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Charles Darwin: The origin of species
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CHAPTER XIII.
GEOGRAPHICAL DISTRIBUTION--continued.
Distribution of fresh-water productions -- On the inhabitants of oceanic
islands -- Absence of Batrachians and of terrestrial Mammals -- On the
relation of the inhabitants of islands to those of the nearest mainland --
On colonisation from the nearest source with subsequent modification --
Summary of the last and present chapters.
FRESH-WATER PRODUCTIONS.
As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more formidable barrier, that they would never have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to different classes, an enormous
range, but allied species prevail in a remarkable manner throughout the
world. When first collecting in the fresh waters of Brazil, I well
remember feeling much surprise at the similarity of the fresh-water
insects, shells, etc., and at the dissimilarity of the surrounding
terrestrial beings, compared with those of Britain.
But the wide ranging power of fresh-water productions can, I think, in most
cases be explained by their having become fitted, in a manner highly useful
to them, for short and frequent migrations from pond to pond, or from
stream to stream, within their own countries; and liability to wide
dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases; of these, some of the
most difficult to explain are presented by fish. It was formerly believed
that the same fresh-water species never existed on two continents distant
from each other. But Dr. Gunther has lately shown that the Galaxias
attenuatus inhabits Tasmania, New Zealand, the Falkland Islands and the
mainland of South America. This is a wonderful case, and probably
indicates dispersal from an Antarctic centre during a former warm period.
This case, however, is rendered in some degree less surprising by the
species of this genus having the power of crossing by some unknown means
considerable spaces of open ocean: thus there is one species common to New
Zealand and to the Auckland Islands, though separated by a distance of
about 230 miles. On the same continent fresh-water fish often range
widely, and as if capriciously; for in two adjoining river systems some of
the species may be the same and some wholly different.
It is probable that they are occasionally transported by what may be called
accidental means. Thus fishes still alive are not very rarely dropped at
distant points by whirlwinds; and it is known that the ova retain their
vitality for a considerable time after removal from the water. Their
dispersal may, however, be mainly attributed to changes in the level of the
land within the recent period, causing rivers to flow into each other.
Instances, also, could be given of this having occurred during floods,
without any change of level. The wide differences of the fish on the
opposite sides of most mountain-ranges, which are continuous and
consequently must, from an early period, have completely prevented the
inosculation of the river systems on the two sides, leads to the same
conclusion. Some fresh-water fish belong to very ancient forms, and in
such cases there will have been ample time for great geographical changes,
and consequently time and means for much migration. Moreover, Dr. Gunther
has recently been led by several considerations to infer that with fishes
the same forms have a long endurance. Salt-water fish can with care be
slowly accustomed to live in fresh water; and, according to Valenciennes,
there is hardly a single group of which all the members are confined to
fresh water, so that a marine species belonging to a fresh-water group
might travel far along the shores of the sea, and could, it is probable,
become adapted without much difficulty to the fresh waters of a distant
land.
Some species of fresh-water shells have very wide ranges, and allied
species which, on our theory, are descended from a common parent, and must
have proceeded from a single source, prevail throughout the world. Their
distribution at first perplexed me much, as their ova are not likely to be
transported by birds; and the ova, as well as the adults, are immediately
killed by sea-water. I could not even understand how some naturalised
species have spread rapidly throughout the same country. But two facts,
which I have observed--and many others no doubt will be discovered--throw
some light on this subject. When ducks suddenly emerge from a pond covered
with duck-weed, I have twice seen these little plants adhering to their
backs; and it has happened to me, in removing a little duck-weed from one
aquarium to another, that I have unintentionally stocked the one with
fresh-water shells from the other. But another agency is perhaps more
effectual: I suspended the feet of a duck in an aquarium, where many ova
of fresh-water shells were hatching; and I found that numbers of the
extremely minute and just-hatched shells crawled on the feet, and clung to
them so firmly that when taken out of the water they could not be jarred
off, though at a somewhat more advanced age they would voluntarily drop
off. These just-hatched molluscs, though aquatic in their nature, survived
on the duck's feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and if blown across the sea to an oceanic island, or to any other
distant point, would be sure to alight on a pool or rivulet. Sir Charles
Lyell informs me that a Dyticus has been caught with an Ancylus (a
fresh-water shell like a limpet) firmly adhering to it; and a water-beetle
of the same family, a Colymbetes, once flew on board the "Beagle," when
forty-five miles distant from the nearest land: how much farther it might
have been blown by a favouring gale no one can tell.
