dicate that climate and other physical causes have, in some cases, a very
powerful effect in modifying specific form and colour, and thus directly
aid in producing the endless variety of nature."

[Illustration: OUTLINES OF WINGS OF BUTTERFLIES OF CELEBES COMPARED WITH
THOSE OF ALLIED SPECIES ELSEWHERE.

Outer outline, _Papilio gigon_, of Celebes. Inner outline, _P. demolion_,
of Singapore and Java.--2. Outer outline, _P. miletus_, of Celebes. Inner
outline, _P. sarpedon_, India.--3. Outer outline, _Tachyris zarinda_,
Celebes. Inner outline, _T. nero_.]

With regard to butterflies of Celebes belonging to different families, they
present "a peculiarity of outline which distinguishes them at a glance from
those of any other part of the world:"[65] it is that the upper wings {86}
are generally more elongated and the anterior margin more curved. Moreover,
there is, in most instances, near the base an abrupt bend or elbow, which
in some species is very conspicuous. Mr. Wallace endeavours to explain {87}
this phenomenon by the supposed presence at some time of special
persecutors of the modified forms, supporting the opinion by the remark
that small, obscure, very rapidly flying and mimicked kinds have not had
the wing modified. Such an enemy occasioning increased powers of flight, or
rapidity in turning, he adds, "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, 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."[66]

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 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 probabl