258 NATURAL HISTORY. 



The skin which unites the toes at their bases acquires, 

 in consequence of the ceaselessly repeated separation 

 of the toes, the power of extension. Thus, in the course 

 of time, are formed the extended webs which unite the 

 toes of ducks, geese, and the like. Similar efforts to 

 swim, that is to say, to progress in a fluid element by 

 striking the water, have in like manner given rise to the 

 extension of the skin between the toes in frogs, turtles, 

 otters, beavers, and such like animals. 



'On the contrary, a bird which is habituated by its 

 mode of life to perch on trees, and which is descended 

 from individuals all of which have had a similar habit, 

 necessarily possesses toes more elongated than, and 

 differently constructed to, those of the aquatic animals 

 just alluded to. Its claws, in course of time, become 

 lengthened, pointed, and curved, so that it can grasp 

 the branches of trees on which it so often reposes. 



' In the same way, a bird frequenting the shore, which 

 does not wish to swim, but which nevertheless is obliged 

 to approach the margin of the water for the purpose 

 of catching its prey, is continually exposed to plunging 

 into the mud. Such a bird, then, being desirous to 

 save its body from becoming wet, makes every effort 

 to extend and lengthen its legs. From the long-continued 

 habit of extending the legs, contracted by this bird and 

 by all of its species, it results that the individuals of 

 the species are now found elevated, as it were, upon 

 stilts, having gradually obtained long naked legs, destitute 

 of feathers up to the thighs or even higher. 



' Again, the same bird, wishing to fish without wetting 

 its body, is obliged to make continual efforts to lengthen 



