DIMORPHISM, the property of assuming two forms. In crystallography (q.v.), two substances which are identical in chemical composition but different in crystalline form, and conse quently in others of their physical properties, are dimorphous.
In biology the word is used when there are two distinct vari eties of an organism which freely interbreed ; thus, in plants, the pin-eyed and thrum-eyed varieties of primrose (Primula) are examples of dimorphism; among animals, some female mimetic butterflies show dimorphism, changing their appearance to re semble a different model (see MIMIcRY) ; some birds, e.g., certain owls, also exhibit dimorphism. In addition, many animals exhibit sexual dimorphism (see SEx) in which the two sexes are sharply differentiated from one another, e.g., deer, birds of paradise, many butterflies, etc.