The great continent of South America, as far as we can judge from the remarkable characteristics of its fauna and the vast depths of the oceans east and west of it, has not during Tertiary, and probably not even during Secondary times, been united with any other continent, except through the intervention of North America. During some part of the Secondary epoch it probably received the ancestral forms of its Edentates and Rodents, at a time when these were among the highest types of Mammalia on the globe. It appears to have remained long isolated, and to have already greatly developed these groups of animals, before it received, in early Tertiary times, the ancestors of its marmosets and monkeys, and, perhaps also, some of its peculiar forms of {82}Carnivora. Later, it received its Camelidæ, peccaries, mastodons, and large Carnivora; and later still, just before the Glacial epoch, its deer, tapir, opossums, antelopes, and horses, the two latter having since become extinct. All this time its surface was undergoing important physical changes. What its earlier condition was we cannot conjecture, but there are clear indications that it has been broken up into at least three large masses, and probably a number of smaller ones; and these have no doubt undergone successive elevations and subsidences, so as at one time to reduce their area and separate them still more widely from each other, and at another period to unite them into continental masses. The richness and varied development of the old fauna of South America, as still existing, proves, however, that the country has always maintained an extensive area; and there is reason to believe that the last great change has been a long continued and steady increase of its surface, resulting in the formation of the vast alluvial plains of the Amazon, Orinoko, and La Plata, and thus greatly favouring the production of that wealth of specific forms, which distinguishes South America above all other parts of our globe.