424 S- J- Holmes 



is a difference perhaps in the mechanical stimuli received, but 

 granting that these start the course of differentiation in different 

 directions, they are entirely inadequate to account for the whole 

 process of differentiation, as Child himself would probably admit. 

 Where the factor of movement is absent. Child has recourse to the 

 supposition of some other form of functional substitution, but 

 he gives no clear account of why the substitution should occur. 

 To say that the end of an arthropod's appendage is regenerated 

 because of the functional activities that occur within it, that mor- 

 phallaxis occurs when the part readily takes on the function of the 

 whole, and that regeneration takes place because the functions 

 of the missing part are imposed upon the new tissue that is 

 developed in its place, may all be very true so far as it goes, but 

 until some principle for the explanation of this functional adjust- 

 ment is brought forward the explanation of regeneration is far 

 from complete. If form regulation is a consequence of functional 

 regulation, as Child and I agree that it is, the interpretation of 

 functional regulation is the next obvious step. The inadequacy 

 of Child's theory is, that it does not contain any general principle 

 of explanation for that functional substitution and equilibration 

 upon which it is assumed that form regulation depends. This, 

 however, is a matter of incompleteness rather than error. But 

 I suspect that when his theory comes to be developed so as to sup- 

 ply this missing element, it will involve, to make it workable, the 

 assumption of some such symbiotic relation between the parts of 

 an organism as I have assumed. It is a strong point in favor of 

 the theory of symbiosis that it affords to a certain degree an expla- 

 nation of physiological adjustment; in fact, it is primarily a the- 

 ory of physiological equilibration. This physiological adjustment 

 brought about through the symbiotic relations of the parts may, 

 as I have attempted to show, be explained, or at least much of it 

 may be explained, as the outcome of a tendency toward chemical 

 equilibration. To the extent that this is true, we have an explana- 

 tion of the regulatory activities of an organism in terms of famil- 

 iar chemical phenomena. 



The conception of something like a symbiotic relation between 

 the parts of an organism which is involved in my own theory 



