GEOGRAPHICAL DISTRIBUTION. 661 
sometimes for hundreds of miles, and in this apparently 
haphazard way islands are, in part at least, supplied with 
their quota of animal life. 
Great rivers, like the Missouri, Mississippi, and the Ama- 
zons, afford means of transportation from one part of a con- 
tinent to another, from the interior to the seaboard, of 
which many fishes, insects, and especially fluviatile mollusks, 
avail themselves. Artificial means of crossing broad rivers 
are offered, to insects especially, by country-roads and bridges 
and railroad bridges, of which the potato-beetle and the 
cabbage-butterfly have fully availed themselves. The Colo- 
rado beetle has advanced steadily eastward, suddenly ap- 
pearing in isolated points in New England, having appar- 
ently been transported by through .grain-cars from Chicago, 
and has been carried to Europe in vessels. The European 
cabbage-butterfly introduced into Quebec spread southward 
into Maine along the Grand Trunk Railroad, into New 
York along the railroads from Montreal to New York, and 
then along the railroads to Washin gton. 
Geological changes, such as the rise and submergence of 
the edges of continents, and also the incoming and wane of 
the glacial period, were still more general and fundamental 
means of the dispersal and rearrangement of faune. 
Division of the Earth into Faune.—When we go from 
Maine to California we shall find that the faunistic features 
of the country radically change three times. Leaving the 
moist, temperate, forest-clad Atlantic region with its char- 
acteristic animals, and entering on the broad, treeless, dry, 
elevated plateau of the Rocky Mountains, we shall notice 
that the Atlantic fauna has been replaced almost wholly by 
anew and strange assemblage ; and when we descend the 
Pacific slope of the Sierra Nevada, there will be found to 
be a second replacement, though much less marked than 
the first. Again, when we pass from Labrador to the Isthmus 
of Panama, we shall find several distinct faune, from an 
arctic one to a purely tropical one. If we pause at Wash- 
ington and analyze the fauna of that point, we shall see 
that it is made up mainly of animals common to the Middle 
Atlantic States, with an infusion of northern and southern 
