1 1 RE GENERA TION 



the theory. We find the process of regeneration taking place not 

 only at a few vulnerable points, but in a vast number of regions, and 

 in each case regenerating only the missing part. The leg of a sala- 

 mander can regenerate from every level at which it may be cut off. 

 The leg of a crab also regenerates at a large number of different 

 levels, and apparently this holds for all the different appendages. If 

 this result had been acquired through the action of natural selection, 

 what a vast process of selection must have taken place in each species 1 

 Moreover, since the regeneration may be complete at each level and 

 in each appendage without regard to whether one region is more 

 liable to injury than is another, we find in the actual facts themselves 

 nothing to suggest or support such a point of view. 



If, leaving the adult organism, we examine the facts in regard to 

 regeneration of the embryo, we find again insurmountable objections 

 to the view that the process of regeneration can have been produced 

 by natural selection. The development of whole embryos from each 

 of the first two or first four blastomeres can scarcely be accounted for 

 by a process of natural selection, and this is particularly evident in 

 those cases in which the two blastomeres can only be separated by a 

 difficult operation and by quite artificial means. If a whole embryo 

 can develop from an isolated blastomere, or from a part of an embryo 

 without the process having been acquired by natural selection, why 

 apply the latter interpretation to the completing of the adult organism ? 



Several writers on the subject of regeneration in connection with 

 the process of autotomy (or the reflex throwing off of certain parts of 

 the body) have, it seems to me, needlessly mixed up the question of 

 the origin of this mechanism with the power of regeneration. If it 

 should prove true that in most cases the part is thrown off at the 

 region at which regeneration takes place to best advantage, it does 

 not follow at all that regeneration takes place here better than else- 

 where, because in this region a process of selection has most often 

 occurred. The phenomenon of regeneration in the arm of the star- 

 fish, that has been described on a previous page, shows how futile is 

 an argument of this sort. If, on the other hand, the autotomy is 

 supposed to have been acquired in that part of the body where regen- 

 eration takes place to best advantage, then our problem is not con- 

 cerned with the process of regeneration at all, but with the origin of 

 autotomy. If the attempt is made to explain this result also as the 

 outcome of the process of natural selection acting on individual vari- 

 ations, many of the criticisms advanced in the preceding pages 

 against the supposed action of this theory in the case of regeneration 

 can also readily be applied to the case of autotomy. In Chapter 

 VIII, in which the theories of autotomy are dealt with, this problem 

 will be more fully discussed. 



