u1 THE SERVICE OF TAILS 75 
that it is enabled to make its long, running leaps, 
which, in the case of the large wallaby, will aver- 
age eight or ten yards at each jump. This is now 
known to be largely an error; the truth (as shown 
by its tracks in the mud and by careful observa- 
tion) is, that the tail only just touches the ground 
now and then; yet it is plain that this heavy mem- 
ber serves a useful purpose in balancing the creat- 
ure. The same must 
have been true of those Ss 
vast reptiles of the , Zz’, 
Mesozoic days, the Ki: 
dinosaurs, ae fu a’ 
rt 
ih, Z 
ie * 
Sa 
=e eo THE JERBOA 
SN Rae KANGAROO. 
tracks are 
impressed so plen- 
tifully upon the 
brownstone rocks 
of the Connecticut 
valley, and of similar animals, in other parts of the 
ancient world, known to have had enormous caudal 
parts —a characteristic of primitive forms. 
One very distinct service the tail of one modern 
marsupial may perform, is illustrated in the be- 
havior of the jerboa kangaroo, which collects the 
