660 THE AMERICAN NATURALIST.  [Vo. XXXII. 
their own through the Cretaceous, where at their decline they 
became, as in Ornithostoma, colossal and toothless. We can 
imagine that the demise of this type was assisted in two ways: 
those with a feebler flight succumbed to the agile, tree-climbing 
dinosaurs; while the avian type, waxing stronger in numbers 
and power of flight and exceeding in intelligence, exhausted the 
food supply of volant insects, and drove their clumsier reptilian 
cousins to the wall, fairly starving them out; just as at the 
present day the birds give the bats scarcely a ratson a’ étre. 
3. Lhe Pacific Coast Revolutions. — It has long been known 
that there are a greater number of insect faunz on the Pacific 
coast, and greater variation of species, with more local varieties, 
than east of the Mississippi River. It has also been shown by 
Gilbert and Evermann, as well as by Eigenmann, to apply to 
the fishes of the Columbia and Frazer River basins. ‘‘ Nowhere 
else in North America,” says the latter, “do we find, within 
a limited region, such extensive variations among fresh-water 
fishes as on the Pacific slope.” He also points out the note- 
worthy fact that the fauna is new as compared with the 
Atlantic slope fauna, and “has not yet reached a stage of stable 
equilibrium.” As previously shown by Gilbert and Evermann, 
“each locality has a variety which, in the aggregate, is different 
from the variety of every other locality ’’; and he adds: “ the 
climatic, altitudinal, and geological differences in the different 
streams, and even in the length of the same stream, are very 
great on the Pacific slope.” 
It is evident that the variations are primarily due to the 
broken nature of the Pacific coast region, and to the isolation 
of the animals in distinct basins more or less surrounded by 
high mountain barriers, with different zones of temperature and 
varying degrees of humidity. 
As brought out by the labors of Le Conte, Diller, and 
Lindgren, the Sierra Nevada region has undergone cycles of 
denudation, and these changes, occurring later than those of 
the Appalachian region, have doubtless had much to do with 
the present diversified and variable fauna. The latest writer, 
N. F. Drake,! states that the western slope of the Sierra Nevada 
1 The Topography of California. Journ. of Geol., vol. v (September and Octo- 
ber, 1897), pp- 563-578. 
