The highly variable brackish-water snail Clithon oualaniensis (Lesson 1831) was first studied in Hong Kong, Ceylon and Malaya--Singapore. Populations within these regions are closely similar, but there are consistent differences between regions. Subsequently, particularly round the Malay Peninsula, orderly changes of variants have come to light. This raises the question of how these have come into being and what is responsible for their apparent stability. Contrary to first impressions, most of the variability of Clithon and of certain species of Umbonium and Nerita is pseudo-polymorphic in nature. Conventional polymorphism seems to make but a small contribution. Quasi-continuous variation and pseudo-polymorphism are essentially the same phenomenon. Both arise by the interaction between polygenic inheritance and environmental conditions. Whether the resulting phenotypes are continuous or discontinuous depends on the developmental physiology of the organs or structures involved and is thus a secondary phenomenon.