42 BIOLOGICAL LECTURES. 



and the diurnal ascending currents complement each other in 

 reducing the extreme of the temperature of the highlands and 

 in increasing the daily variation of the canons and valleys. 



The phenomena discussed in the preceding paragraphs are 

 well known to the meteorologist, and may >e found more or 

 less thoroughly exploited in text-books upon that subject, and 

 the above lengthy consideration of them is given in order that 

 their application to the distribution of life and the limits of 

 zones may be better and more easily comprehended. 



It is presumably a fairly established conclusion with bio- 

 geographers that the northward distribution of southern plants 

 and animals is governed by the sum of the positive tempera- 

 tures for the entire season of growth and reproduction, checked 

 of course by the low temperatures of the winter season. Posi- 

 tive temperatures are computed by adding the excess of tem- 



FIG. 2. Showing drainage of cold air into a valley. 



perature above the point at which plants start into activity in 

 the spring. The season during which the positive temperature 

 would be of moment would embrace only a few weeks of the 

 hottest part of the summer (Merriam, " Life Zones and Crop 

 Zones," Bull. No. 16, U. S. Dept. of Agriculture, Div. of Biol. 

 Survey). The limiting low temperatures might occur at any 

 time in the winter season. 



It is also generally accepted that the southward distribution 

 of northern plants and animals is governed by the mean tem- 

 perature of a brief period during the hottest part of the year. 



Now it needs but a glance to bring us at once to the con- 

 clusion that the valleys have a lower mean temperature and a 

 lower minimum temperature than the adjoining hills, and that 

 they are therefore better suited to the southern advance of 

 northern species than the reverse. Again, the higher minimum 



