PARALLEL EVOLUTION IN OTHER RACES. It is interesting to observe that while the evolution of the horse was progressing during the Tertiary period in North America another group of hoofed animals. now extinct, the Litopterna, in South America evolved a race adapted to the broad plains of Argentina and Patagonia, and singu larly like the horse in many ways. These ani mals likewise lost the lateral toes one after an other, and concentrated the step on the central toe; they also changed the form of the joint-sur faces front ball-and-socket to pulley joints; they also lengthened the limbs and the neck; and they also lengthened the teeth. and compliented their pattern; but, unlike the true horses, they could not form cement on the tooth, and it was by no means so efficient a grinder. This group of ani mals. native to South America. became totally extinct, and were succeeded by the horses, which immigrated from North America. which in their turn became extinct before the appearance of civilized man. Many of the contemporaries of the horse in the Northern Hemisphere, such as the camels in America, the deer, antelopes, sheep.
and cattle in the Old World. were likewise length ening the limbs, lightening and strengthening the feet, and elongating the touth-crowns, to adapt themselves to the changing conditions around them, but none paralleled the horse's evolution quite so closely as did the pseudo-horses of South America.
BIBLIOGRAPHY. Huxley, American Addresses Bibliography. Huxley, American Addresses I London, 18'77) ; Marsh, "Polydaetyl Horses, 'Recent and Extinct," in American Journal of science, vols. xvii., xliii. (New Haven, 1S92); Flower, The Horse: A Study in Natural History I London. 1S91 ) ; Hutchinson. Cr( attires of Other (New York. 159-1); Lucas. Animals Past (New York, 1901) ; Matthew. The Evolu tion of the Horse. No. 9 in series of Popular Guide Leaflets to the American Museum of Nat ural History (New York, 1902).
For technical articles on fossil horses, con sult the bibliography in Woodward. Vertebrate Pahrontology (Cambridge University Press, 1s95) ; and in addition Gidley, in Bulletin of the American Museum of Natural History (New York. 1900, 1901).