192 TROPICAL NATURE, AND OTHER ESSAYS. 



lead to the destruction of all adverse variations, and 

 thus keep up in continually increasing complexity the 

 outward mimicry which now so amazes us. During 

 the long ages in which this process has been going 

 on, and the Danaidae have been acquiring those speciali- 

 ties of colour which aid in their preservation, many 

 a Leptalis may have become extinct from not varying 

 sufficiently 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 num- 

 ber 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 Danaidse 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 Danaidae 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. 1 



Theory of Sexual Colours. In Mr. Darwin's cele- 

 brated 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 



1 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 Heliconidae and Leptalidse, in Hewitson's Exotic Butter/lies, and Felder's 

 Voyage of the " Novara" may be examined. 



