Unstable genotypes are normally recognized in cultivated plants by the appearance of coloured spots or flecking which are easily seen and scarcely affected by the environment. Unstable genes giving variation in characters such as plant mass, height, yield and earliness are unlikely to be recognized because normally it would be virtually impossible to distinguish such variation from environmental variation and other genetic variation normally ascribed to segregating genes, without deliberate search and detailed analysis. Continuous variation may contain a host of instabilities, or their more stable products, which could have had their origins in the many normal processes of differentiation. It is not known how easily heritable changes can be induced by the environment in plant species in general, but in flax and Nicotiana rustica environmentally induced changes giving large relatively stable differences in plant mass, height and flowering time provide a means of studying the behaviour of unstable genotypes affecting these characters. Crosses between environmentally induced flax types and varieties show that the phenotypic differences are not essentially any different from, and could be dispersed unnoticed among, the rest of the genetic and environmental variation. They also show additivity, dominance and gene interaction, and they can be the cause of asymmetric response to selection and inbreeding depression.