v COLOURS OF ANIMALS 368 



Many have only dusky or purplish bands or spots; others 

 have patches of reddish or yellowish brown perhaps the 

 commonest colour among butterflies ; while a considerable 

 number are tinged or spotted with yellow, also a very common 

 colour, and one especially characteristic of the Pieridas, the 

 family to which Leptalis belongs. We may therefore reason- 

 ably suppose that in the early stages of the development of 

 the Danaidse, when they first began to acquire those nauseous 

 secretions which are now their protection, their colours were 

 somewhat plain ; either dusky with paler bands and spots, or 

 yellowish with dark borders, and sometimes with reddish 

 bands or spots. At this time they had probably shorter 

 wings and a more rapid flight, just like the other unprotected 

 families of butterflies. But, so soon as they became decidedly 

 unpalatable to any of their enemies, 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 colour, 

 while all probably had wings adapted for rather quick 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, as 

 well as their slower flight, rendered them noticeable 

 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 Pieridse which happened to resemble some of the 

 Danaidse around them in their yellow and dusky tints and in 

 the general outline of their wings, would be sometimes mis- 

 taken for them by the common enemy, and would thus gain 

 an advantage in the struggle for existence. Admitting this 

 one step to be made, and all the rest must inevitably follow 

 from simple variation and survival of the fittest. So soon as 

 the nauseous butterfly varied in form or colour 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 



