MORPHOLOGY 243 



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 crea- 

 tion of each being, we can only say that so it is; — that 

 it has pleased the Creator to construct all the animals 

 and plants in each great class on a uniform plan; but 

 this is not a scientific explanation. 



Tlie explanation is to a large extent simple on the 

 theory of the selection of successive slight modifications — 

 each modification being profitable in some way to the 

 modified form, but often affecting by correlation other 

 parts of the organization. In changes of this 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 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 relative connec- 

 tion 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 ex- 

 isting 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, mandi- 

 bles, and two pairs of maxillae, these parts being perhaps 

 very simple in form; and then natural selection will 

 account for the infinite diversity in the structure and 



