100 HEREDITY [ch. 



give a parental correlation and thus an intensity of 

 inheritance closely similar to those obtained from 

 the study of various animals and plants. When 

 various non-measurable and less definite characters 

 such as intellectual ability, hand-writing, etc. are 

 investigated by the same methods, a similar intensity 

 of heredity is found, and finally the same is true when 

 the character chosen is liability to certain diseases, 

 notably tuberculosis and insanity, or such abnormal 

 conditions as congenital deafness. Since these latter 

 conditions have been only briefly alluded to, and are 

 of such fundamental importance for the well-being 

 of mankind, the evidence may be referred to rather 

 more fully here. The case of insanity is especially 

 convincing, for it is not open to the objection some- 

 times made with regard to infectious diseases that 

 the cause of the apparent inheritance of the condition 

 is the exposure of the child to infection from the 

 parent. But it must be remembered that there are 

 many kinds of insanity, in one of which at least 

 (chorea), the inheritance appears to be Mendelian, 

 and that of two men with equal tendency to mental 

 aberration, one who is not exposed to strain may 

 remain normal through life, while another under 

 more arduous conditions may break down. But the 

 data of occurrence of insanity among tainted stocks 

 make it certain that ' the insane diathesis is inherited 

 with at least as great an intensity as any physical or 



