CHAPTER XIII 

 THE PRESENT EVOLUTION OF MAN 



The conditions under which infection occurs Inborn immunity develops in 

 the individual under the stimulus of nutriment Acquired immunity under that 

 of use Theories of the causation of variations and the facts of disease The 

 decisive nature of the evidence furnished by disease Malaria Tuberculosis 

 The air-borne diseases. 



W 1 



427. ^["^ J"E. saw in the eleventh chapter that, with the excep- 

 tion of alcohol, opium, and perhaps one or two 

 other narcotics, microbic disease is the only agency 

 which is stringently selective amongst civilized men. Speaking 

 generally, in England, for example, the people who survive and 

 leave offspring are not those who escape contact with the microbes 

 of such prevalent diseases as measles and whooping-cough, but 

 those who are resistant to them. Selection implies, of course, 

 variations in resisting power, and no one will deny that men differ 

 naturally in this particular as in all others. Again, no one will 

 deny that resisting power against one disease does not necessarily 

 imply resisting power against any other. The selection is, in fact, 

 ' specific.' Each disease chooses its own special victims, and is the 

 cause of an evolution only against itself. On the other hand, no 

 one who has studied diseases doubts that, owing to fluctuations in 

 vitality and environment, individuals tend to alter from time to 

 time in their powers of resisting it. The man who is very resistant, 

 may become less resistant through the operation of some cause 

 which lowers his vitality, while he who is very susceptible may be 

 rendered more or less so through changes in health and sur- 

 roundings. Nevertheless, the truth holds that the naturally more 

 resistant tend always, under any condition, to survive in greater 

 numbers than the more susceptible. Lastly, selection is not 

 equally stringent in all environments, nor at all times in the same 

 environments. It has been ascertained, experimentally, that the 

 dosage of microbes plays a large part in the causation of infec- 

 tion, and, therefore, in the causation of acquired immunity or 

 death. The microbes of disease are much more numerous in some 



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