Regcurrafion as Finirfionnl A djitsttiinit 427 



case, It IS not the relation of A to /i-/, that should be niore or less 

 one-sided, but the relation of the tissue in placeofy^ to this complex. 

 This is an important distinction which Child does not seem to have 

 considered. B-F are relatively fixed, the tissue in place of A is 

 young and plastic, and more dependent so far as the direction of 

 Its differentiation is concerned, upon B-F , than these are upon it. 

 We may grant that, when regeneration occurs, the relation of depend- 

 ence between the old parts and the new tissue is more or less one- 

 sided, although the relations of the part removed may not have 

 been. This would naturally result if the parts were relatively 

 stable. They may be in a symbiotic relation, nevertheless, each 

 part contributing in some way to the normal functioning of 

 the others, and dependent to the extent that the removal of one 

 part may alter only to a certain degree the quality and quantity of 

 the activity of the surrounding parts, without producing extensive 

 modifications of structure or function. 



If the parts B-F were more plastic, absence of ^ would natur- 

 ally tend to cause greater changes in them, especially if new tissue 

 were not produced in place of A, which would come to assume 

 some of the missing functions before the modification extended 

 very far. There would be a progressive modification extending 

 from the region of y^, which would tend to become less the farther 

 it extended, but eventually perhaps affecting more or less the entire 

 organism. Functional equilibrium would then be maintained 

 by working over the organism so that all the parts were adjusted 

 to functioning on a smaller scale. The different methods of regu- 

 lation, through morphallaxis, regeneration and the various com- 

 binations'of these processes are, I believe, interpretable according 

 to the symbiotic theory, and the relations of regeneration and 

 morphallaxis to the degree of specialization of the parts which 

 Child has elaborated, are, in fact, exactly what the theory would 

 lead us to expect. 



The difficulty pointed out by Child that the process of differen- 

 tiation of a new part seems to begin at the tip and work back to 

 the base is one which gave me some concern when developing my 

 theory, but I think the difficulty is by no means a fatal one. 

 When a developing limb shows first those structures character- 



