THE WEB OF LIFE 265 



of minute plants or by the accumulation of poisonous waste- 

 products. 



Taking a less artificial instance, we recognize the depend- 

 ence of vegetarian animals on the plants of the given area. 

 When the lemmings of a Scandinavian valley or the voles 

 further south multiply exceedingly in times of plenty, 

 they tend to check their own increase by eating up 

 every green thing. Then the lemmings go on the march 

 and the voles spread from parish to parish. 



There is a necessary proportion to be sustained between 

 herbivorous animals and plants, between carnivorous 

 animals and herbivores, and one of the reasons of the 

 ceaseless struggle for existence is just the clashing of the 

 requirements of different kinds of creatures. The struggle 

 goes on in a more or less inconspicuous sort of way until 

 some environmental cause, such as peculiar weather, 

 brings about a marked disproportion on one side or the 

 other, and then there is a crisis. 



Attention has often been directed to the * beneficent 

 provision of Nature ' that animals which are preyed upon 

 are, on the whole, more prolific than those which prey upon 

 them. Thus, small Rodents tend to be much more prolific 

 than Carnivores. The primary reason for this is probably 

 that less individuated types tend to be more prolific. In 

 a relatively stupid stock the variants in the direction of 

 increased reproductivity will tend to survive. Great 

 reproductivity will become the survival-securing quality 

 of the feeble-minded types. 



Birds keep down insects and small mammals, and they 

 also distribute seeds. It is plain that any sudden reduc- 

 tion in their numbers will bring about disharmony in the 

 order of Nature. Those who make such calculations 



