Seite - 185 - in The Origin of Species

TRANSITIONS OF ORGANIC BEINGS 18S difficult to answer. Yet I think such difficulties have little weight. Here, as on other occasions, I lie under a heavy disadvan- tage, for, out of the many striking cases which I have col- lected, I can give only one or two instances of transitional habits and structures in allied species ; and of diversified habits, either constant or occasional, in the same species. And it seems to me that nothing less than a long list of such cases is sufficient to lessen the difficulty in any particular case like that of the bat. Look at the family of squirrels; here we have the finest gradation from animals with their tails only slightly flat- tened, and from others, as Sir J. Richardson has remarked, with the posterior part of their bodies rather wide and with the skin on their flanks rather full, to the so-called flying squirrels ; and flying squirrels have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to glide through the air to an astonishing distance from tree to tree. 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, 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 became modified and improved in structure in a corresponding man- ner. 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 modi- fication being useful, each being propagated, until, by the ac- cumulated effects of this process of natural selection, a per- fect so-called flying squirrel was produced. Now look at the Galeopithccus or so-called flying lemur, which formerly was ranked amongst bats, but is now believed to belong to the Insectivora. As extremely wide flank-mem-