1 84 STEBBINS 



still possible. On the other hand, the diploid ancestors of some of the amphi- 

 ploids now occur in widely separated areas, so that their geographic ranges 

 must have been different when the hybridization and polyploidy occurred. 



This distributional pattern most probably reflects a recent origin of the 

 complex. The species of Aegilops are weeds, which grow best on the edges 

 of cultivated fields and in overgrazed pastures. They have followed agricul- 

 tural and pastoral man in his wanderings, wherever the climate has been 

 suitable for them. Two of the amphiploids, A. triunciaUs and A. cylindrka, 

 have spread rapidly through parts of the central and western United States 

 during the present century. For this reason the writer is inclined to believe 

 with Kihara that the more recent of the amphiploids were formed in early 

 Neolithic times, when previously uncommon and geographically separated 

 diploid species may have been brought together and hybridized naturally in 

 the cultivated fields and grazed pastures which men were beginning to create 

 in the Middle East at that time. Their subsequent spread throughout the 

 Mediterranean region accompanied the expansion of agriculture and grazing 

 in that region. By observations on the growth of the plants as cultivated in 

 the climate of Japan, Kihara has shown that the amphiploids are far more 

 tolerant of climatic diversity than their ancestral diploids. This fact, along 

 with their efficient means of seed dispersal, accounts for their rapid spread. 



On the other hand, the oldest of the amphiploids probably dates from the 

 Pleistocene epoch, since altered distributions necessary for the hybridization 

 of A. squarrosa and A. comosa, leading to the formation of the amphiploids 

 A. ventricosa and A. crassa, are most easily explained by changed climatic 

 conditions which existed during the pluvial period of the great ice age. The 

 polyploid complex of Aegilops, therefore, has evolved relatively recently. It 

 owes its success partly to the opening up of new ecological niches by climatic 

 changes which accompanied the ice age, but even more to the changes in the 

 earth's vegetation brought about by man's activity. 



A somewhat older type of polyploid complex is represented by the genus 

 Dactylis, or orchard grass. This genus consists of perennials native to the 

 Mediterranean region and adjacent parts of Europe and Asia. Its best-known 

 representative, the common orchard grass of central and northern Europe, 

 was once regarded as a classical example of an autotetraploid derived from a 

 single diploid type, which is represented by D. aschersoniana, found in the 

 forests of the same region. More recently, however, Myers (1948) and 

 Zohary (1955) have found that typical D. glomerata is intermediate in 

 morphological characteristics between D. aschersoniana and another very 

 different diploid type native to the steppes of northeastern Iran and adjacent 

 Turkmenistan, D. woronomi. Zohary and the present writer, furthermore, 

 have found eight other different diploid types. These diploids are mostly 

 distributed around the periphery of the natural geographic range of Dactylis 

 as a whole, and with the exception of D. aschersoniana and its close counter- 



