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Show 162 DIFFICULTIES ON THEORY. [CHAP. VI. Look at the fa1nily of squirrels;. her~ we ha':e the finest gradation fron1 animals wit~ their ~a1ls only shghtly flattened and from others as Su J. Richardson has remarked, 'with the posterio~ part of their bodies rather wide and with the skin on their flanks rather full, to tho so-called flying squirrels; and flr:ing ~quirrels have their limbs and even the base of the ta1l united by a broad expanse of skin, which serve~ as a parach~te. and .allows them to glide throng~ the a1r to an aston1sh1ng 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, or to collect food more quickly, or, as there _is reason to beli~ve, by lessening the danger fro1n occasional falls. But 1t does not follow from this fact that the structure of each squirrel is the best that it is possible to conceive under all natural conditions. Let the climate and vegetation change, let other competing rodents ?r 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 b~can1e modified and ilnproved in structu~·e 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 socalled flying squirrel was produced. Now look at the Galeopithecus or flying lemur, which formerly was falsely ranked amongst bats. It has an extremely wide flank-membrane, stretching from the corners of the jaw to the tail, and including the limbs and the elongated fingers: the flank-membrane is, also, furnished with an extensor muscle. Although no graduated links of structure, fitted for gliding through the air, now con:nect the Galeopithecus with the other Lemuridre, yet I can see no difficulty in supposing that such links formerly existed, and that each had been formed by the same steps as in the case of the less perfectly gliding squirrels; and CHAP. VI.] TRANSITIONAL HABITS. 163 that each grade of structure had been useful to its possessor. Nor can I see any insuperable difficulty in further believing it possible that the membrane-connected fingers and fore-arm of the Galeopithecus might be greatly lengthened by natural selection; and this, as far as the organs of flight are concerned, would con vert it into a bat. In bats which have the wing-membrane extended from tbo top of the shoulder to the tail, including the hind-legs, we perhaps see traces of an apparatus originally constructed for gliding through the air rather than for flight. If about a dozen genera of birds had become extinct or were unknown, who would have ventured to have surmised that birds might have existed which used their wings solely as flappers, like the logger-headed duck (Micropterus of Eyton); as fins in the water and front legs on the land, like the penguin ; as sails, like the ostrich; and functionally for no pu11opose, like the Apteryx. Yet the structure of each of these birds is good for it, under the conditions of life to which it is exposed, for each has to live by a struggle; but it is not necessarily the best possible under all possible conditions. It 1nust not be inferred from these remarks that any of the grades of wing-structure here alluded to, which perhaps may all have resulted from disuse, indicate the natural steps by which birds have acquired their perfect power of flight; but they serve, at least, to show what diversified means of transition are possible. Seeing that a few members of such water-breathing classes as the Crustacea and Mollusca are adapted to live on the land, and seeing that we have flying birds and mam1nals, flying insects of the 1nost diversified types, and fortnerly had flying reptiles, it is conceivable that flyingfish, which no'v glide far through the air, slightly rising and turnin~ by the aid of their fluttering fins, might have been modined into perfectly winged animals. If this had been effected, who 'vould have ever imagined thn.t in an early transitional state they had been inhabitants of the open ocean, and had used their incipient organs of .:flight exclusively, as far as we know, to escape being devoured by other fish ~ |
