THE NUMBERS OF ANIMALS 119 



animals during the summer. Another point is that over- 

 eating of the food-supply usually results in the destruction of 

 the entire population, irrespective of individual merits. There 

 have been instances recorded of the various oak moths (such 

 as Tortrix viridana) eating all the leaves of the trees upon which 

 they were living and then simply dying of starvation, just as 

 the cats did on Berlenga Island.-^^^ 



27. It is usual, therefore, to find that gregarious gluttony 

 of the whole population is avoided by having various other 

 checks which act in two ways, first by aflFecting the chances of 

 reproduction or by Hmiting the number of young produced, 

 and secondly by eliminating the animals themselves. The 

 first method is often fixed more or less permanently by the 

 hereditary constitution of each species ; but there are a great 

 many cases known in which the weather affects mating or 

 breeding, or in which climate or food-supply vary the number 

 of young produced in a brood, or the number of broods born 

 in a year. For instance, the short-eared owl (Asio flammeus) 

 may have twice as many young in a brood and twice as many 

 broods as usual, during a vole plague, when its food is extremely 

 plentiful. ^^'^ But these variations in the reproductive capacity 

 are small compared to the limits which are imposed by the 

 constitution of the animals. 



The second result is brought about mainly by means of 

 predatory enemies, carnivores or parasites, or both, not to 

 speak of other checks such as climatic factors . These influences 

 dispose all the time of a certain margin of the population, so 

 that there are left a certain number of comparatively well-fed 

 and, as it were, well- trained animals ; for these checks act 

 selectively and probably have important effects on the quality 

 as well as the quantity of the population. Starvation only 

 comes in in various indirect ways, as by lowering the resistance 

 to attack by carnivorous enemies or parasites, or to the weather, 

 and so increasing the selective power of these agencies. 



28. The regulation of numbers of most animals would 

 appear, therefore, to take place along the following lines. Each 

 species has certain hereditary powers of increase, which are 

 more or less fixed in amount for any particular conditions. It 



