characters consistently with a belief in the continuity of the
germ-plasm, a theory which implies the rejection of the supposition
that acquired characters can become hereditary? How can the individuals
of any species come to possess various characters which are undoubtedly
hereditary, if all changes which are due to the influence of external
conditions are transient and disappear with the individual in which
they arose? Why is it that individuals are distinguished by innate
characters, as well as by those which I have previously called
transient, and how can deep-seated hereditary characters arise at all,
if they are not produced by the external influences to which the
individual is exposed?
In the first place it may be argued that external influences may not
only act on the mature individual, or during its development, but that
they may also act at a still earlier period upon the germ-cell from
which it arises. It may be imagined that such influences of different
kinds might produce corresponding minute alterations in the molecular
structure of the germ-plasm, and as the latter is, according to our
supposition, transmitted from one generation to another, it follows
that such changes would be hereditary.
Without altogether denying that such influences may directly modify the
germ-cells, I nevertheless believe that they have no share in the
production of hereditary _individual_ characters.
The germ-plasm or idioplasm of the germ-cell (if this latter term be
preferred) certainly possesses an exceedingly complex minute structure,
but it is nevertheless a substance of extreme stability, for it absorbs
nourishment and grows enormously without the least change in its
complex molecular structure. With Nägeli we may indeed safely affirm so
much, although we are unable to acquire any direct knowledge as to the
constitution of germ-plasm. When we know that many species have
persisted unchanged for thousands of years, we have before us the proof
that their germ-plasm has preserved exactly the same molecular
structure during the whole period. I may remind the reader that many of
the embalmed bodies of the sacred Egyptian animals must be four
thousand years old, and that the species are identical with those now
existing in the same locality. Now, since the quantity of germ-plasm
contained in a single germ-cell must be very minute, and since only a
very small fraction can remain unchanged when the germ-cell developes
into an organism, it follows that an enormous growth of this small
fraction must take place in every individual, for it must be remembered
that each individual produces thousands of germ-cells. It is therefore
not too much to say that, during a period of four thousand years, the
growth of the germ-plasm in the Egyptian ibis or crocodile must have
been quite stupendous. But in the animals and plants which inhabit the
Alps and the far north, we have instances of species which have
remained unchanged for a much longer period, viz. for the time which
has elapsed between the close of the glacial epoch and the present day.
In such organisms the growth of the germ-plasm must therefore have been
still greater.
If nevertheless the molecular structure of the germ-plasm has remained
precisely the same, this substance cannot be readily modifiable, and
there is very little chance of the smallest changes being produced in
its molecular structure, by the operation of those minute transient
variations in nutrition to which the germ-cells, together with every
other part of the organism, are exposed. The rate of growth of the
germ-plasm will certainly vary, but its structure is unlikely to be
affected for the above-mentioned reasons, and also because the
influences are mostly changeable, and occur sometimes in one and
sometimes in another direction.
Hereditary individual differences must therefore be derived from some
other source.
I believe that such a source is to be looked for in the form of
reproduction by which the great majority of existing organisms are
propagated: viz. in sexual, or, as Häckel calls it, amphigonic
reproduction.
It is well known that this process consists in the coalescence of two
distinct germ-cells, or perhaps only of their nuclei. These germ-cells
contain the germ-substance, the germ-plasm, and this again, owing to
its specific molecular structure, is the bearer of the hereditary
tendencies of the organism from which the germ-cell has been derived.
Thus in amphigonic reproduction two groups of hereditary tendencies are
as it were combined. I regard this combination as the cause of
hereditary individual characters, and I believe that the production of
such characters is the true significance of amphigonic reproduction.
The object of this process is to create those individual differences
which form the material out of which natural selection produces new
species.
At first sight this conclusion appears to be very startling and almost
incredible, because we are on the contrary inclined to believe that the
continued combination of existing differences, which is implied by the
very existence of amphigonic reproduction, cannot lead to their
intensification, but rather to their diminution and gradual
obliteration. Indeed the opinion has already been expressed that
deviations from the specific type are rapidly destroyed by the
operation of sexual reproduction. Such an opinion may be true with
regard to specific characters, because the deviations from a specific
type occur in such rare cases that they cannot hold their ground
against the large number of normal individuals. But the case is
different with those minute differences which are characteristic of
individuals, because every individual possesses them, although of a
different kind and degree. The extinction of such differences could
only take place if a few individuals constituted a whole species; but
the number of individuals which together represent a species is not
only very large but generally incalculable. Cross-breeding between all
individuals is impossible, and hence the obliteration of individual
differences is also impossible.
