THE RELATIONS OF THE NORTH AMERICAN FLORA TO THAT OF SOUTH AMERICA.
Reviewing the floral relations of North and South America as illustrated in the foregoing instances, we may say that the phenomena of distribution agree fairly with the record of physical conditions which have succeeded each other and those which still exist, and upon which we might almost a priori have predicted an analogous set of distribution phenomena. In this relationship we may distinguish three categories of distribution: (1) Those due to the conditions of human civilization, commerce, etc. This has resulted in placing the same species in similar regions of both continents, as, for example, Fagonia cretica in Lower California and Chile; Munroa squarrosa, western plains of North America, plains of Argentine and high plateaus of Chile and Bolivia; Frankenia grandiflora, Southern California and Arizona, coast lands of Chile; Oxytheca dendroidea, Lastarricea chilensis, and Chorizanthe comrnmissuralis, all in Southern California and Western Chile. (2) Those due to the operation of natural causes acting unde present conditions of climate, geology, etc. Under this head may be cited such species as sida leprosa, hastata, anomala, Cienfugosia sulphurea, Spergulariaplattensis and, in general, elements of Gulf zone distribution; also certain elements which still find a pathway along the continental axis, including some alpine and mountain xerophilous genera. (3) The third category of distribution would include those phenomena due to geological and climatic changes acting through long periods. Under this head are included the elements of greatest significance in the relationsip of the North and South America floras. The endemic boreal flora of the Andes, the equally endemic boreal flora of the Mexican Cordilleras, and genera with sharply distinct species or sub-genera in the arid extra-tropical regions of both continents, which may be called remnant elements.
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