Thrips, flowers which it was intended to fertilise with their own pollen
may sometimes have been afterwards crossed with pollen brought by these
insects from another flower on the same plant; but as we shall hereafter
see, a cross of this kind does not produce any effect, or at most only a
slight one. When two or more plants were placed near one another under
the same net, as was often done, there is some real though not great
danger of the flowers which were believed to be self-fertilised being
afterwards crossed with pollen brought by Thrips from a distinct plant.
I have said that the danger is not great because I have often found that
plants which are self-sterile, unless aided by insects, remained sterile
when several plants of the same species were placed under the same net.
If, however, the flowers which had been presumably self-fertilised by me
were in any case afterwards crossed by Thrips with pollen brought from a
distinct plant, crossed seedlings would have been included amongst the
self-fertilised; but it should be especially observed that this
occurrence would tend to diminish and not to increase any superiority in
average height, fertility, etc., of the crossed over the self-fertilised
plants.

As the flowers which were crossed were never castrated, it is probable
or even almost certain that I sometimes failed to cross-fertilise them
effectually, and that they were afterwards spontaneously
self-fertilised. This would have been most likely to occur with
dichogamous species, for without much care it is not easy to perceive
whether their stigmas are ready to be fertilised when the anthers open.
But in all cases, as the flowers were protected from wind, rain, and the
access of insects, any pollen placed by me on the stigmatic surface
whilst it was immature, would generally have remained there until the
stigma was mature; and the flowers would then have been crossed as was
intended. Nevertheless, it is highly probable that self-fertilised