396 STERNA OF THE LONGIPENNES. 



of the coracoids at their bases, and the form of the 

 lines of their union with the sternum, afford a very 

 firm base in the cross direction. 



The more typical of these birds are not very rapid 

 fliers, but many of them are much on the wing ; and 

 they fly by powerful strokes rather than quickly- 

 repeated ones. Hence they have the sternum broad, 

 and the fibres of the pectoral muscles long, so that, 

 on the principle formerly explained while noticing 

 the wings of the anisodactyli, these wings sweep over 

 large arches and with much power, though slowly as 

 compared with birds that have the sternum narrower 

 and the muscular fibres shorter. 



There is another advantage which this large sweep 

 but slow motion of the wing has over a shorter and 

 quicker one : the wind has much less effect either in 

 fatiguing the bird or in drifting it to leeward ; and 

 hence we find that most sea birds have this long and 

 swinging flight, whether they belong to this family or 

 not. Those of this family are of course not the best 

 fliers among sea birds, as their sternal apparatus is 

 not formed wholly for flight, but for that and resisting 

 the plunge jointly ; and as each of these requires a 

 different structure, each of them must, to some ex- 

 tent at least, weaken the other. 



STERNA OF THE LONGIPENNES. 



The birds which compose this order have some re- 

 semblance to those of the former tribe, and also to 

 those of the grallidae, as might be expected from the 

 birds partaking in part of the habits of both; but 

 they have also habits which are more peculiarly their 

 own, and the sternal apparatus is modified to accord 

 with these. Not only this, but as there is a gradation 

 from the gulls, which feed on the wing, and also, in 



