THE PROBLEM 



7 



brought about by the formation and accumulation in or 

 about the cell of substances not present in the undiffer- 

 entiated cell. Differentiation is of course merely a 

 visible indication of differences of some sort in physio- 

 logical activity in different parts, although physiological 

 differences may exist without visible differentiation. 

 The physiological differences appear to consist at least 

 in large part of specialization in activity, that is, the 

 various fundamental activities of life which are all 

 present to some degree in the unspecialized cell or part 

 become more or less definitely distributed and localized 

 among different parts, a process often called division of 

 labor. Such specialization of parts is a characteristic 

 feature of life in all except the simplest individuals, and 

 even there it is probably present to some extent. 



Physiological specialization and the differentiation 

 which may result from it occur in any orderly way, and 

 in fact constitute the fundamental evidence for the 

 orderly character of the individual. The orderly 

 course of specialization and differentiation proceeds 

 very much as if there were underlying it a plan or scheme 

 characteristic for each kind of individual which is 

 worked out in a regular constant order, as the construc- 

 tion of a building according to a plan follows a regu- 

 lar course. The orderly localization of parts and the 

 orderly sequence in their appearance with reference to 

 certain directions in the developing individual indicate 

 the existence of some sort of ordering capacity under- 

 lying and preceding the stages where the order becomes 

 structurally visible. It is evident that this underlying 

 order, plan, or whatever it may be that determines the 

 developmental and physiological order in the individual 



