DISTRIBUTION IN TIME AND SPACE. 6 1 



animals and plants. These limits are by no means sharply- 

 defined, for wherever the limit is made some species will pass 

 beyond it ; yet the majority found on one side are different 

 from the majority of those on the other. North America (ex- 

 cluding Mexico) forms the Nearctic realm or fauna {Regnum 

 Nearcticuni), and the same boundaries may be used in the limi- 

 tation of our fern flora, although some species from tropical 

 regions invade our borders in Florida, Texas, and Arizona. 

 Leaving out of question the species that are widely distributed 

 over the greater part of our country, many of which are cos- 

 mopolitan species, we may divide the Nearctic realm into five 

 provinces, each of which possesses many species peculiar to itself. 

 139. The provinces* are as follows : 



I. Boreal : inhabiting (with a few exceptions) the northern 

 portion of the United States, extending through Canada and 

 British America, some species even reaching Labrador, Green- 

 land, and Alaska, and nearly all represented also in the north- 

 ern portions of the Old World. 



n. Medial : extending throughout the mountain and hilly 

 region of the States east of the Mississippi, westward to the 

 mountains, and northward into Canada, and in a few instances 

 also inhabiting the Old World. 



in. Occidental : extending along the western border of 

 the continent from British Columbia to California, in a few 

 cases appearing also in the Rocky Mountain region. 



IV. SONORAN : inhabiting the central mountain regions of 

 Western Texas, Arizona, and Colorado, many of the species ex- 

 tending thence into Mexico, and some even to South America. 



V. Austral : inhabiting the border of the Gulf of Mexico, 

 many of the species extending into the West Indies and Tropi- 

 cal America. 



I4-0. Geologic Distribution. — It is well known that the 

 plants and animals now existing on the earth are not the same 

 in kind as those of former ages. Geologists have carefully 

 studied the stony heart of nature, and have drawn therefrom 



* This division is a slight modification of one proposed by John H. Red- 

 field in 1875. Cf. Bulletin Torrey Botanical Club^ VI, 1-7. 



