MORPHOLOGY 391 



one of a bee or bug, and the great jaws of a beetle ? — 

 yet all these organs, serving for such different purposes, 

 are formed by infinitely numerous modifications of 

 an upper lip, mandibles, and two pairs of maxillae. 

 Analogous laws govern 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 inde- 

 pendent 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 mem- 

 brane, 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 con- 

 nection 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 pairs of maxillae, these 



