VARIATIONS IN POWER OF ABSTINENCE. 149 
certain object, and then in their as rapidly dying off. In this re- 
spect they resemble the Fungi among plants. (Borany, § 789.) 
152. There are great variations in the degree of power 
by animals of different species to sustain abstinence 
from food, which appear to be related to their respective 
habits of life ; such as most easily obtain a constant supply 
of food being immediately dependent upon it, and vice versd. 
Thus, among the larve of Insects, those that feed upon vege- 
tables or dead animal matter (in the neighbourhood of which 
their eggs are usually deposited by the parent) speedily die if 
placed out of reach of their aliment ; whilst those that lie in 
wait for living prey, the supply of which is uncertain, are able 
to endure a protracted abstinence, even to the extent of ten 
weeks, without injury. Again, carnivorous Birds and Mam- 
mals are generally able to exist for some time without food ; 
their natural habits leading them to glut themselves upon the 
earcase of the animal they have destroyed, in such a manner 
as to prevent them from requiring any new supply for some 
time : thus the wild cat has been kept twenty days without 
food, the dog has lived for thirty-six days in the same circum- 
stances, and the eagle for a similar period. But some herbi- 
vorous animals, such as the camel and the antelope, whose 
habits are such as to keep them out of the reach of food for 
several days together, are able to endure a similar abstinence ; 
whilst among the insectivorous Mammals, which naturally 
take food often, and but little at a time, the power of absti- 
nence is much less,—the mole, for instance, perishing in 
confinement, if not fed once a day, or even more frequently. 
153. We have next to consider the different substances 
used as food, in regard to their chemical composition ; and to 
inquire for what purposes in the nutrition of the body they are 
respectively destined. The Vegetable tissues are chiefly made 
up of the three components, oxygen, hydrogen, and carbon ; 
the oxygen and hydrogen having the same proportions as in 
water. Their composition being thus nearly the same as that 
of starch, gum, and sugar (into which, indeed, they may for 
the most part be converted by a simple chemical process), 
alimentary substances of this kind form a natural group to 
which we may give the name of Saccharine (sugary).—But in 
many vegetable substances used as food, there is a considerable 
quantity of oily matter, stored up in cells ; and the same kind 
