SELECTION 153 



important part, and periodical seasons of 

 extreme cold have destroyed as many as four- 

 fifths of the bird of an observed area. Epi- 

 demics, too, may occur. In many a species 

 a large stock of individuals is often essential 

 to its continuance. 



Complex and unexpected checks and re- 

 lations exist between organic beings which 

 have to struggle together; witness the pro- 

 found alteration of the flora and fauna of a 

 heath when planted with Scots pine, these 

 again being wholly dependent upon the ex- 

 clusion of cattle. But in several parts of 

 the world insects determine the existence of 

 cattle. Again, red clover depends for fertil- 

 ization upon the humble-bees, these upon 

 immunity from the attacks of field-mice, and 

 thus indirectly upon the number of cats. 

 Hence no bees, no clover, and the more cats, 

 the more clover. 



The struggle will almost invariably be 

 most severe between the individuals of the 

 same species, for they frequent the same 

 districts, require the same food, and are ex- 

 posed to the same dangers. In the case of 

 varieties of the same species, the struggle 

 will generally be almost equally severe, and 

 we sometimes see the contest soon decided 

 (as in the case of varieties of wheat or of 

 sweet pea, of the mountain sheep or of the 



