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Dr. august WEISMANN 










Oxford University Press Warehous 

Amen Corner, E.C. 

(Hew 2)orft 

[2 Fourth Avenue 



The four essays which constitute this Second Volume sup- 
plement those of the First, by bringing forward new facts to 
support the earlier ideas, and by improving and completing the 
latter on the basis of the most recent discoveries. 

The first two essays afford further support to the arguments 
in favour of the non-transmission of acquired characters, inas- 
much as they attempt to prove that these arguments hold in 
certain cases which at first sight appear to refute them. The 
diminution of parts which are no longer used has been ex- 
plained in an earlier essay as the result of the cessation of 
natural selection, i. e. of panmixia. The conception that every 
part of the organism is maintained at the level it has reached 
only by means of the continued activity of natural selection, 
and that any intermission of this activity leads to a gradual 
diminution, has passed through many minds. Darwin himself 
appears to have held this idea, and Romanes, and especially 
Seidlitz, have more or lea^s clearly expressed it. But the 
thought first attained its full significance when we arrived 
at the definite conclusion that the Lamarckian principle of modi- 
fication had no real existence, because acquired characters, and 
hence the decrease which organs suffer by disuse, are not 
inherited. Thus every explanation of the existence of disused 
parts in a rudimentary state fails, except panmixia ; and the 
conclusion was unavoidable that the countless characters which 
enter into our conception of a species can only be maintained 
at their present level by the ceaseless activity of natural 

iv author's preface to second volume. 

The Second Essay is concerned with the problem as to the 
origin of those higher mental powers of civilized races which 
have played no part in the struggle for existence, and the high 
cultivation of which has been entirely independent of natural 
selection. I have chosen the art of music as an example, 
because it presents so deceptive an appearance of improvement 
by the inheritance of the results of practice from one generation 
to another. 

The Third Essay is an answer to the numerous objections 
which Prof. Vines of Oxford has advanced against many of my 
views. The reader of the First Volume will perhaps welcome 
it, as it elucidates some points upon which I have been fre- 
quently misunderstood. 

The Fourth and last Essay is not only the longest, but the 
one to which I attach the chief importance, because my views 
as to the essential meaning of so-called sexual reproduction, 
and the allied process of conjugation in unicellular organisms 
reach their final form in it, having been reconstructed on the 
basis of various new discoveries. I believe that I have solved, 
at any rate as regards the main points, the problem of the 
enigmatical double extrusion of polar bodies from the animal 
egg, and have explained why only a single division of the 
nuclear substance does not take place. I hope, furthermore, 
that I have thus confirmed my views upon the general signifi- 
cance of so-called sexual reproduction, — as a means for pro- 
ducing hereditary individual variations, and for arranging these 
variations in ever fresh combinations. 

My hypotheses have been at times severely handled when 
shown to be incorrect by the discovery of new facts,— even 
when these latter were themselves founded on my views. 
I freely admit that I have made many mistakes ;— my explana- 
tion of the formation of polar bodies by the egg was at first 
wrong, then only partially right, and claims to be correct only 
in the concluding essay. Let who will reproach me : I am not 
ashamed of this error ; on the contrary, I regard it with a cer- 
tain satisfaction, for I believe it pointed the path to truth. I 


have left it unchanged in the essays of the First Volume, not 
only for this reason, but chiefly because it is, I think, of great 
interest to trace the development of a scientific truth. Hypo- 
theses, even when not absolutely right, may be of value in 
advancing our knowledge, if only they are relatively right, 
i.e. when they correspond with the state of existing knowledge. 
They are like the feelers which the short-sighted snail stretches 
forth on its darkened path, testing this way and that, and with- 
drawing them and altering its route as soon as they come across 
any obstacle ; just as an unyielding fact may show that we are 
on a wrong road. 

Rome was not built in a day, and no scientific truth is at once 
revealed without a prolonged previous history made up of 
mingled truth and error. The last word has not yet been 
spoken on the subject dealt with in these essays ; but if we 
remember the complete obscurity which, only ten years ago, 
surrounded everything which is now clearly revealed in the 
final essay, we shall not be able to refrain from an inward 
feeling of satisfaction. Of course, much remains to be done in 
this department of biology ; but a firm foundation has been laid 
on which much may be erected. 

Freiburg i. B. : 
20 August, 1 89 1. 


We have wished to add a few words in order to thank those 
who have kindly helped us in rendering the last Essay on 
Amphimixis, which presented exceptional difficulties, accounting 
lor the delay in the appearance of this volume. Early in the 
present year one of us had the opportunity of consulting Pro- 
tessor Weismann and bringing under his notice all the most 
difficult passages in the four essays. As a result of this, the 
English translation has been modified in some few respects, 
and, to this extent, represents Professor Weismann more 
accurately than the original German. We desire to express 
our warmest thanks to him and to Fraulein Diestel for the 
great trouble they have taken in assisting us. 

In this country our chief thanks are due to Miss Lilian J. 
Gould, who not only translated the First Essay, but carefully 
read through the proof sheets of the Essay on Amphimixis, and 
made manj^ valuable suggestions. We have also been helped 
on special points by Professors E. Ray Lankester, S. H. Vines, 
and F. Gotch, and by Mr. D. G. Ritchie. Other kind assistance 
has been acknowledged in our Preface to the Second Edition, 
in Volume I. 

In conclusion, we venture to express the hope that these new 
essays may deepen the interest already aroused by Professor 
Weismann's earlier writings. If this be so, we shall always 
remember with pleasure the time and work which have been 
devoted to the production of these Essaj^s in their present form. 

E. B. P. 

A. E. S. 
Oxford : May, 1892. 


Translator. Title, Page 

IX. Miss Lilian J. Gould . Retrogressive Development in 

Nature, 1886 i 

X. Frau LiJROTH .... Thoughts upon the Musical 

Sense in Animals and Man, 
1889 . . 31 

XI. A. E. Shipley .... Remarks on Certain Problems 

OF THE Day, 1890 71 

XII. The Editors and others Amphimixis or the Essential 

Meaning of Conjugation and 
Sexual Reproduction, 1891 . 99 

INDEX 223 

Essays in Vol. 11 independently Published in this 

XI. Translated in full in 'Nature/ Vol. XLI, pp. 317-323, by G. H. 

No further translations or abstracts have appeared. 


Retrogressive Developjfient in Nahtre, 

A lecture delivered at the * Akademische Gesellschaft,' 
Freiburg-im-Breisgau, January, 1886. 




Evolution in the animal and vegetable kingdoms is generally 
understood to mean an uninterrupted progress from lower to 
higher forms of life. Such a view is not, however, strictly 
correct ; for retrogression plays an important part in evo- 
lution, as is shown by the fact that an investigation into the 
history of degenerate forms often teaches us more of the causes 
of change in organic nature than can be learnt by the study 
of progressive ones. Such investigation is, therefore, of the 
deepest interest. 

To begin with a well-known instance, we are all aware of 
the existence of birds which cannot fly, and of some among 
them which do not even possess wings. One of these is the 
Apteryx of New Zealand, called by the natives ' Kiwi-kiwi.' 
The most superficial observer would at once remark that this 
bird lacks something, since it reminds one of a man without 
arms ; for the wings are totally absent, and the place where 
they should be, is covered with a close smooth growth of hair- 
like feathers. 

Not very long ago the question why this bird should lack 
wings would have been regarded as sufficiently answered by 
reference to its mode of life. The Kiwi lives in woods, not in 
the trees, but on the ground ; in the day-time it hides in holes 
in the ground, and comes out warily at night to hunt the worms 
and insects which form its prey. It has no need of wings to 
obtain its food, nor does it stand in any fear of native enemies 
on the ground ; for two species of bat are the only representa- 

B 2 


tives of the Mammalia in New Zealand. In former days it 
would have been said that the Kiwi was created without wings, 
because it had no need for them ; but now that we can no longer 
hold the old simple doctrine of special creation, and are com- 
pelled to believe that the animals and plants of every age have 
not been suddenly created out of nothing, but have been de- 
veloped from ancestral forms, such an assumption can have but 
little weight. The idea of such special creation is not compatible 
with the present state of our knowledge ; we cannot suppose 
that the cause of all being called the forms of Hfe into existence 
in their present form by word alone, but rather by the action 
of natural forces upon matter, these working together to pro- 
duce the whole universe of everlasting change, seen in the 
rise and decline of solar systems, no less than in the evolution 
and extinction of species. We do not hold that the Kiwi 
was created out of nothing, but that it was developed from 
older forms, from species of birds very unlike itself. These 
birds again were evolved from lizard-like reptiles, which pos- 
sessed fore- as well as hind-limbs : hence the primitive birds 
must have had these also, and their fore-legs must have been 
gradually changed into wings. It is, therefore, certain that 
the ancestors of the Kiwi possessed wings. Why, then, should 
the Kiwi have lost them ? 

Furthermore we have positive evidence in support of the 
above conclusion that the ancestral form possessed wings, 
which have been eliminated in the existing species — because 
the Kiwi even now bears traces of them as minute rudiments 
hidden under its feathers, and although these no longer serve 
any purpose, the essential structure of the wing is plainly 
recognizable, and there are even some short crooked feathers 
which, with their strong shafts, are very like true primary 

The actual reason why the Kiwi possesses only rudimentary 
wings is, of course, to be found in the fact that, with its present 
structure and habits, they would be useless to it, and so far we 
should be justified in saying that the bird has no wings because 
it has no use for them. The Kiwi is certainly formed for 
terrestrial life ; its short but strong legs and feet are adapted 
for scratching the earth or digging out holes under the roots of 
great trees, and enable it to make its escape swiftly and noise- 


lessly, when pursued by the natives or by one of the few 
indigenous birds of prey. It confines itself almost entirely to 
the food it can find in the earth, especially worms, in searching 
for which it is greatly assisted by the long beak with its delicate 
sense of touch. It drives its bill into the soft damp ground, 
much as the snipe does, and extracts the worms with great 
skill and precision. 

When the species first arose, it was confined to the ground, 
since nothing was to be gained ,by leaving it, and the physical 
structure was therefore adapted to this mode of Hfe, by the 
gradual elimination of the wings. If the species were only 
now being formed, the above-mentioned change would most 
probably not have occurred ; for with the invasion of its 
domain by man, bringing his fire-arms and his cats and dogs, 
the conditions of life of the Kiwi have been considerably 
altered, and wings jnight now stand the helpless bird in good 
stead. But they have been irretrievably lost, and the race of 
Kiwis will consequently soon be extinct, hke the gigantic 
ostrich-like birds, the Moas, which are known to have inhabited 
New Zealand within the memory of man, and the skeletons of 
which, over twelve feet high, arouse our wonder in museum 

As the winged ancestors of the Kiwi adapted themselves 
more and more to life on the ground in the woods, they came 
to use their wings less and less, and we may safely conclude 
that this increasing tendency to disuse of the organs of flight, 
continuing through long generations, affected the organs them- 
selves, and in some indirect way diminished their size, gradually 
reducing them to the insignificant appendages we now find. 

It is easy to understand how it is that degeneration has gone 
further in the case of the Kiwi than in that of the ostrich ; for, 
although the latter does not fly, it still uses its wings as aids in 
running swiftly over the African plains and deserts, while 
such rapid movement across open country is not necessary for 
the Kiwi, living as it does in coverts. Short wings with large 
feathers, like those of the ostrich, would be rather a hindrance 
than otherwise to the Kiwi in moving quickly through thickets 
and among underwood, and therefore its wings have been 
reduced to mere rudiments which are externally altogether 


It is not only among the ostriches that we find degeneration 
of this kind ; certain species of water-birds have become too 
heavy and awkward to rise into the air, and in these too, for 
instance in the penguin, the wings are quite useless as organs of 
flight. But, although useless for flying, they are still of some 
service for motion in water, and therefore have not degenerated 
as completely as those of the Kiwi. They have, however, 
become far smaller than those of flying birds, and, clothed with 
short scale-like feathers, they bear some resemblance to the 
fins of fishes. 

These few instances will suffice to show that nature is pur- 
poseful, not only in adapting recently developed structures to 
her uses, i. e. in fitting them to perform properly the functions 
allotted to them, but, conversely, in removing everything 
superfluous, so that as soon as a structure is no longer required 
it is eliminated. Of course, this ehmination is neither sudden 
nor voluntary, but comes to pass gradually, in accordance with 
certain laws, so that we are often able to watch every stage 
of the transition from the full development of an organ to the 
entire absence of it. 

Such degeneration of once important parts is not only found 
here and there in nature ; it is of frequent, nay, among the 
higher animals, of general occurrence. It is in fact a natural 
consequence of the evolution of the higher animals of to-day 
from earlier and lower forms, which lived under totally different 
conditions, necessitating the possession of parts and organs, 
which, in process of time, have been either altered or com- 
pletely atrophied. If nature had not possessed the power to 
cause the disappearance of superfluous organs, there could 
have been no such thing as the transmutation of species ; for 
primitive structures, when they became superfluous, would have 
been in the way of those in actual use and would have hindered 
their development. Indeed, had the retention of all original 
structures been a necessity from the first, the result would 
have been the production of monsters quite unfit to live. 
Hence the retrogression of superfluous structures is a condition 
of progression. 

Having found disuse to be the immediate cause of the dis- 
appearance of a structure in the course of the development of 
a species, we may further ask how a structure once essential to 


life can fall into disuse. Obviously, this can only happen 
through a change in the conditions under which the animal 
lives. When a bird which has been accustomed to seek its 
food in trees and bushes, finds upon the ground supplies so 
rich as to afford better sustenance, it will gradually come to live 
more and more upon the ground, and less and less in trees, a 
fact which taken alone will entirely alter the conditions of its 
life. It will not require to fly, and will consequently fly less 
and less often, and after the lapse of generations will cease to 
fly altogether. And to bring all this about, the wood in which 
it lives, the climate, the surrounding animals, need not have 
undergone any changes ; merely the adoption of a new habit 
by the bird itself will suffice. 

It is the same with animals removed from their original 
habitat ; they may find themselves in circumstances so essen- 
tially different as to render superfluous some organ which had 
once been indispensable. For instance, if a species which had 
always lived in the light, were to find its way into some new 
habitat where there was complete darkness, its eyes would be- 
come useless to it ; and accordingly we commonly find that in 
such species the eyes have more or less completely atrophied. 

This is the case, for instance, with animals which live in 
dark caves. In the limestone caverns of Carniola and Carinthia 
a blind amphibian, the Proteus, is found in great numbers, and 
there are also bhnd Crustacea (both isopods and amphipods), 
blind insects and snails. In the Mammoth Cave of Kentucky 
among other bhnd animals we find a blind fish and a bhnd 
fresh-water crayfish. It is almost superfluous to offer any 
further proof that these species are descended from ancestors 
which possessed the power of sight, beyond the fact that the 
caverns in question have not existed from the beginnings of 
organic life, and that therefore the animals must have lived in 
the hght before they entered them. Nevertheless, in many of 
these animals direct proof exists in the fact that they still 
possess vestiges of what have once been eyes. The Proteus 
and the blind fish of the Mammoth Cave have small imperfectly- 
developed eyes under the skin, which are no longer of any use 
as organs of sight. In the case of the blind crayfish, the eyes 
have entirely disappeared, although the moveable stalks upon 
which they were placed still remain. 


Caves are not the only places where animals are known to 
live in the dark ; in deep wells and at the bottom of the sea 
and of lakes complete darkness reigns. To Professor Forel 
of Morges we owe the discovery of the depth to which light 
can penetrate. Photographic plates were sunk at night to a 
certain depth, and after being suspended to a buoy, were 
exposed, for a period of from twenty-one to twenty-four hours, 
to such light as could reach them. By this means Forel found 
that even in the transparent water of the Lake of Geneva, 
the light in winter, when the water is clearest, only penetrated 
to a depth of loo metres, and scarcely 50 metres in summer. 
Later experiments by Fol and Sarasin, with more perfect 
apparatus and more highly sensitive plates, proved, however, 
that light penetrates the Lake of Geneva to the greater depth 
of 170 metres. On a bright day there is about as much light 
at such a depth as we are accustomed to see on a starlight 
night, when there is no moon. Below this there is utter 
darkness ; and we find blind animals from these downwards 
to the greatest depths (300 metres), at which, for example, a 
blind isopod and an amphipod exist. In the sea, when the 
water is undisturbed, light penetrates as far as 400 metres, but 
as we now know that animal life exists in the sea at a depth of 
4000 metres, there still remains a vast region in which darkness 
reigns, and in which numberless blind animals are found, — 
blind fish, blind crustaceans of various species, bhnd molluscs 
and worms. Forms nearly related to all these five where the 
light penetrates, and possess eyes. 

Burrowing animals, too, have, for the most part, either poorly- 
developed eyes or none at all. Thus earthworms are sightless, 
while closely-allied pelagic species generally possess eyes, 
often very highly developed, and of complex structure. The 
common mole has indeed eyes, although very small ones, 
completely hidden in its close fur, but in Africa there are 
moles which are devoid of eyes and therefore entirely blind. 

Many other instances might be brought forward to prove 
that the disuse of the organs of vision results in their disappear- 
ance. And the same conclusion holds good for other organs ; 
experience teaches that, as soon as any organ falls into disuse, 
it degenerates and is finally lost altogether. 

We find interesting confirmation of this fact in the other 


organs of special sense, although cases of the disuse of these 
are of less frequent occurrence. Thus the caecilians, tropical 
worm-like or snake-like amphibians, living underground, have 
lost not only the sense of sight, but that of hearing also. They 
possess neither tympanum nor tympanic cavity, and although 
the auditory vesicle, w^hich is buried in the interior of the 
skull, still exists, the auditory nerve, v^hich should be in con- 
nection with it, supplying its sensitive nerve-endings, has 
entirely disappeared. The sense of hearing must have become 
useless to them in their life underground, or the organ would 
not have degenerated ^. They are compensated for the want 
of it by a remarkably keen sense of smell, which is more highly 
developed in these animals than in any other vertebrates. 

Instances are also known of disuse causing degeneration in 
the sense of smell ; thus the whales and dolphins have more or 
less completely lost this organ which is so highly developed in 
the rest of the Mammaha. 

Retrogression is, however, not always carried so far as to do 
away with a structure altogether, although this generally 
happens with the organs of sense, because they can scarcely 
be adapted to other uses. But not infrequently the degenerat- 
ing organ can be turned to account in some other way, and then 

1 It is now known that the above statements as to the existence of a 
rudimentary auditory organ in Caedlia are erroneous. Recent researches 
have shown us that these animals not only possess a complete auditory 
apparatus, but that it is even more perfect than in other Amphibia. In 
their splendid 'Ergebnisse zoologischer Forschungen auf Ceylon,' Heft 4, 
1890, the cousins Sarasin have given an accurate account of the auditory 
organ of a caecilian {Epicrium), and show that it is very far from being in 
a degenerate condition. It possesses all the essential parts, the auditory 
nerve is even larger than usual, and one of the ' maculae acusticae ' present 
is unrepresented in other Amphibia. These writers even prove that, in 
addition to the ordinary apparatus, many accessory auditory organs are 
present in the skin, each of which contains an otolith : these are homo- 
logous with the ' organs of the lateral line ' of other Amphibia and of fish. 

Up to the present time our knowledge of the auditory organ of Caecilia 
has been founded upon the statements of two excellent observers, Pro- 
fessors Retzius and Wiedersheim ; but the material at their disposal was 
restricted to a few badly preserved specimens. 

We must therefore maintain that the organ of hearing as well as that 
of smell has been especially developed in the caecilians as a compensation 
for the want of eyesight. Those conditions of life that would render the 
power of hearing useless do not appear to exist. As a result of these 
recent researches, I am now unable to adduce an example of a rudi- 
mentary auditory organ. — A.W., 1891. 


retrogression either stops just short of actual ehmination, as in 
the case of the wings of the ostrich, or so alters and transforms 
the structure as to fit it for new functions, like the wings of the 
penguin, which aid it in swimming. 

The far-reaching effects, on the development of species, of 
retrogression consequent upon disuse are nowhere to be seen 
more clearly than among parasitic animals. 

Many groups of animals contain certain genera, families, 
or even whole orders, which live at the expense of other 
animals, feeding on their blood or tissues, yet not killing them 
after the manner of beasts of prey. Such are the parasites, 
some of which only seek their unwilling host when impelled 
by hunger, and leave it as soon as they are satisfied ; while 
others take up their abode in or upon it, only to be driven 
thence by its death. The great group of worms includes very 
many parasites, and they are almost as numerous among the 
Crustacea. Most crustaceans are free-swimming or actively 
running inhabitants of the water, especially of the sea, and 
their food is partly of a vegetable nature and partly consists of 
living or dead animals ; but nearly every order includes some 
parasitic form, in which the effects of disuse resulting from 
parasitism are plainly traceable. 

A visit to the fish-market at any European sea-port, and an 
examination of some of the larger fish, will generally lead to 
the discovery of certain segmented animals firmly attached to 
the integument, and bearing some resemblance to wood-lice. 
These parasites, called fish-lice, suck the blood of the fish. 
They are not permanently fixed, but leave their host from 
time to time and seek a fresh one. Now these animals exhibit 
with great clearness the effects of parasitic habits : their legs 
are short, being no longer required for swimming, but chiefly 
for holding on by, and the organs of sense also are somewhat 
degenerate, for parasites scarcely need them. It is, of course, 
necessary for predaceous crustaceans to be able to distinguish 
their prey at a distance, and for this purpose they require keen 
sight and a delicate sense of touch in their antennae ; but para- 
sitic forms, when once attached to their host, do not readily 
leave it, or if they do so, a new host is easily found, since fish 
are mostly gregarious. Hence in these fish-lice the eyes and 
antennae have become small and insignificant. 


This is, however, but the first step in retrogressive develop- 
ment : more marked effects are witnessed in forms which are 
more completely and permanently fixed to their hosts. To 
the same crustacean order belong the Enioniscidae, which are 
internally parasitic upon other crustaceans, especially upon the 
common shore-crab {Carcinus maenas). During their whole 
life, these parasites never leave the host, nor move from the 
position they have once taken up within it. They live attached 
to its liver, sucking the juices ; after growing enormously and 
producing thousands and thousands of eggs, they finally die. It 
is clear that such a mode of life must render superfluous, and 
therefore degenerate, many structures which were essential to 
their free-swimming ancestors. This retrogression takes place 
to such a degree, and the whole structure of the animal is thereby 
so modified and altered that they are scarcely recognizable 
as Crustacea. The characteristic segmentation of the body 
is entirely lost, and the hard exo-skeleton is replaced by a thin 
soft skin. The body lengthens to a vermiform shape, acquires 
peculiar pointed appendages for the reception of the eggs, and 
becomes colourless, like that of all animals which live in the 
dark. All these modifications are quite inteUigible ; the seg- 
mentation of the crustacean body facilitates movement, while the 
hard exo-skeleton serves for the attachment of muscles. The 
eyes and antennae completely disappear, because the animal 
lives in darkness, and does not need to see, and because the 
sense of touch is unnecessary to it after it has once taken up 
its position. Not a vestige remains of certain mouth-organs 
which are well developed in alHed species; and the legs, 
of which free-swimming forms have seven thoracic and six 
abdominal pairs, are reduced in number. The internal organs 
are also reduced, with the single exception of the ovaries, 
which increase so much in size that the animal appears like 
a mere bag of eggs. 

It may now be asked how we know this peculiar vermiform 
being to be a crustacean and an Isopod at all. We know this 
to be a fact because there are many other parasitic Isopods in 
which degeneration has not gone so far, and which present 
well-marked stages of transition from the above-mentioned 
fish-Hce to the Entoniscidae. Furthermore, the descent of the 
Entoniscidae from free-swimming forms is clearly proved by 


the fact that the young still resemble the latter in the 
possession of eyes and antennae, segmented bodies, well 
developed jaws, and numerous legs : in short, in all essential 
points of structure, they exactly resemble the locomotive forms. 
The young of the Entoniscidae are actually free-swimming 
organisms, and it is necessary for the perpetuation of the 
species that they should be so, for how could the parent 
animal, possessing no organs of locomotion, leave its original 
host for a fresh one ? And yet such a change is essential for the 
continuance of the species ; for in course of time the hosts will 
die. Under such circumstances the young Entoniscidae leave 
the mother as perfect Isopods, make their way out of the host, 
and lead a free-moving life in the sea until they find and enter 
another Carcinus maenas : they then undergo a whole series 
of retrograde changes in rapid succession, and finally attain the 
remarkable vermiform shape already spoken of Of course, 
retrogressive development did not reach anything like this 
degree at first ; it was only attained after the lapse of countless 
generations, and a passage through many intermediate forms. 
The original parasitic Isopods lived no doubt, like the fish-lice, 
attached to the external integument of their host ; these were 
followed by others which took up their abode in the internal 
cavities of the body, in the respiratory chamber and the 
alimentary canal. Gradually increasing modification then 
occurred, as the parasites found their way farther and farther 
into the internal organs. The Entoniscidae are not the most 
extreme cases of retrogressive development among the para- 
sitic Crustacea ; there are species in which not only the legs, 
antennae, eyes, and segments of the body, but the whole head, 
and even the stomach, intestines, and mouth disappear ; food 
being taken in through peculiar root-like tubes, which absorb 
the juices of the host in such a manner as to supply ready made 
nourishment which needs no digestion. But the Entoniscidae 
afford sufficient proof of the extraordinary effect of the disuse 
of certain parts in transforming the whole organic structure of 
a species. 

Since we find that disuse of an organ is always followed by 
its gradual disappearance in the course of many generations, 
the supposition naturally arises that this decline is the direct 
consequence of disuse, and that the inactivity of an organ is the 


immediate cause of its degeneration, a view which has hitherto 
actually been held, and which at first seems credible enough 
and even plausible. 

It is, of course, a well-known fact, although perhaps the 
subject has hardly been sufficiently studied, that parts which 
are much used grow larger and more powerful, while those 
which are seldom exercised become small and weak. Constant 
gymnastic exercise will immensely increase the size and strength 
of the muscles of our arms ; while these limbs will lose what 
strength they once possessed if the muscles are never exerted. 
The performances of athletes afford us the best examples of 
the extent to which practice can increase the muscular 
strength and activity of man ; and, on the other hand, those 
who work at occupations entailing a sedentary life and lack of 
exercise plainly show the weakening effects of disuse. Experi- 
ments prove this still more clearly : when the nerve supplying 
a muscle is cut, degeneration of the muscle ensues, because its 
activity is at an end, and the same thing happens with glands, 
when their functions are disturbed by severing the nerves 
which supply them. We may accept the general proposition 
that an organ may be strengthened by exercise, and weakened 
by a long continued state of inactivity. It is not necessary here 
to go into the question of how this is brought about, nor has 
it been as yet completely explained : it is sufficient for our 
present purpose to know that such is the case. 

Since we may take it for granted that disuse of an organ will 
lead to its degeneration, even in the life -time of a single in- 
dividual, may we not also conclude that the gradual disappear- 
ance of a superfluous structure in the course of generations is 
due simply to the tendency to degeneration being handed down 
from one generation to another, and thus gradually intensified 
to the extent of complete elimination? For supposing disuse 
to produce infinitely small effects during the life of each in- 
dividual, yet surely these effects would be cumulative, and in 
course of generations the organ would gradually diminish in 
importance, become smaller and weaker, and ultimately dis- 
appear altogether. 

This explanation, obvious as it may seem to be, cannot be 
the right one, for there are many facts which are quite incom- 
patible with it. 


In the first place, it compels us to assume as a fact what has 
often been asserted, but never yet proved, viz. the hereditary 
transmission of acquired characters. 

It is well known that many mental and physical qualities of 
parents are transmitted to their children, such as the colour of 
the eyes and hair, the shape and size of the finger-nails ; and 
not only these but, as everyone knows, even such minute and 
indefinable physical and mental characteristics as likeness of 
features, bearing, gait, handwriting, a mild and equable or 
passionate and irritable temperament. But all these characters 
are blastogenic, or inherent in the parents ; whether they first 
show themselves early or late, they have existed in the parents 
in a more or less marked degree and in different combinations, 
from the beginning. Characters only acquired by the operation 
of external circumstances acting during the life of the indi- 
vidual, cannot be transmitted. The loss of a finger is not 
inherited ; all the thousand faculties which are gained by the 
exercise of various organs or of the whole body are purely 
personal acquirements, and are not handed down to posterity. 
No case was ever known of a child being able to read without 
being taught, even though the parents had exercised their 
faculties in this direction all their lives. Children do not even 
learn to speak untaught, although not only their parents, but 
countless generations of ancestors, have exercised and per- 
fected the brain and vocal organs by learning and speaking 
a language. It may now be considered as satisfactorily 
established that children of civilized nations, if brought up in 
a wilderness and cut off from all communication with man, 
would make no attempt at speech. For proof of this I need 
not fall back on the not very well authenticated story of the 
Persian monarch, who is said to have made the cruel experi- 
ment of taking twenty new-born children and bringing them 
up together, without ever allowing them to hear a word of 
human speech ; they are supposed never to have made any 
sound resembling speech, but to have imitated with great 
fidelity the bleating of a goat which lived among them. The 
same thing is told in all the well-known cases of young or 
adult persons found living in an utterly wild state in the woods, 
cases which have occurred from time to time up to the last 
century in Germany, France, England and Russia. Nearly all 


these persons are said to have uttered sounds resembHng the 
cries of wild animals with which they had associated, but not 
one was ever known to speak. When we consider the constant 
and unremitting practice in speech which we gain in a life-time, 
whether by speaking aloud or merely by thinking to ourselves, 
and remember that in spite of the effect of this perpetual 
exercise for centuries upon the human brain and vocal organs, 
— the power of speech has not become in the slightest degree 
fixed or intensified by heredity, I think that we are justified 
by this one fact alone in altogether doubting whether acquired 
characters can ever be transmitted in any real sense. Moreover 
their transmission is quite incompatible with the only theory 
of heredity which seems to me to be tenable. 

But if the results of the exercise of an organ are not inherited, 
neither can the effects of disuse be handed down. Hence, if 
this be true, the retrograde changes taking place during the 
lives of individuals cannot possibly be intensified in the course 
of generations ; for the process of retrogression would have to 
begin afresh in each generation successively, and thus would 
never advance any farther than it did in the individuals of the 
first. We must, then, regard this supposition that degenera- 
tion is caused by mere disuse as a mistaken one, and seek a 
more satisfactory explanation of the facts. I think, moreover, 
that such an explanation is to be found in what may be called 
reversed natural selection. 

To state my meaning more clearly, Charles Darwin and 
Alfred Russel Wallace have taught us to understand by 
' natural selection ' that process of elimination effected by 
nature itself without the aid of man. Inasmuch as far more 
individuals are born than can possibly live, only the best are 
enabled to survive, the best being those which are so formed 
as to be the 'fittest,' as we say, for the conditions of life in 
which they are placed. As in each generation only the fittest 
survive and propagate the species, their qualities only are 
transmitted, while the less useful qualities of the weaker 
individuals die out. Each successive generation will therefore 
consist of individuals better organized than those of the pre- 
ceding one, and thus useful characters will be gradually 
intensified from generation to generation, until the greatest 
possible degree of perfection is reached. Probably this theory 


is far from new to many of my readers : it has been so often 
explained in various well-known works and periodicals, that 
any further elucidation is unnecessary. 

What holds good for the individual as a whole also holds 
good for each separate organ, inasmuch as the ability of an 
animal to perform its allotted functions depends on the 
efficiency of each particular organ : hence, by means of this 
perpetual ehmination of the unfit, every organ is brought to the 
greatest perfection. On this hypothesis, and on this only, is it 
possible to explain the wonderful adaptability of the minutest 
details of structure in animals and plants, and the development 
of the organic world through the operation of natural forces. 

If this view be the true one, if adaptation in all the parts of 
living forms be truly the result of natural selection, then the 
same process which produced these adaptations will tend to 
preserve them, and they will disappear directly natural selection 
ceases to act. These considerations show why organs which 
have become superfluous and have fallen into disuse necessarily 
degenerate and ultimately disappear. 

As an example of this, let us take one of the newts, which 
are so common in our swamps and pools in spring. If we 
examine its eyes we find that they are not very large, but very 
highly developed : their structure bears considerable likeness 
to that of the human eye, and they play a very important part 
in the hfe of the animal, which is almost entirety dependent on 
keenness of vision for finding its prey. It detects at once and 
snaps at anything in motion : were it not for its eyes, it would 
infallibly starve. Now. these eyes are extremely delicate and 
complex organs, which have only very gradually, — i. e. in the 
course of countless generations and of almost endless time, — 
reached the degree of perfection attained by them in the living 
newt. The whole series of developmental stages is not indeed 
known to us ; but in other groups of animals we find eyes at 
every grade of development, and from these we can form some 
idea of the way in which the gradual improvement of an 
original simple and imperfect eye took place. The slow but 
steady progress in development from stage to stage is due, 
as I believe, to the fact that the eyes of these animals were 
never all exactly alike, nor all equally keen, and that only those 
individuals survived in each generation in which the develop- 


ment of the eyes was above the average. This process of 
natural selection would not only gradually produce improve- 
ment in the eye, but would also tend to keep the improvement, 
when gained, up to a certain standard. 

Now suppose such a species to have been carried under- 
ground by water into a dark cavern. It would only graduallj'- 
adapt itself to the new conditions and thus be enabled to thrive 
in the cave : but after the lapse of generations the individuals 
would have learnt to live in complete darkness, and to 
distinguish and catch their prey without the aid of sight, and 
this would be rendered possible by an improvement in other 
organs, especially those of touch and smell. Thus in course of 
time a race of newts would be produced perfectly adapted for 
life in the dark, and for finding food by scent alone and not b}^ 
sight ; and this race would make its way farther and farther 
underground, and pass its whole life in utter darkness. It is in 
some such way as this that not only the entrances of caverns, 
but whole series of caves, connected by subterranean streams, 
rivers and lakes, like those in the Karst Mountains, near Trieste, 
have come to be tenanted by animals. 

Directly, however, such cave-dwellers became able to exist 
without using their eyes, degeneration of these organs would 
set in : as soon as they ceased to be essential to the life of the 
animal, natural selection would be powerless to affect them, for 
it would be immaterial whether the eyes of any animal were 
above or below the standard. Hence the individuals with 
weaker sight would no longer be eliminated, but would have an 
equal chance of surviving and propagating their species. 
Crossing would then take place between individuals with strong 
and weak eyes, and the result would be a gradual deterioration 
of the organ. Possibly the process might be accelerated by the 
circumstance that small and degenerate eyes would be rather 
an advantage, because their decrease would involve an increase 
in the powers of other and now more important organs, such 
as those of touch and smell. But even independently of this, 
the eye, directly it ceases to be kept up to a certain standard of 
development by natural selection, will gradually deteriorate, 
the process being very slow at first, but absolutely sure. 

The same simple explanation suffices for all cases of retro- 
gressive development, whether of organs or species. On any 

VOL. II. c 


other theory many facts are incapable of explanation, even 
assuming the possibility of the hereditary transmission of 
acquired characters, such as those produced by disuse. 

It is clear that degeneration as a result of disuse can only 
take place in an organ the activity of w^hich depends upon its 
exercise, so that a real effect is produced b}^ the discharge of 
function. The act of seeing involves certain chemical changes 
in the retina of the eye, and perhaps even in the optic nerve, 
processes which do not take place when the eye is no longer 
exposed to light. Flying involves metabolism in the muscles 
which move the wings, and this also ceases when flight is at an 
end. So that an actual retrogressive influence is exerted on 
certain parts of the eye and on the muscles, by disuse. But 
how can the stamens of a plant be affected by the failure or 
success of their pollen in finding its w^ay to the stigma of 
another flower ? Yet we know that hermaphrodite flowers 
sometimes revert to the original condition in which the sexes 
were separate, and this by the gradual atrophy of the stamens 
in one flower and the style in another. Whether this particular 
case is to be explained by the cessation or by the active 
operation of natural selection, is another question, which we 
may proceed to consider. 

After the anthers, in the course of evolution, have withered 
away and disappeared, their stalks (the filaments) remain, 
and are often of considerable height and thickness. Slowly 
and very gradually these degenerate also : we find them quite 
long in some species, in others short, while in others again 
they have completely disappeared, only reappearing now and 
then in single instances to remind us that they were once of 
normal occurrence. It is true that the filaments are no longer 
useful, but how can this fact have any direct effect in causing 
them to degenerate ? Their structure remains the same, the sap 
circulates in them as before and supplies nourishment to them 
as well as to the petals and the style. From my point of view 
the matter is intelligible enough ; for the bare filaments which 
have lost their anthers are in no way essential to the life of the 
species : natural selection is powerless to affect them and they 
gradually degenerate. 

Even more striking instances are to be found in the animal 
kingdom. Why have most of our domestic animals lost their 


original colouring ? Clearly because colour became of little or 
no importance to them as soon as they were sheltered under 
the protection of man, while in a wild state it was a great 
safeguard against detection by their enemies. 

Similarly the hairy covering has ceased to be of importance 
to certain of the Mammalia — and has disappeared. Thus whales 
and dolphins have a naked skin for the most part entirely 
devoid of hair, although they are unquestionably descended 
from hairy ancestors, and even now rudimentary hairs may be 
detected in certain parts of the body by the aid of the micro- 
scope. Obviously, the disappearance of the hairy covering 
cannot be a direct consequence of disuse, for hair will grow as 
well, whether its protective warmth be useful or of no import- 
ance to the animal. But its disappearance as an indirect 
consequence of disuse is plain ; for as soon as an immense 
thickness of blubber was developed beneath the skin of the 
whale, the warmth of an additional covering was unnecessary : 
the hair becoming superfluous, natural selection ceased to 
affect it, and degeneration at once set in. If anyone is inclined 
to doubt whether the direct action of sea-water may not have 
caused the disappearance of the hair, it is only necessary to 
point to the group of seals, in which all the smaller species 
possess a thick coat of fur, while, among the larger kinds, the 
walrus has but a scanty covering of bristles, because, like the 
whale, it has developed a layer of blubber, which is amply 
sufficient to protect its huge body from cold. 

Examples of an entirely different kind are afforded by those 
animals which hide themselves in cases or houses. The 
hermit-crab partly conceals itself in empty shells, the aquatic 
larvae of caddis-flies [Phryganidae) build cases within which 
their cylindrical bodies are enclosed, and the larvae of certain 
small moths [Psychidae) do the same. Whenever the body of any 
such animal is thus partially enclosed in a case, the protected 
parts are soft and whitish, i. e. more or less colourless, while 
the exposed parts retain the ordinary hard integument of the 
Arthropoda and are variously and strongly coloured. Now wc 
may maintain that, in a certain sense, the hard integument of 
crabs and insects fulfils the ' function ' of protecting the soft 
parts of the animal from injury, but, correctly speaking, this 
defence is not a real function at all, because the exercise of 

c 2 


function implies activity, while the use of the hard integument 
can only be of a passive kind. The horny covering itself is not 
in the least affected, whether it is useful or useless as a defence 
against stings and bites : such assaults are quite immaterial to 
it, nor does its condition in any way depend upon the frequency 
or rarity of attack. Degeneration cannot, then, be the result of 
the protection afforded to the integument. Inasmuch as the 
integument of all the three kinds of animals mentioned above 
only degenerates in those parts which are protected by the 
case, clearly the only explanation must be that the hard cover- 
ing is unnecessary for those parts which are otherwise 
protected, and that consequently natural selection has no 
power to preserve it. 

But the most striking instances are to be found among the 
social insects, especially the ants. The male and female ants 
are winged, and at certain times of the year rise into the air in 
great swarms. Everyone must have seen these swarms filling 
the air in summer and autumn : they may often be seen on the 
top of a hill, or surrounding the summit of some tower, alighting 
on walls and parapets or covering the hats and clothes of people. 
The males and females, however, form the minority in an ant- 
community, the greater number being workers — the common 
wingless ants. Now these workers, in the course of the develop- 
ment of the species, have forfeited their wings as a consequence 
of disuse, because the power of flight would be useless to them, 
and they would be exposed to even greater dangers in the air 
than on the ground. The business of their lives is to forage for 
food-supplies, and to collect building materials for the nest, but 
everything which they seek is obtainable on the ground : they 
have also to feed the larvae and tend the pupae, and to them 
alone belongs the defence of the nest if attacked. All these 
tasks bind them to a life on the ground ; hence, when in former 
days, they were being gradually developed from perfect females, 
they came to use their wings less and less, as they gave them- 
selves up more and more complete^ to the duties allotted to 
them. Now, in this case also, it would at first sight seem prob- 
able that the long continued disuse produced a certain amount 
of degeneration in each individual, that this first retrograde 
changewas inherited by the succeeding generation, and graduall}^ 
intensified by further disuse, and so on. Such a view is, how- 


ever, entirely disposed of by a fact which admits of no dispute 
and cannot be explained away, viz. the fact tJiat the worker's of 
ants are infertile^ and do not propagate their species. Consequently, 
it is impossible that the degeneration caused by disuse during 
individual lives should be handed down, and the elimination of 
the wings is only explicable on the other theory, which ascribes 
it to the cessation of the operation of natural selection which 
ensued when the wings became useless and of no importance. 
It may certainly be objected that the disappearance of the wings 
might have taken place before the workers became infertile; 
but such a supposition cannot be accepted, for reasons which I 
need not enter upon here. The infertility of workers may also 
be regarded as a difficulty from my point of view, but it must 
be remembered that the principle of the ehmination of the un- 
fittest does not act directly on the workers, but on their parents, 
the propagators of the species. In other words, natural selec- 
tion does not affect the workers themselves, but the parents, 
and determines their survival according as they produce perfect 
or imperfect workers. 

The process by which the degeneration of superfluous organs 
takes place may fittingly be called 'universal crossing' (Pan- 
mixia), because it implies that not those individuals only in 
which any particular organ is best developed survive and 
propagate their species, but that survival is quite independent 
of the efficiency or non-efficiency of the organ. This process 
of Panmixia must have had, and must have still great influence 
on the development of the organic world. The changes 
wrought by evolution have been and are innumerable, and 
they by no means always occur in an upward direction, but 
often — as shown in the case of the parasites — in a downward 
one, or perhaps most frequently in both directions at once, the 
change being retrogressive in one part and progressive in 
another. And very often the former change may actually lead 
to the latter. We ourselves could hardly have attained so high 
a degree of intellectual development, had we not forfeited a 
considerable share of the physical advantages possessed by our 
remote ancestors. The savage tribes which depend upon the 
chase, are gifted with a much keener sense of hearing, smell, 
and sight than we are, and this is not merely the result of 
constant training, but is also due to the inheritance of more 


efficient organs. In this respect civilization has caused de- 
generation in us, by means of Panmixia, owing to the fact that 
the well-being of individuals no longer depends upon the 
highest possible development of their sense-organs. At the 
present day we are able to make a living equally well, whether 
our sense of hearing or smell is delicate or the reverse, and 
even keenness of sight is no longer of decisive importance to 
us in the struggle for existence. Ever since the invention of 
spectacles, short-sighted persons— in the higher classes at any 
rate— experience hardly any greater difficulty in getting a 
living, than that endured by people with keen sight. In former 
times a short-sighted soldier or general would have been a 
sheer impossibility, and so would a short-sighted hunter : in all 
grades of society short sight used to be a very real disadvantage 
and an almost complete bar to advancement of an}'- kind. This 
is no longer the case now ; a short-sighted man makes his way 
in life as successfully as any other, and his defect, if congenital, 
will be transmitted to his children, and will therefore tend to 
make hereditary short sight commoner among certain classes. 
Of course short sight may also be an acquired character, and 
in such cases it is, I venture to affirm, not transmitted. But I 
believe that the great prevalence of short sight is not only due 
to the injuries acquired by over-straining the eyes and con- 
tinualty looking at near objects, but also to Panmixia, or cessa- 
tion of the action of natural selection, — a law to which we are 
naturally subject in common with other animals. 

Much might be said of the effects of civilization in causing 
physical degeneration, which indeed appears to be on the 
increase. Consider for a moment the teeth : the art of dentistry 
has been brought to such a pitch of perfection, that artificial 
teeth are now almost to be preferred to natural ones. At any 
rate no one need die now from insufficient nourishment in 
consequence of the inability to masticate food, and there is 
nothing to prevent the transmission of a predisposition to bad 
teeth to any number of descendants. 

Nevertheless we need not fear that civilization will ever lead 
to utter degeneration in man. The antidote is to be found in 
the very process which causes the first deterioration of an 
organ ; for obviously such deterioration can only continue as 
long as it is not injurious to the individual in the struggle for 


existence, and when that point is reached natural selection will 
interfere to prevent further degeneration. To return to our 
former example, it is quite conceivable that the percentage of 
persons with hereditary short sight may steadily increase, 
without seriously affecting the general standard of vision of 
mankind as a whole, or even that of a single nation or class, 
because degeneration below a certain point would become a 
fact of decisive importance to the individual, leading to failure 
in the struggle for existence. Thus we need not fear the com- 
plete loss of our eyes through degeneration, like that which 
has affected the animals living in the dark and the above- 
mentioned parasites ; and we need not anticipate any serious 
diminution of our muscular strength, or powers of endurance, 
or many other qualities. 

Hitherto I have only treated of the degeneration of physical 
characters in consequence of disuse and Panmixia, but the same 
thing takes place with mental qualities, a fact which need not 
surprise us when we remember how close is the connection 
between all mental and physical processes, how the relative 
size and complexity of the brain is a measure of the degree of 
intelligence, and how every instinctive action of an animal pre- 
supposes a corresponding arrangement of the nervous system 
which compels a certain action to follow upon a certain stimulus. 
Hence degeneration of an instinct in an animal must always 
have been preceded by degeneration of that network of nerve- 
cells and nerve-fibres in the brain in which the instinctive 
action had its rise. Retrogression, then, in physical structure 
is not antagonistic to retrogression in instinct and mental 
faculty, but mental and physical degeneration rather go hand 
in hand. Very definite and extensive physical degeneration 
always implies a corresponding mental deterioration. Those 
Entoniscidae which have lost their eyes, antennae, legs, and 
jaws, have also degenerated in intelligence, as is but natural in 
animals which only require to remain still and imbibe nourish- 
ment : the whole nervous system of these Crustacea has been 
reduced to a remarkable degree. 

Certain examples are most interesting as tending to prove 
that retrogression may be confined to one particular instinct, 
leaving the animal and its powers as a whole quite unaffected. 
The loss by domestic animals of the instinct to escape is one of 


these examples. Almost all wild animals, mammals as well as 
birds, possess the instinct to escape : they are not only 
extremely attentive to the slightest sound and smell, and to 
every movement taking place within their field of vision, but all 
of them, the predaceous animals not excepted, are continually 
mindful of their safety, and though not always consciously on 
the watch, are so to a great extent instinctively. A wild bird 
flies away at the least sound ; a hedgehog which has been 
surprised, and has rolled itself up, only unrolls itself to run away 
after the lapse of a considerable time, while the slightest suspi- 
cious sound will make it roll up even more tightly. These acts 
are not the result of reflection, but are purely instinctive, the 
act of rolling-up being always associated with the perception 
of sound, so that the former follows instantaneously upon the 
latter, before the animal has had time to reflect on its meaning, 
just as we shut our eyes the instant that anything touches 
them. In the higher animals these movements are certainly 
under conscious control, i. e. they are capable of suppression, 
and hence it is that animals in a state of captivity lose the 
instinct of being startled and of escaping. This instinct is 
nevertheless deeply implanted in them, and many generations 
must be passed in domestication before the natural timidity is 
lost. I believe that the loss is brought about by cessation of 
the action of natural selection, and a consequent gradual degene- 
ration of the instinct. Of course it is difficult to judge of the 
amount of influence exercised by custom upon the life of the 
individual, but it may at least be considered as certain that the 
young of our domestic fowls, geese, and ducks, have lost much 
of the instinct to escape possessed by their wild ancestors, and 
that they would never become quite wild again even if placed 
under the care of a wild mother from the first. 

The length of time which may be necessary before domes- 
tication can get the better of this passive kind of wildness, as 
the instinct to escape may be called, is seen in the case of the 
guinea-pig. These animals have been domesticated ever since 
the discovery of South America about 400 years ago,^ — a period 
of time which has not sufficed to overcome their natural timidity. 
Any loud noise will make them start violently and seek to 
escape, although they may never in their lives have had any 
experience of real danger : even shortly after birth the same 


thing will happen. In these, as with the various species of 
pheasants which have been domesticated, the young animals 
are the wildest : the instinct to escape has been inherited 
almost unaltered, and the process of taming must begin afresh 
with each individual. The tameness of the adult animal is here 
still an acquired character, i. e. one acquired during the lifetime 
of the individual, and is not inherent, or rather, it is not the re- 
sult of those changes in the potentialities of the germ which 
are gradually produced by universal crossing. The tameness 
comes about just as in wild animals taken young, such as 
foxes, wolves, rats, or finches, all of which are tameable up to 
a certain point, and become accustomed to the absence of 

It is also interesting to note that loss of the instinct which 
impels animals to seek their food may sometimes occur. Both 
food itself and the power of obtaining it are essential to life, and 
the instinct of seeking food may be looked upon as the first and 
earliest developed of any : yet it may be partially or even 
entirely lost. The young of many birds no longer possess the 
instinct ; they open their bills and cry, and they swallow food 
placed in their mouths, but they have no idea of picking it up 
if scattered on the floor of their cage ; the sight of food does 
not result in any impulse to eat. At this early period of life 
such birds have not learned the art of feeding themselves, and 
this is not unnatural ; for they leave the egg in a very unde- 
veloped condition, and their parents feed them by putting food 
into their mouths. A part of the food-seeking instinct has thus 
become superfluous and has disappeared. It may be objected 
that the little creatures are too undeveloped to feed themselves ; 
this is true, and it is the reason why the parents feed them and 
why their instinct is undeveloped. But many other birds, 
fowls, for instance, run about directly they are out of the egg 
and pick up food for themselves ; here the food-seeking in- 
stinct is unimpaired. 

One of the most remarkable cases of degeneration of the 
food-seeking instinct is found in certain ants. It has been 
known ever since the beginning of this century that some 
species of ants keep slaves, for instance, the reddish ant found 
in the meadows of Switzerland and Alsace {Polyergiis ritfescens). 
It is not a large but a strong species, which has adopted the 


habit of sallying forth in troops from time to time, to make 
raids upon and plunder the nests of some weaker species, 
such as the common Formica fusca. The object is, however, 
not to destroy or devour the ants they attack, but merely to 
carry off the pupae to their own nest, where they receive 
every care : the workers hatched from them are then employed 
as servants, or, to use the usual term, as slaves. These slaves, 
fulfil all the duties of the nest, which would otherwise have 
fallen to the share of the red workers ; they feed the larvae, 
build galleries and chambers, bring in food-supplies, and even 
feed their lazy masters ! This is no fable, as was once thought, 
but an ascertained fact, proved to be such early in this century 
by Huber of Geneva, a celebrated observer of ants, and since 
fully confirmed by his pupil and successor Auguste Forel, 
as well as by Sir John Lubbock. I have also convinced myself 
of the truth of the assertion. 

The most curious part of it, however, is that, in consequence 
of being constantly fed b}' their slaves, the red ants have 
entirely forgotten how to procure food for themselves. If they 
are shut up and supplied with honey, which is their favourite 
food, they will not touch it, but will suffer hunger, become 
weak and feeble, and ultimately die of starvation, unless pit}^ 
is taken upon them and they are given one of their dusky 
slaves. Directly this is done, the slave falls to work, eats a 
quantity of the honey, and then proceeds to feed its masters, 
which are perfectly willing to be saved from starvation in this 

Here, then, as in the case of nestlings, the food-seeking 
instinct and the power of distinguishing food by sight have 
degenerated, and clearly in consequence of disuse. Inasmuch 
as a colony of red ants always owns plenty of slaves, the 
food-seeking instinct has become unnecessary, natural selection 
has ceased to affect it, and it has gradually died out. Other 
instincts too have been lost by these red ants in consequence 
of their habit of keeping slaves; they have quite forgotten 
the art of nest-building and in part that of tending their young. 
Other species of ants devote much attention to their pupae, 
moving them about the nest from time to time, and often 
carrying them out into the air and sun, and they feed their 
larvae with the greatest assiduity. But the red slave-making 


ants have no such instincts ; they care nothing for their own 
young, and the species would become extinct, if they were 
suddenly deprived of their slaves. So it is not only among men, 
that there is a curse upon slavery ; even animals become 
degraded by it. 

