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and hardly an island can be named on which our smaller quadrupeds have
not become naturalized and greatly multiplied. It cannot be said, on the
ordinary view of creation, that there has not been time for the creation of
mammals; many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered, and by their tertiary
strata: there has also been time for the production of endemic species be-
longing to other classes ; and on continents it is known that new species of .^^
mammals appear and disappear at a quicker rate than other and lower!
animals. Although terrestrial mammals do not occur on oceanic islands,
aerial mammals do occur on almost every island. New Zealand possesses
two bats found nowhere else in the world: Norfolk Island, the Viti Archj-



GEOGRAPHICAL DISTRIBUTION 293

pelago, the Bonin Islands, the CaroHne and Marianne Archipelagoes, and
Mauritius, all possess their peculiar bats. Why, it may be asked, has the
supposed creative force produced bats and no other mammals on remote
islands? On my view this question can easily be answered; for no terrestrial
mammal can be 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 species have enor-
mous ranges, and are found on continents and on far distant islands. Hence,
we have only to suppose that such wandering species have been modified
in their new homes in relation to their new position, and we can understand
the presence of endemic bats on oceanic islands, with the absence of all
other terrestrial mammals.

Another interesting relation exists, namely, between the depth of the sea
separating islands from each other, or from the nearest continent, and the
degree of affinity of their mammalian inhabitants. Mr. Windsor Earl has
made some striking observations on this head, since greatly extended by
Mr. Wallace's admirable researches, in regard to the great Malay Archi-
pelago, which is traversed near Celebes by a space of deep ocean, and
this separates two widely distinct mammalian faunas. On either side, the
islands stand on a moderately shallow submarine bank, and these islands
are inhabited by the same or by closely allied quadrupeds. I have not as yet
had time to follow up this subject in all quarters of the world; but as far
as I have gone, the relation holds good. For instance, Britain is separated
by a shallow channel from Europe, and the mammals are the same on both
sides; and so it is with all the islands near the shores of Australia. The West
Indian Islands, on the other hand, stand on a deeply submerged bank,
nearly one thousand fathoms in depth, and here we find American forms,
but the species and even the genera are quite distinct. As the amount of
modification which animals of all kinds undergo partly depends on the
lapse of time, and as the islands which are separated from each other, or from
the mainland, by shallow channels, are more likely to have been continuously
united within a recent period than the islands separated by deeper channels,
we can understand how it is that a relation exists between the depth of the
sea separating two mammalian faunas, and the degree of their affinity, a rela-
tion which is quite inexplicable on the theory of independent acts of creation.

The foregoing statements in regard to the inhabitants of oceanic islands,
namely, the fewness of the species, with a large proportion consisting of
endemic forms — the members of certain groups, but not those of other
groups in the same class, having been modified — the absence of certain
whole orders, 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, etc., seem to me
to accord better with the belief in the efficiency of occasional means of
transport, carried on during a long course of time, than with the belief in



294 THE ORIGIN OF SPECIES

the former connection of all oceanic islands with the nearest continent; for
on this latter view it is probable that the various classes would have im-
migrated more uniformly, and from the species having entered in a body,
their mutual relations would not have been much disturbed, and, conse-
quently, they would either have not been modified, or all the species in a
more equable manner.

