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.
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 impassable barrier, that they never would have
extended to distant countries. But the case is exactly the reverse. Not
only have many fresh-water species, belonging to quite different
classes, an enormous range, but allied species prevail in a remarkable
manner throughout the world. I well remember, when first collecting in
the fresh waters of Brazil, 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 this power in fresh-water productions of ranging widely, though so
unexpected, 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; and
liability to wide dispersal would follow from this capacity as an
almost necessary consequence. We can here consider only a few cases. In
regard to
fish, I believe that the same species never occur in the fresh waters
of distant continents. But on the same continent the species often
range widely and almost capriciously; for two river-systems will have
some fish in common and some different. A few facts seem to favour the
possibility of their occasional transport by accidental means; like
that of the live fish not rarely dropped by whirlwinds in India, and
the vitality of their ova when removed from the water. But I am
inclined to attribute the dispersal of fresh-water fish mainly to
slight changes within the recent period in the level of the land,
having caused rivers to flow into each other. Instances, also, could be
given of this having occurred during floods, without any change of
level. We have evidence in the loess of the Rhine of considerable
changes of level in the land within a very recent geological period,
and when the surface was peopled by existing land and fresh-water
shells. The wide difference of the fish on opposite sides of continuous
mountain-ranges, which from an early period must have parted
river-systems and completely prevented their inosculation, seems to
lead to this same conclusion. With respect to allied fresh-water fish
occurring at very distant points of the world, no doubt there are many
cases which cannot at present be explained: but 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. In the second place, 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 fishes confined
exclusively to fresh water, so that we may imagine that a marine member
of a fresh-water group might travel far along the shores of the sea,
and subsequently
become modified and adapted to the fresh waters of a distant land.
Some species of fresh-water shells have a very wide range, and allied
species, which, on my 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 they are immediately killed by
sea water, as are the adults. I could not even understand how some
naturalised species have rapidly spread throughout the same country.
But two facts, which I have observed—and no doubt many others remain to
be observed—throw some light on this subject. When a duck suddenly
emerges from a pond covered with duck-weed, I have twice seen these
little plants adhering to its back; and it has happened to me, in
removing a little duck-weed from one aquarium to another, that I have
quite unintentionally stocked the one with fresh-water shells from the
other. But another agency is perhaps more effectual: I suspended a
duck’s feet, which might represent those of a bird sleeping in a
natural pond, 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 would be sure to alight on a pool or rivulet, if blown
across sea to an oceanic island or to any other distant point. Sir
Charles Lyell also
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 flown with 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 shown, as
remarked by Alph. de Candolle, in large groups of terrestrial plants,
which have only a very few aquatic members; for these latter seem
immediately to acquire, as if in consequence, a very wide range. I
think favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally, though rarely, 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 I can show are the greatest
wanderers, and are occasionally found on the most remote and barren
islands in the open ocean; they would not be likely to alight on the
surface of the sea, so that the dirt would not be washed off their
feet; when making 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 table-spoonfuls of mud from three different points,
beneath water, on the edge of a little pond; this mud when dry weighed
only 6 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 vast distances, and if
consequently the range of these plants was not very great. 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 in pellets or in excrement, many
hours afterwards. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle’s remarks
on this plant, I thought that its distribution must remain quite
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; although I do not know the fact, yet
analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach a
pellet containing the seeds of the Nelumbium undigested; or the seeds
might be dropped by the bird whilst feeding its young, in the same way
as fish are known sometimes to be dropped.
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 individuals of the species,
however few, already occupying any pond, yet as the number of kinds is
small, compared with those on the land, the competition will probably
be less severe between aquatic than between terrestrial species;
consequently an intruder from the waters of a foreign country, would
have a better chance of seizing on a place, than in the case of
terrestrial colonists. We should, also, remember that some, perhaps
many, fresh-water productions are low in the scale of nature, and that
we have reason to believe that such low beings change or become
modified less quickly than the high; and this will give longer time
than the average for the migration of the same aquatic species. We
should not forget the probability of many species having formerly
ranged as continuously as fresh-water productions ever can range, over
immense areas, and having subsequently become extinct in intermediate
regions. But the wide distribution of fresh-water plants and of the
lower animals, whether retaining the same identical form or in some
degree modified, I believe mainly depends on the wide dispersal of
their seeds and eggs by animals, more especially by fresh-water birds,
which have large powers of flight, and naturally travel from one to
another and often distant piece of water. Nature, like a careful
gardener, thus takes her seeds from a bed of a particular nature, and
drops them in another equally well fitted for them.