With respect to plants, it has long been known what enormous ranges many
fresh-water, and even marsh-species, have, both over continents and to the
most remote oceanic islands. This is strikingly illustrated, according to
Alph. de Candolle, in those large groups of terrestrial plants, which have
very few aquatic members; for the latter seem immediately to acquire, as if
in consequence, a wide range. I think favourable means of dispersal
explain this fact. I have before mentioned that earth occasionally adheres
in some quantity to the feet and beaks of birds. Wading birds, which
frequent the muddy edges of ponds, if suddenly flushed, would be the most
likely to have muddy feet. Birds of this order wander more than those of
any other; and are occasionally found on the most remote and barren islands
of the open ocean; they would not be likely to alight on the surface of the
sea, so that any dirt on their feet would not be washed off; and when
gaining the land, they would be sure to fly to their natural fresh-water
haunts. I do not believe that botanists are aware how charged the mud of
ponds is with seeds: I have tried several little experiments, but will
here give only the most striking case: I took in February three
tablespoonfuls of mud from three different points, beneath water, on the
edge of a little pond; this mud when dry weighed only 6 and 3/4 ounces; I
kept it covered up in my study for six months, pulling up and counting each
plant as it grew; the plants were of many kinds, and were altogether 537 in
number; and yet the viscid mud was all contained in a breakfast cup!
Considering these facts, I think it would be an inexplicable circumstance
if water-birds did not transport the seeds of fresh-water plants to
unstocked ponds and streams, situated at very distant points. The same
agency may have come into play with the eggs of some of the smaller
fresh-water animals.
Other and unknown agencies probably have also played a part. I have stated
that fresh-water fish eat some kinds of seeds, though they reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish; they
then take flight and go to other waters, or are blown across the sea; and
we have seen that seeds retain their power of germination, when rejected
many hours afterwards in pellets or in the excrement. When I saw the great
size of the seeds of that fine water-lily, the Nelumbium, and remembered
Alph. de Candolle's remarks on the distribution of this plant, I thought
that the means of its dispersal must remain inexplicable; but Audubon
states that he found the seeds of the great southern water-lily (probably
according to Dr. Hooker, the Nelumbium luteum) in a heron's stomach. Now
this bird must often have flown with its stomach thus well stocked to
distant ponds, and, then getting a hearty meal of fish, analogy makes me
believe that it would have rejected the seeds in the pellet in a fit state
for germination.
In considering these several means of distribution, it should be remembered
that when a pond or stream is first formed, for instance on a rising islet,
it will be unoccupied; and a single seed or egg will have a good chance of
succeeding. Although there will always be a struggle for life between the
inhabitants of the same pond, however few in kind, yet as the number even
in a well-stocked pond is small in comparison with the number of species
inhabiting an equal area of land, the competition between them will
probably be less severe than between terrestrial species; consequently an
intruder from the waters of a foreign country would have a better chance of
seizing on a new place, than in the case of terrestrial colonists. We
should also remember that many fresh-water productions are low in the scale
of nature, and we have reason to believe that such beings become modified
more slowly than the high; and this will give time for the migration of
aquatic species. We should not forget the probability of many fresh-water
forms having formerly ranged continuously over immense areas, and then
having become extinct at intermediate points. But the wide distribution of
fresh-water plants, and of the lower animals, whether retaining the same
identical form, or in some degree modified, apparently depends in main part
on the wide dispersal of their seeds and eggs by animals, more especially
by fresh-water birds, which have great powers of flight, and naturally
travel from one piece of water to another.
ON THE INHABITANTS OF OCEANIC ISLANDS.
