different families, in regard to which Poulton showed that in their
early youth they possess the power of adapting themselves exactly
to the colour of their chance surroundings. It is obvious that the
protection which the caterpillar would gain from being coloured
_approximately_ like its surroundings would be insufficient, for
instance because the surroundings may be very diverse, since
the species lives upon different, variously coloured plants and
plant-parts. Thus a facultative adaptation arose. Selection gave rise
to an extraordinarily specialized susceptibility on the part of the
different cell elements of the skin to differences of light, and the
result of this is that the skin of the caterpillar invariably takes on
the colouring which is reflected upon it in the first few days of its
life from the plants and plant-parts by which it is surrounded. Thus
the caterpillars of one of the Geometridæ, _Amphidasis betularia_, take
on the colours of the twig between and upon which they sit, and they
can be made black, brown, white, or light green quite independently of
their food, according to the colour of the twigs (or paper) among which
they are reared.
Colour-change in fishes, Amphibians, Reptiles, and Cephalopods, depends
upon much more complex adaptations. In their case a reflex-mechanism
is present which conducts the light-stimulus affecting the eye to the
brain, and there excites certain nerves of the skin; these in their
turn cause the movable cells of the skin which condition the colouring
to change and rearrange themselves in the manner necessary to bring
about the harmonization of colour. On this depends the colour-change of
the famous chamæleon, and also the scarcely less striking case of the
tree-frog, which is light green when it sits on trees, but dark brown
when it is kept in the dark. All these are secondary reactions of the
organism in which the external stimulus is, so to speak, made use of
to liberate adaptive variations, either permanently or transitorily.
In the caterpillars colour-changes are permanent, that is, it is only
the young caterpillar which takes on the colour of its surroundings;
later it does not change, even when it is exposed to different light,
or intentionally placed upon a food-plant of a different colour.
In fishes, frogs, and cuttlefishes, on the contrary, the reaction
of the colour-cells to light only lasts a little longer than the
light-stimulus, and it changes with it. The purposiveness of this
difference of reaction is obvious.
We cannot say to what degree the direct influence of external
conditions is effectively operative on the germ-plasm, or how far, by
persistently repeated slight changes, the determinants and the parts
of the body determined by them may be made to vary in the course of
generations; that is to say, how large a part this direct influence
of climate and food may play in the transmutation of species. We can
give no answer from experience, because there is an entire lack of
perfectly satisfactory and clear experiments; we only know in a few
cases how great the variations are which can be brought about in the
body during the individual life by means of any of these factors. In
most cases it is uncertain whether actually hereditary effects play any
part, that is, whether the germ-plasm itself is affected. But if we
wish to be theoretically clear as to how far direct climatic effects
may go, we may say this, that they may operate as long as they cause
no disturbance in the life of the species concerned, for at the moment
that such a direct effect begins to be prejudicial to the species
personal selection will step in, and, by preferring the individuals
which react least strongly to the climatic stimulus, will inhibit the
variation. If in any case this should be physically impossible, the
species would die out in the climate in question. That a species of
plant or animal has climatic limits indicates that individuals which go
beyond these are exposed to influences which make life impossible and
which natural selection is unable to neutralize. We are here brought
face to face with one of the limits to the scope of natural selection.
There is no doubt that the influences of the environment must always
have a powerful effect upon the soma of the individual, but we have
seen, in the case of Alpine plants and of galls, how very far this
effect may go without leaving any trace in the germ-plasm.
LECTURE XXXII
INFLUENCE OF ISOLATION ON THE FORMATION OF SPECIES
Introduction--Isolated regions are rich in endemic species--Is
isolation a condition in the origin of species?--Moriz Wagner,
Romanes--'Amiktic' local forms, the butterflies of Sardinia, of the
Alps, and of the Arctic zone--Periods of constancy and periods of
variation in species--Amixia furthered by germinal selection--The
thrushes of the Galapagos Islands--The intervention of sexual
selection--Humming-birds--Central American thrushes--Weaver-birds
of South Africa--Papilionidæ of the Malay Archipelago--Natural
selection and isolation--Snails of the Sandwich Islands--Influences
of variational periods--Comparison with the edible snail and with the
snail fauna of Ireland and England--Changed conditions do not always
give rise to variation--Summary.
In an earlier lecture I endeavoured to show, by means of Darwinian
arguments and examples, how important for every species, in relation to
its transmutation, is the companionship of the other species which live
with it in the same area. We saw that the 'conditions of life' operated
as a determining factor in the composition of an animal and plant
association quite as momentously as any climatic conditions whatsoever,
and, indeed, Darwin rated the influences of vital association even
more highly, and attributed to them an even greater power of evoking
adaptation than he granted to the physical conditions of life.
