Chap. VI.] TRANSITIONS OF ORGANIC BEINGS. 139 



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

 sional, 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 hat. 



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 

 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 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 indi- 

 viduals 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 

 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 

 'been greatly lengthened by natural selection ; and this, as far as the 



