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Sex, the differentiation of cellular elements, either alone or with other surrounding structures, into male and female, so that their union [REPRODUCTION] results in the stimulus of the latter into a new individual. In its simplest form sex shows itself in the union of similar gametes, reproductive cells, that is, incapable by themselves of giving rise to a new organism. Organisms producing such gametes are termed isogamous; the union of their gametes, conjugation; and its result (among plants at least), a zygospore. Such similar gametes may be free-swimming, ciliated, and pear-shaped, or planogametes, or without cilia or definite form (aplanogametes). Among the Protozoa each organism generally consists of such an undifferentiated conjugating gamete. But little higher in both the algal and the animal series heterogamy,spermatozoid, or sperm-cell, is generally the smaller and more active, being a well-defined, ciliated, free-swimming mass of protoplasm. This form, it has been suggested, is the result of the excess of katabolism (q.v.) in a previously undifferentiated amoeboid cell, finding its outward expression in increased activity of movement. The female gamete, oosphere, ovum, or germ cell, on the other hand, is generally larger, spherical, unciliated and quiescent, the result of an excess of anabolism, i.e. of potential, rather than of kinetic, energy, The union of such heterogamous elements is termed fertilisation. Where male and female organs are borne by the same individual, and on the same branch or body segment, the organism may be termed monoclinous, the term hermaphrodite being unsuitable, as suggesting self-fertilisation. Where male and female organs, though on the same individual, are more distant (as when in distinct flowers), the organism is termed monoecious. Where the organs occur on distinct individuals, it is termed dioecism, a condition which is the rule among the higher animals more than among the higher plants. It is now recognised that the sex of an embryonic organism is determined by the condition of the parents as regards relative age, etc, by the guality and quantity of food supplied to the female parent or larvae, by temperature, and by other similar external conditions. Abundant nourishment tends to produce females; but it is difficult as yet to arrive with certainty at any other law of general application as to sex determination. Among some worms, such as the Rotifera, and some crustaceans, such as the cirripedes, the male becomes degenerated into a mere appendage, or "complemental male," borne by the female, and a similar condition occurs in the algal OEdogonium. Among insects the sexes are most strongly contrasted by secondary sexual characters, such as the smaller size, greater activity, brighter colours, and sound-producing powers of the male; whilst among bees, ants, and other Hymenoptera (q.v.), we may almost be said to have more than two sexes. Among mammals, offensive organs, such as horns and tusks, and ornamental appendages, such as manes and colour-patches, often mark the males; but among birds the contrast of sex is yet more striking in the generally gay plumage and nuptial song of the male. Numerous subsidiary sex questions, such as apogamy, parthenogenesis, etc., are treated separately. The advocacy of Charles Darwin gave importance to his theory of Sexual Selection, according to which the choice of a mate by the female animal has had much to do with the evolution of secondary sexual characters in the male, such as song, gay plumage, colour, wattles, moustaches, etc. Wallace has argned that this process is entirely controlled by natural selection, and other biologists think the part played by it in the origin of species quite subordinate.