“From the general similarity in these females to the appearance assumed for a time by healthy males in July, I am disposed to refer this seasonal change in males, in this and in other species of ducks, to a temporary exhausted state of the male generative organs, and their consequent diminished constitutional influence on the plumage. “A male shut up by himself from early spring to the end of July undergoes no change in his plumage; but if he is allowed to associate with females till their season of incubation commences, he then goes through the change, and this appears to indicate the cause of the partial summer moulting. “The appearance is somewhat different, but yet very interesting in insects and crustacea. In these classes the sexual organs are double and distinct, arranged one on each side of the elongated mesial line. It sometimes happens, that a species in which the sexes are of a different color, or markings, or form has one sexual organ of each sort, male and female, in which case each half of the same insect is developed under the exclusive influence of the sexual organ on its own side. Instances are preserved among our collections of butterflies, mothes and beetles; and I have seen it twice in the common lobster. “Nor is the human race exempt from the operation of the law which prevails in the Mammalia. In women, at an advanced age, hair appears on the chin and upper lip, and the voice alters, becoming deep in its tone. The beard in old men becomes thin and soft, and our own inimitable Shakespeare has told us, * * * his big manly voice Turning again toward childish treble, pipes And whistles in his sound.” Gurney (1888) has recorded several cases in which female birds have assumed male plumage. For instance, he describes a female merganser, _Mergus serrator_, assuming male plumage that showed no signs of disease in the ovary. Mr. Cecil Smith had a female widgeon (_Mareca penelope_) on his ponds near Trenton, which assumed the male plumage some years ago, and which, so far as he knew, had not had young nor laid eggs. “On May 16th, 1887, a chaffinch (_Fringilla cœelebs_) in full male plumage was shot at Chapel Town, near Leeds, in Yorkshire, by the son of Mr. W. L. Jackson, M. P.; it was skinned by G. R. Grassham, assistant to Mr. W. E. Clarke at the Museum, who, much to his surprise, found that it was a female, and contained an egg, ready for laying, of a pale blue, without markings, and another egg in a less forward state. This chaffinch is in every way in perfect male plumage, and I am indebted to Mr. Clarke for his kindness in sending these particulars with the specimen, which he received from Grassham a few hours after the latter had dissected the bird.