The hybrid origin of a plant in a state of nature can be recognised by four tests: first, by its occurrence only where both presumed parent-species exist or have recently existed; and this holds good, as far as I can discover, with the oxlip; but the P. elatior of Jacq., which, as we shall presently see, constitutes a distinct species, must not be confounded with the common oxlip. Secondly, by the supposed hybrid plant being nearly intermediate in character between the two parent-species, and especially by its resembling hybrids artificially made between the same two species. Now the oxlip is intermediate in character, and resembles in every respect, except in the colour of the corolla, hybrids artificially produced between the primrose and the polyanthus, which latter is a variety of the cowslip. Thirdly, by the supposed hybrids being more or less sterile when crossed inter se: but to try this fairly two distinct plants of the same parentage, and not two flowers on the same plant, should be crossed; for many pure species are more or less sterile with pollen from the same individual plant; and in the case of hybrids from heterostyled species the opposite forms should be crossed. Fourthly and lastly, by the supposed hybrids being much more fertile when crossed with either pure parent-species than when crossed inter se, but still not as fully fertile as the parent-species.
For the sake of ascertaining the two latter points, I transplanted a group of wild oxlips into my garden. They consisted of one long-styled and three short- styled plants, which, except in the corolla of one being slightly larger, resembled each other closely. The trials which were made, and the results obtained, are shown in tables 2.14, 2.15, 2.16, 2.17 and 2.18. No less than twenty different crosses are necessary in order to ascertain fully the fertility of hybrid heterostyled plants, both inter se and with their two parent-species. In this instance 256 flowers were crossed in the course of four seasons. I may mention, as a mere curiosity, that if any one were to raise hybrids between two trimorphic heterostyled species, he would have to make 90 distinct unions in order to ascertain their fertility in all ways; and as he would have to try at least 10 flowers in each case, he would be compelled to fertilise 900 flowers and count their seeds. This would probably exhaust the patience of the most patient man.
TABLE 2.14. Crosses inter se between the two forms of the common Oxlip.
Column 1: Illegitimate union. Short-styled oxlip, by pollen of short-styled oxlip: 20 flowers fertilised, did not produce one capsule.
Column 2: Legitimate union. Short-styled oxlip, by pollen of long-styled oxlip: 10 flowers fertilised, did not produce one capsule.
Column 3: Illegitimate union. Long-styled oxlip, by its own pollen: 24 flowers fertilised, produced five capsules, containing 6, 10, 20, 8, and 14 seeds. Average 11.6.
Column 4: Legitimate union. Long-styled oxlip, by pollen of short-styled oxlip: 10 flowers fertilised, did not produce one capsule.
TABLE 2.15. Both forms of the Oxlip crossed with Pollen of both forms of the Cowslip, P. veris.
Column 1: Illegitimate union. Short-styled oxlip, by pollen of short-styled cowslip: 18 flowers fertilised, did not produce one capsule.
Column 2: Legitimate union. Short-styled oxlip, by pollen of long-styled cowslip: 18 flowers fertilised, produced three capsules, containing 7, 3, and 3 wretched seeds, apparently incapable of germination.
Column 3: Illegitimate union. Long-styled oxlip, by pollen of long-styled cowslip: 11 flowers fertilised, produced one capsule, containing 13 wretched seeds.
Column 4: Legitimate union. Long-styled oxlip, by pollen of short-styled cowslip: 5 flowers fertilised, produced two capsules, containing 21 and 28 very fine seeds.
TABLE 2.16. Both forms of the Oxlip crossed with Pollen of both forms of the Primrose, P. vulgaris.
Column 1: Illegitimate union. Short-styled oxlip, by pollen of short-styled primrose: 34 flowers fertilised, produced two capsules, containing 5 and 12 seeds.