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.

I do not deny that there are many and grave difficulties in understanding

how several of the inhabitants of the more remote islands, whether still

retaining the same specific form or modified since their arrival, could

have reached their present homes. But the probability of many islands

having existed as halting-places, of which not a wreck now remains, must

not be overlooked. I will here give a single instance of one of the cases

of difficulty. Almost all oceanic islands, even the most isolated and

smallest, are inhabited by land-shells, generally by endemic species, but

sometimes by species found elsewhere. Dr. Aug. A. Gould has given several

interesting cases in regard to the land-shells of the islands of the

Pacific. Now it is notorious that land-shells are very easily killed by

salt; their eggs, at least such as I have tried, sink in sea-water and are

killed by it. Yet there must be, on my view, some unknown, but highly

efficient means for their transportal. Would the just-hatched young

occasionally crawl on and adhere to the feet of birds roosting on the

ground, and thus get transported? It occurred to me that land-shells, when

hybernating and having a membranous diaphragm over the mouth of the shell,

might be floated in chinks of drifted timber across moderately wide arms of

the sea. And I found that several species did in this state withstand

uninjured an immersion in sea-water during seven days: one of these shells

was the Helix pomatia, and after it had again hybernated I put it in

sea-water for twenty days, and it perfectly recovered. As this species has

a thick calcareous operculum, I removed it, and when it had formed a new

membranous one, I immersed it for fourteen days in sea-water, and it

recovered and crawled away: but more experiments are wanted on this head.

The most striking and important fact for us in regard to the inhabitants of

islands, is their affinity to those of the nearest mainland, without being

actually the same species. Numerous instances could be given of this fact.

I will give only one, that of the Galapagos Archipelago, situated under the

equator, between 500 and 600 miles from the shores of South America. Here

almost every product of the land and water bears the unmistakeable stamp of

the American continent. There are twenty-six land birds, and twenty-five

of these are ranked by Mr. Gould as distinct species, supposed to have been

created here; yet the close affinity of most of these birds to American

species in every character, in their habits, gestures, and tones of voice,

was manifest. So it is with the other animals, and with nearly all the

plants, as shown by Dr. Hooker in his admirable memoir on the Flora of this

archipelago. The naturalist, looking at the inhabitants of these volcanic

islands in the Pacific, distant several hundred miles from the continent,

yet feels that he is standing on American land. Why should this be so? why

should the species which are supposed to have been created in the Galapagos

Archipelago, and nowhere else, bear so plain a stamp of affinity to those

created in America? There is nothing in the conditions of life, in the

geological nature of the islands, in their height or climate, or in the

proportions in which the several classes are associated together, which

resembles closely the conditions of the South American coast: in fact

there is a considerable dissimilarity in all these respects. On the other

hand, there is a considerable degree of resemblance in the volcanic nature

of the soil, in climate, height, and size of the islands, between the

Galapagos and Cape de Verde Archipelagos: but what an entire and absolute

difference in their inhabitants! The inhabitants of the Cape de Verde

Islands are related to those of Africa, like those of the Galapagos to

America. I believe this grand fact can receive no sort of explanation on

the ordinary view of independent creation; whereas on the view here

maintained, it is obvious that the Galapagos Islands would be likely to

receive colonists, whether by occasional means of transport or by formerly

continuous land, from America; and the Cape de Verde Islands from Africa;

and that such colonists would be liable to modification;--the principle of

inheritance still betraying their original birthplace.

Many analogous facts could be given: indeed it is an almost universal rule

that the endemic productions of islands are related to those of the nearest

continent, or of other near islands. The exceptions are few, and most of

them can be explained. Thus the plants of Kerguelen Land, though standing

nearer to Africa than to America, are related, and that very closely, as we

know from Dr. Hooker's account, to those of America: but on the view that

this island has been mainly stocked by seeds brought with earth and stones

on icebergs, drifted by the prevailing currents, this anomaly disappears.

New Zealand in its endemic plants is much more closely related to

Australia, the nearest mainland, than to any other region: and this is

what might have been expected; but it is also plainly related to South

America, which, although the next nearest continent, is so enormously

remote, that the fact becomes an anomaly. But this difficulty almost

disappears on the view that both New Zealand, South America, and other

southern lands were long ago partially stocked from a nearly intermediate

though distant point, namely from the antarctic islands, when they were

clothed with vegetation, before the commencement of the Glacial period.

The affinity, which, though feeble, I am assured by Dr. Hooker is real,

between the flora of the south-western corner of Australia and of the Cape

of Good Hope, is a far more remarkable case, and is at present

inexplicable: but this affinity is confined to the plants, and will, I do

not doubt, be some day explained.

The law which causes the inhabitants of an archipelago, though specifically

distinct, to be closely allied to those of the nearest continent, we

sometimes see displayed on a small scale, yet in a most interesting manner,

within the limits of the same archipelago. Thus the several islands of the

Galapagos Archipelago are tenanted, as I have elsewhere shown, in a quite

marvellous manner, by very closely related species; so that the inhabitants

of each separate island, though mostly distinct, are related in an

incomparably closer degree to each other than to the inhabitants of any

other part of the world. And this is just what might have been expected on

my view, for the islands are situated so near each other that they would

almost certainly receive immigrants from the same original source, or from

each other. But this dissimilarity between the endemic inhabitants of the

islands may be used as an argument against my views; for it may be asked,

how has it happened in the several islands situated within sight of each

other, having the same geological nature, the same height, climate, &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.

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 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.

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.