THE CLASSIFICATION OF BIRDS 163 



characters have not yet put in an appearance, or have done 

 so only to a small extent. There is no memher of the 

 group, wide though it is, in which there is an ambiens, a 

 ■ special bird muscle. It may be that Gadow's discovery of 

 a small independent slip of the rectus femoris, which he 

 interprets as a rudimentary ambiens, is really the beginning 

 of this characteristic muscle. The remarkable peculiarities 

 of this muscle seem to forbid the notion that it is the direct 

 descendant of anything reptilian. And we have the un- 

 doubted fact that, apart from the possible rudiment already 

 referred to, it is present in no pico-passerine bird. If it 

 had disappeared in them there would be here and there a 

 rudiment left. But nothing of the kind has been hinted by 

 Gaeeod and Foebbs, who between them dissected so many 

 of these birds, and who would have been especially on the 

 look-out for such a point. I should therefore be disposed 

 to disagree at once with GtAeeod's opinion that those birds 

 which have lost the ambiens ' may be set down as having 

 possessed the muscle in their ancestral form.'- The ambiens 

 is so purely a bird muscle, though it may doubtless have its 

 homologue among reptiles, that one cannot but think that it 

 was acquired within the class ; and the facts discovered by 

 Mitchell (see above) entirely support this way of look- 

 ing at the . matter, and indeed suggested it. As to the 

 muscular system of the wing, a highly characteristic muscle 

 is the . expansor secundariorum ; this was supposed for 

 some time to be absent in the group under consideration, 

 but it has been found to occur in some of them. In the 

 majority of those in which it does occur its structure is 

 decidedly more rudimentary than in some of the Homalogo- 

 natsB. It is true that Fuebeingbe regards this muscle as 

 the abortive remnant of a reptilian muscle. But this state- 

 ment cannot be made about the patagial muscles, which are 

 essentially ornithic. ■ 



Now it is noteworthy that, with the exception of the 

 colies, not a single bird referable to the great group of Ano- 

 malogonatse has a biceps slip, while in the majority of them 

 the tendon of the . tensor brevis is exceedingly simple, being 



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