Page:Collier's New Encyclopedia v. 08.djvu/417

LEFT SEXTON 361 SEXUAL SELECTION construction depends on the theorem that if a ray of light suffer double reflection the angle between the original ray and its direction after the second reflection is double the angle made by the reflecting surfaces. The instrument of which this theorem is the principle is a brass sector of a circle in outline, the sector being the sixth part of a complete circle, for which reason the instrument is called a sextant. It consists of a graduated limb, forming about the sixth part of a circle. Two mirrors, the index mirror (1) and the lower part of the horizon glass at 5, have for object the reflection of the rays of light coming from the object under observation. The telescope (2) collects and transmits to the eye the rays of light emanating from the horizon glasses. An index and a vernier (13) permit the read- ing, on the graduated limb, of the quan- tity of light which the mirror has turned. When observing altitudes, the instru- ment is held perpendicularly to the hori- zon in the plane of the line joining the two objects. In taking noon observations at sea, to determine the latitude, the ob- server takes his place shortly before meridian, and turning down one or sev- eral of the shades, to prevent his eye being injured by the glare, directs the telescope or site tube to the sun, moving the index so as to bring its reflected image to coincide with the sea horizon; as the sun rises, he gradually advances the limb, clamping it and using the regu- lating screw for this purpose, as the sun's path becomes more nearly horizontal, and slightly rocking the instrument from side to side to insure that it is in a vertical plane at the moment when the sun at- tains its greatest height. The reading of the limb at the moment when the sun begins to dip is noted, and a very simple calculation, adding his declination de- rived from the "Nautical Almanac" to the true zenith distance obtained by ob- servation, gives the latitude. SEXTON, an under officer of the church, whose duty is to take care of the vessels, vestments, etc., belonging to the church, to attend on the officiating min- ister, and perform other duties pertain- ing to the church, to which is, in England, added the duty of digging and filling up graves in the churchyard. SEXUAL SELECTION, a term applied by Darwin to the process of favoring and eliminating which to some extent occurs in the mating of many animals. It is a special case of natural selection, depend- ing on a competition between rival males, in which a premium is set on those quali- ties which favor their possessors in se- curing mates. This competition takes two forms: On the one hand, rival mates, for instance stags and gamecocks, fight with one another, and the conquerors have naturally the preference in mating; on the other hand, rival males sometimes seem to vie with one another in display- ing their attractive qualities before their desired mates, who, according to Darwin, choose those that please them best. Where there is direct competition be- tween males, the weakest will tend to be eliminated, either directly by death or in- jury in the struggle, or indirectly by diminished success in reproduction. In the same way, if a male be lacking in the qualities necessary to find a mate — e. g., in senses acute enough to find out her whereabouts — that male may remain un- reproductive. In regard to the second aspect of sexual selection, in which the females are be- lieved to exercise some choice, giving the preference to those suitors which have brighter colors, more graceful forms, sweeter voices, or greater charms of some kind, there is no little difference of opin- ion. Darwin indeed believed strongly in the female's choice, and referred to this process of selection many of the qualities which distinguish male animals. On the other hand, Alfred Russel Wal- lace maintains a very different position. "There is," he says, "a total absence of any evidence that the females admire or even notice the display of the males. Among butterflies there is literally not one particle of evidence that the female is influenced by color or even that she has any power of choice, while there is much direct evidence to the contrary." The theory of sexual selection is of con- siderable importance in a general theory of evolution. This may be illustrated in reference to the bright plumage of many birds. If we believe that the fe- males are sensitive to the slight excel- lences which distinguished one suitor from another and that their choice of mates is determined by these excellences (which Wallace emphatically denies), then we may say that the greater brightness of male birds may have been evolved by sexual selection. This was Darwin's opinion. Before we can believe that attractively bright ornaments could become charac- teristic of males by sexual selection, or that protectively plain coloring could be- come characteristic of females by natural selection, we must assume that the quali- ties of brightness can be entailed in in- heritance on the males only, and the qualities of plainness on the females only. But this fundamental assumption has not yet been justified by a sufficiently strong body of facts. Wallace has also in his work on "Dar- winism" (1889) worked toward a ra-