252 PHILOSOPHY OF ZOOLOGY. 
well-marked characters. ‘The appetites, so far as we are 
able to judge from experience, observation, and analogy, 
form an essential part of the constitution of every animal. 
The desires, on the other hand, are not so universally dis- 
tributed. They constitute rather the distinguishing cha- 
racters of individuals or species, than of classes, as no one 
desire (except perhaps that of warmth) appears common 
to all. Besides, they differ from the appetites, in the uni- 
formity of their excitements. They do not return after cer- 
tain intervals: and they do not cease to act, upon the at- 
tainment of their object. 
1. Desire of Warmth—Whether the blood be hot or 
cold,—whether the animal resides on the land or in the 
waters, there is a particular degree of heat congenial to the 
feelings, and necessary to the functions of every animal. 
There is a propensity, in animals at least, to secure this 
requisite degree of warmth, and its operations, more, per- 
haps, than any other circumstance, mfluence their physical 
distribution. This instinct is destined to guard animals 
equally against the extremes of heat and cold, and to keep 
them within that range of temperature most conducive to 
their health and comfort. In the fixed animals, this desire, 
if it exists, cannot excite to action, as locomotion is denied. 
In the execution of its important ends, this instinct 
guides animals to particular places, to the open sunshine, or 
the refreshing shade ; to the heated sand-bank, or the cool- 
ing water; to the exposed mountains, or the sheltered plains. 
The standard of temperature differs according to the 
species, and even according to the individuals, and the dis- 
trict of the globe they are destined to dwell in. Hence we 
find individuals of a species in a variety of situations with 
regard to temperature, in the same district, and animals 
thriving in every latitude. 
This desire, like all our powers, is greatly influenced by 
