CHAP. XII.] IN THE NORTH AND SOUTH. 321 



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 species ranges twice or thrice as far, and is 

 twice or thrice as common, as another species within their own 

 homes. 



Various special difficulties also remain to be solved ; for instance, 

 the occurrence, as shown by Dr. Hooker, of the same plants at 

 points so enormously remote as Kerguelen Land, New Zealand, 

 and Fuegia ; but icebergs, as suggested by Lyell, may have been 

 concerned in their dispersal. The existence at these and other 

 distant points of the southern hemisphere, of species, which, 

 though distinct, belong to genera exclusively confined to the 

 south, is a more remarkable case. Some of these species are so 

 distinct, that we cannot suppose that there has been time since 

 the commencement of the last Glacial period for their migration 

 and subsequent modification to the necessary degree. The facts 

 seem to indicate that distinct species belonging to the same 

 genera have migrated in radiating lines from a common genera ; 

 and I am inclined to look in the southern, as in the northern 

 hemisphere, to a former and warmer period, before the commence- 

 ment of the last Glacial period, when the Antarctic lands, now 

 covered with ice, supported a highly peculiar and isolated flora. 

 It may be suspected that before this flora was exterminated during 

 the last Glacial epoch, a few forms had been already widely dis- 

 persed to various points of the southern hemisphere by occasional 

 means of transport, and by the aid as halting-places, of now 

 sunken islands. Thus the southern shores of America, Australia, 

 and New Zealand may have become slightly tinted by the same 

 peculiar forms of life. 



Sir C. Lyell in a striking passage has speculated, in language 

 almost identical with mine, on the effects of great alterations of 

 climate throughout the world on geographical distribution. And 

 we have now seen that Mr. Croll's conclusion that successive 

 Glacial periods in the one hemisphere coincide with warmer 

 periods in the opposite hemisphere, together with the admission 

 of the slow modification of species, explains a multitude of facts 

 in the distribution of the same and of the allied forms of life in 

 all parts of the globe. The living waters have flowed during one 

 period from the north and during another from the south, and in 

 both cases have reached the equator ; but the stream of life has 

 flowed with greater force from the north than in the opposite 

 direction, and has consequently more freely inundated the south. 

 As the tide leaves its drift in horizontal lines, 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 altitude under the 



