CiiAi-. VI, OF TRANSITIONAL VARIETIES. 173 



On the Origin and IW'nsifions of Orr/anlc Heings wifhjjecH' 

 liar Habits and /Structure. 



It has been asked by the opponents of such views as I hold, 

 how, for instance, a land carnivorous animal could have been 

 converted into one with aquatic habits ; for how could tlu; ani- 

 mal in its transitional state have subsisted ? It would be easy 

 to show that within the same group carnivorous animals exist, 

 having' every intermediate grade between truly aquatic and 

 strictly terrestrial habits ; and as each exists by a struggle for 

 life, it is clear that each is well adapted in its habits to its 

 place in Nature. Look at the Mustela vison of North Amei-ica, 

 which has 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 

 frpzen waters, and iM'e3'S, like other polecats, on mice and land 

 animals. If a different case had been taken, and it had 1)een 

 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 imder a heavy disadvan- 

 tage, for, out of the many striking cases which I have collected, 

 I can give only one or two instances of transitional hal)its and 

 structures in closely-allied species of the same genus; and of 

 diversified habits, either constant or occasional, in the satiKj 

 species. And it seems to me that nothing less than a long 

 list of such cases is sufficient to lessen the difficulty in an}- ])ar- 

 ticidar case like that of the bat. 



Ix)ok at the family of squirrels : here we have the finest gra- 

 dation from animals with their tails only slightly flattened, and 

 from others, as Hir J. IJichardson has remarked, with the pos- 

 terior part of their bodies rather wide and with the skin on 

 their flanks rather full, to the so-called flying-squirrels ; and fly- 

 ing-squirrels have their limbs and even the base of the tail united 

 l)y 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 coUect food moie 

 quickly, or, as there is reason to believe, by lessming the dan- 

 ger from occasional falls, lint it does not follow from this fact 

 that the structure of each sfjuirrel is the best that it is possible 



