In view of these facts, it is not strange that no advance in culture is noticeable; and the grounds just mentioned may go far to explain why we catch sight, here and there, of bits of customs, habits, and manners of life which strangely remind us of widely distant people—though it will not explain the presence of words of Malay or Chinese origin which are claimed to exist.30 What is known as the Eskimo trace is quite marked in the physical characters and in the arts of the Californians.31 It is, probably, the continuance of the type of the primitive American race. It would naturally be interesting to know whether any date can be given for the Pliocene Age, and so give us some ideas as to the antiquity of man, if he were really here during that epoch. This, however, is one of the most difficult questions to answer, and in the present state of our knowledge incapable of solution. Approximations have, of course, been made, and, as might be expected, vary greatly in results. When it was acknowledged on all hands that on geological grounds the age of the earth was certainly very great, many times the few thousand years hitherto relied on, it is not strange that popular thought swung to the other extreme, and hundreds of millions of years were thought necessary to explain the series of changes which the geologists unfolded. This demand for a greatly extended time was strengthened when the law of the gradual evolution of life was expounded by the modern school of naturalists, and as great a lapse of time as five hundred millions of years was not deemed an extravagant estimate. Sir William Thompson has, however, demonstrated that the time that has elapsed since the crust of the earth became solidified can not be far from one hundred millions of years, and consequently we know the time since the appearance of life must be greatly less than that number of years. Attempts have been made to estimate the length of time required to form the sedimentary crust of the earth. The results are so divergent on this point that it is best not to adopt any standard at present. Our views on this matter are also dependent on the time that has elapsed since the close of the Glacial Age, which, as we have seen, is not yet a settled point. If it be true that the islands of the Pacific commenced to sink during late Tertiary times, then we have a measure of that time in the growth of coral, which has required at least four hundred thousand years to form reefs the thickness of some that are known to exist.32