Fl.) and the primrose (P. vulgaris, Brit. Fl.), as has been surmised by several botanists. It is probable that oxlips may be produced either from the cowslip or the primrose as the seed- bearer, but oftenest from the latter, as I judge from the nature of the stations in which oxlips are generally found (2/13. See also on this head Hardwicke's 'Science Gossip' 1867 pages 114, 137.), and from the primrose when crossed by the cowslip being more fertile than, conversely, the cowslip by the primrose. The hybrids themselves are also rather more fertile when crossed with the primrose than with the cowslip. Whichever may be the seed-bearing plant, the cross is probably between different forms of the two species; for we have seen that legitimate hybrid unions are more fertile than illegitimate hybrid unions. Moreover a friend in Surrey found that 29 oxlips which grew in the neighbourhood of his house consisted of 13 long-styled and 16 short-styled plants; now, if the parent-plants had been illegitimately united, either the long- or short-styled form would have greatly preponderated, as we shall hereafter see good reason to believe. The case of the oxlip is interesting; for hardly any other instance is known of a hybrid spontaneously arising in such large numbers over so wide an extent of country. The common oxlip (not the P. elatior of Jacq.) is found almost everywhere throughout England, where both cowslips and primroses grow. In some districts, as I have seen near Hartfield in Sussex and in parts of Surrey, specimens may be found on the borders of almost every field and small wood. In other districts the oxlip is comparatively rare: near my own residence I have found, during the last twenty-five years, not more than five or six plants or groups of plants. It is difficult to conjecture what is the cause of this difference in their number. It is almost necessary that a plant, or several plants belonging to the same form, of one parent-species, should grow near the opposite form of the other parent-species; and it is further necessary that both species should be frequented by the same kind of insect, no doubt a moth. The cause of the rare appearance of the oxlip in certain districts may be the rarity of some moth, which in other districts habitually visits both the primrose and cowslip.

Finally, as the cowslip and primrose differ in the various characters above specified,--as they are in a high degree sterile when intercrossed,--as there is no trustworthy evidence that either species, when uncrossed, has ever given birth to the other species or to any intermediate form,--and as the intermediate forms which are often found in a state of nature have been shown to be more or less sterile hybrids of the first or second generation,--we must for the future look at the cowslip and primrose as good and true species.

Primula elatior, Jacq., or the Bardfield Oxlip, is found in England only in two or three of the eastern counties. On the Continent it has a somewhat different range from that of the cowslip and primrose; and it inhabits some districts where neither of these species live. (2/14. For England, see Hewett C. Watson 'Cybele Britannica' volume 2 1849 page 292. For the Continent, see Lecoq 'Geograph. Botanique de l'Europe' tome 8 1858 page 142. For the Alps see 'Annals and Magazine of Natural History' volume 9 1842 pages 156 and 515.) In general appearance it differs so much from the common oxlip, that no one accustomed to see both forms in the living state could afterwards confound them; but there is scarcely more than a single character by which they can be distinctly defined, namely, their linear-oblong capsules equalling the calyx in length. (2/15. Babington 'Manual of British Botany' 1851 page 258.) The capsules when mature differ conspicuously, owing to their length, from those of the cowslip and primrose. With respect to the fertility of the two forms when these are united in the four possible methods, they behave like the other heterostyled species of the genus, but differ somewhat (see Tables 1.8 and 1.12.) in the smaller proportion of the illegitimately fertilised flowers which set capsules.

Charles Darwin

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