sense, as an indispensable preliminary condition to the splitting of
species, but it is a different question whether the mutual sterility so
frequently observable between species has not conversely been produced
by natural selection in order to facilitate the separation of incipient
species. For there can be no doubt that the process of separating two
new forms, or even of separating one new form from an old one, would
be rendered materially easier if sexual antipathy or diminished
fertility of the crossings could be established simultaneously with the
other variations. This would be useful, since pure and well-defined
variations would be better adapted to their life-conditions than
hybrids, and would become increasingly so in the course of generations.
But as soon as it is useful it must actually come about, if that is
possible at all. It may be, however, as we have said before, that the
two divergent forms depend merely upon quantitative variations of the
already existing characters; sexual attraction, whether it depends
upon very delicate chemical substances, or on odours, or on mutual
complementary tensions unknown to us, will always fluctuate upwards and
downwards, and plus or minus determinants, which lie at the root of
these unknown characters in the germ-plasm, must continually present
themselves and form the starting-point for selection-processes of a
germinal and personal kind, which may bring about sexual antipathy and
mutual sterility between the varieties. I therefore consider Romanes's
idea correct in so far that separation between species is in many cases
accompanied by increasing sexual antipathy and mutual sterility. While
Romanes supposed that 'natural selection could in no case have been
the cause' of the sterility, I believe, on the contrary, that it could
only have been produced by natural selection; it arises simply, as
all adaptations do, through personal selection on a basis of germinal
selection, and it is not a preliminary condition of the separation
of species, but an adaptation for the purpose of making as pure and
clean a separation as possible. It is obviously an advantage for both
the divergent tendencies of variation that they should intermingle
as little as possible. This is corroborated by the fact that by no
means all the marked divergences of species are accompanied by sexual
alienation, and that the mutual sterility so frequently seen is not an
inevitable accompaniment of differences in the rest of the organism.
That this is not the case is very clearly proved by our domesticated
animals. The differences in structure between the various breeds of
pigeon and poultry are very great, and breeds of dog also diverge
from one another very markedly, especially in shape and size of body.
Yet all these are fertile with one another, and they yield fertile
offspring. But they are products of artificial selection by man, and
he has no interest in making them mutually sterile, so that they have
not been selected with a view to sexual alienation, but in reference
to the other characters. The segregation of animal species into
several sub-species on the same area is probably usually accompanied
by sexual antipathy, since in this case it would be useful although
not indispensable. But the matter is different in the case of the
transformation of a colony upon a geographically isolated region.
'Amiktic' forms, such as _Vanessa ichnusa_ of Corsica, are hardly
likely to be sexually alienated from the parent form; we have here
to do only with the preponderance of a fortuitous and biologically
valueless variation and its consequent elevation to the rank of a
variety. The new form was not an adaptation, but only a variation, and
as it was of no use, it was not in a position to incite any process of
selection favouring its advancement.
But even adaptive transformations on isolated regions from which the
parent species is excluded are not likely to develop rapidly any sexual
antipathy as regards the parent stock, and I should not be surprised if
experiments showed that there is perfect mutual fertility between, for
instance, many of the species of _Achatinella_ on the Sandwich Islands
or of _Nanina_ on Celebes, or between the species of thrushes on the
different islands of the Galapagos Archipelago, or between these and
the ancestral species on the adjacent continent, if that species is
still in existence. For there was no reason why sexual antipathy to
the parent form should have developed in any of these adaptation forms
which have arisen in isolation, and therefore it has probably not been
evolved.
That our view of the mutual sterility between species, as an adaptation
to the utility of precise species-limitation, is the correct one is
evidenced not only by our domesticated races, but even more clearly by
plants, in regard to which it is particularly plain that the sexual
relations between two species are adaptational. We have already seen
in what a striking way the sensitiveness of the stigma of a flower is
regulated in reference to pollen from the same plant, that some species
are not fertilizable by their own pollen at all, that others yield very
little seed when self-fertilization is effected, and that others again
are quite fertile--as much so as with the pollen of another plant of
the same species. We regarded these gradations of sexual sensitiveness
as adaptations to the perfectly or only moderately well-assured visits
of insects, or to their entire absence. I wish to cite these cases as
well as the heterostylism of some flowers as evidence in support of
the conception of the mutual sterility of species which I have just
outlined. But this only in passing. The point to which I chiefly wish
to direct attention is the mutual fertility of many plant-species.
