212 THE AMERICAN NATURALIST. (VOL. XXXIII. 
of the valley. Thus the total daily variation on the hill was 20° to 
30° F. less than that of the valley. 
Now, the northern advance of southern plants is governed by the 
sum of the positive temperatures, or the sum of the temperatures, 
above that at which plants and animals start into activity in the 
spring, taken throughout the entire season of growth and reproduc- 
tion; and the So0uthward distribution of northern plants is governed 
by the mean temperature of a brief period of a few weeks during the 
hottest part of the summer. It is obvious, therefore, that, as the 
positive temperature of the hills and mesas is greater than that of 
the valleys, the southern plants should find their way farthest north 
along the minor ridges and hills. At the same time the average tem- 
perature of the valleys is lower than that of the ridges, and hence 
the northern flora should reach its southernmost limits down valleys 
heading up in mountains and mesas favorable to the development of 
the greatest effects of inversions of temperature. 
The influence of inversions of temperature is, therefore, to make 
extremely sharp deflections of the zonal boundaries, which may extend 
only a short distance locally, or which may reach over a hundred kil- 
ometers from the general limits of the zone. This conclusion is sup- 
ported by my own observations in Arizona, and by facts concerning 
the flora and fauna of New Mexico and Texas cited by Professor 
Townsend. 
Ascending currents of air also cause changes in humidity which 
exercise an extremely local influence on the distribution of the mois- 
ture-loving forms. As the diurnal warm current ascends the slope 
` of a hill or the walls of a cañon, it expands and loses heat, and at the 
same time the dew point is lowered or the relative humidity is in- 
creased. When the current reaches the level of the highland it flows 
over it as a moist and cool wind. It is gradually warmed again, how- 
ever, and its dew point raised in a few kilometers, progress. As a 
result of this action the area bordering upon a cañon, gulch, or val- 
ley offers a much more humid atmosphere than regions more removed, 
and hence these portions are most suitable for the —— 
species. 
' This soared is beautifully illustrated by the disteibvasion of Raz- 
oumofskya vaginata (Willd.) Kuntze, which is parasitic on Pinus pon- 
derosa var. scopulorum in this region. Although native of a semi-arid 
region, the parasite is most successful in the germination of its seeds 
and the attachment of the seedlings to the host plant in a humid 
atmosphere. While it is found throughout the pine belt, it is most 
