The Origin of Species

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The Origin of Species Page 40

by Charles Darwin

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.

 

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