EXPERIMENTS IN INHERITANCE. 171 
B. Dense and Dilute Colourations. 
Each colour in the epistatic series has its dense and dilute form, 
density and dilution forming an allelomorphic pair. Density may be 
thought of as due to the presence of a factor D and dilution as due to 
the absence of this factor. My investigations concern the dense and 
dilute forms of black and chocolate. 
I was led to investigate this matter through the appearance, 
recorded in my 1908 report, of black, blue, and chocolate young in a 
litter from the mating of two blacks, and the fact that a particular 
yellow mouse threw blacks when mated with chocolate, and blues 
when mated with blue. While working the matter out, Miss Durham’s 
account (1908) of similar experiments appeared. The details I now 
publish confirm her work, while showing that Cuénot’s suggestion that 
chocolate is the dilute form of black is untenable. I am able to add 
some further types of crosses to those recorded by Durham, which give 
the expected results. 
Evidence against Cuénot’s suggestion that chocolate is the dilute 
form of black and in favour of the view :— 
a. That chocolate carries the factor for dense deposition of pigment, 
and that silver-fawn is the condition of chocolate in which this factor 
is absent ; 
b. Thal black carries the factor for dense deposition of pigment, 
and that blue is the condition of black in which this factor is absent. 
Black and chocolate are the two lowest terms in the epistatic 
colour series. 
Chocolate homozygous (DD bb) x blue homozygous (dd BB). 
F, black (Dd Bb). 
F, 15 black : 4 blue : 3 choc. ; 1 sil.-fawn 
1 1 1 0 ex pair blacks of unknown 
parentage. 
16 5 4 1 
44 17 17 8 Durham. 
60 22 21 9 observed. 
63 21 21 7 calculated 9.3.3.1 ratio. 
Black homozygous (DD BB) x silver-fawn (dd bb). 
F, black (Dd Bb). 
F, 5 black : 2 blue : 3 choc. : 1 sil.-fawn 
67 21 20 5 Durham. 
72 23 23 6 observed. 
69°75 23°25 23°25 775 calculated 9.3.3. 1 ratio. 
The above two types of di-hybrid matings substantiate the view above 
stated. Further matings, all in harmony with this view, are: 
Black carrying silver-fawn (Dd Bb) x silver-fawn (dd bb). 
F, 2 black : 1 blue : 0 choc. + 2-sil.-fawn observed. 
1:25 1-25 1:25 1:25 calculated 1.1.1.1 ratio 
