REPTILES AND AMPHIBIANS 143 



every animal knows that the only way of avoiding the 

 fatal bite is to grasp it by the neck ; and as for most 

 animals, and a good many human beings, serpents are 

 always serpents, the dread is extended to the adder. 

 As a result of this we do not find the faculty of 

 regenerating the tail in any serpent. 



Thus we see that the power of regeneration has been 

 developed by natural selection wherever it is necessary 

 where a species is constantly exposed to mutilation. 

 This faculty is increased in proportion to the frequency 

 of the mutilations, and is not found at all where it would 

 be of no avail, where the enemies of a species destroy 

 the whole animal when they catch it. It is on this 

 account that we do not find any faculty of regeneration 

 in the frog ; its enemies do not tear off pieces of it, but 

 kill the whole animal, as we know to be the case with 

 the stork, ringed adder, fox, weasel, hedgehog, and 

 the innumerable other enemies of the frog. There is 

 one animal, however, that mutilates the frog before eating 

 it, and that is the tortoise. These approach the frog 

 from the depth of the pond, as it sits unsuspectingly 

 on the edge, seize it suddenly by the leg, drag it under 

 the water, get the leg as far down the throat as possible, 

 and then cut it off from the body with their sharp claws. 

 When they have swallowed the leg, they tear other 

 parts from their unfortunate victim, until nothing is left 

 but the skeleton. In this case it would certainly be an 

 advantage to the frog if it could escape after the loss 

 of one leg and form a new one. But it is exceptional 

 for a frog to be captured by a tortoise, partly because 

 they live chiefly on insects, snails, worms, fishes, and 



