THE PRESENT EVOLUTION OF MAN— PHYSICAL 239 



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 mul- 

 tiply 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 circum- 

 stances. 



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 inter- 

 mediate in the scale we find the same to be true. 

 Scarlatina, stnall-pox^ and measles, the microbes of 

 which are little capable of existing outside the body, 



^ White, British Medical Jowrnal, Feb. 25, 1893. 



