304 THEORIES OF INHERITANCE AND DEVELOPMENT 



Driesch, and other writers. In discussing them, it should be borne 

 in mind that, although both have been especially developed by the 

 advocates of the pangen-hypothesis, neither necessarily involves that 

 hypothesis in its strict form, i.e. the postulate of discrete self-propa- 

 gating units in the idioplasm. This hypothesis may therefore be laid 

 aside as an open question, and will be considered only in so far as it 

 is necessary to a presentation of the views of individual writers. 



The Roux-Weismann hypothesis has already been touched on at 

 p. 183. Roux conceived the idioplasm (i.e. the chromatin) not as a 

 single chemical compound or a homogeneous mass of molecules, but 

 as a highly complex mixture of different substances, representing 

 different qualities, and having their seat in the individual chromatin- 

 granules. In mitosis these become arranged in a linear series to 

 form the spireme-thread, and hence may be precisely divided by the 

 splitting of the thread. Roux assumes, as a fundamental postulate, 

 that division of the granules may be either quantitative or qualitative. 

 In the first mode the group of qualities represented in the mother- 

 granule is first doubled and then split into equivalent daughter-groups, 

 the daughter-cells therefore receiving the same qualities and remain- 

 ing of the same nature. In ''qualitative division," on the other hand, 

 the mother-group of qualities is split into dissimilar groups, which, 

 passing into the respective daughter-nuclei, lead to a corresponding 

 differentiation in t/te daughter-cells. By qualitative divisions, occur- 

 ring in a fixed and predetermined order, the idioplasm is thus split 

 up during ontogeny into its constituent qualities, which are, as it were, 

 sifted apart and distributed to the various nuclei of the embryo. 

 Every cell-nucleus, therefore, receives a specific form of chromatin which 

 determines the nature of the cell at a given period and its later his- 

 tory. Every cell is thus endowed with a power of self-determination, 

 which lies in the specific structure of its nucleus, and its course of 

 development is only in a minor degree capable of modification through 

 the relation of the cell to its fellows (" correlative differentiation "). 



Roux's hypothesis, be it observed, does not commit him to the 

 theory of pangenesis. It was reserved for Weismann to develop the 

 hypothesis of qualitative division in terms of the pangen-hypothesis, 

 and to elaborate it as a complete theory of development. In his 

 first essay ('85), published before De Vries's paper, he went no fur- 

 ther than Roux. "I believe that we must accept the hypothesis that 

 in indirect nuclear division, the formation of non-equivalent halves 

 may take place quite as readily as the formation of equivalent halves, 

 and that the equivalence or non-equivalence of the subsequently pro- 

 duced daughter-cells must depend upon that of the nuclei. Thus, 

 during ontogeny a gradual transformation of the nuclear substance 

 takes place, necessarily imposed upon it, according to certain laws, 



