5 6 SCIENCE PROGRESS 



cannot be a function of the natural order ; for we find at one 

 end of the scale animals so dissimilar as rats and dogs 

 resembling each other in their high degree of immunity, and, 

 at the other end, guinea-pigs and chimpanzees alike exhibiting 

 marked susceptibility. On the other hand, animals belonging 

 to the same natural order — rodentia — occupy positions at the 

 extreme ends of the scale. The Commissioners themselves do 

 not appear to have recognised fully the complete absence of 

 relationship between natural order and degree of susceptibility, 

 for they remark on p. 14 of the report "the fact that the 

 bacillus of bovine tuberculosis can readily by feeding as well 

 as by subcutaneous injection give rise to generalised tuberculosis 

 in the anthropoid ape, so nearly related to man, . . . has an 

 importance so obvious that it need not be dwelt upon." Yet 

 the experiments clearly establish that natural order has no 

 significance whatever in this connection. 



More importance might be attached to the question of diet, 

 for carnivorous animals appear to have distinctly greater powers 

 of resistance than herbivorous. 



Dr. Archdall Reid has, however, furnished us with an 

 explanation of the different degrees of susceptibility to tuber- 

 culosis exhibited by different branches of the human family, 

 which, when extended to animals, will be found to agree in a 

 remarkable manner with the facts. He has investigated the 

 prevalence of zymotic diseases among a large number of 

 races both of the Old and New Worlds, and has shown con- 

 clusively that a community exhibits a resistance to a disease 

 which is in strict proportion to its past experience of it. The 

 European peoples have suffered from tuberculosis at least since 

 the days of Hippocrates, and they have now, in spite of the 

 high death-rate which they still show, acquired a degree of 

 immunity which can only be measured by observing the effects 

 of the disease upon virgin soil. The history of the peoples of 

 the New World furnishes an opportunity. Tuberculosis was 

 carried thither in the sixteenth century, and among an unpro- 

 tected native populace its effects were tragic. Whole tribes 

 disappeared before it. The Caribbean and Tasmanian are gone. 

 The Red Man only lingers on in spots artificially isolated from 

 the contact of civilisation. 



Such a relationship between experience and disease is fully 

 in accord with the theory of evolution by natural selection. 



