more advanced stage.

At the close of his arboreal life the ape had inherited or acquired the following assets: His vertebrate and mammalian structure had given him a large, vigorous, compact, athletic, adaptable body. The mammalian care of the young had insured their survival, but only at the expense of great strain and risk of the mother. Something at least approaching family life was already attained. Arboreal life with its gymnastic training had moulded the body, differentiated hand and foot, given the possibility of erect posture, emancipating the hand from the work of locomotion and setting it free to become a tool-fashioning and tool-using organ. The ape has keen sense-organs, an eye for distances, and other conditions; and the use of these powers has given him a brain far superior to that of any of his humbler fellows. These are full of great possibilities and opportunities, if he will only use them.

But why did our ancestor descend from his place of safety in the trees and live on the ground, exposed to the attacks of fierce, swift, and well-armed enemies? Very few of the Primates, except the rock and cliff-inhabiting baboons, ever made this great venture. There must have been some quite compelling argument to induce him to take so great a risk. The change took place probably at some time during the latter half of the Cenozoic or Tertiary period, the last great division of geological time, the Age of mammals.5 The earliest Tertiary Epoch, the Eocene, was a time of warm and equable climate, when apes lived far north in Europe, and doubtless in Asia also. Some of these apes were of fair or large size, showing that conditions were favorable and food abundant. The next epoch, the Oligocene, was similar but somewhat cooler. The third, the Miocene, was cooler still and dryer. Palms now forsook northern Europe, being gradually driven farther and farther south. Life became more difficult, food scarcer. Apes could not longer survive in northern Europe, but had to seek a warmer, more favorable, environment farther south, for many of the fruit and food trees had been crowded out and famine threatened.6 But insects and other small and toothsome animals remained on the ground, and were abundant along the shores of rivers and lakes. There, too, were fruits and berries, roots and tubers. There the food supply was still more than sufficient.

Thus far we have glanced at Europe only. But the same changes are taking place in Asia, the cradle and home of most placental mammals, the main area of a huge zoological province of which Europe was but a westward projection, and with which America had direct connection from time to time in the region of Behring’s Straits. Here, during late Miocene and early Pliocene times, in the latter part of the Cenozoic era, a dryer and somewhat harsher climate had been accompanied by the appearance of wide plains fitted for grazing animals, as well as stretches of forest, with all varieties of landscape favoring great diversity as well as abundance of mammalian life. It was, perhaps, the golden age for most mammals, when food was plenty, climate not too severe, and every prospect pleased. This slow and gradual, but fairly steady, lowering of temperature was to culminate in the Great Ice Age of the Pleistocene Epoch, so destructive to mammalian life in the northern hemisphere.

A second climatic change, perhaps even more important than the lowering temperature, was the increase of aridity. Even during the Oligocene Epoch “the flora indicates a lessening humidity and a clearer differentiation of the seasons,”7 The great trough of the inland sea which had stretched from the Mediterranean to the Indian Ocean began to rise, the first uplift taking place along the Pyrenees and western Alps. The Miocene was marked by a series of great movements. The old inland sea was displaced, subsidence gave place to uplift, and the greatest mountain system of the globe, including the Alps and the Himalayas, began to grow through vast repeated uplifts in the crust.8 The continents were elevated and widened. The forest-dwelling types became restricted and largely exterminated, and animals of the plains, in the form of horses, rhinoceroses, and the cloven-hoofed ruminants, expanded in numbers and in species. This profound faunal change implies dryer climate. There was now a lesser area of tropic seas to give moisture to the atmosphere. The mountains were now effective barriers, shutting off the moisture-bearing winds from the interior of the continents.

These changes would have been noticeable in Europe north of the Alps, but were far more so in central Asia along the northern face of the great plateau of Thibet, with its eastern and western buttresses, and its towering rampart of the Himalayas on the south, cutting off the warm moisture of the Indian Ocean. Northward of this vast plateau and westward over the far less elevated Iranian plateau and Afghanistan, forest was fast being replaced by parklands of mingled groves and glades, or by grassy plains, or even by dry steppes. Dessication, aridity of climate, was fast compelling forest and arboreal mammals to migrate or radically change their habits of life.9

Almost all the apes found their old environment and continued their arboreal life by migrating far southward through India or into Africa. But at the rear of the retreating host were forms from the cooler northern regions. They were hardy and vigorous, and probably larger than most of their fellows. Possibly some of them were caught in isolated decreasing areas of forest surrounded by steppe or plain. Some of them, at least, began to descend from the trees, to seek the new food supplies of riversides, glades, and thickets, and thus gradually to become accustomed to life on the ground. It was a very hazardous experiment; only the most hardy and wary and the quickest in perception, wit, and movement survived. Among these were our ancestors, driven like all their forebears by the spur of necessity into a new mode of life under trying conditions.

They were still only apes, with long arms and short legs, and probably scrambled mostly on all fours. They had heavy brows, retreating foreheads, projecting jaws, and a brutal physiognomy. Of the mental life of the man who was to be descended from them there were few signs. They were bundles of very slight possibilities.

But let us not “despise the day of small things.” They were still far from the invisible line between apedom and manhood. Physically they resembled man quite closely. They had hand and foot, and a fair-sized brain, though they had scarcely begun to realize the possibilities of these structures.

Arboreal life could teach them little more; continuance in that school would have meant a very comfortable stagnation. They were now promoted to a new school of vastly more difficult