124 DARWINISM STATED BY DARWIN HIMSELF. 



DIFFICULTIES NOT TET REMOVED. 



I am far from supposing that all the diffi- 

 *^^ ' culties in regard to the distribution and affini- 

 ties 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 can not 

 be indicated. We can not say why certain species and 

 not others have migrated ; why certain species have been 

 modified and have given rise to new forms, while others 

 have remained unaltered. We can not hope to explain 

 such facts, until we can say why one species and not an- 

 other 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 Ker- 

 guelen Land, New Zealand, and Puegia ; 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 can not 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 mi- 

 grated 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 



