ARBOREAL ADAPTATIONS OF HIND-LIMB 6^ 



in a temporary position of uprightness upon a flexed 

 thigh. A dog sitting up to beg, a squirrel eating its nut, 

 or a bear awaiting its bun, are examples of this degree of 

 uprightness. There are many animals which adopt this 

 posture as a life habit, the Kangaroo (Macropus) and the 

 Jerboa (Dipus) are good examples of Mammals which 

 are habitually orthograde as far as their trunk axis is 

 concerned, but in which the thigh is normally flexed so 

 as to be nearly at right angles with the trunk. But the 

 posture which is temporarily assumed by the primitive 

 tree-climber is very different from this, for in the interval 

 which we are picturing its body axis is tending to become 

 carried upright upon a thigh which is more or less ex- 

 tended as the trunk is raised towards the grasping hands, 

 It is tree-climbing which makes this posture a possibility, 

 and even its temporary adoption marks a great step in 

 evolution, since, with the increasing perfection of the 

 arboreal activities, the assumption of this posture is an 

 oft-recurring one. 



With the repetition of this action anatomical changes 

 are brought about in the limb, for many adaptations must 

 take place when the femur is brought into line with the 

 vertebral axis instead of being at right angles to it. These 

 adaptations will show their first manifestations even when 

 the demand for the posture is only occasional. Briefly, 

 the femur becomes capable of a more complete rotation 

 at the hip- joint, so that its extension may be carried 

 through a right angle, and it may take up a position 

 parallel to the axis of the vertebral column. 



The capsule of the joint becomes modified to permit of 

 this extension, and the muscles become accommodated 

 to the new poise. In the completely adapted arboreal 

 animal this posture tends to become more or less habitual. 

 In some of the Lemurs it is almost as well established as 

 in the Anthropoids themselves, and under these con- 

 ditions the anatomical adaptations become more perfect. 

 The fibres of the capsule of the hip-joint take on a per- 



