ENTOMOLOGY OF COLORADO. 319 



period. In America, the whole continent was covered with ice at 

 least as far south as 40°, except a rather problematical area between 

 the eastern and central regions. The ice-sheet went further south 

 in the west than in the east, extending about to San Diego, though 

 the basin of the Colorado in Arizona seems to have been free. 



However, to the northwest there was apparently an unfrozen 

 coast-line, at least as fertile as that of Greenland at the present day. 

 We read that "the warm currents of the Pacific, which now temper 

 the severity of the coast climate of Alaska, seem to have exerted the 

 same influence during the glacial period, for none of the glaciers 

 which descend from the inland range reach the sea, nor do they 

 appear to have done so in glacial times. . . . The shores of British 

 Columbia, on the contrary, are indented by long and deep fiords, 

 through which, as in Norway and Greenland, the old glaciers, now 

 stayed further in, traveled out to sea." 



It can readily be imagined that such a state of affairs would lead 

 to the destruction of a large part of the fauna, the remainder either 

 surviving along the northwest coast-line, or going southwai'd to the 

 Gulf States and Mexico. The eastern fauna, with which we are 

 not now particularly concerned, would largely survive, owing to there 

 being a considerable area of unglaciated territory available. This, 

 indeed, has been the case. The Californian fauna would survive in 

 part to the north, and also in Lower California and the western coast 

 region of Mexico. But the fauna of the central region would be 

 almost annihilated, because the warm winds being cut off by the 

 coast ranges, the country would become exti'emely cold, even far 

 down into the higher lands of Mexico. The arid region where not 

 actually glaciated would be a frozen desert, and the migration of the 

 fauna southward would be far from easy. 



In the eastern province the species of the moist Northern States 

 would find little difficulty in migrating southward into the equally 

 moist Southern States. The isotherms would shift southward over 

 moderately uniform country. In the central region, however, this 

 would not be the case. There is no place available to the South, 

 except the moister coast line, and the interior uplands, which latter 

 were undoubtedly glaciated. The great plains between the Rocky 

 Mountains and the Mississippi would have made an impassable bar- 

 rier for most species, preventing migration in that direction. 



But, it may be urged, at some point to the southward the moun- 

 tains or central uplands would cease to be glaciated^ and why should 



TEANS. AM. ENT. SOC. XX. NOVEMBER, 1893. 



