44 INTROGRESSIVE HYBRIDIZATION 



The first step in the analysis of any highly variable popula- 

 tion is to discover at least two characters that are varying 

 and to devise means for measuring this variation objectively. 

 They should, if at all possible, be characters with no trans- 

 parent dependency upon each other or upon a common 

 factor. Corolla length, leaf length, and internode length, for 

 instance, might be expected to vary more or less together; 

 the same influences that produced a longer leaf on one plant 

 might well produce larger flowers and longer internodes on 

 the stem. 



The second step is to score a number of individual plants 

 simultaneously for these two characters and then to plot the 

 results as a scatter diagram. Let us suppose that in such a 

 population we have found leaves to vary from glabrous to 

 highly pubescent and the flower color to range from very 

 Hght to quite dark. Having turned each of these two char- 

 acters into a set of objective grades and scored 25 plants for 

 both, we then produce a scatter diagram that shows graph- 

 ically the extent to which variation in flower color is con- 

 nected with variation in pubescence. Figures 5 to 8 illus- 

 trate the four different situations we might possibly meet. 



We may find, as in Fig. 5, that the light-colored flowers 

 are all glabrous and that the dark-colored ones, though 

 usually more or less pubescent, may occasionally be almost 

 glabrous. These facts suggest, though they do not prove, 

 that the light- and dark-colored plants are genetically iso- 

 lated from each other, as when two well-isolated species are 

 growing together. Again we may find, as in Fig. 6, that 

 flower color and pubescence vary quite independently of one 

 another. Another possibility is shown in Fig. 7; the two 

 characters are completely correlated. The lightest-colored 

 plants are the most glabrous, and the darkest are the most 

 pubescent. The darker the color, the heavier the pubescence, 

 without exception. Such a situation would result if color 

 and pubescence were affected simultaneously by the same 

 factor, as, for instance, moisture. The drier the site, shall we 

 say, the lighter the color and the less developed the pubes- 



