180 DIFFICULTIES ON THEORY. Chap. VI. 



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. 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, winch 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 all natural conditions. Let the 

 climate and vegetation change, let other competing 

 rodents or new beasts of prey immigrate, or old ones 



