62 



The Rocky Mountain Locust. 



IT CAN NOT PERMANENTLY THRIVE OR PERPETUATE ITSELF 

 IN THE MISSISSIPPI VALLEY. 



The comparatively sudden change from the attenuated 

 and dry atmosphere of five to eight thousand feet or more 

 above the sea level, to the more humid and dense atmos- 

 phere of one thousand feet above that level, does not 

 agree with the species. The first generation hatched in this 

 low country is more or less unhealthy, and those that attain 

 maturity do not breed, but quit the country. At least such 

 is the case in the whole of the Mississippi Valley proper. 

 As we go west or northwest and approach nearer and 

 nearer the insect's native home, the power to propagate 

 itself and become localized, becomes, of course, greater 

 and greater, until at last we reac i the country where it is 

 found perpetually. Thus in the western parts of Kansas 

 and Nebraska, in parts of Colorado and Minnesota, in fact, 

 in all the region indicated by the pink color in Plate I, the 

 progeny from the mountain swarms may multiply to the 

 second or even third generation, and wing their way in 

 more local and feeble bevies to the country east and south. 

 Yet eventually they vanish from that region and perish, 

 unless fortunate enough to be carried back by favorable 

 winds to the higher country where they flourish. There 

 is nothing more certain than that the insect is not autoch- 

 thonous in Texas, West Arkansas, Indian Territory, West 

 Missouri, Kansas, Western Iowa, Nebraska, or even Min- 

 nesota ; aud whenever it overruns any of those States it 

 sooner or later abandons them. The same also is true of 

 parts of Colorado, Montana, Dakota, and even of Manitoba. 



THE CONDITIONS WHICH PREVENT THE PERMANENT SETTLE- 

 MENT OF THE SPECIES IN THE MISSISSIPPI VALLEY. 



The conditions which determine the geographical limits 

 in which a species can exist, are often complex, and it is 



