164 



ANALYSIS OF THE ENVIRONMENT 



showoi for the spruce grouse in Figure 36, 

 in contrast with the slender toes character- 

 istic of the sharp-tailed grouse. These in- 

 stances illustrate the convergent adaptations 

 of animals that Hve on a shifting substra- 

 tum, whether it is sand or snow. 



»"^ 



■>-. 



■»1^J-- 



Fig. 35. The side winder, Crotalus cerastes, 



with tracks on sand (moving from lower right 



to upper left). ( Rephotographed from 

 Mosauer. ) 



The various structural convergences 

 found in the rapid-running mammals of the 

 usually firm-surfaced plains find a logical 

 extreme in the horse and antelope and, in 

 another direction of development, in the 



kangaroo and in the various jumping mice 

 and rats of the dry, open plains. Such ter- 

 restrial forms are to be contrasted with the 

 long-armed, short-legged apes and monkeys 

 that clamber about on the physical sub- 

 strate furnished by tropical trees. 



The solid ground, whether made of rock 

 fragments or soil, furnishes a medium in 

 which diverse kinds of animals hve, as well 

 as being a substratum for varied animal ac- 

 tivities. Dense populations of protozoans 

 and of larger, but still small to tiny, soft- 

 bodied animals, and some larger ones, too, 

 are restricted to living within the soil. 

 Others spend significant parts of their fives 

 in burrows, and many more are casual resi- 

 dents or visitors underground. A brief re- 



Fig. 36. "Snow-shoe" extensions (right) on 

 the feet of the spruce grouse (Dendragapus 

 canadiensis) contrasted with those of the sharp- 

 tailed grouse (Pediocetes phasianellus). (From 

 specimens in the Chicago Museum of Natural 

 History. ) 



view of some of the complex relations of 

 these animals to the soil would be suitable 

 here; such a discussion is reserved how- 

 ever, for Chapter 16, deafing with Ecologi- 

 cal Relations of the Soil. 



11. PHYSICOCHEMICAL AND CHEMICAL PHASES 



The emphasis in the last several chapters 

 has been upon environmental physics. 

 Light, heat, gravity, pressure, currents of 

 air and of water, sound, substratal vibra- 

 tions, mechanical shock, and the substratum 

 in general have been discussed. The extent 

 fro which the life and distribution of animals 



depends upon the action of the physical 

 forces of the world in which they live may 

 come as a surprise. The great influence of 

 water and of carbon dioxide in controlfing 

 the environment (p. 76) is related to the 

 physical as well as to the chemical proper- 

 ties of these important compounds. 



