SEXUAL DIMORPHISM. This is clue to the rise of secondary characters. Such features are the male lion's mane, the horns of the busk, the gay plumage which distinguishes the cock from the hen, and the plumes, colored combs and wat tles. top-knots, brilliant, conspicuous bands and spots, spurs, and those markings or new plum age especially developed during the breeding sea son. Males tend among vertebrates to be larger, they lead the flock, guard the females and young; in character they are more jealous and pugna cious. This is the case not only with mammals and birds, but with reptiles and frogs. The vociferous cries in spring of frogs and toads are mainly from male throats, the females being much less noisy. Certain fishes, as the salmon, during the breeding season are distinguished by bright colors and ornamental appendages. Of the invertebrates only insects, spiders, and crustacsa afford examples. Among coleoptera the stag beetles (Lucanidre) are remarkable for their size and the enormous jaws and horns of the males, and there are two sets of males, those which in lack of armature approach the females, and those which are much larger and remarkably aberrant. In certain spiders the males are gaily colored and their legs greatly modified in shape. Darwin has explained sexual dimorphism by his theory of sexual selection (q.v.) . Sexual dimorphism reaches its acme in the males of certain solitary barnacles; they are minute, very much reduced in structure, living inside the mantle-cavity of the female, where they are anchored by their antennN.
Is PLANTS. The simplest plants give no indi cation of any sexual process, but reproduce by cell division or by non-sexual spores. The gradual transition from the sexless to the sexual condi tion is clearly shown in several groups of alga:. For examplgi,U]othrix, a green alga,consists of a single row of cells, each of which has ordinary vegetative powers. In some cells a few large ciliated swim ming sexless spores are developed by cell division. Other cells produce numerous smaller similar bodies. Both sorts when discharged swim about and either directly form filaments, or they may fuse in pairs, thus producing a new cell, capable of developing a new vigorous individual. Since this fusing is the essence of the sexual process, botanists conclude that sexual cells have been derived from sexless swimming spores.
The sexual cells (gametes) are at first alike, a condition distinguished by special terminology from that in which two sexes are distinct. Thus, the mother cell within which the gametes are developed is called a gametangium ; the condi tion of having similar pairing gametes is i,*og amy; the act of fusion is conjugation, and. the resulting sexually formed spore is a zygospore or zygote. Only the lower alga and fungi are isog anions. Very early in the history of the evo lution of sex in plants the pairing gametes began to differentiate. In one series the gametes be came gradually larger and proportionately less active, until a relatively large and absolutely pas sive cell, the female gamete, egg, or oosphere, was formed. In the other series activity was increased, and size perhaps diminished, resulting in the for mation of the male gamete, sperm, antherozoid, or spermatozoid. This differentiation of sex con tinues from the higher algae throughout the plant kingdom, with the following special terminology. The gametangium which develops the sperms is called an antheridium (q.v), and that which de velops the usually single egg an oogonium among the algae and fungi and an arehegonium in the higher groups. The condition of having dis similar gametes is heterogamy; the process of fusion is fertilization; and the resulting sexual ly formed spore is an oOspore or fertilized egg.
Although isogamy and heterogamy may he re garded as the normal stages in the evolution of sex among plants, there is a special form of sexuality among the red algae (Rhodophycem, q.v.) that deserves mention. In this group, al though a male cell or sperm is developed, as in cases of ordinary heterogamy, the female organ (procarp) develops no distinct egg, but is dif ferentiated into two regions, namely a bulbous base (carpogonium) with a hair-like prolonga tion (trichogyne) with which the male cell fuses, and thus fertilizes the carpogonium, by which, more or less directly, spores are developed. In this ease, therefore, there is a sexual act involving a sperm or its equivalent, but no egg. This sexual does not result in a distinct spore, but in the final formation of a fruit-like struc ture (eystocarp) containing spores. This pecu liar modification of heterogamy may be called carpogamy, which is fertilization of a carpogo nium rather than of an egg.