147 
other influences, such as a rigorous climate or a barren soil. Our 
grain fields include neither the barren desert, the frozen mountain 
tops, nor the ice-clad regions of the far North, but the fertile prairies- 
and valleys over which vegetation naturally grows in great luxuriance 
and profusion, each species if left to itself being kept in its proper 
numerical sphere by natural laws. The agriculturist, however, comes 
upon the scene and incites an insurrection, causing the three species 
before mentioned to not only rebel, but overrun and take possession of 
these broad acres, putting the original inhabitants to death and estab- 
>-^ 
Fig. 2.— The annual cycle of the Hessian Fly (Webster del.). 
lishing themselves in nearly or quite full power. If the contest were 
Avholly a natural one, the interlopers would soon be forced into their 
proper places, and exist only in proportion as they could resist the 
returning encroachments of the natural flora. But the plow and the 
hoe again interpose, and the victors still hold the field. Nature then 
does what is naught but good generalship, brings up her reserves in 
the animal and vegetable enemies of the three usurping species and 
precipitates them upon the foe. It is here that the hand of the hus- 
bandman seems to lose its cunning. He can fight the forests, the 
