 Chapter
12 - Geographical Distribution -- continued
*
Distribution of fresh-water productions *
On the inhabitants of oceanic islands *
Absence of Batrachians and of terrestrial Mammals *
On the relations 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, 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 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 explicable 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 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 view of occasional means of
transport having been largely efficient in the long course
of time, than with the view of all our oceanic islands having
been formerly connected by continuous land with the nearest
continent; for on this latter view the migration would probably
have been more complete; and if modification be admitted,
all the forms of life would have been more equally modified,
in accordance with the paramount importance of the relation
of organism to organism.
I do not deny that there are many and grave difficulties
in understanding how several of the inhabitants of the more
remote islands, whether still retaining the same specific
form or modified since their arrival, could have reached
their present homes. But the probability of many islands
having existed as halting-places, of which not a wreck now
remains, must not be overlooked. I will here give a single
instance of one of the cases of difficulty. Almost all oceanic
islands, even the most isolated and smallest, are inhabited
by land-shells, generally by endemic species, but sometimes
by species found elsewhere. Dr. Aug. A. Gould has given
several interesting cases in regard to the land-shells of
the islands of the Pacific. Now it is notorious that land-shells
are very easily killed by salt; their eggs, at least such
as I have tried, sink in sea-water and are killed by it.
Yet there must be, on my view, some unknown, but highly
efficient means for their transportal. Would the just-hatched
young occasionally crawl on and adhere to the feet of birds
roosting on the ground, and thus get transported? It occurred
to me that land-shells, when hybernating and having a membranous
diaphragm over the mouth of the shell, might be floated
in chinks of drifted timber across moderately wide arms
of the sea. And I found that several species did in this
state withstand uninjured an immersion in sea-water during
seven days: one of these shells was the Helix pomatia, and
after it had again hybernated I put it in sea-water for
twenty days, and it perfectly recovered. As this species
has a thick calcareous operculum, I removed it, and when
it had formed a new membranous one, I immersed it for fourteen
days in sea-water, and it recovered and crawled away: but
more experiments are wanted on this head.
The most striking and important fact for us in regard
to the inhabitants of islands, is their affinity to those
of the nearest mainland, without being actually the same
species. Numerous instances could be given of this fact.
I will give only one, that of the Galapagos Archipelago,
situated under the equator, between 500 and 600 miles from
the shores of South America. Here almost every product of
the land and water bears the unmistakeable stamp of the
American continent. There are twenty-six land birds, and
twenty-five of those 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 modifications;
the principle of inheritance still betraying their original
birthplace.
Many analogous facts could be given: indeed it is an almost
universal rule that the endemic productions of islands are
related to those of the nearest continent, or of other near
islands. The exceptions are few, and most of them can be
explained. Thus the plants of Kerguelen Land, though standing
nearer to Africa than to America, are related, and that
very closely, as we know from Dr. Hooker's account, to those
of America: but on the view that this island has been mainly
stocked by seeds brought with earth and stones on icebergs,
drifted by the prevailing currents, this anomaly disappears.
New Zealand in its endemic plants is much more closely related
to Australia, the nearest mainland, than to any other region:
and this is what might have been expected; but it is also
plainly related to South America, which, although the next
nearest continent, is so enormously remote, that the fact
becomes an anomaly. But this difficulty almost disappears
on the view that both New Zealand, South America, and other
southern lands were long ago partially stocked from a nearly
intermediate though distant point, namely from the antarctic
islands, when they were clothed with vegetation, before
the commencement of the Glacial period. The affinity, which,
though feeble, I am assured by Dr. Hooker is real, between
the flora of the south-western corner of Australia and of
the Cape of Good Hope, is a far more remarkable case, and
is at present inexplicable: but this affinity is confined
to the plants, and will, I do not doubt, be some day explained.
