Change of Aspect with Altitude. 247 



two directly opposing aspects is a difference in insolation. 

 Although precipitation and atmospheric conditions may inter- 

 fere, the same primary cause is oj^erative in change of aspect with 

 altitude. The sim is the direct source of both heat and light, 

 and, aside from precii)itation, modifies to a large extent the 

 moisture supply in both soil and air. A high slope receiving 

 direct rays will in its conditions be equivalent in some measure to 

 a lo\\ slope with oblique rays. 



The following remarks, it is needless to say, are based on 

 much more observation than is detailed above. It has been 

 suggested that some of our species do not occupy as much ground 

 as they are capable of doing, either within or without their 

 geographic areas, and that the way is open for their active ad- 

 vancement and invasion of unoccupied areas. However well 

 this may fit plants under cultivation and a certain class of do- 

 mestic weeds, whose living conditions are made for them, it 

 certainly can not hold true of the wild plants that lend the 

 Southv/estern desert and mountains their true physiognomy. 

 Go where vou will, the recurrence of the same kind of habitat 

 is almost invariably accompanied by the reappearance of the 

 same plants and plant associations. In the desert, the same 

 soil and other features of ph}siography bear quite generally 

 the same societies. In the mountains, without instrument or 

 map, one can determine the altitude within a very few hundred 

 feet by the species he meets on the slopes. If direction should 

 be lost, one can quickly re-orient himself by the plants found 

 on the various aspects. Every similar situation bears a similar 

 coterie of species, and a very few species give any indication of 

 movement, either forward or backward. 



As seen above, many of the variations found in some species 

 are distributed according to the different altitudes and aspects, 

 and also to the geology and soil. Both in form and local dis- 

 tribution, there is constantly apparent a close fitting of the plant 

 to the place ^^here found. Local ranges of the various species, 

 which are usually circumscribed areas within their geographic 

 ranges, are exceedingly well defined, each corresponding to a 

 certain range of habitat conditions. These conditions consist 

 of various components, chief among which are moisture and 

 temperature. As already indicated, in their effects upon the 



