Duplicate genes for capsule-form in Bursa bursa-pasloris. 125 



hand, the possibility that they are sonietiiiies duplicate (shall we say 

 once in a thousand times?), is not to be overlooked. 



The theoretical and practical importance of the conclusions drawn 

 from these studies on quantitative characters makes it essential that 

 the evidence be made as impreg-nable as possible. The extreme diffi- 

 culties to be overcome in the attainment of a fairly complete and de- 

 cisive demonstration of the manner of inheritance of such characters 

 are a challenge for a great deal of intensive work on some single, easily 

 handled quantitative character, with perfectly controlled individual anal- 

 ysis through as many generations as may be required. In anticipation 

 of such future work, and without calling into question the correctness of 

 the conclusions arrived at by any of the investigators whose work has 

 been included in the above list, it may not be out of place to point out 

 several weaknesses in the evidence upon which the conclusions have been 

 based, in order that future work along these lines may be strengthened. 



For a considerable portion of the characters listed in the above 

 table the only evidence yet available that Mendelian segregation has 

 taken place, is the greater variability of the F» compared with the 

 parent strains and their Fi hybrids. Sometimes there is added a small 

 amount of evidence that the F3 families are significantly differentiated. 

 Castle (1912) maintains that other explanations of this increased varia- 

 bility in the F2 are possible. This ought to be gi'anted, but the rather 

 vague hypothesis actually offered by Castle as an alternative seems far 

 less plausible than the hypothesis of segregation of plural Mendelian 

 determiners. 



Already in 1906, Johannsen gave, by a comparison of coeffi- 

 cients of variation, a full demonstration of the fact that the variability 

 of the Fl in regard to several quantitative characters was of the same 

 order as that of the parents. He also expressed the view, based upon 

 some preliminary experiments in the , greenhouse, that segregations of 

 size-characters were represented in the second generation hybrids from 

 his "pure-line" beans. The author (Shull 1910) was perhaps the first 

 to actually bring an Fo coefficient of variability into comparison with 

 the coefficients of variability of the parent strains and of their F, hy- 

 brids, thus giving adequate mathematical proof of the increased varia- 

 bility of an apparently continuous quantitative character in the F,.. 



This greater variability was definitely referred, in my paper, to 

 the segregation of numerous hypothetical Mendelian determiners; but 

 these were not assumed to be plural determiners for the particular 

 character then being studied, namely, the number of rows of grains on 



