THE SURVIVAL OF THE FITTEST 385 



devices for their betterment or as a protection 

 against their enemies. We have at least two spe- 

 cies of the great sulphtir butterflies (CatopsiKa) 

 which love the sunlight and are especially abundant 

 in open .places. Their flight is exceedingly swift 

 and they constantly move in spirals and zig-zags, 

 so that it is difficult for any bird to capture them. 

 There is a small butterfly in our hammocks in 

 considerable numbers in autumn and early winter, 

 one of the Eunicas or violet-wings, E. tatila proba- 

 bly. The upper side of its wings is shaded with 

 magnificent royal purple; both sides of the upper 

 wings are white spotted, and the under side of the 

 lower ones is smoky colored. It almost always 

 aUghts on the smooth, brown bark of small trees, 

 closing the wings at once, but leaving the upper 

 ones raised, and in that position the white spots 

 show plainly. Then it slowly opens its wings; 

 the upper ones drop down behind the lower ones 

 and only the smoky under surfaces of the lower 

 wings show. If the color of the bark on which it 

 has alighted is lighter or darker than that shown 

 by the butterfly it slowly changes its tint until it 

 harmonizes with its environment. Once I saw one 



of these Eunicas alight on a spot where a dark bit 

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