J. T. Cunningham makes an attempt to explain the phenomena of sexual dimorphism on Neo-Lamarckian principles. His theory is set forth in a paper entitled _The Heredity of Secondary Sexual Characters in relation to Hormones_, which was read before the Zoological Society of London, and published in full in the _Archiv für Entwicklungsmechanik der Organismen_. “The significant correlation of male sexual characters,” he writes, “is not with any general or essential property of the male sex, such as katabolism (or the tendency to dissipate energy, as we have called it), but with certain habits and functions confined to one sex, but differing in different animals. . . . In those animals which possess such (_i.e._ secondary sexual) characters, the parts of the soma (_i.e._ the body) affected differ as much as they can differ; any part of the soma may show a sexual difference: teeth in one mammal, skull in another; feathers of the tail in one bird, those of the neck in another, and so on. But in all cases such unisexual characters correspond to their functions or use in habits and instincts which are associated, but only indirectly, with sexual production. These habits are as diverse and as irregular in their distribution as the characters. The cocks of common fowls and of the Phasianidæ generally are polygamous, fight with each other for the possession of the females, and take no part in incubation or care of the young, and they differ from the hens in their enlarged brilliant plumage, spurs on the legs, and combs, wattles, or other excrescences on the head. In the Columbidæ _per contra_ the males are not polygamous, but pair for life, the males do not fight, and share equally with the females in parental duties. “Corresponding with this contrast of sexual habits is the contrast of sexual dimorphism, which is virtually absent in the Columbidæ. “I think, then, the only scientific explanation is that the difference of habits is the cause of the sexual dimorphism, and that the special sexual habits which occur in some species but not in others are the causes of the sexual characters. . . . The habits in question always involve certain definite stimulations applied to those parts of the body whose modification constitutes the somatic sexual characters. The stimulations are confined, as the characters are confined, to one sex, to one period of life, to one season of the year, to those animals which have the characters, to those parts of the body which are modified.” Mr Cunningham believes that these stimulations cause hypertrophy or excessive growth of the part affected, and that this peculiarity is transmitted to the offspring. And thus he supposes all the ornaments and excrescences of the males of various species to have arisen.