64 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



through the stream. I, in order to preserve my watch and other 

 instruments from a ■wetting, seated myself astride on Antonio's 

 shoulders. When we were about halfway across, where the 

 water reached nearly to my bearer's neck, a man appeared on the 

 shore. Seeing him I shouted out, half in jest, ' Are there any 

 crocodiles in the river here "i ' My feelings may be imagined 

 when I received the answer, ' Oh, yes ; there are plenty of croco- 

 diles in the water, but they will not eat men.' Everyone will 

 be reminded by this story of the many similar ones of sharks, 

 alligators, and other animals, which all concur in proving that 

 these creatures exhibit the most remarkable preferences in the 

 choice of their food, and that even individuals of the same species 

 differ widely. 



It will be unnecessary to adduce any further examples or 

 even to investigate the credibility of these current stories 

 regarding crocodiles; for, even without these, the instances 

 given above suiEce to show that polyphagous or monophagoiis 

 habits are not immutable characters, but that, on the contrary, 

 almost every species is able more or less to vary the nature 

 of its food. Hence the dependence of an animal on its 

 nutrition is not absolute, and consequently the selective 

 influence of the nutrition is, as we see, in some degree 

 limited by the animal's capability for accommodating itself, 

 with very various results, to a diet hitherto unknown to it. 

 The selective influence must, at any rate, remain tolerably 

 great, particularly on monophagous creatures ; and it is more 

 than probable that a sudden and rapid change of nutrition, such 

 as may sometimes be forced upon animals by external circum- 

 stances, will inevitably lead to the equally rapid death of most 

 species.'^ 



Now, if we suppose that such a sudden change of nutrition 

 actually were imperative on several species at once — such, for 

 instance, as always occurs in the migration of many marine 

 creatvires, and of all parasites — some species must perish, because 

 they would not be capable of living on the unaccustomed food ; 

 others might sui-vive because they were omnivorous or because, 

 even though monophagous, they were able to adapt their 

 functions promptly to the new conditions of life. In the latter 



