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Show 212 LETTER FROM THE REV. W. HINCKS. [Mai*. 26, the markings are alike or very nearly so, but the former species is considerably 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 character, all our species of Cygnus are in great confusion. Let it only appear 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. buccinator 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 admitted respecting C. passmori were founded on m y 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. buccinator would display the oordiform swelling which I found in old specimens. 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 m y 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 C. passmori cannot, as I once thought possible though very doubtful, 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 m y letter of April 10, 1864, were specimens of C. passmori, and, with m y original specimen and Yarrell's figure which belonged to that species, show the degrees of development of that species, whilst m y young Swan noticed in the last paper I sent over, your two specimens, and m y 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. passmori has the knob marking the vertical bend though much more feebly than in the other species, but has no hori- |