2, FALCONID&. 
and the irides become bright yellow. Young females 
have the whole of the under parts from the throat to 
the under tajl-coverts of a uniform reddish bay, without 
any of the darker-coloured streaks observable in adult 
females. 
Having thus described the last of the British Falconide, 
it may be desirable, before quitting this group, to exhibit a 
representation of the breast-bone, or sternum, of one of the 
types of the Falcons, in order to show in the form and 
magnitude of the principal bone, and the others attached to 
it, the power of flight possessed by these birds, of which 
the breast-bone affords good comparative indication. 
The power of flight is one of the decided characteristics 
of the organization of the class of Birds; and the division 
of the Falconide includes those birds which appear to pos- 
sess this power in the highest degree of perfection. The 
conditions necessary to produce this power in its fullest ex- 
tent are, large and strong pectoral muscles; great extent 
of surface, as well as peculiarity of form, in the wing; and 
feathers of firm texture, strong in the shaft, with the fila- 
ments of the plume arranged and connected to resist pres- 
sure from below. The extent of surface, the form and 
other peculiarities of the wings, have been already noticed, 
and the anatomical part only requires to be briefly de- 
scribed. A certain degree of specific gravity is necessary 
to rapid flight, and this is imparted by large pectoral mus- 
cles ; the power of these muscles may be estimated by the 
depth of the keel, and the breadth of the sides of the 
breast-bone or sternum, as affording extent of surface for 
the attachment of those large muscles by the action of 
which the wings are brought down. 
As an illustration of this form, the figure here inserted 
as a vignette is a representation about one-fourth less than 
