160 TERRESTRIAL CONDITIONS 



together the literature and discussed the methods of study (see especially 

 several papers by Wiesner). The light in which animals live varies from 

 that of the strongest sunlight of mid-day to the darkest recess of soil, etc. 

 Many animals show diurnal migration due to changes in light. 



2. TEMPERATURE 



The temperature of the air varies with light (insolation). Cloudy 

 summer days are about 4 cooler than sunny days. Cloudy winter days 

 are warmer (125, p. 136) than sunny ones. The temperature of the 

 lowest strata of air on sunny days varies in some inverse ratio with the 

 distance from the soil, vegetation, etc. The temperature immediately 

 above bare soil may be very high in summer (see Table XXXV). 



3. PRESSURE 



According to experimental work by Cohnheim and others (126, 127), 

 man is sensitive to variations in atmospheric pressure. Many other 

 animals, such as rabbits, dogs, etc., are probably also sensitive. Bird 

 movements are often correlated with variation in atmospheric pressure. 

 In all cases the pressure, as meteorologically recorded, represents a 

 variation in humidity, etc., and relations to pressure alone have been 

 but little studied. 



4. HUMIDITY 



Atmospheric humidity (128) is very important to animals and 

 determines the sensible temperature and rate of evaporation to a large 

 degree (see under " Evaporation," below). 



5. COMPOSITION OF THE ATMOSPHERE 



(Table II, p. 59) 

 The amount of carbon dioxide varies (125) in different localities but 

 is usually greatest near the ground where decomposition is taking place. 

 Animals living among decaying organic substances probably live in the 

 presence of much more carbon dioxide than animals upon vegetation. 

 Carbon dioxide is probably important to animals because of its effect 

 upon respiratory activity. Carbon dioxide is believed by some physiolo- 

 gists to be a necessary stimulus to the brain to cause all respiratory 

 movements. It is further held by some that mountain sickness (asso- 

 ciated with high altitude) is due to decreased carbon dioxide pressure. 



6. CURRENTS 

 Currents of wind are important in scattering animals and in affecting 

 the rate of evaporation from their bodies. Some animals take up 



