10 THE EVOLUTION THEORY 



is proved by the root-tips which are formed anew when they are 

 injured, and the closing of wounds on the stem by a 'callus.' 



I shall return to plants when we are dealing with the mechanism 

 of regeneration, but I must now direct more attention to animals, 

 inquiring further into the question as to whether the faculty of 

 regeneration is correlated with the degree of liability to injury to 

 which the animal is exposed, and with the biological importance 

 of the injured part, for this must be the case if regeneration be 

 reall}^ regulated by adaptation. 



Hardly any other vertebrate has attained such celebrity on 

 account of its high regenerative capacity as the water-newt, species 

 of the genus Triton. It can regrow^ not only its tail, but the legs 

 and their parts if they are cut off. Spallanzani saw the legs grow six 

 times, after he had cut them off six times. In the blind newt [Proteus) 

 of the Krainer caves, a near relative of the common newt, the leg 

 regenerated only after a year and a half, although the animal stands 

 on a lower stage of organization than the newt, and thus should 

 rather rej^lace lost parts more easily. But Proteus lives sheltered 

 from danger in dark, still caves, while Triton is exposed to numerous 

 enemies which bite off pieces from its tail or legs ; and the legs are 

 its chief means of locomotion, without which it would have difticulty 

 in procuring food. It is different with the elongated eel-like newt 

 of the marshes of South Carolina, Siren lacertina. This animal 

 moves by wriggling its very muscular trunk, after the manner of 

 an eel, and in consequence of the disuse of its hind legs it has almost 

 completely lost them. Even the fore-legs have become small and 

 weak, and possess only two toes, and these do not regrow if they are 

 bitten ofi', or only do so very slowly. 



Earthworms are exposed to much persecution ; not only birds, 

 such as blackbirds and some woodpeckers, but, above all, t]ie moles 

 prey upon them, and Dahl has shown that moles often lay up stores 

 of worms in winter which they have half crippled by a bite, while 

 even Reaumur knew that moles frequently only half devoured earth- 

 worms. It was thus an obvious advantage to earthworms that a part 

 of the animal should be able to regrow a whole, and accordingly we 

 find a fairly well-developed regenerative capacity among them. But 

 it varies greatly in the different species, and it would be interesting 

 if we knew the conditions of life well enough to be able to decide 

 whether the faculty of regeneration rises and falls in proportion 

 to the dangers to whicli the species is exposed. Unfortunately we 

 are far from this as yet: we only know that, in the connnon 

 earthworms of the genera Lumbricus and Allolohophora, the faculty 



