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 Hlephas columti, the bisons, the Hquus 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 Zlephas 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. FLORENTINO AMEGHINO. 
