214 THE SURVIVAL OF THE UNLIKE. [iX. 



tals. I fancy that the future historian will find that 

 the advent of the spray in the latter part of this 

 century marked an important epoch in agricultural 

 pursuits.. Yet this epoch is not disconnected from 

 the era before it. It is but a natural outcome or 

 consequence of the rapid increase of insect and fun- 

 gous enemies, which increase, in turn, is induced by 

 the many disturbing influences of cultivation itself. 

 When we devise effective means of checking the in- 

 cursions of our foes, therefore, we are only keeping 

 pace with the initial progress fostered by the origina- 

 tion of new varieties and the quickening commercial 

 life of our time. Yet the era of spraying is none the 

 less a mark of great achievement, and we have not 

 yet seen the good of which it will ultimately prove 

 to be capable. But a greater achievement than this 

 must be made before we shall have reached the ideal 

 and inevitable method of combating external pests ; 

 we must learn to so control natural agencies that one 

 will counteract another. Nature keeps all her forces 

 and agencies in comparative equilibrium bj' pitting 

 one against another in the remorseless struggle for 

 existence. (See Essay VII.) The introduction of in- 

 sect parasites and predaceans, entomogenous fungi, 

 colonization of insectivorous birds, and the use of 

 strategy in cultivation and in the selection of im- 

 mune species and varieties, and the planning of rota- 

 tions and companionships of plants, will eventually be 

 so skillfully managed that most of our enemies will 

 be kept under measurable control. A short rotation 

 is now known to be the best means of combating 

 wire -worms and several other pests. The first great 

 success in this direction in America is the intro- 



