138 TROPICAL SAVANAS 
the young. In any case the fact that, with rare excep- 
tions, marsupials have only succeeded in surviving 
in the Australian region, suggests that the placental 
method, that practised by the ungulates, has been 
justified in the struggle for existence as against the 
marsupial method, despite the greater risk to the 
mother before birth and at birth in the former method. 
Only one kangaroo is produced at a birth. This is 
an interesting point, because a reduction in the number 
of young is frequent in animals which must escape 
with their young by flight when threatened, and do 
not attempt to defend themselves nor to hide. A similar 
tendency is well marked in the higher ungulates. But 
it must not be supposed that the kangaroo is entirely 
helpless. In the ungulates we find that, though the 
primary purpose of the elongation of the limb is to 
give swiftness, yet secondarily, e.g. in the horse, the 
powerful hind legs can be used as weapons. Similarly 
the kangaroo can use its long hind-legs to deliver heavy 
blows, capable of killing a dog. Like so many denizens 
of open country, the animals are markedly gregarious. 
In regard to the other marsupials we need only note 
that Australia contains various forms showing special 
adaptations to open grassy plains or desert regions, 
these adaptations exhibiting a curious convergent 
resemblance to those which appear among the pla- 
centals. For example, throughout the greater part of 
Australia the open grassy plains are inhabited by the 
so-called pig-footed bandicoot, a small animal with 
long hind-legs, remarkable in showing an ungulate-like 
reduction in the number of the toes. The fore-foot has 
two functional toes, the hind only one; the animals 
take both vegetable and animal food. The jerboas and 
jumping mice of other desert regions are represented 
