ones just
indicated, have probably interfered with the acquisition through natural
selection of structures, which it is thought would be beneficial to
certain species. One writer asks, why has not the ostrich acquired the
power of flight? But a moment's reflection will show what an enormous
supply of food would be necessary to give to this bird of the desert
force to move its huge body through the air. Oceanic islands are
inhabited by bats and seals, but by no terrestrial mammals; yet as some
of these bats are peculiar species, they must have long inhabited their
present homes. Therefore Sir C. Lyell asks, and assigns certain reasons
in answer, why have not seals and bats given birth on such islands to
forms fitted to live on the land? But seals would necessarily be first
converted into terrestrial carnivorous animals of considerable size, and
bats into terrestrial insectivorous animals; for the former there would
be no prey; for the bats ground-insects would serve as food, but these
would already be largely preyed on by the reptiles or birds, which first
colonise and abound on most oceanic islands. Gradations of structure,
with each stage beneficial to a changing species, will be favoured only
under certain peculiar conditions. A strictly terrestrial animal, by
occasionally hunting for food in shallow water, then in streams or
lakes, might at last be converted into an animal so thoroughly aquatic
as to brave the open ocean. But seals would not find on oceanic
islands the conditions favourable to their gradual reconversion into a
terrestrial form. Bats, as formerly shown, probably acquired their
wings by at first gliding through the air from tree to tree, like the
so-called flying squirrels, for the sake of escaping from their enemies,
or for avoiding falls; but when the power of true flight had once been
acquired, it would never be reconverted back, at least for the above
purposes, into the less efficient power of gliding through the air.
Bats, might, indeed, like many birds, have had their wings greatly
reduced in size, or completely lost, through disuse; but in this case
it would be necessary that they should first have acquired the power of
running quickly on the ground, by the aid of their hind legs alone, so
as to compete with birds or other ground animals; and for such a change
a bat seems singularly ill-fitted. These conjectural remarks have been
made merely to show that a transition of structure, with each step