16 THE EVOLUTION THEORY 



we drew from the facts cited above, according to which the faculty 

 of regeneration comes under the law of adaptation. For the 

 disappearance of this faculty must take place very much more slouiy 

 than its grouih. For instance, the development of the tail-fin of the 

 whale has long been an accomplished fact, while the hind-legs of this 

 colossal mammal, which were rendered useless by the development of 

 the tail-fin, still lie concealed in a rudimentary state within the 

 muscles of the trunk. Yet these limbs must have lost their significance 

 for the animal exactly at the time that the tail-fin became more 

 powerful. Thus the retrogression must have taken place more slowly 

 than the progressive transformation. 



It is clear, then, that the faculty of regeneration is not a primary 

 character of living beings occurring uniformly in all species of 

 equally high organization and in all parts of an animal in the same 

 degree ; it is a power which occurs in animals of equal complexity in 

 as varying degrees as in their parts, and which is manifestly 

 regulated by adaptation. Between parts with the faculty of re- 

 generation and parts without it there must be an essential difference ; 

 there must be present in the former something that is wanting in the 

 latter, and, according to our theory, this is the equipment with 

 regeneration-determinants, that is, with the determinants of the parts 

 which are to be reconstructed. 



If this be really so it should be capable of proof, at least in so 

 far that we should be able to establish that the power of completing 

 or re-forming a damaged or lost part is a limited one, localized in 

 certain parts and cell-layers. This can be actually proved, as may be 

 seen from numerous cases in which the faculty of regeneration is 

 associated with autotomy, that is, with the power of breaking off* 

 or dropping off a part of the body. Even in worms we find this 

 power, as we mentioned before in speaking of the high regenerative 

 capacity of Lurtihriculus. This worm reproduces in summer by what 

 is called ' schizogony,' that is, by breaking into two, three, or more 

 pieces, and it does not seem to require a very strong stimulus, such as 

 pressure of the end of the worm by the jaws of an insect larva, to 

 start this rupture; it often follows from quite insignificant friction 

 on the ground. Certainly the power of regeneration is so great in this 

 animal that it is out of the question to talk of localizing the primary 

 constituents of regeneration ; almost every broken surface is capable 

 of regeneration. 



But this localization is well illustrated in Insects and Crustaceans, 

 which possess the power of self-amputation in their appendages, 

 especially in their legs. As far back as i8z6 MacCullock observed 



