THE AMERICAN NATURALIST. [Vol. XXXVIII. 



Epoch upon the Ufe of this and other countries have long been 

 recognized and carefully studied so that numerous distributional 

 phenomena due thereto have thus been referred to their proper 

 cause ; and although modern lines of insect migration are being 

 mapped and their relative importance determined, there is never- 

 theless a vast amount of detail to be worked out in order that 

 certain local problems may be understood. It is with one of 

 these local problems that we have now to do. 



Some years ago, as the result of several seasons' collecting in 

 the Great Basin, I became convinced that the distribution of 

 certain species of Coleoptera therein occurring could not be 

 accounted for by the theory of zonal arrangement of life, but 

 apparently had as ultimate cause some condition or combination 

 of conditions which belonged to the geological rather than the 

 i)resent history of the area under discussion. With the aim of 

 testing this belief, further collections were made and all available 

 data bearing upon the matter were collated, in order that it 

 might be seen whether or not the above conclusion rested upon 

 tangible grounds. 



The Great Basin is a vast area of interior drainage, lying 

 between the Wasatch Mountains and the Sierra Nevadas. The 

 id, the soil in general desert, the vegetation such as 

 s regions of like nature in other parts of the world, 

 bemg marked by the preponderance of a few species of dry 

 dwarfed shrubs and the absence of forests except on certain 

 igier mountain ranges which ridge the basin here and there. 



bodies ''''''^ ^^'^^ ^""P^^ 



ties o water outside the limits, one of the most common 

 methods of introduction of ex 



characteri2 



along the lin 



a-limital species — by migration 

 water-course — is eliminated, and the fauna 

 >s not exposed to this source of modification. The two great 

 mountain chains which form the eastern and,western nms have 

 district"!'! ^^^^ interchange of inhabitants with the 



passes 'fro'"^ ''^y^^nd. though some have doubtless crossed the 

 ahnmH,/r"' southern boundary is less 



tnicts nn " ?; '^"'^ 'P^^^^^ ^^'^^ '^^^^ 



rim i. r^\\ r ^"""^"^ ^^^^ quarter, while the northern 



> of the 



^in and the Columbia 



series of hills that mark the divide between 

 drainage system. 



