and not upon an internal principle of evolution, since that could
take no account whatever of external circumstances, but would cause
variations in the organism altogether independently of these. Thus, in
considering the origin of any of the larger groups of animals, we may
exclude a phyletic power as the guide of its evolution as soon as we
can prove that all its essential structural relations, as far as they
diverge from those of nearly related groups, are adaptations. We may
not be able to do this for nearly all of the animal groups, and it will
hardly be possible in regard to a single group of plants, because our
insight into the _biological_ significance of characters, which means
more than the functional significance of the individual parts, and
their correlation as parts of a whole, is seldom sufficiently intimate
or thorough. But among animals we can do this in regard to some groups;
one of these is the order of whales or Cetaceans.
[Illustration: FIG. 130. Skeleton of a Greenland Whale with the contour
of the body. _Ok_, upper jaw. _Uk_, lower jaw. _Sch_, shoulder-blade.
_OA_, upper arm. _UA_, bones of fore-arm. _H_, hand. _Br_, vestige of
the pelvis. _Fr_, vestige of the femur. _Tr_, vestige of the lower part
of the leg. After Claus.]
Cetaceans, as is well known, belong to the Mammalia, that is to
say, to a class whose structure was built for life on the land. The
ancestors of Cetaceans were similar to the other mammals, and possessed
a coat of hair and four legs, and a body the mass of which was so
distributed that it could be borne by those four legs. But all the
modern Cetaceans live in the sea, and they have therefore entirely
changed their bodily form; they have become spindle-shaped like fishes,
well adapted for cleaving the water, but incapable of moving upon land.
At the same time, their hind-legs have completely disappeared, and can
now be demonstrated only as rudiments within the mass of muscle (Fig.
130, _Br_, _Tr_, _Fr_), while the fore-legs have been transformed
into flippers, in which, however, the whole inherited, but greatly
shortened, skeleton of the mammalian arm is concealed (_OA_, _UA_,
_H_). The skin has lost its covering of hair so completely that in
some cases no traces of it are demonstrable except in the embryo. All
these changes are adaptations to an aquatic life, and could not have
been produced independently of the influence of external conditions.
But there is much more than this. A thick layer of blubber under the
skin gives this warm-blooded animal an effective protection against
being cooled down by the surrounding water, and at the same time gives
it the appropriate specific gravity for life in the sea; an enormous
tail-fin similar to that of fishes, but placed horizontally, forms the
chief organ of locomotion, and for this reason the hind-legs became
superfluous and degenerated. Similarly, the muscles of the ear have
also disappeared, for the hearing organ of this aquatic type is no
longer suited for receiving the sound-waves through an air-containing
trumpet, but receives them by a shorter route from the surrounding
water, directly through the bones of the skull. Remarkable changes
in the respiratory and circulatory organs make prolonged submersion
possible, and the displacement of the external nares from the snout to
the forehead enables the animal to draw breath when it comes up from
the depths to a frequently stormy surface. It would take a long time to
enumerate all that can be recognized as adaptive in these remarkable
aquatic mammals to a life in what to their ancestors would have been a
strange and hostile element. Let us study particularly the case of the
whalebone whales, for instance the Greenland whale, and we are at once
struck by the enormous size of the head, which makes up about a third
of the whole body (Fig. 130). Can this, which has such an important
effect in determining the whole type of animal, be an outcome of some
internal power of development? By no means! It is rather an adaptation
to the mode of nutrition peculiar to this swimming mammal, for it
does not, like dolphins and toothed whales, feed on large fishes and
Cephalopods, but on minute delicate molluscs--Pteropods and pelagic
Gastropods, on Salpæ, and the like, which often cover the surface
of the Arctic Ocean in endless shoals, sometimes extending for many
miles. To enable the whale to sustain life on such minute morsels it
was necessary that it should be able to swallow enormous quantities;
teeth were therefore useless, and they have become rudimentary, and can
only be demonstrated in the embryo as rudiments (dental germs) in the
jaw; but in place of these there hang from the roof of the mouth-cavity
great plates of 'whalebone,' a quite peculiar product of the mucous
membrane of the mouth, the ends of which are frayed into fibres, and
form a sieve-net for catching the little animals which are engulfed
with the sea-water. The mouth-cavity itself has become enormous, so
that great quantities of water at a time can be strained through the
net of whalebone-plates.
When I mention that peculiar changes have occurred also in the internal
organs, that the lungs have elongated longitudinally and thus enable
the animal more readily to lie in the water in a horizontal position,
that peculiar arrangements exist within the nostrils and the larynx
which enable the animal to breathe and swallow simultaneously, and that
the diaphragm lies almost horizontally because of the length of the
lung, I think I have said enough to indicate that not only does almost
everything about the whale diverge from the usual mammalian type, but
that all these deviations are adaptations to an aquatic life. If
everything that is characteristic, that is, typical either of the order
or of the family to which animals belong, depends upon adaptation,
what room is left for the activity of an internal power of evolution?
