332 



the humidity, air movement, light intensity, and other conditions nor- 

 mal to the habitat of the organism in that stage of its life-history. 

 In other words, the developmental unit is the effect produced in one 

 hour by one degree of medial temperature in conjunction with all 

 other phenomena characterizing the standard conditions described 

 above. This phowmcna-dcgrce-hour may be designated as one pheno- 

 hour. 



While relative velocity of development has heretofore been expressed as 

 the arithmetical reciprocal of the time required to complete a stage in the 

 life-cycle, this new method permits a definition of absolute velocity as the num- 

 ber of developmental units per hour. 



The threshold of development is the intensity, or quantity, of 

 any factor immediately above which development begins to be measurable. 

 For example, the temperature threshold is that degree of temperature just 

 above which development begins to be perceptible in amount. It is nut a 

 fixed point but varies, within certain limits, with the humidity and other 

 weather factors and with the generation and the individual. For the 

 larvae in the apple, it varies from -13° to -iS^ F. ; for the pupa and egg, 

 from 4-1° to 49° F. ; and for the hibernated larva, from -13° to 50° F. 



The developmental total for any stage is the sum of develop- 

 mental units for that stage. It is calculated by simply adding together 

 all the developmental units for every hour from the observed (or calcu- 

 lated) beginning of the stage to the observed (or calculated) end of it, 

 using velocity values such as those shown in Table I for hourly combina- 

 tions of recorded temperature and humidity. More briefly, a develop- 

 mental total is obtained directly by summing the hourly velocity values 

 for the known weather conditions throughout the stage. Similarly, a 

 developmental total for a whole life-cycle may be obtained. Develop- 

 mental totals arc not constants but vary with the rainfall of the season 

 and the preceding season, with other weather factors, with the generation, 

 and with the individual. The average, or normal, developmental total for 

 any lot of individuals or for any generation under any set of conditions, 

 is, however, useful in the interpretation of data and in the prediction of 

 appearance. 



Standard time for a stage is the number of hours (or days) 

 calculated from the normal developmental total for average organ- 

 isms under standard conditions. Because of the practical difficul- 

 ties involved, only temperature and humidity are taken into account 

 in the calculation of standard time in this paper. The term substi- 

 tution-quotient is here used to designate one-twenty-fourth of the 

 number of pheno-hours calculated for a stage by the temperature-substitu- 

 tion method as described in PART THREE (pp. 387-393). When 

 correctly calculated, the substitution-quotient is numerically equal to one- 

 twenty-fourth of the normal developmental total for the stage ; and it is 

 used only in establishing standards of development and velocity values. 

 The velocity values (numbers of developmental units per hour 

 for different combinations of temperature, humidity, etc., as shown 

 in Table I) are here regarded as fixed and standard for average 



