VARIATIONS OF GARTER-SNAKES. 77 



the whole area is united into a uniform feature by its treeless con- 

 dition. It is the grass-land area of North America. In spite of this 

 uniformity, however, the division of the area into the plains and 

 prairie regions is a real if not a well-marked one, and seems to be due 

 to differences in the climatic conditions. 



Extending as it does from Texas nearly to the Arctic Circle, and 

 having an extreme width of about 18 degrees of longitude, the prairie- 

 plains region presents a considerable variety of climatic conditions. 

 In general it is characterized by hot summers and mild winters, but 

 extremes of temperature are not rare. The western part of the region 

 is quite arid, for much of the moisture carried by the prevailing 

 westerly winds that is not precipitated on the western slopes of the 

 Pacific Mountains is condensed and • precipitated as the air is again 

 compelled to rise on the western slopes of the Cordilleras, so that in 

 their subsequent course east of the Rockies these winds are desiccating 

 agents. The plains are thus characterized by small rainfall (less than 

 30 inches) and a large proportionate evaporation. Transeau (1905, 

 884) has shown that when the ratios between the rainfall and evapo- 

 ration are mapped for different localities, the plains are marked by a 

 rainfall equal to 20 to 60 per cent of the evaporation called for. This 

 in part accounts for the more xerophytic nature of its flora. Approx- 

 imately between the 97th and 98th meridian the rainfall increases to 

 the eastward above 30 inches, and the rainfall-evaporation ratios rise 

 to between 60 and 80 per cent, and the more favorable conditions are 

 marked by the presence of the more mesophytic grass formations of 

 the prairie region. Where the evaporation ratios rise above 80 per 

 cent the prairie region gives way to the dense forested area of eastern 

 North America. 



Specimens of radix with definite locality labels have been examined 

 as follows: Lake Winnepeg, Rush Lake, Regina, Canada; Pueblo, 

 Greeley, Fort Collins, Colorado; Du Page County, Sycamore, Chicago, 

 Berwyn, Lake County, Palos Park, Mount Carmel, Illinois; Clay 

 County, Palo Alto County, Ames, Iowa; Peabody, Fort Riley, Kansas, 

 Fort Snelling, Mankato, IVIinnesota; St. Charles County, St. Louis, 

 Madison County, Missouri; Threeforks, Miles City, Fort Benton, 

 Montana; Loup Fork, Fort Pierre, Nebraska; Pembina, Turtle 

 Mountains, Mouse River, North Dakota; Ilermosa, South Dakota; 

 Dallas, Texas; Racine, Madison, Kenosha, Wisconsin; "Bridgers 

 Pass," Fort Laramie, Wyoming. 



As we have previously said in the discussion of the geographic con- 

 ditions, the prairie-plains region is not to be distinguished from the 

 plains and prairies of central and western Texas. At the present time, 

 however, we have no evidence that the form enters the latter except 

 possibly in the extreme northern part, in Oklahoma and Indian Terri- 

 tory, although the southern margin of the range of radix, like the 



