382 MORPHOLOGY. [Chap. XIV 



Morphology. 



We have seen that the members of the same class, independently 

 of their habits of life, resemble each other in the general plan of 

 their organisation. This- resemblance is often expressed by the 

 term " unity of type ; " or by saying that the several parts and organs 

 in the different species of the class are homologous. The whole 

 subject is included under the general term of Morphology. This is 

 one of the most interesting departments of natural history, and may 

 almost be said to be its very soul. What can be more curious than 

 that the hand of a man, formed for grasping, that of a mole for 

 digging, the leg of the horse, the paddle of the porpoise, and the 

 wing of the bat, should all be constructed on the same pattern, and 

 should include similar bones, in the same relative positions ? How 

 curious it is, to give a subordinate though striking instance, that 

 the hind-feet of the kangaroo, which are so well fitted for bounding 

 over the open plains, — those of the climbing, leaf-eating koala, 

 equally well fitted for grasping the branches of trees, — those of the 

 ground-dwelling, insect or root eating, bandicoots, — and those of 

 some other Australian marsupials, — should all be constructed on the 

 same extraordinary type, namely with the bones of the second and 

 third digits extremely slender and enveloped within the same skim 

 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 relationship, 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 con- 

 nected 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 homo- 

 logous 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 th9 immensely long spiral proboscis of a sphinx-moth, 



