so-called 'indifferent' characters, whose occurrence is so often used
as evidence against natural selection, as a proof that evolution is
guided essentially by internal forces alone. But this objection was
based on a fallacy, for the fact that the first small variations are
due to internal processes in the germ-plasm does not imply that the
whole further course of evolution is determined by these alone, any
more than the fact that a sleigh requires a push to start it on its
descent down an inclined plane would imply that its rapid descent is
due only to the force of the push, and not at the same time to the
attraction of gravity. But the analogy is not quite sound, for the
processes within the germ-plasm which condition and direct variation
do not merely give them the first shove off; they are associated with
every onward step in the evolutionary path of the species, they impel
it further and further, and without these continual impulses the
progressive movement would cease altogether. We have seen that, for
internal reasons, germinal selection continues to impel the varying
determinants further along the path on which they have started, and
thus gives them cumulative strength, and that it is in this way that
adaptations to the conditions of life are brought about. The evolution
of the character of a species may thus be compared to the course of
a sleigh upon a level snow-surface which it could traverse in any
direction, but it is moved only by the impulses received through
germinal selection. The conditions of life to which the varying parts
have to adapt themselves may be thought of as the distant goal, and
the processes in the germ-plasm which give rise to variation in a
definite direction may be compared to numerous human beings scattered
irregularly over the surface of the snow. If the sleigh receives from
one of these a push which chances to be in the direction of the goal,
it rushes on towards this and ultimately reaches it if the person
pushing continues to push in the same direction. So far, then, it
seems as if the transformation of the part concerned depended upon
germinal selection alone, but we must not forget that the germ-plasm
does not contain only one determinant for every part of the body, but
as many determinants as there are ids. We must therefore increase the
number of our sleighs, and now it is obvious that the pushers of the
sleighs, that is, germinal selection, may push one sleigh on toward
the goal, but others in the opposite or in any other direction. If we
assume that all the sleighs which have taken a wrong direction must
reach dangerous ground, and ultimately plunge into an abyss, but that
from a neighbouring point sleighs were being dispatched to replace all
that came to grief, that these in their turn might attempt to reach
the goal, it would ultimately come about that the requisite number
of sleighs would arrive at the goal--that is to say, that the new
adaptation would be attained.
The abysses represent the elimination of the less favourable
variational tendencies, and the constant replacing of sleighs
represents the intermingling of fresh ids through amphimixis. If all
the sleighs run in the wrong direction they all come to grief, that
is, the individual concerned is eliminated with all the ids of its
germ-plasm--it disappears altogether from the ranks of the species. But
if only a portion of them run in the right direction, care is taken
that in following generations, that is, in the continuation of the
sleigh-race, this portion combines with those of another group which
are also running in the right direction--that is, with the half number
of ids from another germ-plasm in amphimixis.
It is not possible to follow the analogy further, but perhaps it may
serve to illustrate how germinal selection may be the only impelling
force in the organisms, and yet only a small part of its results
are determined by itself, and by far the larger part by external
conditions. We understand how a variation in a definite direction can
exist, and yet it is not that which creates species, genera, orders,
and classes; it is the selection and combination of the variational
tendencies by the conditions of life, which occurs at every step.
There was no variational tendency leading from terrestrial mammals to
Cetaceans, but there was a variational tendency moving the nostrils
upwards towards the forehead, the hind-limbs towards diminution,
the lungs towards lengthening, the tail towards broadening out. But
each of these variational tendencies was always only one of several
possibilities, and that the particular path which led towards the
'goal' was followed was due to the fact that the others plunged into
the abyss to which the wrong paths led, that is to say, they were
weeded out by selection. Thus germinal selection offers a possibility
of reconciliation between Nägeli's and Darwin's interpretations, which
seem so directly contradictory; for the former referred everything to
the hypothesis of an internal evolutionary force, the latter rejected
this, and regarded natural selection as the main, if not the exclusive
factor in evolution. The internal struggles for food, which we have
assumed as occurring in the germ-plasm, represent an internal force,
though not in the sense of Nägeli, who thought of determining influence
operative from first to last, but still an impelling force, which
determines the direction of variation for the individual determinants,
and must therefore do the same for the whole evolution up to a certain
point; for it is only the _possible_ variations of the determinants in
a germ-plasm which can be chosen, selected, combined, and increased by
natural selection, and every germ-plasm cannot give rise to all sorts
of variations; the determinants contained in it condition what is
possible and what is not, and this is an important limitation to the
efficacy of natural selection, and to a certain extent also implies a
guiding and determining power on the part of the internal mainspring,
to wit, _germinal selection_.
