58 
NATURAL SELECTION 
in 
return to their station to devour. If a bird began by capturing 
the slow-flying conspicuous Heliconidse, and found them always 
so disagreeable that it could not eat them, it would after a 
very few trials leave off catching them at all ; and their whole 
appearance, form, colouring, and mode of flight are so peculiar 
that there can be little doubt birds would soon learn to dis- 
tinguish them at a long distance, and never waste any time in 
pursuit of them. Under these circumstances, it is evident 
that any other butterfly of a group which birds were accus- 
tomed to devour would be almost equally well protected by 
closely resembling a Heliconian externally, as if it acquired 
also the disagreeable odour; always supposing that there 
were only a few of them among a great number of the Heli- 
conias. If the birds could not distinguish the two kinds 
externally, and there were on the average only one eatable 
among fifty uneatable, they would soon give up seeking for 
the eatable ones, even if they knew them to exist. If, on the 
other hand, any particular butterfly of an eatable group 
acquired the disagreeable taste of the Heliconias while it 
retained the characteristic form and colouring of its own 
group, this would be really of no use to it whatever ; for the 
birds would go on catching it among its eatable allies (com- 
pared with which it would rarely occur), it would be wounded 
and disabled, even if rejected, and its increase would thus be 
as effectually checked as if it were devoured. It is important, 
therefore, to understand that if any one genus of an extensive 
family of eatable butterflies were in danger of extermination 
from insect-eating birds, and if two kinds of variation were 
going on among them, some individuals possessing a slightly 
disagreeable taste, others a slight resemblance to the Heli- 
conidse, this latter quality would be much more valuable than 
the former. The change in flavour would not at all prevent 
the variety from being captured as before, and it would 
almost certainly be thoroughly disabled before being rejected. 
The approach in colour and form to the Heliconidse, however, 
would be at the very first a positive, though perhaps a slight 
advantage ; for although at short distances this variety would 
be easily distinguished and devoured, yet at a longer distance 
it might be mistaken for one of the uneatable group, and so 
be passed by and gain another day’s life, which might in 
