STEPPES OF ASIA AND AMERICA 65 



a crouching position is adopted, the food being carried 

 to the mouth by the fore-hmbs. As in rodents in 

 general, the food consists largely of roots, seeds, berries, 

 &c., and not whoUy of grass, as in the kangaroo and in 

 most ungulates. The curious elongation of the hind-legs 

 gives such speed that the jerboas can escape their 

 enemies by flight, while on the other hand the bobacs 

 at once seek their burrows on an alarm, and make 

 underground tunnels to the feeding-grounds, so that 

 they are never far from an open burrow. Another very 

 interesting structural point in regard to the jerboas is 

 the reduction of the number of toes from five to three 

 on the hind-foot. Among the ungulates we find a 

 similar reduction, carried however much further, for 

 the horse and its allies have only one toe on each foot. 

 This reduction gives speed, and is an adaptation to 

 swift movement over a relatively hard surface. 



The commonest jerboa of the steppes is Alactaga 

 decumana, the five-toed jerboa as it is called, because 

 the lateral toes are present on the hind-feet, though 

 they are minute and functionless. Another pecuMarity 

 of the hind-foot, which it shares with other jerboas, is 

 the fusion of the bones which form the sole of the foot 

 in man, that is of the metatarsal bones. As a result 

 a very strong bone called the cannon bone is formed, 

 quite comparable to the cannon bone in the antelopes 

 and their allies, though it is formed in a different way. 

 This also is an adaptation to ensure swift movement 

 over firm ground, for it gives the necessary rigidity 

 during the taking of the long leaps. Though the head 

 and body together of this animal only measure some 

 seven inches, it is stated that it cannot be overtaken 

 by a horse, so extraordinarily rapid is its speed. Like 

 the other steppe rodents the jerboas are social, forming 



1404 E 