The essential difference between Darwin's view of the transformation
of forms and my own lies in the fact that Darwin conceived of natural
selection as working only with variations which are not only due to
chance themselves, but the intensification of which also depends in
its turn solely upon natural selection, while, according to my view,
natural selection works with variational tendencies which become
intensified through internal causes, and are simply accumulated by
natural selection in an ever-growing majority of ids in a germ-plasm
through the selection of individuals.
This affects our view of the establishment of a specific type in so
far that my intra-germinal variational tendencies are not necessarily,
and not always due to chance, though they are so in most cases. If
certain determinants are impelled to vary in a particular direction
through climatic or any other influences, as we have seen to be the
case, for instance, with the climatic varieties of many Lepidoptera,
then the corresponding determinants in all the individuals must vary in
the same direction, and thus all the individuals of the species which
are subject to the same influence must undergo the same variation.
Transformations of this kind have exactly the appearance of resulting
from 'an internal evolutionary force,' such as Nägeli assumed, and the
unity of the specific type will not be disturbed by them.
Nor will this occur, as far as I can see, if the transformation of
a species depends solely upon new adaptations and their internal
consequences, for if a particular organism has to adapt itself to
special new conditions, it will usually be able to do so only in _one_
way, and thus natural selection will always allow _the same_ suitable
variational tendencies to survive and reproduce, so that the unity
of the specific type will not be permanently disturbed in this way
either. The more advantageous the new conditions of life prove, and the
more diverse the ways in which they can be utilized, the more rapidly
will the species first adapted to them multiply, and the more will
their descendants be impelled to adapt themselves specially to the
_different_ possibilities of utilizing the new situation, and thus,
from a parent species adapted in general to the new conditions, there
arise forms adapted to its more detailed possibilities. I must refer
again to the previous instance of the Cetaceans which originated from
vegetarian littoral, or fluviatile mammals, and have evolved since the
Triassic period into a very considerable number of species-groups.
All are alike in their _general_ adaptation, and these adaptations to
the conditions of life of aquatic animals--the fish-like form, the
flippers, the peculiarities of the respiratory organs and the organs of
hearing--when once acquired would not and could not be lost again; but
each of the modern groups of whales has its particular sphere of life,
which it effectively exploits by means of subordinate adaptations.
Thus there are the dolphins with their bill-like jaws and the two
rows of conical teeth, their active temperament, rapid movements, and
diet of fish; and the whalebone whales with their enormous gape, the
sieve-apparatus of whalebone-plates, and a diet of small molluscs and
the like. But each of these groups has split up into species, and if we
again regard the principle of adaptation as determining and directing
evolution, we are no nearer being able to prove the assumption in
regard to individual cases, for we know too little about the conditions
of life to be able to demonstrate that the peculiarities of structure
are actually adaptations to these. Theoretically, however, it is quite
easy to suppose that adaptation to a particular sphere of life was
the guiding factor in their evolution, and if this be so--as we have
already proved that it is in regard to the two chief groups and the
whole class--then the harmony of the structure must be due solely to
the continued selection of the fittest. We require no further principle
of explanation for the establishment of a specific type.
This 'type' is thus not reached by any indefinite varying of the parent
form in all directions, but in general it is reached by the most direct
and shortest way. The parent form must indeed have become to some
extent fluctuating, since not only the variational tendencies 'aiming
at the goal,' but others as well, must have emerged in the germ-plasm;
but gradually these others would occur less and less frequently, being
always weeded out afresh by selection, until the great majority of the
individuals would follow the same path of evolution, under the guidance
of germinal selection, which continues to work in the direction that
has once been taken. After a short period of variation, which need not,
of course, involve the whole organism, but may refer only to certain
parts of it, a steady direct progress in the direction of the 'goal,'
that is, of perfect adaptation, will set in, as we have seen in the
case of the _Planorbis_ snails of Steinheim.
We must not forget, however, that natural selection works essentially
upon a basis of sexual reproduction, which with its reduction of the
ids and its continually repeated mingling of germ-plasms, combines the
existing variational tendencies, and thus diffuses them more and more
uniformly among the individuals of a whole area of occupation. Sexual
reproduction, continual intermingling of the individuals selected
for breeding, is thus a very effective and important, if not an
indispensable, factor in the evolution of the specific type.