We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty with respect to
distribution, on the view that not only all the individuals of the same
species have migrated from some one area, but that allied species, although
now inhabiting the most distant points, have proceeded from a single area,
the birthplace of their early progenitors. I have already given my reasons
for disbelieving in continental extensions within the period of existing
species on so enormous a scale that all the many islands of the several
oceans were thus stocked with their present terrestrial inhabitants. This
view removes many difficulties, but it does not accord with all the facts
in regard to the productions of islands. In the following remarks I shall
not confine myself to the mere question of dispersal, but shall consider
some other cases bearing on the truth of the two theories of independent
creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. New Zealand, for instance,
with its lofty mountains and diversified stations, extending over 780 miles
of latitude, together with the outlying islands of Auckland, Campbell and
Chatham, contain altogether only 960 kinds of flowering plants; if we
compare this moderate number with the species which swarm over equal areas
in Southwestern Australia or at the Cape of Good Hope, we must admit that
some cause, independently of different physical conditions, has given rise
to so great a difference in number. Even the uniform county of Cambridge
has 847 plants, and the little island of Anglesea 764, but a few ferns and
a few introduced plants are included in these numbers, and the comparison
in some other respects is not quite fair. We have evidence that the barren
island of Ascension aboriginally possessed less than half-a-dozen flowering
plants; yet many species have now become naturalised on it, as they have in
New Zealand and on every other oceanic island which can be named. In St.
Helena there is reason to believe that the naturalised plants and animals
have nearly or quite exterminated many native productions. He who admits
the doctrine of the creation of each separate species, will have to admit
that a sufficient number of the best adapted plants and animals were not
created for oceanic islands; for man has unintentionally stocked them far
more fully and perfectly than did nature.
Although in oceanic islands the species are few in number, the proportion
of endemic kinds (i.e. those found nowhere else in the world) is often
extremely large. If we compare, for instance, the number of endemic
land-shells in Madeira, or of endemic birds in the Galapagos Archipelago,
with the number found on any continent, and then compare the area of the
island with that of the continent, we shall see that this is true. This
fact might have been theoretically expected, for, as already explained,
species occasionally arriving, after long intervals of time in the new and
isolated district, and having to compete with new associates, would be
eminently liable to modification, and would often produce groups of
modified descendants. But it by no means follows that, because in an
island nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend partly on the species which are not modified
having immigrated in a body, so that their mutual relations have not been
much disturbed; and partly on the frequent arrival of unmodified immigrants
from the mother-country, with which the insular forms have intercrossed.
It should be borne in mind that the offspring of such crosses would
certainly gain in vigour; so that even an occasional cross would produce
more effect than might have been anticipated. I will give a few
illustrations of the foregoing remarks: in the Galapagos Islands there are
twenty-six land birds; of these twenty-one (or perhaps twenty-three) are
peculiar; whereas of the eleven marine birds only two are peculiar; and it
is obvious that marine birds could arrive at these islands much more easily
and frequently than land-birds. Bermuda, on the other hand, which lies at
about the same distance from North America as the Galapagos Islands do from
South America, and which has a very peculiar soil, does not possess a
single endemic land bird; and we know from Mr. J.M. Jones's admirable
account of Bermuda, that very many North American birds occasionally or
even frequently visit this island. Almost every year, as I am informed by
Mr. E.V. Harcourt, many European and African birds are blown to Madeira;
this island is inhabited by ninety-nine kinds, of which one alone is
peculiar, though very closely related to a European form; and three or four
other species are confined to this island and to the Canaries. So that the
islands of Bermuda and Madeira have been stocked from the neighbouring
continents with birds, which for long ages have there struggled together,
and have become mutually co-adapted. Hence, when settled in their new
homes, each kind will have been kept by the others to its proper place and
habits, and will consequently have been but little liable to modification.
Any tendency to modification will also have been checked by intercrossing
with the unmodified immigrants, often arriving from the mother-country.
Madeira again is inhabited by a wonderful number of peculiar land-shells,
whereas not one species of sea-shell is peculiar to its shores: now,
though we do not know how sea-shells are dispersed, yet we can see that
their eggs or larvae, perhaps attached to seaweed or floating timber, or to
the feet of wading birds, might be transported across three or four hundred
miles of open sea far more easily than land-shells. The different orders
of insects inhabiting Madeira present nearly parallel cases.
Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes; thus in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take, or recently took, the place of mammals. Although New Zealand is here
spoken of as an oceanic island, it is in some degree doubtful whether it
should be so ranked; it is of large size, and is not separated from
Australia by a profoundly deep sea; from its geological character and the
direction of its mountain ranges, the Rev. W.B. Clarke has lately
maintained that this island, as well as New Caledonia, should be considered
as appurtenances of Australia. Turning to plants, Dr. Hooker has shown
that in the Galapagos Islands the proportional numbers of the different
orders are very different from what they are elsewhere. All such
differences in number, and the absence of certain whole groups of animals
and plants, are generally accounted for by supposed differences in the
physical conditions of the islands; but this explanation is not a little
doubtful. Facility of immigration seems to have been fully as important as
the nature of the conditions.
