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.
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