50 The Plant World. 



origin and movements of the plants here represented it may be 

 assumed that a large contingent of Sonoran species had its 

 origin in the immediate region, some of them possiblv on the verv 

 ground of the Desert Laboratory domain. Certain other species, 

 relatively few in numbei, belong to genera which exhibit their 

 greatest morphological differentiation and most numerous 

 representation in the Northwestern and Western United States. 

 .Still other plants have a history interwoven with the geology 

 of Western America, which at present can only be stated in 

 general tei ms. At a relatively early day, possibly not later than the 

 Pliocene, there seems to have been a continuous highway, 

 from Southern Mexico, along the Isthmus of Panama, 

 southward to Argentina and Patagonia, by which the creosote 

 bush, various cacti, yuccas, and mxcmbers of other groups repie- 

 sented on the Laboratoiy domain at the present day, made their 

 southward journeys. In the northern hemisphere the later ad- 

 vance of the great ice sheet in the Pleistocene drove southward 

 many plants, some of which are found today on Tumamoc Hill, 

 but in general they have made the mountains their pathway 

 and also their home, and their minor movements can not well 

 be traced. 



Of this paper as a whole it may be said that the study of 

 local distribution has been carried far enough to warrant a 

 number of important conclusions. It was evidently the purpose 

 of the author to lay the foundation for a broader study by as 

 exact and thorough an investigation of a restricted area as could 

 be made, and in the carrying out of this undertaking he has been 

 fortunate in associating with himself a number of collaborators 

 whose work has furnished data of permanent value; but the 

 wider field, indicated by the title of the paper, still remains, in 

 great part, to be worked. 



NOTES AND COMMENT. 



We greatly regret the occurrence of several mistakes in the 

 article by h. R. Waldron on "Heredity in Populations in Pure 

 Lines," in the January number of the Plant World, due to the 

 fact that proof was not submitted to the author. The errors 

 to which reference is made are to be corrected by the omission 



