16 HEEEDITY 



acquirements. In discussing each of these, we are 

 concerned chiefly with bodily or physical characters. 

 But we would do very ill to study only these and 

 there to rest content. It will be necessary thereafter 

 briefly to introduce the consideration of the facts and 

 possibilities of mental heredity, mental variation, 

 mental acquirements. In so doing we not only 

 complete our study of the subject, not only consider 

 matters of great interest in themselves, but are also 

 preparing ourselves for the scientific study of the 

 human mind, the cardinal feature of this study 

 to-day being its recognition of the fact that man's 

 mind is not a prime fact, but has a history. 



One other aspect of this great study of heredity 

 must also gain our attention. The inheritance of 

 disease — disease of body and disease of mind — is a 

 matter of vast practical importance. On none other 

 is the public more constantly misinformed, so influ- 

 enced by the erroneous ideas of, say, forty years ago. 

 Furthermore, the study of inheritance of disease, and 

 also the inheritance of immunity to disease, is of 

 great scientific interest, for it throws light not only 

 upon the nature of heredity itself, but also upon the 

 manner in which the existing types and races of 

 mankind have been affected by the incidence of 

 disease in the past. It has been left to a dis- 

 tinguished medical observer, Mr. Archdall Keid, to 

 show how disease has acted, and is still acting, 

 through heredity, as an instrument of natural selec- 

 tion, in virtue of which mankind is constantly tend- 

 ing to become more and more immune to the attacks 

 of the lowly vegetable organisms which are the active 

 causes of nearly all disease. 



