234 MORPHOLOGY. [Chap. XIV. 



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, becom- 

 ing 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 rela- 

 tive connection 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 con- 

 structed 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 insects, 

 we have only to suppose that their common progenitor 

 had an upper lip, mandibles, and two pairs of maxillte, 

 these parts being perhaps very simple in form; and 

 then natural selection will account for the infinite di- 

 versity in the structure and functions of the mouths 

 of insects. Nevertheless, it is conceivable that the gen- 

 eral 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 multi- 

 plication 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 

 subject; namely, serial homologies, or the comparison 



