CORRELATION OF DISTRIBUTIONAL FEATURES. 527 



importance warrants on account of Merriam's failure to publish the 

 numerical data on which they were based. The climatic extremes have 

 been read from these maps, however, for all of our botanical areas. A 

 consultation of the tables of climatic extremes will show that the 

 absence of the readings for individual stations, and the mere possession 

 of the values for the isoclimatic lines, has made the climatic extremes 

 for these maps (plates 37 and 44) very general in their nature, and has 

 resulted in giving the same extremes for areas which are widely sepa- 

 rated or very unlike. 



The physiological summation of temperatures appears to be a more 

 natural method of securing a measure of the cumulative effects of this 

 factor on plants than either of the remainder summations which we 

 have used, and its resulting figures should bear a closer relation to 

 distributional facts than the figures derived from any other mode of 

 sunmiation thus far suggested. The four charts showing summations 

 by the different methods (plates 37, 38, 39, and 40) have a generic 

 resemblance so far as concerns the general sweep of their isoclimatic 

 lines, although the actual values represented by the lines differ. The 

 amplitude of the conditions expressed by the physiological sunamation 

 is not great for any one of the 6 life zones, although it is not so narrow, 

 in any case except the Transition Zone, as to indicate that it expresses 

 one of the conditions of most vital importance in controlling the 

 position of the limits between the life-zones. When the remainder 

 summation above 32° has been found to have such apparent impor- 

 tance as a controlling condition as to form a leading basis for the 

 delineation of life-zones, it is a matter of surprise that the more logical 

 physiological summation does not show a high degree of importance 

 when correlated with the life-zones. This discrepancy might be 

 attributed to the fact that the boundaries of the zones were partly 

 determined by the temperature of the hottest 6 weeks, particularly in 

 the western or arid zones, if it were not for the fact that it is these 

 zones in which there is the strongest indication that the physiological 

 summation is of importance as a controlling condition. It is at least 

 true, from the indications of our data, that the physiological summa- 

 tion appears to be more important in controlling the zones than is any 

 other temperature factor that we have used, except in the Austro- 

 riparian Zone, where several others are of equally narrow amplitude. 

 As we proceed from the northern toward southern zones the mean 

 temperature of the coldest 14 days of the year is seen to be a controlling 

 condition of increasing importance. 



With a single exception, the moisture conditions of the western and 

 eastern subdivisions of Merriam's transcontinental zones show a very 

 different range of values, as would be expected from the fact that the 

 subdivision was made on a basis of the differences in these conditions. 

 The one exception is the case of the Transition Zone, in which the 



