Chapter XI

Geographical Distribution

Present distribution cannot be accounted for by differences in physical

conditions -- Importance of barriers -- Affinity of the productions of the

same continent -- Centres of creation -- Means of dispersal, by changes of

climate and of the level of the land, and by occasional means -- Dispersal

during the Glacial period co-extensive with the world.

In considering the distribution of organic beings over the face of the

globe, the first great fact which strikes us is, that neither the

similarity nor the dissimilarity of the inhabitants of various regions can

be accounted for by their climatal and other physical conditions. Of late,

almost every author who has studied the subject has come to this

conclusion. The case of America alone would almost suffice to prove its

truth: for if we exclude the northern parts where the circumpolar land is

almost continuous, all authors agree that one of the most fundamental

divisions in geographical distribution is that between the New and Old

Worlds; yet if we travel over the vast American continent, from the central

parts of the United States to its extreme southern point, we meet with the

most diversified conditions; the most humid districts, arid deserts, lofty

mountains, grassy plains, forests, marshes, lakes, and great rivers, under

almost every temperature. There is hardly a climate or condition in the

Old World which cannot be paralleled in the New--at least as closely as the

same species generally require; for it is a most rare case to find a group

of organisms confined to any small spot, having conditions peculiar in only

a slight degree; for instance, small areas in the Old World could be

pointed out hotter than any in the New World, yet these are not inhabited

by a peculiar fauna or flora. Notwithstanding this parallelism in the

conditions of the Old and New Worlds, how widely different are their living

productions!

In the southern hemisphere, if we compare large tracts of land in

Australia, South Africa, and western South America, between latitudes 25

deg and 35 deg, we shall find parts extremely similar in all their

conditions, yet it would not be possible to point out three faunas and

floras more utterly dissimilar. Or again we may compare the productions of

South America south of lat. 35 deg with those north of 25 deg, which

consequently inhabit a considerably different climate, and they will be

found incomparably more closely related to each other, than they are to the

productions of Australia or Africa under nearly the same climate.

Analogous facts could be given with respect to the inhabitants of the sea.

A second great fact which strikes us in our general review is, that

barriers of any kind, or obstacles to free migration, are related in a

close and important manner to the differences between the productions of

various regions. We see this in the great difference of nearly all the

terrestrial productions of the New and Old Worlds, excepting in the

northern parts, where the land almost joins, and where, under a slightly

different climate, there might have been free migration for the northern

temperate forms, as there now is for the strictly arctic productions. We

see the same fact in the great difference between the inhabitants of

Australia, Africa, and South America under the same latitude: for these

countries are almost as much isolated from each other as is possible. On

each continent, also, we see the same fact; for on the opposite sides of

lofty and continuous mountain-ranges, and of great deserts, and sometimes

even of large rivers, we find different productions; though as mountain

chains, deserts, &c., are not as impassable, or likely to have endured so

long as the oceans separating continents, the differences are very inferior

in degree to those characteristic of distinct continents.

Turning to the sea, we find the same law. No two marine faunas are more

distinct, with hardly a fish, shell, or crab in common, than those of the

eastern and western shores of South and Central America; yet these great

faunas are separated only by the narrow, but impassable, isthmus of Panama.

Westward of the shores of America, a wide space of open ocean extends, with

not an island as a halting-place for emigrants; here we have a barrier of

another kind, and as soon as this is passed we meet in the eastern islands

of the Pacific, with another and totally distinct fauna. So that here

three marine faunas range far northward and southward, in parallel lines

not far from each other, under corresponding climates; but from being

separated from each other by impassable barriers, either of land or open

sea, they are wholly distinct. On the other hand, proceeding still further

westward from the eastern islands of the tropical parts of the Pacific, we

encounter no impassable barriers, and we have innumerable islands as

halting-places, until after travelling over a hemisphere we come to the

shores of Africa; and over this vast space we meet with no well-defined and

distinct marine faunas. Although hardly one shell, crab or fish is common

to the above-named three approximate faunas of Eastern and Western America

and the eastern Pacific islands, yet many fish range from the Pacific into

the Indian Ocean, and many shells are common to the eastern islands of the

Pacific and the eastern shores of Africa, on almost exactly opposite

meridians of longitude.

A third great fact, partly included in the foregoing statements, is the

affinity of the productions of the same continent or sea, though the

species themselves are distinct at different points and stations. It is a

law of the widest generality, and every continent offers innumerable

instances. Nevertheless the naturalist in travelling, for instance, from

north to south never fails to be struck by the manner in which successive

groups of beings, specifically distinct, yet clearly related, replace each

other. He hears from closely allied, yet distinct kinds of birds, notes

nearly similar, and sees their nests similarly constructed, but not quite

alike, with eggs coloured in nearly the same manner. The plains near the

Straits of Magellan are inhabited by one species of Rhea (American

ostrich), and northward the plains of La Plata by another species of the

same genus; and not by a true ostrich or emeu, like those found in Africa

and Australia under the same latitude. On these same plains of La Plata,

we see the agouti and bizcacha, animals having nearly the same habits as

our hares and rabbits and belonging to the same order of Rodents, but they

plainly display an American type of structure. We ascend the lofty peaks

of the Cordillera and we find an alpine species of bizcacha; we look to the

waters, and we do not find the beaver or musk-rat, but the coypu and

capybara, rodents of the American type. Innumerable other instances could

be given. If we look to the islands off the American shore, however much

they may differ in geological structure, the inhabitants, though they may

be all peculiar species, are essentially American. We may look back to

past ages, as shown in the last chapter, and we find American types then

prevalent on the American continent and in the American seas. We see in

these facts some deep organic bond, prevailing throughout space and time,

over the same areas of land and water, and independent of their physical

conditions. The naturalist must feel little curiosity, who is not led to

inquire what this bond is.

This bond, on my theory, is simply inheritance, that cause which alone, as

far as we positively know, produces organisms quite like, or, as we see in

the case of varieties nearly like each other. The dissimilarity of the

inhabitants of different regions may be attributed to modification through

natural selection, and in a quite subordinate degree to the direct

influence of different physical conditions. The degree of dissimilarity

will depend on the migration of the more dominant forms of life from one

region into another having been effected with more or less ease, at periods

more or less remote;--on the nature and number of the former

immigrants;--and on their action and reaction, in their mutual struggles

for life;--the relation of organism to organism being, as I have already

often remarked, the most important of all relations. Thus the high

importance of barriers comes into play by checking migration; as does time

for the slow process of modification through natural selection.