The law which causes the inhabitants of an archipelago,
though specifically distinct, to be closely allied to those
of the nearest continent, we sometimes see displayed on
a small scale, yet in a most interesting manner, within
the limits of the same archipelago. Thus the several islands
of the Galapagos Archipelago are tenanted, as I have elsewhere
shown, in a quite marvellous manner, by very closely related
species; so that the inhabitants of each separate island,
though mostly distinct, are related in an incomparably closer
degree to each other than to the inhabitants of any other
part of the world. And this is just what might have been
expected on my view, for the islands are situated so near
each other that they would almost certainly receive immigrants
from the same original source, or from each other. But this
dissimilarity between the endemic inhabitants of the islands
may be used as an argument against my views; for it may
be asked, how has it happened in the several islands situated
within sight of each other, having the same geological nature,
the same height, climate, &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 (having on one
side for the moment the endemic species, which cannot be
here fairly included, as we are considering how they have
come to be modified since their arrival), we find a considerable
amount of difference in the several islands. This difference
might indeed have been expected on the view of the islands
having been stocked by occasional means of transport a seed,
for instance, of one plant having been brought to one island,
and that of another plant to another island. Hence when
in former times an immigrant settled on any one or more
of the islands, or when it subsequently spread from one
island to another, it would undoubtedly be exposed to different
conditions of life in the different islands, for it would
have to compete with different sets of organisms: a plant,
for instance, would find the best-fitted ground more perfectly
occupied by distinct plants in one island than in another,
and it would be exposed to the attacks of somewhat different
enemies. If then it varied, natural selection would probably
favour different varieties in the different islands. Some
species, however, might spread and yet retain the same character
throughout the group, just as we see on continents some
species' spreading widely and remaining the same.
The really surprising fact in this case of the Galapagos
Archipelago, and in a lesser degree in some analogous instances,
is that the new species formed in the separate islands have
not quickly spread to the other islands. But the islands,
though in sight of each other, are separated by deep arms
of the sea, in most cases wider than the British Channel,
and there is no reason to suppose that they have at any
former period been continuously united. The currents of
the sea are rapid and sweep across the archipelago, and
gales of wind are extraordinarily rare; so that the islands
are far more effectually separated from each other than
they appear to be on a map. Nevertheless a good many species,
both those found in other parts of the world and those confined
to the archipelago, are common to the several islands, and
we may infer from certain facts that these have probably
spread from some one island to the others. But we often
take, I think, an erroneous view of the probability of closely
allied species invading each other's territory, when put
into free intercommunication. Undoubtedly if one species
has any advantage whatever over another, it will in a very
brief time wholly or in part supplant it; but if both are
equally well fitted for their own places in nature, both
probably will hold their own places and keep separate for
almost any length of time. Being familiar with the fact
that many species, naturalised through man's agency, have
spread with astonishing rapidity over new countries, we
are apt to infer that most species would thus spread; but
we should remember that the forms which become naturalised
in new countries are not generally closely allied to the
aboriginal inhabitants, but are very distinct species, belonging
in a large proportion of cases, as shown by Alph. de Candolle,
to distinct genera. In the Galapagos Archipelago, many even
of the birds, though so well adapted for flying from island
to island, are distinct on each; thus there are three closely-allied
species of mocking-thrush, each confined to its own island.
Now let us suppose the mocking-thrush of Chatham Island
to be blown to Charles Island, which has its own mocking-thrush:
why should it succeed in establishing itself there? We may
safely infer that Charles Island is well stocked with its
own species, for annually more eggs are laid there than
can possibly be reared; and we may infer that the mocking-thrush
peculiar to Charles Island is at least as well fitted for
its home as is the species peculiar to Chatham Island. Sir
C. Lyell and Mr. Wollaston have communicated to me a remarkable
fact bearing on this subject; namely, that Madeira and the
adjoining islet of Porto Santo possess many distinct but
representative land-shells, some of which live in crevices
of stone; and although large quantities of stone are annually
transported from Porto Santo to Madeira, yet this latter
island has not become colonised by the Porto Santo species:
nevertheless both islands have been colonised by some European
land-shells, which no doubt had some advantage over the
indigenous species. From these considerations I think we
need not greatly marvel at the endemic and representative
species, which inhabit the several islands of the Galapagos
Archipelago, not having universally spread from island to
island. In many other instances, as in the several districts
of the same continent, pre-occupation has probably played
an important part in checking the commingling of species
under the same conditions of life. Thus, the south-east
and south-west corners of Australia have nearly the same
physical conditions, and are united by continuous land,
yet they are inhabited by a vast number of distinct mammals,
birds, and plants.
