Chap. YI. 
TRANSITIONAL HABITS. 
181 
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 corre¬ 
sponding manner. 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 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 flying lemur, 
which formerly was falsely ranked amongst 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 
LemuridsB, 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. Nor can I see 
any insuperable difficulty in further believing it pos¬ 
sible that the membrane-connected fingers and fore¬ 
arm of the Galeopithecus might be greatly lengthened 
by natural selection; and this, as far as the organs 
of flight are concerned, would convert it into a bat. 
In bats which have the wing-membrane extended from 
the top of the shoulder to the tail, including the hind¬ 
legs, we perhaps see traces of an apparatus originally 
constructed for fflidins: through the air rather than 
for flight. 
