48 The Recapitulation Theory and Human Infancy 



"There is certainly no evidence in favor of the view that the 

 organization of the egg need be anything like the organization 

 of the embryo that comes from the egg, although the organiza- 

 tion of the egg may be perfectly definite in its character." 79 



The several "factors" contributing to the differentiation of 

 structure are thus summarized by Jenkinson: 



"Experimental investigation, as far as experiment has at 

 present gone, has shown, first, that a certain complexity of the 

 physical and chemical environment is a necessary condition 

 of normal development; that complexity may, it is true, vary 

 within certain limits, but those limits can only be transgressed 

 under pain of abnormality or death. In the second place, it 

 has been demonstrated that the initial structure of the germ, 

 and the mutual interactions of the parts as they develop are 

 both indispensable factors." 80 



We are thus encouraged to believe that development is neith- 

 er an independent process of self-realization of the germ-cell on 

 the one hand, nor a differentiating effect of a regulative environ- 

 iment upon a neutral protoplasm on the other, but rather an 

 intricate and puzzling interaction or mutual interrelationship 

 of specific external influences and internal conditions. The 

 authoritative primacy of the germ-cell thus becomes greatly re- 

 duced. To recall the usage of von Baer, the egg, the chick, and 

 the hen, the entire ontogeny in fact, is really the bird. The two 

 important ideas for the present discussion of ontogenetic-ances- 

 tral resemblances, to be derived from these considerations, 

 are these: the fate of any embryogeny is in part determined 

 in the course of ontogenetic history, and the succession of 

 events in this history permits a new organization in each phase. 

 This conception of development receives empirical support 

 from the circuitous and indirect course taken by many animals 

 from the germ-cell to the terminal or adult condition. If then 

 the caterpillar is found to have certain describable resemblances 

 to a worm, or a tadpole to a fish, the fact that the subsequent 



79 Regeneration, 1901. 



80 Experimental Embryology, 1909, p. 279. 



For a summary of the results of experimental study of the effect of environmental 

 differences upon the life cycle, see Morgan, Experimental Zoology, 1910, Chaps. 

 XIX-XXIII. 



A recent significant discussion of the effective relationship of environment to or- 

 ganic existence, is to be found in Henderson, Fitness of tho Environment, 1913. 

 A summary statement from this book is the folio whig: ". . . . somehow, be- 

 neath adaptations, peculiar and unsuspected relationships exist between the proper- 

 ties of matter and the phenomena of life." 



