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 
