322 PLANT-BREEDING 



cal qualities, and perhaps even the whole mode of growth 

 and development. 



But if we wish to give a direct proof of this assertion, 

 comparative study is no longer sufficiently reliable. Its 

 units are in reality hypothetical. Their existence may seem 

 to be quite obvious and to be in no need of further proof, but 

 as soon as they are to become the basis for far-reaching con- 

 clusions they ought to be beyond all reasonable doubt. 



Our question is, when one unit is added to or subtracted 

 from a well-known type, what are the changes which are 

 thereby produced ? Now such an addition or subtraction is 

 exactly what we call a mutation, and thus it becomes evident 

 that only directly observed mutations can give a reliable 

 answer. Moreover, mutations are so rare that the chance 

 of two of them occurring together seems too small, or, in 

 other words, that we may confidently assume that each single 

 mutation affects only a single unit. 



Considering the mutations of the evening primroses from 

 this point of view, our conception of the correlative nature 

 of the different changes which each of them produces will 

 at once become clearer in its meaning and win the rank of 

 full experimental proof. Some of them are more easily de- 

 scribed and understood than others, but the same general 

 rules prevail in nearly every single case. 



Let us begin with the short-styled variety or (Enothera 

 brevistylis. Its mutative origin has, as a fact, not been direct- 

 ly observed, but may be deduced from its occurrence on only 

 one native locality and amidst an overwhelming throng of 

 normal primroses. The unity of its character, has, on the 

 other hand, been proven by its behavior in crosses with the 

 parent species. The characteristic mark of this variety lies 

 in the short style which, instead of pushing the stigmas 

 above the anthers, hardly reaches the throat of the tube. 

 Other marks are correlated with this. In the first place, 



