On the Origin of Species by Means of Natural Selection
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Chapter XII
Geographical Distribution--continued
Distribution of fresh-water productions -- On the inhabitants of oceanic
islands -- Absence of Batrachians and of terrestrial Mammals -- On the
relation of the inhabitants of islands to those of the nearest mainland --
On colonisation from the nearest source with subsequent modification --
Summary of the last and present chapters.
As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more impassable barrier, that they never would have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to quite different classes, an enormous
range, but allied species prevail in a remarkable manner throughout the
world. I well remember, when first collecting in the fresh waters of
Brazil, feeling much surprise at the similarity of the fresh-water insects,
shells, &c., and at the dissimilarity of the surrounding terrestrial
beings, compared with those of Britain.
But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having become
fitted, in a manner highly useful to them, for short and frequent
migrations from pond to pond, or from stream to stream; and liability to
wide dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases. In regard to fish, I
believe that the same species never occur in the fresh waters of distant
continents. But on the same continent the species often range widely and
almost capriciously; for two river-systems will have some fish in common
and some different. A few facts seem to favour the possibility of their
occasional transport by accidental means; like that of the live fish not
rarely dropped by whirlwinds in India, and the vitality of their ova when
removed from the water. But I am inclined to attribute the dispersal of
fresh-water fish mainly to slight changes within the recent period in the
level of the land, having caused rivers to flow into each other.
Instances, also, could be given of this having occurred during floods,
without any change of level. We have evidence in the loess of the Rhine of
considerable changes of level in the land within a very recent geological
period, and when the surface was peopled by existing land and fresh-water
shells. The wide difference of the fish on opposite sides of continuous
mountain-ranges, which from an early period must have parted river-systems
and completely prevented their inosculation, seems to lead to this same
conclusion. With respect to allied fresh-water fish occurring at very
distant points of the world, no doubt there are many cases which cannot at
present be explained: but some fresh-water fish belong to very ancient
forms, and in such cases there will have been ample time for great
geographical changes, and consequently time and means for much migration.
In the second place, salt-water fish can with care be slowly accustomed to
live in fresh water; and, according to Valenciennes, there is hardly a
single group of fishes confined exclusively to fresh water, so that we may
imagine that a marine member of a fresh-water group might travel far along
the shores of the sea, and subsequently become modified and adapted to the
fresh waters of a distant land.
Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and must
have proceeded from a single source, prevail throughout the world. Their
distribution at first perplexed me much, as their ova are not likely to be
transported by birds, and they are immediately killed by sea water, as are
the adults. I could not even understand how some naturalised species have
rapidly spread throughout the same country. But two facts, which I have
observed--and no doubt many others remain to be observed--throw some light
on this subject. When a duck suddenly emerges from a pond covered with
duck-weed, I have twice seen these little plants adhering to its back; and
it has happened to me, in removing a little duck-weed from one aquarium to
another, that I have quite unintentionally stocked the one with fresh-water
shells from the other. But another agency is perhaps more effectual: I
suspended a duck's feet, which might represent those of a bird sleeping in
a natural pond, in an aquarium, where many ova of fresh-water shells were
hatching; and I found that numbers of the extremely minute and just hatched
shells crawled on the feet, and clung to them so firmly that when taken out
of the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily drop off. These just hatched molluscs,
though aquatic in their nature, survived on the duck's feet, in damp air,
from twelve to twenty hours; and in this length of time a duck or heron
might fly at least six or seven hundred miles, and would be sure to alight
on a pool or rivulet, if blown across sea to an oceanic island or to any
other distant point. Sir Charles Lyell also informs me that a Dyticus has
been caught with an Ancylus (a fresh-water shell like a limpet) firmly
adhering to it; and a water-beetle of the same family, a Colymbetes, once
flew on board the 'Beagle,' when forty-five miles distant from the nearest
land: how much farther it might have flown with a favouring gale no one
can tell.
