DIFFICULTIES OF THE THEORY 243 



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 exterminated, unless they 

 also became modified and improved in structure in a cor- 

 responding manner. Therefore, I can see no difiiculty, 

 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 among 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 ex- 

 tensor muscle. Although no graduated links of structure, 

 fitted for gliding through the air, now connect the Galeo- 

 pithecus with the other Insectivora, yet there is no diffi- 

 culty 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 mem- 

 brane-connected fingers and forearm of the Galeopithecus 



