NATURAL SELECTION 



protected by their stings, are brilliantly but not 

 in general protectively coloured. Bugs and 

 ground - beetles emit a disagreeable, pungent 

 smell, and they are often conspicuously coloured. 

 But the most wonderful of all are the cases of 

 protective mimicry. The heliconidae are among 

 the most beautiful of South American butter- 

 flies. Being never eaten by birds, on account 

 of a nauseous liquid which exudes from them 

 when touched, they are not only very lazy fly- 

 ers, but have the under sides of their wings 

 as gorgeously tinted as the upper side, so that 

 they can be seen from quite a long distance. 

 From the same cause they are prodigiously 

 numerous, swarming in all the tropical forests. 

 Now it is obvious that if another butterfly, not 

 protected by a disagreeable odour or taste, were 

 to resemble the heliconia in colouring, it would 

 be as efliciently protected as by imitating a dead 

 leaf or dry twig ; provided that there were but 

 few of these butterflies among a large number 

 of heliconias. For, as Mr. Wallace says, " 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.'' Now along with 

 the heliconidae there does, in fact, live a distinct 

 family of butterflies, the pieridae, most of which 

 are white, and which are anatomically as distinct 



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