sufficiently intimately acquainted with any species to be able to
recognize the nature and meaning of all the details of its structure,
in all their relations: and we are still less able to trace the
ancestral history in each case, and to make out the origin of those
structures of which the presence in the descendants depends primarily
upon heredity. But already a fair advance towards the attainment of
inductive proof has been made; for the number of adaptations which have
been established is now very large and is increasing every day. If,
however, we anticipate the results of future researches, and admit that
an organism only consists of adaptations, based upon an ancestral
constitution, it is obvious that nothing remains to be explained by a
phyletic force, even though the latter be presented to us in the
refined form of Nägeli’s self-changing idioplasm.
It will perhaps be useful to illustrate my views by a familiar example.
I choose the well-known group of the whales. These animals are
placental mammals, which, probably in secondary times, arose from
terrestrial Mammalia, by adaptation to an aquatic life.
Everything that is characteristic of these animals and distinguishes
them from other mammals depends upon this adaptation. Their fore-limbs
have been transformed into rigid paddles, only movable at the
shoulder-joint; upon the back and the tail there are ridges with a form
somewhat similar to the dorsal and caudal fins of fishes. The organ of
hearing is without any external ear and without an air-containing
external auditory meatus. The aerial vibrations do not pass, as in
other mammals, from the external auditory passage to the tympanic
cavity and thus to the nerve-terminations of the inner ear; but they
reach the tympanic cavity by direct transmission through the bones of
the skull, which possess a special structure and contain abundant
air-cavities. This arrangement is obviously adapted for hearing in
water. The nostrils also exhibit peculiarities, for they do not open
near the mouth, but upon the forehead, so that the animal can breathe,
even in a rough sea, as soon as it comes to the surface. In order to
facilitate rapid movement in water, the whole body has become extended
in length, and spindle-shaped, like the body of a fish. The hind limbs
are absent in no other mammals, the fish-like _Sirenia_ being alone
excepted. In the whales, as in the _Sirenia_, these appendages have
become useless, owing to the powerfully developed tail-fin; they are
now rudimentary and consist of some small bones and muscles deeply
buried in the body of the animal, which nevertheless, in certain
species, still exhibit the original structure of the hind-limb. The
hairy covering of other mammals has also disappeared, its place having
been taken by a thick layer of fat beneath the skin, which affords a
much better protection against cold. This fatty layer was also
necessary in order to diminish the specific gravity of the animal, and
to thus render it equal to that of sea-water. In the structure of the
skull there are also a number of peculiarities, all of which are
directly or indirectly connected with the conditions under which these
animals live. In the whalebone whales, the enormous size of the face,
the immense jaws, and wide mouth are very striking. Can it be suggested
that this very characteristic appearance is entirely due to the
guidance of some internal transforming force, or to some spontaneous
modification of the idioplasm? Any such suggestion cannot be accepted,
for it is easy to show that all these structural features depend upon
adaptation to a peculiar mode of feeding. Functional teeth are absent,
but rudimentary ones exist in the embryo as relics of an ancestral
condition in which these organs were fully developed. Large plates of
whalebone with finely divided ends are suspended vertically from the
roof of the mouth. These whales feed upon small organisms, about an
inch in length, which swim or float upon the water in countless
numbers; and in order that they may subsist upon such minute animals,
it is necessary to obtain them in immense numbers. This is achieved by
means of the huge mouth which takes in a vast quantity of water at a
single mouthful. The water then filters away through the plates of
whalebone, while the organisms which form the whale’s food remain
stranded in the mouth. Is it necessary to add that the internal
organs—so far as we understand the details of their functions, and so
far as their structure differs from that of the corresponding organs in
other Mammalia—have also been directly or indirectly modified by
adaptation to an aquatic life? Thus all whales possess a very peculiar
arrangement of the nasal passages and larynx, enabling them to breathe
and swallow at the same time: the lungs are of enormous length, and
thus cause the animal to assume a horizontal position in the water
without the exercise of muscular effort: in consequence of this latter
modification, the diaphragm extends in a nearly horizontal direction:
there are moreover certain arrangements in the vascular system which
enable the animal to remain under water for a considerable time, and so
on.
And now, in reference to this special example, I will repeat the
question which I have asked before:—‘If everything that is
characteristic of a group of animals depends upon adaptation, what
remains to be explained by the operation of an internal developmental
force?’ What remains of a whale when we have taken away its adaptive
characters? We are compelled to reply that nothing remains except the
general plan of mammalian organization, which existed previously in the
mammalian ancestors of the _Cetacea_. But if everything which stamps
these animals as whales has arisen by adaptation, it follows that the
internal developmental force cannot have had any share in the origin of
this group.
