264 NATURAL SCIENCE [October 



in the States of Virginia, Georgia, Carolina, and in the whole of the 

 valley of the Mississippi, mingled with the most characteristic 

 representatives of the North American fauna. 



We arrive at the beginning of the Anthropozoic era, and with it 

 ceased the communication between the two Americas, the land 

 which for a long time had united them being again submerged. 

 We see then during the Quaternary times North America invaded 

 by new forms ; the Mastodons were replaced by gigantic elephants, 

 accompanied by various other genera and species from the Old 

 World. We see the Elephas columbi, the bisons, the Equus tau, and 

 E. conversidens descending by the valleys of Mexico and advancing 

 towards the south as far as the isthmus of Panama, but they found 

 it interrupted and were not able to tread the South American 

 soil. 



A last retreat of the ocean which made itself felt over the greater 

 part of the American coasts of the Atlantic left dry great shore 

 banks of marine shells, like those of the neighbourhood of La Plata, 

 which provide material for the building of this beautiful city, made 

 by the genius and energy of our sympathetic rector [of the uni- 

 versity] ; this fresh continental encroachment upon the ocean again 

 united both Americas, when the ElepJtas columbi and the other great 

 mammals which had accompanied it in its migration to the south had 

 already disappeared from the north. The bridge reappeared in the 

 form of a narrow and tortuously long piece of land, which served 

 from that time as a highway to the pre-Columbian peoples of our 

 hemisphere who migrated successively, and backwards and forwards, 

 from north to south, and from south to north, strewing the road with 

 ruins, in which the mixture of a hundred peoples to-day misleads 

 the cleverest investigators of the prehistoric past of the world of 

 Columbus. Florentine Ameghino. 



