218 MISSOURI BOTANICAL GARDEN. 



the Yuccas occur in practically unbroken continuity from the 

 Atlantic coast in the vicinity of Fortress Monroe to Florida, 

 and across the Southwest and Mexico to California in the 

 neigborhood of Monterey, Y. glauca reaching well up on 

 the upper Missouri River, and Y. baccata following the 

 Rocky Mountains northwards into southern Colorado. 

 From this it may be inferred that they have become 

 ■specifically differentiated at a comparatively recent date, 

 an inference which is supported by the fact that notwith- 

 standing various dissemination contrivances which may be 

 held to be of still later acquisition, all of the species 

 except the distinctively West Coast brevifolia and Whipplei 

 may be broadly classed under the same type. The occur- 

 rence of so many species of the same floral type, and of 

 what may be called modern habit of growth, over a 

 common geographical area in the southern part of the 

 continent, while the apparently ancient brevifolia is 

 restricted to the desert region in or adjacent to California, 

 and the seemingly more highly differentiated Hespero- 

 yuccas occur only in the mountains near the latter region, 

 though each has its specific Pronuba quite different from 

 the one associated with the Eastern Yuccas, would lead one 

 to surmise that the geological record, if it had been pre- 

 served, would show that the first of the Yuccas were of 

 wide distribution, probably extending across the continent 

 on the higher parallels while the northern climate was less 

 rigorous than it now is, but receding to the south under the 

 advancing glacial cold. Indeed it seems reasonable to 

 suppose that the ancient type as represented in brevifolia, 

 with an equally ancient type of Pronuba, has persisted in 

 the Pacific region throughout, perhaps owing to the series 

 of circumstances which have led to the preservation of the 

 Sequoias in the same region,* while the more widely 

 distributed species became differentiated and, with their 

 pollinator, passed to the south under new conditions. 



• See Gray, Presidential Address, Dubuque meeting American Asso- 

 ciation for the Advancement of Science, —Proceedings, xxi. 1. 



