uirrels; each grade of structure having been useful to its possessor.
Nor can I see any insuperable difficulty in further believing it
possible that the membrane-connected fingers and fore-arm of the
Galeopithecus might have been greatly lengthened by natural selection;
and this, as far as the organs of flight are concerned, would
have converted the animal into a bat. In certain bats in which the
wing-membrane extends from the top of the shoulder to the tail and
includes the hind-legs, we perhaps see traces of an apparatus originally
fitted for gliding through the air rather than for flight.

If about a dozen genera of birds were to become extinct, who would have
ventured to surmise 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 as 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
conditions. It must not be inferred from these remarks that any of the
grades of wing-structure here alluded to, which perhaps may all be the
result of disuse, indicate the steps by which birds actually acquired
their perfect power of flight; but they serve to show what diversified
means of transition are at least 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, so 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 have
survived to the present day, for they will have