Chap. XIII. MORPHOLOGY. 435 



ferent purposes, are formed by infinitely numerous modi- 

 fications of an upper lip, mandibles, and two pairs of 

 maxilla?. 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 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 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- 

 nexion 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 homo- 

 logous construction of the limbs throughout the whole 

 class. So with the mouths of insects, we have only to 



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