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
beneficial, is a highly complex affair; and that there is nothing strange
in a transition not having occurred in any particular case.
Lastly, more than one writer has asked why have some animals had their
mental powers more highly developed than others, as such development would
be advantageous to all? Why have not apes acquired the intellectual powers
of man? Various causes could be assigned; but as they are conjectural, and
their relative probability cannot be weighed, it would be useless to give
them.
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