42 GENERAL HISTORY OF BIRDS. 



far as all modern birds are involved, the fully-winged condition is the 

 primitive and normal one and the imperfectly-winged forms are derivative 

 ones which have lost the use of their wings from disuse; those members, being 

 no longer exercised, have become more or less curtailed, the extent of re- 

 duction being in some degree the reflection of the length of time of their 

 deviation from the parental stock. 



The power of flight is determined to a large extent by the form of the 

 wings and the relative length of the constituent primary feathers. Birds of 

 swift and prolonged flight have the wings more or less lengthened and pointed 

 and one or more of the outer feathers elongated, while, in those of diminished 

 power, the wings are generally shortened and often rounded, none of the 

 feathers being much extended; the Swallows are typical examples of the 

 former kind and the gallinaceous birds of the latter. The form of the wings 

 is generally considered characteristic of a family ; rarely is there great varia- 

 tion. In one family (the Falcons or Falconids) pointed wings and rounded 

 wings and intermediate conditions are represented by difi'erent genera. 



The most aberrant of wings is developed in the Penguins. In them, the 

 distinction of the alar feathers into primaries, secondaries and tertials, so 

 characteristic of almost all other birds, is entirely obliterated and the feathers 

 have a scale-like appearance andare used as flippers. Any one who has seen 

 a Penguin pursuing its fishy prey under water could not fail to have been 

 struck by the similarity of the bird's movements to those of the mammalian 

 Seal. 



The similarity in the structure of all birds has been claimed to be a 

 necessary consequence of adaptation for flight and attainment of the avian type, 

 and the classification of birds into orders and even sub- classes has been 

 defended for such reason. The entire irrelevancy of such an argument be- 

 comes manifest on the consideration of other adaptations for flight as evinced in 

 the existing mammalian Bats and Colugos and the extinct reptilian 

 Pterodactyles. In the former digits, which are so reduced in birds, are to the 

 number of four greatly elongated and serve for the support of a volant mem- 

 brane; in the Pterodactyles a single finger, corresponding to the little finger of 

 most vertebrates, was excessively elongated and served for the attachment of 

 a membrane subservient to flight. 



Even in extinct primaeval birds Archseopteryx (e. g.), a considerable differ- 

 ence from recent birds existed in alar structure. In fact the potentiality, of 

 difference in birds was practically unlimited and the fact that greater differ- 

 ences do not exist is evidence of the fitness of the avian structure as now known 

 for the struggle of life, and it is a perversion of taxonomy to otherwise repre- 

 sent it by the multiplication of orders which lead the student to suppose that 



