130 TRANSITIONS OF ORGANIC BEINGS. [CHAP. VL 



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 exter- 

 minated, unless they also became 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 

 formerly was ranked amongst 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. Although no graduated links of 

 structure, fitted for gliding through the air, now connect the 

 Galeopithecus with the other Insectivora, yet there is no difficulty 

 in 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 Galeopithecus might have been greatly lengthened by natural 

 selection ; 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 

 api 9,ratus originally fitted for gliding through the air rather than 

 for night. 



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 



