THE PEAIEIES 209 



gument in a much neglected paper (1878) in which he not only 

 accurately described the striking distribution of the forest areas 

 in western Iowa, but considered forest and prairie in their rela- 

 tion to moisture as influenced by prevailing winds, temperature 

 and topography. He reached the conclusion that "the funda- 

 mental condition of forest-groicth is a constant medium humidity 

 of air and soil," the prairie of course developing where this 

 condition does not exist. He placed greater stress on the value 

 of moisture retained in the air and soil than on rainfall and 

 closed the paper with this significant suggestion : ' ' Let us there- 

 fore, while not neglecting our pluviometers look more carcfulli/ 

 to our hygrometers in our study of this subject." 



But in the great majority of the papers cited in which climatic 

 factors are considered they are given prominence individually, or 

 are represented as merely accessory. It will be most convenient 

 to consider these factors separately though by several authors 

 they were variously combined with other factors. 



Rainfall 



Of all the climatic factors rainfall has been given the greatest 

 prominence in discussions of the causes of the distribution of 

 forest and prairie. 



Cooper (1859) fixed upon a precipitation of 15 inches during 

 the growing season as determining the limit of tree-growth ; 

 Shaler (1891) concluded that an annual rainfall of less than 20 

 inches was fatal to trees; and Powell (1896) fixed the limit at 

 20 to -40 inches. But Dana (1866) had noted that prairies occur 

 even where the annual rainfall reaches 45 inches, and Whitney 

 (1876. 1882) showed that forests may persist even where the 

 rainfall falls below 20 to 25 inches. 



Other authors, while less specific, ascribed the cause of the 

 prairie wholly or chiefly to insufficient rainfall. Among these 

 were Vaughn (1856. 1860). Newberr>^ (1860). Hinrichs (1876) 

 and Hitchcock (1898). Dana (1849) considered dry summers 

 the cause; Schimper (1903) ascribed it to moderate rainfall, dry 

 winters and an early moist summer; Warming (1909^ sought it 

 in long winters and hot and dry summers; ]Marsli (1898) in 

 a want of spring and summer rains; and Gleason (1909) in a 



