du de la Societe Entomologique de Belgaue_,
series ii., No. 59, 1878.]

[Footnote 112: _Nature_, vol. xxxiv. p. 547.]

[Footnote 113: _Proceedings of the Zool. Soc. of London_, 1870, p. 369.]

[Footnote 114: _The Naturalist in Nicaragua_, p. 321.]

[Footnote 115: Mr. Belt first suggested this use of the light of the
Lampyridae (fireflies and glow-worms)--_Naturalist in Nicaragua_, p.
320. Mr. Verrill and Professor Meldola made the same suggestion in the
case of medusae and other phosphorescent marine organisms (_Nature_,
vol. xxx. pp. 281, 289).]

[Footnote 116: W.E. Armit, in _Nature_, vol. xviii. p. 642.]

[Footnote 117: _Proc. Ent. Soc._, 1869, p. xiii.]

CHAPTER X

COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX

Sex colours in the mollusca and crustacea--In insects--In
butterflies and moths--Probable causes of these colours--Sexual
selection as a supposed cause--Sexual coloration of birds--Cause
of dull colours of female birds--Relation of sex colour to
nesting habits--Sexual colours of other vertebrates--Sexual
selection by the struggles of males--Sexual characters due to
natural selection--Decorative plumage of males and its effect on
the females--Display of decorative plumage by the males--A
theory of animal coloration--The origin of accessory
plumes--Development of accessory plumes and their display--The
effect of female preference will be neutralised by natural
selection--General laws of animal coloration--Concluding
remarks.

In the preceding chapters we have dealt chiefly with the coloration of
animals as distinctive of the several species; and we have seen that, in
an enormous number of cases, the colours can be shown to have a definite
purpose, and to be useful either as a means of protection or
concealment, of warning to enemies, or of recognition by their own kind.
We have now to consider a subordinate but very widespread
phenomenon---the differences of colour or of ornamental appendages in
the two sexes. These differences are found to have special relations
with the three classes of coloration above referred to, in many cases
confirming the explanation already given of their purport and use, and
furnishing us with important aid in formulating a general theory of
animal coloration.

In comparing the colours of the two sexes we find a perfect gradation,
from absolute identity of colour up to such extreme difference that it
is diffi