118 ANIMAL LIFE 
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- 
