SO-CALLED FORMATIVE SUBSTANCES. iSl 



It is, of course, quite true that the character of the dynamic or 

 functional conditions at any given stage is determined in greater 

 or less degree by the character of the structural consequences of 

 previous activities. These new activities give rise to new struc- 

 tural results until a certain stage is attained, determined doubt- 

 less by the complexity of the physico-chemical constitution of 

 the organism, where the mutual determination of structure and 

 dynamic conditions maintains a more or less exact equilibrium 

 for a time. At this stage development is said to be completed. 

 Any or all of the dynamic conditions may in proper environment 

 give rise to conditions visible as structures, hence there are no 

 good grounds for distinguishing certain groups of them or of the 

 substances or structural complexes in connection with which they 

 arise as formative from other merely " functional " groups. 



HULL ZOOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO, 

 May, 1906. 



