April 2A^^ 1879] 



NATURE 



587 



established beyond doubt. Were this not the case 

 Antiochus would be without a female and Medea without 

 a male. Indeed Fritz Miiller has reared from larvae both 

 sexes of an allied species, Epicalia numilia, which differ 

 from one another to the same extent as do the sexes of 

 E. acontins. In both sexes of this latter species the 

 general ground colour of the wings is black, the male 

 having a broad oblique bar of a bright orange colour 

 extending from about the middle of the inner margin to 

 about the middle of the fore-wing in the direction of the 

 apex. There is a corresponding blotch near the middle 

 of the hind-wing, so that when the wings are extended 

 the bar on the fore-wing is continuous with the blotch on 

 the hind-wing, the whole forming one oblique orange bar. 

 The female {Medea) has two oblique rows of pale yellow 

 spots across the fore-wings running nearly parallel with 

 the costal margin, and two similar rows across the hind- 

 wings ; when the insect rests with outstretched wings, the 

 fore- and hind-wings overlap so that the spots of all four 

 wings form three straight parallel rows which are con- 

 tinued on the body by spots of the same colour. The 

 sexes of -£". numilia differ in a similar manner. 



Further, in the female of E. acontius (as in both sexes 

 of E. numilia), the inner margin of the fore-wing is nearly 

 straight, while it is markedly curved in the male. Both 

 wings in this latter sex are also much broader in propor- 

 tion to their length than is the case with the female, and 

 in consequence of this, the wings of Antiochus overlap 

 each other to such an extent that nearly half the hind- 

 wing is hidden beneath the fore-wing, the space thus con- 

 cealed being fully twice as broad as in Medea. The 

 curvature of the inner margin of the fore-wing of a 

 butterfly when exaggerated on the over-lapping portions 

 of the two wings, is, according to the author, a never- 

 failing indication of the presence of a scent-secreting 

 organ at this spot. Thus, having read in Doubleday and 

 Westwood's " Genera " that in the fore-wing of Ageronia 

 " the inner margin in the male is occasionally dilated," 

 Fritz Miiller caught a male specimen oi A. arethusa, and 

 found a strong odour to be emitted by a scent organ 

 concealed between the wings. Now in Antiochus a 

 similar organ exists, while it is absent in the male of 

 E. numilia, and in this latter the fore and hind wings 

 overlap only to the same extent as in the female. 



When in lepidoptera the sexes of a species differ from 

 one another to any great extent in colour and marking, 

 the female is generally inconspicuous or is coloured 

 gaudily in imitation of some other species (mimicry). 

 Thus in Thecla hemon the male is bright blue, while the 

 female is duU broAvn, while in Dyschema amphissa the 

 male is white, and the female is one of the numerous 

 mimickers of Acrcea thalia. This explanation, however, 

 does not apply to the female of E. acontius, since there 

 is no species marked in a similar manner which might 

 serve as a model for mimicry. On the other hand, the 

 Medea type of marking is to be found in a large number 

 of species of the same and of allied genera (the female of 

 Myscelia orsis, for example). Neither can the coloration 

 of Medea be considered protective, since it is very con- 

 spicuous, and the insect has a habit of sitting with wings 

 fully expanded. 



According to Darwin's theory of sexual selection,^ the 

 ancestor of the present genus Epicalia was probably of 

 the Medea type— the present form of the male having 

 resulted from selection by the female. The author then 

 asks whether Medea has preserved the form of marking 

 common to both sexes of the progenitor, and whether 

 this marking has any present significance; also : "Is the 

 colour ornamental, or for offensive or defensive purposes, 

 or both ?— for the one does not exclude the other." In 

 reply to the latter part of this question, dissent is ex- 

 pressed from Prof. Gustav Jaeger's view, that yellow is 

 asa rule an offensive or defensive coloiir. It is next pointed 

 ' " Descent of Man." L 388. 



out that in the female of E. numilia, the row of spots is 

 replaced by a broad oblique yellow bar, this alteration of 

 pattern being attributed to sexual selection by the males, 

 which must have thus preserved but at the same time 

 slightly modified, the taste of the commo« ancestor of the 

 genera Epicalia and Myscelia, the females of a few spe- 

 cies of which have been made to depart to a much 

 greater extent from their congeners by a greater diver- 

 gence of taste on the part of their mates. The females 

 of most of the species of these genera had, however, " set 

 the fashion" in a completely new direction, and thus 

 brought about the dissimilarity of the males. 



In support of this view the author remarks, that 

 although sexual selection is generally regarded as being 

 exerted by the females, yet, as Haeckel has maintained,^ 

 the selection by the males must have an equal influence 

 on the opposite sex. That such a choice is exerted by 

 butterflies the author has already pointed out.' In the 

 present case we must believe that the two sexes mani- 

 fested completely different tastes,^ just in the same 

 manner as much that we consider physically or intellec- 

 tually superior in woman woxild be considered imfitting 

 for men. 



The acquisition and modification of the Medea type of 

 marking may have occurred at a time when both sexes 

 of the ancestral form were alike. Such peculiar marking 

 could not have been produced by the direct action of 

 external conditions nor by any innate " laws of growth," 

 neither can it be considered as a protective colouring 

 produced by natural selection. Sexual selection is thus 

 the only explanation left open. 



It has been shown by Weismann that the colour and 

 marking of butterflies' wings are undoubtedly affected by 

 external conditions, and in the case of lan-ae markings, 

 which, through such conditions, make their appearance 

 on one segment, not unfrequently extend to other seg- 

 ments (by correlation of growth ?). The same appears to 

 hold good for the wings of butterflies : markings which 

 through any cause appear in any one wing cell tend to be 

 repeated on corresponding places in the other wing-cells. 

 When such markings serve as signs of distastefulness or 

 for other protective purposes, they would be preserved, 

 and even increased in brilliancy and size by the action of 

 natural selection. Thus a striped butterfly might be pro- 

 duced from a simple grey or brown one, and the markings 

 regularly repeated on the corresponding places of the 

 wing-cells would not fail to give us a pleasing impression, 

 although no selection with special regard to beauty had 

 taken place. In such cases, however, it is obviously im- 

 material whether the markings of the fore- and hind- 

 wings harmonise or not. When, however, we have an 

 unbroken bar across both fore- and hind-wings so arranged 

 that the pattern is only complete when the insect sits with 

 outstretched wings, or is in an attitude of flight, while in 

 every other position the bar is broken, it may be safely 

 assumed that the ever-vigilant eye of selection had brought 

 about this result. 



The markings oi Medea are then considered from this 

 point of view. The two rows of yellow spots on each 

 wing, as already described, form three straight rows 

 when the wings are spread out as in flight ; in any other 

 position— if, for instance, the fore- wings are pushed too 

 forward or too far backwards — the symmetry is broken. 

 Special attention is called to the fact that the hindmost 

 rows of spots on the hind-wings have been distorted so 

 as to form a straight bar parallel with the other rows ; 

 this results from the displacement of the spots, each of 

 which, although situated in one wing-cell, does not appear 

 on the corresponding place in each cell ; were this the 

 case, the row would be curved instead of straight. That 

 it was the sense of beauty of a critical eye which straight- 



' " Generelle Morphologic," 1866, LL 244. _ ■ Kosmos, ii. 42 



3 The term "reciprocal sexual selection" might be advantageously 

 applied to such ciasses of cases. — R. M, 



