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

chap, vil] MAMMALIA OF THE NEW WORLD. 153tropical climate, and would thus afford an ample area for thecontinued existence and development of the typical SouthAmerican fauna ; even had glaciers descended in places so lowas what is now the level of the sea, which, however, there is noreason to believe they ever did. It is probable too, that tinslow tract, winch all round the Gulf of Mexico would be of considerablewidth, offered that passage for intermigration betweenNorth and South America, which led to the sudden appearancein the former country in Post-Pliocene times, of the huge Megatheroidsfrom the latter ; a migration which took place in oppositedirections as we shall presently show.The birthplaceand migrations of some mammalian familiesand genera.—We have now to consider a few of those casesin which the evidence already at our command, is sufficientlydefinite and complete, to enable us to pronounce with some confidenceas tothe last movements of several important groups ofmammalia.Primates.—The occurrence in North America of numerousforms of Lemuroidea, forming two extinct families, which arebelieved by American paleontologists to present generalizedfeatures of both Lemuridae and Hapalida?, while in Europe onlyLemurine forms allied to those of Africa have occurred indeposits of the same age (Eocene), renders it possible that thePrimates may have originated in America, and sent one branchto South America to form the Hapalida? and Cebidas, andanother to the Old "World, giving rise to the lemurs and trueapes. But the fact that apes of a high degree of organizationoccur in the European Miocene, while in the Eocene, a monkeybelieved to have relations to the Lemuroids and Cebidse has alsobeen discovered, make it more probable that the ancestral formsof this order originated in the Old World at a still earlier period.The absence of any early tertiary remains from the tropical partsof the two hemispheres, renders it impossible to arrive at anydefinite conclusions as to the origin of groups which were, nodoubt, always best developed in tropical regions.Carnivora.—This is a very ancient and wide-spread group, thefamilies and genera of which had an extensive range in veryVol. I.—12