180 
DIFFICULTIES ON THEOEY. 
Chap. VI. 
dives for and preys on fish, but during the long winter 
it leaves the frozen waters, and preys like other pole¬ 
cats on mice and land animals. 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 diffi¬ 
cult, and I could have given no answer. Yet I think 
such difficulties have very little weight. 
Here, as on other occasions, I lie under a heavy dis¬ 
advantage, 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 closely allied 
species of the same genus; 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 flattened, and from others, as Sir J. Eichardson 
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 asto¬ 
nishing 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, or to collect food more quickly, or, as there 
is reason to believe, by lessening the danger from occa¬ 
sional falls. But it does not follow from this fact that 
the structure of each squirrel is the best that it is pos¬ 
sible to conceive under aU natural conditions. Let the 
climate and vegetation change, let other competing 
