1 86 EVOLUTION OF TO-DAY. 



increased when we learn that no cases are known 

 where the same fresh-water species occur on either 

 side of a high continuous mountain range, except in 

 cases where human agencies have come into play, 

 and one or two instances of a remarkable distribu- 

 tion of some very old species. The sea is, then, not 

 an absolute barrier even for fresh-water animals, 

 and a certain amount of similarity of species in two 

 neighboring continents is not inexplicable. 



The last of the difficulties mentioned, viz., the 

 agreement of Alpine and Arctic flora, receives quite 

 a different explanation. The glacial period once 

 more proves of assistance. At the time when the 

 glaciers swept down over Northern Europe, they, of 

 course, drove in front of them all of the animals and 

 plants of the north, so that finally the southern parts 

 of the continents with an Arctic climate cajne to be 

 inhabited by a species belonging to the more north- 

 ern latitudes. When at last the ice began once 

 more to retreat northward, the climate of the south- 

 ern regions began to grow warmer, and the Arctic 

 species could not endure it. Some of them were 

 probably modified to suit the change in climate, 

 giving rise to new species. Others followed the re- 

 treating ice once more northward, keeping thus con- 

 stantly in an Arctic temperature ; while still others, 

 instead of going north, simply ascended the moun- 

 tains, and in this way kept themselves in a suitable 

 climate. Eventually the mountains and the Arctic 

 regions came to possess similar species. And by 

 extending the explanation, the occasional likeness 

 of species in the northern and southern continents 



