THE OKIGINS OF ZYMOTIC DISEASES 173 



malaria in India and West Africa, it has been found difficult 

 to rear English families in the former country and practically 

 impossible in the latter. Young English soldiers perish 

 more readily of cholera and dysentery than older men. The 

 same is true of Polynesians and others when exposed to 

 tuberculosis. 



291. On the other hand we have no certain evidence that 

 any parasitic disease has ever died out. The chances are 

 all against such an occurrence. When once established as 

 parasites the microbes, owing to the continual growth of 

 human populations, would find a constantly augmented food 

 supply, and constantly increased opportunities of reaching 

 fresh fields of conquest. Preventive measures or other 

 agencies have caused the partial disappearance of leprosy from 

 several countries, but it is still very prevalent in many 

 quarters of the globe. Contagious diseases have spread very 

 widely. Air-borne diseases have become endemic instead of 

 merely epidemic, and almost every child now suffers from 

 measles, chicken-pox, whooping-cough, and small-pox, 

 modified or unmodified. Sanitary science, which has left 

 unaffected the air-borne maladies, has recently diminished 

 earth- and water-borne diseases in some of the more highly 

 civilized countries ; but in the rest of the world they are, 

 apparently, as prevalent as ever. We must conclude, there- 

 fore, that both the total disappearance of old diseases and the 

 appearance of new diseases are events of the greatest rarity 

 in the modern world, if indeed they ever occur. 



292. The analogy of other forms of life renders it tolerably 

 certain that the microbes of each disease, no matter how 

 widely diffused at the present time, originated, each species 

 of them, in a single centre from a single ancestral type of 

 saprophytic organism. It is very improbable, for example, 

 that types so highly specialized as the microbes of syphilis 

 or rabies or measles had multiple origins. The constant 

 tendency in nature is towards differentiation in type, not 

 towards approximation. No one supposes, for example, that 

 the varieties of apples or those of peaches have descended 

 from two or more species which have become alike. On the 

 contrary, our whole evolutionary conception of life leads us 



which malaria and yellow fever are prevalent tend to lose their 

 immunity if they sojourn abroad, and then are apt to contract the 

 disease on their return. Doubtless, when living within the area infected 

 by the pathogenetic organisms they are often infected. A "passive" 

 immunity results, which, however, is not permanent like the "active" 

 immunity which follows actual illness and recovery from some diseases. 