In order to explain the effects of sexual reproduction, we will first
of all consider what happens in monogonic or unisexual reproduction,
which actually occurs in parthenogenetic organisms. Let us imagine an
individual producing germ-cells, each of which may by itself develope
into a new individual. If we then suppose a species to be made up of
individuals which are absolutely identical, it follows that their
descendants must also remain identical through any number of
generations, if we neglect the transient non-transmissible
peculiarities caused by differences of food and other external
conditions.
Although the individuals of such a species might be actually different,
they would be potentially identical: in the mature state they might
differ, but they must have been identical in origin. The germs of all
of them must contain exactly the same hereditary tendencies, and if it
were possible for their development to take place under exactly the
same conditions, identical individuals would be produced.
Let us now assume that the individuals of such a species, reproducing
itself by the monogonic process and therefore without cross-breeding,
differ, not only in transient but also in hereditary characters. If
this were the case, each individual would produce descendants
possessing the same hereditary differences which were characteristic of
itself; and thus from each individual a series of generations would
emanate, the single individuals of which would be potentially identical
with each other and with their first ancestor. Hence the same
individual differences would be repeated again and again, in each
succeeding generation, and even if all the descendants lived to
reproduce themselves, there would be at last just as many groups of
potentially identical individuals as there were single individuals at
the beginning.
Similar cases actually occur in many species in which sexual
reproduction has been entirely replaced by the parthenogenetic method,
as in many species of _Cynips_ and in certain lower Crustacea. But all
these differ from our hypothetical case in one important respect; it is
always impossible for all the descendants to reach maturity and
reproduce themselves. The vast majority of the descendants generally
perish at an early stage, and only about as many remain to continue the
species as reached maturity in the preceding generation.
We have now to consider whether such a species can be subject to the
operation of natural selection. Let us take the case of an insect
living among green leaves, and possessing a green colour as a
protection against discovery by its enemies. We will assume that the
hereditary individual differences consist of various shades of green.
Let us further suppose that the sudden extinction of its food-plant
compelled this species to seek another plant with a somewhat different
shade of green. It is clear that such an insect would not be completely
adapted to the new environment. It would therefore be compelled,
metaphorically speaking, to endeavour to bring its colour into closer
harmony with that of the new food-plant, or else the increased chances
of detection given to its enemies would lead to its slow but certain
extinction.
It is obvious that such a species would be altogether unable to produce
the required adaptation, for _ex hypothesi_, its hereditary variations
remain the same, one generation after another. If therefore the
required shade of green was not previously present, as one of the
original individual differences, it could not be produced at any time.
If, however, we suppose that such a colour existed previously in
certain individuals, it follows that those with other shades of green
would be gradually exterminated, while the former would alone survive.
But this process would not be an adaptation in the sense used in the
theory of natural selection. It would indeed be a process of selection,
but it could form no more than the beginning of that process which we
call natural selection. If the latter could only bring existing
characters into prominence, it would not be worth much consideration,
for it could never produce a new species. A species never includes,
from the beginning, individuals which deviate from the specific type as
widely as the individuals of the most nearly allied species deviate
from it. And it would be still less possible to explain, on such a
principle, the origin of the whole organic world; for, if so, all
existing species would have been included as variations of the first
species. Natural selection must be able to do infinitely more than
this, if it is to be of any importance as a principle of development.
It must be able to accumulate minute existing differences in the
required direction, and thus to create new characters. In our example
it ought to be able, after preserving those individuals with a colour
nearest to the required shade, to lead their descendants onward through
successive stages towards a complete harmony of colour.
But such a result is quite unattainable with the asexual method of
reproduction: in other words, natural selection, in the true meaning of
the term, viz. a process which could produce new characters in the
manner above described, is an impossibility in a species propagated by
asexual reproduction.
If it could be shown that a purely parthenogenetic species had become
transformed into a new one, such an observation would prove the
existence of some force of transformation other than selective
processes, for the new species could not have been produced by these
latter. As already explained, the only selection which would be
possible for such a species, would lead to the survival of one group of
individuals and to the extinction of all others. Thus in our example
that group of individuals would alone survive, the ancestors of which
originally possessed the appropriate colour. But if one group alone
survived, it follows that all hereditary individual differences would
have disappeared from the species, for the members of such a single
group are identical with one another and with their original ancestors.
