DUNN — UNIT CHARACTER VARIATION IN RODENTS 
129 
The. above is a somewhat pretentious introduction to a discussion 
which adds so httle to the matters mentioned in the opening paragraph, 
and yet I hope it has not been without interest to those engaged in the 
study of mammalian variation and evolution. The facts and theories 
discussed are to have an important place in general biology, and one 
may perhaps wish to hear of progress in a field which has tended at 
times to shut itself off from its fellow branches, by the dialect it has been 
forced to use. 
Perhaps the best way of presenting the evidence on unit variation 
in color in the Rodentia is to describe the appearance and genetic 
behavior of each of the principal variations with a short list of the 
species in which it has been studied, and of the species in which a varia- 
tion of similar appearance has been reported.^ Where the inheritance 
of a variation has not been determined by experimental breeding this 
fact is noted by an asterisk. This list makes no claim to completeness 
except in the cases of variations which have been studied experiment- 
ally. The rest of the variations have been reported as occurring in 
the wild or are represented by specimens in the Museum of Com- 
parative Zoology at Harvard University, the Museum of the Boston 
Society of Natural History, or the American Museum of Natural 
History of New York. I am indebted to Dr. Glover M. Allen of the 
Boston Society of -Natural History for help in gathering this part of 
the material, and for helpful suggestions and criticism of this paper. 
All of the variations fisted appear to have arisen, probably by muta- 
tion, from the primitive coat color of all rodents, the dull protective 
grey pattern known as ‘‘agouti.’^ This color, which is actually a mosaic, 
is due to the presence of three pigments, black, brown and yellow, 
distributed uniformly over the dorsal surface of the animal. Each 
dorsal hair is characterized in general by an area of black next to the 
skin in which brown granules are mingled and generally masked by the 
black, followed by a band of diffuse yellow. The apex of the hair 
is typically black. The belly is always of a fighter shade than the 
dorsum, due to a lesser concentration of black pigment and a wider 
area of pale dusky yellow in the hairs. The ^ ‘agouti’^ coat is seen in 
a typical form in the familiar wild house mouse {Mus musculus), the 
common rat of this country {Rattus norvegicus), etc. It characterizes 
the wild type forms of all the species included in the following fist. 
2 This proceeding may be expected to lead to some errors since similarity of 
appearance is not always evidence of similarity in germinal constitution, but in 
the absence of breeding data we must use the only criterion available. 