Widely-ranging species, abounding in individuals, which have already

triumphed over many competitors in their own widely-extended homes will

have the best chance of seizing on new places, when they spread into new

countries. In their new homes they will be exposed to new conditions, and

will frequently undergo further modification and improvement; and thus they

will become still further victorious, and will produce groups of modified

descendants. On this principle of inheritance with modification, we can

understand how it is that sections of genera, whole genera, and even

families are confined to the same areas, as is so commonly and notoriously

the case.

I believe, as was remarked in the last chapter, in no law of necessary

development. As the variability of each species is an independent

property, and will be taken advantage of by natural selection, only so far

as it profits the individual in its complex struggle for life, so the

degree of modification in different species will be no uniform quantity.

If, for instance, a number of species, which stand in direct competition

with each other, migrate in a body into a new and afterwards isolated

country, they will be little liable to modification; for neither migration

nor isolation in themselves can do anything. These principles come into

play only by bringing organisms into new relations with each other, and in

a lesser degree with the surrounding physical conditions. As we have seen

in the last chapter that some forms have retained nearly the same character

from an enormously remote geological period, so certain species have

migrated over vast spaces, and have not become greatly modified.

On these views, it is obvious, that the several species of the same genus,

though inhabiting the most distant quarters of the world, must originally

have proceeded from the same source, as they have descended from the same

progenitor. In the case of those species, which have undergone during

whole geological periods but little modification, there is not much

difficulty in believing that they may have migrated from the same region;

for during the vast geographical and climatal changes which will have

supervened since ancient times, almost any amount of migration is possible.

But in many other cases, in which we have reason to believe that the

species of a genus have been produced within comparatively recent times,

there is great difficulty on this head. It is also obvious that the

individuals of the same species, though now inhabiting distant and isolated

regions, must have proceeded from one spot, where their parents were first

produced: for, as explained in the last chapter, it is incredible that

individuals identically the same should ever have been produced through

natural selection from parents specifically distinct.

We are thus brought to the question which has been largely discussed by

naturalists, namely, whether species have been created at one or more

points of the earth's surface. Undoubtedly there are very many cases of

extreme difficulty, in understanding how the same species could possibly

have migrated from some one point to the several distant and isolated

points, where now found. Nevertheless the simplicity of the view that each

species was first produced within a single region captivates the mind. He

who rejects it, rejects the vera causa of ordinary generation with

subsequent migration, and calls in the agency of a miracle. It is

universally admitted, that in most cases the area inhabited by a species is

continuous; and when a plant or animal inhabits two points so distant from

each other, or with an interval of such a nature, that the space could not

be easily passed over by migration, the fact is given as something

remarkable and exceptional. The capacity of migrating across the sea is

more distinctly limited in terrestrial mammals, than perhaps in any other

organic beings; and, accordingly, we find no inexplicable cases of the same

mammal inhabiting distant points of the world. No geologist will feel any

difficulty in such cases as Great Britain having been formerly united to

Europe, and consequently possessing the same quadrupeds. But if the same

species can be produced at two separate points, why do we not find a single

mammal common to Europe and Australia or South America? The conditions of

life are nearly the same, so that a multitude of European animals and

plants have become naturalised in America and Australia; and some of the

aboriginal plants are identically the same at these distant points of the

northern and southern hemispheres? The answer, as I believe, is, that

mammals have not been able to migrate, whereas some plants, from their

varied means of dispersal, have migrated across the vast and broken

interspace. The great and striking influence which barriers of every kind

have had on distribution, is intelligible only on the view that the great

majority of species have been produced on one side alone, and have not been

able to migrate to the other side. Some few families, many sub-families,

very many genera, and a still greater number of sections of genera are

confined to a single region; and it has been observed by several

naturalists, that the most natural genera, or those genera in which the

species are most closely related to each other, are generally local, or

confined to one area. What a strange anomaly it would be, if, when coming

one step lower in the series, to the individuals of the same species, a

directly opposite rule prevailed; and species were not local, but had been

produced in two or more distinct areas!

Hence it seems to me, as it has to many other naturalists, that the view of

each species having been produced in one area alone, and having

subsequently migrated from that area as far as its powers of migration and

subsistence under past and present conditions permitted, is the most

probable. Undoubtedly many cases occur, in which we cannot explain how the

same species could have passed from one point to the other. But the

geographical and climatal changes, which have certainly occurred within

recent geological times, must have interrupted or rendered discontinuous

the formerly continuous range of many species. So that we are reduced to

consider whether the exceptions to continuity of range are so numerous and

of so grave a nature, that we ought to give up the belief, rendered

probable by general considerations, that each species has been produced

within one area, and has migrated thence as far as it could. It would be

hopelessly tedious to discuss all the exceptional cases of the same

species, now living at distant and separated points; nor do I for a moment

pretend that any explanation could be offered of many such cases. But

after some preliminary remarks, I will discuss a few of the most striking

classes of facts; namely, the existence of the same species on the summits

of distant mountain-ranges, and at distant points in the arctic and

antarctic regions; and secondly (in the following chapter), the wide

distribution of freshwater productions; and thirdly, the occurrence of the

same terrestrial species on islands and on the mainland, though separated

by hundreds of miles of open sea. If the existence of the same species at

distant and isolated points of the earth's surface, can in many instances

be explained on the view of each species having migrated from a single

birthplace; then, considering our ignorance with respect to former climatal

and geographical changes and various occasional means of transport, the

belief that this has been the universal law, seems to me incomparably the

safest.

In discussing this subject, we shall be enabled at the same time to

consider a point equally important for us, namely, whether the several

distinct species of a genus, which on my theory have all descended from a

common progenitor, can have migrated (undergoing modification during some

part of their migration) from the area inhabited by their progenitor. If

it can be shown to be almost invariably the case, that a region, of which

most of its inhabitants are closely related to, or belong to the same

genera with the species of a second region, has probably received at some

former period immigrants from this other region, my theory will be

strengthened; for we can clearly understand, on the principle of

modification, why the inhabitants of a region should be related to those of

another region, whence it has been stocked. A volcanic island, for

instance, upheaved and formed at the distance of a few hundreds of miles

from a continent, would probably receive from it in the course of time a

few colonists, and their descendants, though modified, would still be

plainly related by inheritance to the inhabitants of the continent. Cases

of this nature are common, and are, as we shall hereafter more fully see,

inexplicable on the theory of independent creation. This view of the

relation of species in one region to those in another, does not differ much

(by substituting the word variety for species) from that lately advanced in

an ingenious paper by Mr. Wallace, in which he concludes, that 'every

species has come into existence coincident both in space and time with a

pre-existing closely allied species.' And I now know from correspondence,

that this coincidence he attributes to generation with modification.

