VARIATION AND HEREDITY 15 



But, if the character be recessive, it may lie concealed 

 in the germ cells of some of those who outwardly 

 show no sign of it. Some forms of deaf-mutism 

 seem to be recessive. Hence it is specially dangerous 

 for normal folk who both come from deaf-mute stock 

 to intermarry. The defect is likely to reappear in 

 their children. Some types of mental defect show the 

 same relations. A pair both of whom are afflicted 

 breed " true " ; they produce none but feeble-minded 

 offspring. A pair who are themselves normal but come 

 from weak-minded stocks, will find about one quarter 

 of their children mentally defective. 



It will now be seen how essential it is to extend our 

 knowledge of these forms of Mendelian inheritance. 

 In these cases, we can predict the probable or certain 

 result of a proposed marriage, and foretell whether it 

 should or should not be entered upon. 



We have begun with those cases in which simple 

 Mendelian inheritance already has been made out, 

 because, in the present state of our knowledge, in them 

 alone we can predict the probable average composition 

 of a large family if we know the family history of the 

 parents. But Mendel's laws have been demonstrated 

 as yet for only a few cases of human inheritance, and 

 for most characters we are thrown back upon vaguer 

 methods of inquiry. 



Let us take a typical instance of continuous variation, 

 such as difference in stature. Let us suppose that the 

 average height of the men in a certain race is 5 feet 

 8 inches. Let us select as many fathers as we can find 

 whose height is approximately 6 feet that is, 4 inches 

 more than the average. A measurement of the height 



