III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 83 



variation with survival of the fittest. If, however, the 

 reader thinks that teeth are parts peculiarly qualified for 

 rapid variation (in which view the author cannot concur), 

 he is 'requested to suspend his judgment till he has con- 

 sidered the question of the independent evolution of the 

 highest organs of sense. If this seems to establish the 

 existence of some other law than that of " Natural Selec- 

 tion," then the operation of that other law may surely be 

 also traced in the harmonious coordinations of dental form. 



The other difficulty, kindly suggested to me by the 

 learned professor, refers to the structure of birds, and of 

 extinct reptiles more or less related to them. 



The class of birds is one which is remarkably uniform in 

 its organization. So much is this the case, that the best 

 mode of subdividing the class is a problem of the greatest 

 difficulty. Existing birds, however, present forms which, 

 though closely resembling in the greater part of their struct- 

 ure, yet differ importantly the one from the other. One 

 form is exemplified by the ostrich, rhea, emeu, cassowary, 

 apteryx, dinornis, etc. These are the struthious birds. 

 All other existing birds belong to the second division, 

 and are called (from the keel on the breast-bone) carinate 

 birds. 



Now, birds and reptiles have such and so many points 

 in common that Darwinians must regard the former as 

 modified descendants of ancient reptilian forms. But on 

 Darwinian principles it is impossible that the class of birds 

 so uniform and homogeneous should have had a double rep- 

 tilian origin. If one set of birds sprang from one set of rep- 

 tiles, and another set of birds from another set of reptiles, 

 the two sets could never, by " Natural Selection " only, have 

 grown into such a perfect similarity. To admit such a 

 phenomenon would be equivalent to abandoning the theory 

 of " Natural Selection " as the sole origin of species. 



Now, until recently it has generally been supposed by 



