40 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser, 



4. Evaporation from moist soil. This likewise depresses 

 the temperature and works in harmony with No. 3. The 

 effect is also noticeable around lakes and moist depressions. 

 The influence is sometimes very local. It is even possible 

 that in tall forests the tree species may belong to one life- 

 zone whereas the communities beneath these trees may con- 

 tain herbaceous plants and terrestrial animals from the zone 

 next above. In such cases we must consider, in addition to 

 evaporation, such influences as shading, the movement of 

 cold air, the effect of cold water, etc. 



5. Proximity to large bodies of water. Water is usually 

 an equalizer of temperature, but the presence of unusually 

 cold or warm bodies and ocean currents must be taken into 

 account. 



6. Influence of lingering snow hanks and of glaciers. 

 Because of peculiar topography snow accumulates much more 

 abundantly in some places than in others. These deep snow 

 fields require a longer time for their complete melting, or 

 may persist throughout the summer, thus acting as refriger- 

 ators for the immediate neighborhood. The effect is often 

 very local and gives rise to a "spotting" of life-zones which 

 cannot be accounted for on other grounds. Glaciers may 

 produce similar effects. 



7. Changes in the vegetable covering. Forest fires, ava- 

 lanches, and other denuding agents may result in a change 

 from a previously existing life-zone to the one next below 

 (Merriam, 1899, p. 51). As the laws of plant succession 

 become operative the species will again change to those of 

 the climax formation, and there will be going on in the 

 meantime a nice adjustment between temperature conditions 

 and plant covering, each acting upon the other. In such 

 cases relicts are sometimes present. These may persist for 

 many years side by side with species brought in by the 

 changed conditions. Such phenomena are exceedingly per- 

 plexing to the student of life-zones. 



8. Extent of a mountain area. The larger a mountain 

 mass the less will be the effect of surrounding influences. 

 For example, in a desert region the zones will occur at 



