No. 614] PHYSIOLOGICAL PROBLEMS 139 



terminations must be more numerous and especial care 

 must be exercised to have the temperatures constant. In 

 determining the optimum light for different temperature 

 much more rapid progress can be made by running ex- 

 periments under at least three conditions of this factor 

 for each temperature. Deviations due to factors other 

 than temperature should be shown on such a chart prob- 

 ably in a manner indicated by the broken line on Fig. 3. 

 If the main curve is drawn for shortest time, all devia- 

 tions in light, etc., will increase the so-called total tem- 

 perature, and lines may be drawn for these conditions 

 above the main curve as the facts necessitate. 



Much investigation will be necessary to determine the 

 corrections which must be made in determining mean tem- 

 peratures which must be derived from conditions in which 

 the temperature slowly rises and falls during several 

 hours of each day, within the ranges of temperature 

 where the velocity curve is not a straight line. Tempera- 

 tures outside the straight-line limits should not be mixed 

 with the temperatures of the straight line limits. These 

 outside temperatures must be considered or estimated in 

 terms of units sufficiently small to approach accuracy. 

 In the case of daily temperature fluctuations the tempera- 

 tures outside the straight-line limits must be considered 

 by hours, and suitable corrections made before they can 

 be included in the daily mean. The exact nature of this 

 correction will have to be determined by careful inves- 

 tigation. 



(d) Humidity Threshold 

 The workers thus far cited have studied temperature 

 alone, intending in a general way to keep other factors 

 constant. There is undoubtedly a threshold of develop- 

 ment with reference to each factor which influences devel- 

 opment. Berger found that growth ceased in tenebrionid 

 larva? fed on bran dried at 105 degrees, and that they 

 lived for months with a loss of weight ; doubtless with a 

 very small increase in moisture they could be maintained 

 at the initial weight. More recently Pierce has found 



