196 Miscellanea (v) Heterostylism in natural populations of the Primrose,

TWIT, primrose is one of the heterostylio species of Primula, the flowers being either "thrum " with short style and anthers at the mouth of the tube, or "pin " with long style and anthers in the tube. It is known that the two forms exist in nature in about equal numbers; and that "legitimate " unions between the two types are much more fertile than "illegitimate " unions within a type (Darwin, 1877). Gregory (1915) found that thrum is dominant to pin, all natural thrums examined being heterozygous, so that the union of thrum and pin gave the two types in almost equal numbers (229 thrum, 236 pin), while thrum selfed gave 3 thrum: 1 pin (39 thrum, 13 pin). In counting natural populations I had two objects in view, to see whether the ratio of the two types diverged significantly from unity, and whether individual populations varied significantly from the mean proportion. I usually counted between 100 and 200 plants growing as closely together as possible, so that they might be regarded as a naturally inter-crossing population. Most of the populations were found on roadsides in Wales and southern England. Those at Garreg 1 and Port Meirion were in open woods, that at Garreg 2 in a pasture, while those at Ymstyllynn and Pangbourne were by the sides o