COLOURS OF ANIMALS. 



191 



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 as 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 mistaken 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 diflerences than at first. This woukl 



