242 ENVIRONMENTAL CONDITIONS. 



(I) MERRIAM'S MEAN NORMAL TEMPERATURE FOR HOTTEST SIX WEEKS OF 



YEAR. (PLATE 44.) 



In the same paper (1894) from which we have already made extracts, 

 Merriam calls attention to the fact that, while his summation indices 

 (our plate 37) appear to furnish satisfactory criteria, for relating tem- 

 perature conditions to the northward limits of species distribution, yet 

 these do not seem at all satisfactory in connection with the southward 

 extension of northern forms. This author writes (1894, p. 233) : 



It is evident * * * that the southward range of Boreal species * * * is regulated by 

 some cause other than the total quantity of heat [i. e., his summation indices]. This cause 

 was believed to be the mean temperatm-e of the hottest part of the year, for it is reasonable 

 to suppose that Boreal species in ranging southward will encounter, sooner or later, a 

 degree of heat they are unable to endure. * * * For experimental pm-poses, and without 

 attempting unnecessary refinement, the mean normal temperature of the 6 hottest consecu- 

 tive weeks of summer was arbitrarily chosen and platted on a large contour map of the 

 United States, as in the case of the total quantity of heat. 



We here reproduce in its essentials, as our plate 44, the chart thus 

 obtained — Merriam' s (1894) plate 13 — because of its scarcity and of 

 its interest in connection with our own studies. The marked differences 

 between this chart and that of our plate 37 (also reproduced from 

 Merriam) are practically confined to the Pacific Slope. East of the 

 Sierra Nevada, Cascade, and San Bernardino Ranges the zone with a 

 normal for the hottest 6 weeks of above 79° F. (26° C.) corresponds 

 well with that of the Merriam summation above 18,000 (F.) or 10,000 

 (C.) ; the zone characterized by a 6-weeks normal of from 72° F. (22° 

 C.) to 79° F. (26° C.) corresponds with that having a summation of 

 from 11,500 (F.) or 6,300 (C.) to 18,000 (F.) or 10,000 (C); the zone 

 with a 6-weeks normal of from 64° F. (18° C.) to 72° F. (22° C.) corre- 

 sponds to that with a summation from 10,000 (F.) or 5,500 (C), to 

 11,500 (F.) or 6,400 (C.) ; and a similar correspondence is noted between 

 the zone having a 6-weeks normal below 64° F. (18° C.) and that with 

 a summation of less than 10,000 (F.) or 5,500 (C). On the Pacific 

 Slope, however, no such series of comparisons can be instituted. While 

 the coldest zone of the summation chart does not appear at all on the 

 Pacific Slope of the United States, the zone of the 6-weeks normals, 

 which corresponds to this elsewhere, occupies the whole coast as far 

 south as Los Angeles. Furthermore, the next to the coldest zone of 

 normals extends much farther westward and southward in the region 

 under discussion than does the corresponding zone of summations; the 

 former occupies the coastal area west of the San Bernardino and San 

 Jacinto Mountains, south of Los Angeles. Merriam has drawn im- 

 portant conclusions from these differences, bearing upon the delimitation 

 of his life-zones, a matter which will receive some attention in Part III 

 of the present publication. 



(J) NORMAL MEAN ANNUAL TEMPERATURE. (PLATE 45.) 



The normal mean annual temperature is commonly employed by 

 climatologists for comparing climatic temperature intensities, and it 



