294 THE GERM-PLASM 



Each id of the germ-plasm passes through all the ontogenetic 

 stages ; that is to say, the number of paternal and maternal ids 

 remains constant throughout ontogeny, and every cell is there- 

 fore controlled by an equal number of ids from both parents ; 

 and, moreover, in normal reproduction between individuals of 

 the same species, the same number of paternal and maternal ids 

 take part in the process. 



The fact that the structure of each cell, organ, or part of the 

 body of the offspring is nevertheless not exactly intermediate 

 between that of the corresponding parts of the parents, must be 

 due to the following causes. 



In the first place, an exactly intermediate structure need 

 not necessarily arise even if all the active paternal and 

 maternal determinants of a cell were exactly alike, or homo- 

 dynamous ; for even in such a case, the ' controlling forces ' of 

 the maternal determinants might preponderate over those of the 

 paternal ones, owing to a superiority as regards the rate of assim- 

 ilation and multiplication, or in some other respect. The cell- 

 body would then become filled more rapidly by the biophors of 

 the maternal determinants which pass into it from the nucleus, 

 and the multiplication and distribution of the paternal deter- 

 minants would thus be checked. Hence the controlling force 

 itself may be different with regard to the homologous paternal 

 and maternal determinants. 



The differences are, liowever, still further increased, owing to 

 the fact that the ids, and more especially the homologous 

 determinants derived from either parent, are not always homo- 

 dynamous, but are, in part at any rate, almost always heter- 

 odynamous. The controlling force of the homodynamous 

 determinants must, however, necessarily be cumulative, and the 

 inequality in the force of heredity of the father and mother at 

 any particular stage in ontogeny, is essentially due to the fact 

 that although the number of ids is the same, the number of 

 homodynamous determinants — i.e.^ those which have a cumula- 

 tive effect — is different. 



It was deduced from the consideration of the phylogeny of 

 variation, that the determinants must have varied independently 

 of each other in the different ids of a germ-plasm, so that the 

 homologous determinants may be present in very different 

 variants in their ids ; and that, moreover, these variants of the 

 determinants in the different onto-idic stages may be combined 



