124 CONDITIONS AFFECTING THE 



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 complications 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 requii'es 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 by 

 which I arrive at the result : — 



Plants. Plants. 



1 X 50 in 1st year = 50 



50 X 50 „ 2nd „ = 2,500 



2,500 X 50 „ 3rd „ = 125,000 



125,000 X 50 „ 4tli „ = 6,250,000 



6,250,000 X 50 „ 5th „ = 312,500,000 



332,500,000 X 50 „ 6tli „ = 15,625,000,000 



15,625,000,000 X 50 „ 7th „ = 781,250,000,000 



781,250,000,000 X 50 „ 8th „ = 89,062,500,000,000 



39,062,500,000,000 X 50 „ 9tli „ = 1,953,125,000,000,000 



51,000,000 sq. miles — the dry sur- -j 



face of the eartliX 27,878,400— J =sq.ft. 1,421,798,400,000,000 



tlie number of sq. ft. in 1 sq. mile ) ' 



being 531,326,600,000,000 



square feet less than would be required at the end of the ninth year. 