_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, on the
view that all the individuals both of the same and of allied species
have descended from a single parent; and therefore have all proceeded
from a common birthplace, notwithstanding that in the course of time
they have come to inhabit distant points of the globe. I have already
stated that I cannot honestly admit Forbes’s view on continental
extensions, which, if legitimately followed out, would lead to the
belief that within the recent period all existing islands have been
nearly or quite joined to some continent. This view would remove many
difficulties, but it would not, I think, explain all the facts in
regard to insular productions. In the following remarks I shall not
confine myself to the mere question of dispersal; but shall consider
some other facts, which bear 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. If we look
to the large size and varied stations of New Zealand, extending over
780 miles of latitude, and compare its flowering plants, only 750 in
number, with those on an equal area at the Cape of Good Hope or in
Australia, we must, I think, admit that something quite independently
of any difference in physical conditions has caused 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 under half-a-dozen
flowering
plants; yet many have become naturalised on it, as they have on 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 have not been created on oceanic islands; for man has
unintentionally stocked them from various sources far more fully and
perfectly than has nature.
Although in oceanic islands the number of kinds of inhabitants is
scanty, the proportion of endemic species (_i.e._ those found nowhere
else in the world) is often extremely large. If we compare, for
instance, the number of the endemic land-shells in Madeira, or of the
endemic birds in the Galapagos Archipelago, with the number found on
any continent, and then compare the area of the islands with that of
the continent, we shall see that this is true. This fact might have
been expected on my theory, for, as already explained, species
occasionally arriving after long intervals in a new and isolated
district, and having to compete with new associates, will be eminently
liable to modification, and will 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 on the species which do not become modified
having immigrated with facility and in a body, so that their mutual
relations have not been much disturbed. Thus in the Galapagos Islands
nearly every land-bird, but only two out of the eleven marine birds,
are peculiar; and it is obvious that
marine birds could arrive at these islands more easily 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 one endemic land bird; and
we know from Mr. J. M. Jones’s admirable account of Bermuda, that very
many North American birds, during their great annual migrations, visit
either periodically or occasionally this island. Madeira does not
possess one peculiar bird, and many European and African birds are
almost every year blown there, as I am informed by Mr. E. V. Harcourt.
So that these two islands of Bermuda and Madeira have been stocked by
birds, which for long ages have struggled together in their former
homes, and have become mutually adapted to each other; and when settled
in their new homes, each kind will have been kept by the others to
their proper places and habits, and will consequently have been little
liable to modification. Madeira, again, is inhabited by a wonderful
number of peculiar land-shells, whereas not one species of sea-shell is
confined to its shores: now, though we do not know how seashells are
dispersed, yet we can see that their eggs or larvæ, perhaps attached to
seaweed or floating timber, or to the feet of wading-birds, might be
transported far more easily than land-shells, across three or four
hundred miles of open sea. The different orders of insects in Madeira
apparently present analogous facts.
Oceanic islands are sometimes deficient in certain classes, and their
places are apparently occupied by the other inhabitants; in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take the place of mammals. In the plants of the Galapagos Islands, Dr.
Hooker has shown that the proportional numbers of the different orders
are very different from
what they are elsewhere. Such cases are generally accounted for by the
physical conditions of the islands; but this explanation seems to me
not a little doubtful. Facility of immigration, I believe, has been at
least as important as the nature of the conditions.
