306 NATATORES. 



in the body of the clear water, though they readily enough 

 approach and seize subjects which they find floating on the 

 surface. 



There is another peculiarity in their structure as con- 

 nected with their mode of feeding which is worthy of notice : 

 those which dive, and which have the wings short and 

 rounded so that they may act better under the water, have 

 the enlargement at the pulmonary extremity of the trachea 

 membranous, supported to small fibres of bone ; and those 

 which merely dabble, but always have some portion of the 

 body above water when they are feeding, have it bone, or a 

 cartilaginous flexure. The reason seems partly at least to be, 

 that the bird, which is entirely cut off from contact with the 

 atmosphere, may have a supply of air in a contractile reser- 

 voir, to sustain it while under water, but which is unneces- 

 sary in those which have a portion of the body above water, 

 and can derive, at least, some supply of air through the breath- 

 ing pores in that part, while ordinary respiration through the 

 trachea is suspended. At all events, that is a more rational 

 supposition, than that the enlargement of the trachea is con- 

 nected with voice or with the regulating of the specific gra- 

 vity of the bird. Even the use of the air-bladder in fishes as 

 a means of buoyancy is vague and doubtful, because some 

 species that have it remain habitually near the surface ; and, 

 altogether, the ascending and descending of animals in water, 

 appear to depend chiefly upon muscular effort rather than 

 upon specific gravity.* 



The habits of these birds in the procuring of their food, 

 lead by easy and obvious inference to their principal haunts. 

 They feed only where they can reach the bottom, and conse- 

 quently they cannot be birds of the high seas, and need not 

 generally be looked for on the rock shores, or the beaches of 

 clean sand gravel. The wide sea is adapted for birds that 

 *. See Note, page 430. 



