THE PRESENT EVOLUTION OF MAN — PHYSICAL 203 



that they take it only in a mild form, are at an advan- 

 tage, and tend to survive and leave offspring, who, 

 inheriting their peculiarities, . are likewise able to 

 survive and continue the race in spite of the presence 

 of the disease. It follows, therefore, that the presence 

 of any fatal or serious disease during generations must 

 tend to call forth in the race attacked a gradually 

 increasing power of resistance to it, just as the presence 

 of camivora in any country calls forth in weaker 

 creatures — e.g. hares and antelopes — increasing powers 

 of evading attack. On the other hand, just as the 

 powers of attack in the camivora must, as a condition 

 of their survival, undergo evolution pari passu with 

 the evolution of the powers of evading attack in 

 their prey, so it is probable, that as the powers of 

 resisting microbic disease xmdergo evolution in man 

 and brutes, the powers of attack in the pathogenic 

 organisms must undergo corresponding evolution ; at 

 least in the case of all those pathogenic organisms to 

 which a human or brute prey is essential to existence, — 

 e. g. the micro-organisms of syphilis, rabies, and tuber- 

 culosis — if not in the case of those micro-organisms 

 which, having their normal sources of subsistence else- 

 where, only accidentally or occasionally attack the 

 higher animals — e. g. the micro-organisms of malaria. 



For reasons which will be considered presently, 

 microbic diseases are often limited in their areas of 

 distribution, and therefore, as a further deduction from 

 the foregoing, it follows that the races dwelling within 

 any such areas must become, " by nature," more resist- 

 ant to the action of the prevalent disease than races 

 dwelling outside the area, for the reason that they are 

 descended from an ancestry which was rendered 

 increasingly resistant to the disease by the survival of 

 the fittest ; and therefore if individuals, whose ancestors 

 lived outside the area, migrate into it, they should, if 



