Chap. XI. DURING THE GLACIAL PERIOD. 381 



why certain species have been modified and have given 

 rise to new groups of forms, and others have remained 

 unaltered. We cannot hope to explain such facts, 

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

 comes naturalised by man's agency in a foreign land ; 

 why one ranges twice or thrice as far, and is twice or 

 thrice as common, as another species within their own 

 homes. 



I have said that many difficulties remain to be solved : 

 some of the most remarkable are stated with admirable 

 clearness by Dr. Hooker in his botanical works on the 

 antarctic regions. These cannot be here discussed. I 

 will only say that as far as regards the occurrence of 

 identical species at points so enormously remote as 

 Kerguelen Land, New Zealand, and Fuegia, I believe 

 that towards the close of the Glacial period, icebergs, 

 as suggested by Lyell, have been largely concerned in 

 their dispersal. But the existence of several quite 

 distinct species, belonging to genera exclusively confined 

 to the south, at these and other distant points of the 

 southern hemisphere, is, on my theory of descent with 

 modification, a far more remarkable case of difficulty. 

 For some of these species are so distinct, that we cannot 

 suppose that there has been time since the commence- 

 ment of the Glacial period for their migration, and 

 for their subsequent modification to the necessary 

 degree. The facts seem to me to indicate that pe- 

 culiar and very distinct species have migrated in radi 

 a ting lines from some common centre ; and I am in- 

 clined to look in the southern, as in the northern hemi- 

 sphere, to a former and warmer period, before the com- 

 mencement of the Glacial period, when the antarctic 

 lands, now covered with ice, supported a highly peculiar 

 and isolated flora. I suspect that before tins flora was 

 exterminated by the Glacial ej>och, a few forms were 



