182 
DIFFICULTIES OK THEOKY. 
Chap. VI. 
If about a dozen genera of birds bad become extinct 
or were unknown, who would have ventured to have 
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, 
indicate the natural steps by which birds have ac¬ 
quired 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 conceiv¬ 
able 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 per¬ 
fectly winged animals. If this had been effected, 
who would have ever imagined that in an early tran¬ 
sitional 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 de¬ 
voured 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- 