And yet this very force is said to be the main factor in the
transformation of species, and Nägeli unhesitatingly asserts that both
the animal and vegetable kingdoms would have become very much as they
now are, if there had been no adaptation to new conditions, and no such
thing as competition in the struggle for existence[180].
But even if we admit that such an assumption affords some explanation,
instead of being the renunciation of all attempts at explanation; if we
admit that an organism, the characteristic peculiarities of which
entirely depend upon adaptation, has been formed by an internal
developmental force; we should still be unable to explain how it
happens that such an organism, suited to certain conditions of life,
and unable to exist under other conditions, appeared at that very place
on the earth’s surface, and at that very time in the earth’s history,
which offered the conditions appropriate for its existence. As I have
previously argued, the believers in an internal developmental force are
compelled to invent an auxiliary hypothesis, a kind of ‘pre-established
harmony’ which explains how it is that changes in the organic world
advance step by step, parallel with changes in the crust of the earth
and in other conditions of life; just as, according to Leibnitz, body
and soul, although independent of each other, proceed along parallel
courses, like two chronometers which keep perfect time. And even this
supposition would not be sufficient, because the place must be taken
into account as well as the time: thus the whales could not have
existed if they had first appeared upon dry land. We know of countless
instances in which a species is exclusively and precisely adapted to a
certain localized area, and could not thrive anywhere else. We have
only to remember the cases of mimicry in which one insect gains
protection by resembling another, the cases of protective resemblance
to the bark or the leaves of a certain species of plant, or the
numerous marvellous adaptations of parasitic animals to certain parts
of certain species of hosts.
A mimetic species cannot have appeared at any place other than that in
which it exists: it cannot have arisen through an internal
developmental force. But if single species, or even whole orders like
the _Cetacea_, have arisen independently of any such force, then we may
safely assert that the existence of the supposed force is neither
required by reason nor necessity.
Hence, abstaining from the invocation of unknown forces, we are
justified in carrying on Darwin’s attempt to explain the transformation
of organisms by the action of known forces and known phenomena. I say
‘carry on the attempt,’ because I do not believe that our knowledge in
this direction has ended with Darwin, and it seems to me that we have
already arrived at ideas which are incompatible with certain important
points in his general theory, and which therefore necessitate some
modification of the latter.
The theory of natural selection explains the rise of new species by
supposing that changes occur, from time to time, in those conditions of
life to which an organism must adapt itself if it is to continue in
existence. Thus a selective process is set up which ensures that only
those out of the existing variations are preserved, which correspond in
the highest degree to the changed conditions of life. By continued
selection in the same direction the deviations from the type, although
at first very insignificant, are accumulated and increased until they
become specific differences.
I should wish to assert more definitely than Darwin has done, that
alterations in the conditions of life, together with changes in the
organism itself, must have advanced very gradually and by the smallest
steps, in such a way that, at each period in the whole process of
transformation, the species has remained sufficiently adapted to the
surrounding conditions. An abrupt transformation of a species is
inconceivable, because it would render the species incapable of
existence. If the whole organization of an animal depends upon
adaptation, if the animal body is, as it were, an extremely complex
combination of new and old adaptations, it would be a highly remarkable
coincidence if, after any sudden alteration occurring simultaneously in
many parts of the body, all these parts were changed in such a manner
that they again formed a whole which exactly corresponded to the
altered external conditions. Those who assume the existence of such a
sudden transformation overlook the fact that everything in the animal
body is exactly calculated to maintain the existence of the species,
and that it is just sufficient for this purpose; and they forget that
the minutest change in the least important organ may be enough to
render the species incapable of existence.
It may perhaps be objected that the case is different in plants, as is
proved by the American weeds which have spread all over Europe, or the
European plants which have become naturalized in Australia. Reference
might also be made to the plants which inhabited the plains during the
glacial epoch, and which at its close migrated to the Alpine mountains
and to the far north, and which have remained unaltered under the
apparently diverse conditions of life to which they have been subjected
for so long a time. Similar instances may also be found among animals.