Many remarkable little facts could be given with respect to the
inhabitants of remote islands. For instance, in certain islands not
tenanted by mammals, some of the endemic plants have beautifully hooked
seeds; yet few relations are more striking than the adaptation of
hooked seeds for transportal by the wool and fur of quadrupeds. This
case presents no difficulty on my view, for a hooked seed might be
transported to an island by some other means; and the plant then
becoming slightly modified, but still retaining its hooked seeds, would
form an endemic species, having as useless an appendage as any
rudimentary organ,—for instance, as the shrivelled wings under the
soldered elytra 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, though it would have no chance of successfully
competing in stature with a fully developed tree, when established on
an island and having to compete with herbaceous plants alone, might
readily gain an advantage by growing taller and taller and overtopping
the other plants. If so, natural selection would often tend to add to
the stature of herbaceous plants when growing on an island, to whatever
order they belonged, and thus convert them first into bushes and
ultimately into trees.
With respect to the absence of whole orders on
oceanic islands, Bory St. Vincent long ago remarked that Batrachians
(frogs, toads, newts) have never been found on any of the many islands
with which the great oceans are studded. I have taken pains to verify
this assertion, and I have found it strictly true. I have, however,
been assured that a frog exists on the mountains of the great island of
New Zealand; but I suspect that this exception (if the information be
correct) may be explained through glacial agency. This general absence
of frogs, toads, and newts on so many oceanic islands cannot be
accounted for by their physical conditions; indeed it seems that
islands are peculiarly well 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 known to be immediately killed by sea-water, on my view we
can see that there would be great difficulty in their transportal
across the sea, and therefore why they do not exist on any oceanic
island. 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, but have not finished my search; as yet I 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 connected with the mainland; moreover,
icebergs formerly brought boulders to its western shores, and they may
have formerly transported foxes, as so frequently now happens in the
arctic regions. Yet it cannot be said that small islands will not
support small mammals, for they occur in many parts of the world on
very small islands, if 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 thought that
mammals appear and disappear at a quicker rate than other and lower
animals. Though terrestrial mammals do not occur on oceanic islands,
ærial 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 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 ærial 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 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,
etc., 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
naturalised 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, etc., 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.
This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world of
other species allied to it, 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 it would be
difficult to prove it. Amongst mammals, we see it strikingly displayed
in Bats, and in a lesser degree in the Felidæ and Canidæ. We see it, if
we compare the distribution of butterflies and beetles. So it is with
most fresh-water productions, in which so many genera range over the
world, and many individual species have enormous ranges. It is not
meant that in world-ranging
genera all the species have a wide range, or even that they have on an
_average_ a wide range; but only that some of the species range very
widely; for the facility with which widely-ranging species vary and
give rise to new forms will largely determine their average range. For
instance, two varieties of the same species inhabit America and Europe,
and the species thus has an immense range; but, if the variation had
been a little greater, the two varieties would have been ranked as
distinct species, and the common range would have been greatly reduced.
Still less is it meant, that a species which apparently has 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 on
the view of all the species of a genus having descended from a single
parent, though now distributed to the most remote points of the world,
we ought to find, and I believe as a general rule we do find, that some
at least of the species range very widely; for it is necessary that the
unmodified parent should range widely, undergoing modification during
its diffusion, and should place itself under diverse conditions
favourable for the conversion of its offspring, firstly into new
varieties and ultimately into new species.
In considering the wide distribution of certain genera, we should bear
in mind that some are extremely ancient, and must have branched off
from a common parent at a remote epoch; so that in such cases there
will have been ample time for great climatal and geographical changes
and for accidents of transport; and consequently for the migration of
some of the species into all
quarters of the world, where they may have become slightly modified in
relation to their new conditions. There is, also, some reason to
believe from geological evidence that organisms low in the scale within
each great class, generally change at a slower rate than the higher
forms; and consequently the lower forms will have had a better chance
of ranging widely and of still retaining the same specific character.
This fact, together with the seeds and eggs of many low forms being
very minute and better fitted for distant transportation, probably
accounts for a law which has long been observed, and which has lately
been admirably discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms is, the more widely it is
apt to range.
