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 Washington. 



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 faunae. 



Division ofithe Earth into Faunae. 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 Kocky Mountains, we shall notice 

 that the Atlantic fauna has been replaced almost Avholly by 

 a new 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 fauna?, 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 



