butterfly as an example) the body is weighted behind the insertion of
the wings by the long and heavy abdomen, so as to produce an oblique
position when freely suspended. There is also much more wing surface
behind than before the fulcrum. Now if such an insect produces by
muscular action a regular flapping of the wings, flight must result. At
the downward stroke the pressure of the air against the hind wings would
raise them all to a nearly horizontal position, and at the same time
bend up their posterior margins a little, producing an upward and onward
motion. At the upward stroke the pressure on the hind wings would
depress them considerably into an oblique position, and from their great
flexibility in that direction would bend down their hind margins. The
resultant would be a slightly downward and considerably onward motion,
the two strokes producing that undulating flight so characteristic of
butterflies, and so especially observable in the broad-winged tropical
species. Now all this is quite conformable to the action of a bird's
wing. The rigid anterior margin, the slender and flexible hind margin;
the greater resistance to upward than to downward pressure, and the
slight concavity of the under surface, are all characters common to the
wings of birds and most insects, and, considering the totally different
structure and homologies of the two, I think there is at least an _a
priori_ case for the function they both subserve being dependent upon
these peculiarities. If I remember rightly, it is on these principles
that the Duke of Argyll has explained the flight of birds, in which,
however, there are of course some specialities depending on the more
perfect organisation of the wing, its greater mobility and flexibility,
its capacity for enlargement and contraction, and the peculiar
construction and arrangement of the feathers. These, however, are
matters of detail; and there are no doubt many and important differences
of detail in the mode of flight of the different types of insects which
would require a special study of each. It appeared to me that the Duke