THE STRUGGLE FOR EXISTENCE 285 



ing, and the woodpecker better fitted to extract grubs from 

 the tree. 



In the struggle of species with species one may gain a 

 little one year and another the next, the numbers of each 

 species fluctuating a little with varying circumstances, but 

 after a time, unless disturbed by the hand of man, a point 

 will be reached when the loss will almost exactly balance 

 the increase. This produces a condition of apparent equi- 

 librium. The equilibrium is broken when any individual or 

 group of individuals becomes capable of doing something 

 more than hold its own in the struggle for existence. 



"When the conditions of life become adverse to the exist- 

 ence of a species it has three alternatives, or, better, one of 

 three things happens, namely, migration, adaptation, extinc- 

 tion. The migration of birds and some other animals is a 

 systematic changing of environment when conditions are 

 unfavorable to life. When the snow and ice come, the fur- 

 seal forsakes the islands on which it breeds, and which are 

 its real home, and spends the rest of the year in the open 

 sea, returning at the close of winter. Some other animals 

 migrate irregularly, removing from place to place as condi- 

 tions become severe or undesirable. The Rocky Mountain 

 locusts, which breed on the great plateau along the eastern 

 base of the Rocky Mountains, sometimes increase so rapidly 

 in numbers that they can not find enough food in the scanty 

 vegetation of this region. Then great hosts of them fly 

 high into the air until they meet an air current moving 

 toward the southeast. The locusts are borne by this cur- 

 rent or wind hundreds of miles, until, when they come to 

 the great grain-growing Mississippi Valley, they descend 

 and feed to their hearts' content, and to the dismay of the 

 Nebraska and Kansas farmer. These great forced migra- 

 tions used to occur only too often, but none has taken place 

 since 1878, and it is probable that none will ever occur 

 again. With the settlement of the Rocky Mountain plateau 

 by farmers, food is plenty at home. And the constant fight- 



