STERNA OF PIGEONS. 363 



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 gallinidaB 

 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 



