that we are dealing with the
presence and absence of two factors, a colour developer (C), and a colour
modifier (G), both acting, as it were, upon a substratum of black. The F_2
generation really consists of the four classes agoutis, blacks, albino
agoutis, and albino blacks in the ratio 9 : 3 : 3 : 1. But since in the
absence of the colour developer C the colour modifier G can produce no
visible result, the last two classes of the ratio are indistinguishable,
and our F_2 generation comes to consist of three classes in the ratio
9 : 3 : 4, instead of four classes in the ratio 9 : 3 : 3 : 1.

+-------+-------+-------+-------+
|GCB....|GCB....|GCB....|GCB....|
|GCB....|GcB....|gCB....|gcB....|
|.......|.......|.......|.......|
|.Agouti|.Agouti|.Agouti|.Agouti|
+-------+-------+-------+-------+
|GcB....|GcB |GcB....|GcB |
|GCB....|GcB |gCB....|gcB |
|.......| |.......| |
|.Agouti| Albino|.Agouti| Albino|
+-------+-------+-------+-------+
|gCB....|gCB....|gCB####|gCB####|
|GCB....|GcB....|gCB####|gcB####|
|.......|.......|#|#|
|.Agouti|.Agouti|##BLACK|##BLACK|
+-------+-------+-------+-------+
|gcB....|gcB |gcB####|gcB |
|GCB....|GcB |gCB####|gcB |
|.......| |#| |
|.Agouti| Albino|##BLACK| Albino|
+-------+-------+-------+-------+

FIG. 8.

Diagram to illustrate the nature of the F_2 generation which may arise
from the mating of agouti with albino in mice or rabbits.

This explanation was further tested by experiments with the albinos. In an
F_2 family of this nature there ought to be three kinds, viz. albinos
homozygous for G (GGccBB), albinos heterozygous for G (GgccBB), and albinos
without G (ggccBB). These albinos are, as it were, like photographic plates
exposed but undeveloped. {53} Their potentialities may be quite different,
although they all look alike, but this can only be tested by treating them
with a colour developer. In the case of the mice and rabbits the
potentiality for which we wish to test is the presence or absence of the
factor G, and in order to develop the colour we must introduce the factor
C. Our developer, therefore, must contain C but not G. In other words, it
must be a homozygous black mouse or rabbit, ggCCBB. Since such an animal is
pure for C it must, when mated with any of the albinos, produce only
coloured offspring. And since it does not contain G the appearance