HEREDITY 2 1 7 



two classes, those who are born healthy and of sound constitution, and 

 those who come into the world otherwise. All such conceptions, it should 

 be clearly seen, are in fact untrue. A healthy person is one who has no 

 gross anatomical or physiological defects and enough normal general 

 health to get on with. 



In fact the great progress made by medicine as an art and a science from 

 the dawn of civilization down to today is based on the steadily developed 

 recognition of the infinite complexity and relative nature of the phe- 

 nomena included in the general term "disease." And especially the re- 

 markable progress of the last two centuries is due to the extension of this 

 general principle into the study of particular diseases. Even the most simple 

 (apparently) of abnormal conditions is found on closer scrutiny to be of 

 the utmost complexity. A common boil is spoken of in scientific terms as 

 a simple inflammation and even moderately informed lay people know it 

 as the result of some "germ" getting into an insignificant scratch. In 

 reality the processes are complex far beyond our present understanding. 

 Essentially the same process in the lungs gives rise to the acute and often 

 fatal disease, pneumonia. But when pneumonia is examined, even in the 

 light of our present imperfect knowledge, attention being paid to the 

 particular germ giving rise to the infection, and the qualities and distri- 

 bution of the reaction material in the lungs, it is easy to discriminate more 

 than ten essentially independent kinds of extensive and severe inflamma- 

 tions of the lungs, which would be properly designated by the practicing 

 physician as pneumonia. 



It will readily be understood, therefore, that when an attempt is made 

 to deal with the points of contact and mutual influence of two such all- 

 inclusive and infinitely complex assemblies of phenomena as those of in- 

 heritance and of disease, it cannot profitably be done solely with reference 

 to general principles. Nor would it be useful in this place to attempt a 

 very detailed account of what is known. The plan adopted is to try to 

 give an outline of principles where these are discernible and to illustrate 

 them vv^ith such concrete examples as may be most informing to the general 

 reader. 



INFECTIOUS DISEASES 



As previously pointed out there is in the rigid sense no such thing as the 

 positive inheritance of an infectious disease. This lies in the nature of the 

 case since the impelling incident in such a disease is the entry of an agency: 

 germ, bacterium or protozoan, from the environment. None the less, the 

 inheritance is of very vital significance and within certain limits absolutely 

 controls the prevalence of these diseases. This is true when we approach 

 the question from a wide biological viewpoint, regarding species lines. It 

 then becomes in truth a matter of common knowledge. It is probably 

 quite correct to state that each distinct species of animal or plant has 



