180 THE PRINCIPLES OF HEREDITY 



which men attributed to the wrath of God, their tremendous 

 infectivity and rapid spread, their equally sudden and com- 

 plete departure as of Divine anger assuaged, point rather to 

 air- and water-borne diseases of the types now endemic and 

 comparatively harmless among us ; but still so fearful in their 

 effects on isolated communities. Like the light flashed from 

 a child's mirror on a darkened wall, so they flickered and 

 swept forwards and backwards from end to end of the Old 

 World from the Malay Peninsula to the North Cape of 

 Norway, from Kamschatka to the South point of Africa. A 

 parallel may be found in the recent epidemic of rinderpest 

 amongst the herbivorous animals of Africa. Years might 

 pass, old men might remember, the peoples might sacrifice to 

 their gods ; but when a fresh generation of those who knew 

 not the disease had arisen, when the harvest of the non- 

 immune was ripe and ready, the diseases would return to the 

 dreadful reaping. Behind them the earth was heaped with 

 the dead, and the few and stricken survivors grubbed for 

 roots to satisfy their hunger. To-day sanitation has nearly 

 abolished water-borne disease ; and, in a population largely 

 immune, epidemics of air-borne disease, like a light thrown on 

 a sun-light wall, are but faint simulacra of that which they 

 were in their old days of awful power. 



300. The progress of consumption was different. It was 

 never truly epidemic. Owing to its low infectivity, to its 

 lingering nature, to the fact that no immunity could be 

 acquired against it, it did not spread suddenly when first 

 introduced, but when once established its virulence did not 

 abate within measurable time. In other words, it was endemic 

 from the beginning. It made its home in the hovels of the 

 early settlers on the land. In such situations as in Poly- 

 nesian villages modern Englishmen do not take the disease. 

 But their remote ancestors were more susceptible; they 

 could be infected by a smaller dose of bacilli. Gradually as 

 civilization advanced the conditions grew more stringent, 

 men gathered into larger and denser communities, into ham- 

 lets and villages in which they built houses ill-lighted and 

 worse ventilated. 



301. With the rise of towns, and ultimately of great cities, 

 the stringency of selection continually increased ; and, with 

 it, step by step, the resisting power of the race. To-day 

 Englishmen dwell under conditions as impossible to their 

 remote ancestors as to the modern Red Indians. In fact, no 

 race, especially in cold and temperate climates, is now able to 

 achieve civilization, to dwell in dense communities, unless it 



