16 Journal of the Mitchell Society \_July 



occur more or less abundantly in the Neo-tropical realm to the 

 southward. In reptiles it possesses pit vipers but no true vipers, 

 and among the former the rattlesnakes are exclusively Ameri- 

 can and predominantly North American. In the Amphibia 

 its characteristic species belong to the tailed forms among which 

 the large family Plethodontidse is exclusively North American 

 while the smaller families of Sirenidse, and Amphiumidse con- 

 taining the large eel-shaped salamanders are not found else- 

 where. 



THE LIFE ZONES OF NORTH AMERICA 



These are more or less parallel belts running across the con- 

 tinent from east to west, and are limited mainly by the mean 

 temperature of the region, which in its turn is determined by 

 the two factors of latitude and elevation. Of course other fac- 

 tors play a large part in determining the life of these regions, 

 the most important being the comparative humidity, in fact 

 this last element splits three of our southerly life zones into two 

 distinct portions, an eastern or humid division and a western or 

 arid division. 



These life zones are seven in number, the three northern 

 being cold or boreal in character, while the four southern are 

 warm or austral. Of course each zone grades into both the one 

 above and the one below, so that there is never a hard and fast 

 dividing line between any two contiguous ones, still in spite of 

 this each zone is fairly well characterized by the forms of life 

 or by the combination of forms occurring in it. 



The transcontinental zones are, — 



1. An Arctic Zone, forming the American portion of the 

 Arctic region, including all the country north of the northern 

 limit of trees. 



2. A Hudsonian Zone, including the northern half of the 

 boreal forest region. 



3. A Canadian Zone, including the southern half of the 

 boreal forests. These three zones cover by far the greater part 

 of Canada and enter the United States, mainly along its chief 

 mountain ranges at high elevations. 



