CHAPTER III 



We may now resume the main thread of our 

 argument, and seek to confirm inductively the con- 

 ckision we have ah-eady arrived at deductively ; namely, 

 that the present evolution of man is chiefly against 

 zymotic disease. We have not, however, quite done 

 with deduction. 



Adopting a different classification from that which 

 we have heretofore found useful, zymotic diseases may 

 be divided into three classes. 



The first class includes those diseases of which the 

 pathogenic organisms are entirely parasitic, and are 

 capable of existence only in living tissues, or for a 

 short time, before morbid changes of importance occur, 

 in the previously infected tissues of dead animals, 

 including man — e. g. rabies ; practically, however, since 

 the death of the microbes so soon follows that of the 

 host, the former may be considered as capable of 

 existence only in living tissues. They are, therefore, 

 never earth, air, or water-borne, but are acquired by the 

 healthy only through actual contact with the diseased, 

 and then only under special circumstances, since the 

 cells at the surface of the body are normally quite able 

 to repel the invasions of their microbes. They are 

 therefore contagious in the strictest sense of the word, 

 and therefore — since conditions of temperature, mois- 

 ture, &c. in the bodies of men are much the same all 

 the world over — they are quite independent of dim atic 



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