In lower as well as in higher plants fertile hybrids occur not
infrequently under natural conditions, and cultivated hybrids, such
as the new _Medicago media_, a form made by mingling two species of
clover, may go on reproducing with its own kind for a considerable
time. A number of Phanerogams yield fertile hybrids, and in Orchids
even species of different genera have been crossed and have yielded
offspring which was in some cases successfully crossed with a third
genus.
If these facts prove anything it is that the factors which determine
the mutual sterility of species are quite distinct from their
morphological differences, in other words, from the diagnostic
characters of the specific type. For a long time the verdict on this
matter was too entirely based on observations made on animals, among
which mutual sterility arises relatively easily, even where it was not
intended (_sit venia verbo!_). Even the pairing, but still more the
period of maturity, the relations of maturity in ovum and sperm, and
even the most minute details in the structure of the sperm-cell, the
egg-shell, envelope, &c., have to be taken into account, and these may
bring about mutual or, as Born has shown, one-sided sterility. We know,
through the researches of Strasburger, that a great many Phanerogams,
when pollinated artificially from widely separated species of different
genera and families, will at least allow the pollen-tube to penetrate
down to the ovule, and that in many cases amphimixis actually results.
It follows that we must not lay too great stress upon the mutual
sterility which occurs almost without exception among the higher
animals, but must turn to the plants with greater confidence.
Among plants there is very widely distributed mutual fertility between
species. I doubt, however, whether the observations on this point
are sufficient to warrant any certain conclusion in regard to the
importance of the phenomena in the formation of species. At least it
is not easy to see why the mutual sterility of many species of plants
should not have been necessary or useful in separating species, and why
it was not therefore evolved. We may point to the fact that animals
can move from place to place as the chief reason, and this factor
does undoubtedly play a part, but the widespread crossing of plants
by insects makes up to some extent, as far as sexual intermingling
is concerned, for their inability to move from place to place. I do
not know whether the species of orchid which are fertile with one
another belong to different countries, so that we may assume that they
originated in isolation, or whether fertile orchids from the same area
are fertilized by different insects and are thus sexually isolated.
This and many other things must be taken into consideration. Probably
these relations have not yet been adequately investigated; probably
what is known by some experts has not yet been made available to
all. Future investigations and studies must throw more light upon the
problem.
In any case, however, we can see from the frequency of mutual fertility
among plants that mutual sterility is not a _conditio sine qua non_
of the splitting up of species, and we must beware of laying too
great stress upon it even among animals. Germinal selection is a
process which not only forms the basis of all personal selection, but
which is also able to give rise of itself, without the usual aid of
sexual intermingling, to a new specific type. And we cannot with any
confidence dispute that, even without amphigony, a certain degree of
personal selection may not ensue solely on the basis of the favourable
variational departures originating in the germ-plasm. It would be
premature to express any definite views on this point as yet, but the
diverse cases of purely asexual or parthenogenetic reproduction in
groups of plants rich in species make this hypothesis seem probable.
The most remarkable example of this is probably to be found in the
Lichens, the symbiotic nature of which we have already discussed,
in which--now at least--neither the Fungus nor the Alga associated
with it is known to exhibit sexual reproduction. If this is really
the case, then the existence of numerous and well-marked species of
lichens leads us to the hypothesis just expressed, and we must suppose
that the unity of the specific type is attained in this case solely
by a continual sifting of the useful from the useless variations of
the determinants, and through purely germinal intensification of the
surviving variational tendencies.