We are, therefore, prepared to recognize that even the transference of
a species to a different fauna or flora may cause it to vary, and this
occurs when the species gradually extends the area of its distribution,
so that it penetrates into regions which contain a materially different
association of forms of life. But these migrations are not necessarily
only gradual, that is, due to the slow extension of the original area
of distribution in the course of generations as the species increases
in numbers; they may also occur suddenly, when isolated individuals
or small companies of a species transcend in some unusual manner the
natural boundaries of the old area, and reach some distant new region
in which they are able to thrive.
Species-colonies of this kind may be due to the agency of man, who has
spread many of his domesticated animals and plants widely over the
earth, but who has also intentionally or unintentionally forced many
wild animals and plants from their original area to distant parts
of the earth, as, for instance, when the English humble-bees were
imported into New Zealand with a view to securing the fertilization
of the clover; but such colonies also occur in thousands of cases
independently of man's agency, and the means by which they are brought
about are very diverse. Little singing-birds are sometimes driven
astray by storms, and carried far away across the sea, to find, if
fortune favours them, a new home on some remote oceanic island;
fresh-water snails, which have just emerged from the egg, creep on to
the broad, webbed feet or among the plumage of a wild duck or some
other migratory bird, and are carried by it far over land and sea,
and finally deposited in a distant marsh or lake. This must happen
not infrequently, as is evidenced by the wide distribution of our
Central European fresh-water snails towards the north and south. But
terrestrial snails can also, though more rarely, be borne in passive
migration far beyond limits which are apparently impassable, as is
evidenced by the presence of land-snails on remote oceanic islands.
The Sandwich Islands are more than 4,000 kilometres from the continent
of America; they originated as volcanoes in the midst of the Pacific
Ocean; and yet they possess a rich fauna of terrestrial snails,
the beginnings of which can only have reached them by the chance
importation of individual snails carried by strayed land-birds.
Charles Darwin was the first who attempted to investigate the problem
of the colonization of oceanic islands by animal inhabitants, and the
chapter in _The Origin of Species_ which deals with the geographical
distribution of animals and plants still forms the basis of all the
investigations directed towards this point. We learn from these that
many land-animals, of which one would not expect it on _a priori_
grounds, may be carried away by chance over the ocean, either, as in
the case of butterflies and other flying insects, and of birds and
bats, by being driven out of their course by the wind, or by being
concealed--either as eggs or as fully-formed animals--in the clefts of
driftwood, where they can resist for a considerable time the usually
destructive influence of salt water. Thus eggs of some of the lowest
Crustacea (Daphnidæ), which are contained in large numbers in the mud
of fresh water, may be transported with some of the mud on the feet
of birds, and this may happen also to encysted infusorians and other
unicellulars, and to the much more highly organized Rotifers as well.
In all these cases, and in many others, it may happen occasionally that
single individuals, or a few at a time, may be carried far afield, and
may reach regions from which their fellows of the same species are
entirely excluded. If they thrive there they may establish colonies
which will gradually spread all over the isolated area as far as it
affords favourable conditions of life.
But oceanic islands are not the only cases of isolated regions;
mountains and mountain-ranges which rise in the midst of a plain also
form isolation-areas for mountain-dwelling plants or animals which
have not much power of migrating. In the same way marine animals
may be completely isolated from each other by land-barriers, as the
inhabitants of the Red Sea, for instance, are from those of the
Mediterranean, as has been clearly expounded by Darwin. The idea of an
isolated region is always a relative one, and the region which seems
absolutely insular for a terrestrial snail is not so at all for a
strong-flying sea-bird. There is no such thing as _absolute_ isolation
of any existing colony, for otherwise the colony could never have
reached the region; but the degree of isolation may be _absolute_ as
far as the time of our observation is concerned, if the transportation
of the species concerned occurs so rarely that we cannot observe it in
centuries, or perhaps in thousands of years, or if the extension of its
range could only take place through climatic or geological changes,
such as a subsidence of land-barriers between previously separated
portions of the sea, or, in the case of land animals such as snails,
the elevation of the sea-floor and the filling up of arms of the sea
which had separated two land-areas. But even the transportation of a
species by the accidental means already indicated will occur so rarely,
if the isolated insular region is very distant, that the isolation of
a colony by such a chance may be regarded as almost absolute as far as
the members of the same species in the original habitat are concerned.
If we examine one of these insular regions with reference to the
animal inhabitants which live upon it in isolation, we are confronted
with the surprising fact that it harbours numerous so-called endemic
species, that is to say, species which occur nowhere else upon the
earth, and that these species are the more numerous the further the
island is removed from the nearest area of related species. It looks
at first sight quite as if isolation alone were a direct cause of the
transformation of species.
The facts which seem to point in this direction are so numerous that I
can only select a few of them. The Sandwich Islands, to which we have
already referred, possess eighteen endemic land-birds, and no fewer
than 400 endemic terrestrial snails, all belonging to the family group
of Achatinellinæ, which occurs there alone.
