INTROGRESSION AND EVOLUTION 79 



win and Schultes, 1947). Either accidentally or with in- 

 tent, seedlings from wild trees came up in clearings where 

 they were being used for food. These areas were often out- 

 side the natural range of that species or variety and some- 

 times within pollination distance of other species. Con- 

 sequently these isolated trees tended to be cross-pollinated. 

 Under the primitive agriculture of these areas, clearings were 

 occupied for a time and then deserted. As the disturbed land 

 gradually reverted to jungle there were many opportunities 

 for the hybrid seedlings of the isolated nut trees to germinate 

 and survive. They crossed back to the native species of that 

 vicinity, and thus the process of introgression might have 

 started in hundreds of little clearings in the jungle. The 

 more or less casual use of Hevea for its edible nuts increased 

 the natural introgression between some of the species. When 

 man gradually learned that the latex of Hevea also had its 

 applications, he already had at hand variable, introgressed, 

 semidomesticated populations, in which trees superior in 

 latex were more likely to be found. 



The extent and frequency of introgression must certainly 

 vary greatly with the type of agriculture that is being prac- 

 ticed. Under the jungle-clearing pattern, like that just de- 

 scribed for Hevea, it must have been at a maximmn. Today 

 it can be seen to vary widely between areas of pastoral 

 agriculture and those devoted exclusively to field crops. In 

 the latter, in the so-called cotton belts, wheat belts, and corn 

 belts, the native vegetation is completely removed over wide 

 areas. Alien crop plants are introduced. There are few op- 

 portunities for hybridization and almost no niches in which 

 the hybrid segregates may survive when they do occur. A 

 pastured area is very different. The native vegetation is 

 removed only in part, though natural ecological conditions 

 are drastically changed. The plants introduced in pastures 

 and hayfields are of many kinds. There are new opportu- 

 nities for hybridization between various components of the 

 native vegetation previously isolated, or between them and 

 their close relatives among the introduced plants and weeds. 



