276 NATATOEES, SWIMMING BIRDS. 



Subclass III. AVES AQUATICl], or NATATOEES. 



AQUATIC BIRDS. SWIMMERS. 



This, the third and last series, containing all remaining carinate birds of the 

 present geologic epoch, is a group that may readily be defined upon the principles 

 of adaptive modification already explained under head of Aves Terrestres; 

 although as in the cases of the other two " subclasses," it does not rest upon 

 characters of much morphological significance. The birds composing it are 

 aquatic in a strict sense, fitted to progress upon or through the water, and to derive 

 the greater part of their sustenance from the same source ; many of them are 

 absolutely independent of land, except for the purpose of reproduction. Manifest 

 indications to be fulfilled in adaptation to an aquatic mode of life, are such a con- 

 figuration of the body as will enable the bird to rest upright on the water, boat- 

 like ; and such conformation of the legs as will render them a pair- of paddles 

 rather than simple pillars of support, together with water proof clothing of the 

 body. Accordingly, all swimming birds have a more or less broad and depressed 

 shape, especially flattened underneath. The coat of feathers is compact and 

 impervious to water, either by its close imbrication, or its thickening with broad 

 tracts and abundant down-feathers, or its plentiful lubrication with oil from the well- 

 developed gland on the rump ; in general, these three circumstances conspire to the 

 single result. The modifications of the legs are especiallj' interesting. In general, 

 these limbs are transformed into oars by means of webs stretching from tip to tip 

 of the front toes, and sometimes also from the inner toe to the hallux. This com- 

 plete palmation is so nearly universal that it alone would characterize the Swim- 

 mers, were it not that in one family the same result is effected by means of broad 

 lobes instead of plain webs, and for the fact that a very few genera of waders are 

 more or less completely palmiped. Since these broad webs would interfere in 

 passing each other were the legs as close together and as parallel as they are in 

 higher birds, another feature is introduced. The limbs are widelj^ separated, in 

 swimming, not only by the unusual width of the bodjr, but by an outward obliquity 

 of the members themselves ; divergence begins at the hip-joint in the direction of 

 the axis of the femur, and increases thence to the terminal segments. Greater 

 power being required to pws/i the body through the water than is needed to simply 

 support it, first on one leg and then on the other, as in ordinary walking, the femur 

 is shortened to become rather a fulcrum for advantageous application of power, 



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