While, therefore, there can be little doubt that the class represented by the Ornithorhynchus is the posterity of the Sauropsida (birds and reptiles), yet the imperfect knowledge of the development of the Ornithorhynchus, and the total absence, so far, of fossil remains, make it impossible to designate which particular order of Sauropsida ought to be considered as the progenitor of the Ornithorhynchus, and through it of the rest of the Mammalia. In seeking the origin of the pouch-bearing mammals, we meet with the same difficulties, though not in the same degree. The Marsupialia are intermediate, in many respects, between the Monotremata (Ornithorhynchus) and the ordinary Mammalia. In the present state of our knowledge, it seems more advisable to regard simply some of the Monotremata as the ancestors of the Marsupialia than to attempt to designate which particular one was that ancestor, or exactly the manner of their development. That the Marsupialia came after the Monotremata (Ornithorhynchus, etc.) seems most probable, from the fact of their young, in their transitory condition, offering the arrangement of the viscera so characteristic of the Ornithorhynchus, while in their adult condition they agree with the ordinary Mammalia. The pouch-bearing Mammalia offer examples of meat and vegetal feeders, as well as of the leaping, burrowing, and climbing kinds.
So striking is the parallel between the different kinds of pouch-bearing mammals and the different orders of the ordinary Mammalia that many naturalists seem disposed to consider the different orders of the ordinary mammals as having come directly or indirectly from the corresponding kinds of pouch-bearers; that is, extinct Marsupials, like the grazing and browsing Kangaroo, were the ancestors of the orders represented at the present time by the Pig and Horse. Pouch-bearers like the Opossums, using their big toe as a thumb, gave rise to Monkeys, improperly called four-handed; while the meat-eaters (Dog) are the posterity of
