That wonderfully accurate observer, Sprengel, who first showed how
important a part insects play in the fertilisation of flowers, called
his book 'The Secret of Nature Displayed;' yet he only occasionally saw
that the object for which so many curious and beautiful adaptations have
been acquired, was the cross-fertilisation of distinct plants; and he
knew nothing of the benefits which the offspring thus receive in growth,
vigour, and fertility. But the veil of secrecy is as yet far from
lifted; nor will it be, until we can say why it is beneficial that the
sexual elements should be differentiated to a certain extent, and why,
if the differentiation be carried still further, injury follows. It is
an extraordinary fact that with many species, flowers fertilised with
their own pollen are either absolutely or in some degree sterile; if
fertilised with pollen from another flower on the same plant, they are
sometimes, though rarely, a little more fertile; if fertilised with
pollen from another individual or variety of the same species, they are
fully fertile; but if with pollen from a distinct species, they are
sterile in all possible degrees, until utter sterility is reached. We
thus have a long series with absolute sterility at the two ends;--at one
end due to the sexual elements not having been sufficiently
differentiated, and at the other end to their having been differentiated
in too great a degree, or in some peculiar manner.
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