-Chap. XII. in the North and South. 341 



The same principles apply to the distribution of terrestrial 

 animals and of marine productions, in the northern and southern 

 temperate zones, and on the intertropical mountains. When, 

 during the height of the Glacial period, the ocean-currents were 

 widely different to what they now are, some of the inhabitants of 

 the temperate seas might have reached the equator ; of these a few 

 would perhaps at once be able to migrate southward, by keeping to 

 the cooler currents, whilst others might remain and survive in the 

 colder depths until the southern hemisphere was in its turn sub- 

 jected to a glacial climate and permitted their further progress ; in 

 nearly the same manner as, according to Forbes, 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 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 migrated ; 

 why certain species have been modified and have given rise to new 

 forms, whilst others have remained unaltered. 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 sup- 

 pose 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 centre; and I am inclined 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 



