We have now reviewed the evidence which leads us to believe that man is related to forms of life still extant. This evidence constitutes a presumption which justifies us in the belief that we shall discover the intermediate forms and so partially complete the series of man's descent. The gaps in this series must be filled by the reconstructed skeletons of bone remains of prehistoric man. In consideration of the fact that bone usually decays within a comparatively short time, the chances are slight of finding remains in a sufficient state of preservation to constitute positive evidence. Under certain peculiar conditions, these bone remains are preserved for great periods of time. They must be protected from the action of the air, the corrosive action of water, and from the destructive action of insects and certain plant agencies which cause decay. The necessary conditions are present in dry caves and where natural agencies have deposited layers of sand and gravel. Hence it is that the bone remains of prehistoric man are most frequently found in undisturbed boulder-clays and drift, or imbedded in the floor of some cave of great antiquity. Investigation and discovery have brought to light a considerable number of bone remains of prehistoric man. We shall, therefore, examine this evidence to ascertain how far we may expect to reconstruct the intermediate steps in the descent of man.

In the first place how are we able to tell certainly whether any bone remains which we find are reminis cent of prehistoric man? Is it not possible that they are simply the remains of some relatively modern pathological individual and not of some lower type of man? The associated circumstances are of utmost importance. If the particular part of a skull which we have discovered

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

From Romanes, "Darwin and after Darwin."

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 forms of life existed. It is the Tertiary and the Quaternary, the periods during which the higher 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.14 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).15 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.