LEMMINGS AND GOPHEKS 61 



the remains of Citellus douglasi have been recorded from 

 Potter Creek cave in California, and those of Citellus tride- 

 cemlineatus from the Canard 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 spermophiles in the Cali- 

 fornian cave. The two spermophiles 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 end 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. 



