shall not depreciate the significance of this if we consider that
individual variations are the outcome of the fluctuations in the
equilibrium of the determinant system of the germ-plasm, to which it
is always more or less subject, and that variations of the germ-plasm,
whether towards plus or minus, bear within themselves the tendency
to go on increasing in the direction in which they have begun, and
to become definite variational tendencies. In isolated regions such
variational tendencies must continue undisturbed for a long period,
because they run less risk of being suppressed by mingling with
markedly divergent germ-plasms.
The probability that variational tendencies set up in some ids of the
germ-plasm by germinal selection will persist and increase is obviously
greater the more the germ-plasms combining in amphimixis resemble each
other. For instance, let us call the varying determinants _Dv_, and
assume as a favourable case that these are represented in three-fourths
of all the ids in the fertilized eggs of a butterfly-female which has
been driven astray on to an island, that is, that they are present
in twelve out of sixteen ids; then of 100 offspring of the first
generation it is possible that seventy-five or more will contain the
determinants _Dv_, some of them in a smaller number of ids, some in a
great number than the mother, according as the reducing division has
turned out. If the pairing of the second generation be favourable--and
this again is purely a matter of chance--a third generation must arise
which would contain the variants _Dv_ throughout, and thus the fixation
of this particular variation on this particular island would be begun.
In other words, the possibility would arise, that, if individuals with
a majority of _Dv_ ids predominated, they would gradually come to be
the only ones, since by continual crossing with the minority which
possessed only the determinants _D_, they would mingle the varied ids
with those of the descendants of these last, till ultimately germ-plasm
with only the old ids would no longer occur.
In following out this process it is not necessary to assume that the
first immigrant possessed the variation _visibly_; if determinants
varying in a particular direction occurred in the majority of its ids,
these would, as a consequence of persistent germinal selection, go on
varying gradually until the externally visible variation appeared. This
would not have appeared at all if the animal concerned had remained
in the original habitat of its species, for there it would have been
surrounded by normal germ-plasms, and its direct descendants, even if
they had been as favourably situated for the origin of variations as
we have assumed, would not have reproduced only among themselves, and
therefore even in the next generation the number of _Dv_ ids would have
diminished.
Obviously it is to a certain extent a matter of chance whether in the
isolated descendants the variation or the normal form remains the
victor, for it depends on the number of _Dv_ ids originally present in
the fertilized eggs, then on the chances of reducing divisions, and
finally on the chance which brings together for pairing individuals in
which the similarly varied ids preponderate. The probability of the
conquest of the variation will depend in the main on the strength of
the majority of the varied ids in the fertilized eggs of the parents;
if this be an overwhelming majority, then the chances of favourable
reducing divisions and pairings will also be great. The origin of a
pure amixia variety will thus depend upon the fact that _the same_
variational tendency _Dv_ was present in a large number of the ids of
the ancestral germ-plasm. We need not wonder therefore that of the
numerous diurnal butterflies of Corsica and Sardinia only eight have
developed into endemic, probably 'amiktic,' varieties.
But since we know that so many species in oceanic islands and other
isolated regions are endemic or autochthonous, i.e. of local origin,
there must obviously be some other factor in their evolution in
addition to the mere prevention of crossing with unvaried individuals
of the same species. The variational tendencies which have arisen in
the germ-plasm through germinal selection may--as we have already
seen--gain the ascendancy in various ways; first, by being favoured
by the climatic influences, then by being taken under the protection
of personal selection, whether in the form of natural or of sexual
selection.
As the inhabitants of insular areas are not infrequently subject to
special climatic conditions, we may assume at the outset that many of
the 'endemic' species are climatic varieties, but in many cases this
explanation is insufficient. For instance, special local forms of
mocking-bird live on several of the Galapagos Islands, but this cannot
depend upon differences of climate, for the islands are only a few
kilometres apart, and resemble one another as regards the conditions
of life which they present. But as the differences between these
local forms show themselves especially in the male sex, as colour
variations of certain parts of the plumage, we must take account of
sexual selection, which, though with its basis in germinal selection,
has in many islands followed a path of its own. Sexual selection
operates especially in the case of sporadically occurring characters
which are in any way conspicuous. But it is just such variations as
these that are called into existence by germinal selection, whenever it
is allowed to continue its course undisturbed through a long series of
generations. Characters of this kind, such, for instance, as feathers
of abnormal structure or colour in a bird, or new colour spots in a
butterfly, make their appearance when a group of determinants has been
able to go on varying in the same direction for a long time unimpeded,
that is, without being eliminated as injurious by natural selection or
obliterated by crossing. This is very likely to happen in the case of
an isolated area, and as soon as the conspicuous character thus brought
about makes its appearance, sexual selection takes control of it, and
ensures that all the individuals, that is, all the germ-plasms which
possess it, have the preference in reproduction.
