142 THE ORIGIN OF SPECIES 



If a different case had been taken, and it had been asked how an 

 insectivorous quadruped could possibly have been converted into a 

 flying bat, the question would have been far more difficult to 

 answer. Yet I think such difficulties have little weight. 



Here, as on other occasions, I lie under a heavy disadvantage, 

 for, out of the many striking cases which I have collected, I can 

 give only one or two instances of transitional habits and structures 

 in allied species; and of diversified habits, either constant or oc- 

 casional, in the same species. And it seems to me that nothing less 

 than a long list of such cases is sufficient to lessen the difficulty in 

 any particular case like that of the bat. 



Look at the family of squirrels; here we have the finest gra- 

 dation from animals with their tails only slightly flattened, and 

 from others, as Sir J. Richardson has remarked, with the posterior 

 part of their bodies rather wide and with the skin on their flanks 

 rather full, to the so-called flying squirrels; 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. 

 We cannot doubt that each structure is of use to each kind of squir- 

 rel 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 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 be- 

 come modified and improved in structure in a corresponding man- 

 ner. 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 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 ex- 

 tensor muscle. Although no graduated links of structure, fitted for 

 gliding through the air, now connect the Galeopithecus with the 



