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106 Shi-hllang Zhang and De-qu{m Shiareas is hampered by several problems. Often, exotics offer the only good sources of resistance todiseases, but they are difficult to propagate since most exotics are not adapted to the long-dayconditions in north China. Most of them have tall plants, high ear placement, small ears, too manyleaves, low harvest index, weak root system, lodging, very late flowering, and long anthesis-silkinginterval. Some of the plants can not silk or tend to be sterile (Oyervides-Garcia et al. 1985). Theseweaknesses are in close relationship with photoperiod sensitivity.Selection for improvement of adaptation should be done to adapt tropical or subtropicalgermplasm under the long-day conditions in temperate zones (Goodman, 1985). The sensitivity oftropical and subtropical maize under long day usually conceals most of valuable genetic variation.When exotic gennplasm is undergoing improvement under long day conditions, a large population sizeand low selection intensity is required to maintain the genetic diversity within the population andminimize the possibility of genetic drift due to inbreeding depression. Mass selection meets theserequirements and is usually used for this purpose (Hallauer et a!. 1988).Mass selection is usually effective for improving characters that posses high heritability basedon individual performance (Empig et a!. 1971; Hallauer et aI. 1988). The genetic gain of bi-par~ntalmass selection is usually twice of the half-sib (mono-parent control) mass selection (Empig et al. 1971;Hallauer et a!. 1988). The method of mass selection has been used successfully to improve theadaptation of exotic germplasm (Compton et al. 1979; Hallauer et a!. 1988). The adaptation of tropicaland subtropical maize in temperate zone is always related with early flowering (Hunter 1973; Troyeret al. 1975, 1985). Because heritability of flowering date is usually high, the populations usuallyrespond well to mas