THE PRESENT EVOLUTION OF MAN — PHYSICAL 253 



other districts, where the microbes were less virulent, 

 and from which they subsequently migrated. 



But as regards diseases for the persistence of the 

 microbes of which a human prey is essential, it is clear 

 that not only have the races of men which they have 

 aflflicted undergone evolution in relation to them, but 

 their microbes must also have undergone evolution in 

 relation to man, since these microbes, some species of 

 which are now wholly parasitic, must have descended 

 from ancestors that were originally wholly saprophytic, 

 and which, at a time when man had not yet appeared 

 on the scene, were able to maintain existence perfectly 

 in his absence. This evolution, this passage of the 

 microbes from the saprophytic to the parasitic type, must 

 have occurred at a date long subsequent to man's appear- 

 ance. He cannot have inherited any of them from his 

 brute ancestors, for, so far as we know, they never 

 afflict animals in a wild state, though some species of 

 animals in a domesticated state are afflicted by some 

 of them. Again, nomadic tribes in a sparsely populated 

 country are never afflicted by them, except sometimes 

 when they are infected from more populous countries, 

 and even then they are never afflicted by tuberculosis, 

 the most death-dealing of all diseases, except they 

 contract it under circumstances to which nomadic tribes 

 are not usually subjected — e.g. close dwellings. An 

 abundant and settled population is essential for their 

 evolution and persistence — abundant as regards all of 

 them, because otherwise the scant supply of nutriment 

 would prevent the evolution and persistence of a 

 parasitic species; settled as regards most of them, 

 because otherwise, since they are earth, air, or water- 

 borne, the removal of the nutritive supply would also 

 prevent their evolution and persistence; and this is 

 especially true as regards tuberculosis, the microbes 

 of which require a specially favourable environment. 



