6836 Birds. 



and the same manoeuvres are repeated in an opposite direction. And 

 thus we may observe in the well-known structure of the swallow the 

 utmost care lavished in the most minute particulars that impetus may 

 be economized in every possible way. The flatness of the crown, the 

 puffiness of the loral and gular feathers, leaving no angles about the 

 bill, the peculiar curve of the ventral surface, carried by the lengthening 

 of the under tail-coverts with a clean sweep to the tail, and above all 

 the reduction of the mass of the plumage by the diminution of the size 

 of each feather, which have their exposed surfaces polished, — con- 

 trivances all evidently tending to the same end. What is the distance 

 at which a swallow can see an insect we can of course only surmise, 

 for the ray reflected from these minute points floating in the air is not 

 perceptible to the human eye during ordinary daylight, but if the 

 insect intercept the ray, then its vibrating gauzy wings enable us to 

 see it at a considerable distance. On a bright, calm evening, I find, 

 when looking towards the sun, I can see very small floating insects at 

 ten or twelve yards. May we suppose the eye of the swallow has the 

 same power where the ray is reflected ? And further it may be 

 remarked that, in accordance with this mode of taking prey, swallows 

 very rarely pause in their flight or raise the wings above the plane of 

 the back. On the contrary, the tips of the motionless extended wings 

 are usually rather depressed (in the swifts remarkably so), or having 

 gained the required impetus by a number of vigorous strokes, they 

 shoot along with the wing much bent at the flexure, — a position in 

 which it seems to offer least impediment to the onward rush, and is 

 still available for modulating it in the most delicate manner. 



"The flight of these birds was thus to be adapted to pass in suc- 

 cession through a certain number of floating points. To do this a 

 certain velocity was necessary, as the points are moving in every direc- 

 tion. But this velocity supposes, in such circumstances, the constant 

 acquirement of a certain amount of impetus, — a force very favourable 

 to the bird, for it counteracts other opposing forces and greatly relieves 

 it. Every care is therefore taken to preserve it, by structure and 

 direction of flight. It also leaves the bird the use of its wings to 

 modulate with great exactitude this direction in a greater degree 

 doubtless than with the generality of birds, as we know that a ship 

 with a certain amount of 'way' will steer best. As we have every 

 reason to believe that a certain appreciable time is necessary for a ray 

 of light to make a distinct impression on the organs of other animals, 

 exactly as with our own, it follows that the rays whose direction is 

 parallel to the course of the bird will dwell longest on the eye, and 



