what is known of the geological history of the
marsupials the difficulty vanishes. In the Upper Eocene deposits of
Western Europe the remains of several animals closely allied to the
American opossums have been found; and as, at this period, a very mild
climate prevailed far up into the arctic regions, there is no difficulty
in supposing that the ancestors of the group entered America from Europe
or Northern Asia during early Tertiary times.

But we must go much further back for the origin of the Australian
marsupials. All the chief types of the higher mammalia were in existence
in the Eocene, if not in the preceding Cretaceous period, and as we find
none of these in Australia, that country must have been finally
separated from the Asiatic continent during the Secondary or Mesozoic
period. Now during that period, in the Upper and the Lower Oolite and in
the still older Trias, the jaw-bones of numerous small mammalia have
been found, forming eight distinct genera, which are believed to have
been either marsupials or some allied lowly forms. In North America
also, in beds of the Jurassic and Triassic formations, the remains of an
equally great variety of these small mammalia have been discovered; and
from the examination of more than sixty specimens, belonging to at least
six distinct genera, Professor Marsh is of opinion that they represent a
generalised type, from which the more specialised marsupials and
insectivora were developed.

From the fact that very similar mammals occur both in Europe and America
at corresponding periods, and in beds which represent a long succession
of geological time, and that during the whole of this time no fragments
of any higher forms have been discovered, it seems probable that both
the northern continents (or the larger portion of their area) were then
inhabited by no other mammalia than these, with perhaps other equally
low types. It was, probably, not later than the Jurassic age when some
of these primitive marsupials were able to enter Australia, where they
have since remained almost completely isolated; and, being free from
the competition of higher forms, they have developed into the great
variety of types we now behold there. These occupy the place, and have
to some extent acquired the form and structure of distinct orders of the
higher mammals--the rodents, the insectivora, and the carnivora,--while
still preserving the essential characteristics and lowly organisation of
the ma