Chap. XII.] IN THE NORTH AND SOUTH. 169 



and allied species, which now live so widely separated 

 in the north and south, and sometimes on the inter- 

 mediate 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 difiiculties 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 suppose 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 



