60 
ORIGIN OF LIFE IN AMERICA 
land hare, and the lemmings resemble the creatures we have 
already met in the same country. Yet there are two perfectly 
distinct kinds of lemming in these barren-grounds. In winter 
they are easily discriminated since one of them does not 
change its rusty brown fur, while the other (Dicrostonyx 
hudsonius) becomes white all over. The former we have not 
met with before. It neither occurs in Greenland nor in 
Labrador, and appears to be a more recent immigrant 
to North America. This supposition is strengthened by 
the fact of the occurrence of this lemming (Lemmus 
trimucronatus) from Alaska as far east as Melville Penin¬ 
sula. In Alaska even another species of the same genus 
(Lemmus minusculus) occurs, and on the Pribilof Islands 
still another, the black-footed lemming (Lemmus nigripes) 
Beyond Bering Strait in eastern Asia other kinds make their 
appearance, whilst in Europe the common lemming (Lemmus 
lemmus) was well known long before the days of Linnaeus. 
Its remains having been discovered in many European caves 
along with those of other arctic animals, it is not unreason¬ 
able to assume that the genus Lemmus is of Old World origin 
and one of the many Asiatic immigrants that have invaded the 
North American continent in the past, though only a few of 
the latter have chosen the inhospitable barren grounds for 
their home. 
Another new form which we have not met with before 
is the ground squirrel known as the Hudson Bay 'spermophila 
(Citellus paVryi). It is abundant all over the barren-ground 
region alluded to, the earth being occasionally riddled with 
their burrows. These “ spermophiles ” or “ gophers ” as they 
are sometimes called, occupy a somewhat intermediate posi 
tion between squirrels and marmots. Numerous species are 
known from North America, altogether about forty. In Asia 
and Europe there are fewer kinds of spermophiles, yet they 
do occur, and this circumstance, while tending to prove once 
more that Bering Strait was dry land, renders the search 
for the original home of the genus more difficult. The fact 
that in Europe Citellus(Spermophilus) only inhabits the east, 
having invaded the continent in Pleistocene times, and then 
retreated again towards the country whence it came, points 
to Asia as the home of the spermophiles. On the other hand, 