The Galapagos Islands lie 1,000 kilometres distant from the coast
of South America, and they too harbour twenty-one endemic species
of land-birds, among them a duck, a buzzard, and about a dozen
different but nearly related mocking-birds, each of which is found
in only one or two of the fifteen islands. The group of islands also
possesses peculiar reptiles, and they take their name from the gigantic
land-tortoises, sometimes 400 kilogrammes in weight, which in Tertiary
times inhabited also the continent of South America, but are now found
in the Galapagos Islands only. The islands also possess endemic lizards
of the genus _Tropidurus_, and although the lizards can no more have
been transported across the ocean than the tortoises, but corroborate
the conclusion drawn from geological data, that the islands were still
connected with the mainland in Tertiary times, the occurrence of a
particular species of _Tropidurus_ upon almost every one of the fifteen
islands testifies anew to the mysterious influence of isolation, for
most of these islands are quite isolated regions for the different
species of lizard, even more than for the mocking-birds, which have
also split up into a series of species.
We are thus led to the hypothesis, which was first introduced into the
Evolution Theory by Darwin, that the prevention of constant crossing
of an isolated colony with the others of the same species from the
original habitat favours the origin of new endemic species, and his
conclusion is confirmed when we learn that islands like the Galapagos
group possess twenty-one endemic land-birds, but only two endemic
sea-birds out of eleven, for the latter traverse great stretches of
sea, and crossings with others of the same species on the neighbouring
continental coasts will often take place. The Bermuda Islands also
afford a proof that the development of endemic species is prevented by
regular crossing with other members of the species from the original
habitat, for although they are 1,200 kilometres distant from the
continent of North America--that is, further than the Galapagos Islands
from South America--they possess no endemic species of bird, and we may
undoubtedly associate this with the fact that the migratory birds from
the continent visit the Bermudas every year.
Madeira also confirms our conclusion, for only one of the ninety-nine
species of bird occurring there can be regarded as endemic, and it has
often been observed that birds from the neighbouring African mainland
(only 240 kilometres distant) are driven across to Madeira. Terrestrial
snails, on the other hand, will seldom be carried to Madeira by birds,
and accordingly we find there an extraordinary number of endemic
terrestrial snails, namely, 109 species.
Although these and similar facts indicate strongly that isolation
favours the evolution of new species, it would be erroneous to imagine
that every isolation of a species-colony conditions its transmutation
to a new species, or, as has been maintained, first by Moritz Wagner,
and later by Gulick and by Dixon, that isolation is a necessary
preliminary to the variation of species--that not selection but
isolation alone renders the transmutation of a species possible, and
thus admits of its segregation into several different groups of forms.
Romanes went so far as to regard the natural selection of Darwin and
Wallace as a sub-species of isolation, and isolation in its diverse
forms he regarded as the sole factor in the formation of species. He
assumed that it was only by the segregation of individuals which did
not vary that the constant reversion to the ancestral species could
be prevented, and he regarded the process of selection as essentially
resulting in the 'isolation' of the fittest through the elimination of
the less-fit. The idea is correct in so far, that selection undoubtedly
aids the favourable variation to conquest over the old forms, precisely
because the latter, being less favourably placed in the struggle for
existence, are gradually more completely overcome and weeded out, so
that a constant mingling of the new forms with the old is prevented,
just as it is by isolation of locality. Obviously the new and fitter
forms could not become dominant, could not even become permanent,
if they were always being mingled again with the old. But whether
it serves any useful purpose to bring this under the category of
'isolation,' and to say that mingling with the ancestral form during
transmutation is prevented by natural selection, in that favourably
varying individuals _are isolated_ by their superiority from the
inferior ones, that is, the non-varying individuals which are doomed
to elimination, is somewhat doubtful. For my part, I should prefer to
retain the original meaning of the word, and to call 'isolation' the
separation of a species-colony by spatial barriers.
Whether this factor by itself prevents the mingling with the ancestral
form as effectually as selection does, and whether isolation alone and
by itself can lead to the evolution of new forms, or perhaps must lead
to them, must now be investigated.
I look at this question from exactly the same point of view as I did
nearly thirty years ago, when in a short paper[27] I endeavoured to
show that, under favourable circumstances, an individual variation of a
species may become the origin of a local variety if it finds itself in
an isolated region. Suppose an island had no diurnal butterflies, until
one day a fertilized female of a species from the continent was driven
thither, found suitable conditions of life, laid its eggs, and became
the founder of a colony; the prevention of constant crossing between
this colony and the ancestral continental species would not in itself
be any reason why the colony should develop into a variety. But suppose
that the foundress of the colony diverged in some unimportant detail of
colouring, such as may at any time arise through germinal selection,
from the ancestral species; then this variation would be transmitted to
a portion of her progeny, and there would thus be a possibility that a
variety should establish itself upon the island which would be the mean
of the characters of the surviving progeny. The greater the divergence
was in the first progeny of the mother-colonist, and the stronger this
variational tendency was, the greater also would be the chance that it
would be transmitted further and become a characteristic aberration
from the marking of the original species. I then designated this effect
of isolation as due to _amixia_, that is, to the mere prevention of
crossing with the members of the same species in the original habitat.
