245 
to which larve in different parts of the tree are simultaneously subjected, 
to variations in the character of food, and to individual differences. How 
much of the variations are attributable to each of these causes can not be 
accurately determined from the data at hand. 
The use of the day as the unit will take account of a variation of 27 
-day-degrees each way from the average. If we assume that variations 
below 500 and above 900 are due to errors of observation and make a 
correction of 27 on account of the use of the day as the unit, it still 
leaves a variation of from 527 to 873 to be attributed to other causes. 
Since there is no way of eliminating these causes in practical operations, 
we must expect under natural conditions that some larve will complete 
their period when the effective day-degrees have reached 527 and that 
others will not complete it until 873 effective day-degrees have accumu- 
lated, the average accumulation being 673. 
Table 6 shows the distribution of larve reared in picked apples as 
to total effective day-degrees required for their development. 
TABLE 6—DIsTRIBUTION OF LARVAE REARED IN PICKED APPLES WITH REFERENCE TO 
ACCUMULATIONS OF EFFECTIVE DAY-DEGREES DURING THE PERIOD 
Number of larve : Range of effective day-degrees 
; 3 400-449 
16 450-499 
41 500-549 
% 59 -, 550-599 
40 600-649 
22 650-699 
12 700-749 
5 750-799 
il 800-849 
Following the same course of reasoning with these data as with the 
preceding, the variations arising from factors which can not be eliminated 
under field conditions, range from 477 to 722, the average being 587. 
These results are not very satisfactory for practical use. The aver- 
ages are, no doubt, approximately correct; but the variations from the 
average are too great and the limits of the variations due to unprevent- 
able causes are a little too uncertain to render the data entirely satisfac- 
tory for the purpose of forecasting life-history events. However, these 
are the nearest approximations to the actual accumulation of effective 
