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Show 17~ EFFECTS OF CHANGES OF CLIMATE [Ch.X. the ocean · h more uniform than that of the atmosphere in- IS muc vest·m g t 11 e 1a n d ' so that we may easily suppose that most of th. e testacea, fi sh , a nd other classes ' miOo 'ht pass from the equatonal w· to t h e t emper ate reO'ions if the mean temperature of those 0 ' • • regw· ns were t 1·a nsposcd , although a second . expatriatiOn o. f t 11 e:se speci•e s of tropical orioo· in into the arctic and antarctic circles would probably be impossible. On the principles above explained, if we found. that at some ~ · d as when for example, our carbomferous stmta 1ormer peno , ' were deposited, the same tree-ferns and other pl~nts i~habited the regions now occupied by Europe and Van D1eman s Land, we should suspect that the species in question bad, at some antecedent period, inhabited lands within the tropics, and that an m· crease of tl1e n1ean annual heat had caused them to emi-grate into both the temperate zones. '!'here are no .g~ological data, however, as yet obtained, to warrant th~ opm10~ that such identity of species existed in the two hemispheres m the era in question. . . . Let us now consider more particularly the effect of viciSSl· tudes of climate in causing one species to give way before the increasinO' numbers of some other. 'Vhcn°temperature forms the barrier which arrests the progress of an animal or plant in a particular direction, the individuals are fewer and less vigorous as they approach ~he extreme confines of the geographical range of the species. But these stragglers are ready to multiply ra?idly on the slightest increase or diminution of l~eat that rna~ be favourable to them, just as particular insects mcrease dunng a hot su~mer, and certain plants and animals gain ground after a series of conO'enial seasons. In almost every district, especially if it be mountainous, there are a variety of species the limits of whose habitations are con- termm. ous, some b e·m g unab l e t o proc.e e d farther . w.i thout encounterinO' too much heat, others too much cold. Individual~, 0 which are thus on the borders of t b e regw· ns proper to th.e ir respective species, are like the out-posts of hostile armies, Ch.X.] ON THE DISTRIBUTION OF SPECIES. 173 ready to profit by every slight change of circumstances in their favour, and to advance upon the ground occupied by their neighbours and opponents. The proximity of distinct climates, produced by the inequalities of the earth's surface, brings species possessing very different constitutions into such immediate contact, that their naturalizations are very speedy whenever opportunities of advancing present themselves. Many insects and plants, for example, are common to low plains within the arctic circle, and to lofty mountains in Scotland and other parts of Europe. If the climate, therefore, of the polar regions were transferred to our own latitudes, the species in question would immediately descend from these elevated stations to overrun the low grounds. Invasions of this kind, attended by the expulsion of the pre-occupants, are almost instantaneous, because the change of temperature not only places the one species in a more favourable position, but renders the others sickly and almost incapable of defence. Lamarck appears to have speculated on the modifications to which every variation of external circumstances might give rise in the form and organization of species, as if he had indefinite periods of time at his command, not sufficiently reflecting that revolutions in the state of the habitable earth, whether by changes of climate or any other condition, are attended by still greater fluctuations in the relative condition of contemporary species. They can avail themselves of these alterations in their favour instantly, and augment their ~umbers to the injury of some other species; whereas the supposed transmutations are only assumed to be brought about by slow and insensible degrees, and in a lapse of ages, the duration of which is beyond the reach of human conception Even if we thought it possible that the palm or the elephant, which now flourish in equatorial regions, could ever Jearn to bear the variable seasons of our temperate zone, or the rigours of an arctic winter we should . ' ' wuh no less confidence, affirm, that they must perish before they had time to become habituated to such new circumstances, |