Other species of slave-making ants are known, and have 
been carefully studied, in which the degeneration of the masters 
goes even farther and affects their physical strength. But so 
much remains unexplained in the life-history of these species, 
that I will not treat of them here, remarkable as are the obser- 
vations which have been made about them. All these examples 
afford further support to our theory of retrogressive develop- 
ment as a result of disuse ; for the above-mentioned cases of 
the degeneration of instinct took place in worker-ants, i. e. in 
animals which have not the power of propagating their species. 
Hence the disappearance of the instincts in question cannot 
be due to the hereditary transmission of any degeneration 
acquired by individuals in consequence of the fact that they 
were not required to seek their own living. 

In the cases above quoted, the instinct of feeding has not 
entirely degenerated, but only a part of it has been lost, viz. the 
instinct of seeking food and the power of recognizing it by sight. 
Evidence is, however, forthcoming to show that the whole 
instinct of feeding is sometimes lost, so that actually no hunger 
is felt and no nourishment taken. This may sound very 
strange, but it is an undoubted fact that there are animals 
which absorb as much nourishment in the larval stage as 
will last them during the rest of their life. Many moths, 
especially among the Bomb3^ces, possess very degenerate 
mouth-organs, and so do the Ephemeridae : all these take no 
sort of food. In male Rotifers the alimentary canal is entirely 
wanting ; they have neither mouth, stomach, nor intestine ; their 
lives are of such short duration that the food material with which 
they begin life is sufficient to sustain them throughout it. 
There is no luxury in nature ; no instinct and no organ in the 
body can persist unless absolutely essential to the life of the 
species. Panmixia — in other words, the cessation of the opera- 
tion of natural selection — removes all that is superfluous, only 
leaving that which is absolutely necessary. 

But, of course, if our theory, be the right one, such retro- 


gressive development can only take place very gradually: it 
must require many generations to completely eliminate what 
is superfluous, and we should expect to find in many animals 
vestiges of organs and structures once significant, but now on 
the road to complete obliteration. And this is actually the case, 
as I have shown above. So-called ' rudimentary ' organs are 
present in numberless cases, and in various animals, and give 
us some idea of the vast amount of change which every species 
must have undergone in the course of ages. Of such a kind are 
degenerate ej^es, hidden beneath the skin, as in the Proteus, 
the golden mole, and the Caecilia ; the rudimentary wings of the 
Kiwi, and of many female moths the males of which have well- 
developed wings ; the almost invisible projections near the 
mouth of the Ephemendae, which are nothing less than degene- 
rate jaws ; and a thousand other examples. To the same causes 
are due the numerous cases in which an organ, fully developed 
in the ancestors, is wanting in the adult descendant, although 
present in a rudimentary condition during youth or embryonic 
life. Thus, the workers of ants are, as before mentioned, wing- 
less, but the vestiges of wings are still to be seen in the larvae, 
in the form of small disc-like objects beneath the skin, which 
subsequently disappear. Thus, too, the larvae of bees have 
lost their legs, because they do not need to crawl about, but live 
enclosed in a waxen cell in close proximity to their food : 
although disuse has thus brought them to the condition of foot- 
less grubs, in the egg they nevertheless still exhibit vestiges of 
the legs which their saw-fly-like ancestors must have possessed. 
Examples like these show that retrogression in an organ, which 
degenerates from disuse, takes place first in the mature stage, and 
does not extend to the embryonic stages until much later. An 
organ may persist in the embryo for thousands of generations 
after it has been ehminated from the adult organization. The 
history of evolution affords many wefl-authenticated instances 
of organs which persist in a rudimentary condition and never 
attain a higher development. They are, of course, of the 
greatest importance as throwing light upon the past history of 
a species, and are in themselves sufficient proof of the number 
and diversity of the ancestors of existing species ; they show 
us how intricate and devious are the workings of nature in the 
evolution of the organic world— now progressive, now retro- 


gressive, now concerned with the development of a single 
structure, and now of a whole organism. Everything that 
nature has built up with such elaborate care— highly-developed 
organs of locomotion, limbs fitted to support a certain weight, 
joints with their complex and yet easy movements, the exquisite 
balance of muscular strength required for rapid motion on the 
ground, wings adapted for flying, with all the marvellously 
adjusted organs which overcome gravity and render rising into 
the air a possibilit}^, every one of the adaptations by which 
animals are placed in communication with the outer world 
which surrounds them, — eyes of the most delicate and complex 
structure, organs of hearing and smell so wonderfully formed 
that it has needed long years of the combined researches of all 
the most eminent naturalists to understand their full signi- 
ficance—each one of these is relinquished, is handed over to a 
process of gradual destruction, the moment it ceases to be 
essential to the life of the species. 

It would indeed seem as if such a process of development 
could not justly be called progress, and as far as the individual 
organ undergoing degeneration is concerned the process is of 
course retrogressive ; but the case becomes different when we 
regard the organism as a whole. For the end and purpose of 
all living beings is after all but the existence of each individual : 
the form assumed, the complexity of structure, the degree of 
perfection, are all quite immaterial provided that the species be 
fit to survive : less than fit it cannot be, or it succumbs, neither 
can it be more so, because no means exist which can enable 
it to rise be3rond the point of fitness necessary for survival. 
Schopenhauer's pessimistic view that the world was as bad as 
it could be, and that, if it could grow in the least degree worse, 
it would be annihilated altogether, might be reversed and con- 
verted into an optimistic one : for it would be equally true to 
say that the world is as excellent as it is possible to make it 
WMth the given materials, and that a nearer approach to absolute 
perfection is inconceivable. The organic world teaches us 
that such is the case ; each existing species shows the purpose 
of its being in every detail of its structure, and in its perfect 
adaptation to the conditions under which it lives. But it is 
only adapted so far as is actually necessary, only so far as to 
make it fittest to survive, and not a step further. The eye of 


the frog is but an imperfect organ of vision as compared with 
the eye of the falcon, or that of man, but it is perfect enough to 
enable it to see the crawling fly or the writhing worm : it 
suffices for the needs of the species. Even the eye of the 
falcon is not absolutely perfect as an organ of vision from a 
purely optical point of view, but it serves to enable the bird to 
distinguish its prey with certainty from a great height : such a 
pitch of perfection is all that is essential for the life of the 
species, and all possibility of higher development of the eye, by 
means of natural selection, is therefore precluded. The object 
of all evolution, viz. the survival of the fittest, is not, however, 
always and only attained by the ever-improving, progressive 
development of the organism as a whole, or of particular 
organs : new possessions are not invariably added to the old, 
but the latter are often rendered superfluous in the course of 
time and taken away. Nor does this happen in an ideally 
perfect way, suddenly, as if by magic, but slowly, in accordance 
with existing laws, so that the process remains uncompleted 
through long ages. But ultimately the organ which is no 
longer essential to life is done away with altogether, and the 
balance between the structure of the body and its functions 
is restored, so that, in this sense also, retrogression may in 
truth be said to be a part of progress. 


Thoughts tipon the Musical Sense in 
Animals and Man. 


From the 'Deutsche Rundschau,' October, 1889. 



Modern biology depends, as everyone knows, upon the 
hj^pothesis of a gradual transformation of all forms of life — 
the hypothesis of the origin of species by the slow process 
of evolution, not by a sudden act of creation. Furthermore, 
most people are aware that biological science holds the chief 
agent of this transformation to be the principle of natural 
selection, discovered by Charles Darwin and Alfred Russell 
Wallace. Out of the vast number of offspring born mto the 
world in each generation, only a very small fraction can survive 
long enough to become the parents of the succeeding genera- 
tion ; the rest perish from the attacks of enemies, from the 
inclemency of weather, from hunger or thirst,— in short, they 
succumb in the struggle for existence. No two individuals are 
exactly alike, but every one differs in certain respects from all 
the others : such differences sometimes increase, sometimes 
diminish the power to succeed in the struggle for life. Those 
individuals which possess an increased power of resistance 
will, as a rule, survive and produce offspring, whether their 
advantage be due to greater muscular strength, keener senses, 
thicker fur, greater speed or power of flight, SiC. This selective 
process being repeated in each generation, so that only those 
individuals which possess qualities the most helpful in the 
struggle for life, are enabled to become the parents of off- 
spring, it follows that such qualities will gradually spread over 
all the individuals which make up the species and will grow 
until they have attained the highest perfection. 



In this way is explained the evolution of every useful quality 
and the adaptation which is so manifest in all living beings. 

It is, however, very probable that the animal world is also 
subject to a selective process of another kind, — the sexual 
selection of Charles Darwin. I will devote a few words to 
this principle, inasmuch as our main subject is immediately 
connected with it. 

We are all familiar with the song of the grasshopper and 
cricket. If one walks in the meadows along a little brook on 
a fine June evening, he will often hear a long-sustained note, 
even, subdued, and pleasant, which vibrates powerfully without 
swelling or diminishing, somewhat like that of the nightingale 
in Haydn's 'Toy Symphony.' A cautious approach will 
enable us to see a mole-cricket sitting, apparently motion- 
less, in front of its hole in the ground. More careful examina- 
tion proves that the short wing-covers are in a state of continual 
vibration, producing friction as they move ; and this it is which 
causes the sound. The microscope shows that minute and 
delicate teeth are placed at regular intervals along a vein on 
one of the wing-covers ; when these are struck at a certain 
rate by a vein on the other wing, they emit a whirring note of 
a definite pitch. One vein acts as the bow, the other as the 
string of a violin ; the mole-cricket is a viohnist, and can there- 
fore hold on its note as long as it will. 

It is evident that the power of producing a song can be of no 
value to these animals in the struggle for existence. It neither 
helps them to find food, nor defends them from their enemies ; 
it is therefore impossible that it can have arisen by the opera- 
tion of natural selection. Furthermore, when we enquire into 
its mode of origin we must take into account the fact that only 
the males possess the gift of song. This is also true of all 
other singing insects, such for instance as grasslioppers. The 
ancient Greeks were aware of this, for Xenarchus, in one of his 
comedies, sa3^s, ' Are not the cicadas happy, whose wives have 
not got an atom of voice ^' 

Here then we find the solution of the problem ; the origin of 
the sound-producing apparatus receives a simple explanation 
in the contest between the males for the possession of the 
females. If we take it for granted that the females are pleased 

^ Et"T' doXv ol TeTTiy€s ovK evdaiixovCi '^Clv rals yvuaifiv ov5' otlovv tpo^vrjs evi ; 


by the song — and this may be accepted as proved,— we can 
understand the development of an at first imperfect musical 
apparatus out of the primitive veins of the wing, and its 
gradual improvement up to its present condition. The females 
must, at all times, have preferred the males that sang the best : 
this being the case, according to the law of heredity, the best 
developed apparatus was, in each generation, transmitted to 
the males of the next, so that a gradual improvement in the 
power of performance must have taken place. The continued 
preference for the best singers necessarily led to improvement 
in song and in the sound-producing organ, until the latter 
became incapable of further improvement. 

Let us now briefly consider the song of birds. Here, too, 
the power of song is possessed by the males alone, and its 
origin cannot be explained by natural selection, inasmuch as it 
does not help in the preservation of the species, but is rather 
disadvantageous, for it betrays the presence of the little 
creatures to their enemies at a distance. But it can be well 
explained by sexual selection. The males that sang the best 
being always preferred by the females, we can understand 
how out of the primitive chirp a kind of song arose in the 
course of generations, and how, in certain species, it became 
more and more complex, until at length it developed into songs 
which delight even man by their beauty, such as those of the 
linnet, the blackbird, and the nightingale. Hence sexual 
selection affords a sufficient explanation of the origin of song 
in birds and insects. 

But how can man have acquired the power of making and 
understanding music, and how can we conceive of the agents 
by which such a faculty has been developed ? 

Can these agents be found in the processes of natural and of 
sexual selection ? Undoubtedly man is as completely subser- 
vient to the influence of natural selection as any other animal or 
plant. Man, like every other organism, is variable, is bound 
by the laws of heredity, and wages a constant struggle for 
existence. Therefore, with him as with them the qualities 
which aid m that struggle will be retained and improved, 
while those which are disadvantageous will be lost. And this 
is natural selection. 

It is impossible to doubt that the intelligence of the human 

D 2 


species has been largely increased since the days of primitive 
man. Intelligence is man's chief weapon, — a weapon which 
must have been as important for his existence as physical 
qualities, and this too even in the most primitive times. Think, 
for instance, of a race that depends solely upon the products 
of the chase. In such a case, not only are keen senses and 
bodily strength and endurance essential for the existence of 
the individual, but he also needs intelligence, cunning, and 
astuteness in hunting game; boldness and the gift of working 
in combination in conquering enemies ; wise foresight in pre- 
venting starvation during unfavourable seasons. Any improve- 
ment in these qualities must have given the possessor a greater 
chance of survival and of leaving offspring. Hence these 
beneficial attributes would be slowly intensified in the course 
of generations : the average degree of intelligence would con- 
tinue to increase so long as the difference between life and 
death, between failure and success in begetting offspring, was 
determined by its means. 

There can be no reason why this gradual increase in the 
human intellect should not be going on at the present day : it 
would at least be difficult to bring forward conclusive argu- 
ments against such an opinion. It must be granted that, 
even under the conditions imposed by modern civilization, the 
highly intelligent man, in any calUng, has, ceteris paribus, more 
chance of founding a family than one with less intelligence. 
If this be true, although only when large numbers are con- 
sidered, it must also follow that the average of very many 
cases would show that the mental power of man is increasing, 
although very gradually. It is quite true that we fail to detect 
any historical evidence of this progress, when, for instance, we 
compare the Greek and Latin poets and philosophers with 
those of our own day. But this fact does not conflict with the 
argument, for the leading nations of the present day are not 
descended from the ancient Greeks. The development of 
mankind does not proceed along a straight road, but a very 
interrupted one. The intellectual achievements of the ancient 
Greeks did not pass into their descendants, but into the 
Romano-germanic nations, and these only received the intel- 
lectual achievement, and not the intellectual power. It is also 
to be noted that an increase in the intelligence of mankind may 


not only take place by a rise in the greatest heights attained 
by human intellect, but also by a rise in the general average. 

We will now leave this aspect of our subject : my object was 
merely to show that the human intellect must have been im- 
proved during many thousands of generations by the process 
of selection, and this can hardly be doubted. 

A very different answer must be given if we ask whether it 
is possible to conceive of a similar origin for every kind ot 
talent and faculty possessed by civilized man, if we enquire 
whether the musical, artistic, poetic, and mathematical talents 
can have originated in a similar process of selection, it is 
clear that they did not arise in this way. Such talents may, 
now and then, have been useful or even of decisive importance 
in the struggle for existence, but as a rule they are not so. 
And no one will be prepared to assert that musical or poetic 
gifts mean an unusually good chance of founding a family, 
although this is perhaps more nearly true to-day than it was in 
the times of Schiller, Haydn, and Mozart, or still earlier. But 
even to-day the man with a practical turn of mind stands a 
greater chance of material success than one whose talents are 
of a more visionary kind. Talents for music, art, poetry, and 
mathematics do not contribute towards the preservation of the 
human species, and therefore they cannot have arisen by the 
operation of natural selection. 

Perhaps, however, the development of the musical sense in 
man depends on sexual selection, as we have seen that it does 
in insects and birds. Darwin held this view; he supposed 
that the primitive song of man originated in courtship. I am 
doubtful whether this opinion can be sustained, but the point 
will be referred to further on. If, however, the theory be 
accepted, if we admit that sexual selection played a decisive 
part in the first development of human song, even then we 
have gained very little as an explanation of the origin of our 
own music, because sexual selection is insufficient to explain 
the immense growth which must have taken place in the 
musical sense since the earliest times, if we admit its existence 
in primitive man. 

We might perhaps be inclined to maintain that such a 
growth of the musical sense has actually occurred, when, 
without referring to primitive man, we simply compare the 


music of the savage with the highest achievements of our own 

When Europeans first visited the islands of the Pacific, all 
the natives were found to practise some sort of music. The 
song of the New Zealanders made a profound impression upon 
Cook, and Chamisso found the song of the Hawaians and 
Tc.hit ans extremely pleasant, although often accompanied by 
an orchestra of noisy instruments, such as drums, hollow tubes 
which were struck violently against the ground, and wooden 
sticks which were knocked together. 

The ' music ' was confined within the limits of a very few 
notes, lying between E and G (or, in the case of Tahiti, between 
C and F), although, at the same time, not only semitones but 
quarter-tones (or ' semi-semitones ') were employed. 

The song was pure, and when a hundred sang together, the 
sound was like that of a single voice. In spite of the limited 
compass of their scale, they had a rather large repertory of 
different melodies and themes, which however were always 
characterized by monotony and unceasing repetition : some of 
these were used as the accompaniment of work, others for 
rowing, dancing, marching to battle, and mourning the dead. 

We must however remember that the Polynesians are not in 
a very low state of civilization. Their poetry is by itself suffi- 
cient to prove this, for it is full of feeling and abounds in 
beautiful similes. Hence we can scarcely look upon their 
music as primitive if this expression implies the lowest form 
of musical art. 

And yet, what an enormous difference, when we compare 
this with one of the great musical works of our own time, such 
as Bach's Passion music in all its depth and magnificence, 
Mozart's G-minor Symphony, or one of the nine 'Revelations' 
(so to name them) of Beethoven. One would almost hesitate 
to apply the term ' music ' to the primitive successions of notes 
made use of by ' savages,' so monstrous does the difference 
between the two entities appear. Yet our own music must 
have developed itself from similar beginnings, — there is no 
other way. And, in fact, we find similar elements in both ; 
notes of definite pitch, separated by definite intervals and held 
for diverse lengths of time, that is to say, distinguished by 
differences of rhythm. So that, in this manner, we arrive 


at the musical theme, the melody, the groundwork of all 

Even in its savage form music becomes, to a certain extent, 
the expression of emotion. The funeral dirge is very different 
from the war-song or the festal song. Of course such melodies 
are very far from attaining the marvellous precision with which 
the highest music can not only excite the whole range of human 
feeling, but can also represent every emotion just as a drawing 
represents form. And music can achieve this with such fine 
shades of expression that language is by no means its equal. 

Disregarding for the present those highly gifted minds that 
created such music, and only considering those which enjoy 
it, it is clear that even for the mere understanding, viz. the 
appreciative enjoyment, of one of our great performances, there 
is required a far higher musical sense than is necessary for the 
comprehension of the monotonous song of a negro tribe, or a 
simple Chinese melody, or one of those melodies in octaves 
which played so prominent a part with the ancient Greeks. 
In order to hear in a symphony of Beethoven or in Bach's 
Mass in B-minor anything more than a mere confusion of 
notes, or a roaring, heaving ocean of sound, demands a highly 
developed musical inteUigence. 

Considering these facts, the assumption seems at first almost 
unavoidable that musical talent in man has gradually increased 
from the condition found in the Polynesians up to the level 
reached by the most civiUzed nations ; and if for the moment 
we adopt the Darwinian hypothesis as to the origin of human 
music, it is clear that the amount of increase which has taken 
place during this rise from the condition met with in the living 
savage ought to be sensibly greater than that which took place 
during the development of primitive man into the living savage. 
It is at any rate certain that the amount of increase in the 
musical art itself has been far greater during the second period 
of its development than it can have been during the first. 

Hence we are led back to the question with which we started, 
viz. how and b}^ what means can this increased refinement and 
growth of the musical talent have been produced ? 

Sexual selection cannot possibly afford the required explana- 
tion, even if we admit that it played a part in the origin of the 
primitive song of ancestral man. It is not only true to-day 


but has been true from times immemorial, that the choice of 
husband and of wife are determined by quaUties other than 
musical gifts, viz. by youth, beauty, strength, and not least by 
mental endowments, not to speak of the various external 
inducements which are always apt to intervene. No one will 
be prepared to maintain that men who cannot sing and lack any 
remarkable musical talent, are or ever were at a disadvantage 
in gaining wives. On the contrary, we know that such men 
have no difficulty in finding unmusical partners, and indeed 
that they not uncommonly marry those in whom this taste is 
strongly marked. If this be so any increase of the musical 
talent by means of sexual selection is rendered impossible. 

I feel sure that many will at this point inquire whether it is 
impossible for musical talent to have grown in exact proportion 
to its exercise. We are all familiar with the fact that by constant 
practice every organ is improved and its power increased. We 
cannot doubt this when we think of the marvellous dehcacy of 
touch acquired by the finger-tips of a blind man who attempts 
to make up for the loss of vision by means of the tactile sense. 
Why then should not the musical sense have been increased 
during the course of unnumbered generations in each one of 
which the mind and ear were exercised in the composition of 
music and in its enjoyment? And such exercise appears to 
have actually taken place, for, as far as we are aware, nearly 
all savage nations, not only the Polynesians, but the American 
Indians, negroes, and Asiatic tribes, — possess some sort of 
musical utterance. 

This explanation would certainly be a very simple one, and it 
would be equally useful in many other directions, provided only 
that it were the true one. Up to the present time it has been 
regarded as valid, and many, even now, consider it to be so. 
But the explanation before us involves a supposition vvhich a 
close examination does not allow us to admit,— the supposition 
that those modifications of an organ which are due to its 
exercise during the individual life can be transmitted to oif- 
spring. The supposed increase of the musical sense in the 
course of generations can only have occurred in the manner 
suggested, provided that this supposition be granted. If how- 
ever the results of practice cannot be handed down it is clear 
that the increase of the sense starts in the descendant at the 


very point at which it began in the parent, so that growth in 
the former can only reach as far as it did in the first ancestor, 
and this in spite of practice continued through any number of 

The amount of improvement possible in a life-time is very 
hmited. No athlete can by any amount of practice lift a 
weight of a hundred or even one of twenty hundredweight, 
although he may be able to raise three or four. And, if our 
views on heredity be correct, the son of an athlete will have to 
start at the point at which his father started. For the son, if 
indeed he inherits his father's gifts, inherits only those with 
which his father came into the world and not any increase 
which they may have undergone during his lifetime. Un- 
limited training therefore will only enable the son to lift a 
weight of three or four hundredweight. 

Biological science asserts, with ever increasing clearness, 
that there is absolutely no evidence for the assumption until 
recently so generally received, that acquired characters can be 
transmitted. It was believed that mutilations were occasionally 
inherited, but a searching examination has shown that the 
evidence brought forward will not stand the test of criticism. 
The results of certain recent experiments, in which the tails of 
mice were amputated, showed that the offspring, although 
examined in many hundreds of cases, were invariably normal ^ 

We are therefore compelled to abandon this hypothesis of 
the transmission of acquired characters, at any rate until it has 
been supported in some other way. We lose with this view a 
very convenient principle of explanation, and we must there- 
fore attempt to understand the phenomena without its aid. 

The question before us is : — How is it possible that such an 
increase in the musical sense took place as seems necessary to 
have raised it from the condition met with in the savage up to 
that found among civilized races at the present day ? When we 
examine this question we are led to inquire whether it is correct 
to assume that any increase in musical talent has, as a matter of 
fact, taken place in the course of ages. That such an increase 
has occurred appears to be a matter of course ; for how could 
our highly developed music have arisen unless the musical 
organ had previously become more efficient ? 
> See Vol. I. pp. 444, 445. 


Let us however consider the converse question \—Is it the 
case that highly developed music must appear when high musical 
talent exists? Let us suppose for instance that a child 
endowed with the talent of a Mozart were born among some 
savage nation such as the Samoans before they were influenced 
b}^ European civilization. Would such a child, after reaching 
maturity, compose stringed quartettes and symphonies ? 
Certainly not. If the Samoans possessed the songs which 
they have to-day, our aboriginal Mozart must soon have known 
them all by heart and would have composed new ones. 
Perhaps, being such a unique genius, he might have produced 
a great musical reform, introducing changes of a revolutionary 
character and raising Samoan music to a higher stage. But he 
would not have raised it to the modern symphony. In order 
to attain such a height he would have been obhged first to 
invent the musical notation, and then, rising higher, to pass 
through polyphonic music, until at last he reached the com- 
mencements of that harmonized music to which symphony 
belongs. The greatest change that he could have introduced 
would have been an extension of the scale from three or four 
whole tones to seven, and in association with this, the composi- 
tion of more elaborately constructed melodies, or at the utmost 
the invention of music in two parts, which is known to have 
taken place comparatively recently, viz. in the times of the 

It would have been as impossible for the Samoan Mozart to 
compose symphonies as for one of the great men of science of 
ancient Greece, such as Archimedes, to invent the modern 
dynamo as used for the transmission of energy or for electric 
lighting. To be enabled to construct such a machine, he 
would have had to work his way through more inventions and 
discoveries than could have been made during the life-time of 
the greatest genius who has ever lived. For in ancient times 
nothing was known of electricity except that amber (electron) 
when rubbed attracted little pieces of paper. Before a man 
could arrive at the knowledge by which he could construct a 
fixed electro-magnet in such a manner as to produce currents 
in a rotating coil, many other discoveries in physics had first 
to be made, the investigations of Gray, Dufay, Kleist, Franklin, 
and others were necessary, Galvani and Volta had to discover 


the electric current, Oerstedt electro-magnetism, while it was 
necessary for Seebeck, Ampere, and Faraday to base upon this 
still further discoveries. In like manner most of these dis- 
coveries had to be made before first Soemmering and then 
Gauss and Weber could use the electric current for signalling 
at a distance ; and even then a whole series of practical 
improvements in telegraphy necessarily preceded Hughes' 
printing telegraph. One discovery is ever built upon another ; 
and the history of music is not less a history of inventions than 
that of the electric telegraph. 

It is therefore impossible for even the greatest genius to pass 
directly from simple melody to symphony. 

I should like to suggest the further question whether it is 
quite certain that Mozarts could not have existed in ancient 
times ; in other words, whether the supposed increase in 
musical talent has in reality taken place as a historical fact, 
or w^hether the talent was not inherent in man from the begin- 
ning, while its expression, i. e. music itself, has undergone 
progressive increase and development. 

At first sight the question may appear to be very strange ; 
but I believe that it is perfectly justifiable. Indeed I am of the 
opinion that the suggestion implied in the question is entirely 
valid. I have shown that from the mere fact that symphonies 
are not composed by savages, we are not entitled to conclude 
that Mozarts have not existed among them ; or, to put it still 
more clearly, we are not entitled without further proof to 
infer that savages never possess high musical talents because 
their music is but lowly developed. Such talent might very 
well exist, but could not produce any marked effect, because of 
the low level attained by the musical environment. 

I am satisfied by the proof afforded by numerous facts that 
this is really the case, and that therefore the high musical 
talent which is more or less possessed by civilized man at the 
present time, does not depend upon a gradual increase in the 
musical sense, and that such increase being non-existent does 
not require explanation. No such rise and increase of the 
musical faculty by itself has taken place. The musical sense 
is rather an ancient possession of mankind chiefly depending 
upon the highly developed auditory organ, and this was 
transferred to man from his animal ancestors and has not 


increased at any rate beyond the condition reached by the 
lowest of existing savages. We have definite proofs of the 
occurrence among savages of musical talent capable of the 
same education as our own. We must therefore consider their 
talent to be as high as ours, although it is generally hidden 
because untrained during the life-time of its possessor. 

Negro races are certainly not at a very high stage of civili- 
zation. We see this clearly by their utter carelessness of 
human life, as shown in the dreadful massacres of the King of 
Dahomey and other chiefs, by the state of servitude to which 
women are subjected, and by the lack of real family life. But 
in spite of these proofs of inferiority it has happened on many 
occasions that negroes have attained to the full understanding 
of our highest music. 

Brindis y Salas, a Cuban negro, who travelled as a violinist 
through Europe and America, is a well-known proof of this. 
He was not merely endowed with excellence of ' technique ' 
along with delicacy of ear, but — as I am told by a distinguished 
musician^ — 'he possessed musical abilities of a very high order. 
His playing was that of an artist.' He must therefore have 
had an inborn musical sense, as high in all essentials as that 
of our greatest performers. It is impossible to urge the objec- 
tion that his ancestors had been under European influence for 
centuries, because such a period of time would be far too short 
for the growth of a special part of the brain as the result of 
inherited practice, and also because European music of a high 
order does not reach the negroes of Cuba. 

Another example is afforded by the 'Jubilee Singers,' a 
company of negro men and women, who in 1887 astonished 
Europe by their ' very extraordinary performances in four-part 
singing,' The authority, whose opinion I have already quoted, 
judges from their performances that there is no doubt whatever 
as to ' the talent of the negro nation for our music' 

We also find among European musicians and composers 
many grounds for the belief that musical talent has not been 
increased by practice in the course of civizilation. If this were 
the case, highly gifted musicians would never have arisen 
in families living, remote from the great influences of their 

^ This information was kindly placed at my disposal by Herr Otto 
Lessmann, of Berlin, editor of the ' Allgemeine Musikzeitung.' 


time, in places where the only music consisted of national 
songs accompanied by the guitar or the zither. But, not un- 
commonly, from these very surroundings have come men 
with a highly developed musical sense, and even celebrated 
composers. Martin Luther, who is known to have been a 
composer, was the son of a poor miner. Palestrina was the 
son of a peasant. Jacob Callwitz, a sixteenth century 
composer, was the son of a labourer, and Joseph Fux, who 
composed in the seventeenth century, was the son of a Styrian 
peasant. Cimarosa was the son of a washerwoman near 
Naples : John Gottlieb Naumann, a renowned composer of the 
eighteenth century, was of peasant extraction, as also was 
Joachim Quanz. The first known ancestor of the Bach family 
was born in 1550, in the country near Gotha, and worked all his 
life as a miller in Wechmar, his native place. Joseph Haydn 
was also born in a village, and was the son of a poor wheel- 

In these instances we cannot maintain that all this musical 
genius sprang out of the earth suddenly and without prepara- 
tion. On the contrary, I wish to point, for example, to Haydn, 
whose parents we certainly know to have been musical. They 
sang when they rested from work, and the father accompanied 
on the harp. The above-mentioned founder of the Bachs also 
frequently played on the cythringen, a kind of guitar, which he 
brought home to the mill from his travels. Sebastian Bach says 
that ' this was, as it were, the beginning of the music of his 
descendants.' The highest musical culture of their time was 
entirely without influence on the musical sense of the ancestors 
of these two great musicians ; the talent existed nevertheless, 
and appeared in the descendants, sometimes to an increased 
and sometimes to a diminished extent. 

It is no reaj objection to this argument to urge that only 
a few out of the large number of musicians in recent centuries 
came from the lower orders. A great musician not only needs 
the highest talent, but also stimulus and all the culture that his 
times can bestow. I previously assumed that the invention of 
two-part singing would be the highest achievement possible 
for our supposed Samoan Mozart, and we may safely con- 
clude that Joseph Haydn would never have surpassed his 
father's national songs and harp had he not chanced to become 


a chorister in the Httle town of Hainburg, and had he not after- 
wards entered the music-school in Vienna, of which Reutter, 
the organist of the cathedral, was the head. Haydn possessed 
musical talent of the highest order, but had it not been trained, 
he could never have accomplished by himself the whole deve- 
lopment of modern music from the national song ; he could 
never have risen from the music of his parents to oratorios and 
stringed quartettes. Such cases afford interesting evidence 
that at least a great part of the development of modern music 
can be accomplished in a lifetime, even when all the ancestors 
have been strangers to the higher musical culture, so that it 
was impossible for their musical sense to be raised by it. The 
musical sense is evidently innate in the human brain, and is 
independent of all training and practice undergone by ances- 
tors. The predisposition may be strong or feeble, but even the 
greatest talent does not enable the possessor to climb to the 
height reached by the music of his time without being raised 
by instruction. That so great a height can be reached in a 
life-time by the son of a German peasant, or even by the 
offspring of a savage race, evidently proves that the musical 
sense of to-day has been inherent in man since times imme- 
morial, and that it has not been increased by the development 
of music or by practice. It has nevertheless been brought to a 
higher stage of development in the most civilized races, as we 
shall see further on. 

We have already seen that musical talent exists in every 
stratum of society. And yet the upper classes have produced 
many more eminent musicians than the lower, a fact which we 
can easily understand when we remember that without early 
stimulus, and the constant opportunity of hearing and being 
instructed in the highest music, even the greatest genius must 
remain undeveloped or, as we may say, latent. 

This is proved b}^ many examples : thus out of sixteen 
renowned German musicians of the sixteenth and seventeenth 
centuries, no fewer than eight were the sons of organists : the 
others were the sons of peasants and labourers, but nearly all 
were choristers when boys. Furthermore, twenty-seven of 
the greatest German and Italian composers of the eighteenth 
and nineteenth centuries were the sons of musicians. Ex- 
amples of these are afforded by Mozart, Beethoven, Weber, 


Hummel, Cramer, Abt Vogler, Hasse, Johannes Brahms, 
Robert Volkmann, Czerny, Karl Reinecke, Cherubini, Bellini, 
Rossini, Antonio Lotti, and Scarlatti. In all these cases it is 
clear that a highly-developed musical sense was transmitted 
from father to son, while the talent of the latter was further 
developed than that of the father, because it was trained and 
exercised from earliest youth, although I do not mean to imply 
that it was not also greater from the very beginning. But the 
greater force of the inherited talent does not depend upon the 
weaker talent of the father having been improved by practice 
during his life-time. Many still believe in the hereditary 
transmission of improvement acquired by practice ; but if such 
inheritance could take place so rapidly, in a single generation, 
we should easily find proofs of it in many occupations and 
pursuits — proofs which are as yet entirely wanting. 

I shall, however, be asked : Whence came the increase in 
the talent of Mozart and Beethoven as contrasted with that of 
their fathers ? It is impossible to give any definite answer to 
this question, but I can, perhaps, indicate it by another ques- 
tion : Whence came the high poetic genius of Goethe, whose 
father had no taste for poetry, while his mother without ever 
having written, exhibited, in her whole character, the most 
distinct endowments in this direction ? How could the poetic 
genius of the mother, which had never been exercised, attain 
so high a level in the son ? We must not forget that poetic 
talent is by no means a simple power but a very complex one, 
depending on a happy combination of many intellectual and 
emotional gifts, which in Goethe's case were derived, as he 
himself tells us, partly from the father and partly from the 

'Vom Vater hab' ich die Statur, 

Des Lebens ernstes Fuhren ; 
Vom Mutterchen die Frohnatur, 

Die Lust zum Fabuliren,' &c. 

Similarly, I . should be inclined to explain the genius of 
Mozart as a very complex power made up of the fine ear, the 
strength of will and energy of his father, and the bright and 
cheerful disposition, the gentleness and refinement of feeling 
of his mother. From this constitution may have arisen the 
infinite flexibility of that wonderful mind which, with unwearied 


activity, ever led to fresh combinations of the emotions which 
became the subjects of musical themes. A psychologist might 
be able to show us more of the constitution of this marvellous 
mind. I will not attempt it ; I merely wish to show that the 
increase in the musical faculty, which appears to pass from 
father to son, can be explained, as in so many other cases, 
entirely without the unproved assumption of the inherited 
effects of practice. Even when the musical sense itself is 
transmitted unaltered, viz. without increase, from father to 
son, a considerable increase in the power of composition may 
nevertheless be brought about by the combination of mental 
gifts derived from the mother with the musical sense inherited 
from the father ; and this sense will therefore gain in the son 
a higher expression. There are many highly-gifted people 
who are unable to compose anything original : even remark- 
able musical talent may co-exist with an utter inability to 
produce anything new. Examples of this are perfectly familiar. 
But in the descendant of such person, the strong receptive 
musical talent may be united to such a complete flexibility of 
the mind and temperament, derived from the mother, that new 
combinations of ideas will ever arise. This latter gift will then 
seize upon the musical sense, and ideas which were perhaps 
of an entirely different nature in the mother, will become 
musical ideas in the son. 

The composer not only needs the musical faculty, the gift of 
originality is also indispensable. I believe that an increase in 
the genius for music which passes from father to son depends 
upon a new combination of mental gifts, with which of course 
an increase in the deHcacy of the musical ear itself may be 
united ; for every inherited quality varies, and may be feebler 
or stronger than it was in the parent. 

Let us now return to the argument that some external 
stimulus is necessary for the development of an existing 
musical faculty. Two facts seem to me to favour this opinion ; 
first, that nearly all the renowned composers and singers of the 
present century have come from large towns, and have thus 
been brought up where from earliest youth they have been 
subject to musical influences of all kinds. I have made a list 
of ninety-eight such cases. Secondly, the fact that during the 
nineteenth century the Jewish race first began to take part in 


the development of music. In this century composers of 
Jewish descent first begin to appear, and among them we find 
very great names, such as Meyerbeer, Mendelssohn, Halevy, 
Rubinstein, Moscheles, Felicien David, and others. This fact is 
probably associated with the emancipation of the Jews, which 
afforded them the opportunity of developing the rich musical 
faculty which they possessed by nature. In this we find a 
further proof that it is impossible for the musical sense of 
modern nations to have been raised by practice during earlier 
centuries ; for the Jews were entirely without adequate 
musical training, so long as all the higher music was bound up 
with religious service. The introduction of music into the 
Jewish synagogue is of quite modern date. Throughout the 
eighteen centuries preceding our own, music had played no 
part in Jewish life, and yet this nation possessed the musical 
faculty in a very high degree, and as soon as the Jews 
began to cultivate their talent they were not only able to 
reach the summit of modern musical achievement, but also to 
contribute towards the progress of the art. This is certainly 
clear evidence for the hypothesis that the musical faculty has 
been latent in mankind from times immemorial, at least in 
many races, and that it can be evoked at any time and raised 
to any height. 

But if the mental instrument with which we make— I mean 
invent and enjoy — music, existed at all times, why did not man 
perform symphonies and oratorios in the age of the Pharaohs ? 
The answer is clear — Because music is an invention^ and one 
which could reach its present height only very slowly in the 
course of centuries. And here we meet with the great differ- 
ence between man and animals. Man possesses a tradition ; 
he improves and perfects his performances by passing on the 
gains of each generation to those which follow. The higher 
animals are not entirely devoid of the power of learning from 
preceding generations, but they possess it in a much lower 
degree. A young goldfinch, when brought up by hand, sings 
untaught the song of its kind, but not so perfectly as when it 
has had an accomplished songster for its teacher. It also 
learns by tradition, but the essential basis of the song was 
present in its organization beforehand, and is inherent. The 
bird speaks, even when untaught, the language of its species. 



Sexual selection, as we may suppose, has made this language 
an essential part of its being. 

It is otherwise with man : his language does not exist as a 
perfected faculty, as a part of his physical nature ; but only as a 
possible expression of it which only becomes actual when the 
individual preserves communication with those who preceded 
him, viz. when he is taught to speak. Hence it is that every 
human child can learn any language : hence it is that there is 
not one single human language but hundreds of them, each of 
which has had its own developmental history— its origin, 
climax, and dechne. Each of these different modes of expres- 
sion of the human mind seems, as it were, a distinct mental 
entity, independent of the individual, and possessing its own 
history. And this is not only true of language, but also of the arts 
and sciences. Not one of these could have existed had not man 
possessed that advantage over animals which enables him to 
transmit the knowledge he has gained to his descendants, so 
that these latter are benefited by building, from the very first, 
upon the high level reached by previous generations, from 
which they can rise still higher. 

All this is far from new : it has long been known that the 
chief difference between man and animals consists in the fact 
that man is capable of mental development while animals are 
not. But I doubt whether the exact difference has ever been 
clearly conceived. The statement just made is not a satis- 
factory expression of it ; for common knowledge of the day 
asserts that animals are certainly capable of development 
although in a sense entirely different from that which is 
intended above. We have every reason for the behef that the 
unceasing transformation of species which took place during 
the earlier epochs of the world's history, is also proceeding to- 
day—that to-day, wherever circumstances are favourable, the 
transformation of species is taking place, although slowly and 
insensibly. But such a process of development of one species 
of animal into a new one, even when combined with an 
improvement and increase in efficiency, is entirely different 
from what we mean by the development of mankind. 

The development of animals transforms one species into 
another and changes the physical nature : but what we 
generally understand by the intellectual development of man- 


kind by no means necessarily entails an}^ physical alteration 
even in the brain itself : it is indeed quite independent of any 
such change. Such development represents an increase in the 
intellectual acquirements of mankind as a whole : this is the 
origin of civilization, using the term in its voidest sense and 
applying it to all the numberless directions taken by civilizing 
forces \ Man, availing himself of tradition, is able, in every 
part of the intellectual domain, to seize upon the acquirements 
of his ancestors at the point where they left them, and to 
pursue them further, finally himself leaving the results of his 
own experience and the knowledge acquired during his life- 
time to his descendants, that they may carry on the same 
process. This method of progress is most clearly shown in the 
history of science, and especially in that of natural science, 
which deals with an immense number of facts and experiences 
which have been very slowly acquired, collected, and transmitted 
to descendants during many centuries of civilization ; and in 
this way alone could the present state of our knowledge have 
been reached. The human being of to-day can be easily raised, 
by a short period of training, to this stage from which, if he be 
successful, he may perhaps make one or more onward steps. 

This consideration affords especially clear evidence for the 
assertion upon which I have already laid great emphasis — 
that the development of any ifiental faculty is not necessarily con- 
nected with any elevation of the mental capacity of the individual. 
Hardly any greater power of observation or more acuteness is 
required to observe the development of an Infusorian under 
the microscope, than was needed in Aristotle's time to make 
out the anatomy of a Cuttlefish, with the naked eye and simple 

^ Very similar ideas have been recently expressed by D. G. Ritchie 
in his ' Darwinism and Politics ' (London : 1891). Thus on pp. 100, loi 
he writes as follows. ' Language renders possible the transmission of 
experience irrespective of transmission by heredity. By means of 
language and of social institutions we inherit the acquired experience, 
not of our ancestors only, but of other races in the same sense of "in- 
heritance " in which we talk of people inheriting land or furniture or 
railway shares. Language renders possible an accumulation of experi- 
ence, a storing-up of achievements, which makes advance rapid and 
secure among human beings in a way impossible among the lower 
animals. Indeed, might we not define civilisation in general as the sum 
of those contrivances which enable human beings to advance indepen ■ 
dently of heredity ? '— E. B. P. 

£ 2 


dissecting instruments. The fact that we can now solve more 
difficult problems than at the beginning of this century, or 
in Aristotle's day, does not depend upon any increase in 
the capacity of the human brain or any improvement in the 
delicacy of the faculty of observation ; but it depends upon the 
heritage which we have received from our ancestors, viz. 
higher problems left for our solution together with better 
means and appliances for their investigation. // is as impossible to 
explain the development of music by an increase and perfecting of 
the musical talent, as to explain the superiority of our pianists over 
those of MozarV s time by a recent improvement in the dexterity of 
the human hand. The very hands which, in Bach's day, could 
only give a bald and imperfect performance on the spinet, 
would now, upon a Steinway's or Bechstein's grand piano, 
produce all the enchanting effect of an orchestra. The causes 
of this immense change are manifold. First, a gradual im- 
provement in the instrument, — itself a result of tradition which 
permitted an advance upon the acquirements of earlier 
generations ; secondly, parallel with this advance, the develop- 
ment of appropriate music ; lastly, the immense improvement 
in pianoforte technique which we associate with the names of 
Haydn, Mozart, Clementi, Hummel, Moscheles, Thalberg, and 
Liszt. No one would dream of suggesting that this advance in 
'technique' is due to an improvement, as regards piano- 
playing, in the powers of the human hand, produced by the 
practice of several consecutive generations. Such an origin is 
indeed impossible, because, happily, every one does not play 
the piano, because everj^ pianist is not a performer of eminence, 
and because the children of such performers rarely become 
performers themselves. Liszt's father was a clerk in an 
accountant's office. Among all our living performers I only 
know one, Pauer of London, whose son is a pianist. It is clear 
that in this case also the possibility of higher performance does 
not depend on higher talent, but upon the tradition of improved 
technique which enables the young artist to strive, from the 
very first, after a higher ideal. 

It is the same, I believe, with music itself— nay with all the 
arts. That emotional instrument wherewith we make music, 
whether developed within us or received from without, 
has been innate in man, and has undergone hardly any 


improvement from times immemorial. But in these days we 
know how to employ it more fully because we have trained it 
to higher achievement from the very beginning of life. The 
musical talent, like every other, is capable of vast improvement 
by life-long training. I well remember hearing for the first 
time, as a boy of thirteen, a great performance— the Pastoral 
Symphony of Beethoven. How clear and distinct is the 
meaning of such a composition now that we are accustomed to 
hear far more intricately written orchestral works ! I was even 
then impressed by the mighty ocean of music, and listened 
with the greatest interest ; but I was unable to disentangle the 
theme from the maze of notes and to understand its ideas. 
It was only by practice of my mental sense, through frequently 
listening to this symphony, that my power of musical percep- 
tion acquired the capacity of picking out, and distinguishing, 
particular passages more and more clearly from the totality of 
the composition, and placing those passages into their due 
relation to the swell of the waves of music which surged along 
beside them. 

Although the average musical faculty has not undergone any 
increase, in the course of ages, it must at one time have 
originated ; and the question arises whether we can explain 
this from a scientific standpoint. How can we conceive the 
existence of a musical sense ? 

Attempts in this direction have been repeatedly made, not 
only since the doctrine of evolution has become prevalent, but 
also during past centuries. The able psychologist C. Stumpf 
has recently directed attention to the fact that the question of 
the origin of music greatly occupied men's minds, especially 
in France, during the middle of the last century. Jean Jacques 
Rousseau had already formed the opinion that music originated 
in language, in excited speech, a view that was simultaneously 
brought forward in Germany by Scheibe. This hypothesis 
must have been forgotten later on, or Herbert Spencer would 
never have enunciated and supported it without reference to 
his predecessors. It has met with little acceptance, and has 
been refuted in detail ; it may now be looked upon as an 
abandoned position. This can hardly be said of the hypothesis 
brought forward by Darwin, who held the antagonistic view^ 
that song is older than language, and arose by sexual selection. 


Important objections have however been raised against this 
hypothesis by many writers, and especially by Stumpf. And 
yet I would freely admit that at present it is difficult, nay 
impossible, to decide whether sexual selection has or has not 
had any part in the origin of human song. But even if it has 
played this part, it by no means follows that there was a 
similar origin for the musical sense also : this faculty might 
have been present beforehand. 

It would lead me too far if I were to attempt any detailed 
exposition of the reasons which, as I think, oppose the 
hypothesis of the origin of the musical sense by sexual selec- 
tion. They partly depend upon the above-mentioned fact that 
any increase in this facult}^ has not taken place since the stage 
reached by man in a savage state. Other objections depend 
upon certain considerations of which I will now speak. The 
explanation of the musical sense is to be looked for in an 
entirely different direction ; I do not believe that it originated 
as something independent and as it were intended for the duty 
it performs, but that it is simply a bye-product or accessory of 
the auditory organ. This organ was a necessity in the struggle 
for existence and has therefore been developed by selective 
processes, and raised to the highest pitch of perfection. The 
musical sense is, I believe, a merely incidental production and 
thus in a certain sense, an unintended one. 

No one can believe that the human hand was created for 
playing on the piano,— that it became what it now is in order 
that man might be able to make use of this instrument. It is, 
as we know, fitted for grasping and for the power of delicate 
touch ; and as these are very useful qualities, of high importance 
in the struggle for life, we feel no difficult3^ in explaining the 
gradual perfecting, by processes of selection, of that form of 
hand w^hich the higher animals had already gained. By means 
of selection, the hand became the perfectly articulated, sensitive, 
and mobile structure that we find, not only in ourselves, but in 
the very lowest savages. But we can do many things with our 
fingers which were never intended, if I ma}^ use the expression ; 
we can, for instance, play on the piano, now that this instrument 
has been invented. And furthermore a native African could, if 
trained as a child and under certain conditions, learn all the 
technique of the modern piano as thoroughly as a European. 


I believe it to he much the same with the musical sense and the 
artistic faculty in general. This faculty is, as it were, the mental 
hand with which we play on our emotional nature, — a hand not 
shaped for this purpose, not due to the necessity for the enjoy- 
ment of music, but owing its origin to entirely different require- 

I will give more detailed evidence in support of this view. 
Our musical organization consists of two parts : — first, the 
auditory organ proper, viz. the outer, middle, and inner ear, 
by which the various sounds become nervous stimuli, each 
producing its corresponding nerve-impulse : secondly, that 
part of the brain which transforms the impulses conveyed to 
it by the auditory nerve into sensations of sound ; this is the 
auditory centre of our brain. 

The first part of this twofold organ, the auditory organ proper, 
is, so far as we know, not much higher in organization than 
that of many animals, and it does not possess any peculiarity of 
construction which would justify us in the assumption that the 
power oi hearing music is greater than in animals. The higher 
animals can certainly hear music : the behaviour of my cat is 
sufficient evidence for this, for she comes near whenever the 
piano is plaj^ed and sits quietly near the performer, sometimes 
jumping up into his lap or even upon the keyboard of the 
instrument. I know of a dog, kept by a family in Berlin, which 
always approached when music was played, often coming from 
distant rooms and opening the doors with his paw. I hear, on 
good authority, of a dog which generally stayed at home, but 
wandered about every now and then in order to indulge his 
love of music. This dog could never be kept at home during 
the fair which is held twice a year at Frankfort-on-the-Main. 
As soon as the street bands appeared and began to play the 
dog ran off and followed them through the streets of Frankfort 
from morning till night. This habit was well known by his 
owners who were accustomed to keep dinner for him in the 
evening at the time of the fair. 

It is sufficiently clear that neither cats nor dogs nor any of the 
other animals which hear the music of man were formed with 
a view to the perception of such sounds. I mean that the 
auditory organ which they possess, arising under the guidance 
of natural selection, cannot have assumed its present form in 


order that these animals might perceive music, for such an 
experience confers absolutely no advantage in the struggle for 
existence. Besides, the animals and their auditory organs are 
far older than man and his music. The faculty of hearing 
music possessed hy these animals must be an incidental ac- 
cessory power possessed by an auditory apparatus which 
assumed its present form under the operation of other causes. 

Now I believe that it is the same with man. Man, too, did 
not acquire his power of hearing music as something by itself, 
but he received, by processes of selection, a very delicate and 
highly elaborate auditory organ ; for this organ has been 
necessary in the struggle. And furthermore, it so happens 
that this organ can also be used for hearing music. By the 
assertion that the auditory organ of man was produced by 
natural selection, I do not mean to imply that it was not 
already formed in the pre-human period. We have never 
found the direct ancestors of man, and even if we were fortunate 
enough to meet with their remains it would be impossible to 
make out the minute microscopic structure of the soft tissues 
which, during life, covered the osseous parts of the auditory ap- 
paratus deeply buried in one of the bones of the skull. But it is 
most probable that our direct ancestors possessed an auditory 
organ nearly similar to that which we possess to-day ; for in 
the living caricatures of men, the apes, it reaches almost the 
same degree of perfection. It must be admitted that there are 
no researches into the minute details of the ape's ear hke those 
of Hasse and Retzius on the auditory organ of certain other 
Mammalia. Hence we cannot decide whether the length of the 
scale which can be heard by an ape is as great as that heard by 
a man ; but we may assume that it is nearly the same. 

