“This angular arrangement, of which we find examples in the ship’s prow, the plowshare, the thin edge of a wedge, and a multitude of utensils designed for cleavage, is the most favorable for pushing through the mass of the air with the least fatigue. If in marshaling their flying battalions the goose and the duck had taken counsel of the engineer’s science, they could not have managed better. But they have no need of others’ advice: instructed by their own instinct, they utilized long before we did the principle of the wedge. “Moreover, to divide among all the members of the flock the excess of fatigue incurred by the file-leader in opening a passage through the air by strength of wing, each in turn takes the post of honor, the forward end of the single file or the point of the angle formed by the double file. Its term of service ended, the bird at the head retires to the rear to recuperate, and another leader takes its place. By this equitable division of labor the fatigue does not prove excessive for any one bird, and the flock leaves no stragglers behind.” CHAPTER LVIII CARRIER-PIGEONS Resuming the subject of bird instinct as illustrated by the migratory flock’s unerring precision in finding its way over thousands of miles to a desired nesting-place, Uncle Paul continued as follows: “How is it that so many thousands, even millions, of migrating birds can direct their course through trackless space each to the particular rock or tree or nest left behind six months before, when the yearly removal was decided upon to some southern region a thousand miles or more distant? How, for example, does the frail swallow manage to find again, at the return of spring, its tiny abode in the north when it retraces the long journey of the previous autumn? In order that we may be sure it is the same swallow returned to the same nest we tie a colored string, as I have said, around the bird’s claw; and, lo and behold, when April comes, with it comes our swallow to its dwelling under the eaves. It is indeed our identical bird and no other; it is the very one that fashioned the nest of clay, cherished bit of private property now so eagerly taken possession of once more. The owner’s demonstrations of satisfaction and delight are convincing proof, even were the bit of red thread not there to dispel all doubt. “If the swallow is able to find its nest again upon returning in the spring from the land of the negroes, still more will it be able to find it after being removed merely from its native canton to the neighboring one.