Chap. VI. TKANSITIONAL HABITS. 181 



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 corre- 

 sponding 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 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 connect the Galeopithecus with the other 

 Lemuridse, 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 per- 

 fectly gliding squirrels ; and that each grade of structure 

 had been useful to its possessor. Nor can I see any in- 

 superable 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 convert it into a bat. In bats which 

 have the wing-membrane extended from the top of the 

 shoulder to the tail, including the hind-legs, we per- 

 haps 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 



