transported across a wide space of sea, but bats can fly across. Bats have
been seen wandering by day far over the Atlantic Ocean; and two North
American species either regularly or occasionally visit Bermuda, at the
distance of 600 miles from the mainland. I hear from Mr. Tomes, who has
specially studied this family, that many of the same species have enormous
ranges, and are found on continents and on far distant islands. Hence we
have only to suppose that such wandering species have been modified through
natural selection in their new homes in relation to their new position, and
we can understand the presence of endemic bats on islands, with the absence
of all terrestrial mammals.
Besides the absence of terrestrial mammals in relation to the remoteness of
islands from continents, there is also a relation, to a certain extent
independent of distance, between the depth of the sea separating an island
from the neighbouring mainland, and the presence in both of the same
mammiferous species or of allied species in a more or less modified
condition. Mr. Windsor Earl has made some striking observations on this
head in regard to the great Malay Archipelago, which is traversed near
Celebes by a space of deep ocean; and this space separates two widely
distinct mammalian faunas. On either side the islands are situated on
moderately deep submarine banks, and they are inhabited by closely allied
or identical quadrupeds. No doubt some few anomalies occur in this great
archipelago, and there is much difficulty in forming a judgment in some
cases owing to the probable naturalisation of certain mammals through man's
agency; but we shall soon have much light thrown on the natural history of
this archipelago by the admirable zeal and researches of Mr. Wallace. I
have not as yet had time to follow up this subject in all other quarters of
the world; but as far as I have gone, the relation generally holds good.
We see Britain separated by a shallow channel from Europe, and the mammals
are the same on both sides; we meet with analogous facts on many islands
separated by similar channels from Australia. The West Indian Islands
stand on a deeply submerged bank, nearly 1000 fathoms in depth, and here we
find American forms, but the species and even the genera are distinct. As
the amount of modification in all cases depends to a certain degree on the
lapse of time, and as during changes of level it is obvious that islands
separated by shallow channels are more likely to have been continuously
united within a recent period to the mainland than islands separated by
deeper channels, we can understand the frequent relation between the depth
of the sea and the degree of affinity of the mammalian inhabitants of
islands with those of a neighbouring continent,--an inexplicable relation
on the view of independent acts of creation.
All the foregoing remarks on the inhabitants of oceanic islands,--namely,
the scarcity of kinds--the richness in endemic forms in particular classes
or sections of classes,--the absence of whole groups, as of batrachians,
and of terrestrial mammals notwithstanding the presence of aerial
bats,--the singular proportions of certain orders of plants,--herbaceous
forms having been developed into trees, &c.,--seem to me to accord better
with the view of occasional means of transport having been largely
efficient in the long course of time, than with the view of all our oceanic
islands having been formerly connected by continuous land with the nearest
continent; for on this latter view the migration would probably have been
more complete; and if modification be admitted, all the forms of life would
have been more equally modified, in accordance with the paramount
importance of the relation of organism to organism.
I do not deny that there are many and grave difficulties in understanding
how several of the inhabitants of the more remote islands, whether still
retaining the same specific form or modified since their arrival, could
have reached their present homes. But the probability of many islands
having existed as halting-places, of which not a wreck now remains, must
not be overlooked. I will here give a single instance of one of the cases
of difficulty. Almost all oceanic islands, even the most isolated and
smallest, are inhabited by land-shells, generally by endemic species, but
sometimes by species found elsewhere. Dr. Aug. A. Gould has given several
interesting cases in regard to the land-shells of the islands of the
Pacific. Now it is notorious that land-shells are very easily killed by
salt; their eggs, at least such as I have tried, sink in sea-water and are
killed by it. Yet there must be, on my view, some unknown, but highly
efficient means for their transportal. Would the just-hatched young
occasionally crawl on and adhere to the feet of birds roosting on the
ground, and thus get transported? It occurred to me that land-shells, when
hybernating and having a membranous diaphragm over the mouth of the shell,
might be floated in chinks of drifted timber across moderately wide arms of
the sea. And I found that several species did in this state withstand
uninjured an immersion in sea-water during seven days: one of these shells
was the Helix pomatia, and after it had again hybernated I put it in
sea-water for twenty days, and it perfectly recovered. As this species has
a thick calcareous operculum, I removed it, and when it had formed a new
membranous one, I immersed it for fourteen days in sea-water, and it
recovered and crawled away: but more experiments are wanted on this head.
The most striking and important fact for us in regard to the inhabitants of
islands, is their affinity to those of the nearest mainland, without being
actually the same species. Numerous instances could be given of this fact.
