56 



THE AMERICAN NATURALIST 



[Vol. LIIl 



SO must be the migratory ones. In detail there are in- 

 numerable animal peculiarities which influence migra- 

 tion, depending on the stage of development of the ani- 

 mal, its physiological and ecological conditions and char- 

 acteristics, and the status of its environment. The large 

 number of factors involved in this is no doubt an impor- 

 tant conservative influence and checks the speed of in- 

 teraction. 



The word migration is used in several senses, so that 

 for our purpose it is now necessary to define this more 

 definitely. By migration is here meant the movement of 

 animals from one place to another, and this includes, not 

 only the causes and conditions of their migration, but 

 their methods as well and the immediate result upon the 

 animal. If all migrating animals perished at the end of 

 their journey the study of its influence would be rela- 

 tively simple. 



In deference to those who are mainly interested in the 

 animal and who are less interested in the environment it 

 has been customary in many zoological writings to dis- 

 cuss animals first, and their environment later. But as 

 zoology progresses and as explanations are resolved 

 more and more into the sciences upon which it rests, 

 greater and greater prominence is given to the physical 

 causes and conditions of the environment. .Viewed 

 broadly, zoology should be made to fit into the general 

 world system in such a manner as will best aid in under- 

 standing it, irrespective of our traditional habits of mind. 

 For this reason this phase of the discussion will begin 

 with the environment, as a factor in passive migration or 

 transportation, and will lead up to the animal as a factor 

 in its owTi migrations. 



In the orientation of the major features of the world 

 Powell recognized : the atmosphere, the hydrosphere, the 

 lithosphere and the biosphere. These self-explanatory, 

 relatively homogeneous, physically and mechanically dis- 

 tinct, and interacting systems, furnish the medium in 

 wliich animals live and perform their migrations. We 