The previous remarks on 'single and multiple centres of creation' do not

directly bear on another allied question,--namely whether all the

individuals of the same species have descended from a single pair, or

single hermaphrodite, or whether, as some authors suppose, from many

individuals simultaneously created. With those organic beings which never

intercross (if such exist), the species, on my theory, must have descended

from a succession of improved varieties, which will never have blended with

other individuals or varieties, but will have supplanted each other; so

that, at each successive stage of modification and improvement, all the

individuals of each variety will have descended from a single parent. But

in the majority of cases, namely, with all organisms which habitually unite

for each birth, or which often intercross, I believe that during the slow

process of modification the individuals of the species will have been kept

nearly uniform by intercrossing; so that many individuals will have gone on

simultaneously changing, and the whole amount of modification will not have

been due, at each stage, to descent from a single parent. To illustrate

what I mean: our English racehorses differ slightly from the horses of

every other breed; but they do not owe their difference and superiority to

descent from any single pair, but to continued care in selecting and

training many individuals during many generations.

Before discussing the three classes of facts, which I have selected as

presenting the greatest amount of difficulty on the theory of 'single

centres of creation,' I must say a few words on the means of dispersal.

Means of Dispersal. -- Sir C. Lyell and other authors have ably treated

this subject. I can give here only the briefest abstract of the more

important facts. Change of climate must have had a powerful influence on

migration: a region when its climate was different may have been a high

road for migration, but now be impassable; I shall, however, presently have

to discuss this branch of the subject in some detail. Changes of level in

the land must also have been highly influential: a narrow isthmus now

separates two marine faunas; submerge it, or let it formerly have been

submerged, and the two faunas will now blend or may formerly have blended:

where the sea now extends, land may at a former period have connected

islands or possibly even continents together, and thus have allowed

terrestrial productions to pass from one to the other. No geologist will

dispute that great mutations of level have occurred within the period of

existing organisms. Edward Forbes insisted that all the islands in the

Atlantic must recently have been connected with Europe or Africa, and

Europe likewise with America. Other authors have thus hypothetically

bridged over every ocean, and have united almost every island to some

mainland. If indeed the arguments used by Forbes are to be trusted, it

must be admitted that scarcely a single island exists which has not

recently been united to some continent. This view cuts the Gordian knot of

the dispersal of the same species to the most distant points, and removes

many a difficulty: but to the best of any judgment we are not authorized

in admitting such enormous geographical changes within the period of

existing species. It seems to me that we have abundant evidence of great

oscillations of level in our continents; but not of such vast changes in

their position and extension, as to have united them within the recent

period to each other and to the several intervening oceanic islands. I

freely admit the former existence of many islands, now buried beneath the

sea, which may have served as halting places for plants and for many

animals during their migration. In the coral-producing oceans such sunken

islands are now marked, as I believe, by rings of coral or atolls standing

over them. Whenever it is fully admitted, as I believe it will some day

be, that each species has proceeded from a single birthplace, and when in

the course of time we know something definite about the means of

distribution, we shall be enabled to speculate with security on the former

extension of the land. But I do not believe that it will ever be proved

that within the recent period continents which are now quite separate, have

been continuously, or almost continuously, united with each other, and with

the many existing oceanic islands. Several facts in distribution,--such as

the great difference in the marine faunas on the opposite sides of almost

every continent,--the close relation of the tertiary inhabitants of several

lands and even seas to their present inhabitants,--a certain degree of

relation (as we shall hereafter see) between the distribution of mammals

and the depth of the sea,--these and other such facts seem to me opposed to

the admission of such prodigious geographical revolutions within the recent

period, as are necessitated on the view advanced by Forbes and admitted by

his many followers. The nature and relative proportions of the inhabitants

of oceanic islands likewise seem to me opposed to the belief of their

former continuity with continents. Nor does their almost universally

volcanic composition favour the admission that they are the wrecks of

sunken continents;--if they had originally existed as mountain-ranges on

the land, some at least of the islands would have been formed, like other

mountain-summits, of granite, metamorphic schists, old fossiliferous or

other such rocks, instead of consisting of mere piles of volcanic matter.

I must now say a few words on what are called accidental means, but which

more properly might be called occasional means of distribution. I shall

here confine myself to plants. In botanical works, this or that plant is

stated to be ill adapted for wide dissemination; but for transport across

the sea, the greater or less facilities may be said to be almost wholly

unknown. Until I tried, with Mr. Berkeley's aid, a few experiments, it was

not even known how far seeds could resist the injurious action of

sea-water. To my surprise I found that out of 87 kinds, 64 germinated

after an immersion of 28 days, and a few survived an immersion of 137 days.

For convenience sake I chiefly tried small seeds, without the capsule or

fruit; and as all of these sank in a few days, they could not be floated

across wide spaces of the sea, whether or not they were injured by the

salt-water. Afterwards I tried some larger fruits, capsules, &c., and some

of these floated for a long time. It is well known what a difference there

is in the buoyancy of green and seasoned timber; and it occurred to me that

floods might wash down plants or branches, and that these might be dried on

the banks, and then by a fresh rise in the stream be washed into the sea.

Hence I was led to dry stems and branches of 94 plants with ripe fruit, and

to place them on sea water. The majority sank quickly, but some which

whilst green floated for a very short time, when dried floated much longer;

for instance, ripe hazel-nuts sank immediately, but when dried, they

floated for 90 days and afterwards when planted they germinated; an

asparagus plant with ripe berries floated for 23 days, when dried it

floated for 85 days, and the seeds afterwards germinated: the ripe seeds

of Helosciadium sank in two days, when dried they floated for above 90

days, and afterwards germinated. Altogether out of the 94 dried plants, 18

floated for above 28 days, and some of the 18 floated for a very much

longer period. So that as 64/87 seeds germinated after an immersion of 28

days; and as 18/94 plants with ripe fruit (but not all the same species as

in the foregoing experiment) floated, after being dried, for above 28 days,

as far as we may infer anything from these scanty facts, we may conclude

that the seeds of 14/100 plants of any country might be floated by

sea-currents during 28 days, and would retain their power of germination.

In Johnston's Physical Atlas, the average rate of the several Atlantic

currents is 33 miles per diem (some currents running at the rate of 60

miles per diem); on this average, the seeds of 14/100 plants belonging to

one country might be floated across 924 miles of sea to another country;

and when stranded, if blown to a favourable spot by an inland gale, they

would germinate.

