HEREDITY IN MAN 359 



correlation in respect to eye colour is 0-495, and the average 

 fraternal correlation is 0-475. With regard to ability the fraternal 

 correlation is 0-46, the correlation between sisters 0-47, and that 

 between brother and sister 0-44. 



All that has been said with regard to the nature of fertilization 

 and development lead to a similar conclusion. In the processes 

 of maturation and fertilization we can detect a mechanism whereby 

 the bearers of inherited qualities are transmitted from parent to 

 child. In development we see how, when the appropriate stimulus 

 plays upon the fertilized egg, an adult member of the species 

 grows up. It is only when we suppose that within the fertilized 

 egg there are certain predispositions in respect of every character 

 which are derived from the parents that we can in any w^ay 

 understand how adult individuals come into being,^ And it must 

 be emphasized that, how^ever much opinions may differ regarding 

 the precise nature of the mechanism of inheritance, there is no 

 difference regarding the fact of inheritance. All biologists are 

 agreed on this subject up to a point ; the matters in debate are 

 not strictly relevant here. 



3. There are certain points, however, which, in view of the 

 form which the discussion in the following chapters will take, 

 require further treatment. As regards physical characters, it is 

 sufficient to think of all such characters whether great or small 

 as inherited. But with regard to disease it may be as well to 

 consider further to what degree it can be said to be inherited. 



What we call disease falls under two headings : disease due to 

 the attacks of parasites and disease due to structural defects or 

 weaknesses. Parasitic infections are of post-conceptional ac- 

 quirement ; they may be acquired, it is true, before birth, but 

 such an acquirement is strictly analogous to the ' catching ' of 

 a disease after birth. In the sense, therefore, that parasites are 

 not transmitted from the parent to the ovum, as the germinal 

 constitution is transmitted, disease is never inherited. 



It has been shown, however, that in the case of certain diseases, 

 such as tuberculosis, a definite susceptibility to contract the 

 disease is inherited. In other words, given an equal exposure to 

 infection, some men do not ' catch ' the disease, some only 



' The fact that the germinal constitution of any one individual belonging to 

 a species, in which biparental reproduction is taking place, differs almost always 

 from that of any other member of the species is due on the Mendelian hj'pothesis 

 to the chance mixture of factors in the zygote. 



