Mr. R. F. Tomes on White’s Thrush. 
383 
ii hi 
Greatest breadth of the intestine .... 03 
Length of the caeca.0 4 
„ of the cloaca.14 
Breadth of do. 06 
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 f 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 
others 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. 
