364 TROPICAL NATURE v 
increasing complexity the outward mimicry which now so 
amazes us. During the long ages in which this process has 
been going on, and the Danaide have been acquiring those 
specialities of colour which aid in their preservation, many a 
Leptalis may have become extinct from not varying sufii- 
ciently in the right direction and at the right time to keep 
up a protective resemblance to its neighbour ; and this well 
accords with the comparatively small number of cases of true 
mimicry, as compared with the frequency of those protective 
resemblances to vegetable or inorganic objects whose forms 
are less definite and colours less changeable. About a dozen 
other genera of butterflies and moths mimic the Danaide in 
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 Danaide first 
acquired their protective secretions, happened outwardly to 
resemble some of them, and which have, by concurrent varia- 
tion aided by a rigid selection, been able to keep up that 
resemblance to the present day. 
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 trans- 
mission 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 
1 For fuller information on this subject the reader should consult Mr. 
Bates’ 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,” etc., 
already referred to ; and, in the absence of collections of butterflies, the plates 
of Heliconide and Leptalide, in Hewitson’s Exotic Butterflies ; and Felder’s 
Voyage of the “ Novara,” may be examined. 
