DUNN 93 



demojistrate it to he invalid (see Johajuisev, p. 20). The application 

 of the mutation theory to the evolutioji of natural popidatiotis, how- 

 ever, rested solely on hypothesis and extrapolation, until the appear- 

 ance of Du7m's study. Dobzhansky , i?i "Gejietics and the Origin of 

 Species'' (1941, p. 60) says, "-Dmin . . . was the first to apply the 

 above genetic ifiterprctations to the abcrra?it individuals in wild 

 species. . . . The aberrations found in Jiature resemble well-k?iow7i 

 breeds of domestic species (rabbits, mice, and guinea pigs), in which 

 these characteristics are inherited as Mendelian recessives.'" 



The paper, the?i, 7mght be regarded as the begi?nii?ig of efforts to 

 utilize the knowledge gained from laboratory genetics in the in- 

 terpretation of phyloge?iy and relationships betwee?i natural popida- 

 tions. It demofistrates the applicability of genetic theory and evi- 

 de?ice i?i the expla?iatio?i of evolutionary eve?its, and this has become 

 one of the major co?itributio?is to biology to arise from the study of 

 genetics. 



Recent progress ^ in the study of 

 the inheritance of coat colors in several 

 species of rodents has revealed a rather 

 striking similarity in the variations 

 which have arisen in distinct species of 

 that order. This similarity is not only 

 a matter of appearance, which is famil- 

 iar to all students of mammals, but ex- 

 tends as well to the manner of inherit- 

 ance, and most recently has been 

 found to characterize the localization 

 of the determinants or genes for similar 

 variations in two species. Such identity 

 of cause of the same variation in two 

 or more species indicates that such 

 variations are homologous, and that 

 the species which give rise to them 

 have a relationship of a somewhat dif- 

 ferent and more intimate kind than 

 that implied in the theory of relation- 

 ship by common descent. 



1 In this paper, which is to be regarded as 

 a cursory survey leading to a consideration 

 of one or two special points rather than as 

 an authoritative exposition of variation in 

 rodents, I have not felt it essential to furnish 

 a detailed bibliography. The necessary refer- 

 ences may be found in Castle (1920) and 

 Morgan (1919) as noted in the bibliography, 

 and an excellent survey in Wright (1917), 

 which also considers the physiological and 

 chemical aspects of color variation and in- 

 heritance. 



Before detailing the conditions in 

 the species of rodents which have been 

 studied, some explanation of the evi- 

 dence and reasoning which underlie 

 the localization of genes is due to the 

 general reader. It is probably recog- 

 nized by all students of biology that 

 heritable variations arising generally 

 bv mutation are transmitted to the off- 

 spring in accordance with certain 

 definite rules, known familiarly as 

 Mendel's laws of inheritance. The 

 chief of these laws states that heritable 

 characters are transmitted as discrete 

 units which segregate in the formation 

 of the germ cells. A second principle 

 asserts that the segregation of the units 

 is independent, resulting in a random 

 distribution of the characters of the 

 parents among their gametes, such that 

 when two units are involved the 

 chances are equal that they will go to- 

 gether or separately. The first princi- 

 ple probably applies to all inheritance. 

 Evidence has been brought forward to 

 show that the factors or genes which 

 represent the visible characters are not 

 discrete but variable, and capable of 

 change by selection, but this evidence 

 has now been found to indicate not 

 variability but plurality of units. The 



