140 



ANIMAL LOCOMOTION. 



forms the fundamental and distinguishing feature in flight, 

 and must be taken into account by all who seek to solve 

 this most involved and interesting problem by artificial means. 

 The importance of the twisted configuration or screw-like 

 form of the wing cannot be over-estimated. That this 

 shape is intimately associated with flight is apparent from 

 the fact that the rowing feathers of the wing of the bird are 

 every one of them distinctly spiral in their nature ; in fact, 

 one entire rowing feather is equivalent — morphologically and 

 physiologically — to one entire insect wing. In the wing of 

 the martin, where the bones of the pinion are short and in 

 some respects rudimentary, the primary and secondary feathers 

 are greatly developed, and banked up in such a manner that 

 the wing as a whole presents the same curves as those dis- 

 played by the insect's wing, or by the wing of the eagle where 

 the bones, muscles, and feathers have attained a maximum 

 development. The conformation of the wing is such that it 

 presents a waved appearance in every direction — the waves 

 running longitudinally, transversely, and obliquely. The 

 greater portion of the pinion may consequently be removed 

 without materially aff'ecting either its form or its functions. 

 This is proved by making sections in various directions, and 

 by finding, as has been already shown, that in some instances 

 as much as two-thirds of the wing may be lopped ofl" without 

 visibly impairing the power of flight. The spiral nature of 

 the pinion is most readily recognised when the wing is seen 

 from behind and from beneath, and when it is foreshortened. 

 It is also well marked in some of the long-winged oceanic 

 birds when viewed from before (figs. 82 and 83, p. 158), and 

 cannot escape detection under any circumstances, if sought 

 for, — the wing being essentially composed of a congeries of 

 curves, remarkable alike for their aj^parent simplicity and the 

 subtlety of their detail. 



The TFing during its action reverses its Planes^ and describes a 

 Figitre-of-S trccck in space. — The twisting or rotating of the 

 wing on its long axis is particularly observable during exten- 

 sion and flexion in the bat and bird, and likewise in the 

 insect, especially the beetle, cockroach, and such as fold 

 their wings during repose. In these in extreme flexion 



