That the bird is _descended from animals_ which had the fingers and palm bones less firmly united is shown by comparing the hands of a _chick_ and of an _adult_ fowl (Figs. 287, 288). The wrist also solidifies with age, the five carpals of the chick being reduced to two in the fowl (Figs. 287, 288). The thumb or first digit has a covering of skin separate from that of the other digits, as may be seen in a plucked bird. The degenerate hand of the fowl is of course useless as a hand (what serves in its place?) but it is well fitted for firm support of the feathers in flying. The two bones of the forearm are also firmly joined. There are eighteen movable joints in our arm and hand. The bird has only the three joints which enable it to fold its wing. The wrist joint is the joint in the forward angle of the wing. [Illustration: FIG. 289.—BREASTBONE AND SHOULDER BONES OF CASSOWARY. ] [Illustration: FIG. 290.—A FOSSIL BIRD (_archæopteryx_) found in the rocks of a former geological epoch. =Question=: Find two resemblances to reptiles in this extinct bird absent from skeletons of extant birds. ] Since the fore limbs are taken up with locomotion, the =grasping function= has been assumed by the _jaws_. How does their shape adapt them to this use? For the same reason the _neck_ of a bird surpasses the necks of all other animals in what respect? Is the trunk of a bird flexible or inflexible? There is thus a _correlation_ between structure of neck and trunk. Explain. The same correlation is found in which of the reptiles? (Why does rigidity of trunk require flexibility of neck?) Why does the length of neck in birds correlate with the length of legs? Examples? (See Figs. 314, 315, 332.) Exceptions? (Fig. 324.) Why does a swan or a goose have a long neck, though its legs are short? [Illustration: FIG. 291.—QUILL FEATHER. _D_, downy portion. ] To make a firm support for the wings the vertebræ of the back are immovably joined, also there are three bones in each shoulder, the collar bone, the shoulder blade, and the coracoid bone (Fig. 286). The collar bones are united (why?) and form the “wishbone” or “pulling bone.” To furnish surface for the attachment of the large flying muscles there is a prominent ridge or keel on the breastbone (Fig. 286). It is lacking in most birds which do not fly (Fig. 289).