58 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



fat or meat, salt, water, and stimulants must be obtainable in 

 certain proportions .which may be designated as the optimum 

 of nutrition if they are actually to produce all the effects 

 proper to themselves and beneficial to the human organism. 



We are certainly justified in supposing that similar relations 

 exist between the various constituents of the food of the other 

 polyphagous animals. But we know nothing or very little on 

 this subject, although it would be very interesting to learn 

 whether similar relations in the admixture of these constituents 

 subsist for the lower animals as for man ; or, on the other hand, 

 quite different ones for low forms of polyphagous animals, for 

 instance, as Insects, Crustaceans, and Molluscs. At present, 

 therefore, an enumeration of polyphagous animals has no 

 interest, since we cannot learn from it anything as to the 

 dependence of the polyphagous animals on any definite mixture 

 of food, or as to their absolute independence of it. 



Many cases of polyphagy are of the highest interest as con- 

 sidered from another point of view. In connecting and compar- 

 ing the physiological activity of an animal with its position in 

 the general system, we might perhaps expect to find that all the 

 species of a genus, and still more all the individuals of a species, 

 would be equally dependent on the same mixture of food ; and 

 we should be particularly inclined to this assumption in all those 

 cases in which, as we know, the consumption of food directly 

 depends on the presence of one particular organ of definite 

 structure and action. Such a conclusion would nevertheless be 

 wholly unjustified. We will for the present postpone the ques- 

 tion as to how far different individuals of the same species may 

 be capable of varying their nutrition, and will here only investi- 

 gate those cases which show that many polyphagous species are 

 found in genera which otherwise contain none but monophagous 

 Carnivora or Herbivora. 



The greater number of Parrots are, as is well known, vege- 

 table feeders live, that is to say, on grains and fruits. Many, 

 however, eat insects eagerly, and even meat ; and it seems to be 

 a tolerably general custom in zoological gardens to add a 

 certain proportion of fat to the vegetable food of the larger 

 parrots. The Lizards of the Eastern hemisphere are, almost with- 



