342 ON THE ORIGIN OF SPECIES 



difficulties remain to be solved. I do not pretend to 

 indicate the exact lines and means of migration, or 

 the reason why certain species and not others have 

 migrated ; 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 becomes 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 dis- 

 tinct 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 can- 

 not suppose that there has been time since the com- 

 mencement of the Glacial period for their migration, 

 and for their subsequent modification to the necessary 

 degree. The facts seem to me to indicate that peculiar | 

 and very distinct species have migrated in radiating ! 

 lines from some common centre ; and I am inclined 

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

 sphere, to a former and warmer period, before the 

 commencement of the Glacial period, when the ant- 

 arctic lands, now covered with ice, supported a highly 

 peculiar and isolated flora. I suspect that before this 

 flora was exterminated by the Glacial epoch, a few 

 forms were widely dispersed to various points of the 

 southern hemisphere by occasional means of transport, 



