THE METAZOA 



most of them sick, light also enfeebles 

 them or kills many of them outright, clean- 

 liness, for the same reason, he elevates 

 into a religious duty, and he is very par- 

 ticular in his inquiries about foods, espe- 

 cially milk, which he insists must be steril- 

 ized, or in other words have the bacteria 

 in it destroyed. Chemistry and physics he 

 never thinks of except as adjuncts in a 

 purely vital war. 



In practical medicine this subject of 

 vitality never leaves us. Serious diseases 

 are not always due to micro-organisms. 

 Different physical causes may lead, for 

 example, to progressive heart and kidney 

 disease, in which the physician vainly tries 

 to arrest the downward course. Finally 

 things take a turn which has led medical 

 authorities to make the seemingly para- 

 doxical statement that but few people die 

 of their diseases. It is because the slow, 

 devitalizing effects of previous disease on 

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