ADAPTATION FOR LAND-LIFE 



49 



still organs of locomotion. To combine the advan- 

 tages of short limbs for support, and of long ones 

 for speed, is only one of the problems that press for 

 solution. To relieve the muscles of the body's weight, 

 a vertical pillar-like leg is necessary. To gain the 

 leverage needed for propulsion, a bent leg capable 

 of straightening out is requisite. Hence we find that 

 great size in quadrupeds involves columnar limbs, with 

 the segments placed vertically, as in elephants ; but 

 in smaller animals, such as horses, the fore-limb only 

 has this linear character, and bears greater weight, 

 whilst the hind-limb is thrown into a zigzag form 

 when at rest, and into a straighter line in movement. 

 Movement and rest in an erect position, as in man, 

 some apes, and birds, require a still greater change 

 in the disposition of the weight, muscles, and ligaments. 

 The mass of the body now falls upon the hind-limbs, 

 which therefore become straighter than in quadrupeds, 

 and the spine is sustained by ligaments that relieve 

 the back muscles of an otherwise constant strain. The 

 muscles of the shank become more strongly developed, 

 not only to sustain but also to propel the weight now 

 thrown altogether upon them. For these purposes 

 also the foot is not elongated into a slender organ 

 delicately poised on the toe-tips, as in many active 

 quadrupeds, but is flattened and the heel brought 

 down to the ground. Lastly, as an additional prop for 

 support, the tail may be retained and thickened, as 

 in the kangaroo. 



A survey of the vertebrates shows how the changes 



E 



