Figs. 10–13.—10. Fore-limb of bat. 11. Bird. 12. Archæopteryx. 13. Pterodactyl. (Lettered as in previous figures; grouped from various sources.)
2. The coracoid (c), it is seen, is a small, beak-like process of the blade-bone (scapula) in man and mammals; but in birds ([Fig. 11]) and reptiles ([Figs. 14], [18]) it is a separate bone as large as the blade-bone itself, jointed with the latter at the shoulder and with the breast-bone (sternum) in front, thus making together a strong shoulder-girdle for the attachment of the fore-limb. This was undoubtedly the condition in the original or earliest walking animal, viz., reptiles. It was inherited and retained by birds, because necessary for powerful action of the wings in flight. In mammals it gradually dwindled and became united with the blade-bone as a process. In one mammal, the lowest and most reptilian living—the ornithorhynchus—the coracoid is much like that of reptiles—a large, flat bone, separated from the blade-bone and articulated with the breast-bone. It is a significant fact that, in the mammalian embryo, it is first developed as a separate bone and afterward united with the scapula.
Figs. 14–17.—14. Fore-limb of turtle. 15. Mole. 16. Whale. 17. Fish.
3. In man, monkeys, bears, and some other mammals, the limb is fairly free from the body and the elbow half-way down the limb; while in herbivores ([Figs. 8, 9]), such as the horse, ox, and deer, etc., the elbow is high on the side of the body, and the limb is free only from the elbow downward. Perhaps in these cases most observers do not recognize it as an elbow at all. All gradations between these extremes are easily traced. The free condition of the limb is evidently the original one, the condition in herbivores being an extreme modification associated with another modification mentioned under 5.
4. In man and in many mammals, and in all reptiles and birds, there are two bones in the forearm (radius and ulna). In the more specialized forms of hoofed animals (ungulates), such as horse and ruminants ([Figs. 8, 9]), there is apparently but one. Two is the normal and original number; but one of them, the ulna, has gradually become smaller and smaller, and finally is reduced to a short splint, and consolidated with the radius as a process extending backward to form the point of the elbow. In the horse family every step of this reduction and consolidation may be traced in the course of its geological history.
Fig. 18.—Mosasaur.
5. The wrist of many mammals and all birds differs in structure from that of man, chiefly in containing a smaller number of bones. The normal number, as in man, seems to be eight. The decrease takes place mainly by consolidation of two or more into one. In such cases usually the embryo will show the bones still separate, thus revealing the ancestral condition. Again, the position of the wrist is noteworthy. In man, monkeys, the bear family, and several other mammalian families, and in all reptiles, the hand bends forward at the wrist, so that the tread is on the whole palm (palmigrade). But, in all the most specialized mammals, the wrist can not bend in this direction, and therefore this joint can not be brought to the ground. The tread is therefore on the toes (digitigrade), and the wrist is high up above the ground. In the horse ([Fig. 9]), the ox, and many other mammals, for example, the wrist is so high that it is not usually recognized as a wrist, and is often called the fore-knee. Now, homologous parts ought to have the same scientific name; but to use the word “hand” in the case of lower animals might produce confusion and misconception. Therefore it has been agreed among comparative anatomists to use instead the Latin word “manus” for all that corresponds, in any animal, to the hand of man—i. e., all from the wrist downward. The manus of a horse is about fifteen inches long. The manus of a pterodactyl, such as that found by Marsh in the cretaceous strata of the West, with an expanse of wings of twenty-five feet, was probably not less than seven or eight feet long.