The power of appreciating the interval between musical notes 
depends, as we know, upon a wonderfully complex apparatus 
placed in the so-called cochlea. This structure called after its 
discoverer, Corti's Organ, consists of thousands of cells which 
form the terminations of auditory nerve-fibres : each cell can 
only be made to vibrate by a single note of a certain pitch. 
This is brought about by the fact that each cell rests upon part 
of an elastic membrane of microscopic delicacy which passes 
across the cavity of the cochlea, just as upon a stretched string 
which only vibrates with a particular note. If Helmholtz's in- 


terpretation of the apparatus be correct, we can judge of the 
delicacy of any auditory apparatus by the number of such cells. 
The greater the number of cells the more delicate will be the 
hearing of the animal and the wider will be its range. The exact 
measurement and enumeration of Retzius have shown us that 
the human cochlea contains 15,500 such cells, that of the cat 
12,500, that of the rabbit 7,800. Hence man has a more perfect 
sense of hearing than either of these two animals, but we can- 
not determine with certainty whether he can better appreciate 
minute differences, or whether he can hear more notes : pro- 
bably he is superior in both these respects. There are also 
individual differences in the number of cells in the human 
species, although perhaps only within narrow limits. Such 
differences explain why some individuals do not hear so well, 
or cannot distinguish so many deep or high notes, as others. 
I myself possess a rather fine ear, but I can never hear the 
high notes of certain species of grasshoppers, even when 
hundreds of them chirp together, although others can hear 
them easily. 

If then the apparatus by which music is heard in the cat and 
the rabbit be essentially the same as that of man, only differing 
in degree, the following question is naturally suggested : — 
Knowing that nothing can arise unless it be useful, how has it been 
possible for this apparatus to originate ? The power of hearing 
music must have been utterly useless to those animals which 
do not make music, and hence the origin of their auditory 
apparatus must have proceeded from other necessities. What 
can these necessities be ? 

Why has it been useful to Mammalia in the struggle for 
existence to hear with distinctness all the large number of 
notes for which their auditory apparatus is fitted, and which 
renders the hearing of music a possibility ? This question has 
probably never been asked before, and I must admit that the 
answer is by no means easy ; at any rate if a complete and 
detailed explanation be expected. But I believe that it is easy 
to understand in a general way how the ear of these animals 
could have been elaborated and raised to so high a pitch by 
natural selection. Wild animals stand in need of a very fine 
ear. Beasts of prey, such as cats, must in the first place be 
able to hear and distinguish between all the sounds made by 


their prey. But this means that they must hear a scale of 
considerable length ; that, for instance, of the cat must pass 
through all the interval between the cooing of the wood-pigeon 
the call of the cuckoo, and the notes produced by the blackbird, 
the chaffinch, the linnet, the siskin, the thrush, and the pheasant. 
But the wild animal must also be able to hear the sounds made 
by its enemies and distinguish them from others. And not only 
is this the case with the animal sought after by many enemies, 
such as the rabbit, but the enemy itself must also be upon its 
guard against other enemies which endanger its life and that of 
its young. It must distinguish the howl of the hungry wolf 
from the bark of the fox or dog, the deep note of the eagle owl 
from the cry of the eagle and vulture. We need not here take 
man into account, because his existence only began long after 
the development of the auditory organ in these animals, and 
because his influence upon them has been annihilating rather 
than transforming. 

It was therefore necessary for the auditory organs of these 
animals to have a very extensive range, stretching from rather 
low notes on the one side to very high ones on the other. It was 
essential that the organ should be adapted for a continuous 
scale without breaks ; for otherwise the position of the various 
notes could not have been accurately estimated. Indeed we 
feel a sense of admiration and wonder when we see the exceed- 
ingly high development of the cochlea adapted for hearing a 
continuous scale in the mammalian ear, and we can only under- 
stand it when we realize how completely the yery existence of 
wild animals depends on the utmost delicacy of their organs of 
special sense. It is absolutely essential for them to know with 
certainty whether any particular sound proceeds from an enemy 
or from their prey. While a single mistake might be fatal to 
them, one often repeated would be inevitably punished with 
death. If they mistook the sound made by an enemy for that 
of their prey they would of course go to certain destruction, but 
the opposite mistake would also be fatal ; for the food of a 
beast of prey is nearly always scarce, and if many opportunities 
were missed the animal would die of starvation. It is not in 
vain that the fox roves about by night and day searching for 
food, listening for the faintest sound, and ever ready to rush 
upon its prey or to fly; it is not in vain that the hare is so 


timid ; it needs to be extremely sensitive to every sound if it is 
to continue to exist as a species. Hence w^e can perhaps to 
some extent understand why the rabbit has 7800 cells in its 
auditory organ, although this implies the most astonishing 
delicacy of ear. We must not how^ever assume that each of 
these cells is set to a different note, but rather that the four 
cells of each transverse row are fitted to receive the same vibra- 
tion. There remains, however, a surprisingly large number 
of different note-sensations, i. e. nearly 2000. We can realize 
how very delicate hearing must be, which can appreciate only 
1000 different notes, when we remember that a concert grand 
piano contains only 87 different notes. If we reckon that the 
auditory organ can appreciate a somewhat longer scale, namely 
that of a hundred notes situated at the distance of semitones, it 
follows that the interval between two consecutive semitones 
would contain nearly 19 intermediate sounds. The human 
ear, when very highly trained, can distinguish nearly 30 inter- 
mediate notes between A and B-flat, a rather larger number 
than the difference between the numbers of their respective 
vibrations in a second, — (A = 440, B-flat = 467-5). 

If then the mammalian auditory organ must attain so high 
a pitch of perfection lest it should be inadequate in the struggle 
for life, it is clear that the part of the brain by which notes are 
perceived, the auditory centre, must possess a corresponding 
degree of organization. We may indeed assume it to be 
certain that a corresponding degree of development is found 
in those layers of nerve-cells and nerve-fibres in the auditory 
centre, the so-called ' field of memory,' which serve as the 
material basis of the memory of auditory perceptions. Aristotle 
was quite correct in maintaining that ' animals devoid of memory 
would be unable to perceive even the difference between two 
successive notes \' But an elaborate auditory organ would be 
of little or no value to such animals ; they would be unable to 
discriminate between the sound of an enemy and that of their 
prey, for they could not compare the note they were hearing 
with that previously heard, the latter having wholly faded from 
their consciousness. 

It is much to be regretted that we can know with certainty 
in but few cases how far an animal is capable of perceiving 

^ I quote from C. Stumpf, ' Tonpsychologie,' Bd. i, p. 279. 


music. The capacity seems to be present in a tolerably high 
degree ; for it is known that cavalry horses often recognize 
the signals as well as their riders and begin the appropriate 
movements before being directed. 

The evidence is especially clear in the case of certain birds, 
far below the above mentioned mammals in mental power, 
that music may be heard and properly understood by organisms 
which cannot have acquired their auditory apparatus for this 
purpose. I am here referring to those birds which either have 
no song of their own or a very simple one, but which are 
nevertheless capable of imitating the more beautiful song of 
other birds or even the melodies of human music. 

This is especially remarkable in the case of parrots, which 
can learn to sing short melodies quite correctly. It is therefore 
certain that they possess the apparatus necessary for hearing 
music, although they do not sing unless taught. 

Hence the supposition appears to be well founded that man 
possessed the auditory apparatus necessary for music before 
he made music, and that the apparatus did not, by making 
music, attain the degree of development it has reached. It is 
not necessary to assume that the capacity of hearing music 
was a primitive faculty acquired for its own sake ; it may 
rather be conceived of as a secondary, an 'unintended,' ac- 
cessory, as a mere incident in the evolution of the auditory 
organ which reached its high development b}^ ministering to 
other necessities. 

It might perhaps be objected that neither the minute struc- 
ture of the cochlea nor the power of hearing an extensive scale 
proves that music is perceived as music, or that we do as a 
matter of fact hear the third or fifth which is sounded. It 
might be conceived that the musical sense depends upon yet 
another and unknown peculiarity of the auditory apparatus, a 
peculiarity which has been added to the function of hearing and 
the origin of which therefore demands some special explanation. 
But this objection will not hold, because animals such as the 
horse and parrot, can as a matter of fact hear music, although 
we cannot assume thatthej^possess any special contrivance for it. 
The basis on which this objection rests is nevertheless sound, 
for we can never explain the faculty of hearing music by the 
knowledge of our auditory apparatus alone. But to use this 


undoubted fact as an argument for the conclusion stated above, 
would be like maintaining that the hand was specially created 
in order to play the piano, because we can never explain, by a 
mere examination of its structure, the infinitely rapid move- 
ments made by a performer. It might be argued that inasmuch 
as the hand and fingers were never required to make such swift 
mov ments when man existed in a primitive state, they could 
not have been originally capable of such movements, and that 
therefore the faculty which they now possess must have 
depended upon sexual selection or the results of inherited 

The same might be said with regard to the swift movements 
of the fingers in writing. Such arguments depend upon a 
mistaken application of the principles of utility, a principle which 
certainly excludes the possibility of raising an organ by the 
process of selection above the highest point of actual utility, 
but which by no means prevents it from acquiring new uses as 
the result of life-long practice. 

A more serious objection maybe derived from the considera- 
tion of those who are utterly unmusical. We cannot doubt that 
many such people exist, even if most of them are to be accounted 
for by want of training at the right time. Those who are totally 
devoid of the faculty of music, can apparently hear sounds and 
notes of every kind as fully as musical people, but they are 
unable to discern the intervals, or to perceive and reproduce a 
melody, much less to analyse a harmony. If then their 
auditory organ be normally developed we are apparently con- 
fronted with the proof that musical hearing is different from 
ordinary hearing, and has been superadded to the latter,— that 
therefore it cannot be merely an inevitable accessory, but has 
sprung from a source which demands some special explana- 

This argument appears to be sound, but I do not believe that it 
is so. The assumption that the hearing of unmusical people is 
as highly developed as that of the musical is utterly unproved, 
and I believe that it is most improbable. It is to be regretted 
that there are no sufficiently exact researches into the ordinary 
hearing of unmusical persons, and that we have even less 
knowledge of the minute structure of their auditory apparatus. 
But from what we know of musical hearing it follows that the 


ordinary hearing of such people must be imperfect and their 
auditory apparatus abnormal in structure. 

The meaning of the word 'unmusical' is merely relative. 
Mozart possessed such a wonderful memory for absolute pitch 
that he once remarked, directly he began to play his own violin, 
that it was tuned half of a quarter-tone higher than one he had 
played two days before. But many people, although admitted 
to be very musical, have the feeblest memory, or almost none 
at all, for absolute pitch. They cannot tell whether the per- 
formance they are listening to is in the key of A, C, or F : their 
memory deals with intervals alone, and they are satisfied if 
only the relations of the notes in any piece of music are correct. 
This is certainly often due to want of practice, and it is also 
connected with the important part played by the pianoforte in 
the musical education of mankind. The note A is much more 
firmly fixed in the mind of a violinist and has a far more indi- 
vidual character for him than any particular note of the pianoforte 
scale has for the pianist. But it is equally certain that there are 
also differences of talent as regards thememory for absolute pitch. 
Leaving the greatest heights of musical genius, we find that the 
perception of intervals may also be deficient, and that such 
deficiency increases gradually in different individuals until we 
reach a case like that described by Grant Allen in which the 
notes sounded by two successive keys on the piano seem to be 
absolutely the same. Such defects in hearing can only be 
explained by some imperfection in the structure of the auditory 
organ, in this case in the organ of Corti. Hence such an auditory 
organ would not represent what we may suppose to have been 
the primitive ear of man before he began to be musical ; it is 
merely an example of degeneration. A perfectly normal 
auditory organ must always be musical, and this not only with 
regard to the perception of intervals, but* also to the recognition 
of absolute pitch. For even animals must possess the power of 
distinguishing a note as higher or lower than some other note 
of which the pitch is retained in their memory, and if they were 
incapable of this they would be exposed to countless dangerous 
mistakes. We certainly cannot regard the ear of Mozart as 
the primitive normal ear of mankind ; we must rather regard 
it as an abnormaUty as much above the average as the ear of 
a moderately unmusical person is below it. But even Grant 


Allen's extreme case proves that the perception of absolute 
pitch is retained by civilized man ; for this individual distin- 
guished high and low notes, although he could not perceive any 
difference between the successive notes of the scale when he 
played it. 

Hence the different degrees of imperfection in the musical 
faculty seem to me to be traceable to defects in the structure of 
the auditory organ, to a more or less complete degeneration from 
its original and normal state. Defect and degeneration are, as 
everyone knows, apt to occur in any part of the body, and 
should occasion the least surprise in an organ which, like the 
human ear, no longer plays a decisive part in the preservation 
of the species,— a part which it must certainly have played ages 
ago when man lived under more natural conditions. In such 
times he needed a perfect ear just as wild animals need it now. 
The civilized man of the present day no longer depends on the 
acuteness and perfection of this sense ; it is, to a certain extent, 
of no importance whether he has or has not the full number of 
15,500 cells in his cochlea. But those persons in whom the 
number or perhaps the minute structure of these cells is below 
the average, or in whom the tension of the membranes is 
abnormal, will probably be unable to perceive musical intervals 
correctly or may be unable to perceive them at all ; such persons 
are unmusical. 

I do not mean this statement to imply that defects in Corti's 
organ are the only cause of a deficient musical faculty. In some 
cases perhaps the cause may lie in the auditory centre, viz. the 
part of the brain where the impulses of nerves, produced by the 
stimuli of sound-waves, are transformed into the perceptions 
which we call notes. Certain kinds of deficiency in the faculty 
even suggest that the auditory organ and centre may be quite 
normal, but that there is merely a less perfect and less complex 
interconnection between this and the other brain-centres, so that 
the mental perception of music is not possible although the 
music itself is correctly heard. It is especially interesting to 
compare such cases with the remarkable and extremely variable 
phenomena witnessed in those who, from the lesion of a small 
part of the brain, have lost, either wholly or in part, the 
faculty of perceiving and producing music, such loss being 
frequently associated with defects of speech. In addition to 


Kussmaul's admirably explained observations, Kast, Knoblauch, 
and Oppenheim, among German pathologists, have offered 
interesting contributions to this difficult and complex subject, 
into which of course I cannot enter upon the present occasion. 

For the present purpose I merely wish to show that deficiency 
in the musical faculty must always depend upon defect in the 
anatomical structure of the auditory apparatus, the auditory 
centre, or their means of connection. If this be so, the existence 
of unmusical people constitutes no objection to the view I have 
propounded as to the origin of the musical sense. 

But must we really admit that the musical talent of primitive 
man was the same as our own ? Can it be conceived that, in 
these remote times, there were born men who, educated in 
one of our schools of music, would have produced a Haydn, 
a Mozart, or Beethoven, or even an ordinary musician of 
to-day ? 

I am quite sure that this admission will never be made. For 
it is clear that the understanding of our highest music not only 
needs the auditory apparatus and auditory centre, together with 
the life-long training of these : something besides is absolutely 
indispensable, a mind that is sensitive, impressionable, and highly 

I will enter rather more fully into this point. The frequently 
mentioned auditory centre is not a mere supposition ; it is known 
with tolerable certainty. When a certain part of the temporal 
lobe of the cerebrum is destroyed in a dog or monkey, deafness 
ensues, although the auditory apparatus remains uninjured. 
Such animals do not suffer greatly in health ; they continue to 
live, but remain permanently deaf And all the while the 
sound-waves are still converted into nerve-impulses by the 
auditory apparatus, and the impulses corresponding to the several 
notes are still conveyed to the brain by the fibres of the auditory 
nerve. But in the brain that organ is wanting by which these 
impulses are transformed into sensations and are brought into 
relation with consciousness ; the animal is 'psychically deaf,' as 
the technical expression goes. 

If on the other hand we were able to remove every part of 
the cerebrum except the auditory centre, then the mechanical 
conditions necessary for the production of sound-sensations 
would still remain, but the animal or the man would neverthe- 


less be unable to hear, because nothing capable of becoming 
conscious of sound-sensations would be left in the brain. In 
removing nearly the whole cerebrum the mind would be lost 
together with all its accessory powers, thought, imagination, 
will, and self-consciousness. The ' soul ' would be wanting, and 
hence even the most beautiful of the sound-sensations produced 
in the auditory centre could not be perceived because there 
would be nothing capable of perception. 

I have only mentioned this hypothetical case in order to 
show that the way in which music is perceived depends not 
onl}^ upon the auditory centre, but quite as fully upon the 
organ which lies behind, receives the sound-pictures, and 
allows them to have their full effect upon it. If, as in the case 
supposed above, there be no mind, then not a single sound- 
image can be perceived ; but with a highly developed human 
mind of infinite freedom and flexibility and rich in ideas, the 
' parts ' of a polyphonic composition which run through each 
other, and proceed by contrary movement, can be perceived as 
the most charming musical architecture ; they make up an artistic 
structure of rich form, the several parts of which exhibit the 
most significant relationship, rising from and returning into 
each other, and ever presenting in each of its separate parts 
fresh features and new and interesting combinations. But the 
case is very different with the comparatively lowly organized 
brain of an animal such as a parrot ; for the power of mind is 
insufficient to take in such an elaborate sound-picture, and the 
animal can only perceive a confusion of notes, although 
perhaps a pleasing one. Even after constant practice the 
parrot would be unable to follow the movements of the ' parts ' 
cf the composition, because it lacks the necessary intelligence. 
We know by its whistling that it can hear music, but even in 
this it makes but httle progress, and can only repeat short 
pieces, because it does not understand the connection between 
the parts. There is of course a very marked difference between 
the musical perception of a parrot's brain and that of a man. 
But a comparison between the two is perhaps on this very 
account best qualified to render evident the conclusion with 
which we are here concerned, viz. that one and the same 
auditory organ together with its auditory centre must produce an 
entirely different effect upon the mind according as this is more 



highly or loidy organized. The 'soul' is, as it were, played 
upon like an instrument by the musical nerve-vibrations of the 
auditory centre. The more perfect this instrument is the 
greater is the effect produced. The perception of music by 
the highest animals, such as the dog, cat, or horse, must be very 
imperfect as regards the purely formal relation between 
chords and successions of simple notes, because their mind is 
lowly developed, because their intellect cannot find any 
interest in following the manifold intricacies of the progress of 
' parts.' It is not keen and acute enough even to perceive the 
varying distinctions between one ' timbre ' of sound and 
another, for it has no purely mental interests. Only in the 
most crude and general manner are the souls of animals open 
to the emotional effects of music. Music impresses them as 
agreeable or disagreeable, and attracts them entirely irrespec- 
tive of what we call the ' character ' of a performance. The 
above-mentioned dog which followed the music of the fair was 
probably agreeably affected by every performance of the street 
band, whether it was in a major or minor key, whether it was 
a polka or a funeral march. So far as the dog was concerned 
the finer shades of difference, by which we are affected so 
powerfully, had no existence at all ; it was only impressed by 
the sound, the mere pure matter of music, a thing which is of 
no importance to us as compared with the form of it. That 
which we admire most in music, and which chiefly excites our 
interest, is the originality and richness of musical forms, as 
Hanslick has so admirably shown in his interesting essay on 
' The Beautiful in Music ^' We are able to enjoy a sj^mphony 
in a pianoforte arrangement, or, with sufficient practice, by 
merely reading the notes ; and we appreciate not merely its 
formal relationship, but also its emotional effect and significance. 
By reading it we can be sent into a happy or a m'elancholy 
frame of mind, and we can fancy that we see in the composition 
the representation of moods of mind as distinguished from par- 
ticular ' feelings.' Everyone will admit that, at any rate as re- 
gards this latter effect of music, even the highest animal can 
never have any idea, even though its hearing and its auditory 
centre were practised for the whole of its life ; and this must be 

^ See also ' Sensation and Intuition ' by James Sully, and ' The Power 
of Sound ' by Edmund Gurney. 


so because behind its auditory and musical sense there lies 
no correspondingly developed mind. 

The same thing holds, although not to an equal extent, 
between the varied degrees of development reached by the 
human mind. If primitive man did not possess a mind like 
that of his descendants, if his intellect and every dependent 
power became far keener and deeper as the struggle for life 
went on through the course of ages, it follows that the faculty 
of perceiving music must also have been augmented. 

It is therefore impossible that a lost Beethoven ever existed 
among primitive man, nay, I should even doubt whether one 
could be found among existing Australians or negroes. For 
the production of a Beethoven there is needed not onl}^ a 
highly developed musical sense, but also a rich and great soul, 
one that is infinitely sensitive ; and we know by experience 
that such a nature is only to be found among the very highest 
intellects. But I will go further ; I do not believe that the child 
of primitive man, if he were alive to-day, could be raised by 
education to the same level of musical understanding as that 
reached by our own children. He would fail for want of 
inherent power of mind. 

Of course these opinions can never be confirmed, because 
primitive man is not to be found. But we still have the 
Australian native, although, so far as I am aware, the neces- 
sary investigations have never yet been made. But even if 
they were never carried out, it would nevertheless be certain 
that primitive man must have possessed lower mental faculties 
and especially a humbler intellect than civilized man : this 
conclusion is commonly accepted, and it is sufficient for my 

Hence we may assume that susceptibility to music must 
have increased during the intellectual evolution of mankind, so 
long, in fact, as the essential nature of the human mind was 
capable of being raised. It is impossible to decide upon the 
precise period in the history of a certain nation or group of 
nations at which the climax was reached ; for we are by no 
means sure that the human intellect is not even now under- 
going slow and imperceptible development. But as a mere 
suggestion, without any pretence to exactness, I will state that 
the people of 'antiquity/ viz. the ancient civilized nations of 

F 2 


the Mediterranean, had already, at the very dawn of their 
history, attained the highest level of intellectual development. 
If any further growth has occurred since then in European 
nations, it has certainly been so imperceptibly small that it 
could cause no sensible difference in the susceptibility of the 
human soul to music. The times which produced such legis- 
lators as Moses and Solon, poets like Homer and Sophocles, 
philosophers and men of science like Aristotle, Plato, and 
Archimedes, — times which created the Egyptian temples and 
pyramids and the statues of Greek gods, most undoubtedly 
display the achievements of the human intellect at its best. 
And an age which produced the gentle and forgiving Christian 
philosophy shows us that, as regards character and feeling, the 
human mind had attained the highest development. 

We may therefore safely assume that the nations of ' anti- 
quity ' possessed a capacity for music in all respects equal to 
our own, and that the times during which the human intellect 
was raised, at least to any considerable extent, lie far behind 

The fact however that the music of antiquity was so poor, 
depends, as we have seen, upon the complete distinction 
between music and musical talent : the latter is due, and due 
only, to the nature of the individual body and mind, while the 
former is also due to a slow process of development by means 
of tradition. Music is an invention and rests upon tradition,— 
the power on which depend the entire growth of culture, the 
development of language, of the sciences and their practical 
applications, and of every kind of art. 

Painting and sculpture also have not been developed, viz. 
increased and perfected, because of any growth in the physical 
means by which we practise them. The human eye and the 
corresponding part of the brain, the visual centre, have certainly 
not been improved since the age of the lowest culture, or even 
since the times of primitive man. But the artistic acquirements 
of generations have been built one upon another until there 
arose the great art-palace of the present day with all its varied 
chambers. In this case it is even easier to prove that the 
instrument by which art has been invented existed in all its 
present perfection long before the invention had been made, 
and that it did not originate for the sake of art, but to be used as 


a weapon in the great struggle for life. It is evident that the 
keenest vision is of vast importance for the preservation of the 
human species. Hence the arts of painting and sculpture are, 
in the sense above mentioned, merely the incidental accessory 
performance of a faculty never intended for such a purpose. 

It is quite true that the beginnings of art can be traced far 
back to the times of the cave-dwellers ; but whenever it began 
an immense period was required for its development, — a period 
w^hich must have been especially long in the case of music. 

It is almost impossible to realize that men with such high 
aesthetic instincts as those possessed by the ancient Greeks could 
have enjoyed the unisonal effect of accompaniment in the octave ; 
and we can hardly believe that they were unable to invent 
music in two parts. And yet a long time elapsed before the 
gallant troubadours of Provence hit upon the idea of letting the 
melody be accompanied by another deeper-pitched voice, 
originally moreover in succession of fourths and fifths, so very 
unpleasant to us at the present day, but which may even now 
be occasionally heard in the street music of Brittany. 

It is not my intention to follow the slow and gradual evolution 
of music; for this has been clearly shown by the excellent work 
of other writers. But in concluding I wish to repeat that this 
evolution does not depend upon any increase of the musical 
faculty or any alteration in the inherent physical nature of man, 
but solely upon the power of transmitting the intellectual 
achievements of each generation to those which follow. 

This, more than anything, is the cause of the superiority of 
man over animals— this, and not merely human faculty, although 
it may be admitted that the latter is much higher than in 
animals. And even if we were compelled to believe that human 
faculty has reached its limits and can never be increased again, 
even then we need not despair of the almost boundless progress 
of mankind. For each generation always starts from the 
acquirements of the preceding one ; and the living child placed 
from the very first by tradition upon a somewhat greater height 
of intellectual achievement than that of his predecessors, is 
then able, with the same powers, to climb yet higher up the 
steep slope of the most advanced civilization. Hence, even if 
our intellectual powers have reached the highest possible stage, 
human civilization will nevertheless advance, however far we 


may look forward, — the conquests of the mind of man will never 

Lastly I trust that the scientific man ma}^ be excused if, in 
this essay, he has entered into what may appear to be a very 
distant region. Nevertheless it was a purely scientific question 
which led him into this mquiry — the question of the hereditar}- 
transmission of acquired characters. He attempted to explain, 
without any transmission of the results of practice, the existence 
of those human faculties which cannot be explained by the 
process of selection. This led to the explanation of the origin 
of the musical sense which has been adopted in this essay. 
Perhaps the opinions of a biologist may not be altogether 
devoid of interest for the philosopher and the musical critic. 
The questions treated of lie on the boundary between science 
and philosophy, and can hardly be solved from either province 


Remarks on Certain Problems of 
the Day. 


From the ' Biologisches Centralblatt." Bd. X., Nr. i and a, 
pages r and 33 : March, 1890. 



The following essay was originally intended as an answer 
to the criticisms which Professor Vines ^ brought forward 
against certain of my views, shortly after the pubHcation, 
in an English form, of a collected edition of those essays of 
mine which appeared in Germany during the years 1881-1889 ^ 

This answer has been published in German because similar 
objections have been urged by German writers, and I further 
hope that this essay may perhaps serve to render clearer some 
of the problems with which it deals. Much might have been 
added on the points here referred to, but the occasion, and the 
nature of the essay itself, called for a certain amount of restric- 
tion, and enforced a concentrated treatment of the most impor- 
tant subjects. 

Professor Vines commenced his article by a criticism of that 
attribute of immortality which I have claimed both for unicel- 
lular organisms and for the reproductive cells of multicellular 
beings. If I rightly understand the English professor, he does 
not contest the truth of this view, but he fails to find in my 
book a satisfactory explanation of the process by which the 
immortal organisms gave rise, in the course of their phyletic 
development, to mortal descendants. The first difficulty which 
presents itself is to understand how the mortal heteroplas- 
tides can have been evolved from the immortal monoplastides 
or homoplastides. The explanation of this process, given 
in my book, is the only one which seems applicable to the 

^ ' Nature,' Oct. 24, 1889, p. 621 et seqq. 
^ See Vol. I of the present Edition. 


origin of the more complex forms of organic life, namely, that, 
in accordance with the principle of division of labour, the cell- 
body of the unicellular ancestor divided into two dissimilar 
halves, which differed from each other both in structure and 
function. From a single cell which was capable of performing 
all functions, a group of cells arose and shared the various 
kinds of work between them. According to my theory, the 
primitive division produced two kinds of cells, the mortal cells 
of the body proper (soma) and the immortal germ-cells. Un- 
doubtedly Professor Vines beheves, as I do, in the principle of 
division of labour, and in the role which this principle plays in 
the development of the organic world ; but the division of a 
unicellular being into somatic and reproductive cells appears to 
him impossible, and my explanation of the process as due to 
unequal cell-division does not satisfy him ; he holds that ' it is 
absurd to say that an immortal substance can be converted into 
a mortal substance \' 

At first sight indeed this may appear as a great difficulty ; it 
is in reality, however, caused by a confusion between two dis- 
tinct ideas, namely, immortality and eternity. The immortalit}' 
of unicellular beings and of the reproductive cells of multicel- 
lular organisms is, I believe, a fact which does not admit of dis- 
pute. As soon as it is once made clear that the fission of a 
monoplastid is in no way bound up with the death of either 
half, there can be no further dispute about the unlimited per- 
sistence of the individual. But this is very far from affirming 
that such individuals are endowed with eternal life ; on the 
contrary, we always assume that the organic life on our earth 
once had a beginning. The conception of eternity involves the 
past as well as the future, for eternity is without beginning and 
without end ; but it is obvious that such a conception does not 
concern us here. Eternity is at best but an artificial idea ; in 
reality it is no true idea at all, since we cannot conceive it ; it is 
only the negation of an idea, being in fact the negation of that 
which passes away. When we begin to discuss eternity, we 
see that from the point of view of Natural Science, nothing is 
eternal except the ultimate particles of matter and their forces ; 
for no one of the thousandfold phenomena and combinations 
under which matter and force present themselves to us can 
1 ' Nature,' Oct. 1889, p. 623. 


be eternal. The immortality of unicellular organisms and 
of germ-cells is, as I said years ago, not absolute, but po- 
tential ; for they are not, like the gods of ancient Greece, 
compelled to live for ever. Thus we are told that Ares 
received a wound which would have proved fatal to any 
mortal, but although he roared as loud as ten thousand bulls, 
he could not die. The organisms in question can, and the ma- 
jority of them do die, but a part of each lives on. But is it one 
and the same substance which continues to live ? Does not life, 
here and everywhere else, depend on assimilation, that is on 
a constant change of material ? What then is immortal ? Appa- 
rently not a substance at all, but a certain form of motion. The 
protoplasm of unicellular beings possesses such an arrange- 
ment in its chemical and molecular structure, that the cycle of 
material which makes up life is ever repeating itself, and can 
always begin afresh so long as the external conditions remain 
favourable. In this respect it may be compared to the circu- 
lation of water on the earth. Water evaporates, is condensed 
into cloud, falls to the earth as rain, only once more to evapo- 
rate, and thus the cycle repeats itself. And just as there exists 
no inherent cause in the physical and chemical nature of water, 
which interrupts this circulation, so in the physical nature of 
the protoplasm of unicellular beings there is nothing which 
puts an end to the cycle of existence, —that is fission, growth 
by assimilation, and then fission again. It is this property 
which I have called immortality, and in organic nature it is the 
only real immortality to be met with. It is a purely biological 
conception, and must be distinguished from the immortality of 
non-living, that is of inorganic, matter. 

If then this real immortality is simply a cyclical movement 
conditional on certain physical properties of protoplasm, why 
should it be inconceivable that this property, under certain cir- 
cumstances, should alter to some extent, so that the phases of 
metabolic activity should not exactly repeat themselves, but 
after a certain number of cycles should come to an end, result- 
ing in death t All living matter varies, and why is it incon- 
ceivable that variations of protoplasm should arise which, while 
fulfilhng better certain functions advantageous to the indivi- 
dual, should be associated with a metabolism that does not 
exactly repeat itself a metabolism that sooner or later comes 


to a stand-still ? To my mind the descent of the immortal to the 
condition of mortality, is less to be marvelled at than the fact 
that monoplastids and germ-cells have remained immortal. 
The slightest change in the properties of living matter might 
involve such a descent, and certain essential peculiarities in 
the composition of this substance must be most rigidly main- 
tained, in order that the metabolic cycle may sweep on w^ith 
perfect smoothness, and raise no obstacle against its own per- 
sistence. Even if we know nothing further of these essential 
peculiarities of structure, we may at least maintain that the 
rigorous and unceasing operation of natural selection is neces- 
sary to maintain them. Any deviation from this standard ends 
in death. I believe that I have shown that organs which have 
ceased to be useful become rudimentary, and ultimately dis- 
appear owing to the principle of panmixia alone, — not be- 
cause of the direct effect of disuse, but because natural selec- 
tion no longer maintains them at their former level. What is 
true of organs is also true of their functions ; for function is 
but the expression of certain peculiarities of structure, whether 
we can directly perceive the connection or not. If then the 
immortality of unicellular beings rests on the fact that the struc- 
tural arrangement of their substance is so accurately adjusted 
that the metabolic cycle always comes back to the same point, — 
why should, or rather, how could this property of the proto- 
plasm, which is the cause of immortality, be retained when it 
ceased to be necessary ? And clearly it is no longer of use in 
the somatic cells of heteroplastids. From the moment that 
natural selection relaxed its hold upon this property of the 
protoplasm, the power of panmixia began to be felt, and ulti- 
mately led to its disappearance. Prof Vines will probably ask 
how this process can be conceived. I answer, quite simply. 
Let us suppose that certain individuals appeared among the 
monoplastids with such variation of the chemical or molecular 
characters, that the continuous recurrence of their metabolic 
cycle came to an end, so that natural death became a necessity. 
These individuals could never give rise to a persistent variety. 
But if individuals with a similar variation in their somatic cells 
arose among the heteroplastids, no detriment would be felt by 
the species : the body-cells would indeed die, but the undying 
germ-cells would secure the continuance of the species. By 


means of the distinction between somatic and germ-cells, 
natural selection was enabled to direct its attention, to speak 
metaphorically, to the immortality of the germ-cells, and to 
an entirely different range of properties among the somatic 
cells, such as the capacity for movement, irritability, increased 
powers of assimilation, &c. &c. We do not yet know whether 
an increase in these properties is directly connected with a 
change of constitution involving the loss of immortality, but it is 
not impossible that this maybe the case ; and, if so, the somatic 
cells would have ceased to be immortal more quickly than if 
panmixia were the only agency at work. 

I have adduced in my fourth essay ^ the cases of the Volvo- 
cinean genera, Volvox and Pandorina, as examples of the 
differentiation of the lowest heteroplastids from the homo- 
plastids. All the cells of Pandorina are similar and perform 
similar functions. Volvox, on the other hand, consists of 
somatic and germ-cells, and it is here that we should expect 
the introduction of natural death. Dr. Klein's recent observa- 
tions ^ show that this, as a matter of fact, takes place : as soon 
as the germ-cells are matured, and have left the body of the 
Alga, the flagellate somatic cells begin to shrink, and in one or 
two days are all dead. This is all the more interesting because 
the somatic cells fulfil nutritive functions for the aggregate. 
It is true that they are not alone in performing the office of 
assimilation, for the germ- cells also contain chlorophyll; but 
the immense size which the latter attain in Volvox can only be 
explained on the supposition that they receive nutriment from 
the somatic cells. These cells are so constituted that they 
assimilate, but when once the spherical colony has attained its 
definite size they have ceased to grow. By means of a fine 
protoplasmic network the body-cells pass on to the germ-cells 
ail the nutriment they acquire from the decomposition of 
carbon dioxide and water, and when the reproductive cells are 
mature they die. In this case adaptation for supplying nutri- 
ment to the germ-cells may have hastened the introduction of 
death among the somatic cells, inasmuch as some structure may 

1 See Vol. I, p. 163. 

2 Ludwig Klein, ' Morphologische und Biologische Studien uber die 
Gattung Volvox.' Pringsheim's Jahrbucher fur wissenschaftliche Botanik, 
Bd. XX. 1889. 


have arisen in the latter which rendered possible more 
energetic assimilation, but which was accompanied by an 
expenditure of nutriment, and which, after the lapse of a certain 
time, involved the complete cessation of assimilation, and con- 
sequently the death of the organism. 

The conception of a change in the protoplasm which involves 
the loss of immortality is to my mind no more improbable or 
more difficult than the commonly received view of the differen- 
tiation of somatic cells which gradually takes place in their 
phylogeny, by which they are enabled to assume various 
natures, i. e. absorptive, secretory, muscular, nervous, &c. An 
unchangeable immortal protoplasm does not exist, only an 
immortal ' form of activity ' in organic matter. 

Thus my former statement, that unicellular organisms and 
the reproductive cells of higher forms do not suffer natural 
death, is maintained in its entirety ; and I know of no better 
way to give expression to this idea than to say that such 
structures possess immortality, that is real, true immortality, 
not the phantastic, visionary immortality of the old Greek gods. 
If then death from internal causes has no existence for the 
organisms and structures in question, we can nevertheless 
maintain with absolute certainty that every one of them will 
come to an end, not indeed by the operation of forces from 
within, but because the external conditions which are necessary 
for the constant renewal of vital activity must at some future 
time themselves cease to be. The physicist predicts that the 
circulation of water on the earth will at some time inevitably 
cease, not because of any change in the nature of water, but 
because external conditions will render impossible this kind of 
movement of its particles. 

Professor Vines then attacks my views on embryogeny. He 
finds it ' not a little remarkable that Professor Weismann should 
not have offered any suggestion as to the conception which he 
has formed of the mode in which the conversion of germ-plasm 
into somatoplasm can take place, considering that this assump- 
tion is the key to his whole position \' He finds in this the 
same difficulty as in the phyletic development of multicellular 
from unicellular organisms. He concludes his objection with 
the words, 'There is really no other criticism to be made on an 
1 ' Nature,' Oct. 1889, p. 623. 


unsupported assumption such as this, than to say that it in- 
volves a contradiction in terms \' By this Professor Vines 
means that the eternal cannot, from its very nature, pass into 
the mortal, as it must do, if the perishable soma is derived from 
undying germ-cells. It is obvious that this objection rests upon 
the same confusion between immortality and eternity which 
has been already rendered clear. I do not wish to reproach 
Professor Vines with regard to this confusion ; some years ago 
I encountered the same objection, and did not at once see 
where the answer lay. We have hitherto been without a 
scientific conception of immortality : we must understand by 
this term — not life without beginning or end— but life which, 
when it has once originated, continues without limit, accom- 
panied or unaccompanied by modification (viz. specific changes 
in unicellular organisms, or in the germ-plasm of multicellular 
forms). This immortality is a movement of organic material, 
which always recurs in a cycle, and is associated with no force 
that tends to arrest its progress, just as the motion of planets 
is associated with nothing which tends to arrest their move- 
ment, although it had a beginning and must at some future 
time, by the operation of external causes, come to an end. 

Further on, Professor Vines says, 'I understand Professor 
Weismann to imply that his theory of heredity is not— like, for 
instance, Darwin's theory of pangenesis — "a provisional or 
purely formal solution 2" of the question, but one which is 
applicable to every detail of embryogeny, as well as to the 
more general phenomena of heredity and variation ^' I have 
indeed, in contradistinction to my own attempt to give a 
theoretical basis to heredity, spoken of Darwin's pangenesis 
as a purely formal solution of the question ; and perhaps I 
may be allowed to give a short explanation of the expression, 
for I fear that, not only Professor Vines, but many other 
readers of my essays may have misunderstood me. On the 
one hand I am afraid that they may have found in my words 
a tacit objection to Darwin's pangenesis, an objection which 
I did not at all intend, and, on the other, that I was inclined 
to overstate the value of my own theor}^ 

There are, I think, two kinds of theory which may be con- 

1 ' Nature,' Oct. 1889, p. 623. 2 gee Vol. I, p. 168. 

3 ' Nature,' Oct. 1889, p. 623. 


veniently distinguished as ideal and real. Practically it is found 
that they are seldom sharply discriminated ; often both kinds 
occur combined in one and the same theory : nevertheless they 
should be clearly distinguished. The ideal theory seeks to 
explain phenomena by any arbitrarily chosen principle, quite 
apart from the question whether the principle has any actual 
existence or not ^ The ideal theory only seeks to show that 
there are hypotheses on which the phenomena in question are 
explicable. Real theories however are not content with 
plausible hypotheses, but endeavour to include only those 
which possess some degree of probability : they attempt to 
give not merely a formal solution, but, if possible, the correct 
one. Sir WiUiam Thomson has attempted to explain the dis- 
persion of rays of light, by imagining the existence of molecules 
which are composed of concentric hollow spheres, arranged one 
inside the other and connected together by springs. But this 
distinguished physicist never for a moment believed in the 
existence of real molecules, provided with springs ; he wished 
to show that existing conceptions were capable of rendering 
intelligible the phenomena of dispersion. Obviously Darwin's 
pangenesis was conceived in this spirit, and was therefore called 
by him ' provisional ' ; although in later life he may have come to 
attach real worth to the theory. I consider the gemmules to 
be a deliberate invention, like Sir William Thomson's mole- 
cules provided with springs, which have no claim to reality : 
the gemmules merely serve to show the sort of suppositions 
we must make in order to understand the phenomena of 

Ideal theories are by no means useless. They are the first 
and often the indispensable steps which we must take on our 
way to the understanding of complex phenomena. They form 
the foundation upon which real theories can gradually be raised. 
Above all, they supply the impulse to re-examine again and 
again the phenomena they attempt to explain. I should pro- 
bably never have been led to deny the inheritance of acquired 
characters, if Darwin's pangenesis had not shown me that 
the belief in such transmission involved an assumption so 

^ The two philosophers Herbart and Lotze have named these two 
types of theory '■fiction ' and ' hypothesis ' : the former term agrees with 
ideal in expressing the consciousness of unreality. 


difficult to realize as that of the giving off, circulation, and ac- 
cumulation of gemmules. 

I do not even now assert that Darwin's pangenesis may not 
possibly contain a nucleus of truth. De Vries, in his recent 
exceedingly interesting work\ has shown that the ideal (im- 
possible) pangenesis of Darwin may be modified into a real 
(possible) theory, by making a few, although very profound, 
modifications. He accepts my contention that acquired or 
somatogenetic changes cannot be inherited, and thus dismisses 
precisely that part of pangenesis, which, in my opinion, lies 
outside the limits of possibility, namely the throwing off, cir- 
culation and collection of the gemmules. The future will decide 
whether the assumption of modified gemmules furnishes a better 
explanation of the facts of heredity than my hypothesis. 

But under any circumstances, I do not in any way presume 
to consider that the whole problem of heredity is solved. I 
have undertaken investigations on some of the more important 
points raised by the problem, and consequently have been led 
to formulate certain fundamental principles in order to explain 
some of the phenomena of heredity ; but no one knows more 
thoroughly than I do how far we still are from definitely and 
completely understanding, not only every detail of embryology, 
but the more general phenomena also. My endeavour has been 
to substitute a ' real ' theory for the ' ideal ' theory which has 
existed hitherto ; and I therefore took pains in thinking out 
conceptions which should, as far as possible, correspond with 
the results of actual observations. There is undoubtedly a 
material basis of heredity in the ^gg^ which can with equal 
certainty be transmitted from nucleus to nucleus, and it may be 
modified, or may remain unchanged in the process. Further- 
more, the supposition that this substance is able to impress a 
specific character on the cell involves nothing that appears to 
be impossible or non-existent. So far from this being the case, 
we are even now able to prove that the character is thus actually 
stamped upon the cell, although we cannot understand the 
way in which the process happens. Finally, my view that 
germ-plasm in an inactive condition potentially contains certain 
tendencies of the somatic cells which are ultimately derived 
from it, stands upon a firm basis, for we know that ancestral 

^ Hugo de Vries, * Intrazellulare Pangenesis,' Jena, 1889. 


characters can be inherited in a latent state, and we also know- 
that the process of inheritance is associated with a certain 
substance, the idioplasm of the germ-cell. Such idioplasm must 
therefore be in an inactive state during the period of latency. 

If it can be demonstrated that such principles suffice to 
explain the phenomena of heredity, we have made an essential 
advance beyond the ideal theory of pangenesis, which is built up 
on suppositions which do not correspond with realities. Per- 
haps the path which I have struck out may by degrees lead to 
a satisfactory solution of the numerous questions connected 
with heredity ; perhaps further investigation may show that 
we are on the wrong track and must abandon it ; what the 
future of the question may be no one can foretell. My thoughts 
upon heredity are not final, but rather serve as a starting-point 
for further thought ; they constitute no complete theory of 
heredity which claims to have satisfied all sides of this most 
complex subject ; they are rather ' researches ' which, if fortune 
favours, will, sooner or later, directly or indirectly, lead to the 
formation of a real theory. I have expressly stated this in the 
Preface to the English Edition of my collected essaj^s. 

In the same place I have emphasized the fact that my book 
did not originate as a whole, but is made up of a series of 
researches, each of which, I hope, marks some advance, each 
of which is built up on the foundation provided by the previous 
one. It contains to some extent the history of the development 
of my views as they have gradually shaped themselves in the 
course of nearly ten years' work. It is therefore unreasonable 
to extract ideas from the earlier essays and to make use of 
them against the later views. All the essays have been left 

unchanged, and ' certain errors of interpretation left 

uncorrected \' because otherwise the intimate connection which 
exists between the essays could not have been distinctly traced. 

The objections which Professor Vines urges against my 
theory of the Continuity of the Germ-plasm entirely depend, in 
my opinion, on an unintentional confusion of my ideas ; for he 
applies the views of the second essay to the ideas in some of 
the later ones, with which they do not harmonize. I will at- 
tempt to explain this in few words : in my second essay '^ (1883) 

^ See Author's Preface to First Edition, Vol. I, p. iv. 
2 See Vol. I, p. 67. 


I contrasted the body {soma) with the germ-cells and explained 
heredity by the supposition of a material basis residing in the 
germ-cells ; i. e. the germ-plasm, which is continuously passed 
on from one generation to another. When the essay was being 
written, I was not aware that this germ-plasm existed only in 
the nucleus of the egg-cell, and I was therefore able to contrast the 
entire substance of which the egg-cell consists, or the germ-plasm, 
with the substance which composes the body-cells, hence called 
somatoplasm. In the fourth essay ^ (1885) I expressed my con- 
viction, which agreed with that shortly before expressed by 
Strasburger and O. Hertwig, that the substance of the egg- 
nucleus, or, more precisely, the chromatin of the nuclear loops, 
formed the material basis of heredity, the body of the cell being 
only nutritive and capable of being moulded by forces ema- 
nating from the nucleus, but in no way formative. Together 
with the two above-mentioned writers, I transferred the con- 
ception of idioplasm — introduced at that time by Nageli, although 
defined by him in an essentially different manner, — to the ma- 
terial basis of heredity in the egg-nucleus, and submitted that 
not only in the ovum but in every cell the chromatin of the 
nuclear thread was the idioplasm which dominated the whole 
cell, and impressed its own specific character upon the origin- 
ally indifferent cell-body. From this time I no longer spoke of 
the cells of the body as simply somatic protoplasm (somato- 
plasm), but in each cell I distinguished, first, between the 
idioplasm, or substance which gives to the nucleus its power 
of predisposition, and the body of the^ cell or cytoplasm ; and, 
secondly, I distinguished between the idioplasm of the egg- 
nucleus and that of the nucleus of somatic cells. The idioplasm 
of the germ- or sperm-cell alone was called germ-plasm (idio- 
plasm of reproductive cells), while the idioplasm of the somatic 
cells was called somatic idioplasm. Embryogeny, in my opinion, 
depends only on changes in the idioplasm of the egg-nucleus, 
i. e. changes in the germ-plasm. In my fourth essay there is a 
description of the manner in which the idioplasm of the egg- 
nucleus divides, in many species, at the first segmenta- 
tion, each half undergoing certain regular modifications of 
nuclear substance, so that neither daughter-cell possesses 
the collective hereditary tendencies of the species, but one 
^ See Vol. I, p. 163. 
G 2 


contains those of the ectoderm, and the other those of the 
endoderm. The later stages of embryogeny depend on a con- 
tinuance of such regular modifications of idioplasm. Each fresh 
division sorts out fresh predispositions, previously mixed in 
the nucleus of the mother-cell, until at length the full number 
of embryonic cells have come into existence, each with an 
idioplasm in its nucleus which stamps the specific histological 
character upon the cell. 

I fail to understand why this idea presents such remarkable 
difficulties to Professor Vines. In most species the separation 
of the sexual cells takes place late in the embryogeny. Now in 
order to maintain the continuity of germ-plasm from one gene- 
ration to another, I have supposed that, at the first division of 
the ovum, not all the germ-plasm (i.e. idioplasm of the first 
ontogenetic stage) becomes changed into idioplasm of the 
second stage, but that a minute portion of it persists unchanged 
included in one or other of the daughter-cells, where it remains 
inactive, intermingled with the nuclear idioplasm ; I have further 
assumed that in this condition it is transmitted through a longer 
or shorter series of cell-generations until at length it reaches 
certain cells on which it impresses the characters of germ-cells, 
and in these it resumes its activity. This view is not entirely 
devoid of support ; for it is in some degree confirmed by actual 
observations, especially by those on the remarkable wanderings 
through which the germ-cells of Hydroids pass, after starting 
from their original place of formation ^ 

But let us leave the consideration of the degree of probability 
which my theory may possess, and consider only its logical 
accuracy. Professor Vines says, ' The fate of the germ-plasm 
of the fertilized ovum is, according to Professor Weismann, to 
be converted in part into the somatoplasm (!) of the embryo, 
and in part to be stored up in the germ-cells of the embryo. 
This being so, how are we to conceive that the germ-plasm 
of the ovum can impress upon the somatoplasm (!) of the de- 
veloping embryo, the hereditary character of which it (the 
germ- plasm) is the bearer ? This function cannot be discharged 
by that portion of the germ-plasm of the ovum which has be- 

^ Weismann, ' Die Entstehung der Sexualzellen bei den Hydromedusen,' 
Jena, 1883. 


come converted into the somatoplasm (!) of the embryo, for the 
simple reason that it has ceased to be germ-plasm, and must there- 
fore have lost the properties characteristic of that substance. 
Neither can it be discharged by that portion of the germ-plasm 
of the ovum which is aggregated in the germ-cells of the em- 
bryo, for under these circumstances it is withdrawn from all 
direct relation with the developing somatic cells. The question 
remains without an answer '.' 

I believe, however, that the answer is to be found above. I 
know nothing of the ' somatoplasm ' of Professor Vines : my 
germ-plasm, or idioplasm of the ist ontogenetic stage, is not 
modified into the ' somatoplasm ' of Professor Vines, but into 
idioplasm of the 2nd ontogenetic stage, and then into that of 
the 3rd, 4th, 5th, and so on up to the looth and loooth stage ; 
and each stage of idioplasm confers its own specific character 
upon the cell in the nucleus of which it lies. 

Professor Vines also criticises my views as to the idioplastic 
nature of the nuclear substance (the chromatin granules in the 
nuclear loops, &c.). He maintains that it is as easy to speak of 
the continuity of the cell-body as the continuity of the nuclear 
substance, and that hereditary peculiarities can be as well 
transmitted to the offspring by the former as by the latter. 

I can quite understand why a botanist should take this view, 
and indeed, in bringing it forward. Professor Vines does not 
stand alone. Waldeyer^ maintained, in 1888, that established 
facts did not justify us in regarding the nuclear loops as 
possessing an idioplastic nature. Among other zoologists, 
Whitman^ has pronounced very decidedly against the idio- 
plastic nature of the nucleus, and in their recent work, Geddes 
and Thomson ^ have done the same. 

The facts which suggested to my mind that the nuclear loops 
are the material basis of heredity, — in fact the idioplasm, — are 
enumerated in my fourth essay ^ They were chiefly the obser- 
vations of Van Beneden on the process of fertilization in the 

1 'Nature,' Oct. 1889, p. 623. 

2 Waldeyer, ' Ueber Karyokinese und ihre Beziehung zu den Befruch- 
tungsorganen,' Archiv fur Mikr. Anatomic, Bd. XXXII. 1888. 

