172 WONDERS OF ORGANIC LIFE. 



plumage and its fish-feeding habits. In summer, 

 as is well known, this bird frequents our inland 

 rivers, breeding in holes on the bank ; but 

 " when winter sets in, and drives the fish from 

 shallows to deep and sheltered bottoms, freezes 

 the mill-dams, or coats with ice the sluggish 

 basin, worked out by the current in rich allu- 

 vial soil, these birds wander from the interior 

 to the coast, and frequent the mouths of rivulets 

 entering large navigable rivers, dykes near the 

 sea, and similar places, especially on the 

 southern portion of our island." 



Passing from imperfect or limited migration, 

 conducted with regularity, we next advert to an 

 irregular migration, performed by various ani- 

 mals as circumstances may influence them, and 

 of which among mammalia the lemming affords 

 us an example. This little rodent, a sort of field 

 rat, is a native of northern Eussia, Norway, 

 Sweden, and Lapland, etc., and has been long 

 notorious for its incursions upon the cultivated 

 districts bordering its native mountain home. 

 The ordinary food of the lemming consists of grass, 

 the reindeer lichen, the catkins of the dwarf 

 birch, etc., to which they add animal food when 

 obtainable, sometimes even destroying and 

 devouring the weaker of their own species, im- 

 pelled, as we believe, by extreme hunger. It 

 would appear that they accumulate at times in 

 their native fastnesses, and are prompted by 

 instinct to migrate from an over-populated 

 territory. This migration takes place once or 

 twice in the course of fifteen or twenty years. 





