158 
DARWINISM 
CHAP. 
But an insect visiting, first, a long-styled plant, would deposit 
the pollen on the stigma of another plant of the same kind if 
it were next visited; and this is probably the reason why the 
wild short-styled plants were found to he 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 