^ Whitman, * The Seat of formative and regenerative Energy,' Boston, 

^ Geddes and Thomson, ' The Evolution of Sex,' London, 1889. 
5 See Vol. I, p. 163. 


f::^g of A scan's megalocephala^ the observations of Strasburger 
on the fertilization of the egg-cell in phanerogams by means of 
the nucleus alone, and the experiments of Nussbaum and 
Gruber on the artificial division of Infusoria. To these may be 
added certain other considerations of essential importance, viz. 
the occurrence of karyokinesis, and the fact that the formation 
of polar bodies by the ova of animals can be rendered intel- 
ligible only on the assumption that the idioplasm resides in the 
nucleus. The formation of polar bodies involves the division 
of the nuclear substance of the egg into two halves similar in 
quantity, but the cell-body itself is divided into two entirely 
dissimilar portions, the relative sizes of which differ in different 
species. The essential part of this expulsion of polar bodies 
from the ovum, must lie in the division of the nuclear substance, 
and not in the division of the cell. These facts and considera- 
tions, in conjunction with others, completely convinced me that 
the nuclear substance is the sole carrier of hereditary tenden- 
cies : the view which I expressed ten years earlier (1873), of 
the physiological equality (Homodynamy) of the nuclei of both 
male and female germ-cells, became to my mind a certainty, 
and I then advanced the theory of fertilization which is to be 
found in my fourth essay. No one, as far as I know, w4th the 
single exception of Strasburger, has expressed similar views 
on the essential nature of fertilization, at any rate with regard 
to the homodynamy of the sexual nuclei. The distinguished 
observer E. van Beneden, to whom we owe so much of our 
knowledge of the processes of fertilization, has maintained his 
belief in the old view which looks upon fertilization as the union 
of two elements which are essentially opposed to each other. 
He is unable to free himself from the dominant idea, so firmly 
embedded in the biological mind, that sexual difference is 
something fundamental, and an essential principle of life itself. 
To him, the fertilized ovum is a ' hermaphrodite ' being, which 
unites in itself both male and female entities,— an idea which 
has commended itself to man}^ authorities, but an idea of which 
the logical outcome forces us to regard all the cells of the body 
as hermaphrodite. Van Beneden was at the same time swayed 
by the opinion, which is shared by so many workers in other 
lands, that fertilization is a process of rejuvenescence, without 
which terrestrial life could not continue. Many observers still 


cling to this view, and Maupas^ has recently claimed to have 
found a proof of its soundness by showing that it is essential 
for Infusoria to conjugate (sexual reproduction) from time to 

This contention forms a striking example of the difficulty 
with which even scientifically trained minds can shake off 
deeply rooted convictions. Although it must be clear to every 
one that unicellular organisms are immortal, although Maupas 
has himself produced superabundant proofs that the repro- 
duction of Infusoria by fission can go on without ceasing, and 
although he maintains that ' les cycles evolutifs des Cilies 
peuvent se succeder a I'infini ' (p. 437), nevertheless the 
power of the old tradition of the necessity of death is so strong 
in him that he is incapable of recognizing this simple fact. 
Rather than adopt the views propounded by others, he prefers 
to accept the hypothesis that unicellular organisms are really 
mortal and are subject to natural death, but that this is kept 
in abeyance and postponed by the influence of conjugation. 

If we ask, whence comes this idea of the necessity of death, 
we receive the answer, — from our experience of man and the 
higher animals and plants. If we further ask, why has it hitherto 
been entirely overlooked that among these organisms certain 
parts of the body (the reproductive cells) are endowed with 
immortality, the answer is,— because we have only recently 
come to know and completely appreciate the facts of reproduc- 
tion, and therefore have only just arrived at a correct estimate 
of them, and are now for the first time able to recognize in our 
reproductive cells, the undying parts of our individuality. 

For how long then will reproduction be regarded as a dy- 
namical process, as a stimulus, as ' the spark in the powder 
cask,' or in biological language the vitalizing of the Q.%g ? This 
conception is directly derived from the old vital force of earlier 
times, and it is the unrecognized reflection of this latter idea 
which influences many writers, and which, proteus-like, con- 
tinually appearing in new forms, evokes the belief in a necessity 
for the rekindling of life. 

If we lay aside preconceived notions and simply review the 

^ E. Maupas, ' Le rajeunissement karyogamique chez les Cilies,' Arch. 
Zool. exper. et generale, 2 ser., Tom. vii. Nr. i, 2, et 3, 1889. 


facts of the case, we see, on the one side, unicellular animals 
which continually increase by division, and, on the other, multi- 
cellular animals which are differentiated into somatic and 
germ-cells, — animals in which the body dies, while the re- 
productive cells possess the same power of unhmited increase 
by division that is possessed by unicellular beings. But what 
leads us to consider that the capacity for continuous reproduction 
is rendered possible by the fusion of the essential material of 
one organism with that of another, such as we see in both 
conjugation and fertilization ? Nothing but the unconscious 
tradition of the inevitability of death. Maupas thinks that he 
has proved the existence of natural death among the Infusoria, 
since he has shown by his investigations, — excellent as far as 
observation is concerned, — that, from time to time, conjugation 
must make its appearance, or the colony would die out ; but he 
forgets that as a matter of fact under natural conditions, the 
possibility of conjugation is granted, and that thus the so- 
called natural death does not appear more often in nature than 
in the case of those metazoan ova which fail to meet with a 
spermatozoon. The Infusorian which has not conjugated 
gradually disappears, like the animal &gg which remains un- 
fertilized; and the so-called ' senile degeneration ' (Maupas) of 
the former exactly corresponds to the gradual decomposition 
and dissolution of the latter, a process which was described 
long ago, in a species oi Moina, in one of my memoirs on the 
Daphnids. Conjugation, no less than fertilization, is un- 
doubtedly a process of vast importance ; but I believe that its 
significance lies in the maintenance and continual intermingling 
of individual variations, or it may be that some other advan- 
tage is conferred which acts for the preservation of the species. 
In any case nature attaches great importance to it, and seeks 
to ensure it, for each species, to the greatest possible extent. 
For this purpose she has made provision that the periodical 
recurrence of the process should affect as many individuals as 
possible. If however, in spite of every provision, unfavourable 
circumstances should bring it about that certain individuals have 
no part in the process of conjugation, is it to be wondered at 
that nature should care nothing for their preservation ? Or, to 
speak less figuratively, we must not be surprised to see that 
means are taken to prevent the unlimited increase of those 


individuals which are less favourably placed for the continuation 
of the species. How, in fact, can this be otherwise, since, in 
Infusoria, the unlimited continuance of life is bound up with 
conjugation, just as in the ova or spermatozoa of higher 
organisms, it is dependant on fertilization. It might be objected 
that the cases are different, inasmuch as the germ-cells which 
fail to be fertilized perish for lack of nourishment, while the 
Infusoria which fail to conjugate experience no such difficulty : 
when therefore they come to an end after a certain number of 
generations, their death must be due to the working of other 
causes. But in the above-mentioned Daphnid Moina rectirosiris 
when copulation has not taken place the unfertiUzed ^gg is not 
laid at all. It retains the very position in the ovary which it 
would occupy during development, and it is placed under the 
most favourable conditions of nutrition. For some time it 
retains its vitality, but if still unfertilized, it ultimately dies and 
undergoes dissolution, being finally completely reabsorbed by 
the surrounding epithelial cells of the ovary. The &gg is so 
constituted that it remains alive for a certain time awaiting 
fertilization, and then, in spite of the most favourable conditions 
of nutrition, it perishes. If copulation be delayed in the nearly 
allied Moina paradoxa, the unfertilized eggs are laid and die at 
once, so that their material is lost to the animal. It is obvious 
that the arrangement in Moina rectirosiris is a special adapt- 
ation enabling the organism to utilize the material of the large 
eggs which, unless fertilized, are incapable of further develop- 
ment. We do not know what kind of an arrangement it is 
which involves the death of the egg although surrounded by 
such favourable conditions of nutrition, any more than we 
know what causes the fate of the unconjugated Infusorian : the 
facts however show that some arrangement must exist to 
produce such results. The continued life of an egg requiring 
fertilization, is dependant on fertilization ; that of an Infusorian 
needing conjugation, on conjugation. 

The experiments of Maupas seem to show that Infusoria are 
adapted for fertilization, that periodical conjugation is one of 
the conditions of their life, like food and oxygen. But it is a 
fallacy, only explicable on the ground of deep-rooted prejudice, 
to argue from this that they are really mortal, and that their 
actual immortality depends on the magic of conjugation. One 


might just as well maintain that food is the cause of Infusorian 
immortality, inasmuch as death ensues when food is withheld. 
I believe that the essential, fundamental, and original peculiarity 
of living matter was the power to assimilate and to grow with- 
out limit. On this depends the existence of the whole organic 
world : it is a primary power, not a secondary one, and cannot 
have been conjured up afterwards in the organism by any 
refined artifice, call it conjugation, fertilization, or anything else. 
It must have been present from the very beginning of life on the 
earth. How otherwise could life have persisted up to the first 
appearance of conjugation or fertilization.'' For there can be 
scarcely any doubt that neither of these processes is found in 
the lowest organisms at present known to us. I therefore think 
that the loss of this fundamental power of unlimited growth 
must be regarded as a secondary adaptation, called forth by 
certain special circumstances which rendered it necessary for 
achieving the combination of different individual hereditary 
tendencies. When, therefore, certain writers speak of these 
processes of conjugation and fertilization as a rejuvenescence, in 
the sense of a renewal of vital energy, I can onl}^ believe that they 
are upholding a long-vanquished and mystical principle. It is 
quite otherwise if we speak of the conjugation of Infusoria as a 
rejuvenescence in the sense of a dissolution and re-formation of 
many parts : this is a process which may depend throughout 
on well-known natural forces, and which makes its appearance 
not only in conjugation but in division also. I have no objec- 
tion to raise against this kind of rejuvenescence ; in fact the 
continual repetition of such regeneration among these undying 
organisms, exposed, as they are, to constant wear and tear, be- 
comes a necessary assumption. 

In my fourth essay, the idea of fertilization being regarded as 
a process of rejuvenescence, in the sense of a renewal of vital 
force, is opposed, and the converse view is clearly enunciated. 
To condense my argument into a sentence, — we ought not to 
speak, as formerly, of the two conjugating nuclei of the germ- 
cells as male and female, but as paternal and maternal; they are 
not opposed to each other, but are essentially alike, differing 
only as one individual differs from another of the same species. 
Fertilization is no process of rejuvenescence, it is nothing more 
than a mingling of the hereditary tendencies of two individuals. 


These tendencies are exclusively contained in the nuclear 
loops ; the cell-bodies of the spermatozoon and ovum are in 
this respect indifferent, and serve only as the nutritive mate- 
rial w^hich is formed and transformed in a definite way by the 
dominating idioplasm of the nucleus, as clay is moulded by the 
hand of a sculptor. That the &gg and the spermatozoon differ 
so greatly in appearance and function, and that they mutually 
attract each other, depend on secondary adaptations, which 
ensure that they shall find each other, that their idioplasm or 
nuclear substance shall come into contact, while, at the same 
time, a certain amount of nutriment shall be provided for the 
embryogeny, &c. &c. And just as the differentiation of cells 
into male and female reproductive elements is secondary, so 
is that of male and female individuals : all the numerous dif- 
ferences in form and function which characterize sex among 
the higher animals, all the so-called ' secondary sexual charac- 
ters,' affecting even the highest mental qualities of mankind, 
are nothing but adaptations to bring about the union of the 
hereditary tendencies of two individuals. 

These are briefly the ideas on fertilization which I indicated 
in the year 1873, and which I pubhshed in a detailed and 
definite form in 1885, after the discoveries of Van Beneden on 
the morphological processes which take place during the 
fertilization of the ^g^ of Ascaris'^. Towards the end of the 
essay I used these words, ' If it were possible to introduce the 
female pronucleus of an ^g^ into another ^^g 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 would be fur- 
nished. Unfortunately 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 com- 
pensated for by the fact, ascertained by Berthold, that in 
certain Algae {Edocarpus and Scytosiphon) there is not only 
a female, but also a male parthenogenesis ; for he shows that 
in these species the male germ-cells may sometimes develop 
into plants, which however are very weakly^.' 

^ See Vol. I, Essay iv, p. 163. ^ ggg YqJ j^ pp 2^2, 253. 


Since then I have made the attempt to fertilize the ovum 
of a frog w^ith the nucleus of another ; the experiment did not 
succeed, and we could scarcely expect it to do so, considering 
the very considerable amount of injury caused by transferring 
a nucleus into another ^gg. 

Boveri ^ has been more fortunate ; for he succeeded in finding 
an object which permitted the converse of my experiment. 
Adopting the method of R. Hertwig, he separated, by shaking, 
the nucleus from the ovum of an Echinus, and succeeded in 
rearing such denucleated eggs by the introduction of sperma- 
tozoa. A regular segmentation nucleus was formed from the 
spermatozoon which penetrated the &gg, embr3^ogeny followed 
its usual course, and the &gg gave rise to a perfect but rather 
small larva, which swam freely about in the water, and lived 
until the seventh day. 

This experiment is by itself sufficient to prove that the views 
on fertilization adopted b};- Strasburger and myself are correct, 
viz. that the nucleus of the spermatozoon can pla}^ the part of 
the nucleus of the ^gg^ and vice versa, and that the older view 
to w^hich Professor Vines ^ adheres, must be given up. 

An interesting and important modification of Boveri's ex- 
periment, affords further support to the results obtained by 
him, and confirms — if indeed confirmation be necessar}^ — the 
view which looks upon the nuclear substance as idioplasm, as 
maintained by O. Hertwig, Strasburger, and mj^self ^. 

If the eggs of Echinus niicrotubercitlatus, artificially deprived 
of their nuclei, be fertilized, not with the spermatozoa of their 
own species, but with those of another, Sphaerechinus granularis, 
larvae are developed with the true characters of the last species only, 
that is to say, nothing is inherited from the mother but every- 
thing from the father. The nuclear substance is the sole bearer 
of hereditary tendencies and by it the cell is governed. 

I have explained the first polar body of the metazoan egg 
as the carrier of ovogenetic idioplasm which must be removed 

^ Boveri, ' Ein geschlechtlich erzeugter Organismus ohne miitterliche 
Eigenschaften.' Gesellsch. f. Morph. u. Physiol. Munchen, i6 Juli, 

^ S. H. Vines, * Lectures on the Physiology of Plants.' Cambridge, 
1886, pp. 638 681. 

^ Cf. Kolliker, ' Die Bedeutung der Zellenkerne fur die Vorgange der 
Vererbung.' Z. f. W. Z. Bd. 42, 1885. 


in order that the germ-plasm may become dominant. It is 
possible that this explanation may be incorrect. The latest 
observations on the conjugation of Infusoria, as recorded in the 
excellent works of Maupas and R. Hertwig, are opposed to my 
explanation, although the idea upon which it was formed is 
justified. Since it is the nuclear substance which gives to the 
cell its specific character, the egg-cell must before fertilization 
be dominated by an idioplasm distinct from that of the sperm- 
cell, for they are, up to this point, of different form and function. 
As soon however as fertilization is accomplished they both 
contain the same idioplasm, namely germ-plasm. Hence the 
earlier dominant idioplasm must be different from the later. 

This fundamental idea upon which my interpretation of 
the first polar body was founded appears to be sound. One 
might perhaps imagine that the idioplasm of the Qgg was 
originally different from that of the spermatozoon, but that both 
possessed the power of changing into germ-plasm. But this 
would leave wholly unexplained the fact that parthenogenetic 
eggs extrude one polar body. Both facts become clear, if ova 
and spermatozoa are dominated until they reach maturity 
by different histogenetic idioplasmata, with which a small 
amount of germ-plasm is mingled, and if when the former are 
removed, the germ-plasm governs both cells. This process is in 
no way extraordinary and unparalleled ; for entirely analogous 
divisions of the idioplasm into halves of unequal quality, must 
take place hundreds of times in every embryogeny. However, 
I wilHngly admit that on this question the last word has not 
yet been spoken, and would merely add that my theory of 
heredity is not concerned thereby. As regards my theory, the 
significance of the second polar body, and not that of the first, 
is decisive. Even if it be demonstrated that my interpretation 
of the first polar body is erroneous, this would not interfere 
with the conception of the second as halving the number of 
ancestral germ-plasmata. I should then look upon the first 
division as merely leading up to the second, as a first step 
necessary for the reduction of the ancestral plasmata, although 
the reason for its necessity is not at present quite clear 
to us. 

The occurrence of regular changes in the idioplasm during 
ontogeny, which I have urged, and which has been attacked 


by so many writers, particularly by Kolliker^ now appears to be 
justified. If the nucleus of a spermatozoon is capable of con- 
veying to the body of an ovum which has lost its nucleus, the 
hereditary tendencies contained in itself, and of producing an 
organism with paternal characters only, — then we can scarcely 
conceive of ontogen^^ except as a series of regular changes of 
the idioplasm, advancing from cell-division to cell-division, 
and giving its special character to the body of every cell at 
every stage of growth, not only in respect to form, but also to 
function, and especially with regard to the rhythm of cell- 

Professor Vines raises a further objection against m}^ views on 
the origin of variation. In the fifth essay '^ I looked for the signi- 
ficance of sexual reproduction in the fact that it alone could 
have called forth that multiplicity of form which is met with 
among the higher plants and animals, and that constantly 
varying combination of individual variations, which natural 
selection required for the creation of new species. I still hold 
to the view that the origin of sexual reproduction in reality 
depends on the assistance which it affords to the working of 
natural selection, and I am entirely convinced that the higher 
development of the organic world was only rendered possible 
by the introduction of sexual reproduction. On the other hand, 
I am inclined to believe that Professor Vines is right in his 
contention that sexual reproduction is not the or\\y factor which 
maintains the variability of the Metazoa and Metaphyta. I 
might have pointed out in the English translation of my essays 
that my views on this point had somewhat altered since the 
appearance of the German originals. My lamented friend. 
Professor De Bary, too early lost to science, had already 
directed my attention to those fungi which propagate them- 
selves parthenogenetically, and which Professor Vines justly 
cites against this part of my view ; but I wished on the grounds 
mentioned above to make no alteration in the essays. At the 
time when I wrote the essay in question (1886), I was well 
aware that my views on the causes of individual variation were 

' Kolliker, ' Das Karyoplasma und die Vererbung : eine Kritik der 
Weismann'schen Thcorie von der Kontinuitat des Keimplasmas.' Z. f. 
W Z. Bd 44, p 228, 1886. 

^ See Vol. I, p. 257. 


possibly incomplete, and in order to expose the correctness of 
my view to the widest available test, I carried out its logical 
consequences as thoroughly as possible, and laid down the 
principle that species which propagate parthenogenetically 
have no power to develop into new species. Furthermore, 
about the same time, I began a series of experiments to test 
the truth of this statement as to the capacity for variation pos- 
sessed by parthenogenetic species ; these have been continued 
up to the present time, and on some future occasion I hope to 
make them public. 

But even if, as seems at present very probable, sexual repro- 
duction is not the only origin of individual variability in the 
Metazoa, no one will deny that it is the chief means of increas- 
ing these variations and of continuing them in favourable pro- 
portions. In my opinion, the importance of the role which 
sexual reproduction plays in shaping the material for the 
process of selection is scarcely diminished, even if we con- 
cede that some amount of individual variability can be called 
forth by direct influences on the germ-plasm. Even Professor 
Vines considers it probable 'that the absence of sexuality in 
these plants (the parthenogenetic higher Fungi) may be just the 
reason why no higher forms have been evolved from them ; for 
in this respect they present a striking contrast to the higher 
Algae in which sexuality is well marked \' 

But when Professor Vines says ' there can be no doubt that 
sexual reproduction does very materially promote variation ^,' 
he does not intend to imply that this statement is self-evident ; 
for it is well known to him that prominent investigators, like 
Strasburger ^, see in sexual reproduction the reverse action, 
' that of preserving the constancy of specific characters.' I accept 
with pleasure his agreement with my view, confirming the 
chief result of my fifth essay, which may be expressed as 
follows : — Sexual reproduction has arisen by and for natural 
selection, as the only means by which the individual variations 
can be united and combined in every possible proportion. 

Again, with respect to the problem of the inheritance of ac- 

1 ' Nature,' Oct. 1889, p. 626. 2 i Nature,' Oct. 1889, p. 626. 

^ Strasburger, ' Neue Untersuchungen uber den Befruchtungsvorgang 
bei den Phanerogamen als Grundlage fur cine Theorie der Zeugung.' 
Jena 1884, p. 140. 


quired (somatogenic) characters, Professor Vines finds himself 
opposed to me ; for he regards such inheritance as possible. 
I have denied this because it did not appear to me self-evident, 
as had been previously assumed by every one, but rather utterly 
unproven ; and because I believe that completely unproved as- 
sumptions of such importance should not be made, when they 
need such a number of improbable hypotheses to make them 
intelligible. I have tested, as accurately as possible, all the 
available evidence for such inheritance and have found that 
they possess no value as proofs. There is no inheritance of 
mutilations, and, up to the present time, these form the only 
real basis for the assumption of the hereditary transmission of 
somatogenic variations. If, in my last essay', I did not directly 
deny all possibility of such inheritance, Professor Vines should 
interpret that in my favour and not to my discredit : it is not the 
business of an investigator to maintain that a proposition, which 
he sets forth in accordance with the present state of our know- 
ledge, must be accepted as an infallible dogma. Professor 
Vines finds the ' statements of opinion so fluctuating that it is 
difficult to determine what his position exactly is,' but he could 
have easily arrived at my views, if he had judged them by the 
last essay, instead of promiscuously contrasting isolated pas- 
sages from eight essay's, which occupied eight years in their 
production. The last essay is especially concerned with 'the 
supposed transmission of mutilations,' and, at the end of it, my 
verdict on the state of the problem of the inheritance of ac- 
quired (somatogenic) characters, is set forth as follows, 'The 
true decision as to the Lamarckian principle [hes in] the ex- 
planation of the observed 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'-.' 

The distinguished botanist, De Vries, has shown that certain 
constituents of the cell-body, for example the chromatophores 
of Algae, pass directly from the germ- cell of the mother into the 
daughter organism, whilst, as a rule, the male germ-cell contains 
no chromatophores. This appears to be a possible case of the 
^ See Vol. I, p. 431. ^ See Vol. I, p. 461. 


inheritance of somatogenic variations. In these low plants the 
difference between somatic and reproductive cells is slight, and 
the body of the egg-cell does not require to undergo a complete 
change in its chemical and structural characters in order to 
develope into the body of the somatic cells of the daughter 
individual. But what has this to do with the question whether, 
for instance, the skill of a pianist's fingers, acquired through 
practice, can be transmitted to his descendants ? How does 
the result of this practice reach the germ-cells ? Here lies the 
real problem which those who maintain the inheritance of 
somatogenic characters must solve. 

The above-mentioned observations of Boveri on the ova of 
Echinoderms deprived of their nuclei, prove that the body of 
the egg-cell contributes nothing to inheritance. If then the 
inheritance of somatogenic characters takes place, it can only 
be by means of the nuclear substance of the germ-cells, that is 
through the germ-plasm, and that not in its patent, but in its 
latent state. 

To abandon the Lamarckian principle certainly does not 
facilitate the explanation of phenomena, but what we need is 
not a merely formal explanation of the origin of species, 
although it may be the most convenient, but an attempt to 
discover the real and genuine explanation. We must endeavour 
to explain the phenomena without this principle, and I believe 
I have made a beginning in this direction. I have lately 
investigated the phenomena in a case where one would not 
expect to be able to dispense with the principle of modification 
through use, viz. in the case of artistic endowment ^. I pro- 
pounded to myself the question whether the musical faculty 
in man could be conceived of as arising without the increase of 
the original faculty by use. And I arrived at the conclusion 
that not only was this principle unnecessary, but that use has 
actually taken no share in the evolution of the musical sense. 

^ ' Gedanken iiber Musik bei Thieren und beim Menschen.' Deutsche 
Rundschau, October, 1889. Translated as the tenth essay, — the second 
in the present volume. 



Amphimixis or the Essential Meaning of 
Conjttgation a^td Sexual Reproduction. 

With twelve Figures. 

H 2 





The present treatise brings to a conclusion a series of essays 
upon biological problems which have appeared in the course of 
the last ten years. They commenced with an enquiry into the 
duration of life, which led on to the question of the biological 
origin of death, and then turned to certain phenomena of 
inheritance and reproduction. They endeavoured to ascertain 
with certainty and to elucidate the real conditions of these 
phenomena, and to search out their essence and significance. 

I shall attempt to explain, as clearly as possible, the close con- 
nexion existing between certain apparently isolated problems 
and the subject of this essay, which, although mainly concerned 
with so-called ' sexual reproduction,' is in reality the keystone 
of the whole structure. My object is to express more fully 
than before, the thought that the process which we are 
accustomed to regard as reproduction, is not reproduction only, 
but contains something sui generis^ something which may be 
connected with reproduction proper, and in the higher plants 
and animals is so connected, but which is entirely separate in 
the lower organisms. I shall show that its significance does 
not lie in the maintenance of life but in the mingling of 


To attain this object it will be necessary first to consider the 
remarkable morphological processes which accompany the 
maturation of reproductive cells, and, as far as possible, to seek 
for a true interpretation in the results of the most recent 
researches. Furthermore, it will be necessary to apply the 
ideas thus gained to the problem of conjugation, and to bring 
within the scope of the enquiry many other phenomena, such 
as the various kinds of reproduction, certain phases of the 
question of heredity, and the immortality of unicellular organ- 
isms, because these are most intimately connected and indeed 
mutually dependent. 

Thus the thoughts which run through the previous essays 
resemble tangled threads which are gradually unravelled and 
are ultimately all woven together. I will only add the wish 
that the new conceptions to which these researches have led 
may prove a fruitful field for further investigations. 


LiNDAU, Lake of Constance, 
Septetnber 12, 1891. 




Introductiox 105 

I. The SIGNIFICA^'CE of the Process of Maturation of the Germ- 

cells 114 

The Maturation of the Ovmn 114 

The Maturation of the Spermatozoon . . . . . .117 

The Double Division of the Nuclear Substance in the Formation of 

Germ-cells . . . . . . . . . .123 

Other Types of Maturation of Germ- cells ..... 138 

Objections 146 

II. Inheritance in Parthenogenetic Reproduction . . . 150 

The Processes of Maturation in Pat-thenogenetic Eggs and their 

Meaning .......... 150 

Observations on Inheritance in Parthenogenesis .... 159 

The Origin of Parthenogenetic Eggs from those which require 

Fertilization 17° 

III. Amphimixis as the Significance of Conjugation and Fer- 

tilization 176 

The Facts of Conjugation 176 

Meaning of the Phenomena ....... 182 

Objections 187 

The Deeper Significance of Conjugation . . . . .189 

Amphimixis in all Unicellular Organisms I93 

The Theories of Rejuvenescence and of Mingling . . . . 195 
Does Natural Death occur in Unicellular Organisms ? . . 203 
The Appearance of Amphimixis in the Organic World . .210 


Fig. I. Formation of spermatozoa in Ascans megalocephala 

II. Formation of ova ,, ,, ,, 

III. Behaviour of idants during the development of germ-cells 

IV. Formation of spermatozoa in Pyrrhocoris 

V. Diagram of a double idant ,, ,, 

VI. Formation ,, ,, .. ,, 

VII. Wreath of four idants .... 

VIII. Maturation of parthenogenetic egg , 

IX. Germinal vesicle of parthenogenetic egg oi Artemia 

X. The two varieties of Cypris reptans, A and B. 

XI. Conjugation oi Paramaecititn ..... 

XII. Diagram of the conjugation oi Colpidhtnt 


1 20 







For more than a decade biological enquiry has been 
engaged, with renewed energy, upon the problem of fertili- 
zation. When the brothers Hertwig, and Fol had taught and 
demonstrated the fusion of the nuclei of ovum and sperma- 
tozoon, and had further shown that, before fertilization, the 
egg undergoes certain preparatory changes resulting in the 
extrusion of the previously well-known polar bodies,— an 
attempt was made to understand the significance of these pro- 
cesses. What can this substance be that it requires to be 
thrown out from the ovum before fertilization ? The first 
answer to this question depended on the then commonly 
received, although never clearly formulated opinion, that ferti- 
lization consisted in the union of two opposed forces, — of what 
may be described as a male and a female principle which, 
by their fusion, kindled anew that life which, without such 
rejuvenescence, must gradually come to an end. Considering 
the dominant theory as to the significance of fertilization, it was 
certainly justifiable to endeavour to look upon these bodies, 
expelled from the egg, as the bearers of one of the two antithe- 
tical forces, which were previously united in a single ovum, but 
which required to be separated in order to render the egg capable 
of fertilization. The polar bodies were thus looked upon as 
bearers of the male principle, by the removal of which the 
ovum became for the first time sexually differentiated, i.e. 
became female. This idea was not merely ingenious, it was 
the legitimate result of those indefinite ideas as to the essential 
nature of fertilization, which up to the present day have held 


the field. Such a view must inevitably have been brought 
forward, if we were ever to arrive at a solution of the phenomena. 
I should certainly be the last to reproach the three savants 
who developed this hypothesis, although I have perhaps 
contributed something towards the proof of its unsoundness. 
The path to truth often lies through inevitable error. 

I was, from the first, predisposed against the view of Minot, 
Balfour, and Edouard van Beneden, being influenced not only 
by certain isolated phenomena of inheritance, phenomena which 
were at a later time and with perfect justice, urged against it, 
but by the facts of inheritance taken as a w^hole, and by that 
conception as to the nature of fertilization to which I had even 
then been driven by these very facts, although unable to prove 
to myself, or to others, the soundness of my views. 

We recognize tw^o phenomena in amphigonic reproduction : — 
first, fertilization in its strictest sense, i. e. the fact that the ovum 
can only develope into a new being when it has united with the 
spermatozoon, after which union a ' vitalization of the egg' 
takes place (Hensen) ; secondly, the mingling of two hereditary 
tendencies. From the very oldest times it must have been 
observed that the peculiarities of the father as well as of the 
mother, may appear, and to an equal extent, in the children. 
Such transmission was conceived by some writers in a material 
sense ; for they imagined a part of the substance of the mother 
or of the father as the basis of the body of the offspring ; but it 
was also looked upon by others as simply the transmission of an 
impulse. Thus according to Aristotle the father confers the im- 
pulse to movement, while the mother contributes the material. 
Lowenhoek and the other ' spermatists ' held that the semen 
alone forms the substance of the embryo, while his opponents, 
Swammerdam and Malpighi, the so-called 'ovists,' returned 
to Aristotle's view in so far that they believed that the mother 
gives rise to the substance, that is the ovum, while the male 
influence is limited to an 'aura seminalis,' which at the same 
time acts as the transmitter of movement. 

Some writers regard inheritance by means of fertilization 
as a purely immaterial occurrence : thus Harvey, in his 
remarkable and minutely thought-out theory of heredity, 
imagined conception as a mental process, the folds of the 
mucous membrane lining the uterus corresponding to the 


convolutions of the brain, and giving rise to the foetus under the 
influence of the semen ; just as the brain, under the influence of 
external impressions, gives rise to thoughts. The term ' con- 
ception,' when figuratively applied to mental processes,— a term 
which has been obviously derived from conception on the part 
of a woman,— is here reversed, and used to explain the very 
process from which it is itself derived. 

The same fundamental idea runs through all theories of 
fertilization up to the present time— the idea that the fertili- 
zation, i.e. the ' vitalization of the ^%g' is the important part, 
or, as we may say, the true purpose of sexual reproduction. 
The other side of this mode of reproduction has been com- 
paratively neglected ; the fact that two different predispositions, 
on the one hand that of the father, and on the other that of 
the mother, are by fertilization united in a single organism, 
has appeared as a secondary, but it is clear to some extent 
as an inevitable result of fertilization. Although this view is 
nowhere directly expressed, it is implied in all the utterances 
of both older and more recent writers. It must be admitted 
that so long as biologists were acquainted with no method of 
reproduction except the sexual, it was impossible to regard 
fertilization in any other light ; it seemed that the co-operation 
of two individuals was indispensable in order to call a third 
to life, and it can scarcely have been surprising for this new 
organism to resemble its progenitors more closely than any 
other living being. But, even in recent times, when other 
methods of reproduction among plants and animals gained 
recognition, they did not at first cause any alteration in that 
view which regards fertilization as a process of vitalization, a 
calling forth of new life. In the case of all those higher beings 
which do not possess the power of asexual reproduction, it 
became evident that a certain complexity of organization ex- 
cluded this method of increase. But then the asexual repro- 
duction of the lower organisms is by no means always sufficient 
to fulfil every condition necessary for the maintenance of the 
species, and hence the origin of new individuals from unicellular 
germs capable of fertilization must have appeared as an essen- 
tial advantage. 

The first fact which tended to throw doubt on the view that 
fertilization is a renewal of life was the discovery of parthe- 


nogenesis hy C. Th. von Siebold ^ and Rudolph Leuckart ^. 
When it was understood that, under certain circumstances, an 
e.g'g could develope into a new individual without fertilization, 
this fact by itself was sufficient to show that a ' vitalization of the 
germ ' could not be the object of fertilization, and could not be 
the cause of its appearance among living beings. 

But it w^as long before the facts of parthenogenesis were ge- 
nerally accepted : indeed, in some circles they are not received 
at the present day. Only ten years ago, a prominent physio- 
logist, Pfluger, held them to be unproved, and most botanists 
were inchned to doubt their existence among plants as well as 
animals ; for at that time parthenogenesis appeared to be 
wanting in plants and to have been erroneously believed in at 
an earlier date. Even when de Bary and Farlow had proved its 
undoubted existence in certain ferns, and others had found it in 
certain fungi, the Basidiomycetes, and the existence of parthe- 
nogenesis among some plants and many animals could no 
longer be denied, the attempt w^as made to crush the pheno- 
mena in the Procrustean bed of the received conception of 
fertilization. The ingenious French savant Balbiani had pre- 
viously propounded the view that a certain occult and hitherto 
undiscovered fertilization took place at the seat of origin of the 
germs, in the ovaries and testes ; this fertilization was supposed 
to be in addition to the regular, recognized process, and, in cases 
of parthenogenesis, to compensate for it. So deeply rooted was 
the idea that new hfe could only arise by means of fertilization. 

Even those investigators who no longer doubted the reality of 
parthenogenesis could not immediately and completely rid 
themselves of the received view, but endeavoured to make the 
new facts harmonize wdth the old ideas. Probably the most 
interesting attempt of this kind proceeded from Hensen, 
who indeed recognized that the 'views on sexual reproduction 
held up to that time had been overthrown' by means of partheno- 
genesis, inasmuch as the fundamental proposition as to sexual 
propagation had failed, viz., that one of the two sexual cells is 
by itself incapable of development. He nevertheless believed 

^ C. Th. von Siebold, ' Wahre Parthenogenesis ' ; Leipzig, 1856. 
2 Rudolph Leuckart, ' Zur Kenntniss des Generationswechsels und der 
Parthenogenesis bei den Insekten'; Frankfurt, 1858. 


that we must ' not, on account of these isolated cases, under- 
estimate the fact that the necessity for fertihzation is predomi- 
nant, and controls, to their most secret depths, the sources of 
life in animals and plants.' (Phys. d. Zeug. p. 160.) Hensen 
takes as his starting-point the fact that, among many animals, 
e. g. bees and wasps, parthenogenetic ova give rise to male 
individuals only, while in others, namely Psyche and Solenobia 
among Lepidoptera, and Apits, Artejiiia, and Limnadia among 
Crustacea, only females are thus produced ; further, that in many 
Lepidoptera, as Liparis, single eggs possess a power of develop- 
ing without fertilization, but only into male insects, or into 
caterpillars which afterwards die, or in other cases only as far 
as some earlier or later phase of embryonic life. From this he 
concludes that we are here ' dealing with a graduated series of 
phenomena,' ' with a gradation in the powers of development 
and of reproduction, that is of qualities which can be con- 
veniently included in the term ' sexual force. ^ Hence at that 
time V. Hensen considered, if I have rightly understood him, 
that the ' sexual force,' it is true, ordinarily reaches the egg by 
fertilization, but that it may, under certain conditions and in 
varying degrees, be included in the female germ-cells alone. 
Such ova can then undergo embryonic development without 
fertilization, and, according to the amount of contained ' sexual 
force,' can pass through a longer or shorter period of develop- 
ment ; many reaching only a certain stage of segmentation, 
others the entire larval stage, while finally some may attain the 
condition of imago, with mature sexual organs. There are 
moreover various degrees of 'sexual force ' ; for Hensen con- 
siders that male offspring are produced by a smaller force 
than females. Eggs from which, without fertilization, males 
only can arise (bees), possess, in his opinion, a smaller 
'sexual force' than those which without fertilization produce 
females. This view ultimately depends on the conception 
of the life-preserving effect of fertilization, since males alone 
cannot perpetuate the species ; and hence eggs which, without 
fertilization, give rise to males, are unable to maintain the con- 
tinuity of life, and would finally result in the disappearance of 
the species, just as eggs of still smaller ' sexual force ' lead to 
the disappearance of the individual in the larval or even earlier 
embryonic stages. 


A question arising out of this view, and one which Hensen 
doubtfull}^ propounds, is 'whether the "sexual force" could 
increase to such an extent that males should become super- 
fluous,' and whether parthenogenesis, like sexual reproduction, 
could continue, not only for a limited number of generations, but 
for an unending series. 

As regards an answer to these questions Hensen was quite 
unbiassed and awaited the decision of facts ; moreover, from 
his point of view, no theoretical impossibility attended any such 
increase in the female ' sexual force.' He was, at that time, far 
nearer to the most recent views on fertilization than those 
numerous investigators who held parthenogenesis to be the 
consequence of fertilization which had taken place in earlier 
generations, and who considered that its effect could never last 
through an unlimited series of generations, but that the vitalizing 
or rejuvenating effect of fertilization must be renewed from time 
to time, or the power of reproduction would be lost. On these 
fundamental views as to the ' vitalizing of the germ by fertiliza- 
tion ' depends the reluctance of nearly all writers to recognise 
the submitted facts of a continuous and purely parthenogenetic 
reproduction, as for example in the case of the Ostracoda. It 
is certainly true that absolute proofs of the indefinite duration 
of this mode of reproduction cannot be obtained ; for unlimited 
time and innumerable generations are not within the limits of 
observation ; but who doubts whether the sexual method, with 
which we are so completely familiar, and which is for this reason 
spoken of as the usual mode of reproduction, — who doubts 
whether this can endure without limit ? And yet this assump- 
tion is as incapable of proof by appeal to experience as the other. 
It appears to be very difficult to get rid of the deeply rooted idea 
that fertilization is a vitalizing process, a ' rejuvenescence of life,' 
although we are quite unable to explain the nature of the renewal 
which is supposed to take place. The old idea of ' vital force ' 
unconsciously bears a part in this view, an idea which certainly 
does not gain any scientific justification because, as Bunge has 
rightly said, we are to-day very far from laying bare the 
deepest roots of any one of the processes of life and explaining it 
by the operation of known forces. I hardly think that we shall 
ever reach this point, but until the explanation of vital processes 
by means of the well-known chemical and physical properties 


of matter is proved to be impossible, it will, in my opinion, be 
unjustifiable for science to relinquish the attempt. The con- 
ception of vital force and the conception of fertilization as a 
renewal of life hang more closely together than we are in the 
habit of thinking. 

The facts of the transmission of hereditary tendencies from 
both parents to the child, together with the facts of partheno- 
genesis, induced me, at an early date, to look for the essence of 
fertilization, neither in the vitalization of the ^%g^ nor in the 
union of two opposed polar forces, but rather in the fusion of 
two hereditary tendencies, — in the mingling of the peculiarities 
of two individuals. The substances which come together in 
fertilization, from the male and from the female, are not funda- 
mentally different but essentially similar, differing only in 
points of secondary importance. This is what I meant by the 
statement, made shortly after the discovery of the fundamental 
phenomena of fertilization, that the two germ-cells which unite 
together, are in the proportion of one to one, that is that they 
are essentially alike. 

If this conception be valid, the above-mentioned view as to 
the extrusion of polar bodies, propounded by Minot, Balfour, 
and E. van Beneden, must be erroneous ; for a male principle 
such as their theory demands has no existence, and cannot 
therefore be expelled from the ovum. There is no male or 
female principle, but only a paternal and maternal substance. 
If, on the other hand, Minot's Gonoblastid Theory be sound, it 
follows that my view, which finds the essence of fertilization in 
the union of the different hereditary tendencies of two indivi- 
duals, must be abandoned. 

It seemed to me possible to settle the question by means of 
parthenogenesis. If parthenogenetic eggs develope without 
first expelling polar bodies, then Minot's theory, the ' compensa- 
tion theory' as O. Hertwig has recently called it, receives 
material support : if however polar bodies are formed by them, 
it is impossible that such bodies can represent the male 
principle of the ^g%. I succeeded in proving the existence of a 
polar body, first in the ovum of a parthenogenetic Daphnid, 
Polyphemus oculus, and later, in conjunction with Ischikawa, in 
the parthenogenetic eggs of various other species of Daphnids, 
and also in some of the Ostracoda and Rotifera. Blochmann 


showed the existence of a polar body in the parthenogenetic 
ova of the Aphidae, and there is now no doubt that polar bodies 
are formed in most if not in all parthenogenetic eggs. The 
' compensation theory ' must therefore be given up, and the 
question arises as to the theory which can take its place. 

Before the existence of polar bodies in parthenogenetic ova 
had been completely established, I had endeavoured to find, in 
opposition to the ' compensation theory,' another meaning in 
the polar bodies. The history of our earliest knowledge of 
the processes of nuclear division, by the work of Auerbach, 
Butschli, Flemming, and others, is well known : the exist- 
ence of most remarkable and excessively minute arrangements 
for cell-division were shown to exist in the mysterious ' chro- 
matin substance ' of the nucleus, the so-called nuclear loops, 
which are accurately divided in a longitudinal plane, the halves 
then entering the two daughter nuclei which are being formed. 
These chromatin rods acquired a new significance when E. van 
Beneden first showed that they were contained in equal numbers 
in both the male and female reproductive cells, and that they 
arrange themselves side by side, to build up the chromatin 
substance of the embryonic nucleus. Considering this and 
certain other facts, it became more and more probable that the 
chromatin rods were the essential factors in fertilization, the sub- 
stance which was contributed by the parents and fused together 
in the offspring, and which was therefore, in all probability, the 
bearer of hereditary tendencies. Strasburger, O. Hertwig, and 
V. Kolliker also gave expression to this view for which I had 
contended. We regarded the nuclear loops as that idioplasm 
which Nageli had been led, by his acute reasoning, to suggest ; 
a substance which is not fluid, but organized, which possesses 
an extremely complex structure, and is transmitted from one 
generation to another. 

But this view did not decide the question whether the ovum 
was not, after all, vitalized by fertilization. O. Hertwig was 
obviously still under the influence of this idea when in 1885 he 
maintained in the above-mentioned work, that ' the fertilizing 
substance transmits, at one and the same time, those peculiarities, 
which children inherit from their parents.' Such an explanation 
is, in a certain sense, defensible, and we may speak of a ' fer- 
tilizing substance,' in so far as the amounts of nuclear material 


which unite during fertilization seem to be necessary to determine 
the commencement of development. But this refers only to the 
restoration of a certain amount of nuclear substance, rendering its 
quantity sufficient for development, and parthenogenesis shows 
us that when the second polar body is absent this quantity can be 
supplied by a single sexual cell. In the precise meaning of the 
word, as it is ordinarily used, there is no such thing as a 
fertilizing substance, and the progress in thought from the old 
to the new doctrine of fertilization can only take place when the 
idea of such a substance in the old sense is completely aban- 
doned, and when it is recognized that fertilization has no signifi- 
cance except the union in the single offspring of the hereditary 
substance from two individuals. 

The advance which has occurred is due to Strasburger's 
writings as well as my own : the former agreed with O. Hertwig 
and me as to the essential similarity, as regards their chief 
constituents, of the two sexual cells, and as to the secondary 
nature of their differences : Strasburger in fact went so far as to 
say that all differentiations of sex were simply the means 
adapted to bring together the two cell-nuclei which were 
necessary for the sexual act. With this view I not only entirely 
agreed, but totally rejected the pre-existing dynamic theory 
of fertilization, in as much as I could not recognize the object 
of fertilization as the ' vitalization of the germ ' or the ' reju- 
venescence of vital processes,' but regarded it as simply the union 
of the different hereditary tendencies of two individuals. This union, 
which has hitherto been regarded, to some extent, as merely 
a necessary consequence, has become the important feature, while 
the ' vitalization of the germ ' by the interaction of two opposed 
sexual cells,— formerly looked upon as the essential part of the 
process,— has dechned from this high position and is regarded 
as only the means by which the process is effected. 

I was, at that time, so completely convinced that the facts 
warranted no other explanation, that I maintained that the 
nucleus of an ovum might be fertilized as fully by the nucleus 
of another ovum, — i. e. might be rendered equally capable of 
development, — as by the nucleus of a spermatozoon. The passage 
in which I advocated this view runs as follows : — ' If it were 
possible to introduce the female pronucleus of an ^g% into 
another ^^g of the same species, immediately after the transfor- 



mation 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 
would be furnished.' ^ Boveri succeeded in accompHshing this 
a few years later, although he made use of the nuclei of two 
spermatozoa instead of those of the ova. 

I also hold, in opposition to the rejuvenescence theory, that 
there is no polar antithesis, and that, in the union which is the 
essence of fertilization, the nuclear loops contribute neither 
male nor female principle, but a paternal and maternal substance, 
and that the significance of fertilization is nothing more nor less 
than a minghng of the hereditary tendencies of father and 

I. The Significance of the Process of Maturation of 
THE Germ-cells. 

The Maturation of the Ovum, 

Relying on the views set forth above, I have made the attempt 
to substitute a new explanation of the formation of polar bodies 
in the animal ovum for that which has hitherto found acceptance. 
If that substance which is expelled from the ripe ovum in the 
polar bodies be not the male principle, what can it be ? 

The cellular nature of the polar bodies has been demonstrated 
by Giard, Butschli, and O. Hertwig ; van Beneden has shown 
that they contain chromatin, and that at each of the two divi- 
sions which give rise to the two polar bodies, half of the chro- 
matosomes leave the ^gg in the nucleus of a polar body. If 
then the chromatin be the idioplasm, the material basis of here- 
dity, or, in other words, that substance which determines the 
nature and essence of the cell and its descendants, then cells of 
different kinds must contain correspondingly different varieties 
of idioplasm. Hence my theory of germ-plasm may be ex- 
pressed as follows : — The fertilized ovum contains germ-plasm 
in its nucleus, i. e. idioplasm endowed with the collective 
hereditary tendencies of the species : at each of the cell- 
divisions by means of which the ovum developes into the 
organism, this idioplasm splits into two quantitatively similar 

1 Vol. I. pp. 252, 253. 


halves in order to form the nuclei of the daughter-cells. But 
these halves are not always qualitatively alike ; they are only 
so when they are to give' rise to similar cells : when the cells 
which arise by division have a different significance in develop- 
ment, their idioplasm also differs in quality. The germ-plasm 
of the ovum is thus contimially undergoing change during ontogeny, 
inasmuch as the developmental tendencies are being split up, and 
become more and more distributed among the members of successive 
cell generations, until finally each kind of cell in the body contains 
only that developmental tendency which corresponds with its 
specific histological character. Each specific cell is thus domi- 
nated by a specific idioplasm. 

As soon as I had arrived at this conclusion, it was easy and 
indeed inevitable to refer the differences between spermatozoon 
and egg-cell to a specific idioplasm which had stamped its 
peculiarities upon each cell. But since both male and female 
germ-cells contain the substance which fuses during fertilization 
to form the segmentation nucleus, and therefore germ-plasm, I 
concluded that a part of this true germ-plasm which forms the 
nuclear substance, splits off at the first ontogenetic stage, as 
specific sperm or egg idioplasm, which controls the germ-cell 
during its growth, and confers upon it a specific histological 
character. I sought for the meaning of the cell-division which 
results in the separation of the polar bodies, in the suggestion 
that by this means the spermogenetic or ovogenetic idioplasm, 
rendered superfluous after the attainment of the specific form, 
was removed from the germ-cell, while the germ-plasm, grown 
in the mean time to a larger mass, remained behind in the cell. 
I therefore recognised in the cutting-off of the polar bodies the 
removal of histogenetic idioplasm from the germ -cells. 

While I was busy working out these interpretations, I dis- 
covered new facts which caused a modification of this view and 
led to the conclusion, which up to the present time appears to 
be sound, that the formation of polar bodies is a process for the 
reduction of the hereditary substance. 

The fact which led to this conclusion was the law of the 
number of polar bodies, — the discovery that all animal eggs which 
require fertilization expel two polar bodies, one after the other, 
while all true parthenogenetic eggs give rise to one only. Now 
the ovogenetic idioplasm cannot, at the most, occupy more 


than the first polar body ; the second must have some other 
meaning, for if I had been correct in assuming the necessity of 
the separation of the specific nucleoplasm from the egg, it 
follows that this substance must be separated as fully and com- 
pletely from the parthenogenetic as from the sexual egg. The 
second polar body must therefore possess a different meaning. 
In the fifth of the essays here collected \ I first indicated that 
this meaning is a reduction in the substance which forms the 
material basis of heredity, in that the number of the contained 
ancestral plasms are diminished by one-half during the halving 
of the nuclear substance to form the two daughter nuclei. By 
the term ancestral plasms, I referred to the separate kinds 
of germ-plasms from different ancestors which, according to 
my view, must be contained in the germ-plasm of each indi- 
vidual at the present day. If, before the introduction of sexual 
reproduction, the germ-plasm of each living being contained 
the developmental tendencies of one individual only, its structure 
would be altered by sexual reproduction ; for after fertihzation 
the different germ-plasms from two individuals would meet in 
the nucleus of the egg ; furthermore, the number of these dif- 
ferent kinds or units of germ-plasm must necessarily have been 
doubled with each succeeding generation, so long, at least, as 
they could have divided, preparatory to fertilization, without 
relinquishing the power of giving rise, collectively, to the 
whole organism, — that is, until the units had reached the mini- 
mal limits of their mass. From this point onwards sexual re- 
production could only have been rendered possible either by a 
doubling of the nuclear substance, or since this was impossible, 
by a halving of the germ-plasm of both germ-cells before each 
act of fertilization, a halving which was not only quantitative, 
but was above all a separation of the contained individual units, 
a separation of ancestral germ-plasms, or briefly of ancestral 

Hence, after the discovery of the law of the number of polar 
bodies, I interpreted the first division of the nucleus as the re- 
moval of ovogenetic idioplasm from the egg, and the second as 
a halving of the number of ancestral units contained in the 
germ-plasm. Such halving must have occurred, or the number 
of ancestral units would have been doubled. It necessarily 
^ Vol. I. pp. 257-342. 


followed from this view that the ancestral units contained in the 
spermatozoa must also have undergone a diminution by half. 
I postulated therefore a reduction of the spermatozoa by divi- 
sion, and, to my mind, there was 'no doubt' that this process 
occurred in them 'at some time and by some means \' although 
not perhaps in the same manner as in the ova. I even said from 
the very first- that 'it is quite conceivable' that this division 
might occur in a manner entirely different from that of the &gg, 
since in the former case both daughter-cells might be of similar 
size and might become spermatozoa, in which case neither of 
them would shrink and become polar bodies. 

The Maturation of the Spermatozoon. 

I have not been able to make out, by my own investigations, 
the facts which confirm the soundness of these views as to the 
spermatozoa ; my impaired eyesight, which has so often put a 
stop to microscopic investigations, has again rendered the con- 
tinuation of this research impossible. But Oscar Hertwig ' has 
recently given us an account of the development of the sperma- 
tozoa of Ascaris megalocephala, which not only proves the 
reduction of the male germ-cells by division, but also shows 
that it takes place in precisely that way, which from the first I 
had regarded as most likely. 

Since these new facts affect our conclusions with regard to 
many aspects of the process of fertilization, they are here shortly 
abstracted. They may possibly enable us to penetrate still 
more deeply into the meaning and significance of the processes 
by which the nuclei of germ-cells are reduced in size. 

