170 ALTERNATE GLACIAL PERIODS. [Chap. XU, 



be suspected that before this flora was exterminated 

 during the last Glacial epoch, a few forms had been al- 

 ready widely dispersed to various points of the southern 

 hemisphere 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 pecuhar 

 forms of life. 



Sir C. Lyell in a strildng passage has speculated, in 

 language 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 conclusion 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 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 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 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. 



