30 Where did Life Begin? 



The impassability of mountain ranges for 

 most plants is shown by the fact that strongly 

 marked varieties in great numbers and many 

 distinct species occur upon the eastern slopes 

 of the Rocky Mountains', the Sierra Nevadas, 

 the Alleghanies, and even lower ranges, which 

 are not found at all upon their western sides, 

 and vice versa. Such a condition of things, 

 incompatible as it is with an eastern and 

 western migration, is quite consistent, how- 

 ever, with a north and south movement. For 

 all the climatic conditions, especially that of 

 rain-fall, are so different on the opposite sides 

 of all long mountain ranges, that the same 

 variety split and separated by the northern 

 extremities of these ranges would, in moving 

 southward along their eastern and western 

 sides, and encountering such diverse condi- 

 tions, have become in the course of time, under 

 the laws of adaptation, distinct varieties, and 

 probably different species. 



It may be well now to examine some of the 

 conditions assisting this movement. Hot air 

 being lighter than cold, the heated air of the 

 northern equatorial belt has always risen and 



