INTRODUCTION. 
In the series of Land-Birds, properly so called, the Or- 
ders and Families have been disposed according to a method 
in some degree corresponding with the nature of their food, 
those which are carnivorous being placed first, the insecti- 
vorous next, and lastly the granivorous and graminivorous. 
In describing the Water-Birds, strictly so called, or such as 
are web-footed, one might follow a similar order, placing the 
piscivorous kinds opposite the carnivorous terrestrial species, 
those which live chiefly on Crustacea opposite the insectivo- 
rous, and the graminivorous Geese and Ducks opposite the 
Pheasants and Grouse. Or the order might be reversed, the 
gramivorous species being placed first, and the piscivorous 
last. But, in reality, an arrangement in strict conformity 
with the nature of the food, would prove very unnatural, in- 
asmuch as, in many very natural families, species are found 
which are carnivorous, others insectivorous, and others fru- 
givorous. Even in the same genus, Corvus, for example, 
one, the Baven, is carnivorous, another, the Jackdaw, insec- 
tivorous, and a third, the Rook, insectivorous and occasion- 
ally frugivorous. Nevertheless, it will be found that, in a 
certain degree, regimen corresponds with external form and 
internal organization ; and I have, therefore, in the se- 
ries of swimming birds, placed the grass-eating birds by 
themselves, under the name of Cribratrices, although some 
of them also eat worms and mollusca, or even fishes. The 
