The physical as well as the human history of the world has its mythical age, lying dim and vague in the morning mists of creation, like that of the heroes and demigods in the early traditions of man, defying all our ordinary dates and measures. But if the succession of periods that prepared the earth for the coming of man, and the animals and plants that accompany him on earth, baffles our finite attempts to estimate its duration, have we any means of determining even approximately the length of the period to which we ourselves belong? If so, it may furnish us with some data for the further solution of these wonderful mysteries of time, and it is besides of especial importance with reference to the question of permanence of Species. Those who maintain the mutability of Species, and account for all the variety of life on earth by the gradual changes wrought by time and circumstances, do not accept historical evidence as affecting the question at all. The monuments of those oldest nations, all whose history is preserved in monumental records, do not indicate the slightest variation of organic types from that day to this. The animals that were preserved within their tombs or carved upon their walls by the ancient Egyptians were the same as those that have their home in the valley of the Nile today; the negro, whose peculiar features are unmistakable even in their rude artistic attempts to represent them, was the same woolly-haired, thick-lipped, flat-nosed, dark-skinned being in the days of the Rameses that he is now. The Apis, the Ibis, the Crocodiles, the sacred Beetles, have brought down to us unchanged all the characters that superstition hallowed in those early days. The stony face of the Sphinx is not more true to its past, nor the massive architecture of the Pyramids more unchanged, than they are. But the advocates of the mutability of Species say truly enough that the most ancient traditions are but as yesterday in the world's history, and that what six thousand years could not do sixty thousand years might effect. Leaving aside, then, all historical chronology, how far back can we trace our own geological period, and the Species belonging to it? By what means can we determine its duration? Within what limits, by what standard, may it be measured? Shall hundreds, or thousands, or hundreds of thousands, or millions of years be the unit from which we start?