32 2 THE GERM-PLASM 



homodynamous determinants liave a cumulative effect, while 

 heterodynamous determinants tend to counteract one another. 

 The same must be true of the groups of determinants, — the 

 ids and idants ; similar idants must increase the effect produced, 

 while dissimilar ones will interfere with one another to a greater 

 extent the more diversified their composition out of ids and 

 determinants is. Let us confine our attention for the present 

 to the groups of determinants for the flowers. The two species 

 of Datura certainly contain determinants which tend to form 

 white flowers, and we might therefore be disposed to infer that 

 these are homodynamous, and that their effect must be cumu- 

 lative. But this conclusion would be erroneous, for it is quite 

 possible that these determinants only correspond as regards the 

 production of a ' white ' colour^ and differ widely in respect of 

 manv other characteristics of the cells, such as those relating to 

 their size and minute structure. On the other hand, the 'blue ' 

 determinants are actually homodynamous. and correspond not 

 only with regard to the colour to which they give rise, but also 

 in respect of all the other characters of the cells of the flowers, 

 for they are derived from the common ancestral species. When 

 the hybrid begins to develop flowers, the structure of the cells 

 in the latter will depend on the determinants of the two white 

 species and of the blue ancestral form. Although the 'blue' 

 determinants are in the minority in the idioplasms of either 

 parent, they may nevertheless, if they all combine, possess a 

 greater power of transmission than the ' white " ones, if the 

 latter are not homodynamous. i.e., do not possess an exactly 

 similar force of heredity, and consequently cannot produce an 

 intermediate effect. They interfere with one another in their 

 action, as they act in different directions to a greater or less 

 extent. Many cases of reversion can be understood — even 

 though only in principle — by means of this law. They can, 

 moreover, easily be rendered comprehensible in individual cases 

 if we have recourse to figures. 



Let us suppose that the 'blue' ancestral determinants are not 

 only contained in individual ids, but are present in all the ids of 

 entire idants : a minority of old ancestral idants would then be 

 opposed to a majority of modern ones, half of which, however, 

 would correspond to the type of D. Icrvis and half to that of D. 

 ferox. VVe assume that there are in all sixteen idants, six of 

 which are ' blue " and ancestral, and ten • white.' Since, therefore, 