Of course it is possible that the mutual adaptation of the Algæ and
Fungi in the evolution of species of Lichens took place very long ago,
at a time when sexual reproduction still existed, at least in one of
the associated organisms, the Fungus. The Ascomycetes, to which most
of the Lichens belong, do not at present usually exhibit the process
of amphimixis, as I have already noted; but it may perhaps be still
possible to decide whether they must have exhibited it, or at least
could have exhibited it at an earlier stage in their evolution. As the
group of Thallophytes is a very ancient one, it is not inconceivable
that the modern species of Lichens have existed for a long time, and
that they had their origin in the remote past with the assistance of
amphimixis.
Nor need it be objected to this supposition that it has been found
possible to _make new Lichens_ by bringing together Fungi and Algæ
which had not previously been associated with one another; for in
the first place both were already adapted to partnership with other
species, and, moreover, so far no one has succeeded in rearing these
artificial lichens for any length of time, still less in seeing them
evolve into specific forms persistent in natural conditions.
But if this supposition should prove to be not only improbable, but
actually erroneous, then the existence of Lichens would afford a clear
proof that the 'type' of the species does not depend essentially upon
the constant intermingling of individuals, but upon a process which
we may best designate _uniformity of adaptation_. We have simply to
suppose that under similar external influences similar variational
tendencies were started by germinal selection in all the individuals
of the two parent species of a lichen, and set a-going by germinal
selection, just as a warmer climate gives rise to a black variety
in the butterfly _Polyommatus phlæas_, because similar determinants
of the germ-plasm of all the individuals were impelled to vary in
the same manner and direction. This would then give rise to quite
definite variations, and since only the suitable variational tendencies
could survive, primitive though never complicated adaptations would
arise. But we cannot assume that the lichens are not adapted to the
conditions of their life as well as all other organisms. We cannot
judge how far even their shape is to be regarded as an adaptation,
whether the formation of encrusting growths, of tree-like forms, of
cup or bush-lichens, may not be regarded as adaptations towards a full
utilization of the conditions of their life--but even if this is not
the case, the formation of soredia remains an undoubted adaptation to
the symbiosis of those lichens which exhibit them. The soredia cannot
depend upon the direct effect of the conditions of life, for they are
reproductive bodies which did not exist before the existence of the
lichen, and only originated to facilitate their distribution.
Thus there is still a great deal that is doubtful in our theories as to
the transformations of organisms, and much remains still to be done.
But even though we may doubt whether adaptations could come about in
multicellular organisms without amphigony, we may be quite certain of
the converse, that is, that the specific type can be changed in every
individual feature by natural selection on the basis of amphigony,
even as regards invisible features which only express themselves in
altered periods of growth. Even when there is no isolation whatever
and no mutual sterility, and when a mobile species is uniformly
distributed over a large area, a splitting up into races in regard to
one particular character may occur, _simply through adaptation_ to the
spatially different climatic conditions of the area inhabited.
Early in these lectures we discussed the twofold protective value of
the coloration of the 'variable hare' (_Lepus variabilis_), which is
distributed over the Arctic zone of the Old and New World, and also
occurs in the higher regions of the Alps. Wherever there is a sharp
contrast between winter and summer the variable hare exhibits the
same specific type, being brown in summer and white in winter, but in
regard to this very character of colour-change it forms races to some
extent, for it is white for a longer or shorter time according to the
length of the winter--in Greenland for the whole twelve months of the
year, in Northern Norway only for eight or nine months, in the Alps
for six or seven months, but in the south of Sweden and in Ireland
not at all. There it remains brown in winter like our common hare
(_Lepus timidus_). This is not a question of the direct effect of cold;
if it were the species would become white in Southern Sweden also,
for there is no lack of severe cold there, but the ground is not so
uninterruptedly covered with snow, and so the white colour of the hare
would be as often, probably oftener, a danger than a safeguard, and
the more primitive double coloration has therefore been done away with
by natural selection. The change of colour is thus hereditarily fixed,
as is proved by the fact that the Alpine hare, if caught and kept in
the valleys below, puts on a white dress at the usual time, which the
common hare never does.
