48 INTROGRESSIVE HYBRIDIZATION 



ever, there was also conspicuous and measurable variation 

 in exsertion of the stamens, in the color pattern of the sepal, 

 and in the size and proportion of the stylar appendages. As 

 is shown in the diagrams that accompany Chapter 6, it can 

 be demonstrated that all these characters tend to be some- 

 what correlated with redness of corolla and size of sepal. 

 Scores, if not hundreds, of genes are involved. The only 

 knowTi mechanism that would explain their tendency to go 

 together (which is far from absolute) is their having been 

 introduced together into the population. These complexes 

 of characters, which are statistically demonstrable, are the 

 visible results of linkage systems and of other cohesive 

 forces. 



When, by the methods outlined in Chapter 6, one can work 

 over the facts of correlation tendencies in these introgressed 

 populations and produce exact, technical descriptions of the 

 introgressing species, even w^hen it is unknown to the ob- 

 server, the proof of the underlying assumptions is as absolute 

 as one might ever hope for in scientific work. The methods 

 are still crude; it takes experience to use them effectively; 

 but they have already advanced to the stage where they can 

 be given to a group of graduate students as a class exercise. 

 Such a group of students, given representative mass col- 

 lections (Anderson, 1941) of a hybrid population, can rea- 

 sonably be expected to draw up a technical description of the 

 original hybridizing entities that produced the population. 



