MAN AS A MAKER OF NEW PLANTS — ANDERSON 



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its significance stressed or experimental work undertaken. One other 

 example demonstrates the role of man's operations on the habitat. 

 Riley (1938) studied the hybridization of two species of Iris on the 

 lower delta of the Mississippi in a neighborhood where the land-use 

 pattern had produced something as demonstrable and convincing as 

 a laboratory experiment (Anderson, 1949; see fig. 4). Property lines 

 ran straight back from the river; the farms were small, only a few 

 hundred yards wide, and very narrow. Under these conditions it was 

 easy to see that the hybrids between these two irises were virtually 



Figure 4. — A demonstration of man's unconscious role in creating new plants. (From 

 Riley, 1938.) At the far right one of the minor bayous of the lower Mississippi Delta. 

 At right angles to it and running nearly across the figure is the abandoned channel of a 

 former stream, now drained by a ditch. The natural levees of the stream are slightly 

 higher than the surrounding country. Their sharp inner edges are indicated on the map 

 by hachures. The road has been run along the lower levee, and houses have been built 

 along the opposite one. The property lines (as in many old French settlements) produce 

 a series of long narrow farms, which for our purposes serve as so many experimental 

 plots. Each farm has its house on a low ridge with a long entrance drive connecting it 

 across a swale to the public road on the opposite ridge. The farms (including a score of 

 others which are out of sight to the left of the figure) were originally essentially similar. 

 At the point where the ditch joins the bayou is a large population of Iris hexagona giganti- 

 caerulea. Behind the levee on which the houses were built, /. fulva grows on the lower 

 ground as well as farther upstream along the ditch. The key fact to be noted is that 

 the hybrids are on only one farm, that they are abundant there, and that they go up to 

 the very borders of the property on either side. Nature is evidently capable of spawn- 

 ing such hybrids throughout this area, but not until one farmer unconsciously created 

 the new and more or less open habitat in which they could survive did any appear in this 

 part of the delta. (See Anderson, 1949, pp. 1-11, 94-98, for a more complete discussion.) 

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