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THE AMERICAN NATURALIST [Vol. XLIX 



they have been subdivided into numerous "geographic races" and 

 "subspecies." Thus P. maniculatus, which ranges from the Arctic 

 Circle to the Isthmus of Tehuantepec, remains constant only where 

 the environment is identical, hence it is represented by definable 

 "subspecies" in almost every faunal area which it enters (see Fig. 3). 



These observations of Osgood may be compared with 

 the taxonomic results of Miller on the one hand and with 

 the observations of paleontologists on the other : 



First: we note that in the " species" of the subgenus 

 Peromyscus and in the " subspecies" of Peromyscus man- 

 iculatus among the vast number of characters enumer- 

 ated there is not a single distinction recorded in numerical 

 or presence and absence characters ; every single charac- 

 ter recorded is either in the proportions of the skull, ears, 

 feet, and tail, or in the intensities and distribution of the 

 color areas — all characters of degree. The field and 

 museum work of Osgood thus independently accords with 

 the taxonomic work, of Miller, namely, " proportional 

 characters" are universal and abundant, "numerical 

 characters" are less frequent and of a higher or different 

 taxonomic order because much more gradual in evolution. 



Second: the evidence for continuity in the origin of pro- 

 portional characters is absolute. 



Third: in a broad way continuity is also the mode of 

 origin of the so-called numerical characters for it is posi- 

 tively observed except in the case of the mammae, and 

 there is no apparent reason, remarks Osgood, why the 

 mammae also may not have developed in the same way as 

 the more trivial characters. In other words, there is 

 almost complete continuity between groups which many 

 taxonomists would regard as different "genera." The 

 numerical differences in the plantar tubercles on the soles 

 of the feet have not been sufficiently studied, but it is clear 

 that the change from 5 to 6, or vice versa, has come through 

 the gradual reduction or growth of one tubercle and not 

 through any sudden change. Most interesting also is the 

 fact that the 5-tubercled Peromyscus shows decided simi- 

 larity to the genus Oiiychomys, which is 4-tubercled and 

 closely allied to Peromyscus. 



