1877.] 



President's Address. 



445 



us to be threefold, and to be intermixed throughout the continent — an 

 endemic American, a European, and an Asiatic : it seemed that the flora 

 was a ternary compound, so to speak, while that of the temperate Old 

 World was, in a continental point of view, binary — Europe and Asia 

 having many types in common, but very few representatives of the strictly 

 American flora. The distribution of North- American plants, unlike the 

 European, is mainly in a meridional direction, the difference of the floras 

 of the Eastern, Central, and Western States being wonderfully great—far 

 greater than those of similarly situated regions in the Old World. The 

 European components extend over the whole breadth of the continent, 

 diminishing, however, to the westward. The American components present 

 many localized genera, inhabiting the Eastern, Central, and Western States 

 respectively; they increase in numbers and peculiarity, as also in restriction 

 of range, towards the west. The Asiatic components are found both in the 

 Eastern and Western States, but hardly at all in the Central ; and some 

 of them are common to both the east and west, while others are peculiar to 

 each. But whereas the European components prevail on the side towards 

 Europe, the maximum of Asiatic representation is on that remote from 

 Asia. This has been conspicuously shown by Gray's discovery, in the 

 Eastern States, of single representatives of Japanese genera previously 

 supposed to be monotypic ; and what is most noteworthy is, that such 

 representatives are in some cases extremely rare and local plants, found 

 in single and very restricted areas, indicating a dying- out of the Asiatic 

 representation in America. 



The evidences of climatic changes in past eras of the existing flora 

 of the continent are seen in the prevalence of arctic and northern 

 species of plants in the alpine zones of the meridional mountain-chains, 

 the Appalachian, Eocky Mountains, and Sierra Nevada, even as far south 

 as the 33rd parallel. These plants had spread southwards during a period 

 of cold, and on its subsequent mitigation had retired to the lofty situations 

 they now inhabit. To the former existence of a warmer cliinate we may 

 partly look for the extension of Mexican types to the dry regions west 

 of the Rocky Mountains up to the 41st parallel ; and to it may be attri- 

 buted the remarkable northward extension of the Cacti in a very narrow 

 meridional belt, scarcely one hundred miles broad, along the eastern flanks 

 of the same mountains, from their head-quarters in New Mexico, in the 

 33rd, almost to the 50th parallel. 



Of existing influences that determine the development in amount of 

 the vegetation of a country, and the extension in various directions of its 

 components, none are so powerful as the distribution of rainfall and of 

 vapour in the atmosphere. This subject will repay a careful study in 

 America, especially in connexion with the presence or absence of wood- 

 lands and forests, an excellent map of which by Professor Brewer, of 

 Newhaven, was published in 1873 by the Supreme Government, in which 



