REGENERA TION 



far as the individual's well-being is concerned, but entirely useless from 

 the point of view of the continuance of the species, is found in the 

 development, in the earthworm, of a new head after the removal of 

 the anterior end, including the reproductive region. New reproductive 

 organs are not formed, and, although, in virtue of the regeneration of 

 a new head, the individual is capable of carrying on its existence, yet 

 the race of earthworms is not thereby benefited. The production of 

 two tails in lizards, or of two or more lenses in the eyes of newts, are 

 examples of the regeneration of superfluous structures. 



If, however, it is claimed that in the large majority of cases the 

 process of regeneration is for the welfare of the individual, and for 

 the race also, this must be admitted, and it is this fact which has made 

 a deep impression on the minds of many biologists. 



From the causal point of view, we may look upon the formative 

 changes as the necessary outcome of strictly causal principles, and 

 we may suppose that they take place without respect to the final 

 result. But the question before us is rather to explain, if possible, 

 why the changes that take place are in so many cases useful ones. 

 That they are not always useful must be admitted, that they sometimes 

 are must be granted, and it is the latter alternative that has attracted 

 special attention. Now it is undoubtedly the simplest solution to 

 claim that the scientist has nothing to do with the adaptiveness of the 

 response, that his whole problem lies in a study of the causal phe- 

 nomena involved in each process, but it is unquestionably true that 

 scientists have not been satisfied to confine their hypotheses to this 

 side of the question. The widespread interest in the theory of natu- 

 ral selection is, I think, due to the fact that it appears to offer an 

 explanation of the formation of adaptive processes not that it 

 pretends to explain the origin of the adaptive structures or processes 

 themselves, but that it seems to account for the adaptiveness of the 

 fully formed product, i.e. the organism. For it will be seen that if 

 only those forms (variations) survive that are useful, and survive 

 either because the environment selects them (and exterminates the 

 others), or because new forms that arise find a new place in nature 

 where they can remain in existence, then the adaptiveness of the 

 form to its surroundings would seem to be accounted for. In this 

 case we can see how the causal processes that take place in the organ- 

 ism need have no causal connection with the environment, except 

 in the sense that the environment has acted as a selective agent, and 

 appears, therefore, in the light of a teleological factor. But, as has 

 been said before, the question is not so much that organisms are 

 adapted, as that organisms respond adoptively to changes to which 

 they can never have been subjected before. It is for the latter fact 

 that a solution is to be sought. 



