5. The absolute date of the first appearance of man cannot perhaps be fixed within a few years or centuries, either by human chronology or by the science of the earth. It would seem, however, that the Bible history, as well as such hints as we can gather from the history of other nations, limits us to two or three thousand years before the Deluge of Noah, while some estimates of the antiquity of man, based on physical changes or ancient history, or on philology, greatly exceed this limit. If the earliest men were those of the river gravels and caves, men of the 'mammoth age,' or of the 'palæolithic' or palæocosmic period, we can form some definite ideas as to their possible antiquity. They colonised the continents immediately after the elevation of the land from the great subsidence which closed the pleistocene or glacial period, in what has been called the 'continental' period of the post-glacial age, because the new lands then raised out of the sea exceeded in extent those which we have now. We have, as stated in a previous chapter, some measures of the date of this great continental elevation, and know that its distance from our time must fall within about eight thousand years. Many indications, both in Europe and America, lead to the belief that it is physically impossible that man could have colonised the northern hemisphere at an earlier date than this geologically recent continental period. 6. There is but one species of man, though many races and varieties; and these races or varieties seem to have developed themselves at a very early time and have shown a remarkable fixity in their later history. There is reason to believe, however, from various physiological facts, that this is a very general law of varietal forms, which are observed to appear rapidly or suddenly, and then in favourable circumstances to be propagated continuously. It would seem also to apply to the introduction of forms regarded as species, since it is not unusual to find a genus at or near its origin represented by its maximum number of specific forms.