branches of trees.

Objections of the same nature as the foregoing have been advanced by many
writers. In each case various causes, besides the general 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