Closely associated with the evidence of the immensity of the force of volcanic action and the infinitude of time elapsed, Darwin had testimony of the multitude of plant and animal species, some gigantic, others almost infinitely small, some living, others extinct. We know that his thought was greatly affected by his discovery in Uruguay and Patagonia of the fossil remains of extinct mammals, all the more so because they seemed to bear relationship to particular living species and at the same time to show likeness to other species. The Toxodon (bow-tooth), for example, was a gigantic rodent whose fossil remains were discovered in the same region where Darwin found living the capybara, a rodent as large as a pig; at the same time the extinct species showed in its structure certain affinities to the Edentata (sloths, ant-eaters, armadillos). Other fossils represented gigantic forms distinctly of the edentate order and comparable to the Cape ant-eater and the Great Armadillo (_Dasypus gigas_). Again, remains were found of a thick-skinned non-ruminant with certain structural likeness to the Camelidæ, to which the living species of South American ruminants, the _guanacos_, belong. Why have certain species ceased to exist? As the individual sickens and dies, so certain species become rare and extinct. Darwin found in Northern Patagonia evidence of the _Equus curvidens_, an extinct species of native American horse. What had caused this species to die out? Imported horses were introduced at Buenos Ayres in 1537, and so flourished in the wild state that in 1580 they were found as far south as the Strait of Magellan. Darwin was well fitted by the comprehensiveness of his observations to deal with the various factors of extinction and survival. He studied the species in their natural setting, the habitat, and range, and habits, and food of the different varieties. Traveling for three years and a half north and south on the continent of South America, he noticed one species replacing another, perhaps closely allied, species. Of the carrion-feeding hawks the condor has an immense range, but shows a predilection for perpendicular cliffs. If an animal die on the plain the polyborus has prerogative of feeding first, and is followed by the turkey buzzard and the gallinazo. European horses and cattle running wild in the Falkland Islands are somewhat modified; the horse as a species degenerating, the cattle increasing in size and tending to form varieties of different color. The soil being soft the hoofs of the horse grow long and produce lameness. Again, on the mainland, the niata, a breed of cattle supposed to have originated among the Indians south of the Plata, are, on account of the projection of the lower jaw, unable to browse as effectually as other breeds. This renders them liable to destruction in times of drought. A similar variation in structure had characterized a species of extinct ruminant in India.