78 THEIR ORIGIN. 



renders it absolutely needful that a part should leave their 

 homes. This conjecture again appears less probable, for 

 if their wanderings were solely dependent on excess of 

 numbers, they would occur more regularly than is the case. 

 But what is the fact ? " In later times," says M. Soderhjelm, 

 who writes in the year 1833 from Serna in Dalecarlia, 

 " the migrations of the lemmings into these parts have 

 only taken place during the years 1789, 1807, 1808, 1813, 

 and lastly in 1823 that is, an interval of eighteen years 

 occurred between the first known migration and the 

 second, one year between the second and the third, five 

 years between the third and the fourth, ten between the 

 fourth and the fifth ; and since the last, nine years have 

 already elapsed." 



There are those, however, who entertain a somewhat more 

 plausible theory than either of the foregoing: " The lemmings," 

 they say, " which live on vegetable matter, make holes in the 

 earth for the purpose of getting at roots, &c. ; and during 

 the winter time they form runs beneath the snow. When, 

 therefore, immediately after a rapid thaw, accompanied by 

 copious rains, a severe frost of 10 to 12 of Reaumur, has 

 suddenly set in, by which the surface becomes covered with 

 an Is-sJcorpa, or crust of ice, the lemmings are prevented 

 from obtaining access to their food, and are compelled in 

 consequence to seek it in other quarters : but because near at 

 home this is not to be found so much to their liking as 

 their native mountain herbs, they continue their march to 

 distant districts, in the hope that sooner or later they may 

 meet with that of which they are in search. This would 

 seem to be the secret of their migrations." 



Leaving naturalists to settle the cause of these won- 



