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SCIENCE 



[N. S. Vol. XLIII. No. 1100 



and thus encountered differences of climate, 

 which are among the most effective barriers 

 to the spread of mammals, while between 

 North America and the Old World the mi- 

 grations followed the lines of latitude and 

 therefore, to all appearances, were accom- 

 plished much more quickly. (5) Finally, 

 it is not to be supposed that the fossils al- 

 ready discovered make up anything like a 

 complete list. Doubtless, many things still 

 remain to be found, and others, because of 

 rarity or some other unfavorable circum- 

 stance, failed altogether of preservation, 

 and the actual progress of diffusion may 

 well have been more rapid and effective 

 than the observed facts would lead us to 

 suppose. 



In the middle and later Pliocene and still 

 more in the Pleistocene the intermigratiou 

 had proceeded so far that the two conti- 

 nents possessed a very considerable num- 

 ber of mammalian genera in common, as 

 immediately appears from a comparison of 

 the faunal lists. The movement culminated 

 in the Pleistocene, where the number of 

 southern forms in the northern continent 

 and of northern forms in the southern 

 lands reached its maximum. In both con- 

 tinents the Pleistocene mammalian fauna 

 was a very much richer and more varied 

 assemblage than the modern one, for the 

 great and mysterious extinctions which 

 came late in the epoch and at its close, de- 

 vastating more than three fifths of the 

 land surface of the earth, were especially 

 severe in North America and left that con- 

 tinent in a zoologically impoverished state. 

 Beside almost all of the existing mam- 

 mals which still continue to inhabit the re- 

 gion. Pleistocene North America had mas- 

 todons, three species of elephants, several 

 tapirs and ten species of horses, ranging 

 in size from small ponies to species which 

 exceeded the largest modern draught horses. 

 Peccaries spread as far north as Pennsyl- 



vania and from ocean to ocean and were 

 accompanied by great herds of camels and 

 llamas. Seven species of bison, some of 

 them far surpassing the existing buffalo, 

 so-called, were distributed from Florida to 

 Alaska, while musk-oxen and allied forms 

 extended far to the south and along the 

 Pacific coast into California. Modern 

 types of deer and antelopes were associ- 

 ated with, several extinct types, some of 

 which must have been very grotesque in 

 appearance. Especially remarkable is the 

 discovery by Gidley in western Maryland 

 of an antelope which is hardly distinguish- 

 able from the recent African Eland. Al- 

 most all the existing North American 

 beasts of prey have been found in the Pleis- 

 tocene, but there we;-e many more that are 

 now extinct and were of extraordinary 

 size and power ; giant wolves, lions, the terri- 

 ble saber-toothed tigers and the huge short- 

 faced bears are Samples of these vanished 

 forms, the disappearance of which made 

 life much easier for early American Man. 

 All of the animals so far enumerated are 

 typically Aretogsan in character and were 

 either immigrants from the Old "World, of 

 various geological dates of arrival, or were 

 of indigenous North American ancestry 

 and development, but mingled with those 

 were many South American mammals, the 

 history and evolution of which can be fol- 

 lowed in satisfactory detail in the succes- 

 sive Tertiary formations of that continent. 

 The strangest and most conspicuous of 

 those migrants from the south were the 

 great edentates. The huge, elephantine 

 ground-sloths, a group now entirely ex- 

 tinct, but very abundant in the Pleisto- 

 cene of both North and South America, 

 ranged all across the continent from Penn- 

 sylvania to California. The genus Mega- 

 lonyx, which, it is interesting to note, was 

 first named by Thomas Jefferson from some 

 bones found in Virginia, would seem to 



