10 MR. A. R. WALLACE ON THE PAPILIONID^ 



albinoism and melanism may, as I have already stated, be considered as analogous facts ; 

 and I met with one case of a bird, a species of Lory {Eos fuscata, Blyth), clearly existing 

 under two forms, since I obtained both sexes of each from a single flock. 



The fact of the tAvo sexes of one species differing very considerably is so common, that 

 it attracted but little attention till Mr. DarAvin showed how it could in many cases be 

 explained by what he termed sexual selection. For instance, in most polygamous animals 

 the males fight for the possession of the females, and the victors, always becoming the 

 progenitors of the succeeding generation, impress upon their male offspring their own 

 sxiperior size, strength, or unusually developed offensive weapons. It is thus that we can 

 account for the spurs and the superior strength and size of the males in Gallinaceous 

 birds, and also for the large canine tusks in the males of fruit-eating Apes. So the 

 superior beauty of plumage and special adornments of the males of so many birds 

 can be explained by supposing (what there are many facts to prove) that the females 

 prefer the most beavitiful and perfect-plumaged males, and that thus slight accidental varia- - 

 tions of form and colour have been accumulated till they have produced the wonderful 

 train of the Peacock and the gorgeous plumage of the Bird of Paradise. Both these 

 causes have no doubt acted partially in insects, so many species possessing horns and power- 

 ful jaws in the male sex only, and still more frequently the males alone rejoicing in rich 

 colours or sparkling lustre. But there is here another cause which has led to sexual 

 differences, viz. a special adaptation of the sexes to diverse habits or modes of life. This 

 is well seen in female Buttei-flies (which are generally weaker and of slower flight), often 

 having colours better adapted to concealment ; and in certain South American species 

 {Papilio torquatus) the females, which inhabit the forests, resemble the ^neas group, 

 which abound in similar localities, while the males, which frequent the sunny open river- 

 banks, have a totally different coloration. In these cases, therefore, natural selection 

 seems to have acted independently of sexual selection ; and all such cases may be con- 

 sidered as examples of the simplest dimorphism, since the offspring never offer interme- 

 diate varieties between the parent forms. 



The distinctive character therefore of dimorphism is this, that the union of these dis- 

 tinct forms does not produce intermediate varieties, but reproduces them unchanged. 

 In simple varieties, on the other hand, as well as when distinct local forms or distinct 

 species are crossed, the offspring never resembles either parent exactly, but is more or 

 less intermediate between them. Dimorphism is thus seen to be a specialized result of 

 variation, by which new physiological phenomena have been developed ; the two should 

 therefore, whenever possible, be kept separate*. 



3. Local form, or variety. — This is the first step in the transition from variety to species. 



* The phenomena of dimorphism and polymorphism may be well illustrated by supposing that a blue-eyed, flaxen- 

 haired Saxon man had two wives, one a black-haired, red-skinned Indian squaw, the other a woolly-headed, sooty- 

 skinned negress— and that instead of the children being mulattoes of brown or dusky tints, mingling the separate 

 characteristics of their parents in varying degrees, all the boys should be pure Saxon boys like their father, while the 

 girls should altogether resemble their mothers. This would be thought a sufficiently wonderful fact ; yet the phe- 

 nomena here brought forward as existing in the insect-world are still more extraordinary ; for each mother is capablg 

 not only of producing male offspring like the father, and female like herself, but also of producing other females 

 exactly like her fellow-wife, and altogether differing from herself. If an island could be stocked with a colony of 

 human beings having similar physiological idiosyncrasies with Papilio Pammon or Papilio Ormenus, we should see 



