yearly or daily range of temperature unfavorably affects the life ofyan 
area by weeding out the forms most sensitive to such changes, on the 
principle of the “survival of the fittest”. 
The heat of a locality is affected by its latitude, altitude, direction 
of the prevailing winds, height above base level and slope exposure. 
Increase of latitude and altitude produce similar climatic effects, the 
higher area having a similar climate to that of the lower area 
situated a certain distance further from the equator. In other words, 
a traveler passing from the tropics toward the poles at sea level finds 
the climate steadily becoming colder; in climbing a mountain the 
same change is observed. 
If the area of high altitude is great it is warmer than a small 
similar area at the same height and latitude, for the reason that the 
greater area conserves the greater amount of heat as daily received 
from the sun. It sometimes happens that the base level on one 
side of a mountain range is higher than that on the other side; in 
this case the higher level tends to raise the temperature and therefore 
the life zones on that side. A good illustration is the Himalaya 
Mountain range. ‘The plain on the south side is several thousand 
feet higher than the plateau on the north side; in consequence of 
this difference of base level on the two sides the timber line and 
snow line are about three thousand feet higher on the north than on 
the south side. This is in direct opposition to the effect of latitude 
which would tend to lower the snow line on the north side.  ‘I‘he 
Sierra Nevada Mountains are another illustration. ‘The plateau on 
the eastern side is from three to four thousand feet higher than 
the San Joaquin and Sacramento valleys on the west side, and in 
consequence all the life zones are higher on the east side than on 
the west. 
Slope exposure is another disturbing cause. A slope directly fac- 
ing the sun is warmer than one facing away from it. his is very 
noticeable in many canyons running east and west in semi-arid parts 
of California, in which case the timber will be found growing con- 
siderably lower down on the side receiving the least amount of direct 
sunshine. 
Prevailing winds coming directly from large bodies of water tend 
to cool the region contiguous and therefore lower the life zones. 
The next most important agent in the distribution of life is mois- 
4 . . . 
ture. ‘The greater or lesser amount of moisture present in air and 
