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Ovarian cycles and individual reproductive potentials of Cnemidophorus tigris in Colorado and Texaswere studied by counts and measurements of yolked ovarian follicles, corpora lutea, and oviducal eggs. Colorado lizards lay one clutch averaging 3.4 eggs annually between May and August; Texas lizards average two clutches, each averaging 2.2 eggs, between April and mid-August.Geographic variation in reproductive activity has been recognized, mainly by inference, in North American lizards of several families. In species with extensive latitudinal ranges, several clutches of eggs may be produced annually in the southern part of the range; only a single clutch in the northern part (1). There is also evidence that the average number of eggs in each clutch is greater in the northern part of the range of a species (2). The relation of these two variables in a single species has not been established, nor has the importance of this variation in the adaptation of populations of a species been generally recognized. Our studies reveal significant geographic variation both in number of clutches per season and in average number of eggs per clutch within the range of Cnemidophorus tigris (3). Cnemidophorus tigris ranges from southeastern Oregon well into northern Mexico, occurring in all the major deserts of North America. One of us (C.J.M.) studied populations of C. t. septentrionalis in Colorado National Monument, Mesa County, Colorado (4); the other (G.A.H.) studied C. t. marmoratus near Kermit, Winkler County, Texas (5). These study areas are about 950 km apart, near the northeastern and southeastern limits of the range of the species. Each of us took weekly samples of female lizards through two complete activity seasons (1962-63in Colorado; 1963-64 in Texas) and determined ovarian cycles, clutch size, and reproductive potential from counts and measurements of yolkfilled ovarian follicles (Fig. 1), oviducal eggs, and corpora lutea (178 in Colorado; 257 in Texas ) in reproductive lizards.In Colorado, C. tigris leaves hibernation around 1 May, and mature females are active for 75 to 90 days each sum- 30 DECEMBER 1966 mer. Enlarged ovarian follicles (3 to 4 mm in diameter) are visible at emergence; 20 to 25 days later the enlarged follicles begin vitellogenesis and are ready for ovulation within 20 days, when 11.5 to 12.0 mm in diameter. After fertilization and shell deposition the oviducal eggs are laid-all within 2 weeks of ovulation. The median date of egg-laying in 1962 was 21 June. The corpora lutea disappear immediately after ovoposition, and the ovaries remain inactive (all follicles less than 2 mm in diameter) for the remainder of the season.In Texas, the season of major activity starts in early April, although a few individuals may be active in February and March. Mature females become fully active before the end of April, remaining so until mid-August. Newly emerged females have enlarged ovarian follicles which start yolk deposition immediately and reach ovulatory (maximum average) size by 21 May. After ovulation, a...