CHAPTER III 



NATURAL SELECTION 



IN the animal kingdom, and in primitive races of men, 

 want of strength, agility or fleetness is often fatal. 

 These qualities consequently have selective value in 

 the struggle for survival of the fittest. But among 

 the civilized nations of mankind, especially among 

 urban populations, it is probable that the most effective 

 agent in the process of natural selection is disease. A 

 host of infantile disorders sweeps off a hecatomb of 

 victims every year. Those of specially weakly con- 

 stitution, and those specially susceptible to the specific 

 diseases which are rife, succumb more easily and more 

 rapidly. Fewer of them live to maturity, and fewer of 

 them live to hand on their hereditary qualities to off- 

 spring. Thus the race is gradually purified from the 

 taint of general weakness of constitution, and from 

 special liability to the attacks of specific diseases. 



It is impossible to doubt that comparative suscepti- 

 bility to and immunity from definite diseases have 

 played a great part in the history of mankind. Before 

 the world was made one by facility of communication, 

 different races dwelt in comparative isolation. Each 

 race had its own maladies, and nature took every pains 



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