314 



HABIT AND INTELLIGENCE, 



[chap. 



Extract 



from 



Darwin. 



Flying 

 squirrels. 



Flying 

 lemur. 



tension of the fingers, and of tlie skin over the fingers, 

 which constitutes the wing. 



The bat's wing, when in the nascent state, probably 

 resembled the parachute of the flying squirrels. I quote 

 the following from Darwin : ^ " Look at the family of 

 squirrels : here we have the finest gradation from animals 

 with their tails only slightly flattened, and from others, as 

 Sir J. Eichardson has remarked, with the posterior part of 

 their bodies rather wide and the skin on their flanks 

 rather full, to the so-called flying sqiiirrels ; and flying 

 squirrels have their limbs and even the base of the tail 

 united by a broad expanse of skin, which serves as a 

 parachute, and allows them to glide through the air to 

 an astonishing distance from tree to tree. . , . Let the 

 climate and vegetation change, let other competing rodents 

 or new beasts of prey immigrate, or old ones become modi- 

 fied, 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 im- 

 proved in structure in a corresponding manner. Therefore 

 I can see no difficulty, more especially under changing 

 conditions of life, in the continued preservation of indi- 

 viduals having fuller and fuller flank-membranes, each 

 modification being useful, and each being propagated, 

 until, by the accumulated eff'ects of this process of natural 

 selection, a perfect so-called flying squirrel was formed. 

 Now, look at the Galeopithecus, or flying lemur, which 

 formerly was falsely reckoned among 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-membrane 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 Lemuridte, 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. ISTor can I 

 1 Origin of Species, p. 208. 



