136 TROPICAL SAVANAS 



we have various stages in the elongation of the hind- 

 limbs, and therefore in the development of the power 

 of leaping, so among the kangaroos we have forms like 

 the rat-kangaroos (Potorous) in which the elongation 

 of the hind-hmbs is much less marked, and which run 

 on aU fours, and cannot deUver the powerful kicks which 

 are so important a part of, e.g., the giant kangaroo's 

 means of defence. In the scrub of Queensland the five- 

 toed kangaroo {Hypsiprymnodon moschatus) occurs, and 

 in the structure and shortness of its hind-feet approaches 

 the phalangers, showing that the true kangaroos have 

 been evolved in response to the special conditions which 

 prevail on the open plains, just as the jerboas on the 

 Asiatic steppes have been evolved from ordinary 

 rodents, or as the long-legged antelopes and giraffe 

 have been evolved from the ancestral short-legged 

 ungulates, in response to similar conditions. The nature 

 of these special conditions has been already emphasized, 

 but we may repeat that if an animal of the open plains 

 cannot burrow, or defend itself passively in some way, 

 it must be swift to escape its enemies, it must have 

 a wide range of vision in order to note their advance, 

 it must have acute senses. We saw also in connexion 

 with the ungulates that there is a marked tendency to 

 reduction in the number of digits, to a fusion of Hmb 

 bones, and so on, in order to give the necessary strength 

 and rigidity for rapid movement. That the same need 

 produces the same results is obvious when we note that 

 the kangaroo has four toes on its hind-foot instead of 

 five, and that of those four two are rudimentary, like^he 

 useless side toes in antelope or deer, though in the 

 kangaroo the rudimentary toes have apparently a sub- 

 sidiary use, for they are said to be employed in cleaning 

 the fur. 



