218 TRANSITIONS OF ORGANIC BEINGS. [Chap. VL 



ened 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-mem- 

 brane 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 transi- 

 tion 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 fluttering 

 fins, might have been modified into perfectly winged 

 animals. If this had been effected, who would have 



