(12/7. 'Des Varieties' 1865
page 30.)
This extraordinary diversity in the means for favouring or preventing
cross- and self-fertilisation in closely allied forms, probably depends
on the results of both processes being highly beneficial to the species,
but directly opposed in many ways to one another and dependent on
variable conditions. Self-fertilisation assures the production of a
large supply of seeds; and the necessity or advantage of this will be
determined by the average length of life of the plant, which largely
depends on the amount of destruction suffered by the seeds and
seedlings. This destruction follows from the most various and variable
causes, such as the presence of animals of several kinds, and the growth
of surrounding plants. The possibility of cross-fertilisation depends
mainly on the presence and number of certain insects, often of insects
belonging to special groups, and on the degree to which they are
attracted to the flowers of any particular species in preference to
other flowers,--all circumstances likely to change. Moreover, the
advantages which follow from cross-fertilisation differ much in
different plants, so that it is probable that allied plants would often
profit in different degrees by cross-fertilisation. Under these
extremely complex and fluctuating conditions, with two somewhat opposed
ends to be gained, namely, the safe propagation of the species and the
production of cross-fertilised, vigorous offspring, it is not surprising
that allied forms should exhibit an extreme diversity in the means which
favour either end.
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