SHARE OF THE PARENTS IN BUILDING UP THE OFFSPRING 47 



As is well known this does happen not infrequently, althouoh 

 it is difficult or impossible to demonstrate it precisely. In plant- 

 hybrids proof is easier, and it has been established that b}- far the 

 greater number of hybrids maintain a medium between the characters 

 of the two ancestral species. This proves that our assumption of 

 equal strength of the ids of both species must be correct in general, 

 for we know definitely in this case, as I shall show later, that the 

 paternal and the maternal ids are equivalent as regards the characters 

 of the species. This is the case, for instance, with the hybrids between 

 the two species of tobacco-plant, Nkotkma rustica and jV. i)aniculata, 

 which were reared by Kolreuter as far back as the eighteenth century, 

 and which then, as now, maintained a fairly exact medium between 

 the two ancestral species, and did so in all the individuals. Both 

 species thus strive to stamp their own character on the young plant, 

 and in both the hereditary power is equally great; in both it is 

 contained in the same number of i<ls, that is, in the half, for both 

 kinds of sex-cell have undergone reducing division. We have here, 

 then, strict proof that the half number of ids suffices to reproduce 

 in the offspring the type of the species, or, more generally, of the 

 parents. 



If we apply these results to the inheritance of individual 

 differences in Man, we may say, that those germ-cells, to which at 

 the reducing division the same combination of ids has been handed 

 on as that which already determined the type of the parent, will 

 endeavour to impress this image again on the child. If a female 

 cell of this kind combine with a male which likewise contains the 

 facies-combination of the parent, in this case the father, the same 

 thing will happen which we described in the case of the plant-hybrids, 

 that is, a medium form between the type of the two parents will arise. 



Not infrequently, however, there is a marked preponderance of 

 the one parent in the type of the child, and we have to inquire 

 whether the theory gives us any help witli regard to such a case. 



One might be inclined to assume a difference in the determining 

 power of the paternal and maternal ids, but if we cannot show to 

 what extent and for what reason this power may be different such 

 an assumption remains rather an evasion than an explanation. 

 Moreover, it would not always apply to the conditions in Man, for 

 if, for instance, the ids of a particular mother were in general stronger 

 than those of the father, all the chiMren of the pair in question 

 would necessarily take after the mother; but it happens not infre- 

 quently that one child resembles the father preponderantly, and 

 another the mother. Moreover, the ids pass continually from tlie 



