III.] INDKrENDENT SIMILARITIES OF STIIUCTUUE. 101 



side, and of the Moluccas on tlie other, are ahnost entirely 

 absent from Celebes. Their place seems to be supplied by 

 the caterpillar-catchers, of which six or seven species are 

 known from Celebes, and are very numerous in individuals. 

 Wc have no positive evidence that these birds j)ursue but- 

 terflies on the wing, but it is highly probable that they do 

 so when other food is scarce. Mr. Bates suggested to me 

 that the larger dragon-flies prey upon butterflies, but I did 

 not notice that they were more abundant in Celebes than 

 elsewhere." ** 



Now, every opinion or conjecture of Mr. Wallace is 

 worthy of respectful and attentive consideration, but the 

 explanation suggested and before referred to hardly seems 

 a satisfactory one. What the past fauna of Celebes may 

 liave been is as yet conjectural. Mr. Wallace tells us that 

 now there is a remarkable scarcity of fly-cat<?hers, and that 

 their place is supplied by birds of which it can only be said 

 that it is "highly probable" that they chase butterflies 

 " when other food is scarce." The quick eye of Mr. Wal- 

 lace failed to detect them in the act, as also to note any 

 unusual abundance of other insectivorous forms, which 

 therefore, considering Mr. Wallace's zeal and powers of 

 observation, we may conclude do not exist. Moreover, 

 even if there ever has been an abundance of such, it is by 

 no means certain that they would have succeeded in pro- 

 ducing the conformation in question, for the effect of this 

 peculiar curvature on flight is by no means clear. Wc have 

 here, then, a structure hypothetically explained by an un- 

 certain property induced by a cause the presence of which 

 is only conjectural. 



Surely it is not unreasonable to class this instance with 

 the others before given, in which a common modification of 

 form or color coexists with a certain geographical distribu- 

 tion quite independently of the destructive agencies of ani- 



»" Natural Selection," p. 177. 



