CiFAP. XI. OF THE NORTH AND SOUTH. 349 



southward, by kpcpiiiG^ to the cooler currents, Avhile others 

 might remain and survive in the cooler depths, until the south- 

 ern hemisphere was in its turn subjected to a glacial climate 

 and permitted of their lurtlier progress ; in nearly the same 

 manner as, accortling to Forbes, isolated spaces inhabited by 

 Arctic productions exist to the present day in the deeper parts 

 of the temperate seas. 



I am far from supposing that all dilhculties in regard to 

 the distribution and ailinities of the identical and allied spe- 

 cies, 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 mi- 

 gration cannot be indicated. We cannot say why certain spe- 

 cies and not others have migrated ; why certain species have 

 been modified and have given rise to new forms, while others 

 liave remained unaltered. We cannot hope to explain such 

 facts, until we can say why one species and not another be- 

 comes naturalized l^y man's agency in a foreign land ; Avhy 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 in- 

 stance, the occurrence, as shown by Dr. Hooker, of tiie same 

 plants at points so enormously remote as Kerguelen Land, 

 New Zealand, and Fuegia; but icebergs, as suggested by Ly- 

 cll, may have l)een concerned in tlieir 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 me to indicate that dis- 

 tinct species belonging to the same genera have migrated in 

 radiating lines from a common centre ; and I am inclined to 

 look in tiie southern, as in the northern heniisphere, to a former 

 and warmer ])erio(l, before the commencement of the Ghicial 

 period, when the Antarctic lands, now covere^l with ice, sup- 

 ported a liighly pecuhar 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 vari- 

 ous points of the southern hemisphere l)y occasional means of 

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

 islands. Thus tlie southern shores of America, Australia, and 



