No. 



EVOLUTION 69 



environment as truly as complex ones. It is known that 

 many animals decline in vigor if kept in the same medium, 

 but if the medium is kept fresh, or the animal moves 

 about freely and secures a fresh medium, it thrives. 

 Freedom of movement thus permits the animal to move 

 away from influences which interfere with its system, and 

 thus minimizes the disturbance. The continuation of 

 this process tends, with freedom, to bring the animal ul- 

 timately into favorable non-interfering conditions if 



such exist. With these ideas in mind we need to recall 

 that the atmospheric, hydrospheric, lithospheric and 

 vegetational pressure all combine to encroach upon the 

 animal, and to interefere or reinforce its activities and 

 movements. The normal movements of the animal, and 

 the ordinary routine of environmental changes, are thus 

 in process of continual adjust iiuiit. Tims with the mi- 

 gration of the animal habitat, wlicthci' caused })y a change 

 in the atmosphere, hydrosphere, lithosphere, vegetation, 



