In any event, species for which we have data demonstrating that they regularly oversummer south of their breeding ranges are probably adapted to having a part of their populations refrain from breeding each year. Whether this phenomenon can be explained solely in terms of selection at the level of individual birds (Lack, 1954) or involves selection of an adaptive response of the population as a whole (Wynne-Edwards, 1955; see also Taylor, 1961, concerning _Rattus_) is a problem that cannot be resolved at this time. We may note that the species involved ordinarily breed in arctic and subarctic regions, and it would seem advantageous (as set forth below) for nonbreeders to remain well south of such high latitudes. The numbers of oversummering individuals may fluctuate with over-all population density, possibly as a result of crude density, but possibly also as a result of emigration of individuals in excess of optimal density on breeding grounds (see Wynne-Edwards, 1959). One aspect of this phenomenon not explicitly discussed by Wynne-Edwards is the possibility that some individuals never move north to breeding grounds at all, perhaps as a result of a behavioral character genetically-grounded and mediated by delayed maturation of the neurohumoral "clock." This certainly would be an economical means by which population numbers could be regulated, for there would be a saving of energy in that some individuals not only would not move north, but also would not participate in the behavioral interactions involved in territorial spacing. Occurrence of these birds throughout southern North America, Middle America, and northern South America may thus reasonably be understood. LITERATURE CITED ALDRICH, J. W., and DUVALL, A. J. 1958. Distribution and migration of races of the mourning dove. Condor, 60:108-128. AMADON, D., and ECKELBERRY, D. R. 1955. Observations on Mexican birds. Condor, 57:65-80. AMERICAN ORNITHOLOGISTS' UNION CHECK-LIST COMMITTEE 1957. Check-list of North American Birds. Lord Baltimore Press, Baltimore, Maryland. xiii-691 pp. AXTELL, R. W. 1954. The systematic relationships of certain lizards in two species groups of the genus _Holbrookia_. M. A. Thesis, Department of Zoology, Univ. Texas. 55 pp. AXTELL, R. W., and WASSERMAN, A. O. 1953. Interesting herpetological records from southern Texas and northern Mexico. Herpetologica, 9:1-6. BAILEY, V. 1905. Biological survey of Texas. North Amer. Fauna, 25:1-222. BAKER, R. H., and LAY, D. W. 1938. Notes on the mammals of Galveston and Mustang islands, Texas. Jour. Mammal., 19:505. BEHLE, W. H., and SELANDER, R. K. 1953. The plumage cycle of the California gull (_Larus californicus_) with notes on color changes of soft parts. Auk, 70:239-260. BLAIR, W. F. 1952. Mammals of the Tamaulipan Biotic Province in Texas. Texas Jour. Sci., 4:230-250. BROWN, J. L.