646 MURIDA—MUS 
naturally, he succeeded in impregnating, by artificial insemination, 
a female M. musculus by a male Apodemus sylvaticus ; unfortunately, 
the mouse either aborted or else ate her young. As Hagedoorn 
points out, it is quite possible that if the determiner 7 came originally 
from another species, that other species may not have been a yellow 
animal at all. We are inclined to think that in connection with this 
problem Mendelians might profitably try to cross AZ. musculus with 
spicilegus, or one of the other truly wild species of Mus. 
Albinism results either from the absence of the colour factor (C), 
or from the absence of all the colour determiners. Albinos lacking C 
may lack all the colour determiners also, or they may carry certain 
of or all the colour determiners, either in a dilute or a saturated 
condition. Albino mice are thus of many distinct kinds, although 
these kinds cannot usually be distinguished by inspection; appropriate 
breeding-tests, however, reveal the constitutional differences clearly. 
The pied types of mice are less definite than those of rabbits and 
rats, but their coat-pattern is also known to follow Mendel’s law in 
inheritance. Pied mice frequently behave as recessives to whole or 
self-coloured animals, and Cuénot was led to conclude that the pied 
forms with more white are recessive to those with less. Miss Durham, 
however, found that certain pied mice behaved as dominants when 
crossed with self-coloured mice, being in this respect analogous to 
the “English patterned” rabbits. This occurrence of both dominant 
and recessive piedness in tame House Mice affords an interesting 
parallel to the similar occurrence of dominant and recessive yellow 
mice discussed above. 
Although further remarks upon coloration and coat-pattern, from 
a more general point of view, must be reserved for the introduction 
to this work, it is necessary to state here that Mendelian factors are by 
no means simple things as a rule. Each factor is perhaps to be 
regarded as the physiological expression of the sum of a multitude 
of characters assembled in a definite combination. When one or 
more of these characters drop out of, or others enter the complex, 
the latter is disturbed, and by readjustment a new combination, more 
or less different from-its parent, is formed; this new combination 
betrays itself by producing a more or less well-marked modification 
of colour or pattern; and thus we become aware of the fact that 
“factor X” or “colour determiner Y” are mosaics, and not units.! 
1 Reference may be made to the following literature for details and further 
references :—Bateson, Proc. Zool. Soc., 1903, ii.. 71; Mendel’s Principles of 
Heredity, Cambridge, 1909; Cuénot, Arch. Zool. Expér. et Gén. Notes et Rev., 
1902, XxVil. } 1903, Xxxill. ; 1904, xlv.; 1905, cxxili.; 1907, i.; Bull. Mens. Réunion 
Biol. Nancy, 1904, 1050; and Briinn Verh. Naturfor. Ver., 49, 214; Castle and 
Little, Sczence, N.S., 32, 868, 1910; Darbishire, Bzometrika, ii., 1902, 101, 165, and 
