98, THE GENESIS OF SPECIES. [CHAP. 



of Borneo and Java on the one side, and of the Moluccas on 

 the other, are almost 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. We have no positive 

 evidence that these birds pursue butterflies 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." l 



Now, every opinion or conjecture of Mr. Wallace is 

 worthy of respectful and attentive consideration, but the 

 explanation suggested and above referred to hardly seems 

 a satisfactory one. What the past fauna of Celebes may 

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

 now there is a remarkable scarcity of fly-catchers, and that 

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

 said that it is " highly probable " that they chase butter- 

 flies " when other food is scarce." The quick eye of 

 Mr. Wallace 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 far from clear. We have 

 here, then, a structure hypothetically explained by an 

 uncertain property induced by a cause the presence of 

 which is only conjectural. 



Surely it is not unreasonable to class this instance with 



1 " Natural Selection, " p. 177. 



