James Geikie[55], under the heading, "Reliable and Unreliable estimates of geological time," points out that the absolute duration of the Pleistocene cannot be determined, but such investigations as those of Penck "enable us to form some conception of the time involved." He accepts as a rough approximation Penck's opinion that "the Glacial period with all its climatic changes may have extended over half a million years, and as the Chellean stage dates back to at least the middle of the period, this would give somewhere between 250,000 and 500,000 years for the antiquity of man in Europe. But if, as recent discoveries would seem to indicate, man was an occupant of our Continent during the First Interglacial epoch, if not in still earlier times, we may be compelled greatly to increase our estimate of his antiquity" (p. 303).
W. J. Sollas, on the other hand, is content with a far more contracted measure. Basing his calculations mainly on the investigations of de Geer, he concludes that the interval that separates our time from the beginning of the end of the last glacial episode is 17,000 years. He places the Azilian age at 5500 B.C., the middle of the Magdalenian age somewhere about 8000 B.C., Mousterian 15,000 B.C., and the close of the Chellean 25,000 B.C.[56]
But when all the changes in climate are taken into consideration, the periods of elevation and depression of the land, the transformations of the animals, the evolution of man, the gradual stages of advance in human culture, the development of the races of mankind, and their distribution over the surface of the globe, this estimate is regarded by many as insufficient. Allen Sturge claims "scores of thousands of years" for the neolithic period alone[57], and Sir W. Turner points out the very remote times to which the appearance of neolithic man must be assigned in Scotland. After showing that there is undoubted evidence of the presence of man in North Britain during the formation of the Carse clays, this careful observer explains that the Carse cliffs, now in places 45 to 50 feet above the present sea-level, formed the bed of an estuary or arm of the sea, which in post-glacial times extended almost, if not quite across the land from east to west, thus separating the region south of the Forth from North Britain. He even suggests, after the separation of Britain from the Continent in earlier times, another land connection, a "Neolithic land-bridge" by which the men of the New Stone Age may have reached Scotland when the upheaved 100-foot terrace was still clothed with the great forest growths that have since disappeared[58].
One begins to ask, Are even 100,000 years sufficient for such oscillations of the surface, upheaval of marine beds, appearance of great estuaries, renewed connection of Britain with the Continent by a "Neolithic land-bridge"? In the Falkirk district neolithic kitchen-middens occur on, or at the base of, the bluffs which overlook the Carse lands, that is, the old sea-coast. In the Carse of Gowrie also a dug-out canoe was found at the very base of the deposits, and immediately above the buried forest-bed of the Tay Valley[59].
That the neolithic period was also of long duration even in Scandinavia has been made evident by Carl Wibling, who calculates that the geological changes on the south-east coast of Sweden (Province of Bleking), since its first occupation by the men of the New Stone Age, must have required a period of "at least 10,000 years[60]."
Still more startling are the results of the protracted researches carried on by J. Nüesch at the now famous station of Schweizersbild, near Schaffhausen in Switzerland[61]. This station was apparently in the continuous occupation of man during both Stone Ages, and here have been collected as many as 14,000 objects belonging to the first, and over 6000 referred to the second period. Although the early settlement was only post-glacial, a point about which there is no room for doubt, L. Laloy[62] has estimated "the absolute duration of both epochs together at from 24,000 to 29,000 years." We may, therefore, ask, If a comparatively recent post-glacial station in Switzerland is about 29,000 years old, how old may a pre- or inter-glacial station be in Gaul or Britain?
The early History of Man a Geological Problem.
From all this we see how fully justified is J. W. Powell's remark that the natural history of early man becomes more and more a geological, and not merely an ethnological problem[63]. We also begin to understand how it is that, after an existence of some five score millenniums, the first specialised human varieties have diverged greatly from the original types, which have thus become almost "ideal quantities," the subjects rather of palaeontological than of strictly anthropological studies.
The Human Varieties the Outcome of their several Environments.
And here another consideration of great moment presents itself. During these long ages some of the groups—most African negroes south of the equator, most Oceanic negroes (Negritoes and Papuans), and Australian and American aborigines—have remained in their original habitats ever since what may be called the first settlement of the earth by man. Others again, the more restless or enterprising peoples, such as the Mongols, Manchus, Turks, Ugro-Finns, Arabs, and most Europeans, have no doubt moved about somewhat freely; but these later migrations, whether hostile or peaceable, have for the most part been confined to regions presenting the same or like physical and climatic conditions. Wherever different climatic zones have been invaded, the intruders have failed to secure a permanent footing, either perishing outright, or disappearing by absorption or more or less complete assimilation to the aboriginal elements. Such are some "black Arabs" in Egyptian Sudan, other Semites and Hamites in Abyssinia and West Sudan (Himyarites, Fulahs and others), Finns and Turks in Hungary and the Balkan Peninsula (Magyars, Bulgars, Osmanli), Portuguese and Netherlanders in Malaysia, English in tropical or sub-tropical lands, such as India, where Eurasian half-breeds alone are capable of founding family groups.