io8 Plant Genetics 



These response variations, usually called fluctuations, 

 vary with different individuals, but the hereditary 

 capacity of all of them remains the same. A selection on 

 the basis of fluctuations within a pure line, therefore, 

 should not result in any permanent improvement; in 

 fact, it has been demonstrated many times that no such 

 improvement can be effected in this way. When selec- 

 tion is made, however, among varying doses of cumula- 

 tive factors, an entirely different situation is faced, for 

 in such a case we are not dealing with a pure line. 



The significance, therefore, of Castle's results may 

 be realized. He bred his original pure line for many 

 generations and found that it varied only within very 

 narrow limits; and these slight variations therefore he 

 regarded as mere fluctuations. Furthermore, he found 

 that the character of his pure line developed in crossing 

 as a simple unit character and that no complex factors 

 were involved. With this evidence he should not have 

 been able to effect any permanent changes by selection; 

 but this is exactly what he did. Selecting in opposite 

 directions he developed two new strains, the boundaries 

 of the new strains being distinct from one another and 

 distinct from the boundaries of the original strain, that 

 is, the non-selected pure type. 



Castle's next step was significant. He crossed 

 each of his new strains with the same wild race, the 

 result being that each of his new strains behaved as a 

 simple recessive unit, giving a 3:1 ratio among the 

 grandchildren. 



The logical conclusion from this series of experiments 

 may be given in Castle's words, as follows: "The 

 conclusion seems to me unavoidable that in this case 



