94 INTROGRESSIVE HYBRIDIZATION 



method has since been considerably refined. It will be il- 

 lustrated below from the data presented in Riley's paper on 

 introgression in Iris (Riley, 1938). 



A portion of the data from Tables 1, 2, 3, and 4 of Riley's 

 paper were presented (page 3) in Table 1 in a slightly sim- 

 plified form. The figures for sepal lengths have been rounded 

 off to the nearest centimeter. In Riley's paper the method 

 of attack was to examine the two species first, and from a 

 study of them attempt to analyze what was taking place in 

 the hybrids. Using the method of extrapolated correlates, 

 we shall demonstrate from these same data how one may 

 work backwards from the introgressants, to the species from 

 which they were derived. For the purposes of the illustration, 

 therefore, let us suppose that only Iris hexagona var. giganti- 

 caerulea is known to us and that we have come upon Colony 

 H-2, which is much like that species on the whole yet is more 

 variable and shows several variants outside the ordinary 

 range of that species. In the discussion below, following the 

 convention established in Chapter 1, we shall use HGC to 

 designate Iris hexagona var. giganti-caerulea and Fulva to 

 represent 7m fulva. 



For the analysis, what we need is some simple method of 

 determining for the whole population what characters are 

 tending to stay together and in what patterns. We shall 

 work with pictorialized scatter diagrams, choosing for the 

 horizontal and vertical scales two characters each of which 

 can be measured fairly exactly in a series of grades. In 

 Riley's data these conditions are met by petal length and by 

 color of sepal blade. The latter, thanks to the particular 

 chart used by Riley, was scored in a series arranged with in- 

 creasing redness from violet blue through blue violet, violet, 

 and red violet to red. Diagramming increasing redness on 

 the vertical axis and petal length on the horizontal axis, we 

 produce the dots of Figs. 20 and 21 for a population of HGC 

 and for our problem population H-2. From an inspection of 

 these dots it is apparent that redness and petal size are 

 tending to stick together, particularly in those individuals 



