DARWIN AND NATURAL SELECTION 49 



greater than that generally seen in passing from one 

 soil to one quite different. Twelve species of plants 

 flourished in the plantations which could not be found 

 on the heath. The effect on the insects must have 

 been still greater, for six insectivorous birds were very- 

 common in the plantation, which were not to be seen 

 on the heath. . . . Certain plants require the 

 visits of certain insects to fertilise them. Thus the 

 visits of bees are necessary for the fertilisation of 

 clover, and Jumble bees alone visit clover. What 

 would happen if bumble bees became extinct or very 

 rare in England? Red clover, whose reproduction 

 depends upon them, would also become very rare or 

 wholly disappear. The number of humble bees de- 

 pends in a great measure upon the number of field- 

 mice which destroy their nests, and the number of 

 mice is dependent on the number of cats. Thus the 

 number of cats in a district might determine the fre- 

 quency of red clover in that district. 



Everywhere we observe close interdependence and 

 struggle; individuals struggle not only for life but 

 for reproduction. The main factor in that struggle , 

 a necessary condition of natural selection, is the 

 competition between individuals of one giv-en-*pecies 

 which results in the survival of those best armed for 

 the fray. This struggle is most stubborn, for all the 

 individuals live under the same conditions, fight over 

 the same food and invade the same grounds. And 



