herbaceous plant may in multiplying be an indirect helper, by enabling
the herbivora on which the carnivore preys to get more food, and thus
to nourish the carnivore more abundantly; the direct helper may be best
illustrated by reference to some parasitic creature, such as the
tape-worm. The tape-worm exists in the human intestines, so that the
fewer there are of men the fewer there will be of tape-worms, other
things being alike. It is a humiliating reflection, perhaps, that we
may be classed as direct helpers to the tape-worm, but the fact is so:
we can all see that if there were no men there would be no tape-worms.

It is extremely difficult to estimate, in a proper way, the importance
and the working of the Conditions of Existence. I do not think there
were any of us who had the remotest notion of properly estimating them
until the publication of Mr. Darwin's work, which has placed them
before us with remarkable clearness; and I must endeavour, as far as I
can in my own fashion, to give you some notion of how they work. We
shall find it easiest to take a simple case, and one as free as
possible from every kind of complication.

I will suppose, therefore, that all the habitable part of this
globe--the dry land, amounting to about 51,000,000 square miles,--I
will suppose that the whole of that dry land has the same climate, and
that it is composed of the same kind of rock or soil, so that there will
be the same station everywhere; we thus get rid of the peculiar
influence of different climates and stations. I will then imagine that
there shall be but one organic being in the world, and that shall be a
plant. In this we start fair. Its food is to be carbonic acid, water
and ammonia, and the saline matters in the soil, which are, by the
supposition, everywhere alike. We take one single plant, with no
opponents, no helpers, and no rivals; it is to be a "fair field, and no