XII 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 
351 
and New Guinea, while it is entirely unknown in Asia, Africa, 
or Europe. It reappears in America, where several species of 
opossums are found; and it was long thought necessary to postu¬ 
late a direct southern connection of these distant countries, 
in order to account for this curious fact of distribution. When, 
however, we look to 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 inexistence 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 marsu¬ 
pials were able to enter Australia, where they have since 
