UTILITY OF BIRDS IN NATURE. 7 
in a few years would be carpeted with crawling caterpillars, 
and the moths in flight would cover the earth like a blanket 
of fog. But under natural conditions the caterpillars that 
hatch from the eggs of the moth are destroyed by birds, 
mammals, insects, or other animals, by disease or the action 
of the elements, so that in the end only one pair of moths 
succeeds another. If every Robin should produce five young 
each year, and each Robin should live fifteen years, in time 
every square foot of land on this continent would be packed 
with Robins; but the surplus Robins are killed and eaten 
by various other birds or by mammals, each striving to 
maintain itself; so that, eventually, the number of Robins 
remains about the same. 
Thus we see that, while birds, insects, other animals, and 
plants are constantly striving to increase their numbers, the 
creatures that feed upon them operate continually to check 
this undue multiplication. The Hawk preys upon the smaller 
birds and mammals. The smaller birds and mammals feed 
on insects, grass, seeds, leaves, and other animal and vege- 
table food, each virtually endeavoring to gain strength and 
increase the numbers of its race at the expense of other 
living organisms. 
There is a competition among various dissimilar organisms, 
also, in seeking certain kinds of food. Grazing mammals, 
such as cattle, sheep, and deer, eat grass. Grass is eaten 
also by birds, mice, and insects. If any one kind of these 
creatures should be left without check, and become too 
numerous, it might consume the food supply of all. 
In the great struggle for existence, each perpetuating 
form of life that we call a species is really an expansive 
force, that can be restrained and kept in its proper place 
only by the similar expansive forces (other species) by 
which it is surrounded. It is as if the whole field of ani- 
mal and vegetable life consisted. of a series of springs, each 
exerting a pressure in all directions, and each held in place 
only by the similar expansion of the springs surrounding it. 
This action and reaction of natural forces constitute what is 
known as the balance of nature. Any serious disturbance 
of this balance is always fraught with serious consequences. 