[27] _Ueber den Einfluss der Isolirung auf die Artbildung_, Leipzig,
1872.
We have examples of this from the Mediterranean islands, Sardinia and
Corsica, which possess in common nine endemic varieties of butterflies,
most of which diverge from the species of the continent in a quite
inconsiderable degree, though quite definitely and constantly. Thus
there flies in these islands a variety (_Vanessa ichnusa_) of our
common little _Vanessa urticæ_ in which the two black spots on the
anterior wing exhibited by the original species are wanting. The large
tortoise-shell (_Vanessa polychloros_) also occurs there, but it has
not varied and still exhibits the black spots. Our little indigenous
butterfly (_Pararga megæra_), which is abundant on warm, stony slopes,
quarries, and roads, flies about in Sardinia, but as a variety
(_tigelius_), which is distinguished from the original species by the
absence of a black curved line on the posterior wings.
That of two nearly related and similarly marked species, like the large
and small tortoise-shell, one should remain unvaried, while the other
has become a variety, shows us that amixia alone does not necessarily
lead to the evolution of varieties in every case. It might of course be
objected that one species may have migrated to the islands at a much
earlier period than the other, and that it might be a direct effect
of the climate which found expression in this way. But we have other
similar cases in which one of two species has varied in an isolated
region, while the other has not, and in regard to which we can prove
definitely that both were isolated at the same time.
An instance of this kind is to be found in Arctic and Alpine
Lepidoptera, which inhabited the plains of Europe during the Glacial
period, and subsequently, when the climate became milder again,
migrated some to the north into countries within the Arctic zone, and
some to the south to the Alps to escape in their heights from the
increasing warmth. There are many diurnal Lepidoptera which now belong
to both regions, and of these some have remained exactly alike, so that
the Arctic form cannot be distinguished from the Alpine form; others
show slight differences, so that we can distinguish an Arctic and an
Alpine variety. To the former category belong, for instance, _Lycæna
donzelii_ and _Lycæna pheretes_, _Argynnis pales_, _Erebia manto_,
and others; to the second category belong, for instance, _Lycæna
orbitulus_, Prun., _Lycæna optilete_, _Argynnis thore_, and some
species of the genus _Erebia_.
This cannot be an instance of the direct effect of general climatic
influences, for in that case all the nearly related species of a genus
would have varied or not varied; nor have we to do with adaptations,
for the differences in marking are seen on the upper surfaces of
the wing, which do not exhibit protective colouring, at least in
these Lepidoptera. It can only have been the prevention of crossing
that has fixed the existing variational tendencies in the isolated
colonies--variations which would have been swamped and obliterated if
there had been constant crossing with all the rest of the members of
the species.
But there is another factor to be considered. Those Alpine Lepidoptera,
for instance, which have not remained exactly the same in the far
north, have formed local varieties in the rest of the area of their
distribution also, while species which have remained quite alike
in isolated regions, such as the Alps and the north, exhibit no
aberrations in other isolated regions, such as the Pyrenees, in
Labrador, or in the Altai. Thus one species must have had a tendency
in the Glacial period to form local varieties, and the other had not;
and I have already attempted to explain this on the hypothesis that the
former at the time of their migration and segregation into different
colonies were at a period of dominant variability, the latter at a
period of relatively great constancy. Leaving aside the question of
the causes of this phenomenon, we may take it as certain that there
are very variable and very constant species, and it is obvious that
colonies which are founded by a very variable species can hardly ever
remain exactly identical with the ancestral species; and that several
of them will turn out differently, even granting that the conditions of
life be exactly the same, for no colony will contain all the variants
of the species in the same proportion, but at most only a few of
them, and the result of mingling these must ultimately result in the
development of a somewhat different constant form in each colonial area.
If we were to try to imitate this 'amixia' artificially we should
only require to take at random from the streets of a large town a
number of pregnant bitches, and place each of them upon an island
not previously inhabited by dogs, and then a different breed of dog
would arise upon each of these islands, even if the conditions of life
were exactly similar. But if, instead of these variable bitches, the
females of a Russian wolf were placed on the islands, the developing
wolf-colonies would differ as little from the ancestral species as the
various Russian wolves do from one another--similar climate and similar
conditions of life being presupposed.