THE ENDOSKELETON 165 



developed to subserve the needs of an aquatic life, and disap- 

 pear forever with the assumption of a terrestrial environment, 

 although, in secondarily aquatic forms, similar organs are de- 

 veloped from other sources. The paired iins, on the other hand, 

 useful in fishes, assume their highest importance on land, and 

 become the two pairs of free limbs. To these more than to 

 any other organ, the higher vertebrates owe their extraordi- 

 nary development, and their high degree of success in occu- 

 pying so many sorts of environment, since by this means they 

 have been able to possess the surface of the earth, to occupy 

 the trees, to burrow in the ground, to return to the sea and 

 even to conquer the problem of aerial navigation. It is, more- 

 over, probable that the emancipation of the fore limbs from 

 the function of locomotion and the acquirement by them of 

 prehensible powers, which enable them to seize objects and 

 bring them to the immediate attention of the sense organs, 

 have been the chief causes of the excessive brain development 

 which has achieved for the Primates the greatest success thus 

 far attained in the domination of the world. 



Corresponding to the great variety of functions of which 

 the paired limbs are capable, there is an equally vast series of 

 modifications of structure, presenting an army of forms which 

 include various sorts of ambulatory limbs, paddles, grasping 

 organs, tools for excavating the earth, and wings of several 

 sorts. Among the modifications there must be included also 

 the numerous cases of limb reduction, which may affect the 

 fore or hind limbs or both, and exhibit every stage of reduc- 

 tion down to total loss. Thus the amphibian Siren has lost 

 its hind legs totally, while retaining its fore legs, and simi- 

 larly in the case of the whale the fore legs become developed 

 into enormous flippers or paddles, while the hind legs are re- 

 duced to useless rudiments entirely concealed beneath the 

 skin. The opposite tendency is seen in the ostrich, where the 

 legs are very short and heavy, while the wings are much re- 

 duced and almost without function, and is still better exhibited 

 in the kiwi-kiwi of New Zealand, in which the wings have 

 totally disappeared. In several groups the complete or nearly 