I believe, therefore, that a large number of the endemic species of
birds and butterflies in isolated regions result from amixia based
upon germinal selection, whose results have been emphasized by sexual
selection. Experience corroborates this, as far as I can see, for many
of the endemic species of birds in the Galapagos and other islands
differ from one another solely or mainly in their colouring, and in
many it is especially the males which differ greatly.
As to the humming-birds we may say, without going into details
regarding their sexual characters and their distribution, that the many
endemic species which inhabit the Alpine regions of isolated South
American volcanic mountains differ from one another chiefly in the
males and in the secondary sexual characters of these. The family of
humming-birds is characteristically Neotropical, that is, it has its
centre in the Tropics of the New World, and by far the greater number
of humming-bird species--there are about a hundred and fifty--occur
there only, while a few occur as migrants north of the Tropical zone,
and visit the United States as far north as Washington and New York.
We know that many of the most beautiful species have quite a small
area of distribution, that many are restricted to a single volcanic
mountain, living in the forests which clothe its sides. These species
are isolated there, for they do not migrate; apparently they cannot
endure the climate of the plains, but remain always in their mountain
forests. Without doubt they originated there, chiefly, I am inclined
to think, through the variation of the males due to sexual selection.
Any one who has seen Gould's magnificent collection of humming-birds
in the British Museum in London knows what a surprising diversity of
red, green, and blue metallic brilliance these birds display, what
contrasts are to be found in the diverse colour-schemes, and what
differences they exhibit in the length and form of the feathers of the
head, of the neck, of the breast, and especially of the tail. There
are wedge-shaped, evenly truncate, and deeply forked tails, some with
single long, barbless feathers, and so on. All these characters are
confined to the males, and are at most only hinted at in the female;
in no species does the female even remotely approach the male in
brilliance or decorativeness of plumage.
I do not believe that so many species with very divergent plumage in
the males could have developed if they had all lived together on a
large connected area. But here, distributed over a large number of
isolated mountain forests, the decorative colouring or the distinctive
shape which chances to arise through germinal selection on any of these
terrestrial islands can go on increasing, undisturbed by crossing with
individuals of the ancestral species, and furthered, moreover, by
sexual selection.
In this way, if I mistake not, numerous new species have arisen as a
result of isolation, and it is quite intelligible that several new
species may have arisen from one and the same ancestral species, as we
may see from the nearly related yet constantly different species of
mocking-bird on the different islands of the Galapagos group.
A number of similar examples might be given from among birds. Thus
Dixon calls attention to the species of the thrush genus _Catharus_,
twelve of which live in the mountain forests of Mexico and of South
America as far as Bolivia, all differing only slightly from one another
and all locally separated. They came from the plains, migrated to the
highlands, were isolated there, and then no longer varied together all
in the same direction, but each isolated group evolved in a different
direction according to the occurrence of chance germinal variations:
one developed a chestnut-brown head, another a slate-grey mantle, a
third a brown-red mantle, and so on. From what we have already seen in
regard to the importance of sexual selection in evolving the plumage of
birds, it is probable that this factor has been operative in this case
also.
Another example is afforded by the weaver-birds (_Ploceus_) of South
Africa, those ingenious singing-birds resembling blackbirds in size and
form, whose pouch-shaped nests, hanging freely from a branch, usually
over the water, and with their little openings on the under side, are
excellently protected from almost every form of persecution. These
birds have in South Africa split up into twenty or more species, but
the areas of each are not sharply isolated, and the division into
species cannot, therefore, be due to isolation. But it is not difficult
to guess upon what it depends, when we know that the males alone are
of a beautiful yellow and black colour, while the females are of a
greenish protective colouring all over.
Thus, in my opinion, sexual selection plays a part more or less
important in the origin of the numerous endemic species of diurnal
Lepidoptera which are characteristic especially of the islands of the
Malay Archipelago, and which make the Lepidopteran fauna there so rich
in individuality. A large number, indeed the majority of the types of
Papilionidæ, have a peculiar species, a local form, on most of the
larger islands, which is sharply and definitely distinguished from
those of the other islands, usually in both sexes, but most markedly in
the much more brilliantly coloured males.
Thus each of these types forms a group of species, each of which is
restricted to a particular locality, and has usually originated where
we now find it, although of course the diffusion of one of these
large strong-flying insects from one island to the other is in no way
excluded. As an example we may take the _Priamus_ group, the blackish
yellow _Helena_ group, the blue _Ulyssus_ group, and the predominantly
green _Peranthus_ group.
