26 DARWINISM AND THE PROBLEMS OF LIFE 



their numerous enemies, and will be decimated. When 

 the foxes find less to eat, they will decrease again, and 

 give more chance to the hares, and thus the balance of 

 vital advantage will oscillate between the two species. 

 This will bring about in time an unstable equilibrium : 

 that is to say, the number of the two species will be 

 constantly rising or falling a little above or below a 

 certain level, but will remain at the steady average. 



In reality the situation is rather more complicated, 

 as the fox does not live on hares only, and the hare has 

 other enemies besides the fox. But the fact remains 

 that there is this correlation between the animals of 

 a certain region ; and it must be so, otherwise a species 

 would increase indefinitely. 



Let us suppose that a couple of foxes were left 

 to multiply in peace. As a rule the fox has four or 

 five young ones, and this for several years in succession. 

 But we will take a case in which a couple bring six young 

 ones into the world once for all ; and suppose that three 

 of these are male and three female, and that these three 

 couples have each six young in the following year, so 

 that there are then nine pairs, and so on. In ten years 

 the number of foxes would have grown to 118,098, and 

 this number would be much greater if each couple cast 

 young more than once, as is the case in real life. 



As a rule, however, the animal population of a 

 district remains constant, apart from the interference 

 of extraordinary agencies. If, therefore, each of the 

 couples of foxes in a given wood have five young every 

 year, and this for seven years, or thirty-five in all, it 

 follows that, if the number of foxes is to remain steady, 