Many remarkable little facts could be given with respect to the inhabitants
of oceanic islands. For instance, in certain islands not tenanted by a
single mammal, some of the endemic plants have beautifully hooked seeds;
yet few relations are more manifest than that hooks serve for the
transportal of seeds in the wool or fur of quadrupeds. But a hooked seed
might be carried to an island by other means; and the plant then becoming
modified would form an endemic species, still retaining its hooks, which
would form a useless appendage, like the shrivelled wings under the
soldered wing-covers of many insular beetles. Again, islands often possess
trees or bushes belonging to orders which elsewhere include only herbaceous
species; now trees, as Alph. de Candolle has shown, generally have,
whatever the cause may be, confined ranges. Hence trees would be little
likely to reach distant oceanic islands; and an herbaceous plant, which had
no chance of successfully competing with the many fully developed trees
growing on a continent, might, when established on an island, gain an
advantage over other herbaceous plants by growing taller and taller and
overtopping them. In this case, natural selection would tend to add to the
stature of the plant, to whatever order it belonged, and thus first convert
it into a bush and then into a tree.
ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS.
With respect to the absence of whole orders of animals on oceanic islands,
Bory St. Vincent long ago remarked that Batrachians (frogs, toads, newts)
are never found on any of the many islands with which the great oceans are
studded. I have taken pains to verify this assertion, and have found it
true, with the exception of New Zealand, New Caledonia, the Andaman
Islands, and perhaps the Solomon Islands and the Seychelles. But I have
already remarked that it is doubtful whether New Zealand and New Caledonia
ought to be classed as oceanic islands; and this is still more doubtful
with respect to the Andaman and Solomon groups and the Seychelles. This
general absence of frogs, toads and newts on so many true oceanic islands
cannot be accounted for by their physical conditions; indeed it seems that
islands are peculiarly fitted for these animals; for frogs have been
introduced into Madeira, the Azores, and Mauritius, and have multiplied so
as to become a nuisance. But as these animals and their spawn are
immediately killed (with the exception, as far as known, of one Indian
species) by sea-water, there would be great difficulty in their transportal
across the sea, and therefore we can see why they do not exist on strictly
oceanic islands. But why, on the theory of creation, they should not have
been created there, it would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched the
oldest voyages, and have not found a single instance, free from doubt, of a
terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like
fox, come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank in connection with the mainland at a distance
of about 280 miles; moreover, icebergs formerly brought boulders to its
western shores, and they may have formerly transported foxes, as now
frequently happens in the arctic regions. Yet it cannot be said that small
islands will not support at least small mammals, for they occur in many
parts of the world on very small islands, when lying close to a continent;
and hardly an island can be named on which our smaller quadrupeds have not
become naturalised 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
belonging 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
Archipelago, the Bonin Islands, the Caroline 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
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 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 Archipelago,
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 relation 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
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
immigrated more uniformly, and from the species having entered in a body,
their mutual relations would not have been much disturbed, and
consequently, 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 understanding
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
overlooked. I will specify one difficult case. 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
striking instances of which 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 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 find that
several species in this state withstand uninjured an immersion in sea-water
during seven days. One shell, the Helix pomatia, after having been thus
treated, and again hybernating, 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 country 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 membranous 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 Aucapitaine 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
actually the same. Numerous instances could be given. The Galapagos
Archipelago, situated under the equator, lies at a 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 commonly 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 admirable
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 Archipelagos: 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 Galapagos 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 nearest
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
remote, that the fact becomes an anomaly. But this difficulty partially
disappears on the view that New Zealand, South America, and the other
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 commencement
of the last 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; 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 inhabitants
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
near 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,
in 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
considering 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 Galapagos Archipelago, and are likewise found in other parts of
the world, we find that they differ considerably in the several islands.
