TRANSITIONS OF ORGANIC BEINGS 18S 



difficult to answer. Yet I think such difficulties have little 

 weight. 



Here, as on other occasions, I lie under a heavy disadvan- 

 tage, for, out of the many striking cases which I have col- 

 lected, 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 particular 

 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 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 man- 

 ner. Therefore, I can see no difficulty, more especially under 

 changing conditions of life, in the continued preservation of 

 individuals with fuller and fuller flank-membranes, each modi- 

 fication being useful, each being propagated, until, by the ac- 

 cumulated effects of this process of natural selection, a per- 

 fect so-called flying squirrel was produced. 



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

 which formerly was ranked amongst bats, but is now believed 

 to belong to the Insectivora. As extremely wide flank-mem- 



