and the other round pollen. But when these were crossed a remarkable thing occurred, for the progeny, "instead of being white, were purple, like the wild Sicilian plant from which our cultivated sweet peas are descended." This is apparently a typical case of reversion and its cause was found in the fact that at least two factors are necessary in this case for the production of color, a pigment factor R and a color developer C. One of these was lacking in each of the white parents, their gametic formulæ being Cr and cR, but when these two factors came together in the offspring a purple-flowered type was produced with the gametic formula Cc Rr. These F3 plants produced colored and white F3 plants in the proportion of 9 colored: 7 white and the colored forms were of six different kinds (Fig. 57). For the production 
Fig. 57. Results of Crossing two Different Races (A) and (B) of White Sweet Peas; all the F1 hybrids (C) are purple with blue wings like the wild ancestral stock; in F2 six colored varieties are formed ranging from purple with blue wings (D) to tinged white (I) and several kinds (genotypes) of white varieties (K). (After Punnett.) of these six colored forms five different factors must be present in the gametes, according to Punnett, viz.: (1) a color base (R), (2) a color developer C, (3) a purple factor B, (4) a light wing factor L, (5) a factor for intense color I. When all of these factors are present the result is the purple wild form with blue wings, while the omission of one or more of these factors leads to the production of six forms of colored and various types of white flowered plants of the F2 generation.
Castle found that eight different factors may be involved in producing the coat colors of rabbits; these are:
C a common color factor necessary to produce any color.
B a factor acting on C to produce black.
Br a factor acting on C to produce brown.
Y a factor acting on C to produce yellow.
I a factor which determines intensity of color.
U a factor which determines uniformity of color.
A a factor for agouti, or wild gray pattern, in which the tip of every hair is black, below which is a band of yellow, while the basal part of the hair is gray.
