indirect: in the case of a carnivorous animal,
for example, a particular 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 favour". Now,
I will ask you to imagine further that it shall be a plant which shall
produce every year fifty seeds, which is a very moderate number for a
plant to produce; and that, by the action of the winds and currents,
these seeds shall be equally and gradually distributed over the whole
surface of the land. I want you now to trace out what will occur, and
you will observe that I am not talking fallaciously any more than a
mathematician does when he expounds his problem. If y