Introductory 1*7 



vcy has traced from tlic period of Aristotelian zoology, 

 through that of French comparative anatomy of the late 

 eighteenth and early nineteenth centuries, through what 

 might with propriety be called the period of American em- 

 bryology, now barely ended, holds its unmistakable course 

 on into what we may speak of as a physiological period, 

 in the midst of which we now are. It should be remarked 

 tliat "physiological" as lierc used does not refer so much 

 to pliysiology in the professional sense as to an approach 

 to certain developmental phenomena of the organism from 

 the functional side, since the biologists who are tending to- 

 ward the organism al theory are not primarily physiologists 

 but students of individual development. 



The term most characteristic of this latest outcrop of 

 organ ismalism is correlation, and what is distinctive about 

 the present effort as contrasted with that which marked 

 the idea of correlation held by the French anatomists is 

 tliat now the correlatedness of parts in the organism is being 

 h)oked at from the functional more than from the structural 

 side ; and that the necessity is felt more than it was in the 

 earlier period, of finding a causal explanation of the correla- 

 tions. "Equilibrium" is anotlier term that is frequently 

 used by the biologists whose thinking is of this cast, and 

 the kinship of this to Saint-Hilaire's "balance" will not 

 escape the reader's notice. This doctrine of physiological 

 correlation is receiving its fullest elaboration at the hands 

 of C. M. Child, though numerous investigators are con- 

 tributing importantly to it. K. Goebel, E. Radl, W. Pfeffer, 

 L. Jost, J. Nusbaum, E. Schultz, H. Rand, S. J. Holmes 

 and C. Zeleny may be mentioned as biologists who have 

 dealt more or less directly with the problem. Undoubtedly 

 H. Driesch's "harmonious equipotential systems" ought to 

 be mentioned in this connection, though this author's un- 

 qualified commitment to an extra-natural explanation of 

 biological phenomena will hardly permit us to enroll him 



