96 



one member of each pair into the germ 

 cells which form the next generation. 

 The individual chromosomes are some- 

 times recognizable by peculiarities of 

 shape, etc. More often their constancy 

 is of numbers only. These cytological 

 results have been made possible only 

 by a high development of technique 

 and can provide even when greatly ex- 

 tended only correlative evidence on 

 the localization of genes. The geneti- 

 cist or cytologist no more expects to 

 behold the gene of which his literature 

 is full than the chemist hopes to see 

 the atom of which he speaks with un- 

 abated glibness. The gene remains use- 

 ful as a concept and a notation, doubly 

 so now that it includes an idea of spa- 

 tial definition. 



The above is a somewhat pretenti- 

 ous introduction to a discussion which 

 adds so little 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 mam- 

 malian 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 fel- 

 low 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 ap- 

 pearance 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 

 variation of similar appearance has 

 been reported.- Where the inheritance 

 of a variation has not been determined 



- This proceeding may be expected to lead 

 to some errors since similarity of appearance 

 is not always evidence of similarity in germi- 

 nal constitution, but in the absence of breed- 

 ing data we must use the only criterion 

 available. 



DUNN 



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 experimentally. The rest of the 

 variations have been reported as oc- 

 curring in the wild or are represented 

 by specimens in the Museum of Com- 

 parative Zoology at Harvard Univer- 

 sity, the Museum of the Boston So- 

 ciety 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 gather- 

 ing this part of the material, and for 

 helpful suggestions and criticism of 

 this paper. 



All of the variations listed appear to 

 have arisen, probably by mutation, 

 from the primitive coat color of all 

 rodents, the dull protective grey pat- 

 tern known as "agouti." This color, 

 which is actually a mosaic, is due to 

 the presence of three pigments, black, 

 brown and yellow, distributed uni- 

 formly over the dorsal surface of the 

 animal. Each dorsal hair is character- 

 ized 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 difi^use yellow. The apex of 

 the hair is typically black. The belly is 

 always of a lighter 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 (Aiiis 

 musnihis), the common rat of this 

 country {Rattiis jwrvegims), etc. It 

 characterizes the wild type forms of 

 all the species included in the follow- 

 ing list. 



ALBINO 



From this wild type distinct graded 

 losses of pigment have taken place, 