With respect to plants, it has long been known what enormous ranges many
fresh-water and even marsh-species have, both over continents and to the
most remote oceanic islands. This is strikingly shown, as remarked by
Alph. de Candolle, in large groups of terrestrial plants, which have only a
very few aquatic members; for these latter seem immediately to acquire, as
if in consequence, a very wide range. I think favourable means of
dispersal explain this fact. I have before mentioned that earth
occasionally, though rarely, adheres in some quantity to the feet and beaks
of birds. Wading birds, which frequent the muddy edges of ponds, if
suddenly flushed, would be the most likely to have muddy feet. Birds of
this order I can show are the greatest wanderers, and are occasionally
found on the most remote and barren islands in the open ocean; they would
not be likely to alight on the surface of the sea, so that the dirt would
not be washed off their feet; when making land, they would be sure to fly
to their natural fresh-water haunts. I do not believe that botanists are
aware how charged the mud of ponds is with seeds: I have tried several
little experiments, but will here give only the most striking case: I took
in February three table-spoonfuls of mud from three different points,
beneath water, on the edge of a little pond; this mud when dry weighed only
6 3/4 ounces; I kept it covered up in my study for six months, pulling up
and counting each plant as it grew; the plants were of many kinds, and were
altogether 537 in number; and yet the viscid mud was all contained in a
breakfast cup! Considering these facts, I think it would be an
inexplicable circumstance if water-birds did not transport the seeds of
fresh-water plants to vast distances, and if consequently the range of
these plants was not very great. The same agency may have come into play
with the eggs of some of the smaller fresh-water animals.
Other and unknown agencies probably have also played a part. I have stated
that fresh-water fish eat some kinds of seeds, though they reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish; they
then take flight and go to other waters, or are blown across the sea; and
we have seen that seeds retain their power of germination, when rejected in
pellets or in excrement, many hours afterwards. When I saw the great size
of the seeds of that fine water-lily, the Nelumbium, and remembered Alph.
de Candolle's remarks on this plant, I thought that its distribution must
remain quite inexplicable; but Audubon states that he found the seeds of
the great southern water-lily (probably, according to Dr. Hooker, the
Nelumbium luteum) in a heron's stomach; although I do not know the fact,
yet analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach a
pellet containing the seeds of the Nelumbium undigested; or the seeds might
be dropped by the bird whilst feeding its young, in the same way as fish
are known sometimes to be dropped.
In considering these several means of distribution, it should be remembered
that when a pond or stream is first formed, for instance, on a rising
islet, it will be unoccupied; and a single seed or egg will have a good
chance of succeeding. Although there will always be a struggle for life
between the individuals of the species, however few, already occupying any
pond, yet as the number of kinds is small, compared with those on the land,
the competition will probably be less severe between aquatic than between
terrestrial species; consequently an intruder from the waters of a foreign
country, would have a better chance of seizing on a place, than in the case
of terrestrial colonists. We should, also, remember that some, perhaps
many, fresh-water productions are low in the scale of nature, and that we
have reason to believe that such low beings change or become modified less
quickly than the high; and this will give longer time than the average for
the migration of the same aquatic species. We should not forget the
probability of many species having formerly ranged as continuously as
fresh-water productions ever can range, over immense areas, and having
subsequently become extinct in intermediate regions. But the wide
distribution of fresh-water plants and of the lower animals, whether
retaining the same identical form or in some degree modified, I believe
mainly depends on the wide dispersal of their seeds and eggs by animals,
more especially by fresh-water birds, which have large powers of flight,
and naturally travel from one to another and often distant piece of water.
Nature, like a careful gardener, thus takes her seeds from a bed of a
particular nature, and drops them in another equally well fitted for them.
On the Inhabitants of Oceanic Islands. -- We now come to the last of the
three classes of facts, which I have selected as presenting the greatest
amount of difficulty, on the view that all the individuals both of the same
and of allied species have descended from a single parent; and therefore
have all proceeded from a common birthplace, notwithstanding that in the
course of time they have come to inhabit distant points of the globe. I
have already stated that I cannot honestly admit Forbes's view on
continental extensions, which, if legitimately followed out, would lead to
the belief that within the recent period all existing islands have been
nearly or quite joined to some continent. This view would remove many
difficulties, but it would not, I think, explain all the facts in regard to
insular productions. In the following remarks I shall not confine myself
to the mere question of dispersal; but shall consider some other facts,
which bear on the truth of the two theories of independent creation and of
descent with modification.
