452 MOBPn')LOGT. 



parts. The bones of a limb might be shortened and flat- 

 tened 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 aU these modifica- 

 tions would not tend to alter the framework of the bones 

 or the relative 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 

 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 insects, we 

 have only to suppose that their common progenitor had an 

 upper lip, mandibles, and two pairs of maxillae, these parts 

 being perhaps very simple in form; and then natural selec- 

 tion will account for the definite diversity in the structure 

 and functions of the mouths of insects. Nevertheless, it is 

 conceivable that the g^^neral pattern of an organ might 

 become so much obscured as to be finally lost, by the 

 reduction and ultimately by the complete abortion of cer- 

 tain 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 :ea-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 in relative connection — with the elemental parts of a 

 certain number of vertebrae. The anterior and posterior 

 limbs in all the higher vertebrate classes are plainly homo- 

 logous. So it is with the wonderfully complex jaws and 

 legs of crustaceans. It is familiar to almost every one, 

 that in a flower the relative position of the sepals, petals, 

 stamens and pistils, as well as their intimate structure, are 

 intelligible on the view that they consist of metamorphosed 