I will give only one, that of the Galapagos Archipelago, situated under the
equator, between 500 and 600 miles from the shores of South America. Here
almost every product of the land and water bears the unmistakeable stamp of
the American continent. There are twenty-six land birds, and twenty-five
of these are ranked by Mr. Gould as distinct species, supposed to have been
created here; yet the close affinity of most of these birds to American
species in every character, in their habits, gestures, and tones of voice,
was manifest. So it is with the other animals, and with nearly all the
plants, as shown by Dr. Hooker in his admirable memoir on the Flora of this
archipelago. The naturalist, looking at the inhabitants of these volcanic
islands in the Pacific, distant several hundred miles from the continent,
yet feels that he is standing on American land. Why should this be so? why
should the species which are supposed to have been created in the Galapagos
Archipelago, and nowhere else, bear so plain a stamp of affinity to those
created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in the
proportions in which the several classes are associated together, which
resembles closely the conditions of the South
American coast: in fact
there is a considerable dissimilarity in all these respects. On the other
hand, there is a considerable degree of resemblance in the volcanic nature
of the soil, in climate, height, and size of the islands, between the
Galapagos and Cape de Verde Archipelagos: but what an entire and absolute
difference in their inhabitants! The inhabitants of the Cape de Verde
Islands are related to those of Africa, like those of the Galapagos to
America. I believe this grand fact can receive no sort of explanation on
the ordinary view of independent creation; whereas on the view here
maintained, it is obvious that the Galapagos Islands would be likely to
receive colonists, whether by occasional means of transport or by formerly
continuous land, from America; and the Cape de Verde Islands from Africa;
and that such colonists would be liable to modification;--the principle of
inheritance still betraying their original birthplace.
Many analogous facts could be given: indeed it is an almost universal rule
that the endemic productions of islands are related to those of the nearest
continent, or of other near islands. The exceptions are few, and most of
them can be explained. Thus the plants of Kerguelen Land, though standing
nearer to Africa than to America, are related, and that very closely, as we
know from Dr. Hooker's account, to those of America: but on the view that
this island has been mainly stocked by seeds brought with earth and stones
on icebergs, drifted by the prevailing currents, this anomaly disappears.
New Zealand in its endemic plants is much more closely related to
Australia, the nearest mainland, than to any other region: and this is
what might have been expected; but it is also plainly related to South
America, which, although the next nearest continent, is so enormously
remote, that the fact becomes an anomaly. But this difficulty almost
disappears on the view that both New Zealand, South America, and other
southern lands were long ago partially stocked from a nearly intermediate
though distant point, namely from the antarctic islands, when they were
clothed with vegetation, before the commencement of the Glacial period.
The affinity, which, though feeble, I am assured by Dr. Hooker is real,
between the flora of the south-western corner of Australia and of the Cape
of Good Hope, is a far more remarkable case, and is at present
inexplicable: but this affinity is confined to the plants, and will, I do
not doubt, be some day explained.
The law which causes the inhabitants of an archipelago, though specifically
distinct, to be closely allied to those of the nearest continent, we
sometimes see displayed on a small scale, yet in a most interesting manner,
within the limits of the same archipelago. Thus the several islands of the
Galapagos Archipelago are tenanted, as I have elsewhere shown, in a quite
marvellous manner, by very closely related species; so that the inhabitants
of each separate island, though mostly distinct, are related in an
incomparably closer degree to each other than to the inhabitants of any
other part of the world. And this is just what might have been expected on
my view, for the islands are situated so near each other that they would
almost certainly receive immigrants from the same original source, or from
each other. But this dissimilarity between the endemic inhabitants of the
islands may be used as an argument against my views; for it may be asked,
how has it happened in the several islands situated within sight of each
other, having the same geological nature, the same height, climate, &c.,
that many of the immigrants should have been differently modified, though
only in a small degree. This long appeared to me a great difficulty: but
it arises in chief part from the deeply-seated error of considering the
physical conditions of a country as the most important for its inhabitants;
whereas it cannot, I think, be disputed that the nature of the other
inhabitants, with which each has to compete, is at least as important, and
generally a far more important element of success. Now if we look to those
inhabitants of the Galapagos Archipelago which are found in other parts of
the world (laying on one side for the moment the endemic species, which
cannot be here fairly included, as we are considering how they have come to
be modified since their arrival), we find a considerable amount of
difference in the several islands. This difference might indeed have been
expected on the view of the islands having been stocked by occasional means
of transport--a seed, for instance, of one plant having been brought to one
island, and that of another plant to another island. Hence when in former
times an immigrant settled on any one or more of the islands, or when it
subsequently spread from one island to another, it would undoubtedly be
exposed to different conditions of life in the different islands, for it
would have to compete with different sets of organisms: a plant, for
instance, would find the best-fitted ground more perfectly occupied by
distinct plants in one island than in another, and it would be exposed to
the attacks of somewhat different enemies. If then it varied, natural
selection would probably favour different varieties in the different
islands. Some species, however, might spread and yet retain the same
character throughout the group, just as we see on continents some species
spreading widely and remaining the same.
