100 ESSAY ON CLASSIFICATION 



within similar areas the range of this type has been different in dif- 

 ferent epochs. 



Australia, at present almost exclusively the home of Marsupials, 

 has yielded also a considerable number of equally remarkable spe- 

 cies and two extinct genera of that type, all described by Owen in 

 a report to the British Association, in 1844, and in Mitchell's Ex- 

 peditions into the Interior of AiistraliaM^ 



How far similar facts are likely to occur in other classes remains 

 to be ascertained. Our knowledge of the geographical distribution 

 of the fossil remains is yet too fragmentary to furnish any further 

 data upon this point. It is, however, worthy of remark that though 

 the types of the oldest geological periods had a much wider distribu- 

 tion than most recent families exhibit now, some families of fishes 

 largely represented in the Devonian system of the Old World have 

 not yet been noticed among the fossils of that period in America, as, 

 for instance, the Cephalaspids, the Dipteri, and the Acanthodi. 

 Again, of the many gigantic Reptiles of the Triassic and Oolitic peri- 

 ods, none are known to occur elsewhere except in Europe, and it can 

 hardly be simply owing to the less extensive distribution of these 

 formations in other parts of the world, since other fossils of the same 

 formations are known from other continents. It is more likely that 

 some of them at least are peculiar to limited areas of the surface of 

 the globe, as even in Europe their distribution is not extensive. 



Without, however, entering upon debatable ground, it remains 

 evident that before the establishment of the present state of things 

 peculiar types of animals, which were formerly circumscribed within 

 definite limits, have continued to occupy the same or similar grounds 

 in the present period, even though no genetic connection can be 

 assumed between them, their representatives in these different for- 

 mations not even belonging to the same genera. Such facts are in the 

 most direct contradiction with any assumption that physical agents 

 could have anything to do with their origin; for though their occur- 

 rence within similar geographical areas might at first seem to favor 

 such a view, it must be borne in mind that these so localized beings 

 are associated with other types which have a much wider range, and, 

 what is still more significant, they belong to different geological peri- 



"" [T. L. Mitchell, Three Expeditions into the Interior of Eastern Australia ... 2 

 vols., London, 1838.] 



