DIETS 67 



Diets of animals. The feeding habits of animals 

 differ greatly. Some animals, as sheep, cows and horses, 

 live entirely upon plants and on this account are called 

 herbivorous. Others, like wolves, lions and tigers, live 

 only upon the flesh of other animals and are therefore 

 called carnivorous. Still others, as birds, apes and men, 

 live upon a mixed diet of plants and animal flesh and are 

 called omnivorous. 



Selection of diets. The foods chosen by animals 

 have depended upon the kinds obtainable and their lik- 

 ing for them. The food habits of animals as observed 

 to-day are to a large degree the result of a process of 

 natural selection. Those animals which liked the food 

 which made them strong and were fortunate enough to 

 be able to get it, produced strong offspring. These in 

 turn tended to continue the same food habits with the 

 same beneficial results. In the end, their families sur- 

 vived, whereas those which for any reason did not eat 

 wisely perished in the struggle for existence against ene- 

 mies and disease. To-day, therefore, the natural diets 

 of the various species of animals are not the result of 

 accident or of mere individual choice, but are rather 

 the final outcome of a long series of experiments to deter- 

 mine the foods best adapted to their needs. 



Adjustment of teeth to diet. During the long pe- 

 riod of selection, the animal gradually became adapted 

 to the diet chosen. For example, the teeth of herbivorous 

 animals became so changed in order to adapt them to 

 the grinding demanded by their diet, that now they could 

 not eat flesh easily, even if they wanted to. Carnivorous 

 animals, on the other hand, have teeth which are perfect 

 for tearing flesh and crushing bone, but which are quite 

 useless for chewing grass and grinding small grains. 

 Those animals, like the ape and man, whose normal diet 



