y the wild short-styled plants were found to be almost always
most productive of seed, since they must be all fertilised by the other
form, whereas the long-styled plants might often be fertilised by their
own form. The whole arrangement, however, ensures cross-fertilisation;
and this, as Mr. Darwin has shown by copious experiments, adds both to
the vigour and fertility of almost all plants as well as animals.

Besides the primrose family, many other plants of several distinct
natural orders present similar phenomena, one or two of the most curious
of which must be referred to. The beautiful crimson flax (Linum
grandiflorum) has also two forms, the styles only differing in length;
and in this case Mr. Darwin found by numerous experiments, which have
since been repeated and confirmed by other observers, that each form is
absolutely sterile with pollen from another plant of its own form, but
abundantly fertile when crossed with any plant of the other form. In
this case the pollen of the two forms cannot be distinguished under the
microscope (whereas that of the two forms of Primula differs in size and
shape), yet it has the remarkable property of being absolutely powerless
on the stigmas of half the plants of its own species. The crosses
between the opposite forms, which are fertile, are termed by Mr. Darwin
"legitimate," and those between similar forms, which are sterile,
"illegitimate"; and he remarks that we have here, within the limits of
the same species, a degree of sterility which rarely occurs except
between plants or animals not only of different _species_ but of
different _genera_.

But there is another set of plants, the trimorphic, in which the styles
and stamens have each three forms--long, medium, and short, and in these
it is possible to have eighteen different crosses. By an elaborate
series of experiments it was shown that the six legitimate unions--that
is, when a plant was fertilised by pollen from stamens of length
corresponding to that of its style in the two other forms--were all
abundantly fertile; while the twelve illegitimate unions, when a plant
was fertilised by pollen from stamens of a different length from its
own style, in any of the three forms, were either comparatively or
wholly sterile.[52]

We have here a wonderful amount of constitutional difference of the
reproductive organs within a single species, greater than usually occurs
within the numerous distinct species of a genus or group