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. Nueesch 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?