436 THE CAUSES OF THE XI 
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 bea 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 dis- 
tributed 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 