But it is not only in the case of species transformations due to
new adaptations that sexual mingling operates; it does so also in
the case of variations due to purely intra-germinal causes. We have
already seen in discussing Isolation that isolated colonies may come
to have a peculiar character somewhat different from that of the
parent form, because they were dominated by some germinal variational
tendency which occurred only rarely in the home of the parent form, and
therefore never found expression there. On the isolated area this would
indeed be mingled with the rest of the existing germinal variational
tendencies, but the result of this mingling would be different, and the
further development of the tendency in question would probably not be
suppressed.
We need not wonder, therefore, that specific types occur in such
varying degrees of definiteness. If a species is distributed over
a wide connected territory, sporadically, not uniformly, it will
depend partly upon the mutual degree of isolation of the sporadic
areas whether the individual colonies will exhibit the same specific
type or will diverge from one another. If the animal in question is
a slow-moving one like a snail, the intermingling from neighbouring
sporadic colonies will be much slower than in the case of a resident
bird such as a woodpecker. Many interesting results would undoubtedly
be gained if the numerous careful investigations into the geographical
distributions of species and their local races were studied with
special reference to this point, and much light would undoubtedly
be thrown upon the evolution of the specific type. But it would be
absolutely necessary to study carefully all the biological relations
of the animals concerned, to trace back the history of the species as
far as possible, and to decide the period of immigration, the mode and
direction of distribution, and so on.
Nothing shows more plainly the enormous duration of the period of
constancy in species than the wide distribution of the same specific
type on scattered areas or even over different areas absolutely
isolated from one another. If, as we saw, the same diurnal butterflies
live in the Alps and the far North, they must have remained unvaried
since the Glacial period, for it was the close of that period that
brought them to their present habitats, and while other diurnal
butterflies now living on the Alps differ from their relatives in
the Arctic zone (Lapland, Siberia, and Labrador) in some unimportant
spot or line, and must therefore have diverged from one another in
the course of the long period since the Glacial epoch, they have
done so only to a minimal degree, and in characters which possibly
depend solely upon germinal selection and can hardly be regarded as
adaptations.
I should like, however, to cite one of the few cases known to me in
which a slight deviation from the specific type undoubtedly depending
upon adaptation has occurred on an isolated region. The nut-jay
(_Caryocatactes nucifraga_) lives not only upon our Alps and in the
Black Forest, but also in the forests of Siberia, and the birds there
differ from those with us in small peculiarities of the bill, which
is longer and thinner in them, shorter and more powerful in ours.
Ornithologists associate this difference with the fact that in this
country the birds feed chiefly on hard hazel nuts, which they break
open with their bill, and on acorns, beechmast, and, in the Alps,
on cembra-cones, while in Siberia, where there are no hazel nuts,
they feed chiefly upon the seeds of the Siberian cedar, which are
concealed deep down in the cones. Thus we find that in Siberia the
bill is slender, and that the upper jaw protrudes awl-like beyond the
lower, for about 2.5 mm., and probably serves chiefly to pick out the
cedar nuts from behind the cone-scales. In the Alps the birds (var.
_pachyrhynchus_) break up the whole cone of the cembra-pine with their
thick, hard bill, and in the Upper Engadine, where the nut-jay is
abundant, I have often seen the ground underneath the cembra-pines
covered with the débris of its meal. In addition to these differences
between the two races, the Alpine form is stronger in build, the
Siberian form is daintier; in the former the white terminal band on
the tail is narrow (about 18 mm.), in the Siberian form it is broader
(about 27 mm.).
Such cases of variation of individual parts in different areas seem to
me very important theoretically, because they furnish us with an answer
to the view which represents the species as a 'life-crystal,' which
must be as it is or not at all, and which therefore cannot vary as
regards its individual parts. The case of the nut-jay has the further
interest that it is one of the few in which we find the new adaptation
of a single character without variation of most of the other characters.
