diligently searched for, have yet been discovered in digging trenches through the red mud. It is true that, in a few spots, the bones of birds have been met with plentifully in the older tertiary strata, but always in rocks of freshwater origin, such as the Paris gypsum or the lacustrine limestone of the Limagne d'Auvergne. In strata of the same age, in Belgium and other European countries, or in the United States, where no less careful search has been made, few, if any, fossil birds have come to light.

We ought, therefore, most clearly to perceive that it is no part of the plan of Nature to hand down to after times a complete or systematic record of the former history of the animate world. The preservation of the relics, even of aquatic tribes of animals, is an exception to the general rule, although time may so multiply exceptional cases that they may seem to constitute the rule; and may thus impose upon the imagination, leading us to infer the non-existence of creatures of which no monuments are extant. Hitherto our acquaintance with the birds, and even the Mammalia, of the Eocene period has depended, almost everywhere, on single specimens, or on a few individuals found in one spot. It has therefore depended on what we commonly call chance; and we must not wonder if the casual discovery of a tertiary, secondary, or primary rock, rich in fossil impressions of the foot-prints of birds or quadrupeds, should modify or suddenly overthrow all theories based on negative facts.

The chief reason why we meet more readily with the remains of every class in tertiary than in secondary strata, is simply that the older rocks are more and more exclusively marine in proportion as we depart farther and farther from periods during which the existing continents were built up. The secondary and primary formations are, for the most part, marine,—not because the ocean was more universal in past times, but because the epochs which preceded the Eocene were so distant from our own, that entire continents have been since submerged.

I have alluded at p. 299. to Mr. Darwin's account of the South American Ostriches, seen on the coast of Buenos Ayres, walking at low water over extensive mud-banks, which are then dry, for the sake of feeding on small fish. Perhaps no bird of such perfect organization as the eagle or vulture may ever accompany these ostriches. Certainly, we cannot expect the condor of the Andes to leave its trail on such a shore; and no traveller, after searching for footprints along the whole eastern coast of South America, would venture to speculate, from the results of such an inquiry, on the extent, variety, or development of the feathered Fauna of the interior of that continent.

The absence of Cetacea from rocks older than the Eocene has been frequently adduced as lending countenance to the theory of the late appearance of the highest class of Vertebrata on the earth. That we have hitherto failed to detect them in the Oolite or Trias, does not imply, as we have now seen, that Mammalia were not then created. Even in the Eocene strata of Europe, the discovery of Cetaceans has never kept pace with that of land quadrupeds. The only instance cited in Great Britain is a species of Monodon, from the London clay, of doubtful authenticity as to its geological position. On the other hand, the gigantic Zeuglodon of North America (see p. 207.), occurs abundantly in the Middle Eocene strata of Georgia and Alabama, from which as yet no bones of land-quadrupeds have been obtained.

Professor Sedgwick states in a recent work[xxi-A], that he possesses in the Woodwardian Museum, a mass of anchylosed cervical vertebræ of a whale which he found near Ely, and which he believes to have been washed out of the Kimmeridge clay, a member of the Upper Oolite; but its true geological site is not well determined. It differs, says Professor Owen, from any other known fossil or recent whale.

In the present imperfect state then of our information, we can scarcely say more than that the Cetacea may have been scarce, in the secondary and primary periods. It is quite conceivable that when aquatic saurians, some of them carnivorous, like the Ichthyosaurus, were swarming in the sea, and when there were large herbivorous reptiles, like the Iguanodon, on the land, such reptiles may, to a certain extent, have superseded the Cetacea, and discharged their functions in the animal economy.

The views which I proposed originally in the Principles of Geology in opposition to the theory of progressive development may be thus briefly explained. From the earliest period at which plants and animals can be proved to have existed, there has been a continual change going on in the position of land and sea, accompanied by great fluctuations of climate. To these ever-varying geographical and climatal conditions the state of the animate world has been unceasingly adapted. No satisfactory proof has yet been discovered of the gradual passage of the earth from a chaotic to a more habitable state, nor of a law of progressive development governing the extinction and renovation of species, and causing the Fauna and Flora to pass from an embryonic to a more perfect condition, from a simple to a more complex organization.

The principle of adaptation above alluded to, appears to have been analogous to that which now peoples the arctic, temperate, and tropical regions contemporaneously with distinct assemblages of species and genera, or which independently of mere temperature gives rise to a predominance of the marsupial tribe of quadrupeds in Australia, and of the placental tribe in Asia and Europe, or to a profusion of reptiles without mammalia in the Galapagos Archipelago, and of mammalia without reptiles in Greenland.[xxii-A]

This theory implies, almost necessarily, a very unequal representation at successive periods of the principal classes and orders of plants and animals, if not in the whole globe, at least throughout very wide areas. Thus, for example, the proportional number of genera, species, and individuals in the vertebrate class may differ, in two different and distinct epochs, to an extent unparalleled by any two contemporaneous Faunas, because in the course of millions of ages, the contrast of climate and geographical conditions may exceed the difference now observable in polar and equatorial latitudes.

I shall conclude by observing, that if the doctrine of successive development had been paleontologically true, as the new discoveries above enumerated show that it is not; if the sponge, the cephalopod, the fish, the reptile, the bird, and the mammifer had followed each other in regular chronological order—the creation of each class being separated from the other by vast intervals of time; and if it were admitted that Man was created last of all, still we should by no means be able to recognize, in his entrance upon the earth, the last term of one and the same series of progressive developments. For the superiority of Man, as compared to the irrational mammalia, is one of kind, rather than of degree, consisting in a rational and moral nature, with an intellect capable of indefinite progression, and not in the perfection of his physical organization, or those instincts in which he resembles the brutes. He may be considered as a link in the