l^.t TKANSITIOXS Chat. VI. 



lo conceive under all natural conditions. Lot the climate and 

 vegetation change, let other coTnpcting rodents or new beasts 

 of prey immigrate, or old ones become modified, and all anal- 

 ogy 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 cor- 

 responding manner. Therefore, I can see no difficulty, more 

 especially under changing conditions of life, in the continued 

 preservation of individuals with fuller and fuller flunk-mem- 

 branes, each modification being useful, each being propagated, 

 until, by the accumulated efl'ects of this process of natural selec- 

 tion, a perfect so-called flying-squirrel was produced. 



Now look at the Galeopithecus, or flying lemur, which for- 

 merly was ranked among 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 stracture, fitted for gliding 

 through the air, now connect the Galeopithecus with the other 

 LemuridcTJ, yet there is 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 squurels ; and 

 that each grade of structure was useful to its possessor. Nor 

 can I see any insuperable difficulty in further belie\-ing it possi- 

 ble that the membrane-connected fingers .and forearm of the 

 Galeopithecus might be greatly lengthened by natural selec- 

 tion ; and this, as far as the organs of flight 'are concerned, 

 would convert it into a bat. In certain bats which have the 

 wing-mem])rane extended from the top of the shoulder to the 

 tail, including the hind legs, we perhaps yet see actual traces 

 of an apparatus originally fitted 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 surmise that birds 

 might have existed which used their wings solely as flappers, 

 like the logger-headed duck (Micropterus of Eyton) ; as tins in 

 the water and front legs on the land, like the penguin; as 

 sails, like the ostrich ; and functionallj' for no purpose, 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 must not be inferred 

 from these remarks tliat any of the grades of wing-structure 



