Mr. R. F. Tomes on White's Thrush. 383 



// /// 

 Greatest breadth of the intestine .... 03 



Length of the cseca 4 



„ of the cloaca 14 



Breadth of do 6 



For the synonymy of this species^ I refer the reader to the 

 late Prince C. L. Bonaparte's Monograph of the genus Oreocincla 

 in the 'Revue et Magasin de Zoologie/ No, 5, 1857. The 

 presence of fourteen feathers in the tail, determines the species. 



Without having gone into the subject with sufficient care to 

 give a decided opinion, it has always appeared to me that the 

 genera into which many families of birds have been divided are 

 based upon a series of very minute differences of a purely 

 external nature, without sufficient care having been first exercised 

 in endeavouring to ascertain how far they may be referable only 

 to the peculiar habits of the species. Assuming, by way of 

 argument, that such has in some instances been the case, it 

 must be pretty clear that the value of such divisions will depend 

 almost entirely on the number of species contained in each. 

 For instance, were we to take any genus of birds as given by the 

 older writers, we should find it to consist of an assemblage 

 having considerable diversity in the details of external parts ; 

 but perhaps the difference might be so slight that we should 

 hesitate before dividing it into a series of new genera. A 

 considerable number of the species would possibly be possessed 

 of absolutely the same characters, differing only in such as we 

 should not hesitate to name as purely specific. From the 

 othei's these might differ in an inconsiderable but constant 

 manner ; but their separation would in this case be a very proper 

 one : whether of generic or subgeneric rank would not affect 

 the question. Clearly the differences, however minute, could 

 not be regarded as specific ; and in this manner a character, of 

 apparently small value in itself, might become highly serviceable 

 as a means of classification. Indeed its constancy might out- 

 weigh a peculiarity of more marked aspect when of limited 

 occurrence, since it might be difficult in this case to determine 

 what were specific, and what generic characters. No such dif- 

 ficulty could occur in the former of these supposed instances. 



