60 



THE INFLUENCE OF INANIMATE SUBKOUNDINGS. 



denly attack quite Jiealthy living speoimens of Triton toeniatns, 

 overcome them and devour them, although the aquarium was 

 full of luxuriantly growing plants, on which these water-snails 

 usually feed. 



These instances'^ will, I think, suffice to warn us to be 

 cautious when, from the systematic position of an animal and 

 the structure of its organs, we are called upon to determine 

 what may be its mode of life and nutrition ; they further teach 

 us that a polyphagous animal can occasionally be easily trans- 



Fig. 14.— ifl/'itf argentatuSt one of the species of gull experimented on by John 

 Hunter. 



formed into a monophagous one without suffering any serious 

 injury. 



Thus, in general, polyphagous animals are less dependent on 

 their food than monophagous species, and hence food can exert 

 only a weaker selective influence on the former than on the latter. 

 Assuming, for instance, that there were an animal which, up to 

 the present time, had been fitted to use a certain species of 

 animal or plant as food, and that it were suddenly transferred to 

 a foreign country where such food was lacking, or that the 

 animal or plant serving it for food were extirpated, while the 

 creature itself was not ; in, either of these cases the continuance 



