64 INTROGRESSIVE HYBRIDIZATION 



land by natural causes. In these areas species of pines that 

 were previously isolated have been brought together in a 

 newly emerged area in which somewhat diverse floras were 

 in the process of settling down into a new, and supposedly 

 more stable, equilibrium. Hybridization and introgression 

 under such conditions might be able to play a much greater 

 role than in a stabilized community of which all the members 

 have long been selected for their ability to interlock effec- 

 tively. 



Woodson (1947) has presented data on the introgression 

 between three well-differentiated geographical races of 

 Asclepias tuber osa (butterfly weed). One of these is centered 

 upon peninsular Florida, a region that was an island, or 

 series of islands, in Tertiary times and was later connected 

 with the mainland. Through introgression, the fusion of 

 these two varieties has now become a gradual process, ex- 

 tending over an intermediate zone hundreds of miles in 

 depth. The infiltration of the two varieties is so gradual as 

 to be imperceptible to anything less acute than refined sta- 

 tistical methods. From what is generally known about the 

 flora of northern Florida and the Gulf and Atlantic coastal 

 plains it seems probable that the introgression of these two 

 varieties of Asclepias is rather typical of that area. For 

 genus after genus in the flora of the eastern states, there are 

 well-differentiated species or varieties in southern and cen- 

 tral Florida and equally well-differentiated entities on the 

 Coastal Plain. In northern Florida there is centered an inter- 

 mediate zone in which various transitions between the typical 

 coastal-plain type and the typical peninsular type make up 

 the bulk of the populations. It would seem as if, when 

 '^ Orange Island '^ was united to the mainland for the last time, 

 two rather differentiated floras may have met in this inter- 

 mediate zone. Under these unusual conditions, not only 

 would there have been special opportunities for hybridiza- 

 tion, but also, with two sets of plants readjusting themselves 

 into new communities, some of the backcrosses would have 

 been at a selective advantage. Thus introgression would 



