TUBERCULOSIS AMONG ANIMALS 57 



A zymotic disease which persists from generation to generation 

 and is not merely of occasional occurrence, eliminates in each 

 generation those individuals who are most susceptible to it, 

 or who are unable to develop adequate powers of resistance 

 when attacked. Hence a progressively increasing degree of 

 immunity and a continuously falling death-rate. Many other 

 instances of this phenomenon have been investigated by 

 Dr. Archdall Reid. Malaria is deadly to Europeans, but 

 comparatively innocuous to West African natives who have 

 undergone evolution against it ; measles causes a high mortality 

 among the Polynesians ; Esquimaux die from vaccination, while 

 Europeans usually recover from small-pox. Alcoholism, which 

 he regards as governed by the same influences, is least among 

 the ancient peoples round the Mediterranean who have had 

 alcohol longest ; greater among northern Europeans ; and 

 rampant among the primitive peoples of Australia and America 

 to whom the poison is new. 



If, with these facts in mind, we now look again at the list 

 of animals arrived at above, we see at once that it corresponds 

 exactly with that of the degree of closeness of their association 

 with man, i.e. with the opportunities they have had in the 

 past of acquiring tuberculosis from him. Rats and mice, which 

 come first, though not ordinarily regarded as domesticated, 

 are nevertheless very highly so from the scientific point of 

 view. They are constantly and intimately associated with man. 

 They inhabit his cellars and his sewers ; they feed upon his 

 refuse and the infected sweepings of his streets and pavements, 

 which he persists in regarding as the appropriate place for 

 expectoration. In such a habitat they must have undergone 

 stringent natural selection against tuberculosis, the effects 

 of which are seen in the extraordinary degree of immunity 

 they now possess. 



Dogs and cats come next. These animals have for long 

 been closely associated with man, but not under such adverse 

 conditions as rats. They inhabit the better ventilated parts 

 of his dwellings, and are cleaner feeders. They are follow- 

 ing the same path as the rats, but have not yet developed 

 the high powers of resistance possessed by the latter. The 

 greater susceptibility shown by the cat as compared with the 

 dog is probably to be explained by their past histories. It 

 is not possible to say positively which has been domesticated 



