With dredged samples from an isolated bank in Loch Lomond providing standards, data are presented on the interpopulation variations in growth and reproduction which occur in five species of freshwater snails. Infraspecific differences between populations in Planorbis albus and Valvata piscinalis involve only growth rates and the time and intensity of the breeding season. In Physa fontinalis, Lymnaea peregra and Ancylus fluviatilis, interpopulation differences involve the seasonal course of the reproductive cycle (and the number of generations per year), as well as growth rates. A series of the known life cycles of freshwater pulmonates is then considered: two patterns of simple annual cycle, and two patterns each involving a second generation annually, are described, besides certain more exceptional life cycles. All four patterns have been found in different populations of P. fontinalis and L. peregra.
Variations in breeding are dependent both on environmental factors, water temperature being most important, and on endogenous causes involving the growth of the snails; variation in mortality may involve environmental limitation. Most interpopulation variations can be environmentally evoked, but some differences seem genetically determined. It is suggested that adaptive plasticity, as found in several aspects of the physiology of these snails, is of fundamental selective value, given the temporal and spatial peculiarities of the freshwater environment.