Chap. XI. 
DURING THE GLACIAL PERIOD. 
381 
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 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 
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 
hemisphere, to a former and warmer period, before the 
commencement of the Glacial period, when the antarctic 
lands, now covered with ice, supported a highly peculiar 
