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.*_ 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.” ? 
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 
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. 
? 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 
larvee as they hatch. 