It is only in an essentially different sense that we can compare the
species, like any other vital unit, to a crystal, in so far as its
parts are harmoniously related one to another, or, as I expressed
myself years ago, are in a state of equilibrium, which must be brought
about by means of intra-selection. This analogy, however, only applies
to the actual adjustment of the parts to a whole, and not to their
casual adjustment. Species are variable crystals; the constancy of a
species in all its parts must be regarded as something quite relative,
which may vary at any time, and which is sure to vary at some time
in the course of a long period. But the longer the adaptation of
a species to new conditions persists the more constant, _ceteris
paribus_, and the more slowly variable will it become, and this for
two reasons: first, because the determinants which are varying in a
suitable direction are being more and more strictly selected, more
and more precisely adapted, and are thus becoming more like each
other; and secondly, because, according to our theory, the homologous
determinants of all the ids do not vary in the required direction, and
a portion of the unvaried ancestral ids is always carried on through
the course of the phylogeny, and only gradually set aside by the
chances of reducing division. But the more completely these unvaried
ids are eliminated from the germ-plasm the less likely will they be
to find expression in reversions or in impurities of the new specific
characters. I may recall the reversions of the various breeds of
pigeons to the rock-dove, those of the white species of _Datura_ to the
blue form, and the _Hipparion_-like three-toed horse of Julius Cæsar,
and so on. The unvaried ancestral ids, which in these cases find only
quite exceptional expression, will make the new 'specific character'
fluctuating, as long as they are contained in the germ-plasm in
considerable numbers, but they must become more and more infrequent in
the germ-plasm as successive generations are passed through the sieve
of natural selection, and the oftener these germ-plasms, to which the
chances of reducing division and amphimixis have assigned a majority
of the old determinants, are expelled from the ranks of the species by
personal selection. The oftener this has occurred in a species the less
frequently will it recur, and the more constant, _ceteris paribus_,
will the 'type' of the species become.
If we add to this idea the fact that adaptations take place very
slowly, and that every variation of the germ-plasm in an appropriate
direction has time to spread over countless hosts of individuals, we
gain some idea of the way in which new adaptations gradually bring
about the evolution of a more and more sharply-defined specific type.
So far, however, we have only explained the morphological aspect of the
problem of the nature of species, but there is also a physiological
side, and for a long time this played an important part in the
definition of the conception of species. Until the time of Darwin it
was regarded as certain that species do not intermingle in the natural
state, and that, though they could be crossed in rare cases, the
progeny would be infertile.
Although we now know that these statements are only relatively correct,
and that in particular there are many higher plants which yield
perfectly fertile hybrids, it is nevertheless a striking phenomenon
that among the higher animals, mammals, and birds the old law holds
good, and hybrids between two species are very rarely fertile. The two
products of crossing between the horse and the ass, the mule and the
hinny, are never fertile _inter se_, and very rarely with a member of
the parent stock.
We have to ask, therefore, what is the reason of this mutual sterility
of species; whether it is a necessary outcome of the morphological
differences between the species, or only a chance accessory phenomenon,
or perhaps an absolutely necessary preliminary condition to the
establishment of species.
The last was the view held by Romanes. He believed that a species could
only divide into two when it was separated into isolated groups either
geographically or physiologically, that is, when sexual segregation
in some form is established within the species, so that all the
individuals can no longer pair with one another, but groups arise which
are mutually sterile. It is only subsequently, he maintained, that
these groups come to differ from one another in structure. To this
hypothetical process he gave the name of 'physiological selection.'
This view depends--it seems to me--upon an underestimate of the power
of natural selection. Romanes believed that when a species began to
split up, even the adaptive variations would always disappear again
because of the continual crossing, and that only geographical isolation
or sexual alienation, that is, physiological selection, would be
able to prevent this. But even the fact that there are dimorphic and
polymorphic species proves sufficiently that adaptation to two or even
several sets of conditions can go on on the same area. In many ants we
find many kinds of individuals--the two sexual forms, the workers, and
the soldiers, and these last are undoubtedly distinguished by adaptive
characters which must be referred to selection. The same is true of
the caterpillars, whose coloration is adapted to their surroundings
in two different ways. If the individuals of one and the same species
can be broken up into two or more different forms and combinations
of adaptations, while they are mingling uninterruptedly with one
another, natural selection must undoubtedly be able, notwithstanding
the continual intermingling of divergent types, to discriminate between
them and to separate them sharply from one another. Assuredly then a
species can not only exhibit uniform variation on a single area, but
may also split up into two without the aid of physiological selection.
Theoretically it is indisputable that of two varieties which are
both equally well suited to the struggle for existence, a mixed form
arising through crossing may not be able to survive. Let us recall, for
instance, the caterpillars, of which some individuals are green and
some brown, and let us assume that the brown colour is as effective
a protection as the green, then the two forms would occur with equal
frequency; but though a mixed hybrid form which was adapted neither to
the green leaves nor to the brown might occasionally crop up, it would
always be eliminated. It would occur because the butterflies themselves
are alike, whether they owe their origin to green caterpillars or to
brown, and thus at first, at least, all sexual combinations would be
equally probable.