After this a marked sexual dimorphism sets in.
The iris is in most young birds at first brown or dull-coloured, but with maturity attains often very bright tints which add considerably to the charm of the bird; sexual dimorphism is in this respect of common occurrence.
The phenomenon of seasonal dimorphism is of especial moment for the plankton dwellers.
Molluscs are usually of separate sexes, but sexual dimorphism is seldom highly developed.
In some species of Dinophilus there is pronounced sexual dimorphism (the male being small and without gut) as in the Rotifera.
Skua, is not known to exhibit the remarkable dimorphism to which the two preceding are subject.
In association with antimonious and arsenious sulphides, silver sulphide forms many important minerals, which sometimes present dimorphous forms, reflecting the dimorphism of silver sulphide; moreover, the corresponding arsenious and antimonious compounds are frequently isomorphous.
Where the sexes are distinct, a more or less marked dimorphism often exists.
Dimorphism or heterophylly is fairly common.
Dimorphognathus, from West Africa, is the unique example of a sexual dimorphism in the dentition, the males being provided with a series of large sharp teeth in the lower jaw, which in the female, as in most other members of the family, is edentulous.
To allure and attract them to visit the flower the odoriferous secretions and gay colours are developed, and the position and complicated structure of the parts of the flower are adapted to the perfect performance of the process, It is comparatively rare in hermaphrodite flowers for self-fertilization to occur, and the various forms of dichogamy, dimorphism and trimorphism are fitted to prevent this.