Abstract
In Iceland, the polymorphic land snail Cepaea hortensis Mull. is confined to a relatively small area in the extreme south. Sampling 19 populations revealed the presence of a considerable range of phenotypes, the occurrence and frequency of which varies from population to population. A method of expressing the total genetic diversity of the phenotypes in different populations is suggested. The effective absence of the song thrush Turdusphilomelos Brehm, a bird that is known ol prey selectively on Cepaea elsewhere, suggests that other factors promote and maintain the polymorphism. In one area rodents were found to be feeding upon hibernating snails, and it is possible that they exert a considerable selection pressure.
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