MULTIPLE FACTORS 257 



factor, there was no known process whereby a 

 result of this kind would be expected. Castle found, 

 on performing the experiment, that when the minus 

 hooded of first extraction (-0.38) were crossed to 

 normal, the minus hooded of the second extraction 

 averaged +1.01, and when these were again crossed, 

 the minus of third extraction averaged +2.55, and 

 did not range below +1.00. The plus race, after 

 the second extraction, was apparently no further 

 modified than before, being +3.34, there was even 

 some change in the reverse direction; but this 

 result seemed due principally to the peculiar fac- 

 torial composition which one aberrant family hap- 

 pened to have received; after the third extraction 

 further change was again noticeable, the average 

 being +3.04. Moreover, some of the famihes were 

 of almost exactly the same grade as some of those 

 in the third extracted minus series. As a result of 

 these experiments. Castle has reversed his earlier 

 conclusion and states that the case is after all one 

 of multiple modifying factors. 



In favor of the view that factors are constant are 

 the convincing experiments of Johannsen on the size 

 of the Princess beans. The material is highly favor- 

 able for work of this kind, not only because exact 

 measurements may be taken, but because the stocks 

 i-eproduce by self-fertilization and were found to be 

 homozygous. Johannsen's results (Fig. 62) show 

 that no matter how many factors influence the size 

 of the bean, so long as the bean is homozygous, 

 selection of plus and minus variants produces no 



