30 
BOTANICAL GEOGRAPHY, 
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 gnany similar forms offered by the 
tribes of Ranunculaeca, 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? 
