The same law governs the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of
final causes. The hopelessness of the attempt has been expressly admitted
by Owen in his most interesting work on the "Nature of Limbs." On the
ordinary view of the independent creation of each being, we can only say
that so it is; that it has pleased the Creator to construct all the animals
and plants in each great class on a uniform plan; but this is not a
scientific explanation.
The explanation is to a large extent simple, on the theory of the selection
of successive slight modifications, each being profitable in some way to
the modified form, but often affecting by correlation other parts of the
organisation. In changes of this nature, there will be little or no
tendency to alter the original pattern, or to transpose the parts. The
bones of a limb might be shortened and flattened to any extent, becoming at
the same time enveloped in thick membrane, so as to serve as a fin; or a
webbed hand might have all its bones, or certain bones, lengthened to any
extent, with the membrane connecting them increased, so as to serve as a
wing; yet all these modifications would not tend to alter the framework of
the bones or the relative connexion of the parts.
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