that they take it only in a mild form, are at an advantage, 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 carnivora 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 carnivora 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 undergo 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 tuberculosis—if not in the case of those micro-organisms which, having their normal sources of subsistence elsewhere, 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 resistant 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
