What holds good of the Chinese now, probably holds good of their ancestors and the race from which they parted company in Central Asia five thousand years ago, when they pierced their way eastwards through the savage aborigines of Thibet and Mongolia, pushing aside tribes which closed in again behind them, so as to intercept their return or communication with their mother country--a country which may have been equally careless of elaborating stupendous and permanent works of architecture such as other nations glory in possessing, and which, like the pyramids of Egypt and of Central America, stand forth for thousands of years as landmarks of the past. We must, therefore, not be surprised if we do not immediately discover the vestiges of the people of ten, fifteen, or twenty thousand years ago. With an ephemeral architecture (which, as we have seen, is all that a highly populous and long civilized race actually possess), the sites of vast cities may have become entirely lost to recollection in a few thousands of years from natural decay, and how much more so would this be the case if, as we may reasonably argue, minor cataclysms have intervened, such as local inundations, earthquakes, deposition of volcanic ashes from even distant sources, the spread of sandy deserts, destruction of life by exceptionally deadly pestilence, by miasma, or by the outpour of sulphurous fumes. We have shown in another chapter how the process of extinction of species continues to the present day, and from the nature of this process we may deduce that the number of species which became extinct during the four or five thousand years preceding the era of exact history must have been considerable. The less remarkable of these would expire unnoticed; and only those distinguished by their size, ferocity, and dangerous qualities, or by some striking peculiarity, would leave their impress on the mythology of their habitat. Their exact history would be lost as the cities of their epoch crumbled away, and during the passage through dark ages of the people of their period and their descendants, and by conquest or catastrophes such as we have referred to elsewhere; while the slow dispersion which appears to have obtained among all nations would render the record of their qualities the more confused as the myth which embalmed it spread in circling waves farther and farther from its original centre.