It would thus appear as if there must be (or once have been) in the island of Celebes, some peculiar enemy to these larger-sized butterflies which does not exist, or is less abundant, in the surrounding islands. Increased powers of flight, or rapidity of turning, was advantageous in baffling this enemy; and the peculiar form of wing necessary to give this would be readily acquired by the action of “natural selection” on the slight variations of form that are continually occurring. Such an enemy one would naturally suppose to be an insectivorous bird; but it is a remarkable fact that most of the genera of Fly-catchers of Borneo and Java on the one side (_Muscipeta_, _Philentoma_), and of the Moluccas on the other (_Monarcha_, _Rhipidura_), are almost entirely absent from Celebes. Their place seems to be supplied by the Caterpillar-catchers (_Graucalus_, _Campephaga_), 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[7]. However this may be, the fauna of Celebes is undoubtedly highly peculiar in every department of which we have any knowledge; and though we may not be able to trace it satisfactorily, there can, I think, be little doubt that the singular modification in the wings of so many of the butterflies of that island is an effect of that complicated action and reaction of all living things upon each other in the struggle for existence, which continually tends to readjust disturbed relations, and to bring every species into harmony with the varying conditions of the surrounding universe. Footnote 7: Mr. Bates has suggested that the larger Dragon-flies (_Æshna_, &c.) prey upon butterflies; but I did not notice that they were more abundant in Celebes than elsewhere.