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 (Dicrosfconyx 

 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 spermophile 

 (Citellus partryi). 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, 



