We cannot doubt that each structure is of use to each kind of
squirrel in its own country, by enabling it to escape birds or beasts of
prey, or to collect food more quickly, or, as there is reason to believe,
to lessen the danger from occasional falls. But it does not follow from
this fact that the structure of each squirrel is the best that it is
possible to conceive under all possible conditions. Let the climate and
vegetation change, let other competing rodents or new beasts of prey
immigrate, or old ones become modified, and all analogy would lead us to
believe that some, at least, of the squirrels would decrease in numbers or
become exterminated, unless they also become modified and improved in
structure in a corresponding manner. 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.
Pages:
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280