93 



an animal lives in the open or in the forest, would be unwarranted in 

 the light of the preceding discussion of the effect of vegetation upon 

 air temperatures, winds, humidity, relative evaporating power of the 

 air, and corresponding changes in the soil. Animal life is most 

 abundant in a narrow vertical layer above the earth's surface, by far 

 the most of it is within a few inches or feet of the surface ; and above 

 the level of the forest-crown it diminishes with great rapidity. Be- 

 low the surface of the soil the same general law holds; most of the 

 ground animals are within the first few inches of soil, only a small 

 number extending a few feet below the surface, and those found at 

 greater depths being indeed very few. The rate of decline is many 

 times more rapid below the surface than it is above it. There is, then, 

 above and below the surface a rapid and progressive attentuation of 

 the favorable conditions for animals and plants, and the animals do 

 not establish thriving communities far from those physical conditions 

 which are also favorable to vegetation. Animals are dependent upon 

 plants for food, but both are dependent upon a certain complex of 

 physical conditions near the surface of the earth. 



It is well to recall at this point how the influence of the climate and 

 the vegetation exemplify certain general laws which operate in all hab- 

 itats. The differentiation of habitats upon the earth is primarily due 

 to temperature and the specific heat relations of the earth, which re- 

 sult in the several media — gases, liquids, and solids. With a higher 

 temperature all would be gas, and with a lower one all would be solidi- 

 fied. The present intermediate conditions, therefore, permit the pres- 

 ent differentiation. These media are further differentiated by tem- 

 perature about as follows : Since the source of solar energy, heat, and 

 light, and the oxygen supply, are above the surface of the earth, the 

 vertical attenuation of these influences is one of the most striking 

 peculiarities of animal habitats, both in water (where the causes have 

 long been recognized) and upon land. Any covering of the earth, 

 even the surface layer of vegetation, soil and water, tends to shut off 

 heat, light, and oxygen. At the same time such a layer tends to shut 

 in those influences which originate primarily in or below it. Thus car- 

 bonic acid originating under the cover, by organic decay, breathing 

 animals, or bacteria, or washed in by the rain, tends to be shut in. 

 Furthermore, heat once reaching here, either in water or on land, tends 

 toward slow radiation. Thus we may look upon the surface layer as a 

 partition which is under pressure from both sides, and through which 

 constant interchange is in progress, as the process of dynamic equili- 

 bration operates. 



