HEREDITY, VARIATION 121 



they will sit upright, with their back against a stone or other object, 

 hissing or showing fight in a very determined manner. (See 

 Fig. 12.) 



" The circumstance which has given more popular interest to the 

 Lemming than to a host of other species of the same order of 

 animals is that certain districts of the cultivated lands of Norway 

 and Sweden, where in ordinary circumstances they are quite 

 unknown, are occasionally and at very uncertain intervals, varying 

 from five to twenty or more years, literally overrun by an army of 

 these little creatures, which steadily and slowly advance, always in 

 the same direction, and regardless of all obstacles, swimming across 

 rivers and even lakes of several miles in breadth, and committing con- 

 siderable devastation on their line of march by the quantity of food 

 they consume. In their turn they are pursued and harassed by a 

 crowd of beasts and birds of prey, as bears, wolves, foxes, dogs, wild 

 cats, stoats, weasels, hawks, and owls, and are never spared by 

 man ; even the domestic animals not usually predaceous, as cattle, 

 goats, and reindeer, are said to join in the destruction, stamping 

 them to the ground with their feet, and even eating their bodies. 

 Numbers also die from diseases apparently produced by overcrowd- 

 ing. None ever return by the course over which they have come, 

 and the onward march of the survivors never ceases until they reach 

 the sea, into which they plunge, and swimming outwards in the same 

 direction as before, perish in the waves. ... So extraordinary was 

 the sudden appearance of these vast bodies of Lemmings to the 

 Norwegian peasants, that they supposed they must have fallen from 

 the clouds. 



" The principal really ascertained facts regarding these migrations 

 seem to be as follows : When a combination of favourable circum- 

 stances has occasioned a great increase in the numbers of Lemmings in 

 their ordinary dwelling-places, a movement necessarily occurs at the 

 edge of the elevated plateau, and a migration towards the low-lying 

 land begins. The whole body slowly moves forward, advancing in 

 the same general direction in which they started, but following more 

 or less the course of the great valleys. They only travel by night, 

 and they also stay in congenial places for weeks or months, so 

 that, with unaccustomed abundance of food, notwithstanding all the 

 destructive influences to which they are exposed, they multiply 

 excessively during their journey, having families still more numerous 

 and more frequently than in their usual homes. The progress may 

 last from one to three years, according to the route taken and the 

 distance to be traversed until the sea coast is reached, which, in a 

 country so surrounded by water as the Scandinavian peninsula, 

 must be the ultimate goal of such a journey. This may be either 

 the Atlantic or the Gulf of Bothnia, according as the migration has 

 commenced from the west or east side of the elevated plateau. 



