116 ELEMENTARY BOTANY. 



determined by temperature are manifest. The tropics, the 

 temperate and frigid zones, and the mountain sides having 

 corresponding temperatures, have their peculiar species. The 

 numerous Rusts, Smuts, Leaf-mildews, Blights, and other para- 

 sitic Fungi are necessarily limited to the regions in which their 

 hosts grow. 



9. In widely separated regions, but whose soil, climate and 

 other conditions are similar, different species of plants gener- 

 ally occur. Exceptionally it may be found, however, that a 

 given species occurs in two or more such regions, between 

 which now are great distances and impassable barriers. The 

 floras of the two regions must therefore have had a common 

 origin. Recognizing that the geologic or past distribution of 

 species has been an important factor in determining the geo- 

 graphical or present distribution, the area of the globe ma}' 

 be separated into four botanical divisions, namely : (1) North 

 Extra-tropical, (2) Paleotropical, (3) Neotropical, and (4) 

 Southern or Antarctic. Each of these divisions has been 

 separated into regions ; each region is divided into prov- 

 inces and each province into zones. 



10. The North Extra-tropical division coincides with the 

 area of glacial action. Its southern limit is bounded by a 

 line running through the northern part of Africa, crossing 

 Arabia, passing along the shores of the Persian Gulf, thence 

 extending northward above Afghanistan and India, following 

 the south range of the Himalaya Mountains, passing through 

 China and Japan (including most of the latter), then continu- 

 ing north of Mexico and south of the Southern States, emerg- 

 ing between Florida and Cuba. Throughout this region in 

 both the eastern and western hemispheres fossil plants have 

 been found identical with those now living, for example Bald 

 Cypress (Taxodium distichwn). Several have received different 

 names but cannot be separated from living species ; for ex- 

 ample Sequoia langsdorfii (fossil) is not separable from Sequoia 

 sempervirens (the Redwood). And while such fossil species as 

 Carpinus grandis, Corylus mac-qunrrii, Alnus kcfersteinii, do not 

 wholly coincide with the living Water Beech (Carpinus ameri- 



