INTRODUCTION. 29 



inheritance of these can only be determined by quantitative 

 means. The importance of a thorough study of the inheritance 

 of characters, which can be accurately measured, instead of 

 depending on observation alone, has been pointed out by many 

 writers.^' - Until recently, most of the investigations on inheri- 

 tance, undertaken from the Mendelian standpoint, have dealt 

 with characters of a qualitative nature, that is, they have 

 dealt with characters which depend on the presence or absence 

 of a particular attribute, such as colour, hairs or awns. Forms in 

 which these attributes are absent occur and can be used as 

 anah^sers, and the observations are restricted to a detection of 

 the presence of the character involved. By a judicious use of the 

 analysers and a careful analysis of the progeny, it is generally 

 possible b}" observation alone to determine the principles under- 

 lying the inheritance. Characters connected with the size of 

 organs present more difficulty. In the first place, no absence of 

 the character is possible and no analysers exist. Take, for 

 instance, the height of a plant or the length of a leaf, a plant 

 without height or a leaf without length is inconceivable. Thus 

 in such characters we are dealing with difference in degree only ; 

 observation is insufficient and measurement must be emplo3^ed. 

 In the second place, such characters are generally very sensitive 

 to changes in environment and show marked fluctuating 

 variability. Such fluctuating variability may be inherent in 

 the plant, or may be directly due to the influence of the environ- 

 ment on the character under consideration, or indirectly to the 

 effect of the environment on the general vigour of the plant. 

 Should the changes due to fluctuating variability be greater or 

 almost as great as the differences in the characters under investi- 

 gation, they may obscure or entirely mask the effects of inheri- 

 tance. East^ was one of the first to point out that so-called 



^ Tammes, Recueil des Travanx Botaniques Neerlandais, Vol. VIII, 1912 



* Nilsson-Ehle, Kreuzungsuntersuchungoi cdi Hafer und Weizen, Lund, 1909. 

 Kreuzungsuntersitchungen II, Lund, 1911. 



3 East, The American Naturalist, Vol. XLIV, 1910 



