58 LAND MAMMALS IN THE WESTERN HEMISPHERE 



are brought forth in a very immature state and, with the excep- 

 tion of one genus (Perameles) the foetus is not attached by 

 a special structure, the placenta, to the womb of the mother. 

 Like the Prototheria, the Marsupials, which were once spread 

 all over the world, are at present almost entirely confined to 

 Australia and the adjoining islands, the Opossums of North 

 and South America, and one small genus (Ccenolestes) in the 

 latter continent being the exceptions to this rule of distribution. 

 The second and vastly larger infraclass, the Monodelphia, 

 is characterized by the placenta, a special growth, partly 

 of foetal and partly of maternal origin, by means of which the 

 unborn young are attached to the mother and nourished during 

 the foetal period ; they are born in a relatively mature state 

 and are generally able to walk immediately after birth and 

 resemble their parents in nearly all respects. 



The vast assemblage of placental mammals, which range 

 over all the continents, are divided into numerous orders, most 

 of which appear to be natural groups of truly related forms, 

 while some are but doubtfully so and others again are clearly 

 unnatural and arbitrary. As has already been pointed out, 

 the mutual relationships of these orders, as expressed in 

 groups of higher than ordinal rank, offer a much more difficult 

 problem, chiefly because our knowledge of the history of mam- 

 mals is most deficient just where that history is most important 

 and significant, namely, in its earlier portion. In many in- 

 stances, the evolution of genera and families may be followed 

 out within the limits of the order in a very convincing way, 

 but very rarely can the origin of an order be demonstrated. 

 When the history began to be full and detailed, the orders had 

 nearly all been established, and, until the steps of their diver- 

 gence and differentiation can be followed out, their mutual 

 relationships can be discussed only from the standpoint of 

 their likenesses and differences. In the valuation of these, there 

 is much room for difference of opinion, and such difference 

 is not lacking. On the other hand, concerning the number 



