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-limbs is much less marked, and which run 
on all fours, and cannot deliver 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 limb 
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 the 
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. 
