2 74 THE GERM-PLASM 



also represented in the majority of the ids in almost all these 

 individuals. Conversely^ those characters which are beginning to 

 become useless to the species must be contained only in a gradually 

 increasing minority of the ids ; the number of the latter must 

 gradually decrease, until it finally becomes so small that it can 

 no longer give rise to the character in question in consequence 

 of an overwhelming majority of the other ids. 



The above conception of the germ-plasm enables us to un- 

 derstand why the force of heredity may vary /;/ the course of 

 ontogeny in the case of crosses between two species, and why 

 the paternal tendency may dominate in one character and the 

 maternal in another. For, quite apart from the possible degree 

 in which the force of heredity may be present in the individ- 

 ual determinants, and assuming it to be equal in the two 

 species, the nujuber of ids which contain homodynamous 

 determinants will nevertheless vary according to the age of 

 the character in question. A greater number of ids containing 

 homodynamous determinants will indicate a greater force of 

 heredity. If the form of the flower in a species A was acquired 

 long ago, while that of the leaf is a new acquisition, and if the 

 reverse is true in the case of a species B, the hybrid which 

 would be obtained by crossing these two species would resemble 

 species A in the form of its flowers and species B in that of its 

 leaves. A larger number of homodynamous determinants of 

 species A, for the rudiments of the flowers, will be opposed to 

 a smaller number of homodynamous determinants of the species 

 B, which, on the other hand, will contain a larger number of 

 homodynamous determinants for the rudiments of the leaves. 

 The importance of the whole principle will be made still more 

 evident in the following section. 



c. The Struggle of Individual Characters 



The question of individual characters in the two parents has 

 not been taken into account in our previous investigations : in 

 crosses between different species they may be looked upon as 

 insignificant when compared with the specific characters. We 

 must now consider those cases in which the two parents differ in 

 respect of slight individual characters only, confining ourselves 

 to the human race, the individual characters of which we can 

 recognise most clearly. 



