rapidity necessary to
produce the various elevations, folds, invaginations, etc., in which
the different organs and tissues have their origin, and from which
finally the organism itself arises? There can be no doubt that
the causes of all these phenomena lie within the cells them-
selves; that in the ovum and the cells which are immediately
derived from it, there exists a tendency towards a certain determined
(I might almost say specific) mode and energy of. cell-multiplica-
tion. And why should we regard this inherited tendency as con-
fined to the building up of the embryo? why should it not also
exist in the young, and later in the mature animal? The pheno-
mena of heredity which make their appearance even* in old age
afford us proofs that a tendency towards a certain mode of cell-
multiplication continues to regulate the growth of the organism
during the whole of its life.

30 THE DURATION OF LIFE.

The above-mentioned considerations show us that the degree
of reproductive activity present in the tissues is regulated by
internal causes while the natural death of an organism is the
termination—the hereditary limitation—of the process of cell-
division, which began in the segmentation of the ovum.

Allow me to suggest a further consideration which may be com-
pared with the former. The organism is not only limited in time,
but also in space: it not only lives for a limited period, but it can
only attain a limited size. Many animals grow to their full size
long before their natural end: and although many fishes, reptiles, and
lower animals are said to grow during the whole of their life, we do
not mean by this that they possess the power of unlimited growth
any more than that of unlimited life. There is everywhere a
maximum size, which, as far as our experience goes, is never sur-
passed. The mosquito never reaches the size of an elephant, nor
the elephant that of a whale.

Upon what does this depend? Is there any external obstacle
to growth? Or is the limitation entirely imposed from within?

Perhaps you may answer, that there is an established relation
between the increase of surface and mass, and it cannot be denied
that these relations do largely determine the size of the body.
A beetle could never reach the size of an elephant, because, co"\-
stituted as it is, it would be incapable of existence if it attair:d
such dimensions. But nevertheless the relations between surface
and mass do not form the only reason why any given individual
does not exceed the average size of its species. Each individ tal
does not strive to grow to the largest possible size, until vhe
absorption from its digestive area becomes insufficient for its mass ;
but it ceases to grow because its cells cannot be sufficiently nourished
in consequence of its increased size. The giants which occasionally
appear in the human species prove that the plan upon which man
is constructed can also be carried out on a scale which is far larger
than the normal one. If the size of the body chiefly depends upon
amount of nutriment, it would be possible to make giants and
dwarfs at will. But we know, on the contrary, that the size of
the body is hereditary in families to a very marked extent; in fact
so much so that the size of an individual depends chiefly upon
heredity, and not upon amount of food.

+ These observations point to the conclusion that the size of the

THE DURATION OF LIFE, 3l

individual is in reality pre-determined, and that it is potentially
contained in the egg from which the individual developes.

We know further that the growth of the individual depends
chiefly upon the multiplication of cells and only to a slight extent
upon the growth of single cells. It is therefore clear that a
limit of growth is imposed by a limitation in the processes by
which cells are increased, both as regards the number of cells
produced and the rate at which they are formed. How could we
otherwise explain the fact that an animal ceases to grow long
before it has reached the physiologically attainable maximum of its
species, without at the same time ‘suffering any loss of vital
energy ? | )

In many cases at least, the most important duty of an organism,
viz. reproduction, follows upon the attainment of full size—a fact
which induced Johannes Miiller to reject the prevailing hypothesis
which explained the death of animals as due to ‘the influences
of the inorganic environment, which gradually wear away the life
of the individual.’ He argued that, if this were the case, ‘ the
organic energy of an individual would steadily decrease from the
beginning,’ while the facts indicate that this is not so’.

If it is further asked why the egg should give rise to a fixed
nymber of cell-generations, although perhaps a number which
vavies widely within certain limits, we may now refer to the opera-
tion of natural selection upon the relation of surface to mass, and
upgn other physiological necessities which are peculiar to the species.
Because a certain size is the most favourable for a certain plan
of erganization, the process of natural selection determined that
such a size should be within certain variable limits, characteristic
of each species. This size is then transmitted from generation to
generation, for when once established as normal for the species, the
most favourable size is potentially present in the reproductive cell
from which each individual is developed.

If this conclusion holds, and I believe that no essential objection
can be raised against it, then we have in the limitation in space
a process which is exactly analogous to the limitation in time,
which we have already considered. The latter limitation—the
duration of life—also depends upon the multiplication of cells, the

* Johannes Miiller, ‘ Physiologie,’ Bd. I. p. 31, Berlin, 1840.

32 ; THE DURATION OF LIFE.

rapid increase of which first gave rise to the characteristic form of
the mature body, and then continued at a slower rate. In the
mature animal, cell-reproduction still goes on, but it no longer
exceeds the waste; for some time it just compensates for loss, and
then begins to decline. The waste is not compensated for, the
tissues perform their functions incompletely, and thus the way for
death is prepared, until its final appearance by one of the three
great Atria mortis.

I admit that facts are still wanting upon which to base this

hypothesis. It is a pure supposition that senile changes are due to
a deficient reproduction of cells: at the same time this supposition
gains in probability when we are enabled to reduce the limitations
of the organism in both time and space to one and the same
principle. It cannot however be asserted under any circumstances
that it is a pure supposition that the ovum possesses a capacity
for cell-multiplication which is limited both as to numbers produced
and rate of production. The fact that each species maintains an
| average size is a sufficient proof of the truth of this conclusion.
- Hitherto I have only spoken of animals and have hardly men-
tioned plants. I should not have been able to consider them at
all, had it not happened that a work of Hildebrand’s! has recently
appeared, which has, for the first time, provided us with exact
observations on the duration of plant-life.

The chief results obtained by this author agree very well seith
the view which I have brought before you to-day. Hildebrand
shows that the duration of life in plants also is by no means
completely fixed, and that it may be very considerably altered
through the agency of the external conditions of life. He shows
that, in course of time, and under changed conditions of life, an
annual plant may become perennial, or vice versa. The external
factors which influence the duration of life are here however essen-
tially different, as indeed we expect them to be, when we remember
the very different conditions under which the animal and vegetable
kingdoms exist. During the life of animals the destruction of
mature individuals plays a most important part, but the existence
of the mature plant is fairly well secured; their chief period of
destruction is during youth, and this fact has a direct influence

1 See Appendix, note 12, p. 65.

THE DURATION OF LIFE. 83

upon the degree of fertility, but not upon the duration of life.
Climatic considerations, especially the periodical changes of summer
and winter, or wet and dry seasons, are here of greater importance.

It must then be admitted that the dependence of the duration of
life upon the external conditions of existence is alike common to
plants and animals. In both kingdoms the high multicellular
forms with well-differentiated organs contain the germs of death,
while the low unicellular organisms are potentially immortal.
Furthermore, an undying succession of reproductive cells is possessed
by all the higher forms, although this may be but poor consolation to
the conscious individual which perishes. Johannes Miiller is there-
fore right, when in the sentence quoted at the beginning of my
lecture, he speaks of an ‘appearance of immortality’ which passes
from each individual into that which succeeds it. That which
remains over, that which persists, is not the individual itself,—
not the complex aggregate of cells which is conscious of itself,—
but an individuality which is outside its consciousness, and of a low
order,—an individuality which is made up of a single cell, which
arises from the conscious individual. I might here conclude, but
I wish first, in a few words, to protect myself against a possible
misunderstanding.

I have repeatedly spoken of immortality, first of the unicellular ©

organism, and secondly of the reproductive cell. By this word
I have merely intended to imply a duration of time which appears
to be endless to our human faculties. I have no wish to enter into
the question of the cosmic or telluric origin of life on the earth.
An answer to this question will at once decide whether the power
of reproduction possessed by these cells is in reality eternal or only
immensely prolonged, for that which is without beginning is,
and must be, without end.

The supposition of a cosmic origin of life can only assist us
if by its means we can altogether dispense with any theory of
spontaneous generation. The mere shifting of the origin of life
to some other far-off world cannot in any way help us. A truly
cosmic origin in its widest significance will rigidly limit us to
the statement—omue vivum e vivo—to the idea that life can only
arise from life, and has always so arisen,—to the conclusion. that
organic beings are eternal like matter itself.

Experience cannot help us to decide this question; we do not

D

84 THE DURATION OF LIFE.

know whether spontaneous generation was the commencement of ~
life on the earth, nor have we any direct evidence for the idea
that the process of development of the living world carries the
end within itself, or for the converse idea that the end can only
be brought about by means of some external force.

I admit that spontaneous generation, in spite of all vain efforts
to demonstrate it, remains for me a logical necessity. We cannot
regard organic and inorganic matter as independent of each other
and both eternal, for organic matter is continually passing, without
residuum, into the inorganic. If the eternal and indestructible are
alone without beginning, then the non-eternal and destructible must
have had a beginning. But the organic world is certainly not
eternal and indestructible in that absolute sense in which we
apply these terms to matter itself. We can, indeed, kill all organic
beings and thus render them inorganic at will. But these changes
are not the same as those which we induce in a piece of chalk
by pouring sulphuric acid upon it; in this case we only change
the form, and the inorganic matter remains, But when we pour
sulphuric acid upon a worm, or when we burn an oak tree, these
organisms are not changed into some other animal and tree, but
they disappear entirely as organized beings and are resolved into
inorganic elements., But that which can be completely resolved
into inorganic matter must have also arisen from it, and must
owe its ultimate foundation to it. The organic might be con-
sidered eternal if we could only destroy its form, but not its nature.

It therefore follows that the organic world must once have arisen,
and further that it will at some time come to an end. Hence we
must speak of the eternal duration of unicellular organisms and
of reproductive cells in the Metazoa and Metaphyta in that par-
ticular sense which signifies, when measured by our standards, an
immensely long time.

Yet who can maintain that he has discovered the right answer to
this important question? And even though the discovery were
made, can any one believe that by its means the problem of life
would be solved? If it were established that spontaneous genera-—
tion did actually occur, a new question at once arises as to the
conditions under which the occurrence became possible. How can
we conceive that dead inorganic matter could have come together
in such a manner as to form living protoplasm, that wonderful

THE DURATION OF LIFE. 85

and complex substance which absorbs foreign material and changes
it into its own substance, in other words grows and multiplies?
And so, in discussing this question of life and death, we come at
last—as in all provinces of human research—upon problems which
appear to us to be, at least for the present, insoluble. In fact it
is the quest after perfected truth, not its possession, that falls to
our lot, that gladdens us, fills up the measure of our life, nay!

hallows it.

APPENDIX.

Note 1. Tue Duration or Lire amonea Brrps.

Ture is less exact knowledge upon this subject than we might
expect, considering the existing number of ornithologists and
ornithological societies with their numerous publications. It has
neither’ been possible nor necessary for my purpose to look up all
the widely-scattered references which are to be found upon the
subject. Many of these are doubtless unknown to me; for we are
still in want of a compilation of accurately determined observations
in this department of zoology. I print the few facts which I have
been able to collect, as a slight contribution towards such a com-
pilation.

Small singing birds live from eight to eighteen years: the
nightingale, in captivity, eight years, but longer according to
some writers: the blackbird, in captivity, twelve years, but both
these birds live longer in the natural state. A ‘ half-bred nightin-
gale built its nest for nine consecutive years in the same garden’
(Naumann, ‘ Végel Deutschlands,’ p. 76).

Canary birds in captivity attain an age of twelve to fifteen
years (l.c., p. 76).

Ravens have lived for almost a hundred years in captivity
(1. c., Bd. I. p. 125).

Magpies in captivity live twenty years, and, ‘ without doubt,’
much longer in the natural state (1. ¢., p. 346).

Parrots ‘in captivity have reached upwards of a hundred years’
(l.¢., p. 125).

A single instance of the cuckoo (alluded to in the text) is men-
tioned by Naumann as reaching the age of thirty-two years (l.c.,
p- 76).

Fowls live ten to twenty years, the golden pheasant fifteen years,
the turkey sixteen years, and the pigeon ten years (Oken, ‘ Natur-
geschichte, Vogel,’ p. 387).

APPENDIX. 37

A golden eagle which ‘died at Vienna in the year 1719, had been
captured 104 years previously’ (Brehm, ‘ Leben der Vogel,’ p. 72).

A falcon (species not mentioned) is said to have attained an age
of 162 years (Knauer, ‘ Der Naturhistoriker, Vienna, 1880).

A white-headed vulture which was taken in 1706 died in the
Zoological Gardens at Vienna (Schonbrunn) in 1824, thus living
118 years in captivity (1. ¢.).

The example of the bearded vulture, mentioned in the text, is
quoted from Schinz’s ‘ Végel der Schweiz,’ p. 196.

The wild goose must live for upwards of 100 years, according to
Naumann (1. ¢., p. 127). The proof of this is not, however, forth-
coming. A wild goose which had been wounded reached its
eighteenth year in captivity.

Swans are said to have lived 300 years(?), (Naumann, l.c., p.127).

It is evident that observations upon the duration of life in wild
birds can only rarely be made, and that they are usually the result
of chance and cannot be verified. It is on this account all the
more to be desired that every ascertained fact should be collected.

If the long life of birds has been correctly interpreted as com-
pensation for their feeble fertility and for the great mortality. of
their young, it will be possible to estimate the length of life in a
species, without direct observation, if we only know its fertility and
the percentage of individuals destroyed. This percentage can, how-
ever, at best, be known only as an average. If we consider, for
example, the enormous number of sea birds which breed in summer
on the rocks and cliffs of the northern seas, and if we remember that
the majority of these birds lay but one, or at most two eggs yearly,
and that their young are exposed to very many destructive agencies,
we are forced to the conclusion that they must possess a very long
life, so that the breeding period may be many times repeated.
Their number does not diminish. Year after year countless num-
bers of these birds cover the rocks, from summit to sea’ line;
millions of them rest there, and rise in the air like a thick cloud
whenever they are disturbed. Even in those localities which, are
every year visited by man in order to effect their capture, the
number does not appear to decrease, unless the birds are disturbed
and are therefore prompted to seek other breeding-places. From
the small island of St. Kilda, off Scotland, 20,000 young gannets
(Suda) and an immense number of eggs are annually collected ;

38 THE DURATION OF LIFE.

and although this bird only lays a single egg yearly and takes
four years to attain maturity, the numbers do not diminish}.
30,000 sea-gulls’ eggs and 20,000 terns’ eggs are yearly exported
from the breeding-places on the island of Sylt, but in this case
it appears that a systematic disturbance of the birds is avoided
by the collectors, and no decrease in their numbers has yet taken
place*. The destruction of northern birds is not only caused by
man, but also by various predaceous mammals and birds. Indeed
the dense mass of birds which throng the cliffs is a cause of
destruction to many of the young and to the eggs, which are
pushed over the edge of the rocks. According to Brehm the foot
of these cliffs is ‘always covered with blood and the dead bodies of
fledglings.’

Such birds must attain a great age or they would have been
exterminated long ago: the minimum duration of life necessary for
the maintenance of the species must in their case be a very
high one.

Note 2. THe Duration or Lire amona Mammats.

The statements upon this subject in the text are taken from
many sources; from Giebel’s ‘Siugethiere,’ from Oken’s ‘ Natur-
geschichte, from Brehm’s ‘TIllustrirtem Thierleben,’ and from an
essay of Knauer in the ‘ Naturhistoriker,’ Vienna, 1880.

Note 3. Tue Duration or Lire amone Marvre Insects.

A short statement of the best established facts which I have been
able to find is given below. I have omitted the lengthening of
imaginal life which is due to hybernation in certain species. In
almost all orders of insects there are certain species which emerge
from the pupa in the autumn, but which first reproduce in the
following spring. The time spent in the torpid condition during
winter cannot of course be reckoned with the active life of the
species, for its vital activity is either entirely suspended for a time by
freezing (Anadiosis: Preyer *), or it is at any rate never more than
a vita minima, with a reduction of assimilation to its lowest point.

1 Oken, ‘Naturgeschichte, Stuttgart, 1837, Bd. IV. Abth. r.

? Brehm, ‘ Leben der Vigel,’ p. 278.

_ §% *Naturwissenschaftliche Thatsachen und Probleme,’ Populire Vortrige, Berlin,
1880; vide Appendix.

APPENDIX. © ‘ 89

The following account does not make any claim to contain all or
even most of the facts scattered through the enormous mass of
entomological literature, and much less all that is privately known
by individual entomologists. It must therefore be looked upon as
merely a first attempt, a nucleus, around which the principal facts
can be gradually collected. It is wnnecessary to give any special
information as to the duration of larval life, for numerous and exact
observations upon this part of the subject are contained in all ento-
mological works.

I. ORtTHOPTERA.

Gryllotalpa. The eggs are laid in June or July, and the young
are hatched in from two to three weeks; they live through the
winter, and become sexually mature in the following May or June.
‘When the female has deposited her eggs, her body collapses, and
afterwards she does not survive much longer than a month.’
‘ According as the females are younger or older, they live a longer
or shorter life, and hence some females are even found: in the
autumn’ (Résel, ‘Insektenbelustigungen,’ Bd. II. p. 92). Résel
believes that the female watches the eggs until they are hatched,
and this explains the fact that she outlives the process of ovi-
position by about a month. It is not stated whether the males die
at an earlier period.

Gryllus campestris becomes sexually mature in May, and sings from
June till October, ‘when they all die’ (Oken, ‘ Naturgeschichte,’
Bd. II. Abth. iii. p. 1527). Itis hardly probable that any single
individual lives for the whole summer ; probably, as in the case of
Gryllotalpa, the end of the life of those individuals which first
become mature, overlaps the beginning of the life of others which
reach maturity at a later date.

Locusta viridissima and L. verrucivora are mature at the end of
August; they lay their eggs in the earth during the first half
of September and then die. It is probable that the females do
not live for more than four weeks in the mature state. It is not
known whether the males of this or other species of locusts live for
a shorter period.

I have found Locusta cantans in plenty, from the beginning of
September to the end of the month. In captivity they die after
depositing their eggs: the males are probably more short-lived, for

40 THE DURATION OF LIFE.

towards the middle and end of September they are much less
plentiful than the females.

Acridium migratorium ‘dies after the eggs are laid’ (Oken,
‘Naturgeschichte’),

The male Zermes probably live for a short time only, aidvonehi
exact observations upon the point are wanting. The females ‘seem
sometimes to live four or five years,’ as I gather from a letter from
Dr. Hagen, of Cambridge, Mass., U.S.A.

Ephemeridae. Résel, speaking of Ephemera vulgata (‘ Insekten-
belustigungen,’ Bd. II. der Wasserinsekten, 2 Klasse, p. 60 et seq.),
says:—‘Their flight commences at sunset, and comes to an end before
midnight, when the dew begins -to fall.’ ‘The pairing generally
takes place at night and lasts but a short time. As soon as the in-
sects have shed their last skin, in the afternoon or evening, they fly
about in thousands, and pair almost immediately ; but by the next
day they are all dead. They continue to emerge for many days, so
that when yesterday’s swarm is dead, to-day a new swarm is seen
emerging from the water towards the evening.’ ‘They not only drop
their eggs in the water, but wherever they may happen to be,—on
trees, bushes, or the earth. Birds, trout and other fish lie in wait
for them.’

Dr. Hagen writes to me—‘It is only in certain species that
life is so short. The female Padingenia does not live long enough
to complete the last moult of the sub-imago. I believe that a
female imago has never been seen. The male imago, often half in its
sub-imago skin, fertilizes the female sub-imago and immediately
the contents of both ovaries are extruded, and the insect dies. It
is quite possible that the eggs pass out by rupturing the abdominal
‘segments.’

Libellula. All dragon-flies live in the imago condition for some
weeks ; at first they are not capable of reproduction, but after a few
days they pair. :

Lepisma saccharima. An individual lived for two years in a pill-
box, without any food except perhaps a little Lycopodium dust.

“e
II. Nevroprera.

Phryganids ‘live in the imago stage for at least a week and prob-

ably longer, apparently without taking food’ (letter from Dr. Hagen).

+ «Entomolog. Mag.,’ vol. i. p. 527, 1833.

APPENDIX. 41

According to the latest researches Phryganea grandis* never con-
tains food in its alimentary canal, but only air, although it contains
the latter in such quantities that the anterior end of the chylific
ventricle is dilated by it.

III. Srreprsrprera.

The larva requires for its development a rather shorter time than
that which is necessary for the grub of the bee into the body of which
it has bored. The pupa stage lasts eight to ten days. The male,
which flies about in a most impetuous manner, lives only two to
three hours, while the female lives for some days. Possibly the
pairing does not take place until the female is two to three days old.
The viviparous female seems to produce young only once in a life-
time, and then dies: it is at present uncertain whether she also pro-
duces young parthenogenetically (cf. Siebold, ‘Ueber Paedogenesis
der Strepsipteren,’ Zeitschr. f. Wissensch. Zool., Band. XX, 1870).

IV. Hemrerera.

Aphis. Bonnet (‘ Observations sur les Pucerons,’ Paris, 1745) had
a parthenogenetic female of Aphis ewonymi in his possession for
thirty-one days, from its birth, during which time it brought forth
ninety-five larvae. Gleichen kept a parthenogenetic female of
Aphis mati fifteen to twenty-three days.

Aphis foliorum ulmi. The mother of a colony which leaves
the egg in May is 2” long at the end of July: it therefore lives
for at least two and a half months (De Geer, ‘ Abhandlungen zur
Geschichte, der Insekten,’ 1783, III. p. 53).

Phylloxera vastatriz. The males are merely ephemeral sexual
organisms, they have no proboscis and no alimentary canal, and
die immediately after fertilizing the female.

Pemphigus terelinthi. 'The male as well as the female sexual in-
dividuals are wingless and without a proboscis; they cannot take
food and consequently live but a short time,—far shorter than the
parthenogenetic females of the same species (Derbés, ‘ Note sur les
aphides du pistachier térébinthe, Ann. des sci. nat., Tom. XVII, .
1872).

Cicada. In spite of the numerous and laborious descriptions of

* Imhof, ‘ Beitriige zur Anatomie der Perla maxima,’ Inaug. Diss., Aarau, 1881.

42 THE DURATION OF LIFE.

the Cicadas which have appeared during the last two centuries, I
can only find precise statements as to the duration of life in the
mature insect in a single species. P. Kalm, writing upon the
North American Cicada septemdecim, which sometimes appears in
countless numbers, states that ‘six weeks after (such a swarm had
been first seen) they had all disappeared.’ Hildreth puts the life of
the female at from twenty to twenty-five days. This agrees with
the fact that the Cicada lays many hundred eggs (Hildreth states a
thousand); sixteen to twenty at a time being inserted into a hole
which is bored in wood, so that the female takes some time to lay
her eggs (Oken, ‘Naturgeschichte,’ 2** Bd. 3% Abth. p. 1588 et seq.).

Acanthia lectularia. No observations have been made, upon the
bed bug from which the normal length of its life can be ascer-
tained, but many statements tend to show that it is exceedingly
long-lived, and this is advantageous for a parasite of which the food
(and consequently growth and reproduction) is extremely precarious.
They can endure starvation for an astonishingly long period, and
can survive the most intense cold. Leunis (‘Zoologie, p. 659)
mentions the case of a female which was shut up in a box and or-
gotten: after six months’ starvation it was found not only alive
but surrounded by a circle of lively young ones. Goze found
bugs in the hangings of an old bed which had not been used for
six years: ‘they appeared white like paper. I have myself ob-
served a similar case, in which the starving animals were quite
transparent. De Geer placed some bugs in an unheated room in
the cold winter of 1772, when the thermometer fell to —33°C:
they passed the whole winter in a state of torpidity, but revived
in the following May. (De Geer, Bd. III. p. 165, and Oken,
‘ Naturgeschichte,’ 2" Bd. 3% Abth. p. 1613.)

V. Diptera.

Pulex irritans. Oken says of the flea (‘ Naturgeschichte, Bd. IT.
Abth. 2, p. 759) that ‘death follows the deposition of the eggs in
the course of two or three days, even if the opportunity of sucking
blood is given them.’ The length of time which intervenes between
the emergence from the cocoon and fertilization or the deposition
of eggs is not stated. :

Sarcophaga carnaria. The female fly dies ten to twelve hours
after the birth of the viviparous larvae; the time intervening

APPENDIX. 43

between the exit from the cocoon and the birth of the young is
not given (Oken, quoting Réaumur, ‘ Mém. p. s. a l’hist. Insectes,’
Paris, 1740-48, IV). |

Musca domestica. In the summer the common house-fly begins
to lay eggs eight days after leaving the cocoon: she then lays
several times. (See Gleichen, ‘Geschichte der gemeinen Stuben-
fliege,’. Nuremberg, 1764.)

Eristalis tenax. The larva of this large fly lives in liquid
manure, and has been described and figured by Réaumur as the rat-
tailed larva. I kept a female which had just emerged from the
cocoon, from August 30th till October 4th, in a large gauze-covered
glass vessel. The insect soon learnt to move freely about in its
prison, without attempting to escape; it flew round in circles, with
a characteristic buzzing sound, and obtained abundant nourish-
ment from a solution of sugar, provided for it. From September
12th it ceased to fly about, except when frightened, when it would
fly a little way off. I thought that it was about to die, but
matters took an unexpected turn, and on the 26th of September it
laid a large packet of eggs, and again on the 29th of the same
month another packet of similar size. The flight of the animal
had been probably impeded by the weight of the mass of ripe eggs
in its body. The deposition of eggs was probably considerably
retarded in this case, because fertilization had not taken place.
The fly died on the 4th of October, having thus lived for thirty-five
days. Unfortunately, I have been unable to make any experiments
as to the duration of life in the female when males are also present.

VI. LerrpoptTera.

I am especially indebted to Mr. W. H. Edwards’, of Coalburgh,
W. Virginia, and to Dr. Speyer, of Rhoden, for valuable letters
relating to this order.

The latter writes, speaking of the duration of life in imagos
generally :—‘ It is, to my mind, improbable that any butterfly can
live as an imago for a twelvemonth. Specimens which have lived
through the winter are only rarely seen in August, even when the
summer is late. A worn specimen of Vanessa cardui has, for

1 Mr. Edwards has meanwhile published these communications in full; cf. ‘On
the length of life of Butterflies,’ Canadian Entomologist, 1881, p. 205.

44 THE DURATION OF LIFE.

instance, been found at this time’ (‘Entomolog. Nachrichten,’
1881, p. 146).

In answer to my question as to whether the fact that certain
Lepidoptera take no solid or liquid food, and are, in fact, without
a functional mouth, may be considered as evidence for an adapta-
tion of the length of life to the rapid deposition of eggs, Dr. Speyer
replies :—‘ The wingless females of the Psychidae do not seem to
possess a mouth, at any rate I cannot find one in Psyche unicolor
(graminella), 'They do not leave the case during life, and certainly

. do not drink water. The same is true of the wingless female of.

Heterogynis, and of Orgyia ericae, and probably of all the females of
the genus Orgyia ; and as far as I can judge from cabinet specimens,
it is probably true of the males of Heterogynis and Psyche. I have
never seen the day-flying Satwrnidae, Bombycidae, and other Lepi-
doptera with a rudimentary proboscis, settle in damp places, or
suck any moist substance, and I doubt if they would ever do this.
The sucking apparatus is probably deficient.’

In answer to my question as to whether the males of any species
of butterfly or moth are known to pass a life of different length
from that of the female, Dr. Speyer stated that he knew of no ob-
servations on this point.

The following are the only instances of well-established direct
observations upon single individuals, in my possession !:—

Pieris napi, var. bryoniae 8 and 2, captured on the wing: lived
in confinement ten days, and were then killed.

Vanessa prorsa lived at most ten days in confinement.

Vanessa urticae lived ten to thirteen days in confinement.

Papilio ajax. According to a letter from Mr. W. H. Edwards,
the female, when she leaves the pupa, contains unripe eggs in her
body, and lives for about six weeks—caleulating from the first
appearance of this butterfly to the disappearance of the same
generation®. The males live longer, and continue to fly when very
worn and exhausted. A worn female is very seldom seen;—‘I
believe the female does not live long after laying her eggs, but
this takes some days, and probably two weeks.’

Lycaena violacea. According to Mr. Edwards, the first brood of
this species lives three to four weeks at the most.

* When no authority is given, the observations are my own.
? In the paper quoted above, Edwards, after weighing all the evidence, reduces
the length of life from three to four weeks.

APPENDIX. 45

Smerinthus titae. A female, which had just emerged from the
pupa, was caught on June 24th; on the 29th pairing took place ;
on the 1st of July she laid about eighty eggs, and died the following
day. She lived nine days, taking no food during this period, and
she only survived the deposition of eggs by a single day.

Macroglossa stellatarum. A female, captured on the wing and
already fertilized, lived in confinement from June 28th to July 4th.
During this time she laid about eighty eggs, at intervals and
singly; she then disappeared, and must have died, although the
body could not be found among the grass at the bottom of the
cage in which she was confined.

Saturnia pyri. A pair which quitted the cocoons on the 24th or
25th of April, remained in coitu from the 26th until May 2nd—
six or seven days; the female then laid a number of eggs, and died.

Psyche grammella. The fertilized female lives some days, and
the unfertilized female over a week (Speyer).

Solenobia triquetrella. ‘The parthenogenetic form (I refer to
the one which I have shown to be parthenogenetic in Oken’s ‘ Isis,’
1846, p. 30) lays a mass of eggs in the abandoned case, soon after
emergence. The oviposition causes her body to shrivel up, and
some hours afterwards she dies. The non-parthenogenetic female
of the same species remains for many days, waiting to be fertilized ;
if this does not occur, she lives over a week.’ ‘The parthenogenetic
female lives for hardly a day, and the same is true of the partheno-
genetic females of another species of Solenobia’ (S. inconspicuella ?).
Letter from Dr. Speyer.

Psyche calcella, O. The males live a very short time; ‘those
which leave the. cocoon in the evening are found dead on the
following morning, with their wings fallen off, at the bottom of
their cage.’ Dr. Speyer.

Eupithecia, sp. (Geometridae), ‘when well-fed, live for three to four
weeks in confinement; the males fertilize the females frequently,
and the latter continue to lay eggs when they are very feeble, and
are incapable of creeping or flying.’ Dr. Speyer.

The conclusions and speculations in the text seem to be suffi-
ciently supported from this short series of observations. There
remains, as we see, much to be done in this field, and it would
well repay a lepidopterist to undertake some exact observations
upon the length of life in different butterflies and moths, with

46 THE DURATION OF LIFE.

reference to the conditions of life—the mode of egg-laying, the
degeneracy of the wings, and of the external mouth-parts or the
closure of the mouth itself. It would be well to ascertain whether
such closure does really take place, as it undoubtedly does in certain
plant-lice.

VII. Cotroprera.

Melolontha vulgarie. Cockchafers, which I kept in an airy cage
with fresh food and abundant moisture, did not in any case live
longer than thirty-nine days. One female only, out of a total
number of forty-nine, lived for this period ; a second lived thirty-
six days, a third thirty-five, and a fourth and fifth twenty-four
days; all the rest died earlier. Of the males, only one lived as
long as twenty-nine days. These periods are less by some days
than the true maximum duration of life, for the beetles were cap-
tured in the field, and had lived for at least a day; but the differ-
ence cannot be great, when we remember that out of forty-nine
beetles, only three females lived thirty-five to thirty-nine days, and
only one male twénty-nine days. Those that died earlier had
probably lived for some considerable time before being caught.

Exact experiments with pupae which have survived the winter
would show whether the female really lives for ten days more than
the male, or whether the results of my experiment were merely
accidental. I may add that coitus frequently took place during
the period of captivity. One pair, observed in this condition on
the 17th, separated in the evening; they paired again on the
morning of the 18th, and separated in the middle of the day.
Coitus took place between another pair on the 22nd, and again on
the 26th.

I watched the gradual approach of death in many individuals:
some days before it ensued, the insects became sluggish, ceased to ~
fly and to eat, and only crept a little way off when disturbed: they
then fell to the ground and remained motionless, apparently dead,
but moved their legs when irritated, and sometimes automatically.
Death came on gradually and imperceptibly; from time to time
there was a slow movement of the legs, and at last, after some
hours, all signs of life ceased.

In one case only I found bacteria present in great numbers in
the blood and tissues; in the other individuals which had recently

APPENDIX. 47

died, the only noticeable change was the unusual dryness of the
tissues.

Carabus auratus. An experiment with an individual, caught on
May 27th, gave the length of life at fourteen days; this is
probably below the average, since the beetles are found, in the wild
state, from the end of May until the beginning of July.

Lucanus cervus, Captured individuals, kept in confinement, and
fed on a solution of sugar, never lived longer than fourteen days,
and as-a rule not so long. The beetles appear in June and July,
and certainly cannot live much over a month. As is the case with
many beetles appearing during certain months, the length of the
individual life is shorter than the period over which they are found.
Accurate information, especially as to any difference between the
lengths of life in the sexes, is not obtainable.

Isolated accounts of remarkably long lives among beetles are to
be found scattered throughout the literature of the subject. Dr.
Hagen, of Cambridge, Mass., has been kind enough to draw my
attention to these, and to send me some observations of his own.

Cerambyx heros. One individual lived in confinement from
August until the following year 1.

Saperda carcharias. An individual lived from the 5th of July
until the 24th of July of the next year 1.

Buprestis splendens. A living individual was removed from a
desk which had stood in a London counting-house for thirty years ;
from the condition of the wood it was evident that the larva had
been in it before the desk was made?.

laps mortisaga. One individual lived three months, and two
others three years.

Blaps fatidica. One individual which was left in a box and for-
gotten, was found alive when the box was opened six years after-
wards.

Blaps obtusa. One lived a year and a half in confinement.

Lleodes grandis and LE. dentipes, Hight of these beetles from
California were kept in confinement and without food for two years
by Dr. Gissler, of Brooklyn; they were then sent to Dr. Hagen
who kept them another year.

Goliathus cacicus. One individual lived in a hot-house for five

months.
* *Entomolog. Mag.,’ vol. i. p. 527, 1823.

48 THE DURATION OF LIFE.

In addition to these cases, Dr. Hagen writes to me: ‘Among the
beetles which live for more than a year,—Blaps, Pasimachus, (Cara-
bidae)—and among ants, almost thirty per cent. are found with the
cuticle worn out and cracked, and the powerful mandibles so greatly
worn down that species were formerly founded upon this point.
The mandibles are sometimes worn down to the hypodermis.’

From the data before me I am inclined to believe that in certain
beetles the normal length of life extends over some years, and this
is especially the case with the Blapidae. It seems probable that in
these cases another factor is present,—a vila minima, or apparent
death, a sinking of the vital processes to a minimum in consequence
of starvation, which we might call the hunger sleep, after the ana-
logy of winter sleep. The winter sleep is usually ascribed to cold
alone, and some insects certainly become so torpid that they appear
to be dead when the temperature is low. But cold does not affect
all insects in this way. Among bees, for example, the activity of
the insects diminishes to a marked extent at the beginning of
winter, but if the temperature continues to fall, they become active
again, run about, and as the bee-keepers say, ‘try to warm them-
selves by exercise’; by this means they keep some life in them.
If the frost is very severe, they die. In the tropics the period of
hibernation for many animals coincides with the time of maximum
heat and drought. This shows that the organism can be brought
into the condition of a vita minima in various ways, and it would
not be at all remarkable if such a state were induced in certain in-
sects by hunger. Exact experiments however are the only means
by which such a suggestion can be tested, and I have already com-
menced a series of experiments. The fact that certain beetles live
without food for many years (even six) can hardly be explained on
any other supposition, for these insects consume a fair amount of
food under normal conditions, and it is inconceivable that they
could live for years without food, if the metabolism were carried on
with its usual energy.

A very striking example, showing that longevity may be induced
by the*lengthening of the period of reproductive activity, is com-
municated to me by Dr. Adler in the following note: ‘Three years
ago I accidentally noticed that ovoviviparous development takes
place in Chrysomela varians,—a fact which I afterwards discovered
had been already described by another entomologist.

APPENDIX. 49

‘The egg passes through all the developmental stages in the
ovary; when these are completed the egg is laid, and a minute or
two afterwards the larva breaks through the egg-shell. In each
division of the ovary the eggs undergo development one at a time ;
it therefore follows that they are laid at considerable intervals, so
that a long life becomes necessary in order to ensure the develop-
ment of a sufficiently long series of eggs. Hence it comes about
that the females live a full year. Among other species of Chryso-
mela two generations succeed each other in a year, and the duration
of life in the individual varies from a few months to half a year.

VIII. Hymenoptera.

Cynipidae. Ihave been unable to find any accurate accounts of
the duration of life in the imagos of saw-flies or ichneumons; but
on the other hand I owe to the kindness of Dr. Adler, an excellent
observer of the Cynipidae, the precise accounts of that family which
are in my possession. I asked Dr. Adler the general question as to
whether there was any variation in the duration of life among the
Cynipidae corresponding to the conditions under which the deposi-
tion of eggs took place; whether those species which lay many
eggs, or of which the oviposition is laborious and protracted, lived
longer than those species which lay relatively few eggs, or easily and
quickly find the suitable places in which to deposit them.

Dr. Adler fully confirmed my suppositions and supported them
by the following statements :—

‘The summer generation of Neuwroterus (Spathegaster) has the
shortest life of all Cynipidae. Whether captured or reared from the
galls I have only kept them alive on an average for three to four
days. In this generation the work of oviposition requires the
shortest time and the least expenditure of energy, for the eggs are
simply laid on the surface of a leaf. The number of eggs in the
ovary is also smaller than that of other species, averaging about
200. This form of Cynips can easily lay 100 eggs a day.

‘The summer generation of Dryophanta (Spathegaster Taschenbergt,
verrucosus, ete.) lives somewhat longer; I have kept them in con-
finement for six to eight days. The oviposition requires a consider-
able expenditure of time and strength, for the ovipositor has to
pierce the rather tough mid-rib or vein of a leaf. The number of
eggs in the ovary averages 300 to 400.

E

50 THE DURATION OF LIFE.

‘The summer generation of Audricus, which belongs to the exten-
sive genus Aphilotrix, have also a long life. I have kept the smaller
Andricus (such as A. nudus, A. cirratus, A. noduli) alive for a week,
and the larger (A. injflator, A. curvator, A. ramuli) for two weeks.
The smaller species pierce the young buds when quite soft, but the
larger ones bore through the fully grown buds protected by tough
scales. The ovary of the former contains 400 to 500 eggs, that of
the latter over 600.

‘The agamic winter generations live much longer. The species of
Neuroterus have the shortest life; they live for two weeks at the
outside; on the other hand, species of Aphilotriv live quite four
weeks, and Dryophanta and Biorhiza even longer. I have kept
Dryophanta scutellaris alive for three months. The number of eggs
-in these agamie Cynipidae is much larger: Dryophanta and Aphilotria
contain 1200 and Newroterus about 1000,’

It is evidently, therefore, a general rule that the duration of life
is directly proportional to the number of eggs and to the time and
energy expended in oviposition. It must of course be understood
that, here as in all other instances, these are not the only factors
which determine the duration of life, but many other factors, at
present unknown, may be in combination with them and assist in
producing the result. For example, it is very probable that the
time of year at which the imagos appear exerts some indirect
influence. The long-lived Biorhiza emerges from the gall in the
middle of winter, and at once begins to deposit eggs in the oak
buds. Although the insect is not sensitive to low temperature, for
I have myself seen oviposition proceeding when the thermometer
stood at 5° R., yet very severe frost would certainly lead to inter-
ruption and would cause the insect to shelter itself among dead
leaves on the ground. Such interruptions may be of long duration
and frequently repeated; so that the remarkably long life of this
species may perhaps be looked upon as an adaptation to its winter
life.

Ants. Lasius flavus lays its eggs in the autumn, and the young
larvae pass the winter in the nest. The males and females leave
the cocoons in June, and pair during July and August. The males
fly out of the nest with the females, but they do not return to it;
‘they die shortly after pairing.’ It is also believed that the females
do not return to’ the nest, but found new colonies; this point is

APPENDIX. 51

however one of the most uncertain in the natural history of ants.
On the other hand it is quite certain that the female may live for
years within the nest, continuing to lay fertilized eggs. Old
females are sometimes found in the colony, with their jaws worn
down to the hypodermis.

Breeding experiments confirm these statements. P. Huber! and
Christ have already put the life of the female at three to four years,
and Sir John Lubbock, who has been lately occupied with the
natural history of ants, was able to keep a female worker of Formica
sanguinea alive for five years; and he has been kind enough to write
and inform me that two females of Formica fusca, which he captured
in a wood together with ten workers, in December 1874, are still
alive (July 1881), so that these insects live as imagos for six and a
half years or more ”.

1 «Recherches sur les mceurs des Fourmis indigenes,’ Geneve, 1810.

2 These two female ants were still alive on the 25th of September following Sir
John Lubbock’s letter, so that they live at least seven years. Cf. ‘ Observations on
Ants, Bees, and Wasps,’ Part VIII. p. 385; Linn. Soc. Journ. Zool., vol. xv. 1881.

[Sir John Lubbock has kindly given me further information upon the duration of life
of these two queen ants. Since the receipt of his letter, the facts have been published
in the Journal of the Linnean Society (Zoology), vol. xx. p. 133. I quote in full
the passage which refers to these ants :—

‘ Lonceviry.—It may be remembered that my nests have enabled me to keep ants
under observation for long periods, and that I have identified workers of Lasius niger
and Formica fusca which were at least seven years old, and two queens of Formica
fusca which have lived with me ever since December 1874. One of these queens,
after ailing for some days, died on the 30th July, 1887. She must then have been
more than thirteen years. old. I was at first afraid that the other one might be
affected by the death of her companion. She lived, however, until the 8th August,
1888, when she must have been nearly fifteen years old, and is therefore by far the
oldest insect on record.

‘Moreover, what is very extraordinary, she continued to lay fertile eggs. This
remarkable fact is most interesting from a physiological point of view. Fertilization
took place in 1874 at the latest. There has been no male in the nest since then,
and, moreover, it is, I believe, well established that queen ants and queen bees are
fertilized once for all. Hence the spermatozoa of 1874 must have retained their life
and energy for thirteen years, a fact, I believe, unparalleled in physiology.’

: * * * * * * *

‘I had another queen of Formica fusca which lived to be thirteen years old, and
I have now a queen of Lasius niger which is more than nine years old, and still lays
fertile eggs, which produce female ants.’

Both the above-mentioned queens may have been considerably older, for it is im-
possible to estimate their age at the time of capture. It is only certain (as Sir John
Lubbock informs me in his letter) that ‘they must have been at least nine months
old (when captured), as the eggs of F. fusca are laid in March or early in April.’
The queens became gradually ‘somewhat lethargic and stiff in their movements

E2

52 THE DURATION OF LIFE.

On the other hand, Sir John Lubbock never succeeded in keeping
the males ‘alive longer than a few weeks.’ Both the older and
more recent observers agree in stating that female ants, like
queen bees, are always protected as completely as possible from
injury and danger. Dr. A. Forel, whose thorough knowledge of
Swiss ants is well known, writes to me,—‘ The female ants are only
~ once fertilized, and are then tended by the workers, being cleaned
and fed in the middle of the nest: one often finds them with only
three legs, and with their chitinous armour greatly worn. They
never leave the centre of the nest, and their only duty is to lay
eggs.’

With regard to theworkers, Forel believes that their constitution
would enable them to live as long as the females (as the experiments
of Lubbock also indicate), and the fact that in the wild state they
generally die sooner than the females is ‘certainly connected with
the fact that they are exposed to far greater dangers.’ The same
relation seems also to obtain among bees, but with them it has not
been shown that in confinement the workers live as long as the
queens. .

Bees. According to von Berlepsch! the queen may as an excep-
tion live for five years, but as a rule survives only two or three
years. The workers always seem to live for a much shorter period,
generally less than a year. Direct experiments upon isolated or
confined bees, or upon marked individuals in the wild state, do not
prove this, but the statistics obtained by bee-keepers confirm the
above. Every winter the numbers in a hive diminish from
1 2,000—20,000 to 2000-3000. The queen lays the largest number
of eggs in the spring, and the workers which die before the winter
are replaced by those which emerge in the summer, autumn or
during a mild winter. The queen lays eggs at such a variable
rate throughout the year that the above-mentioned inequality in
numbers is explained. The workers do not often live for more than
six to seven months, and at the time of their greatest labour, (May
to July), only three months. An attempt to calculate the length
of life of the workers and drones by taking stock at the end of

(before their death), but there was no loss of any limb nor any abrasion.’ This last
observation seems to indicate that queen ants may live for a much longer period in
the wild state, for it is stated above that the chitin is often greatly worn, and some
of the limbs lost (see pp. 48, 51, and 52).—E. B. P.]

1 A. von Berlepsch, ‘ Die Biene und ihre Zucht,’ etc., 3rd ed.; Mannheim, 1872.

—_-

APPENDIX. 53

summer, gives six months for the former and four months for the
latter *.

The drones do not as a rule live so long as four months, for they
meet with a violent death before the end of this period. The well-
known slaughter of the drones is not, according to the latest obser-
vations, brought about directly by means of the stings of the
workers, but by these latter driving away the useless drones from
the food so that they perish of starvation.

Wasps. It is interesting that among these near relations of the
bees, the life of the female should be much shorter, corresponding
to the much lower degree of specialization found in the colonies.
The females of Polistes gallica and of Vespa not only lay eggs but
take part in building the cells and in collecting food; they are
therefore obliged to use all parts of the body more actively and
especially the wings, and are exposed to greater danger from
enemies.

It is well known from Leuckart’s observations, that the so-called
‘workers’ of Polistes gallica and Bombus are not arrested females
like the workers of a bee-hive, but are females which although
certainly smaller, are in every way capable of being fertilized and
of reproduction. Von Siebold has nevertheless proved that they
are not fertilized, but reproduce parthenogenetically.

The fertilized female which survives the winter, commences to
found a colony at the beginning of May: the larvae, which hatch
from the first eggs, which are about fifteen in number, become
pupae at the beginning of June, and the imagos appear towards the
end of the same month. These are all small ‘workers, and they
perform such good service in tending the second brood, that the
latter attain the size of the female which founded the colony ; only
differing from her in the perfect condition of their wings, for by
this time her wings are greatly worn away.

The males appear at the beginning of July; their spermatozoa
are mature in August, and pairing then takes place with certain
‘special females: which require fertilization’ which have in the
meantime emerged from their cocoons. These are the females which
live through the winter and found new colonies in the following
spring. The old females of the previous winter die, and do not live

1 E. Bevan, ‘ Ueber die Honigbiene und die ee ihres Lebens;’ abstract in
Oken’s ‘ Isis,’ 1844, p. 506,

54 THE DURATION OF LIFE.

beyond the summer at the beginning of which they founded
colonies. At the first appearance of frost, the young fertilized
females seek out winter quarters; the males which never survive
the winter, do not take this course, but perish in October. The
parthenogenetic females, which remain in the nest during the
nuptial flight, also perish.

The males of Polistes gallica do not live longer Nea three
months—from July to the beginning of October; the partheno-
genetic females live a fortnight longer at the outside—from the
middle of June to October, but the later generations have a shorter

life. The sexual females alone live for about a year, including the
_ winter sleep.

A similar course of events takes place in the genus Vespa, In
both these genera the possibility of reproduction is not restricted to
a single female in the nest, but is shared by a number of females.
In the genus Apis alone is the division of labour complete, so that
only a single female (the queen) is at any one time capable of re-
production, a power which differentiates it from the sterile workers.

Nore 4. Tue Douration or Lire or tat Lower Marine ANIMALS.

I have only met with one definite statement in the literature of
this part of the subject. It concerns a sea anemone,—which is a
solitary and not a colonial form. The English zoologist Dalyell, in
August, 1828, removed an Actinia mesembryanthemum from the sea
and placed it in an aquarium’. It was a very fine individual,
although it had not quite attained the largest size; and it must
have been at least seven years old, as proved by comparison with
other individuals reared from the egg. In the year 1848, it was
about thirty years old, and in the twenty years during which it had
been in captivity it had produced 334 young Actiniae. Prof.
Dohrn, of Naples, tells me that this Actinia is still living to-day,
and is shown as a curiosity to those who visit the Botanical Gardens
in Edinburgh, It is now (1882) at least sixty-one years old*.

1 Dalyell, ‘Rare and Remarkable Animals of Scotland,’ vol. ii. p. 203; London, ~
1848.

(? Mr. J. S. Haldane has kindly obtained details of the death of the sea anemone
referred to by the author. It died, by a natural death, on August 4, 1887, after
having appeared to become gradually weaker for some months previous to this date.
It had lived ever since 1828 in the same small glass jar in which it was placed by
Sir John Dalyell. It must have been at least 66 years old when it died.—E.B.P.]

APPENDIX. 55

Nore 5. Tue Duration or Lire in INDiceNovus TERRESTRIAL
AND FrREsH-waTer Mo .wusca.

I am indebted to Herr Clessin—the celebrated student of our
mollusea—for some valuable notes upon our indigenous snails and
bivalves (Lamellibranchiata). I could not incorporate them in the
text, for a number of necessary details as to the conditions of life
are at present entirely unknown, or are at least only known in a
very fragmentary manner. No statistics as to the amount of de-
struction suffered by the young are available, and even the number
of eggs produced annually is only known for a few species. I
nevertheless include Herr Clessin’s very interesting communica-
tions, as a commencement to the life statistics of the Mollusea.

_ (1) ‘ Vitrinae are annual ; the old animals die in the spring, after
having produced the spawn from which the young develope. These
continue to grow until the following spring.’ |

(2) ‘The Suecineae are mostly biennial ; Succinea putris probably
triennial. Fertilization takes place from June till the beginning of
August, and the young develope until the autumn. Swuccinea Pfeif-

- feri and 8. elegans live through the winter, and the fact is proved
by very distinct annual markings. Reproduction takes place in
July and August of the following year, and they die in the autumn.
They continue to grow until their death.’

(3) ‘The shells of our native species of Pupa, Clausilia, and Buli-
mus (with the exception of Bulimus detritus) show but faint annual
markings. They can hardly require more than two years for their
complete development. The great number of living individuals
with full-sized shells belonging to these genera, as compared with
the number which possess smaller shells, makes it probable that
these animals live in the mature condition longer than our other
Helicidae. I have always found full-sized shells present in at
least two-thirds of the individuals of these genera characterized by

-much-coiled shells—a proportion which I have never seen among
our larger Helicidae. Nevertheless direct observations as to the
length of life in the mature condition are still wanting,

(4) ‘The Helicidae live from two to four years ; Hela sericea, H.
hispida, two to three years; H. hortensis, H. nemoralis, H. arbustorum,
as a rule three years; H. pomatia four years. Fertilization is not
in these species strictly confined to any one time of year, but in the

56 THE DURATION OF LIFE.

case of old animals takes place in the spring, as soon as the winter
sleep is over; while in the two-year-old animals it-also happens
later in the summer.’

(5) ‘The Hyalineae are mostly biennial: they seldom live three
years, and even in the largest species such an age is probably
exceptional, The smallest Hyalineae and Helicidae live at most two
years. The length of life is dependent upon the time at which the
parents are fertilized, for this decides whether the young begin to
shift for themselves early in the summer or later in the autumn,
and so whether the first year’s growth is large or small.’

(6) ‘The species of Limuaeus, Planorbis, and Ancylus live two
to three years, that is they take two to three years to attain the full
size. J. auricularis is mostly biennial, L. palustris and L. pereger
two to three years: I have found that the latter, in the mountains
at Oberstorf in the Bavarian Alps, may exceptionally attain the
age of four years, that is, it may possess three clearly defined annual
markings, whilst the specimens from the plain never showed more
than two.’

(7) ‘The Paludinidae attain an age of three or four years.’

(8) ‘The smaller bivalves, Pisidiwm and Cyclas, do not often live
for more than two years: the larger Najadae, on the other hand,
often live for more than ten years, and indeed they are not full
grown until they possess ten to fourteen annual markings. It is

possible that habitat may have great influence upon the length of —

life in this order.’

‘ Unio and Anodonta hedges sexually mature in the third to the
fifth year.’

As far as I am aware but few statements exist upon the length
of life in marine mollusca, and these are for the most part very
inexact. The giant bivalve Zridacna gigas must attain an age of
60 to 100 years’. All Cephalopods live for at least over a year,
and most of them well over ten years; and the giant forms,
sometimes mistaken for ‘ sea-serpents, must require many decades
in which to attain such a remarkable size. LL. Agassiz has deter-
mined the length of life in a large sea snail, Natica heros, by
sorting a great number of individuals according to their sizes: he
places it at 30 years”.

? Bronn, ‘Klassen und Ordnungen des Thierreichs,’ Bd. III. p. 466; Leipzig.
? Bronn, l. c.

APPENDIX. 57

I am glad to be able to communicate an observation made at
the Zoological Station at Naples upon the length of life in
Ascidians, The beautiful white Cionea intestinalis has settled in
great numbers in an aquarium at the Station, and Professor Dohrn
tells me that it produces three generations annually, and that
each individual lives for about five months, and then reproduces
itself and dies. External conditions accounting for this early
death have not been discovered.

It is known that the freshwater Polyzoa are annual, but it is
not known whether the first individuals produced from a colony
in the spring, live for the whole summer. The length of life
is also unknown in single individuals of any marine Polyzoon.

Clessin’s accurate statements upon the freshwater Mollusca, pre-
viously quoted, show that a surprisingly short length of life is the
general rule. Only those forms of which the large size requires that
many years shall elapse before the attainment of sexual maturity,
live ten years or over (Unio, Anodonta); indeed, our largest
native snail (Helia pomatia) only lives for four years, and many
small species only one year, or two years if the former time is in-
sufficient to render them sexually mature. These facts seem to
indicate, as I think, that these molluscs are exposed to great de-
struction in the adult state, indeed to a greater extent than when
they are young, or, at any rate, to an equal extent. The facts
appear to be the reverse of those found among birds. The
fertility is enormous; a single mussel contains several hundred
thousand eggs; the destruction of young as compared with the
number of eggs produced is distinctly smaller than in birds, there-
- fore a much shorter duration of the life of each mature individual
is rendered possible, and further becomes advantageous because the
mature individuals are exposed to severe destruction.

However it can only be vaguely suggested that this is the case,
for positive proofs are entirely absent. Perhaps the destruction of
single mature individuals does not play so important a part as the
destruction of their generative organs. ‘The ravages of parasitic
animals (Zematodes) in the internal organs of snails and bivalves
are well known to zoologists. The ovaries of the latter are often
entirely filled with parasites, and such animals are then incapable
of reproduction. |

Besides, molluscs have many enemies, which destroy them both

58 THE DURATION OF LIFE.

on land and in water. In the water,—fish, frogs, newts, ducks and
other water-fowl, and on land many birds, the hedgehog, toads, etc.,
largely depend upon them for food.

If the principles developed in this essay apply to the freshwater
Mollusca, we must then infer that snails which maintain the
mature condition—the capability of reproduction—for one year,
are in this state more exposed to destruction from the attacks of
enemies than those species which remain sexually mature for two
or three years, or that the latter suffer from a greater proportional
loss of eggs and young.

Nore 6. Uneqguat Leneru or Lire in THE Two SExzs.

This inequality is frequently found among insects. The males
of the remarkable little parasites infesting bees, the Strepsiptera,
only live for two to three hours in the mature condition, while
the wingless, maggot-like, female lives eight days: in this case,
therefore, the female lives sixty-four times as long as the male.
The explanation of these relations is obvious; a long life for the
male would be useless to the species, while the relatively long life
of the female is a necessity for the species, inasmuch as she is
viviparous, and must nourish her young until their birth.

Again, the male of Phyllowera vastatria lives for a much shorter
period than the female, and is devoid of proboscis and stomach, and ~
takes no food: it fertilizes the female as soon as the last skin has
been shed and then dies.

Insects are not the only animals among which we find inequality
in the length of life of the two sexes. Very little attention has
been hitherto directed to this matter, and we therefore possess
little or no accurate information as to the duration of life in.
the sexes, but in some cases we can draw inferences either from
anatomical structure or from the mode of development. Thus,
male otifers never possess mouth, stomach, or intestine, they
cannot take food, and without doubt live much shorter lives
than the females, which are provided with a complete alimentary
canal, Again, the dwarf males of many parasitie Copepods—
low Crustacea—and the ‘complementary males’ of Cirrhipedes
(or barnacles) are devoid of stomach, and must live for a much
shorter time than the females; and the male Hntoniscidae (a family

APPENDIX. 59

of which the species are endo-parasitic in the larger Crustacea),
although they can feed, die after fertilizing the females; while
the latter then take to a parasitic life, produce eggs, and continue
to live for some time. It is supposed that the dwarf male of
Bonellia viridis does not live so long by several years as the hun-
dred times larger female, and it too has no mouth to its alimentary
canal. These examples might be further increased by reference to
zoological literature.

In most cases the female lives longer than the male, and this
needs no special explanation; but the converse relation is con-
ceivable, when, for instance, the females are much rarer than the
males, and the latter lose much time in seeking them. The above-
mentioned case of Aglia tau probably belongs to this category.

We cannot always decide conclusively whether the life of one
sex has been lengthened or that of the other shortened; both
these changes must have taken place in different cases. There is
no doubt that a lengthening of life in the female has arisen in
the bees and ants, for both sexes of the saw-flies, which are be-
lieved to be the ancestors of bees, only live for a few weeks. But
among the Strepsiptera the shorter life of the male must have been
secondarily acquired, since we only rarely meet with such an ex-
treme case in insects. |

Nore 7. Buss.

Tt has not been experimentally determined whether the workers,
which are usually killed after some months, would live as long as
the queen, if they were artificially protected from danger in the
hive; but I think that this is probable, because it is the case
among ants, and because the peculiarity of longevity must be
latent in the egg. As is well known, the egg which gives rise to
the queen is identical with that which produces a worker, and
differences in the nutrition alone decide whether a queen or a
worker shall be formed. I¢ is therefore probable that the duration
of life in queen and worker is potentially the same.

Note 8. Dratu or tHe CELis IN HIGHER ORGANISMS.

The opinion has been often expressed that the inevitable appear-
ance of normal ‘death’ is dependent on the wearing out of the

60 THE DURATION OF LIFE.

tissues in consequence of their functional activity. Bertin says,
referring to animal life! :—‘ L’observation des faits y attache Vidée
d’une terminaison fatale, bien que la raison ne découvre nullement
les motifs de cette nécessité. Chez les étres qui font partie du
régne animal l’exercise méme de la rénovation moléculaire finit
par user le principe qui l’entretient sans doute parceque le hve
vail d’échange ne s’accomplissant pas avec une perfection mathé-
matique, il s’établit dans la figure, comme dans la substance de
létre vivant, une déviation insensible, et que l’accumulation des

écarts finit par amener un type chimique ou morphologique in- |

compatible avec la persistance de ce travail.’

Here the replacement of the used-up elements of tissue by new
ones is not taken into account, but an attempt is made to show
that the functions of the whole organism necessarily cause it to
waste away. But the question at once arises, whether such a result
does not depend upon the fact that the single histological elements,
—the cells,—are worn out by the exercise of function. Bertin
admits this to be the case, and this idea of the importance of
changes in the cells themselves is everywhere gaining ground.
But although we must admit that the histological elements do, as
a matter of fact, wear out, in multicellular animals, this would not
prove that, nor explain why, such changes must follow from the
nature of the cell and the vital processes which take place within

it. Such an admission would merely suggest the question :—how -

is it that the cells in the tissues of higher animals are worn out
by their function, while cells which exist in the form of free and
independent organisms possess the power of living for ever? Why
should not the cells of any tissue, of which the equilibrium is
momentarily disturbed by metabolism, be again restored, so that the
same cells continue to perform their functions for ever :—why cannot
they live without their properties suffering alteration? I have not
sufficiently touched upon this point in the text, and as it is obviously
important it demands further consideration.

In the first place, I think we may conclude with certainty from the
unending duration of unicellular organisms, that such wearing out
of tissue cells is a secondary adaptation, that the death of the cell,
like general death, has arisen with the complex, higher organisms.
Waste does not depend upon the intrinsic nature of the cells, as the

? Cf. the article ‘ Mort’ in the ‘Encyclop. Scienc. Méd.’ vol. M. p. 520.

a ——

APPENDIX. 61

primitive organisms prove to us, but it has appeared as an adapta-
tion of the cells to the new conditions by which they are surrounded
when they come into combination, and thus form the cell-republic
of the metazoan body. The replacement of cells in the tissues must
be more advantageous for the functions of the whole organism than
the unlimited activity of the same cells, inasmuch as the power
of single cells would be much increased by this means. In certain
eases, these advantages are obvious, as for example in many glands
of which the secretions are made up of cast-off cells. Such cells
must die and be separated from the organism, or the secretion would
come to an end. In many cases, however, the facts are obscure,
and await physiological investigation. But in the meantime we
may draw some conclusions from the effects of growth, which are
necessarily bound up with a certain rate of production of new cells.
In the process of growth a certain degree of choice between the old
cells which have performed their functions up to any particular
time, and the new ones which have appeared between them, is as it
were left to the organism.

The organism may thus, figuratively speaking, venture to demand
from the various specific cells of tissues a greater amount of work
than they are able to bear, during the normal length of their life,
and with the normal amount of their strength. The advantages
gained by the whole organism might more than compensate for
the disadvantages which follow from the disappearance of single
cells. The glandular secretions which are composed of cell-de-
tritus, prove that the cells of a complex organism may acquire
fanctions which result in the loosening of their connexion with the
living cell-community of the body, and their final separation from
it. And the same facts hold with the blood corpuscles, for the
exercise of their function results in ultimate dissolution. Hence it
is not only conceivable, but in every way probable, that many
other functions in the higher organisms involve the death of the
cells which perform them, not because the living cell is necessarily
worn out and finally killed by the exercise of any ordinary vital
process, but because the specific functions in the economy of the
cell community which such cells undertake to perform, involve the
death of the cells themselves. But the fact that such functions
have appeared,—involving as they do the sacrifice of a great num-
ber of cells,—entirely depends upon the replacement of the old

62 THE DURATION OF LIFE.

by newly formed cells, that is by the process of reproduction in
cells?.

‘We cannot a priori dispute the possibility of the existence of
tissues in which the cells are not worn out by the performance of
function, but such an occurrence appears to be improbable when
we recollect that the cells of all tissues owe their constitution
to a very far-reaching process of division of labour, which leaves
them comparatively one-sided, and involves the loss of many pro-
perties of the unicellular, self-sufficient organism. At any rate we
only know of potential immortality in the cells which constitute
independent unicellular organisms, and the nature of these is such
that they are continually undergoing a complete process of re-
formation.

If we did not find any replacement of cells in the higher
organism, we should be induced to look upon death itself as the
direct result of the division of labour among the cells, and to con-
clude that the specific cells of tissues have lost, as a consequence
of the one-sided development of their activities, the power of un-
' ending life, which belongs to all independent primitive cells. We
should argue that they could only perform their functions for a
certain time, and would then die, and with them the organism whose
life is dependent upon their activity. The longer they are occupied
with the performance of special functions, the less completely do
they carry out the phenomena of life, and hence they lead to the
appearance of retrogressive changes. But the replacement of cells
is certain in many tissues (in glands, blood, etc), so that we can
never seek a satisfactory explanation in the train of reasoning in-
dicated above, but we must assume the existence of limits to the
replacement of cells. In my opinion, we can find an explanation
of this in the general relations of the single individual to its
species, and to the whole of the external conditions of life; and this
is the explanation which I have suggested and have attempted to
work out in the text.

1 Roux, in his work ‘Der Kampf der Theile im Organismus,’ Jena 1881, has
attempted to explain the manner in which division of labour has arisen among the
cells of the higher organisms, and to render intelligible the mechanical processes
by which the purposeful adaptations of the organism have arisen.

APPENDIX. 63

Norse g. Dxatn sy Suppen Suocx.

The most remarkable example of this kind of death known to me,
is that of the male bees. It has been long known that the drone
perishes while pairing, and it was usually believed that the queen
bites it to death. Later observations have however shown that
this is not the case, but that the male suddenly dies during copulation,
and that the queen afterwards bites through the male intromittent
organ, in order to free herself from the dead body. In this. case
death is obviously due to sudden excitement, for when the latter is
artificially induced, death immediately follows. Von Berlepsch
made some very interesting observations on this point, ‘If one
catches a drone by the wings, during the nuptial flight, and holds
it free in the air without touching any other part, the penis is pro-
truded and the animal instantly dies, becoming motionless as
though killed by a shock. The same thing happens if one gently
stimulates the dorsal surface of the drone on a similar occasion.
The male is in such an excited and irritable condition that the
slightest muscular movement or disturbance causes the penis to
be protruded!’ In this case death is caused by the so-called
nervous shock. The humble-bees are not similarly constituted, for
the male does not die after fertilizing the female, ‘ but withdraws
its penis and flies away.’ But the death of male bees, during
pairing, must not be regarded as normal death. Experiment has
shown that these insects can live for more than four months?.
They do not, as a matter of fact, generally live so long; for—
although the workers do not, as was formerly believed, kill them
after the fertilization of the queen, by direct means—they prevent
them from eating the honey and drive them from the hive, so that
they die of hunger ®.

We must also look upon death which immediately, or very
quickly, follows upon the deposition of eggs as death by sudden
shock. The females of certain species of Psychidae, when they re-
produce sexually, may remain alive for more than a week waiting
for a male: after fertilization, however, they lay their eggs and
die, while the parthenogenetic females of the same species lay their.

1 yon Berlepsch, ‘ Die Biene und ihre Zucht,’ etc.

2 Oken, ‘ Isis,’ 1844, p. 506.
’ von Berlepsch, 1. c., p. 165.

64 THE DURATION OF LIFE.

eggs and die immediately after leaving the cocoon; so that while
the former live for many days, the latter do not last for more than
twenty-four hours. ‘The parthenogenetic form of Solenobia tri-
quetrella, soon after emergence, lays all her eggs together in the
empty case, becomes much shrunken, and dies in a few hours.’
(Letter from Dr. Speyer, Rhoden.)

Norte 10. INTERMINGLING DURING THE Fission oF UNICELLULAR
OrGaNisms |,

Fission is quite symmetrical in Amoebae, so that it is impossible
to recognise mother and daughter in the two resulting organisms.
But in Luglypha and allied forms the existence of a shell
introduces a distinguishing mark by which it is possible to
discriminate between the products of fission ; so that the offspring
can be differentiated from the parent. The parent organism,
before division, builds the parts of the shell for the daughter form.
These parts are arranged on the surface of that part of the proto-
plasm, external to the old shell, which will be subsequently separated
as the daughter-cell. On this part the spicules are arranged and
unite to form the new shell. The division of the nucleus takes
place after that of the protoplasm, so that the daughter-cell is for
some time without a nucleus. Although we can in this species
recognise the daughter-cell for some time after separation from
the parent by the greater transparency of its younger shell, it is
nevertheless impossible to admit that the characteristics of the two
animals are in any way different, for just before the separation of
the two individuals a circulation of the protoplasm through both
shells takes place after the manner described in the text, and
there is therefore a complete intermingling of the substance of
the two bodies.

The difference between the products is even greater after trans-
verse fission of the Jnfusoria, for a new anus must be formed at
the anterior part and a new mouth posteriorly. It is not known
whether any circulation of the protoplasm takes place, as in Eu-
glypha. But even if this does not occur, there is no reason for

1 Cf. August Gruber, ‘Der Theilungsvorgang bei Euglypha alveolata,’ and ‘ Die
Theilung der monothalamen Rhizopoden,’ Z. f. W. Z., Bd. XXXV. and XXXVL.,
p- 104, 1881.

APPENDIX. 65

believing that the two products of division possess a different dura-
tion of life.

The process of fission in the Diatomaceae seems to me to be
theoretically important, because here, as in the previously-mentioned
Monothalamia (Euglypha, ete.), the new silicious skeleton is built
up within the primary organism, but not, as in Huglypha, for the
new individual only, but for both parent and daughter-cell alike ?.
If we compare the diatom shell to a box, then the two halves of
the old shell would form two lids, one for each of the products of
fission, while a new box is built up afresh for each of them. In
this case there is an absolute equality between the products of
fission, so far as the shell is concerned.

Note 11. REGENERATION.

A number of experiments have been recently undertaken, in
connection with a prize thesis at Wiirzburg, in order to test the
_ powers of regeneration possessed by various animals. In all essen-
tial respects the results confirm the statements of the older
observers, such as Spallanzani. Carriére has also proved that
snails can regenerate not only their horns and eyes, but also part
of the head when it has been cut off, although he has shown that
Spallanzani’s old statement that they can regenerate the whole
head, including the nervous system, is erroneous ?.

Note 12. Tue Duration or Lire in Pants.

The title of the work on this subject mentioned in the Text is
‘ Die Lebensdauer und Vegetationsweise der Pflanzen, ihre Ursache
und ihre Entwicklung, F. Hildebrand, Engler’s botanische Jahr-
biicher, Bd. IT. 1. und 2. Heft, Leipzig, 1881.

Nore 13.

[Many interesting facts and conclusions upon the subject of this
essay will be found in a volume by Professor E. Ray Lankester,
On comparative Longevity in Man and the lower Animals, Mac-
- millan and Co., 1870.—E. B. P.]

1 Cf. Victor Hensen, ‘ Physiologie d. Zeugung,’ p. 152.
2 Of. J. Carritre, ‘Ueber Regeneration bei Landpulmonaten,’ Tagebl. der 52.
Versammlg. deutsch. Naturf. pp. 225-226.

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ON HEREDITY.

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PREFACE.

Tue following essay was my inaugural lecture as Pro-Rector of
the University of Freiburg, and was delivered publicly in the hall
of the University, on June 21, 1883; it first appeared in print in
the following August. Only a few copies of the first edition were
available for the public, and it is therefore now reprinted as a second
edition, which only differs from the first in a few not unimportant
improvements and additions.

The title which I have chosen requires some explanation. I do
not propose to treat of the whole problem of heredity, but only
of a certain aspect of it—the transmission of acquired characters
which has been hitherto assumed to occur. In taking this course
I may say that it was impossible to avoid going back to the
foundation of all the phenomena of heredity, and to determine the
substance with which they must be connected. In my opinion
this can only be the substance of the germ-cells; and this sub-

stance transfers its hereditary tendencies from generation to ge-

neration, at first unchanged, and always uninfluenced in any corre-

sponding manner, by that which happens during the life of the
individual which bears it. If these views, which are indicated
rather than elaborated in this paper, be correct, all our ideas upon
the transformation of species require thorough modification, for the
whole principle of evolution by means of exercise (use and disuse),
as proposed by Lamarck, and accepted in some cases by Darwin,
entirely collapses.

The nature of the present paper—which is a lecture and not
an elaborate treatise—necessitates that only suggestions and not

70 ON HEREDITY.

an exhaustive treatment of the subject could be given. I have also
abstained from giving further details in the form of an appendix,
chiefly because I could hardly have attempted to complete a treat-
ment of the whole range of the subject, and I hope to refer
again to these questions in the future, when new experiments and
observations have been made.

I am very glad to see that such an important authority as
Pfliiger! has in the meantime come to the same opinion, from an
entirely different direction—an opinion which forms the founda-
tion of the views here brought forward, namely, that heredity
depends upon the continuity of the molecular substance of the

rm from generation to generation.
ge om gene 0 generatio Be 2

1 Pfliiger, ‘ Ueber den Einfluss der Schwerkraft auf die Theilung der Zellen und
auf die Entwicklung des Embryo,’ Arch. f. Physiol. Bd. XXXII, p. 68, 1883.

IL.

ON HEREDITY.

Wirn your permission I wish to bring before you to-day my
views on a problem of general biological interest—the problem of
heredity.

Heredity is the process which renders possible that persistence
of organic beings throughout successive generations, which is
generally thought to be so well understood and to need no special
explanation. Nevertheless our minds cannot fail to be much per-
plexed by the multiplicity of its manifestations, and to be greatly
puzzled as to its real nature. A celebrated German physiologist
' says?, ‘Although many hands have at all times endeavoured to
break the seal which hides the theory of heredity from our view,
the results achieved have been but small; and we are in a certain
degree justified in looking with little hope upon new efforts under-
taken in this direction. We must nevertheless endeavour from
time to time to ascertain how far we have advanced towards a
complete explanation.’

Such a course is in every way advisable, for we are not dealing
with phenomena which from their very nature are incomprehensible
by man. The great complexity of the subject has alone rendered it
hitherto insuperable, but in the province of heredity we certainly
have not reached the limits of attainable knowledge.

From this point of view heredity bears some resemblance to cer-
tain anatomical and physiological problems, e. g. the structure and
function of the human brain. Its structure—with so many millions
of nerve-fibres and nerve-cells—is of such extraordinary complexity
that we might well despair of ever completely understanding it.
Each fibre is nevertheless distinct in itself, while its connection
with the nearest nerve-cell can be frequently traced, and the function
of many groups of cell elements is already known. But it would
seem to be impossible to unravel the excessively complex network

1 Victor Hensen in his ‘ Physiologie der Zeugung, Leipzig, 1881, p. 216.

72 ON HEREDITY.

into which the cells and fibres are knit together; and hence to
arrive at the function of each single element appears to be also
beyond our reach. We have not however commenced to untie
this Gordian knot without some hope of success, for who can say
how far human perseverance may be able to penetrate into the
mechanism of the brain, and to reveal a connected structure and
a common principle in its countless elements? But surely this
work will be most materially assisted by the simultaneous in-
vestigation of the structure and function of the nervous system in
the lower forms of life—in the polypes and jelly-fish, worms and
Crustacea. In the same way we should not abandon the hope of
arriving at a satisfactory knowledge of the processes of heredity,
if we consider the simplest processes of the lower animals as well
as the more complex processes met with in the higher forms.

The word heredity 1 in its common acceptation, means that pro-
perty of an organism by which its peculiar nature is transmitted to.
its descendants. From an eagle’s egg an eagle of the same species
developes; and not only are the characteristics of the species
transmitted to the following generation, but even the individual
peculiarities. The offspring resemble their parents among animals
as well as among’ men.

On what does this common property of all organisms pa oras

Hackel was probably the first to describe reproduction as ‘an over-
growth of the individual,’ and he attempted to explain heredity as a
simple continuity of growth. This definition might be considered
as a play upon words, but it is more than this ; and such’ an inter-
pretation rightly applied, points to the only path which, in my
opinion, can lead to the comprehension of heredity.

Unicellular organisms, such as Rhizopoda and Infusoria, increase
by means of fission. Each individual grows to a certain size, and
then divides into two parts, which are exactly alike in size and
structure, so that it is impossible to decide whether one of them
is younger or older than the other. Hence in a certain sense these
organisms possess immortality: they can, it is true, be destroyed,
' but, if protected from a violent death, they would live on in-
definitely, and would only from time to time reduce the size of
their overgrown bodies by division. Each individual of any such
unicellular species living on the earth to-day is far older than man-
kind, and is almost as old as life itself.

ON HEREDITY. 73

From these unicellular organisms we can to a certain extent
‘understand why the offspring, being in fact a part of its parents,
must therefore resemble the latter. The question as to why the
part should resemble the whole leads us to a new problem, that of
assimilation, which also awaits solution. It is, at any rate, an
undoubted fact that the organism possesses the power of taking up
certain foreign substances, viz. food, and of converting them into
the substance of its own body.

Among these unicellular organisms, heredity depends upon the
continuity of the individual during the continual increase of its
body by means of assimilation.

But how is it with the multicellular organisms which do not
reproduce by means of simple division, and in which the whole
body of the parent does not pass over into the offspring ?

In such animals sexual reproduction is the chief means of mul-~—
- tiplication. In no case has it always been completely wanting,
and in the majority of cases it is the only kind of reproduction.

In these animals the power of reproduction is connected with \
certain cells which, as germ-cells, may be contrasted with those
which form the rest of the body; for the former have a totally
different réle to play; they are without significance for the life of
the individual !, and yet they alone possess the power of preserving
the species. Each of them can, under certain conditions, develope
into a complete organism of the same species as the parent, with
every individual peculiarity of the latter reproduced more or less
completely. How can such hereditary transmission of the characters
of the parent take place ? how can a single reproductive cell repro-
duce the whole body in all its details ?

Such a question could be easily answered if we were only con-
cerned with the continuity of the substance of the reproductive cells
from one generation to another; for this can be demonstrated
in some cases, and is very probable in all. In certain insects
the development of the egg into the embryo, that is the segmen-
tation of the egg, begins with the separation of a few small cells
from the main body of the egg. These are the reproductive cells,
and at a later period they are taken into the interior of the
animal and. form its reproductive organs. Again, in certain
small freshwater Crustacea (Daphnidae) the future reproductive

Pd

1 That is for the preservation of its life.

74 ON HEREDITY.

cells become distinct at a very early period, although not quite
at the beginning of segmentation, i.e. when the egg has divided
into not more than thirty segments. Here also the cells which are
separated early form the reproductive organs of the animal. The
separation ‘of the reproductive cells from those of the body takes
place at a still later period, viz. at the close of segmentation, in
Sagitta—a pelagic free-swimming form. In Vertebrata they do
not become distinct from the other cells of the body until the
embryo is completely formed. Thus, as their development shows,
a marked antithesis exists between the substance of the undying
reproductive cells and that of the perishable body-cells, We
cannot explain this fact except by the supposition that each re-
productive cell potentially contains two kinds of substance, which
at a variable time after the commencement of embryonic develop-
ment, separate from one another, and finally produce two sharply
contrasted groups of cells. |
It is evidently unimportant, as regards the question of heredity,

whether this separation takes place early or late, inasmuch as the
molecular constitution of the reproductive substance is determined
before the beginning of development. In order to understand the
growth and multiplication of cells, it must be conceded that all
protoplasmic molecules possess the power of growing, that is of
assimilating food, and of increasing by means of division. In the
same manner the molecules of the reproductive protoplasm, when
well nourished, grow and increase without altering their peculiar
nature, and without modifying the hereditary tendencies derived
from the parents. It is therefore quite conceivable that the re-
productive cells might separate from the somatic cells much la
than in the examples mentioned above, without changing the
hereditary tendencies of which they are the bearers. There may
be in fact cases in which such separation does not take place until
after the animal is completely formed, and others, as I believe that
I have shown!, in which it first arises one or more generations
later, viz. in the buds produced by the parent. Here also there is
no ground for the belief that the hereditary tendencies of the repro-
ductive molecules are in any way changed by the length of time
which elapses before their separation from the somatic molecules.

* Compare Weismann, ‘ Die Entstehung der Sexualzellen bei den Hydromedusen,’
Jena, 1883.

ON HEREDITY. 75

And this theoretical deduction is confirmed by observation, for from
the egg of a Medusa, produced by the budding of a Polype, a
Polype, in the first instance, and not a Medusa arises. Here the
molecules of the reproductive substance first formed part of the
Polype, and later, part of the Medusa bud, and, although they
separated from the somatic cells in the bud, they nevertheless
always retain the tendency to develope into a Polype.

We thus find that the reproduction of multicellular organisms is”
essentially similar to the corresponding process in unicellular forms ;
for it consists in the continual division of the reproductive cell ;
the only difference being that in the former case the reproductive
cell does not form the whole individual, for the latter is composed
of the millions of somatic cells by which the reproductive cell is
surrounded. The question, ‘How can a single reproductive cell
contain the germ of a complete and highly complex individual ?’
must therefore be re-stated more precisely in the following form,
‘ How can the substance of the reproductive cells potentially con- |
tain the somatic substance with all its characteristic properties ?’

The problem which this question suggests, becomes clearer when
we employ it for the explanation of a definite instance, such as the
origin of multicellular from unicellular animals. There can be
no doubt that the former have originated from the latter, and that
the physiological principle upon which such an origin depended, is
the principle of division of labour. In the course of the phyletic |
development of the organized world, it must have happened that
certain unicellular individuals did not separate from one another
immediately after division, but lived together, at first as equivalent
elements, each of which retained all the animal functions, including
that of reproduction. The Magosphaera planula of Hackel proves that
such perfectly homogeneous cell-colonies exist 1, even at the present
day. Division of labour would produce a differentiation of the single
cells in such a colony: thus certain cells would be set apart for ob-
taining food and for locomotion, while certain other cells would be
exclusively reproductive. In this way colonies consisting of somatic
and of reproductive cells must have arisen, and among these for

1 It is doubtful whether Magosphaera should be looked upon as a mature form ;
but nothing hinders us from believing that. species have lived, and are still living, in
which the ciliated sphere has held together until the encystment, that is the re-
production, of the constituent single cells.

76 ON HEREDITY.

the first time death appeared. For’in each case the somatic cells
must have perished after a certain time, while the reproductive
cells alone retained the immortality inherited from the Protozoa.
We must now ask how it becomes possible that one kind of cell
in such a colony can produce the other kind by division? Before
the differentiation of the colony each cell always produced others
similar to itself. How can the cells, after the nature of one part
_ of the colony is changed, have undergone such changes in their
nature that they can now produce more than one kind of cell?
Two theories can be brought forward to solve this problem. We
may turn to the old and long since abandoned xisus formativus,
or adapting the name to modern times, to a phyletie force of
development which causes the organism to change from time to
time. This vis a tergo or teleological force compels the organism to
undergo new transformations without any reference to the external
conditions of life. This theory throws no light upon the numerous
adaptations which are met with in every organism ; and it possesses
no value as a scientific explanation.
f Another supposition is that the primary reproductive cells are
\- influenced by the secondary cells of the colony, which, by their
. pit adaptability to the external conditions of life, have become somatic
gy cells: that the latter give off minute particles which entering into
\ the former, cause such changes in their nature that at the next
succeeding cell-division they are compelled to break up into dissimilar
parts.
At first sight this hypothesis seems to be quite reasonable. It is
not only conceivable that particles might proceed from the somatic
o the reproductive cells, but the very nutrition of the latter at the
xpense of the former is a demonstration that such a passage
ctually takes place. But a closer examination reveals immense
difficulties. In the first place, the molecules of the body devoured
are never simply added to those of the feeding individual without
undergoing any change, but as far as we know, they are really as-
similated 1, that is, converted into the molecules of the latter. We
cannot therefore gain much by assuming that a number of mole-
cules can pass from the growing somatic cells into the growing
reproductive cells, and can be deposited unchanged in the latter, so

* Or is an exception perhaps afforded by the nutritive cells of the egg, which
occur in many animals?

ON HEREDITY. 77

that, at their next division, the molecules are separated to become
the somatic cells of the following generation. How can such a
process be conceivable, when the colony becomes more complex,
when the number of somatic cells becomes so. large that they
surround the reproductive cells with many layers, and when at the
same time by an increasing division of labour a great number of
different tissues and cells are produced, all of which must originate
de novo from a single reproductive cell? Each of these various
elements must, ew hypothesi, give up certain molecules to the re-
productive cells; hence those which are in immediate contact with
the latter would obviously possess an advantage over those which
are more remote. If then any somatic cell must send the same
number of molecules to each reproductive cell1, we are compelled to
suspend all known physical and physiological conceptions, and
must make the entirely gratuitous assumption of an affinity on the
part of the molecules for the reproductive cells. Even if we admit
the existence of this affinity, its origin and means of control remain
perfectly unintelligible if we suppose that it has arisen from
differentiation of the complete colony. An unknown controlling
force must be added to this mysterious arrangement, in order to
marshal the molecules which enter the reproductive cell in such a
manner that their arrangement corresponds with the order in
which they must emerge as cells at a later period. In short, we
become lost in unfounded hypotheses.

Tt is well known that Darwin has attempted to explain the
phenomena of heredity by means of a hypothesis which corresponds
to a considerable extent with that just described. If we substitute
gemmules for molecules we have the fundamental idea of Darwin’s
provisional hypothesis of pangenesis. Particles of an excessively
minute size are continually given off from all the cells of the body;
these particles collect in the reproductive cells, and hence any
change arising in the organism, at any time during its life, is repre-
sented in: the reproductive cel] *. Darwin believed that he had by
this means rendered the transmission of acquired characters in-
telligible, a conception which he held to be necessary in order to

1 Or more precisely, they must give up as many molecules as would correspond to
the number of the kind of cell in question found in the mature organism.

2 See Darwin, ‘The Variation of Animals and Plants under Domestication,’ 1875,
vol. ii, chapter xxvii. pp. 349-399-

78 ON HEREDITY.

explain the development of species. He himself pointed out that
the hypothesis was merely provisional, and that it was only an ex-
pression of immediate, and by no means satisfactory knowledge of
these phenomena.

It is always dangerous to invoke some entirely new force in
order to understand phenomena which cannot be readily explained

_by the forces which are already known.

I believe that an explanation can in this case be reached by an
appeal to known forces, if we suppose that characters acquired (in the
true sense of the term) by the parent cannot appear in the course
of the development of the offspring, but that all the characters
exhibited by the latter are due to primary changes i rm.

' This supposition can obviously be made with regard to the
above-mentioned colony with its constituent elements differentiated
into somatic and reproductive. cells. It is conceivable that the
differentiation of the somatic cells was not primarily caused by a
change in their own structure, but that it was prepared for by

_ changes in the molecular structure of the reproductive cell from

pe ES

which the colony arose.

The generally received idea assumes that changes in the external
conditions can, in connection with natural selection, call forth per-
sistent changes in an organism; and if this view be accepted it
must be as true of all Metazoa as it is of unicellular or of homo-
geneous multicellular organisms. Supposing that the hypothe-
tical colonies, which were at first entirely made up of similar cells,
were to gain some advantages, if in the course of de@pment, the
molecules of the reproductive cells, from which each colony arose
became distributed irregularly in the resulting organism, there
would be a tendency towards the perpetuation of such % change,
wherever it appeared as the result of individual variability. As a
result of this change the colony would no longer remain homo-
geneous, and its cells would become dissimilar from the first,
because of the altered arrangement of the molecules in the repro-
ductive cells. Nothing prevents us from assuming that, at the
same time, the nature of a part of the molecule may undergo still
further change, for the molecules are by nature complex, and may
split up or combine together.

If then the reproductive cells have undergone such changes that
they can produce a heterogeneous colony as the result of continual

ee)

ON HEREDITY. 79

division, it follows that succeeding generations must behave in
exactly the same manner, for each of them is developed from a
portion of the reproductive cell from which the previous generation
arose, and consists of the same reproductive substance as the
latter.

From this point of view the exact manner in which we imagine
the subsequent differentiation of the colony to be potentially pre-
sent in the reproductiye cell, becomes a matter of comparatively
small importance. It may consist in a different molecular arrange-
ment, or in some change of chemical constitution, or it may be due
to both these causes combined. The essential point is that the dif-
ferentiation was originally due to some change in the reproductive

cells, just as this change itself produces all the differentiations which

appear in the ontogeny of all species at the present day. No one
doubts that the reason why this or that form of segmentation takes
place, or why this or that species finally appears, is to be found in
the ultimate structure of the reproductive cells. And,as a matter of
fact, molecular differentiation and grouping, whether present from
the beginning or first appearing in the course of development,
plays a rdle which can be almost directly observed in certain
species. The first segmentation furrow divides the ege of such
species into an opaque and a clear half, or, as is often the case
among Medusae, into a granular outer layer and a clear central
part, corresponding respectively with the ectoderm and endoderm
which are formed ata later period. Such early differentiations are
only the visible proofs of certain highly complex molecular re-
arrangements in the cells, and the fact appears to indicate that we
cannot be far wrong in maintaining that differentiations which
appear in the course of ontogeny depend upon the chemical and
physical constitution of the molecules in the reproductive cell.

At the first appearance of the earliest Metazoa alluded to above,
only two kinds of cells, somatic and reproductive, arose from the
seomentation of the reproductive cell. The reproductive cells thus
formed must have possessed exactly the same molecular structure

_as the mother reproductive cell, and would therefore pass through

precisely the same developmental changes. We can easily imagine
that all the succeeding stages in the development of the Metazoa
have been due’ to the same causes which were efficient at the

earliest period. Variations in the’ molecular structure—of the

ee

80 ON HEREDITY.

reproductive cells would continue—-te-appear;and—these—would_he
_increased_and_rendered permanent by means of natural selection,
when their results, in the alteration of certain cells in the body,
were advantageous to the species. The only condition necessary
for the transmission of such changes is that a part of the repro-

ductive substance (the germ-plasm) should always remai
during segmentation and the subsequent building up of the body,

or in other words, that such unchanged substance should pass into
the organism, and after the lapse of a variable period, should reappear
as the reproductive cells. Only in this way can we render to some
extent intelligible the transmission of those changes which have
arisen in the phylogeny of the species; only thus can we imagine
the manner in which the first somatic cells gradually developed
in numbers and in complexity.

It is only by supposing that these changes arose from molecular
alterations in the reproductive cell that we can understand how the
reproductive cells of the next generation can originate the same
changes in the cells which are developed from them; and it is
impossible to imagine any way in which the transmission of changes,
produced by the direct action of external forces upon the somatic
cells, can be brought about ?.

The difficulty or the impossibility of rendering the transmission
of acquired characters intelligible by an appeal to any known force
has been often felt, but no one has hitherto attempted to cast doubts
upon the very existence of such a form of heredity.

There are two reasons for this: first; observations have been
recorded which appear to prove the existence of such transmission ;
and secondly, it has seemed impossible to do without the supposition
of the transmission of acquired characters, because it has always
played such an important part in the explanation of the trans-
formation of species.

It is perfectly right to defer an explanation, and to hesitate

1 To this class of phenomena of course belong those acts of will which call forth
the functional activity of certain groups of cells. It is quite clear that such im-
pulses do not originate in the constitution of the tissue in question, but are due to the
operation of external causes. The activity does not arise directly from any natural
disposition of the germ, but is the result of accidental external impressions. A
domesticated duck uses its legs in a different manner from, and more frequently than
a wild duck, but such functional changes are the consequence of changed external
conditions, and are not due to the constitution of the germ.

ON HEREDITY. 81

before we declare a supposed phenomenon to be impossible, because
we are unable to refer it to any of the known forces. No one can
believe that we are acquainted with all the forces of nature. But,
on the other hand, we must use the greatest caution in dealing
with unknown forces; and clear and indubitable facts must be
brought forward to prove that the supposed phenomena have a real
existence, and that their acceptance is unavoidable.

It has never been proved that acquired characters are trans-
mitted, and it has never been demonstrated that, without the aid
of such transmission, the evolution of the organie world becomes
unintelligible.

The inheritance of acquired characters has never been proved,
either by means of direct observation or by experiment’. It must
‘be admitted that there are in existence numerous descriptions of
eases which tend to prove that such mutilations as the loss of
fingers, the scars of wounds, ete., are inherited by the offspring,
‘but in these descriptions the previous history is invariably obscure,
cand hence the evidence loses all scientific value.

As a typical example of the scientific value of such cases I may
mention the frequently quoted instance of the cow, which lost its
left horn from suppuration, induced by some ‘ unknown cause,’ and
which afterwards produced two calves with a rudimentary left horn
in each case. But as Hensen? has rightly remarked, the loss of
the cow’s horn may have arisen from a congenital malformation,
which would certainly be transmitted, but which was not an ac-
quired character.

The only cases worthy of scientific discussion are the well-known
experiments upon guinea-pigs, conducted by the French physiologist
Brown-Séquard. But the explanation of his results is, in my
opinion, open to discussion. In these cases we have to do with
the apparent transmission of artificially produced malformations.
The division of important nerves, or of the spinal cord, or the

1 Upon this subject Pfliiger states—‘I have made myself accurately acquainted
with all facts which are supposed to prove the inheritance of acquired characters,—
_ that is of characters which are not due to the peculiar organization of the ovum and
spermatozoon from which the individual is formed, but which follow from the in-
cidence of accidental external influences upon the organism at any time in its life. —
Not one of these facts can be accepted as a proof of the transmission of acquired
characters.’ 1. c. p. 68.
2 «Physiologie der Zeugung.’

82 . ON HEREDITY.

removal of parts of the brain, produced certain symptoms which
reappeared in the descendants of the mutilated animals. Epilepsy
was produced by dividing the great sciatic nerve; the ear became
deformed when the sympathetic nerve was severed in the throat ;
and prolapsus of the eye-ball followed the removal of a certain
part of the brain—the corpora restiformia. All these effects were
said to be transmitted to the descendants as far as the fifth or sixth
generation.

But we must inquire whether these cases are really due to here-
dity and not to simple infection. In the case of epilepsy, at
any rate, it is easy to imagine that the passage of some specific
organism through the reproductive cells may take place, as in —
the case of syphilis. We are, however, entirely ignorant of the
nature of the former disease. This suggested explanation may
not perhaps apply to the other cases: but we must remember that
animals which have been subjected to such severe operations upon
the nervous system have sustained a great shock, and if they
are capable of breeding, it is only probable that they will produce
weak descendants, and such as are easily affected by disease. Such
a result does not however explain why the offspring should suffer
from the same disease as that which was artificially induced in
. the parents. But this does not appear to have been by any means
invariably the case. Brown-Séquard himself says, ‘The changes
in the eye of the offspring were of a very variable nature, and
were only occasionally exactly similar to those observed in the
parents.’ |

There is no doubt, however, that these experiments demand
careful consideration, but before they can claim scientific recogni-
tion, they must be subjected to rigid criticism as to the precautions
taken, the number and nature of the control experiments, ete.

Up to the present time such necessary conditions have not been
sufficiently observed. The recent experiments themselves are only
described in short preliminary notices, which, as regards their accu-
racy, the possibility of mistake, the precautions taken, and the exact
succession of individuals affected, afford no data upon which a
scientific opinion can be founded. Until the publication of a com-
plete series of experiments, we must say with Du Bois Reymond ?,
‘The hereditary transmission of acquired characters remains an

1 See ‘Ueber die Uebung,’ Berlin, 1881.

ON HEREDITY. 83

unintelligible hypothesis, ik is only deduced from the facts
which it attempts to explain.’

We therefore naturally ask whether the hypothesis is really
necessary for the explanation of known facts.

At the first sight it certainly seems to be necessary, and it
appears rash to attempt to dispense with its aid. Many phenomena
only appear to be intelligible if we assume the hereditary trans-
mission of such acquired characters as the changes which we
ascribe to the use or disuse of particular organs, or to the direct
influence of climate.. Furthermore, how can we explain instinct
as hereditary habit unless it has gradually arisen by the accumula-
tion, , through heredity, of habits which were practised in succeeding
generations ?

I will now attempt to prove that even these cases, so far as they
depend upon clear and indubitable facts, do not force us to accept
the supposition of the transmission of acquired characters. |

It seems difficult and well nigh impossible to deny the transmis-
sion of acquired characters when we remember the inflaence which

use and disuse have exercised upon certain special organs. It is |
well known that Lamarck attempted to explain the structure of |

the organism as almost entirely due to this principle alone. Accord- |

ing to his theory the long neck of the giraffe arose by constant \

stretching after the leaves of trees, and the web between the toes
of a water-bird’s foot by the extension of the toes, in an attempt
to oppose as large a surface of water as possible in swimming. There
ean be no doubt that those muscles which are frequently used
increase in size and strength, and that glands which often enter
into activity become larger and not smaller, and that their func-
tional powers increase. Indeed, the whole effect which exercise
produces upon the single parts of the body is dependent upon the
fact that frequently used organs increase in strength. This con-
clusion also refers to the nervous system, for a pianist who per-
forms with lightning rapidity certain pre-arranged, highly complex,
and combined movements of the muscles of his hands and fingers
has, as Du Bois Reymond pointed out, not only exercised the
muscles, but also those ganglionic centres of the brain which deter-
mine the combination of muscular movement. Other functions
of the brain, such as memory, can be similarly increased and
strengthened by exercise, and the question to be settled is whether
G 2

;

|
:

:

ee

84 " ON HEREDITY.

characters acquired in this way by exercise and practice can be
transmitted to the following generations. Lamarck’s theory
assumes that such transmission takes place, for without it no
accumulation or increase of the characters in question would be
possible, as a result of their exercise during any number of successive
generations.
f~ Against this we may urge that whenever, in the course of
nature, an organ becomes stronger by exercise, it must possess a
certain degree of importance for the life of the individual, and when
this is the case it becomes subject to improvement by natural
selection, for only those individuals which possess the organ in its
most perfect form will be able to reproduce them. The perfection
of form of an organ does not however depend upon the amount
of exercise undergone by it during the life of the organism, but
primarily and principally upon the fact that the germ from which
the individual arose was predisposed to produce a perfect organ.
The increase to which any organ can attain by exercise during a
single life is bounded by certain limits, which are themselves fixed
by the primary tendencies of the organ in question./ We cannot
by excessive feeding make a giant out of the germ destined to
form a dwarf; we cannot, by means of exercise, transform the
muscles of an individual destined to be feeble into those of a
Hercules, or the brain of a predestined fool into that of a Leibnitz
or a Kant, by means of much thinking. With the same amount
of exercise the organ which is destined to be strong, will attain
a higher degree of functional activity than one that is destined to
be weak. Hence natural selection, in destroying the least fitted
individuals, destroys those which from the germ were feebly dis-
posed ./ Thus the result of exercise during the individual life does
not : acquire so much importance, for, as compared with differences
in predisposition, the amount of exercise undergone by all the
individuals of a species becomes relatively uniform./ The increase
of an organ in the course of generations does not depend upon
the summation of the exercise taken during single lives, but
upon the summation of more favourable predispositions in the
gemma 208 TT FY

In criticizing these arguments, it may be questioned whether the
single individuals of a species which is undergoing modification do,
as a matter of fact, exercise themselves in the same manner and to

ON HEREDITY. ' 85

the same extent. But the consideration of a definite example
-clearly shows that this must be the case. When the wild duck
became domesticated, and lived in a farm-yard, all the individuals
were compelled to walk and stand more than they had done
previously, and the muscles of the legs were used to a correspond-
ingly greater degree. The same thing happens in the wild state,
when any change in the conditions of life compels an organ to be
more largely used. No individual will be able to entirely avoid
this extra use, and each will endeavour to accommodate itself to
the new conditions according to its power. The amount of this
power depends upon the predisposition of the germ; and natural
selection, while it apparently decides between individuals of various
degrees of strength, is in truth operating upon the stronger and
weaker germs.

' But the very conclusions which have been drawn from the
increase of activity which has arisen from exercise, must also be
drawn from the instances of atrophy or degeneration following from
the disuse of organs.

Darwin long ago called attention to the fact that the degeneration
of an organ may, under certain circumstances, be beneficial to the
species. For example, he first proved in the instance of Madeira,
that the loss of wings may be of advantage to many beetles inhabit-
ing oceanic islands. The individuals with imperfectly developed or
atrophied wings have an advantage, because they are not carried
out to sea by the frequent winds. The small eyes, buried in fur,
possessed by moles and other subterranean mammals, can be similarly
explained by means of natural selection. So also, the complete dis-
appearance of the limbs of snakes is evidently a real advantage to
animals which creep through narrow holes and clefts ; and the de-
generation of the wings in the ostrich and penguin is, in part,
explicable as a favourable modification of the organ of flight into
an organ for striking air or water respectively.

But when the degeneration of disused organs confers no benefits.
upon the individual, the explanation becomes less simple. Thus:
we find that the eyes of animals which inhabit dark caves (such
as Insects, crabs, fish, Amphibia, ete.) have undergone degeneration ;
yet this can hardly be of direct advantage to the animals, for they:
could live quite as well in the dark with well-developed eyes. But
we are here brought into contact with a very important aspect of

86 ON HEREDITY.

natural selection, viz. the power of conservation exerted by it.
Not only does the survival of the fittest select the best, but it also»
maintains it’. The struggle for existence does not cease with the
foundation of a new specific type, or with some perfect adaptation
to the external or internal conditions of life, but it becomes, on the
contrary, even more severe, so that the most minute differences of
structure determine the issue between life and death.

The sharpest sight possessed by birds is found in birds of prey,
but if one of them entered the world with eyes rather below
the average in this respect, it could not, in the long run, escape
death from hunger, because it would always be at a disadvantage as
compared with others.

Hence the sharp sight of these birds is maintained by means of .
the continued operation of natural selection, by which the indi-
viduals with the weakest sight are being continually exterminated.
But all this would be changed at once, if a bird of prey of a certain
species were compelled to live in absolute darkness. The quality of
the eyes would then be immaterial, for it could make no difference
to the existence of the individual, or the maintenance of the species.
The sharp sight might, perhaps, be transmitted through numerous
generations; but when weaker eyes arose from time to time, these
would also be transmitted, for even very short-sighted or imperfect
eyes would bring no disadvantage to their owner. Hence, by con-
tinual crossing between individuals with the most varied degrees
of perfection in this respect, the average of perfection would gradu-
ally decline from the point attained before the species lived in the |
dark.

