apted 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.
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 exclusively
for one of its several different habits. But it is difficult to tell,
and immaterial for us, whether habits generally change first and
structure afterwards; or whether slight modifications of structure lead
to changed habits; both probably often change almost simultaneously. Of
cases of changed habits it will suffice merely to allude to that of the
many British insects which n