DIFFICULTIES ON THEORY 163 



the squirrels would decrease In numbers or become 

 exterminated, 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 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-mem- 

 brane 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 Lemuridae, yet I 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 that each 

 grade of structure was 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 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 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 purpose, like the 



