THE INHERITANCE OF DISEASE 5II 



will be worth while to develop by further studies. Up to now 

 the interest has chiefly centered on recognizing certain 

 anatomical types of people and trying to correlate with these 

 the diseases from which they have suffered. The recent work 

 of Draper who approached the question by taking typical 

 cases of certain diseases and studying the physical conforma- 

 tion seems to promise more definite results. Of similar import 

 and carrying even greater suggestion of future interest are 

 observations indicating that the blood grouping, a functional 

 inheritable manifestation developed under definite conditions 

 between the blood cells and the blood serum, is associated in 

 the inheritance with the natural immunity to diphtheria 

 toxin or with the capacity to be immunized against this 

 poison. 



It is quite within the bounds of possibihty that tracing 

 such relationships as have here been outhned may make it 

 possible to trace resistance factors, themselves intangible, 

 through their frequent association with other characters 

 more easily recognizable. 



Sunburn as an Illustration. It is somewhat curious that much 

 of what we know of the principles of the inheritance of 

 disease can be quite well illustrated by a critical con- 

 sideration of simple sunburn. The following paragraphs 

 about this condition may well serve as a summary of the 

 main features developed in the preceding discussion of 

 more serious diseases and defects. 



1. The effect commonly known as sunburn is pathologically 

 closely related to, but not identical with, effects produced by 

 heat rays, roentgen rays, acids and some other chemical 

 agents. Burning is a property of the sun's rays, particularly 

 those of a portion of the ultraviolet region of the spectrum. 

 A rather common type of disease is thus induced by a highly 

 speciahzed and particular agency. 



2. A number of environmental conditions must be 

 observed in order that the injury may be produced. These 

 conditions need not be enumerated in detail here since they 

 relate to the fact that ultraviolet rays are at a threshold 

 level in sunlight as it reaches the earth and only reach 

 burning intensity under clear skies, summer, high altitude, 

 and other favoring circumstances. 



