SELECTION 153 



germinating on ground already occupied and 

 from animals. The amount of food, of course, 

 gives the extreme limit of numbers ; very 

 frequently, however, the check is given by 

 the attacks of enemies, as on game by 

 " vermin." Changes of climate play an im- 

 portant part, and periodical seasons of extreme 

 cold have destroyed as many as four-fifths of 

 the birds of an observed area. Epidemics, 

 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, 



