XXVI 
INTRODUCTION. 
with teeth, the food undergoes no preparation in the mouth, 
but is swallowed in unbruised and untasted morsels. Yet there 
is reason to believe that the first action of the stomach, or its 
preparatory venliiculits, affords in some degree the ruminating 
gratification of taste, as after swallowing food, in some insectiv- 
orous and carnivorous birds, the motion of the mandibles, ex- 
actly like that of ordinary tasting, can hardly be conceived to 
exist without conveying some degree of gratifying sensation. 
The clothing of birds varies with the habits and climates 
they inhabit. The aquatic tribes, and those which live in 
northern regions, are [jrovided with an abundance of plumage 
and fine down, — from which circumstance often we may form a 
correct judgment of their natal regions. In all climates, aqua- 
tic birds are almost equally feathered, and are provided with 
posterior glands containing an oily substance for anointing 
their feathers, which, aided by their thickness, prevents the 
admission of moisture to their bodies. These glands are less 
conspicuous in land birds, — unless, like the fishing Eagles, their 
habits be to plunge in the water in pursuit of their prey. 
The general structure of feathers seems purposely adapted 
both for warmth of clothing and security of flight. In the 
wings of all birds which fly, the webs composing the vanes, or 
plumy sides of the feather, mutually interlock by means of reg- 
ular rows of slender, hair-like teeth, so that the feather, except 
at and towards its base, serves as a complete and close screen 
from the weather on the one hand, and as an impermeable oar 
on the other, when situated in the wing, and required to catch 
and retain the impulse of the air. In the birds which do not 
fly, and inhabit warm climates, the feathers are few and thin, 
and their lateral webs are usually separate, as in the Ostrich, 
Cassowary, Emu, and extinct Dodo. In some cases feathers 
seem to pass into the hairs, which ordinarily clothe the quadru- 
peds, as in the Cassowary, and others ; and the base of the 
bill in many birds is usually surrounded with these capillary 
plumes. 
The greater number of birds cast their feathers annually, and 
appear to suffer much more from it than the quadrupeds do 
