266 THE GERM-PLASM 



species do not interbreed ; but the principles which may be 

 deduced from these imaginary cases may be apphed to the 

 production of hybrids. Homologous determinants and ids co- 

 operate, while heterologous ones do not ; and the larger the num- 

 ber of heterologous determinants present in the homologous 

 ids of the parents, the more do the hereditary tendencies of the 

 latter tend to nullify one another. 



We must now attempt to give a theoretical explanation of the 

 first of the three ki?ids of cojubination of parental characters in 

 the offspring metitioned above, viz., that in which "a mean between 

 both parents is maintained in all parts of tJie plant ."^ 



If we assume that the two parental species are so closely allied 

 that each determinant in the one corresponds to a homologous 

 determinant in the other, a form exactly intermediate between 

 the two species must result, supposing that the number of ids in 

 either parent is the same, and that a similar controlling force 

 acts on the homologous determinants. 



If the idioplasm of one parent is represented by a larger 

 number of ids, its controlling force must obviously be greater ; 

 and as regards the controlling force of the individual determi- 

 nants, we may state as follows: — the control of the cells is in 

 our opinion effected by the disintegration of the determinant 

 into biophors, which, like the pangenes of de Vries, migrate 

 into the cell-body, multiply at its expense, and give rise to 

 definite cell-structures. This multiplication must take place with 

 a certain amount of energy, the degree of which varies in the 

 different kinds of biophors. Thus, whenever such controlling 

 biophors, possessing the energy of growth in different degrees, 

 migrate into the same cell-body, a struggle of the parts (Roux) 

 must ensue, in which the stronger part will be successful, 

 and the weaker will be more or less, or even completely, 

 suppressed. 



The formation of structures which are strictly intermediate 

 between those of the two parents, implies that the homologous 

 determinants pos.sess a similar controlling force. All the deter- 

 minants of any two species are, however, never homologous : 

 this follows from the fact that the number of cells in homologous 

 parts is often very different. The characters of the species do 

 not by any means depend only on the histological nature of the 

 individual cells, but, as already stated, are due in almost a 

 greater degree to the number and arrangement of the cells, to 



