VII GEOGRAPHICAL RANGE 27 
the Marsupials as primitive creatures. It implies on the whole 
that the Marsupials have sprung from a stock with an allantoic 
placenta. The alternative is to assume the mdependent develop- 
ment of an allantoic placenta in both groups of the Mammalia ; 
unless indeed the genus Perameles is to be held to be the most 
primitive race of Marsupials hving, a hypothesis which does not 
appear on the face of it likely. So long as it was believed that 
the mammary pouch of the Monotremes was the equivalent of 
the marsupium of the Marsupials, the persistence of this structure 
seemed to be a bond of union between the groups. But it is now 
known that the marsupium is a special organ confined to the 
Marsupials, an argument which is rather in favour of their beme 
a lateral development of the mammalian stem. It is to be re- 
marked also that the marsupium is feeblest in the Polyproto- 
donts, which may perhaps be looked upon as. the most primitive 
of the Marsupials, owimg to their more numerous teeth and other 
points to be referred to immediately. 
Not only are the Marsupials interesting from the point of 
view of their structure; their present and past distribution is of 
equal interest. During the Mesozoic epoch they occurred in 
Europe and North America; but not, so far as negative evidence 
means anything, in Australia, which is now their headquarters. 
In Europe Marsupials lingered on into the Tertiary period, when 
they finally became extinct. In America, of course, the group 
has persisted to the present day. Now it is important to notice 
that the two main subdivisions of the Marsupials, the Poly- 
protodontia and the Diprotodontia, exist to-day in both Austraha 
and South America. These two divisions, it should be explained, 
differ principally in that one has numerous, the other rarely 
more than two, incisors in the lower jaw. It is perhaps the 
more widely distributed opinion that the Polyprotodontia are the 
more archaic group; this opinion rests upon one or two facts In 
addition to the absence of specialisation in the incisor teeth. 
Among the Polyprotodontia the total number of teeth is greater— 
a Clearly primitive character; secondly, the general form of the 
body of these animals, with four subequal limbs and carnivorous 
or omnivorous diet, contrasts with the purely vegetarian and much 
specialised Kangaroos at any rate. Finally—and sufficient stress 
1 When there are more than two, éwo are especially developed. See Figs. 76, 77 
(pp. 149, 150). 
