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 sho
rt 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.
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.