Page:Popular Science Monthly Volume 82.djvu/399

Rh egg laid by the other. In a case like this, we have the result of a Mendelian experiment performed by nature. The different phases are represented by interchangeable units, and interbreeding normally occurs. Hence, an extreme case of polymorphism, in which all the alternative forms which have been preserved are at present favored in the struggle for existence. Of those which, in the ages past, have disappeared, we have now no trace, but theoretically we should expect some non-mimetic recessive combinations to occur as occasional aberrations. This, I believe, accords with the facts.

Those who examine remarkably adapted forms are always impressed by their striking characters, and find it hard to believe that they have arisen by gradual steps from the more ordinary types. They do not appreciate the ages during which these forms have been evolving, and the multitudes that have perished. Among insects, however, the number of surviving species is usually much greater than in any other group of animals, so that it is possible in a certain sense to compare a specialized type with its ancestors, or at least with contemporaneous species having many of the marks of its ancestors. For this reason insects are exceptionally valuable for the study of evolution; though hardly equal to mammals, which have changed so rapidly within comparatively recent times, and have left us such admirable fossil remains. It would be a useful contribution to the theory of evolution to take up a number of the cases of mimetic or otherwise peculiar insects and show how they are connected by many steps with the more ordinary forms. This has, indeed, been done in part, but it has been difficult, requiring immense and carefully worked out collections. In the Lepidoptera, where these studies are most interesting, the work is being immensely facilitated by the publication of Dr. Seitz's magnificent volumes on the Macrolepidoptera of the world, which place descriptions and good colored figures of all the principal larger Lepidoptera at the service of any one who can afford the very moderate price charged.

We may consider, for example, the "Aristolochia Papilios." These splendid butterflies feed in the larva state on Aristolochia, rarely on allied plants. They occur on both sides of the world, and are doubtless, as a group, of great antiquity. They are strong-smelling and apparently distasteful to most predatory animals; the other two groups of Papilio, not thus protected, frequently produce species which closely imitate them, so much so that "until quite recently models and mimics have often been regarded as closely allied." The great Indo-Australian series of Aristolochia Papilios shows the largest size and extraordinary sexual dimorphism in the Ornithoptera series, usually treated as a distinct genus. The great diversity of the sexes, both in form and color, is extremely impressive in view of what we now know about sex-inheritance. The bright colors are most commonly orange, often green,