50 THE WANDERINGS OF ANIMALS [OH. 



likewise extensive lowlands. The Biota of such a 

 mountain will be a compound from two perhaps very 

 different sources. 



Or the mountain itself may have an original Biota 

 of its own, especially if the mountain is due to a 

 process of folding or other rising of the crust of the 

 globe, instead of being a piled-up volcano, which 

 would of .course begin as an absolutely barren terrain. 



Lastly, the most complicated conditions prevail 

 if the mountain forms part of a long range extending 

 into distant countries. In this case it is an impor- 

 tant consideration, whether such a range runs from 

 north to south like the chief systems in America, or 

 from west to east as they do in the Old World. 



Whilst west to east, latitudinal or zonal ranges 

 often act as powerful barriers, shutting off a southern 

 from a northern Biota, a north to south, longitudinal 

 or meridional range more often acts as a conductor 

 in the exchange of southern members to the north 

 and for northern migrants into the south. This is 

 especially the case, when, according to the law of 

 latitudinal equivalent in altitude, such a range 

 begins low in the north and increases in height 

 towards the south. An excellent illustration is the 

 great central plateau of Mexico which begins at the 

 northern frontier with a few thousand feet elevation 

 and steadily rises to 7500 feet near Mexico City and 

 there forming the northern base of some of the giant 



