The common racer and the several species of whip snakes (Masticophis) were revised by Ortenburger (1928). More recently with much larger series of specimens, Auffenberg (1955) again revised the classification of C. constrictor, but his study was concentrated in Florida and neighboring southeastern states with relatively little attention devoted to populations of the western and central United States. As the species occurs throughout most of the United States and south through the coastal lowlands of eastern Mexico to Guatemala, it is found over a wide range of environmental conditions. Various characters are subject to geographic variation, and some of them follow clines that are maintained over extensive areas. Such characters as the number of hemipenial spines, and the enlargement of one or more basal spines into hooks, the shape of the premaxillary bone, the number of maxillary teeth, the numbers of ventrals and caudals, color of eye, number of dorsal saddle-marks and of ventral spots in juveniles, and ratios of body proportions including tail length to total length have been used to characterize the subspecies.
Also important is the shade of coloration of adults. The subspecies constrictor, priapus and helvigularis that are characteristic of forested habitats in the eastern United States are black dorsally and have their ventral surfaces suffused to a large extent with dark or dusky coloring. Farther westward the ground color becomes progressively paler, greenish, grayish or light brown, and the ventral surface is yellow (white on the throat and neck). The same tendency appears in C. c. paludicola of the Everglades. The populations of arid climates in southern Texas and in the far western states are relatively pale colored. The species thus conforms to Gloger's Rule in changing from a pallid coloration in arid climates to a dark pattern with eumelanins predominating in a humid climate.
Perhaps the most important character that is subject to geographic variation in the racer, and certainly one of the most neglected, is body size. With information now available it is not possible to compare the sizes of different populations except in a general way. The best sources of information concerning size in several subspecies other than flaviventris, are the publications of Auffenberg (1949 and 1955). Comparison of these data with my own is not entirely satisfactory because Auffenberg did not indicate clearly differences in size between the sexes, nor indicate the boundary line between young and adults. Also, his measurements are of overall length. For the purpose of comparison I have subtracted 22.5 per cent, an approximation of ratio of average tail length, from Auffenberg's figures. He stated (1955:98) that the series of specimens on which measurements were based were those "with a uniform coloration," that is to say they had lost the juvenal pattern and were probably sexually mature. Whether the same statement applied to the large series of stejnegerianus in the same author's earlier paper (1949:55) is doubtful.
C. c. constrictor: 34 New York specimens averaged 806 millimeters (Auffenberg, 1955:96).
C. c. flaviventris: 100 Kansas specimens averaged 791 millimeters (males, 746; females, 836).
C. c. priapus: 171 Florida specimens averaged 713 millimeters (Auffenberg, 1955:96).
C. c. stejnegerianus: 291 Texas specimens averaged 664 millimeters (Auffenberg, 1949:54).
C. c. mormon: 94 West Coast specimens (Museum of Vertebrate Zoology) averaged 563 millimeters (males 515, females 600).
C. c. anthicus: 35 northern Louisiana specimens estimated to average 582 millimeters (Clark, 1949:249—the author did not present individual measurements, but indicated the numbers in several size groups in his sample).
The 100 flaviventris in the above list were recorded in June, July and August, a season when the young of the preceding year are still small, and these young were not included. In a fall sample 63 males averaged 729 millimeters and 65 females averaged 886 millimeters, but with the nearly grown young (44 males and 40 females) included, the averages were changed to 651 and 768 respectively. Maximum length perhaps express differences between the subspecies as well as, or even better than, the averages listed above. The following figures indicate some of the maximum overall length measurements in inches that have been published by various authors. These measurements pertain to females unless otherwise indicated.
C. c. constrictor: 743/4, 741/4, 73, 651/2 (Auffenberg, 1955:96).
C. c. flaviventris: 71 (Pope, 1944:172), 72 (Conant, 1958:147).
C. c. priapus: 521/2 (♂) (Auffenberg, 1955:98).
C. c. stejnegerianus: 37 (Auffenberg, 1949:54), 40 (Conant, 1958:148).
C. c. mormon: 511/2 (Museum of Vertebrate Zoology).
C. c. anthicus: 70 (Conant, 1958:149).
In my own study, the largest racers recorded from the Reservation and Rockefeller Tract had the following overall lengths: 59, 571/4, 551/2, 58 ♂ (projected from stub tail).
These sets of figures show that compared with the blue racer in Kansas, with which my own study was concerned, the black racer of the northeastern states reaches a larger size, while the racers of the Southeast and especially those of the far West and of southern Texas, are dwarfed. These size differences are almost certainly correlated with behavioral and ecological differences among the snakes involved. Adaptations to exploit certain types of prey and to utilize most efficiently certain habitats and types of cover, have led to divergent trends in different parts of the range.