Ever since Edouard van Beneden's classical researches on the 
process of fertilization, it has been well known that Ascaris 
megalocephala is one of the most favourable objects for the 
observation of the minute arrangements and changes occurring 
in the nuclei of germ-cells. The nuclear loops are not only 
relatively very large, but are also very few in number. Boveri 
was the first to show that, as regards this number, two varieties 
of the species exist, one containing two nuclear loops in the 
young germ-cells, the other containing four. O. Hertwig then 

1 Vol. I, p. 381. • 2 Vol. I. p. 385. 

^ O. Hertwig, ' Ueber Ei- und Samenbildung bei Nematoden/ Archiv f. 
mikr. Anat. 1890. 


proved, as might have been expected, that this difference in the 
number of loops in the youngest germ-cells exists also in the 
male sex. He called the variety w^hich produces two loops 
Var. univalens, and that which produces four Var. bivalens. Since 
the development of the spermatozoa in both varieties differs 
only in the number of nuclear loops which are formed, I will, 


Fig. I. 

Formation of spermatozoa in Ascm-is inegalocephala, var. bivalens (modi- 
fied from O. Hertwig). A. Primitive sperm-cells. B. Sperm-mother-cells. 
C. First 'reducing division.' D. The two daughter-cells. E. Second 
' reducing division.' F. The four grand-daughter cells, — the sperm-cells. 

in the following account, deal with only one of them, the Var. 

The formation of the spermatozoa falls into three stages ; the 
first is that of the ' primitive sperm-cells ' : these youngest male 
germ-cells then proceed to increase by means of successive 
divisions. The division of the nucleus is effected by kar3'^okinesis 
after the usual manner ; the four nuclear loops split longitudinally 
and the halves form the two daughter-nuclei. After this process 


of multiplication has lasted for a considerable time, the cells pass 
into the second stage, — that of the ' mother-cells of sperma- 
tozoa.' They cease to multiply, grow considerably, and their 
nuclei pass into the resting condition, viz. the condition of a 
nuclear network into which the loops break up. When these 
cells have reached their full size they enter upon the preparation 
for fresh divisions, which are only two in number and rapidly 
follow each other. As soon as these are over, the whole 
development is complete. It is this last stage which brings 
about the ' reducing division ' which I had predicted. The finely 
divided chromatin bodies contained in the nuclear network build 
up eight long, thin rods or threads, which afterwards shorten 
and form thicker rods, arranged by means of the pole-cor- 
puscles or centrosomata, which act in such a manner that four 
rods are turned toward one pole and four toward the other. A 
division of the nucleus and of the cell now follows resulting in 
the formation of two daughter-cells, each of which contains as 
many nuclear loops as the original sperm-cells, i. e. four. This 
division is followed immediately by another on the same plan, 
but without any intervening resting stage : the number of 
nuclear rods is therefore again halved, so that each daughter- 
cell of the second order contains but two. 

Hence the number of nuclear rods is at first increased from 
four to eight, and then by two consecutive divisions, this latter 
number is first halved and then quartered, the final result being 
a halving of the number of rods in the original sperm-cells. 

It is well known that precisely the same results are brought 
about by those divisions of the ovum which give rise to the polar 
bodies. In the egg the nuclear rods are first doubled and then, 
by two consecutive divisions, reduced to half their original 
number. In all essentials, the development of the ovum passes 
through precisely the same process as that of the spermatozoa. 
The first two stages, described by O. Hertwig, in the develop- 
ment of the spermatozoa I also find in the formation of the Qgg. 
The primitive ova correspond to the primitive sperm-cells, the 
mother-cell of the ova, or the mature full-sized ^gg, immediately 
before reduction by division, corresponds to the mother-cell of 
the spermatozoa, the only difference being that the &gg in this, 
the second stage, has, as a rule, attained its definite shape and 
size and is surrounded by its membranes, and that the two last 


divisions, which are togetherspoken of as the 'reducing divisions,' 
generally take place after the ^gg has been laid or has, at any 
rate, left the ovary. This probably explains, as I have already 
maintained, why the division is so unequal, and why all the 
daughter-cells cannot become ova, but only the largest of them, 
viz. that one which alone contains the food-material necessary 
for the building up of the embryo. 

Fig. II. 
Formation of ova in Ascaris rnegaloccphala, var. hivalens. 

In other respects the formation of the polar bodies corres- 
ponds with the two divisions of the mother-cells of sperma- 
tozoa : in both cases there are two successive cell-divisions, 
and furthermore in the ^gg both daughter-cells of the first 
generation divide again— not only the larger one, the ovum, but 
also the smaller or first polar body — for it is well known that 


the latter body generally splits into two secondary polar bodies, 
and the significance of this apparently aimless division has 
hitherto been sought in vain. But now we see that it depends 
on the persistence of a phyletic stage of development, on the 
survival of an earlier condition, in which the original egg-cells 
underwent a ' reducing division,' hke that of the spermatozoa, 
producing four cells, each of which was potentially an ovum. 

Moreover in another, and obviously a decisive point, the ' re- 
ducing divisions ' of ova and spermatozoa are in correspondence ; 
— in the manner and method of the division of the nuclear rods 
in the daughter-nuclei. The process of karyokinesis here differs 
from any other mode of nuclear division, in that there is no 
longitudinal splitting or doubling of the nuclear rods, bringing 
about a contribution from each rod in the equatorial plate to both 
daughter-nuclei ; instead of this, half the whole number of rods 
passes to one pole of the nuclear spindle, and half to the other. 
Furthermore, there is no resting-stage between the two divi- 
sions, during which the rods break up into the nuclear network, 
but the two divisions follow each other without any interval. If 
the ' reducing division,' for which I have argued, has any exist- 
ence, we must look for it here ; for, so far as proofs can be 
afforded by observation, they are forthcoming. The number 
of nuclear rods is reduced to half, and hence the mass of the 
nuclear substance is certainly halved. And if we must concede 
that the rods in a nucleus are not absolutely alike, but are 
derived from the differing germ-plasms of various ancestors 
(viz. that the rods consist of such different kinds of germ- 
plasm), it follows that a reduction of the ancestral germ-plasms 
is admitted. 

The new facts discovered by O. Hertwig leave only one point 
obscure. We see indeed that, in the case of the spermatozoon 
as in that of the ovum, the nuclear rods are reduced to half, but 
we ask in vain why two successive divisions are necessary to 
bring about this reduction, when it seems that a single one 
would suffice. I had formerly concluded that since partheno- 
genetic eggs expel only one polar body, instead of the two which 
separate from all ova requiring fertilization, the first division 
must have a different significance from the second. I regarded 
the second division alone as the ' reducing division,' and this was 
a perfectly sound and logical conclusion, so long as it remained 


unknown that the mother-cells of ova contain twice as many 
nuclear rods as existed in the primitive egg-cells. Until this was 
known, the ' reducing division ' was only required to effect a 
halving of the nuclear substance, and for this purpose one 
division would be sufficient. We now know that a second 
division is rendered necessary because the number of the rods 
is doubled before the process of reduction has begun. The 
object served by this doubling remains an obscure point 
upon which even the spermatogenesis of Ascaris does not at 
present enlighten us. My previous interpretation of the first 
polar body as the removal of ovogenetic nucleoplasm from the 
^%g must fall to the ground : about this there is no possible 
doubt, but how can we better explain the necessity for two 
divisions? Why should the nuclear substance be doubled, 
only to be halved again ? O. Hertwig has also propounded 
this question, but so far without being able to supply an answer. 
He hopes that a more accurate study of the manner and method 
of the arrangement of the chromatin elements in the two succes- 
sive divisions will ultimately lead to a deeper knowledge of the 
essence of the whole process of maturation. I also hope the 
same. The processes which bring about the doubling of the 
chromatin rods in the resting nuclei of ova and sperm-mother- 
cells, contain, without doubt, the key to an understanding of the 
necessity for this increase in number, which at present appears 
to be so mj^sterious and superfluous. 

Whether unaided observation will ever succeed in making 
clear the accessory processes, in other words, whether morpho- 
logical events can be followed in minute detail so far that we 
can wrest from them the secret of their meaning, we cannot 
say. Without some guiding idea, it is scarcely possible that the 
observations of investigators could be directed to the most 
^sential part of the process, especially in this case, where 
differences of substance are probably present — differences 
which might be invisible, but are perhaps capable of being 
inferred by processes of reasoning. 

Thus it may be possible, on the basis of Hertwig's observa- 
tions, to penetrate somewhat deeper into the meaning of the 
remarkable processes which attend the ' reducing divisions,' if 
only the subject be attacked from the point of view of the theory 
of ancestral germ-plasms. 


The Double Division of the Nuclear Substance in the Formation 
of Germ-cells. 

With regard to the egg, the following question can be for- 
mulated — What is the meaning of the first division in the 
formation of polar bodies, since the second alone would suffice 
to halve the nuclear substance? With regard to the sperm- 
mother-cell however the question must run, — why should 
division take place twice, when its single occurrence would have 
sufficed to reduce the nuclear loops by one half? The simplest 
answer to these two questions is to be found in the fact that the 
number of nuclear rods is doubled at the beginning of the 
reducing process, and must therefore be quartered if a diminution 
to one half the normal number be the ultimate necessity. This 
leads us to enquire why the preliminary doubling of the nuclear 
rods is necessary. 

Regarding spermatogenesis only, it might be maintained that 
here we are simply dealing with a process for increasing the 
number of spermatozoa as far as possible, but if we attempt 
hus to explain a fourfold instead of a twofold increase, com- 
parison with the egg-mother-cell, producing four descendants 
of which one only undergoes development, renders any further 
discussion of this idea superfluous. 

In attempting to explain the phenomenon I start from the 
conception which led me to the idea of a ' reducing division,' 
i. e. the building up of the germ-plasm, that is the active sub- 
stance of the nuclear rods, from innumerable ancestral units 
As I explained on the first statement of this idea, it is a view 
which is necessarily suggested if we accept certain premisses, 
the chief of which is, that the hereditary substance from the two 
parents does not altogether become one during the fusion 
which occurs at fertihzation, but that each retains a certain 
independence. This agrees with observed facts in so far that, 
as a result of fertilization, the paternal and maternal rods come 
to lie close to one another in the same nucleus, but undergo no 
true fusion into a single mass. If we assume that this remains 
true during the whole ontogeny, we can only suppose that half the 
nuclear rods of every cell are paternal and half maternal and that 
both these simultaneously influence the cell. We do not yet 
understand how this takes place, and must for the present dis- 


miss the question ; we do know however that it is an actual fact. 
We know that the paternal no less than the maternal nuclear 
rods of the fertilized ovum possess the developmental tendencies 
of the species, and that either of them alone, that is in the 
absence of the other, are present in sufficient numbers to 
regulate the development of the ^g%^ each set containing all 
that is necessary to originate a mature individual of the species. 
And the same fact holds good for each successive stage of 
embryogeny, with just this difference, that the potentiality of 
stages to come, and not of those passed through, is contained in 
the embryonic cells. Furthermore every cell contains the 
separate paternal and maternal nuclear rods, and either set is 
capable of producing all the subsequent stages. This remains 
true throughout the whole course of development, from the 
fertilized ovum which produces the parent, to the male and 
female germ-cells of the offspring. No real fusion of the two 
nuclear substances into a single mass ever takes place, so that 
the corresponding predispositions of the two parents are ar- 
ranged together, but the hereditary substance contributed by the 
father remains separate from that contributed by the mother. 
These substances are made up of units of which each contains 
those collective predispositions which are indispensable for the 
building up of an individual, but each possesses an individual 
character, i. e. they are not entirely alike. I have called such 
units ancestral plasms, and I conceive that they are con- 
tained, in larger or smaller number, in the chromatin of the 
mature germ-cells of living organisms, viz. that the parental 
nuclear rods are made up of a certain number of these. 

I have thus briefly called to mind the manner in which I 
conceive that many such ancestral plasms are collected together 
in a single nuclear mass, and the consequent necessity for a 
' reducing division.' It is not perhaps superfluous to return to 
this subject once more. Each of the parental germ-plasms which, 
at the phyletic origin of sexual reproduction, for the first time 
fused together in the segmentation nucleus of the offspring 
must have contained the potentiality of one individual only, 
and must have been, in a certain sense, completely homogeneous. 
Naturally, such a statement by no means excludes the existence 
of a very complicated structure, in which a number of different 
predispositions, or of different parts, are collected together, but 


it limits each such predisposition to being present only once, and 
in only one variety. I conceive of this primitive germ-plasm, as 
of one single ancestral unit of an existing species, only perhaps 
relatively larger, its separate predispositions not having been 
yet reduced to the present minimum. 

All this however was altered in the germ-cells of the first 
sexually produced individual, in which the nuclear rods of the 
two parents came together, and together composed the heredi- 
tary substance of the child. If now, as has been argued above, 
the paternal and maternal hereditary substances did not fuse 
but only arranged themselves side by side, there will be found, 
in the germ-cells of the child, two substances similar as regards 
the species but dissimilar as regards the individual. If the 
mass of nuclear substance cannot be increased, both kinds of 
nuclear substance must be reduced by one half. If we imagine 
the nuclear material of one such germ-cell to consist of a single 
thread, one half of it would be made up of paternal and the 
other half of maternal germ-plasm. 

I call to mind the diagram by which, in an earlier essay ^, I 
endeavoured to make intelligible how the number of ancestral 
plasms of various kinds which meet together in the germ- 
plasm are doubled in each successive generation, and how, 
in the formation of the germ-cells of each generation, the 
germ-plasms must be reduced to half their size, or their 
united mass would be doubled in every generation. But in 
time a limit to this continuous diminution of the ancestral 
plasms must have been set, and this would occur when the 
amount of substance necessary to contain all the predispositions 
of the individual had reached its minimum. Obviously these 
units cannot become infinitely minute ; however small they 
may be they must always retain a certain size. This follows 
from the extremely complicated structure which we must with- 
out any doubt ascribe to them. These units which make up the 
germ-plasm of living animals I have called ancestral plasms, 
but my views about them have been misunderstood, and I have 
been treated as though I had applied the term to the ultimate 
biological units of idioplasm. Nothing was further from my 
mind : I look upon the single ancestral plasms as extremely 
complex, and built up of countless biological units. I have 
^ Vol. I. p. 369. 


retained the conception in its original form, as it is indispensable 
for the understanding of the ' reducing division.' When I main- 
tained that the units of the germ-plasm are indivisible, I did not 
refer to mechanical divisibility, but to that division which a 
unit cannot undergo without losing its original character. If we 
divide a dog into two parts, neither part is a dog ; and so, accord- 
ing to my views, half an ancestral plasm is not an ancestral 
plasm, is not an hereditary unit capable of calling forth a com- 
plete individual ; or, to express this with reference to its minute 
structure, a half would no longer contain all the predispositions 
necessary for the whole individual. The number of these units 
would be doubled as the result of each fresh fertilization if the 
preliminary halving did not occur. Hence the necessity for 
such halving, which I have attem.pted to render clear by the 
foregoing train of thought. 

Taking my stand upon this, I argued that a ' reducing division' 
of the nuclear material takes place before fertilization in both 
germ-cells, — that is a division contrary to the ordinarj'^ method, in 
that it does not divide the collective ancestral plasms in two equal 
and similar groups between the daughter-nuclei, as in ' equal 
divisions^,' but halves their number so that one daughter 
nucleus receives one set and the other another set of ancestral 
plasms. In the ovum I recognised the necessary ' reducing 
division ' in the formation of the second polar body, for it had 
then been shown by the careful observations of van Beneden and 
Carnoy upon Ascaris megalocephala that two out of the four 
nuclear rods pass into the second polar body while the other 
two compose the nucleus of the ovum. 

The idea of a ' reducing division,' as I then conceived it, seems 
to have met with but little acceptance among the German 
biologists. Except Platner and recently O. Hertwig and 
Henking, I know of no one who has accepted it. The first- 
named employed the expression, but without indicating 
whether he used it in my sense. This cannot be taken for 
granted, as the simple halving of the chromatin mass may 
be so designated. All that we can see is a reduction in 
mass, . and the discoveries of Platner and Hertwig do not 
directly teach us more than that in the division of the mother- 

^ For a further account of these methods of division see Vol. I. pp. 


cell the number of nuclear rods, and therefore the mass of 
hereditary substance, is reduced to one-half. O. Hertwig 
appears to accept my views as set forth above : at any rate he 
thinks that I am ' on the right road in regarding the process by 
which the second polar body is formed as a reducing process, 
by means of which an amount of germ-plasm is removed, equal 
to that which is afterwards conferred by the nucleus of the 
spermatozoon.' Furthermore, his own account of the significance 
of the process seems to agree with mine when he says — ' In 
this very simple way it is brought about that the fusion of the 
two nuclei resulting from the sexual act, — a union of the chro- 
matin substance and the chromatin elements, — does not form 
double the mass which is normal for the species concerned.' 

When, however, we remember that O. Hertwig rejects the 
theory of ancestral plasms, and takes the antagonistic view of a 
complete mingling of maternal and paternal germ-plasm,we must 
be convinced that the reducing process, in the sense in which 
I have spoken of it, has no existence for Hertwig, and that, 
from his standpoint, the only conceivable theory is that of 
a simple reduction of mass. And yet obviously such is not his 
view, for he speaks of chromatin elements ; and hence the 
question arises as to the kind of elements which these can be if 
they are not ancestral plasms. It seems to me that the reducing 
process only acquires a meaning when taken in connection with 
the supposition of ancestral plasms, unless indeed it is merely 
a matter of reduction of mass. But it is most improbable 
that a mere reduction in mass is the object of this very remark- 
able double division of the nuclear substance, which is never 
again repeated in the whole developmental history of the 
animal. First, the mass of nuclear substance is doubled, and 
then reduced by two divisions to one-half its original bulk. 
Obviously it would have been simpler if this process had been 
omitted, and if the nuclear substance of the ^gg and sperm- 
mother-cell had, during its growth, merely stopped short at 
the requisite size. It may perhaps be objected that the growth 
of the ovum and sperm-mother-cell and their histological 
structure necessitate such a mass of nuclear substance. We 
know little or nothing about the relationship of the mass of 
nuclear matter to the mass of the cell-body, but it must 
be doubted whether in this case the relation is fixed, because 


as a rule ova and spermatozoa differ so enormously in size, 
and above all because the ova of different species vary so 
greatly in this respect. Moreover, Boveri has shown us 
that in one and the same species two otherwise indistinguish- 
able germ- cells exist, one of which contains twice as many 
nuclear rods as the other, and therefore as far as we can tell 
twice the amount of nuclear substance. Hence the ' reducing 
division ' cannot be a mere division of mass. 

There remain for consideration the ' chromatin elements ' of 
O. Hertwig. What are these elements ? Are they the smallest 
possible portions of living matter, something like the pangenes 
of de Vries ? This distinguished botanist in his highly sug- 
gestive and thoughtful writings has developed the idea that the 
nuclear substance of the fertilized ovum is composed of count- 
less very minute particles, called by him pangenes. He thus 
recalls Darwin's pangenesis, with which his theory has some- 
thing in common. These pangenes however do not, like the 
gemmules of Darwin, give rise to cells, but they are the bearers 
of the various qualities of cells. If we now assume with de 
Vries that the nuclear substance of germ-cells consists of innu- 
merable kinds of such pangenes, we may regard these either as 
uniformly mixed together without any kind of arrangement, 
or as arranged in a definite order. In the first case, each 
division (reduction) of the mass would only result in a diminu- 
tion, and the components of both halves would remain the 
same : the various kinds of ' chromatin elements ' would not by 
this means be reduced to half, but all the elements would be 
contained in each portion. But if these pangenes were arranged 
in a regular order in the germ-plasm, and if with Hertwig we 
designate the groups of these as predispositions, without ex- 
pressing in any way how such predispositions can be conceived, 
it follows that a halving of the mass of germ-plasm or nuclear 
substance would give rise to two halves, neither of which 
would contain all the predispositions necessary for the con- 
struction of an individual, although both might contain many 
double predispositions. Hertwig imagines that the predisposi- 
tions which according to his view (loc. cit. p. no) are present 
in the germ-plasm of the paternal and maternal germ-cells, 
mingle together, and de Vries has also assumed this. Hertwig 
states that 'it is not improbable,' that in the complete union 


and mingling of the parent nuclear substances presupposed by 
him, 'similar predispositions would arrange themselves closer 
to one another than dissimilar ones, and from the similar but 
varying paternal and maternal predispositions an intermediate 
form might arise by mutual influence.' I have printed the 
words ' intermediate form ' in italics because it appears that so 
much depends upon it ; for obviously the intermediate form of 
predisposition must be looked upon as one and no longer as 
two separate predispositions. Hence, according to Hertwig, 
the fusion of two parental germ-plasms produces an inter- 
mediate form of germ-plasm in which each predisposition is not 
doubled, but remains single. Furthermore, this germ-plasm 
could grow, and could be represented by a larger or smaller 
mass, but it is impossible that it could be halved without losing 
its character as germ-plasm, except it were first doubled 
in size, and all its predispositions were doubled and sym- 
metrically arranged on each side of the plane of division like 
the antimeres in a bilaterally symmetrical animal. But even in 
this last case a ' reducing division,' that is a putting on one side 
of half the number of the corresponding but individually distinct 
chromatin elements, is impossible because both halves would 
contam precisely similar predispositions. O. Hertwig deceives 
himself in believing that he can assume a halving of the number 
of chromatin elements while his conception of the composition 
of the germ-plasm only admits of a halving of mass. In his 
germ-plasm, made by the fusion of paternal and maternal 
predispositions, there are no differing predispositions of one 
and the same part or organ : the parental differences have, 
according to his view, neutralized each other, and each predis- 
position is present as a single intermediate variety. Whence 
comes the necessity or the possibility of any reduction ? What 
are the units which are to be reduced in number ? 

It is clear that we cannot avoid the assumption of higher 
units of germ-plasm, each one of which contains, collected 
together, the varied predispositions of the species, whether called 
by my term ancestral plasm, or by any other name. I shall 
attempt to explain elsewhere that this conception is not only in- 
dispensable for our understanding of the ' reducing division,' but 
that it is even rendered necessary by the phenomena of heredity. 
At present I do not propose to do more than show that the 



idea of a ' reducing division ' presupposes the multiplication of 
the equivalent but individually characterized units in the germ- 
plasm of the fertilized ^gg, and that, without this presupposition, 
the ' reducing division ' is entirely devoid of meaning. 

If we may now feel greater certainty than ever before in 
regarding the double division of the egg and sperm-mother-cell 
as a ' reducing division/ we gain at the same time further proofs 
that the germ-plasm is composed'of ancestral plasms, that is of 
hereditary units of a higher order, each one of which, if it 
alone dominated the ovum, would be capable of guiding the 
whole ontogeny and of producing a complete individual of 
the species. 

Before I attempt to show how these fundamental views throw 
new light on the discoveries of recent years, I will say a few 
words on the independence of the maternal and paternal 

According to the views which I have expressed, the nuclear 
rods are built up of a series of ancestral plasms, which are not 
intimately connected together, but so far as mere position is 
concerned, are arranged next to one another. A rod does 
not represent a kind of ' individuality ' (Boveri), by which term 
there is certainly implied the existence gf an internal relation- 
ship of parts to one another, according to certain laws, a 
relationship which prevents any mechanical division of the 
whole into equivalent parts capable of living and performing, 
their functions. 'Individualities' as defined above are to be 
found in my ancestral plasms, or as I propose to call them 
shortly, the ^Ids ' ^ These cannot be divided without losing the 
power of building up an individual, while, according to my 
view, the series of ancestral plasms which compose the rods 
or ^ Idmits'^ might quite conceivably be removed bodily, by 
division occurring at any spot, and replaced by others, without 
any loss of the essential force which controls the ontogeny 
of the species in question. The only result of such replace- 
ment would be to cause a more or less marked alteration in 

^ The expressions /fi? and Idant serve to recall Nageli's 'idioplasm,' 
from which they are derived. I think it is very necessary to substitute 
some short expressions for the clumsy ' ancestral plasms ' and ' chromato- 
somes,' or the frequently inappropriate ' nuclear rods ' and ' nuclear 

^ See the preceding note. 


the individuality of the being which is produced by this 

There is therefore, in my opinion, nothing inadmissible in the 
idea of the breaking up of the chromatin rods or idants, during 
each nuclear resting stage, if only the single ids remain un- 
changed ; but certain facts in heredity, to be mentioned imme- 
diately, support the view that the specific hereditary substance 
from one or both parents can be contained in the germ-cells 
of the child, and this presupposes that it is at least possible and 
perhaps the rule, for the order and arrangement of the ids in 
the idants to remain unchanged from the germ-cells of the 
parent to those of the offspring. I would, then, assume that, at 
least on the way from germ-cell to germ-cell, the views of van 
Beneden and Boveri are upon the whole correct, viz. that the 
chromatosomes (idants) only apparently break up during the 
nuclear resting stage, but in reality persist. I imagine that, after 
the period of the resting stage, they are generally composed of 
the same ids, for the most part arranged in series similar to those 
which existed before the preceding nuclear division. We are 
already acquainted with such astonishingly delicate mechanical 
arrangements in cells, that the existence of special provision 
for maintaining the original arrangement of the rod elements 
(ids) might be looked upon as very far from an impossibility. 
Even if direct observation should fail to answer this question in 
the future, some certainty might be reached by those indirect 
means which often lead us to a final decision in such excessively 
minute biological questions — viz. the means provided by an 
examination of the facts of heredity. Even now there is, I think, 
one such fact, supporting the idea of a continuity of the idants ; 
I mean the frequently observed fact that a child may pre- 
dominantly or even exclusively resemble one of its parents alone. 
If the elements of the chromatin rods, i. e. the ancestral plasms, 
were irregularly mingled together in each nuclear resting stage, 
to be rearranged at random in the idants, it would scarcely 
ever happen that the scattered ids would come together in a 
series like that which existed in the original paternal or 
maternal idants. The individual stamp of a nuclear rod (idant) 
must entirely depend upon its construction out of particular ids. 
Nevertheless, we must not regard this constitution as for ever 
unchangeable. The universally observed change of indivi- 

K 2 


duality which takes place in the course of generations, and the 
fact that one and the same individuality is never twice repeated, 
suggest to my mind an occasional change in the arrangement 
of ids within the idants — a change which, if it does not occur at 
every opportunity afforded by reconstruction, will at any rate 
take place in the course of generations. 

I will not now enter more fully into the foundation of such 
protracted, and, to a certain extent, secular changes of the idants, 
but will turn at once to the problem propounded above as to the 
meaning and significance of the fact, which has been firmly 
established by the researches of O. Hertwig upon Ascaris, that 
a double division of nucleus and cell is rendered necessary by 
that reduction of idioplasmic elements which is required by my 
theory in both ovum and sperm-cell ; in other words, to explain 
the fact that the number of idants is doubled before being halved. 

Inasmuch as two primary polar bodies are formed, so far 
as we know, by all eggs which require fertilization, we may con- 
clude that the significance of the double division of the sperm- 
mother-cell of Ascaris megalocephala is typical and far- 
reaching, rather than merely accessory or secondary. 

If, as I have shown above, the significance of the original 
increase of the chromatin rods to double their number does 
not lie in the needs of the growing ovum or spermatozoon, 
it must be sought for in some other direction. // lies, as 
I believe, in the attempt to bring about as intimate a mixture as 
possible of the hereditary units of both father and mother'^. 

If the first object of sexual reproduction is to combine the 
hereditary tendencies of two individuals, and not in a mere 
transitory manner (viz. in the single individual proceeding 
from one act of fertilization), but permanently, because such 
a combination affects also the germ-cells of each single indi- 
vidual, and therefore of all succeeding generations,— if this 
be its object, then we must admit that it is mechanically pos- 
sible for a combination of paternal and maternal idants to 
exist side by side in the mature germ-cells of the individual. 
This is obviously conceded if the ' reducing division ' makes 

^ Histologists may perhaps object that the doubling of the idants 
simpl}'^ depends upon a postponement of the normal longitudinal fission 
until the time at which the spindle is formed. This is probably correct, 
but it only explains the existence of the doubling and not its significance. 


no difference between the maternal and paternal nuclear rods, 
but leads to a halving of their number in such a manner 
that the most varied combinations can arise ; so that if ^ + ^, 
and c -\- d represent four rods, there will be found in the 
mature germ-cell not only the paternal group a -^ b and the 
maternal c -^ d, but also the combination a -^ c and b -\- d or 
a + d and b + c, that is combinations of any paternal with any 
maternal element. 

Now it is clear that only very few distinct combinations 
can be brought about in this way, — in the above-mentioned 
case of four rods, only six combinations. But if, as actually 
happens, each of the rods is doubled before their number 
is halved, there are a greater number of possible combinations, 
viz. in the above case, ten. Hence an individual of such a 
species could produce ten kinds of eggs or spermatozoa with 
differing hereditary tendencies. At the fertilization of one 
of these eggs by a spermatozoon of another individual of the 
same species, two different idants would meet each other. 
Each parent produces ten different kinds of germ-cells, hence 
as many different children can proceed from such a union, 
as there are possible combinations between the ten kinds of 
spermatozoa of the father and the ten kinds of ova of the 
mother, namely ten times ten or a hundred. I therefore believe 
that the significance of the longitudinal splitting of the idants, 
and the consequent doubling of their number, is an increase in 
the number of possible combinations. 

It may be doubted whether the increase which is thus 
rendered possible is sufficient to explain certain phenomena 
of heredity. So far as our knowledge extends, it has never 
happened that two children of the same family born succes- 
sively have had that resemblance to each other which is 
familiar in the case of identical twins. Precisely similar germ- 
plasm never seems to be twice formed in the unions of the 
same parents ; it only occurs in those exceptional cases in 
which a fertilized ovum produces two children, when the 
germ-plasm which gives to both of them proceeds from a 
single Qgg and a single spermatozoon. Now a hundred different 
combinations of germ-plasms can occur under the given con- 
ditions, while a human pair can scarcely produce more than 
thirty children : but if only ten were born, one of the hundred 


possible combinations might repeat itself. From this point 
of view, it might therefore be doubted whether the doubling 
of the idants in the germ-mother-cells, together with the suc- 
ceeding two ' reducing divisions,' are sufficient to explain the 
fact that identical children only appear in the form of twins 
developed from a single ovum. 

It may however be urged that the assumption of only four 
idants may not hold for the human species, and that in such 
animals as Ascaris megalocephala bivaleits, which undoubtedly 
possess only four idants, we cannot appreciate the phenomena 
of heredity when applied to the minutest individual differences, 
as we can in the case of man. It is quite conceivable that 
many fertiHzed ova of this species of Ascaris contain precisely 
the same kind of germ-plasm, that is the same combination 
of ids ; we do not however know that this is the case. 
We are unfortunately ignorant of the number of idants which 
is typical for man, and can only assert that it is probably 
higher than four. But the number of possible combinations 
increases very rapidly with an increase in the number of 
idants. Certain Mollusca, as Carinaria and Phyllirhoe, possess 
thirty-two idants, and in Crustacea the number is even higher. 
Eight idants, without doubling, would render possible seventy 
combinations, doubled, they would produce 266 : similarly, 
without and with doubling twelve idants would yield 924 and 
8074 combinations respectively ; sixteen would yield 12,870 
and 258,570 ; twenty would yield 184,756 and 8,533,606. With 
thirty-two idants doubling increases the number of combina- 
tions about 500 fold ^. 

If we now remember that an equal number of idants from 
each parent meet together during fertilization, and that each 
of the parental groups of idants represents only one of the 
numerous combinations which are possible for the species, 
it is evident that the number of possible variations of germ- 
plasm which a single pair is capable of producing must be 
extremely great, for it is a number obtained by multiplying 
together the maternal and paternal number of combinations. 
Thus twelve idants yield 8074 x 8074 variations. Although 
even this large number of combinations does not exclude the 

^ For these figures I am indebted to the kindness of my mathematical 
friend, Professor Liiroth of Freiburg im Breisgau. 


possibility of a repetition (two or more times) of the same 
combination, and the further possibihty of the development of 
those very germ-cells which contain identical germ-plasm — 
the probability of such an occurrence is so excessively remote 
that it may be considered practically non-existent, and we 
have no reason for wondering that identical individuals have 
never been observed among the children successively born 
in any human family. 

To my mind the doubUng of the idants before the ' reducing 
division ' possesses this very significance : — it renders possible 
an almost infinite number of different kinds of germ-plasm, 
so that every individual must be different from all the rest. 
And the meaning of this endless variety is to afford the material 
for the operation of natural selection. 

It might perhaps be objected that sufficient variety could 
have been attained without the doubling of the rods, and that, 
although the difference between the numbers of combinations 
produced with and without doubling is certainly very con- 
siderable, the number of rods would have been large enough 
without increase, since, as a matter of fact, the number of 
descendants developed is always smaller than the number 
of possible combinations. Eight idants without doubling 
give seventy combinations ; these multiplied by the seventy 
combinations of the other parent yield 4900 varieties of 
germ-plasm in the fertilized ova, and potentially an equal 
number of different offspring. We might suppose that this 
number would suffice in all cases ; for when the germ-cells are 
far more numerous (many animals producing 100,000 or even 
upwards of 1,000,000 ova, not to mention spermatozoa) only 
a very small percentage can enter upon development, and 
of these but very few can arrive at maturity and reproduce 
themselves. It would be sufficient, we might think, if there 
were only a few more thousand combinations of germ-plasm 
than of individuals which attain maturity. 

There is, however, much to be said on the other side. If 
we are not able to determine by calculation the number of 
differing individuals which are necessary in order that the 
development of the species may be guided by natural selec- 
tion, we can scarcely fail to recognize that it is only by the 
widest possible choice that the best possible adaptation of 


all parts and organs can be ensured in every case. The 
extraordinary superfluity of individuals in each generation is 
indispensable for that intense selective process which must 
have operated without ceasing if it is to afford the explanation 
of universal adaptation. And if among the thousands of germs, 
which sooner or later succumb in the struggle for existence, 
there were alwa3's a hundred which contained the same com- 
bination of individual characters, // is clear that this number 
would not count for more than one, as material for natural 
selection. It is just because each fertilized germ, and the 
individual arising from it, are different from others as regards 
the combination of characters, that the completeness of adapta- 
tion is rendered possible. It follows from this arrangement 
that the highest possible number of combinations of germ- 
plasm are offered for the operation of natural selection. 

It must furthermore be borne in mind that the full number 
of possible combinations, which is mathematically calculable, 
is, in practice, very far from being attained. We must assume 
in the calculation that the nuclear rods possess a limitless 
power of combination ; but this is neither proved, nor is it 
probable. We are on the safe side in assuming that certain 
combinations are formed more readily than others, and are 
for this reason of more frequent occurrence. And it must 
not be forgotten that identical ancestral units (ids) and identical 
idants may be present in the germ-plasm. Widely different ids 
are not contained in every individual of a species, and perhaps 
never occur in the same individual. In many cases the two 
parents are in some degree blood relations, and would con- 
tain similar or similarly composed idants. Although direct 
observation can tell us nothing on this point, it can still be 
shown that identical idants may be found in one and the same 
nucleus. This is proved by the doubling of the rods which 
takes place before the ' reducing division,' and it can be inferred 
with equal certainty from other conditions. 

The two idants which arise in the mother-cell of the ovum, 
by the longitudinal splitting of a single one, must contain similar 
combinations of ancestral units. If this were not so, it would 
follow that each of the two daughter nuclear rods would contain 
different ids, and hence the number of ids in each single idant 
would necessarily be diminished by half. But this cannot be 


the case, or the two successive 'reducing divisions' would lessen 
the total number of ids, in each germ-cell, to one quarter. Two 
idants are the normal number in Ascaris in. univalens, and they 
are increased to four, by longitudinal fission : a single idant is 
contained in each mature spermatozoon or ovum which is 
formed by the two successive 'reducing divisions.' Hence 

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o ^ ^ 

(h b r 

Fig. III. 













Diagram showing the behaviour of the idants in the various stages of 
the development of the germ-cells in Ascaris megalocephala, var. univalens. 

A shows the actual behaviour of the idants, the final result {IV) of 
which is a halving of the number of ids present in the first stage (/). 
B shows that the arrangement of ids as a double row within the idants 
would cause the final number ilV) to be a quarter of that present in the 
first stage (7). Each of the four groups of figures in both^ and 5 repre- 
sents the idants of a single cell of the corresponding stage. 

these mature germ-cells must contain half the total number of 
ids contained in the two idants of the original germ-cell. If 
this be so it is clear that the ids in the mother-idants are 
doubled in number by longitudinal fission. The small letters 
«, ^, c, &c. in the diagrammatic figure III, represent the ids 
which compose the idants. The numbers I-IV. represent the 
idants of each of the four stages,— the primitive germ-cells, 


the mother-cells of the first and second order, and the germ- 
cells. The series A represents eight ids in each of the 
two idants of a primitive germ-cell, arranged as a single 
row; whereas in series B they form two rows. In A the 
idants of stage / give rise, by longitudinal fission, to the four 
idants of stage //, that is to two pairs of identical idants : in series 
B the two original idants similarly produce the four idants 
of stage //, each of which is different from the others and 
contains only four ids. In consequence of this, in series B, the 
two successive * reducing divisions ' diminish the total number 
of ids in the cell, first (stage ///) from 16 to 8, and then 
(stage IV) from 8 to 4 — i. e. to one quarter the normal number 
of ids ; in series A, on the other hand, the corresponding 
divisions lead to ^hat halving of the normal number of ids 
which is in accordance with theory — i. e. from the 16 of stage / 
to the 8 of stage IV. 

It should be regarded as certain that many identical ancestral 
units may be present in the germ-plasm of a germ-cell, and that 
identical nuclear rods may exist side by side. Furthermore, 
during fertilization, as has been mentioned above, identical 
nuclear rods from the two parents must meet together, the 
frequency of this depending upon the amount of interbreeding 
(using the term in its widest sense) that has occurred, or in 
other words upon the limit set to the number of individuals in 
any given area, and upon the restriction in the number of 
ancestors of the species. Such considerations enable us to 
understand why nature has provided such superabundant 
variations in the germ-plasm of the reproductive cells of a 
single individual. It is the same with the more obvious prodi- 
gality that she lavishes in the millions of germ-cells brought 
forth by every iudWidudilAscaris or sturgeon. We now know that 
this apparent waste is necessary in order to ensure that, on the 
average, at least one or two germs may reach maturity, and 
that thus the species may be maintained. 

Other Types of Maturation of Germ-Cells. 

I would here repeat that, before O. Hertwig, Platner had 
shown that an entirely similar process occurs in the double 
' reducing division ' of the mother-cells of the spermatozoon in 
both the butterfly and the snail. He observed the original 


•doubling of the idants (chromatosomes) and their subsequent 
reduction to half. Furthermore, the observations of Flemming 
on the formation of spermatozoa in the salamander prove that 
there is an initial increase of the nuclear loops to double the 
normal number. These facts enable us to recognize a relation- 
ship, which Hertwig has already propounded in his account of 
the type of ' reducing division ' met with in Ascaris. Platner 
had previously recognized the homology between the formation 
of spermatozoa and of ova, between the two divisions of the 
sperm-mother-cell and the formation of the two polar bodies. 
Inasmuch as these homologies have been proved to exist in a 
worm, in insects, and in a vertebrate, and since also that 
double division which leads to the extrusion of the two primary 
polar bodies is certainly a character common to the Metazoa, 
we may well believe that we are dealing with a process of 
general significance, and one which is repeated during the 
formation of the sexual cells of, at any rate, all the higher 
Metazoa, in essentially the same way. 

Hence, after writing the remarks which appear above, I was 
much astonished by Henking's pamphlet on the formation of 
ova and spermatozoa in an insect, Pyrrhocoris apterus, in which 
the process is described as following an entirely different plan. 
The observations are clearly exact and trustworthy, and if the 
author's explanation be valid, it is impossible to attach to the 
processes of maturation in this insect a meaning similar to that 
found in the other animals which have been studied. I believe, 
however, that Henking's interpretation is erroneous on one 
point, and that the apparently profound differences can be 
reconciled, in fact that they are beautifully adapted to make 
clear the essential parts of the process. 

The difference between the formation of spermatozoa in 
Pyrrhocoris and Ascaris depends upon the fact that, in the 
former, there is no doubling of the idants before the first divi- 
sion of the sperm-mother-cell, yet the first division takes place 
as it does in Ascaris, so that the existing number (24) of idants 
is halved, twelve passing to each daughter-nucleus. The latter 
then enters upon the second division in the usual manner, each 
of the twelve idants splitting longitudinally, and their halves 
passing into the grand-daughter-nuclei. These last grand- 
daughter-cells constitute the sperm-cells, and the final result of 


the process is the same as in other cases ; for the mature 
germ-cells contain only half the number of idants which are 
normally found in the species. 

Henking interprets his corresponding observations upon the 
development of the ova, in the following manner : — The first 
division of the mother-cell is the ' reducing division' suggested 
by me, for this alone reduces the idants to half their normal 
number : the second division is that which I have called the 
' equal division,' i. e. the means by which a number of ids, equal 
to that present before this division commences, passes into each 
daughter-nucleus ; and this is rendered possible because the 
longitudinal splitting of the idants depends upon a doubling 
of the ids by division. 

If this explanation be valid, the interpretation offered above 
of the doubling of the idants in the mother-cells oiAscaris must 
fail, and I doubt whether anj?- other feasible explanation is to be 
found. Henking attempts to reconcile the discrepancy between 
the two observations by altogether doubting the doubling of the 
nuclear rods oiAscaris. I have, however, convinced myself, by 
an examination of the preparations of my pupil, Herr Arnold 
Spuler, that the doubling cannot be denied. Furthermore, it 
was this very process which first afforded an explanation of 
the double division of the mother-cells. Why then should 
there be this universal double division of which we are so com- 
pletely assured by the general occurrence of the two polar 
bodies of the ovum ? Regarding spermatogenesis only, we 
might perhaps be inclined to be satisfied with the answer that 
the number of sperm-cells must be four times that of the 
mother-cells. But, as I have indicated above, the mere increase 
of the spermatozoa might be brought about, and to any extent, 
by additional division of the original sperm-cells; and when 
we remember that the mother-cell of the ovum undergoes this 
double division, whereby three out of the four daughter-cells 
simply disappear as polar bodies, it becomes clear that the 
process is controlled by some deeper necessity. And if any- 
one doubts this, and is inclined to think, with Lameere and 
Boveri, that the polar bodies are merely a phyletic reminis- 
cence, he should remember that rudimentary organs and pro- 
cesses always tend to vary, and that it is inconceivable that, in 
all sexually reproduced Metazoa, these two nuclear divisions 


la. 1^ ^^• 


Fig. IV. 

Formation of spermatozoa in Pyrrhocoris (modified from Henking). 
I. Primitive sperm-cell containing nuclear spindle preparatory to divi- 
sion : a. The equatorial plate as seen from the side, b. as seen from above. 
2. Sperm-mother-cell. 3. Sperm-mother-cell preparatory to the ' reducing 
division.' 4. The same, aftef division of the chromatin-wreaths into 24 
double idants. 5. First ' reducing division.' 6. Second 'reducing division.' 


should have been retained if obsolete, and should have shrunk to 
only one, as soon as regular parthenogenesis commenced. 

The double division must have some meaning, and one 
which is the same in the formation of both spermatozoa and 

I accept the meaning which has been indicated above, and 
believe that Henking's observations can be easily brought into 
accord with the plan of formation of sexual cells observed in 
other species. Henking looks upon the first division of the 
mother-cell as a ' reducing division,' the second as an * equal 
division,' and considers that he uses these terms in the sense in 
which I have employed them. But this is not quite the case. 
I understand by a ' reducing division,' one in which the number 
of ids present in the passive nucleus is reduced to half in 
each of the daughter-nuclei : I understand by an ' equal division ' 
one in which each daughter-nucleus is provided with the full 
number of ids present in the passive nucleus of the mother- 
cell. In the latter case, the daughter-nuclei will contain similar 
ids, but, in the former, this can only occur when the ids of the 
mother-cell are precisely identical. I have never maintained 
that these two contrasted modes of division must be invariably 
recognizable and distinguishable by external characters, and I 
have never identified the chromatosomes of authors with my 
ancestral units. But only when such an identification is as- 
sumed does the reduction of the number of ids by one-half 
(i. e. a ' reducing division ' in my sense of the term) necessarily 
imply a reduction in the number of chromatosomes as well. 
The types of ' reducing ' and ' equal divisions,' as I propounded 
them in 1887 \ are so conceived that the first involves a halving 
of the number of idants, while the second does not. But 
I expressly added — ' I do not mean to imply that it is im- 
possible to imagine any other form in which they [viz. these 
modes of division] may occur 2.' It then seemed to me that the 
form of nuclear division which is accompanied by a longitudinal 
splitting of the idants arranged in the equatorial plate of the 
spindle, can scarcely be conceived of as other than an ' equal 
division,' but even then I added the words * as far as I can see I* 
If we assume the linear arrangement of ids in a single row in 

^ See Vol. I. pp. 366-379, and especially pp. 375-377. 
2 See Vol. I. p. 375. 2 ggg YqI j p ^75, 


the idant, the longitudinal splitting of the latter certainly in- 
volves an ' equal division.' It appears doubtful, however, 
urhether this arrangement is universally present, and I should 
be incHned to question its existence in the second division of 
the mother-cells of Pyrrhocoris, and to believe, on the other 
hand, that the ids are arranged in two rows, and that the idant is in 
reality double. This arrangement would then lead to a new and 
different tj^pe of reducing division.' If the letters ab c, &c. — m. 
in Fig. V, represent the ids, and the vertical line drawn through 
A, the plane of splitting, it is clear that division of the idant 
would result in a reduction of the total number of ids to half in 
each of the daughter-nuclei, as is shown in B. 





-i ^ 

-. /•■ 



Fig. V. 

A. One of the double idants from the equatorial plate of the nuclear 
spindle of the first 'reducing division.' B. The same, showing its 
position after the occurrence of the first ' reducing division ' in the equa- 
torial plate of the nuclear spindle of the second ' reducing division.' 
(Compare Fig. IV, 5 and 6.) 

In support of this assumption there is not only the impossi- 
bility of conceiving the universal occurrence of a second 
division which is not also an essential change in the nuclear 
substance, but, as will be afterwards shown, there is in addition 
the evidence derived from the figures of the process which 
Henking has published. 

The equatorial plate of the nuclear spindle of the first ' re- 
ducing division ' is composed of two sets of twelve idants 
arranged in two wreaths opposite to each other (see Fig. IV. 5). 
Twelve then pass to one and twelve to the other pole, com- 
pleting the first ' reducing division.' Now it can be clearly 
seen that each idant is double from the very first, consisting of 
two halves which are arranged side by side in the spindle 


of the first ' reducing division' (see Fig. IV. 5). In the second 
' reducing division ' they are tv^isted so that the two halves of 
each idant come to lie upon each other, and between them 
passes the plane of division which confers upon each daughter 
nucleus its predetermined half (Fig. IV. 6). If then, these two 
halves, which are prepared so early, contain similar ids, we 
have to do with an ' equal division '; but, in my opinion, there 
is little to be said in favour of this assumption and much for the 

If we enquire as to the origin of the double idants in the 
equatorial plate of the first ' reducing division,' we find that 
deeply staining strands and granules of chromatin separate out 
from the passive nucleus of the mother-cell (Fig. IV. 2) and 
arrange themselves in the very remarkable likeness of a series 
of wreaths^ (Fig. IV. 3), of which there appear to be twelve. 
The full number may not be visible at the same time, because 
one or more is as yet incomplete or is already broken up. 
Each wreath then divides into two similar halves, which by 
contracting become spheres and give rise to the twenty-four 
spherical idants in the equatorial plate of the first ' reducing 
division ' (Fig. IV. 4 and 5). There is, indeed, good cause for 
regarding a process of so definite a character as by no means 
devoid of meaning, and we naturally ask for the significance of 
this wreath-formation. We cannot expect to find the answer 
by direct observation alone, but when we seek assistance from 
the suggestive conception of the idioplasm, as built up of ids, a 
certain meaning is seen to underlie the process. 

During the resting-stage the ids are scattered through the 
nucleus ; they then collect together again into idants, as I 
assume, in an order nearly the same as that previously taken ; 
the idants then grow and double t/iemselves without any separa- 
tion of the halves from each other (Fig. VI. i). 

These double idants unite together in pairs, forming wreaths 
(Fig. VI. 2 and 3), and each of the latter divides into two similar 
halves (Fig.VI. 4), giving rise to two newdouble idants (Fig.VI. 5), 
which may be different from those of the original pair. For 
the adjoining Fig. VII shows that according to the position of 

^ The term ' wreath ' or ' rosette ' is sometimes given to the equa- 
torial plate of Flemming (see Klein ' Atlas of Histology,' p. 442). This 
is of course entirely different from the wreaths mentioned above. — E.B.P. 


the plane ot division {x — x) the halves of the wreaths may be 
built up of different combinations of ids. 

Hence, according to this hypothesis, in the first ' reducing 
division,' we find in the equatorial plate of the nuclear spindle, 
twenty-four double idants, the halves of which lie over each 
other in two rows (Fig. IV. 5), and, which separating into single 
idants, bring about the second ' reducing division ' (Fig. IV. 6). 



Fig. VI. 
Formation of double idants in Pyrrhocons. 

Fig. VII. 

A wreath, formed of the four idants A, A, B, B, about to divide, 
through the moveable plane x — .r, into tw^o double idants. The small 
letters denote the ids, of which only four are shown in each idant. 

For some years I have imagined to myself the grouping of 
the ids into idants, by the arrangement of the former in the 
figure of a wreath, a form which renders possible a moveable 
plane of division. It would seem that this arrangement actually 
obtains in the ' reducing division,' and that nature produces a 
form which I had only conceived as a diagram. 

The formation of wreaths by the idioplasm, during the 



' reducing division' of the germ-cells, is not confined to Pyrrho- 
coris] for Flemming long ago described an entirely similar 
ring-like structure in the salamander, and my assistant, Dr. 
Hacker, has recently observed the formation of wreaths of 
idioplasm in the egg-mother-cells of certain Copepoda. The 
development of these latter does not, however, altogether agree 
with that of the wreaths of Pyrrhocoris, although the same 
purpose is served — viz. the arrangement of the ids in fresh 


The objection may be raised to my interpretation of the 
processes of maturation in Ascaris, that, although it corresponds 
sufficiently well with the variety bivalens and with all other 
animals possessing four or more idants, it does not apply to 
those with only two, such as the variety univalens. When the 
mother-cells contain only two idants, the mature germ-cells 
contain only one, and hence it is a matter of indifference whether 
the ' reducing divisions ' are preceded by the doubling of the 
idants or not. It might be maintained that this doubling and 
the consequent necessity for two divisions, are not explained 
by my interpretation. 

For this variety of Ascaris megalocephala, the objection is 
certainly valid ; but the question arises whether this is by 
itself sufficient to undermine the whole attempt at explanation. 

In the first place, in no other living being have so small 
a number of idants been found as in this variety of Ascaris 
megalocephala. Even so few as four idants occur but rarely ; 
and in the nearest relatives of the species, for instance in 
Ascaris lumbricoides, twelve idants are found ; in other Nema- 
todes, according to Carnoy, there are eight to sixteen ; in Sagitta, 
according to Boveri, eighteen; and the same number m Echinus', 
in a Medusa, Tiara, twenty-eight ; and in three different genera 
of molluscs thirty-two. Ascaris m. univalens is in this respect 
an exception, and should perhaps be dealt with from this point 
of view, especially as the variety bivalens, with four idants, 
appears to be the more common. We know nothing about the 
phenomena of heredity in this parasite of the horse, and cannot 
decide whether the descendants of the variety bivalens are not 


perhaps a really different species from those of the variety 
umvalens. In any case bivalens would be the ancestral form. 

