342 ALTERNATE GLACIAL PERIODS. [Chap. XII. 



been already widely dispersed to various points of the southern hemi- 

 sphere by occasional means of transport, and by the aid as halting- 

 places, of now sunken islands. Thus the southern shores of America, 

 Australia, and New Zealand, may have become slightly tinted by 

 the same peculiar forms of life. 



Sir C. Lyell in a striking passage has speculated, in language 

 almost identical with mine, on the effects of great alternations of 

 climate throughout the world on geographical distribution. And 

 we have now seen that Mr. Croll's conclusion that successive Glacial 

 periods in the one hemisphere coincide with warmer periods in the 

 opposite hemisphere, together with the admission of the slow modifi- 

 cation of species, explains a multitude of facts in the distribution of 

 the same and of the allied forms of life in all parts of the globe. The 

 living waters have flowed during one period from the north and 

 during another from the south, and in both cases have reached the 

 equator : but the stream of life has flowed with greater force from 

 the north than in the opposite direction, and has consequently more 

 freely inundated the south. As the tide leaves its drift in hori- 

 zontal linGS, 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 lowlands to a great 

 altitude 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. 



