The facts first to be reviewed concern the syrinx of certain of the Anatidæ. It is noteworthy that each of the three divisions of this group—the Swans, Geese and Ducks—contains species in which either the syrinx or the windpipe has acquired some singular feature. In the surface-feeding Ducks, modifications of the syrinx are most frequently found. Commonly, as in the Mallard, this takes the form of a spherical bony case; in the diving Ducks this bony chamber has enormously increased in size. Furthermore it has conspicuously changed both in form and character: for it is now roughly trihedral in form, and its walls present large _fenestræ_ closed only by delicate membrane, suggesting that the increased size of the chamber has not been accompanied by a corresponding increase of bony tissue for its construction. Hence all that is available is used for the construction of girders to form supports for the now membranous chamber walls. Some species seem to show that this fenestration has been pushed to excess, leaving only vestiges of this singular chamber, as is shown in PI. 21. In some species the bronchi are much swollen, and the syringeal chamber has entirely disappeared: in others, as in the Merganser and Goosander, a large syringeal chamber is supplemented by dilatations of the windpipe. [Illustration: Plate 21. GRADES OF EVOLUTION IN THE SYRINX OR ORGAN OF VOICE IN THE MALES OF SURFACE FEEDING AND DIVING-DUCKS. 1. Wigeon. 2. Common Sheldrake. 3 and 4. Red-crested Pochard. 5. Red-crested Merganser. 6 and 7. Long-tailed Duck. 8. Steller’s Eider. 9. Common Scoter. Face Page 126.] Save in the case of the Goosander, these peculiar structures are found only in the male, but in the species first named the male, in addition to the syringeal chamber, has two fusiform swellings in the windpipe, one above the other: in the female one of these swellings is present, but there is no syringeal box. This box is generally, and probably correctly, regarded as a sort of musical instrument. Nevertheless the males are far less vociferous than the females which have no such voice resonator. One has only to listen to, and compare the notes of the Mallard drake and duck to discover this fact. Here, then, we seem indeed to have a case of “Hypertely.” Before, however, we build too much on this we must discover whether the sibilant sounds uttered by the males do, or do not, play an important part in arousing the sexual passions of the females.