PLEISTOCENE PERIOD 203 



have assumed appalling dimensions in any part of the 

 continent. 



By the upheaval in the Pliocene of the land now known 

 as the Isthmus of Panama the animal life of South America 

 had been greatly enriched. And horses, mastodons, tapirs, 

 lamas, and other animals had come to mingle with the native 

 fauna. The exports of South America fell far short of her 

 imports, for, beyond some armadillos and ground-sloths, 

 she seems to have sent but little in return to her northern 

 neighbour. Great gain, moreover, had resulted to her from 

 the migrations of North American life consequent on the 

 great glaciation. As the glaciation relaxed, counter-migra- 

 tions led, no doubt, to some losses in her fauna. She had, 

 however, been permanently enriched. Horses, mastodons, 

 peccaries, and mule-deer did not wholly desert her ; whilst 

 some forms of life notably lamas never recrossed the 

 isthmus, and became lost to North America. 



Some of her horses now exhibited certain differences from 

 their emigrant ancestors of the last Period. The most 

 notable of these modified forms were about the size of Shetland 

 ponies ; and their heads were disproportionately large for 

 their bodies (Hippidium, Onohippidium). The most striking 

 feature of the animals was the elongation of the nasal bones, 

 giving the head greater depth than that of the other horses. 

 And the nostrils, it may be assumed, were somewhat drawn 

 back. The scenes which these animals frequented were 

 probably different from those of their relations living on grassy 

 plains. Mr. Lydekker considers that the nasal modification 

 was probably a special adaptation to desert life ; and that by 

 its means particles of sand were prevented from reaching 

 the organ of smell. Towards the close of the Period horses 

 became extinct in South America. Professor Fairfield 

 Osborn considers this to have been caused by some wide- 

 spread epidemic. 



South America, as has been seen, was not merely a collector 

 of animals from other lands ; and some of her whilom native 

 products have acquired considerable posthumous fame. 



Her ground-sloths and armadillos first in view in the 

 Miocene Period had gone on steadily from strength to 