We do not at present know of any bird living in perfect
darkness, and it is improbable that such a bird will ever be found; .
but we are acquainted with blind fish and Amphibia, and among
these the eyes are present it is true, but they are small and hidden
under the skin. I think it is difficult to reconcile the facts of the
case with the ordinary theory that the eyes of these animals have
simply degenerated through disuse. If disuse were able to bring
about the complete atrophy of an organ, it follows that every trace
of it would be effaced. We know that, as a matter of fact, the
olfactory organ of the frog completely degenerates when the olfactory

1 This principle was, I believe, first pointed out by Seidlitz. Compare Seidlitz,
‘ Die Darwin’sche Theorie,’ Leipzig, 1875, p. 198.

ON HEREDITY. a

nerve is divided; and that great degeneration of the eye may be
brought about by the artificial destruction of the optic centre in
the brain. Since, therefore, the effects of disuse are so striking in
a single life, we should certainly expect, if such effects can be trans-
mitted, that all traces of an eye would soon disappear from a species
which lives in the dark.

The caverns in Carniola and Carinthia, in which the blind Proteus
and so many other blind animals live, belong geologically to the
Jurassic formation ; and although we do not exactly know when for
example the Proteus first entered them, the low organization of this
amphibian certainly indicates that it has been sheltered there for
a very long period of time, and that thousands of generations of
this species have succeeded one another in the caves.

Hence there is no reason to wonder at the extent to which the
degeneration of the eye has been already carried in the Proteus ;
even if we assume that it is merely due to the cessation of the
conserving influence of natural selection?

But it is unnecessary to depend upon this assumption alone, for
when a useless organ degenerates, there are also other factors which
demand consideration, namely, the higher development of other
organs which compensate for the loss of the degenerating structure, |
or the inerease in size of adjacent parts. If these newer develop-
ments are of advantage to the species, they finally come to take
the place of the organ which natural selection has failed to
preserve at its point of highest perfection.

In the first place, a certain form of correlation, which Roux*
calls ‘the struggle of the parts in the organism, plays a most
important part. Cases of atrophy, following disuse, appear to be
always attended by a corresponding increase of other organs: blind
animals always possess very strongly developed organs of touch,
hearing, and smell, and the degeneration of the wing-muscles of
the ostrich is accompanied by a great increase in the strength of
the muscles of the leg. If the average amount of food which an
animal can assimilate every day remains constant for a considerable
time, it follows that a strong influx towards one organ must be
accompanied by a drain upon others, and this tendency will increase,
from generation to generation, in proportion to the development of

1 W. Roux, ‘Der Kampf der Theile im Organismus,’ Leipzig, 1881.

en

88 ON HEREDITY.

the growing organ, which is favoured by natural selection in its
increased blood-supply, ete.; while the operation of natural selection
has also determined the organ which can bear a corresponding loss
without detriment to the organism as a whole.

Without the operation of natural selection upon different indi-
viduals, the struggle between the organs of a single individual
would be unable to encourage a predisposition in the germ towards
the degeneration or non-development of a useless organ, and it
could only limit and degrade the development of an organ in the
lifetime of the individual. / If, therefore, acquired characters are not
transmitted, the disposition to develope such an organ would be
present in the same degree in each successive generation, although

the realization would be less perfect. The complete disappearance

of a rudimentary organ can only take place by the operation of
natural selection; this principle will lead to its elimination, inas-
much as the disappearing structure takes the place and the nutriment
of other useful and important organs. Hence the process of natural
selection tends to entirely remove the former. The predisposition
towards a weaker development of the organ is thus advantageous,
and there is every reason for the belief that the advantages would
continue to be. gained, and that therefore the processes of natural
selection would remain in operation, until the germ had entirely
lost all tendency towards the development of the organ in question.
The extreme slowness with which this process takes place, and the
extraordinary persistence of rudimentary organs, at any rate in
the embryo, together with their gradual but finally complete dis-
appearance, can be clearly seen in the limbs of certain vertebrates
and arthropods. The blind-worms have no limbs, but a rudi-
mentary shoulder-girdle is present close under the skin, and the
interesting fact has been quite recently established’ that the fore-
limbs are present in the embryo in the form of short stumps, which
entirely disappear at a later stage. In most snakes all tracés of
limbs have been lost in the adult, but we do not yet know for
certain whether they are also wanting in the embryo. I might
further mention the very different stages of degeneration witnessed
in the limbs of various salamanders; and the anterior limbs of
Hesperornis—the remarkable toothed bird from the eretaceous rocks

* Compare Born in ‘ Zoolog. Anzeiger,’ 1883, No. 150, p. 537-

ON HEREDITY. 89

—which, according to Marsh’, consists only of a very thin and
relatively small humerus, which was probably concealed beneath the
skin. The water-fleas (Daphnidae) possess in the embryonic state
three complete and almost equal pairs of jaws, but two of these
entirely disappear, and do not develope into jaws in any species.
In the same way, the embryo of the maggot-like legless larva of
bees and wasps possesses three pairs of ancestral limbs.

There are, however, cases in which, apparently, acquired variations
of characters are transmitted without natural selection playing any
active part in the change. Such a case is afforded by the short-
sightedness so common in civilized nations.

This affection is certainly hereditary in some cases, and it may
well have been explained as an example of the transmission of
acquired changes. It has been argued, that acquired short-sighted-
ness can be in a slight degree transmitted, and that each successive
generation has developed a further degree of the disease by habitu-
ally holding books ete. close to the eyes, so that the inborn pre-
disposition to short-sightedness is continually accumulating.

But we must remember that variations in the refraction of the
human eye have been for a long time independent of the pre-
serving control of natural selection. In the struggle for existence,
a blind man would certainly disappear before those endowed with
sight, but myopia does not prevent any one from gaining a living.

A short-sighted lynx, hawk, or gazelle, or even a short-sighted
Indian, would be eliminated by natural selection, but a short-sighted
European of the higher class finds no difficulty in earning his
bread.

- Those fluctuations on either side of the average which we call
myopia and hypermetropia, occur in the same manner, and are due to
the same causes, as those which operate in producing degeneration in
the eyes of cave-dwelling animals. If, therefore, we not infrequently
meet with families in which myopia is hereditary, such results m: ;
be attributed to the transmission of an accidental disposition on tlic
part of the germ, instead of to the transmission of acquired short-
sightedness. A very large proportion of short-sighted people do
not owe their affliction to inheritance at all, but have acquired it
for themselves; for there is no doubt that a normal eye may be

1 QO. C. Marsh, ‘Odontornithes, a Monograph on the extinct toothed Birds of
North America’ Washington, 1880.

90 ON HEREDITY.

rendered myopic in the course of a life-time by continually looking
at objects from a very short distance, even when no hereditary
predisposition towards the disease can be shown to exist. Such a
change would of course appear more readily if there was also a
corresponding predisposition on the part of the eye. But I should
not explain this widely spread predisposition towards myopia as
due to the transmission of acquired short-sightedness, but to the
greater variability of the eye, which necessarily results from the
cessation of the controlling influence of natural selection.

This suspension of the preserving influence of natural selection
may be termed ‘Panmivia’ for all individuals can reproduce them- |
selves and thus stamp their characters upon the species, and not
only those which are in all respects, or in respect to some single organ,
the fittest. In my opinion, the greater number of those variations
which are usually attributed to the direct influence of external
conditions of life, are to be ascribed to panmixia. For example, the
great variability of most domesticated soci essentially depends
upon this principle.

A goose or a duck must , possess strong powers of flight in the
natural state, but such powers are no longer necessary for obtaining
food when it is brought into the poultry-yard, so that a rigid selec-
tion of individuals with well-developed wings, at once ceases among
its descendants. Hence in the course of generations, a deterioration
of the organs of flight must necessarily ensue, and the other
members and organs of the bird will be similarly affected.

This example very clearly indicates that the degeneration of an
organ does not depend upon its disuse ; for although our domestic
poultry very rarely make use of their wings, the muscles of flight
have not disappeared, and, at any rate in the goose, do not seem
to have undergone any marked degeneration.

The numerous and exact observations conducted by Darwin
upon the weight and measurement of the bones in domestic fowls,
seem to me to possess a significance beyond that winels he attributed
to them.

If the weight of the wing-bones of the domestic duck bears -
a smaller proportion to the weight of the leg-bones than in the
wild duck, and if, as Darwin rightly assumes, this depends not
only upon the diminution of the wings, but also upon the increase
of the legs, it by no means follows that this latter increase in

ON HEREDITY. 91

organs which are now more frequently used, is dependent upon
hereditary influences alone.

It is quite possible that it. depends, on the one hand, upon the
suspension of natural selection, or panmixia (and these effects would
be transmitted), and on the other hand upon the direct influence
of increased use during the course of a single life. We do not
yet know with any accuracy, the amount of change which may be
produced by increased use in the course of a single life. If it is
desired to prove that use and disuse produce hereditary effects
without the assistance of natural selection, it will be necessary to
domesticate wild animals (for example the wild duck) and preserve
all their descendants, thus excluding the operation of natural selec-
tion. If then all individuals of the second, third, fourth and later
generations of these tame ducks possess identical variations, which
increase from generation to generation, and if the nature of these

changes proves that they must have been due to the effect of use _

or disuse, then perhaps the transmission of such effects may be-
admitted ; but it must always be remembered that domestication

itself influences the organism,—not only directly, but also indirectly,

by the increase of variability as a result of the suspension of natural

selection. Such experiments have not yet been carried out in

sufficient detail 1.

It is usually considered that the origin and variation of instincts
are also dependent upon the exercise of certain groups of muscles and
nerves during a single life-time; and that the gradual improve-_
. ment which is thus caused by practice, is accumulated by hereditary
transmission. I believe that this is an entirely erroneous view,
and I hold that all instinct is entirely due to the operation of natural
selection, and has its foundation, not upon inherited experiences,
but upon the variations of the germ.

Why, for instance, should not the instinct to fly from enemies
have arisen by the survival of those individuals which are naturally
timid and easily startled, together with the extermination of those
which are unwary? It may be urged in opposition to this explana-
tion that the birds of uninhabited islands which are not at first

shy of man, acquire in a few generations an instinctive dread of .

. him, an instinct which cannot have arisen in so short a time

1 C, Darwin, ‘Variation of Animals and Plants under Domestication” Vol. I.

92 ON HEREDITY.

by means of natural selection. But in this case are we really
dealing with the origin of a new instinct, or only with the addition
of one new perception (‘Wahrnehmung,’ Schneider), of the same
kind as those which incite to the instinct of flight—an instinct
which had been previously developed in past ages but had never
been called forth by man? Again, has any one ascertained whether
the young birds of the second or third generation are frightened
by man? May it not be that the experience of a single life-time
plays a great part in the origin of the habit? For my part, I am
inclined to believe that the habit of flying from man is developed
in the first generation which encounters him as a foe*. We see
how wary and cautious a flock of birds become as soon as a few
shots have been fired at them, and yet shortly before this occur-
rence they were perhaps playing carelessly close to the sportsmen.
Intelligence plays a considerable part in the life of birds, and
it by no means follows that the transmission of individual habits
explains the above-mentioned phenomena. The long-continued
operation of natural selection may very well have been necessary
before the perception of man could awake the instinct to flee in
young, inexperienced birds. Unfortunately the observations upon
these points are far too indefinite to enable us to draw conclusions.
There is again the frequently-quoted instance of the young
pointer, ‘which, untrained, and without any example which might
have been imitated, pointed at a lizard in a subtropical jungle, just
as many of its forefathers had pointed at partridges on the plain of
St. Denis,’ and which, without knowing the effect of a shot, sprang
forward barking, at the first discharge, to bring in the game. This
conduct must not be attributed to the inheritance of any mental
picture, such as the effect of a shot, but to the inheritance of a
certain reflex mechanism. The young pointer does not spring
forward at the shot because he has inherited from his forefathers a

1 Compare ‘ Der thierische Wille,’ Leipzig, 1880.

? Steller’s interesting account of the Sea-cow (Rhytina Stellert) proves that this
suggestion is valid. This, large mammal was living in great numbers in Behring
Strait at the end of the last century, but has since been entirely exterminated by
man. Steller, who was compelled by shipwreck to remain in the locality for a whole
‘year, tells us that the animals were at first without any fear of man, so that they
could be approached in boats and could thus be killed. After a few months how-
ever the survivors became wary, and did not allow Steller’s men to approach them,
so that they were difficult to catch.—A. W., 1888.

ON. HEREDITY. , 93

certain association of ideas,—shot and game,—but because he has
inherited a reflex mechanism, which impels him to start forward
on hearing a report. We cannot yet determine without more ex-
periments how such an. impulse due to perception (“Wahrnehmung-
strieb, Schneider) has arisen; but, in my opinion, it is almost in-
conceivable that artificial breeding has had nothing to do with it ;
and that we are here concerned—not with the inheritance of the
effects of training—but with some pre-disposition on the part of the
germ, which has been increased by artificial selection.

The necessity for extreme caution in appealing to the supposed
hereditary effects of use, is well shown in the case of those
numerous instincts, which only come into play once in a life-
time, and which do not therefore admit of improvement by practice.
The queen-bee takes her nuptial flight only once, and yet how
many and complex are the instincts and the reflex mechanisms
which come into play on that occasion. Again, in many insects
' the deposition of eggs occurs but once in a life-time, and yet
such insects always fulfil the necessary conditions with unfailing
accuracy, either simply dropping the eggs into water, or carefully
fixing them on the-surface of the earth beneath some stone, or
laying them on a particular part of a certain species of plant ; and
in all these cases the most complicated actions are performed. It
is indeed astonishing to watch one of the Cynipidae (Rhodites
vosae) depositing her eggs in the tissue of a young bud. She first
carefully examines the bud on all sides, and feels it with her legs
and antennae. Then she slowly inserts her long ovipositor between
the closely-rolled leaves of the bud, but if it does not reach exactly
the right spot, she will withdraw and re-insert it many times, until
at length, when the proper place has been found, she will slowly
bore deep into the very centre of the bud, so that the egg will
reach the exact spot, where the necessary conditions for its develop-
ment alone exist.

But each Cynips lays eggs many times, and it may be argued that
practice may have led to improvement in this case; we cannot
however, as a matter of fact, expect much improvement in a process
which is repeated, perhaps a dozen times, at short intervals of
time, and which is of such an excessively complex nature.

It is the same with the deposition of eggs in most insects. How
can practice have had any influence upon the origin of the instinct

94, ‘ ON HEREDITY.

which leads one of our butterflies—( Vanessa levana)—to lay its green
eggs in single file, as columns, which project freely from the stem
or leaf, so that protection is gained by their close resemblance to the
* flower-buds of the stinging-nettle, which forms the food-plant of
their caterpillars ?

Of course the butterfly is not aware of the advantage which
follows from such a proceeding; intelligence has no part in the
process. The entire operation depends upon certain inherent. ana-
tomical and physiological arrangements:—on the structure of the
ovary and oviducts, on the simultaneous ripening of a certain
number of eggs, and on certain very complex reflex mechanisms
which compel the butterfly to lay its eggs on certain parts of
certain plants. Schneider is certainly right when he maintains that
this mechanism is released by a sensation, arising from. the percep-
tion (whether by sight or smell, or both together) of the particular
plant or part of the plant upon which the eggs are to be laid?
At any rate, we cannot, in such cases, appeal to the effects of ©
constant use and the transmission of acquired characters, as an
explanation ; and the origin of the impulse can only be understood
as a result of the process of natural selection.

The protective cocoons by which the pupae of many insects are
surrounded also belong to the same category, and improvement by
practice is entirely out of the question, for they are only constructed
once in the course of a life-time. And yet these cocoons are often
remarkably complex: think, for instance, of the cocoon spun by the ~
caterpillar of the emperor moth (Saturnia carpini), which is so
tough that it can hardly be torn, and which the moth would be
unable to leave, if an opening were not provided for the purpose ;
while, on the other hand, the pupa would not be defended against
enemies if the opening were not furnished with a circle of pointed
bristles, converging outwards, on the principle of the lobster pot, so
that the moth can easily emerge, although no enemy can enter.
The impulse which leads to the production of such a structure can
only have arisen by the operation of natural selection—not, of
course, during the history of a single species, but during the de-
velopment of numerous, consecutive species—by gradual and un-
ceasing improvements in the initial stages of cocoon-building.

.
* Compare Schneider, ‘ Der thierische Wille.’

ON HEREDITY. : : 95

A number of species exists at the present day, of which the cocoons
can be arranged in a complete series, becoming gradually less and
less complex, from that described above, down to a loosely-con-
structed, spherical case in which the pupa is contained.

The cocoon spun by the larva of Saturnia carpini differs but little
in complexity from the web of the spider, and if the former is con-
structed without assistance from the experience of the single
individual—and this must certainly be admitted—it follows that the
latter may be also built without the aid of experience, while there is
neither reason nor necessity for appealing to the entirely unproved
transmission of acquired skill in order to explain this and a thousand
other operations.

It may be objected that, in man, in addition to the instincts
inherent in every individual, special individual predispositions are
also found, of such a nature that it is impossible that they can
have arisen by individual variations of the germ. On the other
hand, these predispositions—which we call talents—cannot have
arisen through natural selection, because life is in no way dependent
upon their presence, and there seems to be no way of explaining
their origin except by an assumption of the summation of the skill
attained by exercise in the course of each single life. In this case,
therefore, we seem at first sight to be compelled to accept the
transmission of acquired characters.

Now it cannot be denied that all predispositions may be improved
by practice during the course of a life-time,—and, in truth, very
remarkably improved. If we could explain the existence of great
talent, such as, for example, a gift for music, painting, sculpture, or
mathematics, as due to the presence or absence of a special organ in
the brain, it follows that we could only understand its origin and
increase (natural selection being excluded) by accumulation, due to
the transmission of the results of practice through a series of
generations. But talents are not dependent upon the possession of
special organs in the brain. They are not simple mental dis-
positions, but combinations of many dispositions, and often. of
a most complex nature: they depend upon a certain degree of
irritability, and a power of readily transmitting impulses along the
_ nerve-tracts of the brain, as well as upon the especial development
of single parts of the brain. In my opinion, there is absolutely no
trustworthy proof that talents have been improved by their exercise

96 ON HEREDITY.

through the course of a long series of generations. The Bach
family shows that musical talent, and the Bernoulli family that
mathematical power, can be transmitted from generation to genera-
tion, but this teaches us nothing’ as to the origin of such talents.
In both families the high-water mark of talent lies, not at the end
of the series of generations, as it should do if the results of practice
are transmitted, but in the middle. Again, talents frequently
appear in some single member of a family which has not been
previously distinguished. :

Gauss was not the son of a mathematician; Handel’s father was
a surgeon, of whose musical powers nothing is known; Titian was
the son and also the nephew of a lawyer, while he and his brother,
Francesco Vecellio, were the first painters in a family which pro-
duced a succession of seven other artists with diminishing talents.
These facts do not, however, prove that the condition of the nerve-
tracts and centres of the brain, which determine the specific talent,
appeared for the first time in these men: the appropriate condition

surely existed previously in their parents, although it did not ~
achieve expression. They prove, as it seems to me, that a high
degree of endowment in a special direction, which we call talent,
cannot have arisen from the experience of previous generations, that
is, by the exercise of the brain in the same specific direction.

It appears to me that talent consists in a happy combination of
exceptionally high gifts, developed in one special direction. At
present, it is of course impossible to understand the physiological
conditions which render the origin of such combinations possible,
but it is very probable that the crossing of the mental dispositions
of the parents plays a great part in it. This has been admirably
and concisely expressed by Goethe in describing his own charac-

teristics—
Vom Vater hab’ ich die Statur
Des Lebens ernstes Fiihren,
Vom Miitterchen die Frohnatur
Die Lust zum Fabuliren, etc.

The combination of talents frequently found in one individual,
and the appearance of different remarkable talents in the various
branches of one and the same family, indicate that talents are only
special combinations of certain highly-developed mental dispositions
which are found in every brain. Many painters have been admir-
able musicians, and we. very frequently find both these talents

ON HEREDITY. 97

developed to a slighter extent in a single individual. In the
Feuerbach family we find a distinguished jurist, a remarkable philo-
sopher, and a highly-talented artist ; and among the Mendelssohns
a philosopher as well as a musician.

The sudden and yet widespread appearance of a particular talent
in correspondence with the general intellectual excitement of a
certain epoch points in the same direction. How many poets arose
in Germany during the period of sentiment which marked the
close of the last century, and how completely all poetic gifts seem
to have disappeared during the Thirty Years’ War. How numerous
were the philosophers that appeared in the epoch which succeeded
Kant; while all philosophic talent seemed to have deserted the
German nation during the sway of the antagonistic ‘exact science,’
with its contempt for speculation.

Wherever academies are founded, there the Schwanthalers,
Defreggers, and Lenbachs emerge from the masses which had
shown no sign of artistic endowment through long periods of time 1.
At the present day there are many men of science who, had they
lived at the time of Burger, Uhland, or Schelling, would probably
have been poets or philosophers. And the man of science also can-
not dispense with that mental disposition directed in a certain course,
which we call talent, although the specific part of it may not be
so obvious: we may, indeed, go further, and maintain that the
Physicist and the Chemist are characterized by a combination of
mental dispositions which differ from those of the Botanist and the
Zoologist. Nevertheless, a man is not born a physicist or a
botanist, and in most cases chance alone determines whether his
endowments are developed in either direction.

Lessing’) has asked whether Raphael would have been a less
distinguished artist had he been born without hands: we might
also enquire whether he might not have been as great a musician
as he was painter if, instead of living during the historical high-
water mark of painting, he had lived, under favourable personal
influences, at the time of highly-developed and widespread musical
genius. A great artist is always a great man, and if he finds the
outlet for his talent closed on one side, he forces his way through
_on the other.

From all these examples I wish to show that, in my opinion,

_ [} The author refers to the Academy of Arts at Munich. S.S.]
H

Vy

98 ON HEREDITY.

“talents do not appear to depend upon the improvement of any

special mental quality by continued practice, but they are the
expression, and to a certain extent the bye-product, of the human
mind, which is so highly developed in all directions.

But if any one asks whether this high mental development,
acquired in the course of innumerable generations of men, is not
dependent upon the hereditary effects of use, I would remind him
that human intelligence in general is the chief means and the
chief weapon which has served and still serves the human species
in the struggle for existence’, .Even in the present state of
civilization—distorted as it is by numerous artificial encroachments
and unnatural conditions—the degree of intelligence possessed by
the individual chiefly decides between destruction and life; and
in a natural state, or still better in a state of low civilization, this
result is even more striking.

Here again, therefore, we encounter the effects of natural selection,
and to this power we must attribute, at any rate, a great part of
the phenomena we have been discussing, and it cannot be shown
that—in addition to its operation—the transmission of characters
acquired by practice plays any part in nature.

I only know of one class of changes in the organism which is
with difficulty explained by the supposition of changes in the
germ; these are the modifications which appear as the direct
consequence of some 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: and for this reason,
I do not propose to consider the subject in detail.

These changes—such, for example, as are produced by a strange
climate—have been always looked at under the supposition that
they are transmitted and intensified from generation to generation,
and for this reason the observations are not always sufficiently
precise. It is difficult to say whether the changed climate may
not have first changed the germ, and if this were the case the
accumulation of effects through the action of heredity would pre-
sent no difficulty. For instance, it is well known that increased
nourishment not only causes a plant to grow more luxuriantly,
but it alters the plant in some distinct way, and it would be

* Compare Darwin’s ‘ Descent of Man,’

ON HEREDITY. 99

wonderful indeed if the seeds were not also larger and better furnished
with nutritive material. Ifthe increased nourishment be repeated
in the next generation, a still further increase in the size of the
seed, in the luxuriance of the plant, and in all other changes which
ensue, is at any rate conceivable if it is not a necessity. But this
would not be an instance of the transmission of acquired characters,
_ but only the consequence of a direct influence upon the germ-cells,
and of better nourishment during growth.

A similar interpretation explains the converse change. When™
horses of normal size are introduced into the Falkland Islands,
the next generation is smaller in consequence of poor nourishment
and the damp climate, and after a few generations they have de-
teriorated to a marked extent. In such a case we have only to
assume that the climate which is unfavourable and the nutriment
which is insufficient for horses, affect not only the animal as a
whole, but also its germ-cells. This would result in the diminution
in size of the germ-cells, the effects upon the offspring being still
further intensified by the insufficient nourishment supplied during
growth. “But such results would not depend upon the transmission
by the germ-ceils of certain peculiarities due to the unfavourable
climate, which only appear in the full-grown horse.

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 investi-
gated one such case’, and even now I cannot explain the facts
‘otherwise than by supposing the passive acquisition of i aaaoeny
produced by the direct influence of climate.

It must be remembered, however, that my experiments, which
have been repeated upon several American species by H. W.
Edwards, with results confirmatory of my own in all essential
respects, were not undertaken with the object of investigating the
question from this point of view alone. New experiments, under
varying conditions, will be necessary to afford the trué explana-
tion of this aspect of the question; and I have already begun to
undertake them.

_Leaving on one side, for the moment, these doubtful, and

+ ¢Studien zur Descendenztheorie, I. Ueber den Saison-Dimorphismus der
Schmetterlinge.’ Leipzig, 1875. English edition translated and edited by Professor
Meldola, ‘ Studies in the Theory of Descent,’ Part I.

H 2

100 ON HEREDITY.

insufficiently investigated cases, we may still maintain that the
assumption that changes induced by external conditions in the
organism as a whole, are communicated to the germ-cells after the
manner indicated in Darwin’s hypothesis of pangenesis,—is wholly
unnecessary for the explanation of these phenomena. Still we
cannot exclude the possibility of such a transmission occasionally
occurring, for, even if the greater part of the effects must be attri-
buted to natural selection, there might be a smaller part in certain
cases which depends on this exceptional factor.

A complete and satisfactory refutation of such an opinion cannot
be brought forward at present: we can only point out that such
an assumption introduces new and entirely obscure forces, and that
innumerable cases exist in which we can certainly exclude all
assistance from the transmission of acquired characters. In most
cases of variation in colour we have no explanation but the survival
of the fittest 1, and the same holds good for all changes of form
which cannot be influenced by the will of the animal. Very
numerous adaptations, such, for instance, as occur in the eggs of
animals,—the markings, and appendages which conceal them from
enemies, the complex coverings which prevent them from drying
up or protect them from the injurious influence of cold,—must
have all arisen entirely independently of any expression of will,
or of any conscious or unconscious action on the part of the
animals. I will not mention here the case of plants, which as
every one knows are unconscious, for they are beyond my province.
In this matter, there can be no suggestion of adaptation depending
upon a struggle between the various parts of the organism (Roux).
Natural selection cannot operate upon the different epithelial cells
which secrete the egg-shell of Apus, since it is of no consequence to
the animal which secretes the egg-shell whether a good or a bad
shell is produced. Natural selection first operates among the off-
spring, and the egg with a shell incapable of resisting cold or
drought is destroyed. The different cells of the same individual
are not selected, but the different individuals themselves.

In all such cases we have no explanation except the operation
of natural selection, and if we cannot accept this, we may as well

' The colours which have been called forth by sexual selection must also be in-
cluded here.
2 Wilhelm Roux, ‘ Der Kampf der Theile im Organismus.’ Leipzig, 1881.

ON HEREDITY. 101

abandon any attempt at a natural explanation. But, in my
opinion, there is no reason why natural selection should be con-
sidered inadequate to the task. It is true that the objection has
been lately urged, that it is inconceivable that all the wonderful
adaptations of the organism to its surroundings can have arisen
through the selection of individuals; and that for this purpose an
infinite number of individuals and infinite time would be required ;
and stress is laid upon the fact that the wished-for useful changes
can only arise singly and very rarely among a great number
of individuals.

This last objection to the modern conception of natural selec-
tion has apparently some weight, for, as a matter of fact, useful
variations of a conspicuous kind seldom appear, and are often
entirely absent for many generations. If we expect to find
that qualitative changes take. place by sudden leaps, we can
never escape this difficulty. But, I think, we must not look
for conspicuous variations—such as occur among domesticated
animals and plants—in the process of the evolution of species
as it goes on in nature.; Natural selection does not deal with
qualitative but quantitative changes in the individual, and the
latter are always present. .

A simple example will make this clearer. Let us suppose that
it was advantageous to some species—for instance the ancestors of
the giraffe—to lengthen some part of the body, such as the neck:
this result could be obtained in a relatively short time, for the
members of the species already possessed necks of varying length,
and the variations which form the material for natural selection
were already in existence. Now all the organs of every species
vary in size, and any one of them will undergo constant and
progressive increase, as soon as it acquires exceptional usefulness.
But not only will the organ fluctuate as a whole, but also the
parts composing it will become larger or smaller under given con-
ditions, will increase or diminish by the operation of natural selec-
tion. I believe that qualitative variations always depend upon
differences in the size and number of the component parts of
the whole. A skin appears to be naked, when it is really covered
with a number of small fine hairs: if these grow larger and increase
in number, a thick covering is formed, and we say that the skin
is woolly or furry. In the same way the skin of many worms and

102 ._ ON HEREDITY.

Crustacea is apparently colourless, but the microscope reveals the
presence of a number of beautiful pigment spots; and not until
these have increased enormously does the skin appear coloured
to the naked eye. The presence or absence of colour and its
quality when present are here dependent upon the quantity of
the most minute particles, and on the distance at which the
object in question is observed. Again, the first appearance of
colour, or the change from a green to a yellow or red colour
depends upon slight variations in the position or in the number
of the oxygen atoms which enter into the chemical combination
in question. Fluctuations in the chemical composition of the mole-
ecules of a unicellular organism (for example) must continually
arise, just as fluctuations are always occurring in the number of
pigment granules in a certain cell, or in the number of pigment
cells in a certain region of the body, or even in the size of the
various parts of the body.

All these quantitative relations are exposed to individual fluetua-
tions in every species ; and natural selection can strengthen the
fluctuations of any part, and thus cause it to develope further in
any given direction.

From this point of view, it becomes less astonishing and less
inconceivable that organisms adapt themselves—as we see that they
obviously do—in all their parts to any condition of existence, and —
that they behave like a plastic mass which can be moulded into
almost any imaginable form in the course of time.

If we ask in what lies the cause of this variability, the answer
must undoubtedly be that it lies in the germ-cells. From the
moment when the phenomena which precede segmentation com-
mence in the egg, the exact kind of organism which will be
developed is already determined—whether it will be larger or
smaller, more like its father or its mother, which of its parts will
resemble the one and which the other, even to the minutest detail.
In spite of this, there still remains a certain scope for the influence
of external conditions upon the organism. But this scope is
limited, and forms but a small area round the fixed central point
which is determined by heredity. Abundant nourishment can
make the body large and strong, but can never make a giant out
of the germ-cell destined to become a dwarf. Unhealthy seden-
tary habits or insufficient nourishment makes the factory-hand pale

ON HEREDITY. 108

and stunted; life on board ship, with plenty of exercise and sea
air, gives the sailor bodily strength and a tanned skin; but when
once the resemblance to father or mother, or to both, is established
in the germ-cell it can never be effaced, let the habit of life be
what it will.

But if the essential nature of the germ-cell dominates over the
organism which will grow from it, so also the quantitative in-
dividual differences, to which I referred just now, are, by the same
principle, established in the germ, and—whatever be the cause
which determines their presence—they must be looked upon as
inherent init. It therefore follows that, although natural selection
appears to operate upon the qualities of the developed organism
alone, it in truth works upon peculiarities which lie hidden in the
germ-cells. Just as the final development of any predisposition
in the germ, and just as any character in the mature organism
vibrates with a certain amplitude around a fixed central point,
so the predisposition of the germ itself fluctuates, and it is on
this that the possibility of an increase of the predisposition in
question, and its average result, depends.

If we trace all the permanent hereditary variations from
generation to generation back to the quantitative variations
of the germ, as I have sought to do, the question naturally
occurs as to the source from which these variations arose in
the germ itself. I will not enter into this subject at any length
on the present occasion, for I have already expressed my opinion
upon it’.

I believe however that they can be referred to the various ex-
ternal influences to which the germ is exposed before the com-
mencement of embryonic development. Hence we may fairly
attribute to the adult organism influences which determine the
phyletic development of its descendants. For the germ-cells are
contained in the organism, and the external influences which affect
them are intimately connected with the state of the organism in
which they lie hid. If it be well nourished, the germ-cells will
have abundant nutriment; and, conversely, if it be weak and

sickly, the germ-cells will be arrested in their growth. It is even

1 Consult ‘Studien zur Descendenztheorie, IV. Uber die mechanische Auffassung
der Natur,’ p. 303, etc. Translated and edited by Professor Meldola; see ‘ Studies
in the Theory of Descent,’ p. 677, &c.

104 ON HEREDITY.

possible that the effects of these influences may be more specialized ;
that is to say, they may act only upon certain parts of the germ-
cells. But this is indeed very different from believing that the
changes of the organism which result from external stimuli can
be transmitted to the germ-cells and will re-develope in the next
generation at the same time as that at which they arose in the
parent, and in the same part of the organism.

“ We have an obvious means by which the inheritance of all
transmitted peculiarities takes place, in the continuity of the
substance of the germ-cells, or germ-plasm. If, as I believe, the
substance of the germ-cells, the germ-plasm, has remained in per-
petual continuity from the first origin of life, and if the germ-
plasm and the substance of the body, the somatoplasm, have always
occupied different spheres, and if changes in the latter only arise
when they have been preceded by corresponding changes in the
former, then we can, up to a certain point, understand the principle

of heredity; or, at any rate, we can conceive that the human.
mind may at some time be capable of understanding it. We may
at least maintain that it has been rendered intelligible, for we
can thus trace heredity back to growth ; we can thus look upon
reproduction as an overgrowth of the individual, and can thus
distinguish between a succession of species and a succession of
individuals, because in the latter succession the germ-plasm remains
similar, while in the succession of the former it becomes different.
Thus individuals, as they arise, are always assuming new and more _
complex forms, until the interval between the simple unicellular ~
protozoon and the most complex of all organisms—man himself—
is bridged over.

I have not been able to throw light upon all sides of the question
which we are here discussing. There are still some essential points
which I must leave for the present; and, furthermore, I am not
yet in a position to explain satisfactorily all the details which
arise at every step of the argument. But it appeared to me to be
necessary to state this weighty and fundamental question, and to
formulate it concisely and definitely; for only in this way will it
be possible to arrive at a true and lasting solution of the problem.

; We must however be elear on this point—that the understanding
of the phenomena of heredity is only possible on the fundamental

supposition of the continuity of the germ-plasm. The value of

ON HEREDITY. 105

experiment in relation to this question is somewhat doubtful.
A careful collection and arrangement of facts is far more likely
to decide whether, and to what extent, the continuity of germ-
plasm is reconcilable with the assumption of the transmission of
acquired characters from the parent body to the germ, and from
the germ to the body of the offspring. At present such trans-
mission is neither proved as a fact, nor has its assumption been
shown to be unquestionably necessary.

LIFE AND DEATH.

—+4—

PREFACE.

Tur following paper was first printed as an academic lecture in
the summer of the present year (1883), with the title ‘Upon the
Eternal Duration of Life’ (‘Uber die Ewigkeit des Lebens’). In
now bringing it before a larger public in an expanded and improved
form, I have chosen a title which seemed to me to correspond
better with the present contents of the paper.

The stimulus which led to this biological investigation was
given in a memoir by Gétte, in which this author opposes views
which I had previously expressed. Although such an origin has
naturally caused my paper to take the. form of a reply, my inten-
tion was not merely to controvert the views of my opponent, but
rather—using those opposed views as a starting-point—to throw
new light upon certain questions which demand consideration ; to
give additional support to thoughts which I have previously ex-
pressed, and to penetrate, if possible, more deeply into the problem
of life and death.

If, in making this attempt, the views of my opponent have been
severely criticized, it will be acknowledged that the criticisms do
not form the purpose of my paper, but only the means by which
the way to a more correct understanding of the problems before us
may be indicated.

A. W.

FREIBURG I. BREISGAU,
Océ. 18, 1883.

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ITI.

LIFE AND DEATH.

In the previous essay, entitled ‘The Duration of Life, I have
endeavoured to show that the limitation of life in single individuals
by death is not, as has been hitherto assumed, an inevitable phe-
nomenon, essential to the very nature of life itself; but that it is
an adaptation which first appeared when, in consequence of a certain
complexity of structure, an unending life became disadvantageous to
the species. I pointed out that we could not speak of natural
death among unicellular animals, for their growth has no termina-
tion which is comparable with death. The origin of new indivi-
duals is not connected with the death of the old; but increase by
division takes place in such a way that the two parts into which
an organism separates are exactly equivalent one to another, and
neither of them is older or younger than the other. In this way
countless numbers of individuals arise, each of which is as old as ©
the species itself, while each possesses the capability of living on
indefinitely, by means of division.

I suggested that the Metazoa have lost this power of unending
life by being constructed of numerous cells, and by the consequent
division of labour which became established between the various
cells of the body. Here also reproduction takes place by means
of cell-division, but every cell does not possess the power of
reproducing the whole organism. The cells of the organism are
differentiated into two essentially different groups, the reproductive
cells—ova or spermatozoa, and the somatic cells, or cells of the
body, in the narrower sense. The immortality of the unicellular
organism has only passed over to the former; the others must die,
and since the body of the individual is chiefly composed of them,

it must die also,
_ [have endeavoured to explain this fact as an adaptation to the
general conditions of life. In my opinion life became limited in

112 LIFE AND DEATH.

its duration, not because it was contrary to its very nature to be
unlimited, but because an unlimited persistence of the individual
would be a luxury without a purpose. Among unicellular organisms
natural death was impossible, because the reproductive cell and
the individual were one and the same: among multicellular animals
it was possible, and we see that it has arisen.

Natural death appeared to me to be explicable on the principle of
utility, as an adaptation.

These opinions, to which I shall return in greater detail in a
later part of this paper, have been opposed by Gétte 1, who does not
attribute death to utility, but considers it to be a necessity in-
herent in life itself. He considers that it oecurs not only in the
Metazoa or multicellular animals, but also in unicellular forms of
life, where it is represented by the process of encystment, which is
to be regarded as the death of the individual. This encystment is a
process of rejuvenescence, which, after a longer or shorter interval,
interrupts multiplication by means of fission. According to Gétte,
this process of rejuvenescence consists in the dissolution of the
specific structure of the individual, or in the retrogression of the
individual to a form of organic matter which is no longer living
but which is comparable to the yolk of an egg. This matter is, by
means of its internal energy, and in consequence of the law of
growth which is inherent in its constitution, enabled to give rise to
a new individual of the same species. Furthermore, the process of
rejuvenescence among unicellular beings corresponds to the forma-
tion of germs in the higher organisms. The phenomena of death
were transmitted by heredity from the unicellular forms to the
Metazoa when they arose. Death does not therefore appear for
the first time in the Metazoa, but it is an extremely ancient
process which ‘goes back to the first. origin of organic beings’
(Lc. p. 81).

It is obvious, from this short résumé, that Gotte’s view is totally
opposed to mine. Inasmuch as only one of these views can be
fundamentally right, it is worth while to compare the two; and
although we cannot at present hope to explain the ultimate physio-
logical processes which involve life and death, I think nevertheless
that it is quite possible to arrive at definite conclusions as to the
general causes of these phenomena. At any rate, existing facts

1 «Ueber den Ursprung des Todes,’ Hamburg and Leipzig, 1883.

LIFE AND DEATH. 113

have not been so completely thought out that it is useless to con-
sider them once more.
- The question—what do we understand by death ? must be de-
cided before we can speak of the origin of death. Gdtte says, ‘ we
are not able to explain this general expression quite definitely and
- In all its details, because the moment of death, or perhaps more
exactly the moment when death is complete, can in no case be pre-
cisely indicated. We can only say that in the death of the higher
animals, all those phenomena which make up the life of the indivi-
dual cease, and further that all the cells and elements of tissue which
form the dead organism, die, and are resolved into their elements.’
This definition would suffice if it did not include that which is
to be defined. For it assumes that under the expression ‘ dead
organism’ we must include those organisms which have brought to
an end the whole of their vital functions, but of which the component
cells and elements may still be living. This view is afterwards
more accurately explained, and in fact there is no doubt that the
cessation of the activity of life in the multicellular organism rarely -
implies any direct connection with the cessation of vital functions in
all its constituents. The question however arises, whether it is right
or useful to limit the conception of death to the cessation of the
functions of the organism. Our conceptions of death have been
derived from the higher organisms alone, and hence it is quite
possible that the conception may be too limited. The limitation
might perhaps be removed by accurate and scientific comparison
with the somewhat corresponding phenomena among: unicellular
organisms, and we might then arrive at a more comprehensive
definition. Science has without doubt the right to make use of
popular terms and conceptions, and by a more profound insight to
widen or restrict them. But the main idea must always be retained,
so that nothing quite new or strange may appear in the widened
conception. The conception of death, as it has been expressed with
perfect uniformity in all languages, has arisen from observations on
the higher animals alone ; and it signifies not only the cessation of
the vital functions of the whole organism, but at the same time
the cessation of life in its single parts, as is shown by the impossi-
bility of revival. The post-mortem death of the cells is also part
of death, and was so, long before science established the fact that
an organism is built up of numerous very minute living elements,
I

114 LIFE AND DEATH.

of which the vital processes partially continue for some time after
the cessation of those of the whole organism. It is precisely this
- incapacity on the part of the organism to reproduce the phenomena
of life anew, which distinguishes genuine death from the arrest of
life or trance ; and the incapacity depends upon the fact that the
death of the cells and tissues. follows upon the cessation of the
vital functions as a whole. I would, for this reason, define death
as an arrest of life, from which no lengthened revival, either of the
whole or any of its parts, can take place; or, to put it concisely,
as a definite arrest of life. I believe that in this definition I have
. expressed the exact meaning of the conception which language has
sought to convey in the word death. For our present purpose, the
cause which gives rise to this phenomenon is of no importance,—
whether it is simultaneous or successive in the various parts of the
organism, whether it makes its appearance slowly or rapidly. For
the conception itself it is also quite immaterial whether we are
able to decide if death has really taken place in any particular
ease; however uncertain we might be, the state which we call
death would be not less sharply and definitely limited. We might
consider the caterpillar of Huprepia flavia to be dead when frozen
in ice, but if it recovered after thawing and became an imago, we
should say that it had only been apparently dead, that life stood
still for a time, but had not ceased for ever. It is only the irre-
trievable loss of life in an organism which we call death, and we
ought to hold fast to this conception, so that it will not slip from
us, and become worthless, because we no longer know what we
mean by it.

We cannot escape this danger if we look upon the post-mortem ~
death of the cells of the body as a phenomenon which may
accompany death, but which may sometimes be wanting. An
experiment might be made in which some part of a dead animal,
such as the comb of a cock, might be transplanted, before the
death of the cells, to some other living animal: such a part might
live in its new position, thus showing that single members may
survive after the appearance of death, as I understand it. But
the objection might be raised that in such acase the cock’s
comb has become a member of another organism, so that it would
be lost labour to insert a clause in our definition of death which
would include this phenomenon. The same objection might be

LIFE AND DEATH. 115

raised if the transplantation took place a day or even a year before
the death of the cock.

Goétte is decidedly in error when he considers that the idea of
death merely expresses an ‘arrest of the sum of vital actions in
the individual, without at the same time including that definite
* arrest which involves the impossibility of any revival. De-
composition is not quite essential to our definition, inasmuch as
death may be followed by drying-up’, or by perpetual entombment
in Siberian ice (as in the well-known case of the mammoth), or by
digestion in the stomach of a beast of prey. But the notion of a
dead body is indeed inseparably connected with that of death,
and I believe that I was right in distinguishing between the division
of an Infusorian into two daughter-cells, and the death of a Metazoon,
which leaves offspring behind it, by calling attention to the absence
of a dead body in the process of fission among Infusoria?. The
real proof of death is that the organized substance which previously
gave rise to the phenomena of life, for ever ceases to originate
such phenomena. ‘This, and this alone, is what mankind has
hitherto understood by death, and we must start from this definition
if we wish to retain a firm basis for our considerations.

We must now consider whether this definition, derived from
observation of higher animals, may be also applied without altera-
tion to the lower, or whether the corresponding phenomena which
arise in these latter, differ in detail from those of the higher
animals, so that a narrower limitation of the above definition is
rendered necessary.

Gotte believes the process of encystment which takes place in
so many unicellular animals (Monoplastides) to be the analogue of
death. According to this authority, the individuals in question,
not only undergo a kind of winter sleep—a period of latent life—
but when surrounded by the cyst they lose their former specific
organization ; they become a ‘homogeneous substance,’ and are
resolved into a germ, from which, by a process of development,
a new individual of the same species once more arises. The
division of the contents of the cyst, viz. its multiplication, is,
according to this view, of secondary importance, and the essential

* As in the case of the bodies of monks on the Great St. Bernard, or the dried-up
bodies in the well-known Capuchine Monastery at Palermo.
? See below.

I2

116 LIFE AND DEATH.

feature in the process is the rejuvenescence of the individual. This
rejuvenescence however is said to not only consist in the simple

transformation of the old individual, but in its death, followed

by the building up anew of another individual. ‘The parent
organism and its offspring are two successive living stages of the
same substance—separated, and at the same time connected, by the
condition of rejuvenescence which lies between them’ (l.c., p. 79).
An ‘absolute continuity of life does not exist’; it is only the dead
organic matter which establishes the connection, and the ‘ identity
of this matter ensures heredity.’

It is certainly surprising that Gétte should identify encystment
with a cessation of life, and we may well inquire for the evidence
which is believed to support such a view. The only evidence lies
in a certain degree of degeneration in the structure of the individual,
and in the cessation of the visible external phenomena of life, such
as feeding and moving. Does Gdtte really believe that it is an
incorrect interpretation of the facts to assume that a vita minima
continues to exist in the protoplasm, after its complexity has
diminished? Are we compelled to invoke a mystical explanation
of the facts, by an appeal to such an indefinite principle as Gotte’s
rejuvenescence? Would not the oxygen, dissolved in the water,
affect the organic substance the life of which it formerly maintained,
and would it not cause its decomposition, if it were in reality dead ?

I, too, hold that the division of the encysted mass is of
secondary importance, and that the encystment itself, without the
resulting multiplication, is the original and essential part of the
phenomenon. But it does not follow from this that the encyst-
ment should be considered as a process of rejuvenescence. What
is there to be rejuvenated? Certainly not the substance of the
animal, for nothing is added to it, and it can therefore acquire no
new energy; and the forms of energy which it manifests cannot be
changed, since the form of the matter is just the same after quitting
the cyst as it was before. Rejuvenescence has also been mentioned
in connection with the process of conjugation, but this is quite
another thing. It is quite reasonable, at least in a certain sense, to

maintain the connection of rejuvenescence with conjugation ; for

a fusion of the substance of two individuals takes place, to a
greater or lesser extent, in conjugation, and the matter which
composes each individual is therefore really altered. But in simple

LIFE AND DEATH. 117

encystment, rejuvenescence can only be understood in the sense in
which we speak of the fable of the Phcenix, which, when old, was
believed to be consumed by fire, and to rise again from its own
“ashes as a young bird. I doubt whether this idea is in agreement
with the physiology of to-day, or with the laws of the conservation
of energy. It is easy to pull down an old house with rotten beams
and crumbling walls, but it would be impossible to build it anew
with the old material, even if we used new mortar, represented in
Gotte’s hypothesis by water and oxygen. For these reasons I con-
sider the idea of rejuvenescence of the encysted individual to be
contrary to our present physiological knowledge.

It is much more simple and natural to regard encystment as .
adapted for the protection of certain individuals in a colony from
destruction by being dried up or frozen, or for the protection of the
individual during multiplication by division, when it is helpless,
and would easily fall a prey to enemies, or to secure advantages in
some other way'. The case of Actinosphaerium, mentioned by
Gotte, clearly demonstrates that rejuvenescence of the individual is
not the only event which happens during. encystment, for this
would scarcely require six months. The long duration of latent
life, from summer to the next spring, clearly proves that encystment
is of the highest importance for the species, in order to maintain the
life of the individual through the dangers of an unfavourable season ?,

1 Professor Gruber informs me that among the Infusoria of the harbour of
Genoa, he has observed a species which encysts upon one of the free-swimming
Copepoda. He has often found as many as ten cysts upon one of these Copepods,
and has observed the escape of their contents whenever the water under the cover-
glass began to putrefy. Here advantage is probably gained in the rapid transport of
the cyst by the Crustacean.

? The views of most biologists who have worked at this subject agree in all
essentials with that expressed above. Biitschli says (Bronn’s‘ Klassen und Ordnungen
des Thierreichs,’ Protozoa, p. 148): ‘The process of encystment does not appear to
have originally borne any direct relation to reproduction: it appears on the contrary
to have taken place originally,—as it frequently does at the present day,—either for
the protection of the organism against injurious external influences, such as desicca-
tion or the fatal effects of impure water, etc.; and also to enable the organism,
after taking up an unusually abundant supply of food, to assimilate it in safety.’
Balbiani (‘ Journ. de Micrographie,’ Tom. V. 1881, p. 293) says in reference to the
Infusoria, ‘Un petit nombre d’espéces, au lieu de se multiplier & l’état de vie active,
se reproduisent dans une sorte d’état de repos, dit état d’enkystement. Ces sortes
de kystes peuvent étre désignés sous le nom de kystes de reproduction, par opposition
avec d’autres kystes, dans lesquels les Infusoires se renferment pour se soustraire &

des conditions devenues défavorables du milieu qu’ils habitent, le manque d’air, le
desstchement, etc.—ceux-ci sont des kystes de conservation .. .”

118 LIFE AND DEATH.

When in this case, the specific organization degenerates to
a certain extent, such changes depend in part upon the endeavour
to diminish as far as possible the size of the organism—the pseu-
dopodia being drawn in, while the vacuoles contract and com-_
pletely disappear. The degeneration may also, perhaps, depend in
part upon the secretion of the cyst itself, which implies a certain
loss of substance’. But degeneration chiefly depends upon the
fact that the encystment is accompanied by reproduction in the
way of fission, which seems to begin with a simplification of the
organization, that is, with a fusion of the numerous nuclei. It is
well known that many unicellular animals contain several nuclei—
in other words, that the nuclear substance is scattered in small
parts throughout the whole cell. But when the animal prepares
for division, these pieces of nuclear substance fuse into a single
nucleus which itself undergoes division into two equal parts? during
the division of the animal. It is evident that the equal division
of the whole nuclear substance only becomes possible in this way.

There are, however, numerous cases which prove that the bodies
of encysted animals may retain, during the whole process, exactly
the same structure and differentiation, which were previously
characteristic of them. Thus the large Infusorian Ti//ina magna, de-
scribed by Gruber, can be seen through the thin-walled cyst to
retain the characteristic structure of its ectoplasm, and the whole of
its organization. Even the movements of the enclosed animal
do not cease; it continues to rotate actively in the narrow eyst,
as do the two or four parts into which it subsequently divides.
Such observations prove that Gétte’s view that ‘every characteris-
tic of the previous organization is lost,’ is quite out of the question*®
(l.¢., p. 62).

1 This is of importance in so far as single individuals might be thus compelled to
encyst even when the existing external conditions of life do not require it. The
substance which Actinosphaerium, for. example, employs in the secretion of its thick
siliceous cyst must have been gradually accumulated by means of a process peculiar to
the species. We can scarcely be in error if we assume that the silica accumulated
in the organism cannot increase to an unlimited extent without injury to the other
vital processes and that the secretion of the cyst must take place as soon as the
accumulation has exceeded a certain limit. Thus we can understand that encyst-
ment may occur without any external necessity. Similarly, certain Entomostraca
(e.g. Moina) produce winter-eggs in a particular generation, and these are formed
even when the animals are kept in a room protected from cold and desiccation.

* Upon this point Professor Gruber intends to publish an elaborate memoir.

5 This view has not even been proved for Actinosphaerium, upon which Gotte

LIFE AND DEATH. 119

For this reason I must strongly oppose Gdtte’s view that
an encysted individual is a germ, viz. an organic mass still un-
organized which can only become an adult individual by means of
a process of development. I believe that an encysted individual is
one possessing a protective membrane, in structure more or less
simplified as an adaptation to the narrow space within the cyst, and
to a possible subsequent increase by division, in short one in which
active life is reduced to a minimum, and sometimes even completely
in abeyance, as happens when it is frozen.

Tt is evident from the above considerations that encystment in
no way corresponds with that which every one, including myself,
understands by death, because during encystment one and the same
being is first apparently dead and then again alive ; and we merely
witness a condition of rest, from which active life will again
emerge. ‘This would remain true even if it were proved that life is,
in reality, suspended for a time. But such proof is still wanting,
and Gétte was apparently only influenced by theoretical considera-
tions, when he imagined that death intervened where unprejudiced
observers have only recognised: a condition of rest. He apparently
entirely overlooked the fact that it is possible to test his views; for
all unicellular beings are in reality capable of dying: we can.
kill them, for example, by boiling, and they are then really dead and
cannot be revived. But this state of the organism differs chemically
and physically from the encysted condition, although we do not
know all the details of the difference. . The encysted animal, when
placed in fresh water, presently originates a living individual, but
the one killed by boiling only results in decomposition of the dead
organic matter. Hence we see that the same external conditions
give rise to different results in two different states of the organism.
It cannot be right to apply the same term to two totally different
states. There is only one phenomenon which can be called death,
although it may be produced by widely different causes. But if
the encysted condition is not identical with the death which we
ean produce at will, then natural death, viz. that arising from
internal causes, does not exist at all among unicellular organisms.

These facts refute Gdtte’s peculiar view, which depends on the

chiefly relies. The observations which we now possess merely indicate that the
animal contracts to the smallest volume possible. Compare F. E. Schulze, ‘ Rhizo-
podenstudien,’ I, Arch. f. mikr. Anat. Bd. 10, p. 328; and Karl Brandt, ‘ Ueber
Actinosphaerium Eichhornii,’ Inaug. Diss. ; Halle, 1877.

120 LIFE AND DEATH.

existence of natural death among the Monoplastid organisms;
upon proof of the contradictory, his whole theory collapses. But
there is nevertheless a certain interest in following it further, for
we shall thus reach many ideas worthy of consideration.

First, the question arises as to how death could have been
transmitted from the Monoplastides ! to the Polyplastides, a process
which must have taken place according to Gétte. I-will for the
present omit the fact that I cannot accept the supposition that the
process of encystment represents death. We may then inquire

whether death has taken the place of encystment among the

Polyplastides, or, if this is not the case, whether any process com-
parable to encystment exists among the Polyplastides.

Gotte believes that death is always connected with reproduction,
and is a consequence of the latter in both Protozoa and Metazoa.
Reproduction has, in his opinion, a directly ‘fatal effect, and the
reproducing individual must die. Thus the may-fly and the
butterfly die directly after laying their eggs, and the male bee dies
immediately after pairing ; the Orthonectides expire after expelling
their germ-cells, while Magosphaera resolves itself into germ-cells,
and nothing persists except these elements. It is but a step from
this latter organism to the unicellular animals which transform
themselves as a whole into germ-cells; but in order to achieve
this they must undergo the process of aa dah vances which Gitte

assumes to be the same as death.
_ These views contain many fallacies quite apart from the sound-
ness or unsoundness of their foundation. The process of encystment,
as Goétte thinks, represents, in the Monoplastides, true reproduction
to which multiplication by means of division has been secondarily
added. This encystment cannot be dispensed with, for internal
causes determine that it must occasionally interrupt the process of

multiplication by simple division. But, on the other hand, Gétte—

also considers the division of the contents of the cyst to be a
secondary process. The essential characteristic of encystment is a
simple process of rejuvenescence without multiplication. Hence
‘we are forced to accept a primitive condition in which simple
division as well as the division of the encysted individual were

1 The conception of Protozoa and Metazoa does not correspond exactly with that

of unicellular and multicellular beings, for which Gitte has proposed the names
Mono- and Polyplastides.

LIFE AND DEATH. 121

absent, and in which reproduction consisted only in an often-
repeated process of rejuvenescence among existing individuals,
without any increase in their number. Such a condition is in-
conceivable because it would involve a rapid disappearance of the
species, and the whole consideration clearly shows us that division
of un-encysted individuals must have existed from the first, and
that this, and not a vague and mysterious rejuvenescence, has
always been the real and primitive reproduction of the Mono-
plastides. The fact that encystment does not always lead to the
division of the contents of the cyst proves, in my opinion, that not
reproduction but preservation against injury from without, was the
primitive meaning of encystment. Itis possible that at the present
time there are but few Monoplastides which are able to go through
an infinite number of divisions without the interposition of the
resting condition implied by encystment ; although it has not yet
been demonstrated for all species’. But it is not right to conclude
from this that there is an internal necessity which leads to encyst-
ment, that is to say to identify this process with rejuvenescence. It
is much more probable that encystment is merely an adaptation
to continual changes in the external ¢onditions of life, such as
drought and frost, and perhaps also the want of food which arises
from the over-population of small areas. 'The same phenomenon is
known in certain low Crustacea—the Daphnidae—which possess
an ephippium or protective case for their winter-eggs. This case is
only developed after a certain definite number of generations has
been run through, an event which may happen at any time in
the year in species living in pools which are liable to be often
dried-up ; but only in the autumn in such as live in lakes which
are never dry. No one ever doubted that the periodical formation
of the ephippium in certain generations was an adaptation to
changes in the external conditions of life.

Even if the process of rejuvenescence in the Monoplastides were
really equivalent to the death of the higher animals, we could
not conclude from this that it is necessarily associated with re-
production. Encystment alone is not reproduction, and it first

1 Among the Rhizopoda encystment is only known in fresh-water forms, and

not in a single one of the far more numerous marine forms which possess shells (see
' Biitschli, ‘ Protozoa,’ p. 148); the marine Rhizopoda are not exposed to the effects

of desiccation or frost, and thus the strongest motives for the process of encystment
do not exist, at least among forms possessing a shell.

122 LIFE AND DEATH.

becomes a form of reproduction when it is associated with the
division of the encysted animal. Simple division was the true
and original form of reproduction in Monoplastides, and even now
it is the principal and fundamental form.

Hence we see that among the Monoplastides reproduction is not
connected with death, even if we accept Goétte’s view and allow
that encystment represents death. I shall return later on to the
relation between death and reproduction in the Metazoa; but the
question first arises whether encystment, if it is not death, has any
analogue in the higher animals, and further whether death takes
that place in their development which is occupied by encystment in
- the Monoplastides.

Among the higher Metazoa there can be no doubt as to what
we mean by death, but the precise nature of that which dies is not
equally evident, and the popular conception is not sufficient for us.
It is necessary to distinguish between the mortal and the im-
mortal part of the individual—the body in its narrower sense
(soma) and the germ-cells. Death only affects the former; the
germ-cells are potentially immortal, in so fur as they are able,
under favourable circumstances, to develope into a new individual,
or, in other words, to surround themselves with a new body
(soma) }.

But how is it with the lowest Polyplastides in which there is no
antithesis between the somatic and germ-cells, and among which
each of the component cells of the multicellular body has retained
all the animal functions of the Monoplastides, even including re-
production ?

Gitte believes that the natural death of these organisms (which
he rightly calls Homoplastides) consists in ‘the dissolution of
the cell-colony.’ As an example of such dissolution Gitte takes
Hickel’s Magosphaera planula, a marine free-swimming organism
in the form of a sphere composed of a single layer of ciliated cells,

1 I trust that it will not be objected that the germ-cells cannot be immortal, be-
cause they frequently perish in large numbers, as a result of the natural death of the
individual. There are certain definite conditions under which alone a germ-cell can
render its potential immortality actual, and these conditions are for the most part
fulfilled with difficulty (fertilization, etc.). It follows from this fact that the germ-
cells must always be produced in numbers which reach some very high multiple of
the necessary number of offspring, if these latter are to be ensured for the species.
If in the natural death of the individual the germ-cells must also die, the natura
death of the soma becomes a cause of accidental death to the germ-cells.

LIFE AND DEATH. 123

imbedded in a jelly. (For figure see below.) This organism
cannot however be ‘considered as a genuine perfect Polyplastid,
for at a certain time the component cells part from one another
and then continue to live independently in the condition of Mono-
plastides.’ These free amoebiform organisms increase considerably
in size, encyst, and finally undergo numerous divisions—a kind of
segmentation within the cyst. The result of the division is a
sphere of ciliated cells similar to that with which the cycle began.
In fact, Magosphaera is not a perfect Polyplastid, but a transitional

DEVELOPMENT OF MAGOSPHAERA PLANULA (after Hickel).

1. Encysted amoeboid form. 2 and 3. Two stages in the division of the same.
4. Free ciliated sphere, the cells of which are connected by a gelatinous mass. 5.
One of the ciliated cells which has become free by the breaking up of the sphere.
6. The same in the amoeboid form. 7. The same grown to a larger size.

form between Polyplastides and Monoplastides, as the discoverer of
the group of animals of which it is the only representative, indi-
cated, when he named the group ‘ Catallacta.’

According to Gétte, the natural death of Magosphaera consists,
. as in the undoubted Protozoa, in a process of rejuvenescence by
encystment. ‘The dissolution of the ciliated: sphere into single cells
‘cannot be identical with natural death. For the regular and

124 LIFE AND DEATH.

complete separation of the Magosphaera-cells proves that their in-
dividuality has not been completely subordinated to that of the
whole colony, and it proves that the latter is not completely
individualised 1.’

Nothing can be said against this, if we agree in identifying
death with the encystment of the Monoplastides. Now we could,
as Gotte rightly remarks, derive the lower forms of Polyplastides
from Magosphaera if ‘the connection between the cells of the
ciliated sphere were retained until encystment, viz. until the re-
production of the single cells had taken place*.’ After this had
been accomplished, Gétte considers that death would consist ‘in the
complete separation of the cells from one another, accompanied in
all probability by their simultaneous change into germ-cells.’
The fallacy in this is evident; if death is represented in one
case by the encystment during which single cells change into
germ-cells, then this must apply to the other case also, for nothing
has changed except the duration of the cell-colony. The nature
of encystment cannot be affected by the fact that the cells separate
from one another a little earlier or a little later. If it is true
that death is represented by encystment among the Monoplastides,
then the same conclusion must also hold for the Polyplastides ; or
rather death must be represented in them by the process of re-
juvenescence, which Gétte considers to be the essential part of
encystment. Gdétte ought not to identify death with the dissolu-
tion of the cell-colony of which the lowest and highest Poly-
plastides are alike composed; but he should seek it in the process
of rejuvenescence which takes place within the germ-cells. If it is
essential to the nature of reproduction that the cells set apart for
that purpose should pass through a process of rejuvenescence, which
is equivalent to death, then this must be true for the reproductive
cells of all organisms. If these conclusions hold good, there is
nothing to prevent us from assuming that such a process of rejuve-
nescence actually occurs in the higher animals. Gdtte evidently
holds this view, as is plainly shown in the last pages of his essay.
He there attempts to bring his views of the death and rejuve-
nescence of the germ into harmony with his previously developed
idea of the derivation of death among the Polyplastides from the
dissolution of the cell-colonies. Gdétte still clings to the view

'1c., p. 78. 2 lic. p- 47.

a

‘ mn

LIFE AND DEATH. 125

which he propounded in describing the development of Bombinator,
according to which the egg-cell of the higher Metazoa must pass
through a process of rejuvenescence representing death, before it
can become a germ.

According to Gitte’s! idea ‘the egg of a Bombinator igneus before
fertilization cannot be considered to be a cell either wholly or in
part ; and this is equally true of it at its origin and after its complete
development; it is only an essentially homogeneous organic mass
enclosed by a membrane which has been deposited externally.’
This mass is ‘unorganised and not living *, and ‘during the first
phenomena of its development all vital powers must be excluded.’
In this way the continuity of life between two successive in-
dividuals is always interrupted; or, as Gétte says in his last
essay :—‘ The continuity of life between individuals of which one
is derived from the other by means of reproduction, exists neither
in the rejuvenescence of the Monoplastides nor in the condition
of the germ among the Polyplastides—a condition which is derived
from the former °.

This is quite logical, although in my opinion it is both un-
proved and incorrect. But, on the other hand, it is certainly
illogical for Gétte to derive the death of the Metazoa in a totally
different way, i.e. from the dissolution of their cell-colonies. It is
quite plain that the death of the Metazoa does not especially
concern the reproductive cells, but the individual which bears them ;
Gotte must therefore seek for some other origin of death—an
origin which will enable it to reach the body (soma)—as opposed
to the germ-cells, If there still remained any doubt about the
failure to establish a correspondence between death and the encyst-
ment of the Monoplastides, we have here, at any rate,a final demon-
stration of the failure!

But there is yet another great fallacy concealed in this derivation
of the death of the Polyplastides.

Among the lowest Polyplastides, where all the cells still remain
similar, and where each cell is also a reproductive cell, the dissolu-
tion of the cell-colony is, according to Gitte, to be regarded as death,
inasmuch as ‘the integrity of the mother-individual absolutely

«Entwicklungsgeschichte der Unke,’ Leipzig, 1875, p. 65.
2 Id.,p. 842.
3 * Ursprung des Todes,’ p. 79.°

126 LIFE AND DEATH.

comes to an end’ (l.¢., p. 78). The dissolution of a cell-colony into

its component living elements can only be called death in the most

figurative sense, and can have nothing to do with the real death of

the individuals ; it only consists in a change from a higher to a

lower stage of individuality. Could we not kill a Magosphaera

by boiling or by some other artificial means, and would not the

state which followed be death? Even if we define death as an

arrest of life, the dissolution of Magosphaera into many single cells”
which still live, is not death, for life does not cease in the organic

matter of which the sphere was composed, but expresses itself in

another form. It is mere sophistry to say that life ceases because

the cells are no longer combined into a colony. Life does not in

truth cease fora moment. Nothing concrete dies in the dissolution

of Magosphaera; there is no death of a cell-colony, but only of a

conception.. The Homoplastides, that is cell-colonies built up of
equal cells, have not yet gained any natural death, because each of
their cells is, at the same time, a somatic as well as a reproductive

cell: and they cannot be subject to natural death, or the species

would become extinct.

It is more to the purpose that Gdtte has sought for an illus-
tration of death among those remarkable parasites, the Ortho-
nectides, because in them we do at any rate meet with real
death. They are indeed very low organisms; but nevertheless
they stand far above Magosphaera, even if the latter were hypo-
thetically perfected up to the level of a true Homoplastid, for the
cells which compose the body of the Orthonectides are not all similar,
but are so far differentiated that they are even arranged in the
primitive germ-layers, and a form results which has rightly been
compared with that of the Gastrula. It is true they are not quite
so simple as Gétte! figures them, for they not only consist of ecto-
derm and germ-cells, but, according to Julin®, the endoderm is
arranged in two layers—the germ-cells and a layer which forms
during development a strong muscular coat; and in the second
female form the egg-cells are surrounded by a tolerably thick
granular tissue. There is nevertheless no doubt that in the first
female form, when sexually mature, the greater part, not only of the

16.5 eds
? “Contributions & l’histoire des Mesozoaires. Recherches sur l’organisation et
le développement embryonnaire des Orthonectides,’ Arch, de Biologie, vol. iii. 1882.

LIFE AND DEATH. 127

endoderm but. of the whole body, is made up of ova, so that the
animal resembles a thin-walled sac full of eggs. The ova escape
by the bursting of the thin ectoderm, and when they have all

A

Aaa i \' I
12) |

\\
R
SAN

ORTHONECTIDES (after Julin).

8. First female form: the cap-like anterior part has become detached and the egg-
cells (ez) are escaping. 9. Second female form: eiz=egg-cells; outside these are
the muscular layer (m) and the ectoderm (ekt). 10 and 11. Two fragments of such a
‘female broken to pieces by spontaneous division: the egg-cells are embedded in
a granular mass, and undergo embryonic development in it at a later period; the
whole is surrounded by ciliated cells. 12. Male discharging the spermatozoa by the
breaking up of the ectoderm (ek#); sp spermatozoa; m muscle.

128 LIFE AND DEATH.

escaped, the thin disintegrated membrane, composed of ciliated
cells, is no longer in a condition to live, and dies at once. This is
the course of events as described by Gotte, and he is probably
correct in his interpretation. This is the real death of the Ortho-
nectides, and if we regard them as low primitive forms (Mesozoa),
here for the first time in the ascending series we meet with natural
death. But the causes of this are scarcely so clear as Gétte seems
to think when he ascribes it to the effect of reproduction—an
effect which is ‘not only empirically necessary, but absolutely
unavoidable.’ Such a necessity is explained by the fact that the
endoderm consists entirely of germ-cells. Now the life of the
organism, being dependent upon the mutual action of both layers,
must cease as soon as the whole endoderm is extruded during repro-
duction.

Arguments such as these pass over the presence of a mesoderm ;
but apart from this omission, it does not appear to me so self-
evident from a purely physiological standpoint, that the ectodermal
sheath with its muscle layer must die after the extrusion of the
germ-cells. .

In those females to which Gitte refers in this passage, the whole
sheath remains at first quite uninjured, with the exception of a
small cap at the anterior end, which is pushed off to give exit to
the ova; and inasmuch as the sheath continues to swim about in
the nutritive fluids after this has taken place, the proof is at any
rate wanting that it cannot support itself quite as well as before,
although it has lost the germ-cells. ;

Then why does it die? My answer to this is simple :—because
it has lived its time; because its length of life is limited to a
period which corresponds with the time necessary for complete
reproduction. The physical constitution of the body is so regulated
that it remains capable of living until the extrusion of the repro-
ductive cells, and then dies, however favourable external conditions
may be for its further support.

The correctness of this explanation is shown by a consideration
of the males and the second form of females; for in these cases the
body falls to pieces, not as a consequence of reproduction, but as a
preparation for it!

Gotte only mentions the second female form in a note, in which
he says, it appears ‘that in the second female form of these animals

LIFE AND DEATH. 129

the whole body breaks into many pieces, and the superficial layer
gradually atrophies, so that it dies before the eggs are extruded.’
In Julin’s account !, upon which Gétte bases his statements, there
are, however, some not unimportant differences. or instance, the
egos are not extruded at all, but embryonic development takes
place within the body of the mother, which has previously under-
gone spontaneous division into several pieces. In this case, the eggs
differ from those of the other female form, inasmuch as they do not
constitute the whole of the endoderm, but are embedded (as was
stated above) in a fairly voluminous granular mass at the expense
of which, or at least by means of which, they are nourished’; for
they increase considerably in size during their development. But
not only this granular mass, but ell the layers of the body of the
mother, even the ectoderm, persist during the embryonic develop-
ment of the offspring. Indeed, the ectoderm must continue to
grow during the division of the mother animal, for it gradually
covers in the products of division on all sides, and, by means of
its cilia, causes the animal to swim about in the fluids of its host.
After some time the cilia are lost, and the separate parts into
which the mother animal has divided, fix themselves upon some part
of the body-cavity of the’ host; the young become free, and the re-
mains of the body of the mother probably disappear by dissolution
and resorption *. In this case the remains of the mother animal seem
to be, to some extent, consumed by the embryos,—a process which
sometimes, although very rarely, happens elsewhere. We can
scarcely consider this as a primitive arrangement, or look upon
it as a proof that ‘reproduction’ has a necessarily fatal effect upon
the Polyplastid organism.

In the male, the mass of spermatozoa does not swell out the
body to such an extent that its walls must give way and thus
permit an exit, but the large ectoderm cells atrophy spontaneously
at the time of maturity, and as they fall off, exit is given to the
spermatozoa here and there. In this instance also the dissolution
of the body is not a consequence of reproduction, but reproduction can
only take place when the dissolution of the body has preceded it!

cet: ON Fog aie
_ ® Julin does not enter into further details on this point, and it is not quite clear at
what precise time the cells of the ectoderm atrophy; but this is irrelevant to the
origin of death, since the granular mass surrounding the egg-cells at any rate belongs
to the soma of the mother.

K

-

130 LIFE AND DEATH.

In this remarkable arrangement we cannot discern anything
except an evident adaptation of the life of the body-cells to repro-
ductive purposes, and this adaptation was rendered possible beeause,
after the evacuation of the sexual cells, the body ceased to be of any
value for the maintenance of the species.

But even if we assume, that the death of the Orthonectides is,
in Gétte’s sense, a consequence of reproduction, inasmuch as, in the
two forms of females as well as in the male, the extrusion of a mass
of developed germ-cells or embryos deprives the organism of the
physiological possibility of living longer, how can we explain the
necessity of death as a direet consequence of reproduction in
all Polyplastides? Is the body—the soma—of the Metazoa so im-
perfectly developed, as compared with the reproductive cells, that
the extrusion of the latter involves the death of the former? Asa
matter of fact in the majority of cases the relations are reversed ;
the number of body-cells usually exceeds the germ-cells a hundred-
or a thousand-fold, and the body is, as regards nutrition, so com-
pletely independent of the reproductive cells, that it need not be
in the least disadvantageously affected by their extrusion. And
if the Orthonectid-like ancestors of the Metazoa were compelled
to give up their insignificant somatic part after the extrusion
of their germ-cells, because it could now no longer support itself,
does it therefore follow that the somatic cells had for ever lost the
power of surviving, even when their phyletic descendants were sur-
rounded by more favourable conditions? Had they to inherit ‘the
necessity of death’ for all time ? Whence came this great change in
the nature of organisms which, before the differentiation of Homo-
plastids into Heteroplastids, were endowed with the immortality of
unicellular beings ?

And it must be remembered that it is only an assumption which
places the Orthonectides among the lowest Metazoa (Heteroplastids).
I do not intend to greatly emphasize this point, but the formation
of the Gastrula by embole, and the absence of a mouth and ali-
mentary canal, shows that these parasites are extremely degenerate,
and the same may be said of almost all endoparasites. The Gas-
trula, as an independent organism, was without doubt primitively
provided with both mouth and stomach, and the mass of ova
filling the female Orthonectid is an adaptation to a parasitic life,
which on the one side renders the possession of a stomach a super-

LIFE AND DEATH. 131

fluity, and on the other demands the production of a great number
of germ-cells’. It is certain that the Orthonectides, as at present
constituted, cannot have lived in the free condition, and also that
their adaptation to parasitism cannot have arisen at the beginning
of the phyletic development of Metazoa, because they inhabit star-
fishes and Nemertines—both relatively highly developed Metazoa.
Hence it is, at any rate, doubtful whether the Orthonectides can
claim to pass as typical forms of the lowest Heteroplastids, and
whether their reproduction can be looked upon ‘as typical for the
unknown ancestors of all Polyplastids’ (l.¢., p. 45). If, however, we
accept some organism resembling these Orthonectides as the most
ancient Heteroplastid, being a free-living organism, it must have
had a stomach, and the cells surrounding it must—as a whole or in
part—have possessed the power of digesting ; at any rate, they
cannot all have been germ-cells, and therefore it is improbable that
death would be the direct result of the extrusion of the germ-cells.
Let us now consider the manner in which Gotte has endeavoured
to explain the transmission of the cause of death—-which first
appeared in the Orthonectides—from these organisms to all later
Metazoa, until the very highest forms are reached. Exact proofs
of this supposition are unfortunately wanting, and the evidence is
confined to the collection of a number of cases in which death and
reproduction take place nearly or quite simultaneously. These
would prove nothing, even if post hoc were always propter hoc; and
there are, opposed to them, a number of cases in which reproduction
and death take place at different times. In obtaining evidence for
‘the fatal influence of reproduction, is it possible to point to every
ease of sudden death after the act of oviposition or fertilization ?
These cases occur among many of the higher animals, especially in
Insects, and were collected by me in an earlier work’. It is

* Leuckart finds such a great resemblance between the newly born young of
Distoma and the Orthonectides, that he is inclined to believe that the latter are
Trematodes, ‘ which in spite of sexual maturity have not developed further than the

_ embryonic condition of the Distoma’ (‘ Zur Entwicklungsgeschichte des Leberegels,’
Zool. Anzeiger, 1881, No. 99). In reference to the Dicyemidae, which resemble the
Orthonectides in their manner of living and in their structure, Gegenbaur has stated
his opinion that they belong to a ‘stage in the development of Platyhelminthes’
(Grundriss d. vergleich. Anatomie). Giard includes both in the ‘phylum Vermes,’
and regards them as much degenerated by parasitism ; and Whitman—the latest inves-
tigator of the Dicyemids—speaks of them in a similar manner in his excellent work
‘ Contributions to the Life-history and Classification of Dicyemids’ (Leipzig, 1882).

? “Dauer des Lebens;’ translated as the first essay in this volume.
K 2

132 LIFE AND DEATH.

obvious that such cases are exceptional, but in a restricted sense it
is quite true, as far as these individual instances are concerned,
that death appears as a consequence of reproduction. The male bee,
which invariably dies while pairing, is undoubtedly killed in con-
sequence of a very powerful nervous shock ; and the female Psychid,
which has laid all her eggs at once, dies of ‘ exhaustion’—however
we may attempt to explain the term on physiological principles.
Can we conclude from these cases that the effects of reproduction
are, in Gdtte’s sense, universally fatal; that reproduction is the
positive and ‘exclusive explanation of natural death’? (I. ¢., p. 32.)
I need not linger over these isolated examples, but I turn at once
to the foundation of the whole conclusion—a foundation which is
obviously unable to support the superstructure erected on it.
Gotte formally derives the idea that death is a necessary condition
of reproduction, from a very heterogeneous collection of facets.
When we examine this collection we find that the process which is
taken to be death is not the same thing in all these instances,
while the same is true of the influence of reproduction by which
death is supposed to be caused. The whole conception arises out of
the process of encystment, which is regarded as the building-up of
reproductive material—that is, as true reproduction ; and since, ac-
cording to Gitte’s view, the formation of germs is always inti-
mately connected with an arrest of life, and since, by his own
definition, this stand-still of life is equivalent to death, it follows
that, with such a theory, reproduction, in its essential nature, must
be inseparably connected with death. It is necessary at this juncture
to remember what Gétte means by the process of rejuvenescence,
and to point out that he is dealing with something quite different
from ‘the fatal influence of reproduction,’ which was just now men-
tioned with regard to insects. ‘ Rejuvenescence,’ bound up as it is
with eneystment and reproduction, is, according to Gdtte, ‘a re-
coining of the specific protoplasm, by means of which the identity
of its substance is fixed by heredity,’ a ‘marvellous process in which
phenomena the most important in the whole life of the animal,
and in fact of all organisms—reproduction and death—have their
roots’ (l.¢., p. 81). Whether such re-coining really takes place or
not, at any rate I claim to have shown above that it does not cor-
respond with death in the Metazoa, and—if it is represented at all
in these latter—that it ought to be looked for in the reproductive

a

LIFE AND DEATH. 133

cells; and indeed, in another passage, Gotte himself has placed the
process in these cells.

While, among the Monoplastids, according to Gétte, the causes
of the supposed death lie hidden in this mysterious change of the
organism into reproductive material, Gotte asserts that among the
Polyplastids (such as Magosphaera, hypothetically perfected so as
to form a genuine Polyplastid), the causes of death operate so
that the organism breaks up into its component cells, all these
being still reproductive cells—a process which, unlike ‘ rejuvenes-
cence,’ has nothing mysterious about it, and which is certainly not
genuine death. In the Orthonectid-like animals death does not
occur as a consequence of the dispersal of the reproductive cells,
but rather because the part of the animal which remains is so
small and effete that, being unable to support itself, it necessarily
dies. From this point at least the object of death and the con-
ception of it remain the same, but now the idea of reproduction
undergoes a change. When the Rhabdite females of Ascaris are
eaten up by their offspring, is this mode of death connected with
the ‘rejuvenescence of protoplasm’? (l.¢., p. 34.) Is there any
deep underlying relationship between such an end and the essential
nature of reproduction? The same question may be asked with
regard to the ‘Redia or the Sporocyst of Trematodes, which are
converted into slowly dying sacs during the growth of the Cer-
cariae within them.’ We cannot speak of the ‘fatal influence
of reproduction’ among tape-worms just because ‘in the ripe seg-
ments the whole organization degenerates under the influence of
the excessive growth of the uterus.’ It certainly degenerates, but
only so far as the developing mass of eggs demands. In fact, at
a sufficiently high temperature, death does not occur, and such
mature segments of tape-worms creep about of their own accord.
‘We cannot fail to recognize that in this as well as in the above-
mentioned cases we have to do with adaptation to certain very
special conditions of existence—an adaptation leading to an im-
mense development of reproductive cells in a mother organism which
ean no longer take in nourishment, or which has become entirely
superfluous because its duty to its species is already fulfilled. If
this is an example of death inherent in the essential nature of re-
production, then so is the death of a mature segment of a tape-
worm in the gastric juices of the pig that eats it.

134 LIFE AND DEATH.

With Gotte, the conception of reproduction, like the conception of
death, is a protean form, which he welcomes in any shape, if only
he can use it as evidence. If death is a necessary consequence of
reproduction, its cause must be always essentially the same, and
might be expressed in one of the following suggestions :—(1) in
the necessity for a ‘re-coining’ of the protoplasm of the germ-
cells ; but here death could only affect the germ-cells themselves:
(2) perhaps in the withdrawal of nourishment by the mass of
developing reproductive material, just as death occurs sometimes
among men by the excessive drain on the system caused by morbid
tumours: (3) or in consequence of the development of the off-
spring in the body of the mother; this however would only affect
the females, and could therefore have no deep and general signifi-
cance: (4) from the extrusion of the sexual cells,—ova or sper-
matozoa,—and in the impossibility of further nourishment which is
consequent upon this extrusion—(Orthonectides ?): or (5) finally in
an excessively powerful nervous shock brought about by the ejection
of the reproductive cells.

But no one of these alternatives is the universal and inevitable
cause of death. This proves irrefutably that death does not proceed
as ah intrinsic necessity from reproduction, although it may be
connected with the latter, sometimes in one way and sometimes
in another. But we must not overlook the fact that in many
cases death is not connected with reproduction at all; for many
Metazoa survive for a longer or shorter period after the repro-
ductive processes have ceased.

In fact, I believe I have definitely shown that no process exists
among unicellular animals which is at all comparable with the
natural death of the higher organisms. Natural death first ap-
peared among multicellular beings, and among these first in the
Heteroplastids. Furthermore, it was not introduced from any
absolute intrinsic necessity inherent in the nature of living matter,
but on grounds of utility, that is from necessities which sprang
up, not from the general conditions of life, but from those special
conditions which dominate the life of multicellular organisms.
If this were not so, unicellular beings must also have been en-
dowed with natural death. I have already expressed these ideas
elsewhere, and have briefly indicated how far, in my opinion,

1 See the first essay upon ‘ The Duration of Life,’ p. 22 et seq.

LIFE AND DEATH. 135

natural death is expedient for multicellular organisms. I found
the essential reason for confining the life of the Metazoa to a
fixed and limited period, in the wear and tear to which an indi-
vidual is exposed in the course of a life-time. For this reason,
‘the longer the individual lived, the more defective and crippled
it would become, and the less perfectly would it fulfil the purpose
of its species’ (l.c., p. 24). Death seemed to me to be expedient

‘since ‘worn-out individuals are not only valueless to the species,

but they are even harmful, for they take the place of those which
are sound’ (l.c., p. 24).

I still adhere entirely to this explanation; not of course in the
sense that an actual physical struggle has ever taken place between
the mortal and immortal varieties of any species. If Gétte under-
stood me thus, he may be justified by the brief explanations given
in the essay to which I have alluded; but when he also attributes
to me the opinion that such hypothetically immortal Metazoa had
but a very limited period for reproduction, I fail to see what part
of the essay in question can be brought forward in support of his
statement. Only under some such supposition can I be reproached
with having assumed the existence of a process of natural selection
which could never be effective, because any advantage which accrued
to the species from the shortening of the duration of life could not
make itself felt in a more rapid propagation of the short-lived
individuals. The statement ‘that in this and in every other case
it is a sufficient explanation of the processes of natural selection
to render it probable that any kind of advantage is gained’? is
indeed erroneous. The explanation ought rather to be ‘that the
forms in question would for ever transmit their characters to a
greater number of descendants than the other forms.’ I have not
however as yet attempted to think out in detail such processes of
natural selection as would limit the somatic part of the Metazoan
body to a short term of existence, and I only wished to emphasize
the general principle lying at the basis of the whole process, with-
out stating the precise manner in which it operates.

If I now attempt to take this course, and to reconstruct theo-
retically the gradual appearance of natural death in the Metazoa,
I must begin by again alluding to Gdtte’s criticisms in reference
to the operation of natural selection.

+ * Ursprung des Todes,’ p. 29.

136 LIFE AND DEATH.

I consider death as an adaptation, and believe that it has arisen
by the operation of natural selection. Gdotte 1, however, concludes
from this that ‘the first origin of hereditary and consequently
(for the organization in question) necessary death, is not explained
but already assumed.’ ‘The operation and significance of the
principle of utility consists in selecting the fittest from among
the structures and processes which are at hand, and not in directly
creating new ones. Every new structure arises at first, quite
independently of any utility, from certain material causes present
in a number of individuals, and when it has proved useful and is
transmitted, it extends, according to the laws of the survival of the
fittest, in the group of animals in which it appeared. This exten-
sion will undergo further increase with every advance in utility
which results from further structural changes, until it extends
over the whole group. So that usefulness effects the preservation

and the distribution of new structures, but has nothing whatever

to do with the causes of their primary origin and their consequent
transmission to all other individuals. Indeed, on these hereditary
causes the necessity of the structures in question depends, so that
their usefulness in no way explains their necessity.’

‘These conclusions, when applied to the origin of natural death
called forth by internal causes, would show that it became inevitable
and hereditary in a number of the originally immortal Metazoa,
before there could be any question as to the benefits derived from its
influence. Such influence must have consisted in the fact that more
descendants survived the struggle for existence and were able to
enter-upon reproduction among the individuals which had inherited
- the predisposition to die than among the potentially immortal
beings which would be damaged in the struggle for existence,
and would therefore be exposed to still further injuries. The exist-
ing necessity for natural death in all Metazoa might therefore be
derived in an unbroken line of descent from the first mortal
Metozoan, of which the death became inevitable from internal
causes, before the principle of utility could operate in favour of its
dissemination.’

In reply to this I would urge: that it has been very often
maintained that natural selection can produce nothing new, but
can only bring to the front something which existed previously to

tT} os; DPIBe

.

”

LIFE AND DEATH. . 137

the exercise of choice ; but this argument is only true in a very
limited sense. The complex world of plants and animals which we
see around us contains much that we should call new in comparison
with the primitive beings from which, as we believe, everything
has developed by means of natural selection. No leaves or flowers,
no digestive system, no lungs, legs, wings, bones or muscles were
present in the primitive forms, and all these must have arisen
from them according to the principle of natural selection. These
primitive forms were in a certain sense predestined to develope
them, but only as possibilities, and not of necessity; nor were they
preformed in them. The course of development, as it actually took
place, first became a necessity by the action of natural selection,
that is by the choice of various possibilities, according to their
usefulness in fitting the organism for its external conditions of life.
If we once accept the principle of natural selection, then. we must
admit that it really can create new structures, instincts, ete., not
suddenly or discontinuously, but working by the smallest stages
upon the variations that appear. These changes or variations must
be looked upon as very insignificant, and are, as I have of late
attempted to show 1, quantitative in their nature; and it is only
by their accumulation that changes arise which are sufficiently
striking to attract our attention, so that we call them ‘new’
organs, instincts, ete.

These processes may be compared to a manon a journey who pro+
ceeds from a certain point on foot by short stages, at any given time,
and in any direction. He has then the choice of an infinite number
of routes over the whole earth. Ifsuch a man begins his wanderings
in obedience to the impulse of his own will, his own pleasure or
interest,—proceeding forwards, to the right or left, or even back-
wards, with longer or shorter pauses, and starting at any particular
time,—it is obvious that the route taken lies in the man himself and
is determined by his own peculiar temperament. His judgment,
experience, and inclination will influence his course at each turn
of his journey, as new circumstances arise. He will turn aside
from a mountain which he considers too lofty to be climbed; he
will incline to the right, if this direction appears to afford a better
passage over a swollen stream; he will rest when he reaches
a pleasant halting-place, and will hurry on when he knows that

See the preceding essay ‘On Heredity.’

138 LIFE AND DEATH.

enemies beset him. And in spite of the perfectly free choice open
to him, the course he takes is in fact decided by both the place and
time of his starting and by circumstances which—always occur-
ring at every part of the journey—impel him one way or the
other; and if all the factors could be ascertained in the minutest
detail, his course could be predicted from the beginning.

Such a traveller represents a species, and his route corresponds
with the changes which are induced in it by natural selection. The
changes are determined by the physical nature of the species, and
by the conditions of life: by which it is surrounded at any given
- time. A number of different changes may occur at every point,
but only that one will actually develope which is the most useful,
under existing external conditions. The species will remain
unaltered as long as it is in perfect equilibrium with its surround-
ings, and as soon as this equilibrium is disturbed it will commence
to change. It may also happen that, in spite of all the pressure
of competing species, no further change occurs because no one
of the innumerable very slight changes, which are alone possible
at any one time, can help in the struggle; just as the traveller who
is followed by an overpowering enemy, is compelled to suecumb
when he has been driven down to the sea. A boat alone could
save him, without it he must perish ; and so it sometimes happens
that a species can only be saved from destruction by changes of
a conspicuous kind, and these it is unable to produce.

And just as the traveller, in the coursé of his life, can wander an
unlimited distance from his starting-point, and may take the most
tortuous and winding route, so the structure of the original
organism has undergone manifold changes during its terrestrial
life. And just as the traveller at first doubts whether he will ever
get beyond the immediate neighbourhood of his starting-point,
and yet after some years finds himself very far removed from it—
so the insignificant changes which distinguish the first set of
generations of an organism lead on through innumerable other
sets, to forms which seem totally different from the first, but which
have descended from them by the most gradual transition. All
this is so obvious that there is hardly any need of a metaphor to
explain it, and yet it is frequently misunderstood, as shown by the
assertion that natural selection can create nothing new: the fact
being that it so adds up and combines the insignificant small de-

LIFE AND DEATH. 139

viations presented by natural variation, that it is continually pro-
ducing something new.

If we consider the introduction of natural death in connection
with the foregoing statements, we may imagine the process as
taking place in such a way that,—with the differentiation of Hetero-
plastids from Homoplastids, and the appearance of division of
labour among the homogeneous cell-colonies,—natural selection not
only operated upon the physiological peculiarities of feeding, moving,
feeling, or reproduction, but also upon the duration of the life of
single cells. At this developmental stage there would, at any rate,
be no further necessity for maintaining the power of limitless
duration.. The somatic cells might therefore assume a constitution
which excluded the possibility of unending life, provided only that
such a constitution was advantageous for them.

It may be objected that cells of which the ancestors possessed
the power of living for ever, could not become potentially mortal
(that is subject to death from internal causes) either suddenly or
gradually, for such a change would contradict the supposition which
attributes immortality to their ancestors and to the products of their
division. This argument is valid, but it only applies so long as
the descendants retain the original constitution. But as soon as
the two products of the fission of a potentially immortal cell ac-
quire different constitutions by unequal fission, another possibility
arises. Now it is conceivable that one of the products of fission
might preserve the physical constitution necessary for immortality,
but not the other; just as it is conceivable that such a cell—
adapted for unending life—might bud off a small part, which
would live a long time without the full capabilities of life pos-
sessed by the parent cell; again, it is possible that such a cell
might extrude a certain amount of organic matter (a true excre-
tion) which is already dead at the moment it leaves the body.
Thus it is possible that true unequal cell-division, in which only
one half possesses the condition necessary for increasing, may take
place; and in the same way it is conceivable that the constitution
of a cell determines the fixed duration of its life, examples of
which are before us in the great number of cells in the higher
Metazoa, which are destroyed by their functions. The more spe-
cialized a cell becomes, or in other words, the more it is intrusted
with only one distinct function, the more likely is this to be the

140 LIFE AND DEATH. A

ease: who then can tell us, whether the limited duration of life was
brought about in consequence of the restricted functions of the
cell or whether it was determined by other advantages!? In either
case we must maintain that the disadvantages arising from a
limited duration of the cells are more than compensated for by
the advantages which result from their highly effective specialized
functions. Although no one of the functions of the body is ne-
cessarily attended by the limited duration of the cells which per-
form it, as is proved by the persistence of unicellular forms, yet
any or all of them might lead to such a limitation of existence
without in any way. injuring the species, as is proved by the
Metazoa. But the reproductive cells cannot be limited’in this
way, and they alone are free from it. They could not lose their
immortality, if indeed the Metazoa are derived from the immortal
Protozoa, for from the very nature of that immortality it cannot
be lost. From this point of view the body, or soma, appears in
a certain sense as a secondary appendage of the real bearer of
life,—the reproductive cells.

Just as it was possible for the specific somatic cells to be differen-
tiated from among the chemico-physical variations which presented
themselves in the protoplasm, by means of natural selection, until
finally each function of the body was performed by its own special
kind of cell; so it might be possible for only those variations to
persist the constitution of which involved a cessation of activity
after a certain fixed time. If this became true of the whole mass
of somatic cells, we should then meet with natural death for the
first time. Whether we ought to regard this limitation of the
life of the specific somatic cells as a mere consequence of their
differentiation, or at the same time as a consequence of the powers
of natural selection especially directed to such an end,—appears
doubtful. But I am myself rather inclined to take the latter view,
for if it was advantageous to the somatic cells to preserve the un-
ending life of their ancestors—the unicellular organisms, this end

1 The problem is very easily solved if we seek assistance from the principle of
panmixia developed in the second essay ‘On Heredity.’ As soon as natural selec-
tion ceases to operate upon any character, structural or functional, it begins to dis-
appear. As soon, therefore, as the immortality of somatic cells became useless they
would begin to lose this attribute. The process would take place more quickly,
as the histological differentiation of the somatic cells became more useful and com-
plete, and thus became less compatible with their everlasting duration.— A.W. 1888.

ve

a

t
.
ee

LIFE AND DEATH. 141

might have been achieved, just as it was possible at a later period,
in the higher Metazoa, to prolong both the duration of life and of
reproduction a hundred- or a thousand-fold. At any rate, no reason
ean be given which would demonstrate the impossibility of such an
achievement.

With our inadequate knowledge it is difficult to surmise the
immediate causes of such a selective process.. .Who can point out
with any feeling of confidence, the direct advantages in which
somatic cells, capable of limited duration, excelled those capable
of eternal duration? Perhaps it was in a better performance of
their special physiological tasks, perhaps in additional material
and energy available for the reproductive cells as a result of this
renunciation of the somatic cells; or perhaps such additional
power conferred upon the whole organism a greater power of
resistance in the struggle for existence, than it would have had,
if it had been necessary to regulate all the cells to a corresponding
duration.

But we are not at present able to obtain a clear conception of
the internal conditions of the organism, especially when we are
dealing with the lowest Metazoa, which seem to be very rarely
found at the present day, and of which the vital phenomena we
only know as they are exhibited by two species, both of doubtful
origin. Both species have furthermore lost much of their original
nature, both in structure and function, as a result of their parasitic
mode of life. Of the Orthonectides and Dicyemidae we know
something, but of the reproduction in the single free non-parasitie
form, discovered by F. E. Schulze and named by him Trichoplax
adhaerens, we know nothing whatever, and of its vital phenomena
too little to be of any value for the purpose of this essay.

At this point it is advisable to return once more to the derivation
of death in the Metazoa from the Orthonectides, as Gitte en-
deavoured to derive it, when he overlooked the fact that, according
to his theory, natural death is inherited from the Monoplastids and
cannot therefore have arisen anew in the Polyplastids. According
to this theory, death must necessarily have appeared in the lowest
Metazoa as a result of the extrusion of the germ-cells, and by con-
tinual repetition must have become hereditary. We must not how-
ever forget that, in this case, the cause of death is exclusively
external, by which I mean that the somatic cells which remained

142 LIFE AND DEATH.

after the extrusion of the reproductive cells, were unable to feed
any longer or at any rate to an adequate extent ; and that the cause
of their death did not lie in their constitution, but in the unfavour-
able conditions which surrounded them. This is not so much a
process of natural death as of artificial death, regularly appearing
in each individual at a corresponding period, because, at a certain
time of life, the onganism becomes influenced by the same un-
favourable conditions. It is just as if the conditions of life in-
variably led to death by starvation at a certain stage in the life
of a certain species. But we know that death arises from purely
internal causes among the higher Metazoa, and that it is antici-
pated by the whole organisation as the normal end of life. Hence
nothing is gained by this explanation founded on the Ortho-
nectides, and we should have to seek further and in a later stage
of the development of the Metazoa, for the internal causes of true
natural death.

Another theory might be based upon the supposition that natural
death has been derived, in the course of time, from an artificial
death which always appeared at the same stage of each individual
life—as we have supposed to be the case in the Orthonectides. I
cannot agree with this view, because it involves the transmission of
acquired characters, which is at present unproved and must not be
assumed to occur until it has been either directly or indirectly de-
monstrated+. I cannot imagine any way in which the somatic
cells could communicate this assumed death by starvation to the
reproductive cells in such a manner that the somatic cells of the
resulting offspring would spontaneously die of hunger in the same
manner and at a corresponding time as those of the parent. It
would be as impossible to imagine a theoretical conception of such
transmission as of the supposed instance of kittens being born
without a tail after the parent’s tail had been docked ; although
to make the cases parallel the kittens’ tails ought to be lost at the
same period of life as that at which the parent lost hers. And
in my opinion we do not add to the intelligibility of such an
idea by assuming the artificial removal of tails through hundreds
of generations. Such changes, and indeed all changes, are, as |
think, only conceivable and indeed possible when they arise from
within, that is, when they arise from changes in the reproductive

1 See the preceding essay ‘ On Heredity.’

LIFE AND DEATH. 143

cells. But I find no difficulty in believing that variations in these
cells took place during the transition from Homoplastids to Hetero-
plastids, variations which formed the material upon which the un-
ceasing process of natural selection could operate, and thus led to
the differentiation of the previously identical cells of the colony
into dissimilar ones—some becoming perishable somatic cells, and
others the immortal reproductive cells.

It is at any rate a delusion to believe that we have explained
natural death, by deriving it from the starvation of the soma of the
Orthonectides, by the aid of the unproved assumption of the trans-
mission of acquired variations. We must first explain why these
organisms produce only a limited number of reproductive cells
which are all extruded at once, so that the soma is rendered help-

less. Why should not the reproductive cells ripen in succession as

they do indirectly among the Monoplastides, that is to say in a
succession of generations, and as they do directly in great num-
bers among the Metazoa? ‘There would then be no necessity for
the soma to die, for a few reproductive cells would always be pre-
sent, and render the persistence of the individual possible. In
fact, the whole arrangement—the formation of reproductive cells
at one time only, and their sudden extrusion,—presupposes the
mortality of the somatic cells, and is an adaptation to it, just as
this mortality itself must be regarded as an adaptation to the
simultaneous ripening and sudden extrusion of the generative cells.
In short, there is no alternative to the supposition stated above,
viz. that the mortality of the somatic cells arose with the differ-
entiation of the originally homogeneous cells of the Polyplastids
into the dissimilar cells, of the Heteroplastids. And this is the
first beginning of natural death. |

Probably at first the somatic cells were not more numerous than
the reproductive cells, and while this was the case the phenomenon
of death was inconspicuous, for that which died was very small.
But as the somatic cells relatively increased, the body became of
more importance as compared with the reproductive cells, until
death seems to affect the whole individual, as in the higher
animals, from which our ideas upon the subject are derived. In
_ reality, however, only one part succumbs to natural death, but it is

a part which in size far surpasses that which remains and is im-
mortal,—the reproductive cells.

