GEOGRAPHICAL DISTRIBUTION 177 



ward, by keeping to the cooler currents, while others 

 might remain and survive in the colder depths until the 

 southern hemisphere was in its turn subjected to a gla- 

 cial climate and permitted their further progress; in 

 nearly the same manner as, according to Forbes, iso- 

 lated 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 sometimes on the intermediate 

 mountain-ranges, are removed on the views above given. 

 The exact lines of migration cannot be indicated. We 

 cannot say why certain species and not others have mi- 

 grated; why certain species have been modified and have 

 given rise to new forms, while others have remained un- 

 altered. We cannot hope to explain such facts, until we 

 can say why one species and not another becomes natu- 

 ralized by man's agency in a foreign land; why one spe- 

 cies 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 

 t dispersal. The existence at these and other distant points 

 ijof the southern hemisphere, of species, which, though 

 jdistinct, belong to genera exclusively confined to the 

 jsouth, is a more remarkable case. Some of these species 

 tore so distinct that we cannot suppose that there has 



