168 LECTUEES ON BIOLOGY 



enormous number of different species. Animals must have 

 some kind of colour an,d marking why should there not be 

 some who closely resemble other insects or dead objects ? 



Even proofs apparently so convincing as the mimicry between 

 venomous and non-venomous snakes, or the mimicking of the 

 sea-snake (Platurus) by a fish (Ophichthys) create considerable 

 doubt. As regards the snake and the fish, the latter has in its 

 powerful teeth such an excellent weapon that most enemies 

 would be chary of attacking it. It does not, therefore, stand 

 in need of specific protection, and it seems rather that the 

 inconvenient similarity would render it more difficult for it 

 to gain a livelihood. Probably we find here, as in many other 

 cases, ' the phenomenon of convergence/ of the cause of which 

 we know very little, but of the existence of which there are 

 many proofs. Numerous fishes of the Southern Seas, in par- 

 ticular the inhabitants of the coral reefs, have a uniformly 

 striking appearance. Cats, goats, sheep, cattle and dogs which 

 inhabit Angora and Thibet are distinguished by long, silky hair. 

 We might even regard the light colour of the Polar animals as 

 a case of convergence, for there exists no fundamental difficulty 

 in understanding that similar external influences may produce 

 similarity of appearance. That the white colour of many animals 

 is actually subject to influences of temperature is proved by the 

 lemmings. In their Northern home these animals assume, at 

 the approach of autumn, a white coat, but when in captivity if 

 they are kept in a warm room they retain their grey fur through- 

 out the winter. No sooner, however, are they exposed to cold 

 than they will again grow a white fur. 



As regards the venomous snakes and their non-venomous 

 mimickers, there is first of all, as far as is known, no snake-eating 

 animal which distinguishes between venomous and harmless 

 snakes. The common hedgehog devours with equal gusto the 

 harmless ringed snake and the poisonous adder, the digestion of 

 the poison glands causing it apparently not the least discomfort ; 

 even to the bite of the viper the hedgehog appears to be quite 

 insusceptible : as we have already seen, this is due to the fact 

 that there are immunizing substances in the blood of the hedgehog 

 which neutralize the effect of the poison. The guinea-pig dies 

 quickly from the bite of the adder, but an injection of blood-serum 



