Let us now consider to what extent the posterior margin of the wing yields in _an upward direction_ when the wing descends. Borelli does not state the exact amount. The Duke of Argyll, who believes with Borelli that the posterior margin of the wing is elevated during the down stroke, avers that, “whereas the air compressed in the hollow of the wing cannot pass through the wing owing to the closing upwards of the feathers against each other, or escape forwards because of the rigidity of the bones and of the quills in this direction, it passes backwards, and in so doing _lifts by its force the elastic ends of the feathers_. In passing backwards it communicates to the whole line of both wings a corresponding push forwards to the body of the bird. The same volume of air is thus made, in accordance with the law of action and reaction, _to sustain the bird and carry it forward_.”[112] Mr. Macgillivray observes that “to progress _in a horizontal direction_ it is necessary that the downward stroke should be modified _by the elevation in a certain degree of the free extremities of the quills_.”[113] [112] “Reign of Law”--Good Words, February 1865, p. 128. [113] History of British Birds. Lond. 1837, p. 43. _Marey’s Views._--Professor Marey states that during _the down stroke_ the posterior or flexible margin of the wing yields in _an upward direction_ to such an extent as to cause the under surface of the wing _to look backwards_, and make a backward angle with the horizon of 45° _plus_ or _minus_ according to circumstances.[114] That the posterior margin of the wing yields in a slightly upward direction during the down stroke, I admit. By doing so it prevents shock, confers continuity of motion, and contributes in some measure to the elevation of the wing. The amount of yielding, however, is in all cases very slight, and the little upward movement there is, is in part the result of the posterior margin of the wing rotating around the anterior margin as an axis. That the posterior margin of the wing never yields in _an upward direction_ until the under surface of the pinion makes a backward angle of 45° with the horizon, as Marey remarks, is a matter of absolute certainty. This statement admits of direct proof. If any one watches the horizontal or upward flight of a large bird, he will observe that the posterior or flexible margin of the wing never rises during the down stroke to a perceptible extent, so that _the under surface of the wing_ on no occasion looks backwards, as stated by Marey. On the contrary, he will find that _the under surface of the wing_ (during the down stroke) invariably _looks forwards_--the posterior margin of the wing being inclined _downwards and backwards_; as shown at figs. 82 and 83, p. 158; fig. 103, p. 186; fig. 85 (_a b c_), p. 160; and fig. 88 (_c d e f g_), p. 166.