NATURAL SELECTION 395 



egg laid by the other. In a case like this, we have the result of a Men- 

 delian experiment performed by nature. The different phases are rep- 

 resented 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 be- 

 lieve, 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 num- 

 ber 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 com- 

 paratively 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 Lepi- 

 doptera, where these studies are most interesting, the work is being 

 immensely facilitated by the publication of Dr. Seitz's magnificent vol- 

 umes 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 appar- 

 ently 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 Orniihoptera series, usually treated as a dis- 

 tinct genus. The great diversity of the sexes, both in form and color, 

 is extremely impressive in view of what we now know about sex-inheri- 

 tance. The bright colors are most commonly orange, often green, 



