42 THE GENESIS OF SPECIES. [CHAP. 



tacked by the birds, reptiles, or insects, which prey upon 

 other lepidoptera. 



Now it is a curious fact that very different South Amer- 

 ican butterflies put on, as it were, the exact dress of these 

 offensive beauties and mimic them even in their mode of 

 flight. 



In explaining the mode of action of this protecting re- 

 semblance Mr. Wallace observes : " " Tropical insectivorous 

 birds very frequently sit on dead branches of a lofty tree, 

 or on those which overhang forest-paths, gazing intently 

 around, and darting off at intervals to seize an insect at a 

 considerable distance, with which they generally return to 

 their station to devour. If a bird began by capturing the 

 slow-flying conspicuous Heliconidas, 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, coloring, and mode of flight, is so 

 peculiar, that there can be little doubt birds would soon 

 learn to distinguish them at a long distance, and never 

 waste any time in pursuit of them. Under these circum- 

 stances, it is evident that any other butterfly of a group 

 which birds were accustomed to devour, would be almost 

 equally well protected by closely resembling a Heliconia 

 externally, as if it acquired also the disagreeable odor ; 

 always supposing that there were only a few of them among 

 a great number of Heliconias." 



" The approach in color and form to the Heliconidas, 

 however, would be at the first a positive, though perhaps a 

 slight, advantage ; for although at short distances this va- 

 riety would be easily distinguished and devoured, yet at a 

 longer distance it might be mistaken for one of the uneat- 

 able group, and so be passed by and gain another day's 

 life, which might in many cases be sufficient for it to lay a 

 quantity of eggs and leave a numerous progeny, many of 

 11 Loc. cit., p. 80. 



