was found imbedded many feet below the surface in undisturbed beds of sand or gravel and geologists tell us the age of the sand bed, the age of the remains must be at

FIGURE 13. An infant, three weeks old, supporting its
own weight for over two minutes. The attitude of
the lower limbs, feet, and toes is strikingly simian.

least as old as the sand bed. Geologists are able to estimate with approximate accuracy the age of certain deposits of sand or gravel by determining the rate at which similar beds are being formed at the present day through the agency of rivers or glaciers. In this way we may be certain of the age of these remains within a negligible error. It is to be remembered that geologists measure

time in thousands and millions of years.13 Geologists divide the time of the earth's development from an uninhabitable sphere to its present state, into several great epochs in accordance with the type of rock formation existing. The Paleozoic or Primary and the Mesozoic or Secondary, cover the vast epochs when only the most rudimentary form of life existed. It is the Tertiary and the Quaternary, the periods during which Mammals appeared, that are of interest to us. As will be seen from the diagram, the early or lower quaternary began about three-quarters of a million years ago. The first fossil remains of prehistoric man come from the geological formation of this period.

Before we may hope to have a clear understanding of the antiquity of man there is some further evidence of a geological nature which we shall have to examine.

It is important to know the main facts and theories of the glacial periods because it is the duration and frequency of occurrence of these ice ages which give us the most trustworthy evidence of the antiquity of man. If we can learn the age of certain glacial deposits by measuring their rate of formation, we are in a position to say something definite as to the age of human remains and implements found beneath them. It is a generally accepted fact that many ages ago there were vast sheets of continental ice mantling large portions of Europe and North America, just as Greenland is mantled to-day.11 But it is not so generally known that the cold was not constant during the age of ice. The first southward advance of the Arctic ice-sheet was followed by a period of retreat during which temperate conditions prevailed. In 13 See figure 14 for the comparative magnitude of geologic time and the age of human remains. 14 See figure 15.

regions once inhabited by cold climate fauna one finds creatures whose structure and habits show them to have been denizens of warmer lands. There were climatic fluctuations with alternate advance and retreat of the ice. Modern geologists count four glacial epochs covering the

FIGURE 15. Europe during the Period of Maximum Glaciation. period of the Pleistocene (see quaternary in diagram)." The causes of these great climatic fluctuations which brought about the ice ages are variously explained by geologists in accordance with three hypotheses.16 In the course of these ice ages the glaciers pushed southward 15 Lull, R. S.-"Glacial Man," The Yale Review, vol. 1, N. S., 1912, p. 377.

16 See Chamberlain and Salisbury,-Geology, vol. iii, pp. 424-446.