stock. Increased size or greater brilliancy of the flower, more abundant
nectar, sweeter odour, or adaptations for more effectual
cross-fertilisation would all be preserved, and thus would be initiated
some form of specialisation for insect agency in cross-fertilisation;
and in every different species so circumstanced the result would be
different, depending as it would on many and complex combinations of
variation of parts of the flower, and of the insect species which most
abounded in the district.

Species thus favourably modified might begin a new era of development,
and, while spreading over a somewhat wider area, give rise to new
varieties or species, all adapted in various degrees and modes to secure
cross-fertilisation by insect agency. But in course of ages some change
of conditions might prove adverse. Either the insects required might
diminish in numbers or be attracted by other competing flowers, or a
change of climate might give the advantage to other more vigorous
plants. Then self-fertilisation with greater means of dispersal might be
more advantageous; the flowers might become smaller and more numerous;
the seeds smaller and lighter so as to be more easily dispersed by the
wind, while some of the special adaptations for insect fertilisation
being useless would, by the absence of selection and by the law of
economy of growth, be reduced to a rudimentary form. With these
modifications the species might extend its range into new districts,
thereby obtaining increased vigour by the change of conditions, as
appears to have been the case with so many of the small flowered
self-fertilised plants. Thus it might continue to exist for a long
series of ages, till under other changes--geographical or biological--it
might again suffer from competition or from other adverse circumstances,
and be at length again confined to a limited area, or reduced to very
scanty numbers.

But when this cycle of change had taken place, the species would be very
different from the original form. The flower would have been at one time
modified to favour the visits of insects and to secure
cross-fertilisation by their aid, and when the need for this passed
away, some portions of these structures would remain, though in a
reduced or rudimentary condition. But when insect agency became of
importance a second time, the new modifications would start from a
different or more advanced basis, and thus a more complex result might
be produced.