We thus reach the conclusion that monogonic reproduction can never
cause hereditary individual variability, but that, on the other hand,
it is very likely to lead to its entire suppression.
But the case is very different with sexual reproduction. When once
individual differences have begun to appear in a species propagated by
this process, uniformity among its individuals can never again be
reached. So far from this being the case, the differences must even be
increased in the course of generations, not indeed in intensity, but in
number, for new combinations of the individual characters will
continually arise.
Again, assuming the existence of a number of individuals which differ
from one another by a few hereditary individual characters, it follows
that no individual of the second generation can be identical with any
other. They must all differ, not only actually but also potentially,
for their differences exist at the very beginning of development, and
do not solely depend upon the accidental conditions under which they
live. Moreover, no one of the descendants can be identical with any of
the ancestors, for each of the former unites within itself the
hereditary tendencies of two parents, and its organism is therefore, as
it were, a compromise between two developmental tendencies. Similarly
in the third generation, the hereditary tendencies of two individuals
of the second generation enter into combination. But since the
germ-plasm of the latter is not simple, but composed of two
individually distinct kinds of germ-plasm, it follows that an
individual of the third generation is a compromise between four
different hereditary tendencies. In the fourth generation, eight; in
the fifth, sixteen; in the sixth, thirty-two different hereditary
tendencies must come together, and each of them will make itself more
or less felt in some part of the future organism. Thus by the sixth
generation a large number of varied combinations of ancestral
individual characters will appear, combinations which have never
existed before and which can never exist again.
We do not know the number of generations over which the specific
hereditary tendencies of the first generation can make themselves felt.
Many facts seem to indicate however that the number is large, and it is
at all events greater than six. When we remember that, in the tenth
generation, a single germ contains 1024 different germ-plasms, with
their inherent hereditary tendencies, it is quite clear that continued
sexual reproduction can never lead to the re-appearance of exactly the
same combination, but that new ones must always arise.
New combinations are all the more probable because the different
idioplasms composing the germ-plasm in the germ-cells of any individual
are present in different degrees of intensity at different times of its
life; in other words, the intensity of the component idioplasms is a
function of time. This conclusion follows from the fact that children
of the same parents are never exactly identical. In one child the
characters of the father may predominate, in another those of the
mother, in another again those of either grand-parent or
great-grand-parent.
We are thus led to the conclusion that even in a few sexually produced
generations a large number of well-marked individuals must arise: and
this would even be true of generations springing from our hypothetical
species, assumed to be without ancestors, and characterised by few
individual differences. But of course organisms which reproduce
themselves sexually are never without ancestors, and if these latter
were also propagated by the sexual method, it follows that each
generation of every sexual species is in the stage which we have
previously assumed for the tenth or some much later generation of the
hypothetical species. In other words, each individual contains a
maximum of hereditary tendencies and an infinite variety of possible
individual characters (see Appendix VI, p. 326).
In this manner we can explain the origin of hereditary individual
variability as it is known in man and the higher animals, and as it is
required for the theory which explains the transformation of species by
means of natural selection.
Before proceeding further, I must attempt to answer a question which
obviously suggests itself. For the sake of argument, I have assumed the
existence of a first generation, of which the individuals were already
characterised by individual differences. Can we find any explanation of
these latter, or are we compelled to take them for granted, without any
attempt to enquire into their origin? If we abandon this enquiry, we
can never achieve a complete solution of the problems of heredity and
variability. We have, it is true, shown that hereditary differences,
when they have once appeared, would, through sexual reproduction,
undergo development into the diverse forms which actually exist; but
this conclusion affords us no explanation of the source whence such
differences have been derived. If the external conditions acting
directly upon an organism can only produce transient (viz.
non-hereditary) differences in the latter, and if, on the other hand,
the external influences which act upon the germ-cell can only produce a
change in its molecular structure after operating over very long
periods, it seems that we have exhausted all the possible sources of
hereditary differences without reaching any satisfactory explanation.
I believe, however, that an explanation can be given. The origin of
hereditary individual variability cannot indeed be found in the higher
organisms—the Metazoa and Metaphyta; but it is to be sought for in the
lowest—the unicellular organisms. In these latter the distinction
between body-cell and germ-cell does not exist. Such organisms are
reproduced by division, and if therefore any one of them becomes
changed in the course of its life by some external influence, and thus
receives an individual character, the method of reproduction ensures
that the acquired peculiarity will be transmitted to its descendants.
