QUATERNARY 673 



circle in the Yukon territory. They probably became extinct on this 

 continent before the "close of the epoch. 



Bisons of many species roamed over North America during the 

 Pleistocene, some of which were of great size, if the horns can be taken 

 as a measure, one pair of horns measuring more than six feet from tip 

 to tip. Wolves, musk oxen, bears, and rodents (among which is a 

 giant beaver, Castoroides), were also present. 



Although Europe was not invaded by the sloth tribe, it was, never- 

 theless, the meeting place of many animals, those of the tropics and 

 those of the Arctic regions coming at different times and even mingling 

 at others. 



REFERENCES FOR HORSES AND CAMELS 



Camel: International Encyclopedia. 

 Osborn, H. F., — Age of Mammals, p. 484; and others. 



Scott, W. B., — A History of Land Mammals in the Western Hemisphere, pp. 291-308; 

 386-402. 



Birds. — The Pleistocene birds of Europe and America were not 

 of exceptional size, nor did they differ to any important degree from 

 those now living, but in New Zealand and Madagascar, gigantic 

 flightless birds were abundant during that epoch. The name moa 

 includes, in a general way, 20 to 25 species of these New Zealand 

 birds, the largest of which stood 10 feet high, or from two to three 

 feet higher than an ostrich, while the smallest were about the size of a 

 turkey. In all of these, wings are entirely wanting. The develop- 

 ment of flightless birds on these islands seems to be the indirect result 

 of the absence of carnivorous enemies. With an abundance of food 

 throughout the year and no powerful enemies, the New Zealand Pleis- 

 tocene birds had not the usual incentives to flight. Under such con- 

 ditions, some of them increased in bodily size until flight was impos- 

 sible (25 or 30 pounds seems to have been the limit of the weight of 

 flying animals). Once the power of flight was lost, the larger and 

 more powerful the bird the better was the chance of its preservation 

 as long as food was abundant, and great size resulted. A change in 

 climatic conditions, however, was fatal to these bulky birds, since 

 having lost the power of flight they were unable to migrate, and were, 

 therefore, forced to depend upon the food of the islands. Their ex- 

 tinction appears to have been due partly to the cold of the Glacial 

 Period and partly to man who, it is thought, completed their exter- 

 mination about 500 years ago. 



