Mentioning the flight of the pigeon leads me to consider that of other birds. All other animals have one determinate mode of progression, which in every kind is always the same; birds alone have two modes of moving—on the ground, and in the air. Some of them walk, such as the crow, for instance; some hop, as the sparrow and the blackbird; some run, as the partridge and the woodhen; while others throw one foot before the other, like the stork and the crane. Then again, in their flight, some birds expand their wings, and poising themselves in the air, only move them from time to time; others move them more frequently, but then only at the extremities; while others expand them so as to expose the whole of the side. On the other hand, some fly with the greater part of the wings kept ............