276 NATATORES, SWIMMING BIRDS. 
Subclass III]. AVES AQUATICA, or NATATORES. 
AQUATIC BIRDS. SWIMMERS. 
Tuts, 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 haye 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 especially 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 widely separated, in 
swimming, not only by the unusual width of the body, 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 push 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, 
a 
