70 
ORIGIN OF LIFE IN AMERICA 
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 Region, in the far distant Alaska, 
there lives another porcupine very closely related to the 
Canadian species. This yellow-haired porcupine (Erethizon 
epixanthmn), as it is called, ranges from Alaska through 
the Rocky 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, 