If we inquire into the causes of this divergence of forms and their
condensation into numerous species, we shall find that their roots lie
in this case, as in that of all transformations, in germinal selection
and the variational tendencies resulting therefrom, but we must
_regard their fixation us the result of isolation_, which prevented
the variational tendencies which happened to develop on any one island
from being neutralized and swamped by mingling with the variations of
other islands. But that sexual selection took control of these striking
colour-variations and increased them still further is obvious from
the rarely absent dimorphism of the sexes. Even if the females do not
consciously select mates from among the males, they will more readily
accept as a mate the one among several suitors which excites them most
strongly. And that will be the one which exhibits the most brilliant
colours or exhales the most agreeable perfume, for we know from their
behaviour in regard to flowers how sensitive butterflies are to both
these influences.
Although isolation has an important rôle in the formation of all
these species, it seems to me an exaggeration to maintain, as many
naturalists do, that the splitting up of a species is impossible
without isolation. Certainly the splitting up of species is, in
numerous cases, facilitated by isolation, and indeed could only have
been brought about in its present precision by that means, but it is
underestimating the power of natural selection not to credit it with
being able to adapt a species on one and the same area to _different_
conditions of life, and we shall return to this point later on in a
different connexion. But in the meantime it must suffice to point out
that the polymorphism of the social insects affords a proof that a
species may break up into several forms in the same area through the
operation of natural selection alone.
I am therefore of opinion, with Darwin and Wallace, that adaptation to
new conditions of life has, along with isolation, had a material share
in the evolution of the large number of endemic species of snail on the
oceanic islands. This brings us to the co-operation of natural selection
and isolation. If, thousands of years ago, by one of the rarest
chances, an _Achatina_-like snail was carried by birds to the Sandwich
Islands, it would spread slowly, at first unvaried, from the spot
where it arrived over the whole of the snailless island. But during
this process of diffusion it would frequently come in contact with
conditions of life which would not prevent it from penetrating further,
but to which it was imperfectly adapted, and in such places a process
of transformation would begin, which would consist in the fostering of
favourably varying individuals, and which would run its course quietly
by means of personal selection, based upon the never-ceasing germinal
selection, and unhindered by any occasional intrusion of still unvaried
members of the species from the original settlement on the island.
But these new conditions were not merely different from those of the
ancestral country; the island region itself presented very diverse
conditions, to which the snail immigrant had to adapt itself in the
course of time, as far as its constitution allowed. Terrestrial snails
are almost all limited to quite definite localities with quite definite
combinations of conditions; none of our indigenous species occurs
everywhere, but one species frequents the woods, another the fields;
one lives on the mountains, another in the valleys; one on gneiss
soil, another on limy soil, a third on rich humus, a fourth on poor
river-sand; one in clefts and hollows among damp moss, and another in
hot, dry banks of loess, and so on. Although we cannot see in the least
from the structure of the animal why this or that spot should be the
only suitable one for this or that species, we may say with certainty
that each species remains permanently in a particular place because
its body is most exactly adapted to the conditions of life there, and
therefore it remains victorious in the competition with other species
in that particular spot.
In this way the immigrants to the Sandwich Islands must have adapted
themselves in the course of time to their increasingly specialized
habitats, and in doing so have divided up into increasingly numerous
forms, varieties, and species, and indeed into several genera.
But this alone is not sufficient to explain the facts. According to
Gulick's valuable researches there live on one little island of the
Sandwich group no fewer than 200 species of Achatinellidæ, with 600-700
varieties! This remarkable splitting up of an immigrant species is
regarded by him as a result of the isolation of each individual species
and variety, and I do not doubt that this is correct as far as a
portion of these forms is concerned, and that isolation plays a certain
part in regard to them all. Gulick, who lived a long time upon the
island, attempts to prove that the habitats of all these nearly related
varieties and species are really isolated as far as terrestrial snails
are concerned; that intermingling of the snails of one valley with
those of a neighbouring one is excluded, and that the varieties of the
species diverge more markedly from one another in proportion as their
habitats are distant. On the other hand, species of different genera
of Achatinellidæ often live together on the same area; but they do not
intermingle.
