646 MURID^— 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 / 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 M. musculus with 

 spicilegus, or one of the other truly wild species of Mus. 



Albinism results either from the absence of the colour factor (Q, 

 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 Cu6not 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 F" are mosaics, and not units.^ 



' 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 ; Cuenot, Arch. Zool. Expdr. ei GSn. Notes et Rev., 

 1902, xxvii. ; 1903, xxxiii. ; 1904, xlv. ; 1905, cxxiii. ; 1907, i. ; Bull. Mens. Reunion 

 Biol. Nancy, 1904, 1050 ; and Briinn Verh. Naturfor. Ver., 49, 214 ; Castle and 

 Little, Science, N.S., 32, 868, 1910 ; Darbishire, Biometrika, ii., 1902, loi, 165, and 