Subsequently to my experiments, M. Martens tried similar ones, but in a

much better manner, for he placed the seeds in a box in the actual sea, so

that they were alternately wet and exposed to the air like really floating

plants. He tried 98 seeds, mostly different from mine; but he chose many

large fruits and likewise seeds from plants which live near the sea; and

this would have favoured the average length of their flotation and of their

resistance to the injurious action of the salt-water. On the other hand he

did not previously dry the plants or branches with the fruit; and this, as

we have seen, would have caused some of them to have floated much longer.

The result was that 18/98 of his seeds floated for 42 days, and were then

capable of germination. But I do not doubt that plants exposed to the

waves would float for a less time than those protected from violent

movement as in our experiments. Therefore it would perhaps be safer to

assume that the seeds of about 10/100 plants of a flora, after having been

dried, could be floated across a space of sea 900 miles in width, and would

then germinate. The fact of the larger fruits often floating longer than

the small, is interesting; as plants with large seeds or fruit could hardly

be transported by any other means; and Alph. de Candolle has shown that

such plants generally have restricted ranges.

But seeds may be occasionally transported in another manner. Drift timber

is thrown up on most islands, even on those in the midst of the widest

oceans; and the natives of the coral-islands in the Pacific, procure stones

for their tools, solely from the roots of drifted trees, these stones being

a valuable royal tax. I find on examination, that when irregularly shaped

stones are embedded in the roots of trees, small parcels of earth are very

frequently enclosed in their interstices and behind them,--so perfectly

that not a particle could be washed away in the longest transport: out of

one small portion of earth thus completely enclosed by wood in an oak about

50 years old, three dicotyledonous plants germinated: I am certain of the

accuracy of this observation. Again, I can show that the carcasses of

birds, when floating on the sea, sometimes escape being immediately

devoured; and seeds of many kinds in the crops of floating birds long

retain their vitality: peas and vetches, for instance, are killed by even

a few days' immersion in sea-water; but some taken out of the crop of a

pigeon, which had floated on artificial salt-water for 30 days, to my

surprise nearly all germinated.

Living birds can hardly fail to be highly effective agents in the

transportation of seeds. I could give many facts showing how frequently

birds of many kinds are blown by gales to vast distances across the ocean.

We may I think safely assume that under such circumstances their rate of

flight would often be 35 miles an hour; and some authors have given a far

higher estimate. I have never seen an instance of nutritious seeds passing

through the intestines of a bird; but hard seeds of fruit will pass

uninjured through even the digestive organs of a turkey. In the course of

two months, I picked up in my garden 12 kinds of seeds, out of the

excrement of small birds, and these seemed perfect, and some of them, which

I tried, germinated. But the following fact is more important: the crops

of birds do not secrete gastric juice, and do not in the least injure, as I

know by trial, the germination of seeds; now after a bird has found and

devoured a large supply of food, it is positively asserted that all the

grains do not pass into the gizzard for 12 or even 18 hours. A bird in

this interval might easily be blown to the distance of 500 miles, and hawks

are known to look out for tired birds, and the contents of their torn crops

might thus readily get scattered. Mr. Brent informs me that a friend of

his had to give up flying carrier-pigeons from France to England, as the

hawks on the English coast destroyed so many on their arrival. Some hawks

and owls bolt their prey whole, and after an interval of from twelve to

twenty hours, disgorge pellets, which, as I know from experiments made in

the Zoological Gardens, include seeds capable of germination. Some seeds

of the oat, wheat, millet, canary, hemp, clover, and beet germinated after

having been from twelve to twenty-one hours in the stomachs of different

birds of prey; and two seeds of beet grew after having been thus retained

for two days and fourteen hours. Freshwater fish, I find, eat seeds of

many land and water plants: fish are frequently devoured by birds, and

thus the seeds might be transported from place to place. I forced many

kinds of seeds into the stomachs of dead fish, and then gave their bodies

to fishing-eagles, storks, and pelicans; these birds after an interval of

many hours, either rejected the seeds in pellets or passed them in their

excrement; and several of these seeds retained their power of germination.

Certain seeds, however, were always killed by this process.

Although the beaks and feet of birds are generally quite clean, I can show

that earth sometimes adheres to them: in one instance I removed twenty-two

grains of dry argillaceous earth from one foot of a partridge, and in this

earth there was a pebble quite as large as the seed of a vetch. Thus seeds

might occasionally be transported to great distances; for many facts could

be given showing that soil almost everywhere is charged with seeds.

Reflect for a moment on the millions of quails which annually cross the

Mediterranean; and can we doubt that the earth adhering to their feet would

sometimes include a few minute seeds? But I shall presently have to recur

to this subject.

As icebergs are known to be sometimes loaded with earth and stones, and

have even carried brushwood, bones, and the nest of a land-bird, I can

hardly doubt that they must occasionally have transported seeds from one

part to another of the arctic and antarctic regions, as suggested by Lyell;

and during the Glacial period from one part of the now temperate regions to

another. In the Azores, from the large number of the species of plants

common to Europe, in comparison with the plants of other oceanic islands

nearer to the mainland, and (as remarked by Mr. H. C. Watson) from the

somewhat northern character of the flora in comparison with the latitude, I

suspected that these islands had been partly stocked by ice-borne seeds,

during the Glacial epoch. At my request Sir C. Lyell wrote to M. Hartung

to inquire whether he had observed erratic boulders on these islands, and

he answered that he had found large fragments of granite and other rocks,

which do not occur in the archipelago. Hence we may safely infer that

icebergs formerly landed their rocky burthens on the shores of these

mid-ocean islands, and it is at least possible that they may have brought

thither the seeds of northern plants.

