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 in inverse ratio with the 
distance from the soil, vegetation, etc. The temperature immediately 
above bare soil may be very high in summer (see Table XX XV). 
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 IT, 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 