While studying the last of O. Hertwig's works, the thought 
occurred to me whether the fresh combination of ids in 
Ascaris umvalens might not be brought about in a manner 
different from that of the simple rearrangement of idants, and 
I will take this opportunity of expressing the idea, in order that 
its accuracy may be tested by the facts. The material for such 
proof or disproof is not at present accessible to me ; for the 
variety univalens does not seem to occur in south-west Ger- 

In the sperm-mother-cells of Ascaris m. univalens four long 
thin threads are formed from the chromatin distributed in the 
nuclear network of the resting-stage ; these threads are arranged 
so that they cross each other at one point and are there joined 
together by means of a connecting cement-substance (' Linin '). 
Thus they form an Ophiurid-like figure in which the body of 
the Echinoderm represents the place where crossing occurs, 
while the paired arms represent the halves of threads. 
According to O. Hertwig, each of the threads then gradually 
shortens itself until at length it resembles a short thick rod. 
The four rods arrange themselves in two pairs, all four bases 
being closely apposed, the spindle of the first ' reducing division' 
is formed, and finally each daughter-nucleus receives one of 
the pairs. 

Naturally, O. Hertwig was unable to follow these processes 
directly, but he inferred them by combining the very numerous 
stages observed. I should be inclined to look for a somewhat 
different interpretation of the figures given by him, and would 
ask whether the four threads which take the form of an 
Ophiurid, are converted into the rods, not merely by shorten- 
ing, but by the simultaneous /ws/ow of two half threads just as 
if the paired arms of the Ophiurid, which lie side by side, were 
to grow together. Many details support this view. First, the 
connecting cement-substance at the point where the threads 
cross certainly possesses some significance. If, however, the 
nuclear rods arise by the shortening of the long threads only, 
it would appear to have no meaning. Only if we consider that 
it arises from the coupling together of different halves of threads, 
would it possess a meaning, as will be immediately seen. If the 

L 2 


halves of threads, representing the arms of the Ophiurid, are 
directed to each other by the activity of the achromatin nuclear 
network as they are moved hither and thither, it is essential for . 
them to have a central point of support, i. e. the part represent- 
ing the body of the Ophiurid. No conclusive objection can be 
raised against the view that the shortening process is by itself 
sufficient to convert a long thread into a short thick rod ; for 
we know that nuclear threads are subject to great shortening. 
But Hertwig himself seems to have had some doubts as to the 
validity of this explanation which he offers. In support of it 
he reminds us of ' the considerable shortening undergone by 
the threads in the spermatozoa of the salamander,' but he adds 
that this amount is very far below that required in the case of 
Ascaris if his interpretation is to be accepted. 

The bifid form of the rods indicates the longitudinal fusion 
of two threads with their points left free, and finally the 
position of the rods with their bases apposed, and thus 
standing as it were, back to back, is more intelligible when 
we suppose that adjacent arms of the Ophiurid are fused 
together, rather than that each of the long chromatin threads 
has shortened to a rod. If the latter were true we should 
expect that the rods would lie in the middle of the mass of 
'linin' representing the Ophiurid body \ and this, according to 
Hertwig's figures, does not seem to be the case. 

We may very properly be asked for the observations which 
support this view of a fusion between the halves of threads. 
So careful an observer as O. Hertwig can scarcely have 
overlooked these stages, if they have any existence. This 
I freely acknowledge ; but in Plate I he shows a series of 
figures in which two arms of the Ophiurid are approaching 
each other, and are more or less fused together. Perhaps Figs. 
27, 28, 29 should be understood in this way, and we might then 
conclude that the threads only begin to fuse after they have 
already undergone considerable shortening, and further that 
the fusion commences at the position of crossing and proceeds 

^ Because the middle of each long thread passes through the centre 
of the ' linin,' while the gradual shortening of the two ends would 
finally reduce the thread to this greatly thickened middle part. If ad- 
jacent halves fused together there would be no such arrangement : 
they would tend to radiate away from the mass of ' linin ' in which their 
bases alone would lie. 


towards the ends, so that at last only the two points are left 
free. Of course all this can only be tested by the preparations 
themselves, and O. Hertwig is in the best position, from the 
great number of his sections, to decide whether his interpre- 
tation or that which I have offered, is the right one. 

Should my surmise be confirmed, it follows that even in so 
small a number of idants as exist in the variety univalens, 
a number of combinations would be possible, inasmuch as 
halving the rods doubles the number of units capable of com- 
bination, and, of course, any two half rods might fuse in the 
manner described above. 

It would be very easy to explain the fresh combinations of 
germ-plasm in all species, Ascaris m. imivalens included, if we 
might assume that the idants were freshly built up of irregularly 
distributed ids after each resting-stage of the nucleus. But the 
above-mentioned facts concerning hereditary transmission from 
one parent alone, which have already been used as evidence, 
are opposed to this view. 

It is self-evident that I am far from claiming to have found 
the correct interpretation of the details in every case. When 
other workers have tested anew the processes with which my 
attempted explanation deals, and when new facts have been 
discovered, we shall gradually arrive at greater certainty. 
I chiefly look for progress from the comparative investi- 
gation of corresponding processes in many different groups of 
animals. For the present we may well rest satisfied, if at any 
rate the meaning and significance of the two nuclear divisions 
are, upon the whole, recognized as true. 

The future will teach us whether this is the case. In the 
meantime it promises well that, under the guidance of this 
thought, the apparently irreconcilable processes in Ascaris and 
Pyrrhocoris can be brought together under a common point of 
view. From this standpoint the two divisions of the germ- 
mother-cell signify a period of reduction and of reconstruction of 
the idioplasm. If reduction alone were needed— i. e. a dimmution 
of the number of ids by half — a single division would have suf- 
ficed ; but the second was rendered necessary in order to 
attain the greatest possible diversity in the germ-plasm. The 
accomplishment of these two ends is not always brought about 
by precisely the same course, but nature pursues somewhat 


different routes, which however always meet at the principal 
stations, viz. the two nuclear divisions. We have learnt two 
of these routes, on the one hand from O. Hertwig, on the other 
from Henking: the observations of Flemming on the formation 
of spermatozoa in the salamander may possibly point to a 
third, those of Hacker to a fourth, but all agree in leading to 
the same end. 

II. Inheritance in Parthenogenetic Reproduction. 

The Processes of Maturation in Parthenogenetic Eggs and their 


It has for some years been recognized that the characteristic 
development of an Qgg into a fully formed individual is chiefly 
dependent on the nuclear substance, in so far as it is this which 
compels distinct differentiation in a cell-body which was pre- 
viously, at any rate to some extent, indifferent, and which 
communicates to the total product of the egg-cell distinct 
modes of multiplication and development. When this became 
known it was obvious that the amount of nuclear substance 
possessed some significance, and that a certain mass of it was 
essential for the commencement of embryogeny in an egg-cell. 
I have therefore for some time agreed with Strasburger in 
seeking for the power of development without fertilization 
possessed by many ova, in the assumption that they contain an 
amount of germ-plasm which is twice as great as that present 
in eggs requiring fertilization, or that they can give rise to this 
amount by means of some process of growth. When the proof 
was afterwards afforded that parthenogenetic eggs produce 
only one polar body instead of two, I concluded, as is men- 
tioned above, that the formation of the second polar body alone 
signified the halving of the number of ids which was required 
by the theory ; for we could not assume that such a halving 
took place in parthenogenetic eggs. I looked upon the first 
halving of the nuclear substance, common to both kinds of 
eggs, as the removal of some nuclear substance which had no 
further use in either case, and the omission of the second 
nuclear division in parthenogenetic eggs I regarded as the 
means for retaining the amount of germ-plasm necessary for 
the egg to complete its course of embryogeny. 


As I have already stated, that part of my former view of the 
significance of the polar divisions, which interprets the first as 
an extrusion of a specific ovogenetic nucleoplasm, must be 
abandoned. The facts of spermatogenesis, as we have recently 
learnt them from the researches of O. Hertwig, have overthrown 
these views, inasmuch as they prove that the nuclear idioplasm 
of all polar bodies, as well as that which is retained in the egg, 
must be germ-plasm. The polar divisions of the ^g% corre- 
spond exactly with the two divisions of the sperm-mother-cell, 
as will be seen at once by comparing Figs. I and II. By this 
means, four sperm-cells arise from the sperm-mother-cell, and 
of these four each contains half the number of idants character- 
istic of the species (see Fig. I, F). By means of the two polar 
divisions the egg-mother-cell similarly gives rise to the ^gg 
(Fig. II. DEF, i), and the three polar cells (Fig. II. D E F, 
2, 3, and 4), each of which contains the same number of idants, 
viz. two. As it cannot be doubted that the idioplasm of the 
four sperm-cells is germ-plasm, it must also follow that the 
same is true of the three polar bodies as well as of the ovum. 

If then one polar body is always formed in regular partheno- 
genetic eggs, it might seem that an explanation is to be found 
by regarding it as a mere phyletic reminiscence. The question 
arises whether such a view is a just one, and in order to gain 
as clear a solution as is possible at the present time, I have 
added this chapter on parthenogenesis to' the essay. 

Spermatogenesis undoubtedly teaches us that the two ' re- 
ducing divisions ' of the female germ-cell originally performed 
the primary duty of producing four distinct germ-cells from 
each mother-germ-cell. But spermatogenesis at the same time 
shows us that a very remarkable reduction of the idants accom- 
panies these two divisions. The normal number of idants 
present in the mature spermatozoon is by this means reduced 
to half that in the primitive sperm-cell, and the result is reached 
by a most circuitous route, for the original number is first 
increased to double, and then, by two successive divisions, finally 
diminished to half 

When, however, we recognize that in normal parthenogenesis 
one of the two ' reducing divisions ' is absent, while the other 
persists, we can hardly regard the latter as the meaningless 
reminiscence of a process which was full of significance in an 


earlier phyletic stage : we cannot offer such an interpretation 
because this single polar division is found in all regular par- 
thenogenetic eggs in which it has hitherto been sought for. It 
has been found, it is true, in eighteen species only, but these 
belong to different groups of the animal kingdom, viz. in eight 
Daphnids, a Branchiopod, two Ostracodes, three Rotifers, and 
four Insecta. In each of these a single polar body corresponding 
to those of the other seventeen, is expelled, and in each we 
must conclude that an apparently useless doubling of the idants 
takes place, together with an ensuing diminution to half, as is 
shown in the accompanying diagram (Fig. VIII), in which the 
normal number of idants has been fixed at four, in order to 
facilitate the comparison with Figs. I. and II. In view of the 


Fig. VIII. 
Diagram of the maturation of the parthenogenetic q.%^. 

regular occurrence of the phenomena in all the cases which 
have been examined, it is worth while enquiring whether a 
meaning and significance are not to be attributed to these most 
unexpected processes. 

In the first chapter the attempt was made to show that the 
significance of the two ' reducing divisions,' in male and female 
germ-cells, is a double one, first, the diminution of the ids to 
half, and, secondly, the arrangement of the idants in fresh com- 
binations. The first object might be gained by a single nuclear 
division, but the second would be attained only very incom- 
pletely, because a fresh combination of the idants occurs most 
readily, when associated with a previous doubling in number. 


But this latter process renders two ' reducing divisions ' neces- 
sary, — that is if the normal number of idants must be reduced 
to one half. 

That there is no such reduction in regular parthenogenesis 
may be inferred from the large number of idants present in the 
parthenogenetic eggs of Artemia salina, viz. twenty-four or 
twenty-six. If a diminution to one half of the original number 
of idants normal for the species took place at each maturation, 
it is obvious that in each successive generation the idants would 
be reduced to half, and we should at the present day find only a 
single one left in Artemia. Either this polar division is not a 
' reducing division,' or it is preceded by a doubling of the number 
of idants, just as in ova which require fertilization. 

If this latter be true, it follows that in parthenogenesis we 
meet with a simple retention of the first of the two polar divi- 
sions which occur in other ova. 

It is unfortunate that direct observation has not hitherto led 
to an entirely certain decision upon the point. Dr. Otto vom 
Rath has had the great kindness to examine, with this object 
in view, many of my old sections ^ of the parthenogenetic ova 
of Artemia salina, in order to find out those parts of them 
which were most important in this respect. From my earlier 
researches, conducted upon the same material, I was already 
aware that the germinal vesicle, after having approached the 
surface, contains a large number of chromatin granules, which 
are distributed with almost complete regularity. It was evident 
that these granules had not yet become the definite chromato- 
somes or idants, but that they were smaller and more numerous 
(Fig. IX. i). In one germinal vesicle I counted 115 of them ; 
in another, which was already changing into a spindle, I also 
found 115, all lying in the equatorial plane (Fig. IX. 2) ; in a 
third, 77 ; in a fourth, 70 ; and in a fifth, 57. Now in the equa- 
torial plate of the polar spindle, from 48 to 52 spherical idants 
are always arranged in a double wreath (Fig. IX. 3 a). These 
must therefore have arisen from the fusion of several of the 
primary chromatin granules, and the great variation in the 
number of the latter must depend on the fact that the fusion was 

^ Weismann und Ischikawa, ' Weitere Untersuchungen zum Zahlen- 
gesetz der RichtungskOrper ; ' Zoologische Jahrbucher, Bd. III. p. 575, 


much further advanced in some of the germinal vesicles examined 
than it was in others. Half the number of 48 or 52 idants in the 
equatorial plate pass to one, and the other half to the other pole. 
If a diminution in the ids be characteristic of a ' reducing divi- 
sion,' it follows that this term can only be applied to the process 
which has just been described in Artemia, if the whole number 
of 48-52 idants have arisen directly from the primary chromatin 
granules: if, on the other hand, only 24-26 idants were so 
derived, and the equatorial plate was at first composed of a 



© ® © @ © 
® ® © @ O 

Fig, IX. 

Artemia. Germinal vesicle of the parthenogenetic ^g^ before and during 
polar division : partially diagrammatic, from my own preparations. 

I. Numerous chromatin granules scattered through the w^hole thick- 
ness of the germinal vesicle. 2. Numerous chromatin granules (115) 
collected in the equatorial plane. 3 a. The same arranged in a double 
wreath of 52 idants in the polar spindle. 3 6. A part of the double 
wreath with the separate idants indicated by letters. 

single wreath which was subsequently doubled by division ot 
the idants, it would follow that the process would be an ' equal 
division.' In the latter case, the two idants lying over each 
other would be identical, i. e. composed of similar ids, and 
identical idants would pass into each daughter-nucleus. If, 
however, as in the former supposition, the two adjacent idants 
were independently derived, and therefore composed of dif- 
ferent separate ids (chromatin granules), it is clear that the 


idioplasmic construction of the two daughter-nuclei must be 

Inasmuch as we cannot see whether the chromatin granules 
are made up of similar or different idioplasm, it follows 
that direct observation cannot conclusively settle whether we 
are dealing with an ' equal ' or a ' reducing division.' Perhaps, 
however, we may succeed in decisively answering the question 
by other means, and investigations have already been under- 
taken with this special object ; for the present we must rest 
content with conclusions based upon probability. Before 
everything w^e must make certain that the first division in 
eggs requiring fertilization is, in all cases, a ' reducing division.' 
At the present time Artemia reproduces sexually in many of 
its colonies, and hence in parthenogenetic colonies in which 
the eggs have lost the second polar division but have retained 
the first, it may be regarded as probable that the latter has 
kept its original form, i. e. that of a ' reducing division.' 

Still further support for the above conclusions is found in 
the fact that Dr. vom Rath could never find single idants in 
the equatorial plate of the polar spindle of Artemia^ but only 
double ones, each having the form of two large round bodies 
lying over each other (Fig. IX. 3 a). 

If we now further consider that, at the commencement of 
the change of the germinal vesicle into the spindle, the 
chromatin granules lie scattered through the whole thickness 
of the former (Fig. IX. i), and that they then fuse with one 
another, arranging themselves as a single layer in the equa- 
torial plane of the spindle, in the form of an oval disc and not 
that of a simple wreath (Fig. IX. 2), and if we remember that 
they then pass into the arrangement of a double wreath (Fig. 
IX. 3 «), we are led to conclude that no two idants of this 
double wreath have arisen from the doubling by division of 
a single idant, as is the case in the usual ' equal division ' ; 
but that the idants of the oval equatorial plate, which arose 
independently of one another, have subsequently come to place 
themselves one upon the other in the form of a double wreath. 
If this conclusion be sound we have to do with a true ' reducing 

Hence we are justified in assuming as the most probable 
conclusion that a ' reducing division ' takes place, and further- 


more a division which is preceded by a doubling of the 

If this be so, we cannot doubt that the effect of the process 
must be similar to that which follows the corresponding 
processes in eggs which require fertilization, viz. the arrange- 
ment of idants in fresh combinations, as I attempted to show 
in the first chapter. We are thus led to the view that in 
parthenogenetic as well as in sexual eggs a change may take 
place in the constitution of the germ-plasm during successive 

If we start from that point in phjdetic development at 
which parthenogenesis was first established, each idant in 
the original egg-cell was at that time composed of a series 
of different ids. Then, for the first time, these idants were not 
diminished to half the total number by two polar divisions, but, 
after being doubled in the egg-mother-cell and again reduced to 
half by the first polar division, their number in the mature ovum 
became the same as in the original egg-cell (see Fig. VIII). B}^ 
this means a fresh combination was rendered possible and 
indeed unavoidable, unless we assume that the constituents of 
each pair of similar idants, which arose from the doubling of 
the previous idants, separated and united with those of the other 
pairs, forming two exactly similar groups which then respectively 
entered the two daughter-nuclei. This would be the result 
of an ' equal division ' of the nucleus. Such a division is 
attained and ensured precisely because the doubling and 
division of the idants only takes place when they have already 
become arranged in the equatorial plate ; but whenever the 
doubling has occurred beforehand, as is the case here, the 
two halves of an idant may indeed be occasionally shared 
between the two daughter nuclei, but they may also, just 
as readily, both pass into one and the same daughter-nucleus. 
From this freedom in the distribution of the idants follow 
the fresh combinations produced by the ' reducing division ; ' 
and the difference between an ordinary nuclear division, and 
the ' reducing division ' which here takes place, depends 
essentially on the fact that, in the latter, there is a shifting of 
the time at which the doubling of the idants occurs. 

Hence, if a species of Artemia, which had hitherto reproduced 
bisexually, were now to become parthenogenetic, then in spite 


of the cessation, for all future time, of the mingling of the 
idants of the ovum with those of the spermatozoon, it would 
by no means follow that the offspring of a female would 
necessarily become 'identical twins.' With twenty different 
idants, if there are not the 377 million different combinations 
which calculation indicates, there would be nevertheless such 
a vast number of different combinations of idants, that two ova 
produced by the same mother could only rarely be identical. 
Among all the possible combinations, that very one might arise 
which existed in the original egg-cell of the mother herself 
and became expressed in her somatic cells. Such a combination 
would contain one idant of every kind, and such an ovum would 
give rise to an individual ' identical ' with the mother, that is, to 
one similar to the mother in all respects except as regards 
those modifications of the inherited developmental tendencies, 
which are called forth by external circumstances. 

We need not consider the unlikely suggestion, that all com- 
binations are equally probable ; if only it be conceded that 
any degree of difference is possible for the combinations of 
the germ-plasm, remarkable consequences follow. In the 
first place it appears that, in persistent pure parthenogenesis, 
the number of different idants contained in the idioplasm must 
steadily diminish, although perhaps at a very slow rate. If the 
number did not diminish new combinations could never arise, 
and that of the first parthenogenetic mother {A) would be re- 
tained indefinitely, — thus if there were twenty different idants 

{a, b, c, d, e /) the whole series would persist unchanged. 

If, however, another combination arose in the daughter (5), for 

example a a b c d e /, this would be brought about by one 

of the idants {a in this instance) becoming double, and then 
inasmuch as the total number of idants must remain the same, 
it follows that one of the others must be absent (for example /), 
or the number would be twenty-one instead of twenty. As a 
result of this the idant / would be wanting in all the descendants 
of B. If now we suppose that such a new combination, arising in 
this way by the omission of one idant and the reduplication of 
another, would not be formed in each generation, but only in 
every tenth, it follows that at the end of each series of ten 
generations, a fresh combination will arise by another omission 
and another reduplication, and so on, so that after a hundred 


generations the number of different idants would have been 
diminished from twenty to ten, and the whole group would con- 
sist often pairs, for instance ««, bb, cq dd, ee,ff, gg, hh, it, kk, the 
idants in each pair being identical. In the course of later 
generations the number of different idants might be diminished 
still further, although more gradually. 

We are thus led to believe that, in persistent parthenogenesis 
unbroken by bisexual reproduction, a great uniformity of germ- 
plasm will at length arise, and, as a result, a great uniformity 
of individuals. We cannot doubt this if we consider that each 
fresh simplification of the germ-plasm, when it has once ap- 
peared, is unable to revert towards complexity because fertili- 
zation, i. e. the introduction of foreign idants, is excluded. As 
soon as the 'reducing division' causes a single one out of 
the twenty maternal idants in the segmentation nucleus of 
the egg to become double, it has been shown above that 
one of the other idants must be irretrievably lost not onl}'- 
to the maternal germ-plasm and to the daughter, but also to the 
descendants of every generation. Among all the numerous 
possible combinations there is only one which leads to no 
diminution in the number of different idants, viz. the above- 
mentioned arrangement a, b, c, d, e, /, and this is an exact 

repetition of the maternal combination. Hence the diminution 
in the number of different idants is far more probable than the 
maintenance of the complete series, and this probability will be 
repeated in each successive generation, until only two kinds of 
idants remain in the germ-plasm. When, however, this point 
is reached ^, the reverse becomes true ; for the probability that 
idants a alone, or b alone, would be left in the egg-nucleus by 
the ' reducing division' is much less than that both kinds would 
exist side by side. 

This becomes clear if we consider a definite case. Instead 
of the twenty idants which have been assumed hitherto, let us 
take only half as many, viz. ten, and let us suppose that they 
have been already reduced to two different kinds, a and b. 
These double themselves in the mother-egg-c-ell to twenty — ten 
a and ten b. The following combinations are then possible for 

^ Even before this point is reached the probabihty begins to change. — 
A. W. 1892. 


the germ-nucleus ^ of the ^%% produced by the ' reducing 
division,'— 10 rt ; 9<7 + iZ>;8^ + 2Z>; 7« + 3Z>;6« + 4Z>;5« + 5^; 
4<^ + 66; 3« + 7<^; 2rt + 8Z>; 1^ + 9^; ioZ>. 

Hence we see that out of eleven possible combinations there 
are only two which contain one kind of idant alone : all others 
contain both. In the case of twenty idants there are only two 
out of forty-one combinations which contain either a ov b alone ; 
with forty idants, only two out of eighty-one. 

Naturally this does not imply that the diminution to one kind 
of idant is improbable, but only that it would always remain 
largely in the minority, i.e. it would be found in relatively very 
few cases among the numerous eggs of the same mother. 
This must, however, change in the course of generations ; for 
only in one out of the eleven combinations are a and b present in 
equal numbers, and only in the descendants of this single 
variety will the germ-plasm be chiefly made up o^a and b in equal 
proportions : in all the other ten combinations, either a ov b 
preponderates, and according to the extent of preponderance is 
the probability of a greater or less number of eggs which contain 
only a or only b. We may therefore maintain that, by continued 
parthenogenesis, the germ-plasm becomes ever simpler as 
regards its composition oat of ids until it comes to consist of 
only two kinds of idants, but when once this composition has 
been reached it may be retained through long periods of time, 
during which there will be a changing majority, sometimes 
of one and sometimes of the other kind. Among the eggs of 
such a female there would always be some in which the germ- 
plasm would contain only one kind of idant. 

Observations on Inheritance in Parthenogenesis. 

When I developed the idea that the essential meaning of 
sexual reproduction was to ensure that amount of individual 
variability which is necessary for the phyletic development of 
the organic world by means of natural selection, I inferred that 
uninterrupted parthenogenetic reproduction would prevent the 

^ I have employed Strasburger's term ' germ-nucleus ' instead of 
'■ segmentation nucleus ' which has been commonly used up to this time, 
as a general term for the nucleus of the mature egg from which embryonic 
development proceeds, whether parthenogenetic or amphigonic. 


adaptation of a species to new conditions of life \ I argued that, 
the repeated minghng of two individualities being requisite to 
supply the process of selection with the necessary choice of 
combinations of individual qualities, — it follows that a choice 
of sufficient range will not be supplied when one and the same 
set of combinations are passed on by parthenogenesis, through 
long series of generations, to an ever increasing number 
of individuals. A number of ' identical ' individuals would 
thus arise, that is individuals which contain a precisely 
similar fundamental stock of hereditary predispositions, and 
which, at most, can only be distinguished by transient pecu- 
liarities, viz. by those which are the consequence of external 
influences of various kinds upon the body during its pro- 
gress towards maturity or after maturity has been reached. 
When writing on this subject, I expressed the opinion 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-.' I stated this 
conclusion in the strongest possible way although I thought 
that it might perhaps require subsequent modification, because, 
even at that time, I had already considered the possibility that 
the consequences of sexual reproduction of ancestors might 
affect their purely parthenogenetic descendants. But whether 
a simple rearrangement of the ids within the idants would 
suffice to call forth a fresh combination of individual peculiar- 
ities, appeared to me very doubtful ; and 3^et this would have 
been the only alteration in the germ-plasm which we could 
have been led to suggest by the state of our knowledge at the 
time ; for a ' reducing division ' could not have been supposed 
to take place in parthenogenetic eggs, because we did not know 
that the number of the idants doubles before the occurrence ot 
the first polar division, and because a halving of the number 
of idants, without any previous doubling, would necessarily, 
in a few generations, diminish their number to one. But now 
the case is different, and we may affirm that in parthenogenetic 

^ * Die Bedeutung der sexuellen Fortpflanzung.' Jena, 1886, p. 58. 
Translated as the fifth essay. See Vol. I. p. 298. 
2 See Vol. I. p. 298. 


generations, the combination of idants in the different germ- 
cells of one and the same mother can vary. We can therefore 
attribute even to parthenogenetic species a certain power of 
varying, although not to anything like the same extent as in 
bisexual species. 

By the year 1884 I had commenced a series of experiments 
to decide the question of variability in purely parthenogenetic 
species. These experiments are still being carried on, and I hope 
that I may ultimately be able to make a more complete com- 
munication upon the subject. I chose for the purpose a species 
of Cypris (Ostracoda), which was characterized by striking and 
easily seen markings on the shell. I had at my disposal two 
very differently marked varieties of the species in question 
{Cypris reptans), which had been found in the natural state. 
The species appears to be purely parthenogenetic in this 
locality ; at any rate I have never found a male, nor a female 
with spermatozoa in the receptaculum seminis \ The latter fact 
conclusively proves the complete absence of males ; for in 
colonies of those species of Cypris which possess males, we 
always find the receptacula seminis of mature females filled with 
spermatozoa. Even if it were a mere coincidence that of 
the many hundreds of individuals examined, all proved to be 
females, the presence of spermatozoa in their receptacula would 
still have shown the presence of males, if any had existed in 
the locality. But the receptacula were, without exception, empty, 
at all times of the year, and under all the external conditions 
which obtained during my investigation of the colony. 

My two sub-species are distinguished as follows (see Fig. X) : 
variety A is lighter in colour, and there are only a few dark 
green spots of small size on the clay yellow ground-colour of 
the shell. Variety B appears dark green because the spots 
are so much larger that they expose only a little of the clay 
yellow ground-colour of the shell. In both varieties the spots 
agree precisely as to number and position ; the difference 
between them is entirely quantitative, but it is considerable, so 
that the lighter A can be distinguished from the darker B with 
the naked eye at the first glance. 

The experiment was conducted in the following way: I 

^ Compare my earlier paper ' Parthenogenese bei den Ostracoden;* 
Zool. Anzeiger, Bd, III. p. 81, 1880. See also Vol. I. p. 301, note 2. 



placed a solitary individual in a small aquarium, and allowed it 
to multiply until the whole vessel was full of mature, egg- 
producing descendants. All the individuals of the colony were 
then passed in review, and the greater number were killed and 
preserved, one or more having been selected for breeding, and 
these were placed separately in fresh aquaria. In this way, 
in the course of seven j^ears, many thousand individuals have 
passed through my hands ; for the animals breed very rapidly 
and at all times of the year. 

A B 


FiG. X. 
Cypris reptans^ Varieties A and B. 

The first and most remarkable result is the fact that the de- 
scendants of any one individual hear a very close resemblance to each 
other and to their ancestor. I was not able to find any individuals 
which were precisely alike, although at first sight it often 
seemed that such was the case : minute differences, however, 
invariably existed as far as my observations reached, although 
they were often so small as to lead to the doubt whether they 
were due to different predispositions or to different nutriment, 
etc. And indeed no two individuals, not even ' identical ' human 


twins, can be exactly alike in this latter respect. Furthermore, 
as a rule, no changes made their appearance in course of the 
numerous generations during which the examination lasted, 
with an exception which will be immediately described. I now 
possess colonies of A, as well as of B, which cannot be dis- 
tinguished from their ancestors in 1884, and which have there- 
fore retained precisely the same markings as those of the 
original animals. If we reckon six generations to the year, — 
a number by no means excessive for breeding which took 
place in a room, — about forty generations will have been passed 
through since 1884. 

I attempted at first to produce the two forms by artificial 
selection, breeding from the darkest individual of a colony of 
the variety A, and from the lightest of a colony of B, in the hope 
that, perhaps, in the course of generations, one variety might 
be changed into the other. But I obtained no decisive results, 
perhaps because I did not make my selection rightly ; for the 
individuals are so very similar that it is often difficult and 
indeed hardly possible to decide upon those which possess the 
larger spots : perhaps also I mistook transient differences for 
inherited ones, — a confusion which, naturally enough, cannot 
be avoided. 

I was therefore all the more astonished to find, in 1887, 
some individuals of the dark green variety B in the same 
aquarium with the light variety A, and therefore side by side 
with typical, light, clay-coloured individuals. At first I thought, 
although it was most improbable, that these had been accident- 
ally introduced, but the greatest care had always been exercised 
in all these experiments. Furthermore, after the most pains- 
taking precautions against such accidents, precautions which 
prevented all possibility of the eggs being misplaced, there 
presently appeared another similar case in a different aquarium 
containing the variety A, and, later on, yet another. In this 
last case it was possible to find in the aquarium intermediate 
forms between the two varieties, which had been wanting 
on the previous occasions. Again, in May of the present year, 
189T, another case was observed in which a single animal, dis- 
tinctly belonging to the dark sub-species, suddenly appeared 
among 540 mature Cyprides of the light variety. Five descen- 
dants of this individual closely resembled their mother. 

M 2 


For a long time I waited in vain for the converse result, viz. 
the appearance of light individuals of the variety A among 
those of the dark sub-species B, and I was coming to the 
opinion that the latter was the original form of both varieties, 
when, in the winter of 1890-91, a few typical individuals of ^ 
were found in a colony of the sub-species B, which had bred 
true for many years. This colony had arisen from a single 
dark individual which, in the course of seven years, had pro- 
duced many hundreds of descendants all of the typical dark 

We might perhaps refer to the changing influence of external 
circumstances as an explanation of these divergences from the 
type, but any such interpretation is entirely excluded, because 
both forms made their appearance side by side in the same 
aquarium and under precisely the same external conditions. 
These remarkable phenomena must certainly be ascribed to 
internal causes, viz. to changes in the composition of the germ- 
plasm. The required explanation is by no means difficult 
when the subject is studied from the point of view afforded by 
the theory of idants : in fact these observations seem to me 
almost a proof of the validity of the opinion expressed above 
that a ' reducing division ' occurs in parthenogenetic develop- 
ment, and that by its means a fresh combination of idants is 
brought about. 

The fact that the variety A passes into B and conversely, 
B into A, leads to the conclusion that both types originated at 
a time when parthenogenesis was not the exclusive method of 
reproduction : had this been the case, the ids a could not have 
been included in the germ-plasm of animals of the type B, and 
conversely the ids b could not have existed in the type A. The 
explanation of the existence, side by side, of both kinds of ids, 
is only to be found in sexual reproduction which must have 
taken place at no very distant time. 

Let us assume the simplest possible relationship, viz. that 
there are only four idants in the germ-plasm, of which three are 
wholly composed of ids of the type A, and one of ids of the 
type B. The four idants, aaab^ of the primitive germ-cell 
become doubled in the mother-germ-cell by^ngitudinal split- 
ting, and give rise to the eight idants, aaa aa abb. Let us 
further assume the most favourable case for reversion towards 


the variety B, a reversion which would be possible in an egg 
in which the ' reducing division ' takes place so that the com- 
bination of idants, aaaa^is, removed in the polar body, while 
the combination, a abb, remains in the germ-nucleus of the 
ovum. The primitive germ-cells of the next generation con- 
tain the same combination, a abb, which is doubled in the 
mother-germ-cells to aaaabbbb, and it is now clear that a 
'reducing division' might occur, which would bring the four 
idants, b b b b, together into the germ-nucleus of an ovum, and 
from an egg containing germ-plasm with this constitution there 
must undoubtedly arise an individual of the variety B. 

In this illustration, which is of course far too simple, rever- 
sion to the other variety might happen in the third generation. 
In those cases, however, — and they are the usual ones, — in 
which the number of idants is larger, and the proportion of 
variety b much smaller, the exclusive predominance of the 
latter can only take place far more slowly, and, as a rule, in 
much fewer cases; for it depends upon the chance of a com- 
bination of several idants b arising in certain ova, and of the 
survival to maturity of the individuals which develope from 
such eggs,— and these naturally must be far rarer than those 
with a largely predominating number of idants a. Further- 
more, there is no certainty that, among the eggs produced 
by such individuals, any with an increased proportion of idants 
b would find a place. 

These theoretical considerations harmonize well with the 
results of experiment. Variety A can give rise to descendants 
belonging to variety B, but this does not happen in all broods, 
and often only after the lapse of numerous generations. And 
the same is true of variety B in relation to the production of 
variety A. In both cases, relatively few individuals change 
into the other variety, and never all the descendants of one 
mother. In the aquarium in which such a transformation has 
occurred numerous individuals of the original form were in- 
variably present, — a proof that it is always a rare exception for 
such extreme combinations of germ-plasm to be formed. When, 
however, this combination had once arisen, then such an in- 
dividual gave rise, in all the cases observed, to offspring of her 
own type. Thus a mother which arose from variety A, but has 
passed over to variety B^ behaves as though her ancestors had 


belonged to the latter type. She produces ofTspring of the 
variety B, and the type is retained for many generations. 
In the illustration described above the type B would be retained 
indefinitely ; for I assumed that only four idants were present, 
and that all these became of the variety B. In reality, however, 
this would occur but seldom, since the constitution of the germ- 
plasm must be far more complex : not only are the idants more 
numerous, but their composition out of ids does not remain 
entirely the same throughout long periods of time, as I have 
attempted to show in the first part of this essay. 

If the idants are not entirely unchangeable in this respect, if, 
when they are freshly formed out of ids scattered through 
the nuclear network, there is an occasional alteration in the 
arrangement, we might then even assume that, by such dis- 
placements, a germ-plasm a which contains no purely b idants, 
but only a few ids belonging to the latter variety included 
within the a idants, could, nevertheless, in course of generations, 
undergo reversion to the variety B. But these are niceties, 
which it is as yet too early to consider ; for w^e are only on the 
threshold of knowledge concerning hereditary phenomena in 

But something at any rate has been proved ; for we can 
safely affirm that in parthenogenesis individual variation exists, 
which, as in bisexual reproduction, has its foundation in the compo- 
sition of the germ-plasm itself, and thus depends on heredity, and is 
itself inheritable. I thus erred in former times, in believing that 
purely parthenogenetic species entirely lacked the capability of 
transformation by means of selection ; they do possess this 
power to a certain extent. I was, however, right upon the 
main point ; for their capability of transformation must be 
much smaller than in bisexual species, as is evident from the 
observations described above as well as from theoretical con- 
siderations. The latter indicate that, in the course of genera- 
tions, the constitution of the germ-plasm must ever become 
simpler ; while the observations confirm this suggestion, inas- 
much as they prove that a remarkable similarity exists between 
the descendants throughout numerous generations. The ad- 
vantages of that complex intermingling of many individual 
predispositions which was brought about in the amphigonic 
ancestors of parthenogenetic species become gradually lost, 


and we may maintain that purely parthenogenetic species lose the 
capability of modifying themselves, more completely, the longer the 
pure parthenogenesis has continued. So far as we can at present 
decide, this conclusion is in agreement with facts ; inasmuch 
as no highly developed group of the zoological system, rich 
in species, is ever entirely composed of purely partheno- 
genetic species. In the animal kingdom, the Phyllopods and 
Ostracodes, among the Crustacea, are especially remarkable 
for the frequency of parthenogenetic reproduction. But pure 
parthenogenesis only occurs in isolated species, as in the above 
mentioned Cypris reptans and many other species of the same 
genus. Among the Phyllopods I only know of one species, 
Limnadia Hermanni, in which a male has never been found, 
and it is this very species which seems to have become ex- 
tremely rare. In the other parthenogenetic species, in addition 
to the purely parthenogenetic colonies, there are always some 
which are composed of both sexes, as in Apus cancriformis ; or 
else a regular alternation of parthenogenetic with bisexual 
generations takes place in the colony, as in almost all known 
species of Daphnids. The rich development of these groups of 
the zoological S3''stem has arisen under the uninterrupted 
influence of amphigonic reproduction, by means of which 
variations have been mingled together. It is just the same 
with the Aphidae (plant-lice and bark-lice), and with the Cyni- 
pidae. All these groups of animals contain a great variety 
of species, but, in all, a combination of individual characters 
takes place from time to time through the fertilization of ova, 
even though, as is often the case, many purely parthenogenetic 
generations intervene between the bisexual ones. 

I believe that we find, in the tenacious retention of amphigonic 
reproduction by such species as the Phylloxera, a strong support 
of the validity of my theory as to the meaning of sexual repro- 
duction. Those who still recognize in fertilization a renewal of 
vital strength, a rejuvenescence, do not require this conception 
of amphigony as an ever springing well of hereditary individual 
variation in order to understand its remarkable persistence. 
But those who agree with me in believing that the partheno- 
genesis of Cypris reptans which endures for forty consecutive 
generations is the refutation of any such idea of rejuvenescence, 
will hardly find another explanation of this tenacious persist- 


ence. Thus, let us call to mind Phylloxera and its allies, in 
which many purely parthenogenetic generations follow one 
another every year and bring about an immense increase of 
individuals, to be finally succeeded by a single sexual genera- 
tion of insignificant wingless males and females without mouth 
appendages, which have nothing to do but pair immediately 
after birth in order to produce the fertilized ova. Thus, sexual 
reproduction is retained in spite of the fact that no increase, 
but rather a decrease, in the number of individuals is, in these 
cases, brought about by its means, just as in the conjugation of 
the lower unicellular organisms. Some great advantage must 
therefore follow from its retention. 

It may, however, be lost, and we cannot at present decide 
whether the immediate advantages which pure parthenogenesis 
affords are sufficiently important to justify the disappearance of 
those arrangements by which the power of increasing variation 
is guaranteed. We cannot penetrate far enough into the details 
of the struggle for existence to be able to determine whether 
a species can in any way fall into so critical a position that 
its survival can only be brought about by that excessively rapid 
rate of multiplication which is rendered possible by pure par- 
thenogenesis. In such a case amphigony would have to be 
abandoned, for the only choice would be that between extinction 
and parthenogenesis, and the future of the species would be 
to some extent sacrificed to its temporary maintenance. But 
I do not by any means wish to imply that this is the only way 
in which the omission of sexual reproduction can be understood. 
The question is only opened, and wx cannot yet claim to have 
answered it satisfactorily. 

: We must now turn our attention for a short time to the vegetable 
world. Unfortunately, there are not, as far as I am aware, any 
available observations on plants which give us rehable informa- 
tion as to those processes of maturation of male and female sexual 
cells which have now been described in the animal kingdom. 
Certainly Strasburger and others years ago described cell- 
divisions of mother-cells, both male and female, which resemble 
the ' reducing divisions ' of mother-cells in animals ; but 
whether, in this case also, a doubling of the idants precedes 
their twice-repeated division into halves, appears to be un- 
known. If we may assume that such a result is by some 


means ensured, that the number of ids is halved, and that their 
fresh grouping is thereby provided for, we cannot at any rate 
predict whether the process is conducted in precisely the same 
way as in animals. We ought perhaps rather to expect that 
some deviation from the reducing methods customary among 
animals would here be met with, a deviation which would render 
the meaning and significance of the latter even clearer and more 

We are justified, however, in believing that, in the cases of 
plant parthenogenesis, the amount of variation will diminish, 
together with the capability of adaptation by the operation of 
natural selection. Adaptations caused by direct influence on 
the germ-plasm are naturally conceivable in these as in other 
cases, but at present we know so little about such changes, 
whether produced by climatic or nutritive conditions, that it is 
impossible to determine how much may be implied by them. 

Ten years ago parthenogenesis was doubted by botanists, or 
at any rate was regarded as very rare, and only to be found in 
cultivated plants, such as Pferis eretica, in which a certain ten- 
dency to degenerate was recognizable, or, at any rate, in which 
the structural and functional arrangements were no longer 
subject to the operation of natural selection. But we now 
recognize that a whole group of fungi, the Saprolegniae, ' in- 
cluding several genera and many species, are parthenogenetic' 
Among the Ascomycetes ' it is admitted that many genera and 
species . . . are certainly asexual.' Amphigonic reproduction 
in the iEcidiomycetes is ' extremely doubtful,' while the Ba- 
sidiomycetes ' afford an example of a vast family of plants, of 
the most varied form and habit, including hundreds of genera 
and species, in which, so far as minute and long-continued 
investigation has shown, there is not, and probably never has 
been, any trace of a sexual process ^' 

If the last statement be correct, it is impossible to maintain 
the existence of parthenogenesis in the Basidiomycetes ; for 
this method implies the sexual reproduction of ancestors 
as its origin. Parthenogenesis is virgin reproduction, and 
signifies a power of development without fertilization pos- 
sessed by female germ-cells. Parthenogenesis has arisen from 
bisexual reproduction by the elimination of the male and 
^ See Vines in ' Nature,' 1889 (Oct. 24), p. 626. 


the male germ-cells ; with the knowledge we now possess 
there can be no doubt upon this question. Not every unicellular 
germ is phyletically an ovum. We ought to recognize and 
apply to the botanical world the difference between partheno- 
genesis and asexual reproduction from unicellular germs. This 
distinction has not been made with any completeness, as we 
see in the passages quoted above from Professor Vines, and 
hence it is impossible to draw any safe conclusions from the 
asexual reproduction of the above-named fungi and from the 
fact of the phyletic development of numerous genera and 
species,— as to the amount of variation provided by heredity in 
parthenogenetic reproduction. The conditions of life among 
fungi are well known to differ markedly from those of most 
other plants, and it is not inconceivable that these may be 
associated with the disappearance or absence of amphigony ; 
for the peculiar conditions of life may exercise an unusually 
strong direct influence upon the germ-plasm, and may thus 
render it variable. We know that variability is induced in 
other plants when they are submitted to very favourable 
nutritive conditions. But the researches of botanists must not 
be anticipated by these conjectures. 

The Origin of Parthenogenetic Eggs front those vuhich require 

As I have already stated, parthenogenesis must have arisen 
from sexual reproduction. Those cells which develope partheno- 
genetically are female germ-cells which have gained the power 
of producing new organisms without fertilization. We must 
now enquire how this change has been brought about. 

I must first allude to the gonoplastid theory, of which the 
principle has been proved to be untenable, but which is never- 
theless correct in certain aspects, at least in the form in which 
Balfour conceived it. This thoughtful writer expressed the 
idea that the arrangement of polar bodies might have been 
brought about by nature, in order to prevent parthenogenesis. He 
therefore imagined that parthenogenetic development would 
ensue if the polar bodies, containing the supposed ' male prin- 
ciple,' remained in the Qgg. If, however, the facts are somewhat 
different, in so far as the polar divisions of the egg have been 


from the first an adaptation to fertilization, they have at any 
rate the effect of checking the power of development in the 
egg, so that, in a certain sense, we may maintain that their 
expulsion prevents parthenogenesis. On the other hand, we 
are now aware that a polar body is expelled from the partheno- 
genetic egg, while the difference between this and the egg 
requiring fertilization lies in the fact that a second polar body is 
expelled from the latter; but the correct idea nevertheless 
remains that something indispensable for the power of develop- 
ment is removed from the egg. According to our present 
views this is not the unknown ' male principle,' but a certain 
quantity of germ-plasm. 

When we begin to enquire into the manner in which the 
power of parthenogenetic development was gained by an egg 
which required fertilization from the most remote time at which 
multicellular beings existed, the first thought that strikes us is, 
— might not this have been brought about by the suppression of the 
second polar division ? If this happened, the first polar division 
would cause a diminution to the normal number of the pre- 
viously doubled idants, and the second polar division being 
absent, the egg-cell would retain precisely as much nuclear 
material as it would have contained if fertilization had followed 
the expulsion of the second polar body. Since, then, regular 
parthenogenetic eggs invariably possess only one polar body, 
this supposition attains a high degree of probability. There are, 
however, facts which show that parthenogenesis may be 
acquired in another way. 

Blochmann has observed, as is well known, that when the 
egg of the queen-bee is deposited in the cell of a drone, the 
same course of maturation is pursued as when it is laid in a 
female cell. In both cases two polar nuclei are formed, in both 
the nuclear substance is halved twice successively. In the case 
of the unfertilized male egg, the nucleus which remains after 
the second division possesses the power of becoming the germ- 
nucleus, and of developing ; while the female egg is only able 
to enter upon embryogeny after the fusion of its nucleus with 
that of the fertilizing spermatozoon. 

The eggs of Lepidoptera behave in a somewhat similar way ; 
in the great majority of cases they require fertilization, but some 
can develope parthenogenetically. In the case of Liparis dispar, 


Platner found that such parthenogenetic eggs, Hke those which 
require fertihzation, expel two primary polar bodies. 

From this it is clear that parthenogenesis is possible, even 
when the quantity of germ-plasm in the ^%^ has been reduced 
to half. Rolph, in his day, attributed parthenogenesis to better 
nourishment ; Strasburger surmised, in adapting these thoughts 
to the significance of nuclear substance, which had in the mean- 
time been recognized, that ' favourable conditions of nutrition 
counterbalanced the deficiency of nuclear idioplasm.' He as- 
sumed that the nucleoplasm was reduced to half, even in 
parthenogenetic eggs, and that ' the egg-nucleus after its reduc- 
tion to half was unable to initiate the processes of develop- 
ment in the cell-body.' It was in these very cases of exceptional 
parthenogenesis in single ova that I expressed the definite 
opinion that the difference between eggs which are capable of 
parthenogenetic development and those which are not, must be 
quantitative and not qualitative ^. I concluded from the facts 
connected with exceptional parthenogenesis, that a certain 
amount of germ-plasm must be contained in the egg-nucleus if 
it is to be in the position of entering upon embryogeny, and of 
completing it, and that, in these exceptional cases of partheno- 
genetic development, the germ-plasm in the ^g^, after having 
been reduced to half its normal amount, possesses, in some 
unusual way, the power of increasing to double. I am well 
aware that many facts subsequently discovered appear to be 
opposed to this suggestion, but I beheve they only appear to be 
so. For example, my views as to the two varieties of A scan's 
megalocephala might be cited in opposition ; of these varieties 
one possesses two idants in the segmentation nucleus, the other 
four. We might conclude from this that the amount of nuclear 
matter does not control entrance upon development, but some 
other cause,— perhaps those * spheres of attraction ' and the 
central-bodies which E. van Beneden discovered lying in them, 
and which Boveri has called the centrosomata. I do not dispute 
the significance of these remarkable bodies in relation to the 
commencement of nuclear division, but do we know whence 
they come, and whether they are not perhaps, on their part, 
controlled by the nuclear idioplasm (germ-plasm) ? 

^ ' Continuity of Germ- plasm.' Jena, 1885, p. 90. Translated as the 
fourth essay ; see Vol. I. p. 231. 


I hold that this is not only possible, but even probable. The 
difference between the embryogenies of two allied species not 
only depends upon the characteristic differentiation of the single 
cells which compose the body, but also equally upon their 
number, both relatively and absolutely, in all parts of the body. 
One and the same part of the body may be long in one species, 
and short in another : more cells will be required for the con- 
struction of the former than for the latter, or, in other words, 
the earliest embryonic cells of this part of the body must 
multiply more rapidly in one species than the other. If now 
this mode of cell-division is determmed by the specific nature 
of the above-named centrosomata of such cells, it follows that 
embyrogeny must be essentially controlled by the centrosoma, 
i. e. by a part which lies in the cell-body, and which we have 
hitherto regarded as a part of it. 

We do not however know that this is really the case ; pos- 
sibly the centrosoma may have been originally derived from 
the nucleus. But even if we admit that it is, not only in posi- 
tion but also in origin, a part of the cell-body, we must never- 
theless believe that its activity is dependent on the nucleus and 
nuclear substance. The centrosomata form the active, and thus 
the chief part of that remarkable mechanism which controls 
nuclear division. If this mechanism is once set in motion, it 
completes the division in the manner described above, just as a 
spinning machine twists its numerous threads, but that the 
apparatus is put in motion, does not depend upon itself, but 
obviously upon the internal conditions of the cell, which react 
upon the mechanism for division, so that it is compelled to 
enter upon activity. How can we otherwise understand Flem- 
ming's recent discovery that the centrosoma is always present 
in the cell-bod}^, but only periodically initiates the nuclear 
division ? Now the internal condition of the cell is, as we are 
aware, primarily determined, in all its qualities, by the nuclear 
substance, and consequently the centrosoma and the dependent 
mechanism for division are ultimately controlled by the nuclear 
substance, which regulates the rhythm of cell-division and 
dominates the whole structure of the organism. If it were 
otherwise, this nuclear material could not be the hereditary 
substance — the material basis of hereditary qualities \ 

^ Fol's recent observation that the centrosomata of ovum and spermato- 


We know little at present about the detail of processes going 
on in the cell, and mediating between nucleus and cell-body 
and between this latter and the centrosoma ; but I believe that 
this at any rate may be regarded as certain, viz. that everything 
which occurs in the cell, including the rhythm and the manner 
of its multiplication, depends upon the nuclear substance. But 
if this be so we cannot neglect its quantity : there must be a 
minimum amount of tiudear substance below which the control over 
the vital processes of the cell cannot be completely exercised. If this 
be correct, we shall be justified in explaining the cases of excep- 
tional parthenogenesis by the assumption, that the nucleoplasm 
of certain eggs possesses a greater power of growth than that 
of the majority of eggs of the same species ; while in the case 
of the bee, every ovum possesses a power of growth sufficient 
to double its nuclear substance, after reduction to half,— that is, 
when it is not raised to the full amount by means of fertilization. 

