THE EOLE OF ISOLATIO]^^ IN THE FORMATION 

 OF A NARROWLY LOCALIZED RACE OF 

 DEER-MICE (PEROMYSCUS).* 



DR. F. B. SUMNER 

 SCRIPPS Institution, La Jolla, Calif. 



No one who has critically examined large numbers of 

 specimens, belonging to such a widely distributed and 

 diversified genus as Peromyscus, can fail to be impressed 

 with two facts. First, the differences upon which the 

 so-called ''subspecies" are based are real and obvious 

 ones. But, secondly, the actual subspecies which are 

 recognized and named are necessarily highly artificial 

 groups. On the one hand, each subspecies intergrades 

 with others to such an extent that the assignment of a 

 given specimen to one or the other group is often quite 

 arbitrary. And on the other hand, even these "sub- 

 species ' ' themselves are far from being elementary. They 

 are composite groups, comprising, in many cases, a num- 

 ber—perhaps a great number— of distinguishable local 

 types. The word distinguishable is here used in a quali- 

 fied sense. It is likely that the distinctions would com- 

 monly be obvious just in proportion as the collections 

 were made at points which were remote from one another. 



Indeed, it has been said by one who has monographed 

 this genus of mice^ that ''classification becomes . . . like 

 dividing the spectrum and depends largely upon the 

 standards set, for, theoretically at least, the possibilities 

 of subdivision are unlimited (p. 17)." 



None the less, it is generally believed that where well- 

 marked physical or other barriers are interposed between 

 two groups of individuals, this continuous intergradation 



*Eead before Ecological Society of America, San Diego meeting, August, 

 1916. 



1 Osgood, "Revision of the Mice of the American Genus, Peromyscus," 

 North American Fauna, No. 28. Washington, 1909. 



173 



