54 SENESCENCE AND REJUVENESCENCE 



own experimental investigations have forced me to the conclusion 

 that the organic individual consists of a dominant and of sub- 

 ordinate parts and that dominance and subordination in their 

 simplest terms depend upon rate of metabolism (see chap. ix). 

 Not only does the evidence indicate that this is the case, but it is 

 impossible to conceive of a definite, orderly process of differentia- 

 tion attaining a definite constant result in a complex physico- 

 chemical system without some sort of dominance and subordination 

 in the processes involved. In a complex system consisting of co- 

 ordinate parts the process of differentiation must differ widely in 

 character according to conditions, and the orderly character of 

 development and constancy of result which we find in organisms 

 would be impossible. 



Most theories of the constitution of the organism have failed to 

 recognize the necessity for such a relation of dominance and sub- 

 ordination between parts as a fundamental feature; consequently 

 they have failed to account satisfactorily for the orderly course and 

 definite result of differentiation. Driesch is one of the few who 

 have seen clearly that the organic individual is impossible without 

 a controlhng and ordering principle of some sort, and not finding 

 any physico-chemical basis for such a principle, he has vested the 

 control in entelechy. As regards plants, the dominance of the 

 vegetative tip over other parts has been clearly demonstrated, 

 but no such relation of parts in animal development has been 

 generally recognized by zoologists. Nevertheless such a relation 

 exists and must exist, for without it development, as we know it, 

 is impossible. 



Dedifferentiation. — -Dedifferentiation is a process of loss of 

 differentiation, of apparent simplification, of return or approach to 

 the embryonic or undifferentiated condition. Zoologists have been 

 slow to admit its occurrence. According to Weismann — and many 

 agree with him — development proceeds always in one direction 

 and dedifferentiation is impossible. Whenever a new development 

 of a part or a whole occurs, it originates from cells or parts of cells 

 which have not undergone differentiation beyond the stage at which 

 the new development begins. Whenever cells which are visibly 

 differentiated give rise to new wholes or parts, as they often do in 



