48 COMPARATIVE ZOOLOGY. 



mals and the gluten of wheat ; oleaginous, as animal fat 

 and vegetable oil ; or saccharine, as starch and sugar. The 

 iirst 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 we 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 tlie 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, Eeptiles, 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, 



