234 MORPHOLOGY. [Chap. XIV. 



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 liave 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. If 

 we suppose that an early progenitor — the archetype as 

 it may be called — of all mammals, birds, and reptiles, 

 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 class. So 

 with the mouths of bisects, we have only to suj)pose 

 that their common progenitor had an upper lip, 

 mandibles, and two paii's of maxillse, these parts 

 being perhaps very simple in form ; and then natural 

 selection will account for the infinite diversity in the 

 structure and functions 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 reduction and ultimately by the 

 complete abortion of certain parts, by the fusion 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 gigantic 

 extinct sea-lizards, and in the mouths of certain 

 suctorial crustaceans, the general pattern seems thus 

 to have become partially obscured. 



There is another and equally curious branch of our 