144 LIFE AND DEATH.

_ Gétte combats the statement that the idea of death necessarily
implies the existence of a corpse. Hence he maintains that the
cellular sac which is left after the extrusion of the reproductive
cells among the Orthonectides, and which ultimately dies, is not
a corpse; ‘for it does not represent the whole organism, any
more than the isolated ectoderm of any other Heteroplastid’
(l.c., p. 48). But it is only a popular notion that a corpse must re-
present the entire organism. In cases of violent death this idea
is correct, because then the reproductive cells are also killed. But
"as soon as we recognise that the reproductive cells on the one side,
and the somatic cells on the other, form respectively the immortal
and mortal parts of the Metazoan organism, then we must acknow-
ledge that only the latter—that is, the soma without the re-
productive cells —suffers natural death. The fact that all the
reproductive cells have not left the body (as sometimes happens)
before natural death takes place, does not affect this conception.
Among insects, for instance, it may happen that natural death
occurs before all the reproductive cells have matured, and these
latter then die with the soma. But this does not make any differ-
ence to their potential immortality, any more than it modifies the
scientific ‘conception of a corpse. The idea of natural death in-
volves that of a corpse, which consists of the soma, and when the
latter happens to contain reproductive cells, these do not suecumb
to a natural death, which can never apply to them, but to an acci-
dental death. They are killed by the death of the soma just as
they might be killed by any other accidental cause of death.

The scientific conception of a corpse is not affected, whether the
dead soma remains whole for some time, or falls to pieces at once.
I cannot therefore agree with Gétte when he denies that an Ortho-
nectid possesses ‘the possibility of becoming a corpse’ (in his sense
of the word) because ‘its death consists in the dissolution of the
structure of the organism.’ When the young of the Rhabdites
form of Ascaris nigrovenosa bore through the body-walls of their
parent, cause it to disintegrate and finally devour it, the whole
organism disappears, and it would be difficult to say whether a
corpse exists in the popular sense of the word. But, scientifically
speaking, there is certainly a corpse; the real soma of the animal
dies, and this, however subdivided, must be considered as a corpse.
The fact that natural death is so difficult to define without any

LIFE AND DEATH. 145

accurate conception of what is meant by a corpse, proves the neces-
sity for arriving at a scientific idea as to the meaning of the latter.
There is no death without a corpse—whether the latter be small
or large, whole or in pieces.

If we compare the bodies of the higher Metazoa with those of
the lower, we see at once that not only has the structure of the
body increased in size and complexity as far as the soma is con-
cerned, but we also see that another factor has been introduced,
which exercises a most important influence in lengthening the
duration of life. This is the replacement of cells by multipli-
eation. Somatic cells have acquired (at any rate in most tissues)
the power of multiplying, after the body is completely developed
from the reproductive cells. The cells which have undergone
histological differentiation can increase by fission, and thus supply
the place of those which are being continually destroyed in the
course of metabolism. The difference between the higher and
lower Metazoa in this respect lies in the fact that there is only
one generation of somatic cells in the latter, and these are used
up in the process of metabolism at almost the same time that the
reproductive cells are extruded, while among the former there are
successive generations of somatic cells. I have elsewhere en-
deavoured to render the duration of life in the animal kingdom
intelligible by the application of this principle, and have attempted
to show that its varying duration is determined in different species
by the varying number of somatic cell-generations!. Of course,
the varying duration of each cell-generation materially influences
the total length of life, and experience teaches us that the duration
of cell-generations varies, not only in the lowest Metazoa as com-
pared with the highest, but even in the various kinds of cells
in one and the same species of animal.

We must, for the present, leave unanswered the question—upon
what changes in the physical constitution of protoplasm does
the variation in the capacity for cell-duration depend; and what
are the causes which determine the greater or smaller number of
cell-generations. I mention this obvious difficulty because it is
the custom to meet every attempt to search deeper into the com-
mon phenomena of life with the reproach that so much is still
left unexplained. If we must wait for the explanation of these

? See the first essay on ‘The Duration of Life.’
L

146 LIFE AND DEATH.

processes until we have ascertained the molecular structure of cells,
together with the changes that occur in this structure and the con-
sequences of the changes, we shall probably never understand either
the one or the other. The complex processes of life can only be
- followed by degrees, and we can only hope to solve*the great
problem by attacking it from all sides.

Therefore it is, in my opinion, an advance if we may assume that
length of life is dependent upon the number of generations of
somatic cells which can succeed one another in the course of a
single life; and, furthermore, that this number, as well as the
duration of each single cell-generation, is predestined in the germ
itself. This view seems to me to derive support from the obvious
fact that the duration of each cell-generation, and also the number
of generations, undergo considerable increase as we pass from the
lowest to the highest Metazoa.

In an earlier work! I have attempted to show how exactly the
duration of life is adapted to the conditions by which it is sur-
rounded ; how it is lengthened or shortened during the formation
of species, according to the conditions of life in each of them; in
short, how it is throughout an adaptation to these conditions. A
few points however were not touched upon in the work referred
to, and these require discussion ; their consideration will also throw
some light upon the origin of natural death and the forms of life
affected by it. =.

I have above explained the limited duration of the life of
somatic cells in the lower Metazoa—Orthonectides—as a pheno-
menon of adaptation, and have ascribed it to the operation of
natural selection, at the same time pointing out that the existence
of immortal Metazoan organisms is conceivable. If the Mono-
plastides are able to multiply by fission, through all time, then their
descendants, in which division of labour has induced the antithesis ,
of reproductive and somatic cells, might have done the same. If
the Homoplastid cells reproduced their kind unintérruptedly, equal
powers of duration must have been possible for the two kinds of
Heteroplastid cells ; they too might have been immortal so far as im-
mortality only depends upon the capacity for unlimited reproduction.

But the capacity for existence possessed by any species is not
only dependent upon the power within it; it is also influenced

1 See the first essay on ‘The Duration of Life.’

LIFE AND DEATH. 147

by the conditions of the external world, and this renders neces-
sary the process which we call adaptation. Thus it is just as in-
conceivable that either a homogeneous or a heterogeneous cell-colony
possessing the physiological value of a multicellular individual
should continue to grow to an unlimited extent by continued cell-
division, as it is inconceivable that a unicellular being should
increase in size to an unlimited extent. In the latter case the
process of cell-division imposes a limit upon the size attained by
growth. In the former, the requirements of nutrition, respiration,
and movement must prescribe a limit to the growth of the cell-
colony which constitutes the individual of the higher species, just
as in. the case of the unicellular Monoplastides, and it does not
affect the argument if we consider this limitation to be governed
by the process of natural selection. It would only be possible to
regulate the relations of the single cells of the colony to each other
by fixing the number of cells within narrow limits. During the
development of Magosphaera—one of the Homoplastides—the cells
arrange themselves in the form of a hollow sphere, lying in a
gelatinous envelope. But the fact that reproduction does not follow
the simple unvarying rhythm of unicellular organisms is of more
importance ; for a rhythm of a higher order appears, in which each
cell of the colony separates from its neighbours, when it has
reached a certain size, and proceeds by very rapid successive
divisions to give rise to a certain number of parts which arrange
_ themselves as a new colony. The number of divisions is controlled
by the number of cells to which the colony is limited, and at first -
this number may have been very small. With the introduction of
this secondary higher rhythm during reproduction, the first germ
of the Polyplastides became evident; for then each process of fission
was not, as in unicellular organisms, equivalent to all the others ;
" for in a colony of ten cells the first fission differs from the second,
third, or tenth, both in the size of the products of division and also
in remoteness from the end of the process. This secondary fission
is what we know as segmentation.

It seems to me of little importance whether the first' process of
segmentation takes place in the water or within a cyst, although it
is quite possible: that the necessity for some protective structure |
appeared at a very early period, in order to shield the segmenting
cell from danger.

: L2

148 LIFE AND DEATH.

It is impossible to accept Gétte’s conception of the germ (Keim),
and at this point the question arises as to its true meaning. I
should propose to include under this term every cell, eytode, or
group of cells which, while not possessing the structure of the
mature individual of the species, possesses the power of developing
into it under certain circumstances. The emphasis is now laid
upon the expression development, which is something opposed to
simple growth, without change of form. A cell which becomes
a complete individual by growth alone is not a germ but an
individual, although a very small one. For example, the small
encapsuled Heliozoon, which arises as the product of multiple
fission, is not a germ in our sense of the word. It is an individual,
provided with all the characteristic marks of its species, and it has
only to protrude the retracted processes (pseudopodia) and to take
in the expelled water (formation of vacuoles) in order to become
capable of living in a free state. In this sense of the word, germs
are not confined to the Polyplastides, but are found in many Mono-
plastides. There is nevertheless, in my opinion, a profound and
significant difference between the germs of these two groups. And
this lies not so much in the morphological as in the develop-
mental significance of these structures. As far as I have been able
to compare the facts, I may state that the germs of the Mono-
plastides are entirely of secondary origin, and have never formed
the phyletie origin of the species in which they are found. For
instance, the spore-formation of the Gregarines resulted from a
gradually increasing process of division, which was concentrated
into the period of eneystment; and it was induced by a necessity
for rapid multiplication due to the parasitic life and unfavourable
surroundings of these animals. If Gregarines were free-living
animals, they would not need this method of reproduction. The
encysted animal would probably divide into eight, four, or two
parts, or perhaps, like many Infusoria!, it would not divide at all,

1-These assumptions can be authenticated among the Infusoria. The encysted
Colpoda cucullus, Ehrbg, divides into two, four, eight, or sixteen parts; Otostoma
Carteri, into two, four, or eight; Tillina magna, Gruber, into four or five; Lagynus
sp. Gruber, into two; Amphileptus meleagris, Ehrbg. into two or four. The last two
species and many others frequently do not divide at all during the encysted con-
dition. But while any further increase in the number of divisions within the cyst
does not occur in free-swimming Infusoria, the interesting case of Ichthyophthirius
multifiliis, Fouquet, shows that parasitic habits call forth a remarkable increase in

a

LIFE AND DEATH. 149

so that. the whole reprodtiction would depend on simple fission
_ alone during the free state.

The original mode of reproduction among the Monoplastides was
undoubtedly simple fission. This became connected with encyst-
ment, which originally took place without multiplication ; and only |
when the divisions in the eyst became excessively numerous did
such minute plastids appear that a genuine process of development
had to be undergone in order to produce complete individuals.
Here we have the general conception of the germ as I defined it.
Its limitations are naturally not very sharply defined, for it ‘is
impossible to draw an absolute distinction between simple growth
and true development accompanied by changes in form and
structure. For instance, Hickel’s Protomyxa aurantiaca divides
within its cyst into numerous plastids, which might be spoken of
as germs. But the changes of form which they undergo before
they become young Protomyxae are very small, and for the most
part depend upon the expansion of the body, which existed in the
capsule as a contracted pear-shaped mass. It is therefore more
correct to speak only of the simple growth of the products of the
fission of the parent organism, and to look upon these products
as young Protomyxae rather than germs. On the other hand, the
young animals which creep, out. of the germs (the ‘spores’) of
Gregarina gigantea, described by E. van Beneden, differ essentially
from the adult, and pass through a series of developmental stages.
before they assume the characteristic form of a Gregarine,

This is true development+. But sucha method of germ-formation
and development are found most frequently, although not ex-
clusively, among the parasitic Monoplastides, and this fact alone
serves to indicate their secondary origin. It isa form of ontogenetic
development differing from that of the Polyplastides in that it does
not revert to a phyletically primitive condition of the species, but,
on the contrary, exhibits stages which first appear in the phyletic

the number of divisions. This animal divides into at least a thousand daughter in-
dividuals.

1 True development also takes place in the above-mentioned Ichthyophthirius.
While in other Infusoria the products of fission exactly resemble the parent, in
Ichthyophthirius they have a different form; the sucking mouth is wanting while
provisional clasping cilia are at first present. In this case therefore the word germ
may be rightly applied, and Ichthyophthirius affords an interesting example of the
phyletic origin of germs among the lower Flagellata and Gregernions Cf. Fouquet,
‘Arch, Zool. Expérimentale,’ Tom. V. p. 159. 1876.

150 LIFE AND DEATH.

development of the specific form. The Psorosperms were only
formed after the Gregarines had become established as a group. The
amoeboid organisms which creep out of them are in no way to be
regarded as the primitive forms of the Gregarines, even if the
latter may have resembled them, but they are coenogenetic forms
produced by the necessity for a production of numerous and very
minute germs. The necessity for a process of genuine develop-
ment perhaps depends upon the small amount of material contained
in one of these germs, and on other conditions, such as change of
host, change of medium, ete. It therefore results that the funda-
mental law of biogenesis does not apply to the Monoplastides ; for
these forms are either entirely without a genuine ontogeriy and
only possess the possibility of growth, or else they are only endowed
with a coenogenetic ontogeny 1. .

Some authorities may be inclined to limit the above proposition,
and to maintain that we must admit the possibility that we are
likely to occasionally meet with an ontogeny of which the stages
largely correspond with the most important stages in the phyletic
development of the species, and that the ontogenetic repetition of
the phylogeny, although not the rule, may still occur as a rare
exception in the Protozoa.

A careful consideration of the subject indicates, however, that
the occurrence of such an exception is very improbable. Such an
ontogeny would, for instance, occur if one of the lowest Mono-
‘plastides, such as a Moneron, were to develope into a higher form,
such as one of the Flagellata, with mouth, eye-spot, and cortical
layer, under such external conditions that it would be advantageous
for the existence of its species that it should no longer reproduce
itself by simple fission, but that the periodical formation of a cyst
(which was perhaps previously part of the life-history) should be
associated with the occurrence of numerous divisions within the cyst
itself, and with the formation of germs. We must suppose either
that these germs were so minute that the young animals could not

1 Biitschli, long ago, doubted the application of the fundamental law of bioge-
nesis to the Protozoa (cf. ‘ Ueber die Entstehung der Schwiirmsprisslings der Podo-
phrya quadripartita,’ Jen. Zeit. f. Med. u. Naturw. Bd. X. p. 19, Note). Gruber has
more recently expressed similar views, and in fact denies the. presence of develop-
ment in the Protozoa, and only recognizes growth (‘ Dimorpha mutans, Z. f. W. Z.’ Bd.
XXXVII. p. 445). This proposition must however be restricted, inasmuch as a de-
velopment certainly occurs, although one which is coenogenetic and not palingenetic.

LIFE AND DEATH. 151

become Flagellata directly, or that it was advantageous for them to
move and feed as Monera at an early period, and to assume the
more complex structure of the parent by gradual stages. In other
words, the phyletic development would proceed hand in hand with
the ontogeny corresponding to it, although not from any in-
ternal cause, but as an adaptation to the existing conditions of
life. But the supposed transformation of the species also depended
upon these same conditions of life, which must therefore have been
of such a nature as to bring about simultaneously, by an inter-
calation of germs and by a genuine development, the evolution of
the form in question in the last stage of its ontogeny, and the
maintenance of its original condition during the initial stage.
Such a combination of circumstances can have scarcely ever
happened. Against the occurrence of such a transformation as we
have supposed, it might be argued, indeed, that the assumed pro-
duction of very numerous germs does not occur among’ free-living
Monoplastides. Those which have acquired parasitic habits must be
‘younger phyletic forms, for their first host—-whether a lowly or
a highly organized Metazoon—must have appeared before they
could gain access to it and adapt themselves to the conditions of
a parasitic life, and by this time the Flagellate Infusoria were
already established. It is by far less probable that the persistence
or rather the intercalation of the ancestral form would occur in an
ontogenetic cycle, consisting of a series of stages, and not of
two only, as in our example. For as soon as reproduction can be
effected by the simple fission of the adult, not only is there no
reason why the earlier phyletic stages should be again and
again repeated, but such recapitulation is simply impossible.
We cannot, therefore, conclude that the anomalous early stages of
a Monoplastid such as Acimeta correspond with an early form of
phyletic development.

Supposing, for instance, that the Acinetaria were derived from
the Ciliata, then this transformation must have taken place in the
course of the continued division of the ciliate ancestor—partially:
connected with encystment, but for the most part independently of
it. Of the myriads of generations which such a process of develop-
ment may have occupied, perhaps the first set moved with suctorial
processes, while the second gradually adopted sedentary habits, and
throughout the whole of the long series, each succeeding generation

152 LIFE AND DEATH.

must have been almost exactly like its predecessor, and must
always have consisted of individuals which possessed the characters
of the species.

This does not exclude the possibility that in spite of an assumed
sedentary mode of life, the need for locomotion and for obtaining
food in fresh places may have arisen at some period of life. But
whenever formation of swarm-spores takes place instead of simple
fission, this does not depend upon the persistence of an ancestral
form in the ontogenetic cycle, but is due to the intercalation of an
entirely new ontogenetic stage, which happens to resemble an
ancestral form, in the possession of cilia, ete.

I imagine that I have now sufficiently explained the above
proposition, that the repetition of the phylogeny in the ontogeny
does not and cannot occur among unicellular organisms.

With the Polyplastides the opposite is the case. There is no
species, as far as we know, which does not—either in each in-
’ dividual, or after long cycles which comprise many individuals ~
(alternation of generations)—invariably revert to the Monoplastid ~
state. This applies from the lowest forms, such as Magosphaera and
the Orthonectides, up to the very highest. In the latter a great
number of intermediate phyletic stages always occur, although
some have been omitted as the result of concentration in the
ontogeny, while others have sometimes been intercalated.

Sexual reproduction is the obvious cause of this very important
arrangement. Even if this is an hypothesis rather than a fact
we must nevertheless accept it unconditionally, because it is a
method of reproduction found everywhere. It is the rule in every
group of the animal kingdom, and is only absent in a few species in
which it is replaced by parthenogenesis. In these latter instances
sexual reproduction may be local, and entirely absent im certain
districts only (Apus), or it may be only apparently wanting; in some
cases where it is undoubtedly absent, it is equally certain that it
was present at an earlier period (Limnadia Hermanni). We cannot
as yet determine whether its loss will not involve the degeneration
and ultimate extinction of the species in question.

If the essential nature of sexual reproduction depends upon the
conjugation of two equivalent but dissimilar morphological elements,
then we can understand that a multicellular being can only attain
sexual reproduction when a unicellular stage is present in its

LIFE AND DEATH. 1538

development ; for the coalescence of entire multicellular organisms
in such a manner that fusion would only take place between equi-
valent cells, would seem to be impracticable. In the necessity
for sexual reproduction, there is therefore also implied the ne-
cessity for reverting to the original condition of the Polyplastides— —
that of a single cell—and upon this alone depends the fundamental
law of biogenesis. This law is therefore confined to the Poly-
plastides, and does not apply to the Monoplastides; and Gotte’s
suggestion that the latter fall back into the primitive condition
of the organism during their encystment (rejuvenescence), finds
no support in this aspect of the question.

I have on a previous occasion ! referred the utility of death to the
ultimate fact that the unending life of the Metazoan body would
be a useless luxury, and to the fact that the individuals would
necessarily become injured in the course of time, and would be
therefore ‘not only valueless to the species, but ...even harmful,
for they take the place of those which are sound’ (1. ¢., p. 24). I
might also have said that such damaged individuals would sooner
or later fall victims to some accidental death, so that there would
be no possibility of real immortality. I now propose to ex-
amine this statement a little more closely, and to return to a
question which has already been alluded to before.

It is obvious that the advantages above set forth did not form
the motive which impelled natural selection to convert the im-
mortal life of the Monoplastides into the life of limited duration
possessed by the Heteroplastides, or more correctly, which led to the
restriction of potential immortality to the reproductive cells of the
latter. It is at any rate theoretically conceivable that a struggle
might arise between the mortal and immortal individuals of a
certain Metazoan species, and that natural selection might secure
the success of the former, because the longer the immortal in-
dividuals lived, the more defective they became, and as a result gave
rise to weaker offspring in diminished numbers. Probably no one
would be bold enough to suggest such a crude example of natural
selection. And yet I venture to think that the principle of
natural selection is here also to be taken into account, and even
plays, although in a negative rather than a positive way, a very
essential part in determining the duration of life in the Metazoa.

1 See the first essay on ‘The Duration of Life,’ p. 23 e¢ seg.

154 LIFE AND DEATH.

' When the somatic cells of the first Heteroplastides ceased to be
immortal, such a loss would not in any way have precluded them
from regaining this condition. Just as, with the differentiation of
the first somatic cells of the lowest Heteroplastides, their duration
was limited to that of a single cell-generation,—so it must have
been possible for them, at a later period and if the necessity arose,
to lengthen their duration over two, three, or more generations.
And if my theory of the’duration of life in the Metazoa is well
founded, these cells have as a matter of fact increased their duration,
to an extent about equal to that of the organism to which they
belong. There is no ground whatever for the assumption that it _
is impossible to fix the number of cell-generations at infinity,—as
actually happens in the case of the reproductive cells,—but on the
other hand it has already been shown to be obvious that such an
extension is opposed to the principle of utility. It could never be
to the advantage of a species to produce crippled individuals, and
therefore the infinite duration of individuals has never reappeared
among the Metazoa. So far the limited duration of Meta-
zoan life may be attributed to the worthlessness or even the
injurious nature of individuals, which although immortal, were
nevertheless liable to wear and tear. This fact explains why im-
mortality has never reappeared, it explains the predominance of
death, but it was not the single primary cause of this phenomenon.
The perishable and vulnerable nature of the soma was the reason
why nature made no effort to endow this part of the individual
with a life of unlimited length.

Gétte considers that death is inherent in reproduction, and in
a certain sense this is true, but not in the general way supposed by
him.

I have endeavoured to show above that it is most advantageous
for the preservation of the species among the lowest Metazoa, that
the body should consist of a relatively small number of cells, and
that the reproductive cells should ripen simultaneously and all
escape together. If this conclusion be accepted, the uselessness of
a prolonged life to the somatic cells is obvious, and the occurrence
of death at the time of the extrusion of the reproductive cells is
explained. In this manner death (of the soma) and reproduction
are here made to coincide.

This relation of reproduction to death still exists in a great num-

LIFE AND DEATH. 155

ber of the higher animals. But such an association, together with
the simultaneous ripening of the reproductive cells, has not been
maintained continuously in the past. As the soma becomes larger
and more highly organized, it is able to withstand more injuries,
and its average duration of life will extend: part passu with these
changes it will become increasingly advantageous not only for the
number of reproductive cells to be multiplied, but also for the time
during which they are produced to be prolonged. In this manner
a lengthening of the reproductive period arises, at first continuously
and then periodically. It is beyond my present purpose to consider
in detail the conditions upon which this lengthening depends, but
I would emphasize the fact that a lengthening of life is connected
with the increase in the duration of reproduction, while on the
other hand there is no reason to expect life to be prolonged
beyond the reproductive period; so that the end of this period is
usually more or less coincident with death.

A further prolongation of life could only take place when the
parent begins to undertake the duty of rearing the young. The
most primitive form of this is found among those animals, which
do not expel their reproductive cells as soon as they are ripe but
retain them within their bodies, so that the early stages of develop-
ment take place under the shelter of the parent organism. Associ-
ated with such a process there is frequently a necessity for the
germs to reach a certain spot, where alone their further development
can take place. Thus a segment of a tapeworm lives until it
has brought the embryos into a position which affords the possibility
of their passive transference to the stomach of their special host.
But the duration of life is first materially lengthened when the off-
spring begin to be really tended, and as a general rule the increase
in length is exactly proportional to the time which is demanded by
the care of the young. Accurately conducted observations are
wanting upon this precise point, but the general tendency of the
facts, as a whole, cannot be doubted. Those insects of which the
care for their offspring terminates with the deposition of eggs at the
appropriate time, place, etc., do not survive this act; and the dura-
tion of life in such imagos is shorter or longer according as the
_ éggs are laid simultaneously or ripen gradually. On the other hand,
insects—such as bees and ants—which tend their young, have a life
which is prolonged for years.

156 LIFE AND DEATH.

But thé lengthening of the reproductive period alone may result
in a marked increase in the length of life, as is proved by the queen-
bee. In all these cases it is easy to imagine the operation of
natural selection in producing such alterations in the duration of
life, and indeed we might accurately calculate the amount of in-
crease which would be produced in any given case if the necessary
data were available, viz. the physiological strength of the body, and
its relations to the external world, such as, for instance, the power
of obtaining food at various periods of life, the expenditure of energy
necessary for this end, and the statistics of destruction, that is, the
probabilities in favour of the accidental death of a single individual at
any given time. These statistics must be known both for the imagos,
larvae, and eggs; for the lower they are for the imagos, and the
higher for the larvae and eggs, the more advantageous will it be,
ceteris paribus, for the number of eggs produced by the imago to
be increased, and the more probable it would therefore be that a
long reproductive period, involving a lengthening of the life of the
imago, would be introduced. But we are still far from being able
to apply mathematics to the phenomena of life; the factors are too _
numerous, and no attempt has been made as yet to determine them
with accuracy.

But we must at least admit the principle that both the lengthen-
ing and shortening of life are possible by means of natural selection,
and that this process is alone able to render intelligible the exact
adaptation of the length of life to the conditions of existence.

A shortening of the normal duration of life is also possible; this
is shown in every case of sudden death, after the deposition of the
whole of the eggs at a single time. This occurs among certain
insects, while nearly allied forms of which the oviposition lasts over
many days therefore possess a correspondingly long imago-life. The
Ephemeridae and Lepidoptera afford many examples of this, and in an
earlier work I have collected some of them!. The humming-bird
hawk-moth flies about for weeks laying an egg here and there, and,
like the allied poplar hawk-moth and lime hawk-moth, probably
dies when it has deposited all the eggs which can be matured with
the amount of nutriment at its disposal. Many other Lepidoptera,
such as the majority of butterflies, fly about for weeks depositing’
their eggs, but others, such as the emperor-moths and lappet-

1 See Appendix to the first essay on ‘The Duration of Life,’ pp. 43-46.

LIFE AND DEATH. 157

moths, lay their eggs one after another and then die. The eggs of
the parthenogenetic Psychidae are laid directly after the imago has
left the cocoon, and death ensues immediately, so that the whole life
of the imago only lasts for a few hours. No one could look upon
this brief life as a primitive arrangement among Lepidoptera, any
more than we do upon the absence of wings in the female Psychidae;
shortening of life here is therefore clearly explicable.

In such cases have we any right to speak of the fatal effect of
reproduction? We may certainly say that these insects die of
exhaustion ; their vital strength is used up in the last effort of
laying eggs, and in the case of the males, in the act of copulation.
Reproduction is here certainly the most apparent cause of death, |
but a more remote and deeper cause is to be found in the limita-
tion of vital strength to the length and the necessary duties of
the reproductive period. The fact that there are female Lepi-
doptera which, like the emperor-moths, do not feed in the
mago-state, proves the truth of this statement. They still
possess a mouth and a complete alimentary canal, but they have
no spiral ‘tongue, and do not take food of any kind, not even a
drop of water. They live in a torpid condition for days or weeks
until fertilization is accomplished, and then they lay their eggs and
die. The habit of extracting honey from flowers—common to most
hawk-moths and butterflies—would not have thus fallen into
disuse, if the store of nutriment, accumulated in the form of the fat-
bodies, during the life of the caterpillar, had not been exactly
sufficient to maintain life until the completion of oviposition. The
fact that the habit of taking food has been thus abandoned is a
proof that the duration of life beyond the reproductive period would
not be to the advantage of the species.

The protraction of existence into old age among the higher
Metazoa proves that death is not a necessary consequence of repro-
duction. It seems to me that Gdtte’s statement ‘that the
appearances of senility must not be regarded as the general cause
of death’ is not in opposition to my opinions but rather to those
which receive general acceptance. I have myself pointed out that
‘death is not always preceded by senility or a period of old
age},

The materials are wanting for a comprehensive investigation of

1 See the first essay on ‘The Duration of Life,’ p. 21.

158 LIFE AND DEATH.

the causes which first introduced this period among the higher
Metazoa; in fact the most fundamental data are absent, for we do
not even know the part of the animal kingdom in which it first
appeared: we cannot even state the amount by which the duration
of life exceeds that of the period of reproduction, or what is the
value to the species of this last stage in the life of the individual.

It is in these general directions that we must seek for the sig-
nificance of old age. It is obviously of use to man, for it enables
the old to care for their children, and is also advantageous in enabling
the older individuals to participate in human affairs and to exer-
cise an influence upon the advancement of intellectual powers, and
thus to influence indirectly the maintenance of the race. But as
soon as we descend a step lower, if only as far as the apes, accurate
facts are wanting, for we are, and shall probably long be, ignorant
of the total duration of their life, and the point at which the period
of reproduction ceases.

I must here break off in the midst of these considerations, rather
than conclude them, for much still remains to be said. I hope,
nevertheless, that I have thrown new light upon some important
points, and I now propose to conclude with the following short
abstract of the results of my enquiry.

I. Natural death occurs only among multicellular beings; it
is not found among unicellular organisms. The process of eneyst-
ment in the latter is in no way comparable with death. ‘

II. Natural death first appears among the lowest Heteroplastid —
Metazoa, in the limitation of all the cells collectively to one
generation, and of the somatic or body-cells proper to a restricted
period : the somatic cells afterwards in the higher Metazoa came to
last several and even many generations, and life was lengthened to
a corresponding degree.

III. This limitation went hand in hand with a differentiation
of the cells of the organism into reproductive and somatic cells,
in accordance with the principle of division of labour. This diffe-
rentiation took place by the operation of natural selection.

IV. The fundamental biogenetic law applies only to multi-
cellular beings; it does not apply to unicellular forms of life.
This depends on the one hand upon the mode of reproduction by
fission which obtains among the Monoplastides (unicellular or-

LIFE AND DEATH. 159

ganisms), and on the other upon the necessity, induced by sexual
reproduction, for the maintenance of a unicellular stage in the
development of the Polyplastides (multicellular organisms).

V. Death itself, and the longer or shorter duration of life,
both depend entirely on adaptation. Death is not an essential
_ attribute of living matter; it is neither necessarily associated with
reproduction, nor a necessary consequence of it.

In conclusion, I should wish to call attention to an idea which i8
rather implied than expressed in this essay :—it is, that reproduc-
tion did not first make its appearance coincidently. with death.
Reproduction is in truth an essential attribute of living matter, just
as is the growth which gives rise to it. It is as impossible to
imagine life enduring without reproduction as it would be to
conceive life lasting” without the capacity for absorption of food
and without the power of metabolism. Life is continuous and
not periodically interrupted: ever since its first appearance upon
the earth, in the lowest organisms, it has continued without break ;
the forms in which it is manifested have alone undergone change.
Every individual alive to day—even the very highest—is to be
derived in an unbroken line from the first and lowest forms.

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IV.

THE CONTINUITY OF THE GERM-PLASM AS THE ©
FOUNDATION OF A THEORY OF HEREDITY.

1885.

CONTINUITY OF THE GERM-PLASM, &c.

PREFACE.

Tue ideas developed in this essay were first expressed during
the past winter in a lecture delivered to the students of this Uni-
versity (Freiburg), and they were shortly afterwards—in February
and the beginning of March—written in their present form.
I mention this, because I might otherwise be reproached for a
somewhat partial use of the most recent publications on related
subjects. Thus I did not receive Oscar Hertwig’s paper—‘ Contri-
butions to the Theory of Heredity’ (Zur Theorie der Vererbung),
until after I had finished writing my essay, and I could not there-
fore make as much use of it as I should otherwise have done.
Furthermore, the paper by Kélliker on ‘The Significance of the
Nucleus in the Phenomena of Heredity’ (Die Bedeutung der Zell-
kerne fiir die Vorgiinge der Vererbung), did not appear until after
the completion of my manuscript. The essential treatment of the
subject would not, however, have been altered if I had received the
papers at an earlier date, for as far as the most important point—the
significance of the nucleus—is concerned, my views are in accord-
ance with those of both the above-named investigators ; while the
points upon which our views do not coincide had already received

attention in the manuscript.
A. W:;
FREIBURG I. BREISGAU,
June 16, 1885.

(6%

CONTINUITY OF THE GERM-PLASM, &c.

CONTENTS.

PAGE

INTRODUCTION . : ‘ : ‘ ; ‘ : : : y 4 . 165
I. Tur GERM-PLASM . i . “274
1. Historical development of the theory as sto the losallcation of the ger.

plasm in the nucleus * eye 4

2. Nigeli’s ‘idioplasm’ is not identical with Weiueupaats oe patina 7. 180
3. A retransformation of somatic idioplasm into germ-idioplasm does not
take place 7 183
4, Confirmation of the theley. as to ‘the wiraiicauas of ie BES bale
stance afforded by Nussbaum’s and Gruber’s experiments on re-
“generation in Infusoria . . - 185
5. The nucleoplasm changes during eenaay medoraiione ie a ovine May . 186
‘6. The identity of the daughter-nuclei produced by indirect nuclear
division, as assumed by Strasburger, is not necessary for my theory 187
7. The gradual decrease in complexity of the structure of the nucleus.

during ontogeny : ‘ - I90
8. Niigeli’s view on the germs ( ‘Makigiers j in the idicasladiny ; is Re ey
9. The manner in which germ-cells arise from somatic cells. ‘ - 194
10. ‘Embryonic’ cells in the mature organism . : - 196

11. The rule of probability is against a retransformation of smenatig idio-
plasm into germ-plasm . 198

12. The regular cyclical decclonment of ihe. Satna founded wah
phylogeny by Nageli ‘ : 199

13. It follows from phyletic oonniatientont that the gemcel ave ‘ot
arisen at the end of ontogeny = 201

14. They originally arose at the beginning of Suicuays but “a a “Jaber
period the time of their origin was displaced . : : ; + 202
15. A continuity of the germ-cel/s does not now exist in most cases . + 205
16. But there is a continuity of the germ-plasm. : : sUaOR

17. Strasburger’s objection to my supposition that the rae Ae passes
along distinct routes ; + 209

18. The cell-body may remain iithanged Aa the siescieant is thanged «210
19. It is conceivable that all somatic nuclei may contain some germ-plasm 211

II. THE SIGNIFICANCE OF THE PoLAR BODIES . - . 2 : 2 - 212
1. The egg-cell contains two kinds of idioplasm ; germ-plasm and histo-
genetic nucleoplasm . - 213
2. The expulsion of the polar liddink signifies ‘te secnoval of the tae
genetic nucleoplasm . . ; . + 3214
8. Other theories as to the significance of ihe polar idice : ; ah

M 2

164 CONTINUITY OF THE GERM-PLASM, &c,—CONTENTS.

PAGE
4. The modes of occurrence of polar bodies. ; : ; a.) aaa
5. Their possible occurrence in male germ-cells - 3 : - <a
6. There are two kinds of nucleoplasm in the male germ-cells . - « ia
7. Polar bodies in plants . * : ; : . - 3 - 222
8. Morphological origin of polar bodign Thee ; ; - : - 223
ITI. ON THE NATURE OF PARTHENOGENESIS 225
1. The phenomena exhibited in the snaturation of ie es are identical
in parthenogenetic and sexual development . : 225

2. The difference between parthenogenetic and sexual cells ater. be oh a

quantitative nature . >. Siig Dee
3. The quantity of the germ-plasm Kitectalvive Aeealupeaads . 227
4. The expulsion of polar bodies depends upon the es bebo

germ-plasm and ovogenetic nucleoplasm . : . + 230

5. Fertilization does not act dynamically. . . ee
6. An insufficient quantity of germ-plasm arrests daonlopeaant’ b + 232
7. Relation of the nucleus to the cell sid 3. - 234
8. The case of the bee does not constitute any objeation > my theory + 234
9. Strasburger’s views upon parthenogenesis . : +, 237
10. Parthenogenesis does not depend upon abundant nasties s > + 239"
11. The indirect causes of sexual and parthenogenetic reproduction . a7 aM
12. The direct causes . : : : : ; a See
13. Explanation of the Hardctin of nibetitve calla F 4 3 je . 243
14. Identity of the germ-plasm in male and female germ-cells . - . 246

IV.

THE CONTINUITY OF THE GERM-PLASM AS THE
FOUNDATION OF A THEORY OF HEREDITY.

INTRODUCTION.

Wuen we see that, in the higher organisms, the smallest
structural details, and the most minute peculiarities of bodily and
mental disposition, are transmitted from one generation to another ;
when we find in all species of plants and animals a thousand
characteristic peculiarities of structure continued unchanged through
long series of generations; when we even see them in many cases
unchanged throughout whole geological periods ; we very naturally
ask for the causes of such a striking phenomenon: and enquire how
it is that such facts become possible, how it is that the individual is
able to transmit its structural features to its offspring with such
precision. And the immediate answer to such a question must be
given in the following terms :—‘ A single cell out of the millions
of diversely differentiated cells which compose the body, becomes
specialized as a sexual cell; it is thrown off from the organism
and is capable of ‘reproducing all the peculiarities of the parent
body, in the new individual which springs from it by cell-division
and the complex process of differentiation.’ Then the more precise
question follows : ‘ How is it that such a single cell can reproduce
the tout ensemble of the parent with all the faithfulness of a
portrait ?’

The answer is extremely difficult; and no one of the many
attempts to solve the problem can be looked upon as satisfactory ;
no one of them can be regarded as even the beginning of a solution or
as a secure foundation from which a complete solution may be
expected in the future. Neither Hiickel’s!, ‘Perigenesis of, the
Plastidule,’ nor Darwin’s ? ‘ Pangenesis, can be regarded as such a
beginning. The former hypothesis does not really treat of that

1 Hickel, ‘ Ueber die Wellenzeugung der Lebenstheilchen etc.,’ Berlin, 1876.
? Darwin, ‘The Variation of Animals and Plants under Domestication,’ vol. ii.

1875, chap. xxvii. pp. 344-399.

166 THE CONTINUITY OF THE GERM-PLASM AS THE

part of the problem which is here placed in the foreground, viz.
the explanation of the fact that the tendencies of heredity are
present in single cells, but it is rather concerned with the question
as to the manner in which it is possible to conceive the trans-
mission of a certain tendency of development into the sexual cell,
and ultimately into the organism arising from it. The same may
be said of the hypothesis of His’, who, like Hickel, regards heredity
as the transmission of certain kinds of motion. On the other hand,
it must be conceded that Darwin’s hypothesis goes to the very root
of the question, but he is content to give, as it were, a provisional
or purely formal solution, which, as he himself says, does not claim
to afford insight into the real phenomena, but only to give us
the opportunity of looking at all the facts of heredity from a
common standpoint. It has achieved this end, and I believe it

has unconsciously done more, in that the thoroughly logical ap-
plication of its principles has shown that the real causes of
heredity cannot lie in the formation of gemmules or in any
allied phenomena. The improbabilities to which any such theory
would lead are so great that we can affirm with certainty
that its details cannot accord with existing facts. Further-
more, Brooks’? well-considered and brilliant attempt to modify
the theory of Pangenesis, cannot escape the reproach that it
is based upon possibilities, which one might certainly describe as
improbabilities. But although I am of opinion that the whole
foundation of the theory of Pangenesis, however it may be modified,
must be abandoned, I think, nevertheless, its author deserves
great credit, and that its production has been one of those indirect
roads along which science has been compelled to travel in order to
arrive at the truth. Pangenesis is a modern revival of the oldest
theory of heredity, that of Democritus, according to which the
sperm is secreted from all parts of the body of both sexes during
copulation, and is animated by a bodily force; according to this
theory also, the sperm from each part of the body reproduces the
same part *. | :

1 His, ‘ Unsre Kérperform etc.,’ Leipzig, 1875.
- ? Brooks, ‘The Law of Heredity,’ Baltimore, 1883.

* Galton’s experiments on transfusion in Rabbits have in the mean time really
proved that Darwin’s gemmules do not exist. Roth indeed states that Darwin has

never maintained that his gemmules make use of the circulation as a medium, but
while on the one hand it cannot be shown why they should fail to take the

a

4
~

_-- -: a —

FOUNDATION OF A THEORY OF HEREDITY. 167

If, according to the received physiological and morphological
ideas of the day, it is impossible to imagine that gemmules pro-
duced by each cell of the organism are at all times to be found in
all parts of the body, and furthermore that these gemmules are col-
lected in the sexual cells, which are then able to again reproduce in
a certain order each separate cell of the organism, so that each
sexual cell is capable of developing into the likeness of the parent
body; if all this is inconceivable, we must enquire for some other
way in which we can arrive at a foundation for the true under-
standing of heredity. My present task is not to deal with the
whole question of heredity, but only with the single although
fundamental question—‘ How is it that a single cell of the body
can contain within itself all the hereditary tendencies of the whole —
organism?’ I am here leaving out of account the further ques-
tion as to the forces and the mechanism by which these ten-
dencies are developed in the building-up of the organism. On
this account I abstain from considering at present the views of
Nigeli, for as will be shown later on, they only slightly touch this
fundamental question, although they may certainly claim to be of
the highest importance with respect to the further question alluded
to above.

Now if it is impossible for the germ-cell to be, as it were,
an extract of the whole body, and for all the cells of the organism
to despatch small particles to the germ-cells, from which the
latter derive their power of heredity; then there remain, as it
seems to me, only two other possible, physiologically conceivable,
theories as to the origin of germ-cells, manifesting such powers as
we know they possess. Either the substance of the parent germ-
cell is eapable of undergoing a series of changes which, after the
building-up of a new individual, leads back again to identical germ-
cells; or the germ-cells are not derived at all, as far as their
essential and characteristic substance is concerned, from the body of

favourable opportunities afforded by such a medium, inasmuch as they are said to be
constantly circulating through the body ; so on the other hand we cannot understand
how the gemmules could contrive to avoid the circulation. Darwin has acted very
wisely in avoiding any explanation of the exact course in which his gemmules
circulate. He offered his hypothesis as a formal and not as a real explanation.
Professor Meldola points out to me that Darwin did not admit that Galton’s ex-
periments disproved pangenesis (‘ Nature,’ April 27, 1871, p. 502), and Galton also
admitted this in the next number of ‘ Nature’ (May 4, 1871, p. 5).—A. W. 1889.

168 THE CONTINUITY OF THE GERM-PLASM AS THE

the individual, but they are derived directly from the parent germ-
cell.

I believe that the latter view is the true one: I have expounded
it for a number of years, and have attempted to defend it, and to
work out its further details in various publications. I propose to
call it the theory of ‘ The Continuity of the Germ-plasm,’ for it is
founded upon the idea that heredity is brought about by the trans-
ference from one generation to another, of a substance with a defi-
nite chemical, and above all, molecular constitution. I have called
this substance ‘germ-plasm, and have assumed that it possesses
a highly complex structure, conferring upon it the power of de-
veloping into a complex organism. I have attempted to explain
heredity by supposing that in each ontogeny, a part of the specific
germ-plasm contained in the parent egg-cell is not used up in the
construction of the body of the offspring, but is reserved unchanged
for the formation of the germ-cells of the following generation.
It is clear that this view of the origin of germ-cells explains the
phenomena of heredity very simply, inasmuch as heredity becomes
thus a question of growth and of assimilation—the most funda-
mental of all vital phenomena. If the germ-cells of successive
generations are directly continuous, and thus only form, as it were,
different parts of the same substance, it follows that these cells
must, or at any rate may, possess the same molecular constitution,
and that they would therefore pass through exactly the same stages
under certain conditions of development, and would form the same
final product. The hypothesis of the continuity of the germ-plasm
gives an identical starting-point to each successive generation, and
thus explains how it is that an identical product arises from all of
them. In other words, the hypothesis explains heredity as part of
the underlying problems of assimilation and of the causes which aet
directly during ontogeny: it therefore builds a foundation from
which the explanation of these phenomena can be attempted.

It is true that this theory also meets with difficulties, for it
seems to be unable to do justice toa certain class of phenomena, viz.
the transmission of so-called acquired characters. I therefore gave
immediate and special: attention to this point in my first publi-
cation on heredity 1, and I believe that I have shown that the

1 Weismann, ‘ Ueber die Vererbung.’ Jena, 1883; translated in the present volume
as the second essay ‘On Heredity.’

3

FOUNDATION OF A THEORY OF HEREDITY. ~ 169

hypothesis of the transmission of acquired characters—up to that
time generally accepted—is, to say the least, very far from being
proved, and that entire classes of facts which have been interpreted.
under this hypothesis may be quite as well interpreted otherwise,
while in many cases they must be explained differently. I have
shown that there is no ascertained fact, which, at least up to the
present time, remains in irrevocable conflict with the hypothesis of
the continuity of the germ-plasm; and I do not know any reason
why I should modify this opinion to-day, for I have not heard of
any objection which appears to be feasible: E. Roth? has objected
that in pathology we everywhere meet with the fact that acquired
local disease may be transmitted to the offspring as a predispo-
sition ; but all such cases are exposed to the serious criticism that
the very point that first needs to be placed on a secure footing is
incapable of proof, viz. the hypothesis that the causes which in each
particular case led to the predisposition, were really acquired.
It is not my intention, on the present occasion, to enter fully
into the question of acquired characters; I hope to be able to
consider the subject in greater detail at a future date. But in
the meantime I should wish to point out that we ought, above
all, to be clear as to what we really mean by the expression ‘ac-
quired character. An organism cannot acquire anything unless it
already possesses the predisposition to acquire it: acquired cha-
racters are therefore no more than local or sometimes general
variations which arise under the stimulus provided by certain ex-
ternal influences. If by the long-continued handling of a rifle, the
so-called ‘ Exercierknochen’ (a bony growth caused by the pres-
sure of the weapon in drilling) is developed, such a result depends
upon the fact that the bone in question, like every other bone, con-
tains within itself a predisposition to react upon certain mechanical
stimuli, by, growth in a certain direction and to a certain extent.
The predisposition towards an ‘ Exercierknochen’ is therefore already
present, or else the growth could not be formed; and the same
reasoning applies to all other ‘ acquired characters.’
Nothing can arise in an organism unless the predisposition to it
is pre-existent, for every acquired character is simply the reaction
of the organism upon a certain stimulus. Hence I should never
have thought of asserting that predispositions cannot be. trans-
1 E. Roth, ‘ Die Thatsachen der Vererbung.’ 2. Aufi., Berlin, 1885, p. 14.

170 THE CONTINUITY OF THE GERM-PLASM AS THE

mitted, as E. Roth appears to believe. For instance, I freely admit
that the predisposition to an ‘ Exercierknochen’ varies, and that a
strongly marked predisposition may be transmitted from father to
son, in the form of bony tissue with a more susceptible constitution.

But I should deny that the son could develope an ‘ Exercierknochen’

without having drilled, or that, after having drilled, he could develope
it more easily than his father, on account of the drilling through
which the latter first acquired it. I believe that this is as im-
possible as that the leaf of an oak should produce a gall, without
having been pierced by a gall-producing insect, as a result of the
thousands of antecedent generations of oaks which have been pierced
by such insects, and have thus ‘acquired’ the power of producing
galls. I am also far from asserting that the germ-plasm—which, as
Lhold, is transmitted as the basis of heredity from one generation to
another—is absolutely unchangeable or totally uninfluenced by
forces residing.in the organism within which it is transformed
into germ-cells. “Iam also compelled to admit that it is conceiv-
able that organisms may exert a modifying influence upon their
germ-cells, and even that such a process is to a certain extent in-
evitable. The nutrition and growth of the individual must exercise
some influence upon its germ-cells ; but in the first place this in-
fluence must: be extremély slight, and in the second place it cannot
act in the manner in which it is usually assumed that it takes place.
A change of growth at the periphery of an organism, as in the case

of an ‘ Exercierknochen,’ can never cause such a change in the mole-

cular structure of the germ-plasm as would augment the predis-

position to an ‘ Exercierknochen,’ so that the son would inherit an —

increased susceptibility of the bony tissue or even of the particular

bone in question. But any change produced will result from the -

reaction of the germ-cell upon changes of nutrition caused by
alteration in growth at the periphery, leading to some change
-in the size, number, or arrangement of its molecular units. In the
present state of our knowledge there is reason for doubting whether
such reaction can occur at all ; but, if it can take place, as all events
the quality of the change in the germ-plasm can have nothing to
do with the quality of the acquired character, but only with the
_) way in which the general nutrition is influenced by the latter. In

the case of the ‘Exercierknochen’ there would be practically no

change in the general nutrition, but if such a bony growth could

FOUNDATION OF A THEORY OF HEREDITY. 171

reach the size of a carcinoma, it is conceivable that a disturbance of
the general nutrition of the body might ensue. Certain experi-
ments on plants—in which Niigeli showed that they can be sub-
mitted to strongly varied conditions of nutrition for several genera-
tions, without the production of any visible hereditary change—
show that the influence of nutrition upon the germ-cells must be
very slight, and that it may possibly leave the molecular structure
of the germ-plasm altogether untouched. This conclusion is also
supported by comparing the uncertainty of these results with the
remarkable precision with which heredity acts in the case of those

’ characters which are known to be transmitted. In fact, up to the
present time, it has never been proved that any changes in general

nutrition can modify the molecular structure of the germ-plasm,
and far less has it been rendered by any means probable that
the germ-cells can be affected by acquired changes which have no
influence on general nutrition. If we consider that each so-called
predisposition (that is, a power of reacting upon a certain stimulus
in a certain way, possessed by any organism or by one of its
parts) must be innate, and further that each acquired character is

_ only the predisposed reaction of some part of an organism upon

some external influence ; then we must admit that only one of the
causes which produce any acquired character can be transmitted,
the one which was present before the character itself appeared, viz.
the predisposition; and we must further admit that the latter
arises from the germ, and that it is quite immaterial to the follow-
ing generation whether such predisposition comes into operation or
not. The continuity of the germ-plasm is amply sufficient to
account for such a phenomenon, and I do not believe that any
objection to my hypothesis, founded upon the actually observed
phenomena of heredity, will be found to hold. If it be accepted,
many facts will appear in a light different from that which has been
cast upon them by the hypothesis which has been hitherto received,
—a hypothesis which assumes that the organism produces germ-
cells afresh, again and again, and that it produces them entirely
from its own substance. Under the former theory the germ-cells
are no longer looked upon as the product of the parent’s body, at
least as far as their essential part—the specific germ-plasm—is
concerned: they are rather considered as something which is to be
placed in contrast with the tout ensemble of the cells which make

172 THE CONTINUITY OF THE GERM-PLASM AS THE

up the parent’s body, and the germ-cells of succeeding generations
stand in a similar relation to one another as a series of generations
of unicellular organisms, arising by a continued process of cell-
division. It is true that in most cases the generations of germ-cells
do not arise immediately from one another as complete cells, but
only as minute particles of germ-plasm. This latter substance,
however, forms the foundation of the germ-cells of the next genera-
tion, and stamps them with their specific character. Previous to
the publication of my theory, G. Jager}, and later M. Nussbaum %,
have expressed ideas upon heredity which come very near to my
own*. Both of these writers started with the hypothesis that there

1 Jager, ‘ Lehrbuch der allgemeinen. Zoologie,’ Bd. II. Leipzig, 1878.

2M. Nussbaum, ‘Die Differenzirung des Geschlechts im Thierreich,’ Arch. f.
Mikrosk. Anat., Bd. XVIII. 1880.

* I have since learnt that Professor Rauber of Dorpat also expressed similar
views in 1880; and Professor Herdman of Liverpool informs me that Mr. Francis
Galton had brought forward in 1876 a theory of heredity of which the fundamental ’
idea in some ways approached that of the continuity of the germ-plasm (‘ Journal
of the Anthropological Institute, vol. v; London, 1876).—A. W., 1888.

[A less complete theory was brought forward by Galton at an earlier date, in
1872 (see Proc. Roy. Soc. No. 136, p. 394). In this paper he proposed the idea that
heredity chiefly depends upon the development of the offspring from elements directly
derived from the fertilized ovum which had produced the parent. Galton speaks of
the fact that ‘each individual may properly be conceived as consisting of two parts,
one of which is latent and only known to us by its effects on his posterity, 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.’ The following diagram shows clearly ‘that the span of each of the links in
the general chain of heredity extends from one structureless stage to another, and
not from person to person :—

Structureless elements } .. Adult Father ... structureless elements
in Father ... Latent in Father... in Offspring.’
Again Galton states—‘ Out of the structureless ovum the embryonic elements are
taken ...and these are developed (a) into the visible adult individual; on the
other hand..., after the embryonic elements have been segregated, the large
residue is developed (0) into the latent elements contained in the adult individual.’
The above quoted sentences and diagram indicate that Galton does not derive the
whole of the hereditary tendencies from the latent elements, but that he believes
some effect is also produced by the patent elements. When however he contrasts
the relative power of these two influences, he attaches comparatively little importance
to the patent elements. Thus if any character be fixed upon, Galton states that it
‘may be conceived (1) as purely personal, without the concurrence of any latent
equivalents, (2) as personal but conjoined with latent equivalents, and (3) as existent
wholly in a latent form.’ He argues that the hereditary power in the first case is

FOUNDATION OF A THEORY OF HEREDITY. 173

must be a direct connexion between the germ-cells of succeeding
generations, and they tried to establish such a continuity by sup-
posing: that the germ-cells of the offspring are separated from the
parent germ-cell before the beginning of embryonic development,
or at least before any histological differentiation has taken place.
In this form their suggestion cannot be maintained, for it is in
conflict with numerous facts. A continuity of the germ-cel/s does
not now take place, except in very rare instances; but this fact
does not prevent us from adopting a theory of the continuity of
the germ-plasm, in favour of which much weighty evidence can be
brought forward. In the following pages I shall attempt to develope
further the theory of which I have just given a short account, to
defend it against any objections which. have been brought forward,
and to draw from it new conclusions which may perhaps enable us
more thoroughly to appreciate facts which are known, but im-
perfectly understood. It seems to me that this theory of the con-
tinuity of the germ-plasm deserves at least to be examined in all its
details, for it is the simplest theory upon the subject, and the one
which is most obviously suggested by the facts of the case, and we
shall not be justified in forsaking it for a more complex theory
until proof that it can be no longer maintained is forthcoming.
It does not presuppose anything except facts which can be observed
at any moment, although they may not be understood,—such as
assimilation, or the development of like organisms from like germs;
while every other theory of heredity is founded on hypotheses which
cannot be proved. It is nevertheless possible that continuity of
the germ-plasm does not exist in the manner in which I imagine
that it takes place, for no one can at present decide whether all the

exceedingly feeble, because ‘ the effects of the use and disuse of limbs, and those of
habit, are transmitted to posterity in only a very slight degree.’ He also argues that
many instances of the supposed transmission of personal characters are really due
to latent equivalents. ‘The personal manifestation is, on the average, though it
need not be so in every case, a. certain proof of the existence of latent elements.’ .
Having argued that the strength of the latter in heredity is further supported by
the facts of reversion, Galton considers it is safe to conclude ‘ that the contribution
from the patent elements is very much less than from the latent ones.’ In the
later development of his theory, Galton adheres to the conception of ‘gemmules’
and accepts Darwin’s views, although ‘with considerable modification” Together
with pangenesis itself, Galton’s theory must be looked upon as preformational, and
so far it is in opposition to Weismann’s theory which is epigenetic. See Appendix
IV. to the next Essay (V.), pp. 316-319.—E. B. P.]

174 THE CONTINUITY OF THE GERM-PLASM AS THE

ascertained facts agree with and can be explained by it. Moreover
the ceaseless activity of research brings to light new facts every
day, and I am far from maintaining that my theory may not be
disproved by some of these. But even if it should have to be
abandoned at a later period, it seems to me that, at the present time,
it is a necessary stage in the advancement of our knowledge, and
one which must be brought forward and passed through, whether
it prove right or wrong, in the future. In this spirit I offer the
following considerations, and it is in this spirit that I should wish
them to be received.

I. Tue GerM-piasm.

I must first define precisely the exact meaning of the termy
germ-plasm.

In my previous writings in which the subject has been alluded

to, I have simply spoken of germ-plasm without indicating more
" precisely the part of the cell in which we may expect to find this
substance—the bearer of the characteristic nature of the species
and of the individual. In the first place such a course was sufficient
for my immediate purpose, and in the second place the number of
ascertained facts appeared to be insufficient to justify a more exact
definition. I imagined that the germ-plasm was that part of a
germ-cell of which the chemical and physical properties—including
the molecular structure—enable the cell to become, under appro-
priate conditions, a new individual of the same species. I therefore
_ believed it to be some such substance as Nigeli 1, shortly afterwards,
called idioplasm, and of which he attempted, in an admirable
manner, to give us a clear understanding. Even at that time
one might have ventured to suggest that the organized substance
of the nucleus is in all probability the bearer of the phenomena of
heredity, but it was impossible to speak upon this point with any
degree of certainty. O. Hertwig? and Fol* had shown that the
process of fertilization is attended by a conjugation of nuclei, and
Hertwig had even then distinctly said that fertilization generally

1 Niigeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre.’ Miinchen
u. Leipzig, 1884.

2 O. Hertwig, ‘ Beitriige zur Kenntniss der Bildung, Befruchtung und Theilung
des thierischen Eies.’ Leipzig, 1876.

° Fol, ‘ Recherches sur la fécondation, etc.’ Gentve, 1879. )

FOUNDATION OF A THEORY OF HEREDITY. 175

depends upon the fusion of two nuclei; but the possibility of the
co-operation of the substance of the two germ-cells could not be
excluded, for in all the observed cases the sperm-cell was very small
and had the form of a spermatozoon, so that the amount of its cell-
body, if there is any, coalescing with the female cell, could not be
distinctly seen, nor was it possible to determine the manner in
which this coalescence took place. Furthermore, it was for some
time very doubtful whether the spermatozoon really contained
true nuclear substance, and even in 1879 Fol was forced to the con-
clusion that these bodies consist of cell-substance alone. In the
following year my account of the sperm-cells of Daphnidae followed,
and this should have removed every doubt as to the cellular nature
of the sperm-cells and as to their possession of an entirely normal
nucleus, if only the authorities upon the subject had paid more
attention to these statements’. In the same year (1880) Balfour
summed up the facts in the following manner—‘ The act of impreg- .
nation may be described as the fusion of the ovum and spermatozoon,
and the most important feature in this act appears to be the fusion
of a male and female nucleus?.’ It is true that Calberla had already _.
observed in Petromyzon, that the tail of the spermatozoon does not.
penetrate into the egg, but remains in the micropyle; but on the
other hand the head and part of the ‘ middle-piece’” which effect
fertilization, certainly contain a small fraction of the cell-body in
addition to the nuclear substance, and although the amount of the
former which thus enters the egg must be very small, it might never-
theless be amply sufficient to transmit the tendencies of heredity.
Niigeli and Pfliiger rightly asserted, at a later date, that the
amount of the substance which forms the basis of heredity is neces-
sarily very small, for the fact that hereditary tendencies are as
strong on the paternal as on the maternal side, forces us to assume
that the amount of this substance is nearly equal in both male
and female germ-cells. Although I had not published anything
upon the point, I was myself inclined to ascribe considerable

1 Kolliker formerly stated, and has again repeated in his most recent publication,
that the spermatozoa (‘Samenfiiden’) are mere nuclei. At the same time he re-
cognizes the existence of sperm-cells in certain species. But proofs of the former
assertion ought to be much stronger in order. to be sufficient to support so improbable
a hypothesis as that the elements of fertilization may possess a varying morpho-

logical value. , Compare Zeitschr. f. wiss. Zool., Bd. XLIT.
? F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 69.

176 THE CONTINUITY OF THE GERM-PLASM AS THE

importance to the cell-substance in the process of fertilization ;
and I had been especially led to adopt this view because my
investigations upon Daphuidae had shown that an animal produces
large sperm-cells with an immense cell-body whenever the economy
of its organism:permits. All Daphnidae in which internal fertiliza-
tion takes place (in which the sperm-cells are directly discharged
upon the unfertilized egg), produce a small number of such large
sperm-cells (Sida, Polyphemus, Bythotrephes); while all species
with external fertilization (Daphnidae, Lynceinae) produce very
small sperm-cells in enormous numbers, thus making up for the
immense chances against any single cell being able to reach an
egg. Hence the smaller the chances of any single sperm-cell
being successful, the larger is the number of such cells produced,
and a direct result of this increase in number is a diminution
in size. But why should the sperm-cells remain or become so
large in the species in which fertilization is internal? The idea
suggests itself that the species in this way gains some advantage,
which must be given up in the other cases; although such ad-
vantage might consist in assisting the development of the fertilized
ovum and not in any increase of the true fertilizing substance. At
the present time we are indeed disposed to recognize this advantage
in still more unimportant matters, but at that time the ascertained
facts did not justify us in the assertion that fertilization is a mere
fusion of nuclei, and M. Nussbaum! quite correctly expressed the
state of our knowledge when he said that the act of fertilization
consisted in ‘the union of identical parts of two homologous cells.’

Pfliiger’s discovery of the ‘isotropism’ of the ovum was the
first fact which distinctly pointed to the conclusion that the bodies
of the germ-cells have no share in the transmission of hereditary
tendencies. He showed that segmentation can be started in
different parts of the body of the egg, if the latter be permanently
removed from its natural position. This discovery constituted an
important proof that the body of the egg consists of a uniform
substance, and that certain parts or organs of the embryo cannot
be potentially contained in certain parts of the egg, so that they
ean only arise from these respective parts and from no others.
Pfliiger was mistaken in the further interpretation, from which he
concluded that the fertilized ovum has no essential relation to the

1 Arch, f. mikr. Anat., Bd. 23. p. 182, 1884.

* FOUNDATION OF A THEORY OF HEREDITY. 177

organization of the animal subsequently formed by it, and that it
is only the recurrence of the same external conditions which
causes the germ-cell to develope always in the same manner. The
force of gravity was the first factor, which, as Pfliiger thought,
determined the building up of the embryo: but he overlooked the
fact that isotropism can only be referred to the body of the egg,
and that besides this cell-body there is also a nucleus present, from
which it was at least possible that regulative influences might
emanate. Upon this point Born? first showed that the position of
the nucleus is changed in eggs which are thus placed in unnatural
conditions, and he proved that the nucleus must contain a principle
which in the first place directs the formation of the embryo. Roux?
further showed that, even when the effect of gravity is compensated,
the development is continued unchanged, and he therefore concluded
that the fertilized egg contains within itself all the forces necessary
for normal development. Finally, O. Hertwig * proved from observa-
tions on the eggs of sea-urchins, that at any rate in these animals,
gravity has no directive influence upon segmentation, but that the
position of the first nuclear spindle decides the direction which will
be taken by the first divisional plane of segmentation. These
observations were however still insufficient to prove that fertiliza-
tion is nothing more than the fusion of nuclei *.

A further and more important step was taken when E. van ~
Beneden® observed the process of fertilization in Ascaris megalo-
cephala, Like the investigations of Nussbaum ® upon the same sub-

ject, published at a rather earlier date, van Beneden’s observations

did not altogether exclude the possibility of the participation of the
body of the sperm-cell in the real process of fertilization; still the
fact that the nuclei of the egg-cell and the sperm-cell do not

1 Born, ‘ Biologische Untersuchungen,’ I, Arch. Mikr. Anat., Bd. XXIV.

? Roux, ‘ Beitrige zum Entwicklungsmechanismus des Embryo,’ 1884.

3 O. Hertwig, ‘ Welchen Einfluss iibt die Schwerkraft,’ ete. Jena, 1884.

* [Our present knowledge of the development of vegetable,ova (including the
position of the parts of the embryo) is also in favour of the view that it is not in-
fluenced by external causes, such as gravitation and light. It takes place in a
manner characteristic of the genus or species, and essentially depends on other causes
which are fixed by heredity, see Heinricher ‘ Beeinfiusst das Licht die Organanlage
am Farnembryo?’ in Mittheilungen aus dem Botanischen Institute zu Graz, II.
Jena, 1888.—S. S.]

5 fH. van Beneden, ‘ Recherches sur la maturation de l’ceuf,’ etc., 1883.

6 M. Nussbaum, ‘Ueber die Veriinderung oe Genclieoktaprodukte bis zur Ei-
furchung,’ Arch. Mikr. Anat., 1884.

N

178 THE CONTINUITY OF THE GERM-PLASM AS THE

coalesce irregularly, but that their loops are placed regularly opposite
one another in pairs and thus form one new nucleus (the first seg-
mentation nucleus), distinctly pointed to the conclusion that the
nuclear substance is the sole bearer of hereditary tendencies—that
in fact fertilization depends upon the coalescence of nuclei. Van
Beneden himself did not indeed arrive at these conclusions: he was
prepossessed with the idea that fertilization depends upon the union
of two sexually differentiated nuclei, or rather half-nucleithe male
and female pronuclei. He considered that only in this way could a
single complete nucleus be formed, a nucleus which must of course
be hermaphrodite, and he believed that the essential cause of further
development lies in the fact that, at each successive division of
nuclei and cells, this hermaphrodite nature of the nucleus is main-
tained by the longitudinal division of the loops of each mother-
_ nucleus, causing a uniform distribution of the male and female
loops in both daughter-nuclei.

But van Beneden undoubtedly deserves great credit for having
constructed the foundation upon which a scientific theory of heredity
could be built. It was only necessary to replace the terms male
and female pronuclei, by the terms nuclear substance of the male
and female parents, in order to gain a starting-point from which —
further advance became possible. This step was taken by Stras-
burger, who at the same time brought forward an instance in
which the nucleus only of the male germ-cell (to the exclusion of
its cell-body) reaches the egg-cell. He succeeded in explaining
the process of fertilization in Phanerogams, which had been for a
long time involved in obscurity, for he proved that the nucleus of
the sperm-cell (the pollen-tube) enters the embryo-sae and fuses
with the nucleus of the egg-cell: at the same time he came to
the conclusion that the body of the sperm-cell does not pass into
the embryo-sac, so that in this case fertilization can only depend
upon the fusion of nuclei?.

} Eduard Strasburger, ‘Neue Untersuchungen iiber den Befruchtungsvorgang bei
den Phanerogamen als Grundlage ftir eine Theorie der Zeugung.’ Jena, 1884.

[It is now generally admitted that, in the Vascular Cryptogams, as also in Mosses
and Liverworts, the bodies of the spermatozoids are formed by the nuclei of the cells
‘from which they arise. Only the cilia which they possess, and which obviously merely
serve as locomotive organs, are said to arise from the surrounding cytoplasm. It is
therefore in these plants also the nucleus of the male cell which effects the fertilization
of the ovum. See Gobel, ‘Outlines of Classification and Special Morphology,’ trans-

FOUNDATION OF A THEORY OF HEREDITY. 179

Thus the nuclear substance must be the sole bearer of hereditary
tendencies, and the facts ascertained by van Beneden in the case of
Ascaris plainly show that the nuclear substance must not only
contain the tendencies of growth of the parents, but also those of a
very large number of ancestors. Each of the two nuclei which
unite in fertilization must contain the germ-nucleoplasm of both
parents, and this latter nucleoplasm once contained and still contains
the germ-nucleoplasm of the grandparents as well as that of all
previous generations. It is obvious that the nucleoplasm of each
antecedent generation must be represented in any germ-nucleus in
an amount which becomes less as the number of intervening genera-
tions becomes greater; and the proportion can be caleulated after
the manner in which breeders, when crossing: races, determine the ©
proportion of pure blood which is contained in any of the descend-
ants. Thus while the germ-plasm of the father or mother constitutes
half the nucleus of any fertilized ovum, that of a grandparent only
forms a quarter, and that of the tenth generation backwards only
avsz, and so on. The latter can, nevertheless, exercise influence~
over the development of the offspring, for the phenomena of atavism
show that the germ-plasm of very remote ancestors can occasionally
make itself felt, in the sudden reappearance of long-lost characters. .
Although we are unable to give a detailed account of the way in
which atavism happens, and of the cireumstances under which it
takes place, we are at least able to understand how it becomes
possible; for even a very minute trace of a specific germ-plasm
possesses the definite tendency to build up a certain organism, and
will develope this tendency as soon as its nutrition is, for some
reason, favoured above that of the other kinds of germ-plasm present
in the nucleus. Under these circumstances it will increase more

’ rapidly than the other kinds, and it is readily conceivable that a

preponderance in the quantity of one kind of nucleoplasm may
determine its influence upon the cell-body.

Strasburger—supported by van Beneden’s observations, but in
opposition to the opinions of the latter—had already explained, in
a manner similar to that described above, the process by which the
hereditary transmission of certain.characters takes place, and to this

. lated by H. E. F. Garnsey, edited by I. B. Balfour, Oxford, 1887, -p. 203, and

Douglas H. Campbell, ‘ Zur Entwicklungsgeschichte der Spermatozoiden,’ in Berichte
d. deutschen bot. Gesellschaft, vol. v (1887), p. 120.—S. 8.]
N 2

180 THE CONTINUITY OF THE GERM-PLASM AS THE

extent our opinions coincide. The nature of heredity is based upon
the transmission of nuclear substance with a specific molecular con-
stitution. This substance is the specific nucleoplasm of the germ-
cell, to which I have given the name of germ-plasm.

O.. Hertwig! has also come to the same conclusion : at an»earlier
date he had looked upon the coalescence of nuclei as the most
essential feature in the process of fertilization. He now believes
that this former opinion has been confirmed by the recent dis- .
coveries which have been shortly described above.

Although I entirely agree with Hertwig, as far as the main
question is concerned, I cannot share his opinions when he identi-
fies Nigeli’s idioplasm with the nucleoplasm of the germ-cell.
Niigeli’s idioplasm certainly includes the germ-plasm, if I may
retain this expression for the sake of brevity. Niigeli in forming
his hypothesis did indeed start with the germ-cells, but his idio-
plasm not only represents the nucleoplasm of the germ-cells, but
also that of all the other cells of the organism; all these nucleo-
plasms taken together constitute Nigeli’s idioplasm. According
to Niigeli, the idioplasm forms a network which extends through
the whole body, and represents the specific molecular basis which
determines its nature. Although this latter suggestion — the
general part of his theory—is certainly valid, and although it is
of great importance to have originated the idea of idioplasm in this
general sense, in contrast to the somato-plasm (‘ Nihrplasma’), it is
nevertheless true that we are not justified in retaining the details
of his theory.

In the first place the idioplasm does not form a directly con-
tinuous network throughout the entire body; and, secondly, the
whole organism is not penetrated by a single substance of homo-
geneous constitution, but each special kind of cell must contain
the specific idioplasm or nucleoplasm which determines its nature.
There are therefore in each organism a multitude of different
kinds of idioplasm. Thus we should be quite justified in generally
speaking of Niigeli’s idioplasm as nucleoplasm, and vice versa,

It is perfectly certain that the idioplasm cannot form a con-
tinuous network through the whole organism, if it is seated in the
nucleus and not im the cell-body. Even if the bodies of cells are

' O. Hertwig, ‘Das Problem der Befruchtung und der Isotropie des Eies.’ J ena,
1885.

FOUNDATION OF A THEORY OF HEREDITY. 181

everywhere connected by fine processes (as has been proved in animals
by Leydig and Heitzmann, and in plants by various botanists),
they do not form a network of idioplasm but of somato-plasm; a
- substance which, according to Nageli, stands in marked contrast to
idioplasm. Strasburger has indeed already spoken of a ‘cyto-idio-
plasm,’ and it is certainly obvious that the cell-body often possesses
a specific character, but we must in all cases assume that such a
character is impressed upon it by the influence of the nucleus, or,
in other words, that the direction in which the cell-substance is
differentiated in the course of development is determined by the
quality of its nuclear substance. So far, therefore, the deter-
mining nuclear substance corresponds to the idioplasm alone,
while the substance of the cell-body must be identified with the
somato-plasm (‘ Niihrplasma’) of Niigeli. At all events, in practice,
it will be well to restrict the term idioplasm to the regulative
nuclear substance alone, if we desire to retain the well-chosen terms
of Nigeli’s theory.

But the second part of Nigeli’s theory of the idioplasm is also
untenable. It is impossible that this substance can have the same
constitution everywhere in the organism and during every stage
of its ontogeny. If this were so, how could the idioplasm effect
the great differences which obtain in the formation of the various
parts of the organism? In some passages of his work Nigeli
seems to express the same opinion; e.g. on page 31 he says, ‘It
would be practicable to regard—although only in a metaphorical
sense—the idioplasms of the different cells of an individual as them-
selves different, inasmuch as they possess specific powers of pro-
duction: we should thus include among: these idioplasms all the con-
ditions of the organism which bring about the display of specific ac-
tivity on the part of cells.’ It can be clearly seen from the passages
immediately preceding and succeeding the above-quoted sentence,
that Niigeli, in speaking of these changes in the idioplasm, does
not refer to material, but only to dynamical changes. On page 53
he lays special stress upon the statement that ‘the idioplasm
during its growth retains its specific constitution everywhere
throughout the organism,’ and it is only ‘within these fixed
_ structural limits that it changes its conditions of tension and move-
ment, and thus alters the forms of growth and activity which are
possible at each time and place’ Against such an interpretation

182 THE CONTINUITY OF THE GERM-PLASM AS THE

weighty objections can be raised. At present I will only men-
tion that the meaning of the phrase ‘conditions of tension and
movement’ ought to be made clear, and that we ought to be
informed how it is that mere differences in tension can produce
as many different effects as could have been produced by differences
of constitution. If any one were to assert that in Daphnidae, or in
any other forms which produce two kinds of eggs, the power of de-
veloping only after a period of rest, possessed by the winter-eggs,
is based upon the fact that their idioplasm is identical with that
‘of the summer-eggs, but is in another condition of tension, I
should think such a hypothesis would be well worth consideration,
for the animals which arise from the winter-eggs are identical
with those produced in summer: the idioplasm which caused
their formation must therefore be identical in‘its constitution ; and
can only differ in the two cases, as water differs from ice. But
the case is quite otherwise in the stages of ontogeny. How many
different conditions of tension ought to be possessed by one and
the same idioplasm in order to correspond to the thousand different
structures and differentiations of cells in one of the higher organ-
isms? In fact it would be hardly possible to form even an
approximate conception of an explanation based upon mere ‘ con-
ditions of tensions and movement.’ But, furthermore, difference
in effect should correspond, at any rate to some extent, with
difference in cause: thus the idioplasm of a muscle-cell ought to
_ differ more from that of a nerve-cell and of a digestive-cell in
the same individual, than the idioplasm of the germ-cell of one
individual differs from that of other individuals of the same species ;
and yet, according to Niigeli, the latter small difference in the
effect is supposed to be due to difference of quality in the cause—
the idioplasm, while the former fundamental difference in the his-
‘tological differentiation of cells is supposed to follow from mere
difference ‘of tension and movement.’

Nigeli’s hypothesis appears to be self-contradictory; for, al-
though its author recognizes the truth of the fundamental law of
development, and explains the stages of ontogeny as an abbre-
viated recapitulation of phyletic stages, he nevertheless explains
the latter by a different principle from that which he employs to,
explain the former. According to Niigeli, the stages of phylogeny
are based upon true qualitative differences in the idioplasm: the

A on

FOUNDATION OF A THEORY OF HEREDITY. 183

germ-plasm of a worm is qualitatively different from that of Am-
phiowus, a frog, or a mammal. But if such phyletic stages occur
crowded together in the ontogeny of a single species, they are said to
be based upon different ‘conditions of tension and movement’ of one
and the same idioplasm! It seems to me to be necessary to con-
clude that if the idioplasm, in the course of phyletic development,
undergoes any alteration in specific constitution, such alterations
must also take place in ontogeny; so far at least as the phyletic
stages are repeated. Either the whole phyletic development is
based upon different ‘conditions of tension and movement,’ or if
this—as I believe—is impossible, the stages of ontogeny must
be based upon qualitative alterations in the idioplasm. :

Involuntarily the question arises—how is it that such an acute
thinker fails to perceive this contradiction? But the answer is
not far to seek, and Nageli himself indicates it when he adds these
words to the sentence quoted above: ‘It follows therefore that
if a cell is detached as a germ-cell in any stage of ontogenetic
development, and from any part of the organism, such a cell will
contain all the hereditary tendencies of the parent individual.’
In other words, if we are restricted to different ‘conditions of
tension and movement’ as an explanation, it seems to follow as a
matter of course that the idioplasm can re-assume its original
condition, and therefore that the idioplasm of any cell in the
body can again become the idioplasm of the germ-cell; for this to
take place it is only necessary that the greater tension should
become the less, or vice versa. But if we admit a real change
in constitution, then the backward development of the idio-
plasm of the cells of the body into germ-cells appears to be
very far from a matter of course, and he who assumes it must
bring forward weighty reasons. Nigeli does not produce such
reasons, but considers the metamorphosis of the idioplasm in on-
togeny as mere differences in the ‘ conditions of tension and move-
ment.’ This phrase covers the weak part of his theory; and I
look upon it as a valuable proof that Nigeli has also felt that the
phenomena of heredity can only find their explanation in the
hypothesis of the continuity of the germ-plasm; for his phrase is
' only capable of obscuring the question as to how the idioplasm
of the cells of the body can be re-transformed into the idioplasm of
germ-cells. ;

184 THE CONTINUITY OF THE GERM-PLASM. AS THE

I am of the opinion that the idioplasm cannot be re-transformed,
and I have defended this opinion for some years past1, although 1

have hitherto laid especial stress on the positive aspect of the

question, viz. on the continuity of the germ-plasm. I have
attempted to prove that the germ-cells of an organism derive their
essential nature from the fact that the germ-plasm of each genera-
tion is carried over into that which succeeds it ; and I have tried to
show that during the development of an egg into an animal, a part of
the germ-substance—although only a minute part—passes over un-
changed into the organism which is undergoing development, and
that this part represents the basis from which future germ-cells
arise. In this way it is to a certain extent possible to conceive
how it is that the complex molecular structure of the germ-plasm
can be retained unchanged, even in its most minute details, through
a long series of generations.

But how would this be possible if the germ-plasm were formed

anew in each individual by the transformation of somatic idio-—

plasm? And yet if we reject the ‘ continuity of the germ-plasm’
we are compelled to adopt this latter hypothesis concerning its
origin. It is the hypothesis adopted by Strasburger, and we have
therefore to consider how the subject presents itself from his point
of view.

I entirely agree with Strasburger when he says, ‘The specific
_ qualities of organisms are based upon nuclei’; and I further agree
with him in many of his ideas as to the relation between the
nucleus and cell-body : ‘Molecular stimuli proceed from the nucleus
into the surrounding cytoplasm; stimuli whieh, on the one hand,
control the phenomena of assimilation in the cell, and, on the
other hand, give to the growth of the cytoplasm, which depends
upon nutrition, a certain character peculiar to the species. ‘The
nutritive cytoplasm assimilates, while the nucleus controls the
. assimilation, and hence the substances assimilated possess a certain
constitution and nourish in a certain manner the cyto-idioplasm
and the nuclear idioplasm. In this way the ¢ytoplasm takes part
in the phenomena of construction, upon which the specific form of
the organism depends. This constructive activity of the eyto-idio-
plasm depends upon the regulative influence of the nuclei.’. The

1 This opinion was first expressed in my lecture, ‘ Ueber die Dauer des Lebens,’
Jena, 1882, translated as the first essay in the present volume.

—

FOUNDATION OF A THEORY OF HEREDITY. 185

nuclei therefore ‘determine the specific direction in which an
organism developes.’

The opinion—derived from the recent study of the phenomena of
fertilization—that the nucleus impresses its specific character upon
the cell, has received conclusive and important confirmation in the
experiments upon the regeneration of Infusoria, conducted simul-
taneously by M. Nussbaum? at Bonn, and by A. Gruber? at
Freiburg. Nussbaum’s statement that an artificially separated
portion of a Paramaecium, which does not contain any nuclear
substance, immediately dies, must not be accepted as of general
application, for Gruber has kept similar fragments of other In-
fusoria alive for several days. Moreover, Gruber had previously
shown that individual Protozoa occur, which live in a normal
manner, and are yet without a nucleus, although this structure is
present in other individuals of the same species. But the meaning
of the nucleus is made clear by the fact, published by Gruber, that
such artificially separated fragments of Infusoria are incapable of
regeneration, while on the other hand those fragments which con-
tain nuclei always regenerate. It is therefore only under the in-
fluence of the nucleus that the cell substance re-developes into the
full type of the species. In adopting the view that the nucleus ‘is
the factor which determines the specific nature of the cell, we stand
on a firm foundation upon which we can build with security.

If therefore the first segmentation nucleus contains, in its mole-
-eular structure, the whole of the inherited tendencies of develop-
ment, it must follow that during segmentation and subsequent
cell-division, the nucleoplasm will enter upon definite and varied
changes which must cause the differences appearing in the cells
which are produced ; for identical cell-bodies depend, ceteris paribus,
upon identical nucleoplasm, and conversely different cells depend
upon differences in the nucleoplasm. The fact that the embryo
grows more strongly in one direction than in another, that its cell-
layers are of different nature and are ultimately differentiated
into various organs and tissues,—forces us to accept the conclu-
sion that the nuclear substance has also been changed in nature,
and that such changes take place during ontogenetic development

1M. Nussbaum, ‘Sitzungber. der Niederrheinischen Gesellschaft fiir Natur- und
Heilkunde.’ .Dec. 15, 1884.

? A. Gruber, ‘ Biologisches Centralblatt,’ Bd. IV. No. 23, and V. No. 5.

186 |. THE CONTINUITY OF THE GERM-PLASM AS THE

in a regular and definite manner. This view is also held by Stras-
burger, and it must be the opinion of all who seek to derive the
development of inherited tendencies from the molecular structure
of the germ-plasm, instead of from preformed gemmules.

We are thus led to the important question as to the forces by
which the determining substance or nucleoplasm is changed, and
as to the manner in which it changes during the course of onto-
geny, and on the answer to this question our further conclusions
must depend. The simplest hypothesis would be to suppose that,
at each division of the nucleus, its specific substance divides into
two halves of unequal quality, so that the cell-bodies would also
be transformed ; for we have seen that the character of a cell is
determined by that of its nucleus. Thus in any Metazoon the
first two segmentation spheres would be transformed in such a
manner that one only contained the hereditary tendencies of the
endoderm and the other those of the ectoderm, and therefore, at a
later stage, the cells of the endoderm would arise from the one and
those of the ectoderm from the other; and this is actually known
to occur. In the course of further division the nucleoplasm of the
first. ectoderm cell would again divide unequally, e.g. into the
nucleoplasm containing the hereditary tendencies of the nervous
system, and into that containing the tendencies of the external
skin. But even then, the end of the unequal division of nuclei
would not have been nearly reached ; for, in the formation of the
nervous system, the nuclear substance which contains the hereditary
tendencies of the sense-organs, would, in the course of further cell-
division, be separated from that which contains the tendencies of
the central organs, and the same process would continue in the
formation of all single organs, and in the final development of the
most minute histological elements. This process would take place
in a definitely ordered course, exactly as it has taken place through-
out a very long series of ancestors; and the determining and
directing’ factor is simply and solely the nuclear substance, the
nucleoplasm, which possesses such a molecular structure in the germ-
cell that all such succeeding stages of its molecular structure in
future nuclei must necessarily arise from it, as soon as the re-
quisite external conditions are present. This is almost the same
conception of ontogenetic development as that which has been held
by embryologists who have not accepted the doctrine of evolution :

FOUNDATION OF A THEORY OF HEREDITY. 187

_ for we have only to transfer the primary cause of development, from
an unknown source within the organism, into the nuclear sub-
stance, in order to make the views identical.

It appears at first sight that the knowledge which has been
gained by studying the indirect division of nuclei is opposed to
such a view, for we know that each mother-loop of the so-called
nuclear plate divides longitudinally into two exactly equal halves,
which can be stained and thus rendered visible.

In this way each resulting daughter-nucleus receives an equal
supply of halves, and it therefore appears that the two nuclei must
be completely identical. This at least is Strasburger’s conclusion,
and he regards such identity as a fundamental fact, which cannot
be shaken, and with which all attempts at further explanation must
be brought into accord. |

How then can the gradual transformation of the nuclear substance
_ be brought about? For such a transformation must necessarily
take place if the nuclear substance is really the determining factor
in development. Strasburger attempts to support his hypothesis
by assuming that the inequality of the daughter-nuclei arises from
unequal nutrition; and he therefore considers that the inequality
is brought about after the division of the nucleus and of the cell.
Strasburger has shown, in a manner which is above all criticism,
that the nucleus derives its nutrition from the cell-body, but then
the cell-bodies of the two ex hypothesi identical daughter-nuclei
must be different from the first, if they are to influence their nuclei
in different ways. But if the nucleus determines the nature of
the cell, it follows that two identical daughter-nuclei which have
arisen by division within one mother-cell cannot come to possess
unequal! cell-bodies. As a matter of fact, however, the cell-bodies of
two daughter-cells often differ in size, in appearance, and in their
subsequent history, and these facts are sufficient to prove that in
such cases the division of the nucleus must have been unequal.
It appears to me to be a necessary conclusion that, in such an in-
stance, the mother-nucleus must have been capable of splitting into
nuclear substances of differing quality. I think that, in his argu-
ment, Strasburger has over-estimated the support afforded by exact
observations upon indirect nuclear division. Certainly the fact,
discovered by Flemming, and more exactly studied by Balbiani and
Pfitzner, that, in nuclear division, the loops split longitudinally, is

188 THE CONTINUITY OF THE GERM-PLASM AS THE

of great and even of fundamental importance. Furthermore, the
observations, conducted last year by van Beneden, on the process
of fertilization in Ascaris, have given to Flemming’s discovery a
clearer and more definite meaning than could have been at first
ascribed to it. The discovery proves, in the first place, that the
nucleus always divides into two parts of equal quantity, and fur-
ther that in every nuclear division, each daughter-nucleus receives
the same amount of nuclear substance from the father as from the
mother ; but, as it seems to me, it is very far from proving that the
quality of the parent nucleoplasms must always be equal in the
daughter-nuclei. It is true that the fact seems to prove this ; and
if we remember the description of the most favourable instance
which has been hitherto discovered, viz. the process of fertilization
in the egg of Ascaris, as represented by van Beneden, the two
longitudinal halves of each loop certainly impress the reader as
being absolutely identical (compare, for instance, loc. cit. Plate XIX,
figs. I, 4, 5). But we must not forget that we do not see the
molecular structure of the nucleoplasm, but something which we
can only look upon (when we remember how complex this molecular
structure must be) as a very rough expression of its quantity. Our
most powerful and best lenses just enable us to make out the form
of separate stainable granules present in a loop which is about to
divide: they appear as spheres and immediately after division as
hemispheres. But according to Strasburger, these granules, the so-
called microsomata, only serve for the nutrition of the nuclear sub-
stance proper, which lies between them unstainable, and therefore
not distinctly visible. But even if these granules represent the true
idioplasm, their division into two exactly equal parts would give us
no proof of equality or inequality in their constitution: it would
only give us an idea of their quantitative relations. We can only
obtain proofs as to the quality of the molecular structure of the
two halves by their effect on the bodies of the daughter-cells, and
we know that these latter are frequently different in size and
quality.

This point is so important that I must illustrate it by a few more
examples. The so-called polar bodies (to be treated more in detail
below) which are expelled during maturation from the eggs of so
many animals, are true cells, as was first proved by Biitschli in
Nematodes: their formation is due to a process of undoubted cell-

FOUNDATION OF A THEORY OF HEREDITY. 189

division usually accompanied by a typical form of indirect nuclear
division’. If any one is still in doubt upon this point, after the
observations of Fol and Hertwig, he might easily be convinced of
its/ truth by a glance at the figures (unfortunately too little known)
which Trinchese ? has published, illustrating this process in the eggs
of certain gastropods. The eggs of Amphorina coerulea are in every
way suitable for observation, being: entirely translucent, and having
large distinct nuclei which differ from the green cytoplasm in
colour. In these eggs two polar bodies are formed one after the
other: and each of them immediately re-divides: hence it follows
that four polar bodies are placed at the pole of the egg. But how
is it that these four cells perish, while the nucleus, remaining in
the yolk and conjugating with the sperm-nucleus, makes use of the
whole body of the egg and developes into the embryo? Obviously
because the nature of the polar body is different from that of the egg-
cell. But since the nature of the cell is determined by the quality
of the nucleus, this quality must differ from the very moment of
nuclear division. This is proved by the fact that the supernu-
merary spermatozoa which sometimes enter the egg do not con-
jugate with the polar bodies. According to Strasburger’s theory,
the objection might be urged that the different quality of,the nuclei
is here caused by the very different quantity of cytoplasm by which
they are surrounded and nourished ; but on the one hand the small-
ness of the cell-bodies which surround most polar globules must
have some explanation, and this can only be found in the nature of
the nucleus; and on the other hand the quantity of the cell-body
which surrounds the polar globules of Amphorina is, as a matter of
fact, somewhat larger than the sphere of green cytoplasm which
surrounds the nucleus of the egg! The difference between the polar
bodies and the egg-cell can thus only be explained on the suppo-
sition that, in the division of the nuclear spindle, two qualitatively
different daughter-nuclei are produced.

There does not seem to be any objection to the view that the

1 According to the observations of Nussbaum and van Beneden, the egg of Ascaris
departs from the ordinary type, but I think that the latter observer goes too far
when he concludes from the form of the nuclear spindle (of which the two halves are
inclined to each other at an angle) that we have before us a process entirely differere
from that of ordinary nuclear division. ed

2 Trinchese, ‘I primi momenti dell’ evoluzione nei molluschi,’ Atti Acad. Ly~’
(3) vii. 1879, Roma. the

190 THE CONTINUITY OF THE GERM-PLASM AS THE

microsomata of the nuclear loops—assuming that these bodies
represent the idioplasm—are capable of dividing into halves, equal
in form and appearance, but unequal in quality. We know that
this very process takes place in many egg-cells; thus in the
egg of the earth-worm the first two segmentation spheres are
equal in size and appearance, and yet the one forms the endoderm
and the other the ectoderm of the embryo.

I therefore believe that we must accept the hypothesis that,
in indirect nuclear division, the formation of unequal halves may
take place quite as readily as the formation of equal halves, and
that the equality or inequality of the subsequently produced
daughter-cells must depend upon that of the nuclei. Thus during
ontogeny a gradual transformation of the nuclear substance takes
place, necessarily imposed upon it, according to certain laws, by its
own nature, and such transformation is accompanied by a gradual
change in the character of the cell-bodies.

It is true that we cannot gain any detailed knowledge of thé
nature of these changes in the nuclear substance, but we can very
well arrive at certain general conclusions about them. If we may
suppose, with Niigeli, that the molecular structure of the germ-
idioplasm,. or according to our terminology the germ-plasm, be-
comes more complicated according to the greater complexity of
the organism developed from it, then the following conclusions will
also be accepted,—that the molecular structure of the nuclear -
substance is simpler as the differences between the structures
arising from it become less; that therefore the nuclear substance
of the segmentation-cell of the earth-worm, which potentially con-
tains the whole of the ectoderm, possesses a more complicated
molecular structure than that of a single epidermic cell or nerve-
cell. These conclusions will be admitted when it is remembered
that every detail in the whole organism must be represented in
the germ-plasm by its own special and peculiar arrangement of the
groups of molecules (the micellae of Niigeli), and that the germ-
plasm not only contains the whole of the quantitative and qualita-
tive characters of the species, but also all individual variations

exas far as these are hereditary: for example the small depression
bela the centre of the chin noticed in some families. The physical
manyses of all apparently unimportant hereditary habits or struc-
Nemas, of hereditary talents, and other mental peculiarities, must

FOUNDATION OF A THEORY OF HEREDITY. 191

all be contained in the minute quantity of germ-plasm which is
possessed by the nucleus of a germ-cell ;—not indeed as the pre-
formed germs of structure (the gemmules of pangenesis), but as
variations in its molecular constitution; if this be impossible,
such characters could not be inherited. Niigeli has shown in his
work, which is so rich in suggestive ideas, that even in so minute
a space as the thousandth of a cubic millimetre, such an enormous
number (400,000,000) of micellae may be present, that the
most diverse and complicated artangements become possible. It
therefore follows that the molecular structure of the germ-plasm in
the germ-cells of an individual must be distinguished from that
of another individual by certain differences, although these may
be but small; and it also follows that the germ-plasm of any
species must differ from that of all other species.

These considerations lead us to conclude that the molecular
structure of the germ-plasm in all higher animals must be
excessively complex, and, at the same time, that this complexity,
must gradually diminish during ontogeny as the structures still to.
be formed from any cell, and therefore represented in the mole-
cular.constitution of its nucleoplasm, become less in number. I do
not mean to imply that the nucleoplasm contains preformed struc-
tures which are gradually reduced in number as they are given off
in various directions during the building-up of organs: I mean
that the complexity of the molecular structure decreases as the po-
tentiality for further development also decreases, such potentiality
being represented in the molecular structure of the nucleus. The
nucleoplasm, which in the grouping of its particles contains po-
tentially a hundred different modifications of this substance, must
possess far more numerous kinds and far more complex arrange-
ments of such particles than the nucleoplasm which only con-
tains a single modification, capable of determining the character
of a single kind of cell. The development of the nucleoplasm
during ontogeny may be to some extent compared to an army
composed of corps, which are made up of divisions, and these
of brigades, and so on. The whole army may be taken to re-
present the nucleoplasm of the germ-cell: the earliest cell-division
(as into the first cells of the ectoderm and endoderm) may be
represented by the separation of the two corps, similarly formed
but with different duties: and the following cell-divisions by the

192 THE CONTINUITY OF THE GERM-PLASM AS THE

successive detachment of divisions, brigades, ‘regiments, battalions,
companies, etc.; and as the groups become simpler so does their
sphere of action become limited. It must be admitted that this
metaphor is imperfect in two respects, first, because the quantity
of the nucleoplasm is not diminished, but only its complexity, and
secondly, because the strength of an army chiefly depends upon its
numbers, not on the complexity of its constitution. And we must
also guard against the supposition that unequal nuclear division
simply means a separation of part of the molecular structure,
like the detachment of a regiment from a brigade.’ On the con-
trary, the molecular constitution of the mother-nucleus is certainly
changed during division in such a way that one or both halves
receive a new structure which did not exist before their forma+
tion,

My opinion as to the behaviour of the idioplasm during
ontogeny, not only differs from that of Nigeli, in that the latter
maintains that the idioplasm only undergoes changes in its ‘ con-
ditions of tension and movement,’ but also because he imagines
this substance to be composed of the preformed germs of structures
(‘Anlagen’). Nageli’s views are obviously bound up with his
theory of a continuous network of idioplasm throughout the whole
body; perhaps he would have adopted other conclusions had he
been aware of the fact that the idioplasm must only be sought for
in the nuclei. Niigeli’s views as to ontogeny can be best seen in
the following passages: ‘As soon as ontogenetic development
begins, the groups of micellae in the idioplasm which effect the first
stage of development, enter upon active growth: such activity
causes a passive growth‘of the other groups, and an increase in
the whole idioplasm, perhaps to many times its former bulk. But
the intensities of growth in the two series of groups are unequal,
and consequently an increasing tension is produced which sooner
or later, according to the number, arrangement, and energy of the
active groups, necessarily renders the continuation of the process
impossible. In consequence of such disturbance to the equilibrium,
active growth now takes place in the next group, leading to fresh
irritation, and this group then reacts more strongly than all the
others upon the tension which first stimulated its activity. These
changes are repeated until all the groups are gone through, and
the ontogenetic development finally reaches the stage at which

FOUNDATION OF A THEOKY OF HEREDITY. 193

propagation takes place, and thus the original stage of the germ is
reached.’

Hence, according to Nigeli, the different stages of ontogeny arise
out of the activities of different parts of the idioplasm: certain
groups of micellae in the idioplasm represent the germs (‘ Anlagen’)
of certain structures in the organism: when any such germ reacts
under stimulation it produces the corresponding structure. It seems
to me that this hypothesis bears some resemblance to Darwin’s
theory of pangenesis. I think that Niageli’s preformed germs of
structures (‘ Anlagen’) and his groups of such germs are highly
elaborated equivalents of the gemmules of pangenesis, which,
according to Darwin, manifest activity when their turn comes, or,
according to Nigeli, when they react under stimulation. Whena_
group of such germs, by their active growth or by their ‘irritation,’
have caused a similar active growth or a similar irritation in the
next group, the former may come to rest, or may remain in a
state of activity together with its successor, for a longer or
shorter period. Its activity may even last for an unlimited time,
as is the case in the formation of leafy shoots in many plants.’

Here, again, we recognize the fact that Niigeli’s whole hypothesis
is intimately connected with the supposition that the entire mass
of idioplasm is continuous throughout the organism. Sometimes
one part of the idioplasm and sometimes another part is irritated,
and then produces the corresponding organ. But if, on the other
hand, the idioplasm does not represent a directly continuous mass,
but is composed of thousands of single nucleoplasms which only
act together through the medium of their cell-bodies, then we
must substitute the conception of ‘ontogenetic stages of develop-
ment of the idioplasm’ for the conception of germs of structure
(‘Anlagen’). The different varieties of nucleoplasm which arise
during ontogeny represent, as it were, the germs of Nigeli (‘ An-
lagen’), because, by means of their molecular structure, they create
a specific constitution in the cell-bodies over which they have
control, and also because they determine the succession of future
nuclei and cells.

It is in this sense, and no other, that I can speak of the presence

of preformed germs (‘Anlagen’) in the idioplasm. We may sup-
pose that the idioplasm of the first segmentation nucleus is but
slightly different from that of the second ontogenetic stage, viz.

4

194, THE CONTINUITY OF THE GERM-PLASM AS THE

that of the two following segmentation nuclei. Perhaps only
a few groups of micellae have been displaced or somewhat differ-
ently arranged. But nevertheless such groups of micellae were not

the germs (‘ Anlagen’) of a second stage which pre-existed in the
- first stage, for the two are distinguished by the possession of a
different molecular structure. This structure in the second stage,
under normal conditions of development, again brings about the
change by which the different molecular structure of the third
stage is produced, and so on.

It may be argued that von Baer’s well-known and fundamental
law of development is opposed to the hypothesis that the idioplasm -
of successive ontogenetic stages must gradually assume a simpler
molecular structure. The organization of the species has, on the
whole, increased immensely in complexity during the course of
phylogeny: and if the phyletic stages are repeated in the ontogeny,
it seems to follow that’ the structure of the idioplasm must
become more complex in the course of ontogeny instead of becoming
simpler. But the complexity of the whole organism is not repre-
sented in the molecular structure of the idioplasm of any single
nucleus, but by that of all the nuclei present at any one time. It |
is true that the germ-cell, or rather the idioplasm of the germ-
nucleus, must gain greater complexity as the organism which arises
from it becomes more complex ; but the individual nucleoplasms of
each ontogenetic stage may become simpler, while the whole mass
of idioplasms in the organism (which, taken together, represent the
stage in question) does not by any means lose in complexity.

If we must therefore assume that the molecular structure of the
nucleoplasm becomes simpler in the course of ontogeny, as the
number of structures which it potentially contains become smaller,
it follows that the nucleoplasm in the cells of fully differentiated
tissues—such as muscle, nerve, sense-organs, or glands—must.
possess relatively the most simple molecular structure ; for it cannot
originate any fresh modification of nucleoplasm, but can only con-
tinue to produce cells of the same structure, although it does not
always retain this power.

We are thus brought back to the fundamental question as to
how the germ-cells arise in the organism. Is it possible that
the nucleoplasm of the germ-cell, with its immensely complex
molecular structure, potentially containing all the specific pecu-

FOUNDATION OF A THEORY OF HEREDITY. 195

liarities of an individual, can arise from the nucleoplasm of any
' of the body-cells,—a substance which, as we have just seen, has »
lost. the power of originating any new kind of cell, because of the
continual simplification of its structure during development? It
seems to me that it would be. impossible for the simple nucleo-
plasm of the somatic cells to thus suddenly acquire the power of
originating the most complex nucleoplasm from which alone the
entire organism can be built up: I cannot see any evidence for the
existence of a foree which could effect such a transformation.

