The analysis of intraspecific variation in life history of organisms is an important theme of evolutionary biology. Molluscan shells are suited for life history analysis, because they include a number of life history traits. To reveal the relationship between environment and life history, I have examined the geographic variations of shell size and shape in cowry species (Mollusca: Gastropoda: Cypraeidae). In this presentation, we will throw two questions: (1) How is the variation distributed in space ? (2) What is the cause of the variation ? We answer the first problem by measuring the sizes of shells collected from thirty populations of the world. To solve the second riddle, we propose a mathematical model which focuses on the shell ontogeny of cowries.