20 MR. J. BALL ON THE ELORA 



jectural, and in eacli case should be considered separately with 

 reference to the facts of its distribution and its structural affi- 

 nities. 



The class which I have designated American genera, including 

 those which are widely spread through both portions of the con- 

 tinent and, with the exception of a few representatives in North- 

 eastern Asia, do not extend to the Old AVorld, forms a considerable, 



though not a preponderant, constituent in the Andean flora. 

 When we consider that, although subsidence has probably at various 



times separated the two portions of the continent, the highlands 

 of Mexico and Central America have, in all probability, served 

 during long periods as a bridge over which some portions of the 

 mountain vegetation may have been transferred from North to 

 South and vice versa ] we are led rather to feel surprise at the^ 

 amount of separateness now existing than at the presence of 

 many genera and of a few identical species in the floras of the 

 Andes and that of the Eocky Mountains. It is true that I have 

 reclvoncd as Andean genera and species many that extend north- 

 ward as far as Mexico, and it may well be that that region, so 

 rich in varied forms of vegetation, is the original liome of some 

 that now appear to be more fully developed in the mountain 

 ranges of "Western North America. Among the wide-spread 



American types we must note tAvo natural orders whose original 

 home may, with some confidence, be placed in the north-western 

 part of the continent. The Polemoniace(S^ of which about 140 

 species belong to that region, are represented in the Andes by 

 five species of Gilia^ one of CoUomia^ and by the endemic genus 

 Cantua, They have sent to the Old "World two or three species 

 of Phlox in I^orthern Asia, and a single emigrant which has 



reached Britain, the Jacob's Ladder of old-fashioned gardens, 

 which maintains a struggling existence in several isolated spots 

 in Europe. The other specially American family is that of the 

 HydropliifUacece^ of which 12 genera are known in North America, 

 but Avhich is represented in the Andean chain only by four species 

 of Fhacelia. An opposite phenomenon is presented by the 

 natural order of Loasacece^ which we may, with much probability? 

 hold to have had its origin in the mountain regions of South 

 America. The genus Menfzelia, which extends in South America 

 from the plains of Patagonia through the temperate zone of the 

 Andes, must at some remote period have travelled northward to 

 Mexico, and thence to temperate North America. It has ther^ 



