282 MENTAL EVOLUTION IN ANIMALS. 



percentage of individuals destroyed in itself small, but I 

 doubt whether it is much larger than would be the case if 

 these multitudes of animals were segregated over a very much 

 wider area. A stronger case, I think, is afforded by that of 

 the Norwegian Lemming, and therefore I shall consider it at 

 greater length. 



Since Mr. Darwin wrote his remarks on this subject 

 which are presented in the Appendix, further statements with 

 reference to it have been published. These, therefore, I shall 

 quote. 



Mr. Crotch, who has had the opportunity of observing the 

 phenomena for a number of years, thus briefly gives his 

 account of the facts, so far as they concern us. 



" The Lemmings (which are little rodents) certainly do 

 not visit my part of Norway at any recurring period of 

 years ; but every third or fourth year they may be expected 

 with tolerable regularity, though in variable numbers. Thus 

 it is quite probable that some migrations may have so far 

 escaped notice as to give rise to the old idea that they took 

 place every tenth year. 



" They are, however, always directed westwards ; and 

 thus the theory that they are caused by deficiency of food 

 fails so far, that these migrations do not take place in a 

 southerly direction by which a larger supply might be ob- 

 tained. M. Guyne (loc. cit.) suggested that the course fol- 

 lowed was merely that of the watershed. However, this runs 

 east as well as west, and follows valleys which often run 

 north and south for hundreds of miles, whereas the route 

 pursued by the Lemming is due west. At all events this is 

 the case in Norway, where they traverse the broadest lakes 

 filled with water at an extremely low temperature, and cross 

 alike the most rapid torrents and the deepest valleys. 



" With no guiding pillar of fire, they pass on through a 

 ■wilderness by night ; they rear their families on their journey, 

 and the three or four generations of a brief subarctic summer 

 serve to swell the pilgrim caravan. They winter beneath 

 more than six feet of snow during seven or eight weary 

 months ; and with the first days of summer (for in those 

 regions there is no spring) the migration is renewed. At 

 length the harassed crowd, thinned by the increasing attacks 

 of the wolf, the fox, and even the reindeer, pursued by 

 eagle, hawk and owl, and never spared by man himself, yet 



