The difference also between the number of seeds produced by the long-styled and short-styled flowers of the primrose, when both are illegitimately fertilised, is greater than that between the number produced under similar circumstances by the two forms of the cowslip. The long-styled flowers of the primrose when protected from the access of all insects, except such minute ones as Thrips, yield a considerable number of capsules containing on an average 19.2 seeds per capsule; whereas 18 plants of the long-styled cowslip similarly treated did not yield a single seed.
The primrose, as every one knows, flowers a little earlier in the spring than the cowslip, and inhabits slightly different stations and districts. The primrose generally grows on banks or in woods, whilst the cowslip is found in more open places. The geographical range of the two forms is different. Dr. Bromfield remarks that "the primrose is absent from all the interior region of northern Europe, where the cowslip is indigenous." (2/3. 'Phytologist' volume 3 page 694.) In Norway, however, both plants range to the same degree of north latitude. (2/4. H. Lecoq 'Geograph. Bot. de l'Europe' tome 8 1858 pages 141, 144. See also 'Annals and Magazine of Natural History' 9 1842 pages 156, 515. Also Boreau 'Flore du centre de la France' 1840 tome 2 page 376. With respect to the rarity of P. veris in western Scotland, see H.C. Watson 'Cybele Britannica' 2 page 293.)
The cowslip and primrose, when intercrossed, behave like distinct species, for they are far from being mutually fertile. Gartner crossed 27 flowers of P. vulgaris with pollen of P. veris, and obtained 16 capsules; but these did not contain any good seed. (2/5. 'Bastarderzeugung' 1849 page 721.) He also crossed 21 flowers of P. veris with pollen of P. vulgaris; and now he got only five capsules, containing seed in a still less perfect condition. Gartner knew nothing about heterostylism; and his complete failure may perhaps be accounted for by his having crossed together the same forms of the cowslip and primrose; for such crosses would have been of an illegitimate as well as of a hybrid nature, and this would have increased their sterility. My trials were rather more fortunate. Twenty-one flowers, consisting of both forms of the cowslip and primrose, were intercrossed legitimately, and yielded seven capsules (i.e. 33 per cent), containing on an average 42 seeds; some of these seeds, however, were so poor that they probably would not have germinated. Twenty-one flowers on the same cowslip and primrose plants were also intercrossed illegitimately, and they likewise yielded seven capsules (or 33 per cent), but these contained on an average only 13 good and bad seeds. I should, however, state that some of the above flowers of the primrose were fertilised with pollen from the polyanthus, which is certainly a variety of the cowslip, as may be inferred from the perfect fertility inter se of the crossed offspring from these two plants. (2/6. Mr. Scott has discussed the nature of the polyanthus ('Proceedings of the Linnean Society' 8 Botany 1864 page 103), and arrives at a different conclusion; but I do not think that his experiments were sufficiently numerous. The degree of infertility of a cross is liable to much fluctuation. Pollen from the cowslip at first appears rather more efficient on the primrose than that of the polyanthus; for 12 flowers of both forms of the primrose, fertilised legitimately and illegitimately with pollen of the cowslip gave five capsules, containing on an average 32.4 seeds; whilst 18 flowers similarly fertilised by polyanthus-pollen yielded only five capsules, containing only 22.6 seeds. On the other hand, the seeds produced by the polyanthus-pollen were much the finest of the whole lot, and were the only ones which germinated.) To show how sterile these hybrid unions were I may remind the reader that 90 per cent of the flowers of the primrose fertilised legitimately with primrose-pollen yielded capsules, containing on an average 66 seeds; and that 54 per cent of the flowers fertilised illegitimately yielded capsules containing on an average 35.5 seeds per capsule.