474 ORIGIN OF SPECIES 



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 

 modification being profitable in some way to the modified 

 form, but often affecting by correlation other parts of the 

 organisation. 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 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 all these modifications would not 

 tend to alter the framework of the bones or the relative con- 

 nexion 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 construc- 

 tion of the limbs throughout the class. So with the mouths 

 of insects, we have only to suppose that their common pro- 

 genitor had an upper lip, mandibles, and two pairs of max- 

 illae, 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. Never- 

 theless, 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 



