66 THE AMERICAN NATURALIST [Vol. LIII 



which influence active animal migrations are mainly 

 those which are dependent upon: chemical composition 

 (largely oxygen, CO2 and volcanic gases) ; temperature; 

 pressure ; humidity ; and mechanical etfects, as they are 

 combined in climatic changes. All of these influences un- 

 dergo differences which influence, by acceleration or in- 

 terference, the movements and migrations, acclimatiza- 

 tion and ecological attunement of animals. Furthermore, 

 these influences, or their systems, do not act independ- 

 ently but at the same time, so that their laws of inter- 

 action are the main rules of the game. 



{h) Hydrospheric Influence. — The hydrospheric influ- 

 ences are similar to those of the air, depending on : com- 

 position (salts and gases); temperature; pressure; and 

 mechanical effects (waves, tides, circulating currents). 

 In this geological age of stress and diversity, on account 

 of the mobility of this medium, it has transmitted its 

 pressure with slight friction to animals. Upon land the 

 active streams are a direct response to the steep slopes 

 down which they flow, and they visualize at once the 

 reality of this active media which has kept fishes and 

 other animals busy moving up stream for millions of 

 years. Bodies of standing water, by their isolation, ex- 

 cept when connected with streams, tend to retard active 

 animal migration. Such bodies are likely to abound in 

 the early stages of uplift and to decline as drainage lines 

 develop. The diversity of climate into humid and arid 

 as previously mentioned, tends to favor diversity, chem- 

 ically, in bodies of standing water. These inland waters, 

 while discontinuous to many aquatic animals, are not so 

 to many flying and running kinds. 



In the sea, the narrowing of the continental shelf tends 

 to crowd the shore animals, and favors isolation and di- 

 versity of habitat, and retards ready migration for many 

 animals. The hastened erosion, however, tends to in- 

 crease the continental shelf and its continuity. The 

 deeper w^ater shows relative diversity and tends toward 

 stagnation in its currents because of the relative increase 

 of the land area. 