How much is left of the whale when the adaptations are subtracted?
Nothing more than the general scheme of a mammal; but this was implicit
in their ancestors before the whales originated at all. But if what
makes whales what they are, that is, the whole 'scheme' of a whale,
has originated through adaptation, then the hypothetical evolutionary
power--wherever its seat may be--has had no share in the origin of this
group of animals.
I said all this more than ten years ago, but the idea of an internal
directive evolutionary force is firmly rooted in many minds, and
new modifications of the idea are always cropping up, and of these
the most dangerous seem to me to be those which are not clear in
themselves, but suppose that the use of a shibboleth like 'organic
growth' means anything. That organic growth is at the base of the
phyletic evolution of organisms may be maintained from any scientific
standpoint whatsoever, from ours as well as from Nägeli's, for no one
is so extreme and one-sided as to regard the process of evolution as
due _solely_ to internal or _solely_ to external factors. The process
may thus always be compared to the growth of a plant, which likewise
depends on both internal and external influences. But that is saying
very little; we have still to show how much and how little is effected
by these internal and external factors, what their nature precisely
is, and what relation they bear to one another. There is thus a great
difference between believing, with Nägeli, that 'the animal and plant
kingdoms must have become very much what they actually are, even
had there been on the earth no adaptation to new conditions and no
competition in the struggle for existence,' and sharply emphasizing, in
accordance with the facts just discussed, that, in any case, a whole
order of mammals--the Cetaceans--could never have arisen at all if
there had been no adaptation.
The same thing could be proved in regard to the class of Birds, for
in them too we are able to recognize so many adaptive features, that
we may say everything about them that makes them birds depends upon
adaptation to aerial life, from the articulations of the backbone
to the structure of the skull and the existence of a bill; from the
transformation of the fore-limbs into wings, and of the hind-limbs
to very original organs of locomotion on land or swimming organs in
water, to the structure of the bones, the position, size, and number
of the internal organs, down even to the microscopic structure of
numerous tissues and parts. What could be more characteristic of a
class of animals than feathers are of birds? They alone are enough
to distinguish the class from all other living classes; an animal
with feathers can now be nothing but a bird, and yet the feather is
a skin-structure which has arisen through adaptation, a reptilian
scale which has been so transformed that an organ of flight could
develop from its anterior extremity. We find it thus even in the two
impressions of the primitive bird _Archcæopteryx_, which have been
preserved for us in the Solenhofen slate since the Jurassic period in
the history of the earth. And into what detail does adaptation go in
the case of the feathers! Is not the whole structure, with its quill,
shaft, and vane, precisely adapted to its function, although that is
purely passive? What I have just said of the whole class of Birds holds
true for this individual structure, the feather; everything about it
is adaptation, and indeed illustrates adaptation in two directions,
for in the first place the feathers, by spreading a broad, light,
and yet resistant surface with which to beat the air, act as organs
of flight, while they are also the most effective warmth-retaining
covering conceivable. In both these directions their achievements
border on the marvellous. I need only recall the most recent discovery
in this domain, the proof recently given by the Viennese physiologist,
Sigmund Exner, that the feathers become positively electric in their
superficial layer, and negatively electric in their deeper layer,
whenever they rub against one another and strike the air. But they
are rubbed whenever the bird flies or moves, and the consequence
of the contrast in the electric charging of the two layers is that
the covering feathers are closely apposed over the down-feathers,
while, on the other hand, the similar charging of the down-feathers
makes them mutually repel each other, with the result that a layer
of air is retained between them, and thus there is between the skin
and the covering feathers a loose thicket of feathers uniformly
penetrated by air--the most effective warmth-preserver imaginable.
The electric characters of the feathers--and the same is true of the
hairs of animals--are thus not indifferent characters, but with an
appreciable biological importance, and the same is true of the almost
microscopical series of little hooklets which attach the barbs of the
covering feathers to one another, and thus form a relatively firm but
exceedingly light wing-surface which offers a strong resistance to
the air. But as we must regard these hooklets as adaptations, so must
we also regard the electrical characters of the feathers, and we must
think of them as having arisen through natural selection, as Exner
himself has insisted.
