ON CERTAIN MUSCLES OF BIRDS. 211 
genera, these differences are never more considerable than such as 
consist of the absence of one muscle from the typical arrangement of 
the family, or, in other words, from the modification of one element 
of the typical formula. When, therefore, it is found that under any 
accepted arrangement there are subfamilies differing from one another 
by more than a single muscular peculiarity, there is reason to expect 
that these subfamilies would be further separated in a natural arrange- 
ment. The Accipitres furnish an example; the myological formule 
of its subdivisions are subjoined, + and — indicating the presence or 
absence of the ambiens muscle :— 
Falconide A + 
Vulturide A + 
Cathartide A.X Y + or XY + 
Strigide A — 
Serpentariide B.XY +. 
This table makes it evident that the Falconide and Vultwride are 
widely separated from the Cathartide andthe Serpentariide, and that 
it is perfectly impossible to unite in any intimate way these two Page 114. 
latter families with the two former, or with one another. In fact the 
Accipitres, as generally defined, are not a natural group at all; and 
the Cathartide are not the least more nearly related to the Vulturide 
than to the Falconide. 
Respecting families it may therefore be said that myological pecu- 
liarities which do not involve more than a single structural change 
from the typical arrangement of the family are frequent, and that 
further differences indicate a more distinct relationship. 
The various opinions held by different ornithologists as to the 
correct division of the Orders of the Class Aves are so numerous that 
they make it impossible in this stage of the inquiry to discuss the 
myological features which they present. An inspection of Plate 5 
(XVIL.) is sufficient to show that the formula of a bird is not of direct 
value in estimating ordinal characters. 
Looking at the whole subject from another point of view, it may 
now be asked, What does the arrangement in the muscles above 
described teach as to the major divisions of the Class Aves? The 
remainder of this communication will be an attempt to answer this 
question. 
A mere glance at Plate 5 (X VII.) is sufficient to show that the facts 
disclosed by a study of the myology of birds do not, without extraneous 
assistance, place the different families in their true relationship to 
one another. Because the same muscles are present in two families of 
birds, it cannot therefore be said that their kinship is extremely close, 
P2 
