454 PHYSICAL DETERIORATION & MICROBIC DISEASE 



ments as regards light, ventilation and air-space has somewhat 

 reduced its prevalence, and may continue to do so for a time. But 

 the power of resisting tuberculosis is a direct product of evolution. 

 A fall in the mortality due to it implies lessened stringency of 

 selection, which in turn, if carried far enough, implies retrogression. 

 Therefore to procure a permanent fall in the death-rate, it is not 



their opportunity [whatever that may mean] for development, and that individual 

 predisposition is of little account." But how in that case is it possible to explain 

 the fact that consumption tends to run in families even when individuals are 

 separated by continents and generations ? It can hardly be that the members 

 of families of low resisting power are naturally prone to use tuberculous milk. 

 Besides, the very people who suffer most from tuberculosis when exposed to it in 

 infected dwellings are precisely those who have had no opportunities of being 

 infected during infancy ; who have never drunk any milk but their mother's 

 milk ; who, indeed, in many instances, have never seen a cow, let alone an infected 

 cow ; for example, Esquimaux, Tierra del Fuegians, Polynesians, and the like. 

 Deaths from the various forms of tuberculosis occur in about the same proportion 

 in Japan as in Europe ; but, according to Kitasoto, tuberculosis is unknown 

 amongst the Japanese cattle, infants are never fed on other than human milk, 

 and cow's milk is so little used that the daily per capita consumption by the 

 human population averages no more than three-quarters of a teaspoon. In fact, 

 cow's milk is not a general article of diet in Japan. Additional evidence that 

 tuberculosis is an air-borne disease is afforded by the fact that a sufferer with 

 phthisical lungs is apparently a much greater danger to his fellows than one who 

 suffers from tuberculosis of the skin (lupus). It is possible that some human 

 beings, especially infants, in whom the alimentary canal is the principal seat of 

 disease, have contracted the malady from tuberculous milk ; but, since tubercle 

 is always found in the bronchial glands of infected children, it is not at all probable 

 that the majority of sufferers are thus infected. More especially, it is improbable 

 that pulmonary tuberculosis occurring in adults is commonly due to bacilli imbibed 

 in milk during or after infancy. The Royal Commissioners " Appointed to Inquire 

 into the Relations of Human and Animal Tuberculosis," after conducting a long 

 series of experiments, maintain that human and bovine tuberculosis are similar in 

 kind. Since tuberculosis has never been observed in wild animals, it is practically 

 certain that the bovine disease is derived from human sources. Doubtless, there- 

 fore, they are right, but their labour was unnecessary. " The fact that the bacillus 

 of bovine tuberculosis can readily by feeding as well as by subcutaneous injection 

 give rise to generalized tuberculosis in the anthropoid ape, so nearly allied to man, 

 and indeed seems, so far as our few experiments go, to produce this result more 

 readily than in the bovine body itself, has an importance so obvious that it need 

 not be dwelt on " (Second Interim Report, p. 14). But the susceptibility of anthro- 

 poid apes only proves, as might have been expected, that their bodies furnish 

 an environment favourable to bacilli which have undergone evolution in the 

 allied human species. But apes, unlike Englishmen, have undergone no evolution 

 against tuberculosis. They are in the position of Polynesians. It does not 

 follow that Englishmen can be as readily infected as apes. Judging from analogy, 

 we have every reason to believe it probable (i) that bovine tuberculosis is derived 

 from the human disease ; (2) that the microbes of it are becoming adapted to the 

 new environment ; (3) that they are becoming less adapted to the old environ- 

 ment ; and (4) that consequently they are less virulent to human beings than the 

 microbes derived from the human disease. 



