KO. 2226. 



DISTRIBUTION OF ENTOMOSTRACA—DODDS. 



63 



on the slopes above, or extend out over the lakes on the ice, and have 



a great deal to do with keeping the temperature of the lakes low 



throughout the summer. 



Temperature. — Temperature is probably the climatic factor which 



in this region plays the largest part in determining the distribution 



of animal life, and it is the factor which within our area is subject to 



the greatest variation. On the plains we have the conditions which 



are prevalent throughout temperate latitudes, while in the higher 



parts of the mountains there is a close approach to arctic conditions. 



For purposes of comparison between different parts of the area mider 



study, I have made use again of the three stations — Denver, Frances, 



and Corona — the elevations and temperatures of which are shown in 



Table 2. 



Table 2. — Mean annual temperature. 



The conditions recorded at Denver are representative of the plains 

 in general, and those at Corona of the highest lakes studied in this 

 region, so that the difference between these two stations expresses the 

 divergence between the two extremes of lakes. It is seen that the 

 mean annual difference between these two stations is 23.8° F., whicn, 

 allowing 1.35° F. as equivalent to 1° of latitude, corresponds to 17.2° 

 latitude. Thus, though Corona is distant from Denver but 40 miles, 

 it has an annual mean which might be expected 1, 200 miles to the 

 north. It is this steep temperature gradient that gives interest to 

 studies in this region. 



As a matter of fact, the isotherm corresponding to the temperature 

 of Corona does actually pass through these far northern regions 

 as may be seen by reference to map (fig. 4), while that of Frances, 

 though less extreme, also pusses well to the north, at one place touch- 

 ing the Arctic Circle. In this map it is to be noted that the isotherms 

 are drawn as reduced to sea level, so that the effect of elevation is 

 already felt at Denver, where the actual temperature is probably 15° 

 F. below the corresponding sea-level temperature shown on the map. 

 An isotherm map, not reduced to sea level, would show all lines bend- 

 ing far southward over the Rocky Mountain system, as a result of 

 which the isotherm of 26° F. would actually pass through Corona, 

 while at Denver, only 40 miles to the east, would be that of 50° F. 



The mean distribution of temperature throughout the year at these 

 three stations is shown in fig-ure 5. 



