The descendants of these reptiles were transformed into mammals. If we had had the opportunity of studying the early mammals of the Rocky Mountain region with a full appreciation of the possibilities of evolution, we should have perceived that they were essentially of the same stock and ancestral to our modern types. There were little camels scarcely more than twelve inches high, little taller than cotton-tail rabbits and smaller than the jackass rabbits; horses 15 inches high, scarcely larger than, and very similar in build to, the little English coursing hound known as the whippet; it is not improbable that we shall find the miniature deer; there certainly existed ancestral wolves and foxes of similarly small proportions. You have all read your Darwin carefully enough to know that neither camels, horses, nor deer would have evolved as they did except for the stimulus given to their limb and speed development by the contemporaneous evolution of their enemies in the dog family. THE MIDDLE STAGE OF EVOLUTION. A million and a half years later these same animals had attained a very considerable size; the western country had become transformed by the elevation of the plateaux into dry, grass-bearing uplands, where both horses and deer of peculiarly American types were grazing. We have recently secured some fresh light on the evolution of the American deer. Besides the _Palaeryx_, which may be related to the true American deer _Odocoileus_, we have found the complete skeleton of a small animal named _Merycodus_, nineteen inches high, possessed of a complete set of delicate antlers with the characteristic burr at the base indicating the annual shedding of the horn, and a general structure of skeleton which suggests our so-called pronghorn antelope, _Antilocapra_, rather than our true American deer, _Odocoileus_. This was in all probability a distinctively American type. Its remains have been found in eastern Colorado in the geological age known as Middle Miocene, which is estimated (_sub rosa_, like all our other geological estimates), at about a million and a half years of age. Our first thought as we study this small, strikingly graceful animal, is wonder that such a high degree of specialization and perfection was reached at so early a period; our second thought is the reverence for age sentiment. THE AFRICAN PERIOD IN AMERICA. The conditions of environment were different from what they were before or what they are now. These animals flourished during the period in which western America must have closely resembled the eastern and central portions of Africa at the present time.