2 4 o THE POPULAR SCIENCE MONTHLY. 



THE WINGS OF BIEDS.* 



By Peofessob W. II. FLOWEK, F. R.S., 



DIEECTOE OF THE BEITISH NATUBAL UISTOBY MUSEUM. 



THE power of flying through the air is one of the principal charac- 

 teristics of the class of birds. Although some members of the 

 other great divisions of the vertebrates — the bats among mammals, 

 the extinct pterodactyl among reptiles, the flying-fishes among pisces 

 — possess this power in a greater or less degree, these are all excep- 

 tional forms, whereas in birds the faculty of flight is the rule, its ab- 

 sence the exception. Among invertebrates this power is possessed in 

 a very complete degree by the greater number of insects. 



In the normal structure of the vertebrate animals there are two 

 pairs of limbs, anterior and posterior, never more. It often happens, 

 however, that one pair, and sometimes both, are suppressed, being 

 rudimentary, functionless, or entirely absent. Flight is always per- 

 formed by the anterior or pectoral pair, more or less modified for the 

 purpose. The superaddition of wings to arms, as in the pictorial rep- 

 resentations of angels, has no counterpart in nature. The wings of 

 the bird, the bat, the pterodactyl, and flying-fish, are the homologues 

 of the arms of man, the fore-legs of beasts. In the flying-fish the 

 power is gained simply by an enlargement of the pectoral fin, and the 

 function is very imperfect ; in the pterodactyl, by immense elonga- 

 tion of one (the outer) finger, and extension of the skin between it and 

 the side of the body ; in the bats, by elongation of the four outer fin- 

 gers, and extension of a web of skin between them and the body. In 

 the bird the flying organ is constructed mainly of epidermic structures, 

 peculiar outgrowths from the surface, called feathers — modifications 

 of the same tissue which constitutes the hair, horns, scales, or nails of 

 other animals. Feathers are met with only in birds, and are found in 

 all the existing members of the class, constituting the general covering 

 of the surface of the body. 



The framework to which the broad expanse formed by the feathers 

 is attached is composed of bones, essentially resembling those of the 

 fore-limbs of other vertebrates. The distal segment, manus, or hand, 

 in the vast majority of birds, has three metacarpal bones and digits, 

 the former being more or less united together in the adult state. The 

 digits appear to correspond with the pollex, index, and medius of the 

 typical pentadactyl manus ; the second is always the longest. Both 

 it and the pollex frequently bear small horny claws at their extremity, 

 concealed among the feathers and functionless, but very significant 

 in relation to the probable original condition of the avian wing. These 



* Abstract of a lecture delivered at the Royal Institution of Great Britain, February 

 19, 1886. 



