240 MULTIPLE FACTORS 
factors the P, grades do not necessarily stand as 
the extreme limits of the values obtained in F, 
and F,. It will often happen that the F,, and, 
more especially, some of the F, combinations, will 
contain more factors (or rather, a more effective 
combination of factors) for a given character than 
either of the P, 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 F, or F;. 
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 F, forms will reach 
either of the parental grades. In either type of 
case, however, the two characteristic signs of mul- 
