426 ORIGIN OF SPECIES 



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 lan- 

 guage almost identical with mine, on the effects of great 

 alterations of climate throughout the world on geographical 

 distribution. And we have now seen that Mr. Croll's conclu- 

 sion that successive Glacial periods in the one hemisphere 

 coincide with warmer periods in the opposite hemisphere, 

 together with the admission of the slow modification of spe- 

 cies, 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 direc- 

 tion, and has consequently more freely inundated the south. 

 As the tide leaves its drift in horizontal lines, 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 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 serves as a record, full of interest to us, of the former 

 inhabitants of the surrounding lowlands. 



