102 



Muridte — Mus musculus 



Rattus norvegicus 

 *Fiber zibethicus 

 Caviidce — Cavia cobaya 



Specimens of individuals of other 

 species which are much darker than 

 the wild type are often seen in mu- 

 seums, usually labelled "melano" or 

 "melanic variation." Many of these 

 should not, I believe, be assumed to 

 represent the true black variation. 

 Some, as in "melanic" squirrels of 

 various species, prove on close ex- 

 amination to be only very much 

 darkened "agoutis" in which the black 

 portions of each hair have been ex- 

 tended at the expense of the yellow 

 portions. This variation has been 

 studied by Punnett in the rabbit and 

 found to be due to a gene distinct 

 from black which is allelomorphic 

 with the extension-restriction pair 

 of allelomorphs. Much darkened 

 "agoutis" have been produced in mice 

 by crossing intense blacks or black- 

 and-tans (a darkened form of yellow) 

 with wild agouti-colored mice. The 

 darkness is due in this case to a series of 

 modifying genes distinct from either 

 black, "agouti" or yellow which in 

 the presence of these genes bring 

 about an increase in the amount and 

 intensity of the black pigment granules 

 and a reduction in yellow. Such ex- 

 tended or darkened "agoutis" can usu- 

 ally (though not always) be distin- 

 guished from black by the lighter belly 

 which is typical of the "agouti" pat- 

 tern. 



In addition to the coat color and 

 pattern variations discussed above, 

 many others have occurred in rodents 

 which have been bred in the labora- 

 tory, though they are as yet known 

 in too few species to make compari- 

 sons profitable. One of these, dilution, 

 is a unit character in mice and rabbits. 

 In this variation, the pigment granules 

 are clumped and reduced in distribu- 



DUNN 



tion, producing when acting on black 

 varieties the familiar maltese color of 

 blue rabbits and mice. The maltese cat 

 is the result of a similar variation from 

 black. Dilution is a simple Mendelian 

 recessive to full color. The red and 

 black blotching of guinea-pigs, the 

 ticking or banding of the belly hairs in 

 guinea-pigs, and the white bellies of a 

 fancy variety of "agouti" mice are 

 also known to be Mendelian unit char- 

 acters.^ 



SUMMARY 



A summary of the preceding dis- 

 cussion shows that we have examined 

 five of the commonest variations in 

 rodents: albinism, pink-eye, yellow, 

 white-spotting, and black. In all of 

 these the inheritance is known for at 

 least three species, and in general the 

 variant is recessive to the wild type. 

 Dominant variations have occurred 

 often enough (such as "English spot- 

 ting" in rabbits, and black-eyed white- 

 spotting and yellow in mice) to make 

 us chary about drawing any general 

 conclusions concerning the occurrence 

 of evolution purely by loss mutations 

 from type. The generality that does 

 appear is rather the widespread occur- 

 rence in this order of similar variations 

 both under domestication when the 

 animals are saved and bred, and in the 

 wild, when usually only the stuffed 

 specimens are preserved. There is im- 

 plied in this similarity, which in cer- 

 tain cases amounts to a homology, a 

 similarity in that part of the organism 

 which is responsible for the variations, 

 i.e. the germ plasm. It may be that we 

 know in these days as little concerning 

 the causes of variation as did natural- 

 ists in the days of Lamarck or of Dar- 

 win. We do at least know where the 



^ For a fuller discussion of these the inter- 

 ested reader is referred to Chapter XII in 

 Castle's Genetics and Eugenics, 1920, and to 

 the series of papers by Wright cited above. 



