Subspecies are groups of individuals with similar genetic components or are groups of microgeographic races. In instances where the phenotypic expression of these similar genetic factors, or the "characters," are, as a unit, uniformly different from those of animals of the same species in another geographic area, it is convenient to give recognition to the two kinds by separate subspecific name. Intergradation between two geographically adjacent subspecies occurs, directly or where impassable barriers separate them, indirectly by way of one or more other subspecies. Subspecies of mammals are geographic races, which means that to warrant recognition by subspecific name, there must be a logical geographic range in addition to morphological characters. Timofeef-Ressovsky (1932, 1940) advances the theory of harmoniously stabilized gene-complexes to account for the persistence of subspecies. The persistence of subspecies as genetic units has been best explained, I feel, by Sumner (1932: 84-86) who theorizes as follows: 1. The number of young produced by a subspecies is greater than the carrying capacity of the land they occupy, at least at certain times or in some years. 2. Population pressure results, with a tendency of individuals to emigrate outwards, to the border of the range of the subspecies, where the population pressure is less. 3. The outward moving tendency keeps the center of the range of the subspecies genetically "pure." 4. The peripheral wave continues, as long as favorable habitat is encountered, until an oppositely directed wave of another race is encountered. 5. Areas of intergradation represent local mingling of genetic factors and do not affect the "pure" individuals of the central part of the range of the subspecies. Certain aspects of this hypothesis are strongly supported by the distribution of mammals in Washington. Witness the rapid invasion of _Citellus beecheyi_ and _Lepus californicus_ in Washington, and the eastern cottontail in western Washington and the domestic rabbit in the San Juan Islands. The volume of the "wave of population pressure" where no opposing force is met, is scarcely believable. In seven years the eastern cottontails released in southwestern Washington multiplied from a maximum of 12 individuals to a minimum of 40,000. Competition between subspecies where their ranges come into contact seems to be exceptional. _Peromyscus maniculatus oreas_ and _P. m. austerus_ seem to afford an example of this. However, in a few cases subspecies seem to be determined in part by adaptation to restricted environments; each race lives only where local conditions favor its respective adaptations.