ECOLOGICAL BASIS 



17 



new habitats (in addition to the various intermediary and 

 the parental ones) : 



rich loam 



full sun 



no leaf mold 



rich loam 

 full sun 

 leaf mold 



rich loam 

 deep shade 

 no leaf mold 



rocky soil 

 deep shade 

 leaf mold 



rocky soil 

 full sun 

 leaf mold 



rocky soil 

 deep shade 

 no leaf mold 



Imagine what would have to happen to any natural area 

 before such a set of variously intermediate habitats could be 

 provided! It has been very generally recognized that if hy- 

 brids are to survive we must have intermediate habitats for 

 them. It has not been emphasized, however, that, if any- 

 thing beyond the first hybrid generation is to pull through, 

 we must have habitats not only that are intermediate but 

 also that present all possible recombinations of the contrast- 

 ing differences of the original habitats. If the two species 

 differ in their response to light, soil, and moisture (and what 

 related species do not?), we must have varied recombina- 

 tions of light, soil, and moisture for their hybrid descendants. 

 Only by a hybridization of the habitat can the hybrid re- 

 combinations be preserved in nature. 



Seen in the light of the above argument, Riley's and 

 Viosca's detailed reports (see Chapter 1) on the irises of the 

 Mississippi Delta acquire new significance. They demon- 

 strate a close connection between the treatment of the hab- 

 itat and the number and kinds of hybrids that appeared. 

 Though the narrow French farms were as close together as 

 laboratory plots, nearly all the hybrids were concentrated 

 on one farm. The conspicuously segregating Colony H-1 

 was co-extensive with a small piece of wooded pasture that 

 had been repeatedly cut over and subjected to overpasturing. 

 The area in which the hybrids were found went right up to 

 the fence line and stopped there. Though irises were on the 

 neighboring farms, they were not hybrids. Colony H-2, on 



