LEMMINGS AND GOPHERS 
61 
the remains of Citellus douglasi have been recorded from 
Rotter Creek cave in California, and those of Citellus tride- 
cemlineatus from the Conard fissure in Arkansas. Both of 
these deposits are regarded as Pleistocene in age.* If these 
caves were in Europe they certainly would be held to be pre- 
Glacial, for about fifty per cent, of the species contained in 
them are extinct, while Mastodon, Megalonyx and other 
ancient forms accompany the spennophiles in the Cali- 
forpian cave. The two spennophiles alluded to are still 
living in North America and have a wide range. If the genus 
had invaded Alaska from Asia in Pleistocene times, it is 
scarcely credible that there would have been time for the 
development of the two species and their enclosure in these 
distant caves within the Pleistocene Period, especially when 
we consider that the whole of Canada is believed to have been 
buried under a thick mantle of ice. It is possible, of course, 
that there were two distinct invasions into North America of 
the genus Citellus, one having entered from the west, as I 
shall explain later on, in Miocene times, the later one from 
the north-west towards the ehd of the Pliocene Period. 
Among the carnivores of the barren grounds, the barren- 
ground bear (Ursus richardsoni), the wolverine (Gulo luscus), 
and the barren-ground wolf (Canis occidentalis albus) are 
the most noticeable. 
I have scarcely as yet attempted to bring the study of 
the geographical distribution of birds within the scheme of 
this present work, partly because our knowledge of their past 
history is so defective in comparison with that of many other 
groups, and partly because mountain ranges or narrow 
channels of the sea do not as a rule afford effective barriers 
to their dispersal. Occasionally, however, birds are extremely 
circumscribed in their range, and there can be no doubt that a 
study of their geographical distribution is well worthy of care¬ 
ful attention. If I had not a superabundance of better material 
for my purpose, I should gladly dwell a little longer on the 
problems suggested by their range. Many of them, however, 
are quite useless for our purpose, loons and auks, for instance, 
two groups of arctic birds attached to the sea coast. Many 
* Brown, Barnum, “ Conard Fissure,” pp. 166—167. 
