62 THE WANDERINGS OF ANIMALS [CH. 



and the fact that two creatures require more space 

 than one. 



If cattle require three acres of grazing land per 

 head to keep themselves during the year, a herd of 

 one hundred must have 300 acres, and if this herd 

 increases to 1000 head it must have 3000 acres 

 of grass land, to find which, however, they may have 

 to roam over much more than five square miles. 

 Thus they spread. From a broad point of view 

 spreading proceeds peripherally, in epicycles, each 

 pair representing a centre of its own, and since the 

 middle of the area is already occupied, new land is 

 available only outside. This spreading may be a very 

 slow process, but it tends to increase by geometrical 

 progression, and there is practically unlimited time. 

 If a couple of earthworms are sufficient to occupy 

 one square yard per year with their offspring, their 

 descendants long before 30,000 years (a not im- 

 probable estimate of the time since the end of our 

 glaciation) would have choked the whole world 1 . 



1 This time-honoured kind of calculation implies a great fallacy. 

 It IB, to a certain extent, applicable to animals with practically 

 unlimited power of locomotion, for instance to a herd of cattle enter- 

 ing a new, unoccupied country. In the case of earthworms the 

 actual resulting numbers would soon be infinitely less than the 

 calculated result, because of the choked, inner parts of the area, the 

 inhabitants of which could not possibly pass over those living near the 

 periphery, where alone new land and food is available. In a few years 

 the centrifugal velocity of spreading, necessary to cope with these 