The really surprising fact in this case of the Galapagos Archipelago, and
in a lesser degree in some analogous instances, is that the new species
formed in the separate islands have not quickly spread to the other
islands. But the islands, though in sight of each other, are separated by
deep arms of the sea, in most cases wider than the British Channel, and
there is no reason to suppose that they have at any former period been
continuously united. The currents of the sea are rapid and sweep across
the archipelago, and gales of wind are extraordinarily rare; so that the
islands are far more effectually separated from each other than they appear
to be on a map. Nevertheless a good many species, both those found in
other parts of the world and those confined to the archipelago, are common
to the several islands, and we may infer from certain facts that these have
probably spread from some one island to the others. But we often take, I
think, an erroneous view of the probability of closely allied species
invading each other's territory, when put into free intercommunication.
Undoubtedly if one species has any advantage whatever over another, it will
in a very brief time wholly or in part supplant it; but if both are equally
well fitted for their own places in nature, both probably will hold their
own places and keep separate for almost any length of time. Being familiar
with the fact that many species, naturalised through man's agency, have
spread with astonishing rapidity over new countries, we are apt to infer
that most species would thus spread; but we should remember that the forms
which become nat
uralised in new countries are not generally closely allied
to the aboriginal inhabitants, but are very distinct species, belonging in
a large proportion of cases, as shown by Alph. de Candolle, to distinct
genera. In the Galapagos Archipelago, many even of the birds, though so
well adapted for flying from island to island, are distinct on each; thus
there are three closely-allied species of mocking-thrush, each confined to
its own island. Now let us suppose the mocking-thrush of Chatham Island to
be blown to Charles Island, which has its own mocking-thrush: why should
it succeed in establishing itself there? We may safely infer that Charles
Island is well stocked with its own species, for annually more eggs are
laid there than can possibly be reared; and we may infer that the
mocking-thrush peculiar to Charles Island is at least as well fitted for
its home as is the species peculiar to Chatham Island. Sir C. Lyell and
Mr. Wollaston have communicated to me a remarkable fact bearing on this
subject; namely, that Madeira and the adjoining islet of Porto Santo
possess many distinct but representative land-shells, some of which live in
crevices of stone; and although large quantities of stone are annually
transported from Porto Santo to Madeira, yet this latter island has not
become colonised by the Porto Santo species: nevertheless both islands
have been colonised by some European land-shells, which no doubt had some
advantage over the indigenous species. From these considerations I think
we need not greatly marvel at the endemic and representative species, which
inhabit the several islands of the Galapagos Archipelago, not having
universally spread from island to island. In many other instances, as in
the several districts of the same continent, pre-occupation has probably
played an important part in checking the commingling of species under the
same conditions of life. Thus, the south-east and south-west corners of
Australia have nearly the same physical conditions, and are united by
continuous land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants.
The principle which determines the general character of the fauna and flora
of oceanic islands, namely, that the inhabitants, when not identically the
same, yet are plainly related to the inhabitants of that region whence
colonists could most readily have been derived,--the colonists having been
subsequently modified and better fitted to their new homes,--is of the
widest application throughout nature. We see this on every mountain, in
every lake and marsh. For Alpine species, excepting in so far as the same
forms, chiefly of plants, have spread widely throughout the world during
the recent Glacial epoch, are related to those of the surrounding
lowlands;--thus we have in South America, Alpine humming-birds, Alpine
rodents, Alpine plants, &c., all of strictly American forms, and it is
obvious that a mountain, as it became slowly upheaved, would naturally be
colonised from the surrounding lowlands. So it is with the inhabitants of
lakes and marshes, excepting in so far as great facility of transport has
given the same general forms to the whole world. We see this same
principle in the blind animals inhabiting the caves of America and of
Europe. Other analogous facts could be given. And it will, I believe, be
universally found to be true, that wherever in two regions, let them be
ever so distant, many closely allied or representative species occur, there
will likewise be found some identical species, showing, in accordance with
the foregoing view, that at some former period there has been
intercommunication or migration between the two regions. And wherever many
closely-allied species occur, there will be found many forms which some
naturalists rank as distinct species, and some as varieties; these doubtful
forms showing us the steps in the process of modification.