THE COLORS OF ANIMALS AND PLANTS. 



533 



America are so immensely numerous and so 

 greatly varied, not only in color but in structure, 

 that we may be sure they are of vast antiquity 

 and have undergone great modification. A large 

 number of them, however, are still of compara- 

 tively plain colors, often rendered extremely ele- 

 gant by the delicate transparency of the wing- 

 membrane, but otherwise not at all conspicuous. 

 Many have only dusky or purplish bands or spots, 

 others have patches of reddish or yellowish-brown 

 — perhaps the commonest color among butter- 

 flies — while a considerable number are tinged or 

 spotted with yellow, also a very common color, 

 and one especially characteristic of the Pieridae> 

 the family to which Leptalis belongs. We may 

 therefore reasonably suppose that in the early 

 stages of the development of the Danaidae, when 

 they first began to acquire those nauseous secre- 

 tions which are now their protection, their colors 

 were somewhat plain, either dusky with paler 

 bauds and spots, or yellowish with dark borders, 

 and sometimes with reddish bands or spots. At 

 this time they had probably shorter wings find a 

 more rapid flight, just like the other unprotected 

 families of butterflies. But as soon as they be- 

 came decidedly unpalatable to any of their ene- 

 mies, it would be an advantage to them to be 

 readily distinguished from all the eatable kinds ; 

 and as butterflies were no doubt already very 

 varied in color, while all probably had wings 

 adapted for pretty rapid or jerking flight, the best 

 distinction might have been found in outline and 

 habits ; whence would arise the preservation of 

 those varieties whose longer wings, bodies, and 

 antennae, and slower flight, rendered them notice- 

 able — characters which now distinguish the whole 

 group in every part of the world. Now, it would 

 be at this stage that some of the weaker-flying 

 Pieridaa which happened to resemble some of the 

 Danaidas around them in their yellow and dusky 

 tints, and in the general outline of their wings, 

 would be sometimes mistaken for them by the 

 common enemy, and would thus gain an advan- 

 tage in the struggle for existence. Admitting 

 this one step to be made, and all the rest must 

 inevitably follow from simple variation and sur- 

 vival of the fittest. So soon as the nauseous but- 

 terfly varied in form or color to such an extent 

 that the corresponding eatable butterfly no longer 

 closely resembled it, the latter would be exposed 

 to attacks, and only those variations would be 

 preserved which kept up the resemblance. At 

 the same time we may well suppose the enemies 

 to become more acute and able to detect smaller 

 differences than at first. This would lead to the 



destruction of all adverse variations, and thus 

 keep up in continually-increasing complexity the 

 outward mimicry which now so amazes us. Dur- 

 ing the long ages in which this process has been 

 going on, many a Leptalis may have become ex- 

 tinct from not varying sufficiently in the right 

 direction, and at the right time, to keep up a pro- 

 tective resemblance to its neighbor ; and this will 

 accord with the comparatively small number of 

 cases of true mimicry as compared with the fre- 

 quency of those protective resemblances to vege- 

 table or inorganic objects whose forms are less 

 definite and colors less changeable. About a 

 dozen other genera of butterflies and moths mimic 

 the Danaidas 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 Danaidas first 

 acquired their protective secretions, happened 

 outwardly to resemble some of them, and have by 

 concurrent variation, aided by a rigid selection, 

 been able to keep up that resemblance to the 

 present day. 1 



Theory of Sexual Colors. — In Mr. Darwin's 

 celebrated work, " The Descent of Man and Se- 

 lection in Relation to Sex," he has treated of sex- 

 ual color in combination with other sexual char- 

 acters, and has arrived at the conclusion that all, 

 or almost all, the colors of the higher animals 

 (including among these insects and all vertebrates) 

 are due to voluntary sexual selection ; and that 

 diversity of color in the sexes is due, primarily, 

 to the transmission of color-variations either to 

 one sex only, or to both sexes, the difference de- 

 pending 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 color was the 

 need of protection, repressing in the female those 

 bright colors which are nominally produced in 

 both sexes by general laws ; and I have attempt- 

 ed to explain many of the more difficult cases on 

 this principle ("A Theory of Birds' Nests," in 

 " Contributions," etc., p. 231). As I have since 

 given much thought to this subject, and have ar- 

 rived at some views which appear to me to be of 



1 For fuller information on this subject the reader 

 should consult Mr. Bates's original paper. " Contributions 

 to an Insect-fauna of the Amazon Yalley ," in " Transac- 

 tions of the Linnean Society,'" vol. xxiii., p. 495; Mr. Tri- 

 men'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 Heliconidas and 

 Leptalida?, in Hewitson's " Exotic Butterflies," and Fel- 

 der's " Voyage of the Novara," may be examined. 



