78 INTROGRESSIVE HYBRIDIZATION 



ramosus) . These two native American plants were originally 

 quite distinct from one another and had very different eco- 

 logical requirements. Erigeron annuus prefers rich, moist 

 situations ; E. strigosus is a plant of dry, barren areas. In the 

 eastern United States they have introgressed so extensively 

 into each other that somewhat intermediate types are found 

 exclusively over wide areas. Apomictical forms of both 

 annuus and strigosus have occurred, some of which seem to 

 have been very widespread. Weed strains of both species 

 have spread far outside their original habitats and have been 

 carried to other continents. 



In parts of their present ranges the two species have been 

 so extensively blurred that it is difficult to conceive of what 

 they may have been like before the advent of man. In other 

 areas, however, they are well differentiated, though intro- 

 gression is still continuing. Their relationships are quite 

 clear in the northern Ozarks. There Erigeron strigosus forms 

 large and only slightly variable populations in dry, rocky 

 areas, while Erigeron annuus, in essentially pure condition, 

 is limited to rich and fairly moist locations, such as barn- 

 yards and fertile vegetable gardens. Intermediate popula- 

 tions are common throughout the area, the degree of inter- 

 mediacy being proportional to the dryness and sterility of 

 the habitat. Yet this intermediacy is something inherent, 

 since cultures raised in the experimental garden retain the 

 characteristics of the populations from which they were 

 derived. 



With many cultivated plants the nature and degree of 

 introgression have probably changed as man has found new 

 uses for each cultivated plant. The probable histories of 

 cucurbits and of hemp have already been alluded to. Seibert 

 (1947, 1948) has discussed the role of introgression in the 

 domestication of Para rubber (Hevea). The wild-growing 

 species of Hevea are native mostly to alluvial soils, and Sei- 

 bert thinks that there may have been some introgression in 

 these areas before the advent of man. Apparently the species 

 was first cultivated for its edible nuts (Baldwin, 1947; Bald- 



