GEOGRAPHICAL DISTRIBUTION 349 



isolated spaces inhabited by arctic productions exist to the present 

 day in the deeper parts of the northern temperate seas. 



I am far from supposing that all the difficulties in regard to the 

 distribution and affinities of the identical and allied species, which 

 now live so widely separated in the north and south, and some- 

 times on the intermediate mountain-ranges, are removed on the 

 views above given. The exact lines of migration cannot be in- 

 dicated. We cannot say why certain species and not others have 

 migrated; why certain species have been modified and have given 

 rise to new forms, while others have remained unaltered. We can- 

 not hope to explain such facts, until we can say why one species 

 and not another becomes naturalized 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 in- 

 stance, 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 com- 

 mencement of the last Glacial period for their migration and sub- 

 sequent 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 centre; anr I am in- 

 clined to look in the southern, as in the northern hemisphere, to 

 a former and warmer period, before the commencement 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 dispersed to various 

 points of the southern hemisphere by occasional means of trans- 

 port, 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 Gla- 

 cial periods in the one hemisphere coincide with warmer periods 

 in the opposite hemisphere, together with the admission of the 



