62 THE AMERICAN NATURALIST [Vol. LIII 



the most powerful motion of the solid earth is seen in the 

 crustal movements, associated with the cycle of isostacy, 

 which elevates and depresses the surface of the land in 

 relation to sea level. In this is seen an essential condi- 

 tion which has made all land life possible, because with- 

 out such movements all the land would have been washed 

 into the larger dominating sea. The great land eleva- 

 tions, such as those which produce plateaus and moun- 

 tains, have transported whole faunas, covering thou- 

 sands of square miles, upward, and have subjected them 

 to great stress, through long periods of time. Such ele- 

 vations as arise in a region unfavorable to animals, may 

 improve them, as in the case of high mountains, rising on 

 a dry desert, but often such elevations, which are de- 

 partures from the favorable thin surface stratum, are 

 in the direction of unfavorable conditions and of limiting 

 factors. Broadly speaking, depressions below sea level 

 are similarly limiting to marine organisms, and these 

 have operated on a magnificent scale. The mountain 

 tops, like the deeps of the sea, are relatively animal 

 deserts, both are extreme departures from the conditions 

 which are normal to most animals. 



The most rapid physical agency in the transportation 

 of animals on land is the influence of running water and 

 that of the wind. These forces operate in short cycles 

 and intensively, in contrast with the movements of the 

 solid earth. 



Volcanic activity has probably been only a minor factor 

 in the transportation of animals, although in a secondary 

 way, in conjunction with other agencies, as currents of 

 water, porous materials buoyed by air, may act as a raft 

 in their transportation. But indirectly by building moun- 

 tains, islands, etc., it has had an influence similar to that 

 of the crustal movements of the earth in forming new 

 habitats, and has thus had a powerful effect. 



