266 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



of P. O. J. cane have been introduced, resulting in great increases in 

 yields. 



Plowing under crop residues is an effective check for many insects, 

 such as garden insects, European corn borer, wheat stem maggot, 

 Hessian fly, celery leaf tier, and many others. This practice restores 

 humus to the soil and provider for better aeration. It is good agro- 

 nomic practice. 



Crop rotations tend to build up the soil or at least prevent its too 

 rapid depletion and generally thwart insect pests. A large part of 

 the increased yields obtained from a rotation where grains follow 

 legumes and legumes follow grains is due to the reduction in insect 

 damage. 



Creosoting railroad ties and telephone poles to protect them against 

 termites has protected them also from saprophytic fungi. 



CONCLUSION 



The progress of civilization can be interpreted in the changing 

 status and range of many insects. This change is noted chiefly in 

 those of economic importance but, by analogy, one may infer that 

 many rare and unusual forms have been equally affected. However, 

 because of their scarcity or lack of importance with reference to 

 modern agriculture, this change is either unknown or less understood. 

 Every specialist finds the study of species distribution very fascinat- 

 ing. One can never be certain whether unusual distributions or long 

 gaps between localities in which the species occur may not be due 

 to transportation agencies of man. The tendency toward specialized 

 agriculture is favorable to the unbalancing of species of insects re- 

 lated directly or indirectly to the different hosts. This has necessi- 

 tated a fiercer warfare to produce a satisfactory crop. Insects maj^ 

 not have learned nor consciously changed to fit the ever-changing 

 conditions of civilization, but through survival of successful strains 

 have just as fully met each move successfully. A stabilized environ- 

 ment for these creatures is as far away as the end of evolutionary 

 development itself. 



