DEFINITE DIFFERENCES 83 



tinuous series of eight shades, because in doing so he 

 groups together a number of probably definite factors 

 with others which are of an indefinite kind. When the 

 colours of the human eye come to be studied in greater 

 detail, there can be little doubt that they will be found 

 to depend upon some such factors (among others) as 

 the following : 



1. (a) Definite differences in structure, and (b) the 

 definite presence and absence of pigment in certain 

 definite positions ; as well as 



2. (a) Indefinite variations (individual differences) in 

 structure, and (b) in quantity of pigment if, indeed, 

 the quantitative differences are not found to be also 

 definite. 



In the above example a suitable and legitimate 

 object for biometrical investigation would be the 

 differences in amount of a particular pigment. 



But definite differences may also exist in the case of 

 an apparently simple quantitative character. The 

 accompanying figure (Fig. 2) shows the variations in 

 length of the fruits of three different but closely allied 

 species of evening primrose, as measured by de Vries. 

 In this diagram the vertical distances are in each case 

 proportional to the number of individuals having 

 particular lengths of fruit, and the actual length of the 

 fruit is in each case proportional to the horizontal 

 distance from an imaginary vertical line some way to 

 the left of the figure ; the points thus plotted are 

 joined by straight lines, so that a polygonal figure is 

 obtained representing the nature of the variation in 



each particular case. The diagram shows at once that 



62 



