The Causes of Individual Differences 19 



all of which may be alike in every discernible character. 

 If these are planted in a space of a foot apart, it will be 

 found, after two or three weeks, that some individuals 

 are outstripping the others, although all of them came up 

 equally well and were at first practically indistinguishable. 

 This means that, because of a little advantage in food or 

 moisture, or other circumstances, some plants have ob- 

 tained the mastery and are crowding out the less fortunate 

 ones. The theory and practice of agriculture rests on 

 the fact that plants can be modified greatly by the condi- 

 tions in which they grow, after they have become thor- 

 oughly established in the soil. Plants may start equal, 

 but differ widely at the harvest ; and this difference may 

 be controlled to a nicety by the cultivator. Every farmer 

 is confident, also, that the best results for the succeeding 

 year are to be got only when he selects seeds from the best 

 that he has been able to produce this year. So, given 

 uniformity or equality at the start, the operator molds 

 the individual plants largely at his will. 



Conception of biotypes. Most varieties are not as 

 uniform as would at first appear. A careful study of 

 plants, when growing, indicates that they are not only 

 modified in different degrees by environment but the plants 

 themselves are not the same. They have different po- 

 tentialities to begin with. Environment causes direct 

 modifications to appear ; it also allows expression in differ- 

 1 ent degrees of the inherent variability present. Most 

 varieties of plants are polytypic, being composed of many 

 distinct types, or " biotypes" as they have been called by 

 Johannsen. All this is a matter of the commonest ob- 

 servation with the gardener, who is so accustomed to 



