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THE AMEBIC AN NATURALIST [Vol. LIII 



the various highs and lows (Adams, ^Iba, pp. 69-71). 

 These transitional periods illustrate Bancroft's law in 

 the process of establishing new relative equilibria. The 

 dynamic centers are to be looked upon as concentrating, 

 transforming and radiating centers, whose recognition 

 and cycles of activity are an essential part in the applica- 

 tion of Bancroft 's law to the development of atmospheric 

 equilibria. 



5. Hydrospheric Agencies in Transportation 

 The waters of the earth are more dense than the air 

 but are yet quite mobile, and undergo relatively rapid 

 cycles of circulation, both in the sea and in inland waters. 



(a) Marine.— The great currents of the sea, the tides, 

 and the wind-formed waves, are very active agents in the 

 transportation of animals. Not only are marine animals 

 extensively transported, but also, in the long run, large 

 numbers of land kinds, as the animals on oceanic islands 

 testify, as shown by Wallace. And just as the processes 

 of erosion operate upon land, and tend to reduce such 

 areas to sea level, so the sea itself possesses its own 

 cycles of transformation of its bottom and shores, tend- 

 ing to flatten them out to the equilibrium of the deep sea 

 floor, transporting materials and redistributing them in 

 response to its stresses, eroding here, depositing there, 

 and always making changes in the conditions which not 

 only transport animals, but as well, by the migration of 

 the physical conditions, lead animals from one locality 

 to another. Land animals are largely influenced by the 

 surface conditions, while the marine ones are largely by 

 sub-surface conditions. 



(h) Inland Water Bodies.— Bodies of inland standing 

 water, in their broad features, are smaller editions of the 

 seas, as far as their waves, circulation and transport- 

 ing powers are concerned. Their chemical character, 

 whether fresh or saline, has more influence upon animals 

 than the mechanics or their methods of circulation. The 

 most marked influence of the inland waters is their rela- 



