28 
ON THE CLASSIFICATION OP BIRDS. 
without any great alteration of the walking attitude. 
This, in fact, is the reason why the long-legged waders 
have necks and bills proportionate to the elevation of 
their bodies ; for as the habit of wading and of running 
requires this great development of foot, so, had they 
short necks and bills, they could not possibly feed but 
in a constrained and unnatural position. According to 
the habits of the bird, so do we find the same structure 
applied to different uses. Hence, in the swans, and the 
ivhole of the duck family (Anatida;), there is great 
elongation of neck, accompanied by very short feet. 
Now, were these terrestrial birds, it is obvious that 
such disproportion of parts would be anomalous ; but 
they are .all aquatic, and without their long necks they 
would he quite unable to reach the bottom of the 
waters upon which they swim, and where they are 
taught to look for their food. A swan or a duck swims 
along the edge of a reedy stream, and thrusts its long 
neck into every break, dives to the muddy bottom, and 
ferrets out among the roots of plants its appropriate 
food. 1 he length and thinness of the neck makes all 
these habits perfectly easy, which, but for such an 
adaptation of structure to its mode of life, would be 
difficult, if not impossible. Again, all birds which 
seize their prey from a fixed station, without using their 
wings in its capture, have also the neck greatly length- 
ened. The whole family of herons, among the waders, 
and the aquatic genera of Carbo and Plotus, are exi 
amplcs of this structure. VThereas, when the wings 
are also used in the capture of the prey, the neck is 
very short. Ihis opposite extreme is very conspicuous 
in the whole tribe of l^isfnroHreify as the swallows^ 
swifts, goatsuckers, &c. ; and in their representatives, the 
flycatchers, todies, tyrants, &c. It may be taken as 
an invariable rule, that out of the insessorial and nata- 
torial order, all birds have their neck and legs of propor- 
tionate length. 
(37.) The head is generally clothed with feathers ; 
but in very many birds it is more or less naked. What 
