52 DOMESTICATED ANIMALS AND PLANTS 
the north till it should lie upon the ground, it would reach to 
Chicago (360 miles) and twenty-three miles beyond into Lake 
Michigan; that is to say, that the descendants of a single 
corn-root louse at half the maximum rate could in a single sea- 
son, if uninterrupted, reproduce enough to make a solid column 
I acre square and 383 miles long,—a perfectly inconceivable 
number. After this computation it is not difficult to believe the 
truth of the assertion that certain bacteria that can double in 
about twenty minutes would be able in a few days, if unre- 
stricted, to fill all the oceans of the earth. 
With this enormous birth rate it becomes important to study 
carefully the checks to increase, and the various means by which 
living things have been prevented long ago from absolutely 
overrunning the earth, where standing room, to say nothing of 
food, is limited. What, now, are the conditions and mutual 
relations between these immense numbers of diverse species 
as they live together in a state of nature? 
The struggle for existence. In general, it may be said that 
species, are indifferent to each other except when interests clash, 
and then one or the other must go under, for the law of the 
wild is that everything lives not where it chooses to live but 
where it is able to live. When so many more individuals are 
produced than can possibly find food and room to survive, there 
ensues at once a battle for life, which has by common consent 
been called, as Darwin named it, the struggle for existence. 
This is a many-sided struggle,—a kind of three-cornered 
fight, — first against natural conditions in general, then against 
the competition of other species, and, last of all, against the 
competition of its own kind. This elemental warfare, for it is a 
warfare, though generally unknown to the participants and often 
not noticeable except to the trained observer, — this warfare is 
1In this general connection read “ Origin of Species by Means of Natural 
Selection,” by Charles Darwin. It is an old and much misunderstood book, 
rather difficult, it is true, but well worth the careful reading of all students of 
life in the wild, 
