240 MULTIPLE FACTORS 



factors the Pi grades do not necessarily stand as 

 the extreme limits of the values obtained in Fi 

 and F2. It will often happen that the Fi, and, 

 more especially, some of the F2 combinations, will 

 contain more factors (or rather, a more effective 

 combination of factors) for a given character than 

 either of the Pi individuals possessed; for, each of 

 the Pi forms may have contained certain factors 

 for the given character, but different factors in the 

 two cases. This was true in the cross of black with 

 ebony flies reported on page 228. It should com- 

 monly be the result where each parental race has 

 separately been subjected to a process of selection 

 in the same direction for the character under con- 

 sideration, for in each line, factors favoring this 

 character, will be conserved, as they appear, but 

 in the two separate lines different sets of factors 

 for producing such a result will tend to accumulate. 

 In the case of "vigor," just such selective processes 

 usually occur in nature, and as a result either 

 parental limit is often transgressed in Fi or F2. 

 On the other hand, where the two races have been 

 selected in opposite directions, or where only one 

 of them has been subjected to selection — as in the 

 case of the fan-tail pigeons, lop-eared rabbits, and 

 other fancy breeds — practically all of the differential 

 factors tending to produce the given character are 

 commonly present in only one of the parental races, 

 and so only the extremes of the Fo forms will reach 

 either of the parental grades. In either type of 

 case, however, the two characteristic signs of mul- 



