7iV THE NORTU AND SOUTH, 403 



productions exist to the present day in the deeper parts of 

 the northern temperate seas. 



I am far from siipposinoj 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 mount- 

 ain-ranges, are removed on the views above given. The 

 exact lines of migration cannot be indicated. We cannot 

 say why certain species and not otliers have migrated; why 

 certain species have been modified and have given rise to 

 new forms, while others have remained unaltered. We 

 cannot 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 

 instance, the occurrence, as shown by Dr. Hooker, of the 

 same plants at points so enormously remote as Kerguelen 

 Land, ISTew 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 remark- 

 able 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 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 center; 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 been already widely dis- 

 persed to various points of the southern hemisiDhere by oc- 

 casional means of transport, and by the aid, as lial ting- 

 places, of now sunken islands. Thus the soutliern shores 

 of America, Australia, and New Zealand may have become 

 slightly tinted by the same peculiar forms of life. 



