An arrangement in every respect analogous to that described in the bird is found in the wing of the bat, the covering or web of the wing in this instance forming the principal elastic ligament (fig. 17, p. 36).
_Power of the Wing--to what owing._--The shape and power of the pinion depend upon one of three circumstances--to wit, the length of the humerus,[93] the length of the cubitus or forearm, and the length of the primary feathers. In the swallow the humerus, and in the humming-bird the cubitus, is very short, the primaries being very long; whereas in the albatross the humerus or arm-bone is long and the primaries short. When one of these conditions is fulfilled, the pinion is usually greatly elongated and scythe-like (fig. 62, p. 137)--an arrangement which enables the bird to keep on the wing for immense periods with comparatively little exertion, and to wheel, turn, and glide about with exceeding ease and grace. When the wing is truncated and rounded (fig. 96, p. 176), a form of pinion usually associated with a heavy body, as in the grouse, quail, diver, and grebe, the muscular exertion required, and the rapidity with which the wing moves are very great; those birds, from a want of facility in turning, flying either in a straight line or making large curves. They, moreover, rise with difficulty, and alight clumsily and somewhat suddenly. Their flight, however, is perfect while it lasts.
[93] “The humerus varies extremely in length, being very short in the swallow, of moderate length in the gallinaceous birds, longer in the crows, very long in the gannets, and unusually elongated in the albatross. In the golden eagle it is also seen to be of great length.”--Macgillivray’s British Birds, vol. i. p. 30.
The goose, duck (fig. 107, p. 204), pigeon (fig. 106, p. 203) and crow, are intermediate both as regards the form of the wing and the rapidity with which it is moved.
The heron (fig. 60, p. 126) and humming-bird furnish extreme examples in another direction,--the heron having a large wing with a leisurely movement, the humming-bird a comparatively large wing with a greatly accelerated one.
But I need not multiply examples; suffice it to say that flight may be attained within certain limits by every size and form of wing, if the number of its oscillations be increased in proportion to the weight to be raised.