242 THE ORIGIN OF SPECIES 



exists by a struggle for life, it is clear that each must be 

 well 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 wmter it leaves the 

 frozen waters, and preys, like other polecats, 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 ques- 

 tion 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 dis- 

 advantage, for, out of the many striking cases which 1 

 have collected, 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 diffi- 

 culty in any particular case like that of the bat. 



Look at the family of squirrels; hei'e we have the 

 finest gradation from animals with their tails only slightly 

 flattened, and from others, as Sir J. Richardson has re- 

 marked, 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 



