337 



cept between approximately 18° and 29° C. The total carbon dioxide is, 

 moreover, the same at the high temperatures where the sum of the "effec- 

 tive degrees" is too great. The sum of 'developmental units for constant 

 temperatures is easily derived for the straight-line portion of the velocity 

 curve, as it is simply the product of time units and constant-temperature 

 units. This product has a fixed value under a given set of conditions 

 and has been known as the "thermal constant." The mean of temperatures 

 varying within the straight-line limits of the velocity curve but not going 

 outside these limits (approximately 18° and 39° C. for the meal-worm 

 pupa) also gives a constant product when multiplied by the time. This 

 product is smaller than the time-temperature product obtained within 

 the same range for constant-temperature conditions, because development 

 proceeds faster under variable-temperature conditions. In very carefully 

 controlled experiments on animals, the product is remarkably constant 

 for any one set of conditions. (Krogh '14a and '14b.) 



Order of Experimentation. 



In the determination of velocities for any stage of an organism, the 

 first procedure is to run a series of preliminary experiments with constant- 

 temperatures at five-degree intervals from 45° to 100° F., beginning with 

 100% humidity at 45° F. and lowering the humidity about 6% with each 

 five degrees rise in temperature. Such experiments would show, for 

 example, in the case of the codling-moth pupa, that the straight-line 

 limits are from a little below 65° to a little above 85°. These should be 

 followed by: (1) a series of experiments under constant temperatures at 

 five-degree intervals from 45° F. to 100° F. with 95%, 85%,. 75%, 65%, 

 55'/y . 45%, 35% relative humidity; and (2) a series of variable-tempera- 

 ture experiments with daily variations ranging from 65° F. to 85° F. with 

 the following humidities at 65° F. : lOO^f, 90%, 80%, 707f., and 60%,— 

 and one experiment out of doors. This would make 90 experiments, and 

 for the desired results the material should be uniform, and all experiments 

 should be started on the same day. This would require a minimum of 

 •'i.OOO individuals ; 9,000 would be preferable ; and this series of experi- 

 ments should be repeated with each generation for each of three seasons. 



Experimentation on this huge scale could not be done with the 

 facilities available for the work here reported. Moreover, when this work 

 was undertaken, there was no basis in experience showing that such a 

 procedure would be necessary. As a result, the variation in the different 

 stocks caused irregularities in the data, w'hich necessitated much addi- 

 tional calculation. However, our experience indicates that the develop- 

 mental totals, the thresholds, and the velocity values are different for each 

 humidity, and that the developmental totals differ most. 



Interpretation of Experimental Data. 



It has proved more convenient to establish fixed velocity values for 

 average stocks under average weather conditions than to establish a fixed 

 developmental total. This was done by determining the average develop- 



