CHAP. XIV.] MORPHOLOGY 359 



same extraordinary type, namely with the bones of the second 

 and third digits extremely slender and enveloped within the same 

 skin, so that they appear like a single toe furnished with two 

 claws. Notwithstanding this similarity of pattern, it is obvious 

 that the hind feet of these several animals are used for as widely 

 different purposes as it is possible to conceive. The case is 

 rendered all the more striking by the American opossums, which 

 follow nearly the same habits of life as some of their Australian 

 relatives, having feet constructed on the ordinary plan. Professor 

 Flower, from whom these statements are taken, remarks in 

 conclusion: "We may call this conformity to type, without 

 getting much nearer to an explanation of the phenomenon ; " and 

 he then adds, " but is it not powerfully suggestive of true relation- 

 ship, of inheritance from a common ancestor?" 



Geoffroy St. Hilaire has strongly insisted on the high importance 

 of relative position or connexion in homologous parts ; they may 

 differ to almost any extent in form and size, and yet remain 

 connected together in the same invariable 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 sam# 

 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 modifications 

 of an upper lip, mandibles, and two pairs of maxillae. The same 

 law governs the construction of the mouths and limbs of crusta- 

 ceans. 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 

 independent creation of each being, we can only say that so it is ; 

 that it has pleased the CreatoV 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 



