68 



THE AMERICAN NATURALIST 



[Vol. LIII 



varies with the season. The succession of vegetation 

 upon all surfaces, drives out some animals, just as it in- 

 vites others to follow with it, as when, with the develop- 

 ment of forest, the animals of the open find conditions 

 unfavorable. The kind of vegetation, whether herba- 

 ceous, woody, conifer or deciduous, etc., has an impor- 

 tant influence upon the movements of many animals. 

 The cycles of vegetation also change the physical condi- 

 tions, the temperature, humidity, soil conditions, and 

 thus influence animals. 



(e) Animal Migrations.— The individual migrations of 

 animals, caused by their owii spontaneity, and that by 



other animals, aiv .■xcvdiiiuly vari.Ml. All the factors 

 which influence individual niovriHciits, as indicated in the 

 first part of this papci', now :i\)]Ay in detail, and in addi- 

 tion there is the pressure exci-tcd 1>\- aninial> lix in.^' asso- 

 ciated with them. Simple aiiiiuals re(iuire a favorable 



