TRANSFORMATION OF ORGANS OF ANIMALS 51 



to be followed in the second stage by the second 

 toe. Then the fourth began to atrophy until the 

 whole weight of the body was supported by the 

 third toe alone, as in the ostrich. 



2. Adaptation to leaping We now come to the 

 various instances of adaptation to leaping. This 

 adaptation is one necessarily undergone either by an 

 animal which originally lived a terrestrial life, such 

 as the Jerboa, or by one which had ceased to be 

 arboreal in adapting itself to leaping, such as 

 the kangaroo ; or, again, by an animal which 

 formerly lived an arboreal life, and which con- 

 tinued to lead such a life with the additional 

 adaptation to leaping, such as the tarsius ; and, 

 finally, by an animal which first lived and swam 

 in the water, and which continued to do so after 

 becoming adapted to leaping, such as the frog. 



In each of these cases adaptation to leaping has 

 resulted in the fuller development of the hind limbs, 

 the size of which, as well as the functional impor- 

 tance, has greatly increased, which shows that the 

 modification is of a progressive nature. All these 

 modifications are, however, attended by degeneration. 



(a) The Jerboa (Dipus). Some rodents, such as 

 rats, porcupines and squirrels, are five-toed. This 

 five-toed foot represents the primitive one from 

 which, after passing through various still existing 

 phases, was derived the three-toed foot of the jerboa. 



With the hare there is no first toe, so that the 

 foot is functionally four-toed. 



