48 
COMPARATIVE ZOOLOGY. 
trials and the gluten of wheat; oleaginous , as animal fat 
and vegetable oil; or saccharine , as starch and sugar. The 
first is the essential food-stuff; no substance can serve 
permanently for food—that is, can permanently prevent 
loss of weight in the body—unless it contains albuminous 
matter. As stated before, all the living tissues are albu¬ 
minous, and therefore albuminous food is required to sup¬ 
ply their waste. Albumen contains nitrogen, which is 
necessary to the formation of tissue; fats and sugars are 
rich in carbon, and therefore serve to maintain the heat 
of the body, and to repair part of the waste of tissues. 
Warm-blooded animals feed largely on farinaceous or 
starchy substances, which in digestion are converted into 
sugar. But any animal, of the higher orders certainly, 
whether herbivorous or carnivorous, would starve, if fed 
on pure albumen, oil, or sugar. Nature insists upon a 
mixed diet; and so we find in all the staple articles of 
food, as milk, meat, and bread, at least two of these prin¬ 
ciples present. As a rule, the nutritive principles in veg¬ 
etables are less abundant than in animal food, and the 
indigestible residue is consequently greater. The nutri¬ 
ment in flesh increases as w T e ascend the animal scale; 
thus, Oysters are less nourishing than Fish; Fish, less than 
Fowl; and Fowl, less than the flesh of Quadrupeds. 
Many animals, as most Insects and Mammals, live solely 
on vegetable food, and some species are restricted to par¬ 
ticular plants, as the Silk-worm to the white mulberry. 
But the majority of animals feed on one another; such 
are hosts of the microscopic forms, and nearly all the ra¬ 
diated species, marine Mollusks, Crustaceans, Beetles, Flies, 
Spiders, Fishes, Amphibians, Reptiles, Birds, and clawed 
Quadrupeds. 
A few, as Man himself, are omnivorous, i. e ., are main¬ 
tained on a mixture of animal and vegetable food. The 
use of fire in the preparation of food is peculiar to Man, 
