248 COMPARATIVE ZOOLOGY 



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. Many 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 mil^, meat, and 

 bread, at least two of these principles present. As a 

 rule, the nutritive principles in vegetables are less abun- 

 dant than in animal food, and the indigestible residue 

 is consequently greater. The nutriment in flesh in- 

 creases 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 particular plants, as the silkworm to the white mul- 

 berry. But the majority of animals feed on one an- 

 other; such are hosts of the microscopic forms, and 

 nearly all the radiated species, marine mollusks, crusta- 

 ceans, 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, 

 who has been called " the cooking animal." A few of 

 the strictly herbivorous and carnivorous animals have 

 shown a capacity for changing their diet. Thus, the 

 horse and cow may be brought to eat fish and flesh ; the 

 sea birds can be habituated to grain ; cats are fond of 

 alligator-pears ; and dogs take naturally to the plantain. 

 Certain animals, in passing from the young to the 



