INVERSIONS OF TEMPERATURE. 43 



temperatures of the ridges and hills, and the greater sum of their 

 positive temperatures would eminently fit them for the northern 

 advance of the southern species. Given then a great plain, cut 

 by valleys and canons, or the foothill region of a mountain 

 range, it may be definitely asserted that the southern plants 

 reach their northern limit on the highlands, and the northern 

 species reach their southern limit in the lowlands. Stated in 

 another form, it may be said that zonal boundaries are deflected 

 southward in valleys and northward on ridges and highlands. 

 This conclusion applies, of course, to topographical features 

 only which show differences in elevation of not more than 300 

 to 500 meters. Even with this limitation the configuration 

 of the country may be such as to deflect the zonal boundary 

 from its general course 100 kilometers or more ; a fact of very 

 great importance both in biogeography and also in economic 

 operations. 



The conclusions reached in the last paragraph are amply 

 supported by the observations of the writer in the San Fran- 

 cisco Mountain region in northern Arizona, also by numerous 

 facts of the distribution of plants and animals in an adjoining 

 region in New Mexico, which have been kindly cited me by 

 Professor Tyler-Townsend. 



Large numbers of species of northern plants are to be found 

 in the valleys of the San Francisco Mountains, while many 

 southern forms are to be collected beyond the general outline 

 of their zone on the southern mesas, hills, and spurs. The most 

 marked illustration of the descent of the northern flora is to 

 be found in the crater of the San Francisco volcano. The 

 eastern wall of the crater has been broken away at some time 

 in the last stages of its activity in Tertiary times, and the crater 

 is now a U-shaped valley, ten kilometers long and three to 

 five kilometers wide, heading up on the inner slope of the 

 mountain at an elevation of 3500 meters, and opening out to 

 the eastward on the plain at an elevation of about 2500 meters. 

 According to current conclusions the species of the pine or 

 transition zone should advance up this valley, especially along 

 the face of the northern slope or side of the valley which has 

 the most favorable slope exposure. The effect of the slope 



