196 



ANIMAL LOCOMOTION. 



downwards and forwards, it would act at a manifest dis- 

 advantage : — 



\st. Because it would present the back or convex surface of 

 the wing to the air — a convex surface dispersing or dissipating 

 the air, while a concave surface gathers it together or focuses it. 



2d, In order to strike backwards effectually, the concavity 

 of the wing would also require to be turned backwards; and 

 this would involve the depression of the anterior or thick 

 margin of the pinion, and the elevation of the posterior or 

 thin one, during the down stroke, which never happens. 



3(i. The strain to which the pinion is subjected in flight 

 would, if the wang struck backwards, fall, not on the anterior 

 or strong margin of the pinion formed by the bones and 

 muscles, but on the posterior or weak margin formed by the 

 tips of the primary, secondary, and tertiary feathers — which 

 is not in accordance with the structure of the parts. 



4:th, The feathers of the wing, instead of being closed, as 

 they necessarily are, by a downward and forward movement, 



'Mf a bird, by altering the axis of its own body, can direct its wing stroke 

 in some degree forwards^ it wiU have the effect of stopping instead of 

 promoting progression ; and that, " Except for the purpose of arresting 

 their flight, birds can never strike except directly downwards — that is, 

 directly against the opposing force of gravity.''^ — Good Words, Feb. 1865, 

 p. 132. 



Mr. Bishop, in the Cyc. of Anat. and Phys., vol. iii. p. 425, says, In 

 consequence of the planes of the wings being disposed either perpendicularly 

 or obliquely backwards to the direction of their motion, a corresponding im- 

 pulse is given to their centre of gravity." Professor Owen, in like manner, 

 avers that a downward stroke would only tend to raise the bird in the air ; 

 to carry it forwards, the wings require to be moved in an oblique plane, so 

 as to strike backwards as well as downwards."— Comp. Anat. and Phys. and 

 Vertebrates, vol. ii. p. 115. 



The following is the account given by M. E. Liais : — When a bird is about 

 to depress its wing, this is a little inclined from before backwards. When 

 the descending movement corrimences, the wing does not descend parallel to 

 itself in a direction from before backwards ; but the movement is accompanied 

 l)y a rotation of several degrees round the anterior edge, so that the wing 

 becomes more in front than beliiiid, and the descending movement is trans- 

 ferred more and more backwards. . . . When the wing has completely 

 descended, it is both further back and lower than at the commencement of 

 the movement." — ^' 0\\ the Flight of Birds and Insects." Annals of Nat. 

 Hist. vol. XV. 3d series, p. 156. 



