170 ALTERNATE GLACIAL PERIODS. [Chap. XII. 



that before this flora was exterminated during the last 

 .Glacial epoch, a few forms had been already 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 Xew 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 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 direction, 

 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 haA^e 

 the living waters left their livinsr 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 serves as a record 

 full of interest to us, of the former inhabitants of the 

 surrounding lowlands. 



