212 LKTTKR FROM THE REV. W. HINCKS. [MaV. 2C, 



the markings are alike or very nearly so, but the former species is con- 

 siderably larger ; yet no practical observer doubts the species being 

 distinct, their feeding and habits being somewhat different. 



" If the relation of the trachea to the sternum is not a good cha- 

 racter, all our species of Cygnus are in great confusion. Let it only 

 ap|)ear that there is a real difference in the sternum and trachea 

 between two sets of birds, and their specific difference is established. 

 I was very particular in considering how far sex would affect the 

 case ; but' as I had a pair, male and female, of C. buccinator, and the 

 sterna were nearly alike, altogether so in all the important particulars, 

 it is clear that sex does not account for any of the differences. 

 Finally the shape of the pair of bronchial tubes, constant, I believe, in 

 every species, is very apt to vary in different species, and is always 

 worth noticing. Now as to the birds examined in this inquiry. 

 The original specimen of C. passmori had its sternum like that 

 figured by Yarrell for C. buccinator, whilst a large pair of C. bucci- 

 nator compared with it had each a sternum such as is given in my 

 paper (fig. 7), and of which the leading characters are much better 

 expressed in your figures through being taken from younger birds. 

 The expanse of the cordiform elevation is not so great in yours, nor 

 the elevation of the knob in front ; but no doubt can be entertained 

 of your birds belonging to C buccinator. All the doubts I admit- 

 ted respecting Or passmori were founded on my examination of 

 several Swans with essentially the sternum of C. passmori, but with 

 slight variations as to the extent of the advance of the bended trachea 

 within the carina, whilst I had no proof that the young of C. buccina- 

 tor would display the cordiform swelling which 1 found in old speci- 

 mens. Since then I have met with a known young C. buccinator which 

 displayed the cordiform enlargement distinctly, though not fully 

 developed in size and height ; and your birds, which, though dwarfed 

 and deficient in colour, certainly come between my young specimen 

 and the old ones before examined by the intermediate development 

 of the cordiform swelling, confirm the argument. In short, I am 

 now in a position to affirm positively that the tracheal condition of 

 0. passmori cannot, as I once thought possible though very doubt- 

 ful, be a step in the development of C. buccinator. If, then, it does 

 not mark a species, it is a mere variety ; and in that case the forms of 

 the sternum and bends of the trachea afford no true characters— a 

 proposition which I cannot readily admit. It is now evident to me 

 that the two young Swans referred to in my letter of April 10, 1864, 

 were specimens of C. passmori, and, with my original specimen and 

 Yarrell's figure which belonged to that species, show the degrees of 

 development of that species, whilst my young Swan noticed in the 

 last paper I sent over, your two specimens, and my pair of old birds 

 of C. buccinator fully illustrate the progress of that species. The 

 cordiform swelling at the hind part of the sternum (as in C. bewickii), 

 found with the knob at the vertical bone in front of the sternum, 

 indicating both a horizontal and vertical bend of the trachea, mark 

 C. buccinator. C. passmorihas the knob marking the vertical bend, 

 though much more feebly than in the other species, but has no hori- 



