IN THE NORTH AND SOUTH. 379 



distribution. And we have now seen that Mr. CrolFs con- 

 clusion 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 inun- 

 dated the south. As the tide leaves its drift in horizontal 

 lines, rising higher on the shores where the tide rises high- 

 est, 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 fast- 

 nesses of almost every land, which serves as a record, full of 

 interest to us, of the former inhabitants of the surrounding 

 lowlands. 



