Dimorphism (Gr. dis, 'twice; morphē, 'shape or form'), a technical term applied in Biology to cases where an organism appears in two different forms. Thus in a colony of hydroids there may be 'dimorphic persons'—respectively nutritive and reproductive, and yet fundamentally the same in structure and origin. So too when the two sexes are markedly different, the term sexual dimorphism may be used. Or the organisms may be different at certain seasons and in certain conditions, as in summer and winter Butterflies (q.v.), or in the two kinds of common Primrose (q.v.) and cowslip. Sometimes three different forms of butterfly are known in the same species, and to such cases the term trimorphism is conveniently applied. The same is true of the common flower Lythrum salicaria. In hydroid and medusoid colonies (e.g. Hydractinia and Siphonophora) the development of multiple forms resulting from 'Division of Labour' (q.v.) is described as polymorphism. The same term is obviously applicable to the different forms exhibited by some social Ants (q.v.). The same set of terms may also be employed in reference to different forms of the same mineral, &c. See also FLOWERS, FERTILISATION.
An inorganic substance is in a somewhat analogous manner said to be dimorphous when it is capable of crystallising in two different forms or systems (see CRYSTALLOGRAPHY). A large number of substances have this property, of which sulphur, carbon, carbonate of lime, and iodide of mercury are the best examples. Thus carbon, as the diamond, crystallises, in the regular system, as octahedra, while as graphite, or Black Lead (q.v.), it forms hexagonal crystals belonging to the rhombohedral system. In some cases there is a great tendency for the one form of crystal to change spontaneously into the other form. The red iodide of mercury is a striking example of this. When heated, this substance is volatilised, and may be condensed on a glass plate as a yellow crystalline crust, consisting of rhombic plates. When this is rubbed, or even scratched, an immediate change takes place, the rhombic plates becoming broken up into octahedra, while the colour at the same time alters from yellow to a brilliant scarlet. So also when sulphur is crystallised from its solution in bisulphide of carbon, it yields transparent amber-coloured octahedra; but when melted sulphur is cooled, prismatic crystals are the result. In the course of a few days the prisms become opaque, owing to their being broken up into small octahedra, while conversely the octahedra, when heated to 230° (110° C.), are changed into smaller prismatic crystals. Those substances, such as sulphate and selenate of nickel, which crystallise in three systems, are said to be trimorphous.