物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 11. GEOGRAPHICAL DISTRIBUTION. Page 4
Thus, as I believe, a considerable number of plants, a few terrestrialanimals, and some marine productions, migrated during the Glacialperiod from the northern and southern temperate zones into theintertropical regions, and some even crossed the equator. As thewarmth returned, these temperate forms would naturally ascend thehigher mountains, being exterminated on the lowlands; those which hadnot reached the equator, would re-migrate northward or southwardtowards their former homes; but the forms, chiefly northern, which hadcrossed the equator, would travel still further from their homes intothe more temperate latitudes of the opposite hemisphere. Although wehave reason to believe from geological evidence that the whole body ofarctic shells underwent scarcely any modification during their longsouthern migration and re-migration northward, the case may have beenwholly different with those intruding forms which settled themselveson the intertropical mountains, and in the southern hemisphere. Thesebeing surrounded by strangers will have had to compete with many newforms of life; and it is probable that selected modifications in theirstructure, habits, and constitutions will have profited them. Thusmany of these wanderers, though still plainly related by inheritanceto their brethren of the northern or southern hemispheres, now existin their new homes as well-marked varieties or as distinct species.
It is a remarkable fact, strongly insisted on by Hooker in regard toAmerica, and by Alph. de Candolle in regard to Australia, that manymore identical plants and allied forms have apparently migrated fromthe north to the south, than in a reversed direction. We see, however,a few southern vegetable forms on the mountains of Borneo andAbyssinia. I suspect that this preponderant migration from north tosouth is due to the greater extent of land in the north, and to thenorthern forms having existed in their own homes in greater numbers,and having consequently been advanced through natural selection andcompetition to a higher stage of perfection or dominating power, thanthe southern forms. And thus, when they became commingled during theGlacial period, the northern forms were enabled to beat the lesspowerful southern forms. Just in the same manner as we see at thepresent day, that very many European productions cover the ground inLa Plata, and in a lesser degree in Australia, and have to a certainextent beaten the natives; whereas extremely few southern forms havebecome naturalised in any part of Europe, though hides, wool, andother objects likely to carry seeds have been largely imported intoEurope during the last two or three centuries from La Plata, andduring the last thirty or forty years from Australia. Something of thesame kind must have occurred on the intertropical mountains: no doubtbefore the Glacial period they were stocked with endemic Alpine forms;but these have almost everywhere largely yielded to the more dominantforms, generated in the larger areas and more efficient workshops ofthe north. In many islands the native productions are nearly equalledor even outnumbered by the naturalised; and if the natives have notbeen actually exterminated, their numbers have been greatly reduced,and this is the first stage towards extinction. A mountain is anisland on the land; and the intertropical mountains before the Glacialperiod must have been completely isolated; and I believe that theproductions of these islands on the land yielded to those producedwithin the larger areas of the north, just in the same way as theproductions of real islands have everywhere lately yielded tocontinental forms, naturalised by man's agency.
I am far from supposing that all difficulties are removed on the viewhere given in regard to the range and affinities of the allied specieswhich live in the northern and southern temperate zones and on themountains of the intertropical regions. Very many difficulties remainto be solved. I do not pretend to indicate the exact lines and meansof migration, or the reason why certain species and not others havemigrated; why certain species have been modified and have given riseto new groups of forms, and others have remained unaltered. We cannothope to explain such facts, until we can say why one species and notanother becomes naturalised by man's agency in a foreign land; why oneranges twice or thrice as far, and is twice or thrice as common, asanother species within their own homes.
I have said that many difficulties remain to be solved: some of themost remarkable are stated with admirable clearness by Dr. Hooker inhis botanical works on the antarctic regions. These cannot be herediscussed. I will only say that as far as regards the occurrence ofidentical species at points so enormously remote as Kerguelen Land,New Zealand, and Fuegia, I believe that towards the close of theGlacial period, icebergs, as suggested by Lyell, have been largelyconcerned in their dispersal. But the existence of several quitedistinct species, belonging to genera exclusively confined to thesouth, at these and other distant points of the southern hemisphere,is, on my theory of descent with modification, a far more remarkablecase of difficulty. For some of these species are so distinct, that wecannot suppose that there has been time since the commencement of theGlacial period for their migration, and for their subsequentmodification to the necessary degree. The facts seem to me to indicatethat peculiar and very distinct species have migrated in radiatinglines from some common centre; and I am inclined to look in thesouthern, as in the northern hemisphere, to a former and warmerperiod, before the commencement of the Glacial period, when theantarctic lands, now covered with ice, supported a highly peculiar andisolated flora. I suspect that before this flora was exterminated bythe Glacial epoch, a few forms were widely dispersed to various pointsof the southern hemisphere by occasional means of transport, and bythe aid, as halting-places, of existing and now sunken islands, andperhaps at the commencement of the Glacial period, by icebergs. Bythese means, as I believe, the southern shores of America, Australia,New Zealand have become slightly tinted by the same peculiar forms ofvegetable life.
Sir C. Lyell in a striking passage has speculated, in language almostidentical with mine, on the effects of great alternations of climateon geographical distribution. I believe that the world has recentlyfelt one of his great cycles of change; and that on this view,combined with modification through natural selection, a multitude offacts in the present distribution both of the same and of allied formsof life can be explained. The living waters may be said to have flowedduring one short period from the north and from the south, and to havecrossed at the equator; but to have flowed with greater force from thenorth so as to have freely inundated the south. As the tide leaves itsdrift in horizontal lines, though rising higher on the shores wherethe tide rises highest, so have the living waters left their livingdrift on our mountain-summits, in a line gently rising from the arcticlowlands to a great height under the equator. The various beings thusleft stranded may be compared with savage races of man, driven up andsurviving in the mountain-fastnesses of almost every land, which serveas a record, full of interest to us, of the former inhabitants of thesurrounding lowlands.