ILLUSTRATIVE OF NATURAL SELECTION. 175 



similar results ? and have we not a right to call upon 

 our opponents for some proofs of their own doctrine, 

 and for an explanation of its difficulties, instead of 

 their assuming that they are right, and laying upon 

 us the burthen of disproof? 



Let us now see if the facts in question do not them- 

 selves furnish some clue to their explanation. Mr. 

 Bates has shown that certain groups of butterflies have 

 a defence against insectivorous animals, independent 

 of swiftness of motion. These are generally very 

 abundant, slow, and weak fliers, and are more or less 

 the objects of mimicry by other groups, which thus 

 gain an advantage in a freedom from persecution 

 similar to that enjoyed by those they resemble. Now 

 the only Papilios which have not in Celebes acquired 

 the peculiar form of wing, belong to a group which is 

 imitated both by other species of Papilio and by 

 Moths of the genus Epicopeia. This group is of weak 

 and slow flight; and we may therefore fairly conclude 

 that it possesses some means of defence (probably in 

 a peculiar odour or taste) which saves it from attack. 

 Now the arched costa and falcate form of wing is gener- 

 ally supposed to give increased powers of flight, or, as 

 seems to me more probable, greater facility in making 

 sudden turnings, and thus baffling a pursuer. But the 

 members of the Polydorus-group (to which belongs 

 the only unchanged Celebesian Papilio), being already 

 guarded against attack, have no need of this increased 

 power of wing ; and " natural selection " would there- 

 fore have no tendency to produce it. The whole family 



