n] FEATURES OF ENVIRONMENT 45 



course that the same volume of air contains only 

 two-thirds the amount of oxygen. Animals rapidly 

 taken up to high levels find it difficult to adjust 

 themselves to these conditions. The thinness of the 

 air, combined with the cold, causes absence of vapour ; 

 radiation, loss of warmth of the ground heated by 

 the sun's rays, is unchecked. On the other hand 

 moisture in the air diminishes the loss of heat by 

 radiation and directly increases the temperature of 

 the atmosphere, because the warmth given off by the 

 heated ground is largely absorbed by the aqueous 

 vapour. Consequently on a brilliant day, on a high 

 snow field it may be scorching hot in the sun, whilst 

 the temperature of the air, as measured in your own 

 shade, shows several degrees of frost. 



In the temperate and cold regions the differences 

 due to latitude and altitude are greatest in the 

 winter and least in the summer. Under the tropics 

 the differences due to altitude are greatest within the 

 24 hours of day and night. It follows that places of 

 the same mean annual temperature may have widely 

 different summer temperatures ; and conversely that 

 places receiving the same amount of summer heat 

 may have widely different annual means. The sig- 

 nificance of these facts becomes apparent in the 

 study of the distribution of animals and plants. The 

 distribution of the various species is governed in 

 the main by the temperature of the warm season, 



