206 [January 



to the immediate action of heat, &c. on the individual ; when such a 

 variety is geographical, I am inclined to think that it is so. because it 

 has become adapted to some peculiarity in its surroundings which does 

 not exist in other portions of the area of the species where the variety 

 does not occur. It is thus that I would account for the Glaucus vari- 

 ety of the female of Papilio Turnus and all similar cases. 



It is of course easy to ascertain whether it is the female or the male 

 that has diverged from its type in any genus. When it is the male, 

 the females retain the type common to the genus or group to which the 

 species belong, and vice versa. Therefore in the cases of Argynnis 

 Diana, Sagana, Paphia, Papilio Turnus, &c. it is the females which 

 have been brought under the operation of natural selection In the 

 case of Gallinaceous and Humming birds it is the males; the form of 

 selection which has been at work here is called by .Mr. Darwin Sexual 

 Selection. It is very interesting in Butterflies to study those genera 

 which exhibit sexual divergence. In Tropical America there are many 

 such. The genus Epicalia affords one of the best cases. A good series of 

 species of this genus exhibits almost all stages of sexual divergence in a 

 small compass. Epicalia Uyaniris and Ep. Capenaa scarcely differ in 

 the two sexes; the male only having a little more color than his partner. 

 In Eji. Ancra the male shines forth with a brilliant orange belt, denied 

 to his mate, although the latter has a handsome blue belt as good as 

 that of her husband. Further on Ep. Orsis shows a wide divergence, 

 the female being striped with slaty blue and white, whilst the male is 

 silky blue. Afterwards we come to the extreme forms of the genus 

 Ep. Acontius, Ep. Nkimilius arid others. In these the females are 

 nearly all similar to that of Ep. Orsix and to both sexes of Ep. Oyani- 

 ris, but the males are so wonderfully different, not only in color but 

 sometimes {Ep. Numilius) in form, that Entomologists bad always 

 placed them in separate genera, until they were detected in copula. A 

 similar gradation is observable in the Papiliones of the P. ^Eneas group. 



A great deal, however, remains to be done before this interesting 

 subject can be placed in such a light as to become clear to every one. 

 We require good cases where the course of variation and its incipient 

 selection of one of the varieties may be seen iu process, such as that 

 which I think I have detected in the Heliconidse, as illustrating the 

 formation of mimetic forms, and described in the memoirs of which I 

 sent you a copy. Then there is a mystery still to be cleared up in 

 the inheritance of varieties by one sex and not by the other. These 

 phenomena in domestic poultry require careful observation and expe- 