The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. If we look to the large size
and varied stations of New Zealand, extending over 780 miles of latitude,
and compare its flowering plants, only 750 in number, with those on an
equal area at the Cape of Good Hope or in Australia, we must, I think,
admit that something quite independently of any difference in physical
conditions has caused so great a difference in number. Even the uniform
county of Cambridge has 847 plants, and the little island of Anglesea 764,
but a few ferns and a few introduced plants are included in these numbers,
and the comparison in some other respects is not quite fair. We have
evidence that the barren island of Ascension aboriginally possessed under
half-a-dozen flowering plants; yet many have become naturalised on it, as
they have on New Zealand and on every other oceanic island which can be
named. In St. Helena there is reason to believe that the naturalised
plants and animals have nearly or quite exterminated many native
productions. He who admits the doctrine of the creation of each separate
species, will have to admit, that a sufficient number of the best adapted
plants and animals have not been created on oceanic islands; for man has
unintentionally stocked them from various sources far more fully and
perfectly than has nature.
Although in oceanic islands the number of kinds of inhabitants is scanty,
the proportion of endemic species (i.e. those found nowhere else in the
world) is often extremely large. If we compare, for instance, the number
of the endemic land-shells in Madeira, or of the endemic birds in the
Galapagos Archipelago, with the number found on any continent, and then
compare the area of the islands with that of the continent, we shall see
that this is true. This fact might have been expected on my theory, for,
as already explained, species occasionally arriving after long intervals in
a new and isolated district, and having to compete with new associates,
will be eminently liable to modification, and will often produce groups of
modified descendants. But it by no means follows, that, because in an
island nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend on the species which do not become modified
having immigrated with facility and in a body, so that their mutual
relations have not been much disturbed. Thus in the Galapagos Islands
nearly every land-bird, but only two out of the eleven marine birds, are
peculiar; and it is obvious that marine birds could arrive at these islands
more easily than land-birds. Bermuda, on the other hand, which lies at
about the same distance from North America as the Galapagos Islands do from
South America, and which has a very peculiar soil, does not possess one
endemic land bird; and we know from Mr. J. M. Jones's admirable account of
Bermuda, that very many North American birds, during their great annual
migrations, visit either periodically or occasionally this island. Madeira
does not possess one peculiar bird, and many European and African birds are
almost every year blown there, as I am informed by Mr. E. V. Harcourt. So
that these two islands of Bermuda and Madeira have been stocked by birds,
which for long ages have struggled together in their former homes, and have
become mutually adapted to each other; and when settled in their new homes,
each kind will have been kept by the others to their proper places and
habits, and will consequently have been little liable to modification.
Madeira, again, is inhabited by a wonderful number of peculiar land-shells,
whereas not one species of sea-shell is confined to its shores: now,
though we do not know how seashells are dispersed, yet we can see that
their eggs or 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.
Oceanic islands are sometimes deficient in certain classes, and their
places are apparently occupied by the other inhabitants; in the Galapagos
Islands reptiles, and in New Zealand gigantic wingless birds, take the
place of mammals. In the plants of the Galapagos Islands, Dr. Hooker has
shown that the proportional numbers of the different orders are very
different from what they are elsewhere. Such cases are generally accounted
for by the physical conditions of the islands; but this explanation seems
to me not a little doubtful. Facility of immigration, I believe, has been
at least as important as the nature of the conditions.
Many remarkable little facts could be given with respect to the inhabitants
of remote islands. For instance, in certain islands not tenanted by
mammals, some of the endemic plants have beautifully hooked seeds; yet few
relations are more striking than the adaptation of hooked seeds for
transportal by the wool and fur of quadrupeds. This case presents no
difficulty on my view, for a hooked seed might be transported to an island
by some other means; and the plant then becoming slightly modified, but
still retaining its hooked seeds, would form an endemic species, having as
useless an appendage as any rudimentary organ,--for instance, as the
shrivelled wings under the soldered elytra of many insular beetles. Again,
islands often possess trees or bushes belonging to orders which elsewhere
include only herbaceous species; now trees, as Alph. de Candolle has shown,
generally have, whatever the cause may be, confined ranges. Hence trees
would be little likely to reach distant oceanic islands; and an herbaceous
plant, though it would have no chance of successfully competing in stature
with a fully developed tree, when established on an island and having to
compete with herbaceous plants alone, might readily gain an advantage by
growing taller and taller and overtopping the other plants. If so, natural
selection would often tend to add to the stature of herbaceous plants when
growing on an island, to whatever order they belonged, and thus convert
them first into bushes and ultimately into trees.
