182 DIFFICULTIES ON THEOEY. Chap. VI. 



surmised 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 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 con- 

 ditions. It must not be inferred from these remarks 

 that any of the grades of wing-structure here alluded to, 

 which perhaps may all have resulted from disuse, indi- 

 cate the natural steps by which birds have acquired 

 their perfect power of flight ; but they serve, at least, 

 to show what diversified means of transition are 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 

 ever imagined that in an early transitional state they 

 had been inhabitants of the open ocean, and had used 

 then- incipient organs of flight exclusively, as far as we 

 know, to escape being devoured by other fish ? 



When we see any structure highly perfected for any 

 particular habit, as the wings of a bird for flight, we 

 should bear in mind that animals displaying early tran- 

 sitional grades of the structure will seldom continue to 

 exist to the present day, for they will have been sup- 

 planted by the very process of perfection through natural 

 selection. Furthermore, we may conclude that transi- 



