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 sonic 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.
tional interpretation of the variations which he was previously content to pos tulate as facts. For he says that "orna ment is the natural outcome and direct product of superabundant health and vigor," and is "due to the general laws of growth and development." It seems to some that this mode of interpreting characters is of far-reaching importance, and that it affects not only the theory of sexual selection but that of natural se lection as well.
To sum up, the problems involved in sexual selection are (1) what physio logical conditions explain the secondary sexual characters which so often distin guish males and females; (2) to what extent and in what degree of refinement does preferential mating occur; and (3) to what extent has sexual selection guided the differentiation of the sexes alike in distinctive qualities and in xs thetic sensitiveness? Before these prob lems can be adequately solved many more facts must be accumulated.