If we are able to recognize all the more prominent features of
the organization in Cetaceans and Birds as due to adaptation, we
must conclude that, in the rest of the great groups of the animal
kingdom, the main and essential parts of the structure are adaptations
to the conditions of life, even although the relations between
external circumstances and internal organization are not so readily
recognizable. For if there were an internal evolutionary force at all,
we should be able to recognize its operation in the origin of the races
of Cetaceans and Birds; but if there be no such power, then even in
cases where the conditions of life are not so conspicuously divergent
as in Cetaceans and Birds, we must refer the typical structure of the
group to adaptation. Thus everything about organisms depends upon
adaptation, not only the main features of the organization, but the
little details in as far as they possess selection-value; it is only
what lies below this level that is determined by internal factors
alone, by germinal selection; but this is not an imperative force in
the sense in which the term is used by Nägeli and his successors, for
it is capable of being guided; it does not necessarily lead to an
invariable and predetermined goal, but it can be directed according to
circumstances into many different paths. But it is precisely this that
constitutes the main problem of the evolution theory--how development
due to internal causes can, at the same time, bring about adaptation to
external circumstances.
This lecture had been transcribed so far, and was ready for the
press, when I received the first volume of a new work by De Vries, in
which that distinguished botanist develops new views in regard to the
transformation of species, based upon numerous experiments, carried on
for many years on the variation of plants. As not only his views, but
the interesting facts he sets forth, seem to contradict the conclusions
as to the transmutation of organisms which I have been endeavouring to
establish, I cannot refrain from saying something on the subject.
De Vries does not believe that the transformation of species can
depend on the cumulative summation of minute 'individual' variations;
he distinguishes between 'variations' and 'mutations,' and attributes
only to the latter the power of changing the character of a species.
He regards the former as mere fluctuating deviations which may be
increased by artificial selection, and may even, with difficulty,
if carefully and purely bred for a long period, be made use of to
give character to a new breed, but which play no part at all in the
natural course of phylogeny. As regards phylogeny, he maintains that
only 'mutations' have any influence, that is, the larger or smaller
saltatory variations which crop up suddenly and which have from the
very first a tendency to be purely transmitted, that is, to breed true.
The facts upon which these views are mainly based are observations
on and breeding experiments with a species of evening primrose
(_Œnothera_) which was found in quantities on a fallow potato-field
at Hilversum in Holland. It had been cultivated previously in a
neighbouring garden, and had sown itself thence in the field. The
numerous specimens of this _Œnothera lamarckiana_ growing there were
in a state of marked 'fluctuating' variability, but in addition there
grew among them two strongly divergent forms which must have arisen
from the others, and which led De Vries to bring the parent stock
under cultivation, in the hope that it would yield new forms, in the
Botanical Gardens at Amsterdam. This hope was fulfilled; in the second
cultivated generation there were, among the 15,000 plants, ten which
represented two divergent forms, and in the succeeding generations
these forms were repeated several times and in many cases, and five
other new forms cropped up, most of them in several specimens and in
different generations of the original stock. All these new forms, which
De Vries calls 'elementary species,' breed true, that is to say, when
they are fertilized with their own pollen they yield seed which gives
rise to the same 'elementary species.' The differences between the
new forms are usually manifold, and of the same kind as those between
the 'elementary' species of the wild Linnæan species. But, according
to De Vries, what we have been accustomed since the time of Linné to
call a 'species' is a collective category, whose components are these
'elementary' species which De Vries has observed in his experiments
with _Œnothera_. In other species, such as _Viola tricolor_ and _Draba
verna_, true-breeding varieties have long been known to botanists, and
these have been studied carefully and tested experimentally, especially
by A. Jordan, and more recently by De Bary.
All 'species,' according to the Linnæan conception, consist, De Vries
maintains, of a larger or smaller number (in _Draba_ there are two
hundred) of these 'elementary' species, and these arise, as is proved
by the case of _Œnothera_, by saltatory or discontinuous 'variations'
which occur periodically and suddenly break up a species into many
new species, because the variations of the germ-plasm, which are for
a time merely latent, suddenly find expression in the descendants of
one individual or another. According to this view, species must be
the outcome of purely internal causes of development, which reveal
themselves as 'mutations,' that is as saltatory variations, which are
stable and transmissible from the very first, and among which the
struggle for existence decides which shall survive and which shall
be eliminated. For the mutations themselves occur in no particular
direction; they are sometimes advantageous, sometimes indifferent,
sometimes even injurious (for instance, when one sex is left out), and
so it is always only a fraction of the mutations, often only a few,
which prove themselves capable of permanent existence. Thus 'species do
not arise through the struggle for existence, but they are eliminated
by it' (p. 150); natural selection does nothing more than weed out
what is unfit for existence, it does not exercise any selective, in
the sense of directive, influence on the survivors. A difference in
the nature of variations was previously maintained by the American
palæontologist Scott, though for different reasons and also with a
different meaning. He believed that variations in a definite direction
were necessary to explain the direct course of development which many
animal groups, such as the horses and the ruminants, have actually
followed, and which he thought could not be ascribed to cumulative
adaptation to the conditions of life. The 'mutations' of De Vries are
not distinguished from the 'fluctuating' variations by following a
definite direction, but in that they are strictly heritable, that they
'breed true.' It is true that 'fluctuating' individual differences are
also transmissible, and can be increased by artificial selection, but
they lack one thing that would make them component parts of a natural
species, namely, constancy; they do not breed true, and are therefore
never independent of selection, but require to be continually selected
out afresh in order that they may be kept pure. They form 'breeds,' not
species, and if left to themselves they soon revert to the characters
of the parent species, as is well known of the numerous 'ennobled
races' among our cereals. De Vries therefore denies absolutely that a
new species could be developed by natural selection from 'fluctuating'
variations, and not alone because there is no constancy of character,
but also because the capacity of the character for being increased is
very limited. Usually nothing more can be achieved than doubling of
the original character, and then progress becomes more difficult and
finally ceases altogether.
