80 INTROGRESSIVE HYBRIDIZATION 



When hybrids do occur there are various new niches in which 

 some of them may possibly succeed. It is significant that 

 most of the studies of introgression up to the present time 

 have been made in pastures or in heavily pastured areas. 

 Riley's studies of Iris were made in pastured swamplands. 

 Anderson and Hubricht worked in overpastured areas in the 

 Ozarks. It would seem to be significant that New Zealand — 

 where the frequency of hybridization has been the subject of 

 several special investigations (Allan, 1937) — is very largely 

 given over to pastoral agriculture. Such genera as Cra- 

 taegus, in which thousands of new species have been de- 

 scribed in the last century, are nearly all plants of pastures. 

 For Crataegus, Marie Victorin has outlined the main steps 

 in the production of the swarms of these new forms in the 

 pastures of French Canada. The great majority of the species 

 described by the late Charles S. Sargent came from such 

 pastured areas in which opportunities for hybridization and 

 consequent introgression were very high. Crataegus (a 

 genus in which both polyploidy and apomixis are frequent) 

 produced a complicated introgression pattern, which has led 

 to great taxonomic confusion. Without these two complica- 

 tions there would have been a less ruffled gene flow between 

 the original hybridizing entities. 



The demonstration that cultivated plants and weeds are 

 very largely the products of introgression is particularly im- 

 portant for plant genetics. It is almost exclusively upon such 

 plants that the theory of plant genetics has been based. 

 From Mendel's original peas to Blakeslee's Daturas, we have 

 worked chiefly with introgressed germplasms. Some of our 

 marker genes are certainly introgressive segments from an- 

 other germplasm. That does not vitiate their use as marker 

 genes but it does mean that our estimates of the role of the 

 gene in evolution may need a correction factor, because 

 nearly all our evidence comes from plants that are somewhat 

 exceptional. 