Considering that the several above means of transport, and that several

other means, which without doubt remain to be discovered, have been in

action year after year, for centuries and tens of thousands of years, it

would I think be a marvellous fact if many plants had not thus become

widely transported. These means of transport are sometimes called

accidental, but this is not strictly correct: the currents of the sea are

not accidental, nor is the direction of prevalent gales of wind. It should

be observed that scarcely any means of transport would carry seeds for very

great distances; for seeds do not retain their vitality when exposed for a

great length of time to the action of seawater; nor could they be long

carried in the crops or intestines of birds. These means, however, would

suffice for occasional transport across tracts of sea some hundred miles in

breadth, or from island to island, or from a continent to a neighbouring

island, but not from one distant continent to another. The floras of

distant continents would not by such means become mingled in any great

degree; but would remain as distinct as we now see them to be. The

currents, from their course, would never bring seeds from North America to

Britain, though they might and do bring seeds from the West Indies to our

western shores, where, if not killed by so long an immersion in salt-water,

they could not endure our climate. Almost every year, one or two

land-birds are blown across the whole Atlantic Ocean, from North America to

the western shores of Ireland and England; but seeds could be transported

by these wanderers only by one means, namely, in dirt sticking to their

feet, which is in itself a rare accident. Even in this case, how small

would the chance be of a seed falling on favourable soil, and coming to

maturity! But it would be a great error to argue that because a

well-stocked island, like Great Britain, has not, as far as is known (and

it would be very difficult to prove this), received within the last few

centuries, through occasional means of transport, immigrants from Europe or

any other continent, that a poorly-stocked island, though standing more

remote from the mainland, would not receive colonists by similar means. I

do not doubt that out of twenty seeds or animals transported to an island,

even if far less well-stocked than Britain, scarcely more than one would be

so well fitted to its new home, as to become naturalised. But this, as it

seems to me, is no valid argument against what would be effected by

occasional means of transport, during the long lapse of geological time,

whilst an island was being upheaved and formed, and before it had become

fully stocked with inhabitants. On almost bare land, with few or no

destructive insects or birds living there, nearly every seed, which chanced

to arrive, would be sure to germinate and survive.

Dispersal during the Glacial period. -- The identity of many plants and

animals, on mountain-summits, separated from each other by hundreds of

miles of lowlands, where the Alpine species could not possibly exist, is

one of the most striking cases known of the same species living at distant

points, without the apparent possibility of their having migrated from one

to the other. It is indeed a remarkable fact to see so many of the same

plants living on the snowy regions of the Alps or Pyrenees, and in the

extreme northern parts of Europe; but it is far more remarkable, that the

plants on the White Mountains, in the United States of America, are all the

same with those of Labrador, and nearly all the same, as we hear from Asa

Gray, with those on the loftiest mountains of Europe. Even as long ago as

1747, such facts led Gmelin to conclude that the same species must have

been independently created at several distinct points; and we might have

remained in this same belief, had not Agassiz and others called vivid

attention to the Glacial period, which, as we shall immediately see,

affords a simple explanation of these facts. We have evidence of almost

every conceivable kind, organic and inorganic, that within a very recent

geological period, central Europe and North America suffered under an

Arctic climate. The ruins of a house burnt by fire do not tell their tale

more plainly, than do the mountains of Scotland and Wales, with their

scored flanks, polished surfaces, and perched boulders, of the icy streams

with which their valleys were lately filled. So greatly has the climate of

Europe changed, that in Northern Italy, gigantic moraines, left by old

glaciers, are now clothed by the vine and maize. Throughout a large part

of the United States, erratic boulders, and rocks scored by drifted

icebergs and coast-ice, plainly reveal a former cold period.

The former influence of the glacial climate on the distribution of the

inhabitants of Europe, as explained with remarkable clearness by Edward

Forbes, is substantially as follows. But we shall follow the changes more

readily, by supposing a new glacial period to come slowly on, and then pass

away, as formerly occurred. As the cold came on, and as each more southern

zone became fitted for arctic beings and ill-fitted for their former more

temperate inhabitants, the latter would be supplanted and arctic

productions would take their places. The inhabitants of the more temperate

regions would at the same time travel southward, unless they were stopped

by barriers, in which case they would perish. The mountains would become

covered with snow and ice, and their former Alpine inhabitants would

descend to the plains. By the time that the cold had reached its maximum,

we should have a uniform arctic fauna and flora, covering the central parts

of Europe, as far south as the Alps and Pyrenees, and even stretching into

Spain. The now temperate regions of the United States would likewise be

covered by arctic plants and animals, and these would be nearly the same

with those of Europe; for the present circumpolar inhabitants, which we

suppose to have everywhere travelled southward, are remarkably uniform

round the world. We may suppose that the Glacial period came on a little

earlier or later in North America than in Europe, so will the southern

migration there have been a little earlier or later; but this will make no

difference in the final result.

As the warmth returned, the arctic forms would retreat northward, closely

followed up in their retreat by the productions of the more temperate

regions. And as the snow melted from the bases of the mountains, the

arctic forms would seize on the cleared and thawed ground, always ascending

higher and higher, as the warmth increased, whilst their brethren were

pursuing their northern journey. Hence, when the warmth had fully

returned, the same arctic species, which had lately lived in a body

together on the lowlands of the Old and New Worlds, would be left isolated

on distant mountain-summits (having been exterminated on all lesser

heights) and in the arctic regions of both hemispheres.

Thus we can understand the identity of many plants at points so immensely

remote as on the mountains of the United States and of Europe. We can thus

also understand the fact that the Alpine plants of each mountain-range are

more especially related to the arctic forms living due north or nearly due

north of them: for the migration as the cold came on, and the re-migration

on the returning warmth, will generally have been due south and north. The

Alpine plants, for example, of Scotland, as remarked by Mr. H. C. Watson,

and those of the Pyrenees, as remarked by Ramond, are more especially

allied to the plants of northern Scandinavia; those of the United States to

Labrador; those of the mountains of Siberia to the arctic regions of that

country. These views, grounded as they are on the perfectly

well-ascertained occurrence of a former Glacial period, seem to me to

explain in so satisfactory a manner the present distribution of the Alpine

and Arctic productions of Europe and America, that when in other regions we

find the same species on distant mountain-summits, we may almost conclude

without other evidence, that a colder climate permitted their former

migration across the low intervening tracts, since become too warm for

their existence.

If the climate, since the Glacial period, has ever been in any degree

warmer than at present (as some geologists in the United States believe to

have been the case, chiefly from the distribution of the fossil Gnathodon),

then the arctic and temperate productions will at a very late period have

marched a little further north, and subsequently have retreated to their

present homes; but I have met with no satisfactory evidence with respect to

this intercalated slightly warmer period, since the Glacial period.

