The humming-bird represents the perfection of one method, the frigate bird of the other, and in his own line each is unrivaled. These two modes of flight are associated with equally distinct modifications of structure, and just as we have every intermediate state of flight between flapping and soaring so the two structural extremes are merged into one another. The humming-bird flies as the Irishman played the fiddle, by main strength, the frigate bird relies on his skill in taking advantage of every varying current of air, and the skeleton of the one indicates great muscular power while that of the other shows its absence. No other bird has such proportionately great muscles as the humming-bird, the keel of the sternum or breast bone from which these muscles arise runs from one end of the body to the other while at the same time it projects downward like the keel of a modern racing yacht. These muscles drive at the rate of several hundred strokes a minute a pair of small, rigid wings, the outermost bones of which are very long while the innermost are very short, a feature calculated to give the greatest amount of motion at the tip of the wing with the least movement of the bones of the upper arm, to which the driving muscles are attached. Another peculiar feature is that the outermost feathers, the flight feathers or primaries, are long and strong, while the innermost, those attached to the forearm, are few and weak; so far as flight is concerned the bird could dispense with these secondaries and not feel their loss. Finally the heart, which we may look upon as the boiler that supplies steam for this machinery, is large and powerful, as is necessary for such a high-pressure engine as the little humming-bird. It is hardly to him that we would look for aid in constructing a flying machine, the expenditure of force is too great for the results attained, the space required for boiler and engine leaves no room for carrying freight. As just intimated the frigate bird is exactly the reverse of his tiny relative; the body is a mere appendage to a pair of wings, while the breast muscles are so small as to show at a glance that of all flying creatures the frigate bird is the one which has most successfully solved the problem of the conservation of energy and can obtain the greatest amount of power with the least expenditure of muscle. There is also a great difference between the hummer and the frigate bird, or between flapping and sailing birds generally, in the complexity of what may be termed the muscles of adjustment, the little muscles that run from the shoulder to the elbow and forearm and, among other duties, are concerned in keeping free from wrinkles that portion of the wing which lies between the shoulder and the wrist, forming a triangular flap with the base forming the front edge of the wing and the apex lying in the elbow joint.