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 of genera; and
all this diversity appears to have arisen for a purpose which has been
obtained by many other, and apparently simpler, changes of structure or
of function, in other plants.
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