450 PHYSICAL DETERIORATION & MICROBIC DISEASE 



methods of procedure are conceivable: (i) we may attempt, by 

 what may be termed external sanitation^ by rendering the 

 environment external to the human body unfavourable to the 

 microbes, to banish the organisms that cause disease, or (2) we 

 may strive to render human beings more resistant, and so practise 

 what may be termed internal sanitation. If we adopt the latter 

 course we may (A} raise the innate resisting power of the race by 

 altering the germ-plasm by artificial selection, or (B) that of 

 individuals by conferring artificial immunity such as that which is 

 acquired to smallpox through vaccination. Artificial selection, 

 even when pressed to a point to which Natural Selection is never 

 pressed, would merely increase the resisting power of the race. 1 

 That is, it would mitigate the severity of disease, but would not 

 banish it. On the other hand, acquired immunity, if universally 

 acquired, would banish disease ; for the micro-organisms, deprived 

 of their nutritive supply, would become extinct. It must not be 

 forgotten, however, (a) that immunity to one disease does not 

 confer immunity to any other, and (b) that while it is possible to 

 acquire immunity to some diseases (e.g. diphtheria and smallpox), 

 it is not possible to acquire it to others (e.g. tuberculosis and leprosy). 

 Therefore, while we may hope to banish utterly the microbes of 

 such a disease as smallpox, either by external sanitation or by 

 internal sanitation, we can hope to banish the microbes of such 

 diseases as tuberculosis only by external sanitation. It is true that 

 by improving the surroundings we are able to increase the power 

 to resist tuberculosis (and its consequences) in people who have 

 lived under weakening conditions (e.g. in slums) ; yet, since many 

 people living under the best conditions take the disease, it is 

 impossible, merely by making people healthier and stronger, 

 to prevent frequent infection and therefore propagation of the 

 microbes. 



737. Our power of banishing the microbes of the various 

 diseases by external sanitation is largely conditioned by the mode 

 in which they pass from one human being to another. The 

 contagious diseases, ophthalmia, various skin complaints, the 

 venereal maladies, and the like, are, for obvious reasons, particularly 



1 Selection by any ill-condition merely increases the racial power of resisting 

 that ill-condition. It cannot procure absolute immunity for the reason that, 

 when the severity of selection is relaxed, retrogression tends to follow. Absolute 

 racial immunity to any ill-condition results only through selection by agencies 

 other than that condition. Thus the higher animals are not immune to the assaults 

 of spiders because they have undergone thorough selection by spiders. They are 

 immune because they were otherwise^selected. 



