^^O S. J. Holmes 



and determine that it forms in harmony with the rest of the organ- 

 ism. Such a conception is entirely congruous with the symbiotic 

 theory, and is, I beheve, consistent with the various observed 

 facts of regeneration. 



If we explain form regulation as an outcome of functional regu- 

 lation, we make little progress until we have some interpretation 

 of the latter process, and any theory of form regulation which 

 offers nothing in this direction, makes no more than the first step 

 toward an explanation of the phenomenon. In his criticism of 

 the theory of form regulation which I have outlined, Child has 

 advanced arguments which are, I believe, by no means fatal to it, 

 and he has not brought forward any other explanation of func- 

 tional equilibration, which both of us regard as the basis of form 

 regulation. Perhaps this may be supplied in further developments 

 of his theory which Child hints are to be made in the future. 

 While functional adaptation may occur independently of any sym- 

 biotic relations, especially in the direct adaptation of parts to exter- 

 nal conditions, the mutual adaptation of parts which forms so 

 important an element in formative processes are, I believe, for 

 the most part, dependent on symbiotic relations. At present I 

 am unable to see how any general explanation of functional equili- 

 bration among the parts of an organism can be reached unless we 

 assume that the parts are, to a considerable degree, interdependent. 

 Perhaps some other interpretation of functional regulation may 

 be advanced which does not make use of this idea. That remains, 

 of course, to be seen. But the theory of the symbiotic relation of 

 the parts of an organism has the merit of enabling us to interpret 

 form regulation and functional regulation as the outcome of ordin- 

 ary physiological activities, and hence to give, in a measure, a 

 causal explanation of the teleological behavior which is manifested 

 in so striking a degree by formative processes, and which forms the 

 strongest support of some recent vitalistic theories. So. far, at 

 least, I hope it is in the line of progress. 



Zoological Laboratory 

 University of Wisconsin 



