10 PREFACE. 



Birds may be disposed in linear series according to 

 certain affinities, so as to run on in an uninterrupted 

 line; but by such an arrangement numerous relations 

 must be severed, and each particular species stands in 

 its place, retaining, if one may so speak, only a portion 

 of its members, the rest being left connected with other 

 and often distant species. 



Since the affinities in a linear series cannot be de- 

 rived from every part of the organization, those of 

 most importance ought to be selected. 



The plumage in its texture and colouring indicates 

 affinity, but in a low degree ; for in the same single 

 species are often found several varieties of plumage, 

 while in another species of the same genus the plumage 

 may be nearly simple, and in two allied species it may 

 be extremely unlike, while in two most distant species 

 it may be almost or entirely similar. Affinities are in- 

 dicated by the wings and tail more than by other parts 

 of the plumage ; by the former more than by the latter ; 

 but birds, in other respects very dissimilar, may have 

 wings so nearly of the same form that the same de- 

 scription might answer for both. 



The feet indicate affinities better, as being intimately 

 connected with the habits, and with internal organiza- 

 tion, particularly with that of the digestive organs. 

 The general form of the body, also that of the organs 

 specially, afford good characters of comparison. 



Of the various organs, the digestive are best adapted, 



