ADAPTATIONS 157 



water fish which has taken to spending its 

 holidays in the sea, and that all such creatures 

 return to their original habitats for breeding 

 purposes, and no doubt all this is quite true. 

 But pure mechanism does not explain it. It 

 can tell us how the eel gets to the river and 

 back to the sea ; at least it can give us much 

 information on this point, but it certainly 

 cannot tell us why it does it, which, after all, 

 is the main question. Finally, amongst a 

 myriad instances, let us consider the strange 

 case of the Lemmings. These little creatures, Lemmings 

 which are not unlike water-rats, live on the 

 eastern side of Norway but "at irregular periods, 

 varying from three to ten years, a large por- 

 tion of them set forth on the most mysterious 

 of pilgrimages. Their course is directed due 

 west, and thus does not lead them to the 

 regions where more plentiful food might be 

 obtained in the south. They spend more than 

 a year moving resolutely on, turning neither 

 to the right nor to the left for any obstacle, 

 swimming lakes and climbing houses which lie 

 in their path. They winter beneath several 

 feet of snow, and rear families on their journey. 

 All the way along they are accompanied by 

 another crowd of travellers, for whose move- 

 ments their migration is the signal. The Fox, 

 the Stoat and the Hawk find a ready livelihood 



