IMMUNITY TO DISEASE 215 



During the early periods of life, habits of mind are easily 

 formed, and may become almost as strong as instincts. The 

 habit to cultivate is then one of enquiry into the why, 

 wherefore, and how, of everything. Certain forms of religion 

 are opposed to this view on principle. It must not be for- 

 gotten that the truth can but become more obvious upon 

 enquiry, if it really be the truth. 



On the whole, then, the environment, particularly of 

 children, is most important to the race in civilised com- 

 munities. So very much depends upon acquirements, and 

 in some ways so little upon the inborn characters, that in 

 spite of acquired characters not being transmitted, too much 

 attention cannot be given to them. 



The question of disease is also one upon which the facts 

 connected with heredity have a very direct bearing. There 

 can be no doubt that characters tend to disappear more or 

 less rapidly as soon as they cease to be the subjects of 

 selection. To put the case another way, if the environment 

 of a race changes in such a manner as to render useless 

 certain characters that were previously useful, these char- 

 acters will disappear. This happens even in the case of 

 anatomical structures, such as limbs; 1 it happens also in 

 the case of immunity to disease. In dealing with diseases 

 occurring among civilised races of men, this point is worthy 

 of consideration. Immunity to a disease can only be obtained 

 by a race in the presence of selection, that is, the disease 

 must exist in the environment. The degree of immunity 

 existing in any given race will be proportionate to the length 

 of time during which selection has acted, and to the extent 

 of the infection. When the disease is such that every 

 individual, or almost every individual, is certain to be in- 

 fected, a comparatively high degree of immunity will be 

 reached. This is the case with tuberculosis. Practically 

 every one in Northern Europe must be infected with the 

 tubercle bacillus at some time or other, and in towns a 

 large proportion of the population must be infected very 



1 See pp. 66-7. 



