i6 ANATOMY 



knowledge of the anatomy of Birds, to decide which characters and 

 which organs are of extrinsic taxonomic value, and which are 

 not. Nor is it always possible to see why certain organs, fully 

 developed, and exhibiting striking and constant features in one 

 group of Birds, are extremely variable in another otherwise very 

 circumscribed and apparently natural group. Supposing such a 

 character to be absent in a given group, is it absent because it has 

 not yet been developed, or is it because it has been lost ? Has it been 

 lost by the ancestors of this group, or has it been abolished within 

 this group ? In the former case the absence of this character would 

 probably help to decide the relative position of the group ; in the 

 latter case this very same character would be reduced to a dia- 

 gnostic point within the group, and not throw any light upon its 

 relationship or systematic position. It may be very easy to dia- 

 gnose genera or even large groups of birds, but this ability to deter- 

 mine them by the helj) of mechanically arranged "keys" does not 

 necessarily aftbrd us more than an occasional glimpse of the sunk 

 avine tree, at the reconstruction of which we all aim, as the true 

 representation of the natural affinities of Birds. 



It is occasionally insisted upon that " tact " will help us to 

 select and to reject characters, and thus prevent us from falling into 

 glaring errors ; but tact is a personal feeling, often bias, and it is 

 proof, not inclination, that settles scientific cpiestions. The import- 

 ance of these considerations, • often expressed before in abler words, 

 is gaining more and more ground among ornithologists, and "w^ill 

 therefore permit the following illustrations of the ways in which 

 we may or may not apply the study of comparative anatomy to 

 classification. 



The presence of the Ambiens Muscle is a Reptilian feature ; 

 among Birds it exists in the majority of the lower groups, and is 

 absent in most of the higher members of the Class. We conclude 

 that the latter have lost this muscle, and not that it has not yet 

 been developed in them. Its reduction or loss is still going on 

 within some groiips, such as Parrots and Pigeons. This loss takes 

 place independently in widely different groups. It follows, first, 

 that absence of this muscle does not always indicate relationship ; 

 secondly, that we can derive forms that are without it from a 

 group which still possess it ; but that the reversed conclusion is not 

 possible. We know of no organ which has been redeveloped after 

 it has once disappeared in the ancestors of the animals under con- 

 sideration. Therefore the absence of the ambiens muscle in all 

 Owls, which apparently use their hinder extremities in the same 

 way as the Falconidx (which possess this muscle), indicates that 

 the Owls are not developed from the Falconidse, but from a group 

 which, like the Macrochires, had already lost this organ. 



Similar arguments apply to thq C.ECA. It is generally admitted 



