4 68 SCIENCE PROGRESS 



of the individuals will have a greater resistance than the average 

 and these may be classed as + i. Others will have less — these 

 will be — i ; others will be o. In the next generation, however, 

 more children will have been produced and reared by the + i 

 individuals. Offspring inherit their parents' characters with 

 variations but this second generation will vary from a new 

 mean, + i ; some individuals will vary towards a greater, some 

 towards a less and some will inherit the character in the same 

 degree as their parents. We shall, therefore, have a generation 

 of individuals consisting of o, + i and + 2. Obviously, the + 2 

 class will have the greatest chance of surviving and rearing 

 children, so the next generation will vary again from a higher 

 mean, + 2. This process must continue as long as the tubercle 

 bacillus continues in the environment, until a very high degree 

 of immunity is attained by the race. Of course, variations away 

 from the average of racial immunity must continue to appear but 

 the standard of the race is maintained because these unfavourable 

 variations are eliminated. There is undoubted evidence that 

 tuberculosis existed in Egypt about 5000 years ago, 1 so it is 

 practically certain that it occurred also in countries further north 

 which had communication with Egypt, 2 at any rate indirectly, 

 where the conditions are as favourable to the tubercle bacillus as 

 they are unfavourable in Egypt. Northern Europeans have 

 therefore been subjected to selection during several thousand 

 years ; hence comes their comparative immunity. The effect of 

 recent legislation must certainly be to lower the standard of 

 immunity of the race, as the susceptible individuals are to be 

 taken in hand wholesale and kept alive to breed children, who 

 will vary in their immunity from a lower mean than that from 

 which their parents varied. 



It is unfortunately inconceivable that the tubercle bacillus 

 can be eliminated altogether. It is able to survive in a dried-up 

 condition during a very considerable period of time and it is 

 probable that the inhalation of dried tubercle bacilli is a common 

 cause of pulmonary tuberculosis in the case of susceptible 

 individuals. Besides this, tuberculosis is probably as common 

 in cattle and perhaps other animals as it is in man. If, even in 

 spite of this wide distribution, the bacillus were eliminated in 



1 G. Elliot Smith and Wood Jones, Archceological Survey of Nubia, Report for 

 1907-8, vol. ii. (Cairo, 19 10). 

 ' G Elliot Smith, op. cit. 



