ARCHAEOPTERYX. The discovery of the impression of a feather in the lithographic limestone of Solenhofen, Bavaria, has enabled us to trace back the descent of birds of Jurassic times; though it afforded no clue as to the type of bird to which it belonged. This feather was named Archaeopteryx lit/iographica, by Hermann von Meyer, so far back as 1861. Later in the same year, and from the same quarry, a very imperfect skeleton was found, together with the "flight-feathers" of the wings and the feathers of the tail. These remains, it is of importance to notice, were those of a "perching-bird," though of a vastly more primi tive type than any now existing. This specimen is now in the British Museum of Natural History, and was described by Owen under the name Archaeopteryx macroura.
In 1872 a second and nearly perfect specimen was f ound again in the same quarry—and is now in the Berlin Museum. That it represents a distinct species of the genus Archaeopteryx is generally admitted. The adjoining text-cut (fig. I) seems to justify the assumption that it fell to the bottom of some shallow lake, and was slowly covered with a deposit of fine mud, so that the bones, after the disintegration of the flesh, remained undis turbed. The interest and importance of these two specimens cannot easily be exaggerated. They have formed the theme of endless discussion by men of science; though from a lack of familiarity with avian anatomy the conclusions at which they have arrived are sometimes surprising.
That birds and reptiles are derivatives from the same common stock is convincingly demonstrated in the structure of living birds; but in Archaeopteryx we find what the evolutionist would expect : stages in the transformation from reptile to bird, to be seen in modern birds only during embryonic and early post-embryonic life. The jaws were armed with teeth, as were those of Ichthyor nis and Hesperornis of the later Cretaceous epoch. These teeth are reptilian heritages. The tail furnishes still more important evidence. In Archaeopteryx, as in the reptiles, this was formed of a long chain of vertebrae. The last i 2 bore each a pair of stiff shafted feathers, shown in fig. 2, directed backwards and outwards. In modern birds the tail-feathers, precisely similar in character, are arranged fan-wise, and fixed by their bases to a median lamina of bone, the "pygostyle." When this lamina is examined in the embryo it is found to be composed of a number of separate ele ments, answering to from six to seven originally separate verte brae. In other words, the tail of modern birds has been derived by a process of "telescoping" the vertebrae, so that the pairs of feathers they originally supported have come to lie within a semicircle embracing what is now a "pygostyle," but what was once a number of vertebrae in linear series, as seen in fig. 2.
The wing is no less remarkable in this connection. In its es sential features it agrees with the late embryonic and early post embryonic stages of living birds. Thus, in modern birds the three metacarpal bones, answering to the bones of the palm of the band, can be separated only during the early stages of develop ment. In Archaeopteryx they never became welded together. In living birds the first and sometimes the second digit may termi nate in a claw, but the third digit has been found to possess a claw only in one or two cases of embryos of the ostrich. The distal, or second row of wrist-bones in Archaeopteryx were welded together to form a semi-circular nodule of bone closely applied to the bases of the metacarpals. In modern birds a pre cisely similar welding together of the wrist-bones is found, but very early in post-embryonic life, and sometimes before, the fused mass comes to form one common bony tissue with the metacarpals. That the wing of Archaeopteryx was evolved pari passu with the feathers in response to the movements peculiar to the requirements of flight scarcely admits of doubt ; though quite different functions have been claimed for it. The problem of the evolution of the skeleton of the wing cannot be divorced from the study of the indubitable "flight-feathers" which it sup ported. These, as in living birds, are divisible into the outermost, or "primaries," borne by the hand, and the "secondaries," borne by the fore-arm. They are as sharply defined as in modern birds, and this could not have come about save as a response to the same stimuli. In other words, they have the same form and ar rangement because they served the same function. In one par ticular the wing of Archaeopteryx seems to differ from that of living birds, and this has proved a stumbling-block to many who have discussed this theme after no more than a very desultory survey of the facts. This difference lies in the fact that the sec ond and third digits projected beyond the outermost primaries and terminated each in a claw—a condition that has been inter preted to show that the wing was used rather for climbing than as an organ of flight. We have a parallel in the wing of the nest ling Hoatzin, and also in the wings of nestling Gallinaceous birds. In the Hoatzin the thumb and first finger, which is very long, are armed with claws which enable the wing to be used for climbing along the branches supporting the nest. Soon the "flight-feathers" of the hand begin to grow, but the development of the outermost primaries is inhibited till the inner feathers have provided a wing surface sufficient to break the force of a fall. Not till then do the outer primaries make their appearance, and at the same time the claw at the tip of the second digit disappears, while by changes in the rate of growth the hand becomes greatly short ened relatively to the fore-arm. The condition of the wing at the time of the inhibition of the primaries answers to the adult condition of the wing of Archaeopteryx, and suggests that the projecting second and third digits, and their claws, were used as climbing-hooks during the annual moult when all the quills were shed at once, as in the Anatidae, for example, to-day.
The shoulder-girdle of Archaeopteryx is very "reptilian," es pecially in regard to the coracoid—the shaft projecting from the sternum for the support of the wing. Of the sternum we know nothing. The absence of a median keel to the sternal plate has been postulated on what is, at present, mere guesswork. The pelvis, though emphatically avian in type, presents many peculiar and interesting features. Of these one of the most striking is seen in the pubes, which met towards their hinder ends to form an elongated, triangular plate ; restorations which have been made of the pelvic-girdle have left out of account the probable cartilaginous areas of the hinder border of the ilium and ischium, seen in the late embryonic and early post-embryonic pelves of modern birds. When these are added, a much more familiar look is given to the whole structure. The foot of Archaeopteryx is profoundly interesting, since, had it alone been found, it would have been regarded as that of a small Corvine bird. This means that it had already become transformed into the typical "Pas serine" type of foot adapted both for perching and walking.