THE EVOLUTION OF THE PLANT-WORLD 21 



are undoubtedly the dominant type of vegetation to-day. 

 Their attainment of this predominant position follows 

 singularly soon, geologically speaking, upon the first 

 evidence we have of their existence, 



In the interior of the United States there is an enor- 

 mous thickness of beds, mainly fresh-water and lacus- 

 trine, extending in age from Jurassic to Pliocene times, 

 and in area from Long Island to Alabama and from 

 the Mexican border to the Yukon. At the base of these 

 is a series known as the Potomac formation, the lignites 

 in which have yielded several successive assemblages of 

 plants of the greatest possible interest. Unfortunately 

 we have to rely for their affinities mainly upon the 

 outline and veining of fragmentary fossil leaves. Of 

 upwards of 700 named species, nearly half are described 

 as Dicotyledons; but the leaf -characters seem also to 

 clearly indicate the presence of Monocotyledons. The 

 age of the lowest beds containing Angiosperms is appa- 

 rently Neocomian (Lower Greensand), and leaves, also 

 apparently belonging to Angiosperms, have been de- 

 scribed from beds of similar age in Portugal and in 

 England. 



Higher in the Potomac series, in Upper Cretaceous 

 beds, perhaps of Cenomanian (Upper Greensand) age, 

 the Dicotyledons have attained a marked predominance, 

 numbering 80 per cent, of the species, and comprising 

 such genera as Salix, Populns, Quercus, Juglans, Myrica, 

 Magnolia, Liriodendron, Ficus, Sassafras, Eucalyptus, 

 Acer, Ilex, Cczsalpinia, Bauhinia, Colutea, Aralia, and 

 Andromeda. These are mostly deciduous trees indica- 

 tive of a warm temperate climate, and are so largely 

 American as to show the continuity of the American 

 flora to be greater than that of the European. Gamo- 

 petala are few and Composites absent. As Cenomanian 

 plants have also been described from Greenland, com- 

 prising 30 Ferns, 8 Cycads, 27 Conifers, and 97 Dicoty- 

 ledons, also suggesting warm temperate regions, we may 

 say that this flora extends through 30 of latitude in the 

 Northern Hemisphere, which gives rise to a probable 

 inference that its centre of origin and dispersal was in 

 this region. 



In Saxony eight species of Proteacea have been 

 described from beds of this age, and in Bohemia seven- 

 teen species of Araliacece, besides Bombacece, Ebenacece, 



