ADAPTATION OF ORGANISMS 315 



of the limbs. Thus, for example, in the Dogs, Foxes, and 

 Wolves, the effective limb length is increased by raising 

 the wrist and heel from the ground and walking merely upon 

 the digits (DIGITIGRADE) ; ' while in Antelopes, Horses, and 

 hoofed runners in general, the chief limb bones themselves 

 are lengthened, subsidiary ones are suppressed, and the wrist 

 and ankle are raised still further from the ground, so that 

 merely the tips of one or two digits of each limb support 

 the animal (UNGULIGRADE) . Thus the typical cursorial 



FIG. 162. Gymnura. (From Lull, after Horsfield and Vigors.) 



forms represent the culmination of Mammalian adaptation 

 to plains and steppes; regions in which long distances must 

 frequently be traversed in quest of food, and safety is to the 

 swift. (Fig. 163.) 



Another line of adaptive radiation is presented by the 

 tree dwellers : arboreal forms which make their own the / 

 world of foliage high above the ground. Such are, for 

 instance, the Sloths (Fig. 164), which are really tree climbers 

 that walk and sleep upside down suspended from branches; 

 the Man-like Apes that swing among the boughs chiefly 

 by their arms; and the Squirrels that scamper along the 

 branches. Some Squirrels and the so-called Flying Lemurs 



