268 EPIDEMIC AND ENDEMIC DISEASE 



442. Somewhat better than mere guessing is the assumption 

 that mankind, under the influence of constant persecution, has 

 gradually grown more resistant, not only to particular diseases, but 

 to microbic infection in general. Owing to the increase of popu- 

 lation, and to the crowded, insanitary, wretched conditions under 

 which many communities live, the facilities for the evolution of 

 parasitic types are now greater than ever they were. Yet, though 

 we have the whole world under survey, we never hear of a new 

 disease. The so-called new diseases are merely maladies newly 

 observed in their ancient habitats, or complaints which have 

 invaded fresh territories. Presumably, therefore, the alteration of 

 saprophytes into parasites (i.e. the evolution of a new disease) is 

 now a more difficult and unlikely occurrence than it was anciently. 

 This conjecture (that mankind has evolved against microbic infec- 

 tion in general as well as against particular diseases) is confirmed 

 by the fact that children, who recapitulate the life-history, are, as 

 a rule, less resistant to microbic disease than adults, even when the 

 disease is one of which the race has had little or no experience. 

 For example, English children succumb much more readily than 

 older persons of their own race to malaria and dysentery. It 

 must be remembered, however, that since variations occur during 

 all stages of development, the race evolves in the stage of develop- 

 ment reached by the child as well as in the adult stage. On this 

 account African and Indian children succumb much less readily 

 to malaria and dysentery than English children, though more 

 readily than adults of their own race. 1 



443. On the other hand, we have no certain evidence that any 

 disease has ever completely died out. The chances are all against 

 such occurrences in the past. When once established as parasites, 

 the microbes, owing to the continual growth of human populations, 

 found a constantly augmented food-supply as well as increased 

 opportunities of reaching fresh fields of conquest. Sanitary 

 science is still in its infancy, and as yet has accomplished compara- 

 tively little. Under skilled direction it has done something towards 

 diminishing earth-, water-, and insect-borne diseases. Tuberculosis, 

 enteric and yellow fever, and malaria have diminished somewhat 

 in a few parts of the world. Leprosy has disappeared from some 

 countries. But air-borne diseases, which are propagated through 

 a medium the flow of which is too swift and the volume too vast 

 for control, are as common as ever. Almost every child acquires 

 measles, whooping-cough, chicken-pox, mumps, common cold, 



1 See 417. 



