EATIO OF CAENIVOKA TO HEEBIVOHA. 51 



gives rise to various relations between the animals and their 

 surroundings in the following manner. All animals need a 

 certain optimum of food; being compelled to take organic as 

 well as inorganic food, they are dependent on plants, which 

 alone are able to form oi'ganic compounds by the decomposition 

 of carbonic acid ; both the quality and quantity of the food 

 lead to a vast number of very various relations between the 

 animals and inorganic nature on one hand, and living beings on 

 the other ; finally, the organs which are auxiliary to the acqui- 

 sition of food are in direct connection with the animal's mode 

 of life. 



Every modiQcation of these relations once established must 

 necessarily exercise an influence on the animals in contact with 

 it, and in this case, as in all others, this influence may be two- 

 fold : selective or transforming. The great variety, as has been 

 briefly indicated, of these conditions and relations requires us to 

 discuss a few cases of more conspicuous interest, in order to un- 

 derstand how far food does in fact exert a direct or indirect 

 influence on different animal forms. 



Monophagous and polyphagous animals. — Any division of 

 animals into two such groups as are here indicated has obviously 

 none but a purely physiological value, nor is it a thoroughly com- 

 prehensive one, as we shall immediately see ; although a very 

 conspicuous contrast exists, and has a certain value, between 

 Monophagous animals — consuming, that is to say, only one- kind 

 of food — and Polyphagous creatures, which eat a variety of 

 food or even anything that comes in their way. 



If we confine our attention to the distinction between the 

 two kinds of food, vegetable and animal, we may regard all 

 purely carnivorous or purely herbivorous animals as monopha- 

 gous ; but within each of these groups there are animals that 

 are monophagous in the strictest sense of the word, several 

 species being fitted in fact to feed on one kind only of organic 

 food. A closer enquiry into the conditions resulting from this 

 will be of interest. 



In the first place, it is clear that a certain ijiterdependence 

 between flesh- and plant-eating animals must exist, and find 

 its expression in the proportional numbers of individuals of 



