EVIDENCE OF HISTORY 263 



to the acre. And so with corn, potatoes, all 

 crops. These experiments cover widely scat- 

 tered fields, it is true, and varying climatic 

 conditions, but in the same acre the most re- 

 markable variations in fertility may be found 

 under identical treatment and climatic condi- 

 tions. An acre of wheat looks fairly uniform 

 to the naked eye. Yet, if the farmer divides 

 it into twenty-five foot strips and harvests each 

 strip separately he may, probably will, find 

 that the yield of some plats is several times 

 that of others. In one corner he will find 

 more straw, in another more grain, and in a 

 third, barren stalks. 



If, now, instead of subjecting the whole 

 acre to uniform treatment, he used part as 

 check plats and divided the remainder among 

 different systems of fertilizing, would he be 

 justified in accounting for the difference in 

 yields by the systems of fertilizing he used? 

 Would he be justified in saying that a dress- 

 ing of nitrate of soda was the sole cause of a 

 yield of ten per cent, above the average of the 

 other plats? He would not, unless he were 

 in a position to say that all other factors were 

 uniform. And he cannot say that. He does 

 not know, and at this period in the advance 



