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 

 1 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. 1 



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 



1 In 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. 



