180 



ANIMAL LOCOMOTION. 



and forearm, and consequently to the primary and secondary 

 feathers which they bear, the precise angles necessary for 

 flight; it in fact insures that the wing, and the curtain or 

 fringe of the wing which the primary and secondary feathers 

 form, shall be screwed into and down upon the air in ex- 

 tension, and unscrewed or withdrawn from it during flexion. 

 The wing of the bird may therefore be compared to a huge 

 gimlet or auger; the axis of the gimlet representing the 

 bones of the wing, the flanges or spiral thread of the gimlet 

 the primary and secondary feathers (fig. 63, p. 138, and fig. 

 97, p. 176). 



Traces of Design in the Wing of the Bird — the arrangement of 

 the Primary^ Secondary, and Tertiary Feathers^ ^^c— There are 

 few things in nature more admirably constructed than the 

 wing of the bird, and perhaps none where design can be more 

 readily traced. Its great strength and extreme lightness, the 

 manner in which it closes up or folds during flexion, and 

 opens out or expands during extension, as well as the manner 

 in which the feathers are strung together and overlap each 

 other in divers directions to produce at one time a solid 

 resisting surface, and at another an interrupted and compara- 

 tively non-resisting one, present a degree of fitness to which 

 the mind must necessarily revert with pleasure. If the 

 feathers of the wing only are contemplated, they may be con- 

 veniently divided into three sets of three each (on both sides 

 of the wing) — an upper or dorsal set (fig. 61, c?, e,f, p. 136), a 

 lower or ventral set (cya, b), and one which is intermediate. 

 This division is intended to refer the feathers to the bones of 

 the arm, forearm, and hand, but is more or less arbitrary in 

 its nature. The lower set or tier consists of the primary (b), 

 secondary (a), and tertiary (c) feathers, strung together by 

 fibrous structures in such a way that they move in an out- 

 ward or inward direction, or turn upon their axes, at precisely 

 the same instant of time, — the middle and upper sets of 

 feathers, which overlap the primary, secondary, and tertiary 

 ones, constituting what are called the " coverts " and " sub- 

 coverts." The primary or rowing feathers are the longest and 

 strongest (b), the secondaries (a) next, and the tertiaries third 

 (c). The tertiaries, however, are occasionally longer than the 



