Families constituting the Order Passeres. 407 



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. 13ut within those limits this 

 character is of little service, owing to the great uniformity of 

 structure that prevails throughout the whole scries 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 

 coiLstant 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 functionlcss. 

 But although the character of the first primary quill was thus 



