THE STRUGGLE FOR EXISTENCE H9 



ing of the locusts by the farmers, by plowing up their eggs, 

 and crushing and burning the young hoppers, keeps down 

 their numbers. 



Another animal of interesting migratory habits is the 

 lemming, a mouse-like animal nearly as large as a rat, which 

 lives in the arctic regions. At intervals varying from five 

 to twenty years the cultivated lands of Norway and Sweden, 

 where the lemming is ordinarily unknown, are overrun by 

 vast numbers of these little animals. They come as an 

 army, steadily and slowly advancing, always in the same 

 direction, and " regardless of all obstacles, swimming across 

 streams and even lakes of several miles in breadth, and 

 committing considerable devastation on their line of march 

 by the quantity of food they consume. In their turn they 

 are pursued and harassed by crowds of beasts and birds of 

 prey, as bears, wolves, foxes, dogs, wild cats, stoats, weasels, 

 eagles, hawks, and owls, and never spared by man ; even 

 the domestic animals not usually predaceous, as cattle, 

 foals, and reindeer, are said to join in the destruction, 

 stamping them to the ground with their feet and even eat- 

 ing their bodies. Numbers also die from disease apparently 

 produced from overcrowding. None ever return by the 

 course by which they came, and the onward march of the 

 survivors never ceases until they reach the sea, into which 

 they plunge, and swimming onward in the same direction 

 as before perish in the waves." One of these great migra- 

 tions lasts for from one to three years. But it always ends 

 in the total destruction of the migrating army. But the 

 migration may be of advantage to the lemmings which re- 

 main in the original breeding grounds, leaving them with 

 enough food, so that, on the whole, the migration^ results in 

 gain to the species. 



But most animals can not migrate to their betterment. 

 In that case the only alternatives are adaptation or destruc- 

 tion. Some individuals by the possession of slight advan- 

 tageous variations of structure are able to meet the new 



