70 OKIGIN OF LIFE IN AMEEICA 



There is no sign here of a post-Glacial centre of dispersal 

 south of the drift area. The centre of dispersal, on the con- 

 trary, lies in Canada and it is from there that it has spread 

 southward. Yet the species had already come into existence 

 when the sabre-tooth tiger and peculiar kinds of peccaries 

 haunted the forests of Arkansas, for its remains have been 

 found together with these extinct creatures in the Conard 

 fissure.* It likewise lived in Pennsylvania at a time when 

 the great Mylodon, Megalonyx and Mastodon still flourished 

 there. 



Beyond the Mackenzie Eegion, in the far distant Alaska, 

 there lives another porcupine very closely related to the 

 Canadian species. This yellow-haired porcupine (Erethizon 

 epixanthum), as it is called, ranges from Alaska through 

 the Eocky Mountains and westward to the Pacific as far south 

 as northern Mexico, thus exhibiting the same indifference 

 to climatic conditions as its near relative. Both of these 

 North American porcupines have short tails. In Mexico, 

 Central and South America we meet with numerous species, 

 all allied to Erethizon, but with prehensile tails, which 

 considerably assist them in climbing trees. 



Now if the genus to which these tree porcupines belong had 

 originated in Alaska or Canada, we should certainly expect 

 it to have traversed Bering Strait into Asia while the wapiti 

 deer and many other Old World forms poured into America. 

 That it has not done so does not tend to disprove the assump- 

 tion of the former existence of a Bering Strait land bridge. 

 It only implies that the genus Erethizon is of southern 

 origin, and has merely spread northward within recent geo- 

 logical times. The south-western region, that vast country 

 of mountains and plains which contains the most important 

 centre of dispersal in North America, has no doubt given 

 rise to the genus Erethizon. Of its past history we know 

 nothing as far as North America is concerned. South of 

 Mexico, as already stated, all tree porcupines possess pre- 

 hensile tails, and are distinguished by other minor differ- 

 ences from Erethizon. For these reasons they have been 

 placed into the distinct genus Coendu. These southern forms 



* Brown, Barnum, "Conard Fissure," p. 166, 