As in Southern Sweden the winter coloration has been wholly eliminated,
so, conversely, from there to the Arctic zone the summer colouring
has been more and more crowded out, and in the Farthest North it has
totally disappeared from the characters of the species. We thus see
that wherever the species lives the double colouring is regulated,
as regards the duration of the winter coat, in exact harmony with
the external conditions. There is a pure white, a pure brown, and
a colour-changing race, and the latter is subdivided into two--one
wearing the winter dress for six, the other for eight months. Probably
these could be still further subdivided, if the different regions of
the Scandinavian Peninsula were investigated individually from south
to north. That the duration of the winter dress has its roots in
the germ-plasm, and does not depend solely on the earlier or later
period at which the cold sets in, is made clear by the two extreme
forms, the white and the brown _Lepus variabilis_, as well as by the
behaviour of captive animals. The familiar case of Ross's lemming,
which remained brown in the warm cabin, and then suddenly became
white when it was exposed to the cold of winter, only shows that the
cold acts as a liberating stimulus. The preparatory changes in the
pellage are already present, and the stimulus of cold brings them
rapidly to a climax. Here, therefore, the necessary variations of the
relevant germinal parts must have continually presented themselves for
selection, which is intelligible enough, since it is merely a question
of plus- or minus-variations. The fact that the six-months' dress can
be transformed into an eight-months' dress must have its cause in some
minute biological units of the germ-plasm; the determinants of the fur
must be able to vary in such a way that a longer or shorter duration of
the winter's coat is the result. The possibility of the whole variation
depends upon the continual fluctuations of all determinants, now
towards plus, now towards minus, and the necessity and inevitableness
of each adaptation to the duration of the winter lies in the unceasing
personal selection--the inexorable preferring of the better adapted.
LECTURE XXXV
THE ORIGIN AND THE EXTINCTION OF SPECIES
Adaptation does not depend upon chance--The case of eyes--Of
leaf-mimicry--All persistent change depends ultimately on
selection--Mutual sterility without great significance--Relative
isolation (_Lepus variabilis_)--Influence of hybridization--Decadence
of species--Differences in the duration of decadence--Natural
death of individuals--Extinction due to excessive variability
(Emery)?--_Machairodus_ as interpreted by Brandes--Lower types more
capable of adaptation than higher--Flightless birds--Disturbance of
insular fauna and flora by cultivation--The big game of Central Europe.
In the polar hare we have a case in which the adaptations to the life
conditions both of time and space are recognizable as the effect of
definite causes, and thus as a necessity; but the same must be true
everywhere even in regard to the most complex adaptations which seem
to depend entirely upon chance; everywhere adaptation results of
necessity--if it is possible at all with the given organization of the
species--as certainly as the adaptation dress of the hare depends on
the length of the winter, and in point of fact not less certainly than
the blue colour of starch on the addition of iodine. The most delicate
adaptations of the vertebrate eye to the task set for it by life in
various groups have been gradually brought about as the necessary
results of definite causes, just in the same way as the complex
protective markings and colouring on the wing of the _Kallima_ and
other leaf-mimicking butterflies.
That adaptations can be regarded as mechanically necessitated is due
to the fact that in every process of adaptation the same direction of
variation on the part of the determinants concerned is guaranteed,
since personal selection eliminates those which vary in a wrong
direction, so that only those varying in a suitable direction survive,
and they then continue to vary in the same direction. But the greatest
difference between our conception of natural selection and that of
Darwin lies in this: that Darwin regarded its intervention as dependent
upon chance, while we consider it as necessary and conditioned by the
upward and downward intra-germinal fluctuation of the determinants.
Appropriate variational tendencies not only _may_ present themselves,
they _must_ do so, if the germ-plasm contains determinants at all
by whose fluctuations in a plus or minus direction the appropriate
variation is attainable.
