The Policemen of the Ah 



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riitto by \ 1 h ^^^CoIm 

 VgRY YOUNG COTTONTAII, RABBITS IN NEST 



Among other results it was demonstrated 

 that the laws governing the distribution 

 of mammals, birds, reptiles, insects, and 

 plants are essentially the same. Hence a 

 map showing the boundaries of an area 

 inhabited by an association of species of 

 one group serves equally well for the 

 other groups. Comparison of the facts of 

 distribution as noted on this mountain 

 with corresponding facts over the country 

 at large disclosed three important truths : 

 (a) That the several life zones of the 

 mountain could be correlated with cor- 

 responding zones long recognized in the 

 eastern United States; (b) that these 

 same zones are really of transcontinental 

 extent, though never before recognized 

 in the West; and (c) that the faunas and 

 floras of North America as a whole, and, 

 for that matter, of the Northern Hemi- 

 sphere north of the tropical region, are 

 properly divisible into but two primary 

 life regions — a northern, or Boreal, and 



a southern, or Austral (then termed 

 Sonoran), both stretching across the con- 

 tinent from ocean to ocean. 



Subsequently a careful study of the 

 geographic distribution of plants and 

 animals was undertaken, to include the 

 whole of the United States and, where 

 necessary, the reg-ion contisruous.* 



The practical use of zone maps is 

 easily understood. If, for instance, it is 

 ascertained that a certain crop thrives in 

 one part of a particular zone, it is to be 

 expected that elsewhere within the zone, 

 where similar local conditions prevail, 

 the same or a closely allied crop will do 

 well. As each zone includes thousands of 

 square miles, the value of such informa- 

 tion is obvious. 



The final step toward making such 



* The first announcement of the laws of tem- 

 perature control of the geograpliical distribu- 

 tion of terrestrial animals and plants was made 

 in this Magazine, vol. vi, 1894. 



