living beings are not supplied even to the extent he himself requires, is answered in the expression of Mr. Darwin already quoted, namely, "the imperfection of the geological record." No fact of geology is more patent than that, to use Sir Charles Lyell's words, "it is not part of the plan of Nature to write everywhere, and at all times, her autobiographical memoirs. On the contrary," continues this late distinguished scientist, "her annals are local and exceptional from the first, and portions of them are afterwards ground into mud, sand, and pebbles, to furnish materials for new strata." The very process of rock-formation consists in the rearrangement of the particles of previously formed materials, and the manufacture of new strata implies the destruction of the old with the included "fossils" of the latter. The geological series is thus certainly a detached and discontinuous collection of formations, interrupted by gaps of considerable and often undeterminable extent. Of the contemporaneous life-history of the globe, during the periods of time represented by such gaps, we have no record whatever. But even when the materials for forming a detailed history of any past period of our globe are found in tolerable plenty, the record is never complete. "We can never hope," says Lyell in a most emphatic passage on breaks in the sequence of rock formations, "to compile a consecutive history by gathering together monuments which were originally detached and scattered over the globe. For, as the species of organic beings contemporaneously inhabiting remote regions are distinct, the fossils of the first of several periods which may be preserved in any one country, as in America, for example, will have no connection with those of a second period found in India, and will, therefore, no more enable us to trace the signs of a gradual change in the living creation, than a fragment of Chinese history will fill up a blank in the political annals of Europe." Add to these considerations the brief chronicle of a long and important chapter of geological history, namely, that soft-bodied animals and plants are rarely preserved as fossils; that land animals are but sparsely represented in any formations as compared with marine forms; and that even "metamorphism," or the alteration of rocks subsequent to their formation, is known to alter and obliterate their fossil contents, and we find reasons of the most stable and satisfactory kind for the imperfect nature of even the fullest records of rocks and of their fossils that man has been able to obtain.

But in what direction does the positive evidence we have been able to obtain lead? Clearly to the side of evolution, and towards the supply of "missing links" in a fashion which even the most sanguine expectations of scientific ardour could scarcely have hoped to see realised. Bearing in mind that vast tracts of rock-formations are as yet absolutely unexplored, the present subject is seen to be one to which each year brings its quota of new and strange revelations. And at

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the most, any one record of what has been done towards supplying missing links" must be held to be merely provisional, and to serve but as a prelude to the discoveries of a succeeding period. Especially within the last few years, however, has the evidence of the existence of animals which may fairly be deemed "missing links accumulated in a very marked degree, and in some cases in a very astonishing fashion. The reader has but to become informed of recent discoveries amidst the Tertiary rocks of North America, to learn the surprising revelations concerning intermediate forms between existing groups of mammals or quadrupeds, which, chiefly through the researches of Professor Marsh, have been added to the conquests of science. What, for example, is to be said of the zoological position of the huge Dinoceras (Fig. 71) and its allies, creatures as large as existing elephants, and which, from the examination of their skeletal remains, can at the best be regarded as intermediate betwixt the elephants themselves, and the odd-toed Ungulates (or hoofed quad

rupeds), such as the rhinoceroses, &c.? Dinoceras thus possessed two large canine teeth (cc), six small molars (m) on each side, and four horncores (h1h2), besides a pair of similar structures in front of the upper jaw. Or, again, which rank, save that of an intermediate position, and as a veritable group of "missing links," can be assigned to the extinct quadrupeds, included by Marsh under the collective name Tillodontia, the remains of which occur in the Eocene Tertiaries of the United States? For how else should we classify animals with great front teeth like the Rodents or "gnawers," grinders like the Ungulates or hoofed quadrupeds, and a skull and skeleton generally like that of the carnivorous Bears? Or, once more, what can be said of the affinities or relationship of the extinct Toxodonts, also from American deposits, in which the characters of Rodents are united to those of Ungulates and Edentates-the latter being a group of animals represented by the existing sloths, armadillos, and ant-eaters? Nor is the list of extinct quadrupeds which fall into no existing group, but present a union of the characters of several distinct divisions, exhausted with the foregoing brief chronicle. Again drawing upon the well-nigh inexhaustible treasure-house of geological specimens in the recent deposits of the New World, we find the extinct Marauchenia connecting the odd-toed hoofed mammals

with the even-toed division. Passing to the whales and their kin, we find the extinct Zeuglodon with its well-developed teeth—a feature unusual in living whales-appearing to connect the whale tribe with the seals and their

allies. Similarly, the curious Anoplotherium (Fig. 73) of the Eocene Tertiary deposits appears to connect the swine race with the true cud-chewers or Ruminants, just as the Palæotherium (Fig. 72) itself-one of the first animals

FIG. 72.-PALæotherium (restored).