This difficulty has already been appreciated by other writers.
Nussbaum’s! theoretical views, which I have already mentioned,
also depend upon the hypothesis that cells which have once become
ditferentiated for the performance of special functions cannot be
re-transformed into sexual cells: he also concludes that the latter
are separated from all other cells at a very early period of embry-
onic development, before any histological differentiation has taken
place. Valaoritis? has also recognised that the transformation of
histologically differentiated cells into sexual cells is impossible.
He was led to believe that the sexual cells of Vertebrata arise
from the white blood corpuscles, for he looked upon these latter -
as differentiated to the smallest extent possible. Neither of these
views can be maintained. The former, because the sexual cells
of all plants and most animals are not, as a matter of fact, separated
from’ the somatic cells at the beginning of ontogeny; the latter,
because it is contradicted by the fact that the sexual cells of
vertebrates do not arise from blood corpuscles, but from the germinal
epithelium. But even if this fact had not been ascertained we
should be compelled to reject Valaoritis’ hypothesis on theoretical
grounds, for it is an error to assume that white blood corpuscles
are undifferentiated, and that their nucleoplasm is similar to the
germ-plasm. There is no nucleoplasm like that of the germ-
cell in any of the somatic cells, and no one of these latter can be
said to be undifferentiated. All somatic cells possess a certain
degree of differentiation, which may be rigidly limited to one
single direction, or may take place in one of many directions. All
these cells are widely different from the egg-cell from which they
originated : they are all separated from it by many generations of

1 M. Nussbaum, ‘ Archiv fiir Mikroskopische Anatomie,’ Bd. XVIII und XXIII.

* Valaoritis, ‘Die Genesis des Thier-Eies.’ Leipzig, 1882.

02

196 THE CONTINUITY OF THE GERM-PLASM AS THE

cells, and this fact implies that their idioplasms possess a widely
different structure from the idioplasm, or germ-plasm, of the egg- *
cell. Even the nuclei of the two first segmentation spheres cannot
possess the same idioplasm as that of the first segmentation nucleus,
and it is, of course, far less possible for such an idioplasm to be pre-
sent in the nucleus of any of the later cells of the embryo. The
structure of the idioplasm must necessarily become more and more
different from that of the first segmentation nucleus, as the de~
velopment of the embryo proceeds. The idioplasm of the first
segmentation nucleus, and of this nucleus alone, is germ-plasm, and
possesses a structure such that an entire organism can be pro-
duced from it. Many writers appear to consider it a matter of
course that any embryonic cell can reproduce the entire organism,
if placed under suitable conditions. But, when we carefully look
into the subject, we see that such powers are not even possessed by
those cells of the embryo which are nearest to the egg-cell—viz.
the first two segmentation spheres. We have only to remember
the numerous cases in which one of them forms the ectoderm of
the animal while the other produces the endoderm, in order to
admit the validity of this objection.

But if the first segmentation spheres are not able to develope into
a complete organism, how can this be the case with one of the
later embryonic cells, or one of the cells of the fully developed
animal body? It is true that we speak of certain cells as being
‘of embryonic character, and only recently Kolliker* has given a
list of such cells, among which he includes osteoblasts, cartilage
cells, lymph corpuscles, and connective tissue corpuscles: but even
if these cells really deserve such a designation, no explanation of the
formation of germ-cells is afforded, for the idioplasm of the latter
must be widely different from that of the former.

It is an error to suppose that we gain any further insight into
the formation of germ-cells by referring to these cells of so-called
‘embryonic character, which are contained in the body of the
mature organism. It is of course well known that many cells are
characterized by very sharply defined histological differentiation,
while others are but. slightly differentiated ; but it is as difficult to
imagine that germ-cells can arise from the latter as from the
former. Both classes of cells contain idioplasm with a structure

1 Kolliker, ‘ Die Bedeutung der Zelikerne,’ etc.; Zeitschr. f. wiss, Zool. Bd. XLII.

FOUNDATION OF A THEORY OF HEREDITY. 197

different from that which is contained in the germ-cell, and we
have no right to assume that any of them can form germ-cells until
it is proved that somatic idioplasm is capable of undergoing re-
transformation into germ-idioplasm.

The same argument applies to the cells of the embryo itself, and
it therefore follows that those instances of early separation of
sexual from somatic cells, upon which I have often insisted as
indicating the continuity of the germ-plasm, do not now appear to
be of such conclusive importance as at the time when we were not
sure about the localization of the idioplasm in the nuclei. In the
great majority of cases the germ-cells are not separated at the
beginning of embryonic development, but only in some one of the
later stages. A single exception is found in the pole-cells (‘ Pol-
zellen’) of Diptera, as was shown many years ago by Robin! and
myself*, These are the first cells formed in the egg, and accord-
ing to the later observations of Metschnikoff* and Balbiani*, they
become the sexual glands of the embryo. Here therefore the
germ-plasm maintains a true unbroken continuity. The nucleus
of the egg-cell directly gives rise to the nuclei of the pole-cells,
and there is every reason to believe that the latter receive un-
changed a portion of the idioplasm of the former, and with it
the tendencies of heredity. But in all other cases the germ-cells
arise by division from some of the later embryonic cells, and as
these belong to a more advanced ontogenetic stage in the de-
velopment of the idioplasm, we can only conclude that continuity
is maintained, by assuming (as I do) that a small part of the germ-
plasm persists unchanged during the division of the segmentation
nucleus and remains mixed with the idioplasm of a certain series of
cells, and that the formation of true germ-cells is brought about at
a certain point in the series by the appearance of cells in which the
germ-plasm becomes predominant. But if we accept this hypo-
thesis it does not make any difference, theoretically, whether the
germ-plasm becomes predominant in the third, tenth, hundredth,
or millionth generation of cells. It therefore follows that cases
of early separation of the germ-cells afford no proof of a direct

1 «Compt. rend.’ Tom. LIV. p. 150.

? «Entwicklung der Dipteren.’ Leipzig, 1864.

* « Zeitschr. f. wiss. Zool.’ Bd. XVI. p. 389 (1866).
+ *Compt. rend.’ Nov. 13, 1882.

198 THE CONTINUITY OF THE GERM-PLASM AS THE

persistence of the parent germ-cells in those of the offspring ; for a
cell the offspring of which become partly somatic and partly germ-
cells cannot itself have the characters of a germ-cell; but it may
nevertheless contain germ-idioplasm, and may thus transfer the sub-
stance which forms the basis of heredity from the germ of the
parent to that of the offspring.

If we are unwilling to accept this hypothesis, nothing remains
but to credit the idioplasm of each successive ontogenetic stage with
a capability of re-transformation into the first stage. Strasburger
accepts this view; and he believes that the idioplasm of the nuclei
changes during the course of ontogeny, but returns to the condition
of the first stage of the germ, at its close. But the rule of pro-
bability is against such a suggestion. Suppose, for instance, that
the idioplasm of the germ-cell is characterized by ten different
qualities, each of which may be arranged relatively to the others in
two different ways, then the probability in favour of any given

I
1024
that is to say, the re-transformation of somatic idioplasm into germ-
plasm will occur once in 1024 times, and it is therefore impossible
for such re-transformation to become the rule. It is also obvious
that the complex structure of the germ-plasm which potentially
contains; with the likeness of a faithful portrait, the whole in-
dividuality of the parent, cannot be represented by only ten charac-
ters, but that there must be an immensely greater number; it is
also obvious that the possibilities of the arrangement of single
characters must be assumed to be much larger than two; so that we

combination would be represented by the fraction €)" =

, wi
et the formula (~ , where p represents the possibilities, and ~ the
g 2 Pp P

characters. Thus if ~ and p are but slightly larger than we
assumed above, the probabilities become so slight as to altogether
exclude the hypothesis of a re-transformation of somatic idioplasm
into germ-plasm. .

It may be objected that such re-transformation is much more
probable in the case of those germ-cells which separate early
from the somatic cells. Nothing can in fact be urged against
the possibility that the idioplasm of (e.g.) the third generation of
cells may pass back into the condition of the idioplasm of the germ-
cell; although of course the mere possibility does not prove the

FOUNDATION OF A THEORY OF HEREDITY. 199

fact. But there are not many cases in which the sexual cells are
separated so early as the third generation: and it is very rare for
them to separate at any time during the true segmentation of the
egg. In Daphnidae (Moina) separation occurs in the fifth stage of
segmentation !, and although this is unusually early it does not
happen until the idioplasm has changed its molecular structure six
times. In Sagitta? the separation does not take place until the
archenteron is being formed, and this is after several hundred
embryonic cells have been produced, and thus after the germ-
_ plasm has changed its molecular: structure ten or more times. But
in most cases, separation takes place at a much later stage; thus in

Hydroids it does not happen until after hundreds or thousands of °
cell-generations have been passed through ; and the same fact holds~

in the higher plants, where the production of germ-cells frequently
occurs at the end of ontogeny. In such cases the probability of a
re-transformation of somatic idioplasm into germ-plasm becomes
infinitely small.

It is true that these considerations only refer to a rapid and
sudden re-transformation of the idioplasm. If it could be proved
that development is not merely in appearance but in reality a
cyclical process, then nothing could be urged against the occur-
rence of re-transformation. It has been recently maintained by
Minot* that all development is cyclical, but this is obviously
incorrect, for Nageli has already shown that direct non-cyclical
courses of development exist, or at all events courses in which the
earliest condition is not repeated at the close of development. The
phyletic development of the whole organic world clearly illus-
trates a development of the latter kind; for although we may
assume that organic development is not nearly concluded, it is
nevertheless safe to predict that it will never revert to its original
starting-point, by backward development over the same course
as that which it has already traversed. No one can believe that
existing Phanerogams will ever, in the future history of the world,
retrace all the stages of phyletic development in precise inverse
order, and thus return to the form of unicellular Algae or Monera;
or that existing placental mammals will develope into Marsupialia,

1 Grobben, ‘ Arbeiten d. Wien. Zool. Instituts,’ Bd. IL. p. 203.

? Biitschli, ‘ Zeitschrift f. wiss. Zool.’ Bd. XXIII. p. 409.
° ¢Seience,’ vol. iv. No. 90, 1884.

*”

200 THE CONTINUITY OF THE GERM-PLASM AS THE

Monotremata, mammal-like reptiles, and the lower vertebrate forms,
into worms and finally into Monera. But how can a course of
development, which seems to be impossible in phylogeny, occur as
the regular method of ontogeny? And quite apart from the
question of possibility, we have to ask for proofs of the actual
occurrence of cyclical development. Such a proof would be af-
forded if it could be shown that the nucleoplasm of those somatic
cells which (e.g. in Hydroids) are transformed into germ-cells
passes backwards through many stages of development into the
nucleoplasm of the germ-cell. It is true that we can only recognise
differences in the structure of the idioplasm by its effects upon the
cell-body, but no effects are produced which indicate that such
backward development takes place. Since the course of onward de-
velopment is compelled to pass through the numerous stages which
are implied in segmentation and the subsequent buflding-up of the
embryo, ete., it is quite impossible to assume that backward develop-
ment would take place suddenly. It would be at least necessary to
suppose that the cells of embryonic character, which are said to be
transformed into primitive germ-cells, must pass back through at any
rate the main phases of their ontogeny. A sudden transformation
of the nucleoplasm of a somatic cell into that of a germ-cell would

be almost as incredible as the transformation of a mammal into an -

amoeba ; and yet we are compelled to admit that the transforma-

tion must be sudden, for no trace of such retrogressive stages of —

development can be seen. If the appearance of the whole cell gives
us any knowledge as to the structure of its nuclear idioplasm, we
may be sure that the development of a primitive germ-cell proceeds
without a break, from the moment of its first recognizable formation,
to the ultimate production of distinct male gr female sexual cells.

I am well aware that Strasburger has stated that, in the ulti-
mate maturation of the sexual cells, the substance of the nuclei
returns to a condition similar to that which existed at the begin-
ning of ontogenetic development; still such a statement is no
proof, but only an assumption made to support a theory. I am
also aware that Nussbaum and others believe that, in the formation
of spermatozoa in higher animals, a backward development sets in
at a certain stage; but even if this interpretation be correct, such
backward development would only lead as far as the primitive germ-
cell, and would afford no explanation of the further transformation

FOUNDATION OF A THEORY OF HEREDITY. 201

of the idioplasm of this cell into germ-plasm. But this latter
transformation is just the point which most needs proof upon any
theory except the one which assumes that the primitive germ-cell
still contains unchanged germ-plasm. Every attempt to render
probable such a re-transformation of somatic nucleoplasm into germ-
plasm breaks down before the facts known of the Hydroids, in
which only certain cells in the body, out of the numerous so-called
embryonic cells, are capable of becoming primitive germ-cells, while
the rest do not possess this power.

I must therefore consider as erroneous the hypothesis which
assumes that the somatic nucleoplasm may be transformed into
germ-plasm. Such a view may be called ‘the hypothesis of the
eyclical development of the germ-plasm.’

' Nageli has tried to support such an hypothesis on phyletic
_ grounds. He believes that phyletic development follows from an
extremely slow but steady change in the idioplasm, in the direction
of greater complexity, and that such changes only become visible
periodically. He believes that the passage from one phyletic stage
to another is ‘chiefly due to the fact that ‘in any ontogeny, the
very last structural change upon which the separation of germs
depends, takes place in a higher stage, one or more cell-generations
later’ than it occurred in a lower stage. ‘The last structural
change itself remains the same, while the series of structural
changes immediately preceding it is increased. I believe that
Nageli, being a botanist, has been too greatly influenced by the
phenomena of plant-life. It is certainly true that in plants, and
especially in the higher forms, the germ-cells only make their
appearance, as it were, at the end of ontogeny; but facts such
as these do not hold in the animal kingdom: at any rate they
are not true in the great majority of cases. In animals, as I have
already mentioned several times, the germ-cells are separated from
the somatic cells during embryonic development, sometimes even at
its very commencement; and it is obvious that this latter is the
original, phyletically oldest, mode of formation. The facts at our
disposal indicate that the germ-cells only appear, for the first time,
after embryological development, in those cases where the forma-
tion of asexually produced colonies takes place, either with or with-
out alternation of generations; or in cases where alternation of
generations occurs without the formation of such colonies. In

202 THE CONTINUITY OF THE GERM-PLASM AS THE

a colony of polypes, the germ-cells are produced by the later genera-
tions, and not by the founder of the colony which was developed
from an egg. ‘This is also true of the colonies of Siphonophora,
and the germ-cells appear to arise very late in certain instances of
protracted metamorphosis (Echinodermata), but on the other hand,
they arise during the embryonic development of other forms (In-
secta) which also undergo metamorphosis. It is obvious that the
phyletic development of colonies or stocks must have succeeded
that of single individuals, and that the formation of germ-cells in
the latter must therefore represent the original method. Thus
the germ-tells originally arose at the beginning of ontogeny and
not at its close, when the somatic cells are formed:

This statement is especially supported by the history of cer-
tain lower plants, or at any rate chlorophyll-containing organisms,
and I think that these forms supply an admirable illustration of
my theory as to the phyletic origin of germ-cells, as explained in
my earlier papers upon the same subject.

The phyletic origin of germ-cells obviously coincides with the
differentiation of the first multicellular organisms by division of
labour’. If we desire to investigate the relation between germ-
cells and somatic cells, we must not only consider the highly
developed and strongly differentiated multicellular organisms, but
we must also turn our attention to those simpler forms in which
phyletic transitions are represented. In addition to solitary
unicellular organisms, we know of others living in colonies of which
the constituent units or cells (each of them equivalent to a uni-
cellular organism) are morphologically and physiologically identical.
Each unit feeds, moves, and under certain circumstances is capable
of reproducing itself, and of thus forming a new colony by repeated
division. The genus Pandorina (Fig. I), belonging to the natural
order Volvocineae, represents such ‘homoplastid’ (Gétte) organisms.
It forms a spherical colony composed of ciliated cells, all of which
_are exactly alike: they are embedded in a colourless gelatinous
mass. Each cell contains chlorophyll, and possesses a red eye-spot,
and a pulsating vacuole. These colonies are propagated by the

* Among unicellular organisms, encysted individuals are often called germs.
They sometimes differ from the adult organism in their smaller size and simpler
structure (Gregarinidae), but they represent the same morphological stage of in-
dividuality. :

FOUNDATION OF A THEORY OF HEREDITY. 203

sexual and asexual (Fig. II) methods alternately, although in the
former case the conjugating swarm-cells cannot be distinguished
with certainty as male or female. In both kinds of reproduction,
each cell in the colony acts as a reproductive cell; in fact, it
behaves exactly like a unicellular organism.

III. A young individual of Volvox minor (after Stein), still enclosed in the

wall of the cell from which it has been parthenogenetically produced. ‘The constituent cells are divided

into somatic cell (sz), and germ-cells (£2).

I. Pandorina morwm (after Pringsheim), a swarming colony. II. A colony divided into sixteen daughter
all the cells alike.

colonies :

It is very interesting to find in another genus belonging to the
same natural order, that the transition from the homoplastid to the

204 THE CONTINUITY OF THE GERM-PLASM AS THE

heteroplastid condition, and the separation into somatic and repro-
ductive cells, have taken place. In Volvow (Fig. III) the spherical
colony consists of two kinds of cells, viz. of very numerous small cili-
ated cells, and of a much smaller number of large germ-cells without
cilia. The latter alone possess the power of producing a new colony,
and this takes place by the asexual and sexual methods alternately :
in the latter a typical fertilization of large egg-cells by small sper-
matozoa occurs. The sexual differentiation of the germ-cells is not
material to the question we are now considering ; the important
point is to ascertain whether here, at the very origin of heteroplastid
organisms, the germ-cells, sexually differentiated or not, arise from
the somatic cells at the end of ontogeny, or whether the substance of
the parent germ-cell, during embryonic development, is from the first
separated into somatic and germ-cells. The former interpretation
would support Nigeli’s view, the latter would support my own. But
Kirchner ! distinctly states that the germ-cells of Volvow are differ-
entiated during embryonic development, that is, before the escape of
the young heteroplastid organism from the egg-capsule. We cannot
therefore imagine that the phyletic development of the first hetero-
plastid organism took place in a manner different from that

which I have previously advocated on theoretical grounds, before .

this striking instance occurred to me. The germ-plasm (nucleo-
‘ plasm) of some homoplastid organism (similar to Pandorina) must
have become modified in molecular structure during the course of

phylogeny, so that the colony of cells produced by its division was —

no longer made up of identical units, but of two different kinds.
After this separation, the germ-cells alone retained the power of
reproduction possessed by all the parent cells, while the rest only
retained the power of producing similar cells by division. Thus
Volvow seems to afford distinct evidence that in the phyletic
origin of the heteroplastid groups, somatic cells were not, as Nigeli
supposes, intercalated between the mother germ-cell and the daughter
germ-cells in each ontogeny, but that the somatic cells arose
directly from the former, with which they were previously identical,
as they are even now in the case of Pandorina. Thus the con-
tinuity of the germ-plasm i is established at least, for the beginning
‘of the phyletic series of development.

“ 1 Compare Biitschli in Bronn’s ‘Klassen und Ordnungen des Thierreichs,’ Bd. I.
P: 777:

ak

FOUNDATION OF A THEORY OF HEREDITY. 205

The fact, already often mentioned, that in most higher-organisms
the separation of germ-cells takes place later, and often’very late,
at the end of the whole ontogeny, proves that the time at which
this separation of the two kinds of cells took place, must have been
gradually changed. In this respect the well-established instances
of early separation are of great value, because they serve to connect
the extreme cases. It is quite impossible to maintain that the
germ-cells of Hydroids or of the higher plants, exist from the
time of embryonic development, as indifferent cells, which cannot
be distinguished from others, and which are only differentiated at a
later period. Such a view is contradicted by the simplest mathe-
matical consideration ; for it is obvious that none of the relatively
few cells of the embryo can be excluded from the enormous increase
by division, which must take place in order to produce the large
number of daughter individuals which form a colony of polypes.
It is therefore clear that all the cells of the embryo must for a long
time act as somatic cells, and none of them can be reserved as
germ-cells and nothing else: this conclusion is moreover confirmed
by direct observation. The sexual bud of a Coryne arises at a
part of the Polype which does not in any way differ from sur-
rounding areas, the body wall being uniformly made up of two
single layers of cells, the one forming the ectoderm and the other the
endoderm. Rapid growth then takes place at a single spot, and
some of the young cells thus produced are transformed into germ-
cells, which did not previously exist as separate cells.

Strictly speaking I have therefore fallen into an inaccuracy in
maintaining (in former works) that the germ-cells are themselves.
immortal ; they only contain the undying part of the organism—
the germ-plasm; and although this substance is, as far as we
know, invariably surrounded by a cell-body, it does not always
control the latter, and thus confer upon it the character of a
germ-cell, But this admission does not materially change our
view of the whole subject. We may still contrast the germ-cells,
as the undying part of the Metazoan body, with the perishable
somatic cells. If the nature and the character of a cell is deter-
- mined by the substance of the nucleus and not by the cell-body,
then the immortality of the germ-cells is preserved, although only
the nuclear substance passes uninterruptedly from one generation
to another.

206 THE CONTINUITY OF THE GERM-PLASM AS THE

G. Jiger’ was the first to state that the body in the higher
organisms is made up of two kinds of cells, viz., ontogenetic and
phyletic cells, and that the latter, the reproductive cells, are
not a product of the former (the body-cells), but that they arise
directly from the parent germ-cell. He assumed that the formation

of germ-cells takes place at the earliest stage of embryonic life, and’

he thus believed the connexion between the germ-plasm of the
parent and of the offspring had received a satisfactory explana-
tion. As I have previously mentioned in the introduction, Nuss-
baum also brought forward this hypothesis at a later period, and
also based it upon a continuity of the germ-cells. He assumed
that the fertilized egg is divided into the cells of the individual and
into the cells which effect the preservation of the species, and he

supported this view by referring to the few known cases of early ©
’ . separation of the sexual cells. He even maintained this hypothesis

when I had proved in my investigations on Hydromedusae that the
sexual cells are not always separated from the somatic cells during
embryonic development, but often at a far later period. Not ‘only
is the hypothesis of a direct connexion between the germ-cells of
the offspring and parent broken down by the facts known in the
Hydroids, and in the Phanerogams? which resemble them in this
respect, but even the instances of early separated germ-cells quoted
by Jager and Nussbaum do not as a matter of fact support their
hypothesis. Among existing organisms it is extremely rare for the
germ-cells to arise directly from the parent egg-cell (as in Diptera).
If, however, the germ-cells are separated only a few cell-generations
later, the: postulated continuity breaks down; for an embryonic
cell, of which the offspring are partly germ-cells and partly somatic
cells, cannot itself possess the nature of a germ-cell, and its idioplasm

1 Gustav Jager, ‘Lehrbuch der Allgemeinen Zoologie, Leipzig, 1878; IL.
Abtheilung. Probably on account of the extravagant and superficial speculations
of the author, the valuable ideas contained in his book have been generally over-
looked. It is only lately that I have become aware of Jiiger’s above-mentioned hy-
pothesis. M. Nussbaum seems to have also arrived at the same conclusion quite
independently of Jager. The latter has not attempted to work out his hypothesis
with any degree of completeness. The above-mentioned observations are followed
immediately by quite valueless considerations, as, for instance, that the ontogenetic
and phyletic groups are in concentric ratio! The author might as well speak of
a quadrangular or triangular ratio!

(? Facts of the same kind are also known in the Vascular Cryptogams, Muscineae,
Characeae, Florideae, etc.—S. 8.]

—

=
a

FOUNDATION OF A THEORY OF HEREDITY. 207

cannot be identical with that of the parent germ-cell. In order to
prove this, it is only necessary to refer to the arguments as to the
ontogenetic stages of the idioplasm. In the above-mentioned

instances, the continuity from the germ-substance of the parent

to that of the offspring can only be explained by the supposition
that the somatic nucleoplasm still contains some unchanged germ-
plasm. I believe that the fundamental idea of Jiiger and Nuss-
baum is quite correct: it is the same idea which has led me to the
hypothesis of the continuity of the germ-plasm, viz., the conviction
that heredity can only be understood by means of such an hypothesis.
But both these writers have worked out the idea in the form of an
hypothesis which does not correspond with the facts. That this is
the case is also shown by the following words of Nussbaum—‘ the

-cell-material of the individual (somatic cells) can never produce a

single sexual cell.’ Such production undoubtedly takes place, not
only in Hydroids and Phanerogams, but in many other instances.
The germ-cells cannot indeed be produced by any indifferent cell
of embryonic character, but by certain cells, and under circumstances
which allow us to positively conclude that they have been pre-
destined for this purpose from the beginning. In other words,
the cells in question contain germ-plasm, and this alone enables
them to become germ-cells.

As a result of my investigations on Hydroids1, I concluded that
the germ-plasm is present in a very finely divided and therefore in-
visible state in certain somatic cells, from the very beginning of
embryonic development, and that it is then transmitted through

‘innumerable cell-generations, to those remote individuals of the

colony in which sexual products are formed. This conclusion is
based upon the fact that germ-cells only occur in certain localized
areas (‘Keimstitten’) in which neither germ-cells nor primitive
germ-cells (the cells which are transformed into germ-cells at a later
period) were previously present. The primitive germ-cells ‘are also
only formed in localized areas, arising from somatic cells of the
ectoderm. The place at which germ-cells arise is the same in all
individuals of the same species; but differs in different species. It
can be shown that such differences correspond to different phyletic

_ stages of a process of displacement, which tends to remove the

* Weismann, ‘ Die Entstehung der Sexualzellen bei den Hydromedusen.’ Jena,
1883. :

208 THE CONTINUITY OF THE GERM-PLASM AS THE

localized area from its original position (the manubrium of the_

Medusa) in a centripetal direction. For the purposes of the present
enquiry it is unnecessary to discuss the reasons for this change of
position. The phyletic displacements of the localized areas are
brought about during ontogeny by,an actual migration of primitive
germ-cells from the place where they arose to the position at which
they undergo differentiation into germ-cells. But we cannot believe
that primitive germ-cells would migrate if the germ-cells could
be formed from any of the other young cells of indifferent character
which are so numerous in Hydroids. Even when the localized area
undergoes very slight displacement, e.g. when it is removed from
the exterior to the interior of the mesogloea', the change is always
effected by active migration of primitive germ-cells through the
substance of the mesogloea. Although the localized area has been
largely displaced in the course of phylogeny, the changes in posi-
tion have always taken place by very gradual stages, and never
suddenly, and all these stages are repeated in the ontogeny of all
existing species, by the migration of the primitive germ-cells from
the ancestral area to the place where the germ-cells now arise.
Hartlaub? has recently added a further instance (that of Odelia) to
the numerous minute descriptions of these phyletic displacements of
the localized area, and ontogenetic migrations of the primitive germ-
cells, which are given in my work already referred to. The
instance of Obelia is of especial interest as the direction of dis-

placement is here reversed, taking place centrifugally instead of in _

a centripetal direction.

But if displacements of the localized areas can aly take place by
the frequently roundabout method of the migration of primitive
germ-cells, we are obliged to conclude that such is the only manner
in which the change can be effected, and that other cells are unable
to play the réle of the primitive germ-cells. And if other cells are
unable to take this part, it must be because nucleoplasm of a
certain character has to be’ present in order to form germ-cells, or
according to the terms of my theory, the presence of germ-plasm is

[1 I adopt this term, suggested by E. Ray Lankester and G. C. Bourne, as the
name of the supporting lamina of Coelenterata. See ‘ Quart. Journ. Microsc. Sci.’
Jan. 1887, p. 28 —E. B. P.]

? Dr. Clemens Hartlaub, ‘Ueber die Entstehung der Sexualzellen bei Obelia.’
Freiburg, Inaugural Dissertation: see also ‘Zeitschrift fiir wissenschaftliche Zoologie.’
Bd. XLI. 1884.

* we Santee

FOUNDATION OF A THEORY OF HEREDITY. 209

indispensable for this purpose. I do not see how we can escape
the conclusion that there is continuity of the germ-plasm; for
if it were supposed that somatic idioplasm undergoes transforma-
tion, into germ-plasm, such an assumption would not explain
why the displacement occurs }, small stages, and with extreme
and constant care for the preservation of a connexion with cells
of the ancestral area. This fact can only be explained by the hypo-
_ thesis that cell-generations other than those which end in the
production of the cells of the ancestral area, are totally incapable
of transformation into germ-cells.

Strasburger has objected that the transmission of germ-plasm
along certain lines, viz. through a certain succession of somatic
cells, is impossible, because the idioplasm is situated in the nucleus
and not in the cell-body, and because a nucleus can only divide
into two exactly equal halves by the indirect method of division,
which takes place, as we must believe, in these cases. ‘It might
indeed be supposed,’ says Strasburger, ‘that during nuclear division
certain molecular groups remain unchanged in the nuclear sub-
stance which is in other respects transformed, and that these
groups are uniformly distributed through the whole organism ;
but we cannot imagine that their transmission could only be effected
along certain lines.’

I do not think that Strasburger’s objections can be maintained.
I base this opinion on my previous criticism upon the assumed
equality of the two daughter-nuclei formed by indirect division.
I do not see any reason why the two halves must always possess
the same structure, although they may be of equal size and weight.
I am surprised that Strasburger should admit the possibility that
the germ-plasm, which, as I think, is mixed with the idioplasm of
the somatic cells, may remain unchanged in its passage through
the body; for if this writer be correct in maintaining that the
changes of nuclear substance in ontogeny are effected by the
nutritive influence of the cell-body (cytoplasm), it follows that
the whole nuclear substance of a cell must be changed at. every
division, and that no unchanged part can remain. We can only
imagine that one part of a nucleus may undergo change while
the other part remains unchanged, if we hold that the necessary
transformations of nuclear substance are effected by purely internal
causes, viz. that they follow from the constitution of the nucleo-

P

210 THE CONTINUITY OF THE GERM-PLASM AS THE

plasm. But that one part may remain unchanged, and that such
persistence dves, as a matter of fact, occur is shown by the cases
above described, in which the germ-cells separate very early from
the developing egg-cell. Thus in the egg of Diptera, the two
nuclei which are first separated by division from the segmentation
nucleus, form the sexual cells, and this proves that they receive
the germ-plasm of the segmentation nucleus unchanged. But
during or before the separation of these two nuclei, the remaining part
of the segmentation nucleus must have become changed in nature,
or else it would continue to form ‘pole-cells’ at a later period
instead of forming somatic cells. Although in many cases the
cell-bodies of such éarly embryonic cells fail to exhibit any
visible differences, the idioplasm of their nuclei must undoubtedly
differ, or else they could not develope in different directions. It
seems to me not only possible, but in every way probable, that the
bodies of such early embryonic cells are equal in reality as well as
in appearance ; for, although the idioplasm of the nucleus deter-
mines the character of the cell-body, and although every differ-
entiation of the latter depends upon a certain structure of its
nucleoplasm, it does not necessarily follow that the converse pro-
position is true, viz. that each change in the structure of the
nucleoplasm must effect a change in the cell-body. Just as rain
is impossible without clouds, but every cloud does not necessarily
produce rain, so growth is impossible without chemical change, but
chemical processes of every kind and degree need not produce
growth. In the same manner every kind of change in the mole-
cular structure of the nucleoplasm need not exercise a transforming
influence on the cytoplasm, and we can easily imagine that a long
series of changes in the nucleoplasm may appear only in the kind
and energy of the nuclear divisions which take place, the cell-
substance remaining unchanged, as far as its molecular and che-
mical structure is concerned. This suggestion is in accordance
with the fact that during the first period of embryonic develop-
ment in animals, the cell-bodies do not exhibit any visible differ-
ences, or only such as are very slight; although exceptional in-
stances occur, especially among the lower animals. But even
these latter (e.g. the difference in appearance of the cells of the
ectoderm and endoderm in sponges and Coelenterata) perhaps
depend more largely upon a different admixture of nutritive sub-

FOUNDATION OF A THEORY OF HEREDITY. 211

stances than upon any marked difference in the cytoplasm itself. It
is obvious that, in the construction of the embryo, the amount of
cell-material must be first of all increased, and that it is only at a
later period that the material must be differentiated so as to
possess various qualities, according to the principle of division of
labour. Facts of this kind are also opposed to Strasburger’s view,
that the cause of changes in the nucleoplasm does not lie within
this substance itself but within the cell-body.

I believe I have shown that theoretically hardly any objections
can be raised against the view that the nuclear substance of
somatic cells may contain unchanged germ-plasm, or that this
germ-plasm may be transmitted along certain lines. It is true
that we might imagine @ priori that all somatic nuclei contain
‘a small amount of unchanged germ-plasm. In Hydroids such
an assumption cannot be made, because only certain cells in a
certain succession possess the power of developing into germ-cells ;
but it might well be imagined that in some organisms it would
be a great advantage if every part possessed the power of growing
up into the whole organism and of producing sexual cells under
appropriate circumstances. Such cases might exist if 1t were pos-
sible for all somatic nuclei to contain a minute fraction of un-
changed germ-plasm. For this reason, Strasburger’s other objection
against my theory also fails to hold; viz. that certain plants can
be propagated by pieces of rhizomes, roots, or even by means of
leaves, and that plants produced in this manner may finally give
rise to flowers, fruit and seeds, from which new plants arise. ‘It
is easy to grow new plants from the leaves of Begonia which
have been cut off and merely laid upon moist sand, and yet in the
normal course of ontogeny the molecules of germ-plasm would not
have been compelled to pass through the leaf; and they ought
therefore to be absent from its tissue. Since it is possible to raise
from the leaf a plant which produces flower and fruit, it is per-
fectly certain that special cells containing the germ substance
cannot exist in the plant.’ But I think that this fact only proves,
that in Begonia and similar plants, all the cells of the leaves
or perhaps only certain cells contain a small amount of germ-

plasm, and that consequently these plants are specially adapted

- for propagation by leaves. How is it then that all plants cannot

be reproduced in this way? No one has ever grown a tree from
sa

212 THE CONTINUITY OF THE GERM-PLASM AS THE

the leaf of the lime or oak, or a flowering plant from the leaf
of the tulip or convolvulus. It is insufficient to reply that, in the
last-mentioned cases, the leaves are more strongly specialized, and
have thus become unable to produce germ-substance ; for the leaf-
cells in these different plants have hardly undergone histological
differentiation in different degrees. If, notwithstanding, the one
can produce a flowering plant, while the others have not this
power, it is of course clear that reasons other than the degree
of histological differentiation must exist; and, according to my
opinion, such a reason is to be found in the admixture of a minute
quantity of unchanged germ-plasm with some of their nuclei.

In Sachs’ excellent lectures on the physiology of plants, we read
on page 723'—‘In the true mosses almost any cell of the roots,
leaves and shoot-axes, and even of the immature sporogonium,
may grow out under favourable conditions, become rooted, form)
new shoots, and give rise to an independent living plant.’ Since
such plants produce germ-cells at a later period, we have here
a case which requires the assumption that all or nearly all cells
must contain germ-plasm.

‘The theory of the continuity of the germ-plasm seems to me
to be still less disproved or even rendered improbable by the facts
of the alternation of generations. Ifthe germ-plasm may pass on
from the egg into certain somatic cells of an individual, and if it
ean be further transmitted along certain lines, there is no difficulty
in supposing that it may be transmitted through a second, third,
or through any number of individuals produced from the former by
budding. In fact, in the Hydroids, on which my theory of the
continuity of the germ-plasm has been chiefly based, alternation
of generations is the most important means of propagation.

II. Tue SIGNIFICANCE OF THE PoLAR BopIEs.

We have already seen that the specific nature of a cell depends
upon the molecular structure of its nucleus; and it follows from
this conclusion that my theory is further, and as I believe strongly,
supported, by the phenomenon of the expulsion of polar bodies,
which has remained inexplicable for so long a time.

» English translation, by H. Marshall Ward. Oxford, 1887, Clarendon Press.

FOUNDATION OF A THEORY OF HEREDITY. 213

For if the specific molécular structure of a cell-body is caused
and determined by the structure of the nucleoplasm, every kind
of cell which is histologically differentiated must have a specific
nucleoplasm. But the egg-cell of most animals, at any rate during
the period of growth, is by no means an indifferent cell of the
most primitive type. At such a period its cell-body has to
perform quite peculiar and specific functions; it has to secrete
nutritive substances of a certain chemical nature and physical con-
stitution, and to store up this food-material in such a manner
that it may be at the disposai of the embryo during its develop-
ment. In most cases the egg-cell also forms membranes which
are often characteristic of particular species of animals. The
growing egg-cell is therefore histologically differentiated: and
in this respect resembles a somatic cell. It may perhaps be com-
pared to a. gland-cell, which does not expel its secretion, but
deposits it within its own substance’. To perform such specific
functions it requires a specific cell-body, and the latter depends
upon a specific nucleus. It therefore follows that the growing
-egg-cell must possess nucleoplasm of specific molecular struc-
ture, which directs the above-mentioned secretory functions of
the cell. The nucleoplasm of histologically differentiated cells
may be called histogenetic nucleoplasm, and the growing egg-
cell must contain such a substance, and even a certain specific
modification of it. This nucleoplasm cannot possibly be the same
as that which, at a later period, causes embryonic development.
Such development can only be produced by true germ-plasm
of immensely complex constitution, such as I have previously
attempted to describe. It therefore follows that the nucleus of
the egg-cell contains two kinds of nucleoplasm :—germ-plasm
and a peculiar modification of histogenetic nucleoplasm, which
may be called ovogenetic nucleoplasm. This substance must greatly
preponderate in the young egg-cell, for, as we have already seen,
if controls the growth of the latter. The germ-plasm, on the
other hand, can only be present in minute quantity at first, but
_ it must undergo considerable increase during the growth of the
cell. But in order that the germ-plasm may control the cell-

({' Such gland-cells are known in both animals and plants. See W. Gardiner and —
Tokutaro Ito, On the structure of the mucilage-secreting cells of Blechnum occidentale
L., and Osmunda@ regalis L., ‘ Annals of Botany,’ vol. i. p. 49.—8. 8.]

214 THE CONTINUITY OF THE GERM-PLASM AS THE

body, or, in other words, in order that embryonie development
may begin, the still preponderating ovogenetic nucleoplasm must
be removed from the cell. This removal takes place in the same
manner as that in which differing nuclear substances are separated
during the ontogeny of the embryo: viz. by nuclear division,
leading to cell-division. The expulsion of the polar bodies is
nothing more than the, removal of ovogenetic nucleoplasm from
the egg-cell. That the ovogenetic nucleoplasm continues to
greatly preponderate in the nucleus up to the very last, may be
concluded from the fact that two successive divisions of the latter
and the expulsion of two polar bodies appear to be the rule. If in

this way a small part of the cell-body is expelled from the egg, |

the extrusion must in all probability be considered as an inevitable
loss, without which the removal of the ovogenetie nucleoplasm
cannot be effected.

This is my theory of the significance of polar bodies, and I
do not intend to contrast it, 7 extenso, with the theories pro-
pounded by others; for such theories are well known and differ
essentially from my own. All writers agree in supposing that
something which would be an obstacle to embryonic development
is removed from the egg; but opinions differ as to the nature of
this substance and the precise reasons for its removal’. Some ob-
servers (e.g. Minot*, van Beneden, and Balfour) regard the
nucleus as hermaphrodite, and assume that in the polar bodies the
male element is expelled in order to render the egg capable of
fertilization. Others speak of a rejuvenescence of the nucleus,
others again believe that the quantity of nuclear substance must be
reduced in order to become equal to that of the generally minute
sperm-nucleus, and that the proportions for nuclear conjugation are
in this way adjusted.

The first view seems to me to be disproved by the fact that male
as well as female qualities are transmitted by the egg-cell, while

the sperm-cell also transmits female qualities. The germ-plasm of.

the nucleus of the egg cannot therefore be considered as female,

1 Thus in 1877 Bitschli thought that ‘the chief significance of the formation of
polar bodies lies in the removal of part of the nucleus of the egg, whether this
removal is effected by simple expulsion or by the budding of the egg-cell.’ * Ent-
wicklungsgeschichtliche Beitriige ;’ Zeitschrift fiir wissenschaftliche Zoologie, Bd.
XXIX. p. 237, footnote. -

? C. 8. Minot, ‘ Account, ete.;’ Proc. Boston Soc. Nat. Hist. vol. xix. p. 165, 1877.

ted

FOUNDATION OF A THEORY OF HEREDITY, 215

and that of the sperm-nucleus cannot be considered as male: both
are sexually indifferent. The last view has been recently formulated
by Strasburger, who holds that the quantity of the idioplasm
contained in the germ-nucleus must be reduced by one half, and
that a whole nucleus is again produced by conjugation with the
sperm-nucleus. Although I believe that the fundamental idea
underlying this hypothesis is perfectly correct, viz. that the in-
fluence of each nucleus is as largely dependent upon its quantity
as upon its quality, I must raise the objection that the decrease in
quantity is not the explanation of the expulsion of polar bodies. The
quantity of idioplasm contained in the germ-nucleus is, as a matter
of fact, not reduced by one-half but by three-fourths, for two
divisions take place one after the other. Thus by conjugation
with the. sperm-nucleus, which we may assume to be of the same
size as the germ-nucleus, a nucleus is produced which can only
contain half as much idioplasm as was present in the original
germ-nucleus, before division. Strasburger’s view leaves un-
explained the question why the size of the germ-nucleus, before the
expulsion of polar bodies, was thus twice as large; and even if we
neglect the theory of ovogenetic nucleoplasm and assume that this
larger nucleus was entirely made up of germ-plasm, it must be
asked why the egg did not commence segmentation earlier. The
theory which explains the sperm-cell as the vitalizing principle
which starts embryonic development, like the spark which kindles
the gunpowder, would indeed answer this question in a very simple
manner, But Strasburger does not accept this theory, and holds,
as I do, a very different view, which will be explained later on.

If, on the other hand, we assume that the germ-nucleus contains
two different kinds of nucleoplasm, the question is answered quite
satisfactorily. In treating of parthenogenesis, further on, I shall
mention a fact which seems to me to furnish a real proof of the
validity of this explanation; and, if we accept this fact for the
present, it will be clear that the simple explanation now offered
of phenomena which are otherwise so difficult to understand,
would also largely support the theory of the continuity of the
germ-plasm. Such an explanation would, above all, very clearly
demonstrate the co-existence of two nucleoplasms with different
qualities in one and the same nucleus. My theory must. stand
or fall with this explanation, for if the latter were disproved, the

216 THE CONTINUITY OF THE GERM-PLASM AS THE

continuity of the germ-plasm could not be assumed in any instance,
not even in the simplest cases, where, as in Diptera, the germ-cells
are the first-formed products of embryonic development. For even
in these insects the egg possesses a specific histological character
which must depend upon a specifically differentiated nucleus. If
then two kinds of nucleoplasm are not present, we must assume that
in such eases the germ-plasm of the newly formed germ-cells,
which has passed on unchanged from the segmentation nucleus, is at
once transformed entirely into ovogenetic nucleoplasm, and must be
re-transformed into germ-plasm at a later period when the egg is
fully mature. We could not in any way understand why such a
re-transformation requires the expulsion of part of the nuclear sub-
stance.

At all events, my explanation is simpler than all others, in that
it only assumes a single transformation of part of the germ-plasm,
and not the later re-transformation of ovogenetic nucleoplasm into
germ-plasm, which is so improbable. The ovogenetic nucleoplasm
must possess entirely different qualities from the germ-plasm ; and,
above all, it does not readily lead to division, and thus we can better
understand the fact, in itself so remarkable, that egg-cells do not
increase in number by division, when they have assumed their
specific structure, and are controlled by the ovogenetic nucleoplasm.
The tendency to nuclear division, and consequently to cell-division,
is not produced until changes have to a certain extent taken place
in the mutual relation between the two kinds of nucleoplasm
contained in the germ-nucleus. This change is coincident with
the attainment of maximum size by the body of the egg-cell.
Strasburger, supported by his observations on Spirogyra, concludes
that the stimulus towards cell-division emanates from the cell-
body; but the so-called centres of attraction at the poles of the
nuclear spindle obviously arise under the influence of the nucleus
itself, even if we admit that they are entirely made up of cytoplasm.
But this point has not been decided upon, and we may presume
that the so-called ‘ Polkérperchen’ of the spindle (Fol) are derived
from the nucleus, although they are placed outside the nuclear
membrane!, Many points connected with this subject are still in a

1 E. van Beneden and Boveri have recently, quite independently of each other,
made a more exact study of these ‘ Polkiérperchen’ (‘ Centrosoma,’ Boveri). They

show that nuclear division starts from these bodies, although the mode of origin of
the latter is not yet quite clear.—A. W., 1888.

j

FOUNDATION OF A THEORY OF HEREDITY. 217

state.of uncertainty, and we must abstain from general conclusions
until it has been possible to demonstrate clearly the precise nature
of certain phenomena attending indirect nuclear division, which
still remain obscure in spite of the efforts of so many excellent
observers. We cannot even form a decided opinion as to whether
the chromatin or the achromatin of the nuclear thread is the real
idioplasm. But although these points are not yet thoroughly
understood, we are justified in maintaining that the cell enters
upon division under the influence of certain conditions of the
nucleus, although the latter are invisible until cell-division has
already commenced,

I now pass on to examine my hypothesis as to the significance of
the formation of polar bodies, in the light of those ascertained facts
which bear upon it.

If the expulsion of the polar bodies means the removal of the*
ovogenetic nucleoplasm after the histological differentiation of the
egg-cell is complete, we must expect to find polar bodies in all
species except those in which the egg-cell has remained in a
primitive undifferentiated condition, if indeed such species exist.
Wherever the egg-cell assumes the character of a specialized cell,
e.g. in the attainment of a particular size or constitution, in the
admixture of food-yolk, or the formation of membranes, it must also
contain ovogenetic nucleoplasm, which must ultimately be removed
if the germ-plasm is to gain control over the egg-cell. It does not
signify at all, in this respect, whether the egg is or is not destined
for fertilization.

If we examine the Metazoa in regard to this question, we find
that polar bodies have not yet been discovered in sponges +, but this
negative evidence is no proof that they are really absent. In all
probability, no one has ever seriously endeavoured to find them, and
there are perhaps difficulties in the way of the proofs of their exist-
ence, because the egg-cell lies free for a long time and even moves
actively in the tissue of the mesogloea. We might expect that the
formation of polar bodies takes place here, as in all other instances,
when the egg becomes mature, that is, at a time when the eggs
are already closely enveloped in the sponge tissue. At all events
' the eggs of sponges, as far as they are known, attain a specific

1 The existence of polar bodies in sponges has been recently proved by Fiedler :
Zool,, Anzeiger., Nov. 28, 1887.—A. W., 1888.

218 THE CONTINUITY OF THE GERM-PLASM AS THE

nature, in the possession of a peculiar cell-body, frequently con-
taining food-yolk, and of the nucleus which is characteristic of all
animal eggs during the process of growth. Hence we cannot
doubt the presence of a specifie ovogenetic nucleoplasm, and must
therefore also believe that it is ultimately removed in the polar
bodies.

In other Coelenterata, in worms, echinoderms, and in molluses
polar bodies have been described, as well as in certain Crustacea,
viz. in Balanus by Hoek and in Cetochilus septentrionale by Grobben.
The latter instance appears to be quite trustworthy, but there is
some doubt as to the former and also as regards Moina (a Daphnid),
in which Grobben found a body, which he considered to be a polar

body, on the upper pole of an egg which was just entering upon

segmentation. In insects polar bodies have not been described up
to the present time ', and only in a few cases in Vertebrata, as in
Petromyzon by Kupffer and Benecke.

It must be left to the future to decide whether the expulsion of
polar bodies occurs in those large groups of animals in which they

have not been hitherto discovered. The fact, however, that they

have not been so discovered cannot be urged as an objection to my
theory, for we do not know a priori whether the removal of the ovo-
genetic nucleoplasm has not been effected in the course of phylogeny
‘in some other and less conspicuous manner. The cell-body of the
polar globules is so minute in many eggs that it was a long time
before the cellular nature of these structures was recognized? ; and
it is possible that their minute size may point to the fact that
a phyletic process of reduction has taken place, to the end that the
egg may be deprived of as little material as possible. It is at all
events proved that in all Metazoan groups the nucleus undergoes
changes during the maturation of the egg, which are entirely similar
to those which lead to the formation of polar bodies in those eggs
which possess them. In the former instances it is possible that
nature has taken a shortened route to gain the same end.

It would be an important objection if it could be shown that no

* They have now been observed in many species, so that their general occurrence =.

in insects is tolerably certain. Compare bibliography given in Weismann and
Ischikawa, ‘ Weitere Untersuchungen zum Zahlengesetz der Rishtungebtrpaey
‘ Zoolog. Jahrbiicher,’ vol. iii, 1888, p. 593-—A. W., 1888.

? Van Beneden, even in his last work, considers shake bodies to have‘only the value
of nuclei; 1. ¢., p. 394.

_"
er

FOUNDATION OF A THEORY OF HEREDITY. 219

process corresponding to the expulsion of polar bodies takes place
in the male germ-cells, for it is obvious that here also we should,
according to my theory, expect such a process to occur. The great
majority of sperm-cells differ so widely in character from the ordi-
nary indifferent (i. e. undifferentiated) cells, that they are evidently
histologically differentiated in a very high degree, and hence the
sperm-cells, like the yolk-forming germ-cells, must possess a specific
nuclear substance. The majority of sperm-cells therefore resemble
the somatic cells in that they have a specific histological structure,
but their characteristic form has nothing to do with their fertilizing
power, viz. with their power of being the bearers of germ-plasm.
Important as this structure is, in order to render it possible that
the egg-cell may be approached and penetrated, it has nothing to
do with the property of the sperm-cell to transmit the qualities of
the species and of the individual to the following generation. The
nuclear substance which causes such a cell to assume the appearance
of a thread, ora stellate form (in Crustacea), or a boomerang form
(present in certain Daphnids), or a conical bullet shape (Nematodes),
cannot possibly be the same nuclear substance as that which, after
conjugation with the egg-cell, contains in its molecular .structure
the tendency to build up a néw Metazoon of the same kind as that
by which it was produced. We must, therefore, conclude that the
sperm-cell also contains two kinds of nucleoplasm, namely, germ-
plasm and spermogenetic nucleoplasm.

It is true that we cannot say a priort whether the influence
exercised on the sperm-cell by the spermogenetic nucleoplasm
might not be eliminated by some means other than its removal
from the cell. It is conceivable, for instance, that this substance
may be expelled from the nucleus, but may remain in the cell-body,
where it is in some way rendered powerless. We do not yet really
know anything of the essential conditions of nuclear division, and
it is quite impossible to bring forward any facts in support of my
previous suggestion. The germ-plasm is supposed to be present
in the nucleus of the growing egg-cell in smaller quantity than the
ovogenetic nucleoplasm, and the germ-plasm gradually increases in
quantity: thus when the ege has attained its maximum size, the
' opposition between the two different kinds of nucleoplasm becomes
so marked, in consequence of the alteration in their quantitative
relations, that their separation, viz. nuclear division, results. But

220 THE CONTINUITY OF THE GERM-PLASM AS THE

although we are not able to distinguish, by any visible charae~
teristics, the different kinds of nucleoplasm which may be united
in one nuclear thread, the assumption that the influence of each
kind bears a direct proportion to its quantity is the most obvious
and natural one. The tendency of the germ-plasm contained
in the nucleus cannot make itself felt so long as an excess of
ovogenetic nucleoplasm is also present. We may imagine that
the effects of the two different kinds of nucleoplasm are combined
to produce a resultant effect ; but when the two influences exerted
upon the cell are nearly opposed, only the stronger can make
itself felt, and in such a case the latter must exceed the former in
quantity, because part of it is as it were neutralized by the other
nucleoplasm working in an opposite direction. This metaphorical
representation may give us a clue to explain the fact that the
ovogenetic nucleoplasm comes to exceed the germ-plasm in quan-
tity. For obviously these two kinds of nucleoplasm exert oppo-
site tendencies in at least one respect. The germ-plasm tends
to effect the division of the cell into the -two first segmentation
spheres; the ovogenetic nucleoplasm, on the other hand, possesses a
tendency towards the growth of the cell-body without division,
Hence the germ-plasm cannot make itself felt, and is not able to
expel the ovogenetic nucleoplasm until it has reached such a
relative size as enables it to successfully oppose the latter.

Applying these ideas to the sperm-cells we must see whether
the expulsion of part of the nuclear substance, viz. of the spermo-
genetic nucleoplasm, corresponding to the ovogenetic nucleoplasm,
takes place in them also.

As far as we can judge from thoroughly substantiated obser-
vations such phenomena are indeed found in many cases, although
they appear to be different from those occurring in the egg-cell,
and cannot receive quite so certain an interpretation.

The attempt to prove that a process similar to the expulsion of
polar bodies takes place in the formation of sperm-cells has already
been made by those observers who regard such expulsion as the
removal of the male element from the egg, thus leading to sexual
differentiation ; for such a theory also requires the removal of part
of the nuclear substance from the maturing sperm-cell. Thus,
according to E. van Beneden and Ch. Julin, the cells which, in
Ascaris, produce the spermatogonia (mother-cells of the sperm-cells),

FOUNDATION OF A THEORY OF HEREDITY. 221

expel certain elements from their nuclear plate, a phenomenon which

has not been hitherto observed in any other animal, and even in

this instance has only been inferred and not directly observed.

Moreover the sperm-cells have not attained their specific form-
(conical bullet-shaped) at the time when this expulsion takes place

from the spermatogonia, and we should expect that the spermo-

genetic nucleoplasm would not be removed until it has completed

its work, viz. not until the specific shape of the sperm-cell has

been attained. We might rather suppose that phenomena explic-

able in this way are to be witnessed in those sperm-blastophores

(mother-cells of sperm-cells) which, as has been known for a long

time, are not employed in the formation of the nuclei of sperm-

cells, but for the greater part remain at the base of the latter and
perish after their maturation and separation. In this case an in-

fluence might be exerted by these nuclei upon the specific form of
the sperm-cells, for the former arise and develope in the form of
bundles of spermatozoa in the interior of the mother-cell.

It has been already shown in many groups of animals that parts
ot the sperm-mother-cells! perish, without developing into sperm-
cells, as in Selachians, in the frog, in many worms and snails,
and also in mammals (Blomfield). But the attention of observers
has been directed to that part of the cell-body which is not used
in the formation of sperm-cells, rather than to the nucleus; and
the proof that part of the nucleus also perishes is still wanting
in many of these cases. Fresh investigation must decide whether
the nucleus of the sperm-mother-cell perishes as a general rule,
and whether part of the nucleus is rendered powerless in some
other way, where such mother-cells do not exist. Perhaps the
paranucleus (Nebenkern) of the sperm-cell, first described by La
Valette St. George, and afterwards found in many animals of very
different groups, is the analogue of the polar body. It is true that
this so-called paranucleus is now considered as a condensed part of

1-T purposely abstain from using a more precise term, for the complicated ter-
minology employed in’ spermatogenesis hardly contributes anything to the elucida-
tion of the phenomena themselves. Why do we not simply speak of sperm-cells
and spermatoblasts, and distinguish the latter by numbers when they occur in
successive generations of different form? Moreover, all the names which have been
, suggested for successive stages of development, can only be applied to the special
group of animals upon which the observations have been made. Hence great con«

fusion results from the use of such terms as spermatoblasts, spermatogonia, sperma-
tomeres, spermatocysts, spermatocytes, spermatogemmae, etc,

222 THE CONTINUITY OF THE GERM-PLASM AS THE

the cell-body, but we must remember that it has beén hitherto a
question whether the head of the spermatozoon is formed from the
nucleus of the cell or from the paranucleus ; and that the observers
who held the former view were in consequence obliged to regard
the paranucleus as a product of the cell-body. But according to
the most recent investigations of Fol!, Roule?, Balbiani*®, and Will*,
upon the formation of the follicular epithelium in the ovary of
different groups, it is not improbable that parts of the nucleus
may become detached without passing through the process of
karyokinesis. Thus it is very possible that the paranucleus may
be a product of the main nucleus and not a condensed part of
the cell-body. This view is supported by its behaviour with stain-
ing reagents, while the other view, that it arises from the cell-
substance, is not based upon direct observation. Consequently
future investigation must decide whether the paranucleus is to
be considered as the spermogenetic nucleoplasm expelled from the
nucleus. But even if this question is answered in the affirmative,
we should still have to explain why this nuclear substance, remain-
ing in the cell-body, does not continue to exercise any control over
the latter. é

Strasburger has recently enumerated a large number of cases
from different groups of plants, in which the maturation of both
male and female germ-cells is accompanied by phenomena similar
to the expulsion of polar bodies. In this respect the phenomena
occurring in the pollen-grains of Phanerogams bear an aston-
ishing resemblance to the maturation of the animal egg. For
instance, in the larch, the sperm-mother-cell divides three times
in succession, the products of division being very unequal on each
occasion ; and exactly as in the case of polar bodies, the three small
so-called vegetative cells shrink rapidly after separation, and have
no further physiological value. According to Strasburger, the so-
called ‘ventral canal-cell,’ which, in mosses, ferns, and Conifers,

1 Fol, ‘Sur Vorigine des cellules du follicule et de l’ovule chez les Ascidies.’
Compt. rend., 28 mai, 1883.

2 Roule, ‘La structure de l’ovaire et la formation des ceufs chez les Phallusiadées.’
Tbid., 9 avril, 1883.

% Balbiani, ‘Sur l’origine des cellules du follicule et du noyau vitellin de l’euf
chez les Géophiles.’ Zool. Anzeiger, 1883, Nos. 155, 156.

* Will, ‘Ueber die Entstehung des Dotters und der Epithelzellen bei den Amphi-
bien und Insecten.’ Ibid., 1884, Nos. 167, 168,

FOUNDATION OF A THEORY OF HEREDITY. 223

separates from the female germ-cell, reminds us, in every way, of
the polar bodies of animal eggs. Furthermore, the spermatozoids
in the mosses and vascular eryptogams throw off a small vesicle
before performing their functions?. On the other hand the equiva-
lents of ‘ polar bodies’ (the ‘ ventral canal-cells’) are said to be ab-
sent in the Cycads, although these are so nearly allied to Conifers.
Furthermore, ‘no phenomenon occurs in the oospheres (ova) of An-
giosperms which can be compared to the formation of polar bodies.’
Strasburger therefore concludes that the separation of certain parts
from the germ-cells is not in all cases necessary for maturation,
and that such phenomena are not fundamental, like those of
fertilization, which must always take place along the same morpho-
logical lines. He further concludes that the former phenomena are
only necessary in the case of the germ-cells of certain organisms,
in order to bring the nuclei destined for the sexual act into the
physiological condition necessary for its due performance.

Iam unwilling to abandon the idea that the expulsion of the
histogenetie parts of the nuclear substance, during the maturation
ot germ-cells, is also a general phenomenon in plants; for the
process appears to be fundamental, while the argument that it
has not been proved to occur universally is only of doubtful value.
The embryo-sac of Angiosperms is such a complex structure that
it seems to me to be possible (as it does to Strasburger) that ‘ pro-
cesses which precede the formation of the egg-cell have borne
relation to the sexual differentiation of the nucleus of the egg.’
Besides, it is possible that the vegetable egg-cell may, in certain
cases, possess so simple a structure and so small a degree of histo-
logical specialization, that it would not be necessary for ‘it to
contain any specific histogenetic nucleoplasm: thus it would con-
sist entirely of germ-plasm from the first. In such cases, of course,
its maturation would not be accompanied by the expulsion of
somatic nucleoplasm.

I have hitherto abstained from discussing the question as to
whether the process of the formation of polar bodies may require
an interpretation which is entirely different from that which I
have given it, whether it may receive a purely morphological inter-

{? It is almost certain that this vesicle is not derived from the nucleus, but from
the cytoplasm of the sperm-mother-cell. See Douglas H. Campbell, ‘Zur Ent-

wicklungsgeschichte der Spermatozoiden’ in Berichte der deutschen botanischen
' Gesellschaft, vol. v; 1887, p. 122.—S. 8.1].

224, THE CONTINUITY OF THE GERM-PLASM AS THE

pretation. In former times it could only be regarded as of purely
phyleticsignificance: it could only be looked upon as the last remnant
of a process which formerly possessed some meaning, but which is
now devoid of any physiological importance. We are indeed com-
pelled to admit that a process does occur in connexion with the true
polar bodies of animal eggs, which we cannot explain on physio-
logical grounds ; I mean the division of the polar bodies after they
have been expelled from the egg. In many animals the two polar
bodies divide again after their expulsion, so as to form four bodies,
which distinctly possess the structure of cells, as Trinchese observed
in the case of gastropods. But, in the first place, this second division
does not always take place, and, secondly, it is very improbable
that a process which occurs during the first stage of ontogeny,
or more properly speaking, before the commencement of ontogeny,
and which is, therefore, a remnant of some excessively ancient
phyletic stage, would have been retained up to the present day
unless it possessed some very important physiological significance.
We may safely maintain that it would have disappeared long ago
if it had been without any physiological importance. Relying
on our knowledge of the slow and gradual, although certain, dis-
_ appearance, in the course of phylogeny, of organs which have lost
their functions, and of processes which have become meaningless,
we are compelled to regard the process of the formation of polar
bodies as of high physiological importance. But this view does
not exclude the possibility that the process possessed a morpho-
logical meaning also, and I believe that we are quite justified in
attempting (as Biitschli! has recently done) to discover what this
morphological meaning may have been.

Should it be finally proved that the expulsion of polar bodies
is nothing more than the removal of histogenetic nucleoplasm
from the germ-cell, the opinion (which is so intimately connected
with the theory of the continuity of the germ-plasm) that a re-
transformation of specialised idioplasm into germ-plasm cannot
occur, would be still further confirmed ; for we do not find that any
part of an organism is thrown away simply because it is useless:
organs that have lost their functions are re-absorbed, and their
‘ material is thus employed to assist in building up the organism.

1 Biitschli, ‘Gedanken iiber die morphologische Bedeutung der sogenannten Rich-
tungskérperchen,’ Biolog. Centralblatt, Bd. VI. p. 5, 1884.

ne

L
a |

FOUNDATION OF A THEORY OF HEREDITY. 225

III. On toe Naturrt or PARTHENOGENESIS,

It is well known that the formation of polar bodies has been
repeatedly connected with the sexuality of germ-cells, and that it has
been employed to explain the phenomena of parthenogenesis. I
may now, perhaps, be allowed to develope the views as to the
nature of parthenogenesis at which I have arrived under the in-
fluence of my explanation of polar bodies.

The theory of parthenogenesis adopted by Minot and Balfour is
distinguished by its simplicity and clearness, among all other in-
terpretations which had been hitherto offered. Indeed, their ex-
planation follows naturally and almost as a matter of course, if the
assumption made by these observers be correct, that the polar
body is the male part of the hermaphrodite egg-cell. An egg
which has lost its male part cannot develope into an embryo until
it has received a new male part in fertilization. On the other
hand, an egg which does not expel its male part may develope with-
out fertilization, and thus we are led to the obvious conclusion that
parthenogenesis is based upon the non-expulsion of polar bodies.
Balfour distinctly states ‘that the function of forming polar cells
has been acquired by the ovum for the express purpose of prevent-
ing parthenogenesis?.’

It is obvious that I cannot share this opinion, for I regard the/
expulsion of polar bodies as merely the removal of the ovogenetic’
nucleoplasm, on which depended the development of the specific
histological structure of the egg-cell. I must assume that the
phenomena of maturation in the parthenogenetic egg and in the
sexual egg are precisely identical, and that in both, the ovogenetic
nucleoplasm must in some way be removed before embryonic de-
velopment can begin.

Unfortunately the actual proof of this assumption is not so com-
plete as might be desired. In the first place, we are as yet uncer-
tain whether polar bodies are or are not expelled by parthenogenetic
eggs”; forin no single instance has such expulsion been established
beyond doubt. It is true that this deficiency does not afford any

» F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 63.
? The formation of a polar body in parthenogenetic eggs has now been proved: see
note at the end of this Essay; see also Essay VI.—A. W., 1888.

Q

226 THE CONTINUITY OF THE GERM-PLASM AS THE

support to the explanation of Minot and Balfour, for in all eases
in which polar bodies have not been found in parthenogenetic eggs,
‘these structures are also absent from the eggs which require fertiliza-
‘tion in the same species. But although the expulsion of polar
bodies in parthenogenesis has not yet been proved to oceur, we must
assume it to be nearly certain that the phenomena of maturation,
whether connected or unconnected with the expulsion of polar
bodies, are the same in the eggs which develope parthenogenetically
and in those which are capable of fertilization, in one and the same
_ species. This conclusion depends, above all, upon the phenomena
of reproduction in bees, in which, as a matter of fact, the same egg
may be fertilized or may develope parthenogenetically, as I shall
have occasion to describe in greater detail at a later period.

Hence when we see that the eggs of many animals are capable of
developing without fertilization, while in other animals such de-
velopment is impossible, the difference between the two kinds of
eggs must rest upon something more than the mode of transforma-
tion of the nucleus of the germ-cell into the first segmentation
nucleus. There are, indeed, facts which distinctly point to the con-
clusion that the difference is based upon quantitative and not
qualitative relations. A large number of insects are exceptionally
reproduced by the parthenogenetic method, e.g. in Lepidoptera.
Such development does not take place in all the eggs laid by
an unfertilized female, but only in part, and generally a small
fraction of the whole, while the rest die. But among the latter
there are some which enter upon embryonic development without
being able to complete it, and the stage at which development
may cease also varies. It is also known that the eggs of higher
animals may pass through the first stages of seementation without
having been fertilized. This was shown to be the case in the egg
of the frog by Leuckart?, in that of the fowl by Oellacher*, and
even in the egg of mammals by Hensen*.

Hence in such cases it is not the impulse to development, but the

1 R. Leuckart, — article ‘ Zeugung,’ in R. Wagner’s ‘ Handwérterbuch der Phy-
siologie,’ 1853, Bd. IV. p. 958. Similar observations were made by Max Schultze.
These observations appear however to be erroneous, for Pfitiger has since shown that
the eggs of frogs never develope if the necessary precautions are taken to prevent the
access of any spermatozoa to the water.—A. W., 1888.

2 Oellacher, ‘Die Veriinderungen des unbefruchteten Keims des Hiihncheneies.

* Zeitschrift fiir wissenschaftliche Zoologie,’ Bd. XXII. 'p. 181. 1872.
* Hensen, ‘ Centralblatt,’ 1869, No. 26.

FOUNDATION OF A THEORY OF HEREDITY. I2F

power to complete it, which is absent. We know that force is
always bound up with matter, and it seems to me that such
instances are best explained by the supposition that too small an
amount of that form of matter is present, which, by its controlling ~
agency, effects the building-up of the embryo by the transforma-
tion of mere nutritive material. This substance is the germ-plasm
of the segmentation nucleus, and I have assumed above that it is
altered in the course of ontogeny by changes which arise from
within, so that, when sufficient nourishment is afforded by the cell-
body, each succeeding stage necessarily results from the preceding
one. I believe that changes arise in the constitution of the
nucleoplasm at each cell-division which takes place during the
building-up of the embryo, changes which either correspond or
differ in the two halves of each nucleus. If, for the present, we
neglect the minute amount of unchanged germ-plasm which is
reserved for the formation of the germ-cells, it is clear that a great
many different stages in the development of somatic nucleoplasm
are thus formed, which may be denominated as stages I, 2, 3, 4, &e.,
up to”. In each of these stages the cells differ more as develop-
ment proceeds, and as the number by which the stage is denomi-
nated becomes higher. Thus, for instance, the two first segmen-
tation spheres would represent the first stage of somatic nucleo-
plasm, a stage which may be considered as but slightly different
in its molecular structure from the nucleoplasm of the segmentation —
nucleus; the four first segmentation spheres would represent the
second stage ; the succeeding eight spheres the third, and so on. It
is clear that at each successive stage the molecular structure of the
nucleoplasm must be further removed from that of the germ-plasm,
and that, at the same time, the cells of each successive stage must
also diverge more widely among themselves in the molecular
structure of their nucleoplasm. Early in development each cell
must possess its own peculiar nucleoplasm, for the further course of
development is peculiar to each cell. It is only in the later stages
that equivalent or nearly equivalent cells are formed in -large
numbers, cells in which we must also suppose the existence of
equivalent nucleoplasm.

If we may assume that a certain amount of germ-plasm must be
contained in the segmentation nucleus in order to complete the
whole process of the ontogenetic differentiation of this substance ;

Q 2

228 THE CONTINUITY OF THE GERM-PLASM AS THE

if we may further assume that the quantity of germ-plasm in the
segmentation nucleus varies in different cases; then we should be
able to understand why one egg can only develope after fertiliza-
tion, while another can begin its development without fertilization,
but cannot finish it, and why a third is even able to complete its
development. We should also understand why one egg only passes
through the first stages of segmentation and is then arrested, while
another. reaches.a few more stages in advance, and a third de-
velopes so far that the embryo is nearly completely formed. These
differences would depend upon the extent to which the germ-plasm,
originally present in the egg, was sufficient for the development of
the latter ; development will be arrested as soon as the nucleoplasm
is no longer capable of producing the succeeding stage, and is thus
unable to enter upon the following nuclear division.

From a general point of view such a theory would explain many
difficulties, and it would render possible an explanation of the
phyletie origin of parthenogenesis, and an adequate understanding
of the strange and often apparently abrupt and arbitrary manner
of its occurrence. In my works on Daphnidae I have already laid
especial stress upon the proposition that parthenogenesis in insects
and Crustacea certainly cannot be an ancestral condition which has
been transmitted by heredity, but that it has been derived from a
sexual condition. In what other way can we explain the fact that
parthenogenesis is ‘present in certain species or genera, but absent
in others closely allied to them; or the fact that males are entirely
wanting in species of which the females possess a complete apparatus
for fertilization? I will not repeat all the arguments with which
I attempted to support this conclusion’. Such a conclusion may
be almost certainly accepted for the Daphnidae, because partheno-
genesis does not occur in their still living ancestors, the Phyllo-
pods, and especially the Lstheridae. In Daphnidae the cause and
object of the phyletic development of parthenogenesis may be traced
more clearly than.in any other group of animals. In Daphnidae
we can accept the conclusion with greater -certainty than in all
other groups, except perhaps the Ap/idae, that parthenogenesis is
extremely advantageous to species in certain conditions of life ; and
that it has only been adopted when, and as far as, it has been

4 Weismann, ‘ Beitriige zur Naturgeschichte der Daphnoiden,’ Leipzig, 1876-79,
Abhandlung VII, and ‘ Zeitschrift fiir wissenschaftliche Zoologie,’ Bd. XXXITT.

FOUNDATION OF A THEORY OF HEREDITY. 229

beneficial ; and further, that at least in this group parthenogenesis
became possible, and was adopted, in each species as soon as it
became useful. Such a result can be easily understood if it is only
the presence of more or less germ-plasm which decides whether an
egg is, or is not, capable of development without fertilization.

If we now examine the foundations of this hypothesis we shall
find that we may at once accept one of its assumptions, viz. that
fluctuations occur in the quantity of germ-plasm in the segmen-
tation nucleus; for there can never be absolute equality in any
single part of different individuals. As soon therefore as these
fluctuations become so great that parthenogenesis is produced, it may
become, by the operation of natural selection, the chief mode of
reproduction of the species or of certain generations of the species.
In order to place this theory upon a firm basis, we have simply to
decide whether the quantity of germ-plasm contained in the seg-
mentation nucleus is the factor which determines development ;
although for the present it will be sufficient if we can render this
view to some extent probable, and show that it is not in contra-
diction with established facts.

At first sight this hypothesis seems to encounter serious diffi-
culties. It will be objected that neither the beginning nor the end
of embryonic development can possibly depend upon the quantity
of nucleoplasm in the segmentation nucleus, since the amount may
be continually increased by growth; for it is well known that
during embryonic development the nuclear substance increases
with astonishing rapidity. By an approximate calculation I found!
that, in the egg of a Cynips, the quantity of nuclear substance
present at the time when the blastoderm was about to be formed,
and when there were twenty-six nuclei, was even then seven times
as great as the quantity which had been contained in the seg-
mentation nucleus. How then can we imagine that embryonic
development would ever- be arrested from want of nuclear sub-
‘ stance, and if such deficiency really acted as an arresting force, how
then could development begin at all? We might suppose that
when germ-plasm is present in sufficient quantity to start sezmen-
tation, it must also be sufficient to complete the development; for
‘it grows continuously, and must presumably always possess a power

1 Weismann, ‘Beitrage zur Kenntniss der ersten Entwicklungsvorginge im
Insectenei,’ Bonn, 1882, p. 106.

~~ 280 THE CONTINUITY OF THE GERM-PLASM AS THE

equal to that which it possessed at the beginning, and which was .

just sufficient to start the process of segmentation. If at each
ontogenetic stage, the quantity of nucleoplasm is just sufficient to
produce the following stage, we might well imagine that the whole
ontogeny would necessarily be completed.

The flaw in this argument lies in the erroneous assumption that
the growth of nuclear substance is, when the quality of the nucleus
and the conditions of nutrition are equal, unlimited and un-
controlled. The intensity of growth must depend upon the quan-
tity of nuclear substance with which growth and the phenomena of
segmentation commenced, There must be an optimum quantity
of nueleoplasm with which the growth of the nucleus proceeds
most favourably and rapidly, and this optimum will be represented
in the normal size of the segmentation nucleus. Such a size
is just sufficient to produce, in a certain time and under certain
external conditions, the nuclear substance necessary for the construc-
tion of the embryo, and to start the long series of cell-divisions.
When the segmentation nucleus is smaller, but large enough to
enter upon segmentation, the nuclei of the two first embryonic
cells will fall rather more below the normal size, because the
growth of the segmentation nucleus during and after division will
be less rapid on account of its unusually small size. The suecceed-
ing generations of nuclei will depart more and more from the
normal size in each respective stage, because they do not pass into
a resting-stage during embryonic development, but divide again
immediately after their formation. Hence nuclear growth would
become less vigorous as the nuclei fell more and more below the
optimum size, and at last a moment would arrive when they would
be unable to divide, or would be at least unable to control the cell-
body in such a manner as to lead to its division.

The first event of importance for embryonic development is the
maturation of the egg, i.e. the transformation of the nucleus of the

germ-cell into a nuclear spindle and the removal of the ovogenetie °

nucleoplasm by the separation of polar bodies, or by some ana-
logous process. ‘There must be some cause for this separation, and

I have already tried to show that it may lie in the quantitative

relations which obtain between the two kinds of nucleoplasm con-
tained in the nucleus of the egg. I have suggested that the
germ-plasm, at first small in quantity, undergoes a gradual increase,

_

FOUNDATION OF A THEORY OF HEREDITY. 231

so that it can finally oppose the ovogenetic nucleoplasm. I will not
further elaborate this suggestion, for the ascertained facts are in-
sufficient for the purpose. But the appearances witnessed in nuclear
division indicate that there are opposing forces, and that such a
contest is the motive cause of division ; and Roux? may be right
in referring the opposition to electrical forces. However this may
be, it is perfectly certain that the development of this opposition
is based upon internal conditions arising during growth in the
nucleus itself. The quantity of nuclear thread cannot by itself
determine whether the nucleus can or cannot enter upon division ;
if so, it would be impossible for two divisions to follow each other
in rapid succession, as is actually the case in the separation of
the two polar bodies, and also in their subsequent division. In
addition to the effects of quantity, the internal conditions of the
nucleus must also play an important part in these phenomena.
Quantity alone does not necessarily produce nuclear division, or the
nucleus of the egg would divide long before maturation is complete,
for it contains much more nucleoplasm than the female pronucleus,
which remains in the egg after the expulsion of the polar bodies,
and which is in most cases incapable of further division. But the
fact that segmentation begins immediately after the conjugation of
male and female pronuclei, also shows that quantity is an essential
requisite. The effect of fertilization has been represented as ana-
logous to that of the spark which kindles the gunpowder. In the
latter case an explosion ensues, in the former segmentation begins.
Even now, many authorities are inclined to refer the polar repul-
sion manifested in the nuclear division which immediately follows
fertilization, to the antagonism between male and female ele-
ments. But, according to the important discoveries of Flemming
and van Beneden, the polar repulsion in each nuclear division is
- not based.on the antagonism between male and female loops, but
depends upon the antagonism and mutual repulsion between the
two halves of the same loop. The loops of the father and those
of the mother remain together and divide together throughout
the whole ontogeny.

What can be the explanation of the fact that nuclear division
. follows immediately after fertilization, but that without fertilization

1 'W. Roux, ‘ Ueber die Bedeutung der Kerntheilungsfiguren.’ Leipzig, 1883.

232 THE CONTINUITY OF THE GERM-PLASM AS THE

it does not occur in most cases? There is only one possible ex-
planation, viz. the fact that the quantity of the nucleus has been
suddenly doubled, as the result of conjugation. The difference
between the male and female pronuclei cannot serve as an explana-
tion, even though the nature of this difference is entirely unknown,
because polar repulsion is not developed between the male and
female halves of the nucleus, but within each male and each female
half. We are thus forced to conelude that increase in the quantity
of the nucleus affords an impulse for division, the disposition
towards it being already present. It seems to me that this view
does not encounter any theoretical difficulties, and that it is an
entirely feasible hypothesis to suppose that, besides the internal
conditions of the nucleus, its quantitative relation to the cell-body
‘must be taken into especial account. It is imaginable, or perhaps
even probable, that the nucleus enters upon division as soon as its
idioplasm has attained a certain strength, quite apart from the
supposition that certain internal conditions are necessary for this
end. As above stated, such conditions may be present, but division
may, not occur because the right quantitative relation between
nucleus and cell-body, or between the different kinds of nuclear
idioplasm, has not been established. I imagine that such a quan-
titative deficiency exists in an egg, which, after the expulsion of the
ovogenetic nucleoplasm in the polar bodies, requires fertilization in
order to begin segmentation. The fact that the polar bodies were
expelled proves that the quantity of the nucleus was sufficient to
cause division, while afterwards it was no longer auficiont to pro-
duce such a result.

This suggestion will be made still clearer it an example. In
Ascaris megalocephala the nuclear substance of the fémale pro-
nucleus forms two loops, and the male pronucleus does the same ;
hence the segmentation nucleus contains four loops, and this is
also the case with the first segmentation spheres. If we suppose
that in embryonic development, the first nuclear division requires

such an amount of nuclear substance as is necessary for the forma- .

tion of four loops,—it follows that an egg, which can only form
two or three loops from its nuclear reticulum, would not be able to
develope parthenogenetically, and that not even the first division
would take place. If we further suppose that, while four loops
are sufficient to start nuclear division, these loops must be of a

a oe

FOUNDATION OF A THEORY OF HEREDITY. 233

certain size and quantity in order to complete the whole ontogeny
(in a certain species), it follows that eggs possessing a reticulum
which contains barely enough nuclear substance to divide into
four segments, would be able to produce the first division and
perhaps also the second and third, or some later division, but
that at a certain point during ontogeny, the nuclear substance
would become insufficient, and development would be arrested.
This will occur in eggs which enter upon development without
fertilization, but are arrested before its completion. One might
compare this retardation leading to the final arrest of development,
to a railway train which is intended to meet a number of other
trains at various junctions, and which can only travel ‘slowly
because of some defect in the engine. It will be a little behind time
at the first junction, but it may just catch the train, and it may
also catch the second or even the third ; but it will be later at each
successive junction, and will finally arrive too late for a certain
train; and after that it will miss all the trains at the remaining
junctions. The nuclear substance grows continuously during de-
velopment, but the rate at which it increases depends upon the
nutritive conditions together with its initial quantity. The nu-
tritive changes during the development of an egg depend upon
the quantity of the cell-body which was present at the outset, and
which cannot be increased. If the quantity of the nuclear sub-
stance is rather too small at the beginning, it will become more and
more insufficient in succeeding stages, as its growth becomes less
vigorous, and differs more from the standard it would have reached
if the original quantity had been normal. Consequently it will
gradually fall more and more short of the normal quantity, like
the train which arrives later and later at each successive junction,
because its engine, although with the full pressure of steam, is
unable to attain the normal speed.

It will be objected that four loops cannot be necessary for nuclear
division in Ascaris, since such division takes place in the formation of
the polar bodies, resulting in the appearance of the female pronucleus
with only two loops. But this fact only shows that the quantity of
nuclear substance necessary for the formation of four loops ig not

_ necessary for all nuclear divisions; it does not disprove the assump-
tion that such a quantity is required for the division of the seg-
mentation nucleus. In addition to these considerations we must not

234 THE CONTINUITY OF THE GERM-PLASM AS THE

leave the substance of the cell-body altogether out of account, for,
although it is not the bearer of the tendencies of heredity, it must
be necessary for every change undergone by the nucleus, and it
surely also possesses the power of influencing changes to a large ex-
tent. There must be some reason for the fact that in all animal
eggs with which we are acquainted, the nucleus moves to the sur-
face of the egg at the time of maturation, and there passes through
its well-known transformation. It is obvious that it is there sub-
jected to different influences from those which would have acted
upon it in the centre of the cell-body, and it is clear that such an
unequal cell-division as takes place in the separation of the polar
bodies could not occur if the nucleus remained in the centre of
the egg.

This explanation of the necessity for fertilization does not exelude
the possibility, that, under certain circumstances, the substance of
the egg-nucleus may be larger, so that it is capable of forming four
loops. Eggs which thus possess sufficient nucleoplasm, viz. germ-
plasm; for the formation of the requisite four loops of normal size,
(namely, of the size which would have been produced by fertilization),
can and must develope by the parthenogenetic method.

Of course the assumption that four loops must be formed has only
been made for the sake of illustration. We do not yet know
whether there are always exactly four loops in the segmentation
nucleus! I may add that, although the details by which these
considerations are illustrated are based on arbitrary assumptions, the
fundamental view that the development of the egg depends, ceteris
paribus, upon the quantity of nuclear substance, is certainly right,
and follows as a necessary conclusion from the ascertained facts. It
is not unlikely that such a view may receive direct proof in the
results of future investigations. Such proof might for instance be
forthcoming if we were to ascertain, in the same species, the number
of loops present in the segmentation nucleus of fertilization, as
compared with those present in the segmentation nucleus of par- —
thenogenesis.

The reproductive process in bees will perhaps be used as an argu-
ment against my theory. In these insects, the same egg will de-
velope into a female or male individual, according as fertilization

1 We now know that the number of loops varies considerably in different species,
even when they belong to the same group of animals (e.g. Nematodes).— A.W., 1888.

FOUNDATION OF A THEORY OF HEREDITY. 235

has or has not taken place, respectively. Hence, one and the same
egg is capable of fertilization, and also of parthenogenetic develop-
ment, if it does not receive a spermatozoon. It is in the power of
the queen-bee to produce male or female individuals: by an act of
- will she decides whether the egg she is laying is to be fertilized or
unfertilized. She ‘knows beforehand ’! whether an egg will develope
into a male or a female animal, and deposits the latter kind in the
cells of queens and workers, the former in the cells of drones. It
has been shown by the discoveries of Leuckart and von Siebold that
all the eggs are capable of developing into male individuals, and
that they are only transformed into ‘female eggs’ by fertilization.
This fact seems to be incompatible with my theory as to the cause|
of parthenogenesis, for if the same egg, possessing exactly the same}
contents, and above all the same segmentation nucleus, may de- |
velope sexually or parthenogenetically, it appears that the power
of parthenogenetic development must depend on some factor other
than the quantity of germ-plasm.

Although this appears to be the case, I believe that my theory
encounters no real difficulty. I have no doubt whatever, that the
same ego may develope with or without fertilization. From a care-
ful study of the numerous gxcellent investigations upon this point
which have been conducted in a particularly striking manner by
Bessels* (in addition to the observers quoted above), I have come
to the conclusion that the fact is absolutely certain. It must be
candidly admitted that the same egg will develope into a drone
when not fertilized, or into a worker or queen when fertilized. One
of Bessels’ experiments is sufficient to prove this assertion. He
cut off the wings of a young queen and thus rendered her incapable
of taking ‘ the nuptial flight.’ He then observed that all the eggs
which she laid developed into male individuals. This experiment
was made in order to prove that drones are produced by unfertilized
eggs; but it also proves that the assertion mentioned above is correct,
for the eggs which ripen first and are therefore first laid, would have

1 This expression is used by bee-keepers, for instance by the well-known Baron
Berlepsch. Ofcourse, it would be more accurate to say that the queen, seeing the cell
of a drone, is stimulated to lay an unfertilized egg, and that, on the other hand, she
_ is stimulated to lay a fertilized egg when. she sees the cell of a worker, or that of a

queen. : ;
* E. Bessels, ‘Die Landois’sche Theorie widerlegt durch das Experiment.’ -
Zeitschrift fiir wissenschaftliche Zoologie, Bd. XVIII. p. 124. 1868.

236 THE CONTINUITY OF THE GERM-PLASM AS THE

been fertilized had the queen been impregnated. The supposition
that, at certain times, the queen produces eggs requiring fertiliza-
tion, while at other times her eggs develope parthenogenetically, is
quite excluded by this experiment ; for it follows from it, that the
eggs must all be of precisely the same kind, and that there is no
‘difference between the eggs which require fertilization and those
which do not.

But does it therefore follow that the quantity of germ-plasm
in the segmentation nucleus is not the factor which determines
the beginning of embryonic development? I believe not. It
can be very well imagined that the nucleus of the egg, having
expelled the ovogenetic nucleoplasm, may be increased to the
size requisite for the segmentation nucleus in one of two ways:
either by conjugation with a sperm-nucleus, or by simply growing
to double its size. There is nothing improbable in this latter as-
sumption, and one is even inclined to inquire why such growth
does not take place in all unfertilized eggs. The true answer to

this question must be that nature generally pursues the sexual .

method of reproduction, and that the only way in which the
general occurrence of parthenogenesis could be prevented, was by
the production of eggs which remained sterile unless they were
fertilized. This was effected by a loss of the capability of growth
on the part of the egg-nucleus after it had expelled the ovogenetic
nucleoplasm.

The case of the bee proves in a very striking manner that the
difference between eggs which require fertilization, and those which
do not, is not produced until after the maturation of the egg, and
the removal of the ovogenetic nucleoplasm. The increase in the
quantity of the germ-plasm cannot have taken place at any earlier
period, or else the nucleus of the egg would always start embryonic
development, by itself, and the egg would probably be incapable of
fertilization. For the relation between egg-nucleus and sperm-

“nucleus is obviously based upon the fact that each of them is in-
sufficient by itself, and requires completion. If such completion
had taken place at an early stage the egg-nucleus would either
. cease to exercise any attractive force upon the sperm-nucleus, or
else conjugation would be effected, as in Fol’s interesting experi-
ments upon fertilization by many spermatozoa; and, as in these ex-
periments, malformation of the embryo would result. In Daphnidae

FOUNDATION OF A THEORY OF HEREDITY. 237

I believe I have shown! that the summer-eggs are-not only de-
veloped parthenogenetically, but also that they are never fertil-
ized ; and the explanation of this incapacity for fertilization may
perhaps be found in the fact that their segmentation nucleus is
already formed.

We may therefore conclude that, in bees, the nucleus of the ege,
formed during maturation, may either conjugate with the sperm-
nucleus, or else if no spermatozoon reaches the egg may, under the
stimulus of internal causes, grow to double its size, thus attaining
the dimensions of the segmentation nucleus.. For our present pur-
pose we may leave out of consideration the fact that in the latter
ease the individual produced is a male, and in the former case a
female.

It is clear that such an increase in the germ-plasm must depend,
_ to a certain extent, upon the nutrition of the nucleus, and thus in-
directly upon the body of the egg-cell ; but the increase must chiefly
depend upon internal nuclear conditions, viz. upon the capability of
growth. We must further assume that the latter condition plays
the chief part in the process, for everywhere in the organic world
the limit of growth depends upon the internal conditions of the
growing body, and can only be altered to a small extent by differ-
ences of nutrition. The phyletic acquisition of the capability of
parthenogenetic development must therefore depend upon an alter-
ation in the capability of growth possessed by the nucleus of the
ess: |

This theory of parthenogenesis most nearly approaches Stras-
burger’s views upon the subject, for he also explains the non-occur-
rence of parthenogenetic development by the insufficient quantity
of nucleoplasm remaining in the egg after the expulsion of polar
bodies. ‘The former theory differs however in that the occurrence
of parthenogenesis is supposed to be only due to an increase of this
nucleoplasm to the normal size of the segmentation nucleus. Stras-
burger assumes that ‘specially favourable conditions of nutrition
counteract the deficiency of nuclear idioplasm,’ while it seems to
me that nutrition must be considered as only of secondary import-
ance. ‘Thus in bees, as above stated, the same ege may develope
_parthenogenetically or after fertilization, the nucleus being’ subject
to the same conditions of nutrition in both cases. Strasburger?

1 «Daphniden,’ Abhandlung, vi. p. 324. ANE Cs, pe U ROy

238 THE CONTINUITY OF THE GERM-PLASM AS THE

considers that parthenogenesis may be interpreted by one of three
possible explanations. First, he suggests that especially favourable
nutrition may lead to the completion of the nuclear idioplasm.
But if this assumption be made, we must ask why a part of the
idioplasm should be previously expelled, when immediately after-
wards the presence of an equal amount becomes necessary. Such a
view can only be explained by the above-made assumption that the
expelled nucleoplasm has a different constitution from that possessed
by the nucleoplasm which is afterwards formed. It is true that we
do not yet certainly know whether a polar body is expelled in eggs
in which parthenogenesis occurs, but we do know that the egg of
the bee passes through the same stages of maturation whether it
is to be fertilized or not. I can hardly accept Strasburger’s second
suggestion, ‘that under some favourable conditions of nutrition half
[or perhaps better, a quarter] of the idioplasm of the egg-nucleus
is sufficient to start the processes of development in the eyto-idio-.
plasm.’ Finally, his third suggestion, ‘that the eyto-idioplasm,
nourished by its surroundings and thus increased in quantity, com-
pels the nucleus of the egg to enter upon division,’ presupposes that
the cell-body gives the impulse for nuclear division, a supposition
which up to the present time remains at least unproved. The
ascertained facts appear to me to indicate rather that the cell-
body serves only as a medium for the nutrition of the nucleus, and
Fol’s recently mentioned observations, which have been especially
quoted by Strasburger in support of his theories, seem to me to
rather confirm my conclusions. If supernumerary sperm-nuclei
penetrate into the egg, they may, under the nutritive influence of
the cell-body, become centres of attraction, and may take the first
step towards nuclear and cell-division by forming amphiasters.
Such nuclei cannot control the whole cell-body and force it to |
divide, but each one of them, having grown to a certain size at the
expense of the cell-body, makes its influence felt over a certain area.
Strasburger is quite right in considering this process as a ‘ partial
parthenogenesis.’ Such partial parthenogenesis presumably occurs
in all egg-nuclei, but the latter cannot attain to complete partheno-
genesis when, as in Fol’s supernumerary sperm-nuclei, their powers
of assimilation are insufficient to enable them to reach the requisite
size. As before stated, the cell-body does not force the nucleus to
divide, but vice versa. It would, moreover, be quite erroneous to

FOUNDATION OF A THEORY OF HEREDITY. 239

suppose that parthenogenetic eggs must contain a larger amount of
nutritive material in order to facilitate the growth of the nucleus,
The parthenogenetic eggs of certain Daphnidae (Bythotrephes, Poly-
phemus) are very much smaller than the winter-eggs, which require
fertilization, in the same species. It is also an error for Strasburger
to conclude that ‘it has been established with certainty that favour-
able conditions of nutrition cause parthenogenetic development in
Daphnidae, while unfavourable conditions cause the formation of
eggs requiring fertilization.’ It is true that Carl Dising 1, in his
notable work upon the origin of sex, has attempted, in a most
ingenious manner, to prove, from my observations and experiments
on the reproduction of Daphnidae, ‘that winter or summer-egegs are
formed according to the nutritive condition of the ovary.’ I do
not, however, believe that he has succeeded in this attempt, and
at all events it is quite clear that the validity of such conclusions
is not fully established. I have observed that the maturing eggs
break up in the ovaries and are absorbed in those Daphuidae
(Sida) which are starved because sufficient food cannot be pro-
vided in captivity. Hence such animals live, as it were, at the
expense of their descendants; but it would be quite erroneous
to conclude with Diising, from the similarity which such disap-
pearing egeg-follicles bear to the groups of germ-cells which
normally break up in the formation of winter-eggs, that with
a less degree of starvation winter-eggs would have been formed.
Diising further quotes my incidental remark that the formation of
resting-eggs in Daphnia has been especially frequent in aquaria
‘which had been for some time neglected, and in which it was
found that a great increase in the number of individuals had
taken place.’ He is: entirely wrong in concluding that there
was any want of food in these neglected aquaria; and if I had
foreseen that such conclusions would have been drawn, I might

have easily guarded against them by adding that in these very
aquaria an undisturbed growth of different algae was flourishing,
so that there could have been no deficiency, but, on the contrary,
a great abundance of nutritive material. I may add that since
that time I have conducted some experiments directly bearing upon
this question, by bringing virgin females as near to the verge of

* Carl Diising, ‘ Die Regulirung des Geschlechtsverhiiltnisses.’ Jena, 1884.

240 THE CONTINUITY OF THE GERM-PLASM AS THE

starvation as possible, but in no case did they enter upon sexual
reproduction 1.

An author must have been to some extent misled by preconceived
ideas when he is unable to see that the manner in which the two
kinds of eggs are respectively formed, directly excludes the possi-
bility of the origin of sexual eggs from the effects of deficient or

- poor nutrition. The resting eggs, which require fertilization, are

always larger, and require for their formation far more nutritive
material, than the parthenogenetic summer-eggs. In Moia, for
instance, forty large food-cells are necessary for the formation of
a resting egg, while a summer-egg only requires three. And
Diising is aware of these facts, and quotes them. How can the
formation of resting eggs depend upon the effects of poor nutrition
when food is most abundant at the very time of their formation?
In all those species which inhabit lakes, sexual reproduction oceurs
towards the autumn, and in such cases the resting eggs are true
winter-eggs, destined to preserve the species during the winter.
But at no time of the year is the food of the Daphnidae so abundant
as in September and October, and frequently even until late in
November (in South Germany). At this period of the year, the
water is filled with flakes of animal and vegetable matter in a state
of partial decomposition, thus affording abundant food for many
species. It also swarms with a large number of species of Crustacea,
Radiolaria, and Infusoria; and thus such Daphnids as the Poly-
phemidae are also well provided for. Hence there is no deficiency
in the supply of food. Any one who has used a fine net in our fresh
waters at this time of the year must have been at first astonished
at the enormous abundance of the lower forms of animal life ; and
he must have been much more astonished if he has been able to
compare such results with the scanty population of the same
localities in spring. But it is during the spring and summer that
these very Daphnidae reproduce themselves parthenogenetically.
I am far from believing that my experiments on Daphnidae are
exhaustive and final, and I have stated this in my published
writings on the subject ; but it seems to me that I have established
the fact that direct influences, whether of food or of temperature,
acting upon single individuals, do not determine the kind of eggs

1 I intend to publish these experiments elsewhere*in connexion with other
observations.

FOUNDATION OF A THEORY OF HEREDITY. 241

which are to be produced; but that such a decisive influence is to be
found in the indirect conditions of life, and especially in the
- average frequency of the recurrence of adverse cireumstances which
kill whole colonies at once, such as the winter cold, or the drying-
up of small ponds in summer. It is unnecessary for me to contro-
vert Diising in detail, as I have already taken this course in the

case of Herbert Spencer?, who had also formed the hypothesis that

diminished nutrition causes sexual reproduction.

One of my observations seems, indeed, to support such a view, but 3

only when it is considered as an isolated example. I refer to the
behaviour of the genus Moima. Females of this genus which
possess sexual egos in their ovaries, and which would have con-
tinued to produce such eggs if males had been present, enter in
the absence of the latter upon the formation of parthenogenetic
summer-eges, that is, if the sexual eggs have not all been extruded,
but have been re-absorbed in the ovary. At first sight, indeed, such
a result appears to indicate that the increase in nutrition, produced
by the breaking-up of the large winter-egg in the ovary, deter-
raines the formation of parthenogenetic eggs. This apparent con-
clusion seems to be further confirmed by the following fact. The
transition from sexual to parthenogenetic reproduction only occurs
in one species of Moina (I. rectirostris), but in this species it occurs
always and without exception, while in the other species which I
have investigated (IZ. paradoxa), winter-eges, when once formed, are
always laid, and such females can never produce summer-eggs.
But in spite of this fact, Dising is mistaken when he explains the
continuous formation of sexual eggs in the latter species as due to
the absence of any great increase in the amount of nutrition, such
as would have followed if the ege had broken up in the ovary.
In many other Daphnidae which have come under my notice, the
females frequently enter again upon the formation of partheno-
genetic summer-eggs, after having laid fertilized resting eggs,
upon one or more occasions. This is the case, for instance, in all
the species of Daphnia with which I am acquainted, and such
a fact at once proves that the abnormal increase in nutrition
produced by the absorption of winter-eggs cannot be the cause of
_ the succeeding parthenogenesis. It also supports the proof that
‘ Weismann, ‘ Daphniden,’ Abhandlung, VII. p. 329; Herbert Spencer, ‘The

' Principles of Biology,’ 1864, vol. i. pp. 229, 230.
R

24.2 THE CONTINUITY OF THE GERM-PLASM AS THE

a high or low nutritive condition of the whole animal can have
nothing to do with the kind of eggs which are produced, for in
the above-quoted instance, the nutrition has remained the same
throughout, or at .all events has not been increased. It is erroneous
to always look for the explanation of the mode of egg-formation in
the direct action of external causes. Of course there must be
direct causes which determine that one germ shall become a winter-
ege, and another a summer-egg; but such causes do not lie outside
the animal, and have nothing to do with the nutritive condition of
the ovary: they are to be found in those conditions which we are
not at present able to analyze further, and which we must, in the
meantime, call the specific constitution of the species. In the young
males of Daphnidae the testes have precisely the same appearance
as the ovaries of the young females!, but the former will, never-
theless, produce sperm-cells and not ova. In such cases the sex of
the young individual can always be identified by the form of the
first antenna and of the first thoracic appendage, both of which
are always clawed in the male. But who can point to the direct
causes which determine that the sexual cells shall become sperm-
cells in this case, and not egg-cells? Does the determining cause
depend on the conditions of nutrition? Or, again, in the females,
can the state of nutrition determine that the third out of a group
of four germ-cells shall become an egg-cell, and that the others
shall break up to serve as its food?

It is, I think, clear that these are obvious instances of the general
conclusion that the direct causes determining the direction of
development in each case are not to be looked for in external con-
ditions, but in the constitution of the organs concerned.

We arrive at a like conclusion when we consider the quality of
the eggs which are produced. The constitution of one species of
Moina contains the cause which determines that each individual
shall produce winter-eggs only, or summer-eggs only; while in
another species the transition from the formation of sexual eggs to
the formation of summer-eggs ean take place, but only when the
winter-egg remains unfertilized. The latter case appears to me to
be notably a special adaptation, in this and other species, to the
deficiency of males, which is apt to occur. At all events, it is

1 The same fact has since been ascertained in species belonging to several groups
of animals.

FOUNDATION OF A THEORY OF HEREDITY. 243

obvious that it is an advantage that an unfertilized sexual egg
shall not be lost to the organism. The re-absorption of the winter-
egg is an arrangement which, without being the cause, is favourable
to the production of summer-eggs.

This subject is by no means a simple one, as is proved by the
behaviour of the small group of Daphuidae. Thus in some species,
the winter-eggs are produced by purely sexual females, which never
enter upon parthenogenesis ; in others, the sexual females may take
the latter course, but only when males are absent; in others, again,
they regularly enter upon parthenogenesis. In my work on
Daphuidae, I have attempted to show that their behaviour in this
respect is associated with the various external conditions under
which the. different species live; and also that the ultimate
occurrence of the sexual period, and finally the whole cyclical
alternation of sexual and parthenogenetic reproduction, depend
upon adaptation to certain external conditions of life.

With the aid of my hypothesis that the egg-nucleus is com-
posed of ovogenetic nucleoplasm and germ-plasm, I can now
attempt to give an approximate explanation of the nature and
origin of the direct causes which determine the production, at one
time of parthenogenetic summer-eggs, and at another time of
winter-egg's, requiring fertilization. But in such an explanation I
should also wish to include a consideration of the causes which de-
termine the formation of the nutritive cells of the egg and of the
sperm-cells to which I have alluded above.

I believe that the direct cause which determines why the
apparently identical cells of the young testis and ovary in the
Daphnidae develope in such different directions, is to be found in the
fact, that their nuclei possess different histogenetic nucleoplasms,
while, if we neglect individual differences, the germ-plasm remains
precisely the same. In the sperm-cells the histogenetic nucleoplasm
is spermogenetic, in the egg-cells it is ovogenetic. This must be
conceded if our fundamental view is correct, that the specific nature
of the eell-body is determined by the nature of its nucleus.

