That the early condition of mankind was substantially as above indicated is not exclusively a recent, nor even a modern opinion. Some of the ancient poets and philosophers recognized the fact, that mankind commenced in a state of extreme rudeness from which they had risen by slow and successive steps. They also perceived that the course of their development was registered by a progressive series of inventions and discoveries, but without noticing as fully the more conclusive argument from social institutions.
The important question of the ratio of this progress, which has a direct bearing upon the relative length of the several ethnical periods, now presents itself. Human progress, from first to last, has been in a ratio not rigorously but essentially geometrical. This is plain on the face of the facts; and it could not, theoretically, have occurred in any other way. Every item of absolute knowledge gained became a factor in further acquisitions, until the present complexity of knowledge was attained. Consequently, while progress was slowest in time in the first period, and most rapid in the last, the relative amount may have been greatest in the first, when the achievements of either period are considered in their relations to the sum. It may be suggested, as not improbable of ultimate recognition, that the progress of mankind in the period of savagery, in its relations to the sum of human progress, was greater in degree than it was afterwards in the three sub-periods of barbarism; and that the progress made in the whole period of barbarism was, in like manner, greater in degree than it has been since in the entire period of civilization.
What may have been the relative length of these ethnical periods is also a fair subject of speculation. An exact measure is not attainable, but an approximation may be attempted. On the theory of geometrical progression, the period of savagery was necessarily longer in duration than the period of barbarism, as the latter was longer than the period of civilization. If we assume a hundred thousand years as the measure of man's existence upon the earth in order to find the relative length of each period,--and for this purpose, it may have been longer or shorter,--it will be seen at once that at least sixty thousand years must be assigned to the period of savagery. Three-fifths of the life of the most advanced portion of the human race, on this apportionment, were spent in savagery. Of the remaining years, twenty thousand, or one-fifth, should be assigned to the Older Period of barbarism. For the Middle and Later Periods there remain fifteen thousand years, leaving five thousand, more or less, for the period of civilization.