404 ALTERNATE GLACIAL PERIODS. 



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 geograph- 

 ical distribution. And we have now seen that Mr. CroU's 

 conclusion that successive Glacial periods in the one hemi- 

 sphere coincide with warmer periods in the opposite hemi- 

 sphere, together with the admission of the slow modifica- 

 tion of species, explains a multitude of facts in the distri- 

 bution 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 readied 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 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 surviv- 

 ing in the mountain fastnesses of almost every land, which 

 serves as a record, full of interest to us, of the former in- 

 habitants of the surrounding lowlands. 



