No. 624] MIGRATION A FACTOR IN EVOLUTION 67 



(c) Litho spheric Influences.— The interdependence of 

 the physical spheres is so marked that by this time, in the 

 discussion of the air and water, the solid earth has been 

 included in part. The rigidity of the lithosphere is so 

 great that its adjustments to strain are in general rela- 

 tively slow and of long duration. The density of the 

 medium is so great that animals inhabit only a shallow 

 surface stratum, the upper part of the zone of weather- 

 ing processes, in or on the soil. Animals living in the 

 soil are influenced by its density, its physical and chem- 

 ical composition, temperature, and its movements. Those 

 living on it are relatively independent of the quali- 

 ties just mentioned, but are much influenced by the relief 

 of the surface, by the climate and vegetation, and are 

 more truly air rather than earth dwellers. The elevation 

 of the land above sea in itself, and not as modified by 

 climate and topography, probably has little direct influ- 

 ence, except in its degree of stability with regard to 

 erosion. The greater the altitude and the steeper the 

 slope, the greater the physical stress and the rapidity of 

 erosion. Ice and landslides exert pressure and drive 

 animals before them, and are largely dependent on eleva- 

 tion and slope. The cycle of degradation of the land, 

 particularly its topographic diversity, greatly influences 

 the degree of freedom in the movement of land animals. 



{d) Plant Influences.— The stresses of the physical en- 

 vironment in the air, water and earth, impose pressure 

 upon the vegetation. Since the largest number of ani- 

 mals are directly dependent, and a smaller number indi- 

 rectly upon plants, much of this pressure is transmitted 

 to animals. The climatic diversity, seasonal and secular 

 cycles, influence the amount of animal food. Some ani- 

 mals, during adverse seasonal conditions and scanty food 

 supply pass into an inactive state, and tide over such a 

 season, and most animals not possessing such tend to mi- 

 grate. Thus upon the plains the bison wandered with the 

 seasonal changes of pasture, just as mountain sheep and 

 goats migrate up and down the slopes as their pasture 



