In Britain the cowslip and primrose are widely distributed and often grow intermixed or in adjacent habitats. Their flowering times overlap and apparent F1 hybrids between them are well known in the field but are numerically rare; apparent backcross plants have also been reported but are extremely rare. Experimental studies have shown that the artificial F1 hybrid is difficult to obtain and is only partially fertile, but that both the back-cross and the F2 generations are quite vigorous under garden conditions. This being so gene flow between the species is possible and data were collected to determine if it had occurred.

As no obvious hybrid swarms were encountered, some method of classifying populations had to be devised so that those likely to contain unrecognized introgressed individuals were grouped together. To bring this about populations of either species growing in the absence of the other were regarded as pure populations, and those in which the species grew together or in the company of apparent F1 hybrids were regarded as mixed populations. It was assumed that the mixed populations would contain more introgressed plants than the pure populations.

Since the species showed no sign of discordant variation in the mixed populations the usual method of comparing populations diagrammatically by means of radiate indicators was not applicable. Instead, biometrical methods were employed. Three floral dimensions were selected for investigation. The inheritance of these in artifical crosses was also studied so that their behaviour was known. The plants from each population were classified into one of three groups, primroses, cowslips and apparent F1 hybrids. Pure populations and mixed populations were then compared in respect of the means and variances of the three dimensions.