166 TRANSITIONS OF ORGANIC BEINGS. 



air to an astonishing distance from tree to tree. We can* 

 not doubt that each structure is of use to each kind of 

 squirrel in its own country, by enabling it to escape birds 

 or beasts of prey, to collect 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 vegeta- 

 tion 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 become modified and improved in structure in a 

 corresponding manner. Therefore, I can see no difficulty, 

 more especially under changing conditions of life, in the con- 

 tinued 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 was formerly 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 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 forearm of the 

 Galeopithecus might have been greatly lengthened by natural 

 selection ; and this, as far as the organs of flight are con- 

 cerned, would have converted the animal into a bat. In 

 certain bats in which the wing-membrane extends from the 

 top of the shoulder to the tail and includes the hind-legs, we 

 perhaps see traces of an apparatus originally fitted for glid- 

 ing through the air rather than for flight. 



If about a dozen genera of birds were to become extinct, 

 who would have ventured to surmise that birds might have 

 existed which used their wings solely as flappers, like th« 



