30 BOTANICAL GEOOUAPIIV. 



over the whole continent, would form an inte- 

 resting theme, which although partly conjec- 

 tural and never quite exact, might lead to phi- 

 losophical reflections and deductions. Some 

 problems remain to be solved on the subject. 

 Why are our North American trees chiefly of 

 European and Siberian Genera, but unsimilar 

 in species? 



Why are our shrubs still more dissimilar both 

 in genera and species? 



Why are our grasses and cyperacea so often 

 similar in species, while the other monocotyle 

 plants are all different? 



Why are the minute and simple Acotyle 

 plants, such as Algas, Lichens and Fungi, near- 

 ly identic in the two Continents ? 



Why are Mosses and Hepatica so often si- 

 milar, while the ferns are less so, and offer 

 many new forms ? 



Why are many similar forms offered by the 

 tribes of Ranunculacca, Labiate, Juncides, Ro- 

 sacea, Polygonides, Lysimachides, Veronicides, 

 Borragines, Bicornes, Caryophyles, Amenta- 

 cea ? &.c. 



Why are so many dissimilar genera and spe- 

 cies found among the Violacea, Irides, Aspho- 

 delides, Orchides, Gentianides, Apocynea, 

 Corymbifera, Hypericea, Malvacea, &c. of both 

 continents ? 



Why are some families quite lacking in N. 

 America ? Such as Rutacea, Guttifera, Pro- 

 tacea, Epacrides, &c. 



Why do the tropical genera so seldom extend 

 into Florida ? 



Why is the vegetation of Boreal America to- 

 tally unlike that of Austral America? 



