380 PALAEONTOLOGY AND EVOLUTION XI 



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 con- 

 ceive they must have gone through the long 

 series of changes by which they were specialised 

 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 west- 

 ward 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 



