STEPPES OF ASIA AND AMERICA 65 
a crouching position is adopted, the food being carried 
to the mouth by the fore-limbs. As in rodents in 
general, the food consists largely of roots, seeds, berries, 
&c., and not wholly 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 peculiarity 
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 
