CHAPTER III
We may now resume the main thread of our argument, and seek to confirm inductively the conclusion we have already 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, moisture, &c. in the bodies of men are much the same all the world over—they are quite independent of climatic
233