That a horse should grow wings is beyond the limits of the
possibilities of equine variation--there are no determinants which
could present variations directed towards this goal; but that any
multicellular animal which lives in the light should develop eyes lies
within the variational possibilities of its ectoderm determinants, and
in point of fact almost all such animals do possess eyes, and eyes,
too, whose functional capacity may be increased in any direction, and
which are adaptable and modifiable in any manner in accordance with
the requirements of the case. As soon as the determinants of the most
primitive eye came into existence, they formed the fundamental material
by whose plus- or minus-variations all the marvellous eye structures
might be brought about, which we find in the different groups of the
Metazoa, from a mere spot sensitive to light to a shadowy perception
of a moving body, and from that again to the distinct recognition
of a clear image, which we are aware of in our own eyes. And what
wonderful special adaptations of the eye to near and to distant vision,
to vision in the dusk and at night, or in the great ocean-depths, to
recognition of mere movement or the focussing of a clear image, have
been interpolated in the course of this evolution!
All such adaptations are possible, because they can proceed from
variations of determinants which are in existence; and in the same
way it is possible, at every stage of the evolution of organisms, for
eyes to degenerate again, whether they have been high up or low down
in the scale of gradations of this perhaps the most delicate of all
our sense-organs. As soon as a species migrated permanently from the
light into perfect darkness its eyes began to degenerate. We know
blind flat worms, blind water-fleas and Isopods, also blind insects
and higher Crustaceans, and even blind fishes and amphibians, the eyes
of which are now to be found at very different levels of degeneration,
as Eigenmann has recently shown in regard to several species of
cave-dwelling salamanders of the State of Ohio. In all these cases
it is only necessary for the determinants of the eye to continue to
vary in the minus direction, and the disappearance of the eye must be
gradually brought about.
We must picture upward development in quite a similar way. The forest
butterflies of the Tropics could not possibly all have their under
surfaces coloured like a leaf if the protective pattern depended solely
upon the chance of a useful variation presenting itself. It always
presented itself through the fluctuations of the determinants, and
thus the appropriate colourings were not merely able to develop, but
of necessity did so in gradually increasing perfection. If chance
played any part in the matter, it would be quite unintelligible why the
protective colouring should occur only where it acts as a protection,
and why, for instance, it should not appear sometimes upon the upper
surface of the butterfly wing, or upon the posterior wings which are
covered when the butterfly is at rest. We have already studied in
detail the precision with which the coloration is localized on minute
points and corners of the wing: this can only be understood if natural
selection works with the certainty of a perfect mechanism. Chance only
comes into the matter in so far as it depends upon chance whether the
relevant determinants in one id or another are to vary in the direction
of plus or minus; but as the germ-plasm contains many ids, and chance
may decide it differently in each of these, the presence of a majority
of determinants varying in a desirable direction does not depend upon
chance, for if they are not contained in one individual they are in
another. It is only necessary that they should be present in some, and
that these should be selected for reproduction.
We must therefore regard natural selection, that is to say, _personal
selection_, as a mechanical process of development, which begins
with the same certainty and works 'in a straight line' towards its
'goal,' just as any principle of development might be supposed to do.
Fundamentally it is after all a purely internal force which gives
rise to evolution, the power of the most minute vital units to vary
under changing influences, and it is only the guidance of evolution
along particular paths that is essentially left to personal selection,
which brings together what is useful and thus determines the direction
of further evolution. If we bear in mind that even the minutest
variations of the biophors and determinants express nothing more or
less than reactions to changed external conditions in the direction of
adaptation, and that the same is true of each of the higher categories
of vital units, whether they be called cell, tissue, organ, person,
or corm, we see that the whole evolution of the forms of life upon
the earth depends upon adaptations following each other in unbroken
succession, and fitting into each other in the most complex way. The
whole evolution is made possible by the power of variation of the
living units of every grade, and called forth and directed by the
ceaseless changes of the external influences. I said years ago that
_everything_ in organic evolution depended upon selection, for every
lasting change in a vital unit means adaptation to changed external
influences, and implies a preference in favour of the parts of the unit
concerned, which are thereby more fitly disposed.
In this sense we can also say that the species is a complex of
adaptations, for we have seen that it depends upon the co-operation
of different grades of selective processes, that in many cases it is
produced solely by germinal selection, but that in very many more
personal selection plays the chief part, whether in bringing about
sexual adaptations, or adaptations to the conditions of existence.