PEACTICE OF MEDICINE AND HYGIENE 



preserves living things from death, whether in 

 bacteria or men, tends to increase as the species 

 grow older. The parasites are adapting them- 

 selves to the changing conditions of the animals 

 upon which they feed, and are undergoing mu- 

 tations even more rapidly than the more com^ 

 plicated creatures. Nature is impartial. Life 

 is a contest between living things preying upon 

 one another. But man will win, because, to his 

 natural, physical, protective resources, he adds 

 to the contest the ingenuity of his mind. 

 He is studying his enemies by scientific meth- 

 ods, familiarizing himself with their modes of 

 life, and seeking out their vulnerable points. 

 He is carrying on experiments with them, rais- 

 ing them, breeding them, and killing them. As 

 a result of these studies, many are now at his 

 mercy, to destroy when he will; and he is still 

 forging ahead in the battle for his life. 



The parasites causing most of the infective Parasitic 

 diseases have been discovered. The principal diseases, 

 diseases which display the evidences of being 

 due to infective parasites, the causes of which 

 have not yet been found, are: measles, scar- 

 let fever, yellow fever, small pox, typhus fever, 

 and rabies. The principal infective diseases of 

 man which are caused by known bacteria are: 



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