130 Shull. 



increased variability and this increase would appear greater if the two 

 differentiating genes were dominant, than if dominance were absent. 

 This being the case, the point may be emphasized that the mere demon- 

 stration of an increased variability in F 2 does not by itself prove 

 either that several genes of similar nature are involved, that their 

 dominance is lacking, or that all the inheritable size-differences between 

 the Pi individuals are Mendelian, though it does render probable the 

 one essential point, namely, that a Mendelian segregation of some sort 

 has taken place. The latter conclusion is all that has been specif- 

 ically maintained in many cases in which an increased variability has 

 been found in the F 2 and the matter is emphasized here only because 

 the impression might be gained that every demonstration of increased 

 variability in F 2 supports the several assumptions which have been made 

 in the development of the Mendelian explanation of inheritable quantitative 

 differences. 



Attention may be called also to a purely technical manner in which 

 F 2 variability coefficients may be increased. In a number of characters 

 in respect to which the F 2 families of maize (EAST and HAYES 1911) 

 have been reported more variable than the Fi , the data for the parents 

 and the F 2 generation are in each case derived from the progeny of 

 a single mother-plant, while the data for the F 2 are given for the com- 

 bined progenies of 2 5 plants. There is no proof given in these cases 

 that the parents and their Fi hybrids were not as heterogeneous as the 

 F 2 . As the parents were certainly not homozygous some segregation 

 must have taken place in the Fi as well as in the F 2 , so that an Fi 

 progeny grown from a number of ears corresponding with the number 

 used for the F 2 should have shown a larger Fi variability than is re- 

 ported. Consequently in these cases a proper comparison can not be 

 made between the variability of the several generations. It is a pleasure 

 to note, however, that most of HAYES'S data for tobacco (HAYES 1912) 

 and all of EMERSON and EAST'S (1913) excellently handled data for 

 maize, have been derived from strictly individual analyses, and as the 

 results in these cases are not materially different from those reported 

 earlier by EAST and HAYES, it is obvious that the general results of 

 the latter investigators, although in part due to unequal treatment of 

 the several generations, will not on this account need a revision. In 

 supporting the thesis, that near-homozygous types have appeared in the 

 F 2 , by a comparison of F 3 coefficients of variability with the Pi and Fi 

 coefficients, it is likewise important that the several generations to be 

 compared be given like treatment. There should be as many tests of 



