Seite - 186 - in The Origin of Species

186 ORIGIN OF SPECIES brane stretches from the comers of the jaw to the tail, and includes the limbs with the elongated fingers. This flank- membrane is furnished with an extensor muscle. Although no graduated links of structure, fitted for gliding through the air, now connect the Galeopithecus with the other Insec- tivora, yet there is no difficulty to supposing that such links formerly existed, and that each was developed in the same manner as with the less perfectly gliding squirrels ; each grade of structure having been useful to its possessor. Nor can I see any insuperable difficulty in further believing that the membrane connected fingers and fore-arm of the Galeopi- thecus might have been greatly lengthened by natural selec- tion; and this, as far as the organs of flight are concerned, would have converted the animal into a bat. In certain bats in which the wing-membrane extends from the top of the shoulder to the tail and includes the hind-legs, we perhaps see traces of an apparatus originally fitted for gliding through the air rather than for flight. If about a dozen genera of birds were to become extinct, who would have ventured to surmise 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 as front-legs on the land, like the penguin ; as sails, like the ostrich ; and functionally for no purpose, like the Apteryx? Yet the structure of each of these birds is good for it,under the conditions of life towhich it is exposed, for each has to live by a struggle ; but it is not necessarily the best possible under all possible conditions. It must not be inferred from these remarks that any of the grades of wing-structure here alluded to, which perhaps may all be the result of disuse, indicate the steps by which birds actually acquired their perfect power of flight ; but they serve to show what diversified means of transition are at least 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 mammals, fly- ing insects of the most diversified types, and formerly had flying reptiles, it is conceivable that flying-fish, which now glide far through the air, slightly rising and turning by the aid of their fluttering fins, might have been modified ^x^to per-