RELATION OF MAMMALS TO ENVIRONMENT. 435 



bility. Indeed, it is not too imich to avssert that the aesthetic and 

 moral senses of man have developed, wholly or largely, as the result 

 of home constructing which is, from the standpoint of evolution, 

 the first step in acquiring property. 



It will be seen from the foregoing statements that the biological 

 environment is the chief ageney in developing the mental capacity 

 of an animal. It is true that the struggle for existence is with the 

 physical as well as the biological environment, but physical condi- 

 tions are, to a very large extent, beyond the ability of an animal to 

 modify or control. 



Hence the range of the individual, as well as the distribution 

 of the species, is determined principally by physical conditions. 

 Of course the range may be limited by food supply, but no species is 

 found in every place where it might secure food nor does it usually 

 exhaust the supply. Rivers, lakes and smaller bodies of water are 

 often barriers for individuals but not for the species. A mouse 

 or shrew may live along the bank of a small stream and go no 

 farther in one direction than the water, but in time the species will 

 cross it or pass around it. 



But there are other barriers that are less obvious. Certain of 

 the meadow mice live in swampy places, and a low hill will ordi- 

 narily prove a barrier to one of these creatures although not to the 

 species since they occasionally venture on the higher ground and 

 often inhabit it in wet weather. Some of the ground squirrels live 

 only in dry ground and a marsh proves an effective barrier for 

 them. Other meadow mice live in grassy fields, and their food is 

 principally the tender basal portions of the stems of grass and 

 clover. They could secure plenty of food in a pasture or meadow, 

 but they are rarely found in such places because they make tunnels 

 or runways under dry, fallen grass and are seldom found about 

 farms except along fence rows and in places where the grass is 

 allowed to grow up and remain uncut. Here the grass constitutes 

 a shelter and is to be regarded as having a physical rather than a 

 biological relation to the animal. 



Again, there are many animals that live in fields, and a strip of 

 woodland will limit their range. On the other hand, squirrels and 

 other tree dwellers are bounded by areas of prairie or open field. 

 In such cases the species, sooner or later, cross or go around these 

 barriers but this occurs only under unusual conditions, or after 

 the lapse of a long period. To the average individual, living the 

 average life, the limits of activity are much more narrowly cir- 

 cumscribed than they are for the species as a whole. 



