284 INFLUENCES OF GEOGRAPHY. 
once be seen by considering the vast increase of 
living beings as we approach the equatorial regions 
from the poles; but then it must be clearly under- 
stood, that this effect of climate, or rather of heat, 
is observed only in a very general way, and that 
owing tO a great variety of causes, some quite 
incomprehensible, others connected with food, 
situation, &c. the interruptions to this rule of 
increase are both numerous and important ; still, 
on the whole, heat may be considered one of the 
secondary causes that influence the geography of 
animals. The alterations of the seasons, which 
besides bringing an alteration of temperature, in- 
duce considerable difference on the food of animals, 
have a decided influence on their situations, caus- 
ing a variety of movements termed Migrations. 
These changes of place are more immediately de- 
pendent on temperature and the state of the atmos- 
phere, than on food, or other causes. Winds 
frequently affect the Fauna of a country by driving 
aquatic animals to land, or by putting migrating 
animals from their destined courses. The state of 
the atmosphere as regards dryness or humidity, 
together with a continuanee of rains or drought 
will affect the general nature of the climate, and 
thereby the vegetable produce and animals of the 
country in which such conditions occur. Unusually 
hot or fine summers are most likely the causes of 
our receiving certain birds from the southern parts 
of Europe at that season. Long continuance of wet 
and cold at the time of the autumnal migration, will 
influence the period of departure of perhaps all our 
summer visitants. 
Geographical situation, relations, and arrange- 
ment of a country, have considerable power over the 
extent and nature of a Fauna. The adjacency of 
an ocean, or large river in connexion with the sea, 
implies of course the presence of marine produc- 
