SUSCEPTIBILITY AMD IMMUNITY. 227 



Considerable differences as to susceptibility may also exist among 

 adults of the same species. In man these differences in individual 

 susceptibility to infectious diseases are frequently manifested. Of a 

 number of persons exposed to infection in the same way, some may 

 escape entirely while others have attacks differing in severity and 

 duration. In our experiments upon the lower animals we constantly 

 meet with similar results, some individuals proving to be exception- 

 ally resistant. Exceptional susceptibility or immunity may be to 

 some extent a family characteristic or one of race. Thus the negro 

 race is decidedly less subject to yellow fever than the white race, 

 and this disease is more fatal among the fair-skinned races of the 

 north of Europe than among the Latin races living in tropical or sub- 

 tropical regions. On the other hand, small-pox appears to be excep- 

 tionally fatal among negroes and dark-skinned races generally. 



A very remarkable instance of race immunity is that of Algerian 

 sheep against anthrax, a disease which is very fatal to other sheep. 



In the instances mentioned race immunity is probably an ac- 

 quired tolerance due to natural selection and inheritance. If, for 

 example, a susceptible population is exposed to the ravages of small- 

 pox, the least susceptible individuals will survive and may be the pa- 

 rents of children who will be likely to inherit the special bodily char- 

 acters upon which this comparative immunity depends. The ten- 

 dency of continuous or repeated exposure to the same pathogenic 

 agent will evidently be to establish a race tolerance ; and there is 

 reason to believe that such has been the effect in the case of some 

 of the more common infectious diseases of man, which have been 

 noticed to prevail with especial severity when first introduced among 

 a virgin population, as in the islands of the Pacific, etc. 



In the same way we may explain the immunity which carnivor- 

 ous animals have for anthrax and various forms of septicaemia to 

 which the herbivora are very susceptible when the pathogenic germ 

 is introduced into their bodies by inoculation. Prom time immemo- 

 rial the carnivora have been in the habit of fighting over the dead 

 bodies of herbivorous animals, some of which may have fallen a prey 

 to these infectious germ diseases, and in their fighting they receive 

 wounds, inoculated with the infectious material from these bodies, 

 which would be fatal to a susceptible animal. If at any time in the 

 past a similar susceptibility existed among the carnivora, with indi- 

 vidual differences as to resisting power, it is evident that there would 

 be a constant tendency for the most susceptible individuals to perish 

 and for the least susceptible to survive. 



But if we admit this to be a probable explanation of the immu- 

 nity of carnivorous animals from septic infection, we have not yet 

 explained the precise reason for the immunity enjoyed by the 



