94 REGENERATION 



position. His general idea may be gathered from the following 

 quotation : 1 " The dissimilarity, moreover, as regards the power of 

 regeneration in various members of tlie same species, also indicates that 

 adaptation is an important factor in the process. In proteus, which in 

 other respects possesses so slight a capacity for regeneration, the gills 

 grow again rapidly when they have been cut off. In lizards again this 

 power is confined to the tail, and the limbs cannot become restored. 

 In these animals, 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 legs are occasionally 

 met with. The physiological importance of the tail of a lizard con- 

 sists in the fact that it preserves the animal from total destruction, for 

 pursuers will generally aim at the long trailing tail, 2 and thus the 

 animal often escapes, as the tail breaks off when it is firmly seized. 

 It is, in fact, as Leydig was the first to point out, specially adapted for 

 breaking off, 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, pro- 

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

 of wliich is in a certain measure provided for, and not as the outcome 

 of an unknown ' regenerative power ' possessed by the entire animal. 

 This arrangement would not have been provided if the part had been 

 of no, or of only slight, physiological importance, as is the case in 

 snakes and chelonians, although these animals are as highly organized 

 as lizards. The reason that the limbs of lizards are not replaced is, I 

 believe, due to the fact that these animals are seldom seized by the 

 leg, owing to their extremely rapid movements." Overlooking the 

 numerous cases of the regeneration of internal organs that have been 

 known for several years, and basing his conclusion on a small, uncon- 

 vincing experiment of his own on the lungs of a few salamanders, 

 Weismann concludes : " Hence there is no such thing as a general 

 power of regeneration ; in each kind of animal this power is graduated 

 according to the need of regeneration in the part under consideration ; 

 that is to say, the degree in which it is present is mainly in proportion 

 to the liability of the part to injury." 



After arriving at this conclusion the following admission is a 

 decided anticlimax : " The question, however, arises as to whether 

 the capacity of each part for regeneration results from special process 

 of adaptation, or whether regeneration occurs as the mere outcome 



1 The Germ Plasm. Translation by W. Newton Parker, 1893, P a S e IT 6- 



2 There are no facts that show that this statement is not entirely imaginary. T. H. M. 



