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

—154 DISTRIBUTION OF EXTINCT ANIMALS. [part ii.early times. The true bears (Ursus) are almost the onlyimportant genus that seems to have recently migrated. InEurope it dates back to the Older Pliocene, while in NorthAmerica it is Post-Pliocene only. Bears, therefore, seem tohave passed into America from the Palaearctic region in the latterpart of the Pliocene period. They probably came in on thenorth-west, and passed down the Andes into South America,where one isolated species still exists.Ungulata.—Horses are very interesting. In Europe they dateback under various forms to the Miocene period, and true Equusto the Older Pliocene. In North America they are chieflyPliocene, true Equus being Post- Pliocene, with perhaps one ortwo species Newer Pliocene ;but numerous ancestral forms dateback to the Miocene and Eocene, giving a more perfect " pedigreeof the horse " than the European forms, and going back to amore primitivetypeOrohippus.In South America, Equus isthe only genus, and is Post-Pliocene or at most Newer Pliocene.While, therefore, the ancient progenitors of the Equidae werecommon to North America and Europe, in Miocene and evenEocene times, true horses appear to have arisen in the Palasarcticregion, to have passed into North America in the latter part ofthe Pliocene period, and thence tohave spread over all suitabledistricts in South America. They w T ere not, however, able tomaintain themselves permanently in their new territory, and allbecame extinct ;while in their birth-place, the Old World, theycontinue to exist under several varied forms.True tapirs are an Old World group. They go back to theLower Miocene in Europe, while in both North and SouthAmerica they are exclusively Post-Pliocene. They occur inFrance down to the Newer Pliocene, and must, about thattime, have entered America. The land connection by whichthis and so many other animals passed between the Old andNew Worlds in late Tertiary times, was almost certainly in theNorth Pacific, south of Behring's Straits, where, as will be seenby our geueral map, there is a large expanse of shallow water,which a moderate elevation would convert into dry land, in asufficiently temperate latitude.