S TT 
ZOOLOGY. T1 
represented by the Ornithorhynchus is the posterity of the 
Sauropsida (birds and reptiles), yet the imperfect knowl- 
edge 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 Mam- 
malia. 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 des- 
ignate 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 transi- 
tory 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 
