“Secondly, while the same fossils prevail in a particular set of strata for hundreds of miles in a horizontal direction, we seldom meet with the same remains for many fathoms, and very rarely for several hundred yards, in a vertical line, or a line transverse to the strata. This fact has now been verified in almost all parts of the globe, and has led to a conviction, that at successive periods of the past, the same area of land and water has been inhabited by species of animals and plants even more distinct than those which now people the antipodes, or which now coexist in the Arctic, temperate, and tropical zones. It appears that, from the remotest periods, there has been ever a _coming in of new organic forms_, and an _extinction of those which pre-existed on the earth_, some species having endured for a longer, others for a shorter time; while none have ever reappeared after once dying out. The law which has governed the creation and extinction of species seems to be expressed in the verse of the poet— ‘Nature made him, and then broke the die;’ and this circumstance it is which confers on fossils their highest value as chronological tests, giving to each of them, in the eyes of the geologist, that authority which belongs to contemporary medals in history. The same cannot be said of each peculiar variety of rock; for some of these, as red-marl and red sandstone for example, may occur at once upon the top, bottom, and middle of the entire sedimentary series; exhibiting in each position so perfect an identity of mineral aspect, as to be undistinguishable. Such exact repetitions, however, of the same mixtures of sediment, have not often been produced, at distant periods, in precisely the same parts of the globe; and even where this has happened, we are seldom in any danger of confounding together the monuments of remote eras, when we have studied their imbedded fossils and relative position.”[114]