as Enemies of Mankind, 45 



or the attacks of enemies (e.^., carnivora and parasites), the effects 

 of COMPETITION and overcrowding, and, above all, the weather, 

 with its great powers of inflicting injury upon any species, either 

 directly or indirectly (as by curtailing the food-supply). These 

 operate, singly or in combination, at all times on individuals of all 

 ages ; accidents and competition press most hardly, perhaps, upon 

 embryos and young ; the weather destroys chiefly in winter and 

 spring. 



The precise mode in which the surplus is removed varies with 

 the species. Despite the heavy toll levied by accident and com- 

 petition, free-living creatures, like small birds, usually have a 

 large surplus at the end of summer ; and that surplus must be 

 removed by the severe weather of the following seasons. On the 

 other hand, more or less earth-bound animals, like the 3Iuridae, 

 with their habits of constructing warm nests in dry burrows and 

 of amassing stores of provisions, are enabled to withstand all but 

 the most severe weather. In their case the surplus must be 

 removed chiefly by accidents and competition. Their habits render 

 small rodents the easiest of prey ; in fact, they form the staple food 

 of every one of our carnivorous mammals, with the exception of 

 the Otter. Foremost among rat- and mouse-killing mammals are 

 the Stoat and the Weasel ; working incessantly, they kill far more 

 than they devour ; many mature and healthy rodents fall to them, 

 but the young just leaving the nest, with such adults as are weak 

 and sickly, form the great majority of their victims. Among British 

 birds the Barn Owl, Short-Eared Owl, Long-Eared Owl, Tawny 

 Owl, Common and Eough-Legged Buzzards, Kestrel, Eook, Carrion 

 and Hooded Crows, Eaven, Magpie, Heron, and some Gulls may 

 be enumerated as more or less important rat- or mouse-killers ; 

 indeed, in this respect the importance of the Owls and Kestrel 

 can hardly be over-estimated. The stronger and more efficient 

 the carnivora (including in that term all the animals which 

 habitually or occasionally prey upon rats and mice), the more 

 completely and automatically will the surplus rodent population 

 be removed as it arises. 



In a well-balanced or natural fauna, carnivora will never 

 extirpate the species upon which they feed ; the prey is secured 

 only by work ; and physiologically it costs more than it is worth 

 to secure the prey when it becomes scarce. Herein lies the chief 

 natural check upon the numbers of carnivorous animals ; in addition, 

 carnivorous species prey upon each other when opportunity offers. 



