The above account shows the great richness of the vertebrate life during the Pleistocene; furthermore, this abundance evidently existed during the early stages of the epoch. It constituted the materials on which that combination of conditions which we call environment had to work during Pleistocene times. The comparison shows that the result was an impoverishment of the vertebrate fauna. Genera and families, even orders, were wiped out of existence, and these included some of the noblest animals that have graced the face of the earth, the elephants, the mastodons, tapirs, many species of bison, horses, saber-tooth cats, huge tigers, and gigantic wolves. The following nine or ten families became either wholly extinct or continued to exist only in other more hospitable lands: the Megatheriidæ, including several genera of ground-sloths; the Hoplophoridæ or glyptodons; the Caviidæ, which embraced one or more species of huge capybaras; the Elephantidæ, under which are arranged three or four species of elephants and three genera of mastodons; the Equidæ, represented by a dozen or more species of horses; the Camelidæ, of which there were several Pleistocene species and probably three or four genera; the Hyænidæ, of which there appears to have been at least one genus, with one species; the Tapiridæ, including three or four species; and probably the Rhinocerotidæ. Besides these, the subfamily of Felidæ known as Machairodontinæ, embracing those wonderful carnivores the saber-tooth tigers, was suppressed. The Dasypodidæ, which included some armadillos 5 or 6 feet long, are now represented by only one small species in Texas. Of the Tagassuidæ, to which belonged several genera and stately species of peccaries, there exists now in North America north of Mexico but one species, an animal of only moderate size. VIII. ON EVOLUTION DURING THE PLEISTOCENE.