. A history of birds . ternum) is corre-lated with vigorous and long-sustained powers of flight, as inthe case of the Swift, Humming-bird, Sand-grouse and Pigeon,for example ; conversely, a low keel indicates a feeble flight. Inthe Eagles and Steganopodous birds, :, Gannet and Pelican, STRUCTURAL ADAPTATIONS 381 however, the keel is relatively shallow. But the pectoralmuscles in these cases are found to have obtained increased sup-port from the shoulder girdle. In the Gannet and Cormorantand Boatswain-birds {PluPthon), for example, the keel is pro-duced forwards to an extent unknown among o
. A history of birds . ternum) is corre-lated with vigorous and long-sustained powers of flight, as inthe case of the Swift, Humming-bird, Sand-grouse and Pigeon,for example ; conversely, a low keel indicates a feeble flight. Inthe Eagles and Steganopodous birds, :, Gannet and Pelican, STRUCTURAL ADAPTATIONS 381 however, the keel is relatively shallow. But the pectoralmuscles in these cases are found to have obtained increased sup-port from the shoulder girdle. In the Gannet and Cormorantand Boatswain-birds {PluPthon), for example, the keel is pro-duced forwards to an extent unknown among other birds;while the hinder portion of the keel in these birds, and in thelarger Petrels, tapers off to finally die out before reaching theend of the sternal plate. When the sternum assumes this formthe coracoids are either unusually long, and provide specialarticular surfaces for the furcula, such as we have already de-scribed, or they are short and broad, a modification whichreaches its maximum in the larger III. 40.—Shoulder-girdle and Sternum of a Cormorant Economy of material seems to be striven after in the de-velopment of the sterna of birds which fly but little—in suchas the Game-birds, Tinamous and Rails. In the case of thefirst-named, the body of the sternum is generally reducedgreatly by deep notches extending far beyond the middle ofthe breast plate. In the Tinamous all that remains of thesternal plate is a pair of long slender processes arising on eitherside of the anterior border of the keel and running backwardsto its termination, or beyond. In the Rails, in extreme cases,such as the Water-rails, but little of the body of the sternumremains; and in no birds of this order is the sternal plate morethan moderately large. It will have been noticed perhaps that we meet with only arough approximation, and not a complete uniformity, ol adapta- 382 A HISTORY OF BIRDS tion to similar mechanical requirements in the different kindsof flight. That is to say, st
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbirds, bookyear1910
