Il6 THE TiFRM- PLASM 



placed while the animal invariably succumbs to greater injuries/ 

 Fraisse gives similar instances. Thus 'an amputated extremity 

 never grew again to its normal size : merely a somewhat de- 

 formed protuberance was formed on the stump. The tail also 

 was only reproduced to a very slight extent.' * 



With regard to reptiles, Fraisse points out that the regenera- 

 tive capacity obtains to a much slighter extent in some groups 

 than in others. Chelonians, crocodiles, and snakes are unable 

 to regenerate lost parts to any extent, while lizards and geckos 

 possess this capacity in a high degree. 

 f~ The dissimilarity, moreover, as regards the power of regener- 

 ' ation which exists in various members of the same species, also 

 indicates that adaptation is an important factor in this process. 

 In Proteus^ which in other respects possesses so slight a capac- 

 ity for regeneration, the gills grow again rapidly when they 

 have been cut oiT. In lizards, again, this power is confined to 

 the tail, and the limbs cannot become restored : in these ani- 

 mals, however, the tail is obviously far more likely to become 

 mutilated than are the limbs, which as a matter of fact are 

 seldom lost, although individuals with stumps of limbs are occa- 

 sionally met witli. The physiological importance of the tail of a 

 lizard consists in the fact that it preserves the animal from total 

 destruction ; for pursuers will generally aim at the long trail- 

 ing tail, and thus the animal often escapes, as the tail breaks 

 off when it is firmly seized. It is, in fact, as Ley dig was the first 

 to point out, specially adapted for breaking oif", the bodies of the 

 caudal vertebrae from the seventh onward being provided with 

 a special plane of fracture, so that they easily break into two 

 transversely. Now if this capability of fracture is provided for 

 by a special arrangement and modification of the parts of the tail, 

 we shall not be making too daring an inference if we regard the 

 regenerative power of the tail as a special adaptation, produced by 

 selection, of this particular part of the body, the frequent loss of 

 which is in a certain measure provided for, and not as the out- 

 come of an unknown ' regenerative power ' possessed by the 

 entire animal. This arrangement would not have been pro- 

 vided if the part had been of no, or only of slight, physiological 

 importance, as is the case in snakes and chelonians, although 

 these animals are as highly organised as lizards. The reason 



* Loc. cit., p. 152. 



