644. MURIDA—MUS 
up the dark pigments of its segment in turn gradually oxidise and 
become yellow. In some “voles,” e.g., Mzcrotus orcadensis (see p. 458 
above), a similar process appears to take place occasionally. 
Extensive researches upon the inheritance of the coloration, coat 
pattern, and of some physical defects in the House Mouse, have been 
made by the Mendelians. The literature relating to this work has 
become quite voluminous, and no more than a mere outline of the 
broader results can be attempted here. The experiments have mostly 
been made with tame “fancy” mice, but these have been crossed from 
time to time with wild animals. The various conditions investigated 
have been proved for the most part to depend upon the presence or 
absence of certain definite factors, and to obey Mendel’s law in 
inheritance. In the following paragraphs the capital initial signifies 
the presence of a factor as opposed to its absence, denoted by a small 
initial; in general, Y is dominant, + recessive. 
As regards pigmentation, a considerable number of factors appear 
to be involved. Colour is produced by a special factor, C, and if this be 
absent (c) the mouse will be an albino even if it carry all of the other 
pigment factors. The depth of the coloration depends upon another 
factor, D; when this is present with C, the pigment granules are 
developed in full number, and the colour is intense or saturate; in its 
absence (d@), there are fewer granules and the colour is dilute. The 
precise hue of the mouse depends upon a large number of factors, 
known as “colour determiners” ; these determiners stand apparently 
in a certain definite relation to each other. When all the normal 
determiners are present, together with C and J, the animal is in 
appearance an ordinary wild House Mouse, grey or “agouti” in colour. 
Should the grey determiner (G)! be lacking, and the black (4) and 
chocolate (C%) determiners be both present, the mouse will be black— 
the determiner & masking the determiner C%. To be chocolate in 
colour the mouse must not only carry Cz, but it must lack the 
determiners G and &. Grey cannot, however, be called dominant to 
black, because these factors belong to different allelomorphic pairs, 
and Bateson, therefore, introduced the terms “ epzstatic” and “ hypostatic” 
to express the relationship of the different colours; thus, in relation 
to black (4), grey (G) is epistatic, while chocolate (C%) is hypostatic. 
The combined researches of Cuénot, Durham, Castle and Little, 
Hagedoorn, and others, have shown that yellow mice belong to two 
entirely distinct groups, namely :—(1) that in which yellow arises 
1 Strictly, there is no “grey determiner” at all, there being no grey pigment in 
mice. The grey colour is produced by “barring and ticking,” z.e., by the orderly 
arrangement of the three pigments, black, brown, and yellow, in each hair in definite 
bands of restricted extent. This arrangement is brought about, according to the 
Mendelians, by the presence of a special factor (or pair of factors, according to 
Hagedoorn) called the “grey determiner” above. 
