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THE AMERICAN NATURALIST [Vol. LIII 



sist in a given state— may well cause wonder and stimu- 

 late thought. 



In the following analysis of the larger units which in- 

 fluence migration, those agencies will be used which serve 

 as the basis for the smaller systems of action, and some 

 of their main cycles of activity and methods of interac- 

 tion will be indicated briefly. 



2. Atmospheric Agencies in Transportation 

 The instability of the air, its numerous cycles of activ- 

 ity or circulation, hourly, daily, seasonally, annually, and 

 those of longer duration, furnish an agency which has 

 transported animals from one locality to another for 

 ages. Gentle breezes carry small animals, while violent 

 tornadoes carry larger ones. Small eggs, desiccated 

 rotifers, entomostraca, and other small aquatic animals, 

 have been transported long distances by the wind, and 

 have thus found many favorable habitats, otherwise not 

 available to them. The wind, reinforced by streams, 

 even temporary ones, has transported animals long dis- 

 tances, as have the waves of the sea and inland waters. 

 The winds, supplementing the flight of animals, have car- 

 ried them thousands of miles beyond their normal range, 

 as in the case of birds and insects. A vast literature has 

 grown up recording the details of these findings, and yet 

 about the only evolutionary conclusion which can be 

 safely drawn from the multitude of facts is that by these 

 processes animals have tried out and acclimated them- 

 selves to a vast number of isolated habitats which have 

 tended to give them a varied and widespread range, and 

 to that degree it has aided in their perpetuation. 



The most definite evidence of atmospheric influence in 

 evolution is perhaps the direct influence of climate and 

 of climatic changes. Fortunately, for our present pur- 

 pose, and mainly through the researches of Chamberlin 

 (1897-1901) and Huntington ('15, pp. vi-vii), secular 

 climatic cycles have been investigated. Chamberlin has 

 related these intimately to the changes in the hydrosphere 



