I 86 STEBBINS 



feet chromosome pairing. But they constitute only a small fraction of the 

 genus. Over the great bulk of its geographical distribution Dactylis is repre- 

 sented only by tetraploid strains. These tetraploids are abundant, aggressive, 

 and highly variable. In external morphology and ecological adaptation they 

 are mostly intermediate between various ones of the diploids, but some of 

 them are very hard to distinguish from certain diploids in outward appear- 

 ance. They have, nevertheless, the cytological and genetic characteristics of 

 autopolyploids, which is to be expected if they are derived from diploid 

 hybrids similar to the ones made artificially between the existing diploid types. 



Dactylis, therefore, has a pattern of morphological variation typical of that 

 found in many polyploid complexes, and one which is very confusing to 

 taxonomists working on such groups. If only the diploids existed, we would 

 have a series of constant, well-differentiated species, each occupying its own 

 well-defined habitat and geographic area. But the tetraploids, in addition to 

 simulating some of the diploids so closely that they cannot be distinguished 

 from each other except by counting the chromosome number, also form a 

 continuous network of intergrading forms, connecting all the diploids with 

 each other in such a way that no clear lines of distinction can be drawn be- 

 tween them. For this reason, Zohary and the present writer believe that all 

 the diploids and tetraploids together should be grouped as the numerous 

 subspecies of a single highly variable species. 



The evidence from geographic distribution indicates that the polyploid 

 complex of Dactylis is considerably older than that of Aegilops. The forest- 

 loving aschersoniana type of diploid, with a disjunct pattern of distribution 

 in central Europe, the Himalaya, and western China, is probably a relic of 

 the Tertiary period, when a continuous zone of forest stretched across Eurasia. 

 Another probable relic is D. smithii, since the flora of its native home, the 

 Canary Islands, contains many woody species which have close counterparts 

 in the later Tertiary fossil flora of Europe. Furthermore, tetraploid forms of 

 Dactylis having a few unmistakable characteristics of D. smithii exist along 

 the Atlantic coast of Portugal, southern Spain, and North Africa. This 

 phytogeographic evidence that the diploids of Dactylis formerly occupied 

 wider areas than at present is matched by genetic evidence, since the inter- 

 mediate characteristics of most of the tetraploids indicate that their forma- 

 tion was accompanied by extensive hybridization between different diploids. 

 Such hybridization would be impossible at present, because the areas of the 

 diploids do not overlap. 



We can thus reconstruct three phases of the evolutionary history of the 

 polyploid complex of Dactylis. The first phase occurred during the Tertiary 

 period. It involved the divergence of the diploids from each other and their 

 establishment as relatively homogeneous populations inhabiting different 

 ecological niches in the widely divergent climatic zones. The second phase 

 was brought about by the climatic changes which accompanied the Pleistocene 



