Sexual Selection 295 



example. In Hypolimnas itself the females mimic Danainae with 

 patterns very different from those preserved by the non-mimetic 

 males: in the sub-genus Euralia, both sexes resemble the black 

 and white Ethiopian Danaines with patterns not very dissimilar from 

 that which we infer to have existed in the non-mimetic ancestor. 



(7) Although a mclanic form or other large variation may be 

 of the utmost imiwrtance in facilitating the start of a mimetic 

 likeness, it is impossible to explain the evolution of any detailed 

 resemblance in this manner. And even the large colour variation 

 itself may well be the expression of a minute and "continuous" 

 change in the chemical and physical constitution of pigments. 



Sexual Selection (Epigamic Characters). 



We do not know the date at which the idea of Sexual Selection 

 arose in DarAvin's mind, but it was probably not many years after the 

 sudden flash of insight which, in October 1838, gave to him the 

 theory of Natural Selection. An excellent account of Sexual 

 Selection occupies the concluding paragraph of Part i. of Darwin's 

 Section of the Joint Essay on Natural Selection, read July 1st, 1858, 

 before the Linnean Society \ The principles are so clearly and 

 sufficiently stated in these brief sentences that it is appropriate to 

 quote the whole : "Besides this natural means of selection, by which 

 those individuals are preserved, whether in their egg, or larval, or 

 mature state, Avhich are best adapted to the place they fill in nature, 

 there is a second agency at work in most unisexual animals, tending 

 to produce the same efi'ect, namely, the struggle of the males for the 

 females. These struggles are generally decided by the law of battle, 

 but in the case of birds, apparently, by the charms of their song, 

 by their beauty or their power of courtship, as in the dancing rock- 

 thrush of Guiana. The most vigorous and healthy males, imi)lying 

 perfect adaptation, must generally gain the victory in their contests. 

 This kind of selection, however, is less rigorous than the other; it 

 does not rccpiire the death of the less successful, but gives to them 

 feAver descendants. The struggle falls, moreover, at a time of year 

 Avhen food is generally abundant, and perhaps the effect chiefly pro- 

 duced Avould be the modification of the secondary sexual characters, 

 Avhich are not related to the poAver of obtaining food, or to defence 

 fiom enemies, but to fighting Avith or rivalling other males. The 

 result of this struggle amongst the males may be compared in some 

 respects to that produced by those agriculturists Avho pay less 

 attention to the careful selection of all their young animals, and more 

 to the occasional use of a choice mate." 



' Juurit. I'luc. Linn. Sac. Vol. iii. 1859, p. 60. 



