DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 381 



by the axis of the feather occupying an anterior position, the feather when it is 

 made to rotate causing the posterior margin (because of its greater breadth) to 

 move through a greater arc of a circle than the anterior margin. It is owing to 

 the greater travel of the posterior margin as compared with the anterior one, that 

 the feathers of the wing so readily open in flexion and close in extension. The 

 gradation in the length, and strength, and in the degree of overlapping is neces- 

 sitated by the fact that the feathers at the tip of the wing are exposed to a 

 much greater strain than those nearer the root — the former always travelling 

 through a much greater space in a given time than the latter. 



The Wing of the Bird not always Opened Up to the same extent in the Up Stroke. 

 — The elaborate arrangements and adaptations just referred to for increasing 

 the area of the wing, and making it impervious to air during the down stroke, 

 and for decreasing the area and opening up the wing during the up stroke, 

 although necessary to the flight of the heavy-bodied, short- winged birds, as the 

 grouse, partridge, and pheasant, are by no means indispensable to the flight of 

 the long-winged oceanic birds, unless when in the act of rising from a level 

 surface ; neither do the short-winged heavy birds require to fold and open up 

 the wing during the up stroke to the same extent in all cases, less folding and 

 opening up being required when the birds fly against a breeze, and when they 

 have got fairly under weigh. All the oceanic birds, even the albatros, require to 

 fold and flap their wings vigorously when they rise from the surface of the 

 water. When, however, they have acquired a certain degree of momentum, 

 and are travelling at a tolerable horizontal speed, they can in a great measure 

 dispense with the opening up of the wing during the up stroke — nay, more, they 

 can in many instances dispense even with flapping. This is particularly the case 

 with the albatros, which (if a tolerably stiff breeze be blowing) can sail about 

 for an hour at a time without once flapping its wings. In this case the wing is 

 wielded in one piece like the insect wing, the bird simply screwing and un- 

 screwing the pinion on and off the wind, and exercising a restraining influence 

 —the breeze doing the principal part of the work. In the bat the wing is 

 jointed as in the bird, and folded during the up stroke. As, however, the bat's 

 wing, as has been already stated, is covered by a continuous and more or less 

 elastic membrane, it follows that it cannot be opened up to admit of the air 

 passing through it during the up stroke. Flight in the bat is therefore secured 

 by alternately diminishing and increasing the area of the wing during the up 

 and down strokes — the wing rotating upon its root and along its anterior 

 margin during its ascent and descent precisely as in the bird. 



Analysis of the Movements of Extension and Flexion in the Wing of the Gannet. 

 — The changes which the wing undergoes in extension and flexion are seen to 

 great advantage in the gannet (figs. 9, 10, and 11, Pate XII.) 



The pinion of this bird is remarkable for its great length as compared witli 