The arctic forms, during their long southern migration and re-migration

northward, will have been exposed to nearly the same climate, and, as is

especially to be noticed, they will have kept in a body together;

consequently their mutual relations will not have been much disturbed, and,

in accordance with the principles inculcated in this volume, they will not

have been liable to much modification. But with our Alpine productions,

left isolated from the moment of the returning warmth, first at the bases

and ultimately on the summits of the mountains, the case will have been

somewhat different; for it is not likely that all the same arctic species

will have been left on mountain ranges distant from each other, and have

survived there ever since; they will, also, in all probability have become

mingled with ancient Alpine species, which must have existed on the

mountains before the commencement of the Glacial epoch, and which during

its coldest period will have been temporarily driven down to the plains;

they will, also, have been exposed to somewhat different climatal

influences. Their mutual relations will thus have been in some degree

disturbed; consequently they will have been liable to modification; and

this we find has been the case; for if we compare the present Alpine plants

and animals of the several great European mountain-ranges, though very many

of the species are identically the same, some present varieties, some are

ranked as doubtful forms, and some few are distinct yet closely allied or

representative species.

In illustrating what, as I believe, actually took place during the Glacial

period, I assumed that at its commencement the arctic productions were as

uniform round the polar regions as they are at the present day. But the

foregoing remarks on distribution apply not only to strictly arctic forms,

but also to many sub-arctic and to some few northern temperate forms, for

some of these are the same on the lower mountains and on the plains of

North America and Europe; and it may be reasonably asked how I account for

the necessary degree of uniformity of the sub-arctic and northern temperate

forms round the world, at the commencement of the Glacial period. At the

present day, the sub-arctic and northern temperate productions of the Old

and New Worlds are separated from each other by the Atlantic Ocean and by

the extreme northern part of the Pacific. During the Glacial period, when

the inhabitants of the Old and New Worlds lived further southwards than at

present, they must have been still more completely separated by wider

spaces of ocean. I believe the above difficulty may be surmounted by

looking to still earlier changes of climate of an opposite nature. We have

good reason to believe that during the newer Pliocene period, before the

Glacial epoch, and whilst the majority of the inhabitants of the world were

specifically the same as now, the climate was warmer than at the present

day. Hence we may suppose that the organisms now living under the climate

of latitude 60 deg, during the Pliocene period lived further north under

the Polar Circle, in latitude 66 deg-67 deg; and that the strictly arctic

productions then lived on the broken land still nearer to the pole. Now if

we look at a globe, we shall see that under the Polar Circle there is

almost continuous land from western Europe, through Siberia, to eastern

America. And to this continuity of the circumpolar land, and to the

consequent freedom for intermigration under a more favourable climate, I

attribute the necessary amount of uniformity in the sub-arctic and northern

temperate productions of the Old and New Worlds, at a period anterior to

the Glacial epoch.

Believing, from reasons before alluded to, that our continents have long

remained in nearly the same relative position, though subjected to large,

but partial oscillations of level, I am strongly inclined to extend the

above view, and to infer that during some earlier and still warmer period,

such as the older Pliocene period, a large number of the same plants and

animals inhabited the almost continuous circumpolar land; and that these

plants and animals, both in the Old and New Worlds, began slowly to migrate

southwards as the climate became less warm, long before the commencement of

the Glacial period. We now see, as I believe, their descendants, mostly in

a modified condition, in the central parts of Europe and the United States.

On this view we can understand the relationship, with very little identity,

between the productions of North America and Europe,--a relationship which

is most remarkable, considering the distance of the two areas, and their

separation by the Atlantic Ocean. We can further understand the singular

fact remarked on by several observers, that the productions of Europe and

America during the later tertiary stages were more closely related to each

other than they are at the present time; for during these warmer periods

the northern parts of the Old and New Worlds will have been almost

continuously united by land, serving as a bridge, since rendered impassable

by cold, for the inter-migration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as the

species in common, which inhabited the New and Old Worlds, migrated south

of the Polar Circle, they must have been completely cut off from each

other. This separation, as far as the more temperate productions are

concerned, took place long ages ago. And as the plants and animals

migrated southward, they will have become mingled in the one great region

with the native American productions, and have had to compete with them;

and in the other great region, with those of the Old World. Consequently

we have here everything favourable for much modification,--for far more

modification than with the Alpine productions, left isolated, within a much

more recent period, on the several mountain-ranges and on the arctic lands

of the two Worlds. Hence it has come, that when we compare the now living

productions of the temperate regions of the New and Old Worlds, we find

very few identical species (though Asa Gray has lately shown that more

plants are identical than was formerly supposed), but we find in every

great class many forms, which some naturalists rank as geographical races,

and others as distinct species; and a host of closely allied or

representative forms which are ranked by all naturalists as specifically

distinct.

As on the land, so in the waters of the sea, a slow southern migration of a

marine fauna, which during the Pliocene or even a somewhat earlier period,

was nearly uniform along the continuous shores of the Polar Circle, will

account, on the theory of modification, for many closely allied forms now

living in areas completely sundered. Thus, I think, we can understand the

presence of many existing and tertiary representative forms on the eastern

and western shores of temperate North America; and the still more striking

case of many closely allied crustaceans (as described in Dana's admirable

work), of some fish and other marine animals, in the Mediterranean and in

the seas of Japan,--areas now separated by a continent and by nearly a

hemisphere of equatorial ocean.

These cases of relationship, without identity, of the inhabitants of seas

now disjoined, and likewise of the past and present inhabitants of the

temperate lands of North America and Europe, are inexplicable on the theory

of creation. We cannot say that they have been created alike, in

correspondence with the nearly similar physical conditions of the areas;

for if we compare, for instance, certain parts of South America with the

southern continents of the Old World, we see countries closely

corresponding in all their physical conditions, but with their inhabitants

utterly dissimilar.

But we must return to our more immediate subject, the Glacial period. I am

convinced that Forbes's view may be largely extended. In Europe we have

the plainest evidence of the cold period, from the western shores of

Britain to the Oural range, and southward to the Pyrenees. We may infer,

from the frozen mammals and nature of the mountain vegetation, that Siberia

was similarly affected. Along the Himalaya, at points 900 miles apart,

glaciers have left the marks of their former low descent; and in Sikkim,

Dr. Hooker saw maize growing on gigantic ancient moraines. South of the

equator, we have some direct evidence of former glacial action in New

Zealand; and the same plants, found on widely separated mountains in this

island, tell the same story. If one account which has been published can

be trusted, we have direct evidence of glacial action in the south-eastern

corner of Australia.

Looking to America; in the northern half, ice-borne fragments of rock have

been observed on the eastern side as far south as lat. 36 deg-37 deg, and

on the shores of the Pacific, where the climate is now so different, as far

south as lat. 46 deg; erratic boulders have, also, been noticed on the

Rocky Mountains. In the Cordillera of Equatorial South America, glaciers

once extended far below their present level. In central Chile I was

astonished at the structure of a vast mound of detritus, about 800 feet in

height, crossing a valley of the Andes; and this I now feel convinced was a

gigantic moraine, left far below any existing glacier. Further south on

both sides of the continent, from lat. 41 deg to the southernmost

extremity, we have the clearest evidence of former glacial action, in huge

boulders transported far from their parent source.

