OO SKETCH or THE HISTORY OF MAMMALIA. 



in their habits, stealing forth to feed and sport when 

 evening begins to close. They are, however, not alto- 

 gether nocturnal, for Sonnini observed them in broad day 

 playing around the mouths of their subterranean habi- 

 tations, and he particularly noticed that those which he 

 kept delighted to bask in the sun, and were always lively 

 in that situation. The Jerboas are very timid creatures 

 and hasten to their burrows for security on the least 

 noise : if intercepted, they trust to their speed, and seem 

 to fly across the plain ; so great indeed is the rapidity 

 with which they bound along, that a greyhound has 

 some difficulty in the chase. In making each leap they 

 spring from the hind-feet, the impulse being given by 

 the powerful muscles of the thighs, while the tail serves 

 as a balance and rudder. (Fig. 19.) In the act of 

 springing the fore-paws are pressed close to the chest ; 

 they descend, however, upon them, but such is the 

 quickness of the leap, and the celerity with which they 

 recover their due posture and spring again, that the eye 

 is completely deceived, for it appears as if .they never 



20. — Egyptian Jerboa. 



