NATURAL SELECTION 



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 



