216 TRANSITIONS OP ORGANIC BEINGS. [Chap. VL 



adapted to its place in nature. Look at the Mustela 

 vison of North America, which has webbed feet, and 

 which resembles an otter in its fur, short legs, and form 

 of tail. During the summer this animal 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 difficult to 

 answer. Yet I think such difficulties have little 

 weight. 



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

 disadvantage, for, out of the many striking cases which 

 I have collected, I can only give 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 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 squir- 

 rels 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 astonish- 

 ing 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 



