Chap. VI.] TRANSITIONS OF OKGANIC BEINGS. 217 



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 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. 



ISTow 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 



