32 PHILOSOPHY OF ZOOLOGY. 
ence, the condition of the living body bemg suited to the 
place of its residence. But it may be asked, Is there no 
particular degree of heat in which the living principles dis- 
play their energies with the greatest vigour? To this it 
may be answered, That the species of organized beings 
are much more numerous towards the equator, than in 
any of the other districts of the earth ; that their numbers 
decrease as we approach the poles; and that the same de- 
crease may be observed on leaving the sea-shore, and ad- 
vancing towards the mountains. ‘These facts seem to indi- 
cate a mean temperature of about 84° Fahr. the average 
mean heat of the equatorial regions, as the most favourable 
for the increase of living beings; since they decrease in 
number and variety as we approach the poles, with the pro- 
gress of the diminution of temperature. 
The influence of temperature, in modifying the appear- 
ances of organized bodies, is by no means considerable. 
Among plants, the size appears to be influenced by this 
cause, the tallest being found in the warm countries, while 
those of the colder regions are of more humble growth. 
It is probable, however, that, in this case, the agency of 
winds in diminishing the size, is more effectual than the 
decrease of temperature. With regard to animals, size ap- 
pears but little influenced by temperature. The whales of 
the arctic seas are the largest animals in the world ; while 
the land quadrupeds of the warm regions are larger than 
those in temperate climates. But the irregularities which 
present themselves, forbid the establishment of a general 
rule. 
In every case, then, we witness heat necessary, not only 
to enable the vital principle to execute its first movements, 
but the functions of existence in every period. It appears, 
indeed, to be the principal exciting cause of vital energy, 
from the commencement to the termination of its actions. 
