446 Evolution and Distribution of Fishes 



(Didelphys) and Water Opossums {Chironectes) of 

 America, that are spread over both North and South Con- 

 tinents down to Patagonia. These evidently originated 

 somewhere over this wide region, as palaeontological ex- 

 ploration increasingly shows. But other genera of this 

 sub-group occur alike in America and Australasia, so that 

 a common mammalian bond is thus established. The 

 second or Pauciprotodonta exists also in Western America, 

 but can be traced back, as Scott has shown (27^), in fossil 

 types through the Miocene of the Santa Cruz and Pata- 

 gonian beds. The third or Diprotodonta is the highest 

 and most specialized of the great group, and is wholly 

 Australasian. Now the existing distribution of these and 

 of the Australian polyprotodonts indicates — if relative 

 number of species over a given area, and relative simplicity 

 or complexity of structure are used as a criterion — that the 

 entire group passed through Tasmania into south-east 

 Australia, and thence spread westward and northward till 

 highly specialized outliers reached N. E. Australia, N. 

 Guinea, and a few of the E. Indian islands. Their 

 absence from N. Zealand and from Africa would suggest 

 that important and earlier connections once existing be- 

 tween these and a wide southern land, which in turn con- 

 nected with Australia, had already been broken when the 

 marsupials were working eastward. 



Here then, as in so many instances already cited in this 

 work, we have to drop all thought of "Permanency of the 

 great land masses," and have to think rather of a — some- 

 times steadily sometimes suddenly — changing relation of 

 land areas. And this brings us to the question as to when 

 and to what extent a southern land-bridge existed. As 

 clearly appears from the studies of Staunton and Ortmann 

 (275), a large part of lower S. America, including E. 

 Patagonia, was covered by sea during lower and upper 

 Cretaceous times. But a considerable area of W. Pata- 

 gonia existed as a great migrational bridge southward for 

 incomers from the north, alike then and on into Mid- 

 tertiary times. Along this many northern plant genera 

 travelled southward, favored evidently by the mild climate 



