INDIVIDUAL DEVELOPMENT 251 



environment directly on the germ cells. The most fundamental phe- 

 nomena in the unit character theory are unquestionably the segrega- 

 tions of characters that appear in the offspring of hybrids in so-called 

 Mendelian inheritance. In the most typical cases, grandparental 

 characters reappear in definite proportions of the progeny of the hybrid 

 generation. The interpretation, according to the theory of unit 

 characters, is in the hypothesis of purity of the germ cells of the 

 hybrid generation with respect to the segregated characters; which 

 means that the germ cells of the hybrid generation are pure with refer- 

 ence to the contrasting characters united in the soma; in other words, 

 that corresponding contrasted characters can not both remain in the 

 same germ cell, but are segregated in different ones and may thus 

 appear pure in the descendants of a hybrid generation. 3 



We may well doubt that absolute purity of grandparental characters 

 in the offspring of the hybrid generation occurs, and the results un- 

 questionably vary with the environment; but I believe that we have 

 to admit the general principle of segregation. However, the theory 

 of segregation of unit characters in the germ cells is in no way necessary 

 to explain the results ; it is in fact inconsistent with the highly variable 

 result ; if unit characters were segregated in the germ, we should expect 

 very definite constant results. 



If we take our stand on the epigenetic basis and regard the germ 

 cells as no more complex than direct investigation would lead us to 

 suppose, then we have to admit that segregation in the germ cells can 

 involve only constituents of the germ cells themselves. But any 

 variation thus induced in the germ cells would be a factor in each 

 process of the development, and would hence tend to influence every 

 character that appears. Such a hypothesis involves the conception 

 that germ cells contain elements capable of segregation; and this is so. 

 Even if the principle of segregation of characters in inheritance had 

 never been discovered, the principle of segregation of germ-cell elements 

 would still hold, for the two discoveries were made absolutely inde- 

 pendently. 



I refer to the work of Guyer and Montgomery on the chromosomes, 

 which has been followed by a long series of very exact studies. . These 

 studies certainly suggest segregation of parental chromosomes in vary- 

 ing proportions in different germ cells. Indeed, I know of no other 

 interpretation of chromosome behavior that is consistent with the 

 facts. Whatever value we may attribute to the chromosomes in cellular 

 physiology, the variable relations established by their differential segre- 

 gations, even if only quantitative differences are concerned, must involve 

 endless secondary effects in the long series of cell generations that make 



3 Recent modifications of the theory of purity of the germ-cells do not essen- 

 tially modify the argument. 



