THE INHERITANCE OF DISEASE 493 



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. Any refinements of definition must be entirely 

 with reference to some ideal standard which will doubtless 

 change with time and future evolution or achievements. 



In fact the great progress made by medicine as an art 

 and a science from the dawn of civihzation down to today is 

 based on the steadily developed recognition of the infinite 

 complexity and relative nature of the phenomena included 

 in the general term "disease." And especially the remarkable 

 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 insignifi- 

 cant scratch. In reahty 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 hght 

 of our present imperfect knowledge, attention being paid to 

 the particular germ giving rise to the infection, and the 

 quahties and distribution of the reaction material in the 

 lungs, it is easy to discriminate more than ten essentially 

 independent kinds of extensive and severe inflammations of 

 the lungs, which would be properly designated by the prac- 

 ticing physician as pneumonia. 



It will readily be understood, therefore, that when as 

 in this chapter 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 inheritance 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 out- 

 hne of principles where these are discernible and to illustrate 

 them with such concrete examples as may be most informing 

 to the general reader. 



