368 A HISTORY OF BIRDS 



support of the body when on the ground the trunk has become 

 shortened, and this shortening process is still going on. Evi- 

 dence of this may be found by an examination of the skeleton, 

 inasmuch as several pairs of the anterior ribs no longer reach 

 the sternum, and their supporting vertebrae have in consequence 

 been transferred to the cervical series, the length of the thoracic 

 segment of each pair decreasing from behind forwards. A 

 similar suppression of the ribs has taken place at the hinder 

 end of the sternum, thus transferring thoracic vertebrae to the 

 lumbar series. By this arrangement the hind-limb has been 

 brought nearer the centre of gravity, but the carriage of the 

 body is horizontal, as in quadrupedal animals. Birds which 

 swim and dive much differ, however, in this last respect since, 

 in some the trunk is carried obliquely, and in a few, as in the 

 Penguins, vertically, as in man, though in these birds the femur 

 is not markedly short. 



On the other hand, the Accipitres and Striges, when at 

 rest at least, have a vertical carriage ; but this can at any moment 

 be transformed into the more normal horizontal attitude. The 

 mechanics of this question must be discussed later. 



Length of leg and neck largely affect the shape of the 

 body. But there is a by no means universal correlation be- 

 tween these parts in the matter of length. Generally speaking, 

 the length of the neck varies as the length of the legs. A little 

 reflection will show that to a long-legged bird a long neck is 

 essential, at least in the case of ground-feeding or wading 

 types. The short-legged Swans, since much of their food is 

 obtained from the bottom of the streams on which they live, 

 need a long neck ; and where the water is deep the food can 

 only be obtained by a half-diving action, the forepart of the 

 body being thrust down till the tail points vertically upwards. 

 The " Darters " {P lotus) again have short legs and extremely long 

 necks which are used for thrusting, spear-fashion, at the fish 

 on which these birds feed. 



In all birds the neck, whether short or long, possesses ex- 

 treme mobility, more than in any other vertebrate. The fact 

 that there are no long-legged birds with short necks, as in the 

 Mammalia, is to be accounted for by the fact that the fore-limbs, 

 from their specialisation, cannot be used as organs of prehension 

 for conveying food to the mouth, or for the support of the fore- 