The principle which determines the general character of
the fauna and flora of oceanic islands, namely, that the
inhabitants, when not identically the same, yet are plainly
related to the inhabitants of that region whence colonists
could most readily have been derived, the colonists having
been subsequently modified and better fitted to their new
homes, is of the widest application throughout nature. We
see this on every mountain, in every lake and marsh. For
Alpine species, excepting in so far as the same forms, chiefly
of plants, have spread widely throughout the world during
the recent Glacial epoch, are related to those of the surrounding
lowlands; thus we have in South America, Alpine humming-birds,
Alpine rodents, Alpine plants, &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.
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.
In considering the wide distribution of certain genera,
we should bear in mind that some are extremely ancient,
and must have branched off from a common parent at a remote
epoch; so that in such cases there will have been ample
time for great climatal and geographical changes and for
accidents of transport; and consequently for the migration
of some of the species into all quarters of the world, where
they may have become slightly modified in relation to their
new conditions. There is, also, some reason to believe from
geological evidence that organisms low in the scale within
each great class, generally change at a slower rate than
the higher forms; and consequently the lower forms will
have had a better chance of ranging widely and of still
retaining the same specific character. This fact, together
with the seeds and eggs of many low forms being very minute
and better fitted for distant transportation, probably accounts
for a law which has long been observed, and which has lately
been admirably discussed by Alph. de Candolle in regard
to plants, namely, that the lower any group of organisms
is, the more widely it is apt to range.
The relations just discussed, namely, low and slowly-changing
organisms ranging more widely than the high, some of the
species of widely-ranging genera themselves ranging widely,
such facts, as alpine, lacustrine, and marsh productions
being related (with the exceptions before specified) to
those on the surrounding low lands and dry lands, though
these stations are so different the very close relation
of the distinct species which inhabit the islets of the
same archipelago, and especially the striking relation of
the inhabitants of each whole archipelago or island to those
of the nearest mainland, are, I think, utterly inexplicable
on the ordinary view of the independent creation of each
species, but are explicable on the view of colonisation
from the nearest and readiest source, together with the
subsequent modification and better adaptation of the colonists
to their new homes.
Summary of last and present Chapters
In these chapters I have endeavoured to show, that if
we make due allowance for our ignorance of the full effects
of all the changes of climate and of the level of the land,
which have certainly occurred within the recent period,
and of other similar changes which may have occurred within
the same period; if we remember how profoundly ignorant
we are with respect to the many and curious means of occasional
transport, a subject which has hardly ever been properly
experimentised on; if we bear in mind how often a species
may have ranged continuously over a wide area, and then
have become extinct in the intermediate tracts, I think
the difficulties in believing that all the individuals of
the same species, wherever located, have descended from
the same parents, are not insuperable. And we are led to
this conclusion, which has been arrived at by many naturalists
under the designation of single centres of creation, by
some general considerations, more especially from the importance
of barriers and from the analogical distribution of sub-genera,
genera, and families.
With respect to the distinct species of the same genus,
which on my theory must have spread from one parent-source;
if we make the same allowances as before for our ignorance,
and remember that some forms of life change most slowly,
enormous periods of time being thus granted for their migration,
I do not think that the difficulties are insuperable; though
they often are in this case, and in that of the individuals
of the same species, extremely grave.
