The geographical distribution of animals, with a study of the relations ...

40 DISTRIBUTION OF ANIMALS. [part i.nature to affect the larger features of the earth's surface or todetermine the boundaries of great zoological regions.The only other other recent change of great importance whichcan be adduced to illustrate our present subject, is that whichhas taken place between North and South America.marineThe livingshells of the opposite coasts of the isthmus of Panama,as well as the corals and fishes, are generally of distinct species,but some are identical and many are closely allied ; the WestIndian fossil shells and corals of the Miocene period, however,are found to be largely identical with those of the Pacificcoast.The fishes of the Atlantic and Pacific shores of America areas a rule very distinct ; but Dr. Giinther has recently shownthat a considerable number of species inhabiting the seas onopposite sides of the isthmus are absolutely identical. Thesefacts certainly indicate, that during the Miocene epoch a broadchannel separated North and South America ; and it seems probablethat a series of elevations and subsidences have takenplace uniting and separating them at different epochs ;the mostrecent submersion having lasted but a short time, and thus,while allowing the passage of abundance of locomotive fishes,not admitting of much change in the comparatively stationarymollusca.The Glacial Epoch as affecting the Distribution of Animals.—The remarkable refrigeration of climate in the northern hemispherewithin the epoch, of existingspecies, to which the termGlacial epoch is applied, together with the changes of level thataccompanied and perhaps assisted to produce it, has been one ofthe chief agents in determining many of the details of the existingdistribution of animals in temperate zones. A comparisonof the effects produced by existing glaciers with certain superficialphenomena in the temperate parts of Europe and NorthAmerica, renders it certain that between the Newer Pliocene andthe Eecent epochs, a largeportion of the northern hemispheremust have been covered with a sheet of ice several thousandfeet thick, like that which now envelopes the interior of Greenland.Much further south the mountains were covered withperpetual snow, and sent glaciers down every valley ;and all the