KINDS OF MOTION. 139 
cles are only able to continue in exercise for a limited pe- 
riod, during which, their irritability diminishes, and the fur- 
ther exertion of their powers becomes painful. When thus 
fatigued, animals endeavour to place themselves in a condi- 
tion for resting, and fall into that state of temporary lethar- 
gy, denominated sleep. . 
The positions assumed by animals during sleep, are ex- 
tremely various. In the horse, they even differ according 
to circumstances. In the field he lies down, in the stable 
he stands. Dogs and cats form their bodies mto a circle, 
while birds place their heads under thei wings. 
The ordinary time of sleep is likewise exceedingly vari- 
ous in different animals, and in the same animal is greatly 
influenced by habit. It in general depends on circumstan- 
ces, connected with food. 
It is probable, that all animals, however low in the scale, 
have their stated intervals of repose, although we are as yet 
unacquainted either with the position which many of them 
assume, or the periods during which they repose. 
There are many animals in which the muscular filaments 
cease to be perceptible, as im many zoophytes; yet, when 
we see ali the actions of the muscular fibres performed, such 
as contraction and expansion, we admit their existence 
from analogy, and repose the utmost confidence in our con- 
clusions. 
In the classification of animals, the organs of motion are 
very extensively employed. They, however, do not aid us 
in forming divisions of the highest kind, but they assist in 
the construction of subordinate groups. This may appear 
obvious by an examination of the facts stated in this-chap- 
ter. Among Quadrupeds, for example, many walk, some 
fly, others swim, —among Reptiles, some possess feet, others 
