56 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



show that this modification of the foot has been 

 attended by the degeneration of some parts of it, 

 but, as we have to restrict ourselves here to the 

 data with which we are already furnished, it is the 

 degenerative phenomena exhibited by other parts of 

 the limb consequent to adaptation to leaping, which 

 we shall proceed to enumerate. 



There are two leg bones, the fibula, and a larger 

 one, the tibia. These two bones are usually held 

 together by muscles, or by an interosseous membrane, 

 the latter being merely degenerated muscular tissue. 

 As a rule, the tibia and the fibula reach the entire 

 length of the leg and are quite separate. In the 

 process of adaptation to arboreal life, these bones 

 have remained apart, as may be observed in most 

 arboreal animals of the present day. Moreover, in 

 order to promote prehension by the foot, the tibia and 

 fibula in prehensile-footed marsupials may turn upon 

 one another, as is the case with our radius and ulna. 

 With the tarsius it is quite otherwise. Here the 

 lower halves of the tibia and fibula are united, and 

 the muscles which formerly connected the two bones 

 have consequently disappeared. There is further 

 evidence of degeneration in the upper half of the 

 fibula which has become reduced to a mere thread. 

 There is no question that these phenomena are the 

 result of adaptation to leaping, for they are not 

 exhibited among exclusively arboreal animals, while 

 many parallel cases of modification are known to 

 exist amongst other species. 



