INHERITANCE IN MANKIND 47 



blind. But the majority of the normal characters which 

 make up a man are too complex for statement in these 

 terms. His stature is controlled by the total length of 

 many bones, each of which may be separately variable ; 

 his strength is determined by the co-ordination of many 

 factors, associated with different members of his body ; 

 his ability depends on the proper conjunction of a still 

 larger number of attributes : intellect, application, will- 

 power, sense of duty, each of which itself may be highly 

 complex. 



In the present state of our knowledge, it is premature 

 to attempt to trace definite Mendelian inheritance in 

 such complicated characters. Our new familiarity with 

 the rules of simple inheritance, it is true, enables us to 

 catch suggestive glimpses of order from time to time ; 

 but, for the most part, we are driven back at present to 

 vaguer statistical methods. Able parents continue to 

 produce more than their numerical share of able children, 

 whether or no ability ultimately be resolvable into a very 

 large number of dominant Mendelian factors. Occasion- 

 ally we shall find that some " sport " arises, apparently 

 spontaneously, whether we regard it as indicating some 

 process of conjunction of recessive characters not visible 

 in the parents, or prefer to leave it unexplained. 



Even in the inheritance of many diseases we cannot 

 yet prove that exact Mendelian principles hold. Often 

 it is the predisposition to the disease, and not the 

 disease itself, that is inherited ; the disease must then 

 be induced by external conditions. In other cases, 

 the disease may be produced by several causes, only 

 one of which is hereditary. Thus deafness is to some 

 extent hereditary, but doubtless it is also producible by 