This explanation, so far as I can see, is in complete agree- 
ment with all the facts of the case, and especially with the 
observations by which various investigators were, in earlier 
times, enabled to show that the unfertilized eggs of various 
species of animals, e.g. the silk-worm moth {Bombyx mori), 
frequently enter upon an embryonic development which is never 
completed, but is arrested at an earlier or later stage. This 
becomes intelligible if we suppose that the cell is controlled by 
the quantity of nucleoplasm. According as the germ-plasm, 
diminished to half by expulsion of the two polar bodies, 
possesses a weaker or stronger power of growth, it will 
follow that its quantity will be sufficient to bring about the 
first divisions of the ovum, but not to complete the whole em- 

zoon divide during fertilization, and that the halves fuse together to form 
the two pole-bodies of the first segmentation spindle, agrees well with 
this view. Fol, ' La Quadrille des Centres,' Geneve, 1891. Moreover 
this observation does not include anything which need surprise us, 
because nothing takes place except that which precedes every nuclear 
division, viz. the doubling of the centrosoma. The two sexual nuclei 
behave exactly like any other nuclei : even as regards outward appear- 
ance they may retain their independence for a long time in certain 
species, and fusion into a single nucleus only occurs at a later stage of 
segmentation. The evidence for this statement is afforded by observa- 
tions upon Cydopidae by Dr. Ischikawa, communicated to me in letters, 
and independently by the researches of my assistant, Dr. Hacker, upon 
the same animals. 


bryogeny, or, on the other hand, will suffice to bring it to 
completion. In an earher work I have endeavoured to render 
this theoretically intelligible and must here refer to that at- 
tempt \ But I should wish to add in this place that I have, 
since then, convinced myself that the view which I urged is 
correct. In conjunction with Dr. Ischikawa, I have examined 
the eggs of many Lepidoptera as to the power of development 
without fertilization : we observed that, as a matter of fact, 
some eggs entered upon embrj^ogeny, which was, however, 
sooner or later arrested in most of them, only a very few 
reaching the caterpillar stage. Out of about a hundred unfer- 
tilized ova ofAg/m fan, we obtained only a single fully developed 
caterpillar, many eggs shrivelled after a few days, while others 
remained plump : in most of the latter the yolk contained a 
large number of blastoderm cells ; for a whole month these 
eggs developed very slowly and irregularly^, but they finally 
shrivelled and decayed. The ova of one and the same female 
vary in respect to their powers of parthenogenetic development, 
and such individual diiferences cannot lie in the yolk, inasmuch 
as this nutritive material is distributed in the same manner and 
in equal amount in all eggs : they must rather be referred to 
differences in the rate of growth of the germ-plasm ; at any 
rate, I cannot imagine any other cause which might account for 

But this conclusion does not carry the impHcation that par- 
thenogenesis could not have arisen by the method which was 
first indicated, viz. by the suppression of the second polar body. 
Indeed, I am inclined to believe that regular parthenogenesis 
has invariably arisen in this way ; for otherwise the absence of 
the second polar body would not be so common, nor would it 
be without exception. This method cannot however obtain in 
facultative parthenogenesis, because that very egg which is 
capable of parthenogenetic development must also remain 
capable of fertilization. But this latter capability involves that 
reduction of the germ-plasm which occurs by means of the 
second polar division. In those cases in which parthenogenesis 
became necessary, and at the same time the capacity for fertili- 

^ Continuity of Germ-plasm.' Jena, 1885, pp. 92 et seqq. Translated 
as the fourth essay; see Vol. I. pp. 231 et seqq. 

^ The observations were not directed to the details of embryogeny. 


zation had to be retained, there remained nothing except to 
strengthen the ordinary process of egg-maturation and thus 
to endow the retained half of the germ-plasm with increased 
powers of growth. 

III. Amphimixis as the Significance of Conjugation and 

The Facts of Conjugation. 

Biologists have been, for some time, in the habit of comparing 
the conjugation of unicellular organisms with the sexual repro- 
duction of multicellular forms of life, and of regarding them as 
to some extent equivalent. There was an obvious comparison 
between the more or less complete fusion of two of the former, 
and the coalescence of the two sexual cells of the latter ; and this 
conception was strengthened when observation appeared to 
prove that the reproduction of unicellular beings by means of 
fission could not continue indefinitely, unless conjugation took 
place from time to time. Conjugation was looked upon as a 
' fertiHzing ' process which endowed the organism anew with the 
capacity for fission, not once only but repeatedly, just as fertiliza- 
tion in multicellular beings renders possible the production of 
numerous cell-generations, constituting embryogeny. The cell 
material which, in the latter case, is made use of in building up 
the multicellular organism, appears in the former as a succession 
of many generations of unicellular beings ; but, in both cases, 
the capacity for such cell multipHcation depends upon the pre- 
vious occurrence of a fusion of cells, thus originating the life- 
giving force which renders reproduction possible. 

The above sentences form an approximate statement of the 
views which, with some individual differences, have obtained 
among biologists during the decade before the last. Even the 
remarkable discoveries of Biitschli on the conjugation of Infu- 
soria led to no essential modification, although they taught us to 
recognize the mysterious nuclear changes, the analogy of which 
to the processes of fertilization was then unknown. 

However, mainly in consequence of the observations of the 
brothers Hertwig, of Fol and of E. van Beneden, this analogy is 
now recognized, and we may admit that the connection between 


conjugation and fertilization is firmly established, more espe- 
cially since the investigations on the conjugation of Infusoria, 
begun by Biitschli, have been carried to a high degree of com- 
pleteness by the work of Balbiani, Engelmann, Gruber, R. 
Hertwig, and above all by the exhaustive and wonderful in- 
vestigations of Maupas ^ 

But even if we may at length regard the agreement between 
the processes of reproduction and conjugation as firmly estab- 
lished, and the ideas of an earher date confirmed, we cannot, in 
my opinion, retain the former conceptions as to the deeper 
significance of these two processes. Both conjugation and 
fertilization appear in an entirely new light if, — leaving behind 
all ancient prejudices, and without bias — we examine and 
compare them from the standpoint of our present knowledge. 
Each process throws light upon the other, and the true meaning 
of both is thus made clear. 

I will first briefly recapitulate the facts of conjugation as 
established by Maupas and ably confirmed and extended by 
R. Hertwig, and I have therefore appended in Fig. XI. a free 
rendering of Maupas' figures, which illustrate the changes in 
the nucleus accompanying the conjugation of Paramaecium 
caudaium. M indicates the macronucleus, m the micronucleus ; 
m^ and nt^, in figure 3, signify the two daughter-nuclei which 
arise from the first division of the micronucleus ; ni^ — m*, in 
figure 4, the four grand- daughter-nuclei of the same, derived 
from the fission of the daughter-nuclei. In figure 5, three of 
these, m^—m^, are already disintegrating, while the fourth, m*, 
is drawn out into a spindle preparatory to division, and the 
consequent formation of the two reproductive nuclei, Cop^ and 
CopK Figure 6 shows the reciprocal transference of the male 
reproductive nucleus from each animal into the other ; and 

^ We should read the admirable work of Maupas with even greater satis- 
faction if it contained fewer reflexions upon those w^ho have worked in 
the same field. Maupas should not have forgotten that even the ablest 
cannot avoid error, and that it is the fate of all work, even the most 
excellent, to be in time surpassed ; — for upon this the whole advance of 
science depends. We may correct the mistakes of our predecessors 
without forgetting that we stand on their shoulders. The very power 
we possess of improving on them is largely due to the fact that they have 
placed their successors upon a higher level than that from which they 
started themselves, and it is but a poor return for this to label their work 
'superficial/ ' inaccurate/ &c., &c. 




figure 7, the fusion of the male and female nuclei to form the 
germ-nucleus, Kk. 

The essential part of this process is shown even more clearly 
in the annexed diagrammatic representation of the changes 
undergone by the micronucleus, which Maupas has constructed 
for Colpidium triincatum. Fig. XII. illustrates diagrammatically 
the nuclear changes of two conjugating individuals of this 
species. The black spheres represent the persistent nuclei, 
while the circles stand for those which disintegrate and dis- 
appear. Similar processes take place in each individual of the 
conjugating pair. The micronucleus first grows from its pre- 
vious small size, A^, to a considerable bulk, and it is shown in 
^^ as ready for the first fission, producing the two nuclei (B). 
Each of these daughter-nuclei again divides, and thus the four 
grand-daughter-nuclei arise (C). Three of these disintegrate 
and disappear, while one divides and produces two nuclei (D) 
comparable with the sperm- and egg-nuclei of Metazoa. We 
may call these the male and female reproductive nuclei, and may 
regard that as the male which leaves the animal in which it had 
its birth and enters the other organism in order to fuse with 
its female reproductive nucleus. This fusion, represented at E 
in the diagram, leads to the production of the ' combination 
nucleus \' the analogue of the ' germ-nucleus ' of fertilization. 

The old macronucleus disintegrates and is absorbed, but by 
the double division of the ' combination-nucleus ' two new macro- 
and two new micronuclei arise, preliminary to the first fission 
of the whole animal which now commences. 

The essential part of the whole process is the fusion of two 
equal amounts of nuclear substance, the one derived from one 
animal and the other from another, and the formation from this 
nuclear substance, thus derived from two individuals, of the 
nuclei which dominate the animals after conjugation. This 
harmonizes with the process of fertilization in that here also 
two equal masses of nuclear substance, derived from two dif- 
ferent individuals, unite to form the new germ-nucleus. Now 
that we at length recognize that the ' nuclear substance ' is the 
ruHng principle of the cell, that Nageli's 'idioplasm' is the 

^ By this term I mean a nucleus which has arisen by amphimixis, and 
consists of equal amounts of idioplasm from two individuals. 

N 2 


hereditary substance, we are enabled to state that the essence 
of both conjugation and fertiUzation is nothing more than a 
mingling of the hereditary substances of two individuals. I pro- 

hifusorian A 

M M 

Infusorian B 

M M 

¥ ¥ 

I 1 

Fig. XII. 
Diagram of the changes undergone by the micronucleus during the 
canjugation of a Ciliate Infusorian ; (after the diagram given by Maupas 
in the case of Colpidium truncatum) . 

pose to introduce the term Amphimixis to indicate such a 
process of mingling of the idioplasm from two individuals. 
The usefulness and indeed the necessity for some such special 


term will soon be apparent. If we next consider the pheno- 
mena which have been directly observed, we find that, in spite 
of the already mentioned fundamental agreement between the 
two forms of amphimixis (conjugation and fertilization), there 
are some not unimportant differences between them. 

This is partly due to the fact that those Infusoria which have 
supphed the most familiar examples of conjugation, possess two 
kinds of nuclei, the macronucleus and the micronucleus. To 
the former is attributed the vegetative functions, while the latter 
has been termed the 'generative nucleus.' It is certain that 
both nuclei proceed from the same material, viz. from the 
combination-nucleus of the animals after conjugation, that is, 
from a germ-nucleus. It is thus established that their differen- 
tiation depends on the principle of division of labour, and 
Maupas probably comes near the truth when he attributes to 
the macronucleus a ' bon fonctionnement des organes de la vie 
vegetative et a la forme individuelle,'— a conception which does 
not precisely coincide with that of BiitschH, Gruber, and Hert- 
wig, who regard it as an * assimilative nucleus ' only. Ascer- 
tained facts indicate that the micronucleus, in the first place, 
sub-serves amphimixis ; for it and it alone produces the re- 
productive nuclei. But we must beware of restricting its 
activity to this single function. Numerous facts tend to show 
that it has another function, in addition to that which relates to 
the periods of conjugation. In many species there is not one 
micronucleus, but two of them, which are found regularly 
through the whole period of fission, although only one takes 
part in conjugation, while the other disintegrates. In other 
species numerous micronuclei exist, and in Stentor Roeselii there 
are eight-and-twenty regularly distributed through the whole 
animal. This indicates that during the period of multiplication 
of the Infusorian its mass of idioplasm must be greater than 
during the period of conjugation, and this again points to some 
special activity during the former period, I do not know of 
what kind this activity is, and do not care to speculate, since 
the question has no bearing upon our present subject. This 
much, however, is determined, that as regards conjugation, the 
micronuclei bring about the continuity of the germ-plasm. Among 
the Metazoa this continuity is not, in many cases, effected so 
directly and visibly, but it is brought about, as I believe, by 


minute invisible masses of germ-plasm, which arise from the 
^gg and are afterwards carried on, mingled with the contents of 
certain somatic cells. In these cases the origin of such masses 
in the ^gg can only be conjectured, but in conjugation observa- 
tion shows that a part of the idioplasm is, as a matter of fact, set 
apart in the form of micronuclei for the use of the next genera- 
tion. The nuclear substance of the micronucleus alone is 
undying, and continues the vital processes without limit, while 
the macronucleus behaves, in this respect, in an entirely dif- 
ferent manner. 

In the Metazoa the whole cellular structure of the body — the 
soma— is worn out by the processes of life, and suffers natural 
death : in just the same way the Infusorian macronucleus can- 
not continue its functions for unlimited generations, but must be 
renewed from time to time ; and indeed, as we have already 
seen, it is formed anew from the combination nucleus which 
originates in the amphimixis of the two reproductive nuclei. 
During the formation of the new macronucleus the old one is 
destro3^ed and disappears. These are processes which have no 
analogy with fertilization : I shall return to their deeper signifi- 
cance later on. 

A further difference between fertiHzation and conjugation lies 
in the fact that the reproductive nuclei of Infusoria arise from the 
thrice-repeated nuclear division of the micronucleus, while the 
nuclei of the ^gg- and sperm-cells of Metazoa are derived from 
the twice-repeated division of the mother-cell. 

Meaning of the Phenomena. 

It ma}^ appear decidedly premature to attempt an explana- 
tion of the above-mentioned differences and resemblances 
between the two forms of amphimixis ; but I am willing 
to undertake this responsibihty, if only to give a fixed 
direction to further investigation. If I abandon all the theo- 
retical conceptions of fertilization and heredity developed in 
my earlier writings, I do not believe that we need, on this 
account, give up all views upon the processes of conjugation 
as they are known to-day, but rather that future research 
will be more profitable if we endeavour to test some settled 
theory, instead of making observations with no object in view. 

The preparatory divisions of the micronucleus have been 


frequently compared to the formation of polar bodies in the 
animal ^gg. If we consider the physiological significance of 
the two processes, this comparison is certainly striking, but it is 
incorrect to push it so far as the attempt to homologize the 
separate phases ' and to explain them as morphologically equi- 
valent ; for all homology between two living forms depends 
upon their similar origin, and no one can believe that the 
higher animals have originated from the Ciliate Infusoria. 
The kind of conjugation exhibited by the latter is widely re- 
moved from its simplest form, occurring among the lower 
Protozoa, and an}' direct connection between the conjugation 
of Ciliata and the sexual reproduction of Metazoa cannot be 
assumed. Hence any attempt to homologize the separate phases 
of these two kinds of amphimixis must fail, although the pro- 
cesses are in their essence certainly homologous ; for both have 
sprung from the same root, — the conjugation of the lowest forms 
of living beings. 

I shall, however, attempt to show that many of the details of 
the two processes possess a corresponding significance, which 
must therefore be very deeply rooted, inasmuch as similar 
events have not been called forth by a common origin but by 
physiological necessity ; just as the eyes discovered by Semper -^ 
on the back of certain slug-like Molluscs {Oncidium) resemble 
Vertebrate eyes, not because the Molluscs have been derived 
from Vertebrates, or vice versa, but because the necessity for 
eyes has called forth such a structure out of the foundation 
provided by the fundamental nature of hght and the histological 
details of the Oncidium' s dorsal surface. 

I find the foundation of my explanation of the nuclear divi- 
sions accompanying amphimixis in the fact that the micronucleus 
of Infusoria possesses nuclear rods or idants, the proof of which 
we owe to the researches of Pfitzner^, R. Bergh *, Maupas, and 
Balbiani^. This fact indicates that the structure of the idio- 

' A. Giard, ' Sur les globules polaires et les Homologues de ces 
elementfe chez les infusoires cilies.' Paris, 1890. 

- C. Semper, ' Ueber Schneckenaugen vom Wirbelthiert^^pus.' 

^ Pfitzner, ' Zur Kenntniss der Kernthcilung von Opalina ranarum.' 
Morph. Jahrbuch, Bd. XI. p. 454; 1886. 

^ R. Bergh, ' Recherches sur les noyaux de TUrostyla.' Liege, 1889. 

•'' Balbiani, ' Sur la structure intime du noyau de Lo.xophyllunt meleagris.' 
Zool. Anzeiger, No. 329 and 330 ; 1890. 


plasm in Infusoria corresponds with that in Metazoa, and we 
are justified in transferring to these Protozoa the conceptions 
at which we have arrived as to the relation and significance of 
the Metazoan idioplasm, and, above all, the conception of the 
individual difference of nuclear idants. 

R. Bergh's researches upon Urostyla grandis prove that the 
spindle of the micronucleus contains, during division, nine rod- 
like idants (see his fig. 9). Since, however, only one side of 
the spindle is represented in the drawing, the total number of 
idants must be eighteen. All who have observed the phenomena 
of conjugation agree that the first preparatory change in the 
micronucleus consists in a considerable enlargement '^. Maupas'^ 
gives a series of fourteen figures illustrating this increase in 
the size of the micronucleus and its conversion into a spindle, 
and he calculates that, during this period, its original mass is 
multiplied eight-fold. 

Richard Hertwig ^, who has directed special attention to this 
point, found that the micronucleus of a Paramaecimn, imme- 
diately after division, was extremely small, — less than three 
microns* in diameter, while that of the micronucleus of an 
animal previous to conjugation was about seventy-five microns. 

This enormous increase in size largely depends on the growth 
of the achromatin substance which plays a most essential and 
remarkable part in the subsequent divisions, but it does not 
therefore follow that there is no simultaneous increase in the 
idioplasm. I assume that the increase of the micronucleus is 
connected with a doubling of the idants by longitudinal division. 
There is at present no proof of this assumption ; for no one has 

^ Schewiakoff's beautiful observations (' Ueber die karyokinetische 
Kerntheilung der Euglypha alveolata ; ' Morpholog. Jahrbuch, Bd. XIII. 
p. 193; 1888), show that the Infusoria are not the only Protozoa possess- 
ing idioplasm in the form of idants. Not only are idants (chromatosomes) 
shown to exist in the form of loops, but their behaviour during karyo- 
kinesis is so accurately described as to leave no doubt that an ' equal 
division ' is its outcome. The longitudinal splitting of the loops was 
observed not only in microscopic preparations, but in the living animal 
in the act of dividing. It is clear that Euglypha is well adapted for 
observation, and it would be of great value to investigate the relations of 
its nucleus during conjugation from the standpoint of this essay, 

^ Maupas, ' Le rajeunissement karyogamique chez les Cilies.' Archives 
de Zool. exper. et gen. 2 ser. Vol, VII. PI. IX. Figs. 1-14. Paris, 1890. 

•' R. Hertwig, ' Ueber die Conjugation d, Infusorien.' Munich, 1889. 

* A micron or n is the j^Vjt o^ ^ millimetre. 


yet compared the number of the idants in a micronucleus pre- 
paring for conjugation with the number in a micronucleus of 
an Infusorian in the act of fission ; and the few figures which 
we possess, of either of these stages, afford us no reliable 
information on the point. The figures which Maupas gives 
of the micronucleus preparatory to conjugation in Paramaecium 
caudatum and Onychodromus grandis, lend support to my view, 
in so far as the number of idants is very large. In the first 
species I counted twenty-one in the half spindle which is 
figured, giving a total of about forty-two. But I will not lay 
too much stress on this point ; the simplicity of my attempted 
explanation of the changes in the micronucleus appears to me 
to be strongly in favour of the view upon which the explanation 
is based. 

If this assumption be well founded it provides a very simple 
solution of the problem of the complex divisions and repeated 
disintegrations of the micronucleus. The first and second divi- 
sions are reducing divisions which diminish the previously doubled 
idants to half the normal number, corresponding exactly to the 
'reducing divisions' of sperm- and egg- mother-cell. The third 
division, however, which produces the two reproductive nuclei 
(male and female), from one of the four grand-daughter-nuclei 
of the micronucleus, is an ' equal division' which causes each 
daughter-nucleus to contain as many idants as were possessed 
by the parent nucleus. This last division has no analogue in 
Metazoa, simply because their germ-cells are invariably either 
male or female, while the Infusorian micronucleus must give 
rise to both kinds of reproductive nuclei. 

Three out of the four grand-daughter-nuclei of the micro- 
nucleus disappear, only one dividing to form the reproductive 
nuclei [D in the diagram. Fig. XII.). The fact that the others 
disintegrate can be understood in so far that they are super- 
fluous and functionless, just like the polar cells of the animal 
egg. It is more difficult to explain why these three are always 
present, and still harder to find the true reason, the causa 
ejficiens, of their disintegration. 

With regard to this last question, an observation of Maupas 
may put us on the right road. He believes that he has observed 
that, of the four grand-daughter-nuclei derived from the micro- 
nucleus, the one which lies nearest to the bridge connecting the 


two conjugating animals invariably gives rise to the reproductive 
nuclei. This is alone capable of further development, while the 
three which occupy more remote positions are destined to 
disintegrate and disappear. It is only the accident of position 
which fixes upon that one of the four which shall undergo 

If this be true, the causa ejficiens which decides upon that one 
of the grand-daughter-nuclei which shall give rise to the repro- 
ductive nuclei must be sought for in some influence which is 
exercised by the corresponding nucleus of the other animal, 
and which naturally affects most strongl}^ that nucleus which 
lies nearest to it. 

At any rate we are justified in assuming that the idioplasm 
of the four grand-daughter-nuclei of the micronucleus is, apart 
from individual differences, essentially similar, i.e. that each 
contains the same number of idants in the same stage of 
development, and this number will be half that which is normal 
for the species in question. Thus nine would be the number in 
Urostyla grandis, which would be reached in the following 
manner. According to my supposition, during the growth of 
the micronucleus from A^ to A^ (see Fig. XII), the i8 idants are 
doubled by longitudinal fission, becoming 36 ; the two following 
' reducing divisions ' not only diminish the idants from 36 to 18 
in stage B, and from 18 to 9 in stage C, but lead to a fresh 
grouping of the idants, just as in the analogous ' reducing divisions ' 
of the egg- and sperm-cell. Since the 18 idants are doubled, it is 
clear that each one of them will be represented by two idants in 
the enlarged micronucleus of stage A"^, and hence the two 
' reducing divisions ' can originate a number of difterent com- 
binations of 9 idants, just as in the Qgg- and sperm-cell, described 
in the first part of this essay. 

Although in any single individual, only four out of the 
numerous possible combinations would become actual, we may 
perhaps perceive, — in this very fact that there are always at least 
four different possibilities to select from,— the reason why all 
four grand-daughter-nuclei of the micronucleus are formed, 
and why both the daughter-nuclei undergo the second ' reduc- 
ing division,' while the division of one of them alone would 
suffice to ensure the origin of two reproductive nuclei. 



It will be urged against my views that they are based upon 
a method of formation of the reproductive nuclei, which, although 
common among Infusoria, is by no means the only one. As 
a matter of fact, Maupas, whose researches form the only 
foundation for this part of my argument, describes another 
method in the Oxytrichidae. If I neglect the fact that in this case 
two micronuclei are found in the animal preparatory to conju- 
gation, it is because this difference is merely due to the fact that 
two of the grand-daughter-nuclei (instead of only one) undergo 
a second division. Thus two pairs of reproductive nuclei 
arise, of which only one is functional, while the other disin- 
tegrates. But the theoretical explanation is in no way affected 
by these observations. 

The only facts which do not at once harmonize with my 
view is the behaviour of the micronucleus in male Vorticellidae. 
In this case the period of growth of the micronucleus (stages 
A^ — A^) is preceded by its division. I cannot at present 
explain this, unless it simply means that instead of four dif- 
ferent combinations of idants out of which one functional 
reproductive nucleus is to be chosen, eight are in this case 
afforded. A glance at the figure given by Maupas (op. cit. 
p. 364) at once renders this suggestion clear. In any case, the 
extra division must be an ' equal division.' 

Thus the departures from the ordinary modes of division of 
the micronucleus raise no definite objection to my explanation. 

Evidence that the processes which I have explained as 
' reducing divisions ' are really such, is afforded by some of the 
figures given by Maupas, as in figs. 9-13 on Plate XVIII, in 
which the development of the spindle for the nuclear division of 
Onychodromus grandis is represented. The rod-like chromato- 
somes lie longitudinally in the spindle, and appear to be dividing 
transversely. Since we must imagine that the ids are arranged 
lengthwise, the transverse division of the idants must lead to 
a diminution in the number of ids in each rod to half their 
original number. Complete certainty cannot, however, be 
attained by an examination of these figures ; the matter must 
be settled by fresh observations, especially directed to the point. 
The whole mechanism of nuclear division differs in essential 


points from that of the Metazoa, so that without first making 
renewed investigations it is impossible to form a correct idea 
as to what should be regarded as a ' reducing division.' 

According to my view, the explanation of the thrice-repeated 
division of the micronucleus consists, on the one hand, in the 
reduction of the number of idants and their arrangement in new 
combinations, and, on the other hand, in the differentiation of 
the two reproductive nuclei. 

Those who agree with me in looking upon amphimixis as the 
union of idioplasms built up of ids from two individuals, will not 
hesitate to believe that the ids are reduced to half the normal 
number. It is impossible that there can, in this respect, be any 
difference between the amphimixis of unicellular organisms and 
that of Metazoa. It is not equally certain that my view of the 
production of fresh combinations of idioplasm by means of am- 
phimixis can be proved in the Protozoa. It might be objected 
that it is useless for one Protozoon to possess the theoretical pos- 
sibility of producing a great number of individual varieties of idio- 
plasm, because each single animal is only able to utiHze one out 
of many possible combinations. The two animals which com- 
menced conjugation remain at the end of it, and there is no 
increase in number : hence the different nuclei which originated 
from the ' reducing divisions ' cannot be divided among dif- 
ferent animals, as is possible in the case of the four sperm-cells 
which are formed by one sperm-mother-cell, and which contain 
four different combinations of idants. 

This objection is easily met, for exactly the same thing 
happens in the development of the ova in Metazoa. Just as 
only a single &gg, with a single combination of idants, can 
proceed from each egg-mother-cell, while the other three com- 
binations disappear in the polar cells,— so, in this case, three 
grand-daughter-nuclei of the micronucleus disappear, and one 
only persists. The process receives a meaning when we 
remember that countless numbers of egg-mother-cells, con- 
taining precisely similar combinations of idants, are destroyed 
by the process of arranging the idants in fresh groups. The same 
explanation holds among Infusoria; for here also countless indi- 
viduals contain precisely similar combinations, this being true of 
all individuals which are derived from either of the animals 
proceeding from an}' one conjugation. Just as the collective 


egg-cells of a mother would contain identical germ-plasm, it 
they did not undergo the * reducing divisions ' before reaching 
maturity,— so all the descendants of an Infusorian after conju- 
gation would contain similar combinations of idants, if the 
repeated ' reducing divisions ' did not precede the formation of 
the reproductive nuclei. 

Variety of individual character in the hereditary substance is thus 
brought about by means of these divisions. 

The Deeper Significance of Conjugation. 

No one will attempt to oppose the view that the deeper 
meaning of conjugation is closely connected with that of sexual 
reproduction. The process is, in both cases, that of nuclear 
fusion, and, in fact, the formation of a complete nucleus by the 
union of two ' half-nuclei,' as they may be called, that is, two 
nuclei which contain only half the normal amount of hereditary 
substance or idioplasm, and only half the normal number of 
individual hereditary units or ids. From this fusion a new 
nucleus is formed which contains that amount of hereditary 
substance and that number of ids which are normal to the 
species. This is my explanation of the process of fertilization 
in the Metazoa, an explanation which I can extend to the 
Protozoa, now that the long looked for and, indeed, partially 
observed nuclear fusions accompanying conjugation have been 
proved by Maupas to be actual facts. Those who do not accept 
my theory of ids can only maintain that the nuclear fusion of 
conjugation and fertilization leads to the formation of a new 
nucleus by the fusion of two equal masses of individually distinct 
hereditary substance or idioplasm. 

The view which I expressed in 1873, and which has since 
then been established by Strasburger, O. Hertwig, and myself, 
of the essential similarity of the male and female sexual cells, 
can now be confidently extended to conjugation; for Maupas 
has already acknowledged the two reproductive nuclei to be 
essentially similar. They certainly are so, inasmuch as they 
exhibit no traces of the fundamental antagonism which has 
been spoken of as a ' male and female principle ' in the egg- and 

If we may now assume that the nuclear substance which 


arises in the same way in Infusoria and Metazoa bears a similar 
significance in both, we may then proceed to the conclusion set 
forth above that conjugation and fertilization are both essentially 
concerned with the mingling of the hereditary tendencies of two 

At the time when I developed this view, which sought the 
ultimate meaning and true cause of the existence of sexual 
reproduction in the continual supply of fresh combinations of 
hereditary tendencies, I contrasted the Metazoa and Metaphyta 
on the one hand with the Protozoa and Protophyta on the 
other, and maintained that the chief sources of variability in the 
former, the multicellular beings, viz. the external influences 
(including the effects of use and disuse) which alter the body, can 
have no influence on the processes of selection which alter the 
species, because their effects are somatogenic and as such cannot 
be inherited. Only those predispositions can be inherited which 
are contained in the germ-plasm, but these are either entirely un- 
influenced by external agencies, or, if altered at all, only very 
rarely in the same direction as that taken by the somatogenic 
changes which follow the same cause. Although I naturally 
did not assume that the germ-plasm itself was entirely un- 
changed by external influences, the extraordinary persistence 
of heredity taught me that the change was small and could only 
take place by imperceptibly small steps. Such causes might 
well have been the source of the gradual uniform changes in all 
individuals of a species, if the latter were subjected to the same 
modifying influences during long series of generations, but not 
the source of the countless individual differences, ever-varying 
in direction. This protean individual variability is the indispen- 
sable preliminary to all processes of selection, and the unceasing 
mingling of individual hereditary tendencies, which is brought 
about by sexual reproduction, was in my opinion the source of 
this variability. I am now, if possible, more firmly convinced 
than ever of the soundness of this view, and I wish to extend 
it in one direction. 

At the time I have been speaking of, I looked upon unicellu- 
lar beings as organisms in which external influences could 
directly call forth hereditary changes ; for in them reproduction 
involved the fission of the cell so that changes undergone by 
the latter must be transmissible to either half. As an example, 


I selected a Moneron as defined by Hackel, viz. an organism 
without a nucleus. I purposely abstained from considering 
those unicellular beings which possess nuclei, because I was 
then only concerned with bringing forward the general concep- 
tion that sexual reproduction exists in order to ensure indivi- 
dual variability. I was, however, well aware that in the nu- 
cleated Protozoa, and especially in those Infusoria, which 
although unicellular are extremely highly differentiated, such 
a simple transmission of acquired peculiarities was hardly con- 
ceivable. Now that we possess accurate knowledge of the most 
essential points in the process of conjugation, it is possible to 
approach this problem somewhat more closely. 

The fact, as we now know it, that conjugation in Infusoria is 
a mingling of the nuclear substances of two individuals, permits 
the conclusion that, in these animals, the whole individuality of 
the cell, and thus of the cell-body, is contained in the nuclear 
material as predispositions or hereditary tendencies, just in the 
same manner as has been proved in the case of the germ-cells 
of the Metazoa. Nussbaum's experiments upon the artificial 
fission of Infusoria, and those which Gruber undertook at 
my suggestion in the Zoological Institute at Freiburg, prove that 
the nucleus determines the regeneration of the mutilated animal, 
and that it contains, in some way, the essence of the whole or- 
ganism in all its details. Hence we must believe that all those 
variations which appear in Infusoria, in consequence of external 
influences, can only pass on to the products of fission when they 
are accompanied by corresponding changes of the nuclear substance ; 
or, in other words, we come to the conclusion that the heredi- 
tary transmission of somatogenic changes does not, as a rule, 
take place, or only does so when they are accompanied by 
corresponding blastogenic changes. The use of both these ex- 
pressions only implies a correspondence, and not a similarity of 
application, the ' soma ' of Metazoa corresponding to the cell- 
body of Infusoria, the ' germ ' to the nuclear substance. The 
broken bristles of an Infusorian are renewed in the products of 
fission because the predisposition to form them is contained in 
the nuclear substance. Mutilation is no more hereditary here 
than in the Metazoa. Furthermore, all changes in the cell-body 
of an Infusorian are not accompanied by corresponding changes 
in the nuclear substance, and all cannot therefore be inherited ; 


not only is this the case but it seems very questionable whether 
the changes originated by use and disuse are in any way more 
hereditary than they are in Metazoa. There are no direct ob- 
servations to test whether any of the cilia in an Infusorian 
could be strengthened by increased use, either in connexion 
with some new kind of food, or with a struggle against stronger 
currents in water; but we need not doubt that in these or- 
ganisms, small and relatively simple as they are, functional hy- 
pertrophy and atrophy play the same role as in larger and more 
complex beings. I would refer my readers to Wilhelm Roux's 
excellent treatise on this subject in the higher organisms. If 
certain cilia in an Infusorian were to increase in size as the result 
of more active function, how can we conceive the transmission 
of this change to the hereditary substance contained in the 
nucleus? The path is certainly shorter than that from the 
human brain and finger muscles to the reproductive cells ; but 
distance, like all measurements, is only a relative idea, and the 
question arises whether there is any ground for the assumption 
that such increased growth in the cilia causes any corresponding 
change in the nuclear substance of the animal. But if this does 
not occur, any inheritance of acquired characters is as impos- 
sible as it is in man. How, for instance, can an increase of the 
adoral ciliated zone of a Stentor be transmitted to both the 
products of its fission, considering that the hindmost of these 
has to form an entirely new mouth ? It might perhaps be 
pointed out that R. Hertwig believes he has seen the mouth 
of the hinder offspring arise by budding from that of the an- 
terior; but the artificial division of Stentor, as eff'ected by 
Gruber, proves that the mouth of the posterior part is not 
dependent on the existence of the original mouth, but can 
arise quite independently, provided only that a portion of the 
nucleus is present. 

I therefore hold that a belief in the inheritance of acquired 
characters by the highly differentiated Protozoa, as well as by Me- 
tazoa, must be opposed, and I imagine that the phyletic modifications 
of Protozoa arise from the germ-plasm, that is from the idioplasm 
of the nucleus. 

We can now understand why nature has laid so much stress 
on the periodical mingling of the nuclear substances of two 
individuals, — why she has introduced amphimixis among these 


animals. Clearly it has arisen from the necessity of providing 
the process of natural selection with a continually changing 
material, by the combinations of individual characters. 

Amphimixis in all Unicellular Organisms. 

We may extend this conception and enquire w^hether it may 
not, in reality, apply to all unicellular organisms, that is all 
v^hich possess a nucleus and cell-body. The conclusion can 
scarcely be avoided if it be admitted that the nucleus invariably 
bears the same essential significance, and this can hardly be 
doubted. If, as a matter of fact, the lowest, apparently struc- 
tureless unicellular organisms contain a nuclear substance which 
dominates and controls the entire animal, it follows that all 
lasting and therefore hereditary variations of both cell-body 
and nucleus must proceed from the latter, while those direct 
changes of the cell-body which are produced by external in- 
fluences, are as incapable of hereditary transmission as the 
mutilation of the body of an Infusorian. Thus changes in the 
molecular constitution of the cell-body, such as we might 
imagine to be the result of the exercise of particular functions 
(for example, the more powerful movements of an amoeba) would 
probably be transmitted to the immediate offspring, but would 
disappear with the cessation of those causes which rendered 
necessary the increased exercise of the function concerned. 

My earlier views on unicellular organisms as the source of 
individual differences, in the sense that each change called forth 
in them by external influences, or by use and disuse, was sup- 
posed to be hereditary, must therefore be dismissed to some 
stage less distant from the origin of life. I now believe that 
such reactions under external influences can only obtain in 
the lowest organisms which are without any distinction between 
nucleus and cell-body. All variations which have arisen in 
them, by the operation of any causes whatever, must be inherited, 
and their hereditary individual variability is due to the direct 
influence of the external world. Loss of substance must not 
however be included among such individual variations : repair 
would take place by regeneration in these simplest forms of life 
just as in higher Protozoa. At least, I think this is not con- 
tradicted by the fact that the molecular structure of such a 
Moneron, although without the guidance of a nucleus, retains 



a certain external form and limit of size, which it will regain 
after being mutilated. Growth and division are themselves the 
outcome of such tendencies implanted in the molecular structure: 
this, for example, is the case in Bacteria. The whole question 
comes to an end when we reach those lowest of all beings, 
which are entirely formless and have no fixed size, — beings 
which we must regard, little as we know about them, as crossing 
the very threshold of organic life'. 

It is interesting to observe that, from this point of view, the 
nucleus presents itself in a new light. By the agency of con- 
jugation and fertilization it becomes an organ for maintaining the 
constant renewal and transformation of hereditary individual vari- 
ability. Besides this, it plays the part of protecting the species 
from the too powerful effect of transforming external influences 
upon the body, inasmuch as it tends to prevent these from 
becoming hereditary, not indeed actively, but simply because 
every external influence does not cause a corresponding altera- 
tion in the nuclear substance, and thus the latter containing the 
older predispositions tends to restore, after each fission, the older 
condition of the cell-body. It simultaneously acts as a conserv- 
ing and as a progressive principle, exactly as the sexual cells of 
higher beings are, according to my views, supposed to behave. 
The reproductive cells exert a conserving force, inasmuch as 
they retain, with incredible tenacity, the hereditary tendencies 
contained within them, and, above all, because they are unaffected 
by those changes in the soma which are brought about by external 
influences : but they also act progressively by means of amphi- 
mixis and the consequent periodical minghng of the hereditary 
predispositions of two germ-cells, one from each parent, which 
as we have seen, takes place by the removal of half of these 
predispositions and by the arrangement of those which remain 
in fresh combinations. 

If I am correct in my view of the meaning of conjugation as a 
method of amphimixis, we must believe that all unicellular 
organisms possess it, and that it will be found in numerous low 
organisms, in which it has not yet been observed. But it is by 
no means safe to make the a priori assumption that conjugation 

^ Nageli, ' Mechanisch-physiologische Theorie der Abstaramungslehre.' 
Munich, 1884. 


may not also take place in the form of a fusion of two indi- 
viduals among the non-nucleated animals, the Monera : and it 
may be precisely here that a fusion of two whole animals 
with a view to the mingling of characters was first effected. 
We are acquainted with a form of conjugation in certain 
of the Bacillariaceae, and even if it is not absolutely certain 
that the species in question, Cocconeis pediciihts, is without 
a nucleus, many details of the process indicate that the whole 
mass of the organism contains the conjugating idioplasm : 
this is chiefly suggested by the minute size of the conjuga 
ting individuals, which invites comparison with the nucleus, 
diminished by ' reducing divisions,' in order to facilitate amphi- 
mixis. For this reason I believe that we ought not to follow 
Maupas in constructing a general definition of conjugation as 
the fusion of nuclei. 

The Theories of Rejvivenescence and of Mingling. 

I hold that the deeper significance of every form of amphi- 
mixis, — whether occurring in conjugation, fertilization, or in any 
other way, — consists in the creation of that hereditary individual 
variability which is requisite for the operation of the process of 
selection, and which arises from the periodical mingling of two 
individually different hereditary substances. 

That such a mingling is the immediate result of amphimixis 
is no longer open to dispute, and perhaps at no distant date it 
will be admitted that the variabihty I have spoken of must follow 
as a direct consequence. It is well known, however, that many 
and indeed the majority of scientific men, who have expressed 
themselves on the point, hold the opinion that this mingling of 
two hereditary substances is not the one object of amphimixis, 
— its ultimate and most important consequence, and does not ex- 
plain the reason why it was introduced into the organic world. 
It is obvious that my view as to the effect of amphimixis in 
originating variability may be perfectly correct, without the 
essence of fertilization or of conjugation being thereby ex- 
plained. What I regard as its chief object may after all only 
be secondary, and the true significance of the process may lie 
in some consequence unknown to me or which I have over- 

o 2 


We know that, up to the present time, fertihzation has been 
regarded as a vitalizing process, without which the development 
of the ^%g either cannot occur at all, or only exceptionally. I 
need not repeat what I have already said upon this idea in the 
first part of the essay, and it is not necessary to follow the 
gradual modifications which have been introduced ; but I should 
like to submit to a trial the support which the upholders of these 
views have always sought in the process of conjugation, and 
which they are still seeking to-day. 

Maupas, the able investigator of the vital processes of Infu- 
soria, considers that the effect of conjugation is such as to ensure 
the continuation of the species ; it imparts to the animal the 
power ' de renouveler et rajeunir les sources de la vie.' Hence, 
according to this view, the most profound significance of con- 
jugation is to be found in rejuvenescence, an idea which was 
long ago accepted and applied, sometimes to fertilization, some- 
times to conjugation, and sometimes to both together, by Biitschli, 
Engelmann, Hensen, E. van Beneden, and more recently by R. 
Hertwig. Maupas also looks upon these two processes as es- 
sentially similar, and regards both as a 'rejuvenescence,' 
without which life would, sooner or later, come to an end. He 
sharply distinguishes between this somewhat mystical rejuven- 
escence and that which consists in the renewal of many of the 
external parts of the animal, such as mouth, bristles, cilia, &c. 
Such regeneration is certainly connected with conjugation, but 
it also occurs at every fission of an Infusorian and cannot there- 
fore be an essential part of the former process. The rejuven- 
escence which Maupas regards as the essence of conjugation is 
something entirely different, and can hardly be described except 
as a ' renewal of vital force,' using the expression in the sense 
of the old natural philosophers. All other attempted definitions 
of this rejuvenescence are vague and unsatisfactory. It may 
well be doubted whether the return to a certain form of ' vital 
force' is in harmony with the physiology of to-day. On the 
other hand, no period of time has been entirely without an 
advocate of this principle, and quite recently the accomplished 
physiologist Bunge has, although with much reserve, again 
supported the ancient belief in a vital force. In any case we 
could only accept this idea if it were shown that there is no 
chance of explaining the phenomena of life, even in principle. 


without such acceptance. Bunge ^ is certainly correct in main- 
taining that we are not at present in a position to completely 
explain any of the simple processes of life from known 
chemical and physical forces ; but it by no means follows 
that they are inexplicable by such means. All we can say 
is, that everything that we do know about natural processes 
tells against the rejuvenescence of life by conjugation believed 
in by Maupas, as I have already pointed out in an earlier essay. 
To my mind it is difficult to understand how an almost exhausted 
vital force could be raised again to its original state of activity, 
as the consequence of a union with another equally exhausted 
force. Maupas can only reply that we do not understand the 
essence of any ' phenomene primordial' ; but if we cannot follow 
all the details of the chemical processes which for example 
bring about the phenomena of assimilation, because they are 
so extremely complex, and do not admit of our tracing the 
changes which succeed each other through the rapidl}'' shifting 
stages— because this is so, we do not therefore take refuge in the 
assumption that the whole process is unintelligible. But this, 
in my opinion, is the case with the 'rajeunissement karyoga- 
mique ' of which we know neither the beginning — the exhausted , 
condition of the vital force, nor the end — the rejuvenescence, 
nor any intermediate stage. The whole conception is simply 
a fancy, the outcome of earlier deeply rooted convictions as 
to the necessity of death and the 'vitalizing' influence of fer- 
tilization. I do not care, however, to base my opposition to 
Maupas' views on the rejection, as fundamentally untenable, of 
the theory of rejuvenescence ; the argument is superfluous. 

In considering how it is that amphimixis has come to be regarded 
as a renewal or rejuvenescence of vital force ^ the question naturally 
arises— why are we not content to see in this union of two nuclei, 
that which observation shows to us, viz. the union of two nuclear 
substances, and hence the mingling of two individually different 
hereditary predispositions ? Maupas himself admits that this 
occurs, and indeed allows that variability is favoured there- 
by, thus supplying the necessary material for processes of 
selection. Why are we not content with this explanation, why 
do we seek for something further ? 

^ Gustav Bunge, ' Vitalismusund Mechanismus' ; ein Vortrag. Leipzig, 


Obviously for no other reason but that we are saturated 
with the old notion that the egg cannot develope without fer- 
tilization, that fertilization is the same as vitaHzation. But was 
not this view overthrown long ago by facts ? Are we not aware 
that, under certain circumstances, the &gg can develope without 
fertilization ? And is not this often true, for example in the 
Bee and in Apus^ of that very ^g<g which is also capable of 
fertilization? No one would have regarded fertilization as 
the vitalizing of the &%% if the great majority of ova had 
developed parthenogenetically, or if science had first become 
acquainted with parthenogenesis and, later on, with fertilization. 
We should then have said that there must be some advantage 
in the mingling of two hereditary tendencies which has led to 
the introduction of amphimixis. But the facts are otherwise, — 
for centuries mankind has recognized this mingling as the indis- 
pensable antecedent to the development of offspring, and 
now, when we find that, under certain circumstances, an ^gg 
can develope without fertihzation, we are unable to get rid of 
the old prejudice in favour of the view that the mingling 
is something more than a mere preliminary to develop- 
ment,— that it is an accessory force which bears some special 
and entirely peculiar significance. We cling to some supposed 
after-effect of the vitalizing influence of fertihzation, ex- 
tending through many generations, and against such an 
illogical theory even facts fight in vain, for the number of 
generations through which this after-effect is supposed to ex- 
tend, is entirely dependent on the will of the controversialist, 
and keeps pace with the increasing length of the observed series 
of parthenogenetic generations. Maupas himself finds the num- 
ber of 'such generations, which may succeed each other in some 
' rare ' species of Crustacea and Insecta, entirely insufficient to 
justify the conclusion that these agamic generations can con- 
tinue indefinitely. I certainly believe that in most cases they 
are not of unlimited duration, because nature has chiefly fitted 
them for a cj^clical method of reproduction, — for a regular 
alternation of parthenogenetic with sexual increase. But there 
are species like Cypris reptans which I have investigated (see 
Part II of this essay), in which it is certain that no such cycle 
exists, and that parthenogenesis continues without interruption. 
I have observed about forty generations in the case of Cypris 


reptans: this is not an unending series, but we do not know 
of any reproductive cycle which, after forty agamic generations, 
returns to a sexual one. So far as the argument is concerned, 
it does not signify at all whether such cases are rare, as Maupas 
thinks, or common : even their entire failure would afford no 
proof of the theory of rejuvenescence. For the theory of 
minghng,— if I may so designate my hypothesis, — is founded 
on the species-preserving influence of amphimixis, and 
leads us to expect that, wherever it is possible, nature will 
always introduce amphimixis into the reproductive history of 
a species and will render its employment obligatory. We 
should have no ground for wonder if purely agamic repro- 
duction had no real existence. The vitalizing influence of 
amphimixis would not be proved even if this were the case. 

On the other hand, I think a single example of continuous 
agamic reproduction proves that amphimixis is not absolutely 
necessary for the unlimited duration of life. 

But if amphimixis is not absolutely necessary, the rarity of 
purely parthenogenetic reproduction shows that it must have 
a wide-spread and deep significance. Its benefits are not to be 
sought in the single individual ; for organisms can arise by 
agamic methods, without thereby suffering any loss of vital 
energy : amphimixis must rather be advantageous for the 
maintenance and modification of species. As soon as we 
admit that amphimixis confers some such benefits, it is clear 
that the latter must be augmented as the method appears 
more frequently in the course of generations ; hence we are led 
to enquire, how nature can best have undertaken to give this amphi- 
mixis the widest possible range in the organic world. 

The following is an attempt to supply an answer to the 
question. The increase by means of budding and fission 
would be retained in multicellular plants and animals, on 
account of its great advantages, but it would only endure 
for a shorter or longer series of generations. Moreover, 
the expected advent of amphimixis would only take place 
when the collective hereditary tendencies of the individual 
are concentrated in the nucleus of a single cell ; hence the 
mechanism of reproduction must have been associated with 
unicellular germs, and amphimixis became bound up with 
reproduction. I cannot remember that it has ever been 


maintained that the ontogeny of Metazoa and, so far as I 
am aware, of Metaphyta also, primarily depends on the 
necessity for sexual reproduction, or, better still, on the exis- 
tence of unicellular germs. An ontogeny must then follow ; 
for the collective hereditary tendencies of an animal being 
concentrated in a single cell, they must therefore, during 
development, pass through a series of stages very similar 
to those of their phyletic history. But, besides the germs 
destined for sexual reproduction, there are other unicellular 
germs, spores, (S:c. ; and hence it is clear that the unicellular 
condition brings other advantages than those which amphi- 
mixis confers ; but these unicellular agamic germs never 
exhibit any approach to the extent of range witnessed in 
sexual cells, and the origin and universal existence of unicel- 
lular germs are therefore to be sought in the latter, 

I have already shown that the sexual cells, upon their first 
appearance, in some simple cell-colony such as Pandorina, 
would be compelled to undergo a nuclear ' reducing division,' 
after a relatively small number of sexually reproduced genera- 
tions ; because otherwise a continued doubling of the nuclear 
units must have occurred in consequence of the periodically re- 
peated union of the nuclear substance of different individuals. 
This ' reducing division,' which is now securely proved for both 
male and female sexual cells in Metazoa, has, however, 
another meaning. 

I proceed from the assumption that nature aims at the widest 
possible range for amphimixis. How could this be obtained 
more effectually than by rendering the unicellular germs incapable 
of developing alone ? 

The male germ-cells, being specially adapted for seeking and 
entering the ovum, are, as a rule, so ill provided with nutriment 
that their unaided development into an individual would be 
impossible ; but with the ovum it is otherwise, and accordingly 
the ' reducing division ' removes half the germ-plasm, and the 
power of developing is withdrawn. 

What happens in the unicellular organisms ? Here also our 
theory demands that periodic amphimixis should be provided by 
nature. For the attainment of this object it was indispensable 
that, as in Metazoa and Metaphyta, the organisms should, at 
certain periods, arrange themselves in pairs, and that their 


nuclei should be in the state best adapted for fusion, — viz. 
that the mass should be diminished so far as to reduce the 
hereditary units, or ids, to half. And all this as a matter of 
fact takes place. But it is nevertheless insufficient to ensure 
the desired result ; for Maupas' experiments show us that, in 
spite of it, conjugation may be absent. The impulses which 
induce Infusoria to seek one another, and to pair, appear 
at certain periods and under certain external conditions, but 
if the latter are not favourable the impulses are not manifested 
and after the lapse of some time the power of conjugation 
is completely lost in the colony in question. I assume that 
Maupas' observations are correct, and am not criticizing them ; 
but his own results prove, in my opinion, that his interpre- 
tations are erroneous in so far as he endeavours to find 
support for the theory of rejuvenescence by means of the facts 
which he has observed. Those colonies which have passed 
the proper time for conjugation gradually die out. Maupas 
considers that they die a ' naturaP death in consequence of 
old age. He claims to have proved the occurrence of ' physio- 
logical' death in unicellular organisms, and to have refuted 
my views as to their potential immortality. 

But I believe that the facts brought forward by him are 
capable of a different and a more correct interpretation. 

What happens when a colony has passed the appropriate 
period and has therefore lost the power of conjugation ? The 
very same thing which happens to the ovum which has attained 
maturity and has extruded its polar bodies — disintegration, 
preceded by the loss of all power of development. I believe that 
this result proceeds from the same cause in both cases, — 
the reduction of nuclear substance, i. e. in the Infusorian, the 
substance of the micronucleus. The egg disintegrates because 
the nuclear substance is insufficient for the commencement of 
ontogeny, and is imperfectly adapted for its preservation ; 
the Infusorian disintegrates because its macronucleus must be 
renewed periodically, and because this cannot occur after the 
micronucleus has perished. And Maupas informs us that the 
latter disintegrates sooner or later, if the proper time for 
conjugation has passed by. 

If we ask, how is it that the micronucleus disappears when 
conjugation is excluded, Maupas answers that the necessary 


rejuvenescence being absent, the animal grows old (senescence) 
and finall}'' dies a natural death. I do not agree with this 
interpretation. The significant inner changes which take place 
during conjugation were obviously prepared some time before- 
hand, and the micro- and macronuclei of animals which feel 
impelled to conjugate are already in a state which must sooner 
or later lead to profound changes of one or both— and this 
whether conjugation has taken place or not. In either case 
these changes will be essentially the same, — the destruction 
of the macro- and division of the micronucleus. One thing 
alone does not happen, — the coalescence with the nucleus 
of another individual. But we know that all the products 
of the micronuclear division disappear except that which 
gives rise to the reproductive nuclei and that this is 
always the one lying nearest the connecting bridge which 
unites the conjugating animals. If then it is the influence 
of another animal which renders a grand-daughter-nucleus 
capable of further development, we are led to conclude that 
such an influence is lost when conjugation does not occur. 
In this, I believe, lies the cause which leads the vital energies 
to grow weaker and finally to cease, in the descendants of an 
animal which has undergone the changes described above. 
It is the same with the ovum,— the processes of maturation 
which prepare for fertilization, produce changes which prevent 
the future life of the egg-cell, unless it be fertilized. 

