58 DOMESTICATED ANIMALS AND PLANTS 



the atmosphere is useless alike to animals and plants except a 

 very few species of bacteria which constitute, so far as we know, 

 the only means for collecting available nitrogen except the slow 

 and irregular action of electricity. 1 In this way all life, both plant 

 and animal, depends almost absolutely for its nitrogen upon 

 bacteria, the smallest of all organisms, invisible to the naked 

 eye and so exceedingly minute that a hundred of them placed 

 end to end would not reach through the thickness of this sheet 

 of paper. On how slender a thread does the life of the world 

 depend ! 



Every species, therefore, lives wherever it can find suitable 

 food, and does not hesitate to attack another, living or dead, and 

 consume its substance either by the rending of its flesh and 

 the consequent quick destruction of life, by sucking its juices 

 as an external parasite, or even by invading the very body of its 

 prey and consuming its vitals with slow destruction. This is 

 very common among insects, one species laying its egg in the 

 body of another, where it hatches, producing a larva that lives 

 at the expense of the host till death ensues, by which time he is 

 ready to undergo one of his transformations and afterwards 

 "go it alone." 2 



And so it is that food means indiscriminate slaughter by both 

 sudden and lingering methods, so it is that the struggle for 

 existence is chiefly- fought out at this point, and so it is that the 

 food supply is the chief consideration in fixing the prosperity and 

 the life tenure not only of individuals but of species as a whole. 



Competition for room. This is no less real than is competition 

 for food, but it applies to plants rather than to animals, which 

 seldom suffer for mere space. When, however, by chance plants 

 come up too thick for standing room, they are bound to suffer 



1 The electric spark serves to combine nitrogen and hydrogen in small 

 amounts, but the world's supply of nitrogen is supposed to be dependent upon 

 bacterial action. 



2 It is common for wasps to sting a supply of insects, paralyze them, plant 

 an egg in each, and pack them securely away to serve as food for the young 

 larvae as they hatch. 



