96 INTROGRESSIVE HYBRIDIZATION 



may have come in with them, we assume that the pecuH- 

 arities which we find tending to stay together in the two in- 

 dividuals at the upper left of the diagram are doing so be- 

 cause their genes w^re introduced into the population to- 

 gether. Since all seven of these characters are apparently 



5 6 7 8 9 10 11 



Petal size >- 



Fig. 21. Pictorialized diagram of 23 plants from a hybrid colony studied 

 by Riley (see Plate 1). Diagrammed from his data according to the 

 symbols of Fig. 23. The upper-left-hand star-shaped dot represents the 

 hypothetical species responsible for the introgression, as determined by 

 the "method of extrapolated correlates." Further discussion in the text. 



multiple-factor characters, the chances are inconceivably 

 small that the genes for all could vary simultaneously. That 

 redness, smallness, yellow tube color, petal shape, stamen 

 exsertion, a small style appendage, and absence of a crest 

 all are tending to stay together in this population is most 

 readily explained as due to the influx of whole chromosomes 

 or of chromosome segments from a species in which these 

 characters were tied up together. 



From hybrid population H-2 there are indications that 

 these characters are so correlated. By diagramming sim- 



