OCR Text |
Show 332 GEOGRAPHICAL DISTRIBUTION, [CHAP. XL species belonging to genera exclusively confined to the south ~t these and other distant points of the southern hemi~phere is on my theory of descent with modification, a far more' re~arkable 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 modi: fica:tion to the necess~ry degree. Th~ f~cts seem .to me to indicate that pecuhar ana very - d1st1nct SpecieS have migrated in radiating lines from some common centre· and I am inclined to look in the southern, as in the north: ern hemisphere, to a former and warmer period, before the co1nmencement 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 mountainsummits, in a line gently rising from the arctic lowlands CliJ!P. XI.] DURING TH E GLACIA.L PERIOD . 333 to a great height under thee . thus left stranded 1nay be co quatord. The various beings d man, r1· ven up ancl survivin mp. are with savage races of of almost every .land, whict 111 the mountain-fastnesses . interest to us, of the former i heb':~ as a record, full of ing lowlands. n a 1 ants of the surround- |