MUTUAL AFFINITIES OF ORGANIC BEINGS 391 



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 phenome- 

 non;" and he then adds, "but is it not powerfully suggestive of 

 true relationship, of inheritance from a common ancestor?" 



Geoffroy Saint-Hilaire has strongly insisted on the high im- 

 portance of relative position or connection 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 forearm, 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 dif- 

 ferent 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 Creator to construct all the animals and plants in each 

 great class on a uniform plan; but this is not a scientific explana- 

 tion. 



The explanation is to a large extent simple, on the theory of 

 the selection of successive slight modifications, each being profit- 

 able in some way to the modified form, but often affecting by 

 correlation other parts of the organization. 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 connection of the 