The rabbit, which was brought by sailors to the Atlantic island of
Porto Santo, has bred abundantly and remains unchanged in this
locality; the European frogs, which were introduced into Madeira, have
increased immensely and have become almost a plague; and the European
sparrow now thrives in Australia quite as well as with us. But these
instances do not prove that adaptation to external conditions of life
is not of primary importance; they do not prove that an organism which
is adapted to a certain environment will, when unmodified, remain
capable of existence amid new surroundings. They only prove that the
above-mentioned species found in those countries the same conditions of
life as at home, or at least that they met with conditions to which
their organization could be subjected without the necessity for
modification. Not every new environment includes such changed
conditions as will be effective in modifying every species of plant or
animal. The rabbit of Porto Santo certainly feeds on herbs different
from those which form the food of its relations in Europe, but such a
change does not mean an effective alteration in the conditions under
which this species lives, for the herbs in both localities are equally
well suited to the needs of the animal.
But if we suppose that the wild rabbit, occurring in Europe, were to
suddenly lose but a trifle of its wariness, its keen sight, its fine
sense of hearing or of smell, or were to suddenly acquire a colour
different from that which it now possesses, it would become incapable
of existence as a species, and would soon die out. The same result
would probably occur if any of its internal organs, such as the lungs
or the liver, were suddenly modified. Perhaps single individuals would
still remain capable of existence under these circumstances, but the
whole species would suffer a certain decline from the maximum
development of its powers of resistance, and would thus become extinct.
The sudden transformation of a species appears to me to be
inconceivable from a physiological point of view, at any rate in
animals.
Hence the transformation of a species can only take place by the
smallest steps, and must depend upon the accumulation of those
differences which characterise individuals, or, as we call them,
‘individual differences.’ There is no doubt that these differences are
always present, and thus, at first sight, it appears to be simply a
matter of course that they will afford the material by means of which
natural selection produces new forms of life. But the case is not so
simple as it appeared to be until recently; that is if I am right in
believing that in all animals and plants which are reproduced by true
germs, only those characters which were potentially present in the germ
of the parent can be transmitted to the succeeding generation.
I believe that heredity depends upon the fact that a small portion of
the effective substance of the germ, the germ-plasm, remains unchanged
during the development of the ovum into an organism, and that this part
of the germ-plasm serves as a foundation from which the germ-cells of
the new organism are produced[181]. There is therefore continuity of
the germ-plasm from one generation to another. One might represent the
germ-plasm by the metaphor of a long creeping root-stock from which
plants arise at intervals, these latter representing the individuals of
successive generations.
Hence it follows that the transmission of acquired characters is an
impossibility, for if the germ-plasm is not formed anew in each
individual but is derived from that which preceded it, its structure,
and above all its molecular constitution, cannot depend upon the
individual in which it happens to occur, but such an individual only
forms, as it were, the nutritive soil at the expense of which the
germ-plasm grows, while the latter possessed its characteristic
structure from the beginning, viz. before the commencement of growth.
But the tendencies of heredity, of which the germ-plasm is the bearer,
depend upon this very molecular structure, and hence only those
characters can be transmitted through successive generations which have
been previously inherited, viz. those characters which were potentially
contained in the structure of the germ-plasm. It also follows that
those other characters which have been acquired by the influence of
special external conditions, during the life-time of the parent, cannot
be transmitted at all.
The opposite view has, up to the present time, been maintained, and it
has been assumed, as a matter of course, that acquired characters can
be transmitted; furthermore, extremely complicated and artificial
theories have been constructed in order to explain how it may be
possible for changes produced by the action of external influences, in
the course of a life-time, to be communicated to the germ and thus to
become hereditary. But no single fact is known which really proves that
acquired characters can be transmitted, for the ascertained facts which
seem to point to the transmission of artificially produced diseases
cannot be considered as a proof; and as long as such proof is wanting
we have no right to make this supposition, unless compelled to do so by
the impossibility of suggesting a mode in which the transformation of
species can take place without its aid. (See Appendix IV, p. 310.)
It is obvious that the unconscious conviction that we need the aid of
acquired characters has hitherto securely maintained the assumed axiom
of the transmission of such features. It was believed that we could not
do without such an axiom in order to explain the transformation of
species; and this was believed not only by those who hold that the
direct action of external influences plays an important part in the
process, but also by those who hold that the operation of natural
selection is the main factor.
