338 THE PRINCIPLES OF HEREDITY 



whom it may exist unsuspected for months or years. Bacilli 

 in a latent condition may persist, also, in people who to all 

 appearances have quite recovered. Every such person is a 

 focus of infection, the bacilli being disseminated not only by 

 actual expectoration, but in the minute droplets of fluid 

 which are expelled in the acts of sneezing and coughing. 

 We can hardly expect in our climate, and with our social 

 habits and dense population, to render the conditions as 

 unfavourable to the bacilli as they are in tropical Pacific 

 islands, where the natives are becoming extinct. Segregation 

 of consumptives to any degree of completeness is impracticable, 

 not only because of their enormous numbers, and of the 

 difficulty of detecting the disease in its incipient stages, but 

 also because, owing to the long duration of the quarantine 

 necessary, we could not exclude from our shores travellers 

 from less sanitary areas. Apparently, therefore, our only 

 hope of permanently lessening the prevalence of the disease 

 lies in a reduction of the number of people susceptible to it. 

 In other words the problem presented by consumption is one 

 which ultimately will have to be solved if ever it be solved 

 by the student of heredity. Some method will have to be 

 devised to lower the output of children by people predisposed 

 to the complaint, otherwise the mortality from consumption 

 cannot be greatly or permanently reduced. A few States of 

 the American Union have already laws forbidding the 

 marriage of consumptives. It is, however, no part of my 

 present purpose to suggest remedies for the mere discussion 

 of which the community is not as yet prepared. My principal 

 object is to demonstrate that there are certain practical 

 problems of great importance which cannot be dealt with 

 until the public, and especially the medical profession, are in 

 a position to bring an adequate knowledge of heredity to 

 bear on them. The mere existence of such knowledge would 

 be of incalculable benefit, inasmuch as the race would become 

 alive to the dangers which menace it, and improper marriages, 

 meeting, as they should, with universal censure, would 

 become less frequent. 



524. Neither sanitation nor a study of heredity offer any 

 remedy against such air-borne diseases of short duration as 

 measles. At present sanitation has not even diminished 

 them. They are endemic ; if a system of sanitation, efficient 

 beyond anything that seems as yet possible, succeeded in 

 abolishing them from any area, it would render the com- 

 munity liable to epidemics a much greater evil. Since in- 

 born immunity against the more important of these diseases 



