470 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



The plants in the olive orchard had no such history of long associa- 

 tion. The olives were new to California. The societies of micro- 

 organisms in the soil were originally those that go with live oaks, not 

 those accompanying the salvias on the sunny slopes. These must have 

 been greatly changed during the time the olives were cultivated. 

 Furthermore, the olives, being planted at considerable distances from 

 each other, did not re-create either the fairly continuous shade of the 

 oaks or the open sunshine of the upper slopes. The orchard became 

 the site for evolutionary catch-as-catch-can, and under these circum- 

 stances, as we have seen, the new and variable had a decisive advantage. 



Now that we know this much about these salvias, it would be inter- 

 esting to work experimentally with them and the species with which 

 they are associated to determine just what factors allow two different 

 but closely related species to fit together with their associates so per- 

 fectly that all hybrid intermediates are excluded. From experience 

 with other similar problems I would predict that among the most 

 important factors would be fairly specific reactions between some of 

 the other associated plants and these two sages. In our experimental 

 work with sunflowers we have discovered that one of the strongest 

 factors in determining where weed sunflowers may or may not grow 

 is their reaction to grass. Many grasses apparently give off a sub- 

 stance highly toxic to weed sunflowers. The various species of weed 

 sunflowers differ in their sensitivity to this poison. When two such 

 sunflowers hybridize, one of the factors affecting the outcome is the 

 grassiness of the site. Such relationships seem to be very general 

 among plants. On the whole, many species grow where they do, not 

 because they really prefer the physical conditions of such a site, but 

 because they can tolerate it and many other organisms cannot. 



Generally speaking, the plants which follow man around the world 

 might be said to do so, not because they relish what man has done to 

 the environment, but because they can stand it and most other plants 

 cannot. 



Are these salvias weeds? I would put forward the working hy- 

 pothesis that those in the abandoned olive orchard are on the way to 

 becoming weeds. The small exceptional communities of hybridizing 

 colonies similar to this one, which can be found here and there over 

 southern California, are worth considerably more attention than they 

 have hitherto received. They demonstrate the way in which man, 

 the great weed breeder, the great upsetter, catalyzes the formation 

 of new biological entities by producing new and open habitats. 



The Salvia case is not unique. We now have over a score of similar 

 well-documented studies of the connection between hybridization and 

 weedy, disturbed habitats. This relationship had long been known 

 to observant naturalists, though not until the last few decades was 



