424 MORPHOLOGY. 



ordinary view of the independent creation 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. 



The explanation is to a large extent simple, on the theory 

 of the selection of successive slight modifications, each 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 mem- 

 brane, 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 existing 

 general pattern, for whatever purpose they served, we cau 

 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 com- 

 mon progenitor had an upper lip, mandibles, and two pairs 

 of maxillae, these parts being perhaps very simple in form ; 

 and then natural selection will account for the definite 

 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 sub- 

 ject; namely, serial homologies, or the comparison of the 

 different parts or organs in the same individual, and not of 

 the same parts or organs in different members of the same 

 class. Most physiologists believe that the bones of the 

 skull are homologous — that is, correspond in number and 

 relative connection — with the elemental parts of a certain. 



