248 PHILOSOPHY OF ZOOLOGY. 
sity of repose to recruit exhausted strength, and the dif- 
ferent positions assumed by animals in accomplishing this 
object. But we are not left to discover this necessity by 
the slow process of experience; the appetite which we are 
now to consider determines our choice, and in each species 
regulates the different conditions which are necessary ac- 
cording to peculiar plans. 
The time, in which animals sleep, is regulated by the in- 
stinct for food. Those animals which are diurnal feeders are 
nocturnal sleepers, while those which are crepuscular, sleep, 
partly in the night and partly in the day. In general 
there is but one sleep in the course of the natural day, and 
its length varies with the season of the year. In some ani- 
mals, which enjoy sleep during the ordinary periods, the 
condition is assumed and continues permanent during the 
winter season. Such are termed Torpid animals. We 
shall afterwards have occasion to consider the circum- 
stances attending their torpidity. 
Sleep, in general, seizes the frame after the ordinary 
exertion of the body, or of the intellectual or active powers. 
Its approach is accelerated by any extraordinary exer- 
tion, although during the continuance thereof, at what- 
ever period of the day, this instinct is overruled. All ani- 
mals seem to be debilitated by exercise, and to require 
periodical intervals of repose for their renovation. ‘There 
is likewise a tendency to obey the impulses of this instinct, 
whenever the calls of hunger have been satisfied. 
The tendency to sleep is accompanied, in man, with a 
listlessness, and feeling of muscular weakness; the senses 
cease to convey impressions of external objects, so that the 
mind no longer holds its ordinary intercourse with them, 
—attention is relaxed,—memory ceases to recall,—and the 
imagination to arrange, and consciousness itself can scarcely 
be said to be in exercise. At the same time, every instinc- 
