1 8 FALCONID.E.
species under the name of Aquila barthelemii. They constantly
differ from other Golden Eagles, it is said, hy the
presence of a few white feathers among the scapulars. Two
of these birds, taken from the nest in 1857, were sent to
Mr. Gurney, and one of them, never having before shewn
any departure from the ordinary plumage of A. clirysaetus,
was observed in 1864 to have the first scapular on each side
of a pure white. The Norwich Museum possesses a similar
example from Algeria. Young Golden Eagles, before assuming
the fully mature plumage, often have the feathers
of the tarsus white, and in this state some ornithologists
have been inclined to regard them as belonging to a distinct
species.
The foot of the Golden Eagle is so distinctly marked from
that of the White-tailed or Cinereous Eagle, as to afford the
means of deciding between the two at any ag e; and the
three anterior toes of both species are therefore here figured
to shew the distinction. The foot on the left hand is that
of the Golden Eagle, in which the tarsus is clothed with
feathers and each toe is covered with small reticulations as
far as the last phalanx, then with the three broad scales
already referred to. In the foot of the White-tailed Eagle,
represented by the figure on the right hand, the reticulations
are confined to the tarsus, the whole length of each toe being
covered with broad scales.
The figure of the Golden Eagle at the head of this article
was taken from a fine specimen at the garden of the Zoological
Society, where it had lived for eight years.
The power of vision in birds is observed to he very extraordinary;
and in none is it more conspicuous than in the
Eagles, and the Falconiclce generally. I t has been stated
that, probably in the whole range of anatomy, no more perfect
adaptation of structure to function could be adduced than is
to he found in the numerous and beautiful modifications in
the form of various parts of the eyes of different animals,
destined to exercise vision in media of various degrees of
transparency as well as density. The figure on the right
hand of the vignette below represents the circle, composed
of fifteen bony plates, hy which the orb of the eye of the
Golden Eagle is supported. These bony plates are capable
of slight motion upon each other. The figure on the left
hand in the vignette below represents the crystalline lens of
the same b ird ; the lens being subject to great variety of form
in different birds. In the Eagle, the proportion of the axis
to the diameter of the lens is as three and eight-tenths to
five and seven-tenths; in the Eagle-Owl, which seeks its
prey at twilight, the relative proportions of the lens are as
six and seven-tenths to seven and eight-tenths; and in
the Swan, which has to select its food under water, the proportions
of the lens are as three to three and eight-tenths.
Birds have also the power of altering the degree of the
convexity of the cornea. With numerous modifications of
form, aided by delicate muscular arrangement, birds appear
to have the power of obtaining such variable degrees of
extent or intensity of vision as are most in accordance with
their peculiar habits and necessities.