6 INTROGRESSIVE HYBRIDIZATION 



over into a hybrid, the resulting flower may be largely yellow 

 or ivory, depending on its underlying ground color. Along 

 with these recombinations of the color genes go various de- 

 grees of intermediacy between the large flowers of HGC and 

 the small ones of Fulva, between Fulva's floppy petals and 

 the upright ones of HGC. Undoubtedly, there must as well 

 be segregation for some of the basic physiological differences 

 that limit Fulva prevailingly to one kind of a situation, HGC 

 to another. 



Fulva is a wide-ranging species growing in wet clay soils 

 from the Wabash and Ohio River valleys down to the lower 

 delta of the Mississippi. Characteristically it is found in the 

 flat valleys of these large rivers along the edges of the nat- 

 ural levees that they build for themselves. It seems to pre- 

 fer semishade and very often grows along drainage ditches. 

 HGC never gets far from the sea; it is a plant of the lower 

 delta and is found in full sun in the mucky soil of tidal 

 marshes, where the soil is never acid and may be quite 

 alkaline. 



The area where these two species come into contact is, 

 therefore, the lower Mississippi Delta, mostly in the region 

 between New Orleans and the sea. It is flat country where 

 differences of a few inches in the height of the land have 

 more effect on the vegetation than hundreds of feet might 

 have in other parts of the world. (Viosca, 1935.) Here, for 

 thousands of years, the river has been building its delta, 

 splitting itself up into numerous weaving branches, which 

 change their courses constantly and sometimes catastroph- 

 ically. In those rare portions of this rich agricultural region 

 in which man has not greatly altered the natural pattern of 

 the vegetation, Fulva and HGC come into contact whenever 

 a natural levee penetrates a marsh, as, for instance, when a 

 shifting bayou cuts across the course of an abandoned deltaic 

 stream. At such places a few hybrids are sometimes to be 

 found where a natural levee runs into a wide tidal marsh. 

 Hybridization between Fulva and HGC must have been 

 going on occasionally for a very long time. The whole pat- 



