These organs must
have been independently developed, as they could not have been inherited
from a common progenitor; and in the several groups they are formed by the
modification of the fore legs, of the hind legs, of the maxillae or lips,
and of appendages on the under side of the hind part of the body.
In the foregoing cases, we see the same end gained and the same function
performed, in beings not at all or only remotely allied, by organs in
appearance, though not in development, closely similar. On the other hand,
it is a common rule throughout nature that the same end should be gained,
even sometimes in the case of closely related beings, by the most
diversified means. How differently constructed is the feathered wing of a
bird and the membrane-covered wing of a bat; and still more so the four
wings of a butterfly, the two wings of a fly, and the two wings with the
elytra of a beetle. Bivalve shells are made to open and shut, but on what
a number of patterns is the hinge constructed, from the long row of neatly
interlocking teeth in a Nucula to the simple ligament of a Mussel! Seeds
are disseminated by their minuteness, by their capsule being converted into
a light balloon-like envelope, by being embedded in pulp or flesh, formed
of the most diverse parts, and rendered nutritious, as well as
conspicuously coloured, so as to attract and be devoured by birds, by
having hooks and grapnels of many kinds and serrated awns, so as to adhere
to the fur of quadrupeds, and by being furnished with wings and plumes, as
different in shape as they are elegant in structure, so as to be wafted by
every breeze.
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