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 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 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 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.
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