he females of Papilio are more variable and more often tend to
become dimorphic than those of Cosmodesmus, while the latter group has
more often happened to possess a sufficient foundation for the origin of
the resemblance in patterns which, from the start, were common to male
and female.

(5) In very variable species with sexes alike, mimicry can be rapidly
evolved in both sexes out of very small beginnings. Thus the reddish
marks which are common in many individuals of Limenitis arthemis were
almost certainly the starting-point for the evolution of the beautifully
mimetic L. archippus. Nevertheless in such cases, although there is
no reason to suspect any greater variability, the female is commonly a
somewhat better mimic than the male and often a very much better mimic.
Wallace's principle seems here to supply the obvious interpretation.

(6) When the difference between the patterns of the model and presumed
ancestor of the mimic is very great, the female is often alone mimetic;
when the difference is comparatively small, both sexes are commonly
mimetic. The Nymphaline genus Hypolimnas is a good 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 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 I. of Darwin's Section of the Joint Essay
on Natural Selection, read July 1st, 1858, before the Linnean Society.
("Journ. Proc. Linn. Soc." Vol. III. 1859, page 50.) The principles are
so clearly and sufficiently stated in these brief sentences that it
is appropriate to quote the whole: "