122 THE NEOG^IC REALM. [CHAP. 



probably always has been, tropical. Up to the present, it is, 

 accordingly, impossible to account satisfactorily for the disappear- 

 ance of all the larger forms from among the mammalian fauna of 

 South America. 



Returning to the fauna itself, it may be asserted that before 

 the great intrusion of northern forms the mammals of Neogaea 

 were far more distinct from those of the rest of the world than 

 is the case with the fauna of any other region, with the exception 

 of Australasia; and that consequently there can be no hesita- 

 tion in allowing this part of the earth's surface to take rank as a 

 primary realm. At the time when the Santa Cruz fauna was so 

 decidedly marked off there was a much more general similarity 

 between the faunas of all the other regions of the world (exclusive 

 of Notogaea) than is the case at the present day, and it is this 

 antiquity of the differentiation of the Neogaeic fauna that supports 

 so strongly its claim to distinctness. 



It has been suggested that the first land-connection between 

 South and North America was probably of limited extent or short 

 duration ; and some evidence of a later separation between the 

 two areas is afforded by the beds of marine shells already mentioned 

 as occurring in the upper part of the Pampean deposits ; these 

 beds marking an epoch of submergence of a considerable portion 

 of the area 1 . Subsequently to this final submergence of portions 

 of Argentina and Uruguay there was a great upheaval of the 

 country, indicated not only by the upraising of the aforesaid marine 

 beds, but likewise by that of the sand-dunes fringing most parts of 

 the Argentine coast. This upheaval, which has taken place within 

 the human period, certainly resulted in the final union of the two 

 Americas ; and since its date there has probably continued to be 

 a greater and greater admixture of the two originally distinct 

 faunas, so far as climatic conditions have permitted. It is, how- 

 ever, not a little curious that some of the original northern types, 

 such as the vicunas and guanacos, have entirely died out in their 

 original habitat, to flourish only in the southern half of the con- 

 tinent. 



1 There is some evidence to show that the isthmus of Panama was never 

 completely submerged after the Pliocene, but it may have been so narrow as 

 not to allow of much migration of the fauna. 