These are incisive conclusions, based upon an imposing array of weighty
facts. I readily admit that I have rarely read a scientific book
with as much interest as De Vries's _Mutationstheorie_. Nevertheless
I believe that one might be carried away too far by De Vries, for
he obviously overestimates the value of his facts, interesting and
important as these undoubtedly are, and under the influence of what
is new he overlooks what lies before him--the other aspect of the
transmutation of species, to which the attention of most observers
since Darwin and Wallace has been almost exclusively devoted--I mean
the origin of adaptations. Not that he does not mention these, he
assumes in regard to his mutations 'a selection working in a constant
direction,' and seeks to interpret them in terms of it, but as the
mutations occur from purely internal reasons--I mean without any
connexion with the necessity for a new adaptation--and occur only
in a small percentage of individuals, and in no definite direction,
they cannot possibly suffice to explain _adaptation_, which seems to
dominate the whole organic world. But this is precisely the point at
which many botanists cease to understand the zoologists, because among
plants there are fewer adaptations than among animals; or, in any
case, adaptations in plants are not so readily demonstrated as among
animals, which not infrequently seem to us to be entirely built up of
adaptations.
In this book, and in this chapter itself, I have discussed adaptations
and their origin so much already that I need only refer to these pages
for convincing evidence that we cannot think of them as being brought
about by the accumulation and augmentation of individually occurring
saltatory 'mutations.' Not even if we assume that the leaps of mutation
can be increased in the course of generations; in short, even if we
say that mutations are all those variations which breed true and lead
to the development of species, while variations are those which do
not. This would only be playing with words, so let us say that the
fluctuating variations are really different in their nature, that is,
in their causes, from mutations. De Vries lays great stress on the
fact that these two kinds of variations must be sharply distinguished
from one another, and this may have been useful or necessary for the
first investigation of the facts before him, for we must first analyse
and then recombine, but that variations and mutations are in reality
different in nature can assuredly not be assumed, since innumerable
adaptations can only have arisen through the augmentation of individual
variations. These must therefore be able to become 'pure breeding,'
even although they may not have done so in the cases of artificial
selection which have hitherto been observed. How is it possible that
chance mutations, in no particular direction, occurring only rarely and
in a small percentage of individuals, can explain the origin of the
leaf-marking of a _Kallima_ or an _Anæa_--the shifting of the original
wing-nervures to form leaf-veins, and the exact correlation of these
veins across the surfaces of both pairs of wings? And even if we were
to admit that a mutation might have occurred which caused the veins of
the anterior and posterior wings to meet exactly by chance, that would
still not be a leaf-adaptation, for there would still be wanting the
instinct which compels the butterfly, when it settles down, to hold
the wings in such a position that the two pictures on the anterior
and posterior wings fit into each other. Correlated mutations of the
nervous system suited to this end are required, but that is too much to
attribute to happy chance! The same holds true in regard to the whole
leaf-picture on the two wings, for it could not possibly have arisen as
a whole by a sudden mutation. The whole litany of objections which have
been urged throughout several decades against the Darwin-Wallace theory
of natural selection, which were based on the improbability that chance
variations not in a definite direction should yield suitable material
for the necessary adaptations, may be urged much more strongly against
mutations, which make their appearance in much smaller numbers and with
less diversity.
But it is--as we have already seen--in regard to the necessity which
exists almost everywhere for the co-adaptation of numerous variations
of the most different parts, that the 'mutation theory' breaks down
utterly. The kaleidoscopic picture, the mutation, is implicit from
the first, and must be accepted or rejected just as it is in the
struggle for existence; but harmonious adaptation requires a gradual,
simultaneous, or successive purposive variation of all the parts
concerned, and this can be secured only through the fluctuating
variations which are always occurring, and are increased by germinal
selection and guided by personal selection.
Many naturalists, and especially many botanists, regard adaptation
as something secondary, something given to species by the way, to
improve the conditions of their existence, but not affecting their
nature--comparable perhaps to the clothing worn by man to protect
himself from cold; but that is hardly the real state of the matter.