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, &c., 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. Günther 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. Günther 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 larvæ, 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, aërial 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 aërial bats, the singular
proportions of certain orders of plants, herbaceous forms having been
developed into trees, &c., seem to me to accord better with the 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, &c., 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 Felidæ and
Canidæ. 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
aërial 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.