244 THE ORIGIN OF SPECIES 



might have been greatly lengthened by natural selection; 

 and this, as far as the organs of flight are concerned, 

 would have converted the animal into a bat. In certain 

 bats in which the wing-membrane extends from the top 

 of the shoulder to the tail and includes the hind-legs, we 

 perhaps see traces of an apparatus originally fitted for 

 gliding through the air rather than for flight. 



If about a dozen genera of birds were to become 

 extinct, who would have ventured to surmise that birds 

 might have existed which used their wings solely as 

 flappers, like the logger-headed duck (Micropterus of 

 Eyton); as fins in the water and as front legs on the 

 land, like the penguin; as sails, like the ostrich; and 

 functionally for no purpose, like the Apteryx? Yet 

 the structure of each of these birds is good for it, under 

 the conditions of life to which it is exposed, for each has 

 to live by a struggle; but it is not necessarily the best 

 possible under all possible conditions. It must not be 

 inferred from these remarks that any of the grades of 

 wing-structure here alluded to, which perhaps may all be 

 the result of disuse, indicate the steps by which birds 

 actually acquired their perfect power of flight; but they 

 serve to show what diversified means of transition are 

 at least possible. 



Seeing that a few members of such water-breathing 

 classes as the Crustacea and Mollusca are adapted to live 

 on the land; and seeing that we have flying birds and 

 mammals, flying insects of the most diversified types, 

 and formerly had flying reptiles, it is conceivable that 

 flying-fish, which now glide far through the air, slightly 

 rising and turning by the aid of their flattering fins, might 

 have been modified into perfectly winged animals. If 



