Chap. xiv. Morphology. 383 



the curious folded one of a bee or bug, and the great jaws of a 

 beetle? — yet all these organs, serving for such widely different 

 purposes, are formed by infinitely numerous modifications of an 

 upper lip, mandibles, and two pairs of maxilla?. The same law 

 o-overns the construction of the mouths and limbs of crustaceans. 

 So it is with the flowers of plants. 



Nothing can be more hopeless than to attempt to explain this 

 similarity of pattern in members of the same class, by utility or 

 by the doctrine of final causes. The 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 inde- 

 pendent 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 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 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 connexion of the parts. 

 If we suppose that an early progenitor — the archetype as it may 

 he 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 homo- 

 logous 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 maxilla?, 

 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. 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 



