The Causes of Individual Differences 21 



afforded by cultivation, awakened in the plant the inward 

 impulse to the display of all those variations possible 

 within the more or less narrowly circumscribed limits of 

 the species." It is generally agreed by those who have 

 given the matter much thought, that an excess of food 

 above the amount normally or habitually received is one 

 of the very chief, if not the most dominant, causes of in- 

 dividual differences in plants. Certainly every farmer 

 or gardener knows that the richer the soil in available 

 plant-food, the stronger and the more abnormal and 

 unusual his product will be. 



If, then, excess of food supply is a strong factor in the 

 modification of plants, and the one fundamental aim of 

 agriculture is to supply food in excess of natural conditions, 

 it must naturally follow that cultivated plants should be, 

 of all others, the most variable. This is notably true. 

 Now, the first variation that usually comes of this liberal 

 food supply is increase in mere bulk. Probably every 

 plant which has ever been cultivated has increased its 

 stature or the size of some or all of its parts. Moreover, 

 this is generally the direct object of cultivation, to 

 secure larger herbage, fruits, seeds, or flowers. Inci- 

 dentally, we find here an indubitable proof of the truth 

 of the hypothesis of evolution, for if it were impossible for 

 plants to vary or to assume new characters, there would be 

 no cultivation and no agriculture; for there would be 

 little object in cultivating a product if it grew equally well 

 in the wild. 



This variation into mere bigness is more important than 

 it may seem at first. All thoughtful horticulturists agree 

 in thinking that the first thing to be done in ameliorating 



