PDF - Wallace Online
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362 TROPICAL NATUREprotective, serving as a warning of their uneatableness. 1 Onour theory none of these colours offer any difficulty. Conspicuousnessbeing useful, every variation tending to brighterand purer colours was selected ;the result being the beautifulvariety and contrast we find.Imitative Warning Colours the Theory of MimicryWe now come to those groups which gain protection solelyby being mistaken for some of these brilliantly coloured butuneatable creatures, and here a difficulty really exists, and tomany minds is so great as to be insuperable.It will be welltherefore to endeavour to explain how the resemblance inquestion may have been brought about.The most difficult case, and the one which may be takenas a type of the whole class, is that of the genus Leptalis (agroup of South American butterflies allied to our commonwhite and yellow kinds), many of the larger species of whichare still white or yelloAv, and which are all eatable by birdsand other insectivorous creatures. But there are also anumber of species of Leptalis, which are brilliantly red,yellow, and black, and which, band for band and spot forspot, resemble some one of the Danaidse or Heliconidae whichinhabit the same district and which are nauseous and uneatable.Now the usual difficulty is, that a slight approach toone of these protected butterflies would be of no use, while agreater sudden variation is not admissible on the theory ofgradual change by indefinite slight variations. This objectiondepends almost wholly on the supposition that, when the firststeps towards mimicry occurred, the South American Danaidsewere what they are now ;while the ancestors of the Leptalideswere like the ordinary white or yellow Pieridse to which theyare allied. But the Danaioid butterflies of South America areso immensely numerous and so greatly varied, not only in colourbut in structure, that we may be sure they are of vast antiquityand have undergone great modification. A large number ofthem, however, are still of comparatively plain colours, oftenrendered extremely elegant by the delicate transparency ofthe wing membrane, but otherwise not at all conspicuous.1 This has since been found to be the case by Professor Herdman (Trans.Biol. Soc. Liverpool, vol. iv. p. 150).
v COLOURS OF ANIMALS 368Many have only dusky or purplish bands or spots; othershave patches of reddish or yellowish brown perhaps thecommonest colour among butterflies ;while a considerablenumber are tinged or spotted with yellow, also a very commoncolour, and one especially characteristic of the Pieridas, theWe may therefore reason-family to which Leptalis belongs.ably suppose that in the early stages of the development ofthe Danaidse, when they firstbegan to acquire those nauseoussecretions which are now their protection, their colours weresomewhat plain either ; dusky with paler bands and spots, oryellowish with dark borders, and sometimes with reddishbands or spots.At this time they had probably shorterwings and a more rapid flight, just like the other unprotectedfamilies of butterflies. But, so soon as they became decidedlyunpalatable to any of their enemies, it would be an advantageto them to be readily distinguished from all the eatable kinds ;and as butterflies were no doubt already very varied in colour,while all probably had wings adapted for rather quick orjerking flight, the best distinction might have been found inoutline and habits ;whence would arise the preservation ofthose varieties whose longer wings, bodies, and antennae, aswell as their slower flight, rendered them noticeablecharacters which now distinguish the whole group in everypart of the world.Now it would be at this stage that some of the weakerflyingPieridse which happened to resemble some of theDanaidse around them in their yellow and dusky tints and inthe general outline of their wings, would be sometimes mistakenfor them by the common enemy, and would thus gainan advantage in the struggle for existence. Admitting thisone step to be made, and all the rest must inevitably followfrom simple variation and survival of the fittest. So soon asthe nauseous butterfly varied in form or colour to such anextent that the corresponding eatable butterfly no longerclosely resembled the it, latter would be exposed to attacks,and only those variations would be preserved which kept upthe resemblance. At the same time we may well suppose theenemies to become more acute and able to detect smallerdifferences than at first. This would lead to the destructionof all adverse variations, and thus keep up in continually
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362 TROPICAL NATUREprotective, serving as a warning of their uneatableness. 1 Onour theory none of these colours offer any difficulty. Conspicuousnessbeing useful, every variation tending to brighterand purer colours was selected ;the result being the beautifulvariety and contrast we find.Imitative Warning Colours the Theory of MimicryWe now come to those groups which gain protection solelyby being mistaken for some of these brilliantly coloured butuneatable creatures, and here a difficulty really exists, and tomany minds is so great as to be insuperable.It will be welltherefore to endeavour to explain how the resemblance inquestion may have been brought about.The most difficult case, and the one which may be takenas a type of the whole class, is that of the genus Leptalis (agroup of South American butterflies allied to our commonwhite and yellow kinds), many of the larger species of whichare still white or yelloAv, and which are all eatable by birdsand other insectivorous creatures. But there are also anumber of species of Leptalis, which are brilliantly red,yellow, and black, and which, band for band and spot forspot, resemble some one of the Danaidse or Heliconidae whichinhabit the same district and which are nauseous and uneatable.Now the usual difficulty is, that a slight approach toone of these protected butterflies would be of no use, while agreater sudden variation is not admissible on the theory ofgradual change by indefinite slight variations. This objectiondepends almost wholly on the supposition that, when the firststeps towards mimicry occurred, the South American Danaidsewere what they are now ;while the ancestors of the Leptalideswere like the ordinary white or yellow Pieridse to which theyare allied. But the Danaioid butterflies of South America areso immensely numerous and so greatly varied, not only in colourbut in structure, that we may be sure they are of vast antiquityand have undergone great modification. A large number ofthem, however, are still of comparatively plain colours, oftenrendered extremely elegant by the delicate transparency ofthe wing membrane, but otherwise not at all conspicuous.1 This has since been found to be the case by Professor Herdman (Trans.Biol. Soc. Liverpool, vol. iv. p. 150).
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