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 Ewralia, 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 melanic form or other large variation may be 
of the utmost importance 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 Darwin’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 1 of Darwin’s 
Section of the Joint Essay on Natural Selection, read July Ist, 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, which 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 effect, 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, implying 
perfect adaptation, must generally gain the victory in their contests. 
This kind of selection, however, is less rigorous than the other; it 
does not require the death of the less successful, but gives to them 
fewer descendants. The struggle falls, moreover, at a time of year 
when food is generally abundant, and perhaps the effect chiefly pro- 
duced would be the modification of the secondary sexual characters, 
which are not related to the power of obtaining food, or to defence 
from enemies, but to fighting with or rivalling other males. The 
result of this struggle amongst the males may be compared in some 
respects to that produced by those agriculturists who pay less 
attention to the careful selection of all their young animals, and more 
to the occasional use of a choice mate.” 
1 Journ. Proc. Linn. Soc. Vol. 111. 1859, p. 50. 
