If, as Borelli and his successors believe, the posterior margin of the wing yielded to a marked extent in _an upward direction_ during the _down stroke_, and more especially if it yielded to such an extent as to cause the under surface of the wing to make _a backward angle with the horizon of 45°_, one of two things would inevitably follow--either the air on which the wing depends for support and propulsion would be permitted to escape before it was utilized; or the wing would dart rapidly _downward_, and carry the body of the bird with it. If the posterior margin of the wing yielded in an upward direction to the extent described by Marey during the down stroke, it would be tantamount to removing the fulcrum (the air) on which the lever formed by the wing operates. If a bird flies in a horizontal direction the angles made by the under surface of the wing with the horizon _are very slight_, but they _always look forwards_ (fig. 60, p. 126). If a bird flies upwards the angles in question are increased (fig. 59, p. 126). In no instance, however, unless when the bird is everted and flying downwards, is the _posterior margin_ of the wing _on a higher level_ than the anterior one (fig. 106, p. 203). This holds true of natural flight, and consequently also of artificial flight. These remarks are more especially applicable to the flight of the bat and bird where the wing is made to vibrate more or less perpendicularly (fig. 17, p. 36; figs. 82 and 83, p. 158. Compare with fig. 85, p. 160, and fig. 88, p. 166). If a bird or a bat wishes to fly upwards, its flying surfaces must always be inclined upwards. It is the same with the fish. A fish can only swim upwards if its body is directed upwards. In the insect, as has been explained, the wing is made to vibrate in a more or less horizontal direction. In this case the wing has not to contend directly against gravity (a wing which flaps vertically must). As a consequence it is made to tack upon the air obliquely zigzag fashion as horse and carriage would ascend a steep hill (_vide_ figs. 67 to 70, p. 141. Compare with figs. 71 and 72, p. 144). In this arrangement gravity is overcome by the wing reversing its planes and acting as a kite which flies alternately forwards and backwards. The kites formed by the wings of the bat and bird always fly forward (fig. 88, p. 166). In the insect, as in the bat and bird, the posterior margin of the wing never rises above the horizon so as to make an upward and backward angle with it, as stated by Borelli, Marey, and others (_c x a_ of fig. 114, p. 228).