_Wings, their Form, etc.; all Wings Screws, structurally and functionally._--Wings vary considerably as to their general contour; some being falcated or scythe-like, some oblong, some rounded or circular, some lanceolate, and some linear.[75] [75] “The wing is short, broad, convex, and rounded in grouse, partridges, and other rasores; long, broad, straight, and pointed in most pigeons. In the peregrine falcon it is acuminate, the second quill being longest, and the first little shorter; and in the swallows this is still more the case, the first quill being the longest, the rest rapidly diminishing in length.”--Macgillivray, Hist. Brit. Birds, vol. i. p. 82. “The hawks have been classed as noble or ignoble, according to the length and sharpness of their wings; and the falcons, or long-winged hawks, are distinguished from the short-winged ones by the second feather of the wing being either the longest or equal in length to the third, and by the nature of the stoop made in pursuit of their prey.”--Falconry in the British Isles, by F. H. Salvin and W. Brodrick. Lond. 1855, p. 28. [Illustration: FIG. 61.--Right wing of the Kestrel, drawn from the specimen, while being held against the light. Shows how the primary (_b_), secondary (_a_), and tertiary (_c_) feathers overlap and buttress or support each other in every direction. Each set of feathers has its coverts and subcoverts, the wing being conical from within outwards, and from before backwards. _d_, _e_, _f_ Anterior or thick margin of wing. _b_, _a_, _c_ Posterior or thin margin. The wing of the kestrel is intermediate as regards form, it being neither rounded as in the partridge (fig. 96, p. 176), nor ribbon-shaped as in the albatross (fig. 62), nor pointed as in the swallow. The feathers of the kestrel’s wing are unusually symmetrical and strong. Compare with figs. 92, 94, and 96, pp. 174, 175, and 176.--_Original._] All wings are constructed upon a common type. They are in every instance carefully graduated, the wing tapering from the root towards the tip, and from the anterior margin in the direction of the posterior margin. They are of a generally triangular form, and twisted upon themselves in the direction of their length, to form a helix or screw. They are convex above and concave below, and more or less flexible and elastic throughout, the elasticity being greatest at the tip and along the posterior margin. They are also moveable in all their parts. Figs. 61, 62, 63 (p. 138), 59 and 60 (p. 126), 96 and 97 (p. 176), represent typical bird wings; figs. 17 (p. 36), 94 and 95 (p. 175), typical bat wings; and figs. 57 and 58 (p. 125), 89 and 90 (p. 171), 91 (p. 172), 92 and 93 (p. 174), typical insect wings.