56 ARBOREAL MAN 



of attention for its very special adaptations; but it is not 

 a member destined to carry its owner far in the race for 

 mammalian supremacy. 



Only a purely aquatic life could produce an animal in 

 which the hind-limb took no part whatever in the support 

 of the body weight, and in the thoroughly aquatic forms 

 (Sirenia and Cetacea) the hind -limb, deprived of this 

 function, becomes a mere rudiment. Consequently, 

 even in the most primitive of the prototherian Mammals, 

 Ave find that the ideal condition is somewhat widely de- 

 parted from. In the Monotremes the fibula is large, and 

 from its proximal end a process rises above the point of 

 articulation with the tibia, and both tibia and fibula are 

 separated in their whole length: both bones of the second 

 segment are preserved, and some degree of mobility 

 between them still survives the demand for stabilitv. 



ft/ 



The mobility of tibia and fibula upon each other is, how- 

 ever, best retained in the arboreal Metatheria, where, 

 especially in Phascolarctus and some of the Phalangers, 

 the power of rotation rivals that displayed between the 

 radius and ulna. In the arboreal Sloths, again, the fibula 

 is a large and well-formed bone which articulates with 

 the tibia at its two extremities. In the Tree Shrews (Tu- 

 paiadce) the fibula is well developed and entirely sep- 

 arated from the tibia, whereas in many terrestrial Insec- 

 4 f. tivora it has become reduced and fused to its neighbour. 

 In the Primate stock good development, complete 

 V^ V separation, and even slight mobility of the fibula upon 



the tibia, are maintained as a part of the arboreal adap- 

 tation. In all other mammalian orders the fibula tends 

 r^ to undergo the reduction we have noted in the ulna of 

 the fore-limbs of quadrupedal animals; all movement 

 between it and the tibia is lost earlv, and the fibula be- 

 comes a rudiment finally blended into the structure of 

 the dominant tibia. 



No Mammal retains the ideal primitive tarsus, but the 

 same may be said of existing Reptiles and even of their 



X 



