322 



ANIMAL BIOLOGY 



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 (unguli- 

 grade). Thus the typical cur- 

 sorial forms represent the culmi- 

 nation of Mammalian adaptation 

 to plains and steppes; regions in 

 which long distances must fre- 

 quently be traversed in quest of 

 food, and safety is to the swift. 

 (Fig. 203.) 



Another line of adaptive radia- 

 tion 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 

 which are really tree climbers 

 that walk and sleep upside down 

 suspended from branches; the 

 tailless Apes that swing among 

 the boughs chiefly by their arms; 

 and the Squirrels that scamper 

 along the branches. Some Squir- 

 rels and the so-called Flying Le- 

 murs take long soaring leaps sup- 

 ported by wide folds of skin 

 between the sides of the body 

 and the extended limbs. But the 

 Mammals have not left the air 

 untenanted, for truly volant 

 forms are represented by the 

 Bats in which the fore limbs with 

 greatly elongated fingers form 

 the framework of true wings. 

 (Figs. 204, 207.) 



Passing below the surface of the 

 mals. A plantigrade; B, digitigrade; earth FOSSQRIAL Mammals are 

 C, unguhgrade. (*rom Lull, alter 



Fig. 203. - - Foot postures of Mam- 



Pander and D'Alton.) 



found such as the Woodchucks, 