Similarly, the germ-cells of female Daphnidae, which at first do
not exhibit the smallest differences, must really differ in that their
‘nuclei must contain different kinds of nucleoplasm, which are
present in different proportions. Germ-cells which are to produce
a finely granular, brick-red, winter yolk (Moina rectirostris) must

R 2

244 THE CONTINUITY OF THE GERM-PLASM AS THE

possess an ovogenetic nucleoplasm of a somewhat different mole-
cular structure from those germ-cells which have only to form
a few large blue fat-globules, as in the summer-eggs of the same
species. It is further probable that different proportions obtain be-
tween germ-plasm and ovogenetic nucleoplasm, in these two kinds
of germ-cells; and it would be a very simple explanation of the
otherwise obscure part played by the food-cells, if we were to
suppose that they do not contain any germ-plasm at all, and on
this account do not enter upon embryonic development, but are
arrested after growing to a certain size. Such an explanation,
however, would not by itself show why they subsequently undergo
gradual solution in the surrounding fluids. But since we know
that ege-cells also begin to undergo solution as soon as the parent
Daphnid is poorly nourished, we can hardly help also referring the
solution of the food-cells to insufficient nourishment, occurring’ as
soon as the egg-cell, after the attainment of a certain size, exercises
a superior power of assimilation. But hitherto we could not in any
way understand why the third out of a group of germ-cells should
always gain this superior power and become an egg-cell. If it
could be shown that its position: is more highly favoured in respect
of nutrition, we could understand why it outstrips the other three
in development, and thus prevents them from further growth.
But nothing of the kind can be shown to occur with any degree of
probability, as I have previously mentioned in my works on the
subject. At that time, having no better explanation, I adopted
the view in question, although only as a provisional interpreta-
tion. It was not possible for me to seek in the substance of
those four apparently identical cells for the cause of their different
development; but now I am justified in offering the supposition
that during the division of a primitive germ-cell into two, and after-
wards into four germ-cells, an unequal division of the nucleoplasms
takes place, in that one of the four cells receives germ-plasm as
well as ovogenetic nucleoplasm, while the other three receive the
latter alone. Similarly, the fact that the second cell of the group
may occasionally become an egg is also intelligible, although this
fact remained quite inexplicable by my former interpretation. The
fact that true egg-cells, or even the whole ovary with all its germ-
cells, may break up and become absorbed when the animal has been
starved for a certain period of time, seems to me to be no objection

FOUNDATION OF A THEORY OF HEREDITY. 245

to our present view, any more than the fact that an Infusorian may
die from starvation would be an objection to the supposition of the
immortality of unicellular organisms. The growth of an organism
is not only arrested by its constitution, but also by absolute want
of food; but it would be very foolish to explain the differences
in size of the various species of animals as results of the different
conditions of nutrition to which they were subject. Just as
a sparrow, however highly nourished, could never attain the size or
form of an eagle, so a germ-cell destined to become a summer-egg
could never attain the size, form, or colour of a winter-egg. It is
by internal constitutional causes that the course of development is |
determined in both these cases; and in the latter, the cause can
hardly be anything more than the different constitution of the
nucleoplasms.

All these considerations depend upon the supposition that the
egg-nucleus contains two kinds of idioplasm, viz. germ-plasm and
ovogenetic nucleoplasm. I have not hitherto brought forward any
direct evidence in favour of this assumption, but I believe that such
proofs can be obtained.

It is well known that there are certain eggs in which the polar
bodies are not expelled until after the entrance of spermatozoa.
Brooks? has already made use of this fact as evidence against
Minot’s and Balfour’s theory; for he quite rightly concludes that
if the polar bodies really possess the significance of male cells, we
cannot understand why such eggs are unable to develope without
fertilization, when they still possess the male half of the nucleus
necessary for development. But such eggs (e.g. that of the oyster)
do not develope, but always die if they remain unfertilized.

This argument can only be met by a new hypothesis, the con-
struction of which I must leave to the defenders of the above-
mentioned theory. But the observation in question seems to me
to furnish at the same time a proof of the co-existence of two
different nucleoplasms in the egg-nucleus. If the nucleoplasm of
the polar bodies was also germ-plasm, we could not understand
why such eggs are unable to develope parthenogenetically, for at
least as much germ-plasm is contained in the unfertilized egg as
_ would have been present after fertilization.

1 Brooks, ‘The Law of Heredity.’ Baltimore, 1883, p. 73,

246 THE CONTINUITY OF THE GERM-PLASM AS THE

The only objection which can be raised against this conclusion
depends upon the supposition that the nucleoplasm of the sperm-
cell is qualitatively different from that of the egg-cell. I have
already dealt with this view, but I should wish to refer to it again
rather more in detail. Some years ago I expressed the opinion?
that the physiological values of the sperm-cell and of the egg-cell
must be identical; that they stand in the ratio of 1:1. But
Valaoritis ? has brought forward the objection that if we consider
the function of a cell as the measure of its physiological value, it is
only necessary to point to the respective functions of ovum and
spermatozoon in order to show that their physiological values must
be different. ‘The egg-cell alone, by passing more or less com-
pletely through the phyletic stages of the female parent, developes
into a similar organism ; and although the presence of the sperma-
tozoon is in most cases required in order to render possible such a
result, the cases of parthenogenesis prove nevertheless that the
egg can do without this stimulus.’ This objection appeared to be
_ fully justified as long as fertilization was looked upon as the ‘ vital-
ization of the germ,’ and so long as the sperm-cell was considered
as merely ‘the spark that kindles the gunpowder, and further
so long as the germ-substance was believed to be contained in the
cell-body. But now we can hardly give to the body of the egg-
cell a higher significance than that of the common nutritive
soil of the two nuclei which conjugate in fertilization. But
these two nuclei ‘ are not different in nature,’ as Strasburger says,
and as I fully believe. They cannot differ in kind, for they both
consist of germ-plasm belonging to the same species of animal or
plant ; and there cannot be any deeper contrast between them such
as would correspond to the differences between mature individuals.
They cannot, from their essential nature, exercise any special at-
traction upon each other, and when we see that sperm-cell and egg-
cell do nevertheless attract each other, as has been shown in both
plants and animals, such a property must have been secondarily
acquired, and has no other significance than to favour the union of
sexual cells—an arrangement which may be compared to the vi-
brating flagellum of the spermatozoon or the micropyle of the egg,
but which is not fundamental, and is not based upon the molecular

! « Zeitschrift fiir wissenschaftliche Zoologie,’ Bd. XX XIII. p. 107. 1873.
? Valaoritis, 1. ¢., p. 6.

FOUNDATION OF A THEORY OF HEREDITY. 247

structure of the germ-plasm. In lower plants, Pfeffer has proved
that certain chemical stimuli emanate from the egg and attract the
spermatozoid; and according to Strasburger, the synergidae in the
upper part of the embryo-sae of Phanerogams secrete a substance
which is capable of directing the growth of the pollen-tube towards
the egg-cell. In animals it is only known as yet that spermatozoa
and ova do attract each other, so that the former find the latter and
bore their way through its membranes. It has also been shown
that the substance of the egg-body moves towards the pene-
trating spermatozoon (‘cones @’easudation’ in Asteridae: Fol); and
that it sometimes enters upon convulsive movements (Petromyzon).
Here therefore a mutual stimulation and attraction must exist ;
and perhaps we must also assume that there is an attraction be-
tween the two conjugating nuclei, for we cannot readily understand
how the cytoplasm alone could direct the one to the other, as
Strasburger supposes. According to Strasburger’s hypothesis, -we
must suppose that part of the specific cytoplasm of the sperm-cell
continues to surround the nucleus after it has penetrated into the
body of the egg. But however this may be, the assumed attraction
between the conjugating nuclei certainly cannot depend upon the
molecular structure of their germ-plasm, which is the same in both, \
but it must be due to some accessory circumstance. If it were
possible to introduce the female pronucleus of an egg into another
ege of the same species, immediately after the transformation of the
nucleus of the latter into the female pronucleus, it is very probable
that the two nuclei would conjugate just as if a fertilizing sperm-
nucleus had penetrated. If this were so, the direct proof that egg-
nucleus and sperm-nucleus are identical wouid be furnished. Un-
fortunately the practical difficulties are so great that it is hardly
possible that the experiment can ever be made; but such want of
experimental proof is partially compensated for by the fact, ascer-
tained by Berthold, that in certain Algae (Hetocarpus and Scytosi- -
phon) there is not only a female, but also a male parthenogenesis; for
he shows that in these species the male germ-cells may sometimes
develope into plants, which however are very weakly’. Furthermore

1 I quote from Falkenberg, in Schenk’s ‘Handbuch der Botanik,’ Bd. IT. p. 219.
' He further states that these are the only instances hitherto known in which un-
doubted male cells have proved to be capable of further development when they have

been unable to exercise their powers of fertilization. It must be added that the two
kinds of germ-cells do not differ in appearance, but only in behaviour; the female

248 THE CONTINUITY OF THE GERM-PLASM, ETC.

.the process of conjugation may be considered as a proof that this —

view as to the secondary importance of sexual differentiation is
the true one. At the present time there can hardly be any hesita-
tion in accepting the view that conjugation is the sexual repro-
duction of unicellular organisms. In these the two conjugating
cells are almost always identical in appearance, and there is no
evidence in favour of the assumption that they are not also identical
in molecular structure, at least so far as one individual of the
same species may be identical with another. But there are also
forms in which the conjugating cells are distinctly differentiated
into male and female, and these are connected with the former by
a gradual transition: thus in Pandorina, a genus of Volvocineae, we
are unable to make out any differences between the conjugating
eells, while large egg-cells and minute sperm-cells exist in the
closely allied Volvox. If we must suppose that the conjugation of
two entirely identical Infusoria has the same physiological effect as
the union of two sexual cells in higher animals and plants, we can-
not escape the conclusion that the process is essentially the same
throughout: and that therefore the differences, which are perhaps
already indicated in Pandorina and are very distinct in Volvoxr and
in all higher organisms, have nothing to do with the nature
of the process, but are of quite secondary importance. If we further
take into account the extremely different constitution of the two
Kinds of sexual cells in size, appearance, membranes, motile power,
and finally in number, no doubt remains that these differences are
only adaptations which secure the meeting of the two kinds of
conjugating cells: that in each species they are adaptations to the
peculiar conditions under which fertilization takes place.

germ-cells becoming fixed, and withdrawing one of their two flagella, while the male
cells continue to swarm. But even this slight degree of differentiation requires the
supposition of internal molecular differentiation.

NOTE.

Ir is of considerable importance for, the proper appreciation of
the views advanced in the present essay, to ascertain whether a
polar body is or is not expelled from eggs which develope partheno-
genetically. I wish therefore to briefly state that I have recently
succeeded in proving the formation of a polar body of distinctly
cellular structure in the summer-eggs of Daphnidae. I propose to
publish a more detailed account in a future paper.

A. W.
June 22, 1885.

Vv.

THE SIGNIFICANCE OF SEXUAL REPRODUCTION
IN THE THEORY OF NATURAL SELECTION.

1886,

SIGNIFICANCE OF SEXUAL REPRODUCTION, ere.

—_+4+—

PREFACE.

Tue greater part of the present essay was delivered at the first .
general meeting of the Association of German Naturalists, at
Strassburg, on September 18th, 1885, and is printed in the Pro-
ceedings of the fifty-eighth meeting of that Society.

The form of a lecture has been retained in the present publica-
tion, but its contents have been extended in many ways. Besides
many small and a few large additions to the text, I have added
six appendices in order to treat of certain subjects more fully than
was possible in the lecture itself, in which I was often obliged to be
content with mere hints and suggestions. This appears to be all
the more necessary because it is impossible to suppose that many
views and ideas upon which the lecture was based would be well
known to all readers, although they have been described in my
former papers. It was above all necessary to deal with the class of
acquired characters, which, as it seems to me, is easily confounded,
especially by the medical profession, with the much broader class of
new characters generally. Only those new characters can be called
‘acquired’ which owe their origin to external influences, and the
term ‘acquired’ must be denied to those which depend upon the
mysterious relationship between the different hereditary tendencies
which meet in the fertilized ovum. These latter are not ‘acquired’
but inherited, although the ancestors did not possess them as such,
but only as it were the elements of which they are composed.
Such new characters as these do not at present admit of an exact
analysis: we have to be satisfied with the undoubted fact of their
occurrence. The transmission or non-transmission of acquired cha-
racters must be of the highest importance for a theory of heredity,
and therefore for the true appreciation of the causes which lead
to the transformation of species. Any one who believes, as I do,
that acquired characters are not transmitted, will be compelled to

SIGNIFICANCE OF SEXUAL REPRODUCTION, ETC.—PREFACE. 253

assume ‘that the process of natural selection has had a far larger
share in the transformation of species than has been as yet
accorded to it; for if such characters are not transmitted, the
modifying influence of external circumstances in many cases re-
mains restricted to the individual, and cannot have any part in
producing transformation. We shall also be compelled to abandon
the ideas as to the origin of individual variability which have
been hitherto accepted, and shall be obliged to look for a new
source of this phenomenon, upon which the processes of selection
entirely depend.

In the following pages I have attempted to suggest such a
source. '

A. W.

FRrErpure I. Br.,
November 22, 1885.

SIGNIFICANCE OF SEXUAL REPRODUCTION, ere.

CONTENTS.

1, Can we dispense with the principle of natural selection ?
2. Nigeli’s theory of transformation from internal causes :
3. A definite course of development is possible without a i Bi idio-
plasm . ° :
4, Conclusive iipoetenan of * eaipicinena? : : ‘
5, The structure of whales as an example of adhyhebion , .
6. Transformation takes place by the smallest steps
7. The foundation of such minute changes depends upon thidividead ‘variability
8. Difficulty in accounting for variability on the supposition of a continuity of
the germ-plasm ‘ r i
9. Previous theories by which variability has ime aeoounted for
10. Non-transmission of acquired characters
11. Nageli’s and Alexis Jordan’s experiments .
12. Germ-plasm is only altered with great difficulty -
13. The source of individual variation lies in sexual reproduction . 2 .
14. The process of natural selection does not operate when asexual repro-
duction takes place
15. Origin of variability in antostialar ongiolania
16. Sexual reproduction effects combination :
17. E. van Beneden’s and V. Hensen’s sey of sexual reproduction a as a proses
of rejuvenescence . : : 2 ‘ ‘ ie (es .
18. Theoretical objections to such a view .
19. Original significance of conjugation . .
20. Preservation of sexual reproduction by means of heredity .
21. It is lost in parthenogenesis for reasons of utility Jeeps : é 2
22. Parthenogenesis prevents further transformations .
23. It excludes Panmixia and thus prevents disused beoane Shean beconelals
rudimentary . : : é : : : ;
24, Final considerations

APPENDICES.

I. FURTHER CONSIDERATIONS WHICH OPPOSE NAGELI’S EXPLANATION OF
TRANSFORMATION AS DUE TO INTERNAL CAUSES g x
TI. Nicerri’s EXPLANATION OF ADAPTATION . ‘ 3 5 E : :
III. ADAPTATIONS IN PLANTS. “ -
TV. On tHE SuPPOSED TRANSMISSION OF ; AogueED CHARACTERS .
1. Brown-Séquard’s experiments on Guinea-pigs
“2. A case which at first sight appears to prove the ieanasat@nink of seuired
characters ~ s 3
V. ON THE ORIGIN OF Pipeaxnochonkers
VI. W. K. Brooks’ Tueory oF HEREDITY

PAGE
255
256

258
260
261
264
266

266
207
267
269
271
272

274
278

279

282
283
286
287
289
290

291
294

V.

THE SIGNIFICANCE OF SEXUAL REPRODUCTION IN
THE THEORY OF NATURAL SELECTION.

Durine the quarter of a century which has elapsed since Biology
began to occupy itself again with general problems, at least one
main fact has been made clear by the united labours of numerous
men of science, viz. the fact that the Theory of Descent, the idea
of development in the organic world, is the only conception as to
the origin of the latter, which is scientifically tenable. It is not
only that, in the light of this theory, numerous facts receive for the
first time a meaning and significance; it is not only that, under
its influence, all the ascertained facts can be harmoniously grouped
together; but in some departments it has already yielded the
highest results which can be expected from any theory, it has
rendered possible the prediction of facts, not indeed with the abso-
lute certainty of calculation, but still with a high degree of
probability. It has been predicted that man, who, in the adult
state, only possesses twelve pairs of ribs, would be found to have
thirteen or fourteen in the embryonic state: it has been predicted
that, at this early period in his existence, he would possess the
insignificant remnant of a very small bone in the wrist, the so-
called os centrale, which must have existed in the adult condition
of his extremely remote ancestors. Both: predictions have been
fulfilled, just as the planet Neptune was discovered after its ex-
istence had been predicted from the disturbances induced in the
orbit of Uranus.

That existing species have not arisen independently, but have
been derived from other and mostly extinct species, and that on
the whole this development has taken place in the direction of
greater complexity, may be maintained with the same degree of
certainty as that with which astronomy asserts that the earth
moves round the sun; fora conclusion may be arrived at as safely
by other methods as by mathematical calculation.

If I make this assertion so unhesitatingly, I do not make it in
the belief that I am bringing forward anything new nor because

256 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

I think that any opposition will be encountered, but simply because
I wish to begin by pointing out the firm ground on which we
stand, before considering the numerous problems which still remain
unsolved, Such problems appear as soon as we pass from the facts
of the case to their explanation ; as soon as we pass from the state-
ment ‘ The organic world has arisen by development, to the ques-
tion ‘ But how has this been effected, by the action of what forces,
by what means, and under what circumstances ?’ 3

In attempting to answer these questions we are very far from
dealing with certainties ; and opinions are still conflicting. But
the answer lies in the domain of future investigation, that un-
known country which we have to explore.

It is true that this country is not entirely unknown, and if I am
not mistaken, Charles Darwin, who in our time has been the first
to revive the long-dormant theory of descent, has already given a
sketch, which may well serve as a basis for the complete map of the
domain ; although perhaps many details will be added, and many
others taken away. . In the principle of natural selection, Darwin
has indicated the route by which we must enter this unknown land.

But this opinion is not universal, and only recently Carl Niigeli?,
the famous botanist, has expressed decided doubts as to the general
applicability of the principle of natural selection. According to
Niigeli, the co-operation of the external conditions of life with the
known forces of the organism, viz. heredity and variability, are in-
sufficient to explain the regular course of development pursued by
the organic world. He considers that natural selection is at best

= auxiliary principle, which accepts or rejects existing characters,
but which is unable to create anything new: he believes that the
causes of transformation reside within the organism alone. Niigeli
further assumes that organisms contain forces which cause period-
ical transformation of the species, and he imagines that the organic
world, as a whole, has arisen in a manner siniilar to that in which
a single individual arises.

Just as a seed produces a certain plant because it possesses a
certain constitution, and just as, in this process, certain conditions
must be favourable (light, warmth, moisture, &c.) in order that

_ development may take place, ‘although they do not determine the

1 ©, Niigeli, ‘ Mechanisch-physiologische Theorie der Abstammungslehre,’ Miinchen
u. Leipzig, 1884.

IN THE THEORY OF NATURAL SELECTION. 257

kind or the manner of development ; so, in precisely the same way,
the tree of the whole organic world has grown up from the first
and lowest forms of life on our planet, under a necessity arising
from within, and on the whole independently of external influences.
According to Nigeli, the cause which compels every form of living
substance to change, from time to time, in the course of its secular
growth, and which moulds it afresh into new species, must lie
within the organic substance itself, and must depend upon its mole-
cular structure.

It is with sincere admiration and real pleasure that we read the
exposition in which Nigeli gives, as it were, the result of all his
researches which bear upon the great question of the development
of the organic world. But although we derive true enjoyment from
the contemplation of the elaborate and ingeniously wrought-out
theoretical coneeption,—which like a beautiful building or a work of
art is complete in itself,—and although we must be convinced that
its rise has depended upon the progress of knowledge, and that by
its means we shall eventually reach a fuller knowledge ; it is never-
theless true that we cannot accept the author’s fundamental
hypothesis. I at least believe that I am not alone in this respect,
and that but few zoologists will be found who can adopt the hypo-
thesis which forms the foundation of Nigeli’s theory.

It is not my intention at present to justify my own beidialy
different views, but the subject of this lecture compels me to briefly
explain my position in relation to Nigeli, and to give some of the
reasons why I cannot accept his theory of an active force of trans-/
formation arising and working within the organism; and I must
also explain the reasons which induce me to adhere to the theory of
natural selection.

The supposition of such a phyletie force of transformation (see
Appendix I, p. 298) possesses, in my opinion, the greatest defect that
any theory can have,—it does not explain the phenomena. I do not
mean to imply that it is incapable of rendering certain subordinate
phenomena intelligible, but that it leaves a larger number of facts
entirely unexplained. It does not afford any explanation of the
purposefulness seen in organisms: and this is just the main problem

which the organic world offers for our solution. That species are,

from time to time, transformed into new ones might perhaps be

understood by means of an internal transforming force, but that
8

=

258 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

they are so changed-as to become better adapted to the new con-
ditions under which they have to live, is left entirely unintelligible
by this theory. For we certainly cannot accept as an explanation
Nigeli’s statement that organisms possess the power of being
transformed in an adaptive manner simply by the action of an
external stimulus (see Appendix II, p. 300).

In addition to this fundamental defect, we must also note that
there are absolutely no proofs in support of the foundation of this
theory, viz. of the existence of an internal transforming force.

Niigeli has very ingeniously worked out his conception of idio-
plasm, and this conception is certainly an important acquisition
and one that will last, although without the special meaning given
to it by its author. But is this special meaning anything more than
pure hypothesis? Can we say more than this of the ingenious de-
scription of the minute molecular structure of the hypothetical
basis of life? Could not idioplasm be built up in a manner entirely
different from that which Nigeli supposes? And can conclusions
drawn from its supposed structure be brought forward to prove
anything? The only proof that idioplasm must necessarily change,
in the course of time, as the result of its own structure, is to be
found in the fact that Niageli has so constructed it ; and no one
will doubt that the structure of idioplasm might have been so con-
ceived as to render any transformation from within itself entirely
impossible.

But even if it is theoretically possible to imagine that idioplasm
possesses such a structure that it changes in a certain manner, as
the result of mere growth, we should not be justified in thus
assuming the existence of a new and totally unknown principle
until it had been proved that known forces are insufficient for the
explanation of the observed phenomena.

Can any one assert that this proof has been forthcoming? It
has been again and again pointed out that the phyletic development
of the vegetable kingdom proceeds with regularity and according to
law, as we see in the preponderance and constancy of so-called
purely ‘morphological’ characters in plants. The formation of
natural groups in the animal and vegetable kingdoms compels us.
to admit that organic evolution has frequently proceeded for longer
or shorter periods along certain developmental lines. But we are
not on this account compelled to adopt the supposition of un-

IN THE THEORY OF NATURAL SELECTION. 259

known internal forces which have determined such lines of de-
velopment.

Many years ago I attempted to prove? that the constitution or
physical nature of an organism must exercise a restricting in-
fluence upon its capacity for variation. A given species cannot
change into any other species, which may be thought of. A beetle
could not be transformed into a vertebrate animal: it could not
even become a grasshopper or a butterfly; but it could change into
a new species of beetle, although only at first into a species of
the same genus. Every new species must have been directly con-
tinuous with the old one from which it arose, and this fact alone
implies that phyletic development must necessarily follow certain
lines. |

I can fully understand how it is that a botanist has more incli-
nation than a zoologist to take refuge in internal developmental
forces. The relation of form to function, the adaptation of the
organism to the internal and external conditions of life, is less
prominent in ‘plants than in‘ animals; and it is even true that
a large amount of observation and ingenuity is often necessary in
order to make out any adaptation at all. The temptation to
accept the view that everything depends upon internal directing
causes is therefore all the greater. Nigeli indeed looks at the
subject from the opposite point of view, and considers that the
true underlying cause of transformation is in animals obscured by
adaptation, but is more apparent in plants”. Sufficient justification
for this opinion cannot, however, be furnished by the fact that in
plants many characters have not been as yet explained by adapta-
tion. We should do well to remember the extent to which the
number of so-called ‘morphological’ characters in plants has
been lessened during the last twenty years. What a flood of
light was thrown upon the forms and colours of flowers, so often
curious and apparently arbitrary, when Sprengel’s long-neglected
discovery was extended and duly appreciated as the result of Dar-
win’s investigations, and when the subject was further advanced
by Hermann Miiller’s admirable researches! Even the venation
of leaves, which was formerly considered to be entirely without
significance, has been shown to possess a high biological value

? “Ueber die Berechtigung der Darwin’schen Theorie. Leipzig, 1868, p. 27.
2 1. c., Preface, p. vi.

8 2

260 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

by the ingenious investigations of J. Sachs (see Appendix III,
p. 308). We have not yet reached the limits of investigation, and no
reason can be assigned for the belief that we shall not some day
receive an explanation of characters which are now unintelligible?.

It is obvious that the zoologist cannot lay too much stress upon
the intimate connexion between form and function, a connexion
which extends to the minutest details: it is almost impossible to
insist too much upon the perfect manner in which adaptation to
certain conditions of life is carried out in the animal body. In
the animal body we find nothing without a meaning, nothing
which might be otherwise; each organ, even each cell or part of
a cell is, as it were, tuned for the special part it has to perform
in relation to the surroundings.

It is true that we are as yet unable to explain the adaptive
character of every structure in any single species, but whenever
we succeed in making out the significance of a structure, it
always proves to be a fresh example of adaptation. Any one who
has attempted to study the structure of a species in detail, and to
account for the relation of its parts to the functions of the whole,
will be altogether inclined to believe with me that everything
_ depends upon adaptation. There is no part of the body of an
/ individual or of any of its ancestors, not even the minutest and
most insignificant part, which has arisen in any other way than
under the influence of the conditions of life; and the parts of the
body conform to these conditions, as the channel of a river is
shaped by the stream which flows over it.

These are indeed only convictions, not real proofs ; for we are not
yet sufficiently intimately acquainted with any species to be able
to recognize the nature and meaning of all the details of its strue-
ture, in all their relations: and we are still less able to trace the
ancestral history in each case, and to make out the origin of those
structures of which the presence in the descendants depends pri-
marily upon heredity. But already a fair advance towards the
attainment of inductive proof has been made; for the number of
adaptations which have been established is now very large and

1 Since the above was written many other morphological peculiarities of plants
have been rightly explained as adaptations. Compare, for instance, the investiga-
tions of Stahl on the means by which plants protect themselves against the attacks
of snails and slugs (Jena, 1888).—A, W., 1888.

IN THE THEORY OF NATURAL SELECTION. 261

is increasing every day. If, however, we anticipate the results of
future researches, and admit that an organism only consists of
adaptations, based upon an ancestral constitution, it is obvious
that nothing remains to be explained by a phyletic force, even
though the latter be presented to us in the refined form of Nigeli’s
self-changing idioplasm.

It will perhaps be useful to illustrate my views by a familiar
example. I choose the well-known group of the whales. These
animals are placental mammals, which, probably in secondary times,
arose from terrestrial Mammalia, by adaptation to an aquatic life.

Everything that is characteristic of these animals and distin-
guishes them from other mammals depends upon this adaptation.
Their fore-limbs have been transformed into rigid paddles, only
movable at the shoulder-joint; upon the back and the tail there
are ridges with a form somewhat similar to the dorsal and caudal
fins of fishes. The organ of hearing is without any external
ear and without an air-containing external auditory meatus. The
aerial vibrations do not pass, as in other mammals, from the ex-
ternal auditory passage to the tympanic cavity and thus to the
nerve-terminations of the inner ear; but they reach the tympanic
cavity by direct transmission through the bones of the skull,
which possess a special structure and contain abundant air-cavities.
This arrangement is obviously adapted for hearing in water. The
nostrils also exhibit peculiarities, for they do not open near the
mouth, but upon the forehead, so that the animal can breathe,
even in a rough sea, as soon as it comes to the surface. In
order to facilitate rapid movement in water, the whole body has
become extended in length, and spindle-shaped, like the body of
a fish. The hind limbs are absent in no other mammals, the fish-
like Strenia being alone excepted. In the whales, as in the Sirenia,
these appendages have become useless, owing to the powerfully
developed tail-fin; they are now rudimentary and consist of some
small bones and muscles deeply buried in the body of the animal,
which nevertheless, in certain species, still exhibit the original
structure of the hind-limb. The hairy covering of other mam-
mals has also disappeared, its place having been taken by a thick
layer of fat beneath the skin, which affords a much better pro-
tection against cold. This fatty layer was also necessary in order
to diminish the specific gravity of the animal, and to thus render

262 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

it equal to that of sea-water. In the structure of the skull there
are also a number of peculiarities, all of which are directly or m-
directly connected with the conditions under which these animals
live. In the whalebone whales, the enormous size of.the face,
the immense jaws, and wide mouth are very striking. Can it
be suggested that this very characteristic appearance is entirely
due to the guidance of some internal transforming force, or to
some spontaneous modification of the idioplasm? Any such sug-
gestion cannot be accepted, for it is easy to show that all these
structural features depend upon adaptation to a peculiar mode
of feeding. Functional teeth are absent, but rudimentary ones
exist in the embryo as relics of an ancestral condition in which
these organs were fuily developed. Large plates of whalebone
with finely divided ends are suspended vertically from the roof
of the mouth. These whales feed upon small organisms, about an
inch in length, which swim or float upon the water in countless
numbers; and in order that they may subsist upon such minute
animals, it is necessary to obtain them in immense numbers. This
is achieved by means of the huge mouth which takes in a vast
quantity of water at a single mouthful. The water then filters
away through the plates of whalebone, while the organisms which
form the whale’s food remain stranded in the mouth. Is it neces-
sary to add that the internal organs—so far as we understand the
details of their functions, and so far as their structure differs from
that of the corresponding organs in other Mammalia—have also _
been directly or indirectly modified by adaptation to an aquatic
life? Thus all whales possess a very peculiar arrangement of
the nasal passages and larynx, enabling them to breathe and
swallow at the same time: the lungs are of enormous length, and
thus cause the animal to assume a horizontal position in the water
without the exercise of muscular effort: in consequence of this
latter modification, the diaphragm extends in a nearly horizontal
direction: there are moreover certain arrangements in the vascular
system which enable the animal to remain under water for a con-
siderable time, and so on.

And now, in reference to this special example, I will repeat
the question which I have asked before:—‘If everything that
is characteristic of a group of animals depends upon adaptation,
what remains to be explained by the operation of an internal

aa. |
IN THE THEORY OF NATURAL SELECTION. 263

developmental force?’ What remains of a whale when we have
taken away its adaptive characters? We.are compelled to reply
that nothing remains except the general plan of mammalian
organization, which existed previously in the mammalian ancestors
of the Cetacea. But if everything which stamps these animals as
whales has arisen by adaptation, it follows that the internal de-
velopmental force cannot have had any share in the origin of this
group.

And yet this very force is said to be the main factor in the
transformation of species, and Nageli unhesitatingly asserts that
both the animal and vegetable kingdoms would have become very
much as they now are, if there had been no adaptation to new
conditions, and no such thing as competition in the struggle for
existence |.

But even if we admit that such an assumption affords some
explanation, instead of being the renunciation of all attempts at
explanation; if we admit that an organism, the characteristic
peculiarities of which entirely depend upon adaptation, has been
formed by an internal developmental force; we should still be
unable to explain how it happens that such an organism, suited to
certain conditions of life, and unable to exist under other conditions,
appeared at that very place on the earth’s surface, and at that very
time in the earth’s history, which offered the conditions appropriate
for its existence. As I have previously argued, the believers in
an internal developmental force are compelled to invent an auxiliary
hypothesis, a kind of ‘pre-established harmony’ which explains
how it is that changes in the organic world advance step by step,
parallel with changes in the crust of the earth and in other
conditions of life ; just as, according to Leibnitz, body and soul,
although independent of each other, proceed along parallel courses,
like. two chronometers which keep perfect time. And even this
supposition would not be sufficient, because the place must be
taken into account as well as the time: thus the whales could not
have existed if they had first appeared upon dry land. We know
of countless instances in which a species is exclusively and precisely
adapted to a certain localized area, and could not thrive anywhere
else. We have only to remember the cases of mimicry in which
one insect gains protection by resembling another, the cases of

1 lic, pp. 117, 286.

2

264 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

protective resemblance to the bark or the leaves of a certain species
of plant, or the numerous marvellous adaptations of parasitic
animals to certain parts of certain species of hosts.

A mimetie species cannot have appeared at any place other
than that in which it exists: it cannot have arisen through an
internal developmental force. But if single species, or even whole
orders like the Cetacea, have arisen independently of any such force,
then we may safely assert that the existence of the supposed force
is neither required by reason nor necessity.

Hence, abstaining from the invocation of unknown forces, we are
justified in carrying on Darwin’s attempt to explain the trans-
formation of organisms by the action of known forces and known |
phenomena. I say ‘earry on the attempt,’ because I do not believe
that our knowledge in this direction has ended with Darwin, and
it seems to me that we have already arrived at ideas which are in-
compatible with certain important points in his general theory, and
which therefore necessitate some modification of the latter.

The theory of natural selection explains the rise of new species
by supposing that changes occur, from time to time, in those con-
ditions of life to which an organism must: adapt itself if it is to
continue in existence. Thus a selective process is set up which
ensures that only those out of the existing variations are pre-
served, which correspond in the highest degree to the changed
conditions of life. By continued selection in the same direction
the deviations from the type, although at first very insignificant,
are accumulated and increased until they become specific differ-
ences.

I should wish to assert more definitely than Darwin has done,
that alterations in the conditions of life, together with changes in
the organism itself, must have advanced very gradually and by the
smallest steps, in such a way that, at each period in the whole pro-
cess of transformation, the species has remained sufficiently adapted
to the surrounding conditions. An abrupt transformation of a species
is inconceivable, because it would render the species incapable of
existence. If the whole organization of an animal depends upon
adaptation, if the animal body is, as it were, an extremely complex
combination of new and old adaptations, it would be a highly
remarkable coincidence if, after any sudden alteration occurring
simultaneously in many parts of the body, all these parts were

IN THE THEORY OF NATURAL SELECTION. 965

changed in such a manner that they again formed a whole which
exactly corresponded to the altered external conditions. Those who
assume the existence of such a sudden transformation overlook the
fact that everything in the animal body is exactly calculated to
maintain the existence of the species, and that it is just sufficient
for this purpose ; and they forget that the minutest change in the
least important organ may be enough to render the species in-
capable of existence.

It may perhaps be objected that the case is different in plants, as
is proved by the American weeds which have spread all over
Europe, or the European plants which have become naturalized in
Australia. Reference might also be made to the plants which
inhabited the plains during the glacial epoch, and which at its
close migrated to the Alpine mountains and to the far north, and
which have remained unaltered under the apparently diverse con-
ditions of life to which they have been subjected for so long a
time. Similar instances may also be found among animals. The
rabbit, which was brought by sailors to the Atlantic island of Porto
Santo, has bred abundantly and remains unchanged in this locality ;
the European frogs, which were introduced into Madeira, have in-
creased immensely and have become almost a plague; and the
European sparrow now thrives in Australia quite as well as with us.
But these instances do not prove that adaptation to external
conditions of life is not of primary importance; they do not prove
that an organism which is adapted to a certain environment will, .
when unmodified, remain capable of existence amid new surround-
ings. ‘They only prove that the above-mentioned species found
in those countries the same conditions of life as at home, or at
least that they met with conditions to which their organization
could be subjected without the necessity for modification. Not
every new environment includes such changed conditions as will be
effective in modifying every species of plant or animal. The rabbit
of Porto Santo certainly feeds on herbs different from those which
form the food of its relations in Europe, but such a change does
not mean an effective alteration in the conditions under which this
species lives, for the herbs in both localities are equally well suited
_ to the needs of the animal.

But if we suppose that the wild rabbit, occurring in Europe, were
to suddenly lose but a trifle of its warinéss, its keen sight, its fine

266 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

sense of hearing or of smell, or were to suddenly acquire a colour
different from that which it now possesses, it would become in-
capable of existence as a species, and would soon die out. The same
result would probably occur if any of its internal organs, such as
the lungs or the liver, were suddenly modified. Perhaps single
individuals would still remain capable of existence under these cir-
cumstances, but the whole species would suffer a certain decline
from the maximum development of its powers of resistance, and
would thus become extinct. The sudden transformation of a species
appears to me to be inconceivable from a physiological point of
view, at any rate in animals.

Hence the transformation of a species can only take place by the
smallest steps, and must depend upon the accumulation of those
differences which characterise individuals, or, as we call them,
‘individual differences.’ There is no doubt that these differences
are always present, and thus, at first sight, it appears to be simply
a matter of course that they will afford the material by means of
which natural selection produces new forms of life. But the case is
not so simple as it appeared to be until recently; that is if I am
right in believing that in all animals and plants which are repro-
_ duced by true germs, only those characters which were potentially
present in the germ of the parent can be transmitted to the
succeeding generation.

Se I believe that heredity depends upon the fact that a small portion

f the effective substance of the germ, the germ-plasm, remains
unchanged during the development of the ovum into an organism,
and that this part of the germ-plasm serves’as a foundation from
which the germ-cells of the new organism are produced!. There is
therefore continuity of the germ-plasm from one generation to
another. One might represent the germ-plasm by the metaphor of
a long creeping root-stock from which plants arise at intervals,
these latter representing the individuals of successive generations.

Hence it follows that the transmission of acquired characters is an
impossibility, for if the germ-plasm is not formed anew in each
individual but is derived from that which preceded it, its structure,
and above all its molecular constitution, cannot depend upon the
individual in which it happens to occur, but such an individual

* Compare the second and fourth of the preceding Essays, ‘On Heredity’ and ‘ The
Continuity of the Germ-plasm as the Foundation of a Theory of Heredity.’

IN THE THEORY OF NATURAL SELECTION. 267

only forms, as it were, the nutritive soil at the expense of which the
germ-plasm grows, while the latter possessed its characteristic struc-
ture from the beginning, viz. before the commencement of growth.

But the tendencies of heredity, of which the germ-plasm is the
bearer, depend upon this very molecular structure, and hence only
those characters can be transmitted through successive generations
which have been previously inherited, viz..those characters which
were potentially contained in the structure of the germ-plasm. It
also follows that those other characters which have been acquired by
the influence of special external conditions, during the life-time of
the parent, cannot be transmitted at all.

The opposite view has, up to the present time, been maintained,
and it has been assumed, as a matter of course, that acquired
characters can be transmitted ; furthermore, extremely complicated
and artificial theories have been constructed in order to explain how
it may be possible for changes produced by the action of external
influences, in the course of a life-time, to be communicated to the
germ and thus to become hereditary. But no single fact is known
which really proves that acquired characters can be transmitted,
for the ascertained facts which seem to point to the transmission of
artificially produced diseases cannot be considered as a proof; and
as long as such proof is wanting we have no right to make this
supposition, unless compelled to do so by the impossibility of
suggesting a mode in which the transformation of species can take
place without its aid. (See Appendix IV, p. 310.)

It is obvious that the unconscious conviction that we need the
aid of acquired characters has hitherto securely maintained the as-
sumed axiom of the transmission of such features. It was believed
that we could not do without such an axiom in order to explain the
transformation of species; and this was believed not only by those
who hold that the direct action of external influences plays an
important part in the process, but also by those who hold that the
operation of natural selection is the main factor.

Individual variability forms the most important foundation of
the theory of natural selection: without it the latter could not
exist, for this alone can furnish the minute differences by the
~ accumulation of which new forms are said to arise in the course of
generations. But how can such hereditary individual characters _
exist if the changes wrought by the action of external influences,

268 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

during the life of an individual, cannot be transmitted? We are
clearly compelled to find some other souree of hereditary in-
dividual differences, or the theory of natural selection-would collapse,
as it certainly would if hereditary individual variations did not
exist. If, on the other hand, acquired differences are transmitted,
this would prove that there must be something wrong in the
theory of the continuity of the germ-plasm, as above described, and
in the non-transmission of acquired characters which results from
this theory. But I believe that it is possible to suggest that the
origin of hereditary individual characters takes place in a manner
quite different from any which has been as yet brought forward.
To explain this origin is the task which I am about to undertake
in the following pages. |

The origin of individual variability has been hitherto represented
somewhat as follows. The phenomena of heredity lead to the
conclusion that each organism is capable of producing germs, from
which, theoretically at least, exact copies of the parent may arise.
In reality this is never the case, because each organism possesses
the power of reacting on the different external influences with
which it is brought into contact, a power without which it
could neither develope nor exist. Each organism reacting in a
different way must be to some extent changed. Favourable nutri-
tion makes such an organism strong and large; unfavourable nu-
trition renders it small and weak, and what is true of the whole
organism may also be said of its parts. Now it is obvious that
even the children of the same mother meet with influences different
in kind and degree, from the very beginning of their existence, so
that they must necessarily become unlike, even if we suppose them
to have been derived from absolutely identical germs, with precisely
the same hereditary tendencies.

In this manner individual differences are believed to have been
introduced. But if acquired characters are not transmitted the
whole chain of argument collapses, for none of those changes which
are caused by the-conditions of nutrition acting upon single parts
of the whole organism, including the results of training and of the
use or disuse of single organs,—none of these changes can furnish
’ hereditary differences, nor can they be transmitted to succeeding
generations. They are, as it were, only transient characters as far
as the species is concerned,

lt nt ee i ae

ES es eel

IN THE THEORY OF NATURAL SELECTION. 269

The children of accomplished pianists do not inherit the art of
playing the piano; they have to learn it in the same laborious
manner as that by which their parents acquired it; they do not
inherit anything except that which their parents also possessed
when children, viz. manual dexterity and a good ear. Furthermore,
language is not transmitted to our children, although if has been
practised nof only by ourselves but by an almost endless line of
ancestors. Only recently, facts have again been worked up and
brought together, which show that children of highly civilized
nations have no trace of a language they have grown up in
a wild condition and in complete isolation’. The power of speech
is an acquired or transient character : it is not inherited, and cannot
be transmitted : it disappears with the organism which manifests it.
Not only do similar phenomena occur in the vegetable kingdom,
but they present themselves in an especially striking manner.

When Nageli? introduced Alpine plants, taken from their natural
habitat, into the botanical garden at Munich, many of the species
were so greatly altered that they could hardly be recognized: for
instance, the small Alpine hawk-weeds became large and thickly
branching, and they blossomed freely. But if such plants, or even
their descendants, were removed to a poor gravelly soil the new
characters entirely disappeared, and the plants were re-transformed
into the original Alpine form. The re-transformation was always
complete, even when the species had been cultivated in rich garden
soil for several generations.

Similar experiments with identical results were made twenty
years ago by Alexis Jordan *, who chiefly made use of Draba verna
in his researches. These experiments furnish very strong: proofs,
because they were originally undertaken without the bias which
may be given by a theory. Jordan only intended to decide experi-
mentally whether the numerous forms of the plant, as it occurs
wild in different habitats, are mere varieties or true species. He -

* found that the different forms do not.pass into one another, and

1 Compare Rauber, ‘Homo sapiens ferus oder die Zustainde der Verwilderten.’
Leipzig, 1885.

2 *Sitzungsberichte der baierischen Akademie der Wissenschaften,’ vom 18 Nov..
1865. Compare also his ‘ Mechanisch-physiologische Theorie der Abstammungslehre,’
p- 102, ete.

* Jordan, ‘Remarques sur le fait de l’existence en société des esptces végétales
affines.’ Lyon, 1873.

270 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

are in all cases re-transformed after they have been altered by culti-
vation in a soil different from that*in, which they usually grow,
and he therefore assumed that they were true species. All these
experiments therefore confirm the conclusion that external influences
may alter the individual, but that the changes produced are not
transmitted to the germs, and are never hereditary.

Niageli indeed asserts that innate individual differences do not
exist in plants. The differences which we find, for instance, be-
tween two beeches or oaks, are always, according to him, modifi-
cations produced by the influence of varying local conditions. But
it is obvious that Nigeli goes too far in this respect, dithough it
may be conceded that innate individual differences in plants are
much more difficult to distinguish from those which are acquired,
than in animals.

There is no doubt about the occurrence of innate and hereditary
individual characters in animals, and we may find an especially
interesting illustration in the case of man. The human eye can
with practice appreciate the most minute differences between indi-
vidual men, and especially differences of feature. Every one knows
that peculiarities of feature persist in certain families through a
long series of generations. I need hardly remind the reader of the
broad forehead of the Julii, the projecting chin of the Hapsburgs,
or the curved nose of the Bourbons. Hence every one can see that
hereditary individual characters do unquestionably exist inman. The
same conclusion may be affirmed with equal certainty for all our
domestic animals, and I do not see any reason why there should
be any doubt about its application to other animals and to plants.

But now the question arises,—How can we explain the presence
of such characters consistently with a belief in the continuity of the
germ-plasm, a theory which implies the rejection of the supposition
that acquired characters can become hereditary? How can the
individuals of any species come to possess various characters
which are undoubtedly hereditary, if all changes which are due to
the influence of external conditions are transient and disappear
with the individual in which they arose? Why is it that in-
dividuals are distinguished by innate characters, as well as by those
which I have previously called transient, and how can deep-seated
hereditary characters arise at all, if they are not produced by the _
external influences to which the individual is exposed ?

IN THE THEORY OF NATURAL SELECTION. 271

In the first place it may be argued that external influences may
not only act on the mature individual, or during its development,
but that they may also act at a still earlier period upon the germ-
cell from which it arises. It may be imagined that such influences
of different kinds might produce corresponding minute alterations
in the molecular structure of the germ-plasm, and as the latter is,
according to our supposition, transmitted from one generation to
another, it follows that such changes would be hereditary.

Without altogether denying that such influences may directly
modify the germ-cells, I nevertheless believe that they have no
share in the production of hereditary individual characters.

The germ-plasm or idioplasm of the germ-cell (if this latter term
be preferred) certainly possesses an. exceedingly complex minute
structure, but it is nevertheless a substance of extreme stability, for it
absorbs nourishment and grows enormously without the least change
in its complex molecular structure. . With Nigeli we may indeed
safely affirm so much, although we are unable to acquire any direct
knowledge as to the constitution of germ-plasm. When we know
that many species have persisted unchanged for thousands of years,
we have before us the proof that their germ-plasm has preserved
exactly the same molecular structure during the whole period. I
may remind the reader that many of the embalmed bodies of the
sacred Eeyptian animals must be four thousand years old, and that
the species are identical with those now existing in the same
locality. Now, since the quantity of germ-plasm contained in a
single germ-cell must be very minute, and since only a very small
fraction can remain unchanged when the germ-cell developes into
an organism, it follows that an enormous growth of this small
fraction must take place in every individual, for it must be re-
membered that each individual produces thousands of germ-cells.
It is therefore not too much to say that, during a period of four
thousand years, the growth of the germ-plasm in the Egyptian ibis
or crocodile must have been quite stupendous. But in the animals
and plants which inhabit the Alps and the far north, we have
instances of species which have remained unchanged for a much
longer period, viz. for the time which has elapsed between the close

_of the glacial epoch and the present day. In such organisms the
growth of the germ-plasm must therefore have been still greater.

If nevertheless the molecular structure of the germ-plasm has

272 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

remained precisely the same, this substance cannot be readily
modifiable, and there is very little chance of the smallest changes
being produced in its molecular structure, by the operation of those
minute transient variations in nutrition to which the germ-cells,
together with every other part of the organism, are exposed. The
rate of growth of the germ-plasm will certainly vary, but its strue-
ture is unlikely to be affected for the above-mentioned reasons, and
also because the influences are mostly changeable, and occur some-
times in one and sometimes in another direction.

Hereditary individual differences must therefore be derived from
some other source.

I believe that such a source is to be looked for in the form
of reproduction by which the great majority of existing organisms
are propagated: viz. in sexual, or, as Hiackel calls it, amphigonic
reproduction.

It is well known that this process consists in the coalescence of
two distinct germ-cells, or perhaps only of their nuclei. These
germ-cells contain the germ-substance, the germ-plasm, and this

again, owing to its specific molecular structure, is the bearer of the

hereditary tendencies of the organism from which the germ-cell
has been derived. Thus in amphigonic reproduction two groups
of hereditary tendencies are as it were combined. I regard this
combination as the cause of hereditary individual characters, and
I believe that the production of such characters is the true sig-
nificance of amphigonic reproduction. The object of this process is
to create those individual differences which form the material out of
which natural selection produces new species.

At first sight this conclusion appears to be very startling and
almost incredible, because we are on the contrary inclined to believe
that the continued combination of existing differences, which is
implied by the very existence of amphigonic reproduction, cannot
lead to their intensification, but rather to their diminution and
gradual obliteration. Indeed the opinion has already been ex-
pressed that deviations from the specific type are rapidly destroyed
by the operation of sexual reproduction. Such an opinion may be
true with regard to specific characters, because the deviations from
a specific type occur in such rare cases that they cannot hold their
ground against the large number of normal individuals, But the.
case is different with those minute differences which are characteristic

OO ee ee

IN THE THEORY OF NATURAL SELECTION, 278

of individuals, because every individual possesses them, although
of a different kind and degree. The extinction of such dif-
ferences could only take place if a few individuals constituted a
whole species; but the number of individuals which together
represent a species is not only very large but generally incalculable.
Cross-breeding between all individuals is impossible, and hence the

~ obliteration of individual differences is also impossible.

In order to explain the effects of sexual reproduction, we will
first of all consider what happens in monogonic or unisexual re-
production, which actually occurs in parthenogenetic organisms.
Let us imagine an individual producing germ-cells, each of which
may by itself develope into a new individual. If we then suppose
a species to be made up of individuals which are absolutely identical,
it follows that their descendants must also remain identical through
any number of generations, if we neglect the transient non-
transmissible peculiarities caused by differences of food and other
external conditions.

Although the individuals of such a species might be actually
different, they would be potentially identical : in the mature state
they might differ, but they must have been identical in origin.
The germs of all of them must contain exactly the same hereditary
tendencies, and if it were possible for their development to take \
place under exactly the same conditions, identical individuals would
be produced.

Let us now assume that the individuals of such a species, repro-
ducing itself by the monogonic process and therefore without cross-
breeding, differ, not only in transient but also in hereditary cha-
racters. If this were the case, each individual would produce
descendants possessing the same hereditary differences which were
characteristic of itself; and thus from each individual a series of
generations would emanate, the single individuals of which
would be potentially identical with each other and with their
first ancestor. Hence the same individual differences would be
repeated again and again, in each succeeding generation, and
even if all the descendants lived to reproduce themselves, there
would be at last just as many groups of potentially identical

individuals as there were single individuals at the beginning.

Similar cases actually occur in many species in which sexual
reproduction has been entirely replaced by the parthenogenetic
T

274 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

method, as in many species of Cynips and in certain lower Crustacea.
But all these differ from our hypothetical cas¢ in one important
respect ; it is always impossible for all the descendants to reach
maturity and reproduce themselves. The vast majority of the
descendants generally perish at an early stage, and only about as
many remain to continue the species as reached maturity in the
preceding generation.

We have now to consider whether such a species can be subject
to the operation of natural selection. Let us take the case of an
insect living among green leaves, and possessing a green colour as
a protection against discovery by its enemies. We will assume that
the hereditary individual differences consist of various shades of
green. Let us further suppose that the sudden extinction of its
food-plant compelled this species to seek another plant with a
somewhat different shade of green. It is clear that such an insect
would not be completely adapted to the new environment. It
would therefore be compelled, metaphorically speaking, to en-
deavour to bring its colour into closer harmony with that of the
new food-plant, or else the increased chances of detection given to
its enemies would lead to its slow but certain extinction.

It is obvious that such a species would be altogether unable to
produce the required adaptation, for ew hypothesi, its hereditary
variations remain the same, one generation after another. If
therefore the required shade of green was not previously present,
as one of the original individual differences, it could not be pro-
duced at any time. If, however, we suppose that such a colour
existed previously in certain individuals, it follows that those with
other shades of green would be gradually exterminated, while
the former would alone survive. But this process would not be
an adaptation in the sense used in the theory of natural selection.
It would indeed be a process of selection, but it could form no
more than the beginning of that process which we call natural
selection. If the latter could only bring existing characters into
prominence, it would not be worth much consideration, for it could
never produce a new species. A species never includes, from the
beginning, individuals which deviate from the specific type as
widely as the individuals of the most nearly allied species
deviate from it. And it would be still less possible to explain,
on such a principle, the origin of the whole organic world ; for, if

!

IN THE THEORY OF NATURAL SELECTION. 275

so, all existing species would have been included as variations of
the first species. Natural selection must be able to do infinitely
more than this, if it is to be of any importance as a principle of
development. It must be able to accumulate minute existing dif-
ferences in the required direction, and thus to create new characters.
In our example it ought to be able, after preserving those in-
dividuals with a colour nearest to the required shade, to lead their
descendants onward through successive stages towards a complete
harmony of colour.

But such a result is quite unattainable with the asexual method
of reproduction: in other words, natural selection, in the true
meaning of the term, viz. a process which could produce new
characters in the manner above described, is an impossibility in a
species propagated by asexual reproduction.

If it could be shown that a purely parthenogenetic species had
become transformed into a new one, such an observation would
prove the existence of some force of transformation other than
selective processes, for the new species could not have been pro-
duced by these latter. As already explained, the only selection
which would be possible for such a species, would lead to the

_survival of one group of individuals and to the extinction of all
others. Thus in our example that group of individuals would
alone survive, the ancestors of which originally possessed the
appropriate colour. But if one group alone survived, it follows
that all hereditary individual: differences would have disappeared
from the species, for the members of such a single group are
identical with one another and with their original ancestors. We
thus reach the conclusion that monogonic reproduction can never
cause hereditary individual variability, but that, on the other hand,
it is very likely to lead to its entire suppression.

But the case is very different with sexual reproduction. When
once individual differences have begun to appear in a species pro-
pagated by this process, uniformity among its individuals can
never again be reached. So far from this being the case, the
differences must even be increased in the course of generations, not
indeed in intensity, but in number, for new combinations of the

individual characters will continually arise.

Again, assuming the existence of a number of individuals which
differ from one another by a few hereditary individual characters,

T 2

276 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

it follows that no individual of the second generation can be iden-
tical with any other. They must all differ, not only actually but
also potentially, for their differences exist at the very beginning of
development, and do not solely depend upon the accidental conditions
under which they live. Moreover, no one of the descendants can
be identical with any of the ancestors, for each of the former unites
within itself the hereditary tendencies of two parents, and its
organism is therefore, as it were, a compromise between two de-
velopmental tendencies. Similarly in the third generation, the
hereditary tendencies of two individuals of the second generation
enter into combination. But since the germ-plasm of the latter
is not simple, but composed of two individually distinct kinds of
germ-plasm, it follows that an individual of the third generation
is a compromise between four different hereditary tendencies. In
the fourth generation, eight; in the fifth, sixteen ; in the sixth,
thirty-two different hereditary tendencies must come together, and
each of them will make itself more or less felt in some part of the
future organism. Thus by the sixth generation a large number of
varied combinations of ancestral individual characters will appear,
combinations which have never existed before and which can never
exist again. ; :

We do not know the number of generations over which the
specific hereditary tendencies of the first generation can make
themselves felt. Many facts seem to indicate however that the
number is large, and it is at all events greater than six. When
we remember that, in the tenth generation, a single germ contains
1024 different germ-plasms, with their inherent hereditary ten-
dencies, it is quite clear that continued sexual reproduction can
never lead to the re-appearance of exactly the same combination,
but that new ones must always arise.

New combinations are all the more probable because the different
idioplasms composing the germ-plasm in the germ-cells of any
individual are present in different degrees of intensity at different
times of its life; in other words, the intensity of the component
idioplasms is a function of time. This conclusion follows from the
fact that children of the same parents are never exactly identical.
In one child the characters of the father may predominate, in
another those of the mother, in another again those of either
grand-parent or great-grand-parent.

~

“— ww

a: Ye! Se hae

IN THE THEORY OF NATURAL SELECTION. 7E

We are thus led to the conclusion that even in a few sexually
produced generations a large number of well-marked individuals
must arise: and this would even be true of generations springing
from our hypothetical species, assumed to be without ancestors, and
characterised by few individual differences. But of course organ-
isms which reproduce themselves sexually are never without
ancestors, and if these latter were also propagated by the sexual
method, it follows that each generation of every sexual species is
in the stage which we have previously assumed for the tenth
or some much later generation of the hypothetical species. In
other words, each individual contains a maximum of hereditary
tendencies and an infinite variety of possible individual characters
(see Appendix VI, p. 326).

In this manner we can explain the origin of hereditary in-
dividual variability as it is known in man and the higher animals,
and as it is required for the theory which explains the transformation
of species by means of natural selection.

Before proceeding further, I must attempt to answer a question
which obviously suggests itself. For the sake of argument, I
have assumed the existence of a first generation, of which the
individuals were already characterised by individual differences.
Can we find any explanation of these latter, or are we compelled
to take them for granted, without any attempt to enquire into their
origin? If we abandon this enquiry, we can never achieve a com-

plete solution of the problems of heredity and variability. We

have, it is true, shown that hereditary differences, when they have
once appeared, would, through sexual reproduction, undergo de-
velopment into the diverse forms which actually exist; but this
conclusion affords us no explanation of the source whence such
differences have been derived. If the external conditions acting
directly upon an organism can only produce transient (viz. non-
hereditary) differences in the latter, and if, on the other hand, the
external influences which act upon the germ-cell can only produce
a change in its molecular structuré after operating over very long
periods, it seems that we have exhausted all the possible sources
of hereditary differences without reaching any satisfactory ex-
planation.

I believe, however, that an explanation can be given. The origin
of hereditary individual variability cannot indeed be found in the

278 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

higher organisms—the Metazoa and Metaphyta; but it is to be
sought for in the lowest—the unicellular organisms. In these
latter the distinction between body-cell and germ-cell does not exist.
Such organisms are reproduced by division, and if therefore any
one of them becomes changed in the course of its life by some
external influence, and thus receives an individual character, the
method of reproduction ensures that the acquired peculiarity will
be transmitted to its. descendants. If, for instance, a Protozoon, by
constantly struggling against the mechanical influence of currents
in water, were to gain a somewhat denser and more resistent
protoplasm, or were to acquire the power of adhering more strongly
than the other individuals of its species, the peculiarity in question
would be directly continued on into its two descendants, for the
latter are at first nothing more than the two halves of the former.
It therefore follows that every modification which appears in the
course of its life, every individual character, however it may have
arisen, must necessarily be directly transmitted to the two off-
spring of a unicellular organism.

The pianist, whom I have already used as an illustration, may
by practice develope the muscles of his fingers so as to ensure the
highest dexterity and power; but such an effect would be entirely
transient, for it depends upon a modification in local nutrition
which would be unable to cause any change in the molecular
structure of the germ-cells, and could not therefore produce any
effect upon the offspring. And even if we admit that some change
_ might be caused in the germ-cells, the chances would be infinity
to nothing against the production of the appropriate effect, viz.
such a change as would lead to the tbe aaa in the child of
the acquired characters of the parent.

In the lowest unicellular organisms, however, the case is en-
tirely different. Here parent and offspring are still, in a certain
sense, one and the same thing: the child is a part, and usually
half, of the parent. If therefore the individuals of a unicellular -
species are acted upon by any of the various external influences,
it is inevitable that hereditary individual differences will arise in
them ; and as a matter of fact it is indisputable that changes are
thus produced in these organisms, and that the resulting characters
are transmitted. It has been directly observed that individual
differences do occur in unicellular organisms,—differences in size,

IN THE THEORY OF NATURAL SELECTION. 279

colour, form, and the number or arrangement of cilia. It must be
admitted that we have not hitherto paid sufficient attention to
this point, and moreover our best microscopes are only very rough
means of observation when we come to deal with such minute
organisms. Nevertheless we cannot doubt that the individuals
of the same species are not absolutely identical.

We are thus driven to the conclusion that the ultimate origin of
hereditary individual differences lies in the direct action of ex-
ternal influences upon the organism. Hereditary variability can-
not however arise in this way at every stage of organic develop-
ment, as biologists have hitherto been inclined to believe. It can
‘only arise in the lowest unicellular organisms; and when once
individual difference had been’ attained by these, it necessarily
passed over into the higher organisms when they first appeared.
Sexual reproduction coming into existence at the same time, the
hereditary differences were increased and multiplied, and arranged
in ever-changing combinations.

Sexual reproduction can also increase the differences between
individuals, because constant cross-breeding must necessarily and
repeatedly lead to a combination of forces which tend in the
same direction, and which may determine the constitution of any
part of the body. If, for instance, the same part of the body is
strongly developed in both parents, the experience of breeders tells
us that the part in question is likely to be even more strongly
developed in the offspring; and that weakly developed parts will
in the same manner tend to become still weaker. Amphigonic
reproduction therefore ensures that every character which is sub-
ject to individual fluctuation must appear in many individuals
with a strengthened degree of development, in many others with
a development which is less than normal, while in a still larger
number of individuals the average development will be reached.
Such differences afford the material by means of which natural
selection is able to increase or weaken each character according
to the needs of the species. By the removal of the less well-
adapted individuals, natural selection increases the’ chance of
beneficial cross-breeding in the subsequent generations.

Every one must admit that, if a species came into existence
having only a small number of individual differences which
appeared in the different parts of different individuals, the number |

280 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

of differences would increase with each sexually produced generation,
until all the parts in which the variations occurred had received
a peculiar character in all individuals.

Moreover sexual reproduction not only adds to the number of
existing differences, but it also brings them into new combina-
tions, and this latter consequence is as important as the former.

The former consequence can hardly make itself felt in any
existing species, because in them every part already possesses its
peculiar character in all individuals. The second consequence is,
however, more important, viz. the production of new combind-
tions of individual characters by sexual reproduction; for, as
Darwin has already pointed out, we must imagine that not only
are single characters changed in the process of breeding, but that
probably several, and perhaps very many characters, are simul-
taneously modified. No two species, however nearly allied, differ
from each other in but a single character. Even our eyesight,
which has by no means reached the highest pitch of development,
can always detect several, and often very many points of difference;
and if we possessed the powers necessary for making an absolutely
accurate comparison, we should probably find that everything is
different in two nearly allied species.

It is true that a great number of these differences depend upon
correlation, but others must depend upon simultaneous primary
changes. ‘

A large butterfly (Kallima paralecta), found in the East Indian
forests, has often been described in its position of rest as almost
exactly resembling a withered leaf; the resemblance “in colour
being aided by the markings which imitate the venation of a leaf.
These markings are composed of two parts, the upper of which is
on the fore-wings, while the lower one is on the hind wings. The
butterfly when at rest must therefore keep the wings in such a
position that the two parts of each marking exactly correspond,
for otherwise the character would be valueless ; and as a matter of
fact the wings are held in the appropriate position, although the
butterfly is of course unconscious of what it is doing. Hence a
mechanism must exist in the insect’s brain which compels it to
assume this attitude, and itis clear that the mechanism cannot have
been developed before the peculiar manner of holding the wings
became advantageous to the butterfly, viz. before the similarity

a 4, eee

IN THE THEORY’ OF NATURAL SELECTION, 281

to a leaf had made its first appearance. Conversely, this latter
resemblance could not develope before the butterfly had gained the
habit of holding its wings in the appropriate position. Both
characters must therefore have come into existence simultaneously,
and must have undergone increase side by side: the marking
progressing from an imperfect to a very close similarity, while
the position of the wings gradually approached the attitude
which was exactly appropriate. The development of certain
minute structural elements of the central nervous system, and the
appropriate distribution of colouring matter on the wings, must
have taken place simultaneously, and only those individuals have
been selected to continue the species which possessed the favourable
variations in both these directions.

It is, however, obvious that sexual reproduction will readily
afford such combinations of required characters, for by its means
the most diverse features are continually united in the same indi-
vidual, and this seems to me to be one of its most important
results. ;

I do not know what meaning can be attributed to sexual repro-
duction other than the creation of hereditary individual characters
to form the material upon which natural selection may work.
Sexual reproduction is so universal in all classes of multicellular
organisms, and nature deviates so rarely from it, that it must
necessarily be of pre-eminent importance. If it be true that
new species are produced by processes of selection, it follows that
the development of the whole organic world depends on these pro-
cesses, and the part that amphigony has to play in nature, by
rendering selection possible among multicellular organisms, is not
only important, but of the very highest imaginable importance.

But when I maintain that the meaning of sexual reproduction is
to render possible the transformation of the higher organisms by
means of natural selection, such a statement is not equivalent to
the assertion that sexual reproduction originally came into exist-
ence in order to achieve this end. The effects which are now pro-
duced by sexual reproduction did not constitute the causes which
led to its first appearance. Sexual reproduction came into existence
before it. could lead to hereditary individual variability. Its first
appearance must therefore have had some other cause; but the
nature of this cause can hardly be determined with any degree of

282 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

certainty or precision from the facts with which we are at present
acquainted. The general solution of the problem will, however,
be found to lie in the conjugation of unicellular organisms, which
forms the precursor of true sexual reproduction. The coalescence
of two unicellular individuals which represents the simplest and
therefore probably the most primitive form of conjugation, must
have some directly beneficial effect upon the species in which it
occurs.

Various assumptions may be made as to the nature of these bene-
ficial effects, and it will be useful to consider in detail some of
those suggestions which have been brought forward. Eminent
biologists, such as Victor Hensen! and Edouard van Beneden ?,
believe that conjugation, and indeed sexual reproduction generally,
must be considered as ‘a rejuvenescence of life.’ Biitschli also
accepts this view, at any rate as regards conjugation. These author-
ities imagine that the wonderful phenomena of life, of which
the underlying cause is still an unsolved problem, cannot be con-
tinued indefinitely by the action of forces arising from within
itself, that the clock-work would be stopped after a longer or
shorter time, that the reproduction of purely asexual organisms
would cease, just -as the life of the individual finally comes to an
end, or as a spinning wheel comes to rest in consequence of friction,
and requires a renewed impetus if its motion is to continue. In
order that reproduction may continue without interruption, these
writers believe that a rejuvenescence of the living substance is
necessary, that the clock-work of reproduction must be wound up
afresh ; and they recognize such a rejuvenescence in sexual repro-
duction and in conjugation, or in other words in the fusion of two
cells, whether in the form of. germ-cells or of two unicellular
organisms.

Edouard van Beneden expresses this idea in the following words:—
. *Il semble que la faculté que possédent les cellules, de se multiplier
par division soit limitée: il arrive un moment ov elles ne sont plus
eapables de se diviser ultérieurement, 4 moins qu’elles ne subissent
le phénoméne du rajeunissement par le fait de la fécondation.

1 §. Hermann’s ‘ Handbuch der Physiologie,’ Theil II ; ‘ Physiologie der Zeugung,”
by V. Hensen. :

* E. van Beneden, ‘ Recherches sur la maturation de I’ceuf, la fécondation et la
division cellulaire. Gand u. Leipzig, 1883, pp. 404 et seq.

IN THE THEORY OF NATURAL SELECTION. 283

Chez les animaux et les plantes les seules cellules capables d’étre
rajeunies sont: les ceufs; les seules capables de rajeunir sont les
spermatocytes. Toutes les autres parties de l’individu sont vouées
ila mort. La fécondation est la condition de la continuité de la
vie. Par elle le générateur échappe a la mort’ (1. ¢, p. 405).
Victor Hensen thinks it possible that the germ and its products
are prevented from dying by means of normal fertilization: he
says that the law which states that every egg must be fertilized,
was formulated before the discovery of parthenogenesis and cannot
now be maintained, but that we are nevertheless compelled to
assume that even the most completely parthenogenetic species
requires fertilization after many generations (I. ¢., p. 236).

If the theory of rejuvenescence be thoroughly examined, it will
be found to be nothing more than the expression of the fact that
sexual reproduction persists without any ascertainable limit. From
the fact of its general occurrence, the conclusion is, however, drawn
that asexual reproduction could not persist indefinitely as the
only mode of reproduction in any species of animal. But proofs
in support of ‘this opinion are wanting, and it is very probable
that it would never have been advanced if it had been possible —
to explain the general occurrence of sexual reproduction in any
other way,—if we had been able to ascribe any other significance
to this pre-eminently important process. :

But quite apart from the fact that it is impossible to bring
forward any proofs, the theory of rejuvenescence seems to me to
be unsatisfactory in other ways. The whole conception of re-
juvenescence, although very ingenious, has something uncertain
about it, and can hardly be brought into accordance with the
usual conception of life as based upon physical and mechanical
forces. How can any one imagine that an Infusorian, which by
continued division had lost its power of reproduction, could regain
this power by forming a new individual, after fusion with another
Infusorian, which had similarly become incapable of division?
Twice nothing cannot make one. If indeed we could assume that
each animal contained half the power necessary for reproduc-
tion, then both together would certainly form an efficient whole ;
but it is hardly possible to apply the term rejuvenescence to a
process which is simply an addition, such as would be attained
under other circumstances by mere growth; néglecting, for the

284 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

present, that factor which, in my opinion, is of the utmost import-
ance in conjugation,—the fusion of two hereditary tendencies. If
rejuvenescence possesses any significance at all, it must be this,—
that by its means a force, which did not previously exist in the
conjugating individuals, is called into activity. Such a force
would, however, owe its existence to latent energy stored up in
each single animal during the period of asexual reproduction, and
such latent forces would necessarily be of different natures, and of
such a constitution that their union at the moment of conjugation
would give rise to the active force of reproduction. -

The process might perhaps be compared to the flight of two
rockets, which by the combustion of some explosive substance (such
as nitro-glycerine) stored up within themselves are impelled in
such a direction that they would meet at the end of their course,
when all the nitro-glycerine had been completely exhausted. The
movement would then come to an end, unless the explosive material
could have been meanwhile renewed. Now suppose that such a
renewal were achieved by the formation of nitric acid in one of the
rockets and glycerine in the other, so that when they came into
contact nitro-glycerine would be formed afresh equal in quantity and
in distribution on both the rockets to that which was originally
present. In this way the movement would be renewed again and
again with the same velocity, and might continue’ for ever.

Rejuvenescence can be rendered intelligible in theory by some
such metaphor, but considerable difficulties are encountered in
the rigid application of the metaphor to the facts of the case.
In the first place, how is it possible that the motive force can be
exhausted by continual division, while one of its components is
being formed afresh in the same body and during the same time ?
When thoroughly examined the loss of the power of division is
seen to follow from the loss of the powers of assimilation, nutrition,
and growth. How is it possible that such a power can be
weakened and finally entirely lost while one of its components
is accumulated ? |

I believe that, instead of accepting such daring assumptions, it
is better to be satisfied with the simple conception of living
matter possessing as attributes the powers of unlimited assimilation
and capacity for reproduction. With such a theory the mere form
of reproduction, whether sexual or asexual, will have no influence

IN THE THEORY OF NATURAL SELECTION, 985

upon the duration of the capacity: for force and matter under,
simultaneous increase, and are inseparably connected in this as in
all other instances. This theory does not, however, exclude the’
possible occurrence of circumstances under which such an associa-
tion is no longer necessary.

I could only consent to adopt the hypothesis of rejuvenescence, if
it were rendered absolutely certain that reproduction by division
could never under any circumstances persist indefinitely. But this
cannot be proved with any greater certainty than the converse pro-
position, and hence, as far as direct proof is concerned, the facts are
equally uncertain on both sides. The hypothesis of rejuvenescence
is, however, opposed by the fact of parthenogenesis ; for if fertilization
possesses in any way the meaning of rejuvenescence, and depends
upon the union of two different forms of force and of matter, which
- thus produce the power of reproduction, it follows that we cannot
understand how it happens that the same power of reproduction
may be sometimes produced from .one form of matter, alone and
unaided. Logically speaking, parthenogenesis should be as im-
possible as that either nitric acid or glycerine should separately
produce the effect of nitro-glycerine. The supposition has indeed
been made that in the case of parthenogenesis, one fertilization is
_ sufficient for a whole series of generations, but this supposition is
not only incapable of proof, but it is contradicted by the fact that
certain eggs which may develope parthenogenetically are also capable
of fertilization. If, in this case, the power of reproduction were suf-
ficient for development, how is it that the egg is also capable of
fertilization ; and if the power were insufficient, how is it that the
ege can develope parthenogenetically ? And yet one and the same
ege (in the bee) can develope into a new individual, with or with-
out fertilization. We cannot escape this dilemma by making
the further assumption, which is also incapable of proof, that a
smaller amount of reproductive force is required for the development
of a male individual than for the development of a female. It is
true that the unfertilized eggs of the bee produce male individuals,
while the fertilized ones develope into females, but in certain other
species the converse association holds good, while in others, again,
fertilization bears no relation to the sex of the offspring.

Although the mere fact that parthenogenesis occurs at all is, in
my opinion, sufficient to disprove the theory of rejuvenescence, it is

286 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

wéll to remember that parthenogenesis is now the only method of
yeproduction in many species (although we do not know the period
of time over which these conditions have extended), and is neverthe-
less unattended by any perceptible decrease in fertility.

From all these considerations we may draw the conclusion that
the process of rejuvenescence, as described above, cannot be accepted
either as the existing or the original meaning of conjugation, and
the question naturally arises as to what other significance this
latter process can have possessed at its first beginning.

Rolph! has expressed the opinion that conjugation is a form of
nutrition, so that the two conjugating individuals, as it were, devour
each other. Cienkowsky*? also regards conjugation as merely

accelerated’ assimilation. There is, however, not only an essential
difference but a direct contrast between the processes of conjugation
and nutrition. With regard to Cienkowsky’s view, Hensen* has
well said that ‘coalescence in itself cannot be an accelerated nutrition,
because even if we admit that both individuals are in want of
nourishment, it is impossible that the need can be supplied by this
process, unless one of them perishes and is really devoured.’ In
order that an animal may serve as the food of another, it must
perish and must be brought into a fluid form, and finally it must be
assimilated. In the case before us, however, two protoplasmic
bodies are placed side by side and coalesce, without either of them
passing into the liquid state. Two idioplasms unite, together with
all the hereditary tendencies contained in them ; but although it is
certain that nutrition in the proper sense of the word cannot take
place, because neither of the animals receives an addition of liquid
food by the coalescence, yet the consequence of this process must
be in one respect similar to that of nutrition and growth :—the
mass of the body and the quantity of the forces contained in it
undergo simultaneous increase. It is not inconceivable that effects
are by this means rendered possible, which under the peculiar
circumstances leading to conjugation, could not have been otherwise
produced.

I believe that this is at any rate the direction in which we shall
have to seek for the first meaning of conjugation and for its

1 Rolph, ‘ Biologische Probleme.’ Leipzig, 1882.
* Cienkowsky, ‘Arch. f. mikr. Anat.,’ ix. p. 47. 1873.
° Hensen, ‘ Physiologie der Zeugung,’ p. 139.

IN THE THEORY OF NATURAL SELECTION, 287

phyletic origin. This first result and meaning of conjugation may
be provisionally expressed in the following formula :—conjugation
originally signified a strengthening of the organism in relation to
reproduction, which happened when from some external cause, such
as want of oxygen, warmth, or food, the growth of the individual
to the extent necessary for reproduction could not take place.

This explanation must not be regarded as equivalent to that
afforded by the theory of rejuvenescence ; for the latter process is said
to be necessary for the continuance of reproduction, and ought
therefore to occur periodically quite independently of external cir-
cumstances; while according to my theory, conjugation at first
only occurred under unfavourable conditions, and assisted the species
to overcome such difficulties.

But whatever the original meaning of conjugation may have
been, it seems to have become already subordinated in the higher
Protozoa, as is indicated by the changes in the course taken by
this process. The higher Protozoa when conjugating do not as a
rule coalesce completely and permanently’ in the manner followed
by the lower Protozoa, and it seems to me possible, or even probable,
that in the former the process has already gained the full significance
of sexual reproduction, and is to be looked upon as a source of
variability. ‘

Whether this be so or not, I believe it is certain that sexual re-
production could not have been entirely abandoned at any period
since the time when the Metazoa and Metaphyta first arose ; for they
derived this form of reproduction from their unicellular ancestors.

We know that organs and characters which have persisted
through a long series of generations are transmitted with extreme
tenacity, even when they have ceased to be of any direct use to
their immediate possessors. The rudimentary organs in various
animals, and not least in man, afford very strong proofs of the sound-
ness of this conclusion. Another example has only recently been
discovered in the sixth finger, which has been shown to exist in the
human embryo ?, a part which has only been present in a rudimentary

* Coalescence takes place in the so-called bud-like conjugation of Vorticellidae and -
Trichodinidae, etc.

* Compare (1) Bardeleben, ‘ Zur Entwicklung der Fusswurzel,’ Sitzungsber. d. Jen.
Gesellschaft, Jahrg. 1885, Feb. 6; also ‘ Verhandl. d. Naturforscherversammlung

zu Strassburg,’ 1885, p. 203; (2) G. Baur, ‘Zur Morphologie des Carpus und Tarsus
der Wirbelthiere,’ Zool. Anzeiger, 1885, pp. 326, 486.

288 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

form ever since the origin of the Amphibia’. Superfluous organs
become rudimentary very slowly, and enormous periods must elapse
before they completely disappear, while the older a character is,
the more firmly it becomes rooted in the organism. What I have
_ above called the physical constitution of a species is based upon
these facts, and upon them depend the tout ensemble of inherited —
characters, which are adapted to one another and woven together
into a harmonious whole. It is this specific nature of an organism
which causes it to respond to external influences in a manner different
from that followed by any other organism, which prevents it from
changing in any way except along certain definite lines of varia-
tion, although these may be very numerous. Furthermore these
facts ensure that characters cannot be taken at random from the
constitution of a species and others substituted for them. Such a
variation as a mammal wanting the firm axis of the backbone is an
impossibility, not only because the backbone is necessary as a support
to the body, but chiefly because this structure has been inherited
from times immemorial, and has become so impressed upon the
mammalian organization that any variation so great as to threaten
its very existence cannot now take place. The view here set forth
of the origin of hereditary variability by amphigonic reproduction, —
makes it clear that an organism is in a state of continual oscillation
only upon the surface, so to speak, while the fundamental parts of
its constitution, which have been inherited from extremely remote
periods, remain unaffected. |

Thus sexual reproduction itself did not cease after it had existed
in the form of conjugation through innumerable generations of the
vast numbers of species which have been included under the Protozoa;
it did not cease even when its original physiological significance
had lost its importance, either completely or in part. This process,
however, had come to possess a new significance which ensured its
continuance, in the enormous advantage conferred on a species by
the power of adapting itself to new conditions of life, a power which
could only be preserved by means of this method of reproduction.
The formation of new species which among the lower Protozoa
could be achieved without amphigony, could only be attained by
means of this process in the Metazoa and Metaphyta. It was only

1 Tn frogs the sixth toe exists in the hind legs as a rudimentary prehallux. Com-
pare Born, Morpholog, Jahrbuch, Bd. I, 1876,

nn i ess

a

yee a

IN THE THEORY OF NATURAL SELECTION. 289

in this way that hereditary individual differences could arise and
persist. It was impossible for amphigony to disappear, for each
species in which it was preserved was necessarily superior to those
which had lost it, and must have replaced them in the course of
time ; for the former alone could adapt itself to the ever-changing
conditions of life, and the longer sexual reproduction endured, the
more firmly was it necessarily impressed upon the constitution of
the species, and the more difficult its disappearance became.

Sexual reproduction has nevertheless been lost in some cases,
although only at first in certain generations. Thus in the Aphidae
and in many lower Crustacea, generations with parthenogenetic re-
production alternate with others which reproduce themselves by the
sexual method. But in most cases it is clear that this partial loss
of amphigony conferred considerable advantages upon the species by
giving increased capabilities for the maintenance of existence. By
means of partial parthenogenesis a much more rapid increase in the
number of individuals could be attained in a given time, and this —

fact is of the highest importance for the peculiar circumstances

under which these species exist. A species of Crustacean which
inhabits rapidly drying pools, and developes from winter-eggs which
have remained dried up in the mud, has, as a rule, only a very
short time in which to secure the existence of succeeding genera-
tions. The few sexual eggs which have escaped the attacks of
numerous enemies develope immediately after the first shower of
rain ; the animals attain their full size in a few days and reproduce
themselves as virgin females. Their descendants are propagated in
the same manner, and thus in a short time almost incredible num-
bers of individuals are formed, until finally the sexual eggs are
again produced. If now the pool dries up again, the existence of
the colony is secured, for the number of animals which produce
sexual egos is very large, and the eggs themselves are of course
far more numerous, so that in spite of the destructive agencies to
which they are subjected, there will be every chance of the survival
of a sufficient number to produce a new generation at a later
period. Here, therefore, sexual reproduction has not been abandoned
accidentally or from any internal cause, but as an adaptation to

‘certain definite necessities imposed upon the organism by its

surroundings.
It is, however, well known that there are certain instances in
a
U

290 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

which sexual reproduction has been altogether lost, and in which
parthenogenesis is the only form of propagation. In the animal
kingdom, such a condition chiefly occurs in species of which the
closely-allied forms exhibit the above-mentioned alternation between
parthenogenesis and amphigony, viz. in many Cynipidae and Aphidae,
and also in certain freshwater and marine Crustacea. We may
imagine that these parthenogenetic species have arisen from forms
with alternating methods of reproduction, by the disappearance of
the sexual phase.

In any particular case, it may be difficult to point out the motive
by which this change has been determined ; but it is most probable
that the same conditions which originally caused the intercalation
of a parthenogenetic stage have been efficient in causing the
gradual disappearance of the sexual stage. If a species of Crust-
acean, with the above-described alternating method of reproduction
(heterogeny), were killed off by its enemies on a larger scale than
before, it is obvious that the threatened extinction of the species
could be checked by the attainment of a correspondingly greater
degree of fertility. Such increased fertility might well be produced
by pure parthenogenesis (see Appendix V, p. 323), by means of
which the number of egg-producing individuals in all the previous
sexual generations would be doubled.

In a certain sense, this would be the last and most extreme
method by means of which a species might secure continued
existence, for it is a method for which it would have to pay
very dearly at a later period. If my theory as to the causes of
hereditary individual variability be correct, it follows that all species/
with purely parthenogenetic reproduction are sure to die out; not,
indeed, because of any failure in meeting the existing conditions
of life, but because they are incapable of transforming themselves
into new species, or, in fact, of adapting themselves to any new
conditions. Such species can no longer be subject to the process
of natural selection, because, with the disappearance of sexual re-
production, they have also lost the power of combining and in-
creasing those hereditary individual characters which they possess.

All the facts with which we are acquainted confirm this con-
clusion, for whole groups of purely parthenogenetic species or
genera are never met with, as would certainly be the case if
parthenogenesis had been the only method of reproduction through

IN THE THEORY OF NATURAL SELECTION. 291

a successional series of species. We always find it in isolated
instances, and under conditions which compel the conclusion that
it has become predominant in the species in question, and has not
been transmitted from any preceding: species.

There still remains a very different class of facts which, so far as
we can judge, are in accordance with my theory as to the signi-
ficance of sexual reproduction, and which may be quoted in its
support. I refer to the condition of functionless organs in species
with parthenogenetic reproduction.

Under the supposition that acquired characters cannot be trans-
mitted—and this forms the foundation of the views here set forth
—organs which are of no further use cannot become rudimentary
in the direct and simple manner in which it has been hitherto
imagined that degeneration takes place. It is true that an organ
which does not perform any function exhibits a marked decrease of
strength and perfection in the individual which possesses it, but
such acquired degradation is not transmitted to its descendants,
and we must therefore look for some other explanation of the
firmly established fact that organs do become rudimentary through
a series of generations. In seeking this explanation, we shall have
to start from the supposition that new forms are not only created
by natural selection, but are also preserved by its means. In
order that any part of the body of an individual of any species may
be kept at the maximum degree of development, it is necessary
that all individuals possessing it in a less perfect form must be
prevented from propagation—they must succumb in the struggle
for existence. I will illustrate this by a special instance. In
species which, like the birds of prey+, depend for food upon the
acuteness of their vision, all individuals with relatively weak eye-
sight must be exterminated, because they will fail in the competition
for food. Such birds will perish before they have reproduced them-
_ selves, and their imperfect vision is not further transmitted. In.
this way the keen eyesight of birds. of prey is kept up to its
maximum.

But as soon as an organ becomes useless, the continued selection
of individuals in which it is best developed must cease, and
a process which I have termed panmiaia takes place. When this

1 T here make faa of the same illustration which I employed in my first attempt
to explain the effects of panmixia. Compare the second Essay ‘ On Heredity.’

U2

ia
ra
a

292 THE SIGNIFICANCE’ OF SEXUAL REPRODUCTION

process is in operation, not only those individuals with the best-
developed organs have the chance of reproducing themselves, but
also those individuals in which the organs are less well-developed.
Hence follows a mixture of all possible degrees of perfection,
which must in the course of time result in the deterioration of the
average development of the organ. Thus a species which has retired
into dark caverns must necessarily come to gradually possess less
developed powers of vision; for defects in the structure of the
eyes, which occur in consequence of individual variability, are not
eliminated by natural selection, but may be transmitted and fixed
in the descendants!. This result is all the more likely to happen,
inasmuch as other organs which are of importance for the life
of the species will gain what the functionless organ loses in size
and nutrition. As at each stage of retrogressive transformation
individual fluctuations always occur, a continued decline from the
original degree of development will inevitably, although very
slowly, take place, until the last. remnant finally disappears. How
inconceivably slowly this process goes on is shown by the numerous
eases of rudimentary organs: by the above-mentioned embryonic
sixth finger of man, or by the hind limbs of whales buried beneath
the surface of the body, or by their embryonic tooth-germs.
I believe that the very slowness with which functionless organs
gradually disappear, agrees much better with my theory than with
the one which has been hitherto held. The result of the disuse of
an organ is considerable, even in the course of a single individual
life, and. if only a small fraction of such a result were trans-
mitted to the descendants, the organ would be necessarily reduced
to a minimum, in a hundred or at any rate in a thousand genera-
tions. But how many millions of generations may have elapsed
since e.g. the teeth of the whalebone whales became useless, and
were replaced by whalebone! We do not know the actual number
of years, but we know that the whole material of the tertiary rocks
has been derived from the older strata, deposited in the sea, elevated,

{' E. Ray Lankester has suggested (Encycl. Britann., art. ‘ Zoology,’ pp. 818, 819)
that the blindness of cave-dwelling and deep-sea animals is also due to the fact that
‘« those individuals with perfect eyes would follow the glimmer of light and eventually
escape to the outer air or the shallower depths, leaving behind those with imperfect
eyes to breed in the dark place, A natural selection would thus be effected.’ Such
a sifting process would certainly greatly quicken the rate of degeneration due to pan-
mixia alone.—E, B. P.}

IN THE THEORY OF NATURAL SELECTION. 293

and has been itself largely removed by denudation, since that
time.

Now if this theory as to the causes of deterioration in disused
organs be correct, it follows that rudimentary organs can only
‘occur in species with sexual reproduction, and that they cannot be
formed in species which are exclusively reproduced by the partheno-
genetic method: for, according to my theory, variability depends:
‘upon sexual reproduction, while the deterioration of an organ when
disused, no less than its improvement when in use, depends upon
variability. There are therefore two reasons which lead us to
expect that organs which are no longer used will remain aa
reduced in species with asexual reproduction: first, because only,
a very slight degree of hereditary variability can be present, viz.
such a degree as was transmitted from the time when sexual
reproduction was first abandoned by the ancestors; and, secondly,
because even these slight degrees of variability are not combined,
or, in other words, because panmixia cannot occur.

And the facts seem to point in the direction required by the
theory, for superfluous organs do not become rudimentary in
parthenogenetic species. For example, as far as my experience
goes, the receptaculum seminis does not deteriorate, although it is,
_ of course, altogether funetionless when parthenogenesis has become
established. I donot attach much importance to the fact that the
Psychids and Solenobias—(genera of Lepidoptera which Siebold
and Leuckart have shown to include species with parthenogenetic
reproduction)—still retain the complete female sexual apparatus,
because colonies containing males still occasionally occur in these
‘species. Although the majority of colonies are now purely female,
the occasional appearance of males points to the fact that the uni-
sexuality of the majority cannot have been of very long duration.
The process of transformation of the species from a bisexual into
a unisexual form, only composed of females, is obviously in-
complete, and is still in process of development. The case is
similar with several species of Cynipidae, which reproduce by the
parthenogenetic method. In these cases the occurrence of a very
small proportion of males is the general rule, and is not confined to
' single colonies. Thus Adler’ counted 7 males and 664 females in
the common Cynips of the rose.

1 Adler, ‘ Zeitschrift f. wiss. Zool., Bd. XXXV, 1881.

294 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

In some Ostracodes, on the other hand, the males appear to be
entirely wanting: at least, I have tried in vain for years to
discover them in any locality or at any time of the year’.

Cypris vidua and Cypris reptans are such species. Now, although
the transformation of these formerly bisexual species into purely
unisexual female species appears to be complete”, yet the females
still possess a large, pear-shaped receptaculum seminis, with its
long spirally twisted duct, which is surrounded by a thick glandular
layer. This is the more remarkable as the apparatus is very
complicated in the Ostracodes, and retrogressive changes could be
therefore easily detected. Furthermore among insects, in-the genus
Chermes the receptaculum seminis of the females has also remained
unreduced, although the males appear to be entirely wanting, or
at least have never been found, in spite of the united efforts of
several acute observers*. The case is quite different in species which
retain both sexual and parthenogenetic reproduction. Thus, the
summer females of the Aphidae have lost the receptaculum seminis ;
and in these insects sexual reproduction has not ceased, but alter-
nates regularly with parthenogenetic reproduction.

Certainly this proof of the truth of my theory as to the signi-.
ficance of sexual reproduction is far from settling the question: it
only renders the theory highly probable. At present it is im-
possible to do more than this, because we do not yet possess
a sufficient number of facts, for many of them could not have been
sought for until after the theory had been suggested. We are here
concerned with complicated phenomena, into which we cannot
acquire an immediate insight, but can only attain it gradually.

But, nevertheless, I hope to have shown that the theory of

* Compare my paper, ‘ Parthenogenese bei den Ostracoden,’ in ‘ Zool. Anzeiger,’
1880, p. 82. Purely negative evidence, unless on an immense scale, is quite rightly
considered to be of no great value in most cases. But the condition of these animals
renders the accumulation of such evidence unusually easy, because the presence of
males in a colony of Ostracodes can be proved by a very simple indirect test. Thus
if a colony contains any males the receptacula seminis of all mature females are filled
with spermatozoa, and on the other hand we may be quite sure that males are
absent, if after the examination of many mature females, no spermatozoa can be
found in any of their receptacula.

2 We cannot, however, be absolutely certain of this, for it is conceivable that
males may still occur in colonies other than those examined.

* It has now been shown by Blochmann that males appear for a very short time
towards the close of summer, as in the case of Phylloxera.—A.W., 1888.

IN THE THEORY OF NATURAL SELECTION. 295

natural sélection is by no means incompatible with the theory of
‘the continuity of the germ-plasm;’ and, further, that if we
accept this latter theory, sexual reproduction appears in an entirely
new light: it has received a meaning, and has to a certain extent
become intelligible.

The time in which men believed that science could be advanced
by the mere collection of facts has long passed away: we know
that it is not necessary to accumulate a vast number of miscel-
laneous facts, or to make as it were a catalogue of them; but we
know that it is necessary to establish facts which, when grouped
together in the light of a theory, will enable us to acquire a certain
degree of insight into some natural phenomenon. In order to
direct our attention to those new facts which are of immediate
importance, it is absolutely necessary to seek the aid of some
general theory for the arrangement and grouping of those which
we already possess. This has been my object in the present paper.

But it may be perhaps objected that these phenomena are far too
complicated to be attacked at the present time, and that we ought
to wait quietly until the simpler phenomena have been resolved into
their components. It may be asked whether the trouble and
labour involved’in the attempt to solve such questions as heredity
or the transformation of species are not likely to be wasted and
useless.

It is true that we sometimes meet with such opinions, but I
believe that they are based upon a misunderstanding of the method
which mankind has always followed in the investigation of nature,
and which must therefore be founded upon the necessary relations
existing between mankind and nature.

Science has often been compared te an edifice which has been
solidly built by laying stone upon stone, until it has gradually
risen to greater height and perfection. This comparison holds
good up to a certain point, but it leads us to easily overlook the
fact that this metaphorical building does not at any point rest
upon the ground, and that, at least up to the present time, it has
remained floating in the air. Nota single branch of science, not
even Physics itself, has commenced building from below; all
_ branches have begun to build at greater or less heights in the air,
and have then built downwards: and even Physics has not yet
reached the ground, for it is still very uncertain as to the nature of

296 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

matter and force. In no single group of phenomena can we begin
with the investigation of ultimate causes, because at this very point
our means of reasoning stop short. We cannot begin with ulti-
mate phenomena and gradually lead up to those which are more
complicated: we cannot proceed synthetically and deductively,
building up the phenomena from below; but we must as a rule
proceed analytically and inductively, proceeding from above down-
wards.

No one will dispute these statements, but they are often for-
gotten, as is proved by the above-mentioned objection. If we
were only permitted to attack the more complicated phenomena
after gaining a complete insight into the simpler ones, then all
scientists would be physicists and chemists, and not until Physics
and Chemistry were done with should we be permitted to proceed
to the investigation of organic nature. Under these circumstances
we ought not to possess now any scientific theory of medicine ;
for the study of pathological physiology could not be commenced
until normal physiology was completely known and understood.
Yet how great a debt is owing by normal to pathological physio- -
logy! This is an example which enforces the conclusion that it
is not only permissible, but in the highest degree advantageous, for
the different spheres of phenomena to be attacked simultaneously..

Furthermore, if we had been compelled to proceed from the
simple to the complex, what would have become of the Theory of
Descent, the influence of which has advanced our knowledge of
Biology to an altogether immeasurable extent? -

But in this often repeated criticism that we are not yet ready to
attack such complicated phenomena as heredity, is hidden still
another fallacy, for it is implied that facts become less certain in
proportion to the complexity of their causes. But is it less certain
that the egg of an’ eagle developes into an eagle, or that the pecu-
liarities of the father and mother are transmitted to the child, than
that a stone falls to the ground when its support is taken away ?
_ Again, is it not possible to draw a perfectly distinct and certain
conclusion as to the relative quantity of the material basis of
heredity, present in the germ-cells of either parent, from the fact
that: the father and mother possess an equal or nearly equal share
in heredity? But it is really unnecessary to argue in this way:
why should we do more than re-affirm that such a method of pro-

IN THE THEORY OF NATURAL SELECTION. 297

cedure in scientific investigation is the only way by which we can
gradually penetrate the hidden depths of natural phenomena?

No! Biology is not obliged to wait until Physics and Chemistry
are completely finished; nor have we to wait for the investigation -
of the phenomena of heredity until the physiology of the cell is
complete. Instead of comparing the progress of science to a build-
ing, I should prefer to compare it to a mining operation, undertaken
in order to open up a freely branching lode. Such a lode must
not be attacked from one point alone, but from many points
simultaneously. From some of these we should quickly reach the
deep-seated parts of the lode, from others we should only reach its
superficial parts; but from every point some knowledge of the
complex tout ensemble of the lode would be gained. And the more
numerous the points of attack, the more complete would be the
knowledge acquired, for valuable insight will be obtained in every
place where the work is carried on with discretion and perseverance.

But discretion is indispensable for a fruitful result; or, leaving
our metaphor, facts must be connected together by theories, if
science is to advance. Just as theories are valueless without a firm
basis of facts, so the mere collection of facts, without relation and
without coherence, is utterly valueless. Science is impossible with-
out hypotheses and theories: they are the plummets with which
we test the depth of the ocean of unknown phenomena, and thus
determine the future course to be pursued on our voyage of dis-
covery. ‘They do not give us absolute knowledge, but they afford
us as much insight as it is possible for us to gain at the present
time. To go on investigating without the guidance of theories,
is like attempting to walk in a thick mist without a track and
without a compass. We should get somewhere under these cireum-
stances, but chance alone would determine whether we should reach
a stony desert of unintelligible facts or a system of roads leading in
some useful direction; and in most cases chance would decide
against us.

In this sense I trust that the sign-post or compass which I offer
may be accepted. Even though it should be its fate to be replaced
by a better one at a later period, it will have fulfilled its object if
it enables science to advance for even a short distance.

298 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

APPENDICES.

AppEenpix I. FurTHerR CONSIDERATIONS WHICH OPPOSE NAGELI’s
EXPLANATION OF TRANSFORMATION AS DUE TO INTERNAL CAUSES 1,

WueEn I describe Nigeli’s theory of transformation as due to
active causes lying within the organism, as a phyletic foree of
transformation, I do not mean to imply that it is one of those —
mysterious principles which, according to some writers, constitute
the unconscious cause which directs the transformation of species.
Niageli’s idioplasm, which changes from within itself, is conceived as
a thoroughly scientific, mechanically operating principle. This cause
is undoubtedly capable of theoretical conception: the only question
is whether it has any real existence. According to Niageli, the
growing organic substance, the idioplasm, not only represents a
perpetuum mobile rendered possible as long as its substance con-
tinually receives from without the matter and force which are
necessary for continuous growth, but it also represents a per-
petuum variabile due to the action of internal causes*. But this is
just the doubtful point, viz., whether the structure of the idioplasm
itself compels it to change gradually during the course of its growth,
or whether it is not rather the external conditions which compel the
ever slightly varying idioplasm to change in a certain direction by
the summation of small differences. It has been shown above that
we do not gain anything by adopting Niageli’s theory, because the
main problem which organic nature offers for our solution, viz.
adaptation, remains unsolved. Hence this theory does not explain
the phenomena of nature, and I believe that there are also certain
facts which are directly antagonistic to it.

If the idioplasm really possessed the power of spontaneous varia-
bility ascribed to it by Nigeli; if, as a result of its own growth, it
were compelled to undergo gradual changes, and thus to produce
new species, we should expect that the duration of species, genera,

1 Appendix to page 257.
* lL. p. 118.

IN THE THEORY OF NATURAL SELECTION. 299

orders, &e. would be of approximately equal length respectively, at
least in forms of equal structural complexity. The time required
by the idioplasm to undergo. such changes as would constitute
transformation into a new species ought to be always the same at
equal heights in the seale of organization, that is, with equal com-
plexity in the molecular structure of the idioplasm. It appears to_
me to be a necessary consequence of Nigeli’s theory that the causes
of transformation lie solely in this molecular structure of the idio-
plasm. If nothing more than a certain amount of growth, and
consequently a certain period of time during which the organism
reproduces itself with a certain intensity, is required to produce a
change in the idioplasm, then we must conclude that the alteration
in the latter must take place when this certain amount of growth
has been reached, or after this certain period has elapsed. In other
words, the time during which a species exists—from its origin as a
modification of some older species, until its own transformation into
a new one—must be the same in species with the same degree of
organization. But the facts are very far from supporting this con-
sequence of Nigeli’s theory. The duration of species is excessively
variable: many arise and perish within the limits of a single
geological formation, while others may be restricted to a very small
part of a formation; others again may last through several forma-
tions. It must be admitted that we cannot estimate the exact
position of extinct species in the scale of organization, and the
differences may therefore depend upon differences of organization :
or they may be explained by the supposition that certain species
may have become incapable of transformation, and might, under
favourable conditions, continue to exist for an indefinite period.
But this reply would introduce a new hypothesis in direct anta-
gonism to Nigeli’s theory, which assumes that the variability of
idioplasm takes place as the consequence of mere growth, and ne-
cessarily depends upon molecular structure. Niigeli himself asserts
that the essential substance (idioplasm) of the descendants of the
earliest forms of life is in a state of perpetual change, which would
continue even if the series of successive generations were indefinitely
prolonged*. Hence there can be no rest in the process of change
' which the idioplasm must undergo; and this is as true of each
single species as it is of the organic world taken as a whole. We

iL c., p. 118.

300 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

could, perhaps, find shelter in the insufficiency of our geological
knowledge, but the number of ascertained facts is too great for this
to be possible. Thus it is well known that the genus Nautilus has
lasted from Silurian times, through all the three geological periods,
up to the present day; while all its Silurian allies (Orthoceras,
_ Gomphoceras, Goniatites, &c.) became extinct at a comparatively
early period.

