14 



THE EVOLUTION THEORY 



and easily torn animals are exposed to all sorts of injuries, and are, 

 in point of fact, frequently mutilated by enemies which are unable to 

 swallow them whole. Von Graaf not infrequently found examples of 

 marine Planarians (Macrostomuvi) which lacked 'a part of the 

 posterior end or the whole tail region as far as the food-canal,' and of 

 species of Monotus he found ' very often ' in May specimens with the 

 posterior end split or bx-oken off. Probably the persecutors of these 

 flat-worms are some species of Crustacean, but, at any rate, so much 

 is proved, that the Planarians have abundant opportunities of making 

 use of their faculty of regeneration, and that the species gains an 

 advantage from it in respect to its preservation. 



In contrast to this, worms which live within other animals, 

 and are thus secure from mutilation, such as 

 the familiar round-worms [Nematoda), have 

 no power of regeneration at all, and do not 

 survive either longitudinal or transverse 

 division. 



Until recently birds were regarded as 

 possessing a very low degree of regenerative 

 capacity, and, as a matter of fact, they cannot 

 replace a leg or a wing wholly or in part; 

 but, what is otherwise unheard of among 

 higher vertebrates, they can renew the whole 

 anterior portion of the skeleton of the face, 

 the bill, and can indeed almost reconstruct 

 it with new bones and horny parts. Von 

 Kennel communicated a case of this kind in 

 regard to a stork, and for a long time this 

 remained an isolated case, but a few years 

 ago Bordage showed that, in the cocks which 

 are used in the Island of Bourbon for the 

 favourite sport of cock-fighting, the bill is 

 regularly renewed when it has been broken 

 off or shattered. Quite recently Barfurth gave an account of a case 

 of complete renewal of a broken bill in a parrot. Yet it should 

 not astonish us that the bill in birds has such a high regenera- 

 tive power, for of all parts in a bird it is the one that is most readily 

 injured; with it the bird defends itself against its enemies and its 

 rivals, masters its prey, and tears it to pieces, pecks holes in trees 

 (woodpecker), or climbs (parrot), or digs and burrows in the ground, 

 or builds its nest, and so on. That the faculty of regenera- 

 tion could be developed to so high a degree in relation to this 



Fia. 97. A, a Planarian, 

 which has been divided into 

 two by a longitudinal cut. 

 Each half can grow into 

 an entire animal. B, the 

 left half at the beginning 

 of the regenerative process. 

 G, the same completed. After 

 Morgan. 



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