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.
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