Page:Natural History, Birds.djvu/21

8 blood-vessels which supply the vascular nourishing pulp of the barrel. When this pulp has performed its office, and the stalk and other parts of the feather are fully developed, it shrivels up into the well-known substance, which every one finds in a quill when he cuts it for the purpose of making a pen."

As in the Mammalia, the classification of Birds into Orders is founded upon the organs connected with motion and food; that is to say, the characters are taken principally from the beak and feet. The subordinate divisions depend chiefly upon the form of the beak, and pass into each other by almost imperceptible gradations, insomuch that there is no other class of animals in which the genera and sub-genera are so little susceptible of definite limitation.

In the present work we shall adopt the Orders enumerated in the "List of the Genera of Birds" of Mr. G. R. Gray, which are eight in number, viz., Accipitres, Passeres, Scansores, Columbæ, Gallinæ, Struthiones, Grallæ, and Anseres.