136 THE POPULAR SCIENCE MONTHLY 



This supposition will be recognized by many of you as a simplified 

 case of Mendelian inheritance of a unit character due to the presence or 

 absence of a single determiner which can either be or not be in a germ or 

 ovum, and which " segregates." 



No case quite so simple as this can be true of human intellect, but 

 something approximating it has been suggested as perhaps true. 



Suppose, on the other hand, that the germinal basis for intellect con- 

 sists in the presence, in the germ or ovum, of one or more of four deter- 

 miners — I v I 2 , I z and I 4 — contributing amounts 1, 2, 3 and 4 of intel- 

 lectual capacity. The fertilized ovum could then have any one of 256 

 different constitutions ranging from the entire absence of all these deter- 

 miners to the presence of each one "duplex" — i. e., in both germ and 

 ovum. If such duplex presence meant that the two contributions com- 

 bined additively, the original intellect of the individual could range 

 from to 20. Individuals, all of one same original intellect — 10 — 

 might be of very different germinal constitutions, and so of very differ- 

 ent possibilities in breeding. If two individuals, each of original intel- 

 lect 10, were mated, it might be the case that their possible offspring 

 would range in intellect from to 20, or it might be that they could 

 not go below 8 or above 12. 



If the number of germinal determiners of intellect is increased to 

 five or six, the task of telling the constitution of the germs produced by 

 any individual of known original intellectual capacity is enormously 

 increased; and the research needed to guide the best possible breeding 

 of man is very, very much more laborious. Moreover, instead of hoping 

 to bring man to the best possible status (subject to the appearance of 

 new desirable mutations) by a few brilliant rules for marriage, we 

 must then select indirectly and gradually by parental achievement 

 rather than directly by known germinal constitution, just as animal and 

 plant breeders had to do in all cases until recently, and just as they 

 still have to do in many cases. Only after an elaborate system of infor- 

 mation concerning family histories for many generations is at hand, 

 can we prophesy surely and control with perfect economy the breeding 

 for a characteristic which depends on the joint contributions of five or 

 six determiners. For it is just as hard to " breed in " a determiner that 

 raises intellect or morality only one per cent, as it is to " breed in " one 

 which raises it a hundred per cent. — provided, of course, the latter de- 

 terminer exists. And it is thousands of times harder to discover the 

 distribution of a determiner in the human race's germs when it is one 

 of ten that determine the amount of a trait, than when it is one of two. 



The germinal determination of intellect, morality, sanity, energy or 

 skill is, so far as I can judge, much more like the second complex state 

 of affairs than the first simple one. Important observations of the 

 inheritance of f eeble-mindedness and insanity have been made by Daven- 



