POLYMORPHISM (from Gk. polys, much, many + morphe, form). The differ entiation either of one animal into two or more incomplete undetached individuals (pseudindivid uals) or zoiiids, as in the Siphonophores or Phy salia: or that of the animal into two separate sexes, or of the latter into castes, as in the termites and ants. Instead of the functions of the whole animal or plant being equally distributed to the individual organs. some of the organs or parts of the animal are set apart for this or that function. In the hydra the individual is monomorphic, not divided into male and female individuals, but in Hydraetinia, a fixed, vegetative form, there arise sexual or reproductive zol)ids, some female and others male, and also hydra-like or nutritive zooids or incomplete individuals. In the Por tuguese man-of-war (q.v.) we have a still better example of incomplete polymorphism. Thus, as Hertwig states, division of labor leads to greater centralization, "the more polymorphic an animal colony becomes, the more unified it is, the more it gives the impression of being a single animal instead of an aggregation of single animals." In the hydroids alternation of generations (q.v.) has arisen from a division of labor or polymorphism of individuals originally of equiv alent value. in which sonic individuals (the sex ual ones) have separated and acquired a pecu liar structure. Moreover, while alternation of generations has arisen from polymorphism. it can again produce it. Hertwig illustrates this by the case of certain medusie. which. instead of sepa rating, remain permanently attached to the col ony. They then degenerate into 'sporosacs.' in which a mouth, tentacles, and a velum are wanting.
A second kind of polymorphism is that seen in the males and the females of most animals. This
is sexual dimorphism, which may pass into sex ual polymorphism. This is complete polymor phism. Reproduction by budding involves the differentiation of the animal form into three kinds of individual—i.e. males, females. and `neuters,' as among insects. Among the coelente rates and worms the forms reproducing by par thenogenesis (q.v.) are usually larval or imma ture, as if they were prematurely hurried into existence, and their reproductive organs had been elaborated in advance of other systems or organs, for the sudden production, so to speak, of large numbers of individuals like themselves.
Among insects dimorphism is intimately con nected with organic reproduction. Thus the sum mer wingless asexual aphis and the perfect winged autumnal aphis may be called 'dimor phie' forms. The perfect female may assume two Corms, so much so as to be mistaken for two distinct species.
Dimosmusm ix BIRDS. Besides ordinary sex ual dimorphism, depending on sex, and comprised ander the head of secondary sexual characters (see :SEX; :SEXUAL :SELECTION), a few special eases are known, due probably to climate or local causes. Thus, in some species of skua, a parti colored bird may frequently he found mated with a uni-colorous form. either male or female. In the guillemots at nearly every breeding station about one in twenty may be marked with a white circle around the eye. and a white line extending backward from it. these ringed or bridled guillemots being of either sex and apparently paired with birds of normal plumage, no inter mediate forms being known. (See DICHROMA TISM IN BIRDS.) A striking example of dimorph ism in respect to the beak is furnished by *the huia (q.v.).