in] SPREADING 63 



Supposing a family moved, gipsy fashion, only on 

 one day every week, and not more than three miles, 

 then it would cover about 156 miles in one year. 

 For example, the Mongols, crossing Behring's Strait 

 might have arrived at this rate at the Straits of 

 Magellan in about 50 years. 



Let us follow the herd of grazing cattle. They 

 roam about mainly in search of food ; they follow 

 the grass. Where its growth is seasonal, the beasts 

 make seasonal ' migrations,' like the American bison, 

 which moved twice annually between Canada and 

 Mexico. What we usually understand by periodic 

 migrations cannot have sprung into existence sud- 

 denly ; it is more like the cumulative effect of the 

 doings of countless generations. The faculty of 

 shifting the abode was of course always there, the 

 necessity of moving further on was also present, and 

 those members of a species which went in the wrong 

 direction came to grief, whilst the others flourished 

 and could return with their progeny. At first they 

 did not cover great distances, but just enough to find 



conditions, would surpass all reasonable limits. The phenomenon 

 is comparable with the progress of a ' fairy-ring ' except in so far as 

 the inside will contain a population of just comfortable density. If 

 we apply these considerations, the ring of worms will be found to have 

 made astonishingly little progress in a thousand years, and, except at 

 the periphery, the whole area will contain at best only a comfortably 

 dense population, instead of countless millions. 



