LIFE ZONES 455 



to define the zones by these isotherms, but the matter 

 is actually much more complicated. In the first place, 

 temperature is not the only factor; moisture is ex- 

 ceedingly important. The desert and forest may have 

 the same mean annual temperature, may be on the 

 same isotherm, yet they differ entirely in their life, 

 perhaps hardly possessing a single species in common. 

 Then again, it makes a great deal of difference when the 

 moisture falls, and when the cold and warm weather 

 occur. Dr. Merriam laid special emphasis on the 

 total amount of heat received during the growing 

 season of plants, and on the other hand the minimum 

 winter temperature is a decisive factor for many kinds. 

 Even the variations between day and night are very 

 important. Thus in cloudy localities the temperature 

 in the spring may differ comparatively little during 

 the twenty-four hours ; but in the arid Southwest, 

 where the skies are clear, the rapid loss of heat at night 

 may give rise to killing frosts, following uncomfortably 

 warm days. The amount of evaporation .is a factor 

 which cannot be ignored, and it depends upon the 

 moisture in the atmosphere and the movement of the 

 air, as well as on the actual temperature. Finally, in 

 some places we find what is called inversion of tempera- inversion of 

 ture, the tops of the hills or sides of the valleys being 

 actually warmer than the lowlands. In such a locality 

 as Salt River Valley, Arizona, this is due to the fact 

 that cold air is heavier than warm, and so sinks, dis- 

 placing the warmer air much as water would. Growers 

 of oranges know that the sides of the valley are less 

 liable to injurious frosts, and the value of land is 

 affected by these considerations. In the vicinity of 

 San Francisco, California, the same general result is 

 brought about by the sea fogs. Thus the summit of 



