The peat-mosses of Denmark have been appealed to as affording something like a conjectural date for the early neolithic period in that country. These are formed in hollows of the glacial drift, which have been small lakes or ponds in the midst of forests, into which trees have fallen, and which have become gradually converted into peat by the growth of marsh plants. It is clearly established that there have been three successive ages of forest growth, the upper one of beech, below it one of oak, and lowest of all one of fir. The implements and relics found in the beech stratum are all modern, those in the oak stratum are of the later neolithic and bronze ages, and those in the lowest, or fir-horizon, are earlier and ruder neolithic, resembling those found in the older lake-villages and shell-mounds. Now beech has been the characteristic forest tree of Denmark certainly since the Roman period, or for 2000 years, and no one can say for how much longer. If the speculations as to the origin of the Aryan race in Northern Europe are correct, it must have been for very much longer, as the word for beech is common to so many of the dialects into which the primitive Aryan language became divided. The stages of oaks and firs must equally have been of long duration, and the different stages could only have been brought about by slow secular variations of climate during the post-glacial period. Still this affords no reliable information as to specific dates, and we can only take Steenstrup's calculation of from 4000 to 16,000 years for the formation of some of these peat-bogs as a very vague estimate, and this only carries us back to a time when Egypt and Chaldæa must have been already densely peopled, and far advanced in civilization. On the whole, it seems that the neolithic arrow-heads found in Egypt, and the fragments of pottery brought up by borings through the deposits of the Nile, are the oldest certain human relics of the neolithic age which have yet been discovered, and these do not carry us back further than a possible date of 15,000 or 20,000 years B.C.