Chap. XIV.] MORPHOLOGY. 233 



Geoffrey St. Hilaire has strongly insisted on the high 

 importance of relative position or connexion in homo- 

 logous parts; they may differ to almost any extent in 

 form and size, and yet remain connected together in the 

 same iavariable order. We never find, for instance, the 

 bones of the arm and fore-arm, or of the thigh and leg, 

 transposed. Hence the same names can be given to the 

 homologous bones in widely different animals. We see 

 the same great law in the construction of the mouths of 

 insects: what can be more different than the immensely 

 long spiral proboscis of a sphinx-moth, 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 modifica- 

 tions of an upper lip, mandibles, and two pairs of max- 

 illae. The same law governs 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 ad- 

 mitted by Owen in his most interesting work on the 

 ' Nature of Limbs.' On the ordinary view of the in- 

 dependent 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 uni- 

 form 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. 



