BIRD. 



469 



and two notches which appear in the figure. The 

 coracoids, though not very strong, are much better 

 set at their junction with the sternum than those of 

 the parrot. They have not that direct bearing toward 

 the centre of the sternum which we find in the more 

 typical wing birds; but their axes do form an angle 

 with each other as seen in front, and the position of 

 the lines of their union tends also to throw the pres- 

 sure on their heads toward the middle of the sternum. 

 The clavicle, though by no means of the strongest 

 form, is much firmer, in comparison with the quantity 

 of matter that it contains, than the clavicle of the 

 parrot. It is an arch, though an elongated elliptical 

 one, and therefore comparatively weak against lateral 

 pressure on the extremities, though not so much so 

 as a fir with the branches nearly straight, and its 

 process at the middle bearing on the head of the 

 sternum. Thus the sternal apparatus of this order, 

 though far from being so firm as that of those birds 

 of which the air is the principal element, is not so 

 flexible as that of those birds whose chief action is 

 walking upon legs so articulated as to support the 

 axis of the body in a horizontal posture. The pigeons 

 are thus, in their structure, intermediate between air 

 and ground birds ; and they are, on the average, the 

 same in their habits, though, according to this habit, 

 they might perhaps admit of division into four groups. 

 First, those which feed, habitually or occasionally, 

 upon seeds and berries, while these are yet on the 

 plant, the tree, or the bush. Of these the migrant 

 pigeons of the south-east, which are gay in their 

 plumage, and which, though they have perching 

 rather than climbing feet, yet have some resemblance 

 to the parroquets, are the most typical. Secondly, 

 those which, though they feed more on the ground, 

 yet perch and nestle habitually on trees, of which 

 we have British examples in the common ringdove, 

 and in the rarer stock-dove and turtle. Thirdly, those 

 which feed on the ground, and roost and nestle in 

 rocks, of which the rock-dove may be regarded as 

 the type, and all the varieties of pigeon-house and 

 domesticated pigeons as instances. Fourthly, those 

 which bear so much resemblance to the gallinidse 

 that they are popularly called gallinaceous pigeons. 

 Of these one species is as large as the common turkey, 

 or larger. Some of them nestle in trees, and some 

 on the ground, but they are all lower fliers and less 

 discursive in their range than the true pigeon ; they 

 are also much more omnivorous, and on this, as well 

 as on other accounts, they ought in a perfectly natu- 

 ral system to stand as a separate family, if not as a 

 distinct sub-order. 



Before proceeding to notice the sternal apparatus 

 of the gallinaceous tribe, it may not be improper to 

 remark that the flexibility of the lateral parts of 

 sterna produced by notches of the bone, or cartila- 

 ginous continuations, are intimately connected with 

 the power of walking straight forward upon two legs, 

 in birds which have the use of their wings in flight, 

 and which consequently have furcal or clavicular 

 bones, though the form and strength of these vary 

 with the powers of flight. 



It will be readily understood that the clavicle ties 

 the shoulder-joints together, and the coracoids at the 

 same time keep them both at equal distances from 

 the anterior angles of the sternum, so that the line 

 joining the shoulder-joints is immoveable in position 

 with reference to the sternum, except in so fur as the 

 different articulations are loose and admit of play, 



and the change of position produced in this way is 

 very limited. Consequently, if the sternum were 

 inflexible (as it is in birds of powerful wing) in clavi- 

 cled birds which walk by striding with the alternate 

 foot, and not by hopping with both feet at once, the 

 line of the bird's body would twine about like a cork- 

 screw as it marched along, as the advance of each 

 foot would necessarily throw the anterior part of the 

 axis to the opposite side, and give the bird a swinging 

 motion upon its centre of gravity, which would con- 

 siderably increase the fatigue of walking, and thus be 

 a violation of that general law according to which all 

 the natural actions of animals are performed with the 

 smallest possible waste of their energy. We see 

 instances of this swinging of the axis in several flying 

 birds when they walk by steps ; and it is more con- 

 spicuous in proportion as the tarsi are longer, and 

 the tibire more moved in performing the step. But 

 such birds have generally the tarsi rather short, so 

 that on their quick marches upon the ground they 

 take short trundling steps, or they hop, as is the case 

 with most of those passercs which are habitually or 

 occasionally ground feeders. In order that the pro- 

 gress may be by steps of considerable length, it 

 becomes necessary that, if the shoulders require a 

 stiffening clavicle to fit them for flight, the posterior 

 part of the sternum and also the ribs, but especially 

 the former, should be flexible, in order that the one 

 side may contract and the other expand on the ad- 

 vance of the leg, as much as shall keep the axis of 

 the body steady to the line of motion. We have a 

 perfect confirmation of this in the sterna of gallina- 

 ceous birds, all of which are straight-forward runners, 

 and runners without hopping. Many of the long- 

 legged birds also run fast and steadily forward, and 

 never use any action of the feet for that purpose, 

 except the alternate one ; but the particular struc- 

 ture of the bones, by which this is accomplished 

 in them, can be more advantageously explained 

 afterwards. 



Sterna of the Gallmidce. The sternum of these 

 birds has, in its general outline, the lozenge form 

 which characterises that of the pigeon tribe, only it 

 is more elongated in proportion to its breadth. As 

 in these, the anterior part of it is the firmer, but the 

 posterior part is much more flexible, consisting of 

 five processes of bone, all narrow and slender, with 

 the notches next the middle by far the deepest, 

 extending, in fact, fully more than three-fourths of 

 the entire length of the sternum ; and the two lateral 

 processes, though united by cartilaginous membranes 

 to each other and to the central bone, having more 

 the appearance of a forked sternal rib than of portions 

 of the sternum as a continuous bone. The keel is 

 considerably developed, especially in the anterior 

 part, but lower toward the rear, though there it still 

 affords some stiffness to the middle portion of the 

 sternum. The anterior part of the sternum terminates 

 in a triangular process, which is notched at the edges 

 to receive the coracoid bones, and the lateral processes 

 extend forwards nearly as tar as the triangular one 

 in the centre. The coracoids are rather strong, flat- 

 tened on their anterior edges, and with an angular 

 ridire backwards. The clavicle is a semi-ellipse, 

 more elongated, and with the branches much more 

 straight, feeble, and flexible, than in the pigeons. 

 It has always a process at the union of the two 

 branches, often of considi-rublc sizo, and directed 

 toward the anterior pail of the keel U which 



