118 CONDITIONS AFFECTING THE 



same station everywhere ; we thus get rid of the pecu- 

 liar 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 you show that 

 the conditions of your problem are such as may actually 

 occur in nature, and do not transgress any of the known 

 laws of nature in working out your proposition, then 

 you are as safe in the conclusion you arrive at, as is the 

 mathematician in arriving at the solution of his problem. 

 In science, the only way of getting rid of the complica- 

 tions with which a subject of this kind is environed, is 

 to work in this deductive method. What will be the 

 result then ? I will suppose that every plant requires 

 one square foot of ground to live upon ; and the result 

 will be that, in the course of nine years, the plant will 

 have occupied every single available spot in the whole 

 globe ! I have chalked upon the blackboard the figures 

 bv which I arrive at the result : — 