This difference might indeed have been expected if the islands have 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, 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
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 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 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. Nevertheless, 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 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 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, naturalised 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 remember that the species which
become naturalised 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 Candolle, 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 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 species
which inhabit the several islands of the Galapagos Archipelago 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
continuous land, yet they are inhabited by a vast number of distinct
mammals, 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 inhabitants
of oceanic islands, namely, the relation to the source whence colonists
could have been most easily derived, together with their subsequent
modification, 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 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 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 likewise 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 showing us the steps in the process 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
inhabitants 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 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 genus,
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
geological 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 organised 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 themselves
ranging widely--such facts, as alpine, lacustrine, and marsh productions
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 of the same archipelago--are
inexplicable on the ordinary view of the independent creation of each
species, but are explicable if we admit colonisation 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 endeavoured to show that if we make due allowance
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
transport--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
insuperable in believing that all the individuals of the same species,
wherever found, are descended from common parents. And we are led to this
conclusion, which has been arrived at by many naturalists under the
designation 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 allowances
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,
which affected even the equatorial regions, and which, during the
alternations of the cold in the north and the south, allowed the
productions of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world. As showing how
diversified are the means of occasional transport, I have discussed at some
little length the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in the long course
of time all the individuals of the same species, and likewise of the
several species belonging to the same genus, have proceeded from some one
source; then all the grand leading facts of geographical distribution are
explicable on the theory of migration, together with subsequent
modification and the multiplication of new forms. We can thus understand
the high importance of barriers, whether of land or water, in not only
separating but in apparently forming the several zoological and botanical
provinces. We can thus understand the concentration of related species
within the same areas; and how it is that under different latitudes, for
instance, in South America, the inhabitants of the plains and mountains, of
the forests, marshes, and deserts, are linked together in so mysterious a
manner, and are likewise linked to the extinct beings which formerly
inhabited the same continent. Bearing in mind that the mutual relation of
organism to organism is of the highest importance, we can see why two
areas, having nearly the same physical conditions, should often be
inhabited by very different forms of life; for according to the length of
time which has elapsed since the colonists entered one of the regions, or
both; according to the nature of the communication which allowed certain
forms and not others to enter, either in greater or lesser numbers;
according or not as those which entered happened to come into more or less
direct competition with each other and with the aborigines; and according
as the immigrants were capable of varying more or less rapidly, there would
ensue in the to or more regions, independently of their physical
conditions, infinitely diversified conditions of life; there would be an
almost endless amount of organic action and reaction, and we should find
some groups of beings greatly, and some only slightly modified; some
developed in great force, some existing in scanty numbers--and this we do
find in the several great geographical provinces of the world.
On these same principles we can understand, as I have endeavoured to show,
why oceanic islands should have few inhabitants, but that of these, a large
proportion should be endemic or peculiar; and why, in relation to the means
of migration, one group of beings should have all its species peculiar, and
another group, even within the same class, should have all its species the
same with those in an adjoining quarter of the world. We can see why whole
groups of organisms, as batrachians and terrestrial mammals, should be
absent from oceanic islands, whilst the most isolated islands should
possess their own peculiar species of aerial mammals or bats. We can see
why, in islands, there should be some relation between the presence of
mammals, in a more or less modified condition, and the depth of the sea
between such islands and the mainland. We can clearly see why all the
inhabitants of an archipelago, though specifically distinct on the several
islets, should be closely related to each other, and should likewise be
related, but less closely, to those of the nearest continent, or other
source whence immigrants might have been derived. We can see why, if there
exist very closely allied or representative species in two areas, however
distant from each other, some identical species will almost always there be
found.
As the late Edward Forbes often insisted, there is a striking parallelism
in the laws of life throughout time and space; the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see
this in many facts. The endurance of each species and group of species is
continuous in time; for the apparent exceptions to the rule are so few that
they may fairly be attributed to our not having as yet discovered in an
intermediate deposit certain forms which are absent in it, but which occur
above and below: so in space, it certainly is the general rule that the
area inhabited by a single species, or by a group of species, is
continuous, and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by former migrations under different
circumstances, or through occasional means of transport, or by the species
having become extinct in the intermediate tracts. Both in time and space
species and groups of species have their points of maximum development.
Groups of species, living during the same period of time, or living within
the same area, are often characterised by trifling features in common, as
of sculpture or colour. In looking to the long succession of past ages, as
in looking to distant provinces throughout the world, we find that species
in certain classes differ little from each other, whilst those in another
class, or only in a different section of the same order, differ greatly
from each other. In both time and space the lowly organised members of
each class generally change less than the highly organised; but there are
in both cases marked exceptions to the rule. According to our theory,
these several relations throughout time and space are intelligible; for
whether we look to the allied forms of life which have changed during
successive ages, or to those which have changed after having migrated into
distant quarters, in both cases they are connected by the same bond of
ordinary generation; in both cases the laws of variation have been the
same, and modifications have been accumulated by the same means of natural
selection.
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