216 HEREDITARY CHARACTERS 



frequently. 1 When tuberculosis is introduced into a race 

 that has not been subjected to selection, a large part of the 

 population is destroyed, or the race may even be exter- 

 minated. Of course there will be variations away from the 

 racial standard of immunity, even in the most immune 

 races, and the individuals that vary thus will contract the 

 disease. Anything, then, that diminishes the chance of in- 

 fection generally in an immune race will tend to lower the 

 racial standard ; hence it is doubtful whether it is wise to 

 adopt general measures to lessen the risks of infection in 

 the case of those diseases where it is impossible to hope 

 that all risk of infection may be permanently removed. In 

 the case of tuberculosis, it is very doubtful whether anything 

 more than a lessening of the risk of infection could be accom- 

 plished. The bacillus is capable of living for considerable 

 periods in a dried state away from the bodies of animals ; 

 and it is so widely distributed and is so resistant to heat and 

 cold, that its complete extermination seems an impossible 

 achievement. We have already attained a high degree of 

 immunity, and merely lessening the risks of infection without 

 destroying them altogether, would result in a lowering of 

 the resisting power of the race, and might eventually end in 

 disaster. It all depends, of course, whether the race or the 

 individuals is considered. Here the point of view adopted 

 is what is best for the race. 



With some diseases, however, it is quite feasible to pre- 

 vent all chances of infection. Typhoid fever is an example. 

 The micro-organism is almost always water-borne. The 

 precautions necessary to preclude all possibility of infection 

 are very limited, compared to what would have to be done 

 in the case of tuberculosis, at any rate in civilised communi- 

 ties, and the probability is that the preventive measures will 

 improve in the future. On the other hand, measles, chicken- 

 pox, and other similar diseases, to which we as a race have 

 developed a power of acquiring immunity rapidly and easily 

 cannot be dealt with in the same way as typhoid. There 



1 See pp. 106-7. 



