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 commences, the wing does not descend parallel to itself in a direction from before backwards; but the movement is accompanied by a rotation of several degrees round the anterior edge, so that the wing becomes more in front than behind, and the _descending movement is transferred more and more backwards_.... When the wing has completely descended, it is both _further back_ and lower than at the commencement of the movement.”--“On the Flight of Birds and Insects.” Annals of Nat. Hist. vol. xv. 3d series, p. 156.
_1st._ 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.
_3d._ The strain to which the pinion is subjected in flight would, if the wing 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.
_4th._ The feathers of the wing, instead of being closed, as they necessarily are, by a downward and _forward_ movement, would be inevitably opened, and the integrity of the wing impaired by a downward and _backward_ movement.
_5th._ The disposition of the articular surfaces of the wing (particularly that of the shoulder-joint) is such as to facilitate the downward and _forward_ movement, while it in a great measure prevents the downward and _backward_ one.
_6th and lastly._ If the wing did in reality strike downwards and _backwards_, a result the converse of that desired would most assuredly be produced, as an oblique surface which smites the air in a downward and _backward_ direction (if left to itself) tends to depress the body bearing it. This is proved by the action upon the air of free inclined planes, arranged in the form of a screw.