THE STRUGGLE FOR EXISTENCE 119 
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. 
Tn 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 
