PDF - Wallace Online

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