192 MAMMALIA ORDER XLMARSUPIALIA. 



dental succession in the order, the writer may quote the following passage 

 from a^work he has devoted to the group : "The tooth thus replaced has 

 been hitherto generally regarded as corresponding to the last or fourth milk- 

 molar of the higher mammals, while the apparently replacing tooth has con- 

 sequently been identified with the last or fourth pre-molar of the same. 

 Recent researches have, however, tended to show that this is not a case of 

 true replacement at all, and that the tooth, which makes its appearance late 

 in life, is really a retarded pre-molar, which will consequently be the fourth 

 of the full series, while the apparently replaced tooth is really the fifth. Be 

 this as it may, the mode of succession is peculiar and unique ; and it may be 

 still convenient to speak of the replacing tooth as the fourth pre-molar, and 

 the one it replaces as the fourth milk-molar." It should be added that some 

 authorities consider the whole of the teeth of a Marsupial in advance of the 

 molars as corresponding to the milk-series of the Placentals ; and also that, 

 when the full series is developed, there are four pairs of molars in each jaw 

 in the present order. 



That the Pouched Mammals are inferior in their organisation to the 

 Placentals, is admitted by all; and they are also, as shown by their history 

 in past times, among the oldest, if not actually the oldest, representatives 

 of the entire class. Tnere is, however, some difference of opinion among 

 zoologists as to whether this group includes the ancestors from which the 

 higher mammals have originated. 



In regard to their geographical distribution, the Marsupials present some 

 very curious features. At this present day their head-quarters are the 

 Notogfeic realm, comprising Australia, Papua, Celebes, and the other islands 

 lying to the eastward of Wallace's line. And it is here that they attain their 

 maximum development. Whereas, however, they form the great bulk of the 

 mammalian population of Australia and Papua, in Celebes and the adjacent 

 islands they constitute only a small minority of the fauna. Elsewhere, 

 Pouched Mammals are found only in America, where they are represented by 

 the opossums and selvas ; and here they are chiefly restricted to South and 

 Central America, constituting the Neogreic realm, only a single species, which 

 is evidently an immigrant from the south, inhabiting the northern half of 

 the continent. During the Secondary epoch of geological history, Marsupials 

 of extinct generic types were abundant in the northern hemisphere, while in 

 the succeeding Tertiary epoch opossums flourished in Europe and North 

 America. It is thus evident that the ancestral Marsupials were driven from 

 the northern hemisphere by the incoming of the higher forms of mammalian 

 life to find a secure refuge in southern islands. And it further appears that 

 while in South America they have had to compete with numerous types of 

 Eutherians, in Australasia they have had the land pretty much to them- 

 selves, and have there been enabled to attain the wonderful development so 

 especially characteristic of that region. 



The kangaroos and their immediate allies form a large family 



(Macropodidce) belonging to the first of the two great subordinal divisions 



of the Pouched Mammals, technically known as the 



Kangaroo Diprotodontia. This division, which is mainly confined 



Tribe. to the Notogseic realm, is specially characterised by the 



number of the front or incisor teeth, of which there are 



never more than three pairs ; the usual complement being three pairs in the 



upper jaw and one in the lower. The innermost, which are in some cases 



the only pair of incisors in both jaws, are always large, with sharp, cutting 



