_ILLUSTRATIVE OF NATURAL SELECTION. 177 
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 (Grauca- 
lus, Campephaga, &c.), 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 has suggested to me that the larger Dragon- : 
flies (Aishna, &e.) prey upon butterflies ; but I did not 
notice that they were more abundant. in Celebes than 
elsewhere. However this may be, the fauna of Celebes 
is undoubtedly highly peculiar in every department of 
which we have any accurate knowledge; and though we 
may not be able satisfactorily to trace how it has been 
effected, 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. 
But even the conjectural explanation now given fails 
us in the other cases of local modification. Why the 
species of the Western islands should be smaller than 
those further east,—why those of Amboyna should 
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