whose remains were disinterred from Montmartre-connects the pigs and tapirs with the apparently far-removed rhinoceros. The case for the existence of "missing links," wherewith the at present distinct orders and sub-orders of quadrupeds may be connected, would thus seem to be very strong. There would appear to be more than sufficient cause to account for the hopeful spirit of the evolutionist, whose scientific prophecy, that philosophic research into the nature of fossil organisms-begun by Cuvier, in the now classical quarries of Montmartre is destined to powerfully aid his cause, seems likely to be realised. When it lies in the power of the naturalist to point, as well he may, with pride, to the perfect series of forms and missing links which connect the one

toed horse of to-day with the curious three, four, and fivetoed steeds of the past, one may overlook the jubilant tone of the evolutionist in the more silent and deeper satisfaction with which mankind at large is given to welcome the demonstration of a great truth. It is of such a demonstration that Huxley writes: "On the evidence of paleontology, the evolution of many existing forms of animal life from their predecessors is no longer an hypothesis, but an historical fact; it is only," he adds, "the nature of the physiological factors to which that evolution is due which is still open to discussion.”

FIG. 73. RESTORATION OF ANOPLOTHERIUM.

But not merely in the highest class of the animal world have "intermediate forms" been discovered. The case for evolution

grows in interest when we learn that in lower ranks of Vertebrate life, groups of animals, separated apparently by the widest of intervals, are now being linked together by the discovery of intermediate fossil forms. The best-known example of the latter facts is found in the relationship which may be now regarded as being clearly proved to exist between reptiles and birds. Were we to search the whole animal kingdom through for examples of creatures of thoroughly different appearance, habits, and general conformation, no two groups would fall more familiarly to hand than birds and reptiles. There would, indeed, appear to be no similarity or likeness between the secretary bird, which daily devours its quota of snakes, and the prey upon which it lives; or, reversing the comparison, betwixt the unfortunate bird and the serpent whose stony gaze has allured it literally to a living death. Activity of organisation on the one hand would be opposed by a torpidity of action on the other; beauty of form and colour, by appearances frequently grotesque, and often, in popular estimation at least, repulsive. The contrast is one which, in the popular view, would be complete and perfect in every respect. Birds are warm-blooded, and have a four-chambered heart: reptiles possess a slow circulation, a low blood-temperature, and a three-chambered heart, which, however, in

the crocodiles becomes fourchambered. The former class is covered with feathers, the latter with scales, bony plates, or both. The fore-limbs, modified for flight in the bird, are never thus used in reptilesthe so-called "flying lizards" (Fig. 74) possessing no true powers of flight, but being enabled by a parachute-like arrangement of their front ribs to take flying leaps from tree to tree. Birds, as we well know, want teeth; and although in tortoises and turtles, as typical enough reptiles, a dental apparatus is also wanting, the reptilian character tends de

cidedly towards a large and perfect display of teeth.

A closer inspection and comparison of the skeletons of the two groups, such as may be made in a very general review of their bony possessions, would reveal several interesting points of likeness and also of divergence. Thus both classes have a lower jaw which may

be called "compound;" since, unlike the simple two-halved lower jaw of quadrupeds, that of birds and reptiles is composed of numerous pieces united to form the single bone. Then, also, this lower jaw is joined to the skull, not of itself and directly, as in man and quadrupeds, but by a special bone named the quadrate, which, curiously enough, by a wonderful process of alteration and metamorphosis, becomes represented in man and quadrupeds by one (the malleus) of the small bones of the ear (see page 135). Such, among others, are a few points of agreement between reptiles and birds.

But plain grounds of distinction are apparent within the same region of "dry bones." A bird has never more than three fingers (thumb [g], and two next digits [d, e, f]) in its "hand" or wing (Fig. 75); and the supporting bones of these fingers, correspond

ing to our "palm," are united together. The reptile's fingers are never so few as three, and their palm-bones, moreover, are not ossified together. The "merrythought" of the bird (Fig. 76, jj), indissolubly associated with. mystic forebodings of hymeneal nature, consists of the two united "collar-bones;" such a disposition of the collar-bones being unknown in the more prosaic reptilians; and the great "keel" (f) seen on the bird's breast-bone (g) is wanting on that of living reptiles. Next in order, we find that the sacrum, or bone wedged in between the haunch-bones, consists, in birds, of a goodly number of vertebræ or joints of the spine, whereas, in the reptile, one or two vertebræ form the sacrum. In all birds, save the ostrich tribe, the two haunch-bones (Fig. 76, p, r) are not united below or in front in the middle line. In reptiles such a union does take place, this union, indeed, being also seen in man and quadrupeds. In birds, the tail terminates in a "ploughshare-bone" (Fig. 76, d), giving support to the oil gland, the secretion of which is used in preening the feathers. In reptiles no such bone exists, and the joints of the tail simply taper towards the extremity of the appendage. The axis of the thigh-bone (t) in the bird, like that of quadrupeds, lies parallel with the median plane or axis of the body; but in reptiles, the axis of the thigh makes an open angle of varying dimensions with the median plane.

FIG. 75.
SKELETON OF BIRD'S WING.

The ankle of the bird (Fig. 77) is peculiarly formed, inasmuch as the upper half of the ankle, or "tarsus" (a), becomes united to the lower end of the shin-bone or leg (t); whilst the lower half of the ankle unites with the bones corresponding to those of man's instep, the union