V. GRANT 55 



species. Being the dominant plant of the Great Basin and a char- 

 acteristic subdominant in the mountains and high plateaus of 

 western North America, the sagebrush has inevitably received 

 much attention from stockmen, foresters, range specialists, botan- 

 ists, and other naturalists. Nevertheless, despite this general atten- 

 tion, as well as two taxonomic revisions, one in 1923 based on 

 field and herbarium studies, and the other in 1953 utilizing 

 chromosome counts in addition to the classical methods, a classifi- 

 cation of the sagebrushes which accurately represents the natu- 

 rally occurring biological units still eludes us. The recent studies 

 of Ward ( 1953 ) have at least defined the problem and pointed 

 the way toward its eventual solution. There is reason to hope that 

 the continuing investigations of Ward, Beetle, and others will 

 result ultimately in the discovery and delimitation of all the con- 

 stituent species in the Artemisia tridentata complex. The pros- 

 pects that botanists will ever be able to determine correctly 

 many of these species, without field observations, cytological 

 observations, or both, are not, however, encouraging. 



Among the genera listed in Table I, sibling species associated 

 with polyploidy contribute to the taxonomic difficulties in Gilia, 

 Clarkia, and Mentzelia. 



A third cause of obscure species lines in plants is natural hy- 

 bridization including introgression. If two or more species begin 

 to hybridize, and if the derivative forms succeed in establishing 

 themselves as natural populations, the original discontinuities 

 between the species may become utterly blurred. This is the 

 cause of the overlapping ranges of variation of three species of 

 Gilia shown in Fig. 1. It is the most important cause of taxonomic 

 difficulties in the Leafy-stemmed Gilias and the Cobwebby Gilias, 

 the two most critical sections of the genus. The boundary lines 

 between four species of Cobwebby Gilia are obscured by past 

 hybridization of each species with two or more of the other four 

 to such an extent that it is not apparent whether there are four 

 species or one ( Fig. 2 ) . Interspecific hybridization is the cause of 

 the fraction of poorly defined species in Pinus, Quercus, Ceano- 

 thus, Diplacus, and Aquilegia, listed in Table I, and in numerous 

 other genera of higher plants. 



