3402. A Bird's wing is a strange but very instructive composition. It consists, namely, of a Reptile's foot and an Insect's wings.
3403. We saw how the branchiæ of the Insect dried up, separated from the feet, and being liberated as wings, were permeated by tracheæ. In the Bird these wings have remained standing upon the feet and been converted into feathers.
3404. A feather is an Insect's wing.
3405. As the Bird grows out at the thoracic limbs into the Insect's wings, so does it upon the whole body into dried branchial laminæ.
The whole body of the Bird is clothed with branchial laminæ or plates.
3406. The wings of the Insects might be called free tracheæ.
The Bird's feathers are Insect-tracheæ. As in the Insect the wings are a leash or net of tracheæ held together by membranes, so feathers are tracheæ dividing into fibre-like ramules.
3407. The Bird is a Reptilian or Frog's body, beset all over with Insects, like as with parasitic animals.
3408. The highest Insect only attains to the possession of four wings, which in some Moths split again into several feathers. In the Bird a vast number of such wings originates.
3409. An Insect's wing is not more than one feather, and is therefore placed also directly upon the body. These wings must multiply so soon as they occur upon a membered trunk, upon arms. Thus, we need not ask why the Butterfly has four, but the Bird only two wings, seeing that the latter is the nobler animal. The discourse cannot be concerning the wings of the former, for the Butterfly has indeed none, but only four feathers.