407 
Families constituting the Order Passeres. 
other. The form of the sternum has proved to be of the 
greatest importance in separating from the Passeres several 
groups which did not properly belong to it, and we may now 
(since the Humming-birds, the Swifts, and the Todies have 
been separated from it) consider the limits of this great Order 
to be pretty well determined. But within those limits this 
character is of little service, owing to the great uniformity of 
structure that prevails throughout the whole series of Pas¬ 
serine families. An important step was made when it was 
observed that a number of South-American groups differed 
from their Old-World analogues in wanting certain vocal 
muscles; and when it was found that there were correspond¬ 
ing external characters in the wings and feet, the separation 
of these families as a natural series became generally accepted. 
But there are great objections to the use of characters drawn 
from the fleshy parts of birds. It is only in comparatively 
few instances that they have been accurately observed ; and 
they are for this reason of little use to the naturalist who 
possesses even the most extensive collection of skins and ske¬ 
letons. Owing to the paucity of observations, we are also 
unable to determine how far the character in question is a 
constant one ; and there is reason to believe that the larynx, 
the intestines, and the other internal soft parts are liable to 
much modification, even in closely allied forms. In order to 
be practically useful, the characters on which genera, families, 
and groups of families are founded must, whenever possible, 
be drawn from those parts which can be examined in every 
well-preserved skin, supplemented in critical cases by a re¬ 
ference to the sternum , the cranium , or other parts of the 
skeleton. 
Now it is found that the Passeres with imperfect singing 
apparatus are also characterized by having wings with 10 pri¬ 
maries, the first of which is almost always fully developed, or 
very little shorter than those which immediately follow it; 
whereas all other Passeres have either 9 primaries only, or, if 
10, have the first distinctly reduced below its proportionate 
size, and often so small as to be rudimentary and functionless. 
But although the character of the first primary quill was thus 