We do not know that the Glacial epoch was strictly simultaneous at these

several far distant points on opposite sides of the world. But we have

good evidence in almost every case, that the epoch was included within the

latest geological period. We have, also, excellent evidence, that it

endured for an enormous time, as measured by years, at each point. The

cold may have come on, or have ceased, earlier at one point of the globe

than at another, but seeing that it endured for long at each, and that it

was contemporaneous in a geological sense, it seems to me probable that it

was, during a part at least of the period, actually simultaneous throughout

the world. Without some distinct evidence to the contrary, we may at least

admit as probable that the glacial action was simultaneous on the eastern

and western sides of North America, in the Cordillera under the equator and

under the warmer temperate zones, and on both sides of the southern

extremity of the continent. If this be admitted, it is difficult to avoid

believing that the temperature of the whole world was at this period

simultaneously cooler. But it would suffice for my purpose, if the

temperature was at the same time lower along certain broad belts of

longitude.

On this view of the whole world, or at least of broad longitudinal belts,

having been simultaneously colder from pole to pole, much light can be

thrown on the present distribution of identical and allied species. In

America, Dr. Hooker has shown that between forty and fifty of the flowering

plants of Tierra del Fuego, forming no inconsiderable part of its scanty

flora, are common to Europe, enormously remote as these two points are; and

there are many closely allied species. On the lofty mountains of

equatorial America a host of peculiar species belonging to European genera

occur. On the highest mountains of Brazil, some few European genera were

found by Gardner, which do not exist in the wide intervening hot countries.

So on the Silla of Caraccas the illustrious Humboldt long ago found species

belonging to genera characteristic of the Cordillera. On the mountains of

Abyssinia, several European forms and some few representatives of the

peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope a

very few European species, believed not to have been introduced by man, and

on the mountains, some few representative European forms are found, which

have not been discovered in the intertropical parts of Africa. On the

Himalaya, and on the isolated mountain-ranges of the peninsula of India, on

the heights of Ceylon, and on the volcanic cones of Java, many plants

occur, either identically the same or representing each other, and at the

same time representing plants of Europe, not found in the intervening hot

lowlands. A list of the genera collected on the loftier peaks of Java

raises a picture of a collection made on a hill in Europe! Still more

striking is the fact that southern Australian forms are clearly represented

by plants growing on the summits of the mountains of Borneo. Some of these

Australian forms, as I hear from Dr. Hooker, extend along the heights of

the peninsula of Malacca, and are thinly scattered, on the one hand over

India and on the other as far north as Japan.

On the southern mountains of Australia, Dr. F. Muller has discovered

several European species; other species, not introduced by man, occur on

the lowlands; and a long list can be given, as I am informed by Dr. Hooker,

of European genera, found in Australia, but not in the intermediate torrid

regions. In the admirable 'Introduction to the Flora of New Zealand,' by

Dr. Hooker, analogous and striking facts are given in regard to the plants

of that large island. Hence we see that throughout the world, the plants

growing on the more lofty mountains, and on the temperate lowlands of the

northern and southern hemispheres, are sometimes identically the same; but

they are much oftener specifically distinct, though related to each other

in a most remarkable manner.

This brief abstract applies to plants alone: some strictly analogous facts

could be given on the distribution of terrestrial animals. In marine

productions, similar cases occur; as an example, I may quote a remark by

the highest authority, Prof. Dana, that 'it is certainly a wonderful fact

that New Zealand should have a closer resemblance in its crustacea to Great

Britain, its antipode, than to any other part of the world.' Sir J.

Richardson, also, speaks of the reappearance on the shores of New Zealand,

Tasmania, &c., of northern forms of fish. Dr. Hooker informs me that

twenty-five species of Algae are common to New Zealand and to Europe, but

have not been found in the intermediate tropical seas.

It should be observed that the northern species and forms found in the

southern parts of the southern hemisphere, and on the mountain-ranges of

the intertropical regions, are not arctic, but belong to the northern

temperate zones. As Mr. H. C. Watson has recently remarked, 'In receding

from polar towards equatorial latitudes, the Alpine or mountain floras

really become less and less arctic.' Many of the forms living on the

mountains of the warmer regions of the earth and in the southern hemisphere

are of doubtful value, being ranked by some naturalists as specifically

distinct, by others as varieties; but some are certainly identical, and

many, though closely related to northern forms, must be ranked as distinct

species.

Now let us see what light can be thrown on the foregoing facts, on the

belief, supported as it is by a large body of geological evidence, that the

whole world, or a large part of it, was during the Glacial period

simultaneously much colder than at present. The Glacial period, as

measured by years, must have been very long; and when we remember over what

vast spaces some naturalised plants and animals have spread within a few

centuries, this period will have been ample for any amount of migration.

As the cold came slowly on, all the tropical plants and other productions

will have retreated from both sides towards the equator, followed in the

rear by the temperate productions, and these by the arctic; but with the

latter we are not now concerned. The tropical plants probably suffered

much extinction; how much no one can say; perhaps formerly the tropics

supported as many species as we see at the present day crowded together at

the Cape of Good Hope, and in parts of temperate Australia. As we know

that many tropical plants and animals can withstand a considerable amount

of cold, many might have escaped extermination during a moderate fall of

temperature, more especially by escaping into the warmest spots. But the

great fact to bear in mind is, that all tropical productions will have

suffered to a certain extent. On the other hand, the temperate

productions, after migrating nearer to the equator, though they will have

been placed under somewhat new conditions, will have suffered less. And it

is certain that many temperate plants, if protected from the inroads of

competitors, can withstand a much warmer climate than their own. Hence, it

seems to me possible, bearing in mind that the tropical productions were in

a suffering state and could not have presented a firm front against

intruders, that a certain number of the more vigorous and dominant

temperate forms might have penetrated the native ranks and have reached or

even crossed the equator. The invasion would, of course, have been greatly

favoured by high land, and perhaps by a dry climate; for Dr. Falconer

informs me that it is the damp with the heat of the tropics which is so

destructive to perennial plants from a temperate climate. On the other

hand, the most humid and hottest districts will have afforded an asylum to

the tropical natives. The mountain-ranges north-west of the Himalaya, and

the long line of the Cordillera, seem to have afforded two great lines of

invasion: and it is a striking fact, lately communicated to me by Dr.

