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 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 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 



