24 The Rocky Mountain Locust. 



entirely absent. As a rule it is most abundant on open 

 prairies, and in spring and summer is always found in such 

 situations. It matures rather later than spretus, in the 

 same latitude but earlier than femur -rubrum, which is not 

 at all infrequent in timber. 



Whether these three insects, as here defined, are really 

 distinct species, or only races of one and the same, is a 

 question that each individual entomologist will decide for 

 himself, according to his idea of what constitutes a species. 

 All discussion at the present day as to whether we are 

 dealing with species or varieties in the lower classes of ani- 

 mals, is more or less puerile. Naturalists have no fixed 

 standard as to what constitutes a species, and are fast com- 

 ing to the conviction that there is no such thing in nature, 

 and that the term is conventional — an abstract conception. 

 Yet it is the custom, in entomology and botany more par- 

 ticularly, to separate by names, under this term species, 

 forms that are separable and show constant differences ; 

 and the separation of such by the study of large material, 

 and their life-histories is of far more weight and value 

 than that by the examination and description, however 

 detailed, of one or two individuals. As ordinary distinc- 

 tions go, however, there can be no doubt as to the specific 

 distinctness of these three forms, notwithstanding my owni 

 conviction that they merge into one another through excep- 

 tional intermediate individuals. I have little doubt that 

 they will cross with each other and produce fertile progeny, 

 just as many species of plants are known to do ; but such 

 crossing, if it occurs, must be more frequent between 

 femur-rubrum and Atlanis than between either of these 

 and spretus ; because this last is the most effectually sep- 

 arated geographically — a fact proved alike by its dying- 

 out east of the 94th meridian, and by its perishing when 