With respect to the absence of whole orders on oceanic islands, Bory St.
Vincent long ago remarked that Batrachians (frogs, toads, newts) have never
been found on any of the many islands with which the great oceans are
studded. I have taken pains to verify this assertion, and I have found it
strictly true. I have, however, been assured that a frog exists on the
mountains of the great island of New Zealand; but I suspect that this
exception (if the information be correct) may be explained through glacial
agency. This general absence of frogs, toads, and newts on so many oceanic
islands cannot be accounted for by their physical conditions; indeed it
seems that islands are peculiarly well fitted for these animals; for frogs
have been introduced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals and their
spawn are known to be immediately killed by sea-water, on my view we can
see that there would be great difficulty in their transportal across the
sea, and therefore why they do not exist on any oceanic island. But why,
on the theory of creation, they should not have been created there, it
would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched the
oldest voyages, but have not finished my search; as yet I have not found a
single instance, free from doubt, of a terrestrial mammal (excluding
domesticated animals kept by the natives) inhabiting an island situated
above 300 miles from a continent or great continental island; and many
islands situated at a much less distance are equally barren. The Falkland
Islands, which are inhabited by a wolf-like fox, come nearest to an
exception; but this group cannot be considered as oceanic, as it lies on a
bank connected with the mainland; moreover, icebergs formerly brought
boulders to its western shores, and they may have formerly transported
foxes, as so frequently now happens in the arctic regions. Yet it cannot
be said that small islands will not support small mammals, for they occur
in many parts of the world on very small islands, if close to a continent;
and hardly an island can be named on which our smaller quadrupeds have not
become naturalised and greatly multiplied. It cannot be said, on the
ordinary view of creation, that there has not been time for the creation of
mammals; many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is thought that mammals
appear and disappear at a quicker rate than other and lower animals.
Though terrestrial mammals do not occur on oceanic islands, 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.
Besides the absence of terrestrial mammals in relation to the remoteness of
islands from continents, there is also a relation, to a certain extent
independent of distance, between the depth of the sea separating an island
from the neighbouring mainland, and the presence in both of the same
mammiferous species or of allied species in a more or less modified
condition. Mr. Windsor Earl has made some striking observations on this
head in regard to the great Malay Archipelago, which is traversed near
Celebes by a space of deep ocean; and this space separates two widely
distinct mammalian faunas. On either side the islands are situated on
moderately deep submarine banks, and they are inhabited by closely allied
or identical quadrupeds. No doubt some few anomalies occur in this great
archipelago, and there is much difficulty in forming a judgment in some
cases owing to the probable naturalisation of certain mammals through man's
agency; but we shall soon have much light thrown on the natural history of
this archipelago by the admirable zeal and researches of Mr. Wallace. I
have not as yet had time to follow up this subject in all other quarters of
the world; but as far as I have gone, the relation generally holds good.
We see Britain separated by a shallow channel from Europe, and the mammals
are the same on both sides; we meet with analogous facts on many islands
separated by similar channels from Australia. The West Indian Islands
stand on a deeply submerged bank, nearly 1000 fathoms in depth, and here we
find American forms, but the species and even the genera are distinct. As
the amount of modification in all cases depends to a certain degree on the
lapse of time, and as during changes of level it is obvious that islands
separated by shallow channels are more likely to have been continuously
united within a recent period to the mainland than islands separated by
deeper channels, we can understand the frequent relation between the depth
of the sea and the degree of affinity of the mammalian inhabitants of
islands with those of a neighbouring continent,--an inexplicable relation
on the view of independent acts of creation.
All the foregoing remarks on the inhabitants of oceanic islands,--namely,
the scarcity of kinds--the richness in endemic forms in particular classes
or sections of classes,--the absence of whole groups, as of batrachians,
and of terrestrial mammals notwithstanding the presence of 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.
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