STERNA OF THE FLAT-BILLS. 405 



this order is adapted for the bearing up of their 

 bodies while they float upon the water, there is, 

 independently of their different degrees of adaptation 

 for flight, a general character which runs through the 

 whole order. The largest diameter of the body is 

 reversed as compared with that of the grallidae. 

 These have it vertical, so that they may the more 

 easily make their way among the herbage, and also 

 present the least resistance to the air, and give the 

 greatest scope to the legs in walking, wading, or 

 running. But the flat-billed birds have the body 

 more or less flattened horizontally, as well as the bill, 

 by which means they present to the water a large 

 base in proportion to their weight, and thus swim 

 more lightly. The sternum is elongated, and also 

 increased in width, so as to accommodate itself to 

 this form of the body, and upon the average it is so 

 placed that the centre of gravity of the bird is very 

 nearly over its centre of resistance, so that all parts 

 of it are pressed upon with nearly equal force as the 

 bird floats along. 



The habit of floating along the water is, however, 

 in the different genera and species of these birds, 

 combined with so many modifications of other habits, 

 that it is subject to numerous though slight variations. 

 Those habits are greater or less degrees of flying, 

 walking, and diving ; and, from what has been already 

 said respecting birds of which each of these is the 

 principal habit, it will readily be understood, without 

 repetition, how far each must modify the swimming 

 or floating character, which is the principal one of 

 this class. 



They have the sternum elongated, varying in 

 breadth, narrower in the middle than at the extre- 

 mities. The sternal keel is well developed, but 



