various parts of the world, and exactly the same explanation will apply to all of them. They represent those species of each group which, at the time when the Danaidæ first acquired their protective secretions, happened outwardly to resemble some of them, and which have, by concurrent variation aided by a rigid selection, been able to keep up that resemblance to the present day.[19] [19] For fuller information on this subject the reader should consult Mr. Bates’s original paper, “Contributions to an Insect-fauna of the Amazon Valley,” in _Transactions of the Linnean Society_, vol. xxiii. p. 495; Mr. Trimen’s paper in vol. xxvi. p. 497; the author’s essay on “Mimicry,” &c., already referred to; and in the absence of collections of butterflies, the plates of Heliconidæ and Leptalidæ, in Hewitson’s _Exotic Butterflies_, and Felder’s _Voyage of the “Novara,”_ may be examined. _Theory of Sexual Colours._--In Mr. Darwin’s celebrated work, _The Descent of Man and Selection in Relation to Sex_, he has treated of sexual colour in combination with other sexual characters, and has arrived at the conclusion that all or almost all the colours of the higher animals (including among these insects and all vertebrates) are due to voluntary or conscious sexual selection; and that diversity of colour in the sexes is due, primarily, to the transmission of colour-variations either to one sex only or to both sexes; the difference depending on some unknown law, and not being due to natural selection. I have long held this portion of Mr. Darwin’s theory to be erroneous; and have argued that the primary cause of sexual diversity of colour was the need of protection, repressing in the female those bright colours which are normally produced in both sexes by general laws; and I have attempted to explain many of the more difficult cases on this principle. (“A Theory of Birds’ Nests,” in _Contributions, &c._, p. 231.) As I have since given much thought to this subject, and have arrived at some views which appear to me to be of considerable importance, it will be well to sketch briefly the theory I now hold, and afterwards show its application to some of the detailed cases adduced in Mr. Darwin’s work.