Therefore, I can see no difficulty, more especially under changing conditions of life, in the continued preservation of individuals with fuller and fuller flank-membranes, each modification being useful, each being propagated, until, by the accumulated effects of this process of natural selection, a perfect so-called flying squirrel was produced.

Now look at the Galeopithecus or so-called flying lemur, which was formerly ranked among bats, but is now believed to belong to the Insectivora.

An extremely wide flank-membrane stretches from the corners of the jaw to the tail, and includes the limbs with the elongated fingers.

This flank- membrane is furnished with an extensor muscle.