12 THE IMPORTANCE OF BIED LIFE 



dred thousand .species already recognized, and 

 virtually all .are capable of multiplying at an ap- 

 palling rate. 



With this great potentiality for reproduction, 

 then, insects cannot be allowed to go unchecked; 

 otherwise within a short time, the surface of the 

 earth would be a wasted desert. Fortunately 

 nature has pro\ided a multitude of enemies, 

 more, indeed, than birds ever dreamed of having. 

 Thus, the mycelium of certain fungi (the thread- 

 like body of the plants) thrives on the bodies of 

 certain insects; diseases blight them; frost and 

 flood cut short their lives; animals, like the ant- 

 e,ater, make of them a sole source of food; para- 

 sites flourish and grow fat upon them ; they wage 

 continual war among themselves; and birds de- 

 stroy them — ^parasites, predaceous forms, and all. 



But while this form of destruction is perhaps 

 the greatest mission of birds in nature, it also is 

 of the highest economic importance to agri- 

 culture and wiU be discussed more at length in 

 the next chapter. 



4 



Effect on Vegetation 



The next great avian function falls under two 

 apparently opposite heads : the limiting of vege- 

 tation and the spreading of vegetation. 



