_2d_, Because his doctrines have remained unquestioned for nearly two centuries, and have been adopted by all the writers since his time, without, I regret to say in the majority of cases, any acknowledgment whatever. _3d_, Because his views have been revived by the modern French school; and _4th_, Because, in commenting upon and differing from Borelli, I will necessarily comment upon and differ from all his successors. _As to the Direction of the Stroke, yielding of the Wing, etc._--The Duke of Argyll[111] agrees with Borelli in believing that the wing invariably strikes _perpendicularly downwards_. His words are--“Except for the purpose of arresting their flight birds can never strike except _directly downwards_; that is, against the opposing force of gravity.” Professor Owen in his Comparative Anatomy, Mr. Macgillivray in his British Birds, Mr. Bishop in his article “Motion” in the Cyclopedia of Anatomy and Physiology, and M. Liais “On the Flight of Birds and Insects” in the Annals of Natural History, all assert that the stroke is delivered _downwards_ and more or less _backwards_. [111] “Reign of Law”--Good Words, 1865. To obtain an _upward_ recoil, one would naturally suppose all that is required is a _downward_ stroke, and to obtain an _upward and forward_ recoil, one would naturally conclude a _downward and backward_ stroke alone is requisite. Such, however, is not the case. In the first place, a natural wing, or a properly constructed artificial one, cannot be depressed either _vertically downwards_, or _downwards and backwards_. It will of necessity descend _downwards and forwards in a curve_. This arises from its being flexible and elastic throughout, and in especial from its being carefully graduated as regards thickness, the tip being thinner and more elastic than the root, and the posterior margin than the anterior margin. In the second place, there is only one direction in which the wing could strike so at once _to support and carry the bird forward_. The bird, when flying, is a body in motion. It has therefore acquired momentum. If a grouse is shot on the wing _it does not fall vertically downwards_, as Borelli and his successors assume, but _downwards and forwards_. The flat surfaces of the wings are consequently made to strike downwards and forwards, as they in this manner act as kites to the falling body, which they bear, or tend to bear, _upwards and forwards_. So much for the direction of the stroke during the descent of the wing.