CANADIAN TREE PORCUPINE 
71 
of tree porcupines have apparently advanced northward along 
the isthmus of Central America and have reached Mexico 
within quite recent times. There are no grounds, however, 
for the supposition that Erethizon is a modified Coendu. 
When North and South America became joined by the exist¬ 
ing isthmus of Central America in Pliocene times, Erethizon 
no doubt was already an inhabitant of the northern continent. 
Since the two genera belong to the same family Coendidae, 
which differs fundamentally from the Old World family 
Hystricidae, we must assume that long anterior to the Plio¬ 
cene Period North and South America, or such portions of 
those continents which then existed, had already been united 
and then become disconnected again. These theories are 
not founded on zoogeographical data alone. Professor 
Osborn * urges on palaeontological grounds that North and 
South America were joined in Cretaceous and perhaps in 
early Tertiary times, and then separated again until the 
Pliocene. 
All this will be discussed in detail when we come to deal 
with Central America. I only mention the matter now 
because in the Santa Cruz beds of Patagonia the skeleton of 
a peculiar tree porcupine has been discovered and placed 
by Dr. Ameghino into the new genus “ Steiromys.” More 
recently, Professor W. B. Scottf re-examined these Steiromys 
remains, and noted the remarkable fact that they are more 
like the recent Erethizon of North America than any of the 
modern South American tree porcupines. He even argues 
that Steiromys is the direct ancestor of Erethizon. 
As this implies a former land connection between Pata¬ 
gonia and North America, independently of the remainder 
of South America, it largely aided me in the theoretical 
construction of an ancient land bridge between south-western 
North America and Chile.$ It is on this land bridge, I think, 
that the ancestor of the North American Erethizon wandered 
northward from Patagonia in early Tertiary times. All this 
will be more fully explained later on. 
* Osborn, EL. F., “Progress in Mammalian Palaeontology,” p. 99. 
t Scott, W. B., “ Eeport of Princeton Expedition,” V., pp. 413—417. 
J Scharff, E. F., “ Early Tertiary Land-Connection,” p. 521. 