I do not deny that there are many and serious difficulties in understand-
ing how many of the inhabitants of the more remote islands, whether still
retaining the same specific form or subsequently modified, have reached
their present homes. But the probability of other islands having once existed
as halting-places, of which not a wreck now remains, must not be over-
looked. I will specify one difficult case. Almost all oceanic islands, even the
most isolated and smallest, are inhabited by land-shells, generally by en-
demic species, but sometimes by species found elsewhere, striking instances
of whidh have been given by Dr. A. A. Gould in relation to the Pacific. Now
it is notorious that land-shells are easily killed by sea-water; their eggs, at
least such as I have tried, sink in it and are killed. Yet there must be some
unknown, but occasionally efficient, means for their transportal. Would the
just-hatched young sometimes adhere to the feet of birds roosting on the
ground and thus get transported? It occurred to me that land-shells, when
hibernating and having a membraneous diaphragm over the mouth of the
shell, might be floated in chinks of drifted timber across moderately wide
arms of the sea. And I find that several species in this state withstand un-
injured an immersion in sea-water during seven days. One shell, the Helix
pomatia, after having been thus treated, and again hibernating, was put
into sea-water for twenty days and perfectly recovered. During this length of
time the shell might have been carried by a marine current of average
swiftness to a distance of 660 geographical miles. As this Helix has a thick
calcareous operculum I removed it, and when it had formed a new mem-
braneous one, I again immersed it for fourteen days in sea-water, and again
it recovered and crawled away. Baron Aucapitaine has since tried similar
experiments. He placed 100 land-shells, belonging to ten species, in a box
pierced with holes, and immersed it for a fortnight in the sea. Out of the
hundred shells twenty-seven recovered. The presence of an operculum
seems to have been of importance, as out of twelve specimens of Cyclostoma
elegans, which is thus furnished, eleven revived. It is remarkable, seeing
how well the Helix pomatia resisted with me the salt water, that not one of
fifty-four specimens belonging to four other species of Helix tried by Aucapi-
taine recovered. It is, however, not at all probable that land-shells have
often been thus transported; the feet of birds offer a more probable method.

ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO
THOSE OF THE NEAREST MAINLAND

The most striking and important fact for us is the affinity of the species
which inhabit islands to those of the nearest mainland, without being



GEOGRAPHICAL DISTRIBUTION 295

actually the same. Numerous instances could be given. The Galapagos
Archipelago, situated under the equator, lies at the distance of between
500 and 600 miles from the shores of South America. Here almost every
product of the land and of the water bears the unmistakable stamp of the
American continent. There are twenty-six land birds. Of these twenty-one,
or perhaps twenty-three, are ranked as distinct species, and would com-
monly be assumed to have been here created; yet the close affinity of most
of these birds to American species is manifest in every character in their
habits, gestures, and tones of voice. So it is with the other animals, and
with a large proportion of the plants, as shown by Dr. Hooker in his ad-
mirable Flora of this archipelago. The naturalist, looking at the inhabitants
of these volcanic islands in the Pacific, distant several hundred miles from
the continent, 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 plainly the 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
closely resembles 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 the climate, height, and size of the islands, between the Galapagos and
Cape Verde Archipelagoes: but what an entire and absolute difference in
their inhabitants! The inhabitants of the Cape Verde Islands are related
to those of Africa, like those of the Galapagos to America. Facts, such as
these, admit of no sort of explanation on the ordinary view of independent
creation; whereas, on the view here maintained, it is obvious that the Gala-
pagos Islands would be likely to receive colonists from America, whether by
occasional means of transport or (though I do not believe in this doctrine)
by formerly continuous land, and the Cape Verde Islands from Africa; 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 near-
est continent, or of the nearest large island. The exceptions are few, and
most of them can be explained. Thus, although Kerguelen Land stands
nearer to Africa than to America, the plants 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 re-
mote, that the fact becomes an anomaly. But this difficulty partially dis-
appears on the view that New Zealand, South America, and the other



296 THE ORIGIN OF SPECIES

southern lands, have been stocked in part from a nearly intermediate though
distant point, namely, from the Antarctic Islands, when they were clothed
with vegetation, during a warmer tertiary period, before the commence-
ment of the last Glacial period. The affinity, which, though feeble, I am
assured by Dr. Hooker is real, between the flora of the southwestern corner
of Australia and of the Cape of Good Hope, is a far more remarkable case;
but this affinity is confined to the plants, and will, no doubt, some day be
explained.