The relations just discussed,—namely, low and slowly-changing organisms
ranging more widely than the high,—some of the species of
widely-ranging genera themselves ranging widely,—such facts, as alpine,
lacustrine, and marsh productions being related (with the exceptions
before specified) to those on the surrounding low lands and dry lands,
though these stations are so different—the very close relation of the
distinct species which inhabit the islets of the same archipelago,—and
especially the striking relation of the inhabitants of each whole
archipelago or island to those of the nearest mainland,—are, I think,
utterly inexplicable on the ordinary view of the independent creation
of each species, but are explicable on the view of colonisation from
the nearest and readiest source, together with the subsequent
modification and better adaptation of the colonists to their new homes.
_Summary of 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 all
the changes of climate and of the level of the land, which have
certainly occurred within the recent period, and of other similar
changes which may have occurred within the same period; if we remember
how profoundly ignorant we are with respect to the many and curious
means of occasional transport,—a subject which has hardly ever been
properly experimentised on; if we bear in mind how often a species may
have ranged continuously over a wide area, and then have become extinct
in the intermediate tracts, I think the difficulties in believing that
all the individuals of the same species, wherever located, have
descended from the same parents, are not insuperable. And we are led to
this conclusion, which has been arrived at by many naturalists under
the designation of single centres of creation, by some general
considerations, more especially from the importance of barriers and
from the analogical distribution of sub-genera, genera, and families.
With respect to the distinct species of the same genus, which on my
theory must have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life change most slowly, enormous periods of time being thus granted
for their migration, I do not think that the difficulties are
insuperable; though they often are in this case, and in that of the
individuals of the same species, extremely grave.
As exemplifying the effects of climatal changes on distribution, I have
attempted to show how important has been the influence of the modern
Glacial period, which I am fully convinced simultaneously affected the
whole world, or at least great meridional belts. 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 the individuals of the same species, and likewise of
allied species, have proceeded from some one source; then I think all
the grand leading facts of geographical distribution are explicable on
the theory of migration (generally of the more dominant forms of life),
together with subsequent modification and the multiplication of new
forms. We can thus understand the high importance of barriers, whether
of land or water, which separate our several zoological and botanical
provinces. We can thus understand the localisation of sub-genera,
genera, and families; 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 in so mysterious a manner
linked together by affinity, and are likewise linked to the extinct
beings which formerly inhabited the same continent. Bearing in mind
that the mutual relations of organism to organism are 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 new
inhabitants entered one region; 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 in 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 different
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, as we do find, some
groups of beings greatly, and some only slightly modified,—some
developed
in great force, some existing in scanty numbers—in the different 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 of these a
great number should be endemic or peculiar; and why, in relation to the
means of migration, one group of beings, even within the same class,
should have all its species endemic, and another group should have all
its species common to other quarters 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 possess
their own peculiar species of ærial mammals or bats. We can see why
there should be some relation between the presence of mammals, in a
more or less modified condition, and the depth of the sea between an
island 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 likewise be related, but
less closely, to those of the nearest continent or other source whence
immigrants were probably derived. We can see why in two areas, however
distant from each other, there should be a correlation, in the presence
of identical species, of varieties, of doubtful species, and of
distinct but representative species.
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 exceptions to the rule
are so few, that they may
fairly be attributed to our not having as yet discovered in an
intermediate deposit the forms which are therein absent, 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 migration at some former period
under different conditions or by occasional means of transport, and 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, belonging either to a certain
period of time, or to a certain area, are often characterised by
trifling characters in common, as of sculpture or colour. In looking to
the long succession of ages, as in now looking to distant provinces
throughout the world, we find that some organisms differ little, whilst
others belonging to a different class, or to a different order, or even
only to a different family of the same order, differ greatly. In both
time and space the lower members of each class generally change less
than the higher; but there are in both cases marked exceptions to the
rule. On my theory these several relations throughout time and space
are intelligible; for whether we look to the forms of life which have
changed during successive ages within the same quarter of the world, or
to those which have changed after having migrated into distant
quarters, in both cases the forms within each class have been connected
by the same bond of ordinary generation; and the more nearly any two
forms are related in blood, the nearer they will generally stand to
each other in time and space; in both cases the laws of variation have
been the same, and modifications have been accumulated by the same
power of natural selection.