46 THE EVOLUTION THEORY 



resultant of development, the ' type ' of the child. But this is not the 



case. 



Numerous experiments on the hybridization of two species of 



plant have taught us that the descendants of such hybridization 



usually maintain a medium between the ancestral species ; but it is 



not always the case, for in many hyl^rids the character of one 



species, whether paternal or maternal, preponderates in the young 



plant. 



We recognize the same thing still more clearly in Man, whose 

 children by no means always maintain a medium between the 

 characters of the two parents, but frequently resemble one — the 

 father or the mother — much more strongly than the other. 



How can this fact be theoretically explained ? Must we ascribe 

 to the ids of the father or of the mother a greater determining 

 power? Without excluding such an assumption as on a p?'io7'^ 

 grounds inadmissible, I am inclined to believe that we do not 

 require it to explain tliis phenomenon. For, if we take our stand 

 simply on the fact of the preponderance of one parent, it follows 

 directly from this that not all the ids control the type of the 

 child, let the cause of the non-co-operation of some of them be what 

 it may. But if in this case only a portion of the ids contained in 

 the germ -plasm controls the type, this combination of ids suffices 

 to make the child resemble one parent, the father, for instance, 

 and consequently half the number of ids is sufficient in some cir- 

 cumstances to determine the child — taking for granted that the 

 one-sidedness of the inheritance is complete, which never actually 

 happens. But the half number of ids can onl^^ suffice if it includes 

 the same combinations of ids which have determined the type in 

 the case of the father; as soon as one or more ids of this par- 

 ticular combination are replaced by others the paternal germ -plasm 

 alone is not enough to call forth complete resemblance in the child. 



But, at the reduction, a change of arrangement of ids takes place, 

 and a new combination arises, and thus each germ- cell receives its 

 particular group of ids. It may thus happen that, in one particular 

 sperm-cell, exactly the same group of ids is contained as that which 

 determined the type of the father, and that the same is true of a par- 

 ticular egg-cell in regard to the type of the mother. Let us now 

 assume that a sperm-cell and an egg-cell meet, which contain both 

 those groups of ids which had determined the type of the father and 

 of the mother ; if the determining power of the maternal and paternal 

 ids were equal a child would result which would maintain the medium 

 between father and mother. 



