64 THE AMERICAN NATURALIST [Vol. LIII 



The most important character which influences the mi- 

 gration of animals is its own powers of movement. These 

 movements are dependent upon the ecology and the 

 physiology of the animal, its structure and its mode of 

 response. The general principles of response have 

 been discussed in the first part of this paper, where 

 the systems of activity, the cycles of activity, the 

 limits of activity, and the interaction of all systems 

 was emphasized. All of these factors should now 

 be recalled. Animals creep, walk, swim, and fly, accord- 

 ing to the media in which they live, their structure, and 

 their ecology, and the interaction of all these factors put 

 limitations upon animal movements. 



From the standpoint of function, animal movements 

 and migrations have two main influences. By move- 

 ment the animal subjects itself to new conditions, these 

 conditions have a direct influence upon the animal, and 

 change the direction or its internal changes, and it be- 

 comes acclimated or dies; or by its repeated responses 

 and retreating movements, it escapes from the adverse 

 conditions and finally comes to rest in a new relatively 

 favorable condition (Adams, '15, p. 12). This monoton- 

 ous cycle is repeated with all the variations which diver- 

 sity of animals and diversity of conditions can produce, 

 and in its essential features it is the same from Protozoa 

 to man. 



The geological age in which we live is one in which the 

 land surface, relative to the sea, has accumulated uplift 

 from former ages, and has been newly elevated, and as a 

 result there are many high mountains, and the seas are 

 relatively deep. These are conditions of stress, and the 

 processes of adjustment to strain are in full operation. 

 This is a period of relative diversity of the lands and 

 of the seas, which favors diversity, both in the atmos- 

 phoi-o and in the hydrosphere. With the elevation of 

 the land, this diversity is shown both vertically and 

 hoii/ontally. Large areas lie at considerable altitudes 

 and in their departure from the narrow mean surface 



