n] FEATURES OF ENVIRONMENT 41 



dwellers in deserts. Lizards are fond of disporting 

 themselves in the open, to feed, to play and to bask. 

 Their only shelter, if they are not diggers, is afforded 

 by the tussocks and shrubs, to reach which they have 

 to run fast. Desert snakes, excepting those which 

 dig, and have rough oblique rows of scales like Eryx 

 or Echis, are remarkable for the great number of 

 ventral shields and these correspond with the number 

 of vertebrae. To be overtaken in the open, with 

 places of refuge far and wide between, is detrimental ; 

 the most alert and quickest is most likely to escape, 

 or to win if he is the aggressive foe. 



The characteristic feature of desert mammals is 

 their swiftness, and their large ears if they are 

 nocturnal. Toes short and compact, close together, 

 limbs elongated and slender. Witness the gazelles, 

 springboks and other antelopes. Or there is the 

 tendency to elongate the hind limbs, while the fore 

 limbs remain short or even are shortened. This 

 principle leads to the more typical dwellers in plains 

 and semi-deserts, as illustrated by the jumping kan- 

 garoos of Australia ; the same modifications, different 

 in detail, but with precisely the same effect occur 

 in the jumping hares, Pedeies (a rodent) and in 

 the Macroscelides or elephant shrew (Insectivore) of 

 Africa, the little Jerboas Dipus, in South-East Russia 

 and Asia ; and North America has produced similar 

 jumping mice of the genus Zap us. All have a rather 



