Chap. VL] TRANSITIONS OF ORGANIC BEINGS. 217 



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 vege- 

 tation 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 exter- 

 minated, unless they also became modified and improved 

 in structure in a corresponding manner. Therefore, 

 I can see no difficulty, more especially under chang- 

 ing 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 pro- 

 duced. 



Now 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 ex- 

 tremely wide flank-membrane stretches from the corners 

 of the jaw to the tail, and includes the limbs with the 

 elongated fingers. This fiank-membrane is furnished 

 with an extensor muscle. Although no graduated links 

 of structure, fitted for gliding through the air, now con- 

 nect the Galeopithecus with the other Insectivora, yet 

 there is no difficulty in supposing that such links for- 

 merly 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 believ- 

 ing that the membrane connected fingers and fore-arm 

 of the Galeopithecus might have been greatly length- 



