emarks 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 {183} 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 in the water, until their organs of flight had come to a high stage of
perfection, so as to have given them a decided advantage over other animals
in the battle for life. Hence the chance of discovering species with
transitional grades of structure in a fossil condition will always be less,
from their having existed in lesser numbers, than in the case of species
with fully developed structures.

I will now give two or three instances of diversified and of changed habits
in the individuals of the same species. When either case occurs, it would
be easy for natural selection to fit the animal, by some modification of
its structure, for its changed habits, or exclu