D. THE DISTRIBUTION OF LAND ANIMALS 



CHAPTER XX 



ECOLOGICAL FACTORS OF THE LAND AND THEIR 



EFFECT ON ANIMALS 



The characteristics of the air-inhabiting animals which contrast 

 with those of water animals are principally dependent on the chemical 

 and physical properties of the air. We have already discussed above 

 (p. 27) how these conditions determine the selection of those animal 

 groups that live out of the water. At this point certain basic questions 

 must be still more closely investigated. 



Local differences in the chemical condition of the atmosphere are 

 almost entirely unknown except about volcanoes and as they are 

 occasioned by the industries of man. Just as the complete lack of an 

 effective barrier in the ocean makes possible a constant mixing of the 

 waters and thus causes a uniformity in its chemical condition, in con- 

 trast to the diversity of the chemical conditions of the widely separated 

 inland waters, so in the atmosphere a quick and thorough mixing of 

 the much more easily diffusible gases takes place to an even greater 

 degree. In a few isolated and restricted areas the admixture of such 

 gases as sulphur dioxide, hydrogen sulphide, and carbon dioxide (com- 

 pare p. 18), with the air, under natural conditions, makes life im- 

 possible. 1 



The physical differences in the atmosphere are expressed in the 

 climate: in humidity, temperature, pressure, and in the movement of 

 the air, according to their periodic and non-periodic fluctuations during 

 the course of the year. Temperature variations and currents are much 

 more evident in the air than in the water; furthermore, the moisture 

 content of the air also varies decidedly. Hence temporal and local 

 variations in the physical conditions of the air are much greater than 

 those of the water, especially those of the sea, and accordingly the 

 environmental complex under which the air-dwelling animals live is 

 decidedly more varied. 



Humidity. — The humidity of the air is conditioned by evapora- 

 tion, the amount of which is dependent upon the area of exposed 



377 



