162 ON THE ORIGIN OF SPECIES 



webbed feet and which resembles an otter in its fur, 

 short legs, and form of tail ; during 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 insecti- 

 vorous quadruped could possibly have been converted 

 into a flying bat, the question would have been far 

 more difficult, 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, 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, or to collect food more quickly, 

 or, as there is reason to believe, by lessening the 

 danger from occasional falls. But it does not follow 

 from this fact that the structure of each squirrel is the 

 best that it is possible 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 become modified, and all 

 analogy would lead us to believe that some at least oi 



