42 THE DARWINIAN THEORY 



to the entire destruction of the native forests, con- 

 sisting of about a hundred distinct species of trees 

 and shrubs, the young plants being devoured by the 

 goats as fast as they grew up. A famous illustration 

 of the nice balancing of forces between animals and 

 plants is furnished by cats and clover. The red 

 clover is fertilised almost exclusively by humble 

 bees ; and field mice destroy the nests of humble 

 bees in large numbers. Newman estimates that 

 two-thirds of the total number of humble bees' 

 nests in England are thus destroyed. Now, the 

 number of mice depends largely on the number of 

 cats, and hence the abundance of clover depends on 

 the proper supply of cats. Darwin remarks that 

 " battle within battle must be continually recurring 

 with varying success ; and yet in the long run the 

 forces are so nicely balanced that the face of Nature 

 remains for a long time uniform, though assuredly 

 the merest trifle would give the victory to one 

 organic being over another." 



The real struggle is between the most closely 

 allied, and therefore competing forms. The black 

 rat, for example, was the common rat of Europe till 

 the beginning of the eighteenth century, when it was 

 driven out by the larger brown rat. Competition is 

 keener in direct proportion to the closeness of 

 interests, the two covering the same ground. That 

 the struggle for existence is a very real one, and does 

 actually lead to the extermination of less fit forms, is 

 seen in the way in which the Maoris, for instance, 

 are gradually becoming exterminated. The struggle 

 is not necessarily one of actual warfare, the stronger 



