Locust Ravages East of the Mississippi. 201 



nois, the insects were as thick as I found them in many 

 fields around St. Louis, where every step would cause two 

 or three hundred to rise ; and let this migratory instinct 

 be developed, and the mystery of the Illinois flights van- 

 ishes. They are exceptional local phenomena : they are 

 neither so strong nor so long sustained as those of the 

 Rocky Mountain species ; nor are they in any sense to be 

 so much dreaded. 



In short, whenever the climate and conditions in the 

 Mississippi Valley approach those existing in the native 

 home of the Rocky Mountain Locust, some of our native 

 species, and especially those nearest akin to it, also ap- 

 proach it in habit. If the climate of Illinois and Mis- 

 souri were to permanently change in that direction, these 

 species would become permanently modified ; but as there 

 is no immediate danger of such a contingency, the Rocky 

 Mountain Locust is the only species, here considered, that 

 can properly lay claim to the migratory habit. 



LOCUST FLIGHTS EAST OF THE MISSISSIPPI. 



To the unscientific mind there are few things more dif- 

 ficult of apprehension than that species, whether of plants 

 or animals, should be limited in geographical range to 

 areas not separated from the rest of the country by any 

 very marked barriers, or by visible demarcations. Yet such 

 is the fact, known to every naturalist ; and the geograph- 

 ical distribution of species forms at once one of the most 

 interesting and one of the most important studies in natural 

 history. Some species have a very limited, others a very 

 wide range ; and while in the course of time — in the lapse 

 of centuries or ages — the limits have altered in the past 

 and will alter in the future, they are, for all practical pur- 

 poses, permanent in present time. These limits may in 

 fact, for the purpose of illustration, be likened to those 



