INTRODUCTION. 
17 
that procure their food on the land. Birds, therefore, can- 
not strictly he arranged according to their manner of walk- 
ing or flying, any more than according to their food ; hut all 
these circumstances, and others, taken in connexion with 
their organization, lead to the formation of orders and fami- 
lies, which are in themselves more or less natural, that is, 
contain a series of species obviously connected with each 
other more than with other species. 
In this Synopsis of the Birds of England, Scotland, and 
Ireland, the arrangement proposed is into Orders, Families, 
Genera, and Species, the characters of the more comprehen- 
sive divisions alluded to above, are not given ; but as it was 
found expedient to divide it into two volumes, it was thought 
best to give the general title of Land-Birds to the Yolato- 
rial and Gradatorial series, and that of Water-Birds to the 
Grallatorial and Natatorial. 
In the Introduction to the first volume, some necessary 
explanations were given relative to the general structure and 
form of birds, their different parts, their dermal system, in- 
cluding the plumage, mandibles, and claws, and their diges- 
tive organs. On the present occasion it is not requisite to 
continue the subject farther than to say a few words respect- 
ing the structure of the intestinal canal, and the trachea, 
which present very remarkable modifications in the different 
groups, and of which some afford even specific distinctions. 
The width of the mouth corresponds with the size of the 
object to be swallowed; but one cannot always judge of its 
capability of admitting a large mass by its mere appearance, 
for a mouth may seem narrow, which is yet capable, by the 
elasticity of the parts, of being greatly expanded. The 
tongue generally aids in the prehension and deglutition of 
the food, and varies according to its nature, but also has 
evident relation to the form of the cavity in which it is 
lodged. When the tongue is extremely small, as in Cor- 
morants and Gannets, it is useless with reference to prehen- 
sion, and, in such cases, the bird must toss up the object 
