Page:George Archdall Reid 1896 The present evolution of man.djvu/251

Rh an unlimited extent, and while I think we may safely assert the same of other nutrient media normally present in the outside world, it would be too much to assert that no nutrient medium exists, or can exist, in the outside world, in which they are able to multiply to an unlimited extent. Indeed Dr. Koch believes that the mass of decaying organic matter accumulated in the delta of the Ganges presents such a nutrient medium, in which they do multiply to an unlimited extent, and whence they issue forth to cause those epidemics which afflict more favoured regions. It is, however, a question whether, in the total absence of a human population in the basin of the river and its tributaries, the pathogenic organisms would persist, as the microbes of malaria persist under similar circumstances.

It is noteworthy that diseases, the microbes of which are little capable of existing outside the human body, are much more stable in type than diseases, the microbes of which are more capable of so existing. For instance, syphilis at one end of the scale, for people of the same race, does not vary much in relation to time, except when such time is of very long duration; nor in relation to space, even when such space includes the most distant parts of the world. But malaria, at the other end of the scale, varies much in relation to both time and space, since, for people of the same race, in the same locality, it may be of a mild type in one year and of a severe type in the next; or for people of the same race and at the same time, it may be mild in one locality and severe in another. Of diseases intermediate in the scale we find the same to be true. Scarlatina, small-pox, and measles, the microbes of which are little capable of existing outside the body,