The same law which has determined the relationship between the inhabit-
ants of islands and the nearest mainland, is sometimes displayed on a small
scale, but in a most interesting manner, within the limits of the same
archipelago. Thus each separate island of the Galapagos Archipelago is
tenanted, and the fact is a marvellous one, by many distinct species; but
these species are related to each other in a very much closer manner than
to the inhabitants of the American continent, or of any other quarter of
the world. This is what might have been expected, for islands situated so
meat to each other would almost necessarily receive immigrants from the
same original source and from each other. But how is it that many of the
immigrants have been differently modified, though only in a small degree,
the islands situated within sight of each other, having the same geological
nature, the same height, climate, etc.? This long appeared to me a great
difficulty: but it arises in chief part from the deeply-seated error of con-
sidering the physical conditions of a country as the most important; whereas
it cannot be disputed that the nature of the other species 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 the species which inhabit the Gala-
pagos Archipelago, and are likewise found in other parts of the world, we
find that they diflfer considerably in the several islands. This difference
might indeed have been expected if the islands have been stocked by occa-
sional means of transport — a seed, for instance, of one plant having been
brought to one island, and that of another plant to another island, though
all proceeding from the same general source. Hence, when in former times
an immigrant first settled on one of the islands, or when it subsequently
spread from one to another, it would undoubtedly be exposed to different
conditions in the different islands, for it would have to compete with a
different set of organisms; a plant, for instance, would find the ground best
fitted for it occupied by somewhat different species in the different islands,
and would be exposed to the attacks of somewhat different enemies. If,
then, it varied, natural selection would probably favor different varieties in
the different islands. Some species, however, might spread and yet retain
the same character throughout the group, just as we see some species
spreading widely throughout a continent and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago, and
in a lesser degree in some analogous cases, is that each new species, after
being formed in any one island, did not spread quickly to the other islands.
But the islands, though in sight of each other, are separated by deep arms



GEOGRAPHICAL DISTRIBUTION 297

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 deep between the islands, and
gales of wind are extraordinarily rare; so that the islands are far more
effectually separated from each other than they appear on a map. Never-
theless, some of the species, both of those found in other parts of the world
and of those confined to the archipelago, are common to the several islands;
and we may infer from the present manner of distribution that they have
spread from one island to the others. But we often take, I think, an errone-
ous 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 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, both will probably hold their separate places for almost any length
of time. Being familiar with the fact that many species, naturalized through
man's agency, have spread with astonishing rapidity over wide areas, we
are apt to infer that most species would thus spread; but we should remem-
ber that the species which become naturalized in new countries are not
generally closely allied to the aboriginal inhabitants, but are very distinct
forms, belonging in a large proportion of cases, as shown by Alph. de Can-
dolle, to distinct genera. In the Galapagos Archipelago, many even of the
birds, though so well adapted for flying from island to island, differ on the
different islands; 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 and young birds hatched 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
species of 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 colonized by the Porto Santo
species; nevertheless, both islands have been colonized by 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 species which inhabit the several islands of the Galapagos Archi-
pelago not having all spread from island to island. On the same continent,
also, pre-occupation has probably played an important part in checking the
commingling of the species which inhabit different districts with nearly the
same physical conditions. Thus, the south-east and south-west corners of
Australia have nearly the same physical conditions, and are united by con-
tinuous land, yet they are inhabited by a vast number of distinct mammals,



298 THE ORIGIN OF SPECIES

birds, and plants; so it is, according to Mr. Bates, with the butterflies and
other animals inhabiting the great, open, and continuous valley of the
Amazons.

The same principle which governs the general character of the inhabit-
ants of oceanic islands, namely, the relation to the source whence colonists
could have been most easily derived, together with their subsequent modifi-
cation, is of the widest application throughout nature. We see this on every
mountain-summit, in every lake and marsh. For alpine species, excepting in
as far as the same species have become widely spread during the Glacial
epoch, are related to those of the surrounding lowlands; thus we have in
South America, alpine humming-birds, alpine rodents, alpine plants, etc.,
all strictly belonging to American forms; and it is obvious that a mountain,
as it became slowly upheaved, would be colonized from the surrounding
lowlands. So it is with the inhabitants of lakes and marshes, excepting in
so far as great facility of transport has allowed the same forms to prevail
throughout large portions of the world. We see the same principle in the
character of most of the blind animals inhabiting the caves of America and
of Europe. Other analogous facts could be given. It will, I believe, be
found universally true, that wherever in two regions, let them be ever so
distant, many closely allied or representative species occur, there will like-
wise be found some identical species; and wherever many closely allied
species occur, there will be found many forms which some naturalists rank
as distinct species, and others as mere varieties; these doubtful forms show-
ing us the steps in the progress of modification.

