452 PHYSICAL DETERIORATION & MICROBIC DISEASE 



pass from host to host through a medium, the air, which is essential 

 to human existence, and which, blowing where it listeth, cannot 

 be controlled nor efficiently disinfected. Speaking practically, 

 these maladies are independent of climatic or other local conditions 

 except density of population. Since they spread with a rapidity 

 which is proportionate to the number of susceptible individuals, 

 and since immunity to them is conferred by illness and recovery, 

 the more thoroughly they are banished from a community, the 

 greater grows the number of susceptible persons, and therefore the 

 liability to pestilence to disease in its fearful epidemic form. 

 In England, for example, only the endemic prevalence of air-borne 

 disease, and the consequent acquired immunity of the major 

 portion of the population, preserves us from disaster. If air-borne 

 diseases were restricted to a single islet for a generation, the rest 

 of the globe would stand in danger of depopulation. When they 

 broke out from the island habitat they would sweep, as in ancient 

 times, over the whole world not merely in one great and terrible 

 pandemic, but in many recurrent and devastating epidemics, each 

 with its attendant famine, till in the process of ages the epidemics, 

 growing more frequent, but less disastrous, at last settled again 

 into endemic disease. Therefore, even were it possible to banish 

 measles, and whooping-cough, for example, from our midst by 

 external sanitation, it would be disastrous to do so while they yet 

 lingered in any quarter of the globe, and it would always be hard 

 to ascertain with any degree of certainty that some such spot did 

 not exist. In civilized countries unavailing attempts are con- 

 stantly made to restrict the spread of measles and whooping-cough, 

 as by closing schools during such slight epidemics as occur 

 amongst us, but only with the result of aggravating future epidemics. 

 Isolation is quite impracticable except when the great majority of 

 the members of the community have already been infected and 

 thereby have acquired immunity. 1 



741. Clearly, then, in the case of air-borne diseases we have 

 nothing to hope from external sanitation. From artificial selection, 

 also, little or nothing could be gained. Already Natural Selection 

 is very stringent; only people resistant to measles, whooping- 

 cough, influenza, and probably such complaints as bronchitis, 

 pneumonia, and rheumatic fever, survive. Very prolonged and 

 effective artificial selection could only render air-borne maladies 

 somewhat milder to the mass of the population. There remains 

 internal sanitation the production of acquired immunity by means 



1 See 454. 



