Vv COLOURS OF ANIMALS 363 
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, alsoa very common 
colour, and one especially characteristic of the Pieride, the 
family to which Leptalis belongs. We may therefore reason- 
ably suppose that in the early stages of the development of 
the Danaide, 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 antenne, 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 Pieride which happened to resemble some of the 
Danaide 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 
