68 THE DATA OF BIOLOGY. 



If we take, on the one liand, a carnivore living on muscu- 

 lar tissue (for wild carnivores preying upon herbivores which 

 can rarely become fat obtain scarcely any carbo-hydrates) 

 and observe that its food is almost exclusively nitrogenous; 

 and if, on the other hand, we take a graminivorous animal 

 the food of which (save when it eats seeds) contains com- 

 paratively little nitrogenous matter ; we seem obliged to sup- 

 pose that the parts played in the organic processes by the 

 proteids and the carbo-hydrates can in considerable measures 

 replace one another. It is true that the quantity of food 

 and the required alimentary system in the last case, are very 

 much greater than in the first case. But this difference is 

 mainly due to the circumstance that the food of the gramin- 

 ivorous animal consists chiefly of waste-matter — ligneous 

 fibre, cellulose, chlorophyll — and that could the starch, sugar, 

 and protoplasm be obtained without the waste-matter, the 

 required bulks of the two kinds of food would be by no 

 means so strongly contrasted. This becomes manifest on 

 comparing flesh-eating and grain-eating birds — say a hawk 

 and a pigeon. In powers of flight these do not greatly 

 differ, nor is the size of the alimentary system conspicuously 

 greater in the last than in the first; though probably the 

 amount of food consumed is greater. Still it seems clear 

 that the supply of energy obtained by a pigeon from carbo- 

 hydrates with a moderate proportion of proteids is not widely 

 unlike that obtained by a hawk from proteids alone. Even 

 from the traits of men differently fed a like inference may be 

 drawn. On the one hand we have the Masai who, during 

 their warrior-days, eat flesh exclusively; and on the other 

 hand we have the Hindus, feeding almost wholly on vege- 

 table food. Doubtless the quantities required in these cases 

 differ much; but the difference between the rations of the 

 flesh-eater and the grain-eater is not so immense as it would 

 be were there no substitution in the physiological uses of the 

 materials. 



Concerning the special aspects of animal-metabolism, we 



