108 HEREDITARY CHARACTERS 



In some other diseases, however, the tendency seems to 

 be towards complete immunity. 1 



Apparently, then, in the case of immunity to different 

 diseases, we have examples of the process by which new 

 characters may be produced in a race. The selection in 

 the case of contagious disease must necessarily be very 

 stringent, and probably acts more rapidly than in the case 

 of most other characters. The consideration of the pro- 

 duction of immunity to disease in a race is also very useful 

 in making clear that, as regards the race, such characters 

 grow up gradually, and are transmitted from generation to 

 generation, being increased by the slow accumulation of 

 favourable inborn variations occurring in individuals. The 

 change produced by the environment on individual char- 

 acters, as we shall see later, is not a permanent change. 

 The change produced in the race by the selection of inborn 

 variations is, comparatively speaking, a permanent one. 

 The production of characters in the race is, then, the result 

 of natural selection acting upon the inborn variations occur- 

 ring in individuals. Individuals varying in a favourable 

 direction have advantage over those that do not, while those 

 varying in an unfavourable direction are eliminated. It is 

 probable that the new racial characters are derived from 

 individual characters ; individual characters being in their 

 turn the result of individual variations. 



1 This seems to be the case in some zymotic diseases that are generally more 

 serious than measles, c.rj. scarlet fever. Complete immunity appears to exist in 

 some families. My father, although at different times much exposed to con- 

 tagion, never had scarlet fever, neither had any of my uncles. When I was 

 Assistant Medical Registrar at St. George's Hospital, I was exposed to contagion 

 by scarlet fever several times a day for many weeks during a very bad epidemic. 

 I took no special precaution to prevent contagion, but I never had scarlet fever, 

 neither have any of my five brothers or sisters. I have come across several 

 similar instances in other families. 



