TRANSITIONS OF ORGANIC BEINGS. 165 



land animals. If a difEerent case had been taken, and it 

 had been asked how an insectivorous quadruped could pos- 

 sibly have been converted into a flying hat, the question 

 would have been far more difficult to answer. Yet I think 

 such difficulties have little weight. 



Here, as on other occasions, 1 lie under a heavy disad- 

 vantage, 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 occasional, 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 par- 

 ticular case like that of the bat. 



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

 gradation from animals with their tails only slightly flat- 

 tened, 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 can- 

 not doubt tliat 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 exter- 

 minated, unless they also become modified and improved 

 in structure in a correspondir.g manner. Therefore, I can 

 see no difficulty, more especially under changing conditions 

 of life, in the continued preservation of individuals with 

 fuller and fnller 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 pioduced. 



Now look at the Galeopithecus or so-called flying lemur, 