If, for instance, a Protozoon, by constantly struggling against the
mechanical influence of currents in water, were to gain a somewhat
denser and more resistent protoplasm, or were to acquire the power of
adhering more strongly than the other individuals of its species, the
peculiarity in question would be directly continued on into its two
descendants, for the latter are at first nothing more than the two
halves of the former. It therefore follows that every modification
which appears in the course of its life, every individual character,
however it may have arisen, must necessarily be directly transmitted to
the two offspring of a unicellular organism.
The pianist, whom I have already used as an illustration, may by
practice develope the muscles of his fingers so as to ensure the
highest dexterity and power; but such an effect would be entirely
transient, for it depends upon a modification in local nutrition which
would be unable to cause any change in the molecular structure of the
germ-cells, and could not therefore produce any effect upon the
offspring. And even if we admit that some change might be caused in the
germ-cells, the chances would be infinity to nothing against the
production of the appropriate effect, viz. such a change as would lead
to the development in the child of the acquired characters of the
parent.
In the lowest unicellular organisms, however, the case is entirely
different. Here parent and offspring are still, in a certain sense, one
and the same thing: the child is a part, and usually half, of the
parent. If therefore the individuals of a unicellular species are acted
upon by any of the various external influences, it is inevitable that
hereditary individual differences will arise in them; and as a matter
of fact it is indisputable that changes are thus produced in these
organisms, and that the resulting characters are transmitted. It has
been directly observed that individual differences do occur in
unicellular organisms,—differences in size, colour, form, and the
number or arrangement of cilia. It must be admitted that we have not
hitherto paid sufficient attention to this point, and moreover our best
microscopes are only very rough means of observation when we come to
deal with such minute organisms. Nevertheless we cannot doubt that the
individuals of the same species are not absolutely identical.
We are thus driven to the conclusion that the ultimate origin of
hereditary individual differences lies in the direct action of external
influences upon the organism. Hereditary variability cannot however
arise in this way at every stage of organic development, as biologists
have hitherto been inclined to believe. It can only arise in the lowest
unicellular organisms; and when once individual difference had been
attained by these, it necessarily passed over into the higher organisms
when they first appeared. Sexual reproduction coming into existence at
the same time, the hereditary differences were increased and
multiplied, and arranged in ever-changing combinations.
Sexual reproduction can also increase the differences between
individuals, because constant cross-breeding must necessarily and
repeatedly lead to a combination of forces which tend in the same
direction, and which may determine the constitution of any part of the
body. If, for instance, the same part of the body is strongly developed
in both parents, the experience of breeders tells us that the part in
question is likely to be even more strongly developed in the offspring;
and that weakly developed parts will in the same manner tend to become
still weaker. Amphigonic reproduction therefore ensures that every
character which is subject to individual fluctuation must appear in
many individuals with a strengthened degree of development, in many
others with a development which is less than normal, while in a still
larger number of individuals the average development will be reached.
Such differences afford the material by means of which natural
selection is able to increase or weaken each character according to the
needs of the species. By the removal of the less well-adapted
individuals, natural selection increases the chance of beneficial
cross-breeding in the subsequent generations.
Every one must admit that, if a species came into existence having only
a small number of individual differences which appeared in the
different parts of different individuals, the number of differences
would increase with each sexually produced generation, until all the
parts in which the variations occurred had received a peculiar
character in all individuals.
Moreover sexual reproduction not only adds to the number of existing
differences, but it also brings them into new combinations, and this
latter consequence is as important as the former.
The former consequence can hardly make itself felt in any existing
species, because in them every part already possesses its peculiar
character in all individuals. The second consequence is, however, more
important, viz. the production of new combinations of individual
characters by sexual reproduction; for, as Darwin has already pointed
out, we must imagine that not only are single characters changed in the
process of breeding, but that probably several, and perhaps very many
characters, are simultaneously modified. No two species, however nearly
allied, differ from each other in but a single character. Even our
eyesight, which has by no means reached the highest pitch of
development, can always detect several, and often very many points of
difference; and if we possessed the powers necessary for making an
absolutely accurate comparison, we should probably find that everything
is different in two nearly allied species.
It is true that a great number of these differences depend upon
correlation, but others must depend upon simultaneous primary changes.