MUTANTS AND HYBRIDS OF THE OENOTHERAS. 55 



branches of lamarckiana 15.7 1.7 per cent, for the total length of the 

 branches 20.2 db 2.2 per cent, and for the ratio between the width 

 and length of the leaves 9.53 d= 0.22 per cent. 



The great variability of the mutants does not, however, seem to 

 result in any diminution of the gap that separates them from the 

 parent form, and no movement in this direction has been observed in 

 the long period which has elapsed since the new species came into 

 existence. Thus the heights of 0. nanella group themselves about the 

 mean value of 22.81 1.02 cm., with a range from 7 to 35 cm., while 

 those of 0. lamarckiana group themselves about the mean of 88.68 

 0.55 cm., with a range from 77 to 96 cm. The number of branches 

 per individual of lamarckiana ranged from 11 to 25, while that of 

 rubrinervis was 34 to 62. The actual discontinuity is somewhat more 

 fully expressed, however, by a comparison of the numerous features 

 which elude measurements to be seen in Plate XXII, in which leaves 

 from the specimens of lamarckiana and rubrinervis which approached 

 each other most nearly are shown. The actual discontinuity between 

 the retrograde variety, 0. nanella, and its parent in the leading feature 

 of height of stem is even more marked than the gap between the 

 various unit-characters of rubrinervis and lamarckiana. 



Recurring again to the amplitude of the fluctuations in the 

 mutants, it is to be said that it is doubtless much greater in the leaf- 

 forms of the retrograde variety, O. nanella, than in any which have been 

 measured, if the entire mass of foliage is taken into account, since in a 

 certain mid-stage in the rosette it is practically impossible to distinguish 

 it from the parent, although fully distinct as to form and size of the 

 leaves in the very young and very old rosettes. The very range of 

 variation may be in itself a character of the mutants, in which case no 

 reason could be given for its existence, any more than reasons could be 

 given for the existence of any other unit-character. Similar diffi- 

 culties might be encountered in seeking an explanation of the com- 

 parative amplitude of variation of any group of related forms. 