Individual variability forms the most important foundation of the
theory of natural selection: without it the latter could not exist, for
this alone can furnish the minute differences by the accumulation of
which new forms are said to arise in the course of generations. But how
can such hereditary individual characters exist if the changes wrought
by the action of external influences, during the life of an individual,
cannot be transmitted? We are clearly compelled to find some other
source of hereditary individual differences, or the theory of natural
selection would collapse, as it certainly would if hereditary
individual variations did not exist. If, on the other hand, acquired
differences are transmitted, this would prove that there must be
something wrong in the theory of the continuity of the germ-plasm, as
above described, and in the non-transmission of acquired characters
which results from this theory. But I believe that it is possible to
suggest that the origin of hereditary individual characters takes place
in a manner quite different from any which has been as yet brought
forward. To explain this origin is the task which I am about to
undertake in the following pages.
The origin of individual variability has been hitherto represented
somewhat as follows. The phenomena of heredity lead to the conclusion
that each organism is capable of producing germs, from which,
theoretically at least, exact copies of the parent may arise. In
reality this is never the case, because each organism possesses the
power of reacting on the different external influences with which it is
brought into contact, a power without which it could neither develope
nor exist. Each organism reacting in a different way must be to some
extent changed. Favourable nutrition makes such an organism strong and
large; unfavourable nutrition renders it small and weak, and what is
true of the whole organism may also be said of its parts. Now it is
obvious that even the children of the same mother meet with influences
different in kind and degree, from the very beginning of their
existence, so that they must necessarily become unlike, even if we
suppose them to have been derived from absolutely identical germs, with
precisely the same hereditary tendencies.
In this manner individual differences are believed to have been
introduced. But if acquired characters are not transmitted the whole
chain of argument collapses, for none of those changes which are caused
by the conditions of nutrition acting upon single parts of the whole
organism, including the results of training and of the use or disuse of
single organs,—none of these changes can furnish hereditary
differences, nor can they be transmitted to succeeding generations.
They are, as it were, only transient characters as far as the species
is concerned.
The children of accomplished pianists do not inherit the art of playing
the piano; they have to learn it in the same laborious manner as that
by which their parents acquired it; they do not inherit anything except
that which their parents also possessed when children, viz. manual
dexterity and a good ear. Furthermore, language is not transmitted to
our children, although it has been practised not only by ourselves but
by an almost endless line of ancestors. Only recently, facts have again
been worked up and brought together, which show that children of highly
civilized nations have no trace of a language when they have grown up
in a wild condition and in complete isolation[182]. The power of speech
is an acquired or transient character: it is not inherited, and cannot
be transmitted: it disappears with the organism which manifests it. Not
only do similar phenomena occur in the vegetable kingdom, but they
present themselves in an especially striking manner.
When Nägeli[183] introduced Alpine plants, taken from their natural
habitat, into the botanical garden at Munich, many of the species were
so greatly altered that they could hardly be recognized: for instance,
the small Alpine hawk-weeds became large and thickly branching, and
they blossomed freely. But if such plants, or even their descendants,
were removed to a poor gravelly soil the new characters entirely
disappeared, and the plants were re-transformed into the original
Alpine form. The re-transformation was always complete, even when the
species had been cultivated in rich garden soil for several generations.
Similar experiments with identical results were made twenty years ago
by Alexis Jordan[184], who chiefly made use of _Draba verna_ in his
researches. These experiments furnish very strong proofs, because they
were originally undertaken without the bias which may be given by a
theory. Jordan only intended to decide experimentally whether the
numerous forms of the plant, as it occurs wild in different habitats,
are mere varieties or true species. He found that the different forms
do not pass into one another, and are in all cases re-transformed after
they have been altered by cultivation in a soil different from that in
which they usually grow, and he therefore assumed that they were true
species. All these experiments therefore confirm the conclusion that
external influences may alter the individual, but that the changes
produced are not transmitted to the germs, and are never hereditary.
Nägeli indeed asserts that innate individual differences do not exist
in plants. The differences which we find, for instance, between two
beeches or oaks, are always, according to him, modifications produced
by the influence of varying local conditions. But it is obvious that
Nägeli goes too far in this respect, although it may be conceded that
innate individual differences in plants are much more difficult to
distinguish from those which are acquired, than in animals.
There is no doubt about the occurrence of innate and hereditary
individual characters in animals, and we may find an especially
interesting illustration in the case of man. The human eye can with
practice appreciate the most minute differences between individual men,
and especially differences of feature. Every one knows that
peculiarities of feature persist in certain families through a long
series of generations. I need hardly remind the reader of the broad
forehead of the Julii, the projecting chin of the Hapsburgs, or the
curved nose of the Bourbons. Hence every one can see that hereditary
individual characters do unquestionably exist in man. The same
conclusion may be affirmed with equal certainty for all our domestic
animals, and I do not see any reason why there should be any doubt
about its application to other animals and to plants.