P. veris and vulgaris.

The cowslip differs so conspicuously in general appearance from the primrose,
that nothing need here be said with respect to their external characters. (2/2.
The Reverend W.A. Leighton has pointed out certain differences in the form of
the capsules and seed in 'Annals and Magazine of Natural History' 2nd series
volume 2 1848 page 164.) But some less obvious differences deserve notice. As
both species are heterostyled, their complete fertilisation depends on insects.
The cowslip is habitually visited during the day by the larger humble-bees
(namely Bombus muscorum and hortorum), and at night by moths, as I have seen in
the case of Cucullia. The primrose is never visited (and I speak after many
years' observation) by the larger humble-bees, and only rarely by the smaller
kinds; hence its fertilisation must depend almost exclusively on moths. There is
nothing in the structure of the flowers of the two plants which can determine
the visits of such widely different insects. But they emit a different odour,
and perhaps their nectar may have a different taste. Both the long-styled and
short-styled forms of the primrose, when legitimately and naturally fertilised,
yield on an average many more seeds per capsule than the cowslip, namely, in the
proportion of 100 to 55. When illegitimately fertilised they are likewise more
fertile than the two forms of the cowslip, as shown by the larger proportion of
their flowers which set capsules, and by the larger average number of seeds
which the capsules contain. 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.