The relation between the power and extent of migration in certain
species, either at the present or at some former period, and the existence at
remote points of the world of closely allied species, is shown in another
and more general way. Mr. Gould remarked to me long ago, that in those
genera of birds which range over the world, many of the species have very
wide ranges. I can hardly doubt that this rule is generally true, though
difficult of proof. Among mammals, we see it strikingly displayed in bats,
and in a lesser degree in the Felidae and Canidae. We see the same rule in
the distribution of butterflies and beetles. So it is with most of the inhabit-
ants of fresh water, for many of the genera in the most distinct classes range
over the world, and many of the species have enormous ranges. It is not
meant that all, but that some of the species, have very wide ranges in the
genera which range very widely. Nor is it meant that the species in such
genera have, on an average, a very wide range; for this will largely depend
on how far the process of modification has gone; for instance, two varieties
of the same species inhabit America and Europe, and thus the species has
an immense range; but, if variation were to be carried a little further, the
two varieties would be ranked as distinct species, and their range would be
greatly reduced. Still less is it meant, that species which have the capacity of
crossing barriers and ranging widely, as in the case of certain powerfully
winged birds, will necessarily range widely; for we should never forget that
to range widely implies not only the power of crossing barriers, but the



GEOGRAPHICAL DISTRIBUTION 299

more important power of being victorious in distant lands in the struggle
for life with foreign associates. But according to the view that all the species
of a genuSj though distributed to the most remote points of the world, are
descended from a single progenitor, we ought to find, and I believe as a
general rule we do find, that some at least of the species range very widely.

We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for dispersal
and subsequent modification. There is also reason to believe, from geologi-
cal evidence, that within each great class the lower organisms change at a
slower rate than the higher; consequently they will have had a better
chance of ranging widely and of still retaining the same specific character.
This fact, together with that of the seeds and eggs of most lowly organized
forms being very minute and better fitted for distant transportal, probably
accounts for a law which has long been observed, and which has lately been
discussed by Alph. de Candolle, in regard to plants; namely, that the lower
any group of organisms stands, the more widely it ranges.

The relations just discussed — namely, lower organisms ranging more
widely than the higher — some of the species of widely ranging genera them-
selves ranging widely — such facts, as alpine, lacustrine, and marsh produc-
tions being generally related to those which live on the surrounding low
lands and dry lands — the striking relationship between the inhabitants of
islands and those of the nearest mainland — the still closer relationship of
the distinct inhabitants of the islands in the same archipelago — are inex-
plicable on the ordinary view of the independent creation of each species,
but are explicable if we admit colonization from the nearest or readiest
source, together with the subsequent adaptation of the colonists to their
new homes.

SUMMARY OF THE LAST AND PRESENT CHAPTERS

In these chapters I have endeavored to show that if we make due allow-
ance for our ignorance of the full effects of changes of climate and of the
level of the land, which have certainly occurred within the recent period,
and of other changes which have probably occurred — if we remember how
ignorant we are with respect to the many curious means of occasional trans-
port — if we bear in mind, and this is a very important consideration, how
often a species may have ranged continuously over a wide area, and then
have become extinct in the intermediate tracts — the difficulty is not in-
superable in believing that all the individuals of the same species, wherever
found, are descended from common parents. And we are led to this con-
clusion, which has been arrived at by many naturalists under the designa-
tion of single centres of creation, by various general considerations, more
especially from the importance of barriers of all kinds, and from the
analogical distribution of subgenera, genera, and families.

With respect to distinct species belonging to the same genus, which on
our theory have spread from one parent-source ; if we make the same allow-



300 THE ORIGIN OF SPECIES

ances as before for our ignorance, and remember that some forms of life
have changed very slowly, enormous periods of time having been thus;
granted for their migration, the difficulties are far from insuperable ; though
in this case, as in that of the individuals of the same species, they are often
great.

As exemplifying the effects of climatical changes on distribution, I have
attempted to show how important a part the last Glacial period has played,



Online LibraryCharles DarwinThe origin of species → online text (page 40 of 50)