Hooker, that all the flowering plants, about forty-six in number, common to

Tierra del Fuego and to Europe still exist in North America, which must

have lain on the line of march. But I do not doubt that some temperate

productions entered and crossed even the lowlands of the tropics at the

period when the cold was most intense,--when arctic forms had migrated some

twenty-five degrees of latitude from their native country and covered the

land at the foot of the Pyrenees. At this period of extreme cold, I

believe that the climate under the equator at the level of the sea was

about the same with that now felt there at the height of six or seven

thousand feet. During this the coldest period, I suppose that large spaces

of the tropical lowlands were clothed with a mingled tropical and temperate

vegetation, like that now growing with strange luxuriance at the base of

the Himalaya, as graphically described by Hooker.

Thus, as I believe, a considerable number of plants, a few terrestrial

animals, and some marine productions, migrated during the Glacial period

from the northern and southern temperate zones into the intertropical

regions, and some even crossed the equator. As the warmth returned, these

temperate forms would naturally ascend the higher mountains, being

exterminated on the lowlands; those which had not reached the equator,

would re-migrate northward or southward towards their former homes; but the

forms, chiefly northern, which had crossed the equator, would travel still

further from their homes into the more temperate latitudes of the opposite

hemisphere. Although we have reason to believe from geological evidence

that the whole body of arctic shells underwent scarcely any modification

during their long southern migration and re-migration northward, the case

may have been wholly different with those intruding forms which settled

themselves on the intertropical mountains, and in the southern hemisphere.

These being surrounded by strangers will have had to compete with many new

forms of life; and it is probable that selected modifications in their

structure, habits, and constitutions will have profited them. Thus many of

these wanderers, though still plainly related by inheritance to their

brethren of the northern or southern hemispheres, now exist in their new

homes as well-marked varieties or as distinct species.

It is a remarkable fact, strongly insisted on by Hooker in regard to

America, and by Alph. de Candolle in regard to Australia, that many more

identical plants and allied forms have apparently migrated from the north

to the south, than in a reversed direction. We see, however, a few

southern vegetable forms on the mountains of Borneo and Abyssinia. I

suspect that this preponderant migration from north to south is due to the

greater extent of land in the north, and to the northern forms having

existed in their own homes in greater numbers, and having consequently been

advanced through natural selection and competition to a higher stage of

perfection or dominating power, than the southern forms. And thus, when

they became commingled during the Glacial period, the northern forms were

enabled to beat the less powerful southern forms. Just in the same manner

as we see at the present day, that very many European productions cover the

ground in La Plata, and in a lesser degree in Australia, and have to a

certain extent beaten the natives; whereas extremely few southern forms

have become naturalised in any part of Europe, though hides, wool, and

other objects likely to carry seeds have been largely imported into Europe

during the last two or three centuries from La Plata, and during the last

thirty or forty years from Australia. Something of the same kind must have

occurred on the intertropical mountains: no doubt before the Glacial

period they were stocked with endemic Alpine forms; but these have almost

everywhere largely yielded to the more dominant forms, generated in the

larger areas and more efficient workshops of the north. In many islands

the native productions are nearly equalled or even outnumbered by the

naturalised; and if the natives have not been actually exterminated, their

numbers have been greatly reduced, and this is the first stage towards

extinction. A mountain is an island on the land; and the intertropical

mountains before the Glacial period must have been completely isolated; and

I believe that the productions of these islands on the land yielded to

those produced within the larger areas of the north, just in the same way

as the productions of real islands have everywhere lately yielded to

continental forms, naturalised by man's agency.

I am far from supposing that all difficulties are removed on the view here

given in regard to the range and affinities of the allied species which

live in the northern and southern temperate zones and on the mountains of

the intertropical regions. Very many difficulties remain to be solved. I

do not pretend to indicate the exact lines and means of migration, or the

reason why certain species and not others have migrated; why certain

species have been modified and have given rise to new groups of forms, and

others have remained unaltered. We cannot hope to explain such facts,

until we can say why one species and not another becomes naturalised by

man's agency in a foreign land; why one ranges twice or thrice as far, and

is twice or thrice as common, as another species within their own homes.

I have said that many difficulties remain to be solved: some of the most

remarkable are stated with admirable clearness by Dr. Hooker in his

botanical works on the antarctic regions. These cannot be here discussed.

I will only say that as far as regards the occurrence of identical species

at points so enormously remote as Kerguelen Land, New Zealand, and Fuegia,

I believe that towards the close of the Glacial period, icebergs, as

suggested by Lyell, have been largely concerned in their dispersal. But

the existence of several quite distinct species, belonging to genera

exclusively confined to the south, at these and other distant points of the

southern hemisphere, is, on my theory of descent with modification, a far

more remarkable case of difficulty. For some of these species are so

distinct, that we cannot suppose that there has been time since the

commencement of the Glacial period for their migration, and for their

subsequent modification to the necessary degree. The facts seem to me to

indicate that peculiar and very distinct species have migrated in radiating

lines from some common centre; and I am inclined to look in the southern,

as in the northern hemisphere, to a former and warmer period, before the

commencement of the Glacial period, when the antarctic lands, now covered

with ice, supported a highly peculiar and isolated flora. I suspect that

before this flora was exterminated by the Glacial epoch, a few forms were

widely dispersed to various points of the southern hemisphere by occasional

means of transport, and by the aid, as halting-places, of existing and now

sunken islands, and perhaps at the commencement of the Glacial period, by

icebergs. By these means, as I believe, the southern shores of America,

Australia, New Zealand have become slightly tinted by the same peculiar

forms of vegetable life.

Sir C. Lyell in a striking passage has speculated, in language almost

identical with mine, on the effects of great alternations of climate on

geographical distribution. I believe that the world has recently felt one

of his great cycles of change; and that on this view, combined with

modification through natural selection, a multitude of facts in the present

distribution both of the same and of allied forms of life can be explained.

The living waters may be said to have flowed during one short period from

the north and from the south, and to have crossed at the equator; but to

have flowed with greater force from the north so as to have freely

inundated the south. As the tide leaves its drift in horizontal lines,

though rising higher on the shores where the tide rises highest, so have

the living waters left their living drift on our mountain-summits, in a

line gently rising from the arctic lowlands to a great height under the

equator. The various beings thus left stranded may be compared with savage

races of man, driven up and surviving in the mountain-fastnesses of almost

every land, which serve as a record, full of interest to us, of the former

inhabitants of the surrounding lowlands.