66 INTROGRESSIVE HYBRIDIZATION 



of western Oklahoma today. With an increasmgly severe 

 climate and with small populations, opportunities for dif- 

 ferentiation would have been great. As the hot, dry period 

 came to a close and the mesophytic forests moved westward 

 again, these remnants probably first spread out locally and 

 then hybridized with their remote cousins as they came back 

 into the territory. Desmarais (1947) has made an intensive 

 study of the sugar maples which demonstrates something of 

 what took place in that genus. More than one observant 

 naturalist has noted slight regional differences in the Ozark 

 representatives of many other wide-ranging species, which 

 would indicate that the phenomenon may have been a very 

 general one. 



In his studies of introgression in Cistus (1941) Dansereau 

 presented circumstantial evidence that the North African 

 variety of C ladaniferus originated through introgression of 

 C. laurifolius into the typical variety (which is now limited 

 to the Iberian peninsula and southern France). Although 

 he presented no cytological or genetical evidence in support 

 of this hypothesis, he did possess a detailed understanding 

 of the genus Cistus from having monographed it and from 

 having, as a trained ecologist, studied the problem in the 

 field. Furthermore, he made detailed population samples 

 that were analyzed by some of the methods discussed in 

 Chapter 6. His explanation seems to be well established as a 

 working hypothesis. If confirmed, it would be a further 

 demonstration of the role of introgression in differentiating 

 geographical varieties. 



INTROGRESSION AND EVOLUTION UNDER 



DOMESTICATION 



Such disturbances of the habitat as those previously de- 

 scribed certainly must have occurred in prehuman times. 

 It is just as certain that the appearance of man greatly ac- 

 celerated such processes. On the one hand, by moving him- 

 self and his domesticated animals from place to place he re- 



