xir 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 in existence in the Eocene, if not in the preceding 

 Cretaceous period, and as Ave 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 



