222 EVOLUTION AND GENETICS 



Such animals, as might be expected, show adaptations similar to 

 those of the mole but less extreme. The fore-hmbs and claws are 

 well developed, but not extremely. The eyes are, of course, useful 

 above ground, and remain functional, but they are reduced in 

 proportion to the amount of time spent in burrows. The same is 

 true of the ears. Tapering of the body may occur, but is no more 

 extreme than in many animals which do not burrow. 



In addition to the animals which form burrows for conceal- 

 ment there are some that dig for food. The elephant uses his 

 tusks for this purpose. The snout of hogs and of some snakes is 

 turned up at the tip, forming an effective organ for burrowing to 

 slight depths in soft ground. 



Scansorial Adaptations. Such adaptations are chiefly related 

 to the organic environment, but since they are for the purpose of 

 locomotion they may conveniently be treated here. Whether an 

 animal climbs trees or cliffs, the demands upon its body are the 

 same. The appendages and girdles must be strong, to support the 

 weight of the body. In addition to this there must be provided 

 some means of maintaining a safe grip upon the supporting object. 

 Finally the proximal segments of the limbs are seen to be elon- 

 gated in some arboreal vertebrates. This reversal of the condition 

 noted in cursorial forms is due, no doubt, to the fact that intrinsic 

 strength is necessary in the distal parts which are in immediate 

 contact with the support, so that reach must be gained elsewhere 

 if at all. 



Appendages are modified in several ways to enable animals to 

 cling to branches. The claws are usually involved, but in some 

 cases the appendages themselves are prehensile. Animals which 

 climb by clinging to the bark of tree trunks, and run along the 

 upper surface of branches have sharp claws which give them an 

 adequate grip on the surfaces which support them. Squirrels are 

 perhaps the most familiar example, but many birds and some 

 reptiles are similar in habits. The sloths normally suspend them- 

 selves from branches, and are provided with great hook-like claws 

 as an aid to this habit (Fig. 131). The development of the ap- 

 pendages is so extreme that they are scarcely able to move about 

 on the ground. Prehensile appendages are best developed in the 

 primates, where opposability of the thumb and locomotion of the 

 type known as brachiation are found. Adaptations of the last 

 kind have already been discussed under the evolution of man. 



