ix.] PALEONTOLOGY AND EVOLUTION. 211 



which I used to demonstrate the necessity of the existence 

 of all the great types of the Eocene epoch in some ante- 

 cedent period. 



It is this Mesozoic continent (which may well have 

 lain in the neighbourhood of what are now the shores of 

 the North Pacific Ocean) which I suppose to have been 

 occupied by the Mesozoic Monodelphia ; and it is in this 

 region that I conceive they must have gone through the 

 long series of changes by which they were specialized 

 into the forms which we refer to different orders. I 

 think it very probable that what is now South America 

 may have received the characteristic elements of its 

 mammalian fauna during the Mesozoic epoch ; and there 

 can be little doubt that the general nature of the change 

 which took place at the end of the Mesozoic epoch in 

 Europe was the upheaval of the eastern and northern 

 regions of the Mesozoic sea-bottom into a westward 

 extension of the Mesozoic continent, over which the 

 mammalian fauna, by which it was already peopled, 

 gradually spread. This invasion of the land was prefaced 

 by a previous invasion of the Cretaceous sea by modern 

 forms of mollusca and fish. 



It is easy to imagine how an analogous change might 

 come about in the existing world. There is, at present, 

 a great difference between the fauna of the Polynesian 

 Islands and that of the west coast of America. The 

 animals which are leaving their spoils in the deposits 

 now forming in these localities are widely different. 

 Hence, if a gradual shifting of the deep sea, which at 

 present bars migration between the easternmost of these 

 islands and America, took place to the westward, while 

 the American side of the sea-bottom was gradually 

 upheaved, the palaeontologist of the future would find, 

 over the Pacific area, exactly such a change as I am 

 supposing to have occurred in the North -Atlantic area 