As exemplifying the effects of climatal changes on distribution,
I have attempted to show how important has been the influence
of the modern Glacial period, which I am fully convinced
simultaneously affected the whole world, or at least great
meridional belts. As showing how diversified are the means
of occasional transport, I have discussed at some little
length the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that
in the long course of time the individuals of the same species,
and likewise of allied species, have proceeded from some
one source; then I think all the grand leading facts of
geographical distribution are explicable on the theory of
migration (generally of the more dominant forms of life),
together with subsequent modification and the multiplication
of new forms. We can thus understand the high importance
of barriers, whether of land or water, which separate our
several zoological and botanical provinces. We can thus
understand the localisation of sub-genera, genera, and families;
and how it is that under different latitudes, for instance
in South America, the inhabitants of the plains and mountains,
of the forests, marshes, and deserts, are in so mysterious
a manner linked together by affinity, and are likewise linked
to the extinct beings which formerly inhabited the same
continent. Bearing in mind that the mutual relations of
organism to organism are of the highest importance, we can
see why two areas having nearly the same physical conditions
should often be inhabited by very different forms of life;
for according to the length of time which has elapsed since
new inhabitants entered one region; according to the nature
of the communication which allowed certain forms and not
others to enter, either in greater or lesser numbers; according
or not, as those which entered happened to come in more
or less direct competition with each other and with the
aborigines; and according as the immigrants were capable
of varying more or less rapidly, there would ensue in different
regions, independently of their physical conditions, infinitely
diversified conditions of life, there would be an almost
endless amount of organic action and reaction, and we should
find, as we do find, some groups of beings greatly, and
some only slightly modified, some developed in great force,
some existing in scanty numbers in the different great geographical
provinces of the world.
On these same principles, we can understand, as I have
endeavoured to show, why oceanic islands should have few
inhabitants, but of these a great number should be endemic
or peculiar; and why, in relation to the means of migration,
one group of beings, even within the same class, should
have all its species endemic, and another group should have
all its species common to other quarters of the world. We
can see why whole groups of organisms, as batrachians and
terrestrial mammals, should be absent from oceanic islands,
whilst the most isolated islands possess their own peculiar
species of aërial mammals or bats. We can see why there
should be some relation between the presence of mammals,
in a more or less modified condition, and the depth of the
sea between an island and the mainland. We can clearly see
why all the inhabitants of an archipelago, though specifically
distinct on the several islets, should be closely related
to each other, and likewise be related, but less closely,
to those of the nearest continent or other source whence
immigrants were probably derived. We can see why in two
areas, however distant from each other, there should be
a correlation, in the presence of identical species, of
varieties, of doubtful species, and of distinct but representative
species.
As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space:
the laws governing the succession of forms in past times
being nearly the same with those governing at the present
time the differences in different areas. We see this in
many facts. The endurance of each species and group of species
is continuous in time; for the exceptions to the rule are
so few, that they may fairly be attributed to our not having
as yet discovered in an intermediate deposit the forms which
are therein absent, but which occur above and below: so
in space, it certainly is the general rule that the area
inhabited by a single species, or by a group of species,
is continuous; and the exceptions, which are not rare, may,
as I have attempted to show, be accounted for by migration
at some former period under different conditions or by occasional
means of transport, and by the species having become extinct
in the intermediate tracts. Both in time and space, species
and groups of species have their points of maximum development.
Groups of species, belonging either to a certain period
of time, or to a certain area, are often characterised by
trifling characters in common, as of sculpture or colour.
In looking to the long succession of ages, as in now looking
to distant provinces throughout the world, we find that
some organisms differ little, whilst others belonging to
a different class, or to a different order, or even only
to a different family of the same order, differ greatly.
In both time and space the lower members of each class generally
change less than the higher; but there are in both cases
marked exceptions to the rule. On my theory these several
relations throughout time and space are intelligible; for
whether we look to the forms of life which have changed
during successive ages within the same quarter of the world,
or to those which have changed after having migrated into
distant quarters, in both cases the forms within each class
have been connected by the same bond of ordinary generation;
and the more nearly any two forms are related in blood,
the nearer they will generally stand to each other in time
and space; in both cases the laws of variation have been
the same, and modifications have been accumulated by the
same power of natural selection.
|
