Now, this surely is a process quite as nearly akin to the incubation of egg-bearing birds as to the ordinary nursing process of the placental mammals; and on the occult but apparently real principle, that the true arrangement of the animal kingdom is that which we find exemplified by the successive introduction of its various classes and orders in the course of geologic history, should we not anticipate a point of time for the introduction of the marsupiata, intermediate between the widely-distant points at which the egg-bearing birds and the true placental mammals appeared? Ranged at once chronologically, and by their mode of reproduction, the various classes of the vertebrata would run, did we accept the suggested reading, as follows:--First appear cold-blooded vertebrates (fishes), that propagate by eggs or spawn,--chiefly by the latter. Next appear cold-blooded vertebrates (reptiles), that propagate by eggs or spawn,--chiefly by the former. Then appear warm-blooded vertebrates (birds), that propagate by eggs exclusively. Then warm-blooded vertebrates come upon the stage, that produce _eggs_ without shells, which have to be subjected for months to a species of extra-placental incubation. And last of all the true placental mammals appear. And thus, tried by the test of perfect reproduction, the great vertebral division receives its full development in creation. [Illustration: Fig. 70. ANOPLTHERIUM COMMUNE. (_Eocene._)] The placental mammals make their appearance, as I have said, in the earliest ages of the great Tertiary division, and exhibit in the group an aspect very unlike that which they at present bear. The Eocene ages were peculiarly the ages of the Palæotheres,--strange animals of that pachydermatous or thick-skinned order to which the elephants, the tapirs, the hogs, and the horses belong. It had been remarked by naturalists, that there are fewer families of this order in living nature than of almost any other, and that, of the existing genera, not a few are widely separated in their analogies from the others. But in the Palæotheres of the Eocene, which ranged in size from a large horse to a hare, not a few of the missing links have been found,--links connecting the tapirs to the hogs, and the hogs to the Palæotheres proper; and there is at least one species suggestive of an union of some of the more peculiar traits of the tapirs and the horses. It was among these extinct Pachydermata of the Paris basin that Cuvier effected his wonderful restorations, and produced those figures in outline which are now as familiar to the geologist as any of the forms of the existing animals. The London Clay and the Eocene of the Isle of Wight have also yielded numerous specimens of those pachyderms, whose identity with the Continental ones has been established by Owen; but they are more fragmentary, and their state of keeping less perfect, than those furnished by the gypsum quarries of Velay and Montmartre. In these the