A keen and clever controversialist might still bring forward
many objections against such an argument. I do not therefore
place too much dependence upon the geological facts by themselves,
as a disproof of the self-variability of Nigeli’s idioplasm ; for it must
be admitted that the facts are not sufficiently complete for this
purpose. For instance, in the case of Nautilus it might be argued
that we do not know anything about the fossil Cephalopods of
pre-Silurian times, and that it is therefore possible that the above-
mentioned allies of Nautilus may have existed previously for as
long a period as that through which Nautilus has lived in post-
Silurian time. However this may be, it will be at least conceded
that the geological facts do not lend any support to Nigeli’s
theory, for we can see no trace of even an approximately regular
succession of forms.

Apprenpix II. NAGELI’s EXPLANATION OF ADAPTATION 1.

In order to explain adaptation Nigeli assumes that, under certain
circumstances, external influences may cause slight permanent
changes in the idioplasm. If then such influences act continually
in the same direction during long periods of time, the changes in
the idioplasm may increase to a perceptible amount, i. e. to a degree
which makes itself felt in visible external characters*. But such
changes alone could not be considered as adaptations, for the essen-
tial character of an adaptation is that it must be a purposeful
change. Niigeli, however, brings forward the fact that external
stimuli often produce their chief effects at that very part of the
organism to which the stimuli themselves were applied. ‘If the
results are detrimental, the organism: attempts to defend itself
against the stimulus: a confluence of nutrient fluid takes place
towards the part upon which the stimulus has acted, and new fissues

1 Appendix to page 258. 2 1. c., p. 137-

>"

IN THE THEORY OF NATURAL SELECTION. 301

are formed which restore the integrity of the organism by replacing
the lost structures as far as’ possible. Thus in plants the healthy
tissues begin to grow actively around the seat of an injury, tending
to close it up, and to afford protection by impenetrable layers of
cork.’ Purposeful reactions of this kind are certainly common in
the organic world, occurring in animals as well as in plants. Thus
in the human body an injury causes a rapid growth of the surround-
ing tissues, which leads to the closing-up of the wound ; while in the
Salamander even the amputated leg or tail is replaced by growth.
An extreme example of these purposeful reactions is afforded by
the tree-frog (Hy/a), which is of a light-green colour when seated
upon a light-green leaf, but becomes dark brown when transferred
to dark surroundings. Hence this animal adapts itself to the colour
of its environment, and thus gains protection from its enemies.
Admitting this capability on the part of organisms to react under
certain stimuli in a purposeful manner, the question remains
whether such a power is a primitive original quality belonging to
the essential nature of each organism. ‘The power of changing the
colour of the skin in correspondence with that of the surroundings
is not very common in the animal kingdom. In the frog this
power depends upon a highly complex reflex mechanism. Certain
chromatophores in the skin are connected with nerves! which pass
to the brain and are there brought into relation, by means of nerve-
cells, with the nervous centres of the organ of vision. The relation
is of such a kind that strong light falling upon the retina consti-
tutes a stimulus for the production of an impulse, which is conducted,
along the previously mentioned motor nerves, from the brain to
the chromatophores, thus determining the contraction of these
latter and the consequent appearance of a light-coloured skin.
When the strong stimulus (of light) ceases, the chromatophores
expand again, and the skin becomes dark. That the chromato-
phores do not themselves react upon the direct stimulus of light
was proved by Lister?, who showed that blind frogs do not possess
the power of altering their colour in correspondence with that of
their environment. It is quite obvious that in this case we are not
dealing with a primary, but with a secondarily produced character ;

* Compare Briicke, ‘ Farbenwechsel des Chamiileon.’ Wien. Sitzber. 1851. Also
Leydig, ‘Die in Deutschland lebenden Saurier,’ 1872.
* «Philosophical Transactions,’ vol. cxlviii. 1858, pp. 627-644.

802 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

and it has yet to be proved that all the purposeful reactions men-
tioned by Nigeli are not similarly secondary characters or adapta-
tions, and thus very far from being primitive qualities of the
organic substance of the forms in which they occur.

I do not by any means doubt that some of the reactions er
nessed in organisms do not depend upon adaptation, but such
reactions are not usually purposeful. Curiously enough, Nigeli
mentions the formation of galls in plants among his instances of
' purposeful reactions under external stimuli. I think, however,
that it can hardly be maintained that the galls are of any use to —
the plant: on the contrary, they may even be very injurious to it.
The gall is only useful to the insect which it protects and supplies
with food. The recent and most excellent investigations of Adler?
and of Beyerinck* have shown that the puncture made by the
Cynips in depositing its eggs is not the stimulus which produces
the gall, as was formerly believed to be the case, but that such a
stimulus is provided by the larva which developes from the egg.
The presence of this small, actively moving, foreign body stimu-
lates the tissue of the plant in a definite manner, always producing
a result which is advantageous to the larva and not to the plant.
It would be to the advantage of the latter if it killed the in-
truding larva, either enclosing it by woody tissue devoid of nourish-
ment, or poisoning it by some acrid secretion, or simply crushing
it by the active growth of the surrounding tissues. But nothing of
the kind occurs: in fact an active growth of cells (forming the
so-called ‘Blastem’ of Beyerinck) takes place around the embryo,
while it is still enclosed in the egg-capsule ; but the growth is not
such as to crush the embryo, which remains free in the cavity, the
so-called larval chamber, which is formed around it. It would be
out of place to discuss here the question as to how we can conceive
that the plant is thus compelled to produce a growth which is at
any rate indifferent and may be injurious to it ; and which, more-
over, is exactly adapted to the needs of its insect-enemy. But it
is at all events obvious that this cannot be an example of a self-

1 Adler, ‘ Beitriige zur Naturgeschichte der Cynipiden,’ Deutsche entom. Zeitschr.
XXL. 1877, p. 209; and by the same author, ‘ Ueber den Generationswechsel der
Eichen-Gallwespen,’ Zeitschr. f. wiss. Zool., Bd. XXX V. 1880, p. 151.

2 Beyerinck, ‘ Beobachtungen itiber die ersten Entwicklungsphasen einiger Cy-
nipidengallen,’ Verhandl. d. Amsterd. Akad. d. Wiss. Bd. XXII. 1883.

IN THE THEORY OF NATURAL SELECTION, 303

protecting reaction under a stimulus, and that therefore an organism
does not always respond to external stimuli in a manner useful to
itself.

But even if we could accept the suggestion that the purposeful
reaction of an organism under stimulation is a primary and not a
secondarily produced character, such a principle would by no means
suffice for the explanation of existing adaptations. Nigeli attempts
to explain certain selected cases of adaptation as the direct results
of external stimuli. He looks upon the thick hairy coat of mam-
mals in arctic regions, and the winter covering of animals in tem-
perate regions, as a direct reaction of the skin under the influence
of cold. He considers that the horns, claws, and tusks of animals
have arisen directly as reactions under stimuli applied to certain
parts of the surface of the body in attack and defence!. This inter-
pretation is similar to that offered by Lamarck at the beginning of
this century. At first sight such a suggestion appears to be
plausible, for the acquisition of a thick hairy covering by the
mammals of temperate regions is.actually contemporaneous with
the cold season of the year. But the question arises as to whether
the production of a larger number of hairs at the beginning of
winter is not merely another instance of a secondary character, like
the assumption of a green colour by the tree-frog under the stimulus
exerted by strong light.

In the case of the hairy coat it is only necessary to produce a
larger number of structures such as had existed previously; but how
can+ it have been possible for the petals of flowers, with their
peculiar and complex forms, to have been developed from stamens
as a direct result of the insects which visit them in order to obtain
pollen and nectar? How could the creeping of these insects and
the small punctures made by them constitute stimuli for the produc-
tion of an increased rate of growth? And how is it possible in any
way to explain, by mere incréase in growth, the origin of a struc-
ture in which each part has its own distinct meaning and plays
a peculiar part in attracting insects and in the process of cross-
fertilization effected by them? Even if the manifold peculiarities of
form could be explained in this way, how can such an explanation

possibly hold for the colours of flowers? How could the white
- colour of flowers which open at night be explained as the direct

116. p. 144.

304 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

result of the creeping of insects? How can the suggestion of such
a cause offer any interpretation of the fact that flowers which open
by day are tinted with various colours, or of the fact that there is
often a bright or highly coloured spot which shows the way to the
hidden nectary ?

There are, moreover, a large number of very striking adaptations
in form and colour, for which no stimulus acting directly upon the
organism can be found. Can we imagine that the green caterpillar},
plant-bug, or grasshopper, sitting among green surroundings,
is thus exposed to a stimulus which directly produces the green
colour in the skin? Can the walking-stick insect, which resembles
a brown twig, be subject to a transforming stimulus by sitting on
such branches or by looking at them? Or again, if we consider
the phenomena of mimicry, how can one species of butterfly, by
flying about with another species, exercise upon the latter such an
influence as to render it similar to the first in appearance ? In
many cases of mimicry, the mimicked and the mimicking species
do not even live in the same place, as we see in the moths, flies,
and beetles which resemble in appearance the much-dreaded wasps.

The interpretation of adaptation is the weak part of Nageli’s
theory, and it is somewhat remarkable that so acute a thinker
should not have perceived this himself. One very nearly gains
the impression that Nageli does not wish to understand the theory
of natural selection. He says, for instance, in speaking of the
mutual adaptation observable between the proboscis, the so-called
tongue’ of butterflies, and flowers with tubular corolla?:—* Among
the most remarkable and commonest adaptations observable in the
forms of flowers, are the corollas with long tubes considered in re-
lation to the long “ tongues” of insects, which suck the nectar from
the bottom of the long narrow tubes, and at the same time effect
the cross-fertilization of the plant. Both these arrangements have
been gradually developed to their present degree of complexity—
the long-tubed corollas from those without tubes, and from those

{* It is now known that many such caterpillars are actually modified in colour by
their surroundings, but the process appears to be indirect and secondarily acquired by
the operation of natural selection, like that of the change of colour in the chamaeleon,
frogs, fish, eto. ; although the stimulus of light acts upon the eyes of the latter animals
and upon the skin of the caterpillar. See the seventh Essay (pp. 394-397) for a more
detailed account,—E. B. P.]

2 Lc, p. 150.

IN THE THEORY OF NATURAL SELECTION. 805

with short ones, the long “tongues” from short ones. Undoubtedly
both have been developed at the same rate so that the length of
both sets of structures has always remained the same.’

No objection can be raised against these statements, but Nigeli
goes on to say :—‘ But how can such a process of development be
explained by the theory of natural selection, for at each stage in
the process the adaptation was invariably complete. The tube of
the corolla and the “tongue” must have reached, for instance, at
a certain time, a length of 5 or1omm. If now the tube of the
corolla became longer in some plants, such an al#ration would have
been disadvantageous because the insects would be no longer able to
obtain food from them, and would therefore visit flowers with
shorter tubes. Hence, according to the theory of natural selection,
the longer tubes ought to have disappeared. If on the other hand
the “tongue” became longer in some insects, such a change would be
superfluous and should have been given up, according to the same
theory, as unnecessary structural waste. The simultaneous change
in the two structures must, according to the theory of natural
selection, be due to the same principle as that by which Miinch-
hausen pulled himself out of a bog by means of his own pig-tail.’

But, according to the theory of natural selection, the case appears
in a very different light from that in which it is put by Nageli.
The flower and the insect do not compete for the greater length of
their respective organs: all through the gradual process, the flower
is the first to lengthen its corolla and the butterfly follows. Their
relation is not like that between a certain species of animal and
another which serves as its prey, where each strives to be the
quicker, so that the speed of both is increased to the greatest possible
extent in the course of generations. Nor do they stand in the
same relation as that obtaining between an, insectivorous bird and a
certain species of butterfly which forms its principal food ; in such
a case two totally different characters may be continually increased
up to their highest point, e.g. in the butterfly similarity to the
dead and fallen leaves among which it seeks protection when
_ pursued, in the bird keenness of sight. As long as the latter
quality is still capable of increase, so long will it still be advanta-
-geous to any individual butterfly to resemble the leaf a little more
completely than other individuals of the same species; for it will
thus be capable of escaping those birds which possess a rather

x

306 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

keener sight than others. On the other hand, a bird with rather
keener sight will have the greatest chance of catching the better
protected butterflies. It is only in this way that we can explain
the constant production of such extraordinary similarities between
insects and leaves or other parts of plants. At every stage of
growth both the insect and its pursuer are completely adapted to
each other; i.e. they are so far protected and so far successful
respectively, as is necessary to prevent that gradual decrease in the
average number of individuals which would lead to the extermina-
_tion of the species!. But the fact that there is complete adaptation
at each stage does not prevent the two species from increasing
those qualities of protection and of pursuit. upon which they respec-
tively depend. So far from this being the case, they would be
necessarily compelled to gradually increase these qualities so long
as the physical possibility of improvement remained on both sides.
As long as some birds possessed a rather keener sight than those
which previously existed, so long would those butterflies possess an
advantage in which the resemblance to leaf-veining was more dis-
tinct than in others. But from the moment at which the maximum
keenness of eyesight attainable had been reached, at which there-
fore all butterflies resembled leaves so completely that even the
birds with the keenest eyesight might fail to detect them when at
rest,—from this very point any further improvement in the simi-
larity to leaves would cease, because the advantage to be gained from
any such improvement would cease at the same time.

Such reciprocal intensification of adaptive characters appears to
me to have been one of the most important factors in the transfor-
mation of species: it must have persisted through long series of
species during phylogeny: it must have affected the most diverse
parts and characters in the most diverse groups of organisms.

In certain large butterflies of the Indian and African forests
—Kallima paralecta, K. inachis, and K. albofasciata—it has been
frequently pointed out that the deceptive resemblance’ to a leaf is so
striking that an. observer who has received no hint upon the subject
believes that he sees a leaf, even when he is looking at the butter-
fly very closely. The similarity is nevertheless incomplete ; for out

1 In order to make the case as simple as possible, I assume that the insectivorous
bird feeds upon a single species of insect, and that the insect is only attacked by a
single species of bird.

IN THE THEORY OF NATURAL SELECTION, 307

of sixteen specimens in the collections at Amsterdam and Leyden,
I could not find a single one which had more than two lateral veins
on one side of the mid-rib of the supposed leaf, or more than three
upon the other side; while about six or seven veins should have
been present on each side. But from two to three lateral veins are
amply sufficient to produce a high degree of resemblance ; in fact
so much so that it is a matter for wonder as to how it has been
possible for such a relatively perfect copy to have been produced ;
or how the sight of birds can have become so highly developed that
while flying rapidly they could perceive the vein-like markings ; or
to state the case more accurately, that they could detect those indi- ©
viduals with a less number of veins than others. It is possible that
the process of increase in resemblance is still proceeding in the
species of the genus Kallima ; at all events, I was struck by the
rather strong individual differences in the markings of the supposed
leaf.

On the other hand, the cause of the increase in length of the
tubular corolla and of the butterfly’s ‘tongue,’ lies neither in the
flower nor in the butterfly, but it is to be found in those other
insects which visit the flower and steal its honey without being of
any assistance in cross-fertilization. It may be stated shortly, that
non-tubular corollas, with the honey freely exposed—for it must be
assumed the ancestral form was of this kind—gradually developed
into corollas with the honey deeply concealed. The whole process
was presumably first started by the flower, for the gradual with-
drawal of the honey to greater depths conferred the advantage of
protection from rain (Hermann Miiller), while larger quantities of
honey could be stored up, and this would also increase the num-
ber of insects visiting the flower and render their visits more
certain. As soon as this withdrawal occurred, the mouth-parts of
insects began to be subjected to a selective process whereby these
organs in some of them were lengthened at the same rate as that: at
which the honey was withdrawn. When once the process had
begun, its continuance was ensured, for as soon as flower-frequent-
ing insects were divided into two groups with short and with long
mouth-parts respectively, a further increase in the length of the
corolla-tube necessarily took place in all those flowers which were
especially benefited by the assured visits of a relatively small
number of species of insects, viz., those flowers in which cross-

X 2

808. THE SIGNIFICANCE OF SEXUAL REPRODUCTION

fertilization was more certainly performed in this way than by the
uncertain visits of a great variety of species. This would imply
that a still further increase in length would take place, for it is
obvious that the cross-fertilization of any flower would be more eer-
tainly performed by an insect when the number of species of plants
visited by it became less;.and hence the cross-fertilization would
be rendered most certain when the insect became completely
adapted—in size, form, character of its surface, and the manner in
which it obtained the honey—to the peculiarities of the flower.
Those insects which obtain honey from a great variety of flowers
are sure to waste a great part of the pollen by carrying it to the
flowers of many different species, while insects which ean only
obtain honey from a few species of plants must necessarily visit
many flowers of the same species one after the other, and they
would therefore more generally distribute the pollen in an effective
manner.

Hence the tube of the corolla, and the ‘tongue’ of the butterfly
which brings about fertilization, would have continued to increase
in length as long as it remained advantageous for the flower to ex-
clude other less useful visitors, and as long as it was advantageous
for the butterfly to secure the sole possession of the flower. Hence
there is no competition between the flower and the butterfly which
fertilizes it, but between these two on the one side, and the other
would-be visitors of the flower on the other. Further details as to
the advantages which the flower gains by excluding all other
visitors, and the butterfly by being the only visitor of the flower,
and also as to themanifold and elaborate mutual adaptations between
insects and flowers, and as to the advantages and disadvantages
which follow from the concealment of the honey—will be found in
Hermann Miiller’s! work on the fertilization of flowers, in which
all these subjects are minutely discussed, and are clearly explained
in a most admirable manner.

Appenpix III. Apaprations IN PLants’.
It is well known that Christian Conrad Sprengel was the first
to recognise that the forms and colours of flowers are not due to

1 English Edition, translated by D’Arcy W. Thompson, B.A. London, 1883,

P- 509 et seqq.
* Appendix to page 260,

IN THE THEORY OF NATURAL SELECTION. 309

chance, that*they are not the mere sport. of nature, and that they
are not made for the enjoyment of man, but that their purpose
is to attract insects for the performance of cross-fertilization. | It
is also well known that this discovery—which was made at the
end of the last century, and which caused much excitement at that
time—was completely forgotten, and was brought-to light again by
Charles Darwin when attacking the same problem.

In his work entitled ‘The Solution of Nature’s Secret in the
Structure and Fertilization of Flowers’ (‘ Das entdeckte Geheimniss
der Natur im Bau und der Befruchtung der Blumen’), published at
Berlin, in 1793, Sprengel showed, in several hundred cases, that the
peculiarities in the structure and colours of flowers were calculated
to attract insects, and to ensure the fertilization of the flowers by
their instrumentality. But it was due to his successor in this line
of investigation that the whole significance of the cross-fertilization
effected by insects was made clear. Darwin! showed that in many
cases, although not in all, the intention of nature was to avoid
self-fertilization, and he showed that stronger and more numerous
descendants are produced after cross-fertilization.

After Darwin, several investigators, such as Kerner, Delpino
and Hildebrand, have paid further attention to the subject, but it
has been especially studied in a most thorough manner by Her-
mann Miiller *. He looked at the subject from more than one point
of view, and showed by direct observation the species of insects
which effect cross-fertilization in various species of our native
flowers: he also studied the structure of insects in relation to
that of flowers, and attempted to establish the mutual adapta-
tions which exist between them. In this way he succeeded in
throwing mutch light upon the process of transformation in many
species of flowers, and in proving that certain insects, although un-
consciously, are, as it were, breeders of certain forms of flowers. He
not only distinguished the disagreeably smelling, generally in-
conspicuous flowers (‘Ekelblumen’)-produced by Diptera which live
on putrid substances, and the flowers which are produced by butter-
flies ; but he also distinguished the flowers bred by saw-flies, by

* Ch, Darwin, ‘On the fertilization of Orchids by Insects.’ London, 1877.

_ ? Compare Hermann Miiller, ‘Die Befruchtung der Blumen durch Insekten und
die gegenseitigen Anpassungen beider.’ Leipzig, 1873. See also many articles by the

same author in ‘Kosmos,’ and other periodicals. These later articles are included
in the English translation by D’Arcy W. Thompson.

>

310 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

Fossoria, and by bees. He even believes that in certain cases (Viola
calcarata) he can prove that a flower which owed its original form to
being bred by bees, was afterwards adapted to cross-fertilization by
butterflies, when it had migrated into an Alpine region where the
latter insects are far more abundant than the former.

Although there must of course be much that is hypothetical in
the interpretations of the different parts of flowers offered by
Hermann Miiller, the majority of these explanations are certainly
correct, and it is of the greatest interest to be able to recognise the
adaptive character of details, even when apparently unimportant,
in the structure and colours of flowers.

Sachs has offered a very convincing explanation as to the mean-
ing of leaf-veining, and of its significance in relation to the
functions of leaves!. He shows that the venation of a leaf isin
every case exactly adapted for the fulfilment of its purpose. It
has, in the first place, to conduct the nutrient fluid in both diree-
tions, and in the second place to support the thin layers of assimi-
lating chlorophyll cells, and to stretch them out so as to expose as
large a surface as possible to the light; lastly, it has to toughen
the leaf as a protection against being torn. He shows in a very
convincing manner that the whole diversity of leaf venation can be
understood from these three principles. Here, again, we meet with
purposeful arrangements in a class of structures in which it was
formerly thought that there was only a chaos of accidental forms,
or, as it were, the mere sport of nature with form.

ApprnpIx IV. ON THE SUPPOSED TRANSMISSION OF ACQUIRED
. CHARACTERS 2, '

,

When I previously maintained that the proofs of the trans-
mission of artificially produced diseases are inconclusive, I had in
mind the only experiments which, as far as I am aware, can be
adduced in favour of the transmission of acquired characters; viz. _
the experiments of -Brown-Séquard* on guinea-pigs. It is well

1. ‘Lectures on the Physiology of Plants, translated by H. Marshall Ward,
Oxford, 1887, p. 47.

2 Appendix to page 267.

% Brown-Sdquard, ‘Researches on epilepsy; its artificial production in animals
and its etiology, nature, and treatment.’ Boston, 1857. Also various papers by the

same author in ‘Journal de physiologie de l'homme,’ Tome I and III, 1858, 1860,-
and in ‘ Archives de physiologie normale et pathologique,’ Tome I-IV, 1868-1872.

IN THE THEORY OF NATURAL SELECTION. 811

known that’ he produced artificial epilepsy in these animals by
dividing certain parts of the central and also the peripheral nervous
system. The descendants of the animals which acquired epilepsy
sometimes inherited the disease of their parents.

These experiments have been since repeated by Obersteiner ',
- who has described them in a very exact and entirely unprejudiced
manner. The fact itself cannot be doubted: it is certain that some
of the descendants of animals in which epilepsy has been artificially
produced, have also themselves suffered from epilepsy in conse-
quence of the disease of their parents. This fact may be accepted
as proved, but in my opinion we have no right to conclude from it
that acquired characters can be transmitted. Epilepsy is not
a morphological character; it is a disease. We could only speak
of the transmission of a morphological character, if a certain mor-
phological change which was the cause of epilepsy had been pro-
duced by the nervous lesion, and if a similar change had re-appeared
in the offspring, and had produced in them also the symptoms of
epilepsy. But that this really occurs is utterly unproved ; and is
even highly improbable. It has only been proved that many de-_
scendants of artificially epileptic parents are small, weakly, and very
soon die; and that others are paralysed in various parts of the
body, i.e. in one or both of the posterior or anterior extremities ;
while others again exhibit trophic paralysis of the cornea leading
to inflammation and the formation of pus. In addition to these
symptoms, the descendants in very rare cases exhibit upon the
application of certain stimuli to the skin, a tendency towards those
tonic and clonic convulsions together with loss of consciousness
which constitute the features of an epileptic attack. Out of thirty-
two descendants of epileptic parents only two exhibited such symp-
toms, both of them being very weakly, and dying at an early age.

These experiments, although very interesting, do not enable us
to assert that a distinct morphological change is transmitted to
the offspring after having been artificially induced in the parents.
The injury caused by the division of a nerve is not transmitted,
and the part of the brain corresponding to that which was removed
from the parent is not absent from the offspring. The symptoms of
a disease are undoubtedly transmitted, but the cause of the disease
in the offspring is the real question which requires solution. The

1 ‘Oesterreichische medicinische Jahrbiicher.’ Jahrgang, 1875, p. 179.

iene el

312 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

symptoms of epilepsy are by no means invariably transmitted ;
they are in fact absent from the great majority of cases, and the
very small proportion in which they do occur, exhibit the symptoms
of other diseases in addition to those of epilepsy. The offspring
are either quite healthy (thirteen out of thirty cases) or they suffer
from disturbances of the nervous system, such as the above-
mentioned motor and trophic paralysis,—symptoms which are not
characteristic of epilepsy: however in some of the latter epilepsy
is also present. |

If therefore we wish to express the matter correctly we must
not state that epilepsy is transmitted to the offspring, but we must
express the facts in the following manner :—animals which have
been rendered epileptic by artificial means, transmit to some of their
offspring a tendency to suffer from various nervous diseases, viz.
from motor paralysis, to a less degree from sensory, and to a high
degree from trophic paralysis; in rare cases, when the symptoms
of paralysis are very marked, epilepsy is also transmitted.

If we now remember that a considerable number of diseases are
already known to be caused by the presence of living organisms

in the body, and that these diseases may be transmitted from one

organism to another in the form of germs, ought we not to con-
clude from the above-mentioned facts, that the symptoms are due
to an unknown microbe which finds its nutritive medium in the
nervous tissues, rather than to suppose that they are due to
morphological changes, such as a modification of the histological
or molecular structure of certain parts of the nervous system?
At all events, it would be more difficult to understand the trans-
mission of such a structural change, than the passage of a bacillus
into the sperm- or germ-cell of the parent. There is no ascertained
fact which supports the former assumption, but it is very probable
that the transmission of syphilis, small-pox and tuberculosis! is to be
explained by the latter method, although the bacilli have not yet
been detected in the reproductive cells. Furthermore, this method
of transmission has been rigidly proved in the case of the mus-

1 A direct transmission of the germs of disease through the reproductive cells
has lately been rendered, probable in the case of tuberculosis, for the bacilli have
been found in tubercles in the lungs of an eight-months’ foetal calf, the mother being
affected at the time with acute tuberculosis, However it is not impossible that
infection may have arisen through the placenta. See ‘ Fortschritte der Medicin,’
Bd. III, 1885, p. 198.

IN THE THEORY OF NATURAL SELECTION. 313

cardine disease of the silkworm. At all events we can understand
in this way how it happened that the offspring of artificially
epileptic guinea-pigs were affected with various forms of nervous
disease, a fact which would be quite unintelligible if we assume
the occurrence of a true hereditary transmission of .a morpho-
logical character, such as a pathological change in the structure of _
some nervous centre. |

The manner in which artificial epilepsy becomes manifest after
the operation, is also in favour of the explanation offered above.
In the first place epilepsy does not result from any one single
injury to the nervous system, but it may follow from a variety
of different injuries. Brown-Séquard produced it by removing
a portion of the grey matter of the brain, and by dividing
the spinal cord, although the disease also resulted from a trans-
verse section through half of the latter organ, or from the section of
its anterior or posterior columns alone, or from simply puncturing
its substance. The most striking effects appeared to follow when
the spinal cord was injured in the region between the eighth
dorsal and the second lumbar vertebrae, although the results were
sometimes also produced by the injury of other parts. Epilepsy
also followed the division of the sciatic nerve, the internal popliteal,
and the posterior roots of all nerves which pass to the legs. The
disease never appears at once, but only after the lapse of some
days or weeks, and, according to Brown-Séquard, it is impossible
to conclude that the disease will not follow the operation until
after six or eight weeks have passed without an epileptic attack.
Obersteiner did not witness in any case the first symptoms of the
disease for several days after the division of the sciatic nerve.
After the operation, sensibility decreases over a certain area on
the head and neck, on the same side as the injury.. If the animal
be pinched in this region (which is called the epileptic area, ‘ zone
epileptogéne ’) it curves itself round towards the injured side, and
violent scratching movements are made with the hind leg of
the same side. After the lapse of several days or even weeks,
these scratching movements which result from pinching in the
above-mentioned area, form the beginning of a complete epileptic
attack. Hence the changes immediately produced by the division
of a nerve are obviously not the direct cause of epilepsy, but they
only form the beginning of a pathological process which is con-

314 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

ducted in a centripetal direction from the nerve to some centre
which is apparently situated in the pons and medulla oblongata,
although, according to others!, it is placed in the cortex of the
cerebrum. Nothnagel? considers that certain changes, the nature
of which is still entirely unknown, but which may be histological
or perhaps solely molecular in character, must be produced, leading
to an increased irritability of the grey matter of the centres con-
cerned.

Nothnagel thinks it possible or even probable that in those
cases in which the division of nerves is followed by epilepsy, a
neuritis ascendens—an inflammation passing along the nerves in a
central direction—is the cause of the changes suggested by him
in the epileptic centre. All our knowledge of bacteria and of the
pathological processes induced by them, seems to indicate that such
a neuritis ascendens, as is assumed by Nothnagel, would render
important support to the hypothesis that the artificial epilepsy is
due to infection. But when we further consider that the offspring
of artificially epileptic animals may themselves become epileptic,
although in most cases they suffer from a variety of other nervous
diseases (in consequence of trophic paralysis), I hardly see how the
facts can be rendered intelligible except by supposing that in these
cases of what I may call traumatic epilepsy, we are dealing with
an infectious disease caused by microbes which find their nutritive
medium in the nervous tissues, and which bring about the trans-
- mission of the disease to the offspring by penetrating the ovum or
the spermatozoon.

Obersteiner found that the offspring were more frequently dis- ~

eased when the mother was epileptic, rather than the father. This
is readily intelligible when we remember that the ovum contains
an immensely larger amount of substance than the spermatozoon,
and can therefore be more frequently infected by microbes and can
contain a greater number of them.

Of course, I do not mean to assert that epilepsy always depends
upon infection, or upon the presence of microbes in the nervous
tissues. Westphal produced epilepsy in guinea-pigs by striking

1 Compare Unvericht, ‘Experimentelle und klinische Untersuchungen iiber die
Epilepsie.’ Berlin, 1883. With regard to the question of hereditary transmission,
the part of the brain in which the epileptic centre is placed is of no importance.

? Compare Ziemssen’s ‘Handbuch der spec. Pathologie und Therapie.’ Bd. XII.
2. Hiilfte; Artikel ‘Epilepsie und Eklampsie.’ Leipzig, 1877.

: -
re

IN THE THEORY OF NATURAL SELECTION. 315

them once or twice sharply upon the head: the epileptic attack
took place immediately and was afterwards repeated. It is obvious
that the presence of microbes can have nothing to do with such an
attack, but’ the shock alone must have caused morphological and
functional changes in the centres of the pons and medulla oblongata,
identical with those produced by microbes in the other cases.
Nothnagel also distinctly expresses the opinion that epilepsy ‘ does
not depend upon one uniform and invariable histological change,
but that the symptoms which constitute the disease may in all pro-
bability be caused by various anatomical alterations, provided that
they take place in parts of the pons and medulla which are mor-
phologically and physiologically equivalent?” Just as a sensory
nerve produces the sensation of pain under various stimuli, such as
pressure, inflammation, infection with the poison of malaria, etc.,
so various stimuli might cause the nervous centres concerned to
develope the convulsive attack which, together with its after-effects,
we call epilepsy. In Westphal’s case, such a stimulus would be
given by a powerful mechanical shock, in Brown-Séquard’s experi-
ments, by the penetration of microbes.

However, quite apart from the question of the validity of this
suggestion, we can form no conception as to the means by which
an acquired morphological change in certain nerve-cells—a change
which is not anatomical, and probably not even microscopical, but
purely molecular in nature—can be possibly transferred to the
germ-cells: for this ought to take place in such a manner as to
produce in their minute molecular structure a change which, after
fertilization and development into a new individual, would lead to
the reproduction of the same epileptogenic molecular structure of
the nervous elements in the grey centres of the pons and medulla
oblongata as was acquired by the parent. How is it possible for all
this to happen? What substance could cause such a change in the
resulting offspring after having been transferred to the egg or sperm-
cell? Perhaps Darwin’s gemmules may be suggested; but each
gemmule represents a cell, while here we have to do with molecules
or groups of molecules. We must therefore assume the existence
of a special gemmule for each group of molecules, and thus the
innumerable gemmules of Darwin’s theory must be imagined as
increased by many millions. But if we suppose that the theory

2 1.6. ps 350.

316 THE SIGNIFICANCE OF SEXUAL REPRODUCTION.

of pangenesis is right, and that the gemmules really cireulate in
the body, accompanied by other gemmules from the diseased parts
of the brain, and that some of these latter pass into the germ-
cells of the individual,—to what strange results would the further
pursuit of this idea lead? What an incomprehensible number of
gemmules must meet in a single sperm- or germ-cell, if each of
them is to contain a representative of every molecule or group
of molecules which has formed part of the body at each period of
ontogeny. And yet such is the unavoidable consequence of the
supposition that acquired molecular states of certain groups of cells
can be transmitted to the offspring. This supposition could only be
rendered intelligible by some theory of preformation*, such as Dar-
win’s pangenesis; for the latter theory certainly belongs to this
category. We must assume that each single part of the body at
each developmental stage is, from the first, represented in the germ-
cell as distinct particles of matter, which will reproduce each part
of the body at its appropriate stage as their turn for development
arrives.

I will only briefly indicate some of the inevitable contradictions
in which we are involved by such a theory. One and the same
part of the body must be represented in the germ- or sperm-cell
by many groups of gemmules, each group corresponding to a
different stage of development; for if each part gives off gem-
mules, which ultimately reproduce the part in the offspring, it is
clear that special gemmules must be given off for each stage in
the development of the part, in order to reproduce that identical
stage. And Darwin quite logically accepts this conclusion in his
provisional hypothesis of pangenesis. But the ontogeny of each
part is in reality continuous, and is not composed of distinct and

1 Tt is generally known that the earlier physiologists believed in what was called
the ‘ evolutionary theory,’ or the ‘ theory of preformation,’ This assumes that the
germ contains, in a minute form, the whole of the fully-developed animal. All the
parts of the adult are preformed in the germ, and development only consists in the
growth of these parts and their more perfect arrangement, This theory was generally
accepted until the middle of the last century, when Kaspar Friedrich Wolff brought
forward the theory of ‘ epigenesis,’ which since that time has been the dominant one.
This assumes that no special parts of the germ are preformations of certain parts of
the fully-developed animal, and that these latter arise by a series of changes in the
germ, which gradually gives rise to them. In modern times the theory of preforma-
tion has been revived in a less crude form, as is shown by the ideas of Niigeli, and
by Darwin’s ‘ pangenesis.’—A. W., 1888.

IN THE THEORY OF NATURAL SELECTION. 317

separate stages. We imagine these stages as existing in the con-
tinuous course of ontogeny; for here, as in all departments of
nature, we make artificial divisions in order to render possible a
general conception, and to gain fixed points in the continuous
changes of form which have in reality occurred. Just as we dis-
tinguish a sequence of species in the course of phylogeny, although
only a gradual transition, not traversed by sharp lines of demar-
cation, has taken place, so also we speak of the stages of ontogeny,
although we can never point out where any stage ends and another
begins. To imagine that each single stage of a part is present
in the germ, as a distinct group of gemmules, seems to me to be a
childish idea, comparable to the. belief that the skull of the young
St. Laurence exists at Madrid, while the adult skull is to be found
in Rome.

We are necessarily driven to such conceptions if we assume that
the transmission of acquired characters takes place. A theory of
preformation alone affords the possibility of an explanation: an
epigenetic theory is utterly unable to render any assistance in
reaching an interpretation. According to the latter theory, the
germ does not contain any preformed gemmules, but it possesses,
as a whole, such a chemical and molecular constitution that
under certain circumstances, a second stage is produced from
it. For example, the two first segmentation spheres may be re-
garded as such a second stage; these again possess such a con-
stitution that a certain third stage, and no other, can arise from
them, forming the four first segmentation spheres. At each of
these stages the spheres produced are peculiar to a distinct species
and a distinct individual. From the third stage a fourth arises,
and so on, until the embryo is developed, and still later the mature
animal which can reproduce itself. No one of the parts of such
an animal was originally present as distinct parts in the ege
from which it was developed, however minute we may imagine
these parts to be. If now an inherited peculiarity shows itself in
any organ of the mature animal, this will be the consequence of
the preceding developmental stages, and if we were able to inves-
tigate the molecular structure of all these stages as far back as

_ )the egg-cell, we should trace back to the latter some minute
/ difference of molecular constitution which would distinguish it
from any other egg-cell of the same species, and was destined

318 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

to be the cause of the subsequent appearance of the peculiarity
in the mature animal. It is only by the aid. of some such hypo-
thesis that we can conceive the cause of hereditary individual dif-
ferences and the tendencies towards hereditary diseases. Hereditary
epilepsy would be intelligible in this way, that is, when the disease
is congenital and not due to the presence of microbes, as is pre-
sumably the case with artificially induced epilepsy.

_ The question now arises as to whether we can conceive the
communication of such traumatic and therefore acquired epilepsy
to the germ-cells. This is obviously impossible under the epi-
genetic theory of development described above. In what way
can the germ-cells be affected by molecular or histological changes
in the pons varolii and medulla oblongata? Even if we assume,
for the sake of argument, that the central nervous system exercises
trophic influences upon the germ-cells, and that such influences
may consist of something more than variations in nutritive con- -
ditions, and may even include the power of altering the molecular
constitution of the germ-plasm in spite of its usual stability; even
if we concede these suppositions, how is it conceivable that the
_ changes produced would be of the exact nature and in the exact
direction necessary in order to confer upon the germ-plasm the
molecular structure of the first ontogenetic stage of an epileptic
individual? How can the last ontogenetic stage of the ganglion
cells in, the pons and medulla of such an individual, stamp upon
the germ-plasm in the germ-cells of the same animal—not indeed
the peculiar structure of the stage itself—but such a molecular
constitution as will ensure the ultimate appearance of epilepsy
in the offspring? The theory of epigenesis does not admit that
the parts of the full-grown individual are contained in the germ as
preformed material particles, and therefore this theory cannot allow
that anything is added to the germ-plasm; but in accepting the
above-made supposition, we are compelled to assume that the mole-
eular structure of the whole of the germ-plasm is changed to a
slight extent.

Nigeli is quite right in maintaining that the solid protoplasm
alone, as opposed to the fluid part, i.e. that part of the protoplasm
which has passed into solution, can act as the bearer of hereditary
tendencies. This appears to be undoubtedly proved by the fact that
the amount of material provided by the male parent for the de-

IN THE THEORY OF NATURAL SELECTION. 319

velopment of an embryo is in almost all animals far smaller than
the amount provided by the female parent.

In Mammalia the share contributed by the father probably only
forms about one hundred-billionth part of that contributed by the
mother, and yet nevertheless the influence of the former in he-
redity is on an average equal to that exerted by the latter’. Now,
from the point of view of epigenesis, no molecule of the brain of
an epileptic animal can reach the germ-cell except in a state of
solution, and therefore no direct increase in the germ-plasm can
be referred to such molecules, quite apart from the fact that such
addition, even if possible, could not be of any value, because the
last stage of the epileptic tendency must be represented in the
nerve-cells and nerve-fibres of the diseased brain, while the first
stage ought to be represented in the germ-cell.

It may be safely asserted that according to the theory of epigenesis
the germ-cells cannot be influenced except as regards their nutri-
tion. Nutritive changes may be imagined to occur through the
varying trophic influence of the nervous system upon the sexual
organs, but the structure of the germ-plasm cannot be altered by
mere nutritive changes, or at all events it cannot be altered in
that distinct and definite direction which is required by the sup-
posed transmission of acquired epilepsy.

Thus the transmission of artificially produced epilepsy can neither
be explained upon the epigenetic theory, nor upon the theory of
preformation ; it can only be rendered intelligible if we suppose
that the appearance of the disease in the offspring depends upon
the introduction and presence of living germs, viz. of microbes.
The supposed transmission of this artificially produced disease is ©
the only definite instance which has been hitherto brought forward
in support of the transmission of acquired characters. I believe
that I have shown that such support is deceptive, not because there
is any uncertainty about the fact of the transmission itself, but
because it is a transmission which cannot depend upon heredity,
and is in all probability due to infection.

Ever since I began to doubt the transmission of acquired cha-
racters, I have been unable to meet with a single instance which
_ could shake my conviction. There were many instances in which
hereditary transmission was clearly established, but in none of them

1 Nageli, 1. c. p. r10.

320 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

was there any reason to suppose that the characters transmitted were
really acquired. For example, Fritz Miiller has recently informed
me of an instance in which he believes that there can be no doubt
of the transmission of acquired characters. His observations are
so interesting in several respects that I will quote them here. He
says in his letter, ‘Among the bastards of two species of Adutilon,

in which I had never observed hexamerous flowers, there was |

a single plant with a few such blossoms. As these flowers are
sterile with the pollen of the same plant, I was obliged to fer-
tilize it with pollen from another plant bearing only pentamerous
flowers, in order to obtain seeds from the former. For three weeks
I examined all the flowers from a plant grown from such seed,
finding 145 pentamerous, 103 hexamerous, and 13 heptamerous
flowers. I examined similarly the flowers of another plant pro-
duced from seed obtained from pentamerous flowers from the same
parent plants. "There were 454 pentamerous and 6 hexamerous
flowers, and hence only 1°3 per cent. of the latter kind.’

It must certainly be admitted that the large proportion of ab-
normal hexamerous flowers depends upon heredity in the instance
first quoted; but the hexamerous condition is not an acquired

character ; it is merely the first appearance of a new innate»

character. It is not due to the reaction of the vegetable organism

under some external stimulus, for it appeared in a plant exposed to.

conditions similar to those which acted upon the other plant which
only produced the normal pentamerous flowers. It must therefore
have resulted from the tendencies which were present in the germ
from which the plant itself developed, either as a spontaneous
change in the germ-plasm or through the combination of two
parental germ-plasms—a combination which may lead to the
appearance or the reality of a new character. We know that the
germ-plasm of each individual is not a simple substance, but pos-
sesses a very complex composition, for it consists of a number of
ancestral germ-plasms represented in very different proportions.
‘Now, although we cannot learn anything directly about the pro-
cesses of growth of the germ-plasm, and its resulting ontogenetic
stages, yet we do know, chiefly from observations upon man, that
the characters of ancestors appear in the offspring in very different
combinations and in very different degrees of strength. This may,
perhaps, be explained by assuming that in the union of parental

IN THE THEORY OF NATURAL SELECTION. 821

germ-plasms which takes place at fertilization, the contained an-
cestral germ-plasms unite in different ways, and thus come to grow
with different strengths. Certain ancestral germ-plasms will meet
and together produce a double effect: other opposed germ-plasms
will neutralize each other ; and between these two extremes all in-
termediate conditions will occur. And these combinations will not
only take place at fertilization, but also at every stage of the whole
ontogenetic history, for each stage is represented by its idioplasm,
which is itself composed of ancestral idioplasms.

We do not yet know enough to be able to prove in detail
the manner in which new characters may arise from such a com-
bination of different kinds of germ-plasm. And yet it appears to
me that such a view, e. g¢. in the case of the variation of buds, is by
far the most natural. There is indeed a single example in which
we can, to some extent, understand how it is that a new character
may arise by these means. Certain canary-birds have a tuft of
feathers on the head, but if two such birds are paired, their
descendants are generally bare-headed, instead of having larger
tufts’. The formation of a tuft depends upon the fact that the
feathers are scanty and in fact absent from part of the skin of the
head. Now when the scanty plumage of both parents is combined
in the offspring the latter is bare-headed. Hence by the com-
bination of ancestral characters a new character (bare-headedness) is
produced, and one which is hardly likely to have ever occurred in
the ancestors of existing canaries.

We do not know the causes which have been in operation when
a flower possesses one petal more than the usual number, any more
than we can explain why it is that one star-fish has five and
another six rays. We cannot unravel the details of the mysterious
relationship between two parent germ-plasms, each of which is
composed. of a countless number of ancestral germ-plasms from the
first and second back to the wth degree. But we can neverthe-
less maintain in a general way that such irregularities are the
result of this complex struggle between the germ-plasms in the
ovum and the idioplasms in the subsequent stages of the de-
veloping organism, and that they are not the result of external
influences.

* See Darwin, ‘The Variation of Animals and Plants under Domestication.” 1875.
Vol. I. p. 311.

Y

322 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

If, however, acquired characters are brought forward in con-
nexion with the question of the transformation of species, the term
‘acquired’ must only be applied to those characters which do not
arise from within the organism, but which arise as the reaction of
the organism under some external stimulus, most commonly as the
\ consequence of the increased or diminished use of an organ or part.
We have then to learn whether the altered conditions of life, by
forcing an organism to adopt new habits, can by such means lead
directly, and not indirectly through natural selection, to the
transformation of the species ; or whether the effects of increased
or diminished use of certain parts, implied by the new habits, are.
restricted to the individual itself, and therefore powerless to effect
any direct modification of the species.

Fritz Miiller’s observation is also interesting in another re-
spect: it appears to controvert my views upon heredity as expressed
in the theory of the continuity of the germ-plasm. If a single
flower can transmit to its descendants special peculiarities which
were not possessed by its ancestors, we seem to be driven to the
conclusion that the ancestral germ-plasm has not passed into the
flower in question, but that new germ-plasm has been formed,
inasmuch as the new characters are derived from the flower itself,
and not from any of its ancestors. I think, however, that the
observation admits of another interpretation: a specimen of Abu-
tilon with many hundred flowers is not a single individual, but a
colony consisting of numerous individuals which have arisen by
budding from the first individual developed from the seed.

I have not hitherto considered budding in relation to my
theories, but it is obvious that it is to be explained from my point
of view, by supposing that the germ-plasm which passes on into a
budding individual consists not only of the unchanged idioplasm of
the first ontogenetic stage (germ-plasm), but of this substance altered,
so far as to correspond with the altered structure of the individual
which arises from it—viz. the rootless shoot which springs from the
stem or branches. The alteration must be very slight, and perhaps
quite insignificant, for it is possible that the differences between the
secondary shoots and the primary plant may chiefly depend upon
the changed conditions of development, which takes place beneath
the earth in the latter case, and in the tissues of the plant in
the former. Thus we may imagine that the idioplasm, when it

IN THE THEORY OF NATURAL SELECTION. 823

developes into a flowering shoot, produces at the same time. the
germ-cells which are found in the latter. We thus approach an
understanding of Fritz Miiller’s observation ; for if the whole shoot
which produces the flower arises from the same idioplasm which
also forms its germ-cells, we can readily understand why the latter
should contain the same hereditary tendencies which were previously
expressed in the flower which produced them. The fact that varia-
‘tions may occur in a single shoot depends upon the changes
‘explained above, which occur in the idioplasm during the course
of its growth, as a result of the varying proportions in which the
ancestral idioplasms may be contained in it.

Fritz Miller’s observation affords a beautiful confirmation of
this view, for if the flower itself transmitted the hexamerous
condition to its germ-cells, we could not understand why some of
the extremely rare hexamerous flowers were produced by the cross-
ing of two pentamerous flowers, in the control experiment. An
explanation of this fact can only be found in the assumption that
the germ-plasm contained in the mother plant, during its growth
and consequent distribution through all the branches of the colony,
became arranged into a combination of idioplasms, which, whenever
it predominated (as it did at certain places), necessarily led to the
formation of hexamerous flowers. I will not consider here the
question as to whether this combination is to be looked upon as an
instance of reversion, or whether it represents something new. Such
a question is of no importance for our present purpose; but the
hexamerous flowers. of the control experiment prove, in my opinion,
that germ-plasm containing the requisite combination was dis-
tributed in the mother plant and also existed, but in insufficient
amount, in shoots which did not produce any hexamerous flowers.

Appendix V. On tHe OriGin or ParrnEenocEnssis |.

The transformation of heterogeny into pure parthenogenesis has
obviously been produced by other causes as well as by those mentioned
in the main part of this paper: Other and quite different circum-
stances have also had a share in its production. Pure parthenogenesis
may be produced without the intermediate condition of heterogeny.

* Appendix to page 290.
Y2

824 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

Thus, for example, the pure and exclusive parthenogenesis with
which the large Phyllopod crustacean, Apus, is reproduced at most,
of its habitats, has not arisen from the loss of previously existent
sexual generations, but simply from the non-appearance of males,
accompanied by the simultaneous acquisition of the power, on the
part of the females, of producing eggs which do not require
fertilization. This is proved by the fact that males occur in certain
scattered colonies of this species, and sometimes they are even
present in considerable numbers. But even if we were not aware
of these facts, the same conclusions might nevertheless have been
drawn from the fact that Apus produces eggs of only one form
—viz. resting eggs with hard shells. In every case in which par-
thenogenesis has been first introduced in alternation with sexual
reproduction, the resting eggs are produced by the latter genera-
tions, while the parthogenetic generations produce eggs with thin
shells, in which the embryo developes and hatches very rapidly.
In this way parthenogenesis leads to a rapid increase of the colony.
In Apus such increase in the number of individuals is gained in an
entirely different manner, viz. by the fact that all the animals
become females, which produce eggs at a very early age, and con-
tinue producing them in increasing fertility for the whole of their
life. In this manner an enormous number of eggs collects at the
bottom of the pool inhabited by the colony, so that after it has
dried up, in spite of loss from various destructive agencies, there
will still remain a sufficiency of eggs to reproduce a numerous
colony, as soon as the pool has filled again.

This form of parthenogenetic reproduction is especially well
suited to the needs of species inhabiting small pools which entirely
depend upon rain-fall, and which may disappear at any time. In
these cases the time during which the colony can live is often too
short to permit the production of several generations even from
rapidly developing summer-eggs. Under these circumstances the
' pool would often suddenly dry up before the series of parthenogenetic
generations had been run through, and hence before the appear-
ance of the sexual generation and resting eggs. In all such cases
the colony would be exterminated.

This consideration might lead us to think that Crustacea, such
as the Daphnidae, which develope by means of heterogeny, would
hardly be able-to exist in small pools filled by the rain; but here

IN THE THEORY OF NATURAL SELECTION. 825

also nature has met the difficulty by another adaptation. As I
have shown in a previous paper+, the heterogeny of the species of
Daphnidae which inhabit such-pools is modified in such a manner,
that only the first generation produced from the resting eggs
consists of purely parthenogenetic females, while the second includes
many sexual animals, so that resting eggs are produced and laid,
and the continuance of the colony is secured a few days after it has
been first founded ; viz. after the appearance of the first generation.

But it is also certain that in the Daphnidae, heterogeny may
pass into pure parthenogenesis by the non-appearance of the sexual
generations. This seems to have taken place in certain species of
Bosmina and Chydorus, although perhaps only in those colonies of
which the continuance is secured for the whole year; viz. those
which inhabit lakes, water-pipes, or wells in which the water
cannot freeze. In certain insects also (e.g. Rhodites rosae) pure
parthenogenesis seems to be produced in a similar manner, by the
non-appearance of males.

But the utility which we may look upon as the cause of partheno-
genesis is by no means so clear in all cases. Sometimes, especially
in certain species of Ostracoda, its appearance seems almost like a
mere caprice of nature. In this group of the Crustacea, one species
may be purely parthenogenetic, while a second reproduces itself
by the sexual method, and a third by an alternation of the two
methods: and yet all these species may be very closely allied and
may frequently live in the same locality and apparently with the
same habit of life. But it must not be forgotten that it is only
with the greatest difficulty that we can acquire knowledge about
_ the details of the life of these minute forms, and that where we can
only recognize the appearance of identical conditions, there may be
highly important differences in nutrition, habits, enemies and the
means by which they are resisted, and in the mode by which
the prey is captured—circumstances which may place two species
living in the same locality upon an entirely different basis of
existence. It is not merely probable that this is the case ; for the
fact that certain species have modified their modes of reproduction.
is in itself a sufficient proof of the validity of the conclusions which
' have just been advanced.

1 Weismann, ‘ Naturgeschichte der Daphnoiden,’ Zeitschrift f. wiss. Zool. XXIII.
1879.

826 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

The fact that different methods of reproduction may obtain in

different colonies of the same species, although with thoroughly
identical habits, may depend upon differences in the external con-
ditions (as in Bosmina and Chydorus mentioned above), or upon the
fact that the transition from sexual to parthenogenetic reproduction
is not effected with the same ease and rapidity in all the colonies
of the same species. As long as males continue to make their
appearance in a colony of Apus, sexual reproduction cannot wholly
disappear. Although we are unable to appreciate, with any degree
of certainty, the causes by which sex is determined, we may never-
theless confidently maintain that such determining influences may
be different in two widely separated colonies. As soon, however, as
parthenogenesis becomes advantageous to the species, securing its
existence more efficiently than sexual reproduction, it will not only
be the case that the colonies which produce the fewest males will gain
advantage, but within the limits of the colony itself, those females
-will gain an advantage which produce eggs that can develope without
fertilization. When the males are only present in small numbers, it
must be very uncertain whether any given female will be fertilized :
if therefore the eggs of such a female required fertilization in order
to develope, it is clear that there would be great danger of entire failure
in this necessary condition. In other words:—as soon as any females
begin to produce eggs which are capable of development without
fertilization, from that very time a tendency towards the loss of
sexual reproduction springs into existence. It seems, however, that
the power of producing eggs which can develope without fertiliza-
tion is very widely distributed among the Arthropoda.

Aprpenpix VI. W. K. Brooks’ TuHrory or Herepiry?.

The only theory of heredity which, at any rate in one point,
agrees with my own, was brought forward two years ago by W. K.
Brooks of Baltimore*. The point of agreement lies in the fact that
Brooks also looks upon sexual reproduction as the means employed
by nature in order to produce variation. The manner in which he
supposes that the variability arises is, however, very different from

1 Appendix to page 277.

* Compare W. K. Brooks, ‘ The Law of Heredity, a Study of the Cause of Variation,
and the Origin of living Organisms.’ Baltimore, 1883.

IN THE THEORY OF NATURAL SELECTION. 827

that suggested in my theory, and our fundamental conceptions are
also widely divergent. While I look upon the continuity of the
germ-plasm as the foundation of my theory of heredity, and there-
fore believe that permanent hereditary variability can only have
arisen through some direct change in the germ-plasm effected by
external influences, or following from the varied combinations which
are due to the mixture of two individually distinct germ-plasms
at each act of fertilization, Brooks, on the other hand, bases his
theory upon the transmission of acquired characters, and upon the
idea which I have previously called ‘the cyclical development of
the germ-plasm.’

Brooks’ theory of heredity is a modification of Darwin’s pan-
genesis, for Brooks also assumes that minute gemmules are thrown
off by each cell in the body of the higher organisms; but such
gemmules are not emitted always, and under all circumstances,
but only when the cell is subjected to unaccustomed conditions.
During the persistence of the ordinary conditions to which it is
adapted, the cell continues to perform its special functions as part
of the body, but as soon as the conditions of life become unfavour-
able and its functions are disturbed, the cell ‘throws off minute
particles which are its germs or gemmules.’

These gemmules may then pass into any part of the organism ;
they may penetrate the ova in the ovary, or may enter into a bud,
but the male germ-cells possess a special power of attracting them
and of storing them up within themselves.

_ According to Brooks, variability arises as a consequence of the
fact that each gemmule of the sperm-cell unites, during fertiliza-
tion, with that part of the ovum which, in the course of develop-
ment, is destined to become a cell corresponding to that from .
which the gemmule has been derived.

Now, when this cell developes in the offspring, it must, as a
hybrid, have a tendency to vary. The ova themselves, as cells,
are subject to. the same laws; and the cells of the organism will
continue to vary until one of the variations is made use of by
natural selection. As soon as this is the case, the organism
becomes, zpso facto, adapted to its conditions ; and the production
of gemmules ceases, and with it the manifestation of variability
itself, for the cells of the organism then derive the whole of their
qualities from the egg, and being no longer hybrid, have no

‘

828 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

tendency to vary. For the same reason the ova themselves will
also cease to vary, and the favourable variation will be transmitted
from generation to generation in a stereotyped succession, until
unfavourable conditions arise, and again lead to a fresh disposition
to vary.

In this way Brooks! attempts to mediate between Darwin and
Lamarck, for he assumes, on the one hand, that external influences
render the body or one of its parts variable, while, on the other
hand, the nature of the successful variations is determined by
natural selection. There is, however, a difference between the
views of Brooks and Darwin, although not a fundamental difference.
Darwin also holds that the organism becomes variable by the opera-
tion of external influences, and he further assumes that changes
acquired in this way can be communicated to the germ and trans-
mitted to the offspring. But according to his hypothesis, every
part of the organism is continually throwing off gemmules which
may be collected in the germ-cells of the animal, while, according
to Brooks, this only takes place in those parts which are placed
under unfavourable conditions or the function of which is in some
_ way disturbed. In this manner the ingenious author attempts to
diminish the incredible number of gemmules which, according to
Darwin’s theory, must collect in the germ-cells. At the same time
he endeavours to show that those parts must always vary which
are no longer well adapted to the conditions of life.

I am afraid, however, that Brooks is confounding two things
which are in reality very different, and which ought. necessarily
to be treated separately if we wish to arrive at correct conclusions :
viz., the adaptation of a part of the body to the body itself, and
its adaptation to external conditions. The first of these adapta-
tions may exist without the second. How can those parts become
variable which are badly adapted to the external conditions, but
are nevertheless in complete harmony with the other parts of the
body ? If the conditions of life, of the cells which constitute the
part in question must\become unfavourable, in order that the
gemmules which produce variation may be thrown off, it is obvious
that such a result would not occur in the case mentioned above.
Suppose, for example, that the spines of a hedgehog are not suffi-
ciently long or sharply pointed to afford protection to the animal,

1 Lio pe 8ai

IN THE THEORY OF NATURAL SELECTION. 329

~ how could such an unfavourable development afford the occasion
for the throwing off of gemmules, and a resulting variability
of the spines, inasmuch as the epidermic tissue in which these
structures arise, remains under completely normal and favourable
conditions, whatever length or sharpness the spines may attain ?
The conditions of the epidermis are not unfavourably. affected
because, as the result of short and blunt spines, the number of
hedgehogs is reduced to far below the average. Or consider the
ease of a brown caterpillar which would gain great advantage by
becoming green ; what reason is there for believing that the cells
of the skin are placed in unfavourable conditions, because, in
consequence of the brown colour, far more caterpillars are detected
by their enemies, than would have been the case if the colour
were green? And the case is the same with all adaptations.
Harmony between the parts of the organism is an essential con-
dition for the existence of the individual. If it is wanting, the
individual is doomed ; but such harmony between any one part and
all others, i.e. proper nutrition for each part, and adequate per-
formance of its proper function, can never be disturbed by the
fact that the part in question is insufficiently adapted to the outer
conditions of life. According to Darwin, all the cells of the body
are continually throwing off gemmules, and against such an
assumption no similar objection can be raised. It can only he
objected that the assumption has never been proved, and that it
is extremely improbable.

A further essential difference between Darwin’s theory of
pangenesis and Brooks’ hypothesis lies in the fact that Brooks
holds that the male and female germ-cells play a different part,
and that they tend to become charged with gemmules in different
degrees, the egg-cell containing a far smaller number than the
sperm-cell. According to Brooks the egg-cell is the conservative
principle which brings about the permanent transmission of the
true characters of the race or species, while he believes that the
sperm-cell is the progressive principle which causes variation.

The transformation of species is therefore believed to take place,
for the most part, as follows:—those parts which are placed in
unfavourable conditions by the operation of external influences,
and which have varied, throw off gemmules which reach the
sperm-cells, and the latter by fertilization further propagate the

330 THE SIGNIFICANCE OF SEXUAL REPRODUCTION

variation. An increase of variation is produced because the gem-
mules which reach the egg through the sperm-cell may unite or con-
jugate with parts of the former which are not the exact equivalents
of the cells from which the gemmules arose, but only very nearly
related to them. Brooks calls this ‘hybridization, and he con-
cludes that, just as hybrids are more variable than pure species, so
such hybridized cells are also more variable than other cells.

The author has attempted to work out the details of his
theory with great ingenuity, and as far as possible to support his
assumptions by facts. Moreover, it cannot be denied that there
are certain facts which seem to indicate that the male germ-cell
plays a different part from that taken by the female germ-cell in
the formation of a new organism.

For example, it is well known that the offspring of a horse and
an ass is different when the male parent is a horse from what it is
when the male parent is an ass. A stallion and a female ass
produce a hinny which is more like a horse, while a male ass and
a mare produce a mule which is said to be more like an ass!, I
will refrain from considering here the opinion of several authors
(Darwin, Flourens, and Bechstein) that the influence of the ass is
stronger in both cases, only predominating to a less extent when
the ass is the female parent; and I will for the sake of brevity

accept Brooks’ opinion that in these cases the influence of the
father is greater than that of the mother. Were this so in all
cross-breeding between different species and in all cases of normal
fertilization, we should be compelled to conclude that the influ-
ences of the male and female germ-plasms upon the offspring

1 This seems to be the general opinion (see the quotation from Huxley in Brooks’
‘ Heredity, p. 127); but I rather doubt whether there is such a constant difference
between mules and hinnies. Furthermore, I cannot accept the opinion that mules
always resemble the ass more than the horse. I have seen many mules which bore
a much stronger likeness to the latter. I believe that it is at present impossible to
decide whether there is a constant difference between mules and hinnies, because the
latter are'very rarely seen, and because mules are extremely variable. I attempted
to decide the question last winter by a careful study of the Italian mules, but I
- could not come across a single hinny, These hybrids are very rarely produced,
because it is believed that they are extremely obstinate and bad-tempered. I after-
wards saw two true hinnies at Professor Kiihn’s Agricultural Institute at Halle.
These hinnies by no means answered to the popular opinion, for they were quite
tractable and geod-tempered. They looked rather more like horses than asses,

although they resembled the latter in size. In this case it was quite certain that
one parent was a stallion and the other a female ass.—A. W. 1889.

IN THE THEORY OF NATURAL SELECTION. 331

differ at any rate in strength. But this is by no means always
the case, for even in horses the reverse may occur. Thus it is
stated that certain female race-horses have always transmitted
their own peculiarities, while others allowed those of the stallion
to preponderate.

In the human species the influence of the mother preponderates
quite as often as that of the father, although in many families most
of the children may take after either parent. There is nevertheless
hardly any large family in which all the children take after the
same parent. If we now try to explain the preponderating in-
fluence of one parent by the supposition of a greater strength in
hereditary power, without first inquiring after some deeper cause,
I think the only conclusion warranted by the facts before us is
that this power is rarely or never equal in both of the conjugating
germ-cells, but that even within the same species, sometimes the
male and sometimes the female is the stronger, and that the strength
may even vary in the different offspring of the same individuals,
as we so frequently see in human families, The egg-cells of the
same mother which ripen one after the other, and also the sperm-
cells of the same father, must therefore present variations in the
strength of their hereditary power. It is then hardly to be wondered
at that the relative hereditary power of the germ-cells in different
species should vary, although we cannot as yet understand why
this should be the case.

It would not be very difficult to render these facts intelligible
in a general way by an appeal to physiological principles. The
quantity of germ-plasm contained in a germ-cell is very minute,
and together with the idioplasms of the various ontogenetic
stages to which it gives rise, is must be continually increased by
assimilation during the development of the organism. If now this
power of assimilation varied in intensity, a relatively rapid growth
of the idioplasm derived from one of the parents would ensue,
and with it the preponderance of the hereditary tendencies of
the parent in question. Now, it is obvious that no two cells of
the same kind are entirely identical, and hence there must be
differences in their powers of assimilation. Thus the varying
hereditary powers of the egg-cells produced from the same ovary
become explicable, and still more easily the varying powers of the
germ-cells produced in the ovaries or testes of different individuals

332 THE SIGNIFICANCE OF SEXUAL REPRODUCTION, ETC.

of the same species; most easily of all the differences observable
in this respect between the germ-cells of different species.

Of course, this hereditary power is always relative, as may be
easily proved by cross-breeding between different species and races.
Thus when a fantail pigeon is crossed with a laugher, the characters
of the former preponderate, but when crossed with a pouter the
characters of the latter preponderate 1. The facts afforded by cross-
breeding between hybrids and one of the pure parent species,
together with a consideration of the resulting degree of variability,
seem to me to be even more unfavourable to Brooks’ view. They
appear to me to admit of an interpretation different from that
brought forward by him; and when he proceeds to make use of
secondary sexual characters for the purpose of his theory, I believe
that his interpretation of the facts can be easily controverted. It
is hardly possible to conclude that variability is due to the male
parent, because the males in many species of animals are more
variable, or deviate further from the original type, than the females.
It is certainly true that in many species the male sex has taken
the lead in processes of transformation, while the female sex has
followed, but there is no difficulty in finding a better explanation
of the fact than that afforded by the assumption ‘that something
within the animal compels the male to lead and the female to follow
in the evolution of new breeds.’ Brooks has with great ingenuity
brought forward certain instances which cannot be explained with
perfect confidence by Darwin’s theory of sexual selection, but this
hardly justifies us in considering the theory to be generally in-
sufficient, and in having recourse to a theory of heredity which is
as complicated as it is improbable. The whole idea of the passage
of gemmules from the modified parts of the body into the germ-
cells is based upon the unproved assumption that acquired characters
can be transmitted. The idea that the male germ-cell plays a
different part from that of the female, in the construction of the
embryo, seems to me to be untenable, especially because it conflicts ©
with the simple observation that upon the whole human children
inherit quite as much from the father as from the mother.

1 Darwin, ‘ Variation of Animals and Plants under Domestication,’ 1875, Vol. II.
Pp. 41.

Vi.

ON THE NUMBER OF POLAR BODIES AND
THEIR SIGNIFICANCE IN HEREDITY,

1887.

©

- pe 1 *
See PO

ON THE NUMBER OF POLAR BODIES AND
THEIR SIGNIFICANCE IN HEREDITY.

os

PREFACE.

Tue following paper stands in close relation to a series of short
essays which I have published from time to time since the year
1881. The first of these treated of ‘The Duration of Life,’ and the
last of ‘The Significance of Sexual Reproduction.’ The present
essay is most intimately connected with that upon ‘The Continuity
of the Germ-plasm,’ and has, in fact, grown out of the explanation
of the meaning of polar bodies in the animal egg, brought forward
in that essay. The explanation rested upon a trustworthy and solid
foundation, as I am now able to maintain with even greater con-
fidence than at that time. It rested upon the idea that in the egg-
cell, a cell with a high degree of histological differentiation, two
different kinds of nuclear substance exert their influence, one after
the other. But continued investigation has shown me that the
explanation built upon this idea is only correct in part, and that it
does not exhaust the full meaning of the formation of polar bodies.
In the present essay I hope to complete the explanation by the
addition of essential elements, and I trust that, at the same time,
I shall succeed in throwing new light upon the mysterious problems
of sexual reproduction and parthenogenesis.

It is obvious that this essay can only contain an attempt at an
explanation, an hypothesis, and not a solution which is above
criticism, like the results of mathematical calculation. But no
biological theory of the present day can escape a similar fate, for
the mathematical key which opens the door leading to the secrets of
life has not yet been found, and a considerable period of time must
elapse before its discovery. But although I can only offer an hypo-

336 ON THE NUMBER OF POLAR BODIES ETC.

thesis, I hope to be able to show that it has not been rashly adopted,
but that it has grown in a natural manner from the secure founda-
tion of ascertained facts.

Nothing impresses the stamp of truth upon an hypothesis more
than the fact that its light renders intelligible not only those facts
for the explanation of which it has been framed, but also other
and more distantly related groups of phenomena. This seems to
me to be the case with my hypothesis, since the interpretation of
polar bodies and the ideas derived from it unite from very different
points of view, the facts of reproduction, heredity and even the
transformation of species, into a comprehensive system, which
although by no means complete, is nevertheless harmonious, and
therefore satisfactory.

Only the most essential elements of the new facts ih form
the foundation of the views developed in this essay will be briefly
mentioned. My object is to show all the theoretical bearings of
these new facts, not to describe them in technical detail. Such
a description accompanied by the necessary figures will shortly be
given in another place’.

A. W.
Freieure 1. Br., May 30, 1887.

1 See Berichten der Naturforschenden Gesellschaft zu Freiburg i. B., Band IIT.
(1887) Heft 1,‘ Ueber die Bildung der Richtungskérper bei Uhincieshedi Eiern, by
August Weamann and C. Ischikawa.

~_—- *~— T_ - —

ON THE NUMBER OF POLAR BODIES, &c.

CONTENTS.

PAGE

I. PARTHENOGENETIC AND SEXUAL Eca % “380

—The process of the formation of polar bodies ony wiley distributed - 339

The significance of polar bodies pi yi to Minot, Balfour, and van

Beneden ? - 340

——My hypothesis of the removal of the histogenetic part of the nucleus . 341

Confirmation by the discovery of polar bodies in parthenogenetic eggs . 345
Parthenogenetic eggs form only one par body, while eggs requiring

fertilization form two - 346
Parthenogenesis depends upon the fact that the part ‘of the nucleus
which is expelled from sexual eggs i in the second Pee body, remains

in the egg J ; ‘ . 348
History of this discovery : - . F ‘ : hae - 349
Ii. SIGNIFICANCE OF THE SECOND PotaR Bopy . : : : ‘ - 352
Refutation of Minot’s theory : 353

The second ‘division of the nuclear spindle involves a reduction of the
ancestral germ-plasms s , ; ‘ ; wm Bb
The theoretical necessity for such reduction . : : : " - 350
Phyletic origin of the germ-plasms in existing species . 357

The necessary reduction takes place by a special form of nuclear division 353
The division which causes this reduction has every been cra

observed : 360
--Van Beneden’s and Carnoy’ s observations. . ; - 3 ; - 360
Two different physiological effects of karyokinesis . : , ; - 364
Significance of direct nuclear division . 365

Arguments in support of the view that the division of the egg-nucleus
which causes redugtion must occur at the end of ovogenetic develop-

ment ; 367
Such nuclear division is to be found in the formation of the second polar

body : : : : ; : ; - 368
History of the origin of this view. . ‘ : : ; FM cs - 368

III, Tux Forrcoinc CoNnSIDERATIONS APPLIED TO THE MALE GERM-CELLS . 370
The male germ-cells also require division in order to reduce the ancestral ,

germ-plasms. . A 370
The germ-plasms of the parents must be contained in the germ-plasm

of the offspring : 370
Advantages which the egg gains by the late o occurrence of the ‘ reducing

division’ 371
The causes of unequal division i in the formation of polar bodies , Pee Gee:
These causes do not apply to the sperm-cell . “ - Abe te
Different kinds of nuclear division occur in spermatogenesis . * =" 75
Some of these may be interpreted as ‘reducing divisions’ . 375
The paranucleus (‘Nebenkern’) of ee ee contains

the histogenetic nucleoplasm . a : . x B76

IV. THe ForEGoING CONSIDERATIONS APPLIED TO PLANTS . é ; Pas 4
Z

338 ON THE NUMBER OF POLAR BODIES, &¢,—CONTENTS,

PAGE

V. CoNCLUSIONS AS REGARDS HEREDITY . : 378
The germ-cell of an individual contains an 2 tmequal” cotatilanhion of

hereditary tendencies . P ° . 378

Dissimilarity between the offspring of the same parents . : » Bo

Identity of twins produced from a single egg . ; é 3 : - 380

VI. RECAPITULATION ’ . : . . : é * - - 383.

VI.

ON THE NUMBER OF POLAR BODIES AND THEIR
SIGNIFICANCE IN HEREDITY.

I. PARTHENOGENETIC AND SEexvuaL Eaa.

Hirnerto no value has been attached to the question whether an
animal egg produces one or two polar bodies. Several observers
have found two such bodies in many different groups of animals,
both high and low in the scale of organization. In certain ‘species
only one has been observed, in others again three, four, or five (e. g.
Bischoff, in the rabbit). Many observers did not even record the ~
number of polar bodies found by them, and simply spoke of ‘ polar
bodies.’ As long as their formation was looked upon as a process
of secondary physiological importance—as an ‘excretion,’ or a
‘ process of purification, or even as the ‘ excreta’ (!) of the egg, as
a ‘rejuvenescence of the nucleus, or of mere historical interest
as a reminiscence of ancestral processes, without any present
physiological meaning—so long was it unnecessary to attach any
importance to the number of these bodies, or to pay special atten-
tion to them. Of all the above-mentioned views, the one which
explained polar bodies as a mere reminiscence of ancestral processes
seemed to be especially well founded. Ten years ago we were far
from being able to prove that polar bodies occurred in all animal
eggs, and even in 1880, Balfour said in his excellent ‘ Comparative
Embryology, ‘It is very possible, not to say probable, that such
changes [the formation of polar bodies] are universal in the animal
kingdom, but the present state of our knowledge does not justify
us In saying sol,’

Even at the present day we are not, strictly speaking, justified in
making this assertion, for polar bodies have not yet been proved to
occur in certain groups of animals, such as reptiles and birds; but

‘they have been detected in the great majority of the large groups
of the animal kingdom, and wherever they have been looked for
! Vol. I. p. 60,

Z 2

340 ON THE NUMBER OF POLAR BODIES AND

with the aid of our modern highly efficient appliances, they have
been found },

A deeper insight into the process of fertilization has above all
led to a closer study of antecedent phenomena.

O. Hertwig? and Fol® showed that the formation of polar bodies
was connected with a division of the nuclear substance of the egg.
Hertwig and Biitschli* then proved that the body expelled from
the egg possessed the nature of a cell, and thus led the way to the
view that the formation of polar bodies is a process of cell-division,
although a very unequal one. Even then there was no reason for
attaching any special importance to the number of these bodies ;
nor should we have such a reason if we agreed with Minot ®, Bal-
four ®, and van Beneden in ascribing a high physiological signifi-
cance to this process, and assumed that the expelled polar body
is the male part of the previously hermaphrodite egg-cell. We
should not know in what proportion the quantities of the ‘ male’
and ‘female’ parts were present, and it would therefore be impossible |
to decide, a priori, whether the ‘male’ part had to be removed
from the body of the egg-cell in one, two, or more portions.

Even after the view that the nuclear substance is the essential
element in fertilization had gained ground—a view chiefly due to
Strasburger’s investigations on the process of fertilization in Pha-
nerogams—and after Hertwig’s opinion had been confirmed, that
the process of fertilization is essentially the conjugation of nuclei,
even then there appeared to be no reason why the zwmber of
divisions undergone by the nucleus of the mature egg should be
looked upon as an essential feature.

1 The most recent example of this kind is afforded by the excellent work of
O. Schultze, ‘Ueber die Reifung und Befruchtung des Amphibieneies,’ Zeitschr. f,
wiss. Zool., Bd. XLV. 1887. Schultze has proved that two polar bodies are expelled
from the egg of the Axolotl and of the frog, although all previous observers, including
O. Hertwig, had been unable to find them. Thus the latter authority states as the
result of an investigation specially directed towards this point, that the nucleus is
transformed in a peculiar manner (‘ Befruchtung des thierischen Kies,’ IIT. p. 81).

? O. Hertwig, ‘ Beitriige zur Kenntniss der Bildung, Befruchtung, und Theilung
des thierischen Eies,’ Morpholog. Jahrbuch, I, IT, and III. 1875-77.

8 H. Fol, ‘ Recherches sur la fécondation et le commencement de I'hénogénie chez
divers animaux.’ Gentve, Bale, Lyon, 1879.

* Biitschli, ‘Entwicklungsgeschichtliche Beitriige,’ Zeitschr. f. wiss. Zool. Bd.
XXIX. p. 237. 1877.

5 0. S. Minot, ‘ Account, ete.’ Proceedings Boston Soc, Nat. Hist., vol. xix. p. 165,
1877.

* F, M. Balfour, ‘Comparative Embryology.’

THEIR SIGNIFICANCE IN HEREDITY. 341

This was the state of the subject at the time when I first made
an attempt to ascertain the meaning of the formation of polar
bodies. I based my views upon the idea, which was just then
gaining ground, that Niigeli’s idioplasm was to be sought for in
the nucleus, and that the nucleoplasm must therefore contain the
substance which determines the form and functions of the cell.
Hence it followed that the germ-plasm—the substance which de~
termines the course of embryonic development—must be identified
with the nucleoplasm of the egg-cell. The conception of germ-
plasm was brought forward by me before the appearance of Nigeli’s
work! which is so rich in fertile ideas; and germ-plasm does not
exactly coincide with Niigeli’s idioplasm* Germ-plasm is only
a certain kind of idioplasm—viz. that contained in the germ-cell—
and it is the most important of all idioplasms, because all the other
kinds are merely the results of the various ontogenetic stages into
which it developes. I attempted to show that the molecular
structure in these ontogenetic stages into which the germ-plasm
developes would become more and more unlike that of the original
structure of this substance, until it finally attains a highly
specialized character at the end of embryonic development, corre-
sponding to the production of specialized histological elements.
It did not seem to me to be conceivable that the specialized idio-
plasm contained in the nuclei of the tissue cells could re-transform
itself into the initial stage of the whole developmental series—
that it could give up its specialized character and re-assume the
generalized character of germ-substance. I will not repeat the
reasons which induced me to adopt this opinion ; they still seem to
me to be conclusive. But let the above-mentioned theory be once
accepted, and there follows from it another interesting conclusion
concerning the germ-cell, or at least concerning those germ-cells |
which, like most animal eggs, possess a specific histological cha-
racter. For obviously, such a character presupposes the existence
of an idioplasm with a considerable degree of histological special-
ization, which must be contained in the nucleus of the egg-cell.
We know, on the other hand, that when its growth is complete,
after the formation of yolk and membranes, the egg contains

* Nageli, ‘Mechanisch-physiologische Theorie der Abstammungslehre,’ Mtinchen

und Leipzig, 1884.
* See the second and fourth Essays in the present volume.

342 ON THE NUMBER OF POLAR BODIES AND

germ-plasm, for it is capable of developing into an embryo. We
have therefore, as it were, two natures in a single cell, which
become manifest one after the other, and which, according to our
fundamental conception, can only be explained by the presence of
two different idioplasms, which control the egg-cell one after the
other, and determine its processes of development. At first a
nucleoplasm leading to histological specialization directs the de-
velopment of the egg and stamps upon it a specific histological
character; and then germ-plasm takes its place, and compels the
egg to undergo development into an embryo. If then the histo-
genetic or ovogenetic nucleoplasm of the egg-cell can be derived
from the germ-plasm, but cannot be re-transformed into it (for the
specialized can be derived from the generalized, but not the
generalized from the specialized), we are driven to the conclusion
that the germ-plasm, which is already present in the youngest
egg-cell, first of all originates a specific histogenetic or ovogenetic
nucleoplasm which controls the egg-cell up to the point at which
it becomes mature; that its place is then taken by the rest of the
unchanged nucleoplasm (germ-plasm), which has in the meantime
increased by growth; and that the former is removed from the
egg in the form of polar bodies—a removal which has been ren-
dered possible by the occurrence of nuclear division. Hence the
formation of polar bodies signified, in my opinion, the removal of
the ovogenetic part of the nucleus from the mature egg-cell. Such
removal was absolutely necessary, if it is impossible that the ovo-
genetic nucleoplasm can be re-transformed into germ-plasm. Hence
the former substance cannot be made use of after the maturation
of the egg, and it must even be opposed to the commencement of
embryonic development, for it is impossible that the egg can be
controlled by two forces of different kinds in the same manner as
it would have been by one of them alone. I therefore concluded
that the influence of the ovogenetic idioplasm must be removed
before embryonic development can take place. In this way it
seemed to me that not only the ordinary cases of ovogenetic and em-
bryonic development became more easily intelligible, but also the
rarer cases in which one and the same species produces two kinds
of eggs—‘ summer and winter eggs.’ Such eggs not only differ
in size but also in the structure of yolk and membranes, although
identical animals are developed from each of them. This result pre-

THEIR SIGNIFICANCE IN HEREDITY. 843

supposes that the nucleus in both eggs contains identical germ-
plasm, while the formation of different yolks and membranes re-
quires the supposition that their nucleoplasm is different, inasmuch
as the two eggs differ greatly in histological character.

The fact that equal quantities are separated during nuclear
division, led me to conclude further that the expulsion of ovogenetic
nucleoplasm can only take place when the germ-plasm in the
nucleus of the egg-cell has increased by growth up to a point at
which it can successfully oppose the ovogenetic nuclear substance.
But we do not know the proportion which must obtain between the
relative quantities of two different nuclear substances in order that
nuclear division may be induced; and thus, by this hypothesis at
least, we could not conclude with certainty as to the necessity for
a single or a double division of the egg. It did not seem to be
altogether inconceivable that the ovogenetic nucleoplasm might be
larger in amount than the germ-plasm, and that it could only be
completely removed by means of two successive nuclear divisions.
Tadmit that this supposition caused me some uneasiness ; but since
nothing was known which could have enabled us to penetrate more
deeply into the problem, I was satisfied, for the time being, in
having found any explanation of the physiological value of polar
bodies ; leaving the future to decide not only whether such explana-
tion were valid, but also whether it were exhaustive. The explana-
tion seems to have found but little favour with some of our
highest authorities. Hensen does not consider that my reasons for
the distinction between germ-plasm and histogenetic nucleoplasm
are conclusive, and it may be conceded that this objection was
perhaps, at that time, well founded. O. Hertwig does not mention
my hypothesis at all in his work on embryology®, although he
states in the preface: ‘Among current problems I have chiefly
taken into consideration the views which seem to me to be most
completely justified, but I have not left unmentioned the views
which I cannot accept.’ Minot’s hypothesis is discussed by Hert-
wig, but Biitschli’s* is preferred by him, although these two

1 Hensen, ‘Die Grundlagen der Vererbung,’ Zeitschr. f. wiss. Landwirthschaft.
Berlin, 1885, p. 749.
_ # 0. Hertwig, ‘Lehrbuch der Entwicklungsgeschichte des Menschen und der Wir-
belthiere.’ Jena, 1886,
Biitschli, ‘Gedanken iiber die morphologische Bedeutung der sog. Richtungskér-
perchen,’ Biol. Centralblatt, Bd. VI. p. 5. 1884.

344 ON THE NUMBER OF POLAR BODIES AND

hypotheses are not strictly opposed to each other; for the former
is a purely physiological, the latter a purely morphological ex-
planation. I desire to lay especial stress upon the fact, that my
hypothesis is simply a logical consequence from the conclusion that
the nuclear substance determines the nature of a cell. How this
takes place is quite another question, which need not be discussed
here. If it is only certain that the nature of a cell is thus deter-
mined, it follows that a cell with a certain degree of histological
specialization must contain a nucleoplasm corresponding to the
specialization. But the mature egg also contains germ-plasm, and
there are only two possibilities by which these facts can be
explained: either the ovogenetic nucleoplasm is capable of re-
transformation into germ-plasm, or it is incapable of such re-trans-
formation. Now, quite apart from the arguments which might be

advanced in favour of one of these two possibilities, the fact that -

a body is undoubtedly expelled from the mature egg seems to me
of importance, while it is of even greater importance that this body
contains nucleoplasm from the germ-cell.

It may be thought that the process, as supposed by me, is
without analogy, but such a conclusion is wrong, for during every
embryonic development there are numerous cell-divisions in which
unequal nucleoplasms are separated from one another, and in all
these cases we cannot imagine any way in which the process can
take place, except by supposing that the two kinds of nucleoplasm
were previously united in the mother-cell, although their differ-
entiation probably took place only a short time before cell-division.
Perhaps the new facts which will be mentioned presently, and the
’ views derived from them, will make my hypothesis upon the histo-
genetic nucleoplasm of the germ-cells appear in a more favourable
light to the authorities above-named.

My hypothesis has at all events the one merit that it has led
me to fruitful investigations,

If the formation of polar bodies really means ‘the removal of ovo-
genetic nucleoplasm from the mature egg, they must also be found
in parthenogenetic eggs ; inasmuch as the latter possess a specific
histological structure equal to that found in eggs requiring fertiliza-
tion. If, therefore, it were possible to observe the formation of polar
bodies in eggs which develope parthenogenetically, such an observa-
tion would not form a proof of the validity of my interpretation ; but

THEIR SIGNIFICANCE IN HEREDITY. 345

it would be a fact which harmonized with it, and negatived a
suggestion which, if confirmed, would have been fatal to the hypo-
thesis. Minot, Balfour, and-van Beneden, from the point of view
afforded by their theories, were compelled to suppose that polar
bodies are wanting in parthenogenetic eggs; and the facts which
were known at that time favoured such an opinion, for in spite
of many attempts, no one had ever succeeded in proving the
formation of these bodies by parthenogenetic eggs.

During the summer of 1885 I first succeeded in ascertaining
that a single polar body is expelled from the parthenogenetic
summer-ege of one of the Daphnidae,—Polyphemus oculus!, Thus
my interpretation of the process in question received support, while
it seemed to me that Minot’s interpretation of polar bodies had
been refuted ; for if these bodies are formed in the parthenogenetic
eggs of a single species, just as in eggs which require fertilization,
it follows that the expulsion of polar bodies cannot signify the
removal of the male element from the egg.

The desire to throw light upon the significance of polar bodies
has been the only cause of my investigation. At the same time
I hoped by this means to gain further knowledge as to the nature
of parthenogenesis.

In the third part of the essay on ‘The Continuity of the Germ-
plasm’ (see p. 225) I attempted to make clear the nature of
parthenogenesis, and I arrived at the conclusion that the difference
between an egg which is capable of developing without fertilization,
and another which requires fertilization, must lie in the quantity
of nucleoplasm present in the egg. I supposed that the nucleus of
the mature parthenogenetic egg contained nearly twice as much
germ-plasm as that contained in the sexual egg, just before the
occurrence of fertilization ; or, more correctly, I believed that the
quantity of nucleoplasm which remains in the egg, after the ex-
pulsion of the polar bodies, is the same in both eggs, but that the
parthenogenetic egg possesses the power of doubling this quantity
by growth, and thus produces from within itself the same quantity
of germ-plasm as that contained in the sexual ege after the
addition of the sperm-nucleus in fertilization.

This was only an hypothesis, and the considerations which had led

* This observation was first published as a note at the end of the fourth Essay in
the present volume. See p. 249.

346 ON THE NUMBER OF POLAR BODIES AND

to it depended, as far as they went into details, upon assumptions ;
but the fundamental view that the quantity of the nucleus decides
whether embryonic development takes place with or without fer-
tilization seemed to me, even at that time, to be correct, and to be
a conclusion required by the facts of the case. Indeed, I thought
it not unlikely that its validity might be proved by direct means:
I pointed out that a comparison of the quantities of the nuclei in
parthenogenetic and sexual eggs, if possible in the same species,
would enable us to decide the question (/. ¢., p. 234).

I had thus set myself the task of making this comparison. The
result of this investigation was to show that, as already mentioned,
polar bodies are formed in parthenogenetic eggs. But even the
first species successfully investigated revealed a further fact, which,
if proved to be wide-spread and characteristic of all partheno-
genetic eggs, was certain to be of extreme importance :—the matura-
tion of the parthenogenetic egg is accompanied by the expulsion of
one polar body, or, as we might express it in another way, the
substance of the female pronucleus is only once divided, and not
twice, as in the sexual eggs of so many other animals. If this
difference between parthenogenetic and sexual eggs was shown to
be general, then the foundations of my hypothesis would indeed
have been proved to be sound. The quantity of nuclear substance
decides whether the egg is capable of undergoing embryonic
development. This quantity is twice as large in the partheno-
genetic as in the sexual egg. I had, however, been mistaken in
a matter of detail; for the difference in the quantities of nuclear
substance is not produced by the expulsion of two polar bodies, and
the reduction of the nuclear substance to a quarter of-its original
amount, in both eggs, while the parthenogenetic egg then doubles
its nuclear substance by growth; but the difference is produced
because the reduction of nuclear substance originally present is less
in one case than it is in the other. In the parthenogenetic egg
the nuclear substance is only reduced to one-half by a single
division; in the sexual egg it is reduced to a quarter by two
successive divisions. It is an obvious conclusion from this fact, if
proved to be wide-spread, that the significance of the first polar
body must be different from that of the second. Only one polar
body can signify the removal of ovogenetic nucleoplasm from the
mature egg, and the second is obviously a reduction of the germ-

THEIR SIGNIFICANCE IN HEREDITY. B47

plasm itself to half of its original amount. This very point seemed
to me to be of great importance, because, as I had foreseen long
ago, and as will be shown later on, the theory of heredity forces us
to suppose that every fertilization must be preceded by a reduction
of the ancestral idioplasms present in the nucleus of the parent
germ-cell, to one-half of their former number.

But before the full bearing of the phenomena could be considered,
it was necessary to ascertain how far they were of general occur-
rence. ‘There were two ways in which this might be achieved, and
in which it was possible to prove that parthenogenetic eggs expel
only one polar body, while sexual eggs expel two. We might
attempt to observe the phenomena of maturation in both kinds of
egos in a species which reproduces itself by the parthenogenetic
as well as the sexual method. ‘This would be the simplest way in
which the question could be decided, if it were possible to make
such observations on a sufficient number of species. But the other
method was also open, a method which would have been the only
one, if we did not know of any animals with two kinds of repro-
duction. We might attempt to investigate the phenomena of
maturation in a large number of parthenogenetic eggs, if possible
from different groups of animals, and we might compare the results
with the facts which are already certain concerning the expul-
sion of polar bodies from the sexual eggs of so many species.

I have followed both methods, and by means of the second
I have arrived already, indeed some time ago, at the certain con-
clusion that the above-mentioned difference is really general and:
without exception. The first polar body only is formed in all the
parthenogenetic eggs which I investigated, with the valuable
assistance of my pupil, Mr. Ischikawa of Tokio. On the other
hand, an extensive examination of the literature of the subject
convinced me that there is not a single undoubted instance of the
expulsion of only one polar body from eggs which require fertiliza-
tion, and that there are very numerous cases known from almost all
groups of the animal kingdom in which it is perfectly certain that
two polar bodies are formed, one after the other. A number of the
older observations cannot be relied upon, for the presence of two
polar bodies is mentioned without any explanation as to whether
they are expelled from the egg one after the other, or whether
they have merely resulted from the division of a single body after

348 ON THE NUMBER OF POLAR BODIES AND

its expulsion. In parthenogenetic eggs two polar bodies are also
formed in most cases, but they arise from the subsequent division
of the single body which separates from the egg. But such sub-
sequent division is only of secondary importance as far as the egg
itself. is concerned, and is also unimportant in the interpretation of
the process. The essential nature of the process is to be found in
the fact that the nucleus of the egg-cell only divides once when
parthenogenesis occurs, but twice when fertilization is necessary, and
it is of no importance whether the expelled part of the nucleus of
the cell-body atrophies at once, or after it has undergone division.
We have, therefore, to distinguish between primary and secondary
polar bodies. If this distinction is recognized, and if we leave out
of consideration all doubtful cases mentioned in literature, such a
large number of well- established observations remain, that the
existence of two primary polar bodies in sexual eggs, and neither
a smaller nor a larger number, may be considered as proved.

Hence follows a conclusion which I believe to be very significant,
.—the difference between parthenogenetic and sexual eggs lies in
the fact that in the former only one primary polar body is ex-
pelled, while two are expelled from the latter. When, in July,
1886, I published a short note! on part of the observations made
upon parthenogenetic eggs, I confined myself to facts, and did not
mention this conclusion. I took this course simply because I did
not wish to bring it forward until I had made sufficient observa-
tions in the first of the two ways described above. I had hoped
to be able to offer all the proofs that can be obtained before
undertaking to publish the far-reaching consequences which would
result from the above-mentioned conclusion. Unfortunately the
material with which I had hoped to quickly settle the matter,
proved less favourable than I had expected. Many hundred
sections through freshly laid winter-eggs of Bythotrephes longi-
manus were made in vain; they did not yield the wished for
evidence, and although continued investigation of other material
has led to better results, the proofs are not yet entirely com-
plete.

I should not therefore even now have brought forward the above-
mentioned conclusion, if another observer had not alluded to this

1 Weismann, ‘ Richtungskérper bei parthenogenetischen Eieren,’ Zool. Aer
1886, p. 570.

THEIR SIGNIFICANCE IN HEREDITY, 349

idea, referring to my observations and also to a new discovery of
his own, In a recent number of the ‘ Biologische Centralblatt,’
Blochmann? gives an account of his continued observations upon
the formation of polar bodies. It is well known that this careful
observer had previously shown that polar bodies do occur in the
eggs of insects, although they had not been found before. Bloch-
mann proved that they are found in the representatives of three
different orders, so that we may indeed ‘confidently hope to find
corresponding phenomena in other insects. This discovery is
most important, and it was naturally very welcome to me, as I
had for a long time ascribed a high physiological importance
to the process of the formation of polar bodies, and it would not
be in accordance with such a view if the process was entirely
wanting from whole classes of animals. To fill up this gap in
our knowledge, and to give the required support to my theoretical
views, I had proposed to one of my pupils, Dr. Stuhlmann?, that
he should work out the maturation of the eggs of insects ; and it
is a curious ill-luck that he, like many other observers, did not
succeed in observing the expected expulsion of polar bodies, in
spite of the great trouble he had taken. It may be that the
species selected for investigation were unfavourable: at all events,
we cannot now doubt that a division of the egg-nucleus is
quite universal among insects, for Blochmann, in his latest con-
tribution to the subject, proves that the Apidae also form polar
bodies. He examined the winter-eggs of Aphis aceris, and as-
certained that they form two’ polar bodies, one after the other.
Even in the viviparous Apidae, thin sections revealed the presence
of a polar body, though Blochmann could not trace all the stages
of its development. It appears that the polar body is here pre-
served for an exceptional period, and its presence can still be
proved when the blastoderm has been formed, and sometimes
when development is even further advanced. Skilled observers of
recent times, such as Will and Witlaczil, have not been able to
find a polar body in the parthenogenetic eggs of the Aphidae, and

1 Blochmann, ‘Ueber die Richtungskérper bei den Insekteneiern,’ Biolog. Cen-
tralblatt., April 15, 1887.

2 F. Stuhlmann, ‘Die Reifung des Arthropodeneies nach Beobachtungen an
Insekten, Spinnen, Myriapoden und Peripatus,’ Berichte der naturforschenden
Gesellschaft zu Freiburg i. Br., Bd. I. p. 101,

350 ON THE NUMBER OF POLAR BODIES AND

Blochmann’s proof of its existence seems to me to be of especial
value, because the eggs of Aphidae are in many respects so unusually
reduced ; for instance, the primary yolk is absent and the egg-
membrane is completely deficient, so that we might have expected
that if polar bodies are ever absent, they would be wanting in these
animals—that is, if they were of no importance, or at any rate of
only secondary importance.

Hence the presence of polar bodies in Aphidae is a fresh con-
firmation of their great physiological importance. As bearing
upon the main question dealt with in this essay, Blochmann’s
observations have an especial interest, because only one polar body
was found in the parthenogenetic eggs of Aphis, while the sexual
eges normally produce two. The author rightly states that this
result is in striking accordance with my results obtained from the
summer-eggs of different Daphnidae, and he adds the remark,—*‘ It
would be of great interest to know whether these facts are due to
the operation of some general law.’ To this remark I can now
reply that there is indeed such a law: not only in the parthenogenetic
eggs of Daphnidae, but also, as I have since found, in those of
the Ostracoda and Rotifera!, only one primary polar body is
, formed, while two are formed in all eggs destined for fertilization.

Before proceeding to the conclusions which follow from this
fact, I will at once remove a difficulty which is apparently pre-
sented by the eggs which may develope with or without fertiliza-
tion. I refer to the well-known case of the eggs of bees. It might
be objected to my theory that the same egg cannot be prepared
for development in more than one out of the two possible ways ;
it might be argued that the egg either possesses the power of
entering upon two successive nuclear divisions during maturation,

1 In the summer-eggs of Rotifera I have, together with Mr. Ischikawa, observed
one polar body, and we were able to establish for certain that a second is not formed.
The nuclear spindle had already been observed by Tessin, and Billet had noticed
polar bodies in Philodina, but without attaching any importance to their number.
These latter observations were not conclusive proofs of the formation of polar bodies
in parthenogenetic eggs, so long as it was not known whether the summer-eggs of
Rotifera may develope parthenogenetically, or whether they can only develope in
this way. Knowing now that parthenogenetic eggs expel only one polar body, we
may perhaps be permitted to draw the conclusion that the summer-egg of a Rotifer
(Lacinularia) which expelled only one polar body must have been a parthenogenetic
egg. But I may add that we have also succeeded in directly proving the occurrence
of parthenogenesis in Rotifera, as will be described in detail in another paper.

x THEIR SIGNIFICANCE IN HEREDITY. 351

and in this case requires fertilization ; or the egg may be of such a
nature that it can only enter upon one such division and can
therefore form only one polar body, and in that case it is capable of
parthenogenetic development. Now there is no doubt, as I pointed
out in my paper on the nature of parthenogenesis’, that in the bee
the very same egg may develope parthenogenetically, which under
other circumstances would have been fertilized. Bessel’s ? experi-
ments, in which young queens were rendered incapable of flight, and
were thus prevented from fertilization, have shown that all the eggs
laid by such females develope into drones (males) which are well
known to result from-parthenogenetic development. On the other
hand, bee-keepers have long known that young queens which are
fertilized in a normal manner continue for a long time to lay eggs
which develope into females, that is to say, which have been
fertilized. Hence the same eggs, viz. those which are lowest in
the oviducts and are therefore laid first, develope parthenogeneti-
-eally in the mutilated female, but are fertilized in the normal
female. The question therefore arises as to the way in which the
eggs become capable of adapting themselves to the expulsion of
two polar bodies when they are to be fertilized, and of one only.
when fertilization does not take place.

But perhaps the solution of this problem is not so difficult as it
appears to be. If we may assume that in eggs which are capable
of two kinds of development the second polar body is not expelled
‘until the entrance ofa spermatozoon has taken place, the explanation
of the possibility of parthenogenetic development when fertilization
does not occur would be fortheoming. Now we know, from the in-
vestigations of O. Hertwig and Fol, that in the eggs of Hehimus
the two polar bodies are even formed in the ovary, and are therefore
quite independent of fertilization, but in this and other similar cases
a parthenogenetic development of the egg never takes place. There
are, however, observations upon other animals which point to the fact
that the first only and not the second polar body may be formed before
the spermatozoon penetrates into the egg. It can be easily under-
stood why it is that entirely conclusive observations are wanting,
for hitherto there has been no reason for any accurate distinction

1 See Essay IV, Part IIT. p. 225.
2 E. Bessels, ‘ Die Landois’sche Theorie, widerlegt durch das Experiment.’
Zeitschr. f. wiss. Zool. Bd, XVIII. p. 124. 1868.

352 ON THE NUMBER OF POLAR BODIES AND

between the first and the second polar body. Butin many eggs it
appears certain that the second polar body is not expelled until the
spermatozoon has penetrated. O. Schultze, the latest observer of
the egg of the frog, in fact saw the first polar body alone extruded
from the unfertilized egg: a second nuclear spindle was indeed
formed, but the second polar body was not expelled until after
fertilization had taken place. A very obvious theory therefore sug-
gests itself:—that while the formation of the second polar body is
purely a phenomenon of maturation in most animal eggs, and is
independent of fertilization,—in the eggs of a number of other
animals, on the other hand, and especially among Arthropods,
the formation of the second nuclear spindle is the result of a
stimulus due to the entrance of a spermatozoon. If this sug-
gestion be confirmed, we should be able to understand why partheno-
genesis occurs in certain classes of animals wherever the external
conditions of life render its appearance advantageous, and further,
why in so many species of insects a sporadic parthenogenesis is ob-
served, viz. the parthenogenetic development of single eggs (Lepi-
doptera). Slight individual differences in the facility with which
the second nuclear spindle is formed independently of fertilization
would in such cases decide whether an egg is or is not capable of
parthenogenetic development. As soon, however, as the second
nuclear spindle is formed, parthenogenesis becomes impossible.
The nuclear spindle which gives rise to the second polar body, and
that which initiates segmentation, are two entirely different things,
and although they contain the same quantity, and the same kind
of germ-plasm, a transformation of the one into the other is
scarcely conceivable. This conclusion will be demonstrated in the
following part of the essay.

we TI. Tue Sienrricance or tHE. Seconp Poiar Bopy.

