164 ON THE ORIGIN OF SPECIES 



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, 

 indicate 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 con- 

 ceivable 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 their 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 

 transitional grades of the structure will seldom continue 

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

 supplanted by the very process of perfection through 

 natural selection. Furthermore, we may conclude that 

 transitional grades between structures fitted for very 

 different habits of life will rarely have been developed 

 at an early period in great numbers and under many 

 subordinate forms. Thus, to return to our imaginary 

 illustration of the flying-fish, it does not seem probable 

 that fishes capable of true flight would have been 

 developed under many subordinate forms, for taking 

 prey of many kinds in many ways, on the land and iD 