Maupas will reply that it has not yet been proved that such 
changes appear when conjugation is absent : he has never 
observ'^ed them in the Infusoria which he prevented from 
conjugating. He did not make the observation because he 
regarded the changes as phenomena of age. It now remains 
to follow accurately the alterations which appear in the 
macro- and micronuclei, when a colony has been prevented 
from conjugating. The observations will be difficult, because 
they must extend over many generations ; for the end of the 
period favourable for conjugation cannot be foretold with 
certainty and, according to Maupas, is not reached in all the 
animals of a colony at the same time. 

My interpretation does not by any means require that the 
changes in animals prevented from conjugating, should follow 
precisely the same course and pass through exactly the same 


stages as those which occur in conjugated animals. This 
is a priori very improbable. We must not forget that the 
interval between two successive conjugations extends over many 
generations, and that those inner conditions which prepare 
for conjugation are gradually built up, reach their highest 
point, and are then lost. If, when the appropriate period 
has arrived, conjugation takes place, the long-prepared pro- 
cesses of maturation take their normal course ; but if this period 
is passed by, the whole future development is abnormal. The 
animal increases a hundredfold or more, but development 
cannot pursue its normal course, the nucleus degenerates, — 
sometimes the macronucleus being the first, sometimes the 
micronucleus, — and finally neither assimilation, nor the main- 
tenance of the characteristic body-form can be kept up, 
and the animals die one after the other. The irregularity in 
the course of these phenomena, as Maupas describes them, 
points to the fact that we are concerned with an abnormal 

Does Natural Death occur in Unicellular Organisms ? 

Why do some writers regard the process described above 
as the equivalent of the normal death of Metazoa 1 Merely 
because of the traditional dogma which asserts the necessity of 
normal ' physiological ' death. They overlook the fact that in 
Infusoria conjugation is a normal process^ the periodical re- 
currence of which is provided for by nature, and upon which 
the whole vital mechanism of these animals is, to a certain 
extent, regulated. Nature must have amphimixis, and brings 
it about by the internal changes which impel the animals to 
pair, and by those which gradually render them unable to live 
when conjugation is artificially prevented. It is, as I have 
already argued, precisely equivalent to the effects which follow 
the non-occurrence of fertilization. The spermatozoon which 
fails to find an ovum, dies. If anyone finds pleasure in bring- 
ing confusion into ideas which have just become to some extent 
clear, he may speak of this as the ' normal death ' of the sperma- 
tozoon ; I call it an accidental death, although I am well aware 
that this unhappy accident is far more common than the success- 
ful attainment of the normal object of a spermatozoon's life. In 
most animals millions of spermatozoa are lost before a single 


one attains its object ; and these vast numbers are necessary- 
just because the way to the &%g is so very precarious. Must 
we regard this destruction as normal because it is so common ? 
Is not fertihzation the normal aim of the vital processes of the 
spermatozoon ? And does not the destruction of those numerous 
spermatozoa which have missed their aim result from the fact 
that they are not adapted for a long independent hfe,— that 
their vital force is soon expended because no precaution has 
been taken to renew it by food ? But has this lack of food been 
brought about because it could not have been taken however 
desirable it may have been ? I believe that spermatozoa want 
a mouth, and all other adaptations for the absorption of nutri- 
ment, because they do not need them for the attainment of the 
object for which they exist, and that, were it otherwise, they 
would have been adapted for living longer. Useless adaptations 
are never met with. Spermatozoa gone astray are of no 
further value to the species, and they may just as well disappear. 
And so it is with those Infusoria which have failed to conjugate ; 
they are useless to the species, since its maintenance requires 
the periodical crossing of individuals and of this they are no 
longer capable. If Infusoria were not adapted for this crossing 
they could live on for ever without amphimixis, just as a par- 
thenogenetic ^gg and its products live on w^ithout it. But those 
very changes which make an Infusorian capable of conjugation 
remove all possibility of unending life without it, just as the two 
' reducing divisions ' withdraw this possibihty from the ^gg. 
An even closer parallel can be drawn, for Kupffer and Bohm ^ 
have shown, by the case of Petromyzon, that there are animal 
eggs which only undergo the^r^^ polar division before they 
come in contact with the spermatozoon, the second following 
after it has penetrated. Such eggs when unfertilized, contain 
the quantity of germ-plasm required for embryogeny, but are, 
nevertheless, incapable of parthenogenetic development. We 
cannot at present recognize those intimate changes upon which 
this incapability must depend, but we may conclude that it 
is a consequence of changes preparatory to amphimixis. The 
eggs are so completely adapted for this event that their power 

^ Bohm ' Ueber die Befruchtung des Neunaugen-Eies.' Sitzgsber. d. 
math.-phys. Klasse d. bayr. Akad. d. Wissensch., Munich, 1887. 


of independent development is interfered with by the preHmi- 
nary changes. But, just as eggs, in which these internal changes 
have once been carried out, cannot remain indefinitely thus 
prepared, but very soon change so that they are no longer 
adapted for fertilization, and finally decay,— so it is with an 
Infusorian which has passed the time for conjugation ; it be- 
comes incapable of conjugating, and finally, of living. 

As far as I can see there is only one point of view from which 
the gradual dissolution of an Infusorian which has not succeeded 
in conjugating can be rightly regarded as a kind of natural 
death ; viz. if we could prove that its destruction is dependent on 
some adaptation especially directed to this end. Maupas is, naturally 
enough, very far from accepting this point of view ; for he 
clings to the old belief that death is a universal attribute of life, 
and is not a phenomenon of adaptation. From my standpoint 
we might argue as follows : — Conjugation must take place 
periodically because the crossing of individuals is necessary for 
the maintenance and development of the species. If it was im- 
possible to ensure the occurrence of crossing in all or the great 
majority of individuals and colonies, there would be a danger of 
the uncrossed ones getting the upper hand. To prevent this, 
the animals which do not conjugate must be prevented from 
living on indefinitely, in fact natural death must occur, and this 
was ensured by conferring upon the macronucleus of the 
animal such a structure that it was used up during assimila- 
tion, while the micronucleus was so constructed that it under- 
went dissolution in consequence of the divisions preparatory 
to amphimixis, or as we may otherwise imagine it. 

I know of no biological principles which are antagonistic 
to such a view, but I scarcely believe that it is a correct one ; 
analogy with the sexual cells is against it. I do not doubt that 
nature would be quite capable of bringing about a natural death 
for those animals which have escaped conjugation, if it were 
necessary for the maintenance of the species ; but their destruc- 
tion does not appear to be necessary. We should hardly 
maintain that the dissolution of a spermatozoon which has 
missed its mark is dependent on the appearance of natural 
death, especially designed for it. On the contrary, it is ob- 
viously destroyed simply because the vital conditions necessary 
for its continued existence are wanting, viz. fusion with an 


ovum. The latter also dies for a similar reason when it has not 
been fertilized. Some years ago I described the different manner 
in which the eggs of two closely allied species of Crustacea 
behave when they have no prospect of being fertilized ^ If a 
female of Moma paradoxa, bearing winter-eggs in the ovary, be 
separated from the males, it nevertheless deposits its ova in the 
brood-chamber, but they utterly disintegrate in a few hours and 
are washed away by the water as it flows through the chamber. 
It is very different with Moina 7'ectirostris ; the winter-egg, when 
ripe and ready to pass into the brood-chamber, almost occupies 
the entire ovary. When males are absent and fertilization does 
not occur, the ^gg is not laid but is retained by the isolated 
female in her ovary in which it remains apparently un- 
changed for many days, probably quite capable of being fer- 
tilized. Finally it changes in appearance, losing its uniform 
finely granular look, while the fat-globules and particles 
of albumen fuse together into great irregular masses which 
are presentl}'- rather rapidly reabsorbed. Instead of winter- 
eggs the parthenogenetic summer-eggs are now formed, and 
we may maintain that the material of the former is not lost 
to the individual or to the species when fertilization is excluded, 
but is converted into new ova which do not require fertilization. 
No one can doubt that the habit of laying the winter-egg only 
after the stimulus provided by fertilization, is an adaptation ; but 
who would explain in this manner the destruction of the un- 
fertilized egg, which remains in the ovary t This destruction is 
certainly not purposeless ; but there are cases of unintended 
usefulness, and other species of Moina prove that this is one of 
them, for the unfertilized eggs are destroyed in the brood- 
chamber (where their material is lost). The destruction is 
therefore no adaptation but merely a consequence of the con- 
stitution of the egg which is so altered by preparation for the 
fertilization which should have ensued, that it can neither de- 
velope into an embryo nor continue to live. It is just the same, 
if I mistake not, with Infusoria ; the gradual destruction of 
those animals which do not conjugate is no special adaptation, 
but rather an inevitable consequence of the necessary internal 

' Weismann, * Beitrage zur Naturgeschichte der Daphnoiden,' Leipzig, 
1876-79. Abhandlung IV. ' Ueber den Einfluss der Begattung auf die 
Erzeugung von Wintereiern.' i 


changes which lead to conjugation, which could perhaps only 
have been prevented by special means '. 

Therefore we cannot speak of natural death as an adaptation 
to prevent unconjugated individuals from gaining the upper 
hand ; and in any case natural death cannot be admitted to 
obtain among Infusoria in general,, inasmuch as it only occurs in 
those animals which are abnormal in not attaining to conjugation. 

We need not discuss whether the dying out of the uncon- 
jugated animals in an Infusorian colony, is an adaptation, 
specially intended for the removal of these harmful individuals, 
or whether, as I prefer to assume, it follows as a consequence 
of those changes which are preparatory to pairing. But even 
the former assumption affords no support to Maupas ; because 
the natural death presupposed by him is the ver}^ reverse 
of an adaptation, being a fundamental attribute of life itself, — 
the inherent tendency to wear itself out. According to this 
view, Infusoria are predestined to death ; they can however be 
rescued by the magic of conjugation, and thus acquire a new 
span of life. 

Such a view does not admit of direct refutation ; we can 
only show that it has its origin in the old mystic conception of life, 
and that it is superfluous. 

Conjugation was long spoken of as the ' sexual reproduction ' 
of Infusoria before we had a more intimate knowledge of the 
nature of the process. The ' tertium comparationis ' was that fusion 
of two cells into one which occurs at any rate in the original 
form of both fertilization and conjugation. I have been accus- 
tomed for many years to urge, in my lectures, that conjugation 
is not reproduction, but rather its opposite ; for reproduction 
implies an increase of at least one in the number of individuals, 
while conjugation leads to a decrease, two individuals fusing 
into one. It has long been recognized that the processes which 
take place in conjugation and fertilization have in themselves 
nothing to do with reproduction. Maupas admits this and ex- 
presses it quite clearly and correctly when he states that 

^ I am here referring to the interesting facts discovered by R. Hertwig, 
which he explained as an Infusorian parthenogenesis. The subject is 
not, however, sufficiently mature for further consideration in this place. 
See R. Hertwig, ' Ueber die Conjugation der Infusorien.' Munich, 


fertilization in the Metazoa is always associated with repro- 
duction, but that the one process is not necessarily an ac- 
companiment of the other, and that, as a matter of fact, the 
conjugation of Infusoria has nothing to do with reproduction. 
The majority of previous writers believed that conjugation 
revived the exhausted power of multiplying by fission. Mau- 
pas shows that this is not the case, that not only is fission 
deferred for a comparatively long time after the occurrence 
of conjugation, but that animals which have been prevented 
from conjugating continue to divide for a considerable period. 

The view which Maupas thus overthrows was never a 
legitimate inference from accurate scientific observations, but 
was one of those traditional conceptions which gain acceptance 
after having been consciously or unconsciously derived from 
other similar conceptions. The supposed vitalizing force of fer- 
tilization was looked upon, for a long period of time, as the 
condition of all development and reproduction. The opposing 
facts were not at first strong enough to shake the foundation of 
this idea, and the preconceived notion that the magic of fertiliza- 
tion was the sole vitaHzing life-maintaining principle, endured, 
while the facts of asexual and parthenogenetic reproduction 
were, by some evasion or other— the influence of fertilization 
extending over many generations, (S:c. — forced into the Pro- 
crustean bed of the received fundamental conceptions. 

Even Maupas remains half buried in these old ideas. 
Although he has rightly recognized that fertilization and re- 
production are two entirely different and even antagonistic 
processes, that they may be connected, as in the Metazoa, or 
disconnected, as in the Protozoa, he still holds to the old 
view of the vitalizing influence of amphimixis; he speaks of 
it as a 'rajeunissement karyogamique,' and declares it to be a 
means for the kindling afresh of that life which would, without it, 
waste away into death. He quite forgets that this view wholly 
depends upon the facts of fertilization among Metazoa, viz. in the 
inseparable connection between fertilization and reproduction 
which we find in these animals, but which he himself has shown 
to he absent from the Protozoa. He overlooks the consequence 
of this absence, viz. the proof that in this case ^ post hoc' is not 
^propter hoc^ and keeps to the old standpoint which was a 
right one only so long as we were obliged to believe that new 


life could not arise without amphimixis, i. e. that reproduction 
was always associated with fertilization. 

As I have already said, I regard the power of living on 
indefinitely when the vital processes have once begun, as the 
fundamental peculiarity of living matter. But this principle 
fails in so many organisms that its very existence was, for a 
long time, entirely overlooked, and hence the limited duration 
of life, together with its termination in natural death, were 
regarded as laws dominating all living beings. Undoubtedly 
the capability of unending life has been lost in very many 
organisms of greater or less complexity, and it is, I think, 
interesting to trace the causes which have led to this loss, and 
have rendered it necessary and even advantageous. 

I will very briefly recall the manner in which the mortality 
of Metazoa may be explained, for this has been treated in 
earlier essays, and my views on the point have undergone no 
essential change. The immortality of Protozoa was carried 
over to the germ-cells of Metazoa and Metaphyta whether 
they are sexual, i. e. adapted for amphimixis, or not. In either 
case they posses potential immortality, i. e. they can, under 
the conditions imposed upon them by their constitution, con- 
tinue without limit to exhibit the phenomena of life. The 
conditions under which the sexually differentiated germ-cells 
live include the fusion of two in amphimixis, but it is not 
generally included among the conditions imposed upon agamic 
or parthenogenetic germ-cells, and, when imposed, it only 
requires to be fulfilled again after the lapse of a certain 

I will not repeat the reasons which, I believe, explain why 
the Metazoan soma has been permitted to lose, or has been 
compelled to lose, the power of unending life, and why natural 
death has made its appearance. I will only call to mind the fact 
that, according to the principle of panmixia, every faculty must 
disappear as soon as it ceases to be necessary. As soon as differ- 
entiation into soma and germ-cells, — viz. the formation of 
Metazoa and Metaphyta,— took place, this principle began to 
act, for the species could be maintained without the immor- 
tality of single individuals. Whether this immortality is in any 
way compatible with the high differentiation of the Metazoan 
body, and if so, whether it would be useful, are questions 

VOL. II. p 


which may remain unanswered— it is enough that it was un- 

In Protozoa unending life was an inevitable necessity for the 
maintenance of the species. 

Potential immortality is found from the very lowest organ- 
isms to the higher Protozoa and to the germ-cells of Metazoa 
and Metaphyta ; but in the latter cases certain conditions are 
imposed upon it, and these include not only the ordinary 
conditions of nourishment, and of surrounding circumstance, 
but, as a rule, the further condition of amphimixis. 

The Appearance of Amphimixis in the Organic World. 

If we are unable to discover any effect of amphimixis which 
can render its prevalence intelligible, nothing remains but to 
accept the rejuvenescence theory. For not only is amphimixis 
found throughout the whole organic world so far as we know it, 
but the entire form of the latter has been controlled in a most 
fundamental manner, and, without amphimixis, would have been 
utterly different. 

It has been shown above that the occurrence of an ontogeny 
in the Metazoa essentially depends upon the necessity for 
amphimixis ; since this presupposes the concentration of the 
collective hereditary tendencies of a species in the nucleus of 
a single cell. But this is not only true of all the varied kinds of 
direct ontogeny: the complex and changing forms of alternation 
of generation in animals and plants are also, mainly and in the 
most important respects, dependent on the necessity for making 
amphimixis possible. I say ' necessity,' because I hold that 
everything real is also necessary, and that this is true even 
of the things we generally call useful ; for I beHeve that in 
nature the really useful — viz. that which is useful when con- 
sidered in relation with the whole of its consequences and not 
by itself alone — is also invariably necessary. The useful becomes 
necessary as soon as it is possible. In this sense we may regard 
amphimixis as necessary because it obviously involves a deep 
and essential use. 

Its unusually elastic powers of adaptation show how far it 
is from being necessary, viz. essential to life, in the usual sense 
of the word. 

If amphimixis is truly rejuvenescence, i.e. the hindering of an 


otherwise inevitable death, we ought to find it as a fiandamental 
process, occurring without a single exception. It is hardly 
necessary to say that this is not the case. Least of all ought its 
appearance to depend obviously upon external conditions of life. 
But this is certainly the case ; the periodicity of its appearance can 
be proved to depend upon adaptation. 

In many thousands of species of the higher animals amphi- 
mixis invariably makes its appearance at the outset of every 
generation, for no egg can develope without fertilization. This 
is true of the whole Vertebrate sub-kingdom. Isolated excep- 
tions to this general law suddenly begin to appear in the group 
of the Arthropods. Certain eggs, in which we should have 
thought fertilization would be the necessary preliminary to 
development, have gained the power of developing un- 
aided, — viz. the power of producing males alone (bees), 
while the same eggs, if fertilized, would produce females. 
In plant-lice, on the other hand, females emerge from unferti- 
lized ova, and not one generation only, but two, three, and 
even many, succeed each other before a sexual generation 
occurs and, with it, amphimixis. How far this latter is from 
being a process of multiplication, and how superficial is the 
connexion which usually obtains between amphimixis and 
multiplication, are shown in the bark-lice, e. g. Phylloxera. In 
these it has already been mentioned that the sexual generation 
consists of minute animals devoid of mouth and of the power 
of taking food. The female lays a single egg, so that, as in the 
primitive form of conjugation, the number of individuals is not 
increased by reproduction, but diminished by half Nature 
could hardly express with greater clearness the stress which she 
lays on amphimixis ; nor could she argue in a more convinc- 
ing way that increase is distinct from amphimixis, and that the 
quickening of new germs need not be dependent upon the latter. 

If amphimixis were a process of rejuvenescence we could 
hardly believe that its occurrence in the life of a species 
would be so excessively fluctuating, — sometimes taking place 
in each generation, sometimes recurring after a lapse of two, 
three, or even as many as ten generations, sometimes being 
absent for forty generations, as I have proved to be the case in 
Cypris reptans. It might be suggested that the recurrence of 
amphimixis does not depend on the number of generations of 

p 2 


individuals, but on the number of cell-generations, and that 
continuous life is rendered possible by the reappearance of 
amphimixis after each miUion or hundred thousand generations 
of cells. We might also — as I have already mentioned — com- 
pare the ' agamic ' cell-generations of Infusoria, which follow 
each other between two periods of conjugation, with the collec- 
tion of cells composing the person of a Metazoon, and regard 
the ontogenetic cell-series, as a whole, as the equivalent of the 
millions of individuals which make up an Infusorian colony. In 
both these cases the rejuvenating and quickening influence of 
amphimixis may be supposed to endure for a certain number of 
cell-generations. I must admit that I consider such reasoning 
to be bad ' philosophy of nature,' i. e. playing with words 
which convey no distinct meaning. It is contradicted by the 
fact that the cell-cycle of ontogeny in the lowest representative 
of the Vertebrata, the Amphioxus, cannot be compared as 
to length with that of the higher members of the group ; it is 
equally disproved by the phenomena of cyclical development, 
showing that in one case the effects of fertilization may extend 
through one ontogeny, in another through two, three, six, or 
even ten ontogenies, not to mention the case in which forty 
generations have elapsed without the occurrence of amphimixis. 

If we regard amphimixis as an adaptation of the highest 
importance, the phenomena can be explained in a simple way. 
I only assume that amphimixis is of advantage in the phyletic 
development of hfe, and furthermore that it is beneficial in 
maintaining the level of adaptation, which has been once attained, 
in every single organism ; for this is as dependent upon the 
continuous activity of natural selection as the coining of new 
species. According to the frequency with which amphimixis 
recurs in the life of a species, is the efficiency with which the 
species is maintained ; since so much the more easily will it 
adapt itself to new conditions of life, and thus become modified. 

Amphimixis must first have appeared among unicellular or- 
ganisms in the form in which we now find it in most Protozoa 
(Flagellata, Sporozoa, Rhizopoda)— namely, as the complete 
fusion of two entire animals \ 

^ Maupas (op, cit. p. 492) attributes to me the view that conjugation 
bears a different significance in the lower Protozoa from that which it 
possesses in the higher, and he describes this ' maniere de voir ' as 


Since this process is in direct antagonism to reproduction, i.e. 
increase, it can only be repeated after long intervals, lest it 
should prevent the sufficient increase of a colony of such animals. 
Hence we find that conjugation recurs periodically among the 
Protozoa ; and indeed — as Maupas has taught us in the Infuso- 
ria — only repeats itself after a great many (120-300) generations. 

Amphimixis, as we have seen, only became possible among 
Metazoa by concentrating or packing all the predispositions into 
the restricted area supplied by the nuclear substance of a single 
cell,— and this must happen even when the adult body is com- 
posed of millions of cells, differentiated in the most diverse 
directions, and combined to form tissues, organs, and systems. 
The result of this arrangement is seen in a highly complex 
ontogeny ; and it is obvious that many conditions of life may 
arise which render it advantageous that the increase of the 
species should not proceed exclusively by this long and intricate, 
and therefore dangerous road, and that accordingly the origin 
of each new individual should not be necessarily bound up with 
amphimixis. In this way we are able to understand the wide 
distribution and diverse forms of asexual reproduction among 
the lower Metazoa and in plants. 

There is, however, another factor, — the appearance, in the two 
last-mentioned groups, of that complex form of individuality 
known as the stock. This is brought about by the budding or 
division of the person, a form of increase which renders possible 
a continuity of the persons proceeding from one another. Such 
increase is not associated with amphimixis, because the indis- 

' superficielle,' etc. I have never held such a view ; the only passage in 
my writings which can have given rise to such a misapprehension deals 
with the phyletic origin of conjugation (' Bedeutung der sexuellen Fort- 
pflanzung, p. 52, translated in vol. i, see pp. 293-294). Anyone who 
refers to this passage will find a hypothesis, expressed with all reserve, 
suggesting the original significance of the fusion of two unicellular 
organisms. Conjugation must have had some beginning, and although 
I believe that in its present form it signifies a source of variability, it 
must originally have had some other meaning, for two Monera would 
scarcely coalesce in order to ensure variability in their descendants. 
A change of function must have taken place, or, as Dohrn has very 
clearly expressed it, a secondary effect associated with the original main 
effect has, at a later date, usurped the place of the latter. Maupas 
accepts conjugation in the form in which it exists, and makes no attempt 
to understand how it originated. I do not blame him for this, but is it 
really so superficial to investigate the origin of any phenomenon ? 


pensable mechanical conditions are wanting. Hence, in the 
formation of stocks, amphimixis does not appear in everj' 
generation of persons, but only periodically in certain genera- 
tions, and from this follows an alternation between two methods 
of increase, viz. with and without amphimixis, or, as it is called, 
an alternation of generation. Many principles come into action 
in this mode of development, which we cannot stop to consider, 
above all the gradual development of high individuahsation in 
the stock, through the differentiation of its persons on the prin- 
ciple of division of labour, as was expounded many years ago, 
in a most convincing manner, by Rudolph Leuckart. 

We can furthermore understand why a longer or shorter 
series of generations elapses before amphimixis becomes asso- 
ciated with increase : a long interval is the necessary conse- 
quence of the formation of highly differentiated animal stocks. 

I need hardly say that I do not, by any means, intend to 
imply that no change in the method of reproduction can have 
arisen without stock-formation. In the groups of polypes and 
medusae, among which the above-mentioned alternation of 
generation is so widely spread, we find species which do not 
form stocks, and which, after passing through a series of gene- 
rations by fission or budding, return to the method of sexual 
reproduction. It is clear that in such cases, the omission of 
a detailed and dangerous embryogeny, together with the more 
rapid multiplication which accompanies the omission, has been 
the efficient cause which has limited amphimixis to certain 
generations. The fresh-water polype, Hydra, is an example of 
this. The duration of the ' agamic ' period is so regulated by the 
external conditions of life that the concentration of the collective 
predispositions of the species in a single cell, which is associated 
with amphimixis, is at the same time made use of to form a 
resting-egg, which carries the species over the unfavourable 

The adoption of entirely different methods by closely allied 
animals shows how little the existence and duration of the 
periods of asexual reproduction have to do with the number of 
cells composing a single individual. In one and the same group 
of Hydromedusae we find species with long periods of asexual 
reproduction side by side with others in which it has entirel}' 
disappeared, so that every generation proceeds from fertilized 


eggs, and is therefore under the direct influence of amphimixis. 
It is well known that some Medusae are budded off from a 
polype-stock, and constitute the sexual generation of the latter, 
marking the end of a series of asexual generations ; while 
other Medusae invariably arise from fertilized ova, and always 
produce eggs requiring fertilization, or, in other words, adapted 
for amphimixis. 

The degree of organisation is, in yet another way, associated 
with the alternation of asexual with sexual generations, and thus 
with the periodicity of amphimixis. This new relationship 
between organisation and the recurrence of amphimixis, depends 
upon the fact that the asexual methods of reproduction by fission 
or budding are not possible in the highest and most complex 
Metazoa. They are only found in the lower groups of Metazoa, 
— the Coelenterates, Worms, and Echinoderms ; disappearing 
in the Arthropods, Molluscs, and Vertebrates. 

In these latter, we might well suppose that every act of increase 
would be connected with amphimixis ; for, — since the structural 
complexity of the animals in question has rendered fission 
and budding impracticable and has therefore compelled a re- 
version to the unicellular germ and the occurrence of a detailed 
ontogeny in every generation, — it might seem probable that 
nature would not lose the advantage of connecting amphimixis 
with such a method of reproduction. We might therefore expect 
to find no exception to the occurrence of sexual reproduction in 
these groups. In this anticipation we should be deceived, inas- 
much as it only appears in the great majority of cases. In the 
minority, amphimixis is very far from universal, in spite of a 
development from unicellular germs which would so easily have 
permitted it : furthermore, in this minority it was formerly 
connected with reproduction, and has been abandoned in 
different degrees. These cases of development from partheno- 
genetic eggs are, above all others, fitted to prove the importance 
of the principle of utility. The transformation of female sexual 
cells, at first directly adapted for amphimixis, into germs no 
longer requiring fertilization, is an artifice by which nature has 
contrived to avoid amphimixis when a high degree of structural 
complexity has prevented reproduction by fission and budding. 

It may be remarked here that this suggestion supplies the 
answer to a difficulty which I was, for a long time, unable to 


solve —namely, the remarkable limitation of parthenogenesis to 
a few definite groups. It is only found in Crustacea, Insecta, and 
Rotifers, and not among Vermes, Coelenterates, and Echino- 
derms^ : furthermore, it does not exist in the two higher groups 
of Molluscs and Vertebrates. The solution to the problem is 
found in the suggestion that the lower groups of animals 
dispense with parthenogenesis, because it is unnecessary to 
them. Whenever increase without amphimixis became advan- 
tageous, it was more readily and better supplied by fission and 
budding. The absence of parthenogenesis in the higher groups 
of animals may probably be explained on the supposition that no 
force has appeared which would render it advantageous for 
amphimixis to be separated from the existing method of in- 
crease. This is especially clear when we investigate the 
grounds on which it must have become advantageous among 
the Arthropods. 

The periodical occurrence of unfavourable conditions of life 
has often been suggested as the cause of the appearance of 
parthenogenesis in Arthropods and Rotifers. I need onty refer 
to my already quoted work on Daphnidae^ in which this question 
is considered at length. Whenever a species lives scattered over 
a small area subject to rapidly changing external conditions 
which are, for a short time, favourable to life and multiplication, 
and then suddenly become unfavourable or even destructive,— 
it must be a great advantage for the increase of individuals to take 
place with the greatest possible rapidity during the favourable 
periods. As indicated in my former work, the advantage of 
parthenogenesis in such cases lies in the fact that multipHcation 
must become many times more effective when every individual 
is a female, or, to express the thought in more general terms, 
when every single germ-cell can produce a new animal. A 
further acceleration ensues from the omission of that retarda- 
tion of development which is implied by the occurrence of 
copulation and fertilization. 

From this point of view w^e can not only explain the appear- 
ance of parthenogenesis in general, but also its special form in 

^ I am aware that it is believed to occur in some Coelenterates, but it 
seems to me doubtful whether any true parthenogenesis takes place. 
And, in any case, isolated exceptions do not invalidate the significance 
of the rule. 


particular cases. In those Daphnids which, Hke the species of 
Moina, inhabit small rapidly filled, but also rapidly drying 
pools, the number of purely parthenogenetic generations which 
succeed one another after the foundation of the colony, is 
small. In Moina paradoxa and M. redirostris males appear 
in the second generation, and some of the females produce 
resting-eggs which require fertilization. If this did not occur, 
if sexual reproduction, viz. multiplication associated with am- 
phimixis, did not take place very soon after the foundation of 
the colony, it would frequently happen that the latter would be 
destroyed by sudden drought, without the formation of resting- 
eggs to carry life in a latent condition over an unfavourable 
period, and the colony would simply perish. It may be urged 
that parthenogenetic eggs might have been provided with resting 
shells like those which are, as a matter of fact, found in other 
Phyllopods, for example Apus. But clearly the object is to 
confer upon the species the advantage of periodically repeated 
amphimixis, and this is therefore connected with the for- 
mation of resting-eggs, and reproduction is so regulated that 
the number of parthenogenetic generations is determined by the 
average duration of the favourable periods of life. Thus, among 
the marsh-dwelling Daphnids numerous purely parthenogenetic 
generations succeed each other before a sexual generation 
appears, while in those which inhabit lakes and are subject to 
uniform conditions of life interrupted only by the cold of winter, 
the cycle is still longer. In some species amphimixis may be en- 
tirely abandoned, and this seems to occur most readily in those 
which produce but one kind of egg, which must naturally be 
provided with a protective resting shell, rather than in those 
forming two kinds of eggs, of which only one is a resting-egg 
and requires fertilization. Thus it is well known that most of the 
colonies of the common Apus cancriformis are purely partheno- 
genetic, and the same is true of the greater number of fresh- 
water Ostracodes. 

Ten years ago, when I first directed my attention to the 
parthenogenesis of these minute Crustacea \ I was able to dis- 
tinguish three stages of reproduction,— the first was found in 

* Zoologischer Anzeiger, i88o, p. 72. ' Parthenogenese bei Ostra- 


such species as Cyprois monacha, of which every generation 
reproduces sexually ; the second was found in those species in 
which numerous parthenogenetic generations alternate with 
a sexual one ; and finally the third included species in which 
males have not yet been found : in one such species {Cypris 
reptans), forty consecutive purely parthenogenetic generations 
have been observed. 

We cannot yet decide why the advantages of amphimixis 
have been entirely given up in this and other cases. We can- 
not at present solve, or even profitably discuss, every biological 
problem. But it is probable that we are dealing not with 
adaptation alone, but perhaps with a suppression of amphi- 
mixis by means of parthenogenesis. Everything which is 
desirable is not possible ; and after parthenogenesis has once 
been incorporated in the hereditary tendencies of a species, 
circumstances may arise in consequence of which it may be 
transferred, by the power of heredity, to the remaining sexual 
generations also, without the possibilitj^ of any interference 
on the part of natural selection. Whether this explanation 
is in the right direction or not, it is at any rate clear, as 
regards the question under discussion (viz. the true significance 
of amphimixis), that the loss of an advantage may be intel- 
ligible in many ways, while the loss of a process of vital reju- 
venescence must stand in direct opposition to the continuance 
of life. 

It would be of the highest interest to consider more closely 
the various cases of parthenogenesis, from this point of view : 
we do not, however, possess sufficiently accurate knowledge of 
the vital relations of the animals in question to enable us to 
estimate the advantages conferred by the disappearance of 
amphimixis, or rather the introduction of parthenogenesis, in 
a larger or smaller number of generations. I may, nevertheless, 
be permitted to afford some indication of the line of argument. 

Parthenogenesis plays an important part in the group of plant- 
lice and bark-lice, containing very numerous species. The ova 
may be deposited or may undergo embryonic development 
within the body of the mother. In either case the advantage 
of parthenogenesis depends, as in the Daphnids, on the ex- 
traordinary rate of multiplication, which naturally reaches the 
highest point in the viviparous Aphidae, because the offspring 


actually produce embryoes within their own bodies before they 
are born. But here we have to do, not so much with the sudden 
termination of a limited and changeful developmental period, 
as with the greatest possible use of the opportunities afforded 
by an extremely rich nutriment of vegetable juices. The exces- 
sively rapid multiplication ensures the colony, and therefore the 
species, from destruction at the hands of its numerous foes, 
which, just on account of the abundance of food provided by 
the vast increase of their prey, become themselves still more 
numerous, so that the multiplication of these plant parasites 
must be carried on at the highest possible rate. Hence we 
find that many purely parthenogenetic generations succeed 
each other, while amphimixis is ensured by a single generation 
of males and females, appearing towards the close of the period 
in which the richest nutriment is supplied. 

On the other hand, we find that in many Cynipidae a partheno- 
genetic alternates with a sexual generation, and it generally 
happens that the latter appears in the summer, and the former 
in spring or even winter. The often considerable structural 
divergence between these two generations depends upon the 
very divergent conditions of life to which they are respectively 
exposed, and above all upon the fact that the eggs are laid in 
various, differently formed parts of plants, necessitating there- 
fore a corresponding difference in the ovipositing apparatus. 
But such considerations need not detain us here. The benefits 
conferred by the absence of amphimixis from the winter genera- 
tion appear to me to follow from the exceptionally unfavourable 
conditions of life by which it is beset. Many of these small 
Hymenoptera, e. g. Biorhiza aptera, emerge in the very middle 
of winter, on warm days in December or January, and creep 
upon the oak-trees, laying their eggs in the heart of the 
winter buds, having laboriously bored through the hard pro- 
tective scales with the ovipositor. Without taking food, and 
frequently interrupted by cold and the long nights, they carry on 
this work until all their eggs are safely deposited or until death 
from snow or cold puts an end to their labours. It is clear that 
such hard conditions must prove fatal to many of these insects 
before they have fulfilled their task, and it must conduce greatly 
towards the maintenance of the species, not only for all the 
time occupied in selection by the sexes and in fertilization 


to be saved, but also for every survivor in the struggle to be 
capable of laying eggs with the power of developing unaided, 
in other words for every such animal to be a female. 

Much might still be said as to the causes of the omission of 
amphimixis from one or more generations, but a few words 
will suffice to show that the appearance of parthenogenesis 
depends upon adaptation to the conditions of life,— ///«/ reproduc- 
tion without mnphimixis has invariably originated from sexual 
reproduction, whenever it was required in order to gain some 
distinct advantage in the effort to maintain the species. We may 
well assume that the advantages which the appearance of 
parthenogenesis must confer, outweigh the disadvantages in- 
volved in the giving up of amphimixis. Our estimate as to 
the effects of the latter is far less certain and precise than of 
the former. If, however, I am not mistaken in my views on 
the significance of amphimixis as the source of individual varia- 
tion, it follows that its omission from a single generation or 
even from a series of generations may be easily compensated ; 
for it always reappears, and mingles afresh the complex indi- 
vidual predispositions into new combinations. The injury 
caused by its withdrawal would be less as the fertility of 
the species was greater ; with this is connected the fact that 
parthenogenesis is chiefly found in very prolific species. Those 
individuals which sink below the level of organization charac- 
teristic of the species could the more easily be eliminated in the 
struggle for existence without in any way endangering the life 
of the species. Perhaps this explains why, in some few species 
of Crustacea {Cypris) and of Insecta {Rhodites rosae), amphimixis 
has utterly vanished without having caused, up to the present 
time, any trace of degeneration in the species. 

We may safely assume that the entire absence of amphi- 
mixis is to be primarily explained as an adaptation, and that 
the alternation between sexual and asexual multiplication 
met with in Hydromedusae, Cestoda, &c., has arisen from 
the demands made by the conditions of life, — demands similar 
to those which have determined the alternation between mono- 
sexual and bisexual generations found in Insecta, Crustacea, 
&c. In both classes of cases amphimixis has been restricted 
to certain generations because it was not necessary in all of 
them, and because such restriction was a great advantage. 


The means by which this limitation is exercised are different 
in the two classes, not by any means because parthenogenesis 
could not have been introduced among the lower Metazoa, 
but because nature did not require it, but resorted to the far 
more practicable and flexible methods of fission and budding. 
When these ceased to be available, she was compelled to 
alter the sexual cells in such a way that their powers of 
development were no longer connected with amphimixis. 

There are indeed no plants wholly devoid of the power of 
reproduction by buds. Not only the formation of stocks but 
also the copious increase of persons and stocks^ by means 
of buds is everywhere at the disposal of nature, and she has 
made a lavish use of them. With this is probably connected the 
fact that parthenogenesis is unusually rare in plants and only 
occurs in a few groups. I must leave it to abler botanists to 
investigate the grounds upon which unicellular germs, originallj- 
adapted for amphimixis, have been, in certain exceptional cases, 
afterwards transformed into parthenogenetic germs. The alter- 
nation of generation, so prevalent among the lower classes 
of plants, takes a form somewhat different from that found 
in the lower groups of animals, inasmuch as, not only the 
multiplication which is associated with amphimixis, but also 
that which is without, viz. agamic, depends upon unicellular 
germs. Ferns, Mosses, and Lycopodiums produce vast quanti- 
ties of spores, the unicellular nature of which certainly does 
not follow from any former connexion with amphimixis in 
remote ancestors. It is far more probable that the unicellular 
condition has proved necessary in order to confer other advan- 
tages which, as has been suggested above, depend upon a minute 
size :— the lightness which facilitates transport by wind and 
water, and the possibility of production in enormous numbers. 

In conclusion, it has been shown that amphimixis is every- 
where present among the vital phenomena of a species when 
its existence is without injury to other vital interests, — that 
it appears, in the Protozoa, independently of reproduction, 
when a connexion with the latter was possible but unneces- 
sary,— and that, in the Metazoa, it is bound up with reproduction, 
inasmuch as its existence only thus became possible. It has 

^ For a definition of this term see page 213. 


further been shown that its occurrence in the Hfe of a species 
becomes more frequent according as its admission by the vital 
conditions does not entail other disadvantages. When neither 
the formation of stocks nor the most rapid multiplication of 
individuals in the shortest time is required, we find amphimixis 
connected with the origin of every new individual ; but when- 
ever the existence of the species would be endangered if new 
generations could not arise from the old in the most rapid 
succession and without any interval, we find that amphimixis 
is not inseparably associated with every act of reproduction, 
but makes its appearance only in certain generations. All 
this clearly points to the conclusion that amphimixis is no 
indispensable vital condition, no renewal of life or ' rejuven- 
escence,' but a process which has indeed a deep significance, 
although it is not inseparable from the continuance of vital 
processes. This conclusion becomes even more evident when 
we recognize how precisely, in the alternation of agamic and 
sexual reproduction, the number of agamic generations is 
regulated so as to correspond with the conditions of the species. 
The rare or frequent repetition of amphimixis in the life-history 
of a species is not determined by its physical nature but by the 
conditions of life. Its regulation depends upon adaptation ; it 
may be entirely excluded and the life of the species still 
continues. I do not know of any facts which lead us, after 
recognizing all this, to assume that amphimixis is anything 
more than an essential advantage in the maintenance and modifi- 
cation of species. 


Abt Vogler, 47. 

Acquired cliaraeters, transmission 

of, 14, 15, 40, 96. 

Aecidiomycetes, reproduction in, 

Aglia tau, development of, 175. 

Amphigonic reproduction, 106, 

Amphimixis, 99, 176, 193, 218; 
definition of, 180, 199 ; appear- 
ance of, 210. 

Ants, 20, 28 ; slaves of, 25. 

Aphidae, polar bodies in, 112; par- 
thenogenesis in, 167, 218. 

Apteryx, 3. 

Apus, parthenogenesis of, 109, 167, 
198, 217. 

Aristotle, on heredity, 106. 

Artemia, parthenogenesis of, 109, 

153, 155- 

Ascaris megalocephala, fertiliza- 
tion of, 86, 91, 132, 146, 172 ; 
lumbricoides, 146 ; formation of 
spermatozoa, 147 ; development 
of spermatozoa in, 117 ; of ovum, 

Ascomycetes, parthenogenetic, 

Auerbach, on nuclear division, 

Baccillariaceae, fusion amongst, 

Bach, 45. 
Balbiani, on parthenogenesis, 108 ; 

on Infusoria, 177, 183. 
Balfour, on polar bodies, 106, iii. 
Basidiomycetes, parthenogenesis 

in, 169. 

Bees, larvae of, 28 ; partheno- 
genesis of, 109, 171, 198, 211. 

Beethoven, 46. 

Bellini, 47. 

Bergh, on nuclear division in In- 
fusoria, 183. 

Berthold, on parthenogenesis, 91. 

Biorhiza, parthenogenesis in, 219. 

Blochmann, on polar bodies, iii, 

Bohm, on fertilization, 204. 

Bombyces, 27, 174. 

Boveri, on fertilization in Echinus, 
92, 114, 146; on nuclear loops, 
117, 128, 131, 172. 

Brahms, 47. 

Branchiopods, polar bodies in, 

Brindis y Salas, negro musician, 

Bunge, on vital processes, no. 

Biitschli, on nuclear division, 112, 
177, i8r, 196; on polar bodies, 

Caecilia, absence of sense organs 

in, 9, 28, 
Callwitz, 45. 
Carcinus, 11, 
Carinaria, number of idants in, 

Carnoy, on maturation of ovum, 

Gat, influenced by music, 55 ; 

number of cells in cochlea, 57. 
Caves, in Carniola and Carinthia, 

7 ; mammoth of Kentucky, 7 ; 

near Trieste, 17. 



Chamisso, 38. 

Cherubini, 47. 

Cimarosa, 45. 

Clementi, 52. 

Cocconeis, fusion of, 195. 

Cochlea, number of cells in human, 


Colpidium, reproduction in, 179. 

Combination nucleus, 179. 

Conjugation, significance of, 189. 

Cook, Captain, 38. 

Corti, organ of, 56. 

Cramer, 47. 

Crustacea, blind, 7 ; parasitic, 10. 

Cyclopidae, fertilization in, 174. 

Cynipidae, parthenogenesis in, 
167, 219. 

Cypris, variation in, 161, 167; or- 
ganic reproduction, 198, 211, 

Cyprois, reproduction of, 218. 

Czerny, 47. 

Daphnids, polar bodies in, in, 
152 ; parthenogenesis of, 167, 

Darwin, on natural selection, 15, 
33 ; on sexual selection, 34 ; on 
pangenesis, 79, 81. 

Death, of the Protozoa, 201, 203, 

De Bary, on parthenogenesis, 108. 

Degeneration, general, 6, 18, 20, 
22, 27 ; of sense of smell, 9; of 
ear, 9 ; of parasites, 10, 12 ; of 
parts of flower, 18 ; of instinct 
to flee, 24. 

Development, retrogressive, i, 3, 

De Vries, on pangenesis, 81, 96, 

Disuse, 6, 7, 12, 15, 26. 
Dog, influenced by music, 55. 
Dolphin, degeneration of sense of 

smell, 9 ; naked skin, 19. 
Domestic animals, loss of original 

colour of, 19; instinct to escape, 


Echinus, fertilization in, 92, 146, 
Ectocarpus, parthenogenesis in, 

Engelmann, 177, 196. 

Entoniscidae, 11, 12, 23. 
Ephemeridae, 27, 28. 
Epicrium, auditory organ of, 9. 
Eternity, 74, 78. 
Euglypha, division of, 184. 

Farlow, on parthenogenesis, 108. 
Flemming, on nuclear division, 

112, 139, 173. 
Fol, on penetration of light in 

water, 8 ; on fertilization, 105, 

173, 176. 
Forel, on penetration of light in 

water, 8 ; on ants, 26. 
Formica fusca, 26. 
Fux, Joseph, 45. 

Geddes, on nature of nucleus, 

Germ cells, 77, 81. 
Germ nucleus, 159. 
Germ plasm, continuity of, 82, 83, 

Giard, on polar bodies, 114. 
Gonoplastid theory, 170. 
Greeks, music of, 39. 
Gruber, on fission in Infusoria, 

86, 177, 181, 191. 
Guinea-pig, 24. 
Gurney, on 'The Power of Sound,' 


Hanslick, on ' The Beautiful in 
Music,' 66. 

Harvey, on conception, 106. 

Hasse, 47, 56. 

Hawaians, music of, 38. 

Haydn, 45, 52. 

Helmholtz, on the auditory organ, 

Henking, on formation of germ- 
cells, 139, 141. 

Hensen, on fertilization, 106, 108, 

Herbart, 80. 

Heredity, 81. 

Hermit crab, 19. 

Hertwig, O., 83, 92, 105, in, 112, 
114, 119, 126, 132, 147, 176, 189. 

Hertwig, B., 92, 105, 176, i8i, 
184, 196, 207. 

Huber, on ants, 26. 

Hummel, 47, 52. 



Hydroids, migration of germ-cells 
in, 84 ; reproduction in, 214. 

Idants. definition of, 130 ; number 

of, 146. 
Idioplasm, 82, 83, 85, 92, 112, 

Ids, definition of, 130. 
Immortality, 74, 78, 87, 209. 
Infusoria, artificial fission in, 86, 

191 ; conjugation of, 87, 90, 177, 

Insecta, polar bodies in, 152. 
Instincts, 24, 25, 26. 
Ischikawa, on polar bodies, iir. 
Isopods, parasitic, 11, 12. 

Jewish musicians, 49. 

Kiwi-kiwi, 3, 4, 5, 28. 
Klein, on Volvox, 77. 
KoUiker, criticisms of, 92, 94. 
Kupflfer, on fertilization, 204. 

Lepidoptera, parthenogenesis in, 

171, 175- 
Leuckart, on parthenogenesis, 

108, 214. 

Light, penetration of, in water, 8. 
Limnadia, parthenogenesis of, 

109, 167. 

Liparis, parthenogenesis of, 109, 

Liszt, 52. 
Lotti, 47. 
Lotze, 80. 

Lowenhoek, on fertilization, 106. 
Lubbock, on ants, 26, 

Malpighi, on fertilization, 106. 

Martin Luther, 45. 

Maupas, on conjugation, 87, 93, 

177, 183, 189, 196. 
Micronuclei, function of, 181 ; 

meaning of division of, 185. 
Minot, on polar bodies, iii. 
Moa, 5. 

Moina, 88, 89, 206, 217, 
Moles, loss of sight in, 8 
Mollusca, number of idants in, 

Moscheles, 52. 
Mozart, 47, 52, 62. 

Music, origin of, 53, 97. 
Musical sense, 33, 55. 
Mutilations, not inherited, 41. 

Nageli, on idioplasma, 112, 194. 

Natural selection, 15, 21, 23, 33, 

Naumann, 45. 

Negroes, talent for music of, 44. 

Nematodes, number of idants, 

Newts, 16. 

New Zealanders, music of, 38. 

Nucleus, significance of, 194. 

Nussbaum, on division in Infu- 
soria, 86, 191. 

Oncidium, eyes of, 183. 
Onychodromus, conjugation in, 

185, 187. 
Ostracoda, parthenogenesis in, 

no, 161, 167, 217 ; polar bodies 

in. III, 152. 
Ostrich, 5, 6. 
Ovum, maturation of, 114, 122, 

123, 126, 132 ; in parthenoge- 

netic, 150, 
Oxytrichidae, 187. 

Palestrina, 45. 

Pandorina, 77, 200. 

Pangenesis, 79, 81, 128. 

Panmixia, 21, 22, 27, 76, 209. 

Paramaecium, reproduction of, 
177, 185. 

Parasites, 10. 

Parthenogenesis, 91, 108. in, 
150, 156, 167, 218 ; in Fungi, 
94, 169; in Ostracoda, no; in- 
heritance in, 159, 166 ; in Cypris, 
i6r ; in plants, 169 ; origin of, 
170? 175; limitation of, 216. 

Pauer, 52. 

Penguin, 6, 10. 

Petromyzon, fertilization of, 204. 

Pfitzner, on nuclear division in 
Infusoria, 183- 

Pfluger, on parthenogenesis, 108. 

Phryganidae, 19. 

Phyllirhoe, number of idants in, 

Phyllopods, 167, 217. 
Phylloxera, 167, 211. 



Platner, on formation of germ- 
cells, 138, 172. 

Polar bodies, 86, 93, 114, 115, 119, 
170; parthenogenetic, 93, iii. 

Polyergus rufescens, 25. 

Polyphemus, polar bodies of, iii. 

Problems, of the Day, 71. 

Proteus, blind, 7, 28. 

Psyche, parthenogenesis of, 109. 

Psychidae, 19. 

Pteris, parthenogenesis in, 169. 

Pyrrhocoris, on formation of 
germ-cells in, 139, 141. 

Quanz, 45. 

Rabbit, number of cells in cochlea, 


Beineke, 47. 

Bejuvenescence, theory of, 195. 

Betzius, on the cochlea, 57. 

Bhodiets, parthenogenesis in, 

Bolph, explanation of partheno- 
genesis, 172. 

Bossini, 47. 

Botifers, 27 ; polar bodies in, iii, 
152 ; parthenogenesis in, 216. 

Bousseau, on the origin of music, 

Budimentary organs, 28. 

Sagitta, number of Idants, 146. 

Sarasin, on penetration of light in 
water, 8 : on Epicrium, 9. 

Scarlatti, 47. 

Scheibe, on the origin of music, 53. 

Schewiakoff, on division in In- 
fusoria, 184. 

Schopenhauer, 29. 

Scytosiphon, parthenogenesis in, 

Semper, on eyes of Oncidium, 183. 

Sexual selection, 34, 39. 

Siebold, von, on parthenogenesis, 

Solenobia, parthenogenesis of, 

Soma, 74, 79. 

Somatic, cells, 77, 81 ; idioplasm. 


Somatoplasm, 83. 

Spencer, Herbert, on the origin 
of music, 53. 

Spermatozoon, maturation of, 117, 
122, 123, 132. 

Sphaerechinus, fertilization in, 92. 

Stentor, 181, 192. 

Strasburger, 83, 86, 92, 95, 112. 
150, 168, 172, 189. 

Stumpf, onthe origin of music, 53. 

Sully, on '■ Sensation and Intui- 
tion,' 66. 

Swammerdam, on fertilization. 

Tahitians, music of, 38. 
Thalberg, 52. 

Theories, real and ideal, 80. 
Tiara, number of idants in, 146. 
Troubadours, 42, 69. 
Twins, 133. 

Urostyla, division of, 184, 186. 

Van Beneden, on fertilization. 85, 

91, III, 114, 196; on nuclear 
division, 112, 126, 131, 172, 176. 

Variation, origin of, 95 ; in par- 

thenogenetic form, 161, 166. 
Vines, criticisms on Vol. i, 73, 78, 

92, 94. 
Volkmann, 47. 
Vol vox, 77. 

Vorticellidae, behaviour of micro- 
nucleus in, 187. 

"Waldeyer, on nuclear loops, 85. 
"Wallace, on natural selection, 15. 

"Wasps, parthenogenesis of, 109. 
Weber, 46. 
Whales, degeneration of sense of 

smell, 9 ; naked skin, 19. 
Whitman, on nature of nucleus, 


Xenarchus, 34. 




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