94 THE POPULAR SCIENCE MONTHLY 



One need not fail to appreciate its logical completeness, its symmetry, 

 and the skill with which it has been defended, and yet one need not be 

 blind to the fact that it has not been a stimulating guide for its friends. 

 It has been conservative rather than progressive. Founded on a definite 

 morphological conception of the ultimate constitution of living sub- 

 stance, it has not adapted itself plastically to the rapidly changing con- 

 ditions in biological science. The considerable amendment it has 

 received in the last eighteen years has only made it so cumbersome and 

 complex that it is now little more than a mere formulation of the facts 

 it attempts to explain. 



Time will not permit us to explore thoroughly the mass of evidence 

 on which this criticism has been based. While differentiation according 

 to the determinant hypothesis assumes qualitative divisions of the chro- 

 matin in the nucleus, numerous investigations have shown that at least 

 five divisions of the egg in some animals may occur before there is any 

 recognizable difference between the cells thus formed. Each of the 

 first sixteen is competent to develop the entire adult structure. The 

 only way to account for such a result in terms of morphological determi- 

 nants is to assume that a complete outfit passes to each cell with each 

 division of the nucleus, obviously a serious burden for the determinant 

 hypothesis to bear. Further, among these phenomena of development 

 which are conveniently investigated under the head of regeneration, 

 similar difficulties have so constantly recurred, requiring similar as- 

 sumptions of reserve determinants, that the theory has long since ceased 

 to interest investigators in this field. It follows, rather than leads, 

 investigation. Finally, in the field of heredity, just that characteristic 

 of Mendelian inheritance — namely, the segregation of parental charac- 

 ters in second generation bjbrids — which at first seemed to give the 

 strongest support to the conception of a germ plasm composed of mor- 

 phological determinants, has now been resolved far more satisfactorily, 

 because more simply and workably, in terms of chemical substances. 



These cases lay emphasis upon the distinction between morphological 

 and physiological conceptions that defines the essential difference be- 

 tween modern preformation and modern epigenesis. Instead of a con- 

 geries of morphological determinants, the epigenesist finds in the germ 

 a problem in physical and chemical relations. He is interested in the 

 dynamic aspects of development, in the energy transformations. He 

 does not seek to construct a scheme of the ultimate organization of 

 living substance, but he does seek to control its operations, to predict 

 its behavior. 



In this new form, the problem of differentiation presents many 

 interesting aspects and is being encouragingly developed. By way of 

 illustration, recent investigations indicate that color differentiation is 

 based essentially on a well-known chemical process, the oxidation, 



