22 MUTANTS AND HYBRIDS OF THE OENOTHERAS. 



representing pure strains of both parents through goneoclinic forms 

 to intermediates in which the parental characters were more or less 

 equally apparent. 



Although the great difficulties attendant upon a valuation of the 

 characters occurring in hybrids were appreciated, it was concluded 

 that such an estimation expressed in tabulated form would be the best 

 method of expression of the dominance of the various characters. 



Hurst selected twenty anatomical characters which were assigned 

 equal value, and included such features as the habit of growth, and 

 form and shape of the leaves and other organs, and upon the preva- 

 lence of these characters the degree of relationship of the hybrid to 

 the parent was estimated. (Hurst, 1900.) 



Peter made a tabulated list of the external characters of the 

 hybrids of the hawk weed, in which these characters were classed as 

 unilateral, goneoclinic, or intermediate with respect to the corre- 

 sponding features of the two parents. The dominance of an3 r given 

 character was calculated from its occurrence in the entire list of 

 plants examined. (Peter, 1884.) 



Mathematical expressions of the dominance 7 of qualities are in 

 themselves misleading unless based upon actual physical measure- 

 ments taken by statistical methods from a number of individuals to 

 eliminate errors. Then, again, the estimation of the development of 

 a paired character presents one of the greatest difficulties encountered 

 in the study of hybrids. Authors are by no* means agreed as to what 

 constitutes absolute dominance. Until within the last few years a char- 

 acter was regarded as dominant by most writers onl} r when it appeared 

 in the hybrid as an exact reproduction of the parental quality. The 

 most recent discussion upon this subject that has come to notice is 

 that of Correns, who takes the ground that a character apparently 

 representing the parental quality to 75 per cent of its full power may 

 be considered as dominant, when occurring to the extent of 25 to 75 

 per cent as intermediate, and below 25 per cent as recessive. (Cor- 

 rens, 1903.) 



This author calls attention to the fact that fully dominant charac- 

 ters occur but rarely, and cites examples of Hyoscyamus and Bryonia. 



The comparatively small number of individuals (33) of the hybrid 

 between O. lamarckiana and 0. biennis which were brought to ma- 

 turity did not give extended opportunities for statistical work, and the 

 number of characters taken into account included many not suscep- 

 tible of direct physical measurement. It was decided, therefore, to 

 express the descriptions in taxonomic terms arranged in tabulated 

 form to serve as a graphic illustration of the general complexion of 



