CHANGE OF SHAPE IN PLANARIA. 29 1 



When we come to consider the process of development some- 

 what more in detail we find that two complex groups of internal 

 factors must be considered, viz., constitution and correlation. 

 This, of course, is true of any machine : its function depends upon 

 the constitution and correlations of its parts, provided, of course, 

 that the necessary external conditions for function are present. 

 In different organisms and in different features of development 

 the two factors may of course play a very different part. 



2. Functional Regitlation and Form Regulation. 



It follows from what has been said that all form regulation is, 

 according to my definition, either directly or indirectly functional 

 regulation, /. t\, the physiological processess in the structure 

 involved determine what the result of regulation shall be (Child, 

 '064 



In certain cases among the turbellaria I have been able to con- 

 trol, inhibit and modify the process of form regulation to a greater 

 or less extent by controlling and modifying, in most cases indi- 

 rectly, the movement and use of the parts most intimately involved 

 in the regulatory processes (Child, '02, '03, 'o^a-c, 'o$a-cT). 

 This work showed very clearly that movement and use of parts 

 were important factors in certain cases and certain features of reg- 

 ulation : they may even be primary factors in determining certain 

 results. It is just as certain, however, that in many other cases 

 they play no part at all. 1 



point is in reality merely one of definition ; he limits the term " function " to the proc- 

 esses of which I spoke in the first paragraphs of this section, while I regard all proc- 

 esses in the organism as functions. His criticism of my position is of course entirely 

 beside the point since he substitutes his definition for mine. Physiological processes 

 of some kind certainly occur even in the earliest stages of development, and that these 

 are functions of an existing structure cannot be doubted. I believe that they are also 

 the formative factors in development. 



Morgan ('07) calls attention to the fact that the functional idea is not new. This 

 of course is true ; it is the position which every physiologist must hold in one form or 

 another. I have never considered that I was formulating a new hypothesis of de- 

 velopment ; I have merely attempted to apply certain physiological ideas to the phe- 

 nomena of form regulation. Among botanists this view has been very generally held 

 for a long time. 



1 Morgan ('07, p. 374) apparently believes that I regard movement as a universal 

 factor in form regulation, for he calls attention to the fact that movement does not 

 occur in many cases of regeneration. But I have repeatedly asserted that my con- 

 clusions upon the effect of movement concerned only certain species and certain fea- 

 tures of regulation (Child, '02, p. 218, p. 229; 'o^a, p. 131 ; '04/5, p. 467, etc.). 



