382 
GEOGEAPHICAL DISTEIBUTION, 
Chap. XI. 
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 hemi¬ 
sphere by occasional means of transport, and by the aid, 
as halting-places, of existing and now sunken islands. 
By these means, as I believe, the southern shores of 
America, Australia, New Zealand, have become slightly 
tinted by the same peculiar forms of vegetable life. 
Sir 0. Lyell in a striking passage has speculated, in 
language almost identical with mine, on the effects of 
great alternations of climate on geographical distri¬ 
bution. I believe that the world has recently felt one 
of his great cycles of change; and that on this view, 
combined with modification through natural selection, 
a multitude of facts in the present distribution both 
of the same and of allied forms of life can be ex¬ 
plained. The living waters may be said to have flowed 
during one short period from the north and from the 
south, and to have crossed at the equator; but to 
have flowed with greater force from the north so as to 
have freely inundated the south. As the tide leaves 
its drift in horizontal lines, though rising higher on 
the shores where the tide rises highest, so have the 
living waters left their living drift on our mountain- 
summits, in a line gently rising from the arctic low¬ 
lands to a great height under the equator. The various 
beings thus left stranded may be compared with savage 
races of man, driven up and surviving in the mountain- 
fastnesses of almost every land, which serve as a record, 
full of interest to us, of the former inhabitants of the 
surrounding lowlands. 
