12 Life-histories of Northern Animals 



all life conditions are changed suddenly, is the freezing point 

 of water, and the two most important divisions of North Amer- 

 ica are shown by lines drawn across the continent indicating: 



(a) The region in which water never freezes, and 



(b) The region in which water is always more or less 

 frozen; 



Or in other words: 



(a) The south limit of frost, and 



(b) The south limit of perpetual frost in the ground that is 

 exposed to the direct rays of the sun. 



These lines demark respectively the north edge of the 

 Tropical and the south edge of the Arctic Realms; the region 

 between is the Temperate Realm. Thus we have the primary 

 division of the northern hemisphere, into Tropical, Temperate, 

 and Arctic Realms, corresponding with the distribution of plants 

 and animals, and that portion of each which belongs to North 

 America is called a Region. 



I The Tropical Region 



If the earth were flat and without currents of air or water, 

 the north boundary of the Tropical Realm would coincide 

 exactly with the geographical Tropic of Cancer (N. Lat. 23 1°), 

 which is the northernmost limit where at some time each year 

 the sun is direct overhead. It is virtually this line modified by 

 local influences, pushed northward by currents of warm water 

 and southward by cool high uplands. 



This is the region where frost is unknown, where the 

 summer is long and hot, and where there is no cold winter. 

 It is the land of the palm-trees, the parrots and monkeys, the 

 home of the black human races. 



(Bull. U. S. Gcol. and Geog. Survey Terr., 1878, Vol. IV, No. 2), his Geographical 

 Distribution of North American Mammals (Bull. Am. Mus. Nat. Hist., Dec. 29, 1892, 

 read Jan. 26, 1891), Dr. C. Hart Merriam's Geographical Distribution of Life in North 

 America (Proc. Biol. Soc. Wash., April 13, 1892), and Life Zones and Crop Zones of 

 the United States (Bull. No. 10, Biological Survey, 1898); with assistance in California 

 from Joseph Grinnell's maps in Pacific Coast Avifauna, No. 3, 1902. 



I have taken Dr. Allen's nomenclature as a basis, adopted Dr. Merriam's main 

 lines of division, and, for the region north of the northern boundary of the United 

 States, have proposed some new names and subdivisions. 