I have already discussed the physiological importance of the first
-polar body, or rather of the first division undergone by the nucleus
of the egg, and I have explained it as the removal of ovogenetic
nuclear substance which has become superfluous and indeed in-
jurious after the maturation of the egg. I do not indeed know of
any other meaning which can be ascribed to this process, now that
we know of the occurrence of a first division of the nucleus in

THEIR SIGNIFICANCE IN HEREDITY. 3538

parthenogenetic as well as in sexual eggs. A part of the nucleus
must thus be removed from both kinds of eggs, a part which was
necessary to complete their growth, and which then became super-
fluous and at the same time injurious. In this respect the observa-
tions of Blochmann! upon the eggs of Musca vomitoria seem to me
to be very interesting. Here the two successive divisions of the
nuclear spindle arising from the egg-nucleus take place, but true
polar bodies are not expelled, and the two nuclei corresponding to
them (one of which divides once more) are placed on the surface of
the egg, surrounded by an area free from yolk granules; and they
break up at a later period. The essential point is obviously to
eliminate from the eg-g-cell the influence of nucleoplasm which has
been separated from the egg-nucleus as the first polar body; and
this condition is satisfied whether the elimination is brought about
by a process of true cell-division, as is the rule in the eggs of most
animals, or by the division and removal of part of the egg-nucleus
alone. The occurrence of the latter method of elimination cer-
tainly constitutes a still further proof of the physiological im-
portance of the process, and this, taken together with the uni-
versal occurrence of polar bodies in all eggs—parthenogenetic and
sexual—forces us to conclude that the process must possess a
definite significance. ~No one of the various attempts which have
been made to explain the significance of polar bodies generally
is applicable to the jivst polar body except that which I have
attempted.

But the case is different with the significance of the second
nuclear division, or the second polar body. Here it might perhaps
be possible to return to the view brought forward by Minot,
Balfour, and van Beneden, and to consider the removal of this
part of the nucleus as the expulsion of the male part of the pre-
viously hermaphrodite egg-cell. The second polar body is only ex-
pelled when the egg is to be fertilized, and at first sight it appears
to be quite obvious that such a preparation of the egg for fertilization
must depend upon its reduction to the female state. I believe how-
ever that this is not the case, and am of opinion that the process
has an entirely different and much deeper meaning.

How can we gain any conception of this supposed herma-
phroditism of the egg-cell, and its subsequent attainment of the
1 l.c., p. I10,

Aa

354 ON THE NUMBER OF POLAR BODIES AND

female state ? What are the essential characteristics of the male and
female states? We know of female and male individuals, among
both animals and plants: their differences consist essentially in the
fact that they produce different kinds of reproductive cells; in part
they are of a secondary nature, being adaptations of the organism
to the functions of reproduction; they are intended to attract the
other sex, or to ensure the meeting of the two kinds of reproductive
cells, or to enable the fertilized egg to develope and sometimes to
guide the development of the offspring until it has reached a certain
period of growth. But all these differences, however great they may
sometimes be, do not alter the essential nature of the organism.
The blood corpuscles of man and woman are the same, and so are
the cells of their nerves and muscles ; and even the sexual cells, so
different in size, appearance, and generally also in motile power,
must contain the same fundamental substance, the same idioplasm.
Otherwise the female germ-cell could not transmit the male
characters of the ancestors of the female quite as readily as the
female characters, nor could the male germ-cell transmit the female
quite as readily as the male characters of the ancestors of tlie male.
It is therefore clear that the nuclear substance itself is not sexually
differentiated. .

I have already previously pointed out that the above-mentioned
facts of heredity contain the disproof of Minot’s theory, inasmuch
as the egg-cell transmits male as well as female characters. Stras-
burger 1 has also raised a similar objection. I consider this objec-
tion to be quite conclusive, for there does not seem to be any way
in which the difficulty can be met by the supporters of the theory.
The difficulty could indeed be evaded until we came to know that
the essential part of the polar body is nuclear substance, and that the
latter must be regarded as idioplasm,—as the substance which is the
bearer of heredity. It might have been maintained that the male
part, removed from the egg, consists only in a condition, perhaps
comparable to positive or negative electricity; and that this con-
dition is present in the substance of the polar body, so that the
removal of the latter would merely signify a removal of the
unknown condition. I do not mean to imply that any of those
who have adopted Minot’s theory have had any such vague ideas

1 Strasburger, ‘Neue Untersuchungen iiber den Befruchtungsvorgang bei den
Phanerogamen als Grundlage einer Theorie der Zeugung.’ Jena, 1884.

THEIR SIGNIFICANCE IN HEREDITY. 855

concerning this process, but even if any one were ready to adopt it,
he would be unable to make any use of the idea. He would
not be able to support the theory in this way, for we now know
that nuclear substance is removed with the polar body, and this
fact requires an explanation which cannot be afforded hy the
theory, if we are right in believing that the expelled nuclear sub-
stance is not merely the indifferent bearer of the unknown principle
of the male condition, but hereditary substance. I therefore be-
lieve that Minot’s, Balfour’s, and van Beneden’s hypothesis, al-
though an ingenious attempt which was quite justified at the time
when it originated, must be finally abandoned.

My opinion of the significance of the second polar body is
shortly this,—a reduction of the germ-plasm is brought about by its -
formation, a reduction not only in quantity, but above all in the
complexity of its constitution. By means of the second nuclear
division the excessive accumulation of different kinds of hereditary
tendencies or germ-plasms is prevented, which without it would be
necessarily produced by fertilization. With the nucleus of the
second polar body as many different kinds of idioplasm are removed
from the egg as will be afterwards introduced by the sperm-
nucleus; thus the second division of the egg-nucleus serves to
keep constant the number of different kinds of idioplasm, of which
the germ-plasm is composed during the course of generations.

In order to make this intelligible a short explanation is necessary.

From the splendid series of investigations on the process of
fertilization, commenced by Auerbach and Biitschli, and continued
by Hertwig, Fol, Strasburger, van Beneden, and many others,
and from the theoretical considerations brought forward by Pfliiger,
Niageli, and myself, 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, and in that part of it which forms the nuclear
thread, which at certain periods appears in the form of loops or
rods. We may further maintain that. fertilization consists in the
fact that an equal number of loops from either parent are placed —
side by side, and that the sezmentation nucleus is composed in this
‘way. It is of no importance, as far as this question is concerned,
whether the loops of the two parents coalesce sooner or later,
or whether they remain separate. The only essential conclusion

Aa 2

’
356 ON THE NUMBER OF POLAR BODIES AND

demanded by our hypothesis is that there should be complete or
approximate equality between the quantities of hereditary sub-
stance derived from either parent. If then the germ-cells of the
offspring contain the united germ-plasms of both parents, it follows
that such cells can only contain half as much paternal germ-plasm
as was contained in the germ-cells of the father, and half as much
maternal germ-plasm as was contained in the germ-cells of the
mother. This principle is affirmed in a well-known calculation
made by breeders of animals, who only differ from us in their use of
the term ‘blood’ instead of the term germ-plasm. Breeders say that
half of the ‘ blood’ of the offspring has been derived from the father
and the other half from the mother. The grandchild similarly
derives a quarter of its ‘ blood’ from each of the four grandparents,

and so on.

Let us imagine, for the sake of argument, that sexual repro-
duction had not been introduced into the animal kingdom, and
that asexual reproduction had hitherto existed alone. In such a
ease, the germ-plasm of the first generation of a species which
enters upon sexual reproduction must still be entirely homo-
geneous; the hereditary substance must, in each individual, con-
sist of many minute units, each of which is exactly like the other,
and each of which contains within itself the tendency to transmit,
under certain circumstances, the whole of the characters of the
parent to a new organism—the offspring. In each of the offspring
of such a first generation, the germ-plasms of two parents will be
united, and every germ-cell contained in the individuals of this second
sexually produced generation will now contain two kinds of germ-
plasm—one kind from the father, and the other from the mother.
But if the total quantity of germ-plasm present in each cell is to
be kept within the pre-determined limits, each of the two ancestral
germ-plasms, as I may now call them, must be represented by only
half as many units as were contained in the parent germ-cells.

In the third sexually produced generation, two new ancestral
germ-plasms would be added by fertilization to the two already
present, and the germ-cells of this generation would therefore con-
tain four different ancestral germ-plasms, each of which would
constitute a quarter of the total quantity. In each succeeding
generation the number of the ancestral germ-plasms is doubled,
while their quantities are reduced by one half. Thus in the fifth

THEIR SIGNIFICANCE IN HEREDITY. 357

sexually produced generation, each of the sixteen ancestral germ-
plasms will only constitute #; of the total quantity; in the sixth,
each of the thirty-two ancestral germ-plasms, only s';, and so on.
The germ-plasm of the tenth generation would be composed of
1024 different ancestral germ-plasms, and that of the n™ of 2”.
By the tenth generation each single ancestral germ-plasm would
only form zzz of the total quantity of germ-plasm contained in
a single germ-cell. We know nothing whatever of the length of
time over which this process of division of the ancestral germ-
plasms may have endured, but even if it had continued to the
utmost possible limit—so far indeed that each ancestral germ-
plasm was only represented by a single unit—a time would at last
come when any further division into halves would cease to be
possible ; for the very conception of a unit implies that it cannot
be divided without the loss of its essential nature, which in this
case constitutes it as the hereditary substance.

In the diagram represented in Fig. I. I have tried to render
these conclusions intelligible. In generation 1. each paternal
and maternal germ-plasm is still entirely homogeneous, and does
not contain any combination of different hereditary qualities, but
the germ-plasm of the offspring is made up of equal parts of
two kinds of germ-plasm. In the second generation this latter
germ-plasm unites with another derived from other parents, which
is similarly composed of two ancestral germ-plasms, and the re-
sulting third generation now contains four different ancestral germ-
plasms in its germ-cells, and so on. The diagram only indicates the
fusion of ancestral germ-plasms as far as the offspring of the fourth
generation, the germ-cells of which contain sixteen different an-
cestral germ-plasms. If we imagine the germ-plasm units to be
so large that there is only room for sixteen of them in the nuclear
thread, the limits of division would~be reached in the fifth genera-
tion, and any further division into halves of the ancestral germ-
plasms would be impossible.

Now however minute the units may be, there is not the least
doubt that the limits of possible division have been long since
reached by all existing species, for we may safely assume that no
one of them has acquired the sexual method of reproduction within
a small number of recent generations. All existing species must
therefore now contain as many different kinds of ancestral germ-

358 ON THE NUMBER OF POLAR BODIES AND

plasms as they are capable of containing ; and the question arises,—
How can sexual reproduction now proceed without a doubling of
the quantity of germ-plasm in each germ-cell, with every new
generation ?

There is only one possible answer to such a question :—sexual re-
production can proceed by a reduction in the xwmber of ancestral
germ-plasms, a reduction which is repeated in every generation.

Father. Mother. Offspring.

Generation I. Generation II.

» I,

This must be so: the only question is, how and when does the
supposed reduction take place.

Inasmuch as the germ-plasm is seated, according to our theory,
in the nucleus, the necessary reduction can only be produced by
nuclear division ; and quite apart from any observation which has
been already made, we may safely assert that there must be a form
of nuclear division in which the ancestral germ-plasms contained in

THEIR SIGNIFICANCE IN HEREDITY. 359

the nucleus are distributed to the daughter-nuclei in such a way
that each of them receives only half the number contained in the
original nucleus. After Roux’s! elaborate review of the whole
subject, we need no longer doubt that the complex method of
nuclear division, hitherto known as karyokinesis, must be con-
sidered not merely as a means for the division of the total quantity
of nuclear substance, but also for producing a division of the
quantity and quality of each of its single elements. In by far the
greater number of instances the object of this division is obviously
to effect an equal distribution of nuclear substance in the two
daughter-nuclei, so that each of the different qualities contained in
the mother-nucleus is transferred to the two daughter-nuclei. This
interpretation of ordinary karyokinesis is less uncertain than per-
haps at first sight it may appear to be. We cannot, it is true,
directly see the ancestral germ-plasms, nor do we even know the
parts of the nucleus which are to be looked upon as constituting
ancestral germ-plasm; but if Flemming’s original discovery of the
longitudinal division of the loops lying in the equatorial plane of
the nuclear spindle is to have any meaning at all, its object must
be to divide and distribute the different kinds of the minutest
elements of the nuclear thread as equally as possible. It has been
ascertained that the two halves produced by the longitudinal split-
ting of each loop never pass into the same daughter-nucleus, but
always in opposite directions. The essential point cannot therefore
be the division of the nucleus into absolutely equal quantities, but it
must be the distribution of the different qualities of the nuclear
thread, without exception, in both daughter-nuclei. But these dif-
ferent qualities are what I have called the ancestral germ-plasms, i.e.
the germ-plasms of the different ancestors, which must be contained
in vast numbers, but in very minute quantities, in the nuclear thread.
The supposition of a vast number is not only required by the
phenomena of heredity but also results from the comparatively
great length of the nuclear thread: furthermore it implies that
each of them is present in very small quantity. The vast number
together with the minute quantity of the ancestral germ-plasms
permit us to conclude that they are, upon the whole, arranged in a
linear manner in the thin thread-like loops: in fact the longitudinal

1 Wilhelm Roux, ‘Ueber die Bedeutung der Kerntheilungsfiguren.’ Leipzig,
1884.

360 ON THE NUMBER OF POLAR BODIES AND

splitting of these loops appears to me to be almost a proof of the
existence of such an arrangement, for without this supposition the
process would cease to have any meaning.

This is the only kind of karyokinesis which has been observed
until recently; but if the supposed nuclear division leading to
a reduction in the number of ancestral germ-plasms has any real
existence, there must be yet another kind of karyokinesis, in
which the primary equatorial loops are not split longitudinally, but
are separated without division into two groups, each of which forms
one of the two daughter-nuclei. In such a case the required redue-
tion in the number of ancestral germ-plasms would take place, for
each daughter-nucleus would receive only half the number which |
was contained in the mother-nucleus.

Now there is more evidence for the existence of this second kind
of karyokinesis than the fact that it is demanded by my theory ;
for I believe that it has been already observed, although it has not
been interpreted in this sense.

It is very probable that this is true of van Beneden’s ! observation
on the egg of Ascaris megalocephala: he found that the nuclear
division which led to the formation of the polar body differs from
the ordinary course of karyokinesis, in that the plane of division
is at right angles to that usually assumed. Carnoy ? has confirmed
this observation in its main features, and he has made the further
observation that out of the eight nuclear loops which are found at
the equator of the spindle, four are removed with the first polar
body, and that half of the remaining four are removed with the
second polar body. ‘The first of these two divisions would have to
be looked upon as a reduction, if it is certain that each of the eight _
nuclear loops consists of different ancestral germ-plasms ; but this
assumption is impossible, although on the other hand it cannot be
directly disproved: for we are not able to see the ancestral germ-
plasms. But it must nevertheless be maintained that the removal
of the first four loops does not imply a reduction in the number of
ancestral germ-plasms in the nucleus; because, as I have already
argued, two successive divisions of the number of ancestral germ-

1 E. van Beneden, ‘Recherches sur la maturation de loouf, la fécondation et la
division cellulaire. Gand et Leipzig, Paris, 1883.

2 J. B. Carnoy, ‘ La Cytodiérése de l’ceuf, la vésicule germinative et les globules
polaires de l’Ascaris megalocephala.’ Louvain, Gand, Lierre, 1886.

THEIR SIGNIFICANCE IN HEREDITY. 861

plasms into halves is inconceivable; and because the first polar
body is also present in parthenogenetic eggs in which such division
into halves cannot take place. But the karyokinetic process can
readily be looked upon as a removal of ovogenetic nucleoplasm, for
we know from the observations of Flemming and Carnoy, that,
under certain circumstances, subsequent divisions may occur, in-
volving an increase in the number of nuclear loops to double their
number. These subsequent divisions of course take place in the
daughter-nuclei. This fact proves, as I think, that there are nuclei
in which the same ancestral germ-plasm occurs in two different
loops: but such loops, identical as regards the composition of their
ancestral germ-plasms, may very well contain different ontogenetic
stages of this substance. This will be the case in the instance
alluded to, if four loops of the first nuclear spindle are to be looked
upon as ovogenetic nucleoplasm, and the four others as germ-
plasm. It is therefore unnecessary to regard the first division of
the egg-nucleus as a ‘reducing division’: it may be looked upon as
an ‘equal division’! entirely analogous to the kind of division which,
in my opinion, directs the development of the embryo. This con-
clusion would receive direct proof if it were possible to show that
the eight loops of the first division have arisen by the longitudinal
splitting of four primary loops: for a longitudinal splitting of the
nuclear thread would be the means by which the different onto-
genetic stages of the germ-plasm could be separated from one
another, without leading to any reduction in the number of ances-
tral germ-plasms in the daughter-nuclei. Thus I have previously
attempted to prove that the ontogenetic development of the ege
must be connected with a progressive transformation of the nucleo-
plasm during successive nuclear divisions, and this transformation
will very frequently (but not always) occur in such a way that the
different qualities of the nucleoplasm are separated from one another
by the nuclear division. The nucleoplasm of the daughter-nuclei
will be identical if the two daughter-cells are to potentially contain
corresponding parts of the embryo; as for instance the first two
segmentation spheres of the egg of the frog, which according to
Roux? correspond to the right and left halves of the future animal.
1 See p. 364.

* Wilhelm Roux, ‘Beitrige zur Entwicklungsmechanik des Embryo, No. 3,
Breslauer arztliche Zeitschrift, 1885, p. 45.

362 ON THE NUMBER OF POLAR BODIES AND

But the nucleoplasm must be unequal if the products of division
are to develope into different parts of the embryo. In both cases,
however, karyokinesis is connected with a longitudinal splitting of
the nuclear threads, and we may conclude from this fact (which is
also confirmed by the phenomena of heredity) that all such nuclei,
whether they have entered upon the same or different ontogenetic
transformations of their nucleoplasm, are identical as regards the
ancestral germ-plasm which they contain. During the whole pro-
cess of seementation and the entire development of the embryo, the
total number of ancestral germ-plasms which were at first contained
in the germ-plasm of the fertilized egg-cell must still be contained
in each of the succeeding: cells.

Thus no objection can be raised against the view that the four
loops of the first polar body contain the ovogenetic nucleoplasm,
that is to say, an idioplasm which contains the total number of an-
cestral germ-plasms, but at an advanced and highly specialized
ontogenetic stage.

The formation of the second polar body may be rightly considered
as a ‘reducing division, as a division leading to the expulsion of
half the number of the different ancestral germ-plasms, in the form
of two nuclear loops, for no reason can be alleged in support of the
assumption that the four loops of the second nuclear spindle are
made up of identical pairs. Furthermore the facts of heredity re-
quire the assumption that the greatest possible number of ancestral
germ-plasms is accumulated in the germ-plasm of each germ-cell,
and thus that the small number of loops not only means an increase
in quantity but a multiplication in the number of different ancestral
germ-plasms present in each of them. If this conclusion be correct,
there can be no doubt that the second division of the egg-nucleus
means a reduction in the above-mentioned sense.

But there are yet other observations which, if correct, must also
be considered as ‘ reducing divisions.’ I refer to all those cases in
which the longitudinal splitting of the loops is either entirely
wanting, or does not occur until after the loops have left the equator
of the spindle and have moved towards the poles. In both instances
the bearing upon the question would be the same, for only half the
number of primary loops would reach each pole in either case. If
therefore the primary loops are not made up of identical pairs, it
follows that the two daughter-nuclei can only contain half the

THEIR SIGNIFICANCE IN HEREDITY. 363

number of ancestral germ-plasms which were contained in the
mother-nucleus. Whether the loops divide on their way to the
poles or at the poles themselves, no difference will be brought about
in the number of ancestral germ-plasms which they contain, for
this number can neither increase nor diminish.. The quantity of
the different ancestral germ-plasms can alone be increased in this
way. I am here referring to observations made by Carnoy! on
the cells which form the spermatozoa in various Arthropods. It
must be admitted, however, that these divisions cannot be regarded
as ‘reducing divisions, if Flemming’s? suggestion be confirmed,
that in all these observations the fact has been overlooked that the
equatorial loops are not primary but secondary, and that they have
arisen from the longitudinal splitting of the nuclear thread during
previous stages of nuclear division. But this point can only be
decided by renewed investigation. Although many excellent re-
sults have been obtained in the subject of karyokinesis, there is still
very much to be learnt before our knowledge is complete ; and this
is not to be wondered at when we remember the great difficulties in
the way of observation which are chiefly raised by the minute size
of the objects to be investigated. Flemming’s most recent publica-
tions prove that we are still in the midst of investigation, and that
highly interesting and important processes have hitherto escaped
attention. A secure basis of facts is only very gradually obtained,
and there are still many conflicting opinions upon the details of this
process. I should therefore consider it to be entirely useless, from my
point of view, to enter into a critical examination of everything
known about all the details of karyokinesis. I am quite content to
have shown how it may be imagined that the reduction required by
my theory takes place during nuclear division; and at the same
time to have pointed out that there are already observations which
. may be interpreted in this sense. But even if I am mistaken in
this interpretation, the theoretical necessity for a reduction in the
number of.ancestral germ-plasms, a reduction repeated in every
generation, seems to me to be so securely founded that the processes
by which it is effected must take place, even if they are not supplied
by the facts already ascertained. There must be two kinds of karyo-

1 Carnoy, ‘ La Cytodiérése chez les Arthropodes.’? Louvain, Gand, Lierre, 1885.
? Flemming, ‘Neue Beitrige zur Kenntniss der Zelle.’ Arch. f. mikr, Anat.
Bd. XXTX, 1887.

364 ON THE NUMBER OF POLAR BODIES AND

kinesis according to the different physiological effect of the process.
First, a karyokinesis by means of which all the ancestral germ-plasms
are equally distributed in each of the two daughter-nuclei after
having been divided into halves: secondly, a karyokinesis by means
of which each daughter-nucleus receives only half the number of
ancestral germ-plasms possessed by the mother-nucleus. The former
may, be called ‘equal division,’ the latter ‘reducing division. Of
course these two processes, which differ so greatly in their effects,
must also be characterized by morphological differences, but we
cannot assume that the latter are necessarily visible. Just as, during
the division of the first and second nuclear spindle in the egg of
Ascaris megalocephala, karyokinesis takes, upon the whole, the same
morphological course, although we must ascribe different physio-
logical meanings to the two processes of division,—so it may be in
other cases. The ‘reducing division’ must be always accompanied
by a reduction of the loops to half their original number, or by a
transverse division of the loops (if such division ever occurs) ;

although reduction can only occur when the loops are not made up

of identical pairs. And it will not always be easy to decide whether
this is the case. On the other hand, the form of karyokinesis in which
a longitudinal splitting of the loops takes place before they separate
to form the daughter-nuclei must always, as far as I can see, be
considered as an ‘equal division.’ In the accompanying figures II
and III, diagrams are given illustrating these two forms of karyo-
kinesis, but I do not mean to imply that it is impossible to imagine
any other form in which they may occur.

In Figure IT a nuclear spindle is seen at A, and at its equatorial
zone there are twelve primary loops. The transverse cross-lines
and other markings on the loops indicate that they are composed
of different ancestral germ-plasms. The loops are shaded differently
in order to render the diagram clear. At B six of the loops are
seen to have moved to either pole, so that the figure is a repre-
sentation of the ‘reducing division.’ Figure III is a diagrammatic

representation of ‘equal division.’ The six loops at the equatorial

zone of A are shown by different cross-lining and shading to be
composed of different ancestral germ-plasms. The loops split
longitudinally in a direction indicated by the longitudinal line
upon each of them. In B the halves of the loops are seen to
have moved to the opposite poles of the spindle, so that there

THEIR SIGNIFICANCE IN HEREDITY. 3865

are not only six loops at each pole, but also all the six combina-
tions of ancestral germ-plasms.

Perhaps some may be inclined to look upon direct nuclear
division as a ‘reducing division,’ but I believe that such a view

Fias. II, TI.

would be incorrect. It is only approximately true that the nuclear
thread is divided into two halves of equal quantity by direct
division, and exact equality would only happen as it were acci-
dentally; so that we cannot speak of a perfectly equal distribution
of the ancestral germ-plasm in the two daughter-nuclei. But the
‘reducing division’ must obviously effect an exactly regular and

;

366 ON THE NUMBER OF POLAR BODIES AND

uniform distribution of the ancestral germ-plasms, although this does .
not imply that every ancestral germ-plasm of the mother-nucleus
would be represented in each of the two daughter-nuclei. But
if out of e.g. eight nuclear loops at the equatorial plane, four pass
into one, and the other four into the other daughter-nucleus, each of
the latter will contain an equal number of ancestral germ-plasms,
although different ones. This is indeed part of the foundation
of the theory, for the ‘reducing division’ must remove exactly
half of the original number of ancestral germ-plasms, and pre-
cisely the same number must be replaced at a later period by
the sperm-nucleus. This could hardly be achieved with sufficient
precision by direct nuclear division.

I now come to inquire whether the expulsion of the second
polar body is in reality, as I have already maintained, a reduction
in the number of ancestral germ-plasms present in the nucleus of
the egg. The view itself is sufficiently obvious, and it would
supply an explanation of the meaning of the process which is still
greatly wanted ; but it will nevertheless be not entirely useless to
consider other possible theories.

It would be quite conceivable to suppose that the youngest
egg-cells, which multiply by division, may undergo one ‘ reducing
division’ in addition to the ordinary process. Of course this
should occur once only, for if repeated, the number of ancestral
idioplasms in the nucleus of the germ-cell would undergo a decrease
greater than could be afterwards compensated by the increase due
to fertilization. Thus the number of ancestral germ-plasms would
continually decrease in the course of generations,—a process which
would necessarily end with their complete reduction to a single
kind, viz. to the paternal or the maternal germ-plasm. But the
occurrence of such a result is disproved by the facts of heredity.
Although such an early occurrence of the ‘reducing division’
would offer advantages in that nothing would be lost, for both
daughter-nuclei would become eggs, instead of one of them being
lost as a polar body, nevertheless I do not believe that it really
occurs: weighty reasons can be alleged against it.

Above all, the facts of parthenogenesis are against it. If the
number of ancestral germ-plasms received from the parents were
reduced to half in the ovary of the young animal, how then could
parthenogenetic development ever take place? It is true that

THEIR SIGNIFICANCE IN HEREDITY. 367

we cannot at once assert the impossibility of an early ‘ reducing
‘division’ on this account, for as I have shown above, the power to
develope parthenogenetically depends upon the quantity of germ-
plasm contained in the mature egg; the necessary amount might
be produced by growth, quite independently of the number of
different kinds of ancestral germ-plasms which form its constituents.
The size of a heap of grains may depend upon the number'of grains,
and not upon the number of different kinds of grains. But in
another respect such a supposition would lead to an unthinkable
conclusion. In the first place, the number of ancestral germ-plasms
in the germ-cells would be diminished by one half in each new
generation arising by the parthenogenetic method ; thus after ten
generations only yo'sz of the original number of ancestral germ-
plasms would be present.

Now, it might be supposed that the ‘ reducing division ’ of the
"young ege-cells was lost at the time when the parthenogenetic
mode of reproduction was assumed bya species ; but this suggestion
cannot hold, because there are certain species in which the same
eggs can develope either sexually or parthenogenetically (e.g. the
bee). It seems to me that such cases distinctly point to the fact
that the reduction in the number of ancestral germ-plasms must
take place immediately before the commencement of embryonic
development, or, in other words, at the time of maturation of the
ege. It is only decided at this time whether the egg of the bee
is to develope into an embryo by the parthenogenetic or the sexual
method ; such decision being brought about, as was shown above,
by the fact that only one polar body is expelled in the first case,
while two are expelled in the second. But if we are obliged to
assume that reproduction by means of fertilization, necessarily
implies a reduction to one half of the number of ancestral germ-
plasms inherited from the parents,—the further conclusion is
obvious, that the second division of the egg-nucleus and the expul-
sion of the second polar body represent such a reduction, and that
this second division of the egg-nucleus is unequal in the sense
mentioned above, viz. one half of the ancestral germ-plasms re-
mains in the egg-nucleus, the original number being subsequently
' restored by conjugation with a sperm-nucleus; while the other
half is expelled in the polar body and perishes.

I may add that observations, so far as they have extended to

368 ON THE NUMBER OF POLAR BODIES AND

such minute processes, do indeed prove that the number of loops
is reduced to one half. It has been already mentioned that, ac-
cording to Carnoy, such reduction occurs in Ascaris megalocephala,
but the same author also describes the process of the formation
of polar bodies in a large number of other Nematodes, and his
descriptions show that the process occurs in such a way that the
number of ancestral germ-plasms must be reduced by half. Some-
times half the number of primary loops pass into the nucleus of the
polar body, while the other half remains in the egg. In other
cases, as in Ophiostomum mucronatum, the primary nuclear rods
divide transversely,—a process which must produce the same effect.
It is true that these observations require confirmation, and since,
with unfavourable objects, the difficulties of observation are ex-
tremely great, there may have been errors of detail; but I do
not think that there is any reason for doubting the accuracy of
the essential point. And this essential point is the fact that the
number of primary loops is divided into half by the formation of
the polar body.

But even if we could not admit that such a conclusion is securely
founded, it cannot be doubted that the formation of the second polar
body reduces to one half the quantity of the nucleus which would have
become the segmentation-nucleus in.the parthenogenetic develop-
ment of the egg. This is a simple logical conclusion from the
two following facts: first, parthenogenetic eggs expel only one polar
body; secondly, there are eggs (such as those of the bee) in which
it is absolutely certain that the same half of the nucleus—which
is expelled as the second polar body in the egg requiring fertil-
ization—remains in the egg when it is to develope parthenoge-
netically, and acts as half of the segmentation-nucleus. But this
proves that the expelled half of the nucleus must consist of true
germ-plasm, and thus a secure foundation is laid for the assump-
tion that the formation of the nucleus of the second polar body
must be considered as a ‘ reducing division.’

I was long ago convinced that sexual reproduction must be
connected with a reduction in the number of ancestral germ-plasms
to one half, and that such reduction was repeated in each genera-
tion. When, in 1885, 1 brought forward my theory of the conutinuity

1 Carnoy, ‘La Cytodiérése de l’ceuf; la vésicule germinative et les globules
polaires chez quelques Nématodes.’ Louvain, Gand, Lierre. 1886.

THEIR SIGNIFICANCE IN HEREDITY. 369

of the germ-plasm, I had long before that time considered whether
the formation and expulsion of polar bodies must not be inter-
preted in this sense. But the two divisions of the egg-nucleus
caused me to hesitate. The two divisions did not seem to admit
of such an interpretation, for by it the quantity of the nucleus
is not divided into halves, but into quarters. But a division
of the number of ancestral germ-plasms into quarters would have
caused, as was shown above, a continuous decrease, leading to their
complete disappearance; and such a conclusion is contradicted by
the facts of heredity. -For this reason I was led at that time
to oppose Strasburger’s view that the expulsion of the polar bodies
means a reduction of the quantity of nuclear substance by only
half. My objection to such a view was valid when I said that the
quantity of idioplasm contained in the egg-nucleus is not, as a
matter of fact, reduced to one half, but to one quarter, inasmuch
a8 two successive divisions take place. I may add that I had also
considered whether the two successive divisions might not possess
an entirely different meaning,—whether one of them led to the
removal of ovogenetic nucleoplasm, while the other resulted in a
reduction in the number of ancestral germ-plasms. But at that
time there were no ascertained facts which supported the suppo-
sition of such a difference, and I did not wish to bring forward the
idea, even as a suggestion, when there was no secure foundation
for it. The morphological aspects of the formation of the first
and second polar bodies are so extremely similar that such a
supposition might have been considered as a mere effort of the
imagination.

Hensen* also rejected the second part of the supposition that
reduction must take place in the number of the hereditary elements
of the egg, and that such reduction is caused by the expulsion of °
polar bodies, because he believed it to be incompatible with the
fact, which had just been discovered, that polar bodies are formed
by parthenogenetic eggs. He concludes with these words: ‘If
this striking fact be confirmed, the hypothesis which assumes that
the egg must be divided into half before maturation, is refuted,
and there only remains the rather vague explanation that a pro-
cess of purification must precede the development of the embryo.’
1 Hensen, ‘Die Grundlagen der Vererbung nach dem gegenwirtigen Wissens-

kreis,’ Zeitschr. f: wissenschaftl. Landwirthschaft, Berlin, 1885, p. 731.
Bb

370 ON THE NUMBER OF POLAR BODIES AND

Nevertheless Hensen is the only writer who has hitherto taken
into consideration the idea that sexual reproduction causes a
regularly occurring ‘diminution in the hereditary elements of —
the egg.’

III. Tue Forecoinac ConstpERATIONS APPLIED TO THE MALE
Gerrmu-CELLs.

If the result of the previous considerations be correct, and if
the number of ancestral germ-plasms contained in the nucleus of
the egg-cell destined for fertilization must be reduced by one half,
there can be no doubt that a similar reduction must also take
place, at some time and by some means, in the germ-plasms of the
male germ-cells. This must be so if we are correct in maintaining
that the young germ-cells of a new individual contain the same
nuclear substance, the same germ-plasm, which was contained in the
fertilized egg-cell from which the individual has been developed.
The young germ-cells of the offspring must contain this substance
if my theory of the continuity of the germ-plasm be well founded,
for this theory supposes that, during the development of a fer-
tilized egg, the whole quantity of germ-plasm does not pass through
the various stages of ontogenetic development, but that a small
part remains unchanged, and at a later period forms the germ-
cells of the young organism, after having undergone an increase in
quantity. According to this supposition therefore the germ-plasm
of the parents must be found unchanged in the germ-cells of the
offspring. elf this theory were false, if the germ-plasm of the germ-
cells were formed anew by the organism, perhaps from Darwin’s
‘oeemmules’ which pour into the germ-cells from all sides, it
would be impossible to understand why it has not been long ago
arranged that each germ-cell should receive only half the number
of the ancestral gemmules present in the body of the parent.
Hence the expulsion of the second polar body—assuming the
validity of my interpretation—is an indirect proof of the soundness
of the theory of the continuity of the germ-plasm, when contrasted
with the theory of pangenesis. If furthermore, a kind of cyclical
development of the idioplasm took place, as supposed by Stras-
burger, and if its final ontogenetic stage resulted in the re-appear-
ance of the initial condition of the germ-plasm, we should fail to

THEIR SIGNIFICANCE IN HEREDITY. 371

understand how any of the ancestral germ-plasms could be lost
during such a course of development.

Whichever view, the latter or the theory of the continuity of
the germ-plasm, be correct, in either case the male germ-cells of
the young animal: must contain the same germ-plasm as that
which existed in the fertilized maternal egg, that is to say, they
must contain all the ancestral germ-plasms of the father and the
mother. Here therefore a reduction must occur, for otherwise the
number of ancestral germ-plasms would be increased by one half at
every fertilization. The egg-cell would furnish 3, but the sperm-
cell 3 of the total quantity of germ-plasm present in the germ-
cells of the parents. But there is no reason for believing that
the reduction of germ-plasm in the sperm-cell must proceed in
precisely the same way as in the egg-cell, viz. by the expulsion
of a polar body. On the contrary, the processes of spermato-
genesis are so remarkably different from those of ovogenesis that
we may expect to find that reduction is also brought about in a
different manner.

The egg-cell does not expel the superfluous ancestral germ-
plasms until the end of its development, and in a form which
induces the destruction of the separated portion. This is certainly
remarkable, for germ-plasm is a most important substance, and
although it seems to be wasted in the production of enormous
quantities of sperm- and egg-cells, such waste is only apparent,
and is in reality the means which renders the species capable of
existence. It may perhaps be possible to prove that in this case
also the waste is only apparent. Such proof would be forthcoming
if it could be shown that the means by which reduction is brought
about in eggs is advantageous, and therefore also, ceteris paribus,
necessary. We see that everywhere, as far as our observation ex-
tends, the useful is also the actual, unless indeed it is impossible
of attainment or can only be attained by the aid of processes which
“are injurious to the species. And if it be asked why germ-plasm
is wasted in the maturation of egg-cells, the following may per-
haps be a satisfactory answer.

Let us suppose that the necessary reduction of the germ-plasm _
does not take place by the separation of the second polar body, but
that it happens during the first division of the first primitive-germ-
cell which is found in the embryo, so that the two first egg-cells

; Bb2 ;

372 ON THE NUMBER OF POLAR BODIES AND

resulting from this division would already contain only half the
number of ancestral germ-plasms from the father and the mother,
contained in the fertilized egg-cell.. In this case the main object,
the reduction of the ancestral germ-plasms, would be gained by a
single division, and all the succeeding nuclear divisions, causing the
multiplication of these two first germ-cells, might take place by
the ordinary form of nuclear division, viz. ‘equal division, But
perhaps nature not only cares for this one main object alone, but
also secures certain secondary advantages at the same time. In the
case which we have supposed the egg-cells of the mature ovary
would only contain two different combinations of germ-plasm,
which we may call combinations 4 and B. Even if millions of
egg-cells were formed, every one of them would contain either 4
or B, and hence (at least as far as the female pronucleus is con-
cerned) only two kinds of individuals could arise from such eggs—
viz. offspring 4’ and B’. All the offspring 4’ would be as similar
to one another as identical twins, and the same would be true
of offspring 2’.

But if the 1ooth instead of the 1st embryonic germ-cell entered
upon the ‘reducing division,’ a hundred cells would undergo this
division at the same time, and thus two hundred different com-
binations of ancestral germ-plasm would arise, and two hundred
different kinds of germ-cells would be found in the mature ovary.
A still greater number of different combinations of hereditary ten-
dencies would arise if the ‘ reducing division’ occurred still later ;
but undoubtedly the diversity in the composition of the germ-
plasm must be greatest of all when the ‘reducing division’ does
not take place during the period in which the germ-cells undergo
multiplication, but at the end of the entire course of ovarian
development, and separately in each full-grown mature egg ready
for embryonic development. In such a case there will be as many
different combinations of ancestral germ-plasms as there are eggs,
for, as I have shown above, it is hardly conceivable that such a
complex body as the nuclear substance of the egg-cell—composed
of innumerable different units—would ever divide twice in pre-
cisely the same manner. Every egg will therefore contain a
somewhat different combination of hereditary tendencies, and thus
the offspring which arise from the different germ-cells of the same
mother can never be identical. Hence by the late occurrence of the

THEIR SIGNIFICANCE IN HEREDITY. 3738

‘reducing division ’ the greatest possible variability in the offspring
is secured,

If my interpretation of the second polar body be accepted, it
is obvious that the late occurrence of the ‘reducing division’ is
proved. At the same time we receive an explanation of the ad-
vantage gained by the postponement of the reduction of the germ-
plasm until the end of the ovarian development of the egg;
because the greatest possible number of individual variations in
the offspring are produced in this way.

If I am not mistaken, this argument lends additional support to
the idea which I have previously propounded,—that the most
important duty of sexual reproduction is to preserve and con-
tinually call forth individual variability, the foundation upon which
the transformation of species is built 1.

But if it be asked whether the postponement of the ‘reducing —
division ’ to the end of the ovarian development of the egg is incon-
sistent with the preservation of the other half of the dividing nucleus,
I should be inclined to reply that a ‘reducing division’ of the
mature egg, resulting in the production of two eggs, was probably
the phyletic precursor of the present condition.. I imagine that the
division of the mature egg-cell—although it is now so extremely
unequal—was equal in very remote times; but that for reasons of
utility, connected with the specialization of the eggs of animals, it
gradually became more and more unequal. It is now hardly pos-
sible to give in detail the various reasons of utility which have
brought about this condition, but it may be assumed that the
enormous size attained by many animal egg-cells has been espe-
cially potent in producing the change.

A careful consideration of this last point seems to me to be
demanded by a comparison of the egg-cells with the male germ-
cells. Just as the female germ-cells of animals are distinguished
by the attainment of a large size, the male germ-cells are generally
remarkable for their minute proportions. In most cases it would
be physiologically impossible for a large egg-cell, rich in yolk, to
attain double its specific size in order to undergo division into two
equal halves and yet to remain of the characteristic size. Even
without the additional difficulties imposed by the necessity for such

1 See the preceding Essay on ‘The Srgnideanse of Sexual Reproduction in the
theory of Natural Selection.’

374 - ON THE NUMBER OF POLAR BODIES AND

a division, all means—such as cells used as food, or the passage of
food from follicular cells into the ovum, ete.—are employed in
order to bring the egg-cell to the greatest attainable size. Fur-
thermore, the ‘ reducing division’ of the nucleus cannot take place
before the egg has attained its full size, because the ovogenetic
nucleoplasm still controls the egg-cell, and must be removed before
the germ-plasm can regulate its development. By arguments such
as these I should attempt to render the whole subject intelligible.
But the case is entirely different with the sperm-cells, which
are generally minute: here it is quite conceivable that a ‘re-
ducing division’ of the nuclei may take place by an equal division
of the sperm-cells, occurring towards the end of the period of their
formation ; that is to say, in such a way that both products of
division remain sperm-cells, and neither of them perishes like the
polar bodies. But the other possibility also demands consideration,
viz. that the reducing division may occur at an earlier stage in the
development of sperm-cells. At all events, the arguments adduced
above, which proved that the consequence would be a want of vari-
ability in the egg-cells, would not apply to an equal extent in the
case of the male germ-cells. Among the egg-cells it may be very
important that each one should have its special individual cha-
racter, produced by a somewhat different composition of its germ-
plasm, inasmuch as a considerable proportion of the eggs frequently
developes, although this is never the case with all of them. But
the production of sperm-cells is in most animals so enormous that
only a very small percentage can be used for fertilization. If,
therefore, e.g. ten or a hundred spermatozoa contained germ-plasm
with exactly the same composition, so that, as far as the paternal
influence is concerned, ten or a hundred identical individuals would
result if they were all used in fertilization, such an arrangement
would be practically harmless, for only one spermatozoon out of an
immense number would be employed for this purpose. From this
point of view we might expect that the ‘reducing division’ of the
sperm-nucleus would not take place at the end of the development
of the sperm-cell, but at some earlier period. There is no necessary
reason for the assumption that this division must take place at the
end of development, and without some cause natural selection can- |
not operate. It is, of course, conceivable that the causes of other
events may also involve the occurrence of this division at the end

THEIR SIGNIFICANCE IN HEREDITY. 875

of development; but we do not at present know of any such causes.
I should not consider the influence of the specific histogenetic
nucleoplasm, i.e. the spermatogenetic nucleoplasm, to be such a
cause, because the quantitative proportions are very different from
those which obtain in the formation of egg-cells, and because it is
not inconceivable that the small quantity of true germ-plasm
which must be present in the nuclei of the sperm-cells at every
stage in their formation might enter upon a ‘reducing division’
with’ the. spermatogenetic nucleoplasm, even when the latter pre-
ponderated.

As soon as we can recognize with certainty the forms of nuclear
division which are ‘reducing divisions,’ the question will be settled
as far as spermatogenesis is concerned. It has been already estab-
lished that various forms of nuclear division oceur at different

periods of spermatogenesis. I make this assertion, not only from |

my own observations, but also from observations which have been
made and insisted upon by others. Thus, van Beneden and Julin!
stated in 1884 that direct and karyokinetic nuclear divisions
alternate with each other in the spermatogenesis of Ascaris megalo-
cephala. Again, Carnoy? distinctly states that the different cell-
generations in the same testis may not uncommonly exhibit con-
siderable differences as regards karyokinesis. ‘This may go so far
that direct and indirect division may proceed simultaneously.’
Platner®, in his excellent paper on karyokinesis in Lepidoptera,
also points out that the karyokinesis of the spermatocytes is
essentially different from that of the spermatogonia. According
to his description, the latter form may be very well interpreted as
a ‘reducing division, for no equatorial plate is formed, and the
chromatin rods (or granules, as they are better called in this case)
remain from the first on both sides of the equatorial plane, and
finally unite at the opposite poles to form the two daughter-nuclei.
Furthermore, if Carnoy has correctly observed, the form of karyo-
kinesis which I have previously interpreted as a ‘ reducing division’
occurs in the sperm-mother-cells—a karyokinesis in which the

1 E. van Beneden and Julin, ‘ La Spermatogénése chez l’Ascaride mégalocéphale,’
Brussels, 1884.

? Carnoy, ‘ La Cytodiérése chez les Arthropodes.’

® Gustav Platner, ‘ Die Karyokinese bei den Lepidopteren als Grundlage fiir eine
Theorie der Zelltheilung.’ Internation, Monatsschrift f. Anatomie und Histologie,
Bd. III. Heft 10. Leipzig, 1886.

376 ON THE NUMBER OF POLAR BODIES AND

chromatin rods either do not divide longitudinally, or else divide
in this way after they have left the equatorial plate and are
proceeding towards the poles. Carnoy does not himself attach any
special importance to these observations, for he only considers
them as proofs that the longitudinal splitting of the loops may
occur at various periods in different species—either at the equator,
or on the way towards the poles, or even at the poles themselves.
We cannot conclude from the author's statements whether this
form of nuclear division only occurs in a single cell-generation
during spermatogenesis, as it must do if it really represents
a ‘reducing division. Until this point is settled, we cannot
decide with certainty whether the described form of karyokinesis
is to be considered as the ‘reducing division’ for which we are
seeking. Fresh investigations, undertaken from these points of
view, are necessary in order to settle the question. It would be
useless to seek further support for the theory by going into further
details, and by critically examining the numerous observations
upon spermatogenesis which have now been recorded. -

I will only mention that among the various nuclei and other
bodies in different animals which have been considered by different
observers as the polar bodies of the sperm-cells, or the cells which
form the latter—in my opinion the paranucleus (‘ Nebenkern’) of
the ‘spermatides’ described by La Valette St. George! has the
highest claim to be considered as the homologue of a polar body.
But I am inclined to identify it with the first rather than the
second polar body of the egg-cells, and to regard it as the histo-
genetic part of the nucleoplasm which has been expelled or
rendered powerless by internal transformations. There are two
reasons which lead me to this conclusion: first, as I have tried to
show above, it is probable that the ancestral germ-plasms are not
removed by expulsion, but by means of equal cell-division ;
secondly, my theory asserts that the histogenetic nucleoplasm
cannot be rendered powerless until the close of histological differ-
entiation.

The whole question of the details of the transformations under-
gone by the nucleus of the male germ-cells is not ready for the

* La Valette St. George, ‘ Ueber die Genese der Samenkérper.’ Fiinfte Mittheilung.
Die Spermatogenese bei den Siiugethieren und dem Menschen,’ Archiv f. mikrosk.
Anat. Bd. XV. 1878.

THEIR SIGNIFICANCE IN HEREDITY. 377

expression of a mature opinion. From the very numerous and
mostly minute and careful observations which have been hitherto
recorded, we cannot conclude with any degree of certainty when
and how the ‘reducing division’ of the nucleus takes place, nor can
we decide upon the processes which signify the purification of the
germ-plasm from the merely histogenetic part of the nucleoplasm.
But perhaps it has not been without value as regards future in-
vestigation that I have tried to apply to the male germ-cells the
views gained from our more certain. knowledge of the corresponding
structures in the female, and thus to indicate the problems which
now chiefly demand solution.

IV. Tue Forrcornc CoNsIDERATIONS APPLIED TO PLANTS.

It remains to briefly consider the case of plants. Obviously, the
‘reducing division’ of the germ-nuclei, if it takes place at all,
cannot he restricted to the germ-cells of animals. There must
be a corresponding process in plants, for sexual reproduction is
essentially the same in both kingdoms; and if fertilization must
be preceded by the expulsion of half the number of ancestral germ-
plasms from the eggs of animals, the same necessity must hold
in the case of plants.

But whether the process always takes place in the form of polar
bodies, and not perhaps principally, or at any rate frequently, in
the form of equal cell-division, is another question. It is true that
polar bodies occur in numerous plants, as we chiefly know from |
Strasburger’s researches’. Strasburger shows that cells are se-
parated by division from the germ-cells, and perish. But it seems
to me doubtful whether we must always regard their formation as
the removal of half the number of ancestral germ-plasms rather
than the histogenetic nucleoplasm of the germ-cell. It appears to
me that histogenetic nucleoplasm must be present in the highly
differentiated vegetable germ-cells, especially in the male cells, and
also that it must be removed during the maturation of the cell, if
my idea of the histogenetic nucleoplasm be accepted. It is very
possible, as I have already mentioned, that there may be quite
indifferent germ-cells, viz. cells which are entirely without specific
histological structure, and in such cases histogenetic nucleoplasm

1.1. ©, p..92-

378 ON THE NUMBER OF POLAR BODIES AND

would be absent; and during the maturation of such germ-cells
no polar body would be formed for its removal. This view accords
with the fact that polar bodies are absent in many plants. Further-
more, I am. far from maintaining that in the cases where polar
bodies occur, they must have the above-mentioned significance.
I only wish to point out that the reduction assumed to be neces-
sary for the nucleus of the vegetable germ-cells is not necessarily
to be sought for at the close of their maturation, but perhaps even
. more frequently in an equal division of the germ-cells during some
period of their development.

It also seems to me to be not impossible that a number of these
vegetative ‘polar bodies’ may have an entirely different signifi-
cance, viz. to perform some special function accessory to fertiliza-
tion, as in the so-called ‘ ventral canal-cells’ of the higher erypto-
gams and conifers. As we know that even the two polar bodies
of the animal egg are not identical—although externally they are
extremely similar, and although they arise in a precisely similar
manner—I am even more inclined than before to consider that
the very various ‘polar bodies’ of plants possess very different
meanings.

But I do not feel justified in criticizing in detail the results of
botanical investigation. I must leave the decision of such ques-’
tions to botanists, and I only desire to state distinctly that a ‘re-
ducing division’ of the nuclei of germ-cells must occur in plants
as well as in animals,

V. CONCLUSIONS WITH REGARD TO HeErepIry.

The ideas developed in the preceding paragraphs lead to remark-
able conclusions with regard to the theory of heredity,—conclusions
which do not harmonize with the ideas on this subject which have
been hitherto received. For if every egg expels half the number of
its ancestral germ-plasms during maturation, the germ-cells of the
same mother cannot contain the same hereditary tendencies, unless
of course we make the supposition that corresponding ancestral
germ-plasms are retained by all eggs—a supposition which can-
not be sustained. For when we consider how numerous are the
ancestral germ-plasms which must be contained in each nucleus,
and further how improbable it is that they are arranged in

THEIR SIGNIFICANCE IN HEREDITY. 379

precisely the same manner in all germ-cells, and finally how in-
eredible it is that the nuclear thread should always be divided in
exactly the same place to form corresponding loops or rods,—we are
driven to the conclusion that it is quite impossible for the ‘ re-
ducing division’ of the nucleus to take place in an identical manner
in all the germ-cells of a single ovary, so that the same ancestral
germ-plasms would always be removed in the polar bodies. But if
one group of ancestral germ-plasms is expelled from one egg, and a
different group from another egg, it follows that no two eggs can
be exactly alike as regards their contained hereditary tendencies :
they must all differ. In many cases the differences will only be
slight, that is, when the eggs contain very similar combinations of
ancestral germ-plasms. Under other circumstances the differences
will be very great, viz. when the combinations of ancestral germ-
plasms retained in the egg are very different. I might here mention
various other considerations ; but this would lead me too far from my
subject, into new theories of heredity. I hope to be able at some
later period to develope further the theoretical ideas which are
merely indicated in the present essay. I only wish to show that the
consequences which follow from my theory upon the second division
of the egg-nucleus, and the formation of the second polar body,
are by no means opposed to the facts of heredity, and even explain
them better than has hitherto been possible.

The fact that the children of the same parents are never entirely
identical could hitherto only be rendered intelligible by the vague
suggestion that the hereditary tendencies of the grandfather pre-
dominate in one, and those of the grandmother in another, while
the tendencies of the great-grandfather predominate in a third,
and soon. Any further explanation as to why this should happen
was entirely wanting. Others even looked for an explanation
to the different influences of nutrition, to which it is perfectly
true that the egg is subjected in the ovary during its later de-
velopment, according to its position and immediate surroundings.
I had myself referred to these influences as a partial explanation},
before I recognized clearly how extremely feeble and powerless are
the influences of nourishment, as compared with hereditary ten-
dencies. According to my theory, the differences between the

1 Weismann, ‘ Studien zur Descendenztheorie,’ ii. p. 306, Leipzig, 1876, translated
by Meldola ; see ‘ Studies in the Theory of Descent,’ p. 680.

:

380° ON THE NUMBER OF POLAR BODIES AND

children of the same awe become intelligible in a simple manner
from the fact that each maternal germ-cell (I shall speak of ‘the
paternal germ-cells later on) contains a peculiar combination of
ancestral germ-plasms, and thus also a peculiar combination of
hereditary tendencies. These latter by their co-operation also pro-
duce a different result in each case, viz. the offspring, which are
_characterized by more or less pronounced individual peculiarities.
But the theory which explains individual differences by referring
to the inequality of germ-cells, may be proved with a high degree
of probability by an appeal to facts of an opposite kind, viz. by
showing that identity between offspring only occurs when they have
arisen from the same egg-cell. It is well known that occasionally
some of the children of the same parents appear to be almost exactly
alike, but such children are without exception twins, and there is
every reason to believe that they have been derived from the same
egg. In other words, the two children are exactly alike because
they have arisen from the same egg-cell, which could of course only
contain a single combination of ancestral germ-plasms, and there-
fore of hereditary tendencies'. The factors which by their co-

[* The similar conclusion that identical ova lead to the appearance of identical
individuals was drawn from the same data by Francis Galton in 1875. See ‘The
history of the Twins, as a criterion of the relative powers of Nature and Nurture,
by Francis Galton, F.R.S., Journal of the Anthropological Institute, 1875, p. 391;
also by the same author, ‘Short Notes on Heredity, etc. in Twins,’ in the same
Journal, 1875, p. 325. ,

The author investigated about eighty cases of close similarity between twins, and
was able to obtain instructive details in thirty-five of these. Of the latter there were
no less than seven cases ‘in which both twins suffered from some special ailment or
had some exceptional peculiarity ;’ in nine cases it appeared that ‘both twins are
apt to sicken at the same time;’ in eleven cases there was evidence for a remarkable
association of ideas; in sixteen cases the tastes and dispositions were described as
closely similar. These points of identity are given in addition to the more super-
ficial indications presented by the-failure of strangers or even parents to distinguish
between the twins. A very interesting part of the investigation was concerned with
the after-lives of the thirty-five twins. ‘In some cases the resemblance of body and
mind had continued unaltered up to old age, notwithstanding very different con-
ditions of life,’ in the other cases ‘the parents ascribed such dissimilarity as there
was, wholly, or almost wholly, to some form of illness.’

The conclusions of the author are as follows: ‘Twins who closely resembled each
other in childhood and early youth, and were reared under not very dissimilar con-
ditions, either grow unlike through the development of natural characteristics which
had lain dormant at first, or else they continue their lives, keeping time like two
watches, hardly to be thrown out of accord except by some physical jar. Nature is
far stronger than nurture within the limited range that I have been careful to assign

THEIR SIGNIFICANCE IN HEREDITY. 381

operation controlled the construction of the organism were the
same, and consequently the results were also the same. Twins
derived from a single egg are identical: this is a statement which,
although not mathematically proved, may be looked upon as nearly
certain. But there are also twins which do not possess this high
degree of similarity, and these are even far commoner than the
others. The explanation is to be found in the fact that the latter
were derived from two egg-cells which were fertilized at the same
time. In most cases, indeed, each twin is enclosed in its own
embryonic membranes, while much less frequently both twins are
enclosed in the same membranes. In one point only the proof is
incomplete ; for it has not yet been shown that identical twins are
always derived froma single egg, since such an origin, together with
a high degree of similarity, could only be established as occurring
together in a small proportion of the cases. We therefore see that
under conditions of nutriment which are as identical as possible,
two egg-cells develope into unlike twins, ove into identical twias ;
although we cannot yet affirm that the latter result invariably
follows. It is conceivable that the stimulus for the production of
two egos from one may be afforded by the entrance of two sper-
matozoa, but these latter, as was shown above, could hardly contain
identical hereditary tendencies, and thus two identical twins would
not arise. It appears indeed that some cases have been observed

to the latter.’ And again, ‘where the maladies of twius are continually alike, the
clocks of their two lives move regularly on, and at the same rate, governed by their
internal mechanism. Necessitarians may derive new arguments from the life
histories of twins.’

The above facts and conclusions held for twins of the same sex, of which at
any rate the majority are shown by Kleinwichter’s observations to have been
enclosed in the same embryonic membranes, and therefore presumably to have
been derived from a single ovum; but in rarer cases the twins, although also
invariably of the same sex, were marked by remarkable differences, greater than
those which usually distinguish children of the same family. Mr. Galton met with
twenty of these cases. In such twins the conditions of training, etc. had been
as similar as possible, so that the evidence of the power of nature over nurture is
strongly confirmed. Mr. Galton writes, ‘I have not a single case in which my cor-
respondents speak of originally dissimilar characters having become assimilated
through identity of nurture. The impression that all this evidence leaves on the
mind is one of wonder whether nurture can do anything at all, beyond giving in-
struction and professional training.’

The fact that twins produced from a single ovum seem to be invariably of the
same sex is in itself extremely interesting, for it proves that the sex of the individual
is predetermined in the fertilized ovum.—E. B. P.]

382 ON THE NUMBER OF POLAR BODIES AND

in which differences have been exhibited by twins which were en-
closed in the same embryonic membranes; but nevertheless I be-
lieve that two spermatozoa are not necessary to cause the formation
of twins by a single egg. We know, it is true, from thé investi-
gations of Fol!, that multiple impregnation produces the simul-
taneous beginning of several embryos in the eggs of star-fishes.
But several embryos and young animals are not developed in this
way, for embryonic development soon ceases, and the egg dies.

The recent observations of Born? upon the eggs of the frog also
make it very probable that a double development is produced by
the entrance of two spermatozoa into the egg, but here also only
monstrosities, and not twins, were produced. On the other hand, it
has been shown that in birds twins may be produced from the same
egg, and there is no reason for the belief that their production is
due to multiple impregnation. But if it may be assumed that human
twins, when identical, have been derived froma single egg, it seems
to me to be extremely probable that fertilization was also effected by
a single sperm-cell. We cannot understand how such a high degree
of similarity could have been produced if two sperm-cells had been
made use of, for we are compelled to assume that two such cells
would very rarely contain identical germ-plasms.

It is most probable that the egg-nucleus coalesces with the
nucleus of a single spermatozoon, but the resulting segmentation-
nucleus divides together with the cell-body itself, without the

occurrence of those ontogenetic changes in the germ-plasm which -

normally take place. The nucleoplasm of the two daughter-cells
still remains in the condition of germ-plasm, and its ontogenetic
transformation begins afterwards—a transformation which must of
course proceed in the same way in both cells, and must lead to the
production of identical offspring. This is at least a possible ex-
planation which we may retain until it has been either confirmed

or disproved by fresh observations,—an explanation which is more-_

over supported by the well-known process of budding in the eggs
of lower animals.

1 Fol, ‘ Recherches sur la fécondation et le commencement de l’hénogénie.’ Gentve,
Bale, Lyon. 1879.

* Born, ‘ Ueber Doppelbildungen'beim Frosch und deren Entstehung.’ Breslauer
arztl. Zeitschrift, 1882. ,

Se eee Se

THEIR SIGNIFICANCE IN HEREDITY. 883

VI. ReEcaApPITuLATION,

To bring together shortly the results of this essay:—the funda-
mental fact upon which everything else is founded is the fact that
two polar bodies are expelled, as a preparation for embryonic develop-
ment, from all animal eggs which require fertilization, while only
one such body is expelled from all parthenogenetic eggs.

This fact in the first place refutes every purely morphological
explanation of the process. If it were physiologically valueless,
such a phyletic reminiscence of the two successive divisions of the
ego-nucleus must have been also retained by the parthenogenetic
ege. pe
In my opinion the expulsion of the first polar body implies the
removal of ovogenetic nucleoplasm when it has become superfluous
after the maturation of the egg has been completed. The expulsion
of the second polar body can only mean the removal of part of the
germ-plasm itself, a removal by which the number of ancestral
germ-plasms is reduced to one half. This reduction must also
take place in the male germ-cells, although we are not able to
associate it confidently with any of the histological processes of
spermatogenesis which have been hitherto observed.

Parthenogenesis takes place when the whole of the ancestral
germ-plasms, inherited from the parents, are retained in the nucleus
of the egg-cell. Development by fertilization makes it necessary
that half the number of these ancestral germ-plasms must be first
expelled from the egg, the original quantity being again restored by
the addition of the sperm-nucleus to the remaining: half.

In both cases the beginning of embryogenesis depends upon the

presence of a certain, and in both cases equal, quantity of germ-
plasm. ‘This certain quantity is produced by the addition of the
sperm-nucleus to the egg requiring fertilization, and the beginning
of embryogenesis immediately follows fertilization. The partheno-
genetic egg contains within itself the necessary quantity of germ-
plasm, and the latter enters upon active development as soon as the
single polar body has removed the ovogenetic nucleoplasm. The
question which I have raised on a previous occasion—‘ When is the
parthenogenetic egg capable of development ?’—now admits of
the precise answer—‘ Immediately after the expulsion of the polar
body.’

584 ON THE NUMBER OF POLAR BODIES, ETC.

From the preceding facts and considerations the important con-
clusion results that the germ-cells of any individual do not contain
the same hereditary tendencies, but are all different, in that no two
of them contain exactly the same combinations of hereditary ten-
dencies. On this fact the well-known differences between the
children of the same parents depend.

But the deeper meaning of this arrangement must doubtless be
sought for in the individual variability which is thus continuously
kept up and is always being forced into new combinations. Thus
sexual reproduction is to be explained as an arrangement which
ensures an ever-varying supply of individual differences.

VII.

ON THE SUPPOSED BOTANICAL PROOFS

TRANSMISSION OF ACQUIRED CHARACTERS.

1888.

From ‘ Biologisches Centralblatt,’ Bd. VIII. Nr, 3 and 4, pages 65
and 97: April 1888.

VI.

ON THE SUPPOSED BOTANICAL PROOFS
OF THE

TRANSMISSION OF ACQUIRED CHARACTERS.

In a lecture on heredity, delivered in 18831, I first brought
forward the opinion that acquired characters cannot be transmitted;
and I then stated that there are no proofs of such transmission,
that its occurrence is theoretically improbable, and that we must
attempt to explain the transformation of species without its aid.
Since that time many biologists have expressed their opinions
upon the subject, some of them agreeing with me, while others
have taken the opposite side. It is unnecessary to allude to those
who have attacked my opinions without first understanding the
real point in dispute, which turns upon the true meaning of the
phrase ‘acquired character. I think it is now generally admitted
that a very important problem is involved in this question, the
solution of which will contribute in a decisive manner towards
the formation of ideas as to the causes which have produced the
transformation of species. For if acquired characters cannot be
transmitted, the Lamarckian theory completely collapses, and we
must entirely abandon the principle by which alone Lamarck
sought to explain the transformation of species,—a principle of
which the application has been greatly restricted by Darwin in
the discovery of natural selection, but which was still to a large
extent retained by him. Even the apparently powerful factors
in transformation—the use and disuse of organs, the results of
practice or neglect—cannot now be regarded as possessing any
direct transforming influence upon a species. And the same is
true of all the other direct influences, such as nutrition, light,

1 See the second Essay ‘ On Heredity.’
Cc 2

(

888 ON THE SUPPOSED BOTANICAL PROOFS OF THE

moisture, and that combination of different influences which we
call climate. All these, with use and disuse, may perhaps produce
great effects upon the body (soma) of the individual, but cannot
produce any effect in the transformation of the species, simply
because they can never reach the germ-cells from which the
succeeding generation arises. But if—as it seems to me—the
facts of the case compel us to reject the assumption of the trans-
mission of acquired characters, there only remains one principle
by which we can explain the transformation of species—the
direct alteration of the germ-plasm, however we may imagine that
such alterations have been produced and combined to form useful
modifications of the body.

The difficulty of understanding these processes of transformation
is by no means lightened by abandoning the Lamarckian theory.
The difficulty in fact becomes much greater, for we are now com-
pelled to seek a different explanation of many phenomena which
were previously believed to be understood. But this can hardly be
regarded as a reason for not accepting the view: for we are in
want of a correct explanation rather than one which is easy and
convenient. We seek truth, and when we recognize that our
path is leading in a wrong direction, we must leave it and take
another road even if it presents more difficulties.

My theory rests, on the one hand, upon certain theoretical con-
siderations which will be mentioned below, and which I have
attempted to develope in previous papers’. On the other hand, it
rests upon the want of any actual proof of the transmission of acquired
characters. My theory might be disproved in two ways,—either
by actually proving that acquired characters are transmitted, or by
showing that certain classes of phenomena admit of absolutely
no explanation unless such characters can be transmitted. It
will be admitted, however, that we must be very cautious in
accepting proofs of this latter kind, for the impossibility of ex-
plaining a given phenomenon may be merely temporary, and may
disappear with the progress of science. No one could have explained
the useful adaptations so common in animals and plants, before the

1 Consult ‘Ueber die Vererbung, Jena, 1883; ‘Die Kontinuitiit des Keim-
plasmas,’ Jena, 1885; ‘ Ueber die Zahl der Richtungskérper und iiber ihre Bedeu-
tung fiir die Vererbung, Jena, 1887. These papers are translated as the second,
fourth and sixth Essays in the present volume.

' TRANSMISSION OF ACQUIRED CHARACTERS. 389

light of the theory of natural selection had fallen on these pheno-
mena; at that time we should have been far from right if we
had assumed that organisms possess a power which causes them
to respond to external influences by useful modifications, a power
unknown elsewhere, entirely unproved and only supported by the
fact that at that time it did not seem possible to explain the
phenomena in any other way.

Although my theory has not been disproved, I will nevertheless
attempt to bring into further accordance with it certain phenomena
which seem at first sight to oppose it. I first began to take this
course in my paper ‘On Heredity’.’ In that paper I attempted to
show how the fact that disused organs become rudimentary may
be readily explained without assuming the transmission of acquired
characters ; and also that the origin of instincts may in all cases
be referred to the process of natural selection®, although many
observers had followed Darwin in explaining them as inherited
habits,—a view which becomes untenable if the habits adopted
and practised in a single life cannot be transmitted.

Other phenomena which appeared to present difficulties were
also considered and brought into accordance with the theory, and —
I think that I have been successful in showing that adequate and
simple explanations may be given.

There certainly remain many phenomena which seem to be
opposed to my theory and for which a new explanation must be
found. Thus Romanes*, following Herbert Spencer +, has recently
pointed to the phenomena of correlation as a proof of the trans-
mission of acquired characters; but, at no distant time, I hope to
be able to consider this objection, and to show that the apparent
support given to the old idea is in reality insecure and breaks
down as soon as it is critically examined. I believe that I shall
be able to prove that correlation cannot be used as the indirect
proof of an hypothesis, of which all direct evidence is still com-

1 See the second Essay.

[? See R. Meldola in Ann. and Mag. Nat. Hist., 1878, vol. i. pp. 158-161. The
author discusses many cases among insects in which instinct is related to protective
structure or colouring: he also considers that instinct is to be explained by the

principle of natural selection which accounts for the other protective features.—

E. B. P.]

[* See ‘ Nature,’ vol. 36, pp. 491-407.—E. B. P.]

[{* See ‘The Factors of organic Evolution’ in ‘The Nineteenth Century ’ for April
and May 1886.—E. B. P.]

Vw TAA A} AQ

a

890 ON THE SUPPOSED BOTANICAL PROOFS OF THE

pletely wanting. It must not be forgotten that the onus probandi
rests with my opponents: they defend the assertion that acquired
characters can be transmitted, and they ought therefore to bring
forward actual proofs; for the mere fact that the assertion has
been hitherto accepted as a matter of course by almost everyone,
and has only been doubted by a very few (such as His, du Bois-
Reymond, and Pfliiger), cannot be taken as any proof of its validity.
Not a single fact hitherto brought forward can be accepted as
a proof of the assumption. Such proofs ought to be found: facts
ought to be discovered which can only be understood with the
aid of this hypothesis. If, for instance, it could be shown that
artificial mutilation spontaneously re-appears in the offspring with
sufficient frequency to exclude all possibilities of chance, then such
proof would be forthcoming. The transmission of mutilations has
been frequently asserted, and has been even recently again brought
forward, but all the supposed instances have broken down when
carefully examined. I think I may here safely omit all further
reference to the proofs dependent upon transmitted mutilations,
especially as Déderlein? has already, in the most convincing manner,
disposed of the argument derived from the tailless cats which
were so triumphantly exhibited at the last meeting of the Associa-
tion of German Naturalists ?.

I now come to the real subject of this paper—the supposed
botanical proofs of the transmission of acquired changes. The
botanist Detmer has recently brought forward certain phenomena
in vegetable physiology *, as a support for the transmission of such
changes, and although I do not believe that they will bear this
interpretation, the discussion of them may perhaps be useful. I am
even inclined to think that these and a few other phenomena in
vegetable physiology, upon which I shall also touch, are very likely
to throw new light upon the whole question which has been so
frequently misunderstood. I should have preferred to leave this
discussion to a botanist, but I do not know whether my views will
meet with any support from the followers of this subject, and I
must therefore attempt the discussion myself. And perhaps it is
of some assistance in clearing up the question, for one who is not

1 See ‘ Biol. Centralbl.’ Bd. VII. No. 23.

2 See the next Essay (VIII).

8 Detmer, ‘Zum Problem der Vererbung,’ Pfliiger’s Archiv f. Physiologie, Bd. 41,
(1887), p. 203.

TRANSMISSION OF ACQUIRED CHARACTERS. 391

accustomed to the usual botanical views, and is more conversant
with other classes of biological knowledge, to consider the facts
brought to light by modern botany, from a general point of view.
Of course I shall not attempt to question the validity of the obser-
vations, nor even the accuracy with which the facts have been
interpreted. I shall only deal with the conclusions which may be
drawn from the facts, and I do not think that it is absolutely
necessary that such criticism should be made by a_ botanist.
Questions of general biological significance such as that of heredity
cannot be entirely solved within the single domain of either
zoological or botanical facts. Both botanists and zoologists must
give due weight to the facts of the province which is not their
own, and must see whether the views which they have chiefly
gained in the one province can be applied to the other, or whether
phenomena occur in the latter which are in opposition to their
previously formed views and which cause them to be abandoned
or modified.

Detmer begins by bringing forward certain facts which prove, as
he believes, that rather important changes in the organism can be
directly produced by external influences. He is of opinion that I
under-estimate the weight of these influences, and that I make light
of the changes which may thus arise in a single individual life.
But obviously, it is of no importance for the question of the trans-
mission of acquired characters, whether the changes directly pro-
duced by external influences upon the soma of an individual are
greater or smaller: the only question is whether they can be
transmitted. If they can be transmitted, the smallest changes
might be increased by summation in the course of generations,
into characters of the highest degree of importance. It is in this
way that Lamarck and Darwin have supposed that an organism is
transformed by external influences. It is therefore interesting to
see what Detmer considers to be a change which has been directly
effected. We can in this way gain a very distinct appreciation of
the difference in views which is caused by the different spheres of
experience which belong to botany and zoology. It will be useful
to gain a clear idea of the differences which are thus caused.

Detmer first alludes to the dorso-ventral structure of the shoots
of Thwa occidentalis, chiefly shown in the fact that the upper
sides of these shoots contain the green palisade cells, while the

392 ON. THE SUPPOSED BOTANICAL PROOFS OF THE

under sides which are turned away from the light possess greem
spheroidal (isodiametric) cells. Ifthe branches of 7wja are turned
upside down and fixed in this position before the production of new
shoots, it is found that the anatomical structure of the latter, when
developed, is reversed. The side of the shoot which was destined
to become the under side, but which was artificially compelled to
become the upper side, assumes the structure of the upper side and
developes the characteristic palisade parenchyma ; and on the other
hand, the under side which was intended to -become the upper side
developes the spongy parenchyma which is characteristic of the
under side. From these facts Detmer conciudes that the dorso-
ventral structure of the shoots of Zhwa has resulted from the
continual operation of an external force, and that the light must be
considered as the cause of the structural change.

But such a conclusion obviously depends upon a confusion of
ideas. No one will doubt that the light was the stimulus which
led to the reversal of the structures in the shoot, but this is a very
different thing from maintaining that it was the cause which
conferred upon the Zhuja-shoot the power of producing palisade
and spongy parenchyma. When a phenomenon only occurs under
certain conditions, it does not follow that these conditions are the
cause of the phenomenon. A certain temperature is necessary for
the development of a bird in the egg, but surely no one will
maintain that the temperature is the cause of the capacity for
such development. It is obvious that the egg has acquired the
power of producing a bird chiefly as the result of a long phyletic
course of development which has led to such a chemical and physical
structure in the egg and the fertilizing sperm-cell, that after their
union and development, a bird, and only a bird of a particular
species, must be produced. But of course certain conditions must
be fulfilled in order that such development may take place; and a
definite temperature is one of these conditions of development.
Thus we may briefly say that the physical nature of the egg is the
cause of its development into a bird,and we may similarly maintain
that the physical nature of a Z/uja-shoot, and not the influence of
light, is the cause of the development of tissues which are character-
istic of the species. In the development of such a shoot the light
plays precisely the same part which is played by temperature in the
development of a bird: it is one of the conditions of development.

swe

TRANSMISSION OF ACQUIRED CHARACTERS. 393

There is nevertheless a difference between these two cases in that
the Lhuja-shoot possesses the possibility of development in two
different ways instead of only one. The upper side of the shoot
can assume the structure of the under side and vice versa, and this
structural reversal depends upon the way in which the light is
thrown upon the shoot. But even if the light causes the structural
reversal, does this justify us in assuming that the structure itself
is also the direct consequence of the influence of light? . I see no
yeason for rejecting the supposition that the physical nature of part
of a plant may be of such a kind that this or that structure may be
produced according as this or that condition of development pre-
vails. . Thus with stronger light the structure of the upper side of
- the shoot developes ; with weaker light, the structure of the under
side. But this physical nature of the 7iwja-bud depends, like that
of a bird’s egg, upon its phyletic history, as we must assume to be
the case with the germs producing all individual developments.
It is therefore quite impossible to interpret the reversal of the
structure in the 7iwja-shoot as the result of modification produced
by the direct infiuence of external conditions. It is an instance
of double adaptation—one of those cases in which the specific
nature of a germ, an organism, or a part of an organism, possesses
such a constitution that it reacts differently under the incidence of
different stimuli.

An entirely analogous example of reversal occurs in the climbing
shoots of the Ivy, and is described in Sachs’ lectures on the
physiology of plants. Such shoots produce leaves only on the side
directed towards the light, and roots (which are made use of in
climbing) only upon the opposite side. If however the position of
the plant be altered so that the root-bearing side is turned towards
the light, while the leafy side is shaded, a reversal occurs, so
that from that time the former only produces leaves, and the latter
nothing but roots. In other words, the Ivy-shoot reacts under
strong light with the production of leaves and under weak light
with the production of roots, just as litmus-paper becomes red with
an acid and blue with an alkali. The physical nature of the Ivy-
shoot was present before the production of either structure, and was
no more due to the action of light itself, than the physical nature
of litmus-paper is due to an acid or an alkali. But this is quite
consistent with the possession of a physical nature which reacts

894 ON THE SUPPOSED BOTANICAL PROOFS OF THE

differently under the two different conditions afforded by light and
shade.

No one would think of bringing forward the changes in the
colour of the green frog (Hy/a) as a proof of the power of direct
influences in causing structural modifications in the animal body.
The frog is light green when it is resting upon green leaves, but it
becomes dark brown or nearly black when transferred to dark
surroundings. This is an obvious instance of adaptation, for the
changes in the colour of the frog depend upon a complex reflex
mechanism. The changes in the shape of the chromatophores of
the skin are not produced by the direct influence of the different
rays of light upon the body-surface, but in consequence of the
action of these rays upon the retina. Blind frogs do not react
under the changes of light. Hence it is impossible that any one
can maintain that the skin of the frog has gained its green colour
as the direct result of the green light reflected from its usual sur-
roundings. It must be admitted that in this and in all similar
cases, there is only one possible explanation, viz. an appeal to the
operation of natural selection. It may be objected that we are not
here dealing, as in the Ziwa and Ivy, with changes in the course
of ontogenetic development following upon the occurrence of this or
that external condition, but only with the different reactions of
a mature organism. But nevertheless, cases of the former kind
appear to be also present in the animal kingdom.

Thus the very careful and extensive investigations of Poulton? upon
the colours of certain caterpillars have distinctly shown that some
species possess the possibility of development in two directions, and
. that the actual direction taken by the individual is decided by the
influence of external conditions. Poulton surrounded certain larvae
of Geometrae with an abundance of dark branches, in addition to
the leaves upon which they fed. When such conditions prevailed
from the beginning of larval life, the caterpillars as they developed,
gradually assumed the dark colour of the twigs and branches upon
which they rested. When other larvae of the same species (and in

{t Dr. Weismann is here alluding to experiments upon the larvae of Rumia Cra- _

taegata. A short account of the results will be found in the Report of the British
Association at Manchester (1887), and in ‘ Nature,’ vol. 36, p. 594. I have now
obtained similar results with many other species (see Trans. Ent. Soc., Lond. 1888,
p- 553); but many of the results are as yet unpublished.—E. B. P.}

TRANSMISSION OF ACQUIRED CHARACTERS. 395

many experiments hatched from the same batch of eggs) were
similarly exposed to the green leaves of the same food-plant, they
did not indeed become bright green like the leaves, but were in-
variably of a much lighter colour than the other larvae, while
many of them gained a brownish-green tint. The larvae of
Smerinthus ocellatus' also possess the power of assuming different
shades of green and of thus approaching’, to some extent, the green
of the plant upon which they happen to live. It is quite impossible
to explain the phyletic development of the green colour of these
and other caterpillars as due te the direct action upon the skin of
the green light reflected from the leaves upon which they sit.
The impossibility of such an effect was pointed out long ago by
Darwin, and also followed from my own investigations. Here, as
in the other cases, the only possible solution is afforded by natural
selection. The colour of the caterpillars has become gradually more
and more perfectly adapted to the colour of the leaves,—and often
to the particular side of the leaves upon which these animals rest,
—not by the direct effect of reflected light, but by the selection of
those individuals which were best protected. Poulton’s experi-
ments quoted above prove that certain species which occur upon
different plants with different colours (or even in some cases upon
the differently coloured parts of the same plant), present us with
a further complication in the process of adaptation, inasmuch as
each individual has acquired the power of assuming a lighter or
darker colour*. The light which falls upon a single individual

[' See the editorial notes by Raphael Meldola, in his translation of Weismann’s
‘Studies in the Theory of Descent’ (the Essay on ‘The Origin of the Markings of
Caterpillars,’ pp. 241 and 306): also E. B. Poulton, in ‘Proc. Roy. Soc.,’ vol.
xxxviii. pp. 296-314; and in ‘Proc. Roy. Soc.,’ vol. xl. p. 135.—E. B. P.]

[? Professor Meldola first called attention to the scattered instances of the kind
here alluded to by Professor Weismann, in 1873: see ‘ Proc. Zool. Soc.,’ 1873, p. 153-
The author explains the relation of this ‘ variable protective colouring’ to other
protective appearances, and he is strongly of the opinion that the former as well as
the latter is to be explained by the action of the ‘survival of the fittest.’

The validity of Dr. Weismann’s interpretation of these effects as due to adapta-
tion, through the operation of natural selection, is conclusively proved by the follow-
ing facts. The light reflected from green leaves becomes the stimulus for the
production of dark brown pigment in those cases in which the leaves constitute the
surroundings for many months. Under these circumstances the leaves of course
become brown at a relatively early date, and protection is thus afforded for the
remainder of the period, although the dark pigment is produced before the change in
the colour of the leaf. Instances of this kind are seen in the colours of cocoons spun
among leaves by certain lepidopterous larvae (see ‘ Proc. Ent. Soc. Lond.,’ 1887, pp.

896 ON THE SUPPOSED BOTANICAL PROOFS OF THE

caterpillar during the course of its growth determines whether the
lighter or darker colour shall be developed. Here therefore we have ~
a case exactly parallel to that of the Zya-shoot in which the
palisade or spongy parenchyma is developed according to the
position in which the shoot is fixed.

As far as it is possible in the present condition of our knowledge
to offer any opinion upon the origin of sex in bisexual animals, it
may be suggested that this problem is also capable of an essentially
similar solution. Each germ-cell may possess the possibility of
developing in either of two directions, the one ‘esulting in a male
individual, and the other resulting in a female, while the decision as
to which of the two possible alternatives is actually taken may rest
with the external conditions. We must, however, include among the
external circumstances everything which is not germ-plasm. More-
over, this explanation is by no means certain, and I only mention
it as an instance which, if we assume it to be correct, further illus-
trates my views upon the phenomena presented by the 7/wa-shoot.

The two other facts brought forward by Detmer as proofs of
the transforming power of external influences can be explained
in precisely the same manner. These instances are—the fact
that Zropacolum when grown in moist air produces leaves with
anatomical characters different from those produced when: the plant
is grown in dry air; and the differences in the structure of the
leaves of many plants, according as they have been grown in the ©
sun or shade respectively. Such differences do not by any means
afford proof of the direct production of structural changes by means
of external influences. How would such an explanation be con-
sistent with the fact that the leaves are, in all these cases, changed
in a highly purposeful manner? Or is it assumed that these organs
were so constituted froni the beginning, that they are compelled
to respond to external conditions by the production of useful

1, li, and 1888, p. xxviii), the cocoons of the same species being of a creamy white
colour when spun upon white paper.

Conversely, the light reflected from the same surfaces serves as the stimulus for
withholding pigment in the cases alluded to by Dr. Weismann (larvae of R.
Crataegata, &c.), in all of which the organism only remains in contact with the
leaves while they are green, viz. at a time when the dark colour would be dis-
advantageous.

Hence precisely opposite effects are produced by the operation of the same force ;
the nature of the effect which actually follows in any case being solely determined
by the advantage afforded to the organism.—E. B. P.]

TRANSMISSION OF ACQUIRED CHARACTERS. 397

changes? Any one who made such an assertion nowadays, or
who even thought of such a thing as a possibility, would prove
that he is entirely ignorant. of the facts of organic nature, and
that he has no claim to be heard upon the question of the
transformation of species. The very first necessity in any scientific
question is to gain acquaintance with that which has been thought
and said upon the subject. And it has been frequently shown

that whole groups of useful characters cannot by any possibility

have been produced by the direct action of external influences. If
a caterpillar, which hides itself by day in the crevices of the bark,
possesses the same colour as the latter, while other caterpillars
which rest on leaves are of a green colour, these facts cannot be
explained as the results of the direct influence of the bark and
leaves. And it would be even less possible to explain upon the
same principle all the details of marking and colour by which these
animals gain still further protection. If the upper side of the
upper wings of certain moths is grey like the stone on which they
rest by day, while in butterflies the under side of both wings
which are exposed during rest, exhibits analogous protective
colours, these facts cannot be due to the direct influence of the
-surroundings which are resembled, but, if they have arisen in any
natural manner, they must have been indirectly produced by the
surroundings. One may reasonably complain when compelled to
repeat again and again these elements of knowledge and of thought
upon the causes of transformation !

Any one who remembers these things, and is aware of the
countless number of purposeful characters which cannot possibly
depend upon such direct influences, will be very cautious in yielding
to any single instance which at first sight appears to be the direct
consequence of external conditions. If Detmer had been thus
cautious he would hardly have written the following sentence as a

résumé of the physiological experiments on plants which have been »

already alluded to: ‘ In certain cases it is possible, as we have seen,
to artificially modify the anatomical structure of certain parts of
plants. In such cases the relation between the structure and the
external influences is undoubtedly clear: the latter act as the
cause; the anatomical structure of the members of the plant
is the consequence of this cause.’ A little more logic would
have prevented the author from expressing such an_ opinion,

398 ON THE SUPPOSED BOTANICAL PROOFS OF THE

for, as has been already shown, it is founded on a confusion
between the true cause of a phenomenon and one of the conditions
which are necessary for its production. We might as well con-
sider the phenomena of geotropism, hydrotropism, and heliotropism—
which have been established, and investigated in such a brilliant
way by modern vegetable physiologists—as the direct results
of the attraction of the earth, of water, and of light; and it is
not improbable that some botanists are even inclined to make this
assumption. And yet it is perfectly easy to show that this cannot
be the case. By geotropism we mean the power possessed by the
parts of a plant of growing along lines which make certain
angles with the direction of the earth’s attraction. For example,
the chief root grows parallel with the earth’s attraction, viz. towards
the centre of the earth, and it is described as positively geotropie :
conversely the main shoot grows along the same line but in an
opposite direction, and it is negatively geotropic. But geotropism
is not a primitive attribute of the plant, and it is even now absent
from those plants which, like many Algae, have no definite position.
Geotropism cannot have arisen before plants first became fixed in
the earth. If any one were to assume that the direct influence of
gravity, continuous through countless generations, had at length
conferred upon the root the power of growing in a geotropic diree-
tion, how would it be possible to explain the fact that the shoot
which has been under precisely the same influence has acquired the
power of growing in an exactly opposite direction? The character-
istic differences between root and shoot cannot have appeared until
the plant became fixed in the ground, and how can we imagine that
the same influence of gravity has since that time directly produced
the two antagonistic results of positive and negative geotropism,
in two structures, which were originally and essentially similar? It
should also be remembered that it is only the main root which
exhibits true positive geotropism. The lateral roots form angles
with the main root, and do not therefore grow towards the earth’s
centre; and the same is true of the lateral shoots which grow
obliquely, and not perpendicularly upwards, like the main shoot.
Moreover the angles which the lateral roots make with the main
root, and the lateral shoots with the main shoot, are quite different
in different species. How is it possible that all these different
modes of reaction witnessed in the different parts of plants can be

TRANSMISSION OF ACQUIRED CHARACTERS. 399

the direct results of one and the same external force? It is quite
obvious that these are all cases of adaptation. The main root has
not acquired the power of growing perpendicularly downwards
under the stimulus of gravity, because this force has acted upon it
for numberless generations, but because such a direction for such a
part was the most useful to the plant. Hence natural selection has
conferred upon the root the power of reacting under the stimulus of
gravity by growing in a direction parallel to this force. For the
main shoot, the opposite reaction was the most useful and has heen
established by natural selection, while still another reaction has been
similarly established for the lateral roots and another for the lateral
shoots.

Each part of a plant has received its special mode of reacting
under the stimulus of gravity because it was useful for the whole
plant, inasmuch as the position of its different parts relatively to
one another and to the soil became thus fixed and regulated. These
modes of reaction have become different in different species, because
the conditions of life peculiar to each require special arrangements.

The same argument also holds with regard to heliotropism.
The power of growing towards the light possessed by green shoots
cannot be a primitive character of the plant: it must have arisen
secondarily. If it were an essential and original character it could
not be reversed in certain parts of the plant; but the roots are
negatively heliotropic, for they grow away from the light. There
are also shoots, such as the climbing shoots of Ivy, which are simi-
larly negatively heliotropic. Whenever the heliotropic power is
thus reversed in shoots, the change is of a useful kind. Thus the
shoots of the Ivy gain the power of clinging closely to a perpen-
dicular wall or to some horizontal plane’. In this case, however,
it is only the shoot which is negatively heliotropiec, its leaves turn
towards the light ; and the same is true of the flower-bearing shoots
which do not climb. All these are clearly adaptations and not the
results of direct influence. The light only provides the stimulus
which calls forth the characteristic reaction from each part of the
plant, but the cause of each peculiar reaction lies in the specific
nature of the part itself which has not been produced by light, but
as we believe by processes of natural selection. If this explanation

1 Compare Sachs, ‘Lectures on the Physiology of Plants,’ translated by H.
Marshall Ward, p. 710.

400 ON THE SUPPOSED BOTANICAL PROOFS OF THE

does not account for the facts we may as well abandon all attempts
at understanding the useful arrangements in organisms.

Sachs has used the term anzsotropism to express the fact that
the various organs of a plant assume the most diverse directions of
growth under the influence of the same forces. He also states that
anisotropism is one of the most general characteristics of vegetable
organization, and that it is quite impossible to form any idea as to
how plants would appear or how they could live if their different
organs were not anisotropic. Since anisotropism is nothing more
than the expression of different kinds of susceptibility to the action
of gravity, light, &c., it is obvious that the configuration of the
plant is to be traced to such specific susceptibilities.

Now these specific susceptibilities cannot have been produced by
the direct effect of the various external influences (as was shown
above), and the only other possible explanation is to recognise them
as adaptations, and to admit that they have arisen by the opera-
- tion of natural selection upon the general variability of plant
organization.

Simple as these conclusious are, I have failed to meet with them
in any of the writings of botanists, and they may perhaps be of use
in helping to shake the vaguely-felt opinion that the characters of
plants are to be chiefly referred to the direct action of external
influences.

At all events it cannot be maintained that the phenomena of
anisotropism support the opinion mentioned above ; and the mere
assertion that it is highly probable that hereditary characters
arise as the result of external influences, is no more than the ex-
pression of an unfounded individual opinion. It is remarkable that
Detmer should make such an assertion as the outcome of his dis-
cussion of the reversed Thuja-shoot, &e., for even if we admit that
the dorso-ventral structure of the shoot is—as Detmer believes—
the direct and primary effect of the action of light, the experiment
with the reversed shoot would prove that no part of this effect
has become hereditary. Although the upper side of the shoot
has produced the palisade parenchyma under the influence of light
for thousands of generations, there is nevertheless no tendency
towards the establishment of any hereditary effect, for as soon as
the upper side of the growing shoot is artificially transformed into
the under side, its normal structure is at once abandoned. Hence

TRANSMISSION OF ACQUIRED CHARACTERS. 401

so far from lending any support to the assumption that acquired
characters can be transmitted, Detmer’s experiment rather tends to
disprove this opinion.

I think I have sufficiently shown that Detmer’s reproach—that
I have under-estimated the effects of external influences upon an
organism—may be fairly directed against its author. If we can
believe that every structural arrangement in plants, which depends
upon certain external conditions, has been produced in a phyletic
sense by these latter, it becomes very easy to explain the trans-
_ formation of species; but in accepting such an explanation we
are building without any foundation, for the proof that acquired
characters can be transmitted has yet to be given.

As a further disproof of my views Detmer quotes the so-called
phenomena of correlation in plants, and he believes that these
instances help us to conceive how the acquired changes of the body
(soma) of the plant may also influence the sexual cells. If the
apical shoot of a young spruce fir be cut off, one of the lateral
shoots of the whorl next below the section rises and becomes an
apical shoot: it not only assumes the orthotropic growth of such
a shoot, but also its mode of branching. The phenomenon itself is
well known, and I have often observed it myself in my garden
without making any botanical experiments; for this experiment
is not uncommonly made by Nature herself, when the apical
shoot is destroyed by insects (for example the gall-making
Chermes), The change of the lateral into an apical shoot occurs
here in consequence of the loss of the true apical shoot, and is
therefore really dependent upon it. The only difficulty is to
understand how these and many other kindred phenomena can
be considered to prove the transmission of acquired characters.
That correlation exists between the parts of an organism, that cor-
related changes are not only common but nearly always accompany
some primary change, has been perfectly well known since Darwin’s
time, and I am not aware that it has been disputed by any one. I
further believe that hardly any one would maintain that it is im-
possible for the reproductive organs to be influenced by correlation.
But this is very far from the admission that such changes would
occur in the germ-cells as would be necessary for the transmission
of acquired characters. For such transmission to occur it would be
necessary for the germ-plasm (the bearer of hereditary tendencies)

Dd

402 ON THE SUPPOSED BOTANICAL PROOFS OF THE

to undergo a transformation corresponding to that produced by
the external influences ;—such a transformation as would cause the
future organism to spontaneously develope changes similar to those
which its parent had acquired. But since the germ-plasm is not
an organism in the sensé of being a microscopic facsimile which
only has to increase in size in order to become a mature organism,
it is obvious that the developmental tendencies must exist in the
specific molecular structure, and perhaps also in the chemical consti-
tution of the germ-plasm itself. It therefore follows that the changes
in the germ-plasm which would be required for the transmission of
an acquired character must be of an entirely different nature from
the change itself acquired by the body of the parent plant: and
yet it is supposed that the former is produced by the latter as a
result of correlation. I will illustrate this by an example. Let us
suppose that the influence of climate had caused a plant to change
the form of its leaves from an ovate into a lobate shape: now such
a change could not be transferred to the germ-plasm in the pollen
and the ovules, as anything similar to leaves or the form of leaves ;
for such specialized morphological features have no existence in the .
germ-plasm. The only thing which could happen would be changes in
its molecular structure which bear no resemblance to those changes
which are implied by the direct alteration of the form of the leaf
in the parent plant. Any one who clearly appreciates this difficulty
will hesitate in admitting the possibility of the transmission of
acquired characters, because it is possible that the sexual cells may
be affected by correlated influences. If the change in the form of a
leaf exercises any influence at all upon the germ-plasm, why should
it produce a corresponding (in the above-mentioned sense) change
in its molecular structure ? Why should it not produce some other
out of the immense number of possible changes? There must be as
many possible changes in the structure of germ-plasm as there are
possible variations in each part of a plant that arises from it. Why
then should the corresponding change always ocecur,—a change
which had never previously existed in the whole phyletic develop-
ment of the organic world; for the plant with the latest modifica-
tion can have never existed before? The occurrence of a particular
change out of the countless possible changes would be about as
likely as if one out of a hundred thousand pins thrown out of a
window were to balance on its point when it reached the ground.

' TRANSMISSION OF ACQUIRED CHARACTERS. 403

The assumption scarcely deserves to be called a scientific hypothesis,
and yet it must be made by all who accept the transmission of
acquired characters,—that is unless they adopt the hypothesis of
pangenesis, which is quite as improbable, and which even Darwin
did not look upon as a real, but only as a formal explanation.

Detmer is also greatly mistaken when he says that I refuse to
admit the transmission of acquired characters, because I am pre-
judiced in favour of my doctrine of the continuity of the germ-
plasm. This doctrine is either right or wrong, and there is no
middle course: to this extent I-quite admit that I am prejudiced.
But the question as to whether acquired characters can be im-
pressed upon the germ and thus transmitted would not be by any
means settled in this way; for even if we admit that the germ-
plasm is not continuous from one generation to another, but that
it must be produced afresh in each individual, this would by no
means necessarily imply that it would potentially receive and retain
every change produced in every part of the individual, and at any
time in its life. It seems to me that the problem of the transmis-
sion or non-transmission of acquired characters remains, whether the
theory of the continuity of the germ-plasm be accepted or rejected.

I will now proceed to examine the last group of phenomena
which Detmer brings forward in favour of the transmission of
acquired characters. He charges me with not having taken into
account, in discussing the problem of heredity, the very important
facts which are known about the strange phenomena of ‘after-
effect’ in plants. Among these ‘after-effects’ are the following.

If vigorous plants of the sun-flower, grown in the open air, be
cut off close to the ground and transferred to complete darkness,
the examination of a tube fixed to the cut surface of the stem will
show that the escape of sap does not take place uniformly, but
undergoes periodical fluctuation, being strongest in the afternoon
and weakest in the early morning. Now the cause of this daily
periodicity in the flow of sap depends upon the periodical changes
due to the light to which the plant was exposed when it was
growing under normal conditions. When plants which have been
grown in darkness from the first are similarly treated, the flow of
sap does not exhibit any such periodicity.

Another instance is as follows:—it is well known that darkness
accelerates, while light retards the growth of plants, and therefore

pd2

404 ON THE SUPPOSED BOTANICAL PROOFS OF THE

plants usually grow more strongly by night than by day. If now
plants are transferred from the open air into constant darkness, the
periodicity in their growth does not immediately disappear, and
often persists for a long time as a phenomenon of after-effect.

The opening and closing of the leaves of Mimosa pudica also takes
place periodically under natural conditions, the leaves closing at
dusk as a result of changes in the stimulus provided by the light.
In this case also, when the plants are transferred to constant dark-
ness, the periodicity in the movements of the leaves continues for
several days.

All this is certainly very interesting, and it proves that periodical
stimuli produce periodical processes in the plant, which are not imme-
diately arrested when the stimulus is withdrawn, and only become
uniform gradually and after the lapse of a considerable time. But I
* certainly claim the right to ask what connexion there is between these
facts and the transmission of acquired characters. All these peculiar-
ities produced by external influences remain restricted to the indi-
vidual in which they arose; most of them disappear comparatively
soon, and long before the death of the individual. No example of the
transmission of such a peculiarity is known. Although successive
generations of sunflowers have been exposed fur thousands of years
to the daily alternation of light and darkness, the periodicity in the
flow of sap has not become hereditary, and does not take place at
all in plants which have always been kept in darkness. Detmer

specially tells us that we can even reverse the periods of opening _

and closing the leaves in Mimosa pudica by keeping them in dark-
ness during the day, but exposed to light at night; an experiment
which was performed by Pfeffer. Here again we see the proof
that influences which have acted upon countless generations have
left no impression whatever upon the germ-plasm.

Detmer himself admits this when he says that the after-effects
are only witnessed during the life of the individual, but he never-
theless adds that he has been for many years convinced that the
phenomena of heredity and after-effect differ in degree and not in
kind. He even goes so far as to assert that, in spite of the obvious
non-transmission of after-effect, the similarity between the natures

of these two classes of phenomena cannot escape the intelligent

observer.
It seems to me that this question does not demand the attention

TRANSMISSION OF ACQUIRED CHARACTERS. 405

of the observer (for the observations have already been made) so
much as that of the thinker. It is not a correct train of reasoning
to conclude that after-effect and heredity are identical in nature,
from the fact that certain periodical influences, acting upon a single
individual, set up periodical physiological processes which continue
for a time after the influences have ceased to act. We might
almost as well argue that the oscillations of a pendulum, which
continue as after-effects when the pendulum has been set going,
are of an identical nature with the process of heredity. All these
phenomena have indeed this much in common :—a cause which
acted at some time in the past, but which is no longer visible at
the time when the phenomenon appears. But the likeness ends
here, and the supposed identity in nature merely depends upon wild
speculation. One difference is very obvious, for the phenomena of
after-effect gradually cease after the withdrawal of the stimulus,
just like the oscillations of the pendulum, while the pheno-
mena of heredity continue without any interruption. As far as
heredity is concerned the physiological processes of after-effect are
not distinguishable from any of the other well-known acquired cha-
racters which are recognizable as morphological changes. After-
effects are not transmitted, and compared with this fact but little
importance can be attached to the use of vague analogies by Detmer,
who would wish to conclude that heredity is only the after-effect
of processes which had been set going in the parent organism.

At the end of his paper Detmer applies the ideas which he has
gained from the consideration of after-effect to certain phenomena
in the normal life of plants. He suggests that the periodical
change of leaf in trees and shrubs may have been produced by the
direct effect of climate. If branches bearing winter buds are cut
off in the autumn and are placed in a hot-house, with their cut
ends in water, the buds do not at once develope, and months may
often elapse before they begin to break. He argues that this
experiment proves that the annual periodicity of the plant no
longer depends directly upon external influences; these latter pro-
duced the periodicity at some earlier time, but it has been gradually
fixed in the organism by after-effect and heredity (!), so that its
disappearance does not now take place when the stimulus is with-
drawn, and changes would only happen very gradually under the
influence of changed climatic conditions. He considers that this is

406 ON THE SUPPOSED BOTANICAL PROOFS OF THE

proved from the fact that our cherry has become an evergreen in
Ceylon.

Such are Detmer’s opinions, and every one will agree with him
in believing that the periodical change of leaf in temperate climates
has been produced in relation to the recurring alternation of summer
and winter. This is certainly the case, and it cannot be doubted
that the character has become fixed by heredity. Where, however,
is the proof that this hereditary character has been produced by the
direct influence of climate? What right have we to look upon
the hereditary appearance of the character as an after-effect of the
direct influence exerted by changes of temperature upon previous
generations? Such an opinion derives but little support from the
previously described experiments upon after-effect, which showed
that these phenomena were never hereditary.

