ALTERNATE GLACIAL rKKIOUS 425 



the southern hemisphere was in its turn subjected to a glacial 

 cHmate and permitted their further progress; in nearly the 

 same manner as, according to Forbes, isolated spaces inhab- 

 ited 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 remark- 

 able case. Some of these species are so distinct, that we 

 cannot suppose that there has been time since the commence- 

 ment 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 

 genera; 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 .Vnt- 

 arctic 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 



