378 ALTERNATE GLACIAL PERIODS 



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 or 

 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, 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, 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 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 

 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 been already widely dispersed to various 

 points of the southern hemisphere by occasional means of 

 transport, and by the aid, as halting-places, of now sunken 

 islands. Thus the southern shores of America, Australia, 

 and New Zealand may have become slightly tinted by the 

 same peculiar forms of life. 



Sir C. Lyell in a striking passage has speculated, in lan- 

 guage almost identical with mine, on the effects of great 

 alterations of climate throughout the world on geographical 



