Lastly, we must not omit to mention that in the Old World the first true apes, _Oreopithecus_ and _Dryopithecus_, appeared. The first of these united some of the characteristics of apes and monkeys; the second, about the same size, was more closely related to the chimpanzee and gorilla. CHAPTER XXIII THE ICE AGE For some reason or reasons concerning which there has been a great deal of speculation but not a large amount of agreement, the closing stages of the last geologic era which precedes our own and which links the great past to the present, were distinguished by great cold and by widespread fields of ice. Ice-sheets spread over six or eight million square miles of the earth's surface where not long before mild climates had prevailed. Were it not for this great Ice Age and for its far-reaching effects on the conditions under which Man has developed, this period, which is sometimes called the Pleistocene, (from Greek words meaning the "most recent"), would be more properly joined to the era which we have just been discussing, the two periods constituting a single period of great land elevation and of ocean-shrinking. This period, however, is now thought to be much more important than it was formerly, and perhaps longer in duration. More than half the ice-covered land lay in North America and more than half the rest in Europe. The glaciation, therefore, was probably confined to certain parts of the world and did not stretch all over the planet. But the whole world felt its effects; even in tropical regions ice and glaciers occurred on mountains where they did not exist before and do not exist now, and on mountains which now have glaciers the ice descended to levels 5000 feet below the point where it now stops. The southern hemisphere was affected as well as the northern, but to a much less degree. In Patagonia and New Zealand glaciers crept down from the mountains and spread out on the plains. Glaciers formed on the mountainous tracts of Tasmania and Australia where none exist now. Most of the higher mountains of the southern hemisphere bore glaciers. The Antarctic regions were presumably buried beneath ice and snow as they are at present, but of that we are not certain. In Asia ice-fields far greater than those existing to-day affected the higher mountains, and from the Lebanon to the Caucasus and from the Himalayas to Siberia and China traces of glaciers are found where they are not to be seen now. Yet on the plateaux and lowlands of Asia ice-sheets were far less extensive than in Europe and in North America. In Europe there were large glaciers in the southern mountains and extensive ice-sheets on the southern plains. Radiating from the Scandinavian highlands a succession of great ice-sheets crept forth on the lowlands of Russia, Germany, Denmark, Holland, and Belgium, and crossing the shallow basin of the North Sea touched the shores of Great Britain, where they were met by ice radiating from the mountains of these isles.