372 STEENA OF THE DIVERS. 



from the horizontal plane of the body, but strike 

 short and quick, and move most forcibly downwards 

 or upwards, according as the course is ascending or 

 descending. 



The natural transition here is not to the running 

 and wading birds, which resort, at least in most of the 

 species, to the banks, shores, and other humid places, 

 which are not so tangled and rank ; with vegetation 

 as the haunts of the tribe now considered. These 

 carry the type in the general form of the body and 

 the sternal apparatus, though modified by the habit 

 and haunt, from the grebes, which unite considerable 

 powers of flight with swimming and diving, through 

 the divers properly so called, to the penguins, which 

 are incapable of flight ; but those species which use 

 the wings chiefly in the water as a sort of fins have 

 the sternal apparatus, of course, diff'erent from those 

 that use them in the air. 



STERNA OF THE DIVERS. 



There is so much diversity of habit, as well as of 

 sternal structure, among these, that it is impossible 

 to select any single species as an average type. We 

 shall, therefore, give three instances which correspond 

 in some measure to the two extremes and the mean 

 — the grebes, the true divers, and the penguins. 



1. Sterna of the Grebes. The sternum of these 

 birds is intermediate, in its general form, between 

 that of the coots, as the aquatic extreme of the pre- 

 ceding group, and that of some of the diving ducks, 

 or rather those of the eiders and scoters, which are 

 intermediate between the swimmers and divers of the 

 duck tribe. The sternum is short and broad, and 

 much broader in the rear than the front; but the 

 breadth in that part does not consist so much of 