It appears to me that there are certain points in this change of
leaf and its accompanying phenomena, which distinctly indicate
that natural selection has been at work. Can Detmer imagine that
the brown scales which form the characteristic protective covering
of winter buds have been produced by the direct action of the cold?
If, however, the peculiar structure of these buds is to be referred to
the specific constitution of the individual rather than to the direct
effects of climate, would it be so very improbable for their physio-
logical peculiarity of lying dormant for several months to have
been developed simultaneously with the structure, by the operation
of natural selection? And if this explanation be correct, we can at
once see why the character has become hereditary, for natural
selection works upon variations of the germ-plasm, and these are
transferred from one generation to another with the germ-plasm
itself.

But Detmer attempts to establish the converse conclusion, and
he argues that the hereditary change of leaf has been abandoned
under the long-continued effect of changed climatic conditions ;
but this opinion is based upon the single instance of the alteration
in the habit of the European cherry in Ceylon. If it were proved
that our cherry, grown from seed in Ceylon and propagated by
seed for several generations, became evergreen gradually and not
suddenly in the first generation: if, under such circumstances, it
came to retain its leaves in the autumn and ceased to produce the
dormant winter buds:—then indeed the transmission of acquired

TRANSMISSION OF ACQUIRED CHARACTERS. 4.07

characters could hardly be doubted. I am not a botanist, but I
believe I am right in supposing that the wild cherry reproduces
itself by seeds, while the edible domesticated cherry is propagated
by grafting. Grafts are, however, parts of the soma of a previously
existent tree, and we are not therefore concerned, in this method
of propagation, with a succession of generations, but with the suc-
cessive distribution of one and the same individual over many wild
stocks. But no one will doubt that one and the same individual
ean be gradually changed during the course of its life, by the
direct action of external influences. The really doubtful point is
whether such changes can be transmitted by means of the germ-
cells. If, as I presume, the English in Ceylon do not care to eat
wild cherries but prefer the cultivated kinds, it follows that the.
branches which bear fruit in that island have not been developed
from germ-cells, at any time since their introduction, and there is
nothing to prevent them from gradually changing their anatomical
and physiological characters in consequence of the direct influence
of climate.

Hence the instance which Detmer looks upon as plainly con-
clusive, can hardly be accepted in support of such a far-reaching
assumption as the transmission of acquired characters.

It is therefore clear that none of the facts brought forward by
Detmer really afford the proofs which he believes that they offer.
But another botanist, Professor Hoffman of Marburg, well known
for his long-continued experiments on variation, has recently called
attention to certain other botanical facts in support of the trans-
mission of acquired characters. These facts are indeed conclusive,
if we accept the author’s use of the term ‘acquired,’ but it will
be found that they lead to hardly any modification in the state
of existing opinion upon the subject.

_In a short note, dated Jan. 1, 1888, the author communicated to |
this journal (‘ Biologisches Centralblatt ’) the statement that changes
in the structure of flowers caused by poor nutrition can be proved
to be hereditary to a greater or less extent }.

A more elaborate account of the experiments will be found in
several numbers of the ‘Botanische Zeitung, and the author
expresses his final results in the following words (see Bot. Zeit.
1887, p. 773) :—‘ These experiments prove with certainty (1) that

' Compare Biol. Centralbl. Bd. VII. No. 21,

408 ON THE SUPPOSED BOTANICAL PROOFS OF THE

insufficient nutrition may cause considerable morphological changes
(viz. qualitative variations) which are in the first place acquired
by thes exual apparatus of the flower, (2) that the “transient’’ (Weis-
mann) characters acquired by the individual can be transmitted?’

The data upon which Hoffman bases these opinions are certain
experiments conducted upon various plants, in order to determine
the conditions of life under which abnormal flowers or any other
variations occur most frequently: to decide, in short, how far
variations are caused by the change of conditions.

It is obvious that the attention of the author was not at first
directed to the question of the transmission of acquired characters.
His experiments are of a much older date than the present con-
dition and significance of the question before us. Hoffmann has,
in fact, re-examined his former results from the new point of view,
and this explains why his proofs are not always sufficiently con-
vincing when applied to the present issue. But this is of no
great importance, inasmuch as there is no necessity for me to
question the correctness of his assumptions.

The essential details of the experiments to which he directs
attention are as follows. .

Different plants with normal flowers were subjected to greatly
changed conditions of life for a series of generations. They were,
for example, crowded together in small pots. Under these cir-
cumstances the plants were of course poorly nourished, and in the
course of generations, several species produced a variable proportion
of abnormal—viz. double-flowers. This, however, was not always
the case, for such flowers did not appear in Matthiola annua and
Helianthemum polifolium, In other species, such as Nigella damas-
cena, Papaver alpinum and Tagetes patula, they appeared and often
increased in numbers in the course of generations, although this
was not a constant result. For instance, four successive genera-
tions of Nigella damascena, when closely sown, produced the
following results :—

1883. No double flowers.

1884. ” ) Pa ”

1885. 23 typical flowers: 6 double flowers.
1886; ‘to’ =", ppt OL. Lge 3 0 SOS

1 I have used the expression ‘transient’ (‘passant’) in the same sense as
‘acquired,’ in order to enforce the conclusion that they are merely temporary, and
disappear with the individual in which they arise. Since the characters of which

TRANSMISSION OF ACQUIRED CHARACTERS. 409

But it was not always the case that the double flowers continued
to appear after they had been once produced. In Papaver alpinum,
which Hoffman has cultivated in successive generations since
1862, other changes in addition to the doubling of the flowers first
appeared in 1882, viz. a slight variability in the form of the leaf,
and a greater variability in the colours of the flowers. The pro-:
duction of double flowers appeared to be favoured by poor nutrition
caused by crowding the plants. The results as regards the number
of double flowers produced in this species by close sowing, from
1882-1886, have been as follows :—

Experiment XI. 1881. 40 per cent. of double flowers.

1882. 4 ” ” >

1883. 5+3 2? ” ”
Experiment XVII. 1884. 13- ,, 39 a7
1885. 0-0 ,, ” 2

1886. 0-0 ” 2? ”

Although in these and some other series of generations the
double flowers again disappeared in the later generations, yet there
can be hardly any doubt that their first appearance was due to the
abnormal conditions of nutrition. This conclusion is also unaffected
by the fact that double flowers appeared in nearly the same pro-
portions in consequence of cultivation in ordinary garden soil. The
plants which were crowded in pots produced 2879 normal flowers,
and 256 (=8-8 per cent.) abnormal and mostly double ones, while
867 normal and 62 (=7-0 per cent.) abnormal ones were produced
on garden beds. Hoffman will not indeed admit that such a
comparison can be fairly made, for the plants in the garden beds
were raised from seed which was in part taken from the double
flowers, and was therefore, he believed, under a strong hereditary
influence. But this latter assumption is not supported by the
results of his own experiments.

Thus experiment XVIITI., conducted upon Papaver alpinum, is
described in these words,—‘ Seeds yielded by double flowers from
experiment XI. (1883), were sown in pots, and the resulting plants
produced from 1884-1886, fifty-three single flowers and no double
ones.’

Hoffmann speaks are hereditary, the term cannot be rightly applied to them, and

I shall prove later on that they cannot be regarded as acquired characters in the
sense required by the theory of descent.

410 ON THE SUPPOSED BOTANICAL PROOFS OF THE

In the converse experiment XIX. ‘The seeds of single flowers
from different stocks were sown in pots, and the resulting plants
produced in 1885 and 1886 forty-three flowers, of which all were
typical except one ;’ while plants produced in the garden by seed
from the same sources, yielded 166 single and five double flowers.
Hoffman also describes other experiments in which the seeds from
double flowers produced plants which also yielded many double
flowers. Thus, for example, in experiment XXI. seeds yielded by
the double flowers of Papaver alpinum were sown in the garden
and produced numerous plants, which in 1885 and 1886 bore 284
single and twenty-one double flowers, that is 7 per cent. of the
latter.

It will therefore be seen that the transmission of the abnormality
is by no means proved beyond the possibility of doubt, for who can
decide between the effects due to heredity and changed conditions
in the last experiment? I have no doubt however that the results
are at any rate in part due to the operation of heredity, for I do not
see how the phenomena can be otherwise understood. Nevertheless
I cannot admit the transmission of acquired characters on this
evidence, for the changes which have appeared are not ‘acquired’
in the sense in which I use the term and in the sense required by
the general theory of evolution. It is true that they may be
described by the use of this word: inasmuch as they are characters
which the plant has come to possess; we are not however en-
gaged in a mere dispute about terms, but in the discussion of a
weighty scientific question. Our object is to decide whether
changes in the soma (the body, as opposed to the germ-cells)
which have been produced by the direct action of external in-
fluences, including use and disuse, can be transmitted; whether
they can influence the germ-cells in such a manner that the latter
will cause the spontaneous appearance of corresponding changes
in the next generation. This is the question which demands an
answer ; and, as has been shown above, such an answer would decide
whether the Lamarckian principle of transformation must be
retained or abandoned.

I have never doubted about the transmission of changes which
depend upon an alteration in the germ-plasm of the reproductive
cells, for I have always asserted that these changes, and these alone,
must be transmitted. If any one makes the contrary assertion,

TRANSMISSION OF ACQUIRED CHARACTERS. 411

he merely proves that he does not understand what I have said
upon the subject. In what other way could the transformation
of species be produced, if changes in the germ-plasm cannot be
transmitted ? And how could the germ-plasm be changed except
by the operation of external influences, using the words in their
widest sense ; unless indeed we assume with Niigeli, that changes
oceur from internal causes, and imagine that the phyletic develop-
ment of the organic world was planned in the molecular structure
of the first and simplest organism, so that all forms of life were
compelled to arise from it, in the course of time, and would have
arisen under any conditions of life. This is the outcome of Nageli’s
view, against which I have contended for years.

If we now use the term ‘acquired characters’ for changes in the
soma which, like spontaneous abnormalities, depend upon previous
changes in the germ-plasm—it is of course easy to prove that
acquired characters are transmitted ; but this is hardly the way to
advance science, for nothing but confusion would be produced by
such a use of terms’. J am not aware that any one has ever
doubted that spontaneous characters, such as extra fingers or toes,
patches of grey hair, moles, ete., can be transmitted. It is true
that such characters are sometimes called ‘ acquired’ in pathological
works, but His has rightly insisted that such an obviously inaccurate
use of the term ought to be avoided, in order to prevent mis-
understanding. If every new character is said to be ‘ acquired ’

1 Compare a paper by J. Orth, ‘Ueber die Entstehung und Vererbung indi-
vidueller Eigenschaften, Leipzig, 1887. This author considers my theory of the
non-transmission of acquired characters to be incorrect, because he will insist upon
using the term ‘ acquired’ for those characters which are due to spontaneous changes
in the germ; although he considers that they are only indirectly acquired. He also
reproaches me with not having discriminated with sufficient clearness between the
two modes in which new characters are acquired by the body, and with having
altogether failed to take into account the class of characters which are due to
variations in the germ. On the very same page he quotes the following sentence
from my writings :—‘ Every change of the germ-plasm itself, however it may have
arisen, must be transmitted to the following generation by the continuity of the
germ-plasm; and hence also any changes in the soma which arise from the germ-
plasm must be transmitted to the following generation.’ Not only does the trans-
mission of Orth’s ‘indirectly acquired characters’ necessarily follow from this sen-
tence, but it is even distinctly asserted by it. I cannot understand how any one
who is aware of what happened at the meeting of the Association of German
naturalists at Strassburg in 1885, can charge me with the confusion of ideas which
has prevailed since Virchow took part in the discussion of this question.

412 ON THE SUPPOSED BOTANICAL PROOFS OF THE

the term at once loses its scientific value, which lies in the restricted
use. If generally used, it would mean no more than the word
‘new’; but new characters may arise in various ways,—by artificial
or natural selection, by the spontaneous variations of the germ, or
by the direct effect of external influences upon the body, including
the use and disuse of parts. If we assume that these latter
characters are transmitted, the further ‘assumption of complicated
relations between the organs and the essential substance of the
germ becomes necessary’ (His), while the transmission of the
other kinds of characters do not involve any theoretical difficulties.
There is therefore obviously a wide difference between these two
groups of characters as far as heredity is concerned, quite apart
from the question as to whether acquired characters are really
transmitted. It is at all events necessary to have distinct terms
which cannot be misunderstood. His! has proposed to call those
characters which are due to selection ‘changes produced by breeding’
(‘erziichtete Abinderungen’), those which appear spontaneously—
‘spontaneous changes’ (‘ eingesprengte Abinderungen’), and these
two groups of characters would then be opposed to those which he
calls ‘acquired changes’ (‘erworbene Abinderungen’), of course
using the term in the restricted sense. Science has always claimed
the right of taking certain expressions and applying them in a
special sense, and I see no reason why it should not exercise this
right in the case of the term ‘acquired.’ It appears moreover that
this word has not always been used in this vague sense by patho-
logical anatomists, such as Virchow and Orth; for Weigert and
Ernst Ziegler have employed it in precisely the same sense as that
in which it has been used by Darwin, du Bois-Reymond, Pfliger,
His and many others, including myself.

It is certainly necessary to have two terms which distinguish
sharply between the two chief groups of characters—the primary
characters which first appear in the body itself, and the secondary
ones which owe their appearance to variations in the germ, how-
ever such variations may have arisen. We have hitherto been
accustomed to call the former ‘acquired characters,’ but we might
also call them ‘ somatogenic, because they follow from the reaction
of the soma under external influences; while all other characters
might be contrasted as ‘ d/astogenic, because they include all those

? His, ‘ Unsere Kérperform,’ Leipzig, 1874, p. 58.

TRANSMISSION OF ACQUIRED CHARACTERS. 413

characters in the body which have arisen from changes in the
germ. In this way we might perhaps prevent the possibility of
misunderstanding. We maintain that the ‘somatogenic’ characters
cannot be transmitted, or rather, that those who assert that they
can be transmitted, must furnish the requisite proofs. The somato-
genic characters not only include the effects of mutilation, but
the changes which follow from increased or diminished performance
of function, and those which are directly due to nutrition and any
of the other external influences which act upon the body. Among
the dlastogenic characters, we include not only all the changes
produced by natural selection operating upon variations in the
germ, but all other characters which result from this latter
cause.

If we now wish to place Hoffmann’s results in their right
position, we must regard all of them as ‘bdastogenic’ characters,
for no one of them can be considered as belonging to the group
which has been hitherto spoken of as ‘acquired,’ in the literature
of evolution: they are not due to somatogenic but to blastogenic
changes. The body of the plant—the soma—has not been directly
affected by external influences, in Hoffman’s experiments, but
changes have been wrought in the germ-plasm of the germ-cells
and, only after this, in the soma of succeeding generations.

There is no difficulty in finding facts in support of this state-
ment, among Hoffmann’s experiments. The proof chiefly lies in
the fact that in no one of his numerous experiments did any
change appear in the first generation. The seeds of different
species of wild plants, with normal flowers, were cultivated in
the garden and in pots (thickly sown in the latter case), but no
one of the plants produced by these wild seeds possessed a single
double flower. It was only after a greater or less number of
generations had elapsed that a variable proportion of double
flowers appeared, sometimes accompanied by changes in the leaves
and in the colours of the flowers. This fact admits of only
one interpretation;—the changed conditions at first produced
slight and ineffectual changes in the idioplasm of the individual,
which was transmitted to the following generation: in this again
the same causes operated and increased the changes in the idioplasm
which was again handed down. Thus the idioplasm was changed
more and more, in the course of generations, until at last the

414, ON THE SUPPOSED BOTANICAL PROOFS OF THE

change became great enough to produce a visible character in the
soma developed from it, such as, for example, the appearance of
a double flower. Now the idioplasm of the first ontogenetic stage
(viz. germ-plasm) alone passes from one generation to another,
and hence it is clear that the germ-plasm itself must have been
gradually changed by the conditions of life until the alteration
became sufficient to produce changes in the soma, which appeared
as visible characters in either the flower or leaf ?.

In addition to the above-mentioned cases Hoffmann also quotes
some facts of a somewhat different kind. He succeeded in in-
ducing considerable changes in the structure of the root of the
wild carrot (Daucus carota) by means of the changes in nutrition
implied by garden cultivation. These changes also proved to be
hereditary.

Unfortunately, I have not the literature of the subject at hand,
and hence I am unable to read the accounts of these older experi-
ments in extenso; but it is sufficiently obvious that in this ease
we are also concerned with a change which did not become visible
until after some generations had elapsed, and which was therefore
a change in the germ-plasm.

Many instances of a precisely similar kind have been long known,
and one of them is to be found in the history of the garden pansy,
which Hoffmann has succeeded in producing from the wild form,
Viola tricolor, in the course of eighteen years. Darwin some time
ago pointed out in his work upon ‘The Variation of Animals and
Plants under Domestication,’ that, in the case of the pansy and
all other ‘improved’ garden flowers, the wild form remained un-
changed for many generations after its transference to the garden,
apparently uninfluenced by the new conditions of life. At length
single varieties began to appear, and these were further developed
by artificial selection and appropriate crossing, into well-marked
races distinguished by peculiar colours, forms, ete.

In these cases also, changes in the germ-plasm are the first

1 Compare on this point Niigeli in his ‘Theorie der Abstammungslehre.’ This
writer also concludes from similar facts that external influences have wrought in
the idioplasm, changes which were at first ineffectual, and which only increased
during the course of generations up to a point at which they could produce visible
changes in the plant. He does not, however, draw the further conclusion that these
changes only influence the germ-plasm, for he was not aware of the distinction between
germ-plasm and somatoplasm.

TRANSMISSION OF ACQUIRED CHARACTERS. 415

results of the new conditions, and there is no evidence for the
occurrence of acquired characters, using the term in its restricted
sense.

I now come to the last botanical fact brought forward by
Hoffmann in support of the transmission of acquired characters.
He states that specimens of Solidago virgaurea brought from the
Alps of the Valais, commenced flowering in the botanical garden
at Giessen, at a time which differed by several weeks from that
at which specimens from the surrounding country, planted beside
them, began to flower. In other words, the time of flowering
must have been fixed by heredity in the alpine Solidago, for the
external conditions would have favoured a time which was simul-
taneous with that of the Giessen plants.

What conclusions can be drawn from these facts? Hoffmann of
course sees in them the proof of the transmission of acquired
characters, but this presupposes that the time of flowering was
originally an acquired character. Hoffmann indeed appears to
entertain this opinion when he somewhat vaguely states that the
time at which flowering begins has been acquired by accommodation
—that is by the influence of climate—during a long series of
generations, and has become hereditary. But what does Hoffmann
mean by ‘accommodation’? He presumably means that which,
since the appearance of Darwin’s writings, has been generally
ealled adaptation :—that is a purposeful arrangement, suited to
certain conditions. The majority of biologists have followed
Darwin in believing that such adaptations have been produced by
processes of natural selection. Hoffmann seems to imagine that
they have arisen in some other way: perhaps he believes, with
Nageli, that they have been directly produced by external in-
fluences.

The fixation of the time at which flowering begins, is an
adaptation which formerly could have been very well explained
as the direct result of external conditions. The question we have
to decide is whether such an explanation is the true one. We
might imagine that the plant would be forced into quicker develop-
ment by an earlier appearance of the warm season. Hence when
transferred into a warmer climate the plant would at first flower
rather earlier, the habit would then be transmitted, and would
increase in successive generations from the continued influence

416 ON THE SUPPOSED BOTANICAL PROOFS OF THE

of climate, until it advanced as far as the organization of the
plant permitted. But in this explanation, as in so many others
of the same kind, it has unfortunately been forgotten that -the
transmission of acquired characters which is presupposed in the
explanation is a totally unproved hypothesis. It is sufficiently
obvious that by interpreting a phenomenon in a manner which
presupposes the transmission of acquired characters, we cannot
furnish a proof of the existence of such transmission.

It always seemed to me that the fixation of the commencement
of flowering, together with similar physiological phenomena in the
animal kingdom (for example, the hatching of insects from winter
eggs), could be explained very satisfactorily by the operation of
natural selection: and even now this explanation appears to me
to be the simplest and most natural. In Freiburg, where the
vine is largely grown, the harvest is often injured by frosts in
spring, which kill the young shoots, buds and flowers. Accord-
ingly, different kinds of vine, which do not push their buds so
early, have now been planted. Any one, who has seen all the
shoots of the former destroyed by the frosts at the end of April,
while the latter, not having opened their buds, were spared,
would not doubt that the former must have been long ago
exterminated, if they had been compelled to struggle for existence
with the others, under natural conditions. Now the time of
flowering fluctuates slightly in the individuals of every species of
plant, and can therefore be modified by natural selection. It is
therefore difficult. to see why the time at which each plant flowers
should not have been fixed in the most favourable manner for each
habitat, by natural selection alone.

Hoffmann is obviously unaware of the fundamental distinction
between the characters primarily acquired by the soma, and the
secondary characters which follow from changes in the germ-
plasm.

If the author had appreciated this distinction he would not
have attempted to strengthen his opinions by following up the
botanical facts which exclusively belong to the second class of
characters, with the enumeration of certain instances selected from
the animal kingdom (viz., the supposed transmission of mutila-
tions), all of which belong to the first class. I yall not discuss
these latter instances, for most of them are old friends, and they

TRANSMISSION OF ACQUIRED CHARACTERS. 417

are all far too uncertain and inaccurate to have any claim on
scientific consideration.

I believe that I have showr that no botanical facts have been
hitherto brought forward which prove the transmission of acquired
characters (in the restricted sense), and that there are not even
any facts which render such transmission probable.

A. W.

NaPLeEs, ZOOLOGICAL STATION,
Jan, 11, 1888.

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VIII.

THE SUPPOSED TRANSMISSION OF
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1888.

A lecture delivered at the Meeting of the Association of German Naturalists
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VIII.

THE SUPPOSED TRANSMISSION OF
MUTILATIONS.

We know well the manner in which Lamarck imagined that the
gradual transformation of species occurred, when he first made
the attempt to penetrate into the mechanism of the process of
evolution, and to ascertain the causes by which it is produced. In
his opinion, a change in the structure of any part of an organism
was chiefly brought about when the species in question met with
new conditions of life and was thus forced to assume new habits.
Such habits caused an increased or diminished activity, and there-
fore a stronger or weaker development, of certain parts, and the
modified parts were then transmitted to the offspring. Inasmuch
as the offspring continued to live under the same changed conditions,
and kept up the altered manner of using the part in question, the
inherited changes would be increased in the same direction during
the course of their life, and would be further increased in each
successive generation, until the greatest possible change had been
effected.

In this way Lamarck was able to give an apparently satisfactory
explanation of at any rate those changes which consist in the mere
enlargement or diminution of a part; such, for instance, as the
great length of neck in the swan and other swimming birds, which
he believed to have been produced by the habit of stretching after
food at the bottom of the water; or the webbed feet of the same
animals, supposed to be produced by the habit of striking the water
with outspread toes, etc. In this way he was also able to explain
the disappearance of a part after it had ceased to be of use; as, for

422 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

instance, the degeneration of the eyes of animals inhabiting caves
or the sunless depths of lakes or the sea.

But it is obvious that such an explanation tacitly assumes that
changes produced by use or disuse can be transmitted to the
offspring ; ¢¢ asswmes the transmission of acquired characters.

Lamarck made this assumption as a matter of course, and when
half a century later Charles Darwin, his more fortunate successor,
refounded the theory of organic evolution, he also believed that we
could not entirely dispense with the Lamarckian principle of
explanation, although he added the new and extremely far-reaching
principle of natural selection. But he certainly attempted to decide
whether the Lamarckian principle of the effects of use and disuse is
truly efficient, by asking himself the question whether such changes,
as for example those produced by exercise during an individual
life, can be transmitted to the offspring. Many observations ap-
peared to him, if not to prove the transmission’ directly, yet to
render it extremely probable; and he thus came to the conclusion that
there is no sufficient reason for denying the transmission of acquired
changes. Hence, in Darwin’s works, use and disuse still play
important parts as direct factors of transformation, in addition to
natural selection.

Darwin was not only an original genius, but also an extra-
ordinarily unbiassed and careful investigator. Whatever he ex-
pressed as his opinion had been carefully tested and considered.
This impression is gained by every one who has studied Darwin’s
writings, and perhaps it in part explains the fact that doubts as to
the correctness of the Lamarckian principle adopted by him have
only arisen during the last few years, These doubts have, however,
culminated in the decided denial of the assumption that changes
acquired by the body can be transmitted. I for one frankly admit
that I was in this respect under the influence of Darwin for a long
time, and that only by approaching the subject from an entirely
different direction was I led to doubt the transmission of acquired
characters, In the course of further investigations I gradually
gained a more decided conviction that such transmission has no
existence in fact.

Doubts on this point have been expressed not only by me but
also by others, such as du Bois-Reymond and Pfliiger. Indeed,
concerning a certain class of acquired characters, viz. mutilations,

EY Gm? Ranera Te

THE SUPPOSED TRANSMISSION OF MUTILATIONS, 423

the great German philosopher, Kant, has distinctly denied that
transmission can take place’; and in more recent times Wilhelm
His has expressed the same opinion *.

But if the transmission of acquired characters is truly impossible
our theory of evolution must undergo material changes. We
must completely abandon the Lamarckian principle, while the
principle of Darwin and Wallace, viz. natural selection, will gain an
immensely increased importance.

When I first expressed this opinion in my essay ‘On Heredity *,
I was well aware of the consequences of such an idea. I knew well
that apparently insurmountable obstacles would be raised against
any explanation of evolution, from which the principle of the direct
transformation of the species by external influences had been ex-
cluded. I therefore endeavoured to show that these difficulties are
not in reality insurmountable, and that it is quite possible to ex-
plain certain phenomena, such as the degeneration of useless parts,
without the aid of the Lamarckian principle. Furthermore it can
be shown that a not inconsiderable number of instincts, viz. all
those which are exercised only once in ‘a lifetime, cannot possibly
have arisen by transmitted practice. This fact renders it unneces-
sary to make use of the Lamarckian principle for the explanation
of other kinds of instinct. I do not mean to deny the existence
of phenomena for which such an explanation has not yet been
found, or at least has not been brought forward; but on the other
hand it appears to me that it has never been proved that we
cannot dispense with the Lamarckian principle in the explanation
of these phenomena. At any rate, I do not know of any facts
which could induce us to abandon from the first any hope of finding
an explanation without the aid of this hypothesis.

If we are able to prove that we may dispense with the assump-
tion of the transmission of acquired characters in explaining such.
phenomena, of course it by no means follows that we must dispense
with it; or, in other words, it does not follow that the transmission

1 It is true that he based his opinions upon entirely erroneous theories as to the
constancy of species. Compare Brock, ‘ Einige altere Autoren iiber die Vererbung
erworbener Eigenschaften’ in ‘Biolog. Centralblatt,’ Bd. VIII, p. 491 (1888): see
also Hugo Spitzer, ‘ Beitrige zur Descendenz-theorie und zur Methodologie der
Naturwissenschaft,’ Leipzig, 1886, pp. 515 et seq.

2 W. His, ‘ Unsere Kérperform,’ Leipzig, 1875.

* See Essay II in the present volume.

4.24, THE SUPPOSED TRANSMISSION OF MUTILATIONS.

of acquired changes cannot take place. It would be as unsafe’ to
make this assertion as to state of a ship seen at a great distance,
that it is only moving by sails and not by steam simply because the.
movement appears to be explicable by sails alone. We ought first
to attempt to show that the ship does not possess a steam-engine,
or at least that the existence of such an engine cannot be proved.

I believe that I am able to show that the actual existence of the
transmission of acquired characters cannot be directly proved ; that:
there are no direct proofs supporting the Lamarckian principle.

If we ask for the facts which can be brought forward by the
supporters of the theory of the transmission of acquired characters,
if we inquire for the observations which induced Darwin, for in-
stance, to adopt such an hypothesis, or which at least prevented
him from rejecting it,—a very brief answer can be given. There are
a small number of observations made upon man and the higher
animals which seem to prove that injuries or mutilations of the body
can, under certain circumstances, be transmitted to the offspring.

A cow which had accidentally lost its horn, produced a calf with
an abnormal horn; a bull which had accidentally lost its tail, from
that time begat tailless calves; a woman whose thumb had been
crushed and malformed in youth, afterwards had a daughter with a
malformed thumb, and so on.

In a great number of such cases every guarantee for the trust-
worthiness of the statements is entirely wanting, and, as His and
still earlier Kant have already said, they are of no greater value
as evidence than the merest tales. But in other cases this asser-
tion cannot be made without further examination; and a small
number of such observations can indeed claim a scientific in-
vestigation and value. I shall presently discuss this point in
greater detail, but I wish now to lay stress upon the fact that,
as far as direct evidence goes, we cannot bring forward any
proofs in favour of the transmission of acquired characters,
except these cases of mutilations. There are no observations which
prove the transmission of functional hypertrophy or atrophy,
and it is hardly to be expected that we shall obtain such proofs
in future, for the cases are not of a kind which lend themselves to
an experimental investigation. 'The hypothesis that acquired cha-
racters can be transmitted is therefore only directly supported by
the above-mentioned instances of the transmission of mutilations.

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 425

For this reason, the defenders of the Lamarckian principle, who
have come forward in rather large numbers recently’, have en-
deavoured to show that these observations are conclusive, and there-
fore of the highest importance. For the same reason I believe that
it is my duty, as I take the opposite view, to explain what I think
of the value of these apparent proofs of transmitted mutilations.

It can hardly be doubted that mutilations are acquired characters :
they do not arise from any tendency contained in the germ, but’
are merely the reaction of the body under external influences. They
are, as I have recently expressed it, purely somatogenic characters *,
viz. characters which emanate from the body (soma) only, as op-
posed to the germ-cells; they are therefore characters which do
not arise from the germ itself.

If mutilations must necessarily be transmitted, or even if they
might occasionally be transmitted, a powerful support would be
given to the Lamarckian principle, and the transmission of functional
hypertrophy or atrophy would thus become highly probable. For
this reason it is absolutely necessary that we should try to come to
a definite conclusion as to whether mutilations can or cannot be
transmitted.

We will now consider in greater detail the facts which have
hitherto been brought forward upon this point. It is not my
purpose to discuss every single case which has been mentioned
anywhere or by anybody; such a discussion would hardly lead
to any result. I propose to select a small number of such instances,
in order to show why they cannot be maintained as proofs. I
shall chiefly deal with cases which have been brought forward as

{* One of the most remarkable forms of this revival of Lamarckism is the establish-
ment in America of a ‘ Neo-Lamarckian School,’ which includes among its members
many of the most distinguished American biologists. One of the arguments upon
which the founders of the school have chiefly relied is derived from the comparative
morphology of mammalian teeth: The evolution of the various types are believed
to be due to modifications in shape, produced by the action of mechanical forces
(pressure and friction) during the life of the individual. The accumulation of such
modifications by means of heredity explains the forms of existing teeth.

It may be reasonably objected that the most elementary facts concerning the de-
velopment of teeth prove that their shapes cannot be altered during the lifetime of
the individual, except by being worn away. The shape is predetermined before the
tooth has cut the gum. Hence the Neo-Lamarckian School assumes, not the trans-
mission of acquired characters, but the transmission of characters which the parent
is unable to acquire !—E, B. P.]

* See p. 412 of the preceding Essay (VII).

426 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

especially strong proofs by my opponents, and which have been
carefully and completely examined. I shall attempt to show that
these are not conclusive and that they must be explained in an
entirely different manner. The insufficiency of the proof does not
always depend upon the same circumstances, and, according to the
latter, we may distinguish different classes of cases.

First of all we may briefly mention those instances in which the ne-
cessary precautions have not been taken before drawing conclusions.

To this class belong the tailless cats which were shown at last
year’s (1887) Meeting of the Association of German Naturalists,
at Wiesbaden. These cats had inherited their taillessness, or rather
their rudimentary tails, from the mother cat, which ‘ was said’ to
have lost her tail by the wheel of a cart having passed over it.
Not only did the owner of the cats, Dr. Zacharias, consider them as
a proof of the transmission of mutilations, but in a recently-
published work, entitled ‘On the Origin of Species, based upon the
Transmission of acquired characters’ (‘ Ueber die Entstehung der
Arten auf Grundlage des Vererbens erworbener Eigenschaften’),
the author, Prof. Eimer, speaks of these cats in the preface as a
‘valuable’ instance of the transmission of mutilations: these ex-
amples therefore form part of the foundation upon which the author
builds up his theoretical views.

Certainly, the want of tails in young cats, of which the mother
had lost its tail by an accident, would have been well worth
consideration, but unfortunately there is no trustworthy record
as to how the mother cat became tailless. Without absolute
certainty upon this point the evidence becomes utterly worthless ;
and Dr. Zacharias has acted very wisely in afterwards admitting
that this is the case, for inherent taillessness has been known in
cats for a long time. The tailless race of the Isle of Man is
mentioned in the first edition of ‘The Origin of Species’; of
course I am referring to Darwin’s work, and not to the above-
mentioned book of the same name, by Prof. Eimer. As to the first
origin of the tailless Manx breed we know no more than about the
origin of that remarkable race of cats with supernumerary toes,
which E. B. Poulton has recently described from Oxford, and has
traced through several generations’. These are innate mon-

[! See ‘ Nature,’ vol. xxix. p. 20, and vel. xxxv. p. 38. In the latter article nine
generations are recorded, and in both articles figures of the normal and abnormal feet

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 4.27

strosities which have arisen from unknown changes in the germ.
Similar monstrosities have been known for a long time, and no one
has ever doubted that they can be transmitted.

It would be equally justifiable to derive the cats with extra toes
from an ancestor of which the toes had been trodden upon, as to
derive the tailless cats of the Isle of Man from an ancestor of
which the tail had been cut off by a cart passing over it, and thus
to regard the existence of the race as a proof of the transmission of
mutilations,

But even if it were certain that the tail of the mother cat had
been mutilated, such a fact would not necessarily prove that the
rudimentary tails of the offspring were due to transmission from
the mother; they might have been transmitted from the unknown
father. This is probably not the case with Dr. Zacharias’ cat, for
tailless kittens occurred in several families produced by the same
mother; but in other eases the possibility of the possession of
innate taillessness by the father must be taken into account. The
following case is, in this respect, very instructive,

Last summer, my friend, Prof. Schottelius, of Freiburg, brought
me a kitten with an innate rudimentary tail, which he had
accidentally discovered as one of a family of kittens at Waldkirch,
a small town in the southern part of the Black Forest. The
mother of the kitten possessed a perfectly normal tail; the father
could not be identified.

A closer investigation resulted in the Sollovini rather un-
expected discovery. For some years past, tailless kittens have
frequently appeared in the families of many different mother cats
at; Waldkirch, and this fact is explained in the following manner.

are given. Additional generations and many more families have been since observed,
and an account of these observations will shortly be published in the same paper.
The breed originally came from Bristol. In the observations recorded, the ab-
normality of the offspring is an indication of the hereditary strength of the female
parents, while the degree of normality is a similar test of heredity through the male
parents; for the female parents were always abnormal, the male parents always
normal, The most abnormal kitten observed possessed seven toes on each forefoot,
seven toes on the right hind foot (three more than the normal number), and six
on the left hind foot. Kittens with seven toes on the forefeet and six on the hind
were comparatively common, and all intermediate conditions between this and the
normal were of frequent occurrence. Cats with extra toes are, I think, not uncom-
mon in most countries, and the fact that the peculiarity is transmitted is also well
known. The object of the investigation alluded to was to observe the transmission
systematically through many generations,—E. B. P.]

428 THE. SUPPOSED TRANSMISSION OF MUTILATIONS.

A clergyman, who lived for some time at Waldkirch, had married
an English lady who possessed a tailless male Manx cat. .The
probability that all the tailless cats in Waldkirch are more or less
distant descendants of that male cat almost amounts to certainty.
Since a male Manx eat has reached the Black Forest, it might
equally well arrive at some other place.

But we will now leave observations such as these, which do not
prove the transmission of a mutilation, because the mutilation itself
has not been established ; and we will turn to more serious ‘ proofs.’

Let us still consider the tails of domesticated animals. In these
animals a spontaneous and considerable reduction of the tail occurs
not uncommonly, and since the habit of cutting off part of the tail
of young animals prevails in many countries, the coincidence has
been explained as a causal relation, and the question has been
raised whether the disposition towards the spontaneous appearance
of rudimentary tails has not arisen in consequence of the artificial
mutilation practised through many generations. This supposition
appears very plausible at first sight, but the keen scientific criticism
of Déderlein, Richter, and Bonnet, together with careful anatomical
investigations, have shown that, at least in the cases which were
carefully examined, such a causal connection did not exist. It
has been shown that the spontaneous rudimentary tails which
occasionally appear in cats and dogs have an entirely different
origin from the transmission of artificial mutilation. They depend
upon an innate peculiarity of the germ, a peculiarity which is
easily and strongly transmitted. They are monstrosities, like the
sixth finger or toe, or, rather, like the rudimentary fingers and
toes, which also occasionally appear. Bonnet? has shown that the
rudimentary tails of dogs depend upon the absence of several
vertebrae, together with an abnormal ossification, and sometimes
also with a premature coalescence, of the vertebrae of the tail.

Bonnet states that in the two first cases examined by him the
reduction occurred at the distal end of the vertebral column in
the tail, the more or less malformed vertebrae being anchylosed.
A membranous appendage extended beyond the end of the reduced

1 Bonnet, ‘Die stummelschwiinzigen Hunde im Hinblick auf die Vererbung er-
worbener Eigenschaften,’ Anat. Anzeiger, Bd. III, 1888, p. 584; see also ‘ Beitrage
zur patholog. Anatomie und allgem. Pathologie’ by Ziegler and Nauwerck, Bd. IV,
1888.

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 429

caudal vertebrae, as the so-called ‘soft tail. These characters were
shown to have been inherited from the mother and to have under-
gone progressive development as regards the number of missing
vertebrae and the proportion of individuals with rudimentary tails.

Ina third instance Bonnet found that four to seven of the normal
caudal vertebrae were absent, and that the column in the region of
the tail was characterised by a tendency towards premature anchy-
losis along its whole length and not merely in its distal portion.
Furthermore the last three to four vertebrae were distorted and
were either placed transversely to the long axis of the tail, or were
so greatly curved that the tip of the tail was directed forwards.

It is obvious that these changes are not such as we should expect
as a result of the transmission of the mutilation of the tail which
is so commonly practised. If the artificial injury were transmitted
we should not expect that a variable number of the mesial ver-
tebrae would be absent, but rather those of the tip. There would
be no reason why the existing vertebrae should be degenerate as in
the majority of the caudal vertebrae of the dogs examined by
Bonnet.

Entirely similar phenomena have been observed by Déderlein in
the tailless cats which not infrequently occur in Japan. In these
cats the rudimentary vertebrae of the tail were reduced to a short,
thin, inflexible spiral, which formed a knot densely covered with
hair on the posterior part of the animal.

Such a reduction of the tail occurs quite independently of
artificial injury, in individuals of which the parents were not
injured: it is even found in races, such as the dachshund, which, as
far as we know, have never been habitually mutilated.

But the fact is rendered especially interesting because the
reduction of the vertebral column in the region of the tail takes
place in very various degrees. Sometimes only four vertebrae are
absent, sometimes as many as ten. The degree of abnormality in
shape and the degree of coalescence between the vertebrae also
differ greatly. Hence Bonnet rightly concludes that a slow and
gradual process of reduction is going on in these animals, a process
which tends, as it were, to shorten the tail. I intentionally say
‘as it were, for of course the statement must not be taken literally,
and we must not conclude that the process of reduction is a con-
sequence of some hypothetical developmental force seated in the

430 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

organism, of which the purpose is to remove the tail, On the
contrary, this instance shows very clearly that the appearance of
a development guided in a certain direction may be produced
without the existence of any motive developmental force.

The disposition of the tail to become rudimentary, in cats and
dogs, may be explained in the simplest way, by the process which
I have formerly called panmixia. The tail is now of hardly any
use to these animals; and neither dog nor cat would perish because
they possessed only an incomplete tail. Hence natural selection
does not now exercise any influence over these parts, and. an occa-
sional reduction is no longer eliminated by the early destruction
of its possessor: therefore such reduction may be transmitted to
the offspring. ;

The race of tailless foxes which, according to Settegast, existed
during the present century on the hunting-grounds of Prince
Wilhelm zu Solms-Braunfels, very soon disappeared; while cats
and dogs with rudimentary tails have been preserved in many
cases. Such results are to be expected, because in these domesticated
animals the absence of the tail did not cause any inferiority in the
struggle for existence.

But these facts appear to me to be remarkable in another
direction. I previously mentioned the tailless race of Manx cats.
Tradition does not tell us how it happened that the descendants of
the first tailless cat in the Isle of Man were able to increase and
spread in such a manner as to form the dominant race in the
island. But we can easily imagine how it happened, when we
learn that tailless cats are especially prized’ in Japan, because
people think that they are better mousers. Every one in Japan
wishes to possess a tailless cat, and people even cut off the tails of
normal cats when they cannot obtain those with congenital rudi-
mentary tails, because they believe that cats become better mousers
in consequence of taillessness. In Waldkirch the same account of
the superiority of tailless cats is curiously enough also found. We
thus see how a slight but striking variation may at once cause an
energetic process of artificial selection, which helps this variation to
predominance: a hint for us to be careful in passing judgment upon

‘See the interesting remarks by Déderlein on this point, which Dr. Ischikawa of
Japan tells me are quite correct. Déderlein, ‘ Ueber schwanzlose Katzen,’ Zool. An-
zeiger, vol. x. Noy. 1887, No. 265.

EE <<

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 431

sexual selection, for the latter also works upon such functionally
indifferent but striking variations. In the case of the cats, man
has favoured a particular variation, because the novelty rather than
the beauty of the character surprised and attracted him. He has
attached an imaginary value to the new character, and by arti-
ficial selection has helped it to predominate over the normal form.
I see no reason why the same process should not take place in
animals by the operation of sexual selection.

But now, after this little digression, let us return to the trans-
mission of mutilations.

We have seen that the rudimentary tails of cats and dogs, as far
as they can be submitted to scientific investigation, do not depend
upon the transmission of artificial mutilation, but upon the spon-
taneous appearance of degeneration in the vertebral column of the
tail. The opinion may, however, be still held that the customary
artificial mutilation of the tail, in many races of dogs and cats, has
at least produced a number of rudimentary tails, although, perhaps,
not all of them. It might be maintained that the fact of the
spontaneous appearance of rudimentary tails does not disprove the
supposition that the character may also depend upon the trans-
mission of artificial mutilation.

Obviously, such a question can only be decided by experiment :
not, of course, experiments upon dogs and cats, as Bonnet rightly
remarks, but experiments upon animals the tails of which are not
already in a process of reduction. Bonnet proposes that the
question should be investigated in white rats or mice, in which the
length of the tail is very uniform, and the oecurrence of rudi- —
mentary tails is unknown.

Before this suggestion was made, I had already attacked the
problem experimentally. Such a course might, perhaps, have been
more natural to those who maintain the transmission of mutilations,
to which I am opposed. Although I undertook the experiments
expecting to obtain purely negative results, I thought that the
latter would not be entirely valueless; and since the numerous
supporters of the transmission of acquired characters do not seem to
be willing to test their opinion experimentally, I have undertaken

‘the not very large amount of trouble which is necessary in order to
conduct such an experimental test.

The experiments were conducted upon white mice, and were

432 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

begun in October of last year (1887), with seven females and five
males. On October 17 all their tails were cut off, and on Novem-
ber 16 the two first families were born. Inasmuch as the period
of pregnancy is only 22-24 days, these first offspring began to
develope at a time when both parents were without tails. These
two families were together eighteen in number, and every in-
dividual possessed a perfectly normal tail, with a length of 11-
12mm. These young mice, like all those born at later periods,
were removed from the cage, and either killed and preserved, or
made use of for the continuance of the breeding experiments. In
the first cage, containing the twelve mice of the first generation,
333 young were born in fourteen months, viz. until January 16,
1889, and no one of these had a rudimentary tail or even a tail but
slightly shorter than that of the offspring of unmutilated parents.

It might be urged that the effects of mutilation do not exercise
any influence until after several generations. I therefore removed
. fifteen young, born on December 2, 1887, to a second cage, just
after they were able to see, and were covered with hair; their tails
were cut off. These mice produced 237 young from December 2,
1887 to January 16, 1889, every one of which possessed a normal
tail.

In the same manner fourteen of the offspring of this second
generation were put in cage No. 3 on May 1, 1888, and their
tails were also cut off. Among their young, 152 in number,
which had been produced by January 16, there was not a single
one with an abnormal tail. Precisely the same result occurred in
the fourth generation, which were bred in a fourth cage and treated
in exactly the same manner. This generation produced 138 young
with normal tails from April 23 to January 16.

The experiment was not concluded with the fourth generation ;
thirteen mice of the fifth generation were again isolated and their
tails were amputated; by January 16, 1889 they had produced 41
young.

Thus go1 young were produced by five generations of artificially
mutilated parents, and yet there was not a single example of a
rudimentary tail or of any other abnormity in this organ. Exact
measurement proved that there was not even a slight diminution
in length. The tail of a newly-born mouse varies from 10-5 to
12 mm. in length, and not one of the offspring possessed a tail

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 433

shorter than 10-5 mm. Furthermore there was no difference in
this respect between the young of the earlier and later generations.

What do these experiments prove? Do they disprove once for
all the opinion that mutilations cannot be transmitted? Certainly
not, when taken alone. If this conclusion were drawn from these
experiments alone and without considering other facts, it might
be rightly objected that the number of generations had been far
too small. It might be urged that it was probable that the
hereditary effects of mutilation would only appear after a greater
number of generations had elapsed. They might not appear by
the fifth generation, but perhaps by the sixth, tenth, twentieth, or
hundredth generation.

We cannot say much against this objection, for there are actual
phenomena of variation which must depend upon such a gradual
and at first imperceptible change in the germ-plasm, a change
which does not become visible in the descendants until after
the lapse of generations. The wild pansy does not change at
once when planted in garden soil: at first it remains apparently
unchanged, but sooner or later in the course of generations varia-
tions, chiefly in the colour and size of the flowers, begin to appear: ©
these are propagated by seed and are therefore the consequence of
variations in the germ. ‘The fact that such variations wever occur
in the first generation proves that they must be prepared for by a
gradual transformation of the germ-plasm.

It is therefore possible to imagine that the modifying effects of
external influences upon the germ-plasm may be gradual and may
increase in the course of generations, so that visible changes in the
body (soma) are not produced until the effects have reached a certain
intensity.

Thus no conclusive theoretical objections can be brought forward
against the supposition that the hereditary transmission of mutila-
tions requires (e.g.) 1000 generations before it can become visible:
We cannot estimate a priori the strength of the influences which
_ are capable of changing the germ-plasm, and experience alone can
teach us the number of generations through which they must act
before visible effects are produced.

If therefore mutilations really “act upon the germ-plasm as the
causes of variation, the possibility or even probability of the ultimate
appearance of hereditary effects could not be denied.

rf

434. THE SUPPOSED TRANSMISSION OF MUTILATIONS.

Hence the experiments on mice, when taken alone, do not con-
stitute a complete disproof of such a supposition: they would have
to be continued to infinity before we could maintain with certainty
that hereditary transmission cannot take place. But it must be
remembered that all the so-called proofs which have hitherto been
brought forward in favour of the transmission of mutilations assert
the transmission of a single mutilation which at once became
visible in the following generation. Furthermore the mutilation
was only inflicted upon one of the parents, not upon both, as in my
experiments with mice. Hence, contrasted with these experiments,
all such ‘ proofs’ collapse ; they must all depend upon error.

It is for this reason important to consider those cases of
habitual mutilation which have been continually repeated for
numerous generations of men, and have not produced any hereditary
consequences. With regard to the habitually amputated tails of
eats and dogs I have already shown that there is only an ap-
parently hereditary effect. Furthermore, the mutilations of certain
parts of the human body, as practised by different nations from
times immemorial, have, in not a single instance, led to the mal-

‘formation or reduction of the parts in question. Such hereditary

effects have been produced neither by circumcision’, nor the re-
moval of the front teeth, nor the boring of holes in the lips or
nose, nor the extraordinary artificial crushing and crippling of the
feet of Chinese women. No child among any of the nations re-
ferred to possesses the slightest trace of these mutilations when born :
they have to be acquired anew in every generation.

Similar cases can be proved to occur among animals. Professor
Kiihn of Halle pointed out to me that, for practical reasons, the
tail in a certain race of sheep has been cut off, during the last
hundred years, but that according to Nathusius, a sheep of this
race without a tail or with only a rudimentary tail has never been
born. This is all the more important because there are other
races of sheep in which the shortness of the tail is a distinguishing
peculiarity. Thus the nature of the sheep’s tail does not imply
that it cannot disappear.

1 Tt is certainly true that among nations which practise circumcision as a ritual,
children are sometimes born with a rudimentary prepuce, but this does not occur more
frequently than in other nations in which circumcision is not performed. Rather
extensive statistical investigations have led to this result.

he
% 4
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THE SUPPOSED TRANSMISSION OF MUTILATIONS. 435

A very good instance is mentioned by Settegast, although per-
haps with another object in view. The various species of crows
possess stiff bristle-like feathers round the opening of the nostrils
and the base of the beak: these are absent only in the rook. The
latter, however, possesses them when young, but soon after it has
left the nest they are lost and never reappear. ‘The rook digs deep
into the earth in searching for food, and in this way the feathers
at the base of the beak are rubbed off and can never grow again
because of the constant digging. Nevertheless this peculiarity,
which has been acquired again and again from times immemorial,
has never led to the appearance of a newly hatched individual with
a bare face.

Thus there is no reason for the assumption that such a result
would occur in the case of the mice even if the experiments had
been continued through hundreds or thousands of generations.
The supposition of the accumulative effect of mutilation is entirely
visionary, and cannot be supported except by the fact that accu-
mulative transformations of the germ-plasm occur; but of course
this fact does not imply that mutilations belong to those influences
which are capable of changing the germ-plasm. All the ascer-
tained facts point to the conclusion that they have not this
effect. The transmission is all the more improbable because of
the striking form of the mutilation in all cases which are relied
upon as evidence. The only objection which can be raised is to
suppose that the absence of the tail is less easily transmitted than
other mutilations, or that mice possess smaller hereditary powers
than other animals. But there is not the slightest evidence in
favour of either of these suggestions; the supporters of the
Lamarckian principle have, on the contrary, always pointed to the
transmission of mutilated tails as one of their principal lines of
evidence.

The opinion has often been expressed that such transmission need
not occur in every case, but may happen now and then under quite
exceptional conditions with which we are unacquainted: for this’
reason it might be urged that all negative experiments and every
refutation of the ‘ proofs’ of the transmission of mutilations are not
conclusive. Only recently, a clever young zoologist said in reference
to Kant’s statements upon the subject, that perhaps the most de-
cided opponent of the transmission of mutilations would not venture

Ff£2

436 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

nowadays to maintain his view with such certainty, ‘for it must —
be admitted that the transmission of acquired characters may take
place at any rate as a rare exception.’ Similar opinions are often
expressed, especially in conversation, and yet they can mean
nothing except that the transmission of acquired characters has been
proved ; for if such transmission can take place at all, it exists, and
it does not make the least difference theoretically whether it occurs
in rare cases or more frequently. Sometimes heredity has been
called capricious, and in a certain sense this is true. Heredity
appears to be capricious because we cannot penetrate into its depths:
we cannot predict whether any peculiar character in the father will
reappear in the child, and still less whether it will reappear in the
first, second, or one of the later children : we cannot predict whether
a child will possess the nose of his father or mother or one of the
grandparents. But this certainly does not imply that the results
are due to chance: no one has the right to doubt that everything is
brought about by the operation of certain laws, and that, with the
fertilization of the egg, the shape of the nose of the future child
has been determined. The co-operation of the two tendencies of
development contained in the two conjugating germ-cells produces
of necessity a certain form of nose. The observed facts enable us to
know something of the laws under which such events take place.
Thus, for instance, among a large number of children of the same
parents some will always have the form of the nose of the mother or
of the mother’s family; others will have the nose of the father’s
family, and so on.

If we apply this argument to the supposed transmission of muti-
lations, such transmission, if possible at all, must occur a certain
number of times in a certain number of cases: it must occur more
readily when both parents are mutilated in the same way, or when
the mutilation has been repeated in many generations, ete. It is
extremely improbable that it would suddenly oceur in a case where
it was least expected, while it did not occur in 900 cases of the
most favourable kind. Those who recognise in the doubtful cases
of transmission of a single mutilation present in only one of the
parents, proofs of the existence of the disputed operation of heredity,
quite forget that the transmission presupposes a very marvellous
and extremely complex apparatus which if present at all ought,
under certain conditions, to become manifest regularly, and not only

sy

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 437

in extremely exceptional cases. Nature does not create complex
mechanisms in order to leave them unused: they exist by use and
for use. We can readily imagine how complex the apparatus for the
transmission of mutilations or acquired characters generally must
be, as I have tried to show in another place. The transmission of
a scar to the offspring e.g. presupposes first of all that each me-
chanical alteration of the body (soma) produces an alteration in the
germ-cells: this alteration cannot consist in mere differences of
nutrition, only affecting an increased or decreased growth of the
cells; if must be of such a kind that the molecular structure of the
germ-plasm would be changed. But such a change could not in
the least resemble that which occurred at the periphery of the body
in the formation of the scar: for there is neither skin nor the pre-
formed germ of any of the adult organs in the germ-plasm, but
only a uniform molecular structure which, in the course of many
thousand stages of transformation, must tend to the formation of a
soma including a skin. The change in the germ-plasm which would
lead to the transmission of the scar, must therefore be of such a kind
as to influence the course of ontogeny in one of its later stages,
so that an interruption of the normal formation of skin, and the
intercalation of the tissue of the scar, would occur at a certain part
of the body. I do not maintain that equally minute changes of the
germ-plasm could not occur: on the contrary, individual variation
shows us that the germ-plasm contains potentially all the minutest
peculiarities of the individual; but I have in vain tried to under-
stand how such minute changes of the germ-plasm in the germ-
cells could be caused by the appearance of a scar or some other
mutilation of the body. In this respect I think that Blumenbach’s
condition is nearly fulfilled: he was inclined to declare himself
against the transmission of mutilations, but only if it were proved
that such transmission was impossible. Although this cannot be
strictly proved, it can nevertheless be shown that the apparatus
presupposed by such transmission must be so immensely complex,
nay! so altogether inconceivable, that we are quite justified in
doubting the possibility of its existence as long as there are no facts
which prove that it must be present. I therefore do not agree with
the recent assertion' that Blumenbach’s condition cannot be fulfilled
to-day, just as it was impossible at the time when it was first

See Brock, ‘ Biolog. Centralblatt,’ Bd. VIII. p. 497, 1888.

438 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

brought forward. But if nevertheless such a mysterious mechanism
existed between the parts of the body and the germ-cells, by means
of which each change in the former could be reproduced in a different
manner in the latter, the effects of this marvellous mechanism would
certainly be perceptible and could be subjected to experiment.

But at present we have no evidence of the existence of any such
effects ; and the experiments described above disprove all the cases
of the supposed transmission of single mutilations.

Of course, I do not maintain that such cases are to be always
explained by want of sufficient observation. In order to make
my position clear, I propose to discuss two further classes of
observations. First of all, there are very many cases of the ap-
parent transmission of mutilations in which it was not the mutila-
tion or its consequences which was transmitted, but the predis-
position of the part in question to become diseased. Richter? has
recently pointed out that arrests of development, so slight as to be
externally invisible, frequently occur, and that such arrests exhibit
a tendency to lead to the visible degeneration of parts in which
they occur, as the result of slight injuries. Since therefore the
predisposition towards such arrest is transmitted by the germ—
occasionally even in an increased degree—the appearance of a trans-
mitted injury may arise. In this way Richter explains, for in-
stance, the frequently quoted case of the soldier who lost his left
eye by inflammation fifteen years before he was married, and who
had two sons with left eyes malformed (microphthalmic). Micro-
phthalmia is an arrest of development. The soldier did not lose
his eye simply because it was injured, but because it was predis-
posed to become diseased from the beginning and readily became
inflamed after a slight injury. He did not transmit to his sons
the injury or its results, but only microphthalmia, the predis-
position towards which was already innate in him, but which led in
his sons from the beginning, and without any obvious external
_ injury, to the malformation of the eye. I am inclined to explain
the case which Darwin in a similar manner adduced, during the
later years of his life,\in favour of the transmission of acquired
characters, and which seemed to prove that a malformation of the
thumb produced by chilblains ean be transmitted. The skin of a

1 'W. Richter, ‘Zur Vererbung erworbener Charaktere,’ Biolog. Centralblatt, Bd.
VIII. 1888, p. 289.

THE SUPPOSED TRANSMISSION OF MUTILATIONS, 439

boy’s thumbs had been badly broken by chilblains associated with
some skin disease. The thumbs became greatly swollen and re-
mained in this state for a long time; when healed they were mal-
formed, and the nails always remained unusually narrow, short,
and thick. When this man married and had a family, two of his
children had similarly malformed thumbs, and even in the next
generation two daughters had malformed thumbs on both hands.
The case is too imperfectly known to admit of adequate criticism ;
but one may perhaps suggest that the skin of different individuals
varies immensely in its susceptibility to the effects of cold, and that
many children have chilblains readily and badly, while others are
not affected in this way. Sometimes members of the same family
vary in this respect, and the greater or less predisposition towards
the formation of chilblains corresponds with a different constitution
of the skin, in which some children follow the father and others
follow the mother. In Darwin’s instance a high degree of sus-
eeptibility of the skin of the thumb was obviously innate in the
father, and this susceptibility was certainly transmitted, and led to
the similar malformation of the thumbs of the children, perhaps
very early and after the effect of a comparatively slight degree
of cold}.

The last class of cases which I should wish to consider, refer to
observations in which the mutilation of the parent was certain, -
and in which a malformation similar to the mutilation had ap-
peared in the child, but in which exact investigation shows that
the malformations in parent and child do not in reality correspond
to each other.

In this class I include an instance which has only become known
during the present year (1888), and which has been observed as

+ This case was not observed by Darwin himself, but was communicated to him by
J. P. Bishop of Perry, in North America (see ‘Kosmos,’ vol. ix. p. 458). Quite
apart from the fact that it is by no means certain whether the father did not already
possess an innate malformation of the thumb, exact data are wanting as to the
time during which the thumb was diseased, and as to the time when the malforma-
tion of the thumb was first observed in the children and the grandchildren ; whether
at birth or at a later period. For a thorough criticism it would also be necessary to
have figures of the thumbs. I should not have alluded to this case, because of its in-
complete history, if it had not appeared to me to illustrate the ideas mentioned
above. Of course I do not maintain that I have suggested the right explanation in
this particular case. It is possible that the father possessed an inherent malforma-

tion of the thumb which he had forgotten by the time that he came to have children
and grandchildren, and was struck by the abnormality of their thumbs.

440 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

exactly as possible by an anthropologist and physician, who has
worked up the history of the case. Dr. Emil Schmidt communi-
cated to this year’s meeting of the German Anthropologists’ Asso-
ciation at Bonn a case which indeed seems at first sight to prove
that mutilations of the human ear can be transmitted. As Dr.
Schmidt has been kind enough to place at my disposal all the
material which he collected upon the subject, I have been able to
examine it more minutely than is generally possible in such cases;
and I will discuss it in detail, as it seems to me to be of funda-
mental importance in the history of human errors upon this subject. .
In a most respectable and thoroughly trustworthy family, the
mother possesses a cleft ear-lobe upon one side. She quite dis-
tinctly remembers that when playing, between the ages of six and
ten years, another child tore out her ear-ring, and that the wound
healed so that the cleft remained. Later on a new hole was made
in the posterior part of the lobe. She had seven children, and the
second of these, who is now a full-grown man, has a cleft ear-lobe
on the same side as the mother. It is not known whether the
mother possessed an innate malformation of the ear before it was
mutilated, but, judging from the present appearance of the ear,
this is extremely improbable. Furthermore, the existence of an
innate cleft in the ear-lobe has never been previously observed.
The parents of the mother did not possess any malformation of the
ear. The conclusion seemed to be therefore inevitable that the trans-
mission of an artificial cleft in the ear-lobe had really taken place.
But we must not be too hasty in forming an opinion. When
we compare the figures I. and II., representing the two ears, we are
first of all struck by the fact that the malformation of the ear of
the son has an entirely different appearance from that of the mother.
The ear-lobe of the latter is quite normally formed; it is broad
and well-developed, and only exhibits a median vertical furrow which
is the result of the mutilation. The ear-lobe of the son, on the
other hand, is extremely minute, one might even maintain that it
is completely wanting. In my opinion a cleft is not present at
all, but the far higher posterior corner of the ear forms the end of
its posterior margin—the so-called helix. But even if another
opinion were pronounced with regard to the interpretation of this
part, there is one other cireumstance to be taken into account,
which appears to me to be absolutely conclusive, and which com-

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 44.)

Fie. I. :
UrAh
c Lah
“ai J Wiel
Atr- li _ -
ee
Lob se
H
Fie, II.
CrAh
H
Tr.
Ceh
‘Atr
Lob? oe

Hi. Helix. Cr, Ah. Crura anthelicis. Ah. Anthelix. Cch. Concha.
Hi', and Hl*. Holes 1 and 2 for ear-rings. ob. Ear-lobe. Sp. H. Spina
helicis. ne. Incisura intertragica. Tr. Tragus. Atr. Antitragus.

44.2 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

pletely excludes the interpretation of this malformation as the
transmission of a mutilation.

If we compare the ears with each other, that, of the mother with
that of the son, not only the anatomist but every trained observer
will at once be struck by the fact that they are totally different
in their outlines as well as in every detail. The upper margin of
the ear is very broad in the mother, in the son it is quite pointed ;
the so-called crwra anthelicis are normally developed in the mother,
in the son they can hardly be distinguished and open in an an-
terior direction, while in the mother they are directed upwards.
The concha itself, the inciswra intertragica, in short everything in
the two ears, is as different as it can possibly be in the ears of
two individuals.

But this fact obviously proves that the son does not possess the
ear of his mother, but probably that of his father or grandfather.
Unfortunately the father and grandfather have been now dead for
a long time, so that we cannot obtain certain evidence upon this
point. At all events, the son does not possess the ear of his mother,
and it would be very rash to suppose that he has inherited the ear
from the father, but the malformation of the ear-lobe from the mother
—a malformation which, as it seems to me, is certainly quite
different from that of his mother’s ear. I said that this case was of
fundamental importance chiefly because it shows very distinctly,
on the one hand, how difficult it is to bring together the material
which is absolutely necessary for the correct understanding of a
single case, and on the other hand, how carefully the abnormalities
must be compared and examined if we wish to escape wrong con-
clusions, Such precautions have hitherto been rarely taken with
the necessary accuracy; people are in most cases satisfied with the
knowledge that an abnormality is present in the child on the
same part which had been malformed by mutilation in the parent.

But if we are to speak of the transmission of a mutilation, it
must be shown, before everything else, that the malformation of
the child corresponds precisely to the mutilation of the parent.

For this reason the older observations upon this subject are, in
most cases, entirely valueless.

The readiness with which we may be deceived is shown by
the fact that I myself nearly became a victim during the past
year (1888). A friend of mine, in order to convince me of the

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 443

transmission of mutilations, called my attention to a linear scar on
his left ear, which extended from the upper margin of the helix
for some distance upon the posterior part of the anthelix, giving it
the appearance of a small, rather sharp ridge. The scar had been
caused by a cut from a duelling sword, which the gentleman
had received during his residence at the University. Strangely
enough, the left ear of his daughter, who is five years old,
exhibits a similar peculiarity. The posterior part of the ant-
helix forms a rather sharp and narrow ridge like that of the father,
although the scar itself is wanting.

T must admit that I was at first rather puzzled by this fact, but
the mystery was soon solved in a very simple manner. I asked
the father to show me his right ear, and I then saw that this ear
possessed a similar ridge on the posterior part of the anthelix.
Only the scar was absent, which in the left ear brought the crest
of the ridge into still greater prominence. The ridge was there-
fore only an individual peculiarity in the formation of the ear
of the father,—a peculiarity which had been transmitted to one
ear of the child. No transmission of the mutilation had taken
place.

In the same manner, many of the so-called proofs of the trans-
mission of mutilations would be shown, by a careful examination, to
be deceptive. We must not expect to succeed in all of them, for in
most cases the investigation cannot be completed, chiefly because
the condition of the part in question in the ancestors is not known
or is only known in an insufficient manner. This is the reason why
fresh examples of such so-called proofs continue to appear from time
to time,—proofs which do not admit of a searching criticism because
something, and in most cases very much, is invariably wanting.
But it will be admitted that even a very large number of incom-
plete proofs do not make a single complete one. On the other
hand, it may be asserted that a single instance of coincidence
between a mutilation in the parent and a malformation in the
- offspring, even if well established, would not constitute a proof of
the transmission of mutilations. Not every post hoe is also a
propter hoc. Nothing illustrates this better than a comparison
between the ‘proofs’ which are even now brought forward in
favour of the transmission of mutilations and the ‘ proofs’ which sup-
ported the belief in the efficacy of so-called ‘maternal impressions’

441 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

during pregnancy, a belief which was universally maintained up to
the middle of the present century. Many of those ‘ proofs’ were
simply old wives’ fables, and were based upon all kinds of subsequent
inventions and alterations. But it cannot be denied that there are
a few undoubtedly genuine observations upon cases in which some
character in the child reminds us in a striking manner of a deep
psychical impression by which the mother was strongly affected
during pregnancy.

Thus a trustworthy person told me of the following case. A well-
known medical authority cut his leg above the ankle with a knife:
his wife was present at the time and was much frightened. She
was then in the third month of pregnancy: the child when born
was found to have an unusual mark upon the same place above the
ankle. People almost forget nowadays the tenacity with which the
idea of maternal impressions was kept up until the middle of this
century ; but it is only necessary to read the received German text-
book on physiology of fifty years ago, viz. that of Burdach, in order
to be convinced of the accuracy of this statement. Not only does
Burdach give a number of ‘ conclusive’ cases in man and even in
animals (cows and deer), but he also attempts to construct a theoretical
explanation of the supposed process. This is undertaken in the fol-
lowing manner,—‘ Imagination influences the function of organs ;’
but the function of the embryo is the ‘tendency towards development,
and hence the influence [of maternal imagination] can make itself
felt only as variations in the mode of development.’ Thus by ex-
changing the conception of function for that of the development of
organs, Burdach comes to the conclusion that ‘ homologous organs of
the mother and the embryo are in such connexion’ that when the
former are disturbed a corresponding ‘change in the formation of
the latter may arise.’

It seems to be not without value for the appreciation of the
questions with which we are dealing to remember that the idea
of ‘maternal impressions’ was only comparatively recently believed
to be a scientific theory, and that the proofs in support of it
were brought forward in form and language as scientifie proofs.
In Burdach’s book we even meet with detailed ‘ proofs’ that
violent mental shocks produced by maternal impressions may not
only exercise their influence upon one but even upon several children
born successively, although with diminishing strength. ‘A young

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 445

wife received a shock during her first pregnancy upon seeing a child
with a hare-lip, and she was constantly haunted with the idea that
her child might have the same malformation. She was delivered of
a child with a typical hare-lip: her next child had an upper lip with
a less-marked cleft; while the third possessed a red mark instead
of a cleft.’

Now what can be said about such ‘proofs’? We may probably
rightly conjecture that Burdach, who was in other respects a clever
physiologist, was in this subject somewhat credulous: but there are
also instances about which there is not the slightest doubt. I may
remind the reader of a case which has been told by no other than
the celebrated embryologist, Carl Ernst von Baer?.

‘A lady was very much upset by a fire, which was visible at a
distance, because she believed that it was in her native place. As
the latter was seven German miles distant, the impression had
lasted a long time before it was possible to receive any certain
intelligence, and this long delay affected the mind of the lady so
greatly, that for some time afterwards she said that she constantly
saw the flames before her eyes. Two or three months afterwards she
was delivered of a daughter who had a red patch on the forehead
in the form of a flame. This patch did not disappear until the
child was seven years old.’ Von Baer added, ‘I mention this case
because I am well acquainted with it, for the lady was my own
sister, and because she complained of seeing flames before her eyes
before the birth of the child, and did not invent it afterwards as the
“cause” of the strange appearance.’

Here then we have a case which is absolutely certain. Von
Baer’s name is a guarantee for absolute accuracy. Why then has
science, in spite of this, rejected the whole idea of the efficacy of
‘maternal impressions’ ever since the appearance of the treatises by
Bergmann and Leuckart ??

Science has rejected this idea for many and conclusive reasons,
all of which I am not going to repeat here. In the first place,
because our maturer knowledge of the physiology of the body
shows that such a causal connexion between the peculiar characters
of the child and, if I may say so, the corresponding psychical im-

1 See Burdach, ‘Lehrbuch der Physiologie,’ Bd. II, 1835-40, p. 128.

? See Handworterbuch der Physiologie von Rud. Wagner, Artikel ‘ Zeugung,’ von
Rud. Leuckart.

446 THE SUPPOSED TRANSMISSION OF MUTILATIONS.

pressions of the mother, is a supposition which cannot be admitted ;
but also and chiefly because a single coincidence of an idea of the
mother with an abnormality in the child does not form the proof
of a causal connexion between the two phenomena.

I do not doubt that among the many thousands of present and
past students in German Universities, whose faces are covered with
scars, there may be one with a son who exhibits a birth-mark on
the spot where the father possesses a scar. All sorts of birth-marks
oecur, and why should they not sometimes have the appearance of
a scar? Such a case, if it occurred, would be acceptable to the
adherents of the theory of the transmission of acquired characters ;
it would in their opinion completely upset the views of their op-
ponents.

But how could such a case, if it were really established, be
capable of proving the supposed form of hereditary transmission,
any more than von Baer’s case could prove the theory of the
efficacy of ‘maternal impressions’ ?

I am of opinion that the extraordinary rarity of such eases
strongly enforces the fact that we have to do with an accidental
and not a causal coincidence. If scars could be really transmitted,
we should expect very frequently to find birth-marks which cor-
respond to scars upon the face of the father,—viz. in almost all
cases In which the son had inherited the type of face possessed by
the father. If this were so we should have to be seriously con-
cerned about the beauty of the next generation in Germany, as so
many of our undergraduates follow the fashion of decorating their
faces with as many of these ‘honourable scars’ as possible.

I have spoken of ‘ maternal impressions’ because I wished to show
that, until quite recently, distinguished and acute scientifie men
have adhered to an idea, and believed that they possessed the proof
of an idea, which has now been completely and for ever abandoned
by science. But in addition to this, there is a very close con-
nexion between the theory of the efficacy of maternal impressions
and that of the transmission of acquired characters, and sometimes
they are even confounded together.

Last year a popular scientific journal quoted the following case
as a proof of the transmission of mutilations. I do not, however,
wish to imply that the editor must be held responsible for the
errors of a correspondent. ‘In November, 1864, a pregnant merino

THE SUPPOSED TRANSMISSION OF MUTILATIONS. 447

sheep broke its right fore-leg, about two inches above the knee-
joint ; the limb was put in splints and healed a long time before
the following March, when the animal produced young. The
lamb possessed a ring of black wool from two to three inches in
breadth round the place at which the mother’s leg had been broken,
and upon the same leg.’ Now if we even admitted that a ring of
black wool could be looked upon as a character which corresponds
to the fracture of the mother’s leg, the case could not possibly be
interpreted as the transmission of a mutilation, but as an instance
of the efficacy of maternal impressions; for the ewe was already
- pregnant when she fractured her leg. The present state of bio-
logical science teaches us that, with the fusion of egg and sperm-
cell, potential heredity is determined’. Such fusion determines the
future fate of the egg-cell and the individual with all its various
tendencies.

Such tales, when quoted as ‘remarkable facts which prove the
transmission of mutilations, thoroughly deserve the contempt with
which they have been received by Kant and His. When the.
above-mentioned instance was told me, I replied, ‘It is a pity that
the black wool was not arranged in the form of the inscription “'To
the memory of the fractured lee of my dear mother.” ’

The tales of the efficacy of ‘maternal impressions’ and of the
transmission of mutilations are closely connected, and break down
before the present state of biological science. No one can be pre-
vented from believing such things, but they have no right to be
looked upon as scientific facts or even as scientific questions. The
first was abandoned in the middle of the present century, and the
second may be given up now; when once discarded we need not
fear that it will ever again be resuscitated.

It is hardly necessary to say that the question as to the trans-
mission of acquired characters is not completely decided by the un-
conditional rejection of the transmission of mutilations. Although
I am of opinion that such transmission does not take place, and that
we can explain the phenomena presented by the transformation of
species without this supposition, I am far from believing that the
question is settled, simply because the transmission of mutilations
may be dismissed into the domain of fable. But at all events we
have gained this much,—that the only facts which appear to directly

1 See V. Hensen, ‘ Physiologie der Zeugung, Leipzig, 1881.

448 THE SUPPOSED TRANSMISSION OF MUTILATIONS,

prove a transmission of acquired characters have been refuted, and
that the only firm foundation on which this hypothesis has been
hitherto based has been destroyed. We shall not be obliged, in
future, to trouble about every single so-called proof of the trans-
mission of mutilations, and investigation may be concentrated upon
the domain in which lies the true decision as to the Lamarckian
principle, it may be concerned with the explanation of the ob-
served phenomena of transformation.

If, as I believe, these phenomena can be explained without
the Lamarckian principle, we have no right to assume a form of
transmission of which we cannot prove the existence. Only if it
could be shown that we cannot now or ever dispense with the
principle, should we be justified in accepting it. I do not think
that I can represent the state of the subject better than by again
referring to the metaphor of the ship. We see it moving along
with all sails set, we can discern the presence of neither paddles
nor screw, and as far as we can judge there is no funnel, nor any
other sign of an engine. In such a case we shall not be justified
in concluding that an engine is present and has some share in the
movement of the vessel, unless the movement is of such a kind that
it is impossible to explain it as due to the unaided action of the
wind, the current, and the rudder. Only if the phenomena pre-
sented by the progress of organic evolution are proved to be inex-
plicable without the hypothesis of the transmission of acquired
characters, shall we be justified in retaining such an hypothesis.

INDEX.

Abutilon, polymorphic flowers of, pp.
320, 323.

Acanthia lectularia, length of life of,
42.

Acineta, 151.

Acquired characters, meaning of, 169 ;
on supposed botanical proofs of trans-
mission of, 390, 397.

Acridium migratorium, length of life
of, 40.

Actiniamesembryanthemum, length of
life of, 54.

Actinosphaerium, 117, 118.

Activity and length of life, 7, 8.

Adansonia, length of life of, 6.

Adler, on the formation of galls, 302.

After-effects, 403.

Aglia tau, deposition of eggs, 18 ; length
of life of, 18, 59.

Algae, immortality of unicellular, 25.
Amoeba, length of time of fission of, 8 ;
immortality of, 25; fission of, 25, 64.

Amphibia, polar bodies of, 340, 352.

Amphileptus meleagris, fission of, 148.

Amphorina coerulea, polar bodies of,
189.

aia iceds, 25, 38.

Ancylus, length of life of, 56.

Andricus, length of life of summer
generation, 50.

Anisotropism, 400.

Anlagen, 192.

Anodonta, length of life of, 56, 57.

Ants, duration of life of male and female,
18, 48, 50, 51, 52, 59, 156.

Aphilotrix, length of life of imago of,

oO.

Aphis, length of life of, 41; partheno-
genesis of, 228, 289; polar bodies of,
349-

Apis, see Bees.

Apus, 152, 324.

Ascaris, 133, 144; fertilization of, 177;
nuclear division in ovum of, 188, 232,
360 ; spermatogonia of, 220 ; spermato-
genesis of, 375.

Ascidians, length of life of, 57.

Atavism, 179.

Atrophy, of organs, 85, 86.

Auerbach, on fertilization, 355.

Bacteria, in dead Cockchafer, 46.

Baer, von, 194; on the infiuence of
maternal impressions on the offspring,
445-

Balanus, polar bodies of, 218.

Balbiani, on nuclear division, 187; on
pole-cells, 197 ; on origin of ova, 222.

Balfour, on impregnation, 175; on polar
bodies, 214, 225, 339, 345, 353-

Bear, length of life of, 13.

Bees, length of larval life of, 15 ; length of
life of queen of, 18, 52, 156; of drones,
18, 53; of workers, 52, 59, 1563 acti-
vity of, 48 ; oviposition of, 52, 54; death
of male, 63, 120, 1323; loss of limbs in
development of larva, 89; nuptial
flight of, 93; development of eggs of,
226, 234, 285, 351.

Begonia, propagation of, 211.

Beneden, van, on fertilization in Ascaris,
177, 188, 355, 360; on polar bodies,
214, 340, 345, 3533 on Polkérperchen,
216; on spermatogonia in Ascaris, 220,
375; on nuclear division, 231 ; on sexual
reproduction, 282.

Berthold, on male parthenogenesis, 247.

Bessels, on importance of fertilization,
235.

Beyerinck, on the formation of galls,
302.

Biorhiza, length of life of imago of, 50.

Birds, length of life of, 11, 36; factors
in duration of life of, 12.

Blackbird, length of life of, 6, 11, 36.

Blaps, length of life of imago of, 47, 48.

Blastogenic characters, 412.

Blochmann, on polar bodies, 349.

Blow-flies, length of larval life of, 15.

Boar, length of life of, 14.

Bombinator igneus, nature of ovum of,
125.

Bombus, 53.

Bombyces, flight of females impeded by
eggs, 17; habits of, 44.

Bonellia viridis, unequal length of life
of male and female, 59.

Bonnet, on rudimentary tails in dogs,

28.

Born, on position of nucleus in ova,

177; on double impregnation, 382.

Gs

450

Bosmina, parthogenesis of, 325.

Brooks, on heredity, 166, 326.

Brown-Séquard, experiments on guinea-
pigs, 81, 310, 313.

Bulimus, length of life of, 55.

Buprestis splendens, length of life of,

47.

Burdach, on the influence of maternal
impressions on the offspring, 444.

Bitschli, on polar bodies, 188, 214, 224,
340; on sexual reproduction, 282; on
processes of fertilization, 355.

Butterflies, climatic varieties of, 99;
death of, 120.

Bythotrephes, spermatozoa of, 176; sum-
mer eggs of, 239; winter eggs of, 348.

Calberla, on impregnation in Petromy-
zon, 175.

Canary birds, length of life of, 36;
plumage of, 321.

Carabus auratus, length of life of imago
of, 47-

Carnoy, on karyokinesis in ovum of As-
caris, 360, 368; on spermatogenesis,

375-

Carp, length of life of, 6.

Cat, length of life of, 6.

Catallacta, 123.

Caterpillars, length of life of phytopha-
gous, I5.

Cells, renewal of, 21; nourishment of,
29; death of, 59.

Cephalopods, length of life of, 56.

Cerambyx heros, length of life of imago
of, 47.

Cetochilus, polar bodies of, 218.

Chermes, parthogenesis of, 294; galls
of, 401.

Cherry-tree, in Ceylon, 406.

Chrysomela varians, ovoviviparous de-
velopment of, 48, 49.

Chydorus, parthenogenesis of, 325,

Cicada, length of life of, 41, 42.

Cienkowsky, on conjugation, 286.

Cionea intestinalis, length of life of,
57:

Circumcision, 434.

Cirrhipedes, complementary males of,

8.
Clausilia, length of life of, 55.
Cockchafer, length of larval life of, 16;
length of imaginal life of, 46.
Coleoptera, length of life of, 46.
Colpoda cucullus, fission of, 148.
Conjugation, 282, 286.
Continuity of germ-plasm, 104.
Copepods, unequal length of life in the
two sexes of parasitic, 58.
Coryne, origin of sexual bud, 205.
yee ligniperda, length of larval life
of, 15.

INDEX,

Crayfish, length of life of, 6.

Cuckoo, length of life of, 11, 36.

Cyclas, length of life of, 56.

Cynipidae, length of life of, 49; num-
ber of eggs of agamic, 50; deposition
of eggs of, 93 ; number of males of, 293.

Cynips, amount of nuclear matter in egg
of, 229; parthenogenesis. of, 274, 290,
293-

Cypris, 294.

Cyto-idioplasm, 181, 184.

Cytoplasm, 184.

Daphnidae, segmentation of the egg of,
73, 199; loss of jaws in development of,
89; winter eggs of, 121; sperm cells of,
175,176; parthenogenesis of, 228, 325 ;
summer eggs of, 236, 239, 240; polar
bodies in parthenogenetic eggs of, 249,
345, 350-

Darwin, on constancy of number of in-
dividuals in successive generations, 12 ;
on Pangenesis, 77, 370; on atrophy of
organs, 85, 90; on cross fertilization,
309; on effect of external influences,
391, 423.

Daucus, structure of root altered by
cultivation, 414.

Death, origin of, 20, 21, 143; relation to
reproduction, 21, 120, 132, 154; ne-
cessity of, 23, 24, 134, 159; utility of,
24, 112, 135, 1533 an adaptation pro-
duced by natural selection, 24, 28, 60;
not universal, 25, 27, III, 119; by
sudden shock, 63; meaning of, 113;
definition of, 114; of the soma, 154.

Degeneration of organs, see Atrophy.

Detmer, on transmission of acquired
characters in plants, 390.

Development amongst Protozoa, 149.

Diatomaceae, fission of, 65.

Dicyemids, 131, 141.

Diptera, length of life of, 42; pole cells
of, 197, 206, 210, 216; as fertilizers of
flowers, 309.

Déderlein, on tailless cats, 390, 428, 430.

Dragon-flies, length of larval life of, 15 ;
length of life of imago, 17, 40.

Dryophanta, length of life of summer
generation of, 49; of winter genera-
tion, 50.

du Bois Reymond, on the transmission
of acquired characters, 82, 390, 422.

Duration of life governed by needs
of species, 9.

Diising, on origin of sex, 239, 241.

Eagles, length of life of, 11, 37; weight
of, 14.

Echinodermata, origin of germ-cells,
202.

Echinus, polar bodies of, 351.

INDEX.

Ectocarpus, male parthenogenesis in,
247.

Eggs, number laid by various birds, 12,
37; of insects, 17.

Hider-ducks, length of life of, 11.

= on the inheritance of mutilations,
426.

Eleodes grandis, and dentiper, length
of life of imago of, 47.

Elephants, length of life of, 6; gestation
of, 7.

Encystment, relation to death of, 112,
II5, 116, 120, 158; protective, I17,
121; of Rhizopoda, 121.

Entoniscidae, unequal length of life in
male and female, 58.

Ephemeridae, length of life of imago of,
40, 156.

Epigenesis, theory of, 316.

_ Bristalis tenax, length of life of, 43.

Estheridae, 228.

Euglypha, identity of products of fission
of, 26, 64, 65.

Eupithecia, length of life of, 45.

Falcons, length of life of, 11, 37.

Fiedler, on polar bodies in sponges,
217.

Flemming, on nuclear division, 187,
231, 359, 301.

Flourens, on length of life, 7.

Fol, on fusion of nuclei, 174, 189; on
origin of ova, 222; on multiple im-
pregnation, 236, 238, 382; on polar
bodies, 3.40, 351; on process of fertiliza-
tion, 355.

Formica sanguinea and fusca, length
of life of, 51.

Fox, length of life of, 14.

Galls, 302, 401.

Galton, on transfusion in Rabbits, 166;
on heredity, 172 ; on twins, 380.

Gannets, numbers collected each year,
37:

Geotropism, 398.

Germ, meaning of, 148.

Germ-cells, 73; predisposition of the,
84, 102; fluctuations in, 102; not
continuous, 173.

Germ-plasm, 80, 191, 266, 341, 357, 371,
403; continuity of, 104, 168, 173, 184;
definition of, 174.

Goliathus cacicus, length of life of
imago of, 47.

Goose, length of life of the wild, 37.

Gétte, on necessity of death, 112; on
rejuvenescence, 115, 1243 on death of
Metazoa, 125.

Gregarines, 148, 149, 202.

ee on polar bodies of Cetochilus,
218,

451

Gruber, on regeneration amongst In-
fusoria, 185.

Gryllotalpa, duration of life of, 39.

Gryllus campestris, duration of life of,
39-

Hackel, on reproduction, 72; on Peri-
genesis of the Plastidule, 165; on
amphigonic reproduction, 272.

Hare, length of life of, 14.

Hartlaub, on origin of germ-cells in
Obelia, 208.

Hawk-moths, length of life of imago, 17.

Helicidae, length of life of, 55, 56, 57.

Heliotropism, 399.

Hemiptera, length of life of, 41.

Hens, length of life of, 36.

Hensen, on sexual reproduction, 282,
286; on difference between germ-
plasm and histogenetic nucleoplasm,
343; on heredity, 369.

Heredity, 29, 71, 378; defined, 72; de-
pendent on continuity of germ-plasm,
104, 168; dependent on coalescence
of nuclei, 178.

Hertwig, O., on fusion of nuclei, 174 ;
on the influence of gravity in segmenta-
tion, 177, 189; on polar bodies, 340,
351; on process of fertilization, 355.

Hesperornis, rudimentary wing of, 88.

Heterogeny, 325.

Heterogynis, 44.

Heteroplastides, 130, 131, 134, 139,
146, 153, 204.
Hildebrandt, on duration of life in

plants, 32, 65; on cross-fertilization,
309.

His, on heredity, 166, 390, 412; on the
transmission of mutilations, 423.

Hoek, on polar bodies in Balanus, 218.

Hoffman, on transmission of acquired
characters, 407.

Homoplastides, 122, 139, 146, 202.

Horse, length of life of, 6, 7; in the
Falkland Islands, 99.

Humboldt’s Atur Parrot, 12.

Hunter, John, experiments in Ana-
biosis, 25.

Hyalineae, length of life of, 56.

Hybrids, 330.

Hydroids, origin of germ-cells of, 199,
206, 207, 211.

Hyla, 301, 394.

Hymenoptera, length of life of, 49.

Hypermetropia, 89.

Ichneumons, length of larval life of,
15; length of life of imago of, 49.

Ichthyophthirius multifiliis, fission of,
148, 149.

Idioplasm, 174, 184, 192, 341; not
identical with nucleoplasm, 180; rela-

Gg2

452

tion to chromatin, 217; two kinds of,
2453 various combinations of, 276.
Imago, length of life of, 16.
Immortality, injurious to species, 24;
of unicellular organisms, 25, 27, 33.
Infusoria, immortality of, 25, 72; re-
generation of lost parts in, 27, 185;
fission of, 64; encystment of, 117.
Insects, duration of life amongst, 15 ;
duration of larval life of, 15; normal
death of, 22; duration of imaginal life
of, 38; segmentation of egg of, 73;
deposition of eggs, 93 ; origin of germ-
cells of, 202 ; polar bodies of, 218.
Instinct, 83; origin of, 91, 389; used
but once in a lifetime, 93.
Isotropism, of the ovum, 176.
Ivy, climbing shoots of, 393, 399.

Jager, on heredity, 172, 206.
Jordan, on varieties, 269.
Julin, on spermatogenesis in Ascaris,

375-

Kallima, mimicry of, 280, 306.

Kant, on the transmission of mutilations,
423.

Karyokinesis, 359, 375-

Keim, see Germ.

Kirchner, on development of Volvex,
204.

Kélliker, on nature of spermatozoa,
175; on embryonic cells, 196.

Lagynus, fission of, 148.

Lamarck, on use and disuse, 83, 84,
303, 387, 391, 421.

Lamellibranchiata, length of life of,

5.

Lane length of life of, 15.

Lasius flavus, length of life of, 50; L.
niger, 51.

Lepidoptera, length of life of imagos of,
43, 156; parthenogenesis among, 226,
352; spermatogenesis in, 375

Lepisma saccharina, length of life of,
40.

Leuckart, on relation of absorbing sur-
face to size of animal, 7;.on develop-
ment of Bees, 235; on the influence of
maternal impressions on the offspring,
445-

Limnadia Hermanni, 152.

Limnaeus, length of life of, 56.

Lion, length of life of, 13.

Lister, on chromatophores of blind
Frogs, 301.

Locusta, length of life of imago of, 39.

Lotze, on activity in connection with
longevity, 7.

8 yar cervus, length of life of imago
of, 47.

INDEX.

Lycaena violacea, length of life of imago

of, 44.
Lynceinae, spermatazoa of, 176.

Macroglossa stellatarum, length of life
of female of, 45.

Magosphaera planula, 75, 120, 122,
126, 147, 152; figure of, 123.

Magpies, length of life of, 36.

Mammals, duration of life of, 38.

Manx cats, 427, 430.

Maternal impressions, supposed in-
fluence on offspring, 444.

May-flies, length of larval life of, 15;
length of life of imago of, 16; habitat
of larvae of, 17; shortening of life of,
19; death of, 120.

Meldola, 395.

Melolontha vulgaris, see Cockchafer.

Mesozoa, 128.

Metazoa, 27, 28, 111, 145; old age of,
157. ;

Metschnikoff, on pole-cells, 197.

Micellae, 190, 194.

Mimetic forms, 264, 280.

Mimosa, ‘ after effects’ in, 404.

Minot, on cyclical development, 199;
on polar bodies, 214, 225, 340, 345,
353-

Moina, winter eggs of, 118, 240; seg-
ee of, 199; polar bodies of,
218.

Molluses, length of life of, 55; deter-
mined by markings on shell, 14;
enemies of, 58.

Monoplastides, 115, 122, 125, 146, 159;
definition of, 120; reproduction of,

149.
Mouse, length of life of,6; gestation of,

Miller, F., on heredity of acquired cha-
racters, 320, 322.

Miller, H., on colours of flowers, 259;
on nectaries, 307.

Multicellular organisms, division of
labour in, 27.

Musca domestica, length of life of, 43.

Musca vomitoria, polar bodies of, 353.

Myopia, 89.

Niageli, 167, 171, 175; on idioplasm,
174, 182, 190, 201, 340, 414; on in-
herent tendency to vary, 256, 298; on
Alpine plants, 269; on adaptation,
300; on medium of heredity, 318,
355: :

Najadae, length of life of, 56.

Natica heros, length of life of, 56.

Nautilus, persistence of, 300.

Nematodes, polar bodies of, 188; nu-
clear division of ovum of, 234, 368.

Neuroptera, length of life of, 40.

INDEX.

Neuroterus, length of life of summer
generation, 49; of winter generation,

50.
Nigella, production of double flowers of,
08

408.

Nightingale, length of life of, 11, 36.
Nothnagel, on the cause of epilepsy,
314- :

Nuclear plate, 187.

Nuclei, behaviour during fission, 118,
188; connection of heredity with
fusion of, 178; influence of, 184; in-
fluence in regeneration, 185 ; nutrition
of, 187.

Nucleoplasm, 179, 185, 191, 227; his-
togenetic, 213; ovogenetic, 213, 230,
2433 spermogenetic, 220, 243.

Nussbaum, on heredity, 172, 195, 206;
on regeneration amongst Infusoria, 185,
200,

Obelia, origin of germ-cells of, 208.

Obersteiner, on inheritance of epilepsy
in guinea pigs, 311, 313.

Ophiostomum, karyokinesis in ovum of,
368

Orgyia, 44.

Orth, on the transmission of acquired

characters, 411.

Orthonectides, 120, 126; figure of,
127; degeneracy of, 130, 131, 141,
152.

Orthoptera, duration of life of, 39.

Ostracodes, parthenogenesis of,
325; polar bodies of, 350.

Otostoma.Carteri, fission of, 148.

294,

Palingenia, sub-imago stage of, 1g, 40.

Paludinidae, length of life of, 56.

Pandorina, 202, 248; figure of, 203.

Pangenesis, theory of, 77, 165, 166,
193, 316, 327.

Panmixia, principle of, 90, 140, 291,
439°.

Papaver, production of double flowers of,
408.

Paranucleus, 376; of the sperm-cell,
221.

Parrots, length of life of, 36.

Parthenogenesis, the origin of, 225,
290, 323, 339; not ancestral, 228; of
bees, 235; partial, 238; explanation
of, 243; male, 247; of Cynips, 273;
not perpetual, 283, 285.

Pasimachus, length of life of, 48.

Pemphigus terebinthi, length of life of,
41.

Petromyzon, impregnation of, 175, 247;
polar bodies of, 218.

Pfeffer, on chemical attraction of oosphere,
247.

Pfitzner, on nuclear division, 187.

453

Pfliger, on heredity, 70, 175, 3553 on
the inheritance of acquired characters,
81, 390, 422; onisotropism of the ovum,
176.

Phanerogams, fertilization of, 178, 247 ;
development of pollen grains of, 222.
ee the length of life of the golden,

36.

Philodina, polar bodies of, 350.

Phryganea grandis, 41.

Phylloxera vastatrix, length of life of,
41; unequal length of life in two sexes
of, 58 ; parthenogenesis of, 294.

Pieris napi, length of life of, 44.

Pig, length of life of, 6.

Pigeon, length of life of, 363; cross-
breeding of, 332.

Pike, length of life of, 6.

Pisidium, length of life of, 56.

Planorbis, length of life of, 56.

Plants, duration of life of, 32, 65.

Polar bodies, 188, 218, 245; the signi-
ficance of, 212, 225, 339; of Sponges,
217; in parthenogenetic eggs, 249,
345, 383; of Rabbit, 339; number of,
340 ; significance of second, 353, 362 ;
in plants, 377.

Pole-cells, of Diptera, 197.

Polistes gallica, ‘ workers of, 53; length
of life of males and females, 54.

Pollen-grains, 222.

Polyphemus, spermatozoa of, 176 ; sum-
mer eggs of, 239 ; polar bodies of, 345.

Polyplastides, definition of, 120, 122,
125, 159 ; development of, 152.

Polyzoa, length of life of, 57.

Poulton, on colours of caterpillars,
394; on cats with supernumerary toes,
420.

Proteus, 87.

Protomyxa aurantiaca, 149.

Protozoa, development amongst, 150;
conjugation of, 287.

Psorosperms, 150.

Psychidae, length of life of, 16, 44, 45,
157; deposition of eggs of, 18; death of
female, 63, 132; parthenogenesis of,
293.

Pulex irritans, length of life of, 42.

Pupa, length of life of, 55.

Rabbit, polar bodies of, 339.

Rauber, on heredity, 172.

Ravens, length of life of, 36.

Regeneration of lost parts, 65; in In-
fusoria, 185.

Rejuvenescence, 112, 116, 124, 132,
153, 283.

Reproduction, original form of, 122;
effect of monogonic, 273, 275; amphi-
gonic, 279, 281, 287.

Reproductive cells, 27, 28, 111.

454 INDEX.

Rhodites rosae, parthenogenesis of, 325.

Richter, on inheritance of acquired
characters, 438.

Robin, on pole-cells of Diptera, 197.

Rolph, on conjugation, 286.

Romanes, on correlation, 389.

Roth, on heredity, 166, 169.

Rotifera, unequal length of life in two
sexes of, 58; polar bodies of, 350.

Roule, on origin of ova, 222.

Roux, on the struggle of the parts in the
organism, 87, 100; on development in
altered conditions, 177 ; on forces con-
trolling nuclear division, 231, 361; on
karyokinesis, 359.

Rudimentary organs, 88; disappear-
ance of, 291; not found in partheno-
genetic forms, 293.

Rumia Crataegata, 394.

Sachs, on reproduction in Mosses, 212;
on venation, 260, 310; on shoots of
climbing Ivy, 393, 399.

Sagitta, segmentation of egg of, 74, 199.

Saperda carcharias, length of life of
imago of, 47.

Sarcophaga carnaria, length of life of,

42.

Saturnia pyri, length of life of, 45; 8.
carpini, cocoon of, 94.

Saturnidae, habits of, 44.

Saw-flies, ancestors of bees and wasps,
19; length of life of imago of, 49, 59.

Schinidt, on malformations of the ear,
440.

Schneider, on instincts of perception,
92, 94-

Schultze, on polar bodies of Amphibia,
349, 352.

Scytosiphon, male parthenogenesis in,
247.

Sea-gulls’ eggs, 38.

Senility, 20, 21, 32, 157.

Sexes, unequal length of life in the two,

8

58.

Sheep, length of life of, 14.

Sida, spermatozoa of, 176 ; absorption of
ova in, 239.

Siebold, von, on development of Bees,
235-

Siphonophora, origin of germ-cells of,
202,

Sirenia, 261.

Sirex, length of larval life of, 15.

Smerinthus tiliae, length of life of
imago of, 45; ocellatus, 395.

Solenobia triquetrella, length of life
of female of, 45; death of partheno-
genetic forms of, 64, 293.

Solidago, time of flowering changed,
415.

Soma, 122, 125, 130, 140, 144, 154, 155.

Somatic cells, 27, 28, 75, 111, 145, 158.
Somatogenic characters, 412.
Somatoplasm, 104, 180.

‘Spathegaster, 49.

Spencer, Herbert, on relation of absorb-
ing surface to size of animal, 7; on in-
fluence of diminished nutrition, 241;
on correlation, 389.

Spirogyra, on cell-division in, 216.

Sponges, polar bodies of, 217.

Spontaneous generation, 34.

Sprengel, on colours in flowers, 308.

Squirrel, length of life of, 14.

— on protective structures in plants,
260.

Strasburger, on fertilization of Phanero-
gams, 178, 340, 355; on cyto-idioplasm,
181; on influence of nuclei, 184; on

. identity of daughter nuclei, 187; on
nuclei of sexual-cells, 200, 215, 246;
on transmission of germ-plasm, 209 ;
on cell-division in Spirogyra, 216; on
development of pollen-grains, 222; on
parthenogenesis, 237; on direction of
growth of pollen-tube, 247; on here-
dity, 354, 369, 369; on polar bodies in
plants, 377.

Strepsiptera, length of life of, 41; un-
equal length of life in two sexes, 58,

59-
Succineae, length of life of, 55.
Swans, length of life of, 37.

Tagetes, production of double flowers of,
408.

nneaY transmission of, 95; nature of,
96.

Tape-worms, 133, 155.

Termites, duration of life of, 18, 40.

Terns’ eggs, 38.

Thuja, dorso-ventral structure of shoots
of, 391, 396.

Tillina magna, 118, 148.

Toad, length of life of, 6.

Transmission of acquired characters,
73, 80, 169, 267, 407, 411; want of
evidence of the, 81; unnecessary for
the theory of evolution, 83 ; unproved,
105, 142; amongst Protozoa, 278;
supposed botanical proofs of, 387.

Trematodes, parasitic in Mollusca, 57,
131, 133.

Trichodinidae, conjugation of, 287.

Trichoplax adhaerens, 141.

Tridacna gigas, length of life of, 56.

Trinchese, on polar bodies, 189, 224.

Tropaeolum, two kinds of leaves of, 396.

Turkey, the length of life of, 36.

Twins, 380.

Unicellular organisms, immortality of,

3 Ae rd
Unio, length of life of, 56, 57.

:

INDEX.

Valaoritis, on origin of germ-cells,
195 ; on physiological value of, 246.

Vanessa cardui, length of life of, 43; V.
prorsa, 44; V. urticae, 44; V. levana,
deposition of eggs of, 94.

Variations, always present, IOI.

Ventral canal cell, 223.

Vertebrata, late origin of reproductive
cells in, 74.

Vespa, 53.

Viola calcarata, fertilization of, 310;
tricolor, 414.

Vitrinae, length of life of, 55.

Volvocineae, 202, 248.

Volvox, 204, 248.

Vorticellidae, conjugation of buds of,
287.

Vultures, length of life of, 11, 37.

THE

455

Wallace, on constancy of number of
individuals in successive generations,
12; 0n production of death by natural
selection, 23.

Wasps, duration of life of male and
female, 18, 53; loss of embryonic limbs
in development of larva of, 89.

Westphal, on epilepsy in guinea-pigs,

314.

Whales, length of life of, 6; adaptation
in, 261,

White mice, experiments in the trans-
mission of mutilations on, 431.

Will, on origin of ova, 222.

Wolff, on theory of epigenesis, 316.

Zacharias, on the inheritance of mutila-
tions, 426.

END.

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Essays upon heredity and
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A431 Essays upon heredity
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