vii GEOGRAPHICAL RANGE 1 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 independent 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 living, 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 being 

 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, owing 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 Australia 

 and South America. These two divisions, it should be explained, 

 differ principally in that one has numerous, the other rarely 

 more than two, 1 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, two are especially developed. See Figs. 76, 77 

 (pp. 149, 150). 



