y this difficulty: one portion of
these curiously modified forms are admitted to have been produced by
variation and some natural action of local conditions; whilst the other
portion, differing from the former only in degree, and connected with
them by insensible gradations, are said to have possessed this
peculiarity of form at their first creation, or to have derived it from
unknown causes of a totally distinct nature. Is not the _a priori_
evidence in favour of an identity of the causes that have produced such
similar results? and have we not a right to call upon our opponents for
some proofs of their own doctrine, and for an explanation of its
difficulties, instead of their assuming that they are right, and laying
upon us the burthen of disproof?

Let us now see if the facts in question do not themselves furnish some
clue to their explanation. Mr. Bates has shown that certain groups of
butterflies have a defence against insectivorous animals, independent of
swiftness of motion. These are generally very abundant, slow, and weak
fliers, and are more or less the objects of mimicry by other groups,
which thus gain an advantage in a freedom from persecution similar to
that enjoyed by those they resemble. Now the only Papilios which have
not in Celebes acquired the peculiar form of wing, belong to a group
which is imitated both by other species of Papilio and by Moths of the
genus Epicopeia. This group is of weak and slow flight; and we may
therefore fairly conclude that it possesses some means of defence
(probably in a peculiar odour or taste) which saves it from attack. Now
the arched costa and falcate form of wing is generally supposed to give
increased powers of flight, or, as seems to me more probable, greater
facility in making sudden turnings, and thus baffling a pursuer. But the
members of the Polydorus-group (to which belongs the only unchanged
Celebesian Papilio), being already guarded against attack, have no need
of this increased power of wing; and "natural selection" would therefore
have no tendency to produce it. The whole family of Danaidae are in the
same position: they are slow and weak fliers; yet they abound in species
and individuals, and are the objects of mimicry. The Satyridae have also
probably a means of protection--perhaps their keeping always near the
ground and their generally obscure colours; while the Lycaenidae and
Hesperidae may find security in their small size and rapid motions. In
the ex