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 : 
I. (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 (0) 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 
6—2 
