INDIVIDUAL DEVELOPMENTS 239 



THE THEOKY OF INDIVIDUAL DEVELOPMENT 1 



By Professor FRANK R. LILLIE 



UNIVERSITY OF CHICAGO 



ORGANIC development presents two aspects : that of the individual 

 and that of the race, ontogeny and phylogeny (evolution) . These 

 are not two separate and distinct series of phenomena; on the one 

 hand, the individual development is to a certain extent a record of 

 the past history of the race, and the promise of future racial develop- 

 ment; on the other hand, evolution is not a series of completed indi- 

 viduals but a series of individual life, histories ; for the only road from 

 one generation to the next is by way of a complete life history. Indi- 

 vidual development is, therefore, not something distinct from evolution ; 

 it is a part of the process of evolution itself; the development of the 

 individual is a chapter in the history of the race. 



The development of the individual may be pictured as a steadily 

 broadening stream that takes its source in the fertilized ovum and 

 flows on until death. In this analogy the individual would be repre- 

 sented as a cross-section of the stream at whatever stage we were 

 examining. Though such an analogy limps, inasmuch as individual 

 development is never before us as a unit, as a stream may be conceived 

 to be, and can indeed be said to exist only as the successive cross- 

 sections (its past having disappeared and its future yet unborn), never- 

 theless, it represents very well the steady, unbroken progress of de- 

 velopment from the ovum to old age. There may be crises in the 

 development of the individual, as, for instance, when the chick leaves 

 the egg or the pullet lays its first egg, but there are no breaks in its 

 continuity. Successive generations may be pictured as hew streams, 

 each taking its source from a particle — a germ cell — from some cross- 

 section of the preceding generation; and evolution may be represented 

 by placing the new source at a different level than the original. For 

 evolution studies we compare cross-sections of different developmental 

 streams (generations) at comparable distances from the sources, and 

 for evolutionary explanation we must examine the entire series of 

 processes involved in the origin of the new source and in the conditions 

 and inherent character of the new developmental stream. 



We can not be said to have actual experience of any other form 



of development than individual development; evolution or racial de- 



1 One of the series of Darwin Anniversary addresses given under the 

 auspices of the Biological Club of the University of Chicago, February 1 to 

 March 18, 1909. 



