106 DIFFERENTIAL FOKMS. 



But in other points the difference is consider- 

 able, to say nothing of size. They differ in the 

 development and form of the wings, and with 

 this is associated corresponding modifications in 

 the style and character of flight ; they differ in 

 the development of the feet, which in some 

 species are much more robust than in others ; 

 and as a rule, the feet are particularly small in 

 such as are most gifted in the powers of flight. 

 Where the feet are large, the wings are reduced, 

 but the facility of clinging is increased. In 

 some species the tarsi are covered by a full muff 

 of most delicate down. 



The tail, an organ of great importance in 

 flight, presents numberless and oppositive modi- 

 fications of form, of expansiveness, of abbrevia- 

 tion, or elongation, and hereby also is the style of 

 aerial progression affected. Every one who has 

 observed the evolutions of the Swift, the Swal- 

 low, the Martin, and the Sand-martin, birds 

 rapid on the wing, cannot but have remarked 

 that each has its own style of flight, by which, 

 irrespective of anything else, the species may be 

 at once determined. In each of these birds, not 

 only the wings, but the tails also, are differently 

 modified a point on which we need not dilate. 

 But very far greater is the modification of these 

 organs amongst the Humming-birds ; greater, 

 therefore, must be the differences in their style 

 of flight, and its ordinary degree of aerial eleva- 

 tion ; this being brought to harmonize with the 



