382 GEOGEAPHICAL DISTRIBUTION. Chap. XI. 



widely dispersed to various points of the southern 

 hemisphere by occasional means of transport, and by 

 the aid, as halting-places, of existing and now sunken 

 islands, and perhaps at the commencement of the 

 Glacial period, by icebergs. By these means, as I be- 

 lieve, the southern shores of America, Australia, New 

 Zealand have become slightly tinted by the same pecu- 

 liar forms of vegetable life. 



Sir C. 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. 



