THE BtR0 ITSELF 
Birds display, to a very marked extent, that, as we may 
somew'liat crudely put it, plasticity of structure, which ainounts 
broadly speaking^ to the popular conception (or misconception) 
of the overworked phrase "adaptation to environment 
Thus it is that the bill and the feet of birds show almost 
infinite variety in the matter of shape. The bill acts as mouth 
and hand; it seizes the food and if necessary tears it into 
small pieces. Structurally the beak consists of the long jaws 
encased iti a horny sheath. The "jaws" are called mtutdjbhs, 
upper and lower respectively, and almost as many terms have 
been coined for the exact description of the bill as for the 
description of leaves in botanical parlance, A bill may be 
turgid, long, epignathous, dentirostral or a host of other 
things, cadi of these words being but one of a series designed 
to express a certain set of conditions, but with these we are not 
particoiarly interested. 
A moment's thought vvill call to mind any number of bill 
modiftcations admirably suited for the work they have to per- 
form : the waders for instance have a long, thin bill well suited 
to probing in the soft mud, the owls and hawks strong hooked 
weapons very appropriate to birds of rapacious habits and the 
ducks have an excellent instrument for straining food from the 
water. 
The legs and feet again show great variety in shape acd 
relative siie. In ground-living birds such as the ostriches, 
bustards and game-birds they are large and strong but in birds 
of marked aerial habits such as the swallows, swifts and night 
jars they have become very small. In fact quite as much 
diversity is shown as in the bill. The perching-birds have 
three toes in front and one behind — a very convenient device 
for grasping boughs. The zygodactyle or yoke'toed foot of 
the woodpeckers, the webbed feet of aquatic birds and the 
aggressive heavily armed feet of the eagles are but a few of 
the leading modifications of the avian foot. A bird is of 
course digitigrade; that is to say it walks on its toes and not on 
the fiat of the foot as does man. It follows that the joints of 
the birds legs usually referred to as "ankle" and '*knee^* are 
realty "base of the toes** and ''ankle" the true knee being 
well up near the body and covered with feathers. Contrary 
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