root, soil, manure,
minerals, and so little to the real gaseous food stuff of which their
crops are, in fact, composed? Why does Hodge, who is so strong on grain
and guano, know absolutely nothing about carbonic acid? That seems at
first sight a difficult question to meet. But I think we can meet it
with a simple analogy.

Oxygen is an absolute necessary of human life. Even food itself is
hardly so important an element in our daily existence; for Succi, Dr.
Tanner, the prophet Elijah, and other adventurous souls too numerous to
mention, have abundantly shown us that a man can do without food
altogether for forty days at a stretch, while he can't do without
oxygen for a single minute. Cut off his supply of that life-supporting
gas, choke him, or suffocate him, or place him in an atmosphere of pure
carbonic acid, or hold his head in a bucket of water, and he dies at
once. Yet, except in mines or submarine tunnels, nobody ever takes into
account practically this most important factor in human and animal
life. We toil for bread, but we ignore the supply of oxygen. And why?
Simply because oxygen is universally diffused everywhere. It costs
nothing. Only in the Black Hole of Calcutta or in a broken tunnel shaft
do men ever begin to find themselves practically short of that
life-sustaining gas, and then they know the want of it far sooner and
far more sharply than they know the want of food on a shipwreck raft,
or the want of water in the thirsty desert. Yet antiquity never even
heard of oxygen. A prime necessary of life passed unnoticed for ages in
human history, only because there was abundance of it to be had
everywhere.

Now it isn't quite the same, I admit, with the carbonaceous food of
plants. Carbonic acid isn't quite so universally distributed as oxygen,
nor can every plant always get as much as it wants of it. I shall show
by-and-by that a real struggle for food takes place between plants,
exactly as it takes place between animals; and that certain plants,
like Oliver Twist in the workhouse, never practically get enough to
eat. Still, carbonic acid is present in very large quantities in the
air in most situations, and is freely brought by the wind to all the
open spaces which alone man uses for his crops and his gardening. The
most important element in the food of plants is thus in effect almost
everywhere available, especially from the point of view of the mere
practical everyday human agriculturist. The wind that