

CHAPTER 



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trogression in Iris: 

 A Typical Example 



Before we can discuss introgressive hybridization intel- 

 ligently we must know what it is Uke. This first chapter at- 

 tempts to define the phenomenon and then to give a descrip- 

 tion of one particular example. Detailed analyses of hybrid- 

 ization under natural conditions have shown that one of its 

 commonest results is repeated backcrossing of the hybrids 

 to one or both parents. With each successive backcross the 

 partially hybrid nature of these mongrels becomes less ap- 

 parent; the end result of each hybridization is an increased 

 variability in the participating species. The possible im- 

 portance of this gradual infiltration of the germplasm of one 

 species into that of another was suggested by Ostenfeld in 

 1927. The process was specifically discussed in 1938 (An- 

 derson and Hubricht) and named '^introgressive hybridiza- 

 tion.'' Its consequences were described as the '^introgres- 

 sion" of one species into another, this terminology being 

 deliberately chosen because it simplified the discussion of 

 particular cases and avoided needless repetition. Intro- 

 gression has since then been investigated in various genera 

 of the higher plants, and its importance among the verte- 

 brates has been demonstrated, at least for fishes and for 

 Amphibia. Heiser has reviewed the literature on intro- 

 gression critically (1949) and discussed its probable evolu- 

 tionary and taxonomic significance. 



For the purposes of this monograph one of the best ex- 

 amples of introgression is provided by two conspicuous irises 

 of the Mississippi Delta. The scientific data concerning it 

 are widely scattered in genetical, ecological, taxonomic, and 

 horticultural literature, but when they are all assembled they 



agree, even to details. There can be little doubt that the 



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