462 ZOOLOGY 



E. TROPICAL. The region without frost at any 

 time of year. It reaches the mainland of the United 

 States only at the southern extremity of Florida. 

 Parts of southern Arizona and California, in the 

 valley of the Colorado River, have a mean annual 

 temperature which would entitle them to be considered 

 tropical, but they must be excluded, as they have 

 winter frosts. The most characteristic plant of the 

 tropics is, perhaps, the coconut palm, which fringes 

 tropical shores all round the world. 



References 



Cyclopedia of American Agriculture, Vol. I (1907), page 20. For details 

 concerning the distribution of life in North America see especially the 

 Bulletins of the Biological Survey, United States Department of Agri- 

 culture, Washington, D. C. 