Although Gulick's statements are worthy of all confidence, and though
his conclusions have great value as contributions to the theory of
evolution, I do not think that he has exhausted the problem of the
causes of this remarkable wealth of forms among the terrestrial snails
of oceanic islands. It is not that I doubt the relative and temporary
isolation of the snail-colonies at numerous localities in the island of
Oahu. But why have we not the same phenomenon in Germany, in England
or Ireland? Gulick anticipates this objection by pointing out the
peculiar habits of the Oahu snails. Many of the species there are
purely arboreal animals, living upon trees and never leaving them, even
during the breeding season, or in order to deposit eggs, for they bring
forth their young alive. Active migration from forest to forest seems
excluded by the fact that on the crests of the mountains there is a
less dense forest of different kinds of trees, and dry sunny air, which
could not be endured by the species of _Achatinella_ and _Bulimella_,
which love the moist shades of the tropical forests. Active migration
over the open grass-land at the mouths of the valleys is also excluded.
It must be admitted that the isolation of these forest snails in their
valleys is for the time being very complete, and that intermingling
of two colonies which live in neighbouring valleys _does not occur
by active migration, within the span of one or several human
generations_. It will also be admitted that our terrestrial snails in
Central Europe are much less isolated in their different areas, that,
for instance, they could get from one side of a mountain to the other
by active migration; but we must nevertheless repeat the question: how
does it happen that in Oahu every forest, every mountain-crest, and so
on, has its own variety or species, while our snails are distributed
over wide stretches of country, frequently without even developing
sharply defined local varieties? The large vineyard or edible snail
(_Helix pomatia_) occurs from England to Turkey, that is, over a
distance of about 3,000 kilometres, and within this region it is found
in many places which might quite as well be considered isolated as
adjacent forest valleys in Oahu. It occurs also on the islands of the
Channel and of the Irish Sea, and lives there without intermingling
with the members of the species on the mainland. But even on the
Continent itself it would be possible to name hundreds of places in
which they are just as well protected from intermingling with those of
other areas as they are in Oahu. There too the snails must _somehow_
have reached their present habitat some time or other, perhaps rather
in an indirect way, by means of other animals; but this is true also of
the snails of a continent, as we shall show more precisely later on.
In the meantime let us assume that this is so, and that the vineyard
snail (_Helix pomatia_), or some other widely distributed snail,
is relatively isolated. _Why then have not hundreds of well-marked
varieties evolved--a special one for each of the isolated areas?_
Obviously there must have been something in operation in the Sandwich
Islands which is absent from the continental habitats of _Helix
pomatia_, for this species shows fluctuations only in size, but is
otherwise the same everywhere, and the few local varieties of it which
occur are unimportant. I am inclined to believe that this 'something'
depends on two factors, and especially on the fact that the immigrant
snail enters upon a period of variability. This will be brought about
in the first place by the fact that the climate and other changes
in the conditions of life will call forth a gradually cumulative
disturbance in the equilibrium of the determinant system, and thus
a variability in various directions and in various combinations of
characters. To this must be added the operation of natural selection,
which attempts to adapt the immigrant to many new spheres of life, and
thus increases in diverse ways the variational tendencies afforded by
germinal selection. These two co-operating factors bring the species
into a state of flux or lability, just as a species becomes more
variable under domestication, likewise as a direct effect of change
of food and other conditions, such as the consciously or unconsciously
exercised processes of selection. It follows from this that, in the
gradual diffusion of snails all over the island, similar localities
would almost never be colonized by exactly similar immigrants, but
by individuals containing a different combination of the existing
variations, so that in the course of time different _constant forms_
would be evolved through amixia in relatively isolated localities.
But everything would be different in the diffusion of a new species
of snail in a region which was already fully or at least abundantly
occupied by snail-species. Let us leave out of account altogether
the first factor in variation, the changed climate, and we see that
a species in such circumstances would have no cause for variation,
because it would find no area unoccupied outside of the sphere to
which it was best adapted; it would therefore not be impelled to adapt
itself to any other, and in most cases could not do so, because in each
it would have to compete with another species superior to it because
already adapted.
The case would be the same if an island were suddenly peopled with
the whole snail-fauna of a neighbouring continent, with which a land
connexion had arisen. If the island had previously been free from
snails, all the species of the mainland would be able to exist there in
so far as they were able to find suitable conditions of life, but each
species would speedily take complete possession of the area peculiarly
suited to it, so that none of their fellow migrants would be impelled,
or would even find it possible, to adapt themselves to new conditions
and thus to become variable and split up into varieties. If Ireland
were at present free from snails, and if a land connexion between it
and England came about, then the snail-fauna of England would probably
migrate quite unvaried to Ireland, and in point of fact the snail-fauna
of the two islands, which were formerly connected, is almost the same.
For the same reason the fauna of England, as far as terrestrial snails
are concerned, is almost the same as that of Germany.