Chapter XII
Geographical Distribution
(Continued)
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, &c., and at the dissimilarity of the surrounding terrestrial
beings, compared with those of Britain.
2 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.
3 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.
4 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.
5 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.
6 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.
7 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.
8 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.
9 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
larvae, 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.
10 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.
11 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.
12 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.
13 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,
aerial mammals do occur on almost every island. New Zealand possesses two bats
found nowhere else in the world: Norfolk Island, the Viti Archipelago, the
Bonin Islands, the Caroline and Marianne Archipelagoes, and Mauritius, all
possess their peculiar bats. Why, it may be asked, has the supposed creative
force produced bats and no other mammals on remote islands? On my view this
question can easily be answered; for no terrestrial mammal can be transported
across a wide space of sea, but bats can fly across. Bats have been seen
wandering by day far over the Atlantic Ocean; and two North American species
either regularly or occasionally visit Bermuda, at the distance of 600 miles
from the mainland. I hear from Mr. Tomes, who has specially studied this
family, that many 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.
14 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.
15 All the foregoing remarks on the inhabitants of
oceanic islands,--namely, the scarcity of kinds--the richness in endemic forms
in particular classes or sections of classes,--the absence of whole groups, as
of batrachians, and of terrestrial mammals notwithstanding the presence of
aerial bats,--the singular proportions of certain orders of
plants,--herbaceous forms having been developed into trees, &c.,--seem to
me to accord better with the view of occasional means of transport having been
largely efficient in the long course of time, than with the view of all our
oceanic islands having been formerly connected by continuous land with the
nearest continent; for on this latter view the migration would probably have
been more complete; and if modification be admitted, all the forms of life
would have been more equally modified, in accordance with the paramount
importance of the relation of organism to organism.
16 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.
17 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.
18 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.
19 The law which causes the inhabitants of an
archipelago, though specifically distinct, to be closely allied to those of
the nearest continent, we sometimes see displayed on a small scale, yet in a
most interesting manner, within the limits of the same archipelago. Thus the
several islands of the Galapagos Archipelago are tenanted, as I have elsewhere
shown, in a quite marvellous manner, by very closely related species; so that
the inhabitants of each separate island, though mostly distinct, are related
in an incomparably closer degree to each other than to the inhabitants of any
other part of the world. And this is just what might have been expected on my
view, for the islands are situated so near each other that they would almost
certainly receive immigrants from the same original source, or from each
other. But this dissimilarity between the endemic inhabitants of the islands
may be used as an argument against my views; for it may be asked, how has it
happened in the several islands situated within sight of each other, having
the same geological nature, the same height, climate, &c., that many of
the immigrants should have been differently modified, though only in a small
degree. This long appeared to me a great difficulty: but it arises in chief
part from the deeply-seated error of considering the physical conditions of a
country as the most important for its inhabitants; whereas it cannot, I think,
be disputed that the nature of the other inhabitants, with which each has to
compete, is at least as important, and generally a far more important element
of success. Now if we look to those inhabitants of the Galapagos Archipelago
which are found in other parts of the world (laying on one side for the moment
the endemic species, which cannot be here fairly included, as we are
considering how they have come to be modified since their arrival), we find a
considerable amount of difference in the several islands. This difference
might indeed have been expected on the view of the islands having been stocked
by occasional means of transport--a seed, for instance, of one plant having
been brought to one island, and that of another plant to another island. Hence
when in former times an immigrant settled on any one or more of the islands,
or when it subsequently spread from one island to another, it would
undoubtedly be exposed to different conditions of life in the different
islands, for it would have to compete with different sets of organisms: a
plant, for instance, would find the best-fitted ground more perfectly occupied
by distinct plants in one island than in another, and it would be exposed to
the attacks of somewhat different enemies. If then it varied, natural
selection would probably favour different varieties in the different islands.
Some species, however, might spread and yet retain the same character
throughout the group, just as we see on continents some species spreading
widely and remaining the same.
20 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.
21 The principle which determines the general character
of the fauna and flora of oceanic islands, namely, that the inhabitants, when
not identically the same, yet are plainly related to the inhabitants of that
region whence colonists could most readily have been derived,--the colonists
having been subsequently modified and better fitted to their new homes,--is of
the widest application throughout nature. We see this on every mountain, in
every lake and marsh. For Alpine species, excepting in so far as the same
forms, chiefly of plants, have spread widely throughout the world during the
recent Glacial epoch, are related to those of the surrounding lowlands;--thus
we have in South America, Alpine humming-birds, Alpine rodents, Alpine plants,
&c., all of strictly American forms, and it is obvious that a mountain, as
it became slowly upheaved, would naturally be colonised from the surrounding
lowlands. So it is with the inhabitants of lakes and marshes, excepting in so
far as great facility of transport has given the same general forms to the
whole world. We see this same principle in the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given. And it
will, I believe, be universally found to be true, that wherever in two
regions, let them be ever so distant, many closely allied or representative
species occur, there will likewise be found some identical species, showing,
in accordance with the foregoing view, that at some former period there has
been intercommunication or migration between the two regions. And wherever
many closely-allied species occur, there will be found many forms which some
naturalists rank as distinct species, and some as varieties; these doubtful
forms showing us the steps in the process of modification.
22 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 Felidae and Canidae. 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.
23 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.
24 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.
25 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.
26 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.
27 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.
28 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.
29 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 aerial 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.
30 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.