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 so pleased the Creator to construct each animal and plant.

The explanation is manifest on the theory of the natural selection of successive slight modifications,—­each modification being profitable in some way to the modified form, but often affecting by correlation of growth other parts of the organisation.  In changes of this nature, there will be little or no tendency to modify the original pattern, or to transpose parts.  The bones of a limb might be shortened and widened to any extent, and become gradually enveloped in thick membrane, so as to serve as a fin; or a webbed foot might have all its bones, or certain bones, lengthened to any extent, and the membrane connecting them increased to any extent, so as to serve as a wing:  yet in all this great amount of modification there will be no tendency to alter the framework of bones or the relative connexion of the several parts.  If we suppose that the ancient progenitor, the archetype as it may be called, of all mammals, had its limbs constructed on the existing general pattern, for whatever purpose they served, we can at once perceive the plain signification of the homologous construction of the limbs throughout the whole class.  So with the mouths of insects, we have only to suppose that their common progenitor had an upper lip, mandibles, and two pair of maxillae, these parts being perhaps very simple in form; and then natural selection will account for the infinite diversity in structure and function of the mouths of insects.  Nevertheless, it is conceivable that the general pattern of an organ might become so much obscured as to be finally lost, by the atrophy and ultimately by the complete abortion of certain parts, by the soldering together of other parts, and by the doubling or multiplication of others,—­variations which we know to be within the limits of possibility.  In the paddles of the extinct gigantic sea-lizards, and in the mouths of certain suctorial crustaceans, the general pattern seems to have been thus to a certain extent obscured.

There is another and equally curious branch of the present subject; namely, the comparison not of the same part in different members of a class, but of the different parts or organs in the same individual.  Most physiologists believe that the bones of the skull are homologous with—­that is correspond in number and in relative connexion with—­the elemental parts of a certain number of vertebrae.  The anterior and posterior limbs in each member of the vertebrate and articulate classes are plainly homologous.  